Showing papers on "Phenocryst published in 1977"

Compositional variations of young basalts in the Mid-Atlantic Ridge rift valley near lat 36°49′N

Abstract: Fifty acoustically positioned samples of fresh basalt were collected by the submersible Alvin from the median valley of the Mid-Atlantic Ridge during the French American Mid-Ocean Undersea Study (FAMOUS) in the summer of 1974. The samples show regular compositional variations from the center of the rift valley (central lava flows) out to the rift valley walls (flank lava flows). The central lava samples show higher ratios of olivine relative to clinopyroxene and plagioclase phenocrysts and contain chrome spinel. Glasses of the flank lava samples are enriched in SiO 2 , TiO 2 , K 2 O, H 2 O, and FeO/MgO relative to central lava samples. Studies of the thickness of palagonite and manganese crusts indicate that the flank lava flows are considerably younger than the inferred spreading age of the crust on which they occur. Flank lavas are generally older than central lavas, but notable exceptions occur. The composition of the flank lava glass can be derived by the removal of approximately 29 wt percent of analyzed phenocrysts (in the ratio 5.7 plagioclase, 2.5 olivine, 1.8 clinopyroxene) from the central lava glass. In addition, other processes (possibly involving volatile transfer) must enrich the flank lavas in K 2 O, TiO 2 , and H 2 O. A model is proposed whereby this crystal fractionation occurs in a shallow, narrow (6-km-wide) magma chamber underlying the median valley. The chamber is compositionally zoned, and central lavas are fed from dikes tapping its hotter axial zone, whereas flank lavas are fed from the cooler, differentiated melt on the margins. The nature of the chemical variations in the lavas permits an estimate of the composition and thickness of the cumulates forming at the base of the chamber.

Rare earth elements as hydrothermal tracers during the formation of massive sulfide deposits in volcanic rocks

Abstract: Massive, tabular sulfide bodies commonly occur within sequences of subaqueous volcanic rocks They are believed to form as chemical sediments when hydrothermal solutions which derived metals from interaction with the volcanic rocks flow out into sea water Such a water-rock interaction, hydrothermal exhalation model implies that changes in the composition of the hydrothermal solutions result from changing" conditions within the hydrothermal system and that the different metal contents of the resulting chemical sediments are related to different alteration reactions It is suggested that the rare earth elements (REE), because of their coherent and somewhat predictable geochemical properties, can serve as tracers of the alteration reactions which take place within these hydrothermal systems Changes in the reactions should be reflected by the REE pattern of the solution which will in turn influence the REE pattern of the resulting depositIn order to test this hypothesis the REE patterns of massive sulfide deposits and associated rocks from the Bathurst-Newcastle District, New Brunswick, Canada, were determined by instrumental neutron activation analysis (INAA) Interpretation of the REE data was aided by a simple computer model which predicted solution REE patterns resulting from various solution-rock interactionsChlorite-rich samples have REE distributions and abundances that are very similar to those of shales which suggests a detrital source for their REE Other chemical sediment samples, except Cu-rich sulfides, have positive europium anomalies The magnitude of the Eu anomaly and the amount of heavy REE enrichment increase with increasing content of oxide and sulfide minerals Samples with high contents of Pb sulfides have the highest Eu anomalies and the strongest enrichments of heavy REE and these characteristics do not correlate with the abundance of any other mineralThe observed patterns are best explained by an input of hydrothermal solutions to the site of chemical sedimentation Because of the characteristic REE patterns and consistent stratigraphic positions of the chemical sediment zones (Cu-rich sulfides, Pb + Zn-rich sulfides, and iron-formation) the following sequence of events apparently took place within the hydrothermal system: 1 Alteration of ferromagnesian minerals and/or volcanic glass released Cu and Fe to the solution (Cu-sulfide zone) 2 Preferential alteration of feldspar by a very concentrated brine enriched the solution in Pb, Zn, Ba, and heavy REE (Pb + Zn-sulfide zone) 3 Continued removal of Fe from the volcanic rocks without alteration of a large amount of the solid phases (iron-formation) Results of the REE study are consistent with the formation of massive sulfide deposits by precipitation from solutions which derived metals from volcanic rocks during water-rock interactions In fact, the data require that water-rock interactions take place and are incompatible with direct release of magmatic water onto the sea floor Generation of solutions capable of precipitating Pb-rich massive sulfide deposits requires that phenocrysts of feldspar are present in the volcanic rocks, that the rocks are felsic and that conditions within the hydrothermal system favor alteration of feldspar A relation should then exist between the metal contents of sulfide deposits and the composition of the host volcanic pile As expected, deposits in mafic lavas are Cu rich while those in mixed, mafic-felsic piles have Pb + Zn/Cu ratios which increase with the felsic rock content of the volcanic pile

Geochemistry and petrogenesis of a basalt-benmoreite-trachyte suite from the southern part of the Gregory Rift, Kenya

Abstract: In the southern Gregory Rift valley a series of transitional basalt, ferrobasalt, and benmoreite flows (1.65–1.4 Myr) is overlain by flood trachyte lavas (1.3–0.9 Myr). Mass balance calculations for major element compositions of rocks of this suite and their phenocrysts and microphenocrysts suggest that the ferrobasalts and benmoreites formed from magma resembling the most primitive basalt by closed system fractionation of plagioclase, clinopyroxene, olivine, titanomagnetite, and apatite. The trachytes formed from evolved magmas largely by alkali feldspar fractionation. Estimates of phenocryst and liquid densities and Rayleigh-law modelling of trace element contents support these conclusions. From Rayleigh-law modelling, we derived a set of effective distribution coefficients. Partial melting of crustal rocks or volatile transfer processes had no significant effect on the petrogenesis of this suite. The duration of the eruptive cycle, cooling time calculations, and mass balance calculations suggest that fractionation occurred in a magma reservoir with volume of at least 3 × 104 km3 during an interval of about 0.8 Myr. Temperatures during fractionation probably ranged from about 1200 °C to 900 °C, and pressures may have been roughly 5 to 8 Kb. We suggest that rift development was accompanied by large-scale injection of basaltic magma and dilation of the crust, extensive fractionation, preferential eruption of low-density and fluid trachytic flood lavas, and by several episodes of normal faulting.

Picrite basalts and related lavas from the Deccan Traps of Western India

Abstract: Detailed chemical and mineralogical data are given for three sequences of basalts and picrite basalts from bore-holes in Western India. The picrite basalts show bulk compositional variation generated by the fractionation of olivine and chromite. Evolved picrite basalt magma appears to have given rise to basalt by the fractionation of olivine+clinopyroxene, despite the presence of abundant plagioclase phenocrysts. It is suggested that a slow settling rate for plagioclase relative to clinopyroxene and olivine is sufficient to account for this feature. The high degree of equilibrium crystallisation which many of the lavas have apparently undergone is interpreted in terms of the mechanism of compensated crystal settling (Cox and Bell, 1972). Experimentally determined atmospheric pressure phase relations are used to model dyke-like magma chambers in some detail. Finally volumetric and age relationships are used to argue that the picrite basalts, despite their porphyritic nature, crystallised from ultramafic liquids containing in some cases at least 16% MgO.

Evolution of oceanic crust: 3. Petrology and chemistry of basalts from the East Pacific Rise and the Siqueiros Transform Fault

Abstract: Basalt samples obtained from the Siqueiros transform fault/fracture zone and the adjacent East Pacific Rise are mostly very fresh oceanic tholeiite and fractionated oceanic tholeiite with Fe+3/ Fe+2 ∼ 0.25; however, alkali basalts occur in the area as well. The rocks of the tholeiitic suite are ol + pl phyric and ol + pl + cpx phyric basalts, while the alkali basalts are ol and ol + pl phyric. Microprobe analyses of the tholeiitic suite phenocrysts indicate that they are Fo68–Fo86, An58–An75, and augite (Ca34Mg50Fe16). The range of olivine and plagioclase compositions represents the chemical variation of the phenocryst compositions with fractionation. The phenocyrsts in the alkali basalts are Fo81 and An69. The suite of tholeiites comprises a fractionation series characterized by relative enrichment of Fe, Ti, Mn, V, Na, K, and P and depletion of Ca, Al, Mg, Ni, and Cr. The fractionated tholeiites occur on the median ridge (which is a sliver of normal oceanic crust) of the double Siqueiros transform fault, on the western Siqueiros fracture zone, and on the adjoining East Pacific Rise, while the two transform fault troughs contain mostly unfractionated or only slightly fractionated tholeiite. We suggest that the fractionated tholeiites are produced by fractional crystallization of more ‘primitive’ tholeiitic liquid in a crustal magma chamber below the crest of the East Pacific Rise. This magma chamber may be disrupted by the transform fault troughs, thus explaining the paucity of fractionated tholeiites in the troughs. The alkali basalts are found only on the flanks of a topographic high near the intersection of the northern transform trough with the East Pacific Rise.

Mineralogy and glass chemistry of recent volcanic ejecta from Mt Erebus, Ross Island, Antarctica

Abstract: Bombs erupted in 1972 and recent flows of anorthoclase phonolite from Mt Erebus both contain phenocrysts of anorthoclase and microphenocrysts of olivine (Fa46Te3Fo51), clinopyroxene (Wo48Fs15En37), titanomagnetite (USP70), pyrrhotite, and apatite; the associated glass is peralkaline (agpaitic index = 1.06). The composition of glass inclusions in large anorthoclase (Ab65An16Or19) phenocrysts, which form a lag gravel on the summit cone of Mt Erebus, is generally more evolved than that from the groundmass of recent flows and bombs. Glass inclusions from between the core and rim of anorthoclase phenocrysts represent variations with time and show a small and systematic increase of Al2O3 and decrease of FeOT (total iron as FeO), possibly owing to reaction of the glass with the anorthoclase after entrapment. In contrast, whole rock-groundrnass trends show decreasing Al2O3 and increasing FeOT. The whole rock compositions of recent flows and ejecta are similar to older (up to 1 m.y.) anorthoclase phonolit...

The Alkalic Rock Suite of Bogoslof Island, Eastern Aleutian Arc, Alaska

Abstract: The historic eruptions of Bogoslof volcano were characterized by hornblende andesite in AD 1796, and nepheline-normative, hornblende basalt in AD 1883 and 1926-1927 Rare earth element (REE) abundances for the 1796 and 1926-1927 eruptions show enrichment of the light REE relative to the heavy REE, overall higher abundances in the andesite and a slight positive Eu anomaly in the basalt The $$^{87}Sr/^{86}Sr$$ ratios are $$070343 \pm 7$$ for the andesite and $$070296 \pm 7$$ for the 1927 basalt Lead isotopic abundances are similar for the two eruptions and the rocks are more radiogenic than mid-ocean ridge basalts Microprobe data for phenocrysts of potassio ferroan pargasite, calcic augite and plagioclase show a considerable compositional range and degree of compositional overlap Mafic inclusions in basalt and andesite contain the same phases as the host lavas and a similar compositional range, suggesting a syngenetic origin for inclusions and host One inclusion contains quartz and biotite which

Volatiles H2O, CO2, and CO in Silicate Melt Inclusions in Quartz Phenocrysts from the Rhyolitic Bandelier Air-Fall and Ash-Flow Tuff, New Mexico

Abstract: Silicate melt inclusions in quartz phenocrysts from air-fall and ash-flow tuff were studied from a section of Bandolier Tuff in Sandia Canyon, Jemez Mountains, New Mexico. My aim was to estimate the concentration of some major volatile species (H2O, CO2, and CO) in the magma before explosive eruption. On a microscope heating stage, I determined the melting points of the inclusions and, with an electron microprobe, determined the bulk chemical composition. I estimated total H2O, CO2, and CO by difference on the basis of the microprobe analyses and measured H2O, CO2, and CO separately by quantitative mass spectrometry. I found several textural varieties of modified inclusions that correlate with field occurrence and cooling rate: devitrivication, zoning, and resorption of the host mineral are evidence of slow cooling. The modified inclusions are unsuitable for chemical analysis using the microprobe. Unmodified inclusions in the quartz phenocrysts from air-fall pumice are clear and glassy, and presumably con...

The boninite revisited

Abstract: Boninite occurring in Chichi-jima, Bonin Islands, is a rare rock, no exact equivalent known anywhere, and is a vesicular, feldspar-free glassy andesite according to SiO2 content (56-59%), consisting of phenocrysts of olivine (Fo90-85) and bronzite (En89-80), microphenocrysts and microlites of augite, and hydrous glass, with small quantities of Cr-rich spinel (Cr2O3 64-54%). Chemically it is characterized by high contents of MgO (12-6%) Cr (900-200 ppm), and Ni (300-100 ppm) similar to primitive basalts, apparently in ill accord with its intermediate SiO2 content. The boninite was first described in some detail by KIKUCHI (1890) and named by _PETERSEN (1891) after Bonin Islands coming from the Japanese “Bu-nin-sima” for no-man's islands. Surprisingly enough, this peculiar rock has since been touched upon only cursorily. We suggest that the boninite was formed by separation at shallow depths from hydrous mantle and rapid quenching from relatively high temperatures.The Bonin, or Ogasawara Islands, situated some 1000 km SSE of Tokyo, form the outer arc of the Idu-Marianas arc, and are composed of dominant volcanic rocks containing pillow lavas and subordinate sedimentary rocks with fossiliferous limestones, the age of which is Oligocene to early Miocene in Chichi-jima, the largest island, and Eocene in Haha-jima, the second largest one, lying 50 km south of Chichi-jima. The boninite, with subordinate bronzite-andesite, makes up the bulk of the pillow lavas in Chichi-jima, while in Haha-jima it has never been found. Volcanic breccias are composed mainly of hypersthene-andesite and-dacite, The magma of the boninite composition could exist in equilibrium with mantle peridotite at 10-5 kb, 1100-1050°C under water-saturated conditions, based on the experimental results of GREEN (1976) and NICHOLLS (1974). During its rise to the surface, olivine (and chromite) crystallized first from it, joined immediately later by bronzite, and augite was the last phase to crystallize. Before plagioclase is precipitated, the magma was erupted rapidly on sea floor to form the plagioclase-free glassy boninite pillow lavas. Available experimental data suggest that the boninite was quenched rapidly from a temperature of more than 900°C. The bronzite-andesite may represent a slowly cooled, relatively unfractionated part of the boninite magma. Low pressure fractionation involving removal of olivine, bronzite, augite, plagioclase, and magnetite could give rise to the hypersthene-andesite and -dacite from a parental boninite.The boninite bears some resemblance to the clinoenstatite-bearing volcanic rocks from Cape Vogel, Papua, which have crystallized first protoenstatite but not olivine, generated at higher temperatures than the boninite. The boninite may well be closely connected in genesis with magnesian andesites found in some orogenic calc-alkalic suites, suggesting a major role of water rising from the subduction zone.

Analcime phenocrysts in a vitrophyric analcimite — Primary or secondary?

Abstract: A vitrophyric analcimite contains phenocrysts of analcime and olivine in a groundmass of titansahlite, titanomagnetite, apatite, and abundant glass. The chemistry of the analcimite and its vitric residuum, the crystallization history of the host, and experimental data collectively favour an interpretation that the analcime “phenocrysts” are ion-exchanged KAl-Si2O6 leucites. Conversion of leucite to analcime resulted from two-way diffusion involving Na+ and H2O, and K+ across the original leucite-glass interfaces and probably was largely accomplished during hydration of the vitric groundmass.

Petrology and Isotope Geochemistry of Tertiary Lavas from the Northern Flank of the Tweed Volcano, Southeastern Queensland

Abstract: The generalized stratigraphic sequence (20-21.8 m.y.) of the northern flank of the Tweed Volcano is: Beechmont Basalt (base)-Rhyolite (composed of two distinct units, the Springbrook and Binna Burra rhyolites)-Hobwee Basalt. In addition, comendite occurs as a postrhyolite intrusive phase. Chemically and mineralogically, the 'basalts' are tholeiitic andesites, which are conveniently divided into olivine-normative and quartz-normative types. Phenocryst mineralogy is olivine and labradorite (microphenocrystic) in the olivine-normative lavas, and plagioclase plus rare augite in the quartz-normative lavas.Rhyolites (which constitute some 7 vol. per cent of the Tweed Shield volume) are of the potassic two-feldspar type; these are characterized by highly fractionated trace element patterns, which are most extreme in the Binna Burra rhyolites. The latter, for example, have low K/Rb (<100) and La/Yb, highly depleted Eu, Ba, Sr, V, Ni, Cr, and variably enriched Rb, U, Th, Pb, Nb, and Zn. Phenocryst phases are: quartz, oligoclase, sanidine, ilmenite, ferrohypersthene (Springbrook rhyolite), and quartz, sanidine, oligoclase, ilmenite, rare Fe-rich fluor-biotite, and very rare resorbed grains of extremely ulvospinel-rich titanomagnetite (Binna Burra rhyolite). Phenocryst equilibration temperatures are estimated to be in the range {reversed tilde equals}900-1050 °C for the Springbrook rhyolite and {reversed tilde equals}800-950 °C for the Binna Burra rhyolite, at oxygen fugacities in proximity to the WM buffer. The comendites are characterised by sanidine, quartz, fluor-arfvedsonite, minor acmite, and ilmenite.Pb isotopic compositions indicate at least two distinct groups of mafic lavas; certain olivine-normative tholeiitic andesites with compositions less radiogenic than modern oceanridge basalts (possibly indicative of lower crustal contamination), and a second more radiogenic group including the remaining isotopically analysed tholeiitic andesites. Sr isotopes reveal small differences between the Beechmont and Hobwee Groups. Pb and Sr isotopic compositions of the three rhyolitic groups are distinct, and all more radiogenic than the mafic lavas.It is concluded that the tholeiitic andesites represent the fractionation products from an olivine-normative tholeiitic basalt, and calculations suggest that olivine-plagioclase-aluminous clinopyroxene-Fe-Ti oxides were the likely fractionating mineral phases. The potassic rhyolites are interpreted also in terms of fractionation from a basaltic parent, although the geochemistry of the Binna Burra rhyolite has been further modified by continued fractionation at the quartz-feldspar minimum. Chemical and mineralogical data, however, suggest some modification of the rhyolitic magmas by crustal equilibration (possibly lower crust). Although the comendite is isotopically distinct from the exposed rhyolites, various least squares mixing calculations suggest that the comendite may have developed by continued late stage quartz-feldspar ternary minimum fractionation of rhyolitic magma.

The petrology of the Apollo 12 ilmenite basalt suite

Abstract: Mineral chemistry data and petrographic interpretations as well as fractionation modeling determined independently indicate that the bulk chemical variations in the ilmenite suite can be reconciled with differentiation within a single cooling unit Olivine porphyritic basalts, medium-grained basalts with evolved compositions, and coarse-grained olivine cumulates are found in the suite It is suggested that most of the chemical variation within the suite could result from fractionation of olivine equivalent to the composition of phenocrysts in vitrophyre 12008 which serves well as a parental magma for the other textural-chemical variants The relative cooling rate sequence within all three textural-chemical subgroups was determined, and systematic correlations between the degree of olivine addition or subtraction and the relative cooling rate is indicated

Trace element partitioning between phenocrysts of plagioclase, pyroxenes and magnetite and the host pyroclastic matrix

Abstract: Up to 26 trace elements were determined in 9 different samples of plagioclase, orthopyroxene, clinopyroxene and magnetite and the host pyroclastic matrix material using instrumental neutron activation analysis. Mineral to whole-rock partition coefficients were reported and interrelationships between chemical composition and crystal structure of the minerals were discussed, especially in the case of REE in plagioclase.

Mineralogy and proposed P-T paths of basaltic lavas from Rabaul caldera, Papua New Guinea

Abstract: Rabaul caldera is a large volcanic depression at the north-east tip of New Britain, Papua New Guinea. The lavas range in composition from basalt to rhyolite and have a calc-alkalic affinity but also display features typical of tholeiites, including moderate absolute iron enrichment in flows cropping out around the caldera. The basalts contain phenocrysts of plagioclase and clinopyroxene with less abundant olivine and titanomagnetite. In the basaltic andesites olivine is rare, while orthopyroxene and titanomagnetite are common along with plagioclase and clinopyroxene. Orthopyroxene is also found mantling olivine in some of the basalts while in both rock types pigeonitic augite is a fairly common constituent of the groundmass. Plagioclase in both basalt and basaltic andesite often exhibits sieve texture and analysis of the glass blebs show them to be of similar composition to the bulk rock. Phenocrystic clinopyroxene is a diopsidic augite in both basalt and basaltic andesite. Al2O3 content of the clinopyroxene is moderately high (∼4%) and often shows considerable variation in any one grain. Calculations show that the microphenocrysts probably crystallised near the surface, while phenocrysts crystallised at around 7 kb (21 km). Neither the basalts nor the basaltic andesites would have been in equilibrium at any geologically reasonable P and T with quartz eclogite. Equilibration between mantle peridotite and a. typical Rabaul basaltic liquid could have occurred around 35 kb and 1270 °C. A basaltic andesite liquid yields a temperature of 1263 °C and a pressure of 28 kb for equilibration with mantle peridotite.

Age of igneous activity and mineralization, Cerro de Pasco, central Peru

Abstract: K-Ar ages of mineral separates of biotite, plagioclase, and sanidine from one of a group of volcanic domes of quartz latite to dacite composition and two quartz latite dikes, all of which predate mineralization at the Cerro de Pasco mining district, are in the range of 14 to 15 m.y., with an overall average of 14.5 + or - 0.2 m.y. A large (6 cm long) sanidine phenocryst from one of a group of late dikes that cut the major mineral deposits gives a K-Ar age of 15.2 + or - 0.2 m.y. This age, slightly but apparently significantly, greater than those of the older volcanic rocks that predate mineralization suggests that the sanidine crystal contains about 1 X 10 (super -11) moles/g of extraneous radiogenic argon.The radiometric ages establish the time of volcanism and mineralization at Cerro de Pasco as middle Miocene and suggest that the various igneous and hydrothermal events took place within a very short period of geologic time. Available geological and radio-metric data support the concept of an intimate genetic relation between hydrothermal activity and the pulse of magmatic activity represented by the exposed volcanic rocks.

Mineralogy of intrusions in the Diablo Plateau, northern Trans-Pecos magmatic province, Texas and New Mexico

Abstract: Electron probe microanalyses of minerals in the nine intrusive rock types of the Diablo Plateau document compositional changes accompanying magmatic differentiation. Plagioclase occurs only in the least differentiated rock type. Alkali feldspar phenocrysts show little compositional variation. Groundmass feldspar compositions were modified by exsolution and deuteric alteration, particularly during growth of natrolite. Nepheline compositions in these hypabyssal rocks are intermediate between those of volcanic and plutonic nephelines elsewhere. Mafic minerals show wider compositional variations. Clinopyroxene, the most abundant of these, changed from sodian augite toward hedenbergite, and then toward acmite. The more sodic pyroxenes crystallized later than feldspars and nepheline from residual liquids after emplacement at the now-exposed level. Amphiboles and biotites show progressive decreases in Ti, Mg, and tetrahedral Al/Si. In the least differentiated rock type and in some autoliths, the amphibole is kaersutite; in later formed rocks, hornblendes and sodic amphiboles are widespread. Other prominent minerals showing compositional variation are manganoan fayalite, aenigmatite, and eudialyte. For the nepheline syenites, crystallization temperatures are estimated at 700 to 725 °C, with log ( f O 2 ) = −20 and log ( f H 2 O ) = + 1.6 to 2.1.

Lateral variation of phenocryst assemblages in volcanic rocks of the Japanese islands

Abstract: To understand the origin of calc-alkaline rocks and island arc volcanism, knowledge of the lateral variation of the petrological characters of volcanic rocks is essential. The lateral variation of chemical compositions of volcanic rocks has been investigated by several authors1–4 and correlated to the depth of the Benioff zone. The east volcanic zone of Japanese Islands, which contains about 140 discrete Quaternary volcanoes, is the most extensively studied volcanic zone in the world. I have examined the petrological data of these volcanoes, particularly with respect to the phenocryst assemblages in volcanic rocks and their changes during the history of respective volcanoes. The references have been mostly taken from 200 papers compiled by Isshiki et al.5.

Qal'eh Hasan Ali maars, central Iran.

Abstract: A group of craters 120 km southeast of Kerman, the largest 1200 m across and 300 m deep, are typical maars, excavated depression with rims of bedded pyroclastic debris. Most of the crater rims are composed entirely of country rock clasts, but the largest crater yields tephrite, composed of phenocrysts of phlogopite, clinopyroxene, and olivine in a groundmass of anorthoclase, analcime, hauyne, clinopyroxene and magnetite, and a cumulate of phlogopite and clinopyroxene with highly potassic glass. The occurrence of hauyne as a ground mass mineral is a rare or unique feature. The maar field lies in the projection to the south of the Nayband fault and, with some flows near that fault, represents a province of Quaternary alkali basalt volcanism.

Barred olivine 'chondrules' in lunar spinel troctolite 62295

Abstract: Several objects have been found in sections of lunar igneous spinel troctolite 62295 that resemble certain meteoritic barred olivine chondrules. Each consists of an apparently spherical single crystal of Fo90 olivine, approximately 0.6-0.8 mm in diameter, containing a set of approximately 30-40 subparallel stringers of An95 plagioclase, whereas the stringers in ordinary meteoritic chondrules consist of glass. The olivine of the 62295 chondrules is also more magnesian, and is radially zoned, having a relatively iron-rich core and rim and an iron-poor intermediate zone. Several possible origins are proposed: impact-generated melt globules solidified in flight, spherical phenocrysts, and meteoritic chondrules, but none of these seems adequate to explain the detailed observations.

Ca-Fe-RICH PYROXENES IN MIOCENE PERLITES FROM THE TSUGAWA AND TADAMI AREAS, NORTHEAST JAPAN

Abstract: Phenocrysts of Ca-Fe-rich pyroxenes such as ferroaugite and ferrohedenbergite were found in perlite lavas, which occur in the Middle Miocene formations in the Tsugawa and Tadami areas, although many of acid volcanic rocks in the areas consist of phenocrysts of biotite or hornblende. These perlites consist of phenocrysts of plagioclase (andesine-oligoclase), ferroaugite or ferrohedenbergite, altered olivine and titanomagnetite in a pale brown glassy groundmass. Ferroaugite and ferrohedenbergite occur in the British and Icelandic Tertiary acid glass and the late stages of the Skaergaard and Bushveld intrusions. Such pyroxenes also occur in some alkaline rocks. On the Ca-Mg-Fe quadilateral, chemical compositions of Ca-Fe-rich pyroxenes from the perlites are plotted near the trend curve of pyroxenes of tholeiitic rocks and are lower TiO2, A12O3 and K2O contents than those from alkaline rocks.

Supercooling on the lunar surface - A review of analogue information

Abstract: Terrestrial analog studies of the phase petrology of supercooled melts and rapid crystal growth are reviewed for possible light shed on lunar crystallization, supercooling, and petrogenic processes, in particular rapid consolidation of lavas extruded on the lunar surface, and impact liquids. Crystallization of major constituent minerals (olivine, pyroxene, plagioclase) in dendritic or skeletal forms is found much more characteristic of lunar igneous rocks than of terrestrial counterparts. Olivine and pyroxene occur often as skeletal phenocrysts, and their stage of crystallization is crucial to the genesis and cooling history of porphyritic lavas. Widespread occurrence of glass and of immature radiate crystallization, particularly of highly zoned pyroxenes and zoned plagioclase, is noted.

Geology and mineral deposits of the Needle Mountains District, southwestern Colorado

Abstract: The Needle Mountains district is set in highly fractured Precambrian granitic rocks that have intruded and metamorphosed older Precambrian metasedimentary rocks. The Precambrian rocks are overlain by outliers of Paleozoic strata and intruded by an upper Tertiary stock. Past mineral production reportedly was limited to silver and gold ores. Some evidence indicates economic potential for basemetal deposits. Mineral deposits are spatially related to a conspicuously altered composite stock, about 2,600 feet wide and 3,500 feet long. The stock consists mainly of an older porphyry body intruded by a younger porphyry body. The older body is composed of pervasively altered granite porphyry or quartz porphyry and related brecciated rocks. A coarser grained core is gradational with a finer grained, flow-structured outer phase. The younger body is composed of variably altered rhyolite porphyry that contains sanidine, plagioclase, quartz, biotite, and sphene phenocrysts set in an aphanitic to microgranular, flow-structured goundmass. The stock was evidently emplaced at a shallow depth. The older body in the stock has been altered to quartz-sericite-pyrite or quartz-sericite-kaolinite-pyrite assemblages in nearly all exposed rocks. Features suggest Mo-Cu resource potential below the surface in the western part of the stock. In places, fractures in Precambrian rocks have been filledmore » with quartz or quartz-pyrite veins. The area of intersecting structural zones includes the Chicago Basin stock and a concentration of metallized veins. Dump rock from mine workings along veins contains significant quantities of sphalerite, galena, and chalcopyrite, and lesser tetrahedrite in a gangue of quartz pyrite, rhodochrosite, and fluorite. 9 figures, 7 tables.« less

Paleozoic-Mesozoic boundary in the Berry Creek Quadrangle, northwestern Sierra Nevada, California

Abstract: Structural and petrologic studies in the Berry Creek quadrangle at the north end of the western metamorphic belt of the Sierra Nevada have yielded new information that helps in distinguishing between the chemically similar Paleozoic and Mesozoic rocks. The distinguishing features are structural and textural and result from different degrees of deformation. Most Paleozoic rocks are strongly deformed and thoroughly recrystallized. Phenocrysts in metavolcanic rocks are granulated and drawn out into lenses that have sutured outlines. In contrast, the phenocrysts in the Mesozoic metavolcanic rocks show well-preserved straight crystal faces, are only slightly or not at all granulated, and contain fewer mineral inclusions than do those in the Paleozoic rocks. The groundmass in the Paleozoic rocks is recrystallized to a fairly coarse grained albite-epidote-amphibole-chlorite rock, whereas in the Mesozoic rocks the groundmass is a very fine grained feltlike mesh with only spotty occurrence of well-recrystallized finegrained albite-epidote-chlorite-actinolite rock. Primary minerals, such as augite, are locally preserved in the Mesozoic rocks but are altered to a mixture of amphibole, chlorite, and epidote in the Paleozoic rocks. In the contact aureoles of the plutons, and within the Big Bend fault zone, which crosses the area parallel to the structural trends, all rocks are thoroughly recrystallized and strongly deformed. Identification of the Paleozoic and Mesozoic rocks in these parts of the area was based on the continuity of the rock units in the field and on gradual changes in microscopic textures toward the plutons.

Volume increase serpentinization in Archaean quench texture ultramafic rocks near Kalgoorlie, Western Australia

Abstract: Textural and chemical data on altered ‘spinifex’ (i.e. quench texture) ultramafic rocks from Corsair, near Kalgoorlie, Western Australia, have indicated that serpentinisation of contained olivine phenocrysts involved a volume increase of some 60%, due to the addition of both SiO 2 and H 2 O.

Contributions to the Mineral C~emistry of Haw~iian Rocks. VI. Olivines in Rocks from Haleakala and West Maui Volcanoes,

Abstract: Phenocryst and groundmass olivine in 22 rocks of the tholeiitic, alkalic, and nephelinicsuites from Haleakala and West Maui volcanoes were analyzed by electron microprobe. Results' and conclusions: Ranges for average compositions of olivine phenocrysts and groundmass are, respectively. F0 73 _ 85 and F0 61 for the tholeiitic suite (onlyqQe tholeiite contained groundmass olivine); F0 54 _ 83 and F0 15 _ 68 for the alkalis suite; and F0 73 _ 83 and F061 _ 66 for the nephelinic suite. In all suites, zoning extends these ranges substantially. Phenocrysts are usually enriched in Fe, Mn, and Ca at their rims, and coexisting groundmass olivine is richer in these same elements and depleted in Cr and Ni contents. In the differentiated alkalic suite, Fe, Mn, and Ca increase, and Ni decreases, from basalt to trachyte (mainly dependent on majo'i: Flement fraction­ ation). The tholeiitic and nephelinic suites can occasionallype distinguished by higher Ca and 10wer':Ni in olivine of the latter.