Early Miocene Adakitic Granite Porphyry in the East Kunlun Orogen: Implication on Melting of Thickened Ancient Lower Crust During Post-Collision

Cenozoic felsic magmatism along the northern Tibetan Plateau records the evolution of thickened continental crust, yet Miocene intrusions in the East Kunlun Orogen remain poorly constrained. Here we integrate field and petrographic observations with zircon U-Pb geochronology, zircon Lu-Hf isotopes, and whole-rock geochemistry for the Ganquanhe biotite monzogranite porphyry. Sample TW6 contains two zircon 206Pb/238U age populations at 14.8 ± 0.2 Ma and 17.5 ± 0.2 Ma, whereas the coherent TW7 population yields the preferred crystallization age of 17.6 ± 0.2 Ma; these uncertainties include a conservative 1% external reproducibility term. The overlapping εHf(t) ranges of the two TW6 age groups support a common magma source and indicate that the younger dates most likely record Pb loss rather than a separate magmatic event. The high-K calc-alkaline, metaluminous to weakly peraluminous rocks have low Y and Yb, high Sr/Y and (La/Yb)N, fractionated rare earth element (REE) patterns, and weak Eu anomalies. Low MgO, Cr, and Ni contents, negative zircon εHf(t) values, and Mesoproterozoic TDM2 ages indicate derivation mainly from ancient, garnet ± amphibole-bearing lower crust. Whole-rock zircon saturation temperatures of 760–782 °C and Sr/Y- and (La/Yb)N-based crustal-thickness estimates of approximately 72–77 km indicate melting within a warm, strongly thickened crust. Major-element source discrimination further favors an amphibolitic lower-crustal source with possible subordinate graywacke–orthogneiss components. The intrusion therefore records Early Miocene lower-crustal melting within a collisional to post-collisional setting along the northern Tibetan Plateau.