Young Silicic Magmatism of the Greater Caucasus, Russia, with implication for its delamination origin based on zircon petrochronology and thermomechanical modeling
Introduction
Magmatism during the continent-continent collisions is perhaps the least understood source of felsic magmas on Earth today, with no one melting mechanism able to explain all magma production in these environments. In Tibet, the most famous continental collision zone, the production of silicic magmas has been attributed both to radiogenic heating of thickened crust (e.g., Bea, 2012) and to partial melting of hydrated oceanic crust residing in the upper mantle after being subducted prior to collision (Mo et al., 2008). In the Greater and Lesser Caucasus mountains, which comprise the active collision zone between the Arabian and Eurasian plates (Fig. 1), there is also significant syn-collisional volcanism. This magmatism has been suggested by a variety of workers using both geochemical and geophysical evidence to be primarily caused by post-collision slab breakoff and lithospheric delamination leading to mantle decompression and melting (Pearce et al., 1990; Keskin, 2003; Şengör et al., 2008; Koulakov et al., 2012; Sugden et al., 2019). However, mantle melting cannot be the sole source of Caucasus magmas, as Sr and Nd isotopes have been used to demonstrate that melting of the Paleozoic crust must also significantly contribute to erupted material (e.g., Lebedev and Vashakidze, 2014).
At the northern end of the collision zone in the Greater Caucasus, rapid uplift since the Pleistocene has produced world-class exposures of syn-orogenic silicic intrusions and volcanism near Mount Elbrus (at 5642 m, the highest mountain and active volcano in Europe). These include the 2 Ma, 4 × 5 km Eldjurta Granite stock at the Tyrnyauz mining district, 35 km east of Elbrus (one of the world's youngest granites). The Eldjurta Granite is affiliated with world class Mo-W mineralization and is crosscut by brecciated rhyolitic bodies, vitrophyres, and aplites (Milanovsky and Koronovsky, 1973; Kostitsyn and Kremenetsky, 1995; Grün et al., 1999). About 10 km southeast of Tyrnyauz, the ~11 × 15 km-wide, 2.9 Ma Chegem Caldera has been uplifted and dissected by erosion, exposing a 2 km-thick intracaldera tuff and a central granodioritic porphyry which is texturally and compositionally similar to the Eldjurta granite, fueling speculation that the two may be surface expressions of a larger underlying batholith (Fig. 2, Fig. 3; Milanovsky and Koronovsky, 1973; Lipman et al., 1993; Gazis, 1994; Gazis et al., 1995, Gazis et al., 1996). The quality of these exposures has made them the focus of important early debates about the formation of welded ignimbrites and texturally similar lavas (“tufo-lavas”, Levinson-Lessing, 1913; Milanovsky et al., 1962; Koronovsky et al., 1982; Koronovsky and Demina, 2007, Bindeman et al. 2021 in press, Fig. 1 therein).
Section snippets
Objectives of the present study
This study investigates the origins of recent silicic magmatism in the Greater Caucasus via two interrelated goals:
- 1)
The young age and good preservation of Greater Caucasus lavas, tuffs, and plutons presents a unique opportunity to conduct a detailed study of its origins. Newly developed CA-ID-TIMS methods can reduce 2-sigma uncertainties of calculated ages for a single zircon crystal to ~0.1% (e.g., ±2 kyr on a 2 Myr unit). For samples with ages of less than 3 Ma as in the studied region (Fig. 2
Regional background: Silicic magmatism in the Elbrus-Tyrnyauz-Chegem region of the Greater Caucasus
The Greater Caucasus of southern Russia formed as the result of the collision between the Arabian, Anatolian, and Eurasian plates, which has been ongoing since the initial closure of the local Tethys ocean in the Jurassic. The current phase of the collision from 35 Ma to the present has culminated in rapid uplift of the Greater Caucasus which has accelerated in the last 5 million years (Ershov and Nikishin, 2004; Zakariadze et al., 2007; Adamia et al., 2011; Vincent et al., 2020; Fig. 1). The
Sampling
Ignimbrites, volcanic necks, and lavas in the Elbrus (samples ELB-1 to ELB-19) and Tyrnyauz (samples ELB-20, −21) areas are preserved as isolated remnants with thicknesses reaching 50 m. We sampled the most prominent ignimbrites on the northern slopes of modern Elbrus edifice, at Tuzluk Mt. (sample ELB-1), and the Lower (ELB-13, −12) and Upper (ELB-9, −11) Stone Mushroom ignimbrites near the upper Birdjalisu river (Fig. 2). Also at Elbrus, we sampled an additional 10 lavas of varying
U-Pb ages
Zircon ages from Chegem and five other regional ignimbrites, two granites, and two lavas were determined by LA-ICP-MS, SHRIMP and CA-ID-TIMS (Bindeman et al. in press, Tables 2, A1-2 therein). Zircons were generally U-rich, enabling precise dating. The 230Th-corrected 206Pb/238U ignimbrite dates cluster around two age groups: 2.92 Ma for the Chegem units and 1.96–1.98 Ma for the Elbrus and Tyrnyauz/Eldjurta samples (Fig. 4). The Elbrus Upper and Lower ignimbrites, mapped by Gazeev and Gurbanov
Trace elements
The trace element geochemistry of the Chegem rhyolites straddles the boundary between collisional and arc-related magmas (Bindeman et al. in press, Fig. 5 therein; Pearce et al., 1984). Basalts and dacites of the Elbrus, Tyrnyauz, and Kazbek volcanic centers are calc-alkaline with elevated alkalis and show enrichments in large ion lithophile elements such as Rb, Sr, Ba, U, and Pb and depletions in the high field strength elements Ti, Zr, and Nb (Lebedev et al., 2010; Lipman et al., 1993;
Conclusions
We present a broad, multi-disciplinary study of recent volcanism in the Greater Caucasus focusing on the young and well-preserved Chegem, Elbrus and Tyrnyauz volcanic centers. We observe clustering of zircon ages at 2.91 and 1.98 Ma, signifying the presence of two pulses of voluminous silicic magmatism. It is likely that a now-eroded caldera at Tyrnyauz is the source of 1.98 Ma voluminous ignimbrites with the Eldjurta Granite representing its parental magma. Likewise, Chegem and Zayukovo
Author statement
Ilya Bindeman: Fieldwork, O-C-H isotope analyses, zircon extraction, paper writing and editing Dylan Colón: Thermomechanical modeling, paper editing Jörn Wotzlaw: zircon ID TIMS and LA-ICP-MS analysis, data plotting and paper editing. Richard Stern: SIMS analysis of zircons for O isotopes. Massimo Chiaradia: Hf isotope analysis of zircons by LA-ICP-MS.: Marcel Guillong: LA-ICP-MS analysis of zircons].
Declaration of Competing Interest
None.
Acknowledgements
Supported by Russian National Science Fund (RNF) grant #19-17-00241. We thank S. Popov, N. Nekrylov and O.E. Melnik for help with fieldwork. Taras Gerya for access to the EULER computer cluster network and use of his modeling code, Caltech sample repository is thanked for access to sample collection associated with C. C. Gazis and H.P. Taylor et al. 1995 study. Careful and very insightful reviews by Juliana Troch, Kelly Russell, Gerhard Woerner and Shan de Silva and an anonymous reviewer from
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