late magmatic mineral assemblages with siderite and zirconian pyroxene and amphibole in the anorogenic mt gibraltar microsyenite new south wales australia and their petrological implications abstract the mt gibraltar intrusion near mittagong and bowral in new south wales australia lat 34 27 54 s long 150 25 44 is a small intrusive body of hypersolvus microsyenite emplaced into the triassic hawkesbury sandstone of the sydney basin in jurassic time possibly related to extensional faulting the rock itself consists of intermediate alkali feldspar with minor titanomagnetite and interstitial pyroxene ranging from nearly pure hedenbergite to hd 34 aeg 65 in composition it is crosscut by an irregular system of late magmatic veins consisting of homogeneous alkali feldspar ab 50 or 50 clinopyroxene evolving from sodic hedenbergite to zirconium rich aegirine arfvedsonite and siderite during postmagmatic evolution of the veins microcrystalline or amorphous silica precipitated together with calcite filling miarolitic cavities the late magmatic mineral assemblage of the veins indicate crystallisation at assumed 700 bar pressure at t 650 670 c log f o2 22 this corresponds to conditions very close to the magnetite wustite curve zirconium bearing pyroxene has a restricted stability field in the system sio 2 zro 2 feo feo 1 5 nao 0 5 ho 0 5 at moderately elevated peralkalinity and intermediate silica activity under such conditions pyroxene and amphibole will act as effective sinks for zr preventing crystallisation of magmatic zircon or more exotic zr silicates the mt gibraltar microsyenite is therefore a rare example of an igneous rock in which zirconium is camouflaged in pyroxene and amphibole rather than forming its own minerals

oxygen isotope evidence for extensive crustal contamination in the okenyenya igneous complex namibia abstract oxygen isotope data are presented for silicate minerals separated from a variety of rock types from the 130 ma okenyenya igneous complex of northwestern namibia these rock types include a tholeiitic suite ranging from olivine gabbro to quartz monzodiorite and syenite and an alkaline suite including gabbro syenite nepheline syenite and essexite in general the difference in 18 o values between coexisting feldspar pyroxene and biotite are consistent with equilibrium at high temperatures and preclude significant interaction with magmatic or external fluids the 18 o values of pyroxene can be used to model magmatic processes and show a systematic increase from 7 1 9 0 along a transect through a tholeiitic olivine gabbro quartz monzodioritic body corresponding to a systematic decrease in mg pyroxene 18 o values also show a strong correlation with initial strontium and neodymium isotope ratios with the alkaline suite having lower 18 o values lower initial strontium isotope ratios and higher initial neodymium isotope ratios than the tholeiitic suite simple oxygen mass balance calculations suggest that the extremes of isotope composition resulted from at least 60 contamination by material of similar isotope composition to the nearby southern etendeka quartz latites assimilation crystal fractionation models of oxygen strontium and neodymium isotope data suggest that the rate of assimilation was high and may have approached simple mixing the contaminated tholeiitic olivine gabbro quartz monzodioritic body was the earliest to crystallize in the complex and this was followed by relatively uncontaminated silica undersaturated magmas

petrology of nepheline syenite pegmatites in the oslo rift norway zirconium silicate mineral assemblages as indicators of alkalinity and volatile fugacity in mildly agpaitic magma agpaitic nepheline syenites contain complex zirconium silicate minerals as primary magmatic phases agpaitic liquidus mineral assemblages are controlled by compositional parameters such as alkalinity and concentrations of water and halogens in the magma the larvik plutonic complex in the late palaeozoic oslo rift norway consists of hypersolvus monzonite larvikite and different varieties of nepheline syenite the nepheline syenite members have miaskitic mineralogy i e zircon stable except for a suite of late mildly agpaitic pegmatites which contain zirconium silicate minerals of the wohlerite rosenbuschite and eudialyte groups in some cases together with zircon these minerals form part of the liquidus mineral assemblage of the nepheline syenite magma together with fluorite and the rock forming minerals alkali feldspar aegirine augite alkali amphibole biotite nepheline and sodalite low variance mineral assemblages with two or three zirconium silicate minerals fluorite and the major rock forming minerals can be modelled in terms of phase equilibria in a multi component system involving the sodium disilicate component of the melt and the volatile components water fluorine and chlorine a semiquantitative isothermal and isobaric petrogenetic grid in log activity space constructed from the observed mineral assemblages and the compositions of the coexisting minerals indicate that there are at least three trends of evolution leading from a miaskitic to an agpaitic crystallization regime in peralkaline nepheline syenites 1 the increasing alkalis trend typically leading to eudialyte dominated liquidus assemblages and possibly further to hyperagpaitic residual liquids 2 the increasing water trend leading to catapleiite crystallization 3 the increasing fluorine trend giving transitionally agpaitic liquidus assemblages with wohlerite lavenite and hiortdahlite with or without zircon but eventually with magmatic fluorite this type of evolution is applicable to weakly agpaitic systems such as the oslo rift pegmatites the evolution of such magmas will most probably terminate at liquidus boundaries also involving eudialyte and or rosenbuschite and they are unlikely to evolve to more strongly agpaitic compositions

the role of hydrothermal processes in concentrating high field strength elements in the strange lake peralkaline complex northeastern canada abstract the middle proterozoic peralkaline pluton at strange lake quebec labrador comprises hypersolvus to subsolvus phases which are unusually enriched in zr y rees nb be and f as exotic alkali and alkaline earth silicate minerals the highest concentrations of these elements are in subsolvus granite which underwent intense low temperature 200 c hydrothermal alteration involving hematization and the replacement of alkali high field strength element hfse minerals by calcic equivalents this alteration is interpreted to have been caused by meteoric or formational waters high temperature 350 c alteration attributed to orthomagmatic fluids is evident in other parts of the subsolvus granite by the replacement of arfvedsonite by aegirine comparisons of the chemical compositions of fresh and altered rocks indicate that rocks subjected to high temperature alteration were chemically unaffected except for depletion in zr y and hrees these elements were appreciably enriched in rocks that underwent low temperature alteration other elements affected by low temperature alteration include ca and mg which were added and na which was removed available data on hfse speciation in aqueous fluids and the chemistry of the pluton suggest that the hfses were transported as fluoride complexes if this was the case the low temperature fluid could not have been responsible for hfse transport because the high concentration of ca and low solubility of fluorite would have buffered f activity to levels too low to permit significant complexation we propose that hfse mineralization and accompanying alteration were the result of mixing in the apical parts of the pluton of a f rich essentially ca free orthomagmatic fluid containing significant concentrations of hfses with an externally derived meteoric dominated fluid enriched in ca as a result of interaction with calc silicate gneisses and gabbros according to this interpretation the latter fluid was responsible for the exchange of ca for alkalis mainly na in hfse rich minerals and by sharply reducing f activity in the mixed fluid through fluorite precipitation and or increasing ph destabilised the hfse fluoride complexes causing deposition of hfse bearing minerals an important implication of this study is that major hfse enrichment may be restricted to those rare cases where f rich ca free metal leaching environments and ca rich depositional environments are juxtaposed

in situ u pb sr nd and hf isotopic analysis of eudialyte by la mc icp ms abstract eudialytes are a group of complex na ca zirconosilicate minerals that generally occur in peralkaline agpaitic syenites given that eudialytes are easily altered and commonly contain inclusions of earlier crystallized minerals in situ laser ablation is considered as the most suitable method to determine their u pb ages and sr nd and hf isotopic compositions electron microprobe and la icp ms analysis of eudialytes from nepheline syenites from ilimaussaq greenland khibiny and lovozero russia saima china tamazeght morocco kipawa and mont saint hilaire canada pocos de caldas brazil and langesund fjord norway indicate that this mineral typically has high contents of u pb nb ta zr hf and rare earth elements ree analysis of an in house standard eudialyte by both solution and laser ablation methods demonstrates that precise and accurate u pb ages can be obtained from eudialyte after correction for the common pb content the high sr nd and hf contents in eudialyte coupled with the generally low rb sr and lu hf ratios also permit the precise determination of in situ sr nd and hf isotopic ratios by la mc icp ms methods however some eudialytes with rb sr ratio of 0 02 and yb sr 0 1 and lu hf 0 08 or yb hf 0 4 0 5 cannot be used for in situ sr and hf isotopic measurement eudialyte is the only mineral investigated to date for which it is possible to determine simultaneously u pb ages and sr nd and hf isotopic compositions

oxygen isotope constraints on the petrogenesis of volcanic arc magmas from martinique lesser antilles abstract volcanic rocks from the island of martinique in the lesser antilles island arc are highly diverse isotopically values of 18o smow in lavas erupted over the last 5 ma from five stratigraphically discrete and geochemically distinct volcanic centers are morne jacob 7 5to 7 9 pitons du carbet 8 0to 8 9 diamant 6 3to 7 4 piton mt conil 7 2to 7 3 andmt pelee 6 5to 7 4 13 of the 27 samples have been corrected for variable amounts of secondary alteration and low temperature hydration the least18o rich andesites on martinique have 18o values similar to those of the northern lesser antilles 18o 6 0 to 7 3 and other intra oceanic arcs like the marianas 18o 5 5 to 6 8 o isotope ratios are compositionally dependent correlations between 18o and radiogenic isotope ratios 87sr 86sr 143nd 144nd 206pb 204pb and indices of fractionation show that modification of isotopic compositions occurred during magmatic differentiation rather than at source a two stage model is proposed for the origin and differentiation of magmas on martinique primary basaltic melts which are broadly similar in composition to primitive island arc magmas elsewhere e g marianas aleutians south sandwich islands are generated from the subduction modified asthenospheric mantle wedge these magmas ascend into the arc crust and differentiate on martinique crystal fractionation is accompanied by contamination with isotopically distinct crust including a component of terrigenous sediments thus subduction zone processes at the lesser antilles and other island arcs are similar involving only a small albeit significant isotopically volumetric contribution from subducted sediment

lessons from ontake san a comparative analysis of debris avalanche dynamics abstract dynamic aspects of the long runout ontake san debris avalanche are evaluated by a comparison of several models an unsteady numerical model assumes two dimensional flow of an incompressible biviscous or newtonian fluid represented as a continuum with a free surface internal deformation of the flowing mass is considered as well as boundary resistances thus flow thinning and deposit shape as well as flow kinematics may be modeled parameters are adjusted to match observed runout with additional constraints on velocity and emplacement time with abundant constraints for ontake san from careful field investigations by japanese research teams our analysis indicates that a substantial decrease in flow resistance occurred as a function of displacement constant property models that match runout tend to overestimate the peak velocities and to underestimate the emplacement times a staged increase in mobility in both constant volume and variable volume models leads to results consistent with field data runout in a channel overflow area was also modeled qualitatively similar results have been obtained by other researchers using simple sliding block models with empirical parameters a slide block model with rational parameter selection a modified flood simulation and a multi element frictional slide model the relative merits of these models are compared the field mechanisms associated with this mobility increase with displacement are poorly understood but the question is now identified as a target for future research at debris avalanche sites and some plausible mechanisms are considered the main reason probably involves the entrainment of river water and saturated sediment leading to enhanced efficiency of fluid pressure mechanisms with undrained shear in addition progressive shearing reduced the mean particle size and angularity and the cohesion and friction and apparent viscosity of avalanche debris near the wetted perimeter hydroplaning the shearing of water films and slurries may have occurred locally

the mineralogical diversity of alkaline igneous rocks critical factors for the transition from miaskitic to agpaitic phase assemblages geochemically the large family of alkaline plutonic rocks both qtz undersaturated and oversaturated compositions can be subdivided into metaluminous na 2 o k 2 o al 2 o 3 types in this paper we discuss two important aspects of the mineralogical evolution of such rocks with respect to their fe mg phases a major mineralogical transition observed is the precipitation of arfvedsonite or aegirine instead of fayalite or magnetite ilmenite the relative stability of these phases is controlled by oxygen fugacity and na activity in the crystallizing melts if na activity in the melt is high enough arfvedsonite aegirine form a common assemblage in peralkaline rocks under both reduced and oxidized conditions major mineralogical differences within this rock group exist with respect to their high field strength element hfse rich minerals most syenitic rocks known as miaskites contain zircon titanite or ilmenite as hfse rich minerals whereas in agpaites complex na k ca ti zr silicates incorporate the hfse similarly only a small group of peralkaline granites are found to lack zircon titanite or ilmenite but instead contain na k ca ti zr silicates here we present a detailed phase petrological analysis of the chemical parameters na 2 o cao k 2 o that influence the transition from miaskitic to agpaitic rocks based on the occurrence of ti and zr minerals several transitional mineral assemblages are identified and two major evolution trends for agpaites are distinguished a high ca trend which is exemplified by the alkaline rocks of the kola province russia and a ca depletion trend which is displayed by the alkaline rocks of the gardar province south greenland both trends show significant na enrichment during magmatic evolution high ca agpaites evolve from nephelinitic parental melts that did not crystallize large amounts of plagioclase in contrast agpaites showing ca depletion originate by extensive fractionation of plagioclase from basaltic parental melts in some peralkaline granites evolutionary trends are observed that culminate in agpaite like hfse mineral associations in the most evolved rocks

geochemical and isotopic zoning patterns of plagioclase megacrysts in gabbroic dykes from the gardar province south greenland implications for crystallisation processes in anorthositic magmas chemical and sr isotopic zoning patterns in plagioclase megacrysts from gabbroic dykes in the gardar province can be used to elucidate magma chamber and emplacement processes the megacrysts occur either as single crystals or assembled as anorthosite xenoliths the size of the megacrysts varies from 1 cm to 1 m they consist of a large core with variable zonation an58 39 and a relatively small 600 m normally zoned rim an62 27 the contact between core and rim is sharp and marked by a sharp increase in anorthite content which can reach 11 mol an this gap is interpreted as having formed during dyke emplacement due to a sudden pressure release some of the megacryst cores show a fairly constant composition whereas others exhibit an unusual wavy oscillatory zoning which has not been reported elsewhere to our knowledge the oscillatory zoning has wavelengths of up to 2 500 m and a maximum amplitude of 7 mol an it is interpreted as reflecting movements of the crystals in the magma reservoir the sr isotopic composition of one crystal shows a radiogenic inner core 87sr 86sr i 0 7044 and a less radiogenic outer core 87sr 86sr i 0 7039 0 7036 the lack of a significant change between outer core and rim 87sr 86sr i 0 7037 is consistent with formation of the more an rich rim due to pressure release variations in the core may be related to movements of the crystal and or magma mixing a trace element profile across a megacryst shows a small increase in sr and small decreases in ba and la contents of the recalculated melt composition across the core rim boundary whereas p ce nd and eu remain constant melt compositional changes upon emplacement are therefore considered to be of minor importance constant ratios of incompatible trace elements in the megacryst cores indicate a dominant influence of a lower crustal source on trace element budgets

hydrogen and oxygen isotope geochemistry of ascension island lavas and granites variation with crystal fractionation and interaction with sea water lavas and pyroclastics on ascension island contain plutonic blocks that include fluid inclusion bearing peralkaline granite 18o 16o ratios f and cl have been analysed on whole rocks and or minerals for lavas and granites and d h ratios and h2o for comenditic obsidians and granites whole rock 18o 16o ratios of fresh alkali basalt hawaiite trachyandesite trachyte and comendite range from 6 0 to 6 9 with 18o tending to increase with increase in sio2 the 18o values of the granites are from 0 0 to 0 3 depleted in 18o relative to the comendites comenditic obsidians have d 80 4 and h2o 0 3 wt while amphiboles from the granites have d 56 2 the o isotope trend of the lavas is consistent with a crystal fractionation model fresh igneous rocks with 18o values greater than 7 involve processes in addition to crystal fractionation of a basaltic magma the d h ratios and cl contents 3 000 ppm of the h2o poor comenditic obsidians represent undegassed primary magmatic values the h isotope compositions and low h2o and cl 167 ppm contents of the granites are consistent with the major degassing loss of 90 of initial h2o of an h2osaturated magma derived from the interaction of sea or possibly meteoric water with the h2o undersaturated comenditic melt it is proposed that associated with caldera subsidence and stoping water was sucked in around the residual magma before the system had time to be sealed up the h2o undersaturated magma consumed this h2o with possibly some minor partial dehydration and dewatering of the hydrated volcanic roof blocks at a pressure of about 1 5 kb the granites are the plutonic equivalents of rhyolitic pyroclastics and not directly of the comendites granites from oceanic islands may in general be a result of generating an h2o saturated acid melt by such direct or indirect crustal water magma interaction processes

volatiles in a peralkaline system abiogenic hydrocarbons and f cl br systematics in the naujaite of the ilimaussaq intrusion south greenland agpaitic rocks comprise most of the exposed part of the 1 16 ga old 8 17 km large and about 1700 m thick ilimaussaq intrusion in south greenland within these more than 600 m thick sequence of sodalite rich naujaites mainly sodalite arfvedsonite alkali feldspar nepheline eudialyte aenigmatite are interpreted as a sodalite flotation cumulate sodalites show two to three different zones in cathodoluminescence cl and at least two zones in thin sections the cl zones can be related to chemical differences detectable by electron microprobe whereas relations with optical zonations are less obvious compositional trends in sodalite reflect trends in the evolution of volatile contents in the melt the sodalite at ilimaussaq is almost free of ca and closely corresponds to the pure na cl sodalite endmember with about 7 wt of cl s contents reach up to 0 9 wt cl br ratios range from 500 to 1700 raman spectroscopy shows that s is present as so4 2 in sodalite although sphalerite zns is a stable phase in naujaites peralkalinity and fo2 conditions allow s2 and so4 2 to be present contemporaneously the whole naujaite sequence is divided into two parts an upper part with low homogeneous s contents and cl br ratios in the sodalite cores and a lower part with strongly variable and higher s contents and with cl br ratios which are decreasing downwards the details of the s content and the cl br ratio evolution show that sodalite strongly influences the halogen contents of the melt by scavenging cl and br the naujaites were formed from a highly reduced halogen rich magma in equilibrium with magmatic methane at about 800 c which upon ascent cooling and fractionation exsolved an aqueous fluid phase both fluids were trapped in separate inclusions indicating their immiscibility micrometer sized aegirine crystals and primary hydrocarbon bearing inclusions are abundant in the crystal cores the inclusions were trapped at pressures up to 4 kbar although the emplacement pressure of the intrusion is about 1 kbar this indicates growth of the sodalite during melt ascent and a very effective mechanism of trace element scavenging during sodalite growth sodalite rims are devoid of aegirine or primary hydrocarbon inclusions and probably reflect the emplacement stage

a comparison of global rare earth element ree resources and their mineralogy with ree prospects in sri lanka abstract rare earth elements rees a group of 17 elements comprises 15 lanthanides scandium and yttrium are largely attracting the world s attention due to their importance in a wide variety of advanced technological applications global rees production is mainly sourced from resources such as carbonatites alkaline igneous rocks placers laterites and ion adsorption clays recently ree demand has been escalating especially due to the ree applications in renewable energy and defense sectors expecting a worldwide shortage of ree supply in the future therefore rees have been widely accepted as strategic elements in the world which compels to prospect for new and alternative ree resources in this context sri lanka has a favorable geological setting which implies the presence of ree mineralization previous geochemical studies in sri lanka have reported significant concentrations of rees in different geological formations and mineral resources accordingly pulmoddai and other beach placer deposits eppawala carbonatite alluvial placer deposits and pegmatites have been identified as potential ree resources in sri lanka monazite apatite allanite and zircon are the primary rare earth re minerals found in the preceding resources the pulmoddai mineral sand deposit is considered as the most potential ree resource in the island which is enriched in monazite containing more than 61 of light rare earth elements lrees similarly eppawala carbonatite contains high concentrations of lrees however despite their significant ree enrichments to date no attempt has been made to recover these ree prospects which essentially conceals their potential of catering for both local and global ree supply chains

may 18 1980 mount st helens deposits in south coldwater creek washington beginning at 0832 pacific daylight time pdt mount st helens washington produced a rockslide debris avalanche and a lateral explosion blast that generated a laterally directed pyroclastic surge called here a blast surge the resulting deposits are thickest in the east west trending valley of north fork toutle river immediately north of the volcano but are almost as thick in south coldwater creek a small tributary of the toutle river in the lee of an east west trending ridge johnston ridge just north of the north fork toutle river the avalanche surmounted johnston ridge 300 m relief and flowed down the valley of south coldwater creek closely following the avalanche and in part contemporaneously with it the blast surge left thin up to 0 5 m deposits on johnston ridge and thick up to 15 m deposits in south coldwater creek these deposits have a lower layer layer a1 which is friable massive poorer in fines particles 1 16 mm and more poorly sorted than the upper layer layer a2 layer a2 is less friable because it contains more silt and clay sized material beneath layer a1 is a zone containing a small percentage of debris from the blast surge mixed with products eroded from the underlying surface this zone designated as layer a0 is a poorly mixed layer containing schlieren of soil shattered volcanic fragments and considerable quantities of organic material layer a1 is strongly sorted and fines depleted and is interpreted to have been deposited from just behind the turbulent head of the blast surge layer a2 is interpreted to have been deposited from the body of the blast surge which is believed to have been less turbulent than the head layer a0 is interpreted to have formed by shearing at the base of the head of the blast surge in the region between the front and the back of the head of the blast surge it is concluded that a strongly fines depleted basal layer of a pyroclastic flow or surge deposit underlain by a thin sheared zone is the signature of an extremely high velocity particulate density current

a review of the occurrence form and origin of c bearing species in the khibiny alkaline igneous complex kola peninsula nw russia abstract the khibiny complex hosts a wide variety of carbon bearing species that include both oxidized and reduced varieties oxidised varieties include carbonate minerals especially in the carbonatite complex at the eastern end of the pluton and na carbonate phases reduced varieties include abiogenic hydrocarbon gases particularly methane and ethane dispersed bitumens solid organic substances and graphite the majority of the carbon in the khibiny complex is hosted in either the carbonatite complex or within the so called central arch the central arch is a ring shaped structure which separates khibinites coarse grained eudialite bearing nepheline syenites in the outer part of the complex from lyavochorrites medium grained nepheline syenites and foyaites in the inner part the central arch is petrologically diverse and hosts the major ree enriched apatite nepheline deposits for which the complex is best known it also hosts zones with elevated hydrocarbon dominantly methane gas content and zones of hydrothermally deposited na carbonate mineralisation the hydrocarbon gases are most likely the product of a series of post magmatic abiogenic reactions it is likely that the concentration of apatite nepheline deposits hydrocarbon gases and na carbonate mineralisation is a function of long lived fluid percolation through the central arch fluid migration was facilitated by stress release during cooling and uplift of the khibiny complex as a result carbon with a mantle signature was concentrated into a narrow ring shaped zone

crystallization conditions and controls on trace element residence in the main minerals from the pedra branca syenite brazil an electron microprobe and la icpms study abstract major and trace element microanalyses of the main minerals from the 610 ma pedra branca syenite southeast brazil allow inferences on intensive parameters of magmatic crystallization and on the partition of trace elements among these minerals with important implications for the petrogenetic evolution of the pluton two main syenite types make up the pluton a quartz free syenite with tabular alkali feldspar laminated silica saturated syenite lss with na rich augite phlogopite hematite magnetite titanite apatite and a quartz bearing syenite laminated silica oversaturated syenite lso with scarce corroded plagioclase plus diopside biotite hornblende ilmenite magnetite titanite apatite both types share a remarkable enrichment in incompatible elements as k ba sr p and lree apatite saturation temperatures of 1060 1090 c are the best estimates of liquidus whereas the pressure of emplacement based on al in hornblende barometry is estimated as 3 3 to 4 8 kbar although both units crystallized under oxidizing conditions oxygen fugacity was probably higher in lss as shown by higher mg of the mafic minerals and higher hematite contents in hem ilmss in contrast with the ca bearing alkali feldspar from lso which hosts most of the whole rock sr and pb virtually ca free alkali feldspar from lss hosts 50 of whole rock sr and 80 of pb the remainder of these elements being shared by apatite pyroxene and titanite this contrast reflects a strong crystal chemical control whereby a higher proportion of an element with similar ratio and charge ca2 enhances the residence of sr and pb in the m site of alkali feldspar the more alkaline character of the lss magma is inferred to have inhibited zircon saturation zr hf remained in solution until late in the crystallization and were mostly accommodated in the structure of ca na pyroxene and titanite which are one order of magnitude richer in these elements compared to the same minerals in lso where most of zr and hf are inferred to reside in zircon the ree th and u reside mostly in titanite and apatite d ree tit ap raises steadily from 1 to 6 from la to tb then remains constant up to lu in the lso sample these values are about half as much in the lss sample where lower contents of incompatible elements in titanite are attributed to its greater modal abundance and earlier crystallization

rare earth elements in apatite as a monitor of magmatic and metasomatic processes the ilimaussaq complex south greenland abstract textural and compositional variations of apatite from the plutonic ilimaussaq complex in south greenland and its surrounding country rocks track magmatic and metasomatic processes detailed back scattered electron bse imaging reveals various types of apatite textures including i growth zonation concentric oscillatory as well as sector zonation formed during magmatic differentiation ii resorption and overgrowth textures due to fluid melt induced metasomatic overprint of precursor apatite and iii replacement textures indicating the destabilization of apatite in favor of monazite in the least evolved rocks of the ilimaussaq complex apatite incorporates rare earth elements and y rey mainly via the coupled substitution ca 2 p 5 rey 3 si 4 in the more evolved peralkaline rocks and in metasomatically overprinted rocks however the coupled substitution 2 ca 2 rey 3 na becomes increasingly relevant and apatite incorporates successively more lree compared to hree as shown by increasing la gd n and gd yb n ratios similarly at the contact between the ilimaussaq rocks and the granitic country rocks the metasomatic effect of the emplacement of the ilimaussaq melts is preserved in partly resorbed precursor apatite which is overgrown by rey enriched apatite with higher la gd n and gd yb n ratios compared to apatite from granites more distant from the contact this study shows that apatite textures and compositions are suitable to track both primary magmatic and later fluid present metasomatic processes the incorporation of rey in apatite is not only dependent on the amount of rey present but also depends largely on na activity in the apatite precipitating melts and fluids

a reassessment of models for hydrocarbon generation in the khibiny nepheline syenite complex kola peninsula russia although hydrocarbon bearing fluids have been known from the alkaline igneous rocks of the khibiny intrusion for many years their origin remains enigmatic a recently proposed model of post magmatic hc generation through fischer tropsch type reactions suggests a primary co2 fluid this hypothesis is based upon textural data gas bulk and isotopic composition and low entrapment temperature of fluid inclusions that indicate an abiogenic origin for the hc gases however new petrographic microthermometric laser raman bulk gas and isotope data are presented and discussed in the context of previously published work in order to reassess models of hc generation the gas phase is dominated by ch4 with only minor proportions of higher hydrocarbons no remnants of the proposed primary co2 rich fluid are found anywhere in the complex primary and secondary fluid inclusions show similar compositions and entrapment conditions the majority of the fluid inclusions are of secondary nature and trapped in healed microfractures this indicates a high fluid flux after magma crystallisation entrapment condtions for primary fluid inclusions are 450 550 c at 2 8 4 5 kbar these temperatures are too high for hc gas generation through the ft reaction chemical analyses of rims of fe rich phases suggest that they are not the result of alteration but instead represent changes in magma composition during crystallisation furthermore there is no clear relationship between the presence of fe rich minerals and fluid inclusion abundance as reported elsewhere fluid inclusion planes generally start and terminate at mineral boundaries 13c values for methane range from 22 4 to 5 4 confirming also an abiogenic origin for the gas these data support an abiogenic hc origin but are not consistent with a post magmatic ft type reaction the presence of primary ch4 dominated fluid and melt inclusions which contain a methane rich gas phase indicate that a magmatic origin is more likely an increase in methane content together with a decrease in 13c isotope values towards the intrusion margin suggest that magmatically derived abiogenic hydrocarbons may have mixed with biogenic hydrocarbons derived from the surrounding country rocks

petrology of the alkaline core of the messum igneous complex namibia evidence for the progressively decreasing effect of crustal contamination the messum complex of nw namibia a part of the parana introduction etendeka volcanic province consists of a dominantly felsic central the etendeka igneous province in the damaraland core surrounded by older gabbros the igneous rocks of the core region of northwest namibia fig 1 contains numerous can be divided in order of decreasing age into 1 a sub alkaline anorogenic igneous complexes of mesozoic age martin suite 2 an outer quartz syenite suite and 3 an inner silicaet al 1960 piranjo 1994 one of which is the messum undersaturated suite dominated by nepheline syenite compositional complex the range in chemical composition both within differences within the quartz syenite suite can be explained by and between the various complexes is remarkable and fractional crystallization but srand o isotope data indicate that includes both silica oversaturated and silica underthese rocks contain a significant crustal component the younger saturated mafic and felsic rocks including carbonatite central nepheline syenites trend towards the 0 1 gpa phonolite messum was classified as a mixed complex by harris minimum and appear to have evolved by closed system fractional 1995 as it contains substantial amounts of both silicacrystallization of a mantle derived magma whereas earlier nepheline undersaturated and silica oversaturated rock types syenites appear to be contaminated with quartz syenite the dating of several damaraland complexes yields ages decreasing effects of crustal contamination with time may reflect that lie between 137 and 124 ma siedner miller progressive armouring of the magmatic plumbing system by less 1968 milner et al 1995b renne et al 1996 the contaminated material basanite dykes that intrude the plutonic alignment of the complexes along two parallel ne sw rock of the core show trace element and ndand sr isotope trends suggests that they represent a hotspot trace caused characteristics that are similar to those of basanites from tristan by the motion of the african plate over a fixed mantle da cunha this suggests that the mantle derived component of the plume e g rhodes 1971 because there is no systematic age progression along the lineament this view has genmessum core rocks came from the tristan plume erally been discounted marsh 1973 recent ideas that large continental flood basalt cfb provinces such as the parana etendeka province are manifestations of mantle plume activity white mackenzie 1989 has refocused attention on the relationship between plumes and linear arrays of complexes in cfb provinces milner

petrogenesis of peralkaline granite dykes of the straumsvola complex western dronning maud land antarctica peralkaline syenite and granite dykes cut the straumsvola nepheline syenite pluton in western dronning maud land antarctica the average peralkalinity index pi molecular al na k of the dykes is 1 20 n 29 and manifests itself in the presence of the zr silicates eudialyte dalyite and vlasovite and the na ti silicate narsarsukite the dykes appear to have intruded during slow cooling of the nepheline syenite pluton and the petrogenetic relationship of the dykes and the pluton cannot be related to closed system processes at low pressure given the thermal divide that exists between silica undersaturated and oversaturated magmas major and trace element variations in the dykes are consistent with a combination of fractional crystallization of parental peralkaline magma of quartz trachyte composition and internal mineral segregation prior to final solidification the distribution of accessory minerals is consistent with late stage crystallization of isolated melt pockets the dykes give an rb sr isochron age of 171 4 4 ma with variable initial 87sr 86sr ratio 0 7075 0 0032 and have an average e nd of 12 0 quartz phenocrysts have 18o values of 8 4 9 2 which are generally in o isotope equilibrium with bulk rock differences in the 18o values of quartz and aegirine average quartz aegirine 3 5 suggest aegirine formation temperatures around 500 c lower than expected for a felsic magma but consistent with poikilitic aegirine that indicates subsolidus growth the negative e nd 10 and magma 18o values averaging 8 6 assuming quartz magma 0 6 are inconsistent with a magma produced by closed system fractional crystallization of a mantle derived magma by contrast the nepheline syenite magma had mantle like 18o values and much less negative e nd average 3 1 n 3 the country rock has similar 18o values to the granite dykes average 8 0 n 108 this means that models for the petrogenesis of the granites by assimilation are unfeasible unless an unexposed high 18o contaminant is invoked instead it is proposed that the peralkaline syenite and granite dykes formed by partial melting of alkali metasomatised gneiss that surrounds the nepheline syenite followed by fractional crystallization

geology and genesis of the toongi rare metal zr hf nb ta y and ree deposit nsw australia and implications for rare metal mineralization in peralkaline igneous rocks the toongi deposit located in central nsw australia hosts significant resources of zr hf nb ta y and ree within a small ca 0 3 km2 rapidly cooled trachyte laccolith toongi is part of regional late triassic to jurassic alkaline magmatic field but is distinguished from the other igneous bodies by its peralkaline composition and economically significant rare metal content that is homogenously distributed throughout the trachyte body the primary ore minerals are evenly dispersed throughout the rock and include lueshite natroniobite and complex na fe zr nb y ree silicate minerals dominated by a eudialyte group mineral egm the egm occurs in a unique textural setting in the rock commonly forming spheroidal or irregular shaped globules herein called snowballs within the rock matrix the snowballs are often protruded by aegirine and feldspar phenocrysts and contain swarms of fine aegirine and feldspar grains that often form spiral or swirling patterns within the snowball secondary ore minerals include ree carbonates y milarite catapleiite and gaidonnayite that fill fractures and vesicles in the rock based on bulk rock geochemical and nd isotope data and thermodynamic modelling of magma fractionation the alkaline rocks of the region are interpreted to represent extrusive to hyperbyssal products of mantle derived magma that ponded at mid crustal levels ca 0 3 gpa and underwent extensive fractionation under low oxygen fugacity conditions the high na2o peralkaline nature of the toongi deposit trachyte developed via extensive fractionation of an alkali olivine basalt parental magma initially in the mid crust and subsequently at shallow levels ca 0 1 gpa this extended fractionation under low fo2 and relatively low h2o activity conditions limited volatile release and allowed build up of rare metal contents to ore grades we speculate that the ore minerals may have originally formed from rare metal rich sodic silicate melt that formed immiscible globules subsequently crystallized to egm in the magma shortly before emplacement and rapid cooling subsequent hydrothermal alteration caused relatively limited and localized remobilization of some ore metals into fractures and vesicles in the rock

wadeite k2zrsi3o9 an alkali zirconosilicate from the saima agpaitic rocks in northeastern china its origin and response to multi stage activities of alkaline fluids abstract the triassic saima alkaline complex in the liaodong peninsula of northeastern china covers an area of about 20 km 2 and is dominated by nepheline syenite with phonolite at its center and a concealed body of eudialyte bearing nepheline syenite in the northwest of the complex the phonolite has similar features to miaskite while the nepheline syenites are classified in the agpaitic group according to their mineral assemblage and the alkalinity and aluminum saturation indexes zircon is the dominant zr bearing mineral in the phonolite whereas wadeite occurs as the only primary zr bearing mineral in the nepheline syenites the transitional crystallization from zircon to wadeite reveals an increase in alkalis and a high k na ratio as the magmas evolved from the volcanic to the intrusive stage the primary wadeite grains underwent varying degrees of hydrothermal alteration overall the areas of weak medium and strong alteration are characterized by the following respective associations 1 wadeite secondary catapleiite gaidonnayite 2 wadeite secondary catapleiite gaidonnayite zircon and 3 pseudomorphs after wadeite the pseudomorphs are widespread and mainly consist of residual wadeite secondary zircon catapleiite gaidonnayite k feldspar calcite and some zr bearing titanite and vesuvianite all of the secondary zircon grains in the three associations are typically enriched in ca and al compared with the primary ca free zircons of the phonolite the progressive alteration of wadeite suggests that the saima complex underwent multiple episodes of fluid activity during a hydrothermal stage including an initial na metasomatism via alkaline fluids then stages most likely involving progressively co 2 rich fluids and an intensive episode involving a mixture of these fluids with externally derived ca rich fluids

partitioning of cl and f between fluid and hydrous phonolitic melt of mt vesuvius at 850 1000 c and 200 mpa abstract the solubilities of cl and or f bearing fluids in phonolitic melts of vesuvius volcano have been determined experimentally at temperatures of 845 865 c and 1000 c and pressure of 200 mpa at 845 865 c the maximum cl concentration in the melt m cl coexisting with cl rich 35 5 wt fl cl and f free fluid is about 0 5 wt an addition of f to the system increases the cl content of the melt up to 0 7 wt at 39 wt cl and 13 wt f in the fluid s the relationship between m cl and fl cl indicates that low density vapor and dense brine phases might have coexisted in some experiments the relationship between m cl and fl cl at 1000 c is similar to that determined at 845 865 c implying that variations of temperature in the range 850 1000 c have no significant effect on the partitioning of cl between fluid and phonolitic melt the concentration of f in phonolitic melt coexisting with f rich fluid s reaches at least 0 9 wt at 845 865 c and 1 3 wt at 1000 c the comparison of data from experiments conducted in au and pt capsules indicates that f may react with au which influences the quality of the experimental results the partition coefficients of f between bulk fluid and melt fl m d f are higher than 1 in all experiments indicating that fluorine is preferentially incorporated into the fluid phase in phonolitic systems in contrast to fl m d f values determined in rhyolitic systems the influence of cl on fl m d f is not detectable at the conditions of our experiments the experimental data imply that mt somma vesuvius phonolitic magmas from the eruptions between a d 79 and a d 1944 may have coexisted with cl rich and f rich fluid phase s prior to eruption

the effect of titanite and other hfse rich mineral ti bearing andradite zircon eudialyte fractionation on the geochemical evolution of silicate melts abstract we present trace element data for hfse rich minerals titanite eudialyte ti bearing andradite zircon from plutonic rocks of the eocene tamazeght complex high atlas mountains morocco in most lithologies significant amounts of titanite are present and as such titanite fractionation played a major role in the evolution of the hfse budget of the whole complex by combining our data with published titanite melt partition coefficients we explore the effects of titanite fractionation on the evolution of geochemical parameters zr hf nb ta la lu and th u ratios during differentiation of silicate melts in detail based upon this we further discuss the genetic relationships between the various rock types present in the tamazeght complex and show that the above mentioned element ratios can potentially be useful for distinguishing geochemically distinct melt reservoirs within a given plutonic rock suite zircon eudialyte and ti bearing andradite are generally rare or are restricted to localized rock units despite their scarcity each of them can have a significant effect upon the hfse budget in their evolving magmas here we derive first estimations of eudialyte melt partition coefficients for ree application of these data to eudialyte bearing rocks of the ilimaussaq complex south greenland and comparing the results to published estimations for the parental melts of these rocks yields consistent results and demonstrates the reliability of our estimations

continuum simulation of flow failures dynamic aspects of several catastrophic slope failures have been evaluated by unsteady flow modelling assuming two dimensional or quasi three dimensional transient flow of an incompressible bivi

on ferroan a type granitoids their compositional variability and modes of origin we recognize eight types of ferroan granitoid that can be distinguished on the basis of major element chemistry these include alkalic granitoids that may be metaluminous or peralkaline alkali calcic granitoids that may be metaluminous peraluminous or peralkaline calc alkalic granitoids that may be metaluminous or peraluminous and rare calcic ferroan granitoids these granitoids may form by two distinct end member processes extreme differentiation of basaltic melts results in ferroan granitoids that are either peralkaline alkalic and alkali calcic or metaluminous alkalic alkali calcic and calc alkalic with alkalinity increasing with increasing pressure of differentiation partial melting of tonalitic to granodioritic crust produces alkali calcic to calc alkalic granitoids that are metaluminous at low pressures and peraluminous at high pressures it is likely that a combination of these two processes plays some role in the formation of most ferroan granitoids most granitoids that are referred to as ferroan however the term has become more confusing than edifying because it has been applied to a broad spectrum of granitoid compositions with varying petrogenesis for this reason we recommend that the term be discontinued and ferroan used in its stead

multiple pulsed debris avalanche emplacement at mount st helens in 1980 evidence from numerical continuum flow simulations abstract the complex 1980 mount st helens debris avalanche is modeled numerically as a transient biviscous fluid flow several approaches are considered including two step rheologically distinct models for avalanche i and combined avalanches ii iii and a composite flow model consisting of retrogressive slides of identical rheology successively accreted to the main avalanche flow for the two step situation flow rheologies are evaluated separately for the initial avalanche comprising the debris avalanche block facies and an ensuing explosive influenced flow strengths normalized by density as high as 250 m2 s2 and apparent newtonian viscosities as much as 275 m2 s were established for the block facies these parameters for the explosively influenced flow are an order of magnitude lower the distribution of stratigraphic units within flowing model debris compared with field distributions suggests that the higher strength emplacement models are appropriate for debris deposited on johnston ridge and in the upper parts and flanks of the north fork toutle river valley in general models for which constant rheology is assumed throughout the flow process provide lower bound emplacement times and excessive early velocities as compared to the prototype event because model calibration is based on matching runout by trial and error it is therefore biased toward the rheologic parameters essential to achieving that runout these values characterize the flow in its latter stages whereas the actual strength and viscosity may have substantially decreased as a function of displacement two dimensional models predict debris accumulation about twice as thick as that observed at the foot of mount st helens where flow divergence was significant this discrepancy is lessened with a quasi three dimensional modification of the flow model accretionary composite flow models with homogeneous rheology simulate the overriding of early avalanche debris by later debris pulses the accretionary composite model also predicts a thick flow snout down valley since this feature is not indicated in the debris deposit the model lends support to the concept of flow separation by rheology the two step and accretionary composite flow models complement one another and provide model based support for the harry glicken hypothesis for complex emplacement at mount st helens

geology and market dependent significance of rare earth element resources china started to produce rare earth elements rees in the 1980s and since the mid 1990s it has become the dominant producer rare earth element export quotas first introduced by the chinese government in the early 2000s were severely reduced in 2010 and 2011 this led to strong government created disparity between prices within china and the rest of the world industrialized countries identified several rees as strategic metals because of rapid price increases of ree outside of china we have witnessed a world scale ree exploration rush the ree resources are concentrated in carbonatite related deposits peralkaline igneous rocks pegmatites monazite apatite veins ion adsorption clays placers and some deep ocean sediments ree could also be derived as a by product of phosphate fertilizer production u processing mining of ti zr bearing placers and exploitation of olympic dam subtype iron oxide copper gold iocg deposits currently rees are produced mostly from carbonatite related deposits but ion adsorption clay deposits are an important source of heavy ree hree small quantities of ree are derived from placer deposits and one peralkaline intrusion related deposit the ideal ree development targets would be located in a politically stable jurisdiction with a pro mining disposition such as canada and australia ree grade hree light ree lree ratio of the mineralization tonnage mineralogy and permissive metallurgy are some of the key technical factors that could be used to screen potential development projects as rees are considered strategic metals from economic national security and environmental points of view technical and economic parameters alone are unlikely to be used in ree project development decision making recycling of ree is in its infancy and unless legislated in the short term it is not expected to contribute significantly to the supply of ree

diffusion effects on oxygen isotope temperatures of slowly cooled igneous and metamorphic rocks recently obtained data on oxygen diffusion in feldspars quartz and hornblende permit the prediction of the apparent18o16o temperatures that would be measured in a rock that consisted only of those three minerals and cooled slowly from high temperature the computed temperatures would be based on the differences in the18o16o ratios between coexisting pairs of minerals the present calculation takes into account the diffusion rates for oxygen as a function of temperature the cooling rate of the rock the mineral grain sizes and the mode of the rock for mineral grains 1 mm in radius and a cooling rate of 10 c m y the minimum difference in apparent temperature between quartz feldspar and feldspar hornblende pairs will be 115 c despite the assumption of a normal uneventful slow cooling history to room temperature further the apparent quartz hornblende temperature will range over 30 c 590 620 c depending on the mode of the rock for a cooling rate of 1000 c m y the apparent difference in temperature can be as much as 400 c consequently consistency in temperatures obtained by oxygen isotope analysis should not be expected in most high grade metamorphic rocks or igneous rocks which are cooled slowly departures from the pattern of temperatures obtained in this model would imply a very rapid quench from high temperature or a complex history for the rock for some minerals including hornblende the relation between temperature and the equilibrium fractionation of oxygen isotopes between coexisting phases has been derived from observed relations in natural specimens the choice of the specimens used for such calibrations needs to be re evaluated in light of these findings this may result in a change in the equilibrium equation constants an example from the literature the san jose tonalite baja california mexico was modelled and yields 18o concentrations in the minerals that correspond closely with the measured values this suggests that the model used is appropriate that the rock has had a simple thermal history and that it cooled at 100 200 c m y over the temperature range 800 500 c the set of paleotemperatures obtained for a rock will in general yield neither the mineral closure temperatures nor the formation or crystallization temperatures on the other hand the cooling rate of the rock may be derived from the data this in turn may have important tectonic implications with regard to denudation and uplift rates

age of mesozoic igneous rocks in northwesternnamibia and their relationship to continental breakup mesozoic igneous rocks of northwestern namibia comprise continental flood basalts and quartz latites of the parana etendeka province and a suite of central sub volcanic intrusions the damaraland complexes conventional k ar ages published for these rocks range from 377 88 ma with the majority of samples yielding ages of 150 110 ma this large spread of ages is interpreted as being due to disturbance of the k ar system by low temperature alteration which is ubiquitous among the volcanic rocks rb sr and 40 ar 39 ar dating methods which are less susceptible to such disturbance yield a narrower range of ages 132 124 ma with one outlier at 149 ma new rb sr internal isochron ages of 129 8 3 8 ma 126 8 1 3 ma and 126 1 7 3 ma have been determined for an etendeka quartz latite and nepheline syenite intrusions in the messum and okorusu complexes respectively in addition 40 ar 39 ar release spectrum ages of 137 0 0 7 ma 135 0 0 7 ma and 126 9 0 6 ma have been determined for samples from the paresis comendite cape cross nepheline syenite and okorusu nepheline syenite complexes respectively a 13 ma period between 137 and 124 ma is considered to be the best estimate of the timing of mesozoic igneous activity in northwestern namibia the continental flood basalt volcanism was probably limited to the early part of this period and is inferred to have been relatively short lived 1 2 ma magmatic activity among the damaraland complexes was contemporaneous with the onset of flood basalt volcanism but continued for approximately 15 ma and terminated after the onset of sea floor spreading in this area 130 125 ma both the flood basalt volcanism and the magmatic activity of the damaraland complexes are thought to have occurred in response to rifting across the upwelling tristan mantle plume the linear distribution of intrusions in damaraland is interpreted as being due to magmatism focused along a structural discontinuity between the pan african damara terrain to the south and proterozoic cratonic basement to the north

subsolidus deuteric hydrothermal alteration of eudialyte in lujavrite from the pilansberg alkaline complex south africa abstract the most evolved rocks of the pilansberg alkaline complex are aegirine lujavrites in which three varieties of eudialyte are recognized on the basis of textural relationships and composition manganoan eudialyte i is a relict orthomagmatic phase occurring as poikilitic plates or as relict grains in pseudomorphed euhedral phenocrysts late eudialyte ii ranges in composition from manganoan eudialyte through kentbrooksite to taseqite like varieties and is considered to be formed by cation exchange with eudialyte i and alkaline fluids eudialyte iii is a hydrothermal phase replacing eudialyte ii and has either taseqite like 5 7 3 wt sro

melt composition control of zr hf fractionation in magmatic processes zircon zrsio4 and hafnon hfsio4 solubilities in water saturated granitic melts have been determined as a function of melt composition at 800 and 1035 c at 200 mpa the solubilities of zircon and hafnon in metaluminous or peraluminous melts are orders of magnitude lower than in strongly peralkaline melt moreover the molar ratio of zircon and hafnon solubility is a function of melt composition although the solubilities are nearly identical in peralkaline melts zircon on a molar basis is up to five times more soluble than hafnon in peraluminous melts accordingly calculated partition coefficients of zr and hf between zircon and melt are nearly equal for the peralkaline melts whereas for metaluminous and peraluminous melts dhf dzr for zircon is 0 5 to 0 2 consequently zircon fractionation will strongly decrease zr hf in some granites whereas it has little effect on the zr hf ratio in alkaline melts or similar depolymerized melt compositions the ratio of the molar solubilities of zircon and hafnon for a given melt composition temperature and pressure is proportional to the hf zr activity coefficient ratio in the melt the data imply that this ratio is nearly constant and probably close to unity for a wide range of peralkaline and similar depolymerized melts however it changes by a factor of two to five over a relatively small interval of melt compositions when a nearly fully polymerized melt structure is approached for most ferromagnesian minerals in equilibrium with a depolymerized melt dhf dzr typical values of dhf dzr range from 1 5 to 2 5 for clinopyroxene amphibole and titanite because of the change in the hf zr activity ratio in the melt the relative fractionation of zr and hf by these minerals will disappear or even be reversed when the melt composition approaches that of a metaluminous or peraluminous granite it is thus not surprising that fractional crystallization of such granitic magmas leads to a decrease in zr hf whereas fractional crystallization of depolymerized melts tends to increase zr hf there is no need to invoke fluid metasomatism to explain these effects results demonstrate that for ions with identical charge and nearly identical radius crystal chemistry does not alone determine relative compatibilities rather the effect of changing activity coefficients in the melt may be comparable to or even larger than elastic strain effects in the crystal lattice

emplacement and geochemical evolution of highly evolved syenites investigated by a combined structural and geochemical field study the lujavrites of the ilimaussaq complex sw greenland abstract structural mapping and the combined study of magmatic to solid state deformation textures and mineral compositions in highly evolved nepheline syenites lujavrites of the alkaline to peralkaline ilimaussaq complex south greenland reveal detailed insight into the emplacement and geochemical evolution of the melts they crystallized from based on magmatic to solid state flow textures such as foliations and lineations we propose that the investigated rock sequence forms a sill like structure with a steep feeder zone that flattens out over a short distance and intrudes into less evolved overlying units as sub horizontal sheets by roof uplift systematic compositional variation of early magmatic eudialyte group minerals egm in the investigated rock sequence monitors the geochemical evolution of the lujavrite forming melt s the chlorine contents of egm decrease successively upwards within the rock sequence which probably indicates a successive increase of water activity during differentiation consistent with a change from sodic pyroxene aegirine to sodic amphibole arfvedsonite in the mineral assemblage both ree contents and fe mn ratios of egm are promising differentiation indicators which increase and decrease respectively upwards within the sequence due to fractional crystallization this closed system evolution is interrupted by a shift towards less evolved melt compositions in one lujavrite unit for which we assume magma recharge our study demonstrates the strength of a combined structural and petrological approach to understand the petrogenesis of an igneous body in more detail and highlights their close connection

on the controls of oxygen fugacity in the generation and crystallization of peralkaline melts the formation of peralkaline melts is generally recognized to be related to enrichment of the mantle in sodium or potassium during a metasomatic event that predated magma generation the wide range in mineral assemblages mineral compositions and fluid compositions in peralkaline melts are to a large extent a function of the sodium potassium content of these mantle metasomatizing fluids which also governs redox conditions we propose that sodic fluids will reduce the mantle assemblage by depleting ferric iron from garnet to form the aegirine component in pyroxene in contrast potassic fluids would oxidize the mantle assemblage by extracting al2o3 from the garnet to make phlogopite we suggest that during differentiation of peralkaline melts once fe ti oxides have been depleted from the assemblage simple crystallization reactions of common solid phases such as aegirine or arfvedsonite control the oxygen fugacity by equilibria such as

phase relations and liquid lines of descent of an iron rich peralkaline phonolitic melt an experimental study we experimentally investigated the phase relations of a peralkaline phonolitic dyke rock associated with the ilimaussaq plutonic complex south greenland the extremely evolved and iron rich composition magnesium number 2 alkalinity index 1 44 feo 12 wt may represent the parental magma of the ilimaussaq complex this dyke rock is therefore perfectly suited for performing phase equilibrium experiments since in contrast to the plutonic rocks of the complex no major cumulate formation processes complicate defining a reasonable starting composition experiments were carried out in hydrothermal rapid quench cold seal pressure vessels at p 100 mpa and t 950 750 c h2o contents ranging from anhydrous to h2o saturated 5 wt h2o and varying fo2 logfmq 3 to 1 where fmq represents the fayalite magnetite quartz oxygen buffer were applied reduced and dry conditions lead to substantial crystallization of alkali feldspar nepheline hedenbergite rich clinopyroxene fayalite rich olivine and minor amounts of ulvospinel rich magnetite which represent the phenocryst assemblage of the natural dyke rock oxidized and h2o rich conditions however suppress the crystallization of olivine in favor of magnetite and clinopyroxene with less or no alkali feldspar and nepheline formation accordingly combined low fo2 and ah2o force the evolution of the residual melt toward decreasing sio2 increasing feo and alkalinity index up to 3 55 on the contrary high fo2 and ah2o produce residual melts with relatively low feo high sio2 and a relatively constant alkalinity index we show that variations of ah2o and fo2 lead to contrasting trends regarding the liquid lines of descent of iron rich silica undersaturated peralkaline compositions moreover the increase in feo and alkalinity index reduced and dry conditions in the residual melt is an important prerequisite to stabilize late magmatic minerals of the dyke rock for example aenigmatite na2fe5tisi6o20 coexisting with the most evolved melts at 750 c contrary to what might be expected experiments with high ah2o and interlinked high fo2 exhibit higher liquidus t s compared with experiments performed at low ah2o and fo2 for experiments where magnetite is liquidus phase this is because ulvospinel poor magnetite crystallizes at higher fo2 and has a higher melting point than ulvospinel rich magnetite which is favored at lower fo2

fractionation of geochemical twins zr hf nb ta and y ho and hree enrichment during magmatic and metamorphic processes in peralkaline nepheline syenites from norra karr sweden abstract the 1 49 ga norra karr complex sweden consists of deformed and metamorphosed peralkaline nepheline syenites that contain eudialyte group minerals as the major host of high field strength elements and rare earth elements earlier studies revealed the presence of distinct generations of eudialyte group minerals and clinopyroxene of magmatic and metamorphic origin here we present the trace element characteristics of the different generations of rock forming minerals the trace element chemistry of eudialyte group minerals mimic whole rock compositions and display well developed negative eu anomalies and strong sr and ba depletions in chondrite normalized diagrams they imply that the norra karr rocks developed by intense fractional crystallization from an alkali basaltic parental magma our data further illustrate that eudialyte group minerals do not strongly fractionate the geochemical twins zr hf y ho and nb ta during magmatic processes the transition from a magmatic to metamorphic environment is marked in egm by a drop in y ho and nb ta the exceptional enrichment of heavy rare earth elements in late metamorphic eudialyte may result from residual enrichment light rare earth elements and nb were preferentially mobilized to form local secondary light rare earth rich rinkite group mineral assemblages while the heavy ree stayed behind and formed heavy ree rich eudialyte magmatic clinopyroxene shows a clear preference for hf over zr while on a subunit scale a negative correlation of the total ree in cpx and in egm is noticed both indicate that early cpx fractionation would not only increase zr hf of the evolving melt but also affect the ree deportment in major hfse hosts

quartz latite rheoignimbrite flows of the etendeka formation north western namibia the etendeka formation of north western namibia consists of a sequence of interbedded quartz latites and tholeiitic basalts and forms part of the karoo igneous province in southern africa the age of the etendeka formation is approximately 130 135 ma the quartz latites make up a significant proportion of the stratigraphic succession 25 of the total stratigraphic thickness and form as much as 60 of the outcrop area in the southern etendeka apart from some systematic differences between pitchstones and devitrified quartz latite largely explained by alteration processes individual quartz latite units exhibit remarkably uniform compositions with no significant vertical or lateral variation geochemistry can be used as a primary criterion for the correlation of major quartz latite units over much of the southern etendeka area enabling the reconstruction of the etendeka formation stratigraphy in this region individual quartz latite units occur as voluminous 400 2600 km3 widespread up to 8800 km2 sheet like deposits typically between 40 and 300 m thick each unit consists of basal main and upper zones the main zone generally constitutes over 70 of the thickness of the unit and typically consists of texturally featureless devitrified quartz latite in contrast the basal and upper zones of the flow are characterised by flow banding pitchstone lenses and breccia with rare occurrences of pyroclastic textures the quartz latites are sparsely porphyritic 10 phenocrysts with glassy or devitrified groundmass textures the phenocrysts consist of plagioclase pyroxene titanomagnetite and rare ilmenite pyroxene geothermometry indicates high 1000 1100 c temperatures of crystallisation which coupled with the absence or primary hydrous phases indicates that the quartz latites were relatively hot h2o undersaturated magmas the quartz latites display features common to both rhyolite lavas and ignimbrites and are clearly the products of an unusual eruption style the local preservation of pyroclastic textures and the broad areal extent of these units lead to the conclusion that the quartz latites are high temperature rheomorphic ignimbrites i e rheoignimbrites a combination of high eruption temperature and relatively low viscosity helps to explain the often completely welded and homogeneous textures observed in most quartz latite outcrops in the etendeka area

retrograde evolution of eclogites evidences from microstructures and 40ar 39ar white mica dates munchberg massif northern bavaria phengites from eclogites and pegmatites 3t 2m1 coarse grained and recrystallized of the munchberg massif weissenstein and oberkotzau have been dated by the 40ar 39ar method 3t micas from the eclogites yielded plateau and isochron ages of 365 7 ma 2m1 micas show disturbed degassing spectra micas from pegmatites show a slight excess ar component with an isochron age of 353 to 351 3 ma an age component of approximately 300 ma was also detected in combination with age values from the literature the cooling history of the munchberg massif from eclogite facies conditions 390 ma to cooling below 350 c 350 ma is documented the age component of 300 ma is attributed to a low grade stage of mineral growth accompanied by a transitional ductile brittle deformation the petrological effects include formation of pumpellyite prehnite facies minerals frequently precipitated in microcraks and cleavage planes of earlier formed minerals this stage has to be seen in conjunction with the intrusions of the fichtelgebirge granite

the crystallization of shoshonitic to peralkaline trachyphonolitic magmas in a h2o cl f rich environment at ischia italy with implications for the feeder system of the campania plain volcanoes abstract bulk rock and mineralogical characterization of massive samples lava flows lava domes scoria spatter chosen to be representative of different activity periods and of the whole known compositional range of ischia island roman magmatic province campanian district is reported in this paper the rocks vary in composition from shoshonites to peralkaline trachyphonolites crystallization started with al rich chromite inclusions and their host magnesian olivine in shoshonites and latites and ended with lavenite rinkite kochite hiortdahlite hainite mn aenigmatite britholite and ti zr rich aegirine in the groundmass of the peralkaline trachyphonolites removal of feldspar bearing assemblages with calcic to sodic plagioclase in shoshonites and latites sodic sanidine and anorthoclase in trachytes and trachyphonolites is established throughout the compositional range and is accompanied by interaction between variably evolved magmas with their differing phenocryst assemblage mostly in mafic and intermediate compositions this led to enrichment in mn alkalis zr nb ree rb th u cl and f and depletion in mg fe v ca ba sr and eu in the most evolved magmas the ischian rocks have a tendency to sodic affinity and by peculiar mineral compositions and compositional trends which do not indicate extremely oxidizing conditions the ischian rocks thus differ from the phlegrean fields analogues and the ultrapotassic more silica undersaturated rocks of somma vesuvius overall variations in the chemical compositions of the rocks and their trends imply unrelated feeding systems and thus are inconsistent with the hypothesis of a common magma reservoir beneath the main volcanic areas of the campanian plain

crystal mat formation as an igneous layering forming process textural and geochemical evidence from the lower layered nepheline syenite sequence of the ilimaussaq complex south greenland abstract the lower layered nepheline syenite sequence kakortokites of the mesoproterozoic alkaline to peralkaline ilimaussaq complex south greenland shows spectacular rhythmic meter scale igneous layering the 29 exposed units have sharp contacts against each other and each of these units consists of three modally graded layers dominated by arfvedsonitic amphibole eudialyte group minerals and alkali feldspar respectively this study uses field observations on changes in mineral orientation recurrent mineral textures compositional data from eudialyte group minerals and amphibole and settling rate calculations based on a modified stokes equation to explain the igneous layering of the kakortokites we propose that the three major cumulus minerals amphibole eudialyte s l and alkali feldspar were separated from each other by density contrasts resulting in modally graded layers within each unit the densest of these three minerals amphibole formed crystal mats within the cooling magma body these crystal mats acted as barriers that inhibited large scale vertical migration of melts and crystals with increasing effectiveness over time the sub volumes of magma captured in between the crystal mats evolved largely as geochemically independent sub systems as indicated by the observed trends in mineral composition

zirconosilicate phase relations in the strange lake lac brisson pluton quebec labrador canada petrographic observations of subsolvus granites at strange lake indicate that the sodium zirconosilicate elpidite crystallized under magmatic conditions but that the calcium zirconosilicates armstrongite and gittinsite are secondary this interpretation is consistent with the extensive solid solution displayed by elpidite and the restricted compositions of armstrongite and gittinsite both calcium zirconosilicate minerals show textural evidence of having replaced elpidite and in the case of gittinsite with major volume loss in the near surface environment gittinsite plus quartz fill the volume formerly occupied by elpidite at greater depth gittinsite and armstrongite partially replaced elpidite but are not accompanied by quartz and abundant pore space is observed where gittinsite is the principal secondary phase below 70 m elpidite is generallyunaltered replacement of elpidite by armstrongite is interpreted to have been a result of the cation exchange reaction nazzrsi601s 3hzo ca2 cazrsi601s 3hzo 2na elpidite armstrongite

the miaskitic to agpaitic transition in peralkaline nepheline syenite white foyaite from the pilanesberg complex south africa abstract the mesoproterozoic pilanesberg complex south africa is built up by several distinct ring shaped intrusions of syenite and peralkaline nepheline syenite a mildly peralkaline na k al 1 04 1 09 medium to coarse grained nepheline syenite makes up the outermost ring in the southwestern part of the complex matooster type white foyaite in this rock mafic silicate minerals amphibole biotite aegirine and ti bearing minerals ilmenite astrophyllite aenigmatite lorenzenite bafertisite jinshajiangite are interstitial to feldspar and nepheline and define a series of mineral assemblages reflecting a change from a miaskitic crystallization regime with na ca amphibole titanite and ilmenite to increasingly agpaitic conditions with arfvedsonite aegirine astrophyllite aenigmatite lorenzenite the main driving force behind the evolution was an increase in peralkalinity of the trapped liquid mainly by adcumulus growth of alkali feldspar and nepheline which in the later stages of evolution was combined with increases in oxygen fugacity and water activity unlike in most other agpaitic rock complexes zr remained compatible in aegirine and to some extent in amphibole almost to the end of the process when a hydrous zirconium silicate mineral hilairite crystallized as the only mineral in the rock having essential zirconium the presence of minerals such as hilairite bafertisite jinshajiangite and a na ree sr rich apatite group mineral fluorcaphite in the latest assemblages suggests that the last remaining interstitial melt or fluid approached a hyperagpaitic composition the isolated melt pockets in the pilanesberg white foyaite follow a pattern of evolution that can be seen as a miniature analogue of the fractional crystallization processes controlling magma evolution in large alkaline igneous rock complexes

oxygen isotope exchange kinetics of mineral pairs in closed and open systems applications to problems of hydrothermal alteration of igneous rocks and precambrian iron formations the systematics of stable isotope exchange between minerals and fluids are examined in the context of modal mineralogical variations and mass balance considerations both in closed and in open systems on mineral pair 18 o plots samples from terranes that have exchanged with large amounts of fluid typically map out steep positively sloped non equilibrium arrays analytical models are derived to explain these effects these models allow for different exchange rates between the various minerals and the external fluids as well as different fluid fluxes the steep arrays are adequately modelled by calculated isochron lines that involve the whole family of possible exchange trajectories these isochrons have initially steep near vertical positive slopes that rotate toward a 45 equilibrium slope as the exchange process proceeds to completion the actual data point array is thus analogous to the hand of an isotopic clock that measures the duration of the hydrothermal episode the dimensionless ratio of the volumetric fluid flux to the kinetic rate parameter u k determines the shape of each individual exchange trajectory in a fluid buffered system u k 1 the solutions to the equations 1 are independent of the mole fractions of the solid phases 2 correspond to taylor s open system water rock equation and 3 yield straight line isochrons that have slopes that approach 1 f where f is the fraction reacted of the more sluggishly exchanging mineral the isochrons for this simple exchange model are closely congruent with the isochrons calculated for all of the more complex models thereby simplifying the application of theory to actual hydrothermal systems in nature in all of the models an order of magnitude of time in units of kt separates steep non equilibrium arrays e g slope 10 from arrays approaching an equilibrium slope of unity on a diagram because we know the approximate lifetimes of many hydrothermal systems from geologic and heat balance constraints we can utilize the 18 o 16 o data on natural mineral assemblages to calculate the kinetic rate constants k s and the effective diffusion constants d s for mineral h 2o exchange these calculated values k qtz 10 14 k feld 10 13 10 12 agree with experimental determinations of such constants in nature once the driving force or energy source for the external infiltrating fluid phase is removed the disequilibrium mineral pair arrays will either 1 remain frozen in their existing state if the temperatures are low enough or 2 re equilibrate along specific closed system exchange vectors determined solely by the temperature path and the mineral modal proportions thus modal mineralogical information is a particularly important parameter in both the open and closed system scenarios and should in general always be reported in stable isotopic studies of mineral assemblages these concepts are applied to an analysis of 18 o 16 o systematics of gabbros plagioclase clinopyroxene and plagioclase amphibole pairs granitic plutons quartz feldspar pairs and precambrian siliceous iron formations quartz magnetite pairs in all these examples striking regularities are observed on and plots but we point out that plots have many advantages over their equivalent diagrams as the latter are more susceptible to misinterpretation using the equations developed in this study these regularities can be interpreted to give semiquantitative information on the exchange histories of these rocks subsequent to their formation in particular we present a new interpretation indicating that precambrian cherty iron formations have in general undergone a complex fluid exchange history in which the iron oxide magnetite precursor has exchanged much faster with low temperature 400 c fluids than has the relatively inert quartz

bulk and mush melt evolution in agpaitic intrusions insights from compositional zoning in eudialyte ilimaussaq complex south greenland

the peralkaline granite related khaldzan buregtey rare metal zr nb ree deposit western mongolia the khaldzan buregtey rare metal nb zr ree etc deposit in peralkaline granitoids is located in western mongolia 45 km northeast of the town of kobdo the deposit which was discovered by the authors is composed of dome shaped stocks of rare metal peralkaline granite covering 0 85 km and represents the late plutonic phases of the devonian 380 390 ma multiphase khaldzan buregtey peralkaline granite massif the formation of the massif took place during tectonic extension as shown by the development of a major dike complex synchronous with granite emplacement the massif consists of peralkaline granitoids as well as gabbro and basalt and is characterized by a radial circular shape eight intrusive phases are recognized and consist of 1 nordmarkite 2 peralkaline granite 3 dikes of fine grained peralkaline granite 4 dikes of pantellerite 5 rare metal peralkaline granite 6 gabbroids 7 miarolitic peralkaline rare metal granite and 8 syenite and leucite basalt field relationships of the rare metal peralkaline granite bodies indicate an intrusive igneous origin the rare metal peralkaline granites consist of k na feldspar quartz albite arfvedsonite aegirine fluorite and diverse rare metal minerals ranging up to 25 vol percent ore minerals are represented by elpidite gittinsite and zircon pyrochlore nb rare metal flurocarbonate and monazite ree and polylithionite li the zirconium minerals often occur in the endocontact and apical parts of rare metal granite bodies and are reflected in the negative correlation of zr and nb contents in the seventh phase granites the rare metal peralkaline granites of the deposit differ from those of other massifs by their high and variable ca and f contents the granites are rich in zr up to 5 3 wt nb up to 0 8 wt ree up to 0 4 wt y up to 0 3 wt be sn rb and sometimes li pb zn hf and ta they are depleted in sr and ba and have an almost flat ree distribution pattern with a small negative eu anomaly the initial 87 sr 86 sr ratio of fluorite from rare metal peralkaline granite is 0 70433 or 7 the end value for these rocks at 386 ma ranges from 4 3 to 5 7

old model nd ages in namibian pan african rocks the nd isotope analytical technique is a powerful tool for studying many geological processes particularly the evolution of the upper mantle as seen through the isotope and trace element geochemistry of mantle derived volcanic rocks1 3 and magmatic processes along destructive plate margins4 5 however previous studies of continental crust6 8 have been largely restricted to archaean examples and concerned primarily with dating their time of derivation from the upper mantle in the present study we investigated the nd and sr isotope characteristics of rocks involved in a relatively young 800 450 myr orogenic event such events represent critical stages in the evolution of most of the earth s continental crust new material is added from the mantle and pre existing crust is remobilized by erosion and sedimentation deformation and magmatic activity nd and sr isotopes were used to outline the range in age and geochemical characteristics of rock sequences and provinces within both the upper mantle and the pre existing crust which were sampled during the orogeny and hence provided the major components of what is now a stable segment of continental crust

short lived magmatic activity in an anorogenic subvolcanic complex 40ar 39ar and ion microprobe u pb zircon dating of the erongo damaraland namibia abstract the erongo subvolcanic center in namibia is the largest composite bimodal complex in the mesozoic etendeka igneous province of namibia this study of 40ar 39ar and high spatial resolution u pb zircon dating demonstrates that emplacement of the various igneous units at erongo took place within a time span equivalent to or shorter than geochronologically resolvable age differences ca 2 ma and at the peak of regional flood basalt activity in the etendeka parana province the erongo complex comprises a series of felsic volcanic and intrusive units that overlie or intrude basaltic lavas attributed to the etendeka group the stratigraphically oldest rhyodacite erongorus unit yielded u pb zircon ages of 131 8 1 1 ma 1 the overlying ombu rhyodacite was previously dated at 135 0 1 6 ma and our new ages for a resurgent intrusion of the same magma type ombu granodiorite in the vent area are 132 6 1 1 ma 40ar 39ar and 132 3 1 9 ma u pb zircon respectively the youngest felsic magmas at erongo are the high silica ekuta rhyolite and compositionally equivalent erongo granite these units yielded u pb zircon ages of 131 9 2 9 and 130 3 1 4 ma respectively concordant 40ar 39ar biotite ages from two samples of the granite average 132 2 0 8 ma the final stage of magmatism at erongo involved emplacement of basic alkaline plugs and dikes phlogopite and kaersutite from foidite plugs of this series yielded 40ar 39ar ages of 130 8 1 0 and 132 0 1 0 ma in a regional context these age results indicate that silicic magmatism in the largest complexes of the damaraland erongo brandberg paresis messum began simultaneously with the peak of flood basalt effusion at about 132 ma throughout the etendeka province and ceased by about 130 ma the silicic magmas are hybrid with varying contributions of crustal and mantle derived melts and the age constraints suggest that crustal melting was caused by a short lived thermal pulse related to the main flood basalt event basic magmatism in the damaraland complexes continued sporadically thereafter to about 123 ma but lacked the power to create further crustal melting

contrasting trends of agpaitic crystallization in nepheline syenite in the pilanesberg alkaline complex south africa abstract different types of peralkaline nepheline syenite in the pilanesberg alkaline complex show contrasting zr and ti bearing mineral assemblages in some members of the green foyaite suite eudialyte is the dominant zr mineral as is common in many other agpaitic nepheline syenites eudialyte has formed together with aegirine arfvedsonite alkali feldspar s sodalite and nepheline aenigmatite is the main ti bearing mineral but some samples have normandite a ti zr disilicate mineral as a late magmatic mineral other varieties of green foyaite do not have eudialyte but carry lamprophyllite a ti sr disilicate mineral as a prominent early crystallizing mineral in contrast pockets of trapped melt in white foyaite crystallized a series of titanium minerals starting with clearly miaskitic assemblages with titanite and ilmenite through increasingly agpaitic assemblages with astrophyllite aenigmatite and lorenzenite with hilairite forming at the end as the only mineral with essential zr in both green and white foyaite aegirine has percent to sub percent amounts of zro2 chemographic analysis of melt mineral equilibria suggests that the main driving force behind the evolution of mineral assemblages in both white and green foyaite is increasing peralkalinity of the melt in the white foyaite this took place at high ah2o throughout whereas the green foyaite started its evolution at much lower ah2o the different zr and ti mineral assemblages observed in the latter rock type can be explained by differences in water activity during crystallization of the magma reflecting different pre emplacement fractionation and degassing histories of batches of genetically related nepheline syenite magma

zr and ree mineralization in sodic lujavrite from the saima alkaline complex northeastern china a mineralogical study and comparison with potassic rocks abstract the triassic saima alkaline complex on the liaodong peninsula northeastern china consists mainly of potassic phonolite nepheline syenite and sodic lujavrite the lujavrite shows significant zr ree mineralization which is present in the form of early magmatic zr ree enriched clinopyroxene 30 40 titanite 5 and loparite ce and late magmatic to hydrothermal wadeite widespread eudialyte group minerals 5 10 and catapleiite ultimately the fractionation of the alkaline magma leads to the crystallization of mosandrite and hezuolinite textural relations and compositional variation among the characteristic zr ree bearing minerals record that both zr and rees were strongly incompatible in the sodic melt but that zr mineralization preceded ree mineralization the main zr ree mineralization in the saima lujavrite resulted from the high peralkalinity na k ratio and hfse content low oxygen fugacity and the intensive activity of water and volatiles of its evolving magma the discontinuous and abrupt changes in melt composition and mineral assemblage from the potassic nepheline syenite of the complex to the sodic lujavrite suggest that their magma was derived from different episodes of magmatic activity with different physico chemical characteristics rather than from the continuous evolution of a single magmatic event

oxygen isotope geochemistry of the silicic volcanic rocks of the etendeka parana province source constraints oxygen isotope ratios of pyroxene phenocrysts in the silicic volcanic rocks from the cretaceous etendeka parana flood basalt province namibia south america are believed to reflect the delta sup 18 o values of the original magmas the authors recognize a high delta sup 18 o value type delta sup 18 o pyroxene approximately 10 per thousand found in the south of both regions and a low delta sup 18 o value type delta sup 18 o pyroxene approximately 6 5 per thousand found in the north other differences between thee two rhyolite types include higher concentrations of incompatible elements and lower initial sup 87 sr sup 86 sr ratios in the low delta sup 18 o value type the authors suggest that the regional distribution of rhyolite types reflects differences in source composition which can best be explained if the sources are lower crustal late proterozoic mobile belt material high delta sup 18 o and archean lower crust low delta sup 18 o

agpaitic nepheline syenites a potential source of rare elements abstract the agpaitic nepheline syenites may with regard to their elevated contents of a number of rare elements be compared with the complex granite pegmatites both rock types form highly evolved magmas under closed system conditions which prevent volatiles from escaping agpaitic rocks are peralkaline nepheline syenites characterized by the presence of complex rare element minerals such as eudialyte and mosandrite instead of the simple minerals such as zircon and ilmenite uranium deposits have been found in agpaitic rocks in brazil pocos de caldas china saima and greenland ilimaussaq nb and apetite deposits in russia lovozero and khibina the type locality of agpaitic rocks the ilimaussaq complex in south greenland is characterized by elevated contents of elements such as zr ree y be li nb u th and f not only in pegmatites late veins and contact fenites but also in some of the rocks especially lujavrite and kakortokite the most important rare element containing minerals are eudialyte zr ree y nb steenstrupine u th ree rinkite nb ree y u pyrochlore and perovskite nb ta li mica li chkalovite be and villiaumite f occurrences of eudialyte rich kakortokite and lujavrite are presently being explored in the ilimaussaq complex as sources of zr ree y and nb

oxygen isotope geochemistry of the mesozoic anorogenic complexes of damaraland northwest namibia evidence for crustal contamination and its effect on silica saturation fifty six new oxygen isotope analyses of minerals separated from nine mesozoic anorogenic complexes of damaraland in northwest namibia have been used to estimate the 18o values of the original magmas magma these complexes range in composition from nepheline syenite to alkaline and peraluminous granites often with a variety of rock types present at a single centre the silica undersaturated rock types show a relatively small spread of values of magma from 6 0 to 7 0 mean 6 6 which is consistent with their derivation from the mantle with little or no subsequent crustal contamination the silica oversaturated rocks show a wide range of magma values from 4 9 to 12 0 with a mean value of 8 3 the high values of magma in the silica oversaturated rocks can only be explained with considerable involvement of the continental crust in their petrogenesis and those rocks with magma 10 are interpreted as essentially crustal melts it is generally accepted that the damaraland complexes were generated as a result of rifting across the tristan plume with the plume providing both mantle derived magma and with it the heat required for crustal melting in addition to their mantle like oxygen isotope ratios the undersaturated rocks of the damaraland complexes possess the trace element characteristics e g low zr nb ratios of ocean island basalt suggesting that their parental magmas were produced from the plume itself in contrast the oversaturated complexes generally have higher zr nb ratios that are consistent with a larger crustal input the highest values of magma in the damaraland complexes are found in granitic rocks that intrude the central zone of the pan african damara orogen where presumably there is a substantial component of sedimentary origin in the lower to middle crust

compositional variation of apatite from rift related alkaline igneous rocks of the gardar province south greenland textural and compositional variations of apatite from rift related gabbros syenogabbros syenites quartz syenites and nepheline syenites of the mid proterozoic gardar province south greenland are presented and compared to apatite compositions from other rock suites the observed zoning textures of apatite are interpreted to represent 1 primary growth zonation concentric and oscillatory that formed during magmatic differentiation and 2 secondary irregular overgrowths patchy zonation and resorption textures assigned to metasomatic overprinting due to interaction with fluids melts and intra crystalline diffusion compositional variation in the apatites is mainly due to coupled substitutions of ca and p by variable amounts of si na and ree which show increasing concentrations during magmatic differentiation furthermore f concentrations in apatites increase from gabbroic through syenogabbroic to syenitic rocks whereas cl concentrations show the opposite trend compared to apatite compositions from gabbroic dioritic and granitic rocks in general apatites from alkaline rock suites are characterized by exceptionally high contents of ree and si and in some alkaline rocks they attain sr contents comparable to those reported from carbonatites typical low mn and s contents are probably a result of low oxygen fugacity during crystallization at relatively high temperatures

constraints on the origin of damaran granites by rb sr and δ 18 o data the mean 87sr 86sr and mean 87rb 86sr ratios of the intrusive granites from the north and south of the orogen s central zone plot on straight lines these are interpreted as areal isochrons indicating the time of last sr isotope homogenization 526 and 571 ma ago in the respective source rocks

rb sr age determinations of rocks from the okenyenya igneous complex northwestern namibia the okenyenya igneous complex is one of a suite of intrusions which define a prominent northeast trending linear feature in damaraland northwestern namibia precise rb sr internal isochron ages range from 128 6 1 to 123 4 1 4 ma for the major phases of intrusion identified within the complex the tholeiitic gabbros forming the outer rings of the complex and the later alkali gabbros which form the central hills cannot be distinguished in terms of rb sr ages although field relations clearly indicate the younger age of the latter the intrusionsof nepheline syenite and essexite comprising the mountain of okenyenya bergon the northern edge of the complex give ages of 123 4 1 4 and 126 3 1 ma respectively and form the final major phase of intrusion the ages obtained for early and late intrusive phases define a minimum magmatic life span of approximately 5 ma for the complex the determined age of the okenyenya igneous complex 129 123 ma when taken together with the few reliable published ages for other damaraland complexes 130 134 ma suggests that these sub volcanic complexes were emplaced contemporaneously with the widespread etendeka volcanics 130 ma and relate to magmatism associated with the breakup of southern africa and south america with the opening of the south atlantic ocean the linear distributionof intrusions in damaraland is interpreted to be due to magmatism resultingfrom the upwelling tristan plume being focused along a structural discontinuity between the pan african damaran terrain to the south and proterozoiccratonic basement to the north

oxygen isotope fractionation in quartz albite anorthite and calcite laboratory measurements of equilibrium oxygen isotope fractionation in quartz albite anorthite and calcite have been carried out by anhydrous exchange between silicates and calcite at temperatures of 600 c and above exchange in these systems is as rapid as exchange between silicates and water fractionation factors can be summarized in the following equations qc 0 38 106t 2 abc 0 57 106t 2 anc 1 59 106t 2 from which the silicate pair fractionations are readily obtained these results are compared with published theoretical estimates as well as data derived from hydrothermal experiments some significant differences are found in particular it is difficult to reconcile all of the hydrothermal data either with theoretical calculations or with the present experimental data the new experiments provide an internally consistent set of fractionation factors suitable for isotopic thermometry and for test of disequilibrium in natural systems

oxygen and neodymium isotope evidence for source diversity in cretaceous anorogenic granites from namibia and implications for a type granite genesis abstract many of the early cretaceous intrusive complexes in the damaraland of west central namibia are silicic in composition although all have trace element characteristics typical for the so called a type granites major differences in alkali aluminum ratios and isotopic compositions require diverse magma sources this paper presents nd and o isotope data from the five largest silicic complexes paresis erongo brandberg cape cross gross spitzkoppe that provide new constraints on the nature of crustal and mantle sources involved and their relative proportions the paresis complex has an isotopic signature 18o 9 end130 ma 21 indicating a crustal component similar to mesoproterozoic gneisses of the angola craton the other complexes have isotope variations 18o from 8 1 to 10 7 and end130 ma from 1 to 9 that can be explained by a binary mixing model between a mantle and crustal component more importantly this same mixing line also fits the nd o isotope variations reported from the mafic okenyenya and messum complexes and from rhyodacites in the southern etendeka volcanic sequence the uniformity of the crustal component implied by this mixing model suggests lower crustal material in contrast to the geologic complexity of the neoproterozoic damara belt presently exposed at the surface this is consistent with the isotopic data and we interpret the crustal component to be lower crustal metametasediments that were dehydrated and perhaps melt depleted by generation of the s type granites which are widespread in the damara belt the mantle component is interpreted to be dominated by the tristan mantle plume but some involvement of depleted mantle material is needed to explain all of the isotope data the data rule out any significant role for enriched subcontinental mantle lithosphere all silicic damaraland complexes as well as the etendeka rhyodacites classify as a type granites despite their proven source diversity this means that the a type geochemical characteristics of these and by analogy other anorogenic granites does not depend on a particular magma source the unifying feature of the diverse granites in the damaraland is likely to be related to the high temperatures involved which allow partial melting of dehydrated and possibly melt depleted lower crustal lithologies

evidence of crystallization in residual cl f rich agpaitic trachyphonolitic magmas and primitive mg rich basalt trachyphonolite interaction in the lava domes of the phlegrean fields italy the lava domes in the northwestern cuma northern punta marmolite and central accademia parts of the phlegrean fields are the subject of this study the cuma and punta marmolite trachyphonolitic lava domes are among the oldest phlegrean products cropping out the cuma rocks have an agpaitic groundmass with early alkali feldspar fe rich clinopyroxene fe edenite and sodalite and late rosenbuschite fluorite baddeleyite pyrochlore britholite monazite aegirine often zr rich and exceptionally fe mn rich olivine the bulk rock compositions at cuma have some of the highest concentrations of zn mn zr nb th u and lanthanides among the phlegrean fields rocks and some of the lowest mgo p 2 o 5 sr eu and ba the punta marmolite dome is chemically less evolved and lacks characteristic agpaitic minerals but features alkali feldspar sodalite nepheline and relatively na poor fe rich hedenbergite with rare ca rich plagioclase xenocryst cores the accademia dome belonging to the recent activity is latitic to trachytic in composition has highly forsteritic olivine with chromiferous spinel inclusions calcic plagioclase and mg rich diopside phlogopite xenocrysts in an evolved host rock with phenocrysts and microlites of alkali feldspar fe rich clinopyroxene fe rich amphibole magnetite fe rich olivine and accessory baddeleyite zirconolite and fluorite there is clear evidence of open system magma crystallization in the form of interaction between a crystallizing primitive shoshonitic basalt in a reservoir already filled by rather evolved trachytic magma the magmatic evolution towards the evolved compositions is dominated by crystallization of more and more na rich alkali feldspar in a cl f rich and relatively h 2 o poor environment input of mafic magma is evident in many trachytic eruptions of the phlegrean fields and even in the products of the campanian ignimbrite but eruptions having mineral assemblages rich in xenocryst phases as well as eruptions virtually free of mafic magma input are also frequently observed throughout the history this suggests a variable pattern of open and closed system crystallization which may or may not be linked to explosive activity and that can be caused by intermittent supply of basaltic magma from depth

the effect of fluorine on clinopyroxene melt trace element partitioning the impact of fluorine on phase relations and clinopyroxene melt element partitioning has been explored experimentally to better understand the effect of this halogen on the residual enrichment of the ree and hfse during crystallisation of alkaline and peralkaline magmas clinopyroxene was grown from three hbox h 2 hbox o saturated synthetic glasses of tephriphonolite to phonolite composition in a rapid quench internally heated pressure vessel at 650 800 circ c and 200 mpa with delta log f hbox o 2 fixed to ca fmq 1 the fluorine content in the charges was varied and produced quenched melts from fluorine free to 1 6 wt f the experiments yield an assemblage of melt fluid sodic clinopyroxene biotite magnetite k feldspar titanite fluorite and hiortdahlite a na ca ti f sorosilicate addition of fluorine markedly increases the mode of biotite without significantly affecting the mode of clinopyroxene relative to fluorine free compositions experiments with 1 6 wt fluorine in the melt show a strong decrease in clinopyroxene melt partition coefficients for the trivalent ree la dy and y with a lesser decrease for the d mathrm hree and d mathrm hfse 4 the diminished uptake of these metals by clinopyroxene may reflect changes to their speciation in the melt phase consistent with the formation of ree f complexes and with modifications to the medium range structural environment around hbox hfse 4 ions in the melt an increase in the fluorine content of the melt will thus make the ree and hbox hfse 4 progressively less compatible and therefore available for residual enrichment

fluid evolution in the nepheline syenites of the ditrău alkaline massif transylvania romania abstract the ditr u alkaline massif is an intrusion into the bucovina nappe system that is part of the mesozoic crystalline zone located in transylvania romania in the eastern carpathians nepheline syenites are the most abundant rocks in the central and eastern part of the massif and represent the last major intrusion of the complex fluid inclusions in nepheline aegirine and albite were trapped at magmatic conditions on or below the h 2 o saturated nepheline syenite solidus at about 400 600 c and 2 5 5 kbars early nepheline and to a lesser extent albite were altered by highly saline fluids to produce cancrinite sodalite and analcime during this process cancrinite also trapped fluid inclusions the fluids in most cases can be modeled by the h 2 o nacl system with varying salinity however inclusions with more complex fluid composition containing k ca co 3 etc in addition to nacl are common raman spectroscopic analyses of daughter minerals confirm the presence of alkali carbonate fluids in some of the earliest inclusions in nepheline aegirine and albite during crystallization the melts exsolved a high salinity carbonate rich magmatic fluid that evolved to lower salinity as crystallization progressed phases that occur early in the paragenesis contain high salinity inclusions while late phases contain low salinity inclusions the salinity trend is consistent with experimental data for the partitioning of chlorine between silicic melt and exsolved aqueous fluid at about 2 0 kbars the activity of water a h 2 o in the melt increases during crystallization resulting in the formation of hydrous phases during late stage crystallization of the nepheline syenites

magmatic evolution and controls on rare metal enrichment of the strange lake a type peralkaline granitic pluton quebec labrador abstract although it is well known that a type granites are enriched in the rare earth elements ree and other high field strength elements hfse the magmatic processes that concentrate these elements are still poorly understood the 1 24 ga strange lake pluton in northern quebec labrador provides an extraordinary example of hyper enrichment in the ree zr and nb in a peralkaline a type granite the pluton consists of two hypersolvus granite units southern and northern and a transsolvus granite all of which contain perthitic alkali feldspar as the earliest major mineral the transsolvus granite also contains separate albite and microcline crystals arfvedsonite a sodic amphibole occurs exclusively as phenocrysts in the transsolvus granite whereas in the hypersolvus granite it is present as a late interstitial phase the primary hfse minerals are zircon monazite ce gagarinite ce and the pyrochlore group minerals magma evolution was monitored by the alumina content in the bulk rock which decreases from the southern to the northern hypersolvus granite and is lowest in the transsolvus granite alkalinity indices and bulk si fe rb ree zr nb concentrations show the opposite trend alkali feldspar compositions mirror the trend shown by the bulk rock i e decreasing al contents are accompanied by increasing si fe3 ree zr and nb contents the major driving forces for the evolution of the hypersolvus magma prior to emplacement were the early separation of a fluoride melt from the silicate melt and the crystallization of alkali feldspar and hfse rich phases zircon monazite ce pyrochlore group an alkali feldspar rich crystal mush containing lree fluoride melt droplets was emplaced as the least evolved southern hypersolvus granite massive fractionation of alkali feldspar led to a sharp increase in h2o and f activity in the magma chamber that triggered the crystallization of arfvedsonite and was followed by emplacement of the northern hypersolvus granite which contained a higher proportion of lree fluoride melt droplets further evolution in the magma chamber led to a transition from a miaskitic to an agpaitic composition the transsolvus granite was intruded in the form of a low viscosity crystal mush of alkali feldspar quartz arfvedsonite after appreciable crystallization of arfvedsonite and lree fluoride melt droplets upon emplacement arfvedsonite and gagarinite ce crystals segregated as cumulates in response to a combination of flow differentiation and gravity settling the immiscible fluoride melt accumulated in a volatile rich residual silicate magma which migrated to the top of the pluton where it formed the f ree rich cores of highly mineralized pegmatites

fluid inclusion studies of the drammen granite oslo paleorift norway the drammen granite is a subvolcanic complex intruded during the permian igneous activity in the oslo paleorift molybdenite deposits occur within the complex in large discrete quartz veins accompanied by moderate alteration of the granite microscopic and microthermometric studies of fluids trapped in quartz from miarolytic cavities allow characterization of the late magmatic fluids in the granites the fluids from which the cavity quartz precipitated were present during and shortly after the magmatic stage from 750 800 c to ca 500 c the salinity decreased from 20 25 eq w nacl at the earlier stages to ca 5 eq w nacl in the latest fluids while the co2 content increased from 0 2 mol to 6 8 mol the presence of devitrified glass in some samples is direct evidence for the coexistence of a magma and a fluid phase the fluids were under lithostatic pressure 1 300 1 500 kb within most of the granite but were under hydrostatic conditions locally near the contacts where boiling occurred the early separation of a fluid phase implies that only moderate concentration and extraction of metals took place the early saline fluids became diluted probably by introduction of ground water after the first aliquots separated

na and k distribution in agpaitic pegmatites abstract composition and zoning of amphibole in agpaitic pegmatites of the 1 16 ga ilimaussaq complex south greenland record the chemical evolution of the final stages of an already extremely fractionated melt our results show that the general differentiation trends found in the earlier rocks of the complex are continued in the pegmatites albeit with some significant modifications the dominating exchange mechanism of na si ca al in the amphiboles of the magmatic stage changes to k si ca al and k na in some pegmatitic samples na k ratios in amphiboles which generally increase in the course of the ilimaussaq fractionation partly display a reversal during the crystallization of the most differentiated amphiboles the alkali trends are probably related to the buffering of na and k activity by the co crystallization of albite and microcline this buffering favors na in cooling fluids this mechanism is lost when analcime replaces feldspar as a stable phase in the late stages of crystallization e g due to locally elevated h2o activity analcime does not incorporate significant amounts of k and accordingly amphibole incorporates more k in analcime bearing assemblages the na k variation in amphiboles in the ilimaussaq pegmatites allow a detailed view into the late stage evolutionary trends of a textbook agpaitic complex the transition from silicate melt to aqueous fluid is recorded by the change of the dominant alkali ion in the pegmatitic amphiboles from na to k only in the very latest stage virtually k free mineral assemblages in analcime aegirine veins support the existence of a na dominated aqueous fluid

the mineralogy of ba and zr rich alkaline pegmatites from gordon butte crazy mountains montana usa comparisons between potassic and sodic agpaitic pegmatites at gordon butte crazy mountains montana agpaitic nepheline syenite pegmatites intrude potassic alkaline rocks principally malignites and nepheline microsyenites all pegmatite veins are composed predominantly of potassium feldspar nepheline prismatic aegirine barytolamprophyllite wadeite eudialyte loparite ce and altered rinkite vudyavrite embedded in spherulitic and fibrous aegirine well differentiated veins contain pockets filled with calcite fluorapatite mangan neptunite mn ti enriched prismatic aegirine calcium catapleiite and an unidentified ca ti silicate the potassium feldspar corresponds to ba rich sanidine with relatively low na contents the nepheline contains low levels of sio2 and elevated fe contents the compositions of nepheline cluster in the lower portion of the morozewicz buerger convergence field indicating low temperature crystallization and or chemical re equilibration of this mineral the association of sanidine with nearly stoichiometric nepheline is unusual for agpaitic rocks and probably reflects inhibition of al si ordering in the feldspar by ba at least four types of clinopyroxene can be distinguished on the basis of their morphology and composition all these types correspond to al and ca poor aegirine typically 0 6 and 2 6 wt al2o3 and cao respectively the overall evolutionary trend of clinopyroxene in the gordon butte rocks is from fe poor diopside to aegirine augite in the malignites and nepheline microsyenites and culminates with the pegmatitic aegirine this trend is characteristic for potassic alkaline complexes and results from preferential partitioning of fe2 into biotite during the magmatic crystallization barytolamprophyllite in the pegmatites is primary as opposed to deuteric only a few crystals contain a core composed of lamprophyllite the evolutionary history of the gordon butte pegmatites can be subdivided into primary agpaitic and deuteric stages the earliest paragenesis to crystallize included accessory zircon and thorite sr rich loparite also precipitated relatively early serving as a major repository for sr ree and nb during the agpaitic stage diverse titano and zircono silicates barytolamprophyllite eudialyte wadeite and rinkite among others consumed most of the ba sr ti zr and nb still remaining in the melt the final stage in the evolution of the pegmatites involved interaction of the earlier formed mineral assemblages with deuteric fluids in common with the rocky boy pegmatites sr ree na rich fluorapatite ba fe titanates and ree bearing carbonates ancylite calcio ancylite and bastnasite parisite series are chief products of the deuteric stage the alteration of the primary mineral assemblages by deuteric fluids also produced muscovite zeolite pseudomorphs after nepheline replacement of wadeite and eudialyte by catapleiite group minerals re deposition of ba in the form of hyalophane baotite and benitoite and cation leaching from rinkite eudialyte and loparite the mineralogy of the pegmatites from gordon butte other potassic complexes and sodic agpaitic occurrences is compared in detail

a sliding consolidation model for flow slides a model is proposed for the behaviour of flow slides in loose cohesionless materials in which the debris spreads out as a uniform sheet excess pore fluid pressures are assumed to be generated for example by undrained loading arising from the collapse of metastable structure in the source area of the flow slide under the influence of these the leading element of the debris moves downslope by basal sliding consolidating as it does so by single upward drainage as a result the pore pressure at the base of the element eventually decays to a value that brings the element to rest hence defining the run out of the flow slide the model is applicable in principle to subaqueous and subaerial flow slides and to pore fluids consisting of water gas or both of these the predictions of the model are tested against the observed behaviour and measured properties of the 1966 flow slide at aberfan south wales the results show that a sliding consolidation model provides a possible mechanism for that event

ore forming potential of alkaline magmas abstract geochemical models of the genesis of the largest rare metal deposits connected with agpaitic nepheline syenites are discussed in this paper thermodynamic calculations demonstrate that high alkalinity prevents the separation of volatile components into the gas phase the rare earth deposits have been formed at the magmatic stage during the process of crystal differentiation jointly with convection in the magmatic system the saturation of alkaline magma in respect of ore minerals at early stages of crystallization is considered to be the necessary condition for the formation of cumulative magmatic deposits these considerations are proved by data on micro inclusions

the magmatic and fluid evolution of the motzfeldt intrusion in south greenland insights into the formation of agpaitic and miaskitic rocks the 1 275 ga motzfeldt intrusive complex in the gardar failed rift province in south greenland formed from six successively intruding melt batches sm1 sm6 interpreted to be derived from a common magma source at depth five units sm1 sm5 crystallized an alkaline to peralkaline miaskitic mineral assemblage of amphibole clinopyroxene feldspar nepheline fe ti oxides zircon apatite fluorite and rarely olivine the last magmatic batch sm6 is characterized by an agpaitic mineral assemblage of aegirine nepheline alkali feldspar eudialyte and rare fluorite or sodalite coexisting mafic minerals constrain the crystallization conditions of the miaskitic rocks to about 850 6008c whereas solidus temperatures below 5008c are indicated by coexisting alkali feldspars in the agpaitic rocks oxygen fugacities during the orthomagmatic stage are below the fmq fayalite magnetite quartz buffer fmq 0 5t o 2 0 whereas late hematite provides evidence of a higher relative oxygen fugacity during late stage alteration the nd and oxygen isotope compositions of amphiboles and pyroxenes are homogeneous throughout the complex and suggest a common mantle derived magma source for all six units which is comparable with other gardar intrusions the hydrogen isotopic composition of amphiboles dd 99 to 132 indicates lowtemperature fluid rock interaction with low fluid rock ratios fluid inclusion studies indicate that h2o nacl fluids present during the magmatic stages in the miaskitic units had salinities of510 wt nacl eq calcite crystals in fluid inclusions within these rocks suggest that co2 or hco3 was an important component of the original fluid phase in contrast the agpaitic unit is characterized by a ch4 h2o naclfluid the c o h isotope compositionsofthefluid inclusions in all units are consistent with mixing between a small volume of magmatic fluid and a large volume of meteoric water the chemical evolution of the motzfeldt complex is a type example of the connection of the transition from miaskitic to agpaitic mineral assemblages with redox dependent fluid solid equilibria thetransitionfromarelatively oxidized to a relatively reduced fluid is correlated with a change from a more reduced fe 2 bearing miaskitic taramite arfvedsonite zircon sm1 sm5 to a more oxidized fe 3 bearing agpaitic assemblage aegirine eudialyte sm6 we suggest that coupled fluid solid redox equilibria involving fe 2 f e 3 c o2 and ch4 in this case were simply driven by temperature decrease and an overall increase in na k in the melt this observation sheds light on the heavily debated miaskite agpaite transition the combined temperature and compositional effect stabilized fe 3 c h 4 and enhanced the solubility of zr possibly as na zr si o complexes in the latest stage melt sm6 resulting in the crystallization ofan agpaitic mineral assemblage in terms of redox conditions during crystallization the motzfeldt rocks represent an intermediate case in the gardar province between more oxidized co2dominated intrusions such as the syenite carbonatite complex of

distinguishing magmatic and metamorphic processes in peralkaline rocks of the norra karr complex southern sweden using textural and compositional variations of clinopyroxene and eudialyte group minerals

nb zr and lree rich titanite from the alno alkaline complex crystal chemistry and its importance as a petrogenetic indicator titanite occurs as an accessory phase in melteigite ijolite and nepheline syenite from the alno alkaline intrusion and is typically associated with ti andradite and calcite perovskite titanite fr