CRYSTALLIZATION CONDITIONS OF THE CHUGWATER ANORTHOSITE: EVIDENCE FOR COMPLEX MIXING WITHIN THE MAGMA CHAMBER

The Chugwater anorthosite is a 10 km-thick layered anorthositic intrusion in the Laramie Anorthosite Complex. We recognize three major units in the intrusion. 1) The main series has the common ferromagnesian assemblage augite-low Ca pyroxene (Opx at lower levels, Pig at higher)-magnetite-ilmenite. Quartz is present in the highest stratigraphic levels. 2) Leucotroctolites are present as small (usually 100 m in diameter) isolated bodies within the pluton. They are particularly abundant at some stratigraphic horizons. They contain more calcic plagioclase and more magnesian silicates than the main series. 3) Mixed rocks occur within the main series within several 100 meters around the leucotroctolites or are spread out along leucotroctolite-rich horizons. These are similar to the main series anorthosite except that they contain olivine. In the upper 5 km of the stratigraphy the XFe of Opx in the main series increases from around 0.4 to 0.55. There is no consistent trend in the lower 5 Km, suggesting cryptic variation on a finer scale than our sampling. XFe of the Opx in the leucotroctolites=0.31 and in the mixed rocks it is intermediate between that of the main series and the leucotroctolite. Pyroxene thermometry indicates that the pluton had solidified by around 1050°C. QUILF equilibria indicate that the leucotroctolite had a silica activity around 0.7 and an oxygen fugacity about 1 log unit above FMQ. The silica activity of the upper main series increased with stratigraphic height from 0.7 to 1.0 whereas the oxygen fugacity fell from around 1.0 log units above FMQ to slightly below FMQ. These relations indicate that Chugwater anorthosite consisted of two major magmas. The dominant magma of the main series contained abundant entrained plagioclase, was relatively enriched in iron and had relatively high silica activity and low oxygen fugacity. From time to time during its crystallization this magma it was intruded by small volumes of a magnesian magma with relatively low silica activity and high oxygen fugacity.