Fore‐arc basalts and subduction initiation in the Izu‐Bonin‐Mariana system

Abstract

[1] Recent diving with the JAMSTEC Shinkai 6500 manned submersible in the Mariana fore arc southeast of Guam has discovered that MORB-like tholeiitic basalts crop out over large areas. These "fore-arc basalts" (FAB) underlie boninites and overlie diabasic and gabbroic rocks. Potential origins include eruption at a spreading center before subduction began or eruption during near-trench spreading after subduction began. FAB trace element patterns are similar to those of MORB and most Izu-Bonin-Mariana (IBM) back-arc lavas. However, Ti/V and Yb/V ratios are lower in FAB reflecting a stronger prior depletion of their mantle source compared to the source of basalts from mid-ocean ridges and back-arc basins. Some FAB also have higher concentrations of fluid-soluble elements than do spreading center lavas. Thus, the most likely origin of FAB is that they were the first lavas to erupt when the Pacific Plate began sinking beneath the Philippine Plate at about 51 Ma. The magmas were generated by mantle decompression during near-trench spreading with little or no mass transfer from the subducting plate. Boninites were generated later when the residual, highly depleted mantle melted at shallow levels after fluxing by a water-rich fluid derived from the sinking Pacific Plate. This magmatic stratigraphy of FAB overlain by transitional lavas and boninites is similar to that found in many ophiolites, suggesting that ophiolitic assemblages might commonly originate from near-trench volcanism caused by subduction initiation. Indeed, the widely dispersed Jurassic and Cretaceous Tethyan ophiolites could represent two such significant subduction initiation events.

Geodynamic evolution of Upper Cretaceous Zagros ophiolites: formation of oceanic lithosphere above a nascent subduction zone

“…This magmatic evolution, from bottom to top, appears to have been continuous; there is no evidence in any of the ophiolites for a hiatus between lower MORB/IAT-like and upper calc-alkaline/boninite sequences in the form of an unconformity or intervening sedimentary sequence. This evolution is similar to that seen in the IBM fore-arc (Reagan et al 2010).…”

“…They were generated from subduction component-free mantle, perhaps flowing in from beneath Eurasia to fill space created by the initial sinking of the Tethyan plate, as hypothesized by Stern & Bloomer (1992). Such a model is also suggested for the early MORB-like lavas from the IBM fore-arc (Reagan et al 2010). To approximate the proportion of ophiolitic lavas and dykes that are arclike rather than non-arc-like (MORB and E-MORB) we plotted all Zagros ophiolitic lavas and dykes on a Nb/Yb v. Nb/La diagram (Fig.…”

“…Most OB and IB mafic rocks have high Th/Yb and plot above the mantle array near the volcanic arc array, with some exceptions that are MORB-like. For comparison data from the Kizildag ophiolite, Turkey (Dilek & Thy, 2009), Oman lavas (Godard, Dautria & Perrin, 2003) and IBM fore-arc lavas (Reagan et al 2010) are shown.…”

“…Shervais, 1982) for Zagros outer belt (a) and inner belt (b) ophiolites. For comparison, fields for the V1 (lower, Geotimes unit) and V2 (upper, Lasail unit) lavas of Oman ophiolite (data from Alabaster, Pearce & Malpas, 1982) and for Mariana fore-arc basalts and boninites (Reagan et al 2010) are shown.…”

“…Along the Kherran to Sartakht road there is a volcanic pile with sparsely phyric basaltic-andesites at the bottom grading upwards into porphyritic basalts with plagiogranitic veins and then into pillowed lavas ( Boundaries between tholeiitic and calc-alkaline suites (thin solid line) and between low-, medium-and high-Fe suites (thick, dashed lines) are from Miyashiro (1974) and Arculus (2003), respectively. For comparison compositional fields from the Kizildag ophiolite, Turkey (Dilek & Thy, 2009), Oman (Godard, Dautria & Perrin, 2003) and IBM fore-arc lavas (Reagan et al 2010) are shown. are primitive.…”