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 i Nd 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.