| Jadeite-quartzite is a special type of rock in the ultrahigh pressure metamorphic belt of Dabie mountain.The reaserch on the origin and the formation ages is of great significance to the formation and evolution of the daban orogenic belt.Jadeite-quartzite belt of Dabie Mountains,with a length of 40 km and width of 1km,spreads east-westwards along the line from Changpu of Yuexi County to Yezhai of Qianshan County,Anhui Provience,China.We recently discovered new utcrop in the Houchong,Huichong,Jiangjiaban of Qianshan County.Based on field investigation,section drawing,sample collection,By reaserch on petrography,petrogeochemistry, isotope chronology and analysis of these phenomenona,the result obtained are as follows:1.Jadeite-quartzite,marble,biotite plagioclase-gneiss exposed in Houchong,Shangxintian area.Occured as stratified or stratoid beds,with more eclogite rock blocks,it is identited as a typical assemblage of metamorphic sedimentary rocks,which makes the Jadeite-quartzite belt of Dabie Mountain.a four-in-one system of marble,jadeite-quartzite,biotite plagioclase-gneiss,eclogite.The major mineral component of jadeite-quartzite are:jadeite,quartz,garnet,rutile and a little phengite.Coesite occur in both jadeite and garnet.2 The average contents of major element of Jadeitite-quartzite are SiO2 72.73%,Al2O3 11.77%,MgO 1.61%,Fe2O3 5.00%,Na2O 5.45%,K2O 0.22%,TiO2 0.59%.The chemical composition is characterized by high silicon,rich sodium and rich iron.The average contents of trace element of Jadeitite-quartzite are Th 10.12×10-6,U 1.94×10-6,Hf 10.91×10-6,Sc 7.68×10-6,Zr 437.05×10-6,Nb 15.81×10-6,Y 45.36×10-6,Ba 141.88×10-6,Sr 30.07×10-6;The ratio of Th/U,Th/Sc,La/Sc are respectively 5.18,1.56,9.50;the average of?REE is 223.07×10-6,and?La/Yb?N is 7.01,?Eu is 0.61,with marked negative Eu anomalies,enriched LREE.Sm/Nd ratio is 0.180.23,which shows its protolith belongs to sedimentary rock type from upper crust.The result of geochemical protolith discrimination shows that the protolith of jadeite-quartzite is sodium-rich metagraywacke.Reseach on tectonic settings and provenance show that the protolith of jadeite-quartzite from the Dabie Mountains formed in the active continental margin and continenta island arc type tectonic settings.3.By using the LA-ICP-MS,we get U–Pb isotope data for cores of the zircon from jadeite-quartzite date from 1777 to 1876 Ma,inferred that the age of the protolith is Paleoproterozoic,on the contrary,The data of mantle and rims is 231 to 237 Ma,indicates it is the age of metamorphic zircons which formed in the UHP phase. |
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