HIGH-GRADE TECTONIC BLOCKS FROM NEW IDRIA SERPENTINITE BODY, DIABLO RANGE, CALIFORNIA: TWO-STAGE EXHUMATION

Three high-grade tectonic blocks, including jadeite-bearing retrograde eclogite (JEC), pumpellyite-rich retrograde eclogite (PEC), and clinopyroxene-bearing garnet-amphibolite (CGA), were found in the New Idria serpentinite body of the California Coast Ranges. Microtextures, parageneses and chemistries of minerals in the JEC and PEC define two metamorphic stages: a peak eclogite-facies metamorphism (M₁^E), and a blueschist-facies recrystallization (M₂). The M₁^E stage is represented by a mineral assemblage Grt + Omp (Jd_~48) + Rt + Ep + Qtz; garnet is highly chloritized but preserved mineral inclusions of rutile, and rare omphacite, epidote, and quartz. The Grt-Cpx thermometry yields T= ca. 600-650 °C at P> 1.2 GPa for the peak eclogite-facies metamorphism (M₁^E). In the JEC, M₂ stage mineral assemblage, Gln + Chl + Jd + Pmp + Lws + Ttn, is well developed. In particular, M₂ jadeite (up to 94 mol.% Jd) replaces M₁^E omphacite together with other M₂ minerals. In the PEC, M₁^E stage mineral assemblage is extensity replaced by the M₂ assemblage Chl + Pmp ± Lws + Ttn. M₁^E omphacites exsolved abundant diopside lamellae with wide compositional gap; M₁^E garnets are sometimes completely pseudomorphed by chlorite and pumpellyite. Phase equilibria constrain metamorphic conditions of P>0.8 GPa at T= 250-300 °C for the blueschist-facies recrystallization (M₂). In contrast, the CGA contains a peak amphibolite-facies (M₁^A) assemblage Grt + Hbl + Cpx (Jd_~14) + Qtz, and a blueschist-facies overprinting corresponding to the M₂ of JEC and PEC. The Grt-Cpx thermometry suggests T= ca. 580-620 °C at P> 0.8-1.0 GPa for M₁^A. These high-grade blocks may have experienced similar petrotectonic events as other Franciscan high-grade blocks. Consistent blueschist-facies overprinting of different metamorphic blocks suggest both blocks and their enclosing antigorite serpentinites were emplaced through 2-stage exhumation: initial extrusion from mantle to crustal depth along subduction channel similar to the 2-way street model and nearly vertical serpentinite diapirism from shallow depths to the present position.