BEAR PEAK INTRUSIVE COMPLEX, KLAMATH MOUNTAINS, CALIFORNIA: EVOLUTION OF AN ULTRAMAFIC TO SILICIC PLUTONIC SUITE THROUGH CRYSTAL ACCUMULATION, WALL-ROCK ASSIMILATION, AND MULTIPLE MAGMAS

The Bear Peak intrusive complex (BPic), Klamath Mountains, California, is a composite plutonic suite that ranges in composition from ultramafic to silicic. Clinopyroxene- and hornblende-rich ultramafic rocks form a central unit that is complexly intruded by multiple generations of crosscutting gabbroic to dioritic dikes. The ultramafic rocks, interpreted as cumulates, are thus a large-scale intrusion breccia. These complex field relationships suggest that multiple magmas passed through the cumulates and presumably fed higher mafic units in the complex. The bulk of the intrusive complex consists of mappable gabbroic to dioritic to tonalitic/granodioritic units. Enclaves are common in the plutonic units of the BPic, and distinctive migmatitic amphibolite indicates magma temperature high enough to permit dehydration partial melting. The distribution of these mafic host-rock enclaves suggest that much of the gabbroic to dioritic parts of the BPic was emplaced as magmatic sheets that coalesced into mappable, relatively homogeneous units, which grew by piecemeal intrusion. The BPic was emplaced into rocks of the Rattlesnake Creek terrane (RCt) and a dynamothermal contact aureole surrounds the intrusive complex. Contact metamorphism was chiefly at hornblende-hornfels facies conditions and grades into the regional greenschist-facies metamorphism of the RCt. Andalusite and chloritoid form small porphyroblasts in some of the more aluminous metasedimentary rocks indicating low-P contact metamorphism (<4 kb). Al-in-hornblende geobarometry in tonalitic to granodioritic rocks also suggests P conditions of ~4 kb. The BPic was emplaced after regional contraction (Nevadan orogeny), because the Late Jurassic Galice Formation in the footwall of the regional Orleans thrust underwent contact metamorphism, as shown by chiastolite porphyroblasts. The BPic is an example of the extended compositional range characteristic of some oceanic-arc plutonic suites and demonstrates how multiple processes (chiefly magmatic) can yield a broad range in rock composition within a single intrusive complex. Ongoing geochemical studies indicate that multiple magmas were involved in the evolution of the BPic.