Arc peridotite, eclogite and tonalite: a trio of ingredients for craton assembly

Amongst the current models for the formation of cratonic lithosphere, the evidence for an arc-related origin has hardened over the last few years. The trio of fundamental rock types in cratons – extremely depleted peridotite, eclogite originating from ocean crust [1], and abundant tonalitic melts that must be derived by melting of basaltic material – all point to accretion of multiple arcs as the cause. Trace element studies of depleted mantle peridotite xenoliths show that most cpx and garnet was introduced later [2], and now even opx has been shown to inherit trace element patterns from former olivine [3], meaning that the lithosphere at the time of craton formation was more strongly depleted than previously recognized. The same multiple overprinting events also affected eclogite xenoliths [4]. The eclogites differ in mineral chemistry and oxygen isotopes signatures from garnet pyroxenites that would result from crystallization of highpressure melts [5], thus clearly favouring an origin as subducted ocean crust, whereby trace elements implicate arcrelated picrite protoliths [6]. Cratonic peridotites are reminiscent of modern accretion of sub-arc lithosphere, where olivines and spinels with Mg# and Cr# both up to 0.95 witness the extreme degree of depletion [7]. These come from areas of accretion of multiple arcs, such as the closure of Tethys or modern Indonesia. Differences are in the style of enrichment: the involvement of subducted crust of continental origin in the Mediterranean finds no parallel in the tonalitic gneisses of the late Archaean, possibly indicating that most crust formed for the first time during the period 3.0-2.5Ga, and that continental crust production was largely prevented at earlier times by a lack of modern-style subduction processes. [1] Jacob (2004) Lithos 77, 295–316. [2] Rehfeldt et al. (2008) Geochim. Cosmochim. Acta 72, 5722–5756. [3] Rehfeldt et al. (2010) Geochim. Cosmochim. Acta 74, this volume. [4] Jacob et al. (2009) Lithos 112S, 1002–1013. [5] Gonzaga et al. (2010) J. Volc. Geotherm. Res, 190, 235–247. [6] Jacob & Foley (1999) Lithos 48, 317–336. [7] Prelevic & Foley (2007) Earth Planet. Sci. Lett. 256, 120–135.