Crust-mantle interaction during the tectono-thermal reactivation of the North China Craton: constraints from SHRIMP zircon U-Pb chronology and geochemistry of Mesozoic plutons from western Shandong

摘要

Chronological, geochemical and Sr-Nd-Pb isotopic analyses have been carried out on the Mesozoic plutons in western Shandong with the aim of characterizing crustal-mantle evolution during the tectono-thermal reactivation of the craton. Detailed SHRIMP zircon U-Pb dating reveals two main periods of Mesozoic activity with contrasting compositions. The older magmatic pulse is manifested by monzonites and monzodiorites from Tongshi for which zircon rims yield a concordant age of 177 + -4 Ma and the cores have a discordant age of ca. 2.5 Ga. Low MgO and Cr, high Na_2O contents and especially their isotopic compositions (~(87)Sr/~(86)Sr < 0.7042, ~(206)Pb/~(204)Pb < 16.8 and implied_(Nb) by approx -12) are consistent with derivation from late Archean-Paleoproterozoic lower crust. Relatively high HREE contents in these Jurassic plutons indicate a garnet-free source (< 32 km), in contrast to the garnet-bearing source (> 40 km) of the late Mesozoic high Sr and low Y granitoids from the same region. Distinctively different depths of crustal melting suggest dynamic thickening of the crust by magmatic underplating during the Jurassic and Cretaceous. The younger dioritic plutons from Laiwu and Yinan were emplaced at 132-126 Ma and show relatively high MgO and Cr contents and large isotopic variability. They were likely derived from enriched lithospheric mantle source and were subjected to crustal contamination during magma evolution. Early Cretaceous mantle melting is coeval with the widespread late Yanshanian granitic magmatism in North China. Early Cretaceous time may correspond to a critical period when a temperature increase due to lithospheric thinning allowed the intersection of the local geotherm and the wet peridotite solidus. While some mantle-derived magmas were erupted, most were trapped at variable crustal depths, triggering large-scale concomitant melting of the crust. Lithospheric thinning must have continued until the late Cretaceous because of the change in the source of mafic magmas from lithospheric to asthenospheric at that time. It is proposed that removal of the lithospheric keel beneath the North China craton may have been initiated as early as the Jurassic, but with the most intense period in the Cretaceous between 130-75 Ma. Such a relatively long time-scale (approx 100 Ma) emphasizes the role of thermomechanical erosion by convective mantle in lithospheric thinning beneath this region.