Sm-Nd and SHRIMP U-Pb zircon geochronology of high-pressure granulites in the Sanggan area, North China Craton: timing of Paleoproterozoic continental collision

High-pressure (HP) mafic granulites are widely distributed as sheet-like and lenticular bodies within highly deformed tonalitic gneisses in the Sanggan area, North China Craton. They have clockwise isothermal decompression and isobaric cooling P-T paths implying continental collision tectonics. Whole-rock analyses of 15 samples define a Sm-Nd isochron yielding an age of 2547 +/- 87 Ma with epsilon(Nd)(t) = 2.7. Corresponding Nd depleted mantle model ages show a similar range and are interpreted as the age of the magmatic protolith to the high-pressure granulites. A combined Sm-Nd mineral + whole-rock isochron from one of these samples yields an age of 1813 +/- 23 Ma, taken to represent the metamorphic age at an early stage on the cooling path. SHRIMP U-Pb zircon analyses of three HP mafic granulite samples give uniform weighted mean Pb-207/Pb-206 ages of 1817 +/- 12, 1817 +/- 12 and 1819 +/- 16 Ma. These ages are interpreted to date metamorphism of the HP granulites and, with he Sm-Nd mineral + whole-rock result, strongly suggest that the time of continental collision was at ca. 1.82 Ga. An additional sample records two age populations; one at 1803 +/- 9 Ma and an older one at 1872 +/- 16 Ma. The latter may indicate the timing of either an earlier metamorphic event or possibly the earliest stage of a single prolonged metamorphic episode that peaked at similar to 1.82 Ga. These results confirm the Paleoproterozoic age for assembly of the North China Craton. (c) 2004 Elsevier Ltd. All rights reserved.