ROLE OF LIMITED CRUSTAL ASSIMILATION IN THE EVOLUTION OF THREE DISTINCT ANORTHOSITE TYPES IN THE LARAMIE ANORTHOSITE COMPLEX, WYOMING

The Laramie Anorthosite Complex (LAC) contains three anorthosite plutons: the 1436 Ma Chugwater anorthosite, the 1434 Ma Poe Mt. anorthosite, and the Snow Creek anorthosite, which intrudes both of the above. Each pluton has a distinct mineralogy. Both the Chugwater and Poe Mt. contain augite, low Ca-pyroxene (opx in more Mg-rich rocks, pigeonite in the more Fe-rich rocks), and ilmenite and magnetite. The Poe Mt. is generally olivine-saturated. The Chugwater anorthosite is olivine-free except in the vicinity of troctolitic plutons that intruded when the Chugwater was still a crystal mush. Locally (at high stratigraphic levels) the Chugwater is quartz-saturated. The Snow Creek anorthosite is distinguished from the other two by a lack of magnetite and olivine, and local quartz-saturation. Because all plutons contain ferromagnesian minerals with similar Fe/Mg ratios, this change in mineralogy reflects differences in silica activity: lowest in Poe Mountain, highest in Snow Creek. The high silica activity in the Snow Creek is responsible for the absence of magnetite, which was consumed by the reaction silica + ulvospinel=orthopyroxene+ ilmenite.

Each body has distinct Nd and Sr isotopic characteristics. The Chugwater anorthosite has the most mantle-like initial Nd and Sr isotope ratios whereas the Snow Creek has initial ratios indicating assimilation of Archean crust. In addition, Snow Creek anorthosite zircons contain inherited components.

Other anorthosite plutons exhibit the same range in silica activity as is found in the LAC. For example Labrieville is silica-saturated, St. Urbain and Marcy are intermediate, and Harp Lake is olivine-saturated. In the LAC this variation may be due to the amount of crustal rock that has been assimilated. The variation elsewhere may likewise be due to crustal contamination but this is difficult to confirm because the crust typically is isotopically juvenile, unlike in the LAC where the country rock is isotopically distinct from anorthositic magmas.