Microstructures in the Baima layered intrusion, SW China: Constraints on the late-stage magmatic processes during solidification of a crystal mush

Abstract

Magmatic microstructures in layered intrusions record key petrographic evidence for the late-stage magmatic processes that took place during solidification of a crystal mush. Systematic examination of the magmatic microstructures can reveal dynamic evolution of interstitial liquid in crystal mush, which however has been only conducted in a few studies on layered intrusions by far. In order to constrain the late-stage magmatic processes related to the Fe-Ti oxide-bearing layered intrusions, we investigated the magmatic microstructures of the Baima intrusion in the Panxi region (SW China). The intrusion is divided into three major units from the base upwards, including the lower zone (LZ), middle zone (MZ) and upper zone (UZ). Through detailed examination of a series of drill core samples, we found several different types of reactive microstructures, such as replacive symplectites, fish-hook pyroxene/olivine/amphibole and amphibole rim. Replacive symplectites include fine-grained, lamellar intergrowths of An-rich plagioclase + olivine/clinopyroxene/amphibole/phlogopite/biotite that are rooted to Fe-Ti oxides and replacing plagioclase primocrysts (Type 1) and intergrowths of An-rich plagioclase + orthopyroxene/olivine/amphibole that are rooted to olivine and replacing plagioclase primocrysts (Type 2). These microstructures are particularly abundant in the LZ except for the Type 2 symplectites, which are rare throughout the intrusion. An-rich plagioclase in the replacive symplectites and fish-hook textures have An contents up to 96, much higher than those of plagioclase primocrysts. Overall, fish-hook amphibole and amphibole rims contain 0.19−4.24 wt.% TiO2, higher than those of amphibole lamellae in the replacive symplectites that varying from 2.38 wt.% TiO2 to below the detection limit. These microstructures were developed during solidification of the intrusion due to the reaction between plagioclase primocrysts and ambient Fe-rich liquid, accompanying with addition of Fe, Ti and Ca and removal of alkalis and silica in an open system. The abundance of such reactive microstructures in the LZ are therefore attributed to silicate liquid immiscibility within a crystal mush, in which the segregation of the buoyant Si-rich component left behind the dense Fe-rich liquid in the lower part of magma chamber. The common presence of amphibole and phlogopite/biotite in the replacive symplectites indicates that continuous fractionation of the Fe-rich liquid leads to hydration of interstitial liquid during the late-stage solidification.