From magmatic arc to a post-accretionary setting: Late Palaeozoic granitoid plutons in the northwestern Trans-Altai Zone, Mongolia

Abstract

The Trans-Altai Zone in the southern tract of the Central Asian Oceanic Belt is composed of Early Palaeozoic oceanic crust preserved in Ordovician to Devonian ophiolite fragments and Devonian–Carboniferous igneous arcs. The Edren and Baaran subzones at the NW tip of the Trans-Altai Zone were intruded by Late Palaeozoic plutons that have been examined by the combined geochronological and geochemical study. Mississippian subduction-related plutons intruded Devonian and Carboniferous volcano-sedimentary sequences in two magmatic pulses. The older, Tournaisian plutons (dated at 352 ± 1 and 347 ± 4 Ma) occur in both subzones; the younger Visean/Serpukhovian ones (331 ± 1 Ma) are found only at the northern boundary of the Edren Subzone. All Mississippian rocks are high-K calc-alkaline and characterised by a strong enrichment of hydrous fluid mobile lithophile elements over conservative Nb, Ta and Ti relative to normal mid-ocean ridge basalts. Low 87 Sr/ 86 Sr i (~ 0.7035–0.7038) and highly positive ε iNd values (+ 6.6 to + 5.2) suggest a relatively juvenile parental magma source with a short mean crustal residence. This corresponds well with the age of scarce inherited zircons, none of which is older than 530 Ma. The Early Permian post-tectonic plutons intruded the shallow crust of the Baaran Subzone (Devonian–Carboniferous flysch and Early Carboniferous volcanic arc). The prominent concentric body of the Aaj Bogd Pluton is composed of monzodiorites to monzogabbros (284 ± 1 and 294 ± 3 Ma) in its centre, surrounded by granite with syenite (282 ± 1 Ma) in the main mass of the pluton. Whole-rock Sr–Nd isotopic ratios match those of Carboniferous magmatic rocks, while trace-element patterns point to an intra-plate origin influenced by a fertile asthenospheric mantle component. On the other hand, the slightly older (290 ± 1 Ma) quartz syenites to alkali feldspar granites in the Baaran Subzone have spurious arc-like geochemistry inherited from their arc-related crustal source(s). Regional distribution of the numerous oval-shaped Early Permian alkaline post-orogenic plutons, some with A 2 -type granite affinity, follows the major Permian strike-slip zones spanning from the Dulate Arc in the west to the Khan Bogd Pluton in the east. These late, transcurrent zones apparently played an important role in late-orogenic magma generation, ascent and emplacement.