IGNEOUS GARNET GEOCHEMISTRY FROM THE QUARTZ MOUNTAIN STOCK, MANASTASH INLIER, CENTRAL CASCADES, WASHINGTON

The ca. 157 Ma Quartz Mountain stock is one of several Late Jurassic arc complexes located within the central Cascades, Washington. The stock consists of predominantly granodiorite and tonalite, with lesser quartz diorite, diorite, gabbro and trondhjemite. It intrudes metasedimentary rocks of the Lookout Mountain Formation and the informally named Hereford Meadow amphibolite (MacDonald, 2006). The stock contains numerous small cupolas and is slightly zoned along its margin. The geochemistry of the stock is predominantly low-K, calcic, magnesian, and metaluminous. 5 out of 21 samples are peraluminous and 2 additional samples have aluminum saturation indexes just slightly above 1. The whole-rock molar Mn/(Fe+Mg) ratios from the stock are very low (<0.004). Garnet has been reported from numerous phases of the stock and is found in both metaluminous and peraluminous samples.

20 garnets from an intermediate, peraluminous quartz diorite from a large northern cupola were analyzed by EMPA at the Florida Center for Analytical Electron Microscopy. The garnets are subhedral, and are a primary igneous phase. 7 to 10 point transects across 3 garnets displayed minimal to no zoning and no enrichment of MnO along the grain margins. The garnets are grossular-rich almandine, display little substitution of Mn and Mg for Fe²⁺, and almost no substitution of Fe³⁺ and Cr for Al. They plot in the igneous garnet field on the Mn-Fe²⁺-Mg, almandine + spessartine – pyrope - grossular, and CaO-MnO diagrams. The CaO-MnO and spessartine-grossular values are transitional between garnets derived from M- I- and S-type magmas. The Mn values for 12 garnets are less than 10%. This is suggestive of high pressure (>7 kbar) crystallization. However, the CaO values (4.06 to 5.25 wt. %) suggest low pressure crystallization. It is interpreted that assimilation of sedimentary rocks from the Lookout Mountain Formation oversaturated some Quartz Mountain stock magma in Al and resulted in igneous garnet crystallization.