Geochemistry and geochronology of the Miocene adakite-like potassic dikes in Tethyan Himalaya: New insights into Indian lithosphere slab tearing and breakoff

Abstract

Adakitic and potassic magmatic rocks are widespread in the Himalaya and the Tibetan Plateau, and their petrogenesis might provide potential insights into the deep crustal processes. However, the detailed spatiotemporal distributions of these rocks and their controlling mechanisms remain ambiguous. Here we report geochemical data, zircon U-Pb ages and Hf isotopic compositions of east-west (E-W) directed Qunrang monzonite porphyry dikes located in the Tethyan Himalaya. These rocks are acid and potassic with low Mg, low abundances of compatible elements, low Y and HREE, but with high Al, high Sr (822–1386 ppm) and Ba (2027–6091 ppm), high Th (62–84 ppm), U (9–16 ppm) and Th/U ratios (4–7), and thus high Sr/Y and (La/Yb)N ratios. The above geochemical features, combined with their negative whole-rock εHf(t) values (−3.9 to −4) and negative to small positive zircon εHf(t) values (−7.2 to +0.5), suggest that they were derived from partial melting of a thickened lower continental crust consisting mainly of garnet-amphibolite which were triggered and hybridized by underplating enriched mantle-derived shoshonitic melts. Zircon U-Pb ages of the Qunrang monzonite porphyry yielded a weighted mean age of 10.02 ± 0.33 Ma. Based on these observations, synthesized with the spatiotemporal distribution of the Oligocene-Miocene adakitic and potassic rocks in the southern Tibet, we propose that lateral detachment and longitudinal tearing processes might be potentially accounted for the eastward and southward younging trend of these magmatic rocks, whereas slab rollback and hinge advance might result in the unique E-W directed Qunrang adakite-like potassic dikes and the northward reversal younging trend.