Autochthonous origin of the Encruzilhada Block, Dom Feliciano Belt, southern Brazil, based on aerogeophysics, image analysis and PT-paths

Abstract

The Encruzilhada Block lies between the eastern hinterland and western foreland of the Dom Feliciano Belt (southern Brazil) and its tectonic meaning in the orogenic evolution of the belt is still poorly understood. Its low magnetic signal is similar to that of the foreland supracrustal sequences, whereas the high gammaspectrometric signal appears due to the hinterland post-collisional granitoids. Satellite image lineament analysis points to tectonic similarities between the Encruzilhada Block and the foreland, as the structures in both domains show predominantly NNW and NNE trends, while the hinterland mostly presents NE-trending structures. The pre-orogenic connection between the Encruzilhada Block and the foreland is indicated by the correlation between the Várzea do Capivarita and Cerro da Árvore complexes. Both contain 800 – 780 Ma metaigneous rocks with arc-like geochemistry, and parametamorphic rocks with similar sedimentary provenance. Common syn-orogenic deformation history of both complexes is suggested by progressive to P-T o-th E-W transpressive deformation and similar peak metamorphic age (660 – 640 Ma). PT-paths suggest exhumation of the hinterland Várzea do Capivarita Complex from high-T/low-P conditions of 790–820 °C/4.4–4.8 kbar to 660–720 °C/2.5–3.4 kbar at ca. 630 Ma. On the other hand, the foreland Cerro da Árvore Complex was subject to progressive metamorphism from 555 to 565 °C/5.4–5.7 kbar to 560–580 °C/5.8–6.3 kbar, which is interpreted as a result of orogenic thickening. Evidence suggests that both complexes have originated in a similar, if not a single, basin on an attenuated lithosphere with high geothermal gradient, possibly a (back-arc?) rift. Oblique collision has caused basin inversion and thrusting of the lower crust over the rift margin, which may have been accompanied by lithosphere delamination. Progressive transpressional deformation and voluminous post-collisional magmatism (640 – 578 Ma) controlled by major lineaments have blurred the original tectonic contact and finally caused thermal metamorphism in both complexes at ca. 3 kbar, when the autochthonous Encruzilhada Block was stabilized in its present geological configuration. The intracontinental character of major shear zones limiting hinterland and foreland requires a review of the existing evolutionary model for the Dom Feliciano Belt in the context of Western Gondwana amalgamation.