Metamorphic age of the khondalite series in the Helanshan region: Constraints on the evolution of the western block in the North China Craton
贺兰山孔兹岩系的变质时代及其对华北克拉通西部陆块演化的制约

The khondalite series in the Helanshan region is a main part of the Khondalite Belt in the western block of the North China Craton, whose metamorphic time is still unknown. Using SHRIMP zircon U-Pb dating technique, three representative Al-rich gneiss samples from the Helanshan khondalite series were analyzed, including garnet-cordierite-K-feldspar gneiss, garnet-cordierite monzo-gneiss and garnet-biotite-plagioclase gneiss. The results show that the three rocks have nearly same detrital zircon age though they were collected from different strata. Apparent 207Pb/206Pb ages of detrital zircons range from 2.0 to 2.1Ga and weighted mean ages are from 2017Ma to 2040Ma. According to CL image and Th/U value, all the detrital zircons are magmatic origin, indicating a large scale of granitic intrusion at that time, which could provide enough material for deposition of protolith of the khondalite series. A few detrital zircons have yielded ages (2520~2949Ma) older than 2.5Ga, suggesting the Archean magmatic activities in the region. There are typical metamorphic zircons in the garnet-cordierite monzo-gneiss which might be formed by dehydration reaction of biotite. These metamorphic zircons have yielded a concordant mean age of 1950±8Ma which is interpreted as the metamorphic age of the Helanshan khondalite series. This age is the same as the metamorphic time of Wulashan and Daqingshan khondalite series, suggesting that the whole Khondalite Belt was formed by collision between the Yinshan block and Ordos block at about 1.95Ga.