阿拉善地区乌力吉花岗岩地球化学特征及地球动力学意义

Geochemical Characteristics of Wuligi Granite in Alashan Area and Geodynamic Significance

阿拉善地块属于中亚造山带中段的多板块结合部位,是研究古亚洲洋闭合过程及中亚造山带晚古生代演化的理想窗口。阿拉善地块北缘的乌力吉花岗岩处在宗乃山-沙拉扎山构造带中,与恩格尔乌苏蛇绿岩带和查干础鲁蛇绿岩带构成完整的沟弧盆体系。采用LA-ICP-MS-锆石-U-Pb-定年和岩石地球化学分析,对乌力吉岩体的岩石类型、年代学、源岩特征以及构造背景进行了研究,同时为古亚洲洋的拼合碰撞时限提供年代学约束。实验结果表明:样品岩石类型主要为碱性-钙碱性准铝质花岗岩和闪长岩;单颗粒锆石中获得两组相对独立的岩石年龄:(236.1±1.1)Ma和(248.6±0.8)Ma,表明岩体成岩时期主要为印支期早期且具有多期侵入的特征。乌力吉花岗岩源岩主要为I型花岗岩,源于地壳火山弧区和同碰撞区,表明由于洋壳俯冲作用,研究区在早三叠世内发育了岛弧花岗岩侵入体。随着俯冲作用深入,以恩格尔乌苏蛇绿岩带为代表的古亚洲洋在早三叠世-中三叠世内完成闭合碰撞,并在研究区形成同碰撞花岗岩,碰撞时间应早于(236.1±1.1)Ma。

The Alashan block belongs to the multi-plate joint of the middle section of the Central Asian orogenic belt,and is an ideal window for studying the closure process of the ancient Asian Ocean and the Late Paleozoic evolution of the Central Asian orogenic belt.Wuliji granite in the northern margin of alashan block is located in the zongnaishan-salazar mountain tectonic belt,together with engel usu ophiolite belt and charganchulu ophiolite belt,it forms a complete trench arc basin system.LA-ICP-MS-zircon-U-Pb dating and petrogeochemical analysis were used.The rock types,chronology,source rock characteristics and tectonic background of the Wuliji rock mass were studied.The results show that the rock types of the samples are mainly alkali-calc-alkaline quasi-aluminous granite and diorite.Two groups of relatively independent rock ages were obtained from single grain zircon:(236.1±1.1)Ma和(248.6±0.8)Ma,The results show that the diagenetic period of the rock mass is mainly early indosinian and has the characteristics of multiple intrusion.The source rocks of wuliji granites are mainly type I granites,which originate from the crustal volcanic arc area and synclinal collision area,It is concluded that island arc granite intrusions developed in the study area during the early Triassic due to oceanic subduction.With the deepening of subduction,the ancient Asian ocean,represented by the engel usu ophiolite belt,completed closed collision in the early Triassic to the middle Triassic,and formed syn-colliding granite in the study area,which should have collided earlier than(236.1±1.1)Ma.