Geological Characteristics And Study Of Detrital Zircon Geochronology Of Epimetamorphic Rock Series In Zhalantun Area

This study bases on the investigation and evaluation subjection of the National oil-gas resource stratagem precinct:The constraints of regional tectonic on Mesozoic and Cenozoic basins (XQ-2004-07-01), which belongs to the subjection:The evolution and oil-gas perspective of Mesozoic and Cenozoic rift basins in exploration region around Daqing(XQ-2004-07). There are several hundreds Samples of rocks, more than 20 samples for the analyses of whole-rock major element and trcee element and more than 20 samples of zircon grains collected in Zhalantun and its adjacent zone, Northeastern China, during the field geologic survey and the measurement of typical section. Based on the analyses of geochemistry of important samples, the protolith formation and tectonic settings of the epimetamorphic rock series in Zhalantun area have been discussed. By the researches of system mineralogy and interior structure of detrital zircon of epimetamorphic rock series and its adjacent Paleozoic strata in Zhalantun area , and based on the results of U-Pb dating and the analyses of probability density distribution-histogram plot of the ages of detrital zircons, the research methodology of detrital zircon geochronology have been validated, and the formation ages, provenance of ptotolith of the epimetamorphic rock series and interrelated significant geological events have been discussed. These studies have offered some very credible datum for the basic geology research of Xing'an plot, even the Northeast of China,and have provided some constraints on renewedly understanding of paleogeographic, tectonic reconstructions and geologic evolution in the Northeast of China. So they have important meaning.On the basis of previous study, combined with field geological investigation, geochemical analysis and geochronological study of detrital zircons to typical samples of the Zhalantun epimetamorphic rocks, some important data and cognitions are gotten:1. The Zhalantun epimetamorphic rock series were mainly distributed in Woniuhe, Arongqi and Molidawa areas as unequal remnants. The epimetamorphic rock series distributed in Woniuhe and Arongqi areas are mainly composed by mica, chlorite schist with greenschist metamorphism which were undergone intensely ductile deformation. The epimetamorphic rock series distributed in Molidawa area are maily composed by greenschist-low amphibolite facies garnet, andalusite, cordierite mica schist and meta-sandstone formed during thermocantact metamorphism.2. The protolith of Zhalantun epimetamorphic rock series is a suite of continental clastic sedimentary rock, during which the protolith of mica, chlorite schist is the volcano-sedimentary formation of sandstone, greywacke with volcanic interlayers, while the protolith of arnet, andalusite, cordierite mica schist is aluminous-enriched pelite and pelitic siltstones. The metamorphic series were formed in back-arc Marginal Basins during the island arc-active continental marginal background. The sedimentary rocks of lower Silurian-middle Devonian of late Paleozoic were formed in the surroundings of active continental margin, while the Permian sedimentary sandstone was formed in passive continental margin.3. The cognitions to the strata age acquired from the study by geochronology of the detrital zircons are consistent with the results of paleontology and stratigraphy research. It is testified that the chronology of the detrital zircon's theory has rationality and veracity to constrain the sedimentary strata age. 4. The geochronological study of the detrital zircons of Zhalantun epimetamorphic rock series indicates that the oldest sedimentary age of mica, chlorite schist in Zhalantun and Arongqi areas is between 439Ma and 480 Ma, which is belong to late period of Early-Paleozoic; while the oldest sedimentary age of meta-pelite and meta-sandstone in Molidawa area is 391Ma~416Ma belong to early-middle periods of Late-Paleozoic.5. The geochronological study to the detrital zircons of Zhalantun epimetamorphic rocks indicates that there was a continental-continental collision orogenic event around 500Ma in the north Daxing'anling, which reflects the influence of late Pan-African movement on the area. Through the orogeny Xing'an block, Erguna block and Songnen block amalgamated together during early Paleozoic, and developed active continental margin. The cognition coincides with the regional strata and paleontology data. The chronological studies on the detrital zircon in the sedimentary basin also gave some important geological records about formation and break-up of the Rodinia super-continent, which reveals there may be collisional orogeny and continental break-up events during Neoproterozoic in the area. Meanwhile the study also gave pre-Cambrian source information around Xing'an block. Those cognitions provide important constraints on the rebuilding of the geological history and recovery of the paleo-tectonic framework and structure to north Daxing'anling area.