Alain Gourgaud

Neogene ignimbrites of the Nevsehir plateau (Central Turkey): stratigraphy, distribution and source constraints

Abstract In Anatolia (Turkey), extensive calc-alkaline volcanism has developed along discontinuous provinces from Neogene to Quaternary times as a consequence of plate convergence and continental collision. In the Nevsehir plateau, which is located in the Central Anatolian Volcanic Province, volcanism consists of numerous monogenetic centres, several large stratovolcanoes and an extensive, mainly Neogene, rhyolitic ignimbrite field. Vent and caldera locations for the Neogene ignimbrites were not well known based on previous studies. In the Neogene ignimbrite sequence of the Nevsehir plateau, we have identified an old group of ignimbrites (Kavak ignimbrites) followed by five major ignimbrite units (Zelve, Sarimaden Tepe, Cemilkoy, Gordeles, Kizilkaya) and two smaller, less extensive ones (Tahar, Sofular). Other ignimbrite units at the margin of the plateau occur as outliers of larger ignimbrites whose main distributions are beyond the plateau. Excellent exposure and physical continuity of the units over large areas have allowed establishment of the stratigraphic succession of the ignimbrites as, from bottom to top: Kavak, Zelve, Sarimaden Tepe, Cemilkoy, Tahar, Gordeles, Sofular, Kizilkaya. Our stratigraphic scheme refines previous ones by the identification of the Zelve ignimbrite and the correlation of the previously defined ‘Akkoy’ ignimbrite with the Sarimaden Tepe ignimbrite. Correlations of distant ignimbrite remnants have been achieved by using a combination a field criteria: (1) sedimentological characterisitics; (2) phenocryst assemblage; (3) pumice vesiculation texture; (4) presence and characteristics of associated plinian fallout deposits; and (5) lithic types. The correlations significantly enlarge the estimates of the original extent and volume of most ignimbrites: volumes range between 80 km 3 and 300 km 3 for the major ignimbrites, corresponding to 2500–10,000 km 3 in areal extent. The major ignimbrites of the Nevsehir plateau have an inferred source area in the Derinkuyu tectonic basin which extends mainly between Nevsehir and the Melendiz Dag volcanic complex. The Kavak ignimbrites and the Zelve ignimbrite have inferred sources located between Nevsehir and Derinkuyu, coincident with a negative gravity anomaly. The younger ignimbrites (Sarimaden Tepe, Cemilkoy, Gordeles, Kizilkaya) have inferred sources clustered to the south between the Erdas Dag and the Melendiz Dag volcanic complex. We found evidence of collapse structures on the northern and southern flanks of the Erdas Dag volcanic massif, and of a large updoming structure in the Sahinkalesi Tepe massif. The present-day Derinkuyu tectonic basin is mostly covered with Quaternary sediments and volcanics. The fault system which bounds the basin to the east provides evidence that the ignimbrite volcanism and inferred caldera formation took place in a locally extensional environment while the basin was already subsiding. Drilling and geophysical prospecting are necessary to decipher in detail the presently unknown internal structure of the basin and the inferred, probably coalesced or nested, calderas within it.

Pb-Nd-Sr isotope and trace element geochemistry of Quaternary extension-related alkaline volcanism: a case study of Kula region (western Anatolia, Turkey)

Abstract The Quaternary alkaline volcanism of Kula is located in a western Anatolian graben system that resulted from an Aegean extensional regime. The typically silica-undersaturated, alkaline lavas of Kula are distinguished in three different sequences, namely Burgaz, Elekcitepe and Divlittepe, with a range of basanite, tephrite and phonotephrite compositions. To evaluate the source characteristics of the Kula lavas, rare earth elements as well as Sr, Nd and Pb isotopic compositions were determined. The volcanic rocks show significant enrichment in high field strength elements (such as Nb and Ta) and large ion lithophile elements. Furthermore, they exhibit high Nb/Y (>3) ratios, which are typical characteristics of within-plate alkaline volcanic rocks. However, the Rb, Ba, Sr, Nb and Ta contents and Rb/Nb and K/Nb ratios of the Kula lavas are considerably higher than in typical ocean island basalts (OIB), implying a contribution of lithospheric mantle in their genesis. Isotopic compositions of 87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb for the Kula lavas range from 0.703029 to 0.703490, from 0.512773 to 0.512941, from 18.689 to 19.064, from 15.606 to 15.683 and from 38.560 to 39.113, respectively. Assimilation and fractional crystallisation modelling indicates that crustal contamination does not play a significant role in the evolution of the Kula lavas but that they evolved via fractional crystallisation processes. Elemental variations are also related to fractional crystallisation. The geochemical characteristics of the Kula lavas suggest a contribution from two mantle sources: (1) mainly an OIB-like asthenospheric component and (2) a limited contribution from a lithospheric mantle component. The volcanism of Kula is linked to an extensional setting in western Anatolia that developed during Late Miocene–Pliocene. It is correlated to the thinning of the crust in a post-collision period and concomitant with the upwelling of the underlying upper mantle.

Ignimbrites of Cappadocia (Central Anatolia, Turkey): petrology and geochemistry

Abstract In Cappadocia (Central Anatolia, Turkey), high-K calc-alkaline volcanic rocks have a volume of at least 1000 km3 and cover an area of about 40,000 km2. Rhyolitic to dacitic ignimbrites and two andesitic lava flows were erupted from Upper Miocene (11.2 Ma) to Quaternary times, in relation to the collision of the Arabian and Eurasian plates. K-rich rhyolitic and locally Na-rich dacitic ignimbrites are commonly intercalated with lacustrine sediments and, more rarely, with andesitic lava flows. Each ignimbrite exhibits its own mineralogical association and trace-element chemistry that enable stratigraphic correlations (i.e., Rb and Sr or Fe, Mg, Mn and Ti contents of biotite). Geochemical data (major, trace elements and Sr–Nd isotopes) show that the origin of the studied volcanic units can be related to fractional crystallisation of a mantle-derived magma. However, crustal contamination is also thought to be a major process that evolved through time. From Miocene (11.2 Ma) to Quaternary times, the ignimbritic rocks exhibit a drastic decrease of 87 Sr / 86 Sr ratio which may be linked with the transition from collisional to extensional tectonics.

Magma mixing: origin of intermediate rocks and “enclaves” from volcanism to plutonism

Abstract Intermediate magmatic rocks are rarely homogeneous; a common example is the presence of magmatic “enclaves” Volcanic examples show that they are indications of magma mixing processes. Mixing events in the trachyandesitic suites from the Sancy Volcano (France). The volcanic activity from Sancy consists of several brief trachyandesitic cycles. Each of them begins with the eruption of highly heterogeneous benmoreites, followed by more homogeneous mugearites. Light porphyritic trachyandesites enclose numerous inclusions of varied darker lavas. The crenulated geometry of the contacts, the presence of chilled margins, the vesiculation of the core of the basic parts, suggest that these different rock types were magmatic at the same time although under marked thermal disequilibrium. Xenocrysts are common: partly resorbed sanidine rimmed with plagioclase in a basic matrix, reactional olivines in tridymite-bearing trachytes. This shows that the mixing occurred between partially crystallized and fractionated magmas. Chemical compositions are continuously variable from basalts to trachytes within the same eruptive cycle. All these facts might be interpreted as the result of a mechanical intermingling, more or less achieved, between two end-members of contrasting composition in the reservoir. In the Sancy Volcano this phenomenon occurred at least, 4 or 5 times during a 600 000 y span. Mixing evidence in Hercynian granodiorites from France. Similar observations may be described in granodioritic rocks. The more widespread “enclaves” exhibit the same characteristics as their volcanic equivalents. Although most of them are small, more important volumes of basic rocks may be associated in the same massif. In the neighbourhood of such large bodies, swarms of smaller enclaves recall the generation of basic pillows. The nature of the contact between small “microgranular enclaves” and their host rocks, the occurrence of xenocrysts with reaction rims (rapakivi feldspars and quartz ocelli in a basic surrounding, amphibole-pyroxene clots in a granitic matrix) and geochemical variations, again suggest thermal and mineralogical disequilibrium. Such observations indicate the comagmatic character of enclaves with regard to their host rocks. They cannot be interpreted as restites and are best described as co-igneous inclusions resulting from non achieved mixing between coexisting magmas. Photonic rocks are less suitable to demonstrate such petrogenetic processes because they result from slow cooling and crystallization during a longer residence time in a magmatic chamber. The successive mixing episodes, giving rise to different generations of co-igneous inclusions are also difficult to recognize. On the other hand, in volcanism this process is quenched and periodically sampled by eruptions. In conclusion the heterogeneity and the variability of many intermediate rocks cannot be explained only in terms of simple partial fusion and fractional crystallization processes in a closed system. Volcanic examples point to a complex evolution of magmatic reservoirs that were continuously fractionated, periodically tapped, periodically refilled and mixed.

Petrogenetic modelling of Quaternary post-collisional volcanism: a case study of central and eastern Anatolia

Extensive continental collision-related volcanism occurred in Turkey during Neogene–Quaternary times. In central Anatolia, calc-alkaline to alkaline volcanism began in the Middle–Late Miocene. Here we report trace elemental and isotopic data from Quaternary age samples from central and eastern Anatolia. Most mafic lavas from central Anatolia are basalt and basaltic andesite, with lesser amounts of basaltic trachyandesite and andesite. All magma types exhibit enrichment in LILE (Sr, Rb, Ba and Pb) relative to HFSE (Nb, Ta). Trace element patterns are characteristic of continental margin volcanism with high Ba/Nb and Th/Nb ratios. 87Sr/86Sr and 143Nd/144Nd isotopic ratios of central Anatolian lavas range between 0.704105–0.705619 and 0.512604–0.512849, respectively. The Quaternary alkaline volcanism of eastern Anatolia has been closely linked to the collision between the Arabian and Eurasian plates. Karacadag and Tendurek volcanic rocks are represented by alkali basalts and basaltic trachyandesites, respectively. As expected from their alkaline nature, they contain high abundances of LIL elements, but Tendurek lavas also show depletion in Nb and Ta, indicating the role of crustal contamination in the evolution of these magmas. 87Sr/86Sr and 143Nd/144Nd ratios of the Karacadag and Tendurek lavas range from 0.703512 to 0.704466; 0.512742 to 0.512883 and 0.705743 to 0.705889 and 0.512676, respectively. Petrogenetic modelling has been used to constrain source characteristics for the central and eastern Anatolian volcanic rocks. Trace element ratio plots and REE modelling indicate that the central Anatolian volcanism was generated from a lithospheric mantle source that recorded the previous subduction events between Afro-Arabian and Eurasian plates during Eocene to Miocene times. In contrast, The Karacadag alkaline basaltic volcanism on the Arabian foreland is derived from an OIB-like mantle source with limited crustal contamination. Tendurek volcanism, located on thickened crust, north of the Bitlis thrust zone, derived from the lithospheric mantle via small degrees (1.5 %) of partial melting.

The 1984 nuée-ardente deposits of Merapi volcano, Central Java, Indonesia: stratigraphy, textural characteristics, and transport mechanisms

For many centuries Merapi volcano has generated hot avalanches of blocks, lapilli and ashes, derived from the destruction of partially solidified, viscous lava domes (Merapi-type nuées ardentes). On 15 June 1984, at least four nuées ardentes came down the southwest slope of the Merapi, the first and the last being responsible for more than 99% of the deposits which are now exposed. The first nuée ardente, a Merapi-type nuée ardente, was produced by the destruction of the dome, travelled 7 km from the crater, leaving a measured deposit, 2.7 m thick, 4 km from the crater, near its upper depositional limit, regularly increasing to a maximum measured thickness of 12 m at the front of the deposit. The lower contact is sharp, non-erosive, with pines still rooted in the underlying paleosol. The deposit consists of 50% ash, 33% lapilli, and 17% blocks, with two subpopulations (one Rosin and one normal), and is finespoor, with less than 4% of fine ash (d finer than 4 φ). The deposit displays reverse population grading of both vesiculated and massive clasts, and of the maximum grain size. The maximum size significantly increases regularly down-current over most of the exposed length of unit 1, and bed thickness increases for the entire length of the deposit. The deposit of the second nuée ardente is only 6–21 cm thick, and of very limited lateral extent. It is a normally graded, coarse to fine ash, with a finespoor base. The third unit consists of fines-poor, normally graded coarse ash, exposed in low-amplitude (20–40 cm), 12-m-wavelength dunes. The deposit of the fourth nuée ardente rests in sharp erosive contact on the underlying unit, increasing in thickness down-flow. It consists of transitional coarse and fine-grained strata, 6–130 c cm thick, dipping 5–10° down-flow. The deposit, made up of two subpopulations (one Rosin and one normal), is normally graded over the entire bed, but coarsegrained strata are reversely graded. The relative content of vesiculated clasts increases up-bed in both strata types, from 12% at the base to 40% at the top. The characteristics of unit 1 suggest that it accumulated from a concentrated suspension of cohesionless solids exhibiting non-Newtonian behavior, where dispersive pressure played an important role in the suspension of the clasts. Units 2 and 3 were probably deposited from dilute turbulent suspensions, whereas the upper unit (4) is a classic example of deposition from a high-density turbulent suspension leading to the formation of multiple traction carpets driven by the overlying, lower-density, surge. The horizontal distance travelled by a hot rock avalanche may be influenced by its transport mechanism. Debris flows are mobile on very low slopes-as low as 1°-whereas grain flows, even density-modified grain flows, require relatively high slopes-more than 6° at Merapi-to maintain their mobility. If the present Merapi dome were to collapse and produce a debris flow, its present volume coupled with the minimal 1.5 km vertical drop could travel a distance ranging between 15 and 30 km. However, if transport were by grain flow mechanisms, the mass could come to rest as it reaches a 5–10° slope.

Post-collision neogene volcanism of the Eastern Rif (Morocco): magmatic evolution through time

Abstract Neogene volcanism in the Eastern Rif (Morocco) comprises a series of calc-alkaline, potassic calc-alkaline, shoshonitic and alkaline volcanic rocks. According to new stratigraphical, along with new and previous chronological and geochemical data, the orogenic volcanism was successively (1) calc-alkaline (basaltic andesites and andesites: 13.1 to 12.5 Ma, rhyolites: 9.8 Ma), (2) K-calc-alkaline (basaltic andesitic to rhyolitic lavas and granodiorites: 9.0 to 6.6 Ma), and (3) shoshonitic (absarokites, shoshonites, latites, trachytes: ∼7.0 to 5.4 Ma). The later Pliocene volcanism was basaltic and alkaline (5.6 to 1.5 Ma). The calc-alkaline and K-calc-alkaline series exhibit lower K2O (0.7–5.3 wt.%), Nb (8–19 ppm) contents and higher 87 Sr / 86 Sr (0.70773–0.71016) than the shoshonitic series (K2O: 2.4–7.2 wt.%, Nb: 21–38 ppm, 87 Sr / 86 Sr : 0.70404–0.70778). Pliocene alkaline basalts have a sodic tendency (Na2O/K2O: 1.7–3.5), high Nb content (up to 52 ppm), and low 87 Sr / 86 Sr ratio (0.70360–0.70413). The variations through time of K2O, Nb and Sr isotopic ratio reflect different mantle sources: (i) calc-alkaline, potassic calc-alkaline and shoshonitic series are derived from a mantle source modified by older subduction, (ii) alkaline basalts are derived mainly from an enriched mantle source. Through time, incompatible elements such as Nb increased while 87 Sr / 86 Sr decreased, suggesting a decreasing influence of metasomatized mantle (inherited subduction). Such evolution is related to the post-collision regimes operating in this area, and could be linked to the succession of extensional, compressional and strike-slip fault tectonics.

The Hasan Dagi stratovolcano (Central Anatolia, Turkey) : evolution from calc-alkaline to alkaline magmatism in a collision zone

Abstract The Hasan Dagi volcano is one of the two large Plio-Quaternary volcanoes in Cappadocia (Central Anatolia, Turkey). Three stages of edifice construction have been identified for this volcano: Paleovolcano, Mesovolcano and Neovolcano. Most samples from Hasan Dagi volcano are calc-alkaline and define an almost complete trend from basaltic andesite to rhyolite. However, the more recent (Neovolcano) mafic samples are alkaline basalts. The mineralogical and geochemical characteristics of the oldest lavas (Kecikalesi (13 Ma) and Paleo-Hasan Dagi (7 Ma)) are significantly different from those of the younger lavas (Meso- and Neo-Hasan Dagi ( 87 Sr / 86 Sr than the Paleo- and Mesovolcano basalts, whereas the Mesovolcano basalts display more radiogenic Pb than Paleovolcano samples. Magma mixing processes between initially heterogeneous and/or variably contaminated magmas may account for the genesis of the less differentiated and intermediate lavas (48–57% SiO2). Meso- and Neovolcano differentiated lavas (60–68% SiO2) are either derived from the analyzed basalts or from more primitive and more depleted magmas by fractional crystallization±some crustal contamination (AFC). Furthermore, the highly differentiated samples (72–75% SiO2) are not strongly contaminated. The strong calc-alkaline character of Hasan Dagi lavas, in the absence of contemporaneous subduction, must reflect the heritage of the early subduction of the Afro–Arabian plate under the Eurasian plate. The evolution towards alkaline compositions through time is clearly related to the development of extensional tectonics in Central Anatolia in the Late Miocene.

Eruptive history of the Colima volcanic complex (Mexico)

Abstract The evolution of the Colima volcanic complex can be divided into successive periods characterized by different dynamic and magmatic processes: emission of andesitic to dacitic lava flows, acid-ash and pumice-flow deposits, fallback nuees ardentes leading to pyroclastic flows with heterogeneous magma, plinian air-fall deposits, scoriae cones of alkaline and calc-alkaline nature. Four caldera-forming events, resulting either from major ignimbrite outbursts or Mount St. Helens-type eruptions, separate the main stages of development of the complex from the building of an ancient shield volcano (25 × 30 km wide) up to two summit cones, Nevado and Fuego. The oldest caldera, C1 (7–8 km wide), related to the pouring out of dacitic ash flows, marks the transition between two periods of activity in the primitive edifice called Nevado I: the first one, which is at least 0.6 m.y. old, was mainly andesitic and effusive, whereas the second one was characterized by extrusion of domes and related pyroclastic products. A small summit caldera, C2 (3–3.5 km wide), ended the evolution of Nevado I. Two modern volcanoes then began to grow. The building of the Nevado II started about 200,000 y. ago. It settled into the C2 caldera and partially overflowed it. The other volcano, here called Paleofuego, was progressively built on the southern side of the former Nevado I. Some of its flows are 50,000 y. old, but the age of its first outbursts is not known. However, it is younger than Nevado II. These two modern volcanoes had similar evolutions. Each of them was affected by a huge Mount St. Helens-type (or Bezymianny-type) event, 10,000 y. ago for the Paleofuego, and hardly older for the Nevado II. The landslides were responsible for two horseshoe-shaped avalanche calderas, C3 (Nevado) and C4 (Paleofuego), each 4–5 km wide, opening towards the east and the south. In both cases, the activity following these events was highly explosive and produced thick air-fall deposits around the summit craters. The Nevado III, formed by thick andesitic flows, is located close to the southwestern rim of the C3 caldera. It was a small and short-lived cone. Volcan de Fuego, located at the center of the C4 caldera, is nearly 1500 m high. Its activity is characterized by an alternation of long stages of growth by flows and short destructive episodes related to violent outbursts producing pyroclastic flows with heterogeneous magma and plinian air falls. The evolution of the primitive volcano followed a similar pattern leading to formation of C1 and then C2. The analogy between the evolutions of the two modern volcanoes (Nevado II–III; Paleofuego-Fuego) is described. Their vicinity and their contemporaneous growth pose the problem of the existence of a single reservoir, or two independent magmatic chambers, after the evolution of a common structure represented by the primitive volcano.

Petrological and geochemical characteristics of Cenozoic high-K calc-alkaline volcanism in Konya, Central Anatolia, Turkey

Abstract Late Miocene to Pliocene volcanic rocks outcrop west, northwest and southwest of the Konya area in Central Anatolia, Turkey. Volcanic products are lava domes, nuee ardentes and ignimbrite deposits, predominantly andesitic to dasitic in composition, together with rare basalt, basaltic andesite, basaltic trachyandesite and trachyandesite (50.35–69.39% SiO2). The serie exhibits high-K calc-alkaline affinities. Fractional crystallization of pyroxene, plagioclase and Fe–Ti oxides is the main process in the magmatic evolution of Konya volcanic rocks. Volcanic units exhibit typical high-K calc-alkaline character. Their geochemical characteristics (e.g., enrichments in LIL elements such as K, Rb, Ba, Sr, depletion in HFSE such as Ti, Nb, and high Ba/Nb and Low Nb/Y ratios) are consistent with those of active continental margin regions. High 87 Sr/ 86 Sr (0.704841–0.707340) and low 143 Nd/ 144 Nd (0.512390–0.512618) ratios suggest crustal involvement in their petrogenesis. Correlations between 86 Sr/ 87 Sr isotope with Rb, Rb/Nb, Rb/Ba, and Rb/Sr also emphasize the effect of crustal contamination on the andesitic and dacitic magmas. As a consequence, Konya volcanic rocks are products of assimilation and fractional crystallization (AFC) processes of a magma which seems to be linked to the subduction of the African plate underneath the Anatolian plate during Miocene.