On the Morphology and Geochemistry of Hydrothermal Crypto- and Microcrystalline Zircon Aggregates in a Peralkaline Granite

Author(s)

Vlach, Silvio R. F.

• Document type: Journal article
• Source: MINERALS; v. 12, n. 5, p. 17-pg., 2022-05-01.

Abstract

Singular crypto- and microcrystalline hydrothermal zircon aggregates occur in peralkaline granites from the Corupa Pluton of "A-type" granites and syenites in Graciosa Province, Southern Brazil, and are herein characterized for their morphological, textural and geochemical (major, minor and trace elements, and Lu-Hf isotopes) properties. The aggregates were found to present a variety of habits, such as dendritic, oolitic, botryoidal and spherulitic, and they are associated with typical hydrothermal minerals (alkali-feldspars, quartz, fluorite, epidote-group minerals, phyllosilicates and Fe oxides) in micro-fractures and small miarolitic cavities in the host rock. They precipitated directly from a hydrothermal fluid and, compared to magmatic zircon crystals from the host, were found to contain relatively high abundances of the "non-formula" elements (e.g., Fe, Al, and Ca) and HFSEs (High-Field-Strength Elements), particularly the L- and MREEs (Light and Medium Rare Earth Elements), features most typical of hydrothermal zircon, as well as high Th/U ratios, whereas the Lu-Hf isotopic signatures were found to be similar. The formation of the zircon aggregates and the associated epidote-groups minerals was probably due to the interaction between an orthomagmatic, F-bearing, aqueous fluid transporting the HFSEs with the host-rock and/or with an external meteoritic fluid from the country rocks. The preservation of an amorphous-like Zr-silicate compound and crypto-to-microcrystalline zircon varieties is arguably related to the inefficient fluid flux and/or elemental diffusion in a low-temperature oxidizing environment. (AU)

• FAPESP's process: 08/00562-0 - Mantle-crust interaction and the genesis and evolution of the A-type: post-collisional magmatism: geochemical and isotopic records from the Graciosa Province, S-SE Brazil
• Grantee: Silvio Roberto Farias Vlach
• Support Opportunities: Regular Research Grants
• FAPESP's process: 19/17343-4 - Geochemical and textural properties of accessory minerals as petrological and geochronological indicators in granitic rocks and associated greisens
• Grantee: Silvio Roberto Farias Vlach
• Support Opportunities: Regular Research Grants