Geochemistry and origin of tektites from Dalat area,Vietnam

摘要

We measured the major and trace element contents and Rb-Sr isotopic compositions of 9 tektites from the Dalat area,Vietnam.All the samples studied are splash-form tektites which show pitted or grooved surfaces with schlieren structures on some surfaces.The trace element ratios Ba/Rb (avg.4.00),Th/Sm (avg.2.06),Sm/Sc (avg.0.60),Th/Sc (avg.1.25),La/Sc (avg.3.48),Th/U (avg.6.83),and the rare earth elements (REE) contents of tektites of this study are similar to the average upper continental crust.From the chemical composition,it is suggested that tektites in this study are derived from similar parental terrestrial sedimentary deposit which may be related to post-Archean upper crustal rocks.The tektites from the Dalat area have high positive εSr(0) values-ranging from 184.8～196.5 which indicate that the parental material for these tektites have similar Sr isotopic compositions to old terrestrial sedimentary rocks and they were not dominantly derived from recent young sediments (such as soil or loess).The Sr isotopic data obtained by the present study support the conclusion proposed by Blum et al.(1992)[1] that the depositional age of sedimentary target materials is close to 170Ma (Jurassic).Mixing calculations based on the model proposed by Ho and Chen (1996)[2] for various amounts and combinations of target rocks indicate that the best fit for tektites from the Dalat area is a mixture of 39％ shale,29％ sandstone,32％ greywacke,1％ quartzite.