岩石学报  2019, Vol. 35 Issue (8): 2470-2486, doi: 10.18654/1000-0569/2019.08.10   PDF    
华北克拉通南缘官道口群和洛峪群的年代学研究新进展——来自凝灰岩SHRIMP锆石U-Pb年龄的新证据
张恒1, 高林志1, 周洪瑞2, 宋彪1, 丁孝忠1, 张传恒2, 刘昊岗2, 龚成强2     
1. 中国地质科学院地质研究所, 北京 100037;
2. 中国地质大学, 北京 100083
摘要:华北克拉通南缘结晶基底之上广泛发育中-新元古代(1800~542Ma)盖层岩系,但由于缺少精确的同位素年龄数据,其地层划分和对比方案一直存在较大的争议。本次工作在河南省栾川县龙家园组底部和下部各发现了一套凝灰岩,进而获得了1594±12Ma和1541±8Ma的SHRIMP锆石U-Pb年龄,首次精确标定了该地区龙家园组的形成时限,该组底界的时代接近1600Ma。龙家园组沉积时代为中元古代蓟县纪最早期;在河南省汝阳、汝州、鲁山地区的洛峪口组凝灰岩中获得了1596±7Ma、1596±15Ma、1608±17Ma和1620±16Ma的SHRIMP锆石U-Pb年龄,进一步精确标定了洛峪口组的形成时代,洛峪口组形成于长城纪的末期。结合汝阳群云梦山组碎屑锆石U-Pb年龄结果,汝阳群-洛峪群的形成时限约为1700~1600Ma。官道口群应是汝阳群之后的地层系统,而不是早前认为的与之同期的地层。
关键词: 华北克拉通南缘     龙家园组     洛峪口组     SHRIMP     锆石U-Pb    
Chronology progress of the Guandaokou and Luoyu groups in the southern margin of North China Craton: Constraints on zircon U-Pb dating of tuff by means of the SHRIMP
ZHANG Heng1, GAO LinZhi1, ZHOU HongRui2, SONG Biao1, DING XiaoZhong1, ZHANG ChuanHeng2, LIU HaoGang2, GONG ChengQiang2     
1. Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;
2. China University of Geosciences, Beijing 100083, China
Abstract: Meso-to Neoproterozoic (1800~542Ma) sedimentary rocks crop out extensively along the southern margin of the North China Craton, where they locally overlie Archean-Paleoproterozoic crystalline basement. However, due to the lack of accurate isotope age data, the division and correlation of the strata have always been controversial. Two interbedded tuffite layers in the bottom and lower part of the Longjiayuan Formation have been obtained in Luanchuan County, Henan Province, by means of the SHRIMP zircon U-Pb dating, the ages are 1594±12Ma and 1541±8Ma. With these data, the depositional age of the Longjiayuan Formation can be calibrated accurately for the first time, which indicates that the bottom age of this formation should be most likely about 1600Ma. The Longjiayuan Formation should be the earliest strata in the Mesoproterozoic Jixian System. Four interbedded tuffite layers in the Luoyukou Formation have been obtained in Ruyang, Ruzhou, Lushan counties, Henan Province, by means of the SHRIMP zircon U-Pb dating, the ages are 1596±7Ma, 1596±15Ma, 1608±17Ma and 1620±16Ma, respectively. These further confirme the depositional time of the Luoyukou Formation. Combining with the results of detrital zircon U-Pb data from the Yunmengshan Formation of the Ruyang Group, the depositional time span of both the Luoyu Group and the Ruyang Group can be limited roughly into the period from 1700Ma to 1600Ma. The Guandaokou Group should be strata after the Ruyang Group.
Key words: The southern margin of North China Craton     The Longjiayuan Formation     The Luoyukou Formation     SHRIMP     Zircon U-Pb    

通常认为,华北克拉通在1.85~1.80Ga期间固结,形成为稳定的陆块。随后发生系列裂谷事件,其一即为“豫陕裂谷”,分布于豫-陕-晋交界地区,发育广泛的中-新元古界陆源碎屑岩和碳酸盐岩地层,主要包括汝阳群、洛峪群、官道口群、洛南群、栾川群、黄连垛组等(图 1)(河南省地质矿产局, 1989; 高林志和乔秀夫, 1992; 高林志等, 2008a, b, 2009; 周洪瑞等, 1998; 李怀坤等, 2010; 苏文博等, 2010; Lu et al., 2008; Su et al., 2008; Gao et al., 2009)。其下伏广阔的中元古界熊耳群火山岩,其上被罗圈组冰碛岩平行不整合覆盖,为华北中-新元古代相关研究的焦点地区之一(李怀坤等, 2009, 2011, 2014; 和政军等, 2011; 苏文博等, 2012; 张拴宏等, 2013; 田辉等, 2015; Li et al., 2013; Zhang et al., 2015, 2016),也是探讨中-新元古代华北克拉通与哥伦比亚、罗迪尼亚超大陆演化关系和早期生命演化进程的重要地区(武铁山, 1982; 杜汝霖, 1984; 石铨曾等, 1996; 高林志等, 2010; Zhai et al., 2000; Lu et al., 2002; Wilde et al., 2002; Andersen, 2005, 2014; Zhao et al., 2009; Zhao, 2011; Yang et al., 2012; Xiao et al., 2014; Tang et al., 2015)。

图 1 华北克拉通南缘前寒武纪地质简图及采样位置 1-省界;2-地质界线;3-推测界限;4-断层;5-深大断裂;6-登封群(太华群);7-嵩山群;8-熊耳群;9-汝阳群;10-官道口群;11-洛峪群;12-早元古代侵入体;13-栾川群;14-五佛山群;15-震旦系;16-采样点 Fig. 1 Simplified geological map of the southern margin of the North China Craton

由于缺少古生物化石和同位素年龄资料,对华北克拉通南部中-新元古界地层的对比和划分,以及如何与华北中-北部地区进行对比,一直有不同认识和看法(吴瑞棠和关保德, 1988; 周洪瑞等, 1998; 高林志等, 2008a, b, 2009; 苏文博, 2014)。二十世纪末,随着洛峪群和高山河组中一些遗迹化石、微古植物及宏观藻类的相继发现,古生物研究者根据这些遗迹化石、微古植物等组合特征,认为洛峪群、官道口群可能属于中元古代末期(胡建民等, 1991; Xiao et al., 1997; Yin et al., 2005)、新元古代青白口纪(邢裕盛等, 1996; 尹崇玉和高林志, 1999; 高林志等, 2002),或者震旦纪(阎玉忠和朱士兴, 1992; 尹崇玉和高林志, 2000)。在地层年代学方面,一直缺少高精度的锆石U-Pb年龄数据。仅部分学者在洛峪群三教堂组和覆盖在洛峪群之上的董家组获得了一些海绿石和粘土矿物测年数据,由此将该组的顶界粗略地限定为900~800Ma之间(李怀坤等, 2009; 张拴宏等, 2013),成为划归青白口系的年代学依据。直到近期,苏文博等(2012)运用LA-ICP-MS方法,首次在河南汝州阳坡村附近洛峪口组的层凝灰岩中获得了锆石U-Pb年龄为1611±8Ma,汪校锋(2015)在洛峪口组上部和顶部的层凝灰岩中分别获得了1662±20Ma和1640±16Ma的LA-ICP-MS锆石U-Pb年龄结果,李承东等(2017)在洛峪口村和阳坡村附近剖面洛峪口组中的层凝灰岩中分别获得了1638±9Ma、1634±10Ma的LA-ICP-MS锆石U-Pb年龄,彭楠等(2018)在河南汝州阳坡村洛峪口组凝灰岩夹层中获得了1639±3Ma的SHRIMP锆石U-Pb年龄。这些精确的锆石U-Pb年龄数据基本将该套地层包括下部的汝阳群限定为长城纪。但这套裂谷系最底部地层官道口群、汝阳群依旧缺乏可靠的数据。

本次工作在河南省栾川县九里沟剖面龙家园组底部和下部首次获得了2组凝灰岩SHRIMP锆石U-Pb年龄,在汝阳洛峪口村、汝州栗树岭村、阳坡村和鲁山九女洞地区进一步获得4组凝灰岩SHRIMP锆石U-Pb年龄,从而首次精确厘定了官道口群龙家园组的地层时代,并进一步精确限定了洛峪群洛峪口组的地层时代。这些年龄数据对于整个中国华北地区中-新元古界划分、对比以及对认识古生物演化具有非常重要的意义。

1 区域地质背景及样品特征

研究区位于华北克拉通的西南部,分为嵩山-箕山地层小区、渑池-确山地层小区和卢氏-栾川地层小区。以洛南-栾川-确山-固始深大断裂带为界,其南侧为扬子陆块边缘区秦岭造山带。区内发育新太古宇-古元古界TTG片麻岩登封群、太华群和具有紧闭线性褶皱的嵩山群。在古元古代末期,区内曾发生大幅度沉降,沿深大断裂及其北侧发育大量中-基性火山岩,形成近6000km2、厚达8000m、具似盖层性质的中元古界熊耳群。熊耳群构造变形微弱,除脆性断裂构造较发育,褶皱构造多为宽缓的穹隆,核部叠合于基底隆起区,表现为盖层特征。在熊耳群之上发育未变质的中元古界陆源碎屑岩和碳酸盐岩。其中,嵩山-箕山地层小区发育兵马沟组、马鞍山组和五佛山群,渑池-确山地层小区发育汝阳群、洛峪群、黄连垛组、董家组、罗圈组,卢氏-栾川地层小区发育高山河组(群)、官道口群、栾川群等地层单元(图 1)。

(1) 官道口群是劳子强等(1989)在《1:50万河南省地质图》中于卢氏县官道口地区创名,内分高山河组、龙家园组、巡检司组、杜关组、冯家湾组。官道口群主要分布于豫西地区的卢氏、灵宝一带,向西延入陕西省,向东延入方城拐河一带,与下伏熊耳群、上覆栾川群呈平行不整合接触。其中,龙家园组底部为含砂砾白云岩、含砾白云岩,下部为紫红色白云岩,中上部以燧石条带白云岩和厚层白云岩为主,叠层石发育丰富。

① 劳子强等.1989. 1:50万河南省地质图

本次研究所观察的龙家园组剖面位于栾川九里沟-祖师庙-三川公路,地层由老到新依次为官道口群(包括高山河组、龙家园组、巡检司组、杜关组、冯家湾组)、栾川群(包括白术沟组、三川组、南泥湖组和煤窑沟组)、南华系和震旦系。在龙家园组底部发现数层深灰绿色凝灰岩夹层,厚度从1~10cm不等,取样20180726-1(图 2a, b)。镜下观察,该样品为凝灰结构。岩石由玻屑、火山尘组成,玻屑主要呈弧面棱角状、杆状等,与火山尘混杂分布,部分已脱玻为隐晶状长英质并具轻微粘土化等假像(图 3)。在龙家园组下部发现厚层紫红色晶屑凝灰岩,取样20180726-7(图 2a, c)。镜下观察,该样品呈流纹构造,岩石由晶屑、玻屑、火山尘组成,其晶屑和玻屑主要由斜长石和石英组成,占50%以上,火山尘混杂其中(图 3)。

图 2 栾川九里沟-三川剖面龙家园组岩性地层划分(a)及样品20180726-1 (b)和样品20180726-7 (c)的岩层露头 1-厚层白云岩;2-豆状白云岩;3-藻礁白云岩,含隐生叠层石;4-条纹状白云岩,含波状、柱状叠层石及核形石;5-含砾白云岩;6-含燧石团块条纹状白云岩;7-燧石条带白云岩;8-厚层-中厚层石英砂岩;9-页岩;10-凝灰岩;11-安山玢岩;12-采样点 Fig. 2 Schematic stratigraphic section of the Longjiayuan Formation at Jiuligou-Sanchuan section (a) and outcrop of the sampling sites (b, c)

图 3 栾川九里沟-三川剖面龙家园组凝灰岩野外(a、c)及镜下(b、d)照片 层凝灰岩样品20180726-1野外特征(a)和正交偏光镜下特征(b);晶屑凝灰岩样品20180726-7野外特征(c)和正交偏光镜下特征(d) Fig. 3 Outcrop (a, c) and photomicrographs (b, d) of the tuff in the Longjiayuan Formation at Jiuligou-Sanchuan section

(2) 关保德等(1980)将大汝阳群(包括现汝阳群和洛峪群)解体,继而将上部的3个组新建洛峪群,划分为崔庄组、三教堂组和洛峪口组,其广泛出露于豫西一带。洛峪群与下伏汝阳群北大尖组呈整合接触,与上覆黄连垛组呈平行不整合接触。洛峪群中下部主要为石英砂岩和页岩,上部主要为泥质白云岩。其中,洛峪口组命名地点位于河南省汝阳县洛峪口村(关保德等, 1980, 1988)。洛峪口组主要由一套白云岩-白云质灰岩夹泥页岩组成,可见厚度30~220m,主要分布于洛阳-汝阳-确山一带(图 1),呈西北-东南向的带状展布。左景勋等(1997)最早在阳坡村附近剖面洛峪口组二段顶部发现“肉红色沉凝灰岩”,本次工作在汝州阳坡村、栗树岭村、汝阳洛峪口村和鲁山九女洞地区共采集了4个凝灰岩样品,分别为:20161120-1、20161120-4、20161121-3和20180723-8(图 4)。

图 4 洛峪群洛峪口组柱状对比图及采样层位(据左景勋等, 1997修改) 1-白云岩;2-含燧石团块条带白云岩;3-具菊花状砾屑白云岩;4-叠层石白云岩;5-纹层状叠层石白云岩;6-透镜状白云岩;7-透镜状灰岩;8-页岩;9-粉砂质页岩;10-凝灰岩;11-寒武系辛集组;12-震旦系罗圈组;13-中元古代三教堂组;14-中元古代黄连垛组 Fig. 4 The correlation column map of Luoyukou Formation of the Upper Proterozoic Luoyu Group (modified after Zuo et al., 1997)

样品20161120-1采自阳坡村剖面洛峪口组第二段中砖红色层凝灰岩夹层,该段发育多层砖红色层凝灰岩,每层厚度1~20cm不等,岩石坚硬。样品20161120-4采自栗树岭村附近242省道旁的剖面。在该剖面发现两层砖红色凝灰岩,每层厚度5~10cm。由于地表风化,凝灰岩夹层很容易与周围岩层剥离。样品20161121-3采自洛峪口组的命名地洛峪口村附近243省道旁的剖面(图 5a-c)。样品20180723-8采自鲁山县九女洞剖面,为一层灰绿色薄层凝灰岩,厚度约5cm(图 5e)。在显微镜下观察发现,样品20161120-1、20161120-4、20161121-3均主要由玻屑、少量晶屑和火山尘组成,三者呈杂乱分布,粒度均小于0.5mm;玻屑呈各种弧面棱角状,已脱玻化形成隐晶-微粒长英矿物集合体,有的具蒙脱石类粘土矿物蚀变;晶屑为钾长石和少量单晶石英,呈棱角状;火山尘和黏土质矿物充填上述碎屑间,已重结晶为隐晶长英质,粘土岩屑具褐铁矿化(图 5d)。样品20180723-8呈凝灰结构,主要由火山尘组成,约占90%以上,偶见少量玻屑和晶屑,主要由斜长石和石英组成,呈鸡骨状、棱角状等,夹杂于火山尘之中(图 5f),因其火山物质均>90%,应定名为层凝灰岩。

图 5 洛峪口组凝灰岩野外及正交偏光镜下照片 Fig. 5 Outcrop and photomicrographs under CPL of the tuff in the Luoyukou Formation
2 分析方法

锆石单矿物分选在河北廊坊区域地质调查所实验室进行。在无污染的条件下,将原岩样品粉碎成全部通过60目的一次性筛网,淘洗岩石粉末,得到含锆石的重砂。再经重液和电磁分选后得到锆石样品,在双目镜下挑选出锆石颗粒。将待测锆石颗粒依照从大到小的顺序排列,与锆石标样(TEM)粘贴在已粘贴了双面胶的玻璃板上,灌注环氧树脂后制成样品靶。待树脂固结变硬后,打磨和抛光至锆石中心部位暴露。然后拍摄锆石的透、反射光显微图像和阴极发光(CL)图像。测定时选取无裂隙和包裹体且CL图像灰度单一的位置测定。SHRIMP锆石U-Pb同位素测定在中国地质科学院地质研究所北京离子探针中心SHRIMP Ⅱ上完成。详细的分析原理及流程参见相关文献(宋彪等, 2002; Williams, 1998)。一次离子流O2-强度为2.5nA,束斑大小为25~30μm。标准锆石M257(U含量840×10-6)和TEM(206Pb/238U年龄为416.8Ma)的测值分别用于校正待测样品的U含量和206Pb/238U年龄(Black et al., 2003)。根据实测204Pb扣除普通Pb。每个数据点由5次扫描构成,误差为1σ,而年龄加权平均值误差为2σ。数据处理使用1.02版SQUID和2.49版ISOPLOT程序(Stacey and Kramers, 1975; Ludwig, 2001)。单个数据点误差为1σ,加权平均年龄误差为95%置信度。

3 分析结果

通常,>1000Ma的样品,因可能的铅丢失及相对较高的放射性成因铅含量,多采用207Pb/206Pb年龄;对 < 1000Ma,特别是更年轻的样品,因不易得到准确的207Pb/206Pb比值及铅丢失可能较少的原因,采用206Pb/238U年龄。但这不是绝对的,需要根据样品的具体情况来考虑,究竟是使用两者中的哪一个更合理。如U含量很低,即使老的样品,207Pb/206Pb比值也不能准确得到,此时铅丢失观察不到,也应使用206Pb/238U年龄。本研究中的样品,20161120-1、20180723-8的年龄>1000Ma,且含有较高的放射性成因铅,故采用207Pb/206Pb年龄;而20161120-4、20161121-3、20180726-1、20180726-7因U含量低,我们采用206Pb/238U年龄(宋彪等, 2015)。

龙家园组凝灰岩样品20180726-1、20180726-7锆石CL图像显示(图 6),锆石多为长柱状,多发育完整的岩浆生长环带,少部分发育不完整岩浆生长环带。锆石呈自形,晶棱发育。总体表现为典型的酸性岩浆锆石特征。样品20180726-1的锆石粒度多在100~150μm,样品20180726-7的锆石粒度多在50~100μm,样品20180726-1的锆石比样品20180726-7的锆石具有更好的生长环带;洛峪口组砖红色凝灰岩样品20161120-1、20161120-4、20161121-3锆石CL图像显示(图 6),锆石多为短柱状和不规则方形,半自形,晶棱发育,部分发育不完整的生长环带。锆石粒度多小于100μm。锆石的颜色较浅,显示可能受后期热变质影响的特征。洛峪口组灰绿色层凝灰岩样品20180723-8锆石CL图像显示(图 6),锆石多为短柱状,自形,晶棱发育,多发育较完整生长环带。锆石颜色较深,粒度多在100μm左右。洛峪口组灰绿色层凝灰岩样品20180723-8的锆石总体显示比砖红色凝灰岩样品20161120-1、20161120-4、20161121-3的锆石具有更好更完整的晶形,颜色也较深,显示可能为不同的火山岩浆来源。

图 6 龙家园组和洛峪口组凝灰岩样品锆石阴极发光图像 Fig. 6 Cathodoluminescence images of analyzed zircons from tuff samples in the Longjiayuan and Luoyukou formations

(1) 龙家园组深灰绿色层凝灰岩(20180726-1)

共测试了15个数据点,测定时斑点均位于环带区域。U含量变化范围为61×10-6~439×10-6,Th含量变化范围为64×10-6~455×10-6,Th/U值变化范围为0.43~1.37(表 1)。其中4.1和11.1的206Pb/238U年龄较大,分别为1807±18Ma和2320±24Ma;14.1和9.1偏离谐和线。余下11个测点均位于U-Pb谐和线或其附近,其206Pb/238U年龄加权平均值为1594±8Ma(MSWD=0.63),该年龄代表了该层层凝灰岩的形成年龄(图 7)。

表 1 龙家园组凝灰岩SHRIMP锆石U-Th-Pb同位素测定结果 Table 1 SHRIMP U-Th-Pb isotopic compositions of zircons from tuff in the Longjiayuan Formation

图 7 龙家园组和洛峪口组凝灰岩SHRIMP锆石U-Pb谐和图 Fig. 7 SHRIMP zircon U-Pb concordia diagrams of zircons from tuff samples in the Longjiayuan Formation and in the Luoyukou Formation

(2) 龙家园组晶屑凝灰岩(20180726-7)

共测试了18个数据点,测定时斑点均位于环带区域。U含量变化范围为70×10-6~406×10-6,Th含量变化范围为81×10-6~93×10-6,Th/U值变化范围为0.21~1.22(表 1)。从U-Pb谐和图中可以看出,所有测点均位于U-Pb谐和线附近。18个数据的206Pb/238U年龄加权平均值为1541.1±7.9Ma(MSWD=1.17),207Pb/206Pb年龄平均值为1513±12Ma(MSWD=1.00)。206Pb/238U年龄加权平均值1541.1±7.9Ma代表了该套晶屑凝灰岩的形成年龄(图 7)。

(3) 洛峪口组砖红色凝灰岩(20161120-1)

共测试了18个数据点,测定时斑点位于环带区域或附近。U含量变化范围为21×10-6~212×10-6,Th含量变化范围为20×10-6~207×10-6,Th/U值变化范围为0.45~6.46,放射成因206Pb含量为5.28×10-6~40.7×10-6(表 2)。其中4.1的207Pb/206Pb年龄(2509±17Ma)较大,应为继承锆石,或存在继承核的影响。17.1和18.1在U-Pb谐和图中的投影点落在了谐和线的左侧。其余15个数据点具有相近的207Pb/206Pb年龄,其谐和度大多在75%~100%之间,谐和度在90%以上的数据点大于73%。这15个数据点拟合的不一致线与一致线的上交点年龄为1596±15Ma(MSWD=1.4),该年龄代表了该层层凝灰岩的形成年龄(图 7)。

表 2 洛峪口组凝灰岩SHRIMP锆石U-Th-Pb同位素测定结果 Table 2 SHRIMP U-Th-Pb isotopic compositions of zircons from tuff in the Luoyukou Formation

(4) 洛峪口组砖红色凝灰岩(20161120-4)

共测试了18个数据点,测定时斑点位于环带区域或附近。U含量变化范围为31×10-6~492×10-6,Th含量变化范围为24×10-6~558×10-6,Th/U值变化范围为0.44~6.31,放射成因206Pb含量为7.61×10-6~106×10-6(表 2)。9.1的206Pb/238U年龄较大(1812±17Ma),应为继承锆石,或存在继承核的影响。1.1、15.1偏离谐和线。选取谐和度较高且206Pb/238U年龄较集中的8个数据,求得其206Pb/238U年龄加权平均值为1596.5±7.2Ma(MSWD=1.39),该年龄代表了该层层凝灰岩的形成年龄(图 7)。

(5) 洛峪口组砖红色凝灰岩(20161121-3)

共测试了15个数据点,测定时斑点位于环带区域或附近。U含量变化范围为16×10-6~228×10-6,Th含量变化范围为74×10-6~451×10-6,Th/U值变化范围为0.34~10.34,放射成因206Pb含量为4.03×10-6~66.9×10-6(表 2)。其中5.1的206Pb/238U年龄(1893.3±7.8Ma)较大,应为继承锆石,或存在继承核的影响。6.1和8.1的206Pb/238U年龄较小(1105.8±8.1Ma和1113.1±5.2Ma),锆石可能受到了后期热事件影响。测点7.1偏离谐和线和主群较远,且U含量较高。其余11个点的206Pb/238U年龄加权平均值为1608±17Ma(MSWD=2.33),该年龄代表了该层层凝灰岩的形成年龄(图 7)。

(6) 洛峪口组灰绿色层凝灰岩(20180723-8)

共测试了15个数据点,测定时斑点均位于环带区域。U含量变化范围为83×10-6~468×10-6,Th含量变化范围为35×10-6~507×10-6,Th/U值变化范围为0.44~1.12,放射成因206Pb含量为20.9×10-6~65.7×10-6(表 2)。6.1、14.1偏离谐和线和主群较远,7.1的年龄较小(207Pb/206Pb年龄为1585±22Ma)。其余12个点的207Pb/206Pb年龄加权平均值为1620±16Ma(MSWD=0.72),该年龄代表了该层层凝灰岩的形成年龄(图 7)。

4 讨论 4.1 地层时代的精确厘定

20世纪80年代以来,关保德等(1980, 1988)对该区作了系统的地层划分和对比研究。由于缺乏生物证据和可信的同位素年龄数据,部分前寒武纪地质学者根据岩性对比和叠层石资料认为华北克拉通南缘汝阳群和洛峪群归属中元古界,与蓟县剖面中元古界地层对比(李钦仲等, 1985; 曹瑞骥, 1988; 关保德等, 1988; 华洪等, 1999; 薛耀松等, 2001)。随着洛峪群和高山河组中微古植物和宏观藻类的陆续发现,及在部分层位中获得一些海绿石和粘土矿物测年数据,部分学者认为洛峪群、官道口群可能属于中元古代末期或新元古代青白口纪。近期,苏文博等(2012)在洛峪口组中凝灰岩获得了1611±8Ma的LA-ICP-MS锆石U-Pb年龄。汪校锋(2015)李承东等(2017)彭楠等(2018)后续也在洛峪口组中凝灰岩获得了精确的锆石U-Pb年龄,测年数据结果在1662~1634Ma之间。这些年龄数据限定了洛峪口组的地层时代。

但是,在华北克拉通南缘熊耳群之上的其它前寒武纪地层单元中一直未获得精确的锆石U-Pb年龄。本工作首次在龙家园组层凝灰岩和晶屑凝灰岩中获得两组SHRIMP锆石U-Pb年龄(1594±12Ma和1541±8Ma),精确厘定了其中火山事件的年龄,进而标定了龙家园组的形成时代。龙家园组形成于蓟县纪的早期。龙家园组之下发育有高山河组碎屑岩,但目前较多划分意见是将高山河组单独划出,不再归官道口群,可与汝阳群对比(其下部火山岩夹层、含铁建造是很好的对比标志)。本次工作,笔者进一步在汝州阳坡村、栗树岭村、汝阳洛峪口村和鲁山九女洞剖面洛峪口组凝灰岩中获得四组精确SHRIMP锆石U-Pb年龄(1596±15Ma、1596±7Ma、1608±17Ma和1620±16Ma),精确厘定了其中火山事件的年龄,进一步标定了洛峪口组的形成时代。确定洛峪口组形成于长城纪的末期。

4.2 华北南缘中-新元古代地层系统的初步厘定

熊耳群火山岩被认为是华北克拉通南缘结晶基底之上第一套稳定的盖层型沉积,赵太平等(2004)对熊耳群中的英安-流纹斑岩和同期侵入的辉长-闪长岩等,首先进行了SHRIMP锆石U-Pb定年,认为熊耳群形成于1.8~1.75Ga。后续,众多学者对熊耳群火山岩以及同期侵入岩体的年龄进行了测试(Peng et al., 2008; He et al., 2009; Wang et al., 2010),所获得的锆石U-Pb年龄基本都在1.78Ga或1.76Ga左右。因此,熊耳群火山岩的形成时代基本确定为1.8~1.75Ga。熊耳群曾长期成为华北克拉通南缘仅有的获得高精度锆石U-Pb年龄的地层。近期,苏文博等众多学者及笔者先后在洛峪群洛峪口组凝灰岩中获得精确锆石U-Pb年龄,洛峪口组时代基本确定为1.65~1.60Ga。本研究在栾川县官道口群龙家园组中首次获得两组SHRIMP锆石U-Pb年龄(1594±12Ma和1541±8Ma),成为该地区获得的又一精确地层时代数据。

王淼、周洪瑞及笔者对鲁山县大黑潭沟剖面汝阳群云梦山组和九女洞剖面黄连垛组进行了碎屑锆石U-Pb年龄测试(王淼等, 未发表),其中云梦山组碎屑锆石年龄频谱最小峰值年龄为1785±34Ma,年龄最小的一组碎屑锆石共有4颗,其207Pb/206Pb年龄加权平均值为1658±63Ma(MSWD=0.064),指示汝阳群云梦山组沉积年龄不早于1700Ma,即汝阳群的地层时代下限约为1700Ma,或者更小。黄连垛组碎屑锆石年龄频谱最小峰值年龄为1786±60Ma,年龄最小的一批碎屑锆石共有2颗,207Pb/206Pb年龄值分别为:1690±60Ma和1729±56Ma。同时,徐文超等(2015)苏文博(2016)认为黄莲垛组在岩性和叠层石组合上可与官道口群龙家园组—巡检司组对比。因此,结合碎屑锆石年龄信息,黄连垛组应属蓟县纪。高林志等(未发表)在栾川群大红口组玄武安山岩中获得了850±13Ma的SHRIMP锆石U-Pb年龄;阎国翰等(2010)在大红口组变质粗面岩中获得了约860Ma的SHRIMP锆石U-Pb年龄。结合区域地层对比及岩石叠置关系,笔者认为栾川群应属青白口系。综合前人区域地层对比和最新相关研究成果,及以上高精度锆石U-Pb年龄数据,本文给出了华北克拉通南缘晚前寒武系地层对比简表(图 8)。

图 8 华北克拉通南缘晚前寒武纪地层对比简表 Fig. 8 Correlation of the revised Precambrian chronostratigraphic of the southern margin of the North China Craton
4.3 早期宏体生物时代讨论

20世纪90年代以来,在华北克拉通南缘中-新元古界地层中发现了大量的后生动物遗迹化石、具刺疑源类、甚至多细胞宏观藻类等化石(胡建民等, 1991; 阎玉忠和朱士兴, 1992; 邢裕盛等, 1996; Xiao et al., 1997; 尹崇玉和高林志, 1999, 2000; 高林志等, 2002; Yin et al., 2005; Pang et al., 2013)。但是,由于缺少精确的地层年代的约束和广大古生物学者对生物演化阶段的固有认识,这些生物出现的时代多被认为是新元古代。而近期华北克拉通南缘洛峪口组和龙家园组地层时代的精确厘定将洛峪口组、高山河组等地层中的微古植物和宏观藻类的时代提前到1.60Ga或者更早,这些发现将传统上认为其出现的时代极大地往前提了。在华北克拉通北部的中元古界地层串岭沟组、团山子组和高于庄组中也均发现多细胞真核生物和大型宏观藻类等化石(阎玉忠, 1995; 牛邵武, 2002; 梅冥相, 2008; Du et al., 1995; Zhu and Chen, 1995; Lamb et al., 2009; Zhu et al., 2016; Qu et al., 2018)。随着大量高精度测年数据的获得,这些多细胞真核生物和大型宏观多细胞藻类被认为出现在1.63Ga或者更早,也验证了对这些早期宏体生物形成时代的认识。

近期,刘军平等(2018)在扬子板块西南缘东川群鹅头厂组灰黑色炭泥质板岩中同样发现了大量保存较好的宏体多细胞生物化石。东川群时代约在1742~1503Ma之间(孙志明等, 2009; 朱华平等, 2011; 王子正等, 2013; 侯林等, 2013; 郭阳等, 2014; 杨斌等, 2015; Zhao et al., 2010; Wang and Zhou, 2014),因此扬子板块西南缘发现的宏观多细胞生物同样出现在中元古代的早期。在格林兰岛西北部和澳大利亚西北部的中元古界地层中亦有大量大型宏体生物的报道(彭楠等, 2018; Buick and Knoll, 1999; Samuelsson et al., 1999; Butterfield, 2015)。因此,越来越多中元古界早期地层中发现宏体生物化石的报道表明:先前认为这些进化程度较高的属于新元古代的真核生物群或宏观藻类,可能在中元古代早期就已经出现了。中元古代早期可能是早期大型宏体生物演化的重要阶段。这同样启发广大古生物学者探索这一重要生命演化阶段的环境信息。

5 结论

(1) 通过对河南栾川九里沟-三川剖面龙家园组底部和下部的凝灰岩进行SHRIMP锆石U-Pb同位素年代学研究,获得了1594±12Ma和1541±8Ma两组高精度年龄,将龙家园组准确地标定为中元古代蓟县纪最早期沉积岩系;

(2) 通过对河南汝州阳坡村、栗树岭村、汝阳洛峪村和鲁山九女洞剖面洛峪口组中的层凝灰岩进行SHRIMP锆石U-Pb同位素年代学研究,获得了1596±15Ma、1596±7Ma、1608±17Ma和1620±16Ma四组高精度年龄,进一步标定了洛峪口组的形成时代。确定洛峪口组形成于长城纪的末期;

(3) 汝阳群-洛峪群的时限约为1700~1600Ma。官道口群应是汝阳群之后的地层系统,而不是早前认为的与汝阳群同期的地层。

致谢      本论文在成文过程中得到刘燕学、柳永清、耿元生、唐烽等专家的指导和帮助;野外工作得到了河南省地质调查院王世炎教授、左景勋教授级高工、杜凤军教授级高工的支持和指导;采样和实验测试分析得到了王淼、张继彪、倪敬波、张连祥、周星、廖博文、王宏宇等研究生的帮助;国家科技基础条件平台北京离子探针中心的杨淳女士、江南女士、车晓超博士在机时安排、样靶制备、阴极发光图像准备等方面提供了帮助;审稿专家赵太平、王世炎、相振群、祝禧艳提出了建设性的意见;编辑部俞良军老师为本文编排、勘误付出了辛勤工作。在此对他们表示衷心感谢!

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