Middle Paleozoic intermediate-mafic rocks of the Tsoroidog Uul’ accretionary complex, Central Mongolia: Petrogenesis and tectonic implications

Abstract

The Tsoroidog Uul’ accretionary complex is hosted by the Tsetserleg terrane in the southwestern Khangay-Khentey orogenic system (Central Mongolia), which represents the segment of the Central Asian Orogenic Belt and has significant regional implications for its tectonic evolution. This paper reports the results of field investigations and petrography, bulk-rock major and trace element geochemical, as well as SmNd isotopes of Middle Paleozoic intermediate-mafic rocks from the Tsoroidog Uul’ accretionary complex. We investigate a wide range of rock types which can be divided into 4 groups on the basis of their TiO2 and REE characteristics. Group 1 is characterized by moderate TiO2, relatively flat chondrite-normalized REE patterns (La/Smn = 1.0; Gd/Ybn = 1.2). These basalts are divided into two subgroups: (1) Nb/Thpm = 3.6, Nb/Lapm = 0.8, Zr/Nb = 24.6, and Ce/Ybpm = 0.8 (N-MORB type); (2) Nb/Thpm = 1.3, Nb/Lapm = 1.1, Zr/Nb = 11.9, and Ce/Ybpm = 1.6 (E-MORB type). Group 2 shows high TiO2 and LREE (La/Smn = 3.0), differentiated HREE (Gd/Ybn = 2.5), positive Nb anomalies shown in primitive mantle-normalized multi-element patterns (Nb/Thpm = 1.2; Nb/Lapm = 1.1), and low Zr/Nbav. ratios (~6). Group 3 displays low TiO2, high LREE (La/Smn = 3.6), Zr/Nbav. = 24.4, and low Nb (Nb/Lapm = 0.2). Group 4 exhibits moderate TiO2, flat REE patterns (La/Smn = 0.8; Gd/Ybn = 1.1), negative Nb anomalies (Nb/Thpm = 0.3; Nb/Lapm = 0.6) and Zr/Nbav. = 33. The εNd(t) values are positive for Group 1 and Group 2, but negative for Group 3. Based on their petrological and geochemical features, we suggest that the Group 1 and 2 mafic volcanic rocks were formed in an oceanic environment, and represent mid-oceanic ridge basalt (MORB) and oceanic-island basalt (OIB), respectively. In contrast, Group 3 intermediate dikes probably have supra-subduction origin with calc-alkaline features, whereas Group 4 represents arc tholeiite basalt including remnants of the continental volcanic arc. Overall, the Middle Paleozoic intermediate-mafic rocks of the Tsoroidog Uul’ accretionary complex were probably generated from heterogenous mantle sources. Thus, we propose that spatial and temporal changes of the Paleo-Pacific Oceanic lithosphere, which subducted under the continental margin of the Siberian Craton, resulted in the variable composition of the intermediate-mafic rocks of this complex. The accretionary complex of the Tsetserleg terrane, which extends into Ulaanbaatar terrane, was formed by subduction of the Paleo-Pacific Oceanic lithosphere or Mongol-Okhotsk Ocean.