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a b s t r a c tThis article provides laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and secondary ionization mass spectrometry (SIMS) U-Pb and U-Th zircon dates for crystals separated from Late Pleistocene dacitic lava dome rocks of the Ciomadul Volcanic Dome Complex (Eastern Carpathians, Romania). The analyses were performed on unpolished zircon prism faces (termed rim analyses) and on crystal interiors exposed through mechanical grinding an polishing (interior analyses). 206 Pb/ 238 U ages are corrected for Th-disequilibrium based on published and calculated distribution coefficients for U and Th using average whole-rock and individually analyzed zircon compositions. The

10 Abstract Ciomadul is the youngest volcano in the Car-11 pathian-Pannonian region produced crystal-rich high-K 12 dacites that contain abundant amphibole phenocrysts. The 13 amphiboles in the studied dacites are characterized by large 14 variety of zoning patterns, textures, and a wide range of 15 compositions (e.g., 6.4-15 wt% Al 2 O 3 , 79-821 ppm Sr) 16 often in thin-section scale and even in single crystals. Two 17 amphibole populations were observed in the dacite: low-Al 18 hornblendes represent a cold (\800°C) silicic crystal 19 mush, whereas the high-Al pargasites crystallized in a hot 20 ([900°C) mafic magma. Amphibole thermobarometry 21 suggests that the silicic crystal mush was stored in an upper 22 crustal storage (*8-12 km). This was also the place where 23 the erupted dacitic magma was formed during the remo-24 bilization of upper crustal silicic crystal mush body by hot Author Proof We investigate the implications of our study for using 91 amphibole to constrain the subvolcanic plumbing systems 92 of andesitic to dacitic arc volcanoes in general. Finally, we 93 discuss the origin, conditions, and processes lead to com-94 monly observed bimodal amphibole populations in inter-95 mediate mixed magmas erupted at composite arc 96 volcanoes. Our study highlights that different amphibole 97 thermobarometers can produce essentially different results, 98 which may lead to false interpretations on the magma 99 evolution and architecture of the magma storage system 100 without clear textural control and crystal growth stratigra-101 phy. We point out the deficiency of the Ridolfi's thermo-102 barometric model that yields always the same p-T 103 evolution path for amphiboles along their stability curve. Geological setting105 Ciomadul volcano is located at the southeastern edge of the 106 Carpathian-Pannonian region, and this is the youngest 107 volcano of this area (Szakács and Seghedi 1995; Szakács 108 et al. 2002;Vinkler et al. 2007;Harangi et al. 2010; Kar-109 átson et al. 2013). It is found at the southern termination of 110 the Cȃlimani-Gurghiu-Harghita (CGH) andesitic-dacitic 111 volcanic chain (Fig. 1) 120 et al. 1987;Szakács et al. 1993; Mason et al. 1996). Vol-121 canic eruptions in Ciomadul could have started around 122 200 ka, and the last volcanic event occurred at 123 31,000 ± 260 cal BP. Initially, the volcanism was mostly 124 effusive and a lava dome complex developed called here 125 ''old Ciomadul.'' Later, the volcanic activity became more 126 explosive and as a result of successive phreatomagmatic 127 and subplinian eruptions, the edifice of the ''old Ciomadul'' 128 was partially destroyed and two deep explosive craters 129 were formed (Szakács and Seghedi 1995; Karátson et al. 130 2013). The erupted magma remained fairly homogeneous 131 through time and shows high-K dacitic composition (Sza-132 kács and Seghedi 1986; Vinkler et al. 1997). The geody-133 namic background of the CGH volcanism and particularly 134 the volcanic activity of southern Harghita and Ciomadul is 135 still hi...

14Alkaline basaltic volcanism has been taking place in the Carpathian-Pannonian Region 15 since 11 Ma and the last eruptions occurred only at 100-500 ka. It resulted in scattered low-16 magma volume volcanic fields located mostly at the margins of the Pannonian basin. Many of 17 the basalts have compositions close to those of the primitve magmas and therefore can be 18 used to constrain the conditions of the magma generation. Low degree (2-3%) melting could 19 occur in the convective asthenosphere within the garnet-spinel transition zone. Melting started 20 at about 100 km depth and continued usually up to the base of the lithosphere. Thus, the final 21 melting pressure could indicate the ambient lithosphere-asthenosphere boundary. The 22 asthenospheric mantle source regions of the basalts were heterogeneous, presumably in small 23 scale, and included either some water or pyroxenite/eclogite lithology in addition to the fertile 24 to slightly depleted peridotite. Based on the prevailing estimated mantle potential temperature 25 This is reinforced by the detected low-velocity seismic anomalies in the upper mantle beneath 37 the volcanic fields. 38 39Manuscript Click here to download Manuscript: IJES_paper_2014_rev.docx Click here to view linked

Ciomadul is the youngest volcanic system in the Carpathian-Pannonian Region recording eruptive activity from ca. 1 Ma to 30 ka. Based on combined zircon U-Th and (U-Th)/He geochronology, Ciomadul volcanism is divided into two main eruptive periods: Old Ciomadul (1 Ma-300 ka; OCEP) and Young Ciomadul Eruptive Period (160-30 ka; YCEP). OCEP activity comprises Eruptive Epochs 1-3, whereas new ages for eight lava domes and four pyroclastic units belonging to the YCEP lead to its further subdivision into two eruptive epochs: Eruptive Epochs 4 and 5. The extrusion of most of the lava domes occurred between 160 and 90 ka (Eruptive Epoch 4) during three eruptive episodes at ca. 155 ka, 135 ka and 95 ka (Eruptive Episodes 4/1, 4/ 2 and 4/3, respectively) along a NE-SW lineament, which is perpendicular to the regional NW-SE trend of the Călimani-Gurghiu-Harghita volcanic chain. Eruptive Epoch 5 occurred after a ca. 40 kyr of quiescence at ca. 55-30 ka, and is mainly characterized by explosive eruptions with a minor lava dome building activity. All of the dated pyroclastic outcrops, together with the lava dome of Piscul Pietros, belong to the older Eruptive Episode 5/1, with an eruption age of 55-45 ka. The eruption centers of Eruptive Epoch 5 are located at the junction of the conjugated NW-SE and NE-SW lineaments defined by the older eruptive centers. The whole-rock geochemistry of all studied samples is fairly homogeneous (SiO 2 = 63-69 wt%, K 2 O = 3-4 wt%). It also overlaps with the composition of the lava domes of the Old Ciomadul Eruptive Period, implying a monotonous geochemical characteristic for the past 1 Myr. The eruption rates for the Ciomadul volcanism were determined based on the erupted lava dome volume calculations, supplemented with the eruption ages. The activity peaked during the Eruptive Epoch 4 (160-90 ka), having an eruption rate of 0.1 km 3 /kyr. In comparison, these values are 0.05 km 3 /kyr for the YCEP (160-30 ka) and 0.01 km 3 /kyr for the overall Ciomadul volcanism (1 Ma-30 ka). Based on the geochemical characteristics, the quiescence periods and the lifetime of the complex, as well as the relatively small amount of erupted material, this volcanic system can be placed in a subduction-related post-collisional geodynamic setting, which shows strong chemical similarities to continental arc volcanism. The commonly found long repose times between the active phases suggest that the nature of a volcano cannot be understood solely based on the elapsed time since the last eruption. Instead, comprehensive geochronology, coupled with the understanding of the magma storage behavior could be a base of hazard assessment for volcanic fields, where the last eruptions occurred several 10's of thousand years ago and therefore they are not considered as potentially active.

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ABSTRACT. This paper provides new accelerator mass spectrometry (AMS) radiocarbon age data for the last volcanic events in the Carpathian-Pannonian region of eastern-central Europe. The eruption ages were determined on charcoal fragments collected from pumiceous pyroclastic flow deposits at 2 localities of the Ciomadul Volcano. Two charcoal samples from the southeastern margin of the volcano (Bixad locality) set the date of the last volcanic eruption to 27,200 ± 260 yr BP (29,500 ± 260 cal BC). On the other hand, our data show that the Tusnad pyroclastic flow deposit, previously considered as representing the youngest volcanic rock of the region, erupted at ~39,000 yr BP (~41,300 cal BC). Thus, a period of dormancy more than 10,000 yr long might have elapsed between the 2 volcanic events. The different ages of the Tusnad and Bixad pyroclastic flow deposits are confirmed also by the geochemical data. The bulk pumices, groundmass glass, and the composition of the main mineral phases (plagioclase and amphibole) suggest eruption of slightly different magmas. Considering also the assumed long volcanic history (~600 ka) of the Ciomadul, these data suggest that further detailed studies are necessary on this seemingly inactive volcano in order to evaluate the possible renewal of volcanic activity in the future.

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