Zircon U-Pb ages of olivine pyroxenite xenolith from Hannuoba: Links between the 97-158 Ma basaltic under-plating and granulite-facies metamorphism

摘要

U-Pb zircon dating by LA-ICP-MS and SHRIMP for one olivine pyroxenite yields complex age populations including Mesozoic ages of 97-158 Ma and 228 +- 8.7 Ma, Early Paleozoic ages of 418-427 Ma, Paleoproterozoic age of 1844 ± 13 Ma, Neoarchean age of 2541 ± 54 Ma and middle Archean age of 3123 ± 4.4 Ma. The 97-158 Ma and 228 ± 8.7 Ma zircons show typical igneous oscillatory zonation in CL images, suggesting two episodes of magmatic events. Overlapping of the 97-158 Ma ages with that of granulite xenoliths indicates that the Mesozoic granulite-facies metamorphism was induced by heating from the basaltic underplating at the base of the lower crust. Both processes lasted at least from about 158 to 97 Ma. Ages of 418 -427 Ma could be records of the subduction of Mongolia oceanic crust under the North China craton. Ages of 1.84 Ga, 2.54 Ga and 3.12 Ga correspond to the three important crust-mantle evolutionary events in the North China craton, and imply preservation of Precambrian lower crust in the present-day lower crust.