Characterization of anhydrous to hydrous paragenetic sequence from pyroxene-bearing and pyroxene-absent variants of the late Carboniferous Baobei pluton in west Junggar of China

Abstract

The Baobei pluton in west Junggar, composed of an earlier pyroxene-bearing granitoid and a variety of later pyroxene-absent hornblende–biotite-bearing granitoids (quartz diorite, granodiorite, monzogranite), intrudes early Carboniferous volcano–sedimentary rocks. Zircons separated from the granodiorite provide a U–Pb age of 306.5 ± 2.6 Ma, which represents the age of magma emplacement. The pyroxene-bearing granitoid is porphyritic with phenocrysts of orthopyroxene and plagioclase. Two types of orthopyroxene (Mg-rich Opx-1, Fe-rich Opx-2) occur in the pyroxene-bearing granitoid. Both the Opx-1 and Opx-2 show depletions of light REE and enrichments of heavy REE with negative Eu anomalies. The evolved melt in equilibrium with the Opx-2 contains higher light REE concentrations compared with the parental melt in equilibrium with the Opx-1 based on geochemical modeling. The pyroxene-bearing and pyroxene-absent variants of the pluton record the progression from the anhydrous orthopyroxene–clinopyroxene assemblage to the hydrous hornblende–biotite assemblage. These rocks are high-K calc-alkaline, metaluminous to weakly peraluminous and ferroan, and have similar trace element compositions characterized by light REE enriched and constant heavy REE patterns with enrichments of Pb and depletions of Nb and Ta. The melt corresponding to different granitoids was formed by dehydration melting of basaltic rocks at 4.8–9.7 kbar and 1060–1146 °C within a post-collisional geological setting in west Junggar.