Late Mesozoic mafic intrusive complexes in North China Block: constraints on the nature of subcontinental lithospheric mantle

The nature of Mesozoic subcontinental lithospheric mantle (SCLM) has been rarely iscussed in detail even though it might be crucial to understanding of the lithospheric thinning process and mechanism in North China Block (NCB). Late Mesozoic ultramafic to mafic intrusive complexes occurring in west Shandong Province, eastern China, provide a possible approach to constrain the nature of the SCLM in NCB. These mafic rocks are characterized by LILE and LREE enrichment, HFSE depletion and apparently positive Eu. anomalies with EM1-like Sr and Nd isotopic compositions. A long-term metasomatism by carbonatitic fluid can be inferred in accordance to their LREE enrichment, HFSE depletion and low Sr-87/Sr-86 ratios. Additionally, the strong LILE enrichment (such as Rb, Ba and K) and the relatively wider range of epsilon (Nd)(t) values indicate that the mantle source had undergone recent metasomatism by high-potassium OIB-like melt during or shortly before partial melting, resulting in mantle heterogeneity. The geochemical and Sr-Nd isotopic characteristics of the ultramafic to mafic intrusives suggest their origin from enriched lithospheric mantle, which was probably dominated by phlogopite-bearing harzburgites at 80 to 160km depths. A combination of the partial melting of such a heterogeneously enriched mantle with fractional crystallization of olivine and pyroxene during magma ascent can well interpret their petrogenesis. When comparing the nature of the Mesozoic SCLM with that in the early Paleozoic, lithospheric extension or thinning occurred in response to the widespread basaltic generation during the late Mesozoic. It is favorable that the Archaean lithospheric keel in NCB was removed by means of the decompressional melting of the preexistent metasome defined by Menzies et al. (1993). (C) 2001 Elsevier Science Ltd. All rights reserved.