Revisiting Emplacement Depths of the Fine Gold Intrusive Suite, West-Central Sierra Nevada

The Fine Gold Intrusive Suite (FGIS) is a large intrusive complex in the west central portion of the Sierra Nevada Batholith. Portions of the Sierra Nevada Batholith have been well studied for plutonic pressure and crystallization histories (e.g., Ague and Brimhall, 1988, GSAB), whereas the regional depth of emplacement of the FGIS is not well characterized, and in previous work pressure estimates were not corrected for crystallization temperatures. An accurate sense of barometric gradient in the FGIS is important to evaluate the roles of pre-batholithic structural breaks in controlling magma emplacement levels, and also to reconstruct erosional levels within the Sierra Nevada as a whole. In this study, samples from the FGIS, all from the Bass Lake Tonalite, were petrographically characterized to identify those samples that contain mineral assemblage and crystallization textures appropriate for application of the Aluminum-in-Hornblende barometer of Hammarstrom and Zen (1986) re-calibrated by Anderson and Smith (1995). Analysis of these samples and use of the barometer results in both pressure and temperature of crystallization. FGIS amphiboles are typical magnesio-hornblende on average: K_0.2Na_0.1Ca_1.8[Mg_2.4(Al,Fe3+)_(0.2-0.6)]Si_6.7Ti_0.1Al_1.3O₂₂(OH)₂. Plagioclase compositional ranges are Ab_(54-69)An_(30-45)Or_(0-1). Bass Lake Tonalite data of Ague and Brimhall (1988) were re-calculated for typical plagioclase composition in the Bass Lake Tonalite (Ab₆₂An₃₇Or₁), yielding slightly higher crystallization pressures (3.3 to 5.8 kbar) than the original range (2.4 to 4.5 kbar). New FGIS crystallization pressures of 2.6 to 3.5 kbar match the recalculated data well, thus providing larger coverage for estimates of emplacement depth. Apparent temperatures from adjacent amphibole and plagioclase rims were found to be 691 to 767°C. When all barometric data are considered together, and uncertainties of the calibration are factored in, we find that FGIS crystallization pressures generally show only slight variations: they generally decrease from about 3 to 3.5 kbar in the northeastern regions to closer to 2.5 to 3.0 kbar in the southwestern regions. Pressures less than 3.5 kbar are consistent with the widespread occurrence of andalusite and cordierite in contact metamorphosed schists throughout the region. The slight trend toward lower pressures in the southwest, where the FGIS abuts the Foothills belt metamorphic rocks, is consistent with the general westward tilting of the batholith exposing deeper levels eastward in the FGIS. We find no evidence for major changes in emplacement levels of the FGIS associated with structural breaks or with major differences of intrusive age.