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We report our newly obtained geochronological and geochemical data of volcanic rocks in the Tsarigiin gol area, Ereendavaa terrane to better understand of the Early-Middle Paleozoic tectonic evolution of the Kherlen massif, NE Mongolia. LA-ICP-MS zircon U-Pb dating reveals two stages of magmatism in the southern part of the Ereendavaa terrane: one in Late Ordovician (∼462 to 455 Ma) and the other in Early Devonian (∼418 Ma). The Late Ordovician Tsarig volcanic rocks are medium to high-K calc-alkaline and intermediate to felsic in composition and characterized by relative enrichment of LREE and LILEs (e.g., Rb and U) and also by relative depletion of HFSEs (e.g. Nb, Ta, P and Ti), a typical feature of subduction-related magmas. These rocks have mostly positive whole-rock ƐNd(t) (+1.65 to +1.35) and zircon ƐHf(t) (+0.1 to +4.07) values with Mesoproterozoic Nd and Hf model ages of 1.59–1.09 Ga. They were likely generated in a continental arc setting, and their parental magmas were originated from a common source as the product of extensive fractional crystallization of mafic melts and their interaction with arc crust. The Early Devonian Chandmana volcanic rocks consist of rhyolite and minor trachybasalt, displaying a typical bimodal distribution. The trachybasaltic rocks are characterized by low Lan/Smn (2.08–2.60), Tbn/Ybn (1.42–1.50) and La/Yb (7.51–8.96) ratios and relatively flat MREE to HREE patterns with ƐNd(t) values being close to zero (−1.36 to +0.13). The rhyolites are high-K calc-alkaline and possess variably negative ƐHf(t) (−4.96 to −0.29) and ƐNd(t) (−2.14 to −0.57) values, exhibiting a strong affinity to A2-type granites. Considering the distinct compositional gap between the end-members and geochemical and isotopic signature of the bimodal suite, the felsic magma is interpreted to have derived from partial melting of a hybrid crustal source composed of both Pre-Neoproterozoic and juvenile crustal materials, while the mafic rocks were probably derived from a mantle source previously metasomatized by slab-derived melt. The two phases of magmatism in the southern part of the Ereendavaa terrane reflected two stages of tectonic evolution: Early-Middle Paleozoic subduction of the Paleo-Asian Ocean plate that gave rise to the Late Neoproterozoic-Early Cambrian Undur-Khaan island arc and Late Cambrian to Late Ordovician Ereendavaa active continental margin. Silurian amalgamation of the Ereendavaa and Idermeg terranes (Kherlen massif) caused the break-off of the subducted slab and the Early to Middle Devonian extension in the area.

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 Sm Nd 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 eNd(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.