鄂北大洪山晋宁期MORB-like玄武岩的识别与洋内俯冲作用  

The discrimination of Jinningian MORB-like basalt and intra-oceanic subduction in the Dahongshan area, Northern Hubei

作　　者：谢纪海 胡正祥 毛新武 孔令耀 杨青雄 杨成 郭盼 XIE Jihai;HU Zhengxiang;MAO Xinwu;KONG Lingyao;YANG Qingxiong;YANG Cheng;GUO Pan(Wuhan Geomatics Institute,Wuhan 430034,Hubei,China;Hubei Geological Survey,Wuhan 430034,Hubei,China)

机构地区：[1]武汉测绘研究院,湖北武汉430034 [2]湖北省地质调查院,湖北武汉430034

出　　处：《中国地质》2019年第6期1496-1511,共16页Geology in China

基　　金：湖北省地质局科技项目“扬子陆块北缘（湖北段）基底组成、结构、演化及成矿作用研究”（KJ2019-2）;中国地质调查局项目“湖北1∶5万长岗店、均川、客店坡、古城畈、三阳店幅区域地质调查”（12120113012800）联合资助~~

摘　　要：鄂北随州大洪山地区出露大量镁铁质岩(如:辉长岩、辉绿岩、(枕状)玄武岩),它们主要以岩块的形式构造混杂在一套碎屑岩中,表现为典型造山带基质-岩块混杂的特征。大洪山镁铁质岩为拉斑玄武岩系列岩石组合,地球化学方面,不相容元素Rb、Ba、K、Th、U富集,高场强元素Nb、Ta亏损,表现为岛弧玄武岩的特点,而平坦的稀土配分模式(ΣLREE/ΣHREE=1.41~4.48,LaN/YbN=0.76~4.79),Zr/Y=2.65~5.38,Ti/V=29.19~54.97,又可与洋中脊玄武岩对比。因此,我们推测大洪山镁铁质岩属于MORB-like玄武岩(或前弧玄武岩)类岩石组合,其形成于洋内初始俯冲环境,成岩岩浆由俯冲洋板片脱水交代亏损洋中脊地幔减压熔融产生。通过LA-ICP-MS锆石U-Pb测年,分别获得南风垭、绿林寨玄武岩(816.6±7.6)Ma(MSWD=0.47)、(813.1±4.8)Ma(MSWD=0.37)的成岩年龄,结合已经取得的杨家棚辉长岩947 Ma、厂河枕状玄武岩824 Ma、绿林辉绿岩820 Ma的年龄结果,说明大洪山地区的这套前弧镁铁质岩组合大致形成于817~947 Ma,它们可能是多阶段洋内俯冲的产物。大洪山地区这套前弧镁铁质岩的厘定说明扬子地块与桐柏-大别地块之间晋宁期发生过一定规模的洋内-洋陆俯冲和造山运动,二者可能曾在青白口纪晚期拼合到一起。There are numerous mafic rocks e.g.,gabbro,diabase,basalt,pillow basalt,fumarolic-amygdaloidal basalt,in the Dahongshan area,Suizhou City,northern Hubei Province.They are mainly in the form of block structurally mixed in a set of clastic rock,characterized by mélange of exotic blocks and matrix strata,suggesting a typical orogenic belt.The mafic rocks from Dahongshan area show the features of tholeiite series,and are geochemically enriched in incompatible elements such as Rb,Ba,K,Th and U and depleted in high field strength elements such as Nb and Ta,similar to features of island arc basalts.Nevertheless,the features of flat REE patterns(ΣLREE/ΣHREE=1.41-4.48,LaN/YbN=0.76-4.79,Zr/Y=2.65-5.38 and Ti/V=29.19-54.97)are the same as features of mid-ocean ridge basalt.Therefore,the geochemical signatures and regional geological characteristics show that these mafic rocks should be part of MORB-like/fore-arc basalts,formed along intra-ocean arc where the subduction-initiation happened.Their parent magma was produced by the nascent depleted MORB mantle and interacted with the contribution of fluids from the slab sinking plate with decompression melting.The basalts from Nanfengya and Lulinzhai yielded LA-ICP-MS U-Pb zircon ages of(816.6±7.6)Ma(MSWD=0.47)and(813.1±4.8)Ma(MSWD=0.37)respectively,interpreted as their crystallization age.Combined with the previous research results of gabbro in Yangjiapeng(947 Ma),pillow basalt in Changhe(824 Ma),and diabase in Luling(820 Ma),it is held that mass mafic rocks were formed in Jinningian period(817-947 Ma)in the Dahongshan area.They may be the products of multi-stage intra-ocean subduction.The discrimination of Jinningian ore-arc/MORB-like basalt in the Dahongshan suggests that it experienced a certain scale of ocean-ocean to ocean-continent subduction and orogeny between Yangtze block and Tongbai-Dabie block in Jinningian period,and the two blocks might have been aggregated together in late Qingbaikou period.

关 键 词：鄂北大洪山 晋宁期 地球化学 锆石U-PB测年 MORB-like玄武岩 洋内弧 俯冲造山作用 地质调查工程 

分 类 号：P588.145[天文地球—岩石学] P542.4[天文地球—地质学]