sulfur geochemical constraints on mesoproterozoic restricted marine deposition lower belt supergroup northwestern united states abstract despite nearly a century of study by a diverse group of precambrian workers competing interpretations have persisted that favor both marine and nonmarine depositional settings for the mesoproterozoic belt supergroup of the northwestern united states our data for sedimentary pyrite formation in unmetamorphosed organic rich shales of the newland formation of the lower belt supergroup western montana argue that at least a portion of the belt basin during its early history was characterized by restricted marine conditions with only episodic incursions of seawater this model is supported by ratios of organic carbon corg to pyrite sulfur that once corrected for selective diagenetic and thermal loss of corg and the inferred high reactivity of the corg reservoir in the absence of vascular land plant remains suggest degrees of sulfate limitation consistent with a restricted marine setting the isotopically heavy disseminated pyrite that dominates the black shales shows a broad range of 34s values of bacterial origin 34s ranges from 8 7 to 36 3 mean 7 6 n 41 the pyrite data overlap with the 34s range of coeval barite sulfate within the basin and are similar to independent estimates for the sulfate of mesoproterozoic seawater these relationships have allowed us to delineate a record of bacteriogenic pyrite that formed in a modified marine reservoir where the supply of sulfate was limited a bacterial mechanism is further suggested by the morphological character of this early formed pyrite including the ubiquitous framboids after comparing our results with those from present day localities such as the black sea and sites of lacustrine deposition we conclude that the abundant 34s enriched bacteriogenic disseminated pyrite observed in black shales of the newland formation must record a style of sulfate limitation possible only within an isolated marine basin receiving an intermittent flux of seawater although the entire belt basin likely remained a restricted marine setting throughout deposition of the lower belt supergroup sulfur isotope data from the newland formation at two localities separated by more than 150 km in the eastern belt basin reveal stratigraphic trends in the strength of the marine connection specifically the connection evolved such that inputs of seawater sulfate increased progressively in frequency and possibly strength during deposition of the newland formation overall the hypothesized restricted marine setting when viewed in light of independent tectonic sedimentologic and geochronologic evidence suggests syndepositional and likely episodic continental rifting which helps constrain the proterozoic paleotectonic history of western laurentia

how late are k feldspar megacrysts in granites abstract various petrologists have suggested that k feldspar megacrysts grow in granites that are extensively crystallized even at subsolidus conditions however experimental evidence indicates that though k feldspar nucleates relatively late in the crystallization history abundant liquid is available for development of large crystals a great deal of evidence involving many different factors favours a magmatic phenocrystic origin for k feldspar megacrysts in granites namely simple twinning oscillatory zoning euhedral plagioclase inclusions and concentric crystallographically controlled arrangements of inclusions in addition abundant evidence has been presented of 1 mechanical accumulation of k feldspar megacrysts in granites 2 alignment of megacrysts and megacryst concentrations in magmatic flow foliations 3 involvement of megacrysts in zones of magma mixing in granite plutons and 4 occurrence of megacrysts in some volcanic rocks implying that the megacrysts were suspended in enough liquid to be moved without fracturing or plastic deformation detailed trace element and isotopic data also indicate that megacrysts can move between coexisting felsic and more mafic magmas irregular overgrowths on megacrysts are consistent with continued magmatic growth after euhedral megacrystic growth ceased the overgrowths being impeded by simultaneously crystallizing quartz and feldspar grains

is stoping a volumetrically significant pluton emplacement process discussion glazner and bartley 2006 1 cite and interpret five lines of evidence to indicate that magmatic stoping is unlikely to be a volumetrically significant process during pluton emplacement because of the iconoclastic tone of the paper it is useful to examine each line of evidence cited by glazner

the edison magnetite deposits in the context of pre syn and post orogenic metallogenesis in the grenville highlands of new jersey most of the magnetite deposits of the eastern grenville province have been described as precipitates from hydrothermal solutions derived from metamorphic processes from early late or post grenville granites or as metasediments granites host many of these deposits but others including the historic edison iron mines of new jersey are hosted by potassium feldspar gneiss commonly interpreted as meta arkose our new geochemical data indicate however that the protolith of the potassium feldspar gneiss is rhyolite not arkose supporting evidence includes i the absence of an underlying potassic provenance for arkosic sediment ii potassium and sodium contents among host rocks that exceed the range of arkose but are consistent with a type rhyolite and iii a close chemical resemblance of the potassium feldspar gneiss to an a type granite byram granite that is closely associated in time and space as the ore zone through the edison mines is approached the k na ratio of the host rocks undergoes

mafic magma intraplating anatexis and hybridization in arc crust bindal batholith norway distinct with nd and o in the migmatites from 7 6 to 9 6 the dioritic velfjord plutons 448 ma were emplaced into and from 10 9 to 13 5 and in the contact granites regional migmatitic metapelitic and metacarbonate rocks at midfrom 5 2 to 7 5 and from 9 6 to 12 3 respectively crustal levels corresponding to pressures of 700 mpa exthus the contact granites could have a source that is isotopically humation to 400 mpa began while the migmatites were in a distinct from but mineralogically similar to the diatexites or they partly molten state with increasing proximity to the plutons could result from mixing of magma similar to the high k leucosomes regional stromatic migmatites change to diatexite and diatexitic with dioritic magmas mass balance calculations are consistent dikes are common within 500 m of the contacts we interpret these with the latter interpretation with proportions of granitic to dioritic relationships to indicate that heat from the plutons resulted in contact magmas from 7 1 to 7 3 emplacement and solidification of the migmatization in a zone up to 1 km wide typical residual dioritic plutons provided zones of structural anisotropy along which mineralogy in the diatexites is plagioclase quartz biotite high k leucosome magmas and contact granite magmas collected garnet sillimanite k feldspar consistent with biotite these magmas were injected by additional dioritic magma and dehydration melting podand dike like leucosomes consist of two further hybridized because the solidi of the plutons were several types earlier high k granitic ones with mineral assemblages hundred degrees higher than that of the granitic magmas the pluton identical to the migmatites and later low k tonalitic ones in which walls acted as long lived hot rigid surfaces along which magmas sillimanite is sparse and garnet absent the high k leucosome collected and migrated magmas can be explained by biotite dehydration melting at 700 mpa within the aureole mafic magmas were locally injected into and hybridized with the diatexites and the high k leucosome

evolution of the triangle zone in the rocky mountain foothills near coalspur central alberta a triangle zone which commonly occurs along the external margin of a foreland thrust and fold belt with a buried thrust front is underlain by a subhorizontal blind foreland verging thrust that ends against a foreland dipping hinterland verging thrust these contemporaneous thrusts active towards the end of orogenesis enclose an intercutaneous wedge that moved towards the foreland during orogenesis a triangle zone evolves through periodic replacement of faults bounding the active wedge replacements occur in cycles during each of which a lower fault tends to be replaced by one in a lower stratigraphic horizon an upper fault by one farther away from the foreland each cycle ends with the lower fault moving to a younger horizon where it joins a new more external upper fault near coalspur the triangle zone exposes the remnants of several wedges involving upper cretaceous and paleocene molasse most of these wedges developed during the last cycle but one and have a combined displacement of about 5

caught in the act the first few hours of xenolith assimilation preserved in lavas of the rockeskyllerkopf volcano west eifel germany article i nfo article history lithic sedimentary xenoliths in magmatic pyroclastic deposits in the rockeskyllerkopf volcanic complex of the quaternary west eifel volcanic field preserve evidence of the earliest stages of melting reaction with and assimilation into their silica undersaturated basanite host the xenoliths have a 0 5 1 5 mm thick surface glaze of rhyolitic glass and internally have both silica undersaturated and silica saturated glasses that are the result of in situ melting of micas and clay minerals and reaction of these melts with the surrounding quartz matrix mixing between the rhyolite glaze and host basanite resulted in the formation of a clinopyroxene and glass rich hybrid zone the glazed xenoliths are commonly found with either no coating of lava or with void space between them and the host this unusual occurrence is due to vesiculation of the glaze during eruption which led to physical separation of the xenolith from its surrounding melt simple calculations indicate that assimilation occurred at 1180 c over a period of 12 h and that all the textural features including deformation of the partially molten xenoliths occurred during transport and eruption rather than after deposition

generation of trondhjemite from partial melting of dacite under granulite facies conditions an example from the new jersey highlands usa abstract new field and geochemical data place the losee metamorphic suite a tonalite trondhjemite complex of northern new jersey into the context of a major proterozoic continental are represented by a discontinuous belt of northern appalachian metadacite samples of losee rock range from extremely leucocratic trondhjemite locally associated with amphibolite to banded biotite hornblende pyroxene and garnet bearing tonalites the major element and ree composition of the tonalite closely resembles dacite from continental are settings and model melts extracted from an eclogite residue by partial melting at 15 kbar the ree composition of most losee trondhjemite is enriched in ree particularly hree compared with losee tonalite and is interpreted as the product of local anatectic melting of losee tonalite metadacite that occurred in a granulite facies environment during the grenville orogeny

a view from the roof magmatic stoping in the shallow crust chita pluton argentina nearly 2 km of vertical relief and exposure of continuous pluton roof wall corners around the chita pluton northwest argentina provides constraints on the 3 d host rock displacement field attending magma emplacement in the shallow crust the pluton is rectangular in crosssectional view and consists of weakly to non deformed granite to granodiorite structures in surrounding metasedimentary host rocks are truncated at a knife sharp contact and are only weakly deflected from their regional orientations final emplacement occurred by stoping together with minor doming of the earth s surface structural restoration of a cross section indicates that host rocks above the pluton may have been domed upwards during emplacement by as much as a few hundred meters accounting for up to 20 of the exposed pluton volume the remaining 80 of the space for pluton emplacement was made by downward removal of host rock from the present level of exposure although mechanisms such as floor subsidence are permissive late stoping has destroyed evidence for the early emplacement history however geometrical constraints provided by exposure of continuous wall to roof contacts indicate that early emplacement mechanisms at this level in the crust and immediately below were also dominated by the downward displacement of host rocks through a conduit that now is represented by the solidified pluton q 2002 elsevier science ltd all rights reserved

the possible role of benthic microbial mats during the formation of carbonaceous shales in shallow mid proterozoic basins a considerable portion of the upper member of the mid proterozoic newland formation meagher county montana consists of carbonaceous silty shales striped shale facies this type of shale facies is common in proterozoic basins and is host to several major base metal deposits the striped shales were deposited in a subtidal setting basinward of carbonates characterized by cryptalgal laminites mudcracks and flat pebble conglomerates the carbonaceous silty shales are considered remnants of benthic microbial mats irregular internal laminae patterns of particle trapping mechanical deformation during penecontemporaneous soft sediment deformation and filamentous microbiota provide evidence for this interpretation the dolomitic clayey shale contains graded silt clay couplets and these are interpreted as storm layers modern subtidal microbial mats can only survive under special conditions but in the proterozoic it is suggested that benthic microbial mats colonized the shallow seafloor during periods of low sediment input leading to the formation of carbonaceous shales

granitic magma transport by fracture propagation granitic magmas commonly ascend tens of kilometres from their source terranes to upper crustal emplacement levels or to the earth s surface apart from its obvious bearing on the interpretation of the geology and geochemistry of felsic igneous rocks the magma ascent mechanism critically affects any modelling of the metamorphic evolution of the upper lithosphere as well as its rheology we propose that in general granitic magmas ascend via propagating fractures as dykes in extremely short time periods long distance diapiric transport of granitoid magmas through crustal sections is not viable on thermal or mechanical grounds and there is an apparent total absence of field evidence for diapiric rise of such magmas even in supposed type localities neither the shapes nor internal or external characteristics of granitic plutons necessarily reveal anything about the transport of their precursor magmas these are purely arrival phenomena dictated by local structure kinematics and stress states based on existing numerical treatments of the problem we show that granitic magmas are apparently sufficiently inviscid to travel through fractures to high crustal levels without suffering thermal death by freezing for example a 2000 km3 granitic batholith can be inflated by a 1 km 3m 20 km deep dyke system in less than 900 years the model proposed has numerous far reaching petrological and rheological consequences some of which are outlined

a mathematical model of trace element and isotopic behavior during simultaneous assimilation and imperfect fractional crystallization the process of coupled assimilation and fractional crystallization afc is one of the most popular petrogenetic concepts that explains magmatic differentiation conventional geochemical models for this process assume that crystals are removed instantaneously from the magma body as they are produced however recent advances in isotopic microanalysis have clarified that the crystals are suspended within the magma body for a certain period affecting the whole rock composition in response to the intra grain isotopic zoning this study develops a mass balance model for simultaneous assimilation and imperfect fractional crystallization aifc to describe the effects of suspended crystals on the path of magma evolution the mass balance differential equations for the liquid and suspended crystals are solved simultaneously the analytical solution of the aifc equations gives a quantitative account of the evolution paths of trace elements and isotopes within bulk crystals liquid and magma crystals plus liquid the chemical path of the magma differs markedly from that predicted by the conventional afc model

effects of melt viscosity and silica activity on the rate and mechanism of quartz dissolution in melts of the cmas and cas systems the dissolution rate of quartz in melts of the cmas and cas systems at 1 600 c and 1 5 gpa is a function of both the silica activity of the melt and its viscosity in melts with low silica activity quartz dissolves more quickly than in higher asio2 melts regardless of viscosity for melts with equal asio2 dissolution is faster in the low viscosity melt quartz dissolution is controlled by interface kinetics in three of the four melts used in this study for times much greater than predicted by the model of zhang et al in contrib mineral petrol 102 492 513 1989 one melt which was previously shown to adhere to the predicted behaviour at lower temperature shows a significant activation time at higher temperature all the dissolution data indicate that there are likely to be three distinct domains of dissolution behaviour although the details of why a particular melt falls in any one domain require further study although the current database is small the relationship between quartz solubility and the dissolution constant indicate that solubility may be a useful parameter for predicting dissolution rates particularly if silica activity and melt viscosity are also known

carbon sulfur plots to show diagenetic and epigenetic sulfidation in sediments abstract organic carbon vs sulfide sulfur plots are now being used regularly by many geochemists to help understand recent and ancient depositional environments and diagenetic processes usually these plots are useful to recognize nonmarine vs marine environments or oxic vs anoxic vs euxinic depositional environments however c vs s plots can also indicate diagenetic and epigenetic events that produce excess sulfide four new examples are presented and discussed

geology geochemistry and emplacement conditions of the vega intrusive complex an example of large scale crustal anatexis in north central norway

mesoproterozoic graphite deposits new jersey highlands geologic and stable isotopic evidence for possible algal origins deposits within metaquartzite yielded 13 c values of 26 2 1 and graphite from massive deposits within biotite quartz feldspar gneiss yielded 13 c values of 23 4 disseminated graphite from biotite quartz feldspar gneiss country rock was 22 3 indistinguishable from the massive deposits hosted by the same lithology anatectic pegmatite is graphitic only where generated from graphite bearing host rocks one sample gave a 13 c value of 15 the 34 s values of trace pyrrhotite are uniform within individual deposits but vary from 0 to 9 from one deposit to another apart from pegmatitic occurrences evidence is lacking for long range mobilization of carbon during grenvillian orogenesis or post grenvillian tectonism the field petrographic and isotope data suggest that massive graphite was formed by granulite facies metamorphism of proterozoic accumulations of sedimentary organic matter possibly algal mats preservation of these accumulations in the sedimentary environment requires anoxic basin waters or rapid burial anoxia would also favour the accumulation of dissolved ferrous iron in basin waters which may explain some of the metasediment hosted massive magnetite deposits in the new jersey highlands

translation and the resolution of the pluton space problem abstract the late cretaceous mono creek granite has a pronounced nw se elongate shape 60 km long by 10 km wide characteristic of plutons from the eastern sierra nevada batholith an 8 km wide bulge exists on the ne side of this pluton which exhibits evidence of forceful emplacement or in situ ballooning such as deflection of metamorphic wallrock and igneous foliation and the orientation of fracture patterns three dimensional strain analysis indicates that wallrock strains do not provide enough volume to accommodate the emplacement of the bulge a recurring problem in studies of plutonic terranes we suggest that emplacement of the mono creek bulge was accommodated by all components of the three dimensional displacement field including translation rotation and pure strain shape change of the surrounding units classical strain analysis only addresses the rotation and pure strain components and is incapable of quantifying the translation component however our analysis suggests that translation plays the dominant role in the emplacement process a shell model of translation of the surrounding igneous and metamorphic units is proposed for the mono creek bulge which suggests that the translation component decreases dramatically away from the intrusion consistent with the observed geology and finite strain analysis we propose that translation is the solution to the recurring pluton space problem either through tectonically controlled passive or magmatically controlled active movement of the wallrocks translation is generally the neglected component of the displacement field but it may often be evaluated through judicious use of finite strain analysis and tectonic reconstruction

the skaergaard layered series part iii non dynamic layering layering in the skaergaard intrusion has been divided into two introduction general types one produced by magmatic flow and another by recent studies of the structural and textural features of processes resulting from variations of rates of nucleation and crysthe skaergaard intrusion mcbirney nicolas 1997 tallization and in the case of the layered series by compactionhave distinguished two broad types of layering one related processes modal variations caused by shifts of cotectic produced by the dynamic effects of magmatic flow and proportions produce thick layers which in the layered and upper another by processes that operate in situ such as varied border series are diffuse and normally lack strong foliation and nucleation and growth of crystals recrystallization or by lineation in the marginal border series the layers are thinner and compaction related mechanisms we refer to layering sharper possibly because the rate of accumulation was slower formed by these latter processes as non dynamic to oscillatory nucleation may have played a role in producing fineemphasize that it is not the result of fluid dynamic scale cyclic layers but it was less important than solution and processes although most layering combines elements of reprecipitation during slow cooling and ostwald ripening evidence more than one process the contribution of each mechfor compaction is found in deformed plagioclase laths and a relative anism can usually be recognized from its distinctive form deficiency of incompatible elements in rocks formed on the floor and setting layering related to compaction becomes sharper with increasing the distinguishing features of dynamic magmatic layheight in the layered series until it suddenly disappears above the ering have been described in part ii of this series mcbirtrough horizon near the base of upper zone b mechanical sorting ney nicolas 1997 they are best seen near the steep during compaction may have produced crude layering but if it did margins of the layered series in what wager brown the evidence has long since been destroyed by the superimposed effects 1968 referred to as the cross bedded zone where the of solution and reprecipitation when interstitial liquid rose through layering is disrupted by slumping and channeling and the overlying crystals and re equilibrated with them numerical the rocks have a marked foliation and lineation simulations illustrate how small differences of surface energy caused layering not associated with magmatic flow takes a by variations of grain size textural dependence of solubility and variety of forms but we can distinguish two general pressure solution can cause segregation of minerals into layers during end members each of which has a distinct origin and solution and reprecipitation characteristics the first results from varied rates of

equilibrium mineral fluid stable isotope fractionation factors in graphitic metapelites abstract the stable isotopic composition c o h and s of a fluid in equilibrium with the common metapelite assemblage graphite sulfide has been calculated using a modified redlich kwong equation of state at 500 3 5 kbar and 700 c 6 kbar for a range of f o2 and f s2 conditions the four major fluid species under these conditions are h 2 o co 2 ch 4 and h 2 s stable isotope fractionation factors among these fluid species remain significant to large even at temperatures as high as 700 c at 500 c 3 5 kbar 18 o quartz fluid ranges from 3 0 to 3 1 13 c graphite fluid ranges from 10 1 to 4 3 and d biotite fluid x f biotite 0 25 ranges from 31 to 74 depending on the values of f o2 and f s2 at 700 c 6 kbar 18 o quartz fluid ranges from 1 7 to 1 6 13 c gaph te fluid ranges from 7 6 to 1 6 and 34 s biotite fluid x fe biotite 0 25 ranges from 17 to 164 depending on the values of f o2 and f s2 values of j34 s pyrite fluid are small and independent of f o 2 and f s 2 the results have implications for studies using stable isotopic methods to investigate interaction between a fluid and a graphitic metapelite during metamorphism especially during devolatilization and fluid rock interaction equilibrium values of mineral fluid during metamorphism are dependent upon the composition of the fluid which in turn is dependent upon f o2 and f s2

is stoping a volumetrically significant pluton emplacement process magmatic stoping is widely called upon to explain discordant pluton contacts but several lines of evidence indicate that stoping is not volumetrically significant in the emplacement of most plutons first xenoliths rarely make up more than a few percent of most plutons this sparseness even on well exposed pluton floors argues that few xenoliths make it into plutons second piles of rock fragments have sufficiently high porosities 50 that a significant accumulation of stoped blocks on the floor of a magma chamber would trap much if not all of the available magma producing pluton scale magmatic breccias that are not observed third although settling calculations show that large meter scale or larger xenoliths could sink rapidly in magma natural fragmentation processes are likely to produce a fractal distribution of particle sizes with small particles far more abundant than large ones the absence of small xenoliths at pluton contacts where magma cools most rapidly thus argues against stoping fourth although contamination of magmas by anatectic liquids derived from wall rocks and xenoliths is common geochemical data from many plutons clearly rule out significant bulk assimilation of local wall rocks finally there is growing evidence that at least some large plutons never were large molten bodies of magma but grew incrementally stoping cannot be volumetrically important in incrementally emplaced plutons because blocks can only fall through the volume that is molten at any given time where xenoliths are abundant in plutons they may commonly represent isolation of wall rock bodies between successive intrusive increments rather than wall rock fragments that were engulfed by magma however even xenoliths that were engulfed by magma did not necessarily sink from the roof of a magma chamber although stoping undoubtedly occurs during the emplacement of some plutons it is unlikely to be a volumetrically significant process

characteristic mineralogy of arc related cumulate gabbros implications for the tectonic setting of gabbroic plutons and for andesite genesis several mineralogically distinct suites of gabbroic cumulates occur as xenoliths in basalt and andesite from active arc volcanos and in plutons exposed in deeply eroded arcs calcic an85 100 plagioclase and moderately fe rich fo60 80 olivine commonly occur together in arc cumulate gabbros but not in cumulate gabbros from mid ocean ridges ocean islands or tholeiitic layered intrusions compositions of coexisting olivine and plagioclase in gabbro cumulates can establish the tectonic setting of ancient volcano plutonic complexes in deeply eroded arcs gabbroic cumulates typically occur in zoned plutons that also contain gabbros diorites and quartz diorites of basaltic to andesitic composition in many cases it is clear that the cumulate gabbros are chemically and physically related to the gabbro diorite basalt andesite fractionation trends seen in the zoned plutons zoned plutons in deeply eroded arcs provide physical evidence that crystal fractionation plays an important role in andesite petrogenesis

the terminology of structures in thrust belts abstract a review of structures and geometric relationships recognized in thrust belts is presented a thrust is defined as any contractional fault a corollary being that thrusts must cut up section in their transport direction flats are those portions of a thrust surface which were parallel to an arbitrary datum surface at the time of displacement and ramps are those portions of thrusts which cut across datum surfaces ramps are classified on the basis of their orientation relative to the thrust transport direction and whether they are cut offs in the hangingwall or footwall of the thrust lateral variations in the form of staircase trajectories are joined by oblique or lateral ramps which have a component of strike slip movement an array of thrusts which diverge in their transport direction may form by either of two propagation models these are termed piggy back propagation which is foreland directed and overstep propagation which is opposed to the thrust transport direction an array of thrust surfaces is termed an imbricate stack and should these surfaces anastamose upwards a duplex will result the fault bounded blocks are termed horses a duplex is bounded by a higher roof thrust and a lower floor thrust the intersection of any two thrust planes is termed a branch line thrusts can be classified on the basis of their relationship to asymmetric fold limbs which they cut a further classification arises from whether a particular thrust lies in the hangingwall or footwall of another one the movement of thrust sheets over corrugated surfaces or the local development of thrust structures beneath will fold higher thrust sheets these folds are termed culminations and their limbs are termed culmination walls accommodation of this folding may require movement on surfaces within the hangingwall of the active thrust these accommodation surfaces are termed hangingwall detachments and they need not root down into the active thrust this category of detachment includes dip slip hangingwall drop faults which are developed by differential uplift of duplex roofs and out of the syncline thrusts which develop from overtightened fold hinges back thrusts as well as forming as hangingwall detachments may also form due to layer parallel shortening above a sticking thrust or by rotation of the hangingwall above a ramp

magmatic stoping as an important emplacement mechanism of variscan plutons evidence from roof pendants in the central bohemian plutonic complex bohemian massif the presence of numerous roof pendants stoped blocks and discordant intrusive contacts suggests that magmatic stoping was a widespread large scale process during the final construction of the central bohemian plutonic complex bohemian massif the measured total length of the discordant contacts that cut off the regional cleavage and were presumably formed by stoping corresponds to about half of all contacts with the upper crustal host rocks in addition at least some of the straight cleavage parallel intrusive contacts may also have recorded complex intrusive histories ending with piecemeal stoping of thin cleavage bounded host rock blocks into the magma chamber based on the above we argue that the fast strain rates required for emplacement of large plutons of the central bohemian plutonic complex into brittle upper crustal host rocks over relatively short time span could not have been accommodated entirely by slow ductile flow or slip along faults instead the emplacement was largely accommodated by much faster thermal cracking and extensive stoping independent of regional tectonic deformation finally we emphasize that magmatic stoping may significantly modify the preserved structural patterns around plutons may operate as an important mechanism of final construction of upper crustal plutons and thus may contribute to vertical recycling and downward transport of crustal material within the magma plumbing systems in the crust

application of stable carbon isotopes to early biochemical evolution on earth the emergence and early evolution of life on earth have long been intriguing subjects for scientific inquiry and a continuous source of intel lectual excitement current work at this frontier proceeds along a number of different avenues and comprises a variety of disciplinary approaches that together strongly suggest that the earth was inhabited by life as early as almost 4 gyr ago the present state of the field as inferred from the results of a broad interdisciplinary approach has recently been summarized by j w schopf and coworkers cf schopf 1 983 it is with the advantage of such a synoptic view that the present review addresses a single facet of the theme namely the potential isotopic infer ences of the presence of co2 fixing autotrophic organisms for the carbon cycle of the early earth with biologically derived organic carbon as well as carbonates limestone dolomite etc abundantly preserved in sediments relevant isotopic evidence should have been encoded in the geologic record from its onset about 3 8 gyr ago it has been known since the pioneering studies by nier gulbransen 1939 murphey nier 1941 and rankama 1948 that the trans formation of inorganic carbon into living matter entails a marked bias in favor of the light isotope c2c with the heavy species l3c retained in the inorganic reservoir subsequent work performed by many investigators e g craig 1953 park epstein 1960 has confirmed that all common

andean foothills structures in northern magallanes basin argentina the foothills of the northern segment of the magallanes basin between 47 and 49 s latitudes record the development of a complex fold and thrust belt in the patagonian cordillera during the late miocene upper paleozoic sedimentary and metasedimentary rocks and thick sequences of lower cretaceous deposits with well defined source and reservoir rocks are deformed by the andean events the structure is characterized by a triangle zone and a series of related underthrusts which define a nonemergent backthrust system at the mountain front differences in the degree of deformation from south to north reveal the evolution of the triangle zone the stratigraphy of the molasse deposits constrains the beginning of the deformation at least to the early tertiary although the final structure was developed during the late miocene these deformed deposits are covered by extensive late miocene horizontal retro arc basalts analysis of the evolving subduction zone located west of and beneath the study area shows that the formation of a subsequent volcanic gap in the magmatic arc and the development of extensive retro arc basalts are related to the collision of a segment of the chile ridge during the late miocene the final deformation and uplift of the patagonian cordillera fold and thrust belt at these latitudes are closely linked to this collision

temporal variation of mineralogy and petrology in cognate gabbroic enclaves at arenal volcano costa rica gabbroic enclaves ejected during the current eruption phase a 1 and during the latest prehistoric eruption phase a 2 of arenal volcano show systematic variations in texture mineralogy and composition as a function of host rock chemistry and timing of eruption the most differentiated enclaves occur in the more differentiated a 2 lavas enclaves in the a 1 volcanics are consistently less evolved within the current a 1 eruption the most mafic enclaves are amphibole bearing rocks that were erupted during the first 2 3 years of activity 1968 1970 these enclaves occur in the most differentiated a 1 volcanics and are not in equilibrium with their host rocks they crystallized from a hydrous melt that was slightly more mafic than anything erupted during the current cycle we interpret the enclaves as sidewall crystallization products of a melt possibly a high alumina basalt that was immediately parental to the a 1 lavas enclaves that occur in a 1 rocks erupted after 1970 and all of the a 2 enclaves are amphibole free and less mafic than the early a 1 enclaves their chemistry suggests that they formed during the early to intermediate crystallization of their host lavas none of the enclaves contain minerals that might have equilibrated with a primary mantle derived melt geothermometry is consistent with geochemistry with amphibole bearing a 1 enclaves yielding the highest pyroxene temperatures ave 1090 c and a 2 enclaves the lowest ave 1030 c geobarometry suggests mid to upper crustal depths for the crystallization of all enclaves the enclaves are cognate and reflect pre eruptive crystallization of arenal magmas they record evolution from a hydrous basaltic magma to the drier basaltic andesites that characterize the current eruption volatiles appear to have been lost due to depressurization during the slow ascent of the magmas through the upper levels of the crust following the initial explosive eruption volatile loss and depressurization resulted in the destabilization and the progressive resorption of amphibole the a 2 lavas may represent the long term fractionation products of basaltic andesite magmas similar in composition to the a 1 lavas anorthitic plagioclase commonly thought of as a phase stabilized by high ca na and high water pressure continued to crystallize in a system with relatively low ca na and which had dehydrated and or depressurized to the point at which amphibole was no longer stable this suggests that compositional characteristics other than high ca na or high water content may have stabilized the anorthite in the basaltic and basaltic andesite melts at arenal we speculate that the high alumina content of the arenal magmas may be the stabilizing factor

deformation transfer in viscous detachments comparison of sandbox models to the south pyrenean triangle zone deformation transfer in viscous detachments comparison of sandbox models to the south pyrenean triangle zone

triangle zone and displacement transfer structures in the eastern front ranges southern canadian rocky mountains the geometry of a relict triangle zone at the boundary of the foothills and front ranges in the southern canadian rocky mountains is constrained by detailed surface mapping over 700 m of relief and by seismic reflection data the geometry and progressive development of the triangle zone along a strike length of 15 20 km in the displacement transfer zone between the coleman and misty thrusts is illustrated using closely spaced balanced cross sections palinspastic restorations and a sequentially restored cross section structural geometries show that a northeast foreland verging mainly carbonate wedge of mississippian to triassic rock was inserted along a major upper detachment zone in shale near the base of the jurassic cretaceous clastic package this was accompanied by southwest hinterland verging displacements along the upper detachment zone tectonic thickening of the clastic package exceeding 200 and backthrusting later northeast verging deformation slightly modified the triangle zone by steepening structures tightening folds and minor thrusting recognition of relict triangle zones within the fold and thrust belt may document important changes in the rate of thrust front advancement and aid in the delineation of potential hydrocarbon traps similar to those discovered along th present day thrust belt margin in the southern canadian rocky mountains

quartz garnet isotope thermometry in the southern adirondack highlands grenville province new york quartz garnet oxygen isotope thermometry of quartz rich metasedimentary rocks from the southern adirondack highlands grenville province new york yields metamorphic temperatures of 700 800 c consistent with granulite facies mineral assemblages samples from the irving pond quartzite record d 18 o qtz grt 2 68 0 21 1 s d n 15 corresponding to peak metamorphic conditions of 734 38 c this agrees well with the estimates from garnet biotite exchange thermometry similar temperature estimates are obtained from swede pond 682 47 c n 3 and king s station c 700 c the whitehall area records higher temperatures 798 25 c n 3 all of these temperatures are higher than previous regional temperature estimates the c 800 c temperatures near whitehall are consistent with preservation of pre granulite contact temperatures adjacent to anorthosite the preservation of peak metamorphic temperatures in garnet of all sizes is consistent with slow oxygen diffusion in garnet and closure temperatures of at least 730 c peak metamorphic fractionations are preserved in rocks with varying quartz feldspar ratios indicating that the modal percentage of feldspar does not affect retrograde oxygen exchange in these rocks the lack of this correlation suggests slow rates of oxygen diffusion in quartz and feldspar consistent with the results of anhydrous oxygen diffusion experiments

assimilation and crystal accumulation in a mid crustal magma chamber the sausfjellet pluton north central norway the sausfjellet pluton is made up of two intrusive units emplaced into high grade metamorphic rocks of the helgeland nappe complex of the uppermost allochthon in the norwegian caledonides the eastern part of the pluton intruded marble and less voluminous calc silicate and pelitic rocks the western half is hosted predominantly by semi pelitic migmatite with intercalated marble remelting of the migmatite during pluton emplacement occurred in a thermal aureole as much as 1000 m wide the early gabbroic unit forms the southeastern part of the body it consists of hornblende bearing to hornblende rich gabbro and diorite which is thought to have crystallized from an h2o rich andesitic parental magma the younger dioritic unit underlies the central and western parts of the pluton as well as a zone as much as 200 m wide that separates the rest of the pluton from its host rocks herein the annular zone the interior or central zone of the dioritic unit is pyroxene diorite that is locally interlayered with anorthosite the western and annular zones are by comparison mineralogically heterogeneous they range from diorite to quartz monzonite and from biotite bearing two and threepyroxene assemblages to biotite hornblende assemblages neither rock type nor mafic assemblage is correlated with position in the pluton or proximity to a contact stoped blocks of a distinctive coarse grained diorite as well as pyroxenerich calc silicates are present in the gabbroic unit and the central zone of the dioritic unit the few stoped blocks observed in the western zone of the dioritic unit are predominantly quartz rich gneiss chemical variation in the central zone of the dioritic unit is interpreted to result from accumulation of pyroxenes plagioclase from an h2o poor andesitic parent these rocks have approximately constant d 18 o of 6 6f0 2x and lack evidence of in situ assimilation heterogeneities in the western and annular zones of the dioritic unit are reflected in variable anomalously enriched incompatible element contents and in d 18 o which ranges from 6 7x to 8 6x petrologic models indicate that the magma parental to the central zone could also be parental to the western and annular zones if so evolution of the western and annular zone magma was by crystal accumulation and assimilation of metapelitic host rocks as much as 20 of the mass of the western and annular zones can be ascribed to assimilated material which apparently entered the magma by stoping therefore the asymmetrical

evaluating the origin of garnet cordierite and biotite in granitic rocks a case study from the south mountain batholith nova scotia evaluating the origin of garnet cordierite and biotite in granites provides important insight into closed and open system magma evolution we present field textural major and trace element mineral chemical and sr nd whole rock isotopic data on garnet cordierite and biotite rich zones from the peraluminous south mountain batholith we infer that 1 garnet rich zones of decimeter to meter size with 30 vol large subhedral garnet with abundant inclusions of detrital country rock monazite represent partially assimilated metapelitic country rocks where garnet is the incongruent product of biotite dehydration melting 2 cordieriterich zones tens of meters to kilometers in dimension with 5v ol large subhedral to euhedral zoned cordierite formed by crystallization from relatively evolved magmas and subsequent crystal accumulation 3 biotite rich zones with large subhedral to euhedral biotite with abundant euhedral apatite inclusions making up 80 vol centimeter scale or 25 vol kilometer scale of the rocks formed dominantly by fractional crystallization throughout the chemical evolution of the batholith our results suggest that for garnet and cordierite a combination of textural and mineral chemical characterization is probably sufficient to determine their origin in granites however for biotite and other readily equilibrated minerals evaluating both mineral and rock textures and majorelement trace element and isotopic compositions is essential

reply to comments on is stoping a volumetrically significant pluton emplacement process the 188th meeting of the geological society of washington was devoted to a consideration of the methods of igneous intrusion the discussion was opened by mr whitman cross who directed it to the methods by which the large igneous masses such as laccoliths stocks and batholiths have

exploding xenoliths and the absence of elephants graveyards in granite batholiths abstract biotite monzogranite near the eastern margin of the late devonian south mountain batholith at portuguese cove nova scotia surrounds a large 20 m 25 m anisotropic decimetre scale interbedded psammite and semipelite xenolith of the metamorphic regional greenschist thermal cordierite hornfels country rocks of the cambro ordovician meguma group the xenolith has two distinct types of surfaces against the monzogranite i one rounded diffuse and apparently old surface that had prolonged contact with the granite magma and ii two straight sharp and apparently new surfaces that had more recently come into contact with the granite magma more than 375 thin sheets width n 224 0 1 0 5 cm n 66 0 5 1 0 cm n 51 1 0 5 0 cm n 30 5 0 10 0 cm n 3 10 cm n 2 ranging in composition from monzogranite to pegmatite to coarse quartz sericite cut the xenolith parallel to bedding producing a cumulative volume expansion of 14 in the xenolith aspect ratios in some of the thin sheets exceed 1000 1 several sheets cut the xenolith perpendicular to bedding producing a volume expansion of

dynamics and thermodynamics of magma mixing insights from a simple exploratory model the mixing of magmas of distinct temperature bulk composition mineralogy and physical properties plays a central role in explaining the diversity of magma types on earth and in explaining the growth of continental and oceanic crust magma mixing is also of practical importance for example the mixing of distinct magmas has been cited as an important process in creation of economically important horizons in layered intrusions as well as a triggering mechanism for initiation of volcanic eruptions the motivation for better quantifying the dynamics and thermodynamics of magma mixing and its attendant plutonic and volcanic products is clear the degree of magma mixing which spans a continuum from mingling to complete hybridization depends upon initial and boundary conditions magma properties driving forces and time available for mixing magma mingling produces a heterogeneous mixture of discrete clumps of the end member magmas whereas complete hybridization involves the thermodynamic equilibration of two distinct magmas to form a third qualitatively mixing occurs via reduction in the size of compositional heterogeneities i e clumps through stretching and folding by viscous flow followed by homogenization once shear has reduced the size of compositional anomalies to diffusive length scales quantification of this process relies on two statistical measures the linear scale of segregation defined as the spatial integral of the compositional correlation function related to the size distribution of the segregated clumps within the mixture and the intensity of segregation i a measure that quantifies how much the composition at each location differs from the average the mixing dynamics of a layered system are analyzed in terms of the parameters governing mixing rayleigh lewis and buoyancy numbers and viscosity ratio to estimate how the timescale for magma hybridization h compares to solidification recharge diffusive and assimilation timescales this analysis illustrates that hybridization times can be shorter than or comparable to thermal solidification and replenishment timescales thus formation of hybridized or nearly hybridized magmas is one anticipated outcome of mixing the machinery of thermodynamics can be used to compute the hybrid magma state an exploratory model for the thermochemistry of hybridization is developed based on binary eutectic phase relations and thermodynamics eight thermodynamic parameters define the phase diagram and associated energetics and six parameters initial temperatures compositions mass ratio of mixing magmas and an enthalpy parameter are necessary and sufficient to determine the state of hybrid magma uniquely while relevant combinations of 14 thermodynamic and mixing parameters might suggest that the number of mixing outcomes i e products is too high to systematize monte carlo simulations using the exploratory model document how millions of arbitrary initial states evolve into five possible final mixed states such an analysis implies that a magma mixing taxonomy that defines possible mixed product states can be developed and tied to petrologic indicators of mixing additional insights gained from this exploratory model that are supported by independent results from a multicomponent multiphase thermodynamic model of magma mixing magma chamber simulator include 1 the proclivity of invariant point hybrid states which may explain some instances of compositionally monotonous melts associated with mixed magma eruptions 2 a surprising thermal effect such that the temperature of hybridized magma can be significantly less than the initial temperature of either of the mixing magmas this type of magma mixing may result in crystal resorption thus invalidating an assumption that resorption textures in crystals are typically the result of a magma heating event 3 illustration of the differing effects of stoped block temperature and composition on hybrid magma temperature and phase state and 4 illustration of a cessation of crystallization effect that may pertain to the morb pyroxene paradox differences between adiabatic or r hybridization and diabatic or rfc hybridization are also explored the model can be used to elucidate the thermodynamic principles underlying magma mixing in the hybridization limit these principles are of general applicability and carry over to more compositionally complicated systems

carbon isotope thermometry in marbles of the adirondack mountains new york carbon isotope thermometry has been applied to coexisting calcite and graphite in marbles from throughout the adirondack mountains new york eighty nine calcite graphite pairs from the amphibolite grade nw adirondacks change systematically in temperature north westwards from 680 to 640 to 670 c over a 30 km distance reflecting transitions from amphibolite facies towards granulite facies to the north west and to the south east temperature contours based on calcite graphite thermometry in the nw adirondacks parallel mineral isograds with the orthopyroxene isograd falling above 675 c and indicate that regional metamorphic temperatures were up to 75 c higher than temperatures inferred from isotherms based on cation and solvus thermometry bohlen et al 1985 fifty five calcite graphite pairs from granulite grade marbles of the central adirondacks give regional metamorphic temperatures of 670 780 c in general agreement with cation and solvus thermometry data for amphibolite and granulite grade marbles show a 12 oo range in 13ccal and 13cgr a strong correlation between carbon isotopic composition and the abundance of graphite cgr crock indicates that the large spread in isotopic compositions results largely from exchange between calcite and graphite during closed system metamorphism the trends seen in 13c vs cgr crock and 13ccal vs 13cgr could not have been preserved if significant amounts of co2 rich fluid had pervasively infiltrated the adirondacks at any time the close fit between natural data and calculated trends of 13c vs cgr crock indicates a biogenic origin for adirondack graphites even though low 13c values are not preserved in marble delamination of 17 graphite flakes perpendicular to the c axis reveals isotopic zonation with higher 13c cores these isotopic gradients are consistent with new graphite growth or recrystallization during a period of decreasing temperature and could not have been produced by exchange with calcite on cooling due to the sluggish rate of diffusion in graphite samples located 2km from anorthosite show a decrease of 0 5 0 8 oo in the outer 100 of the grains while samples at distances over 8 km show smaller core to rim decreases of c 0 2 oo correlation between the degree of zonation and distance to anorthosite suggests that the isotopic profiles reflect partial overprinting of higher temperature contact metamorphism by later granulite facies metamorphism core graphite compositions indicate contact metamorphic temperatures were 860 890 c within 1 km of the marcy anorthosite massif if samples with a significant contact metamorphic effect cal gr 3 2 oo are not included then the remaining 38 granulite facies samples define the relation 13c cal gr 3 56 106t 2 k

mechanics of triangle zones and passive roof duplexes implications of finite element models abstract finite element models of passive roof duplexes simulate the evolution and distribution of fault slip and deformation as functions of boundary conditions friction on the roof fault cover strength and amount of overburden a foreland buttress is essential for backthrust displacement in the models this foreland buttress is analogous to a pin line between the cover and footwall forelandward of the triangle zone in a balanced cross section and could be caused by a stratigraphic pinchout of the upper detachment or a strong cover in the foreland with boundary conditions that prevent the cover from slipping toward the hinterland relative to the wedge incipient forethrust and backthrust ramps zones of high equivalent plastic strain cut through the cover in the models there is more backthrust displacement with a lower friction coefficient on the upper detachment and with a stronger cover because of pressure dependence both fault strength and material strength increase with increasing overburden therefore the effect of overburden on backthrust displacement is determined by the nature of the pressure dependencies for fault friction and material strength in models with multiple imbricate faults the distribution of equivalent plastic strain adjacent to each fault is similar to that in single fault models with differences resulting from bending strains in the wedge and cover the strain distribution in the cover of the models is similar to that above natural passive roof duplexes characterized by layer parallel shortening above the tip of each horse in the duplex

upward delamination of cascadia basin sediment infill with landward frontal accretion thrusting caused by rapid glacial age material flux the cascadia convergent margin is a first order research target to study the impact of rapid sedimentation processes on the mechanics of frontal subduction zone accretion the near trench part of the accretionary prism offshore washington is affected by strongly increased glacial age sedimentation and fan formation that led to an outstanding quaternary growth rate with landward vergent thrust faulting that is rarely observed elsewhere in accretionary wedges multichannel seismic reflection data acquired on the orwell project allows us to study the structure and dynamics of the atypical frontal accretion processes we performed a kinematical and mechanical analysis of the frontal accretion structures and developed a dynamic coulomb wedge model for the landward verging backthrust formation backthrusting results from heterogeneous diffuse strain accumulation in the mechanically heterogeneous cascadia basin sediment succession entering the subduction zone and strain partitioning along a midlevel detachment that is activated by gravitational loading caused by rapid glacial age sedimentation these complex deformation processes cause the passive upward delamination of the upper turbidite beds from the basal pelagic carbonate section similar to triangle zone formation and passive backthrust wedging in foreland thrust belts caused by rapid burial beneath syntectonic sediment deposits the deformation mechanism at the tectonic front of the cascadia margin is an immediate response to the strongly increased late pleistocene sediment flux rather than to atypical physical boundary conditions as generally thought

sr and nd isotopic compositions age and petrogenesis of a type granitoids of the vernon supersuite new jersey highlands usa abstract voluminous late mesoproterozoic monzonite through granite of the vernon supersuite underlies an area of approximately 1300 km2 in the highlands of northern new jersey the vernon supersuite consists of hastingsite biotite bearing granitoids of the byram intrusive suite bis and hedenbergite bearing granitoids of the lake hopatcong intrusive suite lhis these rocks have similar major and trace element abundances over a range of sio2 from 58 to 75 wt are metaluminous to weakly peraluminous and have a distinctive a type chemistry characterized by high contents of y nb zr lree and ga al ratios and low mgo cao sr and hree whole rock rb sr isochrons of bis granite yield an age of 1116 41 ma and initial 87 sr 86 sr ratio of 0 70389 and of lhis granite an age of 1095 9 ma and initial 87 sr 86 sr ratio of 0 70520 both suites have similar initial 143 nd 144 nd ratios of 0 511267 to 0 511345 bis and 0 511359 to 0 511395 lhis values of end are moderately high and range from 1 21 to 2 74 in the bis and 2 24 to 2 95 in the lhis petrographic evidence field relationships geochemistry and isotopic data support an interpretation of comagmatism and the derivation of both suites from a mantle derived or a juvenile lower crustal parent with little crustal assimilation both suites crystallized under overlapping conditions controlled by p t fh2o lake hopatcong magma crystallized at a liquidus temperature that approached 900 c and a pressure of about 6 kbar and remained relatively anhydrous throughout its evolution initial p t conditions of the byram magma were 850 c and about 5 5 kbar bis magma was emplaced contemporaneous with or slightly preceding lhis magma and both magmas were emplaced during a compressional tectonic event prior to granulite facies metamorphism that occurred in the highlands between 1080 and 1030 ma

the structure of mountain fronts commonly the part of a mountain front which is visible at the surface consists of foreland dipping thrust belt rocks elevated above their regional structural position and overlain more or less conformably by molasse several explanations for their geometry are possible 1 major detachments exist within or beneath the molasse resulting in transport of the foreland basin examples of this geometry come from the swiss molasse plain the southern pyrenees and the mackenzie mountains of canada 2 displacement is lost on major backthrusts beneath the frontal monocline examples cited here are the rockies of alberta the sulaiman ranges of pakistan the mackenzies and the andes in peru 3 thrust sheets travelled over an old land surface and syntectonic molasse contemporaneously offlaps the topographic high of the thrust front this phenomenon occurs along the alpine thrust front in haute provence 4 the frontal fold represents deformation above a large scale thrust tip no unequivocal example of tip line strain at this scale has been recorded although this type of deformation may occur in the brooks range of alaska in many areas mountain fronts show a combination of these idealized geometries

scale of pluton wall rock interaction near may lake yosemite national park ca usa interaction of magma with wall rock is an important process in igneous petrology but the mechanisms by which interactions occur are poorly known the western outer granodiorite of the cretaceous tuolumne intrusive suite of yosemite national park california intruded a variety of metasedimentary and igneous wall rocks at 93 1 ma the may lake metamorphic screen is a metasedimentary remnant whose contact zone exhibits a variety of interaction phenomena including xenolith incorporation disaggregation and partial melting the chemical contrast of these metasedimentary rocks with the invading pluton provides an excellent measure of pluton wall rock interactions wall rock xenoliths mostly pelitic quartzite are predominantly located in an elongate horizon surrounded by a hybridized fine grained granodiorite initial sr and nd isotopic ratios of the hybridized granodiorite indicate significant local incorporation of crustal material major and trace element geochemical data indicate that contamination of the granodiorite occurred via selective assimilation of both high k and low k high silica partial melts derived from pelitic quartzite although the hybridized granodiorite shows significant amounts of contamination adjacent to xenoliths the proportion of contamination is undetectable more than a meter away these results indicate that the chemical and isotopic variability of the tuolumne intrusive suite is not caused by magma contamination via in situ wall rock assimilation

isotopic study of the manaslu granite himalaya nepal inferences on the age and source of himalayan leucogranites a detailed isotopic study of the manaslu leucogranite was carried out a u pb age of 25 ma and a whole rock rb sr age isochron of 18 ma were obtained suggesting that the magmatic activity lasted at least 7 ma initial sr isotopic ratios are very high 0 740 to 0 760 and initial nd isotopic ratios are low nd in 13 to 16 and they show the existence of large isotopic variations even at the metre scale these are not the result of perturbations by fluids but rather they reflect the initial isotopic heterogeneity of the source material which has not been obliterated by magmatic processes e g fusion mixing by convection these results also support the crustal origin of this leucogranite the tibetan slab paragneisses whose sr and nd isotopic ratios are very similar to those of the granite at an age of 20 ma are the most probable parental material nd model ages for both the leucogranite and the gneisses are in the range 1 5 2 ga a model of formation of the manaslu granite by coalescence of different batches of magma is in agreement with the present data

mechanical stability of the triangle zone the backthrust wedge the triangle zone arrangement of faults that commonly develops at the foreland margin of fold and thrust belts is one of the few places in this structural setting that backthrusts of significant dimensions are observed even in this setting the backthrust may be a relatively ephemeral feature as it can be cannibalized by the foreland advance of thrusting in an active fold and thrust system one of the factors that determines the longevity of a specific backthrust and triangle zone in an active system is the mechanical stability of the backthrust wedge the wedge of rock that forms the hangingwall to the backthrust upper and lower stability limits for this backthrust wedge can be determined using the critical wedge concept for a cohesive coulomb material the configuration of the backthrust wedge developed in triangle zones along the southern and central alberta portion of the canadian cordillera agrees well with the stability limits derived for reasonable rock properties and normal fluid pressures in contrast a low taper triangle zone interpretation for northeastern british columbia does not fall within the calculated stability limits even when abnormally high fluid pressures and extreme rock properties are assumed however a similar low taper geometry may be achieved by the shingling of stable backthrust wedges upper crustal delamination and passive roof duplexing are both processes that serve to sustain a mechanically stable backthrust wedge configuration suggesting that they are both viable paths for the progressive advance of the mountain front triangle zone

midcrustal emplacement of the sausfjellet pluton central norway ductile flow stoping and in situ assimilation midcrustal 25 30 km emplacement of the dioritic sausfjellet pluton into rocks of the helgeland nappe complex central norway occurred in two stages stage 1 consists of two pyroxene hornblende gabbro and diorite stage 2 is asymmetrically zoned with a modally layered central zone of diorite and anorthosite and a western annular zone of quartz bearing monzodioritic rocks igneous layering was locally attenuated folded and boudinaged in the hyper solidus state the magmatic foliation trajectory pattern in the pluton defi nes a shallowly southwest plunging synform that crosscuts compositional zones mineral lineations plunge shallowly to moderately to the southwest the pluton intruded a major lithologic boundary within the nappe migmatitic pelitic gneisses are the main host rocks to the western part of the pluton whereas the eastern and central parts are hosted predominantly by metacarbonate rocks calc silicate marble quartzo feldspathic and dioritic xenoliths up to 200 m in length are present throughout the pluton they are most common in stage 1 and the central zone of stage 2 metapelitic xenoliths are conspicuously absent ductile fl ow during emplacement produced an 1 kmwide structural aureole in which host rock structures were defl ected into subparallelism with the steeply inward dipping margin of the pluton tight antiforms developed along the northeastern southeastern and southwestern margins amphibolite grade shear zones in the host rocks preserve plutonside up kinematic indicators in addition to the abundance of dioritic calc silicate and quartzo feldspathic gneiss xenoliths and geochemical evidence for assimilation in the western annular zone regional discordance of the pluton host rock contacts indicates that stoping was also an important emplacement process at midcrustal depths following magma emplacement foundering of the central portion of the chamber combined with possible ca 445 ma regional contraction produced the map scale synform defi ned by magmatic foliations and igneous layers this study demonstrates that stoping and assimilation may occur simultaneously with host rock ductile fl ow during magma chamber evolution at midcrustal levels and offers an explanation of why xenolith preservation may be compositionally dependent

multistage emplacement of the mcdoogle pluton an early phase of the john muir intrusive suite sierra nevada california by magmatic crack seal growth new mapping of the southern mcdoogle pluton an early intrusion in the john muir suite in the central sierra nevada california has resolved important details of incremental assembly by magmatic crack seal the tabular mcdoogle pluton concordantly intruded the steeply dipping mylonitic sawmill lake shear zone around 95 ma the pluton was previously divided into three phases based on wall rock screen abundance and lithology the earliest central phase consists of steep dikes separated by abundant wall rock screens these screens are largely absent from border phases located on either side of the central phase the inferred relative ages of intrusive phases and the wall rock screen distribution signify a change from antitaxial addition at the margins of the central phase to syntaxial addition at the center of each border phase assembly during pluton growth interaction among the effects of preexisting foliation older nondeformed intrusions and synintrusive static recovery of shear zone rocks likely caused the transition from antitaxial to syntaxial growth the trend from antitaxial to syntaxial growth during emplacement has been observed in other intrusions and intrusive suites and may be a general pattern of incremental intrusion associated with the feedback between preexisting deformation and thermal addition in magmatic arcs

copper gold occurrences in the palaeoproterozoic inglefield mobile belt northwest greenland a new mineralisation style abstract inglefield land in northwest greenland is an ice free 7000 km 2 region underlain by the palaeoproterozoic inglefield mobile belt composed of quartzo feldspathic gneisses meta igneous and supracrustal rocks these rocks are unconformably overlain by an unmetamorphosed cover of sedimentary and igneous rocks of the mesoproterozoic thule basin and the lower palaeozoic franklinian basin mineralisation in inglefield land is characterised by a copper gold metal association that can be classified in terms of the hosting rocks namely garnet sillimanite paragneiss hosted orthogneiss hosted and mafic ultramafic hosted the paragneiss hosted mineralisation the topic of this paper is essentially confined within a ne trending structural corridor and consists of bands of sulphide graphite bearing hydrothermally altered quartzo feldspathic gneiss called rust zones these are commonly parallel to the paragneiss main foliation suggesting a close relationship the rust zones have strike lengths from a few metres to more than 5 km and widths ranging from a few centimetres to 200 m sulphides mainly include pyrrhotite pyrite and chalcopyrite the sulphides form disseminations up to 30 by volume but in places they form massive pods or lenses up to 20 30 m and about 0 1 0 5 m wide graphite contents are up to 5 vol rust zones typically consist of a quartz plagioclase mosaic associated with a late generation of red brown biotite sericite chlorite and epidote mylonitic or cataclastic textures are locally recognisable xrd analyses of graphite indicate temperatures of between 650 and 700 c sulphur isotope analyses show 34 s values ranging from 6 2 to 9 3 an ore genesis model is proposed in which the palaeoproterozoic precursor sandstone carbonaceous shale succession is polydeformed and polymetamorphosed to granulite facies quartzo feldspathic and pelitic gneisses with transposition of layering to axial plane of folds followed by ductile shearing and mylonitisation from which future rust zones were derived the mylonitic zones were infiltrated by fluids whose origin can be ascribed to deep penetrating surface waters and or external brines in our ore genesis model we envisage that brines derived from the overlying lower palaeozoic franklinian succession infiltrated the basement into the structural channels provided by the shear mylonitic zones at the regional scale this infiltration was facilitated by a ne trending corridor postulated to be a deep structure

stable isotope and petrologic evidence for the origin of regional marble hosted magnetite deposits and the zinc deposits at franklin and sterling hill new jersey highlands united states mesoproterozoic marble in the new jersey highlands hosts small magnetite deposits that occur in discrete groups along linear trends and are concentrated mainly in the western part of the region magnetite also forms a layer structurally beneath the zn fe mn orebody at the franklin mine marble host rocks are interlayered at fe and zn fe mn deposits with bimodal metavolcanic rocks amphibolite and rhyolitic gneiss that were deposited synchronously in a back arc basin developed on the eastern margin of laurentia between 1 3 and 1 25 ga magnetite in the deposits ranges from massive to disseminated and is characterized by low ti variable mn and locally high s concentrations of as v sb and zn are enriched in magnetite compared to marble host rocks and associated metavolcanic rocks carbon and oxygen isotope ratios of the marbles are consistent with water rock interaction between the marble protolith and a mixture of igneous fluids and seawater regional geology petrology and isotope data indicate that fe was introduced into the carbonate protolith as low temperature fe oxides and hydroxides via hydrothermal fluids discharged along basinal fracture zones on the sea floor related marble hosted zn fe mn deposits at franklin and sterling hill contain the assemblage willemite zincite franklinite stable isotope compositions of the zn deposits are consistent with alteration of host rocks by water rich fluids preserving protolith carbon isotope ratios stable isotope modeling suggests that zn silicate oxides or hydroxides represent an appropriate protolith for the high grade zn ores alteration of metavolcanic rocks at the fe deposits combined with the isotope data and other petrologic and geochemical evidence document the presence of an extensive hydrothermal system of grenville age centered in the western highlands metals precipitating from this hydrothermal system were responsible for fe in the marble hosted magnetite deposits and zn in the deposits at franklin and sterling hill from the same or a related hydrothermal system underplating of the back arc by mafic magma provided the heat to melt the lower crust and drive the hydrothermal system resulting in carbonate hosted fe and zn ore deposits and associated bimodal volcanism

passive roof duplexes under the rocky mountain foreland basin alberta seismic reflection data in the central alberta foothills near edson reveal the presence of small passive roof duplexes in upper cretaceous rocks within the otherwise undeformed foreland basin as much as 40 km northeast of the mountain front monocline the tops and bottoms of the duplexes are defined by backthrusts and sole thrusts which follow bedding planes within upper cretaceous strata overlying the structures is an essentially uncontracted 1 8 km thick section of upper cretaceous and tertiary rocks which is passively uplifted over the thickened duplexes the underlying autochthonous sequence of mesozoic and paleozoic rocks exhibits some minor folding but is also uncontracted our interpretation extends both the upper and lower detachments of the widely accepted triangle zone model more than 30 km farther under the foreland basin than has previously been supposed the seismic data illustrate relatively clearly the form of the leading edge of the last phase of rocky mountain thrusting we expect that similar features will be observed elsewhere in the rocky mountain foothills and probably at other mountain fronts worldwide

a classification of thrust fronts variations in structural style at thrust fronts are examined to determine whether they represent different stages of typical thrust sheet development or unique events as orogenic stresses declined thrust fronts are classified as buried or emergent in which either the sole thrust died out in the subsurface or emerged to the synorogenic surface buried thrust fronts are subdivided into two types type 1 fronts end in broad zones of low strain type 2 fronts end in zones of high strain emergent thrust fronts may display significant horizontal displacement strongly emergent or limited displacements of about 1 km weakly emergent tip stick thrust fronts exhibit high strains and out of sequence thrusting different types of thrust fronts are linked by transitional tip li e anticlines and strike slip zones most thrust front types represent different stages of typical thrust sheet development but those in type 1 display atypical development unique to thrust fronts