Geothermal implications for the Sierra Nevada batholith from vertical and horizontal compositional zoning studies in contrasting pluton types.

In the I-type Sierra Nevada batholith, two fundamentally different heat-producing element (HPE) fractionation trends are evident between the western (tonalite-trondhjemite) plutons and the central-eastern (granodiorite-granite) plutons. The western granitic rocks are intrinsically low in heat production (0.2-1.5 mu Wm^-3) for the compositional range 57-77% SiO₂, thus showing little evidence for significant vertical HPE variation. The central-eastern granitic rocks have inherently higher heat production (1.5-5.0 mu Wm^-3) for compositions between 57 and 77% SiO₂, thus exhibiting some evidence for vertical variation. For compositions with <60% SiO₂, the central-eastern granitic rocks have heat production between 2.0 and 4.0 mu Wm^-3 compared to <1.0 mu Wm^-3 for the western granitic rocks. Therefore, the eastward increase in heat flow across the batholith is due to changing heat production in source materials. It is concluded that there is no geochemical evidence to support an exponential vertical HPE distribution as the cause of a linear heat-flow-heat production relationship in this region. -J.M.H.