Geochemistry and tectonic setting of the Luzon arc, Philippines 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 Manila 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 subduction along fall below the mantle array; they appear to be the Manila Trench for some 1,200 km from the result of sediment source contaminants or GEOLOGIC SETTING the Coastal Range in Taiwan south to Min- a unique mantle composition. Incorporation doro. We suggest calling this the "Luzon of sediments analyzed from eastern China Tectonic Framework arc." There are five distinct segments along and Taiwan into a MORB-type mantle can this arc: Mindoro, Bataan, 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). Earthquake 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 Marinduque 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 South China Sea 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-Palawan 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/101/5/663/3380617/i0016-7606-101-5-663.pdf by guest on 01 October 2021 664 DEFANT AND OTHERS 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, Lingayen-Dingalan fault; 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 Camiguin 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. Central Luzon. 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 (region 1, Fig. 1), exists along the western coast 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). Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/101/5/663/3380617/i0016-7606-101-5-663.pdf by guest on 01 October 2021 GEOCHEMISTRY AND TECTONIC SETTING OF LUZON ARC, PHILIPPINES 665 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 Baguio 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.