Abstract
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 ocean 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 (126–127Ma). 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=8.31–10.2ppm, Nb/La=0.71–1.20, and Nb/U=21.4–37.9; group II has Nb=40.5–52.6ppm, Nb/La=0.84–1.58, and Nb/U=18.8–47.8; and group III has Nb=65.7–105ppm, 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 gradually increases from group I (ENBs) to group II (HNBs) and group III (HNBs), leading to 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 emplaced in an extensional accretionary prism were derived from 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, the Maierze bimodal volcanic rocks as well as the metallogenesis of the DCAD.
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