班公湖-怒江成矿带西段阿翁错地区早白垩世花岗闪长岩的成因及构造背景——来自斜长石和黑云母化学成分的约束

Located at the western part of the Bangong Co-Nujiang metallogenic belt, the Early Cretaceous granodiorites from the Awengcuo area are characterized by the widely developed mafic microgranular enclaves (MMEs) and obvious magma mixing. Plagioclase and biotite, as the main rock-forming minerals, record the material composition and physicochemical conditions of the magmatic system, which are ideal objects for studying the petrogenesis, magmatic evolution and tectonic setting. This paper focuses on the granodiorites (host rock) and MMEs from the aspects of geology, petrography and mineral chemistry. The MMEs are in various shapes and have sharp or transitional contact boundaries with the host rocks. The enclaves often contain back vein. The MMEs and host rocks are commonly interpenetrated with each other, and chilled borders with width of 1–2 mm on the enclaves’ side can be observed occasionally. Many phenocrysts in the enclaves are captured from the host rocks, and acicular apatite formed by quenching is found in both the MMEs and host rocks. The above characteristics suggest that the MMEs are the products of magma mixing and mingling, and material interchanges are common. The results show that the plagioclases in the host rocks have intensive oscillatory normal zoning features with narrow An value range of 48.16 to 59.05, which may represent the plagioclase crystallized before the injection of mafic magma. The An values of the plagioclase with normal zoning features vary widely, with a core An value of 29.86 and a sudden increase to 51.09 outward, showing an overall gradual increase, indicating the addition of calcium-rich magma. Plagioclases of the host rocks show normal and reverse zoning features, with relatively complex core to mantle variations of An values and wide oscillatory zonation. Formation of the wide core to mantle oscillatory zonation of the plagioclase may due to the abrupt changes triggered by magma mixing. With the continuous mixing and gradual cooling of the magma, the plagioclase continued to crystallize in the static environment, and formed a clean edge of normal zonation. Although there is a normal zoning in the inclusion matrix plagioclase, the An values vary significantly from 62.40 in the core to 28.63 in the edge. It is obviously different from the normal zoning characteristics of the host rock plagioclase, which may be the result of the mixing of mafic magma and felsic magma. The core of plagioclases in the matrix of the MMEs with the characteristics of reverse zoning and normal-reverse zoning suffered weak melting erosion, but the range of the An values is relatively small, with the characteristics of intensive oscillatory zoning. It is suggested that the core melting erosion may be caused by the rapid decrease of pressure during the ascent of the mafic magma and formed in a static magma chamber. The host rock biotite and enclave matrix biotite are primary magmatic biotite, with AlVI values of 0–0.16 apfu and 0–0.04 apfu, oxidation coefficients of 0.11–0.63 and 0.16–1.66, and magnesia contents of 0.40–0.47 and 0.42–0.46, respectively, indicating that both of the host rocks and MMEs are calc alkaline I-type granites, whose formation is related to the subduction of island arc. Combined with the regional tectonic setting, it is considered that the early Cretaceous granodiorites in the Awngcuo area were formed under the tectonic setting of the transition from northward subduction to collision of the Shiquanhe-Namucuo Tethys ocean basin. © 2023 Science Press. All rights reserved.

Origin and Tectonic Setting of the Early Cretaceous Granodiorites in Awengcuo Area,Western Bangong Co-Nujiang Metallogenic Belt: Constraints from Mineral Chemistry of Plagioclase and Biotite