甘肃马鬃山地区梧桐井片麻岩年代学的研究

Geochemical Characteristics and Tectonic Implications of Wutongjin Granitic Gneisses in the Mazongshan Area, Gansu Province

摘要: 甘肃北山梧桐井片麻岩体主要为花岗片麻岩,岩石化学成分特点为准铝质(Al₂O₃为11.45%～16.07%),富硅(SiO₂为61.03%～75.28%),偏碱(Na₂O+K₂O为6.74%～8.24%),TFeO/MgO值高,而CaO含量低(0.13%～5.33%);富含REE(ΣREE=102.17×10^-6～372.17×10^-6),Y、Nb等高场强元素(HFSE).这些特点与I型和S型花岗质岩石有明显的区别,是一种典型的A2型花岗岩.梧桐井片麻岩体U-Pb等时线年龄为558Ma,表明北山地区在新元古代末处于造山后拉张裂解环境.

Abstract: The Wutongjin granitic gneisses is exposed in the Beishan area, Gansu Province. The Wutongjin granitic gneisses arem etalum in ous(Al₂O₃=11.45%～16.07%) and characterized by high SiO₂ (61.03%～75.28%),Na₂O+K₂O (6.74%～8.24%) and TFeO/MgO radios, and high REE(102.17×10^-6～372.17×10^-6), Y, Nb, and low CaO (0.13%～5.53%)contents. The characteristics are obviously different from those observed from I-and S-type granites, but quite similar to A2 type granites. The Wutongjin granitic gneisses has given a U-Pb isochron age of 558 Ma. Therefore, it is suggested that a crustal rifting eventmight have started in the Beishan a readuring Neoproterozoic.