摘要
对燕山褶断带内三叠纪都山花岗质岩基进行了详细野外观察,测量了岩体变形的构造要素,获得了锆石U-Pb定年、全岩主微量元素和锆石Lu-Hf同位素分析数据,用以揭示都山复式岩基的成因及构造背景。都山复式岩基包括南部变形较强的都山岩体和北部弱变形的大石柱岩体,锆石LA-ICP-MS分析表明都山岩体和大石柱岩体侵位时代在误差范围内一致,大石柱岩体并非晚二叠世岩体,而是与都山岩体近于同期侵位。都山复式岩基主要由花岗岩、石英二长岩及少量闪长岩组成,岩石具有高SiO2含量,低MgO含量的高钾钙碱性系列岩石,富集大离子亲石元素（如K、Ba、Sr）和轻稀土元素,亏损高场强元素（如Ti、Nb、Ta）和重稀土元素,并表现出埃达克质岩石的特征。锆石εHf（t）值介于-13.69~3.94。主微量元素和Hf同位素组成表明,都山复式岩基岩浆源区包括古老下地壳以及少量富集岩石圈地幔和亏损的软流圈地幔,母岩浆可能经历了岩浆混合。都山复式岩基南部岩体内片麻理具有环状分布特征,并且从岩体中心到边缘逐渐增强,岩体边部包裹大量片麻岩围岩团块。这些特征表明,岩体可能是在区域挤压构造应力场之下通过顶蚀-膨胀的方式主动侵位的。结合区域地质背景,都山花岗质复式岩基可能是在华北克拉通北缘中—晚三叠世岩石圈拆沉和减薄之后的板内伸展构造背景下软流圈上涌引起岩石圈地幔熔融底侵古老下地壳后形成的。中—晚三叠世之交,华北北缘进入板内伸展构造体制,岩石学证据表明,岩石圈经历了破坏和减薄,而上地壳岩石仍然记录了挤压构造变形。因此华北北缘中生代早期构造体制在时间和深度层次上存在解耦,岩石圈伸展的浅部响应存在明显的滞后性。
Detailed field investigation and deformation analyses,in-situ zircon U-Pb and Lu-Hf analysis,and the major and trace element analyses were performed for the Dushan composite batholith in the Yanshan fold and thrust belt to reveal its petrogenesis and tectonic setting.The Dushan composite batholith consists of the southern strong deformed Dushan pluton and the northern undeformed Dashizhu pluton.Zircons LA-ICP-MS U-Pb dating results indicate that the Dushan pluton and the Dashizhu pluton have the same emplacement ages within the limit of error.The Dashizhu pluton is not a late Permian intrusion as previously believed,but synchronous with the Dushan pluton.The Dushan composite batholith consists of granite,quartz monzonite and minor diorite,and is characterized by high concentrations of SiO2,K2O＋Na2O,LILEs（e.g.,K,Ba,Sr）and LREEs,and low concentrations of MgO,HFSEs（e.g.,Ti,Nb,Ta）and HREEs.ZirconεHf（t）falls in between-13.69and 3.94.Whole rock geochemistry and zircon Hf isotopic compositions indicate that the Dushan composite batholith was mainly derived from partial melting of the ancient lower crust of the North China Craton（NCC）,with minor involvement of enriched sub-continental lithospheric mantle and depleted asthenospheric mantle,and the parental magma may have undergone magma mixing.Field mapping shows that the Dushan pluton displays well-developed concentric magmatic foliations and the intensity of magmatic foliation tends to increase towards the external margin of the pluton;host gneiss rocks are widespread as enclaves in the margin of the pluton.All these field evidences support that the Dushan pluton was formed by stoping and expansion in a contraction tectonic stress field during its emplacement.Based on regional geology,the Dushan batholith were likely generated by mixing of magmas from upwelling of asthenosphere-induced mantle melting and ancient lower continental crust melting induced by underplating of basaltic magmas in an intraplate extensional setting and related to lithospheric delamination and thinning of the northern part of the NCC.During the Middle-Late Triassic period,the northern part of the NCC entered into an intraplate extensional setting manifested by lithosphere destruction,thinning and asthenosphere upwelling as shown by magmatic petrogenesis;however,the shallow crust still recorded the contraction deformation.Therefore,the tectonic setting of the northern margin of the NCC during the Early Mesozoic shows the decoupling in the temporal and spatial scale,and the extension in the upper crust level resulting from lithospheric thinning and destruction was likely much later than that in the lower lithosphere mantle level.
出处
《地学前缘》
EI
CAS
CSCD
北大核心
2014年第4期275-292,共18页
Earth Science Frontiers
基金
国家"深部探测技术与实验研究"专项(SinoProbe-08-01-03)
国家自然科学基金项目(40972149)
关键词
燕山褶断带
都山
华北北缘
中生代早期
Yanshan fold and thrust belt
Dushan
northern margin of the North China Craton
Early Mesozoic time
