PRECAMBRIAN GEOLOGY OF YELLOWSTONE NATIONAL PARK (YNP) AND SURROUNDING AREAS: PLUTONIC ROCKS WITH INTERMEDIATE COMPOSITIONS

Plutonic rocks with intermediate composition occur throughout the Precambrian exposures in northern Yellowstone National Park (YNP). These plutonic rocks occur as lensoidal bodies range in scale up from meters to 100s of meters in length, and exhibit clear intrusive contacts with the granitic, granodioritic, and tonalitic plutons that are volumetrically dominant in the area. However, field relations demonstrate that the entire magmatic suite is coeval, as all rock types can be seen cross-cutting all others. Petrographic analysis and point counting confirms that these rocks are dioritiic to tonalitic in composition (QAPF diagram), including the primary magmatic mineral phases: hornblende, plagioclase, biotite, and quartz (in varying amounts. Accessory minerals include zircon, titanite and apatite, and retrograde muscovite and hematite are locally developed. Unaltered phaneritic textures are prevalent in this suite of rocks, and. unlike most of the local granitic and granodioritic bodies, metamorphic and deformational textures are not well-developed.

This suite of intermediate plutonic rocks is similar to those described in the main Beartooth massif to the east. Mueller et al. (2011) have also described the coeval occurrence of dioritic rocks with more felsic granodioritic to granitic plutons (the Long Lake Magmatic Coples, LLMC). They have interpreted this occurrence as being the result of melting different source areas and subsequent mingling of different batches of melting at mid-crustal levels in a continental arc setting at 2800 Ma. The recognition of similar rock types and igneous structures in the current field area significantly extends the occurrence of this magmatic arc. Whole rock geochemistry (XRF, LA-ICPMS) and geochronology (U/Pb zircon) analyses are in progress to determine if these intermediate plutonic rocks are compositionally and temporally related to the Beartooth LLMC.