Early Precambrian Crustal And Magmatic Rocks In The Uralan Area Of The North China Craton: Zircon SHRIMP U-Pb Dating And Geochemical Studies

Wulashan is located in the west of Daqingshan and is an important part of the Khondalite Belt,where metasedimentary rocks widely occur with some meta-intrusive rocks.The Wulashan Rock Group is subdivided into five rock formation,namely hornblende gneiss rock formation,biotite gneiss rock formation,garnet biotite rock formation,garnet leptite rock formation and marble rock formation.Early Precambrian intrusive rocks include TTG rocks,gabbro and other kinds of rocks,all of them underwent strong metamorphism and deformation.We carried out SHRIMP U-Pb zircon dating on 4 garnet biotite gneiss samples,1 mafic granulite sample,1garnet granite sample and 6 meta-intrusive rocks.There may be garnet biotite gneisses of two ages.The garnet biotite gneiss reprsented by samples NM1529 and NM1540 were formed during the middle-late Paleoproterozoic,whereas the garnet biotite gneiss represented by samples NM1551 and NM1562 were formed during the late Neoarchean.The garnet granite was formed as a result of anatexis of the late Neoarchean garnet biotite gneiss.Four meta-intrusive samples,including gneissose diorite,gneissic tonalite,gneissic granodiorite and gneissic charnockite,have zircon U-Pb analyses scattering along concordia,with ²⁰⁷Pb/²⁰⁶Pb ages ranging from?1800Ma to?2500 Ma.Zircon U-Pb analyses of meta-gabbro and augen granite are concentrated in concordia,with ²⁰⁷Pb/²⁰⁶Pb ages ranging from?1800 Ma to?2000 Ma.It is common that the stronger recrystallization the zircon domains show,the younger the ²⁰⁷Pb/²⁰⁶Pb age are.Combined with geochemical data and previous studies,some main conclusions can be drawn as follows.1)The original Wulashan Rock Group can be subdivided in terms of formation ages.The hornblende gneiss rock formation,biotite gneiss rock formation and a pat of the garnet biotite rock formation were formed during the late Neoarchean,the rest of the garnet biotite rock formation,garnet leptite rock formation and marble rock formation were formed during the middle-late Paleoproterozoic.The"Daqingshan supracrustal rocks"in the Daqingshan area,which was once considered to be early Paleoprotetozoic in formation age,should also form in the late Neoarchean.2)In the Wulashan area,there are intrusive rocks of the late Neoarchean and the middle Paleoproterozoic,the middle Paleoproterozoic intrusive rocks were formed in an extending environment.Late Neoarchean-early Paleoproterozoic and late paleoproterozoic tectonothermal events widely occurred in the Wulashan area,similar to or same as those in the Daqingshan area.3)The geological evolution in the Wulashan area can be divided into three stages.a)Late Neoarchean supracrustal rocks and TTG rocks were formed in a plate regime,followed by high-grade metamorphism at the late Neoarchean-earliest Paleoproterozoic;b)mantle addition to crust and crustal recycling occurred during the middle Paleoproterozoic when the area underwent a extentional process;c)late paleoproterozoic Khondalite Belt was formed and underwent high-grade metamorphism which resulted in overprinting of the late Neoarchean rocks.