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Showing papers on "Phenocryst published in 1968"


Journal ArticleDOI
01 Nov 1968-Nature
TL;DR: The typical basalt of the ocean floor and mid-ocean ridges seems to be chemically variable, within a somewhat restricted range1, 4 but most contain olivine and hypersthene in the CIPW norm, and carry sparse phenocrysts of olivines, more frequently accompanied by plagioclase than by clinopyroxene as discussed by the authors.
Abstract: THE typical basalt of the ocean floor and mid-ocean ridges seems to be chemically variable, within a somewhat restricted range1–4, but most contain olivine and hypersthene in the CIPW norm, and carry sparse phenocrysts of olivine, more frequently accompanied by plagioclase than by clinopyroxene1–6. They are olivine-tholeiites7 and their tendency towards high alumina characteristics is reflected in the predominance of olivine + plagioclase phenocryst assemblages.

202 citations


Journal ArticleDOI
Hisashi Kuno1
TL;DR: The hypothesis that andesite magmas originate from basalt through fractionation is supported for the following reasons: 1) A close association of andesites and dacite with basalt in many volcanoes and a complete gradation in chemistry and mineralogy throughout this suite.
Abstract: The hypothesis that andesite magmas originate from basalt magmas through fractionation is supported for the following reasons: 1) A close association of andesite and dacite with basalt in many volcanoes and a complete gradation in chemistry and mineralogy throughout this suite. 2) Formation of andesite magmas from basalt magmas by differentiation in situ of some intrusive and extrusive bodies. 3) Agreement between the calculated compositions of solid materials to be subtracted from basalt magmas to yield andesite magmas and the observed mineralogy of phenocrysts in these rocks. 4) Higher alkali contents in andesite and dacite associated with high-alumina basalt than in those associated with tholeiite. 5) A complete gradation from the high iron concentration trend of basalt magma fractionation (Skaergaard) to the low or noniron concentration trend (the calc-alkali series) which can be ascribed to the difference of the stage of magnetite crystallization. 6) Similarity between the orogenic rock suite and plateau basalts in the preferential eruption of magmas of middle fractionation stage, givin rise to the great volume of andesite in the orogenic belts and iron-rich basalt in the plateau lavas. Petrological and seismic refraction studies suggest that a great volume of gabbroic materials are present in the lower crust underneath the volcanic belts as a complementary material for the andesite lavas. The island arc structure would develop by repeated eruption of andesite on the surface and by thickening of the oceanic crust underneath the arc due to the addition of gabbroic materials. The suitable portion of the lower crust may be subjected to partial melting to produce granitic magma in the later stage of development of the arc, successively changing it to a part of the adjacent continent.

160 citations


Journal ArticleDOI
TL;DR: In this paper, the compositional variability of the groundmass minerals and glass in 10 specimens from a complete 225-foot section of the prehistoric tholeiitic lava lake of Makaopuhi Crater, Hawaii.
Abstract: The electron probe X-ray microanalyzer has been used to determine the compositional variability of the groundmass minerals and glass in 10 specimens from a complete 225-foot section of the prehistoric tholeiitic lava lake of Makaopuhi Crater, Hawaii. The order of beginning of crystallization was: (1) chromite, (2) olivine, (3) augite, (4) plagioclase, (5) pigeonite, (6) iron-titanium oxides and orthopyroxene, (7) alkali feldspar and apatite, and (8) glass. Although the lake is chemically tholeiitic throughout, the occurrence of ferromagnesian minerals is as though there were a gradation from alkali olivine basalt in the upper chill downwards to olivine tholeiite. Groundmass olivine decreases downwards and disappears at about 20 feet. Pigeonite is absent in the uppermost 5±2 feet, then increases in amount down to 20 feet, below which augite and pigeonite coexist in constant 2∶1 proportions. Strong zoning and metastable compositions characterize the pyroxenes of the chilled zones, but these features gradually disappear towards the interior of the lake to give way to equilibrium pyroxenes. Relatively homogeneous poikilitic orthopyroxene (≈ Ca4Mg70Fe26) occurs in the olivine cumulate zone, having formed partly at the expense of pre-existing olivine, augite, and pigeonite (≈ Ca8Mg66Fe26). The growth of orthopyroxene is believed to have been facilitated by the slower cooling rate and higher volatile pressure at depth, and by the rise in Mg/Fe ratio of the liquid due to the partial dissolution of settled olivine. Unlike olivine and pyroxene, feldspar is least zoned in the upper and lower chilled regions. The greatest range of compositional zoning in feldspar occurs at 160 to 190 feet, where it extends continuously from Or1.0Ab22An77 to Or64Ab33An3. The feldspar fractionation trend in the An-Ab-Or triangle gradually shifts with depth toward more “equilibrium” trends, even though the zoning becomes more extreme. The variation with depth in the initial (core) composition of the plagioclase suggests the influence of either slow nucleation and growth (undercooling) or slow diffusion in the liquid, relative to the rate of cooling. Idiomorphic opaque inclusions in olivine phenocrysts are chrome-spinels showing continuous variation from 60 percent chromite to 85 percent ulvospinel and to magnetite-rich spinel. A pre-eruption trend of increasing Al with decreasing Cr can be recognized in chromites from the upper chill. Most of the inclusions show a trend of falling Cr and Al, toward an ulvospinelmagnetite solid solution which is progressively poorer in Usp with depth. This trend was produced by solid state alteration of the chromite inclusions during cooling in the lava lake. Ilmenite (average Ilm91Hm9) coexists with variably oxidized titaniferous magnetite in the basalt groundmass. Estimated oxygen fugacities agree well with other independent determinations in tholeiitic basalt. No sulfide phase has been detected. Fractional crystallization produced a groundmass glass of granitic composition. Average, in percent, is: SiO2, 75.5; Al2O3, 12.5; K2O, 5.7; Na2O, 3.1; CaO, 0.3; MgO, 0.05; total FeO, 1.2; and TiO2, 0.8. Normative Or> Ab. Minor changes in glass composition with depth are consistent with a greater approach towards the granite minimum. Incipient devitrification precluded reliable analysis of glass from the lower half of the section. The SiO2-phase associated with devitrification contains alkalis and Al and is believed to be cristobalite. Needle-like apatite crystals in the groundmass glass are Siand Fe-bearing fluorapatites containing appreciable rare earths (predominantly Ce) and variable Cl. The grain-size and maximum An content of the cores of plagioclase grains were controlled by cooling rate and are at a maximum at the center of the section. The most homogeneous pyroxene (and olivine, Moore and Evans, 1967), most equilibrium pyroxene trends, most abundant alkali feldspar, and most equilibrium feldspar trends are found at 160 to 190 feet, which is appreciably below that part of the lake which was slowest to crystallize. Volatile pressure, increasing with depth, possibly controlled the degree of attainment of equilibrium more than cooling rate. Since they are dependent on cooling history, some of the modal criteria commonly used for recognizing basalt types, such as the absence of Ca-poor pyroxene, presence of groundmass olivine, and the presence of alkali feldspar, should be applied with caution. Petrographic comparison of basalts from one flow, volcano, or province, with another, should recognize the possible variations due to cooling history alone.

159 citations


Journal ArticleDOI
TL;DR: A large number of garnet phenocrysts from Palaeozoic rhyodacites and granodiorite porphyrites from Central and Northeastern Victoria have been analyzed using the electron microprobe as discussed by the authors.
Abstract: A large number of garnet phenocrysts from Palaeozoic rhyodacites and granodiorite porphyrites from Central and Northeastern Victoria have been analyzed using the electron microprobe. These garnets, from an area of several thousand square miles, are very uniform in composition (dominantly almandine, with subordinate pyrope and minor grossular and spessartine). They show minor zoning with a very thin outer rim slightly richer in almandine and spessartine than the remainder of the phenocryst. They are surrounded by a complex intergrowth of cordierite and hypersthene forming a reaction rim. Resorbed quartz phenocrysts are typically associated with the garnet phenocrysts. The uniform composition, the conspicuous size and the subhedral-euhedral form of the garnet phenocrysts indicate that they crystallized directly from the acid calc-alkaline magma at an early stage of its crystallization. High pressure experimental work on a natural garnet-bearing rhyodacite glass demonstrates that almandine-rich garnet and quartz are near-liquidus phases at 18 and 27 kb \(\left( {P_{{\text{H}}_{\text{2}} {\text{O}}} < P_{{\text{LOAD }}} } \right)\), but garnet does not appear until well below the liquidus at 9 kb. A comparison of the composition of the experimentally crystallized garnets with the natural garnets suggests that these acid calc-alkaline magmas began to crystallize at pressures between 9 and 18 kb, i.e. at depths corresponding to the lower crust or upper mantle.

109 citations


Journal ArticleDOI
TL;DR: In this paper, the details of plagioclase zoning were correlated throughout a 3-sq mi quartz-monzonite stock on Mount Pilchuk, Washington, and a typical zone sequence in all phenocrysts is: andesine cores with oscillatory zoning; core-rim boundaries marked by sharp drops of 8 to 15 percent An, and rims normally zoned from An 28 to An 15.
Abstract: Details of plagioclase zoning were correlated throughout a 3-sq mi quartz-monzonite stock on Mount Pilchuk, Wash. A typical zone sequence in all phenocrysts is: andesine cores with oscillatory zoning; core-rim boundaries marked by sharp drops of 8 to 15 percent An, and rims normally zoned from An 28 to An 15 . Discontinuities between rims and cores represent a period of core resorption and non-crystallization caused by pressure decrease during intrusion. Coarse oscillations within cores (3 to 7 percent An) record convection and turbulence in an inhomogeneous magma during the early stages of crystallization. Constant composition of the outermost core zones indicates homogenization of the magma prior to and during intrusion. Zones within the outermost core show low compositional correlation and indicate a high degree of crystal mixing.

94 citations


Journal ArticleDOI
TL;DR: A series of basaltic and andesitic lavas from three centers in the Cascades (Lassen, Medicine Lake, Mt. Shasta) have been investigated as mentioned in this paper.
Abstract: A series of basaltic and andesitic lavas from three centers in the Cascades (Lassen, Medicine Lake, Mt. Shasta) have been investigated. The lavas are weakly porphyritic, containing phenocrysts of plagioclase, augite, and olivine or orthopyroxene; these phases are also found in the groundmass. Titanomagnetite is a groundmass phase in most lavas but it appears to be absent in some. A sub-calcic augite is found in the groundmass in some of the basic lavas. Orthopyroxenes are present only in the salic lavas and show an increase in calcium with increasing iron. The range in composition shown by both phenocryst and groundmass plagioclase is very similar except that the phenocrysts extend to slightly more calcic compositions. The residual glasses in many of the lavas have a rhyolitic composition. However, only those from the Shasta andesites have normative salic constituents that plot near the ternary minimum in the Ab-Or-Qtz system at 500 bars. Both chemical and mineralogical data allow the lavas of the different centers to be distinguished from one another. The most likely origin for the orogenic lavas of the Cascades is by partial melting of the upper mantle.

91 citations


Journal ArticleDOI
TL;DR: In this paper, Pantelleritic lavas are characterized by phenocrystic anorthoclase-sodic minerals, such as sanidine, quartz, sodic ferrohedenbergite, and cossyrite.
Abstract: Recent pantelleritic lavas comprise the whole of the isolated and outlying volcano of Mayor Island. Mineralogically, they are characterised by phenocrystic anorthoclase-sodic27 sanidine, quartz, sodic ferrohedenbergite, and cossyrite. Nine new chemical analyses of the lavas are presented (including one residual glass), confirming their strongly sodic and peralkaline nature. One analysis is also given of trachybasalt, which occurs as common inclusions in the mantling pumice deposits. These inclusions are characterised by abundant feldspar phenocrysts. Detailed trace element data is presented for five of the lava samples, representing the mam volcanic phases and the trachybasalt inclusions. The following conclusions are presented: The data is interpreted to indicate that the pantellerites were derived by crystal differentiation from a postulated mildly alkali olivine basalt parent — feldspar fractionation is considered to have been extremely important in this process. It is shown that the element enrichment occurring in the younger lavas may not be wholely explained by crystallisation differentiation alone — it is possible that some additional process is required. It is also shown that the observed enrichment of sodium in the youngest lavas can only occur during crystal fractionation if quartz, as well as anorthoclase, separate from the magma. This is due to the higher alkali abundances of the anorthoclase phenocrysts, relative to the pantellerite compositions. There is limited evidence that post-eruptive devitrification of some of the lavas has resulted in some modification of the lava chemistry, notably sodium loss.

80 citations


Book ChapterDOI
01 Jan 1968
TL;DR: Partition coefficients between natural melts, as represented by igneous matrix material, and amphibole, augite, olivine and plagioclase phenocrysts have been determined for eight rare-earth elements and barium by a mass-spectrometric stable-isotope-dilution technique as mentioned in this paper.
Abstract: Partition coefficients between natural melts, as represented by igneous matrix material, and amphibole, augite, olivine and plagioclase phenocrysts have been determined for eight rare-earth elements and barium by a mass-spectrometric stable-isotope-dilution technique. Peridotite nodules, crystallization of plagioclase and relationships between basalts are briefly discussed in terms of the partition coefficient data.

78 citations


Journal ArticleDOI
TL;DR: In this article, rare earth abundance data for mafic and plagioclase phenocrysts and their igneous matrix materials indicate that Eu anomalies, while caused by the relative stability of divalent Eu, are controlled largely by crystal-chemistry rather than by the redox conditions in the melt.

73 citations


Journal ArticleDOI
P. E. Baker1
01 Jan 1968-Lithos
TL;DR: In this paper, Petrographic and mineralogical data are given for representative rock-types and twenty-two new chemical analyses are presented for the Mt. Misery lavas and comparisons are made with other calcalkali provinces.

61 citations


Journal ArticleDOI
TL;DR: The early Tertiary intrusive igneous rocks of the Marsco area, Skye, form part of the Western Redhills Complex as discussed by the authors, and they are predominantly granitic in composition, but also include small amounts of gabbro and ferrodiorite.
Abstract: The early Tertiary intrusive igneous rocks of the Marsco area, Skye, form part of the Western Redhills Complex. They are predominantly granitic in composition, but also include small amounts of gabbro and ferrodiorite. The intrusions are arcuate, steep-sided, dyke-like bodies. Portions of the roofs of several successive plutons are preserved on Marsco. The margins, against granite, of the gabbro on top of the mountain give evidence of the chilling of basic against acid magma. Seven high-level granites are described. They show a considerable variety of textures and mineralogy; usually containing feldspar phenocrysts, and ground-mass fayalite, ferrohedenbergite, calciferous or alkali amphibole, biotite and numerous accessory minerals. Mineralogical data and twelve new whole-rock chemical analyses are presented. It is shown that the ferrodiorite is probably contaminated by acid magma. Two granites south of Marsco are truncated by the Western Redhills intrusions, and appear to belong to a separate, possibly earlier, intrusive centre. The six major granites in the Marsco area are classified structurally into three pairs. The second members of each pair are very alike mineralogically and chemically. It is postulated that the acid magmas were produced by partial melting of Lewisian gneisses and Torridonian sediments in the thermal aureole of a large basic intrusion. The phase-chemical relationships of the granites to their supposed source rocks are discussed. A petrogenetic scheme is given, which attempts to explain the compositions and forms of the granitic dykes, and their emplacement in pairs. It is thought that intrusion took place during the tilting of a foundering crustal block.

Journal ArticleDOI
TL;DR: In this article, the authors examined the phenocrystic mineralogy of the rhyolitic lavas associated with the four volcanic centres of the Taupo Volcanic Zone, namely the Mokai Ring Structure, Lake Taupo and Rotorua Caldera, and Okataina Volcanics, and concluded that the crystallisation of quartz and feldspar in the rhyolites of the region can be broadly explained in terms of the Ab-Or-Q-HO system.
Abstract: This account is based on the systematic examination of the phenocrystic mineralogy of the rhyolitic lavas (referred to as rhyolites) associated with the four volcanic centres of the Taupo Volcanic Zone, namely the Mokai Ring Structure, Lake Taupo Volcanic Centre, Rotorua Caldera, and Okataina Volcanic Centre. From the results of over 200 modal analyses, it has been possible to subdivide the Mokai rhyolites into seven groups of varying extent, and the Okataina rhyolites into five complexes. The rhyolites of each group or complex are interpreted to be the eruptive products of the same high-level magma reservoir. Evidence suggests that the foci of eruptions in the Mokai structure have migrated eastwards with time. A close correlation exists between the quartz and plagioclase phenocrysts in the rhyolites from each of the groups and complexes of the Mokai and Okataina centres, and also for the Lake Taupo and Rotorua rhyolites. This is interpreted to indicate that the rhyolites from each group, etc., crystallised under similar water vapour pressure conditions, from magmas of fairly restricted compositions. Differences exist among groups or complexes, in the proportions of plagioclase crystallising before being joined by quartz, this being mainly the result of small differences in magma compositions between these groups or complexes. Significant correlations also exist between the average diameter and modal abundance of both the quartz and plagioclase phenocrysts, and provide additional evidence against a xenocrystic origin of these phenocrysts. It is believed that the rhyolites of the four volcanic centres have similar crystallisation histories before their eruption, with the exception of the Haroharo rhyolites (and their associated pumice deposits) within the Okataina centre. These Haroharo eruptives differ in a number of respects from the other rhyolites of the region, the most interesting difference being the occurrence of phenocrystic cummingtonite (in place of the normal calcic hornblende) in most of the pumice deposits and some of the lavas. It is concluded that the crystallisation of quartz and feldspar in the rhyolites of the Taupo Volcanic Zone, considered in relation to the rhyolite chemistry, can be broadly explained in terms of the Ab-Or-Q-HO system (Tuttle and Bowen, 1958).

Journal ArticleDOI
TL;DR: In this paper, it was shown that the increase of anorthite content does not necessarily imply either increasing Sr in successive fractionated liquids or enrichment of Ca relative to Sr in crystallising plagioclase.

Journal ArticleDOI
TL;DR: In this paper, the contents of U and Th in mineral fractions separated from some volcanic lavas and a peridotite nodule have been measured, and Ionic substitution of Ca in clinopyroxene by u and th under equilibrium conditions is proposed for the possible partition mechanism.

Journal ArticleDOI
TL;DR: In this article, the central alteration zone has gained Al, K, Ti, Si, and water, and lost Fe and Mg, compared to unaltered rock in the inner zone.
Abstract: Quartz porphyries, keratophyres, and dacites are altered to sericite-quartz schists in a discrete envelope around the ore body (earlier arsenopyrite and later pyrite). The varieties of altered rock closest to ore (sericite rock, andalusite rock) also constitute the core of the alteration channel in depth below the ore body. An outer zone of chloritic and pyrite-banded schist surrounds this inner zone at and below the level of the ore body. The distribution density of quartz phenocrysts is the same in altered and unaltered rocks, showing that no significant volume changes are involved. The central alteration zone has gained Al, K, Ti, Si, and water, and lost Fe and Mg, compared to unaltered rock. The outer zone has gained Mg, Fe, water, and S. Both zones have lost Ca and Na. Iron lost from the altered rocks as a whole, particularly in depth, may now be fixed in the ore; Mg and Al distributions also suggest upward migration. Zoning is viewed as the result of continuous action of an ascending solution.

Journal ArticleDOI
TL;DR: In the three areas studied, the McMurdo volcanics have been subdivided into four formations: Melania (oldest, and predominantly basaltic); Aurora Trachyte; Nubian Basalt; and the Trachte Hill Formation as discussed by the authors.
Abstract: In the three areas studied, the McMurdo volcanics have been subdivided into four formations: Melania (oldest, and predominantly basaltic); Aurora Trachyte; Nubian Basalt; and the Trachyte Hill Formation. Tentative correlations have been made among the three areas. In addition to these four divisions, maps are presented showing the lavas subdivided on the basis of their petrography (particularly phenocryst contents). Five broad petrographic types are recognised: Olivine-augite basalts; plagioclase basalts; hornblende basalts; pyroxene trachytes, and hornblende trachytes. Black Island and Brown Peninsula are believed to have been originally stratovolcanoes, similar to the present form of Mt Bird. In each area, this early conebuilding phase (Melania Formation) is predominantly olivine-augite basalt, although locally trachyte and trachybasalt are recognised. This phase was followed, after a period of prolonged quiescence, by the Aurora Trachyte Formation, comprising extensive viscous flows of pyroxen...

Journal ArticleDOI
TL;DR: In this article, the authors investigated the properties of extrusive and pyroclastic rocks of Alban Hills, Vico and Cimino Volcanoes as well as of their minerals such as leucite, sanidine, biotite and zeolites.

Journal ArticleDOI
TL;DR: Tauhara Volcano consists of five chemically and petrographically distinct dacite cumulo-domes as discussed by the authors, which are glomeroporphyritic and contain plagioclase, pyroxene, and amphibole.
Abstract: Tauhara Volcano consists of five chemically and petrographically distinct dacite cumulo-domes. The rocks are glomeroporphyritic. The dominant phenocrysts, occurring in various proportions, are plagioclase (An40), quartz, orthopyroxene, augite, and calcic hornblende (in various states of oxidation). Five types of orthopyroxene are distinguished according to optical properties and habit. Other minerals occurring as phenocrysts are magnesian olivine, biotite, and magnetite. The groundmass mineral assemblage, in crystalline rocks, is plagioclase-orthopyroxene-clinopyroxene-cristobalite-magnetite. The Tauhara dacites are classified petrographically according to the relative percentages of plagioclase, pyroxene, and amphibole. The rocks contain basic cognate xenoliths (enclaves plesiomorphes), and xenocrysts from these, characteristic of calc-alkaline dome forming lavas. Five new chemical analyses show that the Tauhara dacites range in composition from 65.8–68.0% SiO2 (water-free). A chemical analysis ...

01 Jan 1968
TL;DR: The Undercliffe Falls porphyritic adamellite is an even-grained marginal type and a relatively basic phase with white K-feldspar phenocrysts as mentioned in this paper.
Abstract: Igneous rocks from the Undercliffe Falls-Liston-Maryland area of northern New South Wales vary from metadolerites to granites. The main plutonic rock is the Undercliffe Falls porphyritic adamellite and this intrudes Permian "Boorook" strata which, away from contacts, are little-altered sediments and volcanics. The porphyritic adamellite is intruded by the Stanthorpe adamellite which is displaced by the more acidic Ruby Creek granite. These masses are described by means of new chemical, mineralogical and petrographic data, resulting in the division of the Undercliffe Falls and Stanthorpe masses into a number of varieties. Much of the Undercliffe Falls mass (the normal type) is a porphyritic rock with basic xenoliths which have a preferred orientation developed by a primary platy flow imposed when the intrusion was magmatically emplaced. Contacts are discordant in detail. Large pink K-feldspars, some of which are mantled by plagioclase, are found in the normal type, in its contained xenoliths and in metadolerite country rock. Other varieties of the Undercliffe Falls mass include an even-grained marginal type and a relatively basic phase with white K-feldspar phenocrysts. The younger intrusions are even-grained rocks which were probably emplaced as magmas. Compositionally the Ruby Creek granite falls in the low melting portion of the NaAISi3O8- KAISi3O8-SiO2-H2O-(CaAI2Si2O8) system. The Undercliffe Falls porphyritic adamellite probably formed by chemical hybridization involving gabbroic rock (similar to the wall-rock metadolerite found in the area) and acidic magma. Calculations indicate that the Stanthorpe adamellite is a suitable acid parent and it, together with the Ruby Creek mass, may have been derived by crustal melting.

Journal ArticleDOI
TL;DR: The Benue trough is a graben structure containing folded Upper Cretaceous sediments as discussed by the authors, which is dominated by the voluminous and still active vulcanicity of the Cameroun rift.
Abstract: The Benue trough is a graben structure containing folded Upper Cretaceous sediments. Mid-Tertiary to Recent rift-valley type vulcanicity in and near the trough was mainly basaltic (in the north at least), salic lavas being now represented mainly by a few eroded plugs. The Nigerian rocks are regarded as a practically extinct sub-province within a larger Gulf of Guinea alkaline province, dominated by the voluminous and still active vulcanicity of the Cameroun rift. The two structures have the same trend as other graben features further south along Africa's western margin. Large sodic plagioclase crystals in a basalt plug within the Nigerian sub-province have rounded edges and more calcic overgrowths. They probably crystallized at deep crustal levels but are unaccompanied by any of the mafic phases that normally crystallize from basalts under high pressure. However, spinel crystals and olivine nodules found in a nearby plug indicate that the complementary mafic assemblage may be found not far away. Sparse published data on phenocrysts and inclusions in the Jos Plateau lavas offer some support for this suggestion. Crystallization of sodic feldspars in basalt may have a bearing on the independent genesis of salic liquids, while apatite and magnetite inclusions in them suggest a possible link with carbonatites.

Journal ArticleDOI
TL;DR: The Cenozoic volcanic complex in the Santa Rosa Range consists of many widely scattered, independent vents, associated with sequences of flows that reach a maximum thickness of 3000 ft as mentioned in this paper.
Abstract: The Cenozoic volcanic complex in the Santa Rosa Range consists of many widely scattered, independent vents, associated with sequences of flows that reach a maximum thickness of 3000 ft. The predominant rock types are basalt and rhyolite-rhyodacite, but intermediate rock types are abundant and widely distributed. All rocks in the sequence are characterized by a lack of biotite, amphibole, and other hydrous minerals. Olivine is the predominant phenocryst in andesites; olivine and plagioclase are virtually the only phenocrysts in basalts. Xenoliths and xenocrysts are found only in rhyolites, rhyodacites, and dacites. The most primitive rock type in the sequence is a basalt that resembles mid-oceanic tholeiite chemically, and high-alumina basalt from the Oregon plateaus petrographically. In the northern part of the range, basalts, differentiated from this rock type, approach ferro-gabbroic compositions; in the southern part, the major differences between primitive and differentiated basalt are lower CaO and MgO, and much higher alkali percentages in the latter. Siliceous rocks were produced as end products of differentiation in all parts of the range. Petrographic and chemical features suggest that titaniferous magnetite was fractionated in larger amounts in the south than in the north. This very likely prevented iron enrichment in the south, but did not lead to early development of a silica enrichment trend. Apparently labradorite was fractionated in amounts large enough to counterbalance the silica enriching effect of fractionating olivine and magnetite, until a very late stage in magmatic history.


Journal ArticleDOI
20 Apr 1968-Nature
TL;DR: The olivine basalt is a normal alkaline variety, quite fresh, with phenocryst and microphenocryst olivines (close to Fa25), titaniferous augite and titanomagnetite as mentioned in this paper.
Abstract: TWENTY-ONE miles east of Gombe in northern Nigeria, one of the numerous Tertiary–Quaternary olivine basalt plugs of the Benue trough alkaline volcanic province1 contains many conspicuous sub-rectangular white feldspar crystals, 1–3 cm long. The olivine basalt is a normal alkaline variety, quite fresh, with phenocryst and microphenocryst olivine (close to Fa25), titaniferous augite and titanomagnetite. Moderately zoned plagioclase microphenocrysts have labradoritic compositions (An54–62).

Journal ArticleDOI
TL;DR: In this paper, the cross-section of a porphyritic central zone sharply bounded by non-porphyritic margins is described, where plagioclase, augite and olivine phenocrysts are arranged in two main layers.
Abstract: Some thin basaltic intrusive sheets in south-eastern Iceland consist in cross-section of a porphyritic central zone sharply bounded by non-porphyritic margins. Within the porphyritic zone phenocrysts of plagioclase, augite and olivine are arranged in two main layers, an upper layer containing mostly plagioclase phenocrysts, and a lower layer containing concentrations of augite and olivine phenocrysts. The phenocrysts are considered to have been gravitationally sorted during the passage through the sheets of a highly fluid and strongly flowing porphyritic basalt magma.

Journal ArticleDOI
TL;DR: Olivine and augite phenocrysts are set in the intergranular or intersertal groundmass which is composed of plagioclase, augite, olivine, alkali feldspar, magnetite, apatite and glass as mentioned in this paper.
Abstract: Numerous basaltic lavas and dikes of the Pliocene have been known in the western part of Hokkaido, but little petrographic work has been carried out on those rocks so far. The basalts designated as the basin type in this paper, are distributed around the Takikawa-Fukagawa basin and have a unique petrographic feature. Olivine and augite phenocrysts are set in the intergranular or intersertal groundmass which is composed of plagioclase, augite, olivine, alkali feldspar, magnetite, apatite and glass, whereas phenocyrstic plagioclase is completely absent. Chemically those basalts belong to the alkali olvine basalt suite, but are characterized by high alumina content. The degree of silica-undersaturation is moderate, and normative Hy is usually present only in small amount or sometimes normative Ne is calculated instead. On the contrary, the Hamamasu basalts, Etaidake basalts and other basaltic rocks distributed to the west of the basin (Fig. 1), belong to the different type of basalt, slightly silica-understaturated, accompanying the calc-alkali andesite (Oba et al., 1966).

Journal Article
TL;DR: In this paper, the authors investigated the chemical composition of the rhyolites from Dźwirzyno and Kamien Pomorski, and the results of calculations into a, b, c and s parameters, according to A. N. Rost, W. H. Schimanski and W. Zawaricki.
Abstract: SECONDARY RHYOLITES IN THE BALTIC PART OF WEST POMERANIA Summary In the Baltic part of the West Pomeranian area, effusive rooks have so far been pierced at Kamien Pomorski and Dźwirzyno. In addition, clastic material of these rocks has been found to occur in sedimentary deposits encountered at Karcino. The clastic rocks of the upper stage of the Rotliegendes are of small thickness, ranging from 1 to 4 metres. These are red or grey-rusty, coarse-grained, badly sorted sandstones, passing at places into conglomerates. The coarse-clastic material is of autochthonous origin. In bore hole Kamien Pomorski IG-l, effusive rocks of the lower stage of the Rotliegendes have been encountered at a depth of 2721.8 m, but not pierced to a depth of 2810.5 m. The effusive rocks are restricted here to two varieties: perlitic - younger rhyolite, and miocropoikilitic - older rhyolite. Mineral composition of these rocks is given in Table 1. At Dźwirzyno, volcanic rocks have been ascertained at a depth of 2530.5 m. After piercing 47.7 m the bore hole was abandoned. Here, the varieties pierced in the bore hole are as follows: perlitic variety - the youngest one, microcrystalline variety - transitional one, and micropoikilitic variety – the oldest one. The chemical composition of the rhyolites from Dźwirzyno and Kamien Pomorski is given in Table 2, and the results of calculations into a, b, c and s parameters, according to A. N. Zawaricki, in Table 3. This was a basis to ascertain that the younger rhyolites from Kamien Pomorski and Dźwirzyno (analyses 1, 2 and 5) and the older rhyolites from the same localities (analyses 3, 6 and 7) can well be correlated. The thickness of the rhyolites found in the Baltic part at West Pomerania is unknown. However, taking into account the thickness of the eruptive series from North-East Mecklenburg, Rugen and Usnam (K. H. Albrecht, K. Goldbecher, 1964; W. Rost, W. Schimanski, 1967) we should expect that it is also considerable. Three kinds of rhyolites have been distinguished in the rocks of the Baltic part of West Pomerania: older, transitional and youngest ones (at Kamien Pomorski the transitional rhyolite is absent). Differences between the rhyolites consist in various character of phenocrysts, in mode of crystallization of rock body and in the degree of hydrothermal alteration. In the older rhyolites from Kamien Pomorski, minerals of inter telluric phase occur mainly in porphyry forms, in the younger ones - in glomeroporphyry form. Observations of glomeroporphyry textures demonstrate that these are a result of clot-like developed rocks of relict consolidation texture, disturbed during the movement of eruptive mass. In general, two types of extremely different rhyolites have been distinguished in the rocks examined: 1- older rhyolite that consists of dacitic rock body and of acid macrocrystals of albite, quartz and orthoclase. Miaroles and asymmetric amygdaloidal forms show that solidification of eruptive mass took place under quiet conditions, without greater displacements. The rock body is completely crystallized and secondarily altered. 2 - younger rhyolite built up of leukorhyolitic rock mass and of middle-basic glomeroporphyroblasts of oligoclase-andesine (albite) and of olivine, as well as of chloritized mafic mineral of columnar habitus. Asymmetric amygdaloidal forms and protoclased feldspar crystals point that crystallization took place during a violent displacement of eruptive mass. Rock mass is feebly crystallized and locally chloritized. The successive phases of eruption took probably place in short time spans. At Kamien Pomorski, the eruption of perlitic rhyolite was preceded by a period of brecciation and of hydrothermal activity. At Dźwirzyno, this period corresponded to the era of successive eruption phase expressed in the form of transitional rhyolite that, apart from a different development of rock body, represents rather the older rhyolite. As concerns chemical composition, the rhyolites under consideration belong to rhyolites-leukorhyolites of sodium and potassium character. Owing to the complete alteration of the rock body the rhyolite examined might be called palaeorhyolites. On the other hand, due to a predominating influence of the secondary alteration - albitization, thus due to a partial homogenization of chemical and mineral composition, volcanic rocks may be termed albite palaeorhyolites. A comparison of the effusive rocks of Rotliegendes age, from the Baltic area of West Pomerania and of North-East Mecklenburg, Rugen and Usnam shows that the rhyolites from Kamien Pomorski and Dźwirzyno may generally correspond to the quartz porphyries and rhyo-dacites from the southern part of Rugen, and from Greifswald. A different development of perlitic forms of rock body in rhyolites points to different conditions of lava consolidation. The rock masses are of similar chemical composition, since no considerable differences exist in the results of chemical analyses of quartz porphyries from Rugen, vitrophyres and rhyo-dacitic porphyry from Greifswald (D. Korich, 1967) on the one hand, and the rhyolites from Kamien Pomorski and Dźwirzyno from the other. In addition, in both regions the rocks underwent also hydrothermal alteration processes, among which albitization no doubt played a considerable role. 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Journal ArticleDOI
TL;DR: Sokoloff et al. as discussed by the authors found that the porphyroid bodies are the same as those of the regionally predominant fracture zones and that their interbedding with rocks of varying composition and age, their contact alterations, metasomatic phenocrysts, and other geological and petrographic evidence, supplemented by chemical analyses, suggest an imrusive character of these rocks, rather than a sedimentary, pyroclastic, effusive origin, though this was believed by some of the earlier investigators.
Abstract: Orientations of the porphyroid bodies are found to be the same as those of the regionally predominant fracture zones. Their “interbedding” with rocks of varying composition and age, their xenoliths, contact alterations, metasomatic phenocrysts (in carbonaceous-sericitic shales near their contacts with the porphyroids), and other geological and petrographic evidence, supplemented by chemical analyses, suggest an imrusive character of these rocks, rather than a sedimentary, pyroclastic, effusive origin, though this was believed by some of the earlier investigators. – V. P. Sokoloff.