VOLCANIC VISCERA?: USING OXYGEN ISOTOPES IN ZIRCON TO UNDERSTAND THE RELATIONSHIP OF VOLCANIC AND PLUTONIC ROCKS IN THE SIERRA NEVADA BATHOLITH

We present new oxygen isotope data from zircons (Zrc) in Cretaceous volcanic rocks preserved in the Sierra Nevada batholith. We also compare them to published data (Lackey et al. 2005, EPSL) from granitoids in the batholith in order to address the genetic relationship of these two rock types. Zircon was analyzed because it provides the most reliable record of magmatic δ ¹⁸O in igneous rocks, especially those that have been hydrothermally altered, as is common in volcanic rocks of the Sierra Nevada.

In continental arc rocks of the Sierra Nevada batholith, plutonic and volcanic rocks are often considered fundamentally related: volcanic rocks are thought to have erupted from large magma chambers that resided in the Earth's crust. However, recent studies suggest that long-lived magma chambers are difficult to sustain in the crust. Additionally, high precision geochronology suggests that volcanic and plutonic rocks occurred on distinctly different time scales: volcanoes erupted continuously over the life of the Sierran arc (~180 to 80 Ma) whereas plutonic rocks were emplaced in short bursts (on the order of 10-15 Ma) with long quiet periods between. This distinct difference suggests that plutonic and volcanic rocks may come from different sources.

To address whether plutonic and volcanic rocks are equivalent in the Sierra Nevada, and to characterize the geochemical relationship of these rocks, we compare δ ¹⁸O(Zrc) in Cretaceous volcanic rocks in the Ritter Range (Minarets Caldera) and Kings Canyon to those of nearby roughly coeval plutons. Values of δ ¹⁸O(Zrc) in the Ritter Range show significant variation (5.3–6.5‰, avg.=5.9±0.5, n=6); equivalent δ ¹⁸O(WR) = 6.5–8.5‰ We also note a spatial distribution of oxygen isotope ratios with mantle-like values (5.2–5.6‰) occurring in the southwestern part of the Ritter Range and δ ¹⁸O values similar to surrounding granites in the Tuolumne Intrusive Series (avg. =6.3±0.3‰) near Tioga Pass. Higher δ ¹⁸O(Zrc) values (7.4–7.7‰, n=2) occur in Kings Canyon where they are transitional in δ ¹⁸O(Zrc) to surrounding values in coeval granitoids (5.4 - 7.7‰, avg. =6.5±0.8). In general, δ ¹⁸O(Zrc) values from volcanic rocks are distinct from those in associated plutonic rocks. The results of this study have direct bearing on our understanding of the generation and modification of continental crust.