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Journal of Petrology Advance Access originally published online on February 22, 2007
Journal of Petrology 2007 48(4):729-752; doi:10.1093/petrology/egl080
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Published by Oxford University Press (2007).

Lamproitic Rocks from a Continental Collision Zone: Evidence for Recycling of Subducted Tethyan Oceanic Sediments in the Mantle Beneath Southern Tibet

Yongfeng Gao1,*, Zengqian Hou2, Balz S. Kamber3, Ruihua Wei1, Xiangjin Meng4 and Rongsheng Zhao5

1Department of Resources, Shijiazhuang University of Economics, Huaian East Road 136, Shijiazhuang, Hebei 050031, China
2Institute of Geology, Chinese Academy of Sciences, Beijing 100037, China
3Department of Earth Sciences, Laurentian University, Sudbury, Ont., Canada
4Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China
5Hebei Bureau of Land and Resources, Shijiazhuang, Hebei 050051, China

RECEIVED FEBRUARY 9, 2005; ACCEPTED DECEMBER 21, 2006


  Abstract

Major and trace element, Sr–Nd–Pb isotope and mineral chemical data are presented for newly discovered ultrapotassic lavas in the Tangra Yumco–Xuruco graben in southern Tibet. The ultrapotassic lavas are characterized by high MgO, K2O and TiO2, low Al2O3 and Na2O contents, and also have high molar K2O/Al2O3, molar (K2O + Na2O)/Al2O3 and K2O/Na2O ratios. Their high abundances of incompatible trace elements such as large ion lithophile elements (LILE) and light rare earth elements (LREE) reach the extreme levels typical of lamproites. The lamproites show highly radiogenic 87Sr/86Sr (0· 7166–0· 7363) and unradiogenic 143Nd/144Nd (0· 511796–0· 511962), low 206Pb/204Pb (18· 459–18· 931), and elevated radiogenic 207Pb/204Pb (15· 6732–15· 841) and 208Pb/204Pb (39· 557–40· 058) ratios. On the basis of their geochemical and isotopic systematics, the lamproites in south Tibet have a distinct magma source that can be differentiated from the sources of potassic lavas in the east Lhasa and Qiangtang blocks. Their high Nb/Ta ratios (17· 10–19· 84), extremely high Th/U ratios (5· 70–13· 74) and distinctive isotope compositions are compatible with a veined mantle source consisting of partial melts of subducted Tethyan oceanic sediments and sub-continental lithospheric depleted mantle. Identification of the lamproites and the delineation of their mantle source provide new evidence relevant for models of the uplift and extension of the Tibetan plateau following the Indo-Asia collision. Metasomatism by partial melts from isotopically evolved, old sediment subducted on the young Tethyan slab is an alternative explanation for Precambrian Nd and Pb model ages. In this model, differences in isotopic composition along-strike are attributed to differences in the type of sediment being subducted, thus obviating the need for multiple metasomatic events over hundreds of million years. The distribution of lamproites, restricted within a north–south-trending graben, indicates that the initiation of east–west extension in south Tibet started at ~ 25 Ma.

KEY WORDS: lamproites; subducted oceanic sediment; Tibetan active continental collision belt

*Corresponding author. Telephone: 0086 311 87207857. Fax: 0086 311 85882537. E-mail: gaoyf{at}sjzue.edu.cn


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[Abstract] [Full Text] [PDF]


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