The petrology of the Las Canadas volcanoes,Tenerife, Canary islands

Tenerife is the largest of the seven Tertiary to Recent volcanic islands that make up the Canary Archipelago. The island is composed of volcanics belonging to the basanitetrachyte-phonolite assemblage that characterises many Atlantic islands. The most voluminous development of intermediate and salic volcanics has been in the centre of the island where the Las Canadas volcanoes arose upon a basement shield composed mainly of basanite and ankaramite flows, tuffs and agglomerates. The initial post-shield activity built the Vilaflor volcanic complex (Lower and Upper Canadas Series) that originally covered much of the underlying shield volcanics. A vast collapse of the complex, probably during post-Pleistocene times, in the centre of the island has left a large semi-circular wall, and provides an excellent vertical section through the complex. Quaternary volcanism within the collapsed area has built the twin, central-type volcanoes, Viejo and Teide, both of which have attendant satellite vents. That part of the Vilaflor Complex exposed in Las Canadas, together with the Viejo and Teide volcanoes, comprise the Las Canadas volcanoes.Four distinct rock types can be recognised in these volcanoes, basanite, trachybasanite, plagioclase phonolite, and phonolite. Each rock type can be recognised chemically and mineralogically, but there is essentially a gradational series from basanite to phonolite that includes both aphyric and glomerophyric rocks. The volcanics are strongly undersaturated and sodic, and some of the phonolites are mildly peralkaline. Variations in degree of undersaturation, and trace element abundances indicate a number of cycles of activity which would be consistent with the known field relations.Forsteritic olivine occurs in the basanites and trachybasanites but is not a stable phase in the more salic volcanics. Clinopyroxene is ubiquitous, varying in composition from titanaugite in the basanites to slightly sodic augite in the phonolites. Strongly sodic pyroxene is restricted to the groundmass of the microcrystalline phonolites along with aenigmatite and a kataphoritic amphibole. Plagioclase is found only in the groundmass of the basanites, but andesine and potash-oligoclase are common phenocryst minerals in the trachybasanites and plagioclase phonolites respectively, whereas the characteristic feldspar of the phonolites is anorthoclase.The relatively smooth curves of major and trace element variation, the presence of accumulative volcanics at all stages of differentiation, zoning of the mineral phases, and the clustering of the phonolites around the low temperature trough in Petrogeny's Residua System, all indicate that the descent from basanite to phonolite has resulted from fractional crystallisation of a basanite parent magma. The trend of pyroxene crystallisation, and the fairly constant FeO/Fe₂O₃ ratio during fractionation indicate crystallisation under low PO₂ conditions.     

Petrology of volcanic rocks from Kaula Island,Hawaii

A compositionally diverse suite of volcanic rocks, including tholeiites, phonolites, basanites and nephelinites, occurs as accidental blocks in the palagonitic tuff of Kaula Island. The Kaula phonolites are the only documented phonolites from the Hawaiian Ridge. Among the accidental blocks, only the phonolites and a plagioclase basanite were amenable to K-Ar age dating. They yielded ages of 4.0–4.2 Ma and 1.8±0.2 Ma, respectively. Crystal fractionation modeling of major and trace element data indicates that the phonolites could be derived from a plagioclase basanite by subtraction of 27% clinopyroxene, 21% plagioclase, 16% anorthoclase, 14% olivine, 4% titanomagnetite and 1% apatite, leaving a 16% derivative liquid. The nephelinites contain the same phenocryst, xenocryst and xenolith assemblages as the tuff. Thus, they are probably comagmatic. The strong chemical similarity of the Kaula nephelinites and basanites to those from the post-erosional stage Honolulu Group on Oahu, the presence of garnet-bearing pyroxenites in the Kaula nephelinites (which previously, had only been reported in the Honolulu volcanic rocks) and the similar age of the Kaula basanite to post-erosional lavas from nearby volcanoes are compelling evidence that the Kaula basanites and nephelinites were formed during a post-erosional stage of volcanism.Now at Occidental Petroleum, Bakersfield, CA, 93309, USA     

Geochemistry of highly-undersaturated ocean island basalt suites from the South Atlantic Ocean: Fernando de Noronha and Trindade islands

The volcanic rocks of the South Atlantic Ocean islands of Fernando de Noronha and Trindade comprise a diverse magmatic series ranging from nephelinites and basanites to phonolites and, on Fernando de Noronha, trachytes. All rock types are highly silica undersaturated with the exception of Fernando de Noronha trachytes_, and have high abundances of incompatible trace elements and strongly LREE (light rare earth element)-enriched REE patterns. Crystal fractionation of parental basanitic magmas produced evolved phonolites and trachytes which display severe trace-element fractionation, even among trace elements (Nb, Ta, Zr, Hf) which normally behave highly incompatibly during crystallisation of alkaline magmas. Moderately to highly evolved compositions develop strongly MREE (middle REE)-depleted REE patterns, and become increasingly depleted in elements such as Nb and, in particular, Ta. Ratios of Nb/Ta and Zr/Hf are highly fractionated in phonolites (60–65, 64–77 respectively in Fernando de Noronha phonolites) compared to ratios in basanites (14, 45 respectively). The compatibility of Nb, Ta, and the REE, and the strong fractionation of Nb/Ta and Zr/Hf ratios and the MREE, during crystallisation from basanite to phonolite are attributable to the crystallisation of small amounts (<5%) of sphene. Trace-element behaviour is relatively insensitive to the major phenocryst phases, and is controlled by minor phases in highly undersaturated alkaline suites. Incompatible trace-element ratios (e.g. La/Nb, Th/Ta) in nephelinites and basanites from Fernando de Noronha and Trindade are generally comparable with those in basaltic and hawaiitic OIB (ocean island basalt) lavas from other South Atlantic islands, but are distinct from those of Gough and Tristan da Cunha OIB. The mantle source for the highly undersaturated volcanism on Fernando de Noronha and Trindade is similar in trace-element characteristics to the typical OIB source which produces alkaline lavas with significant relative enrichment in Nb and Ta compared to other trace elements (as expressed by low La/Nb, Ba/Nb and Th/Ta ratios). The highly undersaturated nature of the magmas and the slight fractionation of some incompatible-element ratios (elevated Ba/Nb, Ba/Rb, Ba/Th etc.) is consistent with a smaller degree of melting of a typical OIB source, but with residual phlogopite in the source to account for significant K depletion and LIL-element fractionation.     

Sodium betpakdalite and conditions of its formation

Because orthopyroxenes do not occur in nephelinites, the widely used methods of calculating f(O₂) based on the olivine + orthopyroxene + spinel paragenesis are not applicable in this case. The authors present new methods of calculation, three of which are based on published data detailing exchange reactions between two pyroxenes and the melt. The activities of MgSiO₂ and FeSiO₂ are calculated from the composition of the groundmass of effusives in equilibrium with clinopyroxenes, making it possible (considering the activities of olivine and titanomagnetite components of the rocks) to normalize f(O₂) with regard to the quartz-fayalite-magnetite (QFM) buffer. In another version, the activities of enstatite are calculated considering the calcium content in rock olivines, using experimental data on the equilibrium of olivine with the two pyroxenes on which the well-known olivine geobarometer is based (Koehler and Brey, 1990). Still another method involves calculation of aFe.0 using the composition of the groundmass of nephelinites on the basis of equilibria of silicate melts with metallic iron (Ariskin et al., 1992), which then, in conjunction with magnetite, yields f (O₂).

The five methods of estimating f (O₂) by use of different sets of experimental data and thermodynamic constants for various solid phases yield a maximum spread that does not exceed 0.8 logarithmic units. The average value of log₁₀ f(O₂) for phenocryst and microphenocryst associations of nephelinites, obtained using all five methods, ranged from +1.6 to +1.8 above the level of the QFM buffer. These estimates support the conclusion that the mantle is in a relatively oxidized state in regions of intra-plate oceanic islands (Amundsen and Neumann, 1992). Coexisting microlites of titanomagnetite and ilmenite from the nephelinite groundmass point to an appreciably lower relative f(O₂) (below the QFM), which evidently is explained by a drop in the redox potential under the conditions of a relatively closed system, with intensive deposition of titanomagnetite.

The phenocryst associations of phonolites yield f(O₂) values normalized with respect to the QFM buffer that are close to analogous values for the phenocrysts and microphenocrysts of nephelinites (on average, 1.5 logarithmic units above the QFM on the basis of constants of the reaction between the components of titanomagnetite, clinopyroxene, nepheline, K-feldspar, and sphene). In all probability, differentiation in the magma chamber that led to the appearance of phonolite magmas in late stages occurred in a system that was open (with regard to oxygen). In this case, more intensive removal of magnetite resulted in silica activities in the residual magmas that were higher than in a closed system (phenocrysts of sphene are present in phonolites, but perovskite is observed in some cases in the groundmass of nephelinites, with sphene being absent).     

Petrologic variations in quaternary volcanic rocks,British Columbia,and the nature of the underlying upper mantle

Volcanic activity has produced Late Tertiary and Quaternary cinder cones and flows between the Snake River Plain, U.S.A. and the Yukon Territory, Canada. The rock types include basanites, alkali olivine basalts, high-iron basalts, hawaiites, ankaramites, nephelinites, and olivine tholeiites. The alkali olivine basalts, basanites and hawaiites sampled are chemically similar to rocks from the mid-Atlantic islands. Associated with the volcanic rocks are xenoliths of ultramafic rocks, gabbros, granites and granulites.Seismic data indicate that the Moho throughout the region dips eastward at a very shallow angle. The low velocity zone has been located beneath southern British Columbia and displays a topographic high trending northwest-southeast. The nephelinite was erupted from near the crest of this high with less undersaturated lavas erupted from along its flanks.The suite of ultramafic xenoliths spans a greater variety of rock types than can be generated by maximum amounts of partial melting of a uniform source material to produce the lavas in the region. Calculated residual olivine compositions in equilibrium with the magmas at low velocity zone depths and liquidii temperatures are more iron-rich than the typical lherzolite xenolith olivine. This suggests that the residua from the partial melting episodes which produced the volcanic rocks are different from the upper mantle lid above the low velocity zone as represented by the ultramafic xenoliths.     

Lamprophyres of the Tomtor Massif: A result of mixing between potassic and sodic alkaline mafic magmas

The paper presents data on inclusions in minerals of the least modified potassic lamprophyres in a series of strongly carbonatized potassic alkaline ultramafic porphyritic rocks. The rocks consist of diopside, kaersutite, analcime, apatite, and rare phlogopite and titanite phenocrysts and a groundmass, which is made up, along with these minerals, of potassic feldspar and calcite. The diopside and kaersutite phenocrysts display unsystematic multiple zoning. Chemically and mineralogically, the rock is ultramafic foidite and most likely corresponds to monchiquite. Primary and secondary melt inclusions were found in diopside, kaersutite, apatite, and titanite phenocrysts and are classified into three types: sodic silicate inclusions with analcime, potassic silicate inclusions with potassic feldspar, and carbonate inclusions, which are dominated by calcite. Heating and homogenization of the inclusions show that the potassic lamprophyres crystallized from a heterogeneous magma, with consisted of mixing mafic sodic and potassic alkaline magmas enriched in a carbonatite component. The composition of the magmas was close to nepheline and leucite melanephelinite. The minerals crystallized at 1150–1090°C from the sodic melts and at 1200–1250°C from the potassic ones. The sodic mafic melts were richer in Fe than the potassic ones, were the richest in Al, Mn, SO₃, Cl, and H₂O and poorer in Ti and P. The potassic mafic melts were not lamproitic, as follows from the presence of albite in the crystallized primary potassic melt inclusions. The diopside, the first mineral to crystallize in the rock, started to crystallize in the magmatic chamber from sodic mafic melt and ended to crystallize from mixed sodic–potassic melts. The potassic mafic melts were multiply replenished in the chamber in relation to tectonic motions. The ascent of the melts to the surface and rapidly varying P–T parameters of the magma were favorable for multiple separations of carbonatite melts from the alkaline mafic ones and their mixing and mingling.     

A basalt-trachyte-phonolite series from Ua Pu,Marquesas Islands,Pacific Ocean

The paper describes a suite of volcanic and intrusive rocks from Ua Pu, one of the Marquesas Islands, situated in the central Pacific Ocean. The rocks comprise alkali olivine basalts, hawaiites, mugearites, trachytes, and phonolites. Their petrographic characters are briefly described and 24 new chemical analyses presented. The rocks fall into a sodic and a potassic series, since some rocks are richer in K₂O than Na₂O, whilst the phonolites have exceptionally high total alkali contents.The differentiation of the series is discussed and special attention is paid to the phonolites in which icositetrahedral pseudomorphs occur. It is concluded that these pseudomorphs are probably after analcime rather than leucite.     

Nephelinitic and Basanitic Rocks

Basanites (which mineralogically should be characterized byan abundance of plagioclase and only a little feldspathoid)and nephelinitic rocks (which normally lack modal plagioclasebut should contain nepheline) are not easily distinguished inthe field, and both are commonly confused with basalts. Sincethe mineralogical distinction sometimes cannot be applied, asin the case when no modal felsic minerals are apparent in thegroundmass, a chemical distinction between nephelinitic andbasanitic rocks becomes desirable. The best solution is providedby the use of the CIPW norm with basanites recognized as having> 5% normative ab and < 20% normative ne, melanephelinitesas having < 5% normative ab and < 20% normative ne, andnephelinites as having > 20% normative ne. It is proposedthat melanephelinites defined in this manner can be furtherdivided and that rocks formerly termed olivine nephelinite shouldnow be called olivine melanephelinite, and that the pyroxene-phyric,olivine-poor melanephelinites should now be termed pyroxenemelanephelinite.     

Alkaline rocks and carbonatites of Amba Dongar and adjacent areas,Deccan Igneous Province,Gujarat, India: 1. Geology,petrography and petrochemistry

Summary Rift-related, late Eocene ( 60 Ma) alkaline-carbonatitec intrusions cover 1200 km2 south of the town of Chhota Udaipur, and form a subprovince within the alkaline magmatism that accompanies the tholeiitic Deccan Traps. They were emplaced temporally between late Deccan Trap flows and late dykes of basalt and picritic basalt. The subprovince comprises five main geographic occurrences (sectors): (1)Amba Dongar: a ring-complex of Ca-Mg-Fe-carbonatites, nephelinites to tephriphonolites, and fluorite deposits; (2)Siriwasan-Dugdha: an intrusive complex of Ca—carbonatites, nephelinites and trachytic rocks; (3)Phenai Mata: a nepheline syenite plug and dykes, plus dykes of tephrites to phonolites and lamprophyres (intimately associated with a layered tholeiitic gabbro—granophyre intrusion); (4)Panwad-Kawant: dykes and plugs of lamprophyres and tephrites to phonolites; (5)Bakhatgarh-Phulmahal: late basic-ultrabasic dykes only. The alkaline rocks range from ultrasodic to ultrapotassic, but are mostly nonperalkaline. Silica-undersaturated examples show higher incompatible and LIL element contents (Rb, K, Nb, Zr, Sr, Ba, LREE, etc.) than the associated tholeiites. However, the late basic-ultrabasic dykes display an continuum of alkaline-tholeiitic compositions. Possible parent magmas are represented among the primitive undersaturated basic dykes (including lamprophyres). The trachytic rocks are subalkaline, and may be genetically related to a tholeiitic rather than alkaline parent magma.

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Alkaligesteine und Karbonatite von Amba Dongar und Umgebung, Dekkan-Provinz, Gujarat, Indien: 1. Geologie, Petrographie und Gesteinschemie

Zusammenfassung Alkalisch-karbonatitische Intrusionen aus dem Obereozän (60 Ma) erstrecken sich über 1200 km² südlich der indischen Stadt Chhota Udaipur und bilden einen Teil des alkalischen Komplexes, der die tholeiitischen Deccan Traps begleitet. Die Gesteine wurden während eines Rifting Prozesses intrudiert und lagern stratigraphisch zwischen Deccan Trap Tholeiiten und basaltischen bzw. pikritischen Ganggesteinen. Das Gebiet läßt sich in fünf geographische Regionen unterteilen: (1) Amba Dongar: ein ringförmiger Komplex aus Ca-Mg-Fe-reichen Karbonatiten, Nepheliniten und Tephriphonoliten mit Fluorit-Lagerstätten; (2) Siriwasan-Dugdha: eine Ca-reiche Karbonatit-Intrusion mit Nepheliniten und Trachyten; (3) Phenai Mata: Eine Nephelin-syenitische Intrusion und begleitende Ganggesteine aus Tephriten, Phonoliten und Lamprophyren, die mit einer magmatisch geschichteten tholeiitischen Gabbro Intrusion vergesellschaftet sind; (4) Panwad-Kawant: Lamprophyrische Ganggesteine neben Tephriten und Phonoliten; (5) Bakhatgarh-Phulmahal: späte basisch bis ultrabasische Ganggesteine. Die alkalischen Gesteine besitzen eine ultrapotassische bis extrem Na-reiche Zusammensetzung, nur vereinzelte Proben weisen jedoch peralkalischen Charakter auf. Silizium-untersättigte (alkalische) Proben besitzen in der Regel höhere Konzentrationen an Mantel-inkompatiblen Elementen (z.B. LREE, Nb und Zr) und LILE (z.B. Rb, Sr und Ba) als die benachbarten Tholeiite. Die späten basisch bis ultrabasischen Ganggesteine stellen ein Kontinuum von alkalischer bis tholeiitischer Geochemie dar. Die primitiven Silizium-untersättigten Ganggesteine (und Lamprophyre) repräsentieren vermutlich die Ausgangssehmelzen; die subalkalischen Trachyte dürften in genetischem Zusammenhang mit den Tholeiiten stehen.

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With 8 Figures     

Potassic dyke swarm in the Sapucai Graben, eastern Paraguay: petrographical, mineralogical and geochemical outlines

The western side of the Paranà Basin of Brazil extends to central Paraguay, where repeated and widespread magmatic activity developed from Lower Cretaceous to Oligocene, associated with late Mesozoic crustal extension trending NE-SW. In central Paraguay this trend is characterized by a zone of NW-SE normal faults which formed the Asunciòn-Sapucai graben, up to 45 km wide and 200 km long, where alkaline rocks occur as volcanic domes, complexes, lava-flows and dykes. These rocks, 128 Ma aged, are dominantly potassic and ne-normative.

A swarm of at least 200, mainley NW-SE trending, dykes occurs in the Sapucai region and seems to be formed by two main lineages: tephrite to phonolite (and peralkaline phonolite) and alkali basalt to trachyphonolite. They are characterized by ubiquitous diopside to ferrosalite, consistently yielding Al enrichment trends; common olivine, Fo_81-69 in tephrites and alkaline basalts, and up to Fo₆₅ in phonolites; zoned megacrysts of hastingsitic hornblende (core) to kaersutite (rim), associated with accessory groundmass pargasite in tephrites and phonotephrites; K-rich hastingsite and K-rich ferroan pargasite in the phonolites. Accessory groundmass mica falls in the annite-phlogopite range, and consistently yields insufficient (Si + Al) to satisfy the expected T site occupancy of 8.00 a.f.u. Fe---Ti oxides are Ti-magnetite, rarely ilmenite or haematite. Phenocrystal, i.e. xenocrystal plagioclase is An_70-20, and An_74-42 in the tephrites and phonolites, respectively; coexisting groundmass microlites are An_22-14, associated with sodasanidine and sanidine. Feldspathoids include analcimized leucite and nepheline; accessories Ti-andradite and sphene.

The two main lineages, recognized by distinctive mineralogical variations, are consistent with the petrochemical variations. Complex interaction of discrete and independently evolving magma batches are indicated by intra- and/or interphase chemical variations, suggesting multiple equilibrations of the crystallizing phases under shallow level, volcanic pressure regime. The observed geochemical trends are quite similar to those of “Roman Region type magma” with the same negative anomalies of Ta, Nb, Zr and Ti. The most likely mantle source is a garnet-peridotite characterized by different enrichment in incompatible elements and which suffered low degree of partial melting (3–7%), which has geochemical and isotopic features distinct from those of the adjoining tholeiitic basalts (130 Ma) and nephelinites (61-39 Ma).

The similarities of the Sapucai dyke suite with Barton's “Roman Region type magma” supports the view that this magma type may not be formed as a result of orogenic and/or subduction-driven activity in this region. Therefore, a causal relationship of the latter activity with “Roman Region type magma” is not supported and remains questionable.     

Geochemistry and petrogenesis of sodic and potassic mafic alkaline rocks in the Deccan Volcanic Province, Mumbai Area (India)

Summary Major element, trace element, Sr- and Nd-isotopes and mineral chemical data are reported for alkaline rocks (lamprophyres, tephrites, melanephelinites, nephelinites and nepheline syenites) cross-cutting the Deccan Trap lava flows south (Murud-Janjira area) and north of Mumbai (Bassein). These rocks range from sodic to potassic and have a large span in MgO (12–2 wt%). The lamprophyres have high content of incompatible elements (e.g., TiO₂ > 3.8 wt%, Nb > 130 ppm, Zr > 380 ppm, Ba > 1200 ppm), and relatively high initial (at 65 Ma) ¹⁴³Nd/¹⁴⁴Nd (0.5128) and low ⁸⁷Sr/⁸⁶Sr (0.7038–0.7042). They are likely to be small-degree melts (2–3%) of volatile- and incompatible element-enriched mantle sources, similar to other alkaline rocks in the northern Deccan, though slightly more potassium-rich. The nepheline-rich rocks have highly porphyritic textures (up to 57% phenocrysts of diopside ± olivine), and anomalously low contents of incompatible elements (e.g., TiO₂ < 1.3 wt%, Nb < 24 ppm, Zr < 100 ppm) indicating that they could not represent liquid compositions. Moreover, their very low initial ¹⁴³Nd/¹⁴⁴Nd ratios (0.5116–0.5120), at ⁸⁷Sr/⁸⁶Sr = 0.7045–0.7049, are unusual in the rocks related to the Deccan Traps and identify a new end-member in this province, that could be identified as “Lewisian-type” lower crust and/or enriched mantle. The melting episode that generated these alkaline rocks likely occurred close to the base of the ca. 100 km-thick Indian lithosphere, very shortly after the main eruption of the Deccan tholeiites. Received January 14, 2000; revised version accepted September 28, 2001     

The lamprophyres of Afyon stratovolcano,western Anatolia,Turkey: description and genesis

The Afyon stratovolcano exhibits lamprophyric rocks, emplaced as hydrovolcanic products, aphanitic lava flows and dyke intrusions, during the final stages of volcanic activity. Most of the Afyon volcanics belong to the silica-saturated alkaline suite, as potassic trachyandesites and trachytes, while the products of the latest activity are lamproitic lamprophyres (jumillite, orendite, verite, fitztroyite) and alkaline lamprophyres (campto-sannaite, sannaite, hyalo-monchiquite, analcime–monchiquite). Afyon lamprophyres exhibit LILE and Zr enrichments, related to mantle metasomatism.     

Mantle-derived mafic-ultramafic xenoliths and the nature of Indian sub-continental lithosphere

Mantle derived xenoliths in India are known to occur in the Proterozoic ultrapotassic rocks like kimberlites from Dharwar and Bastar craton and Mesozoic alkali igneous rocks like lamrophyres, nephelinites and basanites. The xenoliths in kimberlites are represented by garnet harzburgites, lherzolites, wehrlite, olivine clinopyroxenites and kyaniteeclogite varieties. The PT conditions estimated for xenoliths from the Dharwar craton suggest that the lithosphere was at least 185 km thick during the Mid-Proterozoic period. The ultrabasic and eclogite xenoliths have been derived from depths of 100–180 km and 75–150 km respectively. The Kalyandurg and Brahmanpalle clusters have sampled the typical Archaean subcontinental lithospheric mantle (SCLM) with a low geotherm (35 mW/m²) and harzburgitic to lherzolitic rocks with median X_mg ^olivine > 0.93. The base of the depleted lithosphere at 185–195 km depth is marked by a 10–15 km layer of strongly metasomatised peridotites (X_mg ^olivine > ∼0.88). The Anampalle and Wajrakarur clusters 60 km to the NW show a distinctly different SCLM; it has a higher geotherm (37.5 to 40 mW/m²) and contains few subcalcic harzburgites, and has a median X_mg ^olivine = 0.925. In contrast, the kimberlites of the Uravakonda and WK-7 clusters sampled quite fertile (median X_mg ^olivine ∼0.915) SCLM with an elevated geotherm (> 40 mW/m²). The lamrophyres, basanites and melanephelinites associated with the Deccan Volcanic Province entrain both ultramafic and mafic xenoliths. The ultramafic group is represented by (i) spinel lherzolites, harzburgites, and (ii) pyroxenites. Single pyroxene granulite and two pyroxene granulites constitutes the mafic group. Temperature estimates for the West Coast xenoliths indicate equilibration temperatures of 500–900°C while the pressure estimates vary between 6–11 kbar corresponding to depths of 20–35 km. This elevated geotherm implies that the region is characterized by abnormally high heat flow, which is also supported by the presence of linear array of hot springs along the West Coast. Spinel peridotite xenoliths entrained in the basanites and melanephelinites from the Kutch show low equilibrium temperatures (884–972°C). The estimated pressures obtained on the basis of the absence of both plagioclase and garnet in the xenoliths and by referring the temperatures to the West Coast geotherm is ∼ 15 kbar (40–45 km depth). The minimum heat flow of 60 to 70 mW/m² has been computed for the Kutch xenolith (Bhujia hill), which is closely comparable to the oceanic geotherm. Xenolith studies from the West Coast and Kutch indicate that the SCLM beneath is strongly metasomatised although the style of metasomatism is different from that below the Dharwar Craton.     

First Evidence of Lamprophyric Magmatism from the Konya Region,Turkey: a Genetic Link to High-K Volcanism

In the vicinity of Konya （Turkey）,mafic,micro-porphyritic sub-volcanic rocks intrude into the Mesozoic units,which represents the only example of such a rock type in the region.40Ar/39Ar dating of two whole rock samples from the sub-volcanics gave ages of 13.72±0.13 and 12.40±0.11 Ma,suggesting temporal association to the Late Miocene-Pliocene high-K calc-alkaline volcanism in the region.The mineral chemistry and geochemical data permit us to classify the rocks as ＂minette＂ lamprophyres.They include diopside and phlogopite phenocrysts in a microcrystalline groundmass composed of sanidine,phlogopite,diopside and titano-magnetite.Segregation and ocelli-like globular structures occur commonly in the samples.In terms of major elements,the lamprophyres are calcalkaline,and potassic to ultrapotassic rocks.All the lamprophyres display strong enrichments in LILE （Rb,Ba,K,Sr）,radiogenic elements （Th,U） and LREE （La,Ce） and prominent negative Nb,Ta,and Ti anomalies on primordial mantle-normalized trace element diagrams.Geochemical data suggest that the lamprophyres and high-K calc-alkaline rocks in the region derived from a subduction-modified lithospheric mantle source affected by different metasomatic events.Lamprophyric magmatism sourced phlogopite-bearing veins generated by sediment-related metasomatism via subduction,but high-K calc-alkaline magmas are possibly derived from a mantle source affected by fluid-rich metasomatism.     

Composition and origin of Deccan basalts

The Deccan basalts are essentially composed of saturated tholeiitic lavas. However, undersaturated basalts, nephelinites, carbonatites, intermediate and acid differentiates have also been encountered in parts of western India (Upper Traps). Such unusual rocks are broadly aligned in two major rift zones in western India, the Narbada-Son and Cambay grabens and the faulted west coast. These rocks are younger than the earliest tholeiitic eruptions of central India (Lower Traps), but there are evidences of renewed eruptions of tholeiitic basalts in parts of western India. The earliest eruptions of Deccan basalts of quartz tholeiite composition have been derived by high degrees of partial melting of peridotite at moderate depth (37–41 km). The undersaturated basalts and nephelinites were possibly generated by low degrees of partial melting of garnet peridotite in the low velocity zone along the two tectonically active belts (rifts) in western India. The undersaturated basalts were formed prior to the break-up of Gondwanaland, when a concentration of pressure developed at the tectonically weak zones. The renewed eruptions of saturated tholeiites are thought to be post-tectonic, resulting from the release of stress when the tectonic events had ceased.     

Cretaceous alkaline lamprophyres from northeastern Czech Republic: geochemistry and petrogenesis

The northeast part of the Czech Republic (Moravia) and the adjoining part of Poland host a 100-km-long and 15- to 25-km-wide belt containing numerous isolated bodies (mainly sills) of lamprophyre of Lower Cretaceous age. The lamprophyres range from mafic (melanocratic) to evolved, feldspar-rich (mesocratic) variants. Mineralogically, they are characterized by compositionally zoned kaersutite phenocrysts, biotite and high Al–Ti salitic to diopsidic clinopyroxene. The lamprophyres are typically alkaline as shown by high contents of P₂O₅, TiO₂, alkalies and incompatible trace elements such as light REE, Zr, Nb, Y, Ba and Sr, and by high Ti/V (>50) and chondrite-normalized (La/Yb)_n (8–25) ratios. They resemble alkali basalts, basanites and nephelinites. Major element composition and trace element patterns and Nd–Sr isotopic values (εNd ca. +5.5 to +6.6 and εSr ca. –9.5 to –24.0) indicate that the lamprophyric magma was derived from a mantle source that was compositionally similar to the source of ocean island basalts with HIMU affinities and some continental extension-related alkali basaltic suites. The lamprophyres do not show any subduction imprint. They were generated in the garnet stability field by a variable degree of melting. Evolved lamprophyres were formed by clinopyroxene-dominated fractional crystallization of mafic lamprophyric magma. The lamprophyres are interpreted to have been emplaced along conduits formed during the formation of a basin/graben structure in the Early Cretaceous.     

Characteristics of alkali magma differentiation at the Cape Verde Islands

Parental magmas of the Cape Verde Islands are high-Mg foidites, picrobasalts, and basanites. The rocks can be classified into the following two differentiated series: (1) a high-alkali series, which includes picrites, high-Mg foidites, low-Mg-foidites, and phonolites; and (2) a series of moderate alkalinity, which includes picrobasalts, basanites, tephrites, tephrophonolites, phonotephrites, phonolites, and trachytes.The differentiation of both series is associated with a decrease in the concentrations of Mg, Fe, Ti, and Ca and an increase in the contents of Al, Si, K, and Na. Rare lithophile elements (REE, Zr, Nb, Th, Rb, and U) progressively enrich younger derivatives. The Sr and Ba concentrations pass through a maximum, as is typical of alkaline series, which are characterized by broad crystallization fields of plagioclase and melilite, minerals that have high distribution coefficients of these elements. The analysis of the composition of volcanic rocks from the Cape Verde Islands within the scope of the system olivine-diopside-nepheline shows that the evolution of the parental magmas was controlled by crystallization differentiation in shallow-depth intermediate chambers. This conclusion finds further support in data on inclusions in minerals and a simulated crystallization model in a magmatic chamber.     

Spatial and temporal geochemical variability in basin-related volcanism, northern Israel

Middle Miocene (17-9 Ma) volcanism in northeast Israel migrated from eastern Lower Galilee (Poria, west of the Sea of Galilee) to the southwest (Yizre'el Valley) in association with the development of an extensional basin in that area. The Yizre'el Valley magmas are highly undersaturated in silica (basanites and nephelinites), while those of Poria are alkali-basaltic. Scarce Middle Miocene basalts from the Golan, further to the east, are also alkali-basaltic. Magmas from Kaukab (southeastern Lower Galilee) show a range of compositions from alkali-basalts to basanites. The patterns defined by ratios versus concentrations of incompatible elements in Kaukab basalts (e.g. versus La), as well as the scatter observed in Sr isotope ratios, are interpreted in terms of binary mixing. It is suggested that the Kaukab magmas were derived from veined lithospheric peridotites, melts of the veins and of the peridotites being the two end-members. Accordingly, alkali-basaltic- and basanite-dominated areas are underlain by vein-poor and vein-rich lithosphere, respectively. It is also suggested that melting during the Middle Miocene mainly occurred in response to extension, and that it followed the propagation of the Yizre'el-Galilee Basin from northeast to southwest.     

Origin of Exceptionally Abundant Phonolites on Ua Pou Island (Marquesas, French Polynesia): Partial Melting of Basanites Followed by Crustal Contamination

On the basis of the first systematic mapping of Ua Pou, longknown for its exceptionally abundant phonolites, we estimatethat these rocks cover 65% of the surface of the island whereasmafic lavas cover 27% and intermediate ones 8%. The silica-undersaturatedsuite was erupted in a restricted time span (2·9–2·35Myr), following the emplacement of tholeiites derived from ayoung HIMU-type source at c. 4 Ma. Primitive basanites, derivedfrom a heterogeneous mantle source with a dominant EM II + HIMUsignature, represent likely parental magmas. The series is characterizedby a Daly gap defined by a lack of phonotephrites. We considerthat the most likely model for the origin of evolved lavas ispartial melting at depth of primitive basanites, leaving anamphibole-rich residuum and producing tephriphonolitic magmas.These tephriphonolitic magmas may have evolved by closed-systemfractional crystallization towards Group A phonolites. Threeother groups of phonolites could have been derived from tephriphonoliticmagmas by open-system fractional crystallization processes,characterized respectively by seawater contamination (GroupB), assimilation of nepheline syenite-type materials (GroupC) and extreme fractionation coupled with assimilation of theunderlying oceanic crust (Group D). The prominence of evolvedlavas is a consequence of their origin from partial meltingof mafic precursors followed by crustal contamination. KEY WORDS: Marquesas; French Polynesia; phonolite; partial melting; contamination     

Two distinct Miocene age ranges of basaltic rocks from the Rhön and Heldburg areas (Germany) based on Ar/Ar step heating data

New ⁴⁰Ar/³⁹Ar age data are determined for Cenozoic basaltic rocks from the Thuringian Rhön and Heldburg Gangschar (area also referred to as Grabfeld), integral parts of the Central European Volcanic Province. Applying the incremental heating technique on groundmass and plagioclase separates provided data which considerably specify our knowledge on the eruption ages in these volcanic fields and narrow down the duration of volcanic activity compared to earlier studies. All data but one outlier range between 20 and 14 Ma, being thus similar to those of the neighbouring Vogelsberg volcanic complex. The spectrum of ages is clearly divided into two distinct subsets: the Rhön ages are between 20 and 18 Ma, those of the Heldburg Gangschar are between 16 and 14 Ma. Thus, the present data clearly indicate a striking regional and temporal division of the Thuringian Miocene volcanism. The composition of the volcanic rocks in the two volcanic fields is remarkably diverse, ranging from tholeiitic basalts over alkali basalts and basanites to nephelinites. However, radiometric ages do not correlate with geochemical or petrological characteristics of the volcanics within each volcanic field, indicating that the different magma types erupted broadly contemporaneously.The outlier in age (29 Ma) is from a volcanic dyke of the NE Rhön area close to the NW end of the Thuringian Forest. However, more data are required to approve the significance of this age value, in particular since the rock showed isotopic age disturbance.