In situ monazite (U-Th)-Pb ages from the Southern Brasilia Belt, Brazil: constraints on the high-temperature retrograde evolution of HP granulites

In the southern sector of the Southern Brasilia Belt, late Neoproterozoic arcpassive margin collision resulted in juxtaposition of an arc-derived nappe (the SocorroGuaxupe Nappe) over a stack of passive margin-derived nappes (the Andrelandia Nappe Complex) that lies on top of autochthonous basement of the Sao Francisco Craton. (UTh)Pb monazite ages are reported from the high-grade nappes of the Andrelandia Nappe Complex to better constrain the high-temperature retrograde evolution. For residual HP granulites from the uppermost Tres PontasVarginha Nappe, (UTh)Pb ages of c. 662 and 655 Ma from low yttrium monazite inclusions in the rims of, or associated with garnet are interpreted to date the late-stage close-to-peak prograde evolution, whereas an age of c. 648 Ma from a similar low yttrium monazite inclusion is interpreted to record post-peak recrystallization with melt via factures in garnet. For the same nappe, ages of 640631 Ma retrieved from higher yttrium areas or cores in monazite grains that occur both as inclusions in garnet and in the matrix are interpreted to record growth of monazite either by local breakdown of garnet (+/- older monazite) and mass exchange with a matrix melt reservoir along cracks or growth from residual melt in the matrix as it crystallized during high-pressure, close-to-isobaric cooling close to the solidus, the temperature of which, at a given pressure, varies with bulk composition of the residual granulites. (UTh)Pb ages in the range 620588 Ma from lower yttrium areas in these monazite grains and from matrix-hosted patchy monazite are interpreted to date exhumation, as recorded by close-to-isothermal decompression and subsequent close-to-isobaric cooling. Older monazite ages in this group are interpreted to record late-stage interaction with melt close to the solidus whereas younger monazite ages are interpreted to record recrystallization of monazite by dissolutionreprecipitation owing to ingress of alkali fluid from the Carmo da Cachoeira Nappe beneath as fluid was released by crystallization of in-source melt at the solidus. In the underlying Carmo da Cachoeira Nappe, higher yttrium areas in monazite and one single domain monazite yield chemical ages of 619616 Ma, which are interpreted to date growth as in-source melt crystallized close to the solidus along the high-pressure, close-to-isobaric segment of the retrograde PT evolution. Younger (UTh)Pb ages of 600595 Ma retrieved from lower yttrium areas and one single domain monazite are interpreted to record recrystallization of monazite by dissolutionreprecipitation owing to release of fluid at the solidus during exhumation of this nappe. Monazite from the Carvalhos Klippe, interpreted to be correlative with the uppermost nappe, yields a wide range of (UTh)Pb ages: for two zoned grains, c. 619 and c. 614 Ma from higher yttrium cores, and c. 583 and c. 595 Ma from lower yttrium rims; and, 592580 Ma from single domain grains in one sample, and ages of c. 593 and c. 563 Ma from monazite in a second sample. Ages younger than 605 Ma are interpreted to date a fluid-induced response to the early stages of orogenic loading associated with terrane accretion in the Ribeira Belt to the southeast. The results reported here demonstrate that ages retrieved from monazite that grew close to the solidus in residual granulites from a single tectonic unit will vary from sample to sample according to differences in the solidus temperatures. Further, we show that monazite inclusions may yield ages that are younger than the host mineral and confirm the propensity of mnazite to record evidence of tectonic events that are not always registered by other high-temperature mineral chronometers.