Structural geology and contact metamorphism along the northwestern thermal aureole of the Laramie anorthosite complex, Central Laramie Mountains, Wyoming

Abstract

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] The southeastern margin of the Archean Wyoming craton has undergone a complex history of Proterozoic deformation and metamorphism. The Archean rocks in this area were reworked during two periods Paleoproterozoic deformation and metamorphism between ̃1,780 and 1,750 Ma, followed by the emplacement of the Laramie Anorthosite complex (LAC) at ̃1.430 Ma. This study determined: 1) the pressure-temperature conditions and metamorphic reactions in the country rocks along the northwestern contact aureole of the LAC, and 2) the effects of the contact strains and the contact metamorphism on the previous periods of Paleoproterozoic deformation and metamorphism. The structural study included geologic mapping, analysis of features and fabrics produced during the two Paleoproterozoic events (D2 and D3), and analysis of D2-D3 reorientation into the contact strain aureole of two late granitic components of the LAC, the Sybille intrusion (SI) followed by the Red Mountain pluton (RMP). The key feature used to evaluate the contact strain was the reorientation of S2 foliation planes. During emplacement of the SI, S2 foliation planes dipping to the north outside the contact strain zone were rotated about an easterly plunging ENE axis to shallow dips and then steepened to the SSE into the ENE contact with the SI. Adjacent to the RMP, to the north of the SI, the northerly dipping S2 foliation rotated about a moderately plunging NNE-trending axis into a steeply dipping northerly strike along the western margin of the RMP. In the country rocks adjacent to the contact between the SI and the RMP, S2 orientations display a complex geometry produced by the overprinting of the two contact strain events. No penetrative fabrics were formed during the emplacement of the LAC. The contact metamorphic study defined four zones progressively toward the contact with the RMP, based on mineral assemblages in metapelitic rocks. Garnet-biotite geothermometry and garnet-aluminosilicate-plagioclase-quartz geobarometry were used to determine M4 t