origin of diverse geochemical signatures in igneous rocks from the west philippine basin:...

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Origin of Diverse Geochemical Signatures in Igneous Rocks From the West Philippine Basin:

Implications for Tectonic Models

Rosemary Hickey-Vargas1, Ivan P. Savov2, Michael Bizimis3, Teruaki Ishii4, and Kantaro Fujioka5

The West Philippine Basin (WPB), formed by seafloor spreading between 60 and 35 Ma, provides an excellent case study of relationships between basin tectonics and magma chemistry. At 48 Ma, the Izu-Bonin-Mariana (IBM) arc formed along the basin edge, orthogonal to the active spreading center; thus, WPB development is a key issue for this Margins Subduction Factory focus area. WPB basalts from the main spreading stage are normal to enriched mid-ocean ridge basalt (MORB) with an Indian Ocean MORB isotopic signature. Basalts from the Benham Rise and locations near the western Central Basin Spreading Center (CBSC) at 50–35 Ma are geochemically identical to oceanic island basalts. Late-stage CBSC basalts (35–26 Ma) are isotopically like main spreading-stage MORB, with widely varying and decoupled trace element enrichments. Based on basalt geochemistry, the WPB could be a trapped fragment of ancient Indian/Tethyan ocean ridge, as proposed in some models for the initiation of the IBM arc, or it could be a back-arc basin, provided plate configurations allowed replenishment of sub-Indian Ocean asthenosphere. Ocean island basalts were formed by decompression melting of an enriched source beneath the western CBSC, mixing with normal MORB sources to form enriched MORB. This was a transitory feature (15 Ma) related to spreading, rather than a deep-seated plume, and probably did not affect the early IBM arc. Magma formed in small, deep-seated batches as the extension waned. That CBSC activity continued for 22 Ma after the initiation of the IBM arc indicates that forces related to an additional subduction system influenced the WPB.

1. INTRODUCTION

The West Philippine Basin (WPB) is a well-preserved, extinct, marginal basin formed by seaf loor spreading (Figure 1). The tectonic history of the basin, from the onset of opening through active seaf loor spreading to the waning and cessation of spreading, is understood to various extents for each stage of development, but many interesting puzzles remain to be solved. One important controversy centers on whether the WPB spreading cen-ter originated as a trapped segment of Tethyan/Indian mid-oceanic ridge [Uyeda and Ben Avraham, 1972; Stern, 2004] or alternatively by back-arc spreading within a complex of Cretaceous–Paleocene island arcs

Back-Arc Spreading Systems: Geological, Biological, Chemical, and Physical InteractionsGeophysical Monograph Series 166Copyright 2006 by the American Geophysical Union10.1029/166GM15

1 Department of Earth Sciences, Florida International University, Miami, Florida, USA.

2 Department of Terrestrial Magnetism, Carnegie Institution of Washington, D.C., USA.

3 National High Magnetic Field Laboratory, Isotope Geochemistry, and Department of Geological Sciences, Florida State University, Tallahassee, Florida, USA.

4 Ocean Research Institute, University of Tokyo, Tokyo, Japan.5 Research Program for Plate Dynamics, Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan.

288 CASE STUDY: WEST PHILIPPINE BASIN

[Hall et al., 1995; Deschamps and Lallemand, 2002]. In either case, the present-day eastern edge of the basin was the locus along which the early Izu-Bonin-Mariana (IBM) arc formed. The IBM arc is a focus area for the Margins Subduction Factory Initiative, the objectives of which require an understanding of the tectonomag-matic history of the WPB. Knowledge of WPB history is essential for understanding how subduction was initiated and how subduction magmatism began. Additionally, since WPB lithosphere underlies the present- day IBM arc, its origin and resulting composition and structure inf luence the present-day outputs of the IBM subduction factory. An objective of the Subduction Factory Initiative for the IBM focus area is to understand the mass bal-ance among subduction inputs and outputs, for which the presubduction condition of the sub-arc wedge is an essential parameter.

The geochemical composition of magma extracted from the mantle at spreading centers can yield information

about mantle source materials as well as about melting and extraction processes. When the compositions of marginal basin magmas conform to geochemical trends found in mid-ocean ridge basalts (MORBs) worldwide [e.g., Klein and Langmuir, 1987; Klein et al., 1988] and to oceanic island basalts (OIBs) worldwide [e.g., Zindler and Hart, 1986; Hart et al., 1992], these findings indicate that the magmas have sources and processes that are not specifi-cally linked to a subduction-zone setting. In contrast, the geochemical signature of local subduction inputs dis-tinguishes many back-arc basin basalts (BABBs) from MORBs [e.g., Taylor and Martinez, 2003]. In this paper, we use the WPB as a case study to examine the relation-ship between the tectonic development of a spreading cen-ter and the geochemical character of the magma formed there. We also use geochemical information about the WPB basalts to refine and test competing tectonic models for the development of this basin and their implications for the early IBM arc.

Figure 1. Map of the West Philippine Basin showing locations mentioned in the text. Sites where basalts have been recovered and analyzed are marked with dots. The area depicted in Figure 3 is outlined. Magnetic anomalies assign-ments are from Hilde and Lee [1984]. Ages for normal polarity intervals are 20 (43.8–42.5 Ma), 21 (47.9–46.3 Ma), 22 (49.7–49.0 Ma), 23 (51.7–50.8 Ma), 24 (53.3–52.4 Ma), and 26 (57.9–57.5 Ma) from Cande and Kent [1995].

HICKEY-VARGAS ET AL. 289

2. TECTONIC DEVELOPMENT

The West Philippine Basin was explored on Legs 6, 31, and 58 of the Deep Sea Drilling Project (DSDP); more-over, together with the younger but extinct Parece Vela and Shikoku Basins and the active Mariana Trough (Figure 1), the WPB was one of the type locations used by Karig [1971, 1975] to describe the concept of successive arc rifting and back-arc spreading. In contrast to the younger basins, how-ever, there was and still is considerable controversy concern-ing whether the WPB opened as a back-arc basin or whether it formed as a segment of normal mid-ocean ridge that was trapped by the onset of subduction along bounding transform faults (Figure 2).

The latter interpretation was first proposed by Uyeda and Ben Avraham [1972], who noted that the WPB has greater depth and lower heat flow than adjacent back-arc basins, which is more appropriate for old oceanic crust. Additional evidence is that the Central Basin Fault (CBF), which includes the now-extinct Central Basin Spreading Center (CBSC), is nearly orthogonal to the Kyushu Palau Ridge, the remnant arc marking the original trace of the modern IBM arc (Figures 1 and 2). The proposed cause for the initiation of subduction and entrapment is the change in the direction of Pacific plate motion from N to WNW at about 45 Ma, as indicated by the bend in the Hawaii–Emperor seamount chain. According to Uyeda and Ben Avraham [1972], this shift caused convergence along a transform margin connect-ing two ridge segments. Older oceanic lithosphere to the east began to subduct beneath younger lithosphere to the west, trapping a ridge segment that became the CBSC (Figure 2a) and forming the early IBM arc along the former trans-form. Most recently Stern [2004] has developed this concept further. A compilation of dates for igneous rocks from the proto-IBM arc estimates the onset of IBM arc magmatism to be between 55 and 48 Ma [Cosca et al., 1998].

The age of the WPB was established by Hilde and Lee [1984], who interpreted magnetic anomaly data and con-cluded that the basin had opened between about 60 to 35 Ma, with a rapid spreading stage from 60 to 45 Ma, fol-lowed by a slow-spreading stage with a different orientation from 45 to 35 Ma (Figure 1). Hilde and Lee [1984] also identified magnetic anomalies corresponding to the late Paleocene–Early Eocene in the Daito Basin and surrounding areas in the northernmost WPB and included this section in the syntheses of basin opening and spreading. Exploration and sampling of this area of the WPB, including the Daito and Oki-Daito Ridges (Figure 1) by the DSDP and Japanese Geodynamics Project recovered some igneous lithologies, predominantly basaltic andesites, andesites, and tuffs of probable island-arc origin, together with tonalite, gabbro, and

some metamorphic rocks [Karig, 1975; Shiki et al., 1985]. Ages of dredged arc rocks are Cretaceous to Eocene [Shiki et al., 1985]. Recent detailed bathymetric and magnetic study of the WPB has shown that the spreading fabric and magnetic properties of the basin floor south of the Oki-Daito Ridge, extending southward to the Oki-Daito escarpment (Figure 1), are aligned in N-S direction, unlike the NW-SE fabric of the central basin [Okino et al., 1999]. Thus, the north-ern section of the WPB has an unclear relationship to the extinct CBSC. Some researchers who favor a back-arc basin origin for the WPB propose that it opened behind remnant arcs now marked by the Oki-Daito or Daito Ridges or both

Fig. 2Fig. 2

Figure 2. Contrasting reconstructions of early West Philippine Basin formation. (a) Entrapment of a section of spreading mid-ocean ridge after the initiation of subduction along a transform fault [after Uyeda and Ben Avraham, 1972]; (b) opening as a back-arc basin between two opposed subduction zones [after Deschamps and Lallemand, 2002].


[Karig, 1975; Seno and Maruyama, 1984] or the northern-most Kyushu Palau Ridge [Seno and Maruyama, 1984; Hall et al., 1995; Honza and Fujioka, 2004]. Basin floor north of the Oki-Daito escarpment may have formed at a different spreading center, it could be related to the older Oki-Daito arc complex, or it may be a section of mid-ocean seafloor trapped with older arc crust.

Based on these observations, the WPB can be divided into two regions according to age and character (Figure 1): (1) the Central Basin area, which clearly formed by seafloor spread-ing from the CBSC between about 60 and 35 Ma, and (2) the section north of the Oki-Daito escarpment, which has a dis-tinct spreading fabric, and includes the Oki-Daito and Daito Ridges and Amami Plateau, which have island-arc lithologies of Eocene to Cretaceous age.

At some point during the main spreading stage, an addi-tional source of magma developed at the western end of the WPB spreading center and produced the Benham Rise (Figure 1). Based on magnetic anomaly assignments, biostratigra-phy, and 40Ar/39Ar dating, the age of this large (40,000 km3) feature is 49 Ma [Ozima et al., 1977; Hilde and Lee, 1984]. Younger ages of 37–36 Ma were obtained by 40Ar/39Ar dating of plagioclase in basalt [Hickey-Vargas, 1998a], which could indicate continued magmatism, or a later resetting event. A chain of smaller seamounts oriented NE-SW stretches from

the Benham Rise to just southwest of the CBF (Figure 3) and may have formed as seafloor spreading displaced the Benham Rise away from the CBSC. There is an analogous topographic high on the northeast side of the CBF, the Urdaneta plateau (Figure 1), which has been explored and sampled by dredging (Ohara, personal communication, 2005). Hall et al. [1995] and Deschamps and Lallemand [2002] include this excess magma source in their tectonic reconstruction of the basin and propose that it disrupted normal spreading in the western part of the basin and complicated interpretation of seafloor magnetic anomalies.

The CBF, which includes the now extinct CBSC, is nearly orthogonal to the Kyushu Palau Ridge. This unusual orienta-tion is one of the premises that supports the trapped ridge scenario for formation of the WPB. Detailed study of the fault zone in 1996 [Fujioka et al., 1999b; Deschamps et al., 1999, 2002] revealed a striking sequence in the latest stages of spreading. At about 35 Ma, true seafloor spreading ceased and was replaced by deformation and amagmatic extension of the former ridge area. Small amounts of magma erupted during this stage, such that ages as young as 28–26 Ma are obtained from fault zone basalts [Fujioka et al., 1999b]. During and after this stage, the entire ridge was disrupted by NW-SE shearing, which resulted in crustal thinning and some lateral offset of the former ridge crest rift valley from

Fig. 3Fig. 3

Figure 3. Bathymetry of the Central Basin Fault. Locations of samples described in the text are shown. Areas with depths greater than 6000 m are shaded grey. Map is from Fujioka et al. [1999a].


the present-day fault trace. The end of the first phase of IBM arc volcanism is estimated to be about 27 Ma [Meijer et al., 1983; Lee et al., 1995] and the opening of the Parece Vela and Shikoku basins is dated at 29–15 Ma [Okino et al.; 1998]. Thus, the last stages of WPB activity are coincident with these events. With these dates, it is clear that the early IBM arc and the CBSC coexisted, in a near orthogonal orientation, from the initiation of subduction at 55–48 Ma through the end of CBSC activity at 26 Ma.

For the purposes of geochemical discussion we define three stages of magmatic activity in the WPB:

1. Magmatism associated with the main spreading stage of WPB seafloor growth and defined by magnetic anomalies (60–35 Ma)

2. Magmatism associated with an excess magma source or hotspot (50–35 Ma)

3. Late-stage magmatism associated with the CBF and amag-matic extension and deformation of the WPB (35–26 Ma).

3. GEOCHEMICAL CHARACTER OF BASALTS

Early geochemical work on BABB showed that it is similar to MORB but enriched in some highly incompatible elements [Hart et al., 1972]. Small enrichments of fluid-mobile incom-patible elements—B, Cs, Rb, Ba, Sr, Pb, and U, and H2O—are well documented in fresh BABB from active basins such as the Mariana Trough and Lau Basin, and are attributed to incorpo-ration of subducted materials in the BABB source [Hawkins and Melchior, 1985; Hawkins et al., 1990; Hochstaedter et al., 1990; Danyushevsky et al., 1993; Stolper and Newman, 1994; Gribble et al., 1996, 1998; Newman et al., 2000; Taylor and Martinez, 2003] (Figure 4a). Systematic isotopic differ-ences between MORB and BABB are not uniform, although systems that show large enrichments in H2O and fluid-mobile elements, bordering on the character of related arc magmas, also trend toward arc-magma-like 87Sr/86Sr, 143Nd/144Nd. and Pb-isotope ratios (Figure 5). In some cases (for example, basalts from the Sumisu rift of the northern IBM arc), there is a subdued depletion in Nb and Ta relative to rare earth elements (REE) similar to that in arc basalts [Hochstaedter et al., 1990, 2001] (Figure 4a). In their comparative study of geochemical trends in basalts from many active back-arc basins, Taylor and Martinez [2003] propose that the subduc-tion signature diminishes as seafloor spreading progressively moves the back-arc spreading center away from the trench and subducted lithosphere.

3.1. WPB Basalts From the Main Spreading Stage

Basalts that were formed during the main spreading stage (60–35 Ma) were cored at DSDP Sites 291 and 447

and Ocean Drilling Program (ODP) Site 1201 (Figure 1). Magnetic anomaly ages of these sites are 49, 44, and 49 Ma, respectively [Hilde and Lee, 1984]. The basalts have trace element characteristics ranging from highly depleted normal MORB (N-MORB) to enriched MORB (E-MORB) (Figure 4b). The core from Site 447 was unusual in that it contained well-preserved volcanic glass [Hickey-Vargas, 1991, 1998a]. The glasses are N-MORB-like and lack con-spicuous enrichment in fluid-mobile elements such as Rb, K, U, and Pb (Figure 4b). Holocrystalline basalts were sampled at DSDP Site 291 and ODP Site 1201, and these basalts are generally more altered [Hickey-Vargas 1991, 1998a; Savov et al., 2005]. They display variable enrichment in f luid-mobile elements such as Rb, K, U, and Pb, which could result from either posteruptive seawater alteration or enrichment of magma sources by subducted materials (Figure 4b). In general, the wide variation in abundances of these elements among samples from the same unit; for example, U at Site 1201 (Figure 4b) argues for enrichment of these elements through posteruptive alteration. In addition, estimated H2O contents for these rocks are higher than those found in fresh BABBs from active basins [Hickey-Vargas, 1998a; Savov et al., 2005]. Basalts from DSDP Site 447 and ODP Site 1201 are light REE (LREE)-depleted, like N-MORBs (Figures 4b and 8). Basalts from DSDP Site 291 are enriched in LREE, high-field-strength elements (HFSE: Nb, Ta, Th), and fluid-mobile elements (Rb, Ba, U, Pb, K). These are identical to E-MORBs (Figures 4b and 8).

Isotopically, all WPB MORB-like basalts analyzed to date from the main spreading stage have isotopic characteristics resembling those of MORB from the Indian Ocean and unlike those of Pacific Ocean MORB. These characteris-tics are well-established based on their higher 208Pb/204Pb for a given 206Pb/204Pb (Figure 5b) and lower 143Nd/144Nd for a given 206Pb/204Pb (Figure 5c) compared with Pacific MORB [Hickey-Vargas, 1991, 1998a; Hickey-Vargas et al., 1995]. MORBs from Indian and Pacific Oceans can also be distinguished on a plot of 176Hf/177Hf versus 143Nd/144Nd [Pearce et al., 1999; Woodhead et al., 2001], which shows higher 176Hf/177Hf for a given 143Nd/144Nd in the Indian MORB samples. New Hf-isotope data for WPB MORB-like basalts from DSDP Sites 447 and 291 (Table 1) and ODP Site 1201 [Savov et al., 2005] have an Indian Ocean signature, as does a basalt from DSDP Site 447, reported by Pearce et al. [1999] (Figure 5d). Several authors have argued that the Indian Ocean character of WPB basalt reflects the origin of the basin within the geographic extent of an Indian Ocean asthenospheric (MORB-source) domain [Hickey-Vargas, 1998a; Pearce et al., 1999]. Among present-day MORB, the contact of Indian and Pacific MORB sources is observed along the Southeast Indian Ridge at the Australian–Antarctic

Fig. 4Fig. 4

Fig. 5Fig. 5

Table 1Table 1


Discordance [Klein et al., 1988; Pyle et al., 1992] and along the southern Mid-Atlantic Ridge [Pearce et al., 1999].

3.2. WPB Basalts From an Excess Magma Source

Basalts were recovered from the Benham Rise at DSDP Site 292 (Figure 1). These basalts have trace element and isotopic characteristics that are indistinguishable from the array of OIB erupted worldwide (Figures 5 and 6; Hickey-Vargas, 1998a, 1998b]. In terms of global OIB isotopic end-members, the Benham Rise basalts lie between predominant mantle (PREMA) and EM2 [Zindler and Hart, 1986]. A possible conclusion is that the Benham Rise magma source

was a mantle plume or hotspot, which is consistent with both the chemical composition of the lavas and the excess magma production.

Preliminary geochemical results for basalts recovered from the Urdaneta Plateau show that they are also OIB-like (Ohara, personal communication, 2005). There are several other locations within the WPB where OIB-like basalt has been recovered, but none is associated with a well-defined seamount or other bathymetric high. These location are as follows (Figure 1): (1) a series of sills in sediments within the Daito Basin, cored at DSDP Site 446; (2) basalt drilled at DSDP Site 294 just south of the Oki-Daito Ridge; and (3) Japanese Geodynamics Project dredge Site 21-5 on the Oki-

Figure 4. (a) Normalized trace element abundances in average N-MORB, E-MORB, and BABBs from the Mariana Trough and Sumisu Rift. BABB data are from Hochstaedter et al. [1990, 2001], Gill et al. [1992], Hawkins and Melchior [1985], and Hawkins et al. [1990]; (b) normalized trace element abundances in basalts from the WPB normal spreading stage. Data are from Hickey-Vargas [1998a] and Savov et al. [2005]. Primitive mantle normalizing values and values for average N-MORB and E-MORB are from Sun and McDonough [1989].


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Daito Ridge [Aoki and Ishikawa, 1985; Hickey-Vargas, 1996]. Ages of these basalts are not well established. Ozima et al. [1980] reported whole rock 40Ar/39Ar dates of 57–54 Ma for Site 446 basaltic sills, older than the enclosing early middle Eocene sediments, and Hickey-Vargas [1998a] reported pla-gioclase 40Ar/39Ar ages of 43 and 50 Ma. For Site 294, Ozima et al. [1977] reported a whole rock 40Ar/39Ar date of 49 Ma, consistent with the age of overlying sediments. No ages are reported for Oki-Daito Ridge Site 21-5. As noted earlier, samples recovered from other dredge sites on the Oki-Daito Ridge have diverse lithologies, including arc basalts, andes-ites and dacites, gabbro, and tonalite; thus, the occurrence of OIB at one site does not imply that the Oki-Daito Ridge consists entirely of OIB.

Trace element data for basalts from all OIB-like locations define at least two groups or series. For example, basalts from the Daito Basin sill sequence define distinct tholeiitic and alkaline series [Marsh et al., 1980]. Major and trace element characteristics of the tholeiitic series are nearly identical to those of Benham Rise basalts (Figure 6a). Basalts of the alka-line series have higher LREE/HREE and lower HREE than the tholeiites, and are probably formed by smaller extents of melting of more garnet-rich sources, with some input from the lithospheric mantle [Hickey-Vargas, 1998b]. OIB-like basalts from other areas are most similar to the Daito Basin alkaline basalts (Figure 6b), although the smaller recovery and higher degree of alteration of rock from these sites make a match of geochemical character more difficult. On isotope

plots, WPB OIB-like basalts from the Benham Rise and other sites form a coherent group, with few systematic differences (Figure 5). In addition, on all isotope plots, E-MORBs from DSDP Site 291 plot between N-MORBs from Sites 447 and 1201 and WPB OIB-like basalts (Figure 5).

3.3. Late-Stage WPB Basalts

The CBF coincides roughly with the trace of the extinct and now disrupted spreading ridge that formed the WPB floor [Fujioka et al., 1999b]. Basalts from this area represent the final stages of magma extrusion related to basin exten-sion. Major, trace element, and isotopic characteristics of basalts recovered from sites along the CBF, ranging from 600 to 100 km from the Kyushu Palau Ridge, are reported in Sato et al. [2001] and Hickey-Vargas et al. [2006]. Trace element signatures of these basalts are extremely diverse (Figures 7 and 8). Most have N-MORB to E-MORB trace element signatures, as do basalts from the main spreading stage, but at least two other groups can be defined (Figures 5, 7, and 8): (1) basalts from the site closest to the Kyushu Palau Ridge (KR98-01 D3, Figure 3) are depleted in Nb and Ta, and enriched in fluid-mobile elements, like BABB; and (2) basalts with highly enriched, OIB-like signatures are found at Site 334 (Figure 3).

A surprising feature of these basalts is that the isotopic characteristics are not correlated with their trace element character. Isotopic ratios for all of the basalts are within the

Fig. 6Fig. 6

Table 1. Hf isotope data for basalts from the West Philippine Basin

Samplea 176Hf/177Hf b (2σ of mean) Age (Ma)c Lu/Hf 176Hf/177Hfi

447A-25-1 (9-11) 0.283314 (0.000011) 44 0.297 0.283278

291-5-1 (138-140) 0.283152 (0.000008) 43 0.235 0.283123

292-42-3 (144-146) 0.283081 (0.000010) 36 0.118 0.283069

446A-4-2 (18-20) 0.283002 (0.000006) 50 0.070 0.282992

446A-12-4 (34-36) 0.283107 (0.000013) 50 0.057 0.283099aMajor and trace element data for these samples were reported in Hickey-Vargas [1998a, 1998b]. Pb-isotope data for all samples were reported in Hickey-Vargas [1991, 1998a]. Sr and Nd isotope data, except for sample 447A-25-1, were reported in Hickey-Vargas [1991, 1998a]. Newly measured ratios for sample 447A-25-1, using techniques

of Bizimis et al. [2004], are 87Sr/86Sr = 0.702826 (2σ of mean = 0.000007) relative to 87Sr/86Sr = 0.708000 for the

E&A Sr standard, and 143Nd/144Nd = 0.513147 (2σ of mean = 0.000011) relative to 143Nd/144Nd = 0.511850 for the La Jolla Nd standard. bSamples were prepared using the dissolution and separation techniques described in Bizimis et al. [2004], and measurements were made by using the hot-SIMS technique at the National High Magnetic Field Lab, Florida State University, following methods described by Bizimis et al. [2004] and references therein. The JMC 475 Hf standard was measured at 176Hf/177Hf = 0.282199 + 22 (2 S.D., n = 15) and values are reported relative to the accepted value of 0.282160. Measured total Hf blanks were 20 pg and are negligible for the amount of Hf present in the analyzed samples. cAges for samples from Site 291, 292, and 446A are 40Ar/39Ar dates determined for basalts from these sites reported in Hickey-Vargas [1998a]. The age used for sample 447A-25-1 is the magnetic anomaly age from Hilde and Lee [1984].


range of WPB basalts from the main spreading stage (Figure 5). The OIB-like basalts have Sr, Nd, Pb, and Hf isotope ratios between those of N-MORB and E-MORB from sites formed during the main spreading stage but are much more enriched in incompatible elements (Figures 7 and 8). Basalts with BABB trace element characteristics have Sr, Nd, and Hf isotope ratios like those of main spreading stage basalts and are distinguished only by a lower 206Pb/204Pb compared with other late-stage CBF basalts (Figures 5b and 5c).

In summary, the geochemical characteristics of basalts from the different magmatic stages of WPB development are as follows:

1. Basalts from the main spreading stage are like N-MORB to E-MORB with Indian Ocean isotopic characteristics, and they lack the enrichment features observed in BABB from active basins.

2. Basalts from the Benham Rise have OIB-like trace element and isotopic characteristics, as do basalts from

scattered locations not associated with topographic highs, including sills in the Daito Basin and locations on and to the south of the western Oki-Daito Ridge.

3. Basalts from the late stages of basin opening have trace element characteristics that run the gamut from BABB to N-MORB, E-MORB, and OIB, but with a limited range of isotopic characteristics, similar to the N-MORB to E-MORB formed during the main spreading stage. Samples recovered from the CBF near the Kyushu Palau Ridge are the only WPB basalts with discernable BABB features.

4. DISCUSSION

4.1. How Did Opening and Spreading Begin in the West Philippine Basin?

An ongoing controversy about the WPB is whether it formed by “trapping” a normal mid-ocean ridge segment

Figure 6. Normalized trace element abundances in WPB OIB. (a) Basalt from the Benham Rise and tholeiitic basalt from the Daito Basin; (b) alkali basalt from the Daito Basin, basalts from Sites 294 and 21-5. Data are from Hickey-Vargas [1998a, 1998b]. Primitive mantle normalizing values and values for average OIB and E-MORB are from Sun and McDonough [1989].


(Figure 2a) or whether it opened as a back-arc basin (Figure 2b). The trapping hypothesis, originally proposed by Uyeda and Ben Avraham [1972] and more recently developed by Stern and Bloomer [1992] and Stern [2004], is supported by a number of observations: (1) the spreading center is orthogonal to the most conspicuous remnant arc in the region, the Kyushu Palau Ridge; (2) the WPB is deeper and colder than other back-arc basins and its lithosphere is more typical of ocean floor lithosphere; (3) the basin is larger than other back-arc basins; (4) the onset of volcanism along the proto-IBM arc roughly coincides in time with a westward shift in Pacific Plate motion inferred from the bend in the Hawaii–Emperor seamount chain and provid-ing a mechanism for entrapment; and (5) the entrapment and subduction initiation scenario provides a mechanism for the generation of boninites in the earliest stages of IBM arc history [Stern and Bloomer, 1992; Stern, 2004]. Eruption of boninite, an unusual hydrous, intermediate-SiO2, high-MgO magma, was widespread along the early IBM arc, and possible explanations for this occurrence are an important aspect of tectonic models for IBM arc initia-tion. According to Stern [2004] and based on geodynamic models of Hall et al. [2003], subsidence of the older ocean lithosphere along the transform fault would be accompanied by upwelling and decompression of hot, hydrated astheno-sphere, producing conditions necessary for the generation of boninite magma.

Since the original proposal of Uyeda and Ben Avraham [1972], the identity of the ridge that was trapped has been difficult to explain, particularly after interpretation of paleo-magnetic data indicated that the CBSC was oriented NE-SW before 45 Ma and that the Philippine Sea plate had rotated clockwise to its present NW-SE orientation [Hall et al., 1995]. This rotation means that the CBSC was oriented SW-NE, and that associated NW-SE–oriented transform faults would not be strongly affected by the shift to WNW Pacific plate motion at 45 Ma. Possible candidates for the trapped ridge are the Kula–Pacific Ridge [Uyeda and Ben Avraham, 1972], the North New Guinea–Pacific Ridge [Seno and Maruyama, 1984], or other ridges connecting the Tethyan and Pacific Oceans [Wells, 1989; Stern, 2004]. Recent pro-posals that the bend in the Hawaii–Emperor seamount chain does not reflect a change in Pacific plate motion so much as a southward drift of the Hawaiian hotspot [e.g., Tarduno et al., 2003] diminish arguments for entrapment based on the coincidence between the initiation of IBM arc subduction zone magmatism and Pacific plate motion. Nonetheless, the reason for the unusual oblique orientation of the CBSC relative to the Kyushu Palau Ridge, as well as other features of the WPB such as its depth, size, and temperature, are explained by the entrapment scenario.

Samples of WPB seafloor collected for geochemical anal-yses were found to have formed during the basin’s main spreading stage and spanned about 5 Ma, from 49 Ma (Sites 1201 and 291, on either side of the CBF) to 44 Ma (Site 447). These basalts have trace element and isotopic charac-teristics that are indistinguishable from typical N-MORB and E-MORB, which supports the trapping hypothesis but does not counter the alternative back-arc spreading hypoth-eses, because BABB from well-developed active spreading centers often have MORB-like characteristics [Taylor and Martinez, 2003]. A simple way to extend this line of rea-soning would be to sample and examine for MORB versus BABB geochemical characteristics the 50 to 60 Ma WPB basalt from the southernmost section where Hilde and Lee [1984] identified magnetic anomalies 25 and 26 (Figure 1). If the basin opened initially by back-arc spreading, these basalts are most likely to have a subduction-imprinted geo-chemical signature.

That the WPB basalts have Indian MORB rather than Pacific MORB isotopic characteristics supports the concept that the asthenosphere beneath the spreading center at one time had continuity with the larger Indian/Tethyan Ocean domain. This finding could indicate that the plate originated within the range of circulation of Indian/Tethyan type asthe-nospheric mantle, or that such mantle was able to flow into the area tapped by ridge magmatism [Hickey-Vargas, 1998a]. A third alternative, that the same geochemical signature was developed locally by a nearby subducted or upper mantle contaminant, is unlikely because (1) the isotopic signatures of all Philippine Sea plate basalts, including those from the younger Parece Vela and Shikoku basins, the Sumisu Rift, and the Mariana Trough, are identical to Indian Ocean MORB, and (2) geochemically appropriate local contami-nants are not present [Hickey-Vargas, 1998a]. The fact that the Indian MORB signature continues in basalts from the younger basins supports two conclusions: Indian MORB is not locally produced, and its mantle can be tapped despite the northward motion of the Philippine Sea plate over the past 50 Ma. The concept that Indian Ocean asthenosphere is tapped by Philippine Sea plate basins has been widely accepted [e.g., Hickey-Vargas, 1998a; Pearce et al., 1999; Woodhead et al., 2001; Ishizuka et al., 2003].

Most recent tectonic reconstructions favor the origin of the WPB as a back- or inter-arc basin [Hall et al., 1995; Deschamps and Lallemand, 2002; Honza and Fujioka, 2004]. If opening did begin as early as 60 Ma, as inferred from the identification of magnetic anomalies 25 and 26 in the southernmost WPB, then the presumed related sub-duction system cannot be the proto-IBM arc, but it could be either an older arc aligned with the Oki-Daito or Daito Ridges or an arc within the Philippine Islands that was


parallel to the CBSC. Because the WPB lithosphere is cur-rently subducting beneath the Philippines, the original rifted margin may have been destroyed by this recent development. Seno and Maruyama [1984] depict the WPB opening as a back-arc basin behind arcs of the Oki-Daito province and the northernmost Kyushu Palau Ridge, the later development of the rest of the proto-IBM arc being oblique to the existing basin. Hall et al. [1995] show the basin opening between two opposing arcs, the northern Kyushu Palau Ridge/Oki-Daito province and an arc within the Philippine Islands. More recently, Deschamps and Lallemand [2002] compiled ages for areas surrounding and within the WPB and reinterpreted the geophysical data. On the basis of this examination, they suggested a later onset of rifting, at about 55 Ma, and inter-pret the opening as occurring between two opposing arcs: the northernmost Kyushu Palau Ridge and a paleo-Philippine arc formed by northward subduction of the lithosphere of a marginal basin north of the Australian continent (Figure 2b). According to Deschamps and Lallemand [2002], trench roll-back, combined with a possible excess magma supply from a mantle plume, created the conditions for rapid extension and seafloor spreading at the CBSC, with at least one shift in the orientation of the spreading ridge. During the period 55 to 48 Ma, the eastern margin of the opening basin, where the central proto-IBM arc developed, was a transform boundary with the Pacific plate, just as in the “trapped lithosphere” hypothesis described (Figure 2b).

The back-arc tectonic scenario can also be reconciled with the details of WPB magma geochemistry with some important conditions. First, for Indian Ocean asthenosphere to be tapped at the CBSC as it formed within an older exist-ing island arc complex, there must have been a pathway for upper mantle flow between the Indian/Tethyan Ocean basin proper and the opening WPB. Hickey-Vargas [1998a] proposed that the WPB originated within the Indian Ocean asthenospheric domain south of the equator and north of the northern margin of the Australian plate, as is consistent with paleomagnetic data for DSDP Sites 446 and 292 [Hilde and Lee, 1984]. This author also proposed that northward motion of the Australian plate, carrying thick subcontinental litho-sphere, could be a driving force for northward “extrusion” of a section of Indian Ocean asthenosphere and the Philippine Sea lithospheric plate. If a north-dipping subduction system bounded the WPB on the south, as proposed by Hall et al. [1995] and Deschamps and Lallemand [2002], the subducted lithosphere would form a barrier to mantle flow, and mantle inflow would be restricted to the asthenosphere beneath the proposed transform margins (Figure 2). This scenario sug-gests that the transform margins are an essential feature of the tectonic model, and that they must be sufficiently large to permit mantle replenishment. Second, the absence of even

a mild subduction signature in WPB basalts is unexpected for a back-arc basin affected by subduction from two direc-tions. As discussed, it is possible that the oldest (60–50 Ma) WPB basalts would have a stronger BABB signature but have never been sampled. Otherwise, the lack of a subduc-tion signature further supports the need for a pathway for replenishment by asthenospheric mantle from outside the two subduction systems. Possibly, if trench/slab rollback was driving rapid back-arc spreading, as proposed by Deschamps and Lallemand [2002], chemical exchange between materials from the two downgoing slabs and the replenished back-arc mantle would be minimal throughout basin opening.

In summary, the geochemistry of WPB basalts generated by spreading between 49 and 44 Ma are consistent with an origin of the spreading center either by entrapment of an existing Indian/Tethyan mid-ocean ridge or by back-arc spreading. The major constraints contributed by the geo-chemical data are: (1) that the sub-basin asthenosphere must have had continuity with the sub-Indian/Tethyan Ocean asthenosphere or a pathway for replenishment from this source by upper mantle flow must have existed, and (2) that the sub-basin asthenosphere was not affected by subduction inputs during this time.

4.2. Was There a Plume or Hotspot Within the WPB?

Because basalts with the trace element and isotopic charac-teristics of OIB were recovered from drilling on the Benham Rise, there is little question that this feature has an ocean island affinity and was formed by the melting of an enriched mantle source. In the reconstructions of Deschamps and Lallemand [2002] and Hall et al. [1995], with the WPB only slightly opened at 50 Ma, all locations in the West Philippine Basin where OIB-like basalts are actually found are within about 400 km of the location on the CBSC where the Benham Rise formed (Figure 2). This includes Daito Basin Site 446, Site 294, and the Oki-Daito Ridge dredge Site 21-5. Ages for these sites are also similar (49–48 Ma). The limited spatial extent and similar ages suggest that the eruption of OIB was a short-lived event or episode that began about 5 to 10 Ma after spreading in the basin began. That seamounts extend from the Benham Rise to the CBF (Figure 3) suggests that OIB activity may have continued until the end of spreading at 35 Ma. The young 40Ar/39Ar ages of 37 to 36 Ma obtained for plagioclase from basalts from the Benham Rise [Hickey-Vargas, 1998a] could also indicate continued activity. Thus, it is possible that OIB activity continued until 35 Ma, but no OIBs sampled to date are younger than this age.

The geochemical characteristics of the recovered basalts also indicate a relationship among the different sites. For example, the geochemical similarity of Benham Rise basalts


and tholeiitic series basalts from the Daito Basin supports a common source (Figures 5, 6, and 8). Hickey-Vargas [1998b] compared trace element ratios in OIB-like basalts from the Philippine Sea, East Asian marginal basins, and intraplate settings. The Benham Rise and Daito Basin tholeiite series have distinctive low Ba content and low Ba/Th, Ba/Nb, and Ba/Th ratios that are not found in basalts from surrounding areas. In addition, the isotopic and trace element composi-tion of E-MORB-like basalts from DSDP Site 291, which is located about 400 km to the southeast of the Benham Rise (Figure 1), can be modeled as simple mixtures of N-MORB-type mantle sources, such as those tapped by the WPB main spreading stage, and Benham Rise OIB (Figure 8; Hickey-Vargas, 1998a, 1998b). This relationship is also borne out by new Hf-isotope data (Figure 5d and Table 1). Overall, therefore, the geochemical characteristics of West Philippine Basin OIB support their derivation from a single enriched magma source, which extended to or chemically affected mantle sources as far as 400 km from its position at the western end of the CBSC. Geochemical evidence for the enriched source is not found in basalts from the eastern WPB (Sites 447 and 1201), or in late-stage basalts from the CBF.

A second issue is whether the enriched mantle source was related to a deep-seated or long-lived mantle plume or hotspot or instead to a shallow source related to spreading. Given the WPB location at 50 Ma and using the tectonic reconstruc-tion of Hall [1996], Hickey-Vargas [1998a] suggested that the hypothetical long-lived and fixed mantle plume would be located in the present-day Coral Sea. Correcting for the motion of mantle hotspots relative to the Hawaiian plume, Macpherson and Hall [2001] found that the center of the present-day Manus Basin plume would be located within the WPB at 55 to 50 Ma. These authors proposed that the plume affected the proto-IBM arc and was the source of excess heat needed to cause eruption of boninite. This explanation is an alternative to the proposal of Stern and Bloomer [1992], which relates boninite magmatism to mantle upwelling induced by the initial subsidence of old oceanic lithosphere along a transform plate margin.

The small spatial extent of OIB-like volcanic activity, the short time span that coincides with WPB spreading, and the relatively small volume of OIB-like basalts generated in the WPB are more consistent with extension-induced decom-pression melting of a relatively shallow, enriched mantle source than with an origin from a long-lived, deep-seated mantle plume over which the WPB lithosphere passed. These details explain why the OIB-like magmatism ceased when spreading ended. In the context of the two competing models for the origin of the WPB, the timing and limited extent of the area affected by OIB magmatism are consistent with a scenario in which descending plates, coupled with trench

and slab-rollback, induced the ascent and decompression melting of a region of enriched mantle located at intermedi-ate to shallow depths by counterflow. This mantle source may have given rise to the OIB-like magmas that formed the Benham Rise and continued erupting at a decreasing rate until the cessation of spreading. The existence of this enriched source does not distinguish the origin of the WPB as trapped ocean crust or back-arc basin. The small size of the thermal anomaly and the location of the activity in the western WPB suggest that it was not the source of heat for boninite generation.

4.3. Why Did Spreading Cease and What Kind of Magma Was Produced During the Last Stages of Spreading?

Detailed exploration of the CBF reported by Fujioka et al. [1999b] and Deschamps et al. [1999, 2002] revealed that the final stage of activity of the WPB was complex. As noted by Hilde and Lee [1984], wholesale seafloor spreading ceased at about 35 Ma, but the more recent studies conclude that significant extension and deformation continued after that time. Extremely slow spreading and magmatism, accompa-nied by shearing along the ridge, produced a series of rotated blocks or nanoplates (Figure 3). Ages of basalts recovered from one site are as young as 28–26 Ma. This phase was followed by dextral strike-slip faulting and amagmatic exten-sion, possibly related to the forces that caused rifting of the early IBM arc and opening of the Parece Vela and Shikoku basins [Deschamps and Lallemand, 2002; Deschamps et al., 2002].

The geochemical characteristics of these late-stage CBF basalts are very diverse, but like basalts from the main spread-ing stage, they lack the enrichments characteristic of BABB. Only basalts from one site, closest to the Kyushu Palau Ridge, have an anomalously high La/Nb ratio (Figure 7), a common characteristic of island-arc character. Thus, sub-ducted materials may have influenced magma compositions at the latest stage of central basin spreading within 100 km from the active early IBM arc. However, basalts from this one site provide the only detected sign to date of subduction influence in WPB basalts.

Highly enriched basalts with OIB-like trace element char-acteristics (Figure 7) were recovered from the CBF at Site 334 and do not have the same isotopic signature as OIB from other areas, as discussed. Instead, isotope ratios from Site 334 basalts are more like E-MORB from the main spreading stage (Figures 5 and 8). If it is assumed that the isotope ratios vary as the result of mixing N-MORB and OIB sources, as suggested for E-MORB from Site 291 (Figure 8), one can infer that part of the enrichment of the CBF basalts results from their production by very small extents of melting. For

Fig. 8Fig. 8


example, if E-MORBs from mixed sources form by about 20% melting, the enriched CBF basalts can be produced by 1–2% melting of the same sources, provided garnet is pres-ent in the residue. Such small melting extents are consistent with the kind of melting processes that are inferred for the magma source regions of extremely slow-spreading ridges worldwide [Langmuir et al., 1992]. Melt fractions diminish and small magma batches evolve independently into diverse compositions along the length of the waning ridge. The pres-ence of garnet implies that the beginning of melting occurred at depths greater than about 75 km. In addition to explaining the OIB-like trace element characteristics but E-MORB-like isotopic characteristics of the Site 334 samples, this scenario explains the overall decoupling of isotopic and trace element features in the CBF basalts. Because none of these late-stage basalts have isotope ratios resembling those of the earlier erupted OIB, it is probable that the enriched magma source that produced the Benham Rise ceased activity before the latest stages of CBF magmatism. This finding further sup-ports the idea that melting of the enriched source was related to the forces causing spreading, rather than to deep-seated plume activity.

Throughout this late stage of WPB extension, the adjacent early IBM arc was oriented at right angles to the spreading center and was producing arc magma of tholeiitic and calc-alkaline character [Reagan and Meijer, 1984; Lee et al., 1995; Straub, 2003]. An important tectonic question is why WPB spreading and extension continued for so long after the IBM arc was initiated and why it ultimately ceased. According to the model of Deschamps and Lallemand [2002], spread-ing ceased because the forces producing extension, mainly

subduction along the proposed paleo-Philippine arc to the south (Figure 2b), diminished when the northern part of the Australian continental lithosphere approached the trench. This left the CBSC stranded at an oblique angle to the young IBM arc. In this case, both the orientation of the CBSC and the cause of waning magmatism after 35 Ma are essen-tially unrelated to the IBM arc and Pacific plate subduction. Instead, the dominant forces are related to the south-fac-ing Philippine subduction zone. In the trapped lithosphere scenario for formation of the WPB and the IBM arc, there is no explanation for the continuation of spreading after the initiation of subduction for an additional 22 Ma. According to Stern and Bloomer [1992], the IBM subduction system had shifted from lithospheric subsidence and forearc exten-sion to normal subduction, producing normal tholeiitic and calc-alkaline arc magma by about 10 Ma after the initiation of subduction at 48 Ma. Therefore, although the trapped lithosphere hypothesis is supported by a number of lines of evidence, the continuation of CBSC magmatism concur-rently with IBM arc magmatism is best explained if the ori-entation and timing of activities on the CBSC are controlled by another subduction system, as in a back-arc setting.

5. SUMMARY AND CONCLUSIONS

The geochemical characteristics of basalts from the West Philippine Basin are diverse, and the possible causes of that diversity can be interpreted in the context of tectonic models for the opening and development of the basin. The majority of basalts that formed during the main spreading stage of the Central Basin Spreading Center (60–35 Ma) are normal to

Figure 7. Normalized trace element abundances in late-stage CBF lavas. Data are from Hickey-Vargas et al. [2006]. Primitive mantle normalizing values and values for average N-MORB, E-MORB, and OIB are from Sun and McDonough [1989].


enriched MORB of Indian Ocean isotopic character. These characteristics are consistent with an origin of the WPB as a segment of trapped mid-ocean ridge, and also with an ori-gin by back-arc spreading if the basalts formed at a mature stage when subducted materials were no longer entrained in their asthenospheric sources and if a connection with the larger Indian Ocean upper mantle domain existed. Although subduction and spreading occurred together from 48 to 26 Ma, subducted materials left little geochemical imprint in WPB basalts.

Geochemical evidence supports the existence of an enriched magma source centered on the western CBSC from about 50 to 35 Ma. This source gave rise to OIB-like basalts that formed the Benham Rise seamount and the Urdaneta plateau and erupted at several other locations without bathy-metric expression. The geochemical similarity of these OIB-like basalts to each other and their restricted age of eruption suggest that they are related to a single short-lived source. However, there is no geochemical expression of this source in main spreading-stage basalts from the eastern section of the basin, for example, DSDP Site 447 and ODP Site 1201, or in late-stage basalts from the Central Basin Fault. Based on this evidence, it is most likely that the enriched mantle source was not a long-lived, deep-seated mantle plume, but was instead a region of enriched mantle drawn upward as

the result of extension and spreading. This evidence explains why the volume of enriched magma diminished and OIB-like magmatism ceased when the main spreading stage ended at 35 Ma. Given that the enriched source was centered in the western basin, it is unlikely that it was an important heat source for boninite magmatism in the proto-IBM arc, which formed along the opposite margin of the Philippine Sea plate.

Basalts from the late stage of magmatism that accompa-nied the transition from spreading to amagmatic extension along the CBF (35–26 Ma) exhibit notable decoupling of trace element and isotopic characteristics. Isotopic composi-tions are like those of N-MORB to E-MORB basalts from the main spreading stage, but relative incompatible element enrichments are not correlated with the isotopic composi-tions. This decoupling is a common feature of slow-spread-ing ridges worldwide, where extents of melting can be small and isolated batches of magma evolve independently.

An interesting puzzle is why spreading continued long after the onset of IBM arc magmatism, and how IBM arc magmatism and an orthogonally oriented back-arc spread-ing center coexisted from 48 to 26 Ma. This observation is not explained by a model in which the CBSC is simply a segment of older Tethyan/Indian Ocean Ridge that becomes trapped and isolated by the initiation of lithospheric subsid-

Figure 8. Plot of initial 143Nd/144Nd versus Sm/Nd, showing basalts from the WPB normal spreading stage (filled sym-bols), OIB locations (open symbols), and late-stage CBF lavas (asterisks). E-MORB from DSDP Site 291 plot along a linear mixing line between WPB MORB from the main spreading stage and WPB OIB. Late-stage CBF basalts have widely variable Sm/Nd contents, but limited variation of 143Nd/144Nd, which probably reflects limited mantle mixing but extreme variation in the extent of mantle melting beneath the waning CBSC.


ence and then subduction [Stern and Bloomer, 1992]. The continued activity of the CBSC in its orthogonal orientation after the onset of arc magmatism in the early IBM arc, and even into the initial rifting stages of the early arc at 27 Ma, implies that other forces were involved. The CBSC could have formed as a back-arc spreading center related to a southward-facing paleo-Philippine arc [Deschamps and Lallemand, 2002], or it could have originated by entrap-ment of a Tethyan/Indian Ocean Ridge and then continued spreading because of forces related to the paleo-Philippine subduction system. In these latter cases, spreading probably ended along the CBSC when northward subduction beneath the paleo-Philippine arc slowed. Subduction of the Pacific plate along the early IBM arc then became dominant, and the locus of back-arc extension shifted to a position parallel to the arc in the opening Parece Vela and Shikoku basins [Deschamps and Lallemand, 2002].

There are several geochemical strategies that could be used to further test the competing hypotheses for the origin of the WPB. Samples of basalt from the oldest (60–50 Ma) section of the basin floor can be tested for signs of BABB character, which would be expected in the earliest formed basalts from an opening back-arc basin. Similarly, a more complete knowledge of the age, petrology, and geochemistry of the arc rocks from the Oki-Daito and Daito Ridges and Amami Plateau, as well as Philippine Island arc rocks of similar age, might support or disprove the concept that these are related arc terranes that were disrupted and displaced by opening the WPB. Finally, further investigation of the details of mantle-melting conditions, timing of eruptions, and dis-tribution of oldest IBM arc volcanics will help to constrain the tectonic regime in which they could have formed along the edge of the WPB.

Acknowledgments. RHV gratefully acknowledges support from NSF POWRE grant OCE 0074868 and supplement OCE 0201602, which supported the participation of MB, and from NSF Margins grant OCE 0001826. RHV thanks JAMSTEC for the opportunity to participate in R/V Kairei Cruise KR99-10. The participation of IPS was supported by a Schlanger Ocean Drilling Fellowship from the ODP. The first author’s understanding of the West Philippine Basin has benefited greatly from conversations with Anne Deschamps and Kyoko Okino. Thorough and thoughtful review comments by Robert Stern, Leonid Danyushevsky, and David Christie helped improve the focus and clarity of the manuscript.

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origin of diverse geochemical signatures in igneous rocks from the west philippine basin:...

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