Neoproterozoic to middle Palaeozoic evolution of the Central Asian Orogenic Belt in south-central Mongolia: chronological and geochemical perspectives

Mongolia occupies a central position within the eastern branch of the large accretionary Central Asian Orogenic Belt (CAOB) or Altaids. The present work aims to outline the geodynamic environment and possible evolution of this part of the eastern CAOB, predominantly from the Cambrian to the middle Palaeozoic. The investigation primarily focussed on zircon geochronology as well as whole-rock geochemical and Sm–Nd isotopic analyses for a variety of metaigneous rocks from the southern Hangay and Gobi-Altai regions in south-central Mongolia. The southern slope of the Hangay Mountains in central Mongolia exposes a large NWSE-trending middle Neoproterozoic ophiolitic complex (c. 650 Ma), which is tectonically integrated within an accretionary complex developed between the Precambrian Baydrag and Hangay crustal blocks. Formation of the entire accretionary system along the north-eastern margin of the Baydrag block mainly occurred during the early Cambrian, but convergence within this orogenic zone continued until the early Ordovician, because of on-going southward subduction-accretion of the Baydrag block. An important discovery is the identification of a late Mesoproterozoic to early Neoproterozoic belt within the northern Gobi-Altai that was reworked during the late Cambrian and throughout the late Ordovician/Devonian. Early Silurian low-grade mafic and felsic metavolcanic rocks from the northern Gobi-Altai display subduction-related geochemical features and highly heterogeneous Nd isotopic compositions, which suggest an origin at a mature active continental margin. Early Devonian protoliths of granodioritic and mafic gneisses from the southern Gobi-Altai display geochemical and Nd isotopic compositions compatible with derivation and evolution from predominantly juvenile crustal and mantel sources and these rocks may have been emplaced within the outboard portion of the late Ordovician/early Silurian active continental margin. Moreover, middle Devonian low-grade metavolcanic rocks from the southwestern Gobi-Altai yielded geochemical and Nd isotopic data consistent with emplacement in a transitional arc-backarc setting. The combined U–Pb zircon ages and geochemical data obtained from the Gobi-Altai region suggest that magmatism across an active continental margin migrated oceanwards through time by way of subduction zone retreat throughout the Devonian. Progressive extension of the continental margin was associated with the opening of a backarc basin and culminated in the late Devonian with the formation of a Japan-type arc front facing a southward open oceanic realm (present-day coordinates). nDie Mongolei nimmt eine zentrale Position innerhalb des östlichen Teils des großen Zentralasiatischen Orogens (CAOB), auch Altaiden genannt, ein. Die vorliegende Arbeit beschreibt das geodynamische Umfeld und die Entwicklung dieses Teils des CAOB vom Kambrium bis ins mittlere Paläozoikum. Die Untersuchungen konzentrieren sich auf Zirkondatierungen sowie die Geochemie und Sm–Nd-Isotopie von Gesamtgesteinsproben einer breiten Palette von metamorphen Orthogesteinen der südlichen Hangay und Gobi-Altai Region im südlichen Teil der Mongolei. An der südlichen Flanke des Hangay-Gebirges ist ein umfangreicher NW–SE streichender Ophiolitkomplex aus dem mittleren Neoproterozoikum aufgeschlossen (c. 650 Ma). Dieser wurde tektonisch in einen Akkretionskeil zwischen den präkambrischen Baydrag und Hangay Krustenblöcken integriert. Die Hauptphase der Akkretion im gesamten System liegt im frühen Kambrium, jedoch hält die konvergente Tektonik aufgrund der andauernden südwärts gerichteten Akkretion des Baydrag-Grundgebirges bis ins frühe Oberordovizium an. Von besonderer Bedeutung ist die Entdeckung eines Krustensegmentes innerhalb des nördlichen Altai aus dem späten Mesoproterozoikum bis frühen Neoproterozoikum der im oberen Kambrium und Ordovizium/Devon tektonisch und metamorph überprägt wurde. Untersilurische niedriggradige mafische und felsische Metavulkanite aus dem nördlichen Gobi-Altai zeigen subduktionstypische geochemische Eigenschaften und stark heterogene Nd-Isotopie, die auf eine Entstehung an einem reifen aktiven Kontinentalrand hinweisen. Frühdevonische Protolithe aus granodioritischen und mafischen Gneisen des südlichen Gobi-Altai legen auf Grund ihrer Geochemie und Nd-Isotopie eine Herkunft und Entwicklung aus hauptsächlich juveniler Kruste und Mantel nahe. Die Platznahme dieser Gesteine könnte innerhalb der Randbereiche des spätordovizisch/ frühsilurischen aktiven Kontinentalrandes stattgefunden haben. Darüber hinaus deuten Geochemie und Nd-Isotopie mitteldevonischer niedriggradiger Metavulkanite aus dem südwestlichen Gobi-Altai auf eine Platznahme in einem “transitional back-arc” hin. U–Pb Zirkonalter kombiniert mit geochemischen Daten aus der Gobi-Altai-Region legen nahe, dass durch Zurückschreiten der Subduktionszone während des Devons der Magmatismus über den aktiven Kontinentalrand in Richtung Ozean migrierte. Progressive Extension des Kontinentalrandes war mit der Öffnung eines Back-Arc-Beckens verbunden und gipfelte im späten Devon in der Ausbildung einer “Japan-type arc front” in Richtung eines nach Südwesten offenen Ozeans (nach heutigen Koordinaten)

Devonian arc-related magmatism in the Tseel terrane of SW Mongolia: chronological and geochemical evidence

<p>The Tseel terrane in the Gobi Desert of SW Mongolia consists of a composite crustal segment of imbricated sheets of high- and low-grade metamorphic rocks of predominantly middle Palaeozoic age. New sensitive high-resolution ion microprobe (SHRIMP) zircon ages of 396 ± 3 Ma (2σ) and 397 ± 3 Ma (2σ) for two calc-alkaline metarhyolite samples indicate arc volcanism during the early Devonian. An undeformed feldspar porphyry cross-cutting the early Devonian low-grade sequence yielded an emplacement age of 279 ± 2 Ma (2σ). Geochemical and Nd isotopic data for low-grade mafic metavolcanic rocks are consistent with melting of moderately depleted upper mantle sources in a transitional arc–back-arc setting. In addition, there are high- and low-grade felsic metaigneous rocks with subduction-related chemical signatures and heterogeneous Nd isotopic compositions indicating an origin from mixed sources, including juvenile and Precambrian crustal components. The field observations combined with the geochemical and isotopic data suggest that the Tseel terrane represents a segment of a mature Japan-type arc associated with a juvenile back-arc basin. </p

Devonian arc-related magmatism in the Tseel terrane of SW Mongolia: chronological and geochemical evidence

<p>The Tseel terrane in the Gobi Desert of SW Mongolia consists of a composite crustal segment of imbricated sheets of high- and low-grade metamorphic rocks of predominantly middle Palaeozoic age. New sensitive high-resolution ion microprobe (SHRIMP) zircon ages of 396 ± 3 Ma (2σ) and 397 ± 3 Ma (2σ) for two calc-alkaline metarhyolite samples indicate arc volcanism during the early Devonian. An undeformed feldspar porphyry cross-cutting the early Devonian low-grade sequence yielded an emplacement age of 279 ± 2 Ma (2σ). Geochemical and Nd isotopic data for low-grade mafic metavolcanic rocks are consistent with melting of moderately depleted upper mantle sources in a transitional arc–back-arc setting. In addition, there are high- and low-grade felsic metaigneous rocks with subduction-related chemical signatures and heterogeneous Nd isotopic compositions indicating an origin from mixed sources, including juvenile and Precambrian crustal components. The field observations combined with the geochemical and isotopic data suggest that the Tseel terrane represents a segment of a mature Japan-type arc associated with a juvenile back-arc basin. </p