Abstract
The present study discusses the geochemistry of the Early Cretaceous Shyok and Nubra volcanics from the Shyok Suture Zone (SSZ), Trans-Himalaya. Geochemically, the studied volcanic rocks belong to calc-alkaline magma series and vary in composition from basalt-basaltic andesite-andesite-dacite. The SSZ rocks are enriched in light rare earth elements (LREE: La, Ce, Nd) and large ion lithophile elements (LILE: Rb, Ba, U, K, Pb and Sr) while depleted in high field strength elements (HFSE: Nb, P and Ti), analogous to subduction zone magmatism. Their perturbed LILE concentrations coupled with higher 87Sr/86Sr ratios reflect possible influence of post-crystallization processes. Trace element modelling reflects variable degrees of partial melting (~5–30%) of a metasomatised spinel peridotite mantle source followed by fractional crystallization of clinopyroxene and amphibole along with olivine and plagioclase. The Shyok and Nubra volcanics depict positive εNd(t = 110 Ma) values (+2.3 to +5.2), slightly lower than the contemporary Chalt fore-arc boninites (+6.3 to +8.0) and adjoining intra-oceanic island arc rocks along the Indus Tsangpo Suture Zone (ITSZ) such as Nidar ophiolite (+6.8 to +8.7), Shergol gabbros (+5.1 to +8.4) and Dras volcanics (+5.8 to +7.0), therefore reflecting depleted mantle characteristics. In contrast to the ITSZ rocks, the Shyok and Nubra volcanics have enriched LREE-LILE characteristics, indicating their derivation from isotopically depleted and elementally (LREE-LILE) enriched mantle wedge sources. Thus, based on the geochemical and isotopic signatures of the SSZ rocks and their association with continentally derived sediments, we suggest an Andean-type arc system as opposed to ITSZ rocks and associated oceanic sediments, representing intra-oceanic arc system.
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