岩石学报  2013, Vol. 29 Issue (11): 3968-3980   PDF    
西秦岭宕昌地区晚三叠世酸性火山岩的锆石U-Pb年代学、地球化学及其地质意义
黄雄飞1, 莫宣学1, 喻学惠1, 李小伟1, 丁一1,2, 韦萍1,3, 和文言1     
1. 中国地质大学地质过程与矿产资源国家重点实验室,地球科学与资源学院,北京 100083;
2. 江苏省地质矿产局第四地质大队,苏州 215129;
3. 广西省地质矿产局第四地质队,南宁 530007
摘要: 西秦岭造山带在早中生代发育了以印支早期 (245~234Ma) 和印支晚期 (225~205Ma) 两期高钾钙碱性花岗岩占绝对优势,并伴有少量火山岩的强烈岩浆活动。本文报道的西秦岭宕昌火山岩总体上是一套以流纹岩为主,并含粗面岩和英安岩的酸性火山岩组合。流纹岩锆石U-Pb测年结果为229Ma,限定其喷发时代为晚三叠世早期。宕昌火山岩属于过铝质的高钾钙碱性-钾玄岩系列,以富集Rb、Ba、Th、U、K并相对亏损Nb、Ta、Ti、P为特征,(87Sr/86Sr)i=0.7048~0.7068,εNd(t)=-4.3~-4.0,亏损地幔模式年龄 (tDM) 为1.20~1.24Ga。宕昌火山岩起源于下地壳富钾变基性岩的部分熔融,部分岩浆还可能受到了少量地幔或上地壳物质的混染,源区深度约40km。西秦岭地块存在扬子型的中新元古代基底,但不同地段的基底特征存在较大差异。华北板块与扬子板块在西秦岭地区的初始碰撞可能接近229Ma,宕昌火山岩则形成于以整体挤压为主、局部剪切伸展为辅的地壳逐渐加厚的动力学环境。
关键词: 印支期     西秦岭     宕昌     火山岩     花岗岩     大陆碰撞    
Zircon U-Pb chronology, geochemistry of the Late Triassic acid volcanic rocks in Tanchang area, West Qinling and their geological significance
HUANG XiongFei1, MO XuanXue1, YU XueHui1, LI XiaoWei1, DING Yi1,2, WEI Ping1,3, HE WenYan1     
1. State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Science and Resources, China University of Geosciences, Beijing 100083, China;
2. The Forth Geology Prospecting Team, Jiangsu Geology and Mineral Exploration Bureau, Suzhou 215129, China;
3. The Forth Geology Prospecting Team, Guangxi Geology and Mineral Exploration Bureau, Nanning 530007, China
Abstract: Strong magmatic activities dominated by the Early Indosinian and Late Indosinian high potassic calc-alkaline granitoids (245~234Ma and 225~205Ma, respectively) and a small amount of volcanic rocks were developed in western Qinling orogenic belt in Early Mesozoic. The volcanic rocks from Tanchang area of West Qinling, as reported in this paper, are acid rocks which contain mainly rhyolite with minor trachyte and dacite. These volcanic rocks erupted in the early Late Triassic limited by the zircon U-Pb age of 229Ma of the rhyolite. Tanchang volcanic rocks belonging to the peraluminous, shoshonitic to high-K calc-alkaline series are characterized by enrichment in Rb, Ba, Th, U, K, relative depletion in Nb, Ta, Ti, P, (87Sr/86Sr)i=0.7048~0.7068, εNd(t)=-4.3~-4.0 and depleted mantle model ages ranging from 1.20Ga to 1.24Ga. Tanchang volcanic rocks were generated by partial melting of the K-rich meta-mafic protolith in lower crust while some of them might also be related with the contamination of a very small amount of mantle-derived or upper crustal materials, and the depth of the source was ca. 40km. The West Qinling terrane has Mesoproterozoic-Neoproterozoic crustal basement with a relatively large heterogeneity in different regions showing an affinity to Yangtze block. The time of the initial collision between North China block and Yangtze block in West Qinling might be close to 229Ma. Tanchang volcanic rocks were formed in the crustal thickening geodynamic environment mainly characterized by compression on the whole and shearing-extension in local part.
Key words: Indosinian     West Qinling     Tanchang     Volcanic rocks     Granitoid     Continental collision    
1 引言

作为中国中央造山带的重要组成部分,秦岭造山带是华北地块与扬子地块经过长期汇聚而形成的一条复合型造山带 (Meng and Zhang, 1999张国伟等,2001)。西秦岭属于秦岭造山带的西延,夹持于青藏高原东北缘、扬子西北缘、南祁连和东昆仑多个块体之间 (图 1a),对于理解中国大陆地质演化和探索造山带深部动力学过程具有重要意义 (冯益民等,2003张国伟等,2004)。早中生代,随着阿尼玛卿-勉略洋盆的俯冲闭合和大陆碰撞的开始,西秦岭地区发育了以印支早期 (245~234Ma) 和印支晚期 (225~205Ma) 两期高钾钙碱性花岗岩占绝对优势的强烈岩浆活动,但由于西秦岭地区缺乏与俯冲碰撞相关的蛇绿岩或高压-超高压变质岩,也没有古老基底出露 (Zhang et al., 2007a),导致目前对于这些花岗岩的岩石成因、源区特征及构造背景等多个重要问题仍存在诸多争议 (金维浚等,2005张宏飞等,2006Zhang et al., 2007a, b张成立等,2008Qin et al., 2009Cao et al., 2011Wang et al., 2011Zhu et al., 2011Guo et al., 2012Luo et al., 2012)。与花岗岩相比,西秦岭地区仅发育了非常少量的同期次基性岩浆活动 (Guo et al., 2012) 或火山岩喷发活动 (徐学义等,2008尤继元等,2010Li et al., 2013),因此,对这些三叠纪火山岩的详细研究将为认识西秦岭印支期的地球动力学过程提供新的思路。

图 1 西秦岭造山带构造划分图及研究区地质简图 (a)-西秦岭构造简图 (据冯益民等,2003);(b)-西秦岭印支期岩浆岩分布图 (据冯益民等,2002);(c)-宕昌火山岩地质简图 (据甘肃省地质调查院, 2007) Fig. 1 Tectonic subdivision of West Qinling orogenic belt and simplified geological map of the study area

①甘肃省地质调查院. 2007.中华人民共和国1:25万岷县幅 (I48C002002)

甘肃省宕昌县以北出露一套陆相中酸性火山岩系,前人通过地层对比在该火山岩的划分方案上存在上二叠统、下中三叠统和侏罗系三种不同的认识 (冯益民等,2002),最近的1:25万岷县幅 (甘肃省地质调查院, 2007) 区调中获得该火山岩的全岩K-Ar年龄为170~207Ma,进而认为其形成时代为早中侏罗世 (王伟,2008)。然而,受定年精度所限,该火山岩K-Ar年龄范围跨度太大,且缺乏详细的地球化学数据,影响了对岩浆起源、成因及相关构造环境的认识。本文提供了宕昌火山岩精确的锆石U-Pb定年结果、详细的野外地质观察和系统的岩石学、全岩主微量及Sr-Nd同位素地球化学数据,通过分析该火山岩的岩石成因、源区性质及形成背景,为探讨印支期秦岭造山带的造山过程及壳幔演化提供新的制约。

2 地质背景及样品描述

西秦岭造山带北以贵德-临夏-武山-唐藏断裂与祁连地块相邻,南以玛沁-略阳断裂 (阿尼玛卿-勉略缝合带) 与巴颜喀拉-松潘甘孜地块相接,西以温泉-瓦洪山断裂与东昆仑及柴达木地块相隔,东部则以徽成盆地与东秦岭佛坪穹窿相连 (地理位置上大致以宝成铁路线为界)(图 1b)(张国伟等,2001冯益民等,2003Zhang et al., 2007a)。西秦岭早中生代花岗岩岩浆活动非常发育,花岗岩的空间分布具有平行于近东西向的断裂带呈线性展布的特征 (Luo et al., 2012),由北向南则表现出形成时代年轻化的趋势 (Jiang et al., 2010)。这些花岗岩侵位时间集中于印支早期 (245~234Ma) 和印支晚期 (225~205Ma) 两个阶段,特别是印支晚期的岩浆活动在区内分布极为广泛,中间则似存在一个相对短暂、稳定的岩浆宁静期 (234~225Ma)。此外,据徐学义等 (2008)报导,在西秦岭东北天水尹道寺附近有流纹岩和流纹质火山碎屑岩的酸性火山岩出露,锆石U-Pb定年为211Ma。

研究区位于西秦岭东部,区域出露地层主要有以灰岩及砂岩为主的泥盆系、以砂岩为主的二叠系和以砾岩为主的白垩系。区内印支期侵入岩也十分发育,宕昌北东有著名的“五朵金花”岩体 (教场坝、柏家庄、碌础坝、中川、正沟) 及温泉复式岩体 (225~216Ma,Zhang et al., 2007aCao et al., 2011Zhu et al., 2011)(图 1b)。宕昌火山岩主要出露于宕昌县以北的马建川-彭都,簸箕-路院一带,在理川河、珍桌子、堡子川、梨树下等地也有零星分布 (甘肃省地质调查院, 2007)。该火山岩与下伏二叠系、泥盆系及上覆白垩系均呈不整合接触,出露面积约23km2,厚度达2000m以上,岩石类型以流纹岩为主,其次为安山岩、英安岩,并出现大量安山质凝灰角砾岩、英安质火山碎屑岩、流纹质角砾凝灰岩等 (图 1c)(甘肃省地质矿产局,1989)。此外,该火山岩的产状在一定程度上显示出受断裂带控制的特点 (图 1c)。

本次工作主要对簸箕-路院、彭都-马建川地区的火山岩进行了野外观察和采样,样品显示出一定程度的蚀变,岩性主要为流纹岩和英安岩。流纹岩呈灰白色,略显流纹构造,具有斑状结构,斑晶 (约15%) 主要为石英、钾长石和黑云母,并且钾长石多已发生土化,其次有少量绢云母化的斜长石和暗化的角闪石,基质则主要由微晶长石和石英组成,不含碱性暗色矿物 (图 2a)。英安岩呈深灰色,斑状结构,斑晶 (约25%) 主要由绢云母化斜长石、石英、土化钾长石、暗化角闪石和少量黑云母组成,部分斜长石斑晶边缘还可见被燧石英所替代,基质成分与斑晶基本相似,为隐晶质-微晶结构 (图 2b)。

图 2 宕昌火山岩显微镜下照片 (a)-流纹岩 (LY12-18);(b)-英安岩 (LY12-26).Am-角闪石; Bt-黑云母; Q-石英; Kfs-钾长石; Ser-绢云母 Fig. 2 Micro-photographs of the volcanic rocks in Tanchang area
3 分析方法

锆石分选由廊坊诚信地质服务有限公司利用单矿物常规分离技术完成,制靶后进行透射光、反射光及阴极发光照相。锆石U-Pb同位素测年在天津地质矿产研究所同位素实验室利用LA-ICP-MS方法完成,采用质谱仪为美国Thermo Fisher公司制造的Neptune,激光剥蚀系统为ESI公司生产的UP193-FX ArF准分子激光器,激光束斑直径设定为35μm。数据处理采用ICPMSDataCal程序 (Liu et al., 2008, 2010) 和Isoplot (Ludwig,2003) 程序进行,采用Andersen (2002)方法对普通铅进行校正,详细测试方法及仪器参数见李怀坤等 (2010)。测试分析结果见表 1

表 1 宕昌流纹岩 (LY12-18) 锆石U-Pb定年分析数据 Table 1 U-Pb analytical data of zircons from the rhyolite (LY12-18) in Tanchang area

岩石样品无污染粉碎至200目用于测定全岩主、微量元素及Sr-Nd同位素。主量元素分析在中国科学院广州地球化学研究所同位素年代学和地球化学重点实验室完成,使用Rigaku RIX 2000型荧光光谱仪 (XRF) 测定,分析精度优于5%,详细方法参见Goto and Tatsumi (1996)。微量元素分析在中国地质大学 (武汉) 地质过程与矿产资源国家重点实验室完成,采用Agilent 7500a等离子体质谱仪 (ICP-MS) 测定,分析精度优于5%~10%,详细方法及流程参见Gao et al.(2002)。全岩Sr、Nd的分离提纯在北京大学造山带与地壳演化教育部重点实验室完成,上机测试在天津地质矿产研究所同位素实验室利用Triton热电离质谱仪 (TIMS) 分析完成,测定的87Sr/86Sr和143Nd/144Nd比值分别采用87Sr/86Sr=0.1194和143Nd/144Nd=0.7219进行质量分馏校正,分析期间Sr国际标准NBS987给出87Sr/86Sr=0.710234±0.000008(2σ,n=5),Nd国际标准LRIG给出143Nd/144Nd=0.512192±0.000005(2σ,n=5),实验室监控标准BCR-2给出87Sr/86Sr=0.7104993±0.000006(2σ,n=6),143Nd/144Nd=0.512632±0.000002(2σ,n=6),详细的Sr-Nd同位素分析流程见Niu et al.(2012)。全岩主、微量元素数据列于表 2,Sr-Nd同位素数据列于表 3

表 2 宕昌火山岩主量元素 (wt%)、微量和稀土元素 (×10-6) 分析结果 Table 2 Major element (wt%), trace and REE elements (×10-6) of the volcanic rocks in Tanchang area

表 3 宕昌火山岩Sr、Nd同位素数据 Table 3 Sr and Nd isotopic compositions of the volcanic rocks in Tanchang area
4 分析结果 4.1 锆石U-Pb年代学

本文对1件流纹岩样品 (LY12-18) 进行了锆石U-Pb测年。样品中的锆石多呈自形程度较好的柱状,粒度较大,长度为60~200μm,并且发育明显的岩浆震荡环带 (图 3a)。锆石Th、U含量较高且呈正相关 (图略),Th/U值介于0.3~1.4,明显不同于Th/U<0.1的变质锆石,而属于典型的岩浆锆石 (Hoskin and Black, 2000)。25个测点中,剔除5个年龄偏老的测点 (1779~309Ma) 和1个远离谐和线的测点,其余19个测点的206Pb/238U加权平均年龄为229.3±2.0Ma (图 3b)。因此,宕昌酸性火山岩喷发时代为晚三叠世早期,且恰处于西秦岭印支早、晚两期花岗岩岩浆活动的宁静期,而并非前人所定的晚二叠世或早中侏罗世。

图 3 宕昌流纹岩锆石阴极发光图像 (a) 及U-Pb年龄协和图 (b) Fig. 3 Cathodoluminescence images (a) and U-Pb concordia diagrams (b) of zircons from the rhyolite in Tanchang area
4.2 主量和微量元素特征

11件样品烧失量变化于1.99%~5.49%,去除烧失量重新计算后,该火山岩具有偏高的SiO2和Al2O3(分别为64.38%~71.84%和15.14%~17.92%) 及偏低的MgO (0.52%~1.48%)。为避免次生蚀变作用对K2O、Na2O含量所带来的影响,采用相对稳定的高场强元素Nb和稀土元素Y替代全碱进行岩石分类,并和TAS图解 (图 4a) 进行对比。在SiO2-Nb/Y图中 (图 4b),1个样品落入粗面岩区 (Q<20%),4个样品分布于英安岩区与粗面岩区的边界,与TAS图解结果 (图 4a) 基本一致,而英安岩样品K2O/Na2O比值介于1.78~1.81,属于钾质英安岩;其余6个样品虽投入碱流岩区,但由于斜长石含量很少且不含碱性暗色矿物,而具有高钾特征 (K2O/Na2O=2.45~7.72),因此将其命名为碱长流纹岩更为合适。此外,宕昌火山岩SiO2含量明显低于尹道寺流纹岩 (图 4a)。K2O-SiO2图上 (图 4c),碱长流纹岩及粗面岩均属于钾玄岩系列,钾质英安岩位于高钾钙碱性向钾玄岩系列过渡的区域。A/NK-A/CNK图解 (图 4d) 还显示宕昌火山岩主要为过铝质岩石。在哈克图解中 (图 5),随SiO2递增,在除Fe2O3之外的其他氧化物或微量元素含量上,粗面岩、钾质英安岩及碱长流纹岩三者之间并未显示出一定的岩浆演化特征,表明它们的源区可能存在一定的差异性。

图 4 宕昌火山岩分类图解 (a)-TAS图解 (据Le Bas,1986);(b)-SiO2-Nb/Y图解 (据Winchester and Floyd, 1977);(c)-K2O-SiO2图解 (据Rickwood,1989);(d)-A/NK-A/CNK图解 Fig. 4 Classification diagrams for the volcanic rocks in Tanchang area

图 5 宕昌火山岩元素哈克图解 Fig. 5 Harker diagrams of elements of the volcanic rocks in Tanchang area

宕昌火山岩总体上具有较高的 (La/Yb)N比值和较弱的负Eu异常 (δEu=0.75~0.85)。其中,流纹岩具有最低的稀土元素含量 (ΣREE=147.8×10-6~168.4×10-6) 和最高的 (La/Yb)N值 (25.01~30.04),英安岩具有最高的稀土含量 (ΣREE=199.8×10-6~211.8×10-6) 和最低的 (La/Yb)N值 (19.28~21.13),粗面岩则介于二者之间。在球粒陨石标准化的稀土元素配分图上 (图 6a),宕昌火山岩均表现为轻稀土 (LREE) 富集而重稀土 (HREE) 相对亏损的右倾平滑曲线,其分布型式与具有显著负Eu异常和低LREE/HREE比值的尹道寺流纹岩区别十分明显。

图 6 宕昌火山岩稀土元素球粒陨石标准化配分模式图 (a) 与微量元素原始地幔标准化蛛网图 (b)(标准化值据Sun and McDonough, 1989) Fig. 6 Chondrite-normalized REE patterns (a) and primitive mantle-normalized trace element spider diagrams (b) for the volcanic rocks in Tanchang area (normalization values after Sun and McDonough, 1989)

在原始地幔标准化的蛛网图上 (图 6b),宕昌火山岩均富集大离子亲石元素 (LILE) Rb、Ba、K和高场强元素 (HFSE) Th、U,相对弱亏损Sr和HFSE中的Nb、Ta、Ti、P。其中,英安岩具有相对偏高的不相容元素含量,粗面岩的Sr亏损程度最小。与尹道寺流纹岩相比,宕昌火山岩中Th、U的富集程度和Ba、Sr、Nb、P、Ti的亏损程度均明显偏小,说明尽管这两个地区的酸性火山岩均喷发于印支期,但在源区物质上,宕昌火山岩却与具有北秦岭基底高放射性成因Pb同位素特征的尹道寺流纹岩 (徐学义等,2008) 存在很大差异。

4.3 Sr-Nd同位素特征

宕昌火山岩具有相近的Sr、Nd同位素比值,2件流纹岩样品 (87Sr/86Sr)i=0.7048~0.7050,εNd(t)=-4.3~-4.0,亏损地幔模式年龄 (tDM) 为1.20~1.23Ga;2件英安岩样品 (87Sr/86Sr)i=0.7064~0.7068,εNd(t)=-4.1~-4.0,tDM为1.23~1.24Ga。在εNd(t)-(87Sr/86Sr)i图中 (图 7),宕昌火山岩与西秦岭印支期花岗岩类及松潘-甘孜褶皱带花岗岩类显示出一定差异,却与南秦岭新元古代基底耀岭河群及南秦岭晚三叠世花岗岩类十分相近,说明其源区物质与南秦岭基底可能存在重要联系。另外,宕昌流纹岩与英安岩的εNd(t) 十分相近而 (87Sr/86Sr)i却变化较大,暗示了它们源区成分上的不均一性。

图 7 宕昌火山岩εNd(t)-(87Sr/86Sr)i图解 (据Qin et al., 2010aZhang et al., 2007b) Fig. 7 εNd(t)-(87Sr/86Sr)idiagram for the volcanic rocks in Tanchang area (after Qin et al., 2010a; Zhang et al., 2007b)
5 讨论 5.1 起源深度

宕昌火山岩较高的 (La/Yb)N比值 (平均24.32) 明显高于上地壳 (11.35,Rudnick and Gao, 2003),Yb (0.96×10-6~1.83×10-6) 和Y (9.78×10-6~22.6×10-6) 含量也较低,说明源区部分熔融受到了石榴子石的控制,暗示宕昌火山岩源区相对较深 (Rapp and Wastson, 1995)。不过,该火山岩低Sr (153×10-6~318×10-6) 及Sr/Y比值 (9.1~25.0) 的特征并不同于埃达克质岩石 (Defant and Drummond, 1990Defant et al., 2002Rapp et al., 2002)。另外,该火山岩的δEu和Sr与SiO2之间不存在负相关性 (图 5g,h),说明其Sr的弱亏损与Eu的弱负异常 (δEu平均为0.79) 可能与源区少量斜长石的残留有关。尽管陆壳物质具有类似陨石的相对稳定的Zr/Hf比值 (36.3,Sun and McDonough, 1989),但源区角闪石、单斜辉石及钛铁矿的残余仍会导致熔体Zr/Hf比值的升高 (David et al., 2000Pfänder et al., 2007魏瑞华等,2008),同时由于角闪石具有较高的MREE (中稀土) 和HREE分配系数 (Luhr et al., 1984Klein et al., 1997),所以来自角闪岩相的熔体将具有较高的MREE、较高但分馏较弱的HREE以及较低的 (La/Yb)N比值。宕昌英安岩具有相对偏高的Zr/Hf (39.7~40.2)、HREE (图 6a) 和偏低的 (La/Yb)N(19.28~21.13),粗面岩及流纹岩则具有偏低的Zr/Hf (24.8~37.9) 与HREE含量 (图 6a) 及高的 (La/Yb)N(25.01~30.04),指示宕昌火山岩源区残留有角闪石+单斜辉石,并且英安岩源区角闪石的残余相对较多。由此推断,宕昌火山岩的原岩应该是具有“斜长石+角闪石+石榴石+单斜辉石”矿物组合的下地壳石榴角闪岩相,三种岩石成分上的略微差异与不同深度源区组分的不均一性有关,一致于其Sr同位素的较大变化。

基于实验结果,Wyllie (1984)Deng et al.(1998)先后提出和论证了陆壳岩石在正常地壳厚度或加厚地壳中上部 (<30km) 将产生具有 (显著) 负Eu异常的流纹质 (花岗岩) 岩浆,而在加厚地壳底部则形成粗面质岩浆 (P>2.0Gpa时熔体无负Eu异常,而P=1.5~2.0Gpa时的残余矿物组合为“斜长石+石榴石±角闪石”且熔体出现弱负Eu异常)。根据这个认识可以推断,宕昌火山岩 (如粗面岩) 应起源于相对较厚的地壳下部,结合角闪石榴麻粒岩稳定压力的约束 (Drummond et al., 1996) 初步估算宕昌火山岩的起源深度约为40km,该深度与其低Sr、低Yb的特点也基本吻合 (张旗等,2006)。与宕昌火山岩相比,具有显著负Eu异常 (图 6a) 和较低 (La/Yb)N比值 (平均为15.08) 的尹道寺流纹岩则应该具有更低的源区深度,徐学义等 (2008)研究认为该流纹岩岩浆形成于15~35km,其成因可能与加厚地壳背景下挤压与剪切作用的共同控制有关,这也从侧面证明本文对宕昌火山岩起源深度的推断是合理的。

5.2 源区特征

宕昌火山岩是一套以流纹岩为主,兼有粗面岩和英安岩的酸性岩组合,因此不可能由地幔橄榄岩直接部分熔融形成 (Hirose,1997),并且该岩石组合也不同于玄武质岩浆通过高分异或地壳混染 (AFC) 形成的基性-中性-酸性岩浆系列 (Hildreth and Moorbath, 1988Schmidberger and Hegner, 1999)。根据宕昌火山岩的过铝质特征推测其成因应该与地壳物质的熔融密切相关,虽然中上地壳物质成分可以用于解释该火山岩过铝质和富集Th、U的特点,但样品不具有中上地壳来源的岩浆所具有的Sr、Eu强烈亏损特征 (Ellis and Thompson, 1986),而该火山岩高 (La/Yb)N比值、低 (87Sr/86Sr)i和相对高εNd(t) 等特征则限定其源区只可能是下地壳变基性岩类。Zhang et al.(2007a)研究认为多块体的拼合导致了西秦岭和东秦岭的南秦岭构造单元具有不同的地壳基底组成,并将宝成铁路线作为西秦岭与南秦岭基底组成差异的大致界线。值得注意的是,宕昌火山岩具有类似南秦岭基底而有别于西秦岭印支期花岗岩的Sr-Nd同位素组成 (图 7),并且从全岩Nd模式年龄来看,宕昌火山岩 (~1.2Ga) 也略低于西秦岭印支期花岗岩 (1.3~1.7Ga,Zhang et al., 2007a) 和松潘-甘孜褶皱带花岗岩 (1.4~2.0Ga,Cai et al., 2009),却与扬子北缘及南秦岭的大量新元古代岩浆岩 (0.9~1.2Ga,凌文黎等,2002)、南秦岭晚三叠世花岗岩 (0.9~1.3Ga,Qin et al., 2008, 2010a) 及碧口地块阳坝岩体 (1.1~1.2Ga,Zhang et al., 2007bQin et al., 2010b) 十分相近。因此,以宕昌火山岩为代表所反映的西秦岭东部地壳基底可能属于东、西秦岭基底的过渡,这一特点类似于碧口地块 (Zhang et al., 2007b)。目前,南秦岭构造单元已经被揭示出具有亲扬子的特征 (张国伟等,2001张本仁等,2002),这就意味着西秦岭也存在扬子型的中新元古代地壳基底,但不同地段的基底特征可能还存在较大差异,如何解释这种不均一性可能对于探讨西秦岭地块的构造属性具有重要意义。因此,宕昌火山岩应起源于具有扬子板块中元古代晚期新生地壳属性的下地壳重熔,与扬子克拉通发生在中元古代的地壳增生活动相吻合 (张本仁等,2002)。另外,宕昌流纹岩样品中的1779~1742Ma古老锆石与西秦岭多福屯玄武岩所携带的~1.8Ga继承锆石 (εHf(t) 变化于-8.7~+6.4)(Zheng et al., 2010) 时代基本一致,同时也与西秦岭碎屑锆石所表现出的1.8Ga的Nd模式年龄峰值 (Chen et al., 2008) 十分接近,这进一步暗示了西秦岭地块在~1.8Ga可能发生过不均一古老地壳物质的重熔或壳幔混合作用 (Zheng et al., 2010)。

对基性岩部分熔融的实验表明,玄武质岩石来源的熔体Mg#值通常小于45(Rapp et al., 1999)。宕昌火山岩大多具有较低的Mg#(19~35),但部分流纹岩样品具有相对偏高的Mg#(43~48)、Cr (49.9×10-6~54.4×10-6) 和铝饱和度 (A/CNK=1.57~1.66) 以及偏低的Ba/Th、Ba/La比值,并含有继承锆石,可能与极少量的幔源和上地壳物质混染有关。已有研究表明,幔源物质参与导致了西秦岭印支期花岗岩具有高Mg#(44~65) 特点 (张成立等,2008Qin et al., 2009Guo et al., 2012Zhu et al., 2011Luo et al., 2012)。由此说明,尽管宕昌火山岩中可能有部分岩石受到了幔源成分的混染,但其富K的特征却与岩浆遭受富K地幔物质的混染无直接关系 (岩石低Mg#),并且该火山岩不明显的分离结晶趋势 (图 5) 还暗示这种富K特征也不可能由岩浆演化所致,而是反映了西秦岭下地壳的固有属性,可能与源区富K矿物相 (如单斜辉石或黑云母) 的存在有关 (Rapp et al., 2002Xiao and Clemens, 2007),这与西秦岭下地壳为高K玄武质岩石的推论 (Zhang et al., 2007a) 相一致。

5.3 构造环境

从已有研究来看,多数学者认为阿尼玛卿-勉略洋盆在晚古生代末期开始向北俯冲消减,但对于西秦岭地区该洋盆闭合或大陆碰撞的时间仍存在争议,从而导致对西秦岭印支期造山过程及花岗岩类形成背景的认识产生分歧。一种观点认为西秦岭早中生代花岗岩类 (245~205Ma) 均形成于后碰撞环境,并且印支早期花岗岩形成于俯冲陆壳或洋壳断离环境,而印支晚期花岗岩则形成于地壳加厚导致的岩石圈拆沉环境 (张宏飞等,2006张成立等,2008Luo et al., 2012)。然而,Zhang et al.(2004)综合古沉积相演化、岩浆岩发育及构造变形变质记录认为,标志着古特提斯北支洋洋壳闭合的勉略缝合带的形成时代为中晚三叠世,这种认识得到了对川西北周缘前陆盆地的发育时代和同仁地区麦秀辉石安山岩等研究成果的支持,指示华北板块与扬子板块在西秦岭的初始碰撞可能发生在中三叠世晚期 (Meng et al., 2005Li et al., 2013)。还有观点则认为秦岭造山带大陆同碰撞的开始可能会推迟到晚三叠世的中后期 (Jiang et al., 2010Dong et al., 2012)。从西秦岭印支期侵入岩来看,少数线性分布的印支早期花岗岩类 (245~234Ma),如岗察杂岩体、夏河岩体及冶力关岩体等被认为是大陆边缘弧型花岗岩 (金维浚等,2005Guo et al., 2012),而点状或面状分布的印支晚期花岗岩类 (225~205Ma),如温泉、糜署岭、厂坝、黄渚关、憨班、金厂等岩体,其成因则可能与同碰撞或后碰撞伸展作用有关 (Qin et al., 2009Cao et al., 2011Wang et al., 2011Zhu et al., 2011刘志鹏和李建威,2012)。从区域对比来看,东秦岭和碧口地块罕见印支早期花岗岩类,而在印支晚期 (225~200Ma),东秦岭、扬子西北缘及松潘地区都有强烈的高钾钙碱性岩浆侵位活动以及广泛的幔源物质贡献,并且可能发生了板片断离或岩石圈拆沉活动 (Sun et al., 2002Wang et al., 2007Qin et al., 2008, 2010a, b张成立等,2008Cai et al., 2009李雷等,2012)。以上研究表明,虽然印支期西秦岭地区的大陆初始碰撞时间尚未得到准确限定,但阿尼玛卿-勉略洋盆在该区的俯冲至少可以延至中三叠世,并且在印支晚期,无论西秦岭还是东秦岭,直至松潘带和扬子西北缘均由大陆同碰撞环境逐渐转入了后碰撞伸展环境。

宕昌火山岩的形成时代 (229Ma) 处于印支早、晚两期花岗岩类之间,其动力学背景又应如何?金维浚等 (2005)对西秦岭印支早期的冶力关 (245Ma) 及夏河岩体 (238Ma) 的研究认为,这些花岗岩类具有埃达克岩地球化学特征,并推断其成因与印支早期古特提斯洋俯冲导致的活动陆缘加厚下地壳的熔融有关。然而,印支早期西秦岭是否广泛发育加厚陆壳来源的埃达克岩 (Defant et al., 2002Rapp et al., 2002) 仍需慎重考虑 (例如该岩体Sr=390×10-6~420×10-6,Y=20×10-6~20.4×10-6,Sr/Y=19.5~20.6,(La/Yb)N=11.4~11.8,δEu=0.69~0.71,金维浚等,2005),而事实上,区域上与加厚地壳有关的埃达克质岩石却出现于碧口地块的南一里和阳坝岩体及东秦岭的西岔河和五龙等岩体 (224~208Ma),形成时代均属印支晚期,指示该时期秦岭造山带地壳厚度已经超过50km (张成立等, 2005, 2008Zhang et al., 2007bQin et al., 2008, 2010a)。因此,综合考虑秦岭地区埃达克质岩体的形成时代 (224~208Ma)、勉略带蛇绿混杂岩的变质年龄 (242~220Ma,李曙光等,1996Li et al., 1999)、西秦岭拉丁期后 (235Ma) 发生的深水相突变为含 (粗) 碎屑浅水相的沉积相转换 (冯益民等,2002) 和230~210Ma的快速冷却事件 (郑德文等,2004) 以及松潘北缘诺利期 (228Ma) 发育海相磨拉石建造 (Zhang et al., 2004) 等事实,本文认为,中三叠世末至晚三叠世初,西秦岭在深部动力学体制上发生了伸展向挤压过渡的重大转换,并由此导致地壳开始缩短加厚,这可能暗示了华北与扬子板块之间的大陆同碰撞作用的开始,并在印支晚期逐渐转入后碰撞松弛阶段。同时,这种挤压体制也可以解释西秦岭造山带相对短暂的岩浆宁静期 (234~225Ma) 的出现,因为以强挤压为主的构造环境既不利于大量地壳物质的部分熔融及岩浆上升 (肖庆辉等,2002),也不利于强烈的壳幔物质交换或热场作用传递 (张国伟等,1997)。作为印支晚期大规模花岗质岩浆作用的前锋,宕昌火山 (229Ma) 显示出了同碰撞岩浆的过铝质和富Rb而低Yb等特征 (Pearce et al., 1984),指示西秦岭地区华北与扬子板块的初始碰撞可能接近229Ma,反映了以整体挤压为主、局部剪切伸展为辅的地壳逐渐加厚的深部动力学环境,这与该火山岩规模小 (出露面积23km2)、起源深度大 (~40km)、喷发强度大 (厚2000m且火山角砾多) 且受断裂带控制明显等特点十分吻合,也是大规模岩浆作用的早期岩浆所具有的高温、高挥发分的一种表现。

6 结论

(1) 宕昌酸性火山岩属于过铝质富钾岩石系列,喷发时代为晚三叠世 (229Ma),而非早中侏罗世。

(2) 西秦岭地块存在扬子型的中新元古代地壳基底,但不同地段的基底特征存在较大差异。宕昌火山岩起源于富钾基性下地壳的部分熔融,部分岩石还可能受到了少量地幔或上地壳物质的混染,源区深度约40km,岩石成分上的略微差异可归因于源区组分的不均一性所致。

(3) 西秦岭地区华北板块与扬子板块的初始碰撞可能接近229Ma,并在印支晚期逐渐转入后碰撞伸展环境。宕昌火山岩作为印支晚期大规模花岗质岩浆作用的前锋,形成于以整体挤压为主、局部剪切伸展为辅的地壳逐渐加厚的动力学背景。

致谢 本次研究的野外工作得到了中国地质大学 (北京) 陈永健的协助;实验处理及样品测试方面得到了中科院广州地化所刘颖老师、中国地质大学 (武汉) 陈海红及刘硕老师、北京大学朱文萍老师、天津地质矿产研究所耿建珍及肖志斌老师等人的指导和帮助;在此一并表示诚挚感谢!
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西秦岭宕昌地区晚三叠世酸性火山岩的锆石U-Pb年代学、地球化学及其地质意义
黄雄飞, 莫宣学, 喻学惠, 李小伟, 丁一, 韦萍, 和文言