Geochronology, geochemistry and isotopes of Zaibian diabase in the western margin of Jiangnan orogenic belt, China : Implications for tectonic evolution

The Neoproterozoic mafic-ultramafic rocks are widely developed in the Zaibian area, the southwestern margin of the Jiangnan orogenic belt (JOB) , South China. The petrogenesis and tectonic setting of these mafic-ultramafic rocks remain controversial. In this work, we present petrology, geochemistry, Nd-Hf isotopes and zircon LA-ICP-MS U-Pb geochronology of Zaibian diabase, with the aim to reveal their petrogenesis and tectonic setting. Geochemical data suggest the diabase are characterized by high Mg-# (50 similar to 69 ), high Al2O3 ( 13. 94% similar to 18. 63% ) , low K2O ( 0. 03% similar to 0. 33% ) and low P2O5 ( 0. 04% similar to 0. 28% ) , indicating the characteristics of the low potassium tholeiite series. The rocks are also relatively enriched in Th, U elements and depleted in Nb and Y elements, with a weakly negative Eu anomaly. Their relatively constant Nb/Ta, Zr/Nb ratios suggest that these rocks have not underwent contamination during magma emplacement. Zircon U-Pb dating results yielded U-Pb concordant ages of 823. 1 +/- 9. 5Ma, 821. 1 +/- 9. 2Ma and 820. 7 +/- 3. 0Ma, respectively, suggesting that Zaibian diabases were formed in the Early Neoproterozoic. Zircon Lu-Hf isotopes show the epsilon(Hf) ( t) values varying from - 13. 6 to 7. 7, with t(DM2 )model ages in the range of 1214 similar to 2547Ma. Combined with the whole rock geochemical characteristics, these isotopic features further suggest that the magmas are derived mainly from E-mantle involved some depleted mantle components. Based on geological, geochemical and chronological characteristics from this and previous studies, we propose that the Zaibian area within the southwestern part of JOB was under an intraplate tectonic setting during the Early Neoproterozoic.