LA-ICP-MS U–Pb zircon geochronology and geochemistry of Paleoproterozoic mafic dykes from western Shandong Province: Implications for back-arc basin magmatism in the Eastern Block, North China Craton

Abstract

LA-ICP-MS U–Pb zircon geochronological, elemental and Sr–Nd isotopic data are presented for Paleoproterozoic mafic dykes in western Shandong Province to characterize the Paleoproterozoic tectonic process in the Eastern Block, North China Craton. These dykes are predominantly dolerite with a major mineral assemblage plagioclase + clinopyroxene and yield a LA-ICP-MS U–Pb zircon age of 1841 ± 17 Ma. They are geochemically low- to medium-potassium subalkaline basalts and basaltic andesites. ( 87Sr/ 86Sr)(i) ratios range from 0.702859 to 0.703706, and ɛ Nd( t) values are from +3.82 to +7.42, which are slightly enriched comparable to a MORB source. The arc-like signatures are revealed in these dykes by the enrichment in LILEs and LREEs and depletion in HFSEs and HREEs. Such signatures suggest an origination from a MORB-like source metasomatized by newly subduction-related fluids potentially via source contamination. The geochemical affinity to both MORB- and arc-like sources, together with incompatible elemental ratios (e.g., Zr/Y, Th/Yb, Ta/Yb, Th/Nb and Ba/La), appears to support a development of a Paleoproterozoic intra-continental extension or failed back-arc basin setting within the Eastern Block in response to an upwelling asthenosphere, accompanied by the amalgamation of the Western and Eastern Blocks along the Trans-North China Orogen at ∼1.85 Ga. Such a proposed tectonic setting might be analogue to modern back-arc basins along the western Pacific.