Academic literature 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.

The Harrington Peak Quadrangle is located within the Caribou National Forest of southeast Idaho. Within this quadrangle are outcrops of olivine trachyte of Pliocene(?) age overlying sedimentary rocks of Mississippian to Tertiary age. The region contains thrust faults and later normal faults (generally trending north-south} formed during Basin and Range extension. The Largest outcrop of olivine trachyte (approximately1 1/2 X 3 km) probably formed as the result of a fissure eruption. Two other outcrop areas show evidence of being sites of local extrusion. Whole-rock chemical analyses revealed the olivine trachyte to have moderate amounts of SiO2 and Al2O3, high MgO and CaO, and K2O in excess over Na2O (approximately 2:1). Mineralogical characteristics include microphenocrysts of Mg-rich olivine and diopsidic augite in a groundmass of Ba-rich sanidine, diopsidic augite, Fe-Ti oxides, and less commonly phlogopite and/or plagioclase. The olivine trachyte closely resembles the ciminites from the Viterbo region of Italy and has some petrological and mineralogical similarities to many other continental potassic volcanic rocks. The olivine trachyte may have formed by partial melting of a heterogenous mica peridotite mantle source enriched in incompatible elements during a previous tectonic event.