ORIGIN AND EMPLACEMENT OF ARCHEAN OPHIOLITES OF THE CENTRAL OROGENIC BELT, NORTH CHINA CRATON

Understanding Archean crustal and mantle evolution hinges upon identification and characterization of oceanic lithosphere. We describe a complete, yet dismembered and metamorphosed Archean ophiolite sequence in the North China Craton, in the Dongwanzi-Zunhua structural belt and correlatives in the Wutaishan area. Banded iron formation structurally overlies several tens of meters of variably deformed pillow lavas, mafic flows, and picritic amphibolites. These are in structural contact with a 2 kilometer thick mixed gabbro and dike complex with gabbro screens, exposed discontinuously along strike for more than 20 km. The dikes consist of metamorphosed diabase, basalt, hb-cpx-gabbro, and pyroxenite. Many have chilled margins developed on their NE sides, indicating one-way chilling. The dike/gabbro complex is underlain by several kilometers of mixed isotropic and foliated gabbro, which preserve compositional layering approximately 2 kilometers below the dike complex, and then over several hundred meters merge into strongly compositionally layered gabbro and olivine-gabbro. The layered gabbro becomes mixed with layered pyroxenite/gabbro marking a transition zone into cumulate ultramafic rocks including serpentinized dunite, pyroxenite and wehrlite, and finally into strongly deformed and serpentinized olivine and orthopyroxene-bearing ultramafic rocks interpreted as depleted mantle harzburgite tectonites. A U/Pb zircon age of 2.505 Ga from gabbro of the Dongwanzi ophiolite makes it the world’s oldest recognized, laterally-extensive complete ophiolite sequence. This age is confirmed by a circa 2.6 Ga Re-Os isochron from chromites from the belt, and a number of dated 2.5-2.4 Ga cross-cutting younger igneous units. Characteristics of this remarkable ophiolite may provide the best constraints yet on the nature of the Archean oceanic crust and mantle, and offer insights to the style of Archean plate tectonics and global heat loss mechanisms.

The Dongwanzi ophiolite is one of the largest well-preserved greenstone belts in the Central Orogenic belt that divides the North China craton into eastern and western blocks. More than 1,000 other fragments of gabbro, pillow lava, sheeted dikes, harzburgite, and podiform-chromite bearing dunite occur as tectonic blocks (tens to hundreds of meters long) in a biotite-gneiss and BIF matrix, intruded by tonalite and granodiorite, in the Zunhua structural belt. Blocks in this metamorphosed Archean ophiolitic mélange preserve deeper levels of oceanic mantle than the Dongwanzi ophiolite. The ophiolite-related melange marks a suture zone across the North China Craton, traced for more than 1600 km along the Central orogenic belt. Many of the chromitite bodies are localized in dunite envelopes within harzburgite tectonite, and have characteristic nodular and orbicular chromite textures, known elsewhere only from ophiolites. The ultramafic and ophiolitic blocks in the Zunhua melange are interpreted as dismembered and strongly deformed parts of the Dongwanzi ophiolite. We suggest the name "Dongwanzi-Zunhua ophiolite belt" for these rocks.