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

Abstract

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 ε 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 207Pb/ 206Pb ages of 1817±12, 1817±12 and 1819±16 Ma. These ages are interpreted to date metamorphism of the HP granulites and, with the 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 ∼1.82 Ga. These results confirm the Paleoproterozoic age for assembly of the North China Craton.