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

Abstract

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.

Anatexis of accretionary wedge, Pacific-type magmatism, and formation of vertically stratified continental crust in the Altai Orogenic Belt

“…For such heat flow values, the temperatures at 30 km can attain 770°C–1100°C, matching well with the proposed thermal conditions for the crustal anatexis in the Chinese Altai (Figure ). It should be stressed that analogous large‐scale extensional event associated with extensive Devonian partial melting of metasediments and emplacement of various amounts of gabbroic sheets was recently reported from the adjacent Mongolian Altai [ Han ž l et al ., ]. Altogether, Silurian to Devonian granitoids in the Chinese Altai were probably produced during the same crustal extensional event associated with the upwelling of the asthenospheric mantle (Figure ).…”

Neoproterozoic‐Early Paleozoic Peri‐Pacific Accretionary Evolution of the Mongolian Collage System: Insights From Geochemical and U‐Pb Zircon Data From the Ordovician Sedimentary Wedge in the Mongolian Altai

“…Geological map of the studied areas in SW Mongolia. Modified from Tomurtogoo () and Hanžl et al (). Sample locations are shown.…”

“…The Tugrug formation was affected by postdeposition deformation and metamorphism, resulting in the formation of superposed structures and polymetamorphic fabrics. The rocks of this formation show a penetrative flat greenschist‐facies fabric (Hanžl et al, ), whereas its high‐grade metamorphic equivalents were affected by upper amphibolite‐ to granulite‐facies deformation, followed by exhumation of metamorphic and magmatic domes (Broussolle et al, ; Lehmann et al, ). The prominent tectonothermal event was thought to have taken place in the Middle‐Late Devonian, based on U‐Pb and Pb‐Pb zircon ages of migmatitic gneisses and syntectonic granitoids ranging from circa 360 to 385 Ma (Bibikova et al, ; Hanžl et al, ; Kozakov et al, ; Kröner et al, , ).…”

Geology of the Gobi Altai and Tseel terranes in the central part of the Sagsai River Watershed, SE Mongolian Altai

“…The boundary between these units is lithological in the West and East, and sealed in the South by intrusion of the late Carboniferous mafic Sagsai Pluton (Buriánek et al, 2016). Recent study by Hanžl et al (2016) suggests that these two units represent varying degrees of metamorphism of an early Palaeozoic accretionary wedge.…”

“…This pervasive fabric is refolded by open to closed, upright folds with steep NW-SE trending axial planar cleavage (S 2 ). Kröner et al (2010), Guy et al (2014) and Hanžl et al (2016). MML, Main Mongolian Lineament.…”