柴达木盆地北缘黄绿山奥长花岗斑岩成因及其地球动力学意义

Petrogenesis and continental dynamic significance of Huanglüshan oligoclase granite porphyry in the northern margin of Qaidam Basin

黄绿山地区位于柴达木盆地北缘造山带西段,区内花岗岩广泛分布,相对匮乏同位素的研究成果以及岩浆活动和变质作用的地质记录。本文对前人在地质填图中认为原属于华力西期的奥长花岗斑岩侵入体进行岩相学、年代学、地球化学和全岩Sr-Nd同位素研究,探讨其成因类型和源区,并在此基础上讨论了其地球动力学意义。LA-ICP-MS锆石U-Pb定年结果显示,黄绿山奥长花岗斑岩结晶时代为466±3 Ma,为加里东期中奥陶世的产物。样品具有高SiO_(2)和Na_(2)O,低K_(2)O的特点;在SiO_(2)—K_(2)O图解上,除一个样品落入钙碱性区域之外,其余样品全部落入低钾拉斑系列。样品具有右倾的稀土分配特点((La/Yb)N介于4.41~10.01),具有微弱的Eu异常(0.94~1.09)。样品具有较低的10000Ga/Al(<2.6)以及极低的P_(2)O_(5)(平均为0.06%),说明黄绿山奥长花岗斑岩为Ⅰ型花岗岩;结合其中元古代的Nd二阶段模式年龄(1.03~1.21 Ga),认为黄绿山奥长花岗斑岩为中元古代下地壳再活化部分熔融的产物。在花岗岩构造图解上,样品落入VAG区域,显示其与洋壳俯冲具有密切的联系,结合其与奥陶系滩间山群的伴生关系和具有喜马拉雅型花岗岩的特征,因此认为黄绿山奥长花岗斑岩产于与洋壳俯冲相关的弧后盆地环境中;弧后盆地的存在暗示洋壳俯冲仍未结束,地体碰撞尚未开始,因此柴达木盆地北缘洋盆至少在466 Ma仍未关闭。

Objectives:Huanglüshan area is located in the western part of the orogenic belt of the northern margin of the Qaidam Basin.Granite is widely distributed in the area.It is relatively deficient in isotope research results and geological records of magmatic activity and metamorphism.Methods:In this paper,we carried out some researches of the oligoclase granite porphyry intrusion by petrography,geochronology,geochemistry and whole-rock Sr-Nd isotopes,and explores its genetic type and source area,and discuss its geodynamic significance on this basis.Results:The result of LA-ICP-MS zircon U-Pb dating indicate that the crystallization age of the Huanglüshan oligoclase granite porphyry is 466±3 Ma,which is the product of the Caledonian Middle Ordovician.The samples have the characteristics of high SiO_(2),Na_(2)O and low K_(2)O.On the SiO_(2)—K_(2)O diagram,except for one sample falling into the calc—alkaline region,the rest of the samples fall into the tholeiitic series.The results show enrichment in right-leaning rare earth elements,and((La/Yb)N range between 4.41~10.01).The samples have a low 10000Ga/Al(<2.6)and a very low P_(2)O_(5)(average 0.06%),indicating that the Huanglüshan oligoclase granite porphyry is type Ⅰ granite.Conclusions:Combined with Nd two-stage Nd model ages(1.03~1.21 Ga)of the Mesoproterozoic,the Huanglüshan oligoclase granite porphyry is considered to be the product of the partial melting of the lower crust during the Mesoproterozoic reactivation.In the granite structure diagram,the samples fall into the VAG area,which indicate that formation of these granites is closely related to the subduction of oceanic crust.Combined with its associated relationship with the Ordovician Tanjianshan Group and the characteristics of Himalayan type granite,this paper suggests that the Huanglüshan oligoclase granite porphyry be occurred in the back-arc basin environment related to oceanic crust subduction.The existence of back-arc basin implies that the subduction of oceanic crust was not over,and the terrane collision did not began.Therefore,the oceanic basin in the northern margin of Qaidam Basin was not closed at least 466 Ma.