Tracking the oxygen fugacity of enclave-forming granitic melts through plagioclase trace element signatures

Autor(es)

Alves, Adriana ^[1] ; Janasi, Valdecir de Assis ^[1]

Afiliação do(s) autor(es)

^[1] Univ Sao Paulo, Geosci Inst, Rua Lago 562, BR-05508080 Sao Paulo - Brazil

• Tipo de documento: Artigo Científico
• Fonte: CONTRIBUTIONS TO MINERALOGY AND PETROLOGY; v. 175, n. 7 JUN 29 2020.
• Citações Web of Science: 0

Resumo

Magma mixing and contamination are among the dominant processes in the building of the isotopic diversity of granite rocks. Felsic microgranular enclaves (FMEs) are remnants of magma mixing and they often carry xenocrysts that also record contamination and assimilation in crystallization fronts disrupted during replenishment events. In addition to isotopic changes, contamination may alter the redox state of the intruding magmas depending on the nature of the country rocks. We investigate the role played by different country-rocks on the redox trajectory of the precursor magmas of two Brazilian occurrences: the Maua pluton, which intruded sulfide- and graphite-bearing metasediments, and the Salto rapakivi pluton, which intruded orthogneisses. Oxygen fugacity (fO(2)) was estimated using Eu2+/Eu(3+)ratios retrieved from plagioclase chemistry and literature equation that considers melts chemistry and temperature. The model is demonstrated to replicate thefO(2)from plagioclase-bearing experimental data within a precision usually better than 1 log unit. Results obtained for the Maua pluton indicate that contaminated plagioclase cores are significantly more reduced (Delta QFM - 1.5; Sr-87/Sr-86 similar to 0.713) than the xenocryst rims (Delta QFM + 0.5; Sr-87/Sr-86 similar to 0.710). In contrast, results obtained for the Salto rapakivi pluton vary from Delta QFM + 1.0 to Delta QFM + 2.7 and do not coincide with core to rim variations in Sr-87/Sr-86. MMEs are comparatively more reduced (- 0.1 <= Delta QFM <= + 0.7). Our results imply that the redox path registered by plagioclase crystals from different geological backgrounds reflects the nature of their country rocks and the processes that affected their precursor magmas The main advantage of using plagioclase trace element data to model redox conditions of equilibrium magmas is the possibility of determining fO(2) paths via spatially controlled trace element analyses of crystals rims and cores. Coupled to the possibility of obtainment of Sr isotope data, the model represents a powerful way to unravel the processes responsible for redox paths of magmatic rocks of varied nature. (AU)

• Processo FAPESP: 11/07074-4 - Uso combinado de técnicas de análise textural quantitativa e análises químico-isotópicas in situ no estudo de enclaves microgranulares félsicos: implicações para evolução de câmaras magmáticas
• Beneficiário: Adriana Alves
• Modalidade de apoio: Auxílio à Pesquisa - Regular