Construction, solidification and internal differentiation of a large felsic arc pluton: Cathedral Peak granodiorite, Sierra Nevada Batholith
S. D. Burgess, J. S. Miller, 2008. "Construction, solidification and internal differentiation of a large felsic arc pluton: Cathedral Peak granodiorite, Sierra Nevada Batholith", Dynamics of Crustal Magma Transfer, Storage and Differentiation, Catherine Annen, Georg F. Zellmer
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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) ɛNd(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.
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Magmas are subject to a series of processes that lead to their differentiation during transfer through, and storage within, the Earth’s crust. The depths and mechanisms of differentiation, the crustal contribution to magma generation through wall-rock assimilation, the rates and timescales of magma generation, transfer and storage, and how these link to the thermal state of the crust are subject to vivid debate and controversy. This volume presents a collection of research articles that provide a balanced overview of the diverse approaches available to elucidate these topics, and includes both theoretical models and case studies. By integrating petrological, geochemical and geophysical approaches, it offers new insights to the subject of magmatic processes operating within the Earth’s crust, and reveals important links between subsurface processes and volcanism.