Pb isotope systematics and time-integrated Th/U of SE-Asian continental crust recorded by single K-feldspar grains in large rivers

Abstract

To elucidate time-integrated Th/U and Pb isotope characteristics of the SE-Asian continent, 159 individual K-feldspar grains from the rivers Mekong, Salween, Irrawaddy and Red River were analyzed. Earlier U–Pb dating and Hf isotope results of zircon and baddeleyite from the same river sands have established its crustal growth and recycling history, also showing that these heavy minerals are sufficiently representative for the evolution of large continental regions [Geochim. Cosmochim. Acta 64 (2000) 2067]. We therefore consider K-feldspar populations in these sands to be the best possible way to investigate Pb isotope systematics on the continental scale, and analyzing individual grains overcomes the averaging effect of multi-grain measurements. To test the basic condition that U/Pb-ratios are very low in K-feldspar, Pb concentration were measured in all grains, and U in about 10% of them, corroborating Pb concentrations between 4 and 292 ppm with a mean value of 80±40 ppm, and U and Th well below 0.1 and 0.4 ppm, respectively. In situ decay of U can therefore be neglected in all grains and measured Pb isotope ratios are those acquired by the magmatic or metamorphic host material at the time of K-feldspar crystallization. Relative to the model evolution of 206Pb/ 204Pb in upper continental crust, model ages range between 0.5 and 0.2 Ga, whereas many 207Pb/ 204Pb are significantly more radiogenic than any type of Phanerozoic crust. This confirms important recycling of Precambrian crust into the K-feldspar source material such as already observed by Hf signatures for the heavy mineral source lithologies. An important observation is that none of the K-feldspars has preserved Precambrian Pb isotope signature and in consequence, detrital sediments covering the SE-Asian continent must essentially originate from Phanerozoic orogenic belts, where K-feldspars have been re-equilibrated by metamorphic reactions or crystallization in newly formed melts. To dominate delivery of detrital material to such a large extent on the continental scale, these Phanerozoic orogenies must have generated important high mountains ranges, to be subsequently flattened by erosion; today topographic highs are exclusively due to uplift in relation to the India–Asia collision. Concerning 232Th/ 208Pb and time-integrated Th/U ratios, only 4 out of 159 grains plot on the model curves for continental crust, with all other ratios being significantly more radiogenic in 208Pb/ 204Pb, indicating high Th/U source material. If these time-integrated Th/U ratios are modeled in the frame of the known crust formation events, Th/U ratios lie at 4.15–4.22 plotting significantly higher than Th/U deduced for fine-grained sediments or predicted by classical models for average or upper continental crust.