Recovering P-T-t paths from ultra-high temperature (UHT) felsic orthogneiss: An example from the Southern Brasilia Orogen, Brazil

Autor(es)

Motta, Rafael G. ^{[1, 2, 3]} ; Fitzsimons, Ian C. W. ^[3] ; Moraes, Renato ^[1] ; Johnson, Tim E. ^[3] ; Schuindt, Sheila ^[4] ; Benetti, Beatriz Y. ^[5]

Afiliação do(s) autor(es)

^[1] Univ Sao Paulo, Inst Geociencias, BR-05508080 Sao Paulo, SP - Brazil

^[2] Univ Fed Rio Grande do Norte, Ctr Ciencias Exatas & Terra, Dept Geol, BR-59078970 Natal, RN - Brazil

^[3] Curtin Univ, Sch Earth & Planetary Sci, Inst Geosci Res TIGeR, GPO Box U1987, Perth, WA 6845 - Australia

^[4] Univ Portsmouth, Sch Environm, Portsmouth PO1 3QL, Hants - England

^[5] Univ Torino, Dipartimento Sci Terra, Via Valperga Caluso 35, I-10125 Turin - Italy

• Tipo de documento: Artigo Científico
• Fonte: Precambrian Research; v. 359, JUL 1 2021.
• Citações Web of Science: 2

Resumo

Mineral assemblages that are diagnostic of ultrahigh-temperature (>900 degrees C) metamorphism are generally restricted to Mg- and Al-rich rocks. However, in many metamorphic terrains such compositions do not occur, and the rocks are dominated by orthogneisses that typically contain high-variance mineral assemblages that are not diagnostic of particular pressure-temperature (P-T) conditions. Here we use whole-rock and mineral chemistry, thermodynamic phase equilibrium modelling, and zircon U-Pb-Hf isotopic data to constrain the P-T-time history of granulites from the Socorro-Guaxupe Nappe of the Southern Brasilia Orogen. These rocks occur as part of a stack of Neoproterozoic nappes that were assembled during the formation of West Gondwana. Based on phase equilibrium modelling, the peak assemblage in a garnet-bearing felsic granulite (ortho- and clinopyroxene, garnet, K-feldspar, plagioclase, quartz, ilmenite, and silicate melt) records pressures of around 11.5 kbar and temperatures of similar to 1000 degrees C, consistent with the high-Al content of orthopyroxene. The retrograde path records a small decrease in pressure to 9 kbar marked by orthopyroxene replacing garnet and the exsolution of magnetite from ilmenite followed by near-isobaric cooling that led to the development of coronas of hornblende around pyroxene due to crystallization of the last vestiges of melt. Zircon cores yielding U-Pb ages of ca. 625 Ma are interpreted as dating close-to-peak conditions during high-temperature metamorphism. In one sample, zircon rims and `soccer ball' zircon neoblasts both have high U contents and yield ages of ca. 615 Ma, interpreted to mark the onset of melt crystallization. In two other samples, low-U rims and `soccer ball' zircons grains yield ages of ca. 600-595 Ma, interpreted to date the end of melt crystallization. These data reveal a protracted regional metamorphic event in which the lower crust remained at granulite-facies conditions for at least 30 Myr during collision and assembly of West Gondwana. (AU)

• Processo FAPESP: 16/22627-3 - Metamorfismo e fusão parcial em Orógenos antigos II
• Beneficiário: Renato de Moraes
• Modalidade de apoio: Auxílio à Pesquisa - Regular
• Processo FAPESP: 13/04007-0 - Metamorfismo e fusão parcial em orógenos antigos
• Beneficiário: Renato de Moraes
• Modalidade de apoio: Auxílio à Pesquisa - Regular
• Processo FAPESP: 15/22355-0 - Formação, transformação e evolução da crosta continental inferior: investigando o orógeno brasília meridional
• Beneficiário: Rafael Gonçalves da Motta
• Modalidade de apoio: Bolsas no Brasil - Doutorado
• Processo FAPESP: 16/10276-1 - Formação, Transformação e Evolução da Crosta Continental Inferior: Investigando o Orógeno Brasília Meridional
• Beneficiário: Rafael Gonçalves da Motta
• Modalidade de apoio: Bolsas no Exterior - Estágio de Pesquisa - Doutorado