The Cretaceous adakitic–basaltic–granitic magma sequence on south-eastern margin of the North China Craton: Implications for lithospheric thinning mechanism

Abstract

We present here a combined study of Early Cretaceous granodiorites, granites and mafic rocks from the Bengbu area on south-eastern margin of the North China Craton (NCC), in order to provide insights into the thinning mechanism of lithosphere on the cratonic margins. Zircon U–Pb and Ar–Ar dating reveals an adakitic–basaltic–granitic magma sequence with the adakitic intrusions (granodiorites; 123–115Ma) formed prior to the granites and mafic dykes (both ca.112Ma). The granodiorites are geochemically similar to low-Mg# adakitic rocks and isotopically have enriched Sr–Nd–Hf compositions, low radiogenic Pb (206Pb/204Pbi=16.524–17.017), and mantle-like δ18Ozircon (5.8±0.5‰). They were originated from the thickened lower continental crust (LCC) of the NCC. In contrast, the granites have low Sr/Y (<20), La/YbN, Dy/YbN, and prominent negative Eu anomalies, indicating their origination at the shallow middle-lower crustal levels. The mafic dykes are enriched in LREE relative to HREE, and exhibit negative anomalies of Nb, Ta, Ti and positive anomalies of Pb. They have low Nb/U, Ce/Pb, high La/Nb, Ba/Nb, enriched Sr–Nd isotopic compositions and low radiogenic Pb (206Pb/204Pbi=16.789 to 17.262), indicating LCC materials involved into the mantle sources. Modeling suggests derivation of the mafic dykes from the depleted upper mantle hybridized by 10–15% recycled LCC. The contrasting chemical characteristics of adakitic rocks and granites imply that recycling of the LCC was likely achieved through rapid foundering at 115–112Ma, which resulted in thinning of the overlying crust and hybridization of the underlying mantle with closely coeval generation of granitic and basaltic magmas. Anatexis of the pre-existing thickened LCC at 123–115Ma that attenuated the LCC itself and left the residues denser as a result of felsic (adakitic) melt extraction is proposed to have resulted in gravitational instability and foundering of the lithosphere. Such a temporal sequence of magma generation and processes of lithospheric thinning also occurred on north margin of the NCC and are quite similar to the mountain-root removal beneath the Dabie Orogen in central China. This manifests that previous melting of the LCC is perhaps an important trigger responsible for lithospheric foundering in orogenic belts and cartonic margins, where the lithosphere underwent pre-thickening by continental collisions and had stabilized over a long time.