| Popis: |
The abundances of heat-producing elements increase and of rare-earth elements decrease upward in a 1 km vertical section through a pluton in the eastern Sierra Nevada batholith, California. This distribution results mainly from magmatic processes and not from alteration. For samples containing between 47.7% and 65.5% SiO2, sphene is the predominant whole rock site for U, Th and REE. The variation in vertical heat production enables the characteristic depth scale (D) to be calculated from the linear heat flow-heat production relationship. The calculated value for the pluton, D = 2.2 km, is significantly lower than that of the batholith, D = 10.1 km, calculated from combined heat flow and heat generation. This result implies that plutons within a heat flow province may not follow the same distribution law for heat-producing elements. In the Sierra Nevada batholith two fundamentally different heat-producing element fractionation trends are evident. The western granitoids have inherently low heat production (0.2-1.5μW m−3) for compositions between 57% and 77% SiO2. The central-eastern granitoids have inherently high heat production (1.5–5.0 μW m−3) for compositions between 57% and 77% SiO2. We conclude that there is no geochemical evidence to support an exponential vertical HPE distribution of heat-producing elements as the cause of a linear heat flow-heat production relationship in the Sierra Nevada batholith. |
