Geochronology, mineralogy, and geochemistry of the Late Triassic Xiuwacu biotite granite in the southern Yidun Terrane, southwest China: Insights into the petrogenesis and magmatic fertility

Abstract

The southern Yidun Terrane formed in response to the westward subduction of the Garzê–Litang oceanic lithosphere. The Xiuwacu pluton, consisting of the Late Triassic biotite granite and Late Cretaceous monzogranite porphyry, is located in the northern portion of the southern Yidun Terrane. It hosts the Late Cretaceous quartz‐vein‐type Mo–W deposit genetically related to the intraplate monzogranite porphyry. However, the petrogenesis and magmatic fertility of the biotite granite remain unclear. Sixteen zircons from the biotite granite yield a weighted mean 206Pb/238U age of 217 ± 1.2 Ma, indicating that the biotite granite emplaced synchronously with the arc‐related andesites and porphyries in the southern Yidun Terrane. Unlike these slab melt‐derived adakitic rocks, the Xiuwacu biotite granites show typical I‐type granite affinities, with relatively high SiO2 contents, low Sr/Y ratios, and high Y contents. These granites display depleted zircon Hf isotopic signatures (ƐHf(t) = 0.6–3.8) but enriched whole‐rock Sr–Nd and zircon O isotopic signatures, suggesting that the mixing between juvenile materials and external enriched melts might have contributed to the generation of biotite granite. Moreover, they show relatively high Al2O3/(FeOT + MgO + TiO2), Th/Yb, and Th/Nb ratios, which are collectively indicative of the input of sediment melts extracted from subducting slab into the melt source of biotite granite. The high MgO, TiO2, F, and Cl contents of biotite and the excess Al of plagioclase suggest that the Xiuwacu biotite granites formed under high oxygen fugacities, magmatic water contents, and volatiles components and they might have Cu prospecting potentials.