Petrogenesis and Geological Significance of the Miocene Monzogranite Porphyry in the Chunzhe Area, Middle Gangdese Belt

The Oligocene–Miocene magmatic rocks extensively developed in the Gangdese magmatic belt are key records of the post-collisional tectono-magmatic evolution of the Tibetan Plateau. In this study, petrological, zircon U-Pb geochronological, zircon Hf isotopic and whole-rock geochemical investigations were carried out on two granitic porphyry stocks exposed in the Chunzhe area of the middle Gangdese belt. LA-ICPMS zircon U-Pb dating, cathodoluminescence (CL) images and trace element characteristics indicate that the granitic porphyries were emplaced at 11.8 ± 0.2 Ma (MSWD = 1.1) and 11.5 ± 0.1 Ma (MSWD = 1.2), with a small number of zircon grains yielding 206Pb/238U ages of 51.1~59.5 Ma, 29.8 Ma and 19.4~12.2 Ma, which are interpreted as inherited or captured zircon components. The analyzed samples are monzogranite porphyries composed mainly of quartz, plagioclase and alkali feldspar, with variable secondary white mica/sericite. In whole-rock composition, they display high-K calc-alkaline and weakly peraluminous characteristics. These rocks are enriched in large-ion lithophile elements (LILEs) such as Ba, Sr and Rb, and relatively depleted in Nb-Ta-Ti as well as Cr and Ni. They show light rare earth element (LREE) enrichment and heavy rare earth element (HREE) depletion, with distinctly high chondrite-normalized La/Yb ratios (31.05~71.25) and Sr/Y ratios (35.90~49.07), and a positive correlation between the LREE/HREE ratio and La content, indicating robust adakite-like trace element characteristics. Zircon εHf(t) values of the Miocene magmatic rocks range from −4.44 to 2.41, corresponding to two-stage Hf model ages of 1380~944 Ma, suggesting that the magmas were mainly derived from juvenile continental crust materials with the addition of a small amount of ancient continental crust materials. Combined with the regional geological setting, the Chunzhe Miocene granitic porphyries were most likely generated by partial melting of the thickened lower crust in the Gangdese belt during the late stage of Oligocene–Miocene post-collisional magmatism; local lower-crustal delamination may also have contributed, although this is not uniquely constrained by the present dataset.