PETROLOGY, GEOCHEMISTRY, AND STRUCTURE OF THE CHUGWATER ANORTHOSITE, LARAMIE ANORTHOSITE COMPLEX, SOUTHEASTERN WYOMING

Abstract

The Chugwater Anorthosite is one of several 1.43 Ga intrusions that make up the Laramie Anorthosite Complex in the Laramie Mountains, Wyoming, USA. The southwestern portion of the Chugwater Anorthosite has a mappable magmatic stratigraphy totaling at least 8000 meters, and probably 10,000 m or more. It consists of three major units, each of which comprises a lower, dominantly anorthosite portion (>90 vol.% plagioclase) and an upper, dominantly gabbroic anorthosite section (80–90 vol.% plagioclase). Each anorthosite portion contains layers of gabbroic anorthosite and vice versa , on a variety of scales ranging down to centimeters. Plagioclase is dominantly An 50–55 , although higher and lower values are also found. This “main series” of the Chugwater Anorthosite mainly lacks modal and normative olivine. Oxygen fugacity ranged from FMQ to FMQ + 0.5. We interpret that the “main series” was produced by at least three major injections of mildly hyperfeldspathic magma containing approximately 40% entrained plagioclase megacrysts (tabular crystals at least 5 cm across). The Ti contents of the megacrysts (0.3–0.4 wt% TiO 2 ) suggest that these had crystallized at pressures near 10 kbar, consistent with initial formation in a magma chamber at or near the base of the crust. Emplacement pressure is poorly constrained, but appears to have been near 3.5–4 kbar. However, before the main series had completely solidified, it was repeatedly intruded by at least two (and probably more) leucotroctolitic magmas. Contacts of leucotroctolite are sharp against anorthosite, but commonly are diffuse against gabbroic anorthosite, suggesting that sufficient residual melt remained in the latter to permit local mixing with leucotroctolite. We estimate that 80–85% of the Chugwater Anorthosite is main-series, 10–15% is mixed rock, and 5% is leucotroctolite. Prior to final solidification, the entire Chugwater Anorthosite was domed, probably as a result of gravitational instability of the relatively buoyant plagioclase-rich material. The emplacement of the Chugwater Anorthosite as a series of crystal-rich magmas, followed by continued fractionation in a magma chamber at a mid-crustal level and subsequent doming, are characteristics that make the Chugwater Anorthosite intermediate between the Poe Mountain Anorthosite to the north ( in situ fractionation in a magma chamber) and the classic diapiric emplacement of a crystal-rich mush.