Interstitial Microstructures in the Baima Gabbro Pegmatite, SW China: Constraints on the Late-Stage Evolution Processes in Mafic Pegmatites

Interstitial microstructures in igneous rocks record key evidence of late-stage evolution processes. In order to constrain the late-stage evolution of a Baima gabbro pegmatite (Panxi region, SW (Southwestern Sichuan Province), China), we investigated its mineral compositions and late-stage microstructures. These microstructures include replacive symplectites and fish-hook clinopyroxene. Replacive symplectites include fine-grained lamellar intergrowths of anorthite (An)-rich plagioclase + clinopyroxene/amphibole/biotite that are rooted to Fe-Ti oxides and replacing nearby plagioclase primocrysts (Type I) and intergrowths of An-rich plagioclase + clinopyroxene/amphibole that are rooted to olivine and replacing nearby plagioclase primocrysts (Type II). Rare replacive symplectites composed of biotite + plagioclase are also present. Those replacive symplectites and fish-hook clinopyroxene grew at a late magmatic stage with temperatures of 867–1023 °C. An-rich plagioclase in the replacive symplectites and fish-hook textures have An contents up to 94 mol.%, much higher than those of plagioclase primocrysts. Interstitial microstructures are interpreted to reflect interaction between primocrysts and an Fe-rich residual interstitial liquid, consistent with separation and migration of conjugate immiscible melts in a crystal mush. We propose that the hydrous interstitial melt in the Baima gabbro pegmatite may have undergone silicate immiscibility during late-stage magma crystallization. As the crystal fraction increased, crystal-mush compaction and porous melt migration likely became the primary controls on the evolution of the late-stage interstitial melt, rather than convection or diffusion.