Abstract

In the South Mountains metamorphic core complex of central Arizona, a middle Tertiary pluton exhibits the entire spectrum of ductile-to-brittle structures characteristic of metamorphic core complexes. Apart from minor Cu-enrichments in brecciated samples, original major- and minor-element abundances of the deformed igneous rocks are remarkably undisturbed, and there is little correspondence between chemical composition and deformational state. Similarly, biotite and feldspar behaved almost isochemically during deformation except in the most severely deformed samples, where plagioclase became somewhat more sodic and where biotite lost (Fe + Mg) relative to (Al + Ti) and was partly converted to vermiculite.

The oxygen- and hydrogen-isotope compositions of the samples are also quite uniform and are typical for continental granitoid rocks. The range in whole-rock δ18O values is extremely limited for the entire suite (8.3‰ to 9.9‰ versus V-SMOW), from undeformed granodiorite to mylonite, green breccia, and microbreccia. Although δ18O values for quartz (9.4‰ to 11.2‰) and feldspar (7.8‰ to 9.2‰) are similarly restricted, the highest values occur in the most strongly deformed samples. Variably vermiculitic biotite has mildly and apparently randomly disturbed δ18O values (4.9‰ to 6.0‰), but δD values (-75‰ ± 4‰) are remarkably uniform and are typical for "unaltered" plutonic rocks. The δ18O values of barren, ductilely deformed quartz veins (9.1‰ to 9.7‰) and of late, brittlely deformed gangue quartz from the Max Delta gold mine (12.1‰) are consistent with the persistent circulation of a single, isotopically uniform fluid reservoir at temperatures from ∼600 °C (ductile deformation, barren quartz veins) to ∼400 °C (brittle deformation, mineralized quartz veins). The inferred fluid δ18O (∼+7‰) and δD (∼-35‰) values imply a deep-seated, igneous or high-T metamorphic origin for the fluids.

Fluids present during the entire history of ductile-to-brittle deformation were apparently close to chemical and oxygen/hydrogen-isotope equilibrium with the rocks. The temporal association of deformation with Tertiary plutonism and the evidence for high-temperature fluid/rock near-equilibrium support the interpretation that the fluids were expelled from differentiated, late crystallizing portions of the South Mountains plutonic complex.

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