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, tectonic implication of the widely distributed Paleozoic magmatic rocks remain debated. In this study, a suite of igneous rocks, including high Mg dacite of the Kangbutiebao Formation and the associated gneissic granodiorites, have been studied for their petrogenesis and tectonic implication. The high Mg dacite is dated at 400±2 Ma, which broadly resembles the emplacement age of the high Mg gneissic granodiorites of 406 ± 9 Ma, suggesting that they were generated in the same tectono-magmatic event. Both rocks have predominant zircon inheritances of ~500 Ma, which is consistent with the age of predominant zircon population for the widespread Habahe sedimentary sequence, suggesting 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. Their high A/CNK ratios (1.33-2.05) imply that magma source was dominated by sedimentary rocks. Similarly, their high La/Sm ratios also suggest 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 the Habahe sediments. In order to explain the high Mg# values of these rocks, the participation of mantle-derived melt into the precursor magma is considered. Zircons from this rock association mostly yield εHf(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, and they plot in the fractional crystallization trend of differentiated sanukitoids in Harker diagram, possibly suggesting a petrogenetic process like the typical sanukitoid magma elsewhere. This study shows that similar to the sanukitoids in the Setouchi area, our high Mg rock association also involved melting of subducted sediments in a hot subduction regime. Taking consideration of results from previous studies, a ridge subduction regime is considered to interpret the early Devonian tectonic evolution of the Chinese Altai.