Geochronologic, geochemical and isotopic constraints on the origin of monzonitic and related rocks in the Laramie anorthosite complex, Wyoming, USA

We present new U-Pb zircon ages for monzonitic rocks of the 1.43 Ga Laramie anorthosite complex, a major Proterozoic anorthosite plutonic suite in southeastern Wyoming, USA, associated with contemporaneous reduced rapakivi-type anorogenic granites of the Sherman batholith. Our results constrain (1) the geochemical evolution of Fe-enriched residual liquids to anorthosite, (2) the duration and cooling history of mid-Proterozoic magmatism in the region, and (3) the composition and structure of the crust below the exposed plutonic rocks. Monzonitic intrusions and dikes, ranging in composition from monzodiorite to quartz monzonite, account for nearly one-quarter of the Laramie anorthosite complex, forming a discontinuous rim around the anorthositic intrusions. All of the monzonitic rocks crystallized at high temperatures (>950 °C) and under relatively reducing conditions (fO2<FMQ) from dry, highly-evolved (MgO<2 wt.%) magmas that show coherent geochemical trends with higher MgO (5–6 wt.%) ferrodiorites. Clear, colorless low-U (<100 ppm) zircon, showing little evidence for inherited grains, is an abundant accessory mineral in all monzonitic rocks and ranges in abundance from 1 to 5 vol.%. Such low-U concentrations, even lower than in zircon and baddeleyite from the anorthosites in the complex, appear to be a defining characteristic of monzonitic rocks in Proterozoic anorthosite plutonic suites world-wide, and indicate derivation from low-U parental magmas and sources. The new geochronologic results from the Sybille intrusion (eastern monzosyenite: 1435.6±1.9 Ma; western monzosyenite: 1435.7±2.2 Ma), the Red Mountain pluton (high-Zr quartz monzonite dike: 1432.6±3.3 Ma), the Maloin Ranch pluton (coarse-grained monzosyenite: 1434.3±1.2 Ma) and associated monzonitic dikes (fine-grained monzodiorite: 1435.6±2.5 Ma) reveal that all of the intrusive rocks in the Laramie anorthosite complex were emplaced and crystallized in less than 10 million years (1440–1430 Ma), and perhaps as little as in 6 million years (1434±3 Ma). The inferred cooling history of the complex from >1000 °C was relatively rapid (20–25 °C per million years) during the initial 30 million years following emplacement and slowed considerably once ambient temperatures of about 350 °C were reached. Each of the monzonitic intrusions and all of the monzodioritic dikes within the Laramie anorthosite complex are characterized by a wide range of initial 87Sr/86Sr and limited variation of initial εNd, consistent with simple binary mixing between anorthosite residual liquids and Archean crust. The ages and isotopic compositions of most of the monzonitic and related rocks indicate that their ferrodioritic parents were extracted from anorthosite at depth and that they were contaminated during ascent through Archean crust prior to emplacement within and along the margins of the currently exposed anorthosites in the Laramie Mountains. The combined geochronological and geochemical results of this study demonstrate that Proterozoic anorthosite plutonic suites are characterized by extensive, relatively long-lived (∼10 million years) conduit systems, with intrusions distributed at different depths throughout the crust, which are linked and cut by dikes derived from deeper crustal levels.