PRELIMINARY CHARACTERIZATION OF MAFIC MAGMATIC ENCLAVES IN 57KA TOPAZ RHYOLITE LAVA DOMES IN THE BLACKFOOT VOLCANIC FIELD, SE IDAHO

This study focuses on characterization and petrology of mafic enclaves in two rhyolite lava domes occurring in the China Hat lava dome field. These enclaves generally make up <<1% of the total volume of the rhyolite domes. They are relatively abundant in the tephra cone deposits that underlie China Hat, and in widely scattered swarms in China Cap. Much more rare enclaves occur as isolated individual occurrences in both China Hat and China Cap. Rhyolite at outcrop to hand-sample scale contains 0-3% mafic enclaves. Individual enclaves range from ~1mm to ~14cm across. Swarms of enclaves range from ~10cm to ~1m across. The shapes of the enclaves vary from angular or ellipsoidal, to strung out, boudin-like. Enclaves are black and glassy, to dark grey, to reddish grey and aphanitic. Some of the enclaves are sparsely vesicular. Many contain sparse subhedral plagioclase phenocrysts up to 1mm in size, and some include pyroxene phenocrysts (Dayvault et al. 1984). Based mainly on the shapes and glassy to very fine-grained matrix we infer that most are magmatic or disaggregated magmatic enclaves. Interestingly, degassing rims around some enclaves suggest that the mafic magma was hydrous at the time of incorporation into the degassed rhyolite matrix.