High Grade Metamorphism and Partial Melting in the Bluegrass Creek Suite, Central Laramie Mountains, Wyoming

Abstract

The Bluegrass Creek Suite (BCS) comprises Archean metasupracrustal rocks which suffered at least two periods of high grade metamorphism during the Precambrian. The first, a regional amphibolite grade metamorphism (6.0 kb, 660°C; Grant & Frost, 1986) at approximately 2.6 BY significantly dehydrated the BCS. The latest event, at 1.4 BY, a high grade contact metamorphism related to the protracted intrusion of the Laramie Anorthosite Complex (LAC) at approximately 3 kb, resulted in high temperatures (locally >> 700°C) and partial melting of metapelites within 2 km of the LAC intrusive contact. Contact metamorphic mineral assemblages in the BCS record both increasing temperatures and decreasing X(H2O) towards the LAC contact. Five isograds determined using metapelites are located within 3 km of the contact whereas one mapped isograd is based on metacarbonate equilibria. An isobaric T-X(H2O) diagram (Grant, 1985) shows BCS metapelitic equilibria could exist nearly isothermally at sub-solidus temperatures at varying X(H20). However, textural evidence of partial melting in metapelitic compositions increases toward the intrusive contact. Leucosomes, restites, lathe-shaped plagioclase, orthopyroxene overgrowths, euhedral cordierite and quartz, bimodal biotite grain size and fabric, and oikocrystic K-feldspar and quartz are all consistent with the presence of melt. Phase equilibria and textural evidence (especially inclusion relations and igneous morphology) are consistent with melt-forming reactions emanating from a single isobarically invariant point. Two schematic isobaric liquidus diagrams are presented, one for X(H2O) lower and one for X(H2o) higher than the value for the invariant point. In the Bluegrass Creek suite both the temperature and the composition of the coexisting- vapor phase are important variables for melting relations. Metapelitic compositions melting at different X(H2O) results in different melting relations. These relations provide important clues to the process of anatexis at moderate crustal levels. Although garnet-bearing restites formed by incongruent melting of metapelites probably reflect a lower X(H2O) and/or higher temperature than required to produce orthopyroxene-rich restites, it appears that eutectic crystallization of orthopyroxene requires lower X(H2O) than does garnet. The vapor absent melting reaction (at the invariant point) is potentially the most important based on the scarcity of orthopyroxene coexisting with garnet in the BCS.