Geochemical and Nd isotopic study of Palaeozoic bimodal volcanics in Hainan Island, South China—Implications for rifting tectonics and mantle reservoirs

Abstract

Geochemical and SmNd isotopic results are presented for Carboniferous bimodal metavolcanics from the central part of the Hainan Island, south China. The volcanics are composed of both tholeiites and quartz rhyolites, which are interbedded with clastic metasediments. This volcanic sequence hosts the Shilu FeCoCu Deposit, the richest iron deposit in China. Study of the volcanics is thus important for understanding of both the geochemistry of the subcontinental mantle and the genesis of the Shilu Deposit. Tholeiite samples display large geochemical variations but remarkably uniform Nd isotopic signatures. Overall, at comparable MgO level, tholeiites from the western portion of the volcanic belt have lower TiO 2 and P 2O 5, and higher Al 2O 3 TiO 2 and CaO TiO 2 ratios than those from the eastern portion. A tholeiite sample taken from the Junying area, western portion of the volcanic belt has characteristics of N-type mid-ocean ridge basalts (MORB) in having a LREE-depleted pattern, positive ϵ Nd( T) value (+6.8), as well as relatively high Zr Nb (25), Y Nb (10) and low Zr Y (2.4), La Nb (0.81) ratios. On the other hand, tholeiites occurring in the eastern portion of the volcanic belt are relatively enriched in LIL, HFS and LRE elements. Geochemical features, such as positive Nb anomalies on spiderdiagram and generally high Zr Y ratios, indicate that these rocks are analogous to oceanic island tholeiites (OIT) or initial rifting tholeiites (IRT). However, least contaminated tholeiites in the eastern portion have limited ranges of relatively low La Nb ratios (0.82–1.03) and high ϵ Nd( T) values (+4.2∼+7.6), which are similar to the MORB-type tholeiites in the western portion. The above geochemical features can be best explained in terms of different degrees of partial melting of an isotopically similar depleted source(s). The OIT-like tholeiites from the eastern portion were derived by lower degree partial melting, whilst the N-MORB-like ones from the western portion, by higher degree of partial melting. Some tholeiite samples with lower ϵ Nd( T) values show evidence for crustal contamination during their petrogenesis. Quartz rhyolites from the volcanic belt are enriched in SiO 2 (72–80%), K 2O (4.64–6.42%), Rb, depleted in CaO, Na 2O, Sr, and display strong negative Eu anomalies. Their negative ϵ Nd( T) values (−6.9∼−7.6) are similar to that of the bulk continental crust of south China and suggest a derivation by partial melting of a continental crustal source. We propose that Palaeozoic passive rifting tectonics was responsible for the magmatic activity in this region. The early stage of extensional rifting caused increasing degrees of decompressional partial melting of an upwelling asthenospheric mantle and resulted in the production of tholeiitic magmas. The ascent of these magmas into the crust provided heat which caused partial melting at lower/middle crustal levels and yielded rhyolitic magma. The N-type MORB-like affinities exhibited by tholeiites in the western portion of the belt indicate that the rift in this area may have developed into a small oceanic basin.