Abstract: Fogang(main body)biotite granites are a part of large-scaled Fogang granitic complex of east-west trending.They occupy about 85％ of the complex in area.They are composed of about 2～ 6％ biotite,35～ 41％ quartz,10～ 18％ plagioclase (An10—23)and 36～ 48％perthite,corresponding to biotite syenogranite(3a)on the basis of mineral contents by lUGS classification.The biotite has lower Mg／(Mg+Fe )values(0.17-0.21)and the K-feldspar is usually microcline perthite.Fogang biotite granites have an age of 167.5± 7.5Ma(MSWD =2.4),determineted by Rb-Sr whole rock-minera1 isochron method (emplaced during Middle Jurassic period).Similar to S type granites in south China,Fogang granites are markedly rich in silica(Si02：71.74～77.64％)and K20 relative to Na20 (averaging K2O／Na20=1.66),and are moderately peraluminous (averaging A／NKC = 1.056) and calcic-alkaline to weakly subalkaline series(NK／A=0.797 in average).Based on the trace element and rare earth element data,the granites commonly have higher Rb／Sr(1.98～ 41.71),Rb／Nb(11～23),K/Nb(1498-2976)ratios,lower Nb／Ta ratio(5.94 in average),strong to moderate Eu depletion(Eu／Eu* =0.12～0.46),and higher(La／Sm)N values(1.67～5.39).In Sr,Nd,Pb,O,isotopic composition field,Fogang granites have higher Isr (0.7116±0.0023)and εsr (t)values(96～ 103),showing that South China Upper Crust end member occupies about 66～69％ in their source materials in terms of the model calculation. The granites possess higher δ18O‰(+9.3～ +12.5)and 207Pb／204Pb(15.734)and 206Pb／204Pb(18.968～19.049) indicating that they are consistent with Upper Crust Evolution Line in the Pb-tetonic model diagram.The above mentioned data wholely indicate that Fogang biotite granites were formed by partial melting of source materials consisting of mid to lower crust rocks.Some of dark enclaves of dioritic or granodioritic compositions are found in Fogang biotite granites. This suggests that basalt underplating resulted from lithosphere westward subduction of the Palaeo Pacific plate and northward subduction of the Philippine Block,effected upon the formation of Fogang granites.