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Barry L. Reno, The University of Maryland (United States)
Michael Brown, The University of Maryland (United States)
Philip Piccoli, The University of Maryland (United States)
Rudolph A.J. Trouw, Federal University of Rio de Janeiro (Brazil)
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In southeastern Brazil, the first stage of the assembly of West Gondwana is recorded by events in the Southern Brasília Belt, with subsequent stages recorded by events in the outboard Ribeira and Buzios Belts. The Southern Brasília Belt formed by closure of the Brazilide Ocean and suturing of the Arenópolis Arc?Rio de la Plata Craton with the São Francisco Craton. In the southernmost Brasília Belt, the Andrelndia Nappe Complex comprises a stack of passive-margin-derived nappes that record conditions from greenschist facies at the base to high-pressure granulite facies at the top. At the top, the Socorro?Guaxupé Nappe (SGN) represents part of the magmatic arc from the hanging-wall. Immediately below, the Trs Pontas?Varginha (TPV) and Carmo da Cachoeira (CdC) Nappes, which are composed of metapelites, preserve lenses of retrograded eclogite between nappes, which represent slivers of ocean lithosphere transported along nappe contacts during juxtaposition in the subduction system during terminal collisional orogenesis. To constrain peak P-T conditions for the TPV and the CdC Nappes, we constructed pseudosections for multiple samples with different bulk compositions from individual quarry outcrops and overlaid the phase stability fields for the observed peak assemblages for each bulk composition at each quarry. For the TPV Nappe, we retrieved P of 1.2-1.4 GPa at T of 830-900°C, and for the CdC Nappe we retrieved P of 1.1-1.4 GPa at T of 820-870°C. For the SGN, using average thermobarometry we retrieved P up to 1.1 GPa at T ∼780°C from samples close to the base.
A U-Pb zircon concordia age of 669±15 Ma for retrograded eclogite between the TPV and CdC nappes is interpreted to date detachment of the TPV nappe from the subducting plate. Samples throughout the TPV nappe yield 207Pb/206Pb zircon ages of 660-650 Ma, interpreted to record close-to-peak-T early retrograde zircon crystallization, and (U-Th)?Pb monazite ages of 640-630 Ma, interpreted to record retrograde monazite growth. A younger age of ca. 605 Ma from zircon and monazite throughout the nappe stack, and a Rb-Sr Grt-Bt-Pl+Ksp-WR age of ca. 590 Ma from the TPV nappe, corresponds to emplacement of the SGN. Monazite ages of ca. 575-535 Ma likely record overprinting during the Ribera orogeny. The spread of ages retrieved from eclogite and the high-pressure nappes suggest that nappe formation and juxtaposition during subduction and the early stage of exhumation occurred over several tens of millions of years. These data indicate that high-pressure granulite facies metamorphism in the Southern Brasília Belt occurred 30-40 Ma before the widely quoted 630 Ma age previously inferred from limited data for peak metamorphism within the belt (Reno et al., Geology, in review). Thus, amalgamation of West Gondwana began earlier than previously interpreted and was not completed until the Cambrian.
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