ONE BILLION YEARS OF ARCHEAN CRUSTAL EVOLUTION: BLACK ROCK MOUNTAIN, NORTHEASTERN GRANITE MOUNTAINS, WYOMING

Early to Late Archean rocks (3.6 to 2.65 Ga) are preserved at Black Rock Mountain, northeastern Granite Mountains, central Wyoming. Here a NE-striking supracrustal sequence of metapelitic schist, amphibolite, and mafic and felsic volcanic rocks is in tectonic contact with the UC Ranch orthogneiss on the east and the Sacawee orthogneiss on the west.

U-Pb zircon analyses from the tonalitic UC Ranch orthogneiss have Pb/Pb ages from 3.3 Ga to 3.6 Ga. BSE and CL imaging of these zircon reveals oscillatory zoned cores with homogenous unzoned rims. We interpret the U-Pb data to indicate magmatic crystallization at ~3.6 Ga and metamorphism at ~3.3 Ga. An initial e_Nd of +2 and a depleted mantle Nd model age of 3.62 Ga are interpreted to suggest that the UC Ranch orthogneiss represents juvenile crust.

The ca. 3.3 Ga Sacawee orthogneiss, exposed west of Black Rock Mountain, is part of a basement orthogneiss complex that was intruded by gray gneissic sills prior to deformation. U-Pb zircon analyses from one of these sills define a preliminary age of 2.94 Ga.

An interlayered metarhyolite at Black Rock Mountain yields a U-Pb age of 2834 Ma ± 13 Ma that we interpret as the age of deposition of the supracrustal sequence. Peak metamorphic conditions within the supracrustal sequence at Black Rock Mountain are calculated as 7.5 kbar ± 0.5 kbar and 500-600°C by garnet-amphibole thermobarometry. However, the presence of andalusite-bearing schists may preserve a record of decompressive reactions.

Discrete N-NE trending mylonitic shear zones are found throughout the Black Rock Mountain region, many of which are concentrated along the UC Ranch orthogneiss and supracrustal sequence contact. These shear zones may have formed from tectonic juxtaposition of the UC Ranch orthogneiss against the supracrustal sequence and Sacawee orthogneiss in Late Archean time. They were later reactivated at chlorite grade and cross-cut by ca. 2.65 Ga granitic dikes.

The Black Rock Mountain region exposes rocks recording one billion years of Archean crustal evolution, including the formation of the juvenile UC Ranch orthogneiss at ca. 3.6 Ga, emplacement of the ca. 3.3 Ga Sacawee orthogneiss, younger (ca. 2.94 Ga) intrusions into the basement orthogneiss complex, deposition of Late Archean supracrustal rocks, and tectonic assembly by 2.65 Ga.