GEOCHEMISTRY OF THE LATE JURASSIC QUARTZ MOUNTAIN STOCK, MANASTASH INLIER, CENTRAL CASCADES, WASHINGTON: IMPLICATIONS FOR ITS TECTONIC SETTING

The ~157 Ma Quartz Mountain stock is one of several Late Jurassic arc complexes located within the central Cascades, Washington; these arc complexes have been correlated to Late Jurassic arc plutons within the Klamath Mountains, California-Oregon. The Quartz Mountain stock is weakly zoned, has numerous cupolas, mafic enclaves are common, and xenoliths are rare. Mafic mineral phases crystallized before quartz and feldspar. Major and trace element geochemistry (n= 11) has revealed that this stock consists of 2 magma types: a minor dioritic type that consists of quartz diorite and lesser diorite; and, a more voluminous tonalitic type that consists of tonalite and lesser granodiorite. The dioritic type has SiO₂ between 49 and 55 wt. %, Fe# that increases with increasing Si, and is metaluminous. The tonalitic type has SiO₂ between 54 and 65 wt. %, decreasing Fe# with increasing Si, and mostly metaluminous. One sample is slightly peraluminous. All Quartz Mountain samples are Low-K and have calcic Na₂O+K₂O-CaO values. Trace element concentrations in both types are similar to M-type granitoids displaying generally low abundances of large ion lithophile and high field strength elements.

This geochemistry suggests that the Quartz Mountain stock magma formed as an arc; however, the geologic setting of the stock may not support this interpretation. This stock intrudes two units that are faulted against each other, the Lookout Mountain Formation and an amphibolite unit of unknown age. The Lookout Mountain Fm: consists of volcanically derived turbidites that have a youngest detrital zircon age population of ~160 Ma; may have been deposited in a forearc setting; occurs as xenoliths within the stock; and, underwent Buchan metamorphism. This geologic setting leads us to speculate that the Quartz Mountain stock formed as the result of a previously unreported Late Jurassic ridge subduction event; further, the stock geochemistry could have been produced by melting mafic material underplated at the base of an overriding plate as a ridge was subducted. This scenario is found within the forearc of modern and ancient ridge subduction interactions (e.g., Chugach complex, Alaska).