Early cretaceous lamprophyre dyke swarms in Jiaodong Peninsula, eastern North China Craton, and implications for mantle metasomatism related to subduction

Abstract

Early Cretaceous lamprophyre dykes occur widely in the Jiaodong Peninsula, eastern North China Craton (NCC). These mantle-derived magmatic products allow us to elucidate subduction-related mantle metasomatic events that modified the sub-continental lithospheric mantle of the NCC. The Jiaodong Peninsula lamprophyres display zircon UPb ages of 130 ± 2 to 132 ± 1 Ma, arc-like trace element patterns, high initial 87Sr/86Sr (0.7090 to 0.7115), negative εNd(t) (−17.2 to −13.5) and moderately unradiogenic Pb isotopic compositions (206Pb/204Pb: 17.18–17.47, 207Pb/204Pb: 15.50–15.53, 208Pb/204Pb: 37.99–38.03). Crustal contamination had limited effect on the compositions of these dykes during their emplacement, because of very high contents of trace elements in the relatively primitive lamprophyres and invariable SrNd isotopes with increasing SiO2 from 44 to 54 wt%. Ba/Rb and Dy/Yb ratios indicate that the mantle source was probably composed of veined amphibole-bearing peridotites close to the spinel-garnet transition, resulting from fluid-related metasomatism. The trace element patterns and Sr-Nd-Pb isotopic compositions of the lamprophyres mimic those of Triassic ultrahigh-pressure metamorphic rocks from the Yangtze Craton. Because inherited Neoproterozoic zircons in the lamprophyres were exotic materials of eastern NCC, these geochemical features are interpreted to reflect an isotopically enriched mantle source influenced by subducted continental crustal materials from Yangtze Craton. The lamprophyres share similar arc-like trace element patterns and SrNd isotopic compositions with other, almost coeval, lamprophyre dykes, basalts, gabbros and other mafic rocks in adjacent regions. This variable but marked enrichment indicates extensive and widespread mantle metasomatism by components derived from recycled crustal rocks over a large area of the lithospheric mantle beneath the eastern NCC. A gradual increase in εNd(t) from the southern margin to the interior of the craton likely reflects the waning spatial influence of the subducted Yangtze continental crust. This already enriched lithospheric mantle was subsequently metasomatized by fluids from subducting Paleo-Pacific plate, which promoted the hydration of mantle and provided the high water contents of the Early Cretaceous mafic rocks. After these two-stages of metasomatism, both related to subduction, but from different slabs, variable degrees of melting of the heterogeneously metasomatized lithospheric mantle during extension in the Early Cretaceous resulted in concurrent, but variable mafic rocks inheriting similar trace element and radiogenic isotope characteristics.