GEOCHEMICAL AND PETROGRAPHICAL CHARACTERISTICS OF THE ARCHEAN DONGWANZI OPHIOLITE COMPLEX AND RELATED ROCKS

The circa 2.50 Ga Dongwanzi ophiolite of the North China craton, has been interpreted as the world's oldest well preserved ophiolite (Kusky et al., Science, 2001). The ophiolite consists of three imbricate belts separated by Archean gneiss and Proterozoic rocks of the Mesoproterozoic Changcheng system. Samples collected from the southern and central belts of the DWO were analyzed by x-ray fluorescence to determine major and trace elemental abundances. The southern and lower central belts show calc-alkaline and tholeiitic MORB trends. Trace element analysis of the MORB-suite show low Y and Ti concentrations that are characteristic of suprasubduction zone ophiolites. The upper central belt shows SSZ characteristics as well as high concentrations of P, Ti, and Fe. Field and geochronological investigations reveal that parts of the central belt are intruded by at least four deformed dike swarms (three mafic dike swarms, including pyroxenite dikes, sheared gabbroic dikes and diabase dikes). Mesozoic diorite, gabbro and plagioclase-granite (circa 294-300 Ma, R. Tucker, pers. comm.) also cut the CB, and one undeformed mafic dike swarm that also intrudes the Mesoproterozoic sequence. The possibly older rock units in the CB that may be part of a dismembered and metamorphosed Archean ophiolite suite include serpentinized harzburgite with disseminated chromite, pyroxenite, cumulate ultramafics, olivine-gabbro, hornblende-gabbro, leucogabbro, mafic dikes, sheeted dikes, and rodingite. Furthermore, the cpx-pyroxenite is strongly sheared and dynamically recrystallized. Malachite, pyrite, and epidote are identified within fractures of the metagabbro of CB, possibly associated with oceanic fluid circulation, which is also identified within pillow lava in the SE belt near Shang Yin. We provisionally interpret the young calc-alkaline suite rocks to be part of a Mesozoic continental magmatic arc, intruding the much older Archean MORB-tholeiites of the Dongwanzi ophiolite. The recognition of younger rocks in the center of the central belt of the DWO significantly reduces the original interpreted thickness of the gabbroic section of the DWO, and likely also hides early structural breaks within the sequence. The DWO therefore should be regarded as a dismembered and metamorphosed Archean ophiolite fragment.