Sedimentary provenance constraints on drainage evolution models for SE Tibet: Evidence from detrital K‐feldspar
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Abstract The unusual drainage pattern of the Jinsha River (upper Yangtze) is closely linked to the uplift of the Tibetan Plateau, but when this pattern became established is controversial. In this study we compare the Pb isotopic composition of detrital K‐feldspars from the modern Jinsha River with those from the Plio‐Pleistocene Xigeda Formation, a sequence of fluvial and lacustrine deposits widely distributed along the major rivers draining the Tibetan Plateau, to investigate their relationship and to constrain the drainage evolution of the region. Our results demonstrate that the Jinsha River had achieved its current sediment‐transport pattern by the late Pliocene, indicating a drainage character similar to that operating at present. Our findings not only suggest an upper age limit for formation of the Jinsha River but also shed new light on the evolution of the whole Yangtze system. K‐feldspar grains from the Jinsha River have less radiogenic Pb than those in the Red, Mekong, and Salween Rivers, providing a key test as to whether the Jinsha River was previously connected to the paleo‐Red River. Comparison with published Pb isotopic data shows that these characteristically unradiogenic feldspars are absent in the onshore Cenozoic archive from the Hanoi Basin, suggesting that there has been no connection between the lower Jinsha River and the Red River since the Eocene.Keywords:
Radiogenic nuclide
Early Pleistocene
Radiogenic nuclide
Isotopes of strontium
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Radiogenic nuclide
Alkali feldspar
Hornblende
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K-feldspar, a ubiquitous mineral in igneous and metamorphic rocks, has long been used in K-Ar dating. However, a number of problems exist in K-Ar dating of K-feldspar. For example, K-feldspar usually yields younger ages than the co-existed minerals such as biotite. K-feldspar may also yield older ages in some cases. It is suggested that incomplete extraction of radiogenic argon from K-feldspar is one of the main reasons that leads to a younger age. Several solutions have been put forward to resolve the problem, but none of them has been successfully used in experiments. K-feldspar has low closure temperature (about 130~150℃), which can easily cause radiogenic argon lose; In addition, radiogenic argon from the wall rock as excess argon may also enter the K-feldspar. The low temperature K-feldspar (such as adularia) shows capability to maintain various amounts of radiogenic argon which is related to their textures. The content of radiogenic argon in adularia is decreased with the increase of their triclinities. In summary, the applicabilities of K-feldspars from different geological environments are various in K-Ar dating. K-feldspars (such as micro cline and perthite) from intrusive rocks can not be used in K-Ar dating, while those (sanidine and anorthoclase) from extrusive rocks are very good ones, and the low temperature K-feldspars (adularia) can be used in dating conditionally.
Radiogenic nuclide
K–Ar dating
Sanidine
Isotopes of argon
Isochron dating
Radiometric dating
Geochronology
Closure temperature
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The paper brings geomorphological characteristic of Neogene sediments and Pleistocene terraces of the Berounka (the left tributary to the Vltava). The localities of Neogene fluvial and fluvial-lacustrine sediments of Lower Miocene Age are clean-cut and neotectonically dislocated. The Pleistocene terraces (13 levels in 7 groups), diverging downstream, have been incorporated into the stratigraphical Pleistocene system.
Neogene
River terraces
Early Pleistocene
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