East–west-trending tearing of the Indian slab beneath the eastern Tibetan Plateau: Insights from Eocene–Oligocene potassic adakite-like granites in western Yunnan

Abstract

Large-scale Eocene–Oligocene potassic adakite-like granites predominantly occur between 25° N and 27° N, north of the Ailaoshan suture in western Yunnan, eastern Tibetan Plateau. However, limited interpretations of their spatial characteristics hinder our understanding of the tectonic evolution in this region. Herein, we present new petrographic analyses, zircon UPb ages, whole-rock major and trace element compositions, SrNd isotopic data and zircon Hf isotopic mapping for these granites. Zircon UPb ages indicate that the adakite-like granites were emplaced at approximately 37.1–33.6 Ma. These granites exhibit high-K calc-alkaline compositions, enriched light rare earth elements and significant depletion in heavy rare earth elements, characterised by high Sr contents (667–1795 ppm), Sr/Y ratios (35–158) and La/Yb ratios (13–62), alongside low Y (8.2–23.3 ppm) and Yb (0.71–1.13 ppm) contents. The adakite-like granites possess high initial ⁸⁷Sr/⁸⁶Sr ratios (0.70596–0.70687) and low ε_Nd(t) values (−5.73 to −0.65), with two-stage depleted mantle model ages (T_DM²) ranging from 1.3 to 0.9 Ga. Their zircon ε_Hf(t) values range from −5.1 to +0.7, with Neoproterozoic T_DM² ages of 1.4–1.1 Ga. Notably, these adakite-like rocks in western Yunnan exhibit an eastward decrease in zircon ε_Hf(t) and whole-rock ε_Nd(t) values. By integrating these findings with the Hf isotopic mapping results, we suggest spatial heterogeneity in lower crustal composition in western Yunnan. The lower crust in the western Yangtze block comprises Neoproterozoic arc roots, while the lower crust of the Simao block represents a superposition of Neoproterozoic and Permian–Triassic arc roots. Based on the abrupt changes in angle and depth of the eastward-subducting Indian slab, along with the spatial characteristics of the Eocene–Oligocene magmatism, we propose that east–west-trending tearing of the subducted Indian slab occurs along approximately 26° N beneath the eastern Tibetan Plateau.