Geochemical and isotopic constraints on the age and origin of mafic dikes from eastern Shandong Province, eastern North China Craton

摘要

Post-orogenic mafic dikes are widespread across eastern Shandong Province, North China Craton, eastern China. We here report new U-Pb zircon ages and bulk-rock geochemical and Sr-Nd-Pb isotopic data for representative samples of these rocks. LA-ICP-MS U-Pb zircon analysis of two mafic dike samples yields consistent ages of 118.7 ± 0.25 million years and 122.4 ± 0.21 million years. These Mesozoic mafic dikes arc characterized by high (~(87)Sr/~(86)Sr)_i ranging from 0.7082 to 0.7087, low ε_(Nd)(t) values from -17.0 to -17.5, ~(206)Pb/~(204)Pb from 17.14 to 17.18, ~(207)Pb/~(204)Pb from 15.44 to 15.55, and ~(208)Pb/~(204)Pb from 37.47 to 38.20. Our results suggest that the parental magmas of these dikes were derived from an ancient, enriched lithospheric mantle source that was metasomatized by foundered lower crustal eclogitic materials prior to magma generation. The mafic dikes underwent minor fractionation during ascent and negligible crustal contamination. Combined with previous studies, these findings provide additional evidence that intense lithospheric thinning beneath eastern Shandong occurred at -120 Ma, and that this condition was caused by the removal/foundering of the lithospheric mantle and lower crust.