大别-苏鲁造山带高级变质岩中锆石微区U,Th和Pb化学组成特征统计

Th and Pb characteristics of high-grade metamorphic rocks from Dabie-Sulu orogen.

对大别-苏鲁造山带27个高级变质岩(以榴辉岩和片麻岩为主)中锆石进行了337次微区离子探针U,Th和Pb测定和U-Pb年龄研究。根据测定点年龄的不同,可以把变质岩中锆石分成三种不同成因区域,一是变质岩原岩(继承)锆石区域,它遭受了不同程度的变质重结晶作用,其^(206)Pb/^(238)U年龄值大于277Ma;二是三叠纪高压-超高压变质阶段形成的变质锆石区域,其年龄限定为273～184Ma;三是碰撞造山变质作用后形成的锆石区域,其年龄小于148Ma。对三种类型锆石中U,Th和Pb特征分析发现:片麻岩锆石中U的含量是榴辉岩类锆石中U的丰度的两倍,片麻岩类和榴辉岩类锆石中Th的丰度大致相近。各类岩石变质阶段形成的变质锆石和原岩锆石区域相比,U含量有升有降,而Th含量则大幅下降,下降幅度一般在2～10倍,高者可达20～30倍以上。除石榴石橄榄岩外,各类岩石变质锆石中区域的Th/U比都小于0.1,这是判断高压-超高压变质成因锆石的极重要化学指标。大别山超高压变质阶段后形成的锆石的Th/U比多数类似于其原岩锆石,少数接近变质锆石,说明白垩纪后期热扰动对变质岩中已存在的锆石的影响如同岩浆结晶过程。各类岩石中锆石Ph的含量变化比较复杂,Pb-U正相关性表明锆石中普通铅含量很低,绝大部分为放射成因铅。

337 zircon U, Th and Ph contents, and Ph isotopic compositions of 27 high-grade metamorphic rocks, mainly eclogites and gneisses from Dabie-Sulu orogen were analyzed by using ion micro-probe technique. According to their U/Pb ages, three different genetic zircon domains can be distinguished. First one is protolith ( inherited) domain with different degrees of metamorphic re-crystallized protolith domain. Its Pb-206/(238) U ages are older than 273 Ma. The second one is metamorphic zircon domain formed during Triassic HP-UHP metamorphic event, and its ages are between 273 and 170 Ma. The third domain was formed during post-collision stage with ages younger than 170 Ma. Three kinds of zircon domains all have different U, Th and Pb content characteristics. U concentration of zircon from sialic gneissic rocks is almost double of that of mafic eclogite. Both ecolgite and gneiss zircons have similar Th contents. Zircon U content at UHP metamorphic condition from different rocks could be higher or lower compared with their protolith precursor. Whereas zircon Th contents in metamorphic event are dramatically decreased, with the only exception for garnet peridotite. The decreasing extent could reach usually 2 similar to 10 times, even to above 20 similar to 30 times. The average Th/U ratios for pre-metamorphic protolith ( inherited) zircon domains are variable between 0.15 to 1.19, whereas the average Th/U ratios in metamorphic domains of various rocks are all less than 0.1. This Th/U ratio is the most important and promising chemical index to judge weather the zircon was formed during HP-UHP metamorphic condition. The positive correlation between Th/U ratios and Pb-206/(238) U apparent ages indicates that the decreasing of Th/U ratios is mainly controlled by re-crystallization of protolith zircon. The Th/U ratios of post-collision zircon domains for most samples are same as those of protolith domains, only a few are similar to metamorphic domains. It implies that the infection of late Cretaceous thermal disturbance for existing zircons in metamorphic rocks is resembled to that of magmatic crystallization process. Zircon Pb contents in metamorphic rocks are controlled by many factors. The positive correlations between U and Pb imply that most Pb in zircon is radiogenic, common lead is very low.