Ultrapotassic rocks and xenoliths from south Tibet; contrasting styles of interaction between lithospheric mantle and asthenosphere during continental collision

Widespread Miocene (24-8 Ma) ultrapotassic rocks and their entrained xenoliths provide information on the composition, structure, and thermal state of the sub-continental lithospheric mantle in southern Tibet during the India-Asia continental collision. The ultrapotassic rocks along the Lhasa block...

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Bibliographic Details
Published in:	Geology (Boulder) 2017-01, Vol.45 (1), p.51-54
Main Authors:	Xu Bo, Xu Bo, Griffin, William L, Xiong, Qing, Hou Zengqian, Hou Zengqian, O'Reilly, Suzanne Y, Guo, Zhen, Pearson, Norman J, Gréau, Yoann, Yang Zhiming, Yang Zhiming, Zheng Yuanchuan, Zheng Yuanchuan
Format:	Article
Language:	eng
Subjects:	absolute age alkaline earth metals Asia asthenosphere Cenozoic chain silicates China clinopyroxene Far East genesis geochemistry Geochronology Geology hafnium Hf-177/Lu-176 igneous and metamorphic rocks igneous rocks inclusions isotope ratios isotopes Lhasa Block Lithosphere lutetium magmas major elements mantle Marine metals Miocene Neogene nesosilicates orthosilicates Petrology plate collision plate tectonics pyroxene group rare earths Rocks silicates southern Xizang China Sr-87/Sr-86 stable isotopes strontium Tertiary Tomography trace elements U/Pb ultrapotassic composition whole rock xenoliths Xizang China zircon zircon group
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Summary:	Widespread Miocene (24-8 Ma) ultrapotassic rocks and their entrained xenoliths provide information on the composition, structure, and thermal state of the sub-continental lithospheric mantle in southern Tibet during the India-Asia continental collision. The ultrapotassic rocks along the Lhasa block delineate two distinct lithospheric domains with different histories of depletion and enrichment. The eastern ultrapotassic rocks (89°E-92°E) reveal a depleted, young, and fertile lithospheric mantle (87Sr/86Srt = 0.704-0.707 [t is eruption time]; Hf depleted-mantle model age [TDM] = 377-653 Ma). The western ultrapotassic rocks (79°E-89°E) and their peridotite xenoliths (81°E) reflect a refractory harzburgitic mantle refertilized by ancient metasomatism (lavas: 87Sr/86Srt = 0.714-0.734; peridotites: 87Sr/86Srt = 0.709-0.716). These data integrated with seismic tomography suggest that upwelling asthenosphere was diverted away from the deep continental root beneath the western Lhasa block, but rose to shallower depths beneath a thinner lithosphere in the eastern part. Heating of the lithospheric mantle by the rising asthenosphere ultimately generated the ultrapotassic rocks with regionally distinct geochemical signatures reflecting the different nature of the lithospheric mantle.
ISSN:	0091-7613 1943-2682