北呂宋島菲律賓活動帶蛇綠巖及其上覆濁積巖的放射蟲研究新進展

E-mail: ishidak@ias.tokushima-u.ac.jp.

The basement of the Philippine Mobile Belt(PMB) is mainly composed of ophiolites that are mostly overlain by Paleogene to Miocene turbidites in central Luzon. To clarify the geological development of the PMB with respect to the initial stage of the arc volcanism (eg. Yumul et al., 2003, 2008; Dimalanta and Yumul, 2003; Suzuki et al., 2011), radiolarian dating was examined in siliceous sediments associated with the ophiolites and turbidites. The samples were collected from sites identified with the Zambales and Montalban ophiolites, basic tuff phyllites in NW Dingalan, and their overlying formations.

Zambales Ophiolite Complex (ZOC; BMG, 1981)in western Luzon is one of the most investigated ophiolite complexes in the Philippines. This north-south trending complete ophiolite sequence has traditionally been divided into two terranes – Acoje and Coto – based on their geochemical signatures. The ZOC was previously assigned an Eocene age of formation based on the overlying Eocene Aksitero Formation (Amato, 1965) exposed in the southwestern foothills of the Zambales Range (Schweller et al.,1983). This age is consistent with the whole rock K-Ar ages of a dike in Coto Mine that gave a 44.3 to 46.6 Ma age (Fuller et al., 1991). Recent field investigations, however, led to the recognition of chert blocks neighboring the peridotites of the Acoje block. These cherts are believed to form part of the sedimentary carapace of the ophiolite because of their spatial proximity. An initial study assigned a Late Jurassic –Early Cretaceous age for the radiolarians extracted from the chert blocks. A more recent work, however,yielded a Middle Jurassic - Early Cretaceous age(Ishida et al., 2011: Quea?o et al., 2011). The faunas are dominated by multisegmented nassellarian genera(Archaeodictyomitra,Xitus,Pseudodictyomitra,Pseudoeucyrtisetc.) and are associated withTriactomaandTetraditryma. The chert blocks suffered hydrothermal crystallization, and most of the radiolarian tests have lost details of ornamentations. Such condition prevents the species identification. With regard to the Aksitero Formation, a late Eocene to late Oligocene age has been assigned to this formation based on the foraminiferal assemblage (Amato, 1965; Schweller et al., 1984). The discovery of the trace fossilZoophycusfrom the pelagic limestone member of the Lower Aksitero Formation is suggestive of its ichnofacies, that the tuffaceous turbidites and the pelagic limestones were formed deeper than 1000 m in a subsiding basin adjacent to an active arc system as previously mentioned (Garrison et al., 1979). These contrasting ages merits a re-evaluation of the previous tectonic models on the Zambales Ophiolite genesis. It now appears that the two blocks comprising the ZOC were formed at different time – the Acoje block was formed in Middle Jurassic-Early Cretaceous whereas the Coto block was formed during the Eocene. Their different geochemical signatures also suggest a progressive tectonic shift from an island arc to a backarc environment. Latest Jurassic to Early Cretaceous ra-diolarian faunas were also reported from cherts in the serpentinite mélange of Ilococ Norte in northwestern Luzon (Quea?oet al., 2010).

Southern Sierra Madre at Rizal: The Montalban Ophiolitic Complex (MOC; MGB, 2004) constitutes the basement of southern Sierra Madre. The lower basalts of the ophiolite are MORB character while the upper andesite-basalt has an island arc tholeiite (IAT)signature (Arcilla, 1991). The Kinabuan Formation,as sedimentary cover of the MOC, is topped by a radiolarian-bearing bathyal limestone (Ringenbach,1992) that has been dated Santonian to Early Maastrichtian based on planktonic foraminifera (Reyes and Ordo?ez, 1970; Hashimoto et al., 1979; Haeck, 1987).Tumanda (1994) recovered the Campanian radiolarians from the cherty bands. Turonian age was added by radiolarians and pelagic foraminifera (Arcilla,1991). The Maybangain Formation, which conformably overlies the Kinabuan Formation and is composed of the Masungi Limestone member and the clastic-volcanic member – volcanic breccia, sandstone,siltstone, mudstone, and conglomerate (Pe?a, 2008).Early/middle Paleocene to middle Eocene is adopted for this formation (MGB, 2010). Thin alternation of red cherty mudstone and graded fine volcanic sandstone of the Maybangain Formation yields relatively well-preserved radiolarian fauna. The fauna is characterized by the occurrence ofPodocyrtis mitra,Sethochytris triconiscus,Lithochytris vespertilioandEusyringium fistuligerumwith association of nearly 20 gen. spp. The fauna belongs to the middle EoceneP. mitraZone (ex. Sanfilippoet al., 1985; Nigrini and Sanfilippo, 2001), and is slightly younger than that of the Cabog Formation turbidite along the southern Dingalan Bay, which is correlative with theD. mongolfieriZone/T. triacanthaZone (Ishidaet al., 2011a,b). The radiolarian horizons of these formations are chert/siliceous shale-intercalated distal turbiditic deeper-water lithofacies.

Northern Sierra Madre at NW Dingalan: East side of the Philippine Fault Zone, red chert block in the ophiolitic basic tuff phyllite (Lubingan Formation)was dated as Late Mesozoic (late Jurassic/Early Cretaceous) evidenced by the occurrence ofMictyoditrasp. The phyllites are regarded as a cover of the Cretaceous Dibut Bay Meta-ophiolite (Billedo, et al., 1996)(“Baler Ophiolite”). Spherical tests were deformed into oval shapes.

Paleobiogeographic and ocean-climatic conditions inferred from the radiolarian faunas: Late Mezozoic radiolarian fauna from the Zambales Ophiolitic Complex is characterized by the occurrence ofArchaeodictyomitra,Pseudodictyomitra,Xitus,Triactoma, andTetraditryma.These genera are common in Tethyan pelagic environment (O’Dogherty et al., 2009).Middle Eocene radiolarian fauna from the Maybangain Formation is regarded as warmer current affinity. It suggests the effect of a globally warmer period during the late Paleocene-middle Eocene (eg. Salamy and Zachos, 1999; Slujis et al., 2005).

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Recent Progress in Radiolarian Research for Ophiolites and the Overlying Turbidites, Philippine Mobile Belt, Northern Luzon Island

Keisuke ISHIDA1), Shigeyuki SUZUKI2), Carla DIMALANTA3), Graciano YUMUL JR.4),Karlo QUEA?O5), Decibel FAUSTINO-ESLAVA6), Edanjarlo MARQUEZ7),Noelynna RAMOS3), Rolando PE?A8)
1)Institute of Socio-Arts and Sciences, University of Tokushima, Japan;
2)Department of Earth Sciences, Okayama University, Japan;
3)Rushurgent Working Group, National Institute of Geological Sciences, University of the Philippines, Diliman, Quezon City, Philippines1101;
4)Monte Oro Resources and Energy Inc., Pasig, Philippines;
5)Mines and Geosciences Bureau, Diliman, Quezon City, Philippines;
6)School of Environmental Science and Management, University of the Philippines – Los Ba?os, Laguna, Philippines;
7)Department of Physical Sciences and Mathematics, University of the Philippines – Manila, Philippines;
8)Mines and Geosciences Bureau (MGB), Philippines

Philippine Mobile Belt; Radiolarian fauna; Ophiolitic Complex; Turbidites

10.3975/cagsb.2012.s1.15