Solid Earth, 9, 713–733, 2018 https://doi.org/10.5194/se-9-713-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. Boninite and boninite-series volcanics in northern Zambales ophiolite: doubly vergent subduction initiation along Philippine Sea plate margins Americus Perez1, Susumu Umino1, Graciano P. Yumul Jr.2, and Osamu Ishizuka3,4 1Division of Natural System, Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa, 920-1192, Japan 2Apex Mining Company Inc., Ortigas Center, Pasig City, 1605, Philippines 3Research Institute of Earthquake and Volcano Geology, Geological Survey of Japan, AIST, Tsukuba Central 7, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan 4Research and Development Center for Ocean Drilling Science, JAMSTEC, 2-15 Natsushima, Yokosuka, Kanagawa 237-0061, Japan Correspondence: Americus Perez ([email protected], [email protected]) Received: 25 December 2017 – Discussion started: 31 January 2018 Revised: 7 May 2018 – Accepted: 12 May 2018 – Published: 5 June 2018 Abstract. A key component of subduction initiation rock itudes derived from tilt-corrected sites in the Acoje Block suites is boninite, a high-magnesium andesite that is uniquely place the juvenile arc of northern Zambales ophiolite in the predominant in western Pacific forearc terranes and in select western margin of the Philippine Sea plate. In this scenario, Tethyan ophiolites such as Oman and Troodos. We report, for the origin of Philippine Sea plate boninites (IBM and Zam- the first time, the discovery of low-calcium, high-silica boni- bales) would be in a doubly vergent subduction initiation set- nite in the middle Eocene Zambales ophiolite (Luzon Island, ting. Philippines). Olivine–orthopyroxene microphyric high-silica boninite, olivine–clinopyroxene–phyric low-silica boninite and boninitic basalt occur as lapilli fall deposits and pil- low lava flows in the upper volcanic unit of the juvenile 1 Introduction arc section (Barlo locality, Acoje Block) of the Zambales ophiolite. This upper volcanic unit overlies a lower volcanic As the surface manifestation of a convecting mantle, sub- unit consisting of basaltic andesite, andesite to dacitic lavas duction zones play a fundamental role in material transfer and explosive eruptive material (subaqueous pahoehoe and from the surface to the deep interior and vice versa. Early lobate sheet flows, agglutinate and spatter deposits) form- in Earth’s history, the transition from a stagnant lid regime ing a low-silica boninite series. The overall volcanic stratig- to the present mode dominated by plate forces necessitates raphy of the extrusive sequence at Barlo resembles holes subduction initiation. Numerical models show that plate tec- U1439 and U1442 drilled by IODP Expedition 352 in the tonics or mantle overturn in terrestrial planets can be trig- Izu–Ogasawara (Bonin) trench slope. The presence of de- gered by narrow plumes impinging on weakened crust induc- pleted proto-arc basalts in the Coto Block (45 Ma) (Geary ing gravitational instability and slab descent (Crameri and et al., 1989), boninite and boninite series volcanics in Barlo Tackley, 2016; Gerya et al., 2015). Internally driven mecha- (Acoje Block (44 Ma)) and simultaneous and post-boninite nisms such as plume-induced subduction initiation (Whattam moderate-Fe arc tholeiites in Sual and Subic areas of the and Stern, 2015), however, are fundamentally different from Acoje Block (44–43 Ma) indicate that the observed subduc- end-member models of Cenozoic subduction initiation which tion initiation stratigraphy in the Izu–Ogasawara–Mariana rely on preexisting zones of weakness such as fracture zones forearc is also present in the Zambales ophiolite. Paleolat- and transform faults or require lithospheric collapse (Stern, Published by Copernicus Publications on behalf of the European Geosciences Union. 714 A. Perez et al.: Boninite and boninite-series volcanics in northern Zambales ophiolite 2004). How subduction begins in the oceanic domain is one produced the IBM forearc and the mechanism remains elu- fundamental issue yet to be fully deciphered, thus it is re- sive (Keenan and Encarnación, 2016). garded as a high priority scientific target (Challenge No. 11) Here an alternative approach to exploring subduction ini- of the International Ocean Discovery Program (IODP) for tiation processes in the western Pacific region is presented. 2013 to 2023. Understanding this volcanic and tectonic pro- Following Stern et al. (2012), this study focuses on the mid- cess is hampered by its transient nature, with few examples dle Eocene Zambales ophiolite in the Philippines as it offers in the geologic record with the exception of submarine ac- a ∼ 3200 km2 exposure of Eocene supra-subduction oceanic tive and paleo-forearcs and terrestrial supra-subduction ophi- lithosphere. Most of the Jurassic to Cretaceous ophiolites olites. that form basement complexes in the eastern Philippines are In testing subduction initiation models,the Izu–Ogasawara increasingly recognized as complementary features to Cre- (Bonin)–Mariana (IBM) forearc region remains as one the taceous terranes in the northern Philippine Sea plate (e.g., most appropriate localities; this is due to its temporally Amami Plateau, Daito Ridge) possibly sharing a common distinct volcanic record of oceanic accretion and juvenile history as the overriding plate prior to initiation of subduc- arc magmatism immediately following the onset of sub- tion (Billedo et al., 1996; David et al., 1997; Deschamps duction. After the subduction of the Pacific Plate beneath and Lallemand, 2002; Lallemand, 2016). Therefore, juxta- the Philippine Sea plate commenced at 52 Ma, magma- posed Eocene ophiolites are potential targets to investigate tism progressed from 52 to 48 Ma mid-ocean ridge basalt subduction initiation as a plate-scale process. Regional tec- (MORB)-like proto-arc basalts (or forearc basalts) to 48– tonic reconstructions predict the interaction of a pre-Eocene 44 Ma boninite and boninite-series volcanics, followed by ocean basin located east of Eurasia and the western margin 45–35 Ma arc tholeiites and calc–alkaline lavas (Ishizuka et of the Philippine Sea plate (Wu et al., 2016). Coincidentally, al., 2011; Kanayama et al., 2012; Reagan et al., 2010). The aside from the IBM forearc region, the Dasol–Barlo local- widespread occurrence of subduction initiation rock suites ity in northern Zambales ophiolite is where middle Eocene along the eastern margin of the Philippine Sea plate is sum- boninitic rocks are also reported (Evans et al., 1991; Florendo marized in Fig. 1a. Basalts of similar composition and age and Hawkins, 1992; Hawkins and Evans, 1983). The signifi- range to proto-arc basalts dredged along the Mariana and cance of Zambales ophiolite as an analogue of the IBM fore- Bonin trench slopes were found at IODP Expedition 351 arc has been recognized since the early 1990s (Pearce et al., Site U1438 in the Amami Sankaku Basin (Arculus et al., 1992) yet this connection remains unexplored. Motivated by 2015; Ishizuka et al., 2018). This indicates that the Oligocene how the ophiolite concept progressed through studies of the Kyushu–Palau Ridge (KPR) intra-oceanic arc is built upon IBM forearc, we present field, petrologic and geochemical an igneous basement that formed part of the earliest seafloor characterization of a juvenile arc section in northern Zam- spreading event related to subduction initiation. The subse- bales ophiolite to evaluate subduction initiation and boninite quent late Oligocene opening of the Shikoku–Parece Vela petrogenesis in a regional geodynamic context. Basin resulted in its present configuration with Site U1438 in the back-arc side of KPR. Based on dredging and dive ob- servations in the forearc trench slope, proto-arc basalts are 2 Geologic background found at deeper depths relative to boninites which occur ups- lope and subaerially at Chichijima (type locality) and Muko- Located in western Luzon, the structurally coherent Zam- jima island groups (Ishizuka et al., 2011; Kanayama et al., bales ophiolite is a representative supra-subduction ophio- 2012; Umino and Nakano, 2007). The hypothesis that proto- lite of the western Pacific and the largest in the Philippines arc basalt lies stratigraphically below boninite has been sug- (Fig. 1b). The ophiolite spans the entire Zambales mountain gested by IODP Expedition 352 based on four sites drilled range from 16 to 14.7◦ N where it partly underlies stratovol- in the trench-side slope of the Bonin Ridge (Reagan et al., canoes of the Luzon Arc, including Mt. Pinatubo. It consists 2015, 2017). Both spontaneous and induced initiation have of three fault-bound massifs, namely Masinloc, Cabangan been replicated in numerical models of incipient subduc- and San Antonio, which each preserve complete ophiolite tion at the IBM. Earlier models with imposed plate veloci- internal stratigraphy. The ophiolite is flanked by the Cen- ties generally focused on initiation across a preexisting weak tral Valley basin (CVB) to the east and West Luzon basin to zone, and combined with melting models could reproduce the west. Late Eocene to late Miocene Aksitero and Mori- the observed proto-arc basalt–boninite stratigraphy (Hall et ones formations of the CVB record the transition from a al., 2003; Leng et al., 2012). Subsequently, spontaneous sub- hemipelagic, deep marine setting to a shallower environment duction initiation has been shown to be plausible with com-