High-K Calc-Alkalic Plutons in Northeast Brazil: Origin of the Biotite Diorite/Quartz Monzonite to Granite Association and Implications for the Evolution of the Borborema Province

Abstract

High-K calc-alkalic plutons represent a significant proportion of the abundant magmatic bodies that intruded Borborema province (BP) of northeastern Brazil during the Neoproterozoic Brasiliano (Pan-African) orogeny. They consist of an association of mafic to intermediate (diorites to granodiorites) and felsic rocks (coarse-grained to porphyritic quartz monzonites to granites). Field and petrographic evidence indicates that the felsic and mafic rocks coexisted as contemporaneous melts, and major- and trace-element data favor magma mixing over fractional crystallization as the main petrogenetic process responsible for the petrographic and geochemical variability of these rocks. Major- and trace-element, oxygen-isotope, and radiogenic-isotope (Sr and Nd) data suggest that (1) the main source rocks of the granitoids are lower-crustal amphibolites having rare-earth-element (REE) and isotopie characteristics similar to the associated mafic rocks and (2) the source region of the diorites is the metasomatized subcontinental lithospheric mantle. These inferences imply that crustal growth occurred during the Brasiliano orogeny. Dewatering of the mantle and lower crust and addition of consolidated mafic rocks and I-type granitoids to the middle crust certainly strengthened the entire lithosphere, thus contributing to the final cratonization of the BP. Field evidence indicates that the BP high-K calc-alkalic plutons were emplaced in an intracontinental setting, implying that this magmatism was not subduction-zone related. Although the plutons are spatially associated with transcurrent shear zones, the scale of magmatism is too broad to be assigned to shear heating. 40Ar/39Ar data indicate that large areas of the BP underwent slow cooling, unlike orogenic belts where delamination or convective removal of the lithosphere occurred. Therefore, only large convective instabilities in the sublithospheric mantle may explain the thermal anomaly responsible for melting in the BP. It is proposed that a mantle plume impinging the base of the continental lithosphere under the BP may represent such a laterally extensive and long-lived heat source.