TS7.7

The Mesoproterozoic Karadjilga pluton is a poorly studied fragment of the North Tianshan microcontinent located in the western Central Asian Orogenic Belt. Metasedimentary rocks surrounding the pluton consist of marbles and mica schists of the Mesoproterozoic Ortotau Group. These rocks constitute a major west-northwest trending syncline with steep to subvertical limbs. The hinge of the fold is well expressed in the west part of the syncline and plunges east with 30-40° angle of plunge. Eastern termination of the syncline is cut by faults. Granitoid gneisses and granites of the Karadjilga pluton crop out in the core of the syncline. The contacts of the pluton are sub-parallel to bedding and schistosity in surrounding rocks. Primary magmatic contacts are locally reworked by reverse faults and thrusts. Our detailed mapping and structural study revealed inhomogeneous deformation of rocks of the Karadjilga pluton. The following rock types are identified: 1) undeformed granite 2) foliated granite 3) granite-gneiss and 4) mylonite. Undeformed granites form <25-30% of total volume of the pluton and are most widespread in the northeast part of the pluton. On some geological maps they are shown as Ordovician or Devonian. However, U-Pb dating of 9 zircon grains by SHRIMP-II (VSEGEI, St. Petersburg, Russia) yielded a 1125±5 Ma concordant age. It agrees with previously reported U-Pb SHRIMP ages for deformed granites and gneisses (Degtyarev et al., 2011; Kröner et al., 2013) and indicates that undeformed granites belongs to the same Mesoproterozoic magmatic complex. Foliated granites and gneisses prevail and constitute up to 60-70% of total volume. They form west-northwest trending zones alternating with mylonites or undeformed granite. Mylonites are subordinate and occur mainly along the contacts of the pluton. Shear zones seem to be approximately parallel to the schistosity of deformed granites, but their geometry needs more study and mapping. Shear-sense indicators were studied in the oriented thin sections and are represented mainly by sigma and delta structures and oblique foliation with rare folds and other indicators. In all but one sample only strike-slip displacement has been identified. In the northern part of the pluton sinistral displacement predominates, whereas dextral displacement prevails in the southern part of the pluton. Shear zones are most widespread on the margins of the Karadjilga pluton, but locally also occur in the central part of the pluton, where they form narrow west-northwest trending zones. According to shear-sense indicators, displacement within the Karadjilga pluton occurred mainly in the approximately west-east direction that strongly differs from the north-south sense of displacement in the Paleozoic thrust and fold belts of Tianshan.

The study was supported by the RFBR project 20-05-00252.

How to cite: Kushnareva, A., Khudoley, A., Alexeiev, D., and Petrov, E.: Structure and shear-sense indicators of the Mesoproterozoic basement of the North Tianshan microcontinent: Example of granitoid gneisses of the Karadjilga pluton, NW Kyrgyzstan, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4086, https://doi.org/10.5194/egusphere-egu21-4086, 2021.

The Central Asian Orogenic Belt (CAOB), also known as the Altay orogenic belt, is the largest accretionary orogenic belt in the world. It is situated between the Eastern European, Siberian, Tarim, and North China cratons. The CAOB is a large and complex suture zone formed by amalgamation of diverse geologic units including several microcontinents, ophiolites, island arcs, seamounts and accretionary wedges. The evolution of the Precambrian basement in these microcontinents is central to understanding the accretionary and collisional tectonics of the CAOB as well as the evolution of Rodinia supercontinent. The Tianshan block, an important part of the CAOB, is located in the southwestern CAOB, and subdivided from north to south into North Tianshan, Central Tianshan-Yili blocks, and South Tianshan. The Central Tianshan block, located between the Tarim block, the Junggar block and the Kazakhstan block, is one of numerous microcontinental block within the CAOB that overlie Precambrian basement rocks. Constraining the evolution of these ancient basement rocks is central to understanding the accretionary and collisional tectonics of the CAOB, and its place within the Rodinia supercontinent. However, to date, the timing and tectonic settings in which the basement rocks in the Central Tianshan formed are poorly constrained, with only sparse geochemical and geochronological data from granitic rocks within the central segment of the belt. Here, we present a systematic study combining U-Pb geochronology, whole-rock geochemistry, and the Sr-Nd isotopic compositions of newly-identified granites from the Bingdaban area of Central Tianshan. The analyzed samples yield a weighted mean Neoproterozoic ²⁰⁶Pb/²³⁸U ages of 975-911 Ma. All have affinities with calc-alkaline, weakly-peraluminous, magnesian I-type granites. The samples are enriched in LREE, display relatively flat HREE patterns with negative Eu anomalies, and show a depletion in the high field strength elements (HFSEs) Nb, Ta, and Ti and enrichment in large ion lithophile elements (LILEs) Rb, U, Th and Nd geochemical characteristics indicative of subduction-related magmatism. All samples show initial (⁸⁷Sr/⁸⁶Sr)_(t) ratios between 0.705136 and 0.706745. Values for Ɛ_Nd(t) in the granites are in the range -1.2 to -5.7, corresponding to Nd model ages of 1.6-2.1 Ga, indicating a role for Mesoproterozoic to Paleoproterozoic rocks in the generation of the granitic protoliths. The documented geochemical features indicate the protoliths for the granites had a similar petrogenesis and magmatic source, which may reflect partial melting of thickened crust with the addition of small amounts of mantle-derived material. The Tianshan Block probably constituted part of an exterior orogen that developed along the margin of the Rodinian supercontinent during the early Neoproterozoic, and which underwent a transition from subduction to syn-collision compression at 975-911 Ma. This study reveals that crustal reworking may played a key role in Neoproterozoic crustal evolution in the Central Tianshan block and this block has a tectonic affinity to the Yili block.

This study was financially supported by the National Natural Science Foundation of China (41772059) and the CAS “Light of West China” Program (2018-XBYJRC-003).

How to cite: Song, Y., Liu, X., Zhang, Z., Liu, P., and Xiao, Y.: Neoproterozoic I-type granites geochronology and geochemistry of the Chinese Central Tianshan Block, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10663, https://doi.org/10.5194/egusphere-egu21-10663, 2021.

The Central Asian Orogenic Belt (CAOB), is the largest proliferative orogenic belt in the phanerozoic, located between Siberia and the Tarim north China plate. Its tectonic evolution is closely related to the evolution of the ancient Asian Ocean. The CAOB has an intimate connection with the evolution of Paleo-Asian Ocean (PAO)  which experienced geodynamic processes like seamounts accretion, ridge-trench interaction, the constitution of back-arc basins. Since the Paleozoic era, the PAO has undergone expansion, subduction and closure, and finally formed the current Central Asian orogenic belt. The West Junggar, located in the southwest of the Central Asian orogenic belt, is an accretive Mosaic body on the southern edge of the Siberian Craton. It is an important part of the Palaeozoic orogenic collage of the CAOB, and a composite terrane composed of island arcs, ophiolites, seamounts and a key area for the study of the tectonic evolution of The Central Asian orogenic belt during the Paleozoic era. The ophiolite mélange zone in Karamay and the carboniferous siliceous calcite with great thickness jointly indicate the existence of the late Paleozoic residual ocean basin in Junggar area. This paper presents new zircon geochronolgy and whole rock major and element, and Sr-Nd isotope data for mafic rocks in the Baijiantan ophiolitic mélanges.

The studying area is located in the northeast part of Karamay city, In the substratum of metamorphic peridotite serpentine, the pyroxenite, gabbro, jasper and radiolarite blocks of different sizes are distributed, and the edge of the blocks is fragmented and in contact with the matrix structure. The Baijiantan ophiolitic mélange is covered by a set of late Carboniferous volcanic-sedimentary tectonic unconformities .

The magmatic zircons from a anorthosite in Baijiantan ophiolite yield concordia U–Pb isotope age of 370.1±1.2Ma, which is interpreted as the crystallization age of the anorthosite. The mafic rocks of Baijiantan ophiolite are geochemically belong to tholeiitic basalts with low SiO₂ contents as well as relatively depleted in light rare earth element (LREE) and flat in heavy rare earth element (HREE), while the high-field strength elements (Nb and Ta) display a weak depletion. thus they have a N-MORB-type characteristics. which is similar to those of basalts from back-arc basin. The (⁸⁷Sr/⁸⁶Sr)_i of Baijiantan ophiolite range from 0.704567 to 0.705172, and they have positive ε_Nd(t) with from +8.23 to +8.81, indicating they were derived from a depleted MORB-type mantle source.

To sum up, the Baijiantan ophiolite in the western Junggar was formed in the late Devonian. The mafic rocks are characterized by MORB type of basaltic magma. Their Sr-Nd isotopic compositions indicate they were derived from a depleted asthenospheric mantle, all of these features are similar to the back-arc basin basalts. Thus, we suggest the Baijiantan ophiolite was possibly formed in the back arc oceanic basin in the late Devonian.

Acknowledgments:This work is granted by the National Natural Science Foundation of China (Grant No. 41772059), CAS "Light of West China" Program (2018-XBYJRC-003), Guangxi National Natural Science Foundation (Nos. 2018GXNSFFA281009) and Bagui Scholar Innovation Project of Guangxi Province.

How to cite: Xiao, Y., Liu, X., Zhang, Z., Song, Y., and Liu, P.: Tectonic setting and geochronology of  Paleo-Asian Baijiantan Ophiolite in West Junggar, NW China, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10786, https://doi.org/10.5194/egusphere-egu21-10786, 2021.

    The subduction and closure of the Paleo-Asia Ocean generated the Central Asian Orogenic Belt (CAOB), which extends from the Urals in the west through Kazakhstan, northwestern China, Mongolia, and northeastern China to the Russian Far East. It is generally accepted that the CAOB comprises a complicated and varied collage of terranes, including island arcs, ophiolites, accretionary prisms, seamounts, and microcontinents. The CAOB is the world’s largest accretionary orogen and is also considered a type area for studying Phanerozoic continental growth. The accretionary processes of the orogen might have resulted from either the progressive duplication of a single and long-lived island-arc system or the collision of several island arcs and micro-continents, similar to the complex archipelago systems in the modern southwestern Pacific. West Junggar is located in a key area of the CAOB, has been a focus of studies of the tectonic evolution and crustal growth of the orogenic belt. West Junggar has been considered by some geologists as a paleo-Asian intra-oceanic subduction system, whereas others have variously argued that West Junggar was formed by single subduction, arc–arc collision, or ridge subduction, or by post-collisional processes after the early Carboniferous. An understanding of the Carboniferous tec-tonic setting is critical for determining the evolution of West Junggar. A series of early Carboniferous volcanic and intrusive rocks occur in the southern West Junggar. Our new zircon U–Pb geochronological data reveal that diorite intruded at 334.1 ± 1.1 Ma, and that basaltic andesite was erupted at 334.3 ± 3.7 Ma. These intrusive and volcanic rocks are calc-alkaline, display moderate MgO (1.62–4.18 wt.%) contents and Mg# values (40–59), low Cr (14.5–47.2 ppm) and Ni (7.5–34.6 ppm) contents, and are characterized by enrichment in light rare-earth elements and large-ion lithophile elements and depletion in heavy rare-earth elements and high-field-strength elements, meaning that they belong to typical subduction-zone island-arc magma. The rocks show low initial ⁸⁷Sr/⁸⁶Sr ratios (0.703649 to 0.705008), positive Ɛ_Nd(t) values (+4.8 to +6.2, mean +5.4), and young T_DM Nd model ages ranging from 1016 to 616 Ma, indicating a magmatic origin from depleted mantle involving partial melting of 10%–25% garnet and spinel lherzolite. Combining our results with those of previous studies, we suggest that these rocks formed as a result of northwestward subduction of the Paleo-Asian Junggar oceanic plate, which caused partial melting of sub-arc mantle. We conclude that intra-oceanic arc magmatism was extensive in southern Paleo-Asian Ocean during the early Carboniferous.

This study was financially supported by the National Natural Science Foundation of China (41772059) and the CAS “Light of West China” Program (2018-XBYJRC-003).

How to cite: Liu, P., Liu, X., Zhang, Z., Song, Y., Xiao, Y., and Li, D.: Early Carboniferous Paleo-Asian oceanic plate subduction: Implications from geochronology and geochemistry of early Carboniferous magmatism in southern West Junggar, NW China, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11906, https://doi.org/10.5194/egusphere-egu21-11906, 2021.

Sanukitoid is a type of high-Mg andesite that is distinct from typical andesite in being characterized by elevated MgO contents and/or Mg^#[=100* Mg/(Mg + Fe)]. They represent rare mantle-derived rocks that are preserved in both modern and Archean subduction settings, as well as in accretionary orogenic belts. The Central Asian Orogenic Belt (CAOB) is a giant accretionary orogen and the most important area of Phanerozoic continental growth around the world. It is evolved through a long-lived orogeny involving multiple episodes of subductions and accretions marking a major phase of continental growth during the Paleozoic. The West Junggar is an important component within the core of the CAOB, and is located at the junction between the Siberian, Kazakhstan and Tarim blocks. The rocks in West Junggar preserve the amalgamation of the southern CAOB, and are subdivided into northern and southern parts by the Xiemisitai Fault. The study of Carboniferous magmatism in northern West Junggar plays an important role in understanding the tectonic evolution of that part of the Central Asian Orogenic Belt. In this study, we present petrology, zircon U–Pb geochronology, mineral and whole-rock geochemistry, and the Sr–Nd–Hf–Pb isotope compositions of volcanic rocks from the Hamutusi area of northern West Junggar. LA–ICP–MS zircon U–Pb analysis of a representative andesite yielded an early to late Carboniferous age of 324.4±6.9Ma. The volcanic rocks are calc-alkaline, with high SiO₂ (58.10–59.01 wt%), MgO (6.09–6.99 wt%), Mg^# (60.7–62.2), Cr (147–403 ppm), and Ni (29–119 ppm) contents, and are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE), but depleted in high field strength elements (HFSE), These characteristics are similar to those of typical sanukitoids within the Setouchi volcanic belt in Japan. All samples have radiogenic initial Sr and Pb isotopic compositions, and low ε_Nd(t) and ε_Hf(t) values, indicating the sanukitoids were generated by partial melting of subducting sediments in which the melts interacted with the mantle. Geochemical modeling calculations indicate a proportion of 3-10% sediment melt and slab-derived fluids were mixed with the depleted mantle to produce the bulk of the Hamutusi rocks. We conclude that the studied rocks from Northern West Junggar record the transition from normal subduction to subduction of young and hot oceanic lithosphere between the early and late Carboniferous. 

This study was financially supported by the National Natural Science Foundation of China (41772059) and the CAS “Light of West China” Program (2018-XBYJRC-003)

How to cite: Zhang, Z., Liu, X., Liu, P., Song, Y., Xiao, Y., and Li, D.: Petrogenesis of late Carboniferous sanukitoids from northern West Junggar of China in the Central Asian Orogenic Belt, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10555, https://doi.org/10.5194/egusphere-egu21-10555, 2021.