Boninite and Harzburgite from Leg 125 (Bonin-Mariana Forearc): A Case Study of Magma Genesis during the Initial Stages of Subduction

Abstract

Holes drilled into the volcanic and ultrabasic basement of the Izu-Ogasawara and Mariana forearc terranes during Leg 125 provide data on some of the earliest lithosphere created after the start of Eocene subduction in the Western Pacific. The volcanic basement contains three boninite series and one tholeiite series. (1) Eocene low-Ca boninite and low-Ca bronzite andesite pillow lavas and dikes dominate the lowermost part of the deep crustal section through the outer-arc high at Site 786. (2) Eocene intermediate-Ca boninite and its fractionation products (bronzite andesite, andesite, dacite, and rhyolite) make up the main part of the boninitic edifice at Site 786. (3) Early Oligocene intermediate-Ca to high-Ca boninite sills or dikes intrude the edifice and perhaps feed an uppermost breccia unit at Site 786. (4) Eocene or Early Oligocene tholeiitic andesite, dacite, and rhyolite form the uppermost part of the outer-arc high at Site 782. All four groups can be explained by remelting above a subduction zone of oceanic mantle lithosphere that has been depleted by its previous episode of partial melting at an ocean ridge. We estimate that the average boninite source had lost 10-15 wt% of melt at the ridge before undergoing further melting (5-10%) shortly after subduction started. The composition of the harzburgite (<2% clinopyroxene, Fo content of about 92%) indicates that it underwent a total of about 25% melting with respect to a fertile MORB mantle. The low concentration of Nb in the boninite indicates that the oceanic lithosphere prior to subduction was not enriched by any asthenospheric (OIB) component.The subduction component is characterized by (1) high Zr and Hf contents relative to Sm, Ti, Y, and middle-heavy REE, (2) light REE-enrichment, (3) low contents of Nb and Ta relative to Th, Rb, or La, (4) high contents of Na and Al, and (5) Pb isotopes on the Northern Hemisphere Reference Line. This component is unlike any subduction component from active arc volcanoes in the Izu-Mariana region or elsewhere. Modeling suggests that these characteristics fit a trondhjemitic melt from slab fusion in amphibolite facies. The resulting metasomatized mantle may have contained about 0.15 wt% water. The overall melting regime is constrained by experimental data to shallow depths and high temperatures (1250°C and 1.5 kb for an average boninite) of boninite segregation. We thus envisage that boninites were generated by decompression melting of a diapir of metasomatized residual MORB mantle leaving the harzburgites as the uppermost, most depleted residue from this second stage of melting. Thermal constraints require that both subducted lithosphere and overlying oceanic lithosphere of the mantle wedge be very young at the time of boninite genesis. This conclusion is consistent with models in which an active transform fault offsetting two ridge axes is placed under compression or transpression following the Eocene plate reorganization in the Pacific. Comparison between Leg 125 boninites and boninites and related rocks e...

Thermal structure due to solid-state flow in the mantle wedge beneath arcs

“…These calculations are essentially of three types: (1) analytical approximations including various assumptions about coupling between the subducted crust and the overlying mantle and about convection in the mantle wedge [e.g., Davies, 1999;Molnar and England, 1995;Molnar and England, 1990], (2) purely plate-driven models with uniform viscosity, in which the thermal regime is calculated numerically using analytical expressions for corner flow in the mantle wedge, with model results depending on various input parameters including the thickness of the arc "lithosphere" and the depth of coupling between subducting crust and overlying mantle [e.g., Peacock, 2002;Peacock and Hyndman, 1999;Peacock and Wang, 1999;Iwamori, 1997;Peacock, 1996;Ponko and Peacock, 1995;Peacock et al, 1994;Pearce et al, 1992;Peacock, 1991;Peacock, 1990a;Peacock, 1990b], and (3) dynamic models in which the mantle flow field as well as the thermal regime are calculated numerically, with model results depending on parameters such as thermal buoyancy, chemical buoyancy and mantle viscosity [van Keken et al, 2002;Furukawa and Tatsumi, 1999;Kincaid and Sacks, 1997;Furukawa, 1993a;Furukawa, 1993b;Davies and Stevenson, 1992]. These models differ in many respects, but most agree that subduction of oceanic crust that is more than 20 million years old at down-dip rates greater than 20 km/Myr will not produce temperatures at the top of the subducting plate that are high enough to allow fluid-saturated melting of sediment or basalt.…”

Neoarchean siliceous high-Mg basalt (SHMB) from the Taishan granite–greenstone terrane, Eastern North China Craton: Petrogenesis and tectonic implications

“…This clearly demonstrates that a greater proportion of sedimentary-and/or basaltic-derived slab melts have been involved in their generation Pearce and Peate, 1995;Bédard, 1999). Several studies have established that the enriched nature of most Cenozoic high-Mg rocks, including high-Mg andesites and boninites, can be attributed to a contribution from slab-derived melts (Kay, 1978;Cameron et al, 1983;Pearce et al, 1992;Yogodzinski et al, 1994;Shimoda et al, 1998;Tatsumi, 2001;Bindeman et al, 2005;König et al, 2010). Furthermore, the coeval occurrence of boninites with high-Mg andesite and slab melt-related rocks (e.g., adakites) has been recognized in some present-day arcs (e.g., Tonga, Falloon et al, 2008;IzuBonin-Mariana Fore-arc, Pearce et al, 1992), providing evidence of slab melt-related enrichment.…”

“…In this respect, boninitic magma that forms by partial melting of relatively clinopyroxene-rich sources exhibits low Zr/Sm, whereas melts derived from partial melting of clinopyroxene-poor sources exhibit high Zr/Sm. Accordingly, the characteristics of negative Zr anomalies and low (Zr/Sm) PM ratios (<1, barring the sample with a value of 1.27) in the Taishan SHMBs likely indicate that their mantle source is enriched in clinopyroxene, which is distinctly different from that of typical boninites which are derived from partial melting of clinopyroxene-poor harzburgite (Sun and Nesbitt, 1978;Hickey and Frey, 1982;Crawford et al, 1989;Pearce et al, 1992;Taylor et al, 1994).…”

“…5a and b. That is, the mantle source of the Taishan SHMBs is a highly depleted and refractory peridotite that was metasomatically enriched either by melts and/or hydrous fluids derived from subducted oceanic crust and pelagic sediments prior to, or during, remelting (Sun and Nesbitt, 1978;Hickey and Frey, 1982;Crawford et al, 1989;Pearce et al, 1992;Taylor et al, 1994).…”

Mineralogy, geochemistry and sulfur isotope characterization of Cerro de Maimón (Dominican Republic), San Fernando and Antonio (Cuba) lower Cretaceous VMS deposits: Formation during subduction initiation of the proto-Caribbean lithosphere within a fore-arc

“…According to Finlow-Bates and Stumpfl (1981) (Pearce et al, 1992;Reagan et al, 2010). However, chondrite-normalized patterns of LREE-depleted boninites show indisputable similarity to basalts described as transitional (from fore arc basalts, FAB, to boninites) in the IBM fore-arc basin by Reagan et al (2010).…”