STRUCTURE OF THE SOUTHERN MARGIN OF THE BIG HORN BATHOLITH, WYOMING: A MAJOR ARCHEAN SHEAR ZONE?

Geologic mapping was conducted in the headwaters of Paint Rock Creek in the Cloud Peak Wilderness of the Big Horn Mountains in North Central Wyoming to constrain the lithologies and structures of the rocks along the contact of two Precambrian-aged terranes. The northern part of the Bighorn uplift is composed of the Big Horn Batholith, an “undeformed” composite granitic intrusion, whereas the central and southern area of the range exposes older quartzofeldspathic gneiss complexes as well as minor supracrustal rocks. The boundary between the two regions is marked by a shear zone referred to here as the Upper Paint Rock Shear Zone (UPRSZ). The UPRSZ trends ~N80W, and a ~10-m-wide zone of mylonite with strike-parallel kinematics is present. The field relations north of the UPRSZ are complicated. The dominant lithology to the west is migmatite with a strong vertical fabric that trends parallel to the UPRSZ. This migmatite is rich in amphibolite, mica schist and dacite xenoliths and is intruded by strongly to subtly foliated granite. The structural grain in the granite also is ~E-W. Pegmatite locally occurs along the granite-migmatite contact. Late amphibolite dikes also are present. The most interesting features are two other shear zones within the granites that crosscut both the granite and migmatite and are parallel to the UPRSZ. The shear zones have a brittle overprint, revealed by the presence of silicified breccia, which may be related to the Laramide uplift of the Big Horn Mountains. This study aims to define the nature and extent of the UPRSZ. In particular, we want to verify if this contact is a laterally persistent shear zone or an intrusive contact between the Big Horn Batholith and the southern gneiss terrane. If this boundary is indeed a laterally persistent shear zone, it would be among the most significant tectonostratigraphic boundaries in the Wyoming Archean Province. The kinematics of this boundary will enable us to further unravel the early history of the Wyoming Province before it became part of Laurentia. The fundamental contributions of this project are a detailed geologic map of the study area as well as stereographical projections of the fabrics of both migmatite and granites.