岩石学报  2016, Vol. 32 Issue (2): 505-521   PDF    
早古生代古亚洲洋俯冲记录:来自东天山卡拉塔格高镁安山岩的年代学、地球化学证据
李玮1, 陈隽璐2, 董云鹏1, 徐学义2, 李智佩2, 柳小明1, 何登峰1    
1. 大陆动力学国家重点实验室, 西北大学地质学系, 西安 710069;
2. 中国地质调查局西安地质调查中心, 西安 710054
摘要: 吐哈盆地南缘荒草坡群大柳沟组火山岩出露于大草滩断裂以北的彩霞山、土屋铜矿北和卡拉塔格地区。该组火山岩主要为基性、中酸性熔岩及火山碎屑岩组合。LA-ICP MS锆石U-Pb分析获得安山岩和英安岩的年龄分别为434.8±3.8Ma和438.4±4.9Ma,表明大柳沟组火山岩形成于早志留世。安山岩SiO2含量为54.35%~60.70%;Al2O3含量为11.8%~16.3%, MgO含量为4.94%~8.27%,TiO2含量为0.38%~0.52%,Na2O和K2O的含量分别为0.50%~3.83%和0.08%~1.26%,铝饱和指数(A/CNK)为0.77~1.37,低ΣREE(43×10-6~70×10-6),为具有亏损源区特征的高镁安山岩。英安岩较安山岩SiO2含量高(63.74%~75.35%),具有相似的Al2O3(12.3%~16.0%%)和TiO2(0.32%~0.52%)含量;Na2O和K2O含量分别为2.01%~5.53%和0.16%~2.19%,ΣREE含量较安山岩略高(61×10-6~84×10-6),铝饱和指数(A/CNK)为0.77~5.74。安山岩和英安岩均轻重稀土分异明显,具有中等的Eu负异常;富集大离子亲石元素(LILE),亏损高场强元素(HFSE),具有明显的Nb-Ta-Ti负异常。相似的岩石地球化学特征表明安山岩和英安岩可能为同源岩浆,英安岩是安山质岩浆经过角闪石等矿物结晶分异作用形成的,安山岩的母岩岩浆很可能来自亏损大洋岩石圈地幔。基于该组岩石组合、地球化学特征和区域地质资料,推测研究区高镁安山岩可能形成于洋内弧环境,这表明东天山吐哈盆地南缘卡拉塔格地区在早古生代已经存在古亚洲洋的俯冲。
关键词: 卡拉塔格     高镁安山岩     大南湖岛弧带     古亚洲洋俯冲     东天山    
Early Paleozoic subduction of the Paleo-Asian Ocean: Zircon U-Pb geochronological and geochemical evidence from the Kalatag high-Mg andesites, East Tianshan
LI Wei1, CHEN JunLu2, DONG YunPeng1, XU XueYi2, LI ZhiPei2, LIU XiaoMing1, HE DengFeng1    
1. State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi'an 710069, China;
2. Xi'an Center of Geological Survey, China Geological Survey, Xi'an 710054, China
Abstract: The Kalatag area is located on the southern margin of the Turpan-Hami Basin, northern Xinjiang, Northwest China. The Early Paleozoic Daliugou Formation volcanic rocks in this area are widely distributed in Caixia mountains, Tuwu ore and Kalatag districts, which are located to the north of the Dacaotan fault. They consist of basic, intermediate, acidic lavas, and pyroclastic rock. LA-ICP-MS zircon U-Pb dating yield dacite and andesite age of 434.8±3.8Ma and 438.4±4.9Ma, respectively, which show that the Daliugou Formation volcanic rocks formed in the Early Silurian. Andesite have contents of SiO2(54.35%~60.70%), Al2O3(11.8%~16.3%) with A/CNK of 0.77~1.37, MgO(4.94%~8.27%), TiO2(0.38%~0.52%), Na2O(0.50%~3.83%) and K2O(0.08%~1.26%) with high Na2O/K2O ratios of 3.63~38.3(except one samples), and lower ΣREE(43×10-6~70×10-6). These samples are high-Mg andesites which have the characteristics of depleted source. Dacite have higher contents of SiO2(63.74%~75.35%) than andesite, and similar contents of Al2O3(12.3%~16.0%) with A/CNK of 0.77~5.74, TiO2(0.32%~0.52%), Na2O(2.01%~35.53%) and K2O(0.16%~2.19%), and higher ΣREE(61×10-6~84×10-6). Both andesites and dacites have an obvious differentiation between the light rare earth element(LREE) and heavy rare earth element(HREE), Eu negative anomaly with medium. They are all enriched with large-ion lithophile elements(LILE), but are relatively depleted in high field strength elements(HFSE), and have an obvious negative Nb-Ta-Ti anomaly. These andesite and dacite are likely to come from the same magma chamber based on similar geochemical characteristics. Dacite is likely to be formed after andesitic magma underwent minerals such as hornblende crystallization differentiation. Andesitic magmas were most probably derived from a depleted ocean lithosphere mantle. Based on the volcanic assemblage, geochemical characteristics and regional geology, we speculate that the Early Paleozoic volcanism occurred in an intra-ocean island arc setting. Combined with the regional geological record, comprehensive analysis of the isotope geochronology data indicates that the study area have existed the record of the subduction of the Paleo-Asia Ocean.
Key words: Kalatag     High-Mg andesites     Dananhu arc     Paleo-Asian Oceanic subduction     East Tianshan    
1 引言

中亚造山带是全球显生宙陆壳增生与改造最显著的大陆造山带,其造山过程受控于西伯利亚板块与中朝、塔里木板块之间的古亚洲洋的演 化(Şengör et al., 1993; Jahn et al., 2004; Xiao et al., 200320042008200920102013; Xiao and Santosh, 2014; Kovalenko et al., 2004; Shen et al., 2009; Safonova et al., 2008; Wong et al., 2010; Rojas-Agramonte et al., 2011)。中亚造山带经历了多阶段的构造演化过程,古生代是古亚洲洋演化的重要时期(夏林圻等,2002; Xiao et al., 2003200820102013; Xiao and Santosh, 2014; Dong et al., 20072011),因而成为学者们关注的焦点。

近东西向延伸的天山造山带位于中亚造山带南缘,是中亚造山带的重要组成部分。在中国境内,天山造山带以乌鲁木齐-库尔勒一线为界分为东天山和西天山。北天山和南天山缝合带,又可将其划分为北天山构造带、中天山地块和南天山构造带(车自成等,1994; Gao et al., 19951998; Gao and Klemd, 2003; 董云鹏等, 20012006; Dong et al., 2007; Xiao et al., 20042009)。大量的文献资料显示,新疆天山地区古生代洋陆构造格局演变及其复杂造山作用存在众多科学问题,尤其是沿着北天山和南天山缝合带最终碰撞闭合时限仍然存在分歧(Windley et al., 1990; Allen et al., 1992; Gao et al., 1998; Chen et al., 1999; Gao and Klemd, 2003; Zhao et al., 2003; Xiao et al., 200420082009; de Jong et al., 2009; Konopelko et al., 2007; Wang et al., 2007; Zhang et al., 2009; Su et al., 2010; Biske and Seltmann, 2010; Dong et al., 2011)。由于天山地区早古生代地质记录相对较少,导致该地区古亚洲洋早期演化历史不明。在该背景之下,出露于吐哈盆地南缘的早古生代火山岩为东天山早古生代构造演化历史提供了重要的信息。

卡拉塔格位于东天山北缘的哈尔里克-大南湖岛弧带上(图 1)(Xiao et al., 2013),该岛弧带主要包括奥陶纪、泥盆纪-石炭纪的火山岩、志留纪-二叠纪的侵入岩、火山碎屑岩和由浊积岩、玄武岩、硅质岩以及超基性岩组成的增生复合体(Xiao et al., 20042013)。带内主要发育志留纪、泥盆纪、石炭纪、二叠纪岩浆侵入事件(陈富文等,2005; 李少贞等,2006; 李文铅等,2006; Wang et al., 2014; 王银宏等,2014; 夏芳等,2012),志留纪-石炭纪的侵入岩形成于与岛弧有关构造环境(陈富文等,2005; Wang et al., 2014),二叠纪侵入岩形成于造山后的陆内伸展(Wang et al., 2014)。该带大量的斑岩型铜、钼矿床(Han et al., 2014; Zeng et al., 2015),亦形成于古生代与岛弧有关的构造环境,并且亏损地幔源区为硫化物矿床提供了来源(毛启贵等,2010)。然而,对于该地区的荒草坡群大柳沟组火山岩的时代及其构造属性还没有定论。有关吐哈盆地南缘卡拉塔格地区早古生代火山岩,目前存在广泛的争议性认识。在l︰20万康古尔塔格幅区域地质调查报告中(新疆地矿局,1988),这套火山岩被厘定为下泥盆统,命名为卡拉塔格组;1︰20万大草滩幅区域地质调查报告中(新疆地矿局,1994),将其置于中上志留统,未建组;1︰25万五堡幅区域地质调查报告中(新疆地质调查院,2003),将该组置于中奥陶统荒草坡群大柳沟组。毛启贵等(2010)通过测试卡拉塔格地区硫化物矿床围岩的锆石年龄,认为该套火山岩形成于中晚志留世。

①新疆地矿局. 1988. 康古尔塔格幅1:20万区域地质调查报告. 1-211

②新疆地矿局. 1994. 大草滩幅1:20万区域地质调查报告. 1-28

③新疆地质调查院. 2003. 五堡幅1:25万区域地质调查报告. 1-288

图 1 东天山及邻区构造地质简图(据Xiao et al., 2004;中国地质调查局西安地质调查中心,2006)Fig. 1 Structural geological sketch map of the Eastern Tianshan(after Xiao et al., 2004)

①中国地质调查局西安地质调查中心. 2006.中国天山及邻区地质图(1:100万)修编

本次研究发现,卡拉塔格地区的安山岩是一套形成于早志留世的高镁安山岩,其对揭示地球的分异演化、洋壳与洋脊或陆壳的俯冲、地壳拆沉、流体或熔体-地幔间的相互作用和古亚洲洋的构造演化具有非常重要的意义(唐功建和王强,2010)。本文对卡拉塔格地区火山岩进行了详细的野外调查和样品采集,通过ICPMS锆石U-Pb定年,限定了荒草坡群火山岩的形成时代。并对火山岩进行了详细的地球化学研究,为该地区早古生代亚洲洋的构造格局与演化历史提供了重要的地质制约。 2 地质背景

卡拉塔格地区位于中国境内东天山中部,吐哈盆地南缘。空间上从康古尔塔格到大南湖,大致以大草滩断裂为界,以南分布着大量的晚古生代火山岩和晚古生代酸性侵入岩,早古生代火山岩仅出露在吐哈盆地南缘大草滩断裂以北的彩霞山、土屋铜矿北和卡拉塔格地区(图 2)。

图 2 东天山卡拉塔格地区地质简图(据新疆地质调查院,2003修编)Fig. 2 Regional geology of the Kalatag area in east Tianshan

研究区依次出露荒草坡群大柳沟组、中上志留统红柳峡组(S2-3h)、下泥盆统大南湖组(D1d)、中泥盆统康古尔塔格组(D2k)、下石炭统干墩组(C1g)、上石炭统脐山组(C2qs)、企鹅山组(C2q),中二叠统阿尔巴萨依组(P2a)、上二叠统库莱组(P3k),侏罗系八道湾组(J1b)、三工河组(J1s)、西山窑组(J2x)、头屯河组(J2t)以及新生界。

大柳沟组最初由新疆地矿局第一区调大队杜天星(1995)命名,该套地层主要分布在东准噶尔巴里坤荒草坡地区和吐哈盆地东南缘大草滩断裂以北地区(中国地质调查局西安地质调查中心,2006)。总体为一套中酸性火山岩,夹少量凝灰岩和粉砂岩。研究区荒草坡群大柳沟组分布在康古尔塔格大断裂以北彩霞山及其以东一带和沙尔湖以西卡拉塔格地区(图 2)。在彩霞山以东地区,该组被中-上志留统红柳峡组、中泥盆统康古尔塔格组不整合覆盖,在卡拉塔格地区,该组被中-上志留统红柳峡组、下泥盆统大南湖组不整合覆盖,该组未见底。

沙尔湖以西卡拉塔格一带大柳沟组火山岩下部主要为英安岩,夹安山岩、霏细岩及少量凝灰岩;上部以玄武岩为主,其次为安山岩、英安岩。彩霞山及其以东地区(康古尔塔格大断裂以北),大柳沟组火山岩下部主要为凝灰岩、安山岩及少量英安岩;上部主要为钠长斑岩、流纹岩,其次为凝灰岩及凝灰熔岩。

卡拉塔格地区大柳沟组为一套巨厚的海相火山岩建造,厚度达7440m(新疆地质调查院,2003)。在剖面上(图 3),酸性火山岩占剖面总厚度的56%;中性熔岩(安山岩类)占剖面厚度的17%,基性熔岩占剖面厚度的15%,火山碎屑岩占剖面总厚度的11%。

图 3 卡拉塔格地区大柳沟组火山岩剖面(据新疆地质调查院,2003)Fig. 3 Schematic Ordovician volcanic rocks column in the Kalatag area

在大草滩断裂以北彩霞山及其东北一带该组以中酸性火山岩为主,岩性为英安岩、霏细岩、安山岩,在纵向上(剖面上)彩霞山北剖面和彩霞山东北剖面(图 3)中,酸性熔岩厚度占剖面厚度比例分别为28%、13%;中性火山岩分别为24%、72%,火山碎屑岩(火山角砾岩、凝灰岩)分别为48%、15%;碎屑岩、火山碎屑沉积岩夹层极少,多呈单层或透镜状产于熔岩和火山碎屑岩之间。

大柳沟组总体是以酸性火山岩为主的玄武岩、安山岩、英安岩、流纹岩和火山碎屑岩组合。在海豹滩以东该组以中基性熔岩为主,岩性为:橄榄玄武岩、辉石安山岩及安山质、英安质火山角砾岩,玄武岩厚度占17%,安山岩占41%,火山碎屑岩占42%。 3 样品特征

样品采自卡拉塔格地区的中酸性火山岩,采样位置见图 2,岩石镜下特征见图 4。安山岩呈灰-灰黑色,致密块状构造,斑状结构,斑晶为斜长石、正长石和角闪石。基质为微晶结构,斜长石微晶间隙多为绿泥石和绿帘石所充填。斜长石呈自形-半自形板条状,熔蚀结构发育,可见卡钠复合双晶。正长石可见简单双晶,具熔蚀现象。角闪石呈熔蚀浑圆状。岩石中斜长石蚀变普遍,主要为黝帘石化和绿帘石化。英安岩呈浅灰色或浅红褐色,致密块状,熔蚀筛状结构、斑状结构,块状构造。斑晶为斜长石、正长石和角闪石。斜长石斑晶呈自形晶,双晶隐现,含量达10%~12%。正长石呈自形多晶态,简单双晶隐现,可见熔蚀现象,含量5%~7%。角闪石呈熔蚀浑圆状,可见熔蚀孔中结晶的阳起石,含量8%~10%。基质主要为石英、正长石、绿泥石、黝帘石和绿帘石。石英呈熔蚀筛状,含量25%~18%,正长石呈他形板状晶,节理可见,含量30%~32%。绿泥石呈片状晶体,含量2%~3%;黝帘石多分布于斜长石内部,为斜长石的蚀变产物,含量5%~6%。

图 4 卡拉塔格火山岩显微照片左图为单偏光,右图为正交偏光. Or-正长石;Pl-斜长石;Am-角闪石;Zo-黝帘石;Chl-绿泥石;Ep-绿帘石Fig. 4 Microphotographs for textures of the volcanic rocks from the KalatagThe left for the single polarization,the right for orthogonal polarization. Or-orthoclase; Pl-plagioclase; Am-amphibole; Zo-zoisite; Chl-chlorite; Ep-epidote
4 分析方法 4.1 全岩地球化学分析

岩石主-微量、锆石阴极发光照相及锆石U-Pb同位素分析均在西北大学大陆动力学国家重点实验室完成。主量元素用样品的碱熔玻璃片在RIX2100型X射线荧光光谱仪(XRF)上测试,分析过程中采用BCR-2和GBW07105标样监控,分析精度优于5%,烧失量(LOI)在烘箱中高温(1000℃)烘烤90min后称重获得。微量元素分析在Elan6100DRC型ICP-MS分析完成,采用AGV-1、BCR-1、BHVO-1国际标样监控,分析误差小于5%。详细的样品处理过程见参考文献(刘晔等,2007; Gao et al., 2002)。 4.2 LA-ICP-MS锆石U-Pb定年

U-Pb锆石ICP-MS测年,首先将人工重砂分离出的锆石颗粒随机地固定在环氧树脂表面并抛光,抛光后对待测锆石进行透射光、反射光和阴极发光(CL)照相,以帮助选定最佳的锆石测定部位及数据解释。锆石U-Pb定年工作在连接Geolas-193型紫外激光剥蚀系统的Agilient 7500a型ICP-MS上进行。采用He作为剥蚀物质的载气。激光剥蚀束斑直径为30μm,频率为6Hz。激光采样方式为单点剥蚀,详细的分析流程见参考文献(柳小明等,2007; Yuan et al., 2004)。 5 分析结果 5.1 主量和微量元素

吐哈盆地南缘卡拉塔格地区火山岩的主量和微量元素分析结果见表 1。样品D56-3~D56-10来自卡拉塔格地区大柳沟组火山岩剖面下部的英安岩,样品D58-2~D58-4、D59-2~D59-6来自大柳沟组火山岩剖面上部的安山岩。

表 1 卡拉塔格地区火山岩主量元素(wt%)与微量元素(×10-6)化学组成 Table 1 Major(wt%) and trace(×10-6)element compositions of the volcanic rocks in the Kalatag area,eastern Tianshan

安山岩样品SiO2含量在54.35%~60.70%之间;Al2O3含量为11.8%~16.3%;TiO2含量为0.38%~0.52%;MgO含量为4.94%~8.27%,Mg#=59~70,CaO含量为0.36%~6.76%,Na2O和K2O含量分别为0.50%~3.83%和0.08%~1.26%,除样品D59-6(由于蚀变K2O>Na2O)外,其他样品均具有Na2O>K2O的特征,显示富钠,K2O+Na2O含量在1.76%~4.12%,在全碱-SiO2图解和Zr/TiO2-Nb/Y图解(图 5a,b)中,所有样品均落在亚碱性火山岩系列。英安岩样品SiO2含量在63.74%~70.35%之间;Al2O3含量在12.27%~15.98%;TiO2含量在0.32%~0.52%;K2O含量在0.16%~2.19%,Na2O含量在2.01%~5.53%,Na2O>K2O,K2O+Na2O含量在2.79%~6.58%。英安岩亦属于亚碱性火山岩系列(图 5a,b)。在K2O-SiO2图解(图 5c,d)中,大柳沟组安山岩-英安岩具有钙碱系列-低钾拉斑系列岩石特征,大部分样品显示偏低钾系列。

图 5 火山岩岩石分类图解(a)全碱-SiO2(Le Maitre et al., 1989);(b)Zr/TiO2-Nb/Y图解(Winchester and Floyd, 1977);(c)K2O-SiO2图解(Peccerillo and Taylor, 1976);(d)K2O-SiO2图解(Gill,1981)Fig. 5 The volcanic rock classification diagram(a)SiO2 vs. K2O+Na2O diagram( Le Maitre et al., 1989);(b)Zr/TiO2 vs. Nb/Y diagram(Winchester and Floyd, 1977);(c)K2O vs. SiO2 diagram(Peccerillo and Taylor, 1976);(d)K2O vs. SiO2 diagram(Grill,1981)

安山岩的ΣREE 在43×10-6~70×10-6之间,球粒陨 石标准化的REE配分图解显示稀土分布曲线右倾(图 6a),轻稀土元素(LREE)富集,重稀土亏损。(La/Yb)N=2.73~4.32,(La/Sm)N=2.24~2.57,轻重稀土分异明显;具有Eu负异常,δEu=0.68~0.90,显示岩浆演化过程中可能存在斜长石分离结晶作用。在微量元素原始地幔标准化图解上(图 6b)表现出大离子亲石元素Cs、Sr、Ba、Pb和U明显富集,高场强元素Nb、Ta、Th、P、Ti相对亏损,Ta、Nb和Ti具有明显的负异常。

图 6 球粒陨石标准化稀土元素配分图与原始地幔标准化微量元素蛛网图(标准化值据Sun and McDonough, 1989)Fig. 6 Chondrite-normalized REE patterns and primitive-mantle normalized trace element multi-variation diagrams of volcanic rocks from Kalatag area(normalization values after Sun and McDonough, 1989)

英安岩的ΣREE 在61×10-6~84×10-6之间,球粒陨石标准化的REE配分图解(图 6c)显示与安山岩相似的LREE 富集的REE配分型式。轻重稀土分异明显,(La/Yb)N=3.31~4.71,(La/Sm)N=1.97~3.05;具有Eu负异常,δEu=0.66~0.87,显示岩浆演化过程中存在斜长石分离结晶作用。英安岩微量元素原始地幔标准化蛛网图(图 6d)具有与安山岩相似的特征,区别在于元素Cs亏损,Rb相对安山岩富集。英安岩同样具有元素Nb、Ta和Ti明显亏损的特征。 5.2 锆石U-Pb定年

本次研究在野外火山岩剖面中上部采集了安山岩、英安岩年代学样品各1件。样品D59-1(42°36.294′N,91°55.728′E)为灰色安山岩,致密块状。D56-1(42°35.075′N,92°00.057′E)为浅褐色英安岩,手标本可见长石斑晶。

样品D56-1所选可测试锆石15颗,样品D59-1所选可测试锆石9颗。从锆石形态和锆石阴极发光图像看(图 7),锆石颗粒较自形,发育较清晰的岩浆结晶环带。锆石的Th/U比值均介于0.29~1.27(表 2),综合锆石形态分析为岩浆成因锆石(吴元保和郑永飞,2004)。通过LA-ICP-MS测试,2件样品共获得25点同位素数据(表 2)。其中,样品D56-1激光剥蚀17点,获得7个点的谐和数据(图 8a),其加权平均值为438.4±4.9Ma,MSWD=0.99。样品D59-1激光剥蚀8点,获得5个点的谐和数据(图 8b),加权平均值为434.8±3.8Ma,MSWD=0.12。样品D56来自火山岩剖面下部英安岩,样品D59-1采自火山岩剖面上部安山岩,因而D59-1和D56-1年龄数据与地质剖面显示新老关系一致,在误差范围内年龄基本一致。本文锆石U-Pb MA-ICP-MS年龄结果表明,卡拉塔格地区火山岩形成于早志留世。

图 7 锆石样品阴极发光及相应测试年龄圆圈代表激光剥蚀点,并附有点号及年龄Fig. 7 Zircon sample CL images and the corresponding test ageThe circles show LA-ICP-MS analysis spots with corresponding spot numbers. The 206Pb/238U ages(Ma)for each spot are labeled

表 2 样品LA-ICP-MS锆石U-Pb分析结果Table 2 Zircon U-Pb analytical results for samples form the Ordovician Daliugou Fm. in the Kalatag area

图 8 LA-LCP MS锆石U-Pb年龄谐和图和加权年龄平均值Fig. 8 LA-ICP-MS U-Pb concordia diagrams of zircons and weighted average age

6 讨论 6.1 地层形成时代

卡拉塔格地区大柳沟组为火山岩、火山碎屑岩组合,缺少化石记录。区域地质资料和野外地质观察表明,下泥盆统大南湖组、中上志留统红柳峡组在不同位置不整合于大柳沟组火山岩之上(新疆地质调查院,2003)。一些学者根据在彩霞山地区该套火山岩中获得了Rb/Sr等时线年龄值为517±57Ma(新疆地矿局,1995)而初步将该套火山岩定为中奥 陶世(新疆地调院,2003)。由于其年龄值误差偏大,其年龄可信度较低。另一些学者在研究卡拉塔格地区红海块状物矿床时,取得矿体上盘酸性火山岩年龄为416.3±5.9Ma,同时在矿体下盘获得强绢云母化蚀变围岩的K-Ar年龄为424±7Ma(毛启贵等,2010),认为火山岩形成于晚志留世。上述结论与中上志留统红柳峡组不整合于该火山岩之上的地质事实不符。本研究获得了大柳沟组中酸性火山岩的LA-ICP-MS锆石年龄介于428.8~438.4Ma,依据火山岩U-Pb锆石年龄和区域地层不整合接触关系,限定了卡拉塔格地区大柳沟组火山岩应形成于早志留世。

①新疆地矿局.1995. 1:5万 K46E11004、K46E11005、K46E11006、K46E11007、K46E11008五幅联测区域地质调查报告

6.2 早古生代火山岩成因机制

卡拉塔格地区荒草坡群大柳沟组(图 2)总体是以安山岩(28.6%,不同岩性占剖面总厚度的比例)、英安岩+流纹岩(36.7%)、玄武岩(11.5%)和火山碎屑岩(22.7%)为主的岩石组合。该地区火山岩中侵入一套志留纪中酸性岩体(卡拉塔格岩体),由英云闪长岩、花岗闪长岩和二长花岗岩组成(李文铅等,2006)。该地区的岩石组合类似于陆缘弧或洋内弧的火成岩构造组合(邓晋福等, 20072015)。

两类岩石样品具有一致的或相似的球粒陨石标准化稀土配分图解和原始地幔标准化的微量元素比值蛛网图解,两类岩石形成时代基本一致,具有相似的主量元素和微量元素特征,预示着该地区火山岩具有同源岩浆演化的特征。安山岩原始地幔标准化的微量元素配分型式(图 6)显示,大离子亲石元素K、Cs、Sr和Ba明显富集,高场强元素Nb、Ta和Ti强烈亏损,表明该火山岩可能产出于由于陆壳混染的板内构造环境或者与洋壳俯冲作用有关的构造环境(龙晓平等,2006; Rudnick and Gao, 2003)。一般认为,如果岩浆在上升过程中受到了地壳物质的混染,则La/Sm比值会迅速增高,一般在5以上(Lassiter and DePaolo, 1997),而本区火山岩La/Sm比值(3.39~4.72)均低于5,可见没有明显的地壳物质混染。

陆缘弧火山岩浆一般为高钾和钾玄系列岩浆,较大洋弧高K、Sr、Rb、Ba、Zr、Th和U,ΣREE含量较大洋岛弧高(Sarbas,2002; Wilson,1989)。研究区安山岩与英安岩样品具有近似的稀土总量(安山岩ΣREE:43×10-6~70×10-6,英安岩:61×10-6~84×10-6),并且均为钠质低钾钙碱性-岛弧拉斑系列火山岩(Na2O/K2O=1.78~38.3)(图 5cd),火山岩地球化学特征更接近大洋岛弧特征。卡拉塔格大柳沟组火山岩中,侵入一套同时代的中酸性岩体(卡拉塔格岩体),其形成时代为志留纪(李文铅等,2006),另在该岩体中采集花岗闪长岩岩体的U-Pb锆石年龄为早志留世(张成立测试,尚未发表)。岩体由英云闪长岩、花岗闪长岩和二长花岗岩组成,卡拉塔格岩体具有低的Sr同位素比值(Isr=0.7039~0.7041),143Nd/144Nd在0.512859~0.512953,εNd(t)高达+7.74~+9.13(李文铅等,2006),表明卡拉塔格岩体来源于亏损地幔源区,其形成与俯冲洋壳的部分熔融作用有关,可能形成于洋内岛弧环境。

研究区安山岩具有高的MgO含量(>5%,为5.16%~8.64%,除去烧蚀量),和更低的FeOT/MgO比值(FeOT/MgO=0.83~1.46<1.5),同时,Al2O3含量<16%(11.79%~16.3%),CaO含量<10%(0.36%~6.76%),为典型的高镁安山岩(唐功建和王强,2010; Tatsumi,2001)。依据MgO-SiO2图解(图 9图 10b),卡拉塔格地区安山岩具有低铁钙碱系列高镁安山岩特征(邓晋福等, 20102015)。除了本次发现的卡拉塔格地区早志留世的高镁安山岩外,在天山北缘和青藏高原亦有高镁安山岩/闪长岩的报道(王强等,2006; Wang et al., 2011)。本次研究的安山岩与天山北缘的石碳世高镁安山岩地化属性非常接近,类似于日本西南Setouchi新生代岛弧火山岩带中的赞其岩类的高镁安山岩(图 10)。前人对于高镁安山岩的成因研究表明,高镁安山岩可能是由俯冲的洋壳释放的含水流体加入到地幔楔中而引发地幔橄榄岩熔融形成(Tatsumi and Maruyama, 1989; Hirose,1997),这些异常高镁的安山岩不可能来自于干的地幔橄榄岩的熔融,而只可能来自于橄榄岩-H2O体系的熔融或与其相关的过程,最可能来自于与俯冲共生的岛弧环境(唐功建和王强,2010; 王强等,2006)。这些研究成果表明,俯冲洋壳产生的流体(熔体?)交代上覆的大洋岩石圈地幔,大洋岩石圈地幔发生部分熔融形成具有亏损同位素特征的高Mg安山质岩浆,安山质岩浆在上升过程中发生一定程度的结晶分异(角闪石等矿物)(图 10a)形成英安质岩浆(Drummond et al., 1996; Defant et al., 2002; 邓晋福等, 20102015; Green and Falloon, 2015)。

图 9 镁安山岩的SiO2-MgO图解和SiO2-FeOT/MgO图解(据邓晋福等, 20102015)(a)实线PQ和RS分别为HMA/MA与MA/非MA的边界;(b)直线为钙碱系列(CA)与拉斑系列(TH)分界线;点划线为低Fe钙碱系列(LF-CA)与中Fe钙碱系列的边界Fig. 9 SiO2-MgO and SiO2-FeOT/MgO diagram of Magnesian and esite(after Deng et al., 20102015)(a)the lines PQ and RS mean the boundary of HMA/MA and MA/non-MA respectively;(b)the straight line is the CA/TH boundary. The dot line is the boundary between low Fe(LF-CA) and medium Fe calc-alkaline series

图 10 Sr/Y-Y图解和SiO2-MgO图解(据王强等,2006及其文中参考文献)图中数据源来自王强等(2006)及其中的参考文献和Wang et al.(2011);曲线1表示部分熔融,曲线2表示分离结晶Fig. 10 Sr/Y vs. Y and SiO2 vs. MgO diagrams(after Wang et al., 2006 and references cited therein)The data are from Wang et al.(20062011), and references therein. Curve 1 means partial melting,Curve 2 means fractional crystallization

此外,由于本次研究样品为中酸性火山岩,在(Y+Nb)-Rb图解(图 11a)中,火山岩样品均落入火山岛弧环境,在La/Yb-Sc/Ni图解(图 11b)中,该区火山岩落在大洋岛弧环境。

图 11 卡拉塔格大柳沟组火山岩构造环境判别图解(a)Rb-Y+Nb图解(Pearce et al., 1984);Syn-COLG-同碰撞花岗岩;VAG-火山弧花岗岩;Post-COLG-碰撞后花岗岩;WPG-板内花岗岩;ORG-洋中脊花岗岩;(b)La/Yb-Sc/Ti图解(Bailey,1981)Fig. 11 Discrimination diagrams for tectonic settings for volcanic rocks of the Daliugou formation from the Kalatag area(a)Rb vs.Y+Nb diagram(Pearce et al., 1984); Syn-COLG: syn-collisional granitoids; VAG: volcanic arc granitoids; Post-COLG: post-collisional granitoids; WPG: within-plate granitoids; ORG: ocean ridge granites;(b)La/Yb vs.Sc/Ti discrimination diagram for and esites(Bailey,1981)

综上分析,卡拉塔格地区早古生代火山岩和侵入岩可能为同期岩浆活动的产物,早志留世安山岩具有高Mg、低K、贫稀土等亏损源区的特征,其成因机制可能为:在洋内弧环境下,俯冲洋壳形成的流体交代上覆大洋岩石圈地幔发生的部分熔融,形成的初始高Mg安山质岩浆在上升过程中经历一定程度的结晶分异作用,最终形成高Mg安山岩-英安岩组合。 6.3 早古生代火山岩构造意义

天山造山带是一条复杂的增生型造山带(Xiao et al., 2013),东天山卡拉塔格地区早古生代火山岩和岩浆岩的研究,为早古生代东天山地区构造演化提供了重要的地质信息。

卡拉塔格地区早古生代火山岩是东天山大南湖岛弧带的组成部分(Xiao et al., 2012; 李锦轶等,2006)。在吐哈盆地北缘,为哈尔里克岛弧带,形成于晚奥陶世岛弧环境(马星华等,2015),在吐哈盆地南缘,以大草滩断裂带为界,北侧为志留纪-泥盆纪岛弧带(李锦轶等,2006; 李文铅等,2006; 田纹全等,2005; Xiao et al., 2012; 张兴龙等,2004),南侧为石炭纪岛弧带(王强等,2006),位于大草滩断裂南侧的石炭纪企鹅山组火山岩为岛弧拉斑玄武岩系列(侯广顺等,2005; 张洪瑞等,2010; 王银宏等,2014)。该地区早志留纪-泥盆纪火山岩为吐哈南缘的岛弧环境(Xiao et al., 2012; 李锦轶等,2006; 李文铅等,2006; 田纹全等,2005; 张兴龙等,2004),位于大草滩断裂南侧的石炭纪企鹅山组火山岩为岛弧拉斑玄武岩系列(侯广顺等,2005; 张洪瑞等,2010)。吐哈盆地南缘火山岩岛弧带在时空分布上具有从北向南时代依次变新的趋势,火山岩组合由以中酸性火山岩为主的钙碱性系列向以基性火山岩为主的拉斑玄武岩系列过渡,可作为不同时代岛弧向南依次增生(李锦轶等,2006; Xiao et al., 2012)和古亚洲洋岩石圈向北俯冲的极性。

上述分析表明,古亚洲洋在早古生代(O3-S1)已经开始向北俯冲,不同时代的岛弧带依次向南增生,古亚洲洋最终可能在中晚石碳世闭合。 7 结论

(1)东天山吐哈盆地南缘卡拉塔格地区荒草坡群大柳沟组以中酸性火山岩组合为主,火山岩剖面下部英安岩LA-ICP-MS锆石U-Pb年龄为438.4±4.9Ma(MSWD=0.99),上部安山岩LA-ICP-MS锆石U-Pb年龄为434.8±3.8Ma(MSWD=0.12),代表了大柳沟组火山岩形成时代,表明荒草坡群大柳沟组形成于早志留世。

(2)荒草坡群安山岩具有高MgO(4.94%~8.27%)和低FeOT/MgO比值,低Al2O3(11.8%~16.3%)和CaO(0.36%~6.76%),为低铁钙碱系列高镁安山岩。安山岩和英安岩富钠(Na2O>K2O)、轻重稀土分异明显,具有中等程度Eu负异常。大离子亲石元素富集,高场强元素相对亏损,具有显著的Nb、Ta、和Ti元素的负异常。

(3)卡拉塔格地区荒草坡群火山岩形成于洋内岛弧环境,表明古亚洲洋板块在早古生代已经存在向北俯冲消减作用。

致谢     衷心感谢陕西地质调查院李海平高工在野外工作中提供大力帮助;同时由衷感谢两位匿名评审人给出的建议。
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