Sub-solidus hydrothermal alteration in the Elberton & Stone Mountain granites, Georgia: Waves and Cat's Paws
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SubjectScholarship Sewanee 2011; Elberton granite; University of the South, Sewanee, Tennessee; Undergraduate research; Hydrothermal alteration; Stone Mountain granite; Waves; Cat's Paws; Sub-solidus alteration
AbstractThe Elberton granite is a fine-grained granite (1-2mm grain size) composed of 4-7% biotite, 30% quartz, 30-35% Na-rich plagioclase (An15-27), 30-35% K-feldspar, ~ 1% allanite, and lesser apatite, magnetite, sphene, and zircon. Crystallized at depths of ~13km and P=4Kb, its fine grain size was interpreted by Whitney and others (1980) as having resulted from rapid pressure drop due to loss of exsolved volatiles at this depth. The Stone Mt. granite (90 miles east of Elberton) is a medium-grained (2-5mm), two-mica granite composed of 1-2% biotite, 6-12% muscovite, and sub-equal quartz (29-33%), Na-plagioclase (An10-18) (30-35%), and K-feldspar (20-27%). Both granites have similar U-Pb zircon ages (Stone Mt. 325 Ma, Elberton 320 ± 20 Ma) and both formed from subduction-related magmas of the Alleghenian orogeny. Both granites solidified at ~600oC, but each shows unusual sub-solidus features related to hot hydrothermal fluids, likely vapors, emanated after solidification. The Elberton granite is cut by thin (1-4cm), white, sub-horizontal sheet-like structures called waves, while the Stone Mountain granite contains ovoid black-and-white patches called cat’s paws (3-6 cm wide) with a 2-5 cm “black” center rich in tourmaline + quartz, and a 0.5-1cm wide “white” rim rich in plagioclase, K-spar, and quartz. The goal of this study is to use microscopic point-counting techniques to quantify wave and cat’s paw mineralogy, and to use this quantitative mineralogy to infer a stoichiometry of the responsible chemical reactions. Results of Elberton point counts show that its waves (42-47% plag, 40-41% Kspar, 6-12% qtz, 3-4% biotite) are enriched in plag and Kspar, but depleted in quartz, biotite, and allanite relative to the host granite. Microscopic textural features indicate that the depleted minerals were replaced hydrothermally by plag and Kspar, and chemical stoichiometry suggests the replacement reaction involved qtz-bio-allanite replacement by fluids or vapors rich in Na+ and K+. Stone Mt. point counts show that cat’s paw centers have 13-17% tourmaline, 26-38% plag, 19-23% Kspar, 30-37% quartz, 0-1% muscovite, and trace biotite thus more tourm + qtz, less plag + Kspar, and much less musc + bio than the host granite, while cat’s paw rims are strongly enriched in plag (39-48%) + Kspar (22-27%), but depleted in qtz (20-32%), musc (3-6%), and bio (0%) compared to the host. These data suggest (1) that cat’s paw centers formed from high-temp B2O3 vapors that replaced musc, bio, and plag with tourm + qtz (+ excess K+, Na+, Al+3) and (2) that the excess excess K+, Na+,and Al+3 was transferred outward to paw rims to replace qtz-bio-musc with plag+Kspar.
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