【摘要】 甘肃北山大石山岩体主要由角闪花岗岩和二长花岗岩组成,岩石的化学成分特点为过铝质（Al₂O₃含量为 12.18%～13.37%）、富硅（SiO₂含量为74.22%～78.38%）、偏碱（Na₂O+K₂O 为7.60%～8.50%）,FeO^T/MgO 值高,CaO 和 MgO 含量低（分别为0.01%～0.99%和0.05%～1.15%）,富含 Y、Nb 等高场强元素（HFSE）以及 F、Sr、 Ba 和 REE(∑REE=102.3×10^-6～120.02×10^-6)低。这些特点与本区的 I 型和 S 型花岗岩有明显差别,是一种典型的 A₂型花岗岩。大石山岩体 Rb-Sr 等时线年龄为273 Ma,表明北山地区在海西晚期即进入造山后拉张裂解环境。更多还原

【Abstract】 The Dashishan intrusive in Beishan area of Gansu Province mainly consists of biotite potassium- feldspar granites.The granites of Dashishan are metaluminous（Al₂O₃=12.18%～13.37%）and are character- ized by high SiO₂（74.22%～78.38%）,Na₂O+K₂O（7.60%～8.50%）,FeO^T/MgO ratio,REE(102.3×10^-6 ～120.02×10^-6),Y,Nb and F,and low CaO（0.01%～0.99%）,MgO（0.05%～1.15%）,Sr and Ba.These characteristics are very different from characteristics of I-type and S-type granites in Beishan area,but quite simi- lar to A₂ type granites.The Dashishan intrusive has a Rb-Sr isochron age of 273 Ma.It is therefore suggested that crustal rifting might have started in late Hercynian in that area.更多还原