Eruptive history of the Late Quaternary Ciomadul (Csomád) volcano, East Carpathians, part II: magma output rates

This study, which builds on high-precision unspiked Cassignol-Gillot K-Ar age determinations, presents an advanced DEMbased volumetrical analysis to infer long-term magma output rates for the Late Quaternary Ciomadul (Csomád) dacitic lava dome complex (East Carpathians, Romania). The volcanic field of Ciomadul developed on the erosional surface of Lower Cretaceous flysch and ~ 2 Ma old andesites and experienced an extended eruptive history from ~ 850 to < 30 ka. Predominantly effusive activity took place during the first stage (~ 850 to ~ 440 ka), producing volumetrically minor, isolated, peripheral domes. Subsequently, after a ~ 250 ky repose interval, a voluminous central dome cluster developed in the second stage (~ 200 to < 30 ka). During the youngest phase of evolution (~ 60 to < 30 ka), highly explosive eruptions also occurred, resulting in the formation of two craters (Mohos and St. Ana). The calculated ~ 8.00 ± 0.55 km3 total volume of the lava domes, which includes the related volcaniclastic (1.57 km3 ) as well as erosionally removed (0.18 km3 ) material, is in line with dimensions of other medium-sized dacitic lava domes worldwide. This volume was extruded at an average long-term magma output rate of 9.76 km3 / My (0.0098 km3 /ky). However, most of the domes (7.53 ± 0.51 km3 ) were formed in the 200 to < 30 ka period, implying a significantly increased magma output rate of 37.40 km3 /My (0.0374 km3 /ky), more than 30 times higher than in the first stage. Within these long-term trends, individual lava domes of Ciomadul (e.g. those with volumes between 0.02 and 0.40 km3 ) would have been emplaced at much higher rates over a period of years to tens of years. The active periods, lasting up to hundreds of years, would have been followed by repose periods ~ 30 times longer. The most recent eruption of Ciomadul has been dated here at 27.7 ± 1.4 ka. This age, which is in agreement with radiocarbon dates for the onset of lake sediment accumulation in St. Ana crater, dates fragmented lava blocks which are possibly related to a disrupted dome. This suggests that during the last, typically explosive, phase of Ciomadul, lava dome extrusion was still ongoing. In a global context, the analysis of the volumetric dynamism of Ciomadul’s activity gives insights into the temporal variations in magma output; at lava domes, short-term (dayor week-scale) eruption rates smooth out in long-term (millenia-scale) output rates which are tens of times lower

Eruptive history of the Late Quaternary Ciomadul (Csomád) volcano, East Carpathians, part I: timing of lava dome activity

International audienceLocated at the southern tip of the Intra-Carpathian Volcanic Range in Romania, and composed of a dozen dacitic lava domes, the Ciomadul (Csomad) volcanic complex is the youngest eruptive centre of the Carpatho-Pannonian Region. Whereas, in the last decade, the explosive history of Ciomadul since 50 ka has been well constrained by numerous studies, the chronology of the dome sequence still lacks robust chronological constraints and an extended analysis of all available data. Here, we apply a detailed K-Ar dating approach to refine the chronology of the lava dome eruptions, using the unspiked K-Ar Cassignol-Gillot technique. Our dating focused on the most voluminous central part of the lava dome complex. New eruption ages were determined following a strict separation (of 10g) of groundmass from about 3kg of unaltered sample rocks, thereby isolating material whose cooling was contemporaneous with the eruption. The newly applied methodology, mainly consisting of a double full preparation, first at larger grain size (0.4mm) and then at <100m, provides an appropriate procedure to separate suitable material to obtain the K-Ar age of the eruption, i.e. the sample's groundmass, in which there is no risk of the presence of older, inherited crystals. Our new geochronological data set gives an improved insight into the temporal construction of the Ciomadul volcanic complex, where (due to the method applied here) all ages are younger than those from previous studies that used whole-rock K-Ar ages. Our new results show that Ciomadul's volcanic activity began with the construction of the south-eastern, peripheral domes from ca. 850 ka to 440 ka. After a ca. 250-ky-long repose period, the activity resumed in the northern part at around 200 ka, with subsequent domes emplaced between 200 and 130 ka, aligned roughly north-south in the western-central part of the complex. Following a 30-ky-long quiescence period, the eastern-central domes formed between 100 and 60 ka. In addition to the chronological history of lava dome volcanism, we also investigated the sequence of crystallization of mineral phases present in the lavas with respect to the modification of eruption ages. Ages obtained on pure minerals (plagioclase, amphibole, and biotite) are systematically older than those obtained on groundmass, showing that most of them formed up to 1.85 Myr before eruption in a long-lived, pre-Ciomadul magmatic system. Crystal size distribution (CSD) data support the age contrasts between juvenile groundmass and older inherited minerals. After injection of new magma and convective mixing with crystal clots, ascent of the resulting mixture led to eruptions of material representing contrasting ages

Growth and erosion rates of the East Carpathians volcanoes constrained by numerical models: Tectonic and climatic implications

International audienceThe East Carpathian volcanic range experienced an along-arc, Late Miocene to Quaternary migration of eruptive activity during its ~11 Ma-long activity. Here, a novel and complex methodology is presented that yields new geochronological and geomorphological constraints on the evolution of the 20 volcanic edifices. New unspiked K-Ar ages either constrain their lifespan (6.79 ± 0.10–6.47 ± 0.09 Ma for Seaca-Tătarca; 5.47 ± 0.08–4.61 ± 0.07 Ma for Vârghiş) or date the youngest volcanic activity (central Călimani). In parallel, numerical reconstructions of volcanic paleo-topographies were performed to quantify their shape at the end of their construction stage. The inferred initial volcano size shows a wide range (3 to 592 km3), making up the four main successive volcanic segments (910, 880, 279 and 165 km3 for Călimani, Gurghiu, North Harghita and South Harghita segment, respectively) totalizing 2200 km3 and an average growth rate of 200 km3/Ma at range-scale. At the volcano-scale, with only consideration to their respective time spans (i.e. avoiding repose periods), growth rate is characterized by two major trends: a moderate growth rate (137 km3/Ma) for the older volcanoes (11–3.6 Ma) followed by a lower growth rate (28 km3/Ma) obtained for the Plio-Quaternary volcanoes. Comparing reconstructed and current topographies yielded a total eroded volume of 524 ± 125 km3, defining averaged denudation of 22% and a 20 m/Ma erosion rate. Erosion rates for major climatic periods were computed, which highlight the contrasting climatic contexts since 11 Ma. The highest erosion rate (38 m/Ma) occurred during transitional moderate subtropical-continental climate period (9.5–8.2 Ma). An intermediate erosion rate (14 m/Ma) characterized a moderate continental climate period (8.2–6.8 Ma) when conditions became less humid. The lowest erosion rate (7 m/Ma) reflects the prevailing continental but occasionally semi-arid climate (6.8–5.8 Ma). The highest erosion rate (28 m/Ma) was obtained for Plio-Quaternary times during the interglacial/glacial cycles. Such a quantitative morphometric and geochronological approach demonstrates its efficiency to study volcanic dynamism, including both constructional and erosional processes, through time