Chemical and isotopic compositions of Absaroka granitoids, Southwestern Montana Evidence for deep-seated Archean amphibolite basement in the Beartooth Region (original) (raw)

Contributions to Mineralogy and Petrology

Contents of major and most trace elements of granitoids in three intrusions associated with the Cretaceous Independence volcanic complex, Montana, correlate well with SiO2. Major-element contents in granitoids in each intrusion are accurately modeled as mixtures of minimum melts and phenocryst assemblages (presumably restite). Restite assemblages are hypersthene + augite + plagioclase, hornblende + plagioclase, and biotite + plagioclase + quartz. Residues of melting are granulite or amphibolite. Melts in two of the bodies were LREE-enriched but unfractionated in MREE and HREE. REE patterns are consistent with residues dominated by pyroxene or amphibole and feldspar. Initial 2~176 and 2~176 of granitoids define a line interpreted as a secondary isochron established during crustal homogenization 3.3 Ga ago. The relatively low la of source rocks (8.25) suggests that they did not spend long in U-rich environments. Source regions had variable trace element patterns; Th/Pb and U/Pb were correlated, Rb/Sr and Sm/Nd moderately well correlated, but Rb/Sr and U/ Pb were decoupled. This is consistent with poor correlation of Rb, Sr and Ba with SiO2 in some granitoids and may suggest that minor phases that concentrate these elements were inhomogeneously distributed in source regions. The source probably consisted of LREE-rich, Rb-poor metamorphic rocks. Archean amphibolites, exposed in the Beartooth Mountains, are similar to the postulated source materials. They contain plagioclase, hornblende, minor quartz, biotite, and muscovite, and have low Rb/Sr and high LREE/HREE. Certain trace-element characteristics of the granitoids indicate that the deep crust in this part of Montana may be dominated by metamorphosed marie-intermediate lavas that formed on the sea-floor. Metapelites, intercalated with amphibolites at the surface, were rare in granitoid source regions. This buried supracrustal pile was isotopically homogenized ~ 3.3 Ga ago. Although some material melted ~2.7 Ga ago to form granites that dominate the exposed basement, enough remained fertile that heating by mantle-derived magmas 85 90 Ma ago produced the granitic rocks at Independence.