Origin and evolution of synchronous Miocene leucogranite dykes and porphyritic granitoid pluton in the Tangste region of Pangong metamorphic complex, Ladakh Trans-Himalaya, India: Constraints from geochemistry and zircon U-Pb-Hf isotopes

Abstract

The Pangong Metamorphic Complex (PMC) lies within the Karakoram Fault (KF) zone that separates the Ladakh batholith from the easternmost Karakoram terrain in the Ladakh Trans-Himalaya. The PMC is intruded by an extensive network of leucogranite dykes and veins. K-feldspar megacryst-bearing porphyritic granitoid pluton is exposed widely in Tangste region located to the south of the PMC. Geochemistry, zircon UPb SHRIMP geochronology and Hf isotopes of leucogranite, porphyritic granitoid, and geochemistry of associated orthogneiss (migmatized) and amphibolite (refractory residuum) have been conducted to unravel the source-to-sink melt history during the KF development. All the studied rocks are metaluminous (molar Al₂O₃/CaO + Na₂O + K₂O = 0.46–1.02), except for one peraluminous orthogneiss (A/CNK = 1.16), and dominantly magnesian and calc-alkalic to alkali-calcic. Leucogranite and porphyritic granitoid exhibit adakite-like affinity, while orthogneiss has a magmatic arc signature. The geochemical features support that melting of calc-alkaline orthogneiss and infracrustal magmatic source rocks produced leucogranite and porphyritic granitoid melts in a syntectonic environment. UPb geochronology of leucogranite inherited zircon cores yielded a range of ²⁰⁶Pb/²³⁸U ages from 73.6 ± 0.8 Ma to 46.7 ± 0.5 Ma with a weighted mean age of 72.1 ± 1.9 Ma. The positive ɛHf values (+3.81 to +11.30) of dated inherited zircon cores suggest that the sources for leucogranite melts were juvenile crust-derived magmatic rocks (ca. 73–46 Ma) formed in a subduction zone, similar to the granitoids of the Ladakh-Kohistan and Karakoram batholiths. Leucogranite zircon rims grown over the inherited cores provide a crystallization age of 18.27 ± 0.29 Ma, synchronous to the zircon mean crystallization age of 18.50 ± 0.16 Ma in the porphyritic granitoid. The range of ɛHf (−11.81 to +0.11) of leucogranite zircon rims overlaps remarkably with ɛHf (−10.38 to −0.02) of porphyritic granitoid zircon, which show striking evidence of recycling of a common crustal source (calc-alkaline magmatic rocks) in the formation of leucogranite dyke and porphyritic granitoid pluton. The Miocene leucogranite dyke melts thus likely acted as “feeders” to the porphyritic granitoid plutonism during the development of the KF zone in the PMC of Ladakh Trans-Himalaya.