HORNBLENDE-PLAGIOCLASE THERMOBAROMETRY OF THE YOSEMITE VALLEY AND SONORA PASS INTRUSIVE SUITES, SIERRA NEVADA BATHOLITH, CALIFORNIA

Ague and Brimhall (1988) applied the Al-in-hornblende geobarometer to a wide sampling of plutonic rocks from the Sierra Nevada batholith (SNB), elucidating large-scale trends in emplacement depth. Since then, the barometer has been subjected to several recalibrations which include the effects of temperature and other variables. In this study, we apply the barometer to complement recent studies of two intrusive suites of differing age in the north-central part of the batholith: the >500 km² Yosemite Valley Intrusive Suite (YVIS; 103-98 Ma) and the >1000 km² Sonora Pass Intrusive Suite (SPIS; ca. 95-91 Ma).

Three samples from the YVIS (Mt. Hoffmann Granodiorite, correlating with the El Capitan Granite) and four samples from the SPIS (Topaz Lake and Kinney Lakes Granodiorites) were analyzed for hornblende-plagioclase thermometry (Holland and Blundy, 1994) using rim compositions from 37 and 31 pairs of adjacent phenocrysts, respectively. All samples contain the requisite phases for Al-in-hornblende barometry (assuming equilibrium assemblages in the presence of melt), and pressures were determined by iteration using the calibration of Anderson and Smith (1995).

Calculated temperatures for the YVIS are tightly clustered at 701 ± 14 °C (1 S.D.) and calculated pressures average 4.0 ± 0.3 kbar (1 S.D.). Data from the SPIS form two widely-spaced clusters: two samples from the Topaz Lake Granodiorite yield subsolidus temperatures of 642 ± 17 °C which are low enough to exclude from barometry. In contrast, calculated temperatures from the two samples of the older Kinney Lakes Granodiorite average 699 ± 25 °C and calculated pressures average 3.1 ± 0.2 kbar. For comparison, Ague and Brimhall’s results from these same two units cluster around ~2 kbar (older calibration) with the exception of two anomalous low-P samples from the Topaz Lake Granodiorite.

Together with previously determined barometry of the 101-97 Ma Buena Vista Crest Intrusive Suite (3.0-3.8 kbar; Ratajeski et al., 2015) and the 95-85 Ma Tuolumne Intrusive Suite (~ 2.4 kbar; Memeti et al., 2009), our data define a trend of decreasing pressure (from 4 to 2.4 kbar) or emplacement depth (from 15 to 9 km) during the 16 myr from 103-87 Ma. This temporal change appears to be more important than across-strike gradients in emplacement depth in this part of the SNB.