The Age And Tectonic Attribute Of The Longquan Group:Constraints On The Evolution Of The Cathysia Block

The Longquan Group,which is located in southwestern Zhejiang Province,has been recognized as a part of the Precambrian metamorphic basement of the Cathaysia Block all the time.The research on ages and tectonic attributes of metamorphic basement can offer important insights on tectonic settings and geological evolution process of the Cathaysia Block in this period.In this thesis,I try to research the Longquan Group metamorphic rocks with geochemistry and detrital zircon geochronology,the aim is to reveal its protolith property,ages and formed tectonic setting.Further more,the tectonic evolution process of the Cathaysia Block in Early Paleozoic time is proposed.The main conclusions are listed as follows:(1)The Longquan Group is mainly consisted of schists,metasandstones,gneisses and granulites,most of metamorphic rocks are the para metamorphic rocks.Geochemical characteristrics of major and trace elements and the result of geochemical discrimination diagrams indicates that the protoliths of the para metamorphic rocks are arenaceous and argillaceous sedimentary rocks in the main.More exactly,their primary rocks are mainly greywackes,a few are clay rocks.The protoliths of the meta-sedimentary rocks have different tectonic settings of continental arc and active continental margin.The amphibolite is also founded in the Longquan Group.It displays an enrichment in large ion lithophile elements(e.g.Rb,Th,and K)and depletions in high field strength elements(e.g.Th,Nb,Ta,Zr,Hf and Ti).Geochemical characteristics of the rocks are very similar to island arc basalts.According to the result of discrimination diagrams,Protolith of the amphibolite is island arc calc-alkaline basalt,and formed in a tectonic setting of the volcanic arc in the continental margin related with subduction zones.Geochemical analysis suggests that the northeastern Cathysia Block was in a convergent plate margin environment when the Longquan Group formed.As a result,lithologic units have the characteristics of the tectonic settings of continental arc and active continental margin.(2)LA-ICP-MS U-Pb dating of detrital zircons from three samples of the para sedimentary rocks shows that analyzed zircon grains fall into two major age ranges:1150-900 Ma and 600-540 Ma,with age peaks at 990 Ma and 560 Ma.The youngest ages of detrital zircons from three samples are 541 Ma,523 Ma and 537 Ma,which constraints the maximum depositional age of the source rocks.Therefore,formation age of the Longquan Group is less than 560 Ma,it probably formed in Early-Middle Cambrian.Detailed comparisons among the amphibolites of the Longquan Group and the Chencai Group and the biotite-bearing amphibolite of the Chencai Group indicates that they have similar protoliths and tectonic settings.Thus the source rocks of the amphibolite of the Longquan Group formed in early Paleozoic.(3)The research based on detailed comparisons of zircon age distributions suggests that the detrital zircon age spectrum of the Longquan Group meta-sedimentary rocks displays a lack of age peaks of Precambrian magmatic events of the South China Block.Moreover,the Cambrian sedimentary strata in the Cathysia Block has high similarity of zircon age distributions that displays a characteristic feature of Gondwana.During the Cambrian it is impossible for the South China Block to become major provenance of the Longquan Group or the Cambrian sedimentary strata in the Cathysia Block.By analysis and comparison,the most likely provenance of the area is from Gondwanaland lay to the southeast Cathysia Block at that time.In late Neoproterozoic to Ordovician time,the whole Cathysia Block received sedimentary materials from Gondwanaland continually.(4)The protolith of the amphibolite of the Longquan Group is island arc basalt.Analysis of the protoliths of the amphibolite and the biotite-bearing amphibolite of the Chencai Group formed in early Paleozoic obtains the same result.Therefore,there was a continent arc located in Longquan and Chencai area of Zhejiang province.Combined with the tectonic background of the para metamorphic rocks of the Longquan Group,it is probably that the northeastern Cathysic Block was in a convergent plate margin environment at that time,and experienced ocean-continent subductions.(5)According to this study of protolith property,ages of primary rocks,tectonic setting of the Longquan Group and analysis of detrital zircon age spectrum characteristics and detrital zircon provenance,combined with the previous studying results,a completely tectonic evolution process of the Cathysia Block during late Neoproterozoic to Ordovician time is proposed.Since late Neoproterozoic,as the beginning of the amalgamation between the South China Block and Gondwanaland,the Cathysia Block start its collision with northwestern India.Subsequently the southeastern margin of the Cathysia Block accreted with northern India gradually.In Middle Cambrian while the Longquan Group formed,the Cathysia Block had mostly accreted with northern India,which was a part of Gondwanaland.Gondwanaland was the major provenance of Cambrian sedimentary strata in the whole Cathysia Block.Amalgamation between northeastern Cathaysia and Australia mared the final assembly of the supercontinent.In Cambrian to Ordovician time,Because of the ocean-continent subduction between the Kuunga Ocean and the South China Block,the northeastern Cathysia Block was in a convergent plate margin environment.