Geochemical Characteristics Of Xujiahe Formation Mudstones In The Northeast Sichuan And Their Geological Significance

Elemental geochemical characteristics are of great significance for identifying the original sedimentary environment.In the process of sedimentation and diagenesis,stable geochemical elements are widely used to infer the provenance characteristics,identify the tectonic setting and reconstruct the paleo-sedimentary environment.In addition,Paleo-sedimentary environment could be recovered qualitatively or semi-quantitatively by using elements or element ratios which are sensitive to sedimentary environment in mudstone.Based on previous studies,this dissertation systematically studied the paleoclimate,paleo-sedimentary and paleo-provenance of the mudstone samples in the Qilixia area,northeast Sichuan basin through scanning electron microscopy?SEM?observation,major,trace and rare earth elements and X-ray diffraction?XRD?analysis,and obtained the following understandings:The lithology of the Late Triassic Xujiahe Formation samples in Qilixia area is mainly mudstone,silty mudstone,muddy siltstone and siltstone.The contents of clay minerals in samples are dominated by Illite/Smectite?62%⁸8%?,followed by Chlorite?3%¹9%?,Illite?3%¹1%?,Kaolinite?1%⁹%?,FeO/MnO ratio is28.08¹04.23,and Climate index is in the range of 0.31⁰.91,these features reflected the warm and humid climatic conditions of the Late Triassic Xujiahe Formation in the Qilixia area,in which the climate of the 1^st,3^rd and 5^th members of the sedimentary period are relatively warm and humid,and the climate of the 2^nd and 4^th members of the sedimentary period are slightly arid and hot.Lower Li,Sr,Ni and Ga abundance and Sr/Ba ratio?0.08⁰.24?of the Xujiahe formation revealed that the Paleo-salinity of sedimentary water was low and the overall trend was decreasing;V/?V+Ni?ratio?0.69⁰.85?and Ce_anom?0.26⁰.3?indicated that the reducing environment in Xujiahe sedimentary period,and the reductivity of water from the 1^st to the 5^th member of the Xujiahe formation gradually weakened.The correlations between?Ce and?Eu,?Ce and?REE,CIA and Th/U,CIA and Th/Sc in the samples in the study area are not obvious,which indicate that the rocks are controlled by low-to-moderate chemical weathering and diagenesis;The CIW ratio?81.80⁹7.76?implied that the potassium content of the source rock decreased with the new formation,and ICV ratio?0.84¹.32?implied that the 1^st and 5^thh members of Xujiahe Formation may be deposited in the active tectonic environment,while the 3^rd member was the first deposition under the conditions of recirculation of sediments in the calm tectonic environment or strong weathering conditions.According to the evident negative Eu anomaly?0.63⁰.75?,Th/U ratio?2.0⁵.4?,the features of La/Th-Hf,La/Yb-REE,La/Sc-Co/Th discrimination diagram and the distribution pattern of rare earth element are considered the parent rock debris come predominately from the upper crust,and the parent rocks are mainly pro-granite felsic,and mixed with some basic volcanic rocks.Comparing the peripheral rare earth element distribution pattern,it is inferred that the early provenance(such as the 1^stt and 2^nd members)mainly originated from the mixture of diorite,granite and basalt in the South Qinling area,and some were derived from the granite of Longmen Mountains,the late provenance was mainly from the granite of Micang-Daba mountains.The tectonic setting of source area in the Late Triassic strata belongs to the passive continental margin by analyzing the La-Th-Sc and K₂O/Na₂O-SiO₂discrimination diagrams.