GEOLOGICAL TRANSECT THROUGH THE HANGAY-HENTEY BASIN, MONGOLIA

The Hangay-Hentey basin is an intriguing area to study because it may hold key evidence of the tectonic setting and history of the Mongolia-Okhotsk ocean basin. The Hangay-Hentey Basin is located within central Mongolia and is part of the Mongolia-Okhotsk Belt, which stretches approximately 3000 km from western China to the Gulf of Okhotsk. The Hangay-Hentey basin includes the Mongolia-Okhotsk suture, which marks the boundary between the Siberian and Northern China/Mongol cratons. Due to the tectonic complexity of the region, the history, basin type, closure and location of the Mongolia-Okhotsk ocean remain vague and inconclusive. The many hypothesized tectonic models estimate the age of the closure to span from the Permian to the Jurassic.

A comprehensive field and laboratory study was performed to place the sediment package of the Hangay-Hentey basin into a geological context. Data was collected along a transect from 49oN, 106oE to 47o N, 107o30'E. The lithology and age of the rock units within the basin were Proterozoic to Carboniferous marine sediments as well as intrusive Paleozoic to Mesozoic granitoids. The sediments have been metamorphosed to the greenschist facies and have been highly folded. Geological mapping of the units provide valuable evidence to the location of the Mongolia-Okhotsk suture, which differs from several of the previous studies. Thin section analysis using the Gazzi-Dickinson method indicates that the provenances of the sediments are mostly from a recycled orogen and magmatic arc. Detrital-zircon geochronology using isotope-dilution thermal-ionization mass spectrometry (ID-TIMS) indicates that the Proterozoic and Cambrian sediments were likely deposited in a backarc setting. Ages obtained from the aforementioned analyses are compatible with data obtained from a reconnaissance study of the area. Similarly, most of the post-Cambrian sediments within Hangay-Hentey basin were formed in a forearc setting. Analyses of the sedimentary environments of the rocks within the Hangay-Hentey basin are congruent with the tectonic settings indicated by detrital-zircon geochronology.