The combined Lu-Hf and Sm-Nd mineral-whole rock, and zircon U-Pb, REE and Hf isotope study of ultra high pressure rocks from the CCSD program

选取中国大陆科学钻探（CCSD）超高压（UHP）岩石，结合U-Pb LA-ICP-MS和SHRIMP定年方法，研究了不同锆石区域的Sm-Nd和Lu-Hf同位素体系。初步结果表明，锆石微量元素、U-Th-Pb同位素组成和矿物包裹体PT信息的综合利用，可将锆石U-Pb年龄与变质反应联系起来。根据CL成像、矿物包裹体、微量元素含量和Hf同位素组成，区分了继承和变质锆石。在一个样品中可以识别出多达三个阶段的锆石生长或再结晶。这些年龄分别确定了中三叠世俯冲过程中的超高压变质作用和退变质作用的发生时间，以及晚三叠世折返过程中的退变质作用的发生时间。目前正在进行的研究Hf同位素分析的超高压锆石域从不同的样品承诺在超高压变质过程中的Lu-Hf系统的行为的理解新的见解。初步Lu-Hf和Sm-Nd grt-cpx等时线从CCSD和大别榴辉岩产生令人惊讶的一致的年龄，平均约6-8百万年的UHP锆石年轻。这些年龄的解释对我们来说还不清楚。该提案应用期间的一个主要重点是将不同的计时器（锆石上的U-Pb和石榴石-cpx上的Lu-Hf / Sm-Nd）应用于来自相同样品的矿物。这些研究应该澄清，如果不同的计时器适用于不同的岩石上的不同部分的钻孔产生一致的结果或日期不同的事件。因此，它们将（1）测试分析方法的局限性和鲁棒性，（2）允许区分分析和地质分散，（3）将有助于更好地了解苏禄海的变质演化。

Ultrahigh pressure (UHP) rocks from the Chinese Continental Scientific Drilling (CCSD) program were chosen to study the Sm-Nd and Lu-Hf isotope systems in combination with U-Pb LA-ICP-MS and SHRIMP dating of distinct zircon domains. First results show that the combined use of trace element and U-Th-Pb isotope compositions of zircon and PT information from their mineral inclusions, allow the linkage between U-Pb ages and metamorphic reactions. Inherited and metamorphic zircons were distinguished based on CL imaging, mineral inclusions, trace element contents and Hf isotope composition. Up to three phases of zircon growth or re-crystallisation can be identified in a single sample. The ages are interpreted to date the time of (1) prograde and (2) UHP metamorphism during Middle Triassic subduction, and (3) retrograde metamorphism during Late Triassic exhumation. Ongoing studies of the Hf isotope analyses of UHP zircon domains from different samples promise new insights in the understanding of the behaviour of the Lu-Hf system during UHP metamorphism. Preliminary Lu-Hf and Sm-Nd grt-cpx isochrones from CCSD and Dabie eclogites yield surprisingly uniform ages that are in average about 6-8 Myr younger than UHP zircons. The interpretation of these ages is yet not clear for us. A major focus of the applied period of the proposal will be the application of the different chronometers (U-Pb on zircons and Lu-Hf / Sm-Nd on garnet-cpx) on minerals from the same samples. These studies should clarify if the different chronometers applied to different rocks on various sections of the drill hole yield consistent results or date different events. They will thus (1) test the limitations and robustness of the analytical methods, (2) allow to distinguish between analytical and geological scatter, and (3) will contribute to a better understanding of the metamorphic evolution of the Sulu terrane.