Geochronology,Geochemistry And Structural Significance Of Mid-devonian Meta-sandstones From Boyang-yuanlong Area In The Western Qinling Orogenic Belt

The object of study of the thesis is amphibolite-facies meta-sandstones in Boyang-Yuanlong area of the city Tianshui in the Western Qinling orogenic belt.There are different ideas about the age of the meta-sandstones,and the geological significance of the meta-sandstones is seldom discussed.Based on the study of petrology,geochemistry and clastic zircon LA-ICP-MS U-Pb isotope chronology,the thesis has discussed respectively the geologic feature,mineral feature,rocky feature,age,source area feature,sedimentary characteristics and protolith of the meta-sandstones.Combined with the regional geological data and the existing research results about the Western Qinling orogenic belt,the thesis obtains the following main results.(1)The dark-colored mineral of the meta-sandstones had been formed by regional amphibolites-facies metamorphism of the basic volcanic materials.Because of the high recrystallized quartz content and presence of biotite and amphibole of the meta-sandstones,the thesis concludes that the basic volcanic material in the prolith of the high maturity quartz sandstone have been transformed into dark minerals by amphibolite-facies metamorphism.(2)Derital zircon LA-ICP-MS U-Pb dating of the meta-sandstones gives a long-time span from 407 Ma to 2483 Ma.Both of the minimum ages of the samples in Boyang-Yuanlong area belong to Early Devonian,and the ages of many measuring points fall into Early Devonian interval.So it is suggested that the deposition time of the prolith of the meta-sandstone should be not earlier than Early Devonian.Integrated with the regional geological material,the thesis deduces that the prolith of the meta-sandstone deposited in Middle Devonian.(3)According to clastic zircon LA-ICP-MS U-Pb dating of the meta-sandstones,the Boyang sample and the Yuanyang sample of meta-sandstone give respectively peak ages of 407-563 Ma and 396-587 Ma,and the peak ages correspond to magmatism of north Qinling orogenic belt from Cambrian to Early Devonian.The subordinate peaks ages of 905-1094 Ma and 926-1085 Ma represent the provenance of the Qinling orogenic belt(which is related to the convergence of Rodinia supercontinent)and the Yangtze craton.The lower peak ages of 637-847 Ma and 626-824 Ma should relate to the breakup of the Rodinia supercontinent.The age of recognizable metamorphic zircon of 700-1000 Ma and 1700~1900 Ma represent the metamorphic rocks of Qinling Group.The two eclogitic metamorphic zircon illustrate that the deep subduction of continental crust in Neoproterozoic had formed ultra-high pressure metamorphic belt of eclogite facies.(4)The geochemical characteristics of the meta-sandstones in Boyang-Yuanlong area indicate that its prolith mainly formed in tectonic setting of active continental margin.The provenances of meta-sandstone come from the West Qinling orogenic belt and Yangtze craton.Such provenances suggest that the localy plate subducting probably existed in Mid-Devonian along the Tianshui-Wushan suture zone in the West Qinling orogenic belt.