Geochemical and Sr-Nd Isotopic Characteristics of Late Paleogene Ultrapotassic Magmatism in Southeastern Tibet

摘要

Geochemical and Sr-Nd isotopic data are reported for late Paleogene potassic lamprophyres from western Yunnan, southeastern margin of the Tibetan Plateau. These lamprophyres are mostly ultrapotassic in composition with K_2O/Na_2O =2.1 to 5.2, except for a few samples with shoshonitic affinity showing slightly lower K_2O/Na_2O =1.6 to 1.7. They are characterized by high initial ~(87)Sr/~(86)Sr ratios of 0.70624 to 0.70924; negative epsilon Nd(T) values of -1.7 to -4.6; enrichment in large-ion lithophile elements, light rare-earth elements, and Pb; and depletion in high-field-strength dements, resembling those of high K/Ti and low-Ti potassic magmas formed in subduction-related settings. These lamprophyres were generated by partial melting of a metasomatized, phlogopite-bearing spinel harzburgite lithospheric mantle source, followed by crystal fractionation and varying degrees of crustal assimilation. Relatively constant incompatible trace element ratios, such as Rb/Sr (approx 0.2), Rb/Ba (approx 0.1), La/Sm (approx 5), Th/K (approx 0.0003), and Nb/La (approx 0.2), and limited Sr and Nd isotopic compositions in the ultrapotassic rocks possibly reflect an evenly distributed metasomatized mantle source. With a general similarity in geochemistry, the potassic and ultrapotassic magmas from southeastern (40-30 Ma) and northern (<15 Ma) parts of the Tibetan Plateau display obvious differences in Th/U, Rb/Sr and Sr-Nd isotopes. These differences in geochemistry and Sr-Nd isotopes suggest contrasting subcontinental lithosphere mantle bulk compositions beneath the southeastern and northern parts of the Tibetan Plateau, caused by metasomatism involving sub-ducted sediments from distinct crustal provenances.