Metamorphic P–T conditions and thermal structure of Chinese Continental Scientific Drilling main hole eclogites: Fe–Mg partitioning thermometer vs. Zr‐in‐rutile thermometer
Metamorphic P–T conditions and thermal structure of Chinese Continental Scientific Drilling main hole eclogites: Fe–Mg partitioning thermometer vs. Zr‐in‐rutile thermometer
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DOI：
10.1111/j.1525-1314.2009.00831.x
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发表时间：
2009-12
影响因子：
3.4
通讯作者：
R. Zhang;Y. Iizuka;W. Ernst;J. G. Liou;Z.-Q. Xu;T. Tsujimori;C. Lo;B. Jahn
中科院分区：
文献类型：
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作者：
R. Zhang;Y. Iizuka;W. Ernst;J. G. Liou;Z.-Q. Xu;T. Tsujimori;C. Lo;B. Jahn
中国大陆科学钻探（CCSD-MH）项目位于中国中东部苏鲁超高压盆地南部的主孔（5158 m深）中采集的岩心岩石由榴辉岩、各种片麻岩和少量变橄榄岩堆晶组成;该岩性段经历了俯冲带超高压变质作用。含柯石英榴辉岩主要存在于100-2000米深度之间，但在2000米以下，镁铁质榴辉岩很少。用电子探针分析了100 ~ 2774 m的39个榴辉岩岩芯中金红石的Zr、Nb、Cr、Fe、Si、Mg、Al、Ti等元素和代表性榴辉岩矿物的常量元素。金红石团簇中的锆和铌的含量分别为100-400和200-700 ppm。然而，Zr和Nb含量的金红石强烈退变质榴辉岩比新鲜或退变质程度较低的榴辉岩表现出较大的变化，这意味着某种程度上流体渗透影响金红石化学在退变质作用。石榴子石和绿辉石中金红石包裹体的Zr含量略低于基质金红石中的Zr含量，表明金红石包裹体形成于峰值温度之前或接近峰值温度。CCSD-MH榴辉岩的P-T条件由Fe-Mg交换温度计和Zr-in-金红石温度计以及Grt-Cpx-Phn-Ky地质温压计估算。在40 kbar压力下，使用金红石中锆温度计计算出的最高温度范围为700-811 °C，与使用铁镁交换温度计估算的温度相当。榴辉岩的温度估计在一个3000米厚的部分定义了一个连续的梯度，并没有显示出明显的温度差距，这表明从100至3000米深度的岩石可能属于一个单一的，大规模的超高压板。这些数据与CCSD-MH橄榄岩的P-T计算相结合，得出中朝和扬子板块之间的三叠纪俯冲带的地热较低（1.5 °C km−1）;其传导模型地热为1.30 -35 mW m−2。
Core rocks recovered from the main hole (5158 m deep) of the Chinese Continental Scientific Drilling (CCSD‐MH) project, southern Sulu UHP terrane, east‐central China, consist of eclogites, various gneisses and minor metaperidotite cumulates; this lithological section underwent subduction‐zone UHP metamorphism. Coesite‐bearing eclogites are mainly present between the depths of 100–2000 m, but below 2000 m, mafic eclogites are rare. Selected elements (Zr, Nb, Cr, Fe, Si, Mg, Al & Ti) in rutile from 39 eclogite cores from 100 to 2774 m, and major elements of minerals from representative eclogites were analysed by electron microprobe. Zirconium and Nb concentrations of rutile cluster ∼100–400 and 200–700 ppm respectively. However, Zr and Nb contents in rutile from strongly retrograded eclogites show larger variations than those of fresh or less retrograded eclogites, implying that somehow fluid infiltration affected rutile chemistry during retrograde metamorphism. Zr contents in rutile inclusions in garnet and omphacite are slightly lower than those of the matrix rutile, suggesting that the rutile inclusions formed before or close to the peak temperature. The P–T conditions of the CCSD‐MH eclogites were estimated by both Fe–Mg exchange and Zr‐in‐rutile thermometers, as well as by the Grt–Cpx–Phn–Ky geothermobarometer. The maximum temperature range of 700–811 °C calculated at 40 kbar using the Zr‐in‐rutile thermometer is comparable with temperature estimates by the Fe–Mg exchange thermometer. The temperature estimates of eclogites in a ∼3000 m thick section define a continuous gradient, and do not show a distinct temperature gap, suggesting that the rocks from 100 to 3000 m depth might belong to a single, large‐scale UHP slab. These data combined with P–T calculations for CCSD‐MH peridotites yield a low geotherm (∼5 °C km−1) for the Triassic subduction zone between the Sino‐Korean and Yangtze cratons; it lies ∼30–35 mW m−2 conductive model geotherm.