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

选择中国大陆科学钻探（CCSD）项目的超高压（UHP）岩石，结合U-Pb LA-ICP-MS和SHRIMP定年，研究了不同锆石域的Sm-Nd和Lu-Hf同位素系统。结果表明，结合锆石中微量元素和U-Th-Pb同位素组成及其矿物包裹体中的PT信息，可以将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，对继承锆石和变质锆石进行了区分。锆石生长或再结晶的三个阶段可以识别在一个单一的样品。这些年龄被解释为(1)中三叠世俯冲时的前进变质作用和(2)中三叠世超高压变质作用，以及(3)晚三叠世发掘时的逆行变质作用。来自不同样品的UHP锆石域的Hf同位素分析正在进行的研究为理解UHP变质过程中Lu-Hf系统的行为提供了新的见解。CCSD和大别榴辉岩的初步Lu-Hf和Sm-Nd grt-cpx等时线显示出惊人的均匀年龄，平均比UHP锆石年轻6- 8myr。对这些年代的解释我们还不清楚。该建议适用期间的一个主要重点将是对来自同一样品的矿物使用不同的计时器（锆石上的U-Pb和石榴石上的Lu-Hf / Sm-Nd）。这些研究应该阐明，在钻孔的不同部分，不同的岩石上应用不同的时计，是否会得出一致的结果，还是会得出不同的事件。因此，他们将(1)测试分析方法的局限性和稳健性，(2)允许区分分析和地质散射，以及(3)将有助于更好地理解苏鲁地体的变质演化。

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.