Geological and geochemical characteristics of intermediate-basic intrusions in Funiushan area, southern margin of the North China Craton: Implications for tectonic environment and mantle properties in the Early Cretaceous

Abstract

This study presents new geochronological and geochemical data for Early Cretaceous intermediate–basic rocks located on the southern margin of the North China Craton (SNCC). The petrogenesis and sources of these rocks were analysed, and the tectonic environment and mantle properties of the SNCC were discussed. The examined rocks include Tianqiaogou monzodiorites and quartz monzonites, Hebian gabbros and diorites, and Renjiagou quartz monzodiorites. LA-ICP-MS UPb dating indicates the crystallization ages of these rocks are 124–116 Ma. The Tianqiaogou monzodiorites show low MgO and Mg^#, low (⁸⁷Sr/⁸⁶Sr)_i ratios (0.706149–0.706388), and high ε_Nd(t) values (−2.36 to −2.68), suggesting that they derived from partial melting of the juvenile basaltic lower crust, which originated from the partial melting of the lithosphere mantle. The uneven composition and unbalanced mineral structure of the quartz monzonites indicate a result of mixing between dioritic magmas and granitic melts. The Hebian gabbros, classified as high-Nb basalt, exhibit significant enrichment in large ion lithophile elements (LILEs) such as Rb, Ba and La, with no negative anomalies for Nb and Ta, as well as high contents of FeO, TiO₂, and total trace elements amount. These characteristics combined with more depleted NdHf isotopic compositions, suggest an origin from a lithospheric mantle that has been strongly metasomatized by asthenosphere-derived melts. The Renjiagou quartz monzodiorites, classified as high-Mg diorite, have high Cr and Ni contents, are significantly enriched in LILEs and depleted in Nb, Ta, Ti and P, with enriched Sr-Nd-Hf isotopic compositions, indicating an origin of lithospheric mantle metasomatized by dehydration fluids from the Paleo-Pacific plate. Geochemical and isotopic data reveal that these intrusions formed in a strongly extensional tectonic setting linked to the rollback of the subducting Paleo-Pacific Plate which caused large-scale asthenospheric upwelling and facilitated the melting of the lithospheric mantle. The Early Cretaceous intermediate-basic rocks that appeared along the SNCC directly or indirectly originated from the mantle sources with varying chemical and isotopic compositions, highlighting the heterogeneity of the lithospheric mantle.