The composition of the foundered complement to the continental crust and a re-evaluation of fluxes in arcs

Abstract

Most primitive arc melts are basaltic in composition, yet the bulk continental crust, thought to be generated in arcs, is andesitic. In order to produce an andesitic crust from primitive arc basalts, rocks complementary to the andesitic crust have to be fractionated and subsequently removed, most likely through density sorting in the lower arc crust. The Kohistan Arc in northern Pakistan offers a unique opportunity to constrain the composition and volume of material fluxes involved in this process. In a lower crustal section >10km cumulates (dunites, wehrlites, websterites, clinopyroxene-bearing garnetites and hornblendites, and garnet-gabbros) are exposed that are 0.1–0.3g/cm3 denser than the underlying mantle. The cumulates combine with the andesitic bulk Kohistan Arc crust to reproduce the major and trace element composition of primitive basaltic arc melts. Our petrochemical analysis suggests that fractionation and subsequent foundering of wehrlites+ultramafic hornblende–garnet–clinopyroxene cumulates+garnet-gabbros is a viable mechanism for producing andesitic crust from a calc-alkaline/tholeiitic primitive high-Mg basalt. The mass of the foundered material is approximately twice that of the arc crust generated. For an overall andesitic arc composition, we estimate a magma flux into the arc (11–15km3/yr) about three times the rate of arc crust production itself. Foundering fluxes of cumulates (6.4–8.1km3/yr) are one third to half those of the globally subducted oceanic crust (~19km3/yr). Hence, the delaminate forms a volumetrically significant, albeit refractory and depleted geochemical reservoir in the mantle. Owing to its low U/Pb and high Lu/Hf the foundered material evolves with time to a reservoir characterized by unradiogenic Pb and highly radiogenic Hf isotopes, unlike any of the common mantle endmembers defined by OIB chemistry. The unradiogenic Pb of the foundered arc cumulates could counterbalance the radiogenic Pb composition of the depleted mantle. The predicted highly radiogenic Hf (at rather unradiogenic Nd) of the foundered material can explain the εHf–εNd systematics observed in some abyssal peridotites and mantle xenoliths.