Late Palaeozoic magmatism in the eastern Tseel Terrane of SW Mongolia evidenced by chronological and geochemical data

Abstract

The eastern Tseel Terrane of SW Mongolia, which is an important part of the Central Asian Orogenic Belt (CAOB), contains a segment of imbricated sheets of Late Palaeozoic granites as well as a bimodal volcanic suite. Late Devonian (380–382 Ma) granites that are enriched in Th, U, and K and depleted in Nb, Ta, P, Ba, Sr, and Ti indicate arc magmatism and are low‐K tholeiitic to medium‐K calc‐alkaline series. Zircon εHf(t) values are of +8.30 to +14.1, with crustal model ages of 0.83–0.48 Ga. The bimodal volcanic suite is composed of N‐MORB basalt and 302 ± 2 Ma rhyolite having A2‐type granite affinities, εHf(t) values ranging from +9.9 to +11.9, and crustal model ages from 0.69 to 0.59 Ga. The rhyolite mighthave originated from partial melting of juvenile lower crust, rather than fractional crystallization of coeval basaltic magma. Late Carboniferous (299–303 Ma) granites are medium‐K to high‐K calc‐alkaline series, cross‐cutting carbonaceous siltstone, emplaced into the deformed volcanic rocks and are characterized by enrichment in K, Th, and U accompanied by depletion in Ba, Nb, Ta, Sr, P, and Ti in a primitive mantle‐normalized diagram with εHf(t) values ranging from +6.05 to +10.6 and crustal model ages from 0.70 to 0.65 Ga. Thus, LA‐ICP‐MS zircon ages in combination with geochemical data document a phase of arc‐related magmatism in the Tseel Terrane at 380–382 Ma. These rocks were subsequently deformed under low‐grade metamorphic conditions and intruded by undeformed granites at c. 299–303 Ma. The coeval 302 Ma rhyolite in bimodal volcanic suite shows typical A2‐type granitic geochemical affinities and probably documents a back‐arc basin extensional environment, which was probably related to the roll‐back of the Palaeo‐Asian Oceanic Plate during the northward subduction under the Central Mongolia microcontinent.