Palaeoproterozoic Khondalite Belt in the western North China Craton: New evidence from SHRIMP dating and Hf isotope composition of zircons from metamorphic rocks in the Bayan Ul-Helan Mountains area

Zircom U-Pb age and Hf isotope analyses were made on gneissic granite and garnet-mica two-feldspar gneiss from the Helanshan Group in the Bayan Ul-Helan Mountains area, the western block of the North China Craton (NCC). Zircons from the gneissic granite commonly show core-mantle-rim structures, with magmatic core, metamorphic mantle and rim having ages of 2323±20 Ma, 1923±28 Ma and 1856±12 Ma, respectively. The core, mantle and rim show similar Hf isotope compositions, with single-stage depleted mantle model ages (TDM1) of 2455 to 2655 Ma (19 analyses). Most of the detrital zircons from the garnet-mica two-feldspar paragneiss have a concentrated U-Pb age distribution, with a weighted mean 207Pb/206Pb age of 1978±17 Ma. A few detrital zircons are older (2871 to 2469 Ma). The age for metamorphic overgrown rim was not determined because of strong Pb loss due to their high U content. The zircons show large variation in Hf isotope composition, with TDM1 ages of 1999 to 3047 Ma. In com- bination with previous studies, the main conclusions are as follows: (1) protolith of the khondalite se- ries in the Helanshan Group formed during Palaeoproterozoic rather than the Archaean as previously considered; (2) The results lend support to the contention that there is a huge Palaeoproterozoic Khondalite (metasedimentary) Belt between the Yinshan Mountains Block and the Ordos Block in the Western Block of NCC; (3) The widely-distributed bodies of early Palaeoproterozoic orthogneisses in the Khondalite Belt might be one of the important sources for detritus material in the khondalite series; (4) Collision between the Yinshan Block, the Ordos Block and the Eastern Block occurred in the same tectonothermal event of late Palaeoproterozoic, resulting in the final assembly of the NCC.

Zircom U-Pb age and Hf isotope analyses were made on gneissic granite and garnet-mica two-feldspar gneiss from the Helanshan Group in the Bayan UI-Helan Mountains area, the western block of the North China Craton （NCC）. Zircons from the gneissic granite commonly show core-mantle-rim structures, with magmatic core, metamorphic mantle and rim having ages of 2323±20 Ma, 1923±28 Ma and 1856±12 Ma, respectively. The core, mantle and rim show similar Hf isotope compositions, with single-stage depleted mantle model ages （TOM1） of 2455 to 2655 Ma （19 analyses）. Most of the detrital zircons from the garnet-mica two-feldspar paragneiss have a concentrated U-Pb age distribution, with a weighted mean 2^207Pb/^206Pb age of 1978±17 Ma. A few detrital zircons are older （2871 to 2459 Ma）. The age for metamorphic overgrown rim was not determined because of strong Pb loss due to their high U content. The zircons show large variation in Hf isotope composition, with TOM1 ages of 1999 to 3047 Ma. In com- bination with previous studies, the main conclusions are as follows： （1） protolith of the khondalite series in the Helanshan Group formed during Palaeoproterozoic rather than the Archaean as previously considered; （2） The results lend support to the contention that there is a huge Palaeoproterozoic Khondalite （metasedimentary） Belt between the Yinshan Mountains Block and the Ordos Block in the Western Block of NCC; （3） The widely-distributed bodies of early Palaeoproterozoic orthogneisses in the Khondalite Belt might be one of the important sources for detritus material in the khondalite series; （4） Collision between the Yinshan Block, the Ordos Block and the Eastern Block occurred in the same tectonothermal event of late Palaeoproterozoic, resulting in the final assembly of the NCC.