| The Dabie orogenic belt is a typical continental collision belt by Triassic subduction and collision between Yangtze and Sino-Korean plates. It is characterized by not only the largest exposure of Triassic ultrahigh-pressure (UHP) metamorphic rocks in the word but also the most profound occurrence of post-collisional Cretaceous igneous rocks. The Early Cretaceous intermediate to mafic dykes, which intruded into the high-pressure metamorphic rocks of the Susong terrene of the Dabie orogenic belt, are important for understanding the process of mantle-crustal interaction during the deep subduction of the continental plate and the tectonic respond in the uplift. In this study, we present zircon U-Pb ages obtained by the LA-ICPMS technique and geochemical characteristics of these intermediate-mafic dykes and discuss the petrogenesis and geological implications.The analytical results of zircon U-Pb ages show that zircons of six dyke samples give a wide age spectrum from-2600 Ma to-127 Ma. Apparent U-Pb ages of more than one-hundred zircon grains cluster around Archean (>2500 Ma), Paleoproterozoic (2500-1850 Ma and 1800-1650 Ma), Neoprotcrozoic (850-720 Ma), Paleozoic (450-420 Ma), Early Mesozoic (230-190 Ma), and Cretaceous (140-120 Ma). Zircon grains of Early Cretaceous in age are typically of magmatic origin and yield a weighted mean 206Pb/238U age of about 132 Ma, representing the emplacement time of the dykes. This magmatic event should be related to intense post-collisional magmatism in the Dabie orogenic belt. In combination with morphology, internal structures, and trace element contents of zircons, it can be observed that their age patterns are well correlated with tectonothermal events in the Yangtze block. The inherited zircons older than Mesozoic contained in intermediate to mafic dykes is comparable with the age distribution of zircons from the Yangtze block, likely suggesting the Yangtze affinity of the Susong terrene. From the evidence of the inherited zircons, it is speculated that the intermediate to mafic dykes in the Susong terrene originated from the delamination of thickened lithosphere beneath the Dabie orogenic belt.Whole rock major, trace element are studied, the intermediate-mafic dykes are mainly latite, they have varied major elements compositions (e.g. SiO2 range from 51% to 64%), the basic samples show relative high MgO, Cr and Ni contents. All the dyke-samples display homogeneous trace elemental characteristics, like enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE), evident depletion in high field strength elements (HFSE, e. g., Nb, Zr and Ti). Isotopic compositions indicate the magma source of the intermediate-mafic dykes is relatively enriched. The initial 87Sr/86Sr ratios vary from 0.7063 to 0.7109, εNd values are from -15.1 to -22.7. The radiogenic Pb isotopes are relatively low, (206Pb/204Pb)i=15.8585-17.1967, (207Pb/204Pb)i=15.2072-15.3321 and (208Pb/204Pb)i=36.8144-37.6338. Those characteristics indicate the obvious contribution of the Yangtze lower crust. Moreover, the intermediate-mafic dykes show high Sr, low Y and Yb compositions, which comparable to typical adakites, implying generation of the Early Cretaceous intermediate-mafic dykes would be related to the delamination of thickened orogenic lower crust and relative interaction with upper mantle. |
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