Petrogenesis of the Córrego das Campinas Gabbro-Anorthosite Suite: Characterization of a Neoproterozoic massif-type anorthosite in the Goiás Magmatic Arc and its significance in the evolution of the Brasília belt, Brazil

Córrego das Campinas Anorthosite is a rare example of a massif-type anorthosite developed in the late Proterozoic (∼650 Ma) during the amalgamation of West Gondwana. This undisturbed, oval, and homogeneous massif body primarily consists of labradorite (An49-63) with a porphyritic and cumulatic inequigranular texture. Other rocks are associated with anorthosite, such as a gabbroic body, consisting of olivine gabbro, leucotroctolite and gabbro with granular, ophitic and cumulatic textures. The olivine gabbro presents a more primitive character, showing minerals enriched in Mg and Ca, such as olivine Fo76-78 intercumulates and anorthitic plagioclase (An90-97), albeit with an Al-rich diopside as well. The anorthosite body is also bordered by tonalite and quartz monzodiorite. These leucocratic and granular rocks contain An26-46 plagioclase in tonalite and An12-18 plagioclase in quartz monzodiorite. Given the signature of these rocks, they can be considered residual liquids from anorthosite crystallization, more enriched in Na and Fe. Metamorphosed ferrodiorites, amphibolites and ilmenite-rich levels occur in the form of dikes and veinlets associated with anorthosite and also seem to represent differentiated liquids from anorthosite crystallization. A quartz syenite body also occurs, consisting of perthitic orthoclase cumulates with an albite mantle, whose genesis is also associated with a process of accumulation in a magma chamber. A albite granite plug occurs intrusively. This albite granite presents enriched in enriched in Na, Si, Rb, Th, Ta, Nb, Ce, Zr, Hf, Y and Yb showing typical signature of an anorogenic granite. Zircon U–Pb dating of olivine gabbro, tonalite, quartz monzodiorite, ferrodiorite and albite granite indicate that these rocks are contemporaneous, with crystallization ages ranging from 648 to 661 Ma, showing older zircon grains indicating Neoproterozoic crustal material contamination (∼870–900 Ma) and even older in the genesis of these rocks. Sm–Nd model ages around 0.75 Ga and positive ϵNd(t) suggest that these rocks are juvenile and derive from depleted mantle, whereas another set of results around 1.0 Ga and less positive ϵNd(t) values indicate that these rocks are juvenile but show crustal contamination. The high percentages of plagioclase found in the rocks, combined with high Al2O3, CaO and Na2O concentrations in total rock, indicate that the rocks of this suite derive from calc-alkaline or Al-rich basaltic magmas. The formation of the suite is in accordance with the classical petrological model of plagioclase accumulation in magma chambers and its subsequent diapiric rise. The age of the suite and its positioning indicate a correlation with an important mafic magmatic event that occurred in the Brasília Belt between 670 and 600 Ma. The interpretation of geological data on the Brasília Belt, associated with the present study, indicates that the rocks of this suite initially developed in a back-arc setting during the pre-tectonic stage of an active continental margin and that the bodies ascended to shallower regions of the crust during the post-collisional stage, through pressure relief, at approximately 650 Ma. The evolution of the Brasília Belt was marked by a constantly elevated geothermal gradient and the Córrego das Campinas massif-type anorthosite supports this inference. Therefore, we suggest that the constantly elevated geothermal gradients in the Brasília Belt were caused by multiple, concurrent geodynamic processes during its evolution, such as i) blocked mantle convection currents triggered by oceanic crust subduction, which formed an intraoceanic island arc (Goiás Magmatic Arc) between 900 and 800 Ma; this physical barrier limited the heat release from these currents, generating a “heat trap” and thus accounting for the elevated geothermal gradients; ii) thickened crust caused by collisional and accretionary events; iii) increased heat of Rodinia break-up plume; and iv) elongated microplate of the Goiás Massif.