Augite Phenocrysts Research Articles

Remanent Magnetization of Lower to Middle Eocene Basalt Flows from Oregon

THE Siletz River Volcanic Series of north-western Oregon offers an ideal opportunity to determine the direction of the Earth's magnetic field in the western United States during the early Tertiary. The formation consists of 3,000–5,000 ft. of mainly submarine basaltic flows, flow breccia and tuff. Interbedded clastic sedimentary strata have yielded more than thirty species of fossils including brachiopoda and Turritella andersoni cf. subsp. susanae Merriam, on the basis of which J. Durham correlated this formation with the Capay shale of California1. Additional faunal assemblages, including some foraminifera collected subsequently to those referred to above, confirm this correlation and clearly establish the age of the Siletz River Volcanic Series as early middle Eocene to lower Eocene. Petrographically, the flows are porphyritic in texture, with phenocrysts of plagioclase and augite in a ground mass which consists of volcanic glass, plagioclase laths, and granules of augito and magnetite, the latter constituting as much as 15 per cent of the rock1.

HAWAIIAN PETROGRAPHIC PROVINCE

Research Article| October 01, 1949 HAWAIIAN PETROGRAPHIC PROVINCE GORDON A MACDONALD GORDON A MACDONALD U. S. GEOLOGICAL SURVEY, HAWAIIAN VOLCANO OBSERVATORY, HAWAII NATIONAL PARK, T. H. Search for other works by this author on: GSW Google Scholar GSA Bulletin (1949) 60 (10): 1541–1596. https://doi.org/10.1130/0016-7606(1949)60[1541:HPP]2.0.CO;2 Article history received: 29 Jul 1948 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation GORDON A MACDONALD; HAWAIIAN PETROGRAPHIC PROVINCE. GSA Bulletin 1949;; 60 (10): 1541–1596. doi: https://doi.org/10.1130/0016-7606(1949)60[1541:HPP]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The lavas of the Hawaiian Islands range from mafic picrite-basalts and melilite-nepheline basalts to salic trachytes. Olivine basalt, by far the most abundant type, is regarded as representing the parent magma of the Hawaiian province. Closely associated with the olivine basalts are basalts, and picrite-basalts with many large phenocrysts of olivine. Further differentiation results in eruption of andesine andesite, oligoclase andesite, picritebasalt with abundant large augite phenocrysts, and more rarely trachyte. Following a long period of quiescence there have been erupted on some islands nepheline basanite, nepheline basalt, melilite-nepheline basalt, “linosaite,” and a third type of picrite-basalt. The mineral and chemical composition of the various rock types, and their distribution on the individual islands, are described.Starting with olivine basalt as the parent magma means of deriving the other rock types are considered. It is concluded that crystal differentiation has been the principal process, although assimilation of limestone may also have been important. The parts played by gaseous transfer and selective remelting are difficult to evaluate, though both probably operated to some extent.Comparison of the types of igneous rocks in the Hawaiian province with those recorded from other Pacific islands shows that the rocks throughout the true Pacific Basin are, for the most part, closely similar, and suggests essential uniformity of parent magma and petrogenic processes throughout the Pacific Basin. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

CONTRIBUTION TO THE PETROGRAPHY OF HALEAKALA VOLCANO, HAWAII

The volcanic rocks of Haleakala Volcano comprise, from oldest to youngest, the Honomanu, Kula, and Hana volcanic series. The Honomanu volcanic series consists largely of olivine basalt, with less abundant basalt and picrite-basalt containing large phenocrysts of olivine. In the Kula and Hana volcanic series there are some basalts and olivine basalts, but the characteristic rocks are andesites and picrite-basalt containing abundant phenocrysts of augite as well as olivine. New chemical analyses of six specimens of Kula lavas are given. Five are andesites, and one a picrite-basalt. Each of the six rocks is described in detail. A variation diagram for Haleakala rocks illustrates the alkalic character of the series.

Petrography of the Samoan Islands

Tutuila Islands consists of five overlapping shield volcanoes. The lavas of all are predominantly olivine basalts, but picritic basalts, olivine-poor basalts, and olivine-free basalts also are present. Two flows of oligoclase andesite are found in the easternmost (Olomoana) volcano; both andesine and oligoclase andesites are present in the westernmost (Taputapu) volcano. The pre-caldera lavas of Pago Volcano are predominantly olivine basalts, with less abundant olivine-poor basalts, and picritic basalts rich in olivine phenocrysts. The same types are present in the caldera-filling lavas, but the olivine-poor basalts are more abundant; andesites and olivine-free basalts also are present. The post-caldera lavas include olivine basalts, olivine-poor basalts, picritic basalts, andesine andesites, and oligoclase andesites. The picritic basalts differ from those in the earlier groups in containing numerous augite phenocrysts in addition to those of olivine. During the period of eruption of the caldera-filling and post-caldera lavas, there were extruded several masses of quartz trachyte. The last eruptions on Tutuila were of limburgite-basalt. Specimens from Ofu include olivine basalt, basaltic andesite, and picritic basalt with many phenocrysts of augite. Similar olivine basalts and a picritic basalt were collected on Tau. The oldest (Pliocene?) lavas of Upolu are preponderantly olivine basalt, with smaller amounts of olivine-poor basalt, picritic basalt, and basaltic andesite. At least one mass of trachyte is present. The picritic basalts contain phenocrysts of both olivine and augite. With one possible exception, all the Pleistocene lavas of Upolu are olivine basalts. The exception is a specimen of basaltic andesite which may belong with the older lavas. The Recent lava flows also are olivine basalts. The lavas of Savaii are predominantly olivine basalts, but picritic basalts containing augite and olivine phenocrysts also are known. All available chemical analyses of Samoan rocks are listed. The suite is alkalic, near the boundary of the alkali-calcic group, with an alkali-lime index of 50.5. Differentiation was probably largely by crystal settling.

Igneous rocks of the Highwood Mountains, Montana: Part II. The Extrusive Rocks

Early eruptions of quartz latites, rather rich in potash, built up a volcanic mountain over 30 miles across on an irregular surface of late Cretaceous sediments. Erosion then removed much of the quartz latite. Renewed volcanism formed a volcano made up of basaltic rocks (mafic phonolite) that covered the quartz latites. The phonolites vary in texture and in the kind and proportion of the felsic minerals present. All contain abundant phenocrysts of augite and some of olivine; some have phenocrysts of leucite, others analcime, and many have pseudoleucite. Some have phenocrysts of biotite and barium sanidine. The groundmasses contain about the same minerals as the phenocrysts with more felsic minerals and more biotite in the coarser-grained groundmasses.

Note on corroded ferromagnesian phenocrysts in a Scottish basalt

I. Introduction. The writer has recently made a study of a large number of slices of the Lower Carboniferous porphyritic basalt lavas of North Ayrshire and adjoining areas (MacGregor, 1930,1 pp. 89-99). In these rocks phenocrysts of augite, olivine and plagioclase felspar may often be seen to have sub-rounded outlines, or to have embayments and enclosures of groundmass. One rock-slice in particular, because it contains an exceptionally good example of a honeycombed augite phenocryst, deserves a more detailed description than could be accorded to it in the North Ayrshire Memoir (MacGregor, 1930, p. 97). The rock from which the slice (20,196)2 was cut was collected by Mr J. E. Richey near Craig of Neilston, Renfrewshire, in ground surveyed by him for the Geological Survey. It is an olivine-basalt of Dunsapie type (MacGregor, 1930, p. 91), and contains large phenocrysts of olivine, augite and felspar. II. Description of the Rock. The chief feature of the slice is a large phenocryst of pale fawn-brown augite about 1 cm. across (Plate XLVII., Fig. 1). The phenocryst is traversed by ramifying passages that locally pinch out almost entirely. The material in these cavities has crystallised mainly as felspar, granules of augite, and iron-ore, and closely resembles the groundmass. This material enclosed in the augite is in fact seen to be in communication with the groundmass at the right-hand side of the photograph (Plate, XLVII., Fig. 1). There is a decided tendency for little felspar laths (traversed by numerous delicate apatite needles, and accompanied by

Petrographical Notes on the Geological Collection from the south Central Sahara made by Mr. Francis R. Rodd

There are four volcanic rocks in the collection, two basalts and two trachytes. They all come from the volcanic region north and north-east of Auderas (Central Air), 1 which, from Mr. Rodd's mention of ‘isolated volcanic domes’ and ‘conical peaks’, appears to be of Recent, or, at the earliest, of Kainozoic age. The two basalts are labelled ‘Tighummar’, a locality about 14 miles north of Auderas on Rodd's map. One specimen is of the porous character and pale-grey tint so frequent in Kainozoic and Recent basalts, and shows small phenocrysts of olivine and augite, the former weathering into yellow limonitic spots. The other rock is darker and more slag'gy in appearance, and is devoid of determinable phenocrysts. In thin section the pale-grey rock proves to be an olivinebasalt, with sparse and small phenocrysts of almost colourless augite, sometimes showing a faint purplish tinge, and olivines with thick investments of red iron-ore. The olivine forms some phenocrysts as big as the pyroxenes, but all transitions in size can be traced down to numerous, small, hæmatized crystals which form part of the ground-mass. The ground-mass consists of plagioclaselaths and microlites (labradorite, Ab 2 An 3 ), granular augite, and magnetite-cubes, together with the above-mentioned olivines. There is a small amount of turbid isotropic base. The felspars show well-marked flow orientation, and range in size from microlites to microphenocrysts. The second rock approaches the ultrabasic basalts in composition. It shows numerous small phenocrysts of fresh olivine and augite. The latter is dictinctly publish, but many sections have

II. On Primary Analcite and Analcitization

Although in recent years a large number of rocks containing analcite have been described, there is still some difference of opinion regarding the origin of the mineral. It had often been observed in igneous rocks, but was always considered to be secondary until, in 1890, Lindgren ( cf. 1893, p. 289) concluded that it must be primary in some analcite-basalts. These rocks contained phenocrysts of olivine, augite and analcite in a mesostasis of the two latter minerals. The analcite was separated and analyzed and the composition found to agree with NaAlSi2O6.H2O. In the same year, Hunter and Rosenbusch (1890, p. 455) determined the chemical composition of the “glassy” base of the monchiquites, but Pirsson (1896, pp. 682-3) was the first to identify it as analcite. He considered the mineral in this rock to be primary, as also did Cross (1897, p. 690) in a phonolite, Washington (1898, p. 187) in a tinguaite, and Coleman (1899, p. 436) in heronite. This view, to which most of the British writers who have recently described analcite-bearing rocks have subscribed ( cf. Campbell and Stenhouse, 1908, p. 128, and Flett, 1910, pp. 295-6), has been upheld particularly by Tyrrell (1912, p. 71) in the case of the Permo-Carboniferous rocks of the West of Scotland. Hibsch (1910, p. 427) is of the same opinion with regard to the mineral in nephelite-basalt and monchiquite from Bohemia. Most of the continental writers, however, postulate a secondary origin. Thus Brogger (1890, pp. 223-32) and Ussing (1894, pp. 118, 135) considered This 250-word extract was created in the absence of an abstract