Generation of Neoproterozoic high-silica granites by crystal-melt segregation in the northern Yangtze Block, South China: Implications for the evolution of Precambrian continental crust

Abstract

There is a consensus that the generation of high-silica granites could undergo obvious crystal-melt separation, which is of significance for comprehending the evolution of shallow magmatic system and silicic upper crust. Although voluminous studies have paid more attention to the crystal-melt segregation mechanism acting on the Phanerozoic high-silica granites and/or rhyolites, it remains unclear whether the evolution and emplacement of Precambrian (especially the Neoproterozoic) high-silica igneous rocks are controlled by this mechanism. Herein we therefore presented comprehensive studies on the petrology, mineralogy, whole-rock geochemistry, and zircon U-Pb-Hf isotopes for the Neoproterozoic high-silica granites (biotite granites and K-feldspar granites) from the Huangling batholith in the northern Yangtze Block (South China), in order to trace their magmatic source and evolutional process. Zircon U-Pb ages reveal that the Huangling biotite granites and K-feldspar granites were formed at ca. 812–818 Ma, belonging to the contemporary Neoproterozoic magmatism. They are characterized by variable and high SiO2 (70.2–74.8 wt.%), K2O (1.3–6.4 wt. %), Rb (28–228 ppm), and Th (1.7–35.1 ppm) contents. The obviously enriched whole-rock Sr-Nd isotopes (εNd(t) = -23.4 to -18.4) and zircon Hf isotopes (εHf(t) = -44.3 to -18.4) with old two stage model ages (TNdDM2 = 2641–2987 Ma, THfDM2 = 2852–4442 Ma) indicate that they sourced from the partial melting of ancient Archean continental crust. The Huangling biotite granites display higher Ba (532–1659 ppm), Sr (450–741 ppm) and Eu/Eu* (0.84–1.40) values, but lower Rb (28–58 ppm) and Th (1.8–5.5 ppm) contents than the K-feldspar granites (Ba = 345–904 ppm, Sr = 108–152 ppm, Rb = 170–228 ppm, Th = 9.8–35.1 ppm, and Eu/Eu* = 0.31–0.46), suggesting that the biotite granites and the K-feldspar granites represent the residual cumulations and extracted high-silica melts from the crystal mush, respectively. Massive mineral aggregates of plagioclases and biotite support the residual crystal accumulations of the Huangling biotite granites, and the higher zircon Hf, Nb concentrations and lower zircon Eu/Eu* ratios of the K-feldspar granites further advocate that they were derived from the more evolved melts. This work emphasized that intensive crystal-melt segregation process in the crystal mush reservoir plays a crucial role of the generation of Neoproterozoic high-silica granites, which are therefore significant for the evolutional process of the Precambrian high-silica continental crust in the northern Yangtze Block.