From granite to highly evolved pegmatite: A case study of the Shangkelan rare-metal granite–pegmatite system (Altai, NW China)

Abstract

The Shangkelan granite–pegmatite is known for its internal zonation and tungsten mineralization in the Chinese Altai. However, the genetic relationship between the granite and pegmatite and the W mineralization mechanism in Shangkelan remains unclear. Here we present monazite U–Pb ages and Nd isotopic composition, and whole-rock and muscovite chemical compositions of each zone of the stock, to address these issues. From bottom to top, the stock mainly consists of alkali-feldspar granite (AFG) and granitic pegmatite (GP). Monazite LA-ICP-MS U–Pb dating and Nd isotopic analyses reveal that the AFG and GP have identical ages and similar εNd(t) values, 189 ± 2 Ma and −0.79–+0.38 and 190 ± 1 Ma and −0.70–+0.01, respectively. Whole-rock geochemical results show that the stock is highly fractionated and has a differential trend characterized by increasing Rb, Cs, and Tl contents, decreasing W, Ba, and Sr contents, and Zr/Hf ratios from the AFG to GP. Both primary (magmatic) and secondary (hydrothermal) muscovites have been recognized in Shangkelan. From the AFG to GP, element concentrations and ratios of primary muscovite form well-defined differentiation trends marked by gradual increases in Rb, Cs, and Nb, and decreases in W, K/Rb, and Nb/Ta. Based on the gradational contacts in field, identical monazite ages, and similar Nd isotopic compositions, together with the differentiation trends of incompatible elements (such as Rb and Cs) in bulk-rock and primary muscovite and alkali metal Rayleigh fractionation modeling results, we consider that the AFG is parental to GP and they were produced by continuous fractional crystallization of a common magma source. Additionally, for the monazites Nd isotopes, their two-stage Nd model ages of 1034–939 Ma and slightly negative to positive εNd(t) values (−0.79–+0.38) indicate that the magma sources were mainly derived from the reworking of the Stenian–Tonian basement. Secondary muscovite precipitated from the fluids that exsolved from the parental AFG melt has high contents of F and W. Tungsten may be dissolved as H3WO4F2− in the fluids. The greisenization caused the destabilization of H3WO4F2− and eventually led to the precipitation of wolframite in Shangkelan.