RAN Hao, WANG Gen-hou, ZHENG Yi-long, LIANG Xiao, WEI Yi

(School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China )

Abstract: The Paleogenic volcanic rocks from the Rongma area, Qiangtang terrane, northern Tibet, China, consist of basalt, trachybasalt, basaltic andesite, trachyandesite and dacite. SiO₂ content ranges from 41.56% to 68.35%, Na₂O from 0.20% to 5.06%, K₂O from 0.26% to 4.68% and K₂O/Na₂O ratio from 0.06 to 23.40. The rocks are characterized by an enrichment of LREE, a depletion of HREE and there is little positive or negative Eu anomaly. They are enriched in Rb, K, Ba, U and La, and depleted in Ta, Nb, Sr, P, Ti and other elements. Geochemical features indicate they formed in the tectonic extensional environment within the plate. Some lavas were derived from the special crust-mantle transition zone in the Qinghai-Tibetan Plateau. Others were derived from transitional mantle and enriched mantle sources that were affected by metasomatism of the subducting crust. There may be, not only a special crust-mantle transition zone, but also transitional mantle and enriched mantle related to the upwelling of the asthenosphere caused by intracontinental subduction during the Cenozoic.

Key words: Tibet; Qiangtang terrane; Rongma area; paleogenic volcanic rocks; geochemistry; tectonic significance

ACTA MINERALOGICA SINICA Vol. 34, No. 3, 2014, Page 360-368