Timing of Orogenesis in the Southern Brasília Belt

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Reno, Barry L
Brown, Michael
Piccoli, Philip M
The Neoproterozoic-Cambrian Brasília Orogeny records suturing of the passive margin on the western side of the São Francisco Craton with magmatic arcs and the Paranapanema Block in the south or the Amazon Craton in the north during the assembly of West Gondwana. In the southern sector of the Southern Brasília Belt, the Andrelândia Nappe Complex comprises a stack of passive margin-derived nappes metamorphosed to high-pressure granulite facies during subduction-to-collision orogenesis and overlain by an arc-derived high-pressure granulite facies nappe. I present the results of an integrated petrologic and chronologic study on the metamorphic conditions and the timing and duration of orogenesis in the Andrelândia Nappe Complex. I also report on my implementation of the (U-Th)-Pb monazite EPMA dating technique at the University of Maryland, and propose a protocol for statistical analysis and interpretation of monazite ages. Using the U-Pb zircon chronometer in conjunction with Ti-in-zircon thermometry, REE distribution data and petrologic observations, a minimum age for timing of initial nappe detachment is constrained to 678±29 Ma, whereas just-post-peak-T metamorphism of the uppermost passive margin-derived Três Pontas-Varginha Nappe is interpreted to have occurred at ca. 648 Ma. (U-Th)-Pb monazite ages in the Três Pontas-Varginha and underlying Carmo da Cachoeira Nappes indicate that they were accreted to the hanging wall of the subduction zone at depths corresponding to near-peak-P by 635-624 Ma. High-P granulite facies metamorphism of the arc-derived nappe is dated by a U-Pb zircon age of 622±28 Ma. Two Rb-Sr multimineral-whole rock isochrons record cooling of the Três Pontas-Varginha Nappe below ~700˚C at ca. 590 Ma, suggesting slow cooling from peak-T. 40Ar/39Ar biotite ages between 591.4±6.6 and 568.9±6.1 for the Três Pontas-Varginha Nappe indicate an increased cooling rate interpreted to be due to tectonically-driven exhumation. 40Ar/39Ar biotite ages of ca. 540 Ma in the Carmo da Cachoeira Nappe indicate that this nappe was exhumed after the Três Pontas-Varginha Nappe had cooled below ~300˚C. In the southeast part of the Nappe Complex, the Andrelândia Nappe yields 40Ar/39Ar biotite ages consistent with emplacement directly on the São Francisco Craton and cooling from the bottom up.