Journal of Petrology Advance Access originally published online on March 7, 2006
Journal of Petrology 2006 47(6):1177-1220; doi:10.1093/petrology/egl007
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Post-collisional, Potassic and Ultrapotassic Magmatism of the Northern Tibetan Plateau: Constraints on Characteristics of the Mantle Source, Geodynamic Setting and Uplift Mechanisms
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
RECEIVED JANUARY 10, 2005; ACCEPTED FEBRUARY 6, 2006
Cenozoic, post-collisional, potassic and ultrapotassic igneous rocks in the North Qiangtang, Songpan–Ganzi and North Kunlun terranes of the northern Tibetan Plateau are distributed along a semi-continuous, east–west-trending, volcanic belt, which is over 1200 km in length. Spatially, there is a close association with major strike-slip faults, thrust faults and pull-apart basins. The ages of these magmatic rocks range from 45 Ma to the present (the youngest known eruption occurred in 1951); they are shoshonitic, compositionally similar to K-rich subduction-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 primitive magmatic rocks (MgO >6 wt %) in the North Qiangtang terrane compared with those in the Songpan–Ganzi and North Kunlun terranes; correspondingly, the primitive magmas have higher 87Sr/86Sr and 206Pb/204Pb, and lower 143Nd/144Nd ratios in the North Qiangtang terrane than in the Songpan–Ganzi and North Kunlun terranes. The dominant factors that control the geochemical characteristics of the magmas are an enriched asthenospheric mantle source composition, the degree of partial melting of this source, and the combined processes of crustal assimilation and fractional crystallization (AFC). Enrichment of the asthenosphere is considered to have occurred by incorporation of subducted sediments into the mantle wedge above a subducted slab of Indian lithosphere during India–Asia convergence. Continental lithospheric mantle, metasomatically enriched during earlier episodes of subduction, may have also contributed a source component to the magmas. Trace element modelling indicates that the mantle source of the most primitive magmas in the North Qiangtang terrane contained higher amounts of subducted sediment (0·5–10%) compared with those in the Songpan–Ganzi and North Kunlun terranes (<2%). The degrees of partial melting required to generate the primitive potassic and ultrapotassic magmas from the enriched mantle sources range from <0·1% to 
15% in the three major basement terranes. Energy-constrained AFC model calculations show that the more evolved magmatic rocks (MgO <6 wt %) are the results of AFC processes in the middle crust in the North Qiangtang terrane and the upper crust in the Songpan–Ganzi and North Kunlun terranes. We propose that the ultimate driving force for the generation of the post-collisional potassium-rich magmatism in north Tibet is the continuous northward underthrusting of the Indian continental lithosphere following India–Asia collision. This underthrusting resulted in upwelling of hot asthenosphere beneath north Tibet, squeezed up between the advancing Indian lithosphere and the backstop of the rigid Asian continental lithosphere. Asthenospheric upwelling may have also contributed to uplift of the northern Tibetan Plateau.
KEY WORDS: Tibetan Plateau; potassic and ultrapotassic magmatism; enriched asthenospheric mantle source; EC-AFC modelling; geodynamics
* Corresponding author. Present address: Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, People's Republic of China. Telephone: 0086 10 62007334. Fax : 0086 10 62010846. E-mail: zhengfu{at}earth.leeds.ac.uk; zfguo{at}mail.iggcas.ac.cn
CiteULike 
Connotea 
Del.icio.us What's this?
This article has been cited by other articles:
|
|
 |
|
  Y. Gao, Z. Hou, B. S. Kamber, R. Wei, X. Meng, and R. Zhao Lamproitic Rocks from a Continental Collision Zone: Evidence for Recycling of Subducted Tethyan Oceanic Sediments in the Mantle Beneath Southern Tibet J. Petrology, April 1, 2007; 48(4): 729 - 752. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 D. P. West Jr., P. B. Tomascak, R. A. Coish, M. G. Yates, and M. J. Reilly Petrogenesis of the ultrapotassic Lincoln Syenite, Maine: Late Silurian-Early Devonian melting of a source region modified by subduction driven metasomatism Am J Sci, January 1, 2007; 307(1): 265 - 310. [Abstract] [Full Text] [PDF]
|
|