湖北大别山天堂寨花岗岩成因类型讨论

DISCUSSION ON GENETIC TYPE OF TIANTANGZHAI GRANITE FROM DABIE MOUNTAIN, HUBEI PROVINCE

天堂寨花岗岩切穿大别群,不受区域褶皱轴和最老地层控制,根据岩石的主化学成分、稀土元素成分、铅同应素组分、δ^(18)O、(^(87)Sr/^(86)Sr);值以及黑云母成分和副矿物组分,它不是一个混合交代型的花岗岩体,而是一个Ⅰ型或同熔型的岩浆花岗岩岩体。

For a long time past, the granitc body occurred around Tiantangzhai peak at the boundary between Hubci and Anhui Province has been considered as migmato—metasomatic type or anatcctic type. In this paper, however, the author suggests that genetically it is of I-type or syntectic type on the basis of the following evidences. (1) The occunence of the body cutting Pt1/1—Pt4/1 is not consistant with diroction of wall rocks. (2) Its spatial situation is not governed by regional fold axis and oldeststratum; (3) Al_2O_3/(Na_2O+K_2O+CaO) (molecular number) of whole rock is not bigge, than 1.1. (4) in figure of Al_2O_3-(aCaO+Na_2O+K_2O) (atomicity) vs. 100×(Fe_2O_3+FeO)/(Fe_2O_3+FeO+MgO), the granite plots fall mostly in Ⅲ district related with basalt magma differentiation. (5) MF of the biotites>0.38, M>0.45, FM<3.6. (6) As for its accessoray minerals, magnetite>ilmenite and zircon mainly belongs to Pupin's 4 type. (7) ^(206)Pb/^(204)Pb=16.98, ^(207)Pb/^(204)Pb=38.27, ^(208)Pb/^(204)Pb=15.53, δ^(18)O=7.4‰, (^(87)Sr/^(86)Sr)_i=0.7082. (8) LREE/HREE=16.48～25.21, δEu>0.7, almost no Eu anoma]y is shown. (9) In Q-Ab-Or-H_2O figure the granite plots show relative concentrated instead of scattering.