Multi-isotope and geochemical approach to the magma source and tectonic setting of Proterozoic anorthosite massifs and Anorthosite-Mangerite-Charnockite-Granite (AMCG) suites

Abstract

The occurrence of massif-type anorthosite intrusions is a widespread Proterozoic phenomenon. They are usually associated with gabbroic, charnockitic, and granitic rocks, comprising the so-called anorthosite-mangerite-charnockite-granite (AMCG) suite. Although these rocks have been extensively studied worldwide, several aspects concerning their formation remain unsettled. Among them, the magma source and the tectonic setting are the most important. To evaluate these issues, we first compiled geochemical and isotopic data of Proterozoic anorthosite massifs and AMCG suites worldwide and stored it in a database named datAMCG. This plethora of data allows us to make some important interpretations. We argue that the wide-ranging multi-isotopic composition of this group of rocks reflects varying proportions of juvenile mantle-derived melts and crustal components. We interpret that the precursor magmas of most massive anorthosite bodies and associated mafic rocks have a mantle-dominated origin. However, we highlight that a crustal component is indispensable to generate these lithologies. Adding variable amounts of this material during succeeding multi-stage assimilation-fractional crystallization (AFC) processes gives these intrusions their typical mantle-crustal hybrid isotopic traits. In contrast, a crustal-dominant origin with a complementary mantle component is interpreted for most MCG rocks. In summary, the isotopic information in datAMCG indicates that both sources are necessary to generate AMCG rocks. Therefore, we suggest that hybridized magmas with different mantle-crust proportions originate these rocks. This interpretation might offer a more nuanced and accurate depiction of this phenomenon in future work instead of choosing a single-sourced model as in the past decades. Finally, tectonomagmatic diagrams suggest that the rocks under study were likely generated in a tectonic environment that transitioned between collision and post-collisional extension, sometimes involving subduction-modified mantle sources. This interpretation is supported by geological and geochronological information from most complexes, thus challenging the Andean-type margins as an ideal tectonic setting.