Showing papers on "Phenocryst published in 1970"
TL;DR: In this paper, solid-liquid partition coefficients between phenocrysts and the host lavas have been measured for rare-earth elements by an isotope dilution technique and the consistency of much of the data suggests that most of the phenocryst crystallized under equilibrium, or quasi-equilibrium, conditions.
720 citations
TL;DR: Partition coefficients for K, Rb, Sr, and Ba distributed between plagioclase, K-feldspar, clinopyroxene, orthopyroxenes, mica, hornblende, garnet, and olivine phenocrysts and their igneous matrix materials, have been determined by a mass-spectrometric stable isotope dilution technique.
527 citations
TL;DR: In this article, rare earth element concentrations in zircons and apatites separated from dacites and granites have been measured, and the rare earth patterns of zirons from granites appear to reflect patterns of the liquid phase at earlier stages of crystallization.
337 citations
TL;DR: The compositions of five different coexisting pyroxenes hypersthene, pigeonite and augite in groundmass and bronzite in a tholeiitic andesite from Hakone Volcano, Japan have been determined by the electron probe microanalyser as mentioned in this paper.
Abstract: The compositions of five different coexisting pyroxenes hypersthene, pigeonite and augite in groundmass and bronzite and augite of phenocryst in a tholeiitic andesite from Hakone Volcano, Japan have been determined by the electron probe microanalyser. It is shown that there is a compositional gap of about 25 mole per cent CaSiO3 between groundmass pigeonite and augite, compared with 35 per cent CaSiO3 between phenocrystic augite and bronzite. Subcalcic augite or pigeonitic augite was not found. The groundmass augite, which occurs only as thin rims of pigeonite and hypersthene, is less calcic and more iron-rich than the phenocryst augite. It is also shown that the groundmass pigeonite is 3–4 mole per cent more CaSiO3-rich than the coexisting groundmass hypersthene. The Fe/(Mg + Fe) ratios of these coexisting hypersthene and pigeonite are about 0.31 and 0.33, respectively. It is suggested from these results that a continuous solid solution does not exist between augite and pigeonite of the Fe/(Mg + Fe) ratio at least near 0.3 under the conditions of crystallization of groundmass of the tholeiitic andesite. It is suggested from the Mg-Fe partition and the textural relation that the groundmass augite crystallized from a liquid more iron-rich than that from which groundmass hypersthene and pigeonite crystallized.
201 citations
TL;DR: In this paper, an olivine basalt from the Auckland Islands contains partially resorbed "xenocrysts" of orthopyroxene, clinopyroxenes and minor olivines.
102 citations
TL;DR: The Talasea Peninsula is composed of a chain of Quaternary composite volcanoes and a caldera, with lavas ranging in composition from basalt to rhyolite, although andesite is the dominant variety as mentioned in this paper.
Abstract: The Talasea Peninsula is situated in the New Guinea island arc complex which represents the tectonically active margin of the Australian continent. The peninsula is composed of a chain of Quaternary composite volcanoes and a caldera, with lavas ranging in composition from basalt to rhyolite, although andesite is the dominant variety. The caldera is roughly elliptical in shape, 10 × 13 km, with a volume of collapse of about 75 km 3 . Located within the caldera are two fault-bounded blocks which appear to be remnants of the ancestral mountain. Post-collapse volcanic activity is found at Mt. Makalia, an andesitic volcano which has developed in the central part of the caldera. It last erupted about 80 years ago. The other volcanoes of the peninsula are largely andesitic composite cones, some of which almost certainly have been active in the last few hundred years. A large area of the peninsula is composed of acid extrusions with an average thickness of 100 m. The basalts, andesites, and dacites are often highly porphyntic, containing phenocrysts of plagioclase and diopsidic augite, with olivine in the basic members and hypersthene and a titanomagnetite in the intermediate varieties. The rhyolite and rhyodacite lavas are weakly porphyritic and are characterized by phenocrysts of pyroxene, amphibole, biotite, and quartz. The groundmass of the basic lavas is often mostly glass, but when partly crystalline, it contains plagioclase, hypersthene, and/or pigeonite (usually the latter only), augite, iron-titanium oxides, and accessory tndymite or cristobalite. The Talasea basic lavas are quartz-normative, low in TiO 2 , and show moderate absolute iron-enrichment. The series has high K/Rb ratios and is low in K 2 O in comparison to other New Guinea lava series. Numerical evaluation of crystal fractionation of a possible basaltic parent shows that this process could account for all but the acid lavas of the Talasea series; it is only the dominance of andesite that suggests that the series was generated at a depth where andesite is the low melting composition.
87 citations
TL;DR: Tenerife is composed of volcanics belonging to the basanitetrachyte-phonolite assemblage that characterises many Atlantic islands as mentioned in this paper, and the most voluminous development of intermediate and salic volcanics has been in the centre of the island where the Las Canadas volcanoes arose upon a basement shield composed mainly of basanite and ankaramite flows, tuffs and agglomerates.
Abstract: Tenerife is the largest of the seven Tertiary to Recent volcanic islands that make up the Canary Archipelago. The island is composed of volcanics belonging to the basanitetrachyte-phonolite assemblage that characterises many Atlantic islands. The most voluminous development of intermediate and salic volcanics has been in the centre of the island where the Las Canadas volcanoes arose upon a basement shield composed mainly of basanite and ankaramite flows, tuffs and agglomerates. The initial post-shield activity built the Vilaflor volcanic complex (Lower and Upper Canadas Series) that originally covered much of the underlying shield volcanics. A vast collapse of the complex, probably during post-Pleistocene times, in the centre of the island has left a large semi-circular wall, and provides an excellent vertical section through the complex. Quaternary volcanism within the collapsed area has built the twin, central-type volcanoes, Viejo and Teide, both of which have attendant satellite vents. That part of the Vilaflor Complex exposed in Las Canadas, together with the Viejo and Teide volcanoes, comprise the Las Canadas volcanoes. Four distinct rock types can be recognised in these volcanoes, basanite, trachybasanite, plagioclase phonolite, and phonolite. Each rock type can be recognised chemically and mineralogically, but there is essentially a gradational series from basanite to phonolite that includes both aphyric and glomerophyric rocks. The volcanics are strongly undersaturated and sodic, and some of the phonolites are mildly peralkaline. Variations in degree of undersaturation, and trace element abundances indicate a number of cycles of activity which would be consistent with the known field relations. Forsteritic olivine occurs in the basanites and trachybasanites but is not a stable phase in the more salic volcanics. Clinopyroxene is ubiquitous, varying in composition from titanaugite in the basanites to slightly sodic augite in the phonolites. Strongly sodic pyroxene is restricted to the groundmass of the microcrystalline phonolites along with aenigmatite and a kataphoritic amphibole. Plagioclase is found only in the groundmass of the basanites, but andesine and potash-oligoclase are common phenocryst minerals in the trachybasanites and plagioclase phonolites respectively, whereas the characteristic feldspar of the phonolites is anorthoclase. The relatively smooth curves of major and trace element variation, the presence of accumulative volcanics at all stages of differentiation, zoning of the mineral phases, and the clustering of the phonolites around the low temperature trough in Petrogeny's Residua System, all indicate that the descent from basanite to phonolite has resulted from fractional crystallisation of a basanite parent magma. The trend of pyroxene crystallisation, and the fairly constant FeO/Fe2O3 ratio during fractionation indicate crystallisation under low PO2 conditions.
61 citations
TL;DR: The petrology and geochemistry of East Island have been investigated for the first time in this article, where X-ray fluorescence analyses were made of 43 lavas for the major elements plus Cr, Ni, Rb, Sr, Ba, Pb, and Th and the minerals were analysed by electron microprobe.
Abstract: The petrology and geochemistry of East Island have been investigated for the first time. The island is a deeply dissected remnant of a Pleistocene shield volcano, one of several emerging from an oceanic rise forming part of the southwest branch of the Indian Ocean ridge system. The lavas form a flat-lying sequence of oceanites, ankaramites, olivine basalts and feldsparphyric basalts, the ankaramites containing 1 cm phenocrysts of diopsidic clinopyroxene. X-Ray fluorescence analyses were made of 43 lavas for the major elements plus Cr, Ni, Rb, Sr, Ba, Pb, and Th and the minerals were analysed by electron microprobe. The elements Mg, Cr, and Ni are strongly concentrated in spinel, olivine and clinopyroxene phases and in the ankaramites and oceanite lavas with maximum concentrations of 18% MgO, 1,000 ppm Cr, 380 ppm Ni, while Al, Ti, K, Rb, Ba, Th, Na, P, Sr concentrate in the groundmass and in the feldspathic and aphyric basalts. The elements Si, Ca, Fe and Mn remain virtually constant throughout the series.
61 citations
TL;DR: In this article, eleven new analyses and modes of comendite obsidians are presented, and compared with all available data on similar rocks, most specimens are aphyric or contain only sparse phenocrysts, most commonly alkali feldspar.
Abstract: Eleven new analyses and modes of comendite obsidians are presented, and compared with all available data on similar rocks. Most specimens are aphyric or contain only sparse phenocrysts, most commonly alkali feldspar. The oxides SiO2, Al2O3, Na2O and K2O total over ninety percent by weight in all analyses. Iron, as FeO, is the only other constituent rising above one percent by weight. When the analyses are projected into the system Na2O-K2O-Al2O3-SiO2, oceanic and continental samples group differently. Oceanic specimens have a compositional spread ranging from trachytic to the quartz-feldspar cotectic zone, consistent with derivation through a trachyte magma stem. Continental comendites show a strong correlation with the experimentally determined quartz-feldspar minima along a path of increasing peralkalinity. These differences presumably reflect the contrasting environments of magma generation, and suggest an origin by partial melting within the continental crust for the continental comendite obsidians.
55 citations
TL;DR: The lower Cretaceous Crowsnest Formation of Alberta is predominantly trachytic, but some sodic, analcite-rich, rocks occur as mentioned in this paper, which consists of bedded pyroclastic and epiclastic material with minor flows and dikes.
Abstract: The lower Cretaceous Crowsnest Formation of the Crowsnest Pass area, Alberta, is predominantly trachytic, but some sodic, analcite-rich, rocks occur. It consists of bedded pyroclastic and epiclastic material with minor flows and dikes. Most of the volcanism appears to have been explosive in nature. Almost all the igneous material is porphyry tic; nearly all the phenocrysts are zoned, and evidence of resorption is common. Sanidine phenocrysts show evidence of repeated growth and solution with as many as 60 cycles per crystal. The primary, igneous, analcite phenocrysts of the analcite phonolites and blairmorites crystallized at depth, and were preserved by rapid vertical transport and quenching at the surface. In many cases, however, partial or complete decomposition of the crystals occurred, and thermal discoloration of the analcite can be reproduced in the laboratory.
40 citations
TL;DR: Clinopyroxenes from Apollo 12 rocks were studied by X ray diffraction and electron microprobe methods, noting phenocrysts, chemical composition and crystallization as discussed by the authors.
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01 Jan 1970
TL;DR: In this paper, the authors have identified and described peralkaline acid rocks from Iceland for the first time and discussed the volume, bimodal distribution and diversity of acid and intermediate magmas in Iceland.
Abstract: The tertiary to Mid-Quaternary Setberg volcano in western Iceland consists of two centres. The northern, Centre 1 is tholeiitic and defined by cone sheets, a caldera and a gabbro intrusion. The southern, Centre 2 is chemically transitional between a tholeiitic and an alkalic basalt series, and contains a cone sheet swarm, gabbroic ring features and a major granophyric cone sheet. Each centre can be related to a minor magmachamber at a depth of 2-3 km. General hydrothermal alteration around both centres has resulted in aureoles of epidotization and the development of greenstones, containing garnet in the central areas. Late-Quaternary volcanism in the Setberg area has produced an alkalic basalt succession, ranging from alkalic olivine baalts to benmoreites. This activity is related to the E-W Snaefellsnes volcanic zone and its causes are explained in terms of Icelandic plate tectonics. The tholeiitic seies of centre 1, form a differentiated sequence from olivine tholeiites to rhyolites, contaning a Ca-poor pyroxene and augite in the basic and intermediate rocks, and a large compositional interval devoid of olivine. Plagioclase is the sole feldspar phase. The transistional series of Centre 2 is olivine-bearing throughout, with augite and rare phenocrysts of orthopyroxene in the basalts. Two lineages of differentiated rocks of this series are typified either by iron-enrichment (to icelandites) or by alkali enrichment (to rhyodacites and alkalic-rhyolites). The latter contains anorthoclase and potassic oligoclase or sanidine, as well as sodic pyroxene. Rocks of the alkalic series are olivine and pyroxene-bearing, with anorthoclase appearing in the benmoreites, while phlogopite and hornblende have been found in some of the lavas. Apatite occurs as a phenocryst in members of both the transitional and alkalic series. The origin of the transitional basalt magma is discussed according to two hypotheses. Its formation is accounted for by solidification from above in a magma reservoir of great vertical dimensions, leading, to confinement of the magma to depths where orthopyroxene replaces olivine on the liquidus, resulting in a trend towards undersaturation. The chemistry of the Late-Quaternary alkalic basalts of Setberg and Snaefeilsnes is related to magma generation at greater depths than in the case of the tholeiitic basalts of the Icelandic central volcanic zone. The volume, bimodal distribution and diversity of acid and intermediate magmas in Iceland are discussed. The high proportion of acid to basic volcanic rocks in eastern Iceland is contrasted with 3-4% in the rest of the country. Two types of acid centres are identified: centres of Thingmuli type, where tholeiitic basalts are associated with low-alkalic rhyolites; and alkalic centres, containing comendites, quartz- trachytes, alkalic-rhyolites and associated alkalic basalts or transitional basalts. Peralkaline acid rocks are identified and described from Iceland for the first time. Their genesis is discussed and related to fractionation from alkalic-rhyolites and trachytes by the "plagioclase effect". No evidence has been unearthed in this study which contravenes the hypothesis that the Icelandic acid rocks are a product of fractionation from basic magma.
TL;DR: In this paper, a detailed study of the ultrabasic nodules in alkali basalts of the Minusa and Transbaikalian regions (Siberia, USSR), represented by spinel lherzolites, websterites, clinopyroxenites, plagioclase-clinopyroxene rocks, as well as fragments of large augite crystals and sanidine crystals, was presented.
Abstract: The paper presents a detailed study of the ultrabasic nodules in alkali basalts of the Minusa and Transbaikalian regions (Siberia, USSR), represented by spinel lherzolites, websterites, clinopyroxenites, plagioclase-clinopyroxene rocks, as well as fragments of large augite crystals and sanidine crystals. In the course of this study 8 new chemical analyses of inclusions and 23 chemical analyses of minerals from them have been performed. The results of the investigations imply that lherzolites and websterites are xenoliths from the upper mantle, fragments of large augite and sanidine crystals are high-pressure phenocrysts, while clinopyroxenites are cognate nodules.
TL;DR: The Talasea series is composed of a chain of Quaternary volcanoes whose lavas range from basalt to rhyolite and the most distinctive feature is absolute iron enrichment in some lavas as mentioned in this paper.
Abstract: The Talasea Peninsula is composed of a chain of Quaternary volcanoes whose lavas range from basalt to rhyolite. The peninsula is situated in an orogenic environment and the lavas, while essentially calc-alkaline, show some differences from other orogenic suites on the Pacific rim. The most distinctive feature of the Talasea series is absolute iron enrichment in some lavas. Mineralogically, the andesites are characterized by phenocrysts of plagioclase, orthopyroxene, clinopyroxene and titanomagnetite, while the basalts lack titanomagnetite phenocrysts but contain olivine. The acid rocks have a mineralogy similar to that of the andesites, but also contain quartz, amphibole, biotite and ilmenite. The compositions of coexisting titanomagnetite and ilmenite in the acid lavas indicate equilibration temperatures in the range 920° to 860° C and oxygen fugacities (
$$f_{O_2 } $$
) above those of the fayalitemagnetite-quartz buffer assemblage. The mineralogical evidence supports the hypothesis of a crystal fractionation origin for this series and there is a possibility that the
$$f_{O_2 } $$
was more or less constant during the early stages of its evolution. The iron enriched lavas may be an offshoot from the main line of descent, resulting from near-surface fractionation, with the dominance of plagioclase in the crystal residuum producing an iron-rich liquid.
TL;DR: In this paper, three measures of structural state were used: obliquity, optic angle, and a parameter defined on the basis of Wright's "three-peak method".
TL;DR: Phenocrysts of sodic sanidine from twelve upper Cenozoic units of silicic ash-flow tuff and lava from the Western United States contain from 0.25 to 0.45 the Rb present in the associated groundmass materials as mentioned in this paper.
Abstract: Phenocrysts of sodic sanidine from twelve upper Cenozoic units of silicic ash-flow tuff and lava from the Western United States contain from 0.25 to 0.45 the Rb present in the associated groundmass materials. The ratios of potassium to rubidium in the sanidines are, on the average, about four times greater than those of the groundmass. Separation of phenocrystic sanidine from salic melts provides an efficient method for raising the Rb content and lowering the K/Rb ratio of the melts, although the amount of differentiation probably is limited by continuous reequilibration of the alkalis between crystal and liquid phases through ion exchange. Syenites of cumulate origin will have appreciably lower Rb contents and higher K/Rb ratios than the melts from which they precipitated. Available data on the distribution of Rb between synthetic biotite and K-sanidine demonstrate that the separation of biotite probably will not deplete salic melts in Rb relative to K.
TL;DR: The 1886 lava is primarily aphyric although it does contain a few xenocrysts as mentioned in this paper, and the earlier basic rocks were included as blocks in the Kaharoa eruption of 1020 A.D. and erupted as a tephra deposit on 10 June 1886.
Abstract: Basic rocks were included as blocks in the Kaharoa eruption of 1020 A.D. and erupted as a tephra deposit on 10 June 1886. The 1886 lava is primarily aphyric although it does contain a few xenocrysts. The earlier basic rocks, which presumably formed an intrusion before the Kaharoa eruption, contain olivine and augite phenocrysts and many xenocrysts. Sinking of the olivine and augite phenocrysts within this intrusion has formed a pyroxene dolerite, and assimilation of rhyolitic material by the basic magma of the intrusion has formed a series of bytownite dolerites.
TL;DR: In this article, the authors collected fresh abyssal basalts from the median valley of the Gulf of Aden and found that their chemistry is generally tholeiitic, but differs from most other abyssal basins in showing lower than usual Na 2 O and higher than usual CaO.
TL;DR: In this paper, the compositions of the cores and margins of the plagioclase phenocrysts in glassy acid rocks have been used to derive equilibration temperatures with their enclosing liquids using the Kudo-Weill geothermometer.
Abstract: The compositions of the cores and margins of the plagioclase phenocrysts in glassy acid rocks have been used to derive equilibration temperatures with their enclosing liquids using the Kudo-Weill geothermometer. The equilibration temperatures inferred from the compositions of the feldspar margins at 0.5 kilobars water pressure, or less, agree well with the temperatures obtained from the coexisting iron-titanium oxide microphenocrysts. These low water pressures are supported by the water fugacities deduced from the coexisting biotitemagnetite-sanidine assemblages. Acid magma represented by these rocks has a crystallization interval in excess of 100° C and liquidus temperatures above 900° C.
TL;DR: The Campus Andesite as discussed by the authors is a porphyritic andesite with phenocrysts of plagioclase, biotite, and hornblende with groundmass constituents of plagocase, K-feldspar, and minor quartz and magnetite.
Abstract: The Campus Andesite represents a small igneous pluton located in the Rio Grande valley and crops out on the campus of The University of Texas at El Paso. The igneous mass is post-Cretaceous in age, showing intrusive contacts with shale and marl of the Boquillas Formation and is surrounded by Quaternary lake or alluvial deposits (or both). Absolute age of the pluton, based on a K-Ar date, is 47.1 ± 2.3 m.y. Texturally, the intrusive is porphyritic with phenocrysts averaging 2 to 3 mm in length and set in an aphanitic groundmass. The phenocrysts, composed predominantly of plagioclase (andesine), comprise approximately 40 percent of the rock. The mineralogy of the andesite consists of phenocrysts of plagioclase, biotite, and hornblende with groundmass constituents of plagioclase, K-feldspar, and minor quartz and magnetite. Two chemical analyses of the Campus Andesite indicate a chemical composition intermediate between an andesite and a dacite. Mineralogically, the rock is classified as a porphyritic andesite.
TL;DR: The chemical composition of the pyroxenes and olivines of 12 basaltic rocks and 5 lherzolite nodules was determined quantitatively by electron micro-probe analysis.
Abstract: The chemical composition of the pyroxenes and olivines of 12 basaltic rocks and 5 lherzolite nodules was determined quantitatively by electron micro-probe analysis. The composition of the pyroxenes depends on the type of basalt in which they occur. Tholeiitic basalts with normative quartz contain three pyroxenes: orthorombic pyroxenes, pigeonites and augites. All pyroxene phases are zoned and do not show any exsolution. Their Ti and Al contents (Ca-Tschermaks and Ti-augite molecules) are small. All pyroxene phases were formed under disequilibrium with each other and with the melt because of rapid quenching. The sequence of crystallization: orthopyroxene—pigeonite—augite could be established by their Cr content. The alkali olivine basalts undersatured in SiO2 and the olivine nephelinites are characterized by Ti and Al-rich clinopyroxenes. The distribution of Ti and Al in the pyroxenes of the alkali olivine basalts shows a differentiation trend from the cores of the phenocrysts to their outer zones and to the crystals of the ground mass. Thereby the Ca-Tschermaks molecule is being replaced more and more by the Ti-augite molecule. The Ti content of the pyroxenes of the olivine nephelinites decreases in the last stage of differentiation because simultaneously increasing amounts of titaniferous magnetite crystallize. The pyroxenes of lherzolite peridotite nodules are characterized by high Al and low Ti contents which differ according to the type of basalt (alkali olivine basalt or olivine nephelinite) in which the nodules occur. The homogeneous distribution of the elements within the single grains indicates crystallization under equlibrium conditions. The conditions of their formation are comparable to those of Al-pyroxene peridotites in the upper mantle. The composition of pyroxenes of early accumulates of alkali basaltic melts differ from those of peridotite nodules. Therefore lherzolite nodules can be taken as residues of deeper peridotite masses.
TL;DR: The Seven Springs Formation of the Barrilla Mountains, western Texas, consists of 4 tuff units, 3 ignimbrite units, and 1 basalt unit as discussed by the authors, which are genetically related to the underlying Star Mountain Rhyolite flow in terms of sequential eruption of all units from a single magma chamber in which anorthoclase phenocrysts were crystallizing, settling and being partially resorbed.
Abstract: The Seven Springs Formation of the Barrilla Mountains, western Texas, consists of 4 tuff units, 3 ignimbrite units, and 1 basalt unit. The ignimbrites consist of anorthoclase (Or 35–40 ) phenocrysts in a matrix of glass, alkali feldspar, and quartz. The percentage of anorthoclase phenocrysts, the degree of welding of the matrix, and the grain size of the matrix all increase from the oldest to the youngest ignimbrites. The ignimbrites can be genetically related to the underlying Star Mountain Rhyolite flow in terms of sequential eruption of all units from a single magma chamber in which anorthoclase phenocrysts were crystallizing, settling, and being partially resorbed. All ignimbrites underwent posteruptive loss of Na 2 O.
01 Jan 1970
TL;DR: In this paper, the abundance of petrographically similar volcanic rocks in the Precambrian basement of Kansas, Oklahoma, and Texas suggests that the previously reported low relative abundance of Preambrian ash-flow tuff resulted more from lack of recognition than from progressive crustal differentiation.
Abstract: One of the results of a lack of clearly defined welded shards in ancient silicic volcanic rocks is a lack of recognition that many of these rocks areash-flow tuff. Ash-flow tuff units of Cambrian age in the Wichita Mountains of Oklahoma and of Precam-brian age in the St. Francois Mountains of Missouri contain only a few examples of clearly defined shards. That they are, nevertheless, ash-flow tuff is shown by their widespread occurrences, broken phenocrysts, common compaction foliation, and common snowflake texture. The abundance of petrographically similar rocks in the Precambrian basement of Kansas, Oklahoma, and Texas suggests that the previously reported low relative abundance of Precambrian ash-flow tuff resulted more from lack of recognition than from progressive crustal differentiation.
01 Jan 1970
TL;DR: In this paper, eleven new analyses and modes of comendite obsidians are presented, and compared with all available data on similar rocks, most specimens are aphyrie or contain only sparse phenocrysts, most commonly alkali feldspar.
Abstract: Eleven new analyses and modes of comendite obsidians are presented, and compared with all available data on similar rocks. Most specimens are aphyrie or contain only sparse phenocrysts, most commonly alkali feldspar. The oxides Si02, M~O3, Na~O and K~O total over ninety percent by weight in all analyses. Iron, as FeO, is the only other constituent rising above one percent by weight. When the analyses are projected into the system Na~O- K20-AI~Oa-SiO 2, oceanic and continental samples group differently. Oceanic specimens have a compositional spread ranging from trachytie to the quartz-feldspar cotectic zone, consistent with derivation through a traehyte magma stem. Continental eomendites show a strong correlation with the experimentally determined quartz-feldspar minima along a path of increasing peralkalinity. These differences presumably reflect the contrasting environments of magma generation, and suggest an origin by partial melting within the continental crust for the continental eomendite obsidians.