The Characteristics Of Hydrothermal–exhalative Sedimentary Rocks(HESR)and Its Involved Structure Information Of Middle Permian Lucaogou Formation In Northeast Xinjiang

As an important part of the Central Asian orogenic belt,northern Xinjiang recorded the complex evolutionary information of ocean-land transition.For decades,scientists studied the tectonic evolutionary processes via various tectonic research methods.Different conclusions have been drawn hence lots of debates about tectonic setting of northern Xinjiang during Carboniferous-Permian arise.Previous work showed well laminated hydrothermal-exhalative sedimentary rocks?HESR?,which formed by magmatic-hydrothermal processes,developed in these fine grained sedimentary rocks in target area.The information of deep magmatic activities contained by HSER formation offers a fresh perspective to analyze regional background—sedimentary evidence.We conducted petrographic and mineralogy?on both micro and ultra-micro scale?observation,trace element and isotope geochemical analyses on HESR,aim to explore or reinterpret their source and mechanisms.Then discusses the tectonic background related to the rocks formation.Five main observations and interpretations are as follows:1.Well laminated HESR mainly contains hydrothermal-exhalative rocks and hydrothermal-explosive rocks.The former type formed by hydrothermal fluids precipitation.According to mineral composition differences,it falls into two categories:dolomite dominated type and analcime dominated type.The later type formed by clastic depositing and can be classified into calcite dominated type,dolomite dominated type and diopside dominated type.The same type of hydrothermal-explosive rocks can be further divided into porphyroclastic ones and aggregate ones based on different occurrences.2.Hydrothermal-explosive rocks are dominantly composed of limited kinds of minerals with low compositional and textural maturity.Microfractures are frequently observed in minerals of clastics which are main components of hydrothermal-explosive rocks.The clastics are more likely derived from magmatic-hydrothermal material or fluid with diverse properties in variety depth of earth in an starved lake in intracontinental rift basin,rather than weathered source rocks or hydrothermal precipitation.Those deep-derived materials deposited under various mechanisms and driving forces with the distance from the hydrothermal vents after they upwelling into lake bottom.3.Geochemistry and isotopic data results strongly suggest the clastics are deep-derived?even partly are mantle-derived?origin.Rare earth elements distribution of calcite and dolomite dominated hydrothermal-explosive rocks are similar to that of carbonatite.Th/U,Sr content is very different from that of sedimentary carbonate rocks.Moreover,⁸⁷Sr/⁸⁶Sr and?²⁶ Mg are very close to the mantle value.These indicate the source of clastics has close relationships with magmatism and postmagmatic hydrothermal activities.4.Hydrothermal-explosive rocks show great heterogeneity of petrographic,mineralogical and geochemical features,indicating different types have different sources and forming mechanisms.This dissertation considers the aggregate rocks are more likely to deposit nearby hydrothermal vents,controlled by gravity flow.Whereas the porphyroclastic rocks are highly likely to deposit relatively far from hydrothermal vents and mainly controlled by tractive current.5.Hydrothermal-explosive rocks are commonly observed in small amount but with extensive distribution and are rich in deep-derived clastics which mainly consist of alkaline minerals,indicating their clastic sources have strong genetic relationships with alkaline magma.This illustrates that abundant alkaline magma and hydrothermal activities existed during the depositional time of the Permian Lucaogou Formation in the northeast Xinjiang.This finding provides sedimentary evidence that the study area has entered into intraplate extensional tectonic stage during Permian.The realizing of deep-derived clastic rocks which develops in gray-black,fine-grained sedimentary rocks enrich and deepen researches on fine-grained sedimentary rocks.It also provides lots of information of how does deep-derived material getting involved into the processes of sedimentation and diagenesis of fine-grained sedimentary rocks.In addition,study on deep-derived clastic rocks offers a new insight to analyze regional background and provides sedimentary evidence for tectonic evolution stage judgments.Further studying on hydrothermal-exhalative sedimentary rocks will benefit completing and enriching theories of sedimentation and petroleum generation in continental oil-bearing basins.