摘要
阿拉善地块西南缘在中志留世—早泥盆世究竟是处于古亚洲洋俯冲的构造环境,还是因受控于祁连造山带而处于后碰撞伸展环境?这是探讨该区早古生代构造演化不可回避的重要科学问题。阿拉善地块西南缘出露的早古生代岩浆岩是探讨该问题的重要研究对象,但前人的工作集中于北大山和龙首山的东段。本文在龙首山西段及更西部的合黎山地区选择三个花岗岩体进行了LA-ICP-MS锆石U-Pb定年和元素及同位素地球化学研究。结果表明,龙首山西段的大佛寺碱性长石花岗岩形成于晚志留世(426±2 Ma),主量元素具有高SiO2(78.03%)、富碱(ALK=7.96%)、低CaO(0.60%)、贫MgO(0.13%)的特征,稀土分布呈明显的"V"型,大离子亲石元素(LILEs)高度富集而高场强元素(HFSEs)中等富集,Ba、Sr、Eu负异常显著,属于铝质A2型花岗岩。合黎山地区的五坝黑云母花岗岩和张家窑花岗闪长岩分别形成于晚志留世(421±2 Ma)和早泥盆世(406±4 Ma),都属于高钾钙碱性I型花岗岩,轻、重稀土分异((La/Yb)N=10.69~13.79),Eu负异常显著(δEu=0.56~0.77),LILEs富集,HFSEs亏损,Nb、Ta负异常和Zr、Hf正异常同时出现。此外,三个岩体的岩浆源区均显示富集型Sr-Nd同位素特征,具有中元古代的二阶段Nd模式年龄,暗示中元古代地壳物质在不同温度下的部分熔融。结合区域资料,阿拉善地块西南缘中志留世—早泥盆世具有钙碱性—高钾钙碱性I型花岗岩、铝质A2型花岗岩和碱性—过碱性花岗岩的岩石组合,指示后碰撞伸展环境,而该区早古生代俯冲阶段(中奥陶世—早志留世;ca.461~441 Ma)和后碰撞阶段(中志留世—早泥盆世;ca.432~397 Ma)的划分与祁连造山带的构造演化是可对比的。
The Alxa Block is located between the Central Asian Orogenic Belt to the north and the Qilian Orogenic Belt to the southwest. However, the Early Paleozoic tectonic setting of its southwestern margin has long been debated. Researchers who studied the Early Paleozoic plutons in the eastern Beidashan believe that the subduction of Paleo-AsianOcean had reached the southwestern Alxa Block. Others have proposed the post-collision extensional setting for this area based on the Early Paleozoic magmatic rocks in the eastern Longshoushan. It is noteworthy that there is no large fault between the eastern Beidashan and Longshoushan, and the distances between the western Longshoushan and the two orogenic belts are the same as those for the eastern Beidashan and Longshoushan, indicating same tectonic setting. In order to discuss the Early Paleozoic tectonic setting of the western Longshoushan, the Dafosi alkali-feldspar granite, the Wuba biotite granite and the Zhangjiayao granodiorite were sampled for LA-ICP-MS zircon U-Pb dating and element and isotope geochemical analyses. The results indicate that the Dafosi alkali-feldspar granite was formed during Late Silusian(426±2 Ma), and its major elements are characterized by high SiO2(78.03%) and total alkali(7.96%), low CaO(0.60%) and MgO(0.13%), with prominent V-type rare earth element pattern and negative Ba, Sr, and Eu anomalies, belonging to aluminous A2-type granite. The Wuba biotite granite and the Zhangjiayao granodiorite were formed during the Late Silurian(421±2 Ma) and the Early Devonian(406±4 Ma), respectively, and both are high K calc-alkaline I-type granites. They have enriched LREEs(La/YbN=10.69~13.79), depleted Eu(δEu=0.56~0.77), and simultaneously exhibit apparently negative Nb-Ta anomalies and positive Zr-Hf anomalies.Moreover, all the three plutons have enriched Sr-Nd isotopic features and Mesoproterozoic two-stage Nd model ages. Therefore, they may have originated from the partial melting of Mesoproterozoic crust and indicate post-collisional tectonic setti
作者
王增振
陈宣华
邵兆刚
李冰
丁伟翠
张义平
王永超
张垚垚
徐盛林
秦翔
WANG Zengzhen;CHEN Xuanhua;SHAO Zhaogang;LI Bing;DING Weicui;ZHANG Yiping;WANG Yongchao;ZHANG Yaoyao;XU Shenglin;QIN Xiang(Chinese Academy of Geological Sciences,Beijing,100037;Sino Probe Center,Chinese Academy of Geological Sciences and China Geological Survey,Beijing,100037)
出处
《地质学报》
EI
CAS
CSCD
北大核心
2020年第8期2243-2261,共19页
Acta Geologica Sinica
基金
中国地质科学院基本科研业务费项目(编号YWF201708)
公益性基础地质调查二级项目(编号DD20160083,DD20190011)联合资助的成果。
