柴北缘早古生代埃达克质花岗岩锆石定年及其地质意义

摘要

The Tieshiguan rock mass,located at the west-central segment of the northern margin of Qaidam ba-sin,is composed of adakitic granite that was recently discovered in this area.The granite has high contents of SiO2 (71.36% to 73.70%),Al2 O3 (14.48% to 15.64%),and low content of MgO (0.44% to 0.71%);the trace elements display adakitic affinity,which are characterized by strong enrichment of Sr(>350 ×10 -6 ),de-pletion of Yb(0.67 ×10 -6 -1.58 ×10 -6 )and Y(6.74 ×10 -6 -18.02 ×10 -6 ),and high ratio of Sr/Y (21.32 to 72.49),enrichment of LREE and LILE,and depletion of HREE and HFSE.The Tieshiguan gra-nite has 206 Pb /238 U weighted mean age of (422.0 ±6.4)Ma,representing the magma formation age.Studies have shown that the Tieshiguan granite is most likely formed by re-melting thickened continental crust in post-collisional orogenic stage.%铁石观岩体位于柴达木盆地北缘中偏西段，是该地区新发现的具埃达克岩特点的花岗岩。岩石化学分析结果表明，铁石观岩体 SiO2含量变化于71.36％～73.70％，Al2 O3含量介于14.48％～15.64％之间，MgO 含量为0.44％～0.71％，w（Sr）一般＞350×10-6，w（Yb）变化于0.67×10-6～1.58×10-6，w（Y）介于6.74×10-6～18.02×10-6， w（Sr）/w（Y）＝21.32～72.49，富集 LREE 和大离子亲石元素，亏损 HREE 及高场强元素，与典型的埃达克岩相似。铁石观岩体锆石206 Pb /238 U 年龄加权平均值为（422.0±6.4）Ma，代表了岩浆形成年龄。研究表明，铁石观岩体形成于大陆后碰撞造山阶段，为加厚陆壳重熔形成。