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Showing papers on "Phenocryst published in 1975"


Journal ArticleDOI
TL;DR: The intimate association of basalt, andesite, dacite, and rhyolite within a volcanic center suggests that these rocks are genetically related Individual lava flows that show a gradation in composition may preserve maximum evidence of the magmatic processes producing this association.
Abstract: The intimate association of basalt, andesite, dacite, and rhyolite within a volcanic center suggests that these rocks are genetically related Individual lava flows that show a gradation in composition may preserve maximum evidence of the magmatic processes producing this association One such flow of rhyolite to dacite composition, Glass Mountain in northern California, was formed by contamination of rhyolite magma as it intruded the basaltic flows of the Medicine Lake Highland shield volcano Although dacite flows and domes commonly show less variation in composition than the Glass Mountain flow, many show similar evidence of contamination by basalt by the presence of abundant basaltic inclusions and phenocrysts and phenocryst clots from those inclusions Similarly, many andesite flows contain rhyolitic inclusions, rhyolitic bands, and phenocrysts appropriate to rhyolite These observations indicate that andesite and dacite are hybrid rocks that are formed when rising primary basalt and rhyolite magmas either become contaminated with the glassy debris of the volcanic pile or mix with each other directly Linear variation in bulk composition, phenocryst assemblages of intermediate rock, and frequency distribution of lava compositions in the southern Cascade Range, Chilean Andes, Taupo volcanic zone, and Tongan Islands support this hypothesis It appears that partial melting usually produces magma of rhyolitic and basaltic compositions and that any subsequent fractional crystallization is of limited importance

336 citations


Journal ArticleDOI
TL;DR: The sequence of crystallization in a biotite-granite from the Bohus batholith of Norway and Sweden, deduced from its texture, was determined experimentally at 2 kb in the presence of varying only for H 2O contents below 1.2% by weight.
Abstract: The sequence of crystallization in a biotite-granite from the Bohus batholith of Norway and Sweden, deduced from its texture, was magnetite, plagioclase, microcline, quartz, and finally biotite. Several sequences of crystallization were determined experimentally at 2 kb in the presence of varying only for H_2O contents below 1.2% by weight. The rock was fused to a homogeneous glass, and each experiment included samples of finely crushed rock and glass. The samples were reacted in Ag-Pd capsules with measured H_2O content in coldseal pressure vessels with NNO buffer. With excess H_2O (more than 6.5%) the crystallization interval extends from 865° C to 705° C. In the H_2O-deficient region, the solidus temperature remains unchanged as long as a trace of vapor is present, but the liquidus temperature increases as H_2O content decreases; with 0.8 % H_2O the liquidus temperature is 1125° C, the crystallization interval is 420° C, and a separate aqueous vapor phase is evolved only a few degrees above the solidus at 705° C. The biotite phase boundary increases slightly from 845° C with excess H_2O to 875° C with 1% H_2O, and it intersects the steep phase boundaries for quartz and feldspars; the sequence of crystallization changes at each intersection point. Similar diagrams at various pressures for related rock compositions involving muscovite, biotite and amphibole will provide grids useful in defining limits for the water content of granitic and dioritic magmas. Applications are considered for the Bohus batholith, other granitic rocks, and rhyolites. The Bohus magma could have been formed by crustal anatexis as a mobile assemblage of H_2O-undersaturated liquid and residual crystals with initial total H_2O content less than 1.2%, or it could have been derived by fractionation of a more basic parent with low H_2O content from mantle or subduction zone, but it could not have been derived from a primary andesite generated from mantle peridotite. We consider it unlikely that the H_2O content of large granitic magma bodies exceeds about 1.5% H_2O; these magmas are H_2O-undersaturated through most of their histories. Uprise and progressive crystallization of magma bodies produces H_2O-saturation around margins and in the upper regions of magma chambers. H_2O-saturated rhyolitic and dacitic magmas with phenocrysts can be tapped from the upper parts of the magma chambers.

159 citations


Journal ArticleDOI
TL;DR: The voluminous Pleistocene-Recent Taupo rhyolites typically contain phenocrysts of plagioclase (oligoclase-labradorite), quartz, titanomagnetite, ilmenite, and ferromagnesian silicates as mentioned in this paper.
Abstract: The voluminous Pleistocene—Recent Taupo rhyolites typically contain phenocrysts of plagioclase (oligoclase-labradorite), quartz, titanomagnetite, ilmenite, and ferromagnesian silicates. Ferromagnesian assemblages correlate with well defined Fe-Ti oxide equilibration temperature ranges and allow the rhyolites to be subdivided as follows: (1) Cummingtonite (c)—calcic hornblende (hb)—orthopyroxene (opx); 725–755°C, (2) Hb-opx, 750–825°C, (3) Biotite-hb-(c-opx), 720–765°C, (4) Opx-clinopyroxene (cpx), 860–915°C, (5) Fe olivine-opx-cpx, one sample with temperature of 900°C. Plagioclase and orthopyroxene phenocryst compositions typically exhibit a range of composition up to ∼20 mol.%. Calculated average phenocryst equilibration pressures (P total) range between 0.5–4.9 kb, and average 2.2 kb (∼7–8 km depth), indicating upper crustal crystallization. These calculations are very sensitive to variations in phenocryst composition. Calculated $$/_{{\text{H}}_2 {\text{O}}} $$ for the amphibole and biotite-bearing rhyolites indicate phenocryst equilibration under $$P_{{\text{H}}_2 {\text{O}}} \simeq P_{{\text{total}}} $$ , with $$X_{{\text{H}}_2 {\text{O}}} $$ ∼0.17–0.24 (5–8 wt. %). The precipitation of cummingtonite is thus temperature dependent, the upper limit being close to 760°C. Eruptive mechanisms of the lavas, pumices, and ash-flow deposits are evidently not primarily controlled by temperature, P total, $$P_{{\text{H}}_2 {\text{O}}} $$ , or crystal content of the magmas, and explanations must lie in kinetic and fluid dynamic behavior of the magmas. For the Taupo rhyolites, it is suggested that the critical size of a magma body (i.e. Rayleigh number) is a controlling factor in that it will influence the convective regime; fully turbulent convection is deduced to have occurred within the larger magma bodies. One consequence is intense vesiculation, prior to eruption, within the uppermost zones of these magma chambers, and the voluminous pumice deposits are believed to emanate from such chambers. Oscillatory compositional zoning within pyroxene phenocrysts is consistent with magma convection.

140 citations


Journal ArticleDOI
TL;DR: In this article, the major element compositions of group 1 whole rocks and observed phenocrysts were used to predict the crystallization histories of increasingly residual liquids, and allowed semi-quantitative verification of origin by fractional crystallization of the olivine-basalt parent magma.

72 citations


Journal ArticleDOI
TL;DR: In this article, the oxygen isotope variations among all major phenocryst phases: quartz, sanidine, plagioclase, biotite, augite, and magnetite.
Abstract: Glassy rocks from compositionally zoned ash-flow sheets and cogenetic lava flows, erupted from the late Miocene Timber Mountain–Oasis Valley caldera complex, show systematic oxygen-isotope variations (90 analyses) among all major phenocryst phases: quartz, sanidine, plagioclase, biotite, augite, and magnetite. Oxygen-isotope compositions of each phenocryst phase become lighter in O18 with decreasing age and are interpreted as indicating major interaction between meteoric ground water and batholithic-sized bodies of silicic magma prior to eruption. Each of the ash-flow sheets is compositionally zoned from silicic and phenocryst-poor at the base of more mafic and phenocryst-rich at the top; this pattern represents in inverse order the original compositional zonation in the magma chamber. The changing isotopic fractionations between phenocrysts also reflect crystallization temperatures that varied vertically over several kilometers in the differentiated magma. Posteruption oxidation and cooling effects have not significantly modified oxygen-isotope compositions of phenocrysts from glassy rocks.

67 citations


Journal ArticleDOI
TL;DR: The Cenozoic volcanic rocks of Ross Island and vicinity, Antarctica, are surface flows, tuffs, breccias, and small intrusives as discussed by the authors, and the prominent rock is basanitoid.
Abstract: The Cenozoic volcanic rocks of Ross Island and vicinity, Antarctica, are surface flows, tuffs, breccias, and small intrusives. The prominent rock is basanitoid. Alkali-basalt magma reached the surface over an appreciable length of time and was differentiated to produce a rock series : basanitoid → trachybasalt → phonolite. The alkalic, silica-undersaturated rocks are part of a larger petrologic province composed of volcanic centers in a belt nearly 2,000 km long roughly parallel to the Trans-antarctic Mountains. Two trends of magmatic differentiation are apparent. At depth the fractional crystallization of olivine, clinopyroxene, and opaque oxide minerals, all of which are abundant as phenocrysts in the basanitoid flows, produced trachybasalt magma. At crustal levels, fractional crystallization of clinopyroxene, apatite, opaque oxides, kaersutite, plagioclase, and anorthoclase developed the phonolitic rocks. Alkali enrichment is marked, and the end-member phonolites contain 11% Na₂O, 5.6% K₂O, and 57% SiO...

59 citations


Journal ArticleDOI
TL;DR: The dominant rock type of the Fen carbonite complex is damkjernite, a lamprophyric ultrabasic rock that occurs in diatreme and hypabyssal facies.
Abstract: The dominant rock type of the Fen carbonite complex is damkjernite, a lamprophyric ultrabasic rock that occurs in diatreme and hypabyssal facies. Damkjernite bears no chemical resemblance to kimberlite, but is chemically similar to alnoite, monchiquite and ouachitite. Phenocrysts include Ti-phlogopite, amphibole, acmitic pyroxene and nepheline; albite, nepheline and sphene occur in the groundmass. Calcite silicate textures suggest liquid immiscibility during crystallization. Calculated values of aSiO2 and crystallization T for the groundmass allow equilibration of the damkjernite magma with peridotite at T = 1200–1250°C, P = 15–25 kbar. Lherzolite nodules were accidentally included by the magma on its way to the surface, following its partial crystallization in a subcrustal magma chamber. ( 87 Sr 86 Sr ) 0 of six rocks correlates positively with CO2, Sr and Na Na + K , and inversely with SiO2 and oxidation state, suggesting assimilation of crustally contaminated carbonatite. We suggest that the damkjernite formed by partial melting of peridotite in the presence of a CO2-rich vapor phase. Carbonatite magma separated from the silicate melt in the subcrustal magma chamber, intruded into the crust, and exchanged Sr isotopes with its country rocks. During later intrusion, the damkjernite remelted the carbonatites and was contaminated by them.

45 citations


Journal ArticleDOI
TL;DR: Kimberlite breccias from several South African pipes or pipe-like enlargements on dykes contain abundant fragments of country rocks including huge, often brecciated, xenolithic masses which are located at positions well below their original stratigraphic levels.
Abstract: Kimberlite breccias from several South African pipes or pipe-like enlargements on dykes contain abundant fragments of country rocks including huge, often brecciated, xenolithic masses which are located at positions well below their original stratigraphic levels The xenoliths show no pyrometamorphic effects indicative of relatively high temperatures during their incorporation in the kimberlite A striking petrographic feature of these rocks is an emulsion-like texture resulting from the presence of numerous phlogopite-rich, globular segregations, which are set in a matrix which sometimes consists almost solely of primary calcite but in other instances is comprised of variable proportions of calcite and serpentine Evidence is presented which indicates that in the latter cases serpentine has replaced calcite The clear separation of the mica-rich segregations and the essentially calcitic matrix is attributed to the development of low temperature, immiscible K-rich silicate and carbonatitic liquids during emplacement of these rocks The carbonatitic liquids are considered to have acted as the primary transporting fluids during liquid-solid fluidized intrusion and emplacement of the rocks is considered to have taken place as follows After the crystallization of early phenocrysts the residual, ascending kimberlite magma differentiates at relatively high crustal levels into immiscible silicate and carbonatitic liquids accompanied by a coexisting gaseous phase Continued intrusion is accompanied by the separation of the gaseous phase and its accumulation at the head of the magma column Upon further uprise a stage is reached where the internal gas pressure exceeds the lithostatic load and the diatreme is formed by explosive breaching of the cap rocks The resulting rapid pressure drop is accompanied by the upsurge of partly degassed magma fractions which incorporate explosively disrupted cap rock fragments and material which slumps from the walls of the diatreme

43 citations


01 Jan 1975
TL;DR: In a study of the origin of olivine-bearing lunar rocks, symplectite is grouped into the following types: (1) very abundant blebs in phenocrysts, (2) scattered blebs and elongated masses along grain boundaries in troctolite, (3) scattered mosaic assemblages of coarser minerology along the grain boundaries, (4) highly abundant needle-like arrays, and (5), very abundant grains dispersed in rows as discussed by the authors.
Abstract: In a study of its origin in olivine-bearing lunar rocks, symplectite is grouped into the following types: (1) very abundant blebs in olivine phenocrysts, (2) scattered blebs in dunite, (3) scattered elongated masses along grain boundaries in troctolite, (4) scattered mosaic assemblages of coarser minerology along grain boundaries, (5) very abundant needle-like arrays, and (6) very abundant grains dispersed in rows. Four theories are proposed to explain the various symplectite distributions.

40 citations


Journal ArticleDOI
TL;DR: In this article, the authors studied the Bufumbira Bay area of the Birunga volcanic field, in south-west Uganda, form two distinct chemical series diverging from basanite compositions, one differentiating through phonolitictephrites to a trachyte and the other differentiates through leueitites to a phonolite.
Abstract: Lavas ocouring in the western Rift of Central Africa are characterized by the presence of leucite both as phenocrysts and in the groundmass. Samples studied from the Bufumbira Bay area of the Birunga volcanic field, in south-west Uganda, form two distinct chemical series diverging from basanite compositions. One differentiates through phonolitictephrites to a trachyte and the other differentiates through leueitites to a phonolite. The low-pressure fractionation controls of both series are discussed in terms of the coexisting phases and ultramafic inclusions observed in the lavas. Equilibrium fusion of a mantle phlogopite-bearing peridotite combined with subsequent fractionation is the favoured petro-genetic process to produce a potash-rich basanite liquid, which is considered a likely parent of the Bufumbira lavas.

35 citations


Journal ArticleDOI
TL;DR: Partition coefficients of uranium between phenocrysts and their host groundmass have been determined by fission track mapping as mentioned in this paper, showing that U is strongly partitioned into the liquid and only a small fraction of the total whole-rock U content is present in the major rock-forming minerals.

Journal ArticleDOI
04 Sep 1975-Nature
TL;DR: In this paper, the preferential orientation of elongated olivine phenocrysts in porphyritic chondrules has been observed, together with lineation in the most strongly foliated specimens.
Abstract: IT is more than a century since oriented fabrics were first noted in chondrites (see ref. 1), but since then little work has been done on the topic2,3, the most extensive being that by Dodd4. He found that most of his H- and L-group chondrites possessed foliation, together with lineation in the most strongly foliated specimens. The preferential orientation of elongated olivine phenocrysts in porphyritic chondrules has also been observed5.

Journal ArticleDOI
TL;DR: The Older Series volcanics of Mauritius form a widely differentiated transitional basalt suite, in which two distinct stages of activity can be recognized as discussed by the authors, with the first stage composed principally of alternating picrite-basalt flows and agglomerate; this is followed by a second, evolved stage composed of feldsparphyric basalt, hawaiite, mugearite, and high-level trachytic intrusive rocks.
Abstract: The Older Series volcanics of Mauritius form a widely differentiated transitional basalt suite, in which two distinct stages of activity can be recognized. The first stage, shield-building, is composed principally of alternating picrite-basalt flows and agglomerate; this is followed by a second, evolved stage composed of feldsparphyric basalt, hawaiite, mugearite, and high-level trachytic intrusive rocks. Three suites of nodules, each with specific associations, occur in the volcanic rocks; these are probably derived from layered subvolcanic cumulates. The nodular suites are: (a) dunite and wehrlite (restricted to the picrite basalt); (b) Bytownitic anorthosite (found only in the feldsparphyric basalt); and (c) mafic syenite (exclusive to the trachytic intrusive rocks). There is some evidence, from the extremely calcic nature of the plagioclase in the anorthosite nodules and feldsparphyric basalt and from the presence of kaersutite in evolved lava, that a hydrous period of crystallization developed - probably in the later stages of activity. Variation diagrams indicate clearly the close control of phenocryst mineralogy on bulk-lava chemistry, compositions with more than 5 or 6 percent MgO lying along a pronounced olivine -+- clinopyroxene control line and more evolved lava along a trend developed from fractionation of olivine + clinopyroxene + plagioclase + titanomagnetite. Trachytic compositions show pronounced trace-element trends, probably controlled by anorthoclase fractionation, but it can be demonstrated that they are unlikely to represent successive differentiates from a common trachytic magma. A distinct “silica gap” within the series, reflected in the absence of benmoreite, is tentatively ascribed to secondary boiling phenomena.

Journal ArticleDOI
TL;DR: In this paper, chemical analyses for Na, K, and trace elements of muscovite and potassium feldspar of granitic rocks are given for equilibrium and the distribution of trace elements in coexisting minerals suggests that equilibrium was attained and that muscoveite is a primary mineral.

Journal ArticleDOI
TL;DR: One of the three facies of the newly discovered kimberlite on Somerset Island consists of phenocrysts or xenocrysts of olivine, garnet, phlogopite, enstatite-bronzite, magnetite, magnesian ilmenite, red chrome spinel and chromite as mentioned in this paper.
Abstract: One of the three facies of the newly discovered kimberlite on Somerset Island consists of phenocrysts or xenocrysts of olivine, garnet, phlogopite, enstatite-bronzite, magnetite, magnesian ilmenite, red chrome spinel and chromite, set in a fine-grained groundmass consisting of black chrome spinel, magnetite-spinel-ulvospinel solid solution, perovskite, rutile, pyrite, heazlewoodite, chalcopyrite, dolomite and calcite. Chemical analyses, as well as detailed descriptions of the texture and occurrence of all the major minerals, are given. The texture of the whole rock indicates that an immiscible carbonate liquid was evolved at one stage in the crystallization of the silicate liquid, and that a third sulphide liquid may also have existed. During the period when the two or three liquids existed, some extremely complex epitaxial zonations were formed around oxide minerals which were crystallizing from the silicate liquid. These zonations show that there was a late-stage concentration of TiO 2 in the silicate liquid. The zonations have also been used to reconstruct the history of crystallization of the kimberlite.

Book ChapterDOI
01 Jan 1975
TL;DR: One of the three facies of the newly discovered kimberlite on Somerset Island consists of phenocrysts or xeno-crysts of olivine, garnet, phlogopite, enstatite-bronzite, magnetite, magnesian ilmenite, red chrome spinel and chromite as discussed by the authors.
Abstract: One of the three facies of the newly discovered kimberlite on Somerset Island consists of phenocrysts or xeno-crysts of olivine, garnet, phlogopite, enstatite-bronzite, magnetite, magnesian ilmenite, red chrome spinel and chromite, set in a fine-grained groundmass consisting of black chrome spinel, magnetite–spinel–ulvospinel solid solution, perovskite, rutile, pyrite, heazlewoodite, chalcopyrite, dolomite and calcite. Chemical analyses, as well as detailed descriptions of the texture and occurrence of all the major minerals, are given. The texture of the whole rock indicates that an immiscible carbonate liquid was evolved at one stage in the crystallization of the silicate fiquid, and that a third sulphide liquid may also have existed. During the period when the two or three liquids existed, some extremely complex epitaxial zonations were formed around oxide minerals which were crystallizing from the silicate liquid. These zonations show that there was a late-stage concentration of TiO 2 in the silicate liquid. The zonations have also been used to reconstruct the history of crystallization of the kimberlite.

Journal ArticleDOI
TL;DR: In this article, a comparison between these pyroxenes and similar ones from alkali basalts is made and optical and chemical data are presented and a comparison is made between the two types of aluminous Ca-rich clinopyroxenes.
Abstract: Aluminous Ca-rich clinopyroxenes (5.95-7.63 wt % Al2O3) are next in abundance to leucite in the basic alkaline potassic lavas from Monte Somma and Vesuvius, Italy, and occur as phenocrysts (1 to 7 mm), micropbenocrysts (< 1 mm), and groundmass granules. Zoning of various types is a conspicuous feature in these pyroxenes. Optical and chemical data are presented and a comparison is made between these pyroxenes and similar ones from alkali basalts. It is known from published data that Al2O3 fluctuates strongly in the oscillatory zones of Vesuvian pyroxenes. This can be explained as due to temperature variation in the magma, to magma variation in silica content due to contamination, to oscillations in leucite precipitation, or to a combination of these factors. Of these three factors, oscillations in leucite precipitation, as it appears, would be more effective than the others since it would have a greater control over Si/A1 availability in the magma.

Journal ArticleDOI
TL;DR: In this article, the Nohi rhyolite complex in Central Japan, which is composed mainly of rhyolitic ignimbrite, was examined and the initial Cs/Rb ratio of this magma is 0.030 ± 0.005.
Abstract: Cs/Rb ratio was examined on the Nohi rhyolite complex in Central Japan, which is composed mainly of rhyolitic ignimbrite. The variation in the Cs/Rb ratio of the magma through crystallization differentiation was estimated using a Rayleigh fractionation model and available distribution coefficients between phenocryst minerals and co-existing groundmass. It was found that the K/Rb ratio of this complex decreased from 273 to 125, but the Cs/Rb ratio remained constant through the differentiation process. The initial Cs/Rb ratio of this magma is 0.030 ± 0.005. The value is very close to the average Cs/Rb ratio of the ignimbrite in the Taupo region, Northern New Zealand, but is distinctly heigher than that of ocean floor basalts. To estimate the effect of contamination the Rb-Sr whole rock isochron method was used.

Journal ArticleDOI
TL;DR: The Pureora andesite volcano as discussed by the authors is sited at the junction of the Taupo Volcanic Zone with a line of low-silica andesites extending from Ohakune to Waiheke Island.
Abstract: Pureora andesite volcano is sited at the junction of the Taupo Volcanic Zone with a line of andesite volcanoes extending from Ohakune to Waiheke Island. The lavas are porphyritic, with plagioclase, augite, and hypersthene the dominant phenocrysts. Chemically all samples may be classified as low-silica andesites, and both major and trace element contents are consistent with their derivation by crystal accumulation of pyroxene from a labradorite or labradorite-pyroxene andesite magma.

Journal ArticleDOI
TL;DR: In the St. Mesmin olivine (LL-breccia) as mentioned in this paper, an electron microscopy of the olivines suggests that it is free of submicroscopic inclusions and exsolution lamellae.

Journal ArticleDOI
TL;DR: The chemical affinities of the Knee Lake volcanic rocks change gradually (at a silica content of approximately 55%) from tholeiitic to calc-alkaline with increasing stratigraphic height.
Abstract: Feldsparphyric basalts commonly occur as a distinctive 'marker horizon' 300 to 500 m below the felsic volcanic units of many Archean greenstone belts. Within the Knee Lake greenstone belt, northern Manitoba, glomeroporphyritic basalts, characterized by large irregularly-shaped aggregates of plagioclase phenocrysts, occupy this stratigraphic position within the two lower-most volcanic cycles. A major chemical and/or tectonic break does not occur within the volcanic cycles, but rather, the chemical affinities of the Knee Lake volcanic rocks changes gradually (at a silica content of approximately 55%) from tholeiitic to calc-alkaline with increasing stratigraphic height. The accumulation of plagioclase phenocrysts suggests extensive crystal fractionation of the Knee Lake magmas. The gradual transition in affinity of the Knee Lake lavas from tholeiitic to calc-alkaline can be explained by near-surface fractionation under constant fO2 conditions.

Journal ArticleDOI
TL;DR: In this paper, a comparison of K-Ar and ages based on U-Th-He on young basalts from the San Francisco Volcanic Field near Flagstaff, Arizona, showed that approximately one fifth of the He is retained and thus U-th-He ages are too young.
Abstract: Comparison of K-Ar and ages based on U-Th-He on young basalts from the San Francisco Volcanic Field near Flagstaff, Arizona, shows that approximately one fifth of the He is retained and thus U-Th-He ages are too young. Leaching with weak acid produces large losses of U in only a few minutes, which suggests that He is lost from surficial positions on minerals from radiation damage sites and from interconnecting crystal defects. One sample of large plagioclase phenocrysts showed excess He. It seems unlikely that the U-Th-He method can be used to obtain reliable ages on young basalts. (auth)

01 Jan 1975
TL;DR: In the Apollo 15 pyroxene-phyric basalt group, the predominant trend of differentiation is one of Fe enrichment, which is also reflected in the progressive increase of FeO content in normally zoned plagioclase.
Abstract: Sample 15075 is a holocrystalline gabbroic rock that can be classified as a coarse-grained variant of the Apollo 15 pyroxene-phyric basalt group The phenocrysts of pyroxene show spectacular compositional zoning, from hypersthene-pigeonite, through subcalcic augite, to 'ferropyroxenes' The predominant trend of differentiation is one of Fe enrichment, which is also reflected in the progressive increase of FeO content in the normally zoned plagioclase As in other samples of pyroxene-phyric basalts, the plagioclase in 15075 did not crystallize until the pyroxenes had reached a subcalcic augite composition; the onset of plagioclase and Fe-rich pyroxene coprecipitation is marked by sharp discontinuity in the Ca/(Fe + Mg) and Ti/Al trends of the pyroxene

Journal ArticleDOI
01 Jan 1975-Lithos
TL;DR: An intensely welded ignimbrite has been identified in the upper part of the thick Palaeocene lava pile of Ubekendt Ejland, west Greenland as discussed by the authors, which consists of 2.3 volume present of phenocrysts and 2 percent of lithic fragments (basalt and trachyte) in a compact nitreous matrix of flattened and welded rhyolitic pumice fragments and glass shards.

Book ChapterDOI
01 Jan 1975
TL;DR: The dominant rock type of the Fen carbonite complex is damkjernite, a lamprophyric ultrabasic rock that occurs in diatreme and hypabyssal facies as mentioned in this paper.
Abstract: The dominant rock type of the Fen carbonite complex is damkjernite, a lamprophyric ultrabasic rock that occurs in diatreme and hypabyssal facies. Damkjernite bears no chemical resemblance to kimberlite, but is chemically similar to alnoite, monchiquite and ouachitite. Phenocrysts include Ti–phlogopite, amphibole, acmitic pyroxene and nepheline; albite, nepheline and sphene occur in the groundmass. Calcite silicate textures suggest liquid immiscibility during crystallization. Calculated values of asi02 and crystallization T for the groundmass allow equilibration of the damkjernite magma with peridotite at T = 1200–1250 ° C, P = 15–25 kbar. Lherzolite nodules were accidentally included by the magma on its way to the surface, following its partial crystallization in a subcrustal magma chamber. ( 87 Sr/ 86 Sr) O of six rocks correlates positively with CO 2 , Sr and Na/Na + K, and inversely with SiO 2 and oxidation state, suggesting assimilation of crustally contaminated carbonatite. We suggest that the damkjernite formed by partial melting of peridotite in the presence of a CO 2 -rich vapor phase. Carbonatite magma separated from the silicate melt in the subcrustal magma chamber, intruded into the crust, and exchanged Sr isotopes with its country rocks. During later intrusion, the damkjernite remelted the carbonatites and was contaminated by them.

01 Mar 1975
TL;DR: Humphries, D.J. and Humphries as mentioned in this paper provided by NASA Astrophysics Data System provided by the Lunar and Planetary Institute Provided by the NASA ASTrophysics data system.
Abstract: 0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System ~IALCOLITE CRY STALL1 ZATI ON ETC . O ~ I l a r ~ a , M . J . alld Humphries, D.J. Table 2 R e s u l t s a t 1140+3°~, v a r y i n g c o n t a i n e r s and oxygen f u g a c i t i e s . Key i n Table 1 . R e s u l t s i r l Milo c a p s u l e s i n 80% H2 20% C02 atmosphere, or a s s t a t e d ToC 1281 1269 1253 1229 1202 I )1178 ll77 1Bn 1147 1144 1139 1124 111

Book ChapterDOI
01 Jan 1975
TL;DR: Kimberlite breccias from several South African pipes or pipe-like enlargements on dykes contain abundant fragments of country rocks including huge, often brecciated, xenolithic masses which are located at positions well below their original stratigraphic levels as mentioned in this paper.
Abstract: Kimberlite breccias from several South African pipes or pipe-like enlargements on dykes contain abundant fragments of country rocks including huge, often brecciated, xenolithic masses which are located at positions well below their original stratigraphic levels. The xenoliths show no pyrometamorphic effects indicative of relatively high temperatures during their incorporation in the kimberlite. A striking petrographic feature of these rocks is an emulsion-like texture resulting from the presence of numerous phlogopite-rich, globular segregations, which are set in a matrix which sometimes consists almost solely of primary calcite but in other instances is comprised of variable proportions of calcite and serpentine. Evidence is presented which indicates that in the latter cases serpentine has replaced calcite. The clear separation of the mica-rich segregations and the essentially calcitic matrix is attributed to the development of low temperature, immiscible K-rich silicate and carbonatitic liquids during emplacement of these rocks. The carbonatitic liquids are considered to have acted as the primary transporting fluids during liquid-solid fluidized intrusion and emplacement of the rocks is considered to have taken place as follows. After the crystallization of early phenocrysts the residual, ascending kimberlite magma differentiates at relatively high crustal levels into immiscible silicate and carbonatitic liquids accompanied by a coexisting gaseous phase. Continued intrusion is accompanied by the separation of the gaseous phase and its accumulation at the head of the magma column. Upon further uprise a stage is reached where the internal gas pressure exceeds the lithostatic load and the diatreme is formed by explosive breaching of the cap rocks. The resulting rapid pressure drop is accompanied by the upsurge of partly degassed magma fractions which incorporate explosively disrupted cap rock fragments and material which slumps from the walls of the diatreme.

Journal ArticleDOI
TL;DR: In this article, a small (360 × 180 m) rhyolitic intrusive body in the lower portion of the Portage Lake Lava Series of Michigan's Keweenaw peninsula was mapped and sampled in detail.