Plagioclase-hornblende geothermobarometry of metamafites from the Arroio Grande Ophiolite, Dom Feliciano Belt, southernmost Brazil

Abstract

This paper reports the first geothermobarometry data for the Brazilian sector of the Punta del Este Terrane (southeasternmost Dom Feliciano Belt), based on petrographic features (optical and electron microscopy) and mineral chemistry (electron probe microanalysis) of metamafites from the Arroio Grande Ophiolite, located near the Brazil/Uruguay border. Microtextures found in the studied amphibolite and metagabbro samples suggest that the southern portion of the ophiolite underwent two main metamorphic events, named M1 and M2. Based on field relations and radiometric ages from the literature, the minimum age for the M1 is constrained at ca. 640 Ma (migmatization event in the northern portion of the ophiolite). The M2 is constrained at ca. 610-585 Ma (syn-kinematic granite intrusion and formation of blackwall reaction zones in its southern portion). The P-T conditions of both events are estimated from plagioclase-hornblende geothermobarometry, constraining the M1 between 706 and 750 ± 40 °C and 5.5–6.3 ± 1.5 kbar, and the M2 between 595 and 698 ± 40 °C and 3.4–5.3 ± 1.5 kbar. It suggests that, as early as 640 Ma, fragments of the ophiolite were buried at depths of ca. 20–23 ± 5 km and, at around 610-585 Ma, these rocks were affected by a shear zone system at depths of ca. 15–19 ± 5 km. The P-T estimates presented in this paper, together with the suggested ages for the M1 and M2 events, are in good agreement with data reported for the southern (Cerro Olivo Complex) and northern (Paso del Dragón Complex) sectors of the Punta del Este Terrane in Uruguay. Thus, the present contribution further demonstrates that high-T/low-P conditions, up to the lower-granulite facies, together with partial melting events, are ubiquitous in the southeasternmost sector of the Dom Feliciano Belt during the Late Cryogenian events related to the Brasiliano/Pan-African orogenic cycle and the amalgamation of the Southwest Gondwana paleocontinent.