岩石学报  2014, Vol. 30 Issue (10): 3021-3033   PDF    
扬子地块西南缘大红山群变质基性岩的地球化学研究及构造意义
杨红, 刘平华, 孟恩, 王舫, 肖玲玲, 刘超辉    
中国地质科学院地质研究所, 北京 100037
摘要:扬子地块西缘近年报道了大量古元古代晚期的地层和侵入岩体,这些地层和岩体的构造环境多数被认为与大陆裂谷相关,而位于扬子西缘的~1.7Ga大红山群一直缺少相关的地球化学证据。本研究对大红山群两种变质基性岩——石榴(斜长)角闪岩(A组)和绿帘斜长角闪岩(B组)的主微量元素进行了地球化学分析,并对其构造环境进行了判别。A、B两组变质基性岩基本具有相似的地球化学性质:主量元素SiO2含量集中于46%~50%,MgO含量较低(5%~6%),但A组FeOT(平均14.28%)远高于B组FeOT(平均3.26%),可能与A组后期发生铁矿化作用有关;两组角闪岩的REE配分模式较一致:A组(La/Yb)N=1.52~4.67和B组(La/Yb)N=1.34~4.50,显示LREE轻微富集,Eu异常、Ce异常均不明显:Eu/Eu*=0.82~1.24,Ce/Ce*=0.93~1.05,两组样品稀土元素特征均具有类似富集型洋中脊玄武岩(E-MORB)的特征;二者的微量元素配分模式也基本一致:相容元素Cr、Co、Ni含量变化较大,不相容元素相对富集,Nb、Ta、Ti负异常不明显,Zr、Hf轻微负异常,其微量元素配分模式区别于岛弧玄武岩和OIB,而与E-MORB较一致。大红山群变质基性岩的地球化学性质可与东川群的基性侵入岩进行类比。A、B两组变质基性岩的原岩为拉斑玄武岩,通过不活动元素构造判别图解推断其构造属性为与富集地幔有关的大陆裂谷环境。变质基性岩中未受陆壳混染样品(Nb/La≈1)的微量、稀土元素地球化学特征显示其岩浆源区可能为富集地幔,结合前人已发表的相关εNdt)推测大红山群变质基性岩的岩浆源区为不均匀的岩石圈地幔。同样,据扬子西缘相关火山岩的εNdt)、εHft)值推测,扬子西缘1.8~1.5Ga变质火成岩的岩浆源区可能为不均匀的岩石圈地幔。本研究为扬子西缘1.8~1.5Ga大陆裂谷环境的构造岩浆活动提供了新的地球化学证据,大红山群与其同时代火成岩及地层的存在,共同表明了扬子西缘曾在古-中元古代发生了一系列与大陆裂解有关的构造岩浆事件,这期事件是哥伦比亚大陆裂解在扬子西缘的响应。
关键词变质基性岩     地球化学     大陆裂谷     大红山群     扬子地块    
Geochemistry and its tectonic implications of metabasite in the Dahongshan Group in southwestern Yangtze block
YANG Hong, LIU PingHua, MENG En, WANG Fang, XIAO LingLing, LIU ChaoHui    
Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China
Abstract: A quantity of strata and intrusions aged in Late Paleoproterozoic were reported recently, tectonic settings of which were mostly interpreted to be continental rifting. There has been a long lack of geochemical evidence for the related tectonic environments in the Dahongshan Group in 1.7Ga. This study focused on geochemical characteristics of major elements, trace elements and REE of two groups of metabasite in the Dahongshan Group, including garnet bearing amphibolites and hornblendite (Group A) and epidote amphibolites (Group B). Group A and B have similar geochemical properties. They have similar major components with SiO2 ranging from 46% to 50% and low MgO content of 5%~6%. While FeOT content of Group A (14.28% in average) is much higher than that of Group B (3.26% in average), which may be attributed to the late iron-mineralization of Group A. They have similar flat chondrite-normalized REE patterns with slightly LREE enrichment [(La/Yb)N=1.52~4.67 of Group A and (La/Yb)N=1.34~4.50 of Group B], and no apparent Ce and Eu anomalies (Eu/Eu*=0.82~1.24, Ce/Ce*=0.93~1.05), which resemble the E-MORB pattern but are different from the OIB pattern. The two groups also have similar primitive-mantle normalized spidergrams with enrichment of incompatible elements, not obviously Nb, Ta and Ti anomalies and slightly negative Zr and Hf anomalies, which also resemble the E-MORB pattern but are different from the IAB or OIB pattern. Geochemical characteristics of the Dahongshan Group are similar to those of the mafic dykes intruded into the Dongchuan Group. Protolith of the two groups of metabasite were tholeiite. Through the discrimination diagram with immobile elements, a tectonic setting of continental rift related to enriched mantle was obtained. Geochemical characteristics of the samples without continental crust contamination (Nb/La≈1) and the published involved εNd(t) show that the magma source of metabasite in Dahongshan Group may be the uneven lithospheric mantle. In addition, the published involved εNd(t) and εHf(t) were used to infer that the magama source of the 1.8~1.5Ga metavolcanics in the western Yangtze block may be the uneven lithospheric mantle. This study presents new geochemical evidence for the continental-rift related tectonic magmatism in 1.8~1.5Ga in western Yangtze block. Metabasite of the Dahongshan Group together with other meta-volcanics in the same age in western Yangtze block certify a series of tectonic magmatism related to rifting events in Meso- to Paleoproterozoic, which is in the same period of global Columbia supercontinent cracking event.
Key words: Metabasite     Geochemistry     Continental rift     Dahongshan Group     Yangtze block    
1 引言

扬子地块西缘传统称“康滇地轴”,其地质构造复杂,岩石类型多样。康滇地轴分布的岩石类型主要包括大量较年轻的中新元古代岩浆杂岩、变质地层(耿元生等,2008)和部分古元古代变质地层。其中古元古代变质地层主要包括在拉拉、小关河-东川、大红山等地区出露的三套中-浅变质岩系,分别为河口群、通安组-汤丹-东川群和大红山群(尹福光等,2012)。关于这套古元古代变质地层,最早有学者根据其分布特征、火山岩性质及沉积建造等,认为其构造属性是与全球地壳演化相关的大陆拉张裂谷(华仁民,1990龚琳等,1996)。随着最近几年锆石原位U-Pb测年技术的发展,扬子西缘新近有大批1.8~1.5Ga的岩浆岩测年数据发表(Greentree and Li, 2008; 孙志明等,2009; Zhao et al., 2010; Zhao and Zhou, 2011; 关俊雷等,2011; 周家云等,2011; 朱华平等,2011; 耿元生等,2012; 王冬兵等, 20122013; 杨红等,2012;郭阳等,2012; 侯林等,2013; 叶现韬等,2013; 王子正等,2013),这些数据说明扬子西缘存在大量1.8~1.5Ga的古-中元古代岩浆岩,扬子西缘在1.8~1.5Ga期间发生过一定规模的构造岩浆事件。

关于扬子西缘这期岩浆活动事件的构造属性如何,部分学者通过元素地球化学、同位素地球化学数据对不同的群组进行了探讨,包括河口群(赵彻终等,1999周家云等,2011)、东川群(Zhao et al., 2010; 王生伟等,2013; Wang and Zhou, 2014)、原昆阳群迤纳厂组(杨耀民,2004杨耀民等,2005)、武定海孜环斑花岗岩(王子正等,2013)以及通安组侵入辉长岩(王冬兵等,2013),也基本认为其构造环境与大陆裂谷相关,而与这些地层或岩体同时代的大红山群却一直缺乏相关的地球化学证据。前人曾对大红山群的构造环境进行地化方面的探讨,却一直未有定论,如徐启东(1998)通过元素地球化学认为大红山群变质火山岩形成于洋中脊环境,张鸿翔等(2001)通过元素地球化学分析认为大红山群变质基性岩形成于弧后盆地进一步拉张形成的板内玄武岩。为进一步确定大红山群的构造环境,本文选取大红山群两种变质基性岩进行详细的地球化学研究,通过主量、微量、稀土元素的分析和相应的地球化学特征,来恢复其原岩类型、探讨其形成的构造环境,为揭示扬子西缘的构造格局和演化历史提供新的证据。文中出现的矿物缩写依据Whitney and Evans(2010)

2 地质背景 扬子地块西邻青藏高原,北部与华北地块以秦岭-大别-苏鲁造山带相隔,东南与华夏地块相邻(图 1a)。扬子地块西缘地理上位于中国四川西部和云南中北部。与扬子北缘相比,扬子西缘一直未发现太古宙地层,而是沿绿汁江断裂和小江断裂厘定出大量古元古代岩浆岩并确定了相关地层的形成年代,如河口群形成年龄介于1.8~1.6Ga(关俊雷等,2011周家云等,2011王冬兵等,2012),通安组形成年龄介于1.8~1.5Ga(孙志明等,2009耿元生等,2012王冬兵等,2013),东川群形成于1.75~1.6Ga(Zhao et al., 2010; 朱华平等,2011),大红山群年龄介于1.72~1.6Ga(Greentree and Li, 2008; Zhao et al., 2011; 杨红等,2012),武定迤纳厂组形成年龄介于1.75~1.7Ga(郭阳等,2012侯林等,2013叶现韬等,2013),武定海孜环斑花岗岩年龄~1.73Ga(王子正等,2013)。这些古元古代地层是目前扬子西缘发现的较老地层。

图 1 扬子地块西南缘大红山群露头分布地质简图及采样位置(据Greentree and Li, 2008)(a)-研究区在扬子地块的位置;(b)-大红山群露头分布地质略图Fig. 1 Simplified geological map showing distribution of Dahongshan Group and the sample location in southwestern Yangtze Block(after Greentree and Li, 2008)(a)-simplified tectonic map showing the study area in southwestern Yangtze block;(b)-simplified geological map of the study area

大红山群位于元谋-绿汁江断裂和哀牢山-红河剪切带之间,零星出露于云南新平县和元江县地表,出露面积120km2,以出产“大红山式”铁矿和铜矿著称。大红山群自下而上分为老厂河组、曼岗河组、大红山组、肥味河组和坡头组五个组(钱锦和和沈远仁,1984),老厂河组总厚377m,以具斜层理的钾长石英岩及石榴白云片岩为特征,为陆源碎屑岩夹少量碳酸盐的组合建造;曼岗河组总厚650m,以钠质火山沉积岩为特征,全组构成火山喷发沉积旋回;红山组总厚880m,以火山熔岩(细碧-角斑岩)为标志性特征,是大红山式铁矿产出的主要层位,构成火山喷发环境;肥味河组总厚375m,为浅到中等变质的碳酸盐建造,含火山碎屑及透镜体;坡头组厚度大于626m,为含炭质、砂泥质及碳酸盐的组合建造,为陆源物质沉积变质而成。因此,大红山群整体反映了一个火山喷发-沉积与海水正常沉积作用共同作用的“优地槽”环境。目前大红山群各群组已知的火山岩年龄为:老厂河组1711~1686Ma(杨红等,2012)、曼岗河组1681±13Ma(Zhao et al., 2011)、红山组1675±8Ma(Greentree and Li, 2008),以及侵入至底部三组的粒玄岩岩脉为1659±13Ma。大红山群整体经历了绿片岩相-低角闪岩相变质作用,与同时代的河口群变质程度相当。

① 钱锦和,沈远仁. 1984. 滇中大红山群的层序划分特征及对比. 云南省地矿局科研报告 3 样品采集与岩石学特征 3.1 样品采集

大红山群变质基性岩主要产于曼岗河组、红山组中,老厂河组也可见少量夹层或透镜体。本文样品采自于新平县老厂河剖面(图 1b),所属组段为曼岗河组和老厂河组。

变质基性岩总体经历了绿片岩相-低角闪岩相的低中级变质,其岩性分布也有一定规律性。一种呈层状或透镜状,与变质中酸性岩、变质沉积岩呈互层产出,外观上呈黑绿色、灰黑绿色(图 2),含石榴石、不含或少含绿帘石,岩性主要为石榴斜长角闪岩或石榴角闪石岩,此处归为A组。A组石榴石含量不均,外观上石榴石颗粒变化较大。

图 2 大红山群石榴(斜长)角闪岩野外照片(a)-变质基性岩(石榴斜长角闪岩)、变质沉积岩、变质中酸性岩互层;(b)-变质基性岩(石榴角闪岩)以透镜体形式产于变质中酸性岩中;(c)-石榴石较粗的角闪岩;(d)-石榴石较细的斜长角闪岩Fig. 2 Field photographs of garnet amphibolite or hornblendite of the Dahongshan Group(a)-the metabasite(garnet amphibolite),metasedimentary and meta-intermediate-acidic rocks interbedded with each other;(b)-the metabasite(garnet hornblendite)as lenses preserved within the meta-intermediate-acidic rocks;(c)-hornblendite with coarser grains of garnet;(d)-amphibolite with fine grains of garnet

另一种为含杏仁体的变质基性岩,外观上呈灰绿色,含绿帘石而不含石榴石,岩性主要为绿帘石斜长角闪岩,此处归为B组(图 3)。B组中杏仁体分布并不均匀,密集处如图 3b,杏仁外层为钠质长石、内核为绿帘石。有的绿帘斜长角闪岩发生蚀变,转变为绿帘黑云斜长片麻岩。

图 3 大红山群绿帘斜长角闪岩野外照片(a)-杏仁状绿帘斜长角闪岩夹绿帘石岩透镜体;(b)-绿帘斜长角闪岩中的“杏仁”,黄绿色的绿帘石内核,白色的钠长石边;(c、d)-绿帘斜长角闪岩野外产状Fig. 3 Field photographs of epidote amphibolite of the Dahongshan Group(a)-epidotite as lenses in epidote amphibolite with amygdaloidal structure;(b)-the ‘almonds’ in epidote amphibolite with core of yellowish green epidote and white rim of albite;(c,d)-field occurrence of epidote amphibolite

本文采取A组9件新鲜样品、B组6件新鲜样品进行岩石学和地球化学研究,其中A组样品YJ13-4曾获得变质锆石U-Pb年龄为849±12Ma,YJ13-6的锆石Pb-Pb年龄约为1688Ma(杨红等,2012)。

3.2 岩石学特征

呈层出现的A组变质基性岩主要为石榴(绿帘)斜长角闪岩(图 4a-c)和石榴角闪石岩(图 4d),斑状变晶结构,块状或片状、片麻状构造。石榴斜长角闪岩主要矿物组合为角闪石(>60%)+长石+石英(长英质<30%)+石榴石(5%~10%)+磁铁矿(<5%),部分样品中含有<10%的碳酸盐矿物(多为白云石),可能经历过后期碳酸盐质流体改造。角闪石在单偏光下Ng方向的多色性为蓝绿色,说明变质程度较低,达到低角闪岩相变质程度(常丽华等,2006)。石榴石斑晶的粒径变化较大,约在0.1~1.5cm之间。有的石榴石呈筛状变晶结构,颗粒较大,包裹自形、半自形的斜长石包体;有的颗粒则较均一,呈粒状变晶结构。石榴角闪石岩(图 4d)矿物组成较简单,主要矿物组合为角闪石(80%)+石榴石(10%)+石英(5%)+磁铁矿(5%),未发现斜长石。

图 4 大红山群石榴(斜长)角闪岩的代表性显微照片(单偏光)(a)-样品YJ13-4石榴斜长角闪岩,主要矿物组合为斜长石(Pl)+角闪石(Hbl)+石榴石(Grt)+石英(Qz)+碳酸盐矿物(Cb);(b)-样品XP5-1石榴斜长角闪岩,主要矿物组合为角闪石+斜长石+石英+石榴石;(c)-样品YJ13-6石榴绿帘斜长角闪岩,主要矿物组合为角闪石+斜长石+石榴石+绿帘石(Ep)+石英;(d)-样品XP5-3石榴角闪石岩,主要矿物组合角闪石+石榴石+石英Fig. 4 Photomicrographs of garnet amphibolite or hornblendite in the Dahongshan Group(PPL)(a)-sample YJ13-4 garnet amphibolite with main mineral assemblages of Pl+Hbl+Grt+Qz+Cb;(b)-sample XP5-1 garnet amphibolite with assemblages of Hbl+Pl+Qz+Grt;(c)-sample YJ13-6 garnet epidote amphibolite with assemblages of Hbl+Pl+Grt+Ep+Qz;(d)-sample XP5-3 garnet hornblendite with assemblages of Hbl+Grt+Qz

B组绿帘斜长角闪岩(图 5a-c)外观上呈灰色或灰绿色,斑状或细粒变晶结构,片状或片麻状构造。主要矿物组合为角闪石(50%~60%)+斜长石(30%)+绿帘石(<10%)+磁铁矿(<5%),此外,部分样品中约含2%的后期碳酸盐矿物白云石,或可见细细的碳酸盐脉,说明裂隙曾经历过后期碳酸盐质热液的充填。有的角闪石蚀变为绿泥石,有的则全部蚀变为黑云母+绿泥石。老厂河及曼岗河剖面中均可见含杏仁体的绿帘斜长角闪岩(图 3b),杏仁体的主要组成矿物为绿帘石、长石和碳酸盐矿物,常见到杏仁体中绿帘石作为内核、钠长石作为外边(图 3b图 5d)。杏仁体的大小不一,薄片范围内较大的可达2.5×2.0cm。杏仁体的存在说明其原岩为气孔状玄武岩,后期又被富含Fe、Mg、Ca、Na质的热液充填。

图 5 大红山群绿帘斜长角闪岩的代表性显微照片(单偏光)(a-c)-样品XP2-1、XP2-5、XP4-3绿帘斜长角闪岩,主要矿物组合为斜长石(Pl)+角闪石(Hbl)+绿帘石(Ep)±磁铁矿(Mag);(d)-“杏仁”的绿帘石核部+长石边部Fig. 5 Typical photomicrographs of epidote amphibolite in the Dahongshan Group(PPL)(a-c)-epidote amphibolites(Samples XP2-1,XP2-5 and XP4-3)with mineral assemblages of Pl+Hbl+Ep±Mag;(d)-amygdaloidal structure with epidote core and plagioclase rim
4 分析方法

本研究野外分三次采取15件较新鲜的变质基性岩样品,并据岩性分为A、B两组,其中B组用于化学分析的样品尽量避开杏仁体,蚀变强烈的样品也未参与地化分析。样品在河北省区域地质调查所矿物分选实验室进行无污染粉碎,磨制至200目左右形成全岩粉末样品。全岩主量与微量测试分析在国家地质实验测试中心完成测试。主量元素分析采用3080E型和PW4400型X射线荧光光谱仪(XRF)。其中FeO采用容量滴定法(国家标准GB/T 14506.14—2010监控),CO2采用电导法(国家标准GB 9835—1988监控),H2O+和烧失量(LOI)采用重量法(国家标准GB/T 14506.2—2010和JY/T 1253—1999标准监控)分析完成;微量元素和稀土元素分析采用等离子质谱仪ICP-MS(X-series)完成,采用DZ/T 0223—2001标准监控。

5 岩石地球化学

大红山群变质基性岩的全岩主量、微量及稀土元素分析结果见表 1

表 1 大红山群两组变质基性岩的岩石化学成分(主量元素:wt%;稀土和微量元素:×10-6)Table 1 Chemical compositions of two groups of metabasite in the Dahongshan Group(major elements: wt%; trace elements: ×10-6)
5.1 主量元素特征

研究区A、B两组样品的SiO2、Al2O3除个别样品外变化不大,其中SiO2含量多集中于46.03%~49.90%,符合基性岩45%~52%的含量范围,样品XP4-3稍偏中性、HS17-2偏超基性,也不排除受后期变质改造的影响;Al2O3=11.98%~16.14%,平均为13.99%。MnO和P2O5的含量均较低(<0.5%)。3件样品YJ13-4、YJ13-5、XP4-1的烧失量较高,介于3%~4%,说明后期变质过程中有较多挥发分的加入。

A、B两组样品的TiO2含量有轻微的差别:A组TiO2平均值为2.46%,B组TiO2平均值为1.82%;两组样品的MgO含量相对均偏低(A、B组平均值分别为6.04%、5.03%),FeOT和Mg#(Mg#=Mg/(Mg+Fe))值有明显差别:A组石榴石(斜长)角闪岩FeOT=11.73%~18.20%(平均14.28%)、Mg#=42~57(平均值47),而B组不含石榴石的绿帘斜长角闪岩FeOT=2.54%~3.76%(平均3.26%)、Mg#=72~78(平均值76),B组样品Mg#更高,达到原生玄武质岩石Mg#(约为70)的标准(Dupuy and Dostal, 1984)。两组样品均低钾高钠,CaO、Na2O+K2O的含量也有差别:A组CaO=4.95%~8.86%、Na2O+K2O=1.43%~5.49%,B组CaO=2.84%~6.24%,Na2O+K2O=6.13%~8.05%。

总体来看,大红山群A、B两组变质基性岩的主量元素存在差别,尤其FeOT,A组的FeOT非常高,可能与后期的铁矿化作用有关,因此A组的Mg#并不能说明其分离结晶的程度。而B组可能受铁矿化影响较小,Mg#较高。B组绿帘斜长角闪岩可与同时代东川群的因民-落雪矿区辉长岩(王生伟等,2013)的主量元素进行对比,A、B两组均有低钾富钠的特征,与同时代东川群辉绿岩体相似。

5.2 稀土元素特征 A、B两组变质基性岩的稀土配分模式类似,稀土元素总量(∑REE)稍有差别(图 6a),A组(YJ13-6除外)稀土总量稍高于B组,分别为98.14×10-6~176.2×10-6和64.00×10-6~123.0×10-6;轻、重稀土元素(LREE、HREE)分异不强烈,A组(La/Yb)N=1.52~4.67,B组(La/Yb)N=1.34~4.50,LREE均显示轻微富集,两组变质基性岩总体上具有相对平坦的球粒陨石标准化的稀土配分模式(图 6a)。两组样品轻、重稀土元素内部的分异也较弱,分别显示为:(La/Sm)N=0.89~2.41,(Gd/Yb)N=1.37~1.83(表 1图 6a)。变质基性岩的Eu异常不明显(Eu/Eu*=0.82~1.24),除样品YJ13-4、XP6-3显示较弱的Eu正异常外,多数样品具有较弱的Eu负异常,说明变质基性岩的原岩在形成过程中无明显的斜长石分离结晶作用(Woodhead,1988)。其Ce异常也不明显(Ce/Ce*=0.93~1.05),说明样品未受明显的氧化作用影响。

图 6 大红山群两组变质基性岩的球粒陨石标准化稀土配分图解(a)和原始地幔标准化微量元素配分图解(b)(标准化值据Sun and McDonough, 1989)Fig. 6 Chondrite-normalized REE diagram(a) and primitive-mantle-normalized trace element diagram(b)for two groups of metabasite in the Dahongshan Group(normalization values after Sun and McDonough, 1989)

大红山群变质基性岩轻稀土弱富集、稀土配分模式较平坦及不明显的Eu异常特征,与富集型洋中脊玄武岩(E-MORB)稀土配分模式相似(Sun and McDonough, 1989),而与轻重稀土分异较明显的洋岛玄武岩(OIB)有区别。大红山群变质基性岩与东川群辉绿岩体的稀土配分模式其实具有相似性,只是后者被解释为似OIB模式。

5.3 微量元素特征

表 1所示,高场强元素Nb=4.71×10-6~22.5×10-6、Zr=88.6×10-6~190×10-6、Hf=2.55×10-6~5.11×10-6、Y=23.7×10-6~46.0×10-6。相容元素Cr、Co、Ni含量变化较大,两组样品Cr、Ni与MgO无相关性,仅有Co与MgO具弱的正相关(图略,相关系数约为0.6)。鉴于变质程度高的基性岩中大离子亲石元素的易活动性,原始地幔标准化微量元素配分图中K、Rb、Sr、Ba、U元素未参与讨论(图 6b)。

图 6b中,A组样品(YJ13-6除外)的微量元素总量相对高于B组,但分配模式除个别样品外是相似的,左侧的不相容元素略显富集(图 6b)。多数样品Ti亏损不明显,但YJ13-4显示明显负异常,可能与铁钛氧化物的分离结晶有关(Fowler and Jensen, 1989),YJ13-6表现明显正异常,可能与后期流体过程中磁钛铁矿矿化作用有关。高场强元素Nb、Ta负异常不明显,Zr、Hf显示微弱负异常。大红山群变质基性岩的微量元素特征区别于岛弧环境基性岩,因岛弧基性岩中Nb、Ta、Zr、Hf具有明显的负异常(Woodhead,1988; McCulloch and Gamble, 1991),也区别于Nb、Ta有明显正异常、轻重稀土分异明显的OIB环境,反而与E-MORB的稀土配分模式相似(图 6b)。大红山群变质基性岩比东川群基性岩(Zhao et al., 2010; 王生伟等,2013)的微量元素配分模式相似。

6 讨论 6.1 原岩恢复与构造环境判别

运用地球化学数据对变质岩的原岩性质及构造成因进行探讨,较理想的条件为变质作用是一个等化学过程,但往往在变质过程中,元素的迁移总是发生的。经过前人的研究(Pearce,1975; Wood et al., 1979; Rudnick et al., 1985; Rolllison,1993; Kerrich et al., 1999; Polat and Hofmann, 2003),大离子亲石元素(LILE)和低场强元素(LFSE)在变质过程中易发生迁移,称为活动性元素;而高场强元素(HFSE)、重稀土元素(HREE)以及过渡族元素(Cr、Ni、V)等在变质过程中比较稳定、不易发生迁移,这些元素称为不活动性元素,通常以与Zr的相关性来判断其不活动程度(Barnes et al., 1985; Polat et al., 202)。

在对变质基性岩进行原岩恢复时,Nb、Y、Yb、Zr、Ti、V等不活动性元素组合是判别原岩构造环境的有效工具。大红山变质基性岩经历了低角闪岩相的中级变质作用,关于原岩火山岩的分类,运用Nb/Y-Zr/TiO2(Winchester and Floyd, 1977)判别图解进行分类,样品皆投于亚碱性玄武岩(拉斑玄武岩)区域(图 7)。

图 7 大红山群两组角闪岩的Nb/Y-Zr/TiO2图解(据Winchester and Floyd, 1977)Fig. 7 Nb/Y vs. Zr/TiO2 diagram(after Winchester and Floyd, 1977)for two groups of amphibolite in the Dahongshan Group

几种玄武质成分岩石的判别图解如图 8。在微量元素Y-Cr判别图解(图 8a)上,两种角闪岩共15点主要落在洋中脊(MORB)和板内玄武岩(WPB)区域,其中B组石榴(斜长)角闪岩主要落在MORB,A组石榴角闪岩落在MORB和WPB区域;在Ta/Yb-Th/Yb判别图解(图 8b)上,两种角闪岩A组3点和B组4点落于富集的大洋中脊玄武岩(E-MORB)区域,其它8点沿陆壳混染趋势进入火山弧(VAB)区域,玄武岩类型上主要为拉斑玄武岩,部分为过渡性玄武岩;Th-Hf-Ta三角形判别图解(图 8c)中,两种玄武岩共13点投点在E-MORB和板内拉斑玄武岩(B区);能有效区分板内玄武岩与其它类型玄武岩的Zr-Ti-Y三角形判别图解(Pearce and Cann, 1973; 图 8d)中,两组样品主要落点于WPB区。由以上四种判别图解可以看出,两种样品的微量元素判别投点主要落于E-MORB和板内拉斑玄武岩中,部分样品可能受陆壳混染作用进入到火山弧区域。

图 8 大红山群变质基性岩不活动元素构造环境判别图解(a)-玄武岩的Cr-Y判别图解(据Pearce,1982),其中VAB=火山弧玄武岩,WPB=板内玄武岩,MORB=大洋中脊玄武岩;(b)-玄武岩的Ta/Yb-Th/Yb判别图解(据Pearce,1983),其中E-MORB=富集型大洋中脊玄武岩,N-MORB=正常大洋中脊玄武岩,OIB=洋岛玄武岩,图中E-MORB位置据Sun and McDonough, 1989;(c)-玄武岩的Th-Hf-Ta判别图解(据Wood,1980),A=N-MORB,B=E-MORB+板内拉斑玄武岩,C=碱性板内玄武岩,D=IAB;(d)-区分板内玄武岩和其它类型的Zr-Ti-Y判别图解(据Pearce and Cann, 1973),A=岛弧拉斑玄武岩,B=MORB、岛弧拉斑玄武岩和钙碱性玄武岩,C=钙碱性玄武岩,D=WPBFig. 8 Immobile trace element tectonic discrimination diagrams for metabasite in the Dahongshan Group

关于变质基性岩的构造环境判别,还可运用ln(La/Th)、ln(Sm/Th)、ln(Yb-Th)和ln(Nb/Th)参数的组合相加来判断,尤其适用于区分板块边缘和板内构造环境,具有78%~97%的成功率(Agrawal et al., 2008)。计算得到的判别图解如图 9所示,多数样品投点于大洋中脊玄武岩(MORB),部分样品落点于大陆裂谷玄武岩CRB区域(图 9b,d)。综合图 8的构造判别结果,大红山群变质基性岩的构造环境既具有富集型大洋中脊玄武岩(E-MORB)的特征,又具大陆裂谷玄武岩的特征,推测其原岩构造环境为与富集地幔有关的大陆裂谷。

图 9 大红山群变质基性岩不活动元素(取对数)的构造判别图解(据Agrawal et al., 2008)MORB=大洋中脊玄武岩;IAB=岛弧玄武岩;OIB=洋岛玄武岩;CRB=大陆裂谷玄武岩;正方形代表A组石榴(斜长)角闪岩,圆点代表B组绿帘斜长角闪岩Fig. 9 Log-transformed immobile trace element tectonic discrimination diagrams for metabasite in the Dahongshan Group(after Agrawal et al., 2008)

大红山群红山组以细碧-角斑岩为标志性岩石类型,细碧角斑岩的成因虽未有定论但反映了海相喷发的环境,大红山群其它组中大理岩的出现说明其沉积构造环境并非为深海,而为半深海相。因此,与大红山群相关的大陆裂谷是一个拉伸较开的裂谷。扬子西缘已发表的位于绿汁江断裂与小江断裂之间、与大红山群同时代的火山岩构造环境多被解释为与大陆裂谷相关(赵彻终等,1999; 周家云等,2011; Zhao et al., 2010; 王生伟等,2013; 王冬兵等,2013),因此,大红山群与这些同时代的火山岩可能经历了同一大陆裂谷环境的构造拉伸事件。

6.2 源区性质及成因机制

原始地幔的Nb/La=1(Weaver,1991),大红山群的A组含石榴角闪岩Nb/La为0.67~1.34,B组绿帘角闪岩Nb/La为0.67~0.90,说明两组变质基性岩的岩浆源区不同程度地受到陆壳的混染。A、B两组中Nb/La值接近1(1±0.1)的角闪岩样品(A组XP5-1、XP5-2、XP6-2、XP6-3、XP5-3和B组XP2-2)在微量、稀土元素的配分模式上具有相似性,个别元素(Ce、Eu、Nb、Ta、Zr、Hf)的正负异常上表现相似的特征,在构造判别图解中也具有相似的分布区域,且一直显示为较富集的E-MORB的特征,由此推断A、B两种变质基性岩的岩浆源区应保持一致,其源区可能为富集的岩石圈地幔。A组石榴斜长角闪岩的原岩年龄接近1.7Ga(杨红等,2012),目前虽未获得B组变质基性岩的原岩年龄,但根据两者地球化学性质基本相似的特征,判断B组的原岩也可能形成于古元古代晚期。

Hu et al.(1991)曾对大红山群红山组和曼岗河组变质火山岩包括变钠质熔岩、角闪片麻岩(Sm-Nd等时线年龄1657±82Ma)的εNd(t)进行测定,范围为3.1±1.8,显示~1.7Ga大红山基性岩岩浆源区也可能具有亏损地幔的特征。综合考虑这两种情况,大红山群变质基性岩的岩浆源区可能为不均匀的岩石圈地幔,部分显示为亏损地幔,部分显示为富集地幔。

扬子西缘部分与大红山群同时代火山岩的εNd(t)、锆石εHf(t)值显示其岩浆来源于亏损地幔:如东川群1.69~1.74Ga凝灰岩和粒玄岩岩脉中锆石的εHf(t)=-0.8~10.1(Zhao et al., 2010),东川群古元古代约1.69Ga辉绿岩的εNd(t)=-0.2~3.8(王生伟等,2013),侵入到通安组内1.69Ga辉长岩的锆石εHf(t)=6.2~9.8(王冬兵等,2013)。另外,扬子西缘裂谷相关的岩浆岩源区也有显示富集地幔的例子:成矿年龄为1.5~1.6Ga的武定迤纳厂铜铁矿矿石及成矿期萤石的εNd(t)分别为-3.2、-4.6,说明其成矿作用物质来源于富集地幔(杨耀民等,2005)。由此可见,扬子西缘1.8~1.5Ga火山岩的岩浆源区并不均匀,有的为亏损地幔,有的为富集地幔。

6.3 大地构造意义

古元古代中晚期发生了哥伦比亚超大陆的汇聚和裂解事件(Condie,2002; Rogers and Santosh, 2002; Zhao et al., 2002)。扬子地块与华北地块是中国最大的两个前寒武纪地质构造单元。华北地块在2.1~1.6Ga有着与哥伦比亚超大陆聚合和裂解有关的大规模构造岩浆热事件的记录(Zhai and Liu, 2003; Guo et al., 2005; Peng et al., 2008; Zhao et al., 2009; Zhang et al., 2012)。近年来的研究也发现,扬子地块北缘也存在2.1~1.8Ga与哥伦比亚超大陆聚合和裂解 有关的构造热事件的年代学记录(Zhang et al., 2006; Sun et al., 2008; Wu et al., 2009; Xiong et al., 2009; Peng et al., 2009; Zhang et al., 2011)。扬子西缘近一两年来报道了大量具有1.8~1.5Ga年龄的火山岩和侵入岩,它们的构造属性引起了学者的广泛重视,大部分火成岩经地球化学分析多被推测为与大陆裂谷有关的构造环境(尹福光等,2012),但仍有部分古元古代火成岩仍然缺乏地球化学数据的有力支 持。

本研究通过对大红山群两组变质基性岩进行详细的元素地球化学分析,主微量和稀土元素特征以及构造判别表明,其原岩均形成于~1.7Ga与的大陆裂谷环境,为扬子西缘在1.8~1.6Ga时期的构造岩浆活动提供了新的地球化学证据。大红山群联合其同时代的火成岩及地层,共同证明了扬子西缘曾在古元古代晚期发生了一期构造岩浆事件,且这期构造岩浆事件与大陆裂解相关,可能是哥伦比亚大陆裂解事件在扬子西缘的响应。在扬子西缘和扬子北缘继续开展与哥伦比亚大陆裂解有关的研究,可共同为扬子地块的构造演化历史提供更多的证据。

7 结论

本文通过对大红山群变质基性岩的野外及岩相学观察,在先前年代学工作的基础上,对大红山群两组角闪岩——A组石榴(斜长)角闪岩和B组绿帘斜长角闪岩进行系统的地球化学研究和对比,得到的结论如下:

(1)大红山群石榴(斜长)角闪岩(A组)和绿帘斜长角闪岩(B组)除有细微差别外,具有相似的地球化学性质:两组变质基性岩的主量元素总体相差不大,MgO含量均不高(5%~6%),但A组FeOT值明显高于B组,可能受后期变质或铁矿化影响;A组稀土总量均高于B组,但配分模式保持一致:轻稀土元素轻微富集,Ce异常、Eu异常均不明显,均具有类似E-MORB的特征;A组微量元素总量也稍高于B组,但配分模式基本一致:不相容元素相对富集,Nb、Ta、Ti负异常不明显,Zr、Hf轻微负异常,与E-MORB微量元素配分模式一致。大红山群两组角闪岩的稀土、微量元素配分模式与东川群基性岩相似,显示E-MORB的特征。

(2)大红山群两组变质基性岩的原岩均为拉斑玄武岩,多种构造环境判别图解推测均形成于与富集地幔有关的大陆裂谷环境。两者的岩浆源区可能一致,其岩浆源区均经历过不同程度的陆壳混染。据前人发表的大红山群及扬子西缘同时代火山岩的εNd(t)、εHf(t)值,判断其岩浆源区可能是不均匀的岩石圈地幔。

(3)大红山群变质基性岩的地球化学分析表明其原岩形成于~1.7Ga的大陆裂谷环境,为扬子西缘1.8~1.5Ga时期的构造岩浆活动提供了新的证据。大红山群与其同时代的火成岩及地层,共同证明了扬子西缘曾在古-中元古代发生了一系列与大陆裂解有关的构造岩浆事件,是哥伦比亚大陆裂解事件在扬子西缘的响应。

致谢    本文部分野外工作得到耿元生研究员、杜利林研究员的指导;论文写作过程中与杨崇辉研究员、杜利林研究员和赵子然研究员进行了有益的讨论;审稿人对本文提出了中肯的意见和建议;在此一并表示感谢。

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