U-Pb zircon age constraints on the Dongwanzi ultramafic-mafic body, North China, confirm it is not an Archean ophiolite

In Science May 11 2001 (p. 1142-1145), Kusky et al. reported an ophiolite complex of late Archean age in the Dongwanzi area of the North China Craton, which has subsequently been considered a hallmark for Archean plate tectonics. However, this interpretation has been questioned because no convincing pillow lavas or sheeted dikes have been found in the Dongwanzi complex and, more importantly, as a major part of the ophiolite complex, the Dongwanzi ultramafic-mafic body has not previously been dated and it therefore remains unknown whether it is of Archean age. In this study, we carried out detailed field investigations and SHRIMP U-Pb zircon dating on the rocks of the Dongwanzi ultramafic-mafic body, as recognized by Kusky et al. Our field observations reveal that in several places along the boundaries of the Dongwanzi body, mafic dikes emanating from the main body intrude the country rocks of the unmetamorphosed Mesoproterozoic sedimentary successions of the Changcheng-Jixian 'System'. One of these dikes yielded a SHRIMP U-Pb zircon age of 306 +/- 6 Ma, similar to the SHRIMP U-Pb zircon age of 308 +/- 34 Ma obtained from a plagioclase-bearing pyroxenite from the Dongwanzi ultramafic-mafic body. Four other samples collected from various parts of the ultramafic-mafic body (gabbro, leucogabbro or pyroxenite) record similar SHRIMP U-Pb zircon ages around similar to 300 Ma. Therefore, both field relationships and SHRIMP U-Pb zircon dating results unequivocally demonstrate that the Dongwanzi ultramafic-mafic body is not part of an Archean ophiolite. (c) 2006 Elsevier B.V. All rights reserved.