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.     

Boninite as a possible calc-alkalic primary magma

Boninite is an unusual, plagioclase-free magnesian andesite, occurring as vesicular pillow lavas and hyaloclastites, accompanied by andesites and dacites in Chichi-jima, Bonin Islands. The Bonin Islands belong to the Izu-Mariana arc and consist of dominant volcanic rocks and subordinate sedimentary rocks of late Oligocene-early Miocene age. The chemistry of boninite is characterized by high contents of MgO. Cr and Ni similar to primitive basalts, but apparently in ill accord with its relatively high SiO₂ content of ? 55%. The relation of SiO₂ to total FeO/MgO ratio indicates that boninite belongs to the cale-alkalic rock suite. The mineralogy of boninite consists of olivine (Fo87-90), orthopyroxene (En87-90), clinopyroxene (Wo38-35En37-44Fs25-21), hydrous glass and Cr-spinel, Experimental studies show that the magma of boninite composition could be in equilibrium with upper mantle peridotite at pressures less than 17 kb and temperatures of 1200–1050°C under high P_H2_O. It is suggested that boninite is a sea-floor quenched product (900°C) of a direct partial melt of the upper mantle. Related andesites and dacites are considered to be probably fractional crystallization products from the same magma.     

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.     

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.     

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.     

Fractional crystallization trends in the system Mg2SiO4-CaAl2Si2O8-FeO-Fe2O3-SiO2 over the range of oxygen partial pressures of 10−11 to 10−0.7 atm

A brief report is made of current laboratory investigations on phase relations among olivine, pyroxene, anorthite, magnetite, tridymite, liquid and gas in the system Mg₂SiO₄-CaAl₂Si₂O₈-FeO-Fe₂O₂-SiO₂ over a wide range of oxygen partial pressures. Courses of fractional crystallization under various conditions of oxygen partial pressure are depicted using an anorthite saturation diagram. Starting with a basalt-like composition in the system, fractional crystallization at a moderate oxygen partial pressure (10 atm.) results in an andesite-like residual liquid of composition 55 SiO₂, 14 iron oxide, 6 MgO, 9 CaO, 16 Al₂O₃ at a temperature of 1155°C. With fractional crystallization in a closed system, the end liquid approaches the composition of 45 SiO₂, 38 iron oxide, 6 CaO and 11 Al₂O₃, at a temperature of 1050°C and oxygen partial pressure of about 10^?12 atm. The andesitic final liquid in this system would be expected to further differentiate toward dacitic and rhyolitic compositions if alkalies and water were present in the system. On the basis of these studies, the derivation of liquids of andesitic, dacitic or rhyolitic composition from primary basalts by fractional crystallization seems entirely possible if the oxygen partial pressure is maintained at a moderate or high level.     

Amphibole-bearing basalts from the submarine volcano Kick’em-Jenny in the lesser antilles Island arc

An oceanographic survey by H.M.S. Hecla of the 1974 active submarine volcano (12°18′N and 61°38′W) revealed a crater at 190 m below sea level and bottom-sampling yielded fresh olivine basalt pyroclastics with phenocrysts of olivine, plagioclase and clinopyroxene. Megacrysts of amphilbole, up to 16 modal percent, are subsilicic and nepheline-normative ferroan pargasites. The mineral assemblage Ol+Cpx+Pl+Amph appears to have been in equilibrium in the Kick’em-Jenny melt prior to eruption, although published experimental studies on hydrous (H₂O-saturated) or anhydrous alkali basaltic compositions have not yielded this mineral assemblage at any pressure. Interpolation between the experimentally determined phase relationships for dry and water-saturated alkali basaltic liquids indicates, however, that in an isobaric section at 5 kb the observed assemblage can exist in the water-undersaturated region. The Kick’em-Jenny olivine basalts belong to a suite of variably undersaturated basaltic rocks including alkali picrites and basanites, common in Grenada and the southern Grenadines, but although the available evidence indicates the importance of the presence of water in the genesis of these melts, their origin seems most likely to be associated with partial melting of upper mantle material rather than melting of amphibolite in an underthrust lithospheric slab.     

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.     

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.     

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.     

Volcan ceboruco: A major composite volcano of the mexican volcanic belt

Ceboruco is a major composite volcano at the western end of the Mexican Volcanic Belt, near the junction between the North American and Pacific plates. The volcano is built from successive eruptions of andesite lavas and pyroclastic rocks, and major eruptions during its history have resulted in the formation of two concentric calderas. The youngest volcanic activity has included the extrusion of dacites within the inner caldera and a voluminous flank eruption of andesite during 1870–72. Fumarolic activity persists to the present day. Chemical analyses show that the lavas are of cale-alkaline type and rangs from andesite (SiO₂=58–61%) to acid dacite (SiO₂=68%) in composition. The rate of increase of K₂O relative to SiO is greater than that in volcanic rocks from the Mexican Volcanic Belt as a whole. This indicates that simple models based on the application of such relationships may not be adequate to explain the petrogenesis of calc-alkaline lavas.     

Magma types of volcanic rocks and continental margin of the Taiwan upper Paleozoic Geosyncline

Greenrocks are very common in the Tananao Schist of eastern Taiwan where the known fossils are of Permian in age. Fourty-four greenrock samples were chemically analysed and their magma types studied. The chemical composition of the greenrocks have marked variation common in volcanic rock series. The greater parts of the greenrocks belong to basalt and a smaller portion to basaltic andesite (SiO₂ 53 %–58 %). They are probably isochemical with their original igneous rocks except for volatile components. No striking Fe-enrichment exists in a MgO-ΣFeO-(Na₂O=K₂O) diagram. Based on (Na₂O=K₂O)-Al₂O₃-SiO₂ diagrams afterKuno (1960), the parent magma of the rocks mostly belong to the high-alumina basalt series and only a few to alkali olivine basalt series. The high-alumina basalt can be looked upon as having an incipient trend for the calc-alkaline or the hypersthene series ofKuno (1959). The average K/Rb ratio of 460, the average TiO₂ percentage of 1.5 %, and low K₂O of around 0.5 % seem to warrant a conclusion that the basaltic rocks were poured out in the upper Paleozoic eugeosyncline on an embryonic continental crust. Considering the rock association of amphibolite plus serpentine (dismembered ophiolite), meta-graywacke, metachert, crystalline limestone, metaarkose, and metabasites in the Tananao Schist, the most probable site for the eugeosyncline may been an extensional trough near the fragmented paleo-Asiatic margin.     

Capping volcanic rocks: West central Nevada

Cenozoic capping volcanic rocks in the Nevada portion of the Basin and Range Province of the western United States belong to the high alumina calcalkaline igneous series. Varying proportions of plagioclase (An=85 to 45 percent), pyroxene (augite, pigeonite, and hypersthene), olivine, magnetite, biotite, and oxyhornblende indicate a modal range from olivine basalt to andesite. Major element analyses made on randomly collected samples, as well as on samples from systematically measured stratigraphic sections in localities of minimum erosion show ranges in Al₂O₃ (from 17.5 to 22.5 percent); SiO₂ (from 44.0 to 54.0 percent); MgO (from 3.47 to 8.20 percent) and CaO (from 7.19 to 11.90 percent). Na₂O/K₂O is always greater than 1.0. Ba⁺⁺ and Sr⁺⁺ abundances for some of the rocks are in agreement with those suggested, by workers in the field, for average basalt and andesite derived by melting of mantle or lower crust, but for many of the samples the values found are considerably higher. Although the presence of biotite in the earlier flows and oxyhornblende in the later ones along with the presence of much magnetite in all the rocks examined suggests that in part these rocks were derived by crystallization of a melt under conditions of high partial pressure of oxygen, the available trace element data indicates that contamination of the magma with crustal material was also a factor in their developmental history. The case for a parent magma, subsequently fractionally crystallized and contaminated in part, is strengthened by the occurrence of crystal cumulates and highly altered xenoliths in some of the flows.     

Petrology of filicudi,aeolian archipelago

Volcanic rocks on the island of Filicudi are typical of the cale-alkaline association of the Aeolian archipelago, and range in composition from basalt to high-K andesite. All specimens contain abundant phenocrystic plagioclase (cores of An_82–97, rims becoming progressively more sodic with increasing Differentiation Index), clinopyroxene, and magnetite; the most basic samples contain olivine, which is replaced by orthopyroxene at around D.I.=44. SiO₂ generally increases with time, and this in conjunction with the continuity in mineralogy suggests that the lavas are the differentiation products of a single batch of magma. Least-squares mixing calculations are consistent with a shallow fractional model, as is the variation in trace element abundances.     

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.     

Pegmatitoides in the horizontal basalts (series I) of lanzarote and fuerteventura Islands

Coarse-grained rocks found as veinlets, veins and dykes in the horizontal basaltic formation (Series I) of the Islands of Lanzarote and Fuerteventura (Canarian Archipelago) have been studied petrographycally and chemically. The term « pegmatitoides » as given byLacroix (1928, 1929) has been adopted for these coarse-grained rocks. Petrographycally, they are alkali gabbros, ranging from theralites to essexites and even to nepheline monzonitic types. The degrce of crystallization and the textural relationships vary within wide limits. Basalts that contain pegmatitoides are olivine basalts or even more basic types, such as oceanites. Chemical study of pegmatitoides with relation to their host basalts shows that pegmatitoides are richer in Si₂O, Al₂O₃, Na₂O, K₂O and poorer in CaO and MgO than their hosts-basalts. Hypotheses are presented in this study as to the genesis of these pegmatitoides.     

Lateral variation of basalt magma type across continental margins and Island Arcs

Quaternary basalt magmas in the Circum-Pacific belt and island arcs and also in Indonesia change continuously from less alkalic and more siliceous type (tholeiite) on the oceanic side to more alkalic and less siliceous type (alkali olivine basalt) on the continental side. In the northeastern part of the Japanese Islands and in Kamchatka, zones of tholeiite, high-alumina basalt, and alkali olivine basalt are arranged parallel to the Pacific coast in the order just named, whereas in the southwestern part of the Japanese Islands, the Aleutian Islands, northwestern United States, New Zealand, and Indonesia, zones of high-alumina basalt and alkali olivine basalt are arranged parallel to the coast. In the Izu-Mariana, Kurile, South Sandwich and Tonga Islands, where deep oceans are present on both sides of the island arcs, only a zone of tholeiite is represented. Thus the lateral variation of magma type is characteristic of the transitional zone between the oceanic and continental structures. Because the variation is continuous, the physico-chemical process attending basalt magma production should also change continuously from the oceanic to continental mantle. Suggested explanations for the lateral variation assuming a homogeneous mantle are: 1) Close correspondence between the variations of depth of earthquake foci in the mantle and of basalt magma type in the Japanese Islands indicates that different magmas are produced at different depths where the earthquakes are generated by stress release: tholeiite at depths around 100 km, high-alumina basalt at depths around 200 km, and alkali olivine basalt at depths greater than 250 km. 2) Primary olivine tholeiite magma is produced at a uniform level of the mantle (100–150 km), and on the oceanic side of the continental margin, it leaves the source region immediately after its production and forms magma reservoirs at shallow depths, perhaps in the crust, where it undergoes fractionation to produce SiO₂-oversaturated tholeiite magma, whereas on the continental side, the primary magma forms reservoirs near the source region and stays there long enough to be fractionated to produce alkali olivine basalt magma, and in the intermediate zone, the primary magma forms reservoirs at intermediate depths where it is fractionated to produce high-alumina basalt magma.     

The petrology, phase relations and tectonic setting of basalts from the taupo volcanic zone, New Zealand and the Kermadec Island arc - havre trough, SW Pacific

Volcanism in the Taupo Volcanic Zone (TVZ) and the Kermadec arc-Havre Trough (KAHT) is related to westward subduction of the Pacific Plate beneath the Indo-Australian Plate. The tectonic setting of the TVZ is continental whereas in KAHT it is oceanic and in these two settings the relative volumes of basalt differ markedly. In TVZ, basalts form a minor proportion (< 1%) of a dominant rhyolite (97%)-andesite association while in KAHT, basalts and basaltic andesites are the major rock types. Neither the convergence rate between the Pacific and Indo-Australian Plates nor the extension rates in the back-arc region or the dip of the Pacific Plate Wadati-Benioff zone differ appreciably between the oceanic and continental segments. The distance between the volcanic front and the axis of the back-arc basin decreases from the Kermadec arc to TVZ and the distance between trench and volcanic front increases from around 200 km in the Kermadec arc to 280 km in TVZ. These factors may prove significant in determining the extent to which arc and backarc volcanism in subduction settings are coupled.All basalts from the Kermadec arc are porphyritic (up to 60% phenocrysts) with assemblages generally dominated by plagioclase but with olivine, clinopyroxene and orthopyroxene. A single dredge sample from the Havre Trough back arc contains olivine and plagioclase microphenocrysts in glassy pillow rind and is mildly alkaline (< 1% normative nepheline) contrasting with the tholeiitic nature of the other basalts. Basalts from the TVZ contain phenocryst assemblages of olivine + plagioclase ± clinopyroxene; orthopyroxene phenocrysts occur only in the most evolved basalts and basaltic andesites from both TVZ and the Kermadec Arc.Sparsely porphyritic primitive compositions (Mg/(Mg+Fe²) > 70) are high in Al₂O₃ (>16.5%), and project in the olivine volume of the basalt tetrahedron. They contain olivine (Fo₈₇) phenocrysts and plagioclase (> An₆₀) microphenocrysts. These magmas have ratios of CaO/Al₂O₃, A1₂O₃/TiO₂ and CaO/TiO₂ in the range of MORB and MORB picrites and can evolve to the low-pressure MORB cotectic by crystallisation of olivine±plagiociase. Such rocks may be the parents of other magmas whose evolutionary pathways are complicated by interaction of crystal fractionation, crystal accumulation and mixing processes and the filtering action of crust of variable density and thickness. The interplay of these processes likely accounts for the scatter of data about the cotectic. More evolved rocks from both TVZ and KAHT contain clinopyroxene and orthopyroxene phenocrysts and their compositions merge with basaltic andesites and andesites. Stepwise least-squares modelling using phenocryst assemblages in proportions observed in the rocks suggest that crystal fractionation and accumulation processes can account for much of the diversity observed in the major-element compositions of all lavas.We conclude that the parental basaltic magmas for volcanism in the TVZ and KAHT segments are similar thereby implying grossly similar source mineralogy. We attribute the diversity to secondary processes influencing liquids as they ascended through complex plumbing systems in the sub arc mantle and cross.     

Potassium variation across the New Britain volcanic arc

Late Cainozoic volcanoes of the New Britain island arc overlie an inclined Benioff zone that extends to a depth of at least 580 km. The rocks are tholeiitic basalt, andesite, dacite, and rhyolite. Unlike many other examples of island arcs described in the literature, K₂O contents in rocks with the same SiO₂ content do not increase progressively as depth,h, to the New Britain Benioff zone increases. The most complex relationships between K₂O, SiO₂, andh are shown by volcanoes overlying the deeper part of the Benioff zone. In these, the K₂O contents of rocks containing more than about 60% SiO₂,decrease as depth to the Benioff zone increases. The New Britain volcanic arc provides a striking exception to the generalisation thatK-h relationships are essentially similar in all island arcs.     

The linear relation between temperatures based on the silica content of groundwater and regional heat flow: A new heat flow map of the United States

Application of the silica geothermometer to over 70,000 non-thermal groundwaters from the United States has shown that there is a correlation between the average silica geotemperatures for a region (T SiO₂ in °C) and the known regional heat flow (q in mW m^?2) of the form: 1 $$TSiO_2 = mq + b$$ wherem andb are constants determined to be 0.67°C m² mW^?1 and 13.2°C respectively. The physical significance of ‘b’ is the mean annual air temperature. The slope ‘m’ is related to the minimum average depth to which the groundwaters may circulate. This minimum depth is estimated to be between 1.4 and 2.0 km depending on the rock type. A preliminary heat flow map based on equation (1) is presented using theT SiO₂ for new estimates of regional heat flow where conventional data are lacking. Anomalously high localT SiO₂ values indicate potential geothermal areas.