The present study reports and discusses the genesis of zincian chromite in the ultramafic xenoliths from the Dongripali area,Bastar craton,Central India.The zincian chromite is in the ultramafic xenoliths of Bengpal s...The present study reports and discusses the genesis of zincian chromite in the ultramafic xenoliths from the Dongripali area,Bastar craton,Central India.The zincian chromite is in the ultramafic xenoliths of Bengpal supracrustal rock hosted by Neoarchaean Bundeli gneisses.Compositionally zincian chromite shows a range of Cr_(2)O_(3)(39.69 to 51.66 wt%),Al_(2)O_(3)(05.30 wt%to 08.71 wt%),FeO(21.74 wt%to 27.51 wt%),Fe_(2)O_(3)(10.19 wt%to 19.36wt%)with higher ZnO content ranging from 1.73 wt%to 4.08 wt%.Accordingly,their Cr#[Cr/(Cr+Al)]varies in a narrow range from 0.83 to 0.85.Its calculated melt composition supports metamorphic or post-magmatic nature rather than common occurrences such as inclusion in diamonds,meteorites,and association with any sulfide-rich mineralised belt.This reveals that the post-magmatic processes play a vital role in transforming chromite to zincian chromite.The empirical thermometric calculation from chromite,amphibole,and pyroxene support their metamorphic origin and formed during low-P and high-T amphibolite grade facies of metamorphism(~700℃).The Neoarchaean granitic magmatism has a significant role in generating and transferring the heat during contact metamorphism with hydration of ultramafic xenoliths and further alteration,i.e.,serpentinisation.The olivine is a major repository for Mn,Zn,and Co in peridotite/ultramafic;these elements get mobilised during the metamorphism and serpentinisation.This is a possible reason for the mobilisation of zinc and incorporation in the chromite within altered ultramafic.As a result,chromiterich ultramafic xenolith subjected to metamorphic process gets enrichment of Zn and Fe due to elemental exchange.It converts common chromite into zincian chromite,as reported in altered ultramafics elsewhere.展开更多
A large-scale high-pressure granulite belt (HPGB), more than 700 km long, is recognized within the metamorphic basement of the North China craton. In the regional tectonic framework, the Hengshan-Chengde HPGB is locat...A large-scale high-pressure granulite belt (HPGB), more than 700 km long, is recognized within the metamorphic basement of the North China craton. In the regional tectonic framework, the Hengshan-Chengde HPGB is located in the central collision belt between the western block and eastern block, and represents the deep crustal structural level. The typical high-pressure granulite (HPG) outcrops are distributed in the Hengshan and Chengde areas. HPGs commonly occur as mafic xenoliths within ductile shear zones, and underwent multipile deformations. To the south, the Hengshan-Chengde HPGB is juxtaposed with the Wutai greenstone belt by several strike-slip shear zones. Preliminary isotopic age dating indicates that HPGs from North China were mainly generated at the end of the Neoarchaean, assocaited with tectonic assembly of the western and eastern blocks.展开更多
Four distinct litho-tectonic belts (zones) in the Yinshan area, North China, were identified by pressure-temperature contours and litho-tectonic features, such as the Sanggan granulite belt, Jining metasedimentary bel...Four distinct litho-tectonic belts (zones) in the Yinshan area, North China, were identified by pressure-temperature contours and litho-tectonic features, such as the Sanggan granulite belt, Jining metasedimentary belt, Wulashan-Daqingshan front tectonic zone and Se' eratengshan belt. This area witnessed two important thermo-tectonic events. The older one is c. 2.5 Ga while the younger one c. 1.9 Ga. The Se' ertengshan Neoarchaean terrane features a clockwise PT path with the decompression ranging from > 1500 MPa to 800–1000 MPa in the Se' ertengshan belt, which implies an island arc setting. The Sanggan belt is a Mesoarchaean microcontinent reworked by Neoarchaean magma underplating, which shows an counterclockwise PT path. During the Palaeoproterozoic period, two Archaean continent (arc) collided. The Archaean basement of the Sanggan and Wulashan-Daqingshan belts overthrust northwards, the PTt paths of basement show a decompression from 1000–1200 MPa to 500–700 MPa. The PT paths of the Jining and Erdaowa Groups show different PTt paths: the former shows counterclockwise while the latter clockwise, which indicates that the Jining and Erdaowa groups formed in different thermo-tectonic settings.展开更多
Zircon LA-ICP-MS U-Pb age and whole-rock geochemistry data are presented for the Dongxigao diorite porphyries in Pingyi,western Shandong,to restrict its petrogenesis. The zircons from the Dongxigao diorite porphyries ...Zircon LA-ICP-MS U-Pb age and whole-rock geochemistry data are presented for the Dongxigao diorite porphyries in Pingyi,western Shandong,to restrict its petrogenesis. The zircons from the Dongxigao diorite porphyries exhibit striped absorption and oscillatory growth zoning in the cathodoluminescence images,which have upward-type characteristics of rare earth elements( REEs) with positive anomaly of Ce and negative anomaly of Eu,and have high Th / U ratios( 0. 06--0. 91),indicating its magma origin. Zircon LA-ICP-MS UPb dating indicates that^(207) Pb /^(206) Pb ages of 16 spots of zircons are between 2476 Ma and 2701 Ma,yielding a weighted mean age of 2 481 ± 8 Ma,which represents the formation age of Dongxigao diorite porphyries,i. e.Late Neoarchaean. Geochemically,the samples have low contents of SiO_2( 54. 94% and 54. 86%) and high amounts of MgO( 7. 19% and 7. 58%) and Mg~#values( 66. 47 and 67. 59). Additionally,the Dongxigao diorite porphyries show enrichment of LREEs and prominent negative Eu anomaly( δEu = 0. 89 and 0. 91),and they are characterized by enrichment of large ion lithophile elements( LILEs)( e. g. Cs,Rb and Ba),depletion of high field strength elements( HFSEs)( e. g. Nb,Ta,Zr,Hf and Ti),and positive anomaly of K and Pb. The results imply that the Dongxigao diorite porphyries dominantly derived from partial melting of enriched lithospheric mantle with attributes of arc magma,and the magma source area was mixed with amounts of continental crust material,it suggests that they formed in transformation tectonic setting changing from compression to intraplate extension.展开更多
基金Geological Survey of India,Ministry of Mines,Government of India for funding the opportunity to work in this projects。
文摘The present study reports and discusses the genesis of zincian chromite in the ultramafic xenoliths from the Dongripali area,Bastar craton,Central India.The zincian chromite is in the ultramafic xenoliths of Bengpal supracrustal rock hosted by Neoarchaean Bundeli gneisses.Compositionally zincian chromite shows a range of Cr_(2)O_(3)(39.69 to 51.66 wt%),Al_(2)O_(3)(05.30 wt%to 08.71 wt%),FeO(21.74 wt%to 27.51 wt%),Fe_(2)O_(3)(10.19 wt%to 19.36wt%)with higher ZnO content ranging from 1.73 wt%to 4.08 wt%.Accordingly,their Cr#[Cr/(Cr+Al)]varies in a narrow range from 0.83 to 0.85.Its calculated melt composition supports metamorphic or post-magmatic nature rather than common occurrences such as inclusion in diamonds,meteorites,and association with any sulfide-rich mineralised belt.This reveals that the post-magmatic processes play a vital role in transforming chromite to zincian chromite.The empirical thermometric calculation from chromite,amphibole,and pyroxene support their metamorphic origin and formed during low-P and high-T amphibolite grade facies of metamorphism(~700℃).The Neoarchaean granitic magmatism has a significant role in generating and transferring the heat during contact metamorphism with hydration of ultramafic xenoliths and further alteration,i.e.,serpentinisation.The olivine is a major repository for Mn,Zn,and Co in peridotite/ultramafic;these elements get mobilised during the metamorphism and serpentinisation.This is a possible reason for the mobilisation of zinc and incorporation in the chromite within altered ultramafic.As a result,chromiterich ultramafic xenolith subjected to metamorphic process gets enrichment of Zn and Fe due to elemental exchange.It converts common chromite into zincian chromite,as reported in altered ultramafics elsewhere.
文摘A large-scale high-pressure granulite belt (HPGB), more than 700 km long, is recognized within the metamorphic basement of the North China craton. In the regional tectonic framework, the Hengshan-Chengde HPGB is located in the central collision belt between the western block and eastern block, and represents the deep crustal structural level. The typical high-pressure granulite (HPG) outcrops are distributed in the Hengshan and Chengde areas. HPGs commonly occur as mafic xenoliths within ductile shear zones, and underwent multipile deformations. To the south, the Hengshan-Chengde HPGB is juxtaposed with the Wutai greenstone belt by several strike-slip shear zones. Preliminary isotopic age dating indicates that HPGs from North China were mainly generated at the end of the Neoarchaean, assocaited with tectonic assembly of the western and eastern blocks.
文摘Four distinct litho-tectonic belts (zones) in the Yinshan area, North China, were identified by pressure-temperature contours and litho-tectonic features, such as the Sanggan granulite belt, Jining metasedimentary belt, Wulashan-Daqingshan front tectonic zone and Se' eratengshan belt. This area witnessed two important thermo-tectonic events. The older one is c. 2.5 Ga while the younger one c. 1.9 Ga. The Se' ertengshan Neoarchaean terrane features a clockwise PT path with the decompression ranging from > 1500 MPa to 800–1000 MPa in the Se' ertengshan belt, which implies an island arc setting. The Sanggan belt is a Mesoarchaean microcontinent reworked by Neoarchaean magma underplating, which shows an counterclockwise PT path. During the Palaeoproterozoic period, two Archaean continent (arc) collided. The Archaean basement of the Sanggan and Wulashan-Daqingshan belts overthrust northwards, the PTt paths of basement show a decompression from 1000–1200 MPa to 500–700 MPa. The PT paths of the Jining and Erdaowa Groups show different PTt paths: the former shows counterclockwise while the latter clockwise, which indicates that the Jining and Erdaowa groups formed in different thermo-tectonic settings.
基金Supported by projects of Program for New Century Excellent Talents in University(No.NCET-12-0237)Natural Science Foundation of China(No.41472052)Basic Scientific Research Foundation of Central Universities of China
文摘Zircon LA-ICP-MS U-Pb age and whole-rock geochemistry data are presented for the Dongxigao diorite porphyries in Pingyi,western Shandong,to restrict its petrogenesis. The zircons from the Dongxigao diorite porphyries exhibit striped absorption and oscillatory growth zoning in the cathodoluminescence images,which have upward-type characteristics of rare earth elements( REEs) with positive anomaly of Ce and negative anomaly of Eu,and have high Th / U ratios( 0. 06--0. 91),indicating its magma origin. Zircon LA-ICP-MS UPb dating indicates that^(207) Pb /^(206) Pb ages of 16 spots of zircons are between 2476 Ma and 2701 Ma,yielding a weighted mean age of 2 481 ± 8 Ma,which represents the formation age of Dongxigao diorite porphyries,i. e.Late Neoarchaean. Geochemically,the samples have low contents of SiO_2( 54. 94% and 54. 86%) and high amounts of MgO( 7. 19% and 7. 58%) and Mg~#values( 66. 47 and 67. 59). Additionally,the Dongxigao diorite porphyries show enrichment of LREEs and prominent negative Eu anomaly( δEu = 0. 89 and 0. 91),and they are characterized by enrichment of large ion lithophile elements( LILEs)( e. g. Cs,Rb and Ba),depletion of high field strength elements( HFSEs)( e. g. Nb,Ta,Zr,Hf and Ti),and positive anomaly of K and Pb. The results imply that the Dongxigao diorite porphyries dominantly derived from partial melting of enriched lithospheric mantle with attributes of arc magma,and the magma source area was mixed with amounts of continental crust material,it suggests that they formed in transformation tectonic setting changing from compression to intraplate extension.
