experimental reactions of amphibolite with boron bearing aqueous fluids at 200 mpa implications for tourmaline stability and partial melting in mafic rocks reactions between hornblende plagioclase amphibolite and acidic and alkaline b bearing aqueous fluids have been investigated by experiments at 475 600 c and 200 mpa at 600 c hornblende calcic plagioclase react to form tourmaline danburite clinopyroxene quartz in acidic fluids containing 0 5 1 0 wt b2o3 tourmaline is precipitated directly from acidic fluids and the reaction is driven by neutralization of fluids by na ca derived from the breakdown of reactant solids the concentration of b2o3 in fluids needed to stabilize tourmaline increases as ph increases above approximately 6 0 and tourmaline is unstable in alkaline fluids ph approximately 6 5 7 0 regardless of b concentration in addition to acid base relations tourmaline stability is favored by comparatively higher activity coefficients for al species in acidic fluids the concentrations of al and si in fluid increase with alkalinity with the eventual production of felsic borosilicate melts through partial melting of the plagioclase component of the amphibolite in seeded experiments tourmaline also contributes components to melt partial melting is evident in the range 500 525 c at 200 mpa in experiments with 8wt b2o3 in fluid as na2b4o7 the experimental results are applied primarily to metasomatic reactions between mafic rocks and borate fluids derived from granitic magmas but tourmaline stability and partial melting in mafic regional metamorphic systems are also discussed briefly

corundum cr muscovite rocks at o briens zimbabwe the conjunction of hydrothermal desilicification and lil element enrichment geochemical and isotopic evidence monomineralic domains of chlorite corundum and cr muscovite coexist over a kilometer scale within ultramafic schists of the harare greenstone belt 2 73 ga this exotic lithological association includes the conjunction of some of the most aluminous al2o3 88 wt and potassic k2o 10 wt rocks known the paragenetic sequence developed from chlorite corundum corundum diaspore cr muscovite variably overprinted both the corundum and chloritite domains terminal stages were marked by sporadic production of andalusite quartz and finally margarite

montana seismic network performance an evaluation through the snes method in this paper we analyze the location performance of the montana regional seismic network mrsn using the seismic network evaluation through simulation snes method montana has a high level of seismicity that includes approximately 1500 locatable earthquakes annually the mrsn comprises 38 stations deployed over an area of approximately 50 000 km2 the application of the snes method permits us to evaluate the background noise levels of the network stations and estimate an empirical law that links the variance of p and s travel time residuals to hypocentral distance this in turn permits us to assess the appropriateness of the velocity model used by the mrsn in the location routine we constructed snes maps for m l 1 4 1 6 1 8 2 0 and 2 2 fixing the hypocentral depth at 10 km and at the 95 confidence level through application of the snes method we show that mrsn provides the best monitoring coverage in the flathead valley of northwestern montana with errors for m l 2 that are less than 2 and 6 km for epicenter and hypocentral depth respectively at magnitude 2 2 this seismic network is capable of locating earthquakes as deep as 150 km and provides a threshold of completeness down to magnitude 1 5 for most of western montana we delineate some seismogenic areas of western montana including the central portion of the centennial tectonic belt in extreme southwestern montana that are not adequately covered by the mrsn alone

gold mineralization in the mazowe area harare bindura shamva greenstone belt zimbabwe i tectonic controls on mineralization the mazowe group of mines principal mines mazowe bernheim and stori s golden shaft is situated within the harare bindura shamva greenstone belt of the zimbabwean archaean craton west of the chinamora batholith gold mineralization in the form of quartz sulfide reefs is structurally controlled in reverse shear zones that dip moderately north at mazowe mine and conjugate steep strike slip shear zones striking wnw dextral and ne sinistral at bernheim and stori s golden shaft mines the syn mineralization deformation d2 3 in all the mines has a northerly shortening direction this deformation is compatible with the regional late archaean d2 3 event which agrees with late archaean ages determined for the host rocks and for the mineralization the mineralization cannot be related to any major regional scale shear zones and it is incompatible with strains related to the intrusion of either the chinamora batholith or internal granitoid suites these observations show that significant gold deposits can form in relatively minor deformation events unrelated to either major shear zones or granitoid intrusions

gold mineralization in the mazowe area harare bindura shamva greenstone belt zimbabwe ii genetic relationships deduced from mineralogical fluid inclusion and stable isotope studies and the sm nd isotopic composition of scheelites in the mazowe area some 40 km nw of harare in zimbabwe gold mineralization is hosted in a variety of lithologies of the archean harare bindura shamva greenstone belt in structures related to the late archean regional d2 3 event conspicuous mineralzogical differences exist between the mines the mainly granodiorite hosted workings at mazowe mine are on pyrite rich reefs mines of the bernheim group have metabasalt host rocks and are characterized by arsenopyrite rich ores and stori s golden shaft and alice mine both in metabasalts work sulfide poor quartz veins in contrast to the mineralogical diversity near identical fluid inventories were found at the different mines both h2o co2 ch4 fluids of low salinity and highly saline fluids are present and are regarded to indicate fluid mixing during the formation of the deposits notably these fluid compositions in the mazowe gold field markedly contrast to ore fluids typical of archean mesothermal gold deposits on other cratons stable isotope compositions of quartz from the various deposits 18o 10 8 to 13 2 smow calcite 18o 9 5 to 11 9 smow and 13c 3 2 to 8 0 pdb inclusion water d 28 to 40 smow and sulfides 34s 1 3 to 3 2 cdt are uniform within the range typical for archean lode gold deposits worldwide the fluid and stable isotope compositions support the statement that the mineralization in the mazowe gold field formed from relatively reduced fluids with a metamorphic signature during a single event of gold mineralization microthermometric data further indicate that the deposits formed in the pt range of 1 65 2 3 kbar and 250 380 c ages obtained by using the sm nd and rb sr isotope systems on scheelites are 2604 84 ma for the mineralization at stori s golden shaft mine and 2 40 0 20 ga for mazowe mine the archean age at stori s is regarded as close to the true age of gold mineralization in the area whereas the proterozoic age at mazowe mine probably reflects later resetting

isotopic and geochemical evidence for crust mantle interaction during late archaean crustal growth abstract the charnockites and enderbites of the north marginal zone of the limpopo belt in southern zimbabwe represent a suite of dry magmas of tonalite trondhjemite granodiorite ttg affinity which crystallised directly in the lower crust they were emplaced between 2 6 and 2 7 ga and were derived from a source which showed remarkable lead isotope homogeneity unlike many other lower crustal rocks they have low k rb and low k u ratios and have not experienced depletion of their heat producing elements th u k and rb and thus represent an unusual suite of granulites in this study we have combined the constraints of trace element and isotope geochemistry to characterise the nature of the protolith to the nmz enderbites and charnockites we use a combination of high values calculated lead and strontium isotope initial ratios and old nd model ages to show that the enderbites and chamockites were derived from a protolith which was in part formed more than 500 ma before magma genesis we argue from the very high values that this protolith constituted two separate components an old high low sm nd crustal component and a younger low high sm nd mantle derived component both of which were homogenised by extensive mixing prior to 2 7 ga the presence of an older crustal component contrasts with other archaean ttg suites worldwide which show little evidence for a crustal prehistory the process of mantle crust interaction during late archaean crustal growth in southern zimbabwe invites comparison with modern convergent margins such as the andes

the 1983 borah peak idaho earthquake and its aftershocks abstract the 28 october 1983 borah peak idaho earthquake m s 7 3 occurred in an area of low historic seismicity within east central idaho along a segment of the lost river fault active during the holocene a dense network of portable short period seismographs up to 45 stations station spacings of 2 to 10 km was installed beginning several hours after the main shock and operated for 22 days in addition to records from the portable instrumentation data from permanent seismograph stations operating in idaho utah montana oregon washington and wyoming provide a good regional data base no foreshock activity above m c coda magnitude 2 0 was detected for the 2 month period preceding the main shock the epicenter of the main shock is 14 km south southwest of the end of the surface faulting this relationship suggests unilateral rupture propagating to the northwest the distribution of 421 aftershocks of m c 2 defines an epicentral pattern 75 km 10 km trending north northwest parallel to the surface rupture but displaced laterally southwest by 5 to 10 km aftershocks extend to depths of approximately 16 km and in the central and southeastern portion of the aftershock pattern define a zone dipping approximately 45 southwest that intersects the surface near the fault scarp the entire aftershock zone as observed during the first 3 weeks was active shortly after the main shock occurred fault plane solutions for 47 aftershocks show predominantly normal faulting with varying amounts of strike slip motion tension axis orientations indicate a dominant extension direction of nne ssw during the aftershock sequence there is considerable diversity among the aftershock focal mechanisms even along the central and southeast portions of the fault where the hypocenters appear to outline the main fault break we therefore interpret most of the aftershocks to represent complex fracturing on subsidiary structures adjacent to the main fault

metamorphic evolution of kyanite staurolite bearing epidote amphibolite from the early palaeozoic oeyama belt sw japan early palaeozoic kyanite staurolite bearing epidote amphibolites including foliated epidote amphi bolite fea and nonfoliated leucocratic or melanocratic metagabbros lmg mmg occur in the fuko pass metacumulate unit fpm of the oeyama belt sw japan microtextural relationships and mineral chemistry define three metamorphic stages relict granulite facies metamorphism m1 high p hp epidote amphibolite facies metamorphism m2 and retrogression m3 m1 is preserved as relict al rich diopside up to 8 5 wt al2o3 and pseudomorphs after spinel and plagioclase in the mmg suggesting a medium p granulite facies condition 0 8 1 3 gpa at 850 c an unusually low variance m2 assemblage hbl czo ky st pg rt ab crn occurs in the matrix of all rock types the presence of relict plagioclase inclusions in m2 kyanite associated with clinozoisite indicates a hydration reaction to form the kyanite bearing m2 assemblage during cooling the corundum bearing phase equilibria constrain a qualitative metamorphic p t condition of 1 1 1 9 gpa at 550 800 c for m2 the m2 minerals were locally replaced by m3 margarite paragonite plagioclase and or chlorite the breakdown of m2 kyanite to produce the m3 assemblage at 0 5 gpa and 450 500 c suggests a greenschist facies overprint during decompression the p t evolution of the fpmmay represent subduction of an oceanic plateau with a granulite facies lower crust and subsequent exhumation in a pacific type orogen

crustal structure and thickness along the yellowstone hot spot track evidence for lower crustal outflow from beneath the eastern snake river plain 1 receiver functions from seismic stations about the yellowstone hot spot track are migrated to depth using a vp vs map constructed from stacking of the direct and free surface moho reverberations i e h k analysis and a shear velocity tomogram constructed from surface wave measurements the thickest crust 48 54 km resides in the wyoming province beneath the sampled laramide age blocks and the thinnest crust 32 37 km resides in the montana basin and range province the eastern snake river plain esrp crust is thickest 47 km at its ne end beneath the young calderas and thinnest 40 km at its sw end beneath the older twin falls caldera two esrp crustal thickness domains are found 1 at the older twin falls and picabo calderas the mean esrp crust is 4 km thicker with respect to its margins and 2 adjacent to the heise caldera field the mean esrp crust is 4 km thicker with respect to its se margin crust but no thicker with respect to its nw margin crust this lobe of anomalously thick crust is explained as resulting from lower crustal outflow from beneath the heise caldera field confirmation of these crustal thickness variations is provided by inspection of common conversion point ccp stacks that delineate several secondary features the top of a thick high velocity 3 9 km s lower crust layer within the wyoming province up to 17 km thick and a paired negative and positive amplitude arrival at 12 km depth and 18 km depth beneath the yellowstone caldera this paired arrival would be consistent with a low velocity zone perhaps associated with magma staging beneath the caldera our most important finding is that the magmatic loads injected into the esrp crust over the last 4 12 myr in tandem with the esrp crustal viscosity structure have been sufficient to drive significant outflow of the esrp lower crust

mass transfer along the yellowstone hotspot track ii kinematic constraints on the volume of mantle derived magma abstract this paper evaluates the neogene kinematic evolution of the eastern snake river plain esrp to estimate the volume of mafic magma transferred from mantle to crust during passage over the yellowstone hotspot four major kinematic processes affected the esrp including densification due to emplacement of gabbro into the middle crust subsidence and consequent filling of a 6 km deep basin with volcanic and plutonic rocks 20 extension via dike injection or faulting ductile attenuation and lower crustal flow from esrp to the adjacent basin and range two kinematic models involving these processes are proposed overall the two models are viable only if the initial crustal thickness were 43 47 km thick otherwise the lost mass of the esrp is not balanced by the excess mass in the basin and range in the first model densification eruption and extension by dike injection the esrp crust receives mantle derived gabbro equivalent to a 16 km thick layer but would need to lose 10 km of crust via lower crustal flow in the second model densification eruption and extension by faulting and ductile attenuation the esrp crust receives a 9 4 km equivalent thickness of mantle derived gabbro and would need to lose 6 km of crust via lower crustal flow the cumulative amount of mantle derived gabbro in the esrp crust derived by this kinematic analysis equivalent to a 9 4 16 km thick layer is nearly identical to the amount estimated independently using a mass balance petrologic model suggesting it is a robust result

a discussion of corundum cr muscovite rocks at o briens zimbabwe the conjunction of hydrothermal desilicification and lil element enrichment geochemical and isotopic evidence by kerrich et al

a classification of gem corundum deposits aimed towards gem exploration abstract this classification of gem corundum ruby and sapphire deposits takes into consideration petrographic data and the mode of genesis of the deposits it is aimed at being practical and useful in the field particularly for prospecting purposes deposits are classified into primary and secondary deposits primary deposits include igneous and metamorphic deposits igneous gem corundum deposits are rare and include sapphire bearing syenites from kenya emphasis is put on metamorphic deposits that are subdivided into metamorphic s s metasomatic and anatectic deposits many gem corundum deposits result from metasomatic processes small scale metasomatism mostly involves desilication reactions between silico aluminous rocks pegmatites gneisses etc and silica poor rocks ultramafites meta carbonates and leads to the formation of limited size deposits large scale diffuse metasomatism is usually more difficult to characterize and therefore is not separated from isochemical metamorphism in this classification in metamorphic deposits gem corundum results from transformation of an al rich and or si poor protolith such deposits include ruby bearing mafic granulites ruby bearing meta limestones and ruby sapphire bearing gneisses and granulites an intermediate category includes anatectic deposits secondary deposits encompass sedimentary and volcanic xenoclastic occurrences in sedimentary deposits gem corundum occurs as clasts originating from other lithologies in volcanic deposits gem corundums are xenocrysts that have a range of origins the proposed classification outlines geological environments favorable to the crystallization and distribution of gem corundum thus facilitating prospecting and mining of this gemstone

the nickeliferous archean madziwa igneous complex northern zimbabwe petrological evolution magmatic architecture and ore genesis the nickeliferous madziwa igneous complex consists of several lenticular mafic ultramafic enclaves within late archean gneissic terrain the lenses of igneous rock are locally deformed and metamorphosed to lower greenschist grade they are here interpreted to be remnants of a large composite magmatic structure with two main components 1 a set of narrow 10s 100s of meters wide dikes intruding the gneissic foliation and 2 a large lopolith made up of leuconorite some dikes contain a differentiated and vertically oriented layered suite comprising a central pyroxenite layer plus norite as both a continuous marginal layer and intermittent layers within the pyroxenite the pyroxenite norite suite has a preliminary u pb zircon age of 2684 ma other dikes are made up of diorite and or ferrodiorite in places the dioritic rocks also intrude the pyroxenite norite suite the leuconorite lopolith transects both the dikes and the gneissic country rocks its basal contacts with the pyroxenite norite suite varying locally from intrusive to gradational to magmatic erosional although modified in places by secondary mobilization disseminated ni cu sulfides are primarily hosted within the central ortho to mesocumulate pyroxenite adjacent to internal norite layers whole rock geochemical data establish the comagmatic origin of the principal rock types and indicate an archean d type basaltic source magma with ca 8 mgo the two dioritic rock types representing late immiscible silica and fe ti p rich derivatives of pyroxenite norite leuconorite crystallization in addition to the major rock types peridotites occur in several outlying dikes and may represent fractionates of a komatiitic precursor to the basalt geologic and geochemical evidence points to bulk assimilation of country rock gneiss by the madziwa magma and sulfide segregation triggered by felsic contamination these processes did not occur locally within the dikes but rather in the conduit system prior to the emplacement of the magma charged with sulfide droplets the unusual vertical layering of the pyroxenite norite sequence and the localization of the sulfide ores are attributed to the strong outward cooling gradient across such narrow dikes and to large scale lateral movement of sulfide droplets through the solidifying pyroxene crystal framework ahead of an advancing postcumulus plagioclase crystallization front

seismic reflection profiling in the boulder batholith montana seismic reflection profiling combined with gravity data allows more exact determination of the geometry of the controversial boulder batholith of montana reveals laminated structure of the lower crust beneath the batholith and identifies the moho at a depth of 38 km the batholith has inward dipping contacts the dip being about 50 on the west side on the basis of seismic data and the depth to the batholith floor is constrained between 12 and 18 km indicating a great volume for the batholith the boulder batholith was emplaced between 80 and 70 ma during an eastward thrusting in the fold and thrust belt a presumed basal decollement of the thrust system might coincide with the batholith floor and may correspond to the top of the lower crustal layering at a depth of 18 km

late archaean clastic sedimentary rocks shamvaian group of the zimbabwe craton first observations from the bindura shamva greenstone belt the 2 65 ga old shamvaian group sedimentary rocks occur as a folded succession in the central part of the bindura shamva greenstone belt of zimbabwe the strata comprise distinct shear zone bounded tectonostratigraphic units which may be stratigraphically arranged as follows the lower part of the succession is represented by a transgressive fining upward sequence of alluvial fan conglomerate overlain by fluvial braid plain pebbly sandstone and marine shoreface sandstone detritus was derived from a mid archaean granitoid gneiss terrain situated to the east sediment supply and subsidence rate must have been high shallow shelf sedimentation was followed by deep water sub wave base deposition by turbidity currents giving rise to a thick succession of fine to coarse clastic material the turbidite deposits were locally overlain by shallow marine sandstone and fluvial to alluvial fan conglomerate an upward increase in the abundance of intermediate and felsic volcanic clasts suggests an increase in the proximity of a volcanic terrain such as a volcanic arc deposition was followed by layer parallel shearing during thrust belt style tectonism major shear zones developed preferentially along the contact between shallow and deep marine facies associations basin initiation may have been related to extensional tectonics possibly on rifted continental crust whereas later stages of basin history were characterized by compression suggesting a foreland or fore arc basin setting sedimentary facies stratigraphy and facies distribution are remarkably similar to some late archaean sedimentary sequences of the superior province in canada

crustal structure of the eastern snake river plain determined from ray trace modeling of seismic refraction data ray trace travel time modeling of the seismic refraction record sections for a profile from near soda springs idaho to near mckay idaho was used to derive a crustal model across the eastern snake river plain esrp the derived crustal model is consistent with the velocity structure interpreted from a profile along the axis of the esrp the interpretation also indicates that significant lateral inhomogeneities exist in the upper crust beneath the esrp when compared with the upper crust beneath the adjoining northern rocky mountain and basin and range provinces the most prominent features of the crustal structure inferred by the ray trace modeling are as follows 1 the northwest margin of the esrp was modeled as a fault structure downthrown on the srp side se with an offset of greater than 4 km the southeast margin conversely appears to be downwarped with possible minor faulting paleozoic sedimentary rocks are present beneath the cenozoic volcanics of the esrp possibly to and beyond 40 km from the southeastern margin of the esrp near blackfoot idaho 2 the modeling indicates no abrupt variation in the depth to the top of the lower crustal layer near the margins of the esrp 3 an intermediate 6 5 km s layer occurring beneath the esrp interpreted from a refraction profile located along the axis of the esrp was found to be localized within the esrp margins this layer is interpreted as a pervasive intrusion of higher velocity material from the upper mantle into the highly fractured upper crustal layer in this region 4 a density model of the crust across the esrp was prepared with the densities selected using the interpreted seismic velocities as a constraint the gravity field calculated from this model resulted in a good match to observed gravity data over the eastern snake river plain

vanadio oxy chromium dravite nav3 cr4mg2 si6o18 bo3 3 oh 3o a new mineral species of the tourmaline supergroup vanadio oxy chromium dravite nav3 cr4mg2 si6o18 bo3 3 oh 3o a new mineral species of the tourmaline supergroup

age constraints on the palaeoproterozoic lomagundi jatuli event in zimbabwe zircon geochronology of the magondi supergroup

magmatic nickel sulfide mineralization in zimbabwe review of deposits and development of exploration criteria for prospectivity analysis significant magmatic nickel sulfide deposits in zimbabwe occur in archean greenstone belts and archean to proterozoic mafic ultramafic intrusions such as madziwa and the great dyke here we review exploration criteria for magmatic nickel sulfide deposits taking into account nickel sulfide mineral systems in general zimbabwe s cratonic evolution and the geological setting of the deposits we propose that the essential ingredients for nickel sulfide mineral systems in zimbabwe are i proximity to greenstone belts craton margins and orogenic belts ii large mafic ultramafic intrusions and their feeders iii crustal contamination of basic ultrabasic magma with sulfidic crustal lithologies and iv the vicinity to major fault zones that allowed fertile magma to ascend into the upper crust we suggest that there is potential for new nickel sulfide discoveries both within and outside the greenstone belts including the karoo flood basalt province the mashonaland dolerites and the limpopo magondi and zambezi orogenic belts

enhanced h and f incorporation in borian olivine b containing olivine from the tayozhnoye fe deposit siberia russia has unusually high h and f contents of 0 012 h and 0 026 0 043 f per formula unit of four o atoms there are a minimum of eight infrared o h absorption bands none of which correlates with the known o h band frequencies of hydrous magnesium silicates the o h band at 3672 cm 1 has not been observed in previously published olivine spectra transmission electron microscopy indicates substantial replacement of olivine by clinohumite and disordered mixtures of olivine and humite group minerals near the rims of the olivine grains whereas the interiors of the olivine grains contain regions of unaltered olivine structure h 2o contents calculated from the defect stoichiometry and defect density are approximately one tenth of the total h 2o content estimated from the infrared data the o h bands therefore are believed to reflect the presence of oh within the olivine crystal structure infrared active stretching vibrations at 758 1164 and 1257 cm 1 are consistent with the presence of 4 bo 4 groups within the olivine structure our data represent the first direct evidence of the coupled substitution b f oh si 1 o 1 in orthosilicate minerals as a potential carrier of b f and h in the subducted oceanic lithosphere olivine may play a key role in recycling these elements into the mantle

the bulawayan supergroup a late archaean passive margin related large igneous province in the zimbabwe craton the four principal lithostratigraphic units koodoovale manjeri reliance and zeederbergs of the widespread late archaean bulawayan supergroup in the zimbabwe craton record the early to middle stage development of a major c 250 000 km 2 volcanosedimentary basin the craton wide c 2 70 ga manjeri unit varies systematically from a tens of metres thick dominantly chemogenic basinal sediment in gradational conformable contact with the underlying several kilometres thick diachronous 2 83 2 71 ga felsic koodoovale unit in the north and west to an up to hundreds of metres thick dominantly siliciclastic retrogradational platformal sediment unconformably overlying protocratonic basement in the south and east the intermittent c 2 70 ga reliance unit consists of the flow fields of several discrete komatiitic sill flow complexes scattered throughout the craton and fed by an interlinked system of mid to upper crustal flow through feeder sills the c 2 70 ga zeederbergs unit comprises a submarine flood basalt sequence of several kilometres thickness that erupted from ene trending parallel mafic dykes preserved in the south and east the basalts form a craton wide blanket to the reliance flow fields and intervening manjeri sediments certain structural stratigraphic features rule out primary construction of the bulawayan supergroup by early lateral accretion as proposed by several previous workers instead evolution of the bulawayan basin can best be explained by vertical accretion involving 1 abundant episodic and bimodal plume related volcanism and 2 extension and rapid subsidence of continental lithosphere many aspects of the bulawayan supergroup provide important insights into archaean tectonics mantle plume activity magma generation and lithospheric plumbing

pb isotopes a multi function tool for assessing tectonothermal events and crust mantle recycling at late archaean convergent margins abstract lead isotopes are a powerful tool in studying archaean granitoids and continental growth because 207pb is a sensitive indicator of involvement of lead from old archaean crustal sources this paper aims to demonstrate the significance and promote the use of lead isotopes by reviewing the theory concepts and traditions for interpretations of pb isotope data as well as by introducing a global model based on crust mantle recycling for discriminating late archaean convergent margin settings of granitoids case plots of k feldspar and whole rock data of ca 2 7 ga granitoids from several archaean cratons show that lead isotope systematics when interpreted in the correct geological context can be used for resolving many questions on crustal evolution from magmatic and tectonothermal history of individual intrusions to large scale tectonics and supercontinent correlations

using ancillary information to improve hypocenter estimation bayesian single event location bsel we have developed and tested an algorithm bayesian single event location bsel for estimating the location of a seismic event the main driver for our research is the inadequate representation of ancillary information in the hypocenter estimation procedure the added benefit is that we have also addressed instability issues often encountered with historical nlr solvers e g non convergence or seismically infeasible results bsel differs from established nonlinear regression techniques by using a bayesian prior probability density function prior pdf to incorporate ancillary physical basis constraints about event location p wave arrival times from seismic events are used in the development depth a focus of this paper may be modeled with a prior pdf potentially skewed that captures physical basis bounds from surface wave observations this pdf is constructed from a rayleigh wave depth excitation eigenfunction that is based on the observed minimum period from a spectrogram analysis and estimated near source elastic parameters for example if the surface wave is an rg phase it potentially provides a strong constraint for depth which has important implications for remote monitoring of nuclear explosions the proposed bayesian algorithm is illustrated with events that demonstrate its congruity with established hypocenter estimation methods and its application potential the bsel method is applied to three events 1 a shallow mw 4 earthquake that occurred near bardwell ky on june 6 2003 2 the mw 5 6 earthquake of july 26 2005 that occurred near dillon mt and 3 a deep mw 5 7 earthquake that occurred off the coast of japan on april 22 1980 a strong rg was observed from the bardwell ky earthquake that places very strong constraints on depth and origin time no rg was observed for the dillon mt earthquake but we used the minimum observed period of a rayleigh wave 7 seconds to reduce the depth and origin time uncertainty because the japan event was deep there is no observed surface wave energy we utilize the prior generated from the dillon mt event to show that even in the case when a prior is inappropriately applied high quality data will overcome its influence and result in a reasonable hypocenter estimate

initial pb of the amitsoq gneiss revisited implications for the timing of early archaean crustal evolution in west greenland reply abstract the rapid change in 207 pb 206 pb ratio during early earth history provides a powerful constraint for temporal resolution of archaean crustal evolution new and published data for amitsoq gneiss feldspars and whole rocks define the earth s least radiogenic silicate pb known until now the most important feature of the data is that independent of the exact slope of the regression in a common pb diagram a very well defined intercept with plausible terrestrial pb evolution lines is obtained between 3 63 and 3 66 ga identical to a combined pb pb regression age of 3654 73 ma mswd 17 6 we conclude that the magmatic precursors of the amitsoq orthogneisses separated from a depleted mantle like source at ca 3 65 ga this contrasts sharply with claims based on ion probe u pb zircon dating that the amitsoq gneisses had a prolonged episodic magmatic evolution extending from ca 3 87 to 3 60 ga pb isotopes offer no evidence that any amitsoq gneiss analysed until now began its evolution in the crust as long ago as 3 85 ga we propose that zircons significantly older than 3 65 ga were inherited from chemically evolved rocks as old as ca 3 85 ga geochemical arguments are presented to support this proposal new feldspar and whole rock pb isotope data are also reported for sheets of quartz dioritic gneiss on akilia island that yield the oldest zircon u pb age clusters in the range 3 84 3 87 ga these sheets are discordant to a metasedimentary enclave containing accessory apatite with graphite inclusions whose c isotope ratios are regarded as biogenic in origin as above pb isotopes in the sheets are consistent with a magmatic age of 3 65 ga which we regard as the true minimum age for these waterlain sediments and their possibly biogenic markers additional pb isotope data as well as limited published sm nd and u pb zircon data on the akilia enclaves themselves suggest an age between ca 3 65 and 3 70 ga for their original deposition some 150 200 ma younger than proposed previously thus the question of overlap of earliest life with a bolide impact scenario terminating 3 80 ga as on the moon becomes irrelevant

origin of gem corundum in calcite marble the revelstoke occurrence in the canadian cordillera of british columbia abstract the calcite marble hosted gem corundum ruby sapphire occurrence near revelstoke british columbia canada occurs in the monashee complex of the omineca belt of the canadian cordillera corundum occurs in thin folded and stretched layers with green muscovite ba bearing k feldspar anorthite an0 85 1 phlogopite na poor scapolite other silicate layers within the marble are composed of 1 diopside tremolite quartz and 2 garnet alm0 7 0 5grs0 2 0 4 na rich scapolite diopside tremolite na k amphiboles non silicate layers in the marble are either magnetite or graphite bearing predominantly pink locally red or purple opaque to transparent corundum crystals have elevated cr2o3 0 21 wt and variable amounts of tio2 rare blue rims on the corundum crystals contain higher amounts of tio2 0 53 wt and fe2o3 0 07 wt the associated micas have elevated cr v ti and ba contents petrography of the silicate layers show that corundum formed from muscovite at the peak of metamorphism 650 700 c at 8 5 9 kbar because the marble is almost pure calcite dolomite is very rare the corundum was preserved because it did not react with dolomite to spinel calcite during decompression the scapolite bearing assemblages formed during or after decompression of the rock at 650 c and 4 6 kbar gem quality corundum crystals formed especially on borders of the mica feldspar layers in an assemblage with calcite whole rock geochemistry data show that the corundum bearing silicate mica feldspar layers formed by mechanical mixing of carbonate with the host gneiss protolith the bulk composition of the silicate layers was modified by si and fe depletion during prograde metamorphism high element mobility is supported by the homogenization of 18o and 13c values in carbonates and silicates for the marble and silicate layers the silicate layers and the gneiss contain elevated contents of cr and v due to the volcanoclastic component of their protolith

archean magmatic granulites diapirism and proterozoic reworking in the northern marginal zone of the limpopo belt the northern marginal zone nmz of the limpopo belt southern africa is a high grade gneiss belt dominated by magmatic granulites of the charnoenderbite suite which intruded minor mafic ultramafic and metasedimentary rocks between 2 74 and 2 57 ga the intrusive rocks have crustal and mantle components and occur as elliptical bodies interpreted as diapirs peak metamorphism p 800 mpa t 800 850 c occurred at ca 2 59 ga the highly radiogenic nature of the rocks in the nmz supplemented by heat from mantle melts led to heating and diapirism culminating in the intrusion of distinctive porphyritic charnockites and granites horizontal shortening and steep extrusion of the nmz during which crustal thickening was limited by high geothermal gradients contrast with overthickening and gravitational collapse observed particularly in more recent orogens the granulites were exhumed by the end of the archean the pervasive late archean shortening over the whole of the nmz contrasts with limited deformation on the zimbabwe craton possibly owing to the strengthening effect of early crust in the craton in the southeast of the nmz strike slip kinematic indicators occur within the transition zone and the triangle shear zone where dextral shearing reworked the archean crust at ca 1 97 ga

revised velocity structure of western montana using a one dimensional 1d layered earth approach we determined a new crust and upper mantle velocity model for western montana to improve earthquake hypocenter locations p wave arrival times recorded on 280 stations from 1432 well recorded earthquakes provided input for a sparse damped least squares sensitivity matrix we solved for 1d velocity structure refractor depths station corrections and hypocentral positions by minimizing travel time residuals the new model has three layers with p wave velocities of 5 70 6 12 and 6 53 km sec with corresponding interface depths of 7 0 19 8 and 39 7 km below the surface the upper mantle velocity is 8 00 km sec we determined station corrections and show that the yellowstone caldera is the only regional geologic feature not adequately modeled by our new 1d velocity model use of the new model and station corrections reduced hypocenter location uncertainties and travel time residuals implying improved hypocenter locations for western montana earthquakes online material table of seismographic stations

metamorphism and uplift of alpine schist in the franz josef fox glacier area of the southern alps new zealand geothermometry and geobarometry of 10 garnet oligoclase zone schists in the franz josef fox glacier area southern alps new zealand give temperatures ranging from 415 to 625 c and pressures from 5 2 to 9 2 kbar indicating a t p array of about 50 c kbar and inferred peak temperature conditions over a c 15 km thick section at depths between c 20 and 34 km the present day distribution of the schist samples implies that only about one third of the original crustal section is now exposed the garnet oligoclase zone schists represent the deeper part of a metamorphosed and deformed accretionary complex that was associated with late palaeozoic early mesozoic subduction along the gondwana continental margin partial uplift c 0 2 m ma and erosion of the complex during jurassic cretaceous times rangitata uplift was synchronous with d2 deformation and recrystallization as recorded by the p t array cenozoic kaikoura uplift and exhumation of the schist since c 30 ma to form the southern alps was associated with oblique slip movement on the alpine fault the present day position and steep eastward dip of isograds and d2 structures suggest considerable clockwise rotation during uplift associated with ductile attenuation and tectonic thinning by over two thirds of the original schist sequence largely due to simple shear along schistosity planes as the schist generally shows only incipient greenschist facies retrograde recrystallization an apparently complete although contracted prograde mineral sequence has been preserved by rapid uplift 5 km ma of hot rock and the effects of limited shear heating near the alpine fault

the structural history and mineralization controls of the world class geita hill gold deposit geita greenstone belt tanzania the geita hill gold deposit is located in the archean geita greenstone belt and is one of the largest gold deposits in east africa the geita greenstone belt experienced a complex deformation and intrusive history that is well illustrated and preserved in and around the geita hill gold deposit deformation involved early stages of ductile shearing and folding d1 to d5 during which episodic emplacement of large diorite intrusive complexes sills and dykes occurred these ductile deformation phases were followed by the development of brittle ductile shear zones and faults d6 to d8 the last stages of deformation were accompanied by voluminous felsic magmatism involving the intrusion of felsic porphyry dykes within the greenstone belt and the emplacement of large granitic bodies now forming the margins of the greenstone belt early folded lamprophyre dykes and later lamprophyre dykes crosscutting the folded sequence are common although volumetrically insignificant the gold deposit formed late during the tectonic history of the greenstone belt post dating ductile deformation and synchronous with the development of brittle ductile shear zones that overprinted earlier structural elements the main mineralizing process involved sulfide replacement of magnetite rich layers in ironstone and locally the replacement of ferromagnesian phases and magnetite in the diorite intrusions the intersection between the brittle ductile d6 geita hill shear zone and different structural elements of ductile origin e g fold hinges and the contact between banded ironstone and folded diorite dykes and sills provided the optimal sites for gold mineralization

tabular intrusion and folding of the late archaean murehwa granite zimbabwe during regional shortening the murehwa granite belongs to the late archaean chilimanzi suite of granites that occupy more than 50 of the zimbabwe craton the granite has compositional layers on metre to tens of metre scales with generally shallow dips and dip directions towards the centre of the granite near its margins the layering is systematically folded about sub horizontal east west axes microcline phenocrysts are generally strongly aligned parallel to the layering but locally crosscut the layering because they are axial planar to the folds crystals are not significantly deformed fabrics are thus magmatic the granite was intruded in a tabular sheet that may have been only about 6 km thick and fed by dykes contrary to previous concepts of diapirism ballooning for some archaean granites of the zimbabwe craton the systematic orientation of magmatic folds and axial planar fabrics demonstrate that the granite was intruded during regional north south shortening this shortening is consistent with the orientation of cratonic scale fractures that were intruded by the great dyke within 25 ma of the intrusion of the murehwa granite the granite may have intruded in a relatively short interval of less than 1 26 ma within the longer duration of the entire chilimanzi granite event

tectonic evolution of a greenstone sequence in northern zimbabwe sequential early stacking and pluton diapirism structural metamorphic relationships in the 2 71 2 64 ga harare shamva greenstone belt in northern zimbabwe demonstrate that the archean stratigraphy of the zimbabwe craton can not be regarded as a coherent and nonrepetitive autochthonous succession of ultra mafic and felsic greenstones the greenstone belt shows an early set of amphibolite grade d1 m1 structures and associated synkinematic metamorphic assemblages that were formed during an episode of layer parallel shearing at about 2 64 ga this resulted in sequential west directed imbricate stacking and recumbent folding of distinct tectonostratigraphic greenstone sequences and includes the development of a syntectonic sedimentary succession d2 m2 fabrics strain patterns and contact aureoles overprint d1 geometries and are related to the diapiric emplacement of the chinamora batholith the structural data indicate that granite gneiss terrains can be subdivided into two types 1 structurally emplaced terrains the madziwa gneiss terrain and 2 diapiric domes the chinamora batholith the history of the harare shamva greenstone belt indicates that the late archean 2 7 2 6 ga evolution of the zimbabwe craton must allow for accretion of diverse crustal fragments including oceanic and back arc mafic crust volcanic arc felsic crust continental crust and related sedimentary sequences the process of accretion was probably diachronous across the craton and may have involved concomitant diapirism strike slip faulting magma intrusion and sedimentation

constraints on archaean crustal evolution of the zimbabwe craton a u pb zircon sm nd and pb pb whole rock isotope study the u pb ages of zircons from seven felsic volcanic and plutonic rocks from northern zimbabwe combined with field data and pb pb and sm nd whole rock isotope data constrain the timespan of development of the harare shamva granite greenstone terrain and establish the relative involvement of juvenile mantle derived and reworked crustal material basement cover field relationships and isotope and geochemical data demonstrate that the greenstones were deposited onto 3 2 2 8 ga basement gneisses in ensialic continental basins geodynamic models for the generation of the areally extensive bimodal magmatic products and growth of the pre existing crustal nucleus consistent with our interpretations are rift related 1 intracontinental rifting related to mantle plume activity or 2 rifting in a back arc environment related to a marginal volcanic arc the data in conjunction with field evidence do not indicate the presence and accretion of an older ca 2 70 ga and a younger ca 2 65 ga greenstone sequence in the harare part of the greenstone belt as was recently postulated on the basis of shrimp zircon ages zircon ages for basal felsic volcanics 2715 15 ma and a subvolcanic porphyry 2672 12 ma constrain the initiation and termination of deposition of the greenstone sequence the timespan of deposition of the upper bulawayan part of the greenstone sequence corresponds well with radiometric ages for upper bulawayan greenstones in the central and southern part of the craton and supports the concept of craton wide lithostratigraphic correlations for the late archaean in zimbabwe zircon ages for a syn tectonic gneiss 2667 4 ma and a late syn tectonic intrusive granodiorite 2664 15 ma pinpoint the age of deformation of the greenstone sequence and compare well with a pb pb age of shear zone related gold mineralization 2659 13 ma associated with the latter intrusive phase the intimate timing relation of greenstone deformation and granitoid emplacement but also the metamorphic evidence for a thermal effect of the batholiths on the surrounding greenstone belts and the structural and strain patterns in the greenstone sequence around and adjacent to the batholiths imply that the intrusion of the granitoids had a significant influence on the tectono thermal evolution of the greenstone belt prolonged magmatic activity is indicated by the zircon ages of small post tectonic plutons intrusive into the greenstone belt with a mineralized granodiorite dated at 2649 6 ma and an unmineralized tonalite at 2618 6 ma the 2601 14 ma crystallization age of an external chilimanzi type granite agrees well with existing radiometric ages for similar granites within the southern part of the craton demonstrating a craton wide event and heralding cratonization the similarity between u pb zircon ages and tdm model ages 2 65 2 62 ga and the positive ndt values 3 to 2 for the late syn tectonic and post tectonic intrusive plutons within the greenstone belt indicate magmatism was derived directly from the mantle or by anatexis of lower crustal sources with very short crustal residence times and minor contamination with older crust the rather high model 1 values 8 2 8 6 are unlikely to indicate the involvement of significant amounts of older crust and may be inherited from a high u pb mantle source as was suggested by previous workers for the archaean mantle beneath southern africa the older tdm ages for the felsic volcanics 3 0 2 8 ga and the porphyries 2 8 2 7 ga suggest that these felsic magmas were derived by partial melting of a source that was extracted from the mantle ca 200 ma prior to volcanism or may indicate interaction between depleted mantle derived melts and older crustal material

comparing orthomagmatic and hydrothermal mineralization models for komatiite hosted nickel deposits in zimbabwe using multiple sulfur iron and nickel isotope data trojan and shangani mines are low grade 0 8 ni komatiite hosted nickel sulfide deposits associated with ca 2 7 ga volcano sedimentary sequences of the zimbabwe craton at both mines nickel sulfide mineralization is present in strongly deformed serpentinite bodies that are enveloped by a complex network of highly sheared silicified and sulfide bearing metasedimentary rocks strong polyphase structural metamorphic metasomatic overprints in both the trojan and shangani deposits make it difficult to ascertain if sulfide mineralization was derived from orthomagmatic or hydrothermal processes or by a combination of both multiple s fe and ni isotope analyses were applied to test these competing models massive ores at shangani mine show mass dependent fractionation of sulfur isotopes consistent with a mantle sulfur source whereas s isotope systematics of net textured ore and disseminated ore in talcose serpentinite indicates mixing of magmatic and sedimentary sulfur sources potentially via post magmatic hydrothermal processes a restricted range of strongly mass independent 33s values in ore samples from trojan mine likely reflects high temperature assimilation of sulfur from supracrustal rocks and later superimposed low temperature hydrothermal remobilization iron isotope values for most ni bearing sulfides show a narrow range suggesting that in contrast to sulfur nearly all of iron was derived from an igneous source negative ni isotope values also agree with derivation of ni from ultramafic melt and a significant high temperature fractionation of ni isotopes fe isotope values of some samples from shangani mine are more fractionated than expected to occur in high temperature magmatic systems further suggesting that hydrothermal processes were involved in either low grade ore formation liberation of ni from olivine by sulfur bearing hydrothermal fluids or remobilization of existing sulfides potentially inducing secondary ni sulfide mineralization

orebody geometry in lode gold deposits from zimbabwe implications for fluid flow deformation and mineralization the geometry of some orebodies can be described simply and accurately by three orthogonal axes u v w the ratios between these axes can be expressed as a parameter j u v 1 v w 1 and represented by a graph of u v plotted against v w analogous to the treatment of strain ellipsoids the orientations of orebodies can be plotted simply on projections using the uvw axes measurements of ore bodies from two examples of lode gold deposits from the zimbabwe craton show that most of these orebodies are oblate however orebodies can have significant u v ratios implying a component of pipe like fluid flow during mineralization pipe flow is demonstrated to be orders of magnitude more conductive than flow in planar veins and faults there are significant variations in orebody geometry between deposits and within different sections of a single deposit w values appear to be influenced by host rock more permeable rocks have higher w a negative trend of j value with orebody volume indicates that orebodies do not evolve in a self similar way but tend to more oblate shapes with increasing volume

serpentinization of oceanic peridotites temperature dependence of mineralogy and boron content serpentine mineralogy 18o 16o ratios and boron content were determined in equatorial atlantic serpentinized ultramafic rocks of mantle derivation the oxygen isotope data were used to estimate the temperatures of serpentinization a relationship appears to exist between temperature of serpentinization mineralogy of the serpentinites and their boron content orthochrysotile is the dominant serpentine phase in the 200 120 c range of serpentinization temperatures lizardite and polygonal serpentine are dominant in the 120 80 c range and clinochrysotile and lizardite in the 80 30 c temperature range these results are interpreted in terms of p t stability of serpentine phases and of changes in chemistry of circulating seawater during ascent of the ultramafic bodies in the oceanic crust an inverse relationship was found between temperature of serpentinization and boron content of the serpentinites suggesting that boron is acquired by the serpentinites from seawater at low temperature