兴蒙造山带晚古生代伸展减薄过程:来自内蒙林西地区岩体的地球化学证据

The Late Paleozoic extending and thinning processes of the Xing'an-Mongolia orogenic belt:Geochemical evidence from the plutons in Linxi area,Inner Mongolia

内蒙古林西县双井地区出露一套含暗色包体的花岗质岩体,位于双井片岩和房框子单元花岗质片麻岩接触带,该岩体对厘定古亚洲洋演化阶段、西伯利亚板块和阿穆尔板块最终碰撞缝合时限具有重要意义。本文对该岩体进行了详细的岩相学、岩石地球化学以及锆石U-Pb年代学研究,讨论了该岩体成因、岩浆源区以及研究区造山带地壳演化等问题。岩体中暗色包体和周边花岗岩大部分锆石记录了-285Ma和-270Ma两期年龄,少量锆石记录了-320Ma的较老年龄,表明该区域在晚石炭世就存在岩浆活动,并在-285Ma、-270Ma更为显著,且岩浆活动具有幕式发生的特点,通过锆石的Ti温度计算得岩浆形成温度为-670℃。暗色包体相较周边花岗岩部分更富Al_(2)O_(3)(>13.96%)、CaO(>1.61%)、Na_(2)O(>4.61%)以及Fe_(2)O_(3)(>3.67%)、MgO(>0.62%)、TiO_(2)(>0.28%)、P_(2)O_(5)(>0.08%)等,但贫K2O(<2.10%),但二者发育有一致的变形现象,具有相同的侵位时代(误差范围内一致)、相似的微量元素配分模式、相近的Sr-Nd-Hf同位素组成,指示二者可能为同源岩浆不同演化阶段的产物。此外,暗色包体样品中,Zr/Hf比值介于31.74-37.08之间(接近原始地幔的Zr/Hf比值36.25),黑云母有壳源和壳幔混合源两种成因类型,ISr值介于0.7039-0.7055之间,εNd(t)值介于-7.30--5.87之间,锆石的εHf(t)介于-3.58-6.72之间等地球化学特征,均指示暗色包体的母岩浆在形成过程中有幔源物质的加入,以上特征显示岩体为壳幔混合源岩浆经历不同演化阶段后的产物。岩体中锆石Eu/Eu*与地壳厚度的拟合结果显示研究区在ca.290-270Ma地壳经历了一次显著的减薄过程(ca.50-35km),为区域存在地幔物质上涌提供了动力学证据。结合前人资料,本次研究的含暗色包体的花岗质岩体属于大石寨期,其母岩浆的形成、侵位时代,以及壳幔混合源等地球化学特征精细地约束了该区大石寨期岩浆活动的发育时间及过程,为研究西拉木伦缝合带晚古生代构造演化提供了新证据,同时也清晰地指示出古亚洲洋在晚古生代末期并不处于大洋俯冲阶段,而是造山后的伸展裂解阶段。

A set of granitic plutons with enclaves is exposed in Shuangjing area,Linxi County,Inner Mongolia,which is located in the contact zone between Shuangjing schist and Fangkuangzi Unit granitic gneiss.It is of great significance to determine the evolution stages of the Paleo-Asian Ocean and the final collision and suture time of the North China Craton and Siberia Craton.This paper discusses the petrogenesis,magma sources of the plutons and crustal evolution of the orogenic belt based on their petrography,geochemical characteristics and zircon U-Pb geochronology.Most zircons of the enclaves and surrounding granites recorded two ages of-285Ma and-270Ma,and a small number of zircons recorded an older age of-320Ma,indicating that magmatism existed in the area in the Late Carboniferous,and was more significant at-285Ma and-270Ma,and the magmatism occurred episodically.The data of zircon thermometers show their crystallization temperature was-670℃.The enclaves are richer in Al_(2)O_(3)(>13.96%),CaO(>1.61%),Na_(2)O(>4.61%)and Fe_(2)O_(3)(>3.67%),MgO(>0.62%),TiO_(2)(>0.28%),P_(2)O_(5)(>0.08%)than surrounding granitic domain,but poor in K2O(<2.10%),but they have the same deformation phenomenon,the same emplacement age(consistent within the range of error),similar trace element distribution pattern and the similar Sr-Nd-Hf isotopic compositions indicate that they may be the products of the same magma in different evolution stages.In addition,the results show that the Zr/Hf ratios of the enclave samples are 31.74-37.08,which is close to the Zr/Hf ratios(36.25)of the primitive mantle;the biotite in the enclaves has two genetic types:crustal origin and mixed crust-mantle derived source;the enclaves Isr range from 0.7039 to 0.7055,εNd(t)range from-7.30 to-5.87,εHf(t)in zircons range from-3.58 to 6.72,all of the above may indicate that the contribution of mantle derived materials,and the granitic rock is the product of crust-mantle derived magma after different evolution stages.Combined with previous research data,the granitic pluton with the enclaves in this paper belongs to the Dashizhai phase,the formation and emplacement age of its parent magma and the crust-mantle derived finely constrain the development time and process of the Dashizhai magmatic activity in this area,which provides new evidence for the study of the Late Paleozoic tectonic evolution of the Xar Moron suture zone,and it indicates that the Paleo-Asian Ocean was in the post-orogenic stage of extension,rather than oceanic subduction stage at the end of the Late Paleozoic.