Phonolitic Magma Research Articles

238U- and 232Th-series chronology of phonolite fractionation at Mount Erebus, Antarctica

Uranium, thorium, radium, and barium abundances and 234U 238U and 230Th 232Th isotopic ratios determined by thermal ionization mass spectrometry and ( 228Th) ( 232Th ) activity ratios determined by alpha spectrometry are used to date anorthoclase growth and infer magma chamber residence times of phonolites erupted in 1984 and 1988 from Mount Erebus, Antarctica. The 1984 and 1988 glasses have slightly different ( 230Th) ( 232Th ) ratios but both have a 10% excess of ( 230Th) over ( 238U) and equilibrium ( 228Th) values. By comparing these data and Pb-isotopic data reported in Sun and Hanson (1975) to similar data for oceanic basalts, the duration of differentiation from basanite to phonolite is limited to less than 150,000 years. The anorthoclase separates have ( 230Th) ( 238U ) ratios exceeding those of the associated glasses but have ( 230Th) ( 232Th ) ratios like those of the glasses. Both glasses are depleted in 226Ra with respect to 230Th by about 25%, whereas associated anorthoclase separates have extreme excesses of 226Ra over 230Th and ( 228Th) ( 232Th) = 2.2. On a plot of ( 226Ra)/Ba vs. ( 230Th)/Ba, the glass-anorthoclase pairs produce isochrons averaging 2380 y, which represents the average age of anorthoclase growth in the shallow magma system at Erebus. The implied residence time of phonolite magmas in the shallow magma chamber system of Erebus is about 3000 y. Final crystal growth occurred after intrusion into the converting lava lake less than decades before eruption.

Geochemical and petrographic characteristics of potassium-rich pyroclastic and lava samples from Vulsini volcano, Roman magmatic region, Italy

Abstract Pyroclastic deposits are the dominant eruptive product at Vulsini and at other potassic volcanoes in the Roman magmatic region, central Italy. Geochemical data from 139 K-rich undersaturated Vulsinian samples are used in the first systematic study of both pyroclastic and lava eruptions from a large volcano in this region. The pyroclastic samples span the complete range of rock chemistries; however, compared to the lavas they are concentrated in the more evolved products and thus significantly enlarge the erupted volume of phonolitic and trachytic magma. The pumice and lava major- and trace-element trends generally form a continuum consistent with a fractionated sequence of K-rich magmas as follows: leucite basanite/Mg-rich leucitite→leucitite/tephritic leucitite→leucite phonolite/leucite trachyte/saturated trachyte. Olivine is conspicuous in the primitive basanitic pumice and lava samples and is also present in the less evolved tephritic leucitites. Clinopyroxene crystallizes at all stages of magma evolution along with ±leucite, ± plagioclase, ± mica, ± opaque oxides and ±sanidine. Leucite phenocrysts are prominant in the intermediate to more evolved compositions, particularly in the leucite phonolites where they form large macrophenocrysts. Sanidine is a significant phenocryst phase in the more evolved samples, particularly in the evolved pumices where it dominates the modal assemblage: in these samples increasing Rb and rapidly decreasing Sr and Ba, as well as reducing K2O/Na2O ratios, are linked to sanidine fractionation. High Ni contents (∼ 250 ppm), high Mg numbers (∼ 78) and forsteritic olivine (Fo92) suggest that the basanitic pumice and lava magmas had a mantle origin through partial melting. Similarly, Mg-rich leucitites are also inferred to be mantle partial melts. On the basis of high K, CaO, Rb and F concentrations and high K/Na, CaO/MgO, low K/Rb and high di/ol ratios found in the primary basanites and leucitites, the mantle source is inferred to be rich in phlogopite and clinopyroxene. By comparison with various purported primary alkaline magmas the Vulsinian primary magmas show considerable enrichment of K, Rb and Th, while other elements such as Nb, Zr, Sr, P and Ti are significantly depleted, thus suggesting a decoupling between these groups of incompatible elements in the mantle beneath Vulsini. Variable Nb/Th ratios are a feature of Vulsinian magmas and such variations along with the relative decoupling of incompatible elements are linked to metasomatic processes and the development of a phlogopite- and clinopyroxene-enriched mantle.

Cumulate nodules as evidence for convective fractionation in a phonolite magma chamber

The physical mechanism by which chemical zonation develops in magma chambers has been controversial partly because unambiguous geological constraints have been lacking. The 11,000 years B.P. eruption of Laacher See Volcano produced a zoned tephra deposit and also ejected crystal-rich nodules which provide a snapshot of the materials crystallising at the magma chamber margins. New data on petrography and chemical compositions of nodules, their cumulate minerals and interstitial glasses are used to deduce the chemical evolution of the phonolite melt due to fractional crystallisation of the mineral assemblages. These data, together with those on the vertical zonation of the melt in the bulk of the chamber, are shown to be consistent with a model of stratification of the chamber by convective fractionation, in which a thin boundary layer of residual melt from fractional crystallisation ascends at the chamber side and accumulates at the roof. Crystallisation could have provided buoyancy to drive convection by enriching incompatible volatile components (mainly water) in the residual melt. Available fluid dynamic studies of single- and double-diffusive boundary layers are used to assess convection in the Laacher See chamber. The boundary layer is likely to have been: (1) laminar, which implies that the density gradient in the chamber steepened upwards; (2) in the counterflow regime, in which compositional and thermal layers flow in opposite directions; and (3) thin (≲ 10 cm), estimated from theory for a flat wall, suggesting that wall morphology could be important in determining boundary layer characteristics. Estimates of mass transfer rates due to this mechanism suggest that the chamber could have become stratified in a time of the order of 10 3 years.

Strontium, neodymium and lead isotope characteristics of the Granadilla Pumice, Tenerife: a study of the causes of strontium isotope disequilibrium in felsic pyroclastic deposits

Summary The Granadilla Pumice deposit, the product of a single explosive eruption of phonolitic magma, exhibits stratigraphically systematic internal compositional variations interpreted as the result of fractionation of the observed phenocryst assemblage in the pre-eruptive magma body. Whole-rock 87 Sr/ 86 Sr data exhibit a consistent profile, with the most differentiated compositions having the most radiogenic strontium. The resultant 87 Sr/ 86 Sr versus 1/Sr mixing line has a positive slope, suggesting the addition of a relatively radiogenic strontium component to the existing strontium concentration profile. There is no significant systematic variation, however, in neodymium or lead isotopes. Separated alkali feldspar phenocrysts have 87 Sr/ 86 Sr values of 0.7031–0.7032, identical with Tenerife mafic magmas; the strontium isotopic variation is restricted to the complementary glasses. The more radiogenic component in the glasses is only partially removed by acid leaching and has a strontium isotope ratio of about 0.707, deduced from analysis of the leachant. Strong acid leaching, accompanied by up to 25% sample dissolution, results in greater, but not complete, loss of the more radiogenic component, with a good correlation between pre- and post-leaching glass isotopic compositions. These relationships cannot be easily explained by either post-eruptive alteration or contamination of the magma chamber with high- 87 Sr/ 86 Sr country rock. The data are instead consistent with a model of interaction between vesiculating magma and hydrothermal fluids with high 87 Sr/ 86 Sr, immediately before and/or during explosive eruption, involving limited diffusion of strontium from the fluid into bubble walls before quenching of the magma to form glassy pumice. This process may be common during large explosive eruptions, and we suggest that whole-rock strontium isotope variations found in strontium-poor felsic pyroclastic deposits may not reflect purely magmatic processes, even though they closely mimic plausible magmatic variations.

Crystallisation of nepheline syenite in a subvolcanic magma system: Tenerife, canary islands

The late Quaternary phonolitic pumice deposits of Tenerife, Canary Islands, are the product of a periodically-tapped, periodically-replenished, zoned alkaline magma system. Nepheline syenite blocks occur as lithics in the deposits, and provide solidified representatives of the phonolite magmas. The blocks are considered to have crystallised in the roof zone of the active magma system at a depth of roughly 4 km beneath the Las Cañadas caldera. Conversion of highly-differentiated phonolitic magma to solid nepheline syenite was achieved between 770° and 680°C. Quenched glass of extreme composition in one erupted block provides a sample of the last interstitial liquid; comparison of this sample with phonolitic pumice, and with fully crystallised syenites, allows reconstruction of the magmatic, interstitial and subsolidus crystallisation histories of nepheline syenite. Phases represented by the phenocryst assemblage of the most differentiated phonolitic pumices (sanidine, sodalite, nepheline, Na-poor pyroxene, biotite, sphene and magnetite) formed an initial crystal “mush”, containing abundant trapped liquid, on the walls of the magma chamber. Interstitial crystallisation, involving the conversion of pyroxene to Na-rich compositions, continued growth of felsic phases, and partial to complete consumption of biotite, sphene, magnetite and apatite, was accompanied by extreme sodium enrichment in the residual liquid. Stellate aegirine, lavenite, loparite and Mn-rich ilmenite are among the final products of magmatic crystallisation. Modification of feldspar primocrysts persisted into subsolidus conditions. It is suggested that complete solidification of an open-system alkaline magma chamber results in the formation of an alkaline alkaline ring complex.

Temporal variations in column height and magma discharge rate during the 79 A.D. eruption of Vesuvius

Research Article| August 01, 1987 Temporal variations in column height and magma discharge rate during the 79 A.D. eruption of Vesuvius STEVEN CAREY; STEVEN CAREY 1Graduate School of Oceanography, University of Rhode Island, Kingston, Rhode Island 02881 Search for other works by this author on: GSW Google Scholar HARALDUR SIGURDSSON HARALDUR SIGURDSSON 1Graduate School of Oceanography, University of Rhode Island, Kingston, Rhode Island 02881 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1987) 99 (2): 303–314. https://doi.org/10.1130/0016-7606(1987)99<303:TVICHA>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation STEVEN CAREY, HARALDUR SIGURDSSON; Temporal variations in column height and magma discharge rate during the 79 A.D. eruption of Vesuvius. GSA Bulletin 1987;; 99 (2): 303–314. doi: https://doi.org/10.1130/0016-7606(1987)99<303:TVICHA>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The 79 A.D. plinian eruption of Vesuvius ejected ∼4 km3 (ORE) of phonolitic magma over a period of ∼19 hr. A change in magma composition during the eruption is marked by a sharp transition from white, evolved phonolitic pumice to denser, overlying gray pumice, at mid-level within the fall deposit. Deposition of the upper, gray pumice fall was interrupted six times by the emplacement of pyroclastic surges and flows. Reverse size grading is conspicuous in the fall deposit. Measurements of maximum pumice and lithic diameters have been used to construct isopleths for eight chronostratigraphic levels within the fall deposit. The temporal evolution of eruption column height and magma discharge rate have been evaluated from these isopleths, using a theoretical model of pyroclast dispersal from explosive eruptions. During ejection of the white pumice, the column height rose from 14 to 26 km, as the magma discharge rate increased to 7.7 × 107 kg/s. Shortly after onset of the gray pumice fall, the column reached its maximum altitude of 32 km, with a discharge rate of 1.5 × 108 kg/s. Subsequent generation of surges and pyroclastic flows was associated with fluctuations in column height, supporting an origin by column collapse. At the white-gray boundary in the fall deposit, pumice density increases abruptly from 0.60 g/cm3 in the white pumice to 1.10 g/cm3 at the base of the gray pumice. Higher in the gray fall, the density decreases continuously to 0.60 g/cm3. The variation in pumice density is attributed primarily to differences in volatile content of two magmas which were tapped and mixed in varying proportions during ascent and eruption. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Mid-Proterozoic alkaline magmatism in southern Greenland: the Gardar province

Summary The Gardar province ( c. 1320–1120 Ma old) relates to repetitive rifting within the Mid-Proterozoic continent and the attendant large-scale generation of alkali and transitional olivine basaltic and hawaiitic magmas. Lavas from the earliest episodes are preserved in down-faulted units (the Eriksfjord formation), whereas younger basic magmatism is recorded only as intrusions, mainly dykes. The province shows a compositional continuum from basaltic to salic alkaline compositions. The latter are represented among the early lavas, as dykes and, most prominently, as central-type complexes commonly involving layered (cumulate) syenitic intrusions. Most of the central complexes display either a silica-oversaturated trend (augite syenite-quartz syenite-(A-type) alkali granite) or a silica-undersaturated trend (augite syenite-pulaskite-foyaite). Strongly peralkaline products are present in each association with agpaitic rocks occurring as differentiates at Ilímaussaq and Motzfeldt. The oversaturated and undersaturated complexes, irrespective of age, define southerly and northerly zones respectively in the province. ‘Giant-dyke complexes’, produced in late-Gardar rifting events (contemporaneous with Keweenawan activity in N America) are important in providing a link between simple dilatational dykes and the central-type complexes. An ultramafic lamprophyre-carbonatite association plays a subordinate role in the province, and carbonatites are also associated with some of the salic complexes. The carbonatites at Grønnedal-Ika are thought to be residual from CO 2 -rich phonolitic magmas, whereas carbonate (siderite)-fluoride rocks at Ivigtut crystallized from volatile-rich residues associated with an alkali granite. The salic alkaline magmas were produced from alkali or transitional olivine basaltic parent magmas, with crystal fractionation being the dominant mechanism. Generally low 87 Sr/ 86 Sr (i) ratios in the alkaline complexes suggest little assimilation of old radiogenic crustal materials. The basaltic magmas were characterized by high Al 2 O 3 /CaO and FeO/MgO ratios, and it is inferred that they themselves were residues from more magnesian parent magmas that had undergone extended olivine and clinopyroxene fractionation at sub-crustal levels. The primary magmas may have been derived from garnet lherzolite source rocks with high garnet to clinopyroxene ratios. Late-Gardar basic magmas of the Tugtutôq-Ilímaussaq-Nunataq zone are geochemically distinct from all other Gardar basic magmas in being enriched in P, Ba, Nb and light rare earth elements. This feature may be due to metasomatic enrichment of source rocks along a lineament sub-parallel to the Archaean craton margin. Gardar alkaline magmas were rich in F, Cl and C compounds (oxides and/or hydrocarbons) while being relatively anhydrous. Many crystallized under low f O 2 conditions. The fundamental cause of the repetitive alkaline magma generation may have been the ascent of F-, Cl- and C-rich fluids from the deep mantle, leading to partial melting of the lithospheric mantle close to the boundary between Archaean and Proterozoic crustal units.

Zonation, mixing and eruption of silica-undersaturated alkaline magma: a case study from Tenerife, Canary Islands

AbstractThe mineralogy and petrology of banded pumice clasts from a mixed-magma non-welded ignimbrite on Tenerife, Canary Islands, is described. Phenocrysts, which frequently show signs of resorption, are grouped into three assemblages in mutual disequilibrium; these assemblages were originally precipitated by alkali basalt magma (assemblage 1, titanaugite + olivine + calcic plagioclase + high-Mg Fe–Ti oxides), trachyandesitic magma (assemblage 2, salite + kaersutite + sodic plagioclase + hauyne + medium-Mg Fe–Ti oxides) and phonolitic magma (assemblage 3, aegirine-augite + biotite + sodic sanidine + sodalite + nepheline + sphene + low-Mg magnetite) respectively. Phenocryst types (e.g. clinopyroxene, feldspar) which are common to two or all assemblages are compositionally distinctive in each. The excess silica content of nepheline in assemblage 3 indicates a temperature of 770 °C for phonolitic magma, while Fe–Ti oxides and clinopyroxene-ilmenite give temperatures of 915 °C and 907 °C respectively for the trachyandesite. Assemblage 1 and 2 phenocrysts are set in mafic glass; assemblage 3 phenocrysts are only found in a matrix of salic (phonolitic) glass. Analyses of the volumetrically dominant phonolitic glass define a compositional trend consistent with fractionation of assemblage 3 phenocrysts. Analyses of the mafic glass scatter about a straight-line trend produced by mixing of phonolitic liquid with homogeneous intermediate (trachyandesitic) magma. These relations are interpreted as the result of two distinct magma mixing events, consequent on the invasion of a stably zoned magma chamber containing phonolitic liquid stratified above denser trachyandesitic magma, by intruding basaltic magma. High Ca content of assemblage 1 plagioclase suggests precipitation of the basaltic phenocrysts under locally high partial pressure of water. The preferred model involves, in chronological sequence: (i) generation of a differentiated phonolitic cap above trachyandesitic magma, through wall crystallization and boundary-layer uprise of phonolite; (ii) pooling, convective heat loss, and partial crystallization of dense, hydrous alkali basalt magma injected into the chamber and forming a liquid layer beneath the trachyandesitic magma; (iii) volatile saturation of the residual basalt liquid through crystallization, consequent buoyant uprise into, and efficient mixing with, trachyandesite; (iv) eruption of chamber contents and partial mixing of the mixed basalt–trachyandesite with phonolite, although the precise relative timing of eruption and the second mixing event is not known.

Mineralogical and Chemical Zonation of the Laacher See Tephra Sequence (East Eifel, W. Germany)

The late Quaternary Laacher See Tephra (LLST, MLST, ULST: Lower, Middle, Upper Laacher See Tephra) of the East Eifel volcanic field (W. Germany) is zoned from highly evolved, volatile-rich and crystal-poor phonolite at its base towards a mafic, crystal-rich phonolite at the top of the deposit. This is shown by phenocryst abundances, major and trace element chemistry of whole rocks, matrix-glass and some mineral compositions. MgO content in whole rocks ranges from 0.07 wt. per cent in lower LLST to 0.85 wt. per cent in ULST phonolite. Late mafic hybrid pyroclasts contain up to 7.0 wt. per cent MgO. Na2O shows a reverse trend from 11.7 wt. per cent in LLST to 5.1 per cent in ULST. Trace elements are divided into depleted (compatible) elements (e.g. Sr, Sc, Co), and enriched (incompatible) elements (e.g. Zn, Zr, Nb). ‘Semi-compatible’ elements (Ta, Y) show minimum concentrations at an intermediate stratigraphic level (MLST). All compositional gradients are smooth showing a major compositional interface between LLST/MLST and ULST. Twelve phenocryst phases occur: sanidine, plagioclase, hauyne, amphibole, clinopyroxene, sphene, apatite, Ti-magnetite, biotite, nepheline, cancrinite and zircon. The latter three are restricted to LLST phonolite. There is a steep gradient in mineral abundances from the LLST and MLST to the mafic ULST phonolite. Microprobe analyses define two phenocryst populations: (a) Fe, Na and Mn-rich evolved phenocrysts (hastingsite, aegirine-augite, Ab-rich plagioclase) which predominate within highly differentiated (LLST) phonolite; (b) Mg and Ti-rich and Fe, Na, Mn-poor phenocrysts, which are most abundant in ULST. This compositional zonation is interpreted as the result of continuous eruption from a zoned magma column; highly evolved early magma representing the upper part and mafic late phonolitic magma representing the lower part of a stratified magma chamber emplaced at shallow, crustal levels.

Petrogenesis of the Zoned Laacher See Tephra

The late Quaternary Laacher See phonolitic tephra deposit (East Eifel, W. Germany) is mineral-ogically and chemically zoned from highly evolved, volatile-rich and crystal-poor at its base towards a mafic, crystal-rich phonolite at the top (Wörner & Schmincke, 1984). This zonation is interpreted as the result of a continuous eruption from a zoned magma column. Major and trace element evidence shows that the last erupted mafic ULST (Upper Laacher See Tephra) phonolite can be derived from a basanite parent magma via fractional crystallization of 30 per cent clinopyroxene, 24 per cent amphibole, 4 per cent phlogopite, 3.8 per cent magnetite, 2.5–3.0 per cent olivine and 1 per cent apatite, leaving a derivative of 30 per cent evolved magma. Starting from the mafic (ULST) phonolite as a parent, the zoned sequence is postulated to have been formed by progressive fractional crystallization of the observed phenocryst phases. This model was tested by a series of 7 step-by-step mass balance fractionation calculations. Abundance, modal composition and relative variations of calculated fractionated phases agree well with the observed phenocryst abundances: sanidine followed by plagioclase and minor amounts of mafic phases are to be fractionated to give the observed zoned sequence. The most evolved phonolite, however, cannot be generated by subtraction of phenocrysts from the underlying phonolite. Processes such as liquid-state differentiation may therefore have chemically modified the upper part (cupola) of the Laacher See magma column subsequent to crystal fractionation. The erupted phonolite magma (5.3 km3) was calculated to have started with a volume of 56 km3 of parental basanite magma which fractionated to form 16.6 km3 of mafic phonolite. This magma further differentiated to give a 5.3 km3 zoned (erupted) phonolite column. The non-erupted volume of 50 km3 is postulated to form a cooling cumulate body below the present day Laacher See volcano. The Laacher See magma system represents a complex end-member type of a highly evolved small volume composition ally zoned magma chamber with steep major and trace element gradients, the uppermost volatile rich magma layer resembling the stable roof part of rhyolitic chambers.

The eruptive center of the late quaternary Laacher see tephra

About 5 km3 of phonolite magma were erupted 11,000 years ago in the Laacher See area in Plinian and Vulcanian eruptions. The eruptive vents for all tephra, in dispute for over 200 years, must have been located entirely within the Laacher See basin, because: (1) All deposits are thickest in the tuffring surrounding the basin, with one exception: pyroclastic flow deposits are thickest 2 to 6 km away from the rim of the basin having ponded in radial valleys. Three Plinian fallout layers show minor secondary thickness maxima 12 to 16 km east of Laacher See. (2) Axes of all depositional fans and single tephra sheets converge in the Laacher See basin. (3) Diameters of both lithic and pumice clasts reach their maxima in outcrops close to Laacher See volcano. (4) Directions of asymmetric ballistic impact sags indicate the Laacher See basin as source area. (5) The dimension of Laacher See crater roughly corresponds to the erupted volume of phonolitic tephra (5.3 km3 magma DRE) and lithic clasts (0.7 km3). (6) Xenolith types are consistent with an eruptive center within the Laacher See basin. (7) The systematic chemical and mineralogical zonation of the tephra deposits — from highly differentiated aphyric phonolite at the base to highly phyric mafic phonolite at the top — strongly suggests eruption from a single compositionally zoned magma column. (8) Several lines of evidence indicate migration of the main eruptive focus from the south to the north during the later part of the eruption, both vents however being confined within the Laacher See basin.

Variation in [formula omitted] during differentiation of phonolitic magma, Tenerife, Canary Islands

Variation in Nb Ta ratio through a suite of phonolitic volcanics is reported. Sphene, a phenocryst mineral, shows preferential incorporation of Ta relative to Mb, consistent with the bulk rock variation, where Nb Ta increases with fractionation. Nb in these magmas may be partly present in the Nb 3+ state.

The A.D. 472 “Pollena” eruption: volcanological and petrological data for this poorly-known, plinian-type event at vesuvius

The pyroclastic products of a poorly-known eruption of Vesuvius (ascribed by a combination of historic and radiocarbon data to A.D. 472) have been investigated from both volcanological and petrological points of view. The eruptive sequence starts with pumice-fall deposits (three units can be recognized) that darken upwards where there are sandwave interbeds. Surge deposits cover the pumice-fall bed and thick pyroclastic-flow deposits represent the uppermost levels of the deposit. Isopach maps of both the pumice-fall and pyroclastic-flow deposits led to an estimate of the total volume of tephra of about 0.32 km 3. The eruptive sequence and the distribution of lithic ejecta are similar to those of the major Plinian eruptions of Somma-Vesuvius (although the volume involved is significantly lower) and reflect an increase in the hydromagmatic character of the eruption with time. The products range in composition from phonolites (first-erupted) to phonolitic leucitites with gradual changes upwards. Whole rock chemistry and microprobe mineralogy indicate that the Pollena sequence represents a liquid line of descent towards the phonolitic minimum of petrogeny's residua system. Fractionation occurred within a shallow magma chamber (P H 2 O probably slightly higher than 1 kb) and was mainly controlled by leucite and clinopyroxene. The basic parental magma approached the composition of the recent period (A.D. 1631–1944), tephritic leucitites of Vesuvius. The phonolitic magma can be derived from a leucititic parent by fractionating about 50% solid phases. A two-stage fractionation model is suggested: the first stage occurred during the rise of magma from the deep source and the second within the shallow magma chamber. The rate of magma introduction during the 150 to 200 year repose time preceeding the eruption probably averaged 1.2 to 1.7 × 10 −3 km 3-yr −1. These conditions were probably favorable for the occurrence of magma-mixing within the convecting zone of the magma chamber.

Medium pressure crystallization of a monchiquitic magma — evidence from megacrysts of Drever's block, Ubekendt Ejland, West Greenland

Megacrysts and polymineralic fragments of extraordinary diversity from a Tertiary monchiquitic dyke of Ubekendt Ejland comprise three groups: (1) Cr-diopside-fassaitic diopside + olivine, Fo 90.5−81.5 + CrAl spinels. (II) Fassaitic salite-ferrisalite + KTi-pargasite-ferropargasite + apatite + AlTi-magnetite, (III) Scapolite + hyalophane + potassium feldspar + nepheline + analcime. By comparison with mineralogy and phase relations in the host rock and experimental data from alkaline rocks the megacrysts are related to a sequence of crystallization from primitive monchiquitic to potassic phonolitic magmas rich in H 2O and CO 2 at 5–11 kb. Group I megacrysts formed at temperatures of 1300-1150°C and group II between ? 1150–⪸800° C and f o 2 < 10 −9 bar at the latter temperature. High P co 2 may have stabilized the scapolite in the more evolved liquid and K-feldspar and nepheline began to crystallize at ca. 800°C possibly together with the ferrisalite.

Magma mixing in mafic alkaline volcanic rocks: the evidence from relict phenocryst phases and other inclusions

Green clinopyroxenes, commonly rounded and anhedral and richer in Fe, Na and Mn than the pyroxenes of the surrounding groundmass are a common feature of mafic alkaline volcanic rocks (e.g. basanites, monchiquites, leucitites). Some are accompanied by one or more of the following phases: Fe-rich kaersutite and biotite, anorthoclase, sodic plagioclase, apatite, magnetite, sphene, which are believed to be cognate with the green pyroxenes. We review evidence that these minerals have crystallized from mugearite, trachyte or phonolite magmas, and their presence in mafic alkaline rocks is due to magma mixing. The intermediate and salic magmas may sometimes be generated at mantle depths, possibly by melting of mantle material enriched in Fe, Na and volatiles.

Some constraints on the origin of phonolites from the Gregory Rift, Kenya, and inferences concerning basaltic magmas in the Rift System

Plateau-type phonolites of the Gregory Rift represent magmas with densities of about 2.3 g/cm 3. These magmas must have erupted soon after their formation. The average depth of the bases of the approximately 100 to 300 magma chambers from which they erupted was between about 10 and about 24 km. The average vertical extent of the magma chambers was between about 3 and about 8 km. The aggregate volume of phonolitic magma formed beneath the Rift in Miocene times probably lies in the range of 0.5 to 1×10 5 km 3. Both the crystal fractionation model and the anatectic model for formation of this volume of phonolitic magma require the presence of a large reservoir of basaltic magma, probably picritic in character, with a volume of at least 5×10 5 km 3 and perhaps as much as 20×10 5 km 3. This reservoir presumably is now part of the dense basic intrusive complex along the Rift axis. The Miocene and Pliocene episodes of basaltic volcanism in this region may be related to eruption of evolved liquids from this reservoir.

Chemistry of peralkaline phonolite dykes from the Grønnedal-Íka area, South Greenland

The chemical variation observed in a suite of fifteen aphyric peralkaline phonolite dykes of mid-Gardar age from the vicinity of the Gronnedal-Ika alkaline complex in south-west Greenland is discussed. From relationships in the system Na2O-K2O-Al2O3-SiO2 it is argued that the members of the series are related by the fractionation of feldspar approximating to Ab55Or40An5 in composition, along with augite and lesser amounts of other ferromagnesian minerals. The bearing of these rocks on phase equilibria in the analogous natural system is discussed, and consideration is given to the possible origins of the initial peralkaline phonolite magma.

Neogene Peralkaline Igneous Activity in Eastern Peru

Research Article| August 01, 1971 Neogene Peralkaline Igneous Activity in Eastern Peru JAMES W STEWART JAMES W STEWART Institute de Investigations Geological, Casilla 10465, Santiago, Chile *Technical co-operation United Kingdom–Chile. Search for other works by this author on: GSW Google Scholar GSA Bulletin (1971) 82 (8): 2307–2312. https://doi.org/10.1130/0016-7606(1971)82[2307:NPIAIE]2.0.CO;2 Article history received: 05 Apr 1971 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation JAMES W STEWART; Neogene Peralkaline Igneous Activity in Eastern Peru. GSA Bulletin 1971;; 82 (8): 2307–2312. doi: https://doi.org/10.1130/0016-7606(1971)82[2307:NPIAIE]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Three volcanic necks which cut Tertiary sediments in the Amazon plain represent an apparently isolated igneous event which took place about 5 m.y. ago (K: Ar determinations on cryptoperthite). The principal rock types of these necks are unusual undersaturated alkaline varieties characterized by sodalite and melanite and containing cognate inclusions of jacupirangite. Alkali trachytes also occur.The existence of an underlying plutonic complex containing feldspathoidal syenite, alkali syenite and jacupirangite, is postulated. The presence of jacupirangite and melanite in the volcanic rocks increases the probability that carbonatite might be another constituent of the inferred subvolcanic complex.The intrusion of the phonolites was completely unrelated in time to the emplacement of the known alkaline complexes of Brazil. The phonolitic magma might have formed as a consequence of fusion near the eastern limit of a Benioff zone which dipped landward from the western edge of the South American continent. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.