EVOLUTION OF AN OCEANIC-ARC PLUTONIC SUITE: BEAR MOUNTAIN INTRUSIVE COMPLEX, KLAMATH MOUNTAINS, CALIFORNIA

The Bear Mountain intrusive complex (BMic), Klamath Mountains, California, is a multi-phase plutonic suite ranging in composition from ultramafic to silicic and emplaced into the Rattlesnake Creek terrane during the Late Jurassic (~148 Ma). This intrusive complex was emplaced in a piecemeal fashion and includes five plutonic units in order of decreasing age: 1) elongated, flanking bodies of ultramafic to gabbroic rocks (Blue Ridge, Clear Creek, and Cedar Creek); 2) biotite+two-pyroxene diorite/monzodiorite of the Buck Lake plutonic unit, 3) biotite-bearing hornblende gabbro/diorite of the Punchbowl plutonic unit; 4) biotite+hornblende (±quartz) diorite of the Doe Flat plutonic unit; and 5) minor biotite±hornblende quartz diorite to tonalite/granodiorite of the Wilderness Falls plutonic unit. Inclusions of the older units are found within younger units, whereas dikes related to younger units intrude older units. The 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 and between 3–5 kb. A dynamothermal aureole extends for ~600–1000 m from the intrusive contact. Metabasaltic 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. Adjacent to the intrusive complex, contact-metamorphic conditions were hornblende-hornfels facies and locally reached pyroxene-hornfels facies. The effects of thermal metamorphism apparently extend >2 km from the intrusive contact and are well documented in various mineral assemblages in metaserpentinite. A concentric, margin-parallel magmatic foliation is found throughout the main body of the BMic. Map-scale evidence suggests emplacement of the BMic as a series of sheet-like 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.