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陈蔡早古生代俯冲增生杂岩对华夏与扬子地块拼合过程的指示意义
赵希林1, 姜杨1, 邢光福1, 于胜尧2, 彭银彪2, 黄文成1, 王存智1, 靳国栋1     
1. 中国地质调查局南京地质调查中心, 南京 210016;
2. 海底科学与探测技术教育部重点实验室/中国海洋大学海洋地球科学学院, 山东 青岛 266100
摘要: 华夏地块与扬子地块的拼合时限与方式长期存在争议。本文对出露于浙江诸暨一带的原"陈蔡岩群"进行了详细的露头尺度解剖。野外地质调查表明,原"陈蔡岩群"主要由不同性质的外来岩块与基质组成。其中:代表外来岩块的大理岩及斜长角闪岩的变质年龄分别为(424.7±2.9)和(420.6±1.8)Ma,成岩年龄分别为(479.2±9.5)~(424.7±2.9)Ma和(507.7±7.8)~(420.6±1.8)Ma,斜长角闪岩原岩为具OIB(洋岛玄武岩)特征的碱性玄武岩,大理岩的原岩为海相碳酸盐岩,二者共同构成了洋岛海山组合。代表原地岩块的变长石石英砂岩主要物源区为3 620~1 530 Ma形成于活动大陆边缘和大陆岛弧环境下的古老地壳物质;与之构造混杂接触的斜长角闪岩变质年龄为(438.0±2.5)Ma,其原岩分别为形成于消减带岛弧环境的岛弧拉斑玄武岩、形成于俯冲环境下的富Nb玄武岩和洋岛海山环境下的具OIB特征的碱性玄武岩类。代表基质的含榴黑云斜长片麻岩LA-MC-ICP-MS锆石U-Pb年龄测试结果表明,其变质年龄为(441.0±3.0)Ma,碎屑206Pb/238U年龄多数为840~780 Ma,反映其物源主要来自于新元古代,且最年轻的沉积年龄限定在598 Ma,说明片麻岩原岩可能为早古生代沉积地层。陈蔡地区该套岩石组合的发现表明,原"陈蔡岩群"的构造属性应为早古生代俯冲增生杂岩。结合测区及龙游地区新发现的加里东期麻粒岩和退变榴闪岩,提出扬子与华夏两大地块碰撞于445~420 Ma。
关键词: 俯冲增生杂岩     加里东期     华夏地块     扬子地块     洋岛海山     陈蔡岩群     地球化学    
Chencai Early Paleozoic Subduction-Accretionary and Their Restrictions on Collage Between Cathaysia and Yangtze Block
Zhao Xilin1, Jiang Yang1, Xing Guangfu1, Yu Shengyao2, Peng Yinbiao2, Huang Wencheng1, Wang Cunzhi1, Jin Guodong1     
1. Nanjing Center, China Geological Survey, Nanjing 210016, China;
2. Key Lab of Submarine Geosciences and Prospecting Techniques, Ministry of Education/College of Marine Geosciences, Ocean University of China, Qingdao 266100, Shandong, China
Supported by National Key Research and Development Plan (2016YFC0600205) and Project of China Geological Survey (121201008000150002, 12120113070800, 12120114005801)
Abstract: The timing and model of the collage between Cathaysia block and Yangtze block have been debated for a long time. In this study, a detailed profile anatomy from "Chencai Group" was carried out on outcrop scale in Zhejiang. The field geological survey showed that the "Chencai Group" mainly consists of exotic rocks and matrix. The metamorphic age of the marble and amphibolite from the exotic rocks are (424.7±2.9) and (420.6±1.8) Ma respectively, and their diagenesis ages fall between (479.2±9.5)-(424.7±2.9) Ma and (507.7±7.8)-(420.6±1.8) Ma respectively. The protoliths of amphibolites show the OIB characteristics similar to those of alkaline basalts, and the marble protoliths belong to marine carbonate rocks that together make up the combination of ocean island seamount. The provenance of autochthonous feldspar quartz sandstone was the ancient crust in a continental margin and continental arc environment aged between 3 620-1 530 Ma. The protoliths of amphibolites were island arc tholeiite formed in a subduction zone, and Nb-rich basalts formed in a subduction environment, and alkaline basalts with OIB features in an ocean island seamount environment. The LA-MC-ICP-MS zircon U-Pb data indicated that the metamorphic age of the matrix garnet-bearing biotite-plagioclase gneiss is (441.0±3.0) Ma, with detrital age of 840-780 Ma. These data demonstrated that the source of the gneisses was mainly formed in the Neoproterozoic, with the youngest depositional age of 598 Ma, an Early Paleozoic sedimentary strata. The recognition of the rock units in the Chencai region showed that the "Chencai Group" belonged to the early Paleozoic subduction accretionary complex. In combination with the Caledonian granulite and garnet amphibolite in the Longyou area, the collision between the Cathaysia and Yangtze block occurred during 445-420 Ma.
Key words: subduction-accretionary complex     Caledonian     Cathaysia block     Yangtze block     ocean island seamount     Chencai Group     geochemistry    

0 引言

华南板块由扬子地块和华夏地块沿江山—绍兴断裂带发生拼合[1-4],但关于华夏与扬子两大地块碰撞拼合的时限与方式长期存在争议,严重制约了对华南构造演化的认识。主流观点有两种:发生于0.82~1.00 Ga[1, 4-14];碰撞发生在820 Ma甚至更晚[15-18]。目前来看,只有少数学者认同华夏与扬子地块沿江山—绍兴断裂带在加里东期完成最终拼合的观点[19-25]

最近,董学发等[23]提出浙江龙泉一带构造混杂岩的形成与古大洋地壳的俯冲、消减有关;彭松柏等[24-25]认为扬子地块与华夏地块之间存在早古生代洋盆并发生俯冲-增生碰撞造山作用。为了理清华夏与扬子两大地块碰撞拼合的时限,深化对华南前泥盆纪大地构造格架及演化的认识,本文对出露于浙江省诸暨地区的原“陈蔡岩群”进行了露头尺度的解剖,并对该区斜长角闪岩、大理岩、(含榴)黑云斜长片麻岩、变砂岩、变基性岩等进行了LA-ICP-MS锆石U-Pb年代学测试,并结合地球化学研究,讨论了“陈蔡岩群”的构造属性及其对扬子与华夏两大地块碰撞拼贴的指示意义。

1 “陈蔡俯冲增生杂岩”地质概况

“陈蔡俯冲增生杂岩”原称“陈蔡岩群”,主要分布在浙江省诸暨境内,另外在新昌—嵊州一带也有零星出露。区域上由于白垩纪火山岩发育,加之后期断裂的严重破坏,陈蔡俯冲增生杂岩多以小的断块或构造窗的形式整体呈NE向断块状展布(图 1),前人将陈蔡俯冲增生杂岩自下而上划为捣臼湾组、下河图组、下吴宅组和徐岸组4个组[26],主要岩石类型有含榴黑云斜长片麻岩、斜长角闪岩、大理岩、片岩及变长石石英砂岩等。陈蔡俯冲增生杂岩经受了多期变形变质改造,岩石局部具有较强的混合岩化,说明其原始固有的地层特征已经发生了明显改变。

据文献[22]修编。 图 1 浙江省诸暨地区陈蔡俯冲增生杂岩分布图 Figure 1 Simplified geological map of the Chencai subduction-accretionary in Zhuji, Zhejiang Province
2 “陈蔡俯冲增生杂岩”中的洋岛海山组合 2.1 岩石学特征

该套岩石组合主要出露于诸暨市陈蔡镇下河图村一带,呈NE向条带状展布(图 2)。斜长角闪岩呈黑绿色,似层状与大理岩共生,偶见呈团块状夹于大理岩中,可见少量浅色长英质脉体,应为变质作用过程中出溶作用产物[23], 并见后期花岗质脉体侵入;大理岩多为白色,含少量透辉石等暗色矿物及结晶非常粗大(一般粒径>2 cm)的方解石脉体。可见后期的基性岩脉侵入该变质岩组合中。

图 2 下河图村斜长角闪岩和大理岩组合 Figure 2 A combination of amphibolite and marble in Xiahetu Village

斜长角闪岩黑绿色,细粒块状。镜下呈不等粒、粒状变晶结构,弱片麻状构造。主要矿物共生组合为斜长石(体积分数约30%)、角闪石(体积分数约70%)(图 3a),部分样品中可见少量的黑云母(体积分数不到1%,略有绿泥石化)。斜长石和角闪石多为半自形,粒径一般都小于0.5 mm,部分粒径可达1 mm;角闪石单偏光下呈黄绿色-绿色多色性,正中突起,部分样品中可见一定程度的定向排列,另可见少量不透明矿物,可能为变质作用过程中析出的铁质质点。

Hb.角闪石;Pl.斜长石;Di.透辉石;Qtz.石英;Cal.方解石。 图 3 下河图村斜长角闪岩(a)和大理岩(b)的显微结构特征 Figure 3 Microstructure of the amphibolite (a) and the marble (b) in Xiahetu Village

大理岩呈粒状变晶结构,主要矿物为方解石,矿物颗粒较大,一般都大于0.5 mm。大部分样品中还有细小的石英颗粒,并充填于方解石粒间;零星可见透辉石(图 3b)。

2.2 地质年代学特征

前人对该套岩石(特别是斜长角闪岩)做过较多的年代学研究工作,如:王存智等[22]利用ICP-MS锆石U-Pb法测得斜长角闪岩变质年龄为(420.6±1.8) Ma;Zhao等[27]利用ICP-MS锆石U-Pb法测得陈蔡斜长角闪岩变质年龄为(435±3)、(436±3)、(445±4) Ma。但斜长角闪岩和大理岩的成岩年龄尚未获得可靠的数据,本研究通过高精度的锆石SHRIMP U-Pb测试工作,获得了灰岩(大理岩)中碎屑锆石的最小年龄为(479.2±9.5) Ma,变质时代为(424.7±2.9) Ma,其成岩时代应介于二者之间;同时,获得斜长角闪岩中最小的岩浆锆石年龄为(507.7±7.8) Ma,其成岩年龄应为507.7~445.0 Ma。综合考虑,斜长角闪岩及大理岩原岩的成岩时代可能为479~445 Ma。

2.3 地球化学特征及其原岩

下河图村斜长角闪岩稀土元素和微量元素特征均都表现出类似于OIB(洋岛玄武岩)的不相容元素配分型式(图 4),反映源区特征的Nb/La平均值为1.18,Th/Ta平均值为1.41,与典型OIB的Nb/La、Th/Ta值相近[28]。在Ta/Yb-Nb/Yb [29]和Nb/ Yb-La/Yb [30]图解(图 5ab)中,下河图样品具有类似于OIB特征;在w(V)-w (Ti)/1 000和La/Nb-w(La)图解(图 5cd)中,具有与板内玄武岩和洋岛玄武岩相似的特征,表明斜长角闪岩的原岩可能形成于与洋岛海山有关的环境。

数据来自文献[22]。球粒陨石、原始地幔、OIB的值据文献[28]。 图 4 下河图村斜长角闪岩稀土元素球粒陨石标准化配分模式图(a)和微量元素原始地幔标准化蛛网图(b) Figure 4 Chondrite-normalized REE distribution pattern (a) and primitive-normalized trace element distribution pattern (b) of amphibolites in Xiahetu Village
a.地幔演化区范围据文献[29];b.地幔演化区范围据文献[30],不同类型大洋玄武岩标准值据文献[28];c.底图据文献[31];d.底图据文献[32]。N-MORB.正常洋中脊玄武岩;E-MORB.富集洋中脊玄武岩;OIB.洋岛玄武岩;MORB.洋中脊玄武岩;IAT.岛弧拉斑玄武岩;BAT.弧后盆地拉斑玄武岩;WPB.板内玄武岩;IAB.岛弧玄武岩。 图 5 下河图村斜长角闪岩的Ta/Yb-Nb/Yb图解(a),Nb/Yb-La/Yb图解(b),w(V)-w(Ti)/1 000图解(c)和La/Nb-w(La)图解(d) Figure 5 Ta/Yb-Nb/Yb (a), Nb/Yb -La/Yb (b), w(V)-w(Ti)/1 000(c)and La/Nb-w(La) (d) diagram of the amphibolites in Xiahetu Village

下河图村大理岩的δ13CPDB值为-1.861‰~2.066‰,δ18OSMOW基本大于15‰,暗示其原岩可能为海相碳酸盐,并有较明显的δ18O降低现象[33]。同时,下河图村大理岩不同于陆相沉积低w(Sr)(< 90×10-6)和Sr/Ba值(< 0.2),而相对高w(V)(>110×10-6)和w(Ga)(>18×10-6)的特征,指示了海相沉积环境[34],与文献[35]的结论一致。在δ13CPDB-δ18OSMOW碳氧同位素示踪图解(图 6)中,基本投在海相碳酸盐岩区域并沿受碳酸盐溶解作用影响趋势线分布,推测其为海相碳酸盐岩,其δ18O值较低,可能是受到了变质作用过程中去气作用的影响。

底图据文献[36]。 图 6 下河图村大理岩C-O同位素示踪图 Figure 6 C-O isotopic diagram of the marble in Xiahetu Village

因此,下河图村斜长角闪岩与大理岩的原岩与古洋岛海山组合的岩石组合相似,其原岩应形成于洋岛海山构造环境。

3 陈蔡俯冲增生杂岩中的原地岩块 3.1 岩石学特征

该套岩石主要出露于诸暨市璜山镇萃溪村及东和乡下步溪村,其主体为厚层变砂岩、透镜状/团块状斜长角闪岩(图 7a),另可见少量后期的花岗岩侵入(图 7b)。

a.变砂岩,夹杂数层呈透镜状、团块状产出的斜长角闪岩; b.后期侵入于斜长角闪岩中的花岗岩;c.高大村片理化的变砂岩类。 图 7 下步溪村斜长角闪岩与变砂岩野外照片 Figure 7 Outcrops of the amphibolites and the metamorphic sandstone in Xiabuxi Village

变砂岩主要呈灰白色,中-细粒结构,块状构造。根据不同部位长石含量不同,可细分为变长石石英砂岩、变石英砂岩,在经过一定程度的构造置换和构造变形、变质后,表现为“整体近似无序,而局部有序”。高大村变长石石英砂岩中的石英和长石均沿片理方向强烈的定向拉长,石英条带和长石条带常常“互层”产出,非常容易被误认为是残留的原生层理,其实则为片理(图 7c);石英的亚颗粒化比较明显,彼此之间多呈缝合线状接触,发育波状消光;长石多有泥化和绢云母化;二者常常呈一定程度的定向排列(图 8a)。

a.变长石石英砂岩;b.斜长角闪岩。 图 8 下步溪村变长石石英砂岩和斜长角闪岩镜下特征 Figure 8 Microphotographs of the metamorphic sandstone and the amphibolites in Xiabuxi Village

斜长角闪岩呈黑绿色,细粒块状。镜下呈粒状变晶结构,矿物粒径一般都为0.5 mm左右,略具定向排列(图 8b)。主要矿物共生组合为斜长石(体积分数为35%~40%)和角闪石(体积分数为50%~60%),另含少量绿帘石(体积分数为2%~5%)及石英(体积分数为 < 2%)、黑云母等;副矿物有榍石、磷灰石、锆石及金属矿物等,偶见铁质析出,并可见后期方解石细脉。后期有长英质脉体侵入,部分样品表现出明显的硅化和碳酸岩化。

3.2 地质年代学特征

关于该套岩石前人所做研究工作较少。本次研究对出露于萃溪村和下步溪村的变长石石英砂岩和斜长角闪岩进行了ICP-MS锆石U-Pb研究。

变砂岩  下步溪村变长石石英砂岩样品碎屑锆石具有连续的自新太古代到古元古代的碎屑锆石年龄,其207Pb/206U年龄为1 530~3 620 Ma,谐和线的上下交点分别为(1 705±120)和(3 314±150) Ma。而在碎屑锆石的年龄频率直方图上,显示存在2 021~2 105 Ma和2 450 Ma两个主峰以及2 289 Ma的次级峰。萃溪地区斜长角闪岩样品95个测点的207Pb/206U年龄为1 710~2 932 Ma,且存在1 863和2 464 Ma的主峰,2 308和2 666 Ma的次级峰。根据二者的碎屑锆石年龄谱推测,二者可能均为古-中元古代之交沉积的地层。

斜长角闪岩  下步溪村斜长角闪岩38个锆石分析点206Pb/238U年龄加权平均值为(424.6±2.0) Ma(MSWD=0.05,n=38);而萃溪斜长角闪岩29个分析点的206Pb/238U年龄加权平均值为(438.0±2.5) Ma(MSWD=2.3,n=29),代表了二者的变质年龄。该变质年龄与下河图村斜长角闪岩和大理岩的变质年龄基本一致,推测为同期变质作用的产物。

3.3 地球化学特征及其原岩

斜长角闪岩  下步溪村和萃溪村斜长角闪岩原岩为拉斑玄武质岩(图略)。其稀土元素特征与典型的OIB[28]相类似(图 9a),而Nb元素丰度变化较大((1.38~85.30)×10-6)(图 9b),部分样品原岩属于富Nb玄武岩(w(Nb) >7×10-6)[37-40];在微量元素球粒陨石标准化图中可见明显两种不同类型的样品:一种是明显的Nb、Ta正异常特征(图 9b),另一种是明显的Nb、Ta亏损特征(图 9b);Eu (δEu=0.80~1.18)和Ce (δCe=0.93~1.02)元素无很明显异常。

球粒陨石和原始地幔的数据据文献[37];OIB的数据据文献[28]。 图 9 下步溪村斜长角闪岩球粒陨石标准化稀土元素配分图(a)和原始地幔标准化微量元素蛛网图(b) Figure 9 Chondrite-normalized REE distribution pattern (a) and primitive-normalized trace element distribution pattern (b) of the amphibolites in Xiahetu Country

依据地球化学元素特征,结合稀土元素配分曲线及微量元素原始地幔标准化图解,可以将下步溪村斜长角闪岩分为3类。

第一类明显富集Nb元素(9.82×10-6~85.3×10-6),稀土元素配分曲线呈明显轻稀土富集,类似OIB的配分曲线;微量元素蛛网图上可见明显的Nb、Ta正异常,显示类似于OIB的特征[41],在Th/Yb-Ta/Yb中主要投影于OIB区域(图 10a),在w(TiO2) -w(Zr)图解中主要落入板内玄武岩区(图 10b);其La/Ce值为0.39~0.50,大于原始地幔岩的值(0.387) [42],显示其应来自富集的地幔源区[42]

a底图据文献[42];b底图据文献[43]。DM.亏损地幔。 图 10 下步溪村斜长角闪岩Th/Yb- Ta/Yb图解(a)和w(TiO2)-w(Zr)图解(b) Figure 10 Th /Yb- Ta/Yb(a)and w(TiO2)-w(Zr)(b)diagram of the amphibolites in Xiahetu Country

在La/Nb-w(La)构造环境判别图解(图 11a)中,该类斜长角闪岩全部落入洋岛玄武岩(OIB)区;在Zr/Y- w(Zr)图解和2Nb-Zr/4-Y图解(图 11b, 12a)中,全部落入板内碱性玄武岩和板内拉斑玄武岩区内;结合其微量元素分布特征,推测该类斜长角闪岩的原岩是洋岛玄武岩(OIB),原岩可能形成于与洋岛海山有关的环境。岩浆来自具有OIB特征的富集地幔源区,Nb的富集可能与熔融程度很低的熔体交代作用有关[44]

a底图据文献[45];b底图据文献[46]。 图 11 下步溪村斜长角闪岩La/Nb-w(La)(a)和Zr/Y-w(Zr)构造环境判别图解(b) Figure 11 La/Nb-w(La) (a) and Zr/Y-w(Zr) (b) diagram of the amphibolites in Xiabuxi Village
a. 2Nb-Zr/4-Y图解,底图据文献[45]; b. Ti/100-Zr-3Y图解,底图据文献[46];c. Ti/100-Zr-Sr/2图解,底图据文献[46];d. Hf/3-Th-Ta图解,底图据文献[47]。图a:AI.板内碱性玄武岩;AII.板内碱性玄武岩和板内拉斑玄武岩;B.大洋中脊、岛弧玄武岩及钙碱性玄武岩;C.板内玄武岩和火山弧玄武岩;D.N-MORB玄武岩。图b:A.岛弧拉斑玄武岩;B.大洋中脊、岛弧玄武岩及钙碱性玄武岩;C.钙碱性玄武岩;D.板内玄武岩。图c:A.岛弧拉斑玄武岩; B.钙碱性玄武岩; C.洋中脊玄武岩。图d:A.N-MORB玄武岩; B.火山弧玄武岩; C.板内碱性玄武岩; D.岛弧拉斑玄武岩。 图 12 下步溪村斜长角闪岩构造环境判别图 Figure 12 Trace elements tectonic discriminating diagram of the amphibolites in Xiabuxi Village

第二类同样富集Nb元素(4.79×10-6~12.40×10-6)和TiO2(0.76%~2.62%),微量元素蛛网图(图 9b)中高场强元素(HFSE)Nb、Ta明显亏损,Hf、Zr、Ti弱亏损,而大离子亲石元素Sr富集,类似于消减带岛弧玄武岩(IAB)的特征[48],是受来源于俯冲板片的富LILE和LREE流体交代过的地幔楔橄榄岩的部分熔融所形成的[49]。第一类与第二类斜长角闪岩均是富Nb的玄武岩,这与Nb-Nb/U图解[38](图略)判别的结果相符。

个别样品(201-5-1)的w(MgO)较高(8.05%),Mg#值为58.97,表明该类斜长角闪岩岩浆的Nb、Ta亏损不可能是因板内岩浆受地壳混染所致,可能与俯冲板片部分熔融之后与上覆地幔橄榄岩反应有关[50-53]。在La/Nb-w(La)构造环境判别图解(图 11a)中,全部落入岛弧玄武岩(IAB)区;在Ti/100-Zr-3Y图解(图 12b)中,大部分样品落入岛弧玄武岩、钙碱性玄武岩及板内玄武岩区内;在Ti/100-Zr-Sr/2图解(图 12c)中,大部分样品落入洋中脊玄武岩和钙碱性玄武岩区内;而在Hf/3-Th-Ta图解(图 12d)中,大部分样品落在岛弧拉斑玄武岩区内。推断第二类斜长角闪岩原岩为岛弧玄武岩。

目前对于富Nb玄武岩成因仍存在争议,主流观点有两种:地幔楔橄榄岩被派生于消减大洋岩石圈的熔体(被称作埃达克熔体)交代,导致形成富集HFSE的富Nb岛弧玄武岩[37-40];岛弧之下的地幔是一种“大理石花纹状”或“脉纹状”地幔,在这种地幔中,“似OIB”富集组分以各种大小嵌布于“似MORB”亏损基质之中。上述两种幔源组分间不同程度的混合,产生了富Nb岛弧玄武岩中Nb和其他HFSE富集程度的变化[54]。相比而言,第二种成因模式的岩石地球化学证据更为充分[44]。因此,该类富Nb斜长角闪岩的形成很可能与本区早古生代俯冲事件有关。

第三类斜长角闪岩高场强元素Nb、Ta明显亏损,Hf、Zr、Ti弱亏损,大离子亲石元素Sr富集,地球化学特征类似于第二类斜长角闪岩,但是w(Nb)较低((1.38~4.37)×10-6),指示其原岩为岛弧玄武岩,产生于与俯冲消减作用有关的岩浆活动。通常情况下,与消减作用有关的玄武质岩石亏损Ta、富集Th,Th/Ta值多在4以上,而产生于岛弧环境的玄武岩比值通常大于3 [55-56],下步溪村第二类和第三类斜长角闪岩的Th/Ta平均值为4.65;在w(TiO2) -w(Zr)图解(图 10b)上,该类玄武岩全部落入火山弧玄武岩区内,在Zr/Y-w(Zr)图解(图 11b)中,落入岛弧拉斑玄武岩和板内玄武岩区域;La/Nb-w(La)构造环境判别图解(图 11a)中,第二类和第三类斜长角闪岩大部分落在岛弧玄武岩区内;在Ti/100-Zr-3Y图解和Ti/100-Zr-Sr/2图解中大部分落入岛弧玄武岩和岛弧拉斑玄武岩区内(图 12bc);而在Hf/3-Th-Ta图解中样品大部分落入岛弧拉斑玄武岩区内(图 12d)。因此,该类斜长角闪岩的原岩可能为岛弧拉斑玄武岩,形成于消减带的岛弧环境。

变长石石英砂岩  下步溪村变长石石英砂岩未经历后期流体作用的影响,其地球化学特征能够用来判别构造属性[57]。根据Bhatia[57]提出的常量元素、微量元素构造背景判别图(图 13ab),研究区样品大多落入或者靠近活动大陆边缘区域;在SiO2、CaO、MgO、FeOT、K2O、MnO、A12O3、Na2O和TiO2相关的F1-F2[58]双变量物源构造环境图解(图 13c)中,所有变砂岩样品基本靠近活动大陆边缘或者大陆岛弧的环境,显示总体上该区的变砂岩主要来自活动大陆边缘背景下的产物。

底图据文献[57-58]。 图 13 下步溪村变长石石英砂岩构造环境判别图解 Figure 13 Tectonic discriminating diagrams of the metamorphic sandstone in Xiabuxi Village
4 “陈蔡俯冲增生杂岩”中的基质

陈蔡俯冲增生杂岩的基质可能为片麻岩或者片岩类(图 14a)。

a.片麻岩,深熔和塑性流动特征明显; b.片麻岩中黑云母和矽线石交织共生;c.石榴子石核部包裹长英质矿物和黑云母等而呈筛状结构,代表了早期进变质作用阶段的矿物共生组合;d.石榴子石在退变质作用过程中发生熔融分解,形成黑云母+斜长石的集合体。 图 14 诸暨地区陈蔡含石榴石黑云斜长片麻岩露头及镜下特征 Figure 14 Outcrops and microphotographs of the garnet-bearing biotite plagiogneiss in Chencai area Zhuji
4.1 岩石学特征

片麻岩的主要矿物共生组合为黑云母+石榴子石+石英+斜长石±矽线石±钾长石±白云母。黑云母和矽线石交织共生,沿片麻理弯曲延伸,黑云母中可见有矽线石的雏晶晶出(图 14b)。白云母的产出状态有两种:一种是与黑云母共生(可能是黑云母析出铁质质点而成);另一种与斜长石共生,从斜长石内部或边部产出(可能是长石蚀变的结果)。长英质矿物发生部分熔融而呈文象结构,对应于野外露头所见之片麻岩的深熔现象。石榴子石多含长英质矿物和黑云母包裹体而呈筛状结构,代表了早期进变质作用阶段的矿物共生组合(图 14c);部分石榴子石呈明显的核边结构,核部含较多包裹体,而边部较为干净,反映石榴子石经历了二期生长阶段;另外,部分样品中可见明显的石榴子石发生熔蚀或退变成黑云母+斜长石的组合,但保留了石榴子石的假象或有少量石榴子石残留(图 14d)。

4.2 地质年代学特征

前人对该地区片麻岩类进行了许多地质年代学测试工作,并取得了较多可供参考的数据。其中,片麻岩变质时代为加里东期已经形成共识,如Li等[3]和高林志等[59]利用锆石SHRIMP U-Pb法分别测得石榴片麻岩变质年龄为(447±7) Ma和431 Ma;胡艳华等[60]利用ICP-MS锆石U-Pb法测得片麻岩变质年龄为(435±4) Ma。但该套片麻岩的成岩年龄尚未获得可靠的数据,本研究利用ICP-MS锆石U-Pb法测得片麻岩中岩浆锆石年龄为(598±7) ~(1 469±16) Ma(多数为780~840 Ma),反映其物源主要来自于新元古代;其变质年龄为(441±3) Ma(MSWD=0.35,n=8),因此,该套岩石的成岩年龄应为598~441 Ma。

4.3 地球化学特征及其原岩

在Simonen[61]提出的(al+fm)-(c+alk) -Si判别图解(图 15a)中,所有的样品均落入砂质沉积岩和泥质沉积岩的分界线附近,暗示其原岩主要为砂泥质沉积岩。

底图据文献[61]。alfmcalkSi均为尼格里值。 图 15 陈蔡片麻岩(al+fm)-(c+alk)-Si判别图解(a)、La/Th- w(Hf)判别图解(b)和Th-Co-Zr/10判别图解(c) Figure 15 The(al+fm)-(c+alk)-Si(a), La/Th -w(Hf)(b)and Th-Co-Zr/10(c)diagrams of gneiss in Chencai area

在La/Th-w(Hf)判别图解(图 15b)中,样品基本落在长英质源区的右侧,沿“增加古老沉积物成分”趋势线分布,暗示其原岩主要仍是砂泥质沉积岩,且有大量古老沉积物的加入。全岩地球化学分析表明,样品具有岛弧岩浆岩的地球化学特征,在Th-Co-Zr/10判别图解(图 15c)中,多数落入大陆岛弧区域,暗示其可能是形成于活动大陆边缘或大陆岛弧环境的沉积岩,主要碎屑物质来源于周围的火山弧。

综上所述,片麻岩、片岩的原岩主体为形成于活动大陆边缘或岛弧环境的砂泥质沉积岩类,并有古老碎屑物质的加入;碎屑锆石年龄表明片麻岩原岩的主要物源供给者为新元古代的地质体;最年轻的碎屑锆石时代为598 Ma,暗示其成岩时代在早古生代,经历了加里东期(447~435 Ma)变质作用。

5 地质意义

自20世纪70年代以来,国内外地质学者对陈蔡岩群开展了一系列的研究工作,并取得了一系列丰硕的成果[33, 62-69]。本次研究在诸暨一带的原“陈蔡岩群”新识别出代表洋岛海山系统的大理岩、斜长角闪岩组合,获得其成岩年龄分别为479.2~424.7 Ma和507.7~420.6 Ma;划分了3种不同构造环境的斜长角闪岩类岩块,其原岩分别为形成于消减带岛弧环境的岛弧拉斑玄武岩、形成于俯冲环境下的富Nb玄武岩和洋岛海山环境下的具OIB特征的碱性玄武岩类;获得了代表基质的含榴黑云斜长片麻岩成岩时代为598~441 Ma。

虽然大多数学者认为华南洋在约820 Ma前关闭,加里东运动属于陆内造山[3, 6, 70-74],但越来越多的地质证据表明,扬子与华夏之间可能在加里东期才最终碰撞拼贴。

年代学研究表明,华夏地区基底的变质、变形作用主要发生加里东期,经历了强烈的再造和深熔作用[75-76],局部发生了角闪岩相-麻粒岩相变质作用[3, 76-78]和混合岩化[79];泥盆纪/奥陶纪角度不整合面之下前寒武基底-早古生代地层发生韧性剪切[9]和褶皱冲断变形[70]及广泛的岩浆活动[80],均呈现出碰撞造山带的特征;江山-绍兴断裂带内出露的龙游群和陈蔡群变质岩系呈现顺时针的PTt轨迹[3, 81],指示了碰撞造山作用过程;武夷山和南岭地区加里东期(约440 Ma)的麻粒岩相变质作用峰期温度为750~900 ℃、压力达到1.10~1.15 GPa[77-78],高于陆内造山作用下绿片岩相-角闪岩相的变质作用温压条件(400~500 ℃、0.3~0.8 GPa);江山-绍兴断裂带龙游地区退变榴闪岩变质锆石的年龄约为450 Ma,指示了扬子和华夏在加里东期(450~455 Ma)可能发生了碰撞造山事件[82-83];在华夏地块发现了志留纪辉长岩[73],表明加里东期存在可能的岛弧和幔源岩浆活动,明显区别于板内造山作用的岩浆活动性质;钦杭结合带西南段415 Ma海相火山岩,指示华夏-扬子在加里东期为俯冲增生造山带[84];直到约790 Ma扬子陆块东南缘仍受到强烈的洋壳俯冲,尚未与华夏陆块发生碰撞拼贴[18]

同时,Wong等[85]通过对江山-绍兴断裂带两侧中生代酸性岩的研究认为,这两个块体在新元古代时期可能并未完全拼合;王存智等[86]在赣东北高镁安山岩中获得LA-ICP-MS锆石U-Pb法年龄为(794.8±6.0) Ma,提出该套岩石形成于大洋岛弧(洋内弧)环境,据此提出约800 Ma扬子和华夏两大陆块尚未碰撞拼合;Zhao等[27]通过对陈蔡岩群不同类型岩石的系统研究,认为扬子和华夏在古生代仍然存在大洋,陈蔡岩群为大洋洋壳的残迹。研究区存在479~445 Ma的洋岛海山组合,结合基质(黑云斜长片麻岩)中最年轻的碎屑锆石年龄(598 Ma)以及王存智等[22]和Zhao等[27]的研究结果,我们认为扬子和华夏两个地块碰撞拼合时间应在洋岛海山组合和基质形成之后、变质作用发生之前,因此我们将这一时代限定为445~420 Ma,而陈蔡岩群则可能是碰撞过程中形成的俯冲增生杂岩,其内有洋岛海山组合、洋岛玄武岩、岛弧岩浆岩、远洋沉积物等不同岩性、不同时代、不同构造属性的地质体[23, 26],伴随两大陆块的碰撞拼合而发生区域角闪岩相变质作用。

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http://dx.doi.org/10.13278/j.cnki.jjuese.20170022
吉林大学主办、教育部主管的以地学为特色的综合性学术期刊
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文章信息

赵希林, 姜杨, 邢光福, 于胜尧, 彭银彪, 黄文成, 王存智, 靳国栋
Zhao Xilin, Jiang Yang, Xing Guangfu, Yu Shengyao, Peng Yinbiao, Huang Wencheng, Wang Cunzhi, Jin Guodong
陈蔡早古生代俯冲增生杂岩对华夏与扬子地块拼合过程的指示意义
Chencai Early Paleozoic Subduction-Accretionary and Their Restrictions on Collage Between Cathaysia and Yangtze Block
吉林大学学报(地球科学版), 2018, 48(4): 1135-1153
Journal of Jilin University(Earth Science Edition), 2018, 48(4): 1135-1153.
http://dx.doi.org/10.13278/j.cnki.jjuese.20170022

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收稿日期: 2017-08-25

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