含榴花岗片麻岩Lu-Hf年代学初探

张 超, 程 昊*, J D Vervoort

(1. 同济大学 海洋地质国家重点实验室, 上海 200092; 2. School of Earth and Environmental Sciences, Washington State University, Washington 99164, USA)

0 引 言

20 世纪 90 年代 MC-ICP-MS 的出现[1–2]使 Hf同位素的化学分离与质谱测定大为简化[3], Lu-Hf同位素研究由此进入了快速发展的时期。其中以定年为主要目的的 Lu-Hf同位素年代学研究更是取得了长足的发展。Lu-Hf同位素定年对象涉及陨石[4]、岩浆岩[5]、变质岩[6]和沉积岩[7]。定年的主要对象也从早期的全岩和石榴子石为主, 扩展到了磷灰石[7]、辉石[6]和硬柱石[8]等。由于石榴子石常具有较高的母子体176Lu/176Hf比值, 往往能够构筑高质量的等时线, 石榴子石因此成为Lu-Hf年代学的首选矿物。近年来,石榴子石 Lu-Hf年代学[9]在造山带的研究中体现出了其独特的价值, 获得了许多其他定年体系所没能揭示出的造山带演化的信息[10–25]。国内外研究者对大别-苏鲁造山带这类花岗质片麻岩的成因、形成时代及其构造意义等重要的科学问题还存在较大的分歧, 其中一个核心的争论则是其是否经历了超高压变质作用。诠释该论题的一个直接办法就是对该类岩石进行精细的年代学研究, 并与超高压岩石的变质演化史进行对比。西大别属于大别-苏鲁造山带的西缘, 该区出露的含榴花岗质岩石的年代学工作寥寥无几[26–29], 且仅限于锆石U-Pb定年。单一的定年体系给出的信息往往有所局限, 常需要结合多个同位素体系以期提供更多的时间信息来厘定其精细的演化历程。另一方面, 对于造山带中广泛出露的花岗质岩石, 其狭窄的 Lu/Hf比值变化范围使得构建高精度全岩 Lu-Hf等时线极为困难, 国际上尚未有该类岩石的石榴子石Lu-Hf年龄报道。考虑到石榴子石亲Lu排Hf的特性, 这类特殊的岩石是可能给出高精度的石榴子石 Lu-Hf等时线的。因此, 本文尝试对西大别一个典型的含榴花岗片麻岩进行Lu-Hf年代学研究, 结合锆石U-Pb年代学、岩石学和矿物学研究, 给出首条高精度的花岗质岩石的Lu-Hf等时线, 并对含榴花岗片麻岩的石榴子石成因及Lu-Hf/U–Pb年龄所代表的地质意义进行讨论。

1 区域地质背景和样品

大别-苏鲁造山带是位于华北和扬子两大陆块间的复杂碰撞造山带。西大别又称红安地区, 为大别-苏鲁造山带的西延部分, 东侧以商麻断裂与东大别造山带分开, 西侧由大悟断裂与桐柏相隔(图1)。大量的构造、岩石、年代学和地球化学研究表明西大别记录了秦岭-大别-苏鲁造山带的多期演化历史, 为一典型的复合造山带。这一地区以新县高压-超高压变质地体为核心呈一个构造穹窿, 南北两翼由高压至中低压变质带组成[26]。根据岩石构造特征, 从北到南可分为: (南湾)复理石带、(八里畈)构造混杂岩带、(浒湾-新县-红安)高压-超高压-高压变质带以及(木兰山)蓝片-绿片岩带等六个相带[29](图1)。该区榴辉岩出露面积大, 白垩纪岩浆活动较弱, 并出露大量的蓝片岩-绿片岩相变质岩石, 是研究大别-苏鲁造山带构造演化理想的地区。该地区主要出露岩石为花岗质片麻岩, 榴辉岩以透镜体状产出在这些岩石中。少部分花岗质片麻岩含有石榴子石, 这类含榴花岗片麻岩主要零星分布于高压-超高压变质带内。所研究的含榴花岗片麻岩样品采自湖北省麻城市四道河村附近的一个采石场(31°20.712′N,114°3.051′ E, 图1)。该花岗片麻岩岩体内部可见残留的退变质榴辉岩透镜体(图 2a)。样品为灰白色, 中细粒结构, 弱面理化。石英+斜长石+钾长石组合占95%以上, 副矿物有黑云母(约1%)、石榴子石(约2%)、磁铁矿和榍石等。石榴子石离散分布于岩石中(图2b)。前人对该地区的含榴花岗片麻岩锆石 U-Pb定年结果给出一致的约227 Ma[27–29]的谐和年龄。

图1 西大别四道河含榴花岗片麻岩采样图[29]Fig.1 Simplified geologic map of the West Dabie Mountains, modified after Liu et al.[29], showing the sample locality for the Sidaohe granitic gneiss

2 分析方法

样品经机械粉碎和磁选, 在双目显微镜下挑选锆石及不含可见包裹体的石榴子石。石榴子石和全岩的Lu-Hf同位素分析在美国华盛顿州立大学完成, 化学流程及仪器状态与文献[12]一致。石榴子石微量元素和锆石U-Pb测试在中国地质大学(武汉)地质过程与矿产资源国家重点实验室用LA-ICP-MS法完成。激光束斑直径 32 μm。具体分析条件及流程详见文献[30]。石榴子石主要元素成分分析在该实验室JXA 8100电子探针仪上完成, 分析条件为加速电压15 kV , 电流为20 nA , 束斑直径为2 μm。结果校正采用标准ZAF方法, 使用天然矿物作为标样。

3 结 果

图2 西大别四道河含榴花岗片麻岩Fig.2 Field photographs showing granitic gneiss and enclosed retrograded eclogite lenses (a) and granitic gneiss with eyeball-shaped garnet crystal (b) from Sidaohe, the West Dabie Mountains

图3 西大别四道河含榴花岗片麻岩锆石典型CL图及U-Pb谐和图Fig.3 Zircon U-Pb concordia diagram with selected CL images

四道河含榴花岗片麻岩中锆石为透明短柱状, 自形程度较差, 阴极发光(CL)结构表现为弱分带和雾状分带[27–28](图3)。对这些锆石进行了U-Pb年龄测定,206Pb/238U年龄范围是216.5～228.5 Ma (表1和图3), 加权平均值为(223.2±1.1) Ma (2σ, MSWD = 1.8)。此年龄应为这些锆石形成时间的最佳估计值。四道河含榴花岗片麻岩中石榴子石呈他形变晶结构, 粒径 0.2～1.0 mm。主要元素环带不明显, 仅在颗粒边部较窄的区域出现Mn和Ca含量升高、Fe含量降低的变化趋势(表2和图4a、4b)。重稀土元素则呈单边递增/减的变化趋势(表3和图4c～4f)。石榴子石具有极高的母子体同位素比值(176Lu/177Hf = 约300)。两个石榴子石分析点和一个全岩分析点构筑的 Lu-Hf等时线给出(212.2±0.7) Ma (2σ, MSWD = 0.1)的高精度年龄(表 4和图5)。

表1 四道河含榴花岗片麻岩锆石LA-ICP-MS U-Pb同位素分析结果Table 1 Zircon U-Pb isotopic data for Sidaohe granitic gneiss

表2 四道河含榴花岗片麻岩中石榴子石代表性电子探针数据(%)Table 2 Representative electron microprobe analysis data (%) for garnet from Sidaohe granitic gneiss

图4 西大别四道河含榴花岗片麻岩石榴子石的Mg-Fe-Ca-Mn成分环带(a和b)、稀土元素分布模式(c和d)及Y-Lu成分分带(e和f)Fig.4 Mg-Fe-Ca-Mn elemental zoning (a,b), REE patterns (c,d) and Y/Lu zonation (e, f) for two garnet grains from Sidaohe granitic gneissPrp– 镁铝榴石; Alm– 铁铝榴石; Grs– 钙铝榴石; Sps– 锰铝榴石。石榴子石边界接触矿物为长石-绿帘石-石英-黑云母组合。Prp– pyrope; Alm– almandine; Grs– grossular; Sps– spessartine. Mineral assemblage of plagioclase-epidote-quartz-biotite is in contacted with the garnet grains.

4 讨 论

尽管对大别造山带内出露的含榴花岗片麻岩是否经历过高压/超高压变质作用尚存在争议[26–30,33–35],尚未在四道河含榴花岗片麻岩中找到超高压/高压变质作用的矿物学的直接证据, 然而, 四道河含榴花岗片麻岩中石榴子石不具Eu异常(图 4)可能说明石榴子石与长石结晶的非同时性。石榴子石为富钙铁铝榴石(Grs15-22Alm36-63Sps8-36Prp6-9), 与阿尔卑斯的高压变质花岗岩和高压正片麻岩[36]以及大别东部超高压榴辉岩和超高压变质花岗质正片麻岩所含的石榴子石类似[33]。利用锆石中Ti的含量估算该花岗岩中岩浆锆石和变质锆石的形成温度, 分别为约786 ℃和约674 ℃[28]。后者和与之直接接触的榴辉岩峰期温度约 692 ℃[28]一致。前人获得的(227±2)Ma (16 个点)[28]和(227±5) Ma (4 个点)[27]的变质锆石U-Pb年龄虽略高于我们获得(223.2±1.1) Ma (26个点)的年龄, 但都具有极低的 Th/U 比值(0.01～0.03),且与直接接触的榴辉岩年龄相近[28]。

表3 四道河含榴花岗片麻岩中石榴子石LA-ICP-MS稀土元素分析数据(μg/g)Table 3 LA-ICP-MS rare earth element data (μg/g) for garnet from Sidaohe garnet-bearing granitic gneiss

表4 四道河含榴花岗片麻岩石榴子石和全岩Lu-Hf同位素分析结果Table 4 Lu-Hf isotopic data for Sidaohe garnet-bearing granitic gneiss

图5 西大别四道河含榴花岗片麻岩Lu-Hf等时线Fig.5 Garnet-whole rock Lu-Hf isochron for Sidaohe garnet-bearing granitic gneiss

正确解释石榴子石Lu-Hf等时线/拟合线最有效的途径就是厘定石榴子石生长历程。Cheng et al.[14,37]在研究高压榴辉岩中的珊瑚礁状石榴子石时, 发现具有完好生长环带的石榴子石Lu-Hf的等时线年龄实际代表了退变质阶段流体活动的时间。四道河含榴花岗片麻岩中的石榴子石主要元素成分分带非常微弱, 仅在最边缘部分呈Mn和Ca含量升高、Fe含量降低的变化趋势, 指示溶蚀-再吸收的过程。用Crank的一维扩散模型[38]和Carlson[39]的自扩散系数可以估算出石榴子石边部约 100 μm的溶蚀-再吸收边的 Ca-Fe-Mn-Mg成分环带的均一化时间为9.2×106～5.3×107a, 指示石榴子石重结晶后的快速冷却。结合单边递减/增的非中心对称的微量元素分带特征(图4)以及石榴子石内包体的缺乏, 我们认为该石榴子石经历的是一个典型的溶蚀-再结晶的过程[14,37,40-44]。石榴子石边部的帘石、黑云母和石英可能就是退变质过程中流体沿裂隙活动的产物, 一种可能的机制是通过石榴子石+白云母+H2O→绿帘石+黑云母+石英的反应, 类似的石榴子石也在挪威的西部片麻岩(WGR)的超高压变质岩中发现过[45,46]。

放射性同位素定年的等时线法必须满足同源、同时和封闭这三个基本条件。不满足这些前提的回归拟合线都不是严格意义上的等时线。然而, 当相对于这些条件的偏差是可以忽略或者偏差本身具有明确的地质意义的情况下, 虽无法确定出严格意义上的等时线, 但对矿物和全岩的分析点的回归剖析却能提供诸多有益的地质信息[10–24]。我们把四道河含榴花岗片麻岩的石榴子石-全岩拟合线解释为等时线是因为: (1) 石榴子石经历的流体改造过程(溶蚀-再结晶)保证不同母子体比值石榴子石的同源和同时的条件; (2) 全岩极低的176Lu/177Hf比值(表 1)决定了其176Hf/177Hf比值非常接近体系的初始同位素比(图 5); (3) 由于石榴子石快速生长期间造成基质大量Lu亏损及176Lu较长的半衰期, 石榴子石重结晶期间全岩的初始同位素比值变化可以忽略; (4) 石榴子石-全岩拟合线的平均标准权重偏差(MSWD = 0.1)远小于1个自由度(3个数据点)时2.0的临界值[47]。

由于稀土元素在石榴子石中的较慢扩散速率[48],一般地质条件下石榴子石的Nd封闭温度在700～750℃[49]。石榴子石的Lu-Hf体系封闭温度一般被认为要高于700 ℃[50]。结合四道河含榴花岗片麻岩中石榴子石的微量元素成分分带, 约212 Ma的石榴子石Lu-Hf年龄代表的应该是石榴子石重结晶年龄, 对应一期流体活动时间。这个年龄比约223 Ma的锆石U-Pb年龄要晚约10 Ma, 说明石榴子石和锆石生长不是同期的, 这与锆石陡立的重稀土富集型的稀土元素分布模式[28]是一致的。该年龄与大别山东缘的片麻状变质花岗岩约 215 Ma 的高压榴辉岩相重结晶时间以及约212 Ma的苏鲁-大别榴辉岩中石英脉的形成时间一致[51–53], 可能指示了一期相退变质作用流体活动[54]。含榴花岗片麻岩中石榴子石极高的母子体同位素比值也对Hf-Lu混合稀释剂的配比提出了特殊的要求。

野外采样由王超完成, 采样过程中得到郑永飞老师的悉心指导; 郑曙老师、吴元保老师和陈璐同学在电子探针和 LA-ICP-MS测试过程中和数据处理上给予了指导和协助; 吴元保老师还提供了岩石薄片, 在此一并致谢。

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