AMPHIBOLITES OF THE SALTESE UPLANDS CONSERVATION AREA, SPOKANE COUNTY: HIGH-GRADE METAMORPHIC CORRELATIVES OF THE LOWER BELT SUPERGROUP, MOYIE SILLS

Recent geologic mapping at the Saltese Uplands Conservation Area (SUCA) in eastern Spokane County has delineated numerous, large (10-13 meters wide) concordant amphibolite bodies within the dominant bedrock unit, the Hauser Lake Gneiss. The study area is located south of the Spokane Valley within the southern portion of the Priest River metamorphic complex. Here, the Hauser Lake Gneiss occurs near the crest of the Spokane dome mylonite zone and exhibits pervasive mylonitization. The purpose of this study was to map and describe the lithologic units at SUCA, and to examine the mineralogy and geochemistry of the amphibolite bodies. These data were then used to test correlation of the SUCA amphibolites to diabase sills (Moyie sills) that occur within the Prichard Formation of the Mesoproterozoic Belt Supergroup to the east.

Field mapping of the study area has defined the distribution, extent, and concordant nature of the amphibolite bodies. Both the amphibolites and the host Hauser Lake Gneiss exhibit pervasive foliations (avg. 145^o, 33^o westerly), lineations (avg. 28^o @ 251^o), and amphibolite facies metamorphic assemblages. The dominant mineral assemblage for the Hauser Lake Gneiss consists of quartz, plagioclase, K-spar, biotite, sillimanite, and garnet. For the amphibolites, the dominant assemblage is hornblende, plagioclase, quartz, biotite, +/-K-spar, +/-garnet with accessory titanite, apatite, and opaques. Rare relict clinopyroxene (augitic) was observed in thin section. XRF and ICP-MS geochemical analyses were done on two amphibolite samples to quantify the major and trace element chemistry. The amphibolites plot as tholeiitic basalts and have relatively flat (low slope) REE patterns with very minor Eu anomalies. REE patterns for the SUCA amphibolites match closely to those of several Moyie sill occurrences of northern Idaho and western Montana.

Based on the field and geochemical relations, it appears that the SUCA amphibolites are the high-grade metamorphic equivalents to the Moyie sills within the Prichard Formation of the Belt Supergroup of Idaho and Montana.