Petrology of volcanic rocks from anjouan,comores archipelago

Anjouan is an extinct and substantially denuded volcanic island, belonging to the Comores Archipelago, a chain of volcanic islands in the northern Mozambique Channel. Volcanism was probably controlled by a complex regional stress pattern, closely implicated with the recent geotectonic evolution of East Africa and the western Indian Ocean. On Anjouan a lengthy period of shield construction was succeeded by fissure-controlled eruptions forming three extended peninsulas to the north, south and west. A ‘rejuvenescent’ phase of activity erupted lavas on to an erosional surface formed during a period of quiescence following build-up of the main volcanic edifice. Lavas of the shield-building stage comprise ankaramites, oceanites and olivine basalts, with minor development of hawaiites and trachytes. The fissure controlled eruptions are also mainly basaltic although generally more alkaline, while rejuvenescent lavas comprise basanites and a significant proportion of derivatives following a trend towards phonolite. Coarse-grained xenoliths are found in lavas of the latter two phases, while a gabbroic intrusion (the ‘Tatinga Intrusion’) is exposed in the centrally-situated Cirque de Bambao in the vicinity of N’Tingui (1595 m). The main chemical trends and petrographic characteristics of the Anjouan lavas are related to the eruptive sequence.     

Petrology of basalts from Loihi Seamount,Hawaii

Loihi Seamount is the southeasternmost active volcano of the Emperor-Hawaii linear volcanic chain. It comprises a spectrum of basalt compositional varieties including basanite, alkali basalt, transitional basalt and tholeiite. Samples from four dredge collections made on Scripps Institution of Oceanography Benthic Expedition in October 1982 are tholeiite. The samples include highly vesicular, olivine-rich basalt and dense glass-rich pillow fragments containing olivine and augite phenocrysts. Both quartz-normative and olivine-normative tholeiites are present. Minor and trace element data indicate relatively high abundances of low partition coefficient elements (e.g., Ti, K, P. Rb, Ba, Zr) and suggest that the samples were derived by relatively small to moderate extent of partial melting, of an undepleted mantle source. Olivine composition, MgO, Cr and Ni abundances, and Mg/(Mg+Fe), are typical of moderately fractionated to relatively unfractionated “primary” magmas. The variations in chemistry between samples cannot be adequately explained by low-pressure fractional crystallization but can be satisfied by minor variations in extent of melting if a homogeneous source is postulated. Alternatively, a heterogeneous source with variable abundances of certain trace elements, or mixing of liquids, may have been involved. Data for ³He/⁴He, presented in a separate paper, implies a mantle plume origin for the helium composition of the Loihi samples. There is little variation in the helium isotope ratio for samples having different compositions and textures. The helium data are not distinctive enough to unequivocally separate the magma sources for the tholeiitic rocks from the other rock types such as Loihi alkalic basalts and the whole source region for Loihi may have a nearly uniform helium compositions even though other element abundances may be variable. Complex petrologic processes including variable melting, fractional crystallization and magma mixing may have blurred original helium isotopic signatures.     

Petrology of the hilina formation,Kilauea volcano,Hawaii

The Hilina Formation comprises the oldest sequence of lava flows and tuffs exposed on Kilauea Volcano. These rocks are only exposed in kipukas in younger Puna Formation lavas along cliffs on the south flank of Kilauea Volcano. Locally, tuffs and flows of the Pahala Formation separate the underlying Hilina Formation rocks rom the overlying Puna Formation rocks. Charcoal collected from the base of the Pahala Formation yielded a C¹⁴ age of 22.800±340 years B.P. which defines a minimum age for the Hilina Formation. Hilina Formation lavas crop out over a wide region and probably originated from the summit area and from both rift zones. The Hilina Formation contains both olivine-controlled and differentiated lavas (using the terminology ofWright, 1971). The olivine-controlled lavas of the Hilina Formation are distinguishable mineralogically and geochemically from younger olivine-controlled Kilauea lavas. The younger lavas generally contain discrete low-calcium pyroxene grains. greater glass contents, higher K₂O/P₂O₅ ratios and lower total iron contents. Similar geochemical trends prevail for Manuna Loa lavas, and may typify the early lavas of Hawaiian shield volcanoes. Despite these similarities, the Hilina Formation (and all Kilauea) lavas have higher TiO₂ and CaO, and lower SiO₂ and Al₂O₃ contents than Mauna Loa Lavas. These differences have existed for over 30,000 years. Therefore, it is unlikely that the older lavas of Kilauea are compositionally similar to recent Mauna Loa lavas as was previously suggested. K₂O, TiO₂, Na₂ and Zr contents of lavas from a stratigraphic sequence of Hilina Formation lavas are variable. These variations may be utilized to subdivide the sequence into geochemical groups. These groups are not magma batches. Rather, they represent lavas from batches whose compositions may have been modified by crystal fractionation and magma mixing.     

Petrology of the Rumble seamounts,southern Kermadec Ridge,southwest Pacific

The Rumble seamounts are major Quaternary submarine volcanoes which lie at the southern end of the Tonga-Kermadec island arc. Rocks dredged from three of the volcanoes are olivine and pyroxene phyric basalts and highly por-phyritic plagioclase-rich basaltic andesites. Plagioclase phenocrysts display patterns of iron enrichment similar to those observed in rocks from other parts of the arc. Systematic major and trace element variations indicate that the specimens constitute a closely related suite of low-K arc-type rocks showing characteristic depletion in some large ion lithophile elements. They are closely comparable to basaltic rocks of the Kermadec Islands and their genesis may be linked to the currently active subduction system beneath the arc.     

Petrology of the volcanic rocks of saba,West Indies

Saba is composed of basic and intermediate members of the calc alkaline series with a broad resemblance to volcanic rocks on other islands of the northern Lesser Antilles. The most abundant rock-type is hornblende andesite with relatively high potassium and associated elements. Notable petrographic features are the persistence of magnesian olivine into relatively siliceous whole-rock composition and the prevalence of hornblende even in the more basic rocks. Quartz xenocrysts also occur in the basalts Clinopyroxene phenocrysts are of an gite composition but groundmass grains are sometimes in the range subcalcic augite to magnesian pigeonite. Disequilibrium features in the petrography, together with anomalies in the chemical trends may be due to incorporation of xenocrysts or perhaps in extreme instances to the hybridisation of contrasting magmas.     

Petrology and geochemistry of volcanic rocks of Aegina,Greece

The Plio-Pleistocene cale-alkalic lavas of Aegina are comparable geochemically with similar lavas from elsewhere in the South Aegean arc. At least four differentiation series, largely of basaltic andesite to rhyodacite type, have been recognized in Aegina. Petrographical and geochemical evidence shows that lavas and xenoliths have a common origin. The mineralogy of the rocks suggests an alternation of hydrous and dry conditions during their formation. The chemical composition of the basic rocks, and of basic cognate xenoliths in the lavas, suggest that the parent material of all the lavas was of basic composition. Fractionation of the early formed phases is believed to have been the main process for the formation of the more acidic differentiates.     

Petrology of the volcanic rocks of martinique,West Indies

Volcanological differences between the old and the recent lavas from Martinique, Lesser Antilles, are presented, showing that two volcanic series exist in this island:

Dash

a high-alumina basalt series generally mafic, line-grained, partly pillowed, with clinopyroxene-rich lavas which show iron enrichment tendancies en an A.F.M. plot;

a calc-alkaline (slightly potassic) series much more siliceous as a group, porphyric, predominantly sub-aerially erupted with orthopyroxene-rich lavas which show no iron enrichment.

The high-alumina basalt series is considered as having originated from a differentiation trend by fractionation of olivine, clinopyroxene and plagioclase. Lavas range from olivine basalt to tridymite-rich dacite. The calc-alkaline series probably derives from the contamination of the first suite but the occurence of hornblende-rich cumulates indicates the process of fractionation takes place too. Lavas range from orthopyroxene andesite and hornblende andesite to quartz-hornblende dacite and quartz-biotite dacite.     

Petrology of the Campbell Island Volcanics, Southwest Pacific Ocean

The disposition and petrology of a fractionated alkali olivine basalt—peralkaline rhyolite suite from subantarctic Campbell Island are discussed. These rocks (Campbell Island Volcanics: new name) are flows and high-level intrusions derived from two centres of igneous activity. Their age is Upper Miocene and they evolved over a period of 5 Ma. A gabbro intrusion pre-dates volcanism by 5 Ma. The ages of the flows and high-level intrusions cannot be separated, although the intrusions are chemically distinct as they contain all the intermediate members of the suite (mugearite and benmoreite). Similar La/Ce and Zr/Nb ratios for flows and high-level intrusions suggest a co-magmatic origin. Chemical variations indicate that the suite resulted from low-pressure mafic then felsic-dominated fractional crystallisation, which is substantiated for intermediate members of the suite by least-squares and Rayleigh fractionation modelling. One flow of alkali olivine basalt clearly pre-dates other volcanic rocks, and is thus regarded as being genetically unrelated.Although chemically similar to alkali olivine basalt and hawaiite, variations in the mineral chemistry and modal mineralogy of gabbro indicates a prolonged period of in-situ fractionation and re-equilibration.     

The aeolian dust accumulation curve

This article presents a simple physical concept of aeolian dust accumulation, based on the behaviour of the subprocesses of dust deposition and dust erosion. The concept is tested in an aeolian dust wind tunnel. The agreement between the accumulation curve predicted by the model and the accumulation curve obtained in the experiments is close to perfect and shows that it is necessary to discriminate between the processes of aeolian dust deposition and aeolian dust accumulation. Two important thresholds determine the accumulation process. For wind speeds below the deflation threshold, the aeolian accumulation of dust increases linearly with the wind speed. For wind velocities between the deflation threshold and the accumulation limit, the sedimentation balance is above unity and there is still accumulation, though it rapidly drops once the deflation threshold has been exceeded. At wind speeds beyond the accumulation limit, the sedimentation balance is below unity and there will no longer be an accumulation of dust. The thresholds have been determined in a wind tunnel test at friction velocity u_* = 0·34 m s^?1 (deflation threshold) and u_* = 0·43 m s^?1 (accumulation limit), but these values are only indicative since they depend heavily on the characteristics of the accumulation surface and of the airborne grains. Copyright © 2001 John Wiley & Sons, Ltd.     

Recalibrating aeolian sand transport models

A quality‐controlled data set comprising measurements of aeolian sand transport rates obtained at three disparate field sites is used to evaluate six commonly employed transport rate models (those of Bagnold, Kawamura, Zingg, Owen, Hsu, and Lettau and Lettau) and to recalibrate the empirical constants in those models. Shear velocity estimates were obtained using the von Kármán constant and an apparent von Kármán parameter. Models were recalibrated using non‐linear regression and non‐linear regression with least‐squares lines forced through axes origins. Recalibration using the apparent von Kármán parameter and forced regression reduced the empirical constants for all models. The disparity between the predictions from the different models is reduced from about an order of magnitude to about a quarter of an order of magnitude. The recalibrated Lettau and Lettau model provided the greatest statistical agreement between observed and predicted transport rates, with a coefficient of determination of 0·77. Evaluation of the results suggests that our estimations of threshold shear velocity may be too slow, causing errors in predicted transport rates. Copyright © 2012 John Wiley & Sons, Ltd.     

Petrology of the picrite-basalt flows in the Igatpuri area,Nasik district,Maharashtra

In the course of studying the Deccan Trap Hows around Igatpuri (latitudes 19°38′ and 19°45′: longitudes 73°30′ and 73°42′), picrite-basalts, not hitherto reported from this area, have been found occurring associated with basaltic flows. Thirty-eight flows, of a total thickness of 2200 feet, have been delimited. Of these, 8 flows may be termed picrite-basalts with a thickness varying from 25 to 75 feet. A feature of these basic flows is the abundance of olivine phenocrysts, with a complete absence of pyroxene phenocrysts in two of the flows. Two flows may be termed oceanite, two ankaramite, while four flows have phenocrysts of olivine, pyroxene and lelspar of An 65–70 %. In the oceanite flows the olivine phenocrysts constitute 20 to 30 per cent of the rock. They are mostly fresh, but some are altered to iddingsite. As regards the basaltic flows, about half have scanty olivine phenocrysts, the remainder being devoid of olivine. Eight are coarse enough to be termed olivine-dolerites. In the picrite-basalts the pyroxene phenocrysts have an optic axial angle of from 55° to 60°. In the basaltic flows the angle varies from 45° to 52°. The olivine phenocrysts of the picrite-basalts are highly magnesian, whereas those of the basalts are more ferruginous, as determined by optical methods. West’s view that the origin of these picrite-basalts is due to differentiation by crystal settling followed by freezing and extrusion, seems to be supported by this study.     

Petrology of Hualalai volcano,Hawaii: Implication for mantle composition

Hualalai is one of five volcanoes whose eruptions built the island of Hawaii. The historic 1800–1801 flows and the analyzed prehistoric flows exposed at the surface are alkalic basalts except for a trachyte cone and flow at Puu Waawaa and a trachyte maar deposit near Waha Pele. The 1800–1801 eruption produced two flows: the upper Kaupulehu flow and the lower Huehue flow. The analyzed lavas of the two 1800–1801 flows are geochemically identical with the exception of a few samples from the toe of the Huehue flow that appear to be derived from a separate magmatic batch. The analyzed prehistoric basalts are nearly identical to the 1800–1801 flows but include some lavas that have undergone considerable shallow crystal fractionation. The least fractionated alkalic basalts from Hualalai are in equilibrium with mantle olivine (Fo₈₇) indicating that the Hawaiian mantle source region is not unusually iron-rich. The 1800–1801 and analyzed prehistoric basalts can be generated by about 5–10% partial fusion of a garnet-bearing source relatively enriched in the light-rare-earths. The mantle underlying the Hawaiian Islands is chemically and mineralogically heterogeneous before and after extraction of the magmas that make up the volcanoes.     

Modelling palaeosol preservation in aeolian dunes

In some of the world's desert and desert‐marginal areas (e.g. Simpson/Strzelecki, Australia) dunefields preserve well‐developed palaeosols, whereas in other regions with broadly similar climatic regimes and topography (e.g. southwest Kalahari), the dunes are characterized by very poorly developed internal stratigraphy. It has been postulated that dunes such as those in the Kalahari may never have had conditions conducive to soil formation, or that soils once formed but any evidence of palaeosols has been lost due to reworking. This study develops and applies a one‐dimensional numerical model to simulate dune development, soil formation and soil preservation. Variables in the model allowed experimentation on the influence of sediment supply, the time taken for soil to form, and the additional resistance to erosion offered by the soil. Reduced sediment supply plays a vital role in landscape development during periods of initial pedogenesis. Although re‐exhumed palaeosols influence sediment supply, the effect is minimal. Although under almost all parameterized conditions more than half (and up to 80‐90%) of those soils initially formed are lost due to reworking, evidence of their past formation remains in the large majority of profiles, and the dominant factor in controlling the preservation of palaeosols is the frequency of their formation. The implication is that where dunes are found without palaeosols, the most likely (albeit not certain) inference is that they have never formed. Counter‐intuitively, the limited sediment supply means their additional resistance to erosion becomes almost inconsequential to their preservation, at least until the unit approaches complete invulnerability. Short chronostratigraphic hiatuses around palaeosols are normal, and although long gaps can occur, they are extremely infrequent. Where such gaps are observed in field studies, external forcing factors (e.g. climatic or environmental changes) are implied, as they are highly unlikely to result from stochastic net preservation. Copyright © 2014 John Wiley & Sons, Ltd.     

Petrology,geochemistry, and age of the Spurr volcanic complex,eastern Aleutian arc

The Spurr volcanic complex (SVC) is a calc-alkaline, medium-K, sequence of andesites erupted over the last 250000 years by the eastern-most currently active volcanic center in the Aleutian arc. The ancestral Mt. Spurr was built mostly of andesites of uniform composition (58%–60% SiO₂), although andesite production was episodically interrupted by the introduction of new batches of more mafic magma. Near the end of the Pleistocene the ancestral Mt. Spurr underwent avalanche caldera formation, resulting in the production of a volcanic debris avalanche with overlying ashflows. Immediately afterward, a large dome (the present Mt. Spurr) formed in the caldera. Both the ash flows and dome are made of acid andesite more silicic (60%–63% SiO₂) than any analyzed lavas from the ancestral Mt. Spurr, yet contain olivine and amphibole xenocrysts derived from more mafic magma. The mafic magma (53%–57% SiO₂) erupted during and after dome emplacement from a separate vent only 3 km away. Hybrid block-and-ash flows and lavas were also produced. The vents for the silicic and mafic lavas are in the center and in the breach of the 5-by-6-km horseshoe-shaped caldera, respectively, and are less than 4 km apart. Late Holocene eruptive activity is restricted to Crater Peak, and magmas continue to be relatively mafic. SVC lavas are plag ±ol+cpx±opx+mt bearing. All postcaldera units contain small amounts of high-Al₂O₃, high-alkali amphibole, and proto-Crater Peak and Crater Peak lavas contain abundant pyroxenite and anorthosite clots presumably derived from an immediately preexisting magma chamber. Ranges of mineral chemistries within individual samples are often nearly as large as ranges of mineral chemistries throughout the SVC suite, suggesting that magma mixing is common. Elevated Sr, Pb, and O isotope ratios and trace-element systematics incompatible with fractional crystallization suggest that a significant amount of continental crust from the upper plate has been assimilated by SVC magmas during their evolution.     

Petrology of the rajmahal traps of the Northwestern Rajmahal Hills,Bihar, India

Nine basaltic lava-flows, which vary in thickness between 60 feet and 300 feet, were established in the NW Rajmahals. The flows were, at places, laid down one above the other and, at others, were found to contain intervening intertrappean horizons. All the flows are essentially of basaltic composition and are made up of labradoritie plagioclase, pigeonitic and augitic pyroxene, opaque ore, primary glass and secondary minerals (palagonite, secondary silica, calcite and zeolite). The phenocrystic plagioclase ranges in composition between An₇₂ and An₆₂, while the constituents of the groundmass range between An₅₀ and An₁₇. The microphenocrysts of pyroxene are mainly augitic and occasionally pigeonitic while the constituents of the groundmass are essentially pigeonitic. The opaque minerals are magnetite and ilmenite. Petrographically, the lava-flows are more or less similar to one another. The first three flows are, however, more remarkably porphyritic and a little coarser in grain size than the six overlying flows. The eighth flow is devoid of palagonite. Calcite occurs only in certain portions of the second flow. There is a gradual increase in the percentage of primary glass from the first to the ninth flow with a corresponding fall in the total percentage of plagioclase and pyroxene. Statistical analysis of the grain size variation in the plagioclases was carried out and the results were found to be directly related to the prevailing rates of cooling in the different flows and also in the different horizons of the same flow. Modal analysis of the nine flows (in all, 98 samples) was carried out and this brought out some interesting results. Samples from three of the flows were analysed chemically and the corresponding norms were calculated. The order of crystallisation of the primary constituents was established from petrographic and petrological studies. The basaltic magma, which gave rise to the lava-flows of this region, does not appear to have undergone any significant differentiation during the course of its cooling and consolidation. The only discernible effect of crystallisati on differentiation was an enrichment of silica (and, perhaps, alkalis) in the residual liquids and no noteworthy enrichment of iron appears to have occurred at any stage.     

The termination of the last major phase of aeolian sand movement,coastal dunefields,Denmark

Optically stimulated luminescence (OSL) dating of sand samples from stabilized (or inactive) coastal dunes in Denmark provides information on the age of the termination phase of the last major aeolian activity period. A total of 26 sand samples were taken from four different coastal dunefields around the North Sea, Skagerrak and Kattegat coasts of Denmark. The OSL dates indicate that the last major phase of aeolian activity terminated between ad 1860 and 1905. Most of the dunes examined in this study were active around 1820, during a period documented to have been very stormy. A dune management scheme started around 1792, and this no doubt was a major cause of dunefield stabilization, but an overall decline of storminess, particularly spring and summer storminess, around the end of the 19th century must also have contributed to the increasing inactivity of coastal dunes. The new OSL dates on aeolian sand movement agree well with historical data and data from topographic maps on dune movement. This agreement supports the observation from earlier work that OSL dating of recent aeolian sand movement is accurate over the last few decades to centuries. Copyright © 2005 John Wiley & Sons, Ltd.     

Petrology of the diamond-grade eclogite in the Kokchetav Massif, northern Kazakhstan

Abstract The Kokchetav Massif of northern Kazakhstan is unique because of the abundant occurrence of microdiamond inclusions in garnet, zircon and clinopyroxene of metasediments. In order to determine precise pressure–temperature (P–T) conditions, we have systematically investigated mineral inclusions and the compositions of major silicates in Ti–clinohumite–garnet peridotite and diamond-grade eclogite from Kumdy–Kol. It was found that garnet peridotites from Kumdy–Kol contain assemblages of garnet, olivine, Ti–clinohumite and ilmenite. The garnet contains inclusions that are indicative of both ultrahigh pressure (UHP) and retrograde conditions. Inclusions of hydrous phases such as chlorite, amphibole and zoisite were formed at the post-UHP stage. The study also found that eclogite from Kumdy–Kol contains albite–augite symplectites after omphacitic pyroxene. The core of pyroxene (sodic augite) contains high K₂O (up to 1wt%; average 0.24wt%). Phengite is included in the core. Applying the K₂O-in-augite geobarometry, which is based on recent experiments, and the garnet–clinopyroxene (Grt–Cpx) geothermometer for peak metamorphism, the eclogites yield P–T estimates of > 6 GPa and > 1000 °C, and the diamond-grade eclogites yield lower temperature estimates at 900–1000 °C and 5 GPa.     

Petrology and isotope-geochemistry of San Vincenzo rhyolites (Tuscany,Italy)

Two groups of rhyolites have been recognized at San Vincenzo (Tuscany, Italy). Group A rhyolites are characterized by plagioclase, quartz, biotite, sanidine and cordierite mineral assemblages. They show constant MgO and variable CaO and Na₂O contents. Initial⁸⁷Sr/⁸⁶Sr ratios in group A samples range between 0.71950 and 0.72535, whereas the Nd isotopic compositions are relatively constant (0.51215–0.51222). Group B rhyolites are characterized by orthopyroxene and clinopyroxene as additional minerals, and show textural, mineralogical and chemical evidence of interaction with more mafic magmas. The Sr and Nd isotopic ratios range between 0.71283–0.71542 and 0.51224–0.51227 respectively. Magmatic inclusions of variable size (1 mm to 10 cm) were found in groups B rhyolites. These inclusions consist mainly of diopsidic clinopyroxene and minor olivine and biotite. They are latitic in composition and represent blobs of hybrid intermediate magmas entrained in the rhyolitic melts. These magmatic inclusions have relatively high Sr contents (996–1529 ppm) and Sr and Nd isotope-ratios of 0.70807–0.70830 and 0.51245–0.51252 respectively.⁸⁷Sr/⁸⁷Sr data on minerals separated from both group A and B rhyolites and magmatic inclusions reveal strong isotopic disequilibria due to the presence of both restitic and newly crystallized phases in group A rhyolites and due to interaction of rhyolites with a mantle-de-rived magma in group B rhyolites. Isotopic data on whole rocks and minerals allow us to interpret the group A rhyolites as representative of different degrees of melting of an isotopically fairly homogeneous pelitic source; conversely, group B rhyolites underwent interactions with a mantle-derived magma. The crustal source as inferred from isotopic systematics would be characterized by⁸⁷Sr/⁸⁶Sr and¹⁴³Nd/¹⁴⁴Nd ratios close to 0.7194 and 0.51216 respectively. The sub-crustal magma would have Sr isotopic composition close to 0.7077 and a¹⁴³Nd/¹⁴⁴Nd ratio greater than or equal to 0.51252. These isotopic features are different from those reported for the parental magmas postulated for Vulsini and Alban Hills in the nearby Roman Magmatic Province, and are similar to those of the Vesuvius and Ischia magmas.