大兴安岭南端芝瑞盆地流纹岩年代学、地球化学及岩石成因

解开瑞, 巫建华, *, 祝洪涛, 吴仁贵, 刘 帅



大兴安岭南端芝瑞盆地流纹岩年代学、地球化学及岩石成因

解开瑞1, 巫建华1, 2*, 祝洪涛3, 吴仁贵1, 刘 帅2

(1. 东华理工大学 地球科学学院, 江西 南昌 330013; 2. 东华理工大学 核资源与环境国家重点实验室培育基地, 江西 南昌 330013; 3. 核工业243地质大队, 内蒙古 赤峰 024006)

芝瑞火山盆地位于大兴安岭南端内蒙古克什克腾旗境内, 处于西拉木伦河-长春缝合带以南、赤峰-开源断裂带以北的辽源地块, 盆地中的长英质火山岩系不整合于下二叠统大石寨组之上、新近系汉诺坝组玄武岩之下, 并见有多处花岗斑岩侵入其中。SHRIMP锆石U-Pb年龄表明, 盆地火山岩系中流纹岩锆石的206Pb/238U年龄为(156.9±1.7) Ma, 属于晚侏罗世早期岩浆活动的产物。岩石地球化学资料表明, 流纹岩具富SiO2、K2O和高FeOT/MgO值, 低Al2O3、CaO和MgO的特征, 属于高钾钙碱性系列岩石, ∑REE含量较高, 轻重稀土分馏明显, 具右倾的稀土分布模式, 负Eu异常明显, 具有Ga、Zr、Nb、Y含量高和Ba、Sr含量低的微量元素特征, 相对富集Rb、Th、U、Pb、Zr、Hf, 相对亏损Ba、Sr、P、Ti及Nb、Ta等元素, 具有A型流纹岩和低Sr-Ba流纹岩的微量元素特征; 在Nb-Y-3Ga和Rb/Nb-Y/Nb图解上显示拉张构造环境的A2型花岗岩的特征; 具有较高的Sr初始比值((87Sr/86Sr)i=0.706510~0.709821), 较低的Nd初始比值(Nd() = ‒6.32 ~ ‒4.44)和相对较年轻的Nd模式年龄(1304~1457 Ma)及较低的Pb同位素组成((206Pb/204Pb)t=17.15~17.80、(207Pb/204Pb)t= 15.41~15.48、(208Pb/204Pb)t= 37.38~37.63), 在Sr-Nd同位素示踪图解和Pb同位素构造演化图解上流纹岩同时具有下地壳和富集地幔印记。元素、同位素地球化学示踪指示芝瑞流纹岩可能由起源于富集地幔的中元古代年轻下地壳的部分熔融形成, 且在岩浆上升过程中经历了结晶分异作用。在(Yb+Ta)-Rb和(Y+Nb)-Rb图解上, 芝瑞流纹岩均显示板内拉张构造环境, 结合区域上分布的同时代火山岩及A型花岗岩类可以限定流纹岩形成于伸展构造背景, 并可能与北部的蒙古-鄂霍茨克缝合带的演化有关。

流纹岩; 岩石成因论; 地球化学; Sr-Nd-Pb同位素; 晚侏罗世早期; 芝瑞

0 引 言

近NNE向展布的大兴安岭-燕山中生代火山-侵入岩带位于华北古板块与西伯利亚古板块碰撞形成的兴蒙造山带东段, 该带面积广阔、岩石类型多样、地球化学特征复杂且与金属成矿关系密切, 备受国内外地质工作者所瞩目。年代学研究揭示, 大兴安岭-燕山中生代火山-侵入岩带的岩浆活动可分为七个不同的期次[1‒3], 岩石类型研究表明, 该带的长英质火山-侵入岩包括高Ba-Sr型、低Ba-Sr型、A型、I型、埃达克型等类型[1, 4‒8]。该带的长英质火山-侵入岩的岩石成因至少有以下四种: (1)由不同性质的下地壳物质[8‒14]或是浅部地壳物质[1]及古俯冲蚀变洋壳[15]部分熔融而成; (2)中基性岩浆的结晶分异成因[1, 12]; (3)岩浆由不同时期的地壳物质与不同性质的地幔物质混合[5, 16]或是新老地壳物质混合成因[4]; (4)岩浆混合再结晶分异成因, 如岩石圈地幔部分熔融形成的玄武质岩浆和下地壳部分熔融形成的酸性岩浆混合产物的分离结晶[17]。Sr-Nd-Pb同位素地球化学研究表明, 该带的长英质火山-侵入岩的同位素组成存在空间上差异性: (1)塔河-喜桂图断裂带以北的额尔古纳地块, 长英质火山-侵入岩具有相对较高的(87Sr/86Sr)i、负低且变化大的Nd()、较大的DM2、较高且变化较小铅同位素组成的特征[6,18‒21]; (2)塔河-喜桂图断裂带与贺根山-黑河断裂带之间的兴安地块, 长英质火山-侵入岩具有相对较低的(87Sr/86Sr)i、较正高且变化大的Nd()值、较小DM2、较高且变化区间较大的铅同位素组成的特征[6,18‒22]; (3)贺根山-黑河断裂带与西拉木伦河-长春缝合带之间的松辽地块, 长英质火山-侵入岩具有较高的(87Sr/86Sr)i、高的Nd()、小的DM2、较高的铅同位素组成[4,6,17,21‒25]; (4)赤峰-开源断裂带以南的燕山板內造山带, 长英质火山-侵入岩具有较高且变化区间大的(87Sr/86Sr)i、负低的Nd()、较大的DM2、低的(206Pb/204Pb)i、(207Pb/204Pb)i与(208Pb/204Pb)i特 征[5,16,18,20,24,26]。然而, 西拉木伦河-长春缝合带与华北克拉通北缘赤峰-开源断裂带之间的辽源地块, 长英质火山-侵入岩的年代学格架, 岩石组合组合类型与地球化学特征、物质来源与构造背景, 铀成矿背景的分析研究相对薄弱[26‒27]。本次工作选择芝瑞盆地流纹岩为研究对象, 拟通过年代学、地球化学和Sr-Nd-Pb同位素的系统研究, 对流纹岩的成因进行分析, 为揭示华北克拉通北缘中生代的构造属性提供依据, 进而加深对大兴安岭-燕山中生代火山-侵入岩带成因规律的认识。

1 区域地质背景

自古亚洲洋闭合之后, 东北地区经历了环太平洋构造体系和蒙古-鄂霍茨克构造体系的叠加与改造作用[7,28‒34], 中国东北部中生代火山岩的时空分布与岩石组合的综合研究揭示了两大构造体系演化的多阶段性特点[3,29,30], 以中侏罗世早期和侏罗纪末期两次陆壳加厚过程和中侏罗世晚期-晚侏罗世早期和早、晚白垩世的区域性伸展事件与鄂霍茨克洋关闭和太平洋板块的俯冲及俯冲间歇期匹配[29,33,35]。综合俯冲距离(大于1500 km) 问题以及中生代不同时期的火山岩时空分布特征, 大兴安岭-燕山中生代火山-侵入岩带中生代大规模火山活动属古太平洋板块远程俯冲作用产物的认识[1,30]受到质疑, 特别是对晚侏罗世时期环太平洋构造体系的影响范围存在较大争议[3,29,33‒35]。部分学者认为环太平洋构造体系中生代对东北亚大陆影响的空间范围主要在松辽盆地及以东地区[7,29,33‒35], 蒙古-鄂霍茨克构造体系的影响主要存在于松辽盆地以西包括大兴安岭及额尔古纳地区[14,29,33,35], 并可能扩展至华北北缘及燕山板内地区[33,34,36,37], 这种影响得到松辽盆地以西地区区域发育的的中侏罗世晚期-晚侏罗世早期火山岩研究的支持[1,2,13,29,34,38‒42], 但是, 除去燕山造山带地区, 分布于胶东、辽东半岛, 延边-辽北、朝鲜及华北克拉通南缘地区的中-晚侏罗世火山-侵入岩及垂直于华北克拉通边界的非惟一收缩变形指示着当时可能存在多向构造体系作用[30, 31, 43]。晚侏罗世晚期的挤压及早白垩世西迄蒙古-鄂霍茨克缝合带、东抵太平洋之滨以双峰式火山岩组合、变质核杂岩、A型花岗岩等代表的巨型地壳伸展省, 可能与环太平洋构造体系和蒙古-鄂霍茨克构造体系的双重影响相关联[7,30,32‒34,36,37]。

地处赤峰市克什克腾旗芝瑞乡的芝瑞盆地, 位于西拉木伦河-长春缝合带以南、赤峰-开源断裂带以北的辽源地块上, 属大兴安岭与燕山火山-侵入岩带的结合部位, 是沽源-红山子铀成矿带北端铀矿勘查区。盆地内构造以断裂发育为主, 断裂构造主要有EW、NE、NW、SN向四组, 其中以NE向断裂构造最为发育, 主要见于广兴源-大兴永地区, 芝瑞地区因玄武岩覆盖构造形迹不甚清晰。其中盆地北部受大兴永-南窝铺断裂带控制,中带受百岔河断裂带控制,南部受笔连沟断裂控制, 整体呈NE向展布, 每条带长几至十几km, 宽约1 km。盆地NW为广兴源复式岩体, 岩体主体由二长岩、二长花岗岩、花岗岩和花岗闪长岩组成, 岩体早期的二长岩和侵位于二长岩的花岗闪长岩的SHRIMP锆石U-Pb年龄为(263.8±2.1) Ma和(263.3±2.5) Ma[44]。盆地内以晚中生代火山岩系为主, 不整合于前中生代地层之上, 被新生代地层不整合覆盖。盆地NW及SW处靠近盆地边缘, 剥蚀程度较大、出露有下二叠统大石寨组中-酸性火山岩系或燕山早期花岗岩及多期侵入岩脉, 大石寨组下段为灰-灰褐色英安质晶屑凝灰岩、紫红-褐色熔结凝灰岩, 上段为紫色、灰褐色安山岩, 厚约150~300 m; 在盆地西部边缘广兴源-大兴永一带和芝瑞地区的沟谷中为中生代长英质火山岩系出露地区(图1b), 火山岩系总体走向呈NE25°左右, 可分两部分, 下部为灰色酸性凝灰岩及灰紫色、灰黄色凝灰质粉砂岩夹泥灰岩透镜体及薄层砂岩、砾岩为主, 厚40~200 m, 上部为浅灰-灰紫色流纹岩、流纹质熔结凝灰岩, 厚200~500 m, 见有花岗岩、花岗斑岩等侵入体及次火山岩侵入到火山岩系内部。芝瑞地区火山岩系顶部被汉诺坝组和第四系覆盖, 上新统汉诺坝组(N2h)为黑绿色玄武岩夹薄层砂岩、砾岩、泥煤、柴煤, 第四系(Q)主要为风成黄土及砂、砾石等。

本文研究流纹岩样品取自核工业243大队在芝瑞镇吕家沟门一带实施的多个钻孔, 样品新鲜。流纹岩呈灰紫色, 斑状结构, 流纹构造明显, 斑晶以长石为主, 基质为隐晶质。显微镜下观察(图2), 岩石具有斑状结构, 斑晶为石英及碱性长石, 碱性长石具有卡式双晶, 表面不干净, 可见部分黏土矿化现象, 斑晶发育熔蚀反应边, 少量暗色矿物黑云母已发生绿泥石化。基质具有包含霏细结构, 由石英作主晶包含碱性长石霏细状质点构成, 岩石中发育少量硅质条带定向分布, 少量铁质质点沿石英主晶接触部位分布, 副矿物包括锆石、磷灰石和磁铁矿等。

2 分析方法

定年样品编号为ZR208(42°48′34″N,117°47′19″E), 首先把样品破碎, 经浮选和电磁选等方法后, 淘洗、挑纯挑出锆石晶体。之后将挑选好的待测锆石与标准锆石TEM(年龄为417 Ma)一起粘贴, 制成环氧树脂样品靶。干燥后, 打磨、抛光使锆石中心部分暴露, 然后进行反射光、透射光和阴极发光显微照相及SHRIMP锆石U-Th-Pb分析。锆石挑选由河北省廊坊市诚信地质服务有限公司完成, 锆石SHRIMP U-Th-Pb分析在北京离子探针中心SHRIMP-Ⅱ上完成, 具体分析工作流程及处理流程见宋彪等[45]资料, 芝瑞盆地流纹岩的锆石U-Th-Pb分析结果列于表1。

图1 芝瑞盆地地理位置(a)及地质略图(b)

1‒第四系; 2‒新近系汉诺坝组; 3‒上侏罗统新民组; 4‒下二叠统大石寨组; 5‒晚二叠世花岗闪长岩; 6‒晚二叠世石英二长闪长岩或石英二长岩; 7‒中生代花岗岩; 8‒花岗斑岩; 9‒花岗岩脉; 10‒整合地质界限; 11‒角度不整合地质界线; 12‒断层; 13‒居民点

1‒Quaternary; 2‒Neogene Hannuoba Formation; 3‒Upper Jurassic Xinmin Formation; 4‒Lower Permian Dashizhai Formation; 5‒Late Permian granodiorite; 6‒Late Permian quartz-diorite and quartz monzonite; 7‒Mesozoic granite; 8‒granitic porphyry; 9‒granite vein; 10‒geological boundary; 11‒uncomformity; 12‒fault; 13‒settlement

图2 芝瑞盆地流纹岩显微镜下照片

(a) 流纹构造(-); (b)、(c) 斑状结构, 斑晶为钾长石和石英, 边部可见溶蚀反应边, 基质霏细结构(+); Afs‒碱性长石, Qtz‒石英

(a) rhyotaxitic structure; (b),(c) porphyritic texture, porphyritic texture contains alkali feldspar and quartz, and the matrix has felsitic texture; Afs‒alkali feldspar; Qtz‒quartz

表1 芝瑞盆地流纹岩SHRIMP锆石U-Th-Pb同位素分析结果

注:206Pbc和206Pb＊分别表示普通铅和放射性成因铅; 普通铅根据实测204Pb进行校正; 误差为1σ

样品全岩地球化学分析测试工作在湖北省武汉综合岩矿测试中心完成。除FeO含量采用硫酸-氢氟酸溶矿, 重铬酸钾滴定法测得外, 其余主元素采用X射线荧光熔片法(XRF) 测定, 测试仪器为X荧光光谱仪(Magix-pro2440), 样品采用无水四硼酸锂作为溶剂, 分析精度优于2%; 微量元素Sr、Ba用电感耦合等离子体发射光谱法(ICP-OES)测定, 测试仪器为等离子体发射光谱仪(ICAP6300), 其余微量元素和稀土元素均采用电感耦合等离子体质谱法(ICP-MS)测定、测试仪器为X7型电感耦合等离子体质谱仪(Thermoele-mental X7)。对USGS国际标准样品(BHVO-2)的测定结果表明, 样品测定值和推荐值的相对误差小于10%, 且大多数微量元素的分析误差在5%以内, 各分析方法的实验流程及其误差、精度等详见文献[46]。

Sr-Nd-Pb同位素比值测试工作在中国科学院地质与地球物理研究所重点实验室Finnigan MAT 262固体表面热电离质谱计上完成, Sr、Nd同位素分析采用传统的阳离子交换树脂法分别分离富集Sr和Nd元素, 采用146Nd/144Nd=0.7219和86Sr/88Sr=0.1194标准化校正测得Nd和Sr同位素比值。Nd和Sr同位素国际标样AMS和NBS987的测试值分别为143Nd/144Nd= 0.512139±18 (2σ,=28)和87Sr/86Sr = 0.710255±16 (2σ,=33)。铅同位素采用阴离子交换树脂来分离富集, 铅国际标样NBS981的测定结果为207Pb/206Pb=0.9139±4 (2σ,=65)。

3 分析结果

3.1 锆石U-Pb定年

芝瑞盆地流纹岩中的锆石呈自形短柱状或双锥状, 晶形较完整, 阴极发光图像(CL)显示锆石具有条带结构(图2a), 具有典型岩浆结晶锆石的内部结构特征, 锆石颗粒晶形较完整, 颗粒长度约90~140 μm,长宽比约1.3~1.6, 进行测试时所有分析点均处于岩浆环带部位(图3a), 分析结果显示普通Pb含量为0.52%~4.92%, U含量为51~161 μg/g, Th含量为25~ 92 μg/g, Th/U值变化于0.44~0.77, 平均值为0.58 (＞0.4), 具有典型的岩浆锆石成分特征[47]。13个分析点的206Pb/238U年龄数据中(表1), 除分析点1.1与3.1偏离数据组之外, 其余11个测点年龄为155~161 Ma, 且数据投影点在U-Pb谐和曲线图上均位于谐和线附近(图3b), 剔除点1.1和3.1之后剩余的11个数据给出的206Pb/238U年龄加权平均值为(156.9±1.7) Ma (MSWD=0.37), 该年龄代表了流纹岩的形成时代。

3.2 主元素和微量元素

3.2.1 主元素

芝瑞盆地流纹岩主元素和微量元素分析结果见表2。芝瑞盆地流纹岩具有富硅(SiO273.9%~ 75.8%)、富钾(K2O 4.55%~6.44%), 低MgO、CaO、P2O5的特征, 属于钾质岩石(K2O/Na2O 1.07~2.24, Na2O-270%)、富钾(K2O 4%~6%或更高)、低铝(12%~13%)、贫钙和镁的特点[52]。里特曼指数介于2.08~2.77之间, 平均为2.59, 属于碱性-钙碱性系列岩石(图5a)。

图4 芝瑞盆地流纹岩TAS图解(a)及A/NK-A/CNK图解(b)

TAS图解底图据[48], A/NK-A/CNK底图据[49]

TAS from [48]; A/NK-A/CNK from [49]

3.2.2 微量元素

芝瑞流纹岩稀土元素含量较高, 富集轻稀土元素, 重稀土分馏不明显, Eu强烈亏损。SREE= 465~573 μg/g,SLREE=414~520 μg/g,SHREE= 47~ 54 μg/g, (La/Yb)N=8.62~10.9, (La/Sm)N= 4.12~4.34, (Gd/Yb)N=1.35~1.55, Eu/Eu*= 0.08~0.09(均值0.08)。在球粒陨石标准化稀土元素(REE)分布图解上 (图6a[53]), 流纹岩样品呈“右倾”的“V”字形, 具有与A型花岗岩及A型流纹岩[42,52,54]相似的稀土分布模式, 轻重稀土分馏明显, 轻稀土具有一定程度的分馏, 重稀土呈比较平缓的分布模式, Ce不具备正异常而负Eu异常显著。

流纹岩高场强元素Nb、Ta、Zr、Hf、Ce、Y和大离子亲石元素Rb、Th、U的含量较高, Rb 142~ 240 μg/g (平均值196 μg/g), Th 19.2~26.3 μg/g (平均值23.3 μg/g), Pb 10.7~24.6 μg/g(平均值17.9 μg/g), Zr 700~811 μg/g (平均值为764 μg/g), Hf 18.8~23.2 μg/g(平均值21.2 μg/g); Nb 34.3~50.5 μg/g (平均值44.4 μg/g), Ta 2.24~3.47 μg/g (平均3.05 μg/g); 采用原始地幔成分[53]为标准, 对芝瑞盆地流纹岩样品的微量元素含量进行标准化作图。微量元素蛛网图解(图6b)上可以看出, 流纹岩富集Rb、Th、U、Pb、Zr、Hf等元素, 而Ba、Sr、P、Ti和Nb、Ta都表现出明显的负异常, Ba含量为59.6~104 μg/g (平均值81.4 μg/g), Sr含量为17.8~27.2 μg/g (平均值22.4 μg/g); Ga含量较高, 为23.9~27.5 μg/g (均值为25.2 μg/g), 10000Ga/Al值介于3.73~4.11 (均值为3.87, 大于2.6), Zr+Nb+ Ce+Y含量为987~1156 μg/g (平均1084 μg/g, 大于350 μg/g), 具有典型A型花岗岩微量元素特征[55‒57], 与东北地区A型花岗岩[54]、碾子沟A型花岗岩[55]以及大兴安岭地区A型流纹岩[13, 42, 51]相似。在微量元素Harker图解上(图略), 芝瑞流纹岩Rb、Ba、Zr、Hf、Nb、Th、Yb随SiO2含量的升高表现出降低的趋势, 而Sr、U、Pb随SiO2含量的升高而变化不大。

图5 芝瑞盆地流纹岩岩石系列(Na2O+K2O-CaO)-SiO2图解(a)和(FeOT/MgO)-SiO2图解(b)

(Na2O+K2O-CaO)-SiO2图解底图据[50], 海莫赛格流纹岩数据引自文献[42], 红山子流纹岩数据引自文献[51]

(Na2O+K2O-CaO)-SiO2data are quoted from [50], the data for the Haimosaige and Hongshanzi rhyolites are quoted from [41] and [51], respectively

图6 芝瑞盆地流纹岩稀土元素球粒陨石标准化图解(a)与微量元素原始地幔标准化蛛网图(b)

球粒陨石及原始地幔标准化值据文献[53], 海莫赛格流纹岩、红山子流纹岩及A型花岗岩数据分别引自文献[42,51,54]

Chondrite and primitive mantle data are quoted from reference [53], the data for rhyolites from Haimosaige and Hongshanzi and A-type granite are quoted from references[42, 51, and 54], respectively

表2 大兴安岭芝瑞盆地流纹岩主元素(%)、微量元素(μg/g)分析结果及有关参数

3.3 Sr-Nd-Pb同位素特征

芝瑞盆地流纹岩的Sr-Nd-Pb同位素分析结果列于表3。3个流纹岩样品的(87Sr/86Sr)i变化于0.706510~ 0.709821之间,Sr()值为28.53~75.53(平均51.20), (143Nd/144Nd)i=0.512113 ~ 0.512210,Nd()=–6.32 ~ –4.44, ƒSm/Nd= ‒0.42 ~ ‒0.40, 在‒0.6 ~ ‒0.2之间, 给出的有明确地质意义的DM2值变化于1304~1457 Ma之间; 铅同位素组成较低, (206Pb/204Pb)t、(207Pb/204Pb)t和(208Pb/204Pb)t分别为17.15~17.80、15.41~15.48和37.38~37.63。

表3 大兴安岭芝瑞盆地流纹岩Sr-Nd-Pb同位素分析结果及有关参数

注: Sr、Nd、Pb同位素比值年龄校正时采用U-Pb年龄156 Ma。计算Nd()、Sr()和分馏因子ƒSm/Nd的过程中, (87Sr/86Sr)UR=0.7045, (87Rb/86Sr)UR= 0.0827;(143Nd/144Nd)UR=0.512638, (147Sm/144Nd)CHUR=0.1967;亏损地幔两阶段模式年龄(DM2)的计算公式为:DM2=(1/•ln{1+[(143Nd/144Nd)样品– (143Nd/144Nd)DM–[(147Sm/144Nd)样品–(147Sm/144Nd)CC](e–1)]/[(147Sm/144Nd)cc–(147Sm/144Nd)DM], 式中, 下角标“样品”代表样品测试值, “CC”代表地壳。(147Sm/144Nd)CC=0.118, (143Nd/144Nd)DM=0.51315, (147Sm/144Nd)DM=0.2137;Sr=[(87Sr/86Sr)样品/(87Sr/86Sr)UR–0.7]×104。(206Pb/204Pb)i= (206Pb/204Pb)s–(U/Pb)s×(Pb/U)×(238U/204Pb)s(e238t–1); (207Pb/204Pb)i= (207Pb/204Pb)s–[(U/Pb)s×(Pb/U)×(238U/204Pb)s/137.88](e235t–1); (208Pb/204Pb)I= (208Pb/204Pb)s–(Th/Pb)s×(Pb/Th)×(232Th/204Pb)s(e232t–1)。衰变常数采用(147Sm)=6.54×10‒12/a,(87Rb)=1.42×10‒11/a,238U、232Th、204Pb的相对丰度(%)分别为238U=99.2739%、232Th=100%、204Pb的相对丰度可以计算得出,235U=238U/137.88= 0.7200%; U、Th、Pb的原子量分别为U=238.03、Th=232.08、Pb=206.42;238=1.55125×10‒10/a、235=9.8485×10‒10/a、232=0.49475×10‒10/a

4 讨 论

4.1 形成时代和岩石地层单位归属

最近研究结果显示, 大兴安岭-燕辽火山岩带中侏罗世晚期-晚侏罗世早期火山岩地层以发育浅色沉积岩和火山岩为特征, 呈不整合或平行不整合伏于土城子组紫红色粗碎屑岩之下[2, 27], 其中贺根山-黑河断裂带以北的大兴安岭北部地区及赤峰-开源断裂带以南的燕山板内造山带分别以塔木兰沟组和髫髻山组(蓝旗组)镁铁质火山岩组合为特征, 火山岩的年龄为163~150 Ma[2‒3]; 两者之间的松辽地块及辽源地块为新民组高钾钙碱性流纹岩-碱性流纹岩组合为主的一套酸性火山熔岩, 火山岩的年龄为165~150 Ma[2, 27]。

芝瑞盆地的火山岩系长期被视为流纹岩-粗面岩组合, 并可与燕山板内造山带的张家口组或大兴安岭地区兴安岭群对比[58]。新的研究表明, 张家口组或兴安岭群整合于土城子组紫红色碎屑岩系之上, 时代属早白垩世[2,3,27,59]。芝瑞盆地火山岩系底部以火山岩夹灰、灰黄色沉积岩, 上部为浅灰-灰紫色流纹岩、凝灰岩为特征, 剖面结构与红旗组、新民组构成的剖面结构可以对比, 不同于土城子组及其上火山岩组合特征[25, 58], 流纹岩SHRIMP锆石U-Pb年龄为(156.9±1.7) Ma, 地质时代属于晚侏罗世早期。可见, 芝瑞盆地火山岩的岩石组合和地质时代与大兴安岭南部上侏罗统新民组[2,27,42]一致, 而与冀北早白垩世早期张家口组或大兴安岭地区兴安岭群的岩石组合和地质时代[2,3,59]不同, 应将芝瑞地区火山岩系归入新民组。

4.2 岩石成因

4.2.1 物质来源

(1) 主元素、微量元素制约 芝瑞盆地流纹岩具有弱过铝质特征, 流纹岩高SiO2、FeOT/MgO、Rb和Nb且低MgO和CaO(Al2O3)含量同时具有Eu负异常的特征显示其由地壳浅部钙碱性花岗质脱水熔融而成[60]。Rb/Sr、Ti/Y和Ti/Zr值分别为7.56~11.1 (平均8.75)、19.7~23.7(平均21.0)和1.85~ 2.15(平均1.94), 位于壳源岩浆(Rb/Sr>0.5, Ti/Y<100, Ti/Zr<20)范围[61‒63]内, 是陆壳熔融的产物; 同时, 流纹岩亏损Ba、Nb、Ta而富集Pb、Zr的微量元素特征指示岩浆源于地壳。芝瑞流纹岩具有低Sr、高Yb(7.16~8.90 μg/g)含量的特征, 属于非常低Sr高Yb型即南岭型花岗岩(相当于A型花岗岩), 残留相为斜方辉石+高钙斜长石, 压力小于0.8 GPa, 形成的地壳厚度小于30 km[64]; 本区流纹岩的HREE之间的分馏并不明显, 它们呈比较平缓且略下凹的分布模式, 表明部分熔融后的残留物中含有角闪石而不含石榴子石[65]。根据实验岩石学成果, 角闪石和斜长石作为部分熔融残留相且又不发生反应形成石榴子石的温度条件介于850~1100 ℃, 压力小于1.0 GPa, 表明岩浆起源小于35 km[65‒66]。上述特征表明芝瑞流纹岩的岩浆可能起源于下地壳浅部。

(2) Sr、Nd、Pb同位素制约 芝瑞流纹岩较高的(87Sr/86Sr)i值和负Nd()值(‒6.32 ~ ‒4.44)指示岩浆来源与新生地壳或富集地幔有关[67],Nd()值与富集地幔的Nd()值(‒13 ~ ‒8.0[68‒70])及华北克拉通北缘幔源碱性玄武岩的Nd()值(‒5[26])和燕辽地区由富集地幔物质参与形成的中生代侵入岩的Nd()值[60,71‒74]接近(图7a[75‒76]); 在(143Nd/144Nd)i-(87Sr/86Sr)i图解(图7b[77‒78])上, 样品投影点靠近EMⅠ富集地幔区域, 在铅同位素模式图解 (图8[55,77,78])中, 样品投影点落于北回归线之上的下地壳区域, 并靠近于EMⅠ, 指示来源与下地壳密切相关, 且有富集地幔组分参与形成。但流纹岩Nd()值高于华北克拉通古老下地壳(‒44 ~ ‒32)[76], 低于兴安地块及松辽块体较高的正Nd()值[4,6], 与汉诺坝二辉麻粒岩包体的Nd()值(‒18 ~ ‒8)[75]接近; 同时, 流纹岩二阶段Nd模式年龄(1304~1457 Ma)高于兴安地块及松辽地块的DM2值(520~1180 Ma[6,18]), (206Pb/204Pb)t整体小于兴安地块及松辽地块(17.96~18.58[19,21,24]), 指示流纹岩岩浆源区物质与华北克拉通古老下地壳及兴蒙造山带显生宙新生地壳的关系较远, 可能为中元古代年轻下地壳物质。研究已经证实, 汉诺坝二辉麻粒岩包体是幔源基性岩浆底侵到下地壳底部构成的年轻下地壳的一部分[75,79], 新生代时被汉诺坝玄武岩浆以包体形式带到了地表[9], 这不仅说明年轻的基性麻粒岩下地壳确实存在, 而且为解释芝瑞流纹岩同时带有富集地幔和下地壳物质印记提供了依据, 即源于EMⅠ富集地幔的年轻下地壳经部分熔融形成的岩浆上升到地表形成了芝瑞流纹岩。

对燕山陆内造山带中元古代与富集地幔有关的富碱侵入岩类的研究显示[80‒81], 这些岩体的 (87Sr/86Sr)i多数小于0.7053,Nd()为‒7.5 ~ ‒3.4,206Pb/204Pb< 15.70, 芝瑞流纹岩具有比这些碱性岩类稍高的Sr和Pb同位素组成, 那么这种变化是如何形成的呢？据邵济安等[82]研究, 华北克拉通北缘地壳自中生代开始受到幔源岩浆的改造作用, 这些经受改造的地壳物质以晚三叠世赤峰地区堆晶岩包体和河南营子镁铁质麻粒岩为代表, 堆晶岩包体(87Sr/86Sr)i=0.7056~0.7065;Nd()= ‒6.8~ ‒3.4,DM=1586 ~ 1317 Ma, 麻粒岩Nd()= ‒10 ~ ‒9,DM约为1800 Ma, (206Pb/204Pb)t、(207Pb/204Pb)t和(208Pb/204Pb)t分别为17.29~17.49、15.49~ 15.53、37.44~ 38.00[83‒84]。芝瑞流纹岩同位素组成与这些经受改造的地壳相近, 其比中元古代碱性岩类稍高的Sr、Pb同位素组成可能是这种改造作用引起的。

另外, 芝瑞流纹岩的同位素组成与西拉木伦晚侏罗世碾子沟花岗岩[55]、华北克拉通北缘河坎子铁质正长岩[71]、雾灵山A型侵入杂岩[60]、千层背花岗岩[60]、大少冷花岗岩[73]、对面沟石英二长岩[72]、矾山正长岩[74]、甲山正长岩[16]、东猴顶钾长花岗斑岩[5]等中生代侵入体虽有差异, 但整体相对一致。如何解释这些异同点呢？具体研究显示, 源于富集地幔物质与地壳物质混合之后演化而成对面沟、雾灵山、河坎子、房山等侵入体[9, 60,71,72], 东猴顶和甲山碱性侵入体由源于亏损地幔物质与下地壳物质混合演化形成[5,16]。笔者认为, 两阶段模式能解释这些火成岩的Sr-Nd-Pb同位素特征, 即元古宙不同时期的幔源岩浆与少量古老下地壳物质混合形成年轻下地壳并且在中生代时期受到幔源岩浆改造, 这种经受改造的地壳物质经部分熔融形成了上述火山岩和侵入岩。

图7 芝瑞盆地流纹岩εNd(t)-(87Sr/86Sr)i和(143Nd/144Nd)i-(87Sr/86Sr)i图解

汉诺坝麻粒岩的范围据[75], 华北克拉通下地壳、上地壳的范围据文献[76], DMM、EMⅠ、EMⅡ、HIMU和原始地幔为Hart[77]和Zindler.[78]定义的地幔端元, 燕山中生代侵入岩据文献[59, 71,73], 碾子沟容矿花岗岩据文献[55], 东猴顶钾长花岗斑岩据文献[5]

图8 芝瑞盆地流纹岩207Pb/204Pb-206Pb/204Pb(a)和208Pb/204Pb- 206Pb/204Pb(b)图解

图8a底图据文献[55]。DMM代表亏损地幔端元, EM(I、II)代表富集地幔端元[78]; NHRL代表北半球参考线[77]。对面沟, 碾子沟、东猴顶、甲山侵入岩数据据文献[5,16,55,72]

4.2.2 岩浆过程

在Harker图解(图略)上, 流纹岩全岩主元素、微量元素与SiO2的线性相关性较好, 随着SiO2的增高, Al2O3、MnO、FeOT、MgO、CaO、TiO2表现出明显的负相关性, 指示铁镁矿物和斜长石的结晶分异或是部分熔融时这些矿物作为残留相留在源区。贫钠以及低铝含量表明岩浆发生过斜长石的分离结晶作用或是部分熔融过程中在源区残留了斜长石; 对岩浆分异作用敏感的某些微量元素(Cr、La、Yb、Nb、Th等)与SiO2之间线性相关性较好, 指示岩浆形成过程中分异作用占据重要地位[85]。较低的Sr和Ba含量、极强的Eu负异常及很高 Rb/Sr值和DI值, 说明母岩浆在上升过程中经历过大量的长石分离结晶作用[86], 在La/Sm-La关系图(图略)上样品变化趋势线与分离结晶趋势相一致, 也指示流纹岩演化过程中发生了分离结晶作用。

4.2.3 构造环境

地球化学特征显示芝瑞流纹岩属于钾质岩石, 具备A型流纹岩特征, 锆石饱和温度计[87]研究显示芝瑞流纹岩的锆石饱和温度为936~954 ℃(平均944 ℃), 指示具有A型花岗岩一致的高温特征。在Rb-(Y+Nb)和Rb-(Yb+Ta)判别图(图9a,图9b)上均落入WPG板内花岗岩区, 并且显示为Post-COLG[88]; 其Y/Nb值介于1.3~1.93之间, 平均为1.55(>1.2), 符合A2型花岗岩的化学分类, 在构造环境判别图解Nb-Y-3Ga和Rb/Nb-Y/Nb(图9c、图9d)上, 样品落入A2造山后环境区域[89], 说明流纹岩形成于板内伸展构造背景[56,88,89], 与软流圈上涌和岩石圈伸展-减薄作用有关[37]。一般认为, 拉张背景下幔源岩浆底侵所带来的外部热量是地壳能够发生高温部分熔融的关键[56, 66], 大兴安岭白垩纪A型花岗岩具有高的锆石饱和温度, 其高热的产生被认为可能与伸展构造体制下岩石圈减薄导致的地幔岩浆底侵有关[90], 进一步说明晚侏罗世芝瑞地区处于伸展减薄的环境之下, 可能存在幔源岩浆的底侵作用。

图9 芝瑞流纹岩构造环境判别图解

(a)、(b)底图据文献[88]; VAG代表火山弧花岗岩; WPG代表板内花岗岩; ORG代表洋中脊花岗岩; Syn-COLG代表同碰撞花岗岩; Post-COLG代表后碰撞花岗岩。(c)、(d)底图据文献[89], A1代表非造山环境, A2代表造山后环境

(a), (b) after reference [88]; VAG ‒volcanic arc granite; WPG ‒within-plate granite; ORG – ocean ridge granite; Syn-COLG‒ syn-collision granite; Post-COLG‒ post-collisional granite. (c), (d) after reference [89], A1‒ nonorogenic setting, A2‒ post-orogenic setting

在侏罗纪的中末期, 包括研究区在内的大兴安岭-燕山地区处于“东亚多向汇聚”的构造体制 中[36‒37], 伴随北部西伯利亚与华北-蒙古联合陆块碰撞形成蒙古-鄂霍茨克构造带作用下[36,37,91], 蒙古-鄂霍茨克洋关闭使得北西侧的结晶基底推覆于中侏罗世含煤沉积岩之上[28], 同时形成孙吴地区的中侏罗世白云母花岗岩和埃达克质岩石[91], 并使其南部的燕山构造带在中侏罗世发生南北向挤压运动, 形成冀北-辽西地区广泛发育的自北向南的逆冲构造及区域性地层不整合[33,34,36,37]。中侏罗世末期-晚侏罗世早期, 松辽盆地以西地区进入后造山伸展活动, 加厚陆壳坍塌或拆沉形成火山活动[33‒34], 伴随有相应的侵入岩发育[31‒33], 并且火山活动具有自大兴安岭北部向南部冀北辽西地区变年轻的趋 势[3, 34], 岩石组合上具有北部以塔木兰沟组、中南部以新民组和南部冀北辽西地区以髫髻山组火山岩为代表的中基性-酸性-中基性火山岩为主的南北分带特征[2,3,13,27,29,38‒42,51], 火山岩和侵入岩主要属于碱性-钙碱性系列岩石, 具有A型花岗岩特征[7,13,33,38‒40,42], 是蒙古-鄂霍茨克洋闭合造山后岩石圈伸展环境下的产物[29,33,38,40]。对东北亚地区一些典型变质核杂岩核部侵入岩的研究表明, 由于受到鄂霍茨克不对称造山影响, 这些地区中下地壳层次的伸展可能在中晚侏罗世就已经启动[32]。另外, 内蒙古达来庙钾长花岗岩[35]、海莫赛格地区流纹岩[42]、西拉木伦碾子沟二长花岗岩[55]、半砬山钼矿流纹斑岩[92]、红山子盆地流纹岩[51]及芝瑞流纹岩这些陆续厘定的中-晚侏罗世A型花岗岩/流纹岩不仅为辽源地块在中-晚侏罗世时已经进入板内拉张环境提供了岩石学证据, 而且说明芝瑞流纹岩是晚侏罗世时期华北克拉通北缘与蒙古-鄂霍茨克缝合带演化有关的伸展事 件[29, 32‒35]的重要组成部分, 是区域性伸展环境的产物。综上所述, 芝瑞盆地晚侏罗世流纹岩可能形成于与蒙古-鄂霍茨克缝合带的演化有关的伸展构造环境, 可能受到华北克拉通北缘岩石圈伸展减薄之下幔源岩浆的底侵作用影响。

5 结 论

(1) 大兴安岭南部芝瑞盆地火山岩系中流纹岩SHRIMP锆石U-Pb年龄为(156.9±1.7) Ma, 形成于晚侏罗世早期, 火山岩系以高钾钙碱性流纹岩为主, 依照岩石组合特征并结合已获得的晚侏罗世早期流纹岩年龄, 应将其岩石地层单位归属于新民组。

(2) Nd、Sr、Pb同位素特征、常量、微量和稀土元素特征指示芝瑞盆地流纹岩由两阶段模式形成, 中元古代源于EMⅠ富集地幔的岩浆混染少量下地壳物质形成年轻下地壳并在中生代期间又受到地幔物质的改造影响, 直至晚侏罗世早期板内拉张构造环境下发生部分熔融形成的岩浆在上升过程中经历结晶分异后喷出地表形成此套火山岩, 是华北克拉通北缘岩石圈伸展减薄的产物。

(3) 芝瑞流纹岩是伸展构造环境的产物, 与大兴安岭北部和华北克拉通北缘晚侏罗世A型花岗岩/流纹岩指示的构造环境一致, 不仅为辽源地块在晚侏罗世时已经进入板内拉张环境提供了岩石学证据, 而且说明辽源地块晚侏罗世的伸展事件也可能与蒙古-鄂霍茨克缝合带的演化有关。

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Petrogenesis of early Late Jurassic rhyolites from the Zhirui Basin in southern Daxing’an Range: Their chronologic and geochemical constrains

XIE Kai-rui1, WU Jian-hua1,2*, ZHU Hong-tao3, WU Ren-gui1and LIU Shuai2

1. College of Earth Science, East China Institute of Technology, Nanchang 330013, China; 2. State Key Laboratory Breeding Base of Nuclear Resources and Environment, East China Institute of Technology, Nanchang 330013, China; 3. No. 243 Geological Party, China National Nuclear Corporation, Chifeng 024006, China

The Zhirui volcanic basin is located in Hexigten Banner, Inner Mongolia, at the southern tip of the Daxing'an Range. Its tectonic position lies in the Liaoyuan Block between the Xar Moron - Changchun suture zone and the Chifeng-Kaiyuan suture zone. The felsic volcanic series in the Zhirui Basin occurs unconformablely above the underlying Dashizhai Formation, and below the overlying basalt of the Hannuoba Formation, and has been intruded by granitic porphyry. The SHRIMP zircon206*Pb/238U age estimates for the rhyolite yield weighted an average age of (156.9±1.7) Ma, indicating an early Late Jurassic age. The whole-rock geochemistry of the rhyolites from the felsic volcanic series shows an A-type magmatic affinity, with a typical enrichment in SiO2, K2O, Ga, Zr, Nb, Y and a high ratio of FeOT/MgO but low contents of Al2O3, CaO and MgO, belonging to K-high calc-alkaline series rocks. Furthermore, they also have significantly higher contents of trace elements such as Ga, Zr, Nb, Y but low levels of Ba and Sr. Relative enrichment in Rb, Th, U, Pb, Zr, Hf, but depletion in Ba, Sr, P, Ti and Nb, Ta, and fractionated REE patterns show strong negative Eu anomalies. In the Nb-Y-3Ga and Rb/Nb-Y/Nb diagrams, all of the six samples show the characteristics of A2-type granites that demonstrate an extensionally tectonic setting. The rhyolites share such features as relatively high (87Sr/86Sr)ivaleus (0.706510~0.709821), lowNd() values (Nd()= ‒6.32~ ‒4.44) and young Nd-model ages (1304~1457 Ma). They exhibit low radiogenic Pb isotopic compositions, with (206Pb/204Pb)t=17.15~17.80, (207Pb/204Pb)t=15.41~15.48 and (208Pb/204Pb)t=37.38~37.63. The projection points are locateed between the lower crust and the depleted mantle as shown in the Sr-Ndisotope-tracer diagram and located in the lower crust area close to the enriched mantle as shown in the Pb isotope-tracer diagram.These elemental and isotopic characters argue for parental magmas from partial melting of mid-Mesoproterozoic crust materials which were derived from the enriched mantle and their subsequent fractional crystallization. In the (Yb+Ta)-Rb and (Y+Nb)-Rb diagrams, all the rhyolites indicate that they belong to an extensionally tectonic setting within the plate. As viewed from regional correlations with the coeval volcanic rocks, together with the Late Jurassic A-type intrusions widely scattered in NE China, the Zhirui rhyolites are testified to be asssociated with an extensionally tectonic setting within the plate. It is the product of a regional extensional environent associated with the evolution of northern Mongol-Khotsk suture in the Jurassic period, which may be accompanied by mantle-derived magma underplating.

rhyolite; petrogenesis; elements; Sr-Nd-Pb isotopes; early Late Jurassic; Zhirui

P597; P581

A

0379-1726(2016)03-0249-19

2015-10-16;

2015-12-29;

2016-01-28

国家自然科学基金(41372071); 中国核工业集团公司项目(中核地计[2008]74号)

解开瑞(1990–), 男, 硕士研究生, 矿产普查与勘探专业。E-mail: krxie0818@163.com

WU Jian-hua, E-mail: jhwu@ecit.cn; Tel: +86-791-83897549