NO EXCESSIVE CRUSTAL GROWTH IN THE CENTRAL ASIAN OROGENIC BELT: FURTHER EVIDENCE FROM FIELD RELATIONSHIPS AND ISOTOPIC DATA

We provide new field observations and isotopic data for key areas of the Central Asian Orogenic Belt (CAOB), reiterating that no excessive crustal growth occurred during its ca. 800 Ma long orogenic evolution. Many Precambrian blocks (microcontinents) identified in the belt are exotic and are most likely derived from the northern margin of Gondwana, including the Tarim craton.We provide new field observations and isotopic data for key areas of the Central Asian Orogenic Belt (CAOB), reiterating that no excessive crustal growth occurred during its ca. 800 Ma long orogenic evolution. Many Precambrian blocks (microcontinents) identified in the belt are exotic and are most likely derived from the northern margin of Gondwana, including the Tarim craton

Early Palaeozoic Deep Subduction of Continental Crust in the Kyrgyz North Tianshan: Evidence from Field Relationships and Geochronology

International audienc

Lower Moscovian Limestones of the Bogdashan Range (NW China) as an Indicator of Cessation of Arc Magmatism in the Junggar Region

© 2019, Pleiades Publishing, Ltd. Abstract—: The field revision of the Carboniferous and Lower Permian stratigraphy of the northern Bogdashan (South Junggar, Northwest China) shows that the Lower to Middle Carboniferous island arc volcanic rocks, widely developed in this region, are overlapped everywhere by carbonate and terrigenous-carbonate sediments, containing occasional lava flows and overlain up the section by thick terrigenous series practically devoid of volcanic rocks. The deposition of limestone occurred at the stage of dying off of a volcanic arc, and the question of their age is of fundamental importance for dating this event. Carbonates are represented by facies of lagoons, shoals, and bioherms that formed on the leveled surface of the arc and on the slopes of the last active volcanoes. Bioherms are Waulsortian mounds and are mainly composed of algal limestones and carbonate mud. There are no framestones composed of corals and sponges (chaetetids) typical of the tropical zone. The facies of shallow crinoid-fusulinid limestones typical of the adjacent territories of the Southern Tien Shan and Tarim are poorly represented. Paleogeographically, the position of bioherms corresponds to the northern boundary of the realm of Pennsylvanian reefs. On the basis of foraminifers, brachiopods, and corals, the age of carbonates is early Moscovian (ca. 315–310 Ma). Cessation of island-arc volcanism, followed by the accumulation of limestone in Bogdashan, occurred sub-synchronously with formation of the West Junggar (Bayingou) suture and may reflect docking of the Bogdashan arc to the Yili active margin of the Kazakhstan continent. Further subsidence of Bogdashan and adjacent regions of the Junggar and Turfan basins, which was somewhat slower at the end of the Carboniferous and more intense in the Early and Middle Permian, may reflect the development of the foreland basin that formed along the northern flank of the Tien Shan orogen. Marine facies were locally preserved in this basin until the Artinskian (ca. 285 Ma), and later the Junggar and Turfan basins lost connection to the ocean and developed in continental environments

Early Neoproterozoic fore-arc basin strata of the Malyi Karatau Range (South Kazakhstan): depositional ages, provenance and implications for reconstructions of Precambrian continents

Available online 30 March 2023Multidisciplinary geochronological, isotopic, chemical, and facial studies in the Malyi Karatau Range (MK) of South Kazakhstan elucidate the Precambrian stratigraphic framework and evolution of the Ishim Middle Tianshan microcontinent (IMT) in the western Central Asian Orogenic Belt. Detrital zircon and apatite U-Pb ages for siliciclastic rocks and chemostratigraphic Sr-C isotopic data for carbonates indicate that they deposited during 800– 730 Ma. The sediments are dominated by deeper marine facies in the southwest and shallow marine facies in the northeast. According to paleocurrent indicators, the main provenance located to the west of the basin. Based on the detrital zircon age spectra, the source terrane represented a Paleoproterozoic to Archean crustal block, reworked by an early Neoproterozoic c. 850–720 Ma continental arc. Erosion of Archean and Paleoproterozoic crust is also evidenced by negative εNd values for the sandstones. The petrographic and chemical compositions of the sandstones are consistent with a continental arc source; however anomalously high concentrations of chromium in some layers point to the presence of ultramafic rocks in the source terrane. The ages of metamorphic zircons indicate a high-grade metamorphic event at provenance at c. 2.0 Ga and the ages of detrital apatites suggest a reset of U-Pb isotope system in apatite at c. 1.8–1.9 Ga. In the early Neoproterozoic, the MK located between the continental arc in the west and the oceanic basin in the east and represented a fore-arc basin. The similarity of the Precambrian magmatic and metamorphic histories and sedimentary facies indicates that the IMT, the Tarim and Yangtze cratons constituted a single Precambrian Ulutau-Tarim-Yangtze Continent (UTY). Judging from the ages of continental arcs that evolved on the northern and southern sides of the UTY, it represented an independent continent at c. 950–840 Ma and was incorporated in Rodinia at c. 830–820 Ma.D.V. Alexeiev, A.K. Khudoley, S.A. DuFrane, S. Glorie, I.A. Vishnevskaya, S.A. Semiletkin, E.F. Letnikov

NO EXCESSIVE CRUSTAL GROWTH IN THE CENTRAL ASIAN OROGENIC BELT: FURTHER EVIDENCE FROM FIELD RELATIONSHIPS AND ISOTOPIC DATA

We provide new field observations and isotopic data for key areas of the Central Asian Orogenic Belt (CAOB), reiterating that no excessive crustal growth occurred during its ca. 800 Ma long orogenic evolution. Many Precambrian blocks (microcontinents) identified in the belt are exotic and are most likely derived from the northern margin of Gondwana, including the Tarim craton

Zircon and muscovite ages, geochemistry, and Nd-Hf isotopes for the Aktyuz metamorphic terrane: Evidence for an Early Ordovician collisional belt in the northern Tianshan of Kyrgyzstan

International audienceThe Aktyuz metamorphic terrane in the Kyrgyz northern Tianshan consists of granitoid gneisses and migmatites with subordinate paragneisses, greenschists, presumed meta-ophiolites, and garnet amphibolite dykes that contain HP eclogite relicts. The gneisses and migmatites were previously considered to be Archaean and Palaeoproterozoic in age on the basis of α-Pb and U-Pb multigrain zircon dating. Zircons from a post-tectonic granite were previously dated at 692 ± 15 Ma, constraining the time of main deformation and metamorphism in the Aktyuz terrane to the Precambrian. The chemical characteristics of most granitoid samples are consistent with melting of chemically evolved crustal material, which is supported by Nd and Hf isotopic data. Zircon U-Pb SHRIMP ages were obtained for the main varieties of metamorphic rocks, for a gabbro of a low-grade ophiolite complex and for several post-kinematic igneous rocks. In addition, metamorphic muscovite was dated by the 40Ar-39Ar method, and whole-rock Sm-Nd isotopic systematics were obtained on several granitoid rocks. Our magmatic zircon crystallization ages for granitoid gneisses in the Aktyuz and Kemin Complexes range from 778 ± 6 to 844 ± 9 Ma which we interpret to reflect the time of magmatic emplacement of the gneiss protoliths. These rocks reflect an episode of Neoproterozoic granitoid magmatism, which is also documented in southern Kazakhstan, the Kyrgyz Middle Tianshan, the Chinese Central Tianshan and the Tarim Craton. Nd and Hf isotopic systematics show these rocks to be derived from Mesoproterozoic to Archaean sources. The calc-alkaline composition of these rocks seems compatible with a subduction setting, but is most likely inherited from the source, therefore the tectonic scenario for emplacement of the gneiss protolith remains unknown. Two ages of 562 ± 7 and 541 ± 3 Ma and negative εNd(t)-values for granitoid gneisses document a later crustal melting episode. Muscovite 40Ar/39Ar ages of ca. 470 Ma for Aktyuz gneisses constrain the main fabric-forming metamorphism to the Early Palaeozoic. A migmatitic paragneiss, which was previously interpreted as Palaeoproterozoic, contains detrital zircons with an age spectrum from 503 to 1263 Ma; the youngest grain suggests a maximum Cambrian age of protolith deposition. An ophiolitic metagabbro of the Kemin Complex yielded an Early Cambrian age of 531 ± 4 Ma, which is close to the age of ophiolites in the adjacent Djalair-Naiman belt of Kazakhstan, suggesting a possible genetic link. Two samples of quartz diorite from the post-kinematic Dolpran pluton yielded Early Ordovician zircon ages of 471.9 ± 3.5 and 472.0 ± 3.1 Ma. The presence of a 783 ± 7 Ma xenocrystic zircon points at Precambrian crust at depth, which may explain an earlier, discordant apparent age obtained by multigrain zircon dating of this pluton. Undeformed rhyolite and basalt in the East Kyrgyz Range, previously classified as Neoproterozoic and Cambrian, yielded Late Ordovician ages of 451.9 ± 4.6 and 448.9 ± 5.6 Ma respectively. Our data imply that the Aktyuz terrane is not a single segment of continuous Precambrian continental crust but represents a complex amalgamation of Neoproterozoic continental and Early Palaeozoic ophiolitic slivers, which were stacked together in a subduction and collisional setting. Large fragments of continental crust were subducted to HP eclogite-facies conditions, which led to eclogite metamorphism in the mafic dykes. The main deformational event occurred during the latest Cambrian to earliest Ordovician between 503 and 472 M

Mesoproterozoic (Grenville-age) terranes in the Kyrgyz North Tianshan : zircon ages and Nd-Hf isotopic constraints on the origin and evolution of basement blocks in the southern Central Asian Orogen

The North Tianshan orogenic belt in Kyrgyzstan consists predominantly of Neoproterozoic to early Paleozoic assemblages and tectonically interlayered older Precambrian crystalline complexes and formed during early Paleozoic accretionary and collisional events. One of the oldest continental fragments of late Mesoproterozoic (Grenvillian) age occurs within the southern part of the Kyrgyz North Tianshan. Using SHRIMP zircon ages, we document two magmatic events at ~ 1.1 and ~ 1.3 Ga. The younger event is characterized by voluminous granitoid magmatism between 1150 and 1050 Ma and is associated with deformation and metamorphism. The older event is documented by ~ 1.3 Ga felsic volcanism of uncertain tectonic significance and may reflect a rifting episode. Geochemical signatures as well as Nd and Hf isotopes of the Mesoproterozoic granitoids indicate melting of still older continental crust with model ages of ca 1.2 to 2.4 Ga.The Mesoproterozoic assemblages are intruded by Paleozoic diorites and granitoids, and Nd and Hf isotopic systematics suggest that the diorites are derived from melts that are mixtures of the above Mesoproterozoic basement and mantle-derived material; their source is thus distinct from that of the Mesoproterozoic rocks. Emplacement of these plutons into the Precambrian rocks occurred between 461 and 441 Ma. This is much younger than previously assumed and indicates that small plutons and large batholiths in North Tianshan were emplaced virtually synchronously in the late Ordovician to early Silurian.The Mesoproterozoic rocks in the North Tianshan may be remnants of a once larger continental domain, whose fragments are preserved in adjacent blocks of the Central Asian Orogenic Belt. Comparison with broadly coeval terranes in the Kokchetav area of northern Kazakhstan, the Chinese Central Tianshan and the Tarim craton point to some similarities and suggests that these may represent fragments of a single Mesoproterozoic continent characterized by a major orogenic event at ~ 1.1. Ga, known as the Tarimian orogeny.24 page(s