Post-collisional, Potassic and Ultrapotassic Magmatism of the Northern Tibetan Plateau: Constraints on Characteristics of the Mantle Source, Geodynamic Setting and Uplift Mechanisms |
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Authors: | GUO ZHENGFU; WILSON MARJORIE; LIU JIAQI; MAO QIAN |
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Institution: | 1 INSTITUTE OF GEOLOGY AND GEOPHYSICS, CHINESE ACADEMY OF SCIENCES, P.O. BOX 9825, BEIJING 100029, PEOPLE'S REPUBLIC OF CHINA
2 INSTITUTE OF GEOPHYSICS AND TECTONICS, SCHOOL OF EARTH AND ENVIRONMENT, UNIVERSITY OF LEEDS, LEEDS LS2 9JT, UK |
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Abstract: | Cenozoic, post-collisional, potassic and ultrapotassic igneousrocks in the North Qiangtang, SongpanGanzi and NorthKunlun terranes of the northern Tibetan Plateau are distributedalong a semi-continuous, eastwest-trending, volcanicbelt, which is over 1200 km in length. Spatially, there is aclose association with major strike-slip faults, thrust faultsand pull-apart basins. The ages of these magmatic rocks rangefrom 45 Ma to the present (the youngest known eruption occurredin 1951); they are shoshonitic, compositionally similar to K-richsubduction-related magmas, and range in SiO2 from 44 to 66 wt%. There is a relative enrichment of large ion lithophile elements(LILE) and light rare earth elements (LREE) in the most primitivemagmatic rocks (MgO >6 wt %) in the North Qiangtang terranecompared with those in the SongpanGanzi and North Kunlunterranes; correspondingly, the primitive magmas have higher87Sr/86Sr and 206Pb/204Pb, and lower 143Nd/144Nd ratios in theNorth Qiangtang terrane than in the SongpanGanzi andNorth Kunlun terranes. The dominant factors that control thegeochemical characteristics of the magmas are an enriched asthenosphericmantle source composition, the degree of partial melting ofthis source, and the combined processes of crustal assimilationand fractional crystallization (AFC). Enrichment of the asthenosphereis considered to have occurred by incorporation of subductedsediments into the mantle wedge above a subducted slab of Indianlithosphere during IndiaAsia convergence. Continentallithospheric mantle, metasomatically enriched during earlierepisodes of subduction, may have also contributed a source componentto the magmas. Trace element modelling indicates that the mantlesource of the most primitive magmas in the North Qiangtang terranecontained higher amounts of subducted sediment (0·510%)compared with those in the SongpanGanzi and North Kunlunterranes (<2%). The degrees of partial melting required togenerate the primitive potassic and ultrapotassic magmas fromthe enriched mantle sources range from <0·1% to 15%in the three major basement terranes. Energy-constrained AFCmodel calculations show that the more evolved magmatic rocks(MgO <6 wt %) are the results of AFC processes in the middlecrust in the North Qiangtang terrane and the upper crust inthe SongpanGanzi and North Kunlun terranes. We proposethat the ultimate driving force for the generation of the post-collisionalpotassium-rich magmatism in north Tibet is the continuous northwardunderthrusting of the Indian continental lithosphere followingIndiaAsia collision. This underthrusting resulted inupwelling of hot asthenosphere beneath north Tibet, squeezedup between the advancing Indian lithosphere and the backstopof the rigid Asian continental lithosphere. Asthenospheric upwellingmay have also contributed to uplift of the northern TibetanPlateau. KEY WORDS: Tibetan Plateau; potassic and ultrapotassic magmatism; enriched asthenospheric mantle source; EC-AFC modelling; geodynamics |
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Keywords: | : Tibetan Plateau potassic and ultrapotassic magmatism enriched asthenospheric mantle source EC-AFC modelling geodynamics |
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