TOURMALINE-BEARING LITHOLOGIES OF THE PERALUMINOUS TUSAQUILLAS COMPOSITE GRANITIC BATHOLITH, NW ARGENTINA: EVIDENCE FOR MAGMATIC-HYDROTHERMAL TRANSITION FROM QUARTZ AND TOURMALINE

Textural and chemical features of coexisting quartz and tourmaline supergroup minerals provide evidence of the nature of late igneous crystallization and post-crystallization hydrothermal environment in the shallow-level (<10 km) peraluminous Tusaquillas Batholithic Complex, NW Argentina. A suite of quartz- and tourmaline-bearing samples from the granitic rocks and associated tourmaline-rich orbicules, pegmatites and greisen zones contain evidence for multiple periods of quartz and tourmaline growth and dissolution that span the magmatic and hydrothermal stages of the plutonic system. Optical-CL and SEM-CL images readily reveal three stages of quartz development: (1) magmatic stage-1 quartz with relatively homogeneous CL blue-red color and intensity in pegmatite or with well-developed irregular oscillatory zoning consistent with zoning attributed to magmatic zoning, (2) hydrothermal stage-2 quartz that exhibit complex CL oscillatory zoning and idiomorphic hexagonal patterns, has a orange-red CL color and commonly partially replaces and truncates compositional zoning of the stage-1 quartz and earlier formed generation-1 tourmaline, (3) stage-3 hydrothermal quartz confined to healed fractures cutting earlier stages quartz and exhibits a dull-red CL color. Tourmaline in all samples typically develop two or more generations. Generation-1 tourmaline shows zoning characteristics consistent with both closed- and open-system behavior in a magmatic or hydrothermal environment. Relatively uniform textures and compositions are exhibited by generation-1 tourmaline in the greisens, orbicule and pegmatites and interpreted as magmatic. The highly variable composition found in generation-1 tourmaline in a tourmalinite patch likely reflects open-system development. Generation-1 tourmaline from all lithologies reflects the peraluminous lithologic environments and fractionation trends in the magmatic system i.e. they are highly aluminous (Al_total=6.46-6.95 apfu), are markedly Fe-rich (X_Mg = 0.01-0.16) and have variable F (0.00-0.57 apfu). The hydrothermal generations-2 and -3 tourmalines have distinctive compositions relative to their associated generation-1 tourmaline, broadly schorlitic, and linked to influx of reactive low-Ca and high-Na coexisting aqueous fluids.