Genesis and tectonic significance of Miocene lamprophyres in the Tethys Himalaya
Post-collisional mantle-derived magmas serve as the most direct indicators for tracing deep geodynamics and mantle processes.A large-scale lamprophyre dyke swarm has been discovered within the Mesozoic-Cenozoic strata of the Tethys Himalaya,located on the southern side of the Indus-Yarlung suture zone.Zircon U-Pb dating results indicate the intrusion of these dykes occurred between 15 and 10 Ma.Geochemical analyses reveal that these lamprophyres exhibit a moderate SiO2 content(50.0％～52.9％),high K2O content(2.30％～4.77％),and high Na2O content(3.04％～4.23％),thus classifying them as calc-alkaline lamprophyres.Additionally,these rocks are characterized by high MgO content(5.64％～7.63％)and Mg# value(70～74),high content of Cr(363×106～465×10-6),Co(24.7×106～31.2×10-6),Ni(128×106～269×106)and other transition elements,which are indicative of a mantle-derived origin without significant crustal assimilation.The lamprophyres are enriched in large-ion lithophile and light rare earth elements(such as Th,U,Ba,Pb,Nd)but depleted in high field strength elements and heavy rare earth elements(such as Nb,Ta,Ti).Its origin is associated with an enriched lithospheric mantle related to subduction processes and is the product of low-degree partial melting of phlogopite-bearing dipyroxene peridotite within the stability field of garnet.Spatially,the dykes exhibit an E-W parallel orientation,which suggests the existence of a north-south extensional regime in the region during the Miocene.Consequently,these lamprophyres are suggestive of a localized detachment and delamination of the Indian lithospheric plate.Such a process would have likely promoted the asthenospheric upwelling,culminating in partial melting within the enriched mantle source regions of the Indian lithosphere.
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