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大别超高压变质带榴辉岩中金红石SHRIMP原位U—Pb定年及其年代学意义

2016-06-30侯晨阳杨天水石玉若

地球科学与环境学报 2016年3期
关键词:苏鲁锆石变质

侯晨阳+杨天水+石玉若

摘要:对大别超高压变质带榴辉岩中金红石进行了SHRIMP原位U-Pb定年,得到金红石U-Pb年龄为(219±4)Ma, 与前人利用TIMS方法获得的金红石年龄结果在误差范围内是一致的,验证了金红石用于SHRIMP原位U-Pb定年的可行性。通过对比发现,不同赋存状态的金红石U-Pb年龄无明显差别。根据本次测得的年龄数据,结合前人的研究成果,金河桥榴辉岩中金红石的U-Pb年龄为冷却年龄,代表了大别超高压变质带构造折返阶段的时代。

关键词:年代学;榴辉岩;金红石;SHRIMP;U-Pb年龄;封闭温度;大别超高压变质带

中图分类号:P597 文献标志码:A

Abstract: In-situ SHRIMP U-Pb dating of rutile in eclogite from Dabie ultra-high pressure (UHP) metamorphic belt was studied. The U-Pb age of rutile obtained is (219±4)Ma, consistent with the previous TIMS results of rutiles within the error range. This consistence points to the feasibility of in-situ SHRIMP U-Pb dating of rutile. The dating results indicate no obvious differences for the U-Pb ages of rutiles from different occurrences. Taking together with the previous dating results, it is suggested that the U-Pb age of rutile in Jinheqiao eclogite is the cooling age, representing the tectonic exhumation time of Dabie UHP metamorphic belt.

Key words: geochronology; eclogite; rutile; SHRIMP; U-Pb age; closure temperature; Dabie UHP metamorphic belt

0 引 言

随着大别—苏鲁地区榴辉岩中标志性超高压矿物(柯石英和金刚石)的相继发现[1-5],以及在榴辉岩围岩中特别是各种类型正片麻岩和副片麻岩锆石中柯石英包体的普遍发现[6-10],证明了榴辉岩及其围岩的原岩曾经发生过深俯冲并经历了超高压变质作用。经超高压变质作用而形成的超高压变质岩(包括榴辉岩、硬玉石英岩及超高压片麻岩等)形成于100 km以上的深度,但现今已出露地表,说明超高压变质岩一定经历了快速的抬升折返过程[11]。准确限定抬升折返阶段的时代对于重塑大别超高压变质带的快速俯冲-折返的动力学模式有重要的科学意义。

金红石是陆壳岩石中一种常见的副矿物,其主要化学成分为TiO2。在自然界中,金红石以副矿物形式广泛存在于各级变质岩中。金红石在成岩、风化和各种不同程度的变质过程中均能保持极大的稳定性[12],其作为稳定的重矿物相保存了大量的地质作用信息,如变质温度、变质压力、冷却年龄和源岩性质等[13]。金红石中能够容纳一定量的U,部分U含量较高的样品可用于Pb-Pb和U-Pb定年[14-17]。早期的金红石U-Pb年代学研究主要是利用同位素稀释-热电离质谱法(ID-TIMS)[18-20],该方法的优点在于精度很高,但是对样品的消耗量大,流程繁琐,而且当样品经历了复杂的生成过程时,使用该方法获得的年龄可能会是毫无意义的混合年龄。近年来,二次离子探针质谱法(SIMS)已成功应用于金红石的U-Pb年代学研究中[21-22]。目前,金红石的U-Pb定年一般用于限定变质岩的冷却历史和超高压变质岩折返速率的研究[17,19,22-28]。

本文主要对采自大别超高压变质带的金河桥榴辉岩中金红石进行SHRIMP原位U-Pb定年,并对其年代学意义进行探讨。

1 地质背景及样品采集

大别—苏鲁超高压变质带是由华北板块与扬子板块碰撞而形成的,被郯庐断裂所切错分成大别(西部)和苏鲁(东部)超高压变质带两部分(图1)[29]。其中,大别超高压变质带在东部以郯庐断裂为界,向西延伸至桐柏—秦岭造山带。Wang等把大别超高压变质带进一步划分为北大别地体和南大别地体[30]。在南大别榴辉岩中已经发现了柯石英和金刚石等标志性超高压矿物[1,4-5,30],在榴辉岩的围岩片麻岩锆石中也已识别出柯石英包体,这表明南大别榴辉岩及其围岩的原岩均发生了深俯冲并经历了超高压变质作用[6,10]。

本次用于研究的榴辉岩样品采自安徽省太湖县花凉亭水库东北部的金河桥地区(30°30′59.9″N, 116°15′56″E),构造位置处于南大别超高压变质带的南端,属于超高压榴辉岩(图2)。金河桥榴辉岩为粗粒结构,条带状构造,红色石榴石、绿色绿辉石以及白色石英互为条带[图3(a)],副矿物有金红石、角闪石、锆石、磷灰石、白云母、柯石英等。通过对样品进行镜下观察,发现一部分金红石以包体的形式存在于石榴石内部,其余金红石则存在于石榴石及其他矿物颗粒之间或完全被其他矿物包裹[图3(b)]。

2 分析方法

选取新鲜的榴辉岩样品,清洗干净后将其破碎成毫米级的岩石颗粒,然后在显微镜下挑选出含金红石较多且金红石颗粒较大的岩石颗粒,制成符合SHRIMP测试要求的标准样品靶。

李秋立等利用ID-TIMS方法获得金河桥榴辉岩中金红石U-Pb年龄为(218±1)Ma,并根据文献[35]提出的封闭温度方程和文献[14]给出的参数条件计算出金红石的U-Pb体系封闭温度为(470±50)℃[19]。这一结果与Li等在大别超高压变质带双河地区获得的榴辉岩中多硅白云母的Rb-Sr年龄结果((219±5)Ma)和U-Pb体系封闭温度(500 ℃)[36]相一致。陈振宇等计算得到金河桥榴辉岩中金红石的Zr温度为595 ℃~678 ℃,并指出该温度要高于金红石U-Pb年龄所对应的封闭温度[37]。Liu等对采自大别地区的2件榴辉岩样品和1件正片麻岩样品退变锆石微区进行了SHRIMP U-Pb定年,获得的年龄为(220±1)Ma,并将该年龄结果解释为大别地体构造折返至石英稳定域的退变质年龄[38]。刘福来等对大别超高压变质带双河地区榴辉岩锆石进行了SHRIMP U-Pb定年,获得了(216±6)Ma的退变质年龄[39]。此外,已有的锆石年龄数据显示,大别超高压变质带的峰期变质时代在230 Ma左右[39-41]。在前人研究成果的基础上,结合本次金红石SHRIMP U-Pb定年结果,金河桥榴辉岩中金红石的U-Pb年龄((219±4)Ma)应为超高压榴辉岩的冷却年龄,代表了大别超高压变质带在超高压峰期变质后构造折返事件的时代。

4 结 语

通过对大别超高压变质带金河桥榴辉岩中金红石进行SHRIMP原位U-Pb定年,测得金红石U-Pb年龄为(219±4)Ma,榴辉岩中不同赋存状态的金红石U-Pb年龄未发现存在明显的差异。本次获得的金红石U-Pb年龄为超高压榴辉岩的冷却年龄,代表了大别超高压变质带构造折返阶段的时代。

中国地质科学院地质研究所李琳琳参加了野外采样,车晓超在样品制备和仪器调试方面给予了帮助,李秋立研究员提供了金红石标样,在此一并表示感谢。

参考文献:

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