Development of Tourmaline-Bearing Lithologies of the Peraluminous Tusaquillas Composite Granitic Batholith, NW Argentina: Evidence from Quartz and Tourmaline

Abstract

Textural and chemical characteristics of quartz and tourmaline found in tourmaline-rich orbicules, greisens, pegmatites, and tourmalinite segregations associated with the peraluminous leucogranitic Tusaquillas Batholith Complex of northwest Argentina exhibit both magmatic and hydrothermal features. Imaging of quartz by optical cathodoluminescence and scanning electron microscopy cathodoluminescence shows three stages of development. Stage-1 quartz, considered magmatic, develops as large grains in pegmatites that have optical cathodoluminescence homogeneity; as anhedral relict grains partially replaced by stage-2 hydrothermal quartz in tourmalinite segregations, orbicules, and greisens; and as idiomorphic grains with irregularly spaced oscillatory zoning seen in scanning electron microscopy cathodoluminescence in orbicules. Stage-2 quartz, interpreted as hydrothermal, partially replaces stage-1 quartz and generation-1 tourmaline in most lithologies. Stage-3 quartz, a late hydrothermal stage, occurs in all lithologies as weakly luminescing quartz in healed quartz fractures with abundant fluid inclusions, commonly associated with the crystallization of irregular late-stage tourmaline. Multiple generations of tourmaline span magmatic to hydrothermal phases of development. In all lithologies, generation-1 tourmaline is compositionally similar: highly aluminous (range of average values of Altotal = 6.31–6.95 apfu), markedly Fe- and X□-rich (XMg = 0.01–0.17, X□= 0.21–0.51), and having variable F and WO (F = 0.00–0.57 apfu, WO = 0.00–0.40). Generation-1 tourmaline is interpreted as magmatic with compositions reflecting the chemical environment of the host lithologies and with compositional zoning patterns characteristic of both closed- and open-system behavior, possibly related to the transition to subsolidus conditions. Similar to generation-1 tourmaline, later-stage generations-2 and -3 tourmaline compositions are highly aluminous (range of average values of Altotal = 6.38–6.79 apfu), markedly Fe- and X□-rich (XMg = 0.00–0.20, X□= 0.28–0.40), and variably F- and WO-enriched (F = 0.07–0.57 apfu, WO = 0.00–0.31), but notably poorer in Ca and Ti (<0.01 apfu). The later-stage tourmaline is considered to have developed during the subsolidus hydrothermal conditions. External chemical contributions to tourmaline compositions from the country rocks appear to be minor to nonexistent. The X-site and W-site occupancies of the late-generation tourmaline implies subsolidus invasive alkaline, saline aqueous fluids with high Na but minimal Ca contents derived from the crystallizing leucogranites and related rocks across the solidus-to-subsolidus transition.