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

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。

The Wutongjin granitic gneisses is exposed in the Beishan area, Gansu Province . The Wutongjin granitic gneisses aremetaluminous （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 event might have started in the Eeishan area during Neoproterozoic.