Reconstruction Of Paleozoic Ocean Plate Stratigraphy In The Middle Beishan Area,Gansu Province

The Central Asian Orogenic Belt?CAOB?,located between the Siberia Block to the north and the North China-Tarim Block to the south,is the largest Phanerozoic accretionary orogen in the world.The Beishan Orogenic Collage?BOC?in NW China represents a southern segment of the CAOB and is adjacent to the South Tianshan orogen to the west and the Mongolia-Xing'anling orogen to the east.The BOC is composed of several tectonic units?including micro-continents,arcs and sediments on continental margins and arcs?,among which there outcrop several ophiolitic melanges.Ophiolites represent fragments of upper mantle and oceanic crust that were incorporated into continental margins during continent-continent and arc-continent collisions,ridge-trench interactions,and/or subduction-accretion events.The Niujuanzi ophiolitic mélange?NOM?,located in the middle part of the BOC,is a very important suture zone.Many researches have been constructed on the NOM in recent years and many achievements have been obtained,yet its tectonic attributes,formation ages and the timing of termination of the paleo-ocean are still controveral.In this study,the NOM,the Mazongshan ophiolitic mélange?MOM,to the south of the Mazongshan Conty?and some important stata and intrusions were researched.Based on the NOM and MOM,the Paleozoic Ocean Plate Stratigraphy of the Niujuanzi ocean?southern branch of the Paleo-Asian Ocean in the Niujuanzi Area?was reconstructed and its evolution was explored.In this study,isotopic geochronological,paleontological,petrological and geochemical methods were used and the evolution of the Niujuanzi Ocean was analyzed in respects of ages,facies,provenances and locations.Main conclusions were arrived at as follows:?1?The NOM was maily composed of gabbros,diabases,plagiogranites,greywackes and basalts.These faults are usually sinistral or dextal strike-slip faults dipping S or SW in the southern part of the Niujuanzi section and dipping NW in the northern part.The tectonic pattern may show that the NOM was formed during the biodirectional subduction of the Niujuanzi Ocean.?2?Different compositons of the NOM were dated via LA-ICP-MS.Two gabbros have ages of 433.8±3.1 Ma and 354.0±3.3 Ma,two plagiogranites have ages of 429.8±2 Ma and 448.7±2.0 Ma and a diabase has an age of 433.4±3.2 Ma.One gabbro sample has an age of Carboniferous,which is the youngest age that has been obtained in the Hongliuhe-Niujuanzi-Xichangjing ophiolitic mélange belts and indicates that the expansion of the Niujuanzi Ocean still existed in the Carboniferous.?3?Acritarchs?e.g.,Buedingiisphaeridium cf.balticum,Adorfia firma,Micrhystridium henryi,Lophosphaeridium acinatum and Cymatiosphaera keilaensis?were discovered in the matrices of the NOM.The acritarch assemblages show an duration between the latest Early Ordovician and earlist Late Ordovician.Also,some spore fossils were discovered?the youngest detrital zircons in this sample yield ages of 323 Ma and 338 Ma?,which may indicated that some sediments were deposited in the Late Carboniferous.?4?Spore fossils?i.e.Raistrickia clavata and Raistrickia saetosa?were discovered from the matrices in the MOM and the youngest detrital zircon yielded an age of 358 Ma,indicating that some of the matrices were formed in the Carboniferous.The existence periods of the paleo-oceans represented by the NOM and MOM were quite similar,showing that the NOM and MOM may be formed during the subduction of the same ocean and were divided into different parts in the late time.?5?The gabbros have high Mg# values,while the diabases have relatively low Mg# values.The plagiogranites are calc-alkaline and metaluminous,with high SiO₂ and Na2 O contents and low Al2O3 and K2 O contents.The gabbros and diabases are enriched in LILEs and slightly depleted in HFSEs relative to N-MORB and their trace element characteristics are similar to E-MORB.With respect to REE,they have slightly enriched LREEs relative to HREEs.The majority of the plagiogranite trace elements approximate those of the volcanic arc granite?VAG?.The plagiogranites have obviously enriched LREEs relative to HREEs,with a slightly to strongly negative Eu anomaly,which is similar to ORG but distinct from volcanic arc and within plate granite.The ultra-mafic rocks have approximate contents in LILEs but obviously depleted in HFSEs relative to those of E-MORB.Based on the geochemistry of different units,they were formed in a mid-ocean ridge environment but not in a fore-arc or back-arc basin.?6?The detrital zircon U-Pb geochronology of matrices in the NOM and MOM show that the provenance of sediments deposited in the Carboniferous derived from the volcanic arcs and uplifted accretionary wedge,however the earlier ones derived mainly form the volcanic arcs to the north of the Niujuanzi Ocean.The Paleozoic oceanic crust zircons have ages between 430 and 500 Ma,indicating the main extension periods of the Niujuanzi Ocean.The oldest Paleozoic continental zircons yielded ages of about 470 Ma,representing the start of the subduction of the Niujuanzi oceanic crust.?7?Zircons from the Balegentai Intrusion were analyzed via LA-ICP-MS,which yielded ages of about 410 Ma.The geochemical characteristics of granitic rocks from the Balegentai Intrusion were approximate those of adakites,indicating the particial melting of the Niujuanzi oceanic crust.Sedimentary features of the Permian Hongyanjing Formation showed that they were deposited in Late Permian foreland basin which was formed during the collision between different terranes separated by the Niujuanzi Ocean.?8?In respects of the expanding period,subduction polarity and period,the ages of the subduction-related magmatic rocks and the collisional time of the terranes,it is likely that the Niujuanzi Ocean extended westwards and connected with the South Tianshan Suture Zone.