Long-lived intracontinental deformation associated with high geothermal gradients in the Seridó Belt (Borborema Province, Brazil)

Abstract

The Borborema Province (NE-Brazil) experienced widespread intracontinental deformation associated with low-pressure metamorphism during the Neoproterozoic-Cambrian assembly of West Gondwana. The widespread intracontinental deformation was driven by tectonic stresses derived from two continental collisions reported in the literature at ca. 630–610 Ma and includes the development of the continental-scale Borborema Shear Zone System. In this study, we investigate metasedimentary rocks from the Seridó Belt and eastern Patos Shear Zone, which occur in an intracontinental deformation zone at several hundred kilometers from the recognized collision zones. Using field data, quantitative compositional mapping, thermodynamic modeling, and monazite petrochronology, we constrained the metamorphic conditions and duration of intracontinental deformation in the inner Borborema Province. Monazite from leucosome-rich migmatite portions in the eastern Patos Shear Zone display 206Pb/238U dates spreading from 580 ± 13 to 512 ± 11 Ma (n = 125) with a cluster around ca. 570–550 Ma (n = 70). Staurolite-bearing schists from the western Seridó Belt display a large spread of 206Pb/238U dates between 626 ± 13 and 519 ± 12 Ma (n = 50), with a well-defined cluster between ca. 570 and 550 Ma (n = 37). Garnet-andalusite-cordierite schists from the inner and eastern Seridó Belt display monazite 206Pb/238U dates ranging from ca. 550 to 500 Ma (n = 126). Schists from the NNE-SSW striking Picuí Shear Zone in the eastern Seridó Belt exhibit Crd + And-rich nodules wrapped by the foliation, attesting the persistence of deformation after their generation. Monazite inclusions in cordierite and andalusite have REE patterns similar to matrix monazite and together they yield weighted average ages between ca. 530–525 Ma, interpreted as the age of metamorphism within the Picuí Shear Zone. Iterative thermodynamic modeling using the local bulk composition of a Crd + And-rich nodule yielded optimal P-T conditions of ca. 590 °C and 3.8 kbar. The presented data indicate that intracontinental deformation in the inner Borborema Province was long-lived and lasted until at least 530–525 Ma. The deformation was associated with high geothermal gradient conditions that reduced lithospheric strength allowing the development and maintenance of the Borborema Shear Zone System.