早白垩世华北克拉通东部岩石圈减薄过程和机制:来自辽宁本溪北大山岩体的证据  

Processes and mechanism of lithospheric thinning in the eastern North China Craton during the Early Cretaceous:Evidence from the Beidashan pluton,Liaoning Province

作　　者：骆念岗 高莲凤 张振国 尹志刚[1] 崔建宇 吴君飞 邢杰 丁恺 高晨阳 王月 LUO Niangang;GAO Lianfeng;ZHANG Zhenguo;YIN Zhigang;CUI Jianyu;WU Junfei;XING Jie;DING Kai;GAO Chenyang;WANG Yue(College of Mining Engineering,Liaoning Technical University,Fuxin 123000,China;Liaoning Provincial Geological and Mineral Survey Institute Co.,Ltd.,Shenyang 110031,China;Inner Mongolia Pingzhuang Coal Industry(Group)Co.,Ltd.,Chifeng 204000,China)

机构地区：[1]辽宁工程技术大学矿业学院,辽宁阜新123000 [2]辽宁省地质矿产调查院有限责任公司,辽宁沈阳110031 [3]内蒙古平庄煤业(集团)有限责任公司,内蒙古赤峰204000

出　　处：《地学前缘》2023年第3期340-365,共26页Earth Science Frontiers

基　　金：国家自然科学基金项目(41972004);辽宁工程技术大学学科创新团队资助项目(LNTU20TD-14)。

摘　　要：早白垩世华北克拉通岩石圈经历了巨厚减薄事件,但减薄过程及机制仍存在较大争论。本文对位于华北克拉通东部辽宁本溪地区的北大山岩体进行了野外地质调查、岩相学、地球化学、锆石U-Pb定年和Lu-Hf同位素研究,探讨其形成时代、源区和成因,以期得到关于华北克拉通东部早白垩世岩石圈减薄研究的启示。北大山岩体岩性为似斑状黑云二长花岗岩,锆石LA-ICP-MS U-Pb同位素测年结果显示,花岗岩的^(206)Pb/^(238)U加权平均年龄为(117.1±1.4)Ma,表明为早白垩世岩浆活动的产物。北大山岩体似斑状黑云二长花岗岩具有富硅(w(SiO_(2))=69.15%~69.87%)、富碱(w(Na_(2)O+K_(2)O)=8.46%~8.66%)、贫镁(w(MgO)=0.80%~0.90%)、贫钙(w(CaO)=2.03%~2.04%),准铝质(A/CNK=0.98~0.99)的特征;富集Rb、Th、K、Ba等大离子亲石元素,相对亏损Nb、Ta、Ti、P等高场强元素,稀土元素配分曲线为右倾型,轻稀土较陡,重稀土较平缓,花岗岩熔体的锆石饱和温度(763.14~768.87℃)明显低于A型花岗岩(>800℃),以上特征总体表现出高钾钙碱性I型花岗岩特征。锆石Lu-Hf同位素分析表明,锆石εHf(t)值为-21.49~-19.04,均为负值,二阶段亏损地幔Hf模式年龄(t DM2)=2536~2383 Ma,表明其源岩主要为古元古代古老下地壳基性岩的部分熔融。根据年代学和岩石地球化学研究认为,北大山岩体形成于早白垩世时期古太平洋板块俯冲回撤而导致的区域伸展背景环境下,同时,上涌的软流圈引起华北克拉通深部岩石圈发生大规模的拆沉作用,从而致使华北克拉通岩石圈减薄。The North China Craton(NCC)experienced pervasive lithospheric thinning in the Early Cretaceous,but the thinning process and thinning mechanism are still controversial.To shine a new light on the thinning events this paper focuses on the Beidashan Pluton of Liaoning Province,eastern NCC to conduct detailed field geological survey,combined with laboratory research including petrographical,geochemical,U-Pb zircon chronological and Lu-Hf isotopic analyses,so as to explore the formation age,source area and petrogenesis of the Beidashan pluton.The lithology of the Beidashan pluton is porphyritic biotite monzogranite which has a weighted mean age of 117.1±1.4 Ma by zircon U-Pb dating.The quasi peraluminous porphyritic biotite monzogranite is rich in silicon(69.15%69.87%)and alkali(8.46%8.66%)and poor in magnesium(0.80%0.90%)and calcium(2.03%2.04%),and is enriched in large ion lithophile elements(LILEs)(Rb,Th,K,Ba)and depleted in high field strength elements(HFSEs)(Nb,Ta,Ti).The rare earth element(REE)pattern is right-leaning,showing a steep linear slope for light REE(LREE)and flattening of heavy REE(HREE).The zircon saturation temperature of granite melt(763.14768.87℃)is clearly lower than that of A-type granite(>800℃).The above characteristics generally represent high-k,calc-alkaline I-type granitoids.ZirconεHf(t)values range from-21.49 to-19.04,and the two-stage Hf model ages(t DM2)range between 25362383 Ma,indicating the monzogranite is mainly derived from partial melting of Paleproterozoic ancient basaltic crust.According to the chronological and petrogeochemical analyses,the Beidashan pluton was formed under regional extensional environment caused by slab roll-back of the paleo-Pacific plate during the Early Cretaceous,and the upwelling asthenosphere caused large-scale delamination of the deep lithosphere of the North China Craton resulting in lithospheric thinning.

关 键 词：似斑状黑云二长花岗岩 地球化学 年代学 LU-HF同位素 岩石圈减薄 北大山岩体 

分 类 号：P585[天文地球—岩石学] P586[天文地球—地质学] P597