A STRONTIUM AND NEODYMIUM ISOTOPIC INVESTIGATION OF THE LARAMIE ANORTHOSITES, WYOMING, USA - IMPLICATIONS FOR MAGMA CHAMBER PROCESSES AND THE EVOLUTION OF MAGMA CONDUITS IN PROTEROZOIC ANORTHOSITES

A strontium and neodymium isotopic investigation of plagioclase-rich c umulate rocks in the 1.43 Ga Laramie anorthosite complex (LAG) provide s new insights into the evolution of magma chambers and underlying mag ma plumbing systems during the crystallization of Proterozoic anorthos ites. The 725 km(2) LAC intruded the boundary between Archean rocks of the Wyoming Province in the north and Proterozoic island are terranes in the south, the Cheyenne belt, at pressures of 3-4 kilobars. Initia l strontium and epsilon(Nd) isotopic ratios from the three large compo site anorthositic intrusions (Poe Mountain, Chugwater, and Snow Creek) and late troctolitic intrusions range from 0.7034-0.7055 and +0.9 to -4.9, respectively. The isotopic ratios in the cumulates vary as a res ult of (1) the isotopic composition of the parental mantle-derived hig h-Al gabbroic magmas, (2) the amount of contamination by Archean rocks during ascent through the crust, and (3) mixing of replenishing magma s and resident magmas with different isotopic compositions in the magm a chamber at the final level of emplacement. Strontium isotopic and no rmative An variations across the 5-7 km thick stratigraphic section of layered plagioclase-rich cumulates in the 200 km(2) Poe Mountain anor thosite indicate that multiple inputs of magma are needed to construct even relatively small intrusions in Proterozoic anorthosites. The ear liest and strati graphically lowest intrusion was emplaced as a plagio clase-rich magma that crystallized large volumes of compositionally ho mogeneous anorthosite. At least three subsequent chamber-wide episodes of magma replenishment, followed by mixing, are indicated by abrupt s hifts in I-Sr and/or normative An in the stratigraphically higher leve ls of the Poe Mountain anorthosite. Strontium and neodymium isotopic d isequilibrium between a high-Al clinopyroxene megacryst and the layere d cumulates is consistent with a high-pressure origin for Al-rich pyro xene megacrysts in Proterozoic anorthosites. The megacryst crystallize d in a magma chamber at pressures of 10-12 kilobars from relatively un contaminated basaltic parent magma (high-Al gabbro) and was transporte d through the crust in progressively contaminated anorthositic magma. The range of observed isotopic compositions in the anorthositic rocks is nearly identical to that of high-Al gabbroic dikes in the LAG, supp orting the proposition that high-Al gabbros are parental to anorthosit e (Mitchell et al., 1995). The isotopic data require that the anorthos itic parental magmas were contaminated during ascent through the crust by Archean orthogneisses and/or metapelitic rocks. Decreasing Is, and increasing epsilon(Nd) With a relative decrease in age of the intrusi ons (Snow Creek --> Poe Mountain/Chugwater --> troctolites) indicates that the magma conduit became increasingly insulated from crustal cont amination over time. This study indicates that the strontium and neody mium isotopic systems can be used to distinguish between processes tha t occurred at lower crustal/upper mantle pressures, during the ascent of magma diapirs, and within individual magma chambers in Proterozoic anorthosites, and underscores the need for stratigraphic control when addressing the origin of plagioclase-rich cumulates.