GEOLOGY OF RING MOUNTAIN AND TIBURON PENINSULA, MARIN COUNTY, CALIFORNIA

Detailed geologic mapping and petrographic analysis reveal that Ring Mountain and the adjoining NW-trending Tiburon Peninsula are underlain by three distinct terranes (T). The structurally highest terrane, T1, consists of partially serpentinized harzburgite underlain by a relatively thin serpentine-matrix mélange. The mélange matrix, composed of lizardite with minor antigorite and chrysotile, has a pervasive shear-fracture fabric characterized by anastomosing shear-fracture surfaces enclosing small, disk-shaped serpentinite phacoids. Entrained within, or eroded from, the mélange matrix are blocks of variable size and shape, including blocks of blueschist, amphibolite, and eclogite grade rock. T1 is underlain by T2, which consists of schistose metagreywacke, metachert, greenstone, and minor metaconglomerate units of blueschist grade. Although individual units of T2 are somewhat discontinuous at Ring Mountain, they are more laterally continuous in the central and southern Tiburon Peninsula. Metagraywacke, the dominant rock type of T2, is fine- to medium–grained with a semi-planar foliation and common, foliation-normal quartz veins. Cleavage is defined by anastomosing chlorite, white mica ± stilpnomelane. The phyllosilicates enclose porphyroclasts of relict quartz elongated by deformation and fabric-parallel overgrowths together with neoblasts of the blueschist facies assemblage lawsonite ± albite ± jadeite ± glaucophane. T3, the structurally lowest terrane, is dominated by prehnite-pumpellyite grade meta-litharenite with minor metashale. The meta-litharenite of T3 has modal mineral content and QFL ratios (for 32 litharenite samples) confirming that it represents part of the Alcatraz Terrane of Jayko and Blake (1984). At Ring Mountain, the three distinct, stacked terranes are separated by low-angle faults. Southeast of Ring Mountain, along Tiburon Peninsula, that terrane sequence is offset by later NE- and NW-trending high-angle normal faults that juxtapose portions of the three terranes.