Geochemistry And Zircon U-Pb Geochronology Of The Silurian And Devonian Sandstones In The Northeast Margin Of The North Qilian Belt

The North Qilian tectonic Belt ?NQB? is located between the west block ?Ordos-Alxa? of the North China Craton ?NCC? and the Central Qilian Block ?CQB?. The depositional age of the Silurian Zhaohuajing Formation, sedimentary environment of the Silurian-Devonian clastic rocks in the northeast margin of the NQB and its relationship with the NQB tectonic evolution during the late Early Paleozoic have been controversial issues for long time. Based on the field geological survey and rock sample collection, geochemical and zircon U-Pb geochronologic methods are mainly used to discuss the sediment provenance and the tectono- sedimentary environment of the Silurian-Devonian clastic rocks, as well as its relationship with the NQB tectonic evolution. The main research progress and some new understandings in this study are concluded as follows.?1? The zircon grains from the lower part of Silurian Zhaohuajing Formation rock samples in the Tashitougou section of Tongxin County were carried out by LA-ICP-MS, The analyzed results reveal that young zircons mainly concentrate in 426±6-447±7Ma, and the youngest zircon age of 426±6Ma. Combining with the paleontological data and stratigraphic contact relations, Zhaohuajing Formation is covered by the Upper Silurian Hanxia Formation in erosion unconformity and it unconformably overlying by the Upper Ordovician, to indicate that the depositional age of Zhaohuajing Formation is less than the boundary age of the Middle Silurian ?433.4±0.8Ma?, this age is achieveed from the International Stratigraphic Chronology Standard. Thus, Zhaohuajing Formation should belong to Middle Silurian sedimentary strata unit and don't support the Early-Middle Silurian.?2? The clastic composition and heavy mineral from Silurian and Devonian sandstones in the eastern Hexi Corridor of the northeastern margin of NQB were analyzed. The analysis results reveal that these clastic sediments probably come from the Qilian Mountain Areas. The geochemical results reveal that the clastic sediments of Silurian sandstones in the Tashitougou section mainly derive from the felsic igneous source, however, the clastic sediments of Devonian sandstones derive from the quartzite source and felsic-mafic mixing source. The comparision results of REE distribution patterns with possible provenance further revealing that the clastic sediments mainly derive from the North Qilian Early Paleozoic continental margin accretionary complex belts and the Central Qilian Precambrian bedrock, and part of the clastic sediments probably derive from the western North China Block ?Ordos-Alxa?.?3? The LA-ICP-MS zircon U-Pb dating of Zhaohuajing Formation ?S₂z? and Hanxia Formation ?S₃h? show broadly similar age patterns, mainly include 420-480Ma, 500-660Ma,700-850Ma,890-1450Ma,1600-1910Ma and 2000-2700 Ma, and the corresponding peak ages are 450Ma,560Ma,800Ma,990Ma,1700Ma and 2500Ma. The ancient detrital zircons, with a peak age of nearly 2.5Ga, are less and mainly come from the western North China Block ?Ordos-Alxa?. The relatively more and younger Late Precambrian zircons, with a peak age of nearly 990Ma, are likely to come from the Qilian and Alxa Block. And the largest number and the youngest early Paleozoic zircons, with peak ages of 445Ma and 550Ma, come from the granitoids and mafic rocks in the North Qilian Early Paleozoic continental margin accretionary complex belts.?4? Most zircon Th/U values of Zhaohuajing Formation ?S₂Z? and Hanxia Formation ?S₃h? samples are greater than 0.4, these values and zircon REE pattern curves together reveal that these zircons are magmatic zircons. The results of Hf-U/Yb and Y-U/Yb discrimination figures ?Grimes et al.,2007? show that most zircons are continental zircons ?95%? and few zircons crystallized in crustal oceanic crust. The results of CART source rock type classification with zircon trace elements ?Belousova et al.,2002? further proving that original magmatic rock types mainly include granite?63%? and dolerite?23%?, and few ultrabasic rocks?12%? and alkaline rocks?2%?.?5? Through the comprehensive analysis about the geochronology events of detrital zircon in the Silurian and Devonian, and unconformity tectonic events to explore and define two major tectonic events time and attributes during the Late Caledonian to Early Hercynian in the northeastern margin of NQB. The first tectonic event occurred before the deposition of Silurian Zhaohuajing Formation, approximately corresponding to 445Ma zircon U-Pb peak age event. This event mainly reflectes that Zhaohuajing Formation high angle unconformity overlying on the Middle-Upper Ordovician, and the underlying strata are composed of marine clastic rocks and volcanic formations with strong deformation and low-grade metamorphism. This event mainly reflected the metamorphism and deformation event during Early Paleozoic ocean basin closed and collided in the eastern segment of NQB. The second tectonic event occurred between the end of Silurian and the early Devonian, which mainly performances that the angle unconformity between the Upper Silurian Hanxia Formation and Devonian Laojunshan Formation conglomerate. This event is approximately correspond to 420Ma zircon U-Pb peak age event of North Qilian granitoids and mainly represent the strong orogenic uplift event during late period of Early Paleozoic ocean basin subduction collision in the eastern segment of NQB.