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中—新元古代Rodinia超大陆的聚合和裂解事件是当前地球科学领域研究的重点课题,而扬子板块被认为是Rodinia超大陆的重要组成陆块(Li Zhengxiang et al., 1995, 2008; Zhao Guochun et al., 2012; Cawood et al., 2013, 2017;Li Hongbo et al., 2019),其西北缘的米仓山-汉南微地块、碧口微地块和龙门山构造带存在的中元古代晚期—新元古代火山-沉积岩系和新元古代侵入杂岩体均可能与Rodinia超大陆的聚合和裂解过程相关。已有研究表明,扬子板块西北缘新元古代早期(约950~835Ma)的岩浆岩被认为是形成于与持续俯冲作用相关的活动大陆边缘环境(Zhou Meifu et al., 2002a, 2002b, 2006a, 2006b; Ling Weili et al., 2003; Dong Yunpeng et al., 2012; Li Junyong et al., 2017; Luo Biji et al., 2018; Zhao Junhong et al., 2018, 2019)。但对于扬子板块西北缘新元古代早—中期(约835~720Ma)的岩浆岩所产出的构造背景仍存在争议,大致可以总结为以下两种主要观点:①扬子板块西北缘新元古代早—中期岩浆岩与830~795Ma和780~745Ma两期导致Rodinia超大陆裂解的超级地幔柱活动作用有关(Li Zhengxiang et al., 1995, 2008;徐学义等, 2002; Wang Xuance et al., 2008; Wu Tao et al., 2019; Zou Hao et al., 2020);②扬子板块西北缘835~720Ma的岩浆岩应形成于持续俯冲作用相关的构造背景(Zhou Meifu et al., 2002a, 2002b; Zhao Junhong et al., 2011; Dong Yunpeng et al., 2011, 2012; Luo Biji et al., 2018)。
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前人对扬子板块西北缘新元古代构造演化过程的研究主要集中在岩浆岩方面,且多数研究仅仅集中在扬子板块西缘和北缘新元古代早期岩浆作用,对于扬子板块西北缘最北端碧口微地块中岩浆岩的研究相对较少。扬子板块西北缘碧口微地块中碧口群主体为一套发生低绿片岩相区域变质的火山-沉积岩系,还包括少量的超基性—中性侵入岩,而火山岩则以基性火山岩为主。已有的研究表明,碧口群火山岩的形成时代为850~775Ma左右(闫全人等, 2003; Wang Xuance et al., 2008),但碧口群形成的构造环境尚存在争议,有学者提出碧口群可能形成于陆内裂谷相关的构造环境(丁振举等, 1998; 徐学义等, 2002; Wang Xuance et al., 2008; Xia Linqi et al., 2012),或形成于与岛弧有关的俯冲环境(裴先治等, 1989; 闫全人等, 2003)。赖绍聪等(2007)认为碧口地区董家河超基性岩可能与邻近的辉长岩、堆晶辉长岩和变玄武岩共同构成了董家河洋脊型蛇绿混杂岩,并获得其中的辉长岩锆石U-Pb年龄为839±8Ma,指示扬子板块西北缘碧口地区存在一个消失的新元古代古洋盆。碧口微地块中的新元古代侵入岩主体为中性—基性岩,还包括少量的超基性岩。前人对碧口微地块中关口垭闪长岩体(885Ma)、坪头山闪长岩体(855Ma)(Xiao Long et al., 2007),白雀寺基性—中酸性杂岩体(885Ma)(平先权等, 2014;游军等, 2018)、铜厂闪长岩体(879~824Ma)(叶霖等, 2009;Wang Wei et al., 2012)和二里坝花岗闪长岩体(821Ma)(秦利等, 2021)等新元古代中性—中酸性侵入岩体的研究表明其均具有与俯冲作用相关的岩浆弧属性。然而,具有岛弧地球化学属性的中酸性岩浆岩并不代表其一定形成于岛弧环境。而基性侵入岩通常是上地幔玄武质岩浆结晶的产物,对研究地幔属性及深部动力学背景具有重要意义,但目前对于碧口微地块中的基性侵入岩的研究相对较少。本文以陕南勉略宁地区碧口微地块中的黑木林-硖口驿基性—超基性岩带中的黑木林、硖口驿辉长闪长岩、木瓜园辉长岩和变安山质玄武岩为研究对象,通过详细的野外地质调查和锆石LA-ICP-MS U-Pb年代学、岩石学及岩石地球化学研究,确定其形成时代,探讨大地构造属性,为碧口微地块及扬子板块西北缘新元古代构造演化过程的研究提供新的证据。
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1 区域地质背景与岩石学特征
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碧口微地块大地构造位置属于扬子板块西北缘,呈西宽东窄的NEE向三角形楔形体夹持于勉略构造带、龙门山构造带和松潘-甘孜造山带之间(图1a)。碧口微地块内出露的最古老构造岩石地层为新太古代鱼洞子杂岩(裴先治, 2001)。碧口微地块内部出露的岩石地层主体为新元古界碧口群和横丹群。碧口群主体为一套浅变质火山-沉积岩系,横丹群主体为一套浅变质浊积岩系。新元古界南华系—震旦系沉积盖层主要分布于碧口微地块东部勉略宁三角区的代家坝北—大安北—茶店—何家岩一带以及甘肃文县、四川平武地区的碧口微地块南北两侧。
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研究区位于碧口微地块东段的陕南勉略宁地区,区内主要出露震旦系灯影组和陡山沱组沉积盖层、新元古界碧口群浅变质火山-沉积岩系、新元古代黑木林-硖口驿基性—超基性岩带及新元古代侵入岩体(图1b)。其中灯影组主要以灰色厚层状白云岩、白云质灰岩为主。陡山沱组主要以粉砂质板岩为主,夹薄层长英质细砂岩、含碳硅质板岩、泥质板岩。碧口群主体为低绿片岩相区域变质的火山-沉积岩系,可进一步划分为沿黑木林-硖口驿超镁铁质岩(蛇纹岩)分布的碧口群下部和分布于黑木林-硖口驿基性—超基性岩带南北两侧的碧口群上部,其中碧口群下部岩性主要以变玄武岩-安山质玄武岩和变基性凝灰岩为主,夹少量绢云石英片岩;碧口群上部岩性以变中酸性火山岩为主,局部夹少量变基性火山岩、变中酸性凝灰岩及绢云石英片岩。
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本文所研究的黑木林-硖口驿基性—超基性岩带呈NEE向展布(图1b),根据空间展布位置自东向西划分为东段硖口驿地区、中段庙坝北木瓜园地区和西段黑木林地区三个区段。带内主要发育有超基性岩(蛇纹岩)、辉长岩、辉长闪长岩及变玄武岩-安山质玄武岩等。其中,超基性岩主要由斜方辉石橄榄岩、单斜辉石橄榄岩和纯橄岩组成,多已蚀变为蛇纹岩(裴先治, 1989)。
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图1 碧口微地块构造背景图(a)(据徐学义等, 2014修编)和碧口微地块东段黑木林-硖口驿地区地质简图(b) (据陕西地质矿产勘查开发局综合研究队,1996❶;陕西省地质调查院,2007❷)
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Fig.1 Tectonic setting map of Bikou Terrane (a) (modified after Xu Xueyi et al., 2014) and geological sketch map of Heimulin-Xiakouyi area in the eastern of Bikou Terrane (b) (after Shaanxi Geology and Mineral Exploration and Development Bureau Comprehensive Research Team, 1996❶;Shaanxi institute of Geological survey, 2007❷)
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1 —晚震旦世灯影组;2—早震旦世陡山沱组;3—硅质岩岩块;4—新元古界碧口群下部;5—新元古界碧口群上部;6—新元古代黑木林-硖口驿超基性岩;7—新元古代辉长岩;8—新元古代辉长辉绿岩;9—新元古代辉长闪长岩;10—新元古代二里坝花岗闪长岩体;11—新元古代铜厂闪长岩体;12—地质界线;13—实测不明性质断层;14—逆冲断层;15—平移断层;16—韧性剪切断层;17—断层产状;18—地层产状;19—面理产状;20—同位素年龄采样点;21—剖面位置
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1 —Late Sinian Dengying Formation; 2—Lower Sinian Doushantuo Formation; 3—siliceous rock block; 4—lower parts of Neoproterozoic Bikou Group; 5—upper parts of Neoproterozoic Bikou Group; 6—Neoproterozoic Heimulin-Xiakouyi ultrabasic rock; 7—Neoproterozoic gabbro; 8—Neoproterozoic gabbro diabase; 9—Neoproterozoic gabbro-diorite; 10—Neoproterozoic Erliba granodiorite; 11—Neoproterozoic Tongchang diorite; 12—geological boundary; 13—measured unknown fault; 14—thrust fault; 15—strike-slip fault; 16—ductile shear fault; 17—fault occurrence; 18—stratigraphic occurrence; 19—foliation occurrence; 20—isotopic age sampling location; 21—cross section location
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硖口驿辉长闪长岩岩块走向近EW向,出露宽度约300~900m不等,南侧与硖口驿超基性岩(蛇纹岩)、北侧与震旦系灯影组、新元古界碧口群上部均呈构造接触关系(图1b、2a)。硖口驿辉长闪长岩手标本新鲜面为深灰色,具细粒结构,块状构造。其主要矿物组成为斜长石(55%~60%)和单斜辉石(35%~40%);斜长石多呈自形长板状或长条状,粒度约0.2~0.8mm,可见聚片双晶和卡钠复合双晶(图3b、c),部分斜长石表面高岭土化和绿帘石化;单斜辉石粒度约0.2~0.4mm,以他形为主充填于斜长石大的三角形空隙中,显示辉绿结构,部分单斜辉石发生纤闪石化;副矿物为磁铁矿、锆石等。
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木瓜园辉长岩岩块走向近EW向,南北各有出露,南部出露宽度约为150~200m,与新元古界碧口群下部变玄武岩-安山质玄武岩呈构造接触关系;北部出露宽度约50~60m,与新元古界碧口群上部和黑木林超基性岩(蛇纹岩)均呈构造接触关系(图1b、2b)。辉长岩手标本新鲜面为深灰色,具细粒结构,块状构造。其主要矿物组成为斜长石(50%~55%)和单斜辉石(40%~45%);斜长石呈半自形板状,粒度约0.2~0.6mm,蚀变较强,发生高岭土化、绿帘石化,其中多数颗粒与单斜辉石互嵌显示辉长结构;单斜辉石呈半自形板状,粒度约0.2~0.4mm,部分单斜辉石发生纤闪石化。副矿物为磁铁矿、钛铁矿和磷灰石。
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图2 陕南勉略宁地区硖口驿(a)、木瓜园(b)及黑木林(c)剖面
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Fig.2 Profiles of Xiakouyi (a), Muguayuan (b) and Heimulin (c) in Mianxian-Lueyang-Ningqiang area, southern Shaanxi
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1 —蛇纹岩;2—辉长岩;3—辉长闪长岩;4—花岗闪长岩;5—变玄武岩;6—变安山质玄武岩;7—变英安岩;8—变基性凝灰岩;9—绢云石英片岩;10—硅质岩;11—硅质板岩;12—粉砂质板岩;13—地球化学样品采样点;14—锆石U-Pb年龄样品采样点;15—韧性剪切断层;16—逆冲断层;Z1d —下震旦统陡山沱组;Pt3bk1—新元古界碧口群下部;Pt3bk2—新元古界碧口群上部;ψωPt3—新元古代超基性岩;vPt3—新元古代辉长岩;vβPt3—新元古代辉长辉绿岩;vδPt3—新元古代辉长闪长岩;γδPt3—新元古代花岗闪长岩体;Si—硅质岩岩块;S1—第一期构造片理论
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1 —Serpentinite; 2—gabbro; 3—gabbro-diorite; 4—granodiorite; 5—meta-basalt; 6—meta-andesitic basalt; 7—meta-dacite; 8—meta-basic tuff; 9—sericite quartz schist; 10—siliceous rock; 11—siliceous slate; 12—silty slate; 13—sample locations for geochemistry; 14—sample locations for zircon U-Pb age; 15—ductile shear fault; 16—thrust fault; Z1d —Lower Sinian Doushantuo Formation; Pt3bk1—lower parts of Neoproterozoic Bikou Group; Pt3bk2—upper parts of Neoproterozoic Bikou Group; ψωPt3—Neoproterozoic ultrabasic rock; vPt3—Neoproterozoic gabbro; vβPt3—Neoproterozoic gabbro diabase; vδPt3—Neoproterozoic gabbro-diorite; γδPt3—Neoproterozoic granodiorite; Si—siliceous rock block; S1—the foliation of first deformation stage
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图3 黑木林-硖口驿镁铁质岩样品手标本及显微镜下照片
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Fig.3 The rock specimens and microscopic photographs of Heimulin-Xiakouyi mafic rocks
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(a、b、c)—硖口驿辉长闪长岩手标本与显微镜下照片;(d、e、f)—木瓜园辉长岩手标本与显微镜下照片;(g、h、i)—木瓜园变安山质玄武岩手标本与显微镜下照片;(j、k、l)—黑木林辉长闪长岩手标本与显微镜下照片;Pl—斜长石;Cpx—单斜辉石;Hb—角闪石;Qtz—石英;Cal—方解石;Chl—绿泥石;Ep—绿帘石;(b、c、e、f、i、k、l)—正交偏光照片;(h)—单偏光照片
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(a, b, c)—The rock specimens and microscopic photographs of Xiakouyi gabbro-diorite; (d, e, f) —the rock specimens and microscopic photographs of Muguayuan gabbro; (g, h, i) —the hand specimens and microscopic photographs of Muguayaun meta-andesitic basalt; (j, k, l) —the rock specimens and microscopic photographs of Heimulin gabbro-diorite; Pl—plagioclase; Cpx—clinopyroxene; Hb—hornblende; Qtz—quartz; Cal—calcite; Chl—chlorite; Ep—epidote; (b, c, e, f, i, k, l)—cross-polarized photo; (h)—plane polarized photo
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木瓜园变安山质玄武岩采自木瓜园南辉长岩岩块南北两侧的新元古界碧口群下部地层中(图2b),变安山质玄武岩手标本新鲜面为灰绿色,岩石已发生区域变质,具鳞片状变斑晶结构,弱片理化构造、块状构造(图3h、i),主要组成矿物为斜长石(25%~30%),、绿泥石(40%~45%)和绿帘石(10%~15%),斜长石斑晶粒度约为0.2~0.4mm,斜长石和暗色矿物均发生绿帘石化和绿泥石化,整体蚀变强烈,方解石含量约8%~10%,石英含量约5%~8%,副矿物为磁铁矿、钛铁矿和磷灰石。
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黑木林辉长闪长岩岩块走向近NE向,出露宽度约220~250m,与新元古界碧口群下部呈构造接触关系(图1b、2c)。辉长闪长岩手标本为深灰色,具细粒结构、块状构造。主要组成矿物中浅色矿物以斜长石为主,含量约45%~50%,暗色矿物以斜方辉石、单斜辉石和角闪石为主,含量约40%~50%,并含少量蛇纹石(呈针叶状,可能由橄榄石蚀变而来)。斜长石多呈半自形长板状,粒度约0.3~0.5mm,可见聚片双晶(图3k、l),部分斜长石表面发生高岭土化和绿帘石化;斜方辉石呈半自形板状,粒度约0.2~0.5mm,具Ⅰ级灰色到Ⅰ级黄色干涉色和平行消光;单斜辉石呈半自形板状,粒度约0.2~0.5mm,具Ⅱ级蓝色到Ⅱ级紫色干涉色和斜消光,部分单斜辉石发生绿泥石化;角闪石半自形—他形柱状,粒度约0.1~0.4mm;副矿物为磁铁矿、锆石等。
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2 样品测试方法
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对所采辉长岩、辉长闪长岩及变安山质玄武岩共17件岩石地球化学样品进行了主量元素、微量元素和稀土元素分析测试,测试分析在西北大学大陆动力学国家重点实验室完成。主量元素采用波长色散X射线荧光光谱仪(XRF,Rigaku RIX2100)测定,分析精度和准确度优于2%;微量元素和稀土元素测试使用电感耦合等离子质谱仪(ICP-MS)测定,分析精密度和准确度优于10%。详细的实验原理及测试流程详见文献Chen et al., 2000。
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所采辉长岩、辉长闪长岩锆石U-Pb同位素测年样品的粉碎、锆石分离、挑选和制靶以及阴极发光(CL)显微照相等工作由西安瑞石地质科技有限公司完成。其中,木瓜园南辉长岩样品(ML17012-1、ML17012-2)和黑木林辉长闪长岩样品(ML17013-2)的原位锆石U-Pb同位素分析测试在天津地质矿产研究所同位素实验室的激光剥蚀多接收电感器耦合等离子体质谱仪(LA-ICP-MS)上进行测试,多接收器电感耦合等离子质谱仪为ThermoFisher公司制造的Neptune,激光器为美国ESI公司生产的UP193-FXArF准分子激光器,激光波长为193nm,脉冲宽度为5ns,激光剥蚀的束斑直径为30 μm,锆石年龄计算标样采用GJ-1标准锆石,利用NIST612玻璃标样作为外标。硖口驿辉长闪长岩样品(ML1072-1)和木瓜园北辉长岩样品(ML1123-1)的原位锆石U-Pb同位素分析测试在西北大学大陆动力学国家重点实验室的激光剥蚀电感耦合等离子体质谱仪(LA-ICP-MS)上完成,分析仪器为Elan6100DRC型四级杆质谱仪和Geolas200M型剥蚀系统,激光的剥蚀孔径为30 μm,深度为20~40 μm。锆石年龄计算标样采用GJ-1标准锆石,元素含量采用美国国家标准物质局人工合成硅酸盐玻璃NIST SRM610作为外标、29Si作为内标进行同位素分馏校正。后期数据处理采用ICPMSDataCal软件(Liu Yongsheng et al., 2010),年龄计算及谐和曲线图绘制利用Isoplot3.0软件(Ludwig, 2003)完成。
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3 岩石地球化学特征
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本文对硖口驿辉长闪长岩(5件)、木瓜园辉长岩(5件)、变安山质玄武岩(4件)以及黑木林辉长闪长岩(3件)共17件样品进行岩石地球化学测试,分析结果见附表1(http://www.geojournals.cn/dzxb/ch/reader/view_abstract.aspx?file_no=202200799&flag=1),并对烧失量大的岩石样品(LOI≥4.00%)的测试结果进行干体系换算处理。
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3.1 木瓜园辉长岩和变安山质玄武岩
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木瓜园辉长岩具较高且变化较大的SiO2(45.49%~52.60%)、TFe2O3(8.66%~12.91%)、Al2O3(16.16%~18.56%)、CaO(7.51%~11.28%)含量,中等至较高的MgO(4.12%~7.25%)、TiO2(1.18%~1.54%)含量和低的Mg#值(48~62),全碱(Na2O+K2O)含量为3.26%~5.14%。在Nb/Y-Zr/TiO2×0.0001图解中落入基性火山岩区域(图4a)。在SiO2-TFeO/MgO图解中落入拉斑玄武岩系列区域(图4b)。其稀土元素总量较高(∑REE=92.02×10-6~131.45×10-6),(La/Yb)N为3.55~6.71,轻重稀土元素分异较明显,球粒陨石标准化稀土元素配分曲线为右倾型(图5a)。δEu为0.78~1.06,平均值为0.87,岩石Eu负异常不明显,表明斜长石没有发生明显的分离结晶作用。原始地幔标准化微量元素蜘蛛网图显示其富集大离子亲石元素Ba、Th、Pb、Sr,相对亏损高场强元素Nb、Ta、Zr、Ti(图5b),显示出岛弧岩浆岩的地球化学特征。
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图4 黑木林-硖口驿镁铁质岩Nb/Y-Zr/TiO2×0.0001图解(a) (据Winchester et al., 1977)和SiO2-TFeO/MgO图解(b)(据Miyashiro, 1974)
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Fig.4 Nb/Y-Zr/TiO2×0.0001 (a) (after Winchester et al., 1977) and SiO2-TFeO/MgO (b) (after Miyashiro, 1974) diagrams of Heimulin-Xiakouyi mafic rocks
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木瓜园变安山质玄武岩与木瓜园辉长岩相比,具相对高的SiO2(51.62%~56.23%)、TFe2O3(13.02%~15.18%)、TiO2(1.48%~2.26%)含量和较低的Al2O3(15.29%~16.78%)、MgO(2.43%~5.10%)、CaO(3.92%~7.05%)含量,Mg#值低(28~45),全碱(Na2O+K2O)含量为3.51%~4.19%。在Nb/Y-Zr/TiO2×0.0001图解中落入玄武岩区域(图4a)。综合其略高于玄武岩的SiO2含量及岩石学特征,定名为变安山质玄武岩。在SiO2-TFeO/MgO图解中均落入拉斑玄武岩系列区域(图4b)。其稀土元素总量较高(∑REE=76.06×10-6~153.48×10-6),(La/Yb)N为4.08~8.85,轻重稀土元素分异较明显,球粒陨石标准化稀土元素配分曲线为右倾型(图5c)。岩石Eu异常不明显,δEu为0.85~1.01,平均值为0.94。原始地幔标准化微量元素蜘蛛网图显示其富集大离子亲石元素Ba、Th、Pb,相对亏损高场强元素Nb、Ta和Zr(图5d),显示岛弧岩浆岩的地球化学特征。
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3.2 黑木林、硖口驿辉长闪长岩:类玻安质及过渡类型岩石
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黑木林辉长闪长岩具较高且变化小的SiO2(54.27%~56.79%)含量,高的MgO(11.10%~17.04%)含量,低的Al2O3(7.58%~10.61%)、TFe2O3(7.63%~8.81%)、CaO(5.41%~6.59%)、TiO2(0.08%~0.19%)含量。全碱(Na2O+K2O)含量为1.67%~4.52%, Mg#值高(77~78),具玻安岩的地球化学特征(MgO＞8%,SiO2＞52%,低TiO2＜0.5%)(Fallon et al.,1991;Le Bas,2000)。在Nb/Y-Zr/TiO2×0.0001图解中落入安山岩/安山-玄武岩区域(图4a)。在SiO2-TFeO/MgO图解中落入钙碱性系列区域(图4b)。其稀土元素总量低(∑REE=6.26×10-6~12.24×10-6),球粒陨石标准化稀土元素配分曲线显示较为平坦,与Bonin弧前玻安质岩石类似(图5e)。δEu为0.91~1.03,平均值为0.98,岩石Eu异常不明显。N-MORB(正常洋中脊玄武岩)标准化微量元素蜘蛛网图曲线类似于Bonin弧前玻安质岩石(图5f)。
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硖口驿辉长闪长岩具较高的SiO2(53.01%~57.02%)、Al2O3(14.54%~17.65%)、TFe2O3(8.07%~10.93%)含量,中等的MgO(4.24%~8.91%)含量,低的TiO2(0.26%~0.32%)含量和变化较大的CaO(4.45%~10.16%)含量,Mg#值较高(53~65),全碱(Na2O+K2O)含量为1.89%~5.36%。在Nb/Y-Zr/TiO2×0.0001图解中落入玄武岩和安山岩/安山-玄武岩交界区域(图4a)。在SiO2-TFeO/MgO图解中全部落入钙碱性系列区域(图4b)。其稀土元素总量低(∑REE=13.90×10-6~15.12×10-6),球粒陨石标准化稀土元素配分曲线较为平坦(图5e)。岩石Eu异常不明显,δEu为0.90~1.15,平均值为1.02。N-MORB标准化微量元素蜘蛛网图曲线与Bonin弧前玻安质岩石类似(图5f)。结合其具有低的TiO2含量(0.26%~0.32%)、低的Ti/Zr值(38~56),且其Ti/Zr值与MORB(洋中脊玄武岩)、IAB(岛弧玄武岩)及球粒陨石的Ti/Zr数值明显不同(平均值都在110左右),而接近西太平洋岛弧中玻安岩的Ti/Zr值(23~67)(Hickey et al., 1982; Cameron et al., 1983)等特征,表明硖口驿辉长闪长岩应属于过渡类型的类玻安质岩石。
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图5 黑木林-硖口驿镁铁质岩稀土元素配分曲线图(a、c、e)和微量元素蜘蛛网图(b、d、f)(球粒陨石、原始地幔、N-MORB标准化数据据Sun et al.,1989;IAB数据据Kelement et al., 2007; Bonin岛弧前玻安岩数据据Li Yibing et al.,2013)
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Fig.5 The chondrite-normalized REE patterns(a,c,e)and primitive mantle or N-MORB normalized trace element patterns(b,d,f)of Heimulin-Xiakouyi mafic rocks (chondrite,primitive mantle,N-MORB normalized data after Sun et al.,1989; IAB data after Kelement et al., 2007; Bonin fore-arc boninite data after Li Yibing et al.,2013)
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4 锆石U-Pb年龄
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4.1 硖口驿辉长闪长岩锆石U-Pb年龄
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硖口驿辉长闪长岩年龄样品ML1072-1采自硖口驿镇(图1b),多数锆石颗粒为自形晶,粒径约70~150 μm,长宽比介于1∶1~3∶1之间,锆石颗粒多呈短柱状。锆石CL图像显示(图6a),锆石生长环带不明显。从硖口驿辉长闪长岩样品ML1072-1中挑选14颗锆石进行U-Pb年代学测试,分析结果见附表2(http://www.geojournals.cn/dzxb/ch/reader/view_abstract.aspx?file_no=202200799&flag=1)。所测锆石具有较高的Th/U比值,为0.31~2.15之间见附表2(http://www.geojournals.cn/dzxb/ch/reader/view_abstract.aspx?file_no=202200799&flag=1),应为岩浆成因锆石。剔除谐和度低于90%的数据后,获得8个有效分析点(图6a)。其中测点7和测点12的207Pb/206Pb年龄分别为1767±37Ma和2176±31Ma,测点位于锆石核部,可能代表了捕获锆石的年龄;测点4的206Pb/238U年龄为470±5Ma,可能是铅丢失造成的。其余5个测点206Pb/238U年龄较为集中,加权平均年龄为830±11Ma(MSWD=0.69,n=5)(图7a~c),代表了硖口驿辉长闪长岩的结晶年龄。
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图6 黑木林-硖口驿镁铁质岩代表性锆石阴极发光图像和年龄值
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Fig.6 The CL images and ages of representative zircons from Heimulin-Xiakouyi mafic rocks
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4.2 木瓜园辉长岩锆石U-Pb年龄
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木瓜园北辉长岩样品ML1123-1中锆石颗粒多呈半自形短柱状,粒径约70~150 μm,长宽比介于1∶1~3∶1之间。锆石CL图像(图6b)显示,锆石生长环带不明显,锆石晶体有少量熔蚀。从辉长岩样品ML1123-1中挑选出15颗锆石进行锆石U-Pb年代学分析,分析结果见附表2(http://www.geojournals.cn/dzxb/ch/reader/view_abstract.aspx?file_no=202200799&flag=1)。所测锆石具有较高的Th/U比值,介于0.99~2.40之间(＞0.4),应为岩浆成因锆石。剔除谐和度低于90%的测点的测年结果之后,获得的9个有效分析点的206Pb/238U年龄较为集中,其加权平均年龄为832±9Ma(MSWD=0.036,n=9)(图7d、e),代表木瓜园北辉长岩的结晶年龄。
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木瓜园南辉长岩样品ML17012-1中锆石颗粒多呈浑圆状—短柱状,粒径约50~160 μm,长宽比为1∶1~3∶1。锆石CL图像(图6c)显示,多数锆石生长环带不明显,仅个别锆石显示较宽的生长环带。从辉长岩样品ML17012-1中挑选24颗锆石进行锆石U-Pb年代学分析,分析结果见附表2(http://www.geojournals.cn/dzxb/ch/reader/view_abstract.aspx?file_no=202200799&flag=1),所测锆石具有较高的Th/U比值,为1.26~3.40(＞0.4),应属于岩浆成因锆石。其中,测点23和24两个较小年龄(808±8Ma和805±8Ma)可能是后期岩浆事件造成的。测点1~22的206Pb/238U年龄都比较集中,其206Pb/238U加权平均年龄为835±4Ma(MSWD=0.078,n=22)(图7f、g),代表木瓜园南辉长岩的结晶年龄。
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木瓜园南辉长岩样品ML17012-2中锆石颗粒大多数呈自形—半自形的短柱状或长柱状,粒径约50~160 μm,长宽比为1∶1~3∶1。锆石CL图像(图6d)显示,锆石生长环带不明显,仅个别锆石显示较宽的生长环带。从辉长岩年龄样品ML17012-2中挑选出21颗锆石进行锆石U-Pb年代学分析,分析结果见附表2(http://www.geojournals.cn/dzxb/ch/reader/view_abstract.aspx?file_no=202200799&flag=1),所测锆石中仅有一颗锆石的Th/U比值小于0.1,其余锆石的Th/U比值均大于0.4,介于0.80~5.02之间,应属于岩浆成因锆石。剔除谐和度低于90%的数据后,获得的19个有效分析点的206Pb/238U年龄都比较集中,其206Pb/238U加权平均年龄为832±2Ma(MSWD=1.9,n=19)(图7h、i),代表木瓜园南辉长岩的结晶年龄。
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综合上述,木瓜园南、北辉长岩的结晶年龄十分接近,为835~832Ma,应为同期岩浆作用的产物,故选取三个年龄样品中谐和度高的一组年龄(835±4Ma)作为木瓜园辉长岩的结晶年龄。
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4.3 黑木林类玻安质辉长闪长岩锆石U-Pb年龄
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黑木林辉长闪长岩样品ML17013-2锆石颗粒多呈短柱状或粒状,粒径约60~100 μm,长宽比介于1∶1~1∶3之间。锆石CL图像显示,锆石多呈半透明,发育岩浆振荡环带(图6e)。从辉长闪长岩样品ML17013-2中挑选23颗锆石进行U-Pb年代学测试,测试结果见附表2(http://www.geojournals.cn/dzxb/ch/reader/view_abstract.aspx?file_no=202200799&flag=1)。所测锆石中仅有两颗锆石的Th/U比值小于0.4,其余锆石的Th/U比值均介于0.54~0.90之间(＞0.4),应属于岩浆成因锆石。剔除谐和度低于90%的数据点后,获得15个有效分析点。其中,测点22的207Pb/206Pb年龄为1452±1Ma,可能代表了捕获锆石的年龄;测点8和测点11的206Pb/238U年龄年龄较小,分别为806±2Ma和806±2Ma,测点位于核幔交界区,测试结果可能为混合年龄;其余12个有效分析点的206Pb/238U年龄较为集中,加权平均年龄为830±2Ma(MSWD=0.047,n=12)(图7j、k),代表黑木林辉长闪长岩的结晶年龄。
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5 讨论
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5.1 黑木林-硖口驿镁铁质岩的形成时代
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LA-ICP-MS锆石U-Pb定年结果表明见附表2(http://www.geojournals.cn/dzxb/ch/reader/view_abstract.aspx?file_no=202200799&flag=1),本次研究获得的3件木瓜园辉长岩样品(ML1123-1、ML17012-1和ML17012-2)的年龄谐和性较好,其206Pb/238U加权平均年龄分别为832±9Ma(MSWD=0.036,n=9)、835±4Ma(MSWD=0.078,n=22)和832±2Ma(MSWD=1.9,n=19),形成时代非常接近,故选择835±4Ma年龄(谐和度高)作为木瓜园辉长岩的结晶年龄;硖口驿辉长闪长岩样品(ML1072-1)和黑木林辉长闪长岩样品(ML17013-2)的206Pb/238U加权平均年龄分别为830±11Ma(MSWD=0.69,n=5)和830±2Ma(MSWD=0.047,n=12)。综上可知,本次研究获得的黑木林、硖口驿辉长闪长岩和木瓜园辉长岩的形成时代十分接近,表明黑木林-硖口驿基性—超基性岩带的形成时代应为835~830Ma,为新元古代早期。而黑木林-硖口驿基性—超基性岩带北部的二里坝花岗闪长岩的LA-ICP-MS锆石U-Pb年龄为821±6Ma(秦利等, 2021);南部的铜厂闪长岩的LA-ICP-MS锆石U-Pb年龄为824±5Ma(Wang Wei et al., 2012)。上述结果表明碧口微地块在835~820Ma存在重要的岩浆活动事件。
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图7 黑木林-硖口驿镁铁质岩锆石U-Pb谐和图(a、b、d、f、h、j)及加权平均年龄图(c、e、g、i、k)
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Fig.7 Zircon U-Pb concordia diagrams (a, b, d, f, h, j) and weighted mean ages (c, e, g, i, k) of Heimulin-Xiakouyi mafic rocks
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图8 黑木林-硖口驿镁铁质岩Harker图解(a~f)
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Fig.8 Harker diagrams of Heimulin-Xiakouyi mafic rocks (a~f)
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5.2 岩石成因
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5.2.1 木瓜园辉长岩和变安山质玄武岩
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5.2.1.1地壳混染与分离结晶
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原始地幔标准化(Th/Nb)PM值(＞＞1)(Ormerod et al., 1988)和Nb/La(＜1)(Ernst et al., 2000)是判别地壳混染作用的两个可靠的微量元素指标。木瓜园辉长岩和变安山质玄武岩(Th/Nb)PM值分别为2.46~9.55和4.07~5.46,Nb/La值分别为0.17~0.43和0.17~0.34,表明木瓜园辉长岩和变安山质玄武岩岩浆上升过程中遭受一定程度的地壳混染。而受地壳混染后的岩浆具有较高的Th/Nb值(＞5)、Th/Ta值(＞10)(Wooden et al., 1993; Neal et al., 2002),且其La/Sm比值会增大到5以上(Lassiter et al., 2013),木瓜园辉长岩和变安山质玄武岩具有较低的Th/Nb比值(0.29~1.14,平均值为0.61)、Th/Ta比值(4.80~15.15,平均值为8.07)和La/Sm比值(2.78~5.53,平均值为4.06),微量元素原始地幔标准化蜘蛛网图中亦没有明显的Zr、Hf正异常,表明木瓜园辉长岩和变安山质玄武岩岩浆在上升演化过程中可能仅遭受到较弱的地壳物质混染。
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与地幔橄榄岩原生岩浆的Cr(500×10-6~600×10-6)、Ni(500×10-6~600×10-6)含量相比(Wilson, 1989),木瓜园辉长岩的Cr含量(20.7×10-6~274×10-6)、Ni含量(15.8×10-6~78.8×10-6)和变安山质玄武岩的Cr含量(2.9×10-6~36.9×10-6)、Ni含量(0.9×10-6~28.4×10-6)均低于该范围,且木瓜园辉长岩和变安山质玄武岩的Mg#值均低于原生岩浆的Mg#值 (68~75)(Wilson, 1989), 暗示其在岩浆演化过程中经历了镁铁质矿物的分离结晶作用。Harker图解显示,木瓜园辉长岩和变安山质玄武岩样品的Cr、Ni与MgO呈现出正相关关系,表明岩浆在演化过程中发生了橄榄石、辉石等富含Cr、Ni矿物的分离结晶。木瓜园辉长岩及变安山质玄武岩的TiO2-MgO和TFe2O3-MgO均呈负相关,表明可能发生了钛铁矿的结晶分异。
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图9 黑木林-硖口驿镁铁质岩Th/Zr-Nb/Zr图解(a)(据Kepezhinskas et al., 1997) 和La/Yb-Nb/Y图解(b) (据Hoffer et al., 2008)
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Fig.9 Th/Zr-Nb/Zr (a)(after Kepezhinskas et al., 1997)and La/Yb-Nb/Y(b)(after Hoffer et al., 2008) diagrams of Heimulin-Xiakouyi mafic rocks
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5.2.1.2地幔源区
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镁铁质岩浆一般来源于软流圈或者岩石圈地幔的部分熔融,源于软流圈的玄武岩La/Nb＜1.5,La/Ta＜22,而源于岩石圈地幔的玄武岩恰好与之相反(Thompson et al., 1988)。木瓜园辉长岩的La/Nb值为2.32~6.00(＞1.5),La/Ta值为37.13~79.86(＞22), 木瓜园变安山质玄武岩的La/Nb值为2.98~5.89(＞1.5),La/Ta值为44.71~80.41(＞22),指示木瓜园辉长岩和变安山质玄武岩均起源于岩石圈地幔。在排除遭受大规模地壳混染的因素后,暗示其岩浆源区可能受到俯冲组分的影响,在Th/Yb-Nb/Yb图中亦显示有俯冲组分输入的趋势(图11c)。木瓜园辉长岩和变安山质玄武岩具可变的Th/Zr比值和恒定的Nb/Zr比值,较低的Nb/Y和La/Yb比值(图9a、b),表明木瓜园辉长岩和变安山质玄武岩岩浆源区受到了俯冲流体交代的影响。
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在原始地幔标准化(Tb/Yb)P-(Yb/Sm)P图解(图10)中,石榴子石二辉橄榄岩与尖晶石二辉橄榄岩熔融的熔融轨迹明显不同。木瓜园辉长岩岩浆源区主体应为含石榴子石尖晶石二辉橄榄岩(石榴子石相＜40%的贡献比)经过约1%~5%的部分熔融形成,木瓜园变安山质玄武岩岩浆源区主体应为尖晶石二辉橄榄岩经过约1%~5%的部分熔融形成。
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图10 木瓜园辉长岩和变玄武安山岩(Tb/Yb)P-(Yb/Sm)P图解(据Zhang Zhaochong et al., 2006; 原始地幔数据引自Sun et al., 1989)
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Fig.10 The (Tb/Yb)P-(Yb/Sm)P diagram (after Zhang Zhaochong et al., 2006; primitive-mantal normalized values after Sun et al., 1989)of Muguayuan gabbro and meta-andesitic basalt
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粗线为等熔程度曲线,其上的数值标明了熔体百分比;细线标明了石榴子石相地幔源区贡献的百分比
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The curves of constant melt fraction are shown by the bold lines, and the light lines indicate the percentage of melt contribution from garnet-faces mantle
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5.2.2 黑木林、硖口驿辉长闪长岩:类玻安质及过渡类型岩石
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黑木林辉长闪长岩具有高的Cr(823×10-6~954×10-6)、Ni(115×10-6~367×10-6)和Mg#值(77~82),表现出初始岩浆的特征。硖口驿辉长闪长岩的Cr(28×10-6~257×10-6)、Ni(26×10-6~52×10-6)含量远低于原生岩浆参考值,Mg#值为53~66,稍低于原始岩浆Mg#值(68~75),表明在硖口驿辉长闪长岩岩浆演化过程中可能经历了一定程度的壳内岩浆作用,如地壳混染或分离结晶。硖口驿辉长闪长岩的(Th/Nb)PM值为3.91~4.68,Nb/La值为0.29~0.41,表明硖口驿辉长闪长岩岩浆上升过程中受到一定程度的地壳混染。而硖口驿辉长闪长岩具有较低的Th/Nb比值(0.47~0.56)、Th/Ta比值(4.82~6.34)和La/Sm比值(1.90~2.61),表明其岩浆上升过程中受地壳混染的影响较弱(Wooden et al., 1993; Neal et al., 2002; Lassiter et al., 2013),其成分变化主要受控于结晶分异作用。Harker图解显示,硖口驿辉长闪长岩的Cr、Ni与MgO呈正相关,表明岩浆在演化过程中发生了橄榄石、辉石等富含Cr、Ni矿物的结晶分异作用(图8)。
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大多数学者认为玻安质岩石的地幔源为比MORB或弧玄武岩更难熔的亏损方辉橄榄岩域,在浅层(＜50km)及高温(1150~1350℃)的俯冲环境中,受到俯冲板片流体或熔体的交代发生部分熔融,可产生玻安质岩浆(Crawford et al., 1989; Xia Xiaohong et al., 2012; Li Yinbing et al., 2013)。对于源于亏损地幔源的玻安质岩浆,其不相容元素的变化几乎完全由俯冲组分控制,因此,不相容元素可以提供可靠的信息来判别俯冲组分的来源(Crawford et al., 1989; König et al., 2010)。而硖口驿、黑木林辉长闪长岩具相对较低的Nb/Y比值和La/Yb比值(Plank, 2005)(图9b),且受俯冲流体交代作用产生的岩浆的Th/Yb比值＜1(Woodhead et al., 2001),硖口驿、黑木林辉长闪长岩的Th/Yb比值分别为0.24~0.35和0.41~0.62,表明其岩浆源区受到了俯冲流体交代的影响。
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5.3 构造环境判别
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木瓜园辉长岩和变安山质玄武岩的Nb/La值为0.17~0.34(≤1),Hf/Ta值为6.72~8.95(≥5),La/Ta值为37.13~80.41(＞15),可排除板内玄武岩的可能性,并将其限定为弧玄武岩或N-MORB(Condie, 1989)。而Th/Yb值为0.67~1.58(＞0.1),Th/Nb值为0.45~1.14(＞0.07),Nb/La值为0.17~0.34(≤0.8),Hf/Th值为0.57~1.82(＜8),故可以排除N-MORB的可能,指示木瓜园辉长岩和变安山质玄武岩符合弧岩浆岩的特征(Condie, 1989)。木瓜园辉长岩和变安山质玄武岩在Nb-Nb/Th图解(图11a)中均落入岛弧玄武岩区域;在Hf/3-Th-Nb/16图解(图11b)中均落入火山弧玄武岩区域;在Nb/Yb-Th/Yb图解中均落在大洋岛弧区域(图11c);在NbN/ThN-ZrN/SmN(N代表N-MORB标准化)图解中均落入弧火山岩区域(图11d);指示木瓜园辉长岩和变安山质玄武岩形成于与俯冲相关的岛弧环境。
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目前,关于玻安岩形成的构造环境尚存在弧前环境(弧前初始俯冲的标志)(Crawford et al., 1989; Cluzel et al.,2016)、弧后盆地环境(Fallon et al., 1991; Xia Xiaohong et al., 2012)甚至板内环境(Pearce et al., 2019)不同的认识。弧前环境产出的玻安岩具低的CaO/Al2O3比值(＜0.75)、Ti/Zr比值(23~67)及高的Zr/Y比值(＞3)等特征;而弧后盆地形成的玻安岩具高的CaO/Al2O3比值(＞0.75)和低的Zr/Y比值(＜3)等特征(Crawford et al., 1989)。黑木林、硖口驿辉长闪长岩具低的CaO/Al2O3比值(0.28~0.76)、Ti/Zr比值(18~56)、高的Zr/Y比值(3.84~8.56)及N-MORB标准化微量元素蜘蛛网图与Bonin弧前玻安岩类似等特征。且黑木林、硖口驿辉长闪长岩在Nb-Nb/Th图解(图11a)中均落入岛弧玄武岩区域;在Hf/3-Th-Nb/16图解(图11b)中均落入火山弧玄武岩区域;在Nb/Yb-Th/Yb图解和(图11c)中均落在大洋岛弧区域;在NbN/ThN-ZrN/SmN(N代表N-MORB标准化)图解中均落入弧前区域(图11d);指示黑木林、硖口驿辉长闪长岩形成于弧前环境。
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5.4 黑木林-硖口驿SSZ型蛇绿混杂岩
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蛇绿岩作为古洋壳和上地幔的残余碎片(Coleman, 1977),为恢复古大洋或古大陆边缘盆地演化历史提供了重要证据。近年来有关国际大洋钻探计划及深海钻探计划最新成果研究表明,多数蛇绿岩代表板块俯冲消减带(SSZ)相关的弧前或弧后扩张环境,只有少数蛇绿岩形成于大洋扩张脊(Pearce et al., 1984; Robertson, 2002; Dilek et al., 2011, 2014; Pearce, 2014)。本文所研究的木瓜园辉长岩和变安山质玄武岩均具有与俯冲作用相关的岛弧属性,黑木林、硖口驿辉长闪长岩为类玻安质及过渡类型岩石,与Bonin弧前玻安岩类似,产出于弧前环境,与木瓜园辉长岩和变安山质玄武岩组合在一起显示出岛弧拉斑玄武岩-玻安岩地球化学特征的岩浆活动。结合带内出露的大量超镁铁质岩(蛇纹岩)和变玄武岩、变安山岩和变中酸性火山岩,可能共同组成了新元古代早期(835~830Ma)黑木林-硖口驿SSZ型蛇绿混杂岩带。
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图11 黑木林-硖口驿镁铁质岩微量元素构造判别图解
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Fig.11 Tectonic environment discrimination of the mafic rocks in Heimulin-Xiakouyi area
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(a)—Nb/Th-Nb图解(据李曙光, 1993);(b)—Hf/3-Th-Nb/16图解(据Wood, 1980);(c) —Nb/Yb-Th/Yb图解 (据Pearce, 2008);(d)—ZrN/SmN-NbN/ThN图解(据Godard et al., 2003)
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(a)—Nb/Th-Nb diagram(after Li Shuguang, 1993); (b)—Hf/3-Th-Nb/16diagram(after Wood, 1980);(c)—Nb/Yb-Th/Yb diagram (after Pearce, 2008);(d)—ZrN/SmN-NbN/ThN diagram(after Godard et al., 2003)
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5.5 构造意义
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前人对于扬子板块西北缘新元古代早—中期(约835~720Ma)的岩浆岩所产出的构造背景仍存在争议,地幔柱裂解模型的观点认为,扬子板块西北缘新元古代早—中期火山岩(如碧口微地块中碧口群、汉南地区西乡群和米仓山地区铁船山组火山岩)主体均为大陆裂谷玄武岩或双峰式火山岩,形成于与地幔柱作用相关的大陆裂谷环境(徐学义等, 2002, 2009a, 2009b, 2010; 夏林圻等, 2007; Wang Xuance et al., 2008; Wu Tao et al., 2019)。近年来的研究表明,扬子板块西北缘新元古代早—中期火山岩的岩石成因十分复杂,可能并非形成于大陆裂谷环境。碧口微地块内的关口垭闪长岩体(885Ma)、坪头山闪长岩体(855Ma)(Xiao Long et al., 2007)、铜厂闪长岩体(879~824Ma)(叶霖等, 2009; Wang Wei et al., 2012)和二里坝花岗闪长岩体(820Ma)(秦利等, 2021),以及汉南-米仓山微地块内的碑坝辉长岩和辉长闪长岩(880~870Ma)、天平河花岗岩(860Ma)(Luo Biji et al., 2018)、望江山辉长岩-闪长岩套(819~784Ma)(Zhou Meifu et al., 2002a, 2002b; Dong Yunpeng et al., 2011)、酉水辉长岩和辉绿岩(795~752Ma)、西家坝辉长岩(798Ma)(Dong Yunpeng et al., 2011)等,均具有与俯冲作用相关的岩浆弧属性,表明扬子板块西北缘长期处于与洋壳持续俯冲作用相关的构造背景。扬子板块西北缘新元古代早—中期(约835~720Ma)岩浆作用持续时间要远长于典型的地幔柱相关的岩浆作用,更类似于活动大陆边缘的弧岩浆作用(Dong Yunpeng et al., 2011, 2012; Luo Biji et al., 2018)。Gao Feng et al.(2020a, 2020b)研究认为横丹群的物源区主体为新元古代中性—酸性岩浆岩且具有弧属性,且横丹群最可能于740~717Ma沉积于扬子板块西北缘(现今位置)的弧前盆地,从沉积学的角度约束了扬子板块西北缘在新元古代早—中期(835~720Ma)可能为活动大陆边缘构造环境。综上所述,结合前人关于扬子板块西北缘大量新元古代早—中期与俯冲作用相关的岩浆岩和沉积岩的研究成果,本文认为扬子板块西北缘在新元古代早—中期(950~720Ma)处于长期的洋壳东南向俯冲的活动大陆边缘构造环境,并指示扬子板块西北缘在新元古代早—中期(950~720Ma)发生俯冲增生造山作用(Dong Yunpeng et al., 2011, 2012, 2017; Luo Biji et al., 2018; Ao Wenhao et al., 2019),且新元古代早期(835~830Ma)在扬子板块西北缘发生了洋内俯冲,形成黑木林-硖口驿SSZ型蛇绿混杂岩带,最终增生拼贴就位于碧口微地块中。
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6 结论
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(1)锆石LA-ICP-MS U-Pb同位素测年结果显示,木瓜园辉长岩、黑木林和硖口驿辉长闪长岩的结晶年龄分别为835±4Ma、830±2Ma和830±11Ma,表明黑木林-硖口驿SSZ型蛇绿混杂岩带的形成时代为835~830Ma。
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(2)木瓜园辉长岩和变安山质玄武岩富集大离子亲石元素元素、亏损高场强元素,具有岛弧岩浆岩的地球化学特征;其岩浆在上升演化过程中均经历了一定程度的分离结晶和弱的地壳混染作用。
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(3)黑木林和硖口驿辉长闪长岩地球化学特征显示为类玻安质及其过渡类型岩石,富集大离子亲石元素,亏损高场强元素,其低CaO/Al2O3比值、Ti/Zr比值以及稀土元素配分曲线和N-MORB标准化微量元素蜘蛛网图与Bonin弧前玻安岩类似等特征,表明其形成于弧前环境。
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(4)综合扬子板块西北缘新元古代早—中期构造演化过程的研究成果,本文提出扬子板块西北缘在新元古代早—中期(950~720Ma)处于长期洋壳东南向俯冲的活动大陆边缘构造环境并在发生俯冲增生造山作用,新元古代早期(835~830Ma)的洋内俯冲形成黑木林-硖口驿SSZ型蛇绿混杂岩带,最终增生拼贴就位于碧口微地块。
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致谢:对审稿专家和责任编委提出的宝贵修改意见和建议表示衷心的感谢,感谢天津地质矿产研究所同位素实验室和西北大学大陆动力学国家重点实验室工作人员在锆石LA-ICP-MS U-Pb年龄测定及调试过程中给予的支持和帮助。
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注释
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❶ 陕西地质矿产勘查开发局综合研究队.1996.何家岩镇幅地质图(1∶50000).西安地图出版社.
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❷ 陕西省地质调查院.2007.略阳县幅地质图(1∶250000).西安地图出版社.
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摘要
对于扬子板块西北缘新元古代早—中期(835~720 Ma)岩浆岩形成的构造背景尚存在较大争议。本文对陕南勉略宁地区碧口微地块内黑木林-硖口驿SSZ型(俯冲带上盘型)蛇绿混杂岩中的辉长岩、辉长闪长岩和变安山质玄武岩分别进行了锆石LA-ICP-MS U-Pb年代学、岩石学及岩石地球化学研究。研究结果表明,木瓜园辉长岩和变安山质玄武岩属岛弧拉斑玄武岩系列,稀土元素球粒陨石标准化配分曲线均呈右倾型,具有微弱的Eu负异常,微量元素原始地幔标准化图解显示其富集大离子亲石元素(LILE)和轻稀土元素(LREE)而亏损高场强元素(HFSE,如Nb、Ta等),与岛弧岩浆岩相似。黑木林、硖口驿辉长闪长岩属钙碱性系列。黑木林辉长闪长岩具与Bonin弧前玻安质岩石类似的地球化学特征,具较高的SiO2含量(54.27%~56.79%)、MgO含量(11.10%~17.04%)和较低的TiO2含量(0.08%~0.19%),富集大离子亲石元素Rb、Th、U和Pb,亏损高场强元素Nb、Ti,以及弱的Zr、Hf正异常等特征。硖口驿辉长闪长岩MgO含量较低(4.24%~8.91%),略低于玻安质岩石,其N-MORB(正常洋中脊玄武岩)标准化微量元素蜘蛛网图与Bonin弧前玻安岩类似,富集LILE、亏损HFSE,具低的Ti/Zr比值等特征,属于过渡类型的类玻安质岩石。锆石LA-ICP-MS U-Pb测年结果显示,黑木林-硖口驿SSZ型蛇绿混杂岩的原岩形成时代为835~830左右。其中木瓜园辉长岩、黑木林和硖口驿辉长闪长岩的结晶年龄分别为835±4 Ma、830±2 Ma和830±11 Ma。结合前人关于扬子板块西北缘新元古代早—中期构造演化过程的研究,认为扬子板块西北缘在新元古代早—中期(950~720 Ma)处于长期的洋壳东南向俯冲的活动大陆边缘环境并发育俯冲增生造山作用,且新元古代早期(835~830 Ma)在扬子板块西北缘发生了洋内俯冲,形成黑木林-硖口驿SSZ型蛇绿混杂岩,最终增生拼贴就位于碧口微地块。
Abstract
The tectonic background of the early to middle Neoproterozoic (835~720 Ma) magmatic rocks in the northwestern margin of the Yangtze block is still in debate. We report the zircon LA-ICP-MS U-Pb ages, petrological and geochemical data of the gabbro, gabbro-diorite, and meta-andesitic basalt from the Heimulin-Xiakouyi suprasubduction zone (SSZ) ophiolitic mélange in Bikou Terrane, Mianxian-Lueyang-Ningqiang area, southern Shaanxi Province. The results show that the Muguayuan gabbro and meta-andesitic basalts belong to tholeiitic basalt series with right-dipping chondrite-normalized REE patterns, weak negative Eu anomalies. The primitive mantle normalized incompatible element patterns show the enrichment of LILE and LREE and depletion of HFSE (such as Nb, Ta, etc.), which is similar to island arc magmatic rocks. The Heimulin and the Xiakouyi gabbro-diorite belong to calc-alkaline series. The Heimulin gabbro-diorite displays similar geochemical characteristics to the Bonin fore-arc boninite, with relatively high SiO2 content (54.27%~56.79%), MgO content (11.10%~17.04%), and low TiO2 content (0.08%~0.19%). The trace elements of the whole rock have the characteristics of enrichment of LILE (Rb, Th, U and Pb), depletion of HFSE (Nb, Ti), and weak Zr-Hf positive anomalies. The Xiakouyi gabbro-diorite also shares similar geochemical signatures to boninite, with relatively lower MgO content (4.24%~8.91%). In addition, the N-MORB (normal mid-ocean-ridge basalt) -normalized trace element spider diagrams are similar to Bonin fore-arc boninitic rocks, which is enriched in LILEs and depleted in HFSEs, and show low Ti/Zr ratios, indicating that it could belong to transitional type of boninite-like rocks. Zircon LA-ICP-MS U-Pb dating results show that the age of Heimulin-Xiakouyi SSZ ophiolitic mélange is about 835~830 Ma. The crystallization age of Muguayuan gabbro, Heimulin and Xiakouyi gabbro-diorite are 835±4 Ma, 830±2 Ma and 830±11 Ma, respectively. Combined with previous research on the tectonic evolution of the early to middle Neoproterozoic in the northwestern margin of the Yangtze block, we suggest that the northwestern margin of the Yangtze block may be an active continental margin environment of long-term southeastward subduction of oceanic crust in the early to middle Neoproterozoic (950~720 Ma), and may develop subduction-accretion orogeny. In the early Neoproterozoic (835~830 Ma), intra-oceanic subduction occurred on the northwestern margin of the Yangtze block, forming the Heimulin-Xiakouyi SSZ ophiolitic mélange, and the final accretion collage was located in the Bikou Terrane.
