A trapped Philippine Sea plate origin for MORB from the inner slope of the Izu–Bonin trench

Abstract

Basement outcrops sampled by submersible and dredge from the inner slope of the Izu^Bonin trench at 32³N and 6200^6700 m water depth have a distinct mid-ocean ridge basalt (MORB) chemistry unlike any other rocks previously sampled in the Izu^Bonin arc. They are low K tholeiites with moderate TiO 2 (0.7^1.8 wt%), extremely low Ba (1.5^7 parts per million), low Ba/La (1.2^3) and are depleted in light rare-earth elements. These samples could represent either an accreted piece of subducting Pacific plate or a trapped remnant of Philippine Sea plate on which the Izu^Bonin arc was built. Although their major and trace element chemistry do not help to distinguish their source, the Sr, Nd and Pb isotopes clearly support a Philippine Sea plate origin. The isotopic signature of the inner trench slope samples matches that of Philippine Sea plate lavas, with 87 2) values compared to the Northern Hemisphere reference line (NHRL) and their isotopic signature is distinct from the Mesozoic Pacific MORB being subducted. These are the first samples of trapped Philippine Sea oceanic crust discovered in the IzuB onin^Mariana arc. They require that models for the formation of intra-oceanic arc crust account for pre-existing oceanic crust and that estimates of arc magma production rates are lowered accordingly. ß 1999 Elsevier Science B.V. All rights reserved.

Subduction Zones

“…Earlier ideas that forearcs are largely composed of “trapped” oceanic crust that formed before the pertinent subduction zone [ Dickinson and Seely , 1979] are not generally supported by recent studies. Forearc crust generally seems to have been formed by seafloor spreading about the same time as its subduction zone was initiated [ Bloomer et al , 1995], although a minor amount of trapped oceanic crust has been identified [ DeBari et al , 1999]. This indicates that subduction initiation is associated with early extension, not shortening [ Stern and Bloomer , 1992].…”

Untitled

“…Most significantly, incompatible trace elements derived from subducted crust do not unambiguously affect the source region of most FAB, although some FAB lavas from the Mariana fore arc have Pb isotopic compositions consistent with a weak influence of subducted Pacific crust (Reagan et al, 2010). Differences in isotopic and trace element characteristics between IBM FAB and MORB including that from the Philippine plate strongly imply that FAB does not represent preexisting ocean basin or back-arc basin crust trapped prior to subduction initiation, as originally concluded by Johnson and Fryer (1990) and DeBari et al (1999) for MORB-like tholeiites recovered from the Mariana and Izu inner trench walls.…”

Geochronological and geochemical constraint on the Cenozoic extension of Cathaysian lithosphere and tectonic evolution of the border sea basins in East Asia

“…These Hart, 1984) for mantle derived rocks and ores in the border sea area, East Asia. Data from Meijer, 1976;Sasaki and Sato, 1982;Johnson, 1985;Woodhead and Fraser, 1985;Mukasa, 1987;Woodhead, 1989;Zhu and Wang, 1989a,b;Volpe et al, 1990;Stern et al, 1991;Hickey-Vargas, 1991Tu et al, 1992;Pearce et al, 1992;Van Der Laan et al, 1992;Mcdermott et al, 1993;Hickey-Vargas et al, 1995;Spadea et al, 1996;Castillo and Punongbayan, 1996;Gribble et al, 1996;Knittel et al, 1997;Janney and Castillo, 1997;Castillo, 1997;Zhu et al, 1997;Zhu, 1998;Taylor and Nesbitt, 1998;DeBari et al, 1999; and this paper. geochemical boundaries represent the northeastern margin of the Philippine Sea plate or Gondwana-derived continental blocks.…”

“…Samples from the forearc basement of the IzuBonin Arc (ODP Leg 125) and southeastern Pacific basin to the east of the Mariana Arc plot just below the North Hemisphere Reference Line (NHRL) proposed by Hart (1984) and lie in the area of Pacific MORB ( Fig. 7a and b) (Meijer, 1976;Johnson, 1985;Woodhead and Fraser, 1985;Woodhead, 1989;Volpe et al, 1990;Stern et al, 1991;Pearce et al, 1992;Van Der Laan et al, 1992;Gribble et al, 1996Gribble et al, , 1998Janney and Castillo, 1997;Elliott et al, 1997;Taylor and Nesbitt, 1998;DeBari et al, 1999;Elburg and Foden, 1999). The lead isotopic compositions of basalts from both sides of the Izu -Bonin -Mariana Arc can be Fig.…”

“…Its geochemical boundaries have been well established in several districts (Klein et al, 1988;Pyle et al, 1992Pyle et al, , 1995Mahoney et al, 1998). Abundant Pb, Sr and Nd isotopic data for Cenozoic MORB and OIB and Mesozoic residual MORB from the Philippine Sea, South China Sea, Celebes Sea and Sulu Sea has been obtained (Mukasa et al, 1987;Hickey-Vargas, 1991Tu et al, 1992;Pearce et al, 1992;Mcdermott et al, 1993;Hickey-Vargas et al, 1995;Spadea et al, 1996;Castillo and Punongbayan, 1996;Knittel et al, 1997;DeBari et al, 1999). Sr isotopic systematics are easily disturbed by seawater alteration because of the high strontium concentration and long residence time in sea water.…”