Evolution of the Dzhida Zone of the Paleo-Asian Ocean in the Precambrian - Paleozoic

1. Evolution of the Dzhida Zone of the Paleo-Asian Ocean in the Precambrian - Paleozoic I.V. Gordienko1, M.I. Kuzmin2, A.V. Filimonov1, O. Tomurtogoo3 I.V. Gordienko1, M.I. Kuzmin2, A.V. Filimonov1, O. Tomurtogoo3 1Geological Institute, SB RAS, Ulan-Ude, Russia 2Institute of Geochemistry, SB RAS, Irkutsk, Russia 3Institute of Geology and Mineral Resources, MAS, Ulan-Baatar, Mongolia

2. Allocation scheme of Precambrian-Раlеоzоic орhiоlitе belts оf the Central-Asian fold belt (after I.V. Gordienko, 1987) 1 - sedimentary cover оf Precambrian platforms; 2 - marginal protrusions оf the Siberian Platform basement, including pericratonic troughs (Baikal-Patom etc.); 3 – рrotrusions of the pre-Upper Riphean basement; 4 - predominantly Caledonian and Hercynian structure-material complexes оf various geodynamic origin; 5 - ophiolitе belts: 1 - Terekta, 2 - Kobda, 3 - Tsaganshibetin, 4 - West-Sayan, 5 - ­Dzabkhan,6 - Кhankhukhei,7 - South-Tuva,8 - Kurtushiba, 9 - Kuznetsk Alatau, 10 – Iya; l1 – Il’chir; 12 - Dzhida (Argyingol'), 13 - Bayanulan,14 - Abagin, 15 - Bayangol, 16 - Shilka, 17 - IhBogd (Hantaishir), 18 - Bayank-Hongor, 19 - Central Gobi- (Adatzag), 20 - Kerulen, 21 - Undurshilin, 22 - Baikal-Muya, 23 - Solonker. This slide depicts the widespread Riphean-Paleozoic ophiolite belts that served as basis for the identification of the Paleo-Asian Ocean

3. Location of the Dzhida zone (DZ) in the Central-Asian Fold belt The Dzhida zone of the Central Asian fold belt occurs in the south of the Siberian platform. It is interpreted as the area of the Vendian - Paleozoic ocean, island arc, and sea margin basin complexes which formed on the active margin of the Paleo-Asian Ocean. In this slide the green fields show island arc terranes which were formed on the margin of the Paleo-Asian Ocean.

4. Reconstruction of Riphean-Early Paleozoic of the Paleo-Asian Ocean (after L.P. Zonenshain et al., 1976) This slide shows the first reconstruction of the Paleo-Asian Ocean according to L.P. Zonenshain in 1976. The the Dzhida Zoneis located between Siberian craton and Tuva-Mongol massif (“microcontinent”)

5. Position of the Dzhida zone in the southern frame of the Siberian platform 1- Siberian platform; 2, 3 – Tuva-Mongolian microcontinente: 2 – Precambrian metamorphic basement; 3 – Vend-Cambrian carbonate platform cover; 4, 5, 7 – Precambrian-Paleozoic accretionary-subduction zones of Precambrian-Paleozoic: 4 – Oka zone; 5 – Ilchir zone; 7- Khamsarin zone ; 6 – Khamar-Daban zone of metamorphosed sediments of the Vend – Early Paleozoic passive margin of the Siberian craton; 8 – Dzhida zone; 9 – regional faults. The dark green field in this slide shows the location of the Dzhida zone in the folding framework of the south Siberian platform. The Dzhida zones occurs between the Tuva-Mongolian microcontinent and Khamar-Daban zone. The basement of the microcontinent is composed of Precambrian metamorphic rocks overlapped by the Vendian-Cambrian shelf carbonate sediments. The Khamar-Daban zone represents the area of metamorphosed sedimentary complexes of the Vendian-Cambrian passive continental margin of the Siberian continent. The Dzhida zone obtained its modern position relative to the Siberian platform in the Late Paleozoic due to large strike-slip fault movements.

6. Established structural-material complexes of the main paleogeodynamic settings into Dzhida zone The Dzhida zone represents a complex collisional orogen. Three types of complexes were recognized: an ensimatic mature island arc; seamounts; and flysch of marginal paleo-basins

7. Structural-material complexes of the Russian part Dzhida zone In the Russian part of the Dzhida zone the island-arc sedimentary-volcanogenic formations occur as narrow belts along regional displacement zones with northwestern strike. They form a thrust-related assembly that was intensely deformed by late and post-collisional movements. The gently dipping accretionary and early collisional structures were preserved as relics in tectonic slabs. Their boundaries are marked by island-arc mafic-ultramafic complexes. To the west of the central strike-slip zone in the green field, there are predominantly island-arc plagiogranite-diorite complexes. The Dhzida guyot presents a large allochthonous unit, that is composed: (1) tectonic melange which includes large tectonic blocks and slices of the ophiolite sequence; (2) formation of the tectonic mafic breccias with pseudoclastic structures; (3) formation of the tholeiite basalts; (4) formation of the alkaline basalts; (5) dolomite formation. Sediments of the fore arc basin of the Dzhida suite compose the central part of the Dzhida zone. The relations between the flysch, island-arc and guyot complexes are of tectonic nature.

8. Dzhida ensimatic island arc Basement complex (stratified mafic-ultramafic complex) Bugurictay typical massif The mafic-ultramaflc complexis presented by narrow lenticular bodies of hundreds of meters in width and up to kilometers in length. Most of the ultramafic massifs are composed of serpentinites, serpentine-carbonate, and talc-carbonate rocks. Minor amounts of serpentinized pyroxenite and altered gabbro also occur. Thus, the mafic-ultramafic complex may be interpreted as fragment of the lower part of the ophiolite sequence. The layered complex was found only in the large massifs. We interpret the mafic-ultramafic complex as related to a supra-subduction zone of anisland arc.

9. Evolution of the ensimatic Dzhida island arcEarly stage: island arc basalt formation Two basalt series The basalt formation constitutes the main unit of the island-arc complexes. It consists of basalts and dolerites and small intrusions of gabbro-dolerite, fine-grained gabbro. According to geochemical affinities we associate the tholeiite and calc-alkaline series to the early stages of the evolution of an ensimatic island arc. The age of the basalt series is inferred to be Vendian-Early Cambrian. The tholeiites of the upper part of the island arc ophiolite sequence are synchronous with the mafic-ultramafic complex.

10. Evolution of the ensimatic Dzhida island arcMature stage: island arc plagiogranite-diorit complex The Dzhida plagiogranite - diorite complex composes the main volume of the island-arc and it is related to the east of the Dzhida zone. Intrusive complexes of the Dzhida arc are mainly composed of diorites, quartz-diorites, and tonalities. Gabbro, gabbro-norites, and plagiogranites are subordinate. Gradual transitions between different kinds of rocks are widespread. Gabbro and gabbro-norites form small bodies and occur as xenoliths. The gabbros belong to the tholeiitic rock series. Diorites, quartz-diorites, and tonalites are compositionally similar to the gabbros. The age of the Dzhida samples is Middle Cambrian (504±2 and 506±1 Ma)

11. Evolution of the ensimatic Dzhida island arc Mature stage: tuff formation and limestone formation “Island arc” characteristics of the tuff chemical composition This slide shows the transition from the early to the mature stage of the Dzhida island arc This stage in evolution is characterized by a rhyolite-andesite series and small carbonate platforms. The differentiated rhyolite-andesite series are comagmatic with the plagiogranite - diorite of the Dzhida complex.Thick sequences of andesitic tuffs, tuffites, and tephroides were formed in the final phase of the mature stage. Tuffdeposition took place in sub-aquatic conditions and in regions of strong relief resulting from block tectonics. The existence of large tectonic steps is documented by widespread occurrence of coarse-grained sediment.

12. Accretionary prism of the ensimatic Dzhida island arc (Bayan-Gol River, North Mongolia) This slides shows a segment of the accretionary prism of the Dhzida island arc. At this locality boninites occur in a serpentinite melange which supports an ensimatic origin of the island arc. The initial stage of the island arc evolution is manifested by mafic-ultramafic layered cumulate rocks representing the oceanic crust of the basement of the arc. Primitive island-arc tholeiites represent the initial stage of the island arc evolution. Younger tholeiitic basalts were followed by calc-alkaline basalts, associated with boninites. The calc-alkaline basalt suite was deposited in the deepwater environment of the frontal arc. Differentiated volcanic rocks correspond to the mature stage of the island arc evolution. Contemporaneously (Early Cambrian) a narrow carbonate shelf was formedon the volcanic islands. Some sedimentary structures reached sea level as indicated by carbonate facies rocks formed in the wave activity zone and tidal streams. Middle Cambrian andesite and intrusion of the Dzhida plagiogranite-diorite complex mark the transition from the early to the mature stage of island arc evolution.

13. Reconstruction of the accretionary prism environements of the ensimatic Dzhida island arc (Bayan-Gol River, North Mongolia)

14. Dzhida guyot Dzhida seamount structure-material complex The guyot-related complexes form a large allochtonous unit with melange at the base overlain by mafic breccias and tholeiite lavas of MORE-type. The upper part of the complex is composed of alkaline basalts and dolomites.

15. Composite structure for the allochtonous Dzhida guyot 1 – 5 – subdivision: 1- dolomite formation; 2 – alcaline basalt formation; 3 – tholeiite basalt formation; 4 – formation of the mafic breccias; 5 – basic-ultrabasic complex. 6 – tectonic melange. 7 – cross-sections V III IV VI II I This slide shows a reconstruction of the construction of the Dzhida guyot allochthon as inferred from a compilation of cross sections. The lower allochthon part is composed of tectonic melange which includes large tectonic blocks and slices of the ophiolite sequence (meta-ultramafics, pyroxenites, gabbro, tholeiitic basalts) and tectonic mafic breccias. Tectonites of different composition represent the melange matrix. The middle part of the allochthonous unit is composed of the tholeiitic basalt formation. The upper part of the allochthonous unit comprising alkaline basalts and a dolomite sequence, does not contain melange. It forms large stretched bodies (slices), overlaying the lower parts of the allochthonous unit

16. General model for the formation of the Dzhida guyot The general evolutionof the Dzhida guyot volcanic sequence corresponds to the model of oceanic island formation as related to a mantle plume (hot spot). Pillow-basalts of MORB-type represent the initial stage. The transitional series of tholeiitic ferrobasaltes and andesitic basalts correspond to movement of the island away from the plume-conduit. Tholeiitic volcanism was replaced in time by alkaline volcanism erupted when the island had moved away from the plume conduit. The general trend of volcanism correlates with subsidence of the volcanic structure accompanied by shallow water sedimentation. An idealized model of a structure of the Dzhida guyot is presented in these slide.

17. Dzhida guyot basement Mafic breccias The guyot basement is composed of relics of oceanic crust (tectonic blocks in autoclastic melanges and fragments in coarse-grained sediments, a mafic-ultramafic complex in small separate bodies and massifs of different sizes ranging from tens of meters to few kilometers. The melange zones are composed mostly of apoperidotite serpentinite and talk-carbonate rocks. Pyroxenites and gabbro are rare and present as small bodies in the melange. The mafic breccias consist of serpentinite conglomerates and gabbro-breccias of two types. The first type is represented by fragments of gabbro-norite. In the second type, conglomerate-breccias consist of fragment of isotropic gabbro and dolerite dikes. The unit has a striped structure due to alternation of wide (up to several hundreds meters) slices of altered psephites (conglomerate-breccias, conglomerate-like listwenite, chlorite-carbonate tectonites, etc.). Strike and general bedding of these lithological "stripes" is sub-conformal to the unit boundaries

18. Dzhida guyot basalt formations Three basaltic series Tholeiitic pillow lavas and hyaloclastites The tholeiitic basalt suite consists of high-Cr tholeiite including pillow lavas and sub-ordinate hyaloclastite associated with silicious sediment (chert) and limestones. The variolitic pillow lavas consist of densely packed pillows, with round or in some cases flattened shape. The basalts and variolites belong to the tholeiitic rock series. Hyaloclastites consist of small fragments of palagonitized basaltic glass with inclusions of globules of variolites. The matrix is of secondary composition and represented by aggregates of fibrous quartz and clay minerals. In addition to the hyaloclastites, we found lenses of hyaloclastite up to 10-20 m in size at in some sites in the pillow lavas. The lenses are represented by small fragments (2-3 cm sizes dominate) from pillows and variolite globules. The alkaline basalts unit is composed of sub-alkaline olivine basalt, hawaiite, andesitic basalt and andesite as is typical for oceanic islands. Massive lava flows predominate in the unit. Fragment, blocks and lenses of limestone, silicious sediment and peperite are present. The distal zones are composed of volcanoclastic rocks (tuffs, tuffites, hyaloclastite) of mafic, andesitic, and felsic composition and layers of limestones and silicious sediment

19. Structure of the Dzhida guyot in the basin of the Khasurta River

20. Structure of the Dzhida guyot in the Jukhta Creek basin Tectonics-related complexes (olistostromes, tectonized olistostromes, and tectonic melanges) occupy a considerable portion of the guyot. Their formation started with deposition of carbonate during its subsidence and tectonic "collapse". The age of these complexes is estimated as Late Devonian - Carboniferous as suggested by Devonian spores, Carboniferous conodonts and plants.

21. Dzhida guyot dolomite formation According to the dolomite sequence (association of the dolomites with red aleuropelites, presence of barite, typomorphism of silica minerals in concretions, absence of terrigenous material, etc.), the sedimentary environment may be interpreted as a basin with limited water exchange and high evaporation rates, which is characteristic for a partly dewatered carbonate platforms during the final stages (post-erosion?) of guyots evolution. The high evaporation rated may be due to hot climatic conditions. The age of the dolomite sequence is estimated as Silurian(?) – Late Devonian as suggested by Devonian spores and algaes

22. Tentative temporal mоdel for the Dzhida guyot tectonic evolution (adapted from М.G. Leonov, 1988)

23. Tentative temporal mоdel for the Dzhida guyot tectonic evolution According to the structural features and time of formation (V?-C3), the Dzhida guyot is largely analogous to the guyots in Tien Shan. Therefore, despite the absence of a detailed stratigraphic basis, we use the model of tectonic evolution of guyots, as proposed by M.G. Leonov [Leonov et al., 1988] for the interpretation of the tectonic evolution of the Dzhida guyot. According to the model, at the early stage (V-PZ1), growth of volcanic guyot took place on oceanic crust. In Silurian(?)-Carboniferous, on the top of volcanic structure and due to its collapse and subsidence, reef carbonate structures of a dolomite sequence have been formed. Subsidence of the guyot was probably related to slow and a long process of ductile flow and metamorphism, to gravitational flattening, tectonic stratification, and lateral spread of the mafic basement. Mafic breccias were possibly formed during this stage of evolution. According to the structure and composition, the Dzhida guyot is a good analogue of Early Paleozoic guyots of the Altay Mountains consisting of pillow lavas of oceanic tholeiites changing upward to sub-alkaline basalts. They are overlain by siliceous-carbonate sediment. In both cases the guyot structures are surrounded by olistostromes and Middle Cambrian terrigenous series (Baratal'sky guyot)

24. Urigol guyot

25. Dzhida flysch formation. Main subdivision. Slide showing a subdivision of the flysch sequences in the Dzhida zone. The flysch sequence composes the main part of the Dzhida zone and can be divided into 3 rock associations - terrigenous, carbonate-terrigenous and carbonate

26. Idealized column profiles of the flysch sequences Теrrigenous association Main source of the clastic materil was island arc Carbonate - terrigenous associations Main source of the clastic materil was guyot

27. Principalscheme of the geodynamic development of the Dzhida zone

28. Geodynamic evolution of the Dzhida zone of Paleo-Asian ocean in thePaleozoic The complexes of the Dzhida zone represent all stages of evolution of the Paleo-Asian basin (open basin stage, early and mature island arc stages, two stages of collision, etc.). All of the history is estimated to have lasted about 300 Ma. For the geodynamic evolution of the Dzhida zone the following stages have been distinguished: Upper Riphean. The oceanic stage was characterized by formation of oceanic crust in a large oceanic basin. Fragments of this crust are probably present in the mafic-ultramafic complexes of the basement of the island arcs and guyots. Vendian - Middle Cambrian. At the second half of Vendian, formation of oceanic islands as large volcanoes above "hot spots” and spreading zones consisting oftholeiites of MORB-type. At the beginning of Cambrian, these oceanic rises reached a level of carbonate compensation, and carbonate sediments formed on the guyot complexes. Development of a young ensimatic island arc also started in Vendian with formation of low-Cr primitive tholeiite and boninite changing in time to calc-alkaline basalt volcanisms. The volcanic activity o f this stage was mainly related to a deep-water environment of a frontal arc. From the beginning of Cambrian, island arcs reached the sea level accompanied by formation of carbonate shelves on the islands. Andesitic volcanism occurred at this stage. Middle Cambrian - Ordovician. During this period, a volcanic rise consisting of alkaline series of oceanic islands developed. The rise reached the sea level. From the late Middle Cambrian, the island arcs entered their mature stage, with intrusions of plagiogranite-diorite complexes. In the Upper Cambrian- Ordovician(?) the volumes of andesite volcanism dramatically increased, and a large volcanic structure consisting mainly of volcanoclastic rocks was formed. Probably, during the Ordovician, the island arc also entered its mature stage, manifested on sub-aerial sub-alkaline volcanism in an intra-arc environment. Since the beginning of Upper Cambrian, fragment material from the volcanoclastic structures was transported and deposited as turbidite flows in a deep-water fore-arc (?) basin (flysch formation). Silurian - Late Devonian. The time of the closure of oceanic paleobasins. Volcanism in island arcs stopped. Fore-arc deep-water basin was transformed to residual one. At the same time, volcanic activity related to a "hot spot" stopped. Subsidence of the guyot rise and its collapse caused shallow-water carbonate sedimentation, olistostrome formation. Carboniferous - Permian. The period of transformation of the residual basin into a collisional system. At the initial stage, in a regime of total compression, a series of thrusts consisting of fragments of island arcs, guyot, and flysch, were formed. Relics of gently NNW dipping nappes in the island arc complex and olistostromes of the frontal parts of nappes (olistostrome of flysch association) document the early stage of collision. In the end of Carboniferous, intracontinental collision events with a leading role of shift components dominated. This geodynamic situation was characterized by the development of a complicated, often isoclinal folding and zones of dislocation-related greenschist metamorphism. At the final stage of the Late Paleozoic collision during Late Carboniferous - Early Permian, intrusion of collisional granites took place

29. Paleogeodynamic reconstructions for the Siberian continent margin in the Vendian-Early Cambrian: interpretation of paleomagnetic data (1) Rigid blocks: (S) Siberian continent, (А) Aldan Вlock; (2) Paleoasian Ocean (РАО); (3) island arcs and their fragments: (КТ) Kurtushiba, (NS) Northern Sayan, (GA) Gornyi Altai, (ВА) Batenev, (ZK) Zolotokitatskii, (BG) Bayangol, (EI) Egiingol-Ivanov (ВА and EI are shown arbitrarily); (4) subduction zones; (5) direction of the Siberian continent rotation; (6) assumed direction of the oceanic plate motion; geographic position of ophiolitic and island-arc basalts of the Dzhida zone according to paleomagnetic data: (7) MORB-type tholeiitic basalt, (8) WPOIВ-type subalkaline basalt of oceanic islands. Тhе short dashed line shows the assumed absolute motion of the oceanic plate over а hot spot. Slide showing geodynamic reconstructions for the Vendian-Early Cambrian. We carried out this paleomagnetic research on alkaline and tholeiitc basalts of the Dzhida Zone. It was found that the magnetic poles of sub-alkaline basalts of the guyots were concentrated at two separate paleolatitudes, namely: at 20.3° - 20.6°S and 15.6° S, which suggests the absolute motion direction of the oceanic platetowards subduction zone situated, apparently, near the Siberian continen

30. Paleogeodynamic reconstruction for Early Caledonian stage

31. Concluding remarks • On the example of the evolution of the Dzhida zone it is possible to trace a systematic transformation of an oceanic basin into a thrust-fold collisional structure. The reconstructed paleobasin has all the elements of a typical ocean including spreading- and subduction zones, and oceanic islands (guyots). The develoment of the oceanic basin during Vendian - Cambrian can be compared with that of the Western margin of the Pacific Ocean. The collision of island arcs and their assembly to “microcontinents” determined the beginning of the formation of a thrust-fold collisional structure in the Dzhida zone, with intrusion of granites and appearance of residual basins. During the final stage of intra-oceanic complexes and “microcontinents” collision caused a complete closure of oceanic basin and development of the Dzhida zone into an accretion-collisional margin of the Siberian craton. So, the Dzhida oceanic basin can be regarded as a part of Paleo-Asian ocean, which was situated according to L.P. Zonenshain [Zonenshain, 1976] between the Siberian continent and Gondwanaland.