Paleoaltimetry and paleotectonic reconstruction using triple oxygen and hydrogen isotopes: Depleted δ18O and δD values in ignimbrites of Verkhneavachinskaya caldera record collisional uplift during Miocene arc accretion to Kamchatka

Here we use δ¹⁸O and δD values as tools to investigate the paleo-altitude and the origin of large-volume (120 km³, 10 × 12 km) ignimbrites of Verkhneavachinskaya caldera (cf. Verkhne-Avachinskaya) (VC) in eastern Kamchatka, formed the in Late Miocene 5.8 Ma. The basaltic-andesitic intracaldera ignimbrite deposit exhibits low δ¹⁸O values, reaching −5.03‰, and δD values of −182‰ across a 1.2 km depth range in several sampled sections. The results support a massive meteoric-hydrothermal system throughout the cooling history of the thick intracaldera ignimbrite deposit. Using triple oxygen isotope data we estimate that the δ¹⁸O values of altering meteoric water are as low as −19‰ to −23‰, much lower than modern precipitation of −14‰, or − 16‰ estimated for the 2–3 °C warmer Kamchatka climate of the late Miocene. Therefore, the VC meteoric-hydrothermal system depended on high-altitude precipitation and glaciers, and altitudinal isotopic lapse rates suggest a paleo-altitude of 3.5 km at 5.8 Ma during caldera formation. These elevations exceed the modern by 1.5 km and provide a unique snapshot into an evolving landscape and paleo-environment of eastern Kamchatka at 6–5 Ma as the dynamic outcomes of accretion of the Kronotski arc. In particular, the existence of such a high plateau is in line with contemporary accretionary tectonics: accretion of the Shipunsky peninsula of this accreting arc terrain, ∼120 km to the east of the VC, and evidence of contemporaneous exhumation of high-grade Ganal amphibolites to the west. We conclude that the eruption of large-volume mafic ignimbrites that formed VC caldera was syncollisional or intracollisional in nature, likely requiring delamination of thickened crust to account for both uplift and magmatism.