Decoupling of Lu-Hf and Sm-Nd Isotopic System in Deep-Seated Xenoliths from the Xuzhou-Suzhou Area, China: Differences in Element Mobility during Metamorphism

Abstract

This paper presents whole-rock Hf isotopic data for a suite of eclogite and garnet clinopyroxenite xenoliths hosted in the Early Cretaceous dioritic intrusions from the Xuzhou-Suzhou area along the southeastern margin of the Eastern Block of the North China Craton (NCC). Six of the eight studied xenolith samples plot significantly above the terrestrial Hf-Nd isotopic array and have eHf(0) value up to +60. All the samples define a well correlated 147Sm/144Nd-143Nd/144Nd age of 2 081 Ma, which is considered to record the granulite-facies metamorphism. In contrast, the Lu-Hf isotope system faithfully records the protolith information. The mineralogical assemblage, especially garnet and/or zircon (rutile to some extent) mainly controlled the decoupling of Hf-Nd isotope. The metamorphic modification on protolith characteristics and the differences in element mobility during metamorphism may also reinforce the observed decoupling between the Sm-Nd and Lu-Hf isotope systems; i.e., the absence of the correlations in eNd-eHf and also 87Sr/86Sr-143Nd/144Nd diagram. The Lu/Hf isochron age of 2 424 Ma is similar to the zircon age peak of the studied xenoliths and the dominant age of NCC basement, indicating that the igneous protolith has an affinity to the Archean basement of the NCC. Furthermore, the positive eHf(t) values at 2 500 Ma indicate a crustal growth event of 2 500 Ma in the NCC.