T142 Mapping, Minerals, and Metamorphism—Work Small, Think Big II: A Tribute to the Life of Peter Robinson

Wednesday, 28 October 2020
1:30 PM - 5:30 PM
Meeting Rooms

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

Paper number: T142-6
Program: Technical Sessions
Time: 2:45 PM – 3:00 PM

Authors

Abstract

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 (Altotal=6.46-6.95 apfu), are markedly Fe-rich (XMg = 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.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-355967

© Copyright 2020 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.