Contact Metamorphism Adjacent to the Laramie Anorthosite Complex, Albany County, Wyoming

Abstract

The 1440 Ma old Laramie Anorthosite Complex, in SE Wyoming, intrudes Archean granitic gneisses and supracrustal rocks. In the central part of the complex, gneisses of pelitic and granitic composition are intruded and metamorphosed by the central anorthositic dome and the southwestern-most parts of the Maloin Ranch Pluton. The pelitic rocks exhibit field and microtextural features that suggest partial melting. Disruption of the foliation, subhedral to euhedral cordierite, orthopyroxene and oikociystic feldspars are commonly observed. Textural relations between phases suggest reactions between a mixture of crystals and silicate melt during cooling. The migmatites lack any evidence of prograde metamorphism. The granitic gneiss lacks any evidence of melting and contains garnet near the Anorthosite, and the Maloin Ranch Pluton. Pressure conditions of 3.25 (±0.5) kb were calculated from the displacement of the reaction Orthopyroxene + Sillimanite = Hercynitic Spinel + Cordierite, in the pelites. Garnet-biotite geothermometiy, in pelitic assemblages, gives temperatures that range from 620° to 800° C. The garnet-cordierite geothermometer, from pelitic assemblages, yields temperatures that range from 630° to 750° C. Garnet-biotite compositions from the granitic gneiss are within or close to the limits of the experimental calibration. Calculations yielded a peak metamorphic temperature of 850° C and lower retrograde conditions of 760° C. South of the southern anorthositic dome, mafic pyroxene hornfelses and problematic quartzrich metasediments(?) are encountered. In the enigmatic metasediments the textures indicate recrystallization during cooling. This unit does not retain any evidence of prograde metamorphism. In contrast, the mafic pyroxene hornfelses appear to reflect peak metamorphic conditions. The displacement of the reaction Oinopyroxene + Plagioclase = Gamet + Quartz in the metasediments, gives pressure conditions of 3.25 (±1.9) kb. Pyroxene and Fe-Ti oxide geothermometry, from both units, yields temperatures that range from 730° to 840° C. The 1400 Ma Sherman Granite intrudes the Anorthosite Complex and its aureole. The retrograde assemblages observed in the metamorphic rocks, and their areal distribution suggest that the granitic intrusion occurred while the aureole was cooling at 0.5-3 kb and about 600° C.