David J. Springer, College of the Redwoods, 1211 Del Mar Drive, Fort Bragg, California 95437

1. David J. Springer, College of the Redwoods, 1211 Del Mar Drive, Fort Bragg, California 95437 springer@mcn.org (This poster has been altered to meet the 2Mb file limit set by the GSA. Please contact the author if you have questions, comments, or would like to view any of the missing photos.) amph (a) (a) (b) phlo Figures 3a and 3b. Plane- and cross-polarized photomicrographs of a representative texture from the Eel River meimechite. Anhedral to euhedral olivine (olv), clinopyroxene (cpx), amphibole (amph), and phlogopite (phlo) are enclosed in a matrix (mtx) of altered microlites and glass. All of the olivine has been replaced by serpentine. Two clinopyroxene crystals occupy the lower-center portion of the image. A reaction corona of amphibole has formed where the cpx was in contact with residual melt. A crystal of phlogopite (orange) occupies the center-bottom portion of the image. The phlogopite, clinopyroxene and amphibole, all are Ti-rich varieties. The black opaque mineral has not been identified. Width of view is approximately 4 mm. phlo Figure 2. Close-up of the Eel River outcrop. The meimechite flows stand at a high angle and are disrupted by pervasive cross-faulting, jointing, and flow-parallel shear. The flows have heavily serpentinized chill margins, 1 to 10 mm thick, and are non-vesiculated. A large pillow structure is seen in the upper right corner. The view is approx.12 meters across. Figure 1. Site view of the Eel River meimechite. The block lies within a melange unit of the Franciscan Complex and is surrounded by blocks and boulders of various lithologies and metamorphic grade. The meimechite itself, however, shows only limited evidence of metamorphism other than serpentinization: aragonite, the high-pressure polymorph of CaCO3, has been detected in carbonate veins cutting the northern third of the block. (c) (b) cpx Figures 6a, 6b, and 6c. (a) Chondrite-normalized REE diagram of the Eel River meimechite displaying the fairly steep negative slope typical of OIB magmas. Fractionation of the HREE suggests a garnet-bearing source. (b) MORB-normalized spider diagram showing considerable enrichment in the LIL elements and two of the more incompatible HFS elements, Ta and Nb. The resulting ‘humped’ pattern is typical of an OIB magma source. (c) OIB-normalized plot of the Eel River samples lies below, but subparallel to, ‘typical’ OIB values (red line). The lower position of the plot may result from a high degree of melting rather than from any actual depletion of the trace elements. ABSTRACT 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) and to meimechite lavas and dikes found in the Meymecha River region on the Siberian continental platform (Arndt, et al, 1995). 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 SiO2, bulk-rock composition includes 13.5% Fe2O3, 5.8% Al2O3, 4.5% CaO, 1.2 % TiO2, 0.6% K2O, 0.4% Na2O, 0.2% MnO, and 0.2% P2O5. 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. (a) (b) Figures 4a and 4b. A single, large crystal of Ti-clinopyroxene (cpx) occupies the central portion of these photomicrographs from a meimechite pillow-core. The cpx encloses numerous small grains of partially resorbed olivine. The outer edges of the olivine are replaced by serpentine. A single, small prismatic grain of apatite is seen in the groundmass in the upper left corner of the image. View width is approximately 4 mm. TABLE1. Major and trace element geochemical analyses and isotopic ratios for the Eel River meimechite (b) (a) Figure 7a and 7b. Proposed mantle reservoirs based on isotopic signature (Zindler and Hart, 1986). The Eel River meimechite (red square) plots very close to the PREMA (prevalent mantle) reservoir. The PREMA mantle component is the source for many of Earth’s oceanic islands, including Hawaii, Iceland. Chemical analysis by fusion ICP and ICP/MS performed by ActLabs of Ontario, Canada. TABLE 3. Average major and trace element composition and selected ratios for meimichites from the Eel River, Japan, far east Russia, and Siberia. TABLE 2. Eel River meimechite trace and minor element ratios compared to average values for major oceanic basalts, primitive mantle, and C1 chondrite. COMMENTS AND FURTHER RESEARCH: It is reasonable to conclude from the analysis presented here that the Eel River meimechite is a fragment of an oceanic island. However, several geochemical characteristics of the rock are inconsistent with a strict OIB interpretation and will need to be examined during further research: 1) The Eel River samples show significant depletion in the elements Zr, Hf, Sm, and Ti (Figure 6b). These elements are highly incompatible during mantle melting and fractional crystallization and should be enriched in OIB rather than depleted relative to MORB. 2) Figure 6b also shows a significant negative anomaly at Ce, as well as a small amount of relative depletion in Th. Both of these elements are highly incompatible during mantle melting and are not normally under-enriched in OIB. 3) OIB often display a negative Eu anomaly as the result of plagioclase fractionation. The cause of the slight positive anomaly on the chondrite-normalized plot of the Eel River rocks (Figure 6a) requires additional investigation. The Eel River meimechite is very similar geochemically to meimechites found in accretionary complexes of eastern Asia and to meimechites erupted within the continental platform of Siberia (Table 2). Both of these other occurrences have been attributed to deep mantle melting and superplume activity involving voluminous extrusion of many types of lava and pyroclastic material. The Eel River meimechite, on the other hand, is an isolated fragment of an oceanic island; it has no accompanying related lavas, and no obvious superplume connection. The Eel River block may represent a model of melting and meimechite formation that is fundamentally different from the superplume model. As small and seemingly insignificant as the Eel River meimechite at first appears, further study of its petrogenesis may provide valuable information regarding intra-plate magma formation and mantle geochemistry. Ar-Ar dating of the Eel River rock is in progress. • REFERENCES CITED • Arndt, N., Lehnert, K., and Vasil’ev, Y., 1995, Meimechites: highly magnesian lithosphere-contaminated alkaline magmas from deep subcontinental mantle: Lithos, v. 34, p. 41-59. • Ewart, A., Collerson, D., Reglous, M., Wendt, J., and Niu, Y., 1998, Geochemical evolution within the Tonga-Kermadec-Lau Arc-Backarc system: The role of varyingmantle wedge compoistion in space and time: Journal of Petrology, v. 39, p. 331-368. • Ishiwatari, A. and Ichiyama, Y., 2004, Alaskan-type plutons and ultramafic lavas in far east Russia, Northeast China, and Japan: International Geology Review, v. 46, p. 316-331. • Le Bas, M.J., 2000, IUGS reclassification of the high-Mg and picritic volcanic rocks: Journal of Petrology, v. 41, p. 1467-1470. • Niu, Y. and O’Hara, M., 2003, Origin of ocean island basalts: A new perspective from petrology, geochemistry, and mineral physics considerations: Journal of Geophysical Research, v. 108, p. 5-1 – 5-16. • Pearce, J.A., 1983, Role of sub-continental lithosphere in magma genesis at active continental margins, in Hawkesworhthy, C.J., and Norry, M.J., eds., Continental basalts and mantle xenoliths: Shiva Publishing, Cheshire, England, p. 230-250. • Sun, S.-S., 1980, Lead isotopic sturdy of young volcanic rocks from mid-ocean ridges, ocean islands and island arcs: Philosophic Transactions of the Royal Society of London, A297, p. 409-445. • Sun, S.-S. and McDonough, W.F., 1989, Chemical and isotopic systematics of ocean basalts: Implications for mantle composition and processes: In A.D. • Saunders and M.J. Norry (eds), Magmatism in the ocean basins, Geological Society, London, Special Publication 42, p. 313-345. • Zindler, A. and Hart, S., 1986, Chemical Geodynamics: Annual Review Earth and Planetary Science, v. 14, p. 493-571. • Note: • The discrimination diagrams for this presentation were produced using IgPet for Windows, by Michael Carr, Rutgers University. Data source: based on values of aSun and McDonough (1989); b Sun (1980); c Nui and O’Hara (2003); d Ewart et al. (1994) 1 Number of analyses averaged is shown in parentheses. All samples are meimechites as defined by the IUGS. a Sample data from Ishiwatari, A. and Ichiyama, Y. (2004). b Sample data from Arndt, N., et al. (1995). c Based on single Primorye, Russia sample from Ishiwatari, A. and Ichiyama, Y. (2004). Figure 5. Trace and minor element ratios are useful for inferring the original tectonic setting of a volcanic rock. Each of the discrimination diagrams displayed here is based on well-established geochemical indicators of specific tectonic environments. The Eel River meimechite plots in either the within-plate or OIB field on each diagram. The within-plate field includes oceanic islands and continental flood basalts. GSA 2005 Cordilleran Section Meeting, San Jose, California