A PETROGRAPHIC AND GEOCHEMICAL STUDY OF THE MOUNT SPOKANE PLUTON AND ASSOCIATED DIKE ROCKS AT DART HILL, SPOKANE COUNTY, WASHINGTON

Dart Hill is an exposure of the Cretaceous Mount Spokane batholith, located just north of Spokane, WA in the Dartford 7.5 minute quadrangle. The locality is dominated by medium- to coarse-grained porphyritic biotite-muscovite granite. An extensive outcrop along the east side of Dart Hill on Highway 395 is cross-cut by numerous steeply NW dipping pegmatite and apparently genetically related aplite dikes. These later dikes comprise up to 20 - 30% of the outcrop in some places. Detailed hand sample, petrographic and geochemical analyses of all intrusive units have been performed for comparison. The granite, with biotite and little muscovite and no garnet contrasts with the muscovite + garnet-bearing and essentially biotite-free pegmatite dikes. Graphic intergrowths of feldspar and quartz and large books of muscovite are common in the pegmatite dikes. Red garnets scattered throughout the pegmatites are up to 1 mm in diameter. Aplite veins, located exclusively along pegmatite margins, also lack biotite and have garnets (< 1mm) disseminated throughout but often exhibit distinctive thin (< 1cm) and discontinuous mineralogical layering called garnet line rock. Geochemical analyses of two aplite samples suggest that they are richer in SiO-2, Na2O, and K2O but poorer in CaO relative to the host granites. One particularly garnet-rich aplite dike has nearly 1.7 weight percent MnO, considerably higher than the host granite (0.04 – 0.07). The outcrop is also cross-cut by four younger (Eocene?) vertical lamprophyre dikes (< 0.5 m in thickness). These aphanitic mafic rocks contain tiny crystals of biotite and amphibole in a very fine-grained feldspathic matrix. These rocks are hypersthene normative. Electron microprobe analyses of the minerals in all rock types will further our understanding of the rocks at Dart Hill.