Investigation of juxtaposed high- and low-pressure metamorphic field gradients rocks and its tectonic implications, a case study of Turvo-Cajati Formation, Ribeira Belt, Brazil

Abstract

<p>The Turvo-Cajati Formation (TCF) is a metasedimentary unit composing the Curitiba Terrane, a major segment of the southern Ribeira Belt, SE Brazil. It is composed of rocks of greenschist (Low-TCF), amphibolite (Medium-TCF) and granulite (High-TCF) facies conditions. Previous studies in High-TCF indicates that the unit underwent extensive partial melting under high-pressure conditions (670-810 °C and 9.5-12 kbar), within the kyanite stability field. New data on the metamorphic zoning within Low and Medium-TCF were collected using petrography and thermodynamic modeling in the MnNCKFMASHTO system. Four metamorphic zones were recognized for Low-TCF and Medium-TCF: biotite, garnet, staurolite and sillimanite zones where sillimanite zone prevails. The pressure regime is estimated to be below 8 kbar, as staurolite breaks down straight to sillimanite. Thermodynamic modeling yielded metamorphic peak conditions of ~530-560 °C ,~6-7 kbar (garnet zone) and ~660-690 °C ,~6-7 kbar (sillimanite zone). The metamorphic field gradient is flat and of low to medium pressure, below the typical barrovian-type baric regime. It is inferred that Low and Medium-TCF were metamorphosed in a tectonic setting different from the High-TCF. Probability density plots(pdp) from detrital zircon indicate late-Cryogenian-Ediacaran arc-related and Rhyacian sources for all TCF sub-units, where High-TCF presents forearc depositional setting and Low-Medium-TCF back arc depositional setting. This scenario suggests that the TCF is made up of a collisional juxtaposition of an accretionary wedge (High-TCF) and a back-arc basin (Medium-TCF and Low-TCF) on the border of a microplate that includes a Rhyacian basement microcontinent (Atuba Complex). Available petrological and geochronological data suggest that the TCF comprises a paired low-P and high-P belt, associated with a major Ediacaran suture zone in the southern Ribeira Belt. The high metamorphic gradient recorded in the Medium-TCF and Low-TCF was related to asthenospheric upwelling in the back-arc region, which also produced extensive partial melting in the Atuba Complex basement. Metamorphic ages where previously obtained in High-TCF with ages around 589 ± 12 Ma and 584 ± 4 Ma. Petrochronology will be used to obtain the age of metamorphic events, using monazite and apatite grains from Low and Medium-TCF and compare them to available High-TCF data to understand and adjust the proposed model.</p>