Evolved lavas from the Snake River Plain: Craters of the Moon National Monument,Idaho

Holocene lavas from Craters of the Moon (COM) National Monument are representative of differentiated lavas which occur around the margins of the Snake River Plains (SRP) and they range serially in composition from alkali- and phosphorous-rich ferrobasalts to ferrolatites. Petrographic study indicates that these lavas evolved primarily by cotectic crystallization of olivine, plagioclase, magnetite and apatite in the mafic members of the suite (ferrobasalts), and by cotectic crystallization of plagioclase, magnetite, clinopyroxene and minor olivine in the salic members. Quantitative phase relations in the COM lavas, calculated by means of a leastsquares mixing program, indicate that the observed range in composition among these lavas corresponds to at least 70 percent crystallization of a magma similar to the most mafic COM lavas. Anhydrous one-atmosphere experimental crystallization studies fail to reproduce exactly the inferred phase relations; the discrepancy is attributed to the presence of water in the naturally crystallized magmas. The origin of COM parental magma cannot be unequivocably resolved. Available evidence suggests that COM lavas do not represent melts derived directly from the mantle: (1) high Sr⁸⁷/Sr⁸⁶ ratios (0.708 to 0.712), (2) relatively high Fe/(Fe+Mg) and excluded-element content in even the most mafic COM lavas, (3) occurrence of megacrysts of inferred high-pressure origin in the Lava Creek flow. Megacrysts occur in the Lava Creek flow as clusters of labradorite, aluminous clinopyroxene, and olivine. Analogy with the experiments of Thompson [1] and least squares mixing calculations indicate that intermediate (ca. 8 to 10 Kbar) pressure fractionation of such megacrysts from olivine tholeiite magma may yield derivative COM-type liquids.     

The geochemistry and petrogenesis of K-rich alkaline volcanics from the Batu Tara volcano,eastern Sunda arc

Mineralogical, major and trace element, and isotopic data are presented for leucite basanite and leucite tephrite eruptives and dykes from the Batu Tara volcano, eastern Sunda arc. In general, the eruptives are markedly porphyritic with phenocrysts of clinopyroxene, olivine, leucite ±plagioclase±biotite set in similar groundmass assemblages. These K-rich alkaline volcanics have high concentrations of large-ion-lithophile (LIL), light rare earth (LRE) and most incompatible trace elements, and are characterized by high ⁸⁷Sr/⁸⁶Sr (0.70571–0.70706) and low ¹⁴³Nd/ ¹⁴⁴Nd (0.512609–0.512450) compared with less alkaline volcanics from the Sunda arc. They also display the relative depletion of Ti and Nb in chondrite-normalized plots which is a feature of subalkaline volcanics from the eastern Sunda arc and arc volcanics in general. Chemical and mineralogical data for the Batu Tara K-rich rocks indicate that they were formed by the accumulation of variable amounts of phenocrysts in several melts with different major and trace element compositions. The compositions of one of these melts estimated from glass inclusions in phenocrysts is relatively Fe-rich (100 Mg/(Mg + Fe²⁺)=48–51) and is inferred to have been derived from a more primitive magma by low-pressure crystal fractionation involving olivine, clinopyroxene and spinel. Mg-rich (mg 90) and Cr-rich (up to 1.7 wt. % Cr₂O₃) zones in complex oscillatory-zoned clinopyroxene phenocrysts probably also crystallized from such a magma. The marked oscillatory zoning in the clinopyroxene phenocrysts is considered to be the result of limited mixing of relatively evolved with more primitive magmas, together with their phenocrysts, along interfaces between discrete convecting magma bodies.     

Origin of anorthoclase megacrysts in alkali basalts

Anorthoclase megacrysts commonly occur with low pressure cumulate nodules (olivine + clinopyroxene + kaersutite ± oligoclase) in alkali basalts and their differentiates. The absence of anorthoclase from the cumulate nodules indicates that anorthoclase remained suspended in the magma while the other minerals sank, forming the cumulates, assuming a congeneric origin for megacrysts and nodules. On this basis, density calculations indicate that anorthoclase crystallised from a magma of approximately trachyandesitic composition, while the anorthoclase megacrysts usually occur in magmas more basic than trachybasalt. Thus, the anorthoclase megacrysts and the associated cumulate nodules did not crystallise from the host magma, but were picked up from a high-level partly crystallised magma pool containing a more evolved alkali basaltic differentiate by a later surge of more basic liquid which then carried both anorthoclase megacrysts and fragmented cumulates to the surface.     

Intrusion of basaltic magma into a crystallizing granitic magma chamber: The Cordillera del Paine pluton in southern Chile

The Cordillera del Paine pluton in the southernmost Andes of Chile represents a deeply dissected magma chamber where mafic magma intruded into crystallizing granitic magma. Throughout much of the 10x15 km pluton, there is a sharp and continuous boundary at a remarkably constant elevation of 1,100 m that separates granitic rocks (Cordillera del Paine or CP granite: 69–77% SiO₂) which make up the upper levels of the pluton from mafic and comingled rocks (Paine Mafic Complex or PMC: 45–60% SiO₂) which dominate the lower exposures of the pluton. Chilled, crenulate, disrupted contacts of mafic rock against granite demonstrate that partly crystallized granite was intruded by mafic magma which solidified prior to complete crystallization of the granitic magma. The boundary at 1,100 m was a large and stable density contrast between the denser, hotter mafic magma and cooler granitic magma. The granitic magma was more solidified near the margins of the chamber when mafic intrusion occurred, and the PMC is less disrupted by granites there. Near the pluton margins, the PMC grades upward irregularly from cumulate gabbros to monzodiorites. Mafic magma differentiated largely by fractional crystallization as indicated by the presence of cumulate rocks and by the low levels of compatible elements in most PMC rocks. The compositional gap between the PMC and CP granite indicates that mixing (blending) of granitic magma into the mafic magma was less important, although it is apparent from mineral assemblages in mafic rocks. Granitic magma may have incorporated small amounts of mafic liquid that had evolved to >60% SiO₂ by crystallization. Mixing was inhibited by the extent of crystallization of the granite, and by the thermal contrast and the stable density contrast between the magmas. PMC gabbros display disequilibrium mineral assemblages including early formed zoned olivine (with orthopyroxene coronas), clinopyroxene, calcic plagioclase and paragasite and later-formed amphibole, sodic plagioclase, mica and quartz. The early formed gabbroic minerals (and their coronas) are very similar to phenocrysts in late basaltic dikes that cut the upper levels of the CP granite. The inferred parental magmas of both dikes and gabbros were very similar to subalkaline basalts of the Patagonian Plateau that erupted at about the same time, 35 km to the east. Mafic and silicic magmas at Cordillera del Paine are consanguineous, as demonstrated by alkalinity and trace-element ratios. However, the contemporaneity of mafic and silicic magmas precludes a parent-daughter relationship. The granitic magma most likely was derived by differentiation of mafic magmas that were similar to those that later intruded it. Or, the granitic magma may have been contaminated by mafic magmas similar to the PMC magmas before its shallow emplacement. Mixing would be favored at deeper levels when the cooling rate was lower and the granitic magma was less solidified.     

Petrology of clinopyroxenite ejecta from Somma-Vesuvius and their genetic implications

Summary A suite of clinopyroxenite nodules, megacrysis and associated lavas from Somma-Vesuvius, Italy, has been investigated to establish its possible genetic relationships with the leucitebearing lavas of the Roman Region. The clinopyroxenites are essentially composed of clinopyroxene + mica and subordinate olivine, plagioclase, spinels, apatite and glass. The megacrysts are clinopyroxene fragments. The associated lavas are leucite-tephrites and a tephritic leucitite.The mineralogy of the clinopyroxenites is distinctive but gradational to that of the Somma-Vesuvius lavas and reflects subvolcanic crystallization of a silica-undersaturated, mafic magma. The megacrystic clinopyroxene is probably related to the clinopyroxenites.The chemical composition of the clinopyroxenites shows strong affinites to that of the Somma-Vesuvius lavas and corresponds to leucite basanite compositions. Interstitial glass in the clinopyroxenites represents a residual liquid from clinopyroxenite crystallization. This glass approaches the chemical composition of the Somma tephrites.The experimental melting of two clinopyroxenites at 1 atm demonstrates that the essential assemblage of the Somma-Vesuvius lava, leucite + clinopyroxene, may develop from basanite compositions where olivine disappears by reaction with the liquid to form clinopyroxene. It is concluded that the clinopyroxenites represent basanitic magma crystallized at depth and that the Somma-Vesuvius leucite-bearing lavas are potential derivatives of this magma.

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Petrologie von Klinopyroxenit-Auswürflingen von Somma-Vesuv und ihre genetische Bedeutung

Zusammenfassung Leucit-Tephrite und tephritische Leucitite der Romana enthalten Klinopyroxenit-Einschlüsse sowie Kristalle von Klinopyroxen, Glimmer, und untergeordnet Olivin, Plagioklas, Spinell, Apatit und Glas. Die genetischen Beziehungen zwischen Laven und Einschlüssen wurden an Hand der Ergebnisse petrologischer und geochemischer Untersuchungen überprüft.Die Mineralogie der Klinopyroxenite kann mit der der Somma-Vesuv-Laven korreliert werden und weist auf subvulkanische Kristallisation eines Si-untersättigten, mafischen Magmas hin.Die chemische Zusammensetzung der Klinopyroxenite zeigt deutliche Beziehungen zu den Laven von Somma-Vesuv und entspricht einem leucit-basanitischen Typ. Restschmelze der Klinopyroxenit-Kristallisation ist als Glas auf der Intergranulare erhalten. Die Zusammensetzung dieser Gläser ähnelt der von Somma-Tephriten.Schmelzversuche an zwei Klinopyroxeniten bei 1 atm zeigen, daß die wichtigste Mineralassoziation der Somma-Vesuv-Laven, Leucit und Klinopyroxen, aus einer basanitischen Zusammensetzung abzuleiten sind. Olivin verschwindet dabei durch Reaktion mit der Schmelze und Klinopyroxen wird gebildet. Die Untersuchungen lassen erkennen, daß die Klinopyroxenite Kristallisationsprodukte in der Tiefe erstarrter basanitischer Magmen sind, und daß die leucitführenden Magmen von Somma-Vesuv als mögliche Abkömmlinge dieser Magmen zu sehen sind.

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With 3 Figures     

Primary basalts and magma genesis

A representative Quaternary clinopyroxene leucitite lava from the Alban Hills, Roman comagmatic province, central Italy, has been subjected to anhydrous thermal experiments within its melting range at pressures up to 45 kb. The lava contains 2.2% of leucite and 1.4% of diopside phenocrysts in a fine-grained groundmass, suggesting that these phases were crystallizing on the liquidus of the magma immediately prior to its eruption. This situation is reproduced experimentally at 14 kb and 1260 °C. As there is no evidence that the H₂O content of this lava was appreciable, it is concluded that the anhydrous experimental results give a valid indication that this leucitite equilibrated with its phenocrysts at approximately 50 km depth, about 25 km into the upper mantle, before final rapid uprise. Comparison of the bulk compositions of Alban Hills mafic leucitites with that of the eutectic in the synthetic system Diopside-Leucite as a function of pressure confirms the conclusion of the high-pressure experiments. In contrast, the dilc ratios of other Roman province mafic leucitites indicate that they equilibrated within the upper crust prior to eruption. Published Sr and O-isotope studies show unequivocally that, when the Alban Hills mafic leucitites and their phenocrysts equilibrated, the magmas contained a substantial fraction of melt from crustal rocks. These data are reconciled with the experimental demonstration that the magmas evolved entirely within the upper mantle by postulating that their crustal components were derived from partial fusion of Tyrrhenian ocean-floor sediments, subducted beneath Italy during the anticlockwise rotation of the Corsica-Sardinia lithospheric microplate. The Roman province volcanics show considerable chemical similarities with lavas from converging plate margins elsewhere, together with substantial differences from other occurrences of strongly-potassic rocks. It is concluded that this magma type may be polygenetic.     

Coeval felsic and Mafic Magmas in neoarchean calc-alkaline magmatic arcs,Dharwar craton,Southern India: Field and petrographic evidence from mafic to Hybrid magmatic enclaves and synplutonic Mafic dykes

We present field and petrographic data on Mafic Magmatic Enclaves (MME), hybrid enclaves and synplutonic mafic dykes in the calc-alkaline granitoid plutons from the Dharwar craton to characterize coeval felsic and mafic magmas including interaction of mafic and felsic magmas. The composite host granitoids comprise of voluminous juvenile intrusive facies and minor anatectic facies. MME, hybrid enclaves and synplutonic mafic dykes are common but more abundant along the marginal zone of individual plutons. Circular to ellipsoidal MME are fine to medium grained with occasional chilled margins and frequently contain small alkali feldspar xenocrysts incorporated from host. Hybrid magmatic enclaves are intermediate in composition showing sharp to diffused contacts with adjoining host. Spectacular synplutonic mafic dykes commonly occur as fragmented dykes with necking and back veining. Similar magmatic textures of mafic rocks and their felsic host together with cuspate contacts, magmatic flow structures, mixing, mingling and hybridization suggest their coeval nature. Petrographic evidences such as disequilibrium assemblages, resorption, quartz ocelli, rapakivi-like texture and poikilitically enclosed alkali feldspar in amphibole and plagioclase suggest interaction, mixing/mingling of mafic and felsic magmas. Combined field and petrographic evidences reveal convection and divergent flow in the host magma chamber following the introduction of mafic magmas. Mixing occurs when mafic magma is introduced into host felsic magma before initiation of crystallization leading to formation of hybrid magma under the influence of convection. On the other hand when mafic magmas inject into host magma containing 30–40% crystals, the viscosities of the two magmas are sufficiently different to permit mixing but permit only mingling. Finally, if the mafic magmas are injected when felsic host was largely crystallized (~70% or more crystals), they fill early fractures and interact with the last residual liquids locally resulting in fragmented dykes. The latent heat associated with these mafic injections probably cause reversal of crystallization of adjoining host in magma chamber resulting in back veining in synplutonic mafic dykes. Our field data suggest that substantial volume of mafic magmas were injected into host magma chamber during different stages of crystallization. The origin of mafic magmas may be attributed to decompression melting of mantle associated with development of mantle scale fractures as a consequence of crystallization of voluminous felsic magmas in magma chambers at deep crustal levels.     

The influence of amphibole fractionation on the evolution of calc-alkaline andesite and dacite tephra from the central Aleutians,Alaska

Petrologic studies of tephra from Kanaga, Adak, and Great Sitkin Islands indicate that amphibole fractionation and magma mixing are important processes controlling the composition of calc-alkaline andesite and dacite magmas in the central Aleutians. Amphibole is ubiquitous in tephra from Kanaga and Adak Islands, whereas it is present only in a basaltic-andesite pumice from Great Sitkin. Dacitic tephra from Great Sitkin do not contain amphibole. Hornblende dacite tephra contain HB+PLAG+OX±OPX±CPX phenocrysts with simple zoning patterns, suggesting that the dacites evolved in isolated magma chambers. Andesitic tephra from Adak contain two pyroxene and hornbelende populations, and reversely zoned plagioclase, indicating a more complex history involving mixing and fractional crystallization. Mass balance calculations suggest that the andesitic tephra may represent the complements of amphibole-bearing cumulate xenoliths, both formed during the evolution of high-Al basalts. The presence of amphibole in andesitic and dacitic tephra implies that Aleutian cale-alkaline magmas evolve in the mid to lower crust under hydrous (>4 wt.% H₂O) and oxidizing (Ni–NiO) conditions. Amphibole-bearing andesites and pyroxene-bearing dacites from Great Sitkin indicates fractionation at several levels within the arc crust. Despite its absence in many calc-alkaline andesite and dacite lavas, open system behavior involving amphibole fractionation can explain the trace element characteristies of lavas found on Adak Island. Neither open nor closed system fractionation involving a pyroxene-bearing assemblage is capable of explaining the trace element concentrations or ratios found in the Adak suite. We envision a scenario where amphibole was initially a liquidus phase in many calc-alkaline magmas, but was later replaced by pyroxenes as the magmas rose to shallow levels within the crust. The mineral assemblage in these evolved lavas reflects shallow level equilibration of the magma, whereas the trace element chemistry provides evidence for a earlier, amphibole-bearing, mineral assemblage.     

Petrology and Geochemistry of the Bandas del Sur Formation, Las Canadas Edifice, Tenerife (Canary Islands)

The Bandas del Sur Formation preserves a Quaternary extra-calderarecord of central phonolitic explosive volcanism of the LasCañadas volcano at Tenerife. Volcanic rocks are bimodalin composition, being predominantly phonolitic pyroclastic deposits,several eruptions of which resulted in summit caldera collapse,alkali basaltic lavas erupted from many fissures around theflanks. For the pyroclastic deposits, there is a broad rangeof pumice glass compositions from phonotephrite to phonolite.The phonolite pyroclastic deposits are also characterized bya diverse, 7–8-phase phenocryst assemblage (alkali feldspar+ biotite + sodian diopside + titanomagnetite + ilmenite + nosean–haüyne+ titanite + apatite) with alkali feldspar dominant, in contrastto interbedded phonolite lavas that typically have lower phenocrystcontents and lack hydrous phases. Petrological and geochemicaldata are consistent with fractional crystallization (involvingthe observed phenocryst assemblages) as the dominant processin the development of phonolite magmas. New stratigraphicallyconstrained data indicate that petrological and geochemicaldifferences exist between pyroclastic deposits of the last twoexplosive cycles of phonolitic volcanism. Cycle 2 (0·85–0·57Ma) pyroclastic fall deposits commonly show a cryptic compositionalzonation indicating that several eruptions tapped chemically,and probably thermally stratified magma systems. Evidence formagma mixing is most widespread in the pyroclastic depositsof Cycle 3 (0·37–0·17 Ma), which includesthe presence of reversely and normally zoned phenocrysts, quenchedmafic glass blebs in pumice, banded pumice, and bimodal to polymodalphenocryst compositional populations. Syn-eruptive mixing eventsinvolved mostly phonolite and tephriphonolite magmas, whereasa pre-eruptive mixing event involving basaltic magma is recordedin several banded pumice-bearing ignimbrites of Cycle 3. Theperiodic addition and mixing of basaltic magma ultimately mayhave triggered several eruptions. Recharge and underplatingby basaltic magma is interpreted to have elevated sulphur contents(occurring as an exsolved gas phase) in the capping phonoliticmagma reservoir. This promoted nosean–haüyne crystallizationover nepheline, elevated SO₃ contents in apatite, and possiblyresulted in large, climatologically important SO₂ emissions. KEY WORDS: Tenerife; phonolite; crystal fractionation; magma mixing; sulphur-rich explosive eruptions     

Ultramafic and Mafic Inclusions from Adak Island: Crystallization History, and Implications for the Nature of Primary Magmas and Crustal Evolution in the Aleutian Arc

Xenolithic inclusions in calc-alkaline andesite from Mt. Moffettvolcano, Adak Island, Aleutian arc, reveal a nearly continuousrecord of crystallization of basaltic magmas in the crust, andpossibly upper mantle, of the arc. The record is more detailedand continuous than that obtained from study of calc-alkalinevolcanic rocks in the arc. Cumulate xenoliths form a progressiveseries in modal mineralogy from ultramafic, hornblende-bearingolivine clinopyroxenite to both hornblende-bearing and hornblende-freegabbros. The cumulate hornblende gabbro xenoliths are typicalof those found in island arc andesites worldwide. Xenolithicinclusions without cumulate textures, here termed compositexenoliths, are characterized by forsteritic olivine, zoned Cr-diopsideand hornblende, and are interpreted to have resulted from reactionand chilling upon magma mixing at depth. The olivine and clinopyroxene in both cumulate and compositexenoliths show the largest and the most complete variation trendsfor Ni, Cr, and FeO/MgO ratio yet reported in igneous xenolithsfrom island arc volcanic rocks. Variation of Ni in olivine indicatesthat the parent magmas for the xenoliths had minimum MgO contentsof 9 wt. per cent. Variation of Cr in clinopyroxene indicatesthat the magmas were basaltic rather than picritic, probablyin equilibrium with spinel lherzolite at near Moho depths. Successiveinjections of batches of primary melt into a magma chamber fractionatingolivine and clinopyroxene can reproduce observed compatibleelement depletion trends. A steady-state process of cotecticcrystallization in a magma chamber continually replenished withbasaltic magma is a possible mechanism for producing large accumulationsof olivine and clinopyroxene, suggesting that Alaskan-type ultramaficcomplexes are related to hydrous basaltic magmas in island arcs.This steady-state open-system crystallization process can alsoyield the abundant high-alumina basalt type in the Aleutianarc. Continued crystallization of high-alumina basalt in lowercrustal magma chambers, recorded in a mineralogically coherentseries of pyroxenite to hornblende gabbro xenoliths, can yieldbasaltic to andesitic magmas of the calc-alkaline series. No xenoliths with a sedimentary protolith have been found atMt Moffett, evidence that the arc crust is igneous in origin,with the lower crust formed of gabbro crystallized from mantle-derivedmelts. Ultramafic cumulates may reside in both the lower crustor upper mantle beneath the arc. A model is proposed wherebythe cumulate crystallization products of hydrous, mantle beneaththe arc. A model is define the upper mantle and lower crustof the arc over time.The net composition added to the crustof the arc is that of high-alumina basalt.     

The role of magma mixing/mingling and cumulate melting in the Neapolitan Yellow Tuff caldera-forming eruption (Campi Flegrei,Southern Italy)

Understanding the mechanisms responsible for the generation of chemical gradients in high-volume ignimbrites is key to retrieve information on the processes that control the maturation and eruption of large silicic magmatic reservoirs. Over the last 60 ky, two large ignimbrites showing remarkable zoning were emplaced during caldera-forming eruptions at Campi Flegrei (i.e., Campanian Ignimbrite, CI, ~?39 ka and Neapolitan Yellow Tuff, NYT, ~?15 ka). While the CI displays linear compositional, thermal and crystallinity gradients, the NYT is a more complex ignimbrite characterized by crystal-poor magmas ranging in composition from trachy-andesites to phonolites. By combining major and trace element compositions of matrix glasses and mineral phases from juvenile clasts located at different stratigraphic heights along the NYT pyroclastic sequence, we interpret such compositional gradients as the result of mixing/mingling between three different magmas: (1) a resident evolved magma showing geochemical characteristics of a melt extracted from a cumulate mush dominated by clinopyroxene, plagioclase and oxides with minor sanidine and biotite; (2) a hotter and more mafic magma from recharge providing high-An plagioclase and high-Mg clinopyroxene crystals and (3) a compositionally intermediate magma derived from remelting of low temperature mineral phases (i.e., sanidine and biotite) within the cumulate crystal mush. We suggest that the presence of a refractory crystal mush, as documented by the occurrence of abundant crystal clots containing clinopyroxene, plagioclase and oxides, is the main reason for the lack of erupted crystal-rich material in the NYT. A comparison between the NYT and the CI, characterized by both crystal-poor extracted melts and crystal-rich magmas representing remobilized portions of a “mature” (i.e., sanidine dominated) cumulate residue, allows evaluation of the capability of crystal mushes of becoming eruptible upon recharge.     

Petrochemical evidence of magma mingling and mixing in the Tertiary monzogabbroic stocks around the Bafra (Samsun) area in Turkey: implications of coeval mafic and felsic magma interactions

Miocene aged calc-alkaline mafic host stocks (monzogabbro) and felsic microgranular enclaves (monzosyenite) around the Bafra (Samsun) area within Tertiary volcanic and sedimentary units of the Eastern Pontides, Northeast Turkey are described for the first time in this paper. The felsic enclaves are medium to fine grained, and occur in various shapes such as, elongated, spherical to ellipsoidal, flame and/or rounded. Most enclaves show sharp and gradational contacts with the host monzogabbro, and also show distinct chilled margins in the small enclaves, indicating rapid cooling. In the host rocks, disequilibrium textures indicating mingling or mixing of coeval mafic and felsic magmas are common, such as, poikilitic and antirapakivi textures in feldspar phenocrysts, sieve textured-patchy-rounded and corroded plagioclases, clinopyroxene megacrysts mantled by bladed biotites, clinopyroxene rimmed by green hornblendes, dissolution in clinopyroxene, bladed biotite, and acicular apatite. The petrographical and geochemical contrasts between the felsic enclaves and host monzogabbros may partly be due to a consequence of extended interaction between coeval felsic and mafic magmas by mixing/mingling and diffusion. Whole-rock and Sr-Nd isotopic data suggests that the mafic host rocks and felsic enclaves are products of modified mantle-derived magmas. Moreover, the felsic magma was at near liquidus conditions when injected into the mafic host magma, and that the mafic intrusion reflects a hybrid product formed due to the mingling and partial (incomplete) mixing of these two magmas.     

The dynamics of magma-mixing during flow in volcanic conduits

The production of mixed magmas (streaky pumice) during flow in a volcanic conduit has been modelled in the laboratory by studying the flow of two miscible fluids of differing viscosity passing concentrically through a vertical pipe. In the experiments reported in this paper, the outermost fluid is the more viscous, as would be the case when two magmas are simultaneously tapped from a zoned chamber in which silicic magma overlies mafic magma. At a Reynolds number (Re) which is much less than that required for turbulence in isoviscous pipe flow, the interface between two liquids of different viscosity can become unstable. Growth of the instability and mixing proceed when Re, based on the properties of the inner, less viscous fluid (Re _i), is greater than approximately 3 if between 10% and 90% of the flowing fluid is composed of the more viscous fluid. Outside this range of flow rate ratios, higher Re _i and viscosity ratios are required to ensure mixing. When the viscosity ratio U10 the unstable flow takes the form of an asymmetric, sinusoidal wave and at higher viscosity ratios axisymmetric, bead-like waves are the dominant instability. Entrainment across the boundaries of these wavy interfaces results in the production of streaky mixtures of the two liquids. The degree of mixing increases with Re ₁, U and distance downstream. Application of experimental results to magmatic situations shows that mixing will be possible in eruptions which tap layers of different viscosity from a stratified chamber. If a volcanic feeder is allowed to become lined by silicic magma before a mafic magma layer is drawn up from the chamber then a mixed pumice (or lava) sequence will ensue. Alternatively, if draw-up occurs when the feeder is still propagating away from the chamber, the slower flowing silicic magma may be overtaken by the faster flowing mafic magma. The advancing conduit will then have mafic or hybrid chilled margins enclosing a silicic interior, i.e. the usual arrangement in composite dykes and sills. Simultaneous tapping of silicic and underlying mafic magmas from a chamber can thus lead to magma mixing and to the emplacement of either mixed pumice sequences or composite intrusions, depending on the history of magma withdrawal and the dynamics of flow in the conduit.     

Lamprophyres of the Tomtor Massif: A result of mixing between potassic and sodic alkaline mafic magmas

The paper presents data on inclusions in minerals of the least modified potassic lamprophyres in a series of strongly carbonatized potassic alkaline ultramafic porphyritic rocks. The rocks consist of diopside, kaersutite, analcime, apatite, and rare phlogopite and titanite phenocrysts and a groundmass, which is made up, along with these minerals, of potassic feldspar and calcite. The diopside and kaersutite phenocrysts display unsystematic multiple zoning. Chemically and mineralogically, the rock is ultramafic foidite and most likely corresponds to monchiquite. Primary and secondary melt inclusions were found in diopside, kaersutite, apatite, and titanite phenocrysts and are classified into three types: sodic silicate inclusions with analcime, potassic silicate inclusions with potassic feldspar, and carbonate inclusions, which are dominated by calcite. Heating and homogenization of the inclusions show that the potassic lamprophyres crystallized from a heterogeneous magma, with consisted of mixing mafic sodic and potassic alkaline magmas enriched in a carbonatite component. The composition of the magmas was close to nepheline and leucite melanephelinite. The minerals crystallized at 1150–1090°C from the sodic melts and at 1200–1250°C from the potassic ones. The sodic mafic melts were richer in Fe than the potassic ones, were the richest in Al, Mn, SO₃, Cl, and H₂O and poorer in Ti and P. The potassic mafic melts were not lamproitic, as follows from the presence of albite in the crystallized primary potassic melt inclusions. The diopside, the first mineral to crystallize in the rock, started to crystallize in the magmatic chamber from sodic mafic melt and ended to crystallize from mixed sodic–potassic melts. The potassic mafic melts were multiply replenished in the chamber in relation to tectonic motions. The ascent of the melts to the surface and rapidly varying P–T parameters of the magma were favorable for multiple separations of carbonatite melts from the alkaline mafic ones and their mixing and mingling.     

Petrology of middle proterozoic diabases and picrites from Lake Nipigon,Canada

Mafic rocks at Lake Nipigon provide a record of rift-related continental basaltic magmatism during the Keweenawan event at 1109 Ma. The mafic rocks consist of an early, volumetrically minor suite of picritic intrusions varying in composition from olivine gabbro to peridotite and a later suite of tholeiitic diabase dikes, sheets and sills. The diabase occurs primarily as two 150 to 200 m thick sills with a textural stratigraphy indicating that the sills represent single cooling units. Compositional variation in the sills indicates that they crystallized from several magma pulses.The diabases are similar in chemistry to olivine tholeiite flood basalts of the adjacent Keweenawan rift, particularly with respect to low TiO₂, K₂O and P₂O₅. The picrites have higher TiO₂, K₂O and P₂O₅ than the diabases and are similar to, but more primitive than, high Fe-Ti basalts which erupted early in the Keweenawan volcanic sequence.All of the rocks crystallized from fractionated liquids. The picrites are cumulate rocks derived at shallow crustal depths from a magma controlled predominantly by olivine fractionation. Picritic chills are in equilibrium with olivine phenocrysts of composition Fo₈₀ and are interpreted to represent the least evolved liquids observed. The parental magma of the picrites was probably Fe rich relative to the parental magma of the diabase. The diabase sills crystallized from an evolved basaltic liquid controlled by cotectic crystallization of plagioclase and lesser olivine and pyroxene.The emplacement of dense olivine phyric picritic magmas early in the sequence, followed by later voluminous compositionally evolved magmas of lower density suggests the development of a crustal density filter effect as the igneous event reached a peak. Delamination of the crust-mantle interface may have resulted in the transition from olivine controlled primitive magma to fractionated magma through the development of crustal underplating.     

Experimental constraints on depths of fractionation of mildly alkalic basalts and associated felsic rocks: Pantelleria,Strait of Sicily

Pantelleria, Italy, is a continental rift volcano consisting of alkalic basalt, trachyte, and pantellerite. At 1 atm along the FMQ buffer, the least-evolved basalt (Mg #= 58.5% norm ne) yields olivine on the liquidus at 1,180° C, followed by plagioclase, then by clinopyroxene, and by titanomagnetite and ilmenite at 1,075°. After 70% crystallization, the residual liquid at 1,025° is still basaltic and also contains apatite and possibly kaersutite. A less alkalic basalt shows the same order of phase appearance. Glass compositions define an Fe-enrichment trend and a density maximum for anhydrous liquids that coincides with a minimum in Mg#.During the initial stages of crystallization at 1 atm, liquids remain near the critical plane of silica-undersaturation until, at lower temperatures, Fe-Ti oxide precipitation drives the composition toward silica saturation. Thus the qtz-normative trachytes and pantellerites typically associated with mildly ne-normative basalts in continental rifts could be produced by low-pressure fractional crystallization or by shallow-level partial melting of alkali gabbro. At 8 kbar, clinopyroxene is the liquidus phase at 1,170° C, followed by both olivine and plagioclase at 1,135°. Because clinopyroxene dominates the crystallizing assemblage and plagioclase is more albitic than at 1 atm, liquids at 8 kbar are driven toward increasingly ne-normative compositions, suggesting that higher-pressure fractionation favors production of phonolitic derivatives.Natural basaltic samples at Pantelleria are aphyric or contain 1–10% phenocrysts of plag olcpx or ol>cpx, with groundmass Fe-Ti oxides and apatite. The lack of phenocrystic plagioclase in two of the lavas suggests that crystallization at slightly higher PH₂O may have destabilized plagioclase relative to the 1-atm results, but there is no preserved evidence for significant fractionation at mantle depths as clinopyroxene is the least abundant phenocryst phase in all samples and contains only small amounts of octahedral Al. The liquid line and phenocryst compositions match more closely the 1-atm experimental results than those at 8 kbar.Although major-element trends in natural liquids and crytals reflect low-pressure fractionation, minor- and trace-element concentrations preserve evidence of multiple parental liquids. Scatter in variation diagrams exceeds that attributable to crystal accumulation in these phenocryst-poor rocks, and the large range in concentrations of P and Ti at high MgO contents cannot be produced by polybaric fractionation nor by mixing with coexisting felsic magmas. Sr and O isotope ratios rule out significant interaction with crystalline upper crust, Mesozoic shelf sediments, or Tertiary evaporites. Positive correlations of compatible and incompatible elements suggest that the basalts are not simply related to one another by closed-system fractional crystallization of a single parental magma. Increasing Ce/Yb with Ce suggests that these relations are not a product of mixing within a replenished magma chamber, nor of mixing with more felsic members of the suite, which have smaller Ce/Yb ratios. Low-pressure fractional crystallization of ol+cpx+ plag±oxides from slightly different parental magmas produced by varying degrees of melting of garnet-bearing peridotite is a possible scenario.Small and infrequently replenished magma reservoirs in this continental rift environment may account for the strongly differentiated nature of the Pantellerian basalts. There is no correlation between Mg# and eruptive frequency, in part because concentration of volatiles in residual liquids offsets the effect of Fe-enrichment on melt density, such that strong Fe-enrichment is no hindrance to eruption.     

Samples from the crystallising boundary layer of a zoned magma chamber

The spatial and chemical relationships between the melt occupying the reservoir and the mineral assemblages crystallising at the margins are reconstructed for the magma chamber which produced the 11000 yr.B.P. tephra deposit of Laacher See Volcano. The melt showed vertical chemical zonation immediately prior to eruption, and throughout most of the magma volume only a small fraction of crystals were present. The eruption also ejected crystal-rich nodules, ranging from mafic to felsic in composition, which are samples of the materials crystallising at the boundaries of the chamber. New data on nodule petrography and chemical compositions of whole-rocks, minerals and interstitial glasses are presented. Volume fraction of interstitial glass is not systematically related to mineral assemblage and varies typically between 1 and 20 vol%, i.e. the crystals interlock. One exception is a group of mafic nodules with glass volume fractions between 25 and 40 vol%. Bulk compositions of mafic nodules show strong enrichments or depletions in all major elements relative to the mafic phonolite interstitial melt. Felsic nodules show much less pronounced differences with their interstitial melt. Felsic nodules contain interstitial glasses with a range of compositions similar to that in the zoned bulk of the chamber and were probably derived from different heights on the walls. Mafic nodules have glass compositions similar to those at the base of the zoned liquid column and were probably derived from the floor. Modal mineralogy, glass composition and mineral composition are systematically related in the nodules whereas in individual pumices samples derived from the main body of the chamber, a broader range of mineral compositions are often found. Mineral assemblages were especially diverse in the upper part of the chamber. It is deduced that the whole of the essentially liquid part of the chamber was emptied by the eruption, that strongly contrasting mineral assemblages were forming simultaneously on the walls and floor, that the gradient in crystal content in the crystallisation boundary layer was more gradual at the floor than at the walls, and that the pumice mineralogy is not a simple phenocryst assemblage but is a mixture of crystals which grew from melts separated in space and/or time.     

Water-deficient Calc-alkaline Plutonic Rocks of Northeastern Superior Province, Canada: Significance of Charnockitic Magmatism

Calc-alkaline batholiths of the Archaean Minto block, northeasternSuperior Province, Canada, have pyroxene- and hornblende-bearingmineral assemblages inferred to have crystallized from hot,water-undersaturated magmas at 2·729–2·724Ga. A regional amphibolite- to granulite-facies tectonothermalevent at 2·70 Ga resulted in mild to negligible metamorphiceffects on the dominantly granodioritic units. Geochemical,textural and thermobarometric studies define the crystallizationhistory in compositions ranging from cumulate pyroxenite throughquartz diorite, granodiorite, granite, and syn-magmatic gabbroicdykes. Early magmatic assemblages include orthopyroxene, clinopyroxene,plagioclase, biotite, Fe–Ti oxides and ternary feldspar,indicating crystallization from magmas containing <2 wt %H₂O at 1100–900°C. Water enrichment in the residualmelt induced hornblende crystallization at 5 ± 1 kbar,800–600°C. Characterized by a continuum of large ionlithophile element (LILE)-enriched, high field strength element(HFSE)-depleted compositions, the I-type suite resembles moderncontinental arc batholiths in composition and size but not primarymineralogy. Magmatic arcs produced between 2·75 and 1·85Ga commonly have charnockitic components, possibly because slab-derivedfluids interacted with mantle wedges at ambient temperatureshigher by 100°C than at present, producing large volumesof water-deficient magma. KEY WORDS: granitoid rocks; igneous pyroxenes; water-undersaturated magma; charnockite     

Kaersutite-peridotite inclusions and kindred megacrysts in basanitic lavas,Grand Canyon,Arizona

Essentially two types of ultramafic inclusions occur in the basanitic lavas and ejecta deposits of the northwestern Grand Canyon, Arizona. Abundant, olivine-rich nodules contain an emerald green, chrome-rich diopside and chrome-rich spinels. A much less common group of inclusions generally containing poikilitic kaersutite have more variable modal compositions, more variable but iron-rich and chrome-poor mineral compositions, and are characterized by the presence of a titaniferous clinopyroxene which appears black in hand specimen. The nature and petrologic significance of these black clinopyroxene-bearing inclusions, together with megacrysts of kaersutite and black clinopyroxene, are discussed in this paper.Petrographic aspects indicate an origin as cumulates of fractionating basaltic magma. Compositions of pyroxenes suggest high pressures of crystallization. The co-precipitation of orthopyroxene, clinopyroxene, olivine and Mg-spinel from what in all probability was under-saturated magma, together with the total absence of feldspar as a cumulate or intercumulate phase, is compatible with crystallization near 10 kb, on the basis of quite limited experimental data on anhydrous basaltic compositions. Pressures of this sort are attained at depths close to the mantle-crust boundary in the western Grand Canyon. By way of comparison, cumulate-textured inclusions from central Nevada containing rare orthopyroxene, widespread plagioclase, and more Fe-enriched clinopyroxenes, kaersutites, olivines and spinels are postulated to have crystallized at lower temperatures (or at a more advanced stage of fractionation) and possibly at lower pressures.Numerous occurrences, worldwide, of kaersutite-bearing inclusions, always in undersaturated host rocks, have recently been reported. Compositionally, the kaersutites are quite uniform, whether coexistent with pyropic garnet-clinopyroxene (Kakanui, New Zealand), with ortho-pyroxene-clinopyroxene-olivine-Mg spinel (Grand Canyon), or with plagioclase-clinopyroxene-olivine-magnetite. The last assemblage is found in shallow-seated igneous bodies of alkalic, mafic composition, as well as in inclusions within basaltic rocks. These occurrences imply the precipitation of kaersutite amphibole over a broad range of pressures, and as high as those prevailing in the upper mantle.     

Mafic xenoliths from Egyptian Tertiary basalts and their petrogenetic implications

Mineralogical data, coupled with whole-rock major and trace element data of mafic xenoliths from two occurrences of the Egyptian Tertiary basalts, namely Abu Zaabal (AZ) near Cairo and Gabal Mandisha (GM) in the Bahariya Oases, are presented for the first time. Chemically, AZ basalts are sodic transitional, while those of GM are alkaline. In spite of the different petrographic and geochemical features of the host rocks, mafic xenoliths from the two occurrences are broadly similar and composed essentially of clinopyroxene, plagioclase, alkali feldspar, and Fe–Ti oxides. The analytical results of host rocks, xenoliths and their minerals suggest that the xenoliths are cognate to their host magmas rather than basement material. The mafic xenoliths are olivine-free and contain alkali feldspar contrary to the phenocryst assemblage of the host rocks, confirming that they are not cumulates from the host magma. The geochemical and mineralogical characteristics show that the precursor magmas of these xenoliths are more fractionated and possibly contaminated compared to those of the host rocks. Estimated crystallization conditions are  1–3 kbar for xenoliths from both areas, and temperature of  950–1100 °C vs. 920–1050 °C for AZ and GM, respectively. These cognate xenoliths probably crystallized from early-formed, highly-fractionated anhydrous magma batches solidified in shallow crustal levels, possibly underwent some AFC during their ascent, and later ripped-up during fresh magma pulses. The xenoliths, although rare, provide an evidence for the importance of crystal fractionation at early evolution of the Egyptian Tertiary basalts.