HR: 16:15h
AN: V44C-02    [Abstracts]
TI: Where are the Dunite-Wehrlite Cumulates in Arcs?
AU: * Arculus, R J
EM: Richard.Arculus@anu.edu.au
AF: Richard J. Arculus, Department of Earth and Marine Sciences, Australian National University, Canberra, ACT 0200 Australia
AB: Currently, the volumetrically most significant magma flux from the mantle wedge in suprasubduction zone systems is subalkaline basalt containing variable amounts of water but typically <6 wt%. The critically important feature of the crystallization of these comparatively wet magmas is the sequence spinel-olivine-clinopyroxene-plagioclase, wherein the earliest cumulate fraction is dominated by dunites and wehrlites, unlike the sequences/assemblages at mid-ocean ridges where troctolites are associated with dunites because of the reversal of clinopyroxene and plagioclase crystallization order. A persistence in spinel fractionation accompanied by delay in plagioclase crystallization (plus the shift to Si-poor, Ca-rich anorthitic compositions once saturation is reached) are important in generating the low-FeO/MgO magmatic evolutionary trends, and relatively large volumes of continental crust-building, dacitic-rhyolitic fractionates in arc systems. The anticipated large volumes of dunite-wehrlite cumulates are not obvious in the seismically well-determined cross-sections of intra-oceanic arcs but given the seismic wave velocities of dunite-wehrlite cumulates are relatively high (Vp>8 kms-1), these cumulates are assigned to sub-Moho depths or >20 km in the case of the Izu-Bonin Arc. In terms of overall magma flux calculations, these cumulates should be included together with the supra-Moho volumes. Generally, there are few examples of these cumulates world-wide in exhumed arc crust-mantle sections. In addition to parts of the sequences exposed in Talkeetna (Alaska) and Ladakh (Pakistan), other examples include the Greenhills and Luxmore Complexes of southern New Zealand. Hot cumulates dominated by spinel-olivine-clinopyroxene (+ variably amphibole and phlogopite) at sub-Moho depths are plausible protoliths of the relatively common albeit volumetrically minor ankaramitic arc magma type. More generally however, it is proposed that dunite-wehrlite cumulates are incorporated into the advective motions of the mantle wedge, and recycled deeper into the mantle. Rare examples of such cumulates that have been exhumed with ultra-high pressure blueschist-eclogite sequences following subduction possibly occur in the North Qaidam Mountains of China.
DE: 8410 Geochemical modeling (1009, 3610)
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005