New Geochemical Data of Basalts in the Tsoroidog Area, Central Mongolia

Abstract

At present, geochemical data are widely used for reconstructing geodynamic settings, especially, volcanic rocks of mafic composition, i.e., basalts, because they are widespread in many orogenic belts and are indicative of different geodynamic environments. In general, we propose the reconstruction of the tectonic settings of basalts according to their relationships with associated ocean plate stratigraphy (OPS) sediments, their petrogenesis and their geochemical features.In this paper, we present new data on geochemistry of basalts from the Tsoroidog and Bumbattolgoi formations in the Tsetserleg accretionary wedge terrane in southwestern part of the Khangai-Khentey orogenic system [Tomurtogoo, 2012], which have been identified as the key region in the Central Asian Orogenic Belt (CAOB), the world largest accretionary orogeny. Detailed study of their geochemistry of volcanic rocks allowed us to recognize two types of chert-basalt association.Middle Silurian -Lower Devonian Tsoroidog formation is mainly composed of basaltic rocks with thin layers/lenses of thin-bedded, red-whitish color chertquartzite and slates. Basaltic rocks are characterized by medium TiO2 and high Zr/Nb ratio, Mg#=36-47, which is depleted in REE, Nb and Ti. Concentrations of TiO 2 , Fe 2 O 3 and P 2 O 5 decrease when MgO is increased in the binary diagram, whereas Al 2 O 3 is normal, furthermore, Nb, La and Sm decrease when MgO increases to compare with the rare elements. Nb is relatively less than La and Th. (Nb/La) PM value less than 1 is generally indicated in the MORB [Haase, 2002].Lower Devonian Bumbattolgoi formation mainly consists of basaltic rocks associated with shale, sandstone and red color cherts, which is represented by