Geology of the bear mountain intrusive complex, Klamath Mountains, California

The Bear Mountain intrusive complex, Klamath Mountains, California, is a multiphase, ultramafic to silicic plutonic suite emplaced into the Rattlesnake Creek terrane during the Late Jurassic (ca. 151-147 Ma). The intrusive complex includes five plutonic units: (1) elongated, flanking bodies of ultramafic to gabbroic rocks (Blue Ridge, Clear Creek, and Cedar Creek intrusions); (2) biotite + two-pyroxene diorite/ monzodiorite of the Buck Lake plutonic unit; (3) biotite-bearing hornblende ± pyroxene gabbro/diorite of the Punchbowl plutonic unit; (4) biotite + hornblende ± pyroxene (± quartz) diorite of the Doe Flat plutonic unit; and (5) minor biotite ± hornblende quartz diorite to tonalite/granodiorite of the Wilderness Falls plutonic unit. Units 2, 3, and 4 constitute the Bear Mountain pluton. The petrogenesis of these units involved emplacement of distinct batches of magma into the same magmatic center during a complex, multistage intrusion history. Inclusions of older units are found within younger units, and dikes related to younger units intrude older units. Mineral assemblages and Al-in-hornblende barometry indicate early crystallization of pyroxene at >1100 C° and late crystallization of hornblende and biotite at >700 C° between 3 and 5 kb. A dynamothermal aureole extends for -600-1000 m from the external intrusive contact of the Bear Mountain pluton. Metabasic rocks within this dynamothermal aureole have been strongly deformed and recrystallized to well-foliated and lineated hornblende schist or fine-grained amphibolite and exhibit a "pluton-down" sense of shear (i.e., the Bear Mountain pluton). Adjacent to the intrusive complex, contact-metamorphic conditions were hornblende-hornfels facies and locally reached pyroxene-hornfels facies. At some localities in the inner aureole, anatexis occurred, forming migmatitic amphibolite gneiss. The effects of thermal metamorphism apparently extend >2 km from the intrusive contact and are evidenced by various mineral assemblages in metaserpentinite. A concentric, margin-parallel magmatic foliation is found throughout the main body of the Bear Mountain pluton. Map-scale evidence suggests emplacement of the Bear Mountain pluton as a series of sheetlike intrusive bodies that subsequently sagged into an oval-shaped pluton due to negative buoyancy and overall isostatic downward displacement. The three-dimensional shape of the complex is poorly known, but it may be funnel-shaped with a dense root of ultramafic rocks. The ultramafic rocks of the Bear Mountain intrusive complex are primarily cumulates from H ₂ O- and MgO-rich basaltic parental magmas. Differentiation of these magmas gave rise to H ₂ O- and Al ₂ O ₃ -rich basalt/basaltic andesite from which rocks of the Punchbowl unit accumulated. Further differentiation of this magma type resulted in the development of the parental magma for the Doe Flat unit. Magmas parental to the Buck Lake unit were similar to the Punchbowl parent, except that Buck Lake magmas were H ₂ O poor. All of these magmas have trace-element, Sr, Nd, and oxygen isotope values characteristic of a supra-subduction zone tectonic setting. The late-stage tonalite/granodiorite magmas have geochemical signatures consistent with an origin by partial melting of metabasic crustal rocks with residual garnet; therefore, they cannot be explained by local, in situ anatexis of host rock metabasites.