General Tectonic Setting of the Archipelago

The Philippine island arc system (Fig. 1) is a product of the interaction of at least three plates, i.e., the Sundaland Plate in the west, the Philippine Sea Plate in the east, and the Indo- Australian Plate in the south. This archipelago is made up of the aseismic Palawan continental block and the seismically-active Philippine Mobile Belt (Yumul et al., 2003b). The latter is a composite terrane made up of fragments with continental, oceanic and island arc affinities.

Ophiolitic and metamorphic complexes (Tamayo et al., 2004) serve as the basement of magmatic arcs, which range in age from Cretaceous to recent. Subduction and large scale faulting resulting into collision, suturing, magmatism and arc polarity reversals (Yumul et al., 2003b) characterise the Cenozoic evolution of the archipelago (Hall, 1996; Sibuet et al., 2002;

Pubellier et al., 2004a, 2000b).

The island arc system is presently bounded to the west by the east-dipping Early Miocene Manila Trench, Middle Miocene Negros Trench, Late Miocene to Pliocene Sulu and Cotabato Trenches. This western boundary marks the subduction of the Early Oligocene to Early Miocene South China Sea (along the Manila Trench), the Early to Middle Miocene Sulu Sea basin (along the Negros and Sulu Trenches) and the Eocene Celebes Sea basin (Cotabato Trench). On the eastern boundary of the archipelago, the Eocene West Philippine Sea plate is being consumed through oblique subduction along the west-dipping East Luzon Trough- Philippine Trench. The left-lateral strike-slip Philippine Fault Zone takes up whatever stress that cannot be accommodated by the surrounding subduction zones (Fig. 1).

- 39 -

Fig. 1 Schematic map of the Philippines island arc system showing the location of the studied areas. The different trenches, plates and marginal seas bordering the archipelago are indicated, as well as the Philippine Fault Zone straddling the whole arc system. The locations of the representative adakites mentioned in Table 1 are also plotted. EVC: Eastern Volcanic Chain; WVC: Western Volcanic Chain.

- 41 - 3. Adakite Occurrences and Previous Works

Adakites are rather common in the Philippines, and can be found in all the presently active subduction zones related to the South China Sea, the Sulu Sea, the Celebes Sea and the Philippine Sea (Fig. 1), as well as in the Central Mindanao post-collision zone which results from the closure of the northern part of the Molucca Sea (Pubellier et al., 1991). However, large adakitic or mostly adakitic volcanoes like those of Ecuador or the Chilean Austral Volcanic Zone are uncommon, with the exception of Mount Pinatubo (Newhall et al., 1996).

In most volcanic edifices, adakites are intricately associated with normal calc-alkaline lavas, and all the transitions between them can be found: these have been termed either transitional adakites (Sajona et al., 2000a) or adakite-linked andesites (Sajona and Maury, 1998). In addition, the geochemical signatures of basaltic or basaltic andesitic lavas often suggest that their mantle source contain an adakitic component, e.g., in Mt. Arayat in the Eastern Volcanic Chain (EVC) of Luzon (Bau and Knittel, 1993) or in Batan Island (Maury et al., 1998).

3.1. Batan Island

Batan is located in the northern part of the Luzon arc (Fig. 1) resulting from the subduction of the South China Sea beneath the Philippine Sea plate. A double arc has been identified in this region, the two parts of which merge at the level of Batan, and the subducting slab may have been torn beneath this island (Yang et al., 1996). The geology and petrology of Batan has been studied in detail (Richard et al., 1986). The youngest basaltic andesites of Mt. Iraya derive from a metasomatised mantle source bearing an important sedimentary component (Defant et al., 1989, 1990; McDermott et al., 1993; Fourcade et al., 1994; Marini et al., 2005). They contain abundant metasomatised deformed mantle xenoliths (Vidal et al., 1989; Maury et al., 1992), in which adakitic melt inclusions have been identified (Schiano et al., 1995) in primary (deformed) olivine. Their compositions (Table 1) are characterised by very high Sr/Y and La/Yb ratios. They have been interpreted as melts derived from the subducted slab and having reacted with the island arc mantle wedge (Schiano et al., 1995). Besides these inclusions, two andesitic lavas from Batan have previously been identified as adakites (B41 and B42, Sajona et al., 2000a). The K-Ar age of sample B42, 1.09 Ma, is similar to those of the amphibole and phlogopite-rich ultramafic

Table 1 Major and trace element compositions of selected adakitic rocks from the Philippines.

See text for the analytical methods. EVC: Eastern Volcanic Chain. WVC: Western Volcanic Chain.

References: a: Sajona et al., 2000a (major elements); b: this study (major and trace elements, or trace elements only); c: Schiano et al., 1995; d: Margoum, 2002 (major elements); e: Prouteau et al., 1999 (major elements); f: Yumul et al., 2000 (major elements); g: Rae et al., 2004; h: Castillo et al., 1999; i:

Sajona et al., 1996 (major elements); j: Maury et al., 1996 (major elements) and k: Sajona, 1995 (major elements).

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xenoliths containing adakitic glass inclusions, but its isotopic composition evidences a strong sedimentary component (Marini et al., 2005). Due to their fractionated REE patterns with low HREE and Y contents, a number of young basaltic andesites and andesites from Mt. Iraya have been considered as derived from a mantle source modified by slab melts (Maury et al., 1998).

3.2. Northern and Central Luzon

Adakites and adakitic rocks have been sampled in the Mankayan and Baguio Districts in Northern Luzon (Sajona and Maury, 1998; Bellon and Yumul, 2000, 2001; Imai, 2002).

These adakites form part of the Mio-Pliocene Northern Luzon Central Cordillera volcanoplutonic arc complex exposed in the core of this area. This complex is related to the subduction of the South China Sea Plate along the Manila Trench. The substratum of the Mankayan and Baguio Districts is ophiolitic, and interpreted as formed in a Cretaceous arc- marginal basin setting (Tamayo et al., 2004; Pubellier et al., 2004a, 2000b). These ophiolitic units are overlain by transitional island arc tholeiite - mid-ocean ridge basalt metavolcanics.

The Central Cordillera has been uplifted and eroded following the intrusion of Middle Miocene to Pleistocene arc silicic plutons and hypovolcanic intrusives, some of which are adakitic (e.g., the Monglo intrusive, Table 1).

Adakites also occur in Central Luzon within the two parallel volcanic chains straddling over the area (Yumul et al., 2000, 2003a; Bellon and Yumul., 2001). The Eastern Volcanic Chain (EVC), is made up of the less than 1 Ma old Mts. Balungao, Cuyapo, Amorong and Bangcay silicic plugs (Yumul et al., 2000, 2003a) and the Mt. Arayat volcano (Bau and Knittel, 1993). It is considered as a back-arc volcanic chain with respect to the Manila Trench which lies west of the EVC. The Western Volcanic Chain (WVC) which includes among others Mts. Pinatubo, Mariveles, Samat, Natib, is characterized by volcanic rocks generated in the fore-arc – main volcanic arc region with respect to the Manila Trench.

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Silicic plugs of adakitic composition (e.g., Sta. Elena, Poelis) are found both within the fore arc and the main volcanic region. The lava compositions of both the EVC and WVC span the range of arc tholeiitic through calc-alkaline all the way to adakitic. The WVC is directly floored by a supra-subduction zone ophiolite (Yumul et al., 2003a), the Zambales Ophiolitic Complex. The EVC lavas crosscut or overlie the Central Luzon sedimentary basin, which is also believed to overlie an ophiolitic basement (Yumul et al., 2000, 2003a).

The Northern and Central Luzon calc-alkaline lavas seem clearly connected to the eastward-dipping subduction of the Oligocene to Middle Miocene (34-17 Ma) South China Sea oceanic crust (Defant et al., 1989, 1990). The associated adakites have been interpreted as derived from the partial melting of the lower Luzon crust (Yumul et al., 2000, 2003a; Bellon and Yumul, 2001), although their links with fractionation, magma mixing and/or AFC processes are not discarded (Yumul et al., 2003a). The high MgO, Ni and Cr contents of some of these adakites have been attributed to their interaction with mantle peridotites (Yumul et al., 2003a). The ophiolitic composition of the North Luzon substratum and the occurrence within the Monglo adakite (Baguio District) of ophiolite-derived dunitic, gabbroic and amphibolitic xenoliths are consistent with the interaction of these adakitic rocks with their mafic-ultramafic basement.

The cummingtonite- and hornblende-bearing dacite emitted during the 1991 eruption of Mt. Pinatubo (WVC) fulfills several of the petrologic and geochemical criterions for being termed an adakite (Bernard et al., 1996), despite its moderately high Sr/Y (<90) and La/Yb (<12) ratios. Its origin has been attributed either to partial melting of the arc lower crust (Bernard et al., 1996) or to high pressure (1.2 GPa) fractionation of hydrous mafic magmas involving fractionation of garnet and amphibole (Prouteau and Scaillet, 2003). In addition, there is also ample evidence for magma mixing between dacite and basalt during the 1991 eruption (Pallister et al., 1996).

3.3. Negros

The island of Negros in the Central Philippines is bounded to the west by the Negros Trench (Fig. 1), along which the Miocene (20-17 Ma) Sulu Sea back-arc basin is subducting.

The basement of Negros is made of Paleogene volcanic and sedimentary arc sequences intruded by Oligocene dioritic plutons and overlain by Mio-Pliocene detrital sediments, carbonate rocks and arc volcanics. Four Upper Pliocene to Quaternary calc-alkaline to

adakitic volcanoes occur in the island. They are, from north to south: Mts. Silay, Mandalangan, Canlaon (active) and Cuernos de Negros. The latter volcano includes a number of andesitic to dacitic domes and associated pyroclastic flow deposits. A few studies have dealt with the petrology and geochemistry of lavas in Negros (Von Biedersee and Pichler, 1995; Castillo, 1995; Sajona et al., 2000a; Rae et al., 2004). Adakites have been identified by Sajona et al. (2000a) and Rae et al. (2004), especially in Mt. Cuernos de Negros. They have been considered as melts from the young subducted Sulu Sea slab, formed through low (<10%) degrees of partial melting of a N-MORB garnet amphibolitic source (Sajona et al., 2000a). Some samples from the other volcanoes have been termed “transitional adakites” by Sajona et al. (2000a) who consider them as derived from the melting of an upper mantle metasomatised by adakitic melts, although they do not exclude the hypothesis of magma mixing between adakitic and mafic melts.

3.5. Camiguin and Mindanao

The small island of Camiguin, located 12 km north of Central Mindanao, is composed of four medium-K calc-alkaline stratovolcanoes which are from the SE to the NW: Mts.

Ginsiliban, Butay, Mambajao, and Hibok-Hibok, the latter possessing a parasitic cone, Mt.

Vulcan. Hibok-Hibok is the only active volcano (Pelean eruption in 1951). Mts Mambajao, Hibok-Hibok and Vulcan are mainly andesitic to rhyolitic in composition. Castillo et al.

(1999) showed that some of the most evolved lavas in Camiguin, especially from Mt.

Mambajao, present chemical features similar to those of adakites, especially from Central Mindanao ones. However, they interpreted them as derived from mafic melts from a mantle source metasomatised by hydrous fluids, through crustal AFC processes involving low- pressure fractionation of amphibole and accessory phases such as titanite and apatite (but not garnet). The validity of these fractionation models has been questioned by Sajona et al.

(2000b).

Mindanao is the second largest island of the Philippine archipelago and by far the most complex from a tectonic point of view. It is surrounded by three trenches installed during the last 4-3 My (Pubellier et al., 1991, 1996; Fig. 1): the Philippine Trench to the east, along which the Eocene Philippine Sea basin is subducting; the Sulu Trench to the west, corresponding to the subduction of the Miocene Sulu Sea; and the Cotobato Trench to the southwest, linked to the subduction of the Eocene Celebes Sea basin. Three volcanic arcs,

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which will be labelled the Eastern Mindanao (EM), Southern Mindanao (SM) and Western Mindanao (WM; or Zamboanga) arcs, respectively are associated to these three subductions, and each of them contains adakites (Sajona et al., 1993, 1994). In addition, the central part of Mindanao (CM) is covered by the most voluminous volcanic field of the Philippines, which extends northwards to Camiguin island. It cannot be related to any of the above-mentioned subduction zones, as seismic and tomographic data indicate that none of the corresponding slabs extends beneath Central Mindanao (Rangin et al., 1999). Instead, it is underlain by a ca.

300 km deep slab which is interpreted as the stalled Molucca Sea slab (Rangin et al., 1999).

The collision between Eastern and Western Mindanao started 4-5 Ma ago and is still active (Pubellier et al., 2004a, 2000b). The Central Mindanao volcanic field is thus to be regarded as post-collisional (Sajona et al., 1994, 2000b).

All the documented Mindanao adakites have been interpreted as melts from the subducted slabs by Sajona and co-workers (Sajona, 1995; Sajona et al, 1993, 1994, 1997, 2000b). However, the cause of the slab melting event is regarded as variable from one area to another, given the differences in the tectonic setting and/or the ages of the subducted oceanic slab. Slab melting is considered as linked to the subduction of the Miocene oceanic lithosphere beneath WM (Zamboanga), which preserves a high thermal gradient (Sajona et al., 1993). Due to the older age of the Philippine Sea and Celebes Sea basins, this explanation cannot be extended to EM and SM adakites, and Sajona et al. (1993, 1994) proposed that the high thermal regime needed for slab melting occurred during the initiation of subduction along the East Philippines and Cotobato Trenches. A similar transient thermal regime, but due to the thermal rebound following the end of the Molucca Sea subduction, is deemed responsible for the slab melting event generating CM and Camiguin adakites (Sajona et al., 2000b). The occurrence of “transitional adakites” and magnesian andesites in CM has been attributed to the melting of mantle having interacted with slab melts (Sajona et al., 2000b).