华南地块是东亚一个重要的前寒武纪地体,它主要由北西部的扬子克拉通以及东南部的华夏地块构成,两者在新元古早期(1000~820Ma)沿着江南造山带拼合在一起(Li et al., 2009; Wang et al., 2007a; Zhou and Zhu, 1993; Chen et al., 1991; Zhao et al., 2011)。两者的前寒武基底组成、分布及演化各不相同(Zhao and Cawood, 2012; Li et al., 2002; Wang et al., 2007a; Zhao et al., 2011; Yu et al., 2010,2012)。扬子地块具有太古代-古元古代的结晶基底,出露在扬子地块北部崆岭地区的2.9~3.2Ga TTG片麻岩以及广泛分布在元古代表壳岩之下的太古代岩石构成了扬子地块的古陆核(焦文放等,2009;Qiu et al., 2000; Zheng et al., 2006; Zhang et al., 2006)。新元古代岩浆活动和沉积作用分布在扬子板块东南部(江南造山带)以及西部、北部(攀西-汉南褶皱带)。这些围绕于扬子地块周缘的新元古代岩浆活动以花岗质岩石为主,少量镁铁质岩石,主要形成于830~820Ma和770~720Ma(Li et al., 2003a,b; Zhao and Cawood, 2012; Zheng et al., 2004,2007,2008;郑永飞和张少兵,2007)。华夏地块以发育大量显生宙岩浆岩为特征,前寒武纪基底岩石出露较少。对于华夏板块是否存在古老基底一直存在争议。于津海等(2007a)和Yu et al.(2008,2010)认为华夏地块主要由新元古代基底组成,少量古元古代(1890~1815Ma)花岗岩以及表壳岩石出露在浙西南以及闽北地区,而少量的中元古岩石仅在海南西部出露。华夏的古老基底受到了后期构造热事件的强烈改造(Zhao and Cawood, 2012; Zheng et al., 2004; Yu et al., 2010; Yao et al., 2011; 于津海等,2006b)。部分学者则认为华夏地块属于大洋板块俯冲而形成的加里东造山带(马瑞士,2006; 杨树锋等,1995)。邓平等(2002)和舒良树(2006)根据华夏地块前寒武纪岩石的出露地区,将华夏基底组成划分为浙东南-闽西北、赣中-赣南和云开大山三个地块; 而Yu et al.(2010)根据基底岩石组成的差异将华夏的前寒武纪基底划分为武夷地块和南岭-云开地块;但Xu et al.(2007a)是以政和-大埔断裂为界将华夏划分为东华夏与西华夏。无论哪种划分,云开地块都被认为是华夏地块西南部的一个重要的前寒武纪变质基底出露区。但是,一些学者认为郴州-临武-四会-吴川断裂是华夏地块与扬子地块的分界线(图 1a),从而将断裂西侧的云开地块划归于扬子地块(Wang et al., 2003b,2008a; 王剑等,2003; Zhang and Wang, 2007)。最近随着对华南地块研究的深入,关于扬子地块与华夏地块的界线问题(特别是西南部的分界)一直受到广泛的关注。因此,对云开地块前寒武纪基底的组成、演化、时代归属、构造属性的研究不仅有助于更好的了解华南前寒武地壳的形成与演化,而且能为厘清云开地块与扬子和华夏的关系,限定二地块的界线提供重要约束 。
![]() | 图 1 华南前寒武变质基底分布简图(a,据Zhao and Cawood, 2012修改)和云开地区地质简图及采样位置(b,据Wan et al., 2010修改) Fig. 1 Simplified distribution map of Precambrian basement metamorphic rocks in South China(a,modified after Zhao and Cawood, 2012) and geologic map of Yunkai terrane and sampling locations(b,modified after Wan et al., 2010)area |
云开地块位于华南地块西南部的粤西-桂东南地区,呈北东-南西向跨越两广地区。其东部以吴川-四会断裂构造带为界,西侧以岑溪-博白断裂为界,北至罗定-广宁断裂带,南至东南向遂溪断裂。前人曾对云开地块的变质岩系和岩浆活动做过大量研究(Wan et al., 2010; Wang et al., 2007c,2011,2012a; Li et al., 2010),但对云开地块前寒武基底的组成和形成时代却有很大争议(南颐,1994;张业明和彭松柏,2000;黄圭成等,2001;邝永光等,2001; Wan et al., 2010; Wang et al., 2007c; Yu et al., 2010)。早年的研究大多认为云开地块的变质基底具双层结构特点:主要由分布于云开地块中心的高级变质岩(高州杂岩)和围绕在其周围并与其呈断层接触的中级-低级变质的云开群组成,其上被未变质的泥盆纪地层不整合覆盖(广东省地质矿产局,1988; 张业明和彭松柏,2000)。地块内还发育不同阶段的强过铝花岗岩(Wan et al., 2010; Zhang et al., 2012a; Wang et al., 2011,2012a)。高州杂岩和云开群曾被认为具有不同的形成年龄(广东省地质矿产局,1988;钟增球等,1996;邝永光等,2001; 覃小锋等, 2006,2007),但这些数据多是根据传统的锆石Pb-Pb蒸发法获得,仅少数为原位锆石U-Pb同位素定年结果。因而前人对高州杂岩和云开群的研究得出的年龄范围相当大,从古元古-早古生代均有报道(广东省地质矿产局,1988; 覃小峰等,2006;黄圭成等,2000;邝永光等,2001;南颐,1994;钟增球等,1996; Zhang et al., 1997;Yu et al., 2010; Zhang et al., 2012b; Wang et al., 2011)。根据对云开地区花岗岩和变质岩的锆石U-Pb SHRIMP 定年结果以及全岩Nd同位素分析,Wan et al.(2010)认为云开群和高州杂岩实际上同时形成于新元古代晚期-古生代早期,并将其统称为云开表壳岩系。
深层次的高州杂岩原岩多为泥砂质陆源碎屑岩,变质程度多达角闪岩相,由副片麻岩、片岩、石英岩以及大理岩构成的变质表壳岩石及混合岩组成,局部达麻粒岩相(Wan et al., 2010; 张业明和彭松柏,2000)。浅层次基底云开群,原岩陆源碎屑岩夹基性-酸性火山岩组合,变质程度多为绿片岩相,局部可达角闪岩相。大量镁铁-超镁铁质岩石呈透镜状存在于层间,经历了强烈变形变质。云开群分上下两个亚群。上亚群变质程度较低,由变质粉砂岩、千枚岩及石英片岩组成。下亚群由含石墨副片麻岩、绿片岩、混合岩、大理岩及硅质岩构成(Zhang et al., 2012b)。
本次研究对粤西云开地区的罗定、信宜、高州一带地区出露的基底变质岩进行了野外考察(图 1b)。共采集新鲜样品27件,对其中的13件样品进行了化学分析。分布于云开中心的信宜以东早古生代花岗岩附近的样品(YK-13-2、YK-13-3、YK-15、YK-16-1、YK-17、YK-19-1)主要为片岩、片麻岩,矿物成分以石英、黑云母、斜长石为主,个别样品(YK-13-2)中含有石榴子石。总体变质程度为绿片岩相到低角闪岩相。罗定西南罗嚹村附近样品(YK-4、YK-5-2、YK-6、YK-10-1)大多以浅变质的沉积岩石,样品岩性为浅变质的细砂岩、粉砂岩、长石石英砂岩以及板岩、片岩等。矿物以石英、长石、绢云母以及细小的黑云母为主,部分岩石见细小白云母定向分布。以上样品均采自云开群,我们对样品YK-10-1(变质长石石英砂岩)、YK-13- 3(黑云母斜长片麻岩)、YK-16-1(条纹状的黑云母片麻岩)以及YK-19-1(眼球状黑云母片麻岩)进行了锆石U-Pb定年分析,其中对YK-13-3中锆石还进行了Hf同位素分析。而分布于云开南部高州附近的样品(YK-21-1、YK-24-3、YK-25-2)位于高州杂岩与古生代花岗岩的接触带上,变质程度为高角闪岩相-麻粒岩相,样品岩性主要为混合岩、黑云母变粒岩和长英质麻粒岩。麻粒岩样品(YK25-2)中可见特征变质矿物紫苏辉石。石榴子石在混合花岗岩岩(YK-21-1)和麻粒岩中均有发现。 2 分析方法
全岩的主微量元素的分析均在内生金属矿床成矿机制研究国家重点实验室完成,主量元素的检测仪器为ARL9800XP+XRF荧光光谱仪,分析精度好于2%。微量元素的分析在ICP-MS分析实验室的Finnigan ElementⅡ上完成,分析精度好于10%,且绝大多数好于5%。
富集锆石的样品是经过岩石的粉碎、淘洗、磁选、重液分离分选等传统技术获得的。在双目镜下挑选出具有晶形、透明度、颜色等不同特征的锆石,用环氧树脂将其固定后,磨蚀并抛光至锆石中心出露,最后制成LA-ICPMS分析的锆石靶。锆石的背散射成象(BSE)分析、锆石U-Pb定年以及Hf同位素分析均是在澳大利亚Macquarie大学GEMOC/CCFS国家研究中心进行。背散射图像是用CAMECA-SX100电子探针拍摄的,工作时所需电压15kV、光束15~20nm。进行锆石U-Pb定年的仪器为Agilent HP 4500型 ICPMS加上New Wave Research LUV213nm激光剥蚀系统共同完成。详细的分析程序、分析精度和准确性见Griffin et al.(2004)描述。为了增加运送物质的效率、提供更稳定的信号以及增加U/Pb分异的重现性,激光剥蚀物质采用He气传输。并将激光蚀点的直径设定为45μm。每个测试单元包含了10个未知样品点的分析以及前后各两次的标样GJ-1的分析。锆石91500和/或Mud Tank被插入其中分析以检测分析结果的可靠性。测试结果由GLITTER4.4软件进行处理。U-Pb同位素测定数据采用Anderson(2002)方法进行普通Pb校正。校正后的数据由ISOPLOT程序进行处理。对于207Pb/206Pb年龄小于900Ma的锆石采用经普通Pb校正过的206Pb/238U年龄(Yu et al., 2010),而大于900Ma的采用未经普通Pb校正207Pb/206Pb年龄。锆石Hf同位素原位分析,使用New Wave Research LUV213nm激光发射器与Nu Plasma MC ICPMS组成的仪器上完成。使用标样91500与Mud Tank检验数据精确度与准确度,典型激光溶蚀时间为80~120s,溶蚀点直径50~60μm,剥蚀深度40~50μm,详细的测试方法及步骤参见Griffin et al.(2000)。
Hf模式年龄(tDM)采用亏损地幔比值176Hf/177Hf=0.28325,176Lu/177Hf=0.0384计算(Vervorrt et al., 1999)。εHf(t)的计算采用球粒陨石比值176Hf/177Hf=0.282772、176Lu/177Hf=0.0332、λ176Lu=1.865×10-11(Blichert-Toft and Albarède,1997; Scherer et al., 2001)。二阶段Hf模式年龄(tDMC)用平均大陆地壳176Lu/177Hf=0.015计算(Griffin et al., 2002)。 3 岩石地球化学 3.1 主量元素
全岩的地球化学分析见表(表 1;含YK-27数据(Yu et al., 2010))。运用Shaw(1972)的判别式对样品进行DF值计算,结果表明除样品YK-17-1(DF=0.1)外,其它样品DF值皆小于0,显示副变质岩特征。在Si-(al+fm)-(c+alk)原岩类型判别图上(图 2),几乎所有样品均落入沉积岩范围。其中混合花岗岩YK-21-1含石榴子石,其ACNK指数大于1.1,表明它是富铝质变质沉积岩部分熔融形成。在砂岩分类图解中绝大部分样品投影于杂砂岩区(图 3),与南岭一带的同期变质沉积岩类似(魏震洋等,2009;王鹏鸣等,2012)。13个样品的主量元素含量变化范围较大,SiO2含量变化在59.51%~85.09%之间,Al2O3为10.48%~19.1%。个别样品Al2O3为6.87%~9.01%,与其高的SiO2含量(81.66%~85.09%)相关。研究区这些样品的平均化学组成具有比上地壳平均成分(Rudnick and Gao, 2003)更高的SiO2、以及相对更低的Al2O3、CaO、Na2O,而TiO2、Fe2O3T的含量相差不大,表明样品中石英含量相对较高,斜长石含量相对较低。样品的SiO2/Al2O比值变化为3.12~12.38,平均为5.89,显示岩石中等的成熟度。除了1个长石石英砂岩样品(YK10-1)成熟度较高外其余样品之间相差不大,成熟度相近。与南岭地区(平均5.26),扬子南缘梵净山(平均3.68)、四堡地区(平均4.41)的样品进行对比,云开地区沉积岩的成熟度总体上相似南岭地区,而较扬子南缘新元古代沉积岩高。云开地区样品的K2O/Na2O比值变化大(0.52~19.55),平均为4.89。除一个样品外其他样品的K2O/Na2O比值均大于1,表明原岩均受到了强烈风化使得Na不同程度的流失,而K相对于Na较多的保留,其中粘土矿物可能控制了主量元素的组成。不同地区沉积岩的K2O/Na2O比值存在差异,云开地区以及四堡地区(平均8.21)沉积岩的K2O/Na2O比值总体偏低,而梵净山(平均20.4)以及南岭地区(平均16.0)具有相对较高的K2O/Na2O比值(魏震洋等,2009;王丽娟,2010;Wang et al., 2012b)。
| 表 1 云开地块样品主量元素(wt%)及微量元素(×10-6)组成 Table 1 Major(wt%) and trace(×10-6)element compositions of basement rocks in Yunkai terrane |
![]() | 图 2 Si原岩恢复图解(据Winkler et al., 1976) Fig. 2 Protolith discrimination diagram of Niggli index(al+fm)-(c+alk)vs. Si(after Winkler et al., 1976) |
![]() | 图 3 砂岩分类图解(据McLennan et al., 1993) Fig. 3 Classification diagram of s and stone types(after McLennan et al., 1993) |
各个样品中微量元素含量变化范围较大。在后太古代平均页岩(PAAS)标准化蛛网图中(图 4),大多数大离子亲石元素的平均含量与PAAS相近,而Sr以及相容元素Cr,Ni等强烈亏损,高场强元素Nb,Ta轻度亏损。大部分样品Pb明显亏损但有少数显示富集,而大多数样品的Zr、Hf、Th、U等高场强元素轻度富集,也说明沉积物的源区以长英质组分为主或主要由再循环物质组成。少数样品具有更高的Sr、Sc和亏损的U含量,说明这部分沉积岩的源区相对富镁铁组分。样品的稀土元素总量变化大(REE=127×10-6~318×10-6)。球粒陨石标准化稀土配分模式显示大多数样品具有与PAAS相似的稀土分布模式(图 5),为轻重稀土中等分异以及弱-中等的Eu负异常(δEu=0.52~0.90)。
![]() | 图 4 后太古代页岩(PAAS)标准化微量元素蛛网图(标准化值据Taylor and McLennan, 1985) Fig. 4 Post-Archean shale normalized trace element distributions for studied samples in Yunkai terrane(normalized data from Taylor and McLennan, 1985) |
![]() | 图 5 球粒陨石标准化稀土配分模式图(标准化值据Taylor and McLennan, 1985) Fig. 5 Chondrite-normalized rare earth element patterns for basement rocks in Yunkai terrane(normalized data from Taylor and McLennan, 1985) |
本次研究选择了4个样品的锆石进行BSE成像以及精确的LA-ICPMS U-Pb定年分析。对比这4个样品的年龄特征,它们大致可以被能分为2组,第一组包括YK-10-1、YK-16-1以及YK-27(Yu et al., 2010),第二组包括YK-19-1和YK-13-3(表 2)。
| 表 2 云开地块变质沉积岩中碎屑锆石U-Pb定年结果 Table 2 U-Pb dating results of detrital zircons from meta-sedimentary rocks in Yunkai area |
样品中的碎屑锆石大多呈次圆、磨圆的短柱状或不规则状,少部分锆石较自形长柱状,晶棱圆滑边界平直(图 6a-g)。大多数锆石的BSE图像多呈灰白色,亮度较暗。一部分内部结构简单具弱分带到无内部结构(图 6a-c)。另一部分锆石内部可见明显的韵律环带(图 6d,e)。少数具有不规则状的继承核的锆石,BSE亮度较暗,增生边具有均匀到宽的分带(图 6f)。少量锆石具有一条较薄的增生边,具有较高的BSE亮度(图 6g)。这些锆石的Th/U比通常较低,显示出变质锆石的特征。该样品中锆石所含的Th大多变化于10×10-6~953×10-6,U大多变化于15×10-6~1914×10-6,77%的锆石颗粒具有Th/U>0.3,表明大多数锆石具有岩浆成因。
![]() | 图 6 云开地区变质沉积岩样品中锆石的背散射(BSE)图像 Fig. 6 BSE images of zircons from metasedimentary rocks in Yunkai terrane |
对55颗锆石的57次U-Pb同位素分析显示,除一个锆石明显不谐和外,其他锆石的协和度大多大于90%,它们大多落在谐和曲线上或附近(图 7a)。这些碎屑锆石的年龄变化较大,最年轻的协和锆石为600Ma。这颗碎屑锆石具有极高的U含量,导致极低的Th/U比值(0.04),指示其变质成因。此年龄限定了沉积时代晚于600Ma。岩石中含量最多的为新元古代-中元古代晚期(1236~738Ma)锆石,年龄峰在~980Ma,表明原岩主要为Grenville期碎屑物质组成。样品中还含有少量晚古元古代(1831~1600Ma)和早古元古代-晚新太古代(2584~2271Ma)两组碎屑锆石,分别在~1660Ma和~2415Ma出现2个弱峰(图 8a)。样品中最老的一颗碎屑锆石的谐和年龄为3591Ma,表明云开地区存在少量古太古代的地壳物质。
![]() | 图 7 云开地区变质沉积岩样品中锆石U-Pb年龄谐和图 Fig. 7 U-Pb concordia plot of detrital zircons from metasedimentary rocks in Yunkai terrane |
![]() | 图 8 碎屑锆石U-Pb年龄谱(由各样品中谐和度>80%的锆石组成) Fig. 8 U-Pb age spectra of detrital zircons(data used are those with concordance > 80%) |
样品中的锆石大多为圆形至不规则柱状(图 6h-l),多数锆石颜色较深,成分较为均一的锆石占大多数,少数具有弱的环带,个别韵律环带较明显。一些锆石可见变质增生边以及内部的继承核。锆石的Th、U含量变化较大,Th大多集中于14×10-6~2351×10-6,而U主要变化于92×10-6~2129×10-6。74%的锆石的Th/U大于0.3,亦表明锆石以岩浆成因为主。成分环带的不明显可能是由于后期受流体或热事件的影响,离子扩散使得成分均一化的原因。
对样品中51颗锆石进行的U-Pb同位素分析显示,90%的锆石谐和度大于90%,它们都落在谐和曲线上或附近(图 7b)。少数锆石的谐和度较差(79%~87%),而只有一颗锆石是明显的不谐和(谐和度48%),表明这些锆石发生了不同程度的Pb丢失,它们的真实年龄应该老于它们的207Pb/206Pb的表面年龄。该样品中锆石的年龄范围在2849~524Ma,年龄谱与样品YK10-1的较为类似(图 8b),具有1个主峰,2个弱的年龄峰。第一组锆石年龄集中在新元古代-中元古代晚期(1188~765Ma),主峰值在~981Ma。另外两组年龄变化于1865~1324Ma和2849~2394Ma,它们的峰值大致在1662Ma和2404Ma。该样品最年轻锆石的谐和年龄是524Ma,指示原岩的沉积时代不早于524Ma。 4.3 眼球状黑云母片麻岩(YK-19-1)
该样品的锆石大多呈椭圆-圆滑的柱状,部分锆石较为自形、边部平直呈棱角状,表明多数的锆石经历受了长距离的搬运、磨蚀或多相再循环(图 6m-s)。锆石的内部结构形态更为多样,不均匀的内部结构以及亮斑反应了因变质而产生的脱晶化作用。部分继承核的外部可见变质增生边或者环带构造(图 6r-s)。除一个锆石的Th/U比值小于0.1,有63%的锆石颗粒的Th/U比值大于0.3,表明这些锆石大多为岩浆结晶的。
对40颗锆石进行的43次分析显示所有锆石的协和度>80%,84%锆石具有谐和度90%以上(图 7c)。43个分析的年龄变化于3505~415Ma之间,大致可以分成三组。最大一组的锆石获得了最年轻的年龄(467~415Ma)。除去2个偏离的颗粒,余下的9个分析的加权平均年龄为450Ma。第二组锆石年龄集中于新元古代(983~760Ma),峰值在788Ma。第三组年龄变化于2701~2377Ma,峰值为~2494Ma(图 8d)。样品中最老的一颗锆石年龄为3505Ma,稍小于样品YK-10-1中的那颗老锆石,但谐和度较差(89%),说明有一定的发射成因Pb的丢失。样品中最年轻的碎屑锆石年龄为517Ma,指示原岩沉积作用晚于这个时代。本样品锆石年龄谱与上述2个样品有较大差别,碎屑物质主要由新元古代(~788Ma)和晚新太古代(~2.5Ga)的碎屑组成,而缺少Grenville期和~1.6Ga的碎屑物质,而且本样品年龄谱的主峰在450Ma。这些锆石的Th/U大多小于0.20,多为较自形的颗粒,显示变质增生锆石的特点,因此它们很可能是后期加里东变质作用叠加形成的变质(深熔)锆石。
4.4 黑云母斜长片麻岩(YK-13-3)
锆石多为次圆状-不规则柱状,形态多样,有的锆石具有自形晶形。大多数锆石均具有继承核,部分锆石幔部具有明显振荡环带(图 6t-u),部分锆石有细小包裹体、亮斑以及不规则斑块(图 6v-w),具有无-弱分带或变质增生边(图 6x)。多样的锆石反应了成因及来源的多样性。成分分析显示85%的锆石具有大于0.3的Th/U比值,少部分锆石的Th/U比值甚至大于1,指示以岩浆成因锆石为主。
对样品YK13-3中的77颗锆石进行了84次U-Pb同位素分析,结果显示大多数锆石是谐和的,在谐和图上落在谐和曲线上和附近(图 7d),尤其是<1000Ma的年轻锆石。不谐和的锆石主要是那些古元古代和太古代的碎屑锆石。84个年龄分布于2494~439Ma(图 7d),集中在5个年龄段。最主要的一组年龄是新元古代(880~732Ma),其峰值在~795Ma;其他四组年龄分布在早古元古代(2494~2365Ma)、晚古元古代(2037~1719Ma)、早中元古代(1596~1448Ma)和早古生代(544~439Ma)。其中前三组的锆石在谐和图上都各自沿一条不一致曲线分布(图 7d),获得三组上交点和下交点年龄分别是2478±11Ma和440±24Ma、1864±15Ma和448±34Ma、1600±23Ma和464±140Ma。五组锆石的前四组锆石的Th/U大多都大于0.4,指示它们岩浆成因,而第五组早古生代锆石的Th/U值大多小于0.1,而且这些年龄都测自锆石颗粒增生边部,证实它们属于变质年龄。这组锆石的加权平均年龄为445Ma,上述三个不一致曲线的下交点年龄也与这个年龄相近,表明那三组锆石的放射性Pb丢失都可能是早古生代变质事件造成的。说明早古生代的加里东运动对云开地区产生了强烈的影响,也造成了大量~430Ma花岗岩的形成(Wan et al., 2010; Wang et al., 2007c,2011)。
样品YK-13-3的年龄谱与样品YK-19的类似(图 8d),具有丰富的新元古代锆石而缺乏Grenville期锆石,而且也含有大量早古生代(544~439Ma)的变质锆石。另外,样品YK-13-3的最年轻碎屑锆石年龄是489Ma,较样品Yk-10-1和YK-16-1的年轻,而与样品YK-19的相似。
对该样品中的69颗锆石进行了Hf同位素分析。结果显示它们具有较宽的176Hf/177Hf比值(0.280985~0.282709),对应的εHf(t)变化于-22.15~13.07。70%的锆石具有负的εHf(t)值(表 3),表明这些锆石的母岩浆大多数起源于古老壳源物质。图 9显示样品中最主要的新元古代锆石具有最大的Hf同位素变化。176Hf/177Hf比值最高的新元古代锆石几乎与亏损地幔的176Hf/177Hf一致(εHf(t)=13.07),说明这些锆石结晶于幔源岩浆或新生地壳的重熔岩浆。而Hf同位素最低的新元古代锆石具有εHf(t)=-22.15,对应的Hf模式年龄是~3.1Ga,与最古老的一组锆石(~2.5Ga)的模式年龄相似,指示它们的母岩浆起源于相同的 源区。几乎所有晚古元古代锆石都具有负的εHf(t)值(图 9),说明这一时期的岩浆活动主要是古老地壳再循环的产物,而早中元古代(~1.6Ga)锆石大多是正εHf(t)值,指示这一时期很可能有新生地壳的形成。3个早古生代(446~439Ma)变质锆石具有相似的Hf同位素组成,与所有Hf同位素分析的平均值(类似全岩的Hf同位素)相似,也证明这些锆石是新生的变质增生锆石。另一颗489Ma锆石具有明显低的Hf同位素(εHf(t)=-21.34)和新太古代的二阶段模式年龄,也指示了它与其他早古生代变质锆石不同的成因。
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图 9 锆石Hf同位素与U-Pb年龄图解 数据来源:YK-27(Yu et al., 2010);南岭(于津海等, 2006a,2007a;王丽娟等,2008;Yu et al., 2010);梵净山(Zhou et al., 2009;Wang et al., 2010a);四堡(Wang et al., 2012b);广西平南、平乐基性岩中捕获锆石(Zheng et al., 2011) Fig. 9 Diagram of initial 176Hf/177Hf ratios vs. U-Pb age of detrital zircons Data sources: YK-27(Yu et al., 2010); Nanling(Yu et al., 2006a,2007a,2010; Wang et al., 2008b); Fanjingshan(Zhou et al., 2009; Wang et al., 2010a); Sibao(Wang et al., 2012b); xenocrystic zircons in Pingnan and Pingle mafic rocks(Zheng et al., 2011) |
| 表 3 云开地块样品YK-13-3中锆石Hf同位素组成 Table 3 Hf isotope compositions of detrital zircons from sample YK-13-3 in Yunkai terrane |
云开地块一直被认为是华南一个重要的前寒武纪变质地体(广东省地质矿产局,1988;南颐,1994;黄圭成等,2001;覃小锋等,2006;舒良树,2012)。但近年来的锆石U-Pb定年数据显示,尽管在云开地块的一些地区可能存在新元古早期(1000~920Ma)的花岗岩、正片麻岩、镁铁质岩石,但是很多变质沉积岩的形成于新元古中期(800~709Ma)甚至更晚到早古生代(Wang et al., 2013a,2014; Zhang et al., 2012b; Yu et al., 2010; Wan et al., 2010)。本次研究显示云开地区的基底变质岩的原岩多为沉积岩,而且4个定年样品中最年轻的碎屑锆石协和年龄分别为:600±5Ma,524±5Ma、517±5Ma、489±7Ma。从而限定它们的沉积时代不会早于这些年龄。此外,在黑云母斜长片麻岩(YK13-3)和眼球状黑云母片麻岩(YK-19)中还获得较多465~439Ma的变质成因锆石,这与大面积侵入于这些变质沉积岩的早古生代(454~420Ma)花岗岩的年龄以及这些基底变质岩多被古生代-中生代沉积岩覆盖的事实相吻合(广东省地质矿产局,1988;钟增球等,1996;Wan et al., 2010; Wang et al., 2007c,2011)。综合前人可靠的年代学资料,我们认为云开地区基底变质岩主要形成于新元古代中晚期-早古生代早期,少量形成于新元古早期。晚新元古-早古生代的沉积岩在华南大面积分布,与南岭地区的增城、寻乌、粤东北龙川、兴宁、以及闽西地区等地的基底变质岩的原岩形成时代一致(陈忠权等,2001;于津海等, 2005,2006a,b,2007a;王丽娟等,2008;Yu et al., 2010),稍晚于扬子地块南缘的梵净山地区下江群、桂北-桂东北地区丹洲群以及湘北等地的新元古代板溪群沉积岩(王鹏鸣等,2012; Wang et al., 2007a,2010a,2012b)。说明在新元古中晚期-早古生代,云开、南岭大部分地区、以及扬子南缘存在着广阔的沉积盆地。证实了华夏地块主要是由中晚新元古代到早古生代的基底岩石组成,中元古代和古元古代基底变质岩仅见于海南岛西部和华夏东部的武夷地区(刘锐等,2009;Li et al., 2002,2010,2014; Liu et al., 2014;Yu et al., 2009,2012)。 5.2 沉积盆地构造背景
本文试图通过对云开地区新元古晚期-早古生代沉积岩的地球化学特征来分析其沉积时的构造背景。在Bhatia and Crook(1986)所建立的微量元素构造环境判别三角图解中(图 10b,c),云开地区新元古代-早古生代变质沉积岩多分布在大陆岛弧以及其与被动大陆边缘和活动大陆边缘三者的交汇地带。而在Roser and Korsch(1986)的构造环境判别图中(图 11),云开地区大多数的沉积岩投影于被动大陆边缘区域,少量位于活动大陆边缘区域。其中,第一组样品都落入被动大陆边缘区域,而第二组样品落入活动大陆边缘或分界线附近(图 11)。样品在两种不同的判别图解中的大洋岛弧区均未有分布。然而由于两个判别图的划分依据存在差异,因而根据不同的元素特征以及不同判别图解可能得出不同的结论。Bhatia and Crook(1986)认为不同的大地构造环境具有一定的物源特征,在板块构造过程中具有特定的沉积过程。其所建立的构造判别图将构造背景分为了四种:大洋岛弧、大陆岛弧、活动大陆边缘和被动大陆边缘。而Roser and Korsch(1986)按构造位置及矿物成熟度划分了三种沉积盆地构造环境,并未划分出大陆岛弧的区域。他们所定义的活动大陆边缘背景是指与俯冲相关的盆地和大陆碰撞盆地(包括海沟、弧前、弧间以及弧后盆地)以及与走滑断层有关的拉分盆地,沉积物中石英含量中等富集。但实际上这一定义就包含Bhatia and Crook(1986)所定义的大陆弧构造背景。另一方面,由于微量元素化学性质稳定且具有良好的继承性,因而更多的反映了原岩的地球化学特征。而主量元素能更多屏蔽原岩成分的影响,从而我们认为根据主量元素建立的图解能更好的对沉积盆地的板块构造环境作出区分。
![]() | 图 10 Th-Hf-Co、La-Th-Sc图解(a,b,据Jahn and Condie, 1995修改;平均成分据Condie,1993)和微量元素构造环境判别图(b,c,据Bhatia and Crook, 1986) Fig. 10 Diagram of Th-Hf-Co and La-Th-Sc(a,b,after Jahn and Condie, 1995; reference compositions from Condie,1993) and discrimination diagram of tectonic setting(b,c,after Bhatia and Crook, 1986) |
![]() | 图 11 K2O/Na2O-SiO2构造环境判别图(据Roser and Korsch, 1986) Fig. 11 K2O/Na2O-SiO2 discrimination diagram of tectonic setting(after Roser and Korsch, 1986) |
在大陆岛弧以及活动大陆边缘背景下的沉积物主要来自新发生的岩浆活动的产物,具有独特的化学成分。而被动大陆边缘构造上极其稳定缺少岩浆活动,沉积盆地多为大陆岩石圈通过裂解拉张、伸展变薄而成,其物源更加复杂,沉积物既可能是再循环的古老地壳物质,也可能包含未改造的早期大陆岛弧和活动大陆边缘的成分(柏道远等,2007;魏震洋等,2009)。已有的研究曾报道在云开地区可能存在Grenville期的洋盆或弧岩浆活动(Zhang et al., 2012b;Wang et al., 2013a,2014; 覃小锋等, 2005,2007)。此外,在扬子南缘发育新元古代花岗岩和基性火山岩(周金城,2003;Wang et al., 2004,2006; Li,1999; Li et al., 2003a,b; Zheng et al., 2008),而且其新元古沉积物中也发现丰富的新元古碎屑锆石(图 12),说明这些沉积物的源区曾发生强烈的花岗质岩浆活动。这些事件的发生均在云开地区基底沉积岩的沉积作用发生之前(>200Ma)。而华夏地块新元古晚期-早古生代(>500Ma)基本没有岩浆活动。因而,作者认为云开地区这些新元古晚期-早古生代变质沉积岩更可能沉积于被动大陆边缘的沉积环境,只是源区含有未经明显改造的古老岩浆物质。这与华夏地块粤北的南雄、粤东兴宁-梅县、闽西南的桃溪、桂东北以及湘东南等地新元古代中晚期沉积岩的沉积环境相同(柏道远等,2007;魏震洋等,2009;王鹏鸣等, 2012,2013;王丽娟,2010;Zhou et al., 2009)。而在扬子南缘新元古板溪群、下江群、四堡群、丹洲群沉积岩中常常夹有火山岩(高林志等, 2010,2011; 葛文春等,2001;王剑等,2003;Wang et al., 2004),且被835~800Ma花岗岩、镁铁质岩石侵入(周金城,2003;顾雪祥等,2003; Xu et al., 2007b; Wang et al., 2006,2007a,2013b;Li,1999)。这些沉积岩的地球化学特征也显示其形于活动大陆边缘环境。现有研究表明在新元古中晚期整个华南开始进入裂谷阶段,大约从700Ma开始转变为壳内尺度伸展环境,形成相对较为持续而稳定的板内沉积与构造背景(舒良树,2012;Wang et al., 2012a)。
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图 12 碎屑锆石年龄谱对比图 数据来源:南岭地区(于津海等,2007a; 王丽娟等,2008; Yu et al., 2008,2010; Xu et al., 2005,2007a);梵净山(Zhou et al., 2009;Wang et al., 2010a); 四堡地区(Wang et al., 2007a,2012b); 广西平南、平乐捕获锆石(Zheng et al., 2011).各图中选取的锆石均为谐和度大于80%的年龄 Fig. 12 Comparison of U-Pb age spectra of detrital zircons Data sources: Nanling(Yu et al., 2007a; Wang et al., 2008; Yu et al., 2008,2010; Xu et al., 2005,2007a); Fanjingshan(Zhou et al., 2009; Wang et al., 2010a); Sibao(Wang et al., 2007a,2012b); xenocrystic zircons in Pingnan and Pingle mafic rocks(Zheng et al., 2011). U-Pb age spectra of detrital zircons used in those figures with concordance >80% |
稀土元素(REE)、高场强元素(Zr、Hf、Ti、Th、U)以及一些过渡元素Co、Cr和Sc等化学性质稳定抗风化能力强,它们能较好的将原岩的地球化学信息继承到碎屑岩中(Taylor and McLennan, 1985; Nesbitt and Young, 1982; Winchester and Max, 1989; Cullers et al., 1987,1988; McLennan et al., 1990; Wronkiewicz et al., 1987),因而成为解释碎屑沉积岩物源区特征的最有效的工具之一。研究表明长英质源区通常含有较多的Th、Zr、Hf、La,而镁铁质-超镁铁质源区则相对含有较多的Co、Sc、Cr(Jahn and Condie, 1995)。在Th-Hf-Co、La-Th-Sc三角图中(图 10a,b),本研究的样品大多聚集在长英质到中性成分之间,暗示沉积岩的碎屑物质主要来自花岗质岩石、长英质火山岩或类似TTG的源岩(图 10a,b)。云开各样品中Cr/Zr比值总体较低为0.02~0.09,平均为0.05,而Th/Sc比值相对较高为0.66~4.29,平均为1.7(>1.5),亦说明源区成分以长英质为主,镁铁质组分含量较少,这与华夏地块南岭地区的新元古代沉积岩较为类似。而北部扬子地块江南造山带雪峰山和梵净山等地区新元古代沉积岩则相对含有更多的中基性物质(顾雪祥等,2003; Xu et al., 2007b; 魏震洋等,2009;Wang et al., 2010a,2012b)。在Roser and Korsch(1988)根据主量元素所建立的物源区指纹判别图解中(图 13),云开地区沉积物的源区与华夏地块新元古的沉积岩相似,主要为石英质沉积物源区以及长英质火成物源区,少量落入中-基性火成物源区。说明碎屑物质以古老的再循环物质为主,进一步证实了一个被动大陆边缘的沉积环境。沉积岩中锆石的形态多样、部分磨圆度较高,大的Hf同位素以及年龄变化,且εHf(t)以负值占多数等特点都与上述结果相符合。
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图 13 F1-F2主量元素源区特征判别图解(据Roser and Korsch, 1988) 华夏和扬子南缘沉积岩范围(顾雪祥等,2003; 魏震洋等2009; 王鹏鸣等,2012) Fig. 12 F1- F2 discrimination diagram of sediment provenance(after Roser and Korsch, 1988) Data of Cathaysia and Yangtze blocks from(Gu et al., 2003,Wang et al., 2009,Wang et al., 2012c) |
运用沉积岩中碎屑锆石的形态学特征、年龄谱以及Hf同位素特征来限定源区性质已成为现今较普遍且行之有效的一种方法(Yu et al., 2010; Yao et al., 2011; Xu et al., 2007a; Wang et al., 2010a; 王鹏鸣等,2013; Fedo et al., 2003; Greentree et al., 2006; Kuznetsov et al., 2014)。将本次研究的四个样品以及前人获得的碎屑锆石的年龄谱与华夏地块的南岭地区和扬子地块南缘梵净山、四堡等地区新元古代沉积岩的碎屑锆石年龄谱进行比对(图 12),可以发现云开地区新元古代-早古生代沉积岩的碎屑锆石年龄谱大致可以分成两组。第一组样品包括YK-10-1、YK-16-1、YK-27(Yu et al., 2010)、YK-75(Wang et al., 2007c)和G0103-1、G0104-1(Wan et al., 2010),第二组包括YK-19-1、YK-13-3。
第一组样品以富含Grenville期碎屑锆石为特征,同时含有相当数量的~2.5Ga和中元古代碎屑锆石(图 12)。这种年龄谱相似于华夏地块的南岭地区新元古代-早古生代沉积岩的年龄分布特征。说明第一组变质沉积岩样品的物源区很可能与南岭地区类似,来自华夏地块本身或与其相邻的其他板块。大量的Grenville期碎屑锆石表明云开地区与南岭地区一样,在新元古代晚期时曾靠近一个Grenville造山带,这一造山带很可能发育在华夏的南缘(王丽娟等,2008; Yu et al., 2008)。此外格林威尔期的岩浆岩在华夏亦有出露,在云开等地区也有过997~978Ma与岛弧相关的镁铁质火山岩的报道(舒良树等,2008; 舒良树,2012; Zhang et al., 2012b; Li et al., 2014; Wang et al., 2013a)。中元古-晚古元古(1800~1400Ma)的碎屑锆石在云开地区较为丰富,年龄峰在~1659Ma。这组碎屑物质在南岭同时代的沉积岩中亦非常丰富(Xu et al., 2005; Yu et al., 2008)。在整个华南并未发现这期岩浆岩出露,而在华夏东部的武夷地区发育古元古代(1855~1890Ma)花岗岩(刘锐等,2009;Yu et al., 2009; Li et al., 2010,2014; Liu et al., 2014;Zhao et al., 2014),时代早于本组年龄,说明这些碎屑物质不是来自武夷地块。已有的研究显示在Rodinia超大陆中华南处于印度与东南极地块之间,且华南的西南部更靠近印度(Yu et al., 2008)。华夏地块很可能在1.8~1.0Ga期间从北印度分离而向东印度移动(Yu et al., 2008,2012)。Wang et al.(2013b)对北印度以及东印度的年龄谱统计显示它们具有与南岭和云开第一组样品相似的峰值,因而印度地块很可能成为华夏地块这些中元古代(~1659Ma)碎屑物质最重要的物源区。
第二组样品以富含新元古750~900Ma碎屑锆石为特征(图 12),二个样品的主年龄峰分别为800Ma、866Ma,而Grenville期(960~1100Ma)的锆石很少。另外二个次峰分别为新太古代晚期(~2.5Ga)和古元古代(1.6~2.0Ga)(图 12)。这种碎屑锆石年龄谱特征与扬子地块南缘江南造山带新元古代沉积岩中的非常相似,表明它们具有相似的源区。新元古代岩浆活动在扬子地块南缘江南造山带广泛发育(Li et al., 2002,2003a,2003b; Li,1999; Wang et al., 2006; Zheng et al., 2008),晚新太古代基底在扬子地块深部可能普遍存在(Zheng et al., 2006; Wang et al., 2010a),而古元古代(2.0~1.8Ga)岩石在扬子地块北部崆岭地区存在,因此,这些沉积岩的碎屑物质最有可能来自扬子板块。中元古早期到古元古代晚期(1.75~1.5Ga)的岩石只在扬子西缘的一些地区有出露(孙志明等,2009;Greentree and Li, 2008; Zhao et al., 2010; Guan et al., 2011)。但是这个时代的碎屑物质在扬子地块沉积岩中普遍存在(Greentree and Li, 2008; Liu et al., 2008; Sun et al., 2009; Wang et al., 2010a,2012d)。Wang et al.(2010a)曾对扬子南缘梵净山群、四堡群沉积岩中碎屑锆石源区进行对比研究认为其中古老的碎屑锆石多来自于扬子内部被覆盖的地壳基底。
在锆石Hf同位素特征上,第一组样品(以YK-27为代表)以及第二组样品(以YK-13-3为代表)中碎屑锆石Hf同位素特征亦分别显示出与南岭地区以及扬子南缘地区的相似性(图 9)。在第一组样品中也有少量新元古代碎屑存在,但它们大多具有负的εHf(t)值,相同于南岭地区新元古代锆石,而不同于扬子地块新元古代锆石的特征(图 9)。第二组样品的中元古早期-古元古晚期碎屑锆石εHf(t)也以正值为主,相似于扬子南缘新元古代沉积岩(图 9)(Wang et al., 2010a,2012b),而不同于华夏武夷地块晚古元古代锆石的Hf同位素。
Zheng et al.(2011)曾对广西东部幔源岩石中的捕虏锆石开展研究工作,这些锆石能更好的揭示地壳深部的信息。在云开地块向北东延伸至广西平南地区的新生代玄武岩中获得的大量捕虏锆石显示出与云开第一组样品以及南岭地区新元古代沉积岩相似的年龄分布特征(图 12),并且它们的Hf同位素也与南岭地区较为相近(图 9)。而在云开北部桂东北平乐地区发现的中生代云煌岩中的大量捕虏锆石则显示出与云开地块第二组样品以及扬子南缘新元古代沉积岩中碎屑锆石相似的年龄谱以及Hf同位素(图 9、图 12)。表明广西中东部平南地区的深部存在与南岭地区相似的基底组成,而广西东北部平乐地区的深部则存在与扬子南缘相似的基底组成。
研究表明,与南岭地区具有亲缘性的第一组各样品中最年轻的碎屑锆石年龄范围在830Ma(样品G0103-1)到522Ma(样品YK-75)之间,沉积时代相对较老;而与扬子南缘具有亲缘性的样品的沉积时代总体相对较年轻,从517Ma(样品YK-19-1)到489Ma(样品YK-13-3),沉积时代可能更晚。不同时代沉积岩物源区变化的原因可能有二种。其一是因为云开地区位于扬子板块与华夏板块的分界线附近,在新元古代-寒武纪早期<830~522Ma云开地区的沉积盆地主要接受来自华夏南岭地块的碎屑物质,而从寒武中晚期517~489Ma以后盆地开始接受来自扬子南缘的碎屑物质。碎屑物质组成的变化大致发生在522Ma~<517Ma之间。这一变化时间与前人对湖南境内沉积岩的研究成果相吻合(王鹏鸣等, 2012,2013),说明很可能存在一期构造运动使得云开地体沉积盆地的物源区发生了变化。第二种可能是第二组变质沉积岩代表的上构造层原始形成于扬子南缘,后期在加里东期或印支期构造作用影响下被向南推覆滑移至云开地区的华夏地块基底之上。然而,前人对云开地区显生宙的各期构造变形事件进行了详细的研究,结果显示云开地块的构造变形以印支期为主,主要表现为早中生代NE向的逆冲推覆构造同期产生角闪岩相-绿片岩相的变质作用,而加里东期的变形主要发育上部向NW的韧性剪切,同期产生角闪岩相的变质作用以及地壳熔融(林伟等,2011;Wang et al., 2007b;Lin et al., 2008)。另一方面,第二组岩石样品受到了较强的加里东变质作用影响,形成了较多早古生代变质锆石,而扬子南缘新元古代沉积岩普遍只受到低绿片岩相变质影响,说明它们在早古生代变质作用之前已经存在于云开地区。然而云开地区并没有发育强烈的522~517Ma岩浆活动以及变形变质作用,且也没有发现明显的角度不整合。直到寒武纪末-奥陶纪初才在粤西地区广泛发育有角度不整合(郁南运动),并逐渐加强,形成了遍布华夏地块的早古生代(加里东期)强烈的变形变质以及花岗岩的侵入(以S型花岗岩为主)(彭少梅等,1995;Li et al., 2010;Wan et al., 2010;Wang et al., 2011a,2012a)。本次研究在云开地区所采集的变质沉积岩样品中也记录了这一时期构造热事件。这次主要发生在武夷-云开等地区的造山事件被认为可能是印度-澳大利亚板块与华夏板块沿着冈瓦纳北缘拼合作用而产生的远程响应(于津海等,2007b;Wang et al., 2010b,2011,2012a)。而云开地区沉积岩组成变化所暗示的构造运动早于这期构造事件,很可能是这次造山时间的初始阶段,只造成了升降作用,而没有强烈的变质变形和岩浆活动。 5.4 源区前寒武地壳演化
云开地区基底变质沉积岩中存在大量的前寒武纪地壳物质。通过对云开地区两组样品中锆石的U-Pb-Hf同位素分析,我们可以更清楚地了解云开地区或源区的前寒武纪地壳形成和演化历史(图 9)。
云开地区亲华夏的第一组样品(以YK-27为代表)的源区存在4期主要的岩浆事件:2700~2400Ma、1800~1400Ma、1150~900Ma、850~700Ma(图 9)。这4期的岩浆事件均涉及古老地壳物质的再循环以及新生地壳的形成。其中最古老的一期岩浆事件中具有负Hf(t)值的锆石占55%,它们的母岩浆大多起源于中太古-古太古(3.55~3.24Ga)的地壳物质。云开地区沉积岩中最古老的谐和锆石年龄为3.6~3.5Ga,南岭部分地区的沉积岩中也存在相似年龄的锆石(Xu et al., 2007a; Yu et al., 2008,2010),都证实古太古代物质的存在。一颗中元古代(1321Ma)谐和锆石具有4.1Ga的tDMC值,表明源区可能存在少量始太古代的地壳残余。在云开隆起西南延伸的雷州半岛的新生代火山岩中也发现有模式年龄为3.8Ga的新太古代锆石捕掳晶(赵焕和郑建平,2013)。而45%具有正εHf(t)值的锆石的模式年龄变化于3.1~2.8Ga,表明可能存在新太古代幔源物质的加入,即有新生地壳的生成。1800~1400Ma的锆石大多具有负的εHf(t)值,除了2颗锆石。说明这期岩浆主要起源于3.24~2.39Ga的古老地壳,只有少量幔源组分的加入。Grenville期的岩浆活动也主要涉及古老地壳的再循环,它们的母岩浆主要起源于2.6~2.1Ga的地壳组分。但是也有30%的锆石具有正的εHf(t)值,其中两颗锆石的εHf(t)=8.39~9.04,对应的模式年龄为1.30~1.26Ga,接近锆石U-Pb年龄,说明有一定数量新生地壳的形成。新元古代(0.82~0.70Ga)的岩浆事件与Grenville期的相似,6颗分析的锆石中有2颗具有很高的εHf(t)值(+13.8,+9.1),模式年龄与锆石U-Pb年龄相似,指示寄主岩浆起源于新元古代新生地壳。而其他4颗负εHf(t)值的锆石寄主岩浆很可能起源于2.5~1.9Ga古老的壳源物质,与大多数Grenville期岩浆的源区相似。从图 9a可以看到亲华夏地壳的最主要生长发生在新太古代和Grenville期,古元古代晚期-中元古代早期(~1.6Ga)和中太古代也有少量新生地壳生长。
相比之下,与扬子南缘具有亲缘性的第二组样品(以YK-13-3为代表)的分析结果表明源区存在与亲华夏的样品不同的4期主要的岩浆事件:2600~2350Ma、2000~1750Ma、1700~1500Ma、900~750Ma。其中新太古代晚期-古元古代早期(~2.5Ga)的锆石中εHf(t)为负值的锆石占75%,它们的tDMC为3.5~3.1Ga,指示其主要来自古太古代地壳物质再循环。2颗具有正εHf(t)值的锆石落于亏损地幔演化线附近,说明源区存在少量新太古代新生地壳。在古元古-中元古代所发生的2期岩浆事件中,早期2000~1750Ma的锆石大多具有负εHf(t)值,它们的母岩浆主要是新太古代(2.9~2.7Ga)地壳部分熔融形成,幔源物质的贡献较少。其中一颗1980Ma的锆石具有非常低的εHf(t)值(-13.98),其tDMC值为~3.5Ga,与那些晚新太古代的锆石相似(图 9),也证实古太古代物质的存在。而晚期1700~1500Ma的锆石中60%的锆石具有正εHf(t),则显示源区的新生地壳物质贡献较大。新元古代锆石的年龄集中在900~750Ma,而其εHf(t)值范围较广,既具有非常演化富集的(εHf(t)=-22.2),也具有强烈亏损的(εHf(t)=13.1),说明新生代岩浆活动既涉及古老地壳物质的再循环,也伴随有大量新生地壳的形成。大量幔源物质的加入很可能与新元古代Rodinia大陆的裂解有关。在扬子周缘的广大地区都存在这期基性岩浆活动(周金城等,2003;Zhao and Cawood, 2012; Wang et al., 2004,2006,2007a; Zhou et al., 2002,2006; Li et al., 2003b,2006; Dong et al., 2012),证实新元古代是扬子地块一期重要的新生地壳生长时期。 5.5 云开地块归属性
云开地块位于扬子板块与华夏板块的交界部位,地质构造演化复杂,厘清云开地块的归属性对揭示华南大地构造演化、钦杭带构造意义和研究区的成矿规律都具有重要意义。综合各种地质证据,作者认为云开地块属于华夏板块的一部分。(1)虽然晚期沉积岩具有亲扬子的特征,但是云开地块早期地壳组成皆具有亲华夏的特征,且第一组岩石比例远远超过第二组。本文数据以及前人对云开地区沉积岩的碎屑锆石研究表明,在830~522Ma很长的时间段内云开地区均接受来自华夏地块的碎屑物质。(2)广泛分布的早古生代岩浆作用以及丰富的稀有金属矿床是华夏地块的一个典型特征。加里东期和印支期构造热事件被认为华夏内部的两个重要造山事件,尽管印支期的构造岩浆事件可以影响到扬子南缘。这二期的岩浆活动主要发生于华夏,而相关的高级变质作用也仅限于华夏地块。造成这种差异的原因很可能是因为扬子地块与华夏地块基底组成的差异。扬子地块有古老的结晶基底,表现为是一个刚性地质体,而华夏地块主要是由新元古代以来的沉积岩组成(除了武夷地块的古元古代基底)(于津海等,2006b; Yu et al., 2010),因此其刚性程度明显较低。当受到外来构造作用时华夏地块基底物质将更容易发生变质变形作用和随之发生部分熔融。从华夏地块到扬子地块,二期岩浆作用和变质作用明显减弱(于津海等, 2005,2006b,2007b;孙涛,2006;Wan et al., 2010; Wang et al., 2007b,2011,2012a)。云开地区出露大面积加里东期花岗岩和混合岩以及强烈的印支期变质变形作用(Lin et al., 2008;Wang et al., 2007b),显示出与华夏地块其他地区相似的特征。(3)云开地区晚新元古代-早古生代沉积岩形成于被动大陆边缘,此沉积环境与南岭地区的南雄、粤东、桃溪等地的沉积环境相同(魏震洋等,2009),而扬子地块南缘在新元古代时期显示为活动大陆边缘环境(Wang et al., 2012b)。(4)震旦纪到奥陶纪,扬子南缘广大地区面型展布的稳定碳酸岩地台,而华夏地区的大部分地区为笔石相碎屑岩,以韵律状泥沙质岩层为特征。整个粤西地区(包括云开)震旦-寒武纪浅变质沉积岩显示出与华夏地块相似的沉积特点(舒良树,2012; 广东省地质矿产局,1988;Wang et al., 2010b;Wan et al., 2010;Li et al., 2010),也以泥砂质碎屑岩为主。(5)沿着在扬子地台南缘广泛发育的长达数千千米的早寒武纪富Mo-Ni多金属硫化物的黑色页岩系在华夏以及云开地区均未有发现(广东省地质矿产局,1988; Jiang et al., 2006,2007,2009)。(6)新元古代晚期2次雪球事件而形成的冰碛岩在扬子板块内广泛发育,其特征明显,层位与厚度稳定,单其南缘就广泛分布铁丝坳组(750~700Ma,Sturtian冰期)、南沱组(654~635Ma,Marinoan冰期)地层。而在华夏地块内至今未有确切新元古代晚期冰碛岩的报道(Zhou et al., 2004; Zhang et al., 2003,2008)。同样在云开地区的同时代地层中也未存在冰碛岩的记载(广东省地质矿产局,1988; 庞保成和王丽娟,2006)。所有这些都进一步证实了在新元古中-晚期云开地体在位置上属于华夏板块,与扬子地块相似的沉积物为后期剥蚀搬运而来。云开地块的构造归属的确定进一步限定了扬子板块与华夏板块的分界线至少在云开地块以西或以北。由于平乐与平南两地深部物质组成分别与扬子南缘及南岭地块具有亲缘性(图 12),因此,我们认为界限很可能就在在平乐与平南之间。此外在界限附近的平南、平乐、恭城、蒙山附近以及云开地区断续出露有代表幔源物质的岩石及包体,很可能说明深部存在隐伏的断裂(界线)(陈凌云和张忠伟,2003;覃小锋等,2007)。 6 结论
(1)云开地块基底变质岩主要由副变质岩组成。根据本文新的年代学数据以及前人的研究成果,这些变质沉积岩主要形成于新元古代至早古生代(<830~>465Ma),并受到了早古生代(加里东期)的变质作用。
(2)这些新元古代-早古生代沉积岩具有比上地壳平均成分(Rudnick and Gao, 2003)更高的SiO2和相对更低的Al2O3、CaO、Na2O,岩石成熟度中等。它们的微量元素与PAAS相似,但Sr、Cr、Ni等强烈亏损,高场强元素Nb、Ta轻度亏损,而大多数样品的Zr、Hf、Th、U等轻度富集,说明源区更富集长英质组分而贫镁铁组分。
(3)这些沉积岩形成于被动大陆边缘,该时期在南岭及扬子南缘大部分地区存在一个广泛的沉积盆地。
(4)在新元古代晚期-早古生代(830~522Ma)云开地区的沉积盆地主要接受来自华夏南岭地块的的碎屑物质,到了517Ma以后盆地开始接受来自扬子南缘的碎屑物质。即大致在<517~522Ma之间,云开地块沉积盆地的物源区发生了变化,暗示在此期间沉积盆地受到了构造运动影响,使得沉积物源区由华夏地块变成扬子地块。
(5)云开地块亲华夏的样品的源区主要有四次岩浆作用,均涉及到古老地壳再循环以及新生地壳的加入,但以再循环的物质为主。最主要的新生地壳生长发生在新太古代和Grenville期。而亲扬子沉积物的源区表现为不同是四次岩浆事件,新太古代晚期-古元古早期岩浆主要来自古老基底再循环。古元古晚期-中元古早期岩浆大多起源于新生地壳物质,而新元古代是最重要的新生地壳生长期,尽管这期岩浆活动也涉及大量古老地壳物质的再循环。
(6)云开属于华夏地块的一部分,扬子地块与华夏地块的界线在云开地块以北,很可能从平南与平乐之间通过。
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2015, Vol. 31













