Geochemistry and origin of tektites from the Penglei area, Hainan province, southern China

Abstract

Ten tektites from the Penglei area in Hainan province were analyzed for the abundances of major elements, 31 trace elements, and the RbSr isotopic composition. These tektites can be divided into two compositional categories, which are distinguished by high (> 80 wt %) and low (<76 wt %) SiO 2 contents. Tektite TK-1 (high SiO 2) shows a vesicular massive body and has lower refractive index, density, major and trace elements. Its chemical composition closely resembles the average composition of Muong Nong-type indochinites. The others with low SiO 2 either pitted or grooved surfaces, with schlieren structures on some surfaces, and splash-form have similar chemical compositions to those of indochinites. From the chemical composition, it is suggested that the tektites within the Indochina and the Hainan subfields are derived from similar parental material and are similar to the post-Archean upper crustal rocks. In addition, the tektites from Hainan (Hainanites) have large positive ϵ Sr(0) ratios, indicating that the parent material for these tektites resembles old terrestrial sedimentary rocks. From the Sr isotopic data, it is interpreted that the hainanites do not originate from continental material recently derived from the mantle or recent young sediments such as soil or loess. Based on RbSr isotopic data, it has been suggested by Blum et al. Geochim. Cosmochim. Acta 56, 483–492, 1992 that the depositional age of sedimentary target materials is close to 170 Ma (Jurassic). Mixing calculations for various amounts and combinations of target rocks indicate that the best fit for sample TK-1 tektite is a mixture of 2% shale, 38% sandstone, 50% greywacke and 10% quartzite, and the other splash-form tektite is a mixture of 41% shale, 2% sandstone, 20% greywacke and 37% quartzite.