Geochemistry and geochronology of the Paleozoic sedimentary rocks in the Shar Khutul area, Central Mongolia

Abstract

The study area is located in the central part of Tsetserleg terrane in the southwestern margin of the Khangai-Khentey orogenic system. The paper presents new data on geochemistry and geochronology of sedimentary rocks from the Shar Khutul area, where the Tsetserleg terrane consists of Silurian-Devonian oceanic plate stratigraphic unit and Carboniferous shallow water sediment. The Upper Silurian to Middle Devonian Erdenetsogt Formation (S₃-D₂er), which is an oceanic plate stratigraphic unit, is mainly composed of siliceous siltstone, volcanites, tuffs, quartzite, and cherts. The shallow water sediments are divided into Upper Devonian to Lower Carboniferous Tsetserleg Formation (D₃-C₁cc) and Lower–Middle Carboniferous Dzargalant Formation (C_1-2dz). The Tsetserleg Formation (D₃-C₁cc) consists of only sedimentary rocks such as bluish-grey sandstones and siltstones, and Lower–Middle Carboniferous Dzargalant Formation (C_1-2dz) is principally composed of medium- to coarse-grained, brown-greenish grey sandstones with thin-layers of dark siltstones and gravelites.

The SiO₂ content of the Shar Khutul area sandstones ranges from 63.85 to 67.95 wt.% and the average content of TiO₂ is 0.72 wt.% and Al₂O₃ content is 14.38 wt.%. The Chemical Index of Alteration (CIA) value ranges from 48.71 to 56.94 and the range of Index of compositional variations (ICV) is from 0.98 to 1.24. Moreover, the samples studied show that most of the sandstones are generally immature and were derived from weakly weathered source rocks.

The ratios of Eu/Eu* (0.83), La/Sc (3.81), La/Co (5.30), and Cr/Th (13.81) indicate that the derivation of the Shar Khutul area sandstones from felsic rock sources and confirm the signatures of a felsic igneous provenance and suggest an active continental margin tectonic setting of the source area.

The clastic zircons from the medium grained sandstone (Erdenetsogt formation) yield ages between 2.5 Ga and 236 Ma and the detrital zircons exhibit four peak ages at 1.7-2.5 Ga (n = 13), 455-499 Ma (n = 6), 337-382 Ma (n = 13) and 236–250 Ma (n = 5).