Abstract
The oxygen isotope systematics of Tertiary volcanic rocks of east-central Nevada and of plutonic and metamorphic rocks of the Ruby Mountains-East Humboldt Range core complex provide complementary evidence for major 18O-depletion and 18O/ 16O homogenization of mid-crustal rocks during metamorphism and magmatism. The δ 18O value of crustal source material for silicic volcanic rocks decreased from between +9 and +11‰ to between +7 and +8‰ over ∼ 5 Ma. Mid-crustal metasedimentary and granitic rocks in the East Humboldt Range have δ 18O values very similar to the volcanic rocks and values are lower and more homogeneous at deeper structural levels. Exchange with deep-seated mantle-derived igneous rocks, or fluids derived therefrom, is the most plausible 18O-depletion mechanism. Intrusion of these mafic magmas promoted crustal melting and fluid migration. Homogenization of 18O/ 16O resulted from migration of high-temperature fluids and melts at mid-crustal levels, and was less effective at higher structural levels where the crust was dominated by less permeable carbonate rocks.
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