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.     

The composition and differentiation of the volcanic rocks of Carriacou,grenadines, West Indies

Tertiary volcanic rocks of Carriacou occupy two-thirds of the island. The volcanics include volcaniclastics, lava flows and dome lavas and range in composition from basalts to andesites. Carriacou basalts fall into two petrographic types (a) clinopyroxene-plagioclase-phyric basalts and (b) olivine microphyric basalts; the latter having higher MgO and lower Al₂O₃ than the clinopyroxene basalts. Both types are unusually rich in mafic minerals compared with Lesser Antilles basalts in general, although similar types have been reported from the nearby island of Grenada. The potash to silica ratios are relatively high and confirm the similarity between Carriacou and Grenada basalts and the differences between these basalts and basalts from other islands of the Lesser Antilles. The basaltic andesites and andesites from Carriacou correspond closely in mineralogical and chemical composition with typical andesites found elsewhere in the Lesser Antilles. The geochemistry of the volcanics shows that the olivine microphyric basalts display tholeiitic affinities whereas the clinopyroxeneplagioclase-phyric basalt, basaltic andesites and andesites are calcalkaline. The compositional gradation in both the geochemistry and mineralogy of these volcanics suggests that fractional crystallization played an important role in the derivation of the various magma.     

Transitional basalts from the eastern Australian tertiary province

Study of Tertiary volcanic rocks from Mt. Tamborine in S. E. Queensland, Australia, suggests that some basalts from the large Tweed vent are intermediate in character between true tholeiites and well documented alkaline basalts of the Australian Tertiary province. Geochemical evidence and comparison with synthetic systems suggests that the transitional lavas are derived by continuous fractional crystallization during the ascent of a parent magma produced by substantial partial melting of the upper mantle at moderate depths and pressures. Alkaline basalts that underlie the transitional basalts are the result of relatively rapid, intermittent extrusion of magma generated by a smaller degree of partial melting of the upper mantle. This interpretation resolves the difficulties posed by an apparent inversion of the usual stratigraphic relationship of tholeiites capped by alkali basalts, and is in accord with recent seismic studies of the continental margin of eastern Australia.     

Petrology of tholeiitic lavas from Tanna Island (New Hebrides): Importance of cumulative processes in Island arc magmatism

Tanna, one of the southernmost islands of the New Hebrides volcanic arc, is made of Late Pliocene to Recent island arc tholeiitic basalts and andesites, with SiO₂ contents ranging from 45 to 57%. These lavas are highly porphyritic (30–50% in volume): phenocrysts of plagioclase are the most abundant, together with olivine and clinopyroxene. The groundmass contain plagioclase, augite, olivine, magnetite and glass; pigeonite, tridymite, sanidine and, rarely, biotite may also occur. The olivines and clinopyroxenes show an iron enrichment from the cores of phenocrysts to their rims and the groundmass crystals, but their compositional variations are not correlated with the Mg/Fe ratio of bulk host rocks, the most Fe-rich compositions being found in Mg-rich lavas. Plagioclase compositions range from An₉₅ to An₆₀ in the basalts and An₆₀ to An₅₀ in the andesites, but, within each group, they are not correlated with SiO₂ or Na₂O contents of host lavas. Consequently, the bulk major element compositions of Tanna volcanic rocks cannot be considered as primarily controlled by crystal separation from successive liquids. The oxyde-SiO₂ variations diagrams, and the modal compositions and mineral chemistry show that crystal accumulation is the predominant mechanism accounting for bulk rock compositions. However, this does not exclude fractional crystallization: the variation of the calculated groundmass mineralogy strongly suggest the occurrence of crystal removal mainly clinopyroxene and magnetite.     

Geochemistry and origin of basaltic lavas from Society Islands,French Polynesia (south central Pacific Ocean)

The Society Islands of French Polynesia (south-central Pacific Ocean) are formed predominatly by basaltic lavas of alkaline affinities. These intraplate rocks are either Ne- or Hy-normative. Ne-normative basaltic lavas are lower in Si and heavy rare earth elements and higher in Ti, V and Sr than Hy-normative. The differences are attributed to the contrasting behaviour of amphibole during melting of the upper mantle, which, in turn, may be related to the variation in the depth of melting. Compared to pyrolite, the upper mantle source of the lavas was enriched in incompatible elements.     

Geochemistry of the volcanics of central Mexico

The Tertiary and Recent volcanics of Mexico occur in two provinces. The Cordillera Province is made up of about 1700 m of ignimbrite sheets overlain and intercalated in the upper part by olivine basalt and basaltic andesite. The Rio Lerma Province extends transversely across Mexico and in the Valley of Mexico the lavas consist mainly of andesite and dacite, 68 % of those analysed having 62 | 4.7 % SiO₂. A total of 108 chemical analyses were made for the major elements, 90 of these including determinations of Cr, Ni, Cu, Zn, Rb, Sr, Ba, Th and Pb for two areas, the Valley of Mexico in the Rio Lerma Province and the Guadalajara region which lies at the intersection of the two provinces. Computer constructions of normative components in the basalt tetrahedron and other projections support an origin of partial melting of tholeiitic to pyrolitic material for the production of andesite. The Guadalajara lavas have consistently higher K/SiO₂ and K/Rb ratios and lower Mg/SiO₂ ratio than the Valley of Mexico rocks suggesting generation at greater depth.     

Geochronology and magmatic character of the pliocene-pleistocene volcanism in Sardinia (Italy)

The Pliocene-Pleistocene subaerial volcanic activity of the island of Sardinia developed from about five million years ago to the Pleistocene. Volcanism was mainly fissural, related to rifting of the Sardinian crustal block and connected to intraplate tensional tectonics involving at the same time the area of the Tyrrhenian Sea. Areally the most abundant rocks are basic, ranging in serial character from alkaline to subalkaline types. In some areas intermediate and salic lavas also occur; trachytic and phonolitic rocks are mainly associated with basalts of alkaline affinity, whereas rhyolites and dacites are mainly related to subalkalic basalts. K/Ar data show that lavas of different serial character (from alkalic to subalkalic) are produced on the island within the same time range, from about four to two million years; it is to be noted, however, that the early products (about 5 million years) are mildly alkalic in character whereas most of the youngest products (0.6–0.14 million years) are strongly alkalic.     

Volcanic rocks of the Witu Islands,Papua New Guinea: The origin of magmas above the deepest part of the New Britain Benioff zone

The Witu Islands are Quaternary volcanoes that overlie the deepest (about 300–580 km) part of the New Britain Benioff zone. The islands are about 100 km south of the transcurrent-divergent plate boundary that crosses the Bismarck Sea, and they surmount the southeastern end of the Willaumez-Manus Rise. The rocks are olivine- and quartz-normative tholeiitic basalts, low- and high-SiO₂ andesites, dacites, and rhyolites. Alkaline rocks that overlie the deep (greater than 300 km) parts of other Benioff zones have not been found in the Witu Islands. Compared to the Witu Islands rocks, those with similar SiO₂ contents from New Britain volcanoes that overlie progressively shallower parts of the Benioff zone to the south, are, for example, generally poorer in Na+K, Ti, and P, and higher in Ca and Al. There are similar progressive changes in trace-element abundances, but Zr and Nb contents are distinctly richer in Witu Islands rocks.⁸⁷Sr/⁸⁶Sr values range between 0.70311 and 0.7038, which are typical for rocks from New Britain as a whole and from other island arcs in the southwest Pacific. Two¹⁴³Nd/¹⁴⁴Nd values of 0.512211 and 0.512271, taken together with the Sr isotopic results, define a source region equivalent to those for oceanic-island basalts; there is no evidence for sea-water contamination of the sources. Perhaps the most striking feature of the Witu Islands rocks is their compositional diversity. Basalts range from olivine tholeiites similar to marginal-basin basalts from other areas, to quartz tholeiites similar in most respects to those typical of island arcs, and to incompatible-element-enriched tholeiites that are close to silica-undersaturation. Andesites on Unea Island have a strong island-arc signature, but the andesites, dacites, and rhyolite of Garove Island have some features that may be more in common with the silica-oversaturated rocks of oceanic areas. The mineralogy of Witu Islands basalts is characterised by phenocrysts of olivine (Fo_89-54), plagioclase (An_90-55) and Ca-augite. Cr-Al-rich spinels and aluminous magnetites are present as inclusions in some olivine phenocrysts. Groundmass fayalite, alkali feldspar, and dacitic to rhyolitic glasses high in K/Na are found in a few samples. In contrast to basalts from volcanoes above the shallower parts of the New Britain Benioff zone, those of the Witu Islands are characterised by rarity of low-Ca pyroxene, as phenocrysts or in the groundmass. Rocks richer in SiO₂ are characterised by the presence of orthopyroxene phenocrysts and lesser amounts of olivine. Hydrous minerals appear to be absent. Groundmass Fe-Ti oxides define crystallisation temperatures (about 800–1050°C) and oxygen fugacities (fO₂) corresponding to those of the Ni-NiO buffer, but up to two fO₂ log units above it. The suite as a whole is phenocryst-poor compared to most New Britain volcanic rocks. A significant degree of mantle heterogeneity is inferred by the chemical variability of the Witu Islands rocks. However, there are no compelling reasons in support of the interpretation that source heterogeneity is due to the effect of a slab-derived component. The cause of the heterogeneity is unclear, but may be due to mantle differentiation processes related to an anomalous tectonic setting.     

Variation trends in basaltic rocks of the canary islands

More than two hundred new analyses of basaltic rocks from the Canary Islands are presented. The available data show that the earlier successions have similar variation trends throughout the archipelago; these trends represent all the intermediate types between basic differentiates (oceanites, ankaramites) and more salic differentiates (trachy-basalts, hawaiites). In the more recent volcanic series, the study shows that there is a magmatic diversification with different variation trends in the Eastern than in the Central Canary Islands. In the latter the alkaline character becomes stronger in the successive periods of volcanic activity which have been established in each island. In the Eastern islands the basaltic evolution is, on the contrary, towards basaltic rocks with tholeiitic affinities.     

The composition of basaltic lavas from Bayuda,Sudan and their place in the cainozoic volcanic history of north-east Africa

Six new analyses of young basaltic rocks from the Bayuda field show the predominant rock types to be strongly undersaturated basanites and nepheline trachybasalts. Both types are believed to represent magmas of deep-seated origin. Similar rocks are widely distributed in north-east Africa but mildly alkaline to tholeiitic basalts were erupted along the eastern margin of the continent in early and late Cainozoic times, whereas along the Tripoli-Tibesti zone to the west mildly alkaline basalts were probably confined to the early Tertiary. The Tripoli-Tibesti zone was one of uplift and strongly tensional tectonics in the late Mesozoic and early Cainozoic, and at this time may have been a line of potential lithospheric rifting, but a period of quiescence followed and resurgence of activity in the late Cainozoic produced weaker tensional structures and more strongly alkaline basic magmas. The region between these two main zones of activity was characterized throughout by intermittent alkaline volcanicity and weak tectonism. Neverthless, fracture zones which apparently controlled the volcanicity are beginning to be recognized in this area. It is argued that African volcanic activity is related to linear, rather than circumscribed, areas of mantle activity. Possible connections with epeirogenic movements within the Alpine orogenic belt appear to have been neglected in the debate on the causes of African igneous activity.     

Calc-alkaline volcanic rocks from NW Sardinia: Evaluation of a fractional crystallization model

The volcanic centre of Monte Seda Oro, N. W. Sardinia, representative of a Cenozoic calc-alkaline andesitic suite of rocks is composed of a variety of rocks ranging from high alumina basalts to dacites. The minerals of basaltic, andesitic and dacitic rocks show only limited variation in chemical composition. The geochemical data suggest that the various rock-types are related by a crystal-liquid fractionation. Least-squate numerical calculations, using major element data, support the derivation of andesites with SiO₃ content ranging from 53.8 to 59.0% from basalts having about 48.7% of SiO₂ by low pressure crystal fractionation of the phenocryst phases present in these rocks. However, the origin of dacites cannot be readily explained by this mechanism.     

Significance of basic and ultramafic rock inclusions in the basalts of Canary Islands

This paper deals with the mineralogical, textural and chemical features of the dunite, peridotite, pyroxenite and gabbroic inclusions present in the Quaternary basalts of the Canary Islands. The mineralogical composition, structure and texture of the inclusions show that most of them have been formed as crystal cumulates from a nonalkaline basaltic magma in the earlier stages of its fractional crystallization. There are no co-genetic relationships between the inclusions and the host basalts, since the latter have a very strong alkaline-olivine character, although there are also some types with tholciitic aflinities. The study of the data leads to the conclusion that these inclusions can be considered as xenoliths from the basic and ultrabasic complexes that form the substratum and which outcrop in some of the Canary Islands. Attention is called to the fact that in many other volcanic zones of the world there has been a previous emplacement of basic and ultramafic layered complexes and is the question opened whether the association between stratiform-complexes and active basaltic volcanism is more frequent than has been assumed up to now.     

Volcanic Markers of the Post-Subduction Evolution of Baja California and Sonora, Mexico: Slab Tearing Versus Lithospheric Rupture of the Gulf of California

The study of the geochemical compositions and K-Ar or Ar-Ar ages of ca. 350 Neogene and Quaternary lavas from Baja California, the Gulf of California and Sonora allows us to discuss the nature of their mantle or crustal sources, the conditions of their melting and the tectonic regime prevailing during their genesis and emplacement. Nine petrographic/geochemical groups are distinguished: ??regular?? calc-alkaline lavas; adakites; magnesian andesites and related basalts and basaltic andesites; niobium-enriched basalts; alkali basalts and trachybasalts; oceanic (MORB-type) basalts; tholeiitic/transitional basalts and basaltic andesites; peralkaline rhyolites (comendites); and icelandites. We show that the spatial and temporal distribution of these lava types provides constraints on their sources and the geodynamic setting controlling their partial melting. Three successive stages are distinguished. Between 23 and 13 Ma, calc-alkaline lavas linked to the subduction of the Pacific-Farallon plate formed the Comondú and central coast of the Sonora volcanic arc. In the extensional domain of western Sonora, lithospheric mantle-derived tholeiitic to transitional basalts and basaltic andesites were emplaced within the southern extension of the Basin and Range province. The end of the Farallon subduction was marked by the emplacement of much more complex Middle to Late Miocene volcanic associations, between 13 and 7 Ma. Calc-alkaline activity became sporadic and was replaced by unusual post-subduction magma types including adakites, niobium-enriched basalts, magnesian andesites, comendites and icelandites. The spatial and temporal distribution of these lavas is consistent with the development of a slab tear, evolving into a 200-km-wide slab window sub-parallel to the trench, and extending from the Pacific coast of Baja California to coastal Sonora. Tholeiitic, transitional and alkali basalts of subslab origin ascended through this window, and adakites derived from the partial melting of its upper lip, relatively close to the trench. Calc-alkaline lavas, magnesian andesites and niobium-enriched basalts formed from hydrous melting of the supraslab mantle triggered by the uprise of hot Pacific asthenosphere through the window. During the Plio-Quaternary, the ??no-slab?? regime following the sinking of the old part of the Farallon plate within the deep mantle allowed the emplacement of alkali and tholeiitic/transitional basalts of deep asthenospheric origin in Baja California and Sonora. The lithospheric rupture connected with the opening of the Gulf of California generated a high thermal regime associated to asthenospheric uprise and emplaced Quaternary depleted MORB-type tholeiites. This thermal regime also induced partial melting of the thinned lithospheric mantle of the Gulf area, generating calc-alkaline lavas as well as adakites derived from slivers of oceanic crust incorporated within this mantle.     

Geochemistry and petrogenesis of the kilimanjaro volcanic rocks of the Amboseli area,Kenya

The paper summarizes the geochemical and petrogenetic aspects of an investigation of late Tertiary to Recent volcanic rocks in the Amboseli area of southern Kenya. A study of chemical variations in the Amboseli and Kilimanjaro lavas distinguishes a mildly alkaline series (alkali olivine basalts, trachybasalts/trachyandesites, trachytes, rhomb porphyries and phonolites) from a strongly alkaline series embracing subordinate nephelinitic, phonolitic and tephritic lavas. The two series probably evolved independently from a source in the mantle. A comparison of Kilimanjaro with other East African volcanoes shows that the focus of strongly alkaline volcanicity moved from eastern Uganda and western Kenya to northern Tanzania at the end of Miocene times. The Pliocene to Recent centres near the Kenya-Tanzania border show evidence of decreasing alkalinity from a western zone of nephelinite-phonolite volcanoes to an eastern region in which central volcanoes are characterized by the association of strongly and mildly alkaline suites.     

Petrochemical affinities of volcanic rocks from the tonga — Kermadec island arc,Southwest Pacific

Eruptive suites from Tonga (tholeiitic), Raoul Island (tholeiitic) and Macauley Island (high-alumina) are characterised by low alkalis, an absence of andesites in the range 56–65% silica, and restricted acidity for minor glassy differentiates (SiO₂=65–68 %). These volcanics form a chain of islands overlying a seismic zone which extends from Tonga to the central volcanic region of North Island, New Zealand where a calc-alkaline series contains basaltic, andesitic and rhyolitic members in that order of increasing abundance. Within this continental suite, tholeiitic and high-alumina phases are recognised as closely similar to the intra-oceanic Tonga-Kermadec magma types and show petrochemical gradation into the medium-silica andesites, apparently by sialic assimilation.     

Petrology of seamounts northwest of Samoa and their relation to Samoan volcanism

Petrological and geochemical data on dredged samples from five submarine volcanos northwest of Samoa indicate that three of these volcanos belong to the Samoan volcanic province (Field, Lalla Rookh, and Combe banks), and two belong to separate magmatic zones (Wallis Islands and Alexa Bank). The Samoan volcanic province increases in age westward and both shield-building tholeiitic and alkalic lavas (Combe Bank) and strongly undersaturated (post-erosional?) melilitites or nephelinites and ankaramites (Field and Lalla Rookh banks) are present. The age progression and petrochemical character of these rocks is consistent with a fixed hotspot beneath eastern Samoa. Slightly askew from this trend is Alexa Bank where dredged lavas are ocean-island tholeiites; however, its radiometric age and compositional characteristics apparently preclude its association with Samoa by a fixed-hotspot model. Dredged volcanic rocks from near the Wallis Islands are geochemically, petrologically, and temporally different from Samoan volcanism, but are similar in these respects to Quaternary volcanism in Rotuma and Fiji and may be related to plate reorganization accompanying opening of the North Fiji Basin.     

Petrography and petrochemistry of the basalt sequences around mahape,Mumbra and Kalyan,Maharashtra, India

The volcanic sequences in three vertical sections (starting from m.s.l.) of the Deccan trap flows around Mahape, Mumbra and Kalyan have been demarcated into zones, mainly based on the megascopic texture and the degree of abundance of vesicles and amygdules. Petrographic, mineralogical and modal study of twelve samples indicate that the basalts are tholeiitic and are characterised by lateral inhomogeneities. Petrochemical data are suggestive of limited differentiation, a hyperferric iron enrichment, a Hawaiian trend and an affinity to alkalic and high-alumina basalt.     

Major-element petrochemistry of some extrusive rocks from the volcanically active Mariana Islands

Volcanic rocks from six of the currently or recently active volcances of the Mariana Island are show little variation in major element abundances. SiO₂ content averages 51.5 wt.%. The flows are high in Al₂O (mean 17.7 wt.%) and Fe oxides (mean 10.1 wt.% calculated as FeO only), and moderate in MgO content (mean 4.7 wt.%), Na₂O (mean 2.7 wt.%), and K₂O (mean 0.7 wt.%). Only the rocks from Farallon de Pajaros, the northernmost of the Mariana Islands, deviate slightly from the average of the analyses. Three analyses from this island are slightly higher in SiO₂ (about 54 wt.%) and Al₂O₃, and are lower in total Fe oxides and MgO. According to preferred classification, the lavas of the Mariana Islands can be termed mela-andesites, high-alumina basalts, or calc-alkaline (orogenic) basalts. The K₂O values (mean 0.7 wt.%) obtained from lavas of the Mariana Islands are significantly higher than the K₂O values (about 0.33 wt.%) from volcanics of the Izu chain to the north. Inasmuch as the substantial scatter in location of earthquake foci beneath both arcs prevents accurate delineation of the upper boundary of the Benioff zone, it presently cannot be determined whether this discrepancy in K₂O values reflects a difference in depth from the volcanic are to the dipping seismic zone or relates to other phenomena. The older volcanic islands within the Mariana-Bonin island chain apparently defined an island arc system during Eocene to Miocene time. This indicates that the present plane of convergence between the Pacific plate and the Philippine Sea plate has defined the convergence between these plates since Eocene time.     

Two types of alkaline rocks-two types of upper mantle

1) Petrochemical studies of volcanic rocks shows that alkaline rocks of continents and oceans are different genetically in spite of their mineralogical and chemical similarity. 2) Oceanic rocks develop according to the following type: tholeiitic basalt — olivine basalt — alkaline rocks. 3) Continental alkaline rocks are derivatives of initially alkaline basalts and are connected by gradual transitions with calc-alkaline rocks of island arcs. 4) The source of all volcanic rocks is the upper mantle. Therefore the existence of two main types of rocks — oceanic and continental — reflects basic heterogeneities in composition and structure of the upper mantle.     

Agrigan: An introduction to the geology of an active volcano in the Northern Mariana Island arc

Agrigan is the tallest (965 m a.s.l.) and largest (44 km²) of the volcanoes of the northern Mariana Islands. Its slopes are asymmetric to the east; a small caldera (4 km²) dominates the interior. The volcanic edifice has been disrupted along three sets of faults: 1) exterior slump faults, 2) radial faults, and 3) interior faults related to caldera-collapse. The rocks of the volcano are characterized by porphyritic clinopyroxene-olivine-plagioclase basalts and subordinate andesites. Cumulate xenoliths composed of Fo₈₁, An₉₅ and diopside are common in the basalts. Development of the volcano began with 3–4 km of submarine growth. The earliest recognizable flows are the result of fissural Hawaiian- and Strombolian-type eruptions. These were followed by the eruption of more viscous lavas from above the present summit. Flank eruptions of basalt and andesite preceded voluminous outpourings of andesitic pyroclastics contemporaneous with caldera-collapse. Subsequent magmatic resurgence is localized along a N10E rift zone. Violent ejection of lapilli and ash occurred in 1917.