Intermediate-mafic dikes in the East Kunlun Orogen, Northern Tibetan Plateau: A window into paleo-arc magma feeding system

Abstract

The contemporaneous mafic and intermediate dikes of a continental magmatic arc provide a window into the magma feeding system at depth. Here we integrate data on the elemental and Sr-Nd-Hf isotope geochemistry, petrology, mineralogy, and zircon geochronology of late Permian dikes in the East Kunlun Orogen, northern Tibetan Plateau, discuss the petrogenesis of the dikes, and reconstruct the nature and relationship of different magma reservoirs. The dikes are porphyritic diabases, lamprophyres and diorite porphyries that crystallized between 259 and 255 Ma, coeval with their host granodiorites. Geochemical and Sr-Nd-Hf isotopic data indicate the parental magma of the porphyritic diabases and lamprophyres was derived from enriched lithospheric mantle. This magma underplated the crust and underwent varying degrees of magma recharging, crustal assimilation, and fractional crystallization dominated by olivine, pyroxene, and hornblende. This evolved basaltic magmas ascended through the crust, undergoing further differentiation, and some magma entered mid-crustal reservoirs occupied by felsic crystal mushes, rejuvenating the mushes. The subsequent mixing of mafic and felsic materials formed the precursor magmas of the intermediate dikes. Our new data reveal that the magmas stagnated mainly in reservoirs at two levels, 26–32 and 9–18 km, based on hornblende barometry. The underplating, assimilation, and replenishment of basaltic magmas in the lower crust, and their eventual emplacement and differentiation in a mid-crustal reservoir, controlled the mineralogy and geochemistry of the mafic dikes, while the rejuvenation of crystal mushes, the mixing of mafic and felsic materials, and differentiation account for the compositional features of the diorite porphyries. This study shows that the trans-crustal magma feeding system, which controls the compositions of several dispersed but interconnected magma reservoirs, is the key to understanding the compositional diversity and igneous petrogenesis in continental arcs.