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    SHRIMP U–Pb dating, trace elements and the Lu–Hf isotope system of coesite-bearing zircon from amphibolite in the SW Sulu UHP terrane, eastern China [查看] FulaiLiu、AxelGerdes、LingsenZeng、HuaiminXue
    In this study, we link mineral inclusion data, trace element analyses, U–Pb age and Hf isotope composition obtained from distinct zircon domains of complex zircon to unravel the origin and multi-stage metamorphic evolution of amphibolites from the Sulu ultrahigh-pressure (UHP) terrane, eastern China. Zircon grains separated from amphibolites from the CCSD-MH drill hole (G12) and Niushan outcrop (G13) were subdivided into two main types based on cathodoluminescence (CL) and Laser Raman spectroscopy: big dusty zircons with inherited cores and UHP metamorphic rims and small clear zircons.Weakly zoned, grey-white luminescent inherited cores preserve mineral inclusions of Cpx + Pl + Ap ± Qtz indicative of a mafic igneous protolith. Dark grey luminescent overgrowth rims contain the coesite eclogite-facies mineral inclusion assemblage Coe + Grt + Omp + Phe + Ap, and formed at T = 732–839 C and P = 3.0–4.0 GPa. In contrast, white luminescent small clear zircons preserve mineral inclusions formed during retrograde HP quartz eclogite to LP amphibolite-facies metamorphism (T = 612–698 C and P = 0.70–1.05 GPa). Inherited zircons from both samples yield SHRIMP 206Pb/238U ages of 695–520 Ma with an upper intercept age of 800 ± 31 Ma. The UHP rims yield consistent Triassic ages around 236–225 and 239–225 Ma for G12 and G13 with weighted means of 229 ± 3 and 231 ± 3 Ma, respectively. Small clear zircons from both samples give 206Pb/238U ages around 219–210 Ma with a weighted mean of 214 ± 3 Ma, interpreted as the age of retrograde quartz eclogite-facies metamorphism. Matrix amphibole from both samples indicate Ar–Ar ages of 209 ± 0.7 and 207 ± 0.7 Ma, respectively, probably dating late amphibolite-facies retrogression. The data suggest subduction of Neoproterozoic mafic igneous rocks to UHP conditions in Middle Triassic (~230 Ma) times and subsequent exhumation to an early HP(~214 Ma) and a late LP stage (~208 Ma) over a period of ~16 and 6 Myr, respectively. Thus, early exhumation from a mantle depth of 120–100 km to about 60 km occurred at an average rate of 0.3 cm/y, while subsequent exhumation to a middle crustal level took place at approximately 0.54 cm/y. These exhumation rates are considerably slower than those obtained for UHP rocks in the Dora Maira and Kokchetav massifs (2–3 cm/y). Based on similar P–T estimates and trace element and Hf isotope compositions, Sulu amphibolites can be identified as retrograde UHP eclogites. The eHf(800) of +8 implies a significant input from the depleted mantle to the Sulu–Dabie terrane during the middle Neoproterozoic. Overgrown rims are characterized by a distinct trace element composition with low Lu/Hf and Th/U and significantly higher 176Hf/177Hf ratios than inherited cores, consistent with formation during/after garnet (re-)crystallization and fractionation of the Lu–Hf system during UHP metamorphism. The combined dataset suggests homogenization of the 176Hf/177Hf ratio within the metamorphic mineral assemblage and during protolith formation. Observed variations are explained by mixing of material from both domains during laser ablation, e.g., due to partial recrystallization of inherited cores.
    大别超高压变质带的进变质、超高压和退变质时代的准确限定：以双河大理岩中榴辉岩锆石SHRIMPU-Pb定年为例 [查看] 刘福来、薛怀民、许志琴、梁风华、AxelGerdes
    在大别超高压变质带的双河地区存在一种特殊类型的榴辉岩，该类榴辉岩主要以似层状、条带状以及不规则透镜体赋存于大理岩中。矿物组成主要为石榴石、绿辉石以及少量的金红石、白云石、菱镁矿等。沿石榴石和绿辉石边缘常退变为角闪石+斜长石等，有的岩石完全退变为斜长角闪岩。激光拉曼和阴极发光综合分析表明，该类榴辉岩中的锆石可划分为两种类型：继承性碎屑锆石和变质锆石。继承性碎屑锆石十分少见，阴极发光图像具有明显的双层结构，即强发光的核和弱发光的边，核部和边部的包体矿物分别为Pl+Ap和Qtz+Pl。SHRIMP U-Pb定年结果表明，继承性碎屑锆石和核部记录的207Pb/206Pb年龄为2701±15Ma，Th/U比值明显偏高为1.05，稀土元素配分模式显示重稀土明显富集，具有典型岩浆结晶锆石的特点；边部记录的207Pb/206Pb年龄为1801±12Ma~1735±22Ma，Th/U比值则明显偏低，为0.19~0.22之间，稀土元素配分模式显示重稀土相对平坦，具有典型变质锆石的特点。上述特征表明该类继承性碎屑锆石可能来源于太古代的基底，并经历了早元古代变质热事件的改造。新生的变质锆石无论是矿物包体还是阴极发光图象均与继承性碎屑锆石存在明显差异。有的变质增生拾石具有弱发光的核(阴极发光图象呈灰色)和强发光的边(阴极发光图象呈白色)。核部包体矿物组合为Qtz+Grt+Omp+Phe+Dol+Ap，具有典型石英榴辉岩相矿物组合特征，而边部则保存含柯石英的超高压包体矿物组合Coe+Grt+Omp+Mgs+Arg+Ap，表明该类锆石的核部和边部分别形成于俯冲进变质阶段和超高压变质阶段。另一部分变质增生锆石具有强发光的核(阴极发光图象呈白色)和弱发光的边(阴极发光图象呈黑色)。核部保存的标志性超高压包体矿物组合为Coe+Grt+Omp+Mgs+Arg+Ap，边部则保存Qtz+Cal等退变质矿物组合，有的则缺乏矿物包体，表明该类锆石自超高压变质阶段开始生长，并经历了后期退变质作用的改造。从不同微区矿物包体组合的性质及其转变特征可以明显看出，自石英榴辉岩相进变质阶段到超高压峰期变质阶段存在下列转变反应：Qtz→Coe和Dol→Mgs+Arg；而自超高压峰期变质阶段到后期退变质阶段则存在下列退变反应：Coe→Qtz和Arg→Cal。SHRIMP U-Pb定年结果表明，含石英榴辉岩相矿物包体的锆石微区记录的206Pb/238U年龄为249~241Ma，加权平均值为244±4Ma，代表了深俯冲石英榴辉岩相进变质阶段的变质年龄；含柯石英等超高压矿物包体的锆石微区记录的206Pb/238U年龄为239~231Ma，加权平均值为234±3Ma，代表超高压阶段的峰期变质年龄；而含石英和方解石的退变边记录的206Pb/238U年龄为219~211Ma，加权平均值为216±6Ma，应代表后期折返阶段的角闪岩相退变质年龄。上述两类变质增生锆石微区的Th/U比值和稀土元素配分模式十分相似，Th/U比值变化于0.02~0.18之间，稀土元素配分模式显示重稀土相对平坦，稀土元素总量明显低于继承性碎屑锆石，具有典型变质锆石的特点。根据锆石微区矿物包体的化学成分，采用Grt-Omp和Grt-Omp-Phe温压计，结合前人的变质反应实验资料的综合分析，确定榴辉岩的原岩在深俯冲过程的石英榴辉岩相进变质阶段的变质温压条件为T=588~668℃，P=1.7~1.8GPa；超高压峰期阶段的温压条件为T=784~849℃，P＞5.5GPa；而构造折返过程中角闪岩相退变质阶段的温压条件为T=550~720℃，P=0.8~1.4GPa。由此可见，大别超高压变质岩的原岩——元古代（部分可能是太古代）的陆壳物质在早三叠纪发生俯冲至55~60km深处，并经历了石英榴辉岩相变质作用。随后这些变质岩石继续深俯冲至165~175km的地慢深处，于中三叠纪发生了超高压变质作用，石英榴辉岩相矿物组合转变为超高压榴辉岩相矿物组合。最后这些超高压变质岩石发生构造折返，至晚三叠纪抬升到约30km的中下地壳深度，并经历了角闪岩相退变质作用的改造，超高压榴辉岩相矿物组合退变为角闪岩相矿物组合。由此推断，大别超高压变质带俯冲和折返速率分别为11~12km Myr-1和7.5~8.1km Myr-1。该项成果不仅确定了大别超高压变质地体的石英榴辉岩相进变质一超高压榴辉岩相峰期变质一角闪岩相退变质的年代谱系，而且对于重塑大别超高压变质地体的快速俯冲一折返的动力学模式有着重要的科学意义。
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