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, K 2 O and TiO 2 , low Al 2 O 3 and Na 2 O contents, and also have high molar K 2 O/Al 2 O 3 , molar (K 2 O + Na 2 O)/Al 2 O 3 and K 2 O/Na 2 O 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 87 Sr/ 86 Sr (0· 7166–0· 7363) and unradiogenic 143 Nd/ 144 Nd (0· 511796–0· 511962), low 206 Pb/ 204 Pb (18· 459–18· 931), and elevated radiogenic 207 Pb/ 204 Pb (15· 6732–15· 841) and 208 Pb/ 204 Pb (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.
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