Characteristics of the mantle source region of sodium lamprophyres and petrogenetic tectonic setting in northeastern Hunan, China

Abstract

A set of unique sodium lamprophyres is developed in the Cenozoic intraconti-nental extensional zone of northeastern Hunan. These lamprophyres are significantly different in major and trace elements and Sr, Nd isotopes from ordinary potassic lamprophyres. The rocks are characterized as being enriched in Na2O and high in TiO2 and weakly enriched in Nb, Ta, Nd and LREE with no negative Eu anomaly. The trace elements and Sr, Nd isotopic compositions are typical of the mantle source region of oceanic island basalts (OIB). The average initial 87Sr/86Sr ratio is 0.705332, and the average initial 143Nd/144Nd ratio is 0.512650, with ɛNd(t) being +3.5–+3.9, marking a mantle source region of unique sodium lamprophyres. The lamprophyres were formed by metasomatism of the primitive mantle at the bottom of lithosphere by vola-tile-containing fluids/melts from the asthenosphere. The measured Rb-Sr isochron age of sodium lamprophyre is 136.61 Ma, representing a period in which the tectonic setting changed from compressive to extensional. Sodium lamprophyres were formed in a mantle plume tectonic set-ting characterized by mantle upwelling from the asthenosphere within the continent. As-theno-spheric mantle upwelling is the principal geodynamic factor leading to the formation of sodium lamprophyres and constraining the Yanshanian intracontinental extensional activity in northeast-ern Hunan, China.