柴北缘超高压变质带的冷却历史:来自副片麻岩中锆石、金红石的U-Pb年代学和温度信息

Cooling history for North Qaidam UHPM belt: Constraints from zircon,rutile U-Pb dating and thermometry in paragneiss

本文运用LA-ICP-MS和SIMS对柴北缘超高压变质带中东端沙柳河剖面中的副片麻岩进行了锆石和金红石U-Pb年代和微量元素分析。锆石边部的变质时代为425±6Ma,所对应的锆石Ti含量温度计计算出的温度为689±14℃。金红石U-Pb定年给出的年龄为414.0±6.3Ma,代表了副片麻岩在折返过程中冷却到金红石U-Pb封闭温度约570℃的时代。而金红石Zr含量温度计给出同锆石边部较一致的温度685±9℃,代表了峰期变质时代的温度条件。根据锆石的变质时代和变质温度以及金红石的冷却年龄和封闭温度所限定的T-t轨迹,可以得出此副片麻岩在折返过程中的冷却速率约为11℃/Myr。

We analyzed zircon and rutile U-Pb ages and trace elements using LA-ICP-MS and SIMS of paragneiss from Shaliuhe cross-section,North Qaidam ultra-high pressure metamorphic belt. Inherited zircon cores record older protolith ages of 〉700 Ma.While metamorphic zircon rims give weighted average U-Pb age of 425 ± 6Ma,and corresponding TTi-in-zircon= 689 ± 14℃. The rutile give a U-Pb age of 414. 0 ± 6. 3Ma,which may represent the time of the paragneiss cooled down to the closure temperature of U-Pb in rutile（ here,taken as about 570℃）. The rutile thermometry of Zr-in-rutile gives same temperature of 685 ± 9℃ as the zircon thermometry. This indicates both rutile and zircon may preserve the peak metamorphic temperature information. We can construct a T-t path that starts at conditions and time given by the zircon temperature and U-Pb age,and ends with passage through the rutile closure temperature and the rutile age. A cooling rate of about 11℃ /Myr is obtained through this T-t path. Compared with the literature rutile data of eclogites（ Yuka and North Dulan） from this belt,country paragneiss rocks experienced similar cooling history as the enclosed eclogites.