| The Qamdo coal-bearing belt in eastern Tibet is the most important coal resource enrichment area in the Qinghai-Tibet Plateau.which accounts for nearly 70%of the coal resources in the Qinghai-Tibet Plateau.There are three sets of coal-bearing strata in the Qamdo coal-bearing belt,which contain the Machala Formation of Early Carboniferous,Tuoba Formation of Late Permian and Bagong Formation of Late Triassic,in which the Bagong Formation is the most widely distributed and accounts for a area proportion of 73%and a coal resource proportion of 60%of the whole coal-bearing belt.However,due to the long evolution history of the "Sanjiang" Tethys tectonic domain,the Qamdo area experienced multi-period plate divergences and collisions,the uplift of the Qinghai-Tibet Plateau and multi-cycle of the tectonic evolution.And more,the development of faults mand folds makes the original paleogeographic features and paleo-structural patterns of the coal-forming period unrecognizable,and the current occurrence of coal-bearing strata is far from the original sedimentary environment.The identitication of coal basin prototype and its dynamic background are the key to revealing the current regularity of coal resource occurrence.The location of the plate where the coal-bearing strata were deposited,the property of the plate edge.the crust's attribution and the gcodynamic effects of the basin formation process directly determine the climatic conditions for the growth of coal-forming plants and restrict the formation and distribution of peat swamps.Therefore,exploring the controlling effect of the coal basin prototype and the geodynamic background of the coal-forming period of the Qamdo coal-bearing belt can promote the understanding of the occurrence of coal resources in the Qamdo area and even the understanding of the Tethys evolution process in the Sanjiang area.This thesis takes the coal measures of the Qamdo basin as the research object,and starts with the occurrence characteristics of the coal measures.Through field geological survey,real measured geological section,sample collection,laboratory test analysis,and related map compilation,comprehensive disciplinary application of structural geology,sedimentology.coal geology and geochemistry,the prototype of coal-forming basins in the Qamdo area was discussed,and the controlling mechanism of different geodynamic backgrounds on coal formation was revealed.(1)Through data collection,field geological survey,measured geological section and hand specimen observation,it is shown that the Machala Formation of Lower Carboniferous is developed in the west side of the Qamdo coal-bearing belt and is affected by the early sedimentary paleotopography and late structural damage,so that Machala Formation is distributed along the NNW direction with the depositional environment being marine-continental?.The coal seam is formed in the lagoon-barrier island environment of the transgressive system.The number and the thickness of coal seams varies with different areas with the decreasing trend being from north to south.The Machala anticline is the main coal-controlling structure of Machala Formation.The secondary folds and faults developed in the anticline wing had cut off the coal seams into some fragmented blocks and formed a fault-fold type.The Tuoba Formation of Late Permian was limited in the study area and was only exposed in the middle of the Qamdo coal-bearing belt with the distribution direction being NNW.The coal-bearing properties of Tuoba Formation is good in the middle but is rapidly deteriorated in the north and south of Qamdo coal-bearing belt.The coal seams of Tuoba Formation belongs to the transgression of the lagoon phase of shoreland environment.The Toba Formation is affected by the eastern reverse faults,and the secondary fold deformation under fault control is the development pattern of coal-bearing strata.The Bagong Formation of the Upper Triassic is widely developed throughout the Qamdo area,and the coal-bearing strata are generally distributed in the direction of NW and NWW.The Qingnidong-Gongjue paleo-uplift in the central part divides the coal-bearing belt into two parts:The thickness of coal seams of the Bagon Formation in the west is basically similar,and the coal-sedimentary sediments of marine-continental facics are developed at the top of Bagong Formation,which belongs to the lagoon-barrier island environment.The thickness of the Bagong Formation in the eastern varies greatly,and there is no obvious regularity of the distribution.While in the Waxu area,there is a deposition of marlstone representing the deep-water environment,and no coal seams are exposed.The Qamdo-Mangkang synclinorium is the most important coal-control structure of Bagong Formation.The coal-bearing strata are developed in the wing of the synclinorium,while the secondary folds and faults in the wing had cut off the Bagong Formation,some coal seams are spread in the type of Imbricate fan.(2)The study on the major trace elements of clastic rocks in coal-bearing rock series shows that the Early Carboniferous Machala Formation,the Late Permian Tuoba Formation and the Late Triassic Bagon Formation have LREE enrichment,while Eu and HREE are relatively deficient.The total amount of REE in the Machala and Bagong formation is higher than the total amount of rare earth elements in the Toba Formation(Samples collected from the Machala Formation:?REE=176.23×10-6?293.47×10-6 Mean value:232.76×10-6 LREE/HREE=5.05?7.61 Mean value:6.29.Mean average.La/Yb=10.04?15.62,mean value:13.17.Eu elements show weak negative anomalies,?Eu=0.55?0.71,Mean value:0.61.Samples collected from the Tuoba formation:?REE=56.90×10-6?64.45×10-6,Mean value:59.32×100-6,Small change.LREE/HREE=4.71?5.43.Mean value:5.09.La/Yb=10.24?11.76,Mean value:10.82.Eu elements show weak negative anomalies,5Eu=0.69.Samples from the Bagong formation:?REE=140.54×10-6?281.30×10-6,Mean value:201.99×10-6,LREE/HREE=8.09?12.01,Mean value:9.61.La/Yb=12.10?17.68,Mean value:15.02.Eu elements show weak negative anomalies,?Eu=0.59?0.70,Mean value:0.66)Three sets of trace elements of coal measures are compared with the REE characteristics of typical tectonic environments summarized by Bhatia.and combine the structural discrimination diagrams of Zr-Th,Th-Co-Zr/10,Th-Sc-Zr/10,and La./Yb-?REE,La/Th-Hf source area composition discriminant diagram comprehensive analysis of the Machala formation deposited in the post-land arc environment from the active continental margin to the continental margin;fhe regional tectonic setting of the Toba Formation is between the oceanic island arc and the continental island are,which is similar to the western Pacific island arc and the Andean continental margin arc tectonic setting.According to the evidence of the basic rocks in the Jinshajiang suture zone,it is believed that the Nanlishan basic rock mass is formed between the Qamdo micro-terrane and the Huaxia plate in the Early Permian Jinshajiang ocean basin subduction.With the extension of the subduction,the post-arc basin Ascending,sea level declines from the original ocean island arc to the continental island arc environment,in the context of the western Pacific island arc and the Andean continental margin arc.The regional tectonic pattern of the Bagong Formation has undergone a major transformation.The Lancang river ocean basin and the Jinsha river ocean basin on both sides were closed in the early Triassic and successively,and the whole area is in an intraplate environment.Sediment sources are multi-sourced,including acidic island arcs(volcanic arc orogenic belts),sedimentary rocks,mixed felsic and matrix rocks.They arc characterized by a variety of provenances,reflecting the deposition of the Triassic coal-bearing strata.The transition zone of different geodetic background transitions,The regional stress background is complex.(3)By analyzing the characteristics of volcanic-magmatic activity in the study area,the calc-alkaline basalt of the Early Carboniferous Machala Formation has higher Al2O3,lower Na2O,K2O.Although the trace element characteristics of the sample have obvious Th and P negative anomalies,as well as weak Ta and Ti anomalies,indicating the participation of the oceanic crust material,no Nb,Ta,Ti are negatively abnormal,and the lgZr/Y-lgZr structure identification diagram shows that it is formed in the plate environment,and the Early Carboniferous study area is under the background of the post-arc tension.In the Late Permian,the calc-alkaline neutral and meso-basic volcanic rocks developed in the eastern margin of the basin,there is no obvious Fu negative anomaly in the sample,and the enrichment of the large ion lithophile elements Rb,Th,and the loss of high field strength element Ba,Nb,Sr,Ti elements,with typical volcanic are basalt characteristics.Which is reflected that the volcanic activity on both sides of the Late Permian research area is formed by the two-way subduction of the Lancangjiang basin and the Jinshajiang ocean basin.The study area is under the geodynamic background of the two-way contraction extrusion.In the Late Triassic.there is no volcanic activity in the western margin of the basin.The characteristics of the main elements of the intrusive rock show high content of K and A1 and low content of Ti.The rare earth element analysis shows obvious anomaly content of Eu,enriched on K,Rb,Th,Ce relatively depleted content of Ba,Nb,P,geochemical characteristics show that the intrusive rock is a potassic granite formed during the subduction process of the Minjiang ocean basin.The magma source is mainly shell source,and the reflection area is mainly composed of compression stress.The volcanic activity of the western margin of the basin is frequent in this period.The main elements of volcanic rocks show that the proportion of SiO2 in the basite and in the acidic rock type with bimodal characteristics is between 42.38%and 55.97%,68.49%and 76.54%,respectively.The rock geochemical characteristics are shown as a typical intraplate rift-type bimodal volcanic rocks are mainly composed of tensile stress in the extended background.(4)Based on the study of sedimentary facies in the study area,the characteristics of plane distribution and longitudinal filling of the coal seam in the third-order sequence stratigraphic framework of the coal-bearing strata in the study area are analyzed:the lower Carboniferous Machala group is only at the basin edge with strip-like deposits.coal seam continuity is poor,which has the characteristics of enrichment at isolated aggregation center,The peat bath in the coal-forming period is confined to a narrow strip which parallel to the source area.Combined with the analysis of regional sedimentary facies,the Early Carboniferous study area is generally a fault-type marine basin controlled by the Wuqi-Yujiang fault.The overall paleogeographic features are that the Qingnidong paleo-uplift and extensive marine,Sea-land interactions are deposited on the Devonian carbonate platform.The upper Permian coal seam has good continuity and has a unified coal accumulation center.The peat bath in the coal-forming period has good continuity,which shows that the basement of the coal-forming basin is relatively flat,and the space in the middle of the basin can be changed little,which can make the peat bath develop continuously in a large scale.Combined with the analysis of regional sedimentary facies,the Late Permian research area belongs to the inherited basin formed on the basis of Carboniferous lithologic basin.which is a marine fault depression basin between the Wuqi-Liangjiang and Qingyandong faults.The Upper Triassic coal seam has different development rules with Qingnidong-Gongjue as the boundary.The west edge of the coal-bearing belt,the coal seam has good continuity,has a unified coal accumulation center,and has similar development characteristics with the Upper Permian coal seam.The reaction showed that the basement of the western margin of the coal-forming basin was relatively flat.and the space in the basin can be changed little,which allowed the peat bath to develop in a wide range.The eastern edge of the coal-bearing belt,the coal seam continuity is poor,and is only locally isolated.At the same time,the Bakeng formation in this area has a large sedimentary thickness,which shows stratigraphic and coal seam development characteristics is similar to those of the Lower Carboniferous.It reflects the eastern margin of the coal-forming basin,and the basement may be controlled by faults and the terrain is undulating.Combined with analysis of regional lithofacies.the Late Triassic basin is generally under the background of overall uplift,and the overall reference surface of the western margin of the coal-forming basin is lower,providing a good environment for the development of peat swamp in shallow w ater environment,while the changing of the east(5)Based on the comprehensive analysis of the three types of coal forming prototype basins in the study area,combined with the results of regional geodynamic settings research:It is controlled under the extensional background in the post-arc coal forming basin,the coal scam is controlled by the dustpan-shaped faulte and spreads out in a belt at the margin of the basin.Generally,regional sedimentation rate and deposition rate are stable at the initial stage of extension-tension,and the low capacity space can be maintained for a long time,which is conducive to the development of coal seam.However,as the west edge of the coal-forming basin is a chain island arc with continuous distribution,the base topography in the extension direction of the island arc fluctuates,resulting in large changes in the volume space.The coal seams vary greatly in different positions in the coal bearing zone.The coal forming basin under the control of compression environment,the basement of the basin is usually flat,wide and gentle,and the volume space changes not much,which provides a good place for coal-bearing strata deposition,and peat bogs can be formed in a large range within the basin.Generally,the coal seam has the characteristics of continuous type and uniform coal center.However,the study area is affected by the bidirectional subduction of the lancang river basin and the jinsha river basin,with frequent tectonic activities,which,is difficult to maintain stable sedimentary rate and basin sedimentation rate.In the repeated tectonic activities,sea level changes frequently,and the coal seam is characterized by multiple layers and thin thickness. |
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