Yingde Jiang

THE 390 Ma HIGH-T METAMORPHIC EVENT IN THE CHINESE ALTAI: A CONSEQUENCE OF RIDGE-SUBDUCTION?

High-grade rocks occur in the Chinese Altai, but the timing of metamorphism is poorly constrained, which hinders our understanding of the thermo-tectonic history of the region. Representative high-grade samples from the sillimanite zone extending from Hanas to Fuyun were selected for zircon U-Pb dating and temperature estimation. LA-ICP-MS analyses of zircon overgrowth rims and recrystallized domains give consistent ages of ∼390 Ma, which is interpreted to record a regional metamorphic event. Temperature (T) estimations using the amphibole-plagioclase-quartz (Amp-Pl-Qtz) and garnet-biotite (GB) geothermometers give relatively high temperatures ranging from 650 to 700 °C. The zircon metamorphic rims yield temperature estimates of ∼720 °C by using the Ti-in-zircon thermometer. These data suggest that a high-temperature metamorphic event took place in the Chinese Altai in the Middle Devonian, and may imply a tectonic environment involving an unusually elevated heat flux from a deep-seated source. Our data support possible ridge subduction around 390 Ma that caused upwelling of the hot asthenosphere and triggered the high-T metamorphism. This model can also account for coeval volcanic activity with a range of geochemical characteristics, diverse mafic intrusions and extensive hydrothermal mineralization in the Altai orogen.

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

Granitoid magmatism and its role in differentiation and stabilization of the Paleozoic accretionary wedge in the Chinese Altai are evaluated in this study. Voluminous Silurian-Devonian granitoids intruded a greywacke-dominated Ordovician sedimentary succession (the Habahe Group) of the accretionary wedge. The close temporal and spatial relationship between the regional anatexis and the formation of granitoids, as well as their geochemical similarities including rather unevolved Nd isotopic signatures and the strong enrichment of large-ion lithophile elements relative to many of the high field strength elements, may indicate that the granitoids are product of partial melting of the accretionary wedge rocks. Whole-rock geochemistry and pseudosection modeling show that regional anatexis of fertile sediments could have produced a large amount of melts compositionally similar to the granitoids. Such process could have left a high-density garnet- and/or garnet-pyroxene granulite residue in the deep crust, which can be the major reason for the gravity high over the Chinese Altai. Our results show that melting and crustal differentiation can transform accretionary wedge sediments into vertically stratified and stable continental crust. This may be a key mechanism contributing to the peripheral continental growth worldwide.

Geophysical and geochemical nature of relaminated arc-derived lower crust underneath oceanic domain in southern Mongolia

The Central Asian Orogenic Belt (CAOB) in southern Mongolia consists of E-W trending Neoproterozoic 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/m3) 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.

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†

Neoproterozoic to early Paleozoic accretionary processes of the Central Asian Orogenic Belt have been evaluated so far mainly using the geology of ophiolites and/or magmatic arcs. Thus, the knowledge of the nature and evolution of associated sedimentary prisms remains fragmentary. We carried out an integrated geological, geochemical and zircon U–Pb geochronological study on a giant Ordovician metasedimentary succession of the Mongolian Altai Mts. This succession is characterized by dominant terrigenous components mixed with volcanogenic material. It is chemically immature, compositionally analogous to graywacke and marked by significant input of felsic to intermediate arc components, pointing to an active continental margin depositional setting. Detrital zircon U–Pb ages suggest a source dominated by products of early Paleozoic magmatism prevailing during the Cambrian–Ordovician and culminating at ca. 500 Ma. We propose that the Ordovician succession forms an ‘Altai sedimentary wedge’, the evolution of which can be linked to the geodynamics of the margins of the Mongolian Precambrian Zavhan-Baydrag blocks. This involved subduction reversal from southward subduction of a passive continental margin (early Cambrian) to the development of the ‘Ikh-Mongol Magmatic Arc System’ and the giant ‘Altai sedimentary wedge’ above a north-dipping subduction zone (Late Cambrian–Ordovician). Such a dynamic process resembles the tectonic evolution of the peri-Pacific accretionary Terra Australis Orogen. A new model reconciling the Baikalian metamorphic belt along the southern Siberian Craton with peri-Pacific Altai accretionary systems fringing the Mongolian microcontinents is proposed to explain the Cambro–Ordovician geodynamic evolution of the Mongolian collage system.

Arc magmatism associated with steep subduction: Insights from trace element and Sr–Nd–Hf–B isotope systematics

Subduction zones are the major sites for elemental cycling via slab dehydration and subsequent mantle metasomatism and melting. However, the nature of slab fluids associated with steep subduction remains largely unknown. To clarify this issue, we present an integrated study for Late Paleozoic (318–312 Ma) intermediate dykes from the Beishan orogenic collage, NW China. The dykes consist mainly of dioritic and granodioritic rocks. The dioritic dykes exhibit typical subduction-like geochemical signatures, together with relatively high Mg#, high eNd(t) and eHf(t), and low initial Sr isotopes, suggesting that they originated probably from a subduction-modified mantle. The granodioritic dykes exhibit high Mg#, high Sr/Y, La/Yb, and Na2O/K2O ratios, low Y and Yb contents, and mid-ocean ridge basalt-like Sr–Nd isotopes and high zircon eHf(t), similar to slab-derived adakite, indicating that they were likely formed by partial melting of subducted oceanic crust. The coeval adakitic and normal dioritic dykes reflect a thermal anomaly that was probably caused by rollback of subducted oceanic slab. The dioritic dykes have δ11B values from −7.7 to −6.4‰, whereas the adakitic dykes have relatively high δ11B values from −6.9 to −4.4‰. The δ11B values of adakitic dykes are lower than those of typical altered oceanic crust, in agreement with the expected loss of 11B from subducted oceanic slab during early subduction. Results of a mixing model suggest that the mantle source of the dioritic dykes has been hybridized by 11B-depleted fluids expelled from a highly dehydrated slab at deep depth, owing to the high-angle dip of the subducting oceanic slab.

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

ABSTRACT A geological map is an indispensable tool for understanding the structure of the Earth’s crust but high-quality geological maps are usually lacking in remote areas of mountainous Central Asia covered by vast deserts. The progress in remote sensing and geographical information system (GIS), as well as the advancement in analytical methods, have generated new challenges in producing modern geological maps in such regions. The presented 1:50,000 geological map along the Sagsai River summarizes new and more accurate geological data from the geologically interesting region at the contact of the supracrustal and deep crustal Tugrug and Tseel units forming the metamorphosed accretionary wedge on the S and SW slopes of the Mongolian Altai. These geological units are formed by the lower Palaeozoic volcano-sedimentary sequences affected by Devonian Barrovian metamorphism ranging from lower greenschist to granulite facies. This metamorphic basement was subsequently intruded by the post-orogenic late Carboniferous Sagsai Pluton. The presented map shows complex relationships between different crustal levels of the metamorphosed accretionary wedge and post-tectonic intrusion evaluated using a GIS, numerical processing of remote sensing data as well as field documentation and laboratory studies.

MELTING OF ACCRETIONARY WEDGE AND BUILDING MATURE CONTINENTAL CRUST: INSIGHTS FROM THE MAGMATIC EVOLUTION OF THE CHINESE ALTAI OROGEN, CENTRAL ASIA

Tectonic-magmatic reworking of accretionary wedges is a key process responsible for differentiation and stabilization of continental crustal in accretionary orogens. This generic problem can be exemplified by magmatic evolution of the Chinese Altai which represents a high-grade core of the worlds largest accretionary system, namely the Central Asian Orogenic Belt (CAOB). In the Chinese Altai, voluminous SilurianDevonian granitoids intruding a greywacke-dominated Ordovician flysch sequence. These intrusions are classically interpreted to originate from predominant (70‒90 %) juvenile (depleted mantle-derived) magma. However, their close temporal and spatial relationship with the regional anatexis of flysch rocks, allows us to examine the possibility that they were mainly derived from flysch rocks.

Hybrid accretionary/collisional mechanism of Paleozoic Asian continental growth: new plate tectonic perspective

Continental crust is formed above subduction zones by well-known process of “juvenile crust growth”. This new crust is in modern Earth assembled into continents by two ways: (i) short-lived collisions of continental blocks with the Laurussian or later Eurasian continent along the “Alpine Himalayan collisional/interior orogens” in the heart of the Pangean continental plates realm; and (ii) long lived lateral accretion of ocean-floor fragments along “circum-Pacific accretionary/peripheral orogens” at the border of the PaleoPacific and modern Pacific oceanic plate.