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Petrology and multimineral fingerprinting of modern sand generated from a dissected magmatic arc (Lhasa River, Tibet)

By
Eduardo Garzanti
Eduardo Garzanti
Laboratory for Provenance Studies, Department of Earth and Environmental Sciences, University of Milano-Bicocca, 20126 Milano, Italy
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Mara Limonta
Mara Limonta
Laboratory for Provenance Studies, Department of Earth and Environmental Sciences, University of Milano-Bicocca, 20126 Milano, Italy
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Giovanni Vezzoli
Giovanni Vezzoli
Laboratory for Provenance Studies, Department of Earth and Environmental Sciences, University of Milano-Bicocca, 20126 Milano, Italy
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Wei An
Wei An
School of Resources and Environmental Engineering, Hefei University of Technology, 230009 Hefei, China
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Jiangang Wang
Jiangang Wang
State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, 100029 Beijing, China
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Xiumian Hu
Xiumian Hu
State Key Laboratory of Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, 210023 Nanjing, China
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Publication history
05 February 201815 October 2018

ABSTRACT

High-resolution sand petrography and heavy mineral analyses help to frame U-Pb age and Hf isotope data from zircon grains, integrated in turn with geochemical data from detrital apatite, rutile, garnet, and monazite, and with Raman spectroscopy data from detrital amphibole, pyroxene, and epidote-group minerals. This multitechnique approach, including stream-profile analysis, was used to characterize components of the sediment flux and define erosion patterns across the Lhasa block, a complex continental-arc terrane caught in the Himalayan collision. Litho-feldspatho-quartzose detrital modes and hornblende-dominated heavy mineral assemblages suggest that the majority (four fifths) of the sand bed load in the Lhasa River catchment is derived from erosion of granitoid batholiths. Gravel composition, however, is markedly different and dominated by volcanic pebbles in the trunk river, as in all of its four major tributaries, testifying to an order-of-magnitude difference in apparent erosion rates between granitoid batholiths and arc lavas. This marked contrast, partly explained by wide exposures of granitoid rocks in the rugged Nyainqêntanglha Range characterized by active incision, is notably amplified by the high sand-generation potential of granitoid rocks, which, in contrast to dense joint blocks of andesitic lavas, tend to disintegrate to sandy grus upon weathering. Sedimentary strata, making up a good half of exposed rocks, are also underrepresented in sand bed load, suggesting selective mechanical breakdown of nondurable shale/slate grains. This exposes a serious bias affecting estimates based on sand only, and it highlights the necessity for taking into account the entire size spectrum from mud to gravel in order to improve the accuracy of sediment budgets. Provenance analysis should involve multiple methods applied to multiple minerals, rather than be based solely on a single rare mineral, even if it is exceptionally laden with potential provenance information, such as zircon.

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Contents

GSA Special Papers

Tectonics, Sedimentary Basins, and Provenance: A Celebration of the Career of William R. Dickinson

Geological Society of America
Volume
540
ISBN electronic:
9780813795409

GeoRef

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