| Neoprotozoic strata exhumed sporadically in regions like Fuyang,Pujiang,Jiande,Quzhou,Kaihua.Limited to scarce of the signs of discriminations of the strata,arguments persist to exist that how to differentiate them and which category of palaeogeography did them fall into.Petrologic,chronologic and sedimentologic approches are taken to compare differences of strata sequences,sedimental characteristics,detrital zircon dating,clay minerals and heavy minerals among Luojiamen Fm.(Fuyang),Hongchicun Fm.(Fuyang)and Chentangwu Fm.(Pujiang).We can get some thoughts about the depositional environment,tectonic setting and evolution history of the Neoprotozoic basin in Pujiang-Mengshan area,and propose a probabal evolution mode.As follows:1.Chentangwu Fm.is composed of lithic sandstone,fine sandstone,muddy siltstone,mudstone and so on,and developes rhythmic bedding,ripple bedding,horizontal bedding,etc.Luojiamen Fm.is mainly composed of sandstone,siltstone and mudstone,and it also developes rhythmic bedding,horizontal bedding and ripple bedding.Although,Chentangwu Fm.overlies parallelly on Mengshan Fm.(Pujiang)formed by spilite,and Luojiamen Fm.exists alluvial-proluvial conglomerate at bottom.However,the sedimentary rocks combination,sedimentary structure and sedimentary sequence of Chentangwu Fm.is similar with that of Luojiamen Fm..2.XRD and IR spectrum tests indicate that the mudstone of Chentangwu Fm.,Luojiamen Fm.and Hongchicun Fm.has the same constitution.They are mostly composed of clay mineral like illite and chamosite,including some detrital minerals like quartz and albite.The Chamosite,which formed in shallow sea,is authigenic in the study.According to the sedimentary rocks combination,Mengshan Fm.formed in a hemipelagic environment while Chentangwu Fm.formed in a turbidite-shallow environment.Luojiamen Fm.formed in alluvial-proluvial,littoral and trubidite-shallow environment.Hongchicun Fm.formed in beach-tidal-flat environment.3.Heavy mineral combination of Chentangwu Fm.contains zircon,rutile,anatase,garnet,apatite,pyrrhotite,sphalerite.Fuyang Fm.contains hematite,zircon,pyrrhotite,sphalerite,garnet,epidote and Hongchicun Fm.contains zircon,apatite,hematite,anatase,sphalerite,which indicate their provenance are all igneous rocks.The result of the sandstone clastic compositions indicate that the structure characteristics of provenance was magmatic arc both in Chentangwu Fm.and Luojiamen Fm.and this magmatic arc probably belongs to Shuangxiwu Group.4.The age of detrital zircons from bottom sandstone of Chentangwu Fm.spread from 830Ma to 870Ma,and the youngest age is 829Ma.The age of detrital zircons from bottom sandstone of Luojiameng Fm.is in 840～890Ma,and the youngest age is 824Ma whilst Hongchicun Fm.is in 830～930Ma.They are all igneous zircons.The youngest detrital zircon age can indicate when a basin opens,which demonstrates Pujiang basin and Fuyang basin have the identical time when they start to deposit.5.In combination with the sedimentary constitute of the two strata in Pujiang Fm.and Luojiamen Fm.,and even considered features of clay mineral,heavy mineral,provenance and sedimentary facies analysis,the two stratas may be the same one.They both formed in Fuyang-Pujiang basin,which is a Neoproterozoic basin opened in about 840～835Ma(indicated by the age of the spilite)and deposited in about 830～824Ma.Therefore,we propose an evolution model:the crust was stretched by the extension about 835Ma ago,and then rifts formed;the extension was lasting,the turbidite rift basin came to its shape meanwhile exsisting magmatism under the rifting environment;the extension ended until 800Ma ago and the basin was fully deposited with Hongchicun Fm.;during 800Ma～760Ma,massieve bimodal volcanic rocks erupted,which represented the end of rift activity. |
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