Cognate Mafic Enclaves Formed by Cumulated Mush Convection in a Granitic Magma Chamber: Evidence from Mineral Chemistry of the Triassic Zhashui Pluton, Qinling Orogen

Abstract

Mafic enclaves in granites are generally considered to represent coeval mafic melts that derived from metasomatized mantle, which can provide valuable information about crust–mantle interaction. Exploring the genetic links between the mafic enclaves and their host monzogranite from the Triassic Zhashui Pluton, Qinling orogenic belt. The enclaves (220 ± 4.6 Ma) and the monzogranite (220 ± 2.8 Ma) display identical zircon U-Pb ages, and they also share similar trace element and zircon Lu-Hf isotopes, indicating a cognate source. The monzogranite displays zircon ε_Hf(t) values of –0.99 to +1.98, while the mafic enclaves show similar values of –0.45 to +3.26; however, differences in mineral chemistry reveal different crystallization conditions. The amphibole from the mafic enclaves has higher temperature and pressure (757°C; 2.65 kbar) compared to those of the host monzogranite (733°C; 1.96 kbar), suggesting that mafic minerals in the enclaves crystallized at an early stage. Moreover, apatite in the mafic enclaves displays slightly higher volatile contents (0.72 wt%) than those of the monzogranite (0.66 wt%), indicating a volatile-rich condition. These results suggest that the mafic enclaves represent early hydrous mafic cumulates in the granitic chamber, and subsequent magma convection would have led to the formation of the mafic enclaves.

Keywords

mafic enclaves / granitic chamber / mafic cumulate / convection / Qinling orogen

RIGHTS & PERMISSIONS

2024 Geological Society of China