PRECAMBRIAN GEOLOGY OF YELLOWSTONE NATIONAL PARK (YNP) AND SURROUNDING AREAS: MYLONITES AND SHEAR ZONES

A ductile shear zone marked by a series of mylonitic rocks that crop out along the Yellowstone River in northern Yellowstone National Park (YNP) defines a boundary between two geologically distinct regions: Garnet Hill to the west and Junction Butte and Slough Creek to the east. Garnet Hill, the eastern most area of the Jardine Metasedimentary Sequence, is dominated by pelitic schist and gneiss. The Garnet Hill schist formed at 560-615ºC and 4-5.5 kbar and are cut by plutonic rocks that crystallized ~2800 Ma. The gray gneiss of Junction Butte formed at 3200 Ma and at peak metamorphic conditions of ~800ºC and 7-8 kbar. The voluminous plutonic rocks (granite to diorite) of the Slough Creek area crystallized at ~8kbar.

The rocks in the ductile shear zone display a range of deformation fabrics and include gneiss, augen gneiss, mylonite, and locally ultramylonite. The least deformed rock suite is a biotite granitic gneiss with a foliation of 008, 47 E. A more intense fabric is seen in a biotite tonalite augen gneiss, with large potassium feldspar porphyroclasts ranging 1-3 cm. Foliation in the augen gneiss (022, 36 SE) is defined by the preferred orientation of biotite, and quartz-rich rods define a lineation (127, 27 SE). The augen show perthitic texture, tartan and Carlsbad twinning, myrmekite, encased inclusions of matrix minerals, recrystallized tails with120º grain boundaries, and margins with abundant sub-grains. The potassium feldspar augen are asymmetric indicating top to the northwest thrusting. The mylonites and ultramylonites have a well-defined foliation (017, 38 SE) with evidence for grain size reduction, recrystallization via grain boundary migration and perhaps grain boundary sliding.

Progressive deformation from gneiss to mylonite and the deformation textures preserved in the mineral grains imply that this shear zone occurred at relatively high temperature. The asymmetry of the potassium feldspar sigma clasts imply that Junction Butte/Slough Creek was emplaced over the Garnet Hill pelitic schists during top to the northwest thrusting. Based on significant geochronological, metamorphic, and pressure differences (2-4 kbar) across the ductile shear zone, the zone is interpreted as a terrane boundary between Garnet Hill and Junction Butte/Slough Creek.