Back-arc basins are key sites for oceanic lithosphere formation and consumption at convergent plate boundaries,and their formation and subduction processes can be highly variable.The tectonic setting and evolution of ...Back-arc basins are key sites for oceanic lithosphere formation and consumption at convergent plate boundaries,and their formation and subduction processes can be highly variable.The tectonic setting and evolution of the Meso-Tethys Shiquanhe-Jiali ophiolite sub-belt(SJO sub-belt)within BangongNujiang Suture Zone(BNSZ),central xizang,are disputed for the complex rock composition and ages.In this paper,we present geochronology,geochemistry and field observations on the Shiquanhe ophiolite,providing a representative ophiolite example in the western end of SJO.Based on investigation of the petrogenesis and tectonic setting of different rock types,combined with the U-Pb dating,we propose a twostage subduction model for explaining the tectonic evolution of SJO as well as the wither away of a backarc basin.Geochemical and geochronological data indicate that the ca.183 Ma LAN(north of Lameila)gabbros formed in the forearc setting and represent the early-stage subduction of the Bangong MesoTethys.This subduction induced the back-arc spreading recorded in the ca.170 Ma gabbros and lower pillow basalts of PL-SDN(Pagelizanong-Shiquanhe Dam Nan)ophiolitic fragments in the Shiquanhe ophiolite.The basaltic lavas overlying the lower basalts,represented by the ca.168–164 Ma diabasic and boninite dikes have forearc characteristics,and they represent the back-arc basin subduction initiation at a late stage.This work thus recovered the multiple tectonic evolution of SJO sub-belt and emphasise the importance of the back-arc basin subduction in the evolution of ancient oceans.展开更多
High-Mg andesite/diorite(HMA)is useful for identifying subduction-related processes in orogenic belts,including the identification of ophiolites formed in suprasubduction zone(SSZ)environments.The E'rentaolegai hi...High-Mg andesite/diorite(HMA)is useful for identifying subduction-related processes in orogenic belts,including the identification of ophiolites formed in suprasubduction zone(SSZ)environments.The E'rentaolegai high-Mg diorite from the Diyanmiao ophiolite in central Inner Mongolia,North China,has been investigated revealing low-K tholeiitic-calc-alkaline characteristics and have SiO_(2)contents of 53.44-54.92 wt%,MgO contents of 8.44-9.54 wt%,and Mg~#of 54.35-57.60,with variable Fe_(2)O_(3)(7.51-8.61 wt%),Al_(2)O_(3)(11.95-15.09 wt%),and Na_(2)O(3.42-3.94 wt%)contents,low K_(2)O(0.34-0.97 wt%),TiO_(2)(0.35-0.67 wt%),and P2O5(0.12-0.15 wt%)contents,and high Ni(43-193 ppm)and Cr(189-556 ppm)contents.Samples collected have low total rare earth element(REE)contents(30.58-77.80 ppm),with flat or slightly right-dipping REE patterns(La_(N)/Yb_(N)=2.19-3.11)and a lack of pronounced Eu anomalies.The samples are also enriched in large-ion lithophile elements(LILEs,e.g.,K,Rb,Ba,U,and Sr)and depleted in high field strength elements(e.g.,Ta,Nb,Ti,and P).The E'rentaolegai high-Mg diorite has characteristics typical of HMA,and are similar to those of sanukites from the Setouchi Arc in SW Japan.They also display high positiveε_(Nd)(t)values(+6.32 to+7.80),comparable to the values of their host rocks.Petrogenetic analyses suggest that the E'rentaolegai HMA was probably formed by the interaction of partial melts and aqueous fluids from subducted sediments with mantle peridotite.Zircon U-Pb dating reveals that the high-Mg diorite crystallized at 313.6±2.4 Ma,i.e.,late Carboniferous.Combining our data with the temporal and spatial distribution of the Diyanmiao SSZ-type ophiolite,we propose that the eastern Paleo-Asian Ocean had not closed by the late Carboniferous,but intra-oceanic subduction was ongoing.A new model of the initiation of subduction in the eastern PAO during the late Paleozoic.展开更多
Fragments of Proto-Tethyan oceanic lithosphere are well-preserved along the southern belt of the North Qilian suture,and the origin and emplacement of these ophiolites have become subjects of intense debate.In this st...Fragments of Proto-Tethyan oceanic lithosphere are well-preserved along the southern belt of the North Qilian suture,and the origin and emplacement of these ophiolites have become subjects of intense debate.In this study,we integrate field observations,mineralogical and geochemical analyses,zircon U-Pb dating,and isotopic data to investigate the Yanglong ophiolite.The Yanglong ophiolitic rocks are found as tectonic slices resting on the Neoproterozoic sedimentary and volcanic rocks.These rocks are composed of Cambrian serpentinized peridotite,gabbro,dolerite,and rodingite.The spinels in the serpentinized peridotites have variable Cr^(#)values(21,38-46,and 59-61)and display affinity to those in abyssal and forearc peridotites.The dolerites show slight enrichment in Th and have elevated(La/Sm)_(N) ratios(1.19-2.01),indicating a subduction-related geochemical affinity.The Yanglong ophiolitic rocks have positive zirconεHf(t)values(+10.3 to+18.4)and whole-rockεNd(t)values(+5.3 to+6.7)indicating derivation from partial melting of a depleted mantle source.These results,together with the regional geology,collectively suggest that the Yanglong ophiolite was generated in a forearc setting during the Early Cambrian northward intra-oceanic subduction.It was emplaced onto the Central Qilian Block during the subsequent arc-continent collision,no later than the Early Ordovician.展开更多
The ultramafic massif of Feragen,which belongs to the eastern ophiolitic belt of Norway,has abundant amounts of chromite ores.Recent studies have revealed a complex melt evolution in a supra-subduction zone(SSZ)enviro...The ultramafic massif of Feragen,which belongs to the eastern ophiolitic belt of Norway,has abundant amounts of chromite ores.Recent studies have revealed a complex melt evolution in a supra-subduction zone(SSZ)environment.This study presents new whole-rock major element,trace element,and platinum-group element chemistry to evaluate their petrogenesis and tectonic evolution.Harzburgites have high CaO,Al_(2)O_(3),TiO_(2),MgO,and REE contents corresponding to abyssal peridotites,whereas dunites have low CaO,Al_(2)O_(3),TiO_(2),MgO,and REE contents corresponding to SSZ peridotites.The Cr^(#)and TiO_(2) of chromian spinels in the harzburgites suggest as much as about 15%–20%melting and the dunites are more depleted with>40%melting.The harzburgites and the dunites and high-Cr chromitites represent,respectively,the products of low-degree partial melting in a back-arc setting,and the products of melt-rock interaction in a SSZ environment.The calculated fO_(2) values for dunites and high-Cr chromitites(-0.17–+0.23 and+2.78–+5.65,respectively and generally above the FMQ buffer)are also consistent with the interaction between back-arc ophiolites with oxidized boninitic melts in a SSZ setting.展开更多
The debate regarding whether the Yarlung-Zangbo ophiolite(YZO)on the south of the Qinghai-Tibet Plateau,formed in a mid-ocean ridge(MOR)or a supra-subduction zone(SSZ)setting has remained unresolved.Here we present pe...The debate regarding whether the Yarlung-Zangbo ophiolite(YZO)on the south of the Qinghai-Tibet Plateau,formed in a mid-ocean ridge(MOR)or a supra-subduction zone(SSZ)setting has remained unresolved.Here we present petrological,mineralogical,and geochemical data associated with modeling melting geodynamics of the mantle peridotites from the Purang ophiolite in the western segment of the Yarlung-Zangbo Suture Zone(YZSZ)to explore its tectonic environment.The Purang lherzolites are characterized by the protogranular texture and have abyssal-peridotite-like mineral compositions,including low Cr^(#)(20-30)and TiO_(2)contents(<0.1wt%)in spinel,high Al_(2)O_(3)(2.9wt%-4.4wt%)and CaO(1.9wt%-3.7wt%)contents in orthopyroxene and LREE-depletion in clinopyroxene.Compositions of these lherzolites can be modeled by~11%dynamic melting of the DMM source with a small fraction of melt(~0.5%)entrapped within the source,a similar melting process to typical abyssal peridotites.The Purang harzburgites are characterized by the porphyroclastic texture and exhibit highly refractory mineral compositions such as high spinel Cr^(#)(40-68),low orthopyroxene Al_(2)O_(3)(<2.2wt%)and CaO(<1.1wt%)contents.Clinopyroxenes in these harzburgites are enriched in Sr(up to 6.0 ppm)and LREE[(Ce)N=0.02-0.4],but depleted in Ti(200 ppm,on average)and HREE[(Yb)N<2].Importantly,the more depleted samples tend to have higher clinopyroxene Sr and LREE contents.These observations indicate an open-system hydrous melting with a continuous influx of slab fluid at a subduction zone.The modeled results show that these harzburgites could be formed by 19%-23%hydrous melting with the supply rate of slab fluid at 0.1%-1%.The lower clinopyroxene V/Sc ratios in harzburgites than those in lherzolites suggest a high oxidation stage of the melting system of harzburgites,which is consistent with a hydrous melting environment for these harzburgites.It is therefore concluded that the Purang ophiolite has experienced a transformation of tectonic setting from MOR to SSZ.展开更多
Neotethyan ophiolites evolved in multiple seaways separated by Gondwana–derived ribbon continents within an eastward widening, latitudinal oceanic realm(Neotethys) throughout the Mesozoic. Opening and closure of thes...Neotethyan ophiolites evolved in multiple seaways separated by Gondwana–derived ribbon continents within an eastward widening, latitudinal oceanic realm(Neotethys) throughout the Mesozoic. Opening and closure of these seaways were diachronous events, resulting in E–W variations in the timing of oceanic crust production and ophiolite emplacement. The Neotethyan ophiolites are highly diverse in their crustal–mantle structures and compositions, isotopic fingerprints, and sedimentary cover types, pointing to major differences in their mantle melt sources and tectonic and paleogeographic settings of magmatic construction(Dilek and Furnes, 2019). The Jurassic Western Alpine and Ligurian ophiolites in Europe and their counterparts in southern and northern Iberia formed in a narrow basin(Western Tethys) that developed between Europe and North Africa–Adria–Iberia. Their peridotites represent exhumed, continental lithospheric mantle, and the ophiolites display a Hess–type oceanic crustal architecture with MORB geochemical signatures(Dilek and Furnes, 2011). All these ophiolites were incorporated into continental margins from the downgoing oceanic lithosphere of the Western Tethys. Triassic, Jurassic and Cretaceous ophiolites east of Adria formed in different Neotethyan seaways(Dilek et al., 1990), and their rift–drift, seafloor spreading and suprasubduction zone(SSZ) magmatic construction involved multiple episodes of melting, depletion and refertilization of previously or actively subduction metasomatized mantle sources. Deep mantle recycling processes through subduction zone tectonics and/or plume activities played a major role in their melt evolution, and in the incorporation of mantle transition zone(MTZ) materials into their peridotites(Fig. 1;Dilek and Yang, 2018;Xiong et al., 2019). Tectonic mélanges structurally beneath these ophiolites include Permo–Triassic, OIB–type extrusive rocks, indicating that the initial dismantling of the Pangea supercontinent that led to the opening of the Triassic and Jurassic ocean basins within the Neotethyan realm was associated with plume magmatism(Dilek, 2003 a;Yang and Dilek, 2015). This plume signature is absent in the Permo–Triassic magmatic record of the Western Tethys to the west. The Cretaceous ophiolites around the Arabia(Dilek et al., 1990;Dilek and Delaloye, 1992;Dilek and Eddy,1992) and India sub-continents(Fareeduddin and Dilek, 2015) occur discontinuously along a ~9000-km-long belt from SW Anatolia to SE Tibet and Indo-China. The majority of these ophiolites have a Penrose–type oceanic crustal architecture(Dilek, 2003 b) and display SSZ geochemical affinities, complete with a MORB–IAT–BON progression of their chemo-stratigraphy(Fig. 1;Dilek and Thy, 1998;Dilek et al., 1999;Dilek and Furnes, 2014;Saccani et al., 2018). They evolved above a N–dipping, Trans–Tethyan subduction–accretion system that was situated in sub-tropical latitudes within the Neotethyan realm. The Trans–Tethyan subduction–accretion system was segmented into two major domains(Western and Eastern domains) by the NNE–SSW–oriented, sinistral Chaman–Omach–Nal transform fault plate boundary. This Cretaceous intraoceanic arc–trench system was analogous to the modern Izu–Bonin–Mariana(IBM) and Tonga arc–trench systems in the western Pacific in terms of its size. Diachronous collisions of the Arabia and India sub-continents with this segmented Trans-Tethyan arc–trench system resulted in the southward emplacement of the SSZ Neotethyan ophiolites onto their passive margins in the latest Mesozoic(Dilek and Furnes, 2019). A separate N–dipping subduction system, dipping beneath Eurasia to the north during much of the Jurassic and Cretaceous, was consuming the Neotethyan oceanic lithosphere and was responsible for the construction of a composite magmatic arc belt extending discontinuously from Southern Tibet to Northern Iran. Slab rollback along this northern subduction system produced locally developed forearc–backarc oceanic lithosphere that was subsequently collapsed into the southern margin of Eurasia. The existence of these two contemporaneous, Ndipping subduction systems within Neotethys led to its rapid contraction and the fast convergence of India towards Eurasia during the late Mesozoic–early Cenozoic(Dilek and Furnes, 2019). It was the collision with Eurasia of the India sub-continent with the accreted ophiolites around its periphery in the Late Paleogene that produced the Himalayan orogeny.展开更多
In the Galicia Region of the NW Iberian Massif several allochthonous complexes(Cabo Ortegal,Órdenes and Malpica-Tui)contain a rootless Variscan suture that can be traced along the belt,from Iberia to the Bohemian...In the Galicia Region of the NW Iberian Massif several allochthonous complexes(Cabo Ortegal,Órdenes and Malpica-Tui)contain a rootless Variscan suture that can be traced along the belt,from Iberia to the Bohemian Massif in Central Europe.Within these allochthonous complexes are several ophiolite zones bounded by two different continental terranes.There exist in NW Iberia two different ophiolite groups with different chemical compositions,isotopic signatures and structural positions.The Bazar and Vila de Cruces ophiolites,characterized by c.500 Ma protolith ages,represent the Lower Group,whereas the Careón,Purrido and Moeche ophiolites containing 395 Ma maficultramafic sequences represent the Upper Group.This younger group constitutes the most widespread ophiolites in the Variscan Belt.A thick serpentinite mélange(Somozas Mélange)occurring at the base of the Cabo Ortegal Complex also belongs to the ophiolite zones of the Variscan suture.In this paper we describe the Galician ophiolites of the Variscan suture and discuss their tectonic setting of formation.We interpret the generation of the Galician ophiolites within the geodynamic and paleogeographic evolution of the Rheic Ocean and the Pangea supercontinent.展开更多
The Zedang and Luobusa ophiolites are located in the eastern section of the Yalung Zangbo ophiolite belt,and they share similar geological tectonic setting and age.Thus,an understanding of their origins is very import...The Zedang and Luobusa ophiolites are located in the eastern section of the Yalung Zangbo ophiolite belt,and they share similar geological tectonic setting and age.Thus,an understanding of their origins is very important for discussion of the evolution of the Eastern Tethys Ocean.There is no complete ophiolite assemblage in the Zedang ophiolite.The Zedang ophiolite is mainly composed of mantle peridotite and a suite of volcanic rocks as well as siliceous rocks,with some blocks of olivinepyroxenite.The mantle peridotite mainly consists of Cpx-harzburgite,harzburgite,some lherzolite,and some dunite.A suite of volcanic rocks is mainly composed of caic-aikaline pyroclastic rocks and secondly of tholeiitic pillow lavas,basaltic andesites,and some boninitic rocks with a lower TiO2 content （TiO2 ＜ 0.6％）.The pyroclastic rocks have a LREE-enriched REE pattern and a LILE-enriched （compared to HFSE） spider diagram,demonstrating an island-arc origin.The tholeiitic volcanic rock has a LREE-depleted REE pattern and a LILE-depleted （compared to HFSE） spider diagram,indicative of an origin from MORB.The boninitic rock was generated from fore-arc extension.The Luobusa ophiolite consists of mantle peridotite and mafic-ultramaflc cumulate units,without dike swarms and volcanic rocks.The mantle peridotite mainly consists of dunite,harzburgite with low-Opx （Opx ＜ 25％）,and harzburgite （Opx ＞ 25％）,which can be divided into two facies belts.The upper is a dunite-harzburgite （Opx ＜ 25％） belt,containing many dunite lenses and a large-scale chromite deposit with high Cr203; the lower is a harzburgite （Opx ＞25％） belt with small amounts of dunite and lherzolite.The Luobusa mantle peridotite exhibits a distinctive vertical zonation of partial melting with high melting in the upper unit and low melting in the lower.Many mantle peridotites are highly depleted,with a characteristic U-shaped REE pattern peculiar to fore-arc peridotite.The Luobusa cumulates are composed of wehrlite and olivine-pyroxenite,of the P-P-G ophiolite series.This study indicates that the Luobusa ophiolite was formed in a fore-arc basin environment on the basis of the occurrence of highly depleted mantle peridotite,a high-Cr2O3 chromite deposit,and cumulates of the P-P-G ophiolite series.We conclude that the evolution of the Eastern Tethys Ocean involved three stages：the initial ocean stage （formation of MORB volcanic rock and dikes）,the forearc extension stage （formation of high-Cr203 chromite deposits and P-P-G cumulates）,and the islandarc stage （formation of caic-alkaline pyroclastic rocks）.展开更多
One of the major topics of debate in ophiolite geology is the original tectonic setting of ophiolites. New studies show that most ophiolites are formed more frequently in a suprasubduction zone(SSZ) environment and th...One of the major topics of debate in ophiolite geology is the original tectonic setting of ophiolites. New studies show that most ophiolites are formed more frequently in a suprasubduction zone(SSZ) environment and that only a very small number of ophiolites have formed in an oceanic range(MOR). The Masirah ophiolite is one of the few oceanic ridge ophiolites that have been preserved, and the evidence that was formed in a subduction environment is missing(Moseley and Abbotts 1979, Dilek and Furnes, 2011;Rollinson, 2017). Masirah Island, the Batain and Ras Madrah areas of eastern Oman are almost entirely composed of a well-developed ophiolite, known as the Masirah ophiolite(Fig. 1), which is, however, completely unrelated to the nearby Semail Ophiolite in the northern Oman Mountains(Fig. 2). The Masirah ophiolite is Jurassic in age and represents oceanic lithosphere derived from the Indian Ocean, but is about 15–20 Myr later than emplacement of midCretaceous Semail ophiolite in northern Oman. The presence of basaltic to rhyolitic lavas of calc-alkaline affinity and boninites in the lava sequence of the Semail ophiolite led several researchers to propose a back-arc basin model for this ophiolite(e.g. Tamura and Arai, 2006;Godard et al., 2008;Rollinson and Adetunji, 2015). The Masirah Ophiolite shows close affinities with MORB peridotites in general. Most of the olivine from the Masirah harzburgites show Fo contents that are similar to those of olivine from MORB. Both pyroxenes in these harzburgites have similar Mg# values, Al2O3 and Cr2O3 contents to those of pyroxenes from MORB peridotites. The observed primitive mantlenormalized REE patterns showing enrichment in LREEs indicate that the Masirah peridotites have been modified by fluids or melts enriched in LREEs in a MORB environment. Podiform chromitites housed in ophiolites today interpreted as magmatic deposits formed during the reaction of molten rock in environments spike in the middle of the ocean(MOR) or suprasubduccion zone(SSZ)(Arai and Matsukage, 1998;Rollinson and Adetunji, 2015). The Masirah chromitites has a mineral chemistry similar to the mineral chemistry of chromite crystallized from MOR magmas. The Cr# values of chromite in the Masirah chromatite are similar to those of MOR peridotites. These findings suggest that the ultramafic and mafic cumulate rock assemblages overlying the upper mantle peridotites in the Masirah ophiolite represent the products of magma evolution in a MOR initiation stage within the proto Indian Ocean. Coexisting high-and low-Cr# associations of chromitite and dunite have been found in the Semail ophiolite, which illustrates the common situation of ophiolites having both SSZ and MOR geochemical signatures. Cr# varies from 40–60 for shallow chromite bodies, and over the range 70–80 for the deep locations. This diversity of chromitite types suggests two stages of magmatic activity were responsible for the chromitite genesis, in response to a switch of tectonic setting. The first is residual from lower degree, partial melting of peridotite, which produced lowCr# chromitites at the Moho transition zone, possibly in a midocean-ridge setting. The second chromitite-forming event involves higher degree partial melting, which produced high-Cr# discordant chromitite in the upper mantle, possibly in a suprasubduction zone setting. Assemblages of mono-and poly-phase silicate inclusions(including olivine, orthopyroxene, clinopyroxene, amphibole, phlogopite, serpentine, native Fe, FeO, alloy, sulfide, calcite, laurite, celestine and halite) within chromite have been observed in the low Cr# podiform chromitites from the Semail and Masirah ophiolites. The existence of hydrous silicate inclusions in the chromite calls for a role of hydration during chromite genesis. High-T bright green hornblende–edenite included in the chromites is evidence of the introduction of water in the magma at the end of the chromite crystallization. Such paragenesis points to the presence of hydrous fluids during the activity of the shear bands.展开更多
We present an overview of the internal structure of the ophiolite massifs along the Yarlung Zangbo suture zone(YZSZ)in southern Tibet with a focus on the geochemical character and tectonic evolution of the Ocean Islan...We present an overview of the internal structure of the ophiolite massifs along the Yarlung Zangbo suture zone(YZSZ)in southern Tibet with a focus on the geochemical character and tectonic evolution of the Ocean Island Basalt(OIB)and mafic alkaline rock assemblages associated with these ophiolites.The Jurassic–early Cretaceous lavas,massive diabase and gabbroic rocks are either tectonically intercalated with the early Cretaceous,subduction-influenced ophiolitic units,or occur as thrust sheets or blocks with an early Cretaceous mélange and in a Jurassic-Cretaceous flysch unit structurally beneath these ophiolites.They display uniform chondrite-normalized REE patterns with light rare earth element(LREE)enrichment and heavy rare earth element(HREE)depletion,no obvious Eu anomalies or negative Nb,Ta and Ti anomalies,and primitive mantle normalized trace element patterns with significant large-ion lithophile element(LILE)enrichment,similar to those of modern OIB and the Hawaiian alkaline basalts.These mafic alkaline rock assemblages represent OIB-and Plume-type(P-type)oceanic crustal rocks(with no subduction influence)that formed from magmas produced by partial melting of plume–metasomatized asthenospheric mantle source during the early stages of the opening of a Neotethyan seaway between Proto-India and Eurasia.展开更多
We present an overview of the geology,geochemistry and petrogenesis of continental margin ophiolites(CMO),which represent the lithospheric remnants of riftgenerated paleo ocean–continent transition zones(OCTZ)in orog...We present an overview of the geology,geochemistry and petrogenesis of continental margin ophiolites(CMO),which represent the lithospheric remnants of riftgenerated paleo ocean–continent transition zones(OCTZ)in orogenic belts.The igneous stratigraphy and geochemical signatures of Neotethyan CMOs reflect the extent of geochemical heterogeneity,partial melting degrees,and melt evolution patterns in the continental lithospheric mantle prior to the onset of seafloor spreading in rifted margins.Basaltic rocks of the Jurassic CMOs in the External Ligurian units of the Northern Apennines have N-MORB and G-MORB affinities with strong HREE/MREE depletion,and represent the products of partial melting of a heterogeneous subcontinental lithospheric mantle containing small volumes of garnet pyroxenite layers.These extrusive rocks were erupted directly on the exhumed fertile spinel lherzolites of Adria during its OCTZ evolution.Volcanic rocks of the Triassic CMOs in the Albanide-Hellenide orogenic belt are represented by calc-alkaline suites;alkaline basalts and subordinate trachybasalts,trachyandesites,and trachytes;transitional to sub-alkaline plume-type PMORB basalts;sub-alkaline enriched,E-MORB basalts;and,sub-alkaline N-MORB basalts.Upper mantle peridotites are not exposed.展开更多
The tectonic setting of podiform chromitite formation still remains highly debated. There is a close correlation between tectonic settings and oxygen fugacity(fO2)(e.g., Ballhaus, 1993;Dare et al., 2009;Parkinson and ...The tectonic setting of podiform chromitite formation still remains highly debated. There is a close correlation between tectonic settings and oxygen fugacity(fO2)(e.g., Ballhaus, 1993;Dare et al., 2009;Parkinson and Arculus, 1999). Here we present results of fO2 of chromites determined by M?ssbauer spectroscopy from both the Luobusha and Dazhuqu areas along Yarlung Zangbo suture zone, Southern Tibet. The fO2 values(-1.02~0.04 log units against the FMQ buffer) and Cr#(22~54%) in chromites from lherzolites and harzburgites of both areas are similar to those of abyssal peridotites, indicating that they may be residues after partial melting at spreading centers. However, both dunite envelopes and chromitites from Luobusha have high fO2 values(0.04~2.25 log units) and Cr#(73~84%), showing an affinity to boninitic melts, and thus form in a suprasubduction zone. Dazhuqu dunites show diverse fO2 values(-0.22~2.19 log units) and Cr#(22~82%), indicating that they form in distinct settings. Chromitites and chromite dunites from Dazhuqu have low fO2 values(-0.3~0.71 log units) and Cr#(16~63%), both of which are similar to those of MORB-like basalts, inferring that they form in an extensional setting. Both high-Cr and high-Al chromitites from other typical podiform chromite ore deposits, such as Kempirsai, Oman, and Albania ophiolites, also show high fO2 values(e.g., Chashchukhin and Votyakov, 2009;Melcher et al., 1997;Quintiliani et al., 2006;Rollinson and Adetunji, 2015), while the distribution-limited small chromitites and chromite dunites from Dazhuqu exhibit low fO2 values. The phenomenon infers that the suprasubduction zone is more beneficial to the formation of podiform chromitites.展开更多
The Qilian orogenic belt is the northernmost orogen of the Tethyan domain and connects the Altaids to the north. It contains an assembly of Precambrian micro-continental fragments, early Paleozoic island arcs, accreti...The Qilian orogenic belt is the northernmost orogen of the Tethyan domain and connects the Altaids to the north. It contains an assembly of Precambrian micro-continental fragments, early Paleozoic island arcs, accretionary complexes, ophiolites, forearc and backarc basins, and high-pressure(HP) metamorphic rocks, indicating a long history of accretionary processes. Spatially, this orogen is adjacent to the Tarim, Qaidam, and North China blocks, which also extends into accretionary orogenic belts to the east and SW such as the Qinling and Kunlun belts. Abundant ophiolites in this orogen record the closure of an early Tethyan Ocean and amalgamations between micro-continents of North China, Qaidam, and Tarim. Thus, the ages and tectonic settings of these ophiolites within this belt provide important information regarding evolution of the Proto-Tethys Ocean and assembly of micro-continental blocks, which aids understanding of the spatial and temporal relationship of this orogen within the Tethyan realm. Dismembered ophiolites sporadically crop out along the northern margin of the South Qilian belt, and, from east to west, are locally referred to as the Lajishan, Gangcha, Muli, and Dadaoerji ophiolites. Much attention had been paid to these ophiolites, and several competing models for the tectonic evolution of this belt have been suggested. Considerable disagreement remains in respect of the temporal and spatial framework of the Qilian Orogen and details such as timing of subduction(s) and associated polarities, early collision events, and final closure of oceanic basins. In particular, the formation age and tectonic setting of Muli arc-ophiolite complex remains unknown, which limits understanding of the tectonics of the South Qilian belt and the history of the Proto-Tethys Ocean. The Muli arc-ophiolite complex is distributed over 20 km^2 west of the township of Muli in the western segment of the South Qilian Belt and consists of serpentinite, dunite, cumulate gabbro, basalt, plagiogranite, and chert. Field mapping results demonstrate that these units have been largely destroyed by faulting and generally occur as blocks/slices. They are tectonically interlayered with Upper Ordovician – Lower Silurian siliciclastic turbidite. Arc-ophiolite rocks are intruded by 470–450 Ma subduction-related granitoid plutons and are unconformably overlain by shallow marine to non-marine sediments of Permian-Jurassic age. Basalts show typical subduction-related calc-alkaline geochemical affinity, representing portions of an island arc. Geochemical results for plagiogranites and spinels from serpentinite demonstrate that the Muli arc-ophiolite complex represents a super-subduction zone(SSZ)-type ophiolite. U-Pb zircon data indicate formation associated with southward subduction of the Proto-Tethyan Ocean during a short interval between 539–522 Ma. Voluminous Late Ordovician-Early Silurian deep-water marine siliciclastic and volcaniclastic turbidites and volcanic arc rocks are exposed to the south of the Muli arc-ophiolite complex, whereas fluvial coarse-grained sandstones and conglomerates unconformably overlie the Cambrian-Middle Ordovician ophiolite-arc systems in the eastern South Qilian Belt. These indicate that closure of the Proto-Tethys Ocean was diachronous during the early Paleozoic.展开更多
The Late Jurassic Monviso ophiolite in the Western Alps is a multiply deformed,eclogite-facies metaophiolite that represents a remnant of the Alpine Tethyan oceanic lithosphere.The recent recognition of a pre-Alpine d...The Late Jurassic Monviso ophiolite in the Western Alps is a multiply deformed,eclogite-facies metaophiolite that represents a remnant of the Alpine Tethyan oceanic lithosphere.The recent recognition of a pre-Alpine detachment fault in the Lower Tectonic Unit of this ophiolite has led to the discovery of an oceanic core complex,which developed during the initial stages of the tectonic evolution of the Alpine Tethys.The NNWstriking,20–25-km-long shear zone(Baracun Shear Zone)contains ductilely to cataclastically deformed blocks and clasts of Fe-Ti and Mg-Al metagabbros in a matrix made of mylonitic serpentinite and talc-chlorite schist with high Ni–Cr concentrations and high Cl contents.Intensely sheared ophicarbonate rocks and brecciated serpentinite within this shear zone are deformed by the Alpine-phase S1 foliation and D2 folds,providing a critical age constraint for the timing of its formation.Metabasaltic–metasedimentary rocks in the hanging wall increase in thickness away from the shear zone,characteristic of syn-extensional rock sequences in supradetachment basins.展开更多
The recently identified Huashan ophiolitic mélange was considered as the eastern part of the Mianlüe suture in the Qinling orogenic belt. SHRIMP zircon U-Pb geochronology on gabbro from the Huashan ophiolite...The recently identified Huashan ophiolitic mélange was considered as the eastern part of the Mianlüe suture in the Qinling orogenic belt. SHRIMP zircon U-Pb geochronology on gabbro from the Huashan ophiolite and granite intruding basic volcanic rocks indicates crystallization ages of 947±14 Ma and 876±17 Ma respectively. These ages do not support a recently proposed Hercynian Huashan Ocean, but rather favor that a Neoproterozoic suture assemblage （ophiolite） is incorporated into the younger （Phanerozoic） Qinling orogenic belt.展开更多
The subduction of the Bangonghu-Nujiang Meso-Tethys and the collision between the Lhasa and Qiangtang blocks were important events in the growth of the Tibetan crust. However, the timing of collision initiation and cl...The subduction of the Bangonghu-Nujiang Meso-Tethys and the collision between the Lhasa and Qiangtang blocks were important events in the growth of the Tibetan crust. However, the timing of collision initiation and closure timing, as well as nature and structure of the Bangonghu ocean basin, are still poorly constrained. The Lagkor Tso ophiolite, located in the south of Gerze County, Tibet, is one of the most completed ophiolites preserved in the southern side of the Bangonghu- Nujiang suture zone. This study discussed the tectonic evolution of the Bangonghu-Nujiang suture zone as revealed by the Lagkor Tso ophiolite investigated by field investigations, petrology, geochemistry, geochronology and tectonic analysis methods. We present new LA-ICP-MS zircon U-Pb and 39Ar/4~Ar ages for the Lagkor Tso ophiolite, in addition to geochemical and platinum-group element （PGE） data presented for the Lagkor Tso ophiolite in Tibet. It is suggested that the ancient Lagkor Tso oceanic basin split in Middle Jurassic （161.2 ± 2.7 Ma - 165.4 ± 3.5 Ma）, and experienced a second tectonic emplacement during the Early Cretaceous （137.90 ± 6.39 Ma）. The Lagkor Tso ophiolite likely developed in an independent suture zone. The Bangonghu-Nujiang ocean subducted southwards, and the dehydration of the subducting oceanic crust materials caused partial melting of the continental mantle wedge, which formed the second-order expanding center of the obduction dish. This led to inter-arc expansion, followed by the formation of inter-arc and back-arc basins with island arc features, which are represented by ophiolites around the Shiquanhe-Lagkor Tso -Yongzhu region. The tectonic environment presently can be considered to be similar to that of the current Western Pacific, in which a large number of island arc-ocean basin systems are developed.展开更多
The Bayan Gol ophiolite fragment is a portion of the North Tianshan Early Carboniferous ophiolite belt. This ophiolite belt represents a geological record of an Early Carboniferous 'Red Sea type' ocean basin t...The Bayan Gol ophiolite fragment is a portion of the North Tianshan Early Carboniferous ophiolite belt. This ophiolite belt represents a geological record of an Early Carboniferous 'Red Sea type' ocean basin that was developed on the northern margin of the Tianshan Carboniferous-Permian rift system in northwestern China. The late Early Carboniferous Bayan Gol ophiolite suite was emplaced in an Early Carboniferous rift volcano- sedimentary succession of shallow-marine to continental facies (Volcanics Unit). Ophiolitic rocks in the Bayan Gol area comprise ultramafic rocks, gabbros with associated plagiogranite veins, diorite, diabase, pillow basalts and massive lavas. The Early Carboniferous rifting and the opening process of the North Tianshan ocean basin produced mafic magmas in composition of tholeiite and minor amounts of evolved magmas. Compositions of trace elements and Nd, Sr and Pb isotopes reveal the presence of two distinct mantle sources: (1) the Early Carboniferous rift mafic lavas from the Volcanics Unit were generated by a relatively low degree of partial melting of an asthenospheric OIB-type intraplate source; (2) younger (late Early Carboniferous, ~324.8 Ma ago) mafic lavas from the Ophiolite Unit were formed in a relatively depleted MORB-like mantle source, located in the uppermost asthenosphere and then gradually mixed with melts from the asthenospheric OIB-like mantle. A slight interaction between asthenosphere-derived magmas and lithospheric mantle took place during ascent to the surface. Subsequently, the most depleted mafic lavas of the ophiolite assemblage were contaminated by upper-crustal components (seawater or carbonate crust).展开更多
Various combinations of diamond, moissanite, zircon, quartz, corundum, rutile, titanite, almandine garnet, kyanite, and andalusite have been recovered from the Dangqiong peridotites. More than 80 grains of diamond hav...Various combinations of diamond, moissanite, zircon, quartz, corundum, rutile, titanite, almandine garnet, kyanite, and andalusite have been recovered from the Dangqiong peridotites. More than 80 grains of diamond have been recovered, most of which are pale yellow to reddish-orange to colorless. The grains are all 100-200 μm in size and mostly anhedral, but with a range of morphologies including elongated, octahedral and subhedral varieties. Their identification was confirmed by a characteristic shift in the Raman spectra between 1325 cm^-1 and 1333 cm^-1, mostly at 1331.51 cm^-1 or 1326.96 cm^-1. Integration of the mineralogical, petrological and geochemical data for the Dongqiong peridotites suggests a multi-stage formation for this body and similar ophiolites in the Yarlung-Zangbo suture zone. Chromian spinel grains and perhaps small bodies of chromitite crystallized at various depths in the upper mantle, and encapsulated the UHP, highly reduced and crustal minerals. Some oceanic crustal slabs containing the chromian spinel and their inclusion were later trapped in suprasubduction zones（SSZ）, where they were modified by island arc tholeiitic and boninitic magmas, thus changing the chromian spinel compositions and depositing chromitite ores in melt channels.展开更多
The A＇nyemaqen （阿尼玛卿） ophiolite belt along the southern margin of the East Kunlun （昆仑） Mountains marks the suture formed by the closure of paleo-Tethys. The Dur＇ngoi ophiolite in the eastern part of this b...The A＇nyemaqen （阿尼玛卿） ophiolite belt along the southern margin of the East Kunlun （昆仑） Mountains marks the suture formed by the closure of paleo-Tethys. The Dur＇ngoi ophiolite in the eastern part of this belt consists of meta-peridotite, mafic-ultramafic cumulates, sheeted dikes and basaltic lavas. The meta-peridotites consist of dunite, harzburgite, lherzolite, feldspar-bearing lherzolite and garnet-bearing lherzolite and contain residual spinel with Cr# [100×Cr/（Cr＋Al）] ranging from 30 to 57 and Mg# [100×Mg/（Mg＋Fe2＋）] ranging from 50 to 75, indicating an Al- and Mg-rich series. The meta-peridotites have a relatively narrow range of composition with Mg# of 89.2-92.6, Al2O3 contents of （1-4） wt.% and slightly depleted chondrite normalized REE patterns, indicating that they represent relict mantle material that has undergone intermediate to low degrees of partial melting. Garnets in the lherzolite are andradite, enriched in Ca and Fe and depleted in Mg and AI （And=95-97, Pyr=0.3-5, Gro=0-3）, indicating a metamorphic origin. The cumulate rocks mainly consist of dunite, wehrlite, pyroxenite and gabbro. A well-layered gabbro-pyroxenite complex is defined by modal variations in plagioclase and pyroxene. Blocks of garnet-pyroxenite or rodingite are locally present in the meta-peridotites. Garnets in the cumulate rocks are grossular （Gro=69-90, And=9-19, Br=l-12）, also metamorphic origin. The diabase dikes are moderately depleted in LREE [（La/Sm）N=0.5-0.8] and HREE resulting in slightly convex chondrite-normalized patterns with slightly positive Eu anomalies （δEu=1.1-1.3）. The basaltic lavas have REE patterns similar to those of MORB with （La/Sm）N ratios of 0.5-1 and small negative Eu anomalies. They appear to have been derived from a depleted mantle source and to have undergone little or no differentiation during crystallization. SHRIMP U-Pb dating of zircons from the basalts yields 286pb/238U ages of 276-319 Ma （average 308.0±4.9 Ma）. The Dur＇ngoi ophiolite is interpreted as a dismembered fragment of paleo-oceanic crust emplaced during closure of the paleo-Tethyan Ocean basin. Three other suites of oceanic lavas are recognized in the area： island arc volcanic （IAV） rocks, possible back arc basin （BAB） basalts and possible post-collisional volcanic （PCV） and plutonic rocks. The distribution of these rocks suggests north-directed subduction. Opening of the A＇nyemaqen oceanic basin started at least as early as Late Carboniferous （308 Ma） and the basin probably closed during the Early Triassic. The IAV formed in Late Permian （260 Ma）, the BAB in Early-Middle Triassic, and the PCV in Late Triassic. Several large scale, ductile, sinistral strike-slip fault zones, extending hundreds to thousands kilometers, formed along or north of the suture during the Early-Late Triassic, e.g., they are the south margin fault zone of East Kunlun （200-220 Ma）, the Altyn Tagh fault （220-230 Ma）, and the North Qaidam fault zone （240-250 Ma）. These strike-slip faults were probably generated by oblique subduction and closure of the paleo-Tethyan Ocean basin, possibly during exhumation of the subducted plate or uplift of the overriding plate, coincident with post-collisional magmatism.展开更多
The ultrahasic rocks in the Mianxian-Lueyang ophiolite belt are mainly harzburgite and dunite, and exhibit two types of REE distribution pattern: (1) LREE depletion, with remarkable Eu enrichment; (2) slight enrichmen...The ultrahasic rocks in the Mianxian-Lueyang ophiolite belt are mainly harzburgite and dunite, and exhibit two types of REE distribution pattern: (1) LREE depletion, with remarkable Eu enrichment; (2) slight enrichment of LREE, with Eu deficiency. Both gabbros (accumulative gabbros) and diabasic dike swarms show a LREE enrichment but a slight Eu anomaly. The ratios of w (Ti) /w (V), w (Th) /w (Ta). w (Th) /w (Yb) and w (Ta) /w (Yb) indicate that the mid-ocean ridge basalts in this region originated from depleted mantle of asthenosphere, implying an association of MORB-type ophiolite and an ancient ocean basin between Qinling and Yangtze plates during Middle Paleozoic-Early Mesozoic era. The island arc volcanic rocks can be divided into tholeiitic and cafe-alkaline associations, which originated from the upper mantle wedge above slab by partial melting.展开更多
基金supported by the Guangdong Basic and Applied Basic Research Foundation,China(Grant Nos.2024A1515010439,2025A1515010724)National Nature Science Foundation of China(Grant Nos.41972049,41472054,42072229,41977231)+4 种基金Young Innovative Talent Project of Department of Education of Guangdong Province(Natural ScienceGrant No.2022KQNCX184)Natural Research Project of Guangdong Polytechnic of Industry&Commerce(Grant No.2022-ZKT-01)China State Scholarship Fund of visiting scholar(Grant No.20170638507)High-level Talent Special Support Program of Guangdong Polytechnic of Industry&Commerce(Grant No.2023-gc-03).
文摘Back-arc basins are key sites for oceanic lithosphere formation and consumption at convergent plate boundaries,and their formation and subduction processes can be highly variable.The tectonic setting and evolution of the Meso-Tethys Shiquanhe-Jiali ophiolite sub-belt(SJO sub-belt)within BangongNujiang Suture Zone(BNSZ),central xizang,are disputed for the complex rock composition and ages.In this paper,we present geochronology,geochemistry and field observations on the Shiquanhe ophiolite,providing a representative ophiolite example in the western end of SJO.Based on investigation of the petrogenesis and tectonic setting of different rock types,combined with the U-Pb dating,we propose a twostage subduction model for explaining the tectonic evolution of SJO as well as the wither away of a backarc basin.Geochemical and geochronological data indicate that the ca.183 Ma LAN(north of Lameila)gabbros formed in the forearc setting and represent the early-stage subduction of the Bangong MesoTethys.This subduction induced the back-arc spreading recorded in the ca.170 Ma gabbros and lower pillow basalts of PL-SDN(Pagelizanong-Shiquanhe Dam Nan)ophiolitic fragments in the Shiquanhe ophiolite.The basaltic lavas overlying the lower basalts,represented by the ca.168–164 Ma diabasic and boninite dikes have forearc characteristics,and they represent the back-arc basin subduction initiation at a late stage.This work thus recovered the multiple tectonic evolution of SJO sub-belt and emphasise the importance of the back-arc basin subduction in the evolution of ancient oceans.
基金supported by the Ministry of Science and Technology of the People’s Republic of China(Grant No.2022FY101704)the National Natural Science Foundation of China(Grant No.41972061)+1 种基金Science and Technology Innovation Team of Hebei GEO University(30801071)the China Geological Survey(Grant Nos.1212011120701 and 1212011120711)。
文摘High-Mg andesite/diorite(HMA)is useful for identifying subduction-related processes in orogenic belts,including the identification of ophiolites formed in suprasubduction zone(SSZ)environments.The E'rentaolegai high-Mg diorite from the Diyanmiao ophiolite in central Inner Mongolia,North China,has been investigated revealing low-K tholeiitic-calc-alkaline characteristics and have SiO_(2)contents of 53.44-54.92 wt%,MgO contents of 8.44-9.54 wt%,and Mg~#of 54.35-57.60,with variable Fe_(2)O_(3)(7.51-8.61 wt%),Al_(2)O_(3)(11.95-15.09 wt%),and Na_(2)O(3.42-3.94 wt%)contents,low K_(2)O(0.34-0.97 wt%),TiO_(2)(0.35-0.67 wt%),and P2O5(0.12-0.15 wt%)contents,and high Ni(43-193 ppm)and Cr(189-556 ppm)contents.Samples collected have low total rare earth element(REE)contents(30.58-77.80 ppm),with flat or slightly right-dipping REE patterns(La_(N)/Yb_(N)=2.19-3.11)and a lack of pronounced Eu anomalies.The samples are also enriched in large-ion lithophile elements(LILEs,e.g.,K,Rb,Ba,U,and Sr)and depleted in high field strength elements(e.g.,Ta,Nb,Ti,and P).The E'rentaolegai high-Mg diorite has characteristics typical of HMA,and are similar to those of sanukites from the Setouchi Arc in SW Japan.They also display high positiveε_(Nd)(t)values(+6.32 to+7.80),comparable to the values of their host rocks.Petrogenetic analyses suggest that the E'rentaolegai HMA was probably formed by the interaction of partial melts and aqueous fluids from subducted sediments with mantle peridotite.Zircon U-Pb dating reveals that the high-Mg diorite crystallized at 313.6±2.4 Ma,i.e.,late Carboniferous.Combining our data with the temporal and spatial distribution of the Diyanmiao SSZ-type ophiolite,we propose that the eastern Paleo-Asian Ocean had not closed by the late Carboniferous,but intra-oceanic subduction was ongoing.A new model of the initiation of subduction in the eastern PAO during the late Paleozoic.
基金the National Natural Science Foundation of China(Grant Nos.42230308,42072266,42102032,42362006)Bureau of Geological Exploration and Development of Qinghai Province(Grant Nos.[2023]33,[2022]32)China Geological Survey(Grant Nos.DD20221649,DD20242935,12120114079701).
文摘Fragments of Proto-Tethyan oceanic lithosphere are well-preserved along the southern belt of the North Qilian suture,and the origin and emplacement of these ophiolites have become subjects of intense debate.In this study,we integrate field observations,mineralogical and geochemical analyses,zircon U-Pb dating,and isotopic data to investigate the Yanglong ophiolite.The Yanglong ophiolitic rocks are found as tectonic slices resting on the Neoproterozoic sedimentary and volcanic rocks.These rocks are composed of Cambrian serpentinized peridotite,gabbro,dolerite,and rodingite.The spinels in the serpentinized peridotites have variable Cr^(#)values(21,38-46,and 59-61)and display affinity to those in abyssal and forearc peridotites.The dolerites show slight enrichment in Th and have elevated(La/Sm)_(N) ratios(1.19-2.01),indicating a subduction-related geochemical affinity.The Yanglong ophiolitic rocks have positive zirconεHf(t)values(+10.3 to+18.4)and whole-rockεNd(t)values(+5.3 to+6.7)indicating derivation from partial melting of a depleted mantle source.These results,together with the regional geology,collectively suggest that the Yanglong ophiolite was generated in a forearc setting during the Early Cambrian northward intra-oceanic subduction.It was emplaced onto the Central Qilian Block during the subsequent arc-continent collision,no later than the Early Ordovician.
基金financially supported by the National Natural Science Foundation of China(92062215,41720104009,42172069)the China Geological Survey(DD20221886,DD20221817,DD20221657,DD20230340,DD20221630)+1 种基金the Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering Guangdong Laboratory(Guangzhou)(GML2019ZD0201)the Second Tibetan Plateau Scientific Expedition and Research Program(2019QZKK0801)。
文摘The ultramafic massif of Feragen,which belongs to the eastern ophiolitic belt of Norway,has abundant amounts of chromite ores.Recent studies have revealed a complex melt evolution in a supra-subduction zone(SSZ)environment.This study presents new whole-rock major element,trace element,and platinum-group element chemistry to evaluate their petrogenesis and tectonic evolution.Harzburgites have high CaO,Al_(2)O_(3),TiO_(2),MgO,and REE contents corresponding to abyssal peridotites,whereas dunites have low CaO,Al_(2)O_(3),TiO_(2),MgO,and REE contents corresponding to SSZ peridotites.The Cr^(#)and TiO_(2) of chromian spinels in the harzburgites suggest as much as about 15%–20%melting and the dunites are more depleted with>40%melting.The harzburgites and the dunites and high-Cr chromitites represent,respectively,the products of low-degree partial melting in a back-arc setting,and the products of melt-rock interaction in a SSZ environment.The calculated fO_(2) values for dunites and high-Cr chromitites(-0.17–+0.23 and+2.78–+5.65,respectively and generally above the FMQ buffer)are also consistent with the interaction between back-arc ophiolites with oxidized boninitic melts in a SSZ setting.
基金supported by the Second Tibetan Plateau Scientific Expedition and Research(SETP)(2019QZKK0806-02)the National Natural Science Foundation of China(42121003,42122024)+2 种基金CAS“Light of West China”Program(xbzg-zdsys-202310)Guizhou Provincial High level Innovation Talent program(GCC[2023]057)Guizhou Provincial 2021 Science and Technology Subsidies(No.GZ2021SIG)
文摘The debate regarding whether the Yarlung-Zangbo ophiolite(YZO)on the south of the Qinghai-Tibet Plateau,formed in a mid-ocean ridge(MOR)or a supra-subduction zone(SSZ)setting has remained unresolved.Here we present petrological,mineralogical,and geochemical data associated with modeling melting geodynamics of the mantle peridotites from the Purang ophiolite in the western segment of the Yarlung-Zangbo Suture Zone(YZSZ)to explore its tectonic environment.The Purang lherzolites are characterized by the protogranular texture and have abyssal-peridotite-like mineral compositions,including low Cr^(#)(20-30)and TiO_(2)contents(<0.1wt%)in spinel,high Al_(2)O_(3)(2.9wt%-4.4wt%)and CaO(1.9wt%-3.7wt%)contents in orthopyroxene and LREE-depletion in clinopyroxene.Compositions of these lherzolites can be modeled by~11%dynamic melting of the DMM source with a small fraction of melt(~0.5%)entrapped within the source,a similar melting process to typical abyssal peridotites.The Purang harzburgites are characterized by the porphyroclastic texture and exhibit highly refractory mineral compositions such as high spinel Cr^(#)(40-68),low orthopyroxene Al_(2)O_(3)(<2.2wt%)and CaO(<1.1wt%)contents.Clinopyroxenes in these harzburgites are enriched in Sr(up to 6.0 ppm)and LREE[(Ce)N=0.02-0.4],but depleted in Ti(200 ppm,on average)and HREE[(Yb)N<2].Importantly,the more depleted samples tend to have higher clinopyroxene Sr and LREE contents.These observations indicate an open-system hydrous melting with a continuous influx of slab fluid at a subduction zone.The modeled results show that these harzburgites could be formed by 19%-23%hydrous melting with the supply rate of slab fluid at 0.1%-1%.The lower clinopyroxene V/Sc ratios in harzburgites than those in lherzolites suggest a high oxidation stage of the melting system of harzburgites,which is consistent with a hydrous melting environment for these harzburgites.It is therefore concluded that the Purang ophiolite has experienced a transformation of tectonic setting from MOR to SSZ.
文摘Neotethyan ophiolites evolved in multiple seaways separated by Gondwana–derived ribbon continents within an eastward widening, latitudinal oceanic realm(Neotethys) throughout the Mesozoic. Opening and closure of these seaways were diachronous events, resulting in E–W variations in the timing of oceanic crust production and ophiolite emplacement. The Neotethyan ophiolites are highly diverse in their crustal–mantle structures and compositions, isotopic fingerprints, and sedimentary cover types, pointing to major differences in their mantle melt sources and tectonic and paleogeographic settings of magmatic construction(Dilek and Furnes, 2019). The Jurassic Western Alpine and Ligurian ophiolites in Europe and their counterparts in southern and northern Iberia formed in a narrow basin(Western Tethys) that developed between Europe and North Africa–Adria–Iberia. Their peridotites represent exhumed, continental lithospheric mantle, and the ophiolites display a Hess–type oceanic crustal architecture with MORB geochemical signatures(Dilek and Furnes, 2011). All these ophiolites were incorporated into continental margins from the downgoing oceanic lithosphere of the Western Tethys. Triassic, Jurassic and Cretaceous ophiolites east of Adria formed in different Neotethyan seaways(Dilek et al., 1990), and their rift–drift, seafloor spreading and suprasubduction zone(SSZ) magmatic construction involved multiple episodes of melting, depletion and refertilization of previously or actively subduction metasomatized mantle sources. Deep mantle recycling processes through subduction zone tectonics and/or plume activities played a major role in their melt evolution, and in the incorporation of mantle transition zone(MTZ) materials into their peridotites(Fig. 1;Dilek and Yang, 2018;Xiong et al., 2019). Tectonic mélanges structurally beneath these ophiolites include Permo–Triassic, OIB–type extrusive rocks, indicating that the initial dismantling of the Pangea supercontinent that led to the opening of the Triassic and Jurassic ocean basins within the Neotethyan realm was associated with plume magmatism(Dilek, 2003 a;Yang and Dilek, 2015). This plume signature is absent in the Permo–Triassic magmatic record of the Western Tethys to the west. The Cretaceous ophiolites around the Arabia(Dilek et al., 1990;Dilek and Delaloye, 1992;Dilek and Eddy,1992) and India sub-continents(Fareeduddin and Dilek, 2015) occur discontinuously along a ~9000-km-long belt from SW Anatolia to SE Tibet and Indo-China. The majority of these ophiolites have a Penrose–type oceanic crustal architecture(Dilek, 2003 b) and display SSZ geochemical affinities, complete with a MORB–IAT–BON progression of their chemo-stratigraphy(Fig. 1;Dilek and Thy, 1998;Dilek et al., 1999;Dilek and Furnes, 2014;Saccani et al., 2018). They evolved above a N–dipping, Trans–Tethyan subduction–accretion system that was situated in sub-tropical latitudes within the Neotethyan realm. The Trans–Tethyan subduction–accretion system was segmented into two major domains(Western and Eastern domains) by the NNE–SSW–oriented, sinistral Chaman–Omach–Nal transform fault plate boundary. This Cretaceous intraoceanic arc–trench system was analogous to the modern Izu–Bonin–Mariana(IBM) and Tonga arc–trench systems in the western Pacific in terms of its size. Diachronous collisions of the Arabia and India sub-continents with this segmented Trans-Tethyan arc–trench system resulted in the southward emplacement of the SSZ Neotethyan ophiolites onto their passive margins in the latest Mesozoic(Dilek and Furnes, 2019). A separate N–dipping subduction system, dipping beneath Eurasia to the north during much of the Jurassic and Cretaceous, was consuming the Neotethyan oceanic lithosphere and was responsible for the construction of a composite magmatic arc belt extending discontinuously from Southern Tibet to Northern Iran. Slab rollback along this northern subduction system produced locally developed forearc–backarc oceanic lithosphere that was subsequently collapsed into the southern margin of Eurasia. The existence of these two contemporaneous, Ndipping subduction systems within Neotethys led to its rapid contraction and the fast convergence of India towards Eurasia during the late Mesozoic–early Cenozoic(Dilek and Furnes, 2019). It was the collision with Eurasia of the India sub-continent with the accreted ophiolites around its periphery in the Late Paleogene that produced the Himalayan orogeny.
基金support has been provided by the Spanish project CGL2012-34618(Ministerio de Economía y Competitividad).
文摘In the Galicia Region of the NW Iberian Massif several allochthonous complexes(Cabo Ortegal,Órdenes and Malpica-Tui)contain a rootless Variscan suture that can be traced along the belt,from Iberia to the Bohemian Massif in Central Europe.Within these allochthonous complexes are several ophiolite zones bounded by two different continental terranes.There exist in NW Iberia two different ophiolite groups with different chemical compositions,isotopic signatures and structural positions.The Bazar and Vila de Cruces ophiolites,characterized by c.500 Ma protolith ages,represent the Lower Group,whereas the Careón,Purrido and Moeche ophiolites containing 395 Ma maficultramafic sequences represent the Upper Group.This younger group constitutes the most widespread ophiolites in the Variscan Belt.A thick serpentinite mélange(Somozas Mélange)occurring at the base of the Cabo Ortegal Complex also belongs to the ophiolite zones of the Variscan suture.In this paper we describe the Galician ophiolites of the Variscan suture and discuss their tectonic setting of formation.We interpret the generation of the Galician ophiolites within the geodynamic and paleogeographic evolution of the Rheic Ocean and the Pangea supercontinent.
基金jointly supported by the Geological Survey Project of Chinese (Grant No.1212010911070 and No.12120113093900)National Science Foundation of China (Grant No. 41072167)Institute of Geology, Chinese Academy of Geological Sciences (Grant No.J1309)
文摘The Zedang and Luobusa ophiolites are located in the eastern section of the Yalung Zangbo ophiolite belt,and they share similar geological tectonic setting and age.Thus,an understanding of their origins is very important for discussion of the evolution of the Eastern Tethys Ocean.There is no complete ophiolite assemblage in the Zedang ophiolite.The Zedang ophiolite is mainly composed of mantle peridotite and a suite of volcanic rocks as well as siliceous rocks,with some blocks of olivinepyroxenite.The mantle peridotite mainly consists of Cpx-harzburgite,harzburgite,some lherzolite,and some dunite.A suite of volcanic rocks is mainly composed of caic-aikaline pyroclastic rocks and secondly of tholeiitic pillow lavas,basaltic andesites,and some boninitic rocks with a lower TiO2 content （TiO2 ＜ 0.6％）.The pyroclastic rocks have a LREE-enriched REE pattern and a LILE-enriched （compared to HFSE） spider diagram,demonstrating an island-arc origin.The tholeiitic volcanic rock has a LREE-depleted REE pattern and a LILE-depleted （compared to HFSE） spider diagram,indicative of an origin from MORB.The boninitic rock was generated from fore-arc extension.The Luobusa ophiolite consists of mantle peridotite and mafic-ultramaflc cumulate units,without dike swarms and volcanic rocks.The mantle peridotite mainly consists of dunite,harzburgite with low-Opx （Opx ＜ 25％）,and harzburgite （Opx ＞ 25％）,which can be divided into two facies belts.The upper is a dunite-harzburgite （Opx ＜ 25％） belt,containing many dunite lenses and a large-scale chromite deposit with high Cr203; the lower is a harzburgite （Opx ＞25％） belt with small amounts of dunite and lherzolite.The Luobusa mantle peridotite exhibits a distinctive vertical zonation of partial melting with high melting in the upper unit and low melting in the lower.Many mantle peridotites are highly depleted,with a characteristic U-shaped REE pattern peculiar to fore-arc peridotite.The Luobusa cumulates are composed of wehrlite and olivine-pyroxenite,of the P-P-G ophiolite series.This study indicates that the Luobusa ophiolite was formed in a fore-arc basin environment on the basis of the occurrence of highly depleted mantle peridotite,a high-Cr2O3 chromite deposit,and cumulates of the P-P-G ophiolite series.We conclude that the evolution of the Eastern Tethys Ocean involved three stages：the initial ocean stage （formation of MORB volcanic rock and dikes）,the forearc extension stage （formation of high-Cr203 chromite deposits and P-P-G cumulates）,and the islandarc stage （formation of caic-alkaline pyroclastic rocks）.
基金granted by the Sultan Qaboos University Internal Grant(Grant No.IG/DVC/ESRC/18/01)
文摘One of the major topics of debate in ophiolite geology is the original tectonic setting of ophiolites. New studies show that most ophiolites are formed more frequently in a suprasubduction zone(SSZ) environment and that only a very small number of ophiolites have formed in an oceanic range(MOR). The Masirah ophiolite is one of the few oceanic ridge ophiolites that have been preserved, and the evidence that was formed in a subduction environment is missing(Moseley and Abbotts 1979, Dilek and Furnes, 2011;Rollinson, 2017). Masirah Island, the Batain and Ras Madrah areas of eastern Oman are almost entirely composed of a well-developed ophiolite, known as the Masirah ophiolite(Fig. 1), which is, however, completely unrelated to the nearby Semail Ophiolite in the northern Oman Mountains(Fig. 2). The Masirah ophiolite is Jurassic in age and represents oceanic lithosphere derived from the Indian Ocean, but is about 15–20 Myr later than emplacement of midCretaceous Semail ophiolite in northern Oman. The presence of basaltic to rhyolitic lavas of calc-alkaline affinity and boninites in the lava sequence of the Semail ophiolite led several researchers to propose a back-arc basin model for this ophiolite(e.g. Tamura and Arai, 2006;Godard et al., 2008;Rollinson and Adetunji, 2015). The Masirah Ophiolite shows close affinities with MORB peridotites in general. Most of the olivine from the Masirah harzburgites show Fo contents that are similar to those of olivine from MORB. Both pyroxenes in these harzburgites have similar Mg# values, Al2O3 and Cr2O3 contents to those of pyroxenes from MORB peridotites. The observed primitive mantlenormalized REE patterns showing enrichment in LREEs indicate that the Masirah peridotites have been modified by fluids or melts enriched in LREEs in a MORB environment. Podiform chromitites housed in ophiolites today interpreted as magmatic deposits formed during the reaction of molten rock in environments spike in the middle of the ocean(MOR) or suprasubduccion zone(SSZ)(Arai and Matsukage, 1998;Rollinson and Adetunji, 2015). The Masirah chromitites has a mineral chemistry similar to the mineral chemistry of chromite crystallized from MOR magmas. The Cr# values of chromite in the Masirah chromatite are similar to those of MOR peridotites. These findings suggest that the ultramafic and mafic cumulate rock assemblages overlying the upper mantle peridotites in the Masirah ophiolite represent the products of magma evolution in a MOR initiation stage within the proto Indian Ocean. Coexisting high-and low-Cr# associations of chromitite and dunite have been found in the Semail ophiolite, which illustrates the common situation of ophiolites having both SSZ and MOR geochemical signatures. Cr# varies from 40–60 for shallow chromite bodies, and over the range 70–80 for the deep locations. This diversity of chromitite types suggests two stages of magmatic activity were responsible for the chromitite genesis, in response to a switch of tectonic setting. The first is residual from lower degree, partial melting of peridotite, which produced lowCr# chromitites at the Moho transition zone, possibly in a midocean-ridge setting. The second chromitite-forming event involves higher degree partial melting, which produced high-Cr# discordant chromitite in the upper mantle, possibly in a suprasubduction zone setting. Assemblages of mono-and poly-phase silicate inclusions(including olivine, orthopyroxene, clinopyroxene, amphibole, phlogopite, serpentine, native Fe, FeO, alloy, sulfide, calcite, laurite, celestine and halite) within chromite have been observed in the low Cr# podiform chromitites from the Semail and Masirah ophiolites. The existence of hydrous silicate inclusions in the chromite calls for a role of hydration during chromite genesis. High-T bright green hornblende–edenite included in the chromites is evidence of the introduction of water in the magma at the end of the chromite crystallization. Such paragenesis points to the presence of hydrous fluids during the activity of the shear bands.
基金supported by the National Nature Science Foundation of China(41303027)Special Fund for Basic Scientific Research of Central Colleges,Chang’an University(310827153506,310827153407)to G.-X.Yang.Y.Dilek’s research in Tibet and on the Yarlung-Zangbo suture zone ophiolites has been funded by the Chinese Academy of Geological Sciences(Beijing,China).
文摘We present an overview of the internal structure of the ophiolite massifs along the Yarlung Zangbo suture zone(YZSZ)in southern Tibet with a focus on the geochemical character and tectonic evolution of the Ocean Island Basalt(OIB)and mafic alkaline rock assemblages associated with these ophiolites.The Jurassic–early Cretaceous lavas,massive diabase and gabbroic rocks are either tectonically intercalated with the early Cretaceous,subduction-influenced ophiolitic units,or occur as thrust sheets or blocks with an early Cretaceous mélange and in a Jurassic-Cretaceous flysch unit structurally beneath these ophiolites.They display uniform chondrite-normalized REE patterns with light rare earth element(LREE)enrichment and heavy rare earth element(HREE)depletion,no obvious Eu anomalies or negative Nb,Ta and Ti anomalies,and primitive mantle normalized trace element patterns with significant large-ion lithophile element(LILE)enrichment,similar to those of modern OIB and the Hawaiian alkaline basalts.These mafic alkaline rock assemblages represent OIB-and Plume-type(P-type)oceanic crustal rocks(with no subduction influence)that formed from magmas produced by partial melting of plume–metasomatized asthenospheric mantle source during the early stages of the opening of a Neotethyan seaway between Proto-India and Eurasia.
文摘We present an overview of the geology,geochemistry and petrogenesis of continental margin ophiolites(CMO),which represent the lithospheric remnants of riftgenerated paleo ocean–continent transition zones(OCTZ)in orogenic belts.The igneous stratigraphy and geochemical signatures of Neotethyan CMOs reflect the extent of geochemical heterogeneity,partial melting degrees,and melt evolution patterns in the continental lithospheric mantle prior to the onset of seafloor spreading in rifted margins.Basaltic rocks of the Jurassic CMOs in the External Ligurian units of the Northern Apennines have N-MORB and G-MORB affinities with strong HREE/MREE depletion,and represent the products of partial melting of a heterogeneous subcontinental lithospheric mantle containing small volumes of garnet pyroxenite layers.These extrusive rocks were erupted directly on the exhumed fertile spinel lherzolites of Adria during its OCTZ evolution.Volcanic rocks of the Triassic CMOs in the Albanide-Hellenide orogenic belt are represented by calc-alkaline suites;alkaline basalts and subordinate trachybasalts,trachyandesites,and trachytes;transitional to sub-alkaline plume-type PMORB basalts;sub-alkaline enriched,E-MORB basalts;and,sub-alkaline N-MORB basalts.Upper mantle peridotites are not exposed.
基金granted by the China Geological Survey(Grant No.121201102000150069)
文摘The tectonic setting of podiform chromitite formation still remains highly debated. There is a close correlation between tectonic settings and oxygen fugacity(fO2)(e.g., Ballhaus, 1993;Dare et al., 2009;Parkinson and Arculus, 1999). Here we present results of fO2 of chromites determined by M?ssbauer spectroscopy from both the Luobusha and Dazhuqu areas along Yarlung Zangbo suture zone, Southern Tibet. The fO2 values(-1.02~0.04 log units against the FMQ buffer) and Cr#(22~54%) in chromites from lherzolites and harzburgites of both areas are similar to those of abyssal peridotites, indicating that they may be residues after partial melting at spreading centers. However, both dunite envelopes and chromitites from Luobusha have high fO2 values(0.04~2.25 log units) and Cr#(73~84%), showing an affinity to boninitic melts, and thus form in a suprasubduction zone. Dazhuqu dunites show diverse fO2 values(-0.22~2.19 log units) and Cr#(22~82%), indicating that they form in distinct settings. Chromitites and chromite dunites from Dazhuqu have low fO2 values(-0.3~0.71 log units) and Cr#(16~63%), both of which are similar to those of MORB-like basalts, inferring that they form in an extensional setting. Both high-Cr and high-Al chromitites from other typical podiform chromite ore deposits, such as Kempirsai, Oman, and Albania ophiolites, also show high fO2 values(e.g., Chashchukhin and Votyakov, 2009;Melcher et al., 1997;Quintiliani et al., 2006;Rollinson and Adetunji, 2015), while the distribution-limited small chromitites and chromite dunites from Dazhuqu exhibit low fO2 values. The phenomenon infers that the suprasubduction zone is more beneficial to the formation of podiform chromitites.
基金granted by the National Natural Science Foundation of China(Grant No.41872241 and 41672221)
文摘The Qilian orogenic belt is the northernmost orogen of the Tethyan domain and connects the Altaids to the north. It contains an assembly of Precambrian micro-continental fragments, early Paleozoic island arcs, accretionary complexes, ophiolites, forearc and backarc basins, and high-pressure(HP) metamorphic rocks, indicating a long history of accretionary processes. Spatially, this orogen is adjacent to the Tarim, Qaidam, and North China blocks, which also extends into accretionary orogenic belts to the east and SW such as the Qinling and Kunlun belts. Abundant ophiolites in this orogen record the closure of an early Tethyan Ocean and amalgamations between micro-continents of North China, Qaidam, and Tarim. Thus, the ages and tectonic settings of these ophiolites within this belt provide important information regarding evolution of the Proto-Tethys Ocean and assembly of micro-continental blocks, which aids understanding of the spatial and temporal relationship of this orogen within the Tethyan realm. Dismembered ophiolites sporadically crop out along the northern margin of the South Qilian belt, and, from east to west, are locally referred to as the Lajishan, Gangcha, Muli, and Dadaoerji ophiolites. Much attention had been paid to these ophiolites, and several competing models for the tectonic evolution of this belt have been suggested. Considerable disagreement remains in respect of the temporal and spatial framework of the Qilian Orogen and details such as timing of subduction(s) and associated polarities, early collision events, and final closure of oceanic basins. In particular, the formation age and tectonic setting of Muli arc-ophiolite complex remains unknown, which limits understanding of the tectonics of the South Qilian belt and the history of the Proto-Tethys Ocean. The Muli arc-ophiolite complex is distributed over 20 km^2 west of the township of Muli in the western segment of the South Qilian Belt and consists of serpentinite, dunite, cumulate gabbro, basalt, plagiogranite, and chert. Field mapping results demonstrate that these units have been largely destroyed by faulting and generally occur as blocks/slices. They are tectonically interlayered with Upper Ordovician – Lower Silurian siliciclastic turbidite. Arc-ophiolite rocks are intruded by 470–450 Ma subduction-related granitoid plutons and are unconformably overlain by shallow marine to non-marine sediments of Permian-Jurassic age. Basalts show typical subduction-related calc-alkaline geochemical affinity, representing portions of an island arc. Geochemical results for plagiogranites and spinels from serpentinite demonstrate that the Muli arc-ophiolite complex represents a super-subduction zone(SSZ)-type ophiolite. U-Pb zircon data indicate formation associated with southward subduction of the Proto-Tethyan Ocean during a short interval between 539–522 Ma. Voluminous Late Ordovician-Early Silurian deep-water marine siliciclastic and volcaniclastic turbidites and volcanic arc rocks are exposed to the south of the Muli arc-ophiolite complex, whereas fluvial coarse-grained sandstones and conglomerates unconformably overlie the Cambrian-Middle Ordovician ophiolite-arc systems in the eastern South Qilian Belt. These indicate that closure of the Proto-Tethys Ocean was diachronous during the early Paleozoic.
基金“ex 60%–2013 and 2014”Universitàdegli Studi di Torino and PROGEO Piemonte(Universita degli Studi di Torino and Compagnia San Paolo)to A.Festa and G.Balestrothe Italian Ministry of University and Research Cofin-PRIN 2010/2011(“GEOPROB–GEOdynamic Processes of Oceanic Basins”)to A.Festa and P.Tartarotti“Subduction and exhumationof continental lithosphere:Implications on orogenic architecture,environment and climate”to G.Balestro.
文摘The Late Jurassic Monviso ophiolite in the Western Alps is a multiply deformed,eclogite-facies metaophiolite that represents a remnant of the Alpine Tethyan oceanic lithosphere.The recent recognition of a pre-Alpine detachment fault in the Lower Tectonic Unit of this ophiolite has led to the discovery of an oceanic core complex,which developed during the initial stages of the tectonic evolution of the Alpine Tethys.The NNWstriking,20–25-km-long shear zone(Baracun Shear Zone)contains ductilely to cataclastically deformed blocks and clasts of Fe-Ti and Mg-Al metagabbros in a matrix made of mylonitic serpentinite and talc-chlorite schist with high Ni–Cr concentrations and high Cl contents.Intensely sheared ophicarbonate rocks and brecciated serpentinite within this shear zone are deformed by the Alpine-phase S1 foliation and D2 folds,providing a critical age constraint for the timing of its formation.Metabasaltic–metasedimentary rocks in the hanging wall increase in thickness away from the shear zone,characteristic of syn-extensional rock sequences in supradetachment basins.
基金the National Natural Science Foundation of China （Grant No. 40234045, 40473030）.
文摘The recently identified Huashan ophiolitic mélange was considered as the eastern part of the Mianlüe suture in the Qinling orogenic belt. SHRIMP zircon U-Pb geochronology on gabbro from the Huashan ophiolite and granite intruding basic volcanic rocks indicates crystallization ages of 947±14 Ma and 876±17 Ma respectively. These ages do not support a recently proposed Hercynian Huashan Ocean, but rather favor that a Neoproterozoic suture assemblage （ophiolite） is incorporated into the younger （Phanerozoic） Qinling orogenic belt.
基金supported by the National Nature Science Foundation of China (grant No.41372208)China Geological Survey (grant No.1212011221105 and 1212011121259)
文摘The subduction of the Bangonghu-Nujiang Meso-Tethys and the collision between the Lhasa and Qiangtang blocks were important events in the growth of the Tibetan crust. However, the timing of collision initiation and closure timing, as well as nature and structure of the Bangonghu ocean basin, are still poorly constrained. The Lagkor Tso ophiolite, located in the south of Gerze County, Tibet, is one of the most completed ophiolites preserved in the southern side of the Bangonghu- Nujiang suture zone. This study discussed the tectonic evolution of the Bangonghu-Nujiang suture zone as revealed by the Lagkor Tso ophiolite investigated by field investigations, petrology, geochemistry, geochronology and tectonic analysis methods. We present new LA-ICP-MS zircon U-Pb and 39Ar/4~Ar ages for the Lagkor Tso ophiolite, in addition to geochemical and platinum-group element （PGE） data presented for the Lagkor Tso ophiolite in Tibet. It is suggested that the ancient Lagkor Tso oceanic basin split in Middle Jurassic （161.2 ± 2.7 Ma - 165.4 ± 3.5 Ma）, and experienced a second tectonic emplacement during the Early Cretaceous （137.90 ± 6.39 Ma）. The Lagkor Tso ophiolite likely developed in an independent suture zone. The Bangonghu-Nujiang ocean subducted southwards, and the dehydration of the subducting oceanic crust materials caused partial melting of the continental mantle wedge, which formed the second-order expanding center of the obduction dish. This led to inter-arc expansion, followed by the formation of inter-arc and back-arc basins with island arc features, which are represented by ophiolites around the Shiquanhe-Lagkor Tso -Yongzhu region. The tectonic environment presently can be considered to be similar to that of the current Western Pacific, in which a large number of island arc-ocean basin systems are developed.
基金the Land and Resources Survey Project of China(Grant Nos.200113000022,200313000063) the National Namral Science Foundation of China fGrant No.40472044).
文摘The Bayan Gol ophiolite fragment is a portion of the North Tianshan Early Carboniferous ophiolite belt. This ophiolite belt represents a geological record of an Early Carboniferous 'Red Sea type' ocean basin that was developed on the northern margin of the Tianshan Carboniferous-Permian rift system in northwestern China. The late Early Carboniferous Bayan Gol ophiolite suite was emplaced in an Early Carboniferous rift volcano- sedimentary succession of shallow-marine to continental facies (Volcanics Unit). Ophiolitic rocks in the Bayan Gol area comprise ultramafic rocks, gabbros with associated plagiogranite veins, diorite, diabase, pillow basalts and massive lavas. The Early Carboniferous rifting and the opening process of the North Tianshan ocean basin produced mafic magmas in composition of tholeiite and minor amounts of evolved magmas. Compositions of trace elements and Nd, Sr and Pb isotopes reveal the presence of two distinct mantle sources: (1) the Early Carboniferous rift mafic lavas from the Volcanics Unit were generated by a relatively low degree of partial melting of an asthenospheric OIB-type intraplate source; (2) younger (late Early Carboniferous, ~324.8 Ma ago) mafic lavas from the Ophiolite Unit were formed in a relatively depleted MORB-like mantle source, located in the uppermost asthenosphere and then gradually mixed with melts from the asthenospheric OIB-like mantle. A slight interaction between asthenosphere-derived magmas and lithospheric mantle took place during ascent to the surface. Subsequently, the most depleted mafic lavas of the ophiolite assemblage were contaminated by upper-crustal components (seawater or carbonate crust).
基金funded by grants from Sinoprobe-05-02 from the Ministry of Science and Technology of China,the NSF China(Nos.41502062,40930313,40921001,41202036)the China Geological Survey(Nos.12120115027201,12120114061801,2014DFR2127C)from the Institute of Geology,Chinese Academy of Geological Sciences(J1526)
文摘Various combinations of diamond, moissanite, zircon, quartz, corundum, rutile, titanite, almandine garnet, kyanite, and andalusite have been recovered from the Dangqiong peridotites. More than 80 grains of diamond have been recovered, most of which are pale yellow to reddish-orange to colorless. The grains are all 100-200 μm in size and mostly anhedral, but with a range of morphologies including elongated, octahedral and subhedral varieties. Their identification was confirmed by a characteristic shift in the Raman spectra between 1325 cm^-1 and 1333 cm^-1, mostly at 1331.51 cm^-1 or 1326.96 cm^-1. Integration of the mineralogical, petrological and geochemical data for the Dongqiong peridotites suggests a multi-stage formation for this body and similar ophiolites in the Yarlung-Zangbo suture zone. Chromian spinel grains and perhaps small bodies of chromitite crystallized at various depths in the upper mantle, and encapsulated the UHP, highly reduced and crustal minerals. Some oceanic crustal slabs containing the chromian spinel and their inclusion were later trapped in suprasubduction zones（SSZ）, where they were modified by island arc tholeiitic and boninitic magmas, thus changing the chromian spinel compositions and depositing chromitite ores in melt channels.
基金supported by China Geological Survey (Nos. 1212010918013 and 1212010610105)
文摘The A＇nyemaqen （阿尼玛卿） ophiolite belt along the southern margin of the East Kunlun （昆仑） Mountains marks the suture formed by the closure of paleo-Tethys. The Dur＇ngoi ophiolite in the eastern part of this belt consists of meta-peridotite, mafic-ultramafic cumulates, sheeted dikes and basaltic lavas. The meta-peridotites consist of dunite, harzburgite, lherzolite, feldspar-bearing lherzolite and garnet-bearing lherzolite and contain residual spinel with Cr# [100×Cr/（Cr＋Al）] ranging from 30 to 57 and Mg# [100×Mg/（Mg＋Fe2＋）] ranging from 50 to 75, indicating an Al- and Mg-rich series. The meta-peridotites have a relatively narrow range of composition with Mg# of 89.2-92.6, Al2O3 contents of （1-4） wt.% and slightly depleted chondrite normalized REE patterns, indicating that they represent relict mantle material that has undergone intermediate to low degrees of partial melting. Garnets in the lherzolite are andradite, enriched in Ca and Fe and depleted in Mg and AI （And=95-97, Pyr=0.3-5, Gro=0-3）, indicating a metamorphic origin. The cumulate rocks mainly consist of dunite, wehrlite, pyroxenite and gabbro. A well-layered gabbro-pyroxenite complex is defined by modal variations in plagioclase and pyroxene. Blocks of garnet-pyroxenite or rodingite are locally present in the meta-peridotites. Garnets in the cumulate rocks are grossular （Gro=69-90, And=9-19, Br=l-12）, also metamorphic origin. The diabase dikes are moderately depleted in LREE [（La/Sm）N=0.5-0.8] and HREE resulting in slightly convex chondrite-normalized patterns with slightly positive Eu anomalies （δEu=1.1-1.3）. The basaltic lavas have REE patterns similar to those of MORB with （La/Sm）N ratios of 0.5-1 and small negative Eu anomalies. They appear to have been derived from a depleted mantle source and to have undergone little or no differentiation during crystallization. SHRIMP U-Pb dating of zircons from the basalts yields 286pb/238U ages of 276-319 Ma （average 308.0±4.9 Ma）. The Dur＇ngoi ophiolite is interpreted as a dismembered fragment of paleo-oceanic crust emplaced during closure of the paleo-Tethyan Ocean basin. Three other suites of oceanic lavas are recognized in the area： island arc volcanic （IAV） rocks, possible back arc basin （BAB） basalts and possible post-collisional volcanic （PCV） and plutonic rocks. The distribution of these rocks suggests north-directed subduction. Opening of the A＇nyemaqen oceanic basin started at least as early as Late Carboniferous （308 Ma） and the basin probably closed during the Early Triassic. The IAV formed in Late Permian （260 Ma）, the BAB in Early-Middle Triassic, and the PCV in Late Triassic. Several large scale, ductile, sinistral strike-slip fault zones, extending hundreds to thousands kilometers, formed along or north of the suture during the Early-Late Triassic, e.g., they are the south margin fault zone of East Kunlun （200-220 Ma）, the Altyn Tagh fault （220-230 Ma）, and the North Qaidam fault zone （240-250 Ma）. These strike-slip faults were probably generated by oblique subduction and closure of the paleo-Tethyan Ocean basin, possibly during exhumation of the subducted plate or uplift of the overriding plate, coincident with post-collisional magmatism.
文摘The ultrahasic rocks in the Mianxian-Lueyang ophiolite belt are mainly harzburgite and dunite, and exhibit two types of REE distribution pattern: (1) LREE depletion, with remarkable Eu enrichment; (2) slight enrichment of LREE, with Eu deficiency. Both gabbros (accumulative gabbros) and diabasic dike swarms show a LREE enrichment but a slight Eu anomaly. The ratios of w (Ti) /w (V), w (Th) /w (Ta). w (Th) /w (Yb) and w (Ta) /w (Yb) indicate that the mid-ocean ridge basalts in this region originated from depleted mantle of asthenosphere, implying an association of MORB-type ophiolite and an ancient ocean basin between Qinling and Yangtze plates during Middle Paleozoic-Early Mesozoic era. The island arc volcanic rocks can be divided into tholeiitic and cafe-alkaline associations, which originated from the upper mantle wedge above slab by partial melting.
