Magmatic processes during the interaction between alkaline mafic-ultramafic rocks and intruding syenites near the Jabaquara Beach, north of the São Sebastião Island, São Paulo, Brazil

• Ano de defesa: 2022
• Autor(a) principal: Oliveira, Arthur Vicentini de
• Tipo de documento: Dissertação
• Tipo de acesso: Acesso aberto
• Idioma: eng
• Instituição de defesa: Biblioteca Digitais de Teses e Dissertações da USP
• Palavras-chave em Português: Assimilation reaction; Brecha magmática; Contaminação magmática; Magma contamination; Magmatic breccia; Reações de assimilação
• Link de acesso: https://www.teses.usp.br/teses/disponiveis/44/44143/tde-05122022-101321/

Magma mixing is a common process that ranges from purely physical mingling to complete hybridization of chemically distinct magmas. Some of the typical pieces of evidence for mixing are magmatic enclaves and crystal disequilibria, however, assimilation of wall-rock is also a candidate for contamination of a magma chamber. A coastal outcrop on the northside of the São Sebastião Island (SE Brazil) displays an array of blocks and enclave swarms of alkaline mafic-ultramafic rocks and melasyenite brecciated by syenitic rocks and veinlets. Melasyenite also hosts mafic enclaves that exhibit corroded contact and disintegration. Mafic enclaves are commonly associated with the injection of hot basic magma into silicic magma, but the described enclaves are the dispersed fragments of an older mafic cumulate rock. This feature is unusual in the alkaline rocks of the South American Platform and its effects on magmatic diversity are poorly understood. This dissertation shows that the fallen mafic wall-rock reacted with the water-saturated syenite magma and its reactions serve as proxy to comprehend the formation of the melasyenite. Two main reaction zones were identified with the aid of petrography, mineral chemistry, and X-ray compositional maps, a clinopyroxene-laden seam around the enclave, and its rinds rich with biotite. Mineral-only geothermometers suggest that the hot syenite magma partially melted the border of the enclave and assimilated its crystals. As the magma cooled and crystallized, the fluid fraction increased. This colder, K-rich fluid percolated the enclaves and replaced amphibole with biotite. The results suggest that these processes also occurred when an earlier pulse of syenitic magm intruded and fragmented the crystallizing mush of the mafic-ultramafic body, assimilating its crystals. The mafic-derived clinopyroxene reequilibrated with the syenitic melt, with later hydration crystallization forming amphibole at ~900° C and biotite at ~700 °C. Assimilation reactions commonly mask and eliminate the resulting processes and original composition of a material, but the studied melasyenite still preserve clues of its formation. This may represent a window into contamination by assimilation of country rock and the diversification of alkaline magmas.