阿拉善北大山早石炭世花岗斑岩年代学、地球化学及地质意义

Geochronology,geochemistry and geological significance of the Early Carboniferous granite porphyry from the Beidashan area,Alxa block

中亚造山带是世界上最大的增生型构造带之一,其形成与古亚洲洋关系密切,然而古亚洲洋的构造演化和闭合时限还存在争议。阿拉善地块位于中亚造山带的南侧,是揭示上述问题的重要区域。本文对阿拉善地块北大山次井子一处花岗斑岩体进行了岩相学、全岩地球化学、锆石U-Pb年代学和Hf同位素地球化学研究,探讨其岩石形成时代、成因及区域构造背景。次井子岩体SiO_(2)含量为75.04%~76.97%、MgO含量为0.15%~0.24%、TFe_(2)O_(3)含量为1.62%~1.76%,显示富硅、贫镁铁的特征。岩石相对富集轻稀土元素,亏损重稀土元素,Eu强烈亏损,Rb、Th、U、K元素富集,Ba、Sr、P、Ti元素亏损,具有A型花岗岩的特征。锆石U-Pb测年显示,花岗斑岩的侵位年龄为326.3±1.2 Ma。锆石的ε_(Hf)(t)值为-3.53~-1.24,二阶段模式年龄(T_(DM2))在1561~1414 Ma之间,表明岩体的源区主要为古老的地壳物质。结合前人研究,阿拉善地块石炭纪和二叠纪—早三叠世岩浆岩形成环境存在明显的转变,这可能与古亚洲洋的最终闭合有关。

The Central Asian Orogenic Belt is one of the largest accretive tectonic belts in the world and its formation is closely related to the tectonic evolution of the Paleo-Asian Ocean.However,the tectonic evolution and closure time of the paleo-Asian Ocean are still controversial.The Alxa block,located in the south of the Central Asian Orogenic Belt,is a key area to reveal the above problem.We report results of zircon U-Pb dating,Hf isotopic analysis,and whole-rock geochemical data for the Cijingzi granite porphyry in Beidashan area,Alxa block.Our aim is to decipher its age,genesis and tectonic setting.The contents of SiO_(2),MgO and TFe_(2)O_(3)in the Cijingzi granite porphyry are 75.04%~76.97%,0.15%~0.24%and 1.62%~1.76%,respectively,indicating that the granite porphyry is rich in silicon and poor in magnesium and iron.The Cijingzi granite porphyry is relatively enriched in LREE,deficient in HREE,strongly deficient in Eu,enriched in Rb,Th,U and K,and deficient in Ba,Sr,P and Ti.All these characteristics indicate that it is A-type granites.Zircon U-Pb age-dating shows that the emplacement age of the granite porphyry is 326.3±1.2 Ma.Theε_(Hf)(t)values of zircon range from-3.53 to-1.24,and the second-stage model ages(T_(DM2))range from 1561 Ma to1414 Ma.Zircon Hf isotope indicates that the source of granite porphyry is mainly old crustal materials.Combined with previous studies,there is an obvious transition in the formation environment between Carboniferous and Permian-Early Triassic magmatic rocks in the Alax block,which is most likely related to the final closure of the Paleo-Asian Ocean.