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.     

Geochemistry of the andesite flank lavas of three composite cones within the Atitlán Cauldron,Guatemala

Three composite cones have grown on the southern edge of the previously existing Atitlán Cauldron, along the active volcanic axis of Guatemala. Lavas exposed on the flanks of these cones are generally calc-alkaline andesites, but their chemical compositions vary widely. Atitlán, the largest and most southerly of the three cones, has recently erupted mainly pyroclastic basaltic andesites, while the flanks of San Pedro and Tolimán are mantled by more silicic lava flows. On Tolimán, 74 different lava units have been mapped, forming the basis for sequential sampling. Rocks of all three cones are consistently higher in K₂O, Rb, Ba and REE than other Guatemalan andesites. Atitlán’s rocks and late lavas from Tolimán have high Al₂O₃ content, compared to similar andesites from other nearby cones. All major and trace element data on the rocks are shown to be consistent with crystal fractionation involving phases observed in the rocks. If such models are correct, significant differences in the relative proportions of fractionation phases are necessary to explain the varied compositions, in particular higher Al₂O₃ rocks have fractionated less plagioclase. We speculate that inhibition of plagioclase fractionation could occur in chambers where PH₂O is greater and when repose intervals are shorter. The distribution of volcanic vents throughout Guatemala which show this postulated «inhibition of plagioclase fractionation» is systematic with such vents lying just to the south of the main axis. The andesites of the three cones cannot be simply related to the late-Pleistocene rhyolites which are apparently associated with cauldron formation, because unlike the andesites, the rhyolites have markedly depleted heavy REE abundances. Recent dacitic lavas from vents south of San Pedro volcano and silicic pyroclastic rocks which mantle the slopes the San Pedro may reflect residual post-cauldron rhyolitic volcanism.     

The chemical composition of soil from the Apollo 16 and Luna 20 sites

The concentrations of the rare earth elements (REE), K, Rb, Sr, Ba, U, Zr and Cr for the Luna 20 soil and four different Apollo 16 soils are reported. These trace element abundances imply: (1) that the lunar highlands consist of a mixture of rocks rich in large ion lithophile (LIL) elements and LIL-element impoverished anorthosites; or (2) that the bulk of the aluminum-rich crust did not originate by upward segregation of plagioclase in a primitive liquid shell. The Luna 20 soil is distinguished from the Apollo 16 soil by lower aluminum and LIL element abundances.     

Rhyolitic ignimbrites in the region of Afyon (Central Anatolia)

Occurrence and field relations of an extensive ignimbrite sheet near Afyon in Central Anatolia are described. These rhyolitic ignimbrites are part of the important Neogene volcanic activity in Turkey and belong to the alignment of volcanic complexes along the inner border of the Taurian ranges. In close stratigraphical connection and in the same tectonic position as the rhyolites there occurs an assemblage of high-potassic, intermediate to basic volcanic rocks (alkali trachytes, mela-trachytes, latites and leucite-bearing rocks). Petrological and magmatological considerations led to the conclusion that the rhyolites and the potassic series, in spite of the close geological connection, are not related by processes of magmatic differentiation. Arguments in favour of an anatectic origin of the rhyolitic melt are presented. The occurrence of garnet and allanite as accessory minerals and as inclusions in the salic minerals of the ignimbrite are interpreted as relictic witnesses of a sialic parent rock. However, the trace elements, especially high Rb connected with low Sr, Ba and Zr and K/Rb ratios below 100 give a pattern generally explained by strong fractionation processes.     

Genesis of the Angra dos Reis and other achondritic meteorites

Major, minor and trace element abundances were determined in seven Angra dos Reis samples including whole rocks, fassaite (clinopyroxene), olivine and whitlockite separates via sequential instrumental neutron activation analysis. The chondritic normalized rare earth element (REE) abundance pattern for the Angra dos Reis clinopyroxene separates shows a concave downward shape with a small negative Eu anomaly. The strong fractionation between the light and the heavy REE in olivine separates could be attributed to the presence of islands of kirschsteinite in the olivines. The large-ion lithophile trace elements were highly enriched in the whitlockite separate as expected (e.g. La ≈ 370 ppm). The lower Hf and Sc abundances in whitlockite compared to that in the equilibrium “magma” could be the result of favorable partitioning of Hf and Sc in baddeleyite, which may have crystallized prior to or with whitlockite in the interstitial liquid. Comparison of whole rock with mineral separate data shows the presence of ～3% olivine, ～2.6% spinel and small amounts of metallic Ni-Fe and troilite in the whole rock.The trace element abundances in the derivative magma from which the Angra dos Reis clinopyroxene crystallized were estimated from the clinopyroxene data and the clinopyroxene mineral-liquid partition coefficients. From the derivative magma, the trace element abundances in the possible parent magmas were calculated by assuming that these parent magmas have undergone different degrees of clinopyroxene fractional crystallization to yield the Angra dos Reis derivative magma. Using the trace element abundances in these possible parent magmas, a two-stage crystal-liquid fractionation model with source material containing olivine, orthopyroxene and clinopyroxene is presented for the genesis of Angra dos Reis. Possible combinations of the degree of equilibrium non-modal partial melting, the source mineral composition and the initial element abundances required to generate possible Angra dos Reis parent magmas are calculated by the multilinear regression analysis method. Favorable solutions for this two-stage crystal-liquid fractionation model could be that Angra dos Reis crystallized at ～70% fractional crystallization of clinopyroxene from magmas generated by reasonable degrees of equilibrium partial melting (～7–10%) of deep-seated primitive source materials (olivine ～54–30%, orthopyroxene ～33–53%, and clinopyroxene ～13–17%) with trace element (Ba, Sr, REE and Sc) abundances ～3.5–4.7 × chondrites. These calculated REE abundances in the Angra dos Reis parent body are very similar to those suggested for the primordial moon (～3–5 × chondrites).Possible genetic relationships between Angra dos Reis and other achondrites, especially cumulate eucrites and nakhlites, are studied. Apparently, the unique Angra dos Reis could not be related to those achondrites by crystal-liquid fractionation of the same parent body.     

An outline of the geology and geochemistry,and the possible petrogenetic evolution of the volcanic rocks of the Tonga-Kermadec-New Zealand island arc

The Pleistocene-Recent volcanism of this arc extends nearly linearly NNE from northern New Zealand for some 2800 km. Along its western margin lies an active marginal basin (Lau Basin and Havre Trough) which has its southern termination in the Taupo volcanic zone (TVZ, New Zealand). The New Zealand arc segment is developed within a continental crust, whereas the Tonga-Kermadec segments are developed on a ridge system within the oceanic basin. Submarine morphology suggests that the Kermadec volcanoes represent a less advanced stage of evolution relative to those of Tonga.Magmas erupted within the TVZ are dominantly rhyolitic (≈16,000 km³) with subordinate andesites and rare high-alumina tholeiites and dacites. The Kermadec Islands are dominated by tholeiites and basaltic andesites, with subordinate andesites and dacites. The Tongan Islands are dominated by basaltic andesites, with locally developed andesites and dacites. These Tonga-Kermadec lavas are characterised by subcalcic groundmass clinopyroxenes, whereas the younger group of TVZ andesites contain groundmass hypersthene and augite.Geochemically, the TVZ andesites are systematically enriched (relative to those of Tonga-Kermadec) in “incompatible” elements (e.g. K, Rb, Cs, Ba, light REE, U, Th, Zr, Pb), are less Fe-enriched, and contain more radiogenic Sr and Pb (excepting certain ²⁰⁷Pb/²⁰⁴Pb compositions). The evidence points to crustal equilibration of the TVZ andesites prior to eruption.A complete overlap of major and trace element chemistry (including TiO₂) is observed between the Kermadec-TVZ tholeiites and basaltic andesites, and the ocean floor tholeiites of the Lau Basin. Compared to the Tongan lavas, those of the Kermadecs exhibit a greater degree of chemical variability, also reflected in the greater heterogeneity in their Pb isotopic compositions. Moreover, many of the Tonga-Kermadec basaltic andesites exhibit more depleted “incompatible” trace element abundances than the Kermadec and TVZ tholeiites.The “primary” magmas of this arc are interpreted to be of basaltic andesite type, derived from Benioff zone melting (essentially anhydrous), but extensively modified by low-pressure crystal fractionation processes. The Kermadec tholeiites are explained as products of relatively shallow upper mantle partial fusion induced during the earlier stages of diapiric rise of Benioff zone-derived magmas, which are sufficiently hot to intersect the peridotite solidus. This should result in the production and intermixing of a series of magmas extending from olivine tholeiite to basaltic andesite composition. The voluminous rhyolites of TVZ are interpreted as the products of crustal fusion involving Mesozoic sediments.     

South Aegean volcanic arc: Geochemical variations and geotectonic implications

The Aegean volcanic arc is one of the most important geological structure of the Mediterranean area. It is a belt of volcanic centers consisting of products ranging from basaltic, andesitic, dacitic to rhyolitic in composition, all of them displaying a typical calc-alkaline chemical character. The most abundant rock types are represented by andesites and dacites. Minor amounts of basalts and rhyolites occur mainly in the central-eastern sector of the arc. The REE, Rb, Sr, Ba, Th, Ta, Hf, Zr, Ni, Co, V and Cr abundances determined in 27 representative samples from different centers suggest that: 1) the intermediate and acidic terms are products of crystal/liquid fractionation processes starting from basic parent magmas: 2) large variations in incompatible elements occur in the most basic samples that are interpreted as evidence for heterogeneously LIL element-enriched mantle source; 3) plagioclase played a role in the evolution of the volcanic centers of the eastern and central arc different from that played in the volcanoes of the western sector. Along the arc, the differences in the distribution of lithological types, in the volumes of erupted material, in the volcanological characteristics of the different centers as well as in the patterns of trace element distribution in the volcanites are considered to be connected with the prevailing tectonic regime affecting the various sectors of the arc.     

New insights on the origin of troctolites from the breakaway area of the Godzilla Megamullion (Parece Vela back‐arc basin): The role of melt‐mantle interaction on the composition of the lower crust

The troctolites and olivine‐gabbros from the Dive 6 K‐1147 represent the most primitive gabbroic rocks collected at the Godzilla Megamullion, a giant oceanic core complex formed at an extinct spreading segment of the Parece Vela back‐arc basin (Philippine Sea). Previous investigations have shown that these rocks have textural and major elements mineral compositions consistent with a formation through multistage interaction between mantle‐derived melts and a pre‐existing ultramafic matrix. New investigations on trace element mineral compositions basically agree with this hypothesis. Clinopyroxenes and plagioclase have incompatible element signatures similar to that of typical‐MORB. However, the clinopyroxenes show very high Cr contents (similar to those of mantle clinopyroxene) and rim having sharply higher Zr/REE ratios with respect to the core. These features are in contrast with an evolution constrained by fractional crystallization processes, and suggest that the clinopyroxene compositions are controlled by melt‐rock interaction processes. The plagioclase anorthite versus clinopyroxene Mg#[Mg/(Mg + Fe^Tot)] correlation of the Dive 6 K‐1147 rocks shows a trend much steeper than those depicted by other oceanic gabbroic sections. Using a thermodynamic model, we show that this trend is reproducible by fractionation of melts assimilating 1 g of mantle peridotite per 1 °C of cooling. This model predicts the early crystallization of high Mg# clinopyroxene, consistent with our petrological observation. The melt‐peridotite interaction process produces Na‐rich melts causing the crystallization of plagioclase with low anorthite component, typically characterizing the evolved gabbros from Godzilla Megamullion.     

Normative composition and classification of lunar igneous rocks and glasses,I. Lunar igneous rocks

678 major element analyses and all available trace element determinations of lunar rocks with igneous textures were collected from the literature. Rittmann norms were calculated by an ALGOL program. The norm values, grouped according to increasing clinopyroxene contents, were plotted into quartz-plagioclase-orthopyroxene and olivine-plagioclase-orthopyroxene triangles, respectively. The plots indicate that all lunar rocks form a compositional continuum that starts from rocks very high in plagioclase and continues, with increasing clinopyroxene, to plagioclase-poorer and orthopyroxene-richer rocks containing partly quartz, partly olivine.According to apparent clusters in the plots, and taking into account lunar rock types defined by previous authors, the continuum of normative compositions was subdivided into five major rock groups (I to V). The averages of these groups can be characterized by clinopyroxene contents and plagioclase/orthopyroxene ratios (I: 3% cpx, plag/opx = 30; II: 4% cpx, plag/opx = 7; III: 8% cpx, plag/opx = 2; IV: 26% cpx, plag/opx = 0.8; V: 34% cpx, plag/opx = 1.8). According to the contents in K-feldspar, from groups III and V K-rich subgroups were separated. Average contents of major elements and trace elements were calculated for main groups and subgroups.For the normative groups of lunar igneous rocks, names are proposed which conform to the nomenclature of terrestrial rocks.     

Petrology and geochemistry of potassic rocks in the Gölcük area (Isparta, SW Turkey): genesis of enriched alkaline magmas

The Lower Pliocene volcanic rocks occurring in the Gölcük area of SW Turkey exhibit alkaline major element trends with a general potassic character. The development of volcanism can be divided into 2 major stages such as trachytic ancient lavas/domes and tephriphonolitic, trachyandesitic to trachytic Gölcük eruptions (ignimbrites, lava/dome extrusions, phreatomagmatic deposits, and finally, young domes). Volcanic rocks consist primarily of plagioclase, clinopyroxene (which ranges in composition from diopside to augite and are commonly zoned), biotite, and phlogopite. Amphibole phenocrysts are restricted to the pyroclastic deposits. Pseudoleucites are also seen only in the lava/dome extrusions. Oxides and apatites are common accessory phenocryst phases. As would be expected from their potassic–alkaline nature, the volcanic rocks of the Gölcük area contain high amounts of LILE (Ba, Sr, Rb and K), LREE, and Zr. Concentrations of compatible elements such as Cr, Ni and V are very low, possibly indicating fractionation of olivine and clinopyroxene. Correlation of SiO₂, Rb/Sr and MgO with ⁸⁷Sr/⁸⁶Sr (0.703506–0.704142) exhibit an increasing trend in the direction of crustal contamination. However, the isotopic compositions of Sr are not as high to indicate a high level of crustal contamination. Geochemical data are consistent with the derivation of Gölcük volcanic rocks from a metasomatized and/or enriched lithospheric mantle source during crustal extension in the area. This metasomatism was probably occurred by fluids released from the northward subduction between African and Eurasian plates during Tertiary, as the Gölcük volcanic rocks display features of island-arc magmas with having high Ba/Nb (>28) ratios, and Nb and Ti depletions. Lower Pliocene volcanism in the Gölcük was response to extensional tectonics.     

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.     

The gabbroic rocks of the Peninsular ranges batholith, Southern California: cumulate rocks associated with calc-alkalic basalts and andesites

Gabbroic rocks occur only in the west, and are the oldest intrusions in the Peninsular Ranges Cordilleran batholith. They comprise an olivine-pyroxene gabbronorite series and an amphibole gabbro series both of which contain abundant plagioclase and amphibole. They formed by crystal accumulation and in situ differentiation, in multiple intrusive complexes, and are not considered to be related by fractionation to the granitoid rocks of the batholith.Pure mineral separates of plagioclase, olivine, clinopyroxene, orthopyroxene, and amphibole were obtained by magnetic and heavy-liquid methods from a representative suite of gabbroic rocks. Their major- and trace-element contents were determined by X-ray fluorescence, and the data used to test hypotheses on the genesis and fractionation of the gabbros.The plagioclases range from An₉₈ to An₆₅ in composition, olivines, Fo₇₉ to Fo₇₀, occur in rocks where An>₃₆. All clinopyroxenes are augite with Mg #'s varying from 81.1 to 64.7. Orthopyroxene occurs where An<₉₂, and is generally inverted pigeonite or bronzite, and has Mg #'s ranging from 77.9 to 52.1. The amphiboles include tschermakite, tschermakitic hornblende, pargasite, pargasitic hornblende, ferroan pargasite, magnesio-hornblende, and magnesio-taramite, Mg #'s range from 80.4 to 62.5. Systematic chemical and mineralogical changes confirm that differentiation, controlled by mineral assemblages of plagioclase, olivine, spinel, and clinopyroxene initially, and orthopyroxene, amphibole, and magnetite later, took place between intrusive episodes and in situ.The highly clacic plagioclase coexisting with olivine and amphibole suggests that the gabbros were formed from hydrous mafic magmas. The modal mineralogy of the gabbros, and the chemistry of the minerals is very similar to that of the cumulate blocks of the Lesser Antillean volcanoes. These features confirm that the gabbros were derived from a hydrous mafic magma, with high Al₂O₃ and low TiO₂ contents, typical of orogenic environments.Cumulate minerals from the gabbros show little or no zoning and are considered to have formed in equilibrium with the evolving melts. Selected trace-element contents and distribution coefficients are used to calculate the compositions of the melts. The calculations show that the melts in equilibrium with the olivine-pyroxene gabbronorite series contain approximately 100–200 ppm Ba, 200–400 ppm Sr, 30-10 ppm Ni, 20-10 ppm Co, and 300-100 ppm V. K/Rb ratios of the melts, derived from post-cumulus and prismatic amphiboles, are generally in the range 550-250. These values are typical of calc-alkalic basalts and andesites, and it is suggested that they may have erupted at the surface to form a coeval calc-alkalic volcanic sequence.     

Fractionation paths of Atka (Aleutians) high-alumina basalts: Constraints from phase relations

An Aleutian high-alumina basalt from the island of Atka at one atmosphere crystallizes plagioclase (1275°C) followed by olivine (1170°C) and clinopyroxene (1115°C). At oxygen fugacities along NNO, magnetite crystallizes below 1070°C, but its liquidus increases to at least 1175°C at an oxygen fugacity two log units above NNO. Phase relations at two kilobars pressure of melts containing small amounts of water are similar, although orthopyroxene and magnetite are observed to follow clinopyroxene. Amphibole crystallizes at near-liquidus temperatures only at water contents of melts approaching 4.5%. Amphibole assumes the liquidus in melts containing 5% water.Anhydrous melts crystallize plagioclase to 19 kbar, where garnet and clinopyroxene assume the liquidus. Olivine yields to clinopyroxene as the highest-temperature subliquidus phase at about 9 kbar.The array of compositions of basaltic Atka rocks, as displayed on appropriate pseudoternary projections, can be interpreted as a crystal fractionation path at moderate pressure (8 kbar) and small melt-water contents. The interpreted fractionating minerals are olivine, clinopyroxene, plagioclase, and (probably) magnetite. (The actual phenocrysts in Atka basalts like AT-1, which lacks phenocrystic clinopyroxene, must have crystallized at pressure less than 8 kbar, however.) The compositions of two-pyroxene andesites from Atka can be interpreted to lie on a lower-pressure fractionation trend at melt water contents of 2–3%. Such water contents are consistent with the complete absence of amphibole in any Atka rocks and are suggestive that water contents of the basaltic magmas, if the basalts are parental to the andesites, were 1–2%.     

Bimodal tholeiitic and mildly alkalic basalts from Bhir area,central Deccan Volcanic Province,India: Geochemistry and petrogenesis

Bimodal tholeiitic and mildly alkalic basalts occur near Bhir, in the central part of Deccan Volcanic Province (DVP). Major and trace element concentrations show that, of the ten flows, nine are tholeiitic and one is an alkalic basalt. The Bhir basalts have a wide range of chemical composition. Geochemical variations in the stratigraphic section define three distinct phases of evolution (zones 1 to 3). Crystal fractionation of plagioclase, clinopyroxene, olivine and Fe–Ti oxide expanded the compositional range. Low Mg#s (39–55), low concentrations of Ni and Cr and high Zr suggest the evolved nature of the Bhir basalts. Fractionation modeling suggests about 42% fractional crystallization.In spite of the dominant role of fractional crystallization in the evolution of Bhir basalts, some other processes must be sought to explain the chemical variations. Crustal contamination, magma mixing and degree of partial melting are suggested to explain the observed chemical variations. Resorption, reverse zoning and compositional bimodality in plagioclase phenocrysts indicate magma mixing. Samples of flows one and four suspected of being contaminated all have enriched SiO₂ and LILE (K, Rb, and Ba) contents and depletion in Ti and P, believed to be due to ‘granitic’ crustal contamination.As compared to tholeiitic basalts, the alkalic basalts are characterized by low SiO₂ and high TiO₂, Na₂O, K₂O and P₂O₅. Alkalic basalts are richer in LILE (Rb and Ba), HFSE (Nb, Zr, and Y) and REE than the tholeiitic basalts. The alkalic basalt occurrence is important from a petrogenetic point of view and also suggests that the sources of alkalic basalt magmas may be of variable ages under different parts of the DVP. Based on major, trace and rare earth element distributions it is suggested that asthenospheric mantle having affinities with the source of OIB was the source material of the magmas and the range in the composition of tholeiitic and alkalic basalts was probably controlled by different degrees of melting and/or inhomogeneities in the mantle source.     

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

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.     

Rhyolite volcanism in the Krafla central volcano,north-east Iceland

At the Krafla central volcano in north-east Iceland, two main phases of rhyolite volcanism are identified. The earlier phase (last interglacial) is related to the formation of a caldera, whereas the second phase (last glacial) is related to the emplacement of a ring dike. Subsequently, only minor amounts of rhyolite have been erupted. The volcanic products of Krafla are volumetrically bimodal. Geochemically, there is a series of basaltic to basalto-andesitic rocks and a cluster of rhyolitic rocks. Rocks of intermediate to silicic composition (icelandites and dacites) show clear signs of mixing. The rhyolites are Fe-rich (tholeiitic), and aphyric to slightly porphyritic (plagioclase, augite, pigeonite, fayalitic olivine and magnetite). They are minimum melts on the quartz-plagioclase cotectic plane in the granite system (Qz-Or-Ab-An). The rhyolites at Krafla were produced by near-solidus, rather than nearliquidus fractionation. They are interpreted as silicic minimum melts of hydrothermally altered crust, mainly of basaltic composition. They were primarily generated on the peripheries of an active basaltic magma chamber or intrusive domain, where sufficient volumes of crust were subjected to temperatures favorable for rhyolite genesis (850–950° C). The silicic melts were extracted crystal-free from their source in response to crustal deformation.     

Geochemistry of calc-alkaline volcanics in northwestern Nigeria,and a possible Pan-African suture zone

Major and trace element characteristics of dacites and rhyolites overlying and intruding basement rocks in northwestern Nigeria most closely resemble those of intracontinental orogenic volcanic associations. REE patterns point to a deep-seated source for the magmas, perhaps involving garnet fractionation at mantle depths and low-pressure plagioclase fractionation. The occurrence of calc-alkaline volcanics, small basic-ultrabasic complexes and major transcurrent faulting, is consistent with the presence of a Pan-African suture zone in northwestern Nigeria.     

The Natash alkaline volcanic field,Egypt: geochemical and mineralogical inferences on the evolution of a basalt to rhyolite eruptive suite

Extensive lava flows were erupted during the Upper Cretaceous in the Wadi Natash of southern Egypt. The lavas are mainly of alkaline (sodium dominated) composition and include alkali olivine basalt (AOB), hawaiite, mugearite, and benmoreite that intruded with acidic volcanics of trachytic to rhyolitic composition. Abundances of major oxides and trace elements including the REE vary systematically through this compositional spectrum. The gradual decrease of CaO with decreasing MgO is consistent with the dominance of phenocrysts of labradoritic plagioclase (An_75–62) and Mg-rich olivine (Fo_84–80) in the AOB and hawaiite. Olivine phenocrysts are normally zoned with cores consistent with crystallization from a magma having the bulk-rock composition. The sharp decrease of alkalis at low MgO contents (∼0.4% MgO) indicates significant alkali feldspar fractionation during the evolution of trachytes and rhyolites. All Natash lavas show steep chondrite-normalized REE patterns with considerable LREE/HREE fractionation and a regular decrease in La/Lu ratios from the least to the most evolved lavas (La/Lu_n=12.5−9.5). The low absolute abundances of HREE in basic members reflects residual garnet in the source. The basic lavas have experienced compositional modifications after they segregated from the source as evidenced by lower averages of Mg^# (51), Ni (134) and Cr (229) in the AOB. Much of this variation can be explained by variable degrees of polybaric fractional crystallization. Petrographic and geochemical data supported by quantitative modelling suggest the evolution of the Natash Lavas from a common AOB parent in multiple, short-lived magma chambers. In agreement with the phenocryst mineralogy of the Natash lavas, the geochemical models suggest that with increasing degree of differentiation, Mg-rich olivine, calcic plagioclase, and augite are joined and progressively substituted by ferrohedenbergite, alkali feldspars and magnetite. The OIB (ocean island basalt)-like nature of the AOB and hawaiite lavas suggests that the volumetrically dominant source component is the asthenospheric mantle. A mantle-plume source is suggested for the Natash basaltic lavas, with the lavas being generated by partial melting of a garnet peridotite in the asthenosphere.     

Melting relations of a magnesian abyssal tholeiite and the origin of MORBs

Melting relations of a glassy magnesian olivine tholeiite from the FAMOUS area have been studied within the pressure range 1 atm to 15 kbar. From 1 atm to 10 kbar, olivine is the liquidus phase, followed by plagioclase and Ca-rich clinopyroxene. Above 10 kbar, Ca-rich clinopyroxene appears on the liquidus, followed by orthopyroxene and spinel. Near 10 kbar, olivine, orthopyroxene, clinopyroxene, spinel and plagioclase crystallize within 10°C of the liquidus. This indicates that a liquid of this magnesian olivine tholeiite composition could coexist with mantle peridotite at about 10 kbar. This result is in agreement with the geochemistry of Ni; the Ni concentration of the studied sample corresponds to the theoretical concentration in a primary magma [14,15].These data suggest that at least some magnesian mid-oceanic ridge basalts (MORBs) could be primary melts segregated from the mantle at depths near the transition zone between plagioclase lherzolite and spinel lherzolite (about 10 kbar). Based on this model, the residual mantle after extraction of MORBs should be lherzolite, not harzburgite.High-pressure (7–10 kbar) fractionation models involving olivine, plagioclase and clinopyroxene, which have been proposed by several workers (e.g. [36]) to explain the varieties of MORBs, were re-emphasized based on this melting study. The rare occurrence of clinopyroxene as a phenocryst phase in MORBs is explained by precipitation in a magma chamber at high pressure, or by dissolution of clinopyroxene formed earlier at high pressure.     

Petrochemistry and origin of pleistocene volcanic rocks from northern Taiwan

The Pleistocene volcanic rocks from northern Taiwan include the Tatun volcano group and the Chilung volcano group. Three rock types occur in this area: Tatun volcano group yield high-alumina basalt and andesites, whereas the chilung volcano group mainly consists of dacites. In addition, amphibole-rich nodules have also been found in different cruptive units of the former volcano group. Around seventy sample of various rock types have been conducted for geochemical studies, including analyses of major elements and trace elements such as Co, Cr, Cu, Li, Ni, Zn, Zr, V, Rb and Sr. Results of Al₂O₃, MnO, TiO₂ total alkali content, MgO/ΣFeO and K₂O/Na₂O ratios and AMF diagram indicate that these Pleistocene volcanic rocks belong to typical calalkaline rock series. Detailed study of the trace elements reveals that these volcanic rocks are closely correlated with rocks of continental margin type with respect to Rb, Cu, Co, Ni, V and Cr contents, and K/Rb and Ni/Co ratios. These rocks are most probably derived from the fractionation of basaltic magma controlled mainly by the crystallization of amphibole and plagioclase with magnetite playing a minor role.