Ordovician to Devonian granitic plutons in the Hangay Range, Central Mongolia: Petrogenesis and insights into the Paleozoic tectonic evolution of the westernmost Mongol-Okhotsk Orogen

Abstract

The pre-Mesozoic magmatic and tectonic evolution in the Mongol-Okhotsk Orogen has been poorly understood. Here we conducted geochronological and geochemical studies on five Paleozoic granitic plutons in the westernmost Mongol-Okhotsk Orogen (Hangay Range), with an aim to understand their petrogenesis and roles in the Paleozoic tectonic evolution of the Mongol-Okhotsk Orogen. Our results demonstrate that two K-feldspar granites yield Ordovician to Silurian zircon U-Pb ages of 469 ± 3 Ma and 440 ± 3 Ma, and three monzogranites give Devonian zircon U-Pb ages (416 ± 4 Ma, 416 ± 4 Ma and 398 ± 4 Ma). Geochemical data show that all plutons in this study have high SiO2 (65.8–77.9 wt%) and total alkali (6.3–8.9 wt%) contents, and are enriched in large-ion lithophile elements (LILEs, e.g. Rb, Th and K) and depleted in high-field-strength elements (HFSEs, e.g. Nb, Ta and Ti). Furthermore, the Ordovician K-feldspar granite has the characteristics of A2-type granitic rocks with high 10000*Ga/Al (3.4–3.6), while Silurian to Devonian granitic plutons belong to I-type granitoids with low A/CNK values (0.8–1.1). The low Na2O/K2O ratios (0.5–0.7) of the Ordovician and Silurian K-feldspar granites suggest that they were generated by partial melting of medium-to-high K basaltic rocks. Three Devonian monzogranites with high Na2O/K2O ratios (1.1–1.7) have an origin of low-K basaltic rocks. Five Ordovician to Devonian granitic plutons have relatively low εNd(t) values (−4.7 to +0.5) and variable zircon εHf(t) values from −2.6 to +5.5, which plot between the evolutional lines of depleted mantle and the Precambrian basement rocks of microcontinents, and are interpreted to result from partial melting of the relatively juvenile mafic rocks that were underplated in response to the Ediacaran to Cambrian subduction. Given the development of accretionary complex in the westernmost Mongol-Okhotsk Orogen during the Paleozoic, we interpreted these plutons to be formed in a supra-subduction zone associated with the consumption of the Mongol-Okhotsk oceanic plate. In particular, the Ordovician K-feldspar granite intruded into the Cambrian to Ordovician accretionary complex, which, together with its A2-type characteristics, suggests an episode of slab rollback at ~469 Ma.