Tectonic development from oceanic subduction to continental collision: Geochemical evidence from postcollisional mafic rocks in the Hong'an–Dabie orogens

Abstract

Continental collision is generally developed from oceanic subduction to continental subduction. Recycling of both oceanic and continental crustal materials is expected to occur in the same subduction zone. This is demonstrated by an integrated study of major-trace elements and stable-radiogenic isotopes in postcollisional mafic igneous rocks from the Hong'an–Dabie orogens, east-central China. Mafic dykes in the Hong'an orogen exhibit OIB-like trace element distribution patterns, high Nb/U and TiO2/Al2O3 ratios, relatively depleted radiogenic isotope compositions with consistently high εNd(t) values of −1.8 to 4.5 and low initial 87Sr/86Sr ratios of 0.7040 to 0.7050. They are significantly different from mafic intrusives in the Dabie orogen that exhibit arc-like trace element distribution patterns, low Nb/U and TiO2/Al2O3 ratios, relatively enriched radiogenic isotope compositions with low εNd(t) values of −2.3 to −20.7 and high initial 87Sr/86Sr ratios of 0.7061 to 0.7114. In addition, whole-rock and zircon Hf isotope compositions are decoupled in the Hong'an mafic dykes but coupled in the Dabie mafic intrusives. Nevertheless, zircon O isotope compositions exhibit similar variations for the two occurrences of mafic rocks. These observations are interpreted as indicating significant differences in the nature of mantle sources between the Hong'an and Dabie mafic rocks. Two types of melt-peridotite reaction are assumed to occur in oceanic and continental subduction channels, respectively, during continental collision. Reaction of juvenile subcontinental lithospheric mantle (SCLM) peridotite with felsic melt derived from subducting oceanic basalt and sediment before continental subduction would generate the mantle source for the Hong'an mafic dykes, whereas reaction of ancient SCLM peridotite with felsic melt from subducting continental crust would generate the mantle source for the Dabie mafic intrusives. While such contrasting types of melt-peridotite reaction at the slab-mantle interface are responsible for the systematic differences in the geochemical compositions of mantle sources, the incorporation of crust-derived felsic melts into the mantle wedge result in the similar lithology of non-peridotite such as pyroxenite and hornblendite. Therefore, the two types of slab-mantle interaction record the tectonic transition from oceanic subduction to continental subduction in the course of continental collision. The nature of slab–mantle interaction in subduction channel is a key to the origin of mantle sources for postcollisional mafic igneous rocks in continental collision orogens.