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  1. Geochemical Features of Philippine Sea Plate Lithosphere and Implications for Early IBM Arc Formation Igneous rocks - Amami/Daito Province Daito Ridge basaltic & andesitic clasts in sediment (60 Ma) E-MORB Rosemary Hickey-Vargas, Department of Earth Sciences, Florida International University, Miami, Florida, USA (hickey@fiu.edu), Michael Bizimis, National High Magnetic Field Laboratory, Isotope Geochemistry, and Department of Geological Sciences, Florida State University, Tallahassee, Florida, USA (bizimis@magnet.fsu.edu), Ivan Savov, Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D.C., USA (savov@dtm.ciw.edu), Teruaki Ishii, IFREE/JAMSTEC, Yokohama, Japan (ishiit@jamstec.go.jp), 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 (fujioka@jamstec.go.jp) Amami Plateau basalts & tonalite (115 Ma) N-MORB I. Introduction: Ongoing geochronological, petrologic and geochemical studies of the Izu-Bonin-Mariana (IBM) subduction factory indicate that the proto-IBM 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. II. Results for the 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 pre-subduction sub-arc mantle wedge, rather than material transferred from the subducted crust, plot just within the field for Indian MORB (Fig. 3). When Nd-isotope 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). 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. IBM Proto-Arc Amami/Daito Province . Fig. 4: Tectonic reconstruction showing the location of the Amami Plateau and Daito Ridge in the Cretaceous, from Honza and Fujioka (2004). 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]. 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). West Philippine Basin basalt field Amami/Daito Province Amami/Daito Province 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. 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. 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) • IV. Suggested future directions to understand the Philippine Sea Plate and proto-IBM arc: • 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. • 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. Fig. 8: Initial Epsilon Nd versus Sm/Nd for basalts from the West Philippine Basin. Data sources are the same as in Fig. 7. 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 byNSF - 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