Petrology of the Composite Snoqualmie Batholith, Central Cascade Mountains, Washington

Abstract

The Miocene epizonal Snoqualmie batholith consists of at least eight intrusive phases emplaced in a mafic to silicic sequence. This magma suite is characterized by the initial injection of olivine-bearing aluminous calcic gabbros followed by diorites, minor quartz diorites, abundant granodiorites, quartz monzonites, and biotite aplitic alaskites. The batholith consists of approximately 80 percent granodiorite, 12 percent quartz monzonite and alaskite, and 8 percent gabbro and diorite. Analyses of the major oxides from 100 rocks conclusively demonstrate the continuous and systematic range of composition of this calc-alkaline trend. An extensive range of composition is shown by CaO-K 2 O-Na 2 O and MgO-FeO(total iron)-alkali plots. Chemical trends closely parallel those of the Bald Mountain batholith, the Guadalupe Complex, the Southern California batholith, and Cascade volcanic lavas. The chemistry, mineralogy, structural features, and regional geologic setting of the batholith are compatible with a parent magma of approximate andesitic composition. The strong zoning of plagioclase, the successive appearance of several mafic mineral phases in most rocks, the sequence of emplacement, and the chemical trends of the intrusive suite suggest the operation of fractional crystallization. Differentiation of the parent magma produced progressively more granitic magmas which were subsequently emplaced in the crust. Partial or complete assimilation of unknown quantities of cognate mafic rocks may have been a modifying process. Both differentiation and assimilation appear to have occurred at deeper levels than are now exposed. Biotite alaskites plotted in the Ab-Or-Q-H 2 O system suggest that they were derived from quartz monzonite magmas at a depth between 4 and 7 km (1 to 2 kb). Stratigraphic reconstruction suggests batholithic emplacement between depths of 4000 to 8000 feet. The roof zone is presently exposed. Rocks of the batholith are directionless, relatively free of inclusions, and in sharp contact with the wall rocks. Emplacement probably occurred by lateral dilation and uplifting of the roof.