Petrogenesis of Late Triassic adakitic plutons in the East Kunlun Orogen, Northern Tibet: Geodynamic implications for the Paleo-Tethyan orogeny and crustal evolution

Abstract

Adakitic rocks found within continental arcs are pivotal in unraveling the geodynamic processes and tectonic evolution of East Kunlun Paleo-Tethyan orogen. We present an integrated study on the petrography, zircon U-Pb geochronology, whole-rock geochemistry, and Sr-Nd-Hf isotopes of newly identified Late Triassic adakitic plutons in the East Kunlun Orogenic Belt (EKOB), Northern Tibet, aiming to constrain their petrogenesis and geodynamic implications. These plutons comprise by mylonitized granite, monzogranite, granodiorite and dioritic porphyrite, exhibiting consistent zircon U-Pb ages of approximately 204–212 Ma. They have high SiO2, K2O and Sr (average of 479 ppm) contents, but low contents of Y (5.6–18 ppm) and Yb (0.5–1.0 ppm) with high Sr/Y ratios (17–63), resembling adakitic rocks. These adakitic rocks can be classified into high-Mg (Mg#= ∼51) and low-Mg groups (Mg#= ∼35), although they all display enriched large ion lithophile elements and light rare earth elements but depleted high field strength elements. The studied low-Mg adakitic rocks have low MgO (∼0.68 wt%) and Cr (∼35 ppm) with enriched zircon Hf isotopes (εHf(t) = − 1.16 t 2.44), suggesting their derivation by partial melting of thickened juvenile crust. Conversely, the high-Mg adakitic rocks exhibit high MgO (∼1.94 wt%) and Cr (∼61 ppm) with relatively high zircon εHf(t) values (−0.68 to 4.95), indicating their derivation from foundered crustal root with subsequent interaction with enriched mantle-derived magma. Tectonic reconstructions suggest that the formation of these Late Triassic adakitic rocks occurred in a post-collisional extension setting and the East Kunlun Paleo-Tethyan Ocean closed no later than Norian.