The results of recent geothermobarometric and geochronological investigations of scarce eclogites of the NW Himalaya (Tso Morari (Ladakh), India and Kaghan Valley, Pakistan) have caused a major rethink of tectonometam...The results of recent geothermobarometric and geochronological investigations of scarce eclogites of the NW Himalaya (Tso Morari (Ladakh), India and Kaghan Valley, Pakistan) have caused a major rethink of tectonometamorphic models for India\|Asia collision. Numerous petrologic studies have been undertaken on the age and origin of metamorphism in the Higher Himalayan Crystallines (HHC) and Lesser Himalaya formations (LH) and their relationship to granite magmatism and movements along the Main Central Thrust (MCT) and South Tibetan Detachment Fault (STDF). However, all of these events are essentially Miocene (or younger) in age and can clearly be distinguished from subduction and exhumation processes undergone by the eclogites which are of Eocene age (Tonarini et al. 1993; Spencer & Gebauer; 1996; de Sigoyer et al. 1999) and relate to the very early stages of the collision. Eclogites of eastern Ladakh are mafic lenses found in granitic gneisses (Ordovician intrusive age: Girard & Bussy 1999) and their surrounding late Pre\|Cambrian to early Cambrian sedimentary units in the Tso Morari dome (see Steck et al. 1998). Detailed petrological and geochronological studies (Guillot et al. 1997; de Sigoyer et al. 1997, 1999) have identified an eclogite facies stage (2000±300)MPa, (580±60)℃ followed by isothermal decompression associated with glaucophane growth at around (1100±200)MPa. Dating of different phases by different methods yielded ages around 55Ma for this stage ((55±17) Ma, U\|Pb, Aln; (55±12) Ma, Lu\|Hf, Grt\|Cpx\|Rt; (55±7) Ma, Sm\|Nd, Grt\|Gln\|Rt). A subsequent amphibolite facies overprint at slightly higher temperature (610±70)℃ was dated at 45～48Ma (metabasite: (47±11) Ma, Sm\|Nd, Grt\|Hbl; metapelite: (45±4) Ma, Rb\|Sr, Mu\|Ap\|WR and (48±2) Ma, Ar\|Ar, Phe). By (30±1) Ma (Ar\|Ar, Bt\|Mu) retrogression into the greenschist facies had occurred (de Sigoyer et al. 1999). These data indicate a two stage history with early exhumation being much faster (>4mm/a) than the later evolution (1～2mm/a)展开更多
Whole rock major and trace element compositions of seven eclogites from the Tso Morari ultra-high pressure(UHP) complex, Ladakh were determined with the aim of constraining the protolith origins of the subducted crust...Whole rock major and trace element compositions of seven eclogites from the Tso Morari ultra-high pressure(UHP) complex, Ladakh were determined with the aim of constraining the protolith origins of the subducted crust. The eclogites have major element compositions corresponding to sub-alkaline basalts. Trace element characteristics of the samples show enrichment in LILE's over HFSEs(Rb, Th, K except Ba) with LREE enrichments((La/Lu)n = 1.28-5.96). Absence of Eu anomaly on the Primitive Mantle normalized diagram suggests the absence of plagioclase fractionation. Positive correlation between Mg# with Ni and Cr suggests olivine fractionation of mantle melts. Narrow range of(La/Yb)n(2.1-9.4) and Ce/Yb(6.2-16.2) along with Ti/Y(435-735) ratios calculated for the Tso Morari samples is consistent with generation of melts by partial melting of a garnet free mantle source within the spinel peridotite field. Ternary diagrams(viz. Ti-Zr-Y and Nb-Zr-Y) using immobile and incompatible elements show that the samples range from depleted to enriched and span from within plate basalts(WPB)to enriched MORB(E-MORB) indicating that the eclogite protoliths originated from basaltic magmas.Primitive Mantle normalized multi element plots showing significant Th and LREE enrichment marked by negative Nb anomalies are characteristic of continental flood basalts. Positive Pb, negative Nb, high Th/Ta, a narrow range of Nb/La and the observed wide variation for Ti/Y indicate that the Tso Morari samples have undergone some level of crustal contamination. Observed geochemical characteristics of the Tso Morari samples indicate tholeiitic compositions originated from enriched MORB(E-MORB) type magmas which underwent a limited magmatic evolution through the process of fractional crystallization and probably more by crustal contamination. Observed geochemical similarities(viz. Zr, Nb, La/Yb, La/Gd,La/Nb, Th/Ta ratios and REE) between Tso Morari eclogites and the Group I Panjal Traps make the trap basalt the most likely protoliths for the Tso Morari展开更多
文摘The results of recent geothermobarometric and geochronological investigations of scarce eclogites of the NW Himalaya (Tso Morari (Ladakh), India and Kaghan Valley, Pakistan) have caused a major rethink of tectonometamorphic models for India\|Asia collision. Numerous petrologic studies have been undertaken on the age and origin of metamorphism in the Higher Himalayan Crystallines (HHC) and Lesser Himalaya formations (LH) and their relationship to granite magmatism and movements along the Main Central Thrust (MCT) and South Tibetan Detachment Fault (STDF). However, all of these events are essentially Miocene (or younger) in age and can clearly be distinguished from subduction and exhumation processes undergone by the eclogites which are of Eocene age (Tonarini et al. 1993; Spencer & Gebauer; 1996; de Sigoyer et al. 1999) and relate to the very early stages of the collision. Eclogites of eastern Ladakh are mafic lenses found in granitic gneisses (Ordovician intrusive age: Girard & Bussy 1999) and their surrounding late Pre\|Cambrian to early Cambrian sedimentary units in the Tso Morari dome (see Steck et al. 1998). Detailed petrological and geochronological studies (Guillot et al. 1997; de Sigoyer et al. 1997, 1999) have identified an eclogite facies stage (2000±300)MPa, (580±60)℃ followed by isothermal decompression associated with glaucophane growth at around (1100±200)MPa. Dating of different phases by different methods yielded ages around 55Ma for this stage ((55±17) Ma, U\|Pb, Aln; (55±12) Ma, Lu\|Hf, Grt\|Cpx\|Rt; (55±7) Ma, Sm\|Nd, Grt\|Gln\|Rt). A subsequent amphibolite facies overprint at slightly higher temperature (610±70)℃ was dated at 45～48Ma (metabasite: (47±11) Ma, Sm\|Nd, Grt\|Hbl; metapelite: (45±4) Ma, Rb\|Sr, Mu\|Ap\|WR and (48±2) Ma, Ar\|Ar, Phe). By (30±1) Ma (Ar\|Ar, Bt\|Mu) retrogression into the greenschist facies had occurred (de Sigoyer et al. 1999). These data indicate a two stage history with early exhumation being much faster (>4mm/a) than the later evolution (1～2mm/a)
基金financial support received from CSIR, New Delhi by means of SRF (9/137/(0499)/2011-EMR-I)BCUD, Savitribai Phule Pune University, Pune for financial support received through BCUD research project grants
文摘Whole rock major and trace element compositions of seven eclogites from the Tso Morari ultra-high pressure(UHP) complex, Ladakh were determined with the aim of constraining the protolith origins of the subducted crust. The eclogites have major element compositions corresponding to sub-alkaline basalts. Trace element characteristics of the samples show enrichment in LILE's over HFSEs(Rb, Th, K except Ba) with LREE enrichments((La/Lu)n = 1.28-5.96). Absence of Eu anomaly on the Primitive Mantle normalized diagram suggests the absence of plagioclase fractionation. Positive correlation between Mg# with Ni and Cr suggests olivine fractionation of mantle melts. Narrow range of(La/Yb)n(2.1-9.4) and Ce/Yb(6.2-16.2) along with Ti/Y(435-735) ratios calculated for the Tso Morari samples is consistent with generation of melts by partial melting of a garnet free mantle source within the spinel peridotite field. Ternary diagrams(viz. Ti-Zr-Y and Nb-Zr-Y) using immobile and incompatible elements show that the samples range from depleted to enriched and span from within plate basalts(WPB)to enriched MORB(E-MORB) indicating that the eclogite protoliths originated from basaltic magmas.Primitive Mantle normalized multi element plots showing significant Th and LREE enrichment marked by negative Nb anomalies are characteristic of continental flood basalts. Positive Pb, negative Nb, high Th/Ta, a narrow range of Nb/La and the observed wide variation for Ti/Y indicate that the Tso Morari samples have undergone some level of crustal contamination. Observed geochemical characteristics of the Tso Morari samples indicate tholeiitic compositions originated from enriched MORB(E-MORB) type magmas which underwent a limited magmatic evolution through the process of fractional crystallization and probably more by crustal contamination. Observed geochemical similarities(viz. Zr, Nb, La/Yb, La/Gd,La/Nb, Th/Ta ratios and REE) between Tso Morari eclogites and the Group I Panjal Traps make the trap basalt the most likely protoliths for the Tso Morari
