Showing papers on "Phenocryst published in 1973"
TL;DR: The basaltic andesites are interpreted to be the product of direct peridotitic fusion within the mantle, presumably in the region overlying the seismic zone, and modified by olivine fractionation during ascent.
Abstract: The Tongan Islands comprise a double island chain, and lie at the northern end of an active island arc extending NNE from northern New Zealand. The eastern chain are limestone covered, with an underlying pre-Upper Eocene gabbro-basalt-andesite series exposed on one island. The active volcanoes, the subject of this paper, form the western island chain, and lie some 100 km above a westward dipping seismic zone. Basaltic andesite dominates the Islands of Hunga Ha'apai and Late (and also Tofua and Kao, which are not described in this paper). Dacite is the main lava of Fonualei and the 1967-68 Metis Shoal eruption. Acid andesites occur only sporadically.Chemically, the lavas are moderately iron-enriched, with relatively low concentrations of K, Rb, Ba, Zr, REE, Pb, and U. K/Rb ratios are 500-700. The lavas are not typically 'calcalkaline'. Phenocryst minerals (normally poorly zoned) are dominated by bytownite with subordinate orthopyroxene, augite, and sometimes pigeonite in the basaltic andesites. Titanomagnetite occurs only in the more silicic andesites and dacites. The pyroxenes show a limited iron enrichment through the series. Coexisting pyroxenes exhibit equilibrium distributions of Mg, Mn, V, Sc, and Ni. Pyroxene partition coefficients for V, Sc. Mn, and Co increase with increasing iron enrichment. Groundmass minerals include intermediate plagioclase, pigeonite, hypersthene, magnetite, plus potash feldspar and quartz in the dacites. The Metis Shoal lava consists of rhyolitic glass with xenocrysts of forsteritic olivine and enstatite and phenocrysts of bytownite, hypersthene, augite, and titanomagnetite.The basaltic andesites are interpreted to be the product of direct peridotitic fusion within the mantle, presumably in the region overlying the seismic zone, and modified by olivine fractionation during ascent. Least squares numerical calculations, using major and trace element data, support the derivation of the more siliceous lava compositions by low pressure crystal fractionation of basaltic andesite Sr/Sr ratios exhibit a small range of variation (0.7036-0.7043), possibly indicative (except for the Metis Shoal lava) of minor limestone (?) contamination within the more silicic lavas during fractionation.
326 citations
TL;DR: The shape of sills and laccolithic intrusions and associated host rock deformation were studied at several locations on the flanks of the Henry Mountains Diorite sills range from 05 to 10 m in thickness, are less than 1 km2 in areal extent, and have blunt terminations.
318 citations
01 Jan 1973
TL;DR: The oxygen fugacity values of lunar samples were measured directly with an improved solid-electrolyte oxygen cell between 1000 and 1200 C with an accuracy mostly better than 0.2 log f(02) unit as discussed by the authors.
Abstract: The oxygen fugacity values of lunar samples were measured directly with an improved solid-electrolyte oxygen cell between 1000 and 1200 C with an accuracy mostly better than 0.2 log f(02) unit. The bulk rock f(02) values of basaltic igneous rocks 12009, 12053, 15058, and 15595 ranged from 10 to -15.4 power to 10 to the -15.7 power at 1000 C and from 10 to the -12.3 power to 10 to the -12.8 power at 1200 C. Those of microbreccia 14321 also fell in this range, but the change of oxygen fugacity with temperature was irregular in the first heating cycle in comparison to the smooth changes observed with the basaltic igneous rocks. Two different samples of rock 14310 also showed similar f(02) values below 1170 C, but exhibited irreversible sudden rise in f(02) at this temperature for reasons yet to be determined. The data on the phenocryst olivine and the groundmass of basalt 12009 do not conclusively indicate progressive reduction of the lunar magma during cooling.
115 citations
TL;DR: In this paper, microprobe analyses of olivine phenocrysts from basanite and nephelinite lavas show strong zoning toward calcium enrichment, which could be interpreted as a response to pressure release during crystallization.
112 citations
TL;DR: In this paper, the authors used analytical data on the rocks and minerals and simple thermodynamic expressions, the pressures and temperatures of equilibration of lavas and xenoliths, megacrysts and phenocrysts have been calculated.
Abstract: Late Pleistocene or Recent lavas from San Quintin, Baja California are basanitoids and alkali basalts. The surface quench temperatures of the lavas average 1 005° C with log \(f_{{\text{O}}_{\text{2}} } \)=−11.4, as deduced from the groundmass Fe-Ti oxides. Spinel lherzolite xenoliths and megacrysts of augite and andesine have been found in lava flows and cinder deposits. Using analytical data on the rocks and minerals and simple thermodynamic expressions, the pressures and temperatures of equilibration of lavas and xenoliths, megacrysts and phenocrysts have been calculated. The lavas could have been in equilibrium with lherzolite at 1 330–1 410° C and 27.5–31.6 kb, the more silica-poor liquid having the higher values. The basanitoid could have equilibrated with the megacrysts at about 10.5 kb and with phenocrysts at about 1.4 kb and 1130° C. The variation in composition of the lavas may be explained by a rising zone of melting within the mantle, the most silica-poor liquid having the deepest source. The source of the San Quintin basalts is probably related to spreading of the ocean floor in the Gulf of California.
97 citations
TL;DR: BASALT lavas dredged from the mid-Atlantic ridge south of Iceland exhibit a conspicuous correlation of minor element chemistry with distance from Iceland as discussed by the authors, which also happens to be a correlation with altitude, those lavas which contain the higher concentration of incompatible elements (and the higher Fe/Mg and Na/Ca ratios) being collected from greater heights above the general level of the ocean floor.
Abstract: BASALT lavas dredged from the mid-Atlantic ridge south of Iceland exhibit a conspicuous correlation of minor element chemistry with distance from Iceland1. This also happens to be a correlation with altitude, those lavas which contain the higher concentration of incompatible elements (and the higher Fe/Mg and Na/Ca ratios) being collected from greater heights above the general level of the ocean floor. The lavas are predominantly quartz-normative and most contain phenocrysts of olivine, augite and plagioclase.
90 citations
TL;DR: In this article, rare earth, Sr and Ba abundances were determined for three basalts, one mugearite, one trachyandesite and five trachytes from Dogo, Oki Islands, Japan and Ca-rich clinopyroxene and feldspar phenocryst samples separated from them.
Abstract: Rare-earth, Sr and Ba abundances were determined for three basalts, one mugearite, one trachyandesite and five trachytes from Dogo, Oki Islands, Japan and Ca-rich clinopyroxene and feldspar phenocryst samples separated from them. It is concluded that rareearth, Sr and Ba abundance patters for basalts and three trachytes could be explained by crystallization differentiation of olivine basalt magma, while those for other samples suggest more complicated process of magma production. Calculated Eu2+ to total Eu ratios in the magmas showed a increasing trend with differentiation. It is found that Yb and Lu partition coefficients were larger than those of lighter rare earths for clinopyroxenes in trachytes suggesting preferential substitution of the heavy rare-earth ions for Mg-site in the clinopyroxenes.
83 citations
TL;DR: Two generations of primary olivine are present in kimberlite, rounded phenocrysts (Fo94 Fo91) and euhedral groundmass olivines (FO91 Fo88-5) are considered to be mantle derived xenocrysts.
76 citations
TL;DR: The Rabaul caldera is at the northeastern tip of the island of New Britain, Papua New Guinea, Australia as mentioned in this paper, and the equilibration temperature of the iron-titanium oxides range from 1035° to 835° C.
Abstract: The Rabaul caldera is at the northeastern tip of the island of New Britain, Papua New Guinea. Unwelded pumice flows and air fall pumice of andesite, dacite and rhyolite drape the caldera. They contain sparse phenocrysts of plagioclase, pyroxene and rarely amphibole, together with microphenocrysts of titanomagnetite and ilmenite; apatite and pyrrhotite are also present. The equilibration temperature of the iron-titanium oxides range from 1035° to 835° C. Estimates of sulphur fugacity are obtained from the composition of the pyrrhotites which contain about 1% Cu and 0.3% Mn. Calculations show that the fugacity of SO2 may be several tens of bars at 1000° C. An estimate of the activity coefficient of Fe3O4 in titanomagnetite was obtained, and within the limits of error, can be taken as unity in the temperature range 835–1035° C and the composition range 22. 6–42.5% ulvospinel. Calculations suggest that the phenocrysts of orthopyroxene and titanomagnetite in the rhyolitic pumice equilibrated at pressures (Ptotal) of between 2.2 and 2.6 kilobars. Estimates of pH2o are unreliable because of the presumed later hydration of the pumice.
66 citations
TL;DR: Waters et al. as mentioned in this paper proposed three models of magma genesis: partial melting of the subducted Juan de Fuca plate; partial melting along a nearly horizontal shear zone at the base of the American plate; and partial melting within the asthenosphere and fractionation during ascent of the magma.
Abstract: Note: This paper is dedicated to Aaron and Elizabeth Waters on the occasion of Dr. Waters' retirement.
Tholeiitic basalt flows and breccias of Miocene age in western Oregon and Washington form three distinct stratigraphic units. Each unit was erupted from coastal vents marked by dikes and sills of the same composition as associated extrusive rocks. The three coastal basalt units are interbedded with predominantly marine sedimentary rocks of middle to late Miocene age. These units are here named, from older to younger, the Depoe Bay Basalt, Cape Foulweather Basalt, and basalt of Pack Sack Lookout. The three units can be distinguished by their petrographic characteristics. The Depoe Bay Basalt is nonporphyritic; Cape Foulweather Basalt has sparse large labradorite phenocrysts; and Pack Sack basalt has labradorite phenocrysts with numerous pyroxene and glass inclusions as well as augite and olivine phenocrysts. Chemical analyses of basalts from these three units show that each has a distinct and uniform composition. The Depoe Bay Basalt is characterized by high SiO2 content; the Cape Foulweather Basalt has high content of total iron, TiO2, and P2O5; and Pack Sack basalt is marked by relatively high MgO and CaO content.
The Depoe Bay Basalt, Cape Foulweather Basalt, and basalt of Pack Sack Lookout on the coast occur in the same stratigraphic order and are essentially the same ages as three basalt units that erupted on the Columbia Plateau. The plateau-derived units are the Yakima and late-Yakima petrographic types of Waters (1961) and the Pomona flow of Schmincke (1967). The virtual identity in chemical composition of the Depoe Bay Basalt and Yakima-type basalt, the Cape Foulweather Basalt and the late-Yakima–type basalt, and the Pack Sack basalt and the Pomona basalt flow indicate that each pair is consanguineous.
Fissure vents for the plateau basalt are located in eastern Oregon and Washington and western Idaho more than 500 km east of the coastal vent areas. Thus, a regional mechanism of magma generation or emplacement is required. Three models of magma genesis considered in this report are: (1) partial melting of the subducted Juan de Fuca plate; (2) partial melting along a nearly horizontal shear zone at the base of the American plate; and (3) partial melting within the asthenosphere and fractionation during ascent of the magma.
59 citations
TL;DR: In this article, it is suggested that the formation of hour-glass zoning is determined by an environment of crystallization where initial crystal growth is rapid relative to ionic diffusion in the melt, resulting in different chemistry for different growth directions within the crystal.
Abstract: Tertiary alkali basaltic rocks from the Southern Highlands, New South Wales, contain two types of clinopyroxene crystals, which exhibit excellent morphological development of hour-glass zoning. These are small, quench clinopyroxene crystals and large, abundant, euhedral phenocrysts. Electron microprobe analyses show that the different zone sectors derive from compositional differences, with variation in TiO2 and Al2O3 contents inversely with SiO2 content being most significant. Increased depth of colour of titanaugites depends on increased coupled substitution of Al for Si in tetrahedral sites and of Ti3− in an M 1 site, while pleochroism is enhanced by increasing Fe content. It is suggested that the formation of hour-glass zoning is determined by an environment of crystallization where initial crystal growth is rapid relative to ionic diffusion in the melt, resulting in different chemistry for different growth directions within the crystal. This implies that, under certain conditions, kinetic factors, as well as the chemical composition of the host magma, may be significant in determining the chemical composition of phases precipitated, particularly where more than one crystallographic direction of growth is possible.
TL;DR: High-alumina basalt occurs at eight localities in the Taupo Volcanic Zone associated with the Okataina, Maroa and Taupo calderas as mentioned in this paper.
TL;DR: In this paper, the authors show that the pyroxene-tholeiites and plagioclase-tholesiites can be differentiated by crystal fractionation of the high-alumina plagioclase phenocrysts and more than 17% Al2O3.
TL;DR: Gabbroic rocks from the Mid-Atlantic Ridge exhibit compositional and mineralogical changes resulting from fractional crystallization and differentiation in a slow-cooling tholeiitic magma as discussed by the authors.
TL;DR: The Hirschberg and Rodern diatremes, within the Permian Saar-Nahe trough, SW Germany, are composed of basaltic tuffs, with associated small intrusions of K-rich tholeiites as mentioned in this paper.
Abstract: The Hirschberg and Rodern diatremes, within the Permian Saar-Nahe trough, SW Germany, are composed chiefly of basaltic tuffs, with associated small intrusions of K-rich tholeiites. Several tholeiite bodies carry 2–20 mm crystals of magnesian clinopyroxene and orthopyroxene, the latter containing up to 5.5% Al2O3 and often extensively resorbed and rimmed by fine-grained olivine and clinopyroxene. Experimental duplication of these pyroxenes has been achieved under conditions of Pload=6–10 kb, T=1280–1080° C and 2–4 wt.-% H2O, confirming that they represent a rare occurence of high pressure phenocrysts in tholeiitic basalts. These conditions of pyroxene crystallization also place constraints on processes of magma generation, indicating that the tholeiites originated by partial melting of unusually hydrous peridotite mantle (0.4–0.8% H2O) beneath a relatively thin continental crust (maximum thickness approximately 30 km). Water present in the mantle at the site of magma generation may have been derived from the dehydration of oceanic lithosphere prior to the formation of the Saar-Nahe trough. This lithosphere probably underwent subduction at the margin of the Palaeozoic European continent during the Hercynian cycle of sedimentation, andesitic volcanism and folding. The termination of this cycle was followed by a period of basin-range type tensional faulting, leading to the formation of the Permian basins of present-day Central Europe, and widespread bimodal basalt/rhyolite volcanism.
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TL;DR: In this paper, foraminiferal evidence was used to assign an age for the accumulation of Late Pliocene or Early Pleistocene Limestone from the Tasman Sea guyots.
TL;DR: Rittmann as discussed by the authors interpreted the uniformity in the petrochemistry of Mt Etna as resulting from the absence of an intermediate magmatic reservoir, in relation with the tectonic history of the volcano.
Abstract: Most petrologists who have worked on Mt Etna have been surprised by the rather uniform character of its lavas (Lacroix 1908; Washington et al. 1926; Di Franco 1930). Except for a slight differentiation from alkali basalts to trachyandesites in the ancient Trifoglietto caldera, almost all the products forming this huge complex edifice have a tephritic basalt composition. They are usually porphyritic, with phenocrysts of calcic plagioclase (zoned from An 85 to An 40), augite, olivine and titaniferous magnetite. All these minerals, except olivine, are present in the groundmass, which also shows notable amounts of cryptocrystalline sanidine (about 10 %) and nepheline (about 5% ), these being determined by X-ray diffraction (Tanguy 1966). This uniformity in the petrochemistry of Mt Etna is interpreted by Rittmann as resulting from the absence of an intermediate magmatic reservoir, in relation with the tectonic history of the volcano (Rittmann 1963, 1973).
Dissertation•
01 Jan 1973
TL;DR: In this paper, a model of variable volumes of Upper Mantle partial melting is proposed to account for the diversity of major, trace and Rare Earth element compositions and strontium isotope ratios of the basalts and picrites.
Abstract: Grenada is the southernmost volcanic island of the Lesser Antilles. A series of volcanic centres ranging from Pliocene to Recent in age are present overlying a folded Lower to Middle Tertiary volcano-sedimentary formation. Eruptions of silica-undersaturated alkali basalt and picrite magmas have occurred repeatedly during the evolution of these centres. Calc-alkaline andesites and dacites show a close field association with the basalts and picrites. Recent activity on the island has been explosive in nature. A model of variable volumes of Upper Mantle partial melting is proposed to account for the diversity of major, trace and Rare Earth element compositions and strontium isotope ratios of the basalts and picrites. Geochemical, petrographic and mineralogical criteria suggest that the andesites and dacites are related to these basic melts by fractional crystallisation processes. In addition the chemical compositions and strontium isotope ratios of the andesites and dacites reflect the diverse compositions of the parental basalt magmas. The petrography and mineralogy of the andesites and dacites is similar to calc-alkaline suites elsewhere in the arc. Some of the basalts and picrites contain abundant olivine and sector and oscillatory zoned clinopyroxene phenocrysts. In some basalts, phenocryst amphibole is present. An origin by partial melting of an Upper Mantle pericotite source is proposed. Alternative sources are examiner: but partial melting of a subducted lithospheric plate does not appear to be a significant petrogenetic process in Grenada, Fractional crystallisation of olivine, clinopyroxene and spinel is mainly responsible for the development of a normal calc-alkaline trend towards increasing silica-saturation in the magmas. Subsequent crystallisation of plagioclase feldspar and then amphibole is also important in the development of a trend towards silica- rather than alkali-enrichment in the residual melts. The significant feature of the Grenada vulcanicity is the occurrence within a restricted geographic range of magmas of contrasted geochemical characteristics. The local volcanic and tectonic history of the southern part of the Lesser Antilles island arc are probably the most important factors in the development of these unusual characteristics.
TL;DR: In this article, a certain evolution of their composition has been observed: the first lavas are phonolitic tephrites, while the last ones are mugearites This evolution can be explained by assuming a pneumatolytic differentiation in the uppermost parts of the magma column and a subtraction of femic phenocrysts by gravitational differentiation in its deeper parts, where the last products originated.
Abstract: Lava samples, collected periodically during the 1971 eruption of Mt Etna, have been analysed A certain evolution of their composition has been observed: the first lavas are phonolitic tephrites, while the last ones are mugearites This evolution can be explained by assuming a pneumatolytic differentiation in the uppermost parts of the magma column and a subtraction of femic phenocrysts by gravitational differentiation in its deeper parts, where the last products originated Furthermore, the analyses of the 1971 lavas are compared with all available data of ancient products of this complex volcano and, particularly, with those of its historical eruptions
TL;DR: In this paper, the authors summarized the petrological evidence for a Galapagos mantle plume, present new data on basalts from the rift valley of the Galapago Ridge and discuss the origin and composition of the Cocos and Carnegie Ridges.
Abstract: Petrological evidence suggesting a mantle plume beneath Iceland has been presented1,2. Schilling1 gives geochemical data which indicate that the incompatible elements TiO2, P2O5, K2O and La decrease regularly southwards from Iceland along the Reykjanes Ridge axis to about 61° N. The rare earth element pattern for Icelandic tholeiites shows a high normalised La/Sm ratio which steadily decreases to less than 1 along the ridge. The crystallisation index (FeO*/ MgO) and the ratio of pyroxene/plagioclase among the phenocrysts also follow this trend. Campsie et al.2 noted that tholeiites all the way from Iceland to the Charlie Gibbs Fracture Zone retain chemical coherence in their major elements (except SiO2) and belong to a single pyroxene-tholeiite magma type. Here we summarise petrological evidence for a Galapagos mantle plume, present new data on basalts from the rift valley of the Galapagos Ridge and discuss the origin and composition of the Cocos and Carnegie Ridges.
TL;DR: A new occurrence of kimberlite located on Somerset Island in the Canadian Arctic is described in this paper, which is in the form of a diatreme showing evidence of fluidization and is emplaced in Ordovician sediments.
Abstract: A new occurrence of kimberlite located on Somerset Island in the Canadian Arctic is described. The intrusion is in the form of a diatreme showing evidence of fluidization and is emplaced in Ordovician sediments. Kimberlitic breccia containing rounded xenoliths of country rock and massive kimberlite are the dominant rock types. The massive kimberlite contains phenocrysts of olivine (Fo90–Fo93), chrome-pyrope (7–12 mol.% uvarovite), and phlogopite together with xenocrysts of pyrope-almandine (28–36 mol.% pyrope, 58–64 mol.% almandine) set in a groundmass of olivine (Fo89.5), magnetite, perovskite, and serpentine. Abundant dolomitic carbonate, which replaces the silicate groundmass, is considered to be primary and to represent a late stage immiscible phase. Emplacement of the diatreme is thought to have been controlled by a crustal lineament defined by the Boothia Uplift. An Upper Ordovician or Lower Silurian age is postulated on the basis of the lack of country rock xenoliths older than Upper Ordovician age...
TL;DR: The titanium trend is contrary with predictions from the Fe-Ti-O-O system and may be explained by a decrease in oxygen fugacity during magma ascent as discussed by the authors, which is in accordance with the Fe/Ti ratio of the titanomagnetite.
Abstract: Three generations of homogeneous titanomagnetite in a hawaiite from Saint-Clement may be defined by differences in size, habit, or reflectance, and are believed to correspond to different crystallization stages of the lava. Chemical compositions were determined by electron microprobe, and by X-ray and thermomagnetic methods. Larger crystals are of intratelluric origin and occur either as inclusions in phenocrysts of early clinopyroxene (generation 1) or separately in the groundmass (generation 2); in the latter case, they always show evidence of resorption. Post-eruptive titanomagnetite (generation 3) is smaller and so highly oxidized that it is better termed titanomaghemite. Since the maghemitization, which is a low temperature process, alters the metallic ratios (especially the Fe/Ti ratio) the metallic contents at the time of the high-temperature crystallization are exactly known only for the intratelluric titanomagnetites. As intratelluric crystallization proceeds Ti content increases greatly (5 to 14%); Mn also increases (but slightly), A1 and Mg decrease, while results for Cr are inconclusive. Apparently these changes go on until the post-eruptive crystallization stage. The titanium trend is contrary to common belief. However, it is in accordance with predictions from the Fe-Ti-O system and may be explained by a decrease in oxygen fugacity during magma ascent.
TL;DR: In this paper, primary mineral phenocrysts from eight different late Quaternary pyroclastic deposits were fractionated for neutron-activation analysis with the purpose of characterizing each of the deposits on the basis of trace and minor element compositions.
TL;DR: The pre-dominate Tertiary (44.2 ± 2.2 m.y.) intrusions of the Sacramento Mountains are thin dikes and sills of fine and coarse-grained camptonite as mentioned in this paper.
Abstract: The predominate Tertiary (44.2 ± 2.2 m.y.) intrusions of the Sacramento Mountains are thin dikes and sills of fine- and coarse-grained camptonite. Both rock types contain phenocrysts of diopsidic augite, hornblende, and plagioclase ($An_{40-45}$) in a fine groundmass of plagioclase ($An_{35-40}$), chlorite, orthoclase, magnetite, and minor apatite. The rocks differ in that the coarse-grained camptonites have more mafic phenocrysts and the phenocrysts are larger (max. size 30 mm vs. max. size 12 mm). These mineralogical and textural differences are the result of flow differentiation. The coarser-grained camptonites formed during high magma discharge from the magma chamber and the finer-grained camptonites during low discharge.
Dissertation•
01 Jan 1973
TL;DR: The Esja volcanic succession is about 2.4km thick, and comprises olivine tholeiites (25%), thole iites (68%), basaltic andesites (5%), icelandites and rhyolites (2%) as mentioned in this paper.
Abstract: The stratigraphy of Esja is described and the chronology of the succession established by means of the geomagnetic time scale. The Esja volcanic succession is about 2.4km thick, and comprises olivine tholeiites (25%), tholeiites (68%), basaltic andesites (5%), icelandites and rhyolites (2%). Volcanism was active in the Esja region for just over one million years, and during this time span, .at least ten glaciations occurred in the region. The stratigraphic succession is therefore characterized by sequences of lava flows intercalated, at intervals, by thick subglacial hyaloclastite units. Two central volcanoes were active in the Esja region; the Kjalarnes volcano was active for about 0.6 million years and was succeeded after a short interval by the Stardalur volcano, which remained active for about 0.3 million years. Flood-basalt volcanism was concomitant with the central volcanism, and most of the olivine tholeiites are considered to have been erupted in fissures and shield volcanoes unrelated to the central volcanoes. Igneous activity apparently migrated eastwards with time, reflecting the westward crustal drift away from the active volcanic zone, which is a subaerial extension of the mid-Atlantic ridge. The volcanics are tilted and downfaulted towards the east. The irregular topography created by the glaciations in Esja repeatedly prevented lavas erupted in the active volcanic zone from spreading over the tectonically less active neighbour- hood, thus producing angular unconformities in the stratigraphic succession from which the tectonic history of the region can be read. The Esja evidence suggests that tectonic activity is chiefly restricted to the active volcanic zone, and that the crust becomes tectonically inactive soon after it has drifted away from the active zone. Intrusive activity in Esja can be divided into three phases. The oldest dykes in the region trend N 25°E and contemporaneous sheets dip towards the Kjalarnes peninsula, where the intrusive activity culminated in the formation of a multiple dolerite sheet. This intrusion may have been preceded by a caldera collapse in the Kjalarnes area. After the intrusion of the Kjalarnes dolerites the regional trend of dykes changed to N 40°E, and a narrow dyke swarm (representing up to 20% dilation) cut across the Kjalarnes central volcano. The dyke swarm was succeeded by cone sheets focussing to the south of Leidhamrar, and the second phase culminated in the intrusion of large dolerite sheets in Þverfell and Lauganipa. Following a brief interval, during which flood-basalt volcanism was dominant in Esja, the Stardalur central volcano became active and, during its life span, minor intrusions were predominantly in sheet form. Caldera collapse in the Stardalur volcano was followed by the intrusion of basic cone sheets, large dolerite sheets, a sill and finally a laccolith within the caldera. Long after the caldera had been filled the caldera fault zone dominated over the regional fault pattern at depth so that basic and acid volcanics alike were erupted concentrically with and parasitically to the Stardalur caldera. Large dolerite intrusions in Esja are found chiefly within or at the boundaries of the thick hyaloclastite units, and there is evidence of dykes cutting straight through lava successions, but spreading out laterally to form sill-like bodies once they enter the less coherent hyaloclastites. A survey of the literature shows that the majority of large basic intrusions .in Iceland are accommodated in relatively soft and "structureless" host rocks, such as tuffaceous hyaloclastites, sediments, vent and caldera agglomerates, hydrothermally propylitized lavas, and "hot" and still partly liquid acid intrusive material. The majority of the large intrusions are in the form of inclined sheets, but sills and laccoliths are formed when the intrusions are emplaced at shallow levels (perhaps less than 1km) in the crust. The coincidence of central volcanoes having a great bulk of shallow level intrusions, with positive gravity anomalies, and the sites of shallow depth to layer 3 in Iceland strongly suggests that crustal layer 3 consists mostly of basic intrusions. A comparison of the densities of primary and secondary minerals of tholeiitic rocks suggests that infilling of vesicles of porous basalt lavas by secondary minerals will not make the rock as dense as a non-porous rock of the same composition. The estimated density difference of 0.2g/cm 3 between crustal layers 2 and 3 can apparently not be ascribed to secondary alteration of subaerial lavas, but can readily be explained by a transition from altered lavas to non-porous intrusives. It is proposed that the sharp boundary between layers 2 and 3 results from the lavas at the base of layer 2 reaching a degree of alteration at which the rock becomes sufficiently incoherent to accommodate large basic intrusions. The "metamorphic boundary" proposed by Palmason (1971) to explain the correlation between the thermal gradient and depth to layer 3 in Iceland is not therefore primarily a density boundary, but a boundary at which the lavas loose their strength as a result of alteration and host voluminous dense intrusives. The large scale features of crustal layer 3 in Iceland can be explained within the framework of this model. There is a complete range in composition from olivine tholeiites to rhyolites in the Esja volcanic succession, and the majority of the rocks contain some phenocrysts. Crystal fractionation appears to be a feasible mechanism to explain the chemical variation within at least the basaltic rocks in Esja, but whether the intermediate and acid rocks are formed by extensive fractionation or by partial melting of crustal material cannot be answered. The apparent coincidence in time of the emplacement of large basic intrusives and the commencement of intermediate and acid volcanism in eastern Esja may suggest that the rise of voluminous basic magmas has raised the thermal gradient sufficiently to produce the intermediate and acid rocks by partial melting at the base of the crust. Positive gravity anomalies associated with the Kjalarnes and Stardalur central volcanoes are attributed to high level intrusives in the core regions of the two centres. Specific gravity measurements of the chemically analysed rocks from Esja show a range of densities from about 2.5 (rhyolite) to about 3.15g/cm 3 (olivine tholeiite). Local gravity anomalies commonly found associated with central volcanoes are probably due both to local concentrations of rocks of different chemistry and to a high percentage of intrusives. A comparison of the average density of the crust in eastern Iceland and that of the crust on the Iceland-Faeroe ridge suggests that a considerable part of the negative Bouguer gravity anomaly of Iceland (Einarsson 1954) can be explained in terms of geochemical differences between the volcanics of Iceland and those of the surrounding areas. The similarity of the gravity profiles from the aseismic Iceland-Faeroe ridge and from the active Reykjanes ridge to the centre of Iceland suggests that, if a hot spot contributes to the bowl-shaped gravity anomaly of Iceland (Bott et al 1971), then it is not connected with layer 4 under the Reykjanes ridge. A positive magnetic anomaly associated with the Stardalur caldera is explained in terms of a thick pile of normal polarity eruptives within the caldera being surrounded essentially by reverse polarity eruptives. The combined effects of a high magnetite content (which may be caused by unusually high partial pressure of oxygen in the melt) and a high palaeofield strength may cause the very high magnetic intensity of lavas, which give rise to a sharp maximum within the Stardalur magnetic anomaly. Assuming a common cause for three other strong magnetic anomalies, which, with Stardalur, lie on a straight line and are separated regularly in space and time, the possibility of a mantle controlled "high partial pressure of oxygen spot" migrating at half the spreading speed along the spreading axis is discussed.
TL;DR: In this article, a hybrid origin, involving assimilation of gabbro by high-temperature syenitic magma, is proposed for potash-rich basaltic rocks of the absarokite-shoshonite-banakite series, where most of the large crystals and aggregates in these rocks are xenocrysts and microxenoliths, not true phenocrysts as was previously thought.
Abstract: Note: This paper is dedicated to Aaron and Elizabeth Waters on the occasion of Dr. Waters' retirement.
Textural and mineralogical features of potash-rich basaltic rocks of the absarokite-shoshonite-banakite series strongly suggest that most of the large crystals and aggregates in these rocks are xenocrysts and microxenoliths, not true phenocrysts as was previously thought. A hybrid origin, involving assimilation of gabbro by high-temperature syenitic magma, is proposed.
TL;DR: In this paper, it was shown that leopard rock is formed by axial segregation of calcic plagioclase phenocrysts within a single body of ascending phenocryst-rich magma via flow differentiation.
TL;DR: The melting and crystallisation relations of some calc-alkaline lavas from the eastern Sidlaw Hills, Perthshire, have been investigated at I atmosphere pressure as discussed by the authors, and it is concluded that fractionation in the Sidlaw lavas was a low pressure phenomenon and probably took place in the presence of water.
TL;DR: Three "conglomerate" channels are exposed in an approximately 12m-thick (40 ft) Tertiary lamprophyre sill in the northern Sacramento Mountains as discussed by the authors.
Abstract: Three “conglomerate” channels are exposed in an approximately 12-m-thick (40 ft) Tertiary (442 ± 22 my) lamprophyre sill in the northern Sacramento Mountains The sill is intruded into the Pennsylvanian Gobbler Formation as two intrusions, the conglomerate occurring in troughs within the lenticular upper sill The lamprophyre contains abundant fine phenocryst (025 to 8 mm) of hornblende, augite, and plagioclase (An40–45) in a groundmass of plagioclase (An35–40), chlorite, magnetite, and minor orthoclase
The bifurcating lenticular conglomerate is ca 90 to 210 m (300 to 700 ft) wide and scours up to 5 m (15 ft) into the lower sill The conglomerate has a chaotic and often nongrain-supported texture of poorly sorted xenoliths and phenocrysts in a groundmass of lamprophyre The xenoliths consist of fragments of Precambrian schist, gneiss, and granite, Paleozoic sedimentary rock, and clasts eroded from the lower sill; they range in size from less than 1 cm to 05 m, with a mean size of 6 to 8 cm The conglomerate channels are the result of local erosion along the top of the lower sill by a dense xenolith-laden magma current during intrusion of the upper sill The chaotic texture of the conglomerates and the lack of any axial migration of the xenoliths and phenocrysts indicate transportation and deposition were by a highly viscous flow similar to a pebbly mudstone deposit
01 Jun 1973
TL;DR: The Carlsbad twins from Loket (loc. swimming pool) are orthoclase perthites of low triclinicity (Δ=0.6-0.8) as mentioned in this paper.
Abstract: The studied, perfectly shaped Carlsbad twins from Loket (loc. swimming pool) are orthoclase perthites of low triclinicity (Δ=0.0–0.3); the sodic phase relates to low albite. All phenocrysts having zonal structure contain small areas of triclinic feldspars (Δ=0.6–0.8) independent on the zonality of low triclinicity. This microclinization is believed to be younger than the host orthoclase. The content of microcline within the phenocryst varies from about 10 to 30 percent.