sulfur isotopic studies of the dunka road cu ni deposit duluth complex minnesota the dunka road deposit is one of a series of cu ni occurrences located within troctolitic rocks of the southern portion of the duluth complex minnesota sulfide minerals principally pyrrhotite chalcopyrite cubanite and minor pentlandite occur interstitially with silicate minerals and average about 2 volume percent in ore grade horizons areas of sulfide concentrations in excess of 1 to 2 percent occur erratically throughout the troctolitic sequence but are in general concentrated near the basal contact with rocks of the metasedimentary virginia formation the delta 34 s values of sulfides in the troctolites range from 0 2 to 15 8 per mil with an average value near 7 5 per mil pyrrhotite in the underlying virginia formation hornfels and pyrite from unmetamorphosed virginia formation are characterized by a similar range in delta 34 s values the high positive delta 34 s values and similarity to values found in the virginia formation strongly suggest that most of the sulfur in the dunka road deposit was derived from the breakdown of pyrite in the virginia formation the sulfur isotopic compositions of minerals in the troctolites show no consistent lateral or vertical trends and are in general characterized by a high degree of variability such fluctuations in delta 34 s values indicate that sulfur isotopic homogenization was not attained throughout the troctolitic sequence several mechanisms may have contributed to the variations in delta 34 s values found within the troctolites including mixing of sedimentary and igneous sulfur assimilation of sedimentary sulfur characterized by variable delta 34 s values fractionation caused by fluctuations in oxidation states during sulfur volatilization and assimilation and periodic introduction of isotopically distinct magma batches in light of the erratic sulfide mineral and delta 34 s distributions the predominant mechanism is thought to be the introduction of both isotopically and spatially variable country rock sulfur into crystallizing troctolitic magma

variations in os isotope ratios of pyrrhotite as a result of water rock and magma rock interaction constraints from virginia formation duluth complex contact zones abstract os isotope ratios in pyrrhotite bearing pelitic rocks of the 1 85 ga virginia formation are variable with perturbations linked to the emplacement of the 1 1 ga duluth complex pyrrhotite in footwall rocks of the contact aureole show evidence for a mixing event at 1 1 ga involving a low 187 os 188 os fluid however because rocks with perturbed pyrrhotite os isotope ratios occur 1 km or more from the duluth complex the fluid is unlikely to have been of magmatic origin fluid inclusions in layer parallel quartz veins provide evidence of the involvement of a boiling fluid at temperatures between 300 and 400 c analyses of fluid inclusions via la icp ms show that the fluids contain up to 1 7 wt na 1 1 wt k 4330 ppm fe 2275 ppm zn and 415 ppm mg the veins also contain pyrite or pyrrhotite plus minor amounts of chalcopyrite bornite pentlandite and sphalerite the re os isotopic ratios of pyrite from the veins indicate that they crystallized from low 187 os 188 os fluids 18 o values of vein quartz range from 7 7 to 9 5 consistent with an origin involving fluid with a relatively low 18 o value between 2 and 5 meteoric water with such a low 18 o value could have interacted with the igneous rocks of the complex and would have acquired os with a low 187 os 188 os ratio strongly serpentinized olivine rich rocks of the complex are commonly characterized by such low 18 o values and we propose that the fluid involved in serpentinization was also responsible for the perturbation of the os isotopic system recorded by pyrrhotite in the virginia formation two important observations are that only pyrrhotite bearing assemblages in the contact aureole show isotopic perturbation and that intervals showing os exchange are spatially restricted and not uniformly distributed os exchange and mixing has occurred only where temperatures were sufficient to convert pyrite to pyrrhotite and where time integrated water rock ratios in the aureole were high enough to provide a supply of os troctolitic and gabbroic rocks of the partridge river intrusion duluth complex are characterized by os isotope ratios that are indicative of variable degrees of crustal contamination os values of 0 543 xenoliths of carbonaceous and sulfidic pelitic rocks of the virginia formation found in the igneous rocks provide evidence that os was released by organic matter and pyrite in the sedimentary rocks and assimilated by mantle derived magma however residual pyrrhotite produced as a result of pyrite breakdown in the xenoliths is characterized by 187 os 188 os ratios that are much lower than anticipated and similar to those of pyrrhotite in the contact aureole the os exchange and addition shown by pyrrhotite in the xenoliths highlight an unusual cycle of re os liberation during devolatilization kerogen maturation and pyrite to pyrrhotite conversion processes that contribute to magma contamination followed by os uptake by pyrrhotite during back reaction involving magma and or fluid characterized by a relatively low 187 os 188 os ratio the extreme os uptake recorded by pyrrhotite in the xenoliths as well as the lesser degree of uptake recorded by pyrrhotite in the contact aureole is in line with the high os diffusivity in pyrrhotite experimentally determined by brenan et al 2000 our data confirm that os isotope ratios in pyrrhotite bearing rocks may be readily perturbed for this reason caution should be exercised in the interpretation of os isotope ratios in rocks where pyrrhotite may be the primary host of os

os nd o and s isotope constraints on country rock contamination in the conduit related eagle cu ni pge deposit midcontinent rift system upper michigan abstract the eagle cu ni pge deposit is hosted by mafic to ultramafic intrusive rocks associated with the marquette baraga dike swarm in northern michigan sulfide mineralization formed in a conduit system during early stages in the development of the 1 1 ga midcontinent rift system the conduit environment represents a prime location for melt rock interaction in order to better assess the extent of country rock contamination in the eagle system a combined trace element nd os o and s isotope study of country rocks sulfide bearing igneous rocks and massive sulfide was undertaken both the eagle and the weakly mineralized east eagle intrusion show trace element patterns that are similar to those of picritic basalts that formed during early stages of rift development the trace element os nd and o isotopic values of the igneous rocks are consistent with os 1100 and e nd 1100 values for the magmas are 34 and 2 18 o values of pyroxene in feldspathic pyroxenite range from 6 5 to 6 6 and provide the only indication that bulk contamination may locally have exceeded 20 sulfur isotopic values of disseminated and massive sulfide in the eagle intrusion range from 0 3 to 4 6 the 34 s values are much lower than those that characterize most of the country rocks but could still be indicative of a contribution of s from country rocks of up to 50 33 s values of the disseminated and massive sulfides range from 0 10 to 0 09 indicating a source in paleoproterozoic country rocks semi massive sulfide in the eagle deposit has 34 s values between 2 2 and 5 3 and 33 s values show a broad range between 0 86 and 0 86 indicating a major contribution from an archean source isotopic data from the eagle deposit strongly indicate that multiple sources of sulfur were involved in the generation of the ni cu pge mineralization and magmas which traversed variable paths through the mantle and crust were focused and utilized the same conduit at the level of the eagle deposit our results emphasize the fact that the sulfur isotopic values of immediate country rocks may not be appropriate as end member values for mixing calculations sulfur derivation from deeper in the conduit system may have involved rocks with very different sulfur isotope ratios in addition sulfur isotopic exchange reactions between passing magma and accumulated crystals or sulfide liquid in the conduit may have led to decreased sulfur isotope variability and 34 s values near those of mantle sulfur

chemical interactions between a sedimentary diapir and surrounding magma evidence from the phepane dome and bushveld complex south africa the phepane dome is a circular outcrop of metasedimentary rock within the eastern lobe of the bushveld complex hypothesized to have formed as a diapir when underlying wallrock rose into the overlying magma body interactions between the metasedimentary rock of the phepane dome and the magma of the bushveld complex were investigated through measurements of oxygen and lithium isotopic compositions determination of mineral modes and major element mineral compositions cathodoluminescence imaging and dihedral angle analysis evidence from cathodoluminescence imaging and dihedral angle analysis suggest that heat transfer during diapirism caused partial melting and complete recrystallization of the phepane dome metasedimentary rock oxygen isotope analysis of samples from traverses spanning the contact between metasedimentary and igneous rocks demonstrates that relatively minimal exchange of oxygen over distance 4 m occurred across the contact between the phepane dome and the surrounding bushveld magma the lithium concentrations and isotopic compositions of metasedimentary rock are significantly different from the associated igneous rocks lithium isotope analysis of samples from traverses across the contact demonstrates exchange of li over somewhat greater distances 60 m than oxygen consistent with evidence that suggests a higher diffusivity of li than most major elements models of oxygen diffusion through intergranular melt and aqueous fluid are used to place maximum constraints on the duration of diffusive exchange across the contact resulting in estimates ranging from 5 kyr to 5 myr these values are consistent with previous estimates of the duration of crystallization of the bushveld complex and phepane diapir development

the midcontinent rift in the lake superior region with emphasis on its geodynamic evolution abstract the midcontinent rift is a middle proterozoic continental rift which records about 15 m y of extension subsidence and voluminous volcanism in the period 1109 1094 ma in the central part of north america during that time the crust was nearly totally separated and as much as 25 km of subaerial basalts accumulated in a deep central depression following extension and volcanism a longer period of subsidence resulted in development of a post rift sedimentary basin in which as much a 8 km of fluvial and lacustrine clastic rocks were deposited partial inversion of the central depression occurred about 30 50 m y after extension to produce the current configuration of a central horst composed mostly of thick volcanic accumulations between shallower flanking basins

an integrated chemical and stable isotope model of the origin of midocean ridge hot spring systems chemical and isotopic changes accompanying seawater basalt interaction in axial midocean ridge hydrothermal systems are modeled with the aid of chemical equilibria and mass transfer computer programs incorporating provision for addition and subtraction of a wide range of reactant and product minerals as well as cation and oxygen and hydrogen isotopic exchange equilibria the models involve stepwise introduction of fresh basalt into progressively modified seawater at discrete temperature intervals from 100 to 350 c with an overall water rock ratio of about 0 5 being constrained by an assumed 18 o h 2o at 350 c of 2 0 per mil h craig personal communication 1984 this is a realistic model because 1 the grade of hydrothermal metamorphism increases sharply downward in the oceanic crust 2 the water rock ratio is high 50 at low temperatures and low 0 5 at high temperatures and 3 it allows for back reaction of earlier formed minerals during the course of reaction progress the results closely match the major element chemistry von damm et al 1985 and isotopic compositions craig et al 1980 of the hydrothermal solutions presently emanating from vents at 21 n on the east pacific rise the calculated solution chemistry for example correctly predicts complete loss of mg and so 4 and substantial increases in si and fe however discrepancies exist in the predicted ph 5 5 versus 3 5 measured and state of saturation of the solution with respect to greenschist facies minerals the calculated d h 2o is 2 6 per mil in excellent agreement with analytical determinations the calculated chemical mineralogic and isotopic changes in the rocks are also in good accord with observations on altered basalts dredged from midocean ridges humphris and thompson 1978 stakes and o neil 1982 as well as with data from ophiolites gregory and taylor 1981 predicted alteration products include anhydrite and clay minerals at low temperatures and a typical albite epidote chlorite tremolite greenschist assemblage at 350 c the models demand that the major portion of the water rock interaction occur at temperatures of 300 350 c interaction at temperatures below approximately 250 c results in negative 18 o h 2o shifts contrary to the observed positive 18 o values of the fluids exiting at midocean ridge vents hydrogen isotope fractionation curves by suzuoki and epstein 1976 lambert and epstein 1980 and liu and epstein 1984 among others are compatible with the model and require d h 2o to increase at all temperatures as a result of seawater basalt interaction

the north american midcontinent rift beneath lake superior from glimpce seismic reflection profiling the midcontinent rift system is a 1 1 b y old structure extending from kansas through the lake superior region and into southern michigan the rift is filled with thick sequences of basaltic volcanic rocks and clastic sediments for most of its extent it is buried beneath paleozoic rocks but can be traced by its strong gravity and magnetic anomalies the rocks of the rift system are exposed only in the lake superior region and comprise the keweenawan supergroup much of the geology of the keweenawan is beneath lake superior and has only been inferred from potential field studies and seismic refraction studies and extrapolation from on shore geology seismic reflection surveys by the great lakes international multidisciplinary program on crustal evolution in 1986 imaged much of the deep structure of the rift beneath the lake in detail the reflection profiles across the rift reveal a deep asymmetrical central graben whose existence and magnitude was not previously documented they show that in addition to crustal sagging documented by previous investigations normal faulting played a major role in subsidence of the axial region of the rift a sequence of volcanic and sedimentary rocks in places greater than 30 km thick fills the graben thinner volcanic and sedimentary units lie on broad flanks of the rift outside of the graben near the axis the prerift crust is thinned to about one fourth of its original thickness apparently by a combination of low angle extensional faulting and ductile stretching or distributed shear the sense of asymmetry of the central graben changes along the trend of the rift documenting the segmented nature of the structure and suggesting the existence of accommodation zones between the segments the location of the accommodation zones is inferred from abrupt disruptions in the bouguer gravity signature of the rift uplift of the central graben occurred when the original graben bounding normal faults were reactivated as high angle reverse faults with throws of 5 km or more in places the midcontinent rift has some striking similarities to some younger passive continental margins we propose that it preserves a record of nearly complete continental separation which had it not been arrested would have created a middle proterozoic ocean basin

mechanisms and patterns of o and h isotopic exchange during hydrothermal alteration of the north shore volcanic group and related hypabyssal sills midcontinent rift system minnesota abstract hydrothermal flow systems developed in volcanic and interflow sedimentary rocks in response to the emplacement of 1 1ga mafic plutonic rocks during intracontinental rifting in the midcontinent region oxygen and hydrogen isotopic studies of the north shore volcanic group nsvg northeastern minnesota and related hypabyssal sills demonstrate the importance of both permeability and temperature in controlling isotopic exchange in the rift related hydrothermal system oxygen isotopic values of mineral separates and volcanic whole rocks range from 5 5 to 17 7 and show a strong correlation with whole rock water content and intensity of hydrothermal alteration in the shallow portion of the stratigraphic sequence elevated 18 o values and mineralogical alteration are confined principally to highly permeable amygdaloidal flow tops and bottoms massive flow interiors show little or no perturbation from 18 o values considered normal for fresh basaltic rocks with increased depth and temperature isotopic exchange becomes more pervasive 18 o values remain elevated and oxygen isotopic profiles show no difference between flow margins and interiors at depths where greenschist facies mineral assemblages are found flow margins show a reversal to 18 o values as low as 6 6 and d values decrease to 18 o and d values enhanced rates for hydrogen isotopic exchange relative to oxygen are suggested by whole rock d values that tend to be uniform throughout flows that are less than 7 m thick d profiles that parallel those of 18 o values are found in thicker flows the correlation between degree of hydrothermal alteration and 18 o values suggests that solution recrystallization was the primary mechanism of isotopic exchange in the hydrothermal system individual feldspar phenocrysts from a single lava flow may show a range of 18 o values from 10 5 to 17 7 and ranges in individual altered phenocrysts may be as large as 4 8 feldspar isotopic heterogeneity suggests that permeability varied on a grain to grain scale and that flow through microfractures was important in controlling isotopic variations coexisting plagioclase and pyroxene from hypabyssal sills show a very steep slope on a 18 o plagioclase 18 o pyroxene plot also suggestive of kinetically dominated exchange assuming a fluid buffered system in the permeable flow margins a duration of isotopic exchange on the order of 40 000 years is calculated this time period is short compared to the lifetime of the intrusive activity in the rift and suggests that fluid flow was either episodic in nature or confined to the late stages of magmatic evolution

mineralogic and oxygen isotopic studies of open system magmatie processes in the south kawishiwi intrusion spruce road area duluth complex minnesota the south kawishiwi intrusion located along the western margin of the duluth complex minnesota is one of several composite intrusions that are found in the complex the duluth complex is the principal exposed plutonic portion of the 1 1 ga midcontineni rift system in the spruce road area the south kawishiwi intrusion is divided into seven distinct units that are part of the broader south kawishiwi troctolite series defined by severson tech rep nrri tr 91 13a natural resources research institute university of minnesota duluth 1994 units may be characterized as follows unit i basal accumulation of heterogeneous gabbro troctolite and norite unit ii norite with abundant inverted pigeonite unit iii troctolite and olivine gabbro with local oxide rich layers unit iv melatroctolite troctolite olivine gabbro unit v increased plagioclase abundance in troctolites and leucocratic troctolites unit vi strongly altered troctolite unit vii similar to unit v troctolite and leucocratic troctolite country rocks in the spruce road area are granodiorite to quartz monzonite of the archean giants range batholith sutfide mineralization consisting of 1 5 vol of disseminated pyrrhotite cubanite chalcopyrite andpentlandite occurs in units i ii iii and vi oxygen isotopic analyses indicate that unit ii has experienced extensive crustal contamination 5o values of unit ii range from 69 to 7 l o and are 0 enriched compared with values of 5 1 6 8 o found in other units silica contamination is indicated based not only on 5 0 values but also by the predominance of orthopyroxene in the unit possible high 0 contaminant rocks include the giants range batholith and pelitic rocks of the lower proterozoic virginia formation or biwabik iron formation mass balance computations suggest that units in the spruce road area may be related through varying degrees offractionation of a high al olivine tholeiite magma modeling of trace element concentrations and variations in mineral chemistry suggest that discontinuities within the major units developed by in situ boundaryaayer equilibrium crystallization of solidification zones 20 50 m in thickness followed by recharge of fresh magma upward enrichment of incompatible elements olivine fa content and plagioclase ab content may be effectively explained by this process 5 0 values of uncontaminated rock types are strongly correlative with modal mineralogy and can also be modeled by boundary layer fractionation a parental magma 5 0 value of 6 3 o is calculated for unit vii based on olivine and plagioclase values and is similar to that of several other large layered mafic intrusives

stable isotopic studies of mafic sills and proterozoic metasedimentary rocks located beneath the duluth complex minnesota abstract thin 3 20 m picritic to noritic sills are found in the proterozoic metasedimentary footwall rocks of the duluth complex minnesota the sills were emplaced prior to the major intrusions that comprise the duluth complex and underwent contact metamorphism along with the country rocks oxygen hydrogen and sulfur isotopic compositions of the sills indicate a varied history of isotopic exchange between minerals melts and hydrothermal fluids in the high temperature environment below the major plutonic bodies of the midcontinent rift system the pre duluth complex sills exhibit a range in 18o values from 4 9 to 14 8 with values between 6 and 7 generally found in sill interiors high 18o values near sill contacts with high 18o metasedimentary rocks of the pelitic virginia formation or biwabik iron formation coupled with a smooth sigmoidal isotopic profile centered at the contact suggest that oxygen diffusion was an important exchange mechanism the elevated 18o values near the center of the thickest sills are thought to reflect the emplacement of isotopically contaminated basaltic magma dehydration reactions in the pelitic rocks of the contact aureole liberated high 18o fluids that enhanced subsolidus diffusive exchange advective displacement of the diffusion profiles toward the sill interior is less than 40 cm and suggests that layer parallel flow dominated in the dehydration of the contact aureole elevated but uniform 18o values 9 7 to 10 5 in thinner sills suggests that oxygen diffusivity was increased relative to country rocks due to enhanced porosity perhaps related to extensive development of microcracks although d values of the pelitic country rocks record a history of dehydration systematic variations of d 64 to 143 and h2o 0 15 to 5 40 wt content are not found in the sills 18o values of coexisting plagioclase and pyroxene from the sills indicate a close approach to isotopic equilibrium and are consistent with a diffusion dominant exchange process at temperatures near 500 c results of diffusion modelling suggest a duration of isotopic exchange that may have extended from tens of thousands of years to 1 4 ma depending on local controls of porosity and permeability as well as rates of fluid production in the contact aureole localized areas of 18o and d depletion in the sills values as low as 4 9 and 143 respectively denote exchange with meteoric water after interaction with the high 18o metamorphic fluids in the contact aureole although all of the elevated 18o samples in the contact environment may have suffered 18o depletion most exchange with meteoric water appears to be spatially localized and is thought to reflect highly channelized fracture controlled fluid flow sulfur isotopic values of the sills are variable 2 7 to 11 2 and indicative of an evolution involving pre emplacement contamination of basaltic magma and sub solidus exchange with an h2s bearing metamorphic fluid sulfur contents exceed 3 0 wt only within troctolitic to melatroctolitic sills and 34s values of 7 8 to 8 3 are strongly suggestive of pre emplacement contamination by sulfur derived from a proterozoic sedimentary unit high 18o and 34s rocks particularly at sill margins are consistent with either hydrothermal precipitation of fine grained sulfide minerals or isotopic exchange between magmatic sulfides and an h2s bearing metamorphic fluid

origin and history of hydrothermal fluids of the reykjanes and krafla geothermal fields iceland the reykjanes and krafla geothermal fields are both examples of active high temperature systems and show similar assemblages of alteration minerals but the fluid at reykjanes is dominantly sea water whereas that at krafla is meteoric oxygen isotope analyses of surface rock and of drill chip samples from different depths are presented together with results for the krafla fluid which is close to local precipitation 18o 11 9 d 86 8

oxygen isotope exchange and mineral alteration in gabbros of the lower layered series kap edvard holm complex east greenland multiple intrusions of gabbros mafic dikes and syenites in the kap edvard holm complex gave rise to prolonged circulation of meteoric hydrothermal solutions and extreme isotope exchange and mineral alteration in the 3600 m thick lower layered series gabbros in the lower layered series 18o of plagioclase varies from 0 3 to 5 8 and it decreases with an increase in the volume of secondary talc chlorite and actinolite in the same gabbros pyroxenes have a more restricted range in 18o from 5 0 to 3 8 and values of 18opyroxeneare independent of the abundance of secondary minerals which ranges from 14 to 30 these relations indicate that large amounts of water continued to flow through the rocks at temperatures of 500 600 c altering the gabbros to assemblages of talc chlorite actinolite epidote albite and causing significant oxygen isotope exchange in plagioclase but not in pyroxene the extensive low temperature secondary mineralization and 18o depletion of plagioclase in the lower layered series are associated with the later emplacement of dikes and gabbros and syenites which created new fracture systems and provided heat sources for hydrothermal fluid circulation this produced subsolidus mineral alteration and isotope exchange in the lower layered series that are distinct from those in the skaergaard and cuillin gabbros of the north atlantic tertiary province but are similar to those observed in some oceanic gabbros

hydrothermal alteration in the babbitt cu ni deposit duluth complex mineralogy and hydrogen isotope systematics hydrothermal alteration at the babbitt cu ni deposit of the duluth complex is widespread but highly variable in terms of intensity alteration styles range from the filling of microfractures in plagioclase and olivine or minor alteration along grain margins to more intense alteration in the form of macroscopic fracture fillings or massive replacement of primary minerals with or without well developed fractures chlorite occurs as a macroscopic fracture filling within microveinlets in plagioclase and as an alteration of pyroxene biotite and rarely olivine along grain margins or fractures mats of chlorite are intergrown with amphibole epidote calcite and prehnite as part of massive replacement of calcic plagioclase greenalite is found in areas of intense alteration particularly in the vicinity of sulfide mineralization iron rich serpentine occurs along fractures in olivine or as a more massive replacement of olivine in picritic units serpentine composition is related to that of the parent olivine as well as to the degree of serpentinization in addition to its involvement in plagioclase alteration calcic amphibole hornblende actinolite is found as a partial replacement of primocryst and interstitial clinopyroxene ca poor amphibole occurs as an alteration product of orthopyroxene the presence of the assemblage chlorite prehnite actinolite epidote suggests that alteration continued to temperatures less than 400 degrees c the delta d values and h 2 o contents of igneous rocks at babbitt range from 56 to 138 per mil and 0 49 and 4 60 wt percent respectively biotite separates are characterized by delta d values of 67 to 86 per mil and delta 18 o values of 4 2 to 6 1 per mil a range considered normal for biotite in mafic igneous rocks because most of the low temperature reactions between fluids and minerals in the cooling igneous system involved hydration h 2 o content is an approximate indicator of the extent of low temperature alteration rocks with h 2 o contents in excess of approximately 1 5 percent are characterized by delta d values greater than 87 per mil except for a sample of highly serpentinized picrite which has a delta d value of 120 per mil modeling of the isotopic composition of a fluid exsolved from interstitial melt indicates that for samples with minor alteration and h 2 o contents less than approximately 1 wt percent secondary mineral assemblages were probably produced as a d depleted magmatic vapor reacted with primary minerals along cooling fractures and grain margins areas of more intense alteration may have formed from localized accumulations of d enriched vapor released from underlying interstitial melt alternatively intensely altered zones may have formed due to the introduction of relatively d enriched fluid derived from devolatilization of metasedimentary xenoliths and footwall the latter interpretation is consistent with previously reported isotopic ripley and taib 1989 and fluid inclusion pasteris 1989 data both oxygen and hydrogen isotope compositions of one serpentinized picrite suggest that a low 18 o low d fluid of probable meteoric origin has been involved in the serpentinization of olivine rich units an insufficient number of samples have been analyzed to determine if a direct positive correlation exists between platinum group element pge content and degree of alteration in the babbitt deposit however the presence of discrete platinum group minerals in intensely altered zones suggests that most pt and pd remobilization occurred via transport by a d enriched fluid a positive correlation between whole rock delta d values and pd content for samples that contain less than 300 ppb pd suggests that magmatic fluids as well as high delta d fluids derived from metasedimentary country rocks may have been involved in local pt pd redistribution

hydrothermal flow systems in the midcontinent rift oxygen and hydrogen isotopic studies of the north shore volcanic group and related hypabyssal sills minnesota abstract rift related lavas of the north shore volcanic group nsvg are intruded by plutonic rocks of the duluth complex along the unconformity between the nsvg and the underlying proterozoic metasedimentary rocks animikie group and archean volcano sedimentary and plutonic rocks heat associated with the emplacement of the mafic intrusions generated fluid flow in the overlying plateau lavas 18o values for whole rocks from the nsvg and hypabyssal sills range from 5 5 to 17 7 and 5 3 to 11 5 respectively and most values are higher than those considered normal for basaltic rocks 5 4 to 6 0 in general there is a positive correlation between whole rock 18o and water content which suggests that elevated 18o values are related primarily to secondary mineral growth and isotopic exchange during hydrothermal alteration and metamorphism 18oh2o values computed from amygdule filling minerals such as smectite chlorite and epidote found in low to high temperature metamorphic zones range from 1 to 6 with an average value of 3 smectite in the lower grade zones gives computed dh2o values between 26 and 83 whereas epidote in the higher grade zones gives dh2o values of 15 to 6 fluid isotopic compositions computed from epidote and smectite values are suggestive of the involvement of at least two fluids during the early stages of amygdule filling fluid d and 18o values determined from epidote at the higher metamorphic grades indicate that seawater dominated the deeper portions of the system where greenschist facies assemblages and elevated 18o values were produced in flow interiors as well as margins smectite isotopic compositions suggest that meteoric water was predominant in the shallower portions of the system the increase in 18o values of massive flow interiors with depth is interpreted as a result of rock interaction with a fluid of constant oxygen isotopic composition with increasing temperature the stable isotopic data are supportive of previous suggestions that seawater was involved in the hydrothermal system associated with the midcontinent rift although the origin of the seawater remains problematic it appears that marine incursions may have occurred during the late stages of portage lake volcanism and periodically thereafter

macroseismic intensity versus epicentral distance the case of central italy this paper refers to the attenuation of macroseismic intensity with epicentral distance a new attenuation law is discussed with particular reference to italian conditions the new formula shows a much better fit compared to traditional attenuation laws a numerical example indicates that a proper choice of the attenuation law is a crucial step in local hazard evaluation

sulfur and oxygen isotope studies of the interaction between pelitic xenoliths and basaltic magma at the babbitt and serpentine cu ni deposits duluth complex minnesota sulfur and oxygen isotope compositions of two pelitic virginia formation xenoliths and surrounding igneous rocks have been measured in drill core intersects at the babbitt and serpentine cu ni deposits duluth complex the xenolith at the babbitt deposit xenolith 1 shows compositional and textural evidence for partial melting whereas the xenolith at the serpentine deposit xenolith 1 may still be in part connected to the footwall and primary sedimentary mineral assemblages are present in its center the delta 34 s values of the xenoliths range from 6 to 27 per mil a well developed vertical profile of upward increasing delta 34 s values in xenolith 2 is suggestive of initial pyrite formation due to bacterial reduction of sulfate in a sedimentary environment where the rate of sulfate reduction was rapid relative to sulfate supply the extreme variation in delta 34 s values over a vertical interval of less than 20 m illustrates the potential variability of delta 34 s values in the virginia formation the delta 34 s values of magmatic sulfides that formed in response to s assimilation may be equally variable depending upon local sites of contamination and convective homogenization in the magma in situ conservation of sulfur is suggested for xenolith 2 where the fe mg ratio of biotite decreases with increasing bulk rock sulfur content sulfidation reactions involving iron bearing silicates or oxide minerals limited the amount of sulfide liberated by the xenolith assimilation of country rock sulfide occurred either at depth within crustal staging chambers or during magma ascent an oxygen isotope gradient is well developed at the upper contact between gabbronorite norite and xenolith 2 oxygen isotope exchange fronts are poorly developed at the lower contact of xenolith 2 and at both contacts of xenolith 1 at the babbitt deposit the igneous rock at the lower contact is a compositionally distinct troctolite that shows little evidence for country rock contamination intrusion of distinct thin magmatic pulses that cool rapidly after previous devolatilization and possible partial melting of the xenoliths can cause the obliteration of well developed isotopic profiles the lack of concentration gradients that coincide with the oxygen isotope gradient together with results of modeling of mass transfer by diffusive processes suggests that the observed delta 18 o profile was produced during subsolidus cooling the observed profile may be produced by oxygen isotope exchange via fluid assisted grain boundary diffusion within 500 000 yr a duration consistent with the 0 5 to 1 m y time span estimated for emplacement of the major intrusions of the duluth complex

the midcontinent rift system one of the most prominent features on gravity and aeromagnetic maps of the united states is a series of major generally linear anomalies that extend from central kansas to lake superior and then turn southward into central michigan figures 1 2 this system of anomalies clearly reflects major geological features and as such it must represent a very significant episode in the history of the north american continental lithosphere the geologic features associated with these anomalies particularly axial basins filled with basalt and immature clastic rocks along with evidence of crustal extension indicate that the episode was probably one of incipient rifting

sulfur and oxygen isotope studies of melt country rock interaction babbitt cu ni deposit duluth complex minnesota the babbitt cu ni deposit formerly known as minnamax occurs within troctolitic and gabbroic rocks of the precambrian duluth complex both massive and disseminated mineralizations are found in the deposit pyrrhotite cubanite chalcopyrite and minor pentlandite mineralization occurs near the basal contact with pelitic metasedimentary rocks of the virginia formation or higher in the igneous sequence associated with abundant country rock xenoliths cloud zones sulfur and oxygen isotope studies have been carried out to evaluate the importance of melt country rock interaction in localizing sulfide mineralization the delta 34 s values of pyrrhotite plus pyrite from the virginia formation located outside the contact aureole range from 5 6 to 13 7 per mil pyrrhotite from footwall virginia formation hornfels or inclusions in the basal zone exhibits delta 34 s values ranging from 1 5 to 6 2 per mil whereas delta 34 s values of pyrrhotite from xenoliths in cloud zones range from 2 6 to 21 4 per mil isotopic values of basal zone cu ni sulfides range from 6 0 to 13 0 per mil with a mode at 8 to 9 per mil two features highlight basal zone delta 34 s values first delta 34 s values of sulfide mineralization are higher than those of pyrrhotite in immediately adjacent virginia formation hornfels second higher grade ore areas 1 cu show 34 s enrichment with delta 34 s values in the range of 9 to 13 per mil sulfides in cloud zones show a wide range in delta 34 s values from 0 7 to 19 7 per mil sulfur isotope data suggest that most of the sulfur found in the deposit has been derived from sedimentary country rocks however an in situ process of sulfur derivation is not indicated sulfur in basal mineralized zones must have been derived at depth within an auxiliary magma chamber or during magma ascent and emplacement isotopic homogenization was not achieved during the sulfide forming process as evidenced by the spatial variability of delta 34 s values oxygen isotope values of both mildly metamorphosed virginia formation and footwall hornfels range from 8 2 to 14 0 per mil inclusions of virginia formation show pronounced isotopic effects due to interaction with melt xenolith margins may have values as low as 7 to 8 per mil and are depleted in 18 o with respect to xenolith interiors the delta 18 o values of igneous rocks from the babbitt deposit range from 3 9 to 12 2 per mil zones of 18 o depletion are rare and suggest that exchange with a low 18 o fluid phase was spatially restricted anomalous delta 18 o values in excess of 7 to 7 5 per mil are common throughout the deposit but are limited to within 3 m of virginia formation xenoliths a model of diffusion controlled 18 exchange between igneous material and xenoliths best describes the delta 18 o distribution devolatilization and desilicification of the xenoliths occurred early in their history when physical mixing in the host magma was possible addition of high 18 o material from these processes into a large volume of melt affected delta 18 o distributions only slightly continued exchange of oxygen after major cation transfer produced the observed delta 18 o profiles in the case of basal zone cu ni mineralization at babbitt isotopic evidence indicates that sulfide saturation was not caused by in situ addition of sulfur silica or oxygen sulfide saturation is thought to have been achieved at depth triggered by assimilation of large quantities of sulfide liberated through devolatilization reactions occurring in country rocks

geologic and geochemical evidence for the nature and development of the middle proterozoic keweenawan midcontinent rift of north america abstract general models of active continental rifting must take into account the large plateau basalt provinces many of which have been precursors of or associated with continental breakup the keweenawan middle proterozoic midcontinent rift mcr of north america 2300 km long is one such major continental rift but it aborted before significant crustal separation was achieved these plateau basalt rifts including the mcr differ from the east african type in being dominated by broad subsidence rather than rift valleys tholeiitic flood basalts rather than alkalic central volcanoes and large positive gravity anomalies geologic and geophysical evidence is reviewed which contradicts some recent models which have been proposed for the development of the mcr the basic causes of the pre rift subsidence the u shaped nature of the structure and the abrupt termination of its activity are obscure a triple junction in the usual sense does not appear to have been formed

petrogenesis of pelitic xenoliths at the babbitt cuni deposit duluth complex minnesota u s a abstract the babbitt deposit consists of disseminated cu fe ni sulfides found within mafic rocks of the duluth complex generally near contacts with underlying metasedimentary rock types host rocks for the deposit include troctolites olivine gabbros gabbronorites norites and occasionally country rock hornfels xenoliths of country rocks are abundant in the deposit and suggest a relationship between sulfide mineralization and country rock contamination country rocks in the babbitt area include those of the middle precambrian biwabik iron formation and both calcareous and non calcareous pelites of the virginia formation xenoliths contain the assemblage cordierite plagioclase biotite orthopyroxene and are thought to have been derived from virginia formation protoliths comparison of protoliths and xenoliths using composition volume element ratio and mass balance techniques suggests that xenoliths have been strongly depleted in volatiles alkalis and si footwall rocks show only a depletion in volatiles neither fluid phase transport nor diffusion through an intergranular fluid can account for the mass of material transferred extensive partial melting of xenoliths with residual enrichment of feo mgo and al2o3 is the most viable transfer process the lack of sio2 concentration gradients around xenoliths and anomalous igneous rock compositions suggest that desilicification occurred at a time when physical mixing of extracted partial melt and host magma was possible and prior to final emplacement sulfide saturation may have been initiated due to si assimilation with an auxiliary magma chamber however the composition of ores in the babbitt deposit is consistent with saturation being achieved by addition of sediment derived volatile sulfur independent of major element assimilation

an oxygen isotopic profile through the upper kilometer of the oceanic crust dsdp hole 504b dsdp hole 504b is the deepest basement hole in the oceanic crust penetrating through a 571 5 m pillow section a 209 m lithologic transition zone and 295 m into a sheeted dike complex an oxygen isotopic profile through the upper crust at site 504 is similar to that in many ophiolite complexes where the extrusive section is enriched in18o relative to unaltered basalts and the dike section is variably depleted and enriched basalts in the pillow section at site 504 have 18o values generally ranging from 6 1 to 8 5 smow mean 7 0 although minor zeolite rich samples range up to 12 7 rocks depleted in18o appear abruptly at 624 m sub basement in the lithologic transition from 100 pillows to 100 dikes coinciding with the appearance of greenschist facies minerals in the rocks whole rock values range to as low as 3 6 but the mean values for the lithologic transition zone and dike section are 5 8 and 5 4 respectively oxygen and carbon isotopic data for secondary vein minerals combined with the whole rock data provide evidence for the former presence of two distinct circulation systems separated by a relatively sharp boundary at the top of the lithologic transition zone the pillow section reacted with seawater at low temperatures near 0 c up to a maximum of around 150 c and relatively high water rock mass ratios 10 100 water rock ratios were greater and conditions were more oxidizing during submarine weathering of the uppermost 320 m than deeper in the pillow section the transition zone and dikes were altered at much higher temperatures up to about 350 c and generally low water rock mass ratios 1 and hydrothermal fluids probably contained mantle derived co2 mixing of axial hydrothermal fluids upwelling through the dike section with cooler seawater circulating in the overlying pillow section resulted in a steep temperature gradient 2 5 c m across a 70 m interval at the top of the lithologic transition zone progressive reaction during axial hydrothermal metamorphism and later off axis alteration led to the formation of albite and ca zeolite rich alteration halos around fractures this enhanced the effects of cooling and18o enrichment of fluids resulting in local increases in 18o of rocks which had been previously depleted in18o during prior axial metamorphism

tectonic setting of the midcontinent rift system abstract the continental lithosphere which hosts the midcontinent rift mcr system comprises a crustal assemblage ranging in age from 3 6 to 1 5 ga the most voluminous igneous activity along the mcr occurred within late archean 2 7 ga crust presumably due to magmatism associated with a rising mantle plume two major arms of the rift extend to the southeast and the southwest where rifting magmatism and sedimentation were confined to a 2000 km long rift that cuts through the various crustal provinces ranging in age from 3 6 to 1 5 ga tensional reactivation of pre existing crustal discontinuities may have influenced the location of parts of the mcr however direct cause and effect are difficult to distinguish for want of sufficient exposures the most active phases of magmatism and rifting occurred between 1109 and 1087 ma a u pb age of 1097 ma for the southern end of the mcr shows that magmatic activity was synchronous throughout its length there is evidence of rifting and magmatism prior to 1109 ma but there is little evidence of magmatism after 1087 ma rifting may have effectively ceased by that time rifting may have ceased as a result of compressional forces being exerted on the foreland during the grenville collisional event spanning 1030 970 ma and compressional reactivation of normal faults may have resulted in partial inversion of the mcr chemical data and u pb and sm nd isotopic data for igneous rocks along the rift are similar to those for mcr rocks in the lake superior area which are best explained in terms of a mantle plume model however since the distal arms of the mcr were developed in proterozoic lithosphere and did not involve nearly as much magmatism as in the lake superior area mantle plume sources may not have been as important in formation of the distal magmas data from the buried arms are also consistent with the mafic magmas being derived by decompression melting of proterozoic lithosphere during rifting

crustal structure of the midcontinent rift system results from glimpce deep seismic reflection profiles interpretation of great lakes international multidisciplinary program on crustal evolution glimpce seismic reflection profiles indicates that the midcontinent keweenawan 1100 ma rift system of volcanic rocks and postvolcanic and interbedded sedimentary rocks extends to depths as great as 32 km about 10 5 s reflection time along profiles crossing western central and eastern lake superior and the northern end of lake michigan this area may overlie the greatest thickness of intracratonic rift deposits on earth times to moho reflections vary along strike from 11 5 to 14 s about 37 46 km depth in the west to 17 s about 55 km in the center and 13 to 15 s about 42 49 km in the eastern end of lake superior the prerift crust however was thinned 25 30 km beneath the central rift compared with its flanks providing evidence for crustal extension by factors of about 3 4 the midcontinent rift system differs from phanerozoic rifts in having total crustal thicknesses equal to or greater than the surrounding presumably unextended regions

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

keweenaw hot spot geophysical evidence for a 1 1 ga mantle plume beneath the midcontinent rift system the proterozoic midcontinent rift system of north america is remarkably similar to phanerozoic rifted continental margins and flood basalt provinces like the younger analogues the volcanism within this older rift can be explained by decompression melting and rapid extrusion of igneous material during lithospheric extension above a broad asthenospheric thermal anomaly which the authors call the keweenaw hot spot great lakes international multidisciplinary program on crustal evolution seismic reflection profiles constrain end member models of melt thickness and stretching factors which yield an inferred mantle potential temperature of 1 500 1 570c during rifting combined gravity modeling and subsidence calculations are consistent with stretching factors that reached 3 or 4 before rifting ceased and much of the lower crust beneath the rift consists of relatively high density intruded or underplated synrift igneous material the isotopic signature of keweenawan volcanic rocks presented in a companion paper by nicholson and shirey this issue is consistent with the model of passive rifting above an asthenospheric mantle plume

the quantification of breakwater armour profiles for design purposes abstract a new approach for the evaluation of block shape effects in both artificial and rock armour layers is proposed topographic profiles based on high resolution sampling frequencies of between 10 and 20 heights per nominal armour block width are considered model surfaces of a glass spheres and b rock aggregate were prepared and examined by topographic profile sampling a preliminary set of 15 statistical parameters or roughness descriptors were evaluated for each of 10 sets of profile height data a statistical comparison between the surfaces of models a and b indicates the potential of roughness descriptors such as mean peak curvature and peak density to discriminate between surfaces generated from blocks of different shapes filtering of height data for discrimination enhancement is also illustrated nomenclature is introduced to distinguish the planar orientations of the principal profiles of the breakwater the ac plane cross cuts the long axis and is the plane observed in the well known studies of s shaped profiles the bc and bd planes parallel the axis and are the planes proposed for analysis it is argued that the new monitoring method of topographic profile sampling and roughness descriptor analysis provides a promising additional means of assessing the relationship between block shape and hydraulic stability for model and prototype armour layers the importance of further development and testing of these techniques for the quantitative evaluation of surface geometry is stressed in view of the many possible future applications of these methods

re os isotopic variations in carbonaceous pelites hosting the duluth complex implications for metamorphic and metasomatic processes associated with mafic magma chambers abstract previous mineralogic and isotopic studies of sulfide mineralization in the 1 1 ga duluth complex have suggested that sulfur and possibly metals have been derived from proterozoic sedimentary country rocks the re os isotopic system offers great potential in the evaluation of crustally derived metals in magmatic ore forming processes but an understanding of the mechanisms that could cause isotopic variations in country rock contaminants is a prerequisite a suite of sulfidic and carbonaceous pelitic rocks of the virginia formation country rock to the duluth complex ranging from unmetamorphosed through strongly recrystallized and partially melted xenoliths have been analyzed for their os isotopic composition sulfide mineral separates mainly pyrrhotite kerogen graphite isolates and whole rocks were analyzed whole rocks kerogen and graphite from metamorphosed samples plot between 1 85 ga and 1 1 ga chondritic reference isochrons sulfide separates from all metamorphosed samples plot nearer the chondritic 1 1 ga reference isochron and require exchange with a fluid or melt characterized by a chondritic os isotopic composition potentially derived from the duluth complex devolatilization and partial melting may perturb isotopic systematics when accompanied by the loss of re and os bearing fluids these processes could in part explain the scatter in isotopic values found in kerogen and metamorphosed whole rocks however metasomatic addition of os via interaction with a fluid or melt is required to explain the sulfide isotopic ratios and is consistent with sulfide minerals being more susceptible to os exchange with a fluid than kerogen the relative ease with which pyrrhotite may be reset during fluid interaction as well as the variable degrees of exchange shown by kerogen graphite suggest that a knowledge of the petrologic and isotopic evaluation of country rocks is necessary before a meaningful assessment of the isotopic variations found in igneous rocks or associated ore deposits can be undertaken

metamorphic grade and porosity and permeability controls on mafic phyllosilicate distributions in a regional zeolite to greenschist facies transition of the north shore volcanic group minnesota the 8 km thick keweenawan metabasaltic north shore volcanic group in minnesota shows a continuous zeolite to greenschist facies progression through five zonal divisions mafic phyllosilicates are ubiquitous and a well developed smectite corrensite chlorite transition is observed this sequence provides an excellent framework in which the influence of regional thermal as opposed to local scale lithology porosity and permeability factors on this transition factors that are actively debated can be comprehensively documented on a regional scale at lowest grades zones 1 and 2 thomsonite scolecite and heulandite stilbite assemblages respectively tri smectitic phyllosilicates dominate and with increase in grade to zone 3 laumontite albite there is a marked step and compositional gap to corrensite with further increase in grade to zones 4 laumontite albite prehnite pumpellyite and 5 epidote albite actinolite chlorite becomes dominant although a spread in the x ray diffraction and microprobe data suggests that the corrensite to chlorite transition is continuous rather than stepwise analytically distinct corrensite and chlorite in one flow suggest a clear compositional gap between these two phases overall the data offer some support for a discontinuous smectite corrensite chlorite transition above the lowest grade zones 1 and 2 approximately 150 c the phyllosilicates vary on a local within flow scale related to morphological contrasts in flow lithology smectite is dominant in the massive flow units low porosity and permeability whereas corrensite or chlorite is dominant in the amygdaloidal flow tops high porosity and permeability these changes provide clear indication that once above a threshold of approximately 150 c fluid rock ratios have a strong control on the metamorphic process at low grade similarities in xmgo between smectite corrensite chlorite and xmgo of the whole rock show that even though smectite is a relict phase remaining in massive zones it continues to undergo crystal chemical change with increasing grade however contrasts between aliv in the same minerals are provisional indicators that other factors perhaps related to kinetic features linked to advective and diffusive modes of fluid transport may be involved

regional and local patterns of low grade metamorphism in the north shore volcanic group minnesota usa the north shore volcanic group in northern minnesota is part of the middle proterozoic keweenawan sequence one of the largest plateau lava provinces in the world the primary geochemistry of the basalts suggests that volcanism occurred in an intracontinental rift environment the subaerial lava flows mainly amygdaloidal olivine tholeiites and tholeiites have undergone low grade metamorphism from zeolite to lower greenschist facies on the basis of alteration phases replacing the primary magmatic minerals infilling amygdales and veins and replacing secondary minerals the following zones have been distinguished 1 thomsonite scolecite smectite 2 heulandite stilbite smectite 3 laumontitechlorite albite 4 laumontite chlorite albite prehnite pumpellyite and 5 epidote chlorite albite actinolite zone in addition to the overall zonation based on mineral parageneses zonations in the composition of the ab content of the newly formed albite replacing primary ca rich plagioclase and of the newly formed mafic phyllosilicates are observed within the sequence and within single flows mafic phyllosilicates in the upper part of the sequence mainly smectites and mixed layer smectite chlorites display high si and ca na k contents whereas in the lower part of the sequence the amounts of si and ca na k are markedly lower mainly chlorites and mixed layer chlorite smectites similar zonations are observed within the individual flows the albite content of the newly formed plagioclase is highest and the si and ca na k content of the phyllosilicates lowest in the amygdaloidal flow top while the opposite is true for the massive flow interior the above features suggest that the overall pattern is one of burial type metamorphism associated with extension in the rift setting in detail the mineral assemblages are controlled not only by the stratigraphic position but also by the flow morphology controlling permeability whose effect on the assemblages is most pronounced in the stratigraphically upper parts this suggests that at the first stages of alteration lowest grade the patterns of fluid flow were important effects in controlling the assemblages at greater burial depth assemblages are more homogeneous perhaps representative of a more even and pervasive flow pattern using the observed assemblages at face value to define grade and or facies different conditions would be assigned within the different morphological flow portions thus at low grade metamorphic conditions it is essential to integrate assemblages from different morphological flow portions in order to define satisfactorily the overall metamorphic conditions