Apatite Textures, Elemental and Isotopic Compositions Unmask the Homogenizing Process in Silicic Magma Chambers

Silicic magmas are the most viscous of all magmas, however some granitic plutons display remarkably homogeneous compositions, which contradicts the hypothesis that mechanical mixing is the main homogenizing process in the magma chamber. Thus much remains controversial about the mechanisms responsible for the homogeneities of silicic plutons. Here, we present textural observations, elemental mapping, and in situ elemental and Nd-O isotopic data of apatites from the compositionally homogeneous Late Permian Yuanyang A-type granitic pluton (SW Yunnan, South China). Apatite grains display oscillatory chemical zonation and resorption-precipitation texture, suggesting incremental growth dominated by co-genetic magma batches injection. The intra-/inter-grain core to rim elemental and Nd-O isotopic variations imply crystal transfer and crystallization from different melt domains within the crystal mush. We propose that rejuvenation events associated with hotter cogenetic intermediate magma batch injection has induced crystal mush reactivation and convective stirring in silicic magma chambers, thereby homogenizing the entire reservoir. Magma mixing between mafic and viscous felsic magmas is usually incomplete, forming heterogeneous structures and geochemical compositions in felsic magma bodies. However, some silicic magma bodies have homogeneous compositions and structures, such as the Late Permian Yuanyang A-type granites (YAGs) in southwestern South China, and the actual homogenizing process in silicic magma chamber is still unclear. Accessory apatite is ubiquitous in felsic plutons, whose growth zoning could reflect magma chamber evolution. Apatite grains collected from the YAGs show complex resorption-precipitation texture. Their core-rim oscillatory elemental and Nd-O isotopic variations suggest compositional perturbations of ambient magma, probably caused by the injection/mixing of multiple magma pulses. Accordingly, we propose that hot magma recharge could reactivate the crystal-mush, causing magma convection and stirring that gradually homogenized the silicic magma chamber. The apatites from the compositionally homogeneous Yuanyang A-type granites are texturally and geochemically heterogeneousThe identified four apatite zoning domains documented magma replenishment, mixing and crystal mush reactivationMagma convection and stirring induced by magma batches injection are key to homogenizing the silicic magma chamber