Young Silicic Magmatism of the Greater Caucasus, Russia, with implication for its delamination origin based on zircon petrochronology and thermomechanical modeling

Highlights

• Greater Caucasus is one of the best-preserved examples of silicic magmatism in a continental collision environments.

• We report results of a multi-method zircon petrochronologic (U--Pb, O--Hf isotopes) and whole rock geochemistry.

• Thermomechanical modeling describe tempo and volume of magmatism.

• Voluminous silicic volcanism at Chegem, Tyrnyauz and Elbrus are due to a relict slab detachment.

Abstract

Several kilometers of rapid uplift in the past 2–3 million years in the Greater Caucasus in Russia has produced world-class exposures of Pliocene-Pleistocene age granites and ignimbrites at the Tyrnyauz and Chegem volcanic centers just to the east of Europe's highest mountain and active volcano, Mount Elbrus. This represents one of the world's best-preserved examples of silicic magmatism in a continental collision environment. We report results of a multi-method zircon petrochronologic (U--Pb, O--Hf isotopes, trace elements) investigation of six ignimbrites, lavas, and associated granodioritic porphyries from these localities. We observe two pulses of magmatism at 2.92 and 1.98 Ma related to Chegem and Tyrnyauz, respectively. High precision CA-ID-TIMS dating of zircons from the top and bottom of the Chegem ignimbrite and its associated porphyry yield indistinguishable age spectra recording 160 kyr of magma assembly and differentiation, with 2.9181 ± 0.0014 Ma eruption age as is constrained by the youngest dated zircon population. Together with overlapping O and Hf isotopic values, this suggests that they represent a large volume of pre-eruptively homogenized magma with ~20% crystals. Zircons have isotopically diverse cores (δ¹⁸O = +3.4–6.7‰, ε_Hf = +0.8 to +5.6) while the rims (+5.75 ± 0.20‰, ε_Hf = 3.3 ± 0.7) are in isotopic equilibrium with associated unaltered glass and major phenocryst phases. The neighboring and younger ignimbrites and granites of the nearby Elbrus and Tyrnyauz centers have overlapping higher δ¹⁸O and lower ε_Hf zircon values and cluster narrowly in age around 1.98 Ma, suggesting a common source for all these ignimbrites, likely in the Tyrnyauz area. Isotopic data for Chegem, Elbrus and Tyrnyauz zircons and rocks require the contribution of a high proportion (25–55%) of Paleozoic crust to magma petrogenesis.

The origin of sudden spikes of voluminous silicic magmatism is further investigated using thermomechanical modeling of the collisional environment. We show that the voluminous silicic volcanism at Chegem, Tyrnyauz and Elbrus is best explained by heating of the lower crust by asthenospheric upwelling after a relict slab detached and sank into the mantle at approximately 5 Ma. This timing is coincident with both the beginning of rapid uplift of the Greater Caucasus and initiation of major volcanism. After this initial melting, continuing delamination of the lithosphere and lower crust from the subducting plate produced broad mantle melting and basaltic volcanism, potentially explaining continued regional-scale magmatism in both the Greater and Lesser Caucasus.

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).