LA-MC-ICPMS U-Pb geochronology of zircons from the Wenduermiao Group and its tectonic significance
温都尔庙群锆石的LA-MC-ICPMS U-Pb年龄及构造意义

Wenduermiao Group is located at central section of Inner Mongolina of China, which may be subdivided in ascending order into the Sangdalaihuduge Formation and Haerhada Formation. It has been usually considered to be an ophiolite suite, but there has been a controversy on its era. According to filed investigation, Wenduermiao Group includes not only ophiolite but also lithological association of intra-ocean island arc: basalt, andesite-basalt and andesite. Therefore Wenduermiao Group is a suit of complex including ocean-crust slices,intra-ocean island arc and otherwise with different ages and geneses. Based on recent dating of the LA-MC-ICPMS U-Pb zircons from Wenduermiao Group, the intra-ocean island arc metamorphic andesite from upper part of Sangdalaihuduge is dated at 470±2Ma, and the main ages of detrital zircons from two samples(10NM142,10NM143)of Haerhada Formation range from 445Ma to 480Ma and the youngest age of detrital zircons from 10NM143 is mainly 424~438Ma, indicating that there are at least some Middle Silurtion strata in the Haerhada Formation. Our results, in conjunction with the available geological data including ophiolite age (497~477Ma), metamorphic times(446±15Ma~453±1.8Ma) of high-pressure metamorphic rocks and evidence that Late Silurian Xibiehe Formation overlies unconformably Wenduermiao Group, suggest that times of Sangdalaihuduge Formation is possibly from Cambrian to Late Ordovician and times of Haerhada Formation from Late Ordovician to Middle Silurtion. It is thus concluded that Wenduermiao Group is a suit of metamorphic complex formed in Cambrian to Middle Silurtion.