Abstract
Potassic volcanic rocks (PVR) are widely distributed in post-collisional orogenic settings and provide key insights into plateau uplift, crust–mantle interaction, and the genesis of porphyry-type mineralization, although their petrogenesis remains debated. This study investigates two coeval PVR variants (black and grey) exposed in the Geji area, western Tibet. Both share broadly similar whole-rock geochemical features, including elevated SiO2 (64.30–66.32 wt%), low MgO (0.84–2.03 wt%), and depleted compatible elements (Cr = 42.3–52.1 ppm; Ni = 8.4–13.8 ppm), pointing to a lower crustal source. However, they differ significantly in mineralogy and mineral geochemistry, reflecting distinct evolutionary pathways of the magmatic system. In the black PVR, some clinopyroxene phenocrysts show zoning with low-Mg# cores (54–59) and high-Mg# rims (64–74), together with evidence of late-stage recharge by ultrapotassic magma, indicating localized magma mixing. Phlogopite exhibits variable initial 87Sr/86Sr ratios (0.7155–0.7469), and a minor population of inherited zircons (<4%) with affinities to the ancient upper crust further points to limited upper-crustal contamination. In the grey PVR, titanite shows a wider isotopic spread (εNd[t] = −17.7 to − 8.8; ΔεNd[t] = 8.9), whereas apatite Sr isotopes are relatively homogeneous (0.73499–0.73574), in sharp contrast to the black PVR (εNd[t] = −13.5 to − 12.1; 87Sr/86Sr = 0.71931–0.73358). Combined with crystallization ages (black PVR: 25.1 ± 0.5 Ma to 24.0 ± 0.3 Ma; grey PVR: 24.6 ± 0.5 Ma to 23.1 ± 0.4 Ma) and phenocryst abundances (>15% vs. <5%), these differences are interpreted to reflect distinct crystallization sites within a shallow magma reservoir: the grey magma crystallized rapidly at the reservoir margin, undergoing stronger crustal contamination and incomplete mixing, whereas the black magma crystallized more slowly near the reservoir center, experiencing weaker contamination but more complete hybridization. Importantly, the data demonstrate that the lower crust comprises both ancient and juvenile components, variably modified by different mantle-derived inputs at different stages. These findings provide a new framework for reconstructing the lithospheric architecture of the region.