Precisely dating Paleozoic kimberlites in the North China Craton and Hf isotopic constraints on the evolution of the subcontinental lithospheric mantle

Kimberlite, a deep-sourced ultramafic potassic rock, carries not only diamond, but also invaluable mantle xenoliths and/or xenocrysts, which are important for tracking the evolution of subcontinental lithospheric mantle (SCLM). However, it is challenging to accurately determine the emplacement age of kimberlite and its compositions of primary magma because of modifications by crustal and/or mantle contamination and post-emplacement alteration. This paper reports emplacement ages of diamondiferous kimberlites in Mengyin and Fuxian of the North China Craton (NCC) using three different dating methods. For Mengyin kimberlite, single-grain phlogopite Rb-Sr dating yields an isochron age of 485 +/- 4 Ma, U-Th-Pb analyses on perovskite give a U-238-Pb-206 age of 480.6 +/- 2.9 Ma and a Th-232-Pb-208 age of 478.9 +/- 3.9 Ma, and baddeleyite yields a Pb-207-Pb-206 age of 480.4 +/- 3.9 Ma. For Fuxian kimberlite, baddeleyite gives a Pb-207-Pb-206 age of 479.6 +/- 3.9 Ma, indicating that the Paleozoic kimberlites in the NCC were emplaced at similar to 480 Ma. Numerous lines of evidence indicate that the studied baddeleyites are xenocrysts from the SCLM, and can be used to constrain Hf isotope compositions (epsilon(Hf)(t)similar to-6) of the SCLM when kimberlite erupted. Combined with data from Mesozoic-Cenozoic mantle-derived rocks and xenoliths, the Hf isotope evolution trend of the SCLM beneath NCC before craton destruction was tentatively constructed, which suggested that the Archean SLCM was enriched by metasomatism at similar to 1.3 Ga. Further Hf isotope investigations on additional SCLM-derived materials could be used to compare with the constructed Hf isotope evolution trend before craton destruction to determine when lithospheric thinning occurred. (C) 2011 Published by Elsevier B.V.