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_at_mcn.org
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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 All of the olivine has been replaced
by serpentine.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 lt 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. Mgs 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 Earths 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 Vasilev, 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 OHara, 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 OHara
(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