呼和浩特热隆伸展构造:华北“变质核杂岩”再认识  

Hohhot thermal upwelling-extensional structure:Recognition of“metamorphic core complexes”in North China

作　　者：张宇 董树文[1,2] 施炜[1] Yu Zhang;Shuwen Dong;Wei Shi(SinoProbe Lab,Chinese Academy of Geological Sciences,Beijing 100037,China;School of Earth Science and Engineering,Nanjing University,Nanjing 210023,China)

机构地区：[1]中国地质科学院,自然资源部深地科学与探测技术实验室,北京100037 [2]南京大学地球科学与工程学院,南京210023

出　　处：《科学通报》2024年第7期939-949,共11页Chinese Science Bulletin

基　　金：中国地质调查局地质调查项目(DD20230232);国家重点研发计划(2017YFC0601402);中国地质科学院基本科研业务费项目(JKY202202)资助。

摘　　要：对于华北克拉通岩石圈破坏,除了岩石圈厚度减薄和地幔性质改变外,最直接的标志是广泛出露的弥散状穹隆构造及伴随的韧性剪切带、低角度正断层和断陷盆地.这些伸展穹隆构造通常被解释为“变质核杂岩”(metamorphic core complex,MCC),但其组成结构、变形机制和深部过程等方面与北美西部盆岭省经典的变质核杂岩存在明显的区别.本文选择华北克拉通北缘的呼和浩特“变质核杂岩”进行剖析和再研究,在前人研究的基础上,聚焦穹隆核、韧性剪切带和低角度滑覆系统之间的时空关系,通过详细的地质填图、构造解析和年代学研究,将一个以简单剪切应变为主的穹型韧性剪切带(变形时间为152~141 Ma)从原伸展系统中剥离,从而揭示出一个以花岗岩为核的热隆-伸展构造.呼和浩特热隆-伸展构造分为3个结构单元.(1)花岗岩核:由早白垩世A型花岗岩(136~127 Ma)组成;(2)纯剪切应变带:分布在核部花岗岩周缘以纯剪切应变为主的高温(>500℃)变形带,面理发育(无线理),可能由核部岩体侵位时体积膨胀导致;(3)低角度滑覆系统:由韧性剪切带、微角砾岩带、低角度正断层和上叠断陷盆地组成,其变形时代为早白垩世(约134~130 Ma),运动学标志指示上部向南东的剪切特征,其演化过程和变形行为受控核部岩体侵位.通过对热隆-伸展构造的厘定,本研究提出热岩石圈背景下由岩浆上侵主导并伴生热变质变形的伸展机制,为进一步理解华北克拉通伸展动力学提供一个新思路.The North China Craton(NCC)underwent significant intracontinental tectonic shortening and crustal thickening in the Mid-Late Jurassic,followed by extensive magmatism,extensional dome generation,graben basins,and large-scale normal faulting in the Early Cretaceous.In the past 30 years,these extensional domes within the NCC have been interpreted as metamorphic core complexes(MCCs)in the North American Cordilleran.However,their structure,deformation mechanism and deep processes are evidently distinct from those in the Basin and Rang Province of North America.In this study,we investigated the Hohhot granite dome in the northern NCC and clarified the spatio-temporal relationship between the granitic core,ductile shear zone,and low-angle detachment system through geological mapping,structural analysis,and geochronology studies to unveil a thermal upwelling-extensional structure.The Hohhot granite dome is situated within the Daqingshan Jurassic orogen located in the eastern Yinshan tectonic belt.It is composed of a Late Jurassic(ca.152 Ma)adakitic coarse-grained monzonitic granite(K1ηγ53),an Early Cretaceous medium-grained monzonitic granite(K1ηγ53)(ca.136 Ma),and an Early Cretaceous fine-grained granite(K1γ53)(ca.127 Ma),distributed concentrically from outer to inner rings,respectively.The footwall granitic complex and metamorphic basement are separated from the hanging wall Early Cretaceous graben basin by an ENE-trending low-angle detachment faults on the southeastern foothill.Our structural mapping reveals a dome-shaped ductile shear zone(S1)with variable foliations around the Early Cretaceous granite core and regionally constant NW-SE lineations,leading to transformed the Late Jurassic adakitic granite and metamorphic basement into mylonite.Kinematic indicators,including S-C fabrics andσ-type feldspar porphyroclasts,suggest a top-tothe-SE shear.The results of kinematic vorticity indicate that the S1 is characterized by simple-shear-dominated shear.In addition,zircon U-Pb ages of granitic mylonite and post-kin

关 键 词：华北克拉通 早白垩世 热隆-伸展构造 岩浆底劈 低角度滑覆系统 

分 类 号：P588.3[天文地球—岩石学] P542[天文地球—地质学]