Geochronology and geochemistry of Late Devonian-Carboniferous igneous rocks in the Songnen-Zhangguangcai Range Massif, NE China: Constraints on the late Paleozoic tectonic evolution of the eastern Central Asian Orogenic Belt

Abstract

This paper presents new zircon U–Pb ages and Hf isotopic data, and whole–rock geochemical data for Late Devonian–Carboniferous igneous rocks from the Songnen–Zhangguangcai Range Massif (SRM), NE China, to constrain the late Paleozoic tectonic evolution of the eastern Central Asian Orogenic Belt and the nature of the crust beneath the SRM. LA–ICP–MS zircon U–Pb dating reveals two stages of magmatism in the Late Devonian (366Ma) and Carboniferous (325–313Ma). The Late Devonian magmatic event produced alkali feldspar granites with high SiO2 and total alkali contents, as well as pronounced negative Eu anomalies, similar to A-type granites. Based on these features, their high zircon εHf(t) values (+9.6 to +13.0), and the early Paleozoic tectonic evolution of the eastern SRM, we conclude that the primary magma for the Late Devonian granites was generated by partial melting of juvenile lower crust in a post-collisional extensional environment. The Carboniferous magmatic event produced a suite of intermediate–felsic igneous rocks. The intermediate rocks, including basaltic andesite, andesite, and quartz monzodiorites, have low SiO2 contents, high Mg#, high Cr, Co, and Ni contents, and display arc-like trace element characteristics. Based on these observations and their high zircon εHf(t) values (+5.8 to +10.7), we suggest their primary magma was derived from partial melting of depleted mantle, metasomatized by subduction-related fluids. The felsic rhyolites and monzogranites have high SiO2 contents, low Mg#, extremely low Cr, Co, and Ni contents, and zircon εHf(t) values of +10.5 to +13.7 and +4.2 to +6.9, respectively. These results indicate that the primary magma for the felsic rocks was derived from partial melting of heterogeneous juvenile crust. The Carboniferous igneous rock assemblages, together with coeval carbonate sedimentary formations, record an intra-plate extensional setting. The zircon εHf(t) data of Paleozoic igneous rocks from the eastern SRM define a temporal trend that is consistent with tectonic evolution. The increase in εHf(t) values during the early late Paleozoic age mirrors changes in magma source, corresponding to a switch from subduction–collision to post-collision tectonic regimes in the SRM. The zircon Hf isotopic compositions of the studied Late Devonian–Carboniferous igneous rocks record crustal accretion in the eastern SRM during the Neoproterozoic–early Paleozoic. Reworking of these crustal materials occurred in a post-collisional extensional environment during the late Paleozoic.