Rb-Sr and Sm-Nd isotopic dating of an Early Precambrian spilite-keratophyre sequence in the Wutaishan area, north China: preliminary evidence for Nd-isotopic homogenization in the mafic and felsic lavas during low-grade metamorphism

Abstract

The spilite-keratophyre sequence in the Wutaishan area, North China has been dated by zircon from the quartzkeratophyre and a volcanic crystallization age of 2522 Ma (Liu Dunyi et al., 1985) was obtained. This volcanic rock sequence experienced a low-grade metamorphism of greenschist facies during 1800–2000 Ma. However, combined Sm-Nd whole rock data from the volcanic rock sequence, if interpreted in terms of an isochron, gives a younger age of 2250±182 Ma. If separately calculated, Sm-Nd isochrons for quartz-keratophyre and spilite yield two consistently younger ages of 1977±385 Ma and 1981±178 Ma. These Sm-Nd isochron ages are consistent with a Rb-Sr age of 1871±98 Ma, and with the time of regional greenschist facies metamorphism. Consequently, we interpret the 2250 Ma alignment on the Sm-Nd diagram as an errorchron. The two Sm-Nd metamorphic ages suggest that partial homogenization of Nd isotopic composition in water-rich spilite-keratophyric meta-volcanics can be achieved during low-grade metamorphism as a result of REE mobility. Using the ϵNd values at the time of metamorphism and the average 147Sm/144Nd ratios of the spilite and the quartzkeratophyre, initial ϵNd values of 2.6±3.5 and 3.0±6.1, respectively are estimated at the time of volcanic eruption (2522 Ma). Similar positive initial ϵNd values for the spilites and quartz-keratophyres suggest that they might have been derived from the same depleted mantle source, or that the quartz-keratophyres are derived from a short-lived mafic source.