Transcript David J. Springer, College of the Redwoods, 1211 Del Mar Drive

David J. Springer, College of the Redwoods, 1211 Del Mar Drive, Fort Bragg, California 95437
[email protected]
Rock/Chondrites
REEs-Sun and McD 89
100
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amph
10
1
(a)
La Ce Pr Nd PmSm Eu Gd Tb Dy Ho Er Tm Yb Lu
(a)
Rock/MORB
100
(b)
phlo and cross-polarized photomicrographs of a representative texture from the Eel River meimechite. Anhedral to euhedral
Figures 3a and 3b. Planeolivine (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olivine has been replaced by serpentine.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
phlo
phlogopite, clinopyroxene and amphibole,
all are Ti-rich varieties. The black opaque mineral has not been identified. Width of view is
approximately 4 mm.
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.
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.
10
10
1
1
(b)
.1
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.
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
SAMPLE SiO[2]
ER9211B 43.36
ER9213B 42.61
ER06971
43.17
ER-00-SSU 41.99
Al[2]O[3] Fe[2]O[3]
6.03
12.31
5.93
14.24
5.83
14.04
5.56
13.57
MnO
0.197
0.193
0.19
0.182
MgO
31.24
30.16
30.13
31.83
CaO
4.74
4.59
4.31
4.28
Na[2]O
0.202
0.24
0.33
0.707
K[2]O
0.52
0.59
0.61
0.53
TiO[2]
1.21
1.19
1.14
1.12
P[2]O[5]
0.21
0.24
0.24
0.23
V
104
135
94
89
Cr
933
1040
771
977
Co
47
105
121
93
Ni
444
893
985
804
Zn
0
98
104
42
Ga
5
10
10
16
Ge
0
1.1
1
0.7
As
0
0
0
-5
Rb
7
7
8
8
Sr
191
197
171
118
Pr
2.31
2.39
2.26
Feb-30
Nd
10.1
10.6
11
10.3
Eu
0.952
1.03
0.87
0.913
Gd
2.54
2.66
2.5
2.45
Tb
0.38
0.4
0.4
0.37
Dy
2.03
2.09
1.9
1.92
Ho
0.35
0.37
0.3
0.35
Er
0.91
0.95
0.9
0.87
Tm
0.118
0.124
0.12
0.107
Yb
0.69
0.7
0.6
0.66
Lu
0.1
0.096
0.08
0.092
Hf
1.5
1.7
1.5
1.5
Y
9.6
10
9
9.2
Zr
60
69
50
53
Nb
10
14.8
16
17
Mo
6
2
0
-2
Sb
0
0.3
0.5
0.3
Cs
2.3
3
3.6
2.3
Ba
115
125
120
257
La
9.6
9.92
9.8
8.86
Ce
18
18.7
18
17.6
Ta
1
1.1
1
1.22
W
0
0.4
0
-0.5
Sc
11
11
10.6
12
Be
0
0
0
-1
Th
0.98
1.02
0.9
1.12
U
0.37
0.38
0.4
0.47
Sm
2.44
2.6
2.6
2.34
LOI
10.44
10.41
9.77
8.98
Mg#
83.43804
80.78087
80.98454
82.31887
{87}Sr/{86}Sr
0
0
0
0.703852
{143}Nd/{144}Nd
0
0
0
0.512958
{206}Pb/{204}Pb
0
0
0
18.438
{207}Pb/{204}Pb
0
0
0
15.509
{208}Pb/{204}Pb
0
0
0
37.835
Sun/McDon. 1989-OIB
(c)
CsRbBaTh UNbK LaCePbPrSr P NdZrSmEuTiDy Y YbLu
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
(b)
.1
Sr K Rb Ba Th Ta Nb Ce P Zr Hf Sm Ti Y Yb
cpx
(a)
Rock/OIB
100
Pearce, 1983
16.0
DMMb
0.5135
15.9
15.8
EMII
DMMa
HIMU
15.7
BSE
15.6
207
204
Pb/ Pb15.5
15.4
HIMU
BSE
DMMa
0.5125
15.3
15.2
144
Nd/ Nd
PREMA
EMI
PREMA
0.5130
143
DMMb
EMI
15.1
15.0
15
(a)
16
17
18
19
204
Pb/ Pb
206
20
21
22
(b)
0.5120
0.701 0.702 0.703 0.704 0.705 0.706 0.707 0.708 0.709
87
86
Sr/ Sr
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.
OIBa
N-MORBa
E-MORBa
K/Ba
Zr/Nb
Nb/Th
La/Yb
Ti/Zr
Eel River
Lava
34
4.2
14.4
14.5
122
34.3
5.8
12
14
62
95.2
31.8
19.4
0.82
102
Th/La
Zr/Y
Sr/Rb
La/Sm
K/Rb
Zr/Rb
0.106
6.1
22.4
3.8
623
7.7
0.108
9.66
21.3
3.7
387
9.0
0.048
2.64
161
.95
1046
132
COMMENTS AND FURTHER RESEARCH:
36.8
8.8
13.8
2.7
82
Island arc
calc-alkalineb
28.8
28.6
1.3
7.5
116
Island arc
tholeiiticc
30.3
43.5
2.6
1.8
85.3
Back arc
basaltsd
85.3
78.4
9.1
1.2
83.2
Primitive
mantlea
36
15.7
8.4
1.4
116
C1
chondritea
226
15.7
8.5
1.4
115
0.095
3.32
30.8
2.4
414
14
0.110
2.67
39.3
3.4
617
2.86
0.119
2.57
34.9
1.6
400
67
0.042
3.8
38.5
1.06
699
27
0.124
2.46
33.2
1.55
394
17.6
0.122
2.46
3.13
1.55
235
1.7
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:
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.
Nb*2
Island- arc A,B
A = N-MORB
A
AI,AII = W P Alk
AI
Ocean-floor B
B = E-MORB
B
C = OIB (Rift)
D
D = Arc-basalts
D
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.
Ti/100
Hf/3
A
B
C
AII,C = W P Th
B = E-MORB
AII
Calc-alkali B,C
D = N-MORB
B
W ithin-plate D
C
C,D = VAB
C
D
Th
Ta
Zr
Y *3
Zr/4
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.
Y
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.
Ar-Ar dating of the Eel River rock is in progress.
GSA 2005 Cordilleran Section Meeting, San Jose, California
Japan (3)a
Accretionary
complex
44.37
2.01
4.58
12.56
0.18
24.77
7.63
0.39
0.11
0.35
Far East Russia (19)a
Accretionary
complex
44.14
1.57
6.60
14.36
0.23
26.34
5.90
0.62
0.16
0.08
Siberia (15)b
Continental
platform
41.38
1.93
2.34
12.87
0.19
36.0
4.69
0.049
0.21
0.25
TiO2/Al2O3
FeO*/MgO
K2O/Na2O
K2O/Al2O3
0.20
0.39
1.51
0.09
0.25
0.69
0.28
0.02
0.24
0.58
0.26
0.02
0.83
0.36
0.43
0.09
K (ppm)
Ti “
Cr “
Ni “
Y
“
Zr “
Nb “
4,646
7,016
930
782
9.4
58
15
912
12,060
1,423
1,134
18.6
138
25
1,300
9,300
2,157c
1,364c
16c
81c
13c
1,743
11,580
2,889
1,799
10.8
149
31
K/Ba
Zr/Nb
Nb/Th
La/Yb
Ti/Zr
Th/La
Zr/Y
Sr/Rb
La/Sm
Nb/Y
Nb/Zr
34
4.2
14.4
14.5
121
0.106
6.1
22
3.8
1.5
0.25
SiO2
Ti02
Al2O3
FeO*
MnO
MgO
CaO
Na2O
K2O
P2O5
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.
Data source: based on values of aSun and McDonough (1989); b Sun (1980); c Nui and O’Hara (2003); d Ewart et al. (1994)
Eel River (4)
Acccretionary
complex
42.78
1.17
5.84
12.18
0.19
30.84
4.48
0.37
0.56
0.23
1
5.6
6.2
87
115
7.4
5.1
1.3
0.18
0.81
0.16
10.4
4.9
10.6
0.5
78
0.047
14.2
36
4.9
2.9
0.21
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).
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.