Sedimentary And Provenance Characteristics Of The Middle Permian-lower Triassic In The Yiyang Area,Southern Margin Of The North China And Its Implications For The Evolution Of The Basin-mountain System

The Qinling Orogenic Belt is a composite continental collision orogen that formed by the convergence with the North China and Yangtze plates,and its orogenic processes is always synchronized in the basin-mountain coupling system with the southern margin of North China basin.Based on the multi-stage granitoid magmatism retained in the Qinling Orogenic Belt,two important orogeny occurred in the Caledonian and Indosinian periods is well qualified.However,due to the lack of the Late Paleozoic magmatic records,the Late Paleozoic-Early Mesozoic tectonic evolution before the comprehensive collision orogeny during the Middle Triassic and the coupling relationship with the southern margin of the North China is still widely disputed.As one part of the Late Paleozoic-Mesozoic superimposed basin in southern North China,the Yiyang area in western Henan province accepted voluminous sediments from surrounding during the basin formation and deposition period.These sediments may record the details both the uplift and denudation from the Qinling Orogenic Belt and the deposition and filling from the southern North China basin.The study focus on the sediments has an important significance for acquainting comprehensive understanding of the orogenic process with the Qinling Orogenic Belt and tectonic transformation with southern margin of the North China.Thus,this study selected the Middle Permian-Lower Triassic as the research object,using sedimentology,petrology,geochemistry and detrital zircon Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry(LA-ICP-MS)U-Pb chronology,to constraint the material sources of sedimentary formation and evolution characteristics in vertical sequence from multiple aspects.In combination with the analysis of coeval sedimentary facies and sedimentary environment,to discuss the control effect from the orogenic belt to the formation and deposition of peripheral basin,and to provide important sedimentary evidence for the basin-mountain tectonic framework coupled evolution process between the Qinling Orogenic Belt and southern margin of the North China during the Late Paleozoic-Early Mesozoic.The main achievements as follow were obtained:(1)The Yiyang area mainly developed delta-lacustrine sedimentary system,and the sedimentary environment experienced four stage evolution with the marine regression,lacustrine transgression,lacustrine regression and second lacustrine transgression.The Shihezi Formation is presented by the transformation from delta front subfacies at the lower member to the delta plant subfacies at the upper member.The Sunjiagou Formation display the lacustrine transgressive sedimentary sequence from braided delta plain-braided delta front-lakeshore-shallow lake subfacies.The Liujiagou Formation show the lacustrine regressive sequence from lake delta front to lake delta plain subfacies.The Heshanggou Formation reveal the invade process from the lakeshore to shallow lake subfacies.The sedimentary characteristics indicate that the Yiyang area had completed the shift from the transitional delta to terrestrial fluviolacustrine sedimentary system during the period of the Sunjiagou Formation.The depositional cycle of lake basin coordinates with the evolutionary process from basin expansion-atrophy-second expansion.(2)Based on petrology,element geochemistry and LA-ICP-MS detrital zircon U-Pb chronology,the sediment source from the Middle Permian-Lower Triassic had been limited.The provenance of the Shihezi Formation in the Yiyang area was mainly from the Inner Mongolia Palaeo-Uplift along the northern margin of the North China.In addition to the provenance from the basement of the North China,the North Qinling Orogenic Belt(NQOB)began to offer the material as the other important source area during the period of the Sunjiagou Formation,and at the middle member accepted the significant source from the NQOB.The Liujiagou Formation originated mainly from the basement of the North China and the granitoid intrusions from the NQOB.The Heshanggou Formation was mainly from the single-directional source of the NQOB.The comprehensive analysis indicates that the source region of the Yiyang area created a transition from the Middle Permian to the Late Permian.(3)There is a new tectonic transformation process in the southern margin of the North China during the Middle Permian to the Late Permian,and the starting time are limited to the period of the Pingdingshan sandstone member in the Sunjiagou Formation.Through this provenance study based on the Middle Permian-Early Triassic sedimentary formation,and combining the Late Carboniferous-Early Permian study from the former scholar,the comprehensive understanding about the tectonic evolution process during the Late Paleozoic-Early Mesozoic was obtained.During the Late Carboniferous-Early Permian,the paleogeographic pattern of the the North China is higher in the south and lower in the north,and the NQOB provides the major source for the sedimentary basins along the southern margin of the North China;During the Early Permian-Middle Permian,the whole North China took place a large-scale tectonic transformation that the north uplift and south descending,and had been completed during the Shihezi Formation,leading the source from the north margin of the North China can able to reach the south margin;During the Middle Permian-Late Permian,the south margin of the North China create the new transformation,the NQOB and south margin of the North China produced the episodic uplift and started to become the new source area.With the intense uplift of the NQOB,the sedimentary and tectonic paleogeographic characteristics of the south North China generate the significance transformation.(4)This reciprocating movements between uplift and subside in the south margin of the North China has a close relationship with the tectonic evolution of the Mianlue Ocean during the Late Paleozoic.The tectonic evolution of the Qinling Orogenic Belt during the Late Paleozoic-Early Mesozoic was influenced by the convergence and contraction from the main continental blocks,and at the same time overlaid the process of the Mianlue Ocean from the gradual expansion to the subduction and closure.We attribute the uplift geodynamics to the Mianlue oceanic crust northward subduction.This study restricts the start-up time of the Mianlue oceanic crust from perspective of the tectonic response of sedimentary records,indicating that the tectonic setting of the Mianlue Ocean shift from the stretch during the Middle Permian to the compression during the Late Permian-Early Triassic.(5)The basin formation and evolution along the southern margin of the North China are controlled by the tectonic process of the Qinling Orogenic Belt,especially the uplift and subsidence process of the NQOB.The two episodic uplift activities of the NQOB during the Middle Permian-Early Triassic has a good corresponding relation with the coeval depositional cycle and filling characteristics that displayed the lacustrine transgressive,lacustrine retreat and second lacustrine transgressive.During the period when the NQOB create the intense orogeny,the Yiyang area mainly develop the lacustrine sedimentary system and show the sequence of lacustrine transgressive,the deposition is mainly filled with the argillaceous source from further orogenic belt;In contrast,during the period when the NQOB stop the intense orogeny,the Yiyang area mainly develop the delta sedimentary system and show the sequence of lacustrine retreat,the deposition is mainly filled with the sandy source from near basin margin.Therefore,the nature of southern margin of the North China may be no longer the single intracontinental depression basin,but begun to be the synorogenic basin that control by the NQOB during this period.