Geochemistry of eclogites of the Tso Morari complex, Ladakh, NW Himalayas:Insights into trace element behavior during subduction and exhumation

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 eclogites.

Geodynamic evolution of the Tethyan lithosphere as recorded in the Spontang Ophiolite,South Ladakh ophiolites(NW Himalaya,India)

The Spontang Ophiolite complex represents the most complete ophiolite sequence amongst the South Ladakh ophiolites and comprises mantle rocks(depleted harzburgites,dunites and minor lherzolites)as well as crustal rocks(basalt,isotropic gabbros,layered gabbros etc.).In the present study,detailed geochemistry(whole rock as well as mineral chemistry)and Sr-Nd isotopic analyses of thirty-six ultramaficmafic samples have been attempted to constraint the evolution and petrogenetic history of the Tethyan oceanic crust.Major,trace-element and REE patterns of the peridotites and their minerals indicate that the lherzolites experienced lower degrees of partial melting resembling abyssal peridotites(at higher temperatures,TREE=$1216℃)than the harzburgites(6%–8%versus 15%–17%).Elevated eNd(t)and variable^(87) Sr/^(86) Sr(t)ratios along with REE patterns suggest that the Spontang mafic rocks display N-MORB affinity with negligible participation of oceanic sediments in their genesis are originated from a depleted upper mantle with little contribution from subduction-related fluids.MORB-type Neotethyan oceanic crust is associated with the earliest phase of subduction(of older Jurassic age)through which a younger intra-oceanic island arc(Spong arc)subsequently developed.Harzburgites REE display typical U-shaped patterns,suggesting that these rocks have been metasomatized by LREE-enriched fluids.On the other side,mafic rocks are characterized by heterogeneous(Nb/La)PMand(Hf/Sm)PMand relatively homogeneous eNd(t),indicating interaction of subduction-related melts with the upper mantle during the initiation of subduction,in Early Cretaceous times.