Showing papers on "Phenocryst published in 1971"

Oxygen Isotope Thermometry of Mafic Igneous Rocks

Abstract: Measurements of $$O^{18}/O^{16}$$ ratios in phenocrysts from basalts of Hawaii, Tahiti, Tristan da Cunha, New Zealand, and Ross Island show that isotopie fractionations between mineral pairs are uniform from rock to rock. Typical values of the fractionations, expressed as 1,000 In a, are: plagioclase-magnetite, 2.1; plagio-clase-augite, 0.6; plagioclase-olivine, 0.9. On the basis of the laboratory-calibrated plagioclase magnetite pair, the isotopic fractionations represent a temperature of 1,020°-1,100° C, in agreement with field observations. For the corresponding mineral pairs in mafic intrusive rocks, the fractionations are about twice as great, indicating temperatures of isotopie equilibration below 800° C, probably as a result of retrograde exchange during slow cooling.

Silica activity and P total in igneous rocks

Abstract: The variation of silica activity with temperature and pressure for a variety of silica buffers (mineral pairs) allows Ptotal to be calculated for a wide range of igneous rocks. The method also depends on evaluating (∂ log aSiO2/∂P)T and (Δ log aSiO2/ΔT)p; the former is equivalent to the partial molar volume of silica in silicate liquids, while the latter is estimated from published experiments on natural melts. Results for calc-alkaline rhyolites with phenocrysts of quartz, olivine or orthopyroxene, and iron-titanium oxides, range from 3.45 to 9.58 kilobars; a pantellerite is intermediate at 7.53 kilobars. At 1327° C, the silicate inclusions in diamond equilibrated at 63.5 kilobars, and the kimberlite crystallisation path intersected the baddeleyite-zircon reaction at 55.7 kilobars. Two trachybasalts would equilibrate with their lherzolite xenoliths at 17.0 and 21.0 kilobars at surface quenching temperatures. Potassic lavas such as orendites and ugandites at 1300° C would be in equilibrium with mantle olivine-orthropyroxene at 35.1 and 69.0 kilobars respectively. Basalts and basaltic-andesites could equilibrate (at 1100° C) with quartz at between 24.9 and 26.8 kilobars; quartz can therefore be considered a possible high pressure “xenocryst” in lavas with low Sr87/Sr86 ratios. Andesites will equilibrate at 1300° C with the mantle at a depth of 75 kilometres; at greater depths andesite will have a basaltic precursor. In general, lavas with low silica activity will equilibrate at greater depths in the mantle than those with higher silica activities.

Iron-Titanium Oxide Phenocrysts in Compositionally Zoned Ash-Flow Sheets from Southern Nevada

Abstract: Electron-microprobe analyses of coexisting magnetite and ilmenite phenocrysts from four compositionally zoned ash-flow sheets of the Paintbrush and Timber Mountain Tuffs, southern Nevada, show systematic variations in Fe-Ti oxide composition that correlate closely with bulk-rock composition and with the inferred position in the source magma chambers. Comparison with experimental data of Buddington and Lindsley (1964) indicates that the Fe-Ti oxides of rhyolitic upper parts of the differentiated magma chamber crystallized at 700° ± 50° C, whereas oxides of quartz latitic lower parts of the magma column crystallized at temperatures as high as 900° C. The trend of temperature and oxygen fugacity is parallel and intermediate to trends for Ni-NiO and $MnO-Mn_{2}O_{3}$ buffers. The low indicated temperatures for the rhyolites suggest that they were saturated, or nearly saturated, with water at the time of crystallization.

An electron microscopic study of a lunar pyroxene

Abstract: Ion-thinned samples of lunar rock 12052 have been examined in the electron microscope. Exsolution textures have been observed in the pyroxene on a finer scale than those found in Apollo 11 rocks, indicating a faster cooling rate. The early stages of the reaction involve modulations on both (001) and (100) which suggest that decomposition may occur by a spinodal mechanism. A two-stage exsolution structure in the augite rim of a phenocryst is consistent with a sudden increase in the cooling rate—possibly when the magma was extruded onto the surface of the moon.

Voluminous low temperature rhyolitic magmas in New Zealand

Abstract: The glassy acid pumices, lavas and ignimbrites of the Pleistocene-Recent Central volcanic region of New Zealand contain iron-titanium oxide microphenocrysts, whose composition has been determined. Thirteen coexisting titanomagnetite-ilmenite pairs (all onephase and homogeneous) give two groups of equilibration temperatures: 735-780° C for amphibole-bearing rhyolites, and 860-890° C for very young non-amphibole-bearing pumices. O/O analyses of coexisting phenocrysts from five amphibole-bearing rhyolites give estimated temperatures in the range 695-860° C, with an average of approximately 780° C. Using Zen's (1971) thermodynamic data on anthophyllite, the fugacity of water has been calculated for two rhyolites with cummingtonite-orthopyroxene-quartz phenocrysts; at 735° C and 745° C, fHO is 1100 and 1300 bars respectively. These values are consistent with that derived for an analysed phenocryst assemblage of biotite-sanidine-magnetite. As all the investigated rhyolites contain phenocrysts of orthopyroxene and magnetite, it is suggested that the small increase in {Mathematical expression} of the low-temperature amphibole assemblages in comparison to the amphibole-free assemblages is caused by higher silica activity, as quartz phenocrysts are absent in the high-temperature amphibole-free rhyolites. The existence of large-scale rhyolitic magmas, with phenocryst contents ranging from 0-40 %, at temperatures in the lower part of the magmatic range, is interpreted to be consistent with previously suggested models of upper crustal anatexis in New Zealand, in which the breakdown of micas contributed the water necessary for partial melting. Details of the occurrence and chemistry of the cummingtonite phenocrysts are given in the appendix.

Altered Basalts from Swallow Bank, an Abyssal Hill in the NE Atlantic, and from a Nearby Seamount

Abstract: Three hundred and fifty-seven specimens dredged from Swallow Bank in 1958 are vesicular hypocrystalline porphyritic basalts. Chemical, thin section and X-ray studies were made. The least altered specimens gave an analysis close to the tholeiitic ocean floor basalt type. The glassy selvage of pillows altered sequentially to palagonite, fibro-palagonite and montmorillonite. Within the flows the mesostasis and pyroxene phenocrysts have been replaced by chlorophaeite, fibro-chlorophaeite and obscure chlorites, the calcic cores of plagioclase phenocrysts by a mesh of orthoclase plates and their sodic rims by (?) montmorillonite. Similar, but more extensive, mineralogical changes have affected the lavas from the seamount. Chemically, at Swallow Bank, the alteration of the flow interior involved increases in K 2 O (from 1.0 to 3.5g/100g), in H 2 O + (from 1.2 to 5.2 %) and in oxidation ratio, Fe 2 O 3 :Fe20 3 + FeO (from 71 to 98%) with concomitant loss in CaO (from 11.1 to 2.3 %) and MgO (from 4.7 to 1.8%). In these rocks radioactivity increased, seismic velocity decreased and intensity of magnetization remained substantially unchanged. Similar altered rocks could be widespread in layer 2 beneath the ocean.

Notes on the chemistry of ferromagnesian phenocrysts from selected volcanic rocks, central volcanic region

Abstract: Thirty-three new partial or complete chemical analyses are presented for pyroxene, amphibole, and biotite phenocrysts separated from three andesitic, three dacitic and thirteen rhyolitic volcanic rocks.The orthopyroxenes are mostly of hypersthene composition, with the rhyolitic hypersthenes generally being slightly more Fe-rich compared to the andesitic types. No detailed correlation, however, is evident between the Fe/Mg ratio of the rhyolitic orthopyroxenes and their parent rocks or co-existing groundmasses. A correlation between Ca content and the type of co-existing ferromagnesian assemblage was found. The clinopyroxenes are augites, and occur most commonly in the andesites and dacites.Two amphiboles are found, a calcic amphibole and cummingtonite. The former range from a tschermakitic hornblende (andesitic) to magnesio-hornblende in the rhyolites. One dacitic ferro-hornblende was found. The data illustrate the increasing substitution of Al and increasing Na in the andesitic hornblende. A close correlation of the Niggli mg ratio, and also MnO, exists between the analysed co-existing orthopyroxenes and hornblendes. Again, however, no detailed correlation between Fe/Mg ratio of hornblende and parent rock (or co-existing groundmass) was found to exist.

Geology of the Binneringie Dyke, Western Australia

Abstract: The geology of the Binneringie Dyke, one of the largest true dyke occurrences in the world — it extends for something like 200 miles (320 km.) from east to west, and has a maximum width at Cowan Hill of two miles (3.2 km.) — is described. The dyke is characterised by strongly chilled margins, but only meagre contact effects on the country rocks. The marginal zone is enriched in magnesium and calcium, due to an overprecipitation of early formed ferromagnesians. The marginal rocks are glomeroporphyritic and non-porphyritic bronzite, olivine-bronzite and olivine-bronzite-quartz gabbros, the bronzite having complex exsolution features reflecting inversion from magnesian pigeonite: the rocks towards the centre of the dyke have a progressively more ferroan character, augite-pigeonite gabbros taking the place of bronzite gabbros (the ferroan pigeonite being univerted). There are minor intermediate and acid phases throughout the length of the dyke: these take the form of irregular granogabbro segregation patches, in which there is an excessive development of residual “red rock”, of granophyric and devitrified acid glass composition. They also take the form of pegmatoid clots, associated with much epidote, amphibole, chlorite, sphene and prehnite. Much of the intermediate and acid material is, however, in the form of sharply bounded dykes and dykelets, within the gabbro: these bodies are chilled against the gabbro, never project through the chilled margin of the main dyke into the country rock and appear to represent infillings by late magmatic phases of shrinkage cracks in the gabbro. Another suite of internal dykes and dykelets has a basaltic character: dykes of basaltic dolerite of volcanic aspect are represented and these reveal idiomorphic olivine phenocrysts, trachytic textures, quartz palimpsests of chalcedony filled vesicles, and devitrified tachylite glass base material.

The Eastern End of the Galway Granite

Abstract: An account of the eastern part of the Galway Granite is presented together with the first detailed geological map. An aphyric medium-grained alkali granite occurs at the margin of the pluton followed by a coarse porphyritic granite, which becomes more basic towards the granite centre, consistent with findings in the western and northern parts of the batholith. Westwards the coarse porphyritic granite develops a foliation in which xenoliths, and occasionally potash feldspar phenocrysts, are aligned. Chemical fractionation trends illustrate the consanguinity of the granites and aplites. Field, petrographic, and chemical evidence suggest that most of the xenoliths are cognate. The zonation of the granite is thought to have resulted from crystallization of a magma in which compositional gradients were set up during the early crystallization period. A temperature gradient, decreasing from the centre of the magma chamber outwards into the country rocks, resulted in the migration of water, accompanied by alkalis and other volatiles, towards the granite margin; also the early-crystallizing minerals displaced the residual magma outwards. Crystallization was followed by shearing in the deeper parts of the pluton to produce the granite foliation. A total of r 66 rocks have been chemically analysed for 31 constituents.

Feldspar relations in Icelandic alkalic rhyolites

Abstract: Alkalic rhyolites with peralkaline affinities, bearing quartz and potassic feldspar phenocrysts, are described from Iceland for the first time. Evidence from electron-probe analyses of feldspar phenocrysts indicates crystallization in or near the thermal valley of the system SiO2-Or-Ab. Icelandic acid volcanic rocks are subdivided into alkalic rhyolites, belonging to transitional and alkalic basalt lineages, and the mildly calc-alkaline rhyolites of tholeiite lineages.

Giant phenocryst basalts in the deccan trap

Abstract: This paper gives an account of some unusual porphyritic basalts containing very large phenocrysts of plagioclase, averaging from 3 cm to 5 cm in length, and at places as large as 10 to 12 cm in length. These flows have been noticed at more than 20 localities, spread all over Western Maharashtra, occurring between levels of 100 feet below sea level (seen in a drill-core) and an elevation of more than 4000 feet above sea level. They appear to have a considerable lateral extent, some of them having been observed to extend for more than ten miles. In vertical extent, the flows were found to vary from 40 to 150 feet.

Glass crust on intratelluric phenocrysts in volcanic ash as a measure of eruptive violence

Abstract: A comparison of each of the volumes of nine deposits of airborne volcanic ash from the Coatepeque Volcano in El Salvador, Central America to the amount of glass crust adhering to intratelluric phenocrysts shows an inverse relationship. As violence, volume, and eruptive abrasion increase, glass crusts decrease. The eruptive violence of a prehistoric eruption and the original volume of ash can thus be calculated from glass-to-crystal ratios observed in erosional remnants.

Excess Rare Gases in a Subaerial Basalt from Nigeria

Abstract: A Nigerian alkaline basalt containing large phenocrysts of plagioclase feldspars1,2 and comprising one of the numerous Tertiary-Quaternary plugs of the Benue Trough province is a fine grained, subaerial rock which seems to be suitable for whole rock K-Ar dating. The age obtained, however, 95 m.y. (Table 1), is geologically unrealistic, for absence of sediment cover, the freshness of the rock and the ages of associated rocks all point to a late Tertiary origin2.

The petrogenesis of the calc-alkaline Borrowdale Volcanic Group, Northern England

Abstract: The Borrowdale Volcanic Group constitutes a major part of the Ordovician succession in the English Lake District. It comprises a suite of lavas, tuffs and ignimbrites with a maximum measured thickness of 5 km. The rocks of the Lake District are folded into a broad anticline which results in the Borrowdale Volcanics being exposed in two main outcrops, one north and the other south of a central core of older Skiddaw Slates. The northern outcrop consists almost entirely of basalts, basaltic andesites and occasional rhyolites, with very few intermediate members. In contrast, the southern outcrop is composed largely of andesites and dacites. Garnet phenocrysts, absent in the northern outcrop, are relatively abundant in the volcanic rocks of the southern outcrop. Analyses of 229 samples of lavas, ignimbrites and associated intrusives are presented together with electron microprobe analyses of selected garnet and augite phenocrysts. The southern outcrop volcanics are of calc-alkaline affinities, whereas those of the northern outcrop are transitional in character between tholeiitic and calc-alkaline. The use of La/Y ratios is shown to be particularly effective in distinguishing between members of the two suites. Detailed analytical studies on the garnet phenocrysts, especially La and Y abundances, show that crystal fractionation of garnet phenocrysts is incompatible with the geochemistry of their host rocks. It is concluded that the southern outcrop magmas evolved by some process other than crystal fractionation. A partial-melting hypothesis is proposed as an alternative, the melt being stored at depth (possibly at the crust mantle interface) long enough for garnet to nucleate, and then transferred rapidly to the surface. In contrast, the northern outcrop lavas are highly porphyritic and present abundant evidence of crystal fractionation. It is suggested that these rocks evolved by the fractionation of a basalt or basaltic and site parent under relatively dry conditions at shallow depth. The Borrowdale Volcanics are compared with the volcanic rocks of modern Island arcs. In particular the southward transition of magna type from tholeiitic to calc-alkaline compares with similar transitions occurring across modern island arcs. It is concluded that the Borrowdale Volcanics were erupted in an ancient Island arc at the margin of a contracting, proto-Atlantic ocean. This hypothesis is consistent with current models for the evolution of the Caledonian Appalachian orogen. The Borrowdale magmas were probably derived by the partial melting of basaltic oceanic crust carried down into the mantle on descending lithosphere plates. In the case of the southern outcrop rocks the magma- was not affected by subsequent ciystal fractionation, whereas the northern outcrop magma has undergone considerable modification by this process. Finally- the partial melting of oceanic crust is examined in the light of recent experimental studies. It is suggested that island arc tholeiitic magmas are generated at shallow depth by reactions involving amphibole breakdown. Calc-alkaline magmas are produced at greater depths by the partial melting of wet eclogite, and between these two extremes a continuum of transitional magma types could be generated.

Mineralogy and Petrology of the Santo Tomas-Black Mountain Basalt Field, Potrillo Volcanics, South-Central New Mexico

Abstract: The Santo Tomas-Black Mountain basalts were erupted during the Quaternary from four centers. Six lava flows are present at Black Mountain, three at Santo Tomas, and one each at Little Black Mountain and San Miguel. The basalts are grouped into three major types of phenocryst mineralogy: (l) plagioclase abundant, (2) olivine abundant, and (3) both olivineand plagioclase abundant. All three types are alkali-olivine basalts, showing high alkali-silica ratios and total alkali content increasing with silica. Seven periods of basaltic extrusion among the centers have been established on the basis offieldevidence, phenocryst mineralogy, and pyroxene-olivineratios. K-Ar dates show the basalts to be less than 0.3 × 10 6 m.y. old. The basalts are thought to have originated from a single small, shallow magma chamber which was under the influence of a high thermal gradient during differentiation.

Twinning and intergrowth of olivine crystals in chondritic meteorites

Abstract: SUMMARY. The angular relationships among sets of olivine plates in barred chondrules were investigated on the universal stage to test a suggestion that such chondrules represent exsolution and inversion from a pre-existing jadeitic pyroxene. In one of sixty-seven cases studied, adjacent plates represent olivine twinning on (IOO); the others vary randomly and correspond to neither olivine nor pyroxene twinning. On this basis and from other evidence, we conclude that the mineralogy and texture of barred chondrules are primary and due to rapid crystallization of ultrabasic magma in a low-pressure environment. OF the many and varied types of chondrules found in chondritic meteorites, the most bizarre are those that consist of optically parallel plates of olivine separated by a glassy, microcrystalline, or (in the more severely metamorphosed stones) crystalline groundmass. In some cases (fig. I) a single set of plates is present; in others (fig. 2), two or more sets are present. Intergrowths of five or more sets are by no means rare. Chondrules of this type, variously termed 'barred' or 'graticulate', comprise roughly a tenth of the chondrules in a given ordinary chondrite (Dodd and Teleky, 1967). Although they are usually discussed as a distinct class, two lines of evidence suggest that they are genetically related to the microporphyritic chondrules: they commonly occur as inclusions in such chondrules, and the phenocrysts in such chondrules occasionally have barred forms (fig. 3). The common occurrence of two or more sets of olivine plates in barred chondrules raises the question: What governs the orientation of these plates? Twinning is one possibility. A second is that the different sets of plates simply represent skeletal growth from several nucleii. Both of these possibilities are consistent with rapid crystallization of the chondrules, which is also suggested by the small sizes of crystals, the presence of a fine-grained to glassy mesostasis, and the Fe-Mg zoning and high calcium contents of the olivine crystals (Dodd, 1968 and I969). LeBas (1966) has suggested a third interpretation of barred chondrules, namely that they represent a complex sequence of inversion and exsolution from a preexisting jadeitic pyroxene. The basis for this suggestion is the resemblance of some complex barred chondrules to hourglass and herringbone-twinned clinopyroxenes, and a major implication is that some or all chondritic material experienced very high pressures early in its development. A high-pressure history has been suggested before

Phenocrystic olivines from the eastern Azores

Abstract: Phenocrystic and xenocrystic olivines and one olivine nodule sample, all from volcanics in the eastern Azores, range in composition from Fo87 to Fo69. Most are phenocrysts from the Nordeste alkali basaltic complex, eastern Sao Miguel Island, in which olivine-bearing rocks range from ankaramite to latite. CaO, MnO, and NiO trends are related to Mg-Fe contents, both for the concentrates as a whole, and also within analysed zoned crystals. It is shown that anomalous plots of Δ2ϑ(220LiF—062olivine) are related to pronounced Mg-Fe zonal gradients and, in samples from some flows, to mixtures of different generations of olivine. Regressions of Δ2ϑ vs. Fo (mol %) and FeO (wt %) for the least-zoned olivines are in close accord with those for Hawaiian olivines reported by Murata and others (1965). MnO percentages increase linearly with FeO; this and the Hawaiian trend are indistinguishable. Limited data for the most magnesian olivines indicate that NiO increases with FeO up to ∼ 15 wt % of FeO; the trend then decreases with further iron enrichment. This trend is apparent from other olivine data, and its relationship to the onset of pyroxene crystallization is discussed. CaO in olivine phenocrysts is inversely related to normative anorthite percentages in host rocks; the trend appears to be controlled largely by co-precipitation and fractionation of calcic plagioclase.

On the relationship of lamprophyres to hydrothermal ore veins in the Bohemian-Moravian Heights (Czechoslovakia)

Abstract: In the territory of the Bohemian-Moravian Heights, numerous lamprophyre, lamproid and basic dikes, as well as younger hydrothermal ore veins of different types occur. The dike rocks display rather high BaO-contents. Some lamprophyres from the area with barite-bearing ore veins even contain sparse barite, mostly as amygdules and in pseudomorphs after pyroxene phenocrysts. Barite evidently originated from the components contained in the magma; it does not represent a separate and independent hydrothermal supply, from which the ore veins, including the barite ones, would have crystallized. The dike rocks under consideration giade through transitional members into diabases. This fact, along with evidence concerning chemical composition of the rock, suggests that these dikes represent tholeiitic magmas contaminated at depth, mainly by a support of alkalies and volatile constituents and perhaps also barium. In the area under consideration, some genetic relations between dikes and ore veins may exist, but they are very vague only.

The petrochemistry of the Bull Arm Formation near Rantam Station, southeast Newfoundland

Abstract: The Bull Arm Formation of late Precambrian volcanic rocks crops out on the Avalon Peninsula and on the Isthmus of Avalon, SE Newfoundland An area of approximately nine square miles on the Isthmus was mapped and sampled in the summer of 1970 The area was chosen because it showed a complete cross section of rock types exposed in fresh roadside cuttings and therefore easily accessible -- The rock types are mainly rhyolitic flows, ignimbrites and tuffs, minor late basaltic flows and scoria, and associated volcanogenic sediments A relative lack of intermediate derivatives makes the distribution essentially bimodal -- Features such as well preserved igneous flow structures, and the presence of ignimbrites and red sediments, suggest a subaerial environment of deposition for the volcanics, with local reworking of deposits in superficial ponds and lakes to give graded and cross-bedded sediments -- The rocks have undergone mild deformation into open folds with associated axial planar cleavage visible in some tuffaceous and sedimentary deposits This deformation appears to be Acadian in age and was accompanied by metamorphism to low greenschist grade In the tuffs and some flows there is a noticeable development of epidote and zoisite as matrix alteration products -- Metasomatic effects mask the original identity of the rocks and both chemically and mineralogically they may now be ascribed to a spilite/keratophyre suite The development of secondary albite 'phenocrysts' visibly accompanies epidotisation in thin section These are thought to be a product of metasomatism In one area, this metasomatism is overprinted by a (contemporaneous?) potassium enrichment which appears to be structurally controlled Here, rhyolites with increased potassium contents have low sodium and show a progressive development of orthoclase at the expense of albite, in the phenocrysts The metasomatism explains the anomalously young age date of 467 ± 30 my and the high Rb/Sr ratios obtained for the Bull Arm Formation by previous workers It may also account for a local barite/galena mineralisation -- Two lines of evidence, Zr values and bulk analyses, suggest an original calc-alkaline affinity for the rocks, but the bimodal distribution of rock types remains difficult to interpret

Thermomagnetic characteristics of minerals as indicators of the sequence of their crystallization in acidic effusives (as in Far Eastern pumice)

Abstract: Titanium magnetite and ilmenite begin to crystallize first, but most of these minerals come down concurrently with ferrohedenbergite, after phenocrysts of olivine have already appeared. Crystallization of the ferromagnetic minerals ceases upon solidification of the glass. Evolution and order of crystallization of the magmatic melt itself are indicated by different states of oxidation and decay of the ferromagnetic minerals captured by phenocrysts of olivine, ferrohedenbergite, and sanidine. A primary magnetic origin of independent grains of ilmenite is suggested by their abundance and shape, but thermomagnetic evidence indicates that some of them are products of degradation-oxidation of titanium magnetite.

Crystallization of alkalic feldspar phenocrysts and origin of perthites in granitoids (as in certain intrusives in Northeast USSR)

Abstract: Magmatic origin of large (up to 5 cm) phenocrysts of K-feldspar is indicated by mineralogical, chemical, and roentgenometric-structural analysis of the crystals. Perthites are found to be metasomatic products of replacement of plagioclase by K-feldspar. This reaction is accompanied by albitization of plagioclase and then by growth of albite after feldspar, in cleavage planes and elsewhere, or by spot-segregations of albite in places formerly occupied by plagioclase. — V.P. Sokoloff