内蒙古黄土窑孔兹岩系的锆石与金红石年龄研究

内蒙古黄土窑及其毗邻地区孔兹岩系已有一些１８００￣１９００Ｍａ的锆石Ｕ－Ｐｂ年龄数据，但因孔兹岩是高级变质的沉积岩，其锆石Ｕ－Ｐｂ年龄往往代表峰期变质年龄而不是原岩沉积年龄，因此孔兹岩系属太古宙还是古元古代长期未决。内蒙古黄土窑是晋蒙孔兹岩系的经典产地之一，对其锆石和金红石研究和年龄测试，得到孔兹岩系的碎屑锆石年龄为２３１０Ｍａ，变质成因锆石年龄为１８７３Ｍａ，金红石代表的冷却年龄为１７９３Ｍａ。

The Ages of Zircon and Rutile (Cooling) from Khondalite in Huangtuyao, Inner Mongolia

Although zircon ages of 1800~1900 Ma have been reported from khondalite in Huangtuyao and adjacent areas, there has still been a debate on the time of the khondalite (Archean vs. early Proterozoic) because zircon of khondalite dates the peak metamorphism rather than sedimentation. On the basis of the content variation of HfO2 and ZrO2 from core to edge in zircon, most of elliptical and round zircons studied in the paper are determined to be metamorphogenic. However , some zircon overgrowths contain a square core with {110} cleavage traces, suggesting that they are remaining magmatogenic zircon. Becuase the protolith of khondalite is sedimentary rocks, the magmatogenic zircon represents ferrigenous detrital material. The information on two age groups is shown by plotting the data of single-zircon fractions on a U-Pb concordia diagram. Taking the aspect of two age groups and Pb-loss into account, it is able to draw more than two discordant lines. The ages given by three upper intercepts with concordia are 1873, 2169 and 2310 Ma, respectively. The first corresponds to the formation age of zircon overgrowth, i. e. the age of peak metamorphism. The second is a mixed age of metamorphogenic zircon with the detrital core. The last corresponds to the age of the detrital zircon, i. e. the age of sedimentation of the khondalite. In addition, metamorphic rutile from the same sample yields a high-precision U-Pb age of 1793 Ma, corresponding to the cooling age of the Huangtuyao high-grade terrain.