Alfred Kröner

Tectonic models for accretion of the Central Asian Orogenic Belt

The Central Asian Orogenic Belt (c. 1000–250 Ma) formed by accretion of island arcs, ophiolites, oceanic islands, seamounts, accretionary wedges, oceanic plateaux and microcontinents in a manner comparable with that of circum-Pacific Mesozoic–Cenozoic accretionary orogens. Palaeomagnetic and palaeofloral data indicate that early accretion (Vendian–Ordovician) took place when Baltica and Siberia were separated by a wide ocean. Island arcs and Precambrian microcontinents accreted to the active margins of the two continents or amalgamated in an oceanic setting (as in Kazakhstan) by roll-back and collision, forming a huge accretionary collage. The Palaeo-Asian Ocean closed in the Permian with formation of the Solonker suture. We evaluate contrasting tectonic models for the evolution of the orogenic belt. Current information provides little support for the main tenets of the one- or three-arc Kipchak model; current data suggest that an archipelago-type (Indonesian) model is more viable. Some diagnostic features of ridge–trench interaction are present in the Central Asian orogen (e.g. granites, adakites, boninites, near-trench magmatism, Alaskan-type mafic–ultramafic complexes, high-temperature metamorphic belts that prograde rapidly from low-grade belts, rhyolitic ash-fall tuffs). They offer a promising perspective for future investigations.

Accretionary orogens through Earth history

Abstract Accretionary orogens form at intraoceanic and continental margin convergent plate boundaries. They include the supra-subduction zone forearc, magmatic arc and back-arc components. Accretionary orogens can be grouped into retreating and advancing types, based on their kinematic framework and resulting geological character. Retreating orogens (e.g. modern western Pacific) are undergoing long-term extension in response to the site of subduction of the lower plate retreating with respect to the overriding plate and are characterized by back-arc basins. Advancing orogens (e.g. Andes) develop in an environment in which the overriding plate is advancing towards the downgoing plate, resulting in the development of foreland fold and thrust belts and crustal thickening. Cratonization of accretionary orogens occurs during continuing plate convergence and requires transient coupling across the plate boundary with strain concentrated in zones of mechanical and thermal weakening such as the magmatic arc and back-arc region. Potential driving mechanisms for coupling include accretion of buoyant lithosphere (terrane accretion), flat-slab subduction, and rapid absolute upper plate motion overriding the downgoing plate. Accretionary orogens have been active throughout Earth history, extending back until at least 3.2 Ga, and potentially earlier, and provide an important constraint on the initiation of horizontal motion of lithospheric plates on Earth. They have been responsible for major growth of the continental lithosphere through the addition of juvenile magmatic products but are also major sites of consumption and reworking of continental crust through time, through sediment subduction and subduction erosion. It is probable that the rates of crustal growth and destruction are roughly equal, implying that net growth since the Archaean is effectively zero.

Neoproterozoic to paleozoic geology of the Altai Orogen, NW China: New zircon age data and tectonic evolution

We present a synthesis and a new account of the geological and tectonic history of the terranes of the Chinese Paleozoic Altai orogen together with new, single zircon ages for granitic and rhyodacitic rocks. A central terrane consists of Neoproterozoic to Silurian, amphibolite facies, metasedimentary rocks, and abundant Devonian‐Carboniferous granites. The presence of Precambrian basement is indicated by Sinian fossils, our xenocryst ages, and published Nd mean crustal residence ages of granites. Felsic arc‐type lavas on the southern margin of the terrane have a mean 207Pb/206Pb zircon age of 505 Ma, reflecting the time of arc volcanism, and the presence of xenocysts with ages between 614 and 921 Ma suggests derivation by intracrustal melting. Accordingly, we suggest that a Cambro‐Ordovician continental magmatic arc was built on the southern margin of the central terrane by northward subduction. A low‐grade Ordovician Andean‐type arc with a continental basement is situated above a normal fault on the northern side of the central terrane, and a low‐grade Late Silurian to Early Devonian island arc on its southern side is succeeded southward by a terrane with Proterozoic basement overlain by Devonian to Carboniferous basins. During continent‐arc collision high‐grade gneisses of the central terrane were thrust southward over the Late Silurian to Early Devonian island arc with formation of inverted, Barrovian‐type metamorphic isograds. The collisional processes caused exhumation of the high‐grade central terrane and consequent emplacement of abundant granites derived by mixed arc‐crust melting. This new model has major implications for the crustal and tectonic evolution of the Altaids.

The most ancient ophiolite of the Central Asian fold belt: U–Pb and Pb–Pb zircon ages for the Dunzhugur Complex, Eastern Sayan, Siberia, and geodynamic implications

Abstract Ophiolitic rocks with a zircon age of ∼1020 Ma occur in the Dunzhugur complex of East Sayan, Siberia, and are part of a Neoproterozoic to early Palaeozoic segment of the Central Asian fold belt. The most spectacular suite is exposed along the Oka and Bokson rivers, where a complete ophiolite sequence with mantle tectonites, a layered sequence composed of dunite, wehrlite, and pyroxenite, a gabbro section, a sheeted diabase dyke complex and basaltic pillow lavas are exposed. Petrologic and geochemical data suggest that all members of the ophiolite originally belonged to the same cogenetic mafic–ultramafic crustal section and support a supra-subduction zone setting in a fore-arc rifting environment for its origin. Two multigrain zircon size fractions from a plagiogranite are both slightly discordant but yielded a combined mean 207Pb/206Pb age of 1020±10 Ma. Evaporation of three additional fractions of three to four grains each from the same sample produced a mean 207Pb/207Pb age of 1019.9±0.7 Ma that we consider to most closely reflect the time of igneous crystallization of the plagiogranite. This is the oldest ophiolite so far dated from the Central Asian fold belt. The southern margin of the Siberian craton and the palaeo-Asian ocean were established at the end of the Mesoproterozoic, at least 1000 Ma ago. During the time interval 1000–570 Ma, one or several large ocean basins existed between Baltica, Siberia, Kazakhstan, Tarim and northern China, and these blocks are therefore unlikely to have been part of the supercontinent Rodinia. Rifting, initiation of subduction, and marginal basin formation began prior to 1000 Ma and continued through 570 Ma. The Dunzhugur ophiolite of Eastern Sayan provides evidence for the early opening of the palaeo-Asian ocean not later than 1000 Ma ago.

Zircon age and occurrence of the Adaatsag ophiolite and Muron shear zone, central Mongolia: constraints on the evolution of the Mongol–Okhotsk ocean, suture and orogen

The Adaatsag ophiolite in eastern Mongolia is situated in the Mongol–Okhotsk suture zone, which extends from central Mongolia through Transbaikalia to the Sea of Okhotsk and separates the Siberian and Amurian (Mongolian) plates. The ophiolite sequence passes upwards from serpentinite mélange and serpentinized dunite and harzburgite, through layered gabbro (with leucogabbro pegmatite dykes), wehrlite and clinopyroxenite, to isotropic gabbro and leucogabbro, sheeted mafic dykes, and olivine-rich basaltic lavas, overlain by red chert and meta-clastic sediments. A single-zircon mean 207Pb/206Pb evaporation age of 325.4 ± 1.1 Ma for a leucogabbro pegmatite dyke records the time of igneous crystallization of the plutonic suite, and thus the time of formation of oceanic crust in the Mongol–Okhotsk ocean that gave rise to the ophiolite. A U–Pb secondary ionization mass spectrometry (SIMS) and evaporation zircon age of 172 Ma for a mylonitized granite provides a maximum age for the left-lateral Muron shear zone, which occurs close to the left-lateral Mongol–Okhotsk suture, and indicates that the suture in eastern Mongolia formed at least by the mid-Jurassic. We review the evidence and models for subduction tectonics that gave rise to major calc-alkaline batholiths along active continental margins of the bordering plates and to extensive, post-collisional, alkaline to peralkaline magmatism. Whereas the magmatic history of the orogen is better known, the age of the ocean, and the time of formation and deformation of the suture zone are not. Our new data on the Adaatsag ophiolite and Muron shear zone provide key constraints on the early and late stages of development of the Mongol–Okhotsk ocean and orogen.

Age and tectonic setting of Late Archean greenstone-gneiss terrain in Henan Province, China, as revealed by single-grain zircon dating

The authors report precise U-Pb zircon ages for single grains of a metarhyodacite from the Late Archean Dengfeng greenstone belt in Henan Province, China, near the southern margin of the North China craton. Most grains belong to an igneous population whose U-Pb isotopic systematics define a straight line intersecting concordia at 2512 +/- 12 Ma, and this is interpreted as the time of crystallization of the original greenstone volcanics. Several grains are distinctly older, between 2576 +/- 9 and 2945 +/- 44 Ma, and the authors interpret the older grains as xenocrysts of pre-greenstone continental crust that provide evidence for crustal derivation or crustal contamination of the original rhyodacitic lava. The xenocrysts suggest evolution of the Dengfeng greenstone belt in a continental environment that may be represented by the Taihua high-grade gneisses bordering the Dengfeng greenstones and for which the authors obtained ages of 2806 +/- 7 and 2841 +/- 6 Ma. The data add evidence to the now widely held concept that most Archean greenstones developed on or near older continental crust and were therefore prone to crustal contamination. In such cases, conventional zircon dating may not always record the precise age of rock formation.

Granulites in the Tongbai Area, Qinling Belt, China: Geochemistry, petrology, single zircon geochronology, and implications for the tectonic evolution of eastern Asia

The Tongbai area of the eastern Qinling belt in China includes granulite-grade metamorphic assemblages (Qinling Complex) which were previously regarded as Archean to early Proterozoic in age and belonging to the southern margin of the North China plate (craton). Our petrological and geochemical data characterize these rocks as two-pyroxene granulites and garnet granulites which formed at temperatures of 757°–840°C and pressures of about 9.5 kbar and are now found as xenoliths in granodioritic gneisses. The protoliths of these rocks were granodiorites and tholeiitic basalt or gabbro. The 207Pb/206Pb ratios derived from evaporation of single zircons yield ages of 470±20 and 470±14 Ma, respectively, for the basic granulites which we interpret to reflect the time of protolith emplacement. These are intruded by a 435±14 Ma granodioritic gneiss post-dating granulite formation. A metaquartzite sample contains detrital zircons as old as 2555±8 Ma. Two samples of granitoid gneiss from the Tongbai Complex S of the Qinling granulites have single-zircon 207Pb/206Pb evaporation ages of 776±8 and 746±10 Ma, respectively, and document late Proterozoic igneous activity. We suggest that the Qinling granulites document an important and hitherto unknown phase of early Silurian crustal thickening following subduction and continental collision and that both the Qinling and Tongbai Complexes were part of the southern margin of the North China craton prior to this event and record late Proterozoic igneous activity.

Ophiolites and the evolution of tectonic boundaries in the late proterozoic Arabian—Nubian shield of northeast Africa and Arabia

Abstract The Arabian—Nubian shield is currently regarded as one of the best examples to demonstrate that processes of lateral crustal growth and modern-type obduction—accretion tectonics have operated since at least late Precambrian times. In Arabia a number of Pan-African volcano-sedimentary/plutonic belts have been identified that display internal evolutionary patterns suggesting a development from primitive intraoceanic arcs some 900–950 Ma ago to mature, andesite-dominated arcs some 640 Ma ago through processes of ocean-crust obduction, arc collision and magmatic crustal thickening. Several ophiolite-decorated sutures are preserved, but many early tectonic boundaries were obliterated during later overthrusting, faulting and shield-wide granitoid plutonism towards the end of Pan-African evolution and stabilization in the earliest Palaeozoic. In southeastern Egypt and in the Red Sea Hills of the Sudan early Pan-African clastic sediments suggest that a passive continental margin was probably separated from several evolving arcs to the east by marginal seas. These arc segments were later thrust over each other, from east to west, during widespread and considerable horizontal shortening and gave rise to spectacular nappe structures and extensive ophiolite melanges. The apparent lack of well-defined accretionary thrust stacks, high-pressure metamorphic assemblages and widespread ophiolitic melanges in Arabia indicates that accretion either did not occur along margins with deep ocean trenches but involved buoyant crust, or extensive overthrusting took place through which the forearc segments were overridden and are now concealed. This, together with the recognition of distinct tectonic belts and isolated fragments of possible ancient continental crust and oceanic plateaus, supports the contention that Arabia may represent a collage of previously independent exotic terranes that accreted by oblique convergence and strike-slip translation during shield evolution. It is suggested that the Arabian shield contains remnants of microcontinents with pre-Pan African (i.e., > 1000 Ma) crustal history and, perhaps, oceanic plateaus and that its evolution bears similarities with aspects of terrane accretion in the North American Cordillera and in the present western Pacific. The evolution in Egypt and in the Sudan, however, seems characterized by the transformation of a passive continental margin into a tectonically active belt along which ophiolites and arc volcanics were thrust over each other at approximately the same time when the exotic terranes and arcs of Arabia accreted farther east. Final stabilization of the shield occurred when the evolving Arabian plate “docked” with Nubia after marginal basin closure and cessation of arc magmatism some 600–640 Ma ago.

Linking growth episodes of zircon and metamorphic textures to zircon chemistry; an example from the ultrahigh-temperature granulites of Rogaland, SW Norway

Abstract In-situ U-Th-Pb analyses by ion-microprobe on zircon in intact textural relationships are combined with backscatter and cathodoluminescence imaging and trace element analyses to provide evidence for growth episodes of zircon. This approach helps: (a) to unravel the polymetamorphic history of aluminous migmatitic and granitoid gneisses of the regional contact aureole around the Rogaland anorthosite-norite intrusive complex; and (b) to constrain the age of M2 ultrahigh-temperature (UHT) metamorphism and the subsequent retrograde M3 event. All samples yield magmatic inherited zircon of c. 1035 Ma, some an additional group at c. 1050 Ma. This suggests that loss of Pb by volume diffusion in non-metamict zircon is not an important factor even under extreme crustal conditions. Furthermore, the identical inheritance patterns in aluminous (garnet, cordierite ± osumilite-bearing) migmatites and orthogneisses indicate a metasomatic igneous instead of a sedimentary protolith for the migmatite. Results for the M1 metamorphic event at c. 1000 Ma BP are consistent in all samples, including those from outside the orthopyroxene-in isograd. The latter do not show evidence for zircon growth during the M2 metamorphic episode. Zircon intergrown with or included within M2 metamorphic minerals (magnetite, spinel, orthopyroxene) give an age of 927 ± 7 Ma (2σ, n = 20). The youngest observed results are found in zircon outside M2 minerals, some overgrown by M3 mineral assemblages (late garnet coronas, garnet + quartz and orthopyroxene + garnet symplectites) and yield a slightly younger pooled age of 908 ± 9 Ma (2σ, n = 6). These textures are relative time markers for the crystallization of zircon overgrowths during discrete stages of the UHT event. These youngest age groups are consistent with the emplacement age of the Rogaland intrusive complex and the last magmatic activity (Tellnes dyke intrusion), respectively. This is direct and conclusive evidence for UHT metamorphism in the regional aureole being caused by the intrusions, and corrects earlier notions that the events are not linked. Trace element behaviour of zircon (Tb/U and Y content) has been tracked through time in the samples and shows variations both within and between samples. This heterogeneous behaviour at all scales appears to be common in metamorphic rocks and precludes the use of ‘rules of thumb’ in the interpretation of zircon chemistry, but chemical tracers are useful for recognition of zircon growth or recrystallization during metamorphism.

Single zircon ages from high-grade rocks of the Jianping Complex, Liaoning Province, NE China

Abstract The high-grade rocks of the Jianping Complex in Liaoning Provi nce, NE China, belong to the late Archaean to earliest Proterozoic granulite belt of the North China craton. Single zircon ages obtained by the Pb–Pb evaporation method and SHRIMP analyses document an evolutionary history that began with deposition of a cratonic supracrustal sequence some 2522–2551 Ma ago, followed by intrusion of granitoid rocks beginning at 2522 Ma and reaching a peak at about 2500 Ma. This was followed by high-grade metamorphism, transforming the existing rocks into granulites, charnockites and enderbites some 2485–2490 Ma ago. The intrusion of post-tectonic granites at 2472 Ma is associated with widespread metamorphic retrogression and ends the tectono–metamorphic evolution of this terrain. A similar evolutionary sequence has also been recorded in the granulite belt of Eastern Hebei Province. We speculate that the Jianping Complex was part of an active continental margin in the late Archaean that became involved in continental collision and crustal thickening shortly after its formation. There is a remarkable similarity between the ∼2500 Ma North China granulite belt and the equally old granulite belt of Southern India, suggesting that the two crustal domains could have been part of the same active plate margin in latest Archaean times.