Geochemistry, geochronology, and Sr–Nd–Os isotopic compositions of early Permian mafic dykes in the Alxa Block and a new tectonomagmatic model for the evolution of the NW North China Craton

Abstract

Mafic dykes intruding continental crustal rocks preserve evidence for the isotopic and geochemical composition of their melt source and hence the chemical makeup of the mantle beneath a continent. We have investigated the geochronology, geochemistry, and Sr–Nd–Os isotopes of recently discovered, Early Permian mafic dyke intrusions in the Alxa Block of the North China Craton to characterize the isotopic composition and evolution of the mantle underlying this continental block. SHRIMP UPb dating results reveal that mafic dykes in the Yamaitu area in eastern Alxa are 292 ± 4 Ma in age, whereas those in the Beidashan area in western Alxa are 280 ± 2 Ma in age. The Yamaitu dykes consist mainly of basalt to trachy-basalt (SiO₂ = 52.61–53.01 wt%), belong to the medium-K to high-K calc-alkaline series, and show crust-like rare earth element and trace element patterns and enriched ε_Nd(t) values (−5.8 to −5.6), and initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7073 to 0.7074. In comparison, the Beidashan dykes in the west consist of basalt to basaltic andesite (SiO₂ = 50.16 to 57.83 wt%) and belong to medium-K calc-alkaline series, characterized by lesser enrichment in large ion lithophile elements, but depletion in Nb, Ta, Zr, Hf contents, as well as in their ε_Nd(t) values (−1.3 to +3.1) and initial ⁸⁷Sr/⁸⁶Sr ratios (0.7051 to 0.7068). Our Os isotopic data reveal that crustal contamination of the magmas of these dyke intrusions was limited to less than 5 %. These geochemical features and contrasting SrNd isotopic compositions of the analyzed mafic dykes reflect a heterogenous mantle source beneath the Alxa Block containing an enriched subcontinental lithospheric mantle (SCLM) as well as a depleted mantle, both of which were variously modified by subduction-derived melts and fluids. In light of our new findings on the nature of the Early Permian composition of the SCLM beneath the Alxa Block, we have also examined in this study the extant data on geochronological, and geochemical–isotopic data from the early and middle Paleozoic granitoids in the Alxa Block in order to decipher their diverse melt origins within the geodynamic framework of the northwestern part of the North China Craton (NCC). Based on our synthesis of the magmatic evolution of the Paleozoic–Early Mesozoic intrusions in the Alxa Block, we present a new tectonic model for the crustal evolution of the Alxa Block and of the NW part of the North China Craton, involving a southward subduction of a Paleo-Asian Ocean (PAO) seaway beneath the Alxa Block during the Ordovician through Silurian, a collision of the Alxa and the South Mogolian Blocks since the Early Devonian, and an extensional tectonic regime for the Alxa Block during the Early Carboniferous through Permian time.