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The redox sensitivity of Ce and Eu anomalies in zircon has been clearly demonstrated by experimental studies, and these may represent an important tool in the exploration for porphyry Cu deposits which are thought to be derived from oxidised magmas. These deposits are significant because they are the source of much of the world's copper and almost all of the molybdenum and rhenium, key elements in many modern technologies. However, Ce and Eu anomalies in zircon are also affected by the co-crystallisation of REE bearing phases, such as titanite. Here, we report the trace element chemistry of zircons from titanite-bearing intrusions associated with mineralisation at the world class Oyu Tolgoi porphyry Cu-Au deposit (Mongolia). Based on these data, we suggest that neither zircon Eu/Eu*, nor Ce4+/Ce3+ are robust proxies for melt redox conditions, because they are both too strongly dependent on melt REE concentrations, which are usually poorly constrained and controlled by the crystallisation of titanite and other REE-bearing phases. In spite of this, Eu/Eu* can broadly distinguish between fertile and barren systems, so may still be an indicator of porphyry magma fertility, and a useful tool for exploration

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The Central Asian Orogenic Belt (CAOB) in southern Mongolia consists of E-W trendingNeoproterozoic cratons and Silurian-Devonian oceanic tectonic zones. Previous study revealed that the Early Paleozoic accretionary wedge and the oceanic tectonic zone are underlain by a layer giving a homogeneous gravity signal. Forward gravity modelling suggests that this layer is not formed of high-density material typical of lower oceanic crust but is composed of low-to intermediate-density rocks resembling continental crust. The nature of this lower crust is constrained by the whole-rock geochemistry and zircon Hf isotopic signature of abundant Late Carboniferous high-K calc-alkaline and Early Permian A-type granitoids intruding the two Early Paleozoic domains. It is possible to explain the genesis of these granitoids by anatexis of juvenile, metaigneous (tonalitic-gabbroic) rocks of Late Cambrian age, the source of which is presumed to lie in the "Khantaishir" arc (520-495 Ma) further north. In order to test this hypothesis, the likely modal composition and density of Khantaishir arc-like protoliths are thermodynamically modelled at granulite-and higher amphibolite-facies conditions. It is shown that the current average density of the lower crust inferred by gravity modelling (2730 ± 20 kg/m 3 ) matches best metamorphosed leucotonalite to diorite. Based on these results, it is now proposed that Mongolian CAOB has an architecture in which the accretionary wedge and oceanic upper crust is underlain by allochthonous lower crust that originated in a Cambrian arc. A tectonic model explaining relamination of allochthonous felsic to intermediate lower crust beneath mafic upper crust is proposed.