The Material Composition And Structural Significance Of The Matrix Of The Qichun Structural Melange Belt In South Dabie

As the most unique tectonic unit in the world,the Dabie Mountain Orogenic Belt has always been a hotspot in geoscience research and has achieved many important results.However,there are few studies on the low-grade metamorphic rock belt on the south side of the Dabie Mountain,and its structural background and evolution process are still unclear.Finding out the tectonic background and exact age of these low-grade metamorphic rocks will play a key role in further understanding the tectonic pattern and evolution process of the Dabie Mountains.On the basis of 1:50000 regional geological survey,petrology and stratigraphy,this paper studies the matrix of metamorphic sedimentary rocks in Qichun structural melange belt.The metamorphic clastic rocks in dalupu section are selected for geochemistry,LA-ICP-MS U-Pb dating and in-situ Hf isotope analysis.The geological characteristics,mineral and petrological characteristics,formation age,provenance and sedimentary environment of the metamorphic clastic rocks are revealed,which provides constraints for the formation and evolution of the structural melange zone.The research gained the following knowledge:(1)LA-ICP-MS U-Pb isotopic dating results of detrital zircons from meta clastic rocks show that the youngest detrital zircon age is 802 Ma,which limits that the sedimentary age of meta clastic rocks should not be earlier than 800 Ma.Moreover,the age data of several measuring points fall in the early Neoproterozoic,which also limits the lower limit of the time limit for the formation(emplacement)of the tectonic melange zone.(2)Detrital zircon age spectrum is mainly concentrated in 802?911 Ma,1331?1536 Ma,1776?2224 Ma,2302?2789 Ma and 2868?3401 Ma,with four obvious characteristic age peaks:?811 Ma,?1385 Ma,?2020 Ma and?2479 Ma.The peak value of?811 Ma corresponds to the Neoproterozoic magmatism in the northern margin of Yangtze,?_Hf(t)values are all negative and relatively concentrated,indicating that the source area is mainly the recycling of ancient crust.It is inferred that the provenance of this period is mainly the Neoproterozoic island arc belt around.The?1385 Ma pluton in the northern margin of Yangtze has not been reported,but the age spectrum of this period is widespread in Neoproterozoic clastic rocks.The provenance of this stage may be the Meso Proterozoic basement of the Yangtze block which has not been exposed or denuded.;The peak ages from meso Neoarchean to Paleoproterozoic are dominant in the samples,indicating that the main provenance is provided by the basement complex from meso Neoarchean to Paleoproterozoic.(3)The low chemical index of alteration reflects the low weathering degree of the parent rock in the matrix provenance,and the high index of composition variability reflects the low recycling degree of the sediment.It is the first deposition near the source area.The mapping results of La-Th-Sc and Th-Sc-Zr/10 show that the matrix of the tectonic melange belt was formed in the tectonic setting of continental margin arc or active continental margin.