Mineralogy and Petrology of Ultrapotassic Lamprophyre Dykes in the Bangbule Area, Xizang, China: Evidence for Open Magma Chamber Fractionation and Mafic Magma Recharge

Ultrapotassic lamprophyre dykes in southern Xizang provide important insight into the deep processes and nature of the source region. In this study, the lamprophyre dykes in the Bangbule area of southern Xizang were investigated to understand their petrogenesis and source mantle characteristics. The lamprophyres, based on their mineral and chemical compositions, are classified into two types: type-1, mainly comprising biotite and clinopyroxene, and type-2, featuring annite, phlogopite, and clinopyroxene as phenocrysts. Mica 40Ar-39Ar dating indicates their formation in the early Oligocene (29.5 ± 0.2 Ma). Geochemically, both lamprophyre types show extremely high Sr [(87Sr/86Sr)i = 0.713936–0.716999] but low and consistent Nd [(143Nd/144Nd)i = 0.511991–0.512012] isotopic ratios. Type-1 lamprophyres are more enriched in SiO2, Al2O3, and total alkalis (Na2O + K2O) but have lower TFe2O3, CaO, and MgO contents, as well as Mg# values, compared to type-2, underscoring the differentiated nature of type-1. Geochemical evidence shows that the Bangbule lamprophyre originated from the partial melting of the lithospheric mantle metasomatized by subducted Indian continental crust-derived melts, followed by fractional crystallization. The phenocrysts of clinopyroxene, phlogopite, and K-feldspar exhibit disequilibrium textures and complex zoning between their cores and mantles, suggesting magma replenishment and mixing during their formation. The cores of the clinopyroxene and phlogopite phenocrysts exhibit low Mg# values, while their mantles show high Mg# values, indicating magma replenishment between later mafic and early evolved (ultrapotassic) magmas. Accordingly, an open magma chamber model that experienced fractional crystallization and replenishment was proposed for the generation of diverse lamprophyre dykes in Xizang.