weakening associated with the diaspore corundum dehydration reaction in metabauxites an example from naxos greece abstract metabauxite lenses embedded in marble on naxos consist of diasporites below the 420 c isograd and dehydrate into corundum rich rocks with increasing grades of metamorphism while the diasporites are essentially undeformed the corundum rich rocks are strongly deformed even though both diasporites and corundum rich rocks are much stronger than the surrounding intensely deformed marbles the observed structures can be explained as an effect of high fluid pressures during the prograde diaspore corundum dehydration reaction which causes dramatic temporary weakening of the metabauxites to a strength comparable to that of the surrounding deforming marbles deformation of the metabauxite is thus largely restricted to the time span the phase transformation occurred allowing the dehydrating bauxite mass to deform together with the surrounding marbles

40ar 39ar and oxygen isotope studies of polymetamorphism from tinos island cycladic blueschist belt greece petrological oxygen isotope and 40ar 39ar studies were used to constrain the tertiary metamorphic evolution of the lower tectonic unit of the cyclades on tinos polyphase high pressure metamorphism reached pressures in excess of 15 kbar based on measurements of the si content in potassic white mica temperatures of 450 500 c at the thermal peak of high pressure metamorphism were estimated from critical metamorphic assemblages the validity of which is confirmed by a quartz magnetite oxygen isotope temperature of 470 c some 40ar 39ar spectra of white mica give plateau ages of 44 40 ma that are considered to represent dynamic recrystallization under peak or slightly post peak high pressure metamorphic conditions early stages in the prograde high pressure evolution may be documented by older apparent ages in the high temperature steps of some spectra eclogite to epidote blueschist facies mineralogies were partially or totally replaced by retrograde greenschist facies assemblages during exhumation oxygen isotope thermometry of four quartz magnetite pairs from greenschist samples gives temperatures of 440 470 c which cannot be distinguished from those deduced for the high pressure event the exhumation and overprint is documented by decreasing ages of 32 28 ma in some greenschists and late stage blueschist rocks and ages of 30 20 ma in the lower temperature steps of the ar release patterns of blueschist micas almost flat parts of ar ar release spectra of some greenschist micas gave ages of 23 21 ma which are assumed to represent incomplete resetting caused by a renewed prograde phase of greenschist metamorphism oxygen isotope compositions of blueschist and greenschist facies minerals show no evidence for the infiltration of a 18o enriched fluid rather the compositions indicate that fluid to rock ratios were very low the isotopic compositions being primarily controlled by those of the protolith rocks we assume that the fundamental control catalysing the transformation of blueschists into greenschists and the associated resetting of their isotopic systems was the selective infiltration of metamorphic fluid a quartz magnetite sample from a contact metamorphic skarn taken near the miocene monzogranite of tinos gave an oxygen isotope temperature of 555 c and calculated water composition of 9 1 the value of 18o obtained from this water is consistent with a primary magmatic fluid but is lower than that of fluids associated with the greenschist overprint which indicates that the latter event cannot be directly related to the monozogranite intrusion

metamorphic evolution of the attic cycladic metamorphic belt on naxos cyclades greece utilizing 40ar 39ar age spectrum measurements 40ar 39ar age spectrum analysis of phengite separates from naxos part of the attic cycladic metamorphic belt in greece indicates that cooling following high pressure low to medium temperature metamorphism m1 occurred about 50 ma ago phengite has 40ar gradients that suggest that part of the scatter observed in conventional k ar ages was caused by diffusion of radiogenic argon from the minerals during a younger metamorphism m2 in central naxos this metamorphism m2 has overprinted the original mineral assemblages completely and is associated with development of a thermal dome excellent 40ar 39ar plateaus at 15 0 0 1 ma 11 8 0 1 ma and 11 4 0 1 ma obtained on hornblende muscovite and biotite respectively from the migmatite zone indicate that relatively rapid cooling followed the m2 event and that no significant thermal overprinting occurred subsequent to m2 toward lower m2 metamorphic grade 40ar 39ar plateau ages of hornblendes increase to 19 8 0 1 ma concomitantly the proportion of excess 40ar in the spectra increases as well we propose that the peak of m2 metamorphism occurred beween 15 0 and 19 8 ma ago k ar ages of biotites from a granodiorite on the west coast are indistinguishable from those found in the metamorphic complex and hornblende k ar ages from the same samples are in the range 12 1 13 6 ma as the latter ages are somewhat younger than most ages obtained from the metamorphic complex intrusion of the granodiorite most likely followed the peak of the m2 metamorphism the metamorphic evolution of naxos is consistent with rapid crustal thickening during the cretaceous or early tertiary causing conditions at which supracrustal rocks experienced pressures in the range 900 1500 mpa transition to normal crustal thicknesses ended the m1 metamorphism about 50 ma ago the m2 metamorphism and granodiorite intrusion occurred during a period of heat input into the crust possibly related to the migration of the hellenic volcanic ar c in a southerly direction through the area

the formation of longitudinal fronts in a coastal plain estuary longitudinal estuarine fronts aligned parallel to the axis of the estuary are characteristic of the york river their time scale for genesis and decay is of the order of hours they are usually located at the inner edge of the shoals or over the main channel field measurements have shown marked intratidal differences in density and current speeds across this estuary these can be correlated with changes in depth and the strength of the advective processes differential advection between the channel and shoal regions when acting upon a constant longitudinal density gradient is shown to be of sufficient magnitude to generate strong lateral density gradients and thus fronts at particular times within the tidal cycle

on the nature of transverse variability in a coastal plain estuary even though estuarine processes are inherently three dimensional most of the previous studies have focused primarily on the axial longitudinal and vertical variabilities recent surveys in the lower delaware bay indicate the presence of significant transverse variability in the salinity distribution and residual circulation pattern conceptual models are developed to examine the mechanisms which might explain the observed transverse structure results indicate that both the gravitational effect and wind forcing are capable of generating substantial transverse structure in drowned river valley estuaries with triangular cross sections

selective weathering of cross bedded layers forming shelters and small caves on akopan tepui venezuela field laboratory and experimental evidence about diagenesis and weathering of the mataui formation arenites roraima supergroup middle proterozoic abstract south american table top mountains tepuis are formed by quartzites and sandstones of mataui formation belonging to the roraima supergroup lower to middle proterozoic field observations in the northern part of akopan tepui in venezuela revealed that unlike the beds with flat bedding the cross bedded layers are commonly affected by selective weathering expressed by depressions shelters and even small caves all the arenites are fine grained most of them are likely of aeolian origin hardness measurements confirmed that the cross bedded layers are softer than the flat bedded or structureless beds occurring above and below them petrographic analysis showed that the rocks are mostly fine grained quartzites to quartz sandstones with original sand grains overgrown by syntaxial quartz cement and bearing signs of compaction manifested by local pressure solution some intergranular spaces are filled by newly formed phyllosilicates paragonite locally intergrown with muscovite in the overlying beds no porosity was observed by microscopic study and under sem whereas in the underlying rocks signs of slight weathering and etching of phyllosilicates was observed together with locally widened spaces along the grain to grain boundaries the samples from cross bedded layers display weathering the degree of which depends on the present access of flowing water where water has periodical access to the arenite it shows dissolution of quartz cement arenization together with complete leaching and removal of the phyllosilicates due to lateritization the intergranular spaces are considerably widened and quartz grains bear signs of strong skeletal etching and pitting samples from the sites where the water access ceased show mostly leaching of phyllosilicates with weak skeletal dissolution of quartz grains without pitting to imitate the origin of structures observed in the field experimental modelling was performed using artificially layered and compacted fine grained sands on which sodium silicate solution was dripped during soaking downward preferential penetration of the solution along the cross bedding laminae funnel flow was observed in the cross bedded layer the resulting structures display good lithification of horizontally layered sands and relatively poorer lithification of the cross bedded sediment between them showing triangular shaped overhangs and shelters which mimic those on the akopan tepui differential weathering of otherwise petrologically very similar arenites indicates that there had to be a difference in the initial porosity in the time of the rock denudation a preserved porosity in the cross bedded layers was necessary for the water rock contact to perform dissolution

formation and modification of metastable intermediate sodium potassium mica paragonite and muscovite in hydrothermally altered metabasites from northern wales

strontium carbon and oxygen isotope compositions of marbles from the cycladic blueschist belt greece the cycladic blueschist belt greece is mostly submerged below sea level and regional correlations are difficult to establish marbles are widespread within the belt and locally used as marker horizons to subdivide monotonous schist sequences however owing to the lack of distinctive petrographic characteristics the marbles have not been used for island to island correlations this study aims to investigate the potential of sr c and o isotope compositions of marbles as a tool for unravelling the litho and or tectonostratigraphic relationships across the cycladic islands and as a proxy for the time of sediment formation for this purpose we have studied metamorphic carbonate rocks from the islands of tinos andros syros sifnos and naxos identical 87sr 86sr values for certain marble horizons occurring on tinos andros and sifnos are interpreted to document coeval regional carbonate precipitation the 87sr 86sr values of the apparently least altered samples intersect the seawater curve multiple times within the most likely time interval of original carbonate precipitation 240 ma as indicated by previously published ion probe u pb zircon data and thus an unequivocal age assignment is not possible very broad temporal correlations are possible but more subtle distinctions are not feasible on andros the overlapping sr isotope values of marbles representing the lowest and highest parts of the metamorphic succession are in accordance with a model suggesting isoclinal folding or thrusting of a single horizon or very fast sedimentation in contrast distinct 87sr 86sr values for samples from tinos representing different levels of the metamorphic succession suggest that these rocks represent a temporal succession and not the tectonic repetition of a single horizon based on sr o and c isotope characteristics alone the time equivalence of marbles occurring on different islands could not be documented unambiguously however by using various combinations of these parameters some occurrences can be discriminated from the overall sample population the new data further accentuate the general potential of coupled sr c and o isotope characteristics for identification of archaeological provenance and complement existing datasets for aegean marbles

compositional zoning and element partitioning in nickeloan tourmaline from a metamorphosed karstbauxite from samos greece blue green nickeloan tourmaline from a micaceous enclave of a marble from samos greece contains unusually high concentrations of ni up to 3 5 wt nio co up to 1 3 wt coo and zn up to 0 8 wt zno the polymetamorphic karstbauxite sample has an uncommon assemblage of nickeloan tourmaline calcite zincian staurolite gahnite zincohogbomite diaspore muscovite paragonite and rutile the complex geologic history is reflected in multi staged tourmaline growth with cores that represent detrital fragments surrounded by two staged metamorphic overgrowths zone 1 metamorphic overgrowths which nucleated next to detrital cores are highly asymmetric and exhibit compositional polarity such that narrow overgrowths of brown schorl developed at the c pole are enriched in mg ti and f and depleted in al fe and x site vacancies x relative to wider gray blue schorl to foitite overgrowths developed at the c pole volumetrically dominant zone 2 overgrowths are strongly zoned nickeloan dravites with a continuous increase in mg co ca and f at the expense of fe zn cr and v from the zone 1 interface to the outermost rim within zone 2 ni reaches a maximum of 0 5 apfu before decreasing in the outer 20 40 m zone 2 overgrowths also exhibit compositional polarity such that at the c pole overgrowths are enriched in mg f na ca and cr relative to overgrowths at the c pole that are in turn enriched in al fe ni co and x element partitioning involving tourmaline rims and coexisting minerals indicates that relative partitioning of ni is tourmaline staurolite gahnite co is tourmaline staurolite gahnite and zn is gahnite staurolite tourmaline

longitudinal transport in a coastal lagoon abstract a two dimensional finite difference numerical model is used to describe longitudinal transport through a transverse cross section into and out of the northern segment of indian river lagoon along the atlantic coast of florida the model uses four layers to represent the atlantic intracoastal waterway and two layers in the shallow waters east and west of the waterway to quantify transport simulations during a 161 day period from late june through late november 1983 include a seasonal shift in the longitudinal component of the wind transport is in the same sense throughout the cross section 32 of the time relatively subtle lateral shear however has a profound effect on long term net transport for most of the study cumulative net transport in the shallow areas on either side of the waterway follows the cumulative along channel windstress but it is directed upwind in the lower half of the intracoastal waterway four numerical experiments examine the relative importance of terms in the momentum equation and components of the database results suggest that time varying baroclinic forcing is an important component of the pressure gradient simulations which ignore water level variations at the open end of the model differ substantially from those which incorporate water level fluctuations

regional distribution of white k mica polymorphs and their phengite content in the central alps some 150 white k micas from the central alps were analysed for their polymorph and phengite content

genetic links between ash flow calderas and associated ore deposits as revealed by large scale thermo mechanical modeling abstract in addition to a number of epithermal ore deposits some porphyry type ore deposits are also hosted by ash flow calderas spatial correlation between ash flow calderas and these deposits is observed in several parts of the world but temporal correlation between the mineralization and the episode of caldera formation remains unclear especially because geochronological data indicate that the mineralization may date to several million years before or after caldera formation because caldera collapse represents only a very short event in a long time scale evolution between a silicic magma chamber and its host rocks we have developed a thermo mechanical model of upper crustal and magmatic system behavior during the pre and post caldera formation stages major petrophysical properties of ash flow caldera related rocks are incorporated into the model which accounts for brittle elasto ductile rheology and heat transfer and reproduces fault initiation and various caldera collapse scenarios large scale mechanisms of initiation of thermal and structural traps for epithermal ore deposits are described in particular we found that the outer sides of caldera border faults are associated with both anomalous heat transfer and a high fracture density the finite size of the insulating intracaldera units forces the accumulated heat excess to be laterally transferred across the border faults in most cases the model predicts y shaped fault geometry which is surprisingly similar to that of mineralized faults in the field variations in lithostatic pressure on the magma chamber roof and in the vicinity of the border faults can reach 10 to 20 mpa with deep overpressured zones close to the inner edge of the chamber corners despite the fact that hydrothermal convection and fluid geochemistry are not included in the model these thermal and mechanical results are believed to constrain the conditions for mineralization processes our results show that in the presence of regional extension the development of caldera border faults may be suppressed to the advantage of deep seated fractures clustered above the central part of the magma chamber thus favoring conditions for the formation of porphyry type ore deposits finally we demonstrate that well before the eruption the gradual emplacement of a hot and large silicic magma chamber may create significant extensional stress in the brittle upper crust resulting in breakup and pre eruptive normal faulting and thus allowing for conditions of pre caldera mineralization

and mg fe si al2 compositional control on argon behaviour in high pressure white micas a continuous laser probe study from the dora maira nappe of the internal western alps italy abstract high pressure white micas from a restricted area in the dora maira nappe of the western alps italy were analyzed by the conventional 40 ar 39 ar bulk sample step heating method and by the step heating and the spot fusion 40 ar 39 ar continuous laser probe techniques on single grains high si ferich phengites from fine grained gneisses display homogeneous 35 40 ma plateau and spot fusion ages consistent with the upper eocene 40 ar 39 ar phengite ages documented over the whole internal western alps in contrast phengite separates and single grains of high si and mg rich to intermediate composition selected from metapelitic and mg rich mica schist samples yield older discordant apparent ages even when sampled in the same outcrop as the fine grained gneisses these local age variations are interpreted to reflect primarily an increasing ar retentivity with increasing mg content in phengites in metapelite derived white micas the isotopic discordance is marked by 1 1 a characteristic bulk sample upward convex spectrum shape 2 2 single grain disturbed age spectra 3 3 a systematic radially outward decrease in spot ages across individual mica cleavage planes and 4 4 strong grain to grain spot age variations in rock sections 1 1 in mg rich mica schists the isotopic discordance is observed as a reproducible upward convex bulk sample age spectrum with high 140 ma intermediate apparent ages 2 2 upward convex single grain age spectra and 3 3 irregular intra and intergrain age gradients on geochemical grounds it is argued that the isotopic discordance occurring in the metapelites is mainly controlled by the compositional changes introduced by the mg fe si al2 celadonitic re equilibration of the high pressure phengites during greenschist overprinting in mg rich phengites this chemical re equilibration may induce ar loss below their presumed higher blocking temperature a model of ar behaviour in high pressure white micas is proposed which explains the age variations recorded throughout the different lithologies in terms of a simple interaction between the mg fe compositional control on ar retentivity and the mg fe si al2 modifications occurring in phengites during progressive overprinting

40ar 39ar dating of white micas from an alpine high pressure metamorphic belt on naxos greece the resetting of the argon isotopic system overprinting of white micas from high pressure low to medium temperature m1 metamorphic assemblages in pelitic schists on naxos during subsequent thermal dome m2 metamorphism ranges from minor in the southeast of the island to complete recrystallization in the amphibolite facies rocks near the migmatites in the centre of the dome the original m1 minerals are phengites si4 6 7 7 0 and the overprinting minerals are muscovites si4 6 0 6 45 40ar 39ar step heating analyses of white mica separates from rocks in the area where phengite and muscovite occur together yield complex age spectra characterized by low apparent ages in the first and the last stages of gas release and high apparent ages in between these upward convex age spectra are shown to be caused by mixing of two generations of micas each of which has a different age spectrum and argon release pattern seemingly good plateaus in some age spectra from white micas of the area must be interpreted as providing meaningless intermediate ages further the upward convex age spectra have been used to trace the isotopic signature of phengites toward increasing m2 metamorphic grade and suggest that as long as phengites can be observed in the rocks upward convex age spectra occur on naxos crystallization of muscovite at the expense of phengite appears to be the main mechanism of resetting argon isotopic ages in white micas however there is also good evidence for argon loss by volume diffusion from phengites simple diffusion calculations suggest that the m2 metamorphism was caused by a shortlived heat source

observations of wind induced subtidal variability in the delaware estuary sea level and current observations made in the delaware estuary during autumn of 1982 were examined for evidence of wind forced subtidal variability the large subtidal sea level fluctuations at the mouth of the delaware were found to be forced primarily by the wind stress component over the continental shelf nearly parallel to shore indicating coastal ekman transport to the right of the wind local wind forcing within the estuary is shown to be insignificant the subtidal sea level in the interior of the estuary was found to be driven by the wind through a combination of two remote forcing mechanisms one acting at the mouth of the delaware through direct estuary shelf coupling and a second acting locally over the chesapeake bay and being transmitted from the upper chesapeake through the chesapeake and delaware canal which links the two estuaries in the upper delaware estuary we found predominantly barotropic subtidal current fluctuations superimposed on a much weaker two layer gravitational circulation these current fluctuations were produced primarily by the local subtidal surface slope generated by the difference between the two remote forcing mechanisms the evidence strongly suggests that the chesapeake and delaware canal plays a critical role in the subtidal circulation of the delaware estuary

the subtidal circulation in delaware bay and adjacent shelf waters surface and seabed drifters were used to study the subtidal circulation in delaware bay and over the adjacent continental shelf of the middle atlantic bight over 7000 drifters were released in eight experiments during the year long study period velocity vector maps representing the study period mean fields for the surface and bottom confirmed the pattern of classical two layer estuarine flow with surface water moving seaward from the bay onto the shelf and bottom water landward over the shelf from at least as far as 40 km seaward of the bay mouth within the bay bottom water moved laterally toward the nearest shore diverging horizontally along a line roughly marking the deep channels the surface speeds were about an order of magnitude larger than the bottom significant variations from this mean pattern were seen between the eight experiments return percentages for surface and bottom drifters correlated inversely over the eight experiments correlations with wind stress were above the level of significance and winds explained about two thirds of the total variance the strong coupling found between the subtidal circulation in the estuary and over the shelf is consistent with recent observational and modelling studies

retrogression of staurolite schists and the sources of infiltrating fluids during metamorphism staurolite kyanite schists from the dalradian of scotland and ireland show two types of retrograde alteration in the irish examples staurolite and kyanite are replaced by muscovite and paragonite often with margarite and sometimes chloritoid plagioclase in these samples may be albitized in contrast one scottish sample shows replacement of both staurolite and plagioclase by muscovite retrogression resulted from infiltration of fluid but while in some samples the large number of retrograde phases internally buffered the fluid composition and cation metasomatism cannot be proven in others the fluid composition was externally controlled and there was cation metasomatism indicating more extensive infiltration it is demonstrated by the use of activity diagrams showing the relationships between al silicate white mica and alkali feldspar that wholesale alteration to muscovite was most likely caused by fluids moving down temperature from a granitic or arkosic source in contrast if growth of retrograde albite is actually accompanied by na metasomatism then the infiltrating fluids are likely to have been moving up temperature through pelitic lithologies large fluid rock ratios of around 10 1 are needed to achieve significant alkali metasomatism except in extreme situations of large temperature changes and highly concentrated fluids

the lateral density distribution in a partially mixed estuary abstract the density distribution across the york river is characterized by distinct inhomogeneities especially in the upper 2 m of the water column the pattern of variability is repetitive and closely correlated with the semi diurnal tidal cycle and water depth in the shallow areas the water column remains vertically well mixed at all times the density differences are greatest at times of minimum currents at slack before flood the least dense water is located in a lens over the main channel towards the end of the flood cycle and at slack before ebb this is reversed with the least dense water situated over the shoals these density differences result in horizontal pressure gradients which at times may be of sufficient strength to generate localized lateral circulations such circulation patterns would form zones of convergence or divergence across the estuary depending on the particular density distribution

a simple model of estuarine subtidal fluctuations forced by local and remote wind stress observations of estuarine low subtidal sea level and current fluctuations have often shown domination by the remote effects of the wind acting on the adjacent coastal ocean over the local surface stress acting on the estuary itself the remote effects are transmitted to the estuary by impressment on its mouth of sea level change indtlced by the onshore component of coastal ekman transport and become increasingly dominant as the frequency decreases a simple barotropic model is developed to investigate the joint action of these two wind forcing mechanisms the relative shortness of most estuaries relative to low subtidal estuarine wavelengths explains the dominance of the remote effect for both sea level and barotropic current fluctuations in the estuary for the same reason however surface slope is dominated by local wind setup for an estuary with axis nearly parallel to the coast the two effects will operate either in concert or in opposition depending on hemisphere and orientation of the estuary axis relative to the coast for the geometries of both chesapeake bay and the delawm e estuary the model predicts opposition with the remote effect dominant at lower frequencies consistent with recent observations

la pacana caldera and the atana ignimbrite a major ash flow and resurgent caldera complex in the andes of northern chile the recently discovered la pacana caldera 60 35 km is the largest caldera yet described in south america this resurgent caldera of pliocene age developed in a continental platemargin environment in a major province of ignimbrite volcanism in the central andes of northern chile at about 23 s latitude collapse of la pacana caldera was initiated by the eruption of about 900 km3 of the rhyodacitic atana ignimbrite the atana ignimbrite was erupted from a composite ring fracture system and formed at least four major ash flow tuff units that are separated locally by thin air fall and surge deposits all four sheets were emplaced in rapid succession about 4 1 0 4 ma ago caldera collapse was followed closely by resurgent doming of the caldera floor accompanied by early postcaldera eruptions of dacitic to rhyolitic lava domes along the ring fractures the resurgent dome is an elongated asymmetrical uplift 48 5 12 km which is broken by a complex system of normal faults locally forming a narrow discontinuous apical graben later postcaldera eruptions produced large andesitic and dacitic stratocones along the caldera margins and dacitic domes on the resurgent dome beginning about 3 5 ma ago and persisting into the quaternary hydrothermally altered rocks occur in the eroded cores of precaldera and postcaldera stratovolcanoes and along fractures in the resurgent dome but no ore deposits are known a few warm springs located in salars within the caldera moat appear to be vestiges of the caldera geothermal system

alpine deformation on naxos greece a detailed structural study of naxos attic cycladic massif greece reveals two major deformation events the first one is associated with large scale thrusting and high pressure low temperature metamorphism during an early alpine subduction episode the second event occurred during continental extension and the associated development of localized thermal domes where lower crustal rocks were brought into contact with upper crustal units along a major shallow dipping shear zone we agree with a model of naxos as a cordilleran type metamorphic core complex however our observations show that the sense of shear was upper plate moving north during the second event calling for a reinterpretation of existing tectonic models of the cyclades

zoning and recrystallization of phengitic micas implications for metamorphic equilibration white micas phengites in the metasediments of the scottish dalradian display a large range of compositions within single samples the variations in the composition of these phengites are strongly controlled by their structural age with early fabrics containing a paragonite poor celadonite rich phengite whereas in later fabrics the micas are generally paragonite rich and celadonite poor retrograde phengite growth identified using back scattered electron imaging occurs as celadonite rich rims on micas within all existing fabrics and appears to be preferentially developed along existing white mica plagioclase grain boundaries the presence of these chemically distinct phengite populations within single samples implies that chemical exchange between the individual micas was inefficient it is proposed that diffusion controlled exchange reactions in phengites have relatively high closure temperatures below which major element exchange is effectively impossible this closed system behaviour of micas questions the ease with which phengites may equilibrate with other phases during prograde greenschist and lower amphibolite facies metamorphism many of the chemical variations preserved in phengites from such metamorphic rocks may reflect deformation recrystallization controlled equilibria

nevado portugueza volcanic center central peru a pliocene central volcano collapse caldera complex with associated silver mineralization nevado portugueza a major center of calc alkalic volcanism on the crest of the western cordillera of the andes in central peru was active during pliocene and perhaps quaternary time a large central volcano composed mostly of intermediate lavas formed about 3 5 to 4 m y ago explosive pyroclastic volcanism 2 4 or 0 2 m y ago produced the atunsulla tuff a rhyodacitic ash flow sheet of moderate volume and created an east northeast elongate collapse caldera about 7 km in maximum diameter steeply dipping fractures in the hydrothermally altered central part of the intracaldera tuff prism host thin veins containing base and precious metal mineralization dated at 1 90 or 0 08 m y this mineralization is probably related to postsubsidence magmatic activity reflected at the surface by a rhyodacite dome emplaced along the ring fracture system 1 96 or 0 1 m y ago tetrahedrite bearing veins which appear to postdate collapse are exposed at lower elevations directly east of the caldera hot water and sulfur present in workings in these veins may reflect episodically continuing activity of the volcanic center limited geochemical and sr isotope data show that the postsubsidence lavas are chemically similar to the atunsulla tuff and suggest that these unevolved late stage silicic rocks were not derived from magmas compositionally similar to the intermediate lavas that make up the earlier central volcano

orogenic evolution of the hellenides new aspects this progress report is based on investigations of the tectonometamorphic development of crystalline complexes and is restricted to a few key problems of the hellenides these results reveal the mechanisms of intracratonic tectonics remobilization of continental crust and intracrustal detachment throughout the evolution of the hellenides

subtidal sea level and current variations in the northern reach of san francisco bay abstract analyses of sea level and current meter data using digital filters and a variety of statistical methods show a variety of phenomena related to non local coastal forcing and local tidal forcing in the northern reach of san francisco bay a partially mixed estuary low frequency variations in sea level are dominated by non local variations in coastal sea level and also show a smaller influence from tidally induced fortnightly sea level variations low frequency currents demonstrate a gravitational circulation which is modified by changes in tidal current speed over the spring neap tidal cycle transients in gravitational circulation induce internal oscillations with periods of two to four days

li rich zincostaurolite and its decompression related breakdown products in a diaspore bearing metabauxite from east samos greece an emp and sims study li h rich zincostaurolite locally formed at a metabauxite marble contact along the eastern coast of samos during early alpine high p low t metamorphism the staurolite which probably grew from gahnite cookeite kaolinite pyrophyllite diaspore and minor fe oxide occurs as prismatic crystals up to several mm long in a calcite matrix it has an unusual composition with 7 5 12 9 wt zno 1 0 4 9 wt feo 0 66 0 82 wt li 2 o up to 2 6 wt nio and 0 59 wt coo and 0 32 wt mgo calculated structural formulae point to high h 2 o contents 4 14 h atoms per 48 oxygen atoms during late alpine uplift of the samos rocks the staurolite was at greenschist facies conditions variably replaced by bluish cobaltoan gahnite x zn 0 82 white na ca li mica ni rich chlorite x ni 0 42 0 59 zincohogbomite x zn 0 68 0 80 diaspore and fe hydr oxide detailed emp work and determination of li in staurolite and mica by sims indicates systematic zn fe mg ni co li partitioning between staurolite and its decomposition products implying local scale chemical equilibrium balanced reaction equations based on mineral chemical data indicate that the breakdown of staurolite occurred largely isochemically with only introduction of water na and ca into the reacting system the breakdown of high p staurolite during uplift and decompression is thought to be related to the sensitivity of its complex crystal chemistry to changes in physico chemical conditions

an emp and tem aem study of margarite muscovite and paragonite in polymetamorphic metabauxites of naxos cyclades greece and the implications of fine scale mica interlayering and multiple mica generations