Transcript ppt - Margins Program

Geochemical Features of Philippine Sea Plate Lithosphere and
Implications for Early IBM Arc Formation
Rosemary Hickey-Vargas, Department of Earth Sciences, Florida International University, Miami, Florida, USA ([email protected]), Michael Bizimis, National High Magnetic
Field Laboratory, Isotope Geochemistry, and Department of Geological Sciences, Florida State University, Tallahassee, Florida, USA ([email protected]), Ivan Savov,
Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D.C., USA ([email protected]), Teruaki Ishii, IFREE/JAMSTEC, Yokohama, Japan
([email protected]), Kantaro Fujioka, Research Program for Plate Dynamics, Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and
Technology (IFREE, JAMSTEC), Yokohama, Japan ([email protected])
II. Results for the Amami/Daito Province:
West
Philippine
Basin basalts
Initial  Hf
20
Indian MORB field
16
12
Pacific MORB field
WPB MORB-like basalts
WPB OIB-like basalts
Central Basin Fault basalts
I/P line (Pearce et al. 1999)
8
4
4
6
8
10
Initial Nd
12
Fig. 7: Initial Epsilon Hf versus Initial Epsilon Nd for basalts from the
West Philippine Basin. Data are from Savov et al. (2005, J. Petrol),
Hickey-Vargas et at. (2006, AGU Geophysical Monograph 166), and
unpublished.
0.5132
West Philippine Basin basalts
Central Basin
Fault basalts
0.5130
Near
KPR
decreased melting
Initial
143
Nd/144Nd
0.5131
Depleted
MORB
source
mantle
0.5129
Source
mixing
0.5128
Plume?
Enriched source
Benham Rise
0.5127
0.15
0.25
0.35
0.45
100
Daito Ridge basaltic & andesitic
clasts in sediment (60 Ma)
E-MORB
10
Amami Plateau basalts & tonalite (115 Ma)
1
N-MORB
0
Rb Ba Th U Nb Ta K La Ce Pb Pr Sr Nd Zr Hf Sm Eu Gd Ti Dy Y Er Yb Lu
Fig. 2: Trace element abundances in igneous rocks from the Amami
Plateau and Daito Ridge, normalized to primitive mantle and compared with
N-MORB and E-MORB. Data from Hickey-Vargas, 2004, The Island Arc,
and unpublished.
25
Amami/Daito
Province
Initial  Hf
Amami/Daito
Province
IBM
Proto-Arc
Indian MORB field
20
West Philippine
Basin basalts
15
10
Pacific MORB field
Amami Plateau
Daito Basin
Amami corrected for slab input
Daito corrected for slab input
.
Pearce et al. 1999
I/P line
5
Fig. 4: Tectonic reconstruction showing the
location of the Amami Plateau and Daito
Ridge in the Cretaceous, from Honza and
Fujioka (2004).
0
Fig. 6: Tectonic reconstruction showing the
location of the Amami Plateau and Daito
Ridge, and the proto-IBM arc, in the middle
Eocene, from Honza and Fujioka (2004).
II. B. Comparison of IBM Proto-arc rocks and the Amami/Daito Province
Volcanic rocks from the IBM proto-arc, which are mostly boninites and low-K tholeiites, have
Nd and Hf isotopic compositions that overlap with fields for: 1) Indian MORB; 2) West
Philippine Basin basalts (see part III), and 3) arc rocks from the Amami/Daito Province (Fig.
5). While this could indicate that the IBM proto-arc and WPB basalts formed by melting of
“Indian” mantle wedge (i.e., mantle wedge that formed within the Indian Ocean domain), it
could be that arc magmas of the IBM proto-arc sampled old Amami/Daito arc wedge
lithosphere. The Amami/Daito Province was in proximity to the site of subduction initiation
(Fig. 6). Remelting of old arc wedge is especially plausible because the low-Ca, highly
depleted boninites characteristic of the proto-arc require harzburgitic mantle sources that
develop as the residue of arc magma extraction. In addition, an old arc wedge source for
low-Ca boninites can explain the apparent paradox of excess Hf and Zr compared with Nd
and Sm in boninites, and Indian (i.e., upper plate rather than subducted) Hf-Nd isotope ratios
[e.g., Pearce et al. 1999]. Fragments of old, low density arc lithosphere within the proto and
early -IBM arc may have caused magma to stall during ascent to the surface, thus enhancing
magmatic differentiation and generation of felsic crust generally [Reagan et al., submitted].
0
5
Initial Nd
15
24
Indian MORB
field
West Philippine
Basin basalt field
20
16
Amami/Daito
Province
12
IBM Northern Proto-Arc
IBM Proto-Arc DSDP 458&9
8
III. West Philippine Basin
All basalts from the West Philippine Basin plot in the field of Indian MORB on a plot of
Epsilon Hf versus Epsilon Nd (Fig. 7), as well as on Pb-isotope plots (not shown). Basalts
from the West Philippine Basin floor just west of the Kyushu-Palau Ridge at ODP Site 1201
(Fig. 1; 49 Ma?) and DSDP Site 447 (Fig. 1; 45 Ma?) are highly depleted in fluid-immobile
incompatible elements (Nb, Th, La), and have low Sm/Nd ratios (Fig. 8). Basalts with
enriched, OIB-like trace element and isotopic compositions are scattered throughout the
western parts of the WPB, most notably at the Benham Rise. The data suggest that a
plume or enriched magma source existed in the opening WPB, but that it is unlikely that it
affected the part of the plate near the early IBM arc. MacPherson and Hall (2001) proposed
that a mantle plume in the WPB provided heat and initiated melting of the harzburgitic
mantle source of boninites (Fig. 9). However, models of IBM arc initiation in which fore-arc
spreading and eruption of boninites and low-K tholeiitic basalts accompany the initial
subsidence of the Pacific Plate beneath the edge of the PSP [e.g., Hall et al., 2002; Ishizuka
et al., 2006; Reagan et al., this conference) are more consistent with the depleted
composition of contemporaneous eastern WPB basalts.
Basalts from the Central Basin Fault (CBF) of the West Philippine Basin have ages as
young as 28-35 Ma, and therefore overlap in age with the last stages of arc volcanism on
the early IBM arc prior to rifting and opening of the Shikoku and Parece Vela Basins. Trace
element characteristics of CBF basalts are highly variable, ranging from normal-MORB-like
to incompatible element enriched OIB-like compositions (Fig. 8), although isotope ratios do
not overlap with other West Philippine Basin OIB-like basalts (Figs. 7 and 8). Subduction of
the Pacific Plate beneath this spreading center apparently did not affect the composition of
the erupted basaltic magma.
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Fig. 3: Initial Epsilon Hf versus Initial Epsilon Nd for igneous rocks from
the Amami/Daito Province compared with Indian and Pacific MORB and
basalts from the West Philippine Basin (see III). Open symbols are
Epsilon Nd values corrected for the contribution of subducted materials to
the mantle wedge and represent the pre-subduction mantle source for the
rocks. Data are from Hickey-Vargas et al. (submitted to EPSL).
Initial  Hf
24
Igneous rocks - Amami/Daito Province
A. Igneous rocks dredged from the Amami Plateau and drilled at DSDP Site 445 on
the Daito Ridge have typical island arc trace element features, such as HFSE
(high field strength element) depletion and LILE (large ion lithophile element)
enrichment relative to REE (rare earth elements) (Fig. 2). The rocks have
moderate depletion of HFSE Zr and Hf compared with REE Nd and Sm (Fig. 2).
Nd and Hf isotopic compositions, which are dominated by contributions from presubduction sub-arc mantle wedge, rather than material transferred from the
subducted crust, plot just within the field for Indian MORB (Fig. 3). When Ndisotope ratios are corrected for input from the slab, using the algorithms of
Pearce et al. (1999), based on the deficiency of Hf relative to Nd, isotope ratios
shift into the field for Pacific MORB (Fig. 3). This means that these arc rocks
may have had a Pacific MORB-type mantle source prior to modification by
subduction zone fluids. This result is broadly consistent with the tectonic model
of Honza and Fujioka (2004), which has the Amami Plateau and Daito Ridge as
a southwest facing arc on a Pacific plate during the Cretaceous (Fig. 4).
Purple:
Amami/Daito
Province island arcs
Red: West
Philippine
Basin MORBlike basalts
Orange: West
Philippine
Basin OIB-like
basalts
Dark orange:
Central Basin
Fault basalts
Green:
IBM Protoarc sites
Fig. 1: Map of Philippine Sea Plate
1000
Rock/primitive mantle
I. Introduction:
Ongoing geochronological, petrologic and geochemical studies of the
Izu-Bonin-Mariana (IBM) subduction factory indicate that the protoIBM arc began to form at about 50 Ma along a margin of the Philippine
Sea Plate (PSP) which is now marked by the Kyushu Palau Ridge.
We conducted parallel studies of rocks from the PSP that are older
than, or contemporaneous with the early IBM arc, in order to
understand the nature of the lithosphere on which the arc formed.
Locations shown on the map are: 1) the Amami/Daito Province
(purple, which are early Cretaceous to Paleocene island arcs; 2) the
West Philippine Basin (WPB: red and orange dots), which opened
between 60 to 35 Ma; and 3) the Central Basin Fault (deep orange
dots), a late stage feature (35-28? Ma). Red dots show locations
where depleted, N-MORB-like WPB basalts were found, orange dots
are locations where enriched, E-MORB or OIB-like WPB basalts were
found. Bright green dots are locations where proto-IBM arc volcanic
rocks have been described.
Pacific
MORB field
IBM Proto-Arc Guam
I/P line (Pearce et al. 1999)
4
4
6
8
Initial Nd
10
Fig. 5: Initial Epsilon Hf versus Initial Epsilon Nd for volcanic rocks from
the proto-IBM arc compared with values for the Amami/Daito Province,
corrected to 50 Ma. Data for proto-IBM arc rocks are from Pearce et al.
(1999) and Reagan et al. (this conference).
Fig. 9: Tectonic
reconstruction
showing the location of
the Central Basin
spreading center
(CBSC), the proto-IBM
arc and a possible
plume in the West
Philippine Basin at 50
Ma, after Deschamps
and Lallemand (2002)
Sm/Nd
Fig. 8: Initial Epsilon Nd versus Sm/Nd for basalts from the West
Philippine Basin. Data sources are the same as in Fig. 7.
IV. Suggested future directions to understand the Philippine Sea Plate and proto-IBM arc:
1) Exploration of the Amami-Sankaku Basin (IBM Complex Drilling Proposal, Site #1) to learn more about the origin and history of the Amami/Daito Province,
and the nature of the pre-subduction upper plate lithosphere.
2) Focused study of Mariana trench wall sequences and deep drilling in the IBM forearc, to better understand the IBM arc basement.
3) Exploration of the juncture of the CBF and KPR, to understand the relationship between the Central Basin spreading center and the early IBM arc.
References
Deschamps, A., S. Lallemand (2002) The West Philippine Basin: an Eocene to Early Oligocene back- arc basin opened between two opposed subduction zones, Jour. Geophys. Res. 107, 2322-2346.
Hall, C.E., M. Gurnis, M. Sdrolias, L.L. Lavier, R.D. Muellar, R.D. (2002) Catastrophic initiation of subduction following forced convergence across fracture zones, Earth Planet. Sci. Lett, 212, 15-30.
Honza, E. and K. Fujioka (2004) Formation of arcs and backarc basins inferred from the tectonic evolution of Southeast Asia since the Late Cretaceous, Tectonophysics 384, 23– 53.
Ishizuka, O., Kimura, J-I., Li, Y.B, Stern, R.J., Reagan, M.K., Taylor, R.N., Ohara, Y., Bloomer, S.H., Ishii, T., Hargrove III, U.S., and Haraguchi, S. (2006) Early stages in the evolution of Izu–Bonin arc volcanism: New age, chemical, and isotopic constraints, Earth Planet. Sci. Lett. 250, 385-401.
Macpherson, C.G., and R. Hall (2001) Tectonic setting of Eocene boninite magmatism in the Izu- Bonin- Mariana forearc, Earth Planet Sci. Lett., 186, 215- 230.
Pearce, J.A., P.D. Kempton, G.M. Nowell, and S.R. Noble (1999) Hf-Nd element and isotope perspective on the nature and provenance of mantle and subduction components in western Pacific arc-basin systems. J. Petrol., 40, 1579-1611, 1999.
Reagan, M.K., Hanan, B.B., Hartman, B.S., Heizler, M.T., and Hickey-Vargas, R. Petrogenesis of volcanic rocks fromSaipan and Rota, Mariana Islands and implications for the evolution of nascent island arcs, (submitted to J. Petrol.)
Reagan, M.K., Ishizuka, O., Hanan, B.B., Hickey-Vargas, R., Heizler, M.T., Woods, M., Kimura, J-I., Ohara, Y., Stern, R.J., Fryer, P., Bloomer, S. (this conference) The petrology, geochemistry, geochronology, and geodynamic setting of early arc magmas from the Mariana fore-arc.
Research by R. Hickey-Vargas was supported by
NSF - OCE 0001826: MARGINS: Collaborative Research: Magma Generation in the early Mariana Arc system revisited
NSF - OCE 0074868: POWRE: Ridge Meets Margin: Geochemical Effects of Subduction on the Central West Philippine Basin Spreading Center (also M. Bizimis)
NSF - OCE 9314400: Tectonic evolution of the West Philippine basin: Evidence from Geochemical Studies
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