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

Abstract

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 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 eNd, 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. © 2003 Elsevier Science B.V. All rights reserved.