Geochemistry and tectonic setting of the arc,

MARC J. DEFANT Department of Geology, University of South Florida, Tampa, Florida 33620 RENE^C^^AUR^Y } laboratoire de Petrologie et G. S. Oceanologie et Geodynamique, Universite de Bretagne Occidentale, 29287 Brest, France JELLE DE BOER Department of Earth and Environmental Sciences, Wesleyan University, Middletown, Connecticut 06457 JEAN-LOUIS JORON Groupe des Sciences de la Terre, Laboratoire Pierre Siie, Centre d'Etudes Nucléaires de Saclay, B.P. 2, 91191 Gif sur Yvette, France

ABSTRACT ment) with the Trench. In contrast, unreported analytical data, along the entire ex- Babuyan segment samples (north) have un- tent of the Luzon arc and to discuss possible Miocene (10 Ma) to Recent volcanism is usual island-arc Nd and Sr isotopic ratios that origins of these compositional variations. associated with eastward along fall below the mantle array; they appear to be the for some 1,200 km from the result of sediment source contaminants or GEOLOGIC SETTING the Coastal Range in south to Min- a unique mantle composition. Incorporation doro. We suggest calling this the "Luzon of sediments analyzed from eastern Tectonic Framework arc." There are five distinct segments along and Taiwan into a MORB-type mantle can this arc: , , Northern Luzon, explain the Nd and Sr isotopic ratios of the The Manila Trench is a broadly arcuate Babuyan, and Taiwan. The Bataan and Min- Babuyan segment. (concave eastward) structure, which extends doro segments in the south are separated by a from about 13° to 21° N (Figs. 1 and 2). It was northeast-southwest zone of volcanoes. The INTRODUCTION studied in relative detail by Ludwig and others Northern Luzon segment consists of Pliocene (1967), Hayes and Ludwig (1967), and Hayes to Pleistocene necks and plugs that run along The Manila Trench extends 900 km north- and Lewis (1984). focal mecha- the entire western section of Northern Luzon. ward from Mindoro to Taiwan. It results from nisms are relatively rare. De Boer and others The Babuyan segment (10 Ma to Recent) is the east-dipping subduction of the South China (1980) computed the convergence to be east- made up of a larger number of active volca- Sea oceanic basin beneath the Luzon terrane northeast-west-southwest (67° ± 30°). The na- noes than are other segments of the Luzon (Fig. 1). This subduction zone is associated with ture of the arc crust is poorly understood. arc. The Taiwan segment is composed en- collision zones at both its southern (Mindoro) Another subduction system occurs east of tirely of extinct volcanoes in the Coastal and northern (Taiwan) edges (Stephan and Luzon, where subduction along the Philippine Range and on the islands of Lanhsu and others, 1986). Volcanism associated with sub- Trench has generated the Bicol arc (Fig. 1). Lutao (late Miocene to 1.8 Ma). duction is more or less geographically continu- Subduction appears to have been reactivated The nature of volcanism throughout the ous from to the Coastal Range of along the East Luzon Trough, but lack of seis- Luzon arc is clearly subduction related with Taiwan (Fig. 1). Most volcanism occurred in the mic activity deeper than 70 km (Hamburger and calc-alkaline affinities; high Th/La ratios; and Quaternary and the late Pliocene, but the oldest others, 1982) suggests that the west-dipping negative Nb, Ta, and Ti anomalies. The earli- lavas were erupted in the late Miocene (ca. 10 subducting plate is presently not deep enough to est phase of volcanism so far recorded (10 Ma; Richard, 1986; Richard and others, 1986). generate volcanic activity. Ma), however, is tholeiitic (island of Calayan The predominant rock type is andesite, although in the Babuyan segment) with mid-oceanic- individual volcanoes can be basaltic or range in Geodynamic History ridge basalt-like (MORB-like) light-rare- composition from basalt to rhyolite. The rocks earth element and large-ion Iithophile element are calc-alkaline (Defant, 1985; Richard, 1986; The oldest available K-Ar ages of calc- concentrations and low Th/U ratios. Jacques, 1987) with the exception of the Ca- alkaline lavas from the Luzon arc are approxi- There are clear differences in Sr and Nd layan island tholeiitic complex. mately 10 m.y. Biostratigraphic correlations isotopic ratios among the samples from the A number of recent studies along segments of suggest that the volcanic activity began approxi- Luzon arc. Samples from the southern seg- the "Luzon arc" have expanded the existing data mately 13-14 Ma in the Coastal Range of Tai- ments (Mindoro through Bataan) fall within base and now allow for an overview and integra- wan (Pelletier, 1985) and approximately 15 Ma the mantle array, with higher Sr and lower tion of the geochemical data (de Boer and oth- in Northern Luzon (P. Maleterre, 1987, personal Nd isotopic values than those of MORB. Pb ers, 1980; Ragland and Defant, 1983; Defant, commun.). These data suggest that subduction isotope data indicate sediment involvement. 1985; Richard, 1986; Jacques and others, 1986; along the Manila Trench immediately followed The isotopic values have been interpreted to Jacques, 1987; Knittel and Defant, 1988; Knit- opening of the oceanic basin result from source contamination by slab- tel and others, 1987, 1988; Defant and Ragland, (ca. 32 to 17 Ma, according to Taylor and derived fluids (Bataan segment) and by 1988; Defant and others, 1988). The objectives Hayes, 1980, and Ru and Pigott, 1986). The subducted crustal material from collision of of this contribution are to compare the geochem- beginning of collision between the Luzon micro- the Mindoro- terrane (Mindoro seg- istry of the volcanic rocks, including previously plate and the continental margins of the South

Geological Society of America Bulletin, v. 101, p. 663-672, 7 figs., 1 table, May 1989.

663

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Figure 1. Schematic map showing the major tectonic features along the Luzon arc (shown by diagonal rul- ing) (tectonic features are after Pelletier, 1985). Major volcanoes are shown as triangles, and necks are shown as circles. 1, Bataan AFV; 2, Bataan BAV; 3, Macolod Corridor; 4, Mindoro segment; 5, Northern Luzon segment; 6, Babuyan segment; 7, Taiwan segment; A, -Dingalan ; B, Philippine fault; C, Tablas fault. The cross pattern (+) illustrates continental blocks.

necks and plugs. This segment is separated from the Bataan segment by the Lingayen-Dingalan fault (A, Fig. 1). The Babuyan segment has the largest number of active volcanoes, compared to the other segments of the Luzon arc. It includes Mount Cagua in Northern Luzon and the islands from to Batan (Fig. 1). The Taiwan segment represents the northernmost volcanism along this arc and is composed entirely of extinct volcanoes in the Coastal Range and on Lanhsu and Lutao. . The spatial distribution of volcanoes throughout the chain of activity asso- ciated with the Manila Trench is somewhat ir- regular. A narrow zone of arcuate volcanism, the Bataan lineament ( 1, Fig. 1), exists along the western of Central Luzon for approximately 130 km. Volcanoes within this lineament (arc-front volcanoes, AFV) are more tightly spaced than elsewhere in the Philippines. Four volcanoes have developed in back of this main lineament and may roughly parallel it (behind-arc volcanoes, BAV) (region 2, Figs. 1 and 2). K-Ar dates indicate that volcanism was initiated in the AFV and later began in the BAV (Defant and others, 1988). Dates within the AFV range from approximately 7 to 0.2 Ma, whereas within the BAV, the dates range from approximately 1.7 to later than 0.1 Ma (Fig. 2). The arcuate nature of the AFV, and perhaps the BAV, is rather abruptly terminated in the vicinity of Mount Taal (Fig. 1). Here, the vol- canic chain trends northeast-southwest and is apparently associated with northeast-southwest China Sea (north) and the Mindoro-Palawan Volcanism in the Luzon Arc normal faulting. This volcanic field, the Ma- terrane (south) probably occurred about 7 Ma colod Corridor, extends approximately 60 km (Stephan and others, 1986) and 10-15 Ma (Mc- The five segments of the "Luzon arc" are across more than half of Central Luzon (region Cabe and others, 1982,1987), respectively. Col- termed Mindoro and Bataan (of Central Luzon), 3, Figs. 1 and 2). Volcanic activity within this lision ceased in Mindoro during the late Northern Luzon, Babuyan, and Taiwan. The corridor occurred from approximately 0.6 Ma to Miocene or Pliocene but is ongoing in Taiwan Mindoro and Bataan segments are divided by the present (there was older activity not asso- (Fig. 1; Stephan and others, 1986; McCabe and the Macolod Corridor. The Northern Luzon ciated with faulting). This may represent a pull- others, 1982, 1987). segment consists of Pliocene to Pleistocene apart zone (Defant and others, 1988).

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AGE OF VOLCANISM (m.y.)

10 9

Coastal Chain Taiwan Segment Lutao

Lanhsu Egg:

Batan

Babuyan de Claro ••""."...... I J Babuyan Segment

1 Camiguin

Cagua

Northern Luzon

Segment

1 Bataan AFV S« Bataan Segment 2 Bataan BAV

Macolod Corridor SgBSBwgifeg

Mindoro Segment Mindoro Segment

SU High-K Medium-K lili Low-K

Figure 2. Ages and potassium contents of volcanic centers of the Luzon arc. Data from de Boer and others (1980), Wolfe (1981), Defant (1985), Richard and others (1986), Richard (1986), Jacques (1987), P. Maleterre (1987, personal commun.), D. Oles (1987, personal commun.), and Defant and others (1988).

The southern extension of volcanic activity due to uplift throughout most of Northern main eruptive phases have been documented; associated with the Manila Trench exists primari- Luzon. All that remains are volcanic plugs and phase 1 (ca. 1.5 Ma) was marked by effusion of ly along a lineament that stretches nearly 60 km necks (see Fig. 1 for the locations of most of the basaltic andesites and basalts. Thick nuée ar- across Mindoro and terminates at the Tablas important volcanic necks). P. Maleterre (1987, dente deposits covered the whole dur- fault (C and region 4, Figs. 1 and 2). This volcan- personal commun.) has sampled and dated two ing phase 2 (ca. 0.6 to 0.3 Ma). Phase 3 ic chain has been termed the "Mindoro linea- of these necks, Lepanto and Baguio, at 8.7 and produced ash flows during recent periods of ment" (AFV) by Defant and others (1988). 1.5 Ma. R. McCabe (1988, personal commun.) phreatomagmatic activity. Northeast of this lineament, there are a few erup- has found a number of late Miocene flows in the Islands North of Luzon. The islands north of tive centers (BAV) that may parallel the AFV. Baguio region. Luzon (Fig. 1) can be separated into three Although only three samples from the Mindoro The only volcanic complex associated with groups: the Babuyan and Batan (not to be con- segment have been dated, the K-Ar ages are Manila Trench subduction studied to any extent fused with the Bataan segment in Central synchronous with those of the Bataan segment. in Northern Luzon is Mount Cagua (Jacques, Luzon) Archipelagoes and the Chinese islands of Northern Luzon. Volcanism related to sub- 1987) (Fig. 1). This stratovolcano may be more Lanhsu and Lutao. The Babuyan duction along the Manila Trench in this region closely related to the volcanoes north of Luzon consists of three islands, Calayan, Camiguin, and of the Philippines has not been studied in detail than to volcanic necks to the south because it Babuyan de Claro. is composed because of extensive erosion of volcanic centers has erupted in the last few thousand years. Three of three pétrographie and geochronologic units

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TABLE I. SELECTED MAJOR, TRACE, AND RARE-EARTH ELEMENT DATA FROM THE LUZON ARC atomic absorption spectrophotometry (Perkin- Elmer 301; analyst P. C. Ragland, Tallahassee; CL-12 C-54 B39 CA-11 B93 120 168 174 A1203, Fe203, MnO, MgO, NazO, K20, P 0 ). Relative standard deviations are si 1% for Si02 53.0 53.8 51.3 50.5 48.5 47.5 47.8 51.1 2 5 Ti02 0.78 1.05 0.81 1.00 1.15 0.61 0.98 0.93 Si02, <2% for the other major elements with A120, 17.74 18.48 18.38 19.57 17.44 13.81 13.71 18.50 Fe20,' 8.09 6.78 9.75 9.14 9.68 10.83 10.92 9.60 concentrations >1 wt. % oxide, and s;5% for MnO 0.15 0.14 0.17 0.19 0.17 0.21 0.18 0.16 MnO and Ti02 at lower than 1 wt. % concentra- MgO 5.97 4.26 5.21 3.82 6.33 10.87 11.27 5.81 CaO 10.06 9.10 10.66 990 12.02 12.62 11.15 9.74 tions. Further information on precision and ac- Na20 2.96 3.37 2.72 2.96 2.67 1.30 2.27 3.19 curacy is given by Defant (1985). K2O 0.50 1.45 0.74 1.12 1.23 0.41 1.01 1.06 P2O5 0.10 0.20 0.10 0.30 0.20 0.22 0.35 0.36 Rare-earth element (REE) results for sample H2O* 1.18 1.26 0.00 0.94 0.17 ND ND ND

H2O 0.26 0.22 0.15 0.59 0.27 ND ND ND B93 were obtained by emission spectrometry combined with an inducted plasma source unit Total 100.10 99.52 99.16 99.25 99.00 98.42 99.60 101.41 (Govindaraju and others, 1976) (analyst J. Leter- Th 0.52 6.4 2.9 3.2 ND 1.4 3.9 7.1 rier, Nancy, France). Analytical error is 10% ex- Ta 0.10 0.19 0.11 0.22 ND ND ND ND Nb ND ND ND ND ND 6 ND 14 cept for contents < 1 ppm. The accuracy of this La 3.3 22 10 11.6 19 5.4 14.9 25.7 method has been tested by isotope dilution meas- Ce 5.9 42.5 20.2 20.6 47.0 10.6 29.6 48.0 Nd ND ND ND ND 24.4 7.1 18.4 24.6 urements, and the agreement between the two Sm 1.5 3.6 2.3 2.6 5.26 1.5 3.8 4.9 methods is within ±5% (Alibert and others, Eu 0.66 1 19 0.85 0.99 1.31 0.5 1.13 1.45 Tb 0.34 0.48 0.33 0.46 ND 0.2 0.4 0.6 1987). REE's and Ta for samples CL-12, C-54, Yb 1.56 2.00 1.55 2.15 1.72 1.4 1.4 1.9 B39, and CA-11 were determined by instrumen- Lu ND ND ND ND 0.27 0.19 0.20 0.25 tal neutron activation (analyst J.-L. Joron, Sa- Note: CL-12, basaltic andesite from Calayan; C-54, basaltic andesite from Camiquin; B39, basalt from Babuyan de Claro; CA-11, basalt from Cagua; B93, basalt clay, France), according to the method described from (Mount Iraya); 120, basalt from Mount (Bataan AFV); 168, basalt from (Bataan BAV); 174, basalt from (Macolod Corridor). by Chayle and others (1973). The accuracy of •Total iron as Fe^O^. the method is approximately equal to 5% for each trace element analyzed. REE's and Th for samples 120, 168, and 174 were obtained by indirect current plasma/mass spectrometer which range in age from 7 to 4 m.y. The oldest made up of two stratovolcanoes, Mount Mata- (SCIEX). Nb for these samples was determined unit (ca. 6-7 Ma), which consists of basaltic rem to the south and Mount Iraya to the north. by X-ray fluorescence (pressed pellet, Philips andesite flows, is exposed in the western part of They are separated by an isthmus where old 1600). Relative standard deviations for the the island. The central section of the island is andesites (ca. 7 to 10 Ma, Richard, 1986) occur. REE's and Th are <30% for values <0.1 ppm, dominated by Mount Calayan (499 m), which is Mount Matarem is Pliocene, ranging in age from 10% for values between 0.1 and 1 ppm, <7% for composed of 5-6 Ma andesitic lava flows. The 5.8 to 1.7 m.y. In contrast, Mount Iraya is Qua- values between 1 and 10 ppm, <5% for values youngest volcanic formation (ca. 4 Ma) is char- ternary and generated a pyroclastic flow deposit between 10 and 50 ppm, and <3% for values acterized by a thick rhyolitic lava flow that is approximately 1480 yr B.P. (Richard and oth- >50 ppm. A relative standard deviation for Nb exposed in the southeastern section of the island ers, 1986). Only preliminary K-Ar dates (H. Bel- is ±2 ppm for the values obtained. (Jacques, 1987). Ion, 1987, personal commun.) of from 2 to 4 Ma Volcanism on Camiguin island began approx- have been obtained from the island of Sabtang. imately 3 Ma with construction of an andesitic The Chinese islands of Lanhsu and Lutao Potassium-Silica Relations volcano. Two subsidiary cones north (Minabul) have been studied by Richard and others (1986) and east (Caanoan) of the central volcano devel- and Lan and others (1986). Calc-alkaline volcan- Volcanism throughout the Luzon arc is oped at 1 and 0.7 Ma when these two cones, ism occurred mainly during the late Miocene clearly subduction related and mostly calc- along with a small crater, and a stratovolcano and early Pliocene in the Coastal Range of Tai- alkaline (de Boer and others, 1980; Ragland and (Mount Camiguin) were emplaced along a wan (youngest K-Ar age: 4.4 m.y.) and on Lan- Defant, 1983; Defant, 1985; Richard, 1986; northeast-southwest fracture zone. This fracture hsu (youngest age: 3.8 m.y.) (Richard and Jacques, 1987; Defant and others, 1988). On

zone is probably related to the active Babuyan others, 1986). The rocks of Lutao island are the K20-Si02 diagram shown in Figure 3, the fault zone (M. W. Hamburger, 1987, personal predominantly Pliocene (Richard and others, Luzon arc volcanic associations range from low- commun.). Historic eruptions on Didicas islet 1986) (1.8 to 4.3 Ma). K (Calayan) through medium-K types (the Ba- (1856 and 1952; Fig. 1) may be related to vol- taan AFV and Camiguin) to high-K types canism associated with this fault zone. GEOCHEMISTRY (Mount Iraya on Batan island and the Bataan Babuyan de Claro island is situated at the BAV), according to the classification of Pecceril- southern end of the Lutao-Babuyan ridge. Five Analytical Methods lo and Taylor (1976). Calayan is distinctly differ- major volcanic centers have been identified: ent from the other associations, displaying most Cayonan cone (ca. 1.7 Ma), Naydi cone (ca. 1.3 Representative data are reported in Table 1. of the mineralogical and chemical characteristics Ma), and the volcanoes of Mount Dionisio, Major elements for samples CL-12, C-54, of an island-arc tholeiitic series (Jacques, 1987; Mount Pangasun (1 to 0.8 Ma), and Mount Ba- B39, CA-11, and B93 were determined by Maury and others, 1988). Figure 2 shows the buyan. Mount Babuyan is a strombolian vol- atomic absorption spectrophotometry (Perkin- time-and-space-related distributions of low-K, cano that last erupted in 1919. Elmer 5000; analyst J. Cotten, Brest, France) medium-K, and high-K types. Low-K series, The Batan Archipelago is located halfway be- and for samples 120, 168, and 174 by X-ray where present, are older than the others (>4 tween North Luzon and Taiwan (Richard, fluorescence (Philips 1410; analyst P. C. Rag- m.y. old). Progressive transitions are observed with time either from low-K to medium-K as- 1986; Richard and others, 1986). Batan island is land, Tallahassee; Si02, Ti02, CaO) and by

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compared to those of MORB. (2) The Bataan AFV samples are intermediate in LREE's, LILE's, and Th/U ratios and fall between Ca- layan and the other islands in the Babuyan seg- ment (Fig. 3). (3) Samples from the Babuyan segment (excluding Calayan and including Cagua, Camiguin, Batan, and Babuyan de O O £ Claro) and the Bataan BAV have high LREE a a o o o - °° * and LILE concentrations typical of high-K calc- alkaline rocks. Volcanoes located closest to the - T^pD Q • Manila Trench (Calayan and the Bataan AFV) are also the volcanoes with the least enrichment in LREE's and LILE's and the lowest Th/U ratios compared with volcanoes farther to the east. De Boer and others (1980), Ragland and Defant (1983), and Defant (1985) have already

SiOr noted the increase in LILE's across the Bataan segment.

Sr and Nd Isotopic Characteristics o Bataan Segment • Calayan The Sr and Nd isotopic data for the Luzon arc û Bataan BAV are presented in Figure 6 (Richard, 1986; Jacques, 1987; Knittel and others, 1988). Super- A Camiguin imposed on the diagram are a MORB field O Iraya (White and Hofmann, 1982); samples from • Babuyan de Claro Oligocene plutons (Knittel and others, 1988) and the Bicol arc (Mukasa and others, 1986), thought to have been generated during west-

-j ward subduction of the plate; 56 60 Cretaceous magmatic rocks from eastern China B SiO„ (Xuan and others, 1986); Cenozoic basalts from eastern China (Peng and others, 1986); granites from Taiwan (Jahn and others, 1986); and sam- Figure 3. A. Diagram of K20 versus Si02. B. Fields from Mount Iraya (on Batan island); Babuyan de Claro, Camiguin, and Calayan from the Babuyan segment; and the Bataan ples from the Ronda Ultramafic Complex, segment and BAV of the Luzon arc. See text for data sources. (Reisberg and Zindler, 1987). There are clear differences among samples from the Luzon arc (Fig. 6). The Mindoro, Ba- sociations (Coastal Range, Lanhsu, and Baguio) Figure 4. Additional characteristics typical of taan, and Macolod Corridor samples fall within or from medium-K to high-K series (Lutao and calc-alkaline magmas are high Th/La ratios and the mantle array but at lower Nd and higher Sr Batan). Quaternary high-K series rocks occur at negative Ti, Nb, and Ta anomalies [low isotopic values than do the Bicol arc samples both ends of the Luzon arc, near the Taiwan and high-field-strength elements (HFSE) to LILE (Knittel and others, 1988; Knittel and Defant, Mindoro collision zones. There is, however, ratios]. 1988; Defant and others, 1988). The Babuyan minor high-K volcanism also in Baguio (Fig. 2). Th/U values are distinctly different among segment of the Luzon arc has unique isotope samples with high LREE contents compared to values compared to other island arcs. Both meta- Trace Elements those with intermediate LREE contents (Fig. 5). somatized xenoliths and volcanic samples fall Samples from Bataan BAV, Babuyan de Claro, below the mantle array (Richard, 1986; P. Vidal Representative data are plotted on an "ex- and Camiguin fall along a trend of higher Th/U and others, unpub. data). tended" chondrite-normalized diagram (Fig. 4, than do samples from the Bataan AFV and Ca- Table 1). Only the most mafic samples were layan. A somewhat arbitrary line (Th/U = 4.6) DISCUSSION chosen, in an attempt to compare the most prim- has been superimposed in Figure 5 to distinguish itive samples from each complex. One charac- the two groups. The Luzon arc has been shown to consist of teristic of the diagram is a distinctive enrichment In summary, there are apparently three dis- several domains or segments. The segments are of the calc-alkaline samples in light-rare-earth tinct geochemical groups of samples within the geochemically distinct from each other, and elements (LREE's) and large-ion lithophile ele- Babuyan, Mindoro, and Bataan segments. (1) some of the segments have rather complex tec- ments (LILE's) relative to chondrite values. The Calayan samples are tholeiitic, have mid- tonic histories. Geochemical variations between sample from Calayan, however, has a nearly flat oceanic-ridge basalt-like (MORB-like) LREE segments may be due to variations in the crust REE pattern, reflecting its tholeiitic nature. and LILE concentrations, and have low Th/U below the arc, subducted material along the Samples from Batan and Camiguin islands show ratios. These samples do, however, have arc Manila Trench, or the composition of the man- the greatest amount of LREE enrichment in characteristics, such as high LILE/LREE ratios tle wedge. Both the Northern Luzon and Taiwan

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segments have only recently begun to be studied B39 Babuyan (51.3) and, therefore, will not be discussed in detail. Interpretations from new data presented from CL12 Calayan (53.0) the Babuyan segment will be a major focus of C54 Camiguin (53.8) this section. CA11 Cagua (50.5) B93 Batan Island (48.5) The Mindoro and Bataan Segments 120 Bataan Segment (47.5) Volcanics of this region, like other island-arc 168 Bataan BAV (47.8) lavas, are enriched in LILE's relative to those of 174 Macolod Corridor (51.1) MORB. The cause of this enrichment may be an enriched mantle source (for example, oceanic island basalt, as suggested by Morris and Hart, 1983), variation in degree of partial melting, material derived from the slab (fluids), crustal assimilation, or material incorporated into the source (for example, sediments). M. J. Defant and others (unpub. data) have concluded that the chemical characteristics of Bataan and Min- doro magmas (relatively low La/Yb ratios and high Sr concentrations) are inconsistent with the hypothesis of partial melting of the slab (Brophy and Marsh, 1986). Enrichment of Bataan and Mindoro magmas by partial melting of an oceanic island basalt (OIB) source seems unlikely because this mech- anism cannot explain the low HFSE/LILE and Th Ta La Ce Nd Sm Ti Eu Tb Yb Lu LREE/LILE ratios of Central Luzon samples Nb (other arguments, such as high Cs contents and Figure 4. Chondrite-normalized LILE and REE diagrams. Si02 contents given in paren- Sr isotopic ratios in island-arc basalts, have also theses. Nb concentrations are reported for two samples (174 and 120), and Ta concentrations been raised against an OIB source; for example, are reported for samples B39, CL-12, C-S4, and CA-11. Chondrite normalizing values are from Perfit and Kay, 1986). The low HFSE/LILE Wakita and others (1971) and Sun and others (1979). The order of the elements is after Sun and LREE/LILE ratios of the Mindoro and Ba- and others (1979). taan samples can be derived from an OIB source

Figure 5. Diagram of Th ver- sus U. A line (Th/U = 4.6) has been arbitrarily superimposed to distinguish the Bataan AFV and Calayan from the Bataan BAV, Babuyan, Cagua, and Camiguin. A line has been drawn through the Bataan AFV and Calayan to show the lower Th/U ratios of these samples compared to those of the Bataan BAV, Babuyan, and Cagua.

• Bataan Segment a Bataan BAV Calayan Babuyan de Claro Cagua • Camiguin

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0.5130

0.5125

T> z <* •t

•o z CO

0.7040 0.7060 0.7080 0.7100 0.7120

87Sr/86Sr

Figure 6. Graph of I43Nd/144Nd versus 87Sr/86Sr for the Luzon arc. Data for Mindoro and Bataan segments and the Macolod Corridor are from Knittel and others (1988). Data for Batan island (Richard, 1986), Babuyan de Claro (Jacques, 1987), Cretaceous magmatic rocks from eastern China (Xuan and others, 1986), Tertiary granites from Taiwan (Jahn and others, 1986), Cenozoic basalts from eastern China (Peng and others, 1986), MORB (White and Hofmann, 1982), Ronda Ultramafic Complex (Reisberg and Zindler, 1987), a MORB mixing point (O'Nions and others, 1977), and pre-Miocene plutonics and volcanics (Knittel and others, 1988), as well as a Bicol arc field (Mukasa and others, 1986) related to westward subduction, are also shown. CI through C5 are mixing curves between the various indicated end members. The mantle array is from Feigenson and Carr (1986).

if a residual titanate phase exists in the source M. J. Defant and others (unpub. data) have but outside the MORB field (Fig. 6). These sam- region (for example, Green, 1981; Morris and believed that the low HFSE/LILE ratios from ples could have had additional components add- Hart, 1983). Ryerson and Watson (1987) found the Luzon arc samples can be attributed to either ed to them during or after partial melting of a that rutile (and related minerals) saturation re- a MORB-type mantle that undergoes high de- MORB-like source. The Bicol arc- quires concentrations well in excess of those grees of partial melting or to a MORB-type Central Luzon plutonics (westward subduction- found in arc lavas. They concluded that deple- mantle that has been previously melted prior to related magmatism) have higher Nd and lower tion in Nb, Ta, and Ti and reduced Nb/U and partial melting in the arc environment (Ewart Sr radiogenic isotopes compared with those of Nb/Th ratios in volcanic rocks erupted at con- and Hawkesworth, 1987). Ryerson and Watson volcanoes along the Luzon arc. There is no rea- vergent plate margins cannot be attributed to (1987) and Davidson (1987) have pointed out son to believe that the mantle wedge is drasti- residual rutile in their source but must be an that the HFSE concentrations are lower in cally different over this relatively short distance inherent property of the source region. Conse- island-arc basalts than in MORB in some between the Luzon and Bicol arcs. M. J. Defant quently, HFSE depletions in the Luzon arc can- instances. and others (unpub. data) have related the iso- not be explained by residual titanate phases, Isotopic compositions of Nd and Sr in the topic differences to fluid-derived components which rules out an OIB source for the Mindoro Macolod Corridor and the Bataan and Mindoro from the westward-subducting Philippine plate and Bataan magmas. segments of the arc fall within the mantle array compared to the fluid-derived components from

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the eastward-subducting South China Sea plate. Palawan continental terrane into the source re- Note, for example, the higher values of La/Sm The variations between the Bicol and Luzon arcs gion appear to be a more favorable origin for and Ce/Yb ratios in the southern section of the could be the result of preferential crustal con- the elevated LILE's and 87Sr/86Sr than does as- arc as compared to the Bataan section, displayed taminants, but there are no obvious variations similation of pre-existing crust directly below in Figure 7. We attribute these variations to between the arc crusts. the arc. There are no obvious differences be- crustal contamination in the Mindoro-Macolod The Bataan AFV have developed on ophio- tween the pre-existing crust below the Bataan Corridor region of the arc. Because there are no lites of the complex. Hawkins and Ev- and Mindoro segments. obvious differences in the crustal makeup below ans (1983) believed that the Zambales terrane Hildreth and Moorbath (1988) have recently the Bataan and Mindoro segments, we must represents obducted ocean crust-upper mantle conducted a study of the effects of continental conclude that the contamination is a source- of a back-arc basin or island-arc basement. S. B. crust on Andean volcanism of central . related phenomenon. Bachman (1983, personal commun.) and Bach- Along this sequence of volcanoes, the age and Although it is difficult to prove, Defant and man and others (1983) believed that most of makeup of the downgoing slab, the subduction others (1988) have suggested that the LILE var- western Luzon south of the Philippine fault zone and composition of sediment, the evolution of iations between the AFV and the BAV of the and north of the southern extension of Manila the volcanoes, and several other parameters re- Bataan segment (across-arc variations) are due is underlain by an ophiolite basement. The main nearly constant. In contrast, the apparent primarily to variations in different degrees of Bataan BAV developed on predominantly clas- thickness and average age of the continental partial melting within the mantle wedge, result- tic sediments of the Central Valley basin. Ac- crust increase continually from south to north. ing from variations in the amount of fluids re- cording to Bachman and others (1983), this They have argued that the geochemical variabil- leased from hydrous phases. Fyfe (1982) and large Cenozoic sedimentary wedge has formed ity along the arc developed as the magmas rose Tatsumi and others (1986) have discussed slab- in the basin of the Central Valley and may be as through this crust. The types of geochemical var- wedge processes in some detail. Any further thick as 10 km below this section of the Bataan iations that Hildreth and Moorbath (1988) have comments concerning the Bataan AFV and segment. The sedimentary units consist primar- seen are similar to the variations between the BAV would simply be conjecture. ily of carbonate sediments, volcanic ash depos- Bataan and Mindoro (including the Macolod its, and clastics derived from the Zambales Corridor) segments. For example, they noted The Babuyan Segment ophiolite complex in the west and the Sierra distinct and impressive increases in 87Sr/86Sr Madre in the east. ratios (along with decreasing 143Nd/144Nd), The Babuyan segment Nd and Sr isotopic K 0 and other LILE's, and Ce/Yb and La/Sm Granitic-type crustal material or sedimentary 2 ratios are unusual. Richard (1986) noted that ratios from south to north as the crust becomes units with high LILE concentrations do not ap- the Batan island samples have a granulite iso- thicker. Likewise, Mindoro and Macolod Corri- pear to be present below either the AFV or BAV topic signature but did not indicate the possible dor samples are significantly more enriched in of Bataan. In fact, the only crustal terrane de- source of this signature. Lower continental crust these parameters than are the Bataan samples. scribed within this region exists in the Mindoro- granulite-facies Nd and Sr isotope ratios plot Palawan continental terrane. Hamilton (1979), de Boer and others (1980), McCabe and others (1982, 1985), and Karig (1983) have described this terrane as being composed of ophiolites and metamorphic complexes, and Knittel and De- fant (1988) have noted granitic rocks with high 87Sr/86Sr ratios (0.71). The highest 87Sr/86Sr and LILE's found in the Luzon arc south of Northern Luzon are those of the Mindoro segment. Knittel and oth- ers (1987, 1988), Knittel and Defant (1988), and Defant and others (1988) have believed that these high values are the result of slivers of con- tinental crust carried into the mantle during col- lision of the Mindoro-Palawan continental terrane with the Manila Trench. They do not rule out the possibility of sediment source con- tamination from the continental terrane. Mu- kasa and others (1987) suggested, based on Pb isotopic results from the Mindoro segment, that "partial melting of continental crust, MORB La/Sm plus subducted pelagic sediments, or upper man- tle wedge plus high-continental crust are all ca- Figure 7. La/Sm versus Ce/Yb. These ratios have been found to be high where crustal pable of producing lavas with the observed contamination is a prominent process (for example, Hildreth and Moorbath, 1988). The Min- isotopic composition." Incorporation of crustal doro segment and the Macolod Corridor are distinctly higher in these ratios than is the Bataan slivers or sediments derived from the Mindoro- segment.

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below the mantle array (for example, Downes suggests to us that the Babuyan samples are not the generation of the magmas than did the and Leyreloup, 1986) and are one of the poten- simply direct partial melts from a mantle similar southern segments because Hayes and Lewis tial end members of the Babuyan volcanics. The to that for the Ronda Ultramafics that Reisberg (1984) have found that the sediments deposited fact that typical metasomatized peridotitic man- and Zindler (1987) believed have been enriched in the South China Sea basin (derived from tle nodules from Batan island (Richard, 1986) in "exotic components." Perhaps, some of the Taiwan) are thicker in the north than in the show the same isotopic characteristics, with low same exotic components that were found in the south. 143Nd/l44Nd ratios, as do their host lavas with- Ronda Ultramafic Complex are showing up in The release of fluids from the slab, as de- out being equilibrated with them (P. Vidal and the Babuyan segment samples. Mixing curves scribed by Tatsumi and others (1986) and ex- others, unpub. data) indicates that the origin of between the Ronda rocks and hypothetical sed- pounded upon by Defant and others (1988) as a the low Nd isotopic ratios is most likely in the iment derived from Cretaceous magmatic rocks possible origin for the cross-arc Bataan LILE mantle wedge and/or the subducted slab and is of eastern China do not go through the Babuyan variations, may also have taken place in the Ba- thus not linked to the characteristics of the arc data set (for example, C4, Fig. 6). A mixing buyan segment. The early fluids released from crust. Moreover, the lead isotope characteristics curve between Tertiary granites from Taiwan the slab below Calayan initiated partial melting of the Batan island samples are consistent with and the Ronda Complex, however, does pass in the mantle, generating the tholeiitic volcan- contamination by sediments (P. Vidal and oth- through the Babuyan data (C5, Fig. 6). This is ism. The slab component was consequently a ers, unpub. data). consistent with sediment from the Asian conti- relatively small portion of the magmatic system It should be pointed out, however, that three nent incorporated into a mantle wedge consist- because of the large percentage of partial melt- of 22 Cenozoic basalts from eastern China (Peng ing of a Ronda exotic component. The mixing of ing. Smaller degrees of partial melting may have and others, 1986) fall in the same field as do the these two components does not fit the Babuyan created the high LILE concentrations in the rest Babuyan samples in Figure 6 (the other 19 Ce- data very well, however, and we believe that of the Babuyan segment when the more stable nozoic basalts fall within the mantle array). Peng there is a better model. hydrous phases broke down at greater depths. and others (1986) suggested that the isotopic Sediment with a distinctive lower-crustal sig- compositions of these samples might best be ex- nature could have been carried down by the Summary plained by source heterogeneity, but they also subduction process and incorporated into the appear to believe that these samples could have mantle wedge. So far, however, a potential The involvement of sediments in the subduc- undergone lower crustal contamination. source for this sediment has not been discovered tion process is implied by the Pb isotopic data in We cannot presently rule out the possibility of in the area. According to Hayes and Lewis both northern (Babuyan segment; P. Vidal and subduction of continental slivers beneath the (1984), Taiwan is believed to be the source of others, unpub. data) and southern (Mindoro Babuyan segment, but it is not supported by the sediment in the Manila Trench. Granite batho- segment; Mukasa and others, 1987) sections of available models of geodynamic evolution of the liths from Taiwan, however, have Nd and Sr the Luzon arc. A contribution of a mixture of Luzon arc (for example, Stephan and others, isotopic values typical of the upper crust (Jahn sediments from the Cretaceous magmatic rocks 1986). North of 19°N latitude, the South China and others, 1986). The mixing curve between a from east China and from the Taiwan granites basin floor consists of transitional and continen- MORB source (O'Nions and others, 1977) and can explain the isotopic peculiarities of the Ba- tal crust of the . A narrow zone of these granites (CI, Fig. 6) is considerably above buyan segment (low Nd ratios). Variation in the sea floor (shown in Fig. 1) is present between the Babuyan segment samples. In addition, the thickness of subducted sediment is a possible this continental mass and the Manila Trench at a upper crustal source used by Faure (1986) does explanation for the occurrences of Quaternary latitude between 19° and 21.5°N (Sea Beam not generate a mixing line through the Babuyan high-K rocks at the two ends of the arc (Fig. 2). profiles, Pelletier, 1985). The age and origin of samples when mixed with a MORB end These two ends, however, are at or near colli- the sea floor in the latter zone are presently un- member (curve not shown). Mixing curves sion zones, and we cannot rule out the hypothe- known. Sr isotopic values reported by Lan and generated between MORB and Cretaceous ses of contribution from subducted continental others (1986) from Lutao and Lanhsu islands magmatic rocks from eastern China (Xuan and material, especially in the south. others, 1986) (for example, C2 and C3, Fig. 6) (Taiwan segment) (0.7048-0.7053) may indi- The elevated LILE's (Fig. 4), Sr isotopic ra- are close to the Babuyan samples in Figure 6 but cate crustal involvement (Lan and others, 1986). tios (Fig. 6), and Th/U ratios (Fig. 5) toward do not go through them. This suggests that sed- A geochemical study of this region is currently the ends of the arc, compared with the Bataan iments generated from samples with reported in progress. segment (AFV), along with the Pb isotope data, Nd and Sr isotopic ratios from eastern China are It is possible, although not likely, that the if nothing else, suggest that these are not the only source of contamination during the variations in geochemistry along the Babuyan unusual. They require enrichment in the source formation of the Babuyan magmas. An adequate segment are the result of mantle source heteroge- region of the magmas, assimilation-fractional mixture of Cretaceous magmatic rocks, from neities. Heretofore undescribed arc mantle with crystallization processes (less likely because of east China and Tertiary granites from Taiwan, low Nd and moderate Sr isotopic values, com- the absence of upper crustal material), a hetero- however, when added to MORB mantle, may pared with other mantle sources world-wide, geneous mantle, or different degrees of partial explain the observed patterns in Figure 6. These could be the source of the Babuyan segment melting. There is no reason to suspect that two kinds of material are the most likely sources magmas. The Nd isotope ratios resemble those different degrees of melting are taking place of the detrital sediments of the South China Sea from the Ronda Ultramafic Complex in south- along north-south transects of the Luzon arc. plate south of Taiwan (Pelletier, 1985). In fact, ern Spain (Fig. 6) (Reisberg and Zindler, 1987). We are left with source enrichment or a hetero- northern segments of the Luzon arc may have The Sr isotopic ratios, however, are significantly geneous mantle to explain the isotopic and had a larger sediment component involved in higher than those of the Ronda Complex. This trace-element variations along the Luzon arc.

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The BAV and Calayan are different in that their Govindaraju, K., Mevelle, G„ and Chanard, C., 1976, Automated optical emis- v. 67, p. 1273. sion spectrochemical bulk analysis of silicate rocks with microwave Mukasa, S. B„ McCabe, R.. and Gill, J. B„ 1987, Pb-isotopic compositions of geochemistries may be explained by different plasma excitation: Analytical Chemistry, v. 48, p. 1325-1331. volcanic rocks in the West and East Philippine island arcs: Presence of Green, T. H., 1981, Island arc and -building ma;jmatism—A review the Dupal isotopic anomaly: Earth and Planetary Science Letters, v. 84, degrees of partial melting related to cross-arc of petrogenic models based on experimental petrology and geochemis- p. 153-164. subduction processes. try: Tectonophysics, v. 63, p. 367-385. O'Nions, R. K., Hamilton, P. J., and Evensen, N. M., 1977, Variations in Hamburger, M. W., Cardwell, R. K„ and Isaacs, B. L., 1982, Seismotectonics ,43Nd/144Nd and 87Sr/86Sr ratios in oceanic basalts: Earth and Plane- of the Northern Philippine island arc, in Hayes, E., éd., The tectonics tary Science Letters, v. 34, p. 13-22. and geological evolution of the Southeast Asian stas and islands (Part Peccerillo, A., and Taylor, S. R., 1976, Geochemistry of Eocene calc-alkaline ACKNOWLEDGMENTS 2): Washington, D.C., American Geophysical Union Geophysical volcanic rocks from the Kastamonu area, northern Turkey: Contribu- Monograph Series, v. 27, p. 1-22. tions to Mineralogy and Petrology, v. 58, p. 63-81. Hamilton, W., 1979, Tectonics of the Indonesian region: U.S. Geological Sur- Pelletier, B., 1985, De la fosse de Manille a la chaine de Taiwan. Etude We would like to thank the Philippine Nation- vey Professional Paper 1078, 345 p. geologique confins d'une subduction et d'une collision actives. Modele Hawkins, J. W., and Evans, J. C., 1983, Geology of the Zambales range, geodynamique (These de Doctorat]: Brest, France, Universite de Bre- al Power Corporation, Ebasco Services, and the Luzon, Philippine islands: Ophiolite derived from an island-arc back-arc tagne Occidental, 268 p. basin pair, in Hayes, D., ed., The tectonics and geological evolution of Peng, Z. C., Zartman, R. E., Futa, K., and Chen, D. G., 1986, Pb-Sr and Nd Philippine Bureau of Mines and Geosciences. the Southeast Asian seas and islands (Part 2): Washington, D.C., Amer- isotopic systematics and chemical characteristics of Cenozoic basalts, We are grateful to Charles R. Bacon, Mark Rea- ican Geophysical Union Geophysical Monograph Series, v. 27. eastern China: Chemical Geology (Isotope Geoscience Section), v. 59. p. 95-123. p. 3-33. gan, Mark D. Feigenson, Robert McCabe, and Hayes, D. E.. and Lewis, S. D., 1984, A geophysical study of the Manila Perfit, M. R., and Kay, R. W., 1986, Comment on "Isotopic and incompatible Trench, Luzon, Philippines: I. Crustal structure, gravity, and regional element constraints on the genesis of island arc volcanics from Cold Bay Ulrich Knittel for critical reviews of this manu- tectonic evolution: Journal of Geophysical Research, v. 89, and Amak Island, Aleutians, and implications for mantle structure" by script. We gratefully acknowledge the contribu- p. 9171-9195. J. D. Morris and S. R. Hart: Geochimica et Cosmochimica Acta, v. 50, Hayes, D. E., and Ludwig, W. J., 1967, The Manila Trench and West Luzon p. 477-481. tion of unpublished data from Paul C. Ragland. Trough, II. Gravity and magnetics measurements: Deep Sea Research, Ragland, P. C., and Defant, M. J., 1983, Silica standardization: A discriminant v. 14, p. 545-560. technique applied to a volcanic arc system: Earth and Planetary Science We also acknowledge support from National Hildreth, W., and Moorbath, S., 1988, Crustal contributions to arc magmatism Utters, v. 64, p. 387-395. Science Foundation Grant EAR87-21418 (to in the of central Chile: Contributions to Mineralogy and Petrol- Reisberg, L., and.Zindler, A., 1987, Extreme isotopic variations in the upper ogy, v. 98, p. 455-489. mantle: Evidence from Ronda: Earth and Planetary Science Letters, Defant). Jacques, D , 1987, Geologie et petrologie de l'archipel Babuyan et des Monte v. 86, p. 29-45. Tabungon et Cagua Nord Luzon, Philippines: Implications magnatolo- Richard, M., 1986, Geologie et petrologie d'un jalon de I'arc Taiwan-Luzon: giques et geodynamiques [These de Doctorat]: Brest, France, Universite I'ile de Batan (Philippines). 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