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摘要
The mafic rocks originating from deep earth are probes for lithospheric mantle evolution. In this paper, it presents a synthesis study of zircon U-Pb chronology, whole-rock elemental and Sr-Nd isotopic geochemistry, and zircon Hf isotope of the mafic intrusive rocks in the Zhongtiao Mountain area. Two periods of magmatism during the Late Triassic (217±2 Ma) and the Early Cretaceous (121±2 Ma) are unraveled. The Late Triassic samples are characterized by low to intermediate SiO2 contents (46.03%-53.87%), high MgO (14.37%-18.61%), Ni (282×10-6-433×10-6) and Cr (619×10-6-1 847×10-6) concentrations, low magmatophile element abundances, and convex rare earth element distribution patterns, indicating a cumulate origin. The existence of a large number of original amphiboles indicates that the parental magma is highly water-rich. All samples have nearly parallel trace element distribution patterns, which implies that their mild LILE-HFSE (large ion lithophile element - high field strength element) differentiation reflects the inherent attributes of mantle source region, and their parental magma probably originated from partial melting of the mantle wedge metasomatized by subducted sediment melt/fluid. The SiO2content of Early Cretaceous mafic intrusive rocks lies between 49.23%-54.99%, while the contents of MgO and Fe2O3T are 4.29%-7.17% and 9.70%-14.79%, respectively. Meanwhile, these rocks are enriched in LILEs and light rare earth elements (LREEs), and depleted in HFSEs and heavy rare earth elements (HREEs). Their formation is ascribed to partial melting of lithospheric mantle metasomatized by subducted continental crust-derived melt. The Late Triassic complex may be related to post-orogenic oceanic slab breakoff caused by collision between the Yangtze plate and the North China craton (NCC), while the Early Cretaceous intrusive rocks may be linked with back arc lithospheric extension triggered by the retreat of Paleo-Pacific plate during its westward subduction. In addition, the relative depletion of whole-rock Nd (εNd(t)=-18.56 to -12.64) and zircon Hf (εHf(t)=-20.2 to +10.4) isotopic compositions in samples compared with that of typical craton lithospheric mantle indicates that the lithospheric mantle nature of the central and southern part of the NCC have changed significantly since the Late Triassic, and the Early Cretaceous craton destruction extends to the central part of the NCC.
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