Banca de QUALIFICAÇÃO: LARISSA PRAXEDES GRANGEIRO

Uma banca de QUALIFICAÇÃO de MESTRADO foi cadastrada pelo programa.
STUDENT : LARISSA PRAXEDES GRANGEIRO
DATE: 13/12/2023
TIME: 14:00
LOCAL: videoconferência https://www.youtube.com/@ppggufrn
TITLE:

Textures and Mineral Chemistry of Zirconosilicates in Peralkaline Granites of the Papanduva Pluton (PR-SC)

KEY WORDS:

agpaitic minerals; autometassomatism; late- to post-magmatic

PAGES: 42
BIG AREA: Ciências Exatas e da Terra
AREA: Geociências
SUMMARY:

The mineral elpidite is commonly found in peralkaline alkali-feldspar granites of the Papanduva Pluton, located in the Graciosa Province (South-Southeast Brazil). Its ideal chemical formula is Na2ZrSi6O15.3H2O, and its crystallization occurs during the late to post-magmatic stages. In addition to elpidite, this granite also contains other zirconosilicates, typically more potassium-rich. Chemical and textural characterization of this mineral provides insights into the mechanisms of formation and evolution in magmatic and post-magmatic environments. Elpidite can be identified in three distinct textural types: isolated subhedral to euhedral crystals, microcrystalline aggregates, and venular aggregates. Chemical analysis of major elements in the different textural types does not indicate a clear temporal differentiation during crystallization. Regardless of the type, crystals tend to aggregate near the final stages of elpidite. In contrast, (K, Na)-zirconosilicates show variations in Na, Zr, and K contents, approaching the ideal compositions of the minerals georgechaoite and melansonite. However, it is not possible to assert the existence of a complete solid solution between these two elements due to substitutions occurring in restricted intervals. The chemical composition of zirconosilicates in the Papanduva Pluton reveals distinct moments during crystallization, indicating a transition from less evolved rocks, where zircon is the primary carrier of zirconium, to more evolved rocks. This transition is associated with an increase in peralkalinity, oxygen fugacity, alkali activity, and water content, favoring the stabilization of zirconosilicates as zirconium-bearing phases. The presence of (K, Na)-zirconosilicates, originating from the destabilization of elpidite, suggests a significant increase in K2O activity in post-magmatic stages, possibly associated with autometasomatic processes in the rock, of a potassium nature.

COMMITTEE MEMBERS:
Presidente - 2042352 - FREDERICO CASTRO JOBIM VILALVA
Externa ao Programa - 1298966 - RAQUEL FRANCO DE SOUZA - nullExterno à Instituição - SILVIO ROBERTO FARIAS VLACH - USP