Geochemistry of Late Mesozoic dioritic porphyries associated with Kiruna-style and stratabound carbonate-hosted Zhonggu iron ores, Middle-Lower Yangtze Valley, Eastern China: Constraints on petrogenesis and iron sources

Several small Mesozoic sodium-rich dioritic porphyries associated with huge iron oxide deposits occur in the Zhonggu iron ore field, in the eastern part of the Middle-Lower Yangtze River Belt (MLYRB) in the Yangtze craton. We present bulk-rock major and trace element and Sr and Nd isotopic compositions of the three representative dioritic porphyries: the Gushan, Hemushan and Baixiangshan dioritic intrusions. These porphyries are significantly enriched in Pb and light rare earth elements (LREE), relative to high field strength elements (HFSE: Nb, Ta and Ti), coupled with the absence of significant Eu anomalies. They exhibit negative epsilon(Nd)(t) (-5.2 to -6.8), and highly radiogenic Pb (Pb-206/Pb-204 = 18.27-18.45). The similar geochemical characteristics of these iron ore-bearing dioritic stocks suggest that they were derived from a common parent basaltic melt, which was produced by partial melting of the enriched lithospheric mantle. However, the parental melt experienced assimilation of Yangtze Block upper crust, accompanying the fractionation of plagioclase and clinopyroxene during ascent to the surface, which led to the extreme enrichment of iron in a highly evolved magma. The contribution of phosphorus and potential CO2 from the country rocks might have been the crucial factor that led to liquid immiscibility and formation of the unique ore-magma type mineralization. (C) 2010 Elsevier B.V. All rights reserved.