Petrogenesis And Tectonic Implications Of The Early Cretaceous Mafic Dyke Swarms In Duolong Ore Concentration Area, Tibet

The intrusion of mafic dykes into a near-trench accretionary prism, and continental margin magmatism with characteristics that differ from those of adjacent arc magmatism, are direct manifestations of the subduction of a spreading ridge and the formation of a slab window. In this paper, we investigated mafic dykes intruded into the accretionary prism that hosts the Duolong porphyry Cu–Au deposit(DCAD) of western Tibet. LA-ICP-MS analysis of U-Pb in zircon indicates that the dykes formed during the Early Cretaceous(126~127 Ma). The dykes are characterized by εHf(t) values from +2.44 to +11.8. Twenty-nine mafic dyke samples were divided into three groups based on their locations and geochemical compositions: group I has Nb = 8.31~10.2 ppm, Nb/La = 0.71~1.20, and Nb/U = 21.4~37.9; group II has Nb = 40.5~52.6 ppm, Nb/La = 0.84~1.58, and Nb/U = 18.8~47.8; and group III has Nb = 65.7~105 ppm, Nb/La = 1.35~2.08, and Nb/U = 36.5~73.8. Group I is classified as Nb-enriched basalts(ENBs), whereas groups II and III are classified as high-Nb basalts(HNBs). Both the ENBs and HNBs were derived from an adakite-metasomatized mantle wedge that subsequently underwent crystallization of olivine and clinopyroxene. The compositional variations of the studied dykes resulted mainly from mantle source heterogeneity. The volume of the slab melts that reacted the mantle wedge gradually increases from group I(ENBs) to group II(HNBs) and group III(HNBs), leading to their gradually increasing incompatible element concentrations. Considering their geochemical characteristics and field relationships, as well as the unique characteristics of continental margin magmatism in the DCAD, we propose that the dykes were emplaced in an extensional accretionary prism that formed during the northward subduction of a spreading ridge in the Bangong Co–Nujiang Tethys Ocean during the Early Cretaceous. The ridge subduction event was also responsible for the generation of coeval adakites, intermediate–felsic intrusions, and the Maierze bimodal volcanic rocks, and for the metallogenesis of the DCAD.