南秦岭佛坪地区地层格架与物源分析:变质沉积岩中碎屑锆石LA-ICP-MS U-Pb定年提供的制约
  岩石学报  2018, Vol. 34 Issue (5): 1484-1502   PDF    
南秦岭佛坪地区地层格架与物源分析:变质沉积岩中碎屑锆石LA-ICP-MS U-Pb定年提供的制约
刘志慧1,2 , 罗敏3 , 陈龙耀1 , 曲玮1 , 刘晓春1     
1. 中国地质科学院地质力学研究所, 北京 100081;
2. 北京大学地球与空间科学学院, 北京 100871;
3. 大庆油田有限责任公司第四采油厂地质大队, 大庆 163511
摘要:南秦岭佛坪地区位于东西秦岭交汇部位,以前寒武纪基底穹状隆升为主要特征,因其特殊的构造位置和变质变形特征而受到广泛关注。为了更好地揭示南秦岭构造带的性质及其在秦岭造山带构造演化中的作用,本文对佛坪地区的前寒武纪基底和沉积盖层进行了系统的碎屑锆石LA-ICP-MS U-Pb年代学研究。佛坪穹隆核部前寒武纪基底中碎屑锆石主要年龄峰值为600~820Ma、2021Ma和2467Ma,其中3个样品给出最年轻的年龄区间约为615~728Ma,沉积时代不早于新元古代,另外1个样品给出的最年轻的年龄峰值为1113Ma,沉积时代不早于中元古代,否定了关于认为其为太古代或古元古代的变质结晶基底的认识,其碎屑物质可能来源于南秦岭构造带和扬子陆块北缘。佛坪穹隆上覆盖层和外围南侧盖层给出了相似的年龄图谱,主要年龄峰值为410~450Ma、650~880Ma和910~950Ma,其中最年轻的年龄区间为344~416Ma,表明其沉积时代晚于泥盆纪,碎屑物质来源为北秦岭构造带和南秦岭构造带。上述研究结果表明,南秦岭构造带(前寒武纪基底)于新元古代已增生为扬子陆块北缘的重要组成部分,于晚古生代(泥盆纪)紧邻华北-北秦岭构造带南缘,并共同为南秦岭大面积的泥盆纪盆地提供碎屑物质。
关键词: 碎屑锆石     LA-ICP-MS U-Pb定年     物质来源     变质沉积岩     佛坪地区    
Stratigraphic framework and provenance analysis in the Foping area, the South Qinling tectonic belt: Constraints from LA-ICP-MS U-Pb dating of detrital zircons from the metasedimentary rocks.
LIU ZhiHui1,2, LUO Min3, CHEN LongYao1, QU Wei1, LIU XiaoChun1     
1. Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China;
2. School of Earth and Space Sciences, Peking University, Beijing 100871, China;
3. Geological Brigade of NO.4 Oil Production Plant, Daqing Oil Field Company, Daqing 163511, China
Abstract: The Foping area in the South Qinling tectonic belt, located at the junction of the East and West Qinling orogen, is characterized by the domal uplifted Precambrian metamorphic basement. Its particular tectonic location and characteristics on metamorphic and deformation have drawn extensive attention. In this paper, systematic LA-ICP-MS U-Pb dating of the detrital zircons from Precambrian basement and Phanerozoic sedimentary covers in the Foping area has been used to reveal the nature of the South Qinling tectonic belt and the role it plays in the tectonic evolution of the Qinling orogen. The age spectrums of detrital zircons from the four Precambrian basement samples in the core of Foping dome show the major populations at 600~820Ma, 2021Ma and 2467Ma. Three of the samples show a youngest population range of 615~728Ma, suggesting the deposition time is no earlier than the Neoproterozoic. While the other one has the youngest population at 1113Ma, suggesting the deposition time is no earlier than the Mesoproterozoic. Thus, the Precambrian metamorphic basement of the Foping dome is not formed in the Archean or Paleoproterozoic, its sediments might derive from the South Qinling tectonic belt and the northern margin of Yangtze Block. Sedimentary covers in Foping dome and its peripheral strata display similar age spectrums. The youngest populations range from 342~416Ma, and the major populations show three ranges of 410~450Ma, 650~880Ma and 910~950Ma. These data suggest that the sedimentary covers deposited after the Devonian, and sediments were probably derived from the North and South Qinling tectonic belt. Based on those analyses above, we propose that the South Qinling tectonic belt accreted to the northern margin of the Yangtze Block in the Neoproterozoic, and was adjacent to the southern margin of the North China Block-North Qinling tectonic belt in the Paleozoic. The North and South Qinling tectonic belt provided the clastic sediments to the widespread Devonian basins.
Key words: Detrital zircons     LA-ICP-MS U-Pb ages     Material source     Metasedimentary rocks     Foping area    

秦岭造山带作为华北陆块与扬子陆块间重要的连接带,其自身的演化历史对于理解中国大陆形成演化起着关键的作用(许志琴等, 1988; 张国伟等, 2001; Ratschbacher et al., 2003; Dong et al., 2011; Wu and Zheng, 2013; Dong and Santosh, 2016)。同时,秦岭造山带作为一个典型的复合型造山带,其自身复杂的物质组成和造山记录对理解大陆的增生、消减具有重要的指示意义。近年来,研究者们在秦岭造山带的形成和演化上进行了大量的岩石学、同位素年代学和地球化学等方面的工作,其显生宙以来的演化历史可以归纳为早古生代增生造山(北秦岭)、中生代碰撞造山(南秦岭)及白垩纪以来的陆内造山过程(许志琴等, 1988; 张国伟等, 2001; Dong et al., 2011; Liu et al., 2013; Wu and Zheng, 2013; Dong and Santosh, 2016),然而,在南秦岭基底时代和性质上长期以来都存在着较大的争议。

南秦岭佛坪地区位于东西秦岭交汇部位,以前寒武纪基底穹状隆升出露和大规模中生代岩浆岩侵入为主要特征,其特殊的构造位置和变质变形特征一直是秦岭造山带地质学研究的焦点地区之一。现阶段,对于该地区岩石组成、岩浆-变质作用和构造解析方面的研究已取得了初步的认识(张国伟等, 1995; 王根宝, 1997; 王根宝和李三忠, 1998; 魏春景等, 1998; 李三忠等, 2003; 张宗清等, 2004; 查显锋等, 2010; Yang et al., 2011; Zhang et al., 2016)。然而,对佛坪地区整体的地层格架、基底和盖层的分布特征研究程度较低,零星的同位素年代学研究给出的年龄结果也不尽相同,严重制约了南秦岭乃至整个秦岭造山带构造演化历史的研究。

本文采用LA-ICP-MS技术对佛坪地区出露的前寒武纪基底和沉积盖层进行了系统的锆石U-Pb测年。试图: 1)建立佛坪地区地层年代学框架;2)揭示佛坪地区基底和盖层的物源区特征;3)探讨南秦岭构造带性质及其在秦岭造山带中的作用。

1 区域地质背景

秦岭造山带是一条横亘于中国大陆中部的大型造山带,北以洛南-栾川断裂为界与华北陆块南缘分开,南以勉略-巴山-襄广断裂为界与扬子陆块相隔(图 1a)。根据板块构造模式,一般以商丹缝合带为界将秦岭造山带的主体分为北秦岭构造带和南秦岭构造带(图 1b)(Mattauer et al., 1985; Hsü et al., 1987;许志琴等, 1988; 张国伟等, 1988; 王清晨等, 1989)。随后,任纪舜(1990, 1991)考虑到南秦岭构造带的复杂性,以山阳断裂为界将其细分为中秦岭构造带和南秦岭构造带。王宗起等(2009)基于系统的岩石学、地球化学、同位素年代学、地层古生物学和区域地质研究将商丹带和勉略带之间的南秦岭总结为中秦岭弧前盆地系和南秦岭增生杂岩带。

图 1 秦岭造山带的大地构造位置(a, 据Dong et al., 2011修改)、秦岭造山带大地构造简图(b, 据Dong et al., 2012; Hu et al., 2017)和佛坪地区地质简图及取样位置(c, 据陕西省地质矿产局, 1999) Fig. 1 The location of Qinling orogenic belt within China (a, modified after Dong et al., 2011), simplified tectonic map of the Qinling orogenic belt showing the tectonic divisions (b, modified after Dong et al., 2012; Hu et al., 2017) and geological sketch of the Foping area with the locations of the studied samples (c)

① 陕西省地质矿产局. 1999.中华人民共和国区域地质调查报告——1:50000佛坪县幅(I48E015024)

北秦岭构造带自北向南可分为宽坪岩群、二郎坪岩群、秦岭岩群和丹凤岩群,彼此之间均为断层接触。宽坪岩群主要由基性火山岩、碳酸岩(大理岩)和泥砂质岩互层组成,经历了绿片岩相-角闪岩相变质(张寿广等, 1991; 闫全人等, 2008; 胡娟等, 2010; Liu et al., 2011)。变质基性岩的锆石U-Pb年龄为943~611Ma(闫全人等, 2008; 第五春荣等, 2010),变质沉积岩碎屑锆石主要峰值年龄为2425Ma、1750Ma、970Ma、790Ma和610Ma(第五春荣等, 2010; Zhu et al., 2011; Liu et al., 2013)。关于宽坪岩群的形成环境主要有华北陆块南缘被动大陆边缘(Xue et al., 1996; Ratschbacher et al., 2003, 2006; Hacker et al., 2004)和弧后盆地(张寿广等, 1991; 陆松年等, 2003; 王宗起等, 2009; 闫全人等, 2009)等认识。二郎坪岩群以早古生代(490~440Ma)基性-酸性火山岩和碳酸盐岩组合与细碎屑岩混杂为特点,经历了绿片岩相-角闪岩相变质,其形成环境被认为是弧后盆地(张国伟等, 1988, 1995, 2001; 孙勇等, 1996; Meng and Zhang, 2000; Dong et al., 2011)或洋内岛弧(Xue et al., 1996; 陆松年等, 2003; Ratschbacher et al., 2003; Hacker et al., 2004; 王宗起等, 2009)。秦岭岩群是北秦岭构造带中最高级别的变质单元,其原岩主要为一套富铝、富炭的沉积岩系,包含若干花岗质片麻岩侵入体。其中,变质沉积岩碎屑锆石的主要峰值年龄为>2400Ma、2300~2100Ma、1900~1500Ma和1000~600Ma(陆松年等, 2006; 时毓等, 2009; 杨力等, 2010; 万渝生等, 2011)。花岗质片麻岩主要年龄介于980~910Ma之间,个别岩体较年轻,为860~840Ma(陆松年等, 2003; 张成立等, 2004; 王涛等, 2005; 时毓等, 2009; Wang et al., 2009, 2013b)。丹凤岩群为一套强烈变形的绿片岩相-低角闪岩相变质的基性火山岩和碎屑沉积岩为主的构造混杂岩带,其间发育少量变质中酸性火山岩,其形成时代为约530~460Ma(陆松年等, 2003; 裴先治等, 2005, 2007; Dong et al., 2011)。在构造属性上有蛇绿岩(许志琴等, 1988; 张国伟等, 1988; Dong et al., 2011)、岛弧火山岩(Xue et al., 1996; Ratschbacher et al., 2003, 2006; Hacker et al., 2004)以及多种环境下形成的岛弧杂岩带(王宗起等, 2009)等不同的认识。整个北秦岭构造带内古生代岩浆作用广泛发育,可划分为507~470Ma、460~422Ma和415~400Ma三个阶段,分别对应北秦岭构造带洋陆俯冲到碰撞、伸展的过程(Wang et al., 2009, 2013b; Dong et al., 2011; 张成立等, 2013; Wu and Zheng, 2013; Dong and Santosh, 2016)。

南秦岭构造带主要由前寒武纪结晶基底和早古生代-三叠纪沉积地层所构成(张国伟等, 2001; Dong et al., 2011)。南秦岭零星出露的前寒武纪地体以陡岭杂岩、耀岭河群、武当山群和佛坪岩群等为代表。其中陡岭杂岩为新太古代(2.5Ga)结晶基底(Hu et al., 2013; Wu et al., 2014; 聂虎, 2016);武当山群出露面积最大,形成于830~750Ma(Ling et al., 2008, 2010);耀岭河群分布范围最广,形成于810~680Ma(Ling et al., 2008; Zhu et al., 2014, 2015);佛坪岩群形成时代争议最大,多被认为是形成于太古代或古元古代的结晶基底(王根宝, 1997; 李海平, 1998; 李亚林等, 2000; 张宗清等, 2004)。早古生代-三叠纪沉积地层主要包括寒武-奥陶纪灰岩、志留纪页岩、泥盆-石炭纪碎屑沉积岩夹灰岩和少量三叠纪浊积岩。早期的观点认为这些早古生代-三叠纪地层形成于扬子陆块北缘被动大陆边缘(Mattauer et al., 1985; 许志琴等, 1988; 张国伟等, 1988; 王清晨等, 1989; 任纪舜, 1990; Meng and Zhang, 1999; 和政军等, 2005)。王宗起等(2009)则认为这些地层属于于寒武纪-三叠纪之间古秦岭洋双向俯冲背景下的弧前盆地沉积和增生杂岩。Yan et al.(2006a, b, 2012, 2015)对泥盆-石炭纪地层碎屑锆石和砂岩地球化学的研究表明其形成北秦岭大陆岛弧南缘弧前盆地环境。Wang et al. (2017b)对志留纪地层(白水江群)碎屑锆石和砂岩地球化学的研究表明其沉积时代为晚古生代,物质来源为北秦岭构造带和扬子陆块北缘。此外,各地层中广泛侵入呈带状近东西向展布的中生代岩体,包括构造带西段的温泉岩体、中川岩体、何家庄岩体、糜署岭岩体,中部的光头山岩体群和五龙岩体群,东段的东江口岩体群,这些花岗类岩体的年龄集中在248~190Ma之间(秦江锋和赖少聪, 2011; Yang et al., 2011, 2012; Dong et al., 2012; Deng et al., 2016)。

佛坪地区位于秦岭造山带最狭窄的蜂腰部位,也是秦岭造山带东西差异分界的关键部位(陕西省地质调查院, 2017)。它主要由穹隆核部的前寒武纪基底杂岩及上覆盖层长角坝岩群以及穹隆外围晚古生代沉积盖层和侵入于其中的中生代岩浆岩所构成。前寒武纪基底杂岩位于佛坪穹隆的核部,由佛坪县城的佛坪岩群和龙草坪片麻岩套两部分组成。佛坪岩群主要由黑云斜(二)长片麻岩、石榴黑云斜(二)长片麻岩岩、角闪斜长片麻岩、夕线石黑云斜长片麻岩和二辉麻粒岩等组成。龙草坪片麻岩套主要由角闪斜长片麻岩-黑云斜长片麻岩组成,包括滴水岩组(黑云二长片麻岩、黑云斜长片麻岩等),南沟岩组(黑云角闪二长片麻岩、角闪黑云斜长片麻岩等)和温泉岩组(黑云斜长片麻岩、角闪黑云钾长片麻岩等)(陕西省地质矿产局, 1999; 陕西省地质调查院, 2017)。同时,二者均经历了角闪岩相-麻粒岩相的变质作用,并发育大量与深熔作用有关的花岗伟晶岩脉和条带(魏春景等, 1998; Zhang et al., 2016)。穹隆上覆盖层在1:5万佛坪县幅中命名为长角坝岩群(陕西省地质矿产局, 1999),主要岩石类型包括石墨大理岩,石英岩,黑云石英片岩,二云石英片岩等,原岩为一套富铝、富炭的沉积岩系。穹隆上覆盖层与基底佛坪岩群之间为脆-韧性断层接触,经历了高绿片岩相-角闪岩相的变质。穹隆基底和盖层遭受了多期多层次构造作用的改造,原始沉积叠覆关系已遭彻底改造破坏,仅表现为构造岩片的叠置关系(陕西省地质调查院, 2017)。穹隆外围盖层自北向南分别为泥盆系和志留系,泥盆系主要由变质砂岩、黑云斜长片麻岩、大理岩及石英岩夹层构成,志留系主要由二(黑)云石英片岩和变质砂岩构成,整体上这两套地层达绿片岩-角闪岩相。佛坪地区出露的中生代侵入体主要为五龙岩体群,包括五龙岩体,老城岩体,龙草坪岩体,华阳岩体和西岔河岩体等,该岩体群是一个分带的多阶段花岗岩类侵入体,从边部的石英闪长岩到中间带的花岗闪长岩,再到岩体中心的二长花岗岩,侵入时代从235Ma持续到207Ma。佛坪地区岩浆-变质作用和构造解析方面的研究表明,五龙岩体群的形成与佛坪穹隆的变质作用及穹隆核部基底杂岩的隆升有密切的时空关系(王居里, 1997; 王娟等, 2008; 杨朋涛等, 2012; Dong et al., 2012; Qin et al., 2013; Zhang et al., 2016)。

2 样品与分析方法 2.1 样品描述

本文研究样品自北向南分别采自长角坝镇龙草坪地区、佛坪县城和洋县等地区。其中样品FP03-2、FP05-1、FP07-1采自龙草坪片麻岩套,样品FP06-1采自龙草坪小南沟一带,样品FP17-1、FP24-5、FP26-3采自佛坪县城的佛坪岩群,样品FP12-1采自长角坝岩群,样品YX01-1和YX13-1则采自洋县一带的晚古生代沉积盖层(图 1c)。这些样品的位置、矿物组合计年龄结果列于表 1中,代表性样品野外照片和镜下照片见图 2

表 1 佛坪地区变质沉积岩样品的位置、岩性、矿物组合和锆石U-Pb定年结果 Table 1 Sampling sites, lithology, mineral assemblages and U-Pb dating results of the metasedimentary rocks in the Foping area

图 2 佛坪地区典型变质沉积岩样品野外及镜下照片 (a、b)细粒黑云斜长麻岩(样品FP03-2); (c、d)绿帘黑云斜长片麻岩(样品FP06-1); (e、f)中粒石榴黑云斜长片麻岩(样品FP26-3); (g、h)石榴夕线二云石英片岩(样品YX13-1) Fig. 2 Photographs and photomicrographs of the representative metasedimentary rock samples in the Foping area (a, b) fine-grain biotite-plagioclase gneiss (Sample FP03-2); (c, d) epidote-bearing biotite-plagioclase gneiss (Sample FP06-1); (e, f) medium-grain garnet-bearing biotite-plagioclase gneiss (Sample FP26-3); (g, h) garnet-sillimanite-bearing two-mica quarts schist (Sample YX13-1)
2.1.1 龙草坪片麻岩套

3个样品FP03-2、FP05-1和FP07-1分别采自佛坪县长角坝镇温泉、大南沟和火地湾一带,岩性为(绿帘)黑云斜长片麻岩,整体呈细粒粒状变晶结构,片麻状构造。主要矿物组合为斜长石(20%~45%)、石英(30%~50%)、黑云母(10%~25%)、绿帘石(5%~8%),副矿物为榍石和不透明金属矿物等。斜长石多成粒状或板状,发育聚片双晶,大小约为1.0~1.5mm,个别绢云母化。石英具有明显的波状消光,可见熔蚀港湾边和包含结构。绿帘石主要呈粒状,粒径0.2~0.5mm,分布不均。除样品FP03-2外,另外2个样品中黑云母强烈定向排列,并构成片麻理。3个样品均经历了绿帘角闪岩相变质作用。其中样品FP03-2和FP07-1采自前人所述典型的温泉片麻岩露头处,野外可见该类型岩石被大量浅色体侵入,并发生褶皱,片理产状220°∠50°。样品FP05-1属南沟片麻岩典型代表,采自长角坝镇大南沟一带,宏观上呈现条带状中细粒片麻岩特点,实际上是黑云斜长片麻岩混合岩化的结果,淡色条带密集,宽者为伟晶岩质。野外及镜下观察认为其总体特征与温泉片麻岩差别不大,测得片理产状为108°∠43°(图 2a, b)。

2.1.2 佛坪岩群

样品FP06-1为绿帘黑云斜长片麻岩,采自长角坝镇小南沟一带。野外测得其片理产状为355°∠35°。粒状片状变晶结构,片麻状构造。黑云母较富集,淡色条带较多,岩石特征与龙草坪地区的大面积的片麻岩套不同,值得注意的是在1:5万佛坪县副中被归为太古代佛坪沉积岩(主体见于佛坪县城一带)。主要矿物组合为绿帘石(5%~10%)、黑云母(25%~35%)、斜长石(35%~40%)、石英(20%~30%),表明其变质程度为绿帘角闪岩相(图 2c, d)。

样品FP17-1为条带状石榴黑云斜长片麻岩,采自佛坪县城南侧东岳殿村一带。岩石被伟晶岩脉沿片麻理方向侵入,测得片理产状为170°∠30°。岩石呈斑状变晶结构,片麻状构造。主要矿物组合为石榴石(5%~10%)、黑云母(20%~23%)、白云母(7%~10%)、长石(40%~45%)和石英(10%~15%)。变斑晶主要由石榴石构成,粒径大者可达4mm,呈筛状变晶,包含有云母及石英等矿物包体,石榴石边缘及黑云母边缘可见绿泥石化。岩石经历了角闪岩相变质。

样品FP24-5为粗粒石榴黑云斜长片麻岩,采自佛坪县城附近的桃园一带。野外露头发育大量的深熔条带。镜下呈现斑状变晶结构,片麻状构造。矿物组合为石榴石(15%~18%)、夕线石(5%~10%)、黑云母(30%~35%)、长石(30%~35%)、石英(10%~15%),表明其经历了高角闪岩相变质。石榴石变斑晶粒度约0.5~1mm,可见石榴石包裹夕线石。夕线石除少量针柱状外,多以纤维状集合体形式组成纤状变晶结构。基质中黑云母向夕线石转化现象明显,推测亦存在白云母向夕线石的转化(白云母无残留)。

样品FP26-3为中粒石榴黑云斜长片麻岩,采自佛坪县城附近的水田沟一带。条带状石榴黑云斜长片麻岩典型露头,有一定数量的淡色体发育,淡色体内也含有石榴石。此层厚度在10m以上,产状平缓且波状起伏,为95°∠10°。斑状变晶结构,片麻状构造。主要矿物组合为石榴石(12%~15%)、夕线石(3%~7%)、黑云母(30%~35%)、长石(25%~30%)和石英(20%~30%),表明岩石变质程度达高角闪岩相。变斑晶石榴石粒径整体 < 0.5mm,发育裂隙但颗粒整体保存较好,夕线石多呈针柱状,与云母伴生,亦可见少量纤维状集合体(图 2e, f)。

2.1.3 长角坝岩群

样品FP12-1为石墨石英岩,采自长角坝镇教场坝村一带,是长角坝岩群较具代表性的样品。野外观测其整体呈厚层条带状,测得片理产状为20°∠65°。镜下观察岩石为粒状变晶结构,厚层状-块状构造。主要矿物组合为石英(80%)、石墨(7%~10%)、白云母(5%~7%)及少量绿帘石(< 3%)。绿帘石颗粒大者粒径可达1.5~2mm,石英发育熔蚀状港湾边,石墨沿矿物间隙定向发育。岩石整体变质程度不高。

2.1.4 晚古生代沉积盖层

样品YX01-1为二云石英片岩,采自洋县的牛角坝一带。片理产状为190°∠35°。主要矿物组合黑云母(25%~30%)、白云母(25%~35%)、石英(30%~40%)和斜长石(5%~10%),其他为少量榍石、锆石等副矿物。黑云母和白云母平衡共生,并强烈定向,岩石成鳞片变晶结构,片状构造。变质程度较低。

样品YX13-1为含石榴夕线二云石英片岩,采自洋县金水镇滩背一带。野外测得片理产状355°∠75°。变余斑状结构,片状构造。变斑晶石榴石含量为5%~8%,粒径0.5~1.5mm都有发育,包裹大量的斑点状物质,可分辨出少量黑云母、白云母、石英和不透明矿物。夕线石多呈毛发状(纤维状集合体),生长在云母周围,亦可见个别发育较好的针柱状夕线石,含量 < 5%,基质中黑云母和白云母强烈定向,构成岩石的片理,其中黑云母含量为15%~25%、白云母含量为10%~15%、石英含量为40%~45%、长石含量为10%~15%。此外还含有少量的石墨、榍石、锆石等副矿物(图 2g, h)。

2.2 分析方法

锆石的单矿物分离在南京宏创地质勘查技术服务有限公司完成。将人工重砂分离出的锆石制成以环氧树脂为基础的样品靶并进行抛光,抛光后对所研究锆石进行透射光、反射光照相,以观察其抛光面是否有裂隙等,然后进行阴极发光(CL)拍照,来反应锆石的内部结构特征,并根据锆石的内部结构特征选取U-Pb同位素测定分析部位。

锆石U-Pb同位素定年在武汉上谱分析科技有限责任公司LA-ICP-MS上完成。详细的仪器参数和分析流程见Zong et al. (2017)。GeolasPro激光剥蚀系统由COMPexPro 102 ArF 193nm准分子激光器和MicroLas光学系统组成,ICP-MS型号为Agilent 7700e。激光剥蚀过程中采用氦气作载气、氩气为补偿气以调节灵敏度,二者在进入ICP之前通过一个T型接头混合,激光剥蚀系统配置有信号平滑装置(Hu et al., 2015)。本次分析的激光束斑主要为24μm,仅样品FP17-1所用激光束班为32μm,所用频率皆为5Hz。U-Pb同位素定年采用锆石标准91500和玻璃标准物质NIST610作外标进行同位素校正。每个时间分辨分析数据包括大约20~30s空白信号和50s样品信号。对分析数据的离线处理(包括对样品和空白信号的选择、仪器灵敏度漂移校正、元素含量及U-Pb同位素比值和年龄计算)采用软件ICPMSDataCal(Liu et al., 2008, 2010)完成。锆石样品的U-Pb年龄谐和图绘制和年龄加权平均计算采用Isoplot/Ex_ver3(Ludwig, 2003)完成。

3 分析结果

除对样品FP24-5的碎屑锆石按顺序分析93个点外,其他样品皆选120个点顺序进行分析(见电子版附表 1)。除因采用上、下交点法分析年龄数据的样品FP26-3外,其他样品皆选取谐和度>90%的数据点进行统计。并按照一般原则,对于≤1000Ma的数据点取206Pb/238U年龄值,>1000Ma的数据点则取207Pb/206Pb年龄值。从锆石的阴极发光(CL)图像来看(图 3),锆石形态大小多样,长柱状、短柱状、次圆状均有发育。绝大部分锆石具有一定的磨圆度,具有明显的震荡环带结构,显示碎屑锆石的特征。锆石颗粒普遍含有狭窄的生长边,个别生长边较宽,亦可作为独立颗粒出现,发育扇状环带或无-弱分带,属于变质成因。具体分析结果如下。

附表 1 佛坪地区变质沉积岩碎屑锆石的LA-ICP-MS U-Pb分析结果 Appendix1 LA-ICP-MS U-Pb analyses of detrital zircons from metasedimentary rocks in the Foping area

图 3 佛坪地区变质沉积岩中碎屑锆石的阴极发光(CL)图像(年龄误差为1σ) 带数字的圆圈为锆石测点位置和分析点号 Fig. 3 Cathodoluminescene (CL) images of detrital zircons from metasedimentary rocks in the Foping area (Ages are given at 1σ) Circles with numbers are analytical spots with their identification numbers

样品FP03-2中锆石颗粒呈现柱状-次圆状(图 3a),粒径在80~170μm之间,长宽比为2.4~1.2,Th/U=0.06~3.69,绝大部分Th/U> 0.4。该样品碎屑锆石中共获得120个年龄分析点,去掉3个谐和度不足90%的分析点后,获得的年龄1个主要年龄峰值为426Ma,另外4个次要年龄峰值分别为758Ma、941Ma、1694Ma和2490Ma。其中最年轻的一颗碎屑锆石年龄为357±8Ma,最小的年龄峰值为392Ma(图 4a图 5a)。

图 4 佛坪地区变质沉积岩中碎屑锆石的U-Pb年龄谐和图 Fig. 4 Zircon U-Pb concordia diagram of the metasedimentary rocks in the Foping area

图 5 佛坪地区变质沉积岩中碎屑锆石U-Pb年龄频率分布直方图 Fig. 5 Relative probability plots of U-Pb ages for detrital zircon from the metasedimentary rocks in the Foping area

样品FP05-1中锆石颗粒呈现柱状-次圆状(图 3b),粒径在80~110μm之间,长宽比为1.9~1.2,Th/U=0.004~2.93。绝大部分Th/U> 0.4。该样品的锆石中共获得120个年龄分析点,排除8个谐和度不足90%的分析点后,分析得到1个主要年龄峰值为446Ma,另外4个次要年龄峰值分别为742Ma、946Ma、1839Ma、2483Ma。除去一颗CL图像显示扇状环带的变质锆石外,最年轻的碎屑锆石颗粒年龄为341±4Ma,最小的年龄峰值为413Ma(图 4b图 5b)。

样品FP06-1中锆石颗粒呈现柱状-次圆状(图 3c),粒径在70~120μm之间,长宽比为2.4~1.2,Th/U=0.004~3.40。绝大部分Th/U> 0.4。该样品的锆石中共获得120个年龄分析点,排除2个谐和度不足90%的分析点后,分析得到主要年龄峰值为712Ma,次要年龄峰值为817Ma和1817Ma。除去一颗CL图像显示弱分带的变质锆石外,最年轻的碎屑锆石颗粒年龄为581±9Ma,最小的年龄峰值为652Ma(图 4c图 5c)。

样品FP07-1中锆石颗粒呈现柱状-次圆状(图 3d),粒径在70~105μm之间,长宽比为2.2~1.1,Th/U=0.03~2.26。绝大部分Th/U> 0.4。该样品的锆石中共获得120个年龄分析点,排除2个谐和度不足90%的分析点后,获得1个主要年龄峰值为448Ma,另外4个次要年龄峰值分别为740~880Ma、1000~1200Ma、2457Ma。除去一颗CL图像显示无分带的变质锆石外,最年轻的一颗碎屑锆石年龄为321±6Ma,最小的年龄峰值为416Ma(图 4d图 5d)。

样品FP12-1中锆石颗粒多呈现长柱状(图 3e),粒径在60~140μm之间,长宽比为2.3~1.3,Th/U=0.01~1.95,除一颗变质锆石Th/U=0.01,其他比值皆> 0.1,且绝大部分Th/U> 0.4。该样品的锆石中共获得39个年龄分析点,排除3个谐和度不足90%的点后,获得1个主要年龄峰值为429Ma,另外4个次要年龄峰值分别为737Ma、790Ma、1650Ma和2450Ma。除去一颗变质锆石,其中最年轻碎屑锆石颗粒年龄为360±5Ma,最小的年龄峰值为400Ma(图 4e图 5e)。

样品FP17-1中锆石颗粒呈现长柱状-短柱状(图 3f),粒径在80~115μm之间,长宽比为2.2~1.3,Th/U=0.25~1.81,除一颗锆石Th/U=0.25,其他锆石均得出Th/U> 0.4。该样品的锆石中共获得120个年龄分析点,排除3个谐和度不足90%的点后,获得1个主要年龄峰值为808Ma,2个次要年龄峰值分别为760Ma和838Ma。其中最年轻的一颗碎屑锆石年龄为605±8Ma,最小的年龄峰值728Ma(图 4f图 5f)。

样品FP24-5中锆石颗粒呈短柱状和次圆状等(图 3g),粒径为65~105μm之间,长宽比为2.5~1.0,Th/U=0.35~3.00。绝大部分Th/U> 0.4。该样品共获得88个年龄分析点,排除3个谐和度不足90%的点后,分析得到1个最主要的年龄区间为600~950Ma。最年轻的碎屑锆石颗粒的年龄为538±12Ma,最小的年龄峰值为615Ma(图 4g图 5g)。

样品FP26-3中锆石颗粒多呈现柱状-圆状(图 3h),粒径在70~100μm之间,长宽比为1.7~1.0,Th/U=0.02~5.02。大部分Th/U> 0.4。该样品锆石的120个年龄分析点中,偏离谐和线及两条不一致线的6个点不用作年龄分析。其他114个有效分析点中,19个点(绿色点圈)位于谐和线上,年龄区间为1000~1800Ma,最小的207Pb/206Pb年龄峰值为1113±59Ma(n=4, MSWD=0.50),其他年龄峰值包括1200Ma、1600Ma和1800Ma。43个点(红色点圈)落在不一致曲线1上,上交点年龄为2021±35Ma(MSWD=2.0)。52个点(蓝色点圈)落在不一致曲线2上,上交点年龄为2647±44Ma(MSWD=3.0)(图 4h)。

样品YX01-1中锆石颗粒呈现长柱状、短柱状、次圆状和圆状等(图 3i),粒径在55~95μm之间,长宽比为2.0~1.0,Th/U=0.02~1.85。大部分Th/U> 0.4。该样品的锆石中共获得120个年龄分析点,排除13个分析点谐和度小于90%外,其余分析点得到1个主要年龄峰值为415Ma,另外4个次要年龄峰值分别为650~900Ma、1100Ma、1600Ma和2550Ma。其中最年轻的一颗碎屑锆石年龄为372±6Ma,最小的年龄峰值为374Ma(图 4i图 5h)。

样品YX13-1中锆石颗粒呈现长柱状、短柱状和次圆状等(图 3j),粒径在60~110μm之间,长宽比为2.2~1.1,Th/U=0.02~2.22。大部分Th/U> 0.4。该样品的锆石中共获得120个年龄分析点,排除20个谐和度小于90%的点后,分析得到1个主要峰值为414Ma,另外3个次要峰值分别为579Ma、718Ma和867Ma。另外存在1000~1200Ma、1500~1800Ma, 2450Ma、2730Ma等年龄记录。其中最年轻的一颗碎屑锆石年龄为338±5Ma,最小的年龄峰值344Ma(图 4j图 5i)。

4 讨论 4.1 佛坪地区地层年代学框架

佛坪地区地层单元包括穹隆核部的前寒武纪结晶基底佛坪岩群及上覆盖层长角坝岩群以及穹隆外围晚古生代沉积盖层,其中佛坪岩群的形成时代一直存在争议。陕西省地质矿产局(1989)将佛坪岩群划归为晚元古代耀岭河群。王根宝(1997)对龙草坪地区的副片麻岩进行锆石U-Pb定年获得1853±114Ma和2284±26Ma的上交点年龄;李亚林等(2000)对佛坪县城地区佛坪岩群中的3个(角闪)黑云斜长片麻岩样品进行锆石U-Pb定年,分别获得了1847±79Ma、2503±40Ma和2506±24Ma的上交点年龄,确定其形成时代为太古代,但他们测试的数据点均较少且测试点几乎全部集中在下交点附近,其上交点的地质意义是否代表原岩年龄值得讨论。张宗清等(2004)对佛坪变质结晶岩系进行SHRIMP锆石U-Pb测年得出约2000Ma、1200Ma、800Ma、400Ma和200Ma的峰值年龄,将2000Ma解释为原岩年龄,其他年龄为后期构造热事件改造的年龄。陕西省地质调查院(2004)根据区域地层对比认为佛坪岩群形成于新太古代。陕西省地质调查院(2017)报道了佛坪县城含石榴黑云斜长片麻岩SHRIMP锆石U-Pb年龄主要区间为809~637Ma、592~454Ma和192~229Ma,和张宗清等(2004)一样将其解释为后期构造热事件的年龄。由此看出,前人对前寒武纪基底时代的限定多依赖地层对比,或者测试的数据点较少无法对其进行合理的解释。本研究采用LA-ICP-MS锆石U-Pb定年的方法对佛坪岩群的4个变质沉积岩的碎屑锆石进行了年代学的测定,进而对其沉积时代的上限进行限定。其中,位于佛坪县城的3个样品FP17-1、FP24-5和FP26-3中碎屑锆石最年轻的年龄峰值分别为728Ma、615Ma和1113Ma,位于龙草坪小南沟地区的样品FP06-1中碎屑锆石最年轻的年龄峰值为652Ma,而龙草坪地区大部分的片麻岩套样品FP03-2、FP05-1和FP07-1中碎屑锆石最年轻的年龄峰值为392Ma、413Ma和416Ma。由此表明,佛坪岩群大部分基底的沉积时代应该不早于新元古代,佛坪县城区域一小部分基底的沉积时代不早于中元古代,而非太古代或古元古代,同时位于龙草坪地区的佛坪岩群可能仅西侧小南沟地区很少一部分属于新元古代基底。龙草坪片麻岩套大部分样品(FP03-2、FP05-1和FP07-1)中碎屑锆石年龄谱图都具有强的420~452Ma的峰值年龄,且最年轻的年龄峰值分别为392Ma、413Ma和416Ma,表明其所代表地层的沉积时代应该晚于泥盆纪。因此,龙草坪片麻岩套并非前人认为的太古代或古元古代结晶基底,而是属于泥盆纪沉积盖层,其中发育的大量混合岩化现象并不是古老基底的证据,而可能是三叠纪岩体的侵入所造成的热变质。

① 陕西省地质调查院. 2004.中华人民共和国区域地质调查报告——1:250000汉中市幅(I48C003004)

长角坝岩群被认为是佛坪穹隆上覆的沉积盖层,1:5万佛坪县幅划为中元古代地层(陕西省地质矿产局, 1999),但是至今尚无直接的年代学证据。本文对长角坝岩群中的1个变质沉积岩样品FP12-1进行了年代学的测定,得到碎屑锆石中最轻年龄的年龄峰值为400Ma,表明其沉积时代应该晚于早泥盆世。因此,长角坝岩群和龙草坪大面积分布的片麻岩(现测定沉积时代晚于泥盆纪)共同构成了佛坪穹隆的上覆盖层岩系。穹隆南侧的外围沉积盖层中2个变质沉积岩样品YX01-1和YX13-1分别取自1:5万佛坪县幅中标注的泥盆系和志留系(陕西省地质矿产局, 1999),然而这两个样品中碎屑锆石给出的最年轻的峰值年龄分别为374Ma和344Ma,表明其沉积时代分别晚于晚泥盆世和早石炭世,因此,佛坪穹隆外围盖层可能并不存在志留纪地层。

综上所述,佛坪地区地层框架可以概括为中-新元古代基底和晚古生代沉积盖层所构成(表 2)。其中佛坪穹隆主要由中-新元古代基底及上覆大面积分布的泥盆纪盖层所构成,二者之间为脆-韧性断层接触,中元古代基底仅分布于佛坪县城,新元古代基底主要出露于佛坪县城和龙草坪西侧小南沟一带,泥盆纪盖层由长角坝岩群和龙草坪地区除新元古代以外的片麻岩系所构成,基底与盖层呈滑脱断层接触。穹隆外围则由泥盆-石炭纪沉积盖层所构成,外围盖层中泥盆系与基底上覆盖层具有相似的时代和碎屑物质组成(详见下文),但是二者在变质程度上则有明显的差别。

表 2 佛坪地区地层时代划分沿革表 Table 2 The evolution table of the geological age and stratigraphic subdivision in the Foping area

① 陕西省地质矿产局. 1965.中华人民共和国区域地质调查报告——1:200000佛坪幅(I-48-XXIV)

4.2 物源分析

佛坪地区主要由新元古代基底和晚古生代沉积盖层构成,新元古代基底的3件变质沉积岩样品的碎屑锆石给出了相似的年龄谱图,主要峰值约为600~820Ma,其他年龄零星散步于1000~2800Ma之间,碎屑物质来源较为单一,同时这一年龄谱图与南秦岭构造带的武当山群具有明显的相似性(Ling et al., 2010)。锆石U-Pb年代学研究表明南秦岭构造带和扬子板块西北缘新元古代的岩浆作用分别发生于850~630Ma和870~710Ma(Ling et al., 2003, 2008, 2010; Dong et al., 2011, 2017; Wang et al., 2013b; 郭现轻等, 2014; Meng et al., 2015; Dong and Santosh, 2016)。因此,600~820Ma年龄段的碎屑物质可能来源于南秦岭构造带和扬子陆块。中元古代基底的变质沉积岩样品FP26-3碎屑锆石给出的主要峰值年龄约为2467Ma、2021Ma,其他年龄散布于1000~1800Ma,这一年龄谱图与扬子陆块北缘神农架地区郑家垭组具有明显的可对比性(邱啸飞, 2012; 徐大良等, 2016)。西秦岭构造带的鱼洞子群和东秦岭构造带的陡岭杂岩中广泛存在2700~2500Ma的锆石记录(秦克令等, 1992; 张宗清等, 2001; 张欣等, 2010; Hu et al., 2013; Wu et al., 2014),约2467Ma年龄段的碎屑物质来源可能与这两个相近的太古代基底具有密切的联系。扬子陆块西北缘的后河杂岩中英云闪长质片麻岩的原岩侵位时代为2081±9Ma(Wu et al., 2012),同时在扬子陆块北部和西南部未变质的盖层岩石中也发现了大量古元古代的碎屑锆石(Greentree et al., 2006; Zheng et al., 2006; Zhao et al., 2010)。因此,约2021Ma年龄段的碎屑物质可能来源于后河杂岩或者扬子陆块未出露的古元古代基底。对于其他散布于1000~1800Ma年龄段的碎屑锆石,两个明显的年龄峰值为1600Ma和1200Ma。现阶段,在扬子陆块尚未见到1000~1800Ma年龄段地质体的报道,仅在华夏地块海南岛抱板群和石碌群有相关年龄的火山岩和花岗岩类的报道(Li et al., 2002, 2008),但是扬子陆块新元古代和显生宙盖层岩石中发现了大量的1600~1200Ma的锆石(Greentree et al., 2006),因此扬子陆块可能存在着未出露或已剥蚀的中元古代基底为其提供碎屑物质。

早期研究者认为南秦岭构造带晚古生代沉积地层是扬子陆块北缘被动大陆边缘沉积,物质来源为扬子陆块和南秦岭构造带(张国伟等, 1988; 王清晨等, 1989; 任纪舜, 1990; Meng and Zhang, 1999; 和政军等, 2005)。王宗起等(2009)则认为南秦岭晚古生代地层形成于弧前盆地环境。随后大量关于南秦岭泥盆纪地层碎屑锆石和砂岩地球化学的研究则表明,其物质来源似乎与北秦岭构造带有着密不可分的关系(闫臻等, 2002, 2007; Yan et al., 2006a, b, 2012, 2015, 2016; 陈义兵等, 2010; Dong et al., 2013; Chen et al., 2014; 陈龙耀等, 2014; 陈浩, 2016; Liao et al., 2017; 李宗耀等, 2017)。本文对佛坪穹隆晚古生代盖层中4个变质沉积岩和穹隆南侧洋县一带晚古生代盖层中2个变质沉积岩进行了碎屑锆石年代学测定,其中,穹隆处盖层的4个样品(FP03-2、FP05-1、FP07-1、FP12-1)与南侧盖层的1个样品(YX01-1)得到的年龄图谱大致相当,其中1个最重要的年龄峰值为410~450Ma,次要年龄峰值为650~880Ma和910~950Ma,其他散布于1000~2500Ma,这一年龄谱图与刘岭群年龄图谱(442Ma、780~850Ma和900~970Ma)基本相当(Dong et al., 2013; 陈龙耀等, 2014; Liao et al., 2017)。南侧盖层另外1个样品YX13-1给出的年龄谱图则有所差别,得到主要的年龄峰值为414Ma、579Ma、718Ma和867Ma,次要年龄峰值为342Ma和1000~1800Ma,其他年龄散布于2000~3000Ma。大量的研究资料表明,北秦岭构造带晚古生代岩浆作用发育于3个时期,分别为507~470Ma、460~422Ma和415~400Ma(Wang et al., 2009, 2013b; Dong et al., 2011; 张成立等, 2013; Dong and Santosh, 2016)。因此,构成晚古生代盖层碎屑物质主体的400~460Ma的碎屑锆石可能来源于北秦岭古生代岩浆作用。虽然南秦岭构造带也有450~400Ma碱性辉绿岩墙和碱性火山岩的报道(王宗起等, 2009; 向忠金等, 2010, 2016; 邹先武等, 2011; Wang et al., 2017a),但是盖层中该年龄段的碎屑锆石多具有花岗质岩浆锆石清晰细密的震荡环带,不具备基性岩浆锆石宽缓环带的结构特点(图 3),因此,该年龄段的碎屑物质应该来源于北秦岭构造带。北秦岭构造带、南秦岭构造带和扬子板块西北缘新元古代的岩浆作用分别发生于950~850Ma、870~630Ma和870~710Ma(Dong et al., 2011, 2017; Wang et al., 2013b; Meng et al., 2015; Dong and Santosh, 2016)。同时,在秦岭岩群中有867~729Ma角闪岩相变质基性岩的报道(李晔等, 2012; 钱加慧等, 2013);在宽坪岩群和秦岭岩群中有730~610Ma的镁铁质-花岗质岩墙的报道(闫全人等, 2008; Wang et al., 2013b);北秦岭构造带官坡等地的榴辉岩的原岩时代约为814~796Ma(陈丹玲和刘良, 2011; Wang et al., 2011, 2013a; Liu et al., 2016)。由此可以看出,次要峰值910~950Ma年龄段的碎屑锆石来源于北秦岭构造带,次要峰值650~880Ma年龄段的碎屑锆石可能来源于北秦岭构造带或南秦岭构造带,或者北秦岭和南秦岭构造带共同为其提供物源。通过对佛坪地区晚古生代盖层和北秦岭构造带中碎屑锆石年龄图谱(Diwu et al., 2012; Wu and Zheng, 2013)对比可以看出,晚古生代盖层中的所有峰值年龄在北秦岭构造带中都能发现,因此,其碎屑物质可能仅来源于北秦岭构造带。然而,佛坪穹隆外围的五龙岩体群中含有大量650~970Ma和少量1048~2332Ma的捕获或者继承锆石(刘树文等, 2011; Yang et al., 2011, 2012; Zhang et al., 2012; Qin et al., 2013),又从侧面反映出南秦岭构造带前寒武纪基底也足以为晚古生代盖层提供相对应的年龄段的碎屑物质。因此,佛坪地区晚古生代盖层的碎屑物质来源可能为北秦岭构造带和南秦岭构造带的前寒武纪基底。这一假设得到了南秦岭泥盆纪碎屑沉积物地球化学研究的证实(Yan et al., 2006a, b, 2012; Wang et al., 2017b),其认为泥盆纪地层主要由成熟度低的硬砂岩组成,物质来源为北秦岭大陆岛弧的大量不成熟长英质碎屑和少量弧基底的成熟性碎屑。

4.3 构造意义

南秦岭构造带的前寒武纪基底的时代和性质一直是地质学界长期重视并具有争议的问题。现阶段,已通过锆石年代学确定的南秦岭前寒武纪基底主要包括新太古代陡岭杂岩、新元古代小磨岭杂岩、新元古代武当-郧西群和耀岭河群火山-沉积岩序列(Ling et al., 2003, 2008, 2010; 刘仁燕等, 2011; Hu et al., 2013, 2016; 郭现轻等, 2014; Zhu et al., 2014, 2015; Dong and Santosh, 2016; Dong et al., 2017)。本文对佛坪地区的前寒武纪基底的锆石U-Pb年代学研究表明,佛坪地区前寒武纪基底佛坪岩群的时代为新元古代,并非太古代或者古元古代,因此,南秦岭构造带显生宙盖层之下的前寒武纪基底主要为新元古代佛坪岩群-武当-郧西群。通过对南秦岭构造带新元古代基性-酸性侵入体和火山岩的研究发现,南秦岭构造带在850~720Ma代表了扬子陆块北缘的俯冲-增生构造带,并于约720~700Ma拼合到扬子陆块北缘(Yan et al., 2010; Yang et al., 2012; Wu et al., 2012; Hu et al., 2016; Dong and Santosh, 2016; Dong et al., 2017)。佛坪地区前寒武纪基底其形成时代晚于615~728Ma,其碎屑锆石主要的峰值年龄为600~820Ma、2021Ma和2467Ma,表明物质来源为南秦岭构造带和扬子陆块北缘。因此,南秦岭构造带在新元古代时期(615Ma之后)可能紧邻扬子陆块北缘,或者已增生到扬子陆块北缘。

现阶段,虽然多数学者认为北秦岭构造带与华北陆块的弧-陆碰撞结束于志留纪末,同时在华北南缘形成了一个安第斯型的大陆边缘(Ratschbacher et al., 2003, 2006; Hacker et al., 2004; Dong et al., 2011; Liu et al., 2011, 2013; Dong and Santosh, 2016),但是,随后华北与扬子陆块陆-陆碰撞前的构造演化过程则存在着很大的争议。其中,一个关键的科学问题是南秦岭构造带晚古生代时期的构造性质如何,这直接影响着秦岭造山带整个构造演化历史的解析。南秦岭地区出露大面积的泥盆纪沉积地层则为探讨南秦岭晚古生代的构造性质提供了很好的窗口。佛坪地区泥盆纪沉积盖层碎屑锆石主要的年龄峰值为410~450Ma、650~880Ma和910~950Ma,物源分析表明北秦岭构造带和南秦岭构造带前寒武纪基底已足以为其提供碎屑物质。同时,结合南秦岭泥盆系碎屑沉积岩地球化学的研究(Yan et al., 2006a, b, 2012; Wang et al., 2017b),可以推测南秦岭泥盆系可能形成于弧前盆地环境,南秦岭构造带于泥盆纪时期可能紧邻北秦岭构造带,并共同为南秦岭大面积的泥盆纪盆地提供碎屑物质。

5 结论

本文对佛坪地区前寒武纪基底和沉积盖层中碎屑锆石进行了LA-ICP-MS U-Pb年代学研究,重新厘定了佛坪地区地层的年代学框架,并讨论了基底和盖层的物质来源,得到如下结论:

(1) 佛坪穹隆主要由中-新元古代基底及上覆泥盆纪盖层构成,二者之间为脆-韧性断层接触,中元古代基底仅分布于佛坪县城,新元古代基底主要分布于佛坪县城和龙草坪西侧小南沟一带。穹隆外围由泥盆纪-石炭纪沉积盖层所构成,外围盖层与穹隆基底上覆盖层具有相似的时代和碎屑物质来源。

(2) 佛坪地区前寒武纪基底中碎屑锆石主要年龄峰值为600~820Ma、2021Ma和2467Ma,物质来源为南秦岭构造带和扬子陆块北缘,泥盆-石炭纪沉积盖层中碎屑锆石主要年龄峰值为410~450Ma、650~880Ma和910~950Ma,物质来源为北秦岭和南秦岭构造带。

(3) 佛坪穹隆基底和盖层的时代和物质来源表明,南秦岭构造带(前寒武纪基底)于新元古代已增生为扬子陆块北缘的重要组成部分,后于晚古生代(泥盆纪)紧邻华北-北秦岭构造带南缘。

致谢 LA-ICP-MS锆石U-Pb测年工作得到了中国地质大学(武汉)宗克青副教授的指导,在此感谢。感谢王宗起研究员和闫臻研究员对本文提出的重要修改意见。
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