PROLONGED NEAR-SOLIDUS HEATING IN THE SAHWAVE INTRUSIVE SUITE RECORDED BY ZIRCON AND SPHENE GEOCHRONOLOGY AND THERMOMETRY

New SHRIMP U-Pb ages coupled with Ti and Zr thermometry for zircon and sphene in the large (> 1000 km²), compositionally zoned, Late Cretaceous Sahwave Intrusive Suite, about 50 km northeast of Reno, Nevada, support field relations that indicate near-solidus conditions were maintained for a substantial duration of intrusive activity. Units of the Sahwave Intrusive Suite are arranged concentrically, becoming more felsic and younger towards the center. The K-feldspar-megacrystic Sahwave Granodiorite intrudes the less porphyritic Granodiorite of Bob Spring, which in turn intrudes the equigranular and more hornblende- and biotite-rich Granodiorite of Juniper Pass along generally gradational contacts. Zircon U-Pb ages (with 2σ errors) for the Sahwave Granodiorite and the Granodiorite of Juniper Pass are 88.5 ± 2.0 and 92.7 ± 1.4 Ma, respectively; whereas sphene U-Pb ages for samples from the same localities are 86.8 ± 1.2 and 87.8 ± 0.8 Ma, respectively. Although the sphene and zircon ages are statistically indistinguishable for the (younger) Sahwave Granodiorite, sphene in the (older) Granodiorite of Juniper Pass appears to be 4.9 ± 2.2 Ma younger than zircon from this unit, suggesting reheating by the younger intrusive activity. Titanium in zircon and zirconium in sphene thermometry for the same sample give temperatures around 750 and 720 °C, respectively. Uncertainty about lead diffusion in sphene makes it more difficult to say what “closure temperature” is actually represented by the sphene age, but it is very likely over 600 °C. Previous heat-flow modeling of similar large, fairly shallow (~ 5–10 km) intrusions has called into question the notion of long-lived magma chambers, but these data suggest the Sahwave Intrusive Suite in fact remained close to near-solidus conditions over a nearly five-million-year period.