Pliocene Volcanism Research Articles

Temporal constraints on magmatic evolution of Acıgöl Bimodal Volcanic Field (Nevşehir, Türkiye)

Neogene to Quaternary post-collisional Central Anatolian volcanism is associated with complex tectonic transition from collisional to extensional regime starting from Miocene. Upper Miocene – Pliocene volcanism is characterized by calc-alkaline effusive products and 10 major ignimbrite sheets. Quaternary represents an evident change in both eruptive styles and the geochemical characteristics of the volcanic products. Pleistocene Acıgöl Bimodal Volcanic Field (ABVF) represents one of the youngest volcanism in Cappadocian Volcanic Province (CPV) and comprises coeval basic and acidic rocks with a Daly Gap between ∼60 % and 72 % SiO2. Pleistocene basaltic volcanism in ABVF is represented by fissural lavas and monogenetic scoria cones, whereas rhyolitic activity is observed as dome/dome flows and pyroclastics including plinian falls and Kumtepe ignimbrite. Basalts of Lower and Middle Pleistocene are silica-undersaturated being nepheline-normative, on the other hand, Upper Pleistocene basic rocks are hypersthene/quartz-normative. Similarly, Lower and Middle Pleistocene rhyolites have higher Sr, Ba, La, than Upper Pleistocene rhyolites which are represented by elevated Y, Th, Nb, Ta and Rb.Petrological data and performed geochemical models show that Lower – Middle Pleistocene primitive basalts can be generated by the partial fusion of an EM-II type spinel peridotite containing amphibole, phlogopite and accessory rutile. Slightly evolved Upper Pleistocene basalts can be formed by crystal fractionation dominated differentiation of earlier basaltic parents. Fractional crystallization of basaltic magmas in high-level segregated chambers with varying amounts of crustal contribution can produce rhyolitic compositions. Furthermore, the amount of crustal contribution diminishes in Upper Pleistocene.

Volcanic crustal structure of the western Hikurangi Plateau (New Zealand) from marine seismic reflection imaging

Seamounts and basaltic basement can influence deformation and mass fluxes within subduction zones. We examined seamounts and volcanic units across the western Hikurangi Plateau, near the Hikurangi subduction margin, New Zealand, with seismic reflection images. Volcanism at the Hikurangi Plateau occurred in at least three phases that we attribute to (1) Early Cretaceous large igneous province formation, the top of which is marked by laterally continuous and dipping wedges of reflections that we interpret as lava flows; (2) Late Cretaceous seamounts and volcaniclastics that erupted onto the crust of the Hikurangi Plateau and make up the majority of seamount volume and basement relief; and (3) late-stage, Pliocene volcanics that erupted through and adjacent to Cretaceous seamounts and younger sediments of the north-central Hikurangi Plateau. The Pliocene volcanoes do not appear to be strongly welded to the plateau basement and may be petit spot volcanoes that are related to the displacement and accumulation of hydrous transition zone melts. Large seamounts and volcaniclastic units are evenly distributed across most of the Hikurangi Plateau near the Hikurangi margin but are absent from the Pegasus Basin. Although faults are imaged throughout the basement of the Pegasus Basin, contemporary normal faulting of the Hikurangi Plateau is uncommon, except for a zone of Quaternary normal faults near the Pliocene volcanics. These trends indicate that the Hikurangi megathrust may be more influenced by volcanic structures in the north and central Hikurangi margin, where plateau rifting and voluminous seamount eruptions have more substantially overprinted the original Early Cretaceous basement.

Provenance of the Papuan Peninsula (Papua New Guinea): Zircon Inheritance from Miocene–Pliocene Volcanics and Volcaniclastics

Plate tectonic reconstructions of Papua New Guinea prior to the late Cenozoic are characterized by a lack of provenance data to constrain the relative origin of the allochthonous terranes. At present, plate tectonic reconstructions of this region infer that the accreted New Guinea terranes at the northern Australian continental margin are likely autochthonous or para-autochthonous in nature. This study presents the results of an investigation into zircons derived from Miocene–Pliocene volcanics and volcaniclastics of the Papuan Peninsula. Results from U-Pb zircon geochronology inform the recent geological history of the Papuan Peninsula, with magmatism active in the late Miocene and early Pliocene, between approximately 9 Ma and 4.5 Ma. More significantly, however, is the recognition of extensive inherited zircon grains within the volcanic and volcaniclastic sequences. These inherited zircon grains are most likely sourced from the Owen Stanley Metamorphics, which form the basement rocks of the Papuan Peninsula. Provenance of the inherited zircon grains imply that the Cretaceous volcaniclastic protolith of the Owen Stanley Metamorphics must have had input from continental detritus, but this cannot be derived from North Queensland, Australia as inferred by current reconstructions. Instead, zircon U-Pb age spectra correlate with probable source regions further to the south, adjacent to the Shoalwater Formation of the Central Queensland margin, and New Caledonia. These findings suggest that late Mesozoic and Cenozoic regional reconstructions of eastern Australia and the Southwest Pacific require major revision and that additional work is undertaken to inform the provenance of such allochthonous terranes.

Pliocene Volcanic Terrain of the South Caucasus: The Missing Link in Eurasian Palaeobiogeography

Pliocene Volcanic Terrain of the South Caucasus: The Missing Link in Eurasian Palaeobiogeography

Volcano‐Tectonic Evolution of the Christiana‐Santorini‐Kolumbo Rift Zone

AbstractLocated on the Hellenic Arc, the Christiana‐Santorini‐Kolumbo (CSK) rift zone represents one of the most active and hazardous volcano‐tectonic systems in the Mediterranean. Although this rift zone has been intensively studied, its tectonic evolution and the interplay of volcanism and tectonism are still poorly understood. In this study, we use high‐resolution reflection seismic imagery to reconstruct the opening of the rift basins. For the first time, we relate the activity of individual faults with the activity of specific volcanic centers in space and time. Our analysis shows a pre‐volcanic NNE‐SSW‐oriented paleo basin underneath the CSK volcanoes, representing a transfer zone between Pliocene ESE‐WNW‐oriented basins, which was overprinted by a NE‐SW‐oriented tectonic regime hosting Late Pliocene volcanism that initiated at the Christiana Volcano. All subsequent volcanoes evolved parallel to this trend. Two major Pleistocene tectonic pulses preceded fundamental changes in the volcanism of the CSK rift including the occurrence of widespread small‐scale volcanic centers followed by focusing of activity at Santorini with increasing explosivity. The observed correlation between changes in the tectonic system and the magmatism of the CSK volcanoes suggests a deep‐seated tectonic control of the volcanic plumbing system. In turn, our analysis reveals the absence of large‐scale faults in basin segments affected by volcanism indicating a secondary feedback mechanism on the tectonic system. A comparison with the evolution of the neighboring Kos‐Nisyros‐Yali volcanic field zone and Rhodos highlights concurrent regional volcano‐tectonic changes, suggesting a potential arc‐wide scale of the observed volcano‐tectonic interplay.

Geochemistry and Petrogenesis of Basaltic Rocks and Enclosed Xenoliths from the Ghab Pliocene Volcanic Field in Northwestern Syria

Basaltic rocks and their content of ultramafic xenoliths are common in the Ghab Pliocene Volcanic Field in northwestern Syria in the form of lava flows, cinder cones and pyroclastic deposits. The rocks occur within the Ghab pull- apart graben that formed by sinister strike slip faults within the zone that defines the boundary between the African and the Arabian plates. Three petrographic types occur: basanite, olivine basalt and more commonly alkali olivine basalt. The peridotite xenoliths are spinel lherzolite and harzburgite. Geochemical analysis indicates that the basalts are mostly alkaline to subalkaline. A distinctive feature of these rocks is the narrow compositional variations in the content of most major oxides and minor elements, SiO2 (44.33 - 46.43 wt%) and MgO (4.01 - 8.28 wt%). Some of the refractory and high field strength elements and incompatible minor elements in the basalts are relatively high (Cr average = 303 ppm and Ni average = 185 ppm) compared with their content in average basalts. These geochemical characteristics reflect crystallization of the Ghab basalts from pristine and primary magmas that have experienced minimal fractional crystallization and crustal contamination. Similarly, chemical compositions of the ultramafic xenoliths vary within a restricted geochemical range. They are compatible with the generation of these rocks from partial melting of a primitive mantle pyrolite to yield the xenoliths. These magmatic processes operated during the Pliocene in a regional transtensional stress environment attending the development of the Dead Sea Rift.

Evolution of the isotopic-geochemical composition of rocks of Uksichan volcano, Sredinnyi range, Kamchatka, and its relations to the tectonic restyling of Kamchatka in the neogene

The paper presents newly acquired data on concentrations of major and trace elements and on Sr, Nd, and Pb isotope composition in Pliocene and Late Pleistocene–Holocene mafic volcanic rocks of the Uksichan volcanic center, one of the largest in the Sredinnyi Range of Kamchatka. Based on these data, the mafic Pliocene volcanics are thought to be produced by the melting of heterogenized mantle material, which had been hybridized by subduction and asthenospheric processes. The behavior of HFSE and Pb isotopic systematics provide evidence of the melting of subducted sediment and origin of pyroxenite segregations in the peridotite matrix. The low ∆8/4Pb values of the Pliocene lavas of Uksichan shield volcano and in modern large volcanic centers in the Central Kamchatka Depression are correlated with the magmatic productivity, which indicates, when considered together with HFSE and HREE behavior, that the Pacific asthenosphere was involved in the magma-generating processes. The Late Pleistocene–Holocene basalt volcanism, which was spatially constrained to the peripheries of the Pliocene shield edifice, developed in an extensional environment as a result of the melting of an enriched mantle source. The attenuation and then complete termination of volcanic activity in the Sredinnyi Range in the Late Pleistocene–Holocene was associated with an increase in the ∆8/4Pb of the mafic lavas, which indicates that the center of the activity related to the oceanic asthenosphere shifted eastward toward the Central Kamchatka Depression. The influence of the oceanic asthenosphere on subduction-related magmatism is not unique to convergence zones alone and should be taken into consideration when models are constructed for the origin of juvenile continental crust.

Miocene–Pliocene Volcanism of Central Armenia: Geochronology and the Role of AFC Processes in Magma Petrogenesis

This paper presents the results from an isotope-geochronological and geochemical-petrological study of young volcanic rocks sampled in central Armenia, which were formed during the first stage of the Late Cenozoic magmatism in the Lesser Caucasus at the Miocene/Pliocene boundary. We have found the boundary of the area where the Miocene to Pliocene volcanism was evolving, determined its scale and total duration (~1 Ma, between 5.7 and 4.7 Ma), and identified phases of magmatic activity: I during 5.56 ± 0.18, II during 5.39 ± 0.04, and III during 4.86 ± 0.20 Ma. Petrologic data show that the volcanic rocks of central Armenia studied here compose a continuous compositional series from potassium trachybasalt to mugearite to (trachy)andesite to trachyte to rhyolite, while the leading process in the petrogenesis of their parent magmas had been fractional crystallization during the entire period of magmatic activity from the Late Miocene to the Early Pliocene. Crustal assimilation was of limited importance: some small influence of this process was only found for some late lavas that were erupted during phase III. The geochemical evolution of lavas during the earlier phases of activity was occurring unidirectionally from basic to acid compositions; the resumed eruptions of trachyandesites and andesites during phase III after a long intermission was caused by replenishment of mantle melts into magma chambers beneath the region. The entire data set for the area of central Armenia was analyzed to identify one volcanic unit that includes all Miocene to Pliocene magmatic rocks that are found there. The mantle source that was responsible for melt generation beneath the better known area of the Lesser Caucasus during the time period of interest was an OIB-type asthenospheric mantle that had been metasomatized by the subduction of the oceanic lithosphere from the Neotethys Basin.

Miocene-Pliocene mantle depletion event in the northern Fossa Magna, western NE Japan

New isotopic and trace element data presented here imply a temporal change in magma sources and thermal conditions beneath the northern Fossa Magna of NE Japan from the Miocene to the Pliocene. Less radiogenic 176Hf/177Hf and 143Nd/144Nd, high Zr/Hf, and little or no Hf anomaly characterize the Early Miocene volcanism in the northern Fossa Magna region. The mantle wedge consisted of chemically heterogeneous mantle source. Based on out isotope proxies, we propose that during the onset of subduction, influx of hot asthenospheric mantle provided sufficient heat to partially melt newly subducting sediment. Geochemical modeling demonstrates that slab-derived melt mixed with mantle wedge produces the observed isotopic and trace elemental characteristics. In the Middle Miocene, the injection of hot and depleted asthenospheric material replaced the mantle beneath the northern Fossa Magna region of NE Japan. This caused the isotopic signature of the rocks to change from enriched to depleted. Then, the mantle wedge was gradually cooled during the Middle Miocene to the Pliocene with back-arc opening ending in the Late Miocene. Slab surface temperatures were still high enough for sediments to melt but not too high (<∼780°C) to lose zircon as a residual phase. The Late Miocene and Pliocene volcanism at the post stage of the back-arc opening is best explained by a partial melting of subducted metasediment saturated with trace quantities of zircon and rutile.

جيو كيمياء البركنة البليوسينية العائدة لمنطقة شهبا، جنوب سوريا

Pliocene volcanic rocks are considered as dominant rocks and form an important part of igneous succession in this region. The architecture type of Pliocene basalt is various as it consists of several diverse lithological formations, which vary from continuous basalts flow to scoria materials. The Pliocene basaltic rocks consist of several lava flows, which are divided into three primary petrographic groups: alkali basalt, augite alkali basalt and the basanite. The three petro-types can be classified geochemically from alkaline to sub-alkaline and termed as alkali olivine basalt. They have similar compositional ranges of major and trace elements. The geochemical composition shows that the basaltic Pliocene rocks consist of both partial melting and crystal fractions signatures. So the geochemical changing components can be considered as a result of crystal fractions that developed after partial melting process. تتكشف تشكيلات البليوسين النارية على مساحات واسعة من جنوب سوريا، وهي تشكل جزء هام من التتابع الناري في المنطقة.تتكون التوضعات البركانية البليوسينية في منطقة شهبا من تشكيلات ليتولوجية مختلفة، تتراوح بين صبات بازلتية دفقية مستمرة الى توضعات خبثية متماسكة سميكة أحيانا،ً تتضمن توضعات بيروكلاستية متنوعة. يُبنى التتابع البليوسيني الناري في منطقة شهبا من عدد متغير من الصبات البازلتية، المكونة بتروغرافيا من ثلاث مجموعات رئيسية هي: البازلت القلوي والبازلت الاوجيتي القلوي والبازانيت. يُظهر التركيب الجيوكيميائي للصخور البليوسينية، أن الصخور البازلتية المدروسة تبدي تغيرات على المحتوى التركيبي، يمكن اعتبارها أنها ناتجة عن عملية تجزؤ بلوري كان قد تطور عن عملية انصهار جزئي.

Late Cenozoic calc-alkaline volcanism over the Payenia shallow subduction zone, South-Central Andean back-arc (34°30′–37°S), Argentina

A series of mesosilicic volcanic centers have been studied on the San Rafael Block (SRB), 300 km to the east of the present-day volcanic arc. K-Ar ages indicate that this magmatic activity was developed in at least two stages: the older volcanic centers (∼15–10 Ma) are located in the central and westernmost part of the SRB (around 36°S and 69°W) and the younger centers (8–3.5 Ma) are located in an eastern position (around 36°S and 69°30′W) with respect to the older group. These volcanic rocks have andesitic to dacitic compositions and correspond to a high-K calc-alkaline sequence as shown by their SiO2, K2O and FeO/MgO contents. Elevated Ba/La, Ba/Ta and La/Ta ratios show an arc-like signature, and primitive mantle normalized trace element diagrams show typical depletions of high field strength elements (HFSE) relative to large ion lithophile elements (LILE). Rare earth element (REE) patterns suggest pyroxene and amphibole crystallization. Geochemical data obtained for SRB volcanic rocks support the proposal for a shallow subduction zone for the latest Miocene between 34°30″–37°S. Regionally, SRB volcanism is associated with a mid-Miocene to early Pliocene eastward arc migration caused by the shallowing of the subducting slab in the South-Central Andes at these latitudes, which represents the evolution of the Payenia shallow subduction segment. Overall, middle Miocene to early Pliocene volcanism located in the Payenia back-arc shows evidence for the influence of slab-related components. The younger (8–3.5 Ma) San Rafael volcanic rocks indicate the maximum slab shallowing and the easternmost extent of slab influence in the back-arc.

Eruption of reverse-zoned upper Tshirege Member, Bandelier Tuff from centralized vents within Valles caldera, New Mexico

Valles caldera (New Mexico, USA) is the type example of a resurgent caldera and source of the Tshirege Member, Bandelier Tuff, which has been long recognized as a normally zoned sequence of ignimbrites. In this paper, we present geologic, stratigraphic, chemical, and mineralogical data from upper flow units of the Tshirege Member obtained at multiple sites within and east of Valles caldera showing that the upper part of the Tshirege Member is reverse-zoned in chemistry and mineralogy. The key to deciphering these compositional changes in zoning is recognition of flow unit Qbt4u, which we informally name the high Ti–Ba unit or HTBU. The HTBU is widespread within the resurgent dome area of the caldera, but is only found in a small area outside and east of the caldera. The HTBU is the most “mafic” unit in the Tshirege Member and is identified by chemical maxima in Ti, Ba, Sr, P, V, and Th, and by unusually high contents of anorthoclase, plagioclase, orthopyroxene, ilmenite, apatite and zircon with respect to other flow units. The HTBU also contains small (≤1cm) enclaves of quenched andesitic magma consisting primarily of plagioclase, orthopyroxene, clinopyroxene, glass and vesicles. Later erupted Tshirege flow units Qbt5l and Qbt5u are chemically more evolved and contain less plagioclase, orthopyroxene, ilmenite, apatite and zircon than the HTBU, and they are more evolved than flow units erupted before the HTBU (approximately 90% of the Tshirege Member).The HTBU is the host ignimbrite of a complex vent breccia exposed for over 3km along the NW faulted face of Redondo Peak, the Valles caldera resurgent dome. Breccia clasts consist of angular to subrounded fragments ≤1m in diameter of Permian red beds, Miocene basin fill sediments, Miocene to Pliocene volcanics and Quaternary ignimbrite (Otowi Member, Bandelier Tuff) that underlie Valles caldera. Flow units Qbt5l and Qbt5u overlie the HTBU vent breccia but these later units do not contain unusual quantities of lithic fragments. Significantly, small mudstone lithic fragments (generally ≤200μm in diameter) in the HTBU vent breccia contain reaction rinds of secondary garnet (andradite) along margins with ignimbrite host. We argue that the tiny andradite crystals (≤50μm) formed by reaction of degassing volatiles in the vent area with relatively Fe–Ca-rich vent breccia fragments at temperatures approaching 800°C and pressures less than a few MPa. Geologic relations and the restricted distribution of upper Tshirege flow units Qbt3t to Qtb5u indicate they were erupted in a short period of time (weeks to months) from a centralized vent near Redondo Peak, and from other buried vents as far east as the eastern ring fracture. We also conclude that late injection of andesitic melt into the residual Bandelier magma chamber stimulated the eruption of upper Tshirege Member pyroclastic flows from different locations and depths of the chamber resulting in a small but significant reversal in chemical and mineralogical zonation.

Geochemical constraints on the relationship between the Miocene–Pliocene volcanism and tectonics in the Palaoco and Fortunoso volcanic fields, Mendoza Region, Argentina: New insights from 40Ar/39Ar dating, Sr–Nd–Pb isotopes and trace elements

New 40Ar/39Ar analyses constrain the formation of the volcanic succession of Sierra de Palaoco in the present back-arc of the Andean Southern Volcanic Zone (SVZ), near 36°S, to the Late Miocene and assigns them to the Huincán II Formation. The composition of major and trace elements, Sr, Nd and Pb isotopes of the Palaoco and nearby Río Grande rocks require a strong arc-like component in the mantle that is absent or weak in both Early Miocene (Fortunoso Group) and Pleistocene alkaline lavas (Llancanelo Group) erupted in the same area. We evaluate the relative roles of varying mantle source compositions and crustal contamination in the generation of geochemically very different lavas from the Palaoco, Fortunoso and Río Grande volcanic fields, north of the Payún Matrú Volcano. The source for the Early Miocene Fortunoso(I) basalts was a OIB-type mantle devoid of subduction zone input. This type of OIB-like volcanic activity terminated due to a change from an extensional to a compressional tectonic regime. Towards the end of the Miocene renewed alkaline volcanism at Fortunoso (II) display a transition to arc-type incompatible element enrichment. Shortly after the calc-alkaline Palaoco volcanism started with a very strong geochemical arc-signature including Ba/La≈60 and La/Nb=2–3. After a quiesence of 1Ma the major part of the voluminous Late Palaoco basalts were erupted around 7.5Ma over a few hundred ka. These are less enriched in Ba and Sr and have compositions like many Holocene rocks of the Southern Volcanic Zone. Isotopically the Fortunoso I and Palaoco rocks are distinct. Regional volcanism of the Charilehue, Huincán I and II mostly has a moderate arc-type enrichment indicating incipient arc developments. However, Palaoco and La Brea at (c. 35°S) show full geochemical arc-signature, and we infer that a frontal arc was established. The subsequent development in the Palaoco-Río Grande area encompasses renewed late Pliocene calc-alkaline low volume volcanic eruptions (Río Grande group) succeeded in the Late Pleistocene by alkaline OIB-type eruptions (Llancanelo group). In the light of the course of volcanism to the east, in the Nevado area, where late Miocene–Pliocene calc-alkaline volcanism was followed by Late Pliocene–Pleistocene alkaline volcanism. We propose a scenario where the Nazca plate developed an eastwards widening flat slab from which the east dipping slab before the Late Pliocene translated from Palaoco to Nevado and subsequently retreated passing Río Grande in the Late Pliocene. Alkaline back-arc volcanism was active east of the arc-volcanism and expanded westwards during the Late Pliocene and Pleistocene.

Cenozoic volcanism in the Sierra Nevada and Walker Lane, California, and a new model for lithosphere degradation

Volcanic rock and mantle xenolith compositions in the Sierra Nevada (western United States) contradict a commonly held view that continental crust directly overlies asthenosphere beneath the Sierran range front, and that ancient continental mantle lithosphere (CML) was entirely removed in the Pliocene. Instead, space-time trends show that the Walker Lane is the principle region of mantle upwelling and lithosphere removal in eastern California, that lithosphere loss follows the migration of the Mendocino Triple Junction (MTJ), and that the processes of lithosphere removal are not yet complete beneath the Sierra Nevada and its range front. Key evidence is provided by volcanic rock compositions. The 87 Sr/ 86 Sr ratios for mafic volcanics of the Sierra (MgO > 8%) are mostly > 0.705, and 143 Nd/ 144 Nd 87 Sr/ 86 Sr 143 Nd/ 144 Nd > 0.5129), but very much like CML. Similarly, Sierran volcanics carry CML-like trace element ratios, with La/Nb > 3 and Th/Nb > 1, values that are significantly higher than asthenosphere-derived melts (La/Nb 87 Sr/ 86 Sr and e Nd ratios that range to 0.7065 and −3.6, respectively. New estimates of melt extraction depths using Si activity and mineralogy-sensitive trace element ratios (Sm/Yb, Lu/Hf) show that CML extends from the base of the crust (40 km) to depths of 75 km beneath the range front, and to 110 km for Pliocene volcanics of the southern Sierra. This means that garnet-bearing lithologies could not have been dislodged from beneath the southern Sierra until after the Pliocene. Only in the Walker Lane do young (0.18 Ma) volcanic rocks, from the Coso volcanic field, approach asthenosphere-like compositions, which occurs only 20 Ma after MTJ arrival. Temporal trends show that MTJ arrival at any given latitude south of 37°N signals lithosphere heating, probably due to asthenosphere that upwells to replace the sinking Farallon plate. Partial melts of the asthenosphere, and perhaps the asthenosphere itself, intrude into and cause heating and partial melting of overlying CML; this culminates after 10 Ma. After 20 Ma, CML becomes highly degraded and asthenosphere-derived melts are dominant. North of 37°N, volcanic rocks approach asthenosphere-derived compositions to the west, not the east, and 87 Sr/ 86 Sr ratios increase from 18 to 0 Ma, indicating that this region has entered a phase of lithosphere heating, but not yet a phase of lithosphere removal. We propose a new model of lithosphere degradation, where asthenosphere or its partial melts pervasively invade CML beneath the Walker Lane. This process is now nearly complete beneath Coso, and is migrating west, so that it is only partly complete at the southern Sierra range front, or within the Sierra Nevada, at any latitude. This model of intermixed asthenosphere and lithosphere better explains the compositions of volcanic rocks and their included xenoliths, and the remarkably consistent S-wave receiver function data, which show a 70-km-thick lithosphere beneath the Sierra Nevada. If the upper mantle is warm CML, permeated by partial melts, this model may also explain low P- and S-wave velocities.

Geochronology of Pliocene volcanism in the Dzhavakheti Highland (the Lesser Caucasus). Part 2: Eastern part of the Dzhavakheti Highland. Regional geological correlation

The results of isotopic-geochronological study of the Pliocene volcanic rocks in reference sections and volcanic edifices of eastern part of the Dzhavakheti Highland (the northwestern Lesser Caucasus) are considered. The isotopic-geochronological data obtained here are correlated with data on western part of the Dzhavakheti Highland, which have been considered in previous part of this work. Based on correlation, time spans of principal volcanic events of the Pliocene in the study region as a whole are determined, and general trends of the young magmatism evolution within the region are established. In sum, the isotopic-geochronological dates evidence that the Pliocene magmatism of the Dzhavakheti Highland developed practically without essential breaks during the period of about 2 Ma long, from 3.75 to 1.75–1.55 Ma ago. The areal basic volcanism that was most widespread at that time is divisible into five discrete phases according to the isotopic dates obtained. Comparatively short pauses, which separated these phases of magmatic activity, were a few hundreds thousand years long, not more. Chemical composition of moderately acidic to silicic volcanics, which are of a limited distribution in the Dzhavakheti Highland, and their age relations with basic lavas of the region suggest that they are most likely the differentiation products of parental basic mantle-derived magmas. The analyzed distribution of volcanic centers, which erupted basic lavas of the Dzhavakheti Highland, evidence that first two phases of basic magmatism were connected here with volcanic activity in southwestern part of the region (northern termination of the Egnakhag Ridge), whereas activity of volcanoes situated on the east, predominantly in water-shed part and on slopes of the submeridional Dzhavakheti Ridge, controlled development of the third and fourth phases. Consequently, magmatic activity of the Pliocene stage in history of the Neogene-Quaternary magmatism of the Dzhavakheti Highland laterally migrated from the west to the east, being controlled by development of regional submeridional extension zones. Volcanic ridges marking the latter are formed by volcanic edifices, which are amalgamated at their bases and have erupted lavas of close age and composition. The migration of volcanic activity can be described in terms of the “domino effect,” when cessation of volcanism in one zone led to formation of the other submeridional zone of extension and magmatic activity displaced from the west eastward in sublatitudinal direction. In general, evolution of the Pliocene magmatism of the Dzhavakheti Highland, was similar, despite the essential regional peculiarities, to the generalized trend of magmatism evolution in the continental rifts and intraplate zones of the “hot-spot” type.

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 Golcuk 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 Golcuk 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 Golcuk 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 Golcuk 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.

Geochronology of Pliocene volcanism in the Dzhavakheti Highland (the Lesser Caucasus). Part 1: Western part of the Dzhavakheti Highland

Isotopic-geochronological study of the Pliocene magmatic activity in western part of the Dzhavakheti Highland (northwestern region of the Lesser Caucasus) is carried out. The results obtained imply that the Pliocene magmatic activity lasted in this part of the highland approximately 2 million years from 3.75 to 1.75–1.55 Ma. As is established, the studied volcanic rocks correspond in composition mostly to K-Na subalkaline and more abundant normal basalts. Time constraints of main phases in development of basic volcanism within the study region are figured out. We assume that individual pulses of silicic to moderately silicic volcanism presumably took place in the Dzhavakheti Highland about 3.2 and 2.5 Ma ago.

Transtensional Deformation of the Brittle Crust: Field Observations and Theoretical Applications in the Coso-China Lake Region, Eastern Margin of the Sierra Nevada Microplate, Southeastern California

The focus of this work is the description of non-plane strain brittle deformation through field observations and theoretical applications. The seismically and volcanically active Coso—China Lake region in the transtensional Eastern California shear zone is an exemplary location for field observation of brittle structure. The region deforms in a constrictional strain field, well-described by earthquake focal mechanisms and GPS measurements, and further demonstrated by our field observations and outcrop-scale mapping. Fundamental geometric relationships in transtensional zones and tectonic reconstruction allows practical applications of theory using real transtension zone parameters, and expands our ability to describe triaxial strain in three dimensions. Theoretical instantaneous strain axes are constrictional and oriented xi WNW, yi NNE, zi vertical, similar to seismogenic and GPS derived instantaneous strain axes, and kinematic analysis of field data. Orientations of brittle structures in the field correspond reasonably well to theoretical predictions, with normal faults striking generally NNE, parallel to yi, dextral faults northwest to NNW, sinistral faults ENE, and fold axes and thrust faults WNW, parallel to xi. We calculate theoretical rates and amounts of shortening, elongation, and rotation for specific structures within the deformation zone; paleomagnetically-derived rotations in Pliocene volcanics fall within our predicted ranges. We subdivide the field area based on geometry and derive shapes and orientations of the instantaneous and finite strain ellipsoids and associated k-values (Northern Coso, k = 6.84; Southern Coso, k = 2.21). Our results indicate a less prolate, more coaxially-dominated, higher strain and strain rate in Southern Coso, and a more prolate, less coaxially dominated, lower strain and strain rate in Northern Coso, part of which is reflected by elevation and topography. We observe varying degrees of three-dimensional strain partitioning of brittle structures depending on scale, as is also seen in seismicity, and possible partitioning of rotations around vertical and horizontal axes. Multi-scale partitioning of strain and rotation, influenced by preexisting and reactivated structures, characterizes transtensional deformation at Coso, and is well described by kinematic analysis of structural field observations and regional theoretical applications and analyses.

Variation in the mantle sources of the northern Izu arc with time and space — Constraints from high-precision Pb isotopes

We present new ages and geochemical data for back-arc lavas from the northern Izu–Bonin arc 33–35° N including high-precision double-spike Pb isotope measurements. The northern part of the Izu–Bonin arc is distinct from the rest of the arc as it lacks active rifting behind the volcanic front but it does have Quaternary volcanoes (e.g. Niijima). However, in common with the rest of the arc the northern section has back-arc seamount chains and NE–SW volcanic ridges. 40Ar/ 39Ar dating of volcanic rocks has revealed that Quaternary volcanism is limited to within 40 km of the volcanic front. Miocene and Pliocene volcanism extended as far as 120 km west of the current volcanic front along the back-arc seamounts and ridges. The chemical characteristics of back-arc volcanism are significantly different in the Pliocene–Quaternary compared to the Miocene. Opx–cpx andesite and hornblende andesite are dominant in Miocene volcanic centres, while Pliocene and Quaternary centres are characterized by basalt and rhyolite. Miocene volcanic centres show a marked correlation between Th/Ce and Pb and Nd isotopes. Generally, these lavas have higher Δ7/4 and lower 143Nd/ 144Nd with increasing Th/Ce. In contrast, the Pliocene and Quaternary lavas show little, if any, Th enrichment relative to potential mantle sources and no correlation with isotopes. These correlations suggest that partial melt of sediment from the subducting slab was an important component in the Miocene, whereas, the Pliocene–Quaternary volcanic centres show little evidence of sediment melt and are restricted to a contribution of fluid from altered oceanic crust and fluid from sediment. Quaternary volcanoes at similar distances from the volcanic front are calculated to have similar compositions and amounts of slab-derived fluid in their sources. However, on Pb–Pb isotope plots, they lie closer to the NHRL towards south (i.e., Δ8/4 decreases towards south). Almost parallel but distinct trends on Pb–Pb plots imply differing mantle sources depending on the northward position within the arc. This variation in mantle wedge composition is seen to continue into the central part of the arc [Ishizuka, O., Taylor, R.N., Milton, J.A., Nesbitt, R.W., Fluid–mantle interaction in an intra-oceanic arc: constraints from high-precision Pb isotopes, Earth Planet. Sci. Lett. 211 (2003a) 221–236]. The observed along-arc isotopic variation can be explained by the presence of two mantle components; a MORB source observed in the back-arc basins of the Philippine Sea Plate (e.g. [Hickey-Vargas, R., Origin of the Indian Ocean-type isotopic signature in basalts from Philippine Sea Plate spreading centers: An assessment of local versus large-scale processes, J. Geophys. Res. 103 (1998a) 20963–20979]) and a more Pacific MORB-like source. In the northern part of the Izu–Bonin arc, Miocene to Pliocene volcanism was active over a much wider back-arc region than present, and these lavas show the greatest influence of sediment melt generated at slab depths of > 250 km. However, in the Quaternary active volcanism was constrained to the vicinity of the volcanic front. This Quaternary magmatism can be related to the release of fluid from the slab at relatively shallow depths (110–200 km). These spatial and temporal chemical variations could be related to either the change of the angle of subducting Pacific Plate and/or changes in the regional stress regime (e.g. resulting from the collision of the Izu–Bonin arc with the Honshu arc or in association with initiation of rifting in the south).

Factors Controlling the Hydrothermal Sulphur Ore Deposition in Keciborlu Mine Area (SW Turkey)

Abstract. Replacement‐type subsurface sulphur ores of Keciborlu are situated in fault controlled and metasomatically altered fragmental ophiolite media between alkaline Pliocene volcanics of Isparta and Afyon regions, SW Turkey. Subsurface volcanic sulphur‐mineralising fluids and gases were brought up along the sulphur‐hosting oblique slip fault zone into the oxidising meteoric water‐saturated media and the sulphur‐ore bodies were developed at more porous, cross‐fault locations.Chemically the sulphotaric alteration of the parent rock and sulphur ore formation is characterised by outward volcanic gas and sulphuric acid leaching and oxidation to sulphur from the crushed porous centres. Consequently, concentric pyritiferous, opaline and argillaceous zones developed adjacent to the sulphur ores. The inner alteration zones progressed and widened toward the outer zones through time and the central sulphur ore developed during a single, continuous phase of mineralisation in the water saturation zone such that the alteration zones do not cross each other.