Late amalgamation in the central part of West Gondwana: new geochronological data and the characterization of a Cambrian collisional orogeny in the Ribeira Belt (SE Brazil)

Abstract

New U–Pb data reveal that during the mid-Cambrian the central part of West Gondwana was still undergoing a high-grade tectonometamorphic event corresponding to collision. The studied area is located in the southeastern part of the Pan-African–Brasiliano Ribeira Belt, in SE Brazil. The area is part of the Cabo Frio Tectonic Domain (CFTD) which is limited to the NW by a major NE–SW striking thrust zone which separates it from the Neoproterozoic “Oriental terrane”, whereas to the SE it is covered by the Atlantic Ocean. The domain comprises a Paleoproterozoic orthogneissic basement tectonically interleaved with younger supracrustal rocks, folded and metamorphosed at upper amphibolite to granulite facies during the mid-Cambrian. The supracrustal rocks are subdivided in two successions: Búzios (Al-metapelites, calcsilicates and amphibolites) and Palmital (quartz–feldspathic metasediments with minor metapelites). These successions were deposited in a deep oceanic environment between ca. 620 and 525 Ma as indicated by SHRIMP U–Pb data for detrital zircons and by T DM model ages. The metamorphic peak, defined by the mineral associations Ky + Kfs in metapelites and Cpx + Grt + Qz in amphibolites, occurred at minimum pressure of 9 kbar and temperature in excess of 780 °C. At this stage migmatites were generated by partial melting in all lithostratigraphic units, including the amphibolites. The metamorphic peak was also contemporaneous with top to the NW thrusting, testified by mineral and stretching lineations related to progressive deformation phases D1 and D2. The metamorphic peak was dated between 525 and 520 Ma, as determined by U–Pb analyses of zircons of leucosomes. During deformation phase D3, large recumbent folds developed with NW–SE axes, parallel to the main direction of movement. The CFTD was juxtaposed at this stage to the “Oriental terrane” by a major NE–SW striking thrust fault. U–Pb dating of monazites from metapelites and of sphenes from amphibolites revealed ages of about 510 Ma for the mineral growth. The sillimanite, aligned as L 3, partially replaced kyanite, indicating a clockwise P– T– t path for the central and eastern areas of the CFTD. After docking into the Ribeira belt, during the late Cambrian, the western limit of CFTD was affected by a transcurrent dextral shear zone that developed a NE–SW stretching lineation related to D4, under amphibolite facies conditions. This is recorded in monazites and zircons within this shear zone with U–Pb ages ranging from 505 to 490 Ma. At this stage, the central and eastern parts of CFTD were already cooling at a rate of 10 °C/Ma. After 480 Ma, the cooling rate diminished to 5 °C/Ma. A 207 Pb/ 206 Pb age in rutile (480±5 Ma) and a U–Pb zircon age in a post-tectonic pegmatite (440±11 Ma) mark the stabilization of the area during the Ordovician–Silurian boundary. This well-constrained Cambrian collisional orogeny is here denominated the Búzios Orogeny and is the youngest tectonometamorphic event recorded in the Brasiliano belts of Brazil. This orogeny is contemporaneous with some other marginal orogenies (e.g. Pampeana; Ross) that probably accompanied the final adjustments of the Precambrian continents to complete the formation of Gondwana in the Ordovician.