U–Pb zircon age, geochemical and Sr–Nd–Pb–Hf isotopic constraints on age and origin of alkaline intrusions and associated mafic dikes from Sulu orogenic belt, Eastern China

Abstract: Post-orogenic alkaline intrusions and associated mafic dikes from the Sulu orogenic belt of eastern China consist of quartz monzonites, A-type granites and associated mafic dikes. We report here U–Pb zircon ages, geochemical data and Sr–Nd–Pb–Hf isotopic data for these rocks. The SHRIMP U–Pb zircon analyses yield consistent ages ranging from 120.3±2.1 Ma to 126.9±1.9 Ma for five samples from the felsic rocks, and two crystallization ages of 119.0±1.7 Ma and 120.2±1.9 Ma for the mafic dikes. The felsic rocks and mafic dikes are characterized by high (87Sr/86Sr)i ranging from 0.7079 to 0.7089, low ε Nd(t) values from −15.3 to −19.2, 206Pb/204Pb=16.54–17.25, 207Pb/204Pb=15.38–15.63, 208Pb/204Pb=37.15–38.45, and relatively uniform ε Hf(t) values of between −21.6±0.6 and −23.7±1.0, for the magmatic zircons. The results suggest that they were derived from a common enriched lithospheric mantle source that was metasomatized by foundered lower crustal eclogitic materials before magma generation. Geochemical and isotopic characteristics imply that the primary magma to these rocks originated through partial melting of ancient lithospheric mantle that was variably hybridized by melts derived from foundered lower crustal eclogite. The mafic dikes may have been generated by subsequent fractionation of clinopyroxene, whereas the felsic rocks resulted from fractionation of potassium feldspar, plagioclase and ilmenite or rutile. Both were not affected by crustal contamination. Combined with previous studies, these findings provide new evidence that the intense lithospheric thinning beneath the Sulu belt of eastern China occurred between 119 and 127 Ma, and that this was caused by the removal of the lower lithosphere (mantle and lower crust). [Copyright &y& Elsevier]