Journal articles on the topic 'Olivine Trachyte'

AbstractThe paper discusses the mineralogy of eruptives from the Nandewar Volcano, which range in composition from hawaiite and trachyandesite to comendite via tristanite and mafic and peralkaline trachyte. Olivine, Ca-rich pyroxene, and amphibole display marked decreases in 100 Mg/(Mg + Fe) ratios in the sequence trachyandesite to comendite, reflecting variation in host rock compositions. The presence of tscher-makitic subcalcic pyroxene and aluminous bronzite megacrysts in several trachyandesites indicates that these experienced intratelluric crystallization at elevated pressures (6–8 kbar). Some titanomagnetite and plagioclase phenocrysts in trachyandesites may also be moderate pressure cognate precipitates. Groundmass pyroxenes of some trachytes and comendites are strongly acmitic. The presence or absence of coexisting alkali amphiboles and aenigmatite appears to reflect stability over a relatively broad range of fO2 conditions. Aenigmatite rims on titanomagnetite and ilmenite microphenocrysts in several peralkaline eruptives provides support for a ‘no-oxide’ field in T-fO2 space. The Fe-Ti oxide compositional data indicate that magmas spanning the spectrum trachy-andesite-comendite crystallized under conditions of decreasing T and fO2 which broadly coincided with the FMQ synthetic buffer curve. However, a voluminous group of slightly older associated rhyolites appear to have crystallized under significantly more oxidizing conditions.

The Itcha Volcanic Complex is the youngest and easternmost felsic shield volcano of the Anahim Volcanic Belt of central British Columbia. The main body of the shield erupted over an area of ~300 km2 forming Itcha Mountain and Mount Downton. Volcanism associated with the Itcha Shield extended 20 km south to the Satah Mountain area, where lavas erupted along a north-northwest – south-southeast fault system and covered an additional area of 250 km2. The Itcha Volcanic Complex is characterized by a bimodal population of volcanic rocks, which are dominated by felsic lavas. There were two stages of volcanism: (i) an early felsic shield-building stage dominated by felsic lavas ranging in composition from phonolite to minor quartz-normative trachytes, which erupted as flows, domes, and pyroclastic deposits to form a low-angled shield; and (ii) a late mafic capping stage, which comprises a thin veneer of hawaiite and more primitive mafic lavas ranging in composition from alkali olivine basalt to basanite. The late mafic capping stage lavas erupted from satellite cinder cones and fissures concentrated on the eastern side of the shield.The hawaiites that dominate the late mafic capping stage cannot have been derived from the more primitive basalts with which they are associated by low-pressure crystal fractionation but may instead have originated from the fractionation of a clinopyroxene-dominated assemblage at high pressures. The presence of mafic xenocrysts in a megacrystic trachyte unit, whose eruption terminated the felsic shield-building stage, and anorthoclase xenocrysts in the most evolved alkali olivine basalts of the mafic capping stage indicate that the mafic and the felsic magmas interacted prior to eruption. An overlap in 87Sr/86Sr ratios and a similarity in the high-field-strength element ratios of the felsic and the mafic lavas suggest that they are genetically related. Elevated ratios of large-ion lithophile elements to high-field-strength elements (e.g., Rb/Zr) in the trachytes, however, indicates that the felsic magmas were not derived by closed-system fractional crystallization from the mafic magmas and may instead suggest the assimilation of a crustal component.

ABSTRACTPalaeocene volcanic activity is represented in west-central Skye, Inner Hebrides, Scotland, by a laterally extensive and thick pile of sub-aerial lavas mainly belonging to the alkali olivine basalt—hawaiite—mugearite—benmoreite—trachyte suite. The lavas are typical of many continental flood basalt suites and were principally fed from fissure eruptions similar to those of present day Iceland. Intercalated with the lavas are rare beds of heterogeneous volcaniclastic material, including breccias, conglomerates, sandstones and mudstones. The sequence forms a major portion of a larger volcanic field preserved within the NNE-SSW-elongated ‘Sea of the Hebrides’ sedimentary basin.Significant hiatuses in the volcanic activity are marked by deep-weathering profiles and thin sedimentary sequences comprising mudstones, ironstones, coals, sandstones and conglomerates. Palaeocurrent indicators and clast lithologies within the clastic sedimentary rocks indicate that erosion of a massif dominated by the Palaeocene Rum Igneous Complex and its roof rocks, c. 20 km to the S, provided abundant detritus to a river system which drained towards the N. Such sedimentary intercalations aid the stratigraphical subdivision of the lava field. Eight lava groups, each most likely with a different focus of fissure eruption, and divisible into mappable formations, together with two sedimentary formations, are recognised.The alkali olivine basalts are typically thin, with a tendency to form compound flows with limited lateral extents, whilst the hawaiites and mugearites are considerably thicker and cover large areas. Only very rarely are flow terminations observed. The original extents of the single benmoreite and rare trachytes cannot be determined from their limited erosional remnants. The more evolved flows tended to occur after brief hiatuses in the volcanic activity, indicated by well-developed lateritic tops to the underlying flows.The youngest preserved lava is a columnar-jointed olivine tholeiite with a MORB-like composition. The flow is at least 120 m thick and apparently ponded in a steep-sided palaeo-valley within the lava field.Three fault trends are recognised: parallel, normal and marginally oblique to the main NW-SEtrending regional dyke swarm, and dissect the lava field into a number of discrete blocks. The more significant of these faults may have been active during the development of the lava field, and in some instances instrumental in controlling the distribution of the flows.Later Tertiary erosion has removed an unknown thickness of material from the upper part of the lava field, the preserved thickness of which is estimated to be about 1·5 km.

AbstractMiddle Miocene volcanic activity in the Afyon volcanic province (eastern part of Western Anatolia) is characterized by multistage potassic and ultrapotassic alkaline volcanic successions. The volcanism is generally related to the northward subduction of the African plate beneath the Eurasian Plate. In Afyon, the Middle Miocene volcanic products consist of melilite leucitite, tephriphonolite, trachyte, basaltic–trachyandesite, phonolite, phonotephrite, tephriphonolite and lamproite rocks. Near-surface emplacement and relatively quiescent subaerial eruptions of lamproitic magma produced different emplacement forms such as dome/plug-shaped bodies and lava flows, showing variation in volume and texture. The mineralogical constituents of the lamproites are sanidine, olivine (77 < Mg no. < 81), phlogopite (74 < Mg no. < 78), K-richterite, clinopyroxene (74 < Mg no. < 78), with accessory apatite, calcite and opaque minerals. Afyon lamproites resemble Mediterranean-type Si-rich lamproites. Their compositional range is 50–52 wt% SiO2, 4–8 wt% MgO, and they display a typical lamproitic affinity. Chondrite-normalized REE patterns exhibit enrichment in LREE relative to HREE ((La/Yb)CN=15.3–17.0). They show extreme enrichment in LILE relative to primitive mantle values and troughs of Nb and Ti. The lamproites give a range of high initial87Sr/86Sr ratios and low143Nd/144Nd ratios. The geochemical and isotopic characteristics suggest that lamproitic magma is derived from highly metasomatized mantle. The enrichment history may include metasomatic events related to subduction, as in other active orogenic areas of the Mediterranean.

Abstract The Cameroon Line was created by the rejuvenation, at the beginning of the opening of the Atlantic Ocean, of a Pan-African N070 degrees E fracture zone [Moreau et al., 1987], which acted as a huge lithospheric crack taping a hot asthenospheric zone [Deruelle et al., 1998; Marzoli et al., 2000]. The Kokoumi anorogenic pluton belongs to the E-W Garoua rift structure, which represents the easternmost extension of the Benue trough. The Garoua rift opened during the Neocomian-Lower Aptian ages [Benkhelil, 1988] through the rejuvenation of Pan-African normal faults. The rift subsided, was partially filled by conglomerates and sandstones, and the ensemble was folded in the Cretaceous period [Guiraud, 1993]. Post-Cretaceous faulting affected these sediments. Intrusion of the Kokoumi anorogenic complex through the Cretaceous sandstones was favoured by N-S, N070 degrees E, E-W and N135 degrees E faults and N030 degrees E extension [Moreau et al., 1987]. The Kokoumi complex was first described by Koch [1959]. It is composed of a plutonic gabbro-nepheline monzosyenite-nepheline syenite series and of lamprophyric dykes (monchiquites and camptonites). One trachyte dyke is also observed. The gabbros are olivine (Fo 70 )-, nepheline-, or kaersutite-bearing gabbros. They also contain Ti-Al-rich diopside, Ti-rich biotite, titanite, ilmenite, Ti-magnetite and apatite. The nepheline monzosyenites contain diopside, Fe-diopside, kaersutite, Fe-kaersutite, titanite and apatite. The nepheline syenites contain aegirine-augite, F-rich arfvedsonite and aenigmatite. Kaersutite and clinopyroxene predominate in the lamprophyres. Monchiquites and gabbros, camptonites and monzosyenites, display respective similar mineralogy. Monchiquites contain carbonate ocelli. The trachyte does not contain ferromagnesian minerals. For gabbros and monchiquites, equilibrium Fe-Ti oxide temperatures are between 650 and 750 degrees C (+ or -40 degrees C) and oxygen fugacities between 10 (super -15) and 10 (super -14) (+ or -0.5 X 10 (super -15) ) atmospheres, according to Spencer and Lindsley [1981]. Nepheline crystallized below 700 degrees C, according to Hamilton [1961]. All the rocks (except the trachyte) are nepheline normative (Ne 6 to Ne 40 ). Major and trace element distributions in MgO-element diagrams for the two series merge together into a single trend, from monchiquites to nepheline syenites. Nevertheless, the monchiquites trends have different slopes. We deduce the evolution from gabbros to nepheline syenites on the one hand and from monchiquites to camptonites on the other from primitive mantle normalized multi-element diagrams. Multi-element diagrams for the trachyte and the nepheline syenite are strictly similar. Patterns for Kokoumi gabbros are similar to those for basalts of the Kapsiki plateau [Ngounouno et al., 2000] and the Garoua rift [Ngounouno et al., 1997] with typical negative K and positive Zr and Ti anomalies. Patterns for nepheline monzosyenites display negative anomalies in Sr, P, Eu and Ti and those for nepheline syenites and trachyte display greater anomalies in these elements and Ba. Compared to gabbros, nepheline monzosyenites are enriched in all REE with a concave upward pattern and no Eu-anomaly. Nepheline syenites have a range of broadly similar REE patterns to nepheline monzosyenites with steep slope from La to Sm, strong Eu negative anomaly (Eu/Eu (super *) nearly equal 0.15) and heavy-REE spoon-shape. REE patterns for monchiquites, camptonites, and trachyte are respectively similar to those for gabbros, monzosyenites, and nepheline syenite. Initial Sr-isotope ratios of 0.7033 (recalculated from the measured ratios for an age of 39 Ma for plutonic rocks and 20 Ma for the lamprophyres and the trachyte) are similar to those obtained for basalts from the continental segment of the Cameroon Line [Halliday et al., 1988; Ngounouno et al., 2000; Demaiffe et al., unpubl.], whereas nepheline syenites and trachyte are distinctly more radiogenic with values between 0.7128 and 0.7251. Amphibole and whole-rock K-Ar analyses (table III) yield 39.0+ or -0.9 Ma and 36.6+ or -0.9 Ma respectively. Since amphibole is a reliable chronometer in K-Ar dating, we propose the first age as the probable time of emplacement of the gabbros. Whole-rock analysis of nepheline syenite 99 displays an age of 33.1+ or -0.5 Ma. Field and geochemical observations suggest that gabbros and nepheline syenite are cogenetic and hence contemporaneous.

An ENE-WSW trending swarm of Gardar dykes, traversing Mellemlandet and G. E Holm Nunataq is principally composed of a 'main series' with compositions ranging from alkali olivine basalt to trachyte and rhyolite, and scarcer phonolitic trachyte associates. The most basic 'main series' magmas were emplaced as several giant dykes up to 650 m wide. Synformally layered gabbroic and anorthositic cumulates are locally developed within these. At Syenitknold internal differentiation within a giant dyke gave rise to syenogabbros, layered syenite cumulates and peralkaline nepheline syenite pegmatites. A large xenolithic mass of exotic feldspathic gabbro within the syenites is ascribed to the foundering of feldspar-rich roofing facies into the underlying magma chamber. Less extreme differentiation in the same giant dyke east of Syenitknold produced syenogabbroic cumulates containing evidence for vigorous convective flow having developed in the cooling intrusion. Smaller (< 40 m wide) and younger dykes are almost invariably of more differentiated character. The commonest dykes ( < 15 m wide) are of benmoreite and trachyte. Dykes with their interiors crowded with plagioclase xenocrysts and anorthositic inclusions are referred to as 'big feldspar dykes' (B.F.D.s). While all compositions from basalt to benmoreite may be involved in the B.F.D.s, the B.F.D. character is typical of the hawaiites and mugearites. Small (typically < 1 m), scarce dykes and sills of highly silica-undersaturated types range from ultramafic lamprophyres to carbonatites. These may be representative of a compositional continuum between 36 and 2 wt % SiO2,. The main swarm is so closely similar to that seen to the WSW, extending through Tugtutoq and the Narssaq and Qagssiarssuk areas, that it is thought to be merelya faulted continuation ofthe latter. Itso, this swarm, c. 15 km across, is at least 140 km long. The magnitude and extent of this alkaline swarm and its individual components, may well be unique: it differs from other swarms (e.g. that of the roughly contemporaneous Nunarssuit-Isortoq swarm) in the size and abundance of the salic dykes within it. It was almost certainly related to extensive fissure eruption of basic to salic lavas. A clockwise change of several degrees between the orientation of early giant dykes and later differentiated dykes is related to a change in the extensional stress direction during the development of the Gardar rift system.

AbstractInaccessible Island is the eroded remnant of an extinct, comparatively small intraplate volcano dominated by flows of alkaline olivine basalt. The oldest stratigraphie unit is a hydrothermally altered basement of somewhat questionable early Pliocene (6.5 Ma) age. This is unconformably overlain by a volcanic superstructure built up during the last three million years. The two formations have different trace element signatures that may be attributed to different mantle sources. Boulders of gabbro are common but the presence of an in situ plutonic intrusion could not be confirmed. Their K-Ar age of 12.8 Ma may be spurious and their possible relationship with the volcano is uncertain. Reliable age determinations of 0.95–0.72 Ma were obtained on lava flows of the second volcanic stage, subdivided into four units or stratigraphie members. The latest unit consists of plugs, sills and flows of an evolved magma fraction (benmoreite and trachyte) of which benmoreite is considered to be the more voluminous. Several dyke swarms of different ages reveal the internal structure of the volcano. It is concluded that the main volcanic centre was located immediately offshore to the northwest and that the edifice was attached to an east–west volcanic rift zone. Apart from marine erosion, massive land-sliding probably took part in shaping the island and its submarine platform.

Extensional tectonics and incipient rifting on the north side of the Iapetus suture were associated with eruption of (mainly) mildly alkaline olivine basalts. Initially in the Tournaisian (Southern Uplands Terrane), magmatic activity migrated northwards producing the Garleton Hills Volcanic Formation (GHVF) across an anomalous sector of the Southern Uplands. The latter was followed by resumption of volcanism in the Midland Valley Terrane, yielding the Arthur's Seat Volcanic Formation. Later larger-scale activity generated the Clyde Plateau Volcanic Formation (CPVF) and the Kintyre lavas on the Grampian Highlands Terrane. Comparable volcanic successions occur in Limerick, Ireland. This short-lived (c. 30 myr) phase was unique in the magmatic history of the Phanerozoic of the British Isles in which mildly alkaline basaltic magmatism locally led to trachytic differentiates. The Bangly Member of the GHVF represents the largest area occupied by such silicic rocks. The most widespread lavas and intrusions are silica-saturated/oversaturated trachytes for which new whole-rock and isotopic data are presented. Previously unrecognized ignimbrites are described. Sparse data from the fiamme suggest that the magma responsible for the repetitive ignimbrite eruptions was a highly fluid rhyolite. The Bangly Member probably represents the remains of a central-type volcano, the details of which are enigmatic.

AbstractA succession of Viséan (mid- to late Holkerian) volcanic rocks up to 340 m thick is preserved in three fault-blocks at the south end of the Isle of Bute in the Firth of Clyde, Scotland. These rocks form part of the Clyde Plateau Volcanic Formation, which, in this area, disconformably overlies sandstones of the lower Millport Member of the Clyde Sandstone Formation. The lower part of the volcanic succession in south Bute,c. 140 m thick, corresponds to the lower Strathgryfe lavas of the Renfrewshire Hills. This part of the succession is composed dominantly of feldspar-macrophyric and feldspar-microphyric basaltic rocks and mugearites. It is present in all three fault-blocks, whereas the succeeding volcanic rocks (middle and upper divisions) are only preserved in the median St Blane's block where they have a combined thickness of about 200 m. The two younger subdivisions are respectively correlative to the Misty Law Trachytic Centre, which forms a lens between the lower and upper Strathgryfe Members, and the upper Strathgryfe Member of the North Ayrshire section. Lavas of the lower division are feldspar-macrophyric and feldspar-microphyric basaltic rocks and mugearites, but those of the middle and upper divisions display a wider compositional spectrum, including feldspar-macro- and microphyric rocks but ranging from olivine-augite-macrophyric and olivine-augite-feldspar-macrophyric basalts to trachytes. The mafic lavas of south Bute have chondrite-normalized multi-element plots similar to those of ocean island basalts, with enrichment in incompatible elements. The trachytic lavas have similar patterns but are strongly depleted in Sr, P and Ti, reflecting fractionation of such minerals as plagioclase, apatite and magnetite/ilmenite during evolution of the parent magmas. Distribution of high field strength elements favours a within-plate origin for the south Bute lavas and supports derivation from a relatively deep (>50 km) mantle source (garnet lherzolite). Chondrite-normalized REE plots for basaltic lavas of the lower division show enrichment in LREEs and lack strong Eu anomalies. Strong positive Eu anomalies in both felsic and mafic lavas of the middle and upper divisions may be attributable to high oxygen fugacities, but hydrothermal activity or feldspar fractionation may also have played a role. Fe-rich weathering profiles attest to intermittent extrusion and intense weathering processes.

AbstractThe lava domes in the northwestern (Cuma), northern (Punta Marmolite) and central (Accademia) parts of the Phlegrean Fields are the subject of this study. The Cuma and Punta Marmolite trachyphonolitic lava domes are among the oldest Phlegrean products cropping out. The Cuma rocks have an agpaitic groundmass, with early alkali feldspar, Fe-rich clinopyroxene, Fe-edenite and sodalite and late rosenbuschite, fluorite, baddeleyite, pyrochlore, britholite, monazite, aegirine (often Zr-rich) and exceptionally Fe–Mn-rich olivine. The bulk-rock compositions at Cuma have some of the highest concentrations of Zn, Mn, Zr, Nb, Th, U and lanthanides among the Phlegrean Fields rocks, and some of the lowest MgO, P2O5, Sr, Eu and Ba. The Punta Marmolite dome is chemically less evolved, and lacks characteristic agpaitic minerals, but features alkali feldspar, sodalite, nepheline and relatively Na-poor, Fe-rich hedenbergite, with rare Ca-rich plagioclase xenocryst cores. The Accademia dome, belonging to the recent activity, is latitic to trachytic in composition, has highly forsteritic olivine (with chromiferous spinel inclusions), calcic plagioclase and Mg-rich diopside (± phlogopite) xenocrysts in an evolved host rock (with phenocrysts and microlites of alkali feldspar, Fe-rich clinopyroxene, Fe-rich amphibole, magnetite, Fe-rich olivine and accessory baddeleyite, zirconolite and fluorite). There is clear evidence of open-system magma crystallization in the form of interaction between a crystallizing, primitive shoshonitic basalt in a reservoir already filled by rather evolved trachytic magma. The magmatic evolution towards the evolved compositions is dominated by crystallization of more and more Na-rich alkali feldspar in a Cl-, F-rich and relatively H2O-poor environment. Input of mafic magma is evident in many trachytic eruptions of the Phlegrean Fields and even in the products of the Campanian Ignimbrite, but eruptions having mineral assemblages rich in xenocryst phases as well as eruptions virtually free of mafic magma input are also frequently observed throughout the history. This suggests a variable pattern of open- and closed-system crystallization, which may or may not be linked to explosive activity, and that can be caused by intermittent supply of basaltic magma from depth.

Upper Triassic volcanic and sedimentary rocks of the Mamonia Complex in southwestern Cyprus are exposed in erosional windows through the post-Cretaceous cover, where the Mamonia Complex is tectonically imbricated with the Troodos and Akamas ophiolitic suites. Most of these Upper Triassic volcanic rocks have been considered to represent remnants of Triassic oceanic crust and its associated seamounts. New Nd and Pb isotopic data show that the whole Mamonia volcanic suite exhibits features of oceanic island basalts (OIB). Four rock types have been distinguished on the basis of the petrology and chemistry of the rocks. Volcanism began with the eruption of depleted olivine tholeiites (Type 1) and oceanic island tholeiites (Type 2) associated with deep basin siliceous and/or calcareous sediments. The tholeiites were followed by highly phyric alkali basalts (Type 3) interbedded with pelagic Halobia-bearing limestones or white reefal limestones. Strongly LREE-enriched trachytes (Type 4) were emplaced during the final stage of volcanic activity. Nd and Pb isotopic ratios suggest that tholeiites and mildly alkali basalts derived from partial melting of heterogeneous enriched mantle sources. Fractional crystallization alone cannot account for the derivation of trachytes from alkaline basalts. The trachytes could have been derived from the partial melting at depth of mafic material which shares with the alkali basalts similar trace element and isotopic compositions. This is corroborated by the rather similar isotopic compositions of the alkali basalts and trachytes. The correlations observed between incompatible elements (Nb, Th) and εNd and Pb isotopic initial ratios suggest that the Mamonia suite was derived from the mixing of a depleted mantle (DMM) and an enriched component of High μ (μ = 238U/204Pb, HIMU) type. Models using both Nd and Pb isotopic initial ratios suggest that the depleted tholeiites (Type 1) derived from a DMM source contaminated by an Enriched Mantle Type 2 component (EM2), and that the oceanic tholeiites (Type 2), alkali basalts (Type 3) and trachytes (Type 4) were derived from the mixing of the enriched mantle source of the depleted tholeiites with a HIMU component. None of the Mamonia volcanic rocks show evidence of crustal contamination. The Upper Triassic within-plate volcanism likely erupted in a small southerly Neotethyan basin, located north of the Eratosthenes seamount and likely floored by oceanic crust.

L'île de Fayal est composée de quatre unités volcano-structurales, le Galégo, le volcan central, le graben et les zones fissurales. Les trois premières d'entre elles se caractérisent chacune par une séquence pétrographique allant de basaltes alcalins porphyriques à plagioclase, à des trachytes. Les variations minéralogiques et géochimiques (majeurs et traces) sont cohérentes avec un modèle de différenciation par cristallisation fractionnée de plagioclase–salite–olivine–titanomagnétite–kaersutite–apatite, opérant à partir de liquides primaires de compositions voisines. Cependant, les benmoréïtes du volcan central, essentiellement post-caldeiriques, correspondent à des mélanges entre un magma basaltique d'origine profonde remonté à la faveur de l'ouverture du graben, et un magma déjà différencié situé dans la chambre magmatique. Les zones fissurales émettent exclusivement des basaltes alcalins et des hawaïtes porphyriques à olivine et clinopyroxène. Les variations des teneurs en Cr, Co, Ni, Sc, MgO et Al2O3 traduisent le fractionnement du couple olivine–clinopyroxène. Les variations du rapport La/Lu montrent que ces laves dérivent de liquides primaires ayant des teneurs en terres rares lourdes légèrement différentes et traduisant des proportions de grenat résiduel variables.Les liquides primaires de ces quatre unités proviennent d'une source mantélique géochimiquement homogène qui se distingue nettement de celles des autres îles. Il est probable que l'évolution volcanologique de Fayal soit étroitement gouvernée par le jeu de la fracture à composante distensive d'orientation N1 15°.

AbstractMagmas in Faial Island, Azores (Portugal), were mostly erupted from two fissure zones and the Caldeira central volcano during overlapping periods. The fissure zones follow extensional trends oriented WNW and ESE and erupted nepheline- to hypersthene-normative basalts and hawaiites. The Caldeira central volcano builds the central part of the island, which is cut by the fissure zones. Ne-normative basalts show similar high-field-strength element (HFSE) concentrations but higher large ion lithophile element (LILE) concentrations than hy-normative equivalents. Primitive melts were generated by small (3–5%) degrees of partial melting of garnet-bearing peridotite, variably enriched in incompatible elements. Overall, basalts from Faial show relatively higher LILE abundances and LILE/HFSE ratios than those of the other islands of the Azores and of many other volcanoes in the Atlantic area. This feature indicates the existence of chemical heterogeneities in the mantle sources characterized by variable degrees of metasomatism, both at local and regional scales. Hawaiites evolved from basalts through 30–40% fractional crystallization of mafic phases plus some plagioclase, in deep reservoirs, at about 430–425 MPa (~ 15 km). The Caldeira central volcano rocks range from basalts to trachytes. Basalts, produced under similar conditions as fissure basalts, evolved to trachytes through large degrees of polybaric fractional crystallization (100–760 MPa; i.e. ~ 3.6–26 km), involving olivine, clinopyroxene, feldspar and minor quantities of amphibole, biotite, apatite and oxides. In contrast, mafic magmas from the fissure zones were erupted directly onto the surface from magma reservoirs mainly located at the crust–mantle boundary.

Abstract The Salpeterkop volcano forms part of what has been referred to as the Upper Cretaceous Sutherland Suite of alkaline rocks, an igneous province composed of olivine melilitites, carbonatites, trachytes and ultramafic lamprophyres. Salpeterkop is a remnant of the summit tuff ring structure that surrounds a crater which is almost 1 km in diameter and is filled with epiclastic strata. Five pieces of silicified wood were collected from the crater filled sediments, sectioned and identified as a new species of Cupressinoxylon, C. widdringtonioides. This is the first example of the fossil genus in South Africa. Only one member of the Cupressaceae s.l. occurs in southern Africa today. From the wide and indistinct growth rings in the fossil wood it can be deduced that the local climate was warm and humid with little or no seasonality, in support of global records of a warm Late Cretaceous. The preservation of the crater further signifies the low level of erosion the region has experienced since its emplacement.

The present study is a multi-analytical approach to the characterization of several basalt stone samples, from Umm el-Jimāl Cultural Heritage site, northern Jordan, which represents ancient trade routes between Arabian Peninsula and Syria. In particular, Micro-X-ray Fluorescence Mapping as new in the mineralogy, X-ray Diffraction Energy Dispersive X-ray spectroscopy are used for the determination of the petrological, geochemical, and mineralogical characteristics of the basalt used in this archeological site for conservation purposes. The results reveal that it belongs to the alkaline trachy-basalt to basanite-tephrite type. With predominant quartz, olivine, pyroxene, and plagioclase (albite) as major elements, the vesicular texture is occupied with secondary minerals such as biotite, gypsum, and calcite.

Lamongan volcano is one of the unique volcanoes in the Sunda Volcano. This volcano has side eruption centers or on the slopes of the volcano. The morphology of parasitic eruptions in this volcanoes complex includes maars and boccas. There are about 64 parasitic eruption centers consisting of 37 volcanic cones (bocca) and 27 ranu (maar). The purpose of this research is to study the characteristics of lithology and petrogenesis of this volcano complex, especially in Ranu Pakis and surrounding areas. The analytical method used is to do geological mapping and petrographic analysis. The lithology found in this research area consists of magmatic and phreatomagmatic eruption deposits. Genetically this lithology includes pyroclastic flow, pyroclastic fall (scoria fall and phreatomagmatic scoria fall/accretionary lapili), tuff (phreatic) and basaltic lava. In some pyroclastic deposits, especially in maar there are fragments of accretionary lapilli, while in bocca there are basaltic lavas. Other fragments present in pyroclastic deposits are basalt scoria blocks and bombs embedded in the groundmass of volcanic ash. The results of petrographic analysis indicate that the volcanic rocks in the study area are calc alkaline affinity consisting of pyroxene andesite, basalt and pyroxene basalt lava. The pyroxene basalt lava is composed by plagioclase, clinopyroxene and little olivine embedded in the volcanic glass. Lavas are structured scoria and textured porphyritic, intersertal, trachytic, aphyric and pilotaxitic. Trachytic texture is found in the basalt fragments of pyroxene from the pyroclastic fall deposits in Ranu Pakis and Ranu Wurung. While pyroxene andesite lavas composed by plagioclase, clinopyroxene embedded in the volcanic glass. Lavas are structured scoria and textured porphyritic, intergranular, pilotaxitic and aphyric.

Abstract The Island of Linosa is a small part of the large submarine volcanic complex which is locatedat the SW edge of the Linosa Graben, Sicily Channel. The island was formed between 1.06±0.10 and 0.53±0.07 Ma, through three main stages of activity: Paleo-Linosa, Arena Bianca and Monte Bandiera. Major and trace element data show that the compositional variability of the three activity stages is limited, with most of the rocks showing basaltic to hawaiitic composition. Evolved benmoreites and trachytes are foundas lithics in some pyroclastic units of Paleolinosa. The mafic rocks of the three stages show porphyritic texture, with phenocryst assemblages characterized by olivine, clinopyroxene and plagioclase. The volume ratio of olivine vs. clinopyroxene decreases from early to late stages of activity in mafic rocks with comparable major element composition. Clinopyroxene phenocrysts from mafic rocks of the three stages have poorly variable composition, clustering in the augite field. Phenocrysts from the first activity stage (Paleo-Linosa), show a slight increase in TiO2, Al2O3 and CaO, and a decrease of Fe2O3 (total) with the increasing SiO2 content of the host rocks. Crystals from the second and the third stage (Arena Bianca and Monte Bandiera) display a slightly more restricted range of FeOtot, frequently with very high MgO, Al2O3 and TiO2 contents. Crystal chemical investigation of clinopyroxenes from rocks of the three stages with comparable degrees of evolution, revealed significant variation of structural parameters, in particular VM1 and Vcell. These show a consistent decrease, passing from clinopyroxenes of the early stage to crystals extracted from the mafic lavas of stages 2 and 3. Given the similar compositional ranges of the host rocks, structural variations of clinopyroxenes are interpreted to reveal modifications of crystallization pressure, which increased, passing from Paleo-Linosa to the Arena Bianca and Monte Bandiera stages. Given this information, the observed crystal-chemical variations provide information on the depth of magma reservoirs and on the evolution of the plumbing system of Linosa volcano.

AbstractThe Maimecha-Kotui province in the North of Siberian platform is the largest province of ultramafic alkaline rocks in the world. The province comprises thirty-seven central-type complexes together with numerous dykes. The majority of dykes are radially disposed around the ultramafic alkaline massifs. Data are presented for dykes of the Dolbykha carbonatite complex, which comprises olivine and melilite nephelinites; nosean, calcite and cancrinite phonolites; calcite trachytes and calcite carbonatites.Some peralkaline phonolitic dykes contain carbonate-bearing globules with sizes of 1−2 mm to 17−20 mm. Globules consist of polycrystalline calcitic aggregates together with albite, phlogopite, apatite, Sr-lueshite, zircon, ancylite, ilmenite and strontianite. The phonolites have phenocrysts of albite, phlogopite and ilmenite. Albite, phlogopite, calcite and nepheline are also present in the groundmass. Analysis of these materials in the light of experimental data on the liquid immiscibility in carbonate-silicate systems suggests that separation of carbonatite from phonolitic melts took place due to immiscibility in the liquid state. I propose that carbonate melts contained originally significantly higher alkali contents which were subsequently lost into the fluid phase due to the incongruent dissolution of calcium-sodium carbonates in aqueous fluid at low temperatures. The discovery of nyerereite in the carbonatite of Polar Siberia confirms this conclusion. I infer that one of the mechanisms for the genesis of carbonatite melt in Polar Siberia was liquid immiscibility in strongly differentiated phonolitic magmas.The generation of the carbonatites was probably controlled by the depth (and PCO2) of the crustal magma chamber where differentiation took place and probably also by the alkalinity of melts, and the rapidity of magma ascent to the surface.

Nd isotopic composition has been determined for 16 igneous rocks, representing the wide geochemical, spatial and temporal range of post-collisional, late Cenozoic magmas in the Aegean area. Nd isotopes are used to further interpret previously published Pb and Sr isotope data. The overall pattern of late Cenozoic volcanism resulted from rapid extension, with thermal effects causing melting of hydrated, enriched, subcontinental lithosphere to produce widespread K-rich magmas. Slab break-off and intrusion of hot asthenosphere caused partial melting of rift-related continental margin basalts at the detachment point to generate adakitic magmas. Further outboard, mafic magma from enriched lithospheric mantle melted thickened lower crust to produce the granitoid plutons of the Cyclades. Nd isotopic variation in these varied rock types correlates with pre-Cenozoic palaeo-geography. Proterozoic subduction-related enrichment in Th and U, together with other large-ion lithophile elements, produced distinctive Pb isotope composition. This was later modified where Mesozoic subduction of terrigenous sediment was important, whereas subduction of oceanic carbonate sediments produced enrichment in radiogenic Sr and low Ce/Sr ratios. Late Cenozoic magmas sourced in eastern Pelagonian zone sub-continental lithospheric mantle have Nd model ages of about 1.0 Ga, and generally high 87Sr/86Sr and high 207Pb/204Pb (∼ 15.68) and 208Pb/204Pb (∼ 39.0) for low 206Pb/204Pb (∼ 18.6), but rocks to the west have more radiogenic Pb and higher Ce/Sr as a result of greater subduction of terrigenous sediment from the northern Pindos ocean. Magmas sourced from sub-continental lithosphere beneath the Apulian continental block were strongly influenced by subduction of oceanic crust and sediments north of the passive margin of north Africa. Subduction of Nile-derived terrigenous sediment in the east resulted in Nd model ages of 0.7 to 0.8 Ga and radiogenic Pb isotopes. Greater subduction of oceanic carbonate in the west resulted in magmas with higher 87Sr/86Sr and lower Ce/Sr. The strongly negative εNd for adakites in the central Aegean rules out a source from subducted oceanic basalt, and the adakite magma was probably derived from melting of hydrated Triassic sub-alkaline basalt of continental origin. Where trachytic rocks are succeeded by nepheline-normative basalts (e.g. Samos), Nd isotope data imply that early partial melting of the enriched subcontinental lithospheric mantle involved hydrous amphibole and phlogopite, but once these minerals were consumed, younger magmas were produced by partial melting dominated by olivine and orthopyroxene.

Abstract The Saint-Gilles breccias, on the western flank of Piton des Neiges volcano, are clearly identified as debris avalanche deposits. A petrographic, textural and structural analysis of the breccias and inter-bedded autochthonous lava flows enables us to distinguish at least four successive flank slides. The oldest deposit sampled the hydrothermally-altered inner parts of the volcano, and has a large volume. Failure was favored by the presence of a deep intensely-weathered layer. The younger deposits are from superficial sources, as their products are rarely hydrothermalized and are more vesicular. The breccia formation, and especially the progressive breaking up occurring during the debris avalanche displacement, indicates the existence of high speed transport. In the Cap La Houssaye coastal area, abrasion and striation of the underlying lava formation, as well as the packing features observed in the breccia, are considered to be deceleration structures. Introduction Huge landslides of volcano flanks, whether or not initiated by magmatic intrusions, have been recognized as catastrophic events since the 1980 Mount St Helens eruption. On oceanic shield volcanoes, the contribution of failure to the edifice-building process was proposed by Moore [1964] and suggested elsewhere for Hawaii [Lipman et al., 1985 ; Moore et al., 1989], Reunion island [Lénat et al., 1989], Etna [McGuire et al., 1991], and Canarias [Carracedo, 1994, 1996 ; Marty et al., 1996]. This contribution is particularly obvious in island volcanoes showing a U-shaped caldera open to the ocean. Several mechanisms inherent to the causes of failure have been proposed, such as dyke intrusion [McGuire et al., 1990 ; Iverson, 1995 ; Voight and Elsworth, 1997], caldera collapse [Marty et al., 1997], or volcanic spreading [Borgia et al., 1992 ; van Wyk de Vries and Francis, 1997]. Invariably, other factors have been proposed as favorable to volcanic destabilization, such as the probable occurrence of deep low-cohesion layers due to the existence of pyroclastic or hyaloclastic layers [Duffield et al., 1982 ; Siebert, 1984] or an old basement. Gravity spreading models are now frequently proposed to explain the destruction of volcanic edifices [Borgia et al., 1992 ; Merle and Borgia, 1996 ; van Wyk de Vries and Borgia, 1996 ; van Wyk de Vries and Francis, 1997], most of them taking into account basal or intra-volcanic weakness zones. We propose that in such a scenario, density heterogeneity should be an important factor governing the slow evolution of the volcanic pile. Clague and Denlinger [1994] proposed a olivine-rich ductile basal layer that influences the stability of volcano flanks. On Reunion island, a large volcanic landslide has been proposed to explain the peculiar morphology of Piton de la Fournaise-Grand Brûlé [Vincent and Kieffer, 1978]. Bathymetric surveys [Bachèlery and Montagionni, 1983 ; Lénat et al., 1989, 1990 ; Cochonnat et al., 1990 ; Lénat and Labazuy, 1990 ; Labazuy, 1991 ; Bachèlery, 1995 ; Ollier et al., 1998] have confirmed the offshore occurrence of debris avalanche deposits. Similar deposits are also known to exist along the western, northern and southwestern submarine flanks of the Piton des Neiges volcano. Unlike other deposits showing inland prolongation, “Saint-Gilles breccias” displays a well-preserved and non-weathered texture and structure. Because of striking analogies between the “Saint-Gilles breccias” and, for example, the Cantal stratovolcano debris avalanche deposits [Cantagrel, 1995], we conclude that these formations are the products of repeated avalanches during the Piton des Neiges basaltic period [Bachèlery et al., 1996]. We propose an interpretation of their origin, emplacement mechanism and their role in the evolutionary process of the western flank of Piton des Neiges. Volcano-structural setting Mechanical instability of oceanic volcanic edifices generates huge flank landslides, with lateral and mainly submarine transport of sub-aerial materials. These landslides participate in the building of the lower submarine slopes of the volcano. Geophysical surveys have detected low cohesion materials in most offshore Reunion island areas [Malengrau et al., 1999 ; de Voogd et al., 1999 ; Lénat et al., 2001] showing that these materials have largely contributed to the construction of offshore Reunion Island. Such deposits are also found in the inner part (“Cirques”) of Piton des Neiges [Maillot, 1999]. On the other hand, electric and electromagnetic soundings have revealed a deep extending conductor within the Piton de la Fournaise volcanic pile [Courteaud et al., 1997 ; Lenat et al., 2000]. Interpretations about the nature and origin of this conductor depend on its location. In the central caldera zone, as revealed by SP positive anomalies [Malengrau et al., 1994 ; Zlotnicki et al., 1994], the hydrothermal and magmatic complex is probably responsible for the observed low resistivities. Along the flanks, such a hypothesis may not be realistic. Courteaud [1996] suggests the occurrence of a deep argilized layer of volcano-detritic origin. In any case, the hydrothermal complex with high fluid pressures and secondary minerals appears as a potential weak zone that may contribute to the volcano’s instability [Lopez and Williams, 1993 ; Frank, 1995]. Chronology and stratigraphy Extent of the debris avalanche deposits The various breccias found at the western end of Reunion island, on the Piton des Neiges volcano flank, cover a 16 km2 area between Cap Marianne and Saint-Gilles (fig. 1). They are overlain upwards (&gt; 250 to 300 m) by trachyandesitic (mugearite) lava flows of Piton des Neiges differentiated series [Billard, 1974]. Some restricted breccia outcrops in deep valleys from Bernica to the north up to l’Hermitage to the south indicate the existence of larger extension of the debris avalanche deposits. Furthermore, breccias with similar “Saint-Gilles” facies appear down the Maïdo cliff to Mafate “Cirque” at an altitude 1300 m, beneath 600 m of mugearite and some olivine basalt flows. Unpublished electromagnetic data (CSAMT soundings) confirm the inland continuity of the “Saint-Gilles breccias” up to the Maïdo along the Piton des Neiges western flank, hidden by mugearitic flows. Available bathymetric surveys offshore Saint Paul – Saint Gilles areas show the obvious underwater prolongation of “Saint-Gilles breccias” : a shallow depth (&lt; 100 m) plateau followed by a slope with hummocky surface down to 2 500 m depth [Bachèlery et al., 1996 and fig. 2]. From this data, the total surface of “Saint-Gilles” debris avalanche deposits is estimated as more than 500 km2. Chronology A coastal cliff, from Ravine Bernica to Boucan Canot, provides the best outcrop of the northern part of “Saint-Gilles breccias”, with a clear inter-bedding of breccia units and lava formations (photo 1and fig. 3). – The lower breccia unit (Br I), of unknown thickness, has a remarkable friable aspect and a grayish color. – The first autochthonous lava formation (L1) consists in thin pahoehoe olivine basalt flows filling large valleys dug into “Br I”. The top of this formation is striated by the overlying “Br II” unit (photo 2). – Breccia unit “Br II” is interbedded between L1 and L2 olivine basalts. More compact and massive, “Br II” is characterized by a reddish matrix and dark blocks, with many curved fracture surfaces. – On “Br II” or directly on L1, picritic basalt flows L2 are found, filling narrow valleys. – Breccia unit “Br III” lies on “Br II” with a striking sheared contact plane visible along the main road (photo 3). It is a typical debris avalanche deposit with large imbricate blocks within a fine-grained beige matrix. – Once again, basaltic flows of lava formation L3 fill a valley dug into “Br III” near Petite Anse river. – Breccia unit “Br IV” rests on L3 at Petite Anse, but its contact with “Br III” elsewhere is not clear. The facies of this unit is very similar to the “Br III”. All the breccia units are covered by basaltic and trachyandesitic flows from the end of the Piton des Neiges basaltic series, and differentiated series. In the Saint-Gilles river, two formations are superposed : picritic basalts (L4) have flowed on the “Br IV” breccia unit, latter aphyric trachy-andesitic (mugearite) flows (L6) overlapped L4 and the breccia landforms, reaching in places the coastal area. To the north, at Plateau Caillou, plagioclase-phyric basalt flows (L5) are found between mugearite and breccias. Elsewhere on Piton des Neiges, such flows are symptomatic of the transition from the basaltic series to the differentiated series [Billard, 1974]. The occurrence of autochthonous basaltic formations L1 to L3, inter-bedded with “Saint-Gilles breccias”, enables us to distinguish at least four superposed breccia units. Although the emplacement age of the lower “Br I” is not known precisely, it is overlain and therefore older than Cap Marianne pahoehoe lavas (L1) dated at 0.452 Ma [Mc Dougall, 1971]. On the other hand, the upper breccia units are younger than the pahoehoe olivine basalt at Cap la Houssaye dated at 0,435 Ma but older than L5 plagioclasic basalts dated at 0.35 Ma. Geological description of the “breccia sequence” In the synthetic lithologic log (fig. 4) of the Saint-Gilles area, autochthonous lava formations are clearly broken into four separate breccia units. Lava formations. – L1 formation consists of numerous thin pahoehoe olivine-rich to aphyric basaltic flows. Both L2 and L3 formations are characterized by a few thicker (decametric) olivine (frequently picritic) basalt flows. Breccia units. – All breccia units display common characteristics such as the universal association of two facies (photo 4) : (i) a matrix – sandy to silty – facies containing a non-sorted mixture of non-stratified heterogeneous materials ranging from granular size to blocky elements, (ii) coherent large blocks and large pieces (‘block’ facies) of various lithology such as lava flow, scorias, pyroclastics or other breccias ; blocks displaying frequent “jigsaw” features. The lower breccia unit “Br 1” (fig. 4) has a more compact but very heterogeneous aspect, with a chaotic distribution of blocks in a less-developed matrix. This unit is characterized by a deep hydrothermal alteration with a lot of zeolites, chlorite, clays, calcite and oxides. The upper breccia units, “Br II” to “Br IV” (fig. 4) are less heterogeneous than “Br I” because their matrix facies are more voluminous and because the matrix clearly separates the bigger blocks. In both facies, a great diversity of fresh lithologic types such as picritic basalt, olivine-phyric basalt, plagioclase-phyric basalt and aphyric more or less vesicular basalts, gabbro, dunite are found, with no or only few slightly zeolitised blocks. Plurimetric to metric blocks are severely fractured, disintegrated into millimetric to decimetric angular pieces. The frequent polygenic aspect is due to block juxtaposition or imbrication. The abundant matrix is composed of crushed rocks and mineral elements, fine-grained (&lt; mm), showing frequent fluidity and bedding marks (photo 5). The very heterogeneous composition of the matrix is confirmed at a microscopic scale. On the contrary, cores of blocks appear as jigsaw-puzzle-like monolithologic pieces of various basaltic rocks. At their edges, disintegration leads to progressive mixing with neighboring blocks that feed the matrix. Discussion Originality of “Saint-Gilles breccias” “Saint-Gilles breccias” constitute one of the few cases [see also Cantagrel et al., 1999] of debris avalanche deposit outcroppings on the sub-aerial part of an oceanic shield volcano. The main part of the deposit is suspected to be offshore. Their hummocky surface in delineating parallel ridges can be compared to the one described offshore the Grand Brûlé area, east of Piton de la Fournaise [Bachèlery et al., 1996]. “Saint-Gilles breccias” were deposited after several Piton des Neiges flank slide events that were separated by basaltic flows. Repeated debris avalanches have also been proposed to explain Piton de la Fournaise offshore deposits [Lenat et al., 1990 ; Labazuy, 1991]. The occurrence of autochthonous interbedded lava formations is essential to interpret the thick piling up of slide material along Reunion volcano flanks as deposits of repeated avalanches at the same place, instead of as being the products of a single huge event. Many structural and textural features noticed in the upper breccia units reveal crucial information on the emplacement mechanism of debris avalanches. For instance, brecciated blocks are typical of progressive break-up during transport processes. Blocks can simply be fractured, or they can be so severely disintegrated that stretching and mixing with other blocks and matrix formation are observed. The observation of such phenomena implies the existence of numerous percussive events between rocks, as well as internal vibrations in the debris avalanche and therefore the existence of high-speed transport. Lava formations L1 underlying upper breccia units are truncated and strongly striated in a seaward direction (photo 2), parallel to the breccia morphological ridges. In the same way, internal contact surfaces between upper breccia units are shear planes underlain by cataclastic layers and lenses (photo 3). Such structures are interpreted as due to drastic deceleration effects of avalanches reaching a topographic leveling out in the coastal area. This concords with the occurrence of sub-vertical contact areas between the blocks and the matrix. These injections of matrix between the blocks are generated bottom-up from the shear plane at the moment of the sudden deceleration of the avalanche. Other fracture planes that are in accordance with the morphology of ridges, are found in “Br III” unit (see fig. 5). They are interpreted as the result of packing effects. Origin of flank failures Although the source area of breccia formations has not yet been clearly identified, it has to be in the central part of Piton des Neiges as seen in the western cliff of “cirque de Mafate”. Furthermore, “Br I” deeply weathered materials evidently come from the hydrothermalized core of the volcano. Though the “Br I” thickness is not known, the volume involved may be considerable and a part of this volume must constitute the main body of Saint-Gilles offshore deposits. The upper breccias units “Br II” to “Br IV” display very similar textures and lithologies, with dominant non-altered basaltic rocks from the “Phase II” building stage of Piton des Neiges [Billard, 1974]. These units are very thin in the coastal area of Cap La Houssaye (see fig. 2) despite a proximal facies (meaning a deposit in the transport zone nearer than the main deposit zone). They obviously originate from shallow flank slides of restricted extent. We suggest that the upper Saint-Gilles deposits are due to repeated events that produced thin high-speed debris avalanches. Emplacement modalities The morphology of “Saint-Gilles breccias”, or submarine deposits offshore Grand Brûlé (east of Piton de la Fournaise volcano), are typical of sliding movements along shallow depth shear planes (several hundred meters up to two kilometers) within the volcanic pile. But several levels of decollement are suggested by seismic refraction and reflection profiles offshore La Reunion, the deepest corresponding to the top of the preexisting oceanic sediments [de Voogt et al., 1999]. Until now, in Reunion Island, only shallow failures affecting the upper parts of volcanic edifices, with deposits on the lower slopes, have been positively identified. Conditions that trigger giant flank landslides affecting oceanic shields remain poorly understood but we can reasonably speculate that weak hydrothermally-altered layers in the inner part of the volcano favor these gravity-driven processes related to repeated dike injections. The “Saint-Gilles breccia” sequence is considered as a multiphase lateral collapse structure whose first event (“Br I”) was apparently the most voluminous. The corresponding deposit displays frequent hydrothermally-altered material symptomatic of originating from the Piton des Neiges core. Within Piton des Neiges, the low cohesive weathered layer is quite extensive [Nativel, 1978 ; Rançon, 1982] possibly reaching down the volcano flanks [Courteaud et al., 1997]. The interpretative scheme that we propose (fig. 6) in our evaluation of the conditions for the emplacement of Saint-Gilles sequence, takes into account the existence of such a mechanical discontinuity within the volcanic pile. We propose that the massive landslide failure of the west flank of Piton des Neiges volcano that produced the “Br I” breccia, provided efficient channels for younger Piton des Neiges lavas to reach the western and southwestern coastline. Morphological features, as well as radiometric data [Mc Dougall, 1971 ; Gillot and Nativel, 1982] and magnetic surveys [Lénat et al., 2001], yield evidence for preferential accumulation of lava during the last 0.5 m.y. (corresponding mainly to the differentiated series) in this part of the volcano. The relative asymmetry of Piton des Neiges was acquired by rift migration in response to the first huge landslide that produced the “Br I” unit of “Saint-Gilles breccia”, in the manner described by Lipman et al. [1990] for Mauna Loa volcano in Hawaii. The later repetition of flank collapses is consistent with similar structures on other oceanic islands. Since the first lateral collapse, the Piton des Neiges edifice was probably characterized by the existence of an asymmetrical steeper western flank where the old zeolite-rich “Br I” deposits possibly act as a detachment surface for later successive landslides which may have occurred recurrently over a short time interval.

Abstract The Quaternary Tarosero volcano is situated in the East African Rift of northern Tanzania and mainly consists of trachyte lavas and some trachytic tuffs. In addition, there are minor occurrences of extrusive basalts, andesites and latites, as well as peralkaline trachytes, olivine trachytes and phonolites. Some of the peralkaline phonolites contain interstitial eudialyte, making Tarosero one of the few known occurrences for extrusive agpaitic rocks. This study investigates the genetic relationships between the various rock types and focuses on the peculiar formation conditions of the extrusive agpaitic rocks using a combination of whole-rock geochemistry, mineral chemistry, petrography, thermodynamic calculations, and major and trace element modelling. The Tarosero rocks formed at redox conditions around or below the fayalite–magnetite–quartz buffer (FMQ). During multi-level magmatic fractionation at depths between ∼40 km and the shallow crust, temperature decreased from &gt;1100 °C at near-liquidus conditions in the basalts to ∼700 °C in the peralkaline residue. Fractional crystallization models and trace element characteristics do not indicate a simple genetic relationship between the trachytes and the other rock types at Tarosero. However, the genetic relationships between the primitive basalts and the intermediate latites can be explained by high-pressure fractional crystallization of olivine + clinopyroxene + magnetite + plagioclase + apatite. Further fractionation of these mineral phases in addition to amphibole and minor ilmenite led to the evolution towards the peralkaline trachytes and phonolites. The eudialyte-bearing varieties of the peralkaline phonolites required additional low-pressure fractionation of alkali feldspar and minor magnetite, amphibole and apatite. In contrast to the peralkaline trachytes and phonolites, the peralkaline olivine trachytes contain olivine instead of amphibole, thus indicating a magma evolution at even lower pressure conditions. They can be modelled as a derivation from the latites by fractional crystallization of plagioclase, clinopyroxene, magnetite and olivine. In general, agpaitic magmas evolve under closed-system conditions, which impede the escape of volatile phases. In the case of the extrusive agpaitic rocks at Tarosero, the early exsolution of fluids and halogens was prevented by a low water activity. This resulted in high concentrations of rare earth elements (REE) and other high field strength elements (HFSE) and the formation of eudialyte in the most evolved peralkaline phonolites. Within the peralkaline rock suite, the peralkaline olivine trachytes contain the lowest HFSE and REE concentrations, consistent with mineralogical evidence for formation at a relatively high water activity. The lack of amphibole fractionation, which can act as a water buffer of the melt, as well as the evolution at relatively low-pressure conditions caused the early exsolution of fluids and loss of water-soluble elements. This prevented a strong enrichment of HFSE and REE before the magma was finally extruded.

AbstractMelt inclusions in clinopyroxenes of olivine foidite bombs from Serra di Constantinopoli pyroclastic flows of the Vulture volcano (Southern Italy) were studied in detail. The rocks contain abundant zoned phenocrysts and xenocrysts of clinopyroxene, scarce grains of olivine, leucite, haüyne, glass with microlites of plagioclase and K-feldspar. The composition of clinopyroxene in xenocrysts (Cpx I), cores (Cpx II), and in rims (Cpx III) of phenocrysts differs in the content of Mg, Fe, Ti, and Al. All clinopyroxenes contain two types of primary inclusion-pure silicate and of silicate-carbonate-salt composition. This fact suggests that the phenomena of silicate-carbonate immiscibility took place prior to crystallization of clinopyroxene. Homogenization of pure silicate inclusions proceeded at 1 225 – 1 190°C. The composition of conserved melts corresponded to that of olivine foidite in Cpx I, to tephrite-phonolite in Cpx II, and phonolite-nepheline trachyte in Cpx III. The amount of water in them was no more than 0.9 wt.%. Silicate-carbonate inclusions decrepitated on heating. Salt globules contained salts of alkali-sulphate, alkali-carbonate, and Ca-carbonate composition somewhat enriched in Ba and Sr. This composition is typical of carbonatite melts when decomposed into immiscible fractions. The formation of sodalite-haüyne rocks from Vulture is related to the presence of carbonate-salt melts in magma chamber. The melts conserved in clinopyroxenes were enriched in incompatible elements, especially in Cpx III. High ratios of La, Nb, and Ta in melts on crystallization of Cpx I and Cpx II suggest the influence of a carbonatite melt as carbonatites have extremely high La/Nb and Nb/Ta and this is confirmed by the appearance of carbonatite melts in magma chamber. Some anomalies in the concentrations and relatives values of Eu and especially Ga seems typical of Italian carbonatite related melts. The mantle source for initial melts was, most likely, rather uniform, undepleted and was characterized by a low degree of melting and probable presence of garnet in restite.

AbstractFernando de Noronha archipelago presents an older Remédios Formation with subvolcanic intrusions, belonging to two different alkaline series, the sodic (undersaturated: basanites, tephrites, essexites, tephriphonolites, phonolites), and potassic ones (mildly undersaturated to silicic, with alkali basalts, basaltic trachyandesites, trachyandesites, trachytes), and lamprophyres. The upper Quixaba Formation presents nephelinite flows and basanites. A third minor unit, São José, is constituted by basanites carrying mantle xenoliths. Magnesian olivines occur in the Remédios basanites and alkali basalts, and in nephelinites. Melilites are present as groundmass grains in melilite melanephelinites (MEM). Clinopyroxenes (cpx) are mostly salites to titaniferous salites (Remédios sodic series), grading into aegirines in the differentiated aphyric phonolites. Cpx in the lamprophyres show disequilibrium textures. In the Quixaba flows, cpx are salites, enriched in Mg (especially in MEM). Amphiboles, remarkably, are common in tephriphonolites and phonolites and in basaltic trachyandesites, sometimes with disequilibrum zoning textures, and a conspicuous phase in lamprophyres. Dark micas are present as groundmass plates in MEM, OLM and PYM (olivine and pyroxene melanephelinites), with compositional variety (enriched in Ti, Ba, Sr) depending on the composition of the parent rock; BaO can be as high as 16–19%. Feldspars crystallize as calcic plagioclases, sanidines and anorthoclases, depending on the rock types, as phenocrysts and in groundmass, both in Quixaba and Remédios rocks; they are absent in nephelinites. Nephelines are found in Remédios sodic series types and Quixaba rocks. Haüyne and noseane are rarely observed in Remédios rocks.

AbstractHigh-MgO lamproite and lamproite-like (i.e. lamprophyric) ultrapotassic rocks are recurrent in the Mediterranean and surrounding regions. They are associated in space and time with ultrapotassic shoshonites and high-K calc-alkaline rocks. This magmatism is linked with the geodynamic evolution of the westernmost sector of the Alpine–Himalayan collisional margin, which followed the closure of the Tethys Ocean. Subduction-related lamproites, lamprophyres, shoshonites and high-K calc-alkaline suites were emplaced in the Mediterranean region in the form of shallow level intrusions (e.g. plugs, dykes and laccoliths) and small volume lava flows, with very subordinate pyroclastic rocks, starting from the Oligocene, in the Western Alps (northern Italy), through the Late Miocene in Corsica (southern France) and in Murcia-Almeria (southeastern Spain), to the Plio-Pleistocene in Southern Tuscany and Northern Latium (central Italy), in the Balkan peninsula (Serbia and Macedonia) and in the Western Anatolia (Turkey). The ultrapotassic rocks are mostly lamprophyric, but olivine latitic lavas with a clear lamproitic affinity are also found, as well as dacitic to trachytic differentiated products. Lamproite-like rocks range from slightly silica under-saturated to silica over-saturated composition, have relatively low Al2O3, CaO and Na2O contents, resulting in plagioclase-free parageneses, and consist of abundant K-feldspar, phlogopite, diopsidic clinopyroxene and highly forsteritic olivine. Leucite is generally absent, and it is rarely found only in the groundmasses of Spanish lamproites. Mediterranean lamproites and associated rocks share an extreme enrichment in many incompatible trace elements and depletion in High Field Strength Elements and high, and positively correlated Th/La and Sm/La ratios. They have radiogenic Sr and unradiogenic Nd isotope compositions, high 207Pb over 206Pb and high time-integrated 232Th/238U. Their composition requires an originally depleted lithospheric mantle source metasomatized by at least two different agents: (1) a high Th/La and Sm/La (i.e. SALATHO) component deriving from lawsonite-bearing, ancient crustal domains likely hosted in mélanges formed during the diachronous collision of the northward drifting continental slivers from Gondwana; (2) a K-rich component derived from a recent subduction and recycling of siliciclastic sediments. These metasomatic melts produced a lithospheric mantle source characterized by network of felsic and phlogopite-rich veins, respectively. Geothermal readjustment during post-collisional events induced progressive melting of the different types of veins and the surrounding peridotite generating the entire compositional spectrum of the observed magmas. In this complex scenario, orogenic Mediterranean lamproites represent rocks that characterize areas that were affected by multiple Wilson cycles, as observed in the Alpine–Himalayan Realm.

AbstractNodules (coarse-grain “plutonic” rocks) were collected from the ca. 20 ka Pomici di Base (PB)-Sarno eruption of Mt. Somma-Vesuvius, Italy. The nodules are classified as monzonite-monzogabbro based on their modal composition. The nodules have porphyrogranular texture, and consist of An-rich plagioclase, K-feldspar, clinopyroxene (ferroan-diopside), mica (phlogopite-biotite) ± olivine and amphibole. Aggregates of irregular intergrowths of mostly alkali feldspar and plagioclase, along with mica, Fe-Ti-oxides and clinopyroxene, in the nodules are interpreted as crystallized melt pockets.Crystallized silicate melt inclusions (MI) are common in the nodules, especially in clinopyroxenes. Two types of MI have been identified. Type I consists of mica, Fe-Ti-oxides and/or dark green spinel, clinopyroxene, feldspar and a vapor bubble. Volatiles (CO2, H2O) could not be detected in the vapor bubbles by Raman spectroscopy. Type II inclusions are generally lighter in color and contain subhedral feldspar and/or glass and several opaque phases, most of which are confirmed to be oxide minerals by SEM analysis. Some of the opaque-appearing phases that are below the surface may be tiny vapor bubbles. The two types of MI have different chemical compositions. Type I MI are classified as phono-tephrite — tephri-phonolite — basaltic trachy-andesite, while Type II MI have basaltic composition. The petrography and MI geochemistry led us to conclude that the nodules represent samples of the crystal mush zone in the active plumbing system of Mt. Somma-Vesuvius that were entrained into the upwelling magma during the PB-Sarno eruption.

Abstract The Zhob Ophiolite is divided into Naweoba, Omzha and Ali Khanzai blocks. Ali Khanzai Block is further divided into ultramafic, mafic, and lava units which are surrounded by sedimentary rocks successions. The ultramafic unit contains ultramafic tectonic and ultramafic cumulate, mafic rock unit consists of foliated and layered gabbros and mafic dykes are doleritic in composition. Volcanic–volcaniclastic–pelagic rocks unit consists of thick volcanic pillow basalt, hyaloclastite, bedded chert, pelagic limestone and hemipelagic mudstone. The metamorphic sole rocks are tectonically high distorted and dismembered, comprising of amphibolite and greenschist facies. They might have formed through the process of early intra-oceanic obduction of the ophiolite. All units make thrusted contacts and are highly deformed. Petrography and geochemical studies divide the Ali Khanzai Block into rock types such as gabbro, olivine gabbro, dolerite, basalt, basaltic andesite and basaltic trachy-andesite, chert, mudstone, and limestone, dunite, serpentinite, harzburgite, and wehrlite. Mafic dyke intrusions crosscut mantle rocks of block. The mantle rocks are altered, deformed, and deeply weathered, maybe residual melting of enriched mantle peridotite. The contact relationship of dolerite dykes with peridotite indicta that they are late magmatic intrusions. The Crustal gabbros are partially chloritic and sericitic and occur within mantle peridotite,, they may have formed from fractional crystallization in a magma chamber. The volcanic-volcaniclastic-pelagic sediments surround mantle and crustal rock units. It might be a mélange in nature is much like that of the Bagh Complex found beneath the Muslim Bagh Ophiolite, and other ophiolites around the world.