ABSTRACT

The modification of the Archaean lithospheric mantle root beneath the eastern North China Craton (NCC) has been noticed. However, the degree of modification and the characteristics of metasomatic agents for the NCC lithospheric mantle are still unclear. Here, we compile the geochemistry and ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd isotope ratios of Cretaceous primitive basalts (Yixian, Sihetun, Fangcheng and Feixian) from the eastern NCC and estimate the inputs of subduction-related elements into the Archaean lithospheric mantle beneath the NCC. These basalts show primitive (high MgO, Cr and Ni contents) and arc-like geochemical features (enrichments in LILEs and depletions in HFSEs) that indicate that the mantle sources were modified by fluids related to subducted crustal materials. Substantial proportions of subduction-mobile elements (e.g. ~95% Ba and ~90% Th) transferred to the basalt sources via hydrous melts. Thus, a large volume of fluids transferred into the lithospheric mantle. The data support the model that the NCC Archaean lithospheric mantle was weakened by hydrous melts, which resulted in a fusible weakened lithospheric mantle. Preliminary lithospheric thinning was induced by the extension of the NCC resulting from trench retreat of the Paleo-Pacific plate at ~140-120 Ma. Then, decompression melting of the lithospheric mantle caused pervasive melts in the weakened lithospheric mantle, resulting in the lithospheric mantle having a low viscosity comparable to the asthenospheric mantle, which principally reduced the lithospheric mantle.

摘要

华北克拉通东部（NCC）下方太古宙岩石圈地幔根系的改造已经引起人们的注意。然而，NCC岩石圈地幔的修饰程度和交代作用的特征仍不清楚。在这里，我们编译地球化学和⁸⁷ Sr/ ⁸⁶ Sr 和¹⁴³ Nd/ ¹⁴⁴华北克拉通东部白垩纪原始玄武岩（义县、四合屯、防城和肥县）的Nd同位素比值，估算俯冲相关元素对华北克拉通下太古代岩石圈地幔的输入。这些玄武岩显示出原始（高 MgO、Cr 和 Ni 含量）和弧状地球化学特征（LILE 富集和 HFSE 贫化），表明地幔源受到与俯冲地壳物质有关的流体的改造。大量俯冲运动元素（例如~95% Ba 和~90% Th）通过含水熔体转移到玄武岩源。因此，大量流体转移到岩石圈地幔中。数据支持了NCC太古代岩石圈地幔被含水熔体弱化的模型，从而导致了可熔弱的岩石圈地幔。岩石圈的初步减薄是由约 140-120 Ma 的古太平洋板块沟槽后退引起的 NCC 的延伸引起的。然后，岩石圈地幔的减压熔融导致弱化的岩石圈地幔中普遍存在熔体，导致岩石圈地幔具有与软流圈地幔相当的低粘度，这主要是减少了岩石圈地幔。