Evolution of the early Permian volcanic-plutonic complex in the western part of the Permian Gobi-Altay rift (Khar Argalant Mts., SW Mongolia)

Abstract

The Lower Permian volcano-sedimentary complexes of the Khar Argalant and Delger Khangay formations in the Khar Argalant Mts. in south-western Mongolia are products of postorogenic within-plate magmatic activity. They consist of terrestrial lavas and pyroclastic flows with local intercalations of clastic sedimentary rocks. Vascular plants in volcanoclastic layers correspond to typical Lower to Middle Permian terrestrial associations formed under dry to intermediate conditions of temperate to colder climatic zones. The plant communities of all three formations show affmities to the Siberia (Angara) - "Cordaitean taiga". Relationships of volcanic rocks suggest simultaneous eruptions of mafic (basalt to trachyandesite) and felsic (trachyte to rhyolite) lavas. The rocks of both formations háve similar major- and trace-element contents as well as volcanological character The granite of the Shar Oroy Massif, with zircon concordia age of 285 ± 1 Ma, was roughly contemporaneous with the volcanic rocks of the Delger Khangay Fm. The Early/Late Permian clastic sedimentary rocks of the Butnaa Khudag Fm. in the hanging wall of the Delger Khangay Fm. postdated the terrestrial volcanic events. Geochemical and structural characteristics suggest that the Shar Oroy Massif and the surrounding Permian volcanic suite represent an eroded, shallow-level plutonic centre and its eruptive cover, which evolved during a crustal extension.