Geochemistry, U-Pb Zircon Ages and Hf Isotopes of Basement Rocks Beneath the Northeastern Margin of the Ordos Basin: Constraints on the Paleoproterozoic Evolution of the Western North China Craton

Abstract

SHRIMP zircon ages, Hf-in-zircon isotopic compositions and whole rock geochemistry were analyzed on basement metamorphic rocks from drill cores collected from the northeastern margin of the Ordos Basin. Geochemical data from four metasedimentary rocks show large variations in major element compositions, but have similar REE patterns and trace element compositions, with ΣREE = 161.80 × 10−6~341.82 × 10−6, δEu = 0.26~0.63 and LaN/YbN = 3.44~25.38. SHRIMP zircon U-Pb dating of granitic gneiss yielded the magmatic zircon ages between 1856 ± 14 Ma and 2188 ± 11 Ma, with the upper intercept age of 2229 ± 88 Ma. The cores have εHf(t) values of −5.46 to +6.57, and Hf model ages vary from 2869 Ma to 2300 Ma. The analyses of metamorphic overgrowths on zircon grains yield an average metamorphic age of 1865 ± 17 Ma (MSWD = 3.5). The zircon cores have εHf(t) values of −3.98 to 1.95, and Hf model ages vary from 2782 to 2416 Ma. Combined with data from earlier studies, we draw the conclusion that the metamorphic rocks in the borehole were formed during middle Paleoproterozoic time and were involved in a major late Paleoproterozoic tectono-thermal event which might be attributable to the collision between the Western and Eastern Block along the TNCO (the Trans-North China Orogen). The basement rocks are petrologically, geochronologically and geochemically comparable with rocks from the Khondalite series. The fact that the material properties of basement rocks beneath the northeastern Ordos Basin are different from those of the Western Block indicates that the extent and exact boundaries of the Ordos Block need to be refined.