The Significance Of Study Of Chronology And Geology Of Bulgen Alkali Granite In Altai

Bulgen alkali granite , the only alkaline pluton in Altai orogen presently,lie on the south edge of the orogen. Many works have been done ,but the age is stillon debate, which concerned several problem about whether there have two or one stagealkaline and regional structure evolution. Based on the predecessors'research,detailed field works, petrology, geochemistry and geochronology of the alkaligranite, we can draw several main conclusions as bellows:The alkali granites are abundant in arfvedsonites, and have graphic textureand drusitic structure. The rocks are enriched in SiO2,Na2O,K2O,Ga,Nb,Zr,Zn,Y with low content of Al2O3,CaO ,MgO ,TiO2,FeOT,TiO2,P2O5 and negative anomalyBa,Sr,P,Ti.They have highεNd(t) value and low Isr. All of them indicate that thealkali granite was probably related to the mantle partial melting, which providedthe melting of ocean crust or arc material with heat source,with magma mixing andstrong crystal fractionation.SHRIMP Zircon U-Pb dating yield a 206Pb/238U age of 358±4Ma,which is interpretedas the Bulgen alkali granite's formation age. This age firstly confirm the lowerCarboniferous alkaline in Altai orogen. It show that palaeozoic magmatic qualitychanged from calcalkalic to alkaline during the lower Carboniferous, which meansthe tectonic regime had been changed and the end of Altai main orogenic movement.It has significance to the formation and evolution of Altai orogen.