MAGMATIC AND STRUCTURAL EVOLUTION OF AN ANORTHOSITIC MAGMA CHAMBER: THE POE MOUNTAIN INTRUSION, LARAMIE ANORTHOSITE COMPLEX, WYOMING

The 200 km(2) Poe Mountain intrusion, part of the 1.43 Ga Laramie anorthosite complex in southeast Wyoming, USA, preserves an exceptional range of primary magmatic and secondary deformation-induced structures that document the sequence of events involved in the emplacement, crystallization, and high-temperature deformation of a Proterozoic anorthosite. The intrusion consists of a steeply dipping (40-90 degrees) olivine leucogabbroic to anorthositic margin of layered cumulates (minimum stratigraphic thickness of 5 km) that contains structures typical of mafic layered intrusions. The layered margin grades inward to a core of granoblastic (recrystallized) anorthosite characteristic of Proterozoic anorthosite plutonic suites worldwide. From the lowest to the highest stratigraphic levels, the layered series is divided into the anorthositic layered zone (ALZ) and leucogabbroic layered zone (LLZ). Igneous layering occurs at all scales from laterally discontinuous cm-thick horizons to distinct mappable packages of layered rocks up to several hundred meters thick. Contacts between the layered zones and internal subsections can be traced similar to 20 km along strike. The layered rocks are strongly laminated, defined by the orientation of tabular plagioclase; the abundance of layers in the ALZ containing blocky megacrysts of plagioclase increases down-section toward the core. Most of the tabular plagioclase appears to have grown within the upper crustal magma reservoir; plagioclase megacrysts were likely entrained from depth. Isolated examples of layer disruption record periodic re-organization of the semi-consolidated cumulate pile in response to slumping. Structures related to the impact of settled blocks, combined with scours and a mixed magma horizon, indicate that layering and lamination formed directly at or near the magma - pile interface, and that a dynamic magma chamber was present throughout the crystallization history of the Poe Mountain intrusion. Because the rejected interstitial liquid in anorthositic cumulates is denser than intermediate-composition plagioclase, the intrusion floor must have been inclined to allow for drainage. A sloping floor formed in the Poe Mountain intrusion owing to the contemporaneous diapiric rise of the relatively buoyant plagioclase-megacryst-rich rocks in the core; diapirism was the driving force for pervasive recrystallization of the core anorthosite and much of the ALZ. The Poe Mountain intrusion demonstrates that the assembly of Proterozoic anorthosites required active, periodically replenished magma chambers where the efficient segregation of plagioclase and removal of dense rejected melt, aided by syn- to post-crystallization deformation, led to the formation of vast expanses of nearly monomineralic cumulates.