The Cihai diabase in the Beishan region, NW China: Isotope geochronology, geochemistry and implications for Cornwall-style iron mineralization

Abstract

Diabase dykes in Cihai, Beishan region, NW China are spatially and temporally associated with ‘Cornwall-type’ iron deposits. U–Pb dating of zircons from a diabase dyke using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) yields an age of 128.5±0.3Ma, indicating an Early Cretaceous crystallization age. Most of the diabases show low Mg-numbers, suggesting evolved magmas. The diabase dykes show typical ophitic or sub-ophitic textures, and are dominantly composed of phenocrysts of plagioclase (40–50%) and clinopyroxene (30–45%), with minor and varying amounts of biotite and hornblende (1–5%), and minor disseminated magnetite (∼5%). Their mineralogy reflects magma differentiation under relatively low oxygen fugacity conditions. The diabase dykes are characterized by minor variation in SiO2 (44.67–49.76wt.%) and MnO (0.14–0.26wt.%), but show a marked range of Al2O3 (10.66–14.21wt.%), total Fe2O3 (9.52–13.88wt.%), TiO2 (0.66–2.82wt.%) and relatively high MgO (4.87–9.29wt.%) with an Mg# value [atomic Mg/(Mg+Fe2+)] of up to 66. The Cihai diabases possibly experienced fractional crystallization of olivine+clinopyroxene and minor crustal contamination during the differentiation process. Prominent negative Nb, Ta and Ti anomalies suggest derivation from subduction-modified mantle. Furthermore, the rocks have relatively unradiogenic Sr- and Nd-isotopic ratios. These characteristics probably reflect partial melting of a subduction component in the source mantle lithosphere through heat input from an upwelling asthenospheric mantle. Such processes probably occurred within an extensional setting during the Early Cretaceous in the Beishan area. The iron-rich fluids were derived from deep sources, and the iron ores were concentrated through a convection cell driven by temperature gradients established by the intrusion of the diabase sills. The combined processes of subduction-related enrichment in the source, shallow depth of emplacement, and the involvement of large-scale circulation of basinal brines from an evaporitic source are inferred to have contributed to the formation of the ‘Cornwall-type’ mineralization in Cihai.