2019, Vol. 35
华南板块是中国大地构造单元中的重要组成部分，其北以鲜水河-广济断裂为界紧临秦岭-大别造山带, 南以哀牢山-松马缝合带为界紧临印支板块, 东接太平洋板块, 西以龙门山断裂为界紧临松潘-甘孜地块(Wang et al., 2013b)。虽然目前普遍认为，华南板块由扬子和华夏两大块体在新元古代拼合而成(图 1a)，但其最终拼合时间与演化模式仍广受争议。近年来，不同学者对华南板块的形成与演化模式及其拼合时间提出了不同的观点：第一种观点为地幔柱模式，指出其最终拼合时间为880Ma(Li et al., 1999, 2002, 2003, 2008, 2009b；Li and Li, 2003);第二种观点为板片-俯冲模式，将最终拼合时间限定在825Ma(舒良树, 1993; Zhou et al., 2002, 2004;周金城等, 2008, 2014; Li et al., 2009a; Wang et al., 2014a; Zhao et al., 2018)；第三种观点为板片-裂谷模式，指出880~860Ma为其最终拼合时间(Zheng et al., 2007, 2008, 2013)；而持多岛弧-盆系统北西向幕式拼贴模式观点的学者认为江南地区新元古代最晚的弧后盆地关闭于~820Ma(Wang et al., 2013c, 2014b, 2018；Zhang et al., 2015, 2016;谭清立, 2017;谭清立等, 2017)；第五种观点是双向俯冲模式，认为在~825Ma时古华南洋盆的关闭标志着扬子与华夏的最终拼合(Zhao, 2015)；最近，Lin et al.(2018)提出了“阿巴拉契亚型”多块体威尔逊旋回模型，认为华南经历了从早新元古代元早期到中生代的4次拼贴事件。
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图 1 华南新元古代地层简图(a, 据Wang et al., 2013b改编)和江南造山带东段陈蔡地区地质概况及采样点位置(b, 据浙江省地质矿产局, 1996改编) 年龄数据来自：1)Yao et al. (2014)；2)Li et al. (2010)；3)Yao et al. (2016)；4)高林志等(2014a) Fig. 1 Simplified geological map of the Neoproterozoic strata in South China (a, modified Wang et al., 2013b) and sketch geological map of Chencai area in eastern Jiangnan Orogen with sample locations (b, modified after BGMRZ, 1996) |
江南造山带东段沿樟树墩-双溪坞一带的新元古代沉积-岩浆岩形成于955~880Ma，其形成环境被认为是活动大陆边缘(章邦桐等, 1990; Li and Li, 2003; Ye et al., 2007; Li et al., 2008, 2009b;陈志洪等, 2009a, b ; 高林志等, 2014a;陈辉等, 2016)。以往对江南造山带东段的研究主要聚焦于赣东北蛇绿岩、双溪坞群及沿樟树墩至浙东北双溪坞一带发育的岩浆岩，因此有学者认为扬子与华夏在东段的最终拼合时间为~880Ma(Li et al., 2002; Ye et al., 2007; Zhao et al., 2011;李献华等, 2012;周金城等, 2014;张玉芝等, 2015)。张玉芝等(2015)和Zhang and Wang (2016)则认为扬子与华夏在江南造山带各区段可能有着差异的拼合时间，其中中段的拼合时间在830~810Ma，明显晚于东段。近年来，陆续有学者在江山-绍兴断裂带、华夏板块东北缘与江南造山带之交的陈蔡群报道了879~838Ma的岩浆岩形成年龄，如Li et al. (2010)以SHRIMP锆石U-Pb法测得的变流纹岩原岩年龄为838±5Ma；高林志等(2014a)以同样的方法测得的含榴夕线黑云斜长片麻岩及含榴黑云斜长片麻岩原岩年龄分别为848±10Ma和845±9Ma；Yao et al.(2014, 2016)采用LA-ICP-MS锆石U-Pb测得的辉长岩和石英闪长岩的结晶年龄分别为879±10Ma和857±5Ma。目前陈蔡群原岩的形成环境也广受争议，有学者认为其形成于裂谷环境(Li et al., 2010; Shu et al., 2011)，也有学者提出其形成于岛弧环境(孔祥生等, 1994)或与俯冲有关的背景(陈绍海等, 1999; Yao et al., 2014, 2016)。陈蔡群879~838Ma的岩浆活动明显晚于樟树墩-双溪坞一带955~880Ma岛弧岩浆活动。
以往对陈蔡群原岩的研究多聚焦于超基性-基性岩，而对中-酸性岩关注不多。特别是有着独特地质意义的埃达克岩关注较少。埃达克岩最早由Defant and Drummond (1990)提出，指SiO2含量大于56%、Al2O3含量大于15%、富Na低K、高Sr、低Y、Yb，来自于年轻(≤25Ma)俯冲大洋板片直接熔融形成的新生代岛弧火山岩。随后的研究发现，增厚下地壳铁镁质岩石的熔融、拆沉下地壳的熔融及幔源岩浆在加厚地壳底部岩浆库的结晶分异和混染等方式也能形成埃达克岩，其形成时代也不局限于新生代(王强等, 2001;张旗, 2008)。埃达克岩因其形成的特殊环境而造成出露较少，构造动力学意义重大而越来越被关注(王焰等, 2000)。目前华南仅少数地方发现有新元古代埃达克岩，如赣东北蛇绿岩中的埃达克型花岗岩(锆石U-Pb年龄为968±23Ma; 李武显和李献华, 2004)和平水地区埃达克质高镁闪长岩(锆石U-Pb年龄为932±7Ma; 陈志洪等, 2009a)。本次研究对陈蔡群中的花岗闪长岩开展了同位素年代学、全岩地球化学和Sr-Nd同位素研究。研究发现陈蔡花岗闪长岩的地球化学特征与埃达克岩相似。高硅埃达克岩是俯冲大洋板片部分熔融形成的玄武质熔体被地幔楔橄榄岩同化和交代的产物(Martin et al., 2005)，是洋壳消减的直接证据。因此本文的研究成果将会为厘清陈蔡群原岩的形成环境提供重要参考，也会对限定扬子与华夏最终拼合时间和演化模式提供重要依据。
1 地质背景与样品描述江南造山带是扬子板块与华夏板块在新元古代的重要拼合带，沿线保留了一系列新元古代沉积和岩浆作用记录，是研究扬子与华夏两大块体在新元古代时期演化的地球动力学过程的理想场所。江山-绍兴断裂带是江南造山带东段与华夏板块分界带。浙西北陈蔡地区位于江山-绍兴断裂带东北缘，处于江南造山带与华夏板块的交接部位，区内出露的地层主要有陈蔡群、上侏罗统诸暨组和第四纪地层(图 1b)。其中，陈蔡群由孔祥生等(1994)划分和定义，指经受了中压角闪岩相区域动力-热液变质作用和多期褶皱变形的基-中性火山沉积岩系，主要由一套中深度变质的片麻岩、片岩、大理岩组成，局部地区形成各种混合岩及混合花岗岩，总厚度约为8494m，主要分布于浙江诸暨陈蔡、上虞章镇、龙泉、龙游溪口及岱山大衢岛等地，从上至下划分为徐岸组、下吴宅组、下河图组及捣臼湾组。捣臼湾组下部为斜长角闪岩夹二长浅粒岩；上部以二长变粒岩为主。下河图组由大理岩、石墨石英片岩等组成，局部见浅粒岩、二长变粒岩、斜长角闪岩和片麻岩等。下吴宅组岩性为灰、灰黑色辉石斜长角闪岩、角闪变粒岩、斜长角闪岩和黑云母二长变粒岩、斜长变粒岩等，原岩下部可能为中基性、中酸性火山岩和火山碎屑岩，上部可能为砂泥岩。徐岸组岩性以深灰、灰黑色夕线铁铝榴石变粒岩为主，局部为铁铝榴石黑云斜长片麻岩。上侏罗统诸暨组地层与陈蔡群呈断层接触和角度不整合接触，其岩性主要为流纹质凝灰岩、流纹岩斑岩夹沉积岩。新元古界里浦石英闪长岩体是区域内出露的最大岩体，与陈蔡群呈断层接触关系，前人在此获得了854±6Ma(LA-ICP-MS锆石U-Pb; Yao et al., 2016)和841±6Ma(SHRIMP锆石U-Pb; Li et al., 2010)的闪长岩结晶年龄，Yao et al. (2016)认为该岩体形成于汇聚板块边缘，并且是江南造山带的一部分，不属于华夏板块。新元古代花岗岩体和燕山期基性岩体在区域内零星出露，如图 1b所示。
本文研究的样品均采自浙江省诸暨市陈家宅村和石头坑村(13WY-139和13WY-141，其GPS坐标分别为29°41′16.1″N、120°25′59.8″E和29°41′51.1″N、120°28′08.3″E)。样品呈灰黑色，具块状、片麻状构造。显微镜下观察样品具碎裂组构(图 2a, b)和糜棱组构(图 2c, d)。在具碎裂组构的样品中(图 2a、b)，矿物主要有半自形粒状-板状斜长石(见聚片双晶，65%~70%)、钾长石(5%~15%)，两者呈定向排列并普遍发生绢云母化、泥化和因动力变质作用形成裂纹，粒径为0.1~2mm；他形粒状石英(10%~15%)，具波状消光，粒径0.1~0.5mm；鳞片状自形黑云母(5%~10%)，发育绿泥石化，晶形0.05mm×0.1mm左右；副矿物主要有锆石、磷灰石和磁铁矿等。在具糜棱组构的样品中(图 2c, d)，碎斑占55%~65%，主要矿物有半自形粒状斜长石(发育聚片双晶，55%~65%)、歪长石(5%~15%)，呈眼球状和透镜状，粒径为0.5~2mm；他形粒状、丝带状石英(20%~25%)，具波状消光，粒径0.1~0.3mm。碎基占45%~55%，主要由细小的黑云母、斜长石、绢云母和绿泥石。其中斜长石和歪长石发生绢云母化(图 2b, c)和绿泥石化蚀变(图 2d)。石英在变形强烈的部分被定向拉长(图 2c)。副矿物主要有锆石、磷灰石和磁铁矿等。
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图 2 江南造山带东段陈蔡地区花岗闪长岩显微镜岩相学特征照片 Pl-斜长石; Kfs-钾长石; Anr-歪长石; Chl-绿泥石; Q-石英; Bt-黑云母; Srt-绢云母 Fig. 2 Photomicrographs of the granodiorite samples from the Chencai area in eastern Jiangnan Orogen Pl-plagioclase; Kfs-K-feldspar; Anr-anorthoclasite; Chl-chlorite; Q-quartz; Bt-biotite; Srt-sericite |
锆石的分选与制靶在河北省廊坊市河北省欣航测绘院进行。采用常规的重力和磁选方法进行锆石分选，然后在双目镜下挑选出晶型完好、相对大颗粒的锆石粘贴在环氧树脂上，对其进行抛光至露出一半，以便于锆石阴极发光(CL)分析。CL分析在中山大学广东省地质过程与矿产资源探查重点实验室的Carl Zeiss CL阴极荧光装置上进行。锆石U-Pb同位素年龄测定在中国科学院广州地球化学研究所同位素地球化学国家重点实验室型号为Resonetics RESOlution M-50-HR+Neptune Plus的LA-ICP-MS完成，激光剥蚀束斑直径24μm，频率为4HZ。采用91500和GJ作为标样。详细步骤参考文献Xia et al. (2011)。年龄数据的处理和谐和图的生成采用软件Isoplot (version 2.49) (Ludwig, 2001)和ICPMSDataCal 8.4 (Liu et al., 2008)进行。
全岩主量元素分析在中国科学院地球化学研究所矿床地球化学国家重点实验室测定，仪器采用帕纳科公司制造的Axios(PW4400)型X射线荧光光谱仪(XRF)，分析精度在1%~3%之间，详细分析步骤见文献李献华等(2002)。全岩微量元素的分析测试在中国科学院地球化学研究所矿床地球化学国家重点实验室的电感耦合等离子体体质谱仪(Bruker M90 ICP-MS)上完成，大部分元素测试的相对误差优于10%，实验步骤详见Qi et al. (2000)。
Sr-Nd同位素分析在中国科学院广州地球化学研究所同位素地球化学国家重点实验室型号为Neptune Plus的MC-ICP-MS进行，实验流程见梁细荣等(2003)和Li et al. (2004)。87Sr/86Sr和143Nd/144Nd比值的校正值分别为0.1194和0.7219。采用的Sr同位素标样为NBS987溶液，87Sr/86Sr比值为0.710246±7(2σ)；Nd同位素的标样为JNDi-1溶液，143Nd/144Nd比值为0.512084±5(2σ)。
3 分析结果 3.1 LA-ICP-MS锆石年龄本文对花岗闪长岩样品13WY-139B-5(29°41′16.1″N、120°25′59.8″E)和13WY-141A(29°41′51.1″N、120°28′08.3″E)进行了LA-ICP-MS锆石U-Pb定年。
从样品13WY-139B-5的锆石CL图像(图 3)可以看出，其具有典型的振荡韵律环带结构，因此为岩浆成因锆石。锆石呈短柱状到长柱状，长宽比约为21~31，颗粒大小约为50~150μm。选取了25个点进行测年，选择的点多位于振荡韵律环带上(图 3)。所测得的锆石Th含量变化于154×10-6~364×10-6，U含量介于169×10-6~280×10-6之间，Th/U比值变化范围为0.63~1.19(表 1)。所测得的22个点中，除点4和点22偏离主群外，其余点均位于谐和曲线上或附近且比较集中，形成一个集群，其谐和度为93%~99%，23个点的206Pb/238U表观年龄为830~846Ma，206Pb/238U加权平均年龄为838±5Ma (MSWD=0.2)，该年龄代表了该花岗闪长岩样品的形成年龄，指示其形成于新元古代。
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图 3 江南造山带东段陈蔡花岗闪长岩锆石U-Pb年龄谐和图和阴极发光图 Fig. 3 Concordia diagrams of zircon U-Pb data and CL images of the granodiorites from the Chencai area in eastern Jiangnan Orogen |
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表 1 江南造山带东段陈蔡花岗闪长岩锆石LA-ICP-MS分析数据 Table 1 LA-ICP-MS zircon U-Pb analytical results of the granodiorites from the Chencai area in eastern Jiangnan Orogen |
从样品13WY-141A选取了22个点进行LA-ICP-MS锆石U-Pb测年(表 1)。锆石CL图像(图 3)显示具典型振荡韵律环带，锆石呈短柱状，长宽比11~31，粒径30~150μm，Th/U介于0.40~0.95，为典型的岩浆成因锆石。22个点位于谐和曲线上，且较为集中，其206Pb/238U表观年龄集中于824~854Ma，206Pb/238U加权平均年龄为836±4Ma (MSWD=0.5)(图 3、表 1)，为其形成年龄，亦表明其形成于新元古代。
3.2 地球化学特征选取了7个代表性样品进行全岩主微量元素分析，分析结果见表 2。实验测得的7个岩石样品的SiO2含量在63.38%~68.97%之间。Al2O3(15.30%~16.70%)、Na2O(3.74%~6.24%)含量较高，K2O(1.30%~2.57%)含量较低，K2O/Na2O比值的范围为0.17~0.64。全碱含量(K2O+Na2O)介于5.82%~7.32%，为中钾钙碱性系列。铝饱和指数(A/CNK)为1.01~1.54，属过铝质。样品的εNd(t)为高正值(见下文)，因此陈蔡花岗闪长岩为钙碱性过铝质花岗闪长岩。MgO含量较低，介于0.64%~1.99%(Mg#(Mg#=Mg2+/(Mg2++Fe2+))为35~50)，P2O5(0.10%~0.15%)含量较低。在哈克图解(图 4)中，TiO2、MgO、FeOT、P2O5含量随SiO2的增加而减少。由于样品受到蚀变影响，因此使用Zr/TiO2-SiO2图解(图 5a)进行分类投图。在该图解中，样品落入花岗闪长岩区域；在QAP图解(图 5b)中，样品均落入花岗闪长岩区域，与野外和镜下观察相一致。
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表 2 江南造山带东段陈蔡花岗闪长岩主量元素(wt%)和微量元素(×10-6)组成 Table 2 Major (wt%) and trace (×10-6) elemental analytical results of the granodiorites from the Chencai area in eastern Jiangnan Orogen |
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图 4 江南造山带东段陈蔡花岗闪长岩哈克图解 Fig. 4 Plots of SiO2 vs. TiO2(a), FeOT(b), MgO (c) and P2O5 (d) for the granodiorites from the Chencai area in eastern Jiangnan Orogen |
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图 5 江南东段陈蔡花岗闪长岩Zr/TiO2-SiO2分类图解(a, 据Middlemost, 1994)及QAP图解(b, 据Streckeisen, 1976) Fig. 5 Classification diagrams of Zr/TiO2×10-4 vs. SiO2 (a, after Middlemost, 1994) and QAP (b, after Streckeisen, 1976) for the granodiorites from the Chencai area in eastern Jiangnan Orogen |
样品的稀土(REE)总量低(ΣREE=52.40×10-6~126.4×10-6)。球粒陨石标准化REE配分图解呈右倾的形式，轻稀土(LREE)较重稀土(HREE)富集((La/Yb)N=8.44~19.1)，LREE及中稀土(MREE)陡倾，重稀土平坦((La/Sm)N=3.39~7.22，(Sm/Dy)N=2.26~2.67，(Gd/Yb)N=1.06~1.41)(图 6a)。无明显的铕异常，δEu为0.86~1.37。在原始地幔标准化微量元素蛛网图上(图 6b)，高场强元素(HFSE)Nb、Ta和Ti呈明显的负异常，Zr和Hf呈正异常；大离子亲石元素(LILE)Rb、Ba、K、Sr及LREE相对富集。
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图 6 江南造山带东段陈蔡花岗闪长岩球粒陨石标准化稀土元素配分曲线图(a, 标准化值据Taylor and McLennan, 1985)和原始地幔标准化微量元素蛛网图(b, 标准化值据Sun and McDonough, 1989) Fig. 6 The chondrite-normalized rare-earth elements (a, normalization values after Taylor and McLennan, 1985) and primitive mantle-normalized incompatible elemental patterns (b, normalization values after Sun and McDonough, 1989) for the granodiorites from the Chencai area in eastern Jiangnan Orogen |
对样品13WY-139B-8和13WY-141A进行了全岩Sr-Nd同位素测试。样品的(87Sr/86Sr)i比值较低，为0.7015~0.7022。(143Nd/144Nd)i比值较高，为0.511777~0.511881，εNd(t=838Ma)为+4.10~+6.13，Nd模式年龄(tDM)为0.99~1.16Ga(表 3)。
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表 3 江南造山带东段陈蔡地区花岗闪长岩同位素组成 Table 3 Sr-Nd isotopic analytical results for the granodiorites from Chencai area in eastern Jiangnan Orogen |
样品的烧失量(LOI)变化于1.67%~3.76%，结合岩石薄片显微镜照片，说明岩石遭受了中低程度的热液变质作用，因此必须评估岩石的地球化学元素是否发生迁移。在LOI与易发生迁移的K2O、Na2O、CaO等氧化物及Rb、Ba、Sr等微量元素的双变量图解中，K2O、Na2O、CaO及LILE(如Rb、Ba和Sr)与LOI表现出或正或负的相关关系(图 7)，说明这些氧化物和LILE在岩石遭受蚀变的过程中发生了迁移，因此它们不适用于岩石成因分析。而HFSE和REE等不易发生迁移的元素的含量则相对恒定(图略)，因此主要使用这些元素用于岩石成因分析。
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图 7 江南东段陈蔡花岗闪长岩LOI对K2O(a)、Na2O(b)、CaO(c)、Ba(d)、Sr(e)和Rb(f)图解 Fig. 7 Plots of LOI vs. K2O (a), Na2O (b), CaO (c), Ba (d), Sr (e) and Rb (f) for the granodiorites from the Chencai area in eastern Jiangnan Orogen |
典型埃达克岩的地球化学特征包括：SiO2≥56%，Al2O3≥15%，低K2O、高Na2O，Sr≥400×10-6，Y≤18×10-6，Yb≤1.8×10-6，Sr/Y≥20，Eu异常不明显(Defant and Drummond, 1990;王强等, 2001;张旗等, 2002; Castillo, 2012)。陈蔡花岗闪长岩的SiO2含量介于65.21%~68.97%之间；K2O/Na2O比值为0.17~0.67。除13WY-139B-8的Sr含量低于400×10-6外，其余均高于400×10-6，平均457×10-6；样品Y含量均低于18×10-6，平均11.12×10-6；Yb含量介于0.77×10-6~1.67×10-6，平均1.19×10-6；Sr/Y比值最小为26，最大为80，平均47；δEu=0.86~1.37。从Sr/Y与SiO2的关系可以看出，陈蔡花岗闪长岩的地球化学特征与埃达克岩类似。在Sr/Y-Y图解(图 8a)和(La/Yb)N-(Yb)N图解(图 8b)上，样品均落入埃达克岩区域。
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图 8 江南造山带东段陈蔡花岗闪长岩样品Sr/Y-Y(a)和(La/Yb)N-YbN(b)图解(底图据Drummond and Defant, 1990) Fig. 8 Plots of Sr/Y vs. Y (a) and (La/Yb)N vs. YbN (b) for the granodiorites from the Chencai area in eastern Jiangnan Orogen (base map after Drummond and Defant, 1990) |
目前提出的埃达克岩成因模式主要有：(1)俯冲板片熔体与地幔楔橄榄岩的相互作用(如Rapp et al., 1999; Carmichael, 2002; Martin et al., 2005; Macpherson et al., 2006; Tang et al., 2010)；(2)加厚玄武质下地壳的部分熔融(Atherton and Petford, 1993; Rudnick, 1995; Petford and Atherton, 1996; Chung et al., 2003; Condie, 2005; Wang et al., 2005, 2007b;王强等, 2006;王智慧等, 2016)；(3)拆沉下地壳的部分熔融(Kay and Kay, 1993; Rudnick, 1995; Xu et al., 2002; Gao et al., 2004; Wang et al., 2006, 2007a)；(4)俯冲陆壳的部分熔融(Wang et al., 2008)；(5)玄武质岩浆的低压分离结晶和同化混染(Castillo et al., 1999; Li et al., 2009a)。
由大陆下地壳部分熔融形成的埃达克岩具有富K低Na的特征，而陈蔡花岗闪长岩的Na2O含量明显高于K2O(王强等, 2001;张旗, 2008)；由古老下地壳部分熔融形成的埃达克岩具有富集的Sr-Nd同位素特征(王强等, 2001;张旗, 2008)，而陈蔡花岗闪长岩的(87Sr/86Sr)i为0.7015~0.7022，εNd(t)为+4.10~+6.13，表现为亏损的Sr-Nd同位素特征，因此其不可能来源于加厚或拆沉大陆下地壳和古老下地壳的部分熔融。新生的加厚玄武质地壳相变为榴辉岩并拆沉至软流圈发生部分熔融，当经过地幔橄榄岩时与橄榄岩反应，最终形成埃达克岩。这种成因的埃达克岩具有高Mg(Mg#>50)的特征，陈蔡花岗闪长岩的Mg#与之不符，因此也不可能来源于新生的玄武质地壳(Huang et al., 2007)。此外，由新生下地壳部分熔融形成的埃达克岩通常形成于碰撞造山环境或后碰撞环境(如Huang et al., 2007;刘建峰等, 2013)，然而陈蔡地区并未发现在新元古代时期由碰撞造山引起的强烈构造变形，其广泛的区域动力变质作用主要发生在早古生代(如Li et al., 2010;胡艳华等, 2011; Yao et al., 2014;高林志等, 2014b)。Wang et al. (2014a)推测扬子与华夏板块的最终拼合可能并非由碰撞造山引起，而是江南地区在860~830Ma期间存在的弧后盆地的关闭导致了统一的华南板块的形成。Wang et al.(2013c, 2014b)更认为华南内部都未在新元古代发生碰撞造山，而是武夷-云开(1000~920Ma)弧后盆地、双溪坞弧后盆地(970~890Ma)及江南弧后盆(860~830Ma)的幕式关闭导致了扬子与华夏地块的逐步拼合。
也有学者指出，一些碰撞造山带的埃达克岩可能形成于俯冲陆壳的部分熔融，其最突出的地球化学特征包括K2O含量高于3%和εNd(t)值低于-3(Wang et al., 2008)。陈蔡花岗闪长岩的K2O含量变化于1.09%~2.57%，εNd(t)为高正值。区域构造演化背景也不支持该地区在该时期存在陆-陆俯冲，因此其来源也不可能是俯冲陆壳的部分熔融。
陈蔡花岗闪长岩也不可能来源于玄武质岩浆的分离结晶与同化混染，因为：(1)在La/Yb-La图解(图 9a)中，样品的变化趋势与玄武质岩浆的分离结晶趋势不相符，而更符合部分熔融的变化趋势；(2)在Zr/Sm-SiO2图解(图 9b)中，除样品13WY-139B-8的Zr/Sm比值异常高外，其余样品的Zr/Sm比值与SiO2正相关，符合部分熔融趋势；(3)La/Sm比值与La含量的正相关关系(图 9c)说明源区的部分熔融对微量元素比值的影响比分离结晶更为重要(图 9c)；(4)在Nb/La-SiO2图解(图 9d)中，Nb/La比值及SiO2含量的变化趋势也与玄武质岩浆的分离结晶及同化混染过程存在差异。
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图 9 江南造山带东段陈蔡花岗闪长岩La/Yb-La(a)、Zr/Sm-SiO2(b)、La/Sm-La(c)和Nb/La-SiO2(d)图解 图中FC、AFC和部分熔融趋势据Wang et al. (2013c) Fig. 9 Plots of La/Yb vs. La(a), Zr/Sm vs. SiO2 (b), La/Sm vs. La(c) and Nb/La vs. SiO2(d) for the granodiorites from the Chencai area in eastern Jiangnan Orogen |
因此陈蔡花岗闪长岩最可能由俯冲玄武质板片熔体被地幔楔橄榄岩同化和交代而成，证据如下：
(1) 陈蔡花岗闪长岩具有高Na2O、低K2O的地球化学特征及低(87Sr/86Sr)i、高εNd(t)的同位素特征，与俯冲板片部分熔融形成的太平洋新生代岛弧埃达克岩的特征相似。
(2) Martin et al. (2005)将埃达克岩分为两类：高硅埃达克岩(HSA，SiO2>60%)；低硅埃达克岩(LSA，SiO2 < 60%)。高硅埃达克岩具有低MgO(0.5%~4%)、低CaO+Na2O (< 11%)和低Sr(< 1100×10-6)含量的特征，而低硅埃达克岩的MgO为4%~9%、CaO+Na2O>10%、Sr>1000×10-6)。Rapp et al. (1999)提出“e=有效的玄武质板片熔体/地幔橄榄岩”的概念，当e大于1时，玄武质板片熔体在经过地幔楔时交代地幔橄榄岩后形成高硅埃达克岩；当e小于1时，地幔楔橄榄岩发生部分熔融形成的熔体被消耗怠尽的板片熔体改造，形成低硅埃达克岩。岩石学实验表明，玄武质和榴辉质岩发生部分熔融形成的熔体的SiO2含量为56%~72%(图 10; Defant and Drummond, 1993)。陈蔡花岗闪长岩的SiO2和MgO含量分别为63.38%~68.97%和0.64%~1.99%，CaO+Na2O为6.47%~8.64%，与高硅埃达克岩的特征相似。在低硅埃达克岩和高硅埃达克岩图解(图 11)上，样品均投入到高硅埃达克岩区域。在MgO-SiO2图解(图 10)中，样品落入俯冲洋壳形成的埃达克岩的范围，MgO与地幔混染的演化趋势相近。在相同SiO2含量的情况下，俯冲洋壳形成的埃达克岩MgO含量高于实验玄武质熔体，这是俯冲大洋板片熔体在经过地幔楔时交代地幔橄榄岩的结果(Rapp et al., 1999; Martin et al., 2005)。岩石学实验研究表明，在玄武质熔体中加入10%的橄榄岩可使Mg#从44增加到55(Rapp et al., 1999)，陈蔡花岗闪长岩样样品的Mg#为35~50，表明其源区只有少量橄榄岩加入到俯冲板片熔体之中。
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图 10 埃达克岩与玄武岩的实验熔融的MgO-SiO2图(据王强等, 2001) 实验产生的玄武岩部分熔融数据来自Rapp et al. (1999)，俯冲洋壳形成的埃达克岩及岛弧包体中的玻璃包裹体数据来自Wang et al.(2005) Fig. 10 MgO vs. SiO2 diagram for adakite rocks and experimental basaltic partial melts (after Wang et al., 2001) |
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图 11 江南造山带东段陈蔡花岗闪长岩低硅埃达克岩和高硅埃达克岩图解(底图据Martin et al., 2005) Fig. 11 Low-SiO2 adakites and high-SiO2 adakites diagrams for the granodiorites from the Chencai area in eastern Jiangnan Orogen (base map after Martin et al., 2005) |
(3) 现有的诸多区域构造背景研究成果表明，陈蔡地区在新元古代可能确实存在洋壳俯冲消减：如Yao et al. (2014)对陈蔡群角闪片麻岩和变辉长岩的全岩地球化学研究表明其形成与俯冲作用有关；陈迪云和徐伟昌(1993)指出，陈蔡群变质岩原岩成岩环境类似于活动大陆边缘；孔祥生等(1994)认为其形成于岛弧环境，陈蔡岛弧地体与双溪坞陆弧地体的碰撞导致了陈蔡群岩石的变质变形；陈绍海等(1999)的研究认为陈蔡群斜长角闪岩形成于洋内俯冲或洋陆俯冲环境。
陈蔡花岗闪长岩亏损HREE(YbN=4.52~9.84)和低Y含量的特征暗示源区有石榴子石残留。高Sr、Eu异常不明显，说明源区无斜长石残留。MREE较HREE富集，指示源区无角闪石残留。Nb、Ta、Ti明显负异常，说明源区可能有钛铁矿物、金红石的残留。因此，源区残留相主要为石榴子石+金红石，指示发生部分熔融的压力>1.5GPa，深度为50~80km(Xiong, 2006;康磊等, 2016)。
4.2 构造启示陈蔡群开始于海底拉斑玄武岩的喷发，以碳酸盐岩和砂泥质岩沉积结束，指示大洋岛弧的形成环境(浙江省地质矿产局, 1996)。本文及前人在陈蔡群的锆石微区定年研究未见继承锆石，说明其原岩在形成的过程中少有陆壳的参与，因此也支持其形成于洋内环境(Li et al., 2010; Yao et al., 2014, 2016;高林志等, 2014a, b )。位于江山-绍兴断裂北西盘，与陈蔡群呈近似对称分布的双溪坞群及其同期的岛弧岩浆岩形成于活动大陆边缘，其岩浆活动时限为955~880Ma(如章邦桐等, 1990; Ye et al., 2007; Li et al., 2008, 2009b;陈志洪等, 2009a, b ; 高林志等, 2014a;陈辉等, 2016)。如前所述，孔祥生等(1994)认为双溪群与陈蔡群的形成过程中在时空上联系紧密，因此厘清两者联系是破解陈蔡花岗闪长岩成岩过程的关键密码。江南造山带东段不整合于双溪坞群之上的地层为河上镇群，由下到上划分为骆家门组、虹赤村组和上墅组。Wang et al. (2013a)在河上镇群骆家门组碎屑锆石获得的最大沉积年龄为863Ma；侵位于上墅组的石耳山花岗岩年龄为779±11Ma(Li et al., 2003)、花岗斑岩年龄为783±18Ma(薛怀民等, 2010)。取骆家门组最大沉积年龄为河上镇群上限年龄，石耳山花岗斑岩年龄为河上镇群下限年龄，则河上镇群形成于863~783Ma。因此双溪坞陆弧地体的形成时限为955~865Ma。目前在陈蔡群报道的最老的新元古代年龄为879±10Ma(Yao et al., 2014)，因此陈蔡地区的洋壳俯冲可能开始于879Ma，远晚于双溪坞群开始活动的时间。结合孔祥生等(1994)提出的演化模式，我们认为扬子板块与华夏板块之间的古洋壳向双溪坞陆弧地体之下俯冲的过程中，俯冲洋壳在50~80km深处发生部分熔融形成熔体，熔体在经过地幔楔时交代地幔橄榄岩，最终形成高硅埃达克质花岗闪长岩。陈蔡花岗闪长岩的原岩形成时间为838~836Ma，暗示此时俯冲消减仍在进行，扬子与华夏仍未最终碰撞。江南造山带西段和中段位于晋宁期不整合面之下的双桥山群及其相当地层和同期火山-侵入岩形成于880~825Ma，而晋宁期不整合面之上的板溪群及其相当地层和同期火山-侵入岩形成于820~780Ma(如周金城等, 2014)。因此扬子与华夏在江南西段和中段的最终拼合可能发生于825~820Ma。最近，笔者在侵入于陈蔡群捣臼湾组的花岗闪长片麻岩获得了816±5Ma(MSWD=0.9，n=16)和803±7Ma(MSWD=1.1，n=6)的LA-ICP-MS锆石U-Pb年龄，其原岩的形成可能与陈蔡地区的新元古代弧后盆地的关闭有关(作者未刊出资料)，因此扬子与华夏在江南东段的最终拼合时间可能为~820Ma，说明江南各区段的拼合时间较为一致。
5 结论(1) 江南东段陈蔡群中的花岗闪长岩测得的锆石LA-ICP-MS 206Pb/238U加权平均年龄为838±4Ma、836±4Ma，表明其形成于新元古代。
(2) 江南东段陈蔡群中的花岗闪长岩具有高硅埃达克岩的地球化学特征，系扬子板块与华夏板块之间的古洋壳向双溪坞陆弧地体俯冲的过程中，俯冲的大洋板片熔体在经过地幔楔时被地幔楔橄榄岩同化和交代的产物。暗示在836Ma时洋壳仍在消减，扬子与华夏仍未最终拼合。
(3) 扬子板块与华夏板块在江南造山带各区段可能有着一致的拼合时间，约在825~820Ma。
致谢 感谢两位匿名审稿专家对论文提出的建议性意见和建议。Zhang YH研究员对本文的野外工作给予了大力指导，刘汇川和甘成势博士对论文思路提出了中肯的意见，张乐老师等为本文的分析测试给予了热情的帮助，在此表示感谢。
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