high al gabbros in the laramie anorthosite complex wyoming implications for the composition of melts parental to proterozoic anorthosite high al gabbro represents one of the latest phases of magmatism in the 1 43 ga laramie anorthosite complex lac in southeastern wyoming this lithology which is mineralogically and geochemically the most primitive in the lac forms dikes and small intrusions that cross cut monzonitic and anorthositic rocks high al gabbro is characterized by high al2o3 15 19 wt ree patterns with positive europium anomalies eu eu 1 2 3 8 and the lowest initial 87sr 86sr as low as 0 7033 and highest initial nd up to 2 in the lac their sr and nd isotopic characteristics indicate a mantle origin followed by crustal assimilation during ascent intermediate plagioclase an50 60 and mafic silicate fo54 63 compositions suggest that they are not primary mantle melts and that they differentiated prior to final emplacement high al gabbros of the lac are similar compositionally to gabbros from several other proterozoic anorthosite complexes including rocks from the harp lake complex and the hettasch intrusion in labrador and the adirondack mountains of new york these gabbros are considered to be parental to their associated anorthositic rocks a theory that is supported by recent experimental work we interpret lac high al gabbros to represent mantle derived melts produced by the differentiation of a basaltic magma in an upper mantle chamber continued evolution of this magma eventually resulted in the formation of plagioclase rich diapirs which ascended to mid crustal levels and formed the anorthositic rocks of the lac because these gabbros intrude the anorthositic rocks they do not represent directly the magma from which anorthosite crystallized and instead are younger samples of magma formed by identical processes

proterozoic accretionary tectonics at the southern margin of the archean wyoming craton the proterozoic cheyenne belt at the southern margin of the archean wyoming craton consists of strongly deformed lithotectonic blocks bound by major mylonite zones from north to south and structurally lowest to highest these blocks include 1 archean crystalline basement and associated early proterozoic miogeoclinal rocks 2 an amphibolite grade orthogneiss terrane of unknown age containing probable rift related mafic intrusive rocks 3 a 1750 1790 ma marginal basin succession consisting of upper amphibolite grade pelitic and volcano genie schist associated peraluminous granite and minor ultramafic rocks and 4 a 1750 1790 ma intermediate to mafic plutonic metamorphic complex interpreted as the deep roots of an island arc system the earliest deformation d1 produced a synmetamorphic penetrative horizontal transposition foliation and associated recumbent folds tectonic blocks were juxtaposed along major mylonite zones during d2 macroscopic anil microscopic structures associated with d2 indicate northward thrusting along low angle mylonite zones of successively deeper crustal blocks over supracrustal rocks of the wyoming craton at about 1750 ma thrusting occurred at minimum temperatures of 475 c and produced an inverted metamorphic gradient in pelitic rocks north of the belt mylonite zones were subsequently steepened and reactivated under greenschist facies conditions during a period of dextral strike slip d3 between 1750 and 1400 ma cataclasis d4 and brecciation associated with the laramide orogeny d5 locally overprint earlier structures oblique convergence between an island arc and the archean continental nucleus may explain accretion crustal shortening and subsequent strike slip the phanerozoic accretionary events of eastern and western north america provide an analog for this model presence of similar lithologies and shear zones south of the cheyenne belt suggest that the southern margin of the wyoming craton may have been a long lived zone of crustal accretion

jotunites from the grenville province quebec petrological characteristics and implications for massif anorthosite petrogenesis abstract we report new results on the geology mineralogy and chemical compositions of jotunites mainly pyroxene monzodiorites from the morin st urbain and labrieville anorthosite complexes all in the grenville province of quebec jotunites occur between anorthosite and mangerite as marginal units to anorthosite or mangerite plutons or as fine grained dikes in either rock type some occurrences are probably cumulates but the dikes indicate the existence of jotunitic liquids jotunites consist predominantly of sodic plagioclase an25 an45 clinopyroxene and orthopyroxene having intermediate xmg e g en60 en40 along with significant ilmenite magnetite and apatite most samples contain small amounts of k feldspar locally mesoperthite and quartz it is noteworthy that plagioclase and pyroxene compositions in jotunites lie between those in associated anorthosite and mangerite jotunites from all areas span a similar range in sio2 content 40 55 wt and are characterized by high levels of fe2o3 t 30 wt tio2 6 wt and p2o5 3 25 wt all samples are alkalic with k2o 3 6 wt increasing in the sequence morin st urbain labrieville they contain high sr up to 1400 ppm and ba up to 4600 ppm and concentrations correlate well with those in associated anorthosite in each case jotunites also are high in ga high ga al zr zn y and ree but low in rb high k rb u th ni cr co sc and v high ti v jotunites can be linked spatially mineralogically and chemically with associated anorthosite and mangerite and we interpret them as transitional rocks in a largely comagmatic sequence from anorthosite to mangerite we postulate a model for andesine anorthosite petrogenesis involving early crystallization and accumulation of plagioclase from parental magmas of intermediate sio2 content broadly dioritic in composition resulting in fe ti and p enrichment in the remaining magma leading to jotunite high o2 table causes early crystallization of fe ti oxides which suppresses fe mg enrichment in pyroxene fractionation of fe ti oxides pyroxene and apatite ultimately results in si and k enrichment in residual liquids leading to quartz mangerite significantly liquids follow an initial trend of si depletion fe enrichment followed by a reversal of this trend this view contrasts strongly with current models that regard anorthosite jotunite and mangerite as unrelated products of bimodal magmatism

geochemical evolution of the massif type anorthosite complex at bolangir in the eastern ghats belt of india granulite facies recrystallization 750 800 c 7 8 kbar a h2o the bolangir massif type anorthosite in the eastern ghats belt of 0 25 during cooling from the igneous to the regional thermal india is composed of homogeneous anorthosite and subordinate regime volumes of schlieric leuconorite in its marginal parts the massif is separated from granulite grade country gneisses by coarse grained garnetiferous granitoids extremely iron rich ferrodiorites feo 39 30 wt sio2 36 43 wt do 7 7 8 0 occur

permian rifting related fayalite syenite in the panxi region sw china abstract within the panxi rift of the permian emeishan large igneous province elip sw china a 20 km 2 quartz bearing fayalite syenitic pluton intrudes the 260 ma baima igneous complex bic the fayalite syenite is dated using shrimp zircon u pb techniques at 252 2 5 ma the panxi fayalite syenite is mainly composed of alkali feldspar ferroan augite fayalite quartz and fe ti oxides and contains 61 8 63 7 wt sio 2 5 0 7 3 wt fe 2 o 3 t and 9 9 12 1 wt total alkalis belonging to an a type variety of ferroan alkalic metaluminous granitoids the fayalite syenite displays positive eu anomalies eu eu 0 9 8 9 and is enriched in lree la yb n 6 6 11 5 the nd values and 87 sr 86 sr initial ratios of the fayalite syenite range from 1 3 to 1 9 and from 0 7035 to 0 7068 respectively the fayalite syenite and the bic had the same mantle source however the 8 myr age gap between them suggests the fayalite syenite formed by remelting of underplated mafic magmas injected into the crust during elip magmatism the ages of the fayalite syenite and bic are coincident with two permian mass extinction events and suggest that elip may have influenced both

derivation of the 1 0 0 9 ga ferro potassic a type granitoids of southern norway by extreme differentiation from basic magmas major and trace elements sr and nd isotopic data as well as mineral compositions are presented for a selection of the 1 0 0 9 ferro potassic a type granitoids bessefjellet rustfjellet verhuskjerringi valle holum svofjell handeland tveit aseral lyngdal gabbronorites that occur close to the mandal ustaoset line mul of southern norway these hornblende biotite granitoids hbg define an extensive differentiation trend ranging from gabbronorites 50 wt sio 2 to granites 77 wt sio2 this trend is interpreted as resulting from extreme fractional crystallization of several basaltic magma batches with similar major and trace elements compositions at 930 ma the hbg suite displays a narrower range in isr 0 7027 0 7056 than in end t 1 97 down to 4 90 suggesting some assimilation of a rb depleted lower crust afc process or and source variability an age of 929 47 ma is given by a rb sr isochron on the holum granite sri 0 7046 0 0006 mswd 1 7 geothermobarometers indicate a low pressure of emplacement 1 3 2 7 kbar and an oxygen fugacity close to nno high liquidus temperatures are given by the apatite saturation thermometer 1005 1054 c and are in agreement with results from other studies the basaltic parent magmas of the hbg suite are partial melts of an hydrous mafic potassic source lying either in the lithospheric upper mantle or in the mafic lower crust derived from it this contrasts with the 930 ma anorthosite mangerite charnockite suite amc suite of the rogaland province for which a depleted lower crustal anhydrous gabbronoritic source has been indicated the present data imply the penecontemporaneous melting of two contrasting sources in southern norway the source duality could result from an increasing degree of metamorphism amphibolite to granulite from east to west an horizontal stratification of the lower crust or from the stratification of the lithosphere melting of the lower crust or upper mantle it may also indicate that the amc and hbg suites formed in two distinct crustal segments the linear alignment of the hbg suite along the mandal ustaoset shear zone suggests that a linear uprise of the asthenosphere following a lithospheric delamination under this structure could be the vector of the mantle heat 2003 elsevier science b v all rights reserved

evolution of the early proterozoic colorado province constraints from u pb geochronology the colorado province represents an addition of a belt of rocks more than 500 km wide to the southern margin of the archean wyoming craton during the early proterozoic between about 1790 and 1660 ma correspondence in ages between metamorphism deformation and plutonism association of volcanic rocks with comagmatic calc alkalic plutons and lack of older basement are all consistent with the interpretation that the rocks of the province are products of arc magmatism and cannibalistic sedimentation along a convergent margin at the southern edge of the craton

post processing removal of correlated errors in grace data 1 gravity fields produced by the gravity recovery and climate experiment grace satellite mission require smoothing to reduce the effects of errors present in short wavelength components as the smoothing radius decreases these errors manifest themselves in maps of surface mass variability as long linear features generally oriented north to south i e stripes the presence of stripes implies correlations in the gravity field coefficients here we examine the spectral signature of these correlated errors and present a method to remove them finally we apply the filter to a model of surface mass variability to show that the filter has relatively little degradation of the underlying geophysical signals we seek to recover

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

a lower crustal origin for massif type anorthosites the origin of batholith sized masses of plagioclase rich crustal rocks mainly proterozoic in age has mostly been ascribed to aluminous magmas of mantle derivation here we propose that such masses are derived by partial melting of an aluminous lower continental crust which has developed following extraction of a granitic upper crust by intra crustal melting this segregation of upper and lower crusts occurred principally in the late archaean as the lower crust was depleted in heat producing elements during the initial melting to produce granites an external source of heat is required to produce proterozoic massif type anorthosites we speculate that this heat resulted from the continental crust moving over a mantle hotspot or line plume

rhyolites in the emeishan large igneous province sw china with implications for plume related felsic magmatism abstract in the emeishan large igneous province elip western yangtze block sw china minor amounts of silicic volcanic rocks 5 are spatially and temporally associated with flood basalts the distribution and origin of these silicic rocks are keys to understanding the processes associated with mantle plume activities in the binchuan and panzhihua areas southwestern elip there are nb ta poor and nb ta rich rhyolites nb ta poor rhyolites interlayered with basalts in the binchuan area occur in the lower part of the emeishan volcanic sequence and have a crustal origin they are characterized by negative nb ta anomalies low end t and high 87sr 86sr i values and zircon saturation temperatures 836 852 c nb ta rich rhyolites are associated with the main phase of the emeishan volcanic sequence in the binchuan and panzhihua areas and are characterized by positive nb ta anomalies oceanic island basalt oib like trace element ratios low 87sr 86sr i ratios positive end t and ehf t values and high a cnk ratios and fe s in addition they have high ga al ratios high field strength elements hfses and high zircon saturation temperatures 920 c typical of aluminous a type silicic rocks similar to the spatially distributed syenitic intrusions of the elip these nb ta rich rhyolites are best interpreted as hybrid products of crystal fractionation of high ti basaltic magmas coupled with minor crustal assimilation 6 14 on the basis of rhyolite melts and ec afc simulations the temporal and spatial distributions of these two types of rhyolite reflect various extents of thermal and mass exchange between mantle derived basaltic magma and crustal material above a mantle plume when the plume head reached the base of the western yangtze block enriched components in the lithospheric mantle were melted and the melts partially ponded near the crust mantle boundary or in the lower crust where they induced partial melting of crustal materials to form nb ta poor rhyolites during the main phase of volcanism large magma chambers and extensive plumbing systems developed due to decompressional melting of the mantle plume in this stage abundant mafic ultramafic and felsic intrusions were formed and nb ta rich rhyolites and a type granites were emplaced

isotopic disequilibrium and lower crustal contamination in slowly ascending magmas insights from proterozoic anorthosites abstract many proterozoic anorthosite massifs show crustal isotopic signatures that have for decades fuelled debate regarding the source of these temporally restricted magmas are these signatures indicative of lower crustal melting or of significant assimilation of crustal material into mantle derived magmas traditional whole rock isotopic tracers sr nd pb and os like other geochemical petrological and experimental tools have failed to identify unambiguously the origins of the crust like signature and resolve the source controversies for these feldspathic cumulate intrusives we make use of high precision sr nd and pb isotopic compositions of mineral phases plag opx mag and comagmatic high pressure orthopyroxene megacrysts as well as whole rock anorthosites leuconorites from the mealy mountains intrusive suite mmis and the nain plutonic suite nps to probe the origin of the crustal isotopic signatures and assess the importance of differentiation at lower crustal depths this selection of samples represents fragments from various stages of the polybaric ascent of the magmas while the study of the mealy mountains intrusive suite and the nain plutonic suite is instructive as each is intruded into crust of significantly different age and isotopic composition we observe marked differences in the whole rock isotopic composition of proterozoic anorthosites and high pressure megacrysts e g e nd t 2 to 10 intruded into crustal terranes of different ages and isotopic compositions evidence for varying degrees of internal isotopic disequilibrium nd sr pb in anorthosites from these different terranes reinforces the notion that crustal contamination and more importantly the nature of the crustal assimilant has a profound influence on the chemical signature of proterozoic anorthosites while most samples from the mmis and nps show significant and measurable nd and pb disequilibrium sr compositions cluster around zero this decoupling in disequilibrium geometries cannot be explained by melting of the lower crust assimilation of crust with distinctive sr nd and pb isotopic compositions does however explain the origin of decoupling in internal mineral isotopic compositions we also find unexpected patterns of internal isotopic disequilibrium such as isotopically radiogenic orthopyroxene relative to plagioclase and differences in plagioclase isotopic disequilibrium between orthopyroxene and olivine bearing samples these various lines of evidence provide strong support for the generation of crustal isotopic signatures through assimilation and not anatexis of the lower crust these isotopic data show that anorthosite petrogenesis likely involves significant differentiation and solidification at lower crustal depths followed by ascent of high crystallinity bodies 50 crystallinity to mid or upper crustal levels we show that protracted lower crustal differentiation imparts a clear chemical and isotopic signature on mantle derived magmas of proterozoic anorthosites and that this process is central in the development of such slowly ascending plagioclase rich magmas

the origin of anorthosites and related rocks from the lofoten islands northern norway i field relations and estimation of intrinsic variables these calculations indicate that the calcic flakstadoy anorthosite introduction and geological fbc cpx ol opx mtss ilmss plag an57 47 setting crystallized under polybaric conditions at pressures between 4 and 9 kbar and at temperatures between 1140 and 1185 c the sodic the commonly observed association of proterozoic maseidsfjord complex cpx opx mtss ilmss plag sif type anorthosites see ashwal 1993 and references an48 44 crystallized at 1100 1135 c at a maximum pressure therein with jotunitic and mangeritic or charnockitic of 7 3 kbar this technique may provide a means to estimate rocks is still not satisfactorily explained some models crystallization pressure and asio2 in many types of intrusive and consider them to be comagmatic and relate these rocks extrusive rocks coeval mangerites and charnockites intruded subby processes of liquid immiscibility philpotts 1981 or a sequently at 4 kbar and temperatures between greater than 925 c specific fractionation trend owens et al 1993 others and 800 c respectively indicated by the succession of the mafic describe them as coeval but not comagmatic duchesne phase assemblages cpx opx cpx opx ol cpx et al 1989 duchesne 1990 a typical proterozoic pig ol cpx ol that reflect continuous fractionation to anorthosite mangerite charnockite association is exhigher fe mg ratios the evidence for polybaric crystallization of posed in the lofoten islands off the nw coast of norway the fbc quantitatively supports the common model that generation which is dominated by mangerites and charnockites in of proterozoic anorthosites involves initial crystallization at depth this paper the first of a series on these rocks we present crust mantle boundary and intrusion as a crystal rich mush the field observations and phase petrology and determine the conditions under which the intrusions were emplaced detailed estimation of intrinsic parameters p t fo2 asio2 fhcl

composition and petrogenesis of oxide apatite rich gabbronorites associated with proterozoic anorthosite massifs examples from the adirondack mountains new york mafic dikes and sheets rich in fe ti oxides and apatite are commonly associated with proterozoic massif anorthosites and are referred to as oxide apatite gabbronorites oagn within the adirondacks field evidence indicates that during middle to late stages of anorthositic evolution these bodies were emplaced as magmas with unspecified liquid crystal ratios sixty whole rock analyses of adirondack oagn and related rocks define continuous oxide trends on harker variation diagrams sio2 37 54 similar trends exist for sr y nb zr and ree and together suggest a common origin via fractional crystallization a representative parental magma plagioclase rich crystal mush has been chosen from this suite and successive daughter magmas have been produced by removal of minerals with compositions corresponding to those determined in actual rocks least squares mass balance calculations of major element trends indicate that removal of intermediate plagioclase an40 50 plus lesser amounts of pyroxene account for the compositional variation of this suite and produce very low sums of the squares of the residuals r2s 0 25 the extracted mineral phases correspond volumetrically and compositionally to those of the anorthositic suite and the model succeeds in accounting for the observed oagn trends the major element model is utilized to calculate trace elejent concentrations for successive magmas and these agree closely with observation we conclude that beginning with a plagioclase rich crystal mush the extraction of intermediate plagioclase an40 50 drives residual magmas to increasingly fe ti and p rich and sio2 poor conditions characteristic of fenner type fractionation the crystallization sequence is plagioclase plagioclase orthopyroxene plagioclase orthopyroxene pigeonite augite fe ti oxides begin to crystallize near the end of the sequence and are followed by apatite and fayalitic olivine which appears in place of pigeonite augitic pyroxene becomes the dominant ferromagnesian phase in late stages of fractionation resultant oagn magmas are injected into congealed anorthosite by filter pressing of liquid rich interstitial fractions varying compositions of the dikes reflect filter pressing at different stages during fractionation and thereby provide information on the fractionation history of proterozoic massif anorthosites

pyroxene megacrysts in proterozoic anorthosites implications for tectonic setting magma source and magmatic processes at the moho abstract proterozoic anorthosites from the 1630 1650 ma mealy mountains intrusive suite grenville province canada the 1289 1363 ma nain plutonic suite nain churchill provinces canada and the 920 949 ma rogaland anorthosite province sveconorwegian province norway all entrain comagmatic cumulate high alumina orthopyroxene megacrysts haoms the orthopyroxene megacrysts range in size from 0 2 to 1 m and all contain exsolution lamellae of plagioclase that indicate the incorporation of an excess ca al component inherited from the host magma at pressures in excess of 10 kbar at or near moho depths 30 40 km suites of haoms from each intrusion display a large range in 147sm 144nd 0 10 to 0 34 making them amenable for precise age dating with the sm nd system sm nd isochrons for haoms give ages of 1765 12 ma mealy mountains 1041 17 ma rogaland and 1444 100 ma nain all of them older by about 80 to 120 m y than the respective 1630 1650 920 949 and 1289 1363 ma crystallization ages of their host anorthosites internal mineral sm nd isochrons between plagioclase exsolution lamellae and the orthopyroxene host for haoms from the rogaland and nain complexes yield ages of 968 43 and 1347 6 ma respectively identical within error to the ages of the anorthosites themselves this age concordance establishes that decompression exsolution in the haom was coincident with magmatic emplacement of the anorthosites 100 m y after haoms crystallization at the moho correspondence of pb isotope ages 206pb 204pb vs 207pb 204pb with sm nd ages and other strong lines of evidence indicate that the older megacryst ages represent true crystallization ages and not the effects of time integrated mixing processes in the magmas nd isotopic evolution curves afc mixing calculations and the age relations between the hoams and their anorthosite hosts show that the haoms are much less contaminated with crustal components and are an older part of the same magmatic system from which the anorthosites are derived modeling of these anorthositic magmas with melts indicates that their ultramafic cumulates would have sunk in the magma and been sequestered at the moho where they may have sunk deeper into the mantle resulting in large scale compositional differentiation the haoms thus represent a rare example of part of a cumulate assemblage that was carried to the upper crust during anorthosite emplacement and together with the anorthosites illustrate the dramatic influence that magma ponding and differentiation at the moho has on residual magmas traveling towards the surface the new geochronologic and isotopic data indicate that the magmas were derived by melting of the mantle forming magmatic systems that could have been long lived e g 80 100 m y a geologic setting that would fit these temporal constraints is a long lived andean type margin

evidence for an enriched mantle source for jotunite orthopyroxene monzodiorite associated with the st urbain anorthosite quebec abstract mafic orthopyroxene monzodiorite jotunite lithologies are exposed in the st urbain plutonic suite as a marginal facies to quartz mangerite and massif anorthosite intrusive bodies and as dikes within a variety of host rocks high concentrations of ti fe p k ba nb la ce zn ga zr and y characterize these rocks and are distinctive of many mafic lithologies associated with anorthosite massifs worldwide characteristically low concentrations of ni and cr in conjuction with low mg numbers have been used by previous investigators as evidence for either partial melting of mafic granulitic lower crust or extensive fractional crystallization of a mantle derived magma in an attempt to distinguish between these competing models we note that jotunite display many features that bear a strong resemblance to continental tholeiitic flood basalts including chemical signatures on normalized multi element spider diagrams ratios of incompatible trace elements and patterns on rare earth and spider diagrams collectively indicate that the jotunite rocks were derived from an enriched rather than depleted mantle source enrichment may have occured by subduction derived fluids or by mixture with a plume component prior to partial melting so that isotopic and trace element compositions are decoupled small amounts of partial melting of mafic granulite has been advanced as an alternative model we show however that the experimental data on which this model is built are not applicable our preferred model begins with partial melting of a trace element enriched mantle source that fractionates olivine at high to moderate pressures increasing concentrations of p and ti eventually caused a contraction of the olivine stability field in favor of orthopyroxene fractional crystallization may yield the series of rocks from anorthosite leuconorite oxide apatite gabbronorite to jotunite mafic magmas emplaced into continental crust are typically attributed to incipient rifting or mantle upwelling which are features common to many models for the genesis of anorthosite and related rocks

petrogenesis of the red mountain pluton laramie anorthosite complex wyoming implications for the origin of a type granite abstract the red mountain pluton the youngest intrusion in the laramie anorthosite complex is one of five monzonitic to granitic a type plutons intruded at 1 43 ga in the laramie mountains of southeastern wyoming the red mountain pluton ranges in composition from fayalite monzonite clinopyroxene quartz monzonite and biotite hornblende quartz syenite to granite the red mountain pluton is petrologically and geochemically distinct from the northern and southern sherman batholith the sybille intrusion and the maloin ranch pluton it has higher feo t feo t mgo and higher k 2 o is more strongly enriched in rees and at any given silica content it has lower abundances of tio 2 feo mgo cao and p 2 o 5 the fayalite monzonite in the red mountain pluton crystallized at temperatures around 1000 c and oxygen fugacities 1 2 log units below that of the fmq buffer similar to the crystallization conditions of the sybille and maloin ranch plutons the biotite hornblende quartz syenite crystallized at oxygen fugacities near or slightly below those of fmq most of the units in the pluton carry mantle like nd sr and pb isotopic compositions similar to the least contaminated anorthositic and ferrodioritic rocks of the lac indicating that the pluton evolved mainly via differentiation substantial crustal assimilation occurred only in the late dikes and the granite we suggest that all of the 1 43 ga a type intrusions in southeastern wyoming are ultimately derived from tholeiites extreme differentiation or partial melting produced monzonitic magmas that have assimilated varying amounts of siliceous crust the differences between the red mountain pluton and other a type granitoids in southeastern wyoming are related to the extent of differentiation prior to crustal assimilation the compositional ranges of a type granitoids in southeastern wyoming suggest that the geochemical spectrum of a type granites worldwide similarly are produced from evolved mantle derived melt that has assimilated varying proportions of crustal material

the geochemical evolution of anorthosite residual magmas in the laramie anorthosite complex wyoming olivine and pyroxene bearing fit enriched dioritic rocks in the 1434 ma laramie anorthosite complex are interpreted to represent variably fractionated and contaminated magmas residual after the crystallization of anorthosite geochemical characteristics of this suite include the following high contents ofti02 fe20s and p2o5 high incompatible trace element contents rare earth element patterns with a large range of eu anomalies and isotopic compositions that reflect the geographic location of individual samples with i r increasing and ejy decreasing from south to north after extraction from anorthosite fractionation offerrodioritic residual magmas resulted in secondary residual monzpdioritic melts and complementary oxide rich ferrodiorite cumulates geographic trends in isotopic composition reflect an increasing archean crustal component from south to north dioritic dikes and cumulates with isotopic compositions similar to associated anorthosites were derived locally large isotopic discrepancies between some diorites and their hosting anorthosites reflect preferential contamination of residual magma during ascent and emplacement of mantle derived plagioclase rich diapirs followed by subsequent extraction and isolation of fe enriched interstitial melt strong isotopic contrasts between anorthosite and associated fe enriched rocks in anorthosite complexes do not preclude a direct relationship between them and reflect the diversity and complexity of processes during their petrogenesis

grenville age a type and related magmatism in southern laurentia texas and new mexico u s a abstract large grenville age plutons are known to be present in the 1340 1370 ma basement in texas and eastern new mexico but their distribution and compositional ranges are poorly known new u pb zircon dates shrimp rg stanford university on subsurface samples have identified widespread presence of compositionally diverse grenville age 1070 1110 ma plutonic rocks in the texas panhandle core from a 175 m thick sill like gabbroic intrusion yielded a date of 1081 8 3 ma in situ differentiation of this tholeiitic magma led to a 7 fold increase in incompatible element concentrations as ree patterns remained essentially flat trace element data suggest an e morb like source to the south alkali feldspar granite with a type affinities was recovered from drill core from the abilene gravity minimum near albany texas it yielded a date of 1078 23 ma similar to undeformed post orogenic granites in the llano uplift further west an anorthosite xenolith from the eocene three sisters intrusion in el paso texas yielded a date of 1068 30 ma which is within error of the 1110 19 ma age determined for the main stage of the nearby a type red bluff granitic suite xenoliths from potrillo maar volcano in the center of the southern rio grande rift include monzonitic xenoliths with granulitic texture which yielded dates of 1072 ma in contrast potrillo maar xenoliths with igneous textures yielded u pb dates of 27 ma development of the granulitic texture is interpreted to reflect granulite facies metamorphism during tertiary time 26 27 ma on the basis of age zoning and u concentrations in the zircon crystals our results show that grenville age magmatism in the texas and new mexico subsurface was widespread and was coeval with syn and post deformation granites in the llano uplift of central texas the compositions of dated samples suggest that mafic magmas were broadly tholeiitic in character and that granitic magmas were a type these magmatic affinities are thought to be associated with crustal extension rather than with subduction moreover the presence of anorthosite xenoliths in the el paso kilbourne hole region the broadly tholeiitic character of mafic rocks and the a type nature of the granitic rocks are broadly similar to amcg anorthosite mangerite charnockite granite suites present in the grenville province of eastern laurentia

geochemical and isotopic characteristics of different types of anorthosite in the namaqua mobile belt south africa abstract anorthosite genesis in the proterozoic namaqua mobile belt of south africa encompasses a range of intrusive histories including massif type as in the occurrences of plat sjambok and possibly koperberg suite and layered type as at oranjekom different compositional ranges at all levels of analytical resolution mineralogical geochemical and isotopical are revealed there is a spectrum from orthopyroxene dominance and extremely high lree koperberg suite to clinopyroxene horn blende dominance with normal ree patterns oranjekom plat sjambok and low iron oranjekom isotopic data for the various complexes rb sr sm nd indicate differences in magma evolution the oranjekom complex represents a depleted mantle origin with low to moderate contamination sr 6 to 9 nd 7 to 9 while the plat sjambok complex reflects a depleted mantle origin with a noticeable contribution from a crustal component sr 40 to 150 nd 5 to 1 in contrast the koperberg suite sr 300 nd 9 if derived from the mantle would require mixing between a highly enriched mantle source and an extremely enriched crustal contaminant high sr 700 to 800 nd 2 resulting in an unusual isotopic trend with a positive slope the waterkop pluton that has in the past been regarded as related to the plat sjambok complex represents a quite different syenitic magma from a slightly enriched mantle source with very little contamination and is not a cogenetic with the latter collectively the analytical data support original magma generation from different mantle sources followed by various intrusive histories ranging from rapid transit between source and emplacement in the upper crust oranjekom to prolonged travel and differentiation involving considerable crustal contamination koperberg suite the fact that the different intrusive types occur in different suspect terranes of the mobile belt may reflect originally contrasting tectonic settings during a late proterozoic continental collision that played a role during the assembly of rodinia

geochemical and radiogenic isotope sr nd characteristics of paleoproterozoic anorthositic and granitoid rocks in the umiakoviarusek lake region labrador canada recent work in the north central labrador has identified paleoproterozoic anorthositic and granitoid rocks that are spatially associated with yet temporally distinct from younger mesoproterozoic intrusions of the nain plutonic suite the umiakoviarusek lake ul region of labrador contains several of these paleoproterozoic intrusions and provides an opportunity to study their geochemical and radiogenic isotope sr nd characteristics geochemically the anorthositic and granitoid rocks have features consistent with contemporary anorthositic and granitoid rocks from other anorthosite mangerite charnockite granite complexes the anorthositic rocks contain elevated contents of al2o3 cao sr and eu with low ba rb k zr total rare earth elements ree and light ree the granitoid rocks on the other hand contain lower concentrations of these elements along with elevated sio2 and k2o isotopic data at 2050 ma for the anorthositic rocks i sr 0 7048 0 7082 e nd 4 1 to 15 9 and granitoid rocks i sr 0 7036 0 7094 e nd 5 1 to 9 7 are consistent with variable crustal and mantle contributions to their genesis the relatively unradiogenic sr and slightly evolved nd isotopic data from the ul granitoid rocks is consistent with a significant juvenile mantle component possibly derived from an underplating mantle plume this component may also be present in the anorthositic rocks the nd and sr isotopic data are also consistent with crustal contamination from archean source materials however based on the existing isotopic database for the nain province gneisses it is not possible to delineate a specific gneiss component furthermore it is also quite possible that an archean source unlike any described at present was a crustal source component in the ul intrusive rocks

petrogenesis of the labrieville alkalic anorthosite massif grenville province quebec the labrieville massif 1010 ma is an eroded dome consisting mineral plagioclase feldspar the circumstances whereby such nearly monomineralic plutons can be produced of three zones 1 a foliated inner core of pink anorthosite an30or13 2100 ppm sr 1300 ppm ba with minor have puzzled geologists for more than a century and massif anorthosites remain petrologic enigmas in adleuconorite 2 a foliated outer core of pink anorthosite an35or10 1900 ppm sr 850 ppm ba with more abundant leuconorite dition to this fundamental aspect of the anorthosite problem see bowen 1917 scoates 2000 the petro 3 a border of foliated to massive green leucogabbro antiperthitic plagioclase orthopyroxene hemoilmenite and small amounts of genesis of massif anorthosites was apparently restricted in time and space specifically virtually all of the world s biotite are present in all rocks the border zone contains additional clinopyroxene magnetite and apatite labrieville is more alkalic supply of massif anorthosite was formed during the proterozoic and much of it is found in the grenville and srand ba rich than almost all other massif anorthosites the widespread presence of biotite and the high hematite content of province of quebec and labrador kranck 1961 anderson 1969 ashwal 1993 hemoilmenite suggest crystallization from a relatively oxidizing and water bearing magma collectively these features imply that despite their similar plagioclase rich nature it is now apparent that massif anorthosites worldwide are minlabrieville originated from a magma that differed considerably from typical tholeiitic liquids majorand trace element variations suggest eralogically and compositionally diverse see xue that all rock types represent mixtures of cumulus minerals with morse 1993 implying involvement of a wide range of negligible trapped melt the compositionally distinct zones imply parental magma compositions or petrogenetic processes differentiation during crystallization although plagioclase comin their formation however comprehensive studies on positions become more evolved inward other indicators e g xmg individual anorthosite complexes that can elucidate their in pyroxene suggest differentiation outward from core to border we petrogenesis combining field and petrographic obfavor the latter interpretation and suggest that the increase in servations with mineral chemical data and with complete plagioclase an content reflects a decline in pressure during crysmajorand trace element data on a wide range of rock tallization types are few in number admittedly the immense size of certain massifs e g lac st jean complex woussen et al 1988 and the remote or inhospitable locations of

in situ u pb and lu hf isotopic studies of zircons from the sancheong hadong amcg suite yeongnam massif korea implications for the petrogenesis of 1 86 ga massif type anorthosite abstract isotopic and geochemical characteristics of proterozoic anorthosite mangerite charnockite granite amcg suite have long been used for tracing the mantle crustal source and magmatic evolution we analyzed lu hf isotopic compositions of zircon from the sancheong hadong amcg complex yeongnam massif korea in order to understand tectonomagmatic evolution of the paleoproterozoic amcg suite occurring at the southeastern margin of the north china craton ncc the anorthositic rocks in this complex associated with charnockitic and granitic gneisses were recrystallized to eradicate magmatic features in situ shrimp sensitive high resolution ion microprobe u pb analyses of zircon from a leuconorite and an oxide bearing gabbroic dyke yielded weighted mean 207 pb 206 pb ages of 1870 2 ma and 1861 6 ma respectively charnockitic granitic and porphyroblastic gneisses yielded weighted mean 207 pb 206 pb zircon ages of 1861 6 ma 1872 6 ma and 1873 4 ma respectively these crystallization ages together with our previous geochronological data for anorthosites 1862 2 ma are indicative of episodic amcg magmatism over an 10 ma interval initial e hf t values of zircon analyzed from five anorthositic rocks and four felsic gneisses range from 2 1 to 6 1 and 0 3 to 5 4 respectively zircon hf isotopic data in combination with available whole rock sr nd isotopic data suggest that anorthositic parental magma was most likely derived from a mantle source and variably affected by crustal contamination this crustal component is also reflected in charnockitic granitic magmas produced primarily by the melting of lower crust taken together the amcg magmatism at 1 87 1 86 ga in the yeongnam massif is most likely a late orogenic product of paleoproterozoic ncc amalgamation tectonically linked to assembly of the columbia supercontinent

localized pluton deformation and linked focused flow of low volume fraction residual melt in deforming plagioclase cumulates shear hosted sub cm to 50 m wide fe ti zr ree ftzr rich melt bands occur exclusively at folded margins of the bolangir anorthosite pluton eastern india transverse to margin parallel foliation in anorthosite the n to nnw trending ftzr melt bands are oriented at a low angle 90 fractionation of anorthosites from crustally contaminated high al gabbro hag parent melt at the high degree of crystal fractionation crystal content higher than the particle locking threshold the ftzr melts during last stages of crystallization were locked within the plagioclase dominated cumulus pile the occurrence of ftzr melt bands exclusively at folded as opposed to straight segments of pluton margin is attributed to melt enhanced strain localization in the pluton induced by the ftzr melts locked within anorthosite further melt induced straining led to focusing of the ftzr melts along dilatant shear zones formed synchronously with the east west shortening deformation the variable widths of the melt bands correspond to the varying sizes of actively deforming magmatic plagioclase in anorthosite the extraction and transport of locked in low volume fraction melt documented here suggest long duration far field stresses may segregate and transport large volumes of low volume fraction melts in planetary interior over considerable distance

petrology and isotope geochemistry of the mesoproterozoic anorthosite and related rocks of the kunene intrusive complex nw namibia abstract the intrusion of the large c 18 000 km2 anorthosite body of the kunene intrusive complex angola namibia which is associated with minor marginal granite bodies and syenite syenodiorite dykes marked the beginning of mesoproterozoic igneous activity in nw namibia anorthosites display an igneous mineral assemblage of dominating plagioclase together with minor olivine orthopyroxene or clinopyroxene fe ti oxides whereas biotite and pargasite are late magmatic phases apatite is a common accessory phase in regionally associated felsic rocks k feldspar plagioclase clinopyroxene and or hastingsite crystallized first followed by interstitial quartz hastingsite fe ti oxides and titanite the general geochemical characteristics for the anorthositic rocks of the kunene intrusive complex i e high al2o3 cao sr and eu intermediate to high mg numbers of 0 37 0 74 positive nd t values of 3 0 to 1 0 and initial sr ratios of 0 7028 0 7041 are in excellent agreement with their derivation from fractionated basaltic liquids the 18o values obtained for plagioclase 5 88 0 19 18o support their development from mantle derived magmas relic crustal xenoliths of calc silicate rocks and sr and nd isotope data however suggest that the anorthosite magma underwent minor crustal contamination which was most prominent during the early intrusion stage a u pb zircon age of 1376 2 ma was obtained for a felsic rock associated with the 1385 25 ma anorthosites which suggests that the two suites were emplaced during the same igneous event the geochemical and isotope signatures of the granites syenites and syenodiorites provide convincing evidence against consanguinity of the anorthositic and the felsic rock suites i e 1 a compositional gap exists between the major and trace element contents of the two suites 2 trace element and sr and nd isotopic data of the felsic rocks point to a crustal source nd 2 0 to 0 4 initial sr ratio 0 7024 0 7063 3 the ree patterns of the felsic rocks do not exhibit ubiquitous negative eu anomalies which would be expected from fractionation products of melts that previously formed extensive plagioclase cumulates and 4 18o values of magmatic feldspar from the felsic rock suite fall in a restricted range 7 20 7 92 that is about 1 6 higher than the average igneous plagioclase 18o of the anorthosites and suggests a formation of the felsic melts by anatexis of the lower crust

a strontium and neodymium isotopic investigation of the laramie anorthosites wyoming usa implications for magma chamber processes and the evolution of magma conduits in proterozoic anorthosites abstract a strontium and neodymium isotopic investigation of plagioclase rich cumulate rocks in the 1 43 ga laramie anorthosite complex lac provides new insights into the evolution of magma chambers and underlying magma plumbing systems during the crystallization of proterozoic anorthosites the 725 km2 lac intruded the boundary between archean rocks of the wyoming province in the north and proterozoic island arc terranes in the south the cheyenne belt at pressures of 3 4 kilobars initial strontium and end isotopic ratios from the three large composite anorthositic intrusions poe mountain chugwater and snow creek and late troctolitic intrusions range from 0 7034 0 7055 and 0 9 to 4 9 respectively the isotopic ratios in the cumulates vary as a result of 1 the isotopic composition of the parental mantle derived high al gabbroic magmas 2 the amount of contamination by archean rocks during ascent through the crust and 3 mixing of replenishing magmas and resident magmas with different isotopic compositions in the magma chamber at the final level of emplacement strontium isotopic and normative an variations across the 5 7 km thick stratigraphic section of layered plagioclase rich cumulates in the 200 km2 poe mountain anorthosite indicate that multiple inputs of magma are needed to construct even relatively small intrusions in proterozoic anorthosites the earliest and stratigraphically lowest intrusion was emplaced as a plagioclase rich magma that crystallized large volumes of compositionally homogeneous anorthosite at least three subsequent chamber wide episodes of magma replenishment followed by mixing are indicated by abrupt shifts in isr and or normative an in the stratigraphically higher levels of the poe mountain anorthosite strontium and neodymium isotopic disequilibrium between a high al clinopyroxene megacryst and the layered cumulates is consistent with a high pressure origin for al rich pyroxene megacrysts in proterozoic anorthosites the megacryst crystallized in a magma chamber at pressures of 10 12 kilobars from relatively uncontaminated basaltic parent magma high al gabbro and was transported through the crust in progressively contaminated anorthositic magma the range of observed isotopic compositions in the anorthositic rocks is nearly identical to that of high al gabbroic dikes in the lac supporting the proposition that high al gabbros are parental to anorthosite mitchell et al 1995 the isotopic data require that the anorthositic parental magmas were contaminated during ascent through the crust by archean orthogneisses and or metapelitic rocks decreasing isr and increasing end with a relative decrease in age of the intrusions snow creek poe mountain chugwater troctolites indicates that the magma conduit became increasingly insulated from crustal contamination over time this study indicates that the strontium and neodymium isotopic systems can be used to distinguish between processes that occurred at lower crustal upper mantle pressures during the ascent of magma diapirs and within individual magma chambers in proterozoic anorthosites and underscores the need for stratigraphic control when addressing the origin of plagioclase rich cumulates

crystal fractionation and the evolution of intra plate hy normative igneous suites insights from their feldspars feldspars and normative feldspar constituents of bulk magma show complexes and anorogenic granites is associated with hotspot and continental rift environments a major chartrends supportive of fractional crystallization in the three main types of hy normative intraplate suites that contain qz oversaturated acteristic of this magmatism is the production of suites of compositionally diverse rocks that are temporally and rocks ocean island and continental alkalic suites ocean island tholeiitic suites and continental tholeiitic suites these suites are spatially associated much is still unknown about the processes that have led to the diversity of magmas within characterized by the presence of a single feldspar in each suite member a shift of this feldspar from plagioclase to alkali feldspar such suites or to the global diversity of intra plate suite types and k enrichment of alkali feldspar with decreasing temperature in the trachytic members the modal feldspars provide evidence for a many ocean island and continental hotspot and continental rift related suites are characterized by rocks with reaction relationship between feldspars and indicate a build up of magmatic volatile content towards saturation with progressive high alkali silica ratios that fall into the alkalic field of irvine baragar 1971 fig 1 regardless of their state fractionation of a parental magma having low initial volatile content the feldspar and normative feldspar evolutionary paths are unique of silica saturation such alkalic suites range from those that are strongly silica undersaturated containing rocks for each of the three suite types but similar for different suites within the same type this characteristic extends to the felsic members such as nephelinites or leucitites in the more potassic making it easy to distinguish between rhyolitic or granitic rocks suites to mildly silica undersaturated suites the kenfrom the different suite types the feldspars in natural volcanic nedy series of miyashiro 1978 containing alkali basalts suites commonly show evidence for a polybaric history particularly silica undersaturated trachytes and phonolites and in the least evolved suite members late stage feldspars of the finally to hy normative suites the coombs trend of intermediate members and feldspars of the most evolved members miyashiro 1978 that include olivine basalts trachytes show paths indicative of significantly lower temperature and pressure and rhyolites hy normative alkalic suites that contain regimes silica oversaturated rocks on ocean islands are widespread and include those from ascension island e g harris 1983 the samoan islands macdonald 1968 terceira azores e g white et al 1979 and clarion island mexico

inclined layer silicate microstructures and their use as a sense of slip indicator for brittle fault zones abstract this short note introduces a new microscale sense of slip indicator for brittle fault zones the microstructure is developed within laminated quartz veins and consists of fault parallel bands of layer silicate minerals with their long axes and 001 planes consistently inclined in the transport direction like other microscale sense of slip indicators this microstructure may prove most useful in situations where mesoscale criteria are either lacking or poorly developed because of its intimate association with fault zone vein material this microstructure may also help in deciphering the motion history of reactivated faults

neodymium strontium and lead isotopes in the maloin ranch pluton wyoming implications for the origin of evolved rocks at anorthosite margins abstract neodymium strontium and lead isotopic data are used in this study to investigate the origin of chemically evolved rocks in the maloin ranch pluton a composite body that borders and intrudes the laramie anorthosite the range of rock types present is similar to that found at the margins of proterozoic anorthosites throughout the world in the maloin ranch pluton these include ferrodiorite at the base of the intrusion overlain progressively by fine grained monzonite monzosyenite and porphyritic granite biotite gabbro and fine grained granitic dikes are present locally at various levels of this sequence the origin of the evolved rocks and their possible relation to associated anorthositic bodies has been much debated in the maloin ranch pluton each rock type has distinct isotopic characteristics which together with trace element data previously reported suggest different source characteristics for each member strontium and neodymium isotopic data for maloin ranch ferrodiorite and laramie anorthositic rocks show considerable overlap consistent with a comagmatic relation biotite gabbro is chemically and isotopically the most primitive rock type in the maloin ranch pluton the data suggest that biotite gabbro has a mantle source but has undergone extensive fractionation in the crust our results suggest that the remainder and bulk of the intrusion formed by partial melting of the lower crust due to the emplacement of the laramie anorthosite maloin ranch fine grained monzonite is chemically more evolved than the underlying ferrodiorite i e lower mg higher silica and alkalis yet has more primitive isotopic characteristics lower isr higher epsilonnd formation of fine grained monzonite by in situ fractionation of ferrodiorite is precluded by both the isotopic data and by mass balance the isotopic data do not rule out formation of fine grained monzonite by fractionation or contamination of ferrodiorite at depth but this model requires that the ferrodiorite is isotopically heterogeneous and that very large volumes of ferrodiorite were present alternatively trace element and isotopic characteristics of the fine grained monzonite are explained by partial melting of mantle derived lower crust added to the margin of the archean wyoming craton at about 1 8 ga neodymium strontium and lead isotope data for maloin ranch monzosyenite and porphyritic granite also suggest a lower crustal source these data indicate a 1 8 ga source component for the maloin ranch monzosyenites but do not require input of archean crust from the wyoming province to the north

the effect of pressure on dsr plag melt and dcr opx melt implications for anorthosite petrogenesis the crystal liquid partition coefficients for sr and cr dsr and dcr have been determined from electron microprobe analyses of plagioclase liquid and orthopyroxene liquid pairs produced in melting experiments run at pressures from 1 bar to 27 kbar on two compositions relevant to anorthosite petrogenesis one is a primitive jotunite hypersthene monzodiorite tj the other is a sample of an anorthositic dyke 500b results indicate that dsr plag liq remains nearly constant with increasing pressure tj 1 7 to 2 6 500b 0 9 to 1 4 this modest variation apparently results from the combined and opposing effects of crystal chemistry and pressure dsr increases with the albite content of plagioclase which itself increases with pressure along a composition s liquidus so pressure must have an intrinsic negative effect the two models for dsr j blundy b wood geochim cosmochim acta 55 1991 193 209 i bindeman a davis m drake geochim cosmochim acta 62 1998 1175 1193 that take into account the strong correlation between dsr and plagioclase composition overestimate dsr at high pressure whereas the two models that ignore plagioclase composition s morse geochim cosmochim acta 56 1992 1735 1738 r nielsen comput geosci 18 1992 773 788 underestimate it moreover since none of these models takes into account any pressure effect the discrepancies between predicted and observed dsr increase with pressure for all models the new results also show that dcr opx liq increases significantly with pressure dcr 2 at 1 atm and 14 2 at 10 kbar these new data confirm earlier less precise determinations of dcr that were used to infer a high pressure origin for al and cr rich orthopyroxene megacrysts the calculated sr and cr concentrations of liquids in equilibrium at 10 kbar with plagioclase and orthopyroxene megacrysts from anorthosite massifs sr 370 to 610 ppm and cr 20 to 130 ppm are in the range of what is observed in high al gabbros and primitive jotunites the inferred parent magmas of massive anorthosites fl 2000 published by elsevier science b v all rights reserved

some phase equilibrium constraints on the origin of proterozoic massif anorthosites and related rocks experimental data in the range of 1 bar to 13 kbar enable us to introduction map the liquidus equilibria relevant to proterozoic massif anorthere is hardly a consensus on the petrogenesis of thosites and related mafic rocks massif anorthosites are widely proterozoic massif anorthosites but many investigators believed to have formed by accumulation of plagioclase into highbelieve in a petrogenetic scheme that involves at least al basaltic liquids mantle derived basaltic liquids fractionating two major stages 1 extensive crystallization asat pressures sufficiently high 10 13 kbar to crystallize the highly similation of a mantle derived magma ponded at or aluminous orthopyroxene megacrysts typically observed in anorthosite near the base of the crust that produces suspensions of massifs reach plagioclase saturation at low normative silica contents plagioclase in fe rich high al gabbroic liquids followed peritectic like equilibria e g liq opx plag cpx sp by 2 intrusion of these suspensions into the mid upper and a thermal divide on the plagioclase pyroxene liquidus surface crust where they form complex magma chambers e g ensure that mantle derived liquids become nepheline normative with duchesne 1984 emslie 1985 ashwal 1993 wiebe further crystallization and crustal assimilation at depth such 1994 a key component of the scheme is the presence liquids cannot produce the full range of troctolitic noritic to trocin most massifs of high al orthopyroxene megacrysts tolitic gabbroic mineral assemblages observed in anorthosite massifs haom emslie 1975 that appear to preserve a record without extensive low pressure granite assimilation conversely the of the high pressure stage maquil duchesne 1984 array of plausible anorthosite parental liquids not only lies along longhi et al 1993 the structures of massifs vary from the trace of the plagioclase two pyroxene cotectic from 10 to 13 those that are composites of multiple diapiric intrusions kbar but also straddles the thermal divide on the plagioclase harp lake emslie 1980 rogaland duchesne 1984 pyroxene liquidus surface this condition requires mafic source to intrusive bodies with well developed layering of regions such as lower continental crust or foundered mafic plutons leucocratic troctolites and leuconorites or leucogabbros for liquids parental to massif anorthosites and associated mafic overlain by masses of anorthosite michikamau emslie intrusions 1970 laramie frost et al 1993 so there may be

the evolution of troctolitic and high al basaltic magmas in proterozoic anorthosite plutonic suites and implications for the voisey s bay massive ni cu sulfide deposit the voisey s bay massive ni cu sulfide deposit is contained within troctolitic plagioclase olivine intrusions of the 1 34 to 1 29 ga nain plutonic suite coastal labrador canada and represents a new setting for magmatic sulfide deposits proterozoic anorthosite plutonic suites consist of vast quantities as much as 20 000 km 2 of cumulate anorthosite leucotroctolite leuconorite intrusions of troctolite and fe enriched dioritic rocks and large granitic batholiths crystallization ages span nearly 1 200 m y within the middle proterozoic 2 12 0 92 ga many proterozoic anorthosite plutonic suites were emplaced into relatively young proterozoic crust and some were intruded along archean proterozoic boundaries e g nain labrador laramie wyoming korosten ukraine nearly all of the largest plutonic suites contain significant areas thousands of km 2 of plagioclase rich cumulate rocks that are olivine bearing olivine anorthosite leucotroctolite troctolitic rocks 20 30 modal olivine occur in proterozoic anorthosite plutonic suites principally as cumulates in layered or massive intrusions e g kiglapait hettasch jonathon nain intruded into anorthositic rocks with the notable exception of the voisey s bay deposit nearly all troctolitic rocks crystallized late in the evolution of individual plutonic suites many of the troctolitic intrusions have fine grained olivine rich marginal rocks that closely represent parental magma compositions numerous fine grained olivine normative dikes with high al basaltic compositions also occur within many plutonic suites nain harp lake laramie adirondack and share compositional characteristics with the troctolitic magmas these olivine normative high al basaltic magmas record the least evolved compositions in proterozoic anorthosite plutonic suites mg 0 43 0 64 an 45 60 have high al 2o 3 15 19 wt and feo total 9 15 wt contents and typically show the least contaminated sr and nd isotope compositions in a given plutonic suite many dikes and marginal rocks have strong positive eu anomalies a feature of the melt composition that is not related to plagioclase accumulation except at laramie the least evolved troctolitic magmas appear to have about 8 wt percent mgo and corresponding ni and s concentrations between 100 to 150 ppm and 0 02 to 0 05 wt percent respectively based on closed system fractionation in the kiglapait intrusion sulfide saturation occurs late in the crystallization history of troctolitic magmas after ni concentrations have been severely depleted by protracted olivine fractionation the optimal conditions for formation of a ni cu sulfide deposit associated with a proterozoic anorthosite plutonic suite require a relatively high mgo parental magma that has not crystallized much prior olivine and an external source of s to promote early sulfide saturation probably through desulfurization reactions as hot troctolitic magmas ascend through the crust an economically important massive ni cu sulfide deposit on the scale of voisey s bay clearly requires large volumes of magma probably involving continuous replenishment of relatively ni rich unfractionated parental magma in staging chambers or mixing between magmas in conduits during ascent a model for the formation of troctolite associated massive ni cu sulfide deposits is proposed depending on the relative degrees of sulfide saturation and fractionation of the mixed magmas mixing between troctolitic magmas in a conduit or magma chamber may either poison the system stopping sulfide segregation or enhance sulfide segregation

a partisan review of proterozoic anorthosites most anorthosites of the massif type crystallized in the episode 1 7 1 2 gyr with a pronounced peak of the age distribution near 1 4 gyr they were emplaced anorogenically at depths as shallow as 7 km where the ambient temperature of country rocks was probably less than 250 c depths of emplacement may have been as great as 25 km or more in rare cases the greater depths of equilibration estimated from granulite facies metamorphism may be incorrectly interpreted as emplacement depths but in any case they are demonstrably not required or characteristic of anorthosite emplacement penetrative deformation and metamorphism of anorthosites are post emplacement accidents of the local geologic history and are not directly caused by the presence of anorthosites granitic rocks mangerite charnockite suite associated with anorthosite are in general later or contemporaneous products of crustal anatexis with chemical and isotopic signatures distinct from the anorthosites and their residua such granitic rocks should not therefore be summed with the anorthositic rocks to obtain bulk compositions the magmas that produced most anorthosites were dry as shown by high temperature mineralogy and anhydrous mineral assemblages in contact aureoles residua from their crystallization are ferrodiorites to ferrosyenites typical of closed system fractionation these residua were locally and frequently ejected into contemporaneously molten granite where they formed pillows and cooled rapidly the overall chemistry of anorthosites and residua is broadly tholeiitic and consistent with derivation from the mantle olivine bearing magmas locally ranged from leucotroctolite anorthosite to later but coexisting picrite or melatroctolite in the same pluton confirming a wide spectrum of magma types a signal feature of troctolitic and noritic magmas is their low augite content implying high content of spinel component large anorthosite complexes such as nain and harp lake consist of many plutons representing repeated injections of separate magma batches with varying chemistry the abundant true anorthosites richer in plagioclase than magmas cosaturated with a mafic phase must represent plagioclase enrichment by either mechanical or chemical processes or both the role of kinetics in nucleation and solidification of such rocks may be centrally important it is proposed that hyperfeldspathic plagioclase supersaturated liquids were generated by quasi isothermal extraction of mafic minerals from tholeiitic magma enroute to and at the site of emplacement and that such a kinetic process was uniquely permitted in an environment of aborted continental rifting anorthositic rocks may have much to say about the episodic versus continuous geochemical evolution of the earth s mantle

petrogenesis of the 2 1 ga lushan garnet bearing quartz monzonite on the southern margin of the north china craton and its tectonic implications abstract the lushan garnet bearing quartz monzonite was emplaced into the western taihua complex at 2134 17 ma on the southern margin of the north china craton ncc the rocks contain sio 2 contents of 56 98 59 05 wt with high k 2 o na 2 o 7 46 9 14 wt and al 2 o 3 15 87 17 58 wt and low cao 2 38 2 77 wt and mgo 1 18 1 50 wt contents and belong to an intermediate shoshonitic series magmatic garnet grains in the rocks are chemically homogeneous with relatively high cao 6 36 7 92 wt but low mno 1 45 1 94 wt and mgo 2 47 3 45 wt contents and high fe mn ratios they contain 61 94 66 39 mol almandine 18 60 23 40 mol grossular 10 06 15 11 mol pyrope and 1 09 4 32 mol spessartine similar to high pressure garnets formed from m type magma but different from those found in s type granites and metapelites phase equilibrium diagrams in combination with trace element distribution coefficients between zircon and garnet suggest whole rock zircon saturation temperatures varying from 891 c to 951 c and the mineral assemblage of garnet plagioclase amphibole or biotite in these rocks is stable at 15 kbar and 890 950 c with h 2 o t 800 c all of the rocks are enriched in lile and lree with low nb la ratios 0 24 0 61 similar to those of lithospheric mantle which has a nb la ratio of 0 5 therefore the garnet and whole rock chemical compositions indicate that the lushan quartz monzonite possibly formed by small degrees of partial melting of a phlogopite bearing lithospheric mantle the variable zircon hf t values 3 5 to 4 1 with t dm c from 2 34 ga to 2 51 ga suggest involvement of crustal materials the rocks have slightly negative nb and ta anomalies characteristics of island arc basalts iab however their high zr 598 926 ppm and zr y ratios 17 1 21 8 suggest an affinity with ocean island basalt oib hence the mixed iab and oib like geochemical signatures are indicative of continental intra plate rifting possibly related to extension and thinning of the lithosphere and upwelling of asthenosphere our study proposes that an intra continental rifting event occurred at 2 2 2 0 ga on the southern margin of the ncc after the final amalgamation of ncc

re os isotopic evidence for a lower crustal origin of massif type anorthosites massif type anorthosites are large igneous complexes of proterozoic age they are almost monomineralic representing vast accumulations of plagioclase with subordinate pyroxene or olivine and fe ti oxides the 930 myr old rogaland anorthosite province in southwest norway1 represents one of the youngest known expressions of such magmatism the source of the magma and geodynamic setting of massif type anorthosites remain long standing controversies in precambrian geology with no consensus existing as to the nature of the parental magmas or whether these magmas primarily originate in the earth s mantle or crust at present massif type anorthosites are believed to have crystallized from either crustally contaminated mantle derived melts that have fractionated olivine and pyroxenes at depth2 or primary aluminous gabbroic to jotunitic melts derived from the lower continental crust3 here we report rhenium and osmium isotopic data from the rogaland anorthosite province that strongly support a lower crustal source for the parental magmas there is no evidence of significantly older crust in southwest scandinavia and models invoking crustal contamination of mantle derived magmas fail to account for the isotopic data from the rogaland province initial osmium and neodymium isotopic values testify to the melting of mafic source rocks in the lower crust with an age of 1 400 1 550 myr

crustal evolution and the temporality of anorthosites abstract types of rock such as komatiite which formed entirely or dominantly during restricted time periods in solar system history are important indicators of how planetary bodies evolved in deep time in this paper we characterize three different types of temporally restricted anorthosites and discuss their significance to the broad scale evolution of planetary processes primordial anorthosites which constitute the bulk of the lunar crust were sampled during apollo and luna missions and have subsequently been identified in our meteorite collections they are characterized by very calcic plagioclase an93 98 and all have magmatic crystallization ages 4 3 ga suggesting that the earliest planetary crust forming processes on the moon and possibly elsewhere involved substantial if not total melting the magma ocean hypothesis which has endured for nearly 50 years argues that the lunar anorthositic rocks represent global flotation cumulates of plagioclase which crystallized after extensive precipitation and sinking of olivine and pyroxene another possibility revitalized by critical filtering of age data involves serial magmatism whereby the lunar crust was constructed by younger smaller episodic magmatic events on earth two distinct types of temporally restricted anorthosites offer opportunities to understand the evolution of terrestrial geodynamics tectonics and magmatic processes archean megacrystic anorthosites formed only between 3 73 and 2 49 ga as small bodies

the cheyenne belt analysis of a proterozoic suture in southern wyoming abstract the cheyenne belt of southeastern wyoming is a major shear zone which separates archean rocks of the wyoming province to the north from 1800 1600 ma old eugeoclinal gneisses to the south miogeoclinal rocks 2500 2000 ma old unconformably overlie archean basement immediately north of the shear zone and were deposited under transgressive conditions along a rift formed continental margin intrusive tholeiitic sills and dikes are interpreted as rift related intrusions and a date of 2000 ma on a felsic differentiate of these intrusions gives the approximate age of rifting there are no known post 2000 ma felsic intrusions north of the cheyenne belt volcanogenic gneisses and abundant syntectonic calc alkaline plutons of the southern terrane are interpreted as island are volcanic and plutonic rocks the volcanics are a bimodal basalt rhyolite assemblage plutons include large gabbroic complexes and quartz diorite 1780 ma syntectonic granitoids 1730 1630 ma and post tectonic anorthosite and granite 1400 ma there is no evidence for archean crust south of the cheyenne belt structural data thrusts in the miogeoclinal rocks vertical stretching lineations and the same fold geometries north and south of the shear zone suggest that juxtaposition of the two terranes took place by thrusting of the southern terrane island arc over the northern terrane craton and miogeocline probably as a continuation of the south dipping subduction which generated calc alkaline plutons of the southern terrane a metamorphic discontinuity across the shear zone with greenschist facies rocks to the north and upper amphibolite facies rocks and migmatites to the south also suggests thrusting of the southern terrane deeper crustal levels over the northern terrane shallower levels the cheyenne belt may be a deeply eroded master decollement perhaps analogous to a ramp in the master decollement in the southern appalachians this interpretation of the cheyenne belt as a proterozoic suture zone provides an explanation for the geologic geochronologic geophysical metallogenic and metamorphic discontinuities across the shear zone

the link between an anorthosite complex and underlying olivine ti magnetite rich layered intrusion in damiao china insights into magma chamber processes in the formation of proterozoic massif type anorthosites mafic ultramafic intrusions comagmatic with proterozoic massif type anorthosites can provide insights into the parental magma from which large volumes of hyper feldspathic rocks are produced recent deep drilling has unveiled a large olivine ti magnetite rich layered intrusion named dawusunangou beneath the damiao massif type anorthosite complex in the north china craton the layered intrusion is composed of alternating olivine ti magnetite rich dark layers and plagioclase rich light layers ca 35 80 plagioclase with the latter also containing pod or lens shaped pyroxene ti magnetite rich aggregates this layered intrusion shows low mg and ree patterns similar to the overlying damiao anorthosite complex baddeleyite pb pb geochronology yielded indistinguishable crystallization ages of ca 1735 ma for both the dawusunangou layered intrusion and the damiao anorthosite complex suggesting coeval emplacement using the average bulk compositions of the two intrusions mass balance calculations assuming 30 40 dawusunangou and 70 60 damiao would give a composition similar to high al basaltic magma collectively these features indicate that the dawusunangou layered intrusion represents the mafic residues after the segregation of the damiao anorthosites from high al basaltic parental magma a short lived magma chamber is thought to have supplied the two intrusions in situ crystallization with variable nucleation rates for plagioclase combined with the mafic minerals crystallizing in equilibrium proportions resulted in the formation of repeated dark and light layers of the dawusunangou layered intrusion the two intrusions are interpreted to have formed by multiple magma injections instead of continuous differentiation of one melt the parental magma was derived from a depleted mantle source with significant crustal contribution during magma evolution the large nd hf isotopic variations suggest contamination by paleoarchean to neoarchean crust

the relationship between a type granites and residual magmas from anorthosite evidence from the northern sherman batholith laramie mountains wyoming usa abstract anorthosite complexes commonly are spatially and temporally associated with a type granite batholiths but their genetic relationship if any has been difficult to determine this study focuses on the northern portion of the 1 43 1 44 ga sherman batholith which is located adjacent to the 1 44 ga laramie anorthosite complex laramie mountains wyoming usa the northern sherman batholith is an ideal place to resolve the relationship of a type granite with anorthosite because of the age of the crust into which it is emplaced most a type granites are intruded into crust only slightly older than the granites themselves and there is little isotopic contrast between potential mantle and crustal source rocks the northern sherman batholith intrudes archean crust that by 1 44 ga had developed nd sr and pb isotopic compositions that differed greatly from contemporaneous mantle the northern sherman batholith is an iron enriched metaluminous alkalic to alkali calcic batholith most of the batholith is composed of fayalite granite with minor volumes of monzodiorite and olivine gabbro hypabyssal dikes of these compositions crop out in the highest structural levels exposed in the area the northern sherman batholith is less differentiated and has assimilated less felsic crust than the southern portions which intrude more fertile proterozoic crust northern sherman batholith rocks are compositionally and isotopically very similar to monzonitic and dioritic rocks of the laramie anorthosite complex none of these rocks have isotopic compositions similar to their archean host rocks precluding derivation solely from such sources instead northern sherman batholith granites like laramie anorthosite complex rocks are related to mantle derived tholeiitic magmas emplaced at the base of the crust the tectonic environment and magmatic processes that lead to the formation of massif anorthosite are also conducive to the formation of voluminous a type granite of which the northern sherman granite is a prime example

the plagioclase magma density paradox re examined and the crystallization of proterozoic anorthosites intermediate composition plagioclase an40 60 is typically less dense than the relatively evolved basaltic magmas from which it crystallizes and the crystallization of plagioclase produces a dens residual liquid thus plagioclase should have a tendency to float in these magmatic systems there is however little direct evidence for plagioclase flotation cumulates either in layered intrusions or in proterozoic anorthosite complexes the layered series of the poe mountain anorthosite southeast wyoming contains numerous anorthosite leucogabbro blocks that constrain density relations during differentiation all blocks are more mafic than their hosting anorthositic cumulates their plagioclase compositions are more calcic and each block is in strong sr isotopic disequilibrium with its host cumulate associated structures disrupted and deformed layering indicate that 1 a floor was present during crystallization and that plagioclase was accumulating and or crystallizing on the floor 2 compositional layering and plagioclase lamination formed directly at the magma crystal pile interface and 3 the upper portions of the crystal pile contained significant amounts of interstitial melt liquid densities are calculated for proposed high al olivine gabbroic parental magmas and fe enriched ferrodioritic and monzodioritic residual magmas of the anorthosites taking into account pressure oxygen fugacity p2o5 estimated volatile contents and variable temperatures of crystallization for all reasonable conditions calculated block densities are greater than those of the associated melt the liquid densities however are greater than those for an40 60 plagioclase which cannot have settled to the floor plagioclase must either have been carried to the floor in relatively dense packets of cooled liquid plus crystals or have crystallized in situ a sloping floor possibly produced by diapiric ascent of relatively light plagioclase rich cumulates is required to allow for draining and removal of the dense interstitial liquid produced in the crystal pile and may be a characteristic feature during the crystallization of many proterozoic anorthosites and layered intrusions

the origin of anorthosites and related rocks from the lofoten islands northern norway ii calculation of parental liquid compositions for anorthosites clinopyroxene orthopyroxene and plagioclase in the proterozoic introduction lofoten anorthosites are inferred to be in equilibrium with ferrodioritic proterozoic massif type anorthosites are huge intrusive melts on the basis of literature partition coefficients for these phases bodies that contain hundreds to thousands of km of in equilibrium with basaltic melts ferrodiorites and high al gabbros nearly pure 90 plagioclase cumulates of intermediate occur as fine grained dykes in spatial association with the 1 8 composition an50 10 ashwal 1993 and references ga lofoten anorthosites they are linked to each other by an fetherein most proterozoic anorthosites are not associated enrichment fractionation trend typical of tholeiites and they are with significant amounts of mafic or ultramafic cumulates characterized by al2o3 contents of 14 18 5 wt and xfe they contain plagioclase that is relatively sodic usually feo feo mgo between 0 5 and 0 8 the association around an50 and this makes them distinct from other of high al gabbros ferrodiorites and anorthosites is common in anorthositic rocks archean anorthosites lunar anormany proterozoic anorthosites we infer that the high al gabbros thosites anorthosites in layered intrusions which contain to ferrodiorites represent the parental liquids from which these more calcic plagioclase proterozoic anorthosite comproterozoic anorthosites formed the lofoten rocks show fractionation plexes contain anorthosite sensu stricto 90 plagioclase trends that are similar to those of high al gabbros and ferrodiorites as well as variable amounts of leuco gabbroic troctolitic from other proterozoic anorthosite complexes we interpret these trends and noritic rocks which respectively may contain up to in terms of incorporation of intermediate composition plagioclase into 35 clinopyroxene olivine or orthopyroxene a typical tholeiitic magma with subsequent fractionation the first the proterozoic anorthosites of the lofoten islands in process produces positive eu anomalies and the high al2o3 contents northern norway heier 1960 green jorde 1971 of the melts and explains the lack of negative eu anomalies in romey 1971 griffin et al 1978 wade 1985 are most of the ferrodiorites from lofoten and laramie associated with large amounts of temporally related intrusive mangerites charnockites and gabbros markl et al 1998 there are two different anorthosite intrusions present one is a relatively calcic complex which occurs on the island of flakstadoy flakstadoy basic complex fbc and the other is a relatively sodic complex which

regional ai fe mafic magmas associated with anorthosite bearing terranes proterozoic anorthosite massifs are nearly monomineralic accumulations of plagioclase feldspar which must result from unusual conditions of crystallization and emplacement1 2 their chemistry is compatible with derivation from a high alumina basalt parent of mantle origin1 and mafic magmas rich in aluminium and iron have long been suspected of having an important connection to anorthosites and their parent magmas1 4 regional examples of mafic magmas clearly compatible with an anorthosite lineage have not been generally recognized3 in part because of the paucity of candidates within the anorthosite massifs themselves samples of such magmas should also be sought in the country rocks surrounding the massifs here we report the widespread occurrence of al fe mafic magmas in the anorthosite bearing adirondack highlands of new york state they are chemically distinct from typical high ai basalt fe basalt and most common magma types identification of similar al fe basaltic magmas in other areas suggests that such magmas are characteristic of anorthosite bearing terranes

geochemical and nd hf isotopic constraints on the origin of the 1 74 ga damiao anorthosite complex north china craton abstract the 1 74 ga damiao complex in the north china craton consists of anorthosite 85 norite 10 mangerite 4 and minor troctolite 75 would have been required to form the parental high al gabbroic magma from the lower crust we propose that the crustal rocks were dragged into the upper mantle during amalgamation of the north china craton where the ambient temperature was high enough 1271 c at 12 kbar to cause extensive melting producing a deep seated magma chamber the melt then evolved through polybaric crystallization producing anorthositic magmas with abundant suspended plagioclase that were finally injected at mid crustal depth to form the damiao complex

oxygen isotope constraints on the petrogenesis of the sybille intrusion of the proterozoic laramie anorthosite complex the origin of monzonitic intrusions that are associated with proterozoic massif type anorthosite complexes is controversial a detailed oxygen isotope study of the sybille intrusion a monzonitic intrusion of the laramie anorthosite complex wyoming indicates that either derivation from a basaltic magma of mantle origin with a metasedimentary component 20 incorporated early in its magmatic history or a partial melt of lower crustal rocks is consistent with the data the oxygen isotope compositions of plagioclase pyroxene and zircon from the sybille monzosyenite the dominant rock type in the sybille intrusion were analyzed in order to establish the isotopic composition of the source of the magma plagioclase 18o values range from 6 77 to 9 17 we interpret the higher plagioclase 18o values average 8 69 0 30 n 19 to be magmatic in origin lower plagioclase 18o values average 7 51 0 44 n 22 to be the result of variable subsolidus alteration and pyroxene 18o values average 6 34 0 38 n 19 to be the result of closed system diffusional exchange during cooling low magnetic zircons which have been shown to retain magmatic oxygen isotope values despite high grade metamorphism and extensive subsolidus hydrothermal alteration have 18o values 7 40 0 24 n 11 which are consistent with our interpretation of the plagioclase and pyroxene results oxygen isotope data from all three minerals indicate that the magmatic oxygen isotope composition of the sybille intrusion is enriched in 18o relative to the composition of average or normal mantle derived magmas this enrichment is approximately twice the oxygen isotope enrichment that could result from closed system fractionation rendering a closed system comag matic petrogenetic model between the sybille intrusion and the mantle derived anorthositic lithologies of the laramie anorthosite complex improbable

two stage models for lunar and terrestrial anorthosites petrogenesis without a magma ocean two stage diapiric models for lunar ferroan anorthosites and terrestrial massif anorthosites are examined the lunar model is developed to explain early lunar differentiation in the absence of a magma ocean if correct the terrestrial model serves as an analog for the development of lunar anorthositic diapirs there is field and textural evidence of transport of mostly crystalline anorthositic material within the terrestrial complexes this evidence combined with the absence of anorthositic lavas and phase equilibrium constraints inhibiting the production of hyperaluminous magmas is consistent with the detachment of plagioclase rich crystalline mushes from large uppermost mantle plutons and multiple diapiric intrusion of these mushes into the upper sialic crust with attendant anatexis rudimentary dynamical calculations suggest that a simple single layer source for the diapirs is improbable either there were several parental magma chambers or there was a single large chamber that was repeatedly replenished the lunar model is a development of wetherill s 1975 suggestion that following accretion the outer portion of the moon consisted of a stack of overlapping layered intrusions and that reheating of these intrusions mobilized their anorthositic layers which intruded upwards to produce the anorthositic lunar crust dynamical calculations show that gravitational instabilities in anorthositic layers 1 km thick could develop into diapirs on a reasonable time scale 50 100 m y only if the outer portion of the moon was partially remelted we suggest that partial melting of the interior due to the decay of long lived radionuclides with the subsequent onset of global convection heated the stack of intrusions from below thus causing melting and allowing the anorthositic diapirs to grow and ascend fairly rapidly as the convecting zone thickened a mass expulsion of anorthositic material out of the lunar interior may have occurred

fe ti v p ore deposits associated with proterozoic massif type anorthosites and related rocks abstract magmatic rocks containing economic concentrations of iron titanium vanadium and phosphorous are commonly associated with massif type anorthosites and related rocks this rock association is part of the anorthosite mangerite charnockite rapakivi granite suites that are restricted to the proterozoic understanding the geochemistry and emplacement mechanisms of ilmenite magnetite and apatite ore deposits is crucial for exploration efficient mining operations and ore processing this review discusses the controlling factors on the grade of an ore its mineralogy and its major and trace element distribution we present petrogenetic models of currently mined deposits lac tio tellnes damiao and discuss the characteristics of minor ore bodies from anorthosite provinces worldwide grenville north china craton east european craton rogaland laramie models of formation of anorthosite and related rocks are presented as well as the nature of the possible parental magmas of the suite a mineralogical classification of fe ti ores is proposed 1 gabbro noritic ilmenite ore apatite magnetite 2 ti magnetite dominated ore 3 nelsonite fe ti oxides apatite and 4 rutile ilmenite ore the stability of ilmenite and magnetite is then critically reviewed and the influence of various factors particularly oxygen fugacity and crystallization pressure is examined we discuss liquidus compositions of fe ti oxides and the behavior of important trace elements such as cr and v both of which are sensitive to fo2 variations post cumulus evolution of both oxides can occur due to re equilibration with trapped liquid re equilibration with ferromagnesian silicates exsolution oxidation reaction between ilmenite and magnetite and metamorphic overprinting these various processes are described and their effects on the oxide geochemistry are emphasized several potential ore forming processes have been invoked and can explain the formation of huge concentration of ilmenite magnetite apatite fractional crystallization can be combined with crystal sorting and plagioclase buoyancy to produce relative enrichment of dense ore minerals silicate liquid immiscibility can segregate conjugate si rich and fe rich melts the latter being enriched in fe ti p magma mixing can produce hybrid magmas located in a single phase field of the phase diagram and precipitate a pure ilmenite cumulate alternative processes are also described such as ejection of fe ti enriched residual melts by filter pressing and compaction solid state remobilization of ilmenite in veins and hydrothermal transport of fe and ti from the host anorthosite followed by concentration in veins and lenticular ore bodies the magnetic properties of fe ti ore deposits present contrasting signatures depending on whether the natural remanent magnetization is dominated by hemo ilmenite or multi domain magnetite micro and macro scale deformation features of ore rocks are intimately correlated with magma emplacement and with ballooning of the anorthosite diapir associated with gravitational sagging of dense ore bodies exploration perspectives show that oxide apatite gabbronorites are interesting targets because ilmenite in these rocks is poorer in cr and mg and because the ti resource may be combined with apatite and vanadiferous magnetite

petrogenesis of the igneous mucajai amg complex northern amazonian craton geochemical u pb geochronological and nd hf o isotopic constraints abstract the ca 1525 ma igneous mucajai anorthosite monzonite granite amg complex in northern brazil is a rare manifestation of mesoproterozoic intraplate magmatism in the northern amazonian craton the complex comprises a two phase rapakivi granite batholith with subordinate quartz fayalite monzonites and syenites and the closely associated repartimento anorthosite zircon u pb id tims geochronology reveals that the anorthosite 1526 2 ma monzonite 1526 2 ma and the main phase biotite hornblende granite 1527 2 ma of the complex intruded the paleoproterozoic 1 94 ga country rocks simultaneously at 1526 ma and that the more evolved biotite granite is marginally younger at 1519 2 ma intraplate magmatism in the mucajai region was relatively short lived and lasted 12 million years 1529 1517 ma at maximum the nd whole rock id tims e nd from 1 9 to 2 8 hf zircon lam icp ms e hf from 2 0 to 3 1 and o zircon sims 18 o from 6 1 to 7 0 isotopic compositions of the studied rocks are fairly uniform but still reveal a small degree of isotopic heterogeneity in the paleoproterozoic crust enclosing the complex the small isotopic differences observed in the two types of rapakivi granites biotite hornblende granite and biotite granite may result either from an isotopically heterogeneous lower crustal source or more likely from contamination of the granitic magma derived from a lower crustal source during prolonged residence at upper crustal levels

quantitative genetic modelling of the turkel anorthosite eastern ghats belt orissa india abstract principal components analysis of the anorthositic and adjoining felsic suite of rocks of the turkel anorthosite complex suggests that these are unlikely to be comagmatic this has been supported by 18 o values of these rocks in synoptic principal component space the compositions of anorthosite leuconorite and ferrodiorite of turkel are compared with the chemistry of coexisting phases based on experimental studies on proterozoic massif type anorthosite and naturally occurring high al gabbros hag such studies in congruence with the experimental observations strongly indicate that the turkel ferrodiorite represents the residual melt after separation of anorthosite and leuconorite major and minor element mass balance computations suggest that an ensemble of 15 32 wt anorthosite 34 74 wt leuconorite and 29 71 wt ferrodiorite may have originated from a melt which was derived from a hag magma through fractional crystallisation of 8 86 wt olivine and 11 37 wt ca poor clinopyroxene consistent with petrological observations a small amount of zircon and apatite assimilation from the granitoid country rocks has been invoked to best explain the observed high concentration of zr and rees in the anorthositic suite of turkel

the jugsaipatna anorthosite complex eastern ghats belt india magmatic lineage and petrogenetic implications abstract the jugsaipatna anorthosite complex jac in the eastern ghats mobile belt india comprises a central anorthosite leuconorite troctolitic norite core mantled by a rim of high al gabbronorite hag small dykes of ferrodiorite occur in the core region of the jac a lensoidal body of eucritic gabbronorite occurs in contact with leuconorite and outer gabbronorite rim baring ferrodiorites the mafic members such as hag troctolitic norite and eucritic gabbronorite are not known in other anorthosite complexes of this high grade belt the hag is characterised by high al2o3 content 17 wt enriched ree and lree absence of eu anomaly and negative sr anomaly and is therefore inferred to be the source melt for the jac suite of rocks the eucritic gabbronorite comprising an rich plagioclase and high al pyroxenes has a komatiitic affinity with high cr ni contents and primitive ree patterns troctolitic norite leuconorite and anorthosite are interpreted to be cumulate rocks fractionated from the hag the ferrodiorite with enriched fe ti p zr and ree abundances and a prominent negative eu anomaly corresponds to the residual melt of the plagioclase pyroxene dominated fractionates that comprise anorthosite leuconorite

a geochemical study of magmatism across a major terrane boundary sr and nd isotopes in proterozoic granitoids of the southern laramie range wyoming proterozoic granitoids in the laramie range of southeastern wyoming crop out on both sides of an inferred suture between the archean wyoming craton and proterozoic terranes to the south these rocks offer an unusual opportunity to test the various hypotheses for the geometry of the suture no difference in major or trace element composition is observed across the suture nor is there a large difference in isotopic composition nd and sr isotopic data indicate that only a small proportion of archean material has contributed to this granitoid suite even in two samples that intrude archean gneiss the isotopic data therefore require a dominantly proterozoic lower crust both beneath the proterozoic rocks and beneath the margin of the archean block this observation is inconsistent with models in which the suture between the archean and proterozoic blocks dips to the south this inconsistency can be reconciled by magmatic underplating or mechanical underthrusting of proterozoic crust beneath the archean block

polybaric fractional crystallization of high alumina basalt parental magmas in the egersund ogna massif type anorthosite rogaland sw norway constrained by plagioclase and high alumina orthopyroxene megacrysts bulk analyses of plagioclase and high alumina orthopyroxene megacrysts decimetreto metre sized crystals with plagioclase lamellae from the late proterozoic egersund ogna massif type anorthosite rogaland anorthosite province swnorway are used to constrain the parental magma compositions and differentiation processes within this 30 km diameter diapir spatial compositional variations show that two types of anorthosite occur high sr 720 1090 ppm andesine anorthosite an48 4 in the centre of the intrusion and low sr 320 620 ppm labradorite anorthosite an59 6 in the margin two populations of orthopyroxene megacrysts are clearly discriminated by their mn and cr contents but display similar ranges of mg number 55 79 interpretation of trace element data and comparison with inversion models suggest that the two types of anorthosite result from crystallization of different parental magma compositions with high alumina basalt affinities and similar mg numbers but different wollastonite sr mn and cr contents a rayleigh fractional crystallization model of the trace element concentrations vs mgo content in orthopyroxene constrains the cotectic orthopyroxene plagioclase proportions to 0 33 0 67 in both magma types before the appearance of fe ti oxide minerals on the liquidus at f 0 7 where f is melt fraction polybaric crystallization is recorded by variable alumina contents in the orthopyroxene megacrysts 2 3 8 5 wt corresponding to crystallization pressures in the 3 13 kbar range as shown by experimental data this implies that the high alumina orthopyroxene megacrysts mainly crystallized en route during diapiric rise of the anorthositic mush modelling the plagioclase compositions with experimentally determined partition coefficients for ca k sr and ba confirms that pressure variation during polybaric crystallization is the main controlling factor on compositional variations in the anorthosite pluton

carbon is not required during crystallization to produce ferrobasalts ferrodiorites ftp rocks experiments were conducted to determine whether the presence of graphite is required to produce the fe ti p enrichment silica depletion trend seen in ferrobasalts and ferrodiorites jotunites a nominally anhydrous snake river plain olivine tholeiite was crystallized at 9 3 kbar in fe pt capsules the composition of the fe pt alloy was empirically chosen such that the charge neither lost nor gained iron the results of these experiments and those of previous experiments conducted in graphite are strikingly similar in both cases the residual liquids were depleted in silica and enriched in fe ti and p we conclude that the presence of carbon is not essential for producing this distinctive trend

proterozoic massif type anorthosites as the archetypes of long lived 100 myr magmatic systems new evidence from the kunene anorthosite complex angola abstract proterozoic massif type anorthosites are voluminous plagioclase dominated plutonic suites with characteristic intermediate compositions an50 10 that represent mantle derived magmas ponded at moho depths and crystallized polybarically until emplacement at mid crustal levels we present new id tims and la icpms u pb ages as well as detailed field observations from the kunene anorthosite complex kac of sw angola our main objective in this work is to contextualize a variety of important petrographic and geochronological features of the kac that have implications for proterozoic anorthosite petrogenesis and the geological evolution of the sw congo craton high precision u pb zircon and baddeleyite ages for anorthositic rocks from across the complex n 11 range between 1376 and 1438 ma a period of 60 myr showing that such systems may be remarkably long lived dating of magmatic zircons from a comagmatic pegmatoidal enclave in the kac which also contains high al orthopyroxene megacrysts haom gives an age of 1500 ma indicating as much as 120 myr between some of the earliest and latest crystallization products of the system these enclaves provide a snapshot of the liquid line of descent of the anorthosites and are compelling evidence for the comagmatic nature of the haom field evidence for block structure magma mingling and extreme textural heterogeneity in the kac supports emplacement as multiple influxes of magma of various volumes carrying variable proportions of crystals and autoliths from below we infer that the kac represents a long lived mushy magmatic system that was likely emplaced in distinct pulses these observations refine our understanding of proterozoic anorthosite petrogenesis they also provide evidence for an array of magmatic processes operating at various levels of crystallinity that may create textural and chemical heterogeneity in evolving magma products at lower to mid crustal levels

kiglapait geochemistry vii yttrium and the rare earth elements abstract based on 51 wholerock analyses by xrf and summation over the layered group the kiglapait intrusion contains 4 7 1 6 1 2 ppm y which resides principally in augite and apatite using liquid compositions calculated by summation the partition coefficient daug ly is 0 95 0 12 from 84 to 97 pcs percent solidified and 1 5 0 4 above 97 pcs for feldspar the most likely value for d is 0 028 0 02 n 6 ree analyses for 13 whole rocks were interpreted with the aid of yttrium models to yield trends for wholerocks and liquids vs pcs summations over the rocks of the layered group gave la 2 5 ce 5 8 nd 3 9 sm 1 0 eu 0 8 tb 0 17 yb 0 37 and lu 0 06 ppm with 2 s d errors near 30 all these elements are highly incompatible until the arrival of augite which affects chiefly the hree and apatite which affects all but more strongly the lree the net result is that after apatite arrival at 94 pcs the liquid compositions are nearly constant hence dwr lree 1 0 these results are compatible with the mineralogy of the intrusion and the estimated partition coefficients for feldspar olivine augite apatite and fe ti oxide minerals for pre apatite liquids dfsp lree vary regularly with the normative di content of the liquid and change by an order of magnitude hence the bulk liquid composition must be considered in any attempt to invert the compositions of feldspars to parent liquids the eu anomaly at first decreases in kiglapait liquids due to plagioclase fractionation but then increases due to removal of augite and apatite with negative eu anomalies the features dominantly responsible for eu partitioning are liquid structure and for monoclinic ternary feldspars crystal structure the former is best monitored by the augite or diopside content of the liquid and the latter by the k content of the feldspar the chondrite normalized ree pattern for the intrusion has lan 7 4 lun 1 6 ce yb n 3 6 and eu eu 2 4 indicating its feldspar rich nature the chilled margin of the nearby hettasch intrusion has a similar but more evolved pattern corresponding roughly to the kiglapait liquid at 70 pcs as with other data those for the ree suggest source differences for the two intrusions rather than a relationship due to fractionation

the proterozoic ahvenisto rapakivi granite massif type anorthosite complex southeastern finland petrography and u pb chronology abstract the ahvenisto complex belongs to the classic 1615 1645 ma wiborg rapakivi granite terrane of southeastern finland the complex covers an area of 350 km2 of which 70 comprises silicic rocks mainly subalkaline biotite hornblende granites and 25 gabbroic rocks mainly leucogabbronorite some olivine bearing gabbroic rocks and rare anorthosite a minor suite of monzodioritic rocks as well as silicic and mafic dike rocks is also found on outcrop the silicic rocks are usually found to cut the monzodioritic rocks whereas the silicic and monzodioritic rocks always cut the gabbroic rocks in terms of petrography and major element geochemistry the gabbroic and monzodioritic rocks of the ahvenisto complex are much like those described from the proterozoic massif type anorthosite occurrences for example the harp lake complex of labrador the ahvenisto complex was emplaced into the paleoproterozoic 1900 ma svecofennian metamorphic crust 1630 1645 ma ago and is thus one of the earliest intrusions in the wiborg terrane in particular our u pb data on zircon and baddeleyite show that a barely measurable age difference exists between a leucogabbronorite 1643 3 ma a gabbroic pegmatite in leucotroctolite 1637 2 ma a quartz monzodiorite 1642 2 ma a hornblende biotite granite 1633 6 ma and a quartz feldspar porphyry dike 1632 5 ma of the complex

monzonorites from rogaland southwest norway a series of rocks coeval but not comagmatic with massif type anorthosites duchesne j c wilmart e demaiffe d and hertogen j 1989 monzonorites from rogaland southwest norway a series of rocks coeval but not comagmatic with massif type anorthosites precambrian res 45 111 128 monzonorites members of the anorthositic suite of rocks are usually considered as residual after the formation of massif type anorthosites in rogaland s w norway they occur as large dykes intrusions and chilled margins to differentiated massifs emplaced during and soon after the main anorthosite massifs in granulite facies conditions ti p and fe are enriched in monzonorites and steadily decrease towards quartz mangerites the feo feo mgo ratios varying slightly during alkali enrichment bowen trend trace element spidergrams show deep troughs in rb th nb ta sr zr hf and ti the ree slightly decrease in the evolution with la yb s ratios about 9 and neutral to positive eu anomalies several occurrences however show highly contrasted trace element features sr isotope ratios st show a wide interval of variation 0 704 0 710 between the various dykes and intrusions without any correlation with the elements indicative of crustal contamination in the tellnes main dyke evolution towards acidic rocks occurs without contamination and variation in isr fractional crystallization with subtraction of apatite bearing noritic cumulates can account for the major and trace element evolution from monzonorite to quartz mangerite but is unable to explain the large differences between occurrences it is concluded that monzonorites cannot be comagmatic though coeval with massif type anorthosites they result from the crystallization of distinct magma batches possibly formed through partial melting of basic to intermediate rocks in the lower crust

magma traps and driving pressure consequences for pluton shape and emplacement in an extensional regime abstract the level of emplacement and final form of felsic and mafic igneous rocks of the wichita mountains igneous province southwestern oklahoma u s a are discussed in light of magma driving pressure lithostatic load and crustal magma traps deposition of voluminous a type rhyolites upon an eroded gabbroic substrate formed a subhorizontal strength anisotropy that acted as a crustal magma trap for subsequent rising felsic and mafic magma intruded along this crustal magma trap are the a type sheet granites length thickness 100 1 of the wichita granite group of which the mount scott granite sheet is typical and smaller plutons of biotite bearing roosevelt gabbro in marked contrast to the subhorizontal granite sheets the gabbro plutons form more equant stocks with flat roofs and steep side walls late diabase dikes cross cut all other units but accompanying basaltic flows are extremely rare in the volcanic pile based on magmastatic calculations we draw the following conclusions concerning the level of emplacement and the shape of these intrusions 1 magma can rise to a depth at which the magma driving pressure becomes negligible magma that maintains a positive driving pressure at the surface has the potential to erupt 2 magma ascent may be arrested at a deeper level in the crust by a subhorizontal strength anisotropy i e crustal magma trap if the magma driving pressure is greater than or equal to the lithostatic load at the depth of the subhorizontal strength anisotropy 3 subhorizontal sheet intrusions form along crustal magma traps when the magma driving pressure greatly exceeds the lithostatic load under such conditions the magma driving pressure is sufficent to lift the overburden to create the necessary space for the intrusion 4 thicker steep sided stocks or batholiths with flat roofs form at crustal magma traps when the magma driving pressure approximates that of the lithostatic load under these conditions the necessary space for the intrusion must be created by other mechanisms e g stoping 5 subvertical sheets i e dikes form when the magma driving pressure is less than the lithostatic load at the level of emplacement

petrogenesis of the 1 43 ga sherman batholith se wyoming usa a reduced rapakivi type anorogenic granite the 1 43 ga sherman batholith southeastern wyoming usa origin that are found throughout the proterozoic anorogenic granite provinces of the southwestern usa adirshows extreme a type petrochemical characteristics compared with other mid proterozoic granite batholiths of north america it ondacks eastern canada southern greenland and the fennoscandian shield anderson 1983 phanerozoic consists of 1 the sherman granite a coarse grained biotite hornblende granite that locally contains fayalite and pyroxenes 2 examples include paleozoic granites of southeastern australia collins et al 1982 and of corsica poitrasson et the lincoln granite a medium grained biotite granite 3 a porphyritic biotite hornblende granite that probably formed by interaction al 1995 and the mesozoic nigerian younger granites jacobsen et al 1958 and the white mountain magma of granitic and mafic magmas and 4 iron enriched mafic dikes and pods the ilmenite series metaluminous sherman granite series foland allen 1991 anorogenic granite refers to plutons that are not associated with compressional exhibits extreme values of feo feo mgo and is rich in structures and that were emplaced long after any known k ree nb and y it crystallized at temperatures exceeding orogenic event anorogenic or a type granites loiselle 900 c and a pressure of 2 5 kbar with water activity of 0 7 wones 1979 are characterized by low h2o and o2 and dlog fo2 of 0 1 to 0 5 the lincoln granite which is fugacities along with high feo feo mgo k2o peraluminous and has less extreme a type geochemical characteristics na2o and k2o contents a type granites typically are rich crystallized at temperatures as low as 750 c and dlog fo2 of in incompatible elements including rare earth elements around 0 5 units above fmq fayalite magnetite quartz the ree zr nb and ta but poor in co sc cr ni ba rocks of the sherman batholith are chemically equivalent to lavas sr and eu minimum magma liquidus temperatures are from the yellowstone hotspot like the yellowstone magmas the 900 1000 c clemens et al 1986 creaser white sherman batholith probably originated by partial melting of under1991 some workers suggest these rocks are derived plated mantle derived mafic rocks from melting of tonalitic or more felsic crust anderson cullers 1978 collins et al 1982 clemens et al 1986 creaser et al 1991 whereas others relate the granites

can crystallization of olivine tholeiite give rise to potassic rhyolites an experimental investigation experiments were conducted to determine whether the rhyolites and basalts of the intraplate silica saturated potassic suites could be genetically related through crystallization extreme crystallization 96 97 of a high mgo 10 62 wt olivine tholeiite from the snake river plain with an initial bulk water content of 0 4 wt at a mid crustal pressure of 4 3 kbar generated potassic rhyolitic liquids similar in major element chemistry to those found in the quaternary rhyolite domes of the snake river plain and their plutonic equivalents in the proterozoic laramie anorthosite complex residual liquids comparable in composition to the bulk rock compositions of intermediate rocks found at the craters of the moon and cedar butte eruptive centers in the snake river plain are also generated along this crystallization path