the ultrapotassic rocks from southeastern spain the ultrapotassic rocks of orenditic affinity from southeastern spain have variable sio2 and mgo contents high k2o p2o5 ni cr ba ce th zr values and high k2ona2o nimgo ratios on the contrary tio2 nb y are moderate and feot cao v sc are low the parent magmas possibly generated from a phlogopite bearing mantle source enriched in many incompatible elements by metasomatic processes and probably they were characterized by very co2 co2 h2o ratio the low tip ratio and the moderate tio2 and nb contents in spite of the high alkaline element contents of the magmas suggest that titanium and niobium were retained in the source of the metasomatic fluids the low na contents may depend in part on the loss of this element via naf separation during the latest stage of crystallization crustal contamination cannot be excluded

observations on the nature and origin of fusain abstract evidence for a wildfire origin of fusain is examined charcoal produced by natural fires and artificial charring resembles fossil fusain both in physical size and shape and in optical reflectance artificially charred leaves and naturally burnt wood are illustrated using the sem reflectance studies on modern charcoals suggest that much semifusinite may be produced by pyrolysis the ecological implications of a wildfire origin of fusain are discussed and it is concluded that fires have been a feature of terrestrial ecosystems from at least the late devonian

crystallization paths of leucite bearing lavas examples from italy the salic phases found in leucite basanites trachytes and phonolites may be used to portray crystallization in the system naalsio 4 kalsio 4 caal 2si 2o 8 sio 2 the phonolite pentahedron only two lavas have been found that contain the assemblage leucite nepheline plagioclase sanidine and liquid a natural pseudo invariant assemblage at 900 c 100 equivalent to the isobaric invariant point of the four component system the diversity of phases in this group of lavas illustrates the role of halogens in controlling their crystallization paths thus the presence of f in the leucite basanites has stabilized magnesian biotite and suppressed sanidine as has been found in other basanitic lavas brown and carmichael 1969 the presence of cl in these same lavas has induced the crystallization of sodalite which takes the place of nepheline in the groundmass however in the leucite trachytes biotite has suppressed olivine and coexists with sanidine and leucite the presence of s may produce hauyne at the expense of nepheline and in general sulphate minerals which include apatite have the role in lavas of low silica activity that pyrrhotite plays in liquids of high silica activity both pyroxenes and titaniferous magnetites in this suite of lavas are very aluminous groundmass crystals of pyroxene may have one fifth of si replaced by al other phases which occur occasionally are melanite garnet and a potassium rich hastingsite but neither ilmenite nor a sulphide mineral has been found phenocryst equilibration temperatures derived from olivine and sr rich plagioclase are generally in the range from 1 050 c to 1 150 c the high content of incompatible elements e g k ba rb f sr p in these lavas suggests that they represent a small liquid fraction from a mantle source which possibly contains phlogopite

impact of sulphur and trace element geochemistry on the utilization of a marine influenced coal case study from the south wales variscan foreland basin abstract incremental channel samples through two high volatile a bituminous coal seams the amman rider and bute seams below and above the vanderbeckei marine band at the westphalian a b boundary have been analysed using proximate petrographic inductively coupled plasma mass spectrometry icp ms x ray fluorescence xrf and scanning electron microscopy with energy dispersive x ray sem edx techniques the results show that both coals are vitrinite rich and indicate a similar history of palaeomire development with a period of falling water tables towards the middle plies of each seam mean sulphur contents in the coal with the marine roof the amman rider seam 4 19 wt are almost 4 times higher than in the coal with a non marine roof the bute seam 1 09 wt implying that high sulphur contents are derived from sea water sulphate in both coals sulphur is predominantly in the form of pyritic sulphur both syngenetic framboidal and euhedral crystals and epigenetic vein and cleat fill pyrite are present relatively constant levels of organic sulphur at around 2 have been recorded in the amman rider seam sulphur levels in the basal and top plies of each seam are at least twice that of the ply with the lowest sulphur content in the seam reflecting the stages in palaeomire development most strongly influenced by sulphate containing waters thorium uranium ratios are below 2 9 in the coal with a marine roof the amman rider seam indicating an enrichment of u from sea water most marked at the top and bottom of the seam the seam without a marine roof shows th u ratios between 2 5 and 4 9 indicating mild marine enrichment of u the concentrations of environmentally significant trace elements eses are correlated with either the ash or sulphur contents or both the correlation between s and pb as cu ni in the coal with a marine roof implies an origin of these eses from sea water and their mode of occurrence in sulphide minerals most eses in this coal are present at levels at least twice that of the global average in coals the exceptions being pb and zn which have concentrations less than half the global average by contrast the coal without a marine roof and with lower sulphur contents has most eses present at concentrations less than a third of the global average the exceptions being ni and cu diagenetic hydrothermal activity is documented in the middle ply of the coal with a marine roof this hydrothermal system redistributed the sulphur pb and as in this ply the principal utilization of bituminous coals in the south wales coalfield is as feed coals in power station boilers the resultant concentration of most eses in the solid residues from power plant coal combustion theoretically can be alleviated by careful blending of feed coals with low and high ese contents

coexisting k rich alkaline and shoshonitic magmatism of arc affinities in the proterozoic a reassessment of syenitic stocks in the southwestern grenville province biotite rich syenitic stocks in the mont laurier area of the southwestern grenville province are shown to belong to the first recorded proterozoic example of an ultrapotassic k rich alkaline and shoshonitic rock association with clear arc affinities the plutons investigated were previously considered mostly syenitic typical of nepheline syenite alkaline suites slightly metamorphosed and late tectonic with respect to the grenville orogeny we find that they postdate the regional metamorphism and comprise a felsic to ultramafic range of rock types belonging to two series 1 a potassic to ultrapotassic silica undersaturated series of biotite rich nepheline bearing syenite syenite monzonite diorite and pyroxenite and 2 a shoshonitic critically silicasaturated series of quartz syenite and amphibole bearing syenite with rare monzonite and diorite the ubiquitous biotite previously regarded as metamorphic is reinterpreted as igneous and diagnostic of the potassic character the shoshonitic and potassic series display the strong enrichment in al ca k and large ion lithophile elements relative to the high field strength elements e g ba nb 722 la yb 45 and the low contents in mg that are characteristic of arc related magmas the syenitic rocks consistently share the distinctive arc related geochemical signature of their mafic counterparts syenites may thus represent a potential source of paleotectonic information for high grade terranes geochemical discriminants nbn tan and hfn tin ratios indicate that the shoshonitic and potassic series are unrelated by closedsystem fractionation processes rather the chemical differences between the two series probably reflect differences in source characteristics and conditions of melting similar plutons occur throughout the central metasedimentary belt of the southwestern grenville province they define a 1089 to 1076 ma 450 km long grenvillian potassic alkaline plutonic pap province the presence of this k rich alkaline province indicates that the scarcity of k rich rocks in the precambrian could be only apparent and a consequence of misidentification of k rich plutons in metamorphosed precambrian terranes these 1 1 ga ultrapotassic to shoshonitic plutonic rocks are geochemically similar to shoshonites and leucitites of the sunda arc this similarity suggests that subduction type enrichment processes were operating in the proterozoic in ways similar to those of modern settings

melting of peridotites through to granites a simple thermodynamic model in the system kncfmashtocr a new set of thermodynamic models is presented for calculating phase relations in bulk compositions extending from peridotite to granite from 0 001 to 70 kbar and from 650 c to peridotite liquidus temperatures in the system k2o na2o cao feo mgo al2o3 sio2 h2o tio2 fe2o3 cr2o3 kncfmashtocr the models may be used to calculate phase equilibria in partial melting of a large range of mantle and crustal compositions they provide a good fit to experimental phase relation topologies and melt compositions across the compositional range of the model compared with the preliminary model of jennings holland 2015 for peridotite basalt melting relations the inclusion of k2o and tio2 allows for better modelling of small melt fractions in peridotite melting and in reproducing rutile bearing eclogite melting at high pressures an improved order disorder model for spinel is now incorporated above 10 kbar pressure wet partial melting relations may be significantly affected by the dissolution of silicates in aqueous fluid so the set of models includes an aqueous low density silicate bearing fluid in addition to a high density h2o bearing silicate melt oxygen fugacity may be readily calculated for the whole range of bulk compositions investigated and the effect of water content on melt fo2 is assessed

eocene melting of precambrian lithospheric mantle analcime bearing volcanic rocks from the challis kamloops belt of south central british columbia abstract potassic silica undersaturated mafic volcanic rocks form a minor portion of the predominantly calc alkaline eocene challis kamloops volcanic belt which extends from the northwestern united states into central british columbia canada their major occurrence is in the penticton group in south central british columbia where they reach a thickness of up to 500 m and form the northwestern edge of the montana alkaline province these analcime bearing rocks 53 52 ma old are typically rhomb porphyries of ternary feldspar an28ab52or20 additional phenocryst phases include clinopyroxene analcime phlogopite and rare olivine the rocks are characterized by high total alkalis particularly k2o 4 5 wt as well as by a distinct enrichment of large ion lithophile elements versus heavy rare earth elements and high field strength elements they have unusual isotopic compositions compared to most other rocks of the challis kamloops belt particularly high negative nd values and elevated but relatively uniform initial 87sr 86sr ratios 0 7065 the potassic silica undersaturated rocks overlie precambrian crust and lithosphere and were at least in part derived from ancient metasomatized subcontinental mantle lithosphere which was modified in a precambrian subduction setting the alkaline rocks of the challis kamloops belt are related to a slab window environment in particular they were formed above the southern edge of the kula plate adjacent to the kula farallon slab window whereas the montana alkaline province situated well to the southeast was formed directly above the kula farallon slab window upwelling of the hotter asthenospheric mantle may have been the thermal trigger necessary to induce melting of fertile and metasomatized lithospheric mantle

the geochemistry of potassic lavas from vulsini central italy and implications for mantle enrichment processes beneath the roman region major and trace element and 143nd 144nd 0 51209 0 51216 and 87sr 86sr 0 70879 0 71105 isotope analyses are presented on a representative group of lavas from the vulsini district of the roman magmatic province three distinct series are identified the high k and low k series are similar to those described from other italian volcanoes while the third is represented by a group of relative ly undifferentiated leucite basanites which are thought to be near primary mantle melts major and trace element variations within the high k series are consistent with fractional crystallisation from a parental magma similar to the most magnesian leucitites crustal contamination resulted in an increase in 87sr 86sr with increasing fractionation but it was superimposed on magmas which had already inherited a range of incompatible element and isotope ratios from enrichment processes in the sub continental mantle these are reviewed using the available results from vulsini roccamonfina and ernici transition element abundances and ta yb ratios indicate that the pre enrichment mantle was similar to that of e type morb and that these elements were not mobilised by the enrichment process mixing calculations suggest that three components were involved in the enrichment process mantle comparable with the source of morb and two other components rich in trace elements one the low k component had high sr nd th ta and ba nb and no europium anomaly while the second had lower sr nd a negative europium anomaly and very high th ta it was also characterised by low nb ba and high rb ba ratios similar to those reported from phlogopite rich peridotite xenoliths the trace element enrichment processes are therefore thought to have occurred in the mantle wedge above a subduction zone with the trace element characteristics of the high k end member reflecting the subduction of sediments and the stabilisation of mantle phlogopite

distinctive crystal chemistry and site configuration of the clinopyroxene from alkali basaltic rocks the nyambeni clinopyroxene suite kenya a crystal chemical investigation of clinopyroxenes from a suite of nepheline bearing lavas located in the nyambeni range of kenya has delineated the polyhedral site configurations and related intracrystalline relationships these are distinct from those determined for the clinopyroxene in an analogous suite of leucite bearing lavas from the sabatini volcanoes in the roman region of italy dal negro et al 1985

petrology and geochemistry of the lamongan volcanic field east java indonesia primitive sunda arc magmas in an extensional tectonic setting new geochemical data are presented from prehistoric and historical multifarious process involving possible inputs from the subducted oceanic lithosphere subducted oceanic sedieruptive products of the lamongan volcanic field lvf east java a region of the sunda arc covering 260 km and containing ments and fluids the asthenospheric and lithospheric portions of the mantle wedge above the subduction zone 90 eruptive vents plus the historically active lamongan volcano lvf lavas include medium k basalts and basaltic andesites from and the arc crust e g tatsumi eggins 1995 following melt generation processes such as crystal fractionation historical eruptions of lamongan and prehistoric eruptions in the eastern lvf along with a high k suite represented by prehistoric accumulation and crustal assimilation during transit to the surface may obscure the true nature of the magma deposits in the western lvf although lacking some of the characteristics of truly primary basalts the least evolved lavas identified source region the effects of these processes on the compositions of erupted lavas must therefore be removed in the lvf have some of the lowest sio2 contents 43 wt sio2 yet reported in sunda arc volcanic rocks mass balance before models of arc magma genesis can be tested for this reason it is desirable to sample magmas that considerations indicate that two chemically distinct lvf magmas may be parental to suites currently being erupted from the neighbouring have suffered minimal modification since segregation from their source region the compositions of the primary volcanoes semeru and bromo lamongan s historical lavas can be related to the medium k andesitic products of semeru by fractional or primitive magmas from different arcs provide a window on the diverse mechanisms of and the components crystallization despite the former s location at the same distance from the trench as bromo a high k volcano extensional tectonics involved in melt production at convergent margins and as such are essential to our understanding of the possibly related to arc segmentation in the region of the lvf creating subduction zone environment conditions that promote the rapid ascent of parental magmas is many studies have highlighted the correlation between probably responsible for this and several other features of the complex extensional tectonics in volcanic arcs and the eruption of lavas with primitive chemical characteristics e g luhr carmichael 1981 knittel oles 1995 bacon et al

potassium rich volcanic rocks of the muriah complex java indonesia products of multiple magma sources abstract the extinct pleistocene volcano muriah situated behind the main pleistocene recent sunda magmatic arc in north central java has erupted at least two contrasted groups of lavas one group forms a well defined compositional series anhydrous series from leucite basanite to tephritic phonolite with olivine and tschermakitic clinopyroxene the main phenocrysts the other group the hydrous series includes compositionally variable tephrites and high k andesites with common plagioclase biotite and amphibole lavas of the anhydrous series are much richer in lil trace elements than the most potassic lavas of neighbouring active volcanoes but relative hfs element enrichment is less pronounced ree patterns have almost constant slopes from la 250 600 times chondrites to yb 5 10 times chondrites while those of lavas of active centres are less light enriched and show flattening in the heavy ree anhydrous series initial 87sr 86sr ratios 0 7043 0 7046 are lower than those of active centres 0 7047 0 7053 hydrous series lavas are intermediate in all these geochemical characteristics the most mafic a series leucite basanite with mg mg fe 2 0 69 140 ppm ni and 620 ppm cr was probably derived from the primary magma for the series by fractionation of only 5 wt olivine its ree pattern suggests derivation from a garnet bearing source experiments on this basanite with up to 10 olivine and 20 orthopyroxene added and in the presence of h2o and h2o co2 mixtures have shown that for all but very high magma water contents the olivine and garnet liquidus fields are widely separated by fields of phlogopite and clinopyroxene there is no liquidus field of orthopyroxene hence if magma production involved an equilibrium melting process alone the most probable sources are of garnet bearing phlogopite clinopyroxenite type alternatively this magma may represent the end product of interaction between a low k basanite magma from a garnet lherzolite source in the asthenosphere and a phlogopite bearing lherzolite zone in the lower lithosphere its production was probably related to crustal doming and extension superimposed on the dominant subduction regime hydrous series magmas may have resulted from mixing between anhydrous series magmas and high k calc alkaline basaltic to andesitic magmas more directly related to subduction processes

1 1 ga k rich alkaline plutonism in the sw grenville province u pb zircon and baddeleyite dating of six syenitic stocks establishes that the ultrapotassic potassic alkaline and shoshonitic magmatism with island arc affinities in the central metasedimentary belt cmb of the southwestern grenville province canada took place between 1089 and 1076 ma along a 400 km long northeast trending plutonic belt these ages indicate that ultrapotassic rocks with arc affinities are not unique to the phanerozoic west to east emplacement ages along a northern and southern cross section of this belt range from 1083 2 ma kensington through 1081 2 ma lac rouge to 1076 1 3 ma loranger in the north and from 1089 3 4 ma loon lake and 1088 2 ma calabogie to 1076 2 ma westport in the south although closely spaced in time in detail these ages suggest a slight younging of this magmatic activity to the southeast integration of the geochronological data with the spatial extent and potassic character of the plutons shows that the k rich alkaline suite is distinct from the nepheline syenite belt of the bancroft terrane and from the syenite monzonite suite of the frontenac terrane of the cmb and it is considered to be a magmatic episode unique to the elzevir terrane and its gatineau segment the timing and the postmetamorphic emplacement of these plutons indicate that the regional greenschist to granulite facies metamorphism of the country rock precise age unknown is older than 1089 ma throughout the entire elzevir terrane the potassic magmatism is interpreted as the initiation of the 1090 1050 ma ottawan orogeny in the elzevir terrane thus the regional metamorphism in this terrane previously assigned to the ottawan orogeny is an earlier event the contemporaneous emplacement of this postmetamorphic plutonic belt with keweenawan volcanism is at variance with current tectonic models which consider the keweenawan rift to be formed at the same time as regional metamorphism in the cmb

in situ raman spectroscopic investigation of the structure of subduction zone fluids 1 in situ raman spectra of synthetic subduction zone fluids kalsi3o8 h2o system were measured to 900 c and 2 3 gpa using a hydrothermal diamond anvil cell the structures of aqueous fluid and hydrous melt become closer when conditions approach the second critical endpoint almost no three dimensional network was observed in the supercritical fluid above 2 gpa although a large amount of silicate component is dissolved suggesting that the physical and chemical properties of these phases change drastically at around the second critical endpoint our experimental results indicate that the fluids released from a subducting slab change from aqueous fluid to supercritical fluid with increasing depth under the volcanic arcs

geochemical and tectonic relationships in the east indonesian arc continent collision region implications for the subduction of the australian passive margin abstract van bergen m j vroon p z and hoogewerff j a 1993 geochemical and tectonic relationships in the east indonesian arc continent collision region implications for the subduction of the australian passive margin in m j r wortel u hansen and r sabadini editors relationships between mantle processes and geological processes at or near the earth s surface tectonophysics 223 97 116 variations in the isotopic signatures of volcanics along the east sunda banda arc reflect changes in the nature and amount of sedimentary material supplied by the northeast indian ocean floor and the adjacent australian passive continental margin which form the two major domains of the indian ocean plate that approach the arc system a compilation of isotopic data for 200 500 km long arc sectors shows that the trend in magmatic signatures follows distinct subduction collision stages reached by the corresponding oceanic and continental margin sections entering the trench system maximum amounts of magma source contamination are inferred for volcanics near an extinct sector north of timor where the australian continent started to collide with the arc first pb nd isotopic source mixing models point to contamination by sediments with variations in composition similar to observed along arc changes in sediments entering the trench the results indicate an increasing contribution of subducted continental material in the direction of the collision region mass balance calculations considering the magmatic output and minimum input of subducted continental material required to generate the composition of the volcanic arc in the collision region are difficult to reconcile with subduction of ocean floor sediments alone thicknesses of sediments presently covering oceanic crust near the margin are close to calculated thicknesses of the sediments fluxed into the trench and magmatically returned to the arc crust but cannot account for the additional volumes of material accreted on the overriding plate in the same period of time it is inferred that leading portions of the australian continental margin have reached magma generation zones in the easternmost sunda arc and western banda arc which implies subduction to depths greater than 100 km

the geochemistry and petrogenesis of k rich alkaline volcanics from the batu tara volcano eastern sunda arc mineralogical major and trace element and isotopic data are presented for leucite basanite and leucite tephrite eruptives and dykes from the batu tara volcano eastern sunda arc in general the eruptives are markedly porphyritic with phenocrysts of clinopyroxene olivine leucite plagioclase biotite set in similar groundmass assemblages these k rich alkaline volcanics have high concentrations of large ion lithophile lil light rare earth lre and most incompatible trace elements and are characterized by high 87sr 86sr 0 70571 0 70706 and low 143nd 144nd 0 512609 0 512450 compared with less alkaline volcanics from the sunda arc they also display the relative depletion of ti and nb in chondrite normalized plots which is a feature of subalkaline volcanics from the eastern sunda arc and arc volcanics in general chemical and mineralogical data for the batu tara k rich rocks indicate that they were formed by the accumulation of variable amounts of phenocrysts in several melts with different major and trace element compositions the compositions of one of these melts estimated from glass inclusions in phenocrysts is relatively fe rich 100 mg mg fe2 48 51 and is inferred to have been derived from a more primitive magma by low pressure crystal fractionation involving olivine clinopyroxene and spinel mg rich mg 90 and cr rich up to 1 7 wt cr2o3 zones in complex oscillatory zoned clinopyroxene phenocrysts probably also crystallized from such a magma the marked oscillatory zoning in the clinopyroxene phenocrysts is considered to be the result of limited mixing of relatively evolved with more primitive magmas together with their phenocrysts along interfaces between discrete convecting magma bodies

oxygen isotope systematics of the banda arc low delta o 18 despite involvement of subducted continental material in magma genesis abstract this study reports new laser fluorination oxygen isotope data for 60 volcanic rocks and 15 sediments distributed over the whole length of the banda arc eastern indonesia the melt oxygen isotope values 18omelt were calculated from phenocryst 18o data using theoretical and empirical mineral melt fractionation factors the 18omelt of individual volcanic centers within the arc varies between 5 57 and 6 54 except for serua 18omelt 6 13 7 48 and ambon 18omelt 8 12 8 38 these 18omelt values are up to 2 lower than new and previously published oxygen isotope data obtained on whole rock powders by conventional methods we attribute this discrepancy to post emplacement low temperature alteration and or to a systematic deviation of the bulk analysis sediment 18owr calculated from the 18o carbonate and silica fractions both measured conventionally range between 12 9 and 24 2 the low 18omelt values excluding serua and ambon overlap with the mantle range and are in agreement with simple two component source mixing models that predict 1 5 addition of subducted continental material to a depleted morb type source in the sub arc mantle this percentage is consistent with previous models based on sr nd pb th he hf isotope data however correlations between incompatible trace element ratios and oxygen isotope systematics requires involvement of partial melts derived from subducted continental material as the major slab component rather than bulk addition the contribution of hydrous fluids from both subducted altered oceanic crust and continental material is probably of minor importance magma mantle wedge interaction models could account for the observed low 18o signatures but predicted effects are difficult to distinguish from models without mantle wedge interaction assimilation of arc crust material is thought to be important for the high 18omelt values of serua and ambon afc modelling suggests up to 20 and 80 assimilation at serua and ambon respectively inclusions of meta sedimentary material and whole rock sr nd isotopes point to assimilation processes at nila but this probably had little effect on the 18o of phenocrysts which record original source values according to radiogenic isotope data magma sources in the banda arc are the most heavily influenced by fluxes of subducted continental material among currently active oceanic island arcs hence the results of this study suggest that high 18o 6 5 in arc lavas are difficult to reconcile with addition of subducted components to magma sources but must reflect assimilation of arc crustal material

integrating puffing and explosions in a general scheme for strombolian style activity strombolian eruptions are among the most common subaerial styles of explosive volcanism worldwide distinctive features of each volcano lead to a correspondingly wide range of variations of magnitude and erupted products but most papers focus on a single type of event at a single volcano here in order to emphasize the common features underlying this diversity of styles we scrutinize a database from 35 different erupting vents including 21 thermal infrared videos from stromboli italy etna italy yasur vanuatu and batu tara indonesia from puffing through rapid explosions to normal explosions with variable ejection parameters and relative abundance of gas ash and bombs using field observations and high speed thermal infrared videos processed by a new algorithm we identify the distinguishing characteristics of each type of activity and how they may relate and interact in particular we record that ash poor normal explosions may be preceded and followed by the onset or the increase of the puffing activity while ash rich explosions are emergent i e with inflation of the free surface followed directly by emission of increasingly large gas pockets overall we see that all strombolian activities form a continuum arising from a common mechanism and are modulated by the combination of two well established controls 1 the length of the bursting gas pocket with respect to the vent diameter and 2 the presence and thickness of a high viscosity layer in the uppermost part of the volcanic conduit

petrogenesis of the ultrapotassic lincoln syenite maine late silurian early devonian melting of a source region modified by subduction driven metasomatism the late silurian early devonian 418 1 ma lincoln syenite exposed discontinuously over a distance of 75 kilometers in south central and mid coastal maine represents one of the most unusual igneous rocks in the appalachian orogen detailed field and petrographic study reveals that most of the exposed syenites have been variably affected by post magmatic recrystallization however a small proportion of the syenites in south central maine preserve original igneous minerals and textures and allow for an unobstructed study of the igneous petrogenesis the syenites are characterized by large dark colored alkali feldspar megacrysts set in a finer grained matrix dominated by alkali feldspar clinopyroxene orthopyroxene biotite oxides the rocks are intermediate in terms of sio2 content 55 59 wt yet are ultrapotassic 6 7 wt k2o and relatively primitive in terms of their mgo mg 67 cr 400 ppm and ni 150 ppm concentrations additionally the rocks exhibit large ion lithophile element enrichment relative to the high field strength elements along with elevated concentrations of light rare earth elements 200 300 chondritic abundances the syenites show limited radiogenic isotopic variability with relatively high initial 87sr 86sr values 0 707 to 0 710 relatively low initial end values 4 8 to 6 7 and initial pb isotopic compositions characteristic of igneous rocks in central maine for example 207pb 204pb 15 577 to 15 639 the geochemical characteristics of these rocks are most consistent with a model involving the partial melting of a mantle wedge previously metasomatized by subduction derived fluids crystallization of this magma was influenced by the presence of significant amounts of water that suppressed plagioclase feldspar crystallization facilitated the crystallization of mg rich biotite and ultimately resulted in the nucleation and growth of the alkali feldspar megacrysts correlation with the geochemically similar parks pond pluton in eastern maine extends the distribution of late silurian early devonian ultrapotassic magmatism in maine to an orogen parallel distance of over 150 km comparisons with rocks of similar composition in known tectonic environments suggest the lincoln syenite and by association the parks pond pluton formed as a result of lithospheric extension superimposed on a sub continental lithospheric mantle source region previously modified by subduction driven metasomatism the timing and structural position of this ultrapotassic magmatism is consistent with a geodynamic model involving slab break off or the convective removal of lithospheric mantle delamination in association with acadian convergence both processes facilitate a brief period of crustal extension and decompression melting of source rocks previously modified by subduction related processes

the geochemistry of young volcanism throughout western panama and southeastern costa rica an overview oblique aseismic subduction below western panama and southeastern costa rica has produced recent arc related volcanism the aseismicity is probably related to the subduction of relatively hot oceanic lithosphere the volcanism throughout this region over the past 2 ma has been quite distinct consisting of felsic magmas andesites to rhyolites but mainly dacites with geochemical signatures suggesting a metamorphosed basaltic source it is believed that the subduction of young oceanic crust sets up conditions under which the slab melts rather than the overlying mantle wedge rocks with slab melt geochemistries and associated with young subducted crust have been termed adakites elsewhere the young adakite melts are sometimes associated with a few rare young high nb basalts but there is no obvious genetic link between them through differentiation high nb basalts may also be derived from the partial melting of the subducted oceanic crust high nb basalt migmatites have been found with pegmatites of adakite compositions in the exposed subduction terrain of the catalina schist california alternatively the high nb basalts may be partial melts of phlogopite rich mantle that has previously reacted with adakite magmas eruption of adakites and high nb basalts was preceded by a 2 3 ma period of relative quiescence prior to this there was a 7 ma period of calc alkaline volcanism typical of the present day magmatism associated with a distinct benioff zone found throughout the central american arc the abrupt transition in volcanism with time from an early calc alkaline sequence to a later adakite high nb basalt sequence may record a change in the tectonic setting of western panama and southeastern costa rica over the past 12 ma

neodymium isotopic composition of quaternary island arc lavas from indonesia abstract 143 nd 144 nd ratios measured in quaternary lavas from java and the banda arc of indonesia range from 0 51242 to 0 51280 and exhibit an inverse correlation with 87 sr 86 sr isotopically the indonesian samples resemble andean rather than island arc lavas the samples from java plot either within or adjacent to the mantle array towards higher 87 sr 86 sr ratios samples from the banda arc and the anomalous calc alkaline volcano papandajan are characterized by relatively low 143 nd 144 nd and high 87 sr 86 sr ratios these characteristics are consistent with the interpretation that subducted terrigenous material was involved in the genesis of these lavas furthermore the banda arc samples appear to lie on a mixing line between isotopic compositions characteristic of the mantle and upper continental crust a high k trachyte from the alkaline volcano muriah java has isotopic characteristics of the mantle 143 nd 144 nd 0 51270 87 sr 86 sr 0 70424 which implies that the extreme enrichment in large ion lithophile elements in its source must have occurred only shortly before its formation the inferred 143 nd 144 nd ratio of the unmodified mantle beneath java and the banda arc is lower than that observed in mid ocean ridge basalt which may have important implications for a better understanding of the geochemical structure of the mantle

the geochemistry and petrogenesis of the lavas of the vulsinian district roman province central italy the vulsinian lavas are dominated by a suite of undersaturated leucite bearing basic to intermediate compositions the remaining lavas are mainly oversaturated and have shoshonitic affinities one hundred and thirty five samples have been analysed for major elements and most for 20 trace elements twenty seven lavas have been analysed for ree they are all perpotassic for the undersaturated lavas k2o na2o 2 8 and have very high lil element concentrations e g rb 400 800 ppm th 25 150 ppm ree reecho c 200 lree hree cho c 20 even in the most basic rocks

mapping magma sources in the east sunda banda arcs indonesia constraints from helium isotopes abstract we report new helium isotope analyses of olivine and clinopyroxene separates from recent lavas for eleven volcanoes from flores in the east sunda arc through the inactive segment between the arcs to banda island at the extreme of the contiguous banda arc in the east sunda arc 3 he 4 he ratios r vary between 4 5 ra r a air 3 he 4 he for the leucitic batu tara volcano to a remarkable low of pure radiogenic helium 0 0075 ra for werung at the southern tip of lomblen island lavas from the inactive zone which represents the locus of collision of the australian continent with the arc have a narrower range in r r a from 3 9 for kisu in the straits of pantar to 1 0 for romang island our one locality banda island in the banda arc gives the highest r r a ratio 3 1 observed along this arc to date the results are consistent with the involvement of crustal material in magma genesis throughout the east sunda banda arcs as far west as iya in central flores we combine these helium isotope results with published and on going strontium isotope studies and show that the source of the helium in the crustal component is unlikely to be terrigenous sediments derived from the australian continent rather degassing of australian continental crust appears to be the dominant process controlling the helium budget the he sr isotope systematics also provide a framework to account for the areal pattern of 3 he 4 he in this part of the arcs the radiogenic crustal component is diluted with mantle helium both in a down dip direction and with increasing lateral distance from the collision zone these factors result in an excellent first order relationship between the 3 he 4 he ratio degree of he sr enrichment relative to the postulated mantle endmember and alkalinity of the erupted lavas such a relationship has a direct bearing on models of the tectonic evolution of the collision zone and on the observation that helium isotopes are decoupled from strontium and other geochemical signatures along the banda arc

mafic potassic lavas of the quaternary west eifel volcanic field major and trace element analyses for 103 volcanoes of the quaternary west eifel volcanic field show the lavas to be dominantly primitive mgo 7 wt and potassic na2o k2o 1 the rocks are divided into 1 a foidite f suite volumetrically dominant and consisting of four types leucitites and nephelinites melilite bearing foidites olivine free foidites sodalite bearing melilite free foidites and 2 a younger olivine nephelinite and basanite onb suite concentrated in the southeastern part of the field dominantly cpx phyric f suite magmas differ from the dominantly ol phyric onb suite mainly in higher k2o na2o and cao al2o3 ratios higher rb cu h2o co2 and lree concentrations and slightly lower sr ni and y contents most magmas have fractionated small amounts of olivine clinopyroxene and minor phlogopite systematic compositional variations within volcanoes or volcano groups are rare five more differentiated volcanoes 2 tephrites 3 phonolites occur in the center of the field their magmas are interpreted to have formed by fractionation within crustal magma chambers

the origin of the potassic rock suite from batu tara volcano east sunda arc indonesia abstract atu tara is an active potassic volcano in the eastern sunda arc its leucite bearing rock suite can be subdivided into two groups one less evolved with th 20 ppm 87sr 86sr 18o and trace element systematics in the less evolved group suggests that existence of parental magmas with different mantle origins the mantle below batu tara is most likely heterogeneous and several source components are involved in magma genesis trace element and isotopic compositions of batu tara and adjacent volcanoes are consistent with the involvement of a subducted sedimentary crustal component as well as morb and oib mantle the latter with geochemical characteristics comparable to the mantle underlying muriah java melt extraction from this complex mixture is envisioned as a two stage process partial melts of the crust contaminated morb mantle mix in the mantle wedge with partial melts of oib domains different mixtures of these two melts provide the parental magmas that enter the volcanic plumbing system where crystallization hybridization and refilling processes occur the calcalkaline volcanoes in the arc segment show stronger signatures for a subducted crustal component than batu tara which displays a greater influence from the oib mantle source the potassium enrichment can therefore be attributed to contributions both from the enriched mantle and from subducted crustal material mantle type 18o values of the batu tara magmas indicate that the mantle wedge below potassic orogenic volcanoes is not necessarily strongly enriched in 18o

late cretaceous crustal growth in the gangdese area southern tibet petrological and sr nd hf o isotopic evidence from zhengga diorite gabbro abstract recent studies of gangdese granitic magmatism demonstrate a mantle contribution to crustal growth in southern tibet during the jurassic early eocene however the specific mechanism for adding such juvenile crust has been disputed owing to a lack of reliable evidence for contemporaneous deep mantle geodynamic processes here we report on the zhengga diorite gabbro suite from the gangdese area they consist of plagioclase labradorite and anorthite amphibole clinopyroxene biotite and minor magnetite epidote zircon and apatite la icp ms zircon u pb dating for two samples gives a ca 94 ma age for the zhengga intrusive rocks i e the late cretaceous apart from one high sio 2 52 2 wt diorite sample with slightly high k 2 o 1 72 wt and initial 87 sr 86 sr 0 7068 and low e nd t 5 6 values the gabbro samples are geochemically characterized by low sio 2 39 8 50 1 wt and k 2 o 0 3 1 1 wt strongly negative nb ta and positive sr anomalies and uniform initial 87 sr 86 sr 0 7043 0 7048 the gabbros can be divided into two groups group i gabbros with relatively low total rare earth element ree rb and sio 2 contents and positive eu anomalies and group ii gabbros with slightly higher total ree rb and sio 2 contents and negative eu anomalies the group i gabbros have e nd t 1 7 to 4 1 and zircon e hf t 6 5 to 11 1 and 18 o 5 89 to 7 24 values which are slightly different to those of the group ii gabbros 0 2 to 2 0 2 9 to 6 5 and 6 24 to 7 05 trace element compositions of amphibole and clinopyroxene grains suggest that the zhengga mafic magmas contained a significant fluid transported component probably released from subducted oceanic lithosphere we suggest that the parental magmas of the zhengga gabbros were generated by the hydrous partial melting of lithospheric mantle metasomatized by sediment melts fluids the group i gabbros were likely generated by the fractional crystallization of olivine or clinopyroxene from such parental magmas with insignificant crustal contamination whereas the group ii gabbros were probably produced by assimilation and fractional crystallization afc processes from mafic magmas that were geochemically similar to the group i gabbros pre collisional underplating of mantle derived mafic magmas likely played an important role in crustal growth and supplied the source materials for some late late cretaceous cenozoic granitoids of the gangdese batholiths this study also demonstrates that the hydrous partial melting of mantle wedge triggered by the dehydration in a subduction setting has a capacity to create significant volumes of juvenile continental crust

geochemical characteristics of the south tuscany italy volcanic province constraints on lava petrogenesis abstract three suites of volcanic rocks from radicofani mts cimini and mt amiata south tuscany italy were analyzed for major and trace element contents and sr isotopic composition all samples are silica saturated hypersthene normative and some are peraluminous samples from radicofani and the less evolved mts cimini samples have high mg numbers 74 5 66 3 consistent with the fosteritic rich nature of their olivine phenocrysts and high bulk rock abundances of ferromagnesian trace elements all samples are highly enriched in light ree 150 315 chondrites compared to heavy ree 9 15 chondrites and have negative eu anomalies eu eu 0 92 0 44 87 sr 86 sr ratios are high and variable radicofani 0 7137 0 7150 mts cimini 0 7128 0 7138 mt amiata 0 7119 0 7131 a distinctive feature of the radicofani samples is that with decreasing age the rocks increase in k2o content but decrease in la ce these south tuscany rocks have geochemical characteristics of the calc alkaline and alkaline series namely in addition to the high k rb cs ba and lree contents they have high ratios of incompatible elements relative to the high field strength ions such as ti zr hf nb and hree when compared to an estimated primitive mantle composition in general the geochemical characteristics of south tuscany volcanics are consistent with mixing between crustal derived material and mantle derived magma the mantle derived component was unusually rich in radiogenic sr and incompatible elements and had geochemical features similar to basic lavas of the high k series in the roman province

crystal chemistry and site configuration of the clinopyroxene from leucite bearing rocks and related genetic significance the sabatini lavas roman region italy

the petrology of tambora volcano indonesia a model for the 1815 eruption abstract tambora is an active volcano in the east sunda arc and is well known for its catastrophic eruption in april 1815 its lavas are of unusual moderately undersaturated k2o rich types ranging from ne trachybasalt to ne trachyandesite they evolved under conditions of high oxygen fugacity about two log units below the m h buffer and crystallised leucite at low pressures the products of the 1815 eruption are black glassy biotite bearing ne trachyandesites together with scoria pumice and tuff of the same composition the parent trachybasalt liquid probably contained about 3 wt h2o and the trachyandesites nearly 6 wt the model presented here suggests that the 1815 eruption which is known to have followed a lengthy period of inactivity was the result of gradual cooling of a hydrous magma in a closed high level magma chamber emplaced at depths between 1 5 and 4 5 km eruption followed the exsolution of a high pressure aqueous fluid phase formed during cooling and crystallisation of the magma second boiling by the time eruption took place overpressures of about 4 5 kbar had been generated and temperatures in the magma chamber ranged from about 850 c to 700 c the catastrophic eruption took place after failure of the roof and the discharge of the vapour phase may have reached velocities of 650 m s 1 calculations indicate that the volume of magma involved in the eruption was of the order of 33 km3 and this may have evolved about 1 2 1027 ergs of energy during the eruption a blast equivalent to 30 000 megatons of tnt