PETROLOGY AND GEOCHEMISTRY OF AN ULTRAMAFIC LAVA FROM THE FRANCISCAN COMPLEX: A MEIMECHITE FROM THE EEL RIVER BASIN NEAR COVELO, CALIFORNIA

Meimechites are rare, high-Ti ultramafic lavas in which Na₂O + K₂O is < 2% (Le Bas, 2000). The block described here shows a strong OIB signature and is geochemically similar to super-plume meimechites and high-Ti picrites reported from accretionary complexes of eastern Asia (Ishiwatari and Ichiyama, 2004).  The Eel River outcrop, measuring ~16 m x 100 m, consists of multiple, smooth-surfaced flows, each ~ 0.5 to 1.0 meters thick. Occasional pillow structures, lack of vesiculation, high MgO (31%), and low SiO₂ (43%) suggest the flows were highly fluid, and were extruded in a high-pressure submarine environment.  Although the block lies within a mélange unit of the Franciscan Complex, it shows no evidence of the high P/T facies characteristic of that unit.  The rock consists of 53% Mg-olivine, 16% Ti-clinopyroxene, 11% Ti-rich phlogopite, and 6% unidentified opaque. Amphibole comprises approximately 2% of the rock, and altered interstitial glass about 12%.  Acicular apatite and masses of microlitic crystals of variable composition are present as minor constituents.  In thin section, the rock is medium grained and inequigranular. Although it is not generally poikilitic, a number of large cpx crystals (2-4 mm) partially to completely enclose anhedral to subhedral olivine. Much of the olivine is altered to serpentine.  In addition to high MgO and low SiO_2, bulk-rock composition includes13.5% Fe₂O₃, 5.8% Al₂O₃, 4.5% CaO, 1.2 % TiO₂, 0.6% K₂O, 0.4% Na₂O, 0.2% MnO, and 0.2% P₂O_5.  Mg#s range from 81 to 83, Ni content varies from 444 to 985 ppm, and Cr from 771 to 1040 ppm. REE patterns show enrichment in the LREE, with (La/Lu)_N values ranging from 10 to 13.  MORB-normalized spider diagrams reveal enrichment in the more incompatible elements and an overall pattern common to OIB.  A garnet-bearing source is suggested by significant depletion in Y and Yb.  Several LILE and HFSE ratios are consistent with an OIB association (K/Ba = 37, Zr/Nb = 6, Nb/Th = 14, and La/Yb = 15), while Nd, Sr, and Pb isotopic ratios (¹⁴³Nd/¹⁴⁴Nd = 0.5129, ⁸⁷Sr/⁸⁶Sr = 0.7038, ²⁰⁶Pb/²⁰⁴Pb = 18.438) point to a mantle source very close to the PREMA mantle component of Zindler and Hart (1986).  The rock also plots within the OIB or WIP field on several discrimination diagrams including Ti-Zr-Y, Th-Hf-Ta, and Zr-Nb-Y. Block location is 39.8287 N, 123.0693 W.