A STUDENT OUTCROP MAPPING PROJECT

This student group project involved mapping and photographic documentation of an outcrop located along the northern boundary of the Spokane Valley of eastern Spokane County, within the southern portion of the Priest River complex. The project involved field examination of a 230 m long outcrop of Hauser Lake Gneiss (HLG), consisting of medium- to coarse-grained, quartzofeldspathic paragneiss with interlayered biotitic schist. Within the gneiss are quartz and feldspar-rich bands, ranging from 3 cm to 1 m in thickness. Throughout the outcrop are discontinuous pods (up to 1.5 m wide) of pegmatite that are parallel to the foliation. Within the eastern portion of the outcrop is a 10 m thick amphibolite body that is concordant to the gneissic foliation. The amphibolite is hosted within a biotite-rich, schistose section of the HLG. The dominant mineral assemblage within the gneiss is consistent with amphibolite-grade metamorphism and contains quartz, plagioclase, potassium feldspar, biotite, sillimanite, and rare garnet. The amphibolite is composed primarily of plagioclase and hornblende with variable amounts of biotite. Coarse almandine garnets are concentrated within the amphibolite along the western contact.

The general structure of the outcrop consists of westerly-dipping, foliated bands of quartz and feldspar-rich gneiss and biotitic schist. The foliation strikes at 207^o and dips 35^o to the west. The dip of foliation increases (up to 55^o) toward the eastern portion of the outcrop. Sillimanite lineations plunge 19^o at an azimuth of 245^o. The HLG at this outcrop exhibits pervasive blastomylonitic fabric, and boudins of pegmatite range from 3 cm up to 1 m in length, with top to the east kinematics.

This banded sequence of quartz-feldspar gneiss and biotite schist is consistent with the protolith lithology of the Prichard Formation (Proterozoic Belt Supergroup) found in northern Idaho and north-western Montana. The concordant nature of the amphibolite is consistent with an intrusive origin, and we interpret this as being a metamorphosed Moyie sill within the Prichard Formation.