The Shangla Complex ophiolite represents a relic of the Neo-Tethyan oceanic lithosphere along the Indus Suture Zone(also known as the Main Mantle Thrust)in northern Pakistan.This section,thrust onto the continental ma...The Shangla Complex ophiolite represents a relic of the Neo-Tethyan oceanic lithosphere along the Indus Suture Zone(also known as the Main Mantle Thrust)in northern Pakistan.This section,thrust onto the continental margin between the Indian and Karakoram(Asian)plates,is predominantly composed of depleted harzburgites,dunites and chromitites.In this study,we conducted a thorough analysis of mineralogy,whole-rock geochemistry(major oxides,trace elements,PGE),and integrated Re-Os isotopic data from mantle-derived peridotites to understand their petrogenesis and melt evolution.These peridotites exhibit a depleted nature,characterized by a low modal composition of clinopyroxene,a wide forsterite content range in olivine(86.5 to 95.2),and a large variation in Cr#values(25.1–91.4).Their diverse whole-rock geochemistry further suggests varying degrees of partial melting.The Cpx-harzburgites show high average Al_(2)O_(3)(1.83 wt.%),CaO(2.27 wt.%),ΣREE(12.9 ppb),and^(187)Os/^(188)Os values between 0.13095 and 0.12571.On the other hand,the depleted harzburgites and dunites exhibit lower average Al_(2)O_(3)(0.57 wt.%and 0.14 wt.%,respectively),CaO concentration(0.59 wt.%and 0.21 wt.%,respectively),and∑REE concentrations,measured at 12.7 ppb and 8.9 ppb,respectively.The^(187)Os/^(188)Os ratios in the depleted harzburgites and dunites range from 0.12643 to 0.11777,indicating they are less radiogenic compared to the Cpx-harzburgites.The spoon-shaped rare earth elements(REE)patterns suggest that the Cpx-harzburgites underwent low degrees of partial melting(∼10%–15%),whereas the depleted harzburgites and dunites indicate somewhat higher degrees of partial melting(additional melting of the Cpx-harzburgites).The PGE abundances in these depleted harzburgites and dunites are linked to the partial melting of Cpx-harzburgites,resulting in a boninitic-like melt.Their low degree of melting and melt extraction suggests that Cpx-harzburgites initially formed at a mid-ocean ridge(MOR)spreading center or a distal fore-arc basin.In contrast,the depleted harzburgites and dunites were formed during a second phase of melting,followed by refertilization,closely associated with a supra-subduction zone(SSZ)setting.The Re-Os isotopic systematics of the Shangla Complex peridotites reveal model age clusters of ca.250 Ma and ca.450 Ma,potentially corresponding to significant tectonic events in the geodynamic evolution of the Neo-Tethyan,Rheic,and Proto-Tethyan oceans.展开更多
Serpentinized rocks closely associated with Paleoproterozoic eclogitic metabasites were recently discovered at Eseka area in the northwestern edge of the Congo craton in southern Cameroon.Here,we present new field dat...Serpentinized rocks closely associated with Paleoproterozoic eclogitic metabasites were recently discovered at Eseka area in the northwestern edge of the Congo craton in southern Cameroon.Here,we present new field data,petrography,and first comprehensible wholerock geochemistry data and discuss the protolith and tectonic significance of these serpentinites in the region.The studied rock samples are characterized by pseudomorphic textures,including mesh microstructure formed by serpentine intergrowths with cores of olivine,bastites after pyroxene.Antigorite constitutes almost the whole bulk of the rocks and is associated(to the less amount)with tremolite,talc,spinel,and magnetite.Whole-rock chemistry of the Eseka serpentinites led to the distinction of two types.Type 1 has high MgO(>40 wt%)content and high Mg#values(88.80)whereas Type 2 serpentinite samples display relatively low MgO concentration and Mg#values(<40 and 82.88 wt%,respectively).Both types have low Al/Si and high Mg/Si ratios than the primitive mantle,reflecting a refractory abyssal mantle peridotite protolith.Partial melting modeling indicates that these rocks were derived from melting of spinel peridotite before serpentinization.Bulk rock high-Ti content is similar to the values of subducted serpentinites(>50 ppm).This similarity,associated with the high Cr contents,spinel-peridotite protolith compositions and Mg/Si and Al/Si ratios imply that the studied serpentinites were formed in a subductionrelated environment.The U-shaped chondrite normalizedREE patterns of serpentinized peridotites,coupled with similar enrichments in LREE and HFSE,suggest the refertilized nature due to melt/rock interaction prior to serpentinization.Based on the results,we suggest that the Eseka serpentinized peridotites are mantle residues that suffered a high degree of partial melting in a subductionrelated environment,especially in Supra Subduction Zone setting.These new findings suggest that the Nyong series in Cameroon represents an uncontested Paleoproterozoic suture zone between the Congo craton and the Sao Francisco craton in Brazil.展开更多
The Zedong ophiolites in the eastern Yarlung-Zangbo suture zone of Tibet represent a mantle slice of more than 45 km2. This massif consists mainly of mantle peridotites, with lesser gabbros, diabases and volcanic rock...The Zedong ophiolites in the eastern Yarlung-Zangbo suture zone of Tibet represent a mantle slice of more than 45 km2. This massif consists mainly of mantle peridotites, with lesser gabbros, diabases and volcanic rocks. The mantle peridotites are mostly harzburgite, lherzolite; a few dike-like bodies of dunite are also present. Mineral structures show that the peridotites experienced plastic deformation and partial melting. Olivine （Fos9.7-91.2）, orthopyroxene （Enss-92）, clinopyroxene （En4-49Wo47-slFS2-4） and spinel [Mg^#=lOOxMg/（Mg＋Fe）]=49.1-70.7; Cr^#=（100xCr/（Cr＋Al）=18.8-76.5] are the major minerals. The degree of partial melting of mantle peridotites is 10%-40%, indicating that the Zedong mantle peridotites may experience a multi-stage process. The peridotites are characterized by depleted major element compositions and low REE content （0.08-0.62 ppm）. Their ＂spoon-shaped＂ primitivemantle normalized REE patterns with （La/Sm）N being 0.50-6.00 indicate that the Zedong ultramafic rocks belong to depleted residual mantle rocks. The PGE content of Zedong peridotites （18.19-50.74 ppb） is similar with primary mantle with Pd/Ir being 0.54-0.60 and Pt/Pd being 1.09-1.66. The Zedong peridotites have variable, unradiogenic Os isotopic compositions with 187Os/18Os=0.1228 to 0.1282. A corollary to this interpretation is that the convecting upper mantle is heterogeneous in Os isotopes. All data of the Zedong peridotites suggest that they formed originally at a mid-ocean ridge （MOR） and were later modified in supra-subduction zone （SSZ） environment.展开更多
Peridotites from the southern Mariana forearc were sampled on the landward trench slope of the Izu-Bonin-Mariana (IBM) subduction zone by dredging.These mantle wedge peridotites underwent hydration by fluid derived fr...Peridotites from the southern Mariana forearc were sampled on the landward trench slope of the Izu-Bonin-Mariana (IBM) subduction zone by dredging.These mantle wedge peridotites underwent hydration by fluid derived from a dehydrated descending slab,and later interacted with seawater after emplacement at or near the seafloor.This study investigates how these two different rock-fluid interaction processes influenced trace element distribution in the southern Mariana forearc peridotites.We measured trace element concentrations of peridotites from the southern Mariana forearc.The southern Mariana forearc peridotites are characterized by a distinct seawater-like REE pattern with an obvious negative Ce anomaly,and La shows good correlation with other REEs (except Ce).In addition,there is a great enrichment of U,Pb,Sr and Li elements,which show a distinct positive anomaly relative to adjacent elements in the multi-element diagram.For the seawater-like REE pattern,we infer that REEs are mainly influenced by seawater during peridotite-seawater interactions after their emplacement at or near the seafloor,by serpentinization or by marine weathering.Furthermore,the anomalous behavior of Ce,compared with other rare earth elements in these samples,may indicate that they have undergone reactions involving Ce (IV) when the peridotites interacted with seawater.Positive U,Pb,Sr and Li anomalies are inferred to be related to seawater and/or fluids released during dehydration of the subducting slab.展开更多
Platinum group elements(PGE)and Re-Os isotopes of mantle peridotites in the Jinshajiang ophiolite(SW China)were investigated in this study,in order to constrain the evolution of the lithospheric mantle beneath the Jin...Platinum group elements(PGE)and Re-Os isotopes of mantle peridotites in the Jinshajiang ophiolite(SW China)were investigated in this study,in order to constrain the evolution of the lithospheric mantle beneath the Jinshajiang-Ailaoshan Ocean,which was a branch of the eastern Paleo-Tethys.The Jinshajiang peridotites have whole-rock compositions(e.g.,MgO=32.7-38.1 wt.%;Al_(2)O_(3)=0.67-1.30 wt.%)and spinels with moderate Cr#values(0.4-0.6)similar to those of abyssal peridotites,which indicate moderate degrees of partial melting(15%-20%).These peridotites exhibit U-shaped chondrite-normalized REE patterns that could be caused by hydrothermal alteration or melt-rock interaction after mantle melting.In addition,Pd concentrations and(Pd/Ir)_(N)ratios of the Jinshajiang peridotites increases with decreasing Al_(2)O_(3) concentrations.These negative correlations cannot be explained by simple partial melting but record a melt-rock reaction event after mantle melting.This study therefore demonstrates the efficiency of PGE in detecting the melt-rock reaction process relative to whole-rock major and trace elements.The suprachondritic^(187)Os/^(188)Os ratios(0.1272-0.1374)further indicate that the later percolating melt derived from a mantle domain with distinct^(187)Os-enriched isotopic compositions.In comparison with peridotites in the Ailaoshan ophiolite belt,which were not significantly affected by melt percolation,this study further highlights that the lithospheric mantle compositions beneath different segments of the same ocean basin are highly variable and might be controlled by distinct mantle processes in response to different rifting mechanisms.展开更多
The Peng Co ophiolite is located to the west of Peng lake in the area of lakes in north Tibet, which belongs to the Baila-Yilashan sub-belt of the the middle Bangong Co-Nujiang ophiolitic belt. The Peng Co ophiolite i...The Peng Co ophiolite is located to the west of Peng lake in the area of lakes in north Tibet, which belongs to the Baila-Yilashan sub-belt of the the middle Bangong Co-Nujiang ophiolitic belt. The Peng Co ophiolite is mainly composed of mantle peridotites, cumulates, diabase dikes. About 70 percent peridotites are harzburgites and 30 percent are lherzolites. Mineral chemistry of the Peng Co lherzolitesare characterized by low Fo contents(88.85–90.33) of olivine and high Al2O3 content(4.26%–7.25%) in pyroxenes. Compared to the primitive mantle, the Peng Co peridotites have relatively higher MgO contents, lower CaO, Al2O3 and TiO2 contents. The total rare-earth element(REE) contents of the lherzolites are 1.11–1.53 ppm, which are lower than those of the primitive mantle. The chondritenormalized REE patterns of the Peng Co peridotites display slight loss in LREE. In the primitive mantle-normalized spider diagram, the Peng Co peridotites exhibit negative Rb and Zr anomalies and intensively positive U, Ta, Sr anomalies. The PGE contents of Peng Co lherzolites are between 22.9–27 ppb. The chondrite-normalized PGE patterns of the Peng Co lherzolites are consistent with that of the primitive mantle. Mineral and whole-rock geochemistry characteristics of the Peng Co lherzolites show an affinity to abyssal peridotites, indicating that it may have formed in the mid-ocean ridge setting. Through quantitative modeling, we conclude that the Peng Co lherzolites formed after 5%–10% degree of partial melting of the spinelphase lherzolite mantle source. The sharp increase of Cr#(56.74–60.84)in Spinel of harzburgites and relatively high Pd/Ir and Rh/Ir ratios suggest that they have experienced melt-rock reaction. The crystallization sequence of Peng Co cumulate is olivine-clinopyroxene-plagioclase. The Mg# value of clinopyroxene in cumulate peridotite ranges from 86.92 to 89.93, and the mean value of Fo is 84.45, which is obviously higher than that of MOR-type ophiolite cumulates. The mineral composition, sequence of magmatic crystallization and mineral components of Peng Co cumulate are similar to those of the cumulate formed by the SSZ-type ophiolite in the subduction zone. Therefore, we can draw a preliminary conclusion that Peng Co lherzolites were formed in an environment of mid oceanic ridge and were remnants of the spinel lherzolite zone which experienced a partial melting of no more than 10%. In the later period, due to the intra-oceanic subduction, it experienced the rock-meltinteraction, and thus formed the SSZ-type cumulate and harzburgite of high Cr value.展开更多
The Beni Bousera peridotite massif(Internal Rif, Morocco),5 km in width and 15 km in length,is formed in a major part of spinel-bearing lherzolite rimed by a layer of garnet-bearing peridotite(100 m thick)which is in ...The Beni Bousera peridotite massif(Internal Rif, Morocco),5 km in width and 15 km in length,is formed in a major part of spinel-bearing lherzolite rimed by a layer of garnet-bearing peridotite(100 m thick)which is in direct contact with HP-HT granulite metamorphic rocks(16 kbar,860℃).According to recent detailed study,the shearing contact between these two formations shows the presence of serpentinite and展开更多
Serpentinites,which contain up to 13 wt%of water,are important reservoirs for chemical recycling in subduction zones.In the past two decades,forearc mantle serpentinites were identified in different locations around t...Serpentinites,which contain up to 13 wt%of water,are important reservoirs for chemical recycling in subduction zones.In the past two decades,forearc mantle serpentinites were identified in different locations around the world.Here,we present petrology and whole rock chemistry of ultramafic and mafic rocks dredged from the Hahajima Seamount,which is located 24–40 km west to the junction of the Izu-Bonin Trench and the Mariana Trench.Nearly all the collected samples are extensively hydrated,and olivine grains in ultramafic rocks are replaced by serpentine minerals,with only one sample preserving remaining trace of orthopyroxene.Our new results show that the Hahajima serpentinized peridotite samples are all MgO-rich(~42 wt%),but have low contents in Al2O3,CaO,rare earth and high field strength elements,which is consistent with the overall depleted character of their mantle protoliths.Model calculations indicate that these Hahajima peridotite samples were derived from 10%–25%partial melting of the presumed fertile mantle source,which is generally lower than those of peridotites from Torishima Forearc Seamount,Conical Seamount and South Chamorro Seamount(mostly>25%).All the serpentinites from these four forearc seamounts show strong enrichment in fluid-mobile and lithophile elements(Li,Sr,Pb and U).In details,Hahajima Seamount serpentinites do not have obvious enrichment in Cs and Rb,and display remarkably high abundances of U.These observations indicate that the serpentinization of Hahajima peridotites occurred by addition of seawater or low temperature seawater-derived hydrothermal fluid,without or with little contribution from slab-derived fluids.The geochemical signature of serpentinites from Hahajima Seamount could be interpreted as the result of the combination of extensive partial melting and subsequent percolation of seawater through the mantle wedge.展开更多
The serpentinized peridotites overlying the subducted zones in the Izu-Bonin-Mariana(IBM) arc system have been interpret as the cause of the low-velocity layer identified beneath the IBM froearc,in turn few earthquake...The serpentinized peridotites overlying the subducted zones in the Izu-Bonin-Mariana(IBM) arc system have been interpret as the cause of the low-velocity layer identified beneath the IBM froearc,in turn few earthquakes occurred along the plate boundary.Chrysotile,which is a low temperature and highly hydrated phase of serpentine with low frictional strength,has been suggested as the low velocity material in the serpentinized peridotites,besides,brucite is inferred to be likely conducive to stable sliding.However,such idea encounters challenging in our serpentinized peridotites from the southern Mariana forearc,which absent both the above minerals.The presence of talc,which characterized by its weak,low-friction and inherently stable sliding behavior,provides new clue.Here we report the occurrence of talc in serpentinized peridotites collected from the landward trench slope of the southern Mariana forearc.We infer that talc is mainly forming as a result of the reaction of serpentine minerals with silica-saturated fluids released from the subducting slab,and talc also occurs as talc veins sometimes.Due to its unique physical properties,talc may therefore play a significant role in aseismic slip in the IBM subduction zone.展开更多
The Pozanti-Karsanti ophiolite(PKO)in Turkey’s eastern Tauride belt comprises mantle peridotites,ultramafic to mafic cumulates,isotropic gabbros,sheeted dikes and pillow lavas.The mantle peridotites are dominated by ...The Pozanti-Karsanti ophiolite(PKO)in Turkey’s eastern Tauride belt comprises mantle peridotites,ultramafic to mafic cumulates,isotropic gabbros,sheeted dikes and pillow lavas.The mantle peridotites are dominated by spinel harzburgites with minor dunites.The harzburgites and dunites have quite depleted mineral and whole-rock chemical composition,suggesting high degrees of partial melting.Their PGEs vary from Pd-depleted to distinct Pd-enriched patterns,implying the crystallization of interstitial sulphides from sulphur-saturated melts(e.g.MORB-like forearc basalt).U-shaped or spoon-shaped REE patterns indicate that the PKO peridotites may have also been metasomatized by the LREE-enriched fluids released from a subducting slab in a suprasubduction zone.Based on the mineral and whole-rock chemical compositions,the PKO peridotites show affinities to forearc peridotites.Chromitites occur both in the mantle peridotites and the mantle-crust transition zone horizon(MTZ).Chromitites from the two different horizons have different textures but similar mineral compositions,consistent with typical high-Cr chromitites.Chromitites hosted by mantle harzburgites generally have higher total platinum-group element(PGE)contents than those of the MTZ chromitites.However,both chromitites show similar chondritenormalized PGE patterns characterized by clear IPGEs,Rh-enrichments relative to Pt and Pd.Such PGE patterns indicate no or only minor crystallization of Pt-and Pd enriched sulphides during formation of chromitites from a sulphur-undersaturated melt(e.g.boninitic or island arc tholeiitic melt).Dunite enveloping chromitite lenses in the ho*s ting harzburgite resulted from melt-rock reaction.We have performed mineral separation work on samples of podiform chromitite hosted by harzburgites.So far,more than200 grains of microdiamond and more than 100 grains of moissanite(Si C)have been separated from podiform chromitites.These minerals have been identified by EDX and Laser Raman analyses.The diamonds and moissanite are accompanied by large amounts of rutile.Additionally,zircon,monazite and sulphides are also common phases within the heavy mineral separates.Both diamond and moissanite have been analyzed for carbon and nitrogen isotopic composition using the CARMECA 1280-HR large geometry Secondary Ion Mass Spectrometer at the Helmholtz Zentrum Potsdam.In total,61δ13CPDB results for diamond were acquired,exhibiting a range from-28.4‰to-18.8‰.31δ13CPDB results for Moissanite vary between-30.5‰to-27.2‰,with a mean value of-29.0‰.Diamond has relatively large variation in nitrogen isotopic composition with 40δ15NAIR results ranging from-19.1‰to 16.6‰.The discovery of diamond,moissanite and the other unusual minerals from podiform chromitite of the Pozanti-Karsanti ophiolite provides new support for the genesis of ophiolitic peridotites and chromitites under high-pressure and ultra-high reducing conditions.Considering the unusual minerals,the high Mg#silicate inclusions,and the needle-shaped exsolutions in the PKO chromitites,the parental melts of these chromitites may have been mixed with deep asthenospheric basaltic melts that had assimilated materials of the descending slab when passing through the slab in a subduction zone environment.We suggest melt-rock reactions,magma mixing and assimilation may have triggered the oversaturation of chromites and the formation of PKO chromitites.展开更多
Ophiolites along the E-W trending Yarlung-Tsangpo Suture(YTS),which separates the Indian plate from the Eurasian plate,have been regarded as relics of the NeoTethyan Ocean.The Xigaze ophiolite in the central YTS
The Jurassic–Cretaceous ophiolites in the Alpine–Himalayan orogenic belt represent fragments of oceanic lithosphere,developed in different seaways separated by Gondwana–derived ribbon continents within a broad
Tectonic setting of the Neoproterozoic ophiolites is poorly understood.Because of extensive serpentinization/metamorphism in the mantle section,accessory chromian spinel has been used as an important geotectonic indic...Tectonic setting of the Neoproterozoic ophiolites is poorly understood.Because of extensive serpentinization/metamorphism in the mantle section,accessory chromian spinel has been used as an important geotectonic indicator.展开更多
The present study deals with peridotites found in the paleo-proterozoic domain of the Leo Man Shield, Séguéla region, west-central of Côte d’Ivoire. Results show that Séguéla peridotites ...The present study deals with peridotites found in the paleo-proterozoic domain of the Leo Man Shield, Séguéla region, west-central of Côte d’Ivoire. Results show that Séguéla peridotites are composed of lherzolites, dunites and harzburgites. However, iherzolites are the most abundant. The phenocrysts in these rocks are olivine most often serpentinised, and pyroxenes, represented by diopside and enstatite. Amphiboles are divided into two groups: magmatic amphiboles and those from the destabilization of clinopyroxene. Spinels have CrO<sub>3</sub> content which varies between 28% and 37%, Al<sub>2</sub>O<sub>3</sub> between 33% - 41% and MgO is equal to 18%;they are poor in TiO<sub>2</sub> and do not contain zinc oxide. Séguéla peridotites are characterised by a negative anomaly in Nb-Ta, associated with an enrichment of lithophilic elements and light rare earth elements with a fractionation rate (La/Yb) which varies between 46.22 and 150.72 and heavy rare earth elements, and HFSE depletion, which may suggest that were formed in an art context, that is, a mantle enriched by fluids from a subduction zone. However, another hypothesis could be considered, that of the interaction between the mantle and magmas in a subduction zone context.展开更多
Taoxinghu metamorphic peridotite is a firstly reported mantle sequence of ophiolite since Longmuco-Shuanghu-Lancangjiang suture zone （LSLSZ） was proposed, and it is also an important discovered for ophiolite studyin...Taoxinghu metamorphic peridotite is a firstly reported mantle sequence of ophiolite since Longmuco-Shuanghu-Lancangjiang suture zone （LSLSZ） was proposed, and it is also an important discovered for ophiolite studying in central Qiangtang. Based on detailed analyses of whole-rock geochemistry of Taoxinghu metamorphic peridotites and contrast to metamorphic peridotites in typical ophiolites worldwide, the paper investigates their petrogenesis and geological implication. The petrologic results show that the protolith of Taoxinghu metamorphic perdotites have the mineral assemblage and texture characteristic of mantle peridotite. Most metamorphic peridotites hav near global abyssal peridotites major elements contents, while the few is similar to SSZ-type peridotites. They exhibit typically U-shaped REE patterns, characterized by slight enrichment of LREE and HREE relative to MREE and a low fractionated LREE to HREE segment. Trace elements contents are low and all samples are strong enrichment in Cs, U, Pb, weak enrichment in Ba and depletion in Th, but negative Nb anomalies are only observed in few samples. That suggests Taoxinghu metamorphic peridotites have depletion mantle and suprasubduction affinities. A two-stage evolution history is considered： Taoxinghu metamorphic peridotites originated as the residue from melting at a ridge with 7%-20% degree of fraction melting and were subsequently modified by interaction with mafic melt and aqueous fluid within mantle wedge on subducted zone. Combined with previous studies, we preliminarily propose Taoxinghu metamorphic peridotites may be the Products of initial rifting of palo-Tethys, forming at middle Ordivician-upper Cambrian, and they may be the direct evidences for spreading of palo-Tethys.展开更多
We present a new dataset on platinum group elements（PGEs）, whole-rock major and trace elements, and mineral chemistry for the peridotites from the Zedang and Luobusa ophiolite suites, Tibet, in an attempt to better ...We present a new dataset on platinum group elements（PGEs）, whole-rock major and trace elements, and mineral chemistry for the peridotites from the Zedang and Luobusa ophiolite suites, Tibet, in an attempt to better constrain the petrogenesis of the Zedang and Luobusa ophiolites and the tectonic evolution of the Neo-Tethys. Plots of chondrite-normalized PGE, PGE vs. Mg#, and PGE vs. Al_2O_3 suggest that the lherzolite and harzburgite from Zedang and Luobusa have similar PGE characteristics. The Zedang and Luobusa peridotites display U-shaped REE patterns and are enriched in some incompatible elements, indicative of melt-rock interaction. The PGE characteristics may be attributed to partial melting and heterogeneous melt-rock interaction. Mineral chemistry and whole rock major and trace elements data suggest that lherzolite and harzburgite from Zedang and Luobusa have similar geochemical properties. On the spinel Mg# vs. Cr# plot, the composition of the Zedang and Luobusa peridotites is consistent with both abyssal and subduction-zone peridotites. This study indicates that the Zedang and Luobusa peridotites have a similar origin and evolution path： they could have originated from a normal mid-ocean ridge environment and got refertilization in a supra-subduction zone setting.展开更多
The mineral chemistry and texture of clinopyroxenes in peridotite from the Kingkong tectonic zone of the Southwest Indian Ridge segment in an effort to constrain mantle melting beneath this slow-spreading ridge are re...The mineral chemistry and texture of clinopyroxenes in peridotite from the Kingkong tectonic zone of the Southwest Indian Ridge segment in an effort to constrain mantle melting beneath this slow-spreading ridge are reported. There are three types of clinopyroxenes in the abyssal peridotites： coarse-grained, intergranu- lar and exsolved. The compositional variations among these three types suggest that the coarse-grained clinopyroxene is a mantle-derived source. The A1, Na and Ti contents and the Na/Ti ratio of the coarse- grained clinopyroxene may be used to monitor the degree of partial melting, combined with the contradis- tinction with Spinel Cr#, which is calculated to be between 7.9% and 14.9%, and may represent low degrees of melting in the global ocean ridge system. The along-axis compositional variations in the coarse-grained clinopyroxene suggest that the degree of partial melting is primarily controlled by the transform faults on both sides of the ridge. Nonetheless, the northwestern side of the ridge may be affected by a hypothesised detachment fault as documented by the calculated P-T conditions. Simultaneously high Na and low Ti con- tents in the coarse-grained clinoovroxene points to mantle heterogeneities along the ridge axis.展开更多
The rock forming temperatures and pressures represent the p T points of the local regions in the lithosphere at a certain age, providing some important information on rock formation. Based on the preliminary sta...The rock forming temperatures and pressures represent the p T points of the local regions in the lithosphere at a certain age, providing some important information on rock formation. Based on the preliminary statistics on the temperatures and pressures for the formation of eclogites, granulites and peridotites in China, the variant ranges are given, in this paper, of temperatures, pressures and linear geothermal gradients of eclogites, granulites and peridotites. In addition, since the eclogite is different from granulite and peridotite in the p T diagram, these three rocks can be classified into two groups: the first group includes eclogites and the second group granulites and peridotites. Then, the p T correlation functions of these two groups of rocks are provided. Finally, the two groups of rocks have different geothermal gradients at the same pressure gradient or have different pressure gradients at the same geothermal gradient. The temperatures and pressures for the formation of the rocks can be calculated from the mineral chemical compositions, but the depths ( H ) for the rock formation can be calculated only under the hypotheses of given p H (or T H ) correlation functions. The explanations for the ultrahigh pressure metamorphism vary obviously with different hypotheses.展开更多
To discuss the nature of the compositional heterogeneity of the peridotite massifs of the Polar Urals(Russia), the geochemical study by LA-ICP-MS of pyroxenes and amphiboles from these mantle formations was performed....To discuss the nature of the compositional heterogeneity of the peridotite massifs of the Polar Urals(Russia), the geochemical study by LA-ICP-MS of pyroxenes and amphiboles from these mantle formations was performed. The trace element compositions in clinopyroxenes indicate the existence of the mantle protolith of two types. The first protolith type, represented by lherzolites and diopside harzburgites,was originated from the partial melting(5%–10%) under the spinel facies conditions, while the second one,represented by diopside harzburgites, was formed under the polybaric partial melting(17%–19%) under garnet and spinel facies conditions. Subsequently, the mantle peridotite protolith was subject to fluidinduced partial melting in the suprasubduction setting that was resulted in the formation of harzburgites.Being affected by penetrating melts and fluids peridotites experienced the refertilization(LREE enrichment of clinopyroxenes) and high-temperature hydratation with subsequent development of pargasite and Mg amphibole. The high-T fluid-induced metamorphism at the subduction zone was accompanied by the formation of metaperidotites with clinochlore and REE-depleted tremolite.展开更多
Swarms of orthopyroxenite and websterite veins are found within Egiingol residual SSZ peridotite massif of Dzhida terrain(Central Asian Orogenic Belt,Northern Mongolia).The process of Egiingol pyroxenite veins formati...Swarms of orthopyroxenite and websterite veins are found within Egiingol residual SSZ peridotite massif of Dzhida terrain(Central Asian Orogenic Belt,Northern Mongolia).The process of Egiingol pyroxenite veins formation is investigated using new major and trace element analyses of pyroxenite minerals,calculations of closure temperatures and composition of equilibrium melt.The pyroxenites show abundant petrographic and geochemical evidence for replacement of the residual peridotite minerals by ortho-and clinopyroxene due to melt-rock interaction.Relics of peridotite olivines are found in pyroxenites,Cr#of spinel increases from peridotites to pyroxenites,and compositions of ortho-and clinopyroxene change from peridotite to pyroxenite.The authors show that calculated equilibrium melts for investigated pyroxenites are very similar to compositions of boninite lavas from the Dzhida terrain.Therefore,formation of pyroxenite veins most likely resulted from percolation of boninite melts through the Egiingol peridotites.Orthopyroxenite veins formed at first,followed by websterite veins.Thus,the authors assume that pyroxenite veins represent the channels for boninitic melts migration in supra-subduction environment.展开更多
基金supported by a grant from the Science and Technology Innovation Program of Hunan Province(Grant No.2021RC4055)Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitoring,during the Postdoctoral research of the first author(Zaheen Ullah,Assistant Professor,University of Swat,Pakistan)at the Central South University,Changsha,China.
文摘The Shangla Complex ophiolite represents a relic of the Neo-Tethyan oceanic lithosphere along the Indus Suture Zone(also known as the Main Mantle Thrust)in northern Pakistan.This section,thrust onto the continental margin between the Indian and Karakoram(Asian)plates,is predominantly composed of depleted harzburgites,dunites and chromitites.In this study,we conducted a thorough analysis of mineralogy,whole-rock geochemistry(major oxides,trace elements,PGE),and integrated Re-Os isotopic data from mantle-derived peridotites to understand their petrogenesis and melt evolution.These peridotites exhibit a depleted nature,characterized by a low modal composition of clinopyroxene,a wide forsterite content range in olivine(86.5 to 95.2),and a large variation in Cr#values(25.1–91.4).Their diverse whole-rock geochemistry further suggests varying degrees of partial melting.The Cpx-harzburgites show high average Al_(2)O_(3)(1.83 wt.%),CaO(2.27 wt.%),ΣREE(12.9 ppb),and^(187)Os/^(188)Os values between 0.13095 and 0.12571.On the other hand,the depleted harzburgites and dunites exhibit lower average Al_(2)O_(3)(0.57 wt.%and 0.14 wt.%,respectively),CaO concentration(0.59 wt.%and 0.21 wt.%,respectively),and∑REE concentrations,measured at 12.7 ppb and 8.9 ppb,respectively.The^(187)Os/^(188)Os ratios in the depleted harzburgites and dunites range from 0.12643 to 0.11777,indicating they are less radiogenic compared to the Cpx-harzburgites.The spoon-shaped rare earth elements(REE)patterns suggest that the Cpx-harzburgites underwent low degrees of partial melting(∼10%–15%),whereas the depleted harzburgites and dunites indicate somewhat higher degrees of partial melting(additional melting of the Cpx-harzburgites).The PGE abundances in these depleted harzburgites and dunites are linked to the partial melting of Cpx-harzburgites,resulting in a boninitic-like melt.Their low degree of melting and melt extraction suggests that Cpx-harzburgites initially formed at a mid-ocean ridge(MOR)spreading center or a distal fore-arc basin.In contrast,the depleted harzburgites and dunites were formed during a second phase of melting,followed by refertilization,closely associated with a supra-subduction zone(SSZ)setting.The Re-Os isotopic systematics of the Shangla Complex peridotites reveal model age clusters of ca.250 Ma and ca.450 Ma,potentially corresponding to significant tectonic events in the geodynamic evolution of the Neo-Tethyan,Rheic,and Proto-Tethyan oceans.
文摘Serpentinized rocks closely associated with Paleoproterozoic eclogitic metabasites were recently discovered at Eseka area in the northwestern edge of the Congo craton in southern Cameroon.Here,we present new field data,petrography,and first comprehensible wholerock geochemistry data and discuss the protolith and tectonic significance of these serpentinites in the region.The studied rock samples are characterized by pseudomorphic textures,including mesh microstructure formed by serpentine intergrowths with cores of olivine,bastites after pyroxene.Antigorite constitutes almost the whole bulk of the rocks and is associated(to the less amount)with tremolite,talc,spinel,and magnetite.Whole-rock chemistry of the Eseka serpentinites led to the distinction of two types.Type 1 has high MgO(>40 wt%)content and high Mg#values(88.80)whereas Type 2 serpentinite samples display relatively low MgO concentration and Mg#values(<40 and 82.88 wt%,respectively).Both types have low Al/Si and high Mg/Si ratios than the primitive mantle,reflecting a refractory abyssal mantle peridotite protolith.Partial melting modeling indicates that these rocks were derived from melting of spinel peridotite before serpentinization.Bulk rock high-Ti content is similar to the values of subducted serpentinites(>50 ppm).This similarity,associated with the high Cr contents,spinel-peridotite protolith compositions and Mg/Si and Al/Si ratios imply that the studied serpentinites were formed in a subductionrelated environment.The U-shaped chondrite normalizedREE patterns of serpentinized peridotites,coupled with similar enrichments in LREE and HFSE,suggest the refertilized nature due to melt/rock interaction prior to serpentinization.Based on the results,we suggest that the Eseka serpentinized peridotites are mantle residues that suffered a high degree of partial melting in a subductionrelated environment,especially in Supra Subduction Zone setting.These new findings suggest that the Nyong series in Cameroon represents an uncontested Paleoproterozoic suture zone between the Congo craton and the Sao Francisco craton in Brazil.
基金supported by the Project IGCP–649the China Geological Survey(No.DD20160023–01)
文摘The Zedong ophiolites in the eastern Yarlung-Zangbo suture zone of Tibet represent a mantle slice of more than 45 km2. This massif consists mainly of mantle peridotites, with lesser gabbros, diabases and volcanic rocks. The mantle peridotites are mostly harzburgite, lherzolite; a few dike-like bodies of dunite are also present. Mineral structures show that the peridotites experienced plastic deformation and partial melting. Olivine （Fos9.7-91.2）, orthopyroxene （Enss-92）, clinopyroxene （En4-49Wo47-slFS2-4） and spinel [Mg^#=lOOxMg/（Mg＋Fe）]=49.1-70.7; Cr^#=（100xCr/（Cr＋Al）=18.8-76.5] are the major minerals. The degree of partial melting of mantle peridotites is 10%-40%, indicating that the Zedong mantle peridotites may experience a multi-stage process. The peridotites are characterized by depleted major element compositions and low REE content （0.08-0.62 ppm）. Their ＂spoon-shaped＂ primitivemantle normalized REE patterns with （La/Sm）N being 0.50-6.00 indicate that the Zedong ultramafic rocks belong to depleted residual mantle rocks. The PGE content of Zedong peridotites （18.19-50.74 ppb） is similar with primary mantle with Pd/Ir being 0.54-0.60 and Pt/Pd being 1.09-1.66. The Zedong peridotites have variable, unradiogenic Os isotopic compositions with 187Os/18Os=0.1228 to 0.1282. A corollary to this interpretation is that the convecting upper mantle is heterogeneous in Os isotopes. All data of the Zedong peridotites suggest that they formed originally at a mid-ocean ridge （MOR） and were later modified in supra-subduction zone （SSZ） environment.
基金Supported by the Pilot Project of Knowledge Innovation Project,Chinese Academy of Sciences (Nos.KZCX2-YW-211, KZCX3-SW-223)the National Natural Science Foundation of China (No.40830849)the Special Foundation for the Eleventh Five-Year Plan of COMRA (No.DYXM-115-02-1-03)
文摘Peridotites from the southern Mariana forearc were sampled on the landward trench slope of the Izu-Bonin-Mariana (IBM) subduction zone by dredging.These mantle wedge peridotites underwent hydration by fluid derived from a dehydrated descending slab,and later interacted with seawater after emplacement at or near the seafloor.This study investigates how these two different rock-fluid interaction processes influenced trace element distribution in the southern Mariana forearc peridotites.We measured trace element concentrations of peridotites from the southern Mariana forearc.The southern Mariana forearc peridotites are characterized by a distinct seawater-like REE pattern with an obvious negative Ce anomaly,and La shows good correlation with other REEs (except Ce).In addition,there is a great enrichment of U,Pb,Sr and Li elements,which show a distinct positive anomaly relative to adjacent elements in the multi-element diagram.For the seawater-like REE pattern,we infer that REEs are mainly influenced by seawater during peridotite-seawater interactions after their emplacement at or near the seafloor,by serpentinization or by marine weathering.Furthermore,the anomalous behavior of Ce,compared with other rare earth elements in these samples,may indicate that they have undergone reactions involving Ce (IV) when the peridotites interacted with seawater.Positive U,Pb,Sr and Li anomalies are inferred to be related to seawater and/or fluids released during dehydration of the subducting slab.
基金financially supported by the National Natural Science Foundation of China(Grant No.41425011)。
文摘Platinum group elements(PGE)and Re-Os isotopes of mantle peridotites in the Jinshajiang ophiolite(SW China)were investigated in this study,in order to constrain the evolution of the lithospheric mantle beneath the Jinshajiang-Ailaoshan Ocean,which was a branch of the eastern Paleo-Tethys.The Jinshajiang peridotites have whole-rock compositions(e.g.,MgO=32.7-38.1 wt.%;Al_(2)O_(3)=0.67-1.30 wt.%)and spinels with moderate Cr#values(0.4-0.6)similar to those of abyssal peridotites,which indicate moderate degrees of partial melting(15%-20%).These peridotites exhibit U-shaped chondrite-normalized REE patterns that could be caused by hydrothermal alteration or melt-rock interaction after mantle melting.In addition,Pd concentrations and(Pd/Ir)_(N)ratios of the Jinshajiang peridotites increases with decreasing Al_(2)O_(3) concentrations.These negative correlations cannot be explained by simple partial melting but record a melt-rock reaction event after mantle melting.This study therefore demonstrates the efficiency of PGE in detecting the melt-rock reaction process relative to whole-rock major and trace elements.The suprachondritic^(187)Os/^(188)Os ratios(0.1272-0.1374)further indicate that the later percolating melt derived from a mantle domain with distinct^(187)Os-enriched isotopic compositions.In comparison with peridotites in the Ailaoshan ophiolite belt,which were not significantly affected by melt percolation,this study further highlights that the lithospheric mantle compositions beneath different segments of the same ocean basin are highly variable and might be controlled by distinct mantle processes in response to different rifting mechanisms.
基金granted by National Natural Science Foundation of China(41720104009)China Geology Survey Project(DD20160023-01)Foundation of MLR(201511022)
文摘The Peng Co ophiolite is located to the west of Peng lake in the area of lakes in north Tibet, which belongs to the Baila-Yilashan sub-belt of the the middle Bangong Co-Nujiang ophiolitic belt. The Peng Co ophiolite is mainly composed of mantle peridotites, cumulates, diabase dikes. About 70 percent peridotites are harzburgites and 30 percent are lherzolites. Mineral chemistry of the Peng Co lherzolitesare characterized by low Fo contents(88.85–90.33) of olivine and high Al2O3 content(4.26%–7.25%) in pyroxenes. Compared to the primitive mantle, the Peng Co peridotites have relatively higher MgO contents, lower CaO, Al2O3 and TiO2 contents. The total rare-earth element(REE) contents of the lherzolites are 1.11–1.53 ppm, which are lower than those of the primitive mantle. The chondritenormalized REE patterns of the Peng Co peridotites display slight loss in LREE. In the primitive mantle-normalized spider diagram, the Peng Co peridotites exhibit negative Rb and Zr anomalies and intensively positive U, Ta, Sr anomalies. The PGE contents of Peng Co lherzolites are between 22.9–27 ppb. The chondrite-normalized PGE patterns of the Peng Co lherzolites are consistent with that of the primitive mantle. Mineral and whole-rock geochemistry characteristics of the Peng Co lherzolites show an affinity to abyssal peridotites, indicating that it may have formed in the mid-ocean ridge setting. Through quantitative modeling, we conclude that the Peng Co lherzolites formed after 5%–10% degree of partial melting of the spinelphase lherzolite mantle source. The sharp increase of Cr#(56.74–60.84)in Spinel of harzburgites and relatively high Pd/Ir and Rh/Ir ratios suggest that they have experienced melt-rock reaction. The crystallization sequence of Peng Co cumulate is olivine-clinopyroxene-plagioclase. The Mg# value of clinopyroxene in cumulate peridotite ranges from 86.92 to 89.93, and the mean value of Fo is 84.45, which is obviously higher than that of MOR-type ophiolite cumulates. The mineral composition, sequence of magmatic crystallization and mineral components of Peng Co cumulate are similar to those of the cumulate formed by the SSZ-type ophiolite in the subduction zone. Therefore, we can draw a preliminary conclusion that Peng Co lherzolites were formed in an environment of mid oceanic ridge and were remnants of the spinel lherzolite zone which experienced a partial melting of no more than 10%. In the later period, due to the intra-oceanic subduction, it experienced the rock-meltinteraction, and thus formed the SSZ-type cumulate and harzburgite of high Cr value.
文摘The Beni Bousera peridotite massif(Internal Rif, Morocco),5 km in width and 15 km in length,is formed in a major part of spinel-bearing lherzolite rimed by a layer of garnet-bearing peridotite(100 m thick)which is in direct contact with HP-HT granulite metamorphic rocks(16 kbar,860℃).According to recent detailed study,the shearing contact between these two formations shows the presence of serpentinite and
基金The National Natural Science Foundation of China under contract Nos 41506047,41876044 and 91858214the Chinese Academy of Sciences’ Strategic Priority Research Program Grant under contract Nos XDB06030103 and XDB06030204
文摘Serpentinites,which contain up to 13 wt%of water,are important reservoirs for chemical recycling in subduction zones.In the past two decades,forearc mantle serpentinites were identified in different locations around the world.Here,we present petrology and whole rock chemistry of ultramafic and mafic rocks dredged from the Hahajima Seamount,which is located 24–40 km west to the junction of the Izu-Bonin Trench and the Mariana Trench.Nearly all the collected samples are extensively hydrated,and olivine grains in ultramafic rocks are replaced by serpentine minerals,with only one sample preserving remaining trace of orthopyroxene.Our new results show that the Hahajima serpentinized peridotite samples are all MgO-rich(~42 wt%),but have low contents in Al2O3,CaO,rare earth and high field strength elements,which is consistent with the overall depleted character of their mantle protoliths.Model calculations indicate that these Hahajima peridotite samples were derived from 10%–25%partial melting of the presumed fertile mantle source,which is generally lower than those of peridotites from Torishima Forearc Seamount,Conical Seamount and South Chamorro Seamount(mostly>25%).All the serpentinites from these four forearc seamounts show strong enrichment in fluid-mobile and lithophile elements(Li,Sr,Pb and U).In details,Hahajima Seamount serpentinites do not have obvious enrichment in Cs and Rb,and display remarkably high abundances of U.These observations indicate that the serpentinization of Hahajima peridotites occurred by addition of seawater or low temperature seawater-derived hydrothermal fluid,without or with little contribution from slab-derived fluids.The geochemical signature of serpentinites from Hahajima Seamount could be interpreted as the result of the combination of extensive partial melting and subsequent percolation of seawater through the mantle wedge.
基金Supported by the Pilot Project of Knowledge Innovation Project,Chinese Academy of Sciences (No KZCX2-YW-211,KZCX3-SW-223)the National Natural Science Foundation of China (No 40830849)the Special Foundation for the Eleventh Five Plan of COMRA (No DYXM-115-02-1-03)
文摘The serpentinized peridotites overlying the subducted zones in the Izu-Bonin-Mariana(IBM) arc system have been interpret as the cause of the low-velocity layer identified beneath the IBM froearc,in turn few earthquakes occurred along the plate boundary.Chrysotile,which is a low temperature and highly hydrated phase of serpentine with low frictional strength,has been suggested as the low velocity material in the serpentinized peridotites,besides,brucite is inferred to be likely conducive to stable sliding.However,such idea encounters challenging in our serpentinized peridotites from the southern Mariana forearc,which absent both the above minerals.The presence of talc,which characterized by its weak,low-friction and inherently stable sliding behavior,provides new clue.Here we report the occurrence of talc in serpentinized peridotites collected from the landward trench slope of the southern Mariana forearc.We infer that talc is mainly forming as a result of the reaction of serpentine minerals with silica-saturated fluids released from the subducting slab,and talc also occurs as talc veins sometimes.Due to its unique physical properties,talc may therefore play a significant role in aseismic slip in the IBM subduction zone.
文摘The Pozanti-Karsanti ophiolite(PKO)in Turkey’s eastern Tauride belt comprises mantle peridotites,ultramafic to mafic cumulates,isotropic gabbros,sheeted dikes and pillow lavas.The mantle peridotites are dominated by spinel harzburgites with minor dunites.The harzburgites and dunites have quite depleted mineral and whole-rock chemical composition,suggesting high degrees of partial melting.Their PGEs vary from Pd-depleted to distinct Pd-enriched patterns,implying the crystallization of interstitial sulphides from sulphur-saturated melts(e.g.MORB-like forearc basalt).U-shaped or spoon-shaped REE patterns indicate that the PKO peridotites may have also been metasomatized by the LREE-enriched fluids released from a subducting slab in a suprasubduction zone.Based on the mineral and whole-rock chemical compositions,the PKO peridotites show affinities to forearc peridotites.Chromitites occur both in the mantle peridotites and the mantle-crust transition zone horizon(MTZ).Chromitites from the two different horizons have different textures but similar mineral compositions,consistent with typical high-Cr chromitites.Chromitites hosted by mantle harzburgites generally have higher total platinum-group element(PGE)contents than those of the MTZ chromitites.However,both chromitites show similar chondritenormalized PGE patterns characterized by clear IPGEs,Rh-enrichments relative to Pt and Pd.Such PGE patterns indicate no or only minor crystallization of Pt-and Pd enriched sulphides during formation of chromitites from a sulphur-undersaturated melt(e.g.boninitic or island arc tholeiitic melt).Dunite enveloping chromitite lenses in the ho*s ting harzburgite resulted from melt-rock reaction.We have performed mineral separation work on samples of podiform chromitite hosted by harzburgites.So far,more than200 grains of microdiamond and more than 100 grains of moissanite(Si C)have been separated from podiform chromitites.These minerals have been identified by EDX and Laser Raman analyses.The diamonds and moissanite are accompanied by large amounts of rutile.Additionally,zircon,monazite and sulphides are also common phases within the heavy mineral separates.Both diamond and moissanite have been analyzed for carbon and nitrogen isotopic composition using the CARMECA 1280-HR large geometry Secondary Ion Mass Spectrometer at the Helmholtz Zentrum Potsdam.In total,61δ13CPDB results for diamond were acquired,exhibiting a range from-28.4‰to-18.8‰.31δ13CPDB results for Moissanite vary between-30.5‰to-27.2‰,with a mean value of-29.0‰.Diamond has relatively large variation in nitrogen isotopic composition with 40δ15NAIR results ranging from-19.1‰to 16.6‰.The discovery of diamond,moissanite and the other unusual minerals from podiform chromitite of the Pozanti-Karsanti ophiolite provides new support for the genesis of ophiolitic peridotites and chromitites under high-pressure and ultra-high reducing conditions.Considering the unusual minerals,the high Mg#silicate inclusions,and the needle-shaped exsolutions in the PKO chromitites,the parental melts of these chromitites may have been mixed with deep asthenospheric basaltic melts that had assimilated materials of the descending slab when passing through the slab in a subduction zone environment.We suggest melt-rock reactions,magma mixing and assimilation may have triggered the oversaturation of chromites and the formation of PKO chromitites.
文摘Ophiolites along the E-W trending Yarlung-Tsangpo Suture(YTS),which separates the Indian plate from the Eurasian plate,have been regarded as relics of the NeoTethyan Ocean.The Xigaze ophiolite in the central YTS
文摘The Jurassic–Cretaceous ophiolites in the Alpine–Himalayan orogenic belt represent fragments of oceanic lithosphere,developed in different seaways separated by Gondwana–derived ribbon continents within a broad
文摘Tectonic setting of the Neoproterozoic ophiolites is poorly understood.Because of extensive serpentinization/metamorphism in the mantle section,accessory chromian spinel has been used as an important geotectonic indicator.
文摘The present study deals with peridotites found in the paleo-proterozoic domain of the Leo Man Shield, Séguéla region, west-central of Côte d’Ivoire. Results show that Séguéla peridotites are composed of lherzolites, dunites and harzburgites. However, iherzolites are the most abundant. The phenocrysts in these rocks are olivine most often serpentinised, and pyroxenes, represented by diopside and enstatite. Amphiboles are divided into two groups: magmatic amphiboles and those from the destabilization of clinopyroxene. Spinels have CrO<sub>3</sub> content which varies between 28% and 37%, Al<sub>2</sub>O<sub>3</sub> between 33% - 41% and MgO is equal to 18%;they are poor in TiO<sub>2</sub> and do not contain zinc oxide. Séguéla peridotites are characterised by a negative anomaly in Nb-Ta, associated with an enrichment of lithophilic elements and light rare earth elements with a fractionation rate (La/Yb) which varies between 46.22 and 150.72 and heavy rare earth elements, and HFSE depletion, which may suggest that were formed in an art context, that is, a mantle enriched by fluids from a subduction zone. However, another hypothesis could be considered, that of the interaction between the mantle and magmas in a subduction zone context.
基金supported by the National Natural Science Foundation of China (Nos.40872146, 41072166, 41272240)the Project of China Geological Survey (No. 1212011086064,1212011221093)
文摘Taoxinghu metamorphic peridotite is a firstly reported mantle sequence of ophiolite since Longmuco-Shuanghu-Lancangjiang suture zone （LSLSZ） was proposed, and it is also an important discovered for ophiolite studying in central Qiangtang. Based on detailed analyses of whole-rock geochemistry of Taoxinghu metamorphic peridotites and contrast to metamorphic peridotites in typical ophiolites worldwide, the paper investigates their petrogenesis and geological implication. The petrologic results show that the protolith of Taoxinghu metamorphic perdotites have the mineral assemblage and texture characteristic of mantle peridotite. Most metamorphic peridotites hav near global abyssal peridotites major elements contents, while the few is similar to SSZ-type peridotites. They exhibit typically U-shaped REE patterns, characterized by slight enrichment of LREE and HREE relative to MREE and a low fractionated LREE to HREE segment. Trace elements contents are low and all samples are strong enrichment in Cs, U, Pb, weak enrichment in Ba and depletion in Th, but negative Nb anomalies are only observed in few samples. That suggests Taoxinghu metamorphic peridotites have depletion mantle and suprasubduction affinities. A two-stage evolution history is considered： Taoxinghu metamorphic peridotites originated as the residue from melting at a ridge with 7%-20% degree of fraction melting and were subsequently modified by interaction with mafic melt and aqueous fluid within mantle wedge on subducted zone. Combined with previous studies, we preliminarily propose Taoxinghu metamorphic peridotites may be the Products of initial rifting of palo-Tethys, forming at middle Ordivician-upper Cambrian, and they may be the direct evidences for spreading of palo-Tethys.
基金supported by the Marine Geological Survey of the 1 : 250 000 Rizhao Sheet and Lianyungang Sheet (No. GZH201400206)
文摘We present a new dataset on platinum group elements（PGEs）, whole-rock major and trace elements, and mineral chemistry for the peridotites from the Zedang and Luobusa ophiolite suites, Tibet, in an attempt to better constrain the petrogenesis of the Zedang and Luobusa ophiolites and the tectonic evolution of the Neo-Tethys. Plots of chondrite-normalized PGE, PGE vs. Mg#, and PGE vs. Al_2O_3 suggest that the lherzolite and harzburgite from Zedang and Luobusa have similar PGE characteristics. The Zedang and Luobusa peridotites display U-shaped REE patterns and are enriched in some incompatible elements, indicative of melt-rock interaction. The PGE characteristics may be attributed to partial melting and heterogeneous melt-rock interaction. Mineral chemistry and whole rock major and trace elements data suggest that lherzolite and harzburgite from Zedang and Luobusa have similar geochemical properties. On the spinel Mg# vs. Cr# plot, the composition of the Zedang and Luobusa peridotites is consistent with both abyssal and subduction-zone peridotites. This study indicates that the Zedang and Luobusa peridotites have a similar origin and evolution path： they could have originated from a normal mid-ocean ridge environment and got refertilization in a supra-subduction zone setting.
基金The National Basic Research Program of China under contract No.2013CB429705the Public Science and Technology Research Funds Projects of Ocean,the State Oceanic Administration of the People's Republic of China under contract No.201005003+1 种基金the National Natural Science Foundation of China under contract Nos 41106051,40906037 and 41176045the Fundamental Research Funds of the State Oceanic Administration of the People's Republic of China under contract No.JT1001
文摘The mineral chemistry and texture of clinopyroxenes in peridotite from the Kingkong tectonic zone of the Southwest Indian Ridge segment in an effort to constrain mantle melting beneath this slow-spreading ridge are reported. There are three types of clinopyroxenes in the abyssal peridotites： coarse-grained, intergranu- lar and exsolved. The compositional variations among these three types suggest that the coarse-grained clinopyroxene is a mantle-derived source. The A1, Na and Ti contents and the Na/Ti ratio of the coarse- grained clinopyroxene may be used to monitor the degree of partial melting, combined with the contradis- tinction with Spinel Cr#, which is calculated to be between 7.9% and 14.9%, and may represent low degrees of melting in the global ocean ridge system. The along-axis compositional variations in the coarse-grained clinopyroxene suggest that the degree of partial melting is primarily controlled by the transform faults on both sides of the ridge. Nonetheless, the northwestern side of the ridge may be affected by a hypothesised detachment fault as documented by the calculated P-T conditions. Simultaneously high Na and low Ti con- tents in the coarse-grained clinoovroxene points to mantle heterogeneities along the ridge axis.
文摘The rock forming temperatures and pressures represent the p T points of the local regions in the lithosphere at a certain age, providing some important information on rock formation. Based on the preliminary statistics on the temperatures and pressures for the formation of eclogites, granulites and peridotites in China, the variant ranges are given, in this paper, of temperatures, pressures and linear geothermal gradients of eclogites, granulites and peridotites. In addition, since the eclogite is different from granulite and peridotite in the p T diagram, these three rocks can be classified into two groups: the first group includes eclogites and the second group granulites and peridotites. Then, the p T correlation functions of these two groups of rocks are provided. Finally, the two groups of rocks have different geothermal gradients at the same pressure gradient or have different pressure gradients at the same geothermal gradient. The temperatures and pressures for the formation of the rocks can be calculated from the mineral chemical compositions, but the depths ( H ) for the rock formation can be calculated only under the hypotheses of given p H (or T H ) correlation functions. The explanations for the ultrahigh pressure metamorphism vary obviously with different hypotheses.
基金the Project IGCP-649 and the IGG UB RAS (No. АААА-А18118052590029-6)
文摘To discuss the nature of the compositional heterogeneity of the peridotite massifs of the Polar Urals(Russia), the geochemical study by LA-ICP-MS of pyroxenes and amphiboles from these mantle formations was performed. The trace element compositions in clinopyroxenes indicate the existence of the mantle protolith of two types. The first protolith type, represented by lherzolites and diopside harzburgites,was originated from the partial melting(5%–10%) under the spinel facies conditions, while the second one,represented by diopside harzburgites, was formed under the polybaric partial melting(17%–19%) under garnet and spinel facies conditions. Subsequently, the mantle peridotite protolith was subject to fluidinduced partial melting in the suprasubduction setting that was resulted in the formation of harzburgites.Being affected by penetrating melts and fluids peridotites experienced the refertilization(LREE enrichment of clinopyroxenes) and high-temperature hydratation with subsequent development of pargasite and Mg amphibole. The high-T fluid-induced metamorphism at the subduction zone was accompanied by the formation of metaperidotites with clinochlore and REE-depleted tremolite.
基金The reported study was funded by RFBR according to the research project(18-35-00535)IGC State Assignment Project(0350-2019-0008).
文摘Swarms of orthopyroxenite and websterite veins are found within Egiingol residual SSZ peridotite massif of Dzhida terrain(Central Asian Orogenic Belt,Northern Mongolia).The process of Egiingol pyroxenite veins formation is investigated using new major and trace element analyses of pyroxenite minerals,calculations of closure temperatures and composition of equilibrium melt.The pyroxenites show abundant petrographic and geochemical evidence for replacement of the residual peridotite minerals by ortho-and clinopyroxene due to melt-rock interaction.Relics of peridotite olivines are found in pyroxenites,Cr#of spinel increases from peridotites to pyroxenites,and compositions of ortho-and clinopyroxene change from peridotite to pyroxenite.The authors show that calculated equilibrium melts for investigated pyroxenites are very similar to compositions of boninite lavas from the Dzhida terrain.Therefore,formation of pyroxenite veins most likely resulted from percolation of boninite melts through the Egiingol peridotites.Orthopyroxenite veins formed at first,followed by websterite veins.Thus,the authors assume that pyroxenite veins represent the channels for boninitic melts migration in supra-subduction environment.
