Making continental crust: origin of Devonian orthogneisses from SE Mongolian Altai

Orthogneiss and meta-rhyolite bodies from different crustal levels of the Tseel Terrane in the Mongolian Altai were examined using multidisciplinary approach involving structural geology, whole-rock geochemistry and U–Pb zircon geochronology. The orthogneisses form sheet-like bodies parallel with dominant sub-horizontal metamorphic fabric which was heterogeneously verticalized along localized zones of deformation at boundaries of lower and middle crustal domains. Three samples of orthogneisses yielded Late Devonian LA-ICP-MS U–Pb zircon ages of 373±3, 377±5 and 379±2 Ma (2σ), which are interpreted as crystallization ages of felsic magmas. The meta-rhyolite displays poorly constrained, older U–Pb zircon ages of 380±4 and 403±5 Ma, which are also considered as intrusive. Whole-rock geochemistry, including relatively little fractionated REE patterns, as well as radiogenic whole-rock Nd and zircon Hf isotopic signatures point to a rather primitive source of the granitic protoliths. The high-K calc-alkaline chemistry and LILE over HFSE enrichments in the NMORB-normalized spider plots indicate an arc-related origin. Juvenile character of the studied rocks was confirmed by Nd and Hf crustal residence ages that are mostly 0.8–0.9 Ga. The origin of the metaigneous rocks is interpreted in terms of partial melting of Neoproterozoic to Cambrian magmatic arc-derived material, probably dominated by immature psammitic sediments (graywackes). This study brings important arguments that the orthogneisses do not represent an old crystalline basement previously assumed in the Mongolian Altai. A model is proposed suggesting formation of mature and layered continental crust by syn-orogenic melting of youthful volcanosedimentary wedge and emplacement of sub-horizontal syn-orogenic magmatic sheets at all crustal levels during crustal-scale vertical shortening. The vertical shortening was probably connected to lithospheric-scale extensional event associated with massive heat influx and emplacement of juvenile magmas at the bottom of the crust. It is suggested that this mechanism represents potentially a viable model for cratonization of accretionary systems worldwide.

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