阿拉善地块北缘石炭-二叠纪火山-碎屑岩系的构造环境及其对中亚造山带中南段拼合时间的制约

The northern Alxa orogenic belt (NAOB) is the central-south segment of the Central Asian Orogenic Belt (CAOB), and is a key tectonic region to link the eastern and western segments of the southern CAOB. However, the NAOB is less studied, and great controversy has occurred on the final amalgamation time of the NAOB, from the time before Carboniferous, some time during the Permo-Carboniferous to the time after Permian. The Permo-Carboniferous volcano-clastic rocks as thick as thousands of meters in this area are pivotal to address the above scientific issue. Several typical Permo-Carboniferous cross-sections in the Quaganqulu to Suoguonao areas across the NAOB will be selected to study in this project. On the basis of detailed stratigraphic and petrologic studies, the zircon U-Pb high-precision dating work will be used to constrain the formation ages of the volcano-clastic rocks. Furthermore, the petrogeochemistry and zircon Hf-O isotopic tracers will be used to constrain their tectonic setting, and their spatio-temporal coupling relationships with the tectonic evolution of the Paleo-Asian Ocean. Based on the above analyses, a comprehensive analysis on the assemblages of different terranes and the geodynamic setting will be used to constrain the amalgamation time of the central-south CAOB. All the above can help to constrain the tectonic evolution of the NAOB and how it connects the eastern and western segments of the southern CAOB.

阿拉善地块北缘（造山带）处于中亚造山带的中南段，是连接中亚造山带南缘东、西两段的关键构造区。然而，该区研究程度相对薄弱，关于其最终拼合时间存在石炭纪前、石炭-二叠纪之间或二叠纪后等不同认识。该区发育千米级厚的石炭-二叠纪火山-碎屑岩系，能够为认识以上问题提供关键地质制约。本项目拟选取横跨研究区各构造分区的查干础鲁至索果淖等地的石炭-二叠系典型剖面开展工作。在详细的地层学、岩石学研究基础上，通过锆石U-Pb高精度定年限定该火山-碎屑岩系形成时代；通过岩石地球化学和锆石Hf-O同位素示踪等手段确定其形成的大地构造环境，限定其与古亚洲洋演化的时空耦合关系。以此为基础，综合分析研究区各地体拼合时代与动力学背景，限定中亚造山带中南段拼合时限，为认识该区构造演化及其与中亚造山带南缘东西段对比连接提供制约。

阿拉善地块北缘（造山带）处于中亚造山带的中南段，是连接中亚造山带南缘东、西两段的关键构造区。然而，该区研究程度相对薄弱，关于其最终拼合时间存在石炭纪前、石炭-二叠纪之间或二叠纪后等不同认识。本项目选取石炭-二叠纪火山-碎屑岩系为主要研究对象，通过地层对比和碎屑岩锆石年龄，结合前人研究，对珠斯楞、杭乌拉、乌力吉、索果淖等关键地区的石炭-二叠系沉积时代进行了限定。物源分析指出，珠斯楞–杭乌拉地区、乌力吉地区上石炭统碎屑岩主要来自前寒武纪至早古生代岩石，具有南北混合源区；全区二叠系碎屑岩石炭-二叠纪锆石年龄信号显著加强，且多具有相似的锆石峰值年龄。上石炭统碎屑岩锆石主要为前寒武纪和早古生代，只有少量接近沉积年龄的石炭纪锆石，其锆石年龄分布特征与伸展盆地相似；二叠纪碎屑岩继承了石炭纪碎屑岩的源区和构造环境，并受同期广泛的火山作用影响。进一步，本项目获得宝日敖包火山岩锆石U–Pb年龄为320.1±2.3 Ma，锆石εHf(t)值为+9.71–+13.00，TDM2为418–629 Ma；获得高必乌兰花岗岩锆石U–Pb年龄为304.7±1.4 Ma，锆石εHf(t)值为+2.84–+8.15，TDM2为717–1055 Ma；获得呼和包斯格火山岩锆石U–Pb年龄为279.5±1.3 Ma，锆石εHf(t)值为-4.71–+1.88，TDM2为1096–1512 Ma；查干塔塔火山岩具有U–Pb年龄278–280 Ma，本项目获得其锆石εHf(t)值为-11.1–-7.3，TDM2为1674–1913 Ma。以上岩体或火山岩绝大多数具有A型花岗岩和后碰撞花岗岩地球化学特征。综合以上分析，结合前人研究，认为该区至少从晚石炭世开始已经拼合，并进入碰撞后伸展阶段。阿拉善地块北缘经历了晚石炭世海侵和陆壳伸展、早中二叠世盆地快速充填和晚二叠世海水退出的构造-沉积演化历史。