岩石学报  2015, Vol. 31 Issue (6): 1589-1605   PDF    
辽东半岛南辽河群锆石U-Pb年代学及其地质意义
李壮1, 陈斌2 , 刘经纬1, 张璐1, 杨川1    
1. 北京大学地球与空间科学学院, 造山带与地壳演化教育部重点实验室, 北京 100871;
2. 合肥工业大学资源与环境工程学院, 合肥 230009
摘要:本文报道了辽东半岛古元古代胶-辽-吉活动带南辽河群中变质火山岩和沉积岩的锆石U-Pb年代学数据。变质流纹岩的锆石具典型的岩浆振荡环带结构和较高的Th/U比值(>0.3),锆石U-Pb年龄为~2.2Ga,该年龄可代表其原岩形成年龄,在误差范围内与古元古代辽吉花岗岩年龄一致,表明辽吉花岗岩并不是辽河群的基底。变质玄武岩的锆石阴极发光强度较弱、弱分带或无分带,同时具较低的Th/U比值(<0.1),为典型的变质成因锆石,锆石U-Pb年龄为~1.9Ga,代表其变质时代。变质沉积岩的碎屑锆石年龄主要介于1981~3520Ma之间:峰期年龄为2033Ma和2092Ma的锆石年龄信息暗示辽东半岛至少存在一期2000~2100Ma的岩浆事件,并且该时期的中酸性岩浆岩是南辽河群沉积岩的一个重要物源;峰期年龄为2155Ma、2446Ma、2509Ma、2594Ma、2668Ma、2790Ma、3356Ma、3467Ma和3520Ma的锆石年龄信息,区域上与古元古代辽吉花岗岩、辽河群火山岩及太古宙基底年龄相吻合,暗示它们为南辽河群沉积岩提供了重要物源。沉积岩中最年轻的碎屑锆石U-Pb年龄为~2.0Ga,可代表其沉积时的最大年龄。所以,辽河群火山-沉积-变质的时限为2.2~1.9Ga,其演化时间约300Myr。结合前人有关辽东半岛前寒武纪岩石的研究成果,本文研究认为胶-辽-吉活动带的形成演化与弧-陆碰撞有关,而不是许多人坚持的裂谷环境。
关键词辽东半岛     南辽河群     年代学     火山-沉积-变质     华北克拉通    
Zircon U-Pb ages and their implications for the South Liaohe Group in the Liaodong Peninsula, Northeast China
LI Zhuang1, CHEN Bin2 , LIU JingWei1, ZHANG Lu1, YANG Chuan1    
1. Key Laborary of Orogenic Belts and Crustal Evolution, MOE, School of Earth and Space Sciences, Peking University, Beijing 100871, China;
2. School of Resources and Environmental Engineering, Hefei University of Technology, Hefei 230009, China
Abstract: Liaodong Peninsula is located in the southeastern part of Liaoning Province, the basement rocks of which consist mainly of the Archean Anshan-Fushun Complex in the north (Liaobei-Jinan Block), Archean Jinzhou Complex in the south (Liaonan Block), and the Paleoproterozoic Liaohe Group and granitoid intrusions (Jiao-Liao-Ji Belt) in between. The Paleoproterozoic Liaohe Group is divided by the Qinglongshan-Zaoerling shear zone into two parts, the South Liaohe Group and North Liaohe Group, with more volcanic rocks in the former and more clastic and carbonate rocks in the latter. The South Liaohe Group consists of three rock units, with a volcanic-rich sequence in the lower part (the Lieryu and Gaojiayu formations), a carbonate-rich sequence in the middle (the Dashiqiao Formation), and a pelitic sequence in the upper part (the Gaixian Formation). We report in the paper the zircon U-Pb age data for two meta-volcanic rocks from the Lieryu Formation and three meta-sedimentary rocks from the Gaixian Formation of the Paleoproterozoic South Liaohe Group in the Liaodong Peninsula. Zircon analyses on the meta-rhyolite (LZ3) yield 207Pb/206Pb ages of ca.2.2Ga, with the range of 2178~2226Ma. These zircons show oscillatory and sector-zoning, and high Th/U ratios (>0.3), typical of magmatic origin; thus the age probably represents the formation age of the protolith of the meta-volcanic rocks, which are regionally comparable to those of the Paleoproterozoic Liaoji granitoids, indicating that the Liaoji granitoids are not the basement of the Liaohe Group sedimentary-volcanic rocks as previously thought. Zircon analyses on the meta-basalt (LZ12) yield 207Pb/206Pb ages of ca.1.9Ga, with the range of 1850~1949Ma. Zircon grains from the meta-basalt show relatively dark CL images, with patchy zoning or irregular structures and low Th/U ratios (<0.1). These features suggest that they should be formed during the process of fluid-present metamorphism, and their U-Pb ages of ca.1.9Ga represent the metamorphic age. The dating result of the detrital zircons from the meta-sedimentary rocks (LC3, LC5 and LB3) shows that the 207Pb/206Pb ages vary from 1981Ma to 3520Ma, with characteristic age probability peaks at ca.2033Ma, 2092Ma, 2155Ma, 2446Ma, 2509Ma, 2594Ma, 2668Ma, 2790Ma, 3356Ma, 3467Ma and 3520Ma. The characteristic age probability peaks of 2033Ma and 2092Ma indicate that more than one magmatic event occurred at 2000~2100Ma in the Liaodong Peninsula seldom recognized before, and that 2000~2100Ma acid rocks are an important contributor to the provenance for the South Liaohe Group. The characteristic age probability peaks of 2155~3520Ma are mainly consistent with the formation of regionally distributed Liaoji granitoids and coeval volcanic rocks in the Jiao-Liao-Ji Belt and the granitoids of the Archean basement, suggesting that these rocks provided the major component of the provenance for the South Liaohe Group. The deposition time of the meta-sedimentary rocks is later than ca.2.0Ga as limited by the youngest detrital zircons. These new zircon ages for the South Liaohe Group reveal that the volcanism-deposition-metamorphism events of the South Liaohe Group occurred at 2.2~1.9Ga, and lasted for at least 300Myr. Integration of our new data with recent studies on lithology, metamorphism, geochemistry and geochronology of Archean-Paleoproterozoic basement rocks is inconsistent with the rifting model, but rather, supports the Paleoproterozoic arc-continent collision model, as follows: (1) The detrital zircon spectra of the South Liaohe Group are similar to that of back-arc basin; (2) There is a lack of rock associations that are typical of the rift, such as OIB, phonolite and carbonatite; (3) The Paleoproterozoic metamorphic volcanic rocks and Liaoji granitoids show geochemical characteristics typical of arc magmas; (4) The South Liaohe and North Liaohe groups have different rock associations and metamorphic histories (P-T-t paths); (5) The Liaobei-Jinan and Liaonan blocks show considerable differences in terms of lithological units, geochronology and metamorphic features. Therefore, we propose that the Paleoproterozoic Jiao-Liao-Ji Belt formed in the arc-continental collision setting, rather than in a continental rifting setting as previously thought by most workers.
Key words: Liaodong Peninsula     South Liaohe Group     Geochronology     volcanism-sedimentary-metamorphism     North China Craton    
1 引言

华北克拉通是中国最古老的克拉通,其演化历史超过3.8Ga(Liu et al., 19922008; Song et al., 1996)。近十年来,许多学者通过基础地质、地球化学、同位素年代学和地球物理等方法对华北克拉通基底岩石进行研究,识别出三条古元古代活动带(孔兹岩带、中央碰撞造山带和胶-辽-吉活动带;图 1)(Zhao et al., 20012005; 赵国春,2009; Zhao and Zhai, 2013)。国内外学者对孔兹岩带和中央碰撞造山带的古元古代大地构造环境和演化过程进行了较多的研究(Zhao et al., 200220052010; Wilde and Zhao, 2005; Wan et al., 20012006ab; Zhai et al., 2005; Xia et al., 2006; Trap et al., 2007; Santosh,2010; Peng et al., 20102011),相比之下,胶-辽-吉活动带虽有悠久的研究历史,但近年来研究进展滞后(Sun et al., 1993; 路孝平等,2004; Luo et al., 20042008; Lu et al., 2006; Li and Zhao, 2007; 赵国春,2009; Zhao and Zhai, 2013)。随着华北克拉通前寒武纪地质问题研究的不断深入,在辽东地区出现了一系列亟待解决的关键地质问题,如辽河群火山-沉积-变质的时代,辽河群与辽吉花岗岩的关系,特别是有关胶-辽-吉活动带的大地构造属性和演化过程一直存在较大争议:一部分学者认为胶-辽-吉活动带的形成演化与陆内裂谷的闭合有关(张秋生等,1988; Li et al., 200520062011; Luo et al., 20042008; Li and Zhao, 2007);另一部分学者则认为该活动带的形成演化与弧-陆碰撞有关(白瑾,1993; 路孝平等,2004; Faure et al., 2004; Lu et al., 2006; 王惠初等,2011; Li and Chen, 2014)。上述问 题的产生主要由于前人的研究成果多集中于辽河群变质沉积岩和辽吉花岗岩的年代学(Luo et al., 20042008; Li and Zhao, 2007; 孟恩等,2013),缺少对古元古代岩石系统的年代学和成因方面研究(Li and Chen, 2014)。鉴于此,本文选择了辽东半岛古元古代南辽河群中代表性岩石类型(2件变质火山岩和3件变质沉积岩)进行详细的岩石学和锆石U-Pb年代学研究,并结合区域地质资料来建立辽河群火山-沉积建造的年代学格架,进而讨论南辽河群的沉积物源和沉积时代及其与辽吉花岗岩的关系,以期为深入揭示辽东半岛古元古代的大地构造演化提供新的 制约。

图 1 华北克拉通构造划分图(据Zhao et al., 2005修改) Fig. 1 Tectonic subdivision of the North China Craton(modified after Zhao et al., 2005)
2 区域构造背景及样品描述

在大地构造位置上,辽东半岛位于郯庐断裂以东、北邻中亚造山带、南邻秦岭-大别造山带,属于华北克拉通东部陆块的东北段(图 1; Zhao et al., 2005)。该区基底岩石主要由北部辽北-吉南陆块的鞍山-抚顺太古宙杂岩和南部辽南陆块的金州太古宙杂岩及二者之间胶-辽-吉活动带的古元古代辽河群和辽吉花岗岩组成(图 2; Li et al., 2004; Li and Zhao, 2007)。辽河群主要由变火山-沉积岩系组成,经历了低绿片岩-角闪岩相变质,以青龙山-枣儿岭断裂为边界划分为北辽河群和南辽河群(图 2; Li et al., 2004; Li and Zhao, 2007)。南辽河群从下至上包括里尔峪组、高家峪组、大石桥组和盖县组,以变质火山岩的大量出露与北辽河群相区别(Zhao et al., 2005; 孟恩等,2013; Li and Chen, 2014)。里尔峪组和高家峪组主要由含硼变质火山-沉积序列,包括了浅粒岩、变粒岩、斜长角闪岩和云母片岩等;大石桥组主要由大理岩组成,夹少量云母片岩和碳质板岩;盖县组主要由变质碎屑岩组成,包括了千枚岩、云母片岩、变质细砂岩和石英岩等(孟恩等,2013; Li and Chen, 2014)。本文选取其中2件变质火成岩样品和3件变质沉积岩样品进行了锆石年代学研究,具体采样点如图 2所示。

图 2 辽东地区地质简图及采样位置(据Li et al., 2004修改) Fig. 2 Geological map of eastern Liaoning Province,including representative sample locations(modified after Li et al., 2004)

样品LZ3采自营口市大石桥市建一镇西北约4km的里尔峪组(坐标:E122°43′45″,N40°32′24″;图 3a),岩性为变质流纹岩(电气石变粒岩),新鲜面呈灰色,细粒粒状变晶结构,块状构造。矿物主要包括石英、斜长石、电气石和少量不透明矿物。其中石英他形粒状,波状消光明显,均匀分布,粒度在0.2~0.4mm之间,含量约45%;斜长石呈半自形到他形粒状,发育聚片双晶,粒度在0.2~0.4mm之间,均匀分布,含量约45%;电气石呈他形粒状,粒度在0.1~0.2mm之间,含量约10%。野外岩性单一,近百米的剖面上未见粒序层理等沉积构造,矿物组成上缺少钾长石,同时地球化学特征上K2O含量(3.45%~3.67%)大于Na2O(2.15%~2.65%),SiO2含量在68.1%~72.2%之间,均说明电气石变粒岩的原岩是流纹岩,并非碎屑岩。

图 3 辽东半岛南辽河群代表性岩石样品显微图像和变质火山岩的野外地质关系
(a)变质流纹岩(电气石变粒岩);(b)变质玄武岩(斜长角闪岩);(c)斜长石英白云母片岩;(d)变质细砂岩;(e)绢云母千枚岩;(f)变质玄武岩与变质流纹岩呈整合接触关系.Pl-斜长石;Hb-角闪石;Q-石英;Tur-电气石;Ms-白云母
Fig. 3 Micrographs of representative samples and a photo showing geological relationship of the meta-volcanic rocks for the South Liaohe Group in the Liaodong Peninsula
(a)meta-rhyolite(tourmaline fine-grained gneiss);(b)meta-basalt(amphibolite);(c)plagioclase quartz muscovite schist;(d)meta-fine grained s and stone;(e)sericite phyllite;(f)a field photo showing the conformable contact between the meta-basalt and meta-rhyolite. Pl-plagioclase; Hb-hornblende; Qtz-quartz; Tur-tourmaline; Ms-muscovite

样品LZ12采自营口市大石桥市建一镇西北约4km的里尔峪组(与样品LZ3位置相同;图 3b),岩性为变质玄武岩(斜长角闪岩),新鲜面呈灰黑色,中细粒粒状变晶结构,块状构造。矿物主要包括角闪石、斜长石和少量不透明矿物。其中角闪石半自形到自形柱状,均匀分布,粒度在0.5~2mm之间,含量约65%;斜长石呈半自形到他形粒状,粒度在0.2~0.5mm之间,均匀分布,含量约35%。

样品LC3采自本溪市草河口镇南约2km的盖县组(坐标:E123°54′27″,N40°51′40″;图 3c),岩性为斜长石英白云母片岩,新鲜面呈灰黑色,中细粒粒状鳞片变晶结构,片状构造。矿物主要包括石英、白云母、斜长石和少量不透明矿物。其中白云母呈片状或鳞片状,定向分布,构成片理,粒度在0.1~1.5mm之间,含量约45%;石英他形粒状,波状消光,均匀分布,粒度变化较大(0.3~1.0mm),含量约30%;斜长石呈他形粒状,聚片双晶发育,粒度在0.3~1.0mm之间,均匀分布,含量约25%。

样品LC5采自本溪市草河口镇南约2km的盖县组(与样品LC3位置相同;图 3d),岩性为变质细砂岩,新鲜面呈灰白色,细粒粒状变晶结构,块状构造。矿物主要包括石英、斜长石和绢云母。其中石英他形粒状,波状消光明显,均匀分布,粒度在0.05~0.2mm之间,含量约60%;斜长石呈他形粒状,粒度在0.05~0.2mm之间,均匀分布,含量约35%;绢云母呈鳞片状,粒度小于0.1mm,含量约5%。

样品LB3采自凤城市西北约10km的盖县组(坐标:E123°52′42″,N40°28′23″;图 3e),岩性为绢云母千枚岩,新鲜面呈灰黑色,细粒粒状鳞片变晶结构,片状构造。矿物主要包括石英、斜长石和绢云母。其中石英和长石他形粒状,均匀分布,粒度在0.05~0.1mm之间,含量约60%;绢云母呈鳞片状,粒度在0.05~0.12mm之间,含量约40%。 3 分析方法

首先将待测年样品经过手工破碎、淘洗、电磁选、重液分选,在双目镜下挑选,得到含包裹体少、无明显裂隙且晶型完好的锆石。然后将锆石置于环氧树脂内研磨,再抛光清洗制成激光样品靶。锆石的阴极发光(CL)图像主要是查明锆石内部结构(图 4),以便准确选点。样品LC3、LC5和LB3的锆石LA-ICP-MS U-Pb原位定年分析在北京大学造山带与地壳演化教育部重点实验室完成。年龄计算时以国际标准锆石91500为外标进行同位素比值校正,以TEM为监控盲样;元素含量以国际标样NIST 610为外标,29Si为内标。样品LZ3和LZ12的锆石LA-ICP-MS U-Pb原位定年分析在天津地质矿产研究所同位素实验室完成。年龄计算时以国际标准锆石GJ-1为外标进行同位素比值校正,以TEM为监控盲样;元素含量以国际标样NIST 610为外标,29Si为内标。实验中采用He作为剥蚀物质的载气。分析时采用10Hz的激光频率,193nm的激光波长,36μm的激光束斑直径,激光预剥蚀时间和剥蚀时间分别为5s和40s,U、Th、Pb元素积分时间为20ms,其它元素积分时间为15ms。测试结果通过GLITTER 4.4软件计算得出,实验获得的数据采用Andersen(2002)的方法进行同位素比值的校正以扣除普通Pb的影响,谐和图的绘制采用ISOPLOT 3.0完成(Ludwig,2001)。详细的实验分析步骤和数据处理方法见文献Yuan et al.(2004),Liu et al.(2008b)和Gou et al.(2012)。所给定的同位素比值和年龄的误差均在1σ水平。

图 4 辽东半岛南辽河群中代表性锆石阴极发光图像
圆圈代表分析点的位置,锆石下方的数字分别代表分析点号和U-Pb年龄.比例尺为100μm
Fig. 4 Cathodoluminescence(CL)images for representative zircons from the South Liaohe Group in the Liaodong Peninsula
The circles on zircons represent analyzed spots, and the numbers below are the spot number and U-Pb age. The scale bar is 100μm
4 分析结果 4.1 变质火山岩

一般来说,很难从玄武质岩石中获得足够数量的岩浆锆石用于定年。野外地质研究表明,变质流纹岩和变质玄武岩呈整合接触关系(图 3f)。因此,变质流纹岩的岩浆锆石年龄可同时代表变质玄武岩和流纹岩的原岩形成时代。此外,我们也从变质玄武岩中获得相当数量的变质锆石(如下),变质锆石年龄可以代表变质玄武岩和流纹岩的变质时代。变质锆石的形成可能与玄武岩变质过程中强烈的流体活动有关(Corfu et al., 2003)。 4.1.1 变质流纹岩

变质流纹岩(LZ3)中30个锆石的U-Pb年龄列于表 1,并图示于图 5a。从图 4a可看出,绝大部分锆石外形为自形或半自形,直径在60~90μm左右,显示典型的振荡环带结构,结合其较高的Th/U(0.3~0.9)比值,指示其岩浆成因特点(Belousova et al., 2002; Hoskin,2001; 刘福来等,2009)。3个稍老分析点的207Pb/206Pb年龄分别为2320Ma、2379Ma和2556Ma;其余27个分析点沿上交点年龄为2207±10Ma的不一致线分布,其207Pb/206Pb加权平均年龄为2201±5Ma,该年龄应代表变质流纹岩的原岩形成时代。

图 5 辽东半岛南辽河群中变质火山岩和沉积岩锆石U-Pb谐和图(a-e)及沉积岩碎屑锆石207Pb/206Pb年龄直方图(f) Fig. 5 Zircon U-Pb ages for meta-volcanic and meta-sedimentary rocks(a-e) and histogram for detrital zircons from the South Liaohe Group in the Liaodong Peninsula
4.1.2 变质玄武岩

变质玄武岩(LZ12)中21个锆石的U-Pb年龄列于表 1,并图示于图 5b。绝大部分锆石外形为半自形-他形,直径在50~100μm左右,显示弱的CL强度、无分带或者弱分带以及不规则的外形(图 4b),结合较低的Th/U比值(<0.1),指示其变质成因特点(Corfu et al., 2003; Boniface et al., 2012)。21个分析点沿上交点年龄为1875±14Ma的不一致线分布,其207Pb/206Pb加权平均年龄为1876±12Ma,该年龄应代表变质玄武岩的变质时代。

综上,变质火山岩的火山活动和变质作用时代分别为~2.2Ga和~1.9Ga。 4.2 变质沉积岩

3个变质沉积岩(LC3、LC5和LB3)的碎屑锆石U-Pb年代学数据于表 1,并图示于图 5c,d,e。锆石颗粒大多为短柱状或棱柱状,长宽比在1:1至3:1之间,LC3、LC5和LB3的锆石长轴直径分别在70~130μm、70~100μm和60~80μm左右,CL图像显示锆石具有明显的振荡环带结构(图 4c,d和e),Th/U比值集中在0.4~5之间(表 1),指示均为岩浆成因锆石(Belousova et al., 2002; Hoskin,2001; 刘福来等,2009)。从数据和年龄频谱图中(图 5f),盖县组变质沉积岩中碎屑锆石来源复杂,但年龄基本可以分为两组:太古宙(2509~3520Ma)和古元古代(2033~2446Ma)。锆石年龄的主要峰值年龄为2033Ma、2092Ma和2155Ma,次峰值年龄为2446Ma、2509Ma、2594Ma、2668Ma、2790Ma、3356Ma、3467Ma和3520Ma(图 5f)。 5 讨论 5.1 辽东半岛南辽河群年代学格架

胶-辽-吉活动带内辽河群的火山-沉积-变质时限一直存在着较大争议(路孝平等,2004)。最早基于K-Ar(40Ar-39Ar)、Rb-Sr和Sm-Nd全岩等时线或者单颗粒锆石蒸发法年龄研究,前人认为辽河群形成于2300~1900Ma(张秋生等,1988)。由于位于胶-辽-吉活动带的南辽河群经历了较为强烈的多期变形和变质作用叠加,不仅会引起K-Ar和Rb-Sr,甚至Sm-Nd同位素体系的开放,而且也会导致锆石不同程度地发生Pb的丢失现象。本文研究结果表明南辽河群年龄组成非常复杂,因此上述研究方法均不能很好制约南辽河群的火山-沉积-变质时限。

辽河群主要由低绿片岩相-角闪岩相变质的火山-沉积岩系组成,以青龙山-枣儿岭断裂为边界划分为南辽河群和北辽河群(图 2; Li et al., 2005; Li and Zhao, 2007)。南辽河群以变质火山岩的大量出露与北辽河群相区别(Zhao et al., 2005; 孟恩等,2013; Li and Chen, 2014)。本文研究结果表明,变质流纹岩的锆石具岩浆振荡环带和高Th/U比值(>0.4)的特征,均指示其岩浆成因特点,说明南辽河群的火山作用时代为~2.2Ga,这一年龄不仅与前人报道的辽东半岛南辽河群不同出露区的火山岩年龄数据相一致(Sun et al., 1993; Wan et al., 2006; Li and Chen, 2014; 陈斌,未发表数据),而且与北辽河群酸性火山岩的结晶年龄一致(Wan et al., 2006ab),说明~2.2Ga的锆石年龄可以代表整个辽河群的火山作用时代。变质沉积岩中最年轻的三颗具有岩浆成因特征的碎屑锆石给出了近谐和207Pb/206Pb年龄1.98±0.02Ga,限定其最大沉积年龄为~2.0Ga,这一年龄在误差范围内与前人报道的辽东半岛南辽河群不同出露区变质沉积岩碎屑锆石的研究结果吻合(Luo et al., 20042008; Lu et al., 2006; Wan et al., 2006a; 孟恩等,2013)。南辽河群和北辽河群变质沉积岩不仅在沉积地层上可以对比(见章节5.3; 贺高品和叶慧文,1998),而且在碎屑锆石年龄组成上一致(Luo et al., 2008; Lu et al., 2006; Wan et al., 2006a),说明本文的碎屑锆石年龄可以限制辽河群的沉积时代。变质玄武岩中的锆石具弱分带和较低Th/U比值(<0.1),指示其变质成因特点,说明辽河群的变质时代为~1.9Ga,与前人通过对辽东半岛辽河群和辽吉花岗岩中变质锆石SHRIMP或者LA-ICP-MS U-Pb原位定年限定的辽东半岛区域变质时代一致(Luo et al., 20042008; Lu et al., 2006; Li and Zhao, 2007; 孟恩等,2013; Li and Chen, 2014)。此外,Xie et al.(2011)对辽河群盖县组特征变质矿物(石榴石和十字石)定年同样获得了辽河群变质峰期年龄为1.93~1.91Ga,进一步说明辽河群变质时代为~1.9Ga。因此,辽河群变质沉积岩的沉积作用应该早于大规模的变质-变形时代~1.9Ga,其沉积时限为2.0~1.9Ga。

表 1 辽东半岛南辽河群样品中锆石LA-ICP-MS U-Pb分析结果 Table 1 LA-ICP-MS zircon U-Pb dating data for the South Liaohe Group in the Liaodong Peninsula

综上所述,南辽河群的火山活动时代为~2.2Ga,与大洋壳俯冲有关,形成大量弧岩浆岩。而后,沉积作用发生于2.0~1.9Ga之间,火山物质为沉积岩提供了重要的物源。最后,火山岩和沉积岩在~1.9Ga弧陆碰撞过程中强烈的变质-变形(详见下文)。南辽河群火山-沉积-变质的时限为2.2~1.9Ga,其演化时间约300Myr。 5.2 辽东半岛南辽河群与辽吉花岗岩的关系

另一个困扰研究者的问题是辽吉花岗岩与辽河群的关系,该问题同样制约了研究者对胶-辽-吉活动带构造历史的认识。以前的地质观察表明,辽吉花岗岩总是出现在辽河群的底部或大型褶皱构造的核部,并在大多数情况下,经历过变形作用的改造,但局部又可见其“侵入”于辽河群之中,从而有辽吉花岗岩在元古宙“重就位”之说(张秋生等,1988)。但是大量野外观察发现,辽河群和辽吉花岗岩均为构造接触(路孝平等,2004),不存在我们可以用以判断岩体形成相对顺序的侵入接触关系,因此解决该问题的关键是要获得辽河群火山-沉积建造和辽吉花岗岩的可靠年代学数据。依据锆石SHRIMP U-Pb定年分析,Lu et al.(2006)和Li et al.(2004,20062011)和Li and Zhao(2007)提出辽吉花岗岩侵位时代为~2160Ma,并认为辽河群的形成时代不早于辽吉花岗岩,即辽吉花岗岩为辽河群沉积的基底岩石。然而,本文及前人的锆石年代学数据表明,辽河群变质火山岩形成时代为~2.2Ga(Sun et al., 1993; Wan et al., 2006; Li and Chen, 2014; 陈斌,未发表数据),在误差范围内与辽吉花岗岩形成时代一致,说明辽吉花岗岩并不是辽河群火山-沉积建造的基底岩石,而是与辽河群火山岩同时或者稍晚形成。 5.3 辽东半岛南辽河群的沉积物源

锆石U-Pb定年结果显示,辽东半岛南辽河群变质沉积岩样品中包括了大量2100~2240Ma的岩浆锆石年龄(表 1),主要峰值年龄为2155Ma(图 5f),区域上与前人有关辽吉花岗岩和辽河群变质火山岩的年代学研究结果相吻合(本文; Sun et al., 1993; Wan et al., 2006; Lu et al., 2006; Li and Zhao, 2007; Li and Chen, 2014),也与集安群和老岭群、辽河群中碎屑锆石的年代学研究结果(2032~2282Ma; Luo et al., 20042008; Lu et al., 2006; 孟恩等,2013)一致,表明这些锆石应直接来源于胶-辽-吉活动带内同时期的花岗质岩石和火山岩。此外,辽东半岛南辽河群变质沉积岩样品中还包括了大量1981~2100Ma的岩浆锆石年龄(表 1),主要峰值年龄为2033Ma和2092Ma(图 5f),明显比辽吉花岗岩和辽河群变质火山岩的时代年轻,说明这些锆石不可能来自辽吉花岗岩和辽河群变质火山岩。这些锆石具有同心状振荡生长环带,表明它们来自中酸性岩浆岩。该组锆石年龄信息意味着辽东半岛除2100~2200Ma辽吉花岗岩和辽河群火山岩的岩浆事件外,至少还存在一期2000~2100Ma中酸性岩浆活动,这期岩浆事件性质的识别对判别胶-辽-吉活动带的形成演化有着重要意义。前人在碎屑岩和深成岩中也获得这组年龄的信息,均表明该期岩浆事件的存在(Luo et al., 20042008; 王惠初等,2011; Lu et al., 2006; 孟恩等,2013)。而峰值年龄为2446Ma、2509Ma、2594Ma、2668Ma和2790Ma的锆石年龄变化于2411~2794Ma,与华北克拉通基底TTG片麻岩(3.80~2.50Ga; Liu et al., 19922008; Song et al., 1996; Wan et al., 2001; Wu et al., 20052008)中锆石年龄结果相类似,这些碎屑锆石具有同心状振荡生长环带和较高的Th/U比值特征,说明其应来源于辽北-吉南陆块或者辽南陆块的岩浆岩。结合样品中古老锆石年龄~3356Ma、~3467Ma、~3520Ma的存在,表明太古宙古老基底(>3000Ma)也为辽河群提供了重要物源。前人的研究已经表明,辽北-吉南陆块和辽南陆块在岩石组合、年代学和变质特征上明显不同。辽南陆块(即狼林陆块)太古宙花岗岩主要以石英闪长岩-英云闪长岩为主,岩性简单且偏基性,形成时代为2440~2500Ma,经历了角闪岩相变质(林强等,1992; 路孝平等,2004; Lu et al., 2006);辽北-吉南陆块(即龙岗陆块)太古宙花岗岩岩石类型复杂,出现的岩石类型以花岗闪长岩和二长花岗岩为主,英云闪长岩和奥长花岗岩仅局部出现,形成时代为2500~3800Ma,经历了角闪岩-麻粒岩相变质,多处出现麻粒岩以及与麻粒岩相变质有关的紫苏花岗岩(Liu et al., 19922008; 林强等,1992; Song et al., 1996; Wu et al., 20052008; Wan et al., 2005)。这些均说明这两个太古宙陆块可能并不是由同一个太古宙克拉通裂解形成,同时也暗示胶-辽-吉活动带的形成演化可能并非与陆内裂谷的闭合有关(张秋生等,1988; Li et al., 20052006; Luo et al., 20042008; Li and Zhao, 2007),弧陆碰撞模式可能更好的解释胶-辽-吉活动带的地质问题(白瑾,1993; 路孝平等,2004; Faure et al., 2004; Lu et al., 2006; 王惠初等,2011; Li and Chen, 2014)。综上,>3000Ma的古老锆石应来自北部的辽北-吉南陆块。一种可能是,来自于辽北-吉南陆块的被动陆缘沉积与来自辽南陆块的弧前沉积在碰撞缝合过程中发生了构造混杂,并随后发生变质而形成辽河群;另一种可能是,辽北-吉南陆块与辽南陆块中间的大洋俯冲在2200~2000Ma结束以后,两个太古宙陆块相当靠近但并未对接,其间为浅海海盆,同时接受来自辽北-吉南陆块与辽南陆块的陆缘碎屑物质。目前我们无法区分上述两个可能。因此,南辽河群变质沉积岩的原岩可能是随大洋俯冲消减过程中不断形成的弧相关沉积。此外,~3520Ma也是迄今为止胶-辽-吉活动带内发现的最古老的锆石年龄记录,目前年龄为~3.5Ga记录在华北克拉通上少见,仅在冀东地区有该年龄的报道(Jahn et al., 1987),因此其地质意义有待进一步研究。 5.4 辽东半岛辽河群形成的构造环境

有关胶-辽-吉活动带内辽河群形成的构造背景一直就存在争议,一部分学者认为它们形成于与裂谷相关的盆地(张秋生等,1988; Li et al., 20052006; Luo et al., 20042008; Li and Zhao, 2007),而另一部分学者认为形成于弧后或者弧前盆地(白瑾,1993; 路孝平等,2004; Faure et al., 2004; Lu et al., 2006; 王惠初等,2011; Meng et al., 2014; Li and Chen, 2014)。根据最新获得的碎屑锆石年龄以及岩石地球化学等资料,我们认为辽河群形成于辽北-吉南陆块向辽南陆块之下俯冲的弧后或者弧前盆地,除上述提及的辽北-吉南陆块和辽南陆块的不同特征外,主要证据如下:

(1)辽河群不同出露区的碎屑锆石年龄图谱具有相似性,其中最主要的源区都是胶-辽-吉带内2000~2200Ma的岩浆岩,而辽北-吉南陆块与辽南陆块太古代的古老岩石则是次要的源岩(少于10%的锆石颗粒)。2000~2200Ma的岩浆岩(辽河群火山岩和辽吉花岗岩)均属于钙碱性岩浆岩(郝德峰等,2004; 路孝平等,2004; Sun et al., 1993; 王惠初等,2011; Meng et al., 2014; Li and Chen, 2014),在初始地幔均一化的蜘蛛网状图解中来自不同地区的岩浆岩显示了相似的Nb、Ta和Ti亏损的特征(Sun et al., 1993; 王惠初等,2011; Meng et al., 2014; Li and Chen, 2014),表明它们是与弧密切相关的。这样一个弧相关岩石和克拉通来源的二元源区组合与典型的弧后盆地沉积如Tonto Basin Supergroup是相似的,其碎屑物质分别来自本地与弧相关的火山岩和北美克拉通(Condie et al., 1992)。Cawood et al.(2012)提出,裂谷盆地的大部分碎屑锆石的年龄远大于沉积的年龄,只有小于5%锆石颗粒与地层沉积年龄相近。与沉积时代相近的锆石年龄可能反映了裂谷相关的岩浆活动,但是在年龄谱中只占很少的一部分,因为这些岩浆活动产生的镁铁质岩浆形成的锆石极少。本文和前人的碎屑锆石年龄图谱均表明,同构造期锆石占有很高的比例,古老锆石很少,明显不同于裂谷盆地(Cawood et al., 2012)。

(2)辽河群的岩浆岩以酸性岩浆占主导特征,明显不同于东非裂谷的岩浆岩岩石组合(玄武岩为主),同时也缺乏具有典型大陆裂谷岩浆作用特征的岩石,如OIB性质的玄武岩、碳酸岩和碱性岩(响岩和碱流岩)等(Wilson,1989; Furman,2007)。

(3)大部分辽吉花岗岩并非A型花岗岩(如牧牛河岩体、哈达碑岩体、虎皮峪岩体等),主要是I型花岗岩(郝德峰等,2004; 路孝平等,2004; Li and Zhao, 2007),主要证据:(a)榍石、磁铁矿和角闪石的广泛存在,富水及钙碱性岩浆特征;(b)辽吉花岗岩中闪长质包体的发现,富水岩浆特征及壳幔岩浆混合作用的岩石学特征;(c)全岩Nd-Sr同位素和锆石Hf同位素变化范围较大,较宽的化学成分(SiO2含量在60%~74%),吻合于壳幔岩浆混合作用的地球化学特征,与弧花岗岩类似。根据最近获得的锆石U-Pb年代学资料,胶-辽-吉活动带内出露的大量A型花岗岩(先前被认为是辽吉花岗岩; 张秋生等,1988),形成时代主要为~1.85Ga(路孝平等,2004; Li and Zhao, 2007; 杨进辉等,2007),少量中生代(Li et al., 2004; 路孝平等,2004),并不存在大规模的~2.16Ga A型花岗岩。前人研究认为,该时期的岩浆活动在地球化学上具有某种“双峰式”分布特征,缺乏安山质岩浆活动(张秋生等,1988; Li et al., 2011),但我们最新研究表明,辽河群中存在安山质火山岩(矿物组成特征,角闪石/辉石(15%~35%)、钾长石(10%~20%)、斜长石(35%~45%)和石英(10%);地球化学特征,SiO2含量变化于55%~62%),但规模大小有待进一步研究,暗示该时期的岩浆活动可能代表一个连续的岩浆序列,并非所谓的“双峰式”岩石组合(张景山,1994; Li and Chen, 2014)。因此,作为裂谷模式主要证据之一的A型花岗岩和双峰式火山岩有待推敲。

(4)南北辽河群表现出不同的岩石组合类型和变质演化史(P-T-t轨迹)(张秋生等,1988;白瑾,1993; 贺高品和叶慧文,1998; Zhao and Zhai, 2013),二者的火山作用时代、沉积时代和变质时代可以对比(上文已述)。北辽河群以大量陆缘碎屑岩-碳酸盐岩出露(稳定的被动大陆边缘沉积)为特征,主要由石英岩、片岩、千枚岩和大理岩组成,原岩建造为碎屑岩-粘土岩-碳酸盐岩建造,沉积旋回明显,下部以碎屑岩为主,夹少量火山岩,中部为粘土岩夹碳酸盐岩,上部为碳酸盐岩(贺高品和叶慧文,1998)。北辽河群常见有石榴石、十字石、蓝晶石等变质矿物,具有近等温减压(ITD)的顺时针P-T-t轨迹,以夕线石取代蓝晶石为特征,属于典型的中压变质作用,峰期达到低角闪岩相(Zhao and Zhai, 2013)。南辽河群以大量火山岩出露为特征,主要由浅粒岩、变粒岩、片麻岩、片岩、大理岩和石英岩组成,原岩建造为火山岩-碎屑岩-碳酸盐岩建造,沉积旋回不明显,下部为火山岩为主,夹少量碎屑岩,中部为碎屑岩夹碳酸盐岩,上部为碳酸盐岩、粘土岩和碎屑岩(贺高品和叶慧文,1998)。南辽河群常见有石榴石、十字石、红柱石、堇青石和夕线石等变质矿物,具有近等压冷却(IBC)的逆时针P-T-t轨迹,以夕线石取代红柱石为特征,属于典型的低压变质作用,峰期达到高角闪岩相(Zhao and Zhai, 2013)。上述不同的特征均很难用裂谷模式去解释。 6 结论

(1)南辽河群的变质火山岩中岩浆锆石和变质锆石指示火山作用和变质作用时代分别为~2.2Ga和~1.9Ga。变质沉积岩中最年轻的碎屑锆石U-Pb年龄为~2.0Ga,指示沉积作用时代介于2.0~1.9Ga之间。结合前人有关辽吉花岗岩及区域构造变形、变质作用等资料,本文认为辽河群火山-沉积-变质的时限为2.2~1.9Ga,其演化时间约300Myr。辽吉花岗岩并不是辽河群沉积的基底,而是与辽河群火山岩同时或者稍晚形成的。

(2)碎屑锆石的年龄频谱特征显示,辽河群的沉积物主要来源于胶-辽-吉活动带内2000~2100Ma的中酸性岩浆岩、辽吉花岗岩和辽河群火山岩,少部分来自周缘的太古宙基底花岗质岩石。胶-辽-吉活动带至少还存在一期2000~2100Ma的中酸性岩浆活动。

(3)辽河群可能并非形成于许多人坚持的裂谷环境,而可能与大洋壳俯冲过程有关。

致谢

感谢北京大学造山带与地壳演化教育部重点实验室和天津地质矿产研究所同位素实验室在锆石LA-ICP-MS U-Pb测试分析中给予的大力帮助;感谢北京大学地球与空间科学学院王盟、詹彦、刘凤麟、许鑫、余黄露、钱加慧、翟俪娜和王志强等研究生在成文过程中的有意探讨;感谢审稿专家李振生教授、胡波博士和孟恩博士提出的宝贵意见和建议;感谢翟明国院士和赵太平研究员提出修改意见。

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