HOW DEEP WAS THE INTRUSIVE SUITE OF BUENA VISTA CREST? CONTRASTING RESULTS FROM HORNBLENDE-PLAGIOCLASE THERMOBAROMETRY OF GRANODIORITES AND ZIRCON GEOCHRONOLOGY OF BROADLY COEVAL VOLCANICS (MINARETS AND MERCED PEAK COMPLEXES), SIERRA NEVADA BATHOLITH, CALIFORNIA

Within the central Sierra Nevada batholith, volcano-plutonic complexes provide a record of Mesozoic volcanism and assembly of intrusive suites in a convergent-margin arc setting. The westernmost and youngest of these complexes is composed of thick, widespread volcanic sequences (dacitic to rhyolitic flows and ignimbrites) of the western Ritter Range pendant which overlie and are locally intruded by granodioritic plutons associated with the Intrusive Suites of Merced Peak (ISMP) and Buena Vista Crest (ISBVC).

Our geochronological data (zircon SIMS) and previous bulk fraction zircon TIMS ages suggest that several of the 100 to 97 Ma volcanic units are broadly coeval with assembly of the underlying intrusive suites, principally with the 101 to 99 Ma Granodiorite of Illilouette Creek of the ISBVC and the 99 to 97 Ma Jackass Lakes Pluton of the ISMP.

Eight samples from the ISBVC were analyzed for hornblende-plagioclase thermometry (Holland and Blundy, 1994) using rim compositions from 40 pairs of adjacent phenocrysts. Calculated temperatures average 715 °C ± 26 °C (1 S.D.); although spanning a large range from 673-804 °C, 80% of the data cluster more narrowly between 690-730 °C. All the samples contain the requisite phases for Al-in-hornblende geobarometry, assuming that they represent equilibrium assemblages. Calculated pressures (determined by iteration; Anderson and Smith, 1995) average 3.3 ± 0.5 kbar (1 S.D.), and span a range from 2.0-4.7 kbar, although most of the data cluster between 3.0-3.8 kbar. These results are very similar to recent data from the nearby Jackass Lakes Pluton (3.0 ± 0.5 kbar; Pignotta et al., 2010) and raise the question as to how to explain such emplacement depths with broadly coeval volcanic rocks nearby. Two possible explanations are that the hornblendes within these intrusive suites either crystallized early at greater depths, and/or crystallized from mixed magmas, and thus do not represent equilibrium conditions at the depth of emplacement. Another contributing factor to consider is the possible foundering of the overlying volcanic rocks, perhaps driven by volcanic and/or plutonic processes.