Contrasting Sr and Nd isotopic behaviour during magma mingling; new insights from the Mannum A-type granite

Abstract

The Mannum granite in South Australia is a classic A-type granite which displays good evidence for magma mingling, such as numerous syn-plutonic mafic enclaves, and rapakivi feldspars in both the granite and enclave. Study of the Sr and Nd isotopes show that the systems are dominantly controlled by plagioclase and titanite, respectively. The contrasting behaviour of these minerals during the development of a bimodal magmatic system has led to a well-defined 482 Ma isochron across a granite–enclave boundary in the Sr isotope system but not in the Nd isotope system. In-situ techniques, involving the development and production of a new titanite glass standard, were developed to resolve this dichotomy. Inferred repeated interaction with mafic magmas has resulted in the destabilization and restabilization of plagioclase. This has produced rapakivi textures, and provides an effective mechanism for efficient Sr isotope equilibration across the range of bulk rock compositions. Despite this interaction, titanite in the host granite retain a range of initial Nd isotope ratios, indicating multiple parental magmas were assembled to produce the final pluton. Decoupling of the Sr and Nd isotope systems in other magmatic systems could be indicative of magma mixing, as opposed to mingling, where physical evidence (e.g. enclaves, xenocrysts etc.) is absent.