Compositional Variation of Amphiboles During Magma Mixing: A Case Study of Huangyangshan A-Type Granite in Kalamaili Metallogenic Belt, East Junggar, China

Chenyang Ye,Gaoxue Yang,Yonggang Feng,Ruxiong Lei

doi:10.3389/feart.2021.650014

Abstract

The Huangyangshan A-type granitic pluton, distributed along the thrust fault in the Kalamaili region of East Junggar, Xinjiang, China, consists of alkaline granite containing abundant dioritic enclaves that formed via magma mixing. Both the host granite and the enclaves contain sodic amphiboles. The textural evidence indicates that amphiboles crystallized as a magmatic phase in both units. We determined major and trace element contents of amphiboles from both units to investigate the compositional variation of the amphiboles during the magma mixing process. The results show that cations of W- and C-site are influenced by chemical compositions of the magma whereas cations of A-, B- and T-site and Al3+ are controlled by crystal structure. Therefore, the variations of W- and C-site cations can reflect magma evolution. The core and rim of the amphiboles show similar trace element patterns, which also suggests that the amphiboles are late-stage phases. Furthermore, the amphibole-only thermometers yield reasonable estimates that are consistent with petrographic evidence. However, thermometers based on partition coefficients and all the currently available amphibole-based barometers that rely on Al contents or DAl cannot be applied to Fe-rich and Al-poor amphiboles.

Highlights

Magma mixing is a common magmatic process during formation of granitoids (Kumar et al, 2004; Liu et al, 2013; Xiong et al, 2020)
Amphiboles are capable of accommodating a variety of elements with variable ionic charges and radii, including alkali elements, rare earth elements (REE), high field strength elements (HFSE), and large ion lithophile elements (LILE) (Siegel et al, 2017)
Four hypotheses have been proposed to explain the genesis of microgranular mafic enclaves (MMEs): 1) Refractory residues of partial melting source (e.g., White and Chappell, 1977; Barbarin and Didier, 1992; White et al, 1999; Chappell et al, 2000); 2) Early cumulus from host magma (e.g., Dodge and Kistler, 1990; Chen et al, 2009; Shellnutt et al, 2010; Huang et al, 2014); 3) Xenolith of wall rocks captured by magma (e.g., Yang et al, 2004, 2006); 4) Magma mixing through mafic magma injected into chambers of less mafic magma (e.g., Wiebe et al, 1997; Kumar and Rino, 2006; Su et al, 2008; Guo et al, 2010; Yang et al, 2010, 2011)

Summary

Introduction

Magma mixing is a common magmatic process during formation of granitoids (Kumar et al, 2004; Liu et al, 2013; Xiong et al, 2020). If mafic magma injects into felsic magma during magma mixing, microgranular mafic enclaves (MMEs) would likely form. During this process, amphiboles are usually the major rock-forming minerals. The T site is commonly occupied by Si4+, Al3+, Ti4+, and the like and is tetrahedrally coordinated, forming a double chain of tetrahedra. The C site is octahedrally coordinated and occupied by cations including Mg2+, Fe2+, Mn2+, Al3+, Fe3+, Mn3+, Ti4+, etc. Anions including OH−, F−, Cl−, and O2− dominate the W-site. This complexity gives rise to a complicated classification for amphiboles (Hawthorne et al, 2012). Amphiboles are sensitive to chemical compositions of the crystallizing melts and may record some processes during magma evolution

Methods

Results

Discussion

Conclusion