THE ROLE OF FRACTIONATION AT THE EMPLACEMENT LEVEL IN THE TUOLUMNE BATHOLITH, SIERRA NEVADA, CA

The Tuolumne Batholith (TB) in the central Sierra Nevada, California represents a protracted record of plutonic assembly and internal differentiation. The TB is normally zoned with compositions ranging from tonalite to granite porphyry locally reaching up to 74 wt% SiO2. While major element and trace element data from Bateman and Chappell (1979) suggested that fractionation was an important process responsible for the internal variation in the TB, isotopic studies revealed that compositional changes are at least partially due to variations at the source region (Kistler, et. al, 1986). Recently published U/Pb ages for the units of the TB demonstrate a 7-8 my emplacement history (Coleman, et al, 2004). These data have sparked controversies surrounding plutonic assembly at the emplacement level and place a renewed importance on the length scales of fractionation as an indicator of the minimum amount of interconnected melt present at one time. We present new major, trace element, and rare earth element data from one transect across a normally zoned 5 km long, 2 km wide lobe of the Half Dome granodiorite in the southern TB, which demonstrate a strong fractionation pattern. SiO2 increases steadily towards the center of the lobe while Al2O3, CaO, MgO, and Fe2O3 vary inversely and linearly with SiO2. Ba and Sr also vary inversely with SiO2, while Rb increases with increasing SiO2. These trace element trends are well correlated with bulk composition of samples. Rare earth elements (REE) reveal a concave shape with all samples depleted in heavy REE relative to light REE. All REE systematically decrease with increasing SiO2. Pending results of isotopic analyses, these results: 1. are permissive of fractionation being the dominant process responsible for compositional variation in the lobe, 2. indicate that the contentious porphyritic Half Dome granodiorite can be the product of fractionation, and 3. suggest a minimum size of pulses/magma chamber possible at the emplacement level.