Late Neoarchean–Paleoproterozoic arc-continent accretion along the Khondalite Belt, Western Block, North China Craton: Insights from granitoid rocks of the Daqingshan–Wulashan area

Abstract

Late Neoarchean-early Paleoproterozoic granitoid rocks are widely exposed in the Daqingshan–Wulashan area in the central segment of the Khondalite Belt (KB), Western Block, North China Craton (NCC), and are characterized by widespread evidence of ductile deformation and anatexis. To improve our understanding of the formation and evolution of the KB, in this study we analyzed zircon U–Pb ages, zircon Lu–Hf isotopes, and whole-rock geochemistry of the granitoid rocks. LA–ICP–MS U–Pb analysis of zircons from eight representative granitoid samples indicates the granitoid rocks were emplaced at ca 2.45 and ca 2.50Ga, and underwent multi-stage metamorphism at ca 1.90–1.95, 2.45, and 2.50Ga. Zircon Lu–Hf isotopic analysis shows that the granitoids have positive zircon εHf(t) values with an average of +4.36, and two-stage depleted mantle zircon Hf model ages (TDM2) clustering around 2.7–3.0Ga, indicative of a juvenile crustal source that formed at ca 2.7–3.0Ga. Furthermore, whole-rock geochemical analysis of 34 granitoid samples reveal them to be magnesian, low- to high-K, calcic to calc-alkalic, and metaluminous to weakly peraluminous (alumina saturation index (ASI)=0.74–1.17). They are characterized by high total rare-earth-element (REE) contents (57.45–346.96ppm), enrichment in light REE, and depletion in heavy REE with (La/Yb)n ratios of 8.33–224.16. They have weak Eu anomalies (Eu/Eu∗=0.37–2.53, average=1.08), high Sr contents (>300ppm), and relatively low Y and Yb concentrations (average=10.55 and 0.96ppm, respectively). Primitive-mantle-normalized trace-element diagrams show that the granitoid samples are depleted in high-field-strength elements (Nb, Ta, U and Ti) and enriched in large-ion lithophile elements (Ba, K, Rb and Sr). In general, the samples have geochemical characteristics consistent with cordilleran I-type granitoids, which form in subduction-related magmatic arcs. Therefore, our results, together with the occurrence of synchronous mafic rocks and other available geological data, indicate the KB contains multiple magmatic arcs and underwent long-term deposition and multi-phase metamorphism, analogous to Phanerozoic accretionary orogens such as the Cordilleran orogen. Consequently, we infer that the KB underwent long-term arc-continent accretion along the southern margin of the Yinshan Block during the late Neoarchean–Paleoproterozoic, and late-stage continent–continent collision between the Yinshan and Ordos blocks at ca 1.90–1.95Ga.