Early-Middle Paleozoic volcanic rocks from the Ereendavaa terrane (Tsarigiin gol area, NE Mongolia) with implications for tectonic evolution of the Kherlen massif

摘要

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 (similar to 462 to 455 Ma) and the other in Early Devonian (similar to 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 epsilon(Nd)(t) (+1.65 to +1.35) and zircon epsilon(Hf)(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 La-n/Sm-n (2.08-2.60), Tb-n/Yb-n (1.42-1.50) and La/Yb (7.51-8.96) ratios and relatively flat MREE to HREE patterns with epsilon(Nd)(t) values being close to zero ( -1.36 to + 0.13). The rhyolites are high-K calc-alkaline and possess variably negative epsilon(Hf)(t) ( - 4.96 to -0.29) and epsilon(Nd)(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.