Identification of intermediate-silicic cumulates in the Emeishan large igneous province, SW China

Abstract

Geochemical signatures of crystal accumulation in silicic igneous rocks are subtle and difficult to discern. However, the presence of silicic cumulates is required to explain the formation of silicic magmas by fractionation-dominated mechanisms. In large igneous provinces, such rock records are scarce if ever reported. In this study, we document a case example of identifying intermediate-silicic cumulates in the end-Guadalupian (∼260 Ma) Emeishan large igneous province, SW China. We demonstrate that syenitic rocks associated with the Panzhihua intrusion have textures and trace element geochemistry consistent with origins involving both crystal accumulation and melt extraction. Micro-X-ray fluorescence mapping shows that the rocks are either supported by a framework of interconnected, relatively large crystals of alkali feldspar, or by a fine- to medium-grained matrix. The former has relatively low SiO₂, relatively high TiO₂, Fe₂O₃, CaO, Na₂O, Sr and Ba, and a characteristic positive Eu anomaly, whereas the latter has essentially the opposite features. All studied rocks have restricted initial Sr-Nd-Pb isotopic ratios that are similar to mafic rocks of the Panzhihua intrusion, requiring minimal contamination by the Precambrian basement of the Yangtze block. Because of delayed zircon saturation, Zr and Zr/Hf are less promising compared with Eu/Eu* and Ba in identifying cumulates in the studied rocks, a view that might hold in other intraplate magmatic systems. Our results, combined with available data, improve the current understanding of volcanic-plutonic connection concerning silicic magma types of the Emeishan large igneous province.