ORIGINAL PAPER

Frank McDermott Æ Francisco G. Delfin Jr.

Marc J. Defant Æ Simon Turner Æ Rene Maury

The petrogenesis of volcanics from Mt. Bulusan and Mt. Mayonin the Bicol arc, the Philippines

Received: 20 March 2004 / Accepted: 12 October 2005� Springer-Verlag 2005

Abstract Pliocene to recent volcanic rocks from theBulusan volcanic complex in the southern part of theBicol arc (Philippines) exhibit a wide compositionalrange (medium- to high-K basaltic-andesites, andesitesand a dacite/rhyolite suite), but are characterised bylarge ion lithophile element enrichments and HFS ele-ment depletions typical of subduction-related rocks.Field, petrographic and geochemical data indicate thatthe more silicic syn- and post-caldera magmas have beeninfluenced by intracrustal processes such as magmamixing and fractional crystallisation. However, theavailable data indicate that the Bicol rocks as a groupexhibit relatively lower and less variable 87Sr/86Sr ratios(0.7036–0.7039) compared with many of the other sub-duction-related volcanics from the Philippine archipel-ago. The Pb isotope ratios of the Bicol volcanics appearto be unlike those of other Philippine arc segments. Theytypically plot within and below the data field for thePhilippine Sea Basin on 207Pb/204Pb versus 206Pb/204Pband 208Pb/204Pb versus 206Pb/204Pb diagrams, implying a

pre-subduction mantle wedge similar to that sampled bythe Palau Kyushu Ridge, east of the Philippine Trench.143Nd/144Nd ratios are moderately variable (0.51285–0.51300). Low silica (<55 wt%) samples that havelower 143Nd/144Nd tend to have high Th/Nd, high Th/Nb, and moderately low Ce/Ce* ratios. Unlike someother arc segments in the Philippines (e.g. the Babuyan-Taiwan segment), there is little evidence for theinvolvement of subducted terrigenous sediment. Instead,the moderately low 143Nd/144Nd ratios in some of theBicol volcanics may result from subduction of pelagicsediment (low Ce/Ce*, high Th/Nd, and high Th/Nb)and its incorporation into the mantle wedge via a slab-derived partial melt.

Introduction

Late Cenozoic magmatism on the island of Luzon, thePhilippines, has produced two volcanic arcs: the Luzonarc in the west and Bicol arc in the south-east (Fig. 1,inset). The Luzon arc and its northern termination, i.e.the Babuyan-Taiwan segment north of Luzon island,has been the subject of numerous studies (de Boer et al.1980; Hayes and Lewis 1984; Mukasa et al. 1987; Knitteland Defant 1988; Knittel et al. 1988; Defant et al. 1988,1989, 1990; Chen et al. 1990; McDermott et al. 1993). Bycontrast, the Bicol arc, a chain of about a dozen volcaniccentres distributed along the length of the Bicol penin-sula (Fig. 1) has received comparatively little attention.The ca. 900 km2 Bulusan volcanic complex (BVC) at thesouthern extremity of the arc (Fig. 1) is the main focusof this study. It comprises the active cone of Mt. Bulu-san, the 11 km wide Irosin caldera, and older dissectedvolcanic centres that include the Gate Mountains at thesouthern tip of the peninsula and Mt. Bintacan to thewest (Fig. 2). Here we present major element, trace ele-ment, and isotope data for the BVC and for a smallnumber of historic lavas from Mt. Mayon in thesouthern part of the Bicol peninsula (Fig. 1).

Editorial Responsibility: Ian Parsons

F. McDermott (&)UCD School of Geological Sciences,University College Dublin, Belfield, Dublin 4, IrelandE-mail: frank.mcdermott@ucd.ie

F. G. D. Jr.School of Policy, Planning, and Development,University of Southern California,Los Angeles, CA 90089, USA

M. J. DefantDepartment of Geology, University of South Florida,Tampa, FL 33620, USA

S. TurnerDepartment of Earth and Planetary Sciences,Macquarie University, 2109 Sydney, NSW, Australia

R. MauryCNRS-UMR 6538, IUEM, Universite Bretagne Occidentale,29280 Plouzane, France

Contrib Mineral Petrol (2005)DOI 10.1007/s00410-005-0042-7

An early petrological and tectonic study of the Bicolarc ascribed its magmatism to partial melting of thesubducted Philippine Sea Plate (PSP) (Divis 1980). Morerecent studies have emphasised the mantle wedge as asource of the arc magmas (Knittel and Defant 1988;Knittel-Weber and Knittel 1990; Delfin et al. 1993;Castillo and Newhall 2004). Previous work on the

isotope geochemistry of the Bicol arc volcanics has beenconfined to a few studies in which the Bicol rocks wereanalysed in a wider regional context. Divis (1980) re-ported 87Sr/86Sr ratios in the range 0.7034–0.7047 forBicol arc andesites, although individual analyses werenot presented. Knittel and Defant (1988) noted thatrhyolite flows from Tiwi on the eastern flank of

LABO

ParacaleIntrusive

123° E

124˚ E

CAAYUNAN

ISAROG

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Albay GulfSan Vicente-Linao Fault

Lagonoy Gulf

Caramoan

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San MiguelBay

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Ryukyu Arc

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utan

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Arayat

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Fig. 1 Map of the Bicol arc showing the major volcanic centreswith historically active volcanoes denoted by a crater symbol (Mt.Mayon, Mt. Bulusan, Mt. Iriga). The study area outlined on thesouthern tip of the peninsula shows the location of the Mt. BVC

and the location of the Bintacan and Gate volcanics. Historic lavasfrom the 1968 and 1984 eruptions from Mt. Mayon have also beenanalysed. Inset map shows the location of the Bicol arc in a regionaltectonic setting

Casiguran

Juban

GUBAT

Ral

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Sharp Peak

MT. BULUSAN

Mt. Taripungso

Mt.Malobago

Mt.Calunan

Mt.Guho

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EXPLANATION: LEGEND:

610

Ral Recent slope wash & alluvial deposits

QKd Talatak Dome

QJd Jormajam Dome

QAa Agoho Andesite

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Taripungso Andesite

Sharp Peak Volcanics

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Flank lahars & Pyroclastics

Stratocone lavas & breccias

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QHb Homahan Basalt

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Gate Volcanics

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QBtv

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Bintacan Volcanics

POST CALDERA GROUP

LATEPLEISTOCENE

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EA

RLY

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E ? PRECALDERA GROUP

PRE BULUSANTGv

Mt.Jormajam

Mt.Tabon-Tabon

Fig. 2 Detailed geological map of the BVC after Delfin et al. (1993). The outline of the Irosin caldera is shown as the heavy dashed line inthe centre of the complex. Detailed descriptions of the units are given in the text

Mt. Malinao (Fig. 1) are characterised by relatively low87Sr/86Sr ratios (0.70377–0.70397), and they argued for adepleted MORB-like mantle wedge prior to subduction-related enrichments. In a regional study that included asingle andesite sample from Mt. Isarog (Fig. 1), Mukasaet al. (1987) interpreted Pb isotope data to indicate aMORB-type source characterised by a Dupal-type(Dupre and Allegre 1983) anomaly. Subsequently, Ca-stillo (1996) argued that the isotopic characteristics ofPhilippine arc lavas do not reflect a true Dupal anomaly,but instead are intermediate between an Indian OceanMORB mantle-source and an enriched OIB-sourcemantle end member. More recently, Castillo and New-hall (2004) analysed a suite of samples from Mt. Mayonvolcano (Newhall 1979) and argued that the Bicol sub-arc mantle is isotopically similar to Indian MORB, buthas been enriched by subducted pelagic sediments andhydrous fluids from the subducted basaltic crust of thePSP.

In a study of the petrology, chronology, volcanologyand major and trace element geochemistry of a largeselection of volcanic rocks from the BVC, Delfin (1991)established a chronological framework for the pre-, syn-,and post-caldera evolution of the complex. Delfin et al.(1993) published major and trace element data for rep-resentative BVC samples, and focused on magmachamber fractional crystallisation and mixing processesthat influenced the composition of the more evolvedrocks (dacites and rhyolites). The present study buildson these results and presents new trace element andisotope data for a subset of the samples studied byDelfin (1991). By focusing on the more primitive sam-ples, our primary objective is to constrain the nature ofthe sub-crustal magma sources that may have contrib-uted to the BVC rocks, and to offer insights into theirpetrogenesis in the context of regional subduction-re-lated magmatism on the Philippine archipelago.

Regional tectonic setting

In a region of considerable tectonic complexity (e.g.Rangin et al. 1985; Stephan et al. 1986), the Bicol arcoffers relative simplicity. It forms part of the Quaternaryeastern Philippine arc, and is generated by westwardsubduction of the PSP along the Philippine trench(Fig. 1) that commenced in the Pliocene (Barrier et al.1991; Cardwell et al. 1980). The oceanic crust of theWest Philippine Basin being subducted at the PhilippineTrench some 210–280 km east of the arc is 40–60 Maold, making it the oldest segment of the PSP (Hickey-Vargas 1991). The subduction rate along the northernsegment of the trench, closest to the Bicol arc has beenestimated at 6.5–8.5 cm year�1 (Barrier et al. 1991), butrecent GPS measurements provide a somewhat lowerrelative convergence value of 5.4 cm year�1 (Pubellieret al. 2003). The Benioff zone defines a fault plane thatdips 24�±10� and extends to depths of 100 km, butapparently not directly beneath the Bicol arc (Cardwell

et al. 1980). Seismic tomography results show a region ofanomalously high seismic velocities close to 100 kmdepth under the Bicol arc (Rangin et al. 1999). Thispatch of seismically fast material may correspond tocold parts of the newly subducted PSP slab. The absenceof deep earthquakes beneath the Bicol volcanoes sug-gests that slip along the Benioff zone in this part of thePhilippine Trench may be aseismic.

The three historically active volcanic centres in theBicol arc are Mt. Iriga, Mt. Mayon, and Mt. Bulusan(Fig. 1). Volcanic deposits of the Bicol arc overliebasement rocks that include Jurassic to Tertiary schists,gneisses and ophiolites that have been intruded by mid-Tertiary dioritic plutons, and are overlain by late Ter-tiary to Quaternary sediments (Geary et al. 1988; Geiseet al. 1986). In the area around the BVC, the oldest non-volcanic rocks consist of sediments of the Lower-MiddleMiocene Gatbo Formation (Travaglia and Baes 1979),while Mesozoic schists outcrop on Rapu-Rapu island tothe north of the BVC (Geary et al. 1988).

Eruptive history and petrology of the BVC

Recent K–Ar age determinations (JICA-MMAJ 1999)support and refine the eruptive history of the BVC de-scribed previously by Delfin et al. (1993). Eruptions ofhigh-K basaltic andesites and andesites dating from atleast 2.64 Ma to about 1.56 Ma form the Gate Moun-tains, a northwest-trending eroded volcanic massif about12 km wide and 46 km long at the southern end of theBVC (Fig. 2). The high-K Gate volcanics are overlain bya medium-K suite formed during successive pre-, syn-,and post-caldera eruptions, and the possible reasons forthe change to medium-K volcanism are discussed below.Medium-K pre-caldera volcanism had commenced by1.10 Ma, forming several andesitic stratocones includingMt. Bintacan, Mt. Tabon-Tabon, and Mt. Calunan tothe north of the Gate Mountains (Fig. 2). This cone-building episode was followed by major eruptions ofdacitic to rhyolitic pyroclastic flows of the IrosinIgnimbrite at approximately 35–40 ka, resulting in theformation of the 11 km wide Irosin caldera (Fig. 2).Following caldera subsidence, a period of volcanic qui-escence and lake sedimentation is indicated by peatdeposits beneath the Irosin plain (Travaglia and Baes1979). Resurgent post-caldera magmatism in the BVCstarted with the formation of the Sharp Peak volcano(Fig. 2) and continues today through the active Bulusanvolcano which dates from at least 6 ka (Delfin et al.1993), with the last eruption in 1994. These post-calderavents developed not only inside the Irosin caldera, butalso northeast of the pre-caldera volcanic axis (Fig. 2),continuing a northeasterly progression of eruptiveactivity in the BVC.

The oldest pre-caldera lavas erupted in this part ofthe southern Bicol arc were basaltic andesite andandesite lavas of the high-K Gate suite containing phe-nocrysts of plagioclase, clinopyroxene, amphibole and

magnetite in a pilotaxitic groundmass. Biotite is rare andmakes up a significant (3–5%) proportion of the phe-nocrysts in one sample only. Plagioclase phenocrytsrange in composition from An56–75 and are usuallynormally zoned. The ubiquitous presence of 1–10%amphibole phenocrysts is a distinctive feature of thehigh-K lavas. These are up to 3.5 mm in diameter,typically anhedral, and often surrounded by opaquerims of pargasite to ferroan pargasite.

Lavas from the medium-K precaldera unit are pet-rographically and mineralogically more diverse. Lavasfrom Mt. Tabon-Tabon (QTv) and Mt. Calunan (QCa)that form the prominent south-eastern wall of the Irosincaldera (Fig. 2) are compositionally similar, with up to�20% subhedral plagioclase, �7–8% clinopyroxene, 3–4% orthopyroxene, and 3–7% magnetite phenocrysts ina felty to pilotaxitic groundmass. The basalts andbasaltic andesites from Mt. Homahan (QHb, Fig. 2)contain olivine (Fo66–73) phenocrysts coexisting withaugite (Mg #=60–73) and calcic plagioclase (An55–76) inan intergranular groundmass of plagioclase microlites,glass, and clinopyroxene. The adjacent Bintacan volcano(QBtv, Fig. 2) contains minor olivine-bearing, slightlyvesicular basaltic andesites, as well as more voluminousporphyritic andesites. The latter is typically made up of30–40% phenocrysts of plagioclase (An60–93), augite(Mg #=65), and bronzite (Mg #=58) hosted by a hy-alopilitic to intersertal matrix.

Dacite and rhyolite pumice from the Irosin Ignim-brite (QIg) are the least porphyritic and most glass-richextrusives in the BVC. Phenocrysts typically compriseabout 5% of the pumice and usually consist of plagio-clase (An31–63), biotite (Mg #=51), and magne-tite ± amphibole. Rare olivine and clinopyroxenexenocrysts are found in a few samples. In thin section,the pumice groundmass is a colourless to dirty-brownvesicular glass showing pronounced flowage texture.Rounded mafic inclusions range in size from severalmillimetres to a few centimetres in diameter, and arefound in the pyroclastic flows. These inclusions arecharacterised by vesicles (�40%), andesitic composition,and higher phenocryst content (�25–30%) comparedwith their silicic host. Phenocrysts in these mafic inclu-sions include complexly zoned subhedral plagioclase,pale green clinopyroxene and orthopyroxene, and dis-seminated anhedral magnetite. The rhyolite dome of Mt.Malobago (QMd), extruded east of the caldera (Fig. 2),is chemically and petrographically similar to the pumiceof the Irosin Ignimbrite except that the former hashigher phenocryst content (�15%) and contains rareK-feldspar and quartz grains.

The Sharp Peak lavas (QSv, Fig. 2) are the earliestpost-caldera magmas erupted, and they exhibit threedistinguishing features relative to other post-calderaBVC lavas. They are more basic in composition, arehighly vesiculated to scoriaceous, and often containrounded fine-grained inclusions. Phenocryst phasestypically include anhedral to subhedral plagioclase,clinopyroxene, often forming glomerocrysts, subhedral

to euhedral orthopyroxene, magnetite, and rare (<1%)olivine and amphibole. Phenocrysts are usually distrib-uted in a hyalopilitic groundmass of dark brown glasswith some plagioclase microlites. Vesicles in the SharpPeak lavas constitute up to 50% of the total volume inthin sections. Dioritic inclusions (>3 mm in diameter)consisting of interlocking euhedral plagioclase, resorbedamphibole, pyroxenes, and magnetite are common inthese lavas.

The younger post-caldera centres such as the Jor-majam (QJd, Fig. 2) and Talatak Domes (QTd, Fig. 2)are more differentiated, less vesicular andesitic lavas. Inboth domes, the lava has a felty matrix hosting a phe-nocryst assemblage made up of plagioclase (An93–60),clinopyroxene (3–7%), orthopyroxene (3–5%), andmagnetite. A variety of textures are observed in TalatakDome plagioclase phenocrysts including corrodedinclusion-rich cores with inclusion-poor rims, discon-tinuous zoning, inclusion-free cores with inclusion-rid-dled rims, and oscillatory zoning. In the JormajamDome, the coexistence of subhedral olivine grains (1%)and 2–3% opaque-rimmed hornblende implies chemicaldisequilibrium in the crystallising magma.

Porphyritic andesitic lavas from the activeMt. Bulusan cone contain about 35–45% phenocrysts ofplagioclase (An70–40), augite (Mg #=65), hypersthene(Mg #=56), and magnetite in a pilotaxitic to felty ma-trix. Anhedral olivine (1–2%) and amphibole (<1–5%)phenocrysts are also observed in some thin sections.

Analytical techniques

Major and trace elements were analysed by XRF atX-RAL, Ontario, Canada and at the Open University,UK. Typical detection limits were 0.01 wt% and 2 ppmfor the major and trace elements, respectively. REEs andselected trace elements were determined by ICP-MSfollowing HClO4/HF digestion for selected samples.REE contents in two samples from the historic eruptionson Mt. Mayon were determined by INAA at the OpenUniversity. Sr and Nd isotopes were analysed by thermalionisation mass spectrometry (TIMS) at the Open Uni-versity and corrected for within-run mass bias to86Sr/88Sr=0.1194 and 144Nd/146Nd=0.7219. All Sr andNd data are reported relative to the following values forNBS 987 (0.71025) and La Jolla (0.51184). Uncertaintiesdetermined from the 2r reproducibility of the NBS 987and J&M standards during the course of analysis were28 ppm for Sr and 21–25 ppm for Nd.

Pb was analysed in temperature controlled runs(1,250�C) in static multi-collector mode on a FinniganMAT 262 at the Open University, and the ratios werecorrected for �1& per atomic mass unit mass-fractionation using the recommended values for NBS981 (Todt et al. 1996). Blanks for Sr, Nd, and Pb weretypically <1 ng, 500 and 300 pg, respectively.

Th, U and 226Ra concentrations and 230Th/232Thisotope ratios were determined on a subset of the

samples at the Open University by TIMS on a highabundance sensitivity Finnigan MAT 262 equipped withan RPQ-II energy filter. Samples were spiked with229Th-, 236U, and 228Ra tracers and dissolved using HF–HCl–HNO3 in Savillex� beakers. Treatment with HCland H3BO3 was used to obtain completely clear solu-tions and to ensure sample-spike equilibration. Detailsof the U–Th separation and analytical techniques can befound in Turner et al. (1998), and the procedures for228Ra purification and analysis were identical to thosedescribed in Turner et al. (2000). U–Th mass spectro-metric procedures were similar to those described in vanCalsteren and Schwieters (1995). U, Th, and 226Raconcentrations were determined to a precision of betterthan 1.0% (2r), and the U and Th isotope ratios have anexternal reproducibility of �1.0% (2r) which wasmonitored using the Open University standard solutionTh¢U¢Std. Reproducibility of (226Ra/230Th) is estimatedto be 1.3% (Turner et al. 2000). Total procedural blankswere typically 100 and 50 pg, respectively, for U and Thand <0.1 fg for 226Ra. All blanks are insignificant rel-ative to the 30–300 ng of U and Th and the 20–200 fg ofRa that were separated and analysed. Determinations ofthe AThO and TML Th standards yielded(230Th/232Th)=1.017±0.005 (n=5) and (230Th/232Th)=1.079±0.005 (n=2) during this period. Repeat deter-minations of the Mt. Lassen Ra standard yielded anaverage of 226Ra=1,065±9 fg g�1 which is within errorof the values quoted by Volpe et al. (1991). Decayconstants used in the calculation of activity ratios werek238U=1.551·10–10 a�1; k234U=2.835·10�6 a�1; k232

Th=4.948·10�11 a�1; k230Th=9.195·10�6 a�1;k226Ra=4.332·10�4 a�1.

Major and trace element chemistry

Most of the Bicol arc samples analysed here are inter-mediate in character and are largely basaltic andesites

and andesites (�53–62% SiO2). On a K2O versus SiO2

diagram (Fig. 3), the BVC rocks can be subdivided into aPliocene to early Pleistocene high-K suite comprised al-most exclusively of the pre-caldera Gate volcanics, and ayounger Pleistocene to Holocene medium-K suite. Theoldest high-K Gate volcanics and the medium-K pre-caldera Bintacan basaltic andesites and andesites are calc-alkaline (sensu-stricto) using the original definition ofPeacock (1931), as discussed recently by Arculus (2003).The syn-caldera eruptives, dominated by the IrosinIgnimbrite and the Malobago Dome rocks, represent themost differentiatedmagma erupted in the BVC, with SiO2

contents in the range 66–76% (Fig. 3). The mafic inclu-sions within the Irosin Ignimbrite are intermediate incomposition, with silica contents in the range 58–59 wt%. In contrast with the pre-caldera rocks, the post-caldera Sharp Peak volcanics and the Agoho andesitetend to be calcic (sensu-stricto), as are the late Pleistoceneto historic basaltic andesites and andesites fromMt. Mayon. Both Jormajam Dome samples are andesitic(c. 60 wt% SiO2), but as discussed below probably rep-resent the product of magma mixing processes.

At a given SiO2 concentration, the high-K Gate vol-canics contain greater Na2O and P2O5, but lower Fe2O3,MgO, and CaO than the younger medium-K eruptives.Although some scatter is noted for the syn-calderasamples, most of the major elements from the medium-K suite show good linear correlation with SiO2 (Delfinet al. 1993). Al2O3, CaO, Fe2O3, MgO, and TiO2 de-crease with increasing SiO2 (not shown). There is greaterscatter for Na2O, MnO, and P2O5, but in general Na2Oincreases while MnO and P2O5 decrease with increasingdifferentiation.

Both of the historic samples analysed from Mt. Ma-yon are basaltic andesites. They plot within the medium-K series on Fig. 3 and define a narrow compositionalrange relative to the larger suite of late Pleistocene tohistoric Mt. Mayon suite analysed by Castillo andNewhall (2004).

0.0

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48 52 56 60 64 68 72 76

Wt % SiO2

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% K

2O Agoho andesite

Bintacan volcanics

Calunan andesite

Gate volcanics

Homahan Basalt

Irosin Ignimbrite

Jormajam dome

Bulusan stratocone

Malobago Dome

Sharp Peakvolcanics

MayonPliocene to early Pleistocene Gate volcanics

Syn-caldera eruptives

Fig. 3 K2O versus SiO2

diagram for the BVC rocks andthe two historic Mayonsamples. The late-Pliocene toearly-Pleistocene Gate volcanicsform a high-K calc-alkalinesuite. The data field enclosed bythe dashed line represents datafor volcanics from Mt. Mayon(Castillo and Newhall 2004).Data symbols arranged in thestratigraphic order of the unitsthat they represent

Chondrite normalised REE patterns are shown inFig. 4. All samples are LREE enriched (Lan/Ybn in therange 4–12), and a high-K andesite from the Gatevolcanics (sample B220) exhibits the highest REEconcentrations. In detail, the Bicol arc rocks exhibithigher Lan/Ybn at a given silica value compared withthose from the Macolod Corridor to the north-west(Fig. 1, inset). The single sample analysed here fromthe pre-caldera Homahan basalt is also LREE en-riched, but that from the pre-caldera Calunan andesitehas a REE pattern similar to the post-caldera rocks.There is a tendency for higher La/Yb ratios in thesilicic syn-caldera samples (Irosin Ignimbrite andMalobago Dome samples), largely reflecting theirlower HREE abundances. Thus, the Irosin Ignimbriteand Malobago Dome samples have Lan/Ybn ratios of10–12 compared with <10 for the other BVC units.Most of the basalts and basaltic andesites have mod-erate negative Eu anomalies (Eu/Eu*=0.83–0.98), butthe dacitic and rhyolitic Irosin Ignimbrite samples(B146A and B1B) have distinctly more negative Euanomalies (Eu/Eu*=0.5–0.65). In common with manyintra-oceanic subduction-related volcanics (e.g. Wood-head 1989; Elliott et al. 1997; George et al. 2003), theBVC samples exhibit small negative cerium anomalies,with Ce/Ce* in the range 0.88–0.97.

N-MORB normalised (Sun and McDonough 1989)trace element abundances for selected samples are

shown in Fig. 5. All samples show characteristic sub-duction-related enrichments in the large ion lithophile(LIL) elements and LREE, with relative depletions inNb and Ti. Unlike some arcs (e.g. parts of the Mari-anas arc, Elliott et al. 1997) there is no evidence in anyof the Bicol rocks analysed for systematic depletion ofthe HFS elements relative to N-MORB. However, theabsence of strong negative HFS anomalies in the Bicolrocks may reflect their higher overall incompatibletrace element contents, and there is no evidence, forexample, for unusually low LREE/HFS ratios (e.g. La/Zr). The high-K Gate volcanics exhibit a distinctivetrace element pattern and are characterised by higherconcentrations of all of the trace elements in Fig. 5,except for Nb which is significantly lower than that ofthe medium-K rocks. The medium-K (mostly basalticandesites and andesites) exhibits trace element patternsthat are typical of subduction-related magmas (Fig. 5a)and are trace element enriched compared with similarsamples from the Marianas arc (Elliott et al. 1997). Bycontrast, the dacites and rhyolites of the IrosinIgnimbrite and syn-caldera Malobago Dome (Fig. 5b)exhibit stronger LIL element enrichments relative tothe more mafic samples, and are also characterised bylower abundances of the middle to heavy REEs and theHFS elements.

Isotope data

87Sr/86Sr ratios in the Bicol arc volcanics are relativelylow (0.70373–0.70399) compared with those from manyof the other arc segments of the Philippine archipelagofor which data are available (e.g. the Babuyan-Taiwanarc segment, the Luzon/Mindoro arcs, and the MacolodCorridor; Fig. 6). 143Nd/144Nd ratios in the Bicol arcrocks are relatively high (Fig. 6), but extend to lowervalues than those of Central Mindanao (Sajona et al.2000) and Camiguin Island in the southern Philippines(Castillo et al. 1999). Data for basalts erupted in theMacolod corridor, an extensional zone to the west ofBicol (Defant et al. 1988, 1989, 1991; Forster et al. 1990)exhibit distinctly higher 87Sr/86Sr and slightly lower143Nd/144Nd isotope ratios relative to data for the Bicolarc (Miklius et al. 1991; Mukasa et al. 1994; Knittel et al.1997). Similarly, basalts from the Bataan segment in thesouth Luzon part of the western arc exhibit a more en-riched character with higher 87Sr/86Sr and lower143Nd/144Nd than in the Bicol rocks. The low143Nd/144Nd ratios of the north Luzon and the Babu-yan/Taiwan segments of the western Philippine arc sys-tem (Fig. 6) have been well documented previously (e.g.Defant et al. 1989; Chen et al. 1990; McDermott et al.1993), and are in sharp contrast with the data presentedhere for the Bicol volcanics.

Previously published Pb isotope data for the Babuyanand Taiwan arc segments (McDermott et al. 1993; Ca-stillo 1996) are characterised by steep arrays displacedaway from the Northern Hemisphere reference line

1

10

100

1000

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb

Bintacan volcanics

Gate volcanics

Homahan basalt

Irosin Ignimbrite

Bulusan stratocone

Agoho andesite

Malobago Dome

Sharp Peak volcanics

Jormajam dome

Ro

ck/C

ho

nd

rite

Fig. 4 Chondrite normalised REE data for the BVC. Data symbolsas in Fig. 3

(NHRL) on 207Pb/204Pb versus 206Pb/204Pb and208Pb/204Pb versus 206Pb/204Pb diagrams, and wereinterpreted to reflect the influence of subducted terrige-nous sediment. Prior to this study Pb isotope data forthe Bicol arc volcanics consisted of a few samples fromMt. Mayon only (Castillo 1996) and a single analysisfrom a late Miocene andesite flow from near Mt. Isarog(Mukasa et al. 1987). An important observation is thatthe new data for the Bicol arc exhibit lower 207Pb/204Pband 208Pb/204Pb at a given value of 206Pb/204Pb com-pared with almost all of the other arc segments in thePhilippine archipelago. The data plot within and slightlybelow the field for the Philippine Sea Basin MORB(Hickey-Vargas 1991, 1998) on a 207Pb/204Pb versus206Pb/204Pb diagram (Fig. 7a), but are clearly below thisfield on a 208Pb/204Pb versus 206Pb/204Pb diagram(Fig. 7b). Thus, the data extend towards and into the

data field for the Palau Kyushu Ridge (PKR), aspreading centre to the east of the Philippine Trench thatis characterised by systematically lower 207Pb/204Pb and208Pb/204Pb compared with other segments of the Phil-ippine Sea Basin (site 448, Hickey-Vargas 1991). Datafor the Mt. Mayon 1968 and 1984 eruptions plot close toor even slightly below the NHRL on the Pb isotopediagrams, although when a larger group of late Pleis-tocene to historic samples from Mt. Mayon are takeninto account (Castillo 1996; Castillo and Newhall 2004),the Mayon data define a steep array on both the207Pb/204Pb versus 206Pb/204Pb and 208Pb/204Pb versus206Pb/204Pb diagrams. There is no evidence for the Du-pal-type Pb isotope anomaly inferred previously for thisarc segment (Mukasa et al. 1987).

The U-series data are presented in Fig. 8. All of theBicol samples exhibit a relatively narrow range in

Fig. 5 N-MORB normalisedtrace element patterns for a thebasalts, basaltic andesites, andandesites from the BVC andMt. Mayon and b the high-silicaIrosin Ignimbrite andMalobago Dome samples. Datasymbols as in Fig. 3. The greyshaded pattern (sample AGR2,Agrigan, Marianas) is shownfor comparison (Elliott et al.1997)

(230Th/232Th) ratios (0.89–1.07), and the ratios aremoderately low compared with subduction-related lavasglobally. Of the published Th isotope data for subduc-tion-related lavas worldwide, only those from the Luzonarc (McDermott and Hawkesworth 1991) and Indonesia(Rubin et al. 1989; Gill and Williams 1990) exhibit sys-tematically lower (230Th/232Th) ratios (Fig. 8). Five ofthe six samples analysed for Th isotopes plot close to,but slightly to the right of the equiline on a (230Th/232Th)versus (238U/232Th) diagram (Fig. 8, inset) indicatingrelatively small but recent (<350,000 years) enrichments(2–6%) in U relative to Th. U enrichments are com-monly observed in subduction-related lavas (e.g. Gilland Williams 1990; McDermott and Hawkesworth 1991;Condomines and Sigmarsson 1993) and are usuallyattributed to preferential release of uranium from thesubducted slab. The sixth sample (B-107B, a basalticandesite from the post-caldera Sharp Peak vent in theMt. BVC) is unusual because it exhibits relatively low(238U/230Th), indicating recent Th enrichment (c. 20%excess Th) relative to uranium. In common with manylow (230Th/232Th) lavas, the Mayon and Bulusansamples have relatively high Th contents (1.7–10.5 ppm), with the highest values occurring in the earlyhigh-K suite (e.g. sample B-220; Fig. 5). The latter is inU–Th secular equilibrium (Table. 1, 2, 3, 4, 5), consis-

tent with its Pliocene age and so no information aboutsubduction-related U–Th fractionation can be derivedfrom its Th isotope data.

226Ra data for the 1968 and 1984 basaltic andesitesfrom Mt. Mayon were included in a study of the226Ra–230Th systematics of subduction-related rocksworldwide (Turner et al. 2001). Both samples exhibitmoderate Ra excess relative to many subduction-relatedmagmas, with 226Ra/230Th ratios of 1.51 and 1.59 for thesamples erupted in 1968 and 1984 AD, respectively. Since226Ra has a relatively short half-life (1,622 years), thedata require that radium addition occurred recentlycompared with the timescales required to achieve secularequilibrium with 230Th (approximately 8,000 years, orfive half-lives of 226Ra).

0.51220

0.51230

0.51240

0.51250

0.51260

0.51270

0.51280

0.51290

0.51300

0.51310

0.51320

0.703 0.704 0.705 0.706 0.70787Sr/86Sr

143N

d/14

4N

d

Luzon Arc(Bataan segment)

Philippine Sea Basin

Macolod Corridor

Mindoro Arc

Mindanao/CamiguinPKR

Fig. 687Sr/86Sr versus 143Nd/144Nd diagram for the Bicol volca-

nics. Also shown are the data fields for basalts from the PhilippineSea Basin (Hickey-Vargas 1991, 1998)). The data field for the PalauKyushu Ridge basalts (PKR) is shown separately as a shaded field.The data field enclosed by the dashed line represents data forvolcanics from Mt. Mayon (Castillo and Newhall 2004). Datasources for Philippine subduction-related rocks are as follows:Mindanao/Camiguin (Sajona et al. 2000; Castillo et al. 1999),Bataan Arc (Knittel et al. 1988; Mukasa et al. 1994), MacolodCorridor (Knittel et al. 1997; Mukasa et al. 1994), Mindoro Arc(Knittel et al. 1988), Taiwan/Babuyan segments (Vidal et al. 1989;Chen et al. 1990; McDermott et al. 1993), North Luzon(McDermott et al. 1993). Typical 2r measurement errors aresmaller than the data symbols

37.50

37.70

37.90

38.10

38.30

38.50

38.70

38.90

39.10

39.30

17.8 18.0 18.2 18.4 18.6 18.8 19.0 19.2

206Pb/204Pb

208 P

b/20

4 Pb

15.40

15.45

15.50

15.55

15.60

15.65

15.70

17.8 18.0 18.2 18.4 18.6 18.8 19.0 19.2

206Pb/204Pb

207 P

b/20

4 Pb

MC

TS

CM

NHRL

BS

Philippine Sea Basin

M

Philippine Sea Basin

PKR

MC

M

TS

CM

SP

BS

CG

NHRL

SP

PKR

a

b

Fig. 7 a207Pb/204Pb versus 206Pb/204Pb isotope ratios for the Bicol

volcanics. b 208Pb/204Pb versus 206Pb/204Pb isotope ratios for theBicol volcanics. Also shown in both diagrams are fields forpublished data for other arc segments of the Philippines archipel-ago. Data for terrigenous sediments from the South China Sea westof the Manila Trench (McDermott et al. 1993) are shown forcomparison (grey diamonds). Data fields: M Mayon (Castillo andNewhall 2004), MC Macolod Corridor (Mukasa et al. 1994), BSBatan segment (McDermott et al. 1993), SP Southern Philippines(Castillo et al. 1999), TS Taiwan segment, CM Central Mindanao(Sajona et al. 2000). Typical (2r) measurement error ellipses areshown in the upper left of each diagram

Discussion

Compositional variations within the Bicol rocks

Delfin et al. (1993) showed that the steep decrease inAl2O3, CaO, Fe2O3, MgO, and TiO2 with increasingSiO2 in the dacites and rhyolites can be modelled as theproducts of crystallisation and removal of plagioclase,pyroxene, olivine, and magnetite, while a sharp changein slope in the Harker diagrams at about 66–68% SiO2

was interpreted to reflect the onset of biotite ± K-feldspar ± quartz fractionation. However, the results ofmass-balance calculations show that the high-K suite(Gate volcanics, Fig. 3) is unlikely to be related, either asparent or daughter magma, to the medium-K suitethrough crystal fractionation (Delfin et al. op. cit.).Trace element systematics also indicate that the high-Ksuite (Fig. 3) cannot be related to the medium-K lavasthrough shallow-level magmatic processes such ascrystal fractionation or crustal assimilation, implying adistinct parental magma (Delfin et al. 1993). Since thereare no obvious differences in Sr, Nd, or Pb isotopesbetween the high- and medium-K suites, one explana-tion is that the incompatible element enriched high-Ksuite reflects small degree melts of the mantle wedge that

were subsequently re-melted to form the parental mag-mas of the medium-K rocks. We note that the high-KGate volcanics were erupted west–south-west of themedium-K volcanics, and hypothesise that trench-ward(eastward) flow in the mantle wedge produced a sourcefor the medium-K suite that previously had small degreemelts extracted to generate the magmas of the high-KGate volcanics.

Magmatic differentiation processes in the crustclearly played a role in the petrogenesis of the syncalderaand post-caldera stages of the BVC, given the wide rangein rock types and the petrographic evidence for dis-equilibrium textures and magma mixing describedabove. Highly vesicular cognate mafic inclusions in theIrosin Ignimbrite provide evidence that basaltic magmawas entrained by the more siliceous melts prior to theignimbritic eruption at 35–40 ka. Although no detailedphase chemistry data are available for the inclusions,their bulk chemistry is andesitic (average SiO2=58%),and their petrography is dominated by acicular crystalsof orthopyroxene and plagioclase, with rare olivineembedded in a highly vesiculated glassy groundmass.These inclusions may have formed when hot basalticrecharge, later erupted as Sharp Peak lavas (SiO2=51–54%), intruded a cooling reservoir of rhyolitic melt,effectively forming a zoned magma chamber. Sub-sequent degassing and rapid crystallisation of thebasaltic blobs at the magmatic interface may havefacilitated mixing that led to the catastrophic caldera-forming eruption (Delfin et al. 1993). Similarly, theJormajam Dome andesites exhibit petrographic evidencefor chemical disequilibrium, probably reflecting magmamixing (subhedral olivine grains and opaque-rimmedhornblende). The REE and other trace element data forthe Jormajam Dome rocks (Figs. 4, 5) are consistentwith mixing between a basaltic magma (similar to thepre-caldera Homahan basalt, for example) and a silicicrock (e.g. Irosin Ignimbrite composition).

The most striking feature of the new Sr isotope datafor the Bicol arc is the narrow range in 87Sr/86Sr, and wenote that even the rhyolitic Irosin Ignimbrite sampleexhibits relatively low 87Sr/86Sr (0.70390). The range in87Sr/86Sr presented here is similar to that for the Putsanrhyolite (0.70377–0.70387) analysed by Knittel and De-fant (1988), and for volcanics from Mt. Malinao(0.70374–0.70397) published by Knittel-Weber andKnittel (1990). The Irosin Ignimbrite sample alsoexhibits high 143Nd/144Nd (0.51291) relative to otherPhilippine arc segments. Thus, despite the occurrence oflate Jurassic schists in the southern part of the Bicolpeninsula (Geary et al. 1988), the present indications arethat if crustal anatexis or crustal contamination was asignificant process in generating the rhyolitic volcanicsin the BVC, it did not result in large shifts in theirradiogenic isotope ratios. Nonetheless, discussions onthe nature of the subduction-related processes respon-sible for the petrogenesis of the Bicol arc rocks (below)will be restricted to the basaltic and basaltic andesitesamples.

0.0

1.0

2.0

3.0

0.0 1.0 2.0 3.0

(238U/232Th)

(230 T

h/23

2 Th

)

Nicaragua

Pacific arcs

Indonesia0.7

0.8

0.9

1.0

1.1

0.7 0.8 0.9 1.0

B220

1984

B107B

1968B148B1B

Fig. 8 (230Th/232Th) versus (238U/232Th) ‘equiline’ diagram for theBicol volcanics (filled circles) in the context of subduction-relatedsuites worldwide. Also shown for comparison are U-series data forthe North Luzon lavas (open circles) after McDermott et al. (1993).The data field for ‘Pacific arcs’ includes data from the Marianas(McDermott and Hawkesworth 1991; Elliott et al. 1997), Tonga-Kermadec (Turner and Hawkesworth 1997), Vanuatu and Indo-nesia (Turner et al. 2003), and the Aleutians (George et al. 2003).The data field for Nicaragua is after McDermott and Hawkesworth(1991) and Reagan et al. (1994) and that for Kamchatka is afterTurner et al. (1998). Inset diagram shows details of the Bicol data.Typical 2r measurement error ellipses are shown in the upper left ofboth diagrams

Evidence for the nature of the Bicol magma sources

On a 207Pb/204Pb versus 206Pb/204Pb diagram (Fig. 7a),data for the BVC rocks analysed here plot entirelywithin and below the field of Philippine Sea MORB,implying that, unlike the Taiwan, Babuyan and N. Lu-zon segments of the Luzon arc (McDermott et al. 1993;Castillo 1996), they do not require that a large propor-tion of their Pb inventory be derived from subductedterrigenous sediment. Data from DSDP Leg 59 site 448on the PKR, the spreading centre to the east of thePhilippine Trench, exhibit unusually low 207Pb/204Pband 208Pb/204Pb ratios (Hickey-Vargas 1998) and plot

just below the Bicol arc data on the Pb–Pb isotopediagrams. Thus, the new Pb isotope data for the Bicolvolcanics permit the involvement of subducted sedimentin their petrogenesis if the pre-subduction mantle wedgehad unradiogenic 207Pb/204Pb and 208Pb/204Pb similar tothat sampled by the PKR to the east of the arc (Fig. 7).The two historic Mt. Mayon basaltic andesites analysedhere (1968 and 1984) exhibit low 207Pb/204Pb and208Pb/204Pb ratios, and plot closest to the data field forthe PKR, implying that their Pb inventory could havebeen derived from a mantle wedge with this isotopiccomposition. A possible alternative explanation, namelythat low 206Pb/204Pb and 207Pb/204Pb signature of the

Table 1 New major element data for selected volcanics from the Bicol arc

Sample SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5

Post-calderaBulusan strata-cone (RQBv)B34 59.1 0.53 17.7 6.49 0.15 2.99 7.16 3.53 1.80 0.18B-125 59.6 0.53 17.8 6.45 0.15 2.92 6.9 3.63 1.76 0.20B-128A 58.1 0.57 17.5 6.88 0.15 3.54 7.43 3.55 1.68 0.20B-120A 59.4 0.51 17.8 6.27 0.14 3.06 6.89 3.64 1.72 0.15

Jormajam dome (QJd)90-02 61.2 0.46 17.6 5.36 0.13 2.43 6.02 3.55 1.93 0.17B-45 59.4 0.51 18.0 6.01 0.14 2.71 6.70 3.37 1.88 0.19

Agoho andesite (Qaa)B-126A 59.8 0.54 17.6 6.51 0.14 3.07 6.84 3.62 1.80 0.17B-116A 57.5 0.59 17.1 7.43 0.15 3.81 7.45 3.38 1.57 0.18B-118A 59.1 0.48 17.9 6.16 0.14 3.07 6.98 3.53 1.73 0.19B-119 59.5 0.54 17.7 6.37 0.14 3.10 6.69 3.59 1.77 0.17

Sharp peak volcanics (QSv)90-18 54.3 0.50 19.7 7.14 0.15 4.00 8.59 3.18 1.06 0.1790-04 50.9 0.58 20.7 8.49 0.20 3.86 6.75 2.57 0.15 0.18

B-107B 53.9 0.51 19.4 7.47 0.16 4.19 7.99 3.32 1.08 0.16

CalderaIrosin Ignimbrite (Qlg)B-1B 75.6 0.17 13.6 0.82 0.08 0.27 1.38 4.39 3.05 0.04B93 65.7 0.28 16.4 2.97 0.12 1.25 3.21 4.02 2.25 0.11

Pre-calderaBintacan volcanics (QBtv)B-140A 59.2 0.52 18.10 6.45 0.17 2.58 6.09 3.83 1.92 0.35B-148 52.5 0.71 20.8 8.09 0.16 3.79 9.73 2.97 0.87 0.22B-169 57.7 0.56 18.4 7.34 0.16 3.02 7.06 3.48 1.70 0.2590-08 53.6 0.68 19.7 7.88 0.16 3.67 5.61 2.90 1.29 0.25

Calunan andesite (Qca)B-184 55.1 0.76 18.0 8.54 0.17 4.02 8.09 3.42 1.20 0.2090-05 56.1 0.66 18.7 7.21 0.15 3.35 8.14 2.96 1.40 0.17

Gate volcanics (TGv)B-168 58.2 0.62 17.8 6.11 0.14 2.45 6.27 3.42 3.11 0.28B-192 57.5 0.55 18.7 6.79 0.14 2.89 6.90 3.71 1.79 0.27B-213A 53.5 0.81 17.9 8.94 0.17 3.49 7.51 2.75 2.06 0.23B-216 60.9 0.56 17.6 5.51 0.13 1.61 4.96 4.57 2.76 0.36B-220 58.3 0.94 18.6 5.56 0.12 2.17 6.12 4.21 2.47 0.3190-10 53.3 0.67 18.7 7.59 0.17 3.75 8.36 3.07 1.90 0.26

Mt. Mayon1968 54.7 0.72 18.6 8.44 0.17 4.33 8.46 3.38 1.12 0.301984 54.7 0.71 18.6 8.31 0.16 4.23 8.45 3.32 1.13 0.30

All values in wt%. Data for samples B34, B-107B, B-148, B93, B-220, B140A, and B-1B were presented previously by Delfin et al. (1993),but are included here because new isotope data and/or trace element data are provided for these samples in Table. 3 and 4. Map units referto those shown in Fig. 2

Bicol volcanics reflects Pb from subducted oceanic crustformed at the PKR is considered less likely for mass-balance reasons, because any sediment componentwould dominate the Pb budget, and therefore the iso-topic composition of slab-derived Pb.

However, other published data for Mt. Mayon lavas(Castillo and Newhall 2004) with a wider compositionalrange than the two samples analysed here, indicate thatvolcanic products from Mayon exhibit a considerablerange in Sr, Nd, and Pb isotopes. This larger datasetstrongly suggests a role for subducted sediment in thegenesis of some Mt. Mayon lavas (Castillo 1996; Castillo

and Newhall 2004). Considering the Pb isotope datasetfor the Philippine archipelago as a whole, however(Fig. 7), it is evident that there are distinct intersegmentvariations in the dataset that probably reflect processesother than sediment subduction. For example, the sub-parallel nature of the Pb isotope data arrays (Fig. 7)implies a relatively large range in 206Pb/204Pb (18.2–18.5)within those samples least likely to have been affected bysubducted sediment-derived Pb (i.e. lowest 207Pb/204Pb).Thus, the clear evidence for coupled systematic latitu-dinal variations in the isotopic character of the sub-ducted sediment signature (McDermott et al. 1993) may

Table 2 Trace element data for selected Bicol arc volcanics

Sample number Ba Cr Nb Ni Rb Sr Y Zr

Post-calderaBulusan strato-coneB-128A 463 26 6 <2 28 511 24 113B-120A 443 23 <2 <2 35 385 15 105B34 437 24 <2 83 41 419 14 116

Jormajam domeB-45 543 20 4 <2 41 452 15 11690-02 556 19 2 2 44 435 11 117

Agoho andesiteB-125 474 23 4 <2 41 453 17 115B-126A 496 27 2 <2 40 419 19 108B-116A 437 15 3 7 36 427 11 96B-118A 452 4 5 <2 39 437 18 108B-119 465 23 7 2 37 427 16 111

Sharp peak volcanicsB-107B 225 22 42 <2 25 392 16 7890-18 233 14 3 13 24 423 12 7390-04 311 27 2 3 4 444 20 92

CalderaIrosin IgnimbriteB-1B 793 21 7 <2 88 175 15 100B93 792 <2 7 <2 64 323 7 109

Pre-calderaBintacan volcanicsB-140A 799 25 <2 <2 35 675 16 131B-148 284 20 3 4 17 519 15 72B-169 652 25 3 <2 34 582 21 10590-08 748 <2 4 4 18 530 46 104

Calunan andesiteB-184 262 30 <2 <2 25 423 22 9290-05 334 8 3 <2 30 405 21 78

Gate volcanicsB-168 833 15 8 <2 85 726 28 231B-192 669 11 <2 <2 50 654 21 123B-213A 573 3 4 <2 48 513 15 112B-216 986 7 4 <2 77 589 35 204B-220 976 4 <2 <2 60 602 42 18390-10 590 28 <2 7 46 752 16 134

Mt. Mayon1968 353 8 4 6 21 713 23 981984 363 12 4 7 20 714 24 99

All values are in ppm. Data for samples B34, B-107B, B-148, B93, B-220, B140A, and B-1B were presented previously by Delfin et al.(1993), but are included here because new isotope data and/or trace element data are provided for these samples in Table. 3 and 4. Mapunits (Fig. 2) for these samples are given in Table 1

be superimposed on mantle domains with distinct iso-topic characteristics along the archipelago (Castillo1996).

Previously published U-series data for five Holocenebasaltic andesites from the Taiwan segment of the Lu-zon arc (McDermott and Hawkesworth 1991) werecharacterised by very high Th contents (14–24 ppm),

low (230Th/232Th) ratios (0.55–0.67), and high208Pb*/206Pb* ratios (0.98–1.02) that were ascribed tomantle source modification by subducted sediment. Bycontrast, the (230Th/232Th) ratios for the historic andHolocene basaltic andesites from Mt. Mayon and theBVC (post-caldera Sharp Peak vent) presented here(Table 5) exhibit higher (230Th/232Th) ratios (0.89–1.07),

Table 3 INAA trace element data for selected Bicol arc lavas

Sample number U Th Hf La Ce Pr Nd Sm Eu Gd Dy Er Yb

Post-calderaBulusan strataconeB-34 1.2 4.3 3.1 14.4 26.8 3.3 13.5 2.9 0.83 3.1 2.5 1.9 1.9

Jormajam dome90-02 1.2 4.3 2.7 16.6 31.5 3.3 13.6 2.7 0.92 2.7 2.2 1.6 1.4

Agoho andesiteB-116A 1.0 2.9 2.7 14.3 26.2 3 13.3 2.8 0.79 3.1 2.8 1.9 1.9

Sharp peakB-107B 0.6 1.5 2.1 9.4 18.1 2.1 9.9 2.5 0.66 2.3 2.4 1.5 1.6

CalderaIrosin IgnimbriteB-1B 2.2 7.4 2.4 21.1 35.2 3.3 11.8 2.3 0.31 1.6 1.3 1.0 1.2B-93 1.6 6.2 1.7 18.7 32.9 3.3 12.0 2.1 0.74 1.9 1.7 1.2 1.2

Pre-calderaBintacan volcanicsB-140A 0.8 2.8 3.2 31.4 56.5 7.1 28.7 4.8 1.47 4.4 3.2 2.1 2.0B-148 0.8 2.6 1.9 13.3 25.3 3 13.8 3.1 0.94 3.6 3.2 2.1 1.9

Gate volcanicsB-220 3.2 12.3 5.1 35.1 64 7.1 31.0 6.2 1.53 6.5 5.8 4.0 4.2

Mt. Mayon1968 0.57 1.8 2.6 16.3 31.8 19.6 4.3 1.30 2.21984 0.60 2.0 2.7 15.8 31.7 19.5 4.3 1.30 2.2

Data for samples B-148, B-220, B-1B, and B-107B were presented previously by Delfin et al. (1993), but are included here because newisotope data are provided for these samples in Table 4

Table 4 New Sr, Nd and Pb isotope data for selected volcanics from the Mt. Bulusan volcanic complex and Mt. Mayon volcanics in theBicol arc

Sample 87Sr/86Sr 143Nd/144Nd 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb

Post-calderaSharp peak volcanicsB-107B 0.70375 0.512892 18.453 15.516 38.220

CalderaIrosin IgnimbriteB-1B 0.70390 0.512908 18.460 15.513 38.263

Pre-calderaBintacan volcanicsB-148 0.70385 0.512870 18.455 15.504 38.227

Gate Mountain volcanics90-10 0.70387 18.521 15.525 38.383B-220 0.70399 0.512937 18.482 15.497 38.320

Mt. Mayon1968 0.70373 0.512872 18.480 15.483 38.1631984 0.70373 0.512909 18.484 15.493 38.175

Map units refer to those shown in Fig. 2

with Th contents lower by about an order of magnitude(1.7–2.0 ppm). The Bicol rocks exhibit markedly lower208Pb*/206Pb* (0.95–0.96) compared with those from theLuzon arc to the north (0.98–1.02), suggesting that theirsources had lower time-integrated Th/U ratios for geo-logically long (>106 year) timescales. The absence oflarge U–Th disequilibria in these samples indicates eitherfluid-related enrichments of the mantle wedge were rel-atively small and/or that fluid-related enrichments oc-curred, but they are relatively old compared with thetimescales required to preserve significant disequilibria(several hundred thousand years). A notable exception issample B107B, a basaltic andesite from the post-calderaSharp Peak volcano in the Mt. BVC that exhibits low(238U/230Th), plotting to the left of the equiline onFig. 8. Paradoxically, this sample is also characterisedby higher Ba/La and Sr/Th ratios than any of the Bicolrocks analysed here. These ratios are often interpreted toreflect a slab-derived hydrous fluid signature, and so theexpectation would be that this sample should have high,not low (238U/232Th). A possible explanation is that theobserved high Ba/La and Sr/Th ratios reflect old(>350,000 year old) modification of its mantle wedgesource by slab-derived fluids, and that the source hadevolved back on to the (230Th/232Th)–(238U/232Th) eq-uiline prior to the late Pleistocene episode of partialmelting that gave rise to its magma. An alternativeexplanation is that sample B107B represents a magmagenerated by partial melting of a garnet-bearing sourcewith DU>DTh. It has been shown that depending ongarnet compositions, Sr can behave more incompatiblythan Th during melting in the presence of residual garnet(van Westrenen et al. 2001). In those circumstances it ispossible that 230Th excess would be accompanied byhigh Sr/Th ratios, as seen in sample B107B.

The 226Ra/230Th systematics for the two historicMayon samples analysed here appear to require recentradium addition and rapid transport of melts to thesurface (e.g. Turner et al. 2001; Bourdon et al. 2003).Several arcs for which 226Ra and 230Th data are avail-able appear to define broad positive correlations on Ba/Th versus (226Ra/230Th) diagrams and an intriguingpossibility is that such arrays may define the Ba/Thratios of the source prior to the recent addition of

(presumably slab-derived) radium (Turner et al. 2001).Such enrichments would be expected to increase U/Thratios and therefore reduce any partial melting related230Th excesses. In the context of the available globaldatabase for subduction-related rocks, the two samplesanalysed here exhibit only moderate levels of radiumenrichment. They are characterised by Ba/Th ratios thatare intermediate between the low values of trace elementenriched arcs such as the Sunda arc and the LesserAntilles, and the high values that characterise the moredepleted arcs such as the Tonga-Kermadec and theMariana systems. Thus, the radium data for the twohistoric Mayon samples require recent (<10,000 yearold) fluid-related trace element enrichment.

At first sight, the relatively high 143Nd/144Nd ratiosobserved in the Bicol rocks analysed thus far (Castilloand Newhall 2004; this study) appear to preclude sig-nificant involvement of low 143Nd/144Nd subductedterrigenous sediment in their mantle source. However, ifas implied by their Pb isotope systematics, the mantlewedge beneath the southern Bicol arc is isotopicallysimilar to that sampled by the PKR, then it had high143Nd/144Nd (c. 0.51310) and low 87Sr/86Sr (c. 0.7034)prior to the introduction of any subduction-related sig-nature (Fig. 6). The observation that the Bicol samplesare displaced to higher 87Sr/86Sr and lower 143Nd/144Ndrelative to their putative Palau Kyushu-type mantlewedge is important, because it permits the involvementof some subducted sediment in their petrogenesis. Sim-ple mass-balance mixing calculations suggest that theinvolvement of a few % subducted sediment could ac-count for the observed Pb, Sr, and Nd isotope ratios inthe Bicol volcanics, although such calculations are nec-essarily rough because the elemental contents of the endmembers are poorly constrained. Thus, we argue thatthe low 87Sr/86Sr and high 143Nd/144Nd exhibited by theBicol samples (relative to other parts of the archipelagosuch as the Macolod Corridor, for example) may in partreflect an isotopically depleted (relative to the mantlesampled by the Philippine Sea Basin basalts) pre-sub-duction mantle wedge, and not necessarily that an iso-topically enriched subduction-component is negligible.

In Fig. 9a, 143Nd/144Nd ratios are plotted againstTh/Nb ratios for all of the subduction-related rocks

Table 5 U-series data for selected volcanics from the southern Bicol arc

Sample Age (years) 226Ra (fg g�1) Th (lg g�1) U (lg g�1) (238U/232Th) (230Th/232Th) (230Th/238U) (226Ra/230Th)

Bulusan volcanic complexB107B 10–20 kaa 2.049 0.598 0.885 1.070 1.209B1-B 35–40 kaa 7.153 2.296 0.974 0.960 0.986B148 >40 <400 kaa 2.177 0.713 0.993 0.962 0.968B220 2.14 Ma 10.54 2.766 0.796 0.793 0.996

Mt. Mayon1968 32 years 306.06 1.726 0.572 1.005 1.003 0.998 1.5121984 16 years 316.75 1.985 0.603 0.921 0.887 0.963 1.589

aDenotes approximate ages based on radiocarbon dating of the Irosin ignimbrite, stratigraphic and geomorphic considerations, whileB220 has been dated using K–Ar (Delfin et al. 1993). The Mt. Mayon samples 1968 and 1984 were erupted in the years 1968 and 1984 AD,

respectively. 226Ra measurements were completed in the year 2000 AD. All data are as measured, i.e. no age corrections have been applied

from the Philippines for which data are available. Alsoincluded for comparison are the data fields for theMarianas arc (Elliott et al. 1997). In the Mayon (Castilloand Newhall 2004) and Macolod Corridor datasets(Defant et al. 1988, 1989; Forster et al. 1990; Defantet al. 1991; Miklius et al. 1991; Mukasa et al. 1994) highTh/Nb ratios are associated with low 143Nd/144Nd ra-tios. The rationale for this diagram is that because boththorium and niobium are relatively insoluble in slab-

derived hydrous fluids, correlated variability such as thatobserved here must reflect either pre-existing melt-re-lated enrichments in the mantle wedge or the influence ofsubducted sediment. Overall, in the Bicol arc data thereis a strong tendency for the highest Th/Nb ratios to bedeveloped in the most silicic samples, reflecting thetendency for Th to behave more incompatibly than Nbduring high-level (crustal) fractionation processes. Forthis reason our interpretations of the causes of high Th/Nb will be restricted to the lower (<55 wt%) silicasamples, and only these lower silica samples are plottedon Figs. 9 and 10. Moreover, in some suites (e.g. theMacolod Corridor data and the Mayon dataset of Ca-stillo and Newhall (2004), the highest Th/Nb ratios areclearly associated with the low silica (basaltic) samples.Unlike some subduction-related volcanics (e.g. Elliottet al. 1997) the Bicol rocks do not exhibit Nb contentslower than that of average N-MORB (Fig. 5a) and theirhigh Th/Nb ratios are largely a reflection of their highTh contents. One explanation is that high Th/Nb ratiosreflect the influence of subducted sediments, and anobjective of Fig. 9 (discussed below) is to test whetherthe data can be explained by bulk addition of subducted

0.5127

0.5128

0.5129

0.5130

0.5131

0.0 0.5 1.0 1.5 2.0

143 N

d/14

4N

d

0.85

0.90

0.95

1.00

0 0.5 1.0 1.5 2.0

Th/Nb

Ce/

Ce*

Marianas

Macolod Corridor

Mayon (low SiO2)

Macolod Corridor

Marianas

Mayon

MayonMayon (low SiO2)

GLOSSAUC

a

b

Fig. 9 a143Nd/144Nd versus Th/Nb ratios for samples from the

Bicol arc. Four of the five Bicol samples have <55 wt% SiO2 (datasymbols as in Fig. 3). Also shown (dashed outlines) are the data forMayon volcano in the Bicol arc (Castillo and Newhall 2004). TheMayon samples fall into two groups and it is the lower silica group(<52.6 wt% SiO2) that exhibits highest Th/Nb ratios. Also shownfor comparison are published datasets for the Marianas (Elliottet al. 1997) and the Macolod Corridor volcanics (Knittel et al.1997). The black shaded area represents typical MORB composi-tions. The vertical arrows on the abscissa mark the Th/Nb ratios ofaverage upper crust (AUC) after Taylor and McLennan (1985), andan estimate of the global average subducting sediment (GLOSS)after Plank and Langmuir (1998). Measurement errors are typicallysmaller than data symbol size. b Ce/Ce* versus Th/Nb ratios for theBicol samples (data symbols as in Fig. 3). Also shown for referenceare the data fields for the Marianas and Macolod Corridor (datasources as in Fig. 9a). Uncertainties in calculated Ce/Ce* areestimated to be up to ±0.02 at the 2r level, reflecting the analyticalerrors associated with the REE analyses and the choice ofchondrite values (error bar in upper left of diagram)

0.5123

0.5125

0.5127

0.5129

0.5131

0.0 0.5 1.0

Th/Nd

143N

d/14

4N

d

N-MORB OIB

GLOSS

Hole801

Mayon

52.5

58.353.9 75.654.7

Upper crust

Marianas

54.7

Fig. 10 143Nd/144Nd versus Th/Nd ratios for samples from theBicol arc with data fields from Mt. Mayon (Castillo and Newhall2004), the Marianas (Elliott et al. 1997), and the Macolod Corridor(Knittell et al. 1997). Also plotted are the values for bulk pelagicsediment from Hole 801 (ODP Leg 129) east of the MarianaTrench (Elliott et al. 1997) and an estimate of the composition ofglobal subducting sediment (GLOSS) after Plank and Langmuir(1998). The Th/Nd ratio of the average upper crust is taken fromTaylor and McLennan (1985) and should be indicative of thoseterrigenous sediments. The two sloping dashed lines are mixingtrajectories between a depleted mantle composition similar to theN-MORB source and pelagic sediment (Hole 801) and a terrige-nous-dominated sediment (GLOSS). Most of the data plot to theright of the mixing lines indicating that the low 143Nd/144Nd endmember must have had higher Th/Nd ratios than that of bulkpelagic or pelagic-terrigenous mixed sediments. Typical 2r mea-surement errors are smaller than the data symbols

sediment or if addition via a sediment–melt componentis required.

An additional line of evidence for sediment involve-ment in the source of the Bicol rocks is that in commonwith a minority of arcs worldwide (e.g. the Marianas,Vanuatu, Aleutians, Guatemala), the Bicol and Maco-lod Corridor volcanics exhibit subtle negative ceriumanomalies. Negative Ce anomalies are often taken to bea signature of subducted pelagic as opposed to terrige-nous sediment, because of their association with bio-genic phosphatic debris (e.g. fish teeth) that inherit REEpatterns from seawater (e.g. Hole et al. 1984; Plank andLangmuir 1998; George et al. 2003). While analyticalerrors on Ce/Ce* ratios are likely to be relatively large(±0.02), the tendency for low Ce/Ce* to be associatedwith high Th/Nb is consistent with the notion thatsubducted sediment, or a melt thereof, is responsible forthe low 143Nd/144Nd, high Th/Nb end member observedin some of the low silica (<55 wt% SiO2) samples inFig. 9. A key question is whether negative Ce anomaliesare robust indicators of subducted pelagic sediment andfor this reason our interpretations remain tentative. Wenote, for example, that in shoshonitic lavas from theTavau volcano, Fiji, negative cerium anomalies wereattributed to interactions between melt and highly oxi-dising fluids (Rogers and Setterfield 1994). In the case ofthe Tavau rocks, however, additional evidence for therole of highly oxidising fluids included the absence of Euanomalies, despite strong petrographic evidence forplagioclase accumulation, and the early appearance oftitanomagnetite in the fractionating assemblage. In theabsence of such evidence for unusually oxidising con-ditions in the Bicol magmas we tentatively interpret thesmall negative Ce anomalies as reflecting subductedpelagic sediment or more likely, a partial melt thereofinvolving a slab-melt component (e.g. Maury et al.1999). As in the Marianas and Aleutians (Elliott et al.1997; George et al. 2003), the Th/Nb ratios of the basaltsand basaltic andesites exceed those of typical pelagicsediments suggesting that bulk sediment addition isunlikely.

Samples with low 143Nd/144Nd ratios also exhibithigh Th/Nd ratios (Fig. 10), and in this diagram binarymixing yields straight-line relationships. Also shown inFig. 10 are various estimates of the likely Th/Nd ratiosof subducted sediment. Shown for reference is a com-monly used estimate of average bulk sediment, socalled Global Subducted Sediment (GLOSS; Plank andLangmuir 1998) that has a Th/Nd ratio of 0.26, lowerthan that of the upper continental crust (0.41, Taylorand McLennan 1985). Pelagic sediments tend to havelower Th/Nd ratios (e.g. 0.124 for bulk pelagic sedi-ment from Hole 801, Elliott et al. 1997). Thus,depending on the ratio of pelagic to terrigenous sedi-ment being subducted, the Th/Nd ratio of the bulksediment is likely to be in the range �0.1–0.4.Assuming that its 143Nd/144Nd ratio is lower thanabout 0.5127, it seems unlikely that the low143Nd/144Nd component of many of the Bicol or

Macolod Corridor rocks could reflect bulk sedimentaddition to the mantle wedge. Instead it appears thatsediment addition occurred via a small degree partialmelt that fractionates the trace element ratios of thebulk subducted sediment, and in particular raises theTh/Nb and Th/Nd ratio of this end member. Samplessuch as B148 (Bintacan volcanics), for example, aretrue basalts (52.5 wt% SiO2) and exhibit Th/Nb ratiosin excess of 0.8, significantly higher than that of anylikely subducted sedimentary end member. Similararguments apply to the two Mayon samples analysedhere and more particularly to the low-silica Mayongroup of Castillo and Newhall (2004), see Fig. 9a.Similarly, many of the Bicol rocks exhibit Th/Nd ratiosthat exceed those of average OIB and estimates of bulksubducted sediment at several sites globally (Elliottet al. 1997; Plank and Langmuir 1998; George et al.2003).

A key remaining issue is whether the sediment com-ponent was added via a slab-derived partial melt thatincluded a small melt contribution from the subductedoceanic crust. Importantly, previous studies (Schianoet al. 1995; Maury et al. 1999) have demonstrated thatmantle xenoliths from northern Luzon arc lavas exhibitenrichments consistent with the presence of a slab-de-rived partial melt that includes a contribution from avery low degree melt of the subducted oceanic crust.Similar processes have been reported in the Kamchatkaarc (e.g. Kepezhinskas et al. 1995, 1996; Kepezhinskasand Defant 1996). In the absence of mantle xenolith datafor the Bicol arc there is at present no strong evidencethat the sediment-related component was added to themantle wedge via a melt that included a melt fractionfrom the subducted oceanic crust. We note however thatthermal modelling (Peacock 1990; Peacock et al. 1994)permits that small amounts of slab melting may occurduring the transient stage accompanying the initiation ofsubduction, a situation that may pertain beneath thispart of the Philippine archipelago where subductionalong the Philippine Trench commenced within the past3–4 Ma (Sajona et al. 1993).

Conclusions

Volcanic rocks from the BVC and from Mayon volcanoin the southern part of the Bicol arc exhibit a widecompositional range. The earliest (Pliocene) eruptivesfrom the BVC (Gate volcanics) are a distinct high-Ksuite characterised by strong incompatible trace elementenrichment. Field, petrographic and geochemical evi-dence indicates that intracrustal processes such asmagma mixing and fractional crystallisation played animportant role in the petrogenesis of the more evolvedsyn- and post-caldera magmas. Volcanics from Mayonappear to be dominated by medium-K basaltic andesitesand andesites (Castillo and Newhall 2004; this study),with LIL enrichments and HFS element depletionstypical of subduction-related rocks. None of the Bicol

rocks display depletions in the HFS elements relative toN-MORB that are a feature of some other arcs. As agroup, the Bicol lavas exhibit lower and less variable87Sr/86Sr ratios (0.7036–0.7039) than those from theBataan arc, the Macolod Corridor, the Mindoro arc,and the Taiwan/Babuyan segments of the Philippinearchipelago. Pb isotope ratios typically plot within andbelow the data field for the Philippine Sea Basin and theNHRL on 207Pb/204Pb versus 206Pb/204Pb and208Pb/204Pb versus 206Pb/204Pb diagrams. We interpretthis to reflect the presence of a pre-subduction mantlewedge of Indian Ocean type, similar to that sampled bythe PKR, east of the Philippine Trench. Previous claimsfor the presence of a Dupal-type Pb isotope signature(Mukasa et al. 1987) are not supported by the new data.Samples with lower 143Nd/144Nd tend to have high Th/Nd, high Th/Nb, and low Ce/Ce* ratios, interpreted toreflect subduction of pelagic sediment. The low143Nd/144N rocks exhibit Th/Nb and Th/Nd ratios thatare higher than those of typical subducted pelagic sedi-ment, indicating that the latter was incorporated into themantle wedge as a slab-derived partial melt rather thanas a bulk sediment mixture. Unlike the Luzon and theBayuban-Taiwan arc where eastward subduction alongthe Manila trench of terrigenous sediments can accountfor much of the isotopic variability, the Bicol arc vol-canics appear to reflect westerly directed subduction ofpelagic sediments along the Philippine Trench. The newdata extend the range of isotopic compositions previ-ously published for the Philippine volcanics, and rein-force suggestions that there are distinct mantle domainsbeneath the Philippine archipelago (Castillo 1996), aswell as distinct regional isotopic signatures in the sedi-ments being subducted along it (McDermott et al. 1993).

Acknowledgements The authors wish to thank Pat Castillo for apre-print of a manuscript on the Mt. Mayon volcanics and forpermission to refer to the data contained therein. FMcD gratefullyacknowledges support from a University College Dublin NewmanScholarship during the early stages of this study. We acknowledgethe thorough and constructive reviews by John Maclennan and ananonymous reviewer that greatly improved the manuscript.

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