Architecture of volcanic plumbing systems inferred from thermobarometry: A case study from the Miocene Gutâi Volcanic Zone in the Eastern Carpathians, Romania

The Gutâi Volcanic Zone (GVZ) in the Eastern Carpathians records one of the most complex and long-lasting periods of Miocene volcanic activity (15.4–7.0 Ma) in the Carpathian–Pannonian region. Two types of volcanism are documented: 1) caldera-related acidic volcanism (ca. 15.4 Ma); and 2) multiphase intermediate volcanism (13.4–7.0 Ma). Two composite volcanoes (Mogoşa - G5, Igniş - G7), six extrusive domes (Dăneşti - G2, Poiana Cremenei - G3, Breze - G6, Pleşca Mare - G9, Gutâi - G10 and Laleaua Albă - G11) and an intrusive complex (Firiza - G12) were selected for analysis to unravel the architecture of the volcanic plumbing systems (VPS) based on the pressure-temperature (P-T) conditions during crystallisation of amphiboles and clinopyroxenes. Using the reliable geothermobarometers for amphiboles, and for clinopyroxenes, P–T conditions were calculated for basalts, basaltic andesites, andesites, dacites/rhyolites, and mafic microgranular enclaves (MMEs) hosted by dacites. The calculated pressures vary from 0.4 to 1.8 kbar for low-Al amphiboles (7.5–9.3 wt% Al2O3, magnesio-ferri-hornblende) and from 6 to 9 kbar for high-Al amphiboles (11.2–15.6 wt% Al2O3, magnesio-hastingsite, ferri-sadanagaite and pargasite). The calculated temperatures vary between 800 and 860 °C for low-Al amphiboles and between 950 and 1010 °C for high-Al amphiboles. The P–T data acquired for the clinopyroxenes (augite and diopside with 62–85 Mg#) are 3.5–8.5 kbar (predominantly >5 kbar) and 1050–1150 °C. A difference of 4–5 kbar is observed between the pressures recorded by low-Al amphiboles and clinopyroxenes in the G2 dacites and G3, G6 and G10 high-silica andesites. The “at equilibrium” high-Al amphiboles and clinopyroxenes from the G5, G11 and G12 samples record similar high pressures (6.0–8.5 kbar) but differences in temperature of 90–140 °C. The different depths constrained by the P–T data for the crystallisation of the amphiboles and clinopyroxenes indicate the presence of magma storage reservoirs scattered throughout the entire thickness of crust, from very deep locations (27–33 km) near the lower crust–lithospheric mantle boundary (MOHO = 33–35 km), up to shallow levels in the upper crust (2–5 km). Most of the volcanic structures show complex, multi-level interconnected VPS. The presence of numerous very deep magmatic reservoirs in several volcanic structures across the entire time-interval of the volcanism (which have high isotopic signatures, such as 87Sr/86Sr values of 0.7070–0.7076, for even the most basic rocks) provides evidence for a main, deep magma reservoir (“deep hot zone”) that acted as a “MASH zone” at the crust–mantle boundary beneath the GVZ. The architecture of the magmatic plumbing systems controlled the geological evolution of the studied volcanic structures. The processes of assimilation fractional crystallisation and magma mingling and mixing were also controlled by the complex VPS, as recorded by a wide range of volcanological, mineralogical–petrographical, and geochemical features. Overall, the GVZ records a complex evolution of volcanic activity that reflects the intricate magmatic plumbing systems. The VPS models and their inferred architectures provide a better understanding of the complex and long-lasting volcanism in the GVZ, and are relevant to other areas of extinct volcanism globally.