ABSTRACT Ferroan granite is a characteristic rock type of the Laurentian margin. It is commonly associated with anorthosite and related rocks. Ferroan granites are strongly enriched in iron, are alkalic to alkali-calcic, and are generally metaluminous. These geochemical characteristics reflect their tholeiitic parental magma source and relatively reducing and anhydrous conditions of crystallization. Their compositions distinguish them from arc magmas, which are magnesian and calcic to calc-alkalic. Ferroan granite magmas are hot, which promotes partial melting of their crustal wall rocks. Assimilation of these silica-rich and peraluminous melts drives the resulting magmas to higher silica and aluminum saturation values. Where Proterozoic ferroan granites intrude Archean crust, their mantle component is readily identified isotopically, but this is more difficult where they intrude relatively juvenile crust. Ferroan granite forms in tectonic environments that allow partial melts of tholeiitic mantle to pond and differentiate at or near the base of the crust. Phanerozoic examples occur in plume settings, such as the Snake River Plain and Yellowstone, or under certain conditions involving slab rollback, such as those that formed the Cenozoic topaz rhyolites of the western United States or ferroan rhyolites of the Sierra Madre Occidental. It is possible that the long-lived supercontinent Nuna-Rodinia, of which Laurentia was a part, formed an insulating lid that raised underlying mantle temperatures and created a unique environment that enabled emplacement of large volumes of mafic melt at the base of the crust. Ascent of felsic differentiates accompanied by variable crustal assimilation may have created large volumes of Proterozoic ferroan granite and related rocks.

Abstract The Laramie anorthosite complex (LAC) is the one of the best-studied and most accessible anorthosite complexes in the world. Its components, which range in age from ca. 1432 to1436 Ma, are exposed over ~800 km 2 in the southern Laramie range of southeastern Wyoming. Although the eastern margin of the LAC has been truncated by Laramide faulting, its western and northern margins expose local areas of contact with the adjacent Archean gneiss and supracrustal rocks that were reworked by Paleoproterozoic deformation and metamorphism. Three major compositional units are recognized in the LAC: (1) anorthositic rocks, which occur as three major domal structures, the Poe Mountain anorthosite, the Chugwater anorthosite, and the Snow Creek anorthosite; (2) leucogabbroic rocks, which occur as the Strong Creek Intrusion and as smaller intrusions into the anorthositic rocks; and (3) monzonitic rocks, which form three major intrusions—the Red Mountain, Sybille, and Maloin Ranch, rimming the anorthositic rocks. The first day of the field trip examines the anorthositic layered zone and leucogabbroic layered zones within the Poe Mountain anorthosite, which is the oldest and northernmost of the LAC anorthositic rocks. The stops illustrate the processes involved in the formation of anorthositic magma chambers at middle to upper crustal levels, and the magmatic structures that form in Proterozoic anorthosites and mafic layered intrusions where plagioclase is a major constituent. The first half of the second day of the trip examines the Sybille monzosyenite intrusion that rims the western margin of the Poe Mountain anorthosite and is in contact with the Archean granitic gneiss and supracrustal rocks to the west and northwest. The stops illustrate mineralogical variations in the Sybille monzosyenite intrusion, a pendant of ferrodiorite and anorthosite, and the role of fluids in stabilizing the mafic mineral assemblages in these rocks. The latter half of the second day will be a walking traverse across the contact aureole on the western margin of the LAC where temperatures in excess of 900 °C were reached. The last day of the trip examines contact metamorphic zonation and local structural relationships in the Archean country rocks adjacent to the Red Mountain pluton (RMP). The stops illustrate the contact effects along the western margin of the RMP where the country rocks contain interlayers of the RMP. The intrusion of the main RMP body reoriented pre-intrusion deformation features in the country rocks, deformed early intrusive veins of the RMP, and produced contact metamorphic zones that overprint assemblages from previous periods of Paleoproterozoic metamorphism. Thermometry and reactions relationships in the RMP thermal aureole indicate temperatures as high as 800 °C near the contact and within interlayered regions to ca. 600 °C 1.5 km from the contact. The trip stops include easily accessible outcrops along Wyoming Highway 34, and less accessible stops on a private ranch property.