Geochemistry of Late Cambrian-Early Ordovician Metasedimentary Rocks from the Tumurt Formation, South-Western Mongolia: Provenance and Tectonic Setting

The Tumurt Range in southwestern Mongolia is located in the western part of the Gobian Tien Shan Block of the South-Gobi Massif. Thick metasedimentary rocks belonging to the Tumurt Formation crop out extensively in the Tumurt Range and adjacent areas. Most workers have regarded these sedimentary rocks as Ordovician (Sinitsin, 1956), Upper Silurian-Lower Devonian (Lkhundev et al., 2013), or Upper Ordovician-Lower Silurian (Erdenechimeg et al., 2018) in age. However, it was recently defined that the metasedimentary strata of the Tumurt Formation were deposited during a period from the Furongian series of the Cambrian throughout the Early Ordovician (Delgerzaya et al., 2024). In this paper, we present the new results of the whole rock geochemical analysis of the metasedimentary rocks of the Tumurt Formation with the aim of constraining the provenances and tectonic setting. The metasedimentary rocks of the Tumurt Formation include mainly fine- to medium-grained metasandstone, metasiltstone, and different paraschists. These rocks have moderate SiO2 (56.1 - 80.2 wt.%), TiO2 (0.22 - 0.78 wt.%), Al2O3 (8.45 - 17.3 wt.%), Fe2O3T (2.28 - 6.61 wt.%), MgO (0.7 - 3.01 wt.%) and CaO (0.47 - 5.6 wt.%) contents and moderate SiO2 /Al2O3 (average 0.72) and Na2O/K2O (average 0.06) ratios, similar to graywackes from the continental island arc or active continental margin (Bhatia & Crook, 1986). Moreover, all samples are enriched in LREE (Lan/Ybn = 3.42 - 13.7) and display relatively flat HREE patterns with obvious negative Eu anomalies (Eu/Eu* = 0.53 - 0.87), indicating a predominantly felsic upper continental crustal provenance in an active continental margin setting. A primitive-mantle normalized trace element spider diagram indicates that these rocks are enriched in LILEs (Cs, Rb, Ba, and K) and depleted in HFSEs (Sr, Nb, Ta, and Ti), consistent with an upper continental crust affinity. Elemental ratios such as La/Th, Th/Sc, and Zr/Sc indicate a mixed provenance, dominated by felsic igneous sources with minor input from recycled sedimentary material.