2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 16
Presentation Time: 8:00 AM-12:00 PM


MCFADDEN, Rory R., Geology and Geophysics, Univ of Wyoming, 1000 E. University Avenue, University of Wyoming, Laramie, WY 82071, BARNES, Calvin G., Geosciences, Texas Tech Univ, Lubbock, TX 79409-1053 and SNOKE, Arthur W., Geology and Geophysics, Univ of Wyoming, 1000 E. University Avenue, Laramie, WY 82071, rory@uwyo.edu

The Bear Peak intrusive complex (BPic) is a Late Jurassic (~144 Ma) composite plutonic suite that ranges in composition from ultrabasic to silicic. The intrusive complex is calcic to calc-alkalic, magnesian, and metaluminous. It consists of five mappable intrusive units, from oldest to youngest: (1) ultramafic-mafic cumulates, (2) hornblende gabbro, (3) biotite-hornblende quartz diorite, (4) biotite tonalite/granodiorite, and (5) synplutonic mafic dikes. Clinopyroxene- and hornblende-rich ultramafic rocks form a central unit of cumulative rocks that are complexly intruded by multiple generations of crosscutting gabbroic to dioritic dikes. These complex field relationships suggest that multiple magma batches passed through the cumulates and presumably fed structurally higher mafic units of the complex. The rest of the BPic consists of gabbroic to dioritic and tonalitic/granodioritic units. Major, trace, and rare-earth element (REE) compositions show that both the ultramafic-mafic cumulates and hornblende gabbro crystallized from a high-Mg, low-Al basaltic parent. Magnesian olivine, diopsidic augite, hornblende, chromian spinel, and calcic plagioclase are cumulate phases in the ultramafic-mafic unit. In contrast, high Al and low Mg contents in the quartz dioritic rocks suggest an evolved basaltic or basaltic andesite parent. This magma could be the result of differentiation of the high-Mg, low-Al basalt, a hypothesis supported by parallel REE patterns among the gabbroic and dioritic rocks. The biotite tonalite/granodiorite rocks have high Sr values (>700 ppm), large Sr/Y and Ba/Y ratios, and reverse J-shaped REE patterns. These features cannot be derived by differentiation from either of the inferred mafic magmas. They are instead characteristic of partial melting of metabasic rocks in which amphibole ± garnet are residual phases. Similar features characterize a number of Early Cretaceous plutons in the province. The extended compositional range of the BPic is a common feature of oceanic-arc plutonic suites derived from multiple magmas. The presence of crustally derived, late-stage tonalite/granodiorite indicates that some of the juvenile basaltic magmas ponded in the lower crust with resultant deep crustal melting.