THE ARCHEAN DONGWANZI OPHIOLITE, NORTH CHINA CRATON: WORLD’S OLDEST KNOWN COMPLETE OPHIOLITE

We report the discovery of a thick and laterally extensive complete Archean ophiolite complex near the village of Dongwanzi, Eastern Hebei, North China Craton. The top of the ophiolite is marked by banded iron formation overlying several hundred meters of deformed pillow lavas. These grade down through a mixed dike/pillow lava section into a 2 km thick 100% sheeted dike complex that extends continuously for at least 5 km, and perhaps 20 km, along strike. The dikes consist of diabase, basalt and gabbro and most have chill margins developed on their NE sides, but not their SW sides, indicating one - way chilling. The sheeted dike complex is associated with plagiogranites both cutting and cut by dikes of the sheeted complex. The dike complex is underlain by several km of isotropic gabbro, which becomes gradually compositionally layered approximately 2 km below the sheeted dikes, and then over several hundred meters merges into strongly compositionally layered gabbro and olivine-gabbro. The layered gabbro becomes mixed with layered pyroxenite/gabbro marking a transition zone that grades into cumulate ultramafic rocks including dunite, pyroxenite and wehrlite, and finally into strongly tectonized and serpentinized OL-OPX rich ultramafic rocks that may be depleted mantle harzburgite tectonites. The top of the Dongwanzi ophiolite is repeated by Mesozoic thrusting. Published geochronology on rocks of the Dongwanzi ophiolite includes a Nd model age of 2756 ±177 Ma, and a 2546 +29/-24 Ma (U-Pb zircon) age on mafic dikes cutting the ophiolite. The base of the ophiolite is strongly deformed and intruded by the 2591+19/-16 Ma Wangchang trondhjemite and circa 2.50 Ga Cuizhanghizi diorite-tonalite complex. We are currently refining these age constraints on the ophiolite. The circa 2756 Ma age of the Dongwanzi ophiolite makes it the world’s oldest recognized, laterally-extensive complete ophiolite sequence, being more than 750 Ma older than the Purtuniq and Jourma ophiolites, previously thought to be the world’s oldest fragments of oceanic lithosphere. The documentation of a complete Archean ophiolite has significant implications for ideas about Precambrian plate tectonics.