Thermochronology of the modern Indus River bedload: New insight into the controls on the marine stratigraphic record

The Indus River is the only major drainage in the western Himalaya and delivers a long geological record of continental erosion to the Arabian Sea, which may be deciphered and used to reconstruct orogenic growth if the modern bedload can be related to the mountains. In this study we collected thermochronologic data from river sediment collected near the modern delta. U-Pb ages of zircons spanning 3 Gyr show that only ∼5% of the eroding crust has been generated since India-Asia collision. The Greater Himalaya are the major source of zircons, with additional contributions from the Karakoram and Lesser Himalaya. The 39Ar/40Ar dating of muscovites gives ages that cluster between 10 and 25 Ma, differing from those recorded in the Bengal Fan. Biotite ages are generally younger, ranging 0–15 Ma. Modern average exhumation rates are estimated at ∼0.6 km/m.y. or less, and have slowed progressively since the early Miocene (∼20 Ma), although fission track (FT) dating of apatites may indicate a recent moderate acceleration in rates since the Pliocene (∼1.0 km/m.y.) driven by climate change. The 39Ar/40Ar and FT techniques emphasize the dominance of high topography in controlling the erosional flux to the ocean. Localized regions of tectonically driven, very rapid exhumation (e.g., Nanga Parbat, S. Karakoram metamorphic domes) do not dominate the erosional record

Thermochronology of mineral grains in the Red and Mekong Rivers, Vietnam: Provenance and exhumation implications for Southeast Asia

Sand samples from the mouths of the Red and Mekong Rivers were analyzed to determine the provenance and exhumation history of their source regions. U-Pb dating of detrital zircon grains shows that the main sources comprise crust formed within the Yangtze Craton and during the Triassic Indosinian Orogeny. Indosinian grains in the Mekong are younger (210-240 Ma) than those in the Red River (230-290 Ma), suggesting preferential erosion of the Qiangtang Block of Tibet into the Mekong. The Red River has a higher proportion of 700-800 Ma grains originally derived from the Yangtze Craton. 40Ar/ 39Ar dating of muscovite grains demonstrates that rocks cooled during the Indosinian Orogeny are dominant in both rivers, although the Mekong also shows a grain population cooling at 150-200 Ma that is not seen in the Red River and which is probably of original Qiangtang Block origin. Conversely, the Red River contains a significant mica population (350-500 Ma) eroded from the Yangtze Craton. High-grade metamorphic rocks exposed in the Cenozoic shear zones of southeast Tibet-Yunnan are minority sources to the rivers. However, apatite and zircon fission track ages show evidence for the dominant sources, especially in the Red River, only being exhumed through the shallowest 5-3 km of the crust since ̃25 Ma. The thermochronology data are consistent with erosion of recycled sediment from the inverted Simao and Chuxiong Basins, from gorges that incise the eastern flank of the plateau. Average Neogene exhumation rates are 104-191 m/Myr in the Red River basin, which is within error of the 178 ± 35 m/Myr estimated from Pleistocene sediment volumes. Sparse fission track data from the Mekong River support the Ar-Ar and U-Pb ages in favoring tectonically driven rock uplift and gorge incision as the dominant control on erosion, with precipitation being an important secondary influence. © 2006 by the American Geophysical Union

From late Visean to Stephanian: pinpointing a two-stage basinal evolution in the Variscan belt. A case study from the Bosmoreau basin (French Massif Central) and its geodynamic implications

International audiencePost-convergence evolution of the Variscan belt is characterized by the development of intramontane coal-bearing basins containing volcano-sedimentary successions. In the French Massif Central, K––Ar ages on clay particles from fine-grained sediments of the Bosmoreau basin (Limousin area), help pinpoint the evolution of the basin. In the lower part of the sedimentary pile, illite in a siltstone underlying a volcanic layer previously dated at 332±4 Ma by the U––Pb method on zircon, yields a consistent K––Ar age of ca. 340 Ma. Upward in the sedimentary succession, illite yields Stephanian K––Ar ages, which can be combined to provide a mean deposition age of 296.5±3.5 Ma. The Bosmoreau basin, albeit mainly filled with Stephanian deposits, was initiated during the late Visean, i.e. ca. 30 Ma earlier than inferred from biostratigraphical constraints. During the Stephanian, the same structure was reactivated and late Visean deposits were eroded and subsequently blanketed by thick clastic sediments. These results emphasise a two-stage evolution for the Bosmoreau basin, which is closely related to extensional tectonics identified on basement country rocks, and they are used to propose a geodynamic evolution of the studied area

Pretogenesis of Devonian lamprophyre and carbonatite minor intrusions, Kandalaksha Gulf (Kola Peninsula, Russia)

Minor magmatic intrusions (dykes and explosion pipes) of lamprophyric and carbonatitic compositions occur on several islands in the Gulf of Kandalaksha (White Sea, Kola Peninsula, Russia). The lamprophyre dykes yielded K-Ar ages of 368 ± 15 Ma and 360 ± 16 Ma, similar to the majority of alkaline rocks from the Kola Alkaline Province. Mineralogical data (presence of perovskite and sodalite, absence of amphibole phenocrysts) and geochemical data (low SiO2 and A12O3, high MgO) indicate an ultramafic lamprophyre affinity for the investigated silicate rocks. The lamprophyres contain a wide variety of xenoliths including hornblende- and biotite-rich cumulate ultramafic rocks. The carbonatite intrusions have ferrocarbonatite affinities and one of them contains xenoliths of coarse-grained Si-rich calciocarbonatites, together with abundant hornblendites and glimmerites which resemble those in the lamprophyres. The calciocarbonatite xenoliths themselves contain fragments of mica- and hornblende-rich rocks. 40Ar-39Ar ages on phlogopite and amphibole from calciocarbonatite and hornblende-rich cumulate xenoliths are between 386 ± 1.0 Ma and 395.6 ± 4.4 Ma, indicating an early Devonian age and suggesting a close relationship between the calciocarbonatite xenoliths and the ultramafic cumulate xenoliths. Thus, the ferrocarbonatite host magma may have disrupted an older calciocarbonatite-hornblendite-glimmerite intrusion at depth and incorporated xenoliths from it. The presence of hornblendite and glimmerite xenoliths with similar parageneses and identical ages in both the ultramafic lamprophyres and ferrocarbonatites suggests a close relationship between the ferrocarbonatite and lamprophyric magmas. REE patterns of the lamprophyric dykes and calciocarbonatite xenoliths show strong similarities, indicating a petrogenetic relationship. The ultramafic lamprophyres have REE patterns which are indistingishable from the contemporaneous kimberlites and melilitites from the nearby Terskii Bereg area, south Kola Peninsula. Age-corrected Sr and Nd isotope compositions demonstrate that the calciocarbonatite xenoliths have close affinities with other Devonian carbonatites from Kola and Karelia, whereas the lamprophyres are similar to the Kola kimberlites and melilitites. Differences between the Sr and Nd isotopic ratios of the silicate and carbonatite magmas throughout the Kola Alkaline Province are probably due to different mantle-source components. The Kola carbonatites mainly show a depleted mantle signature whereas the lamprophyres, melilitites and kimberlites were derived from a more enriched mantle. However, some degree of assimilation of lower continental crust and late-stage hydrothermal alteration of the silicate magmas may also have occurred