Silurian To Devonian Bidirectional Subduction And Closure Of The Eastern North Tianshan Ocean

The North Tianshan Ocean,which was sandwiched between the Kazakhstan and Mongolia Collages,was one of the most significant branches of the Paleo-Asian Ocean.Due to the complexity of accretionary orogeny and the relative lack of key geological,the understanding of some key information of the Northern Tianshan Ocean,such as opening and closing time,tectonic nature and subduction polarity,is still controversial.It has greatly limited the further understanding of the accretionary pattern,tectonic evolution history,geodynamic mechanism and crustal growth in northern Xinjiang and even the entire western segment of the Central Asian Orogenic Belt.Here,in view of the above issues,this paper studies the Silurian to Mississippian sedimentary tectonic facies,the Ordovician to Mississippian provenance and the Devonian coral paleobiogeography of the eastern segment of the North Tianshan Suture,i.e.,the Kanggurtagh-Dacaotan Complex and the continental margin system on both sides of its.Combined with the collection and synthetic analysis of key geological data,this paper elaborates the tectonic implication of the North Tianshan Ocean and its subduction-closure tectonic evolution history during the key period of Silurian to Devonian,and further reveals the multiple accretion and amalgamation history of the western segment of the Central Asian Orogenic Belt.The preliminary conclusions have been obtained as follows:（1）Based on the elaborate studies of the Silurian to Mississippian successions from the Kanggurtagh-Dacaotan Complex and the active continental margin system on both sides of its,and the lithofacies analysis of significant lithostratigraphic unit in some key areas,this paper reconstructs the spatial and temporal pattern of sedimentary tectonic facies related to ocean evolution on the study area,and further reveals the Silurian to Devonian bi-directional subduction-closure tectonic pattern of the North Tianshan Ocean.Detailed studies show that various sedimentary tectonic facies are distributed from north to south on the basement of the Dananhu-Harlik Arc located to the north side of the Dacaotan Complex.The Silurian-Middle Devonian successions in the northern part of this arc show typical transgressive sequence which is mainly composed of pyroclastic rocks,tuffaceous sandstones,limestones and siliceous mudstones.Combined with the geochemical characteristics of volcanic rocks,these successions suggested that the northern part of this arc belonged to the back-arc basin facies during Silurian to Middle Devonian.The Lower to Middle Devonian successions in the central part of this arc comprise widespread volcanic lava rocks,pyroclastic rocks,nearshore turbidite and shallow-water carbonate rocks.Given the spatial distribution of lithofacies,it can be inferred that the central part of this arc belonged to the magmatic arc to intra-arc basin facies during the Early to Middle Devonian.The Upper Silurian to Lower Devonian successions in the southern part of this arc are thick and coarsening upward turbidites without any intercalated volcanic lava or breccia layers,suggesting that they were deposited in a fore-arc basin facies during the Late Silurian to Early Devonian.The turbidites in the slices from the Dacaotan Complex present coarsening upward sequence.Based on the petrographic analysis,these turbidites can be divided into the northern thrust slices rich in volcanic and accretionary-wedge components and the southern thrust slices containing abundant metamorphic lithics,representing the trench-slope basin facies from the northern and southern continental margin of the North Tianshan Ocean,respectively.The Upper Ordovician to lower Upper Devonian successions on the basement of the Tousuquan Arc comprise volcanic lava rocks,pyroclastic rocks and some intercalated nearshore turbidites,indicating a magmatic arc facies.In summary,the spatial and temporal distribution of trench slope basin facies,fore-arc basin facies,magmatic arc to intra-arc basin facies and back-arc basin facies in the north side of the North Tianshan Ocean suture indicates that this ocean experienced a northward roll-back subduction during Silurian to Middle Devonian,whereas the spatial and temporal distribution of trench slope basin facies and magmatic arc facies in the south side of the North Tianshan Ocean suture suggests that this ocean underwent a southward subduction during Silurian to early Late Devonian.During the latest Devonian to Mississippian,the terrestrial alluvial fans,volcanic lava and pyroclastic flow deposits are widely distributed from the southern part of the Dananhu-Harlik Arc to the Tousuquan Arc,whereas the terrestrial,deltaic or marine clastic rocks with minor volcanic lava or pyroclastic rock interlayers are mainly distributed in the central and northern part of the Dananhu-Harlik Arc.Combined with the spatial distribution of successions and the geochemical characteristics of the magmatic rocks,the author suggests that these strata were deposited in a syn-or post-collision setting.The southern part of the Dananhu-Harlik Arc to the Tousuquan Arc belonged to the suture basin facies,whereas the central and northern part of the Dananhu-Harlik Arc belonged to the subduction-collision foreland basin facies.（2）This paper deciphers the evolutionary history of detrital provenance from the active continental margin located in the both sides of the eastern segment of the North Tianshan Ocean,based on the zircon U-Pb dating and in situ Hf isotope analysis of the Ordovician-Mississippian clastic rocks from the Kanggurtagh-Dacaotan Complex and the tectonic units located on the both sides of it.On the Dananhu-Harlik Arc and its trench-slope located on the north side of the North Tianshan Ocean suture,the Ordovician to early Late Silurian（480-425 Ma）clastic rocks yield a same major age peak at ca.500 Ma and Precambrian age spectra and zirconε_Hf（t）values resembling those of several terranes along the southern margin of the Siberian Plate.In contrast,the Late Silurian to Middle Devonian（425-383 Ma）clastic rocks commonly have major age peaks at a range between460 Ma and 380Ma.Most of them also contain secondary age peaks at ca.500 Ma.All the clastic rocks contain a relatively low proportion of Precambrian zircons,and the Silurian and Devonian zircons in them mostly yield high positiveε_Hf（t）values.Hence,it can be inferred that the provenance of clastic rocks in the Dananhu-Harlik Arc successively experienced two stages:the Ordovician to early Late Silurian rocks were mainly derived from the southern margin of the Siberian Plate,whereas the late Late Silurian to Middle Devonian rocks from were sourced from the Dananhu-Harlik Arc.On the Tousuquan-Yamansu Arc and its trench-slope located on the south side of the North Tianshan Ocean suture,the Ordovician to early Late Silurian（480-425 Ma）clastic rocks yield major age peak at ca.450 Ma.Zircons with Precambrian to Cambrian ages are almost absent from this samples.However,the late Late Silurian-early Late Devonian（425-370 Ma）clastic rocks generally contain many Paleozoic detrital zircons with negative or low positiveε_Hf（t）values and considerable Precambrian zircons resembling the zircon age spectra of the basement rocks from the Yili-Central Tianshan Block.Combined with the petrographic characteristics of clastic rocks,it could be inferred that the provenance of clastic rocks on the Tousuquan-Yamansu Arc successively undergone two stages:the Ordovician to early Late Silurian rocks were sourced from Tousuquan-Yamansu Arc,whereas the late Late Silurian to early Late Devonian rocks were derived from the Yili-Central Tianshan Blocks.The latest Devonian to early Mississippian（370-350 Ma）clastic rocks commonly yield major age peaks at ca.370 Ma and similar Paleozoic zircon age spectra.The Devonian to Mississippian zircons in these samples have high positiveε_Hf（t）values,therefore it can be inferred that these clastic rocks were sourced from the uplifted Tousuquan Arc and southern Dananhu-Harlik Arc in the composite arc.In contrast,the Devonian to Mississippian detrital zircons in the middle to late Mississippian（350-330 Ma）clastic rocks yield variableε_Hf（t）values,suggesting that these deposits were sourced from the composite arc composed of Dananhu-Harlik Arc,Tousuquan Arc and Yili-Central Tianshan Block.（3）Based on the detailed studies of the Early to Middle Devonian rugose coral faunas in the Dananhu-Harlik Arc,and the multivariate analysis results of rugose coral faunas from the operational biogeographic units in the Central Asian Orogenic Belt and adjacent areas,this paper reconstructs the regional Early and Middle Devonian rugose coral paleobiogeographic frameworks.As is shown in these researches,all the Early Devonian rugose coral faunas in the Dananhu-Harlik Arc are composed of considerable small,solitary and undissepimented elements.Genus Syringaxon with well-developed aulos is dominant in these faunas.These characteristics are in sharp contrast with the contemporaneous rugose coral faunas from the southern margin of the Central Tianshan Block.However,the Middle Devonian rugose coral faunas which lived in the shallow-water environment of central to southern part of Dananhu-Harlik Arc mainly comprise medium-to large-size solitary rugose corals with well-developed dissepimentarium and tabulae-or tabellae-filled tabularium,resembling the contemporaneous rugose coral faunas from the southern margin of the Central Tianshan Block.Multivariate analysis results of rugose coral faunas from the Central Asian Orogenic Belt and adjacent areas show that the Dananhu-Harlik and East Junggar Arcs belonged to the Boreal Realm during the Early Devonian era,while the Central Tianshan Block to South Tianshan Complex contemporaneously belonged to the Tethyan Realm.These two biogeographic realms are roughly bounded by the suture of North Tianshan-Solonker Ocean.In contrast,the tectonic units on both sides of the North Tianshan Ocean suture belonged to the Tethyan Realm,whereas the East Junggar,West Junggar and Chinese Altai Arcs constituted the Boreal Realm that was isolated and contained endemic elements.Hence,during the Middle Devonian era,these two biogeographic realms are roughly bounded by the Kalamaili Ocean suture,and the North Tianshan Ocean suture no longer had the significance of vicariance.（4）Comprehensively considering the above-mentioned advances and the previous results of magmatic rocks,ophiolites and structural deformation,the author proposes several new understandings on the tectonic implications,subduction polarity,subduction style and closure time of the North Tianshan Ocean,based on the classical theory of accretionary orogeny.The North Tianshan Ocean was the major ocean separating the Mongolia and Kazakhstan Collages,according to the distinct characteristics of the provenance of Silurian to early Late Devonian clastic rocks from the active continental margins on the both sides of the North Tianshan Ocean,and the Silurian to Early Devonian regional paleobiogeographic framework.This ocean contained long-term northward（present coordinates）roll-back subduction beneath the Dananhu-Harlik Arc and southward（present coordinates）subduction beneath the Tousuquan Arc.This geodynamic setting led to several scenarios,including the opening and extension of the Silurian to Devonian Kalamaili back-arc oceanic basin,the separation of Dananhu-Harlik Arc from the southern margin of the Siberian Plate and the amalgamation of Tousuquan-Yamansu Arc and Central Tianshan Block before the Late Silurian.The long-term bi-directional subduction caused that the eastern segment of the North Tianshan Ocean became a remnant ocean in the Middle Devonian.This ocean was terminally closed before the latest Devonian,which could be reflected by the unconformity between latest Devonian to Mississippian molasse or volcanic rocks and underlying strata,the latest Devonian to Mississippian suture basin-foreland basin depositional system,the similar age spectra of the latest Devonian to Mississippian clastic rocks in the composite arc and the Hf isotopic characteristics of detrital zircons indicating the mixture source.Meanwhile,other branches of the Paleo-Asian Ocean were still evolving,and the closure of the North Tianshan Ocean cannot represent the final closure of the western segment of the Paleo-Asian Ocean.