Abstract
Devonian and Carboniferous stratigraphic sections examined at 17 localities in southern Mongolia from the Tsahir Halgyn Range, 180 kilometers southeast of Altai, to Havtgai, 300 kilometers west of Sainshand, record the development of volcanic arcs and associated basins. A definitive tectonic model cannot yet be advanced with the data currently available for this remote area, but certain interpretations are reasonable based on this new stratigraphic data and supplementary provenance, geochemical and geochronologic data.Western sections examined near Tsahir Halgyn, Shine Jinst and the Edrengin Range contain carbonate and siliciclastic marine strata interbedded with bimodal volcanic units. Sedimentary structures and fossil evidence point to shallow water depths. Thick volcanic flows and volcaniclastic sandstones at Shine Jinst indicate proximity to a volcanic center. A back-arc basin or intra-arc basin associated with an intraoceanic arc is interpreted for these sections. Sections measured in the northeast record primarily marine sedimentary facies, but include volcaniclastic boulder and cobble conglomerates at Havtgai and basalt flows near Mandal Ovoo. These attributes probably reflect an intraoceanic arc setting. The Gurvan Sayhan Range in the central portion of the study area contains basalt flows, marine sedimentary facies and serpentinite. The tectonic setting for this area is less clear because the area is structurally complex and metamorphosed; it could represent an accretionary wedge with obducted oceanic crust and fore-arc basin and trench deposits, an intraoceanic island arc, a mature back-arc basin or a combination of all of these. Localities in the southern portion of the field area, from Tost to Nomgon, include marine sandstones, volcaniclastic conglomerates and subaerial volcanic flows. These are interpreted to represent a continental arc or well-developed oceanic arc.