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We report the results of a study on early Miocene to Pleistocene volcanic rocks cropping out west of the Karlıova Triple Junction in Eastern Anatolia (Elazığ, Tunceli, and Bingöl provinces). Here the Eurasia-Arabia convergence resulted in collision, marked by the Bitlis-Zagros suture (~13 Ma), followed by activation of the dextral transform North Anatolian Fault (NAF). At ~6 Ma the formation of the sinistral transform East Anatolian Fault (EAF) marked the separation of the Anatolian block, which became a kinematically independent plate. On the basis of petrographic, geochemical and Sr-Nd-Pb isotopic characteristics, as well as new 40Ar-39Ar age determinations, we distinguish three phases of activity: (1) early-middle Miocene (16·3-15·5 Ma) production of calc-alkaline basaltic trachyandesite to dacites in the Pertek and Mazgirt districts; (2) emplacement of late Miocene (11·4-11·0 Ma) transitional basalts in the Tunceli area; (3) emplacement of Plio-Pleistocene Na-alkali basalts in Karakoçan (4·1 Ma) and Elazığ (1·7 Ma). The oldest samples are characterized by large ion lithophile element (LILE) enrichment (e.g. Ba/Nb = 32-76) with high 87Sr/86Sr (0·7052-0·7065) and low 143Nd/144Nd isotopic ratios (0·51246-0·51262). The late Miocene basalts display variable geochemical characteristics, including large variations in 87Sr/86Sr (0·7039-0·7068) and LILE/HFSE (high field strength element) ratios (e.g. Ba/Nb = 13-36). The Plio-Pleistocene alkali basalts have higher LILE and HFSE contents and lower LILE/HFSE ratios (Ba/Nb = 8-21) compared with the two previous groups, low 87Sr/86Sr (0·7033-0·7038) and high 143Nd/144Nd (0·51270-0·51290), with compositions similar to those of oceanic intraplate magmas. Pb isotopes vary slightly: 206Pb/204Pb ranges from 18·66 to 19·11, 207Pb/204Pb from 15·64 to 15·72 and 208Pb/204Pb from 38·67 to 39·24, with the calc-alkaline early-middle Miocene rocks characterized by higher 207Pb/204Pb and 208Pb/204Pb at a given 206Pb/204Pb. The evolution of volcanic activity is strictly linked to the geodynamic scenario. The early-middle Miocene magmas, emplaced in a convergent setting, indicate derivation from mantle sources modified by subduction components, whereas the late Miocene Tunceli transitional basalts mark the change from compressional to strike-slip tectonics. During the development of the NAF and EAF, passive upwelling of the sub-slab mantle, favoured by the formation of small pull-apart basins, led to the onset of Na-alkali basaltic activity. [ABSTRACT FROM AUTHOR]

Provence (SE France) was affected by two main phases of sporadic igneous activity during the Cenozoic. New 40 Ar/ 39 Ar laser step-heating data constrain the beginning of the oldest phase to late Eocene (40.82 ± 0.73 Ma), with activity present until early Miocene (~ 20 Ma). The products are mainly andesites, microdiorites, dacites and basaltic andesites mostly emplaced in the Agay-Estérel area. Major- and trace-element constraints, together with Sr Nd Pb isotopic ratios suggest derivation from a sub-continental lithosphere mantle source variably modified by subduction-related metasomatic processes. The compositions of these rocks overlap those of nearly coeval (emplaced ~ 38–15 Ma) late Eocene-middle Miocene magmatism of Sardinia. The genesis of dacitic rocks cannot be accounted for by simple fractional crystallization alone, and may require interaction of evolved melts with lower crustal lithologies. The youngest phase of igneous activity comprises basaltic volcanic rocks with mildly sodic alkaline affinity emplaced in the Toulon area ~ 10 Myr after the end of the previous subduction-related phase. These rocks show geochemical and isotopic characteristics akin to magmas emplaced in intraplate tectonic settings, indicating a sub-lithospheric HiMu + EM-II mantle source for the magmas, melting approximately in the spinel/garnet-lherzolite transition zone. New 40 Ar/ 39 Ar laser step-heating ages place the beginning of the volcanic activity in the late Miocene–Pliocene (5.57 ± 0.09 Ma). The emplacement of “anorogenic” igneous rocks a few Myr after rocks of orogenic character is a common feature in the Cenozoic districts of the Central-Western Mediterranean area. The origin of such “anorogenic” rocks can be explained with the activation of different mantle sources not directly modified by subduction-related metasomatic processes, possibly located in the sub-lithospheric mantle, and thus unrelated to the shallower lithospheric mantle source of the “orogenic” magmatism. [ABSTRACT FROM AUTHOR]

The volcanic products of the late Miocene Morron de Villamayor volcano (Calatrava Volcanic Field, central Spain) are known for being one of the few outcrops of leucitites in the entire circum-Mediterranean area. These rocks are important because aragonite of mantle origin has been reported as inclusion in olivine macrocrysts. We use petrographic observations, mineral compositions, as well as oxygen and carbon isotope ratios coupled with experimental petrology to understand the origin of carbonate phase in these olivine-phyric rocks. Groundmass and macrocryst olivines range from δ 18 O VSMOW of +4.8‰, typical of mantle olivine values, to +7.4‰, indicating contamination by sedimentary carbonate. Carbonates are characterized by heavy oxygen isotope compositions (δ 18 O VSMOW >+24‰), and relatively light carbon isotopes (δ 13 C PDB <−11‰), resembling skarn values, and distinct from typical mantle carbonatite compositions. Petrography, mineral compositions such as low Mg# of clinopyroxene and biotite, low Ca# and low incompatible element abundance of the carbonate, and isotopic ratios of O and C, do not support a mantle origin for the carbonate. Rather, the carbonate inclusions found in the olivine macrocrysts are interpreted as basement limestone fragments entrapped by the rising crystallizing magma. Comparison with experimental carbonatitic and silicate-carbonatitic melts indicates that low-degree partial melts of a carbonated peridotite must have a dolomitic rather than the aragonitic/calcitic composition as those found trapped in the Morron de Villamayor olivine macrocrysts. [ABSTRACT FROM AUTHOR]

Abstract: The Cenozoic igneous activity of Sardinia is essentially concentrated in the 38–0.1Myr time range. On the basis of volcanological, petrographic, mineralogical, geochemical and isotopic considerations, two main rock types can be defined. The first group, here defined SR (subduction-related) comprises Late Eocene–Middle Miocene (~38–15Ma) igneous rocks, essentially developed along the Sardinian Trough, a N–S oriented graben developed during the Late Oligocene–Middle Miocene. The climax of magmatism is recorded during the Early Miocene (~23–18Ma) with minor activity before and after this time range. Major and trace element indicators, as well as Sr–Nd–Pb–Hf–Os–O isotope systematic indicate complex petrogenetic processes including subduction-related metasomatism, variable degrees of crustal contamination at shallow depths, fractional crystallization and basic rock partial melting. Hybridization processes between mantle and crustal melts and between pure mantle and crustally contaminated mantle melts increased the isotopic and elemental variability of the composition of the evolved (intermediate to acid) melts. The earliest igneous activity, pre-dating the Early Miocene magmatic climax, is related to the pushing effects exerted by the Alpine Tethys over the Hercynian or older lower crust, rather than to dehydration processes of the oceanic plate itself. The second group comprises volcanic rocks emplaced from ~12 to ~0.1Ma. The major and, partially, trace element content of these rocks roughly resemble magmas emplaced in within-plate tectonic settings. From a Sr–Nd–Pb–Hf–Os isotopic point of view, it is possible to subdivide these rocks into two subgroups. The first, defined RPV (Radiogenic Pb Volcanic) group comprises the oldest and very rare products (~12–4.4Ma) occurring only in the southern sectors of Sardinia. The second group, defined UPV (Unradiogenic Pb Volcanic), comprises rocks emplaced in the remaining central and northern sectors during the ~4.8–0.1Ma time range. The origin of the RPV rocks remains quite enigmatic, since they formed just a few Myr after the end of a subduction-related igneous activity but do not show any evidence of slab-derived metasomatic effects. In contrast, the complex origin of the mafic UPV rocks, characterized by low 206Pb/204Pb (17.4–18.1), low 143Nd/144Nd (0.51232–0.51264), low 176Hf/177Hf (0.28258–0.28280), mildly radiogenic 87Sr/86Sr (~0.7044) and radiogenic 187Os/188Os ratios (0.125–0.160) can be explained with a mantle source modified after interaction with ancient delaminated lower crustal lithologies. The strong isotopic difference between the RPV and UPV magmas and the absence of lower crustal-related features in the SR and RPV remain aspects to be solved. [Copyright &y& Elsevier]

This article investigates how, where, and to what extent the mineralogical and chemical composition of sand-sized sediments is modified by extreme weathering in modern equatorial settings, with the ultimate goal of learning to read climate from the sedimentary record. To single out the weathering effect, we studied the compositional trends of fluvial sands along the western branch of the East African Rift between 5°S and 5°N. The relative durability of different detrital components, as well as potential hydraulic-sorting and grain-size effects, were assessed by comparing samples with similar provenances in different climatic and environmental conditions or of different size classes within the same sample. Sands of equatorial central Africa at the headwaters of the Congo and Nile basins display the full spectrum of petrologic suites characterizing rift-shoulder and volcanic rift provenances. Unlike in arid Arabia, quartzose sands are not restricted to areas where detritus is recycled from prerift sedimentary covers. In a hot humid climate, weathering can effectively obliterate the fingerprint of parent rock lithology and produce a nearly pure quartz residue even where midcrustal basement rocks are being actively uplifted and widely unroofed. In such settings garnet is destroyed faster than hornblende, and zircon faster than quartz. Weathering control on detrital modes is minor only in the rain shadow of the highest mountains or volcanoes, where amphibole-dominated quartzofelicdspathic metamorphiclastic sands (Rwenzori Province) or clinopyroxene-dominated feldspatholithic volcaniclastic sands (Virunga Province) are generated. Our detailed study of the Kagera basin emphasizes the importance of weathering in soils at the source rather than of progressive maturation in temporary storage sites during stepwise transport and shows that the transformation of diverse parent rocks into a quartzose "white sand" may be completed in one sedimentary cycle in hydromorphic soils of subequatorial lowlands. Micas and heavy minerals, which are less effectively diluted by recycling than main framework components, offer the best key to identify the original source-rock imprint. The different behavior of chemical indexes such as the CIA (a truer indicator of weathering) and the WIP (markedly affected by quartz dilution) helps us to distinguish strongly weathered first-cycle versus polycyclic quartz sands. [ABSTRACT FROM AUTHOR]

Geological and geodynamic models of the mantle often rely on joint interpretations of published seismic tomography images and petrological/geochemical data. This approach tends to neglect the fundamental limitations of, and uncertainties in, seismic tomography results. These limitations and uncertainties involve theory, correcting for the crust, the lack of rays throughout much of the mantle, the difficulty in obtaining the true strength of anomalies, choice of what background model to subtract to reveal anomalies, and what cross-sections to select for publication. The aim of this review is to provide a relatively non-technical summary of the most important of these problems, collected together in a single paper, and presented in a form accessible to non-seismologists. Appreciation of these issues is essential if final geodynamic models are to be robust, and required by the scientific observations. [ABSTRACT FROM AUTHOR]

Abstract: Late Miocene basanites of Nosy Be and Nosy Sakatia islands (Nosy Be Archipelago, northern Madagascar) carry spinel-facies anhydrous ultramafic xenoliths (lherzolites, harzburgites and wehrlites). Geothermobarometric estimates indicate that these xenoliths derive from shallow mantle depths of 35–40 km, with those from Nosy Be island showing equilibration T (averages in the range of 945–985 °C) lower than the Nosy Sakatia analogues (averages ranging from 1010 to 1110 °C). One Nosy Sakatia mantle xenolith exhibits relatively fertile lherzolite composition, with trace and major element mineral chemistry consistent with a residual character after low degrees (1–2%) of mafic melt extraction. We interpret this composition as that resembling a depleted mantle (DM)-like lithospheric composition before metasomatic overprints. The other lherzolites and harzburgites display petrochemical characters consistent with variable extent of partial melting (up to 18%), associated with pronounced metasomatic overprints caused by migrating melts, as highlighted by enrichments in highly incompatible trace elements (e.g. light rare earth elements, LREE and Sr), together with the abundant occurrence of wehrlitic lithologies. The variability of petrochemical features points to different styles of metasomatism and metasomatic agents. The estimated composition of the parental melts of wehrlites matches that of host basanites. The combination of this evidence with the petrographic features, characterized by coarse-granular to porphyroclastic textures and by the presence of olivine without kink-banding, suggests that wehrlites are veins or pockets of high pressure cumulates within the mantle peridotite. The same melts also metasomatized via porous-flow percolation some lherzolites and harzburgites. Distinctly, a group of lherzolites and harzburgites was metasomatized by a different alkaline melt having markedly lower incompatible trace element contents. Late infiltration of metasomatic fluids is responsible for the spongy texture of some clinopyroxenes of lherzolites, harzburgites and wehrlites. [Copyright &y& Elsevier]

Abstract: We present a geodynamic reconstruction of the Central–Western Mediterranean and neighboring areas during the last 50Myr, including magmatological and tectonic observations. This area was interested by different styles of evolution and polarity of subduction zones influenced by the fragmented Mesozoic and Early Cenozoic paleogeography between Africa and Eurasia. Both oceanic and continental lithospheric plates were diachronously consumed along plate boundaries. The hinge of subducting slabs converged toward the upper plate in the double-vergent thick-skinned Alps–Betics and Dinarides, characterized by two slowly-subsiding foredeeps. The hinge diverged from the upper plate in the single-vergent thin-skinned Apennines–Maghrebides and Carpathians orogens, characterized by a single fast-subsiding foredeep. The retreating lithosphere deficit was compensated by asthenosphere upwelling and by the opening of several back-arc basins (the Ligurian–Provençal, Valencia Trough, Northern Algerian, Tyrrhenian and Pannonian basins). In our reconstruction, the W-directed Apennines–Maghrebides and Carpathians subductions nucleated along the retro-belt of the Alps and the Dinarides, respectively. The wide chemical composition of the igneous rocks emplaced during this tectonic evolution confirms a strong heterogeneity of the Mediterranean upper mantle and of the subducting plates. In the Apennine–Maghrebide and Carpathian systems the subduction-related igneous activity (mostly medium- to high-K calcalkaline melts) is commonly followed in time by mildly sodic alkaline and tholeiitic melts. The magmatic evolution of the Mediterranean area cannot be easily reconciled with simple magmatological models proposed for the Pacific subductions. This is most probably due to synchronous occurrence of several subduction zones that strongly perturbed the chemical composition of the upper mantle in the Mediterranean region and, above all, to the presence of ancient modifications related to past orogeneses. The classical approach of using the geochemical composition of igneous rocks to infer the coeval tectonic setting characteristics cannot be used in geologically complex systems like the Mediterranean area. [Copyright &y& Elsevier]

Mantle xenoliths hosted in Miocene-Quaternary mafic alkaline volcanic rocks from Sardinia have been investigated with electron microprobe, laser ablation microprobe-inductively coupled plasma-mass spectrometry and thermal ionization mass spectrometry techniques. The xenoliths are anhydrous clinopyroxene-poor lherzolites and harzburgites, plus very rare websterites and olivine-websterites. Glassy pods having thin subhedral to euhedral microlites of olivine, clinopyroxene and spinel have been found in harzburgites and websterites. Clinopyroxene shows trace element variability, with values of (La/Yb) ranging from sub-chondritic (0.01) to supra-chondritic (8.6). The Sr-Nd isotopic ratios of the clinopyroxenes fall mostly in the field of the European lithospheric mantle xenoliths (Sr/Sr from 0.70385 to 0.70568 and Nd/Nd ranging from 0.512557 to 0.512953). The geochemical characteristics of the Sardinian xenoliths testify to the variable degrees of earlier partial melt extraction, followed by metasomatic modification by alkaline melts or fluids. Websterites are considered to represent small lenses or veins of cumulitic (i.e. magmatic) origin within the mantle peridotite. [ABSTRACT FROM AUTHOR]

Abstract: The volcanic activity of Mt. Karacadağ (SE Anatolia) is divided into three major stages: Siverek Stage (∼11–2.7Ma), Karacadağ Stage (∼1.9–1.0Ma) and Ovabağ Stage (0.4–0.01Ma). The magmas are mildly alkaline mafic rocks, mostly basanites, hawaiites and alkali basalts. Detailed geochemical investigation indicates a continuous variation of composition with time, with the oldest products (Siverek Stage) being characterized by average lowest HFSE (Ti, Hf, Zr, Nb, Ta), 143Nd/144Nd, Nb/U, Ta/Yb, Nb/Nb* and the highest ΔQ, La/Nb, Ti/Nb, Zr/Nb, Ba/Nb, Th/Ta, K/La, and the youngest products (Ovabağ Stage) at the opposite end of the trend. The overall incompatible element content of the Karacadağ volcanic rocks resembles closely average HIMU-OIB compositions, with the oldest samples deviating more strongly from typical compositions of “anorogenic” magmas. We interpret these geochemical variations with a process of partial melting of a chemically and mineralogically heterogeneous mantle source rather than with process of variable crustal contamination at shallow depths. During the first stages of mantle melting the volumes with lowest solidus temperature (e.g., the amphibole and phlogopite-rich metasomes, particularly abundant in the Arabia mantle xenolith suite) contributed significantly to the partial melts. “Average” peridotitic matrix is involved in partial melting processes only when these metasomatic volumes start to be exhausted, producing mantle melts with geochemical composition resembling average “anorogenic” mildly alkaline sodic rocks, common in the circum-Mediterranean area. [Copyright &y& Elsevier]

Abstract: The present-day northern Apennines represent an area where two different orogenic cycles took place. The most ancient is the Alpine Orogeny, initiated with a south- to south-eastward subduction of oceanic lithosphere (Early Cretaceous to Late Eocene phase), followed by continental collision. The younger is referred to the Apennines Orogeny, characterized by a west- to north-westward oceanic lithosphere subduction started from Late Eocene and still active in the southernmost sectors of Italy (Calabrian Arc). In this framework, during the Early Oligocene, an ~800m thick conglomeratic succession extremely rich in volcanic material (up to ~90% in volume) was deposited in the Northern Apennines, forming the Aveto-Petrignacola Formation (APF). The volcanic fraction of this succession is made up of basaltic andesites, andesites and dacites, with minor rhyolites, basalts and gabbros found as pebbles ~0.001–0.5m3 in size. Petrographic, mineral chemical, major and trace element analyses, as well as Sr–Nd–Pb isotopic ratios are compatible with calc-alkaline magmatism generated as a consequence of metasomatic modifications related to the subduction of oceanic lithosphere. The large spectrum of lithologies is compatible with processes of fractional crystallization of the main phases observed in thin sections. However, the large range of isotopic ratios (87Sr/86Sr=0.7044–0.71013; 143Nd/144Nd=0.51278–0.51223; 206Pb/204Pb=18.44–18.83; 207Pb/204Pb=15.55–15.67; 208Pb/204Pb=38.05–38.85) suggests the presence of heterogeneous mantle sources, possibly coupled with interaction of melts with crustal rocks at shallow depth. On the basis of field geology constraints, we propose that the subduction-related geochemical and mineralogical characteristics of the APF volcanic rocks reflect ancient modifications of their mantle sources, lacking any evidence for a genetic link with the Apennine subduction system. Subduction-related metasomatism of APF mantle sources likely occurred during the older south-east dipping subduction of the Alpine Tethys under Northern Adria (present day Northern Italy). [Copyright &y& Elsevier]

The composition of the upper mantle bounded by the Canaries, Eastern Anatolia, Libya and Poland is indirectly investigated by means of the chemical composition of igneous rocks with ‘anorogenic’ geochemical characteristics emplaced during the Cenozoic. The relatively homogeneous composition of these products in terms of incompatible trace-element content and Sr–Nd–Pb isotopic composition is unexpected, considering the variable lithospheric structure of this large area and the different tectono-thermal histories of the various districts. In order to reconcile the geochemical characteristics with a statistical sampling model, it would be necessary to propose volumes of the enriched regions much lower than the sampling volumes for each volcano (that is, less than 10 cubic metres), or alternatively, efficient magma blending from larger areas. The data are consistent with a relatively well-stirred and mixed sub-lithospheric upper mantle, in the solid state, which is also hard to understand. This contrasts with the situation under oceans where magma blending from diverse sources and sampling theory can explain the compositional statistics. [ABSTRACT FROM AUTHOR]

Abstract: The central-western Mediterranean area is a key region for understanding the complex interaction between igneous activity and tectonics. In this review, the specific geochemical character of several ‘subduction-related’ Cenozoic igneous provinces are described with a view to identifying the processes responsible for the modifications of their sources. Different petrogenetic models are reviewed in the light of competing geological and geodynamic scenarios proposed in the literature. Plutonic rocks occur almost exclusively in the Eocene–Oligocene Periadriatic Province of the Alps while relatively minor plutonic bodies (mostly Miocene in age) crop out in N Morocco, S Spain and N Algeria. Igneous activity is otherwise confined to lava flows and dykes accompanied by relatively greater volumes of pyroclastic (often ignimbritic) products. Overall, the igneous activity spanned a wide temporal range, from middle Eocene (such as the Periadriatic Province) to the present (as in the Neapolitan of southern Italy). The magmatic products are mostly SiO2-oversaturated, showing calcalkaline to high-K calcalcaline affinity, except in some areas (as in peninsular Italy) where potassic to ultrapotassic compositions prevail. The ultrapotassic magmas (which include leucitites to leucite-phonolites) are dominantly SiO2-undersaturated, although rare, SiO2-saturated (i.e., leucite-free lamproites) appear over much of this region, examples being in the Betics (southeast Spain), the northwest Alps, northeast Corsica (France), Tuscany (northwest Italy), southeast Tyrrhenian Sea (Cornacya Seamount) and possibly in the Tell region (northeast Algeria). Excepted for the Alpine case, subduction-related igneous activity is strictly linked to the formation of the Mediterranean Sea. This Sea, at least in its central and western sectors, is made up of several young (<30Ma) V-shaped back-arc basins plus several dispersed continental fragments, originally in crustal continuity with the European plate (Sardinia, Corsica, Balearic Islands, Kabylies, Calabria, Peloritani Mountains). The bulk of igneous activity in the central-western Mediterranean is believed to have tapped mantle ‘wedge’ regions, metasomatized by pressure-related dehydration of the subducting slabs. The presence of subduction-related igneous rocks with a wide range of chemical composition has been related to the interplay of several factors among which the pre-metasomatic composition of the mantle wedges (i.e., fertile vs. refractory mineralogy), the composition of the subducting plate (i.e., the type and amount of sediment cover and the alteration state of the crust), the variable thermo-baric conditions of magma formation, coupled with variable molar concentrations of CO2 and H2O in the fluid phase released by the subducting plates are the most important. Compared to classic collisional settings (e.g., Himalayas), the central-western Mediterranean area shows a range of unusual geological and magmatological features. These include: a) the rapid formation of extensional basins in an overall compressional setting related to Africa-Europe convergence; b) centrifugal wave of both compressive and extensional tectonics starting from a ‘pivotal’ region around the Gulf of Lyon; c) the development of concomitant Cenozoic subduction zones with different subduction and tectonic transport directions; d) subduction ‘inversion’ events (e.g., currently along the Maghrebian coast and in northern Sicily, previously at the southern paleo-European margin); e) a repeated temporal pattern whereby subduction-related magmatic activity gives way to magmas of intraplate geochemical type; f) the late-stage appearance of magmas with collision-related ‘exotic’ (potassic to ultrapotassic) compositions, generally absent from simple subduction settings; g) the relative scarcity of typical calcalkaline magmas along the Italian peninsula; h) the absence of igneous activity where it might well be expected (e.g., above the hanging-wall of the Late Cretaceous–Eocene Adria–Europe subduction system in the Alps); i) voluminous production of subduction-related magmas coeval with extensional tectonic régimes (e.g., during Oligo-Miocene Sardinian Trough formation). To summarize, these salient central-western Mediterranean features, characterizing a late-stage of the classic ‘Wilson Cycle’ offer a ‘template’ for interpreting magmatic compositions in analogous settings elsewhere. [Copyright &y& Elsevier]

Petrological and geochemical data for basic (alkali basalts and hawaiites) and silicic peralkaline rocks, plus rare intermediate products (mugearites and benmoreites) from the Pleistocene Boseti volcanic complex (Main Ethiopian Rift, East Africa) are reported in this work. The basalts are slightly alkaline or transitional, have peaks at Ba and Nb in the mantle-normalized diagrams and relatively low 87Sr/86Sr (0.7039–0.7044). The silicic rocks (pantellerites and comendites) are rich in sanidine and anorthoclase, with mafic phases being represented by fayalite-rich olivine, opaque oxides, aenigmatite and slightly Na-rich ferroaugite (ferrohedenbergite). These rocks were generated after prolonged fractional crystallization process (up to 90–95 %) starting from basaltic parent magmas at shallow depths and fO2 conditions near the QFM buffer. The apparent Daly Gap between mafic and evolved Boseti rocks is explained with a model involving the silicic products filling upper crustal magma chambers and erupted preferentially with respect to basic and intermediate products. Evolved liquids could have been the only magmas which filled the uppermost magma reservoirs in the crust, thus giving time to evolve towards Rb-, Zr- and Nb-rich peralkaline rhyolites in broadly closed systems. [ABSTRACT FROM AUTHOR]

Major and trace element analyses plus six new 40Ar/39Ar data obtained on feldspar separates for selected igneous rocks belonging to the Oligo-Miocene igneous activity of Sardinia are reported here. The rocks show 'subduction-related' geochemical characteristics, ascribed to the metasomatic modification of a mantle wedge developed above a W/NW directed subduction system involving the recycling of Neo-Tethyan oceanic lithosphere beneath the southern European margin. The 40Ar/39Ar data shift back the beginning of the igneous activity in Sardinia to late Eocene (38.28 ± 0.26 Ma), whereas the end of the igneous activity is possibly shifted forward to late middle Miocene (12.24 ± 0.98 Ma). We therefore propose to rename the Oligo-Miocene igneous phase as the 'late Eocene-middle Miocene' phase. Rare earth element inversion modeling on subduction-related volcanic rocks of Sardinia requires equilibrium with spinel-lherzolite facies, therefore implying depths of formation lower than ∼80 km. To reach these depths, a 45° dipping slab needs ∼11 to ∼4 Myr (with 1 to 3 cm/yr subduction velocity, respectively), reducing the beginning of Apennine subduction to the ∼49-42 Ma interval. The different models proposed in literature to explain the origin and the evolution of the central western Mediterranean basins are here reviewed and critically discussed. [ABSTRACT FROM AUTHOR]

Trace element partition coefficients between clinopyroxenes and associated glassy matrix (Cpx/L D) have been determined for 13 REE, HFSE4+,5+, U, Th, Sr, Pb, Sc and V from combined LA-ICP-MS/EMP analyses in selected trachytes and trachyphonolites from Campi Flegrei. Composition of clinopyroxene and glass is pretty homogeneous in the trachyphonolites, pointing to an overall attainment of the equilibrium conditions. In trachytes, conversely, phases show some compositional heterogeneity (due to the presence of clinopyroxene xenocrysts) that requested a more careful petrographic and geochemical inspection of the samples to assess the equilibrium clinopyroxene composition. In the trachyte clinopyroxenes, REE are compatible from Nd to Lu (Cpx/L D up to 2.9), like Y, Ti, Sc and V. The Cpx/L D for Eu is lower than those of the adjacent REE, highlighting Eu2+ contribution. High D values are also shown by U, Th, Pb, Zr, Hf, Nb and Ta relatively to basaltic and andesitic systems, whereas the DSr is roughly similar to that found for less evolved magmas. Trachyphonolites are characterized by an overall decrease of the Cpx/L D for highly-charged cations (with the exception of V), and by a slight increase of DSr. REE are still compatible from Nd to Lu (Cpx/L D up to 2.1), like Ti, Y, Sc and V. This variation is also predicted for REE and Y by models based on the elastic strain theory, being consistent with the slightly lower polymerization degree estimated for the trachyphonolites. However, the observed Cpx/L D(REE,Y) are matched by the modelled ones only considering very low T (≤825°C), which are believed unlikely. This mismatch cannot be attributed to effects induced by the water-rich composition of the trachyte–trachyphonolite suite, since they would lower the observed Cpx/L D(REE,Y). Moreover, the anomalous inflections of measured Cpx/L D for HREE suggests some crystal-chemical control, such as the entrance of these elements in a site distinct from M2. It is concluded that the large Cpx/L D determined for trachytes and trachyphonolites are likely induced by hitherto unconstrained changes of the Z3+ activities related to the composition of melt and/or solid. All these considerations strongly highlight the importance of a direct characterization of trace element partitioning in natural samples from magmatic systems poorly characterized by experimental studies. [ABSTRACT FROM AUTHOR]

Abstract: The Cenozoic (mostly Neogene) volcanic activity in Syria is part of the extensive magmatism that took place in the Mashrek Region, Middle East, from upper Eocene to Holocene (∼40–0.0005Ma). Samples in western Syria are mostly high TiO2 (TiO2 ∼1.8–3.7wt.%) alkaline mafic rocks (basanites, hawaiites and alkali basalts) plus rare transitional/tholeiitic basalts and basaltic andesites) with within-plate-like trace element signature. On the basis of incompatible trace element content, the volcanic activity in Syria has been divided into two stages: the first lasting from ∼25 to ∼5Ma and the second from ∼5 to recent times. Indeed, the Syrian lavas show incompatible trace element content increasing with decreasing age from ∼25 to ∼5Ma, followed by an abrupt decrease to low values roughly at the Miocene–Pliocene boundary. This temporal shift in composition is related to major tectonic re-organization occurred during upper Miocene. The proposed petrogenetic model invokes three steps: (a) passive upwelling of the shallow asthenosphere during the development of the Dead Sea transform fault system. Different degrees of partial melting were followed by variable extents of fractional crystallization and limited upper crustal contamination; (b) the Miocene–Pliocene boundary tectonic change enhanced passive decompression of the same sources and a consequent increase in degree of partial melting resulting in low incompatible trace element content of the relatively high-volume liquids; (c) after this phase, the incompatible trace element content in the basaltic magmas increased as consequence of fractional crystallization processes. Major and trace element content similarities with the rest of the circum-Mediterranean igneous rocks are consistent with a common relatively shallow origin for the Cenozoic anorogenic magmatism of the entire circum-Mediterranean area (the so-called Common Magmatic Reservoir). Because much of the igneous activity in the studied area is concentrated near the Dead Sea fault, the origin of Cenozoic magmatism in Syria (and in the rest of the circum-Mediterranean area) reflects a strong lithospheric control on the loci of partial melting. Mantle plumes from lower mantle and/or north-westward channelling of the Afar mantle plume is not needed to explain volcanic activity in Syria and the Mashrek area. [Copyright &y& Elsevier]

Abstract: The Earth''s lithosphere can focus basaltic magmatism along pre-existing weakness zones or discontinuities. However, apart from the influence on the geochemistry of magmas emplaced in subduction tectonic settings (mantle wedge metasomatism related to dehydration of the subducting plates) the role of lithosphere as a magma source for intra-plate (both oceanic and continental), continental margin, and mid-ocean ridge magmatism is not yet fully understood. In many cases intra-plate magmatism has been explained with the existence of deep thermal anomalies (mantle plumes) whose origin has been placed near the upper–lower mantle transition zone (660 km discontinuity) or even deeper, near the mantle–core boundary (∼2900 km). Also in many continental flood basalt provinces (mostly initiated at craton margins) an active role for mantle plumes has been invoked to explain the high melt productivity. In these cases, no active role for melt production has been attributed to the lithospheric mantle. Potential contaminations of asthenospheric or even deeper mantle melts are often considered the only influence of the lithosphere (both crust and mantle) in basalt petrogenesis. In other cases, an active role of the lithospheric mantle has been proposed: the thermal anomalies related to the presence of mantle plumes would trigger partial melting in the lithospheric mantle. At present there is no unequivocal geochemical tracer that reflects the relative role of lithosphere and upper/lower mantle as magma sources. In this paper another role of the lithosphere is proposed. The new model presented here is based on the role of lower crustal and lithospheric mantle recycling by delamination and detachment. This process can explain at least some geochemical peculiarities of basaltic rocks found in large and small volume igneous provinces, as well as in mid-ocean ridge basalts. Metamorphic reactions occurring in the lower continental crust as a consequence of continent–continent can lead to a density increase (up to 3.8 g/cm3) with the appearance of garnet in the metamorphic assemblage (basalt→amphibolite→garnet clinopyroxenite/eclogite) leading to gravitative instability of the overthickened lithospheric keel (lower crust+lithospheric mantle). This may detach from the uppermost lithosphere and sink into the upper mantle. Accordingly, metasomatic reactions between SiO2-rich lower crust partial melts and the uprising asthenospheric mantle (replacing the volume formerly occupied by the sunken lithospheric mantle and the lower crust) lead to formation of orthopyroxene-rich layers with strong crustal signatures. Such metasomatized mantle volumes may remain untapped also for several Ma before being reactivated by geological processes. Partial melts of such sources would bear strong lower crustal signatures giving rise to Enriched Mantle type 1 (EMI)-like basaltic magmatism. Basaltic magmatism with such a geochemical signature is relatively scarce but in some cases (e.g., Indian Ocean) it can be a geographically widespread and long-lasting phenomenon. [Copyright &y& Elsevier]

Abstract: The early Cretaceous (∼130 Ma) igneous complex of Valle Chico (SE Uruguay) is made up of felsic plutonic and subordinate volcanic rocks and dykes cropping out over an area of about 250 km2. This complex is strictly linked with the formation of the Paraná–Etendeka Igneous Province and the first stages of the South Atlantic Ocean rifting. The plutonic rocks range from quartz-monzonite to syenite, quartz-syenite and granite. The volcanic rocks and the dykes range from quartz-latite to trachyte and rhyolite; no substantial differences in term of chemical composition have been found between plutonic and volcanic rocks. Only a sample of basaltic composition (with tholeiitic affinity) has been sampled associated with the felsic rocks. The Agpaitic Index of the Valle Chico felsic rocks range from 0.72 to 1.34, with the peralkaline terms confined in the most evolved samples (SiO2>65 wt.%). Initial 87Sr/86Sr(130) of the felsic rocks range from 0.7046 to 0.7201, but the range of 87Sr/86Sr of low-Rb/Sr samples cluster at 0.7083; 143Nd/144Nd(130) ratios range from 0.5121 (syenite) to 0.5117 (granite). The tholeiitic basalt show more depleted isotopic compositions (87Sr/86Sr(130)=0.7061; 143Nd/144Nd(130)=0.5122), and plots in the field of other early Cretaceous low-Ti basaltic rocks of SE Uruguay. The radiogenic Sr and unradiogenic Nd of the Valle Chico felsic rocks require involvement of lower crustal material in their genesis either as melt contaminant or as protolith (crustal anatexis). In particular, most of the Valle Chico (VC) felsic rocks define a near-vertical array in Sr–Nd isotopic spaces, pointing toward classical EMI-type composition; this feature is considered to reflect a lower crust involvement as observed for other mafic and felsic rocks of the Paraná–Etendeka Igneous Province. Decompression melting of the lower crust related to Gondwana continental rifting before the opening of the South Atlantic Ocean or the presence of thermal anomalies related to the Tristan plume may have induced the lower crust to partially melt. Alternative hypothesis considers contamination of upper mantle by a mafic/ultramafic keel composed of lower crust and uppermost mantle after delamination and detachment processes. This interaction may have occurred after the continent–continent collision during the last stages of the Panafrican Orogeny. This “lower crust” model does not exclude active involvement of upper crust as contaminant, necessary to explain the strongly radiogenic 87Sr/86Sr(130) isotopic composition of some VC SiO2-rich rocks. Mineralogical (sporadic presence of pigeonite, Ca–Na and Na clinopyroxene, calcic- and calco-sodic amphibole) and geochemical evidences (major and trace element as well as Sr–Nd isotopic similarities with the felsic early Cretaceous volcanic rocks of the Arequita Formation in SE Uruguay) allow us to propose for the VC rocks a transitional rock series (the most abundant rock types are of syenitic/trachytic composition) preferentially evolving towards SiO2-oversaturated compositions (granite/rhyolite) also with a strong upper crustal contribution as melt contaminant. This conclusion is in contrast with previous studies according which the VC complex had clear alkaline affinity. Many similarities between VC and the coeval Paresis granitoids (Etendeka, Namibia) are evidenced in this paper. The genetic similarities between VC and the rhyolites (s.l.) of SE Uruguay may find counterparts with the genetic link existing between the early Cretaceous tholeiitic-alkaline Messum complex and the quartz latites (s.l.) of the Awahab Formation (Etendeka region, Namibia). [Copyright &y& Elsevier]

Quantitative provenance analysis based on high-resolution bulk-petrography and heavy-mineral data on beach and dune sands, integrated with detrital-zircon geochronology and chemical analyses of pyroxene, garnet and staurolite, demonstrates that sand derived from the Orange River is carried by powerful and persistent longshore currents as far as northern Namibia and southern Angola, 1750 km north of its mouth. This is the longest cell of littoral sand transport documented so far. Compositional forward modeling indicates that ≥ 80% of dune sand in the Skeleton Coast is Orange-derived, the remaining ≤ 20% being supplied by slow erosion of the Damara Orogen chiefly via the Swakop River. A decrease in basaltic rock fragments and pyroxene with relative enrichment in garnet, staurolite, tourmaline and other metamorphic minerals north of Walvis Bay indicates that only one-third of beach sand in the 350 km dune-free gap between the Namib and Skeleton Coast Ergs is Orange-derived, the remaining two-thirds being supplied largely by the Swakop River draining the Damara Orogen. Although volcanic gravel becomes dominant in beaches of the Skeleton Coast north of the Uniab mouth, detritus from Cretaceous Etendeka lavas accounts for only 4% of beach sand, reflecting limited sand generation in the arid catchment. Contributions from the Kunene River to either dune or beach sands is ruled out, indicating that gem diamonds found in placer deposits along the coast of Namibia are all derived from the Orange River. [ABSTRACT FROM AUTHOR]

Abstract: Sourced as the Nile in distant basaltic rift highlands, the Orange River is the predominant ultimate source of sand for the Namib Desert dunes, as proved independently by bulk-petrography, heavy-mineral, pyroxene-chemistry, and U/Pb zircon-age datasets. Additional local entry points of sand do exist at the edges of the desert, and were quantified by comparison with detrital modes and heavy-mineral suites of hinterland-river sediments. After long-distance fluvial transport, Orange sand is washed by ocean waves and dragged northwards by vigorous longshore currents. Under the incessant action of southerly winds, sand is blown inland and carried farther north to accumulate in the Namib erg, a peculiar wind-dominated sediment sink displaced hundreds of kilometres away from the river mouth. And yet changes in sand mineralogy along the way are minor. After a multistep journey of cumulative 3000km from their source in Lesotho, volcanic rock fragments and pyroxene are found in unchanged abundance as far as the northern edge of the desert. Only locally is volcanic detritus slightly depleted and minor but regular enrichment in quartz and garnet is observed, the sole potential effect of prolonged transport or recycling of Tertiary aeolianites. Selective comminution of fragile minerals is thus proved unable to substantially modify sand composition in fluvial, coastal, or aeolian settings. Mechanical processes have a much greater effect on the morphology of detrital grains, which in Namib dunes appear commonly shaped into nearly perfect spheres. Aeolian sorting concentrates denser minerals locally in placer lags, but such effects can be identified and compensated for. This study demonstrates that mechanical breakdown is unable to markedly affect provenance signatures even during long-distance and prolonged multistep transport in high-energy settings. In arid climates, where chemical processes are negligible, high-resolution bulk-petrography and heavy-mineral analyses are thus powerful techniques to quantitatively reconstruct provenance, and to trace sediment sources and dispersal paths over distances up to thousands of kilometres. [Copyright &y& Elsevier]

Abstract: The Karoo volcanic sequence in the southern Lebombo monocline in Mozambique contains different silicic units in the form of pyroclastic rocks, and two different basalt types. The silicic units in the lower part of the Lebombo sequence are formed by a lower unit of dacites and rhyolites (67–80wt.% SiO2) with high Ba (990–2500ppm), Zr (800–1100ppm) and Y (130–240ppm), which are part of the Jozini–Mbuluzi Formation, followed by a second unit, interlayered with the Movene basalts, of high-SiO2 rhyolites (76–78wt.%; the Sica Beds Formation), with low Sr (19–54ppm), Zr (340–480ppm) and Ba (330–850ppm) plus rare quartz-trachytes (64–66wt.% SiO2), with high Nb and Rb contents (240–250 and 370–381ppm, respectively), and relatively low Zr (450–460ppm). The mafic rocks found at the top of the sequence are basalts and ferrobasalts belonging to the Movene Formation. The basalts have roughly flat mantle-normalized incompatible element patterns, with abundances of the most incompatible elements not higher than 25 times primitive mantle. The ferrobasalt has TiO2 ∼4.7wt.%, Fe2O3 t =16wt.%, and high Y (100ppm), Zr (420ppm) and Ba (1000ppm). The Movene basalts have initial (at 180Ma) 87Sr/86Sr=0.7052–0.7054 and 143Nd/144Nd=0.51232, and the Movene ferrobasalt has even lower 87Sr/86Sr (0.70377) and higher 143Nd/144Nd (0.51259). The silicic rocks show a modest range of initial Sr-(87Sr/86Sr=0.70470–0.70648) and Nd-(143Nd/144Nd=0.51223–0.51243) isotope ratios. The less evolved dacites could have been formed after crystal fractionation of oxide-rich gabbroic cumulates from mafic parental magmas, whereas the most silica-rich rhyolites could have been formed after fractional crystallization of feldspars, pyroxenes, oxides, zircon and apatite from a parental dacite magma. The composition of the Movene basalts imply different feeding systems from those of the underlying Sabie River basalts. [Copyright &y& Elsevier]

The small Plio-Quaternary volcanic centre of Cabezo Negro de Tallante in SE Spain includes a thick deposit of polymictic pyroclastic tuff-breccia, whose fragments are agglutinated by a carbonate-rich component. This feature is also observed in other monogenetic volcanic centres cropping out in the Tallante-Cartagena volcanic district. The carbonate fraction has been recently interpreted in literature as representing a mantle component, therefore pointing to the existence of a diffuse carbonatitic activity in the area. Based on detailed sedimentological (presence of pisoids and root remnants), petrographic (presence of plagioclase and absence of euhedral silicate minerals in the calcite plagues), mineral chemistry (Ba-Sr-poor calcite composition), whole-rock chemistry (overall low incompatible element content in the pure carbonate fraction and a monotonous trace element negative correlation with CaO) as well as isotopic constraints (perfect correlations between Sr-Nd-Pb isotopic ratios with CaO in the basaltic and carbonate fraction, as well as heavy δ18O and light δ13C isotopic composition of the carbonate fraction), we propose a secondary origin for the carbonate component, excluding any contribution of mantle carbonatite melts. The presence of carbonates infiltrating the abundant mantle and crustal xenolith fragments found in the pyroclastic breccia is not related to the presence of carbonatitic melts at mantle to lower crustal depths, but to in-situ fragmentation of the Strombolian tuff-breccia deposit, followed by secondary carbonate infiltration. The pyroclastic breccia was indeed affected by an alternation of carbonate precipitation and dissolution in a vadose zone, where the activity of bacteria, fungi, roots and meteoric water led to the formation of a calcrete (caliche)-type deposits. Basaltic rocks (hawaiites and basanites) occur in the area as scoria and lava fragments in the pyroclastic breccia as well as small lava flows. They have been modelled with a low-degree partial melting of an amphibole-bearing peridotitic mantle close to the lithosphere-asthenosphere boundary. The origin of the mildly alkaline sodic basaltic activity in SE Spain post-dates the abundant and long-lasting subduction-related volcanic phase in the Betic Chain. Its origin is explained without requiring the presence of any thermal anomaly, but simply as consequence of the difference of lithospheric depths and edge-driven-type small-scale convection. • Carbonate-rich volcaniclastic breccias have been found in Cabezo Negro de Tallante volcano in SE Spain. • The whitish carbonate matrix of the breccias has been interpreted in literature as a carbonatitic melt. • This work re-interprets this matrix as a sedimentary component of calcrete-type, unrelated to any • carbonatite involvement. [ABSTRACT FROM AUTHOR]