23 Undergraduate Research in the Rocky Mountains (Posters)

Thursday, 10 May 2012
8:00 AM - 5:30 PM
Alvarado D&E

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

Paper number: 23-7
Program: Theme Sessions
Time: 8:00 AM – 5:30 PM

Authors

Abstract

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.