A Possible Mechanism to Thin Lithosphere of the North China Craton: Insights from Cretaceous Mafic Dikes in the Jiaodong Pensinsula

destruction of an ancient craton, in that it records the loss of more than 100 km of ancient refractory lithospheric mantle during the Mesozoic (e.g., Menzies et al., 1993). This fundamental change in lithospheric architecture has attracted considerable attention over the last three decades, but the specific deep-level processes associated with the thinning of the lithosphere are still being actively debated. Thermo-mechanical-chemical erosion and rapid lithospheric delamination are two major hypotheses that have been suggested for the mode of lithospheric thinning. The former represents a slow thinning process lasting at least 100 Ma, whereas the latter marks a short event lasting about 10 Ma (Menzies et al., 2007). Asthenospheric mantle -derived mafic rocks are important indicators related to cratonic destruction, and they help us to resolve the above questions of timing and mechanism of lithospheric thinning. The thinning process is generally accompanied by upwelling of the convective asthenosphere and modification of the thermal structure of the lithosphere. During lithospheric extension, the source of magmas can shift from the lithosphere to the asthenosphere (Xu et al., 2009). Normally, the asthenosphere beneath an old craton cannot melt until the thickness of the lithosphere is markedly thinned (e.g., McKenzie and Bickle, 1988). Two key questions in constraining the mechanism of lithospheric thinning beneath the NCC are: (1) When did the magma source shift from lithospheric to asthenospheric mantle. (2) Was this transition a relatively “slow”, or a “rapid” process (Ma et al., 2014a). The Mesozoic asthenospheric mantle-derived mafic rocks are key to understanding the thinning processes. However, the occurrences of asthenosphere-derived magmas during the Mesozoic in the NCC are extremely limited. Three places have been found recording the transition in magma source from a lithospheric to an asthenospheric mantle, including the Fuxin basalts (107–97 Ma) in the western Liaoning province (Zhang et al., 2003), the Daxizhuang alkali basalts (73 Ma) and Pishikou mafic dikes (86–78 Ma) in the Jiaodong Peninsula (Yan et al., 2003; Zhang et al., 2008). Recently, we reported on earlier (~120 Ma) asthenospheric mantle-derived high-Ti lamprophyres in the Jiaodong Peninsula, clearly demonstrating the asthenosphere mantle melting during the early Cretaceous (Ma et al., 2014a). Here, we report new zircon U–Pb geochronology, elemental geochemistry, and Sr–Nd–Hf isotopic data for lamprophyres and diabaseporphyries of the Jiaodong Peninsula in order to further place constraints on models for lithospheric thinning. Our results show that the lamprophyres and diabase-porphyries are derived from the convective asthenospheric mantle via different degrees of partial melting, and that this mantle source was previously modified by carbonatitic liquids. Zircon LA-ICP-MS U–Pb dating suggests an emplacement age for these rocks of 123–121 Ma, the earliest evidence for asthenospherically-derived melts in the Jiaodong Peninsula so far. This emplacement age indicates that the thickness of the lithosphere in the Jiaodong Peninsula was relatively thin at that time. Co-occurrence of the asthenospheric and lithospheric mantle-derived mafic rocks as well as high-Mg adakites record a rapid transition from lithospheric to MA Liang, JIANG Shaoyong, Albrecht W. Hofmann and XU Yigang, 2016. A Possible Mechanism to Thin Lithosphere of the North China Craton: Insights from Cretaceous Mafic Dikes in the Jiaodong Pensinsula. Acta Geologica Sinica (English Edition), 90(supp. 1): 106-108.