Minerogenesis of Shilu Iron Ores with Special Reference to Sm—Nd Isotope Geochemical Characteristics of Shilu Group Bimodal Volcanic Rocks in Hainan Island

Presented in this paper are Sm-Nd isotope and major, trace and rare-earth element analyses of bimodal volcanic rocks of the Shilu Group and other stratigraphic units in northwestern Hainan Is-land ,South China. It is shown that there are some N-MORB-type basalts(spilites) in the western part of the bimodal volcanic belt, in addition to some E-MORB-type and initial rift-type tholeiites (IRT) in th emiddle and eastern parts.Sm-Nd model ages of these basalts range from 545 Ma to 460Ma .The other extremes of the bimodal volcanics are porphyritic quartz rhyolites, which are characteristic of crustal material source.Sm-Nd model ages of the rhyolites range from 1562 Ma to 1371 Ma .The bimodal volcanic rocks are almost distributed in fifts or faulted depressions,as well as in the Upper Paleozoic rift of Hainan Island.Tholeiites of the Shilu Group can be compared with Cenozoic basalts in the middle and south-ern parts of the Red Sea Rift Belt in petrology, elemental geochemistry and Sm-Nd isotope geology. Shilu iron ores are closely associated with N-MORB-type basalts located in the western bimodal vol-canic belt.It is very interesting to note that the Shilu Fe-Co-Cu deposit can also be compared with Atlantis II Deep in the Red Sea Rift Belt.Therefore ,the present authors believe that the Shilu depos-it is a kind of hydrothermal deposit related to ocean volcanic belt ,where the geotectonic setting be-longs to initial extensional rifts in the oceanic crust.On the other hand, the largest Fe-Co-Cu ore de-posit in China used to be influenced by Hercynian granites after mineralization ,as is clearly observed on both εNd(T)-1/Nd and εNd(T)-^147Sm/^144Nd diagrams.