Origin of Andesitic and Granitic Magmas in the Northern Sierra Nevada, California

摘要

The early magmas of the northern Sierra Nevada, calc-alkaline andesite of island-arc type and its derivatives, all low in potassium, were generated during the Devonian(?) period, possibly along an eastward-dipping sub-duction zone. These magmas could have been derived from mantle peridotite of the continental plate by introduction of water from the descending oceanic plate. Later, during the Permian(?) period, the magmas became basaltic, with potassium-rich silicic derivatives indicating anhydrous conditions and a deeper level of magma generation. Plutonism began in Jurassic time, at the end of a period of intense deformation and metamorphism. The earliest intrusive rocks are gabbro and diorite. At the end of the Jurassic period, large granitic plutons were emplaced. These grade from hornblende quartz diorite at the borders to monzotonalite at the centers. Trondhjemite occurs as the latest product of crystallization differentiation of plutonic magmas. Exchange of elements between plutonic and metamorphic rocks suggests that the plutonic magmas were composite. The partial melts of the downfolded volcanic and sedimentary rocks were modified by partial melts from the mantle and the subducted oceanic lithosphere below. Relative amounts of material contributed by each of the three sources of plutonic magma changed with time, and these changes, along with differentiation processes, were responsible for the diversity in composition of magmas.