Precambrian Geology of Yellowstone National Park (YNP) and Surrounding Areas: Plutonic Rocks of the Slough Creek Area

By Matt Hanson
Whitman College, Walla Walla WA

A Precambrian felsic pluton in the Slough Creek area of northern Yellowstone National Park is exposed in tens to hundred-meter-scale outcrops and exhibits field and petrographic features unique to this locale. This body is a kilometer-scale, heterogeneous, variably gneissic pluton with a deformation fabric defined by preferred mineral orientation of biotite and hornblende. This deformation fabric is present locally and is not pervasive throughout the exposures. The lineated texture may represent flow within the melt. Meter-scale xenoliths of metasedimentary rocks are also present and may be related to the Jardine Metasedimentary Sequence to the west. Felsic plutons contain decimeter-scale inclusions of dioritic rocks; however, felsic rocks are also found as inclusions within dioritic bodies, suggesting comingling of magmas. Finally, leucocratic xenoliths and felsic dikes intrude the pluton, cross-cutting foliation.

Petrographic analysis reveals a major mineralogy of plagioclase (19-51%), quartz (15-48%), K-feldspar (1-50%), biotite (1-18%), ± hornblende (0-13%), and epidote (0-3%). Titanite, zircon, and allanite exist as potentially dateable accessory minerals. Hornblende is locally partially replaced by biotite and epidote. Additional, euhedral epidote, with allanite cores is interpreted as primary, implying emplacement pressures of 8-10 kbar (25-30 km depth). Titanite is relatively common, reaching 4mm in diameter. Quartz is recrystallized showing undulose extinction and subgrain boundary development. Deformation twinning is present in plagioclase and myrmekite formation is extensive in K-feldspar grains.

Petrographic data, in particular the presence of magmatic epidote, suggests a catazonal emplacement for the Slough Creek pluton. This, along with hornblende, implies metaluminous composition and distinguishes the Slough Creek pluton from the nearby peraluminous, epizonal Hellroaring and Crevice plutons. The emplacement style and scale of the Slough Creek intrusion is also noticeably different from the meter-scale sill-like occurrences comprising the neighboring Long Lake Magmatic Complex. Analytical work currently underway involving whole-rock geochemistry (XRF), hornblende thermobarometry (EMPA) and zircon geochronology aims to further explain pluton evolution.