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Título

Evidences for high-pressure granulites in the Brazilian southeastern coast (Armação dos Búzios - RJ) - thermodynamic modeling

Texto do resumo

The Búzios Succession is characterized as a meta-volcano-sedimentary sequence outcropping at the southeasternmost part of the Araçuaí-Ribeira Orogenic System, associated with the last collisional episode on West Gondwana collage, the Ediacaran-Cambrian Búzios Orogeny. This sequence was deposited in a marine realm interleaved with volcanic layers and underwent high-grade metamorphism and high strain during the Ediacaran to Cambrian, with late- to post-tectonic events in the early Ordovician (Schmitt et al. 2016). These rocks can be divided into four different lithotypes: metapelite, predominant, composed of kyanite (and/or sillimanite)-garnet-orthoclase-plagioclase-biotite-quartz gneiss and accessory rutile; metapsammite, characterized by biotite-plagioclase-orthoclase-quartz gneiss with rare garnet; metabasites represented by garnet-diopside-hornblende gneiss and granofels with minor orthopyroxene and titanite; and calc-silicate rocks such as scapolite-wollastonite-garnet-plagioclase gneiss and diopside-garnet-hornblende-plagioclase granofels. The metapelite was chosen for thermodynamic modeling using TheriakDomino software (de Capitani & Petratakis, 2010), considering its suitable mineral assemblage characteristic for high-pressure granulites given by garnet + orthoclase + kyanite with stable rutile. The preferred chemical system for the modeling was Mn-CNKFMASHTO, and the chemistry was extracted using the Local Bulk tool by X-MapTools from a representative compositional map of a thin section. The results show a stable assemblage composed of garnet + orthoclase + biotite + quartz + kyanite + rutile + melt, lying in a large P-T window. Garnet isopleths technique was applied to refine the results, with almandine-grossular and pyrope-rich cores delivering very precise temperature results (around 850 ºC) and pressure variation between 13 and 17 kbars. Peak conditions can be placed in high-pressure granulite facies, nearing eclogitic conditions. Garnet rim isopleths with the addition of spessartine end-member were also applied, trying it as an effort to characterize the retrograde assemblage conditions, characterized by garnet + orthoclase + plagioclase + biotite + quartz + sillimanite + rutile + melt. P-T retrograde conditions were placed around 7.5 kbar and 810 ºC. Similar data using THERMOCALC software was obtained by Vieira et al. 2022 (15 ± 0.8 kbar, 818-785 ºC), and for backing our data, the same methodology was applied to our sample, resulting in conditions of 15.2 ± 0.6 kbar, confirming the high-pressure conditions. To put in evidence the data obtained for Búzios Succession, a metapsammite from Palmital Succession, an associated meta-volcano-sedimentary sequence outcropping at west, was also analyzed, presenting granulite conditions, reaching 10 kbar and near 850 ºC. The pressure difference grading from west to east at the same elevation can be an indication that presence of tectonic thrusts are stacked on the Cabo Frio Tectonic Domain. The authors acknowledge the support of the following institutions and associations: Fundação Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), the Gondwana Project (CENPES-UFRJ), the Petrochronics and the Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ).

Palavras Chave

Búzios Orogeny; Thermobarometry; TheriakDomino; THERMOCALC; X-MapTools