Deciphering high-pressure granulite-facies metamorphism and anatexis within a collisional orogeny can provide crucial constraints on geodynamic evolution and melt activity during subduction and exhumation.Combining pe...Deciphering high-pressure granulite-facies metamorphism and anatexis within a collisional orogeny can provide crucial constraints on geodynamic evolution and melt activity during subduction and exhumation.Combining petrographic observations,mineral chemistry,REE in Grt-Cpx thermobarometry,and previous work,at least four stages are suggested for the metamorphic evolution of the mafic granulites in the South Altun,including the protolith stage,the high-pressure granulite-facies stage(909-1037℃and 17.3-30 kbar),medium-pressure granulite-facies overprint(9.1-11.9 kbar and 753-816℃),and subsequent late amphibolite-greenschist-facies metamorphism.Zircon U-Pb dating shows that the mafic granulites underwent high-pressure granulite-facies metamorphism at 497.2±3.7 Ma,while the leucosome formed at 498.2±2.9 Ma.Thus,the leucosomes from the host mafic granulite may have been formed at the high-pressure granulite-facies metamorphic event.The characteristics of zircon morphology,mineral inclusions,low Th/U values,HREE enrichment,and negative Eu anomalies indicate that these zircons from the leucosome were formed from the metamorphic melts.The characteristics of whole-rock major and trace elements as well as Hf isotopic features of zircons between the leucosomes and the host mafic granulite indicate that the melt may have been generated by the partial melting of the host mafic granulite.展开更多
The conventional view suggests that the subduction of the South China Sea plate beneath Luzon occurred due to the oceanic lithosphere’s high density,facilitating subduction initiation.However,before the South China S...The conventional view suggests that the subduction of the South China Sea plate beneath Luzon occurred due to the oceanic lithosphere’s high density,facilitating subduction initiation.However,before the South China Sea opened,a continental margin likely existed,meaning that Luzon was directly adjacent to the continental margin rather than the oceanic basin.This would make subduction initiation more challenging.Here,we propose a new model suggesting that during the formation of the South China Sea,extensive mafic magmatic underplating occurred along its continental margin.The high-density magmatic additions may have increased the overall density of the continental margin,potentially exceeding that of Luzon,thereby enabling subduction to proceed.展开更多
Distinguishing high-grade mafic-ultramafic rocks originally crystallized from within-plate basaltic magmatism is challenging and crucial because the chemical composition of the igneous rocks has been modified during h...Distinguishing high-grade mafic-ultramafic rocks originally crystallized from within-plate basaltic magmatism is challenging and crucial because the chemical composition of the igneous rocks has been modified during high-grade metamorphism,causing misidentification of the characters of the parental magma.Proterozoic metamorphosed mafic dykes occur throughout the Chhotanagpur Gneissic Complex(CGC)of eastern Indian shield.The E-W trending mafic dykes from the Saltora area in the southeastern CGC underwent metamorphism in two episodes:M1(650 MPa;770℃)and M2(300 MPa;744℃).The metamafics are enriched in LILE,depleted in HFSE,and display strong fractionation of LREE,nearly flat HREE patterns in a chondrite-normalized REE diagram,and show tholeiitic differentiation trend.Their geochemical affinity is towards rift-related,continental within-plate basalts.About 7%–10%melting of the carbonated spinel-peridotite sub-continental lithospheric mantle(SCLM)produced the parental mafic magma.The pre-existing SCLM was metasomatized by slab-derived fluid during the previous subduction.The upwelling of the asthenosphere in a post-collisional tectonic setting caused E-W trending fractures,lithospheric thinning,and gravitational collapse.These dykes were emplaced during crustal extension around 1070 Ma.The remarkable geochemical similarity between the mafic dykes of Saltora and Dhanbad,the ca.1096 Ma Mahoba(Bundelkhand craton),and the ca.1070 Ma Alcurra mafic dykes in Australia supports a genetic link.展开更多
The Bathani volcano-sedimentary sequence(BVSs)is a typical example of volcanic-plutonic complex from the Proterozoic Chotanagpur Granite Gneiss Complex.We conducted petrological and geochemical studies on two domains ...The Bathani volcano-sedimentary sequence(BVSs)is a typical example of volcanic-plutonic complex from the Proterozoic Chotanagpur Granite Gneiss Complex.We conducted petrological and geochemical studies on two domains of the BVSs;one is a granite pluton,while the other is a subvolcanic rhyolite dome.These two domains were intruded by mafic magmas during their evolution leading to magma mixing.The mafic rocks show tholeiitic trend,while calcalkaline affinities are displayed by the felsic rocks.The rocks display enriched LILE and fractionated REE patterns with enrichments in LREE relative to HREE.展开更多
The Beishan area has more than seventy mafic-ultramafic complexes sparsely distributed in the area and is of a big potential in mineral resources related to mafic-ultramafic intrusions. Many mafic-ultramafic intrusion...The Beishan area has more than seventy mafic-ultramafic complexes sparsely distributed in the area and is of a big potential in mineral resources related to mafic-ultramafic intrusions. Many mafic-ultramafic intrusions which are mostly in small sizes have been omitted by previous works. This research takes Huitongshan as the study area, which is a major district for mafic-ultramafic occurrences in Beishan. Advanced spaceborne thermal emission and reflection radiometer（ASTER） data have been processed and interpreted for mapping the mafic-ultramafic complex. ASTER data were processed by different techniques that were selected based on image reflectance and laboratory emissivity spectra. The visible near-infrared（VNIR） and short wave infrared（SWIR） data were transformed using band ratios and minimum noise fraction（MNF）, while the thermal infrared（TIR） data were processed using mafic index（MI） and principal components analysis（PCA）. ASTER band ratios（6/8, 5/4, 2/1） in RGB image and MNF（1, 2, 4） in RGB image were powerful in distinguishing the subtle differences between the various rock units. PCA applied to all five bands of ASTER TIR imagery highlighted marked differences among the mafic rock units and was more effective than the MI in differentiating mafic-ultramafic rocks. Our results were consistent with information derived from local geological maps. Based on the remote sensing results and field inspection, eleven gabbroic intrusions and a pyroxenite occurrence were recognized for the first time. A new geologic map of the Huitongshan area was created by integrating the results of remote sensing, previous geological maps and field inspection. It is concluded that the workflow of ASTER image processing, interpretation and ground inspection has great potential for mafic-ultramafic rocks identifying and relevant mineral targeting in the sparsely vegetated arid region of northwestern China.展开更多
The Naga Hills Ophiolite(NHO) represents one of the fragments of Tethyan oceanic crust in the Himalayan Orogenic system which is exposed in the Phek and Kiphire districts of Nagaland, India. The NHO is composed of par...The Naga Hills Ophiolite(NHO) represents one of the fragments of Tethyan oceanic crust in the Himalayan Orogenic system which is exposed in the Phek and Kiphire districts of Nagaland, India. The NHO is composed of partially serpentinized dunite, peridotite, gabbro, basalt, minor plagiogranite,diorite dyke and marine sediments. The basalts are mainly composed of fine grained plagioclase feldspar, clinopyroxene and orthopyroxene and show quenching and variolitic textures. The gabbros are characterized by medium to coarse grained plagioclase, orthopyroxene and clinopyroxene with ophitic to sub-ophitic textures. The ultramafic cumulates are represented by olivine, Cpx and Opx.Geochemically, the basalts and gabbros are sub-alkaline to alkaline and show tholeiitic features.The basalts are characterized by 44.1-45.6 wt.% of SiO_2 with 28-38 of Mg#, and the gabbros by38.7-43.7 wt.% of SiO_2, and 26-79 of Mg#. The ultramafic rocks are characterized by 37.4-52.2 wt.% of SiO_2, and 80-88 of Mg#. In multi-element diagrams(spidergrams) both basalts and gabbros show fractionated trends with strong negative anomalies of Zr. Nb. Sr and a gentle negative anomaly of P.However, the rare earth element(REE) plots of the basalts and gabbros show two distinct patterns. The first pattern, represented by light REE(LREE) depletion, suggests N-MORB features and can be interpreted as a signature of Paleo-Tethyan oceanic crust. The second pattern, represented by LREE enrichment with negligible negative Eu anomaly, conforms to E-MORB, and may be related to an arc tectonic setting. In V vs. Ti/1000, Cr vs. Y and AFM diagrams, the basalts and gabbros plot within Island Arc Tholeiite(IAT) and MORB fields suggesting both ridge and arc related settings. The ultramafic rocks exhibit two distinct patterns both in spidergrams and in REE plots. In the spidergram, one group displays highly enriched pattern, whereas the other group shows near flat pattern compared to primordial mantle. In the REE plot, one group displays steeper slopes [(La/Yb)N = 4.340-4.341], whereas the other displays moderate to flat slopes [(La/Yb)N = 0.97-1.67] and negative Eu-anomalies. Our study suggests that the ultramafic rocks represent two possible mantle sources(fertile and refractory).展开更多
The Niangzhong diabase dikes,dated at 138.1±0.4 Ma,are located within the outcrop area of the Comei large igneous province(LIP).These diabase samples can be divided into two groups:samples in Group 1 show varying...The Niangzhong diabase dikes,dated at 138.1±0.4 Ma,are located within the outcrop area of the Comei large igneous province(LIP).These diabase samples can be divided into two groups:samples in Group 1 show varying MgO(1.50 wt.%-10.25 wt.%)and TiO_(2)(0.85 wt.%-4.63 wt.%)contents,and enriched initial isotope compositions(^(87)Sr/^(86)Sr(t)=0.7056-0.7112,ε_(Nd)(t)=-0.3-+3.8),with OIB-like REEs and trace elements patterns,resulting from low degree melting of garnet-bearing lherzolite mantle sources;in contrast,samples in Group 2 show limited MgO(4.14 wt.%-7.75 wt.%)and TiO_(2)(0.98 wt.%-1.69 wt.%)contents,and depleted initial isotope compositions(^(87)Sr/^(86)Sr(t)=0.7075-0.7112,ε_(Nd)(t)=+5.5-+6.2),with N-MORB-like REEs and trace elements patterns,resulting from relatively high degree melting of spinel-bearing lherzolite mantle source.Combined with the published representative data about Comei LIP,we summarize that the source components for Comei LIP products include OIB end-member,enriched OIB end-member,and N-MORB end-member,respectively.Melts modeling suggests that magmas in the Comei LIP evolve in a relatively high oxygen fugacity condition,which influenced their fractionation sequences and led to systematic changes of TiO_(2)contents,Ti/Y and Ti/Ti*ratios.From the spatial and temporal distribution of above three end-member samples,deep process of Kerguelen plume during the Comei LIP formation can be interpreted as the interaction among the Kerguelen plume,the overlying lithospheric mantle,and the upwelling asthenosphere.The magmatism of Comei LIP began at~140 Ma and then lasted and peaked at~132 Ma with the progressively lithospheric thinning of eastern Gondwana upon the impact of Kerguelen plume.展开更多
Occurrence of mafic dykes in Himalaya has been intriguing and debated since long because of its difficulty to ascent and emplacement through a thickened crust.Mafic dykes in Kumaun Lesser Himalaya(KLH)of central Indian
Mafic enclaves in granites are generally considered to represent coeval mafic melts that derived from metasomatized mantle,which can provide valuable information about crust-mantle interaction.Exploring the genetic li...Mafic enclaves in granites are generally considered to represent coeval mafic melts that derived from metasomatized mantle,which can provide valuable information about crust-mantle interaction.Exploring the genetic links between the mafic enclaves and their host monzogranite from the Triassic Zhashui Pluton,Qinling orogenic belt.The enclaves(220±4.6 Ma)and the monzogranite(220±2.8 Ma)display identical zircon U-Pb ages,and they also share similar trace element and zircon Lu-Hf isotopes,indicating a cognate source.The monzogranite displays zirconεHf(t)values of−0.99 to+1.98,while the mafic enclaves show similar values of−0.45 to+3.26;however,differences in mineral chemistry reveal different crystallization conditions.The amphibole from the mafic enclaves has higher temperature and pressure(757℃;2.65 kbar)compared to those of the host monzogranite(733℃;1.96 kbar),suggesting that mafic minerals in the enclaves crystallized at an early stage.Moreover,apatite in the mafic enclaves displays slightly higher volatile contents(0.72 wt%)than those of the monzogranite(0.66 wt%),indicating a volatile-rich condition.These results suggest that the mafic enclaves represent early hydrous mafic cumulates in the granitic chamber,and subsequent magma convection would have led to the formation of the mafic enclaves.展开更多
Virtual absence of igneous complexes with ages between1.8 Ga and 0.8 Ga in southern part of the Siberian Craton allowed to Galdkochub et al.(2010)to formulate a hypothesis of long magmatic quiescence.Most reliable
In this paper we have synthesized the published and unpublished geochemical data on the Palaeoproterozoic mafic magmatism in the Indian Shield.Palaeoproterozoic mafic magmatism is widespread in the Indian Shield;it mo...In this paper we have synthesized the published and unpublished geochemical data on the Palaeoproterozoic mafic magmatism in the Indian Shield.Palaeoproterozoic mafic magmatism is widespread in the Indian Shield;it mostly emplaced as dyke intrusions within the cratons and south Indian granulite region and as intrusives/traps in the intra-cratonic basins and the Eastern Ghat Mobile Belt.展开更多
The Bastar craton largely consists of Mesoarchaean orthogneisses with vestiges of supracrustal rocks that have been intruded by Proterozoic granites and mafic dykes.Many regions in the Bastar craton have been subjecte...The Bastar craton largely consists of Mesoarchaean orthogneisses with vestiges of supracrustal rocks that have been intruded by Proterozoic granites and mafic dykes.Many regions in the Bastar craton have been subjected to medium or high grade metamorphic conditions but the nature and timing of regional metamorphism is poorly understood.In this study,metamorphosed mafic igneous rocks collected from two different dyke swarms from the southern part of the Central Indian Bastar craton have been studied to evaluate the timing and conditions of metamorphism.In this region,two distinct metamorphosed dyke swarms are recognized based on differences in petrology and geochemistry:the Meso-Neoarchaean sub-alkaline BD1 swarm and the Neoarchaean boninite-norite(BN)swarm.展开更多
The Proterozoic felsic and mafic magmatism in India in varied tectonic settings is reviewed and discussed based on available geological,geochemical,and geochronological constraints.Neoarchean-Paleoproterozoic magmatis...The Proterozoic felsic and mafic magmatism in India in varied tectonic settings is reviewed and discussed based on available geological,geochemical,and geochronological constraints.Neoarchean-Paleoproterozoic magmatism,as discrete volcanoplutonic complexes and Large Igneous Provinces(LIPs)in the Bastar,Singhbhum and Dharwar cratons and associated mobile belts are also included.Paleoproterozoic magmatism also contributed to the geodynamics of Himalaya.展开更多
Based on K-Ar isotope analyses, Mesozoic mafic (and alkali ultramafic) dikes from western and eastern Shandong Province, China, are dated at 88.2±1.70 Ma to 169.5±3.7 Ma with the majority of ages ranging fro...Based on K-Ar isotope analyses, Mesozoic mafic (and alkali ultramafic) dikes from western and eastern Shandong Province, China, are dated at 88.2±1.70 Ma to 169.5±3.7 Ma with the majority of ages ranging from 90 Ma to 140 Ma. The emplacement of the dikes suggests a major Yanshanian (Cretaceous) crustal extension in Shandong province. Together with other available age data, this study suggests four periods of crustal extension at about 80 Ma, 100 Ma, 120 Ma and 140 Ma, respectively. Besides the effect of collapse of the Yanshanian orogenic belt on the emplacement of the mafic dikes in Shandong in the Cretaceous, the mantle plume and the extensive left-lateral advection and extension of the Tanlu fault also have controlled the crustal extension and the emplacement of the mafic dikes in eastern Shandong and western Shandong, respectively.展开更多
High-precision data on U and Th contents and Th/U ratios of zircon obtained using secondary ion mass spectrometry analysis have been collected from the literature. Zircon in the granitic rocks has median values of 350...High-precision data on U and Th contents and Th/U ratios of zircon obtained using secondary ion mass spectrometry analysis have been collected from the literature. Zircon in the granitic rocks has median values of 350 ppm U, 140 ppm Th, and Th/U=0.52; the recommended zircon-melt partition coefficients are 81 for Du and 8.2 for DTh. In zircon from mafic and intermediate rocks, the median values are 270 ppm U, 170 ppm Th, and Th/U=0.81, and the recommended zirconmelt partition coefficients are 169 for Du and 59 for DTh. The U and Th contents and Th/U ratios of magmatic zircon are low when zircon crystallizes in equilibrium with the melt. Increasing magma temperature should promote higher Th contents relative to U contents, resulting in higher Th/U ratios for zircon in mafic to intermediate rocks than in granitic rocks. However, when zircon crystallizes in disequilibrium with the melt, U and Th are more easily able to enter the zircon lattice, and their contents and Th/U ratios depend mainly on the degree of disequilibrium. The behavior of U and Th in magmatic zircon can be used as a geochemical indicator to determine the origins and crystallization environments of magmatic zircon.展开更多
The Xuhe mafic rocks, located in Ziyang county of Shaanxi Province, are dominated by diabase-porphyrite, gabbro-diabase, diabase, and pyroxene diorite. Primitive mantle-normalized multi-element patterns show that, the...The Xuhe mafic rocks, located in Ziyang county of Shaanxi Province, are dominated by diabase-porphyrite, gabbro-diabase, diabase, and pyroxene diorite. Primitive mantle-normalized multi-element patterns show that, the Xuhe marie rocks are enriched in large ion lithophile elements （LITE）, such as Ba and Pb, depleted in K and Sr for basic rocks, and are depleted in Sr, P and Ti for pyroxene diorite. Chondrite-normalized REE patterns display LREE enrichment （LaN/YbN = 9.34- 13.99） and have normalized patterns for trace element and REE similar to that of typical OIB. Detailed SIMS zircon U-Pb dating yields emplacement ages of 438.4 ± 3.1 Ma for Xuhe mafic rocks. The relatively low MgO （basic rock： 3.11-7.21 wt%; pyroxene diorite： 0.89-1.21 wt% ） and Mg# （0.20- 0.49） for Xuhe mafic rocks suggest that they were possibly originated from an extremely evolved magma. The rising parental mafic magmas underwent pyroxene and plagioclase fractionation. Crustal contamination of pyroxene diorite before emplacement occurred at a higher crustal level compared to other lithology in Xuhe mafic rocks. The degree of partial melt was low （5%-10%） and in garnetspinel transition facies. Sr-Nd isotope of pyroxene diorite and enrichment mantle characteristics for Xuhe mafic rocks suggest that mafic rocks in the North Daba Mountains were derived from a mixture of HIMU, EMII and small amount of EMI components. Furthermore, this study discusses mantle geodynamic significance of Xuhe mafic rocks in the Silurian, which indicates subduction and uplift of magma caused back-arc extension.展开更多
Mafic rocks are widespread on the Liaodong Peninsula and adjacent regions of the North China Craton. The majority of this magmatism was originally thought to have occurred during the Pre-Sinian, although the precise g...Mafic rocks are widespread on the Liaodong Peninsula and adjacent regions of the North China Craton. The majority of this magmatism was originally thought to have occurred during the Pre-Sinian, although the precise geochronological framework of this magmatism was unclear. Here, we present the results of more than 60 U-Pb analyses of samples performed over the past decade, with the aim of determining the spatial and temporal distribution of mafic magmatism in this area. These data indicate that Paleoproterozoic-Mesoproterozoic mafic rocks are not as widely distributed as previously thought. The combined geochronological data enabled the subdivision of the mafic magmatism into six episodes that occurred during the middle Paleoproterozoic, the late Paleoproterozoic, the Mesoproterozoic, the Late Triassic, the Middle Jurassic, and the Early Cretaceous. The middle Paleoproterozoic （2.1-2.2 Ga） mafic rocks formed in a subduction-related setting and were subsequently metamorphosed during a ca. 1.9 Ga arc-continent collision event. The late Paleoproterozoic （ca. 1.87-1.82 Ga） bimodal igneous rocks mark the end of a Paleoproterozoic tectono- thermal event, whereas Mesoproterozoic mafic dike swarms record global-scale Mesoproterozoic rifting associated with the final breakup of the Columbia supercontinent. The Late Triassic mafic magmatism is part of a Late Triassic magmatic belt that was generated by post-coilisional extension. The Middle Jurassic mafic dikes formed in a compressive tectonic setting, and the Early Cretaceous bimodal igneous rocks formed in an extensional setting similar to a back-arc basin. These latter two periods of magmatism were possibly related to subduction of the Paleo-Pacific plate.展开更多
The Napo-Qinzhou Tectonic Belt （NQTB） lies at the junction of the Yangtze, Cathaysia and Indochina （North Vietnam） Blocks, which is composed of five major lithotectonic subunits： the Qinzhou-Fangcheng Suture Zone...The Napo-Qinzhou Tectonic Belt （NQTB） lies at the junction of the Yangtze, Cathaysia and Indochina （North Vietnam） Blocks, which is composed of five major lithotectonic subunits： the Qinzhou-Fangcheng Suture Zone （QFSZ）, the Shiwandashan Basin （SB）, the Pingxiang-Nanning Suture Zone （PNSZ）, the Damingshan Block （DB） and the Babu-Lingma Suture Zone （BLSZ）. On the basis of geochemical compositions, the Permian mafic igneous rocks can be divided into three distinct groups： （1） mafic igneous rocks （Group 1） from the Longjing region in the PNSZ and Hurun region in the BLSZ, which are characterized by intermediate Ti, P and Zr with low Ni and Cr contents; （2） mafic igneous rocks （Group 2） from the Naxiao and Chongzuo region in the DB, characterized by low-intermediate Ti, P and Zr with high Ni and Cr concentrations; and （3） mafic igneous rocks （Group 3） from the Siming region in the Jingxi carbonate platform of the northwestern margin of the NQTB, with intermediate-high Ti, P and Zr and low Ni and Cr contents. The Group 1 rocks yield a weighted mean 206 Pb/ 238 U age of 250.5±2.8 Ma and are geochemically similar to basalts occurring in back-arc basin settings. The Group 2 rocks exhibit geochemical features to those basalts in island arcs, whereas the Group 3 rocks show geochemical similarity to that of ocean island basalts. All three groups are characterized by relatively low ε Nd （t） values （-2.61 to ＋1.10） and high initial 87 Sr/ 86 Sr isotopic ratios （0.705309-0.707434）, indicating that they were derived from a subduction-modified lithospheric mantle and experienced assimilation, fractional crystallization, and crustal contamination or mixing during magmatic evolution. Accordingly, we propose the existence of an arc-back arc basin system that developed along the NQTB at the border of SW Guangxi Province （SW China） and northern Vietnam, and it was formed by continued northwestward subduction of the Cathaysian （or Yunkai） Block under the Yangtze Block, and northeastward subduction of the Indochina Block beneath the Yangtze Block during Permian time.展开更多
Petrological analysis and LA-ICP-MS zircon U-Pb dating were conducted on high- pressure marie granulites, which occured as xenolith within TTG gneisses, from the Nanshankou Village of the Jiaobei terrane, Shandong Pen...Petrological analysis and LA-ICP-MS zircon U-Pb dating were conducted on high- pressure marie granulites, which occured as xenolith within TTG gneisses, from the Nanshankou Village of the Jiaobei terrane, Shandong Peninsula in the north-eastern part of the North China Craton （NCC）. The mafic HP grannlite is composed of garnet, clinopyroxene, orthopyroxene, amphibole and symplectitic clinopyroxene, orthopyroxene, plagioclase, ilmente and magnetite which were formed after the decomposition of porphyroblastic garnet and clinopyroxene. Four stages of metamorphic mineral assemblages for the mafic HP granulites were constrained by detail petrological and mineralogical in- vestigations. The early prograde assemblage is represented by the mineral inclusions within garnet and clinopyroxene porphyroblasts （Opx1＋Pl1＋Qtz1）, recording the metamorphic conditions at -754-757 ℃, 0.63-0.71 GPa; peak metamorphic conditions were determined at -874-891 ℃, 1.32-1.35 GPa with the mineral assemblage of Grt2＋Cpx2＋Amp2＋Pl2＋Qtz2. Retrograde minerals derived from symplectitic assemblage Opx3＋Cpx3＋Amp3＋Pl3＋Qtz3＋Ilm3±Mag3 were formed at 693-796℃, 0.60-0.84 GPa. A final greensehist to sub-greenschist facies event was recorded by the exsolution of actinolite and albite within a retrograded clinopyroxene, as well as the occurrence of prehnite, chlorite and calcite minerals. Accordingly, a clockwise P-T path was concluded on the basis of the different stages of mineral asseblage. Cathodoluminescence imaging, trace element and U-Pb dating of zircons from the mafic HP granulites recorded similar charactistics for three episodes of Paleo-Meso Proterozoic metamorphic events. These are the metamorphic events preserved in mafic and pelitic granulites in the Jiao-Liao-Ji belt （JLJB） with 207 pb/206pb ages of 2.0-1.9 Ga for peak metamorphism and of 1.86-1.84 Ga for decomposing process, followed by a retrograde amphibolite facies metamorphic event related to the post-orogenic extension at the age of 1.76-1.74 Ga, resulting the exhumation of the granulite to the upper crust level.展开更多
The Helanshan tectonic belt(HTB) is a major tectonic divide between the Alxa and Ordos blocks in the North China Craton. The geochronology and petrogenesis of the mafic dykes in the northern HTB are keys to understand...The Helanshan tectonic belt(HTB) is a major tectonic divide between the Alxa and Ordos blocks in the North China Craton. The geochronology and petrogenesis of the mafic dykes in the northern HTB are keys to understanding the tectonic evolution of this belt. The mafic dykes, intruded into the Neoarchean-Paleoproterozoic metamorphic basement, are mainly composed of diabase with a mineral assemblage of plagioclase(45%-60%), pyroxene(25%-35%), minor quartz and Fe-Ti oxides. The LA-ICPMS U-Pb analysis of zircon grains from representative dykes yield a weighted mean age of 206 ± 1.9 Ma, which represents the crystallization age of the dyke. The diabases show high contents of Fe_2 O_3~T(11.88-17.55 wt.%), low contents of SiO_2(45.65-50.95 wt.%) and MgO(3.31-5.50 wt.%) with low Mg#(=100×MgO/(MgO + FeO) atomic ration) of 33-44. They are characterized by enrichment of light rare earth elements(LREEs) and large ion lithophile elements(LILEs)(e.g., Rb, Ba and Pb), and slight depletion of high field strength elements(HFSEs). These features suggest that the magma has undergone extensive fractionation of olivine and pyroxene but only minor crustal contamination during its evolution. Their high Sm contents and La/Sm ratios, and low Sm/Yb ratios indicate that magma from which the dykes formed was derived from low degree(about 5%) partial melting of an enriched garnet + spinel lherzolite mantle source. Together with regional geology, these geochemical and geochronological data suggest that the mafic dykes in the HTB were formed in an intracontinental extensional setting during the late Triassic.展开更多
基金financially supported by the Basic Foundation of Tianjin University of Commercethe Fund from the Key Laboratory of Continental Dynamics of Ministry of Natural Resources(J2306)。
文摘Deciphering high-pressure granulite-facies metamorphism and anatexis within a collisional orogeny can provide crucial constraints on geodynamic evolution and melt activity during subduction and exhumation.Combining petrographic observations,mineral chemistry,REE in Grt-Cpx thermobarometry,and previous work,at least four stages are suggested for the metamorphic evolution of the mafic granulites in the South Altun,including the protolith stage,the high-pressure granulite-facies stage(909-1037℃and 17.3-30 kbar),medium-pressure granulite-facies overprint(9.1-11.9 kbar and 753-816℃),and subsequent late amphibolite-greenschist-facies metamorphism.Zircon U-Pb dating shows that the mafic granulites underwent high-pressure granulite-facies metamorphism at 497.2±3.7 Ma,while the leucosome formed at 498.2±2.9 Ma.Thus,the leucosomes from the host mafic granulite may have been formed at the high-pressure granulite-facies metamorphic event.The characteristics of zircon morphology,mineral inclusions,low Th/U values,HREE enrichment,and negative Eu anomalies indicate that these zircons from the leucosome were formed from the metamorphic melts.The characteristics of whole-rock major and trace elements as well as Hf isotopic features of zircons between the leucosomes and the host mafic granulite indicate that the melt may have been generated by the partial melting of the host mafic granulite.
基金support from the National Natural Science Foundation of China (Grant No. 42276049)。
文摘The conventional view suggests that the subduction of the South China Sea plate beneath Luzon occurred due to the oceanic lithosphere’s high density,facilitating subduction initiation.However,before the South China Sea opened,a continental margin likely existed,meaning that Luzon was directly adjacent to the continental margin rather than the oceanic basin.This would make subduction initiation more challenging.Here,we propose a new model suggesting that during the formation of the South China Sea,extensive mafic magmatic underplating occurred along its continental margin.The high-density magmatic additions may have increased the overall density of the continental margin,potentially exceeding that of Luzon,thereby enabling subduction to proceed.
基金funded by a major research project grant from the UGC (F.-43-367/2014(SR)) and teachers’ research grant from the University of Calcutta awarded to Dr Bapi GoswamiDST-sponsored Women Scientist scheme-A (SR/WOS-A/EA-47/2018 (G) dated 06.05.2019) awarded to Ms. Poulami Roy is highly acknowledged for their financial help
文摘Distinguishing high-grade mafic-ultramafic rocks originally crystallized from within-plate basaltic magmatism is challenging and crucial because the chemical composition of the igneous rocks has been modified during high-grade metamorphism,causing misidentification of the characters of the parental magma.Proterozoic metamorphosed mafic dykes occur throughout the Chhotanagpur Gneissic Complex(CGC)of eastern Indian shield.The E-W trending mafic dykes from the Saltora area in the southeastern CGC underwent metamorphism in two episodes:M1(650 MPa;770℃)and M2(300 MPa;744℃).The metamafics are enriched in LILE,depleted in HFSE,and display strong fractionation of LREE,nearly flat HREE patterns in a chondrite-normalized REE diagram,and show tholeiitic differentiation trend.Their geochemical affinity is towards rift-related,continental within-plate basalts.About 7%–10%melting of the carbonated spinel-peridotite sub-continental lithospheric mantle(SCLM)produced the parental mafic magma.The pre-existing SCLM was metasomatized by slab-derived fluid during the previous subduction.The upwelling of the asthenosphere in a post-collisional tectonic setting caused E-W trending fractures,lithospheric thinning,and gravitational collapse.These dykes were emplaced during crustal extension around 1070 Ma.The remarkable geochemical similarity between the mafic dykes of Saltora and Dhanbad,the ca.1096 Ma Mahoba(Bundelkhand craton),and the ca.1070 Ma Alcurra mafic dykes in Australia supports a genetic link.
基金a CSIR grant vide Project no.24(0317)/12/EMR-II.Bibhuti Gogoi acknowledges a CSIR JRF/SRF fellowship no.09/045(1146)/2011-EMR1.
文摘The Bathani volcano-sedimentary sequence(BVSs)is a typical example of volcanic-plutonic complex from the Proterozoic Chotanagpur Granite Gneiss Complex.We conducted petrological and geochemical studies on two domains of the BVSs;one is a granite pluton,while the other is a subvolcanic rhyolite dome.These two domains were intruded by mafic magmas during their evolution leading to magma mixing.The mafic rocks show tholeiitic trend,while calcalkaline affinities are displayed by the felsic rocks.The rocks display enriched LILE and fractionated REE patterns with enrichments in LREE relative to HREE.
基金supported by the Special Fund for Basic Scientific Research of Central Colleges (Nos. 2014G1271060, 2013G1271103)Chang’an University, China and the High Resolution Earth Observation Systems of National Science and Technology Major Projects
文摘The Beishan area has more than seventy mafic-ultramafic complexes sparsely distributed in the area and is of a big potential in mineral resources related to mafic-ultramafic intrusions. Many mafic-ultramafic intrusions which are mostly in small sizes have been omitted by previous works. This research takes Huitongshan as the study area, which is a major district for mafic-ultramafic occurrences in Beishan. Advanced spaceborne thermal emission and reflection radiometer（ASTER） data have been processed and interpreted for mapping the mafic-ultramafic complex. ASTER data were processed by different techniques that were selected based on image reflectance and laboratory emissivity spectra. The visible near-infrared（VNIR） and short wave infrared（SWIR） data were transformed using band ratios and minimum noise fraction（MNF）, while the thermal infrared（TIR） data were processed using mafic index（MI） and principal components analysis（PCA）. ASTER band ratios（6/8, 5/4, 2/1） in RGB image and MNF（1, 2, 4） in RGB image were powerful in distinguishing the subtle differences between the various rock units. PCA applied to all five bands of ASTER TIR imagery highlighted marked differences among the mafic rock units and was more effective than the MI in differentiating mafic-ultramafic rocks. Our results were consistent with information derived from local geological maps. Based on the remote sensing results and field inspection, eleven gabbroic intrusions and a pyroxenite occurrence were recognized for the first time. A new geologic map of the Huitongshan area was created by integrating the results of remote sensing, previous geological maps and field inspection. It is concluded that the workflow of ASTER image processing, interpretation and ground inspection has great potential for mafic-ultramafic rocks identifying and relevant mineral targeting in the sparsely vegetated arid region of northwestern China.
文摘The Naga Hills Ophiolite(NHO) represents one of the fragments of Tethyan oceanic crust in the Himalayan Orogenic system which is exposed in the Phek and Kiphire districts of Nagaland, India. The NHO is composed of partially serpentinized dunite, peridotite, gabbro, basalt, minor plagiogranite,diorite dyke and marine sediments. The basalts are mainly composed of fine grained plagioclase feldspar, clinopyroxene and orthopyroxene and show quenching and variolitic textures. The gabbros are characterized by medium to coarse grained plagioclase, orthopyroxene and clinopyroxene with ophitic to sub-ophitic textures. The ultramafic cumulates are represented by olivine, Cpx and Opx.Geochemically, the basalts and gabbros are sub-alkaline to alkaline and show tholeiitic features.The basalts are characterized by 44.1-45.6 wt.% of SiO_2 with 28-38 of Mg#, and the gabbros by38.7-43.7 wt.% of SiO_2, and 26-79 of Mg#. The ultramafic rocks are characterized by 37.4-52.2 wt.% of SiO_2, and 80-88 of Mg#. In multi-element diagrams(spidergrams) both basalts and gabbros show fractionated trends with strong negative anomalies of Zr. Nb. Sr and a gentle negative anomaly of P.However, the rare earth element(REE) plots of the basalts and gabbros show two distinct patterns. The first pattern, represented by light REE(LREE) depletion, suggests N-MORB features and can be interpreted as a signature of Paleo-Tethyan oceanic crust. The second pattern, represented by LREE enrichment with negligible negative Eu anomaly, conforms to E-MORB, and may be related to an arc tectonic setting. In V vs. Ti/1000, Cr vs. Y and AFM diagrams, the basalts and gabbros plot within Island Arc Tholeiite(IAT) and MORB fields suggesting both ridge and arc related settings. The ultramafic rocks exhibit two distinct patterns both in spidergrams and in REE plots. In the spidergram, one group displays highly enriched pattern, whereas the other group shows near flat pattern compared to primordial mantle. In the REE plot, one group displays steeper slopes [(La/Yb)N = 4.340-4.341], whereas the other displays moderate to flat slopes [(La/Yb)N = 0.97-1.67] and negative Eu-anomalies. Our study suggests that the ultramafic rocks represent two possible mantle sources(fertile and refractory).
基金supported by the Second Tibetan Plateau Scientific Expedition and Research(No.2019QZKK0702)the National Natural Science Foundation of China(Nos.92055202,41873023,42102059)+3 种基金the China Geological Survey(No.DD20190057)the National Key Research and Development Project of China(No.2016YFC0600304)the Basic Scientific Research Fund of Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources(Nos.J1901-1,J190110)the National Foundation from China Scholarship Council。
文摘The Niangzhong diabase dikes,dated at 138.1±0.4 Ma,are located within the outcrop area of the Comei large igneous province(LIP).These diabase samples can be divided into two groups:samples in Group 1 show varying MgO(1.50 wt.%-10.25 wt.%)and TiO_(2)(0.85 wt.%-4.63 wt.%)contents,and enriched initial isotope compositions(^(87)Sr/^(86)Sr(t)=0.7056-0.7112,ε_(Nd)(t)=-0.3-+3.8),with OIB-like REEs and trace elements patterns,resulting from low degree melting of garnet-bearing lherzolite mantle sources;in contrast,samples in Group 2 show limited MgO(4.14 wt.%-7.75 wt.%)and TiO_(2)(0.98 wt.%-1.69 wt.%)contents,and depleted initial isotope compositions(^(87)Sr/^(86)Sr(t)=0.7075-0.7112,ε_(Nd)(t)=+5.5-+6.2),with N-MORB-like REEs and trace elements patterns,resulting from relatively high degree melting of spinel-bearing lherzolite mantle source.Combined with the published representative data about Comei LIP,we summarize that the source components for Comei LIP products include OIB end-member,enriched OIB end-member,and N-MORB end-member,respectively.Melts modeling suggests that magmas in the Comei LIP evolve in a relatively high oxygen fugacity condition,which influenced their fractionation sequences and led to systematic changes of TiO_(2)contents,Ti/Y and Ti/Ti*ratios.From the spatial and temporal distribution of above three end-member samples,deep process of Kerguelen plume during the Comei LIP formation can be interpreted as the interaction among the Kerguelen plume,the overlying lithospheric mantle,and the upwelling asthenosphere.The magmatism of Comei LIP began at~140 Ma and then lasted and peaked at~132 Ma with the progressively lithospheric thinning of eastern Gondwana upon the impact of Kerguelen plume.
基金supported under a DST (ESS/16/290/2006) grant sanctioned to Santosh Kumar
文摘Occurrence of mafic dykes in Himalaya has been intriguing and debated since long because of its difficulty to ascent and emplacement through a thickened crust.Mafic dykes in Kumaun Lesser Himalaya(KLH)of central Indian
基金supported by the National Natural Science Foundation of China(Grant Nos.42372071,41421002)the Foundation for the Author of National Excellent Doctoral Dissertation of China(Grant No.201324)a research grant of the State Key Laboratory of Continental Dynamics(Grant No.SKLCD-04).
文摘Mafic enclaves in granites are generally considered to represent coeval mafic melts that derived from metasomatized mantle,which can provide valuable information about crust-mantle interaction.Exploring the genetic links between the mafic enclaves and their host monzogranite from the Triassic Zhashui Pluton,Qinling orogenic belt.The enclaves(220±4.6 Ma)and the monzogranite(220±2.8 Ma)display identical zircon U-Pb ages,and they also share similar trace element and zircon Lu-Hf isotopes,indicating a cognate source.The monzogranite displays zirconεHf(t)values of−0.99 to+1.98,while the mafic enclaves show similar values of−0.45 to+3.26;however,differences in mineral chemistry reveal different crystallization conditions.The amphibole from the mafic enclaves has higher temperature and pressure(757℃;2.65 kbar)compared to those of the host monzogranite(733℃;1.96 kbar),suggesting that mafic minerals in the enclaves crystallized at an early stage.Moreover,apatite in the mafic enclaves displays slightly higher volatile contents(0.72 wt%)than those of the monzogranite(0.66 wt%),indicating a volatile-rich condition.These results suggest that the mafic enclaves represent early hydrous mafic cumulates in the granitic chamber,and subsequent magma convection would have led to the formation of the mafic enclaves.
基金supported by Russian Scientific Foundation(grant No 16-17-10068)
文摘Virtual absence of igneous complexes with ages between1.8 Ga and 0.8 Ga in southern part of the Siberian Craton allowed to Galdkochub et al.(2010)to formulate a hypothesis of long magmatic quiescence.Most reliable
基金supported by the grants from the Department of Science and Technology,Government of India to TR(SR/S4/ES-598/2011)and KVK(EMR/2014/000779)forms a part of CSIRES scheme to TR(21(1041)/17/EMR-Ⅱ)UGC-SAP scheme at the SRTM University(No.F.550/3/DRS-Ⅱ/2016/SAP-Ⅰ).
文摘In this paper we have synthesized the published and unpublished geochemical data on the Palaeoproterozoic mafic magmatism in the Indian Shield.Palaeoproterozoic mafic magmatism is widespread in the Indian Shield;it mostly emplaced as dyke intrusions within the cratons and south Indian granulite region and as intrusives/traps in the intra-cratonic basins and the Eastern Ghat Mobile Belt.
基金the Department of Science and Technology,New Delhi for providing research grant for this work(ESS/16/185/2003)。
文摘The Bastar craton largely consists of Mesoarchaean orthogneisses with vestiges of supracrustal rocks that have been intruded by Proterozoic granites and mafic dykes.Many regions in the Bastar craton have been subjected to medium or high grade metamorphic conditions but the nature and timing of regional metamorphism is poorly understood.In this study,metamorphosed mafic igneous rocks collected from two different dyke swarms from the southern part of the Central Indian Bastar craton have been studied to evaluate the timing and conditions of metamorphism.In this region,two distinct metamorphosed dyke swarms are recognized based on differences in petrology and geochemistry:the Meso-Neoarchaean sub-alkaline BD1 swarm and the Neoarchaean boninite-norite(BN)swarm.
文摘The Proterozoic felsic and mafic magmatism in India in varied tectonic settings is reviewed and discussed based on available geological,geochemical,and geochronological constraints.Neoarchean-Paleoproterozoic magmatism,as discrete volcanoplutonic complexes and Large Igneous Provinces(LIPs)in the Bastar,Singhbhum and Dharwar cratons and associated mobile belts are also included.Paleoproterozoic magmatism also contributed to the geodynamics of Himalaya.
文摘Based on K-Ar isotope analyses, Mesozoic mafic (and alkali ultramafic) dikes from western and eastern Shandong Province, China, are dated at 88.2±1.70 Ma to 169.5±3.7 Ma with the majority of ages ranging from 90 Ma to 140 Ma. The emplacement of the dikes suggests a major Yanshanian (Cretaceous) crustal extension in Shandong province. Together with other available age data, this study suggests four periods of crustal extension at about 80 Ma, 100 Ma, 120 Ma and 140 Ma, respectively. Besides the effect of collapse of the Yanshanian orogenic belt on the emplacement of the mafic dikes in Shandong in the Cretaceous, the mantle plume and the extensive left-lateral advection and extension of the Tanlu fault also have controlled the crustal extension and the emplacement of the mafic dikes in eastern Shandong and western Shandong, respectively.
基金supported by the National Natural Science Foundation of China(Grant No. 40972058)the research grants(2008-Ⅰ-02 and 2008-Ⅱ-08)from the State Key Laboratory for Mineral Deposit Research,Nanjing University
文摘High-precision data on U and Th contents and Th/U ratios of zircon obtained using secondary ion mass spectrometry analysis have been collected from the literature. Zircon in the granitic rocks has median values of 350 ppm U, 140 ppm Th, and Th/U=0.52; the recommended zircon-melt partition coefficients are 81 for Du and 8.2 for DTh. In zircon from mafic and intermediate rocks, the median values are 270 ppm U, 170 ppm Th, and Th/U=0.81, and the recommended zirconmelt partition coefficients are 169 for Du and 59 for DTh. The U and Th contents and Th/U ratios of magmatic zircon are low when zircon crystallizes in equilibrium with the melt. Increasing magma temperature should promote higher Th contents relative to U contents, resulting in higher Th/U ratios for zircon in mafic to intermediate rocks than in granitic rocks. However, when zircon crystallizes in disequilibrium with the melt, U and Th are more easily able to enter the zircon lattice, and their contents and Th/U ratios depend mainly on the degree of disequilibrium. The behavior of U and Th in magmatic zircon can be used as a geochemical indicator to determine the origins and crystallization environments of magmatic zircon.
基金granted by the foundamental research funds for central public welfare research institutes(Grant No.K1313)the Natural Sciences Foundation of China(Grant No.41302080)China State Geological Investigation Program(Grant No.1212011121091,12120114009401)
文摘The Xuhe mafic rocks, located in Ziyang county of Shaanxi Province, are dominated by diabase-porphyrite, gabbro-diabase, diabase, and pyroxene diorite. Primitive mantle-normalized multi-element patterns show that, the Xuhe marie rocks are enriched in large ion lithophile elements （LITE）, such as Ba and Pb, depleted in K and Sr for basic rocks, and are depleted in Sr, P and Ti for pyroxene diorite. Chondrite-normalized REE patterns display LREE enrichment （LaN/YbN = 9.34- 13.99） and have normalized patterns for trace element and REE similar to that of typical OIB. Detailed SIMS zircon U-Pb dating yields emplacement ages of 438.4 ± 3.1 Ma for Xuhe mafic rocks. The relatively low MgO （basic rock： 3.11-7.21 wt%; pyroxene diorite： 0.89-1.21 wt% ） and Mg# （0.20- 0.49） for Xuhe mafic rocks suggest that they were possibly originated from an extremely evolved magma. The rising parental mafic magmas underwent pyroxene and plagioclase fractionation. Crustal contamination of pyroxene diorite before emplacement occurred at a higher crustal level compared to other lithology in Xuhe mafic rocks. The degree of partial melt was low （5%-10%） and in garnetspinel transition facies. Sr-Nd isotope of pyroxene diorite and enrichment mantle characteristics for Xuhe mafic rocks suggest that mafic rocks in the North Daba Mountains were derived from a mixture of HIMU, EMII and small amount of EMI components. Furthermore, this study discusses mantle geodynamic significance of Xuhe mafic rocks in the Silurian, which indicates subduction and uplift of magma caused back-arc extension.
基金financially supported by the National Key Basic Research Program of China(Grant No.2012CB416603)the National Natural Science Foundation of China(Grant No.90914001)+1 种基金the Opening Foundation of the Peking University(Grant No. 0000010541)Undergraduates Innovating Experimentation Project of Jilin University(Grant No.2010C61164)
文摘Mafic rocks are widespread on the Liaodong Peninsula and adjacent regions of the North China Craton. The majority of this magmatism was originally thought to have occurred during the Pre-Sinian, although the precise geochronological framework of this magmatism was unclear. Here, we present the results of more than 60 U-Pb analyses of samples performed over the past decade, with the aim of determining the spatial and temporal distribution of mafic magmatism in this area. These data indicate that Paleoproterozoic-Mesoproterozoic mafic rocks are not as widely distributed as previously thought. The combined geochronological data enabled the subdivision of the mafic magmatism into six episodes that occurred during the middle Paleoproterozoic, the late Paleoproterozoic, the Mesoproterozoic, the Late Triassic, the Middle Jurassic, and the Early Cretaceous. The middle Paleoproterozoic （2.1-2.2 Ga） mafic rocks formed in a subduction-related setting and were subsequently metamorphosed during a ca. 1.9 Ga arc-continent collision event. The late Paleoproterozoic （ca. 1.87-1.82 Ga） bimodal igneous rocks mark the end of a Paleoproterozoic tectono- thermal event, whereas Mesoproterozoic mafic dike swarms record global-scale Mesoproterozoic rifting associated with the final breakup of the Columbia supercontinent. The Late Triassic mafic magmatism is part of a Late Triassic magmatic belt that was generated by post-coilisional extension. The Middle Jurassic mafic dikes formed in a compressive tectonic setting, and the Early Cretaceous bimodal igneous rocks formed in an extensional setting similar to a back-arc basin. These latter two periods of magmatism were possibly related to subduction of the Paleo-Pacific plate.
基金Financial support for this study was jointly provided by the Hundred Talents Project of Ministry of Land and Resources of China, the Ten-Hundred-Thousand Talent Project of Guangxi (Grant No. 2006218)Guangxi Science Foundation (Grant No.0728089)+1 种基金Institute of Mineral Resources, Chinese Academy of Geological Sciences (Grant No. K0914)China Geological Survey (Grant No. 201111020 and1212010510805)
文摘The Napo-Qinzhou Tectonic Belt （NQTB） lies at the junction of the Yangtze, Cathaysia and Indochina （North Vietnam） Blocks, which is composed of five major lithotectonic subunits： the Qinzhou-Fangcheng Suture Zone （QFSZ）, the Shiwandashan Basin （SB）, the Pingxiang-Nanning Suture Zone （PNSZ）, the Damingshan Block （DB） and the Babu-Lingma Suture Zone （BLSZ）. On the basis of geochemical compositions, the Permian mafic igneous rocks can be divided into three distinct groups： （1） mafic igneous rocks （Group 1） from the Longjing region in the PNSZ and Hurun region in the BLSZ, which are characterized by intermediate Ti, P and Zr with low Ni and Cr contents; （2） mafic igneous rocks （Group 2） from the Naxiao and Chongzuo region in the DB, characterized by low-intermediate Ti, P and Zr with high Ni and Cr concentrations; and （3） mafic igneous rocks （Group 3） from the Siming region in the Jingxi carbonate platform of the northwestern margin of the NQTB, with intermediate-high Ti, P and Zr and low Ni and Cr contents. The Group 1 rocks yield a weighted mean 206 Pb/ 238 U age of 250.5±2.8 Ma and are geochemically similar to basalts occurring in back-arc basin settings. The Group 2 rocks exhibit geochemical features to those basalts in island arcs, whereas the Group 3 rocks show geochemical similarity to that of ocean island basalts. All three groups are characterized by relatively low ε Nd （t） values （-2.61 to ＋1.10） and high initial 87 Sr/ 86 Sr isotopic ratios （0.705309-0.707434）, indicating that they were derived from a subduction-modified lithospheric mantle and experienced assimilation, fractional crystallization, and crustal contamination or mixing during magmatic evolution. Accordingly, we propose the existence of an arc-back arc basin system that developed along the NQTB at the border of SW Guangxi Province （SW China） and northern Vietnam, and it was formed by continued northwestward subduction of the Cathaysian （or Yunkai） Block under the Yangtze Block, and northeastward subduction of the Indochina Block beneath the Yangtze Block during Permian time.
基金supported by the National Natural Science Foundation of China (No. 41272072)the NSFC/NRF Research Cooperation Programm (No. 41761144061)the SDUST Research Fund (No. 2015TDJH101)
文摘Petrological analysis and LA-ICP-MS zircon U-Pb dating were conducted on high- pressure marie granulites, which occured as xenolith within TTG gneisses, from the Nanshankou Village of the Jiaobei terrane, Shandong Peninsula in the north-eastern part of the North China Craton （NCC）. The mafic HP grannlite is composed of garnet, clinopyroxene, orthopyroxene, amphibole and symplectitic clinopyroxene, orthopyroxene, plagioclase, ilmente and magnetite which were formed after the decomposition of porphyroblastic garnet and clinopyroxene. Four stages of metamorphic mineral assemblages for the mafic HP granulites were constrained by detail petrological and mineralogical in- vestigations. The early prograde assemblage is represented by the mineral inclusions within garnet and clinopyroxene porphyroblasts （Opx1＋Pl1＋Qtz1）, recording the metamorphic conditions at -754-757 ℃, 0.63-0.71 GPa; peak metamorphic conditions were determined at -874-891 ℃, 1.32-1.35 GPa with the mineral assemblage of Grt2＋Cpx2＋Amp2＋Pl2＋Qtz2. Retrograde minerals derived from symplectitic assemblage Opx3＋Cpx3＋Amp3＋Pl3＋Qtz3＋Ilm3±Mag3 were formed at 693-796℃, 0.60-0.84 GPa. A final greensehist to sub-greenschist facies event was recorded by the exsolution of actinolite and albite within a retrograded clinopyroxene, as well as the occurrence of prehnite, chlorite and calcite minerals. Accordingly, a clockwise P-T path was concluded on the basis of the different stages of mineral asseblage. Cathodoluminescence imaging, trace element and U-Pb dating of zircons from the mafic HP granulites recorded similar charactistics for three episodes of Paleo-Meso Proterozoic metamorphic events. These are the metamorphic events preserved in mafic and pelitic granulites in the Jiao-Liao-Ji belt （JLJB） with 207 pb/206pb ages of 2.0-1.9 Ga for peak metamorphism and of 1.86-1.84 Ga for decomposing process, followed by a retrograde amphibolite facies metamorphic event related to the post-orogenic extension at the age of 1.76-1.74 Ga, resulting the exhumation of the granulite to the upper crust level.
基金Financial support for this study was jointly provided by the National Natural Science Foundation of China(Grant Nos.41421002and 41225008)MOST Special Fund from the State Key Laboratory of Continental Dynamics,Northwest UniversityProject for Key Innovative Research Team of Science and Technology in Shaanxi Province(Grant No.2013KCT-18)
文摘The Helanshan tectonic belt(HTB) is a major tectonic divide between the Alxa and Ordos blocks in the North China Craton. The geochronology and petrogenesis of the mafic dykes in the northern HTB are keys to understanding the tectonic evolution of this belt. The mafic dykes, intruded into the Neoarchean-Paleoproterozoic metamorphic basement, are mainly composed of diabase with a mineral assemblage of plagioclase(45%-60%), pyroxene(25%-35%), minor quartz and Fe-Ti oxides. The LA-ICPMS U-Pb analysis of zircon grains from representative dykes yield a weighted mean age of 206 ± 1.9 Ma, which represents the crystallization age of the dyke. The diabases show high contents of Fe_2 O_3~T(11.88-17.55 wt.%), low contents of SiO_2(45.65-50.95 wt.%) and MgO(3.31-5.50 wt.%) with low Mg#(=100×MgO/(MgO + FeO) atomic ration) of 33-44. They are characterized by enrichment of light rare earth elements(LREEs) and large ion lithophile elements(LILEs)(e.g., Rb, Ba and Pb), and slight depletion of high field strength elements(HFSEs). These features suggest that the magma has undergone extensive fractionation of olivine and pyroxene but only minor crustal contamination during its evolution. Their high Sm contents and La/Sm ratios, and low Sm/Yb ratios indicate that magma from which the dykes formed was derived from low degree(about 5%) partial melting of an enriched garnet + spinel lherzolite mantle source. Together with regional geology, these geochemical and geochronological data suggest that the mafic dykes in the HTB were formed in an intracontinental extensional setting during the late Triassic.
