湖北新春花岗岩类锆石SHRIMP年龄:大别山造山带内弱变质—未变质晋宁期花岗岩类的发现  

U-Pb SHRIMP Zircon Ages of the Qichun Granitoids, Hubei Province: Discovery of Neoproterozoic Weakly Metamorphosed-Unmetamorphosed Granitoids in the Dabie Mountains

作　　者：薛怀民[1] 刘敦一[1] 董树文[2] 简平[1] 刘晓春[3] 

机构地区：[1]中国地质科学院地质研究所,北京100037 [2]中国地质科学院,北京100037 [3]中国地质科学院地质力学研究所,北京100081

出　　处：《地质学报》2004年第1期81-88,共8页Acta Geologica Sinica

基　　金：国土资源部科技中国大陆科学钻探工程综合研究项目(编号2002207);国家重点基础研究发展规划项目(编号TG1999075505);中国地质调查局地调项目(编号200013000169;150C002004);中国地质科学院地质研究所大陆动力学实验室资助

摘　　要：蕲春花岗质杂岩体包括斑状二长花岗岩和花岗岩两部分,它们之间在化学性质上存在着很大的差异,前者表现为高Al_2O_3(15.73％)、相对高CaO(2.46％)、Na_2O含量明显高于K_2O(Na_2O/K_2O=1.27),尤以强烈亏损重稀土元素和极强的轻、重稀土元素分馏程度[(La/Yb)_N=46.8]为特征而类似于太古宙高Al_2O_3的TTG岩石。而后者则以较低的Al_2O_3含量(14.05％)、贫CaO(0.82％)、K_2O含量明显高于Na_2O(Na_2O/K_2O=0.81)为特征,轻、重稀土元素的分馏程度[(La/Yb)_N=10.89]也较片麻状二长花岗岩中弱得多。两类岩石中锆石的U-PbSHRIMP年龄分别为824.6±17.6 Ma和784±20 Ma,该时代与大别山造山带内花岗片麻岩的原岩形成年龄类似。大别山造山带内弱变质-未变质晋宁期花岗岩的出现表明扬子板块印支期向北俯冲时,该花岗质杂岩处于俯冲板片的后缘,可代表造山带内扬子基底的原地露头。而岩体周围的高压变质杂岩应是折返上来的无根构造岩片,大别山造山带内高压-超高压变质杂岩的出露不是整体性抬升剥蚀的结果。The Qichun granitoids encompass two types: porphyritic monzonitic granite and granite, which are significantly different in geochemical quality. The porphyritic monzonitic granite is characterized by high A12O3 (15. 73%), relatively high CaO (2. 46%) and Na2O (Na2O/K2O=l. 27), particularly is characterized by strong depletion of HREE and strong fractionation between LREE and HREE [(La/Yb)N = 46. 8], similar to Archaean TTG gneiss. But the granite is characterized by relatively low A12O3 (14.05%), poor CaO (0.82%), higher K2O content compared to Na2O (Na2O/K2O=0. 81). The fractionation degree between LREE and HREE is far weaker in the granite than in the porphyritic monzonitic granite. The U-Pb SHRIMP Zircon ages are 824. 6± 17. 6 Ma and 784±20 Ma respectively in the two rock types. These ages are similar to the protolith ages of granitic gneisses in the Dabie erogenic belt. The existence of weakly rnetamorphosed-unmetamorphosed granitoids in the Dabie orogenic belt shows that the granitoids are situated on the back edge of the subducted plate, maybe the proto-outcrop of the Yangtze basement. And the high-pressure metamorphic rocks around the granitoid body are rootless tectonic microlithons formed by exhumation. The outcrop of high-ultrahigh metamorphic rocks in the Dabie Mountains is not the result of mass uplift and denudation.

关 键 词：造山带 SHRIMP年龄 稀土元素 锆石 花岗岩 大别山 

分 类 号：P597[天文地球—地球化学] P542.2[天文地球—地质学]