Geochemical studies of tektites from East Asia

抄録

Thirty tektites from East Asia (including Wenchang and Penglei of Hainan Island, Maoming of Guandong, China; Khon-Kaen of Thailand; Bao Loc of Vietnam; Rizal of Luzon, Philippines) have been analyzed for major and trace element contents and Rb-Sr isotopic compositions. All the samples studied are splash form tektites. The trace element ratios Ba/Rb (avg. 3.74), Th/Sm (avg. 2.31), Sm/Sc (avg. 0.43), Th/Sc (avg. 0.99) and the rare earth elements (REE) contents of tektites studied are similar to the average upper continental crust. The chemical data of tektites in this study indicate that they were derived from similar target rocks which may be related to post-Archean upper crustal materials. The tektites from East Asia have high positive ε^Sr(O) values-ranging from 164.2 to 198.6, indicating that they were not dominantly derived from recent young sediments, such as soil or loess. The Ar-Ar ages (736.8 ± 55.5∼814.6 ± 24.4 ka) of tektites of the present study are consistent with the age of other Australasian tektites, which indicates that all Australasian tektites were derived from a single impact event. Previous studies (Koeberl, 1992; Blum et al., 1992; Schnetzler, 1992) and the present chemical data suggest that these tektites are the result of melting at a single site, which is most probably located in the southern part of the Thailand-Laos border. Mixing calculations based on the model suggested by Ho and Chen (1996) for various amounts and combinations of target rocks indicate that the best fit for East Asia tektites is a mixture of 61% greywacke, 32% sandstone and 7% shale.