Biotite geochemical fingerprints deciphering magma evolution and compositional diversity of the concentric Zhongchuan Batholith, West Qinling, China

Abstract

Compositional diversity is a common feature of plutons and a frequent topic in earth science publications. The Zhongchuan Batholith, a composite body in the West Qinling Orogen of central China, provides an important example for understanding compositional variations and the genesis of different granitic phases. The batholith consists of three annular rings (medium-coarse grained porphyritic biotite granite, medium grained phenocryst-bearing biotite granite, and medium-fine grained biotite granite) from the outer to inner rings. Biotite is the most common ferromagnesian mineral in diverse granitic phases of the batholith and can serve as key indicators for the geochemical characteristics of the granites and their compositional variations. In this study, we present major and trace element geochemistry of biotite to constrain the physico-chemical conditions and petrogenesis of the various granitic phases in the batholith. Microstructural and alteration studies of biotite from the different granitic phases reveal features consistent with magmatic biotite. The major elements of the biotites indicate crystallisation temperatures of approximately 778 °C, 755 °C, and 726 °C for the outer, intermediate, and inner rings, respectively. These temperatures correspond to pressures of ~210 MPa (7.5 km), 390 MPa (14 km), and 440 MPa (16 km). The observed changes in crystallisation temperatures and pressures are consistent with the evolution of multi-stage and differentiated magma. The oxygen fugacity of biotite varies from −16 to −12, indicating a high magma oxygen fugacity. Major and trace elements of biotite further attest that the batholith is classified as I-type granite, derived from crust-mantle mixing, with a greater contribution of mantle-derived materials in the outer ring. The compositional diversity of the batholith is attributed to high-degree magma mixing without fractional crystallisation during magma evolution.