Finding of high-grade tectonic blocks from the New Idria serpentinite body, Diablo Range, California: Petrologic constraints on the tectonic evolution of an active serpentinite diapir


Three high-grade tectonic blocks, including jadeite-bearing retrograded eclogite, pumpellyite-rich retrograded eclogite, and clinopyroxene-bearing garnetamphibolite, are newly described in the jadeitite-bearing New Idria serpentinite body. Petrologic analyses reveal two contrasting peak metamorphic stages—eclogite facies metamorphism (M1E) characterized by garnet + omphacite (∼48 mol% jadeite) + rutile ± epidote + quartz, and amphibolite-facies metamorphism (M1E) characterized by garnet + hornblende + augite (∼14 mol% jadeite) + rutile + quartz. Both peak metamorphic events are overprinted by very low-T blueschist-facies minerals (M2), which include glaucophane, lawsonite, pumpellyite, jadeitite (up to 94 mol% jadeite), chlorite, and titanite. Garnet-clinopyroxene geothermometry yields T = ∼580–620 °C at P > 1.3 GPa for the ME1 stage and T = ∼630–680 °C at P = ∼0.8–1.0 GPa for the MA1 stage. The jadeite- and lawsonite-bearing phase equilibria constrain metamorphic conditions of P > 1.0 GPa at T = ∼250–300 °C for the M2 stage that is probably synchronous with the formation of nearby jadeitite within serpentinite. The presence of eclogite blocks suggests that the New Idria serpentinite diapir was initiated at mantle depths. The wide range of P-T conditions of tectonic blocks supports the idea that the New Idria serpentinite diapir rose from mantle depths and enclosed tectonic blocks at various mantle-crustal levels during diapiric upwelling and extrusion.

Special Paper of the Geological Society of America, v. 419, p. 67-80, doi:10.1130/2007.2419(03)