PRECAMBRIAN GEOLOGY OF YELLOWSTONE NATIONAL PARK (YNP) AND SURROUNDING AREAS: METAMORPHIC ENCLAVES AND XENOLITHS

Metasupracrustal and metabasic rocks occur as enclaves and xenoliths associated with the voluminous granitic rocks exposed in the Slough Creek area of Yellowstone National Park (YNP). These pre-magmatic inclusions range from km-scale in the Buffalo Creek area to cm-scale as randomly distributed xenoliths. Metasedimentary enclaves in YNP generally consist of medium-to well-foliated schists. Strongly foliated garnet-bearing schists exhibit syn-kinematic growth of garnet and biotite. In some samples the garnet overprints foliation, indicative of post-deformational garnet growth. Within quartz rich layers, biotite grains tend to be oriented at an angle to the dominant fabric annealing of a crenulation cleavage indicative of multistage deformation within a single metamorphism. These metamorphic rocks are of higher grade than the Jardine Metasedimentary Sequence (JMS). Amphibolites typically contain the assemblage hornblende-plagioclase-quartz-magnetite with minor biotite, epidote, and titanite. On the outcrop, rare isoclinal fold hinges are preserved, indicating the foliation has been transposed, but no micro-fold hinges were observed in thin section. No primary sedimentary structures were observed at outcrop scale.

The xenoliths that lie to the east of a major shear zone at Slough Creek have been intruded by metaluminous, catazonal plutonic rocks with an estimated pressure of emplacement of 8 Kb. This study will test if the xenoliths in the Slough Creek area related to the extensive JMS to the west. JMS rocks crop out along a corridor in the Black Canyon of the Yellowstone River and are relatively low-grade with calculated PT conditions ranging from 572-609°C and 3.4-5.9 kbar. U-Pb geochronology of detrital zircons in the JMS and field relationships with 2.8 Ga plutons constrain the age of deposition of the metasediments to 2.8-2.9 Ga. Geochemical data from this study concluded that provenance was most similar to a continental arc setting and recognized them as turbidites based on well-preserved sedimentary structures. Whole-rock geochemical analysis (XRF, LA-ICPMS) is in progress to determine if the metasupracrustal xenoliths in this study are compositionally equivalent to the JMS, and comparative geothermobarometry will determine if they have comparable pressure and temperature conditions.