Showing papers on "Phenocryst published in 1978"

Explosive volcanic activity in the Mediterranean over the past 200,000 yr as recorded in deep-sea sediments

Abstract: As many as 20 air-borne tephra layers have been identified in the upper Quaternary sequence of deep-sea cores from the eastern Mediterranean. Petrographical examination based on refractive index, phenocryst content, and chemical composition of the volcanic glass distinguishes the parent magma types: (1) tephritic, (2) alkalic-trachytic, (3) peralkalic-pantelleritic, and (4) calc-alkalic andesitic to rhyodacitic and alkali-basaltic. Tephra layers could be correlated with the following source volcanoes: Somma-Vesuvius, Roman district, Phlegraean Fields and Ischia, Pantelleria, Aeolian Islands, Mount Etna, and Aegean arc. The distribution for single layers has been traced over more than 2,000 km.

Residual glasses and melt inclusions in basalts from DSDP Legs 45 and 46: Evidence for magma mixing

Abstract: Compositional relations among natural glasses in basalts recovered by Legs 45 and 46 (DSDP) provide powerful constraints on their differentiation histories. Residual glass compositions in the moderately evolved aphyric and abundantly phyric basalts within each site demonstrate that none of the units is mutually related to any other or to a common parent by simple fractional crystallization. At Site 396, where clinopyroxene phenocrysts are absent, progressively more evolved liquids (lower Mg/ (Mg+Fe) and higher TiO2) are characterized by lower calcium-aluminum ratios, which can only be generated by clinopyroxene fractionation. This paradox is amplified by some melt inclusions in olivine phenocrysts that have higher CaO/Al2O3 and lower TiO2 than any residual glasses. The occurrences of these distinctive compositions are correlated with the highly magnesian character of the host olivines (Fo90−89), and the melts are interpreted as trapped primitive liquids, parental to the more fractionated derivatives. Melt inclusions intermediate in composition between the residual glasses and the most primitive olivine melt inclusions are present in the cores of some plagioclase phenocrysts that have had a history of resorption. On the basis of a petrographic and microprobe analysis of the zoning relations in these phenocrysts, the inclusions are inferred to be melts entrapped at the time of extensive corrosion of the host crystals. Interpreted in conjunction with other mineral and geochemical data, the compositional trends in the glasses indicate that magma mixing has played a major role in the genesis of the Leg 45 and 46 basalts. The reality of mixing is demonstrated by extensive disequilibrium textures in the plagioclase phenocrysts and the presence in evolved lavas of refractory plagioclase and olivine phenocrysts bearing primitive melt inclusions. The chemical imprint of clinopyroxene fractionation despite the absence of clinopyroxene phenocrysts is believed to be accomplished by plating of gabbro on to the upper walls of the subvolcanic magma chamber as it evolves between mixing events. Repeated influxes of primitive magma batches will move the resultant hybrids alway from clinopyroxene saturation and generate olivine-plagioclase cotectic magmas. This model provides a physical buffering mechanism that accounts for the volumetric dominance of moderately evolved basalts among ocean floor tholeiites. Major and trace element models based on the combination of mixing and fractional crystallization also explain heretofore enigmatic geochemical characteristics of MORB.

Akahoya Ash-A Holocene Widespread Tephra Erupted from the Kikai Caldera, South Kyushu, Japan

Abstract: A Holocene volcanic ash layer comprising abundant glass shards occurs as near-surface, soil-forming parent materials in south to north Kyushu and in Shikoku. This layer has been given several local names such as “Akahoya”, “Imogo”, “Onji”, etc. by farmers and pedologists. Its remarkable characteristics as a parent material of soil stimulated the interest of many pedologists to study its source, pedological features, distribution, etc. However, opinions on its source and proper identification varied considerably from one author to another.Detailed petrographic observation and accurate determinations of the refractive indices of the glass and several phenocryst phases in the tephra, together with extensive field work, have led to the conclusion that the Akahoya ash is the product of a single major eruption of the Kikai caldera.The ash is dacitic in composition and contains abundant bubble-walled glass shards and plagioclase, hypersthene, augite and opaque minerals as phenocrysts. The refractive index of the glass ranges from 1.505 to 1.514, and that of the hypersthene, from 1.705 to 1.714. The thickness contour of the ash layer and its grain-size distribution clearly indicate that this ash represents ejecta from the Kikai caldera, which is one of the largest calderas in Japan with an approximate diameter of 20km and largely submerged beneath the sea.The formation associated with this widespread tephra consists of three members; (1) a pumice-fall deposit as the earliest stage, (2) pyroclastic-flow deposits as the middle to the latest stages, and (3) an ash-fall deposit approximately contemporaneous with the pyroclastic flow. The 3rd member is assigned to the Akahoya ash and has the most extensive lobe with an axis length of over 1, 000km, covering most of southwest to central Japan and northwest Pacific Ocean. The volumes of the Akahoya ash-fall deposits must be greater than those of the pyroclastic flows.More than twenty-seven radiocarbon dates of the ash have been obtained so far, ranging rather widely from ca. 3, 000y.B.P. to ca. 9, 000y.B.P. However, the average value of the carbonated woods and peaty materials containing in the layer and the stratigraphical relationships with human remains give a probable age of the ash between 6, 000y.B.P. and 6, 500y.B.P. This marker-tephra is thus extremely significant for studies of Holocene climatic changes and sea levels, as well as for the correlation of archaeological sites.

Mineralogy and petrology of Grenada, Lesser Antilles island arc

Abstract: The mineralogy and petrology of volcanic and plutonic rocks from the island of Grenada are described The volcanic rocks include basanitoids, alkalic and subalkalic basalts, andesites and dacites Phenocryst phases in the basanitoids and basalts are olivine (Fo90−71), zoned calcic augite, spinel ranging from ferrian pleonaste through chromite to titaniferous magnetite, and plagioclase Some of the basalts contain pargasitic amphibole Andesites and dacites generally contain hypersthene and augite, and one pigeonite-hypersthene-augite-bearing andesite was found Apatite commonly occurs as a phenocryst in the andesites and dacites and quartz is present in some dacites as well as being a possible xenocryst in both alkalic and subalkalic basalts Plutonic cumulates found as ejected fragments in tuffs and ashes are composed of variable proportions of olivine, magnetite, calcic augite, amphibole and plagioclase One peridotitic (ol-cpx-opx) fragment was found but spinel or garnet peridotitis are absent Despite the alkalic nature of the association, calcalkalic characteristics such as calcic plagioclase, restricted Feenrichment in coexisting pyroxenes and generally low TiO2 content relative to oceanic suites are present in Grenada Estimates of conditions of equilibration of the basanitoids with potential upper mantle materials using the results of high-pressure experiments are compared with estimates from thermodynamic data Equating $$a_{{\text{SiO}}_{\text{2}} } $$ and $$a_{{\text{Al}}_{\text{2}} {\text{O}}_{\text{3}} } $$ basanitoid with hypothetical garnet peridotite assemblages gives a pressure and temperature of equilibration in the region of 35–38 kbar and 1550–1625 ° K Experimental results are not supportive of these estimates

Effects of kinetics on the crystallization of quartz normative basalt 15597 - An experimental study

Abstract: Equilibrium experiments on a residual liquid composition from the dynamic crystallization study on quartz normative basalt (QNB) 15597 are used to evaluate the effects of kinetics on the crystallization of pyroxene-porphyritic basalts. The results indicate that the liquids coexisting with pyroxene phenocryst rims before the precipitation of groundmass plagioclase + pyroxene have left the equilibrium liquid line of descent and metastably penetrated the plagioclase primary phase volume. The precipitation of groundmass plagioclase + pyroxene brings the basalt system closer to equilibrium with respect to the residual liquid and crystals. At rapid cooling rates pyroxene phenocryst growth produces substantial compositional gradients in the residual silicate liquid at the pyroxene/liquid interface. The compositional gradients enhance the nucleation and growth of groundmass phases at the phenocryst interface, because effects related to constitutional supercooling for these phases (e.g., plagioclase, pyroxene, etc.) favor nucleation at the interface.

Skagi and Langjökull Volcanic Zones in Iceland: 1. Petrology and structure

Abstract: The compositional variation of basalts in a 220-km-long profile through the length of the northward extension of the western volcanic zone of Iceland shows an abrupt discontinuity at 65° 10′N, separating high iron-titanium type light-rare-earth-element (REE)-enriched tholeiitic basalts in the Skagi zone (2.5–0.5 m.y.) to the north from Mg-Al-rich olivine tholeiites with REE patterns unfractionated in relation to chondrites in the Langjokull zone (0.7 m.y. to Recent) to the south. Geophysical evidence indicates that the chemical discontinuity may coincide with steepening of presently existing geothermal gradients and thickening of the Iceland crust from south to north. The two basalt groups exhibit striking petrographic contrasts. Skagi basalts contain phenocrysts of augite, plagioclase, olivine, and minor Fe-Ti oxides and have liquidus temperatures ranging from 1150° to 1170°C. Langjokull basalts are predominantly olivine and plagioclase-phyric, with minor chromian spinels and liquidus temperatures near 1230°C. Petrologic mixing calculations indicate that major element dispersion within Skagi can be accounted for by 15–17% extraction of plagioclase, augite, and olivine in proportions of 4:3:1. The observed major element range in the Langjokull group is consistent with 33–55% extraction of these same phases in the proportions 2.5:1.5:1. On the basis of major element chemistry alone the Skagi magma can be derived by 41–43% fractional crystallization of Langjokull magma, ascribed to filtering through a thicker crust. However, variable degrees of melting related to the thermal gradients are also required for genesis of these magmas to account for the REE variations observed (Schilling et al., 1978); combination of both processes is therefore invoked.

Late Cretaceous high-potassium volcanism in eastern Srednogorie, Bulgaria

Abstract: The Srednogorie zone of Bulgaria was the site of extensive Late Cretaceous calc-alkalic and shoshonitic magmatism. The calc-alkalic volcanism developed mainly in the central and western parts, whereas shoshonitic volcanism was the predominant magmatic feature of the eastern part. Leucitic basanites, limburgites, and picrites occur in minor amounts in eastern Srednogorie. The shoshonitic volcanic rocks are intermediate to silicic in composition and are characterized by high K 2 O, K 2 O/Na 2 O around unity, high Rb, Sr, and Ba, and low TiO 2 contents. They are associated with calc-alkalic rocks less strongly enriched in K and incompatible elements. Petrographically, the calc-alkalic rocks are characterized by the presence of hydrous mineral phenocrysts that are not present in the shoshonitic rocks. Shoshonitic and calc-alkalic rocks have some common chemical characteristics such as low TiO 2 , lack of absolute iron enrichment, and large overlap in many trace-element abundances. These are considered evidence for a genetic relationship between their primary magmas, which may have undergone different degrees of enrichment in incompatible elements. The eastward increase of K-rich volcanic rocks relative to calc-alkalic rocks in the Srednogorie zone is believed to be related to distension tectonics connected with the opening of the Black Sea.

Tertiary volcanism and caldera development near Durango City, Sierra Madre Occidental, Mexico

Abstract: Near Durango City, older andesites are overlain by approximately 800 m of rhyolitic volcanic rock, mostly ash-flow tuff of Oligocene age. The rhyolite is unconformably overlain by a few tens of meters of late Miocene basalt in the highlands west of Durango City and by Quaternary basalt and gravel in the Guadiana Valley. By 32 m.y. ago, voluminous rhyolitic ash-flow tuff was being emplaced over a rugged topography developed on the older andesites. Shortly thereafter, sources in the Durango City area became active and produced two major ash-flow sheets. These ash-flow sheets, together with caldera-fill material, constitute the Carpintero Group. Subsidence in their source areas initiated development of the Chupaderos caldera complex. Most ash-flow sheets overlying the Carpintero Group originated from distant sources, but later eruptions from the caldera area produced one major post-Carpintero ash-flow unit with associated collapse of a distinctive inner caldera. Rhyolitic volcanism ceased in the Durango area by 28 m.y. ago, but continued until 23 m.y. farther west. Chemical analyses of volcanic rocks from a strip mapped in detail across the Sierra Madre Occidental show systematic variations. Silicic volcanic rocks to the east are higher in silicon and potassium and lower in sodium, aluminum, and calcium than rocks to the west. The phenocryst mineralogy of volcanic rocks along the strip reflects these chemical variations. Mafic rocks of the Durango area were erupted in three distinct episodes. A small amount of subalkaline basalt is interlayered with the Durango volcanic sequence. Later, alkaline lavas of the Metates Formation were erupted during an episode of late Miocene normal faulting. The large basalt field in the Guadiana Valley is inter-layered with Quaternary gravel, and the youngest flows are Holocene.

Temporal magmatic variation at Boqueron volcano, El Salvador

Abstract: The relative ages of 21 lavas from Boqueron volcano in El Salvador were determined by superposition. The lavas are grey to black, porphyritic basalts, basaltic andesites and andesites with phenocrysts of plagioclase, augite, olivine, and magnetite. The andesitic lavas appear to have evolved from basaltic magma by fractionation of the observed phenocryst phases.

The petrogenesis and origin of the analcime in the volcanic rocks of the Crowsnest Formation, Alberta

Abstract: The Crowsnest Formation consists of trachytes, analcime phonolites and blairmorites, metamorphosed to zeolite facies. The latter rocks contain large analcime phenocrysts variously suggested as bein...

Petrographic evidence of magma mixing in Shirouma-Oike volcano, Japan

Abstract: Detailed petrographic analysis of calcalkaline volcanic rocks of Shirouma-Oike volcano, Japan, reveals that the complex phenocryst assemblage (Ol+Cpx+Opx+Hb+Bt+Qz+Pl+Mt+Hm) in the younger group volcanic rocks can be divided into two groups, a high temperature group (Ol+Cpx±An-rich Pl) and a low temperature group (Op+Hb+Bt+Qz±Ab-rich Pl+Mt+Hm). Compositional zonation of the phenocrystic minerals, normal zoning in olivine and clinopyroxene, and reverse zoning in orthopyroxene and plagioclase, indicate that these two groups of phenocrysts precipitated from two different magmas which mixed before the eruption. The low temperature magma is a stagnant magma in a shallow magma chamber, to which high temperature basaltic magma is intermittently supplied. Magma mixing is also indicated in olivine-bearing two pyroxene andesite of the older group volcanic rocks, by the coexistence of normally zoned Mg-rich clinopyroxene phenocrysts and reversely zoned Fe-rich clinopyroxene phenocrysts, and by reverse zoning in orthopyroxene phenocrysts. It is concluded that magma mixing is an important process responsible for the generation of the disequilibrium features in calc-alkaline volcanic rocks.

Some petrological aspects of the Prairie Creek diamond-bearing kimberlite diatreme, Arkansas

Abstract: Based on modal and chemical composition, the rocks of the Prairie Creek diatreme situated 4 km SSE of Murfreesboro, Pike County, Arkansas, are classified as micaceous kimberlite The K-Ar isotopic analysis of phlogopite from this diatreme yielded an age of 106 ± 3 my (Albian) which is in agreement with stratigraphic relations Electron beam probe data on minerals from kimberlite breccia, one of the three textural types, are presented The breccia is considered as the potential source of the diamonds that have been mined at the diatreme It contains phenocrysts of olivine (Fo90–92) and serpentine pseudomorphs after olivine embedded in a groundmass of serpentine, minor calcite, chrome-diopside, phlogopite (Mg/Mg+Fe = 8415%), perovskite, spinels, and pentlandite Xenoliths of shales, sandstones, and mantle-derived ultramafic material are also present Spinels are rich in Cr, Ti, and Fe and generally low in Al Zoned spinels show enrichments in Ti and Fe towards their rims A positive correlation between 100(Fe3++Ti)/(Cr+Al+Fe3++Ti) and 100 Mg/(Mg+Fe2+) ratios exists in these spinels and probably reflects an oxygen fugacity increase during magma crystallization Occluded gases in diamonds and kimberlites corroborate the hypothesis that the parent magma of the Prairie Creek kimberlite was derived by partial melting of upper-mantle garnet lherzolite under volatile-rich conditions, primarily enriched in H2O and CO2

Precambrian rhyelites and granites in south-central Wisconsin: Field relations and geochemistry

Abstract: Isolated exposures (inliers) of Precambrian rhyolites and granites (1,765 m.y. old) crop out in the Fox River valley and in the Baraboo area of south-central Wisconsin. The geochemical characterization of rock units, in addition to field and petrographic studies, was used to unravel the complex geology of two of these inliers, the Marquette and Marcellon rhyolites, and to determine in a preliminary fashion the geology of the Precambrian igneous terrain between exposures, which is covered by a section of Paleozoic sedimentary rocks and Pleistocene glacial deposits of varying thickness. The rhyolite at Marquette consists of a series of ash-flow tuffs, interbedded with mud-flow breccia, which are broadly folded into a series of normal and overturned asymmetric folds. The rhyolite is cut by a 100-m-thick andesite dike that intruded along a northeast-trending normal fault. The top of the exposed section is a porphyritic rhyolite containing quartz, alkali feldspar, and plagioclase phenocrysts. This unit is underlain by ash-flow tuffs ranging from porphyritic quartz, plagioclase, alkali feldspar rhyolite to rhyolite with only plagioclase phenocrysts. Plagioclase units are phenocryst poor and have Rb/Sr ratios greater than 1; the three mineral units are phenocryst rich and have Rb/Sr ratios less than 1. The rhyolite at Marcellon consists of four ash-flow tuffs folded into a northeast-trending asymmetric antiform. Lithologically, the Marcellon units are spherulitic, flow-banded, brecciated, and massive. Mineralogically, they vary from porphyritic quartz, plagioclase, alkali feldspar rhyolite to plagioclase-bearing rhyolite. Geochemical correlation was used to relate units from one part of the inlier to the other and thus establish the existence of the antiform. This exposure is cut by andesite and basalt dikes. Forty new major- and trace-element analyses for the Fox River valley and Baraboo rhyolites and granites were used to subdivide these rocks into four chemical groups: (1) fine-grained granite at Baxter Hollow and coarse-grained rhyolite dikes at Observatory Hill are characterized by high CaO and low Rb/Sr; these rocks intrude rhyolite flows and may represent an intrusive event that occurred after the major extrusive episode; (2) fine-grained and porphyritic rhyolite at the Marquette exposure and the fine-grained plagioclase-bearing rhyolite at Marcellon have intermediate CaO and Rb/Sr; (3) porphyritic rhyolite and granophyric granite are characterized by low CaO and high Rb/Sr; similarities in chemistry between these two rock types suggest that the granophyric granites are the subvolcanic equivalents of the rhyolites; and (4) rhyolites at the Marcellon and Baraboo exposures are intermediate in chemistry between groups 2 and 3. The four groups are chemically related and show calc-alkalic affinities. Chemical correlation and geologic mapping show that the chemical groups occur geographically as northeast-trending bands across south-central Wisconsin. Granophyric granite lies to the northwest of group 3 porphyritic rhyolites; the porphyritic rhyolites lie in general to the northwest of the texturally variable rhyolites of groups 2 and 4. Structural trends within exposures parallel contacts between chemical groups, thus strengthening the thesis that the chemical trends reflect the geology of the large area of buried Precambrian rock in south-central Wisconsin.

Petrology of a Sequence of Pyroclastic Rocks from the Valley of Taurus-Littrow (apollo 17 Landing Site)

Abstract: Thirteen samples of core 74001/2, collected at Shorty Crater by Apollo 17, have been analyzed. The samples, taken from the core section below 5.5 cm and representing most of the major stratigraphic units described by Nagle (1978), consist entirely of whole and fragmented orange and black droplets. The grain shape, degree of crystallinity, breakage, vesicularity, predominant olivine shapes, and phenocryst content of about 18,000 droplets were investigated. The crystalline droplets were found to contain olivine and ilmenite as major phases and pyroxene, Cr-spinel and metallic Fe as minor phases. The grain shape, degree of crystallization, olivine shape and olivine composition suggest that a wide range of cooling rates is represented. The upper part of the core section (above 5.5 cm) has undergone in situ reworking by micrometeorites. The 74001/2 samples are believed to be from an ultramafic pyroclastic deposit brought to the surface by the Shorty Crater impact and originally formed by a lava fountain active in the Taurus-Littrow valley 3.5 b.y. ago. The sequence represented in the sample was most likely deposited over a very short period of time.

Komatiites and other high-magnesia lavas: some problems

Abstract: Criteria for the recognition of high-magnesia liquids, among which the absence of phenocrysts is the most important, are discussed. Some high-magnesia lava sequences are strongly porphyritic but it can be demonstrated that their character is not due to the accumulation of ferromagnesian phenocrysts in normal basaltic magmas. The term primitive porphyritic magma is introduced to describe the magmas from which such sequences crystallize. Possible origins of primitive porphyritic magmas include advanced crystallization of high-magnesia liquids without loss of phenocrysts. The occurrence of Phanerozoic high-magnesia lavas associated with continental break-up is described and comparison is made with Archaean komatiite vulcanism. Low levels of incompatible elements are characteristic of the Archaean rocks but high CaO/Al 2 O 3 is not a specifically Archaean feature. Phanerozoic liquids with MgO much above 20 % have yet to be identified but may possibly have existed. Major-element data for komatiites are discussed with a view to the constraints they put on the composition of the source material. Several interpretations, which also have widely varying implications for depth of origin and degree of melting, are presently possible. A new model involving the complete mobilization of source material after a comparatively low degree of partial melting is presented. Bulk compositions of magmas produced lie on a mixing line between the composition of the source and the composition of the liquid fraction at the moment of mobilization.

Magma mixing at mid‐ocean ridges: Evidence from legs 45 and 46‐DSDP

Abstract: An integrated petrologic and geochemical study of basalts recovered in Legs 45 and 46 (DSDP) has indicated, on the basis of disequilibrium mineralogy, that these moderately evolved basalts are mixtures of primitive mantle-derived tholeiites with more evolved magmas. Plagioclase phenocrysts are characterized by substantial diversity in composition and zoning pattern. Many olivine and plagioclase phenocrysts are too refractory to be in equilibrium with liquids of the host basalt composition but possess a composition consistent with crystallization from a primitive mantle-derived basalt liquid. On the basis of melt inclusions trapped in the olivine phenocrysts, features of the primitive melt are estimated. It is suggested that subvolcanic magma chambers beneath midocean ridges receive periodic injections of this primitive melt and its attendant phenocrysts which mix with fractionated chamber-bound magmas, resulting in observed moderately evolved lavas.

Petrology and regional significance of the Roxboro Metagranite, North Carolina

Abstract: The pluton located at Roxboro, North Carolina, is predominantly a light gray to medium gray, microphaneritic metagranite. Phenocrysts of plagioclase, quartz, and perthite are accompanied by porphyroblasts of epidote. Relict igneous textural relationships suggest two possible fractional crystallization models, in both of which the order of crystallization was plagioclase, quartz, and then K-rich feldspar. A crude approximation of the composition of the original plagioclase phenocrysts is An 25 to An 40 . Based on the composition of locally present granophyre, the pluton was emplaced under almost dry conditions with a P total of about 350 bars and a temperature in the vicinity of 950 °C. The shallow depth of emplacement suggests that the Roxboro Metagranite represents the root of a volcanic sequence that once unconformably overlay the presently exposed sequence. During the middle Paleozoic, this granitic intrusive was metamorphosed at a minimum pressure of about 3 kb and a temperature of approximately 400 °C. A foliation, as shown by stringers of mainly biotite and epidote, was produced by the deformational phase accompanying regional metamorphism. All K-rich feldspar is now nearly pure microcline (∼Or 97 ), and all plagioclase is now nearly pure low albite (Ab 97–99 ). Such feldspar compositions accompanied by late growth of ferristilpnomelane indicate a re-equilibration under lower grade conditions than those realized during the peak of the major regional metamorphic event.