Sm-Nd isotopic ages for C-type eclogite(243.9

Shuguang Li, Hart, S.R. and Tieshan Wu, 1990. 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. Precambrian Res., 47: 191-203. The spilite-keratophyre sequence in the Wutaishan area, North China has been dated by zircon from the quartz-keratophyre 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 1800-2000Ma. However, combined Sm-Nd whold rock data from the volcanic rock sequence, if interpreted in terms of an isochron, gives a younger age of 2250±180Ma. If separately calculated, Sm-Nd isochrons for quartz-keratophyre and spilite yield two consistently younger ages of 1977±385Ma and 1981±178Ma. These Sm-Nd isochron ages are consistent with a Rb-Sr age of 1871±98Ma, 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 €Nd values at the time of metamorphism and the average 147Sm/144Nd ratios of the spilite and the quartz-keratophyre, initial €Nd 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 €Nd 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.

在大别山带及其东延部分（苏北-胶南隆起）发育一条榴辉岩带，它是华北与扬子陆块在这里碰撞产生高压变质作用的有力证据ψ。最近，人们在一部分榴辉岩中发现了柯石英，它以包裹体形式存在于石榴石和绿辉石矿物晶体中[2，3]1）。这是继挪威、西阿尔卑斯之后，世界上第三个发现柯石英的造山带，因而引起了广泛重视。含柯石英榴辉岩均分布在该造山带南翼（岳西以南）大别群的片麻岩和大理岩中。在大别山北翼的榴辉岩中尚未有发现柯石英的报道。近年来我们曾报道了二个产于大别山北翼、桐柏-磨子潭断裂南侧大别群杂岩中不同类型榴辉岩的Sm-Nd同位素年龄，它们均生成于印支期[4，5]。然而大别山南翼含柯石英榴辉岩的形成时代尚属未知。在大别山带是否存在多期榴辉岩，是否存在元古高压变质带仍是一个有争议的问题[2，6，7]。为此，我们对大别山南翼的含柯石英榴辉岩进行了Sm-Nd同位素定年研究，以确定其形成时代。

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