Mafic microgranular enclaves: early segregation from metaluminous magma (Sierra de Chepes), Pampean Ranges, NW Argentina

Mafic microgranular enclaves (MME) were formed in metaluminous granitoids from Sierra de Chepes, Pampean Ranges, NW Argentina. The granitoids were emplaced in low-pressure conditions during the Lower Ordovician. The MME seem to have played an important role in the evolution of metaluminous magmas, as has been emphasized in several petrological works. Fieldwork, combined with petrographical, geochemical, and microprobe data, leads to the conclusion that the MME result from early crystallization (mainly hydrous mafic minerals, plagioclase, and some accessory minerals). Early segregation of the MME led to differentiation of the parental granitic magma by fractional crystallization. The MME studied here have isotopic compositions remarkably similar to those of their host granitoids, which suggests that they represent cogenetic suites derived from the same lithospheric source, with crustal residence ages ∼1700 Ma. In contrast, the MME lodged in Andean granitoids during the Mesozoic–Tertiary (e.g. Papudo–Quintero complex, Chile) represent cogenetic suites of primitive mantle origin. Thus, the characteristics of the Ordovician granitoids could help define the geotectonic settings in which MME represent early products of crystallization without significant asthenospheric mantle participation.