Geochronological and Lu-Hf Isotopic Study on Detrital Zircons of the Jianshan Formation, Bayan Obo Group in Shangdu Area, Inner Mongolia: Constraints on Precambrian Crustal Evolution of the Western Block, North China Craton

The Precambrian crustal growth of North China Craton (NCC) has been a hot topic for geologists for a long time. This paper presents LA-ICP-MS U-Pb dating and in-suit Lu-Hf isotopic analysis results of detrital zircons from Jianshan Formation, Bayan Obo Group in the northern western block of North China Craton to illustrate its depositional time and provenance as well as the Precambrian crustal growth in this area. The ages of detrital zircons from Sample TM33 cluster at 1872 similar to 2100Ma (n = 64), with a major age peak at similar to 1960Ma and a minor peak at similar to 2000Ma. The in-suit Lu-Hf isotopic analysis of the zircons indicate their epsilon(Hf)(t) values vary from -2. 6 to 7. 9 (except one of -17. 6), with two-stage Hf model ages (t(DM2)) varying between 2240Ma and 4386Ma (averaged at 2749Ma). The ages of detrital zircons from Sample Z1724 are mainly between 1784 -2200Ma (n = 44), with a major age peak at -1965Ma and a minor peak at -2080Ma. The weighted average age of the youngest concordia ages is 1794 +/- 73Ma (n =2). In-suit zircon Lu-Hf isotopic analysis indicate their epsilon(Hf)(t) values vary from - 5. 4 to 3. 2, with two-stage Hf model ages (t(DM2)) between 2584 similar to 3203Ma. Based on the above data and combined with the 1670Ma gabbro intruding into Bayan Obo Group and the carbonatite with whole rock Pb-207/Pb-206 age at 1649Ma, the depositional age for Jianshan Formation is between 1800Ma and 1650Ma. The main provenances for Jianshan Formation are the Neo-Archean basement in Guyang, Xiwulanbulang and Kondalite Belt in Daqingshan, Zhuozi. There might exist three crustal growth events in the northern western block of NCC at 2600 similar to 2750Ma, similar to 2500Ma and 2000 similar to 2100Ma respectively, and the scale of the last one might be much smaller than the other two.