Major crustal detachment faults of Tertiary age in the Picacho metamorphic core complex of southern Arizona demark three tectonic plates in a structural section characterized by the transition from undeformed granitic basement through a mylonitic carapace to brecciated and hydrothermally altered counterparts. A lower detachment fault defines the top of the lower plate and is coplanar with the overprinting of mylonites to chloritic breccias. Variably altered and fractured granite of an overlying middle plate is bounded by an upper detachment fault on which allochthonous Miocene volcanics have been superposed; the volcanics display intense oxidative potassic alteration. This overall transition is accompanied by a 10‰ increase in δ18O and a 400 °C decrease of temperature. Undeformed Oracle Granite of the lower plate (δ18O ≈7.8) retains near-magmatic mineral fractionations, but these are disturbed in mylonitic equivalents, where ambient temperatures were ∼520 °C, δ18OH2O ≈4‰, and conditions of low water/rock ratios prevailed. Chloritic breccias record a whole-rock shift to +8.5‰, accompanied by diminished temperatures of 300 °C, and the infiltration of fluids where δ18OH2O = 3‰ ± 1‰ under conditions of high water/rock ratios. A pronounced isotopic discontinuity occurs at the upper detachment fault where intensely oxidized and K-metasomatized volcanics of the upper plate are shifted to 18‰ at temperatures that diminished to ∼ 150 °C.

Two distinct fluid reservoirs were involved in alteration of the lower two plates and the upper plate, respectively, the tectonic section recording an upward transition from high to low temperature, low to elevated water/rock ratios, ductile creep to brittle fracturing, and an interface of deep “exchanged” fluids with a shallow oxidized surface aqueous reservoir.

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