广东龙口南昆山铝质A型花岗岩的成因

Origin of Nankunshan aluminous A-type granite, Longkou County, Guangdong Province

南昆山花岗岩出露面积约 2 0 0km2 ,为一等轴状岩体。具中粒等粒结构 ,由黑鳞云母、钠长石、条纹长石和石英组成。弱过铝质 ,MgO和CaO含量极低 ,亚碱质 ,碱度率和NK/A值偏高 ;富F贫Cl,具较高F/Cl值 ;具较高的锆石饱和 (液相线 )温度 (81 0～ 847℃ ) ,与澳大利亚A型花岗岩的平均值 (839℃ )基本相当。在微量元素特征上 ,具有高的 1 0 4 Ga/Al值和 (Zr+Nb +Ce +Y)元素组合值 ,分别高于A型花岗岩下限值的 2 .6和 350× 1 0 - 6 。在同位素组成特征上 ,具有较高的εNd(t)值 (- 1 .54～ - 2 .87)和较低的δ1 8O值 (7.9‰～ 9.8‰ )。按幔 -壳混合模型计算 ,其源区地幔组分约占 63 %～ 69%。上述特征表明 ,南昆山花岗岩与国内外典型铝质A型花岗岩 (如苏州花岗岩和澳大利亚Lachlan褶皱带A型花岗岩 )很相似。

Nankunshan aluminous A_type granite occurs in Longkou County of Guangdong Province, with a outcrop area of about 200 km 2. With medium_grained texture, the granite is composed of protolithionite, albite plagioclase, perthite and quartz. The rock usually is weakly peralumious and subalkaline with high alkali ratio range and NK/A values. The granite contains markedly low MgO and CaO and extremely high (FeO+Fe 2O 3) with a high FeO */(FeO *+MgO) ratio. Nankunshan granite is evidently rich in F and poor in Cl, having a high F/Cl ratio range. Based on its high Zr contents, the calculated zircon saturation temperatures of the rock are relatively high (810～847℃), consistent with those of A_type granites in Australia (839℃,on average). In terms of trace element data, the granite has high 10 4 Ga/Al and (Zr+Nb+Ce+Y) values. These values are higher than the low_limited values of 2.6 and 350(×10 -6 ) for aluminous A_type granites in Lachlan Fold Belt of Australia respectively. The rock has higher total REE contents and strongly Eu negative anomalies, showing a sea_gull shaped REE partition patterns on its chondrite_normalized diagrams, and Rb, U, Th, Nb, Ta, Zr, Hf, Sm elements show evidently positive anomalies on the primitive mantle_normalized trace element spider diagrams. The granite possesses relatively high ε Nd ( t ) values (-1.54～-2.87) and low δ 18 O Values (7.9‰～9.8‰), indicating that its source rocks are located in the interaction belts between the mantle and the crust. Mantle component accounts for about 63%～69% in its total source materials, as calculated by the mixing model of the mantle and the crust end members on the basis of the isotopic data. These features are obviously similar to those of aluminous A_type granites elsewhere in the world. The discovery is important for study on the extensional structure of late Yanshanian crust in eastern part of Nanling Mountain.