Geochemical Constraints on Petrogenesis and Tectonics of the Middle Devonian Granitic and Coeval Mafic Magmatism from the Tannuola Terrane (Northern Central Asian Orogenic Belt)

Abstract

The Tannuola terrane, located in the northern Central Asian Orogenic Belt, comprises magmatic rocks, attributed to island-arc and collisional settings during the Early Cambrian to the Late Ordovician. However, zircon U-Pb age, geochemical, and Sr-Nd isotopic constraints demonstrate that there was a short episode of peralkaline A-type granite magmatism in the northeast border area of the Tannuola terrane. The obtained zircon U-Pb age of 387.7 ± 3.3 Ma indicates emplacement of the peralkaline A-type granitic rocks in the Middle Devonian (Eifelian–Givetian period boundary). Petrologically, these rocks are mainly composed of riebeckite granites and aplites, which are approximately synchronous with augite-rich dolerites. The granitic rocks are ferroan and calc-alkalic to alkali-calcic in composition. They are characterized by a high content of SiO2, total alkali, Zr, and total REE. Significant depletion of Ba, Sr, P, Ti, and Eu indicates fractionation of plagioclase and/or K-feldspar. The values of εNd(t) in riebeckite granites range from +5.61 to +6.55, and the calculated two-stage model age ranges between 610 and 520 Ma. Coeval dolerites on the chondrite-normalized REE pattern, (Th/Yb)pm–(Nb/Yb)pm, and Th/Yb–Nb/Yb diagrams show compositional affinity between E-MORB and OIB. They are rich in incompatible elements with high HFSE/LREE ratios (Nb/La > 1), indicating that the primary magma originated from the lithospheric mantle metasomatized by asthenosphere-derived melt. Based on these geochemical characteristics, it can be reasonably inferred that the peralkaline A-type granitic rocks, and the coeval mafic rocks, are anorogenic and were emplaced in an extensional tectonic environment despite slightly higher Y/Nb values, which might be caused by a crustal contamination effect. The geochemistry of mafic rocks suggests that the magma originated from the enriched mantle sources. The results of a zircon-saturation thermometer show high initial magma temperatures between 923 and 1184 °C, with an average of 1030 °C, indicating this rock association might be related to a mantle plume beneath the northern Central Asian Orogenic Belt.