Chalcophile elemental constraints on sulfide-saturated fractionation of Cenozoic basalts and andesites in SE China

Abstract

Cenozoic volcanic rocks are widespread in eastern China. These rocks have variable isotope compositions, reflecting different mantle sources under SE and NE China. In order to explore their PGE variations, chalcophile elements are determined for basalts from Leiqiong and Sanshui and andesites from Lianping in SE China. Basalts from Leiqiong have higher Ni (66–376ppm) and Cu (30–73ppm) contents than those from Lianping and Sanshui. All the samples have extremely low PGE contents, low Cu/Zr (<1) and high Cu/Pd ratios (19,000–475,000), except for one olivine-rich sample with much higher PGE concentrations. All the samples have variable Pd/Ir (6.1–185) and Pt/Rh ratios (1.07–43). They are PGE-depleted relatively to Ni and Cu and have U-shaped and MORB-like primitive mantle-normalized chalcophile element patterns. Positive correlations of Ni and Cr with MgO in basalts from Leiqiong suggest fractionation of olivine and chromite. All samples have low Cu/Zr and high Cu/Pd ratios, implying that sulfides were segregated during magmatic evolution. Quantitative modeling indicates that the amounts of sulfide removed from the samples are smaller than that from MORB. Sanshui basalts are depleted in Ru and Pt, indicating fractionation of chromite with Fe–Pt alloys and/or accumulation of olivine with Ir alloys. A few Leiqiong and Lianping andesites may also have experienced fractionation of Fe–Pt alloys based on Pt depletions relative to Rh and Pd. Alternatively, the Pt depletion could also be an inherited feature of a metasomatic mantle source. Basalts from Sanshui have the highest and andesites from Lianping have the lowest Pd/Ir ratios, also consistent with a metasomatized source for the andesites in Lianping.The volcanic rocks from SE China show no strong Ir-depletions, unlike those from NE China. The absence of strong Ir-depletions is likely due to more oxidized SCLM beneath SE.