Modification of Archean lower crust of the North China Craton by magma underplating during the Mesozoic: Evidence from zircon U-Pb-Hf-O isotopes of granulite xenoliths

Abstract

Previous studies of felsic granulite and pyroxenite xenoliths have confirmed the existence of the Archean lower crust beneath the Fuxin region which experienced magma underplating in the Phanerozoic. U-Pb-Hf-O isotopes of zircons in felsic granulite xenoliths from the Fuxin late Cretaceous basalts in the North China Craton were analyzed to evaluate the role of magma underplating in the modification of the Archean lower crust during the Phanerozoic. The felsic granulites recorded a uniform upper intercept age of 2.5 Ga and a spread of concordant Mesozoic zircons from 177 to 105 Ma, with a peak at 118 Ma. Most Precambrian zircons show discordant U-Pb ages and obvious core-rim structure. The Mesozoic zircons from the granulite xenoliths can be divided into two groups: zircons with an age of >106 and those with an age of ≤106 Ma. Zircons with ages older than 106 Ma are characterized by an unzoned or core-rim structure and enriched Hf-O isotopic compositions, whereas zircons with ages of ≤106 Ma mainly exhibit oscillatory zoning and have depleted Hf-O isotopic compositions. Some >106 Ma zircons with strongly negative εHf(t) values of −40.4 to −24.9 are considered to be age resets of ancient zircons. The relatively higher Hf-O isotopic values for the rest of the >106 Ma zircons indicate that various proportions of underplated magmas with high εHf(t) and δ18O values contributed to their formation. The underplated magmas may have originated from the lithospheric mantle, metasomatized by melts/fluids from subducted Paleo-Asian and Paleo-Pacific oceanic crusts. The ≤106 Ma zircons probably crystallized in a melt from the depleted mantle. These observations suggest that long-lived magma underplating occurred beneath the Fuxin region during the Mesozoic, and the derivation of underplated magmas changed from an enriched lithospheric mantle to a depleted asthenospheric mantle at ∼106 Ma. Combined with our previous studies, we conclude that magma underplating not only provided heat for remelting, but also added exotic material in the modification of ancient lower crust beneath the Fuxin region during the Mesozoic. Furthermore, continuous modifications of cratonic lower crust by magma underplating weaken its refractory and made it more susceptible to chemical erosion and partial melting, which played an important role in the lower crust destruction of the craton.