Tracking crystal-melt segregation and accumulation in mushy reservoirs: Implication for silicic magma formation in tarim large igneous province

Abstract

The genesis of silicic (SiO₂ > 70 wt%) magmas in Large Igneous Provinces (LIPs) has traditionally been related to partial melting of pre-existing crust and/or crystal fractionation of mantle-derived basaltic magma. Recent petrological studies highlight crystal-melt segregation as a significant mechanism for generating silicic magmas and establishing genetic links between volcanic and plutonic rocks. The identification of complementary cumulate residues at the base of crystal mush and the necessary mafic magmas for mush reactivation has proven challenging, leading to the elusive recognition of crystal accumulations in plutonic rocks with intermediate to silicic compositions. The Xiaohaizi complex in the Tarim LIP in NW China consists of plutonic rocks ranging from mafic to silicic (45–75 wt% SiO₂). This complex provides a natural laboratory for investigating the in-situ crystal-melt segregation process in the shallow crust. Rayleigh fractionation modeling of Ba, Sr, Eu, and Rb reveals that quartz syenites and granites embody highly evolved, extracted melts, while amphibole syenites and fayalite syenites correspond to the complementary cumulate residues. The quartz syenite porphyry likely represent well-preserved snapshot of the parental magma's initial chemical characteristics. Mineral textures and crystal size distribution (CSD) patterns effectively differentiate between cumulate residues and extracted melts. Variations in Eu/Eu*, Rb, and Ba in alkaline feldspars support feldspar accumulation in mush. During magmatic evolution, temperature and pressure of the Xiaohaizi intermediate to silicic magmatic rock gradually decrease from 950 °C to 700 °C and 300 MPa to 100 MPa, respectively. High melt water content (H₂O_melt = ∼4 wt%) in these magmas enhances the efficiency of crystal-melt segregation. The injection of deep, high-temperature (1000 °C ∼ 1100 °C, 300–500 MPa) wehrlite magmas can prolong the crystal-melt segregation process. The similarity in magmatic oxygen fugacity and melt water contents among the mafic, intermediate, and silicic rocks suggests a physicochemical inheritance throughout the magma evolution process. This complex can preserve records of crystal-melt separation process to produce silicic magmas in the shallow crust, providing new evidence for mush model during the Permian within the Tarim LIP.