Mineralogy from three peralkaline granitic plutons of the Late Permian Emeishan large igneous province (SW China): evidence for contrasting magmatic conditions of A-type granitoids

Abstract

The Emeishan large igneous province contains a diverse assemblage of igneous rocks including mildly peralkaline granitic rocks of A-type affinity. The granitic rocks from the Panzhihua, Baima and Taihe plutons are temporally, spatially and chemically associated with layered mafic-ultramafic intrusions. Electron microprobe analyses of the major and accessory minerals along with major and trace element data were used to document the magmatic conditions of the three peralkaline plutons. The amphiboles show magmatic/subsolidus trends and are primarily sodic-calcic in composition ( i.e ., ferrorichterite or richterite). Sodic ( i.e ., riebeckite-arfvedsonite) amphiboles are restricted to the Panzhihua and Taihe plutons. The amphiboles from the Panzhihua and Taihe granites are very similar in composition whereas amphiboles from the Baima syenites have higher MgO wt% and lower FeOt wt% and TiO2 wt%. Whole-rock Zr saturation temperature estimates indicate the initial average magma temperatures were ~940 ± 21 °C for the Panzhihua pluton, ~860 ± 17 °C for the Baima pluton, and ~897 ±14 °C for the Taihe pluton. The initial Fmelt(wt%) values were calculated to be 1.1 ± 0.1, 0.8±0.1 and 1.1±0.1 wt% for the Panzhihua, Baima and Taihe plutons, respectively. The estimated Fmelt(wt%) values are higher than what can be accounted for in the Panzhihua and Taihe plutons and indicate that they may have lost F during crystallization. In contrast the Fmelt(wt%) value for the Baima pluton can be accounted for. The presence of titanite +magnetite +quartz in the Baima syenites indicates oxidizing f O2 conditions whereas the presence of aenigmatite and ilmenite in the Panzhihua and Taihe granites indicate that they were relatively reducing. Although the Atype granitoids formed by the same processes ( i.e ., fractional crystallization of mafic magmas), their differences in major element and mineral chemistry are likely related to a combination of initial bulk magma composition and magmatic oxidation state.