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Processes of silicic melt generation |
Department of Geology, San Jose State University, 1 Washington Square, San Jose, CA 95192, USA (e-mail: sburgess{at}mit.edu)
The Tuolumne Batholith (TB), Sierra Nevada Batholith (USA), is an archetypal large, zoned arc intrusion (c. 1200 km2). Previous work proposed that compositional zonation observed in the TB was produced in-situ by inward differentiation of a large magma chamber and/or large-scale, intrachamber magma mixing. Recent geochronology shows that the TB was intruded over 8–9 Ma, making single pulse fractionation or mixing in a magma chamber of TB dimensions unlikely. We examine processes responsible for compositional variation in the Cathedral Peak Granodiorite, which is the largest mapped unit of the TB. New field, geochemical and geochronological work along a roughly contact-perpendicular 5 km transect indicates: (1) magmatic foliation is steeply-dipping (>60°); (2) field evidence for repeated separation of crystals from melt and local magma mixing is observed; (3) U–Pb zircon ages at opposing ends of the transect are indistinguishable within error (c. 87.5 Ma); (4) bulk composition varies only modestly but trace elements show variable degrees of scatter; (5)
Nd(t) and 87Sr/86Sr(i) have small variation compared with that in the whole TB. Geochemical and isotopic data are compatible with fractionation of major silicates and accessory minerals. However, the geochemical spatial variation, minor isotopic variation and field evidence suggest that fractionation was highly disorganized and also involved mixing with new input magma and remobilization of crystal mush as the pluton solidified. Our observations are consistent with the construction of a large and dynamic magma system within the last c. 1 Ma of TB growth.
