U–Pb ages and trace elements in metamorphic zircon and titanite from UHP eclogite in the Dabie orogen: constraints on P–T–t path
U–Pb ages and trace elements in metamorphic zircon and titanite from UHP eclogite in the Dabie orogen: constraints on P–T–t path
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DOI：
10.1111/j.1525-1314.2011.00938.x
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发表时间：
2011-09
影响因子：
3.4
通讯作者：
X. Gao;Y‐F. Zheng;Y.‐X. Chen
中科院分区：
文献类型：
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作者：
X. Gao;Y‐F. Zheng;Y.‐X. Chen
采用激光烧蚀电感耦合等离子体质谱（ICP-MS）技术，对大别造山带副片麻岩中超高压榴辉岩（UHP）中锆石和钛铁矿的U-Pb同位素和微量元素进行了研究。这些结果提供了大陆俯冲带变质作用期间变质年龄和温度之间的直接联系。锆石U-Pb测年给出两组一致年龄，分别为242 ± 2 ~ 239 ± 5 Ma和226 ± 2 ~ 224 ± 6 Ma。三叠纪锆石U-Pb年龄具有平坦的重稀土配分模式，具有变质生长的特征。两代变质锆石的钛-锆石温度测量分别得出697 ± 27至721 ± 8 °C和742 ± 19至778 ± 34 °C的温度。我们解释说，锆石生长的第一幕发生在俯冲超高压变质开始之前，而第二幕在折返阶段从超高压榴辉岩相制度。因此，沉积原岩的大陆俯冲带变质作用在时间上与两个阶段的流体活动有关，分别在超高压变质阶段之前和之后。较低压力下较年轻的锆石的显着高的Ti‐in‐锆石温度表明在峰值超高压变质作用之后的初始“热”折返。有两种类型的钛铁矿。一个显示轻稀土元素（LREE）富集，陡峭的MREE-HREE模式和无Eu异常，并在0.5 GPa下产生551至605 °C的Zr-in-Titanite温度，另一个显示LREE亏损和平坦的MREE-HREE模式，并在2.0 GPa下产生782-788 °C的Zr-in-Titanite温度。前者适合于U-Pb测年，其不一致下交点年龄为252 ± 3 Ma。因此，第一种类型的钛铁矿被解释为在没有石榴石和斜长石的情况下生长，因此在俯冲的早期阶段。与此相反，第二个发生作为轮辋周围的金红石核心，从而在石榴石的存在下生长在“热”折返。因此，在大陆俯冲带变质作用中，锆石和钛铁矿存在多阶段生长。测时和温压的结合研究对大陆碰撞过程中榴辉岩的P-T-t轨迹提供了严格的约束。看来中温超高压榴辉岩相带不仅是由于陆壳在稍低于湿花岗岩固相线的P-T路径上俯冲而形成的，而且在最初的折返过程中经历了减压加热。
Laser ablation inductively coupled plasma mass spectrometry analyses of U–Pb isotopes and trace elements in zircon and titanite were carried out on epoxy mounts and thin sections for ultrahigh‐pressure (UHP) eclogite in association with paragneiss in the Dabie orogen. The results provide a direct link between metamorphic ages and temperatures during continental subduction‐zone metamorphism. Zircon U–Pb dating gives two groups of concordant ages at 242 ± 2 to 239 ± 5 Ma and 226 ± 2 to 224 ± 6 Ma, respectively. The Triassic zircon U–Pb ages are characterized by flat heavy rare earth element (HREE) patterns typical of metamorphic growth. Ti‐in‐zircon thermometry for the two generations of metamorphic zircon yields temperatures of 697 ± 27 to 721 ± 8 °C and 742 ± 19 to 778 ± 34 °C, respectively. We interpret that the first episode of zircon growth took place during subduction prior to the onset of UHP metamorphism, whereas the second episode in the stage of exhumation from UHP to HP eclogite facies regime. Thus, the continental subduction‐zone metamorphism of sedimentary protolith is temporally associated with two episodes of fluid activity, respectively, predating and postdating the UHP metamorphic phase. The significantly high Ti‐in‐zircon temperatures for the younger zircon at lower pressures indicate the initial ‘hot’ exhumation after the peak UHP metamorphism. There are two types of titanite. One exhibits light rare earth element (LREE) enrichment, steep MREE–HREE patterns and no Eu anomalies, and yields Zr‐in‐titanite temperatures of 551 to 605 °C at 0.5 GPa, and the other shows LREE depletion and flat MREE–HREE patterns, and gives Zr‐in‐titanite temperatures of 782–788 °C at 2.0 GPa. The former is amenable for U–Pb dating, yielding a discordia lower intercept age of 252 ± 3 Ma. Thus, the first type of titanite is interpreted to have grown in the absence of garnet and plagioclase and thus in the early stage of subduction. In contrast, the second one occurs as rims surrounding rutile cores and thus grew in the presence of garnet during the ‘hot’ exhumation. Therefore, there is multistage growth of zircon and titanite during the continental subduction‐zone metamorphism. The combined studies of chronometry and thermobarometry provide tight constraints on the P–T–t path of eclogites during the continental collision. It appears that the mid‐T/UHP eclogite facies zone would not only form by subduction of the continental crust in a P–T path slightly below the wet granite solidus, but also experience decompression heating during the initial exhumation.