Texturas e química mineral de zirconosilicatos em granitos peralcalinos do Pluton Papanduva, PR-SC

Abstract

Elpidite (Na₂ZrSi₆O₁₅·3H₂O) and (Na,K)-zirconosilicates are zirconium-bearing minerals identified in the peralkaline granites of the Papanduva Pluton, located in the Graciosa Province in southern Brazil. These minerals crystallized during the late- to post-magmatic stages, providing insights into the formation and evolution processes in complex magmatic environments. Elpidite occurs in three distinct textural types: (1) isolated crystals with subhedral to euhedral forms, (2) granular aggregates, and (3) vein-like aggregates, each exhibiting specific compositional variations. Additionally, two phases of (Na,K)-zirconosilicates, designated as (Na,K)ZrS-I and (Na,K)ZrS-II, were identified, each displaying distinct textural and compositional properties. Chemical analysis reveals that elpidite has a predominantly uniform composition, with variations in Zr, Al, Ca, and K, aligning with the ideal end-member of elpidite. In contrast, (Na,K)-zirconosilicates are enriched in K relative to Na and exhibit diverse compositional trends. The (Na,K)ZrS-II phase shows greater compositional affinity with melansonite, whereas (Na,K)ZrS-I represents a transitional phase between lower and higher K end-members. However, elemental substitutions occurred within a limited range, suggesting the absence of a complete solid solution between these elements. Trace element analysis of elpidite revealed lower concentrations of most elements compared to zircon, including rare earth elements (REE), but with significant enrichments in Ba, Pb, and Sr. The textural relationships and compositional data of zirconosilicates from the Papanduva Pluton suggest a complex evolution of these minerals, reflecting a transition from less evolved rocks, where zircon predominates as the zirconium carrier, to more evolved rocks. Elpidite likely formed during the late magmatic stages, followed by the development of granular and vein-like varieties. Interaction with K-rich hydrothermal fluids, possibly facilitated by sub-magmatic deformation, resulted in significant hydrothermal alteration, chemical remobilization (including HFSE and REE), and element incorporation, forming (Na,K)ZrS-I and (Na,K)ZrS-II phases from a precursor zirconosilicate, ideally elpidite, or directly from the fluids. This transition is associated with increased peralkalinity (alkali activity), oxygen fugacity, and variation in water content, favoring the stabilization of zirconosilicates as zirconium-bearing phases.