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Journal of Petrology | Volume 44 | Number 10 | Pages 1833-1865 | 2003
© Oxford University Press 2003; all rights reserved
Cenozoic Volcanism in Tibet: Evidence for a Transition from Oceanic to Continental Subduction
1 INSTITUTE OF GEOLOGY AND GEOPHYSICS, CHINESE ACADEMY OF SCIENCES, BEIJING 100029, PEOPLE'S REPUBLIC OF CHINA
2 DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF ARIZONA, TUCSON, AZ 85721-0077, USA
* Corresponding author. Telephone: (520) 626-8763. E-mail: pkapp{at}geo.arizona.edu
Geochronological (KAr or 40Ar/39Ar), major and trace element, SrNdPb isotopic and mineral chemical data are presented for newly discovered Cenozoic volcanic rocks in the western Qiangtang and central Lhasa terranes of Tibet. Alkali basalts of 6545 Ma occur in the western Qiangtang terrane and represent primitive mantle melts as indicated by high mg-numbers [100 x Mg/(Mg + Fe)] (5465), Cr (204839 ppm) and Ni (94218 ppm) contents, and relatively low ratios of 87Sr/86Sr (0·70460·7061), 206Pb/204Pb (18·2118·89), 207Pb/204Pb (15·4915·61) and 208Pb/204Pb (38·4238·89), and high ratios of 143Nd/144Nd (0·51240·5127). In contrast, younger volcanic rocks in the western Qiangtang terrane (
30 Ma) and the central Lhasa terrane (
23,
13 and
8 Ma) are potassic to ultrapotassic and interpreted to have been derived from an enriched mantle source. They are characterized by very high contents of incompatible trace elements, negative Ta, Nb and Ti anomalies, and radiogenic Pb isotopic compositions (206Pb/204Pb = 18·4319·10; 207Pb/204Pb = 15·6415·83; 208Pb/204Pb = 39·1439·67). 87Sr/86Sr (0·70880·7092) and 143Nd/144Nd (
0·5122) ratios of the western Qiangtang terrane potassic lavas are similar to those of 4529 Ma potassic volcanic rocks in the northcentral Qiangtang terrane, whereas 87Sr/86Sr (0·71670·7243) and 143Nd/144Nd (
0·5119) ratios of central Lhasa terrane lavas are similar to those of 2516 Ma ultrapotassic volcanic rocks in the western Lhasa terrane. The 6545 Ma alkali basalts in the western Qiangtang terrane, along with widespread calc-alkaline volcanic rocks of this age in the Lhasa terrane, may be related to roll-back of a previously shallow north-dipping slab of Tethyan oceanic lithosphere beneath Tibet. Subduction as opposed to convective thinning of continental lithosphere is favored to explain potassic volcanism in Tibet because of its occurrence in distinct, eastwest-trending belts (4529 Ma in the Qiangtang terrane; 2517 Ma in the northern Lhasa terrane; 168 Ma in the southern Lhasa terrane) and temporal and spatial relationships with major thrust systems.
KEY WORDS: Tibet; geochemistry; Indo-Asian collision; sodic and potassic volcanism; continental subduction
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