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

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

topographically controlled fronts in the ocean and their biological influence headlands islands and reefs generate complex three dimensional secondary flows that result in physical and biological fronts mixing and diffusion processes near these reefs and headlands are quite different from these processes in the open sea and classical advection diffusion models that were developed for the open sea are not valid near shore topographically generated fronts affect the distribution of sediments and they aggregate waterborne eggs larvae and plankton this aggregation influences the distribution and density of benthic assemblages and of pelagic secondary and tertiary predators

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

response of the u pb chronometer and trace elements in zircon to ultrahigh temperature metamorphism the kadavur anorthosite complex southern india abstract zircon from quartzites in the contact aureole and wider environs of the kadavur anorthosite complex se india was studied by laser ablation icp ms to assess the response of the u pb isotope system and trace element concentrations to ultrahigh temperature uht contact metamorphism 1000 c combined cathodoluminescence imaging and la icp ms analyses show that zircon grains contain detrital cores which yield ages between 3 4 ga and 1 8 ga and exhibit a large spread in ree concentrations these cores are associated with one or two rims that provide concordant age populations at 955 16 ma and 810 7 ma 2 and relatively uniform ree patterns the older ages of ca 955 ma record the imprints of regional early neoproterozoic metamorphism related to rodinia assembly the younger age is interpreted to date anorthosite emplacement and its associated contact metamorphism during a second regional metamorphic episode coeval with the intrusion of a type granites in the area zircon grains from the country rocks away from the anorthosite show additional rims yielding ages from 590 to 490 ma corresponding to regional pan african tectonometamorphism the zircon rims are either newly grown domains or represent recrystallized and re equilibrated parts of precursor zircon discordance is only observed in 25 of the detrital cores these cores have relatively high u concentrations 400 ppm which strongly suggests that pb loss occurred under u ht conditions by partial recrystallization annealing of zircon that had become metamict the preservation of old concordant ages and source ree characteristics by the low u i e weakly to non metamict detrital cores shows that pristine zircon is extremely robust to thermal disturbance and that its u pb systematics can remain unaffected despite multiple u ht metamorphic episodes these observations imply that u pb ages in zircon cannot be reset by volume diffusion under any crustal conditions disturbance of the u pb system in individual zircon crystals or parts thereof can only be achieved by recrystallization of radiation damaged zircon

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

measurements of tidal flow around a headland with a shipboard acoustic doppler current profiler a shipboard acoustic doppler current profiler adcp and moored current meters were used to obtain detailed measurements of the spatial structure of the tidal flow around a headland in vineyard sound massachusetts where tidal currents typically range from 50 to 70 cm s 1 eight shipboard surveys were conducted each of which followed one of five trapezoidal tracks in the vicinity of the headland completing 11 12 circuits in the course of the tidal cycle 12 4 hours the measurements from the adcp compare favorably with moored velocity measurements at two locations rms deviations of 3 cm s 1 but the comparison showed more scatter at two other sites rms deviations of 7 cm s 1 the adcp measurements from separate cruises were merged to form a composite spatial representation of the tidal and residual currents providing resolution of the spatial structure of the flow around the headland at scales from several hundred meters to 10 km the semidiurnal tidal flow is relatively uniform in space roughly following the bathymetry around the headland while a residual current field consists of counterrotating eddies on either side of the headland with spatial scales of 5 8 km and velocities as high as 25 cm s 1 the instantaneous current field indicates the formation of transient eddies on the downstream side of the headland during flood and ebb with clear evidence of flow separation near the tip of the headland

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

crustal structure beneath lofoten n norway from vertical incidence and wide angle seismic data summary a high quality multichannel seismic reflection line was acquired in 1987 along a 175 km long profile across the continental shelf off lofoten northern norway a seismic wide angle experiment was performed in 1988 along the same profile using seven three component ocean bottom seismographs obs with 20 25 km spacing and shotpoint intervals of 240 m the study of the data has shown that the combination of the multichannel reflection and the wide angle obs technique provides information about the crustal structure beneath the lofoten shelf that could not have been achieved using only one of the techniques the multichannel reflection data provide a detailed image of the shallow cretaceous structures which represents an important basis for inversion of the obs data the lower crust and the moho are also well mapped in some parts of the area with the multichannel reflection technique the obs data reveal that significant amounts of pre cretaceous sediments exist along almost the entire profile with a maximum thickness of about 5 km in the vestfjorden basin from the obs data the thickness of the lower crust is inferred to decrease from about 11 5 km under the rost high to about 2 km below the lofoten ridge the obs data indicate further that the moho position under the vestfjorden basin is considerably deeper than can be inferred from the reflection data about 10km below moho a strong dipping event is observed in the obs data this upper mantle reflection might be related to a possible seaward dipping master fault and or presence of layers of partially hydrated peridotite

the role of a man made headland in generating patches in coastal waters of ise bay japan patches of saline water were detected near a tip of a reclaimed land a man made headland in the coastal waters of ise bay japan from ctd casts in addition acoustic doppler current profiler adcp observations indicated that the reclaimed land generated local eddies a hydraulic model and a numerical model provided evidence that the patches were formed by vortices shed at the headland in reversing tidal currents

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

oscillatory flow across an irregular boundary 1 laboratory experiments examining oscillatory flow along a coastline or isobath with a series of headlands or ridges reveal a mechanism for transfer of momentum and mass from the boundary into the interior flow through the establishment of organized residual flow patterns particle imaging velocimetry is used to visualize the vorticity field and to quantify large scale residual circulations generated near the region of roughness two flow regimes are observed for differing ratios of along coast tidal excursion l to the length of the roughness region d for l d 1 a weak inflow is produced in the vicinity of the roughness region with a corresponding alongshore flow away from the region for l d 1 a strong outflow jet is established near the center of the roughness region the two flow regimes result from different residual vorticity patterns and rely on eddies being long lived relative to the tidal period thus these results are relevant to flows in which dissipation is weak relative to the mean flow kinetic energy such as for tidal flow in deep water flow past a single roughness element is used as an analog to tidal flow past a headland and is represented by the high l d limit

transient eddy formation around headlands eddies with length scales of 1 10 km are commonly observed in coastal waters and play an important role in the dispersion of water borne materials the generation and evolution of these eddies by oscillatory tidal flow around coastal headlands is investigated with analytical and numerical models using shallow water depth averaged vorticity dynamics eddies are shown to form when flow separation occurs near the tip of the headland causing intense vorticity generated along the headland to be injected into the interior an analytic boundary layer model demonstrates that flow separation occurs when the pressure gradient along the boundary switches from favoring accelerating to adverse decelerating and its occurrence depends principally on three parameters the aspect ratio b a where b and a are characteristic width and length scales of the headland h cda where h is the water depth cd is the depth averaged drag coefficient and uo a where uo and are the magnitude and frequency of the far field tidal flow simulations with a depth averaged numerical model show a wide range of responses to changes in these parameters including cases where no separation occurs cases where only one eddy exists at a given time and cases where bottom friction is weak enough that eddies produced during successive tidal cycles coexist interacting strongly with each other these simulations also demonstrate that in unsteady flow a strong start up vortex forms after the flow separates leading to a much more intense patch of vorticity and stronger recirculation than found in steady flow

multistage metamorphic evolution and nature of the amphibolite granulite facies transition in lofoten vesteralen norway revealed by u pb in accessory minerals abstract the nature of the amphibolite to granulite facies transition zone exposed in the lofoten vesteralen islands of northern norway and the relationships between high grade metamorphism and the genesis of an anorthosite mangerite charnockite and granite suite amcg are studied with the help of id tims u pb dating of zircon monazite and titanite in polyorogenic gneisses the western part of the region is dominated by granulite facies gneisses intruded by a 1800 1790 ma amcg suite and characterized by a strong magnetic and gravity anomaly to the east this zone borders a crustal section with amphibolite facies gneisses migmatites and granites the transition has been considered to reflect the amphibolite to granulite facies metamorphic gradient of a palaeoproterozoic orogenic event the results of this study show that the region underwent multiple high grade events one in the late archean at ca 2640 ma one in the period slightly before

magmatic co 2 and saline melts from the sybille monzosyenite laramie anorthosite complex wyoming there are three populations of fluid inclusions in quartz from the sybille monzosyenite early co2 secondary co2 and rare secondary brines the oldest consist of low density co2 0 70 inclusions that appear to be co magmatic the densities of these inclusions are consistent with the inferred crystallization conditions of the sybille monzosyenite namely 3 kilobars and 950 1000 c the other types of inclusions are secondary they contain co2 0 50 and secondary brine inclusions that form trains radiating out from a decrepitated inclusion the sites of these decrepitated inclusions are now marked by irregularly shaped fluid inclusions and solid inclusions of salt and carbonate rather than fluid inclusions feldspar contain abundant solid inclusions these consist of magmatic minerals hedenbergite hornblende ilmenite apatite and graphite intimately associated with k na chlorides we interpret these relations as follows the sybille monzosyenite formed from a magma that contained immiscible droplets of a halide rich melt along with a co2 vapor phase the salt was trapped along with the other obvious magmatic minerals during growth of the feldspars co2 may have also been included in the feldspars but it probably leaked later during exsolution of the feldspars and was not preserved both the saline melt and the co2 vapor were trapped in the quartz the melt inclusions in the quartz later decrepitated perhaps due to progressive exsolution of fluids to produce the secondary h2o and co2 inclusions these observations indicate that the sybille monzosyenite which is a markedly anhydrous rock was actually vapor saturated rather than being h2o however the vapor was co2 rich and possibly related to an immiscible chloride rich melt

magnetostratigraphy of the vallesian late miocene in the valles penedes basin northeast spain the magnetostratigraphic analysis of the late miocene continental deposits from the valles penedes basin combined with its well documented fossil mammal record has provided a well resolved chronology for the upper basin infill the study is based on the biostratigraphic and magnetostratigraphic cross correlation of 12 sections throughout the alluvial sequences in the western valles area the biostratigraphic framework consists of 21 mammal localities corresponding to the mammal neogene mn9 and mn10 units the composite magnetic polarity sequence is based on 400 paleomagnetic sites correlation with the geomagnetic polarity time scale gpts has led to an absolute dating of the faunal events and a precise chronostratigraphy of the vallesian mammal stage in its type area the hipparion first appearance datum fad at the mn8mn9 boundary is dated at 11 1 ma in the valles penedes basin this age compared to other radiometrically dated localities in europe north africa and turkey is consistent with an isochronous dispersal of this equid through the mediterranean region a possible isochrony at a larger geographical scale old world mesogea must await more reliable ages of the hipparion fad in asia and africa the cricetulodon fad mn9amn9b boundary is dated to 10 4 ma in chron c5n the progonomys fad mn9mn10 boundary corresponding to the intra vallesian faunal crisis is dated at 9 7 ma c4ar 3r the vallesian spans 2 4 myr from 11 1 ma c5r 1n to 8 7 ma c4an and correlates to the early tortonian

shallow water frictional effects in island wakes only one eddy is present in the island wake in the lee of rattray island a 1600 m wide island in shallow coastal waters off australia this observation is contrary to the predictions of a number of two and three dimensional fixed grid numerical models which all predict that two eddies are present one eddy being shed from each of the two tips of the island the discrete vortex model suggests this observation results from the frictional effects in sub grid scale details of the topography close to the tips of the island this model also successfully reproduces the observations of a karman vortex street in the lee of islands in rupert bay canada by contrast fixed grid models predict no such eddy shedding island wakes are thus sensitive to details of the bathymetry near the separation points and these must be either parameterized in fixed grid models whether regular or boundary fitted or explicitly calculated in non fixed grid models such as the discrete vortex model

characterization of apatite resources in norway and their ree potential a review abstract apatite is a necessity for the production of phosphorus fertilizers and presents a potential raw material for the extraction of ree and y a wide spectrum of apatite deposits is found in norway including sedimentary igneous and vein type deposits the igneous deposits which appear to have the greatest potential for exploitation occur associated with alkaline complexes massif type anorthosite complexes and monzonitic complexes one of the most promising is found in the monzonoritic bjerkreim sokndal layered intrusion of the early neoproterozoic rogaland anorthosite province in southwest norway the intrusion hosts three cumulate units with high grade ore zones the most promising resource is confined to mcu iv which is 50 170 m thick and nearly 10 km long with average normative contents of 10 2 apatite 12 4 ilmenite and 7 3 vanadium rich magnetite the late neoproterozoic cambrian carbonatite bearing alkaline complexes are generally of low grade to represent potential resources of apatite as the sole commodity however apatite may represent a byproduct of potential nb and ree y mineralisation in the fen complex in southern norway the late ordovician silurian misvaerdal complex comprising multiple alkali clinopyroxenite intrusions in the uppermost allochthon of the caledonides in northern norway contains 1 1 5 km long and 100 200 m wide ultrapotassic clinopyroxenite dykes with average contents of 7 10 wt apatite and with intermediate levels of trey 0 5 wt however high levels of th in the apatite make it less suitable as a raw material for fertilizer production apatite fe ti oxide ores being characteristic for the monzonitic complexes are especially well developed in the permian oslo igneous province where apatite rich magnetite clinopyroxenite cumulates are found in the larvik plutonic complex the kodal body has an inferred open pit ore reserve calculated to 70 mt with approximately 11 6 wt apatite 3 0 wt ilmenite and 26 5 wt ilmenomagnetite the apatite contains about 1 wt ree comparable types of deposits of palaeoproterozoic age have recently been recognized in the alkali calcic mangeritic to syenitic intrusions in the lofoten vesteralen mangerite complex in northern norway whereas complexes with variable proportions of anorthosites s s jotunites and mangerites occurring in the middle allochthon of the caledonides in south norway also are known to host apatite bearing fe ti oxide deposits some high in apatite these complexes represent potential areas for green field exploration the trey tree y contents of the apatite in the igneous deposits are discussed and comprise very low levels in the bjerkreim sokndal layered intrusion intermediate levels in the carbonatites and pyroxenites of the alkaline complexes as well as nelsonite dykes and mangerite associated deposits to high levels in the monzonite associated deposits

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

timing of paleozoic orogeny and extension in the continental shelf of north central norway as indicated by laser 40ar 39ar muscovite dating metamorphic rocks of lofoten norway represent a leading edge of the baltic craton that was partly subducted beneath laurentia during caledonian orogeny we find that muscovite porphyroblasts in lofoten record a remarkably complete history of cooling and unroofing and yield data about lofoten9s tectonic evolution in the interval from 425 to 265 ma these data confirm that culminating metamorphism of basement and allochthonous rocks in lofoten occurred during the scandian phase of orogeny permian ages for muscovite are the youngest obtained for regional metamorphic rocks anywhere in scandinavia and appear to record significant basement unroofing during extensional evolution of the norwegian greenland seaway regional age variations of muscovite in lofoten can be related to present crustal thickness devonian to permian sedimentation in the norwegian seaway and extensional structures and they indicate the timing of multiple extension events in the evolution of norway9s passive margin the results of this study show that 40 ar closure profiles in porphyroblasts can form even in active tectonic settings characterized by rapid and episodic unroofing

thermobarometry of metamorphosed pseudotachylyte and associated mylonite constraints on dynamic co seismic rupture depth attending caledonian extension north norway abstract the exhumed post caledonian eidsfjord and fiskfjord extensional shear zones of northern norway exhibit evidence of coseismic rupture propagating into the ductile crust as evidenced by the presence of mylonitic and metamorphosed pseudotachylyte geothermobarometric calculations on garnet bearing mineral assemblages in mylonitic gneisses associated with mylonitic pseudotachylyte and in metamorphosed pseudotachylyte permit determination of the depth and ambient temperature of seismogenic low angle ductile extension average pressures from eidsfjord 570 115 mpa at ca 650 c and fiskfjord 1120 220 mpa at ca 650 c correspond to faulting depths of 21 4 km and 41 9 km respectively the fiskfjord results agree with previous thermobarometric calculations on mylonitic cpx grt bearing pseudotachylyte at fiskfjord the calculated depths are 5 25 km below the depth of the standard seismogenic zone these results demonstrate that low angle normal faults may cut through a large portion of continental crust this occurrence of mylonitic pseudotachylyte in an extensional crustal setting is most easily explained by dynamic downward rupture into the ductile regime and or unusually high shear stresses to account for coseismic rupture at such depths implying a direct connection with a seismogenic normal fault in the upper crust

proterozoic eclogites from the lofoten islands northern norway abstract massif lenses and bands of variably retrogressed eclogites occur within the granulite facies proterozoic basement of the lofoten islands in northern norway eclogites are often related to shear and mylonite zones suggesting that deformation and fluid access was important for their formation the eclogitic rocks are restricted to gabbroic or troctolitic rock compositions and show in the least retrogressed samples the typical assemblage garnet omphacite rutile the widespread occurrence of eclogite shows that the whole group of the lofoten islands was affected by high pressure metamorphism eclogites were found on three of the five major islands so far flakstadoy vestvagoy austvagoy thermobarometry reveals minimum conditions during eclogite formation of 680 c 15 kbar on flakstadoy and 540 c 14 kbar on austvagoy geological petrological and geochemical data suggest the following succession of events a intrusion of mafic to mangeritic magmas at about 1 8 ga griffin et al 1978 b incomplete recrystallization under high pressure granulite facies conditions associated with granulitization was the formation of cl rich hydrosilicates granulite formation was incomplete except for zones of intensive deformation c eclogites formed in response to a significant pressure increase in the same high strain zones however scattered eclogite lenses and boudins also occur in gneisses d post eclogite retrogression occurred under middle to high pressure amphibolite facies conditions probably the leknes event at 1 1 ga of griffin et al 1978 eclogite formation was related to crustal thickening during the final stages of early proterozoic magmatism and granulite facies metamorphism

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

island wakes in shallow coastal waters rattray island northeast australia is 1 5 km long 300 m wide and lies in well mixed water approximately 25 m deep its long axis is inclined at about 60 into the direction of the dominant semidiurnal tidal current the length of the wake in the lee of the island as documented by aerial photographs and satellite imagery appears to equal that of the wake behind a flat plate in a two dimensional flow at a reynolds number of about 10 however current metering drogues measurements and temperature mapping indicate internal wake velocities much greater than would be consistent with such a simple low reynolds number model further estimates of the turbulent eddy coefficient suggest an effective reynolds number more in the vicinity of 103 to reconcile these observational differences and to explain the observed upwelling in the core of the wake an ekman pumping model is proposed it is postulated that the ekman benthic boundary layer driven by rotation in the wake allows the vertical vorticity introduced into the water at the tip of the island at the point of separation to be negated by the vorticity of opposite sign introduced at the bottom further it is shown that a large fraction of the kinetic energy of the upstream flow facing the island is used to drive the wake eddies leading to the conclusions that the trapping of water in the lee of islands greatly increases head losses on continental shelves with numerous islands coral reefs and rock outcrops

chemical heterogeneities of caledonian pseudotachylites in the eidsfjord anorthosite north norway the 1 8 1 7 ga eidsfjord anorthosite complex on langoy vesteralen north norway is thrust over monzonitic gneisses in a 100 m wide fault zone the anorthosite contains abundant pseudotachylite which also overprint ductilely formed ultra mylonites in the monzonitic gneisses 40ar 39ar dating of the pseudotachylites indicates that this deformational event took place during the end of the caledonian orogeny pseudotachylite formation by in situ melting is corroborated by a detailed petrological investigation of chemical and textural criteria identity of host rock mineral compositions and modes with pseudotachylite clasts and calculated modes confirm the common notion that transport over more than a few cm did not occur during pseudotachylite formation because the characteristics of the host rocks are inherited by the pseudotachylite pseudotachylite compositions lie in the range of xrf whole rock analyses of deformed and undeformed host rocks demonstrating that no material was added or lost during pseudotachylitisation the pseudotachylites show strong chemical heterogeneity on a scale of tens of micrometers this compositional variation can be described by an approximately binary mixture of plagioclase with a homogenised mafic component comprising orthopyroxene clinopyroxene and fe ti oxides although the pseudotachylite matrix is not homogenised and locally shows schlieren textures the mafic phase in the anorthositic rocks must have homogenised very well before incorporation into the pseudotachylite different viscosities and mixing properties for the al and the fe rich parts of the pseudotachylite melt are inferred