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

Bindeman, I.N.
Colón, D.P.
Wotzlaw, J.-F.
Stern, R.
Guillong, M.
Published in Journal of Volcanology and Geothermal Research. 2021, vol. 412, no. 107173
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 Chegemvolcanic 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 amulti-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 magmatismat 2.92 and 1.98 Ma related to Chegemand Tyrnyauz, respectively. High precision CA-ID-TIMS dating of zircons fromthe top and bottomof 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. Togetherwith 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‰, εHf = +0.8 to +5.6) while the rims (+5.75 ± 0.20‰, εHf = 3.3 ± 0.7) are in isotopic equilibriumwith 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 εHf 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.
Keywords CaucasusEldjurta GraniteZayukovoElbrusTyrnyauzZirconOxygen isotopesHydrogen isotopesThermomechanical modeling
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Research groups Isotope Geochemistry, Geochronology and Thermochronology
Mineral Resources and Geofluids
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BINDEMAN, I.N. et al. Young Silicic Magmatism of the Greater Caucasus, Russia, with implication for its delamination origin based on zircon petrochronology and thermomechanical modeling. In: Journal of Volcanology and Geothermal Research, 2021, vol. 412, n° 107173. doi: 10.1016/j.jvolgeores.2021.107173 https://archive-ouverte.unige.ch/unige:152179

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Deposited on : 2021-06-07

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