Extension-induced edge driven convection and the origin of granitoid batholiths in southern Borborema Province, NE Brazil

A voluminous plutonism characterizes the Brasiliano Orogeny in the Pernambuco-Alagoas Domain and, to a lesser extent, the Sergipano Belt (southern Borborema Province, northeastern Brazil). Most plutons have ages ranging from 610 Ma to 640 Ma and are commonly interpreted as formed in a continental arc setting based on geochemical and isotopic grounds. The main arguments are the presence of Nb and Ta negative anomalies in primitive-mantle normalized trace element diagrams and the dominantly Mesoproterozoic whole-rock Sm-Nd and zircon Hf model ages of the rocks. These characteristics would suggest that the plutons were formed by mixing of a Paleoproterozoic crustal source with a Neoproterozoic juvenile component generated by partial melting of the mantle wedge above a subduction zone. However, the plutons have several features that are unlike those of the typical calc-alkaline plutons in arc settings. They are dominantly composed of monzogranites, syenogranites, quartz syenites, or syenites; have a high-K calc-alkaline to shoshonitic and even ultrapotassic geochemical signature; and are distributed over a wide area. These characteristics are in stark contrast with the linear distribution of dominantly sodic (Na₂O/K₂O ≥ 1), juvenile tonalitic/granodioritic plutons in continental magmatic arc settings. In addition, there is a notable absence of any indication of oceanic crust remnants, high-pressure rocks and calc-alkaline metavolcanic rocks that could support a subduction/collision model.

Gabbros, granitic orthogneisses, and metavolcanic rocks emplaced at 720-640 Ma have within-plate geochemical signatures, indicating extensional conditions prevailed just before the Brasiliano orogeny. A combination of extension and edge-driven convection on the cratonic margin of the São Francisco/Congo Craton is suggested to be the main source of heat that triggered the magmatic activity. The dominantly c. 2.1 Ga crust of southern Borborema Province contrasts with the Paleo- to Mesoarchean age of the northern São Francisco Craton. It is thus expected that these crustal segments had significantly different lithospheric thicknesses in the Neoproterozoic. Far-field -related extension may have induced small-scale convection along the Mesoarchean cratonic boundary and enhanced the upwelling component of the edge-driven cell.  Decompression then led to asthenospheric partial melting concomitant with warming and partial melting of the lithospheric mantle and continental crust that persisted during the contractional phase. The influence of the juvenile mantle component decreased with time due to thickening-related cooling, explaining the decreasing eNd values of the plutons with time.