Osa Ring complex
Osa Ring complex
Late Jurassic, ~146-148 Ma
Semi-circular ring dike complex in southern Sierra Nevada composed of alternating, semi-circular dikes of hornblende-biotite gabbro and diorite emplaced as sheets, from 0.1 to 5 m thick, with each sheet commonly chilled against, and separated by thin (1 to 25 cm) septa of lighter-colored and coarse rock ranging from granite to hornblende-plagioclase pegmatite (Sisson and Moore, 2010). Septa partially molten during sheet injection. At least 28 thicker (5-250 m) conformable granitic sheets are spaced through the gabbroic layers and are increasingly thicker and abundant toward the higher elevation outer edges of the structure. Granite sheets also dip steeply inward, further defining the basin-shaped structure. Ring-dike complex approximately 5,300 m wide, E-W and 10,100 m wide, N-S. Terminated on western margin by a NNE-striking fault. On basis of Late Jurassic zircon U-Pb isotopic age (~146-148 Ma) for both gabbro and granite dikes (Sisson and Moore, 2010; Clemens-Knott and Gevedon, 2025), interpreted as coeval with other units in Independence Dike Swarm. Clemens-Knott and Gevedon (2025) propose that this unit should be combined with two units in the Buck Meadow intrusive suite (unit Gbm) along with coeval volcanic bodies too small to map to make up their Summit igneous complex.