Controls on the PGE distribution of Permian Emeishan alkaline and peralkaline volcanic rocks in Longzhoushan,Sichuan Province,SW China

摘要

The-260-Ma Emeishan large igneous province in SW China and northern Vietnam contains low-Ti tholeiitic basalts,high-Ti alkaline basalts and minor peralkaline volcanic rocks.The flood basalts in Longzhoushan,Sichuan Province,SW China,comprise four units,each of which has volcanic breccia at the base.From the base upward,rocks of Units 1 and 2 are high-Ti basalts with relatively high MgO(7.6-11.0 wt.%),TiO2(1.7-4.5 wt.%)and Ti/Y ratios(420-790).Unit 3 is composed of tephrites with lower MgO(0.87-6.2 wt.%),TiO2(0.96-2.14 wt.%)and Ti/Y ratios(190-530),whereas Unit 4 consists of basaltic andesites with moderate MgO(3.5-4.8 wt.%),TiO2(2.2-3.1 wt.%)and Ti/Y ratios(383-606).The high-Ti basalts may have been derived from an OIB-like mantle source and the basaltic andesites were probably generated by crustal contamination and fractionation of high-Ti basaltic magmas.The tephrites show high Nb/Y(1.2 to 2.7)and Th/Y(0.31 to 0.55)ratios,variable Zr/Hf ratios(40 to 49)and relatively low eNd(r)(-0.7 to+0.28)values,suggesting an asthenospheric mantle contribution modified by carbonate-rich fluids.The high-Ti basalts contain relatively high and variable PGE and their Cu/Pd ratios are relatively constant(compared to the tephrites)and vary from 8,500 to 86,000.We attribute such relative constancy of the Cu/Pd ratios to the presence of residual sulphide in the source of the high-Ti rocks and subsequent crystallization from S-undersaturated melts.The basaltic andesites have relatively high and constant Cu/Pd ratios(130,000-370,492)and the lowest PGE contents(average 0.12 ppb for Pd,0.20 ppb for Pt),suggesting an earlier removal of sulfide due to crustal contamination before emplacement.The tephrites have PGE abundances(0.057 to 1.89 ppb for Pt and 0.11 to 2.10 ppb for Pd)higher than those for basaltic andesites,however,the Cu/Pd ratios(33,300 to 429,000)of the rocks are more variable than those for the basaltic andesites,suggesting that the tephrites may have undergone both S-saturated fractionation during emplacement of the magmas and weak removal of sulfides from the source magmas.Early fractionation of chromite together with removal of laurite and/or Os-Ir-Ru alloys from the parental magmas may be responsible for the negative Ru anomalies of the tephrites,high-Ti basalts and basaltic andesites.