Showing papers on "Phenocryst published in 1974"
TL;DR: Basalts cored on legs 2 and 3 of the Deep-Sea Drilling Project (DSDP) range in sea floor spreading age from 18 to 67×106 yr as discussed by the authors.
Abstract: Basalts cored on legs 2 and 3 of the Deep-Sea Drilling Project (DSDP) range in sea floor spreading age from 18 to 67×106 yr. Although many of the basalts are highly altered, fresh glass is usually present. Except for site 2–10 the fresh glasses are petrographically and geochemically similar to mid-Atlantic ridge (MAR) axial basalts. There are no systematic compositional differences as a function of distance from the MAR axis. Two sites contain basalts with olivine (Fo90) phenocrysts, high Mg/Mg + ΣFe, high Ni and Cr abundances, and very low large ion lithophile (LIL) element abundances. These basalts are the best candidates for primary magma recovered from the sea floor; fractional crystallization of such basalt may yield the more evolved basalts typical of the MAR. More fractionated basalts with clinopyroxene phenocrysts occur at twp other sites, but they retain low LIL element abundances. Site 2-10 contains titaniferous augite and is relatively enriched in LIL elements. It is unlikely that this basalt was derived by fractional crystallization from LIL element depleted tholeiites; instead, the site 2-10 basalt requires a different mantle source. These results imply that the upper Atlantic Ocean basement is dominantly LIL element depleted tholeiite.
549 citations
TL;DR: The volcanic rocks of Iceland are anomalous in their oxygen isotope content. as mentioned in this paper showed that the Icelandic basalts that resemble ridge basalts in δO18 also resemble them in major element chemistry.
143 citations
TL;DR: In this paper, it was shown that peralkaline melts are of low viscosity and high temperature, as inferred from their chemical composition (high iron and alkali-, and low alumina-concentrations).
Abstract: Peralkaline silicic welded ash-flow tuffs differ characteristically in a number of properties from most calc-alkaline welded tuffs, due to their generally lower viscosity and higher temperatures. For example, individual cooling units are relatively small (less than 30 m thick, less than 5 km3 in volume); rocks can be thoroughly welded and crystallized to feldspar, quartz, and mafic minerals; several primary deformational structures (e.g. lineations, stretching of pumice, folds, ramp structures) indicate late stage laminar creep, resulting from the low yield strength of the nearly homogeneous glass of very low viscosity. Theoretical considerations also suggest that peralkaline melts are of low viscosity and high temperature, as inferred from,e.g., their chemical composition (high iron- and alkali-, and low alumina-concentrations). The low viscosity may also be due to trapping of volatiles. Absence or paucity of OH-bearing phenocryst phases, paucity of pyroclastic rocks, other than ash flow tuffs, formed from highly explosive eruptions, and apparently high crystallization temperatures, indicate that peralkaline silicic magmas are comparatively dry. The common occurrence of peralkaline ash-flow tuffs may be due to an increased water content of the magmas, resulting also in amphibole phenocrysts in some welded tuffs, or to specific volcanotectonic conditions. Ash flows of peralkaline composition move as particularly dense particulate flows. This type of flowage and the very rapid welding of the low viscosity glass lead to a high degree of homogenization of the fine glass shards. This in turn inhibits complete degassing of the collapsing ash flow. Semiclosed systems result where gas overpressures can develop and where volatiles play an important role by fluxing crystallization and transporting dissolved matter. Several types of vesicles can form under these conditions: (a) Spherical vesicles within collapsed ash and pumice particles formed after deposition of the ash flow. (b) Round or irregular vesicles transsecting pyroclastic particles, vesicle sheets, and large cavities, several m in diameter, may form in a largely homogenized ash-flow tuff beneath tightly welded layers. (c) Lensoid cavities formed during granophyric crystallization of large pumice particles. Small ash particles of peralkaline composition may assume spherical shapes due to their low viscosity and in some cases, expansion of bubbles. They form during transport and are preserved under low load pressure in the top part of cooling units. Globule lavas and most froth flows are interpreted as welded ash-flow tuffs, some of their unusual features being due to their peralkaline composition.
94 citations
TL;DR: Oxygen isotope analyses of sanidine phenocrysts from rhyolitic sequences in Nevada, Colorado, and the Yellowstone Plateau volcanic field show that δ18O decreased in these magmas as a function of time, which may have been caused by isotopic exchange between the magma and groundwater low in 18O.
Abstract: Oxygen isotope analyses of sanidine phenocrysts from rhyolitic sequences in Nevada, Colorado, and the Yellowstone Plateau volcanic field show that δ18O decreased in these magmas as a function of time. This decrease in δ18O may have been caused by isotopic exchange between the magma and groundwater low in 18O. For the Yellowstone Plateau rhyolites, 7000 cubic kilometers of magma could decrease in δ18O by 2 per mil in 600,000 years by reacting with water equivalent to 3 millimeters of precipitation per year, which is only 0.3 percent of the present annual precipitation in this region. The possibility of reaction between large magmatic bodies and meteoric water at liquidus temperatures has major implications in the possible differentiation history of the magma and in the generation of ore deposits.
93 citations
TL;DR: In this paper, major and trace elements have been determined in monzonoritic rocks (hypersthene-monzodiorite or jotunite) from two intrusions belonging to the South Rogaland anorthositic complex (Norway).
66 citations
TL;DR: The mineral assemblage Ol+Cpx+Pl+Amph appears to have been in equilibrium in the Kick’em-Jenny melt prior to eruption as discussed by the authors.
Abstract: An oceanographic survey by H.M.S. Hecla of the 1974 active submarine volcano (12°18′N and 61°38′W) revealed a crater at 190 m below sea level and bottom-sampling yielded fresh olivine basalt pyroclastics with phenocrysts of olivine, plagioclase and clinopyroxene. Megacrysts of amphilbole, up to 16 modal percent, are subsilicic and nepheline-normative ferroan pargasites. The mineral assemblage Ol+Cpx+Pl+Amph appears to have been in equilibrium in the Kick’em-Jenny melt prior to eruption, although published experimental studies on hydrous (H2O-saturated) or anhydrous alkali basaltic compositions have not yielded this mineral assemblage at any pressure. Interpolation between the experimentally determined phase relationships for dry and water-saturated alkali basaltic liquids indicates, however, that in an isobaric section at 5 kb the observed assemblage can exist in the water-undersaturated region. The Kick’em-Jenny olivine basalts belong to a suite of variably undersaturated basaltic rocks including alkali picrites and basanites, common in Grenada and the southern Grenadines, but although the available evidence indicates the importance of the presence of water in the genesis of these melts, their origin seems most likely to be associated with partial melting of upper mantle material rather than melting of amphibolite in an underthrust lithospheric slab.
64 citations
TL;DR: In this article, a suite of lavas from the Great Ararat is described, and two distinct series can be recognized, each consisting of andesite, dacite, and rhyodacite (designated Low-Y and high-Y series), they are distinguished by significantly different K.
Abstract: A suite of lavas from Great Ararat is described. Analyses are reported for Si, Al, Fe, Mg, Ca, Na, K, Ti, Mn, S, P, Ba, Nb, Zr, Y, Sr, Rb, Zn, Cu, and Ni. Two distinct series can be recognized, each consisting of andesite, dacite, and rhyodacite (designated Low-Y and high-Y series), they are distinguished by significantly different K. Mn, P, Nb, Zr, Y, Rb, Cu, and normative feldspar contents-The high-Y series corresponds closely to a normal calc-alkali series, but the low-Y series is relatively deficient in K, Mn, P, Nb, Zr, Y, and Rb. $$^{87}Sr/^{86}Sr$$ from both series is $$0.7050 \pm 0.0005$$. Petrographi-cally, both series are characterized by three generations of feldspar, all rock types containing labradorite together with more sodic phenocrysts and groundmass plagioclase: orthopyroxene, clinopyroxene, magnetite and ilmenite are present. Quartz and K-feldspar phenocrysts are absent. Suggestions of the former presence of garnet and brown hornblende, together with relevant experimental and geo-chemic...
47 citations
TL;DR: In this paper, the authors used an Electron microprobe and textural data to deduce a crystallization history for plagioclase in the Grand Manan Island (Canada) diabase sheet.
Abstract: SUMMARY. Electron microprobe and textural data are used to deduce a crystallization history for plagioclase in the Grand Manan Island (Canada) diabase sheet. The plagioclase crystals consist of a prominent core (An~9-67) surrounded by a distinctly more calcic zone (Ans2-76), which grades into a normally zoned mantle (An73_40). This zoning pattern is interpreted as reflecting the elevation of a dry diabase magma from at least io to 2o km to near surface conditions. THE crystallization history of an igneous rock unit may be inferred from the compositional zoning of the plagioclase feldspars. Vance (I962, 1965) reviewed the theories affecting compositional zoning and Bottinga et aL (I966) further evaluated the role of diffusion in crystallization from a multicomponent melt. Correlating compositional zones from one crystal to another is difficult because thin sections include crystal sections that are random both in orientation and in relative position within the crystals. Large scale non-cyclical features such as normally-zoned mantles are more easily correlated and reflect conditions that have affected the body as a whole. Small-scale oscillatory zone patterns cannot be correlated with any degree of confidence possibly because these features reflect local conditions. This paper provides a description and interpretation of zoning patterns in plagioclase from a tholeiite sheet. Generalpetrography. Gunter (I967) described the general petrography and chemistry of a 500 ft thick basalt sheet of Triassic age exposed on the west coast of Grand Manan Island, Bay of Fundy, New Brunswick, Canada. The sample chosen for the present study was taken from the 357"9 ft level (134 ft above the base) and consists of seriate plagioclase and pyroxene up to 2 mm in length with mesostasis consisting of smaller feldspar grains and micropegmatite. The modal composition ,is 40 % plagioclase, 40 % clinopyroxene, 5 % orthopyroxene, and 15 % mesostasis and the normative feldspar composition is Ors. sAb34.6An56.6The larger plagioclase crystals (hereafter referred to as phenocrysts) in the central 45o ft of the sheet contain an optically distinct core surrounded by a distinctly more calcic zone, which is the inner portion of a normally zoned mantle. Although the proportion of core to mantle increases with depth, the general compositional pattern is retained. Three lines of evidence indicate that the plagioclase and augite co-crystallized throughout the event that resulted in the major oscillation of the plagioclase crystals: glomerocrysts of plagioclase, augite, and orthopyroxene occur near the
47 citations
TL;DR: Santiago, Pinzon, and Rabida are each single volcanoes that lie along a north-trending line near the center of the Galapagos Archipelago.
Abstract: Santiago, Pinzon, and Rabida Islands are each single volcanoes that lie along a north-trending line near the center of the Galapagos Archipelago. The core volcano of Santiago Island and its flanking lava fields are composed of basaltic to intermediate lava of alkalic parentage. The smaller islands of Rabida and Pinzon are constructed of tholeiitic lava and tuff, ranging in composition from basalt to siliceous trachyte. On each of these two islands, products of eruptive cycles are preserved as tuff-flow sequences of decreasing degree of differentiation and increasing phenocryst abundance up-section, possibly a consequence of tapping successively deeper levels of compositionally zoned magma chambers.
Island age generally increases to the south away from the Galapagos rift. Volcanic activity on Santiago spanned most of the past 0.7 m.y. K-Ar ages of rocks exposed on Rabida range from about 1.0 to 0.7 m.y.; those on Pinzon are approximately 1.2 to 0.8 m.y. in age.
A part of the tectonic history of the central and southeastern Galapagos Islands is recorded in conspicuous, east-trending alignments of volcanic vents on Santiago and parallel normal faults and some aligned vents on six other islands to the southeast. The central islands among this group are also characterized by uplifted submarine rocks. On the basis of geological and geophysical observations, we hypothesize that this uplift, north-south distention, and associated volcanism may have resulted when an east-trending curtain of mantle upwelled beneath the center of the archipelago during the past 0.5 m.y. The east-trending tectonic pattern appears to be superimposed on the older northwest and northeast trends that have been cited for the entire island group.
46 citations
TL;DR: In this paper, the Cretaceous, almandine-bearing rhyolites of the Mt Somers and Mt Misery Volcanics of Canterbury, New Zealand, can be subdivided on the basis of ferro-magnesian phenocryst assemblages as garnet rhyolite, garnet-biotite-bioitite rhymes, and garnet -orthopyroxene rhymes.
Abstract: The Cretaceous, almandine-bearing rhyolites of the Mt Somers and Mt Misery Volcanics of Canterbury, New Zealand, can be subdivided on the basis of ferro-magnesian phenocryst assemblages as garnet rhyolite, garnet-biotite rhyolite, garnet-orthopyroxene-bioitite rhyolite, and garnet-orthopyroxene rhyolite. The garnets are considered to be primary igneous phenocrysts. Garnets from Mt Somers Volcanics are less variable in composition and have consistently higher grossular and spessartine contents and almandine/pyrope ratios than garnets from Mt Misery Volcanics. There is a regional correlation between Fe/Mg ratios in coexisting garnet, biotite, and parent rock. Application of published experimental data to feldspar and ferromagnesium phenocryst assemblages allows interpretation of feldspar crystallisation histories and estimates of physical conditions of crystallisation. The rhyolite magmas were generated as primary melts whose origin and subsequent development were controlled largely by variations i...
44 citations
TL;DR: Dredged porphyritic basalt from the Discovery Tablemount, South Atlantic Ocean, has been described and analysed, and the rare earths determined as mentioned in this paper, and it is suggested that the tablemount may have drifted to its present postion from a point over a fixed mantle hotspot with which it remained in contact through a thermal and hydrodynamical channel.
Abstract: Dredged porphyritic basalt from the Discovery Tablemount, South Atlantic Ocean, has been described and analysed, and the rare earths determined. The basalt is a quartznormative tholeiite but mineralogically and chemically (including trace elements) has affinities with alkali basalt; the pyroxene trend falls between those of alkali basalts and Hawaiian tholeiites. In addition to reverse zoned labradorite phenocrysts, corroded feldspar xenocrysts are present, ascribed to pressure fluctuations. The basalt has an alteration rind showing chemical variations which in general agree with other published data; surprisingly, however, the rind shows a loss of potash. There is noticeable enrichment in the lighter rare earth elements and an unusually high [La/Sm]E.F ratio; when taken together with the tablemount's position near known hotspots, this evidence leads to the suggestion that the basalt derived from a small mantle plume. The age of the basalt capping is 25 m.y., compared with 67 m.y. for the nearest magnetic anomaly. It is suggested that, unless the tablemount was intrusive, it may have drifted to its present postion from a point over a fixed mantle hotspot with which, however, it remained in contact through a thermal and hydrodynamical channel.
TL;DR: In this article, the Apollo 11 basalt was found to have a nearly eutectic character of the crystallization of the high-titanium basalts, which is a mandatory consequence of the hypothesis that the lunar maria are slowly consolidated giant lava lakes.
TL;DR: The Cape Hoskins volcanic island arc as discussed by the authors is composed of basalt and rhyolite, and contains phenocrysts of plagioclase and subordinate augite, hypersthene, and in most samples iron-titanium oxides.
Abstract: The Cape Hoskins volcanoes form part of the Quaternary volcanic island arc that extends from Rabaul in the east to the Schouten Islands in the west, and they overlie the northerly dipping New Britain Benioff Zone. The products of the volcanoes range in composition from basalt to rhyolite, and are normative in quartz and hypersthene. They contain phenocrysts of plagioclase and subordinate augite, hypersthene, and in most samples iron‐titanium oxides; some samples also contain olivine or quartz or both, and some pumice contains hornblende and, rarely, biotite. Chemical analyses of 29 volcanic rocks are presented; 22 were also analysed for 17 minor elements — Rb, Ba, Sr, Pb, Zn, Cu, Zr, Y, Ni, Co, Sc, Cr, V, Ga, B, U, and Th. Chemically the rocks have many of the characteristics of the ‘island arc tholeiitic series’, but do not show a pronounced relative enrichment in iron and appear to be relatively enriched in Sr. Compared with volcanic rocks from the northern part of the Willaumez Peninsula, they...
TL;DR: In this paper, a new type of abyssal tholeiites characterized by CaO Na 2 O ratios lower than those of ordinary abyssal leiites were found on both the north and south walls of the Puerto Rico Trench.
01 Jan 1974
TL;DR: The ion microprobe was used to measure the composition and distribution of trace elements in lunar plagioclase, and these analyses were used as criteria in determining the possible origins of some nonmare lunar samples as discussed by the authors.
Abstract: The ion microprobe was used to measure the composition and distribution of trace elements in lunar plagioclase, and these analyses are used as criteria in determining the possible origins of some nonmare lunar samples. The Apollo 16 samples with metaclastic texture and high-bulk trace-element contents contain plagioclase clasts with extremely low trace-element contents. These plagioclase inclusions represent unequilibrated relicts of anorthositic, noritic, or troctolitic rocks that have been intermixed as a rock flour into the KREEP-rich matrix of these samples. All of the plagioclase-rich inclusions which were analyzed in the KREEP-rich Apollo 14 breccias were found to be rich in trace elements. This does not seem to be consistent with the interpretation that the Apollo 14 samples represent a pre-Imbrium regolith, because such an ancient regolith should have contained many plagioclase clasts with low trace-element contents more typical of plagioclase from the pre-Imbrium crust. Ion-microprobe analyses for Ba and Sr in large plagioclase phenocrysts in 14310 and 68415 are consistent with the bulk compositions of these rocks and with the known distribution coefficients for these elements. The distribution coefficient for Li (basaltic liquid/plagioclase) was measured to be about 2.
Journal Article•
TL;DR: The presence of anorthoclase in these alkali basalts is reasonable because the normative feldspar compositions from these rocks plot within the excluded portion of the Or-Ab-An diagram as discussed by the authors.
Abstract: Feldspar megacrysts from pyroclastic ejecta of four alkaline basaltic centers were analyzed by atomic absorption spectrophotometry. Anorthoclase is present at three localities, oligoclase at two localities, and andesine at one locality. These crystals are interpreted as phenocrysts of the basalt that crystallized under high-pressure conditions. The presence of anorthoclase in these alkali basalts is reasonable because the normative feldspar compositions from these rocks plot within the excluded portion of the Or-Ab-An diagram. This indicates that two types of feldspar, one alkali-rich and the other a more calcic plagioclase, would have crystallized from the melt. The value for D Sr of 5.6 that we measure for the anorthoclase basalt indicates that crystallization of this mineral will significantly deplete the total strontium content of an alkaline magma.
TL;DR: The Berlins Porphyry as discussed by the authors consists of many small plutons of dacite-granodiorite composition as well as a few surface flows, which are inferred to represent the original magma chilled at the time of emplacement.
Abstract: The Berlins Porphyry, exposed in the lower Buller valley, South Island, New Zealand consists of many small plutons of dacite-granodiorite composition as well as a few surface flows. Stratigraphic evidence indicates emplacement and final solidification of the sub-surface plutons at shallow depths, certainly less than two kilometres. Mineralogy, phenocryst abundances, and total rock chemistry all show little variation and indicate that the plutons at present exposed are probably satellites from a larger sub-surface batholith. Many small intrusions and the margins of most of the larger plutons are composed of black glassy dacite (hyalodacite), which is inferred to represent the original magma chilled at the time of emplacement. Calculations using phenocryst and groundmass compositions suggest that T = 790°C, PH2O = 1.5 kbar, and fO2 = 1013.5 when the magma was crystallising at depth. The inner portion of most larger plutons is composed of microgranodiorite grading into granodiorite which completed i...
TL;DR: The origin of massif anorthosites cannot be simply explained by a single magma type as mentioned in this paper, and the parents for any given anorthosite massif can be determined by the rock sequence associated with the massif.
01 Jul 1974
TL;DR: In this article, the authors measured compressional and shear wave velocities to hydrostatic pressures of 6 kb'in representative samples of basalt from each hole and a relatively high density limestone from Site 249.
Abstract: On Leg 25 of the Deep Sea Drilling Project five holes penetrated basement, and a total of approximately 12.7 meters of basalt were recovered. We have measured compressional and shear wave velocities to hydrostatic pressures of 6 kb'in representative samples of basalt from each hole and a relatively high density limestone from Site 249. In addition, compressional wave velocities are reported for two sedimentary rocks recovered from Sites 246 and 249. The three basalts studied from Site 249 have intersertal textures with altered feldspar laths and groundmasses higWy weathered to greenish yellow nontronite(?) and brown iddingsite. Weathering appears to decrease slightly with depth, and Sample 249-33-2, 126-129 cm is higWy vesicular. Samples 239-21-1, 46-49 cm and 239-20-1, 125-128 cm are intersertal to felty textured basalts with some alteration of their groundmasses. Basalt Sample 240-7-1, 120-123 cm is similar to the basalts from Site 239 in both texture and alteration. Sample 245-19-1,37-40 cm is a coarse-grained, relatively fresh basalt with an intergranular texture. Alteration is confined to pyroxenes in the groundmass. The two basalts from Site 248 have intersertal textures and contain large (up to 0.7 cm in length) feldspar and subordinate pyroxene phenocrysts. The deeper of the two samples (248-17-2, 122-125 cm) has less alteration and is slightly coarser grained than Sample 248-15-1, 35-38 em. Sample 249-27-2, 133-136 em is a slightly recrystallized biomicrite. Sample 249-28-2, 56-59 cm is a glauconitebearing mudstone, and Sample 246-10, CC (3 cm) is a volcanic sand cemented with carbonate.
TL;DR: Analyses of 37 samples of the McCartys basalt collected along the length of the flow indicate the existence of considerable longitudinal variation as mentioned in this paper, and the basalt is a quartz-normative tholeiite containing plagioclase phenocrysts.
Abstract: Analyses of 37 samples of the McCartys basalt collected along the length of the flow indicate the existence of considerable longitudinal variation. Near the vent:, the basalt is a quartz-normative tholeiite containing plagioclase phenocrysts; at distances greater than 4 km from the vent, it is an olivine-normative tholeiite containing olivine phenocrysts. Chemically, the quartz tholeiite is richer in silica, alumina, and lime but poorer in magnesia and ferrous oxide than is the olivine tholeiite. Vertical chemical variations are not significant.
Dissertation•
01 Jan 1974
TL;DR: In this paper, it is suggested that the ash-flow tuffs and porphyrites originated by partial fusion of the lower continental crust and that the basaltic rocks were derived by partial melting of an upper mantle source region.
Abstract: Late Precambrian (Hadrynian) volcanic rocks of the western part of the Harbour Main Group near Colliers comprise a thick succession of chiefly rhyolitic ash-flow tuffs overlain by basaltic lavas and intruded by porphyritic sills of intermediate composition. The tuffs form three distinct ash-flow sequences, each composed of a number of simple cooling units. The Bacon Cove sequence at the base of the section, the Weavers Hill sequence in the middle, and the Finn Hill sequence at the top, are each characterised by contrasting phenocryst assemblages consisting of albite + quartz + biotite, albite + quartz, and albite + minor quartz and clinopyroxene, respectively. Concentration of crystals and xenoliths near the base of individual cooling units is a direct result of gravitative settling of the suspended load during ash-flow emplacement together with concomitant winnowing of the fine ash fraction. The majority of the Colliers ash-flew tuffs probably represent the distal deposits of a much larger ash-flow field originally situated to the north of the map area. -- Subsequent devitrification and hydration of volcanic glass have preserved vitroclastic textures with remarkable fidelity. The devitrification textures so formed locally resemble structures produced during the experimental devitrification of natural rhyolite glass. -- Mild structural deformation dated as Late Hadrynian and post-Cambro-Ordovician involved block-faulting and two distinct periods of folding and weak penetrative deformation. Metasomatic alteration largely preceded a regional metamorphism of prehnite-pumpellyite grade which may have accompanied Late Precanbrian folding. -- Electron microprobe analysis of feldspar compositions throughout the complete range of Harbour Main volcanic rocks revealed predominantly albite and K-feldspar of metamorphic origin, and rare igneous anorthoclase and calcic plagioclase. -- The chemical effects of alteration processes on the whole-rock analyses have been examined quantitatively where possible prior to classification of magmatic trends and affinities. Localised metasomatism involved mobility of essentially Na₂O, K₂O, CaO, Rb, and Ba. The least mobile constituents are Al₂O₃, TiO₂, total Fe, Zr, Cr, and Ni. -- The silicic ash-flow tuffs and granitoid rocks of the Harbour Main Group are very similar in composition and characterised by high total alkalies, Ba, and K/Rb, and low CaO and Rb/Sr. Ba/Sr and Rb/Sr ratios indicate that plagioclase was precipitated early in the differentiation history of rhyolitic magmas and that biotite was a comparatively late phase to arrive on the Liquidus. The porphyritic sills or "porphyrites" have relatively high K₂O, Ba, and Rb, and low CaO and Sr, and are very similar in composition to continental interior "andesites". The basaltic rocks are "transitional" to "mildly alkaline" chemical types with typically high Al₂O₃ and low TiO₂. The main rock-types of the Harbour Main Group cannot be related simply by fractional crystallisation of a parental silicate melt. -- The accumulated chemical data and prominent bimodal association strongly suggest that the Harbour Main voltanic suite was emplaced in an environment characterised by rifting and distension of continental lithosphere. It is suggested that the ash-flow tuffs and porphyrites originated by partial fusion of the lower continental crust and that the basaltic rocks were derived by partial melting of an upper mantle source region.
05 Aug 1974-The Journal of the Japanese Association of Mineralogists,Petrologists and Economic Geologists
TL;DR: Esan volcano is composed of the calc-alkalic rocks which are divided into two types as mentioned in this paper : augite-hypersthene andesite of the early to middle stages and quartz-augite-Hypersthenesite, in which quartz phenocrysts, bipyramidal or resorbed in form, are included as much as 2-8% by volume.
Abstract: Esan, a volcano of the Nasu volcanic zone, is situated at the eastern end of the Kameda Peninsula, southwestern Hokkaido (Figs. 1 & 2; Table 1). In the Pleistocene time, the activity of Esan volcano begun with eruption of pyroclastic flows (Fig. 3), followed by extrusion of two lava domes, Kaiko-zan and Todo-yama. Then, a viscous lava erupted again, forming a semicircular ridge which has been called as “somma.” After a long quiescence, probably in the Holocene age, the final activity occurred, erupting nuee ardentes, pumice flows, lava flows and dome lavas (Fig. 4). Esan (618m) is the latest lava dome. During the whole history, the center of eruption have migrated from northwest to southeast. Esan volcano is composed of the calc-alkalic rocks which are divided into two types (Table 2). One is augite-hypersthene andesite of the early to middle stages, and the other is quartz-augite-hypersthene andesite of the final stage, in which quartz phenocrysts, bipyramidal or resorbed in form, are included as much as 2-8% by volume. As shown in Table 3, Esan lavas are intermediate in composition, ranging from 57.75 to 62.69% in silica content. They are similar in chemistry to other calc-alkalic rocks from the Nasu volcanic zone in Hokkaido (Figs. 5, 6 & 7). Mode of occurrence of phenocrystic quartz and experimental results by Wyllie (1971) (Fig. 8) suggest that the quartz phenocrysts in Esan lavas have not derived from the basement rocks, but crystallized from an intermediate magma together with large plagioclase, hypersthene, augite and hornblende prior to the crystallization of ordinary phenocrystic minerals under a certain condition of several kb, 1000-1100°C and 1-2% in water contents. Subsequently, the quartz phenocrysts suffered magmatic resorption by decreasing of pressure due to ascent of the magma. Simultaneously, hornblende phenocrysts resolved into the aggregates of minute grains of plagioclase, hypersthene, augite and magnetite (Table 4).
05 Dec 1974-The Journal of the Japanese Association of Mineralogists,Petrologists and Economic Geologists
TL;DR: The variation of hornblende phenocrysts in a single lava flow has been studied by the determination of optical properties and the partial chemical analyses as discussed by the authors, and the degree of primary post-eruption oxidation seems in general to be a function of the rate of cooling and that of volatile loss, except for a case of reheating.
Abstract: The variation of hornblende phenocrysts in a single lava flow has been studied by the determination of optical properties and the partial chemical analyses. Average 100×Fe3+/total Fe value is estimated to be about 25 for the green common hornblende dominantly contained in the chilled base, whereas it is about 80 for the reddish oxyhornblende scattered in the interior of the lava. Oxidation, developed during the initial cooling, is at a maximum at points approximately 2m. above the base as a whole. The degree of the primary post-eruption oxidation seems in general to be a function of the rate of cooling and that of volatile loss, except for a case of reheating.
TL;DR: The zonal structure of plagioclase phenocrysts in essential lenticules as well as other petrographic features of the Setogawa welded ash-flow sheet, a member of the late Cretaceous Nohi rhyolite complex in central Japan, varies from the bottom to the top of the sheet corresponding to the variation in a reverse order within the magma column prior to the ash flow eruption.
Abstract: The zonal structure of plagioclase phenocrysts in essential lenticules as well as other petrographic features of the Setogawa welded ash-flow sheet, a member of the late Cretaceous Nohi rhyolite complex in central Japan, varies from the bottom to the top of the sheet corresponding to the variation in a reverse order within the magma column prior to the ash-flow eruption. Plagioclase phenocrysts from the upper parts of the sheet, i.e. from the lower levels of the magma column, are normally zoned, while those from the lower parts of the sheet and from pumice lumps of underlying ash-fall layers show strongly oscillated zoning. In the middle parts, the degree of oscillation is intermediate. The origin of the variation in zonal structure is discussed using simplified model of the binary plagioclase system in terms of the shifting of equilibrium relation between the magma and crystals caused by the change of water pressure. The oscillatory zoned plagioclase crystals were formed in the upper levels of the magma column, where sudden drops and gradual recovering of water pressure were caused repeatedly by intermittent volcanic eruptions prior to the ash-flow eruption. The fluctuation of water pressure were small in the deep levels of the magma reservoir where plagioclase with normal zoning was formed.
01 Jan 1974
TL;DR: In this article, a small volcanic pipe, possibly a diatreme, is at the southeast end of the high and consists of a rubbly basalt-like matrix through which are scattered xenoliths of Precambrian crystalline rocks and of various Paleozoic and Mesozoic strata.
Abstract: A northeast-trending magnetic anomaly in parts of Beaverhead and Madison Counties, Mont., and Clark County, Idaho, may reflect the trend, shape, and size of a concealed pluton. The type of rock that forms the pluton(?) is unknown. A small volcanic pipe, possibly a diatreme, is at the southeast end of the high. The pipe, about 92 m (300 ft) in diameter, consists of a rubbly basalt-like matrix through which are scattered xenoliths of Precambrian crystalline rocks and of various Paleozoic and Mesozoic strata. It is uncertain whether the juxtaposition of the pipe and the magnetic high is meaningful or is merely fortuitous. Although no mineralized rock was found in the area underlain by the anomaly, placer gold has been found nearby. Some 113 km (70 mi) to the west, in Custer and Lemhi Counties, Idaho, a similar northeast-trending magnetic high marks the site of the Gilmore mining district. The similarities in trend, shape, and magnitude between the two anomalies suggest that the high in Beaverhead and Madison Counties should be investigated for undetected mineral deposits, possibly by a geochemical survey. An aer©magnetic survey of southeastern Idaho and southwestern Montana (U.S. Geological Survey, 1972) has disclosed an unusually large magnetic high that cuts across the major structural features of the Centennial area, Beaverhead and Madison Counties, Mont., and Clark County, Idaho (Figs. 1, 2A). The anomaly trends about N. 35° E. and forms an angle of about 66^ with both the eastward-trending Centennial MountainsCentennial Valley structural pair and the northwest-trending Madison Range-Madison Valley pair. The high is about Uo km (25 mi) long and 16 km (10 mi) wide, and has a magnetic relief of about 120 gammas. I suspect that this high reflects the trend, shape, and size of a concealed pluton. The type of rock that forms the pluton(?) is unknown. A small volcanic pipe, possibly a diatreme, is along the southeast edge of the high and may be the result of leakage from the buried pluton(?). There is, however, no convincing evidence to substantiate any relation between the pipe and the pluton(?), and their juxtaposition may be fortuitous. The pipe is exposed along the crest of the eastern part of the Centennial Mountains, where it crops out as a mass of dark-gray to black rubbly rock about 92 m (300 ft) in diameter in the center of sec. 1, T. 15 S., R. 1 W. It is clearly intrusive for it cuts both the basement complex of Precambrian crystalline rocks and the overlying bedded sedimentary strata; it seems to represent the upper end of an upward-punching igneous plug. The rubbly material that forms the pipe breaks apart easily and, thus, the pipe lacks topographic relief; it conforms to the contours of the adjacent terrain. The pipe is composed of two kinds of material: a dark-gray to black fine-grained basaltlike matrix, and a chaotic mixture of metamorphic, sedimentary, and igneous xenoliths. The matrix consists of a pilotaxitic mixture of plagiocla.se microlites through which are scattered small, euhedral to subhedral pyroxene grains and some magnetite. A light brown glass fills the spaces between the microlites, Phenocrysts of euhedral to subhedral clinopyroxene (salite, 2Vz "^ 58°, /I y = 1.690) are common; less abundant are plagioclase laths of labradorite (An^). Both the pyroxene and plagioclase phenocrysts are zoned No olivine was noted although several voids have suggestive olivine shapes. Many of the vesicles and voids are rirnmed by lightgreen chalcedony; the centers of these vesicles are filled with carbonate. Xenocrysts of subround quartz and sedimentary chert (?) are common. This basalt is similar to the basalt that forms the dikes and sills exposed farther to the west in the western Centennial Mountains. Table 1 compares the abundance of elements in the pipe's basalt matrix with those listed by Turekian and Wedepohl (1961, table 2) for other basaltic rocks. In general, the basalt of the pipe has a lower titanium content, but larger amounts of barium and strontium. This predominance of barium and strontium, seemingly is a characteristic of many of the mo,fic alkaline-rich rocks of this part of southwestern Montana (Hamilton and Leopold, 1962, p. B-28). The xenoliths include subangular to angular cobbles and small boulders of Precambrian crystalline rocks such as dark-gray amphibolite, lightbrown dolomite, white and light-green quartzite, and brown mica schist. These rocks are identical to the Precambrian crystalline rocks that form the basement in this part of southwestern Montana. Many of the xenoliths are sedimentary clasts of Paleozoic and Mesozoic age; the following formations are represented: Madison Group (Mississippian), Amsden Formation (Pennsylvanian), Quadrant Sandstone (Pennsylvanian), Mnwoody(?) Formation (Triassic), and Aspen(?) Formation (Cretaceous). Some xenoliths are fragments of pyroxene trachyte porphyry, a mafic volcanic rock that makes up the bulk of nearby Sawtell Peak. This trachyte is tentatively considered as Eocene in age (Witkind, 1972), and the trachyte xenoliths imply that the pipe was emplaced at some time after the Eocene. The youngest layered sedimentary rocks intruded by the pipe are lightgray dolomite beds of the Amsden Formation (Pennsylvanian), but nearby is a small patch of bright cherry-red siltstone and sandstone beds which is unrelated to any of the sedimentary rocks exposed in the general area. These red beds, resting unconformably on the Amsden, may be part of the Beaverhead Formation (Honkala, 1953) of PaleoceneCretaceous age (Ryder and Scholten, 1973), or they may be intensely baked and altered beds of the Aspen Formation of Cretaceous age. If they are part of the Beaverhead Formation they are all that is left of what must have been an extensive cover, for comparable beds are not known in the eastern Centennial Mountains. Conversely, if they are part of the Aspen Formation they must have been baked and altered during the intrusion of the volcanic pipe and then let down thousands of metres vertically with little or no disruption. Although no mineralized rock was found in the area underlain by the Centennial anomaly, some placer gold was found near Lakeview (Fig. 2A). Lyden (19*48, p. 10) stated: "The production of 9.^3 ounces of placer gold was reported from Lakeview on Odell Creek in 1935." The source of this placer gold is uncertain; although it may have come from various igneous intrusions in the Centennial Mountains, there is a strong possibility that it came from extensive glacial deposits that once mantled this area. Some 113 km (70 mi) to the west, in Custer and Lemhi Counties, Idaho, a similar magnetic anomaly, also striking northeast, cuts across the Lemhi Range, here trending about N. 35° W. (Figs. 1, 2B_). This magnetic high trends about N. 50° E., is about 17 km (10.5 mi) long and about 10 km (6 mi) wide, and has a magnetic relief of about 1^0 gammas. The high marks the site of the Gilmore mining district, from which lead, zinc, and silver have been extracted. The igneous rocks in the Cilmore mining district are quartz diorite and granodiorite; these rocks are closely related genetically to the ore deposits (Ruppel and others, 1970, p. lU and 15). Both anomalies cut across the regional structure, implying that they may be related. But there are some major uncertainties when the anomalies are compared, chiefly as to the kind of rock responsible for the Centennial high. It may be a basalt somewhat like that which forms the diatreme(?), thus differing sharply from the intermediatetype rocks responsible for the Lemhi anomaly. Despite this possibility, the similarities between the anomalies in trend, and to some extent in shape and magnitude, are marked. This likeness between these two widely separated anomalies suggests to me that the area underlain by the Centennial anomaly should be searched in more detail for as yet undetected mineral deposits. Such a concealed igneous body(?) may contain mineralized rock; the potential seems sufficient to justify a geochemical survey. Fi gu re 1 . M a j o r ph ys io gr ap hi c fe at ur es in pa rt s of so ut he as t Id ah o an d so ut hw es t Mo nt an a. Lo ca ti on s of fi gu re s 2A ^ an d 2j 3 ou tl in ed by ha ch ur es .