Boninite and boninite-series volcanics in northern Zambales ophiolite: Doubly-vergent subduction initiation along Philippine Sea Plate margins
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Abstract. A key component of subduction initiation rock suites is boninite, a high-magnesium andesite that is uniquely predominant in Western Pacific forearc terranes and in select Tethyan ophiolites such as Oman and Troodos. We report the discovery of low-calcium, high-silica boninite in the middle Eocene Zambales ophiolite (Luzon island, Philippines). Olivine-orthopyroxene microphyric high-silica boninite, olivine-clinopyroxene-phyric low-silica boninite and boninitic basalt occur as lapilli fall deposits and pillow lava flows in the upper volcanic unit of the juvenile arc section (Barlo locality, Acoje Block) of Zambales ophiolite. This upper volcanic unit in turn overlies a lower volcanic unit consisting of basaltic andesite, andesite to dacitic lavas and explosive eruptives (subaqueous pahoehoe and lobate sheet flows, agglutinate, and spatter deposits) forming a low-silica boninite series. The overall volcanic stratigraphy of the extrusive sequence at Barlo resembles Holes U1439 and U1442 drilled by IODP Expedition 352 in the Izu-Ogasawara (Bonin) trench slope. The presence of proto-arc basalts in Coto Block (45 Ma), boninite and boninite series volcanics in Barlo, Acoje Block (44 Ma) and simultaneous and post-boninite moderate-Fe arc tholeiites in Sual and Subic, Acoje Block (44–43 Ma) indicate that the observed subduction initiation stratigraphy in the Izu-Ogasawara-Mariana forearc is present in Zambales ophiolite as well. Paleolatitudes derived from tilt-corrected sites in the Acoje Block place the juvenile arc of northern Zambales ophiolite in the western margin of the Philippine Sea Plate. In this scenario, the origin of Philippine Sea Plate boninites (IBM and Zambales) would be in a doubly-vergent subduction initiation setting.Keywords:
Pillow lava
Forearc
Basaltic andesite
The Nidar Ophiolite is located between the North Himalayan nappes and the Indus Suture Zone in NW Himalaya in eastern Ladakh (India). Based mainly on geochemical argument, this ophiolite is classically interpreted as a relic of an intra-oceanic arc (Maheo et al. 2000; Maheo et al. 2004), which developed at around 140 Ma, prior to the collision between the Indian and Eurasian plates (Ahmad et al. 2008). From top to bottom, this ophiolite is composed of various sedimentary rocks (radiolarites, polygenic conglomerates and carbonates), volcanic rocks (pillow lavas, basaltic to andesitic in composition), gabbros (Fe- and layered gabbros, pegmatites and minor troctolites), serpentinites, dunites, pyroxenites and peridotites (mainly harzburgites). The Nidar Ophiolite underwent an anchizonal metamorphism with preservation of primaries structures (layering) and volcanic textures (pillow lavas). This study is mainly focused on new field observations across the ophiolite and the surrounding units. A new detailed geologic map of the ophiolite between the Nidar village and Kyun Tso area is presented. The upper part of the ophiolitic complex is an alternation of volcanic and sedimentary rocks (500- 1000 m thick) and the lower part consists of large outcrops of gabbros (3000m thick). These mafic rocks are separated from the serpentinized ultramafic rocks by a 200m thick ophiolitic breccia and continental Indus Molasse slices. The Nidar Ophiolite is made up of the classical rock type succession (ultramafites, gabbros, pillow basalts, radiolarites), but the internal structure is far more complex than previously suggested. New field data (geologic and structural maps, lithologic sections, etc.) coupled with new geochemical analysis will help to constrain the geodynamic context and deformation history. Ahmad, T., T. Tanaka, H.K. Sachan, Y. Asahara, R. Islam, et P.P. Khanna. 2008. « Geochemical and isotopic constraints on the age and origin of the Nidar Ophiolitic Complex, Ladakh, India: Implications for the Neo-Tethyan subduction along the Indus suture zone ». Tectonophysics 451 (1–4): 206‑ 24. Maheo, Gweltaz, Herve Bertrand, Stephane Guillot, Georges Mascle, Arnaud Pecher, Christian Picard, et Julia De Sigoyer. 2000. « Temoins d’un arc immature tethysien dans les ophiolites du Sud Ladakh (NW Himalaya, Inde) ». Comptes Rendus de l’Academie des Sciences - Series IIA - Earth and Planetary Science 330 (4): 289‑ 95. Maheo, Gweltaz, Herve Bertrand, Stephane Guillot, Igor M. Villa, Francine Keller, et Paul Capiez. 2004. « The South Ladakh ophiolites (NW Himalaya, India): an intra-oceanic tholeiitic arc origin with implication for the closure of the Neo-Tethys ». Chemical Geology 203 (3–4): 273‑ 303.
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The claim by Furnes et al . (Reports, 23 March 2007, p. 1704) that Greenland metavolcanic rocks require Paleoarchean sea-floor spreading is incompatible with their own data. The purported sheeted dikes have the composition of pyroxenitic komatiite and could not have fed the adjacent ferroandesitic pillow lavas. Neither type has ophiolitic analogs, and both are likely ensialic.
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The Lanong ophiolite,located in Jiangcuo,Bange,Tibet,is a key element within the Baila-Lanong ophiolite belt.Characterized by well preserved pillow lavas,the Lanong ophiolite consists of blocks of peridotites,gabbros,diabases and pillow lavas.Trace elements analysis indicates that the pillow lavas are enriched in Sr,Ba and Th,delpleted in Nb,Ta and Ti,clearly displaying compositional characteristics of the island-arc volcanic rocks.On the Chondrite-normalized trace element diagram,all lava samples show flat patterns,with (La/Yb)N and (Ce/Yb)N ratios similar to those of N-MORB.In conclusion,the Lanong ophiolite is of SSZ affinity,and should be the relics of a back arc basin.Combining the terranes' distribution with the discrimination results of geochemical diagrams,a simple tectonic evolution model is suggested:the Neotethys oceanic crust to the north began to southwardly subduct in middle-late Jurrasic,then a spreading sub-centre was formed on the obducting slab south of the subdction zone,which developed into a back arc basin subsequently.As the subdction going on,the back arc basin closed and only portions of the back arc crust were preserved by accretion.
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Abstract New whole‐rock major and trace element geochemistry from the Leka Ophiolite Complex in Norway is presented and compared to the geochemical evolution and proposed tectonomagmatic processes recorded in the Izu‐Bonin‐Mariana system. These data demonstrate that the Leka Ophiolite Complex formed as forearc lithosphere during subduction initiation. A new high‐precision zircon U‐Pb date on forearc basalt constrains the timing of subduction initiation in the “Leka sector” of the Iapetus Ocean to 491.36 ± 0.17 Ma. The tectonomagmatic record of the Leka Ophiolite Complex captures only the earliest stages of subduction initiation and is thereby distinct from some other Appalachian–Caledonian ophiolites of similar age. The diversity of Appalachian–Caledonian ophiolite records may represent differing preservation and exposure of a variable forearc lithosphere.
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