Clinopyroxene In Situ Sr Isotopic Evidence for Crustal Assimilation of Potassic–Ultrapotassic Mafic Magmas in the Southeastern Tibetan Plateau

Abstract Potassic–ultrapotassic magmas in continental collision zones have attracted much attention due to their genetic association with magmatic–hydrothermal Cu–Au ore formation and lithospheric mantle evolution. Although these magmas are typically modified by crustal assimilation during ascent, key aspects of this process, especially the type and amount of assimilated material and the consequent effects on magma composition, remain poorly constrained, hindering a full understanding of their petrogenesis. Clinopyroxene (Cpx) serves as an ideal archive for deciphering crustal assimilation in such magmatic systems, as it crystallizes over a wide range of melt compositions and is relatively rich in elements that can be used as isotopic tracers (such as Sr). Here, we conduct detailed petrographic studies and in situ elemental and Sr isotopic analyses along core-to-rim traverses of Cpx from a suite of coeval potassic–ultrapotassic mafic lavas and dikes sampled at six locations of the southeastern Tibetan Plateau. Two populations of Cpx are identified: phenocrysts and green cores interpreted as recycled crystals from differentiated progenitor magmas related to the host magmas. The phenocryst cores are characterized by high Mg# (up to 90) and elevated Ni and Cr contents, consistent with derivation from primitive mantle-derived melts. Their similarly high initial 87Sr/86Sr ratios, combined with trace-element systematics, suggest that the primitive melts were derived from a common, slightly enriched lithospheric mantle source metasomatized by carbonate melt-dominated agents. Cpx-based thermobarometry indicates that these magmas ascended through the crust and crystallized at depths corresponding to ~0.2–1.0 GPa, defining a transcrustal plumbing system. Normal core-to-rim zoning in Cpx phenocrysts, as shown by decreasing Mg# and Ni and Cr contents, and increasing incompatible element concentrations (e.g. rare earth elements and Zr), reflects fractional crystallization of Cpx, olivine, and/or phlogopite in the magma plumbing system. Concurrently, systematic increases in initial 87Sr/86Sr ratios from core to rim (up to 0.004) record progressive assimilation of crustal materials with high Sr isotopic ratios. The presence of Neoproterozoic zircon xenocrysts, along with Sr content and isotopic data from regional crustal rocks, identifies Neoproterozoic granitic gneiss as the most likely assimilant. Quantitative models support this interpretation and indicate assimilated wall-rock melt masses of <0.2. Our in situ micro-analytical data demonstrate that even crustal assimilation to this limited extent can markedly control the isotopic signature of potassic–ultrapotassic mafic magmas in the southeastern Tibetan Plateau. Given that such magmas significantly contribute to crustal growth by differentiating into voluminous granitoids, this study provides critical insights into how assimilation modulates the isotopic composition of the post-collisional newly formed crust.