Geochemistry and petrogenesis of the Bonanza King mafic intrusive complex, Trinity terrane ophiolite, California

Keith Robert Willse, University of Nevada, Las Vegas


The Bonanza King mafic intrusive complex (MIC) ({dollar}>{dollar}25 km{dollar}\sp2{dollar}) of the Trinity terrane ophiolite, northern California, consists of cogenetic plutonic and dike lithologies. Steep intrusive contacts exist between cumulate pyroxenite, isotropic gabbro, and peridotite country rock. Near vertical east-west trending, bimodal dikes intruded gabbro and are centered within the complex. Geochemical modeling indicates that accumulation/fractionation of a gabbroic parental magma produced lithologic trends of cumulate pyroxenite and fractionated gabbro and dike lithologies. Similarities between all lithologies suggest cogenetic formation of all the elements of the Bonanza King MIC during the Late Silurian (431+/{dollar}-{dollar}3); Parental magma for the Bonanza King MIC formed in an subduction-related tectonic setting by partial melting of a depleted mantle enriched by slab-derived fluids. Mafic dike and gabbro lithologies contain extremely depleted high field strength elements (Nb 0.09-0.58 ppm) suggesting formation early in the evolution of an intraoceanic convergent margin.