Comprehensive Provenance Study And Tectonic Setting Of Sandstones From The Upper Cretaceous Yaojia Formation, Qianjiadian Area, Southern Songliao Basin

In recent years,the Qianjiadian uranium deposits have been discovered in Late Cretaceous Yaojia Formation as a super large sandstone-type uranium deposit in the southern Songliao Basin.Since the discovery of Qianjiadian uranium deposit,predecessors have carried out a series of related studies on it,among which the provenances studies are basically qualitative.For sandstone-type uranium deposits,the reconstruction of provenance system is helpful to reveal the provenance direction of uranium reservoir during sedimentary period,and to further explore the spatial distribution of provenance-sedimentary system.On a larger scale,the sedimentation and uranium mineralization in basins are controlled by regional tectonic setting.Therefore,it is of great significance to elucidate provenance-sedimentary system and regional tectonic setting for regional metallogenic regularity and exploration and prediction of sandstone-type uranium deposits.In view of this,this paper attempts to study the provenance system and regional tectonic setting of the Yaojia Formation sandstones comprehensively by using various methods,revealing the controlling role of regional tectonic background and mineralization,and serving uranium exploration practice.Based on a great number of high-quality well logging data and serial compilation data with small intervals,the Yaojia Formation has been divided according to the principle of the sequence stratigraphy and the isochronous stratigraphy framework has been established.Under the isochronal stratigraphic framework,taking the sandstone of Yaojia Formation as the research object,the cores taken from typical coring wells are collected systematically,the detrital components of sandstone samples are observed under optical microscope,and element geochemistry of a large number of samples are tested,and the test results are screened,counted,analyzed and discussed.The parent rock type and provenance direction of sandstone from Yaojia Formation are preliminarily determined by both heavy mineral composition analysis and detrial composition analysis.Based on the analysis of major,trace and rare earth elements,the parent rock types of sandstone from Yaojia Formation are determined,and the weathering characteristics and paleoclimate of the sedimentary period of Yaojia Formation are restored,and the tectonic setting of the source area of the Yaojia Formation sandstone is systematically and deeply discussed.The main understandings and conclusions are as follows:1.A regional marker bed and six key interfaces have been identified in the study area based on continuous core data,longing data shape and vertical sequence structures.Accordingly,the Yaojia Formation can be divided into a three-sequence(SQ_K2y),including three system tracts(SQ_K2y-LST,SQ_K2y-EST,SQ_K2y-HST).SQ_K2y-EST can be further subdivided into three small sequence groups(SQ_K2y-EST?Pss1?,SQ_K2y-EST?Pss2?,SQ_K2y-EST?Pss3?).SQ_K2y-HST can be further subdivided into two small sequence groups(SQ_K2y-HST?Pss1?,SQ_K2y-HST?Pss2?).Based on this,we track the marker bed and critical interfaces and found the isochronous stratigraphic framework.Under the isochronal stratigraphic framework,the strata of each unit in Yaojia Formation are generally distributed in a NW-NE direction,and the thickness of each unit is decreasing from southwest to northeast.2.Sand distribution system analysis shows that the sand bodies of the Yaojia Formation are generally distributed in the NW-NE direction,and the provenances direction is southwest.The size of sand body decreases gradually with the development of inheritance of sand body from the SQ_K2y-LST to SQ_K2y-HST system tract.3.Braided river and braided delta depositional system were developed in study area.The braided river depositional system was only developed during the depositional stage of SQ_K2y-LST small sequence group,and was distributed in the southwest corner of the study area,with limited regional distribution.The braided delta depositional system were developed in the main body of the study area during the SQ_K2y-EST and SQ_K2y-HST system tract,and the braided river delta plain is the main depositional system.As a whole,the type of depositional system has changed from braided river depositional system to braided river delta depositional system from south-west to north-east.The braided distributary channel is becoming smaller and more bifurcated.4.ZTR index and the stability coefficient of heavy minerals are mainly distributed along the NW-NE direction in the plane distribution,which is consistent with the direction of sand dispersal system.Therefore,it can be judged that the source of the Yaojia Formation sandstone in the study area is from the southwestern part of the basin.5.Rock and mineral identification shows that the grain of the sandstone from Yaojia Formation was generally medium to fine,with medium sorting and poor roundness.The Yaojiasandstones in the study area are mainly consisted of feldspar sandstone and lithic feldspar sandstone,with quartz content averaging 42%,feldspar content averaging 37%,and lithic content averaging 21%.The sandstones of Yaojia Formation generally have heavy mineral assemblages of zircon+titanomagnetite+garnet.Different types of parent rocks have different light and heavy mineral assemblages,referring to the light and heavy mineral assemblages and detrial composition tables of various parent rocks in Songliao Basin by predecessors,show that the parent rocks of the Yaojia Formation sandstones in the study area are mainly acid magmatic rocks or metamorphic rocks.Qt-F-L and Qm-F-Lt ternary diagrams suggest the parent rocks of the Yaojia Formation during the sedimentary period mainly came from the tectonic highlands within the craton.6.The REE distribution patterns of the Yaojia sandstones are uniform and similar to typical post-Archean Australian Shales and the upper continental crust,with LREE enrichment,flat HREE,and moderate negative Eu anomalies?Eu/Eu*=0.55⁰.69,average 0.62?.The paleoclimate discriminant diagram shows that the climate of Yaojia Formation is semi-arid,and the chemical weathering of the source rocks is weak under the semi-arid climate environment,which is consistent with the discriminant results of CIA,CIW and PIA.Based on major element,trace and rare earth elements analysis,in combination with provenance attribute and tectonic setting discrimination diagrams,these characteristics all above suggested that source materials of the Yaojia Formation sandstones were from passive margin environment,and its provenances come from the paleo-uplift of southwestern area.The source rocks are mainly intermediate-acid igneous rocks?granitoids?and ancient recycled sediments.In this paper,geological information such as drilling,core,logging and microscopic characteristics are combined,and the following innovative achievements are achieved in this paper.Based on the sandstone distribution and heavy mineral analysis,it is considered that the source direction of the sandstone in Yaojia Formation is the southwestern part of the basin.Based on the data of detrital component analysis and geochemical analysis,it is considered that the sandstones of Yaojiadian Formation in Qianjiadian area are feldspathic sandstones and lithic feldspathic sandstones.The parent rocks are mainly consisted of intermediate-acid igneous rocks,which come from the tectonic highland in the interior of Yanshan belt.Weathering in the source area is a weak chemical weathering in the semi-arid climate background.The study of tectonic setting shows that the source area of Yaojia Formation sandstone is a passive continental margin tectonic environment.Based on regional tectonic evolution,it is considered that the source area of Yaojia Formation sandstone is an intracontinental rift tectonic environment,and its provenance area is a cratonic highland adjacent to it.The basin-and-range tectonic system under extensional action between sedimentary basin and provenance area is revealed.Long-lasting uplift provides sufficient source of ore-foring materials for sedimentary basins.