Volcanic rocks of methana,South Aegean arc,Greece

Cale-alkalic lavas and associated intrusives and cognate xenoliths, from the peninsula of Methana, are comparable geochemically with similar lavas from elsewhere in the South Aegean arc. At least four differentiation series have been recognized, largely of basaltic andesite to rhyodacite type, in which hydrous and dry conditions alternated. The compositional range of minerals other than plagioclase is limited. The chemical mineralogy of olivines and pyroxenes has been studied in detail. An aluminous bronzite that is found, contrary to previous interpretations, does not appear to be a high pressure phase. The chemical composition of the basic rocks and basic xenoliths suggests that the parent material of these lavas was of high alumina basalt composition. The abundance of accumulative xenoliths and the change of the mafic minerals to more iron-rich compositions towards the more acid end of each series, suggest that phenocryst fractionation has been the major process for the later stages of the evolution of all these rocks.     

Origine des grenats des roches calco-alcalines du Sud-Est de l’Espagne

Tertiary dacites and rhyodacites in SE Spain contain garnets, both as phenocrysts in the lavas and within their enclosed metamorphic venoliths. Microprobe analyses show that calc-alkaline and metamorphic rocks garnets are quite similar in composition (almandine-rich). However, the zoning patterns displayed in the two types of garnets differ. Garnets included in lavas show normal zoning (or a slight enrichment of Mn at the borders), while garnets in xenoliths have Mn-enriched rims or reverse zoning. The differences in zonation, as well as in petrographic features, indicate that the garnets in volcanic rocks crystallized directly from the calc-alkaline magma at an early stage of its crystallization. On the other hand, the uniform composition of dacitic and rhyodacitic garnets, as well as its low pyrope and grossular content, suggest that the conditions in which the garnets crystallized were rapidly exceeded.     

Bimodal volcanism at the Katla subglacial caldera,Iceland: insight into the geochemistry and petrogenesis of rhyolitic magmas

The Katla subglacial caldera is one of the most active and hazardous volcanic centres in Iceland as revealed by its historical volcanic activity and recent seismic unrest and magma accumulation. A petrologic and geochemical study was carried out on a suite of mid-Pleistocene to Recent lavas and pyroclastic rocks originated from the caldera. The whole series is characterised by a bimodal composition, including Fe-Ti transitional alkali basalts and mildly alkalic rhyolites. Variations in trace-element composition amongst the basalts and rhyolites show that their chemical differentiation was mainly controlled by fractional crystallisation and possible assimilation. The petrology and chemistry of the few intermediate extrusive rocks show that they were derived from magma mingling or hybridisation. The absence of extrusive rocks of true intermediate magmatic composition and the occurrence of amphibole-bearing felsic xenoliths support the hypothesis of partial melting of the hydrated basalt crust as the main process leading to the generation of rhyolites. The ¹⁴³Nd/¹⁴⁴Nd and ⁸⁷Sr/⁸⁶Sr values of Katla volcanic rocks fit the general isotopic array defined by late Quaternary to Recent lavas from Iceland. A few rock specimens are distinguished by low ¹⁴³Nd/¹⁴⁴Nd values suggesting assimilation and mixing of much older crustal material. Despite their similar whole-rock chemical compositions, the postglacial rhyolitic extrusives differ from the felsic xenoliths by their glass composition and the absence of amphibole. This, together with the general chemical trend of volcanic glasses, indicates that the postglacial rhyolitic extrusives were probably derived by a process involving late reheating and partial melting of crustal material by intrusion of basaltic magmas.     

Isotope and trace element evidence for late-stage intra-crustal melting in the High Andes

Purico-Chascon is an acid igneous complex less than 1.5 Ma old rising to 5800 m in the North Chilean Andes, and consisting of andesite-dacite cones and dacite domes on an ignimbrite shield. The rocks are subdivided into two groups: those from Chascon appear to exhibit evidence for magma mixing with more basic material now preserved as xenoliths, whereas among those at Purico no xenoliths have been found.⁸⁷Sr/⁸⁶Sr=0.7095?0.7081 at Purico, 0.7079?0.7069 at Chascon, and 0.7061-0.7057 in the xenoliths from the Chascon lavas:¹⁴³Nd/¹⁴⁴Nd=0.51222?0.51236 overall. The Purico lavas are characterised by higher SiO₂, Rb/Sr,⁸⁷Sr/⁸⁶Sr, and REE abundances, and lower Sr/Nd, Sr/Ba and¹⁴³Nd/¹⁴⁴Nd than most Andean igneous suites. There is no indication ofselective crustal contamination of Sr, or any systematic change in isotope ratios during differentiation. Nonetheless the trend of, for example, high Sr/Nd and Sr contents in rocks with low⁸⁷Sr/⁸⁶Sr (0.704, Ecuador) to low Sr/Nd and Sr and high SiO₂ in rocks with⁸⁷Sr/⁸⁶Sr=0.7081?0.7095 at Purico is interpreted as a shift from subduction zone related magmatism to one with greater crustal affinity. The formation of the least evolved Purico lavas (～60%SiO₂) is discussed in terms of bulk assimilation of crustal material, mixing between crustal- and mantle-derived magmas, and partial melting of pre-existing crust. Although such models are still extremely primitive, the simplest explanation of the observed chemical variations is that the Purico rocks evolved from parental magmas derived by crustal anatexies. Thermal considerations suggest that such late-stage crustal anatexis is a predictable response to crustal thickening which in the Andes is thought to have taken place during the Cenozoic.     

A note on the feldspars in the granite xenoliths of the nyiragongo magma

The lavas of the Nyiragongo volcano in Eastern Zaire contain partially fused granite xenoliths. The relictic feldspars found in these xenoliths were studied by microprobe analysis and by X-ray diffraction methods. Some xenoliths represent originally two-feldspar granites, in others only alkali feldspars or those of an anorthoclase composition were detected. All feldspars are homogeneous without perthitic textures detectable under the microscope. In thebc-diagram of Stewart and Wright all feldspars plot on the analbite-high sanidine join. These feldspars have been perfectly disordered through heating in the melilite-nephelinite magma of the volcano.     

Basaltic ignimbrite-like rocks on Saikhan Volcano, northeastern Khangai, Mongolia: Mineralogic and geochemical evidence

Rocks having a pseudofluidal ignimbrite texture have been found on Saikhan Volcano in northeastern Khangai, Mongolia. The rocks have a typically nodular banded texture. The fiamme and the bands vary in width between a few millimeters to a few centimeters. These rocks have the same bulk composition as trachybasalts and do not differ from the ordinary trachybasalts found on this volcano in the form of dikes and lavas. The difference consists in the composition of glasses and minerals, as well as in the concentration of CO₂ (which is higher in the ignimbrite-like rocks). The glasses in the ignimbrite-like rocks show a trend from basaltic trachyandesites to tephriphonolites and foidites, thus indicating the liquidus crystallization of clinopyroxene. The glasses in the lavas and dikes have a trachyte composition, indicating a residual origin following the crystallization of olivine and Ti-magnetite. Much of the pyroxenes (∼20%) in the ignimbrite-like rocks show calculated pressures during their generation to have been in the range of 6.5–14 kbars, while all pyroxenes in the ordinary lavas and dikes crystallized at pressures below 0.3 kbars. It thus follows that the magmas that have produced the ignimbrite-like rocks began crystallizing in the subcrustal magma chamber under fluid-saturated conditions, whence they were rapidly transported to the surface.     

High-pressure inclusions in tholeiitic basalt and the range of lherzolite-bearing magmas in the Tasmanian volcanic province

Spinel-lherzolite xenoliths have been found in olivine tholeiite near Andover in the Tasmanian Tertiary volcanic province. They show a high-pressure mineralogy of predominant olivine (Mg₉₀), with aluminous enstatite (Mg₉₀) and lesser aluminous diopside and chrome-bearing spinel, and resemble lherzolite xenoliths commonly found in undersaturated lavas. Such xenoliths are unusual in tholeiitic basalts and the occurrence directly attests to a mantle origin for at least some tholeiitic magmas.The lherzolites are accompanied by doleritic and pyroxenitic xenoliths and by olivine, orthopyroxene, clinopyroxene and plagioclase xenocrysts. If near-liquidus phases are represented amongst the xenocrysts, then the magnesian number of the host basalt and its xenocryst assemblage provisionally suggest a magma derived by more than 15–20% partial melting of mantle peridotite, before commencing xenocryst crystallisation at pressures between 8–13 kbar.With this new record, lherzolite-bearing lavas in Tasmania now cover an extremely wide compositional range, extending from highly undersaturated olivine melilitite to olivine tholeiite. They also include a considerable number of fractionated alkaline rocks that are only sparsely reported in the literature as lherzolite hosts. This latter group contains representatives of a previously suggested but unestablished alkaline fractionation series based on olivine nephelinite, viz. calcic olivine nephelinite → sodic olivine nephelinite → potassi-sodic olivine nephelinite → mafic nepheline benmoreite → mafic phonolite.Lherzolite and megacryst-bearing lavas are relatively more abundant in peripheral parts to the main basalt sequences in Tasmania. This suggests that they developed in fringing zones of less intense mantle melting which enhanced stagnation and fractionation of magmas within the mantle before eruption. Calculated crustal thicknesses under these areas suggest that the magmas were generated at pressures exceeding 6–11 kbar, with the Andover tholeiitic magma exceeding 9 kbar.     

Depth of origin of the gas content in thermal waters

The gas composition of recent thermal waters is in perfect conformity with their geologictectonic positions. The composition of gas-liquid inclusions in the basic-hyperbasic xenoliths of basaltoid lavas depends on the depth from which xenoliths are supplied. (The depth is determined by using the methods of geological thermobarometers). By comparing available data we came to the conchision that the gas compositions of thermal waters reflect the depth of generation of deep fluid which heats them and depends on the penetration depth of the fluid-conducting fissure zones.     

Pb isotopes in Ascension Island rocks: oceanic origin for the gabbroic to granitic plutonic xenoliths

The Pb isotopic compositions and U and Pb concentrations of the lava series (alkali basalt to comendite) and of their plutonic xenoliths (gabbro to alkaline granite) of Ascension Island are reported. The data are used to evaluate the source of the xenoliths which formed two differentiation suites: the acidic and intermediate xenoliths together with most of the lavas on the one hand, and the gabbroic xenoliths and a basaltic tuff on the other hand. The Pb isotopic compositions imply a mantle origin for the source magmas of the xenoliths and confirm the possibility of generating granitic rocks in an oceanic environment by fractional crystallization of a mantle-derived magma whose geochemical and isotopic characteristics are comparable to the source magmas of oceanic island basalts.     

江西省广丰县李家膨润土矿床成因及找矿标志

广丰县李家膨润土矿床赋存于白垩系上统南雄组第二岩性段酸性火山岩的顶底部。膨润土的主要矿物成分为蒙脱石,矿石结构构造与成矿原岩有关。矿石化学成分与酸性熔岩相近,矿体形态及空间分布受酸性熔岩流相带的控制。因此认为,膨润土矿床成矿原岩为酸性玻质熔岩,矿床是由酸性玻璃质熔岩经水解脱玻化而形成,其成因类型为风化淋滤蚀变岩型矿床。膨润土可自然改型,由原来的钠基膨润土改型成钙基膨润土和偏镁的钙基膨润土。对时代较新的酸性火山岩,在玻质熔岩发育处都有可能找到有利用价值的膨润土矿。由于膨润土常由珍珠岩、沸石脱玻化形成,三者常共生出现,是很好的找矿标志。     

Forecasting the duration of volcanic eruptions: an empirical probabilistic model

Various xenoliths have been found in lavas of the 1763 (“La Montagnola”), 2001, and 2002–03 eruptions at Mt. Etna whose petrographic evidence and mineral chemistry exclude a mantle origin and clearly point to a cognate nature. Consequently, cognate xenoliths might represent a proxy to infer the nature of the high-velocity body (HVB) imaged beneath the volcano by seismic tomography. Petrography allows us to group the cognate xenoliths as follows: i) gabbros with amphibole and amphibole-bearing mela-gabbros, ii) olivine-bearing leuco-gabbros, iii) leuco-gabbros with amphibole, and iv) Plg-rich leuco gabbros. Geobarometry estimates the crystallization pressure of the cognate xenoliths between 1.9 and 4.1 kbar. The bulk density of the cognate xenoliths varies from 2.6 to 3.0 g/cm³. P wave velocities (V _P ), calculated in relation to xenolith density, range from 4.9 to 6.1 km/s. The integration of mineralogical, compositional, geobarometric data, and density-dependent V _P with recent literature data on 3D V _P seismic tomography enabled us to formulate the first hypothesis about the nature of the HVB which, in the depth range of 3–13 km b.s.l., is likely made of intrusive gabbroic rocks. These are believed to have formed at the “solidification front”, a marginal zone that encompasses a deep region (>5 km b.s.l.) of Mt. Etna’s plumbing system, within which magma crystallization takes place. The intrusive rocks were afterwards fragmented and transported as cognate xenoliths by the volatile-rich and fast-ascending magmas of the 1763 “La Montagnola”, 2001 and 2002–03 eruptions.     

Geochemistry and petrology of meta-igneous granulitic xenoliths in Neogene volcanic rocks of the Massif Central,France — implications for the lower crust

Meta-igneous mafic and ultramafic rocks, which constitute about 60% of the granulitic xenoliths enclosed in the Neogene alkali basalts of the Bournac pipe (French Massif Central) have well preserved magmatic trends of element variations. The meta-igneous suite was probably derived from at least two different parental magmas and it may be a part of a gabbroic complex which resembles mafic bodies associated with anorthosites. The xenoliths are also very similar to many other granulitic xenoliths and to meta-igneous mafic granulitic massifs. This indicates that the gabbroic intrusions may be widespread in the lower crust and the close association of gabbroic rocks with meta-sedimentary granulites suggests a model for the composition of the lower continental crust.     

Bay of Islands ophiolite suite,Newfoundland: Petrologic and geochemical characteristics with emphasis on rare earth element geochemistry

Characteristic geochemical features of the ophiolite suite from the Bay of Islands Complex have been determined by major and trace element analyses of 13 rocks. Based on elements, such as rare earth elements (REE), whose abundances are relatively immobile during alteration and metamorphism, we find that (1) the pillow lavas and diabases are relatively depleted in light REE similar to most tholeiites occurring along spreading oceanic ridges, in back-arc basins and comprising the early phases of volcanism in island arcs; (2) the gabbros, composed of cumulate plagioclase and olivine with poikilitic clinopyroxene, have REE contents consistent with formation as cumulates precipitated from magmas represented by the overlying pillow lavas and diabases; (3) as in most harzburgites from ophiolites, the Bay of Islands harzburgite and dunite have relative REE abundances inconsistent with a genetic relationship to the overlying basic rocks — this inconsistency may be primary or it may result from late-stage alteration, contamination and/or metamorphism; (4) some Bay of Islands lherzolites have major and trace element abundances expected in the mantle source of the overlying basic rocks. Overall, the geochemical features of this Bay of Islands ophiolite suite are similar to those from Troodos and Vourinos, but these data are not sufficient to distinguish between different tectonic environments such as deep ocean ridge, small ocean basin or young island arc.     

Sr and Nd isotope composition of the metamorphic,sedimentary and ultramafic xenoliths of Lanzarote (Canary Islands): Implications for magma sources

The lavas produced by the Timanfaya eruption of 1730–1736 (Lanzarote, Canary Islands) contain a great many sedimentary and metamorphic (metasedimentary), and mafic and ultramafic plutonic xenoliths. Among the metamorphosed carbonate rocks (calc-silicate rocks [CSRs]) are monomineral rocks with forsterite or wollastonite, as well as rocks containing olivine ± orthopyroxene ± clinopyroxene ± plagioclase; their mineralogical compositions are identical to those of the mafic (gabbros) and ultramafic (dunite, wherlite and lherzolite) xenoliths. The ⁸⁷Sr/⁸⁶Sr (around 0.703) and ¹⁴³Nd/¹⁴⁴Nd (around 0.512) isotope ratios of the ultramafic and metasedimentary xenoliths are similar, while the ¹⁴⁷Sm/¹⁴⁴Nd ratios show crustal values (0.13–0.16) in the ultramafic xenoliths and mantle values (0.18–0.25) in some CSRs. The apparent isotopic anomaly of the metamorphic xenoliths can be explained in terms of the heat source (basaltic intrusion) inducing strong isotopic exchange (⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd) between metasedimentary and basaltic rocks. Petrofabric analysis also showed a possible relationship between the ultramafic and metamorphic xenoliths.     

Petrology of basaltic xenoliths in andesitic to dacitic host lavas from Martinique (lesser antilles): Evidence for magma mixing

Some recent calc-alkaline andesites and dacites from southern and central Martinique contain basic xenoliths belonging to two main petrographic types:

The most frequent one has a hyalodoleritic texture (« H type ») with hornblende + plagioclase + Fe-Ti oxides, set in an abundant glassy and vacuolar groundmass.

The other one exhibits a typical porphyritic basaltic texture (« B type ») and mineralogy (olivine + plagioclase + orthopyroxene + clinopyroxene + Fe-Ti oxides and scarce, or absent hornblende).

Gradual textural and mineralogical transitions occur between these two types (« I type ») with the progressive development of hornblende at the expense of olivine and pyroxenes. Mineralogical and chemical studies show no primary compositional correlations between the basaltic xenoliths and their host lavas, thus demonstrating that the former are not cognate inclusions; they are remnants of basaltic liquids intruded into andesitic to dacitic magma chambers. This interpretation is strengthened by the typical calc-alkaline basaltic composition of the xenoliths, whatever their petrographic type (« H », « I » or « B »). The intrusion of partly liquid, hot basaltic magma into colder water-saturated andesitic to dacitic bodies leads to drastic changes in physical conditions. The two components; the basaltic xenoliths are quenched and homogeneized with their host lavas with respect to T^o;fO₂ andpH₂O conditions. « H type » xenoliths represent original mostly liquid basalts in which such physical changes lead to the formation of hornblende and the development of a vacuolar and hyalodoleritic texture. The temperature increase of the acid magma depends on the amount of the intruding basalt and on the thermal contrast between the two components. The textural diversity which characterizes the xenoliths reflects the cooling rate of the basaltic fragments and/or their position relative to the basaltic bodies (chilled margins or inner, more crystallized, portions). In addition to physical equilibration (T, fO₂) between the magmas, mixing involves:

mechanical transfer of phenocrysts from one component to another, in both directions;

volatile transfer to the basaltic xenoliths, with chemical exchanges.

It is here demonstrated that a short period of time (some ten hours to a few days) separates the mixing event from the eruption, outlining the importance of magma mixing in the triggering of eruption. The common occurrence of basaltic xenoliths (generally of « H » type) in calc-alkaline lavas is emphasized, showing that this mechanism is of first importance in calc-alkaline magma petrogenesis.     

Geochemistry and geochronology of Late Triassic volcanic rocks in the Chiang Khong region, northern Thailand

The Chiang Khong segment of the Chiang Khong–Lampang–Tak Volcanic Belt is composed of three broadly meridional sub‐belts of mafic to felsic volcanic, volcaniclastic, and associated intrusive rocks. Associated sedimentary rocks are largely non‐marine red beds and conglomerates. Three representative Chiang Khong lavas have Late Triassic (223–220 Ma) laser ablation inductively coupled mass‐spectroscopy U–Pb zircon ages. Felsic‐dominated sequences in the Chiang Khong Western and Central Sub‐belts are high‐K calc–alkaline rocks that range from basaltic to dominant felsic lavas with rare mafic dykes. The Western Sub‐belt lavas have slightly lower high field strength element contents at all fractionation levels than equivalent rocks from the Central Sub‐belt. In contrast, the Eastern Sub‐belt is dominated by mafic lavas and dykes with compositions transitional between E‐mid‐oceanic ridge basalt and back‐arc basin basalts. The Eastern Sub‐belt rocks have higher FeO* and TiO₂ and less light rare earth element enrichment than basalts in the high‐K sequences. Basaltic and doleritic dykes in the Western and Central sub‐belts match the composition of the Eastern Sub‐belt lavas and dykes. A recent geochemical study of the Chiang Khong rocks concluded that they were erupted in a continental margin volcanic arc setting. However, based on the dominance of felsic lavas and the mainly non‐marine associated sediments, we propose an alternative origin, in a post‐collisional extensional setting. A major late Middle to early Late Triassic collisional orogenic event is well documented in northern Thailand and Yunnan. We believe that the paucity of radiometric dates for arc‐like lavas in the Chiang Khong–Lampang–Tak Volcanic Belt that precede this orogenic event, coupled with the geochemistry of the Chiang Khong rocks, and strong compositional analogies with other post‐collisional magmatic suites, are features that are more typical of volcanic belts formed in a rapidly evolving post‐collisional, basin‐and range‐type extensional setting.     

Intracratonic asthenosphere upwelling and lithosphere rejuvenation beneath the Hoggar swell (Algeria): Evidence from HIMU metasomatised lherzolite mantle xenoliths

The mantle xenoliths included in Quaternary alkaline volcanics from the Manzaz-district (Central Hoggar) are proto-granular, anhydrous spinel lherzolites. Major and trace element analyses on bulk rocks and constituent mineral phases show that the primary compositions are widely overprinted by metasomatic processes. Trace element modelling of the metasomatised clinopyroxenes allows the inference that the metasomatic agents that enriched the lithospheric mantle were highly alkaline carbonate-rich melts such as nephelinites/melilitites (or as extreme silico-carbonatites). These metasomatic agents were characterized by a clear HIMU Sr–Nd–Pb isotopic signature, whereas there is no evidence of EM1 components recorded by the Hoggar Oligocene tholeiitic basalts. This can be interpreted as being due to replacement of the older cratonic lithospheric mantle, from which tholeiites generated, by asthenospheric upwelling dominated by the presence of an HIMU signature. Accordingly, this rejuvenated lithosphere (accreted asthenosphere without any EM influence), may represent an appropriate mantle section from which deep alkaline basic melts could have been generated and shallower mantle xenoliths sampled, respectively. The available data on lherzolite xenoliths and alkaline lavas (including He isotopes, Ra < 9) indicate that there is no requirement for a deep plume anchored in the lower mantle, and that sources in the upper mantle may satisfactorily account for all the geochemical/petrological/geophysical evidence that characterizes the Hoggar swell. Therefore the Hoggar volcanism, as well as other volcanic occurrences in the Saharan belt, are likely to be related to passive asthenospheric mantle uprising and decompression melting linked to tensional stresses in the lithosphere during Cenozoic reactivation and rifting of the Pan–African basement. This can be considered a far-field foreland reaction of the Africa–Europe collisional system since the Eocene.     

Magmatic evolution of the basaltic shield volcanoes of Reunion Island

Chemical data are presented for the basic lavas of the two volcanic shields, Piton des Neiges and Piton de la Fournaise, which comprise Reunion Island. In addition, data for cumulate xenoliths have been used to predict mineral/melt distribution coefficient values for the Reunion magmas.The younger volcanic shield, Piton de la Fournaise, comprises two lava sequences, the >0.5−0.2-m.y. B.P. Primary Shield lavas, and the <0.2-m.y. B.P. Caldera Series lavas. Fractional crystallization models for these lavas indicate that olivine is the major fractionating phase during the evolution from the parental basalt composition to the average basaltic liquid. Only during the evolution of the older, Primary Shield lavas has the common fractionation of an ol + cpx + plag + mt assemblage resulted in the eruption of hawaiitic, ankaramitic and feldspar-phyric lavas. The restriction of the Caldera Series liquids predominantly to olivine fractionation and the extensive cotectic fractionation during the evolution of the Primary Shield sequences is interpreted in terms of the maturity of the volcanic center. The younger stages of evolution involve high magma input into a well-developed feeder and reservoir system, thus maintaining the liquids above a cotectic surface. Whereas, during the evolution of the Primary Shield lavas, lower magma input rates into a less well-developed feeder system increased the probability of the fractionating liquid attaining a cotectic surface. Fractional crystallization accounts for all the chemical variation observed for the Piton de la Fournaise basaltic magmas. The analytical data are closely comparable to the rare earth element (REE) and trace element fractionation curves predicted by least-squares calculations, this supports the use of such models in quantitative evaluation of fractional crystallization.A preliminary survey of Sr isotope values indicates that the oldest (>2 m.y. B.P.) lava sequences of Piton des Neiges may be derived from a source which was isotopically distinct from that of the <2 m.y. B.P. lavas of both volcanic shields. These latter sequences are remarkably consistent in both isotopic and trace element abundance implying a homogeneous source material and an invariable partial melting process. Partial melting calculations indicate that the basaltic lavas have been derived by 5–10% melting of a garnet-poor peridotite (cpx/gt 9). Systematic differences in the light- and heavy-REE patterns between similar basaltic provinces are interpreted to be a result of variation in the nature of the phases buffering the entry of light- and/or heavy-REE into the melt during partial fusion.     

Slab melt as metasomatic agent in island arc magma mantle sources, Negros and Batan (Philippines)

Abstract Two new cases of association of adakites with ‘normal’ island arc lavas and transitional adakites are recognized in the islands of Batan and Negros in northern and central Philippines, respectively. The Batan lavas are related to the subduction of the middle Miocene portion of the South China Sea basin along the Manila trench; those of Negros come from the almost aseismic subduction of the middle Miocene Sulu Sea crust along the Negros trench. The occurrence of the Batan adakites is consistent with previous findings showing adakitic glass inclusions within minerals of mantle xenoliths associated with Batan arc lavas. The similarity of adakite ages (1.09 Ma) and that of the metasomatized xenoliths (1 Ma) suggests that both are linked to the same slab‐melting and metasomatic event. Earlier Sr, Pb and Nd‐isotopic studies, however, also reveal the presence of an important sediment contribution to the Batan lava geochemistry. Thus, the role played by slab melts, assumed to have mid‐ocean ridge basalts‐like (MORB) isotopic characteristics, in enriching the Batan subarc mantle is largely masked by the sediment input. The Negros adakites are present only in Mount Cuernos, the volcanic center nearest to the Negros trench. Batch partial melting calculations show that the Negros adakites could be derived from a garnet amphibolitic source with normal‐MORB (N‐MORB) geochemistry. This is supported by the MORB‐like isotopic characteristics of the Mount Cuernos lavas. The volcanic rocks from the other volcanoes consist of normal arc and transitional adakitic lavas that have slightly higher Sr‐ and Pb‐isotopic ratios, probably due to slight sediment input. Mixing of adakites and normal arc lavas to produce transitional adakites is only partly supported by trace element geochemistry and not by field evidence. The transitional adakites can be modeled as partial melts of an adakite‐enriched mantle. Trace element enrichment of non‐adakitic lavas could reflect the interaction of their mantle source with uprising slab melts, as metasomatic mantle minerals scavenge certain trace elements from the adakitic fluids. Therefore, in arcs beneath which thick (up to 2 km) continent‐derived detrital sediments are involved in subduction, like in Batan, the sediment signature can overwhelm the slab melt input. In arcs like Negros where slow subduction could cause a more efficient scraping of thinner (approximately 1 km) detrital sediments, the contribution of slab melts is easier to detect.     

The quaternary caldera volcano emuruangogolak,kenya rift,and the petrology of a bimodal ferrobasalt-pantelleritic trachyte association

Emuruangogolak is a Quaternary basalitrachyte volcano situated in the Suguta graben of the northern Kenva rift, and probably erupted last early in this century. Following the construction of an early trachytic shield volcano, two episodes of caldera collapse occurred. each preceded by explosive pvroclastic activity. Post-calelera volcanism consisted of alternating phases of basalt and trachyte eruption. The basic lavas are high-Ti ferrobasalts of a mildly alkaline ‘transitional’ composition and the trachytes are peralkaline and oversaturated. A distinct compositional bimodality exists and no rocks in the range 49–59°. SiO, have been found. Major and trace element analyses suggest that the trachytes are genetically related to the basalts. Associations of almost identical lavas occur in Ethiopia. Pantelleria and the Azores but with the presence of intermediate terms Fractional crystallization is the mechanism currently preferred to account for the origin of the trachytes. The ‘Daly gap’ may be a consequence of a crystallization process which limits the volume of intermediate magma available at any time. In addition, the physical properties and spatial distribution of the different magmas probably discriminate against the cruption of lavas of intermediate composition.