Two‐Stage Mafic‐Felsic Magma Interactions and Related Magma Chamber Processes in the Arc Setting: An Example From the Enclave‐Bearing Calc‐Alkaline Plutons, Chinese Altai

Abstract

AbstractA systematic data set of petrography, mineralogy, geochronology, and geochemistry is reported for typical arc granitoids and associated mafic microgranular enclaves (MME) from the Chinese Altai, Central Asian Orogenic Belt. Zircon U‐Pb dating indicates that they crystallized coevally at ∼395 Ma. Magma hybridization modeling suggests that the granitoid parental magma formed by mixing between a mafic and a felsic endmember. The sharp decrease of An values from cores to rims (e.g., from ca. 80 to 40) of plagioclase crystals points to polybaric crystallization accompanied by degassing. Petrographic evidence and plagioclase in situ Sr isotopic compositions ((87Sr/86Sr)i = 0.7053–0.7071) confirm the interaction of at least two isotopically different magmas during the mineral crystallization. A model for the formation of the enclave‐bearing calc‐alkaline plutons in arc settings is presented: At depth, mantle and crustal melting and efficient magma mixing controlled the principal chemical compositions of the granitoid intrusions. Then in the higher level chamber, decompression‐dominated crystallization and the mingling and limited mixing between blobs of mafic magma and the enclosing granitoid magma ultimately determined the texture, mineral composition, and enclave morphology. The MME have (87Sr/86Sr)i of 0.7048–0.7062, εNd(t) of −0.5 to +2.6, and zircon εHf(t) of +2.3 to +5.4, while the host granitoids have similar Sr isotopic compositions ((87Sr/86Sr)i = 0.7054–0.7064), but generally lower whole‐rock εNd(t) and zircon εHf(t) values (−2.2 to +0.4 and +0.6 to +4.6, respectively). The petrogenetic analysis and isotopic characters suggest that granitoid emplacement was a means of crustal growth in the Chinese Altai orogen.