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  中国地质 2019, Vol. 46 Issue (6): 1496-1511  
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谢纪海, 胡正祥, 毛新武, 孔令耀, 杨青雄, 杨成, 郭盼. 2019. 鄂北大洪山晋宁期MORB-like玄武岩的识别与洋内俯冲作用[J]. 中国地质, 46(6): 1496-1511.  
Xie Jihai, Hu Zhengxiang, Mao Xinwu, Kong Lingyao, Yang Qingxiong, Yang Cheng, Guo Pan. 2019. The discrimination of Jinningian MORB-like basalt and intra-oceanic subduction in the Dahongshan area, Northern Hubei[J]. Geology in China, 46(6): 1496-1511. (in Chinese with English abstract).  

鄂北大洪山晋宁期MORB-like玄武岩的识别与洋内俯冲作用
谢纪海1, 胡正祥2, 毛新武2, 孔令耀2, 杨青雄2, 杨成2, 郭盼2    
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
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。
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    
中图分类号:P588.145;P542.4            文献标志码:A             文章编号:1000-3657(2019)06-1496-16
The discrimination of Jinningian MORB-like basalt and intra-oceanic subduction in the Dahongshan area, Northern Hubei
XIE Jihai1, HU Zhengxiang2, MAO Xinwu2, KONG Lingyao2, YANG Qingxiong2, YANG Cheng2, GUO Pan2    
1. Wuhan Geomatics Institute, Wuhan 430034, Hubei, China;
2. Hubei Geological Survey, Wuhan 430034, Hubei, China
Abstract: 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.
Key words: Dahongshan area    northern Hubei Province    Jinningian    geochemistry    zircons U-Pb dating    MORB-like basalt    intraocean arc    subduction orogeny    geological survey engineering    

1 引言

鄂北随南大洪山地区位于扬子地块与桐柏—大别地块之间,是研究扬子地块与桐柏—大别地块拼合、造山运动的关键地区之一(任纪舜等, 2017)。早期的1:20万、1:5万区域地质调查认为大洪山地区主要由中元古代打鼓石群稳定陆缘沉积和角度不整合之上的花山群碎屑-火山岩组成➊➋。在后来的研究中,董云鹏对花山群进行解体,将北部以玄武岩为主的基性火山岩定义为构造侵位的蛇绿混杂岩,称其为“花山蛇绿构造混杂岩”,并根据卷入地层的时代推测花山地区存在海西—早印支期初始小洋盆,该洋盆于中三叠世闭合(董云鹏等, 1998, 1999; Dong et al., 1999),并且长期以来,多数学者认为“花山蛇绿构造混杂岩”代表的缝合带向西与勉略缝合带相连(Lai et al., 1999; 董云鹏等, 2003; 张国伟等, 2003, 2004; Dong et al., 2015)。然而部分学者对混杂带内岩浆岩进行同位素年代学研究,不同测试方法获得的年龄结果差异较大(石玉若等, 2003, 2005a, b; 张宗清等, 2006; Shi et al., 2007),使得随南花山地区这套“混杂岩”一直缺少精确、统一的同位素年代学证据。

笔者近年对大洪山地区调查研究发现(图 1),大洪山地区主要由沿土门—小阜—园潭一线的岛弧侵入岩、火山-碎屑岩及南侧的增生杂岩组成,并且分别在岩浆弧和杂岩中获得大量青白口纪的锆石U-Pb同位素年龄,并初步推测杂岩中的镁铁质岩岩块为洋中脊蛇绿岩套的基性端元残块,大洪山地区可能存在一条不同于勉略带的晋宁期缝合带(胡正祥等, 2015a, b)。然而此认识与前人“勉略带东延”的观点有着较大的差异,因此需要更多可靠的地质资料和系统的分析研究来证实大洪山地区物质组合的真实形成环境和形成时代。本文以大洪山地区前南华纪物质建造为研究对象,重点分析其野外产出、地球化学及年代学特点,旨在查明大洪山晋宁期缝合带主要物质组成及成因,为探讨新元古代扬子北缘形成演化过程提供依据。

图 1 大洪山地区地质简图 (图中相关测年位置及数据参考Shi et al., 2007; 胡正祥等, 2015a, 2017; 廖明芳等, 2016; Xu et al., 2016; 陈超等, 2017a, b, 2018 Fig. 1 The geological sketch map of Dahongshan area (Locations and results of zircons U-Pb dating in the map after Shi et al., 2007; Hu Zhengxiang et al., 2015a, 2017; Liao Mingfang et al., 2016; Xu et al., 2016; Chen Chao et al., 2017a, b, 2018)
2 地质背景及岩石学特征

研究区位于秦岭造山带东段、扬子地块与南秦岭(桐柏—大别)地块的结合部位,北界为勉略—青峰—襄(樊)广(济)区域性深大断裂,南侧被南华系莲沱组角度不整合覆盖。区内主要出露一套中元古代—青白口纪的物质建造,少量震旦系、古生界、白垩系在后期构造运动中呈断块混入其中,平面上整体呈“透镜状”分别于北西和南东向尖灭(图 1)。结合前人的研究成果和最新野外地质调查资料,在这长、宽小于25 km×20 km区域内,笔者系统鉴别出了中元古代稳定陆缘沉积——打鼓石群、青白口纪活动大陆边缘岛弧火山岩—侵入岩组合和代表洋盆消亡的俯冲增生杂岩等三套主要物质(胡正祥等, 2015a)。南侧打鼓石群为一套以白云岩、砂岩、(泥)板岩为主的稳定大陆边缘沉积,具有较为稳定的层序和相对较弱的变形特征,其中的凝灰岩夹层SHRIMP锆石U- Pb年龄为约12 Ga(李怀坤等, 2016)。北侧主体为一套以基性为主,夹安山岩、英安岩、流纹岩及少量粉砂岩的岛弧火山-碎屑岩(Dong et al., 2006胡正祥等, 2017),笔者已分别在土门和仓头垭酸性火山岩中获得841 Ma、828 Ma的LA- ICP- MS锆石U- Pb年龄(胡正祥等, 2015a, 2017);其次,三里岗岩体也具有岛弧中—酸性侵入岩的特点,其形成时间应为858~876 Ma(Shi et al., 2007; 廖明芳等, 2016; Xu et al., 2016)。在稳定大陆边缘沉积和岩浆弧之间,主体为一套砂岩、板岩构成的碎屑岩组合,具有较强的变形特征,主要发育一组NNE倾向紧闭同斜(局部倒转)褶皱和逆冲断层(图 2a),共同指示SSW向的逆冲推覆作用。这套碎屑岩中发育大量岩性、规模、形态各异的岩块,包括砾岩岩块、硅泥岩岩块、白云岩岩块、辉长(绿)岩岩块、玄武岩岩块等,它们以构造混杂的形式呈断块夹在碎屑岩中(图 2bd),显示出典型造山带俯冲增生杂岩的特点。前期笔者曾报道绿林镇一带的角砾状玄武岩,其具有碱性、轻稀土元素和不相容元素强烈富集的洋岛玄武岩的特点,且用LAICP-MS法获得817 Ma的成岩年龄,推测为洋岛/海山环境下的产物(陈超等, 2017a)。除此之外,杂岩中还存在大量的镁铁质岩岩块(包括辉长岩、辉绿岩、玄武岩岩块),其岩性组合、岩石系列均与绿林洋岛玄武岩存在一定差异,本文主要讨论其形成环境、时代,镁铁质岩岩块具有如下岩石学特征。

图 2 大洪山俯冲增生杂岩及镁铁质岩野外特征 a—厂河砂岩中发育同斜倒转褶皱;b—南风垭玄武岩、紫红色硅泥质岩、白云岩混杂;c—关口垭粉砂质板岩逆冲到辉长岩之上,二者接触面上发育构造透镜体;d—关口垭绢云母板岩中夹基性火山岩岩块;e—罗家咀硅质条带白云岩中的辉绿岩脉,辉绿岩发生强片理化;f—罗家咀北气孔-杏仁状玄武岩;g—厂河枕状玄武岩;h—姚家冲西枕状玄武岩 Fig. 2 Field photos of the subduction accretionary complex and mafic rocks in Dahongshan a-Synclinal overturned fold developed in the sandstone in Changhe; b-Mélange in Nanfengya composed of basalt, fuchsia siliceous argillaceous rock and dolomite; c-Silty argillaceous thrust up to the gabbro, and tectonic lenses developed between them in the Guankouya; d-Basic volcanic rock mass mixed in the sericite slate in Guankouya; e-Strong foliated doleritic vein developed in the siliceous band dolomite in Luojiaju; f-Amygdaloidal basalts outcropped in the north of Luojiaju; g-Pillow basalts outcrop in Changhe; h-Pillow basalts outcrop in the west of Yaojiachong

辉长岩:灰绿色,变辉长结构,块状构造,主要由斜长石(60%)和辉石(35%)组成,二者自形程度相近,均呈半自形板状,粒度较大,0.8~1.5mm不等,基性斜长石发生较强蚀变,大部分被黏土矿物和钠黝帘石取代,仅保留其晶体轮廓,辉石有一定程度的绿泥石化、蛇纹石化,局部可见角闪石的反应边,岩石整体有较强的钛-磁铁矿化(图 2c图 3a)。

图 3 大洪山镁铁质岩镜下特征 a—关口垭辉长岩中辉石局部蚀变成透闪石、绿泥石,斜长石基本被粘土矿物交代(+);b—罗家咀辉绿岩中的辉绿结构,辉石基本被绿泥石取代(-);c—厂河东气孔-杏仁状玄武岩中的杏仁构造,充填物主要为方解石(+);d—厂河枕状玄武岩间粒结构(+);Am—角闪石,Aug—普通辉石,Cc—方解石,Chl—绿泥石,Pl—斜长石 Fig. 3 Photomicrographs of the mafic rocks in Dahongshan a-Pyroxenes altered into amphibole and chlorite locally, and plagioclases basically replaced by clay minerals in the gabbro in Guankouya (+); b-The diabasic structure in the diabase in Luojiaju, where pyroxene is basically replaced by chlorite (-); c-Almond texture of Changhe basalt, with the filling materials being mainly calcite (+); d-Pillow basalt of Changhe with intergranular texture (+); Am-Amphibole, Aug-Augite, Cc-Calcite, Chl-Chlorite, Pl-Plagioclase

辉绿岩:灰绿色,变辉绿结构,块状构造,主要由斜长石(65%)和辉石(25%)组成,斜长石呈板条状构成架状,表面绢云母化、碳酸盐化强烈,部分保留完整晶形,辉石分布于斜长石矿物间隙中,呈他形粒状,几乎全部被绿泥石、方解石微粒集合体取代,岩石中铁质矿物(10%)较多,以黄铁矿为主,可能有部分磁铁矿或钛铁矿(图 2e图 3b)。

气孔-杏仁状玄武岩:灰绿色、浅绿色,变间粒结构,气孔、杏仁构造,块状构造,主要由斜长石(50%)和基性玻璃质成分(40%)组成,含少量钛磁铁矿(8%)和方解石(2%),斜长石呈半自形板条状,粒径0.1~0. 3mm,部分斜长石发生黏土化,基性玻璃基本被鳞片状绿泥石取代,气孔内主要充填方解石(图 2f图 3c)。

枕状玄武岩:露头尺度上岩石暗绿色,隐晶质结构,枕状构造发育,单个“枕”大小15~40 cm不等,排列紧密,表现为海相火山岩的特点;镜下主要发育玻基交织结构,局部可见气孔、杏仁构造,主要由半自形长条状基性斜长石(62%)和他形粒状角闪石(25)组成,含少量玻璃质成分(10%)和磁铁矿(3%)(图 2gh图 3d)。

3 分析方法

全岩主量元素、微量元素和稀土元素分析均在武汉综合岩矿测试中心化学分析研究室完成。主量元素SiO2、TiO2、Al2O3、TFe2O3、MnO、MgO、CaO、Na2O、K2O、P2O5采用X荧光光谱法(XRF)在X荧光光谱仪(XRF- 1800)上测定,而FeO则是通过湿化学分析方法获得,微量元素和稀土元素分析采用电感耦合等离子质谱法(ICP-MS)在电感耦合等离子体质谱仪(X2)上测定。具体实验方法和步骤参考马天芳等(2011),分析结果见表 1,采用Geokit软件对实验数据的分析、处理(路远发, 2004)。

表 1 大洪山地区镁铁质岩主量元素(%)、微量元素和稀土元素(10-6)地球化学分析数据 Table 1 Mayor elements (%) and trace elements (10-6) compositions of the mafic rocks in Dahongshan area

两件锆石测年玄武岩样品PM02- 15- 1、PM406-54-1分别采自大洪山顶附近南风垭(GPS:112°59′18.796″E,31°28′53.214″N)和绿林寨景区内的公路边(GPS:113°6′36.908″E,31°21′15.246″N),样品重量均大于25 kg。单矿物锆石挑选在廊坊市诚信地质服务有限公司完成,透射光、反射光和阴极发光图像(CL)的拍摄在北京锆年领航科技有限公司完成。LA-ICP-MS锆石U-Th-Pb同位素和微量元素微区分析在湖北省地质试验测试中心完成,测试仪器采用美国Coherent Inc公司生产的GeoLasPro全自动版193 nm ArF准分子激光剥蚀系统(LA)和美国Agilent公司生产的7700X型电感耦合等离子质谱仪(ICP-MS)联用构成的激光剥蚀电感耦合等离子体质谱分析系统(LA-ICP-MS),激光束斑直径为32 μm,用He作为剥蚀物质的载气,哈佛大学标准锆石91500作为外标,29Si作为内标,GJ-1或者Plešovic为监控标样,具体操作详见周亮亮等, (2017),采用ICPMSDataCal(V7.2)软件对同位素比值数据进行处理(Liu et al., 2008),对实验测得的数据用ISOPLOT程序进行谐和图的绘制以及加权平均年龄计算(Ludwig, 2003杨绍等, 2018)。

4 岩石地球化学特征

大洪山地区镁铁质岩岩石手标本和镜下都可看到绿泥石化、碳酸盐化现象,其在后期构造改造、风化剥蚀过程中经历了一定程度的蚀变作用,这种蚀变对活动性元素有影响。因此,在进行相关解释和构造环境判别时,采用高场强元素(HFSE)Nb、Ta、Ti、Zr、Hf、Th和稀土元素(REE)等受蚀变作用影响较小的元素进行岩石分类、成因和形成环境的分析(Polat et al., 2003Hastie et al., 2007; Pearce, 2014)。用于本文分析的地球化学数据除南风垭和姚家冲12件为实测外,其余六里冲、杨家棚、周家湾、厂河、花山共41件样品数据均选自前人研究成果。

六里冲辉长岩、南风垭玄武岩、杨家棚玄武岩和辉长岩、姚家冲玄武岩和辉绿岩、周家湾玄武岩、厂河枕状玄武岩、花山玄武岩具有比较一致的地球化学特征,认为其应为同一构造环境下的产物。大洪山地区镁铁质岩SiO2含量为40.00%~54.45%,属于基性-中性岩范围,Na2O含量为0.07%~5.96%,K2O含量0.04%~4.15%,全碱(Na2O+K2O)含量为2.81%~6.05%,表现为亚碱性玄武岩系列的特点(Le Bas et al., 1986),在Zr/Ti-Nb/Y图中,大洪山地区镁铁质岩样品基本落在亚碱性玄武岩范围内(图 4a)。有着较高的Al2O3(12.49% ~18.52%)、CaO(1.08%~11.28%,平均8.79%)、TiO2(1.16%~3.55%)含量,MgO含量为3.61%~9.54%,全铁FeOT含量8.79% ~16.82%,FeOT/MgO比值为0.99~3.47,在SiO2-FeOT/MgO图解中,整体落在拉斑玄武岩系列中(图 4b)。少量样品SiO2含量大于52%,且有着较高的MgO含量,但在既定的SiO2含量下,同样相对较高的FeOT含量和FeOT/MgO比值使其区别于一般洋内弧(高)镁安山岩(邓晋福等, 2010)(图 4b)。

图 4 大洪山镁铁质岩Nb/Y-Zr/Ti图(a)(底图据Pearce, 2014)和FeOT/MgO-SiO2图(b)(底图据Miyashiro, 1974; 邓晋福等, 2010) LLC—六里冲辉长岩,数据来源于胡正祥等, 2015a;NFY—南风垭玄武岩;YJP—杨家棚玄武岩和辉长岩,数据来源于石玉若等, 2003, 2005b;YJC-姚家冲玄武岩和辉绿岩,ZJW—周家湾玄武岩,数据来源于董云鹏等, 2003;CH—厂河枕状玄武岩,数据来源于Deng et al., 2013;HS—花山玄武岩,数据来源于董云鹏等, 1999, 后文图片中代号与此图一致 Fig. 4 Nb/Y -Zr/Ti (a) (after Pearce, 2014) and FeOT/MgO-SiO2diagram (b)(after Miyashiro, 1974; Deng Jinfu et al., 2010)of the mafic rocks in Dahongshan LLC-Gabbros of Liulichong, data from Hu Zhengxiang et al., 2015a; NFY-Basalts of Nanfengya, YJP-Basalts and gabbros of Yangjiapeng, data from Shi Yuruo et al., 2003, 2005b; YJC-Basalts and diabases of Yangjiapeng, ZJW-Basalts of Zhoujiawan, data from Dong Yunpeng et al., 2003; CH-Pillow basalts of Changhe, data after Deng et al., 2013; HS-Basalts of Huashan, data from Dong et al., 1999. The Abbreviations in the figures below are coincident.

大洪山地区镁铁质岩总稀土元素含量(ΣREE)为31.81×10-6~203.02×10-6,平均87.35×10-6,有着较低ΣREE含量,但明显高出典型洋内弧高镁安山岩ΣREE含量(Mirdita高镁安山岩ΣREE平均含量7.19× 10-6,马里亚纳高镁安山岩ΣREE平均含量8.77 × 10-6)。ΣLREE/ΣHREE比值为1.41~4.48,(La/Yb)N= 0.76~4.79(平均2.67),有着较低的轻、重稀土分异度,在球粒陨石标准化的稀土配分图上表现为平坦的配分模式(图 5a)。δCe=0.75~1.39(平均0.99),基本无Ce异常,δEu=0.68~1.06(平均0.87),有轻微负Eu异常,说明存在斜长石的分离结晶。在N-MORB标准化微量元素蛛网图上(图 5b),六里冲辉长岩、南风垭玄武岩、杨家棚玄武岩和辉长岩、姚家冲玄武岩和辉绿岩、周家湾玄武岩、厂河枕状玄武岩、花山玄武岩具有较为一致的配分模式。都富集大离子亲石元素(LILE)Rb、Ba、K、Pb和高场强元素(HFSE)Th、U,而亏损高场强元素Nb、Ta,而这类型的微量元素配分模式一般与俯冲相关的环境相联系。

图 5 大洪山地区镁铁质岩球粒陨石标准化REE配分图(a,Sun et al., 1989)和N-MORB标准化蛛网图(b,Ishizuka et al., 2009) (图中均为平均值. Mirdita MORB-like和Mirdita Boninite分别代表阿尔卑斯-喜马拉雅造山带西段Mirdita地区的前弧玄武岩和高镁安山岩,数据来源于Dilek and Furnes, 2009a;Mariana MORB-like和Mariana Boninite分别代表西太平岩俯冲带马里亚纳前弧玄武岩和高镁安山岩,数据来源于Reagan et al., 2010) Fig. 5 Chondrite-normalized REE patterns (a, after Sun et al., 1989) and N-MORB-normalized spidergrams (b, after Ishizuka et al., 2009) of the mafic rocks in Dahongshan (Data in the picture stand for average data. Data of Mirdita MORB-like basalt and Mirdita Boninite after Dilek and Furnes, 2009a; Mariana MORBlike basalt and Mariana Boninite after Reagan et al., 2010)
5 LA-ICP-MS锆石U-Pb年龄

南风垭(PM02-15-1)、绿林寨(PM406-54-1)2件玄武岩测年样品中的锆石均呈自形-半自形短柱状,粒径整体较小,40~90μm不等,且整体而言,锆石振荡环带结构不明显,这些都符合玄武岩锆石结晶的特点(吴元保等, 2004),锆石U-Pb测年结果见表 2图 6

表 2 南风垭、绿林寨玄武岩LA-ICP-MS锆石U-Pb测年数据 Table 2 LA-ICP-MS zircon U-Pb isotopic data of the basalt from Nanfengya and Lulinzhai
图 6 南风垭、绿林寨玄武岩锆石阴极发光图和U-Pb年龄谐和图(白色短线代表 50 μm) Fig. 6 Cathodoluminescence images and concordia plot for the basalt from Nanfengya and Lulinzhai (White bats in the figures stand for 50 μm)

南风垭(PM02-15-1)玄武岩18个分析点中有2个分析点(No. 11、No. 12)谐和度小于90%,不参与年龄分析。其余16个分析点的Th含量35.1× 10-6~4263.8×10-6,U含量388.2×10-6~8636.7×10-6,Th/U比值为0.02~1.36,整体表现为岩浆锆石的特点(吴元保等, 2004),但部分锆石从阴极发光图像和Th/U比值可以看出,其后期发生过一定程度的变质和Pb丢失。分析点No. 13锆石颗粒较大,与玄武岩结晶条件不符,206Pb/238U年龄为1790 Ma,应是岩浆上升过程中的捕获锆石。其余分析点形成的谐和图下交点238 Ma(误差较大),与秦岭—大别造山带广泛发育的印支构造事件时间基本一致(王清晨等, 2002; Liu et al., 2015)。上交点附近5个分析点较为集中,206Pb/238U加权平均年龄为(816.6 ± 7.6) Ma (MSWD=0.47),代表南风垭玄武岩的形成年龄。

绿林寨(PM406-54-1)玄武岩30个分析点中有5个分析点(No. 09、No. 11、No. 15、No. 19、No. 22)因谐和度较低而舍弃。其余25个分析点的Th含量22.6×10-6~658.9×10-6,U含量38.5×10-6~803.9×10-6,Th/U比值在0.27~1.16,表现为岩浆锆石的特点(吴元保等, 2004)。分析点No. 06年龄为1756 Ma,应与南风垭玄武岩类似,同为岩浆上升过程中的捕获锆石。分析点No. 01、No. 04、No. 26、No. 27、No. 29年龄值分别为448 Ma、466 Ma、155 Ma、144 Ma、284 Ma,可能为锆石挑选过程中混入的年轻锆石,而分析点No. 02存在一定的Pb丢失,该6个分析点均不参与加权平均年龄计算。其余18个分析点在谐和图中较为集中,206Pb/238U加权平均年龄为(813.1 ± 4.8) Ma (MSWD=0.37),代表绿林寨玄武岩的形成年龄。

6 讨论 6.1 岩石成因

大洪山地区的镁铁质岩为一套拉斑玄武岩系列辉长(绿)岩-玄武岩组合,以岩块的形式混杂于一套碎屑岩基质中,具有典型造山带混杂岩的特点。高TiO2、Al2O3、FeOT,富集不相容元素、亏损高场强元素Nb、Ta的特点使之区别于正常洋中脊玄武岩(Sun et al., 1989);与典型俯冲带高镁安山岩(Boninite)相比,REE、Rb、Ba、Nb、Ta、Hf、Zr、Ti等微量元素几乎都高出一个数量级(Dilek et al., 2009bReagan et al., 2010)(图 5);而相对较低的轻-重稀土分异度(ΣLREE/ΣHREE=1.41~4.48)、Nb含量(1.66× 10-6~13.31×10-6,平均4.27×10-6)和Zr/Y比值(2.65~ 5.38,平均3.59)可区别于一般大陆溢流、裂谷玄武岩(严再飞等, 2010Mattash et al., 2013)。大洪山镁铁质岩Nb/Yb比0.56~4.59,Th/Yb比0.09~0.72,在Nb/Yb-Th/Yb图中,大部分样品投在与俯冲相关的洋内弧和大陆弧重叠且靠近洋内弧部位(图 7a);Ti/ V比值为29.19~54.97,与一般Ti/V比小于20洋内弧岩石组合存在一定的差异,与菲律宾海盆中的洋中脊玄武岩有较大的重合(图 7b);样品Zr/Nb比17.90~44.59,Nb/Th比2.31~35.03(平均5.69),在Zr/ Nb-Nb/Th图中,主要表现为与俯冲相关的岛弧玄武岩相似(图 7c);而Nb/Y-Zr/Y图解中,样品整体投在与地幔柱无关的岛弧和正常洋中脊玄武岩重合部位(图 7d)。

图 7 大洪山地区镁铁质岩环境判别图解 a—Th/Yb-Nb/Yb图(Pearce, 2014);b—V-Ti/1000图(Shervais, 1982; Ishizuka et al., 2014a);c—Zr/Nb-Nb/Th图(Condie, 2003; Velásquez et al., 2011);d-Nb/Y-Zr/Y图(Fitton et al., 1997; Condie, 2003);Troodos UPL-塞浦路斯上部枕状熔岩;Troodos LPL-塞浦路斯下部枕状熔岩;Bonin FAB、Mariana FAB—小笠原、马里亚纳前弧玄武岩,Philippine Sea MORB-菲律宾海盆洋中脊玄武岩;Pacific crust-太平洋洋壳;ARC—与岛弧相关的玄武岩;N—MORB—正常洋中脊玄武岩;OIB—洋岛玄武岩;OPB—洋底高原玄武岩,DM—亏损地幔;EN—富集组分;PM—原始地幔;REC—循环组分;UC—大陆上地壳;DEP—亏损地幔组分;HIMU—高U/Pb比值的地幔;EM1、EM2—富集地幔 Fig. 7 Tectonic discrimination diagram of the mafic rocks in the Dahongshan area a-Th/Yb-Nb/Yb diagram (after Pearce, 2014), b-V-Ti/1000 diagram (after Shervais, 1982; Ishizuka et al., 2014a, ) c-Zr/Nb-Nb/Th diagram (after Condie, 2003; Velásquez et al., 2011), d-Nb/Y - Zr/Y diagram (after Fitton et al., 1997; Condie, 2003). Troodos UPL-Upper pillow basalt of Troodos; Troodos LPL-Lower pillow basalt of Troodos; Bonin FAB-Fore-arc basalt of Bonin, Mariana FAB-Fore-arc basalt of Mariana; Philippine Sea MORB- Mid- oceanic ridge basalt of Philippine Sea; ARC- Basalt associated with the island arc, N- MORB- Normal mid- ocean ridge basalt; OIB- Ocean island basalt, OPB- Ocean floor plateau basalt; DM- Depleted mantle; EN- Enriched components; PM- Primitive mantle; REC- Recirculated components; UC- Upper crust; DEP- Depleted mantle components; HIMU- High U/Pb ratio mantle; EM1, EM2-Enriched mantle 1, enriched mantle 2

现代菲律宾海盆两侧同时发育两种不同的俯冲消减作用,其西侧为琉球-菲律宾洋-陆俯冲体系,发育典型的大陆边缘岛弧和弧后盆地(Savov et al., 2015),而东侧为伊豆-小笠原-马里亚纳(IBM)洋内弧俯冲体系(Reagan et al., 2010; Ishizuka et al., 2014a, b)。对伊豆-小笠原-马里亚纳一带的洋内弧研究发现,其底部为洋壳性质的橄榄岩、辉长岩、辉绿岩,往上依次出现类洋中脊玄武岩(MORBlike),又称其为前弧玄武岩(FAB)(肖庆辉等, 2016)、玻尼岩和高镁安山岩,再往上过渡到岛弧拉斑玄武岩系列和钙碱性系列的火山岩(Ishizuka et al., 2014a),并且在阿尔卑斯—喜马拉雅造山带塞浦路斯(Cyprus)、欧曼(Oman)、土耳其Kizildag等经典蛇绿岩中发现了类似的岩石组合(Dilek et al., 2009a, b; Pearce et al., 2010; Goodenough et al., 2014),洋内弧岩石序列及其地球化学特征均体现了正常洋盆向大陆边缘岛弧逐步演化的洋陆转换过程(邓晋福等, 2015)。洋内弧岩石组合中前弧玄武岩的出现是洋内初始俯冲开始的标志,其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,地球化学组成兼具亏损地幔源区的洋中脊玄武岩(MORB)和流体交代地幔源区的岛弧玄武岩(IAB)的共同特征,表现为二者过渡类型岩石组合的特点(Reagan et al., 2010Ishizuka et al., 2014c; Pearce, 2014)。

大洪山地区土门—三里岗—小阜一带发育大量玄武岩-安山岩-英安岩-流纹岩组合(图 1中Qbt),其应为一套典型岛弧拉斑玄武岩系列到钙碱性系列火山岩(董云鹏等, 1999, 2003石玉若等, 2003),且我们在酸性火山岩中已获得841 Ma、828 Ma的锆石U-Pb年龄(胡正祥等, 2015a, 2017);加之三里岗中-酸性侵入岩具有岛弧TTG组合的特点,其形成时代为858~876 Ma(Shi et al., 2007; 廖明芳等, 2016Xu et al., 2016),二者应可以代表一个晋宁期的陆缘岩浆弧。本文研究的晋宁期镁铁质岩发育于该岩浆弧南侧,多以岩块的形式混杂在一套碎屑岩中,表现为造山带俯冲增生杂岩的特点(胡正祥等, 2015a, b)。且这些镁铁质岩岩块地球化学方面兼具岛弧玄武岩和洋中脊玄武岩的共同特点,其稀土配分模式和微量元素特征都与典型MORB-like玄武岩相似(图 5),我们推测大洪山镁铁质岩属于MORB-like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。

6.2 地质意义

在大洪山地区这套卷入混杂的镁铁质岩组合中,获得最老的可靠年代学证据为杨家棚辉长岩947 Ma(SHRIMP)(Shi et al., 2007),其次为厂河枕状玄武岩中获得的824 Ma(SHRIMP)的年龄(Deng et al., 2013),我们也曾经在厂河北绿林一带获得过辉绿岩820 Ma(LA-ICP-MS法)的证据(胡正祥等, 2015a),加上本文中在南风垭、绿林寨分别获得的817 Ma、813 Ma玄武岩年龄,说明大洪山地区存在一套晋宁期(813~947 Ma)的镁铁质岩组合,它们形成于一种类似伊豆—小笠原—马里亚纳的洋内弧初始俯冲环境,可能是该时间段内多期次洋内俯冲作用的产物。结合土门—三里岗—小阜一带弧火山-侵入岩的产出特点,我们推测晋宁期大洪山地区可能出现类似菲律宾海盆两侧洋内、洋陆两种俯冲作用同时并存的现象,南侧形成的洋内弧岩石组合最终呈残余岩块的形式构造就位于大陆边缘弧前增生楔中(图 8)。这说明桐柏—大别地块与扬子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。但美中不足的是,目前我们并未在大洪山地区发现确切的洋内俯冲作用相关的洋内弧(高)镁安山岩组合,也有可能该类岩石组合在漫长的地质演化中已经剥蚀殆尽。

图 8 大洪山镁铁质岩形成与就位模式图 Fig. 8 Formation process and emplacement mechanism of mafic rocks in Dahongshan
7 结论

(1)鄂北随州大洪山地区大量镁铁质岩以岩块的形式构造混杂在一套碎屑岩中,表现俯冲增生杂岩基质-岩块混杂的特点,这些镁铁质岩为拉斑玄武岩系列岩石组合,兼具岛弧玄武岩Nb、Ta亏损和洋中脊玄武岩REE低分异度的特点,与典型洋内弧MORB-like玄武岩相似,推测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。

(2)年代学方面,LA-ICP-MS锆石U-Pb测年分别获得南风垭、绿林寨玄武岩817 Ma、813 Ma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813~947 Ma,它们可能是多阶段洋内俯冲的产物。

(3)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。致谢:感谢肖庆辉研究员、陆松年研究员、潘桂棠研究员等的技术指导,感谢彭练红教授、邓兴博士等的有益讨论,感谢审稿专家提出的建设性意见。

注释

➊湖北省地质局区域地质测量队. 1982. 1:20万宜城幅(H- 49-V)、随县幅(H-49-M)区域地质调查报告[R].武汉:湖北省地质调查院.

➋湖北省地质矿产局鄂东北地质大队一分队. 1986. 1:5万客店坡东半幅(H49-22-D)、古城畈幅(H49-23-C)、三阳店幅(H49- 35-A)区域地质调查报告[R].武汉:湖北省地质调查院.

致谢: 感谢肖庆辉研究员、陆松年研究员、潘桂棠研究员等的技术指导,感谢彭练红教授、邓兴博士等的有益讨论,感谢审稿专家提出的建设性意见。

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