巴颜喀拉地体POG型花岗岩同位素年代学研究

Sm Nd Ages and Origins of Metamorphic Rocks for the H 9 Formation in the Bayan Obo Ore District,and Relationship with the Ore Forming Event

巴颜喀拉地体位于青藏高原北部、昆仑地体和羌塘地体之间。在地体的中—东部有大量花岗岩出露。黑云母３９Ａｒ－４０Ａｒ和锆石２０７Ｐｂ／２０６Ｐｂ同位素测年结果，显示侵位年龄在２２０～２１２Ｍａ之间，所经历的两次大的后期动力变质变形作用分别发生在１４８Ｍａ和１１０Ｍａ。

The Bayan Obo ore deposit in Inner Mongolia ,China is the largest REE deposit in the world.Many Words on this deposit have been made and vast amount of scientifil data have been accumulated.However,the origin ,especially ore_forming age have been ubder dispute for a long time. The geochronology of potassium_rich slates,dard slates and metabasic volcanitesfor the overling H 9 formation of the main ore body in the Bayan Obo ore deposit has been studied.Their Sm Nd ages have been obtained.The samples of the pottasium_rich slates present a Sm Nd isochron age of 1 096±56(2σ) Ma,with I Nd =0.51100±4(2σ),ε Nd (t) =-4.4±0.7.The Sm Nd isotopic analytical results of the 5 dark slate samples define a very good isochron of t =1 208±123(2σ) Ma with I Nd =0.51102±10(2σ),ε Nd ( t )=-1.1±1.9.The calculated isochron age of the 8 metabasic volcanic samples is 1 337±86(2σ) Ma,corresponding to I Nd =0.51091±6(2σ),ε Nd ( t )=-0.06±1.13. The Sm Nd isochron ages of the metamorphic rocks of the H 9 formation in the Bayan Obo ore district shows that the forming time for the ore deposit should be>1 000 Ma ,it implies ,the ore forming events chiefly took place in the middly Porterozoic. The Opinions that this ore deposit forms in the Calednian time of～400 Ma and the REE mineralization have two forming stages have not been demonstrated.The potassium rich slates,the dark slates and the metabasic volcanites have the variational ε Nd ( t ) values and ε Nd ( t )<0.But calculated ε Nd values are each other very close on condition of the same time and are in range of -0.4～-1.7 when t =1 286 Ma.It indicates that the metamorphic rocks of the H 9 formation in the Bayan Obo ore district, as this ore deposit which Sm Nd age of whole ore samples is 1 286±91(2σ) Ma ,with I Nd =0.51098±4(2σ),ε Nd ( t )=-0.06±0.78,have been derived from the mantle of LREE slight enrichment.