Quaternary Alkaline Research Articles

Solvothermal syntheses, structures and properties of quaternary alkaline earth metal sulfides BaCu6Ge2S8 and BaCuAsS3

Two quaternary alkaline earth metal sulfides, BaCu6Ge2S8(1) and BaCuAsS3 (2), were synthesized under low-temperature solvothermal conditions. Compound 1 features a three-dimensional (3D) [Cu6Ge2S8]n2n- framework comprising CuS3, CuS4, and GeS4 units, with Ba2+ cations occupying the channels. Compound 2 exhibits a one-dimensional (1D) chained structure, with Ba2+ cations situated between the chains. Alkaline earth metal ions exhibit a structural directing effect that is distinct from that of other guest cations. Specifically, Compound 1 presents a rare instance of a 3D Cu-Ge-S framework formed by condensation of two parallel layers. Compound 2 is the first 1D A-Cu-As-S compound (where A represents guest cations). Additionally, compound 2 presents the first example of AⅡ-Cu-As-S (where AⅡ represents alkaline earth metal cations) compounds. Their optical properties and theoretical calculations were also investigated. Our results demonstrate that solvothermal synthesis is a promising method for obtaining multinary alkaline earth metal sulfides with diverse structures and properties.

Long-lived shallow crustal magma reservoirs beneath Ulleung Island volcano (South Korea) and their rejuvenation recorded in zircon

The microanalytical data obtained from zircons collected on Ulleung Island, an emergent peak of a Quaternary alkaline magma system in the southwestern East Sea (Sea of Japan) back-arc basin, provide new insights into the origin of magma, magmatic processes, and the trigger of explosive eruptions. The U-Th ages of zircons extracted from a pumice layer formed during the intense Plinian eruption at ∼11 ka, known as the Ulleung-Oki tephra, show a wide range spanning the pre-eruption equilibrium state (>350 ka) up to the eruption event itself. Zircon grains derived from syenite and gabbro fragments entrained within the tephra yield weighted mean U-Th and U-Pb ages of 34.7 ± 2.3 ka and 1378 ± 22 ka, respectively. Under cathodoluminescence, zircons from the syenite reveal dark, featureless, or oscillatory-zoned cores, containing numerous inclusions of britholite-like Y-rich phases and pyrochlore. These cores are partially or entirely replaced by inclusion-free, brighter domains, suggesting that in situ dissolution and precipitation processes significantly influenced the transformation of zircon. The consistently low δ18O values (5.4–2.6‰) of zircons from Middle Pleistocene trachyte lavas, the Ulleung-Oki tephra, and lithic fragments indicate the persistent presence of shallow crustal magma reservoirs beneath Ulleung Island. These reservoirs have undergone substantial assimilation of hydrothermally altered rocks, with assimilation rates typically around 10–30% and reaching a maximum of approximately 50%. Zircon core-to-rim variations in oxygen isotopic composition, along with textural observations, suggest the rejuvenation of magma reservoirs through the injection of new melt, which likely served as a trigger for the explosive eruptions at Ulleung Island. The similarity in the range of εHf values between the core and rim (+7.4 to −4.3) implies the Hf isotopic consistency of the recharge melt.

Quaternary low-temperature serpentinization and carbonation in the New Caledonia ophiolite

The low-temperature alteration (< 150 °C) of ophiolites by infiltrated meteoric waters removes atmospheric CO2 through mineral carbonation and is assumed to generate H2 and possibly CH4 according to so-called serpentinization reactions. This overall alteration pattern is primarily constrained by the chemical composition of alkaline springs that are issued in several ophiolites worldwide. Here we report on the fingerprint, as veinlet mineralization, of the reactive percolation of such meteoric waters in the New Caledonia ophiolite (Massif du Sud). The mineralization which resulted from carbonation and serpentinization reactions, is young (< 2 Ma) and formed at a temperature of ca. 95 °C. It is mainly composed of lizardite, dolomite, magnetite ± pyroaurite. Thermochemical simulation of mineral–water equilibria shows that the percolating aqueous fluid was alkaline and H2 bearing. The δ13C of dolomite is exceptionally high, between 7.1 and up to 17.3‰, and is interpreted as evidence of low-temperature methanogenesis. Overall, the percolating fluid had a chemical composition similar to that of the waters issued today in the (hyper)alkaline springs of the Massif du Sud. The studied veinlets are thus interpreted as a sample of the plumbing system that fed an ancient Quaternary alkaline spring in the area.

Time constraints on the geological evolution of the main sedimentary basins and tectono-magmatic events on Livingston Island, South Shetland Archipelago, Antarctica: a review

The Lower Cretaceous to Miocene South Shetlands Islands’ volcanic arc is a result of the subduction of the Phoenix Oceanic Microplate (part of the Pacific Plate) beneath the Antarctic Peninsula. The magmatic activity on Livingston Island has been going on for a long time: since the Early Cretaceous to the present. The evolution of the volcanic arc resulted in the formation of various sedimentary basins and paleoenvironments (turbiditic to shallow marine and continental) that formed varied sedimentary successions and tholeiitic to potassium calc-alkaline subduction-related magmatic rocks. Considering the magmatic activity on Livingston Island, a general trend of rejuvenation can be observed from the WNW (Byers Peninsula) to the ESE (to Hannah Point, Point Williams and further to the Hurd Peninsula and the Tangra Mountain). The rejuvenation of the magmatic rocks in the direction of the thrust plate can be explained as an effect of a flattening subduction that was active from the Early Cretaceous (135 Ma) up to around 90–70 Ma. The mixed ages of the dikes and small intrusions in the Hurd Peninsula are most probably due to periods of rotation and probably beginning of а slab-roll back. A regional Paleocene–Eocene (65–47 Ma) compressional episode that caused a low temperature to high-pressure metamorphism is registered on Smith and Elephant islands. The regional metamorphism was followed by a vast extension between 50–30 Ma that culminated with the opening of the Drake Passage around 34–30 Ma. The magmatic response of that regional scale extension was the intrusion of the Eocene Barnard Point Pluton (46–40 Ma) and dikes with youngest ages of around 30 Ma. The major uplift phase and the exhumation of the Tangra Mountain happened between 22–16 Ma as indicated by the Ap FT thermochronology. The last magmatic event on the island produced the contrastingly different in composition Quaternary alkaline mafic rocks that occupy the area between Innot Point and Burdick Peak. This episode is interpreted as related to a process of crustal extension that culminated in asthenospheric upwelling and rifting along the Bransfield Strait. The latter is due to a slab roll-back that led to the break-up of the South Shetlands from the Antarctic Peninsula and consequent subduction termination.

Drainage rearrangement and landscape evolution: Insights from the Moroccan Massif Central

The Moroccan Massif Central, which is the largest massif of Paleozoic bedrock of the Western Meseta, is located in the foreland of the Alpine Rif orogenic belt. The massif underwent little Cenozoic deformation and represents a peripheral plateau between the Atlantic Ocean and the Cenozoic Middle Atlas chain.On the basis of field observations combined with topographic and fluvial analysis, we show that the upstream part of the Oum Rbia River, which originates from the Middle Atlas Mountains, was once flowing to the northwest in continuity with the Bou Regreg River, which drains the waters of the Moroccan Massif Central to the Atlantic Ocean. A Late Pliocene diversion occurred at the transition between the Middle Atlas and the Massif Central leading the Oum Rbia River to change its course to the southwest into the Tadla Basin and leaving the Bou Regreg River isolated and starved of upstream drainage area. The diversion is documented by a fluvial terrace preserved in a paleostream near the drainage divide between the Oum Rbia and the Bou Regreg catchments, as well as by river profiles and drainage divide instability.Owing to a loss in drainage area, the beheaded Bou Regreg River system could no longer equilibrate the sustained dynamic uplift, which is indicated by lithospheric thinning and Quaternary alkaline volcanism, while the neighboring catchments keep incising. Ultimately, this would have led to a steady decrease in surface relief of the Moroccan Massif Central with the formation of low-relief surfaces that characterize the area as expected by an area-loss feedback model. The bedrock uplift in the Moroccan Massif Central could be related to lithospheric folding associated with the Rif belt.

Sr-Nd-Ca isotopic variations of Cenozoic calc-alkaline and alkaline volcanic rocks above a slab tear in Western Anatolia, Turkey

The tectonic mechanisms that may trigger a transition from calc-alkaline to alkaline volcanic activity above a subduction zone are enigmatic. We report major/trace elemental and Sr-Nd-Ca isotopic compositions of a suite of magmatic samples from the Miocene Gördes calc-alkaline dacites and the Quaternary Kula alkaline basalts from Western Anatolia, Turkey. Zircon sensitive high-resolution ion microprobe dating shows that the Gördes dacites formed at ca. 18 Ma. They are characterized by high-K calc-alkaline affinities, enrichment of light rare earth elements and large-ion lithophile elements (Rb, Th, U), positive Pb anomalies and negative Eu and high field strength element (HFSE) (Nb, Ta, and Ti) anomalies with high 87Sr/86Sri (0.70977−0.71010) and low εNd(t) (−7.3 to −7.0). They have small δ44/40Ca (0.59‰−0.69‰) values lower than that of mid-ocean ridge basalt (0.83% ± 0.11%). The Kula basalts exhibit alkaline affinities and are more enriched in incompatible elements (e.g., Rb, Ba, Th, Nb, Ta, Pb, and Sr) than those of typical oceanic island basalts. Their low 87Sr/86Sri (0.70307−0.70343) and high εNd(t) (+4.2 to +6.5) are consistent with an asthenospheric mantle source. Their δ44/40Ca values, ranging from 0.67‰ to 0.81‰, are higher than those of the dacites and lower than that of the bulk silicate earth (0.94‰ ± 0.10‰). Geochemical modeling suggests that variable degrees of partial melting of an asthenospheric mantle involving recycled oceanic crust may have resulted in low δ44/40Ca values of the Kula basalts. Combined with reported mantle tomographic images, we interpret that sub-slab asthenospheric mantle upwelling through a slab tear during subduction roll-back may have played a key role in the Sr-Nd-Ca isotopic variability from Miocene calc-alkaline dacites to Quaternary alkaline basalts in Western Anatolia.

Quaternary post-collisional high Nb-like basalts from Bijar-Qorveh, NW Iran: A metasomatized lithospheric mantle source

The Quaternary Bijar-Qorveh mafic volcanic rocks (BQMVR) occur in the Sanandaj-Sirjan Zone (SSZ) of Iran, a segment of the Alpine-Himalayan Orogeny with a Gondwana-affinity basement. Phases 1 and 2 of BQMVR are silica-undersaturated (basanite, tephrite, and phonotephrite) and form a highly alkaline or HA suite. The last phase, phase 3, of BQMVR is silica-saturated (alkali olivine basalt and hawaiite) and is called the mildly alkaline or MA suite. BQMVR share well-known subduction-related negative Nb Ta anomalies, and their Nb abundances are high enough to be classified as ‘high-Nb like basalts’. However, they do not show geochemical characteristics of typical high-Nb basalts. Trace element abundances and patterns of the MA suite mimic garnet-bearing spinel peridotite partial melts, whereas the HA suite is too rich in incompatible trace elements to match normal mantle partial melts. Enriched trace element patterns of the HA suite are akin to hornblendite experimental partial melts, and are likely derived from a garnet-amphibole peridotite source. Sr-Nd-Pb isotopic characteristics of the BQMVR indicate a subcontinental lithospheric mantle affinity. The HA suite tends toward the EM1 array, whereas the MA suite follows the EM2 array. The highly radiogenic Sr Nd isotopes of the MA suite indicate significant crustal contamination. Lithospheric delamination beneath the SSZ that followed Arabia-Eurasia collision, triggered garnet-amphibole peridotite partial melting that interacted with 10–30% of the asthenospheric component to have formed the HA suite. Mantle melting at shallower lithosphere depths followed and produced the MA suite. MELTS modeling shows the BQMVR underwent fractional crystallization of clinopyroxene ± olivine. Whole-rock thermobarometric calculations suggest derivation of the primary melts at a depth of ~97 km, however the Cpx-thermobarometer indicates that such melts equilibrated at lower crust depths at a depth of 45 km close to the Moho. The present Quaternary volcanostratigraphic study provides substantial clues for major post-collisional alkaline high-Nb mafic volcanism bearing signatures of two distinct magmatic stages; an early phase (phases 1 and 2) rooted in a deeper garnet amphibole peridotite followed by phase 3 rooted in a garnet-poor spinel peridotite. • Quaternary alkaline mafic volcanism in western Iran is post collisional. • Highly alkaline (HA) suite is derived from juvenile amphibole rich mantle. • Mildly alkaline (MA) suite is derived from shallower, ancient lithospheric mantle. • EM1 and EM2 affinities of the HA and MA suites respectively comply with the SCLM. • MELTS and isotopes revealed FC and AFC roles for HA and MA suites respectively.

Petrogenesis of Middle Miocene to Early Quaternary basalts from the Karayazı–Göksu plateau (Eastern Anatolia, Turkey): Implication for the role of pyroxenite and lithospheric thickness

In Eastern Anatolia, the Karayazı–Göksu basaltic plateau is one of the basaltic eruption centers of post-collisional volcanism, located between the convergence zone of the Arabian and Anatolian plates. The geological and geochemical evolution of the plateau is documented using new 40Ar/39Ar age data, major-and trace element abundances, mineral geochemistry, and isotope compositions (Sr–Nd–Hf) for better understanding the petrogenesis and mantle source characteristics. Samples yield whole-rock 40Ar/39Ar ages of 12.53 ± 0.05 and 12.68 ± 0.05 Ma for transitional basalts (Middle Miocene), 5.88 ± 0.20 Ma for tholeiitic lavas (Late Miocene), and 2.28 ± 0.03 and 2.48 ± 0.12 Ma for alkali members (Early Quaternary). The results suggest that the fractional crystallization of olivine and clinopyroxene in transitional, tholeiitic and lower alkaline lavas, and clinopyroxene in upper alkaline lavas, played a crucial role in their magmatic evolution without significant crustal contamination. Clinopyroxene-liquid equilibria indicate that the crystallization of transitional basalts occurred at the Moho and lower crust (temperature range 1157–1215 °C), tholeiitic basalts at the lower crust (temperature range 1154–1195 °C), alkaline basalts at lower to mid-crustal depths (temperature range 1083–1195 °C). The subduction-related metasomatism is in the form of melts derived from subducted sediments and decrease from Middle Miocene transitional to alkaline Early Quaternary basalts, as indicated by the Sr–Nd–Hf isotopes and Ba/La and Th/Yb contents. The Early Quaternary alkaline basalts contain contributions from isotopically depleted but chemically enriched (enriched-MORB-like) asthenospheric mantle domain. Major elements–based parameters and olivine mineral chemistry yield that Karayazı–Göksu basalts could not be derived from a sole peridotite mantle source; the pyroxenite/eclogite-like mantle component would have contributed to the members of the plateau. Three-component mixing results using SrNd isotopes indicate that the altered oceanic crust contributes to 0.5%–1% of the melt (crust vs. sediment about 65:35) for tholeiitic and alkaline basalts and reach up to 4% for transitional basalts (crust vs. sediment about 55:45). Melt equilibration depths of the basalts indicate that the lithospheric thickness of the region has not been changed on a large scale since the Middle Miocene to the Early Quaternary.

ПРОСТРАНСТВЕННО-ВРЕМЕННАЯ СМЕНА ГЛУБИННЫХ ИСТОЧНИКОВ КАЙНОЗОЙСКИХ ВУЛКАНИЧЕСКИХ ПОРОД ВОСТОКА КОРЯКСКОГО НАГОРЬЯ

A spatiotemporal analysis of the trace element ratios and the isotopic composition of Sr, Nd and Pb in Cenozoic post-accretionary volcanic rocks in the east of the Koryak Highlands (Northeastern Russia) has been carried out. The Early Paleogene volcanic complex is shown to be represented by moderately Ti-rich aluminous tholeiites with elevated contents of high field strength elements (HFSE) (except for Ta and Nb), which makes them similar to E-MORBs. Low (La/Yb)n and high Zr/Nb (25–35) ratios characterize one of the source components as depleted and similar to MORBs. At the same time, low ratios of Nb/Ta, Ce/Pb, Nb/La with a high ratio of K/Nb were determined in basalts, which indicates the presence of a subduction component in the source. Low Zr/Hf and (Dy/Yb)n ratios indicate melting of the garnet-free substratum. The second (Miocene) stage is represented by the subalkaline basalt flows and dacite extrusions and dikes forming a bimodal series. Basalts are characterized by low concentrations of large ion lithophile elements (LILE), enrichment in HFSE, and fractionated distribution of REE, with (La/Yb)n ratios varying from the values characteristic of E-MORBs to the values corresponding to intraplate tholeiites. The Nb/Ta, Ce/Pb, La/Ta, and Hf/Th ratios tend to exhibit compositions of intraplate tholeiites. The (Dy/Yb)n and (La/Yb)pm - Ybpm ratios indicate the formation of Miocene basalts during the selective melting of garnet peridotite with varying garnet contents in the source. Dikes of dacites, in comparison with effusive facies, differ in calc-alkaline differentiation trends and other absolute concentrations of a number of elements. Early Quaternary alkaline olivine basalts and basanites of the Navarin area are high in HFSE, LILE, and REE and are compositionally similar to intraplate volcanics of oceanic islands and continental rifts. Data points of these rocks plotted on discriminant diagrams are confined to the field of the garnet-rich intraplate source with low degrees of partial melting. The initial isotopic ratios of Sr, Nd, and Pb in the Cenozoic basaltoids characterize the deep sources of Cenozoic rocks in the eastern part of the Koryak Highlands as depleted. The similarity of isotope ratios in the basalts of the Miocene and Early Quaternary stages of volcanism suggests that the mantle region was isotopically homogenized as a result of local convection.

Geochemical Characterization of Intraplate Magmatism from Quaternary Alkaline Volcanic Rocks on Jeju Island, South Korea

The major and trace elements of Quaternary alkaline volcanic rocks on Jeju Island were analyzed to determine their origin and formation mechanism. The samples included tephrite, trachybasalts, basaltic trachyandesites, tephriphonolites, trachytes, and mantle xenoliths in the host basalt. Although the samples exhibited diversity in SiO2 contents, the relations of Zr vs. Nb and La vs. Nb indicated that the rocks were formed from the fractional crystallization of a single parent magma with slight continental crustal contamination (r: 0–0.3 by AFC modeling), rather than by the mixing of different magma sources. The volcanic rocks had an enriched-mantle-2-like ocean island basalt signature and the basalt was formed by partial melting of the upper mantle, represented by the xenolith samples of our study. The upper mantle of Jeju was affected by arc magmatism, associated with the subduction of the Pacific Plate beneath the Eurasian Plate. Therefore, we inferred that two separate magmatic events occurred on Jeju Island: one associated with the subduction of the Pacific Plate beneath the Eurasian Plate (represented by xenoliths), and another associated with a divergent setting when intraplate magmatism occurred (represented by the host rocks). With AFC modeling, it can be proposed that the Jeju volcanic rocks were formed by the fractional crystallization of the upper mantle combined with assimilation of the continental crust. The xenoliths in this study had different geochemical patterns from previously reported xenoliths, warranting further investigations.

Migration of Late Miocene to Quaternary alkaline magmatism at the Alpine-Pannonian transition area: Significance for coupling of Adria plate motion with the Alpine-Carpathian front

Within-plate migration of alkaline basaltic centers is generally related to translation of the lithosphere with a hot spot above a largely stationary mantle plume. Here, we report, for the first time, a hitherto unrecognized migration of small-sized Late Miocene to Early Quaternary alkali-basaltic volcanic centers at the transition from Eastern Alps to Pannonian Basin. The volcanic centers migrated along the South Burgenland High in a regular sequence over a 95 km distance from NNE to SSW between 11 Ma and 1.7 Ma. The basaltic magmatism was also associated with thinning of the crust and lithosphere and an increase of the thermal gradient. In detail, three stages of migration are recognized as follows: (i) Stage 1 with a 55 km SSW-towards shift of volcanism between 11 and 5 Ma; (ii) Stage 2 with a ~ 35 km-WSW shift and enlargement of the distribution of volcanic centers between ca. 5 and 3.5 Ma; and (iii) Stage 3 with a S-directed shift of ca. 25 km between 3.5 and 1.7 Ma. The Stage 1 shift of alkali-basaltic volcanism can be also observed in the Little Hungarian Plain and South Slovakia–Nógrád volcanic fields, and the Stage 2 in the Balaton-Bakony of the Pannonian Basin (north of Lake Balaton). We propose that this pattern and mechanism of migration of the volcanic centers along the South Burgenland High resulted from thermally induced progressive thinning of the lithosphere over a mantle plume underneath the ALCAPA block, which was moving from SSW to the NNE between 11 and 1.7 Ma. The migration was interrupted by a marked eastward shift between 5 and 3.5 Ma (Stage 2). The migration of volcanic centers correlates with orientation and timing of regional shortening phases and inversion of the Alpine-Carpathian-Pannonian system although the overall amount of shortening remains uncertain. The deformation events allow to correlate Adria motion with deformation along the frontal parts of Eastern Alps, and Western and Eastern Carpathians across the western Pannonian Basin.

Recent volcano-tectonic activity of the Ririba rift and the evolution of rifting in South Ethiopia

The relationships between volcanic activity and tectonics at the southernmost termination of the Main Ethiopian Rift (MER), East Africa, still represent a debated problem in the MER evolution. New constraints on the timing, evolution and characteristics of the poorly documented volcanic activity of the Dilo and Mega volcanic fields (VF), near the Kenya-Ethiopia border are here presented and discussed. The new data delineate the occurrence of two distinct groups of volcanic rocks: 1) Pliocene subalkaline basalts, observed only in the Dilo VF, forming a lava basement faulted during a significant rifting phase; 2) Quaternary alkaline basalts, occurring in the two volcanic fields as pyroclastic products and lava flows issued from monogenetic edifices and covering the rift-related faults. 40Ar/39Ar dating constrains the emplacement time of the large basal lava plateau to ~3.7 Ma, whereas the youngest volcanic activity characterising the two areas dates back to 134 ka (Dilo VF) to as recent as the Holocene (Mega VF). Volcanic activity developed along tectonic lineaments independent from those of the rift. No direct relations are observed between the Pliocene, roughly N-S-trending major boundary faults of the Ririba rift and the NE-SW-oriented structural trend characteristic of the Quaternary volcanic activity. We speculate that this change in structural trend may be the expression of (1) inherited crustal structures affecting the distribution of the recent volcanic vents, and (2) a local stress field controlled by differences in crustal thickness, following a major episode of reorganization of extensional structures in the region due to rift propagation and abandonment.

Geodynamic setting of Upper Miocene to Quaternary alkaline basalts from Harrat al ‘Uwayrid (NW Saudi Arabia): Constraints from K[sbnd]Ar dating, chemical and Sr-Nd-Pb isotope compositions, and petrological modeling

The volcanic field of Harrat ar Rahah-’Uwayrid (NW Saudi Arabia) consists of an older plateau basalt sequence that overlies Cambrian sandstone and younger cinder cones with smaller flows that are concentrated in the central part of this field. Petrographic and whole rock geochemical data indicate that the older plateau basalts are alkali olivine basalts, while the younger volcanic products are basanites and tephrites with some phonotephrites and tephriphonolites. KAr model ages on 22 bulk-rock samples were obtained for variable grain size fractions (125–250, 250–500 μm). These dates range from 8.2 to 0.34 Ma, whereby three phases of volcanic activity during the Upper Miocene, Pliocene and Quaternary can be distinguished. Sr-Nd-Pb isotopic ratios are similar for all age groups with 87Sr/86Sr = 0.70307–0.70324, 143Nd/144Nd = 0.512912–0.512957, 206Pb/204Pb = 19.360–19.717, 207Pb/204Pb = 15.603–15.633 and 208Pb/204Pb = 39.083–39.521 (present-day ratios are indistinguishable from calculated initial ratios) suggesting that chemical differences among the lavas were probably produced by different degrees of partial melting rather than by different source compositions. Trace element ratios indicate an origin of all volcanics by small degrees of partial melting of amphibole-spinel-garnet peridotite. Geochemical and isotopic data of the Harrat ar Rahah-’Uwayrid are similar to those from the small volcanic fields of Harrat Kura (about 200 km SSE of Harrat al ‘Uwayrid) and of Wadi Jizan in SW Saudi Arabia, but clearly more enriched than those of the volcanic fields that are located in the more central parts of the eastern shoulder of the Red Sea Rift. The lithospheric thickness underneath Harrat ar Rahah-’Uwayrid is estimated to about 60 km based on published results from seismic tomography. Our data support the model that the metasomatized lithospheric mantle was the primary source of the magmas, while the asthenospheric mantle or a plume-type component played an unsignificant role. The composition of the lithospheric mantle source was similar to prevalent mantle (PREMA). There is no need to postulate the participation of a HIMU component and/or enriched mantle components (EM-1, EM-2) in the source of the investigated magmas.

Petrogenesis of plio-quaternary intra-plate continental alkaline lavas from the İskenderun Gulf (Southern Turkey): Evidence for metasomatized lithospheric mantle

The Quaternary alkaline volcanic field of Southern Turkey is characterized by intra-continental plate-type magmatic products, exposed to the north of the İskenderun Gulf along a NE-SW trending East Anatolian Fault, to the west of its intersection with the N–S trending Dead Sea Fault zone. The İskenderun Gulf alkaline rocks are mostly silica-undersaturated with normative nepheline and olivine and are mostly classified as basanites and alkaline basalts with their low-silica contents ranging between 43 and 48 wt.% SiO2. They display Ocean Island Basalt (OIB)–type trace element patterns characterized by enrichments in large-ion-lithophile elements (LILE) and light rare earth element (LREE), and have (La/Yb)N = 8.8–17.7 and (Hf/Sm)N = 0.9–1.6 similar to those of basaltic rocks found in intraplate suites. The basanitic rocks have limited variations Sr-Nd isotopic ratios (87Sr/86Sr = 0.70307–0.70324, 143Nd/144Nd = 0.512918–0.521947), whereas the alkali basalts display more evolved Sr-Nd isotopic ratios (87Sr/86Sr = 0.70346-0.70365, 143Nd/144Nd = 0.512887–0.521896). The İskenderun Gulf alkaline rocks also display limited Pb isotopic variations with 206Pb/204Pb = 18.75–19.09 207Pb/204Pb = 15.61–15.66 and208Pb/204Pb = 38.65–39.02, indicating that they originated from an enriched lithospheric mantle source. Calculated fractionation vectors indicate that clinopyroxene and olivine are the main fractionating mineral phases. Similarly, based on Sr-Nd isotopic ratios, the assimilation and fractional crystallization (AFC) modeling shows that the alkali basalts were affected by AFC processes (r = 0.2) and were slightly contaminated by the upper crustal material.The high TiO2 contents, enrichments in Ba and Nb, and depletions in Rb can likely be explained by the existence of amphibole in the mantle source, which might, in turn, indicate that the source mantle has been affected by metasomatic processes. The modeling based on relative abundances of trace elements suggests involvement of amphibole-bearing peridotite as the source material. İskenderun Gulf alkaline rocks can thus be interpreted as the products of variable extent of mixing between melts from both amphibole-bearing peridotite and dry peridotite.

Geochemistry of Rock-Forming Minerals in Mantle Xenoliths from Basalts of Sverre Volcano, Spitsbergen Archipelago

The paper reports the results of SIMS and SEM-EDS study of rock-forming minerals from melt pockets in the central part of a spinel peridotite xenolith taken from Quaternary alkaline basalts of Sverre Volcano in the northwestern part of West Spitsbergen Island. Olivine and clinopyroxene are analyzed to trace changes related to the metasomatic interaction between spinel lherzolite and a carbonate melt with formation of corresponding secondary minerals and silicate glass. It is established that the metasomatic interaction of the carbonate melt with minerals of host spinel lherzolite is accompanied by partial recrystallization of olivine and clinopyroxene, or crystallization of the second generation of these minerals. Percolating carbonate melt caused significant changes in the major, trace, and rare-earth element composition of the considered minerals, thus placing constraints on the use of the composition of these minerals for calculation of P–T parameters, estimating equilibrium, and modeling petrological processes in mantle.

Sol-Gel-Synthesis of Nanoscopic Complex Metal Fluorides.

The fluorolytic sol-gel synthesis for binary metal fluorides (AlF3, CaF2, MgF2) has been extended to ternary and quaternary alkaline earth metal fluorides (CaAlF5, Ca2AlF7, LiMgAlF6). The formation and crystallization of nanoscopic ternary CaAlF5 and Ca2AlF7 sols in ethanol were studied by 19F liquid and solid state NMR (nuclear magnetic resonance) spectroscopy, as well as transmission electron microscopy (TEM). The crystalline phases of the annealed CaAlF5, Ca2AlF7, and LiMgAlF6 xerogels between 500 and 700 °C could be determined by X-ray powder diffraction (XRD) and 19F solid state NMR spectroscopy. The thermal behavior of un-annealed nanoscopic ternary and quaternary metal fluoride xerogels was ascertained by thermal analysis (TG/DTA). The obtained crystalline phases of CaAlF5 and Ca2AlF7 derived from non-aqueous sol-gel process were compared to crystalline phases from the literature. The corresponding nanoscopic complex metal fluoride could provide a new approach in ceramic and luminescence applications.

Paleo-Asian oceanic subduction-related modification of the lithospheric mantle under the North China Craton: Evidence from peridotite xenoliths in the Datong basalts

In-situ major and trace elements and Sr isotopic compositions of peridotite xenoliths of the Datong Quaternary alkaline basalt were analyzed to evaluate the influences of the southward subduction of the Paleo-Asian oceanic plate (PAOP) on the lithospheric mantle transformation of the North China Craton (NCC). These peridotite xenoliths including spinel harzburgites and lherzolites were classified into three groups. The type 1 peridotites have the lowest temperatures (961–1007°C). Clinopyroxenes in these peridotites exhibit LREE-depleted REE patterns and have the lowest 87Sr/86Sr ratios of 0.70243–0.70411. The type 2 and 3 peridotites show higher temperatures (1017–1022°C). Clinopyroxenes in the type 2 peridotite have V-shaped REE patterns and relatively higher 87Sr/86Sr ratios of 0.70418–0.70465. Clinopyroxenes in the type 3 peridotite have concave-downward REE patterns and unusually high 87Sr/86Sr ratios of 0.70769–0.70929. Carbonatitic veinlets are found in the type 1 peridotites. They show steep LREE-enriched REE patterns with enrichment in LILE and depletion in HFSE, and have the highest 87Sr/86Sr ratios of 0.71145–0.71285.The mineral chemistries and modal calculations suggest that the protolith of these peridotites experienced a variable degree of partial melting. The type 2 and 3 peridotites sampled from deeper depth experienced latter cryptic carbonatitic metasomatism. The carbonatitic veinlets have generally consistent trace element patterns and Sr isotopic ratios with the calculated melts equilibrated with clinopyroxenes in the type 3 peridotite, which may represent the percolated carbonatitic melt quickly solidified in the relatively cold and shallow mantle. The remarkable negative Eu anomalies (0.37–0.61) and highly radiogenic Sr isotopic compositions of the calculated metasomatic agents preclude indicate melt derived from carbonated peridotite or carbonated eclogite but point to a crustal sedimentary origin. Considering the tectonic setting and geophysical research, it is speculated that the carbonatitic melt was derived from the carbonated pelite carried by the southward subducted PAOP under the NCC. The PAOP subduction-related carbonatitic melt–peridotite reaction could have contributed to the transformation of the lithospheric mantle beneath the northern margin of the NCC.

Hazard assessment at the Quaternary La Garrotxa Volcanic Field (NE Iberia)

La Garrotxa Volcanic Field (GVF) in the NE Iberian Peninsula is one of the Quaternary alkaline volcanic provinces that form part of the European Cenozoic Rift System. Active over the last 0.7 Ma, the most recent dated eruption in this volcanic zone took place in the early Holocene (11–13 ka). Its volcanic activity has varied from Hawaiian to violent Strombolian, with numerous episodes of phreatomagmatic activity, and is controlled by the main regional normal faults generated during the Neogene extension that affected the area. Despite the potential for future eruptions and the fact that this is a densely populated industrial area, no volcanic hazard assessment of the field has ever been conducted. In this work, we present the first comprehensive evaluation of the volcanic hazard in the GVF via (1) an evaluation of its volcanic susceptibility, (2) a temporal recurrence rate analysis, (3) a simulation of different eruptive scenarios, such as lava flows, pyroclastic density currents and ashfall, and (4) the elaboration of a qualitative hazard map. The final hazard map shows that the GVF can be subdivided into five different hazard levels, knowledge that will be useful for land-use management and the drawing up of emergency plans.

Origin and geodynamic relationships of the Late Miocene to Quaternary alkaline basalt volcanism in the Pannonian basin, eastern–central Europe

Alkaline basaltic volcanism has been taking place in the Carpathian–Pannonian region since 11 Ma and the last eruptions occurred only at 100–500 ka. It resulted in scattered low-magma volume volcanic fields located mostly at the margins of the Pannonian basin. Many of the basalts have compositions close to those of the primitive magmas and therefore can be used to constrain the conditions of the magma generation. Low-degree (2–3 %) melting could occur in the convective asthenosphere within the garnet–spinel transition zone. Melting started at about 100 km depth and continued usually up to the base of the lithosphere. Thus, the final melting pressure could indicate the ambient lithosphere–asthenosphere boundary. The asthenospheric mantle source regions of the basalts were heterogeneous, presumably in small scale, and included either some water or pyroxenite/eclogite lithology in addition to the fertile to slightly depleted peridotite. Based on the prevailing estimated mantle potential temperature (1,300–1,400 °C) along with the number of further observations, we exclude the existence of mantle plume or plume fingers beneath this region. Instead, we propose that plate tectonic processes controlled the magma generation. The Pannonian basin acted as a thin spot after the 20–12 Ma syn-rift phase and provided suction in the sublithospheric mantle, generating asthenospheric flow from below the adjoining thick lithospheric domains. A near-vertical upwelling along the steep lithosphere–asthenosphere boundary beneath the western and northern margins of the Pannonian basin could result in decompressional melting producing low-volume melts. The youngest basalt volcanic field (Persani) in the region is inferred to have been formed due to the dragging effect of the descending lithospheric slab beneath the Vrancea zone that could result in narrow rupture at the base of the lithosphere. Continuation of the basaltic volcanism cannot be excluded as inferred from the still fusible condition of the asthenospheric mantle. This is reinforced by the detected low-velocity seismic anomalies in the upper mantle beneath the volcanic fields.

Nature and Evolution of the Lithospheric Mantle beneath the Hoggar Swell (Algeria): a Record from Mantle Xenoliths

Peridotite xenoliths exhumed by Quaternary alkaline magmatism in the Tahalgha district, southern Hoggar, represent fragments of the subcontinental lithospheric mantle beneath the boundary between the two major structural domains of the Tuareg Shield: the ‘Polycyclic Central Hoggar’ to the east and the ‘Western Hoggar’, or ‘Pharusian Belt’, to the west. Samples were collected from volcanic centres located on both sides of a major lithospheric shear zone at 4°35' separating these two domains. Although showing substantial variations in their deformation microstructures, equilibrium temperatures and modal and chemical compositions, the studied samples do not display any systematic changes of these features across the 4°35' fault. The observed variations rather record small-scale heterogeneities distributed throughout the study area and reflecting the widespread occurrence of vein conduits and metasomatized wall-rocks related to trans-lithospheric melt circulation during the Cenozoic. These features include partial annealing of pre-existing deformation microstructures, post-deformation metasomatic reactions, and trace-element enrichment, coupled with heating from 750–900°C (low-temperature lherzolites) to 900–1150°C (intermediate-T lherzolites and high-T harzburgites and wehrlites). Trace-element modelling confirms that the range of rare earth element (REE) variations observed in the Tahalgha clinopyroxenes may be accounted for by reactive porous flow involving a single stage of basaltic melt infiltration into a light REE (LREE)-depleted protolith. Whole-rock compositions record the final entrapment of disequilibrium metasomatic melts upon thermal relaxation of the veins–wall-rock system. The striking correlations between equilibrium temperatures and trace-element enrichment favor a scenario in which the high-temperature peridotites record advective heat transport along melt conduits, whereas the intermediate- and low-temperature lherzolites reflect conductive heating of the host Mechanical Boundary Layer. This indicates that the lithosphere did not reach thermal equilibrium, suggesting that the inferred heating event was transient and was rapidly erased by thermal relaxation down to the relatively low-temperature present-day geotherm. The low-T (<900°C) deformed lherzolites (porphyroclastic to equigranular) are characterized by only incipient annealing and LREE-depleted clinopyroxene compositions. They were only weakly affected by the Cenozoic events and could represent relatively well-preserved samples from rejuvenated Pan-African lithosphere. Extensive lithospheric rejuvenation occurred either regionally during the Pan-African orogeny, as a result of lithospheric delamination or thermomechanical erosion after thickening, or more locally along the meridional shear zones. The low-T Tahalgha lherzolites are comparable with lherzolites from Etang de Lherz, southern France, interpreted as lithospheric mantle rejuvenated by melt-induced refertilization during a late stage of the Variscan orogeny.