Pleistocene eruptive chronology of the Gölcük volcano, Isparta Angle, Turkey. Chronologie des épisodes volcaniques pléistocènes du volcan Gölcük, Angle d’Isparta, Turquie

In the Eastern Mediterranean region, the Isparta volcanic belongs to the post-collisional alkali-potassic to ultrapotassic magmatism active since the Miocene in this part of the Anatolian peninsula from Afyon to Isparta. In the so-called Isparta Angle (IA) the magmatism is contemporaneous with an extensional regime intiated during Late Miocene and active throughout the Pliocene and Quaternary. Previous K/Ar dating performed on lavas suggested that potassic-ultrapotassic magmatism occurred between 4.7 to 4 Ma. However, a more recent (Quaternary) activity of the Gölcük volcano is evidenced by the present-day morphology and field evidence although it remained undated and poorly studied so far. Field mapping and new radiometric data indicate that the main volcano-forming stages of the Gölcük volcano consist of three main eruptives cycles. (1) Cycle I, represented by more than 200m-thick pyroclastic flow deposits occasionally separated by paleosoils and corresponding to caldera-forming ignimbritic eruptions. (2) Cycle II, consisting of tephriphonolite lava dome-flows extruded throughout the caldera and currently found along the rim of the present crater. (3) Cycle III made up of tuff-ring deposits related to several phreatoplinian eruptions of a maar-type volcanic activity. This youngest cycle ends with trachytic domes protruding within the maar crater. Unspiked 40K/40Ar dating on mesostasis was performed on lavas (tephriphonolites and trachytic domes), and complemented by preliminary 40Ar/39Ar data on tephra deposits (sanidine). Our preliminary results show that the entire activity of Gölcük volcano took place during the Pleistocene and was disconnected from the older Pliocene volcanism. This volcanic activity can be considered as a new volcanic cycle, starting (Cycle I) around 200 ka with major explosive, regional-scale, events represented by at least six ignimbrites sheets. Cycle II occurred between 115 ± 3 ka to 62 ± 2 ka with probably some associated tephra deposits. Tuff-ring of Cycle III formed from 72.7 ± 4.7 ka to 24 ± 2 ka. The associated phreatoplinian eruptions have almost entirely destroyed the previously formed flow-dome. This latest activity corresponds to several volcanic crises as illustrated by the two domes protrusions separated by about 30 ka. The volcanic history of Gölcük ceased around 24 ka ± 2 ka, but the periodicity of eruptive events appears to be long and complex. Currently, the volcano is at rest, but there is no doubt that the Isparta town (more than 120 000 people) built on top of the most recent tephra falls is exposed to a major volcanic hazard in the future.En Méditerranée Orientale, la région active d’Isparta est le siège d’un magmatisme alcalin lié à la distension affectant cette partie de la Péninsule Anatolienne depuis le Miocène supérieur. Le volcanisme Pliocène est alcalin et très potassique, depuis des magmas lamprophyriques à lamproïtiques, jusqu’à des téphriphonolites et des trachytes. La construction du volcan Gölcük au sud d’Isparta marque le début d’un nouveau cycle éruptif après une longue période d’arrêt et d’érosion. L’étude morpho-structurale du volcan couplée aux datations 40K/40Ar sur lave et 39Ar/40Ar sur monograin de feldspath-K indique une histoire éruptive complexe, nettement plus jeune que l’activité antérieure (Pliocène). Ces résultats préliminaires montrent que l’activité volcanique du Gölcük est située dans le Pléistocène supérieur (Paléolithique) entre environ 200 ka et 24 ka. Trois cycles volcaniques majeurs sont reconnus : (1) Cycle I débutant vers 200 ka avec des éruptions ignimbritiques majeures avec un ensemble de coulées pyroclastiques trachytiques comblant les paléo-vallées ouvertes dans les formations sédimentaires et les formations volcaniques d’âge pliocène ; (2) Cycle II avec un épisode effusif de faible importance succède entre 115 ± 3 ka et 62 ± 2 ka à l’activité explosive initiale avec la mise en place d’un édifice central constitué de dômes-coulées téphri-phonolitiques ; (3) Cycle III entre 70 ka et 24 ka, l’activité devient phréatoplinienne et suit de près le cycle précédent. Le dynamisme éruptif phréatomagmatique est celui d’un maar formé d’un large cratère d’explosion entouré d’un anneau de tufs. La dernière crise volcanique se termine par l’extrusion de plusieurs dômes de trachyte dans le cratère et de téphras associés, de nouvelles coulées pyroclastiques se mettent vraisemblablement en place vers le nord-ouest. Les données de terrain et les âges 40Ar/39Ar disponibles indiquent que ces dernières manifestations (construction du maar) sont très récentes et sub-contemporaines du dernier niveau de retombées ponceuses sous les immeubles de la ville et des dômes de lave intra-caldeira. Cet âge récent est confirmé par un âge 14C obtenu sur des bois carbonisés. La morphologie du volcan actuel est relativement bien conservée, malgré l’érosion très active qui remodèle déjà partiellement les pentes. La reprise éventuelle de l’activité du volcan constituerait un risque majeur à l’échelle de la région et en particulier pour la ville d’Isparta établie au pied de l’édifice, notamment sur les coulées pyroclastiques et les retombées ponceuses les plus récentes

Le plutonisme de tendance alcalin potassique de Stara Planina, Bulgarie (étude pétrologique des complexes de Buhovo-Seslavtzi, Svidnya et Shipka)

Le magmatisme du Paléozoïque supérieur est à l'origine des trois plutons d'affinité potassique dans Stara planina, Bulgarie. Le caractère post tectonique des roches alcalines potassiques de Svidnya, Buhovo-Seslavtzi et Shipka est motivé par leur structure isotrope, restée intacte pendant les déformations reliées aux processus orogéniques. La cristallisation du magma des trois plutons s'est déroulée à faible profondeur, à des pressions entre 2 et 3 kbar pour les plutons de Svidnya et Buhovo-Seslavtzi et entre 5 - 7 kbar pour les roches de Shipka. Les roches de trois plutons sont généralement intermédiaires des compositions syénitique et monzonitique. L'évolution est orientée vers la formation d'un résidu fortement hyperalcalin, avec cristallisation des minéraux mafique alcalins. Les roches présentent une tendance potassique nette, avec enrichissement en éléments Th, U Rb, Ba, Sr, Cs très fort. Avec la tendance agpaïtique les teneurs en éléments Zr, Nb, Ta, TR deviennet importantes, Les particularités géochimiques des roches suggèrent la formation de magma produit par un taux de fusion bas dans un manteau à grenat ou spinelle, métasomatiquement modifié. La modification métasomatique de la péridotite mantélique s'est déroulée avec l'introduction de matériaux d'origine crustale et de fluides au cours de la subduction / collision. Les particularités géochimiques des roches et le caractère de la distribution des éléments sur les diagrammes multicomposants indiquent l'aspect orogénique du magmatisme.Upper Palaeozoic magmatism in Stara palnina produced three potassic alkaline plutons: Svidnya, Buhovo-Seslavtzi and Shipka. The post-tectonic character of the rocks is motivated by their isotropic fabrics, stayed unaffected during the deformations related to orogenic processes. The magma crystallization occurred between 2 - 3 kbar for Buhovo-Seslavtzi and Svidnya complexes and between 5 - 7 kbar for Shipka complex. The rocks from the plutons are of intermadiate composition, and monzonite and syenite varieties dominate. The evolution is orientated toward the formation of strongly peralkaline residu, with crystallization of alkaline mafic phases. The rocks show potassic tendency with significant enrichment in Th, U, Rb, Ba, Sr and Cs. With the agpaitic tendency the content of Zr, Nb, Ta, REE becomes important. The geochemical particularities suppose the magma formation by the low degree melting of metasomatised mantle with garnet or spinel. The metasomatic modification of mantle peridotite occurred by the introduction of crustal components and fluids during the subduction/collision. The geochemical particularities of the rocks and the distribution of element in the spider-diagrammes favor the orogenic character of the magmatism.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

De la justesse des âges K-Ar : exemple de la datation de deux dômes trachytiques du Gölcük (Turquie)

International audienceLe Gölcük, situé au sud de la ville d'Isparta (Turquie), est un volcan composite, construit en trois phases durant le Pléistocène (Alici et al., 1998 ; Platevoet et al., 2008 ; Elitok et al., 2010). Les études réalisées par Platevoet et al. (2008) et Schmitt et al. (2014) se sont intéressées en partie à la datation de deux dômes mis en place au sein de la caldeira du Gölcük. Les âges K-Ar (Platevoet et al., 2008) de ces dômes trachytiques (Küçükçesme Tepe : 52 ± 2 ka et Pilav Tepe : 24 ± 2 ka) sont significativement plus vieux que les âges (U-Th)/He sur zircons obtenus par Schmitt et al. (2014) étant respectivement de 14,1 ± 0,5 et de 12,9 ± 0,4 ka. Nous avons testé l'hypothèse proposée par Schmitt et al. (2014) qui est celle d'un excès d' 40 Ar * faussant les âges K-Ar en datant ces mêmes trachytes par la méthode 40 Ar/ 39 Ar. Nos âges 40 Ar/ 39 Ar sur mésostase confirment les âges (U-TH)/He sur zircons et datent l'activité magmatique la plus récente du Gölcük à 13,4 ± 0,3 ka. Nous proposons que l'erreur sur les âges K-Ar soit liée à un excès d'argon probablement porté par des clinopyroxènes associés à la mésostase. ABSTRACT QUESTIONING THE ACCURACY OF K-AR AGES: EXAMPLE OF THE DATING OF TWO TRACHYTIC DOMES FROM THE GÖLCÜK (TURKEY) The Gölcük volcano, located south to the city of Isparta (Turkey), built-up in three stages during the Pleistocene (Alici et al., 1998; Platevoet et al., 2008; Elitok et al., 2010). A reliable and accurate dating of the most recent volcanic stage is crucial to assess the potential hazards for the infrastructures and the populations. In this context, Platevoet et al. (2008) and Schmitt et al. (2014) applied respectively the K-Ar and the (U-Th)/He methods to date the youngest volcanic stage of the Gölcük and obtained contrasted results. Based on the K-Ar dating of two trachytic domes (Kücükcesme Tepe et Pilav Tepe), the post-caldera stage is dated between 52 ± 2 ka and 24 ± 2 ka by Platevoet et al. (2008). When the (U-Th)/He method is applied on zircons from the same trachytic domes, Schmitt et al. (2014) date the post-caldera stage around 14.1 ± 0.5-12.9 ± 0.4 ka. Schmitt et al. (2014) proposed that the K-Ar ages were erroneously too old, probably because of excess 40 Ar*. We have tested this hypothesis by dating groundmass splits of these trachytes using the 40 Ar/ 39 Ar method. Our 40 Ar/ 39 Ar ages reproduce the (U-Th)/He ages on zircons and date the youngest volcanic stage of the Gölcük at 13.4 ± 0.3 ka. Confirming the hypothesis of Schmitt et al. (2014), we propose that the error on the K-Ar ages is related to excess argon probably trapped in small fragments of clinopyroxene attached to the groundmass

Rhyolites of the Mbépit Massif in the Cameroon Volcanic Line: an early extrusive volcanic episode of Eocene age

International audienceThe Mbépit Massif, one of the oldest volcanoes of the Cameroon Volcanic Line, was built up during the Eocene (40K/40Ar ages of 45.5 and 44.0 Ma). Located in the Eastern part of the Noun Plain, North-East of Foumbot, this massif is made up mainly of rhyolitic domes and thick, viscous lava flows. Such felsic lavas are rather rare in the Cameroon Volcanic Line. Most rhyolites are porphyritic and contain phenocrysts of K-feldspars (Or97–89), Na-feldspars (Ab99–88) and quartz. Scarce Fe–Ti oxides comprise titanohaematite and hemo-ilmenite. Owing to their low CaO contents, rhyolite compositions range from metaluminous to peraluminous (with normative corundum) and less commonly, peralkaline (normative aegirine). Chondrite-normalised REE patterns show LREE enrichment; and LaN/YbN ratios between 12.2 and 17.2, with prominent negative Eu anomalies (0.16 < Eu/Eu* < 0.62). The Th/Ta ratios range between 2.6 and 3. Initial 87Sr/86Sr ratios (0.7047 and 0.7050) and 143Nd/144Nd ratios (0.5125 and 0.5126, εNd of −1.04 to +1.22) suggest mantle sources (HIMU (high µ with μ = 238U/204Pb), EM (Enriched Mantle), FOZO (FOcal ZOne) components) of the rhyolites accompanied by little or no crustal contamination

New insights into the ultrapotassic magmatism through xenoliths from the Eğirdir area, West Anatolia, Turkey

International audiencePlutonic xenoliths have been found within a pipe and a related phreatomagmatic leucitite deposit in the Eğirdir lake area, belonging to the Potassic-Ultrapotassic Afyon volcanic Province, West Anatolia. They consist of kamafugite-type, feldsparbearing syenite, pyroxenite, leucitolite, some small-sized melilitolite and garnet-rich xenoliths, and a carbonatite. A new occurrence of kalsilite is described as either homogeneous acicular crystals or tabular two phases-exsolved crystals in the kamafugite-type and melilitolite xenoliths. Rock textures and compositions indicate cumulates and near-liquid composition rocks corresponding to relatively evolved magmas. All the rocks are strongly silica-undersaturated, Ca-, Mg-, and K-rich, and Al-poor. The fractional crystallization model includes clinopyroxene, apatite, phlogopite, melilite and leucite. Fe-Ti oxides and garnet may be also concerned. The P H2O during crystallization and differentiation is not more than 0.8 GPa. Major elements, trace elements, and REE patterns for xenoliths, which indicate near-liquid compositions, are typical of ultrapotassic series in a post-collisional geodynamic context, as it is the case for the Roman and Central ultrapotassic Italian provinces. The stable isotope 13 C and 18 O values of the calcio-carbonatite plot close to the primary carbonatite field, whereas the carbonates of the feldspar-bearing syenite and the peperite matrix suggest a low-T extensive contamination process. The origin of the carbonatite from kamafugite-type magmas by immiscibility or by fractional crystallization remains questionable; an origin by fractionation-melting of a metasomatized mantle source should be tested in the future

Alkali magmatism on a carbonaceous chondrite planetesimal

International audienceRecent isotopic and paleomagnetic data point to a possible connection between carbonaceous chondrites and differentiated planetary materials suggesting the existence, perhaps ephemeral, of transitional objects with a layered structure whereby a metal-rich core is enclosed by a silicate mantle which is itself overlain by a crust containing an outermost layer of primitive solar nebula materials. This idea has not received broad support mostly because of a lack of samples in the meteoritic record that document incipient melting at the onset of planetary differentiation. Here we report the discovery and the petrologic-isotopic characterization of UH154-11, a ferroan trachybasalt fragment enclosed in a CR chondrite. Its chemical and oxygen isotopic compositions are consistent with very low degree partial melting of a CV chondrite from the oxidized subgroup at a depth where fluid-assisted metamorphism enhanced the Na content. Its micro-doleritic texture indicates crystallization at an increasing cooling rate such as would occur during magma ascent through a chondritic crust. This represents the first direct evidence of magmatic activity in a carbonaceous asteroid on the verge of differentiating and demonstrates that some primitive outer solar system objects related to icy asteroids and comets underwent a phase of magmatic activity early in the solar system. With its peculiar petrology, UH154-11 can be considered the long-sought first melt produced during partial differentiation of a carbonaceous chondritic planetary body bridging a previously persistent gap in differentiation processes from icy cometary bodies to fully melted iron meteorites with isotopic affinities to carbonaceous chondrites

Miocene to recent alkaline volcanism between Al Haruj and Waw an Namous (southern Libya)

International audienceUnspiked K-Ar ages, petrological, geochemical and isotopic data are reported on samples from southern Libya (Wan an Namous--Al Haruj area). The Wan an Namous intracaldera cone dated at 0.2 Ma consists of unusually undersaturated foidite, representing the most extreme compositions among Libyan and Tibestian lavas. A basanitic and a basaltic lava flow located north-west of Wan an Namous, and probably belonging to the Al Haruj volcanic field, were dated at 5.1 Ma and 8.1 Ma. These data extend the range of ages previously reported for Al Haruj lavas. REE and multi-element patterns are typical of alkaline intraplate magmas. Sr ratios range from 0.70314 to 0.70812, whereas Nd ratios are very homogeneous (0.51290-0.51293). Pb ratios (19.231 < 206Pb/204Pb < 19.547, 15.607 < 207Pb/204Pb < 15.641 and 38.859 < 208Pb/204Pb < 39.242) are typical of HIMU-FOZO compositions. Such isotope characteristics are very similar to those available on two Gharyan (northern Libya) lavas and largely overlap those of Hoggar and Cameroon Line alkaline rocks. These lavas were produced by low and variable degrees of partial melting of a garnet- and amphibole-bearing mantle source, constraining the depth of melting between 80 and 150 km. Crustal contamination was also probably involved for the oldest sample. Mineral compositions of a dunite-harzburgite xenolith clearly indicate that the lithospheric mantle was affected by partial melting and metasomatic processes by magmatic liquids, probably associated with the genesis of Cenozoic lavas. Lithospheric delamination and asthenospheric upwelling, due to the reactivation of lithospheric megastructures induced by the Africa-Europe convergence, could represent a model for the genesis of Libyan lavas, as in Hoggar