Petrogenesis of the Devonian High-Mg rock association and its tectonic implication for the Chinese Altai orogenic belt, NW China

Abstract:

The Chinese Altai is a key region to decipher the accretionary history of the gigantic CAOB, one of the largest accretionary orogenic belts on the earth. However, its Paleozoic tectonothermal events and tectonic implications remain debated. In this study, a suite of igneous rocks, including high Mg dacite of the Kangbutiebao Formation and the associated gneissic granite, have been studied for their petrogenesis and tectonic implication. The high Mg dacite is dated at 400±2Ma, which broadly resembles the emplacement age of the high Mg gneissic granite of 406±9Ma, possibly suggesting that they were generated in a common tectono-magmatic event. Both rocks have zircon inheritances of ~500 Ma, which is consistent with the age of predominant zircon population for in the widespread Habahe sequence, indicating that their precursor magmas were probably mainly derived from the Habahe sediments. The high-Mg rock association has similar REE and trace element patterns to those of the Habahe sediments and they plot close to the field of the Habahe sediments in Th/Ta versus La/Ta diagram. They have high A/CNK ratios (1.33-2.05), supporting their source was sedimentary rocks. In Ba/Th versus (La/Sm)_N diagram, these rocks are characterized by high La/Sm ratios which suggests a greater contribution of sediments rather than altered oceanic crust in the magma generation. However, the high Mg# values (~51) of the studied rocks argue against derivation purely from remelting of Habahe sediments. In order to explain the high Mg# values (~51) of these rocks, the participation of mantle-derived melt into the precursor magma is considered. Zircons from this rock association mostly have ε_Hf(t) values from +0.85 to +9.71, supporting the involvement of juvenile materials in the magma generation. Samples from the high Mg rock association fall in the same field as the sanukitoids in TiO2 versus Mg# and Sr/Y versus Y diagrams, plot in the fractional crystallization trend of differentiated sanukitoids in Harker diagram. Similar to the sanukitoids in the Setouchi area, our high Mg rock association also involved melting of subducted sediments in a hot subduction regime. Combined with previous work, a ridge subduction regime is used to interpret the early Devonian tectonic evolution of the Chinese Altai.