Abstract

We used Nd isotopes and trace element data to determine the provenance of sedimentary rocks in the Linxia basin, northeastern Tibet, whose Oligocene through Pliocene sedimentation history has been interpreted to reflect deposition in a flexural basin associated with contractional deformation along the northeastern margin of the Tibetan Plateau. Paleozoic–early Mesozoic metasedimentary source rocks from the Kunlun-Qaidam and Songpan-Ganzi terranes have ϵNd values between −11.8 and −16.1, whereas Paleozoic and Mesozoic plutonic source rocks that intrude the metasedimentary rocks have more positive ϵNd values between −3.6 and −11.2. Cretaceous sedimentary source rocks display ϵNd values of −9.7 and −9.9 in the Maxian Shan, north of the Linxia basin, and −15.3 in the plateau margin south of the basin. With ϵNd values that range between −8.4 and −10.4 before ca. 15 Ma, and −6.2 and −11.8 after ca. 14 Ma, sedimentary rocks of the Linxia basin are less negative than metasedimentary rocks, which are dominant source rocks within the margin of the Tibetan Plateau today. The relatively positive ϵNd values of Linxia basin sedimentary rocks could reflect several possible sources, including (1) a mixture of plutonic and metasedimentary rocks within the northeastern margin of Tibet, (2) Cretaceous sedimentary rocks derived from the north, or (3) loess derived from central Asian deserts. A mass balance calculation indicates that plutonic rocks are not volumetrically significant enough to generate the ϵNd values observed in Linxia basin sedimentary rocks through mixing of plutonic and metasedimentary sources.

Rare earth element patterns suggest that Cretaceous rocks were not a dominant source of sediment. The Nd isotopic composition and rare earth element pattern of Quaternary loess are similar to older deposits in the Linxia basin and reflect loess deposited elsewhere in the Loess plateau and the North Pacific (ϵNd = −8.6 to −10.5). In addition, the modern Daxia River, which drains the margin of the plateau today, transports clay and silt with ϵNd values of −10.5 to −10.8 despite the river's source in more negative metasedimentary rocks of the Kunlun-Qaidam and Songpan-Ganzi terranes, which indicates that the modern fine-grained sedimentary budget is dominated by recent loess deposits. Considering the slow sedimentation rates in the Linxia basin, it is likely that loess sources have contributed a significant volume of fine-grained sediment to this basin throughout its history. An increase in the range of ϵNd values at ca. 14 Ma in the Linxia basin may reflect increased unroofing of the northeastern margin of Tibet, which slightly preceded a change in climate between ca. 13 and 12 Ma in the Linxia basin. A 1.5‰ increase in baseline δ18O values of lacustrine carbonates has been interpreted as the result of reorganization of atmospheric circulation and an increase in aridity on the northeastern margin of the Tibetan Plateau, perhaps associated with the plateau having achieved an elevation sufficient to block moisture from the Indian Ocean and/or Pacific Ocean. Similar timing of exhumation and climate change suggests that northeastward and eastward propagation of the plateau margin was responsible for the middle Miocene climate change observed in the Linxia basin.

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