The relationship between the Taihe Fe-Ti oxide ore-bearing layered intrusion and the adjacent peralkaline A-type granitic pluton in SW China: Constraints from compositions of amphiboles and apatite-hosted melt inclusions

Abstract

The Taihe igneous complex, which is located in the inner zone of the Emeishan Large Igneous Province (ELIP), consists of an Fe-Ti oxide ore-bearing layered gabbroic unit and a silicic enclave-bearing peralkaline A-type granitic unit. The apatite-hosted melt inclusions in the gabbroic unit and the amphiboles in both the gabbroic unit and the granitic unit were used to investigate the relationship between the two units. The apatite-hosted melt inclusions have 32.3–68.3 wt% SiO2, 0.51–23.9 wt% FeOt, 0–7.19 wt% TiO2, 8.87–19.0 wt% Al2O3, 2.39–19.5 wt% CaO, 1.70–7.27 wt% Na2O, 0.24–5.60 wt% K2O, and 0.13–6.83 wt% P2O5. Such a large compositional range is difficult to be achieved through fractional crystallization but can be accounted for by silicate liquid immiscibility developed during the differentiation of the gabbroic unit. The weighted bulk composition of the Taihe gabbroic unit is extremely rich in Fe and poor in Si (17.2 wt% FeOt and 36.2 wt% SiO2), indicating that some Si-rich components must have been removed. A significant compositional gap in the amphiboles in the gabbroic rocks and the adjacent peralkaline A-type granitoids, i.e., calcic amphibole in the gabbros and apatite-hosted inclusions and sodic-calcic to sodic amphiboles in the granites and their silicic enclaves, was observed. This cannot be explained by the previously suggested differentiation of a common basaltic parental magma from gabbros to silicic enclaves and granites, indicating a sudden change in the physical-chemical conditions of the related magmas. We propose that the silicic enclaves enclosed in the peralkaline A-type granitic pluton represent the missing Si-rich components of the Taihe layered gabbroic unit. The continuous expulsion of immiscible Si-rich melts from the lower crystal mush into the upper main magma body caused the crystallization of large amounts of Fe-Ti oxides in the gabbroic unit. Then, the residual Si-rich magmas in the upper part of the Taihe mafic magma chamber were drawn into the subsequently intruding peralkaline granitic magmas during the magmatic transgression process, forming the silicic enclaves within the adjacent contemporaneous peralkaline A-type granitic pluton.