Structural and petrologic evolution of the Bear Peak intrusive complex, Klamath Mountains, California

The Bear Peak intrusive complex is a Late Jurassic (ca. 144 Ma) composite plutonic suite that ranges in composition from ultramafic to silicic. Clinopyroxene- and hornblende-rich ultramafic cumulate rocks form an intrusion breccia that is complexly intruded by multiple generations of crosscutting gabbroic to dioritic dikes. The bulk of the intrusive complex consists of mappable gabbroic to quartz dioritic to tonalitic/granodioritic units. The Bear Peak intrusive complex was emplaced into rocks of the Rattlesnake Creek terrane, producing a dynamothermal contact aureole. Contact metamorphism was chiefly at hornblende-hornfels-facies conditions and grades into regional greenschist- facies metamorphism. Andalusite, cordierite, and chloritoid form small porphyroblasts in some of the more aluminous metasedimentary rocks, indicating low-pressure contact metamorphism (<4 kb). Al-in-hornblende geobarometry in quartz dioritic to tonalitic rocks also suggests pressure conditions of -4 kb. Pseudomorphs of original chiastolite porphyroblasts developed during contact metamorphism of pelitic horizons in the Upper Jurassic Galice Formation, which lies in the footwall of the regional Orleans thrust fault, indicate that the Bear Peak intrusive complex was emplaced after regional contraction related to the Nevadan orogeny. The Bear Peak intrusive complex is an example of the extended compositional range characteristic of some oceanic-arc plutonic suites and demonstrates how multiple, chiefly magmatic processes, can yield a broad range of rock compositions within a single intrusive complex. Mafic magmatic enclaves are common in most of the plutonic units of the Bear Peak intrusive complex, and distinctive migmatitic amphibolite enclaves indicate that magma temperatures were sufficient to facilitate dehydration- melting of metabasic rocks. The distribution of host-rock enclaves and screens suggest that much of the gabbroic to quartz dioritic parts of the Bear Peak intrusive complex were emplaced as magmatic sheets that coalesced into mappable, relatively homogeneous units that grew by piecemeal intrusion. Ultramafic-mafic cumulates and hornblende gabbro crystallized from a high-Mg, low-Al basaltic parent, whereas high-Al, low-Mg contents in quartz dioritic rocks suggest an evolved basaltic or basaltic andesite parent. The biotite tonalite/granodiorite rocks have high Sr values (>700 ppm), large Sr/Y and Ba/Y ratios, and reverse J-shaped rare-earth-element (REE) patterns. These features are characteristic of partial melting of metabasic rocks in which amphibole ± garnet are residual phases. Thus, major, trace, and REE compositions indicate at least two batches of magma were involved in the petrogenesis of the Bear Peak intrusive complex. Complex field relationships and geochemical data suggest that multiple magmas passed through the cumulates and presumably fed structurally higher mafic units in the complex.