Calc-alkaline volcanic rocks and zircon ages of the late Tonian: early Cryogenian arc-related Big Naryn Complex in the Eastern Djetim-Too Range, Middle Tianshan block, Kyrgyzstan

Baiansuluu Terbishalieva,Masafumi Sudo,Anja Maria Schleicher,Sarah B Cichy,Alexander Mikolaichuk,Uwe Altenberger,Edward R Sobel,Martin Jan Timmerman,Jiří Sláma

doi:10.1007/s00531-020-01956-z

Abstract

The Big Naryn Complex (BNC) in the East Djetim-Too Range of the Kyrgyz Middle Tianshan block is a tectonized, at least 2 km thick sequence of predominantly felsic to intermediate volcanic rocks intruded by porphyric rhyolite sills. It overlies a basement of metamorphic rocks and is overlain by late Neoproterozoic Djetim-Too Formation sediments; these also occur as tectonic intercalations in the BNC. The up to ca. 1100 m thick Lower Member is composed of predominantly rhyolites-to-dacites and minor basalts, while the at least 900 m thick pyroclastic Upper Member is dominated by rhyolitic-to-dacitic ignimbrites. Porphyric rhyolite sills are concentrated at the top of the Lower Member. A Lower Member rhyolite and a sill sample have LA-ICP-MS U–Pb zircon crystallization ages of 726.1 ± 2.2 Ma and 720.3 ± 6.5 Ma, respectively, showing that most of the magmatism occurred within a short time span in the late Tonian–early Cryogenian. Inherited zircons in the sill sample have Neoarchean (2.63, 2.64 Ga), Paleo- (2.33–1.81 Ga), Meso- (1.55 Ga), and Neoproterozoic (ca. 815 Ma) ages, and were derived from a heterogeneous Kuilyu Complex basement. A 1751 ± 7 Ma 40Ar/39Ar age for amphibole from metagabbro is the age of cooling subsequent to Paleoproterozoic metamorphism of the Kuilyu Complex. The large amount of pyroclastic rocks, and their major and trace element compositions, the presence of Neoarchean to Neoproterozoic inherited zircons and a depositional basement of metamorphic rocks point to formation of the BNC in a continental magmatic arc setting.

Highlights

Volcanic and plutonic rocks are widespread in the central part of the Kyrgyz Middle Tianshan block (MTS) and were long considered as part of a once-continuous continental arc related to eastward subduction of oceanic crust below the MTS (e.g., Alexeiev et al 2011; Safonova et al 2017)
This study presents whole-rock major and trace element analyses of volcanic rocks and sills of the Big Naryn Complex (BNC) to establish their composition and geological setting, and two new U–Pb zircon ages from a rhyolite lava and a rhyolite sill to determine the age of the BNC
Samples with Loss On Ignition (LOI) values (Kyrgyzstan analyses) and CO2 + H2O abundances (Potsdam analyses) less than 0.75 Wt% and SiO2 contents between 48 and 75 Wt% were plotted in the K2O vs. SiO2 diagram of Peccerillo and Taylor (1976) (Fig. 6c), excluding the basalt samples that have SiO2 ≤ 45 Wt%

Summary

Introduction

Volcanic and plutonic rocks are widespread in the central part of the Kyrgyz Middle Tianshan block (MTS) and were long considered as part of a once-continuous continental arc related to eastward subduction of oceanic crust below the MTS (e.g., Alexeiev et al 2011; Safonova et al 2017). Early Paleozoic oblique subduction of oceanic crust and accretion of the STS continent in the Carboniferous (Hegner et al 2010; Alexeiev et al 2011; Terbishalieva 2018 [327–343 Ma]) played an additional role; long distance strike-slip transfer of arc segments has been suggested for the volcanic units in the Djetim-Too Range (Şengör et al 1993; Mikolaichuk et al 2016). Geochemical and age data were not available, the consensus was that the (sub-)volcanic rocks have a subduction-related origin (Bakirov et al 2014)

Results

Discussion

Conclusion