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Lamproitic Rocks from a Continental Collision Zone: Evidence for Recycling of Subducted Tethyan Oceanic Sediments in the Mantle Beneath Southern Tibet |
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| Authors: | Gao Yongfeng; Hou Zengqian; Kamber Balz S; Wei Ruihua; Meng Xiangjin; Zhao Rongsheng |
| Institution: | 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 |
| Abstract: | Major and trace element, Sr–Nd–Pb isotope and mineralchemical data are presented for newly discovered ultrapotassiclavas in the Tangra Yumco–Xuruco graben in southern Tibet.The ultrapotassic lavas are characterized by high MgO, K2O andTiO2, low Al2O3 and Na2O contents, and also have high molarK2O/Al2O3, molar (K2O + Na2O)/Al2O3 and K2O/Na2O ratios. Theirhigh abundances of incompatible trace elements such as largeion lithophile elements (LILE) and light rare earth elements(LREE) reach the extreme levels typical of lamproites. The lamproitesshow 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 thatcan be differentiated from the sources of potassic lavas inthe 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 compositionsare compatible with a veined mantle source consisting of partialmelts of subducted Tethyan oceanic sediments and sub-continentallithospheric depleted mantle. Identification of the lamproitesand the delineation of their mantle source provide new evidencerelevant for models of the uplift and extension of the Tibetanplateau following the Indo-Asia collision. Metasomatism by partialmelts from isotopically evolved, old sediment subducted on theyoung Tethyan slab is an alternative explanation for PrecambrianNd and Pb model ages. In this model, differences in isotopiccomposition along-strike are attributed to differences in thetype of sediment being subducted, thus obviating the need formultiple metasomatic events over hundreds of million years.The distribution of lamproites, restricted within a north–south-trendinggraben, indicates that the initiation of east–west extensionin south Tibet started at  25 Ma. KEY WORDS: lamproites; subducted oceanic sediment; Tibetan active continental collision belt |
| Keywords: | : lamproites subducted oceanic sediment Tibetan active continental collision belt |
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