THE GEOCHEMICAL EVOLUTION OF ANORTHOSITE RESIDUAL MAGMAS IN THE LARAMIE ANORTHOSITE COMPLEX, WYOMING

Olivine- and pyroxene-bearing Fe-enriched dioritic rocks in the 1434 M a Laramie anorthosite complex are interpreted to represent variably fr actionated and contaminated magmas residual after the crystallization of anorthosite. Geochemical characteristics of this suite include the following: high contents of TiO2, Fe2O3T, and P2O5; high incompatible trace element contents; rare earth element patterns with a large range of Eu anomalies; and isotopic compositions that reflect the geographi c location of individual samples, with I-Sr increasing and epsilon(Nd) decreasing from south to north. After extraction from anorthosite, fr actionation of ferrodioritic residual magmas resulted in secondary res idual monzodioritic melts and complementary oxide-rich ferrodiorite cu mulates. Geographic trends in isotopic composition reflect an increasi ng Archean crustal component from south to north. Dioritic dikes and c umulates with isotopic compositions similar to associated anorthosites were derived locally. Large isotopic discrepancies between some diori tes and their hosting anorthosites reflect preferential contamination of residual magma during ascent and emplacement of mantle-derived plag ioclase-rich diapirs, followed by subsequent extraction and isolation of Fe-enriched interstitial melt. Sarong isotopic contrasts between an orthosite and associated Fe-enriched rocks in anorthosite complexes do not preclude a direct relationship between them and reflect the diver sity and complexity of processes during their petrogenesis.