joint inversions of seismic and geodynamic data for models of three dimensional mantle heterogeneity the seismic models of three dimensional 3 d mantle heterogeneity may be interpreted in terms of the density perturbations which drive mantle flow and thus provide important predictions of flow related observables such as the nonhydrostatic geoid the current models of global scale shear velocity heterogeneity provide reasonably good fits to the long wavelength nonhydrostatic geoid data 60 70 variance reductions but rather poor fits to the corresponding free air gravity anomalies 30 40 variance reductions this major difference is due to two factors 1 the very different amplitude spectrum of the gravity anomalies which is nearly flat for degrees l 3 the largest mismatch between the pattern of predicted and observed gravity anomalies is in the southern hemisphere this observation suggests that one reason for the poor overall match between the two fields may be that the global seismic data are not resolving sufficiently well the heterogeneity in the southern hemisphere portion of the deep mantle in contrast the gravity data provide accurate and geographically uniform constraints on the vertically integrated heterogeneity in the mantle we therefore perform a series of experiments in which we simultaneously invert a large set of seismic data which includes long period waveforms ss s and scs s differential travel times and normal mode structure coefficients and the long wavelength gravity anomaly data we thus determine whether it is possible to derive new 3 d heterogeneity models which satisfy both data sets the gravity anomaly data are interpreted in the context of spherically symmetric viscous flow models of the mantle in these inversion experiments we test several radial viscosity and ln ln profiles and thereby assess their plausibility the joint seismic geodynamic inversions reveal that it is indeed possible to greatly improve the fit to the free air gravity anomalies with variance reductions of 80 90 readily accessible while preserving the fit to the seismic data this improvement is achieved with some adjustment to the heterogeneity in the depth range 1500 2500 km where the seismic data constraints appear to be weakest the joint inversions also reveal new structures in the southern hemisphere portion of the lower mantle which apparently are not resolved by the seismic data alone

detection of mantle plumes in the lower mantle by diffraction tomography hawaii abstract a new method is proposed to detect mantle plumes in the lower mantle the method is akin to diffraction tomography and relies on the scattering of long period seismic body waves by nearly vertical heterogeneities the theory is described in a companion paper ying ji h c nataf earth planet sci lett this issue here we apply this method to an actual set of long period digital seismograms recorded on the world wide network of seismic stations between 1980 and 1994 we select seismograms that illuminate the lower mantle in a 20 20 region around hawaii we construct an image of plume like heterogeneities using a cell size of 1 1 by a lsqr inversion of the scattered waves this image shows a strong slow anomaly about 200 km northwest of hawaii although the image is somewhat noisy resolution tests indicate that this feature is fairly robust the amplitude of the anomaly is between 30 and 60 times larger than what we predict from a simple thermal plume model built with a 600 k maximum temperature excess and a gaussian horizontal cross section with an 1 e diameter of 250 km it is the first time that such a method is applied to the mantle while we think that the existence of this anomaly is real we do not know how to explain such a large amplitude if real this observation has a number of important geodynamical consequences it indicates that a mantle plume is indeed responsible for the hawaii hotspot as speculated by morgan w j morgan nature 230 1971 42 43 the plume is nearly vertical it originates from the d region at the base of the lower mantle the amplitude of the anomaly suggests that partial melt or and a chemical anomaly must be present the plume source is to the northwest of its surface expression as predicted by some models of plume advection in the mantle wind

significance of europium anomalies in silicate melts and crystal melt equilibria a re evaluation although europium speciation in silicate melts partly reflects prevailing oxygen fugacities melt composition and structure play the major role in determining eu2 eu3 ratios and europium partitioning into mineral phases experimental evidence by different investigators on the magnitude of the compositional effect on eu2 eu3 provides consistent results only if account is taken of the oxygen buffer system employed in the experiments the medium dependent reduction of europium can be understood in terms of the preferential stabilization of eu2 by a strong aluminosilicate complex in the melt phase and to a much lesser degree by metasilicate complexes the stability of these complexes increases as the field strength of the associated cation species decreases hence eu2 aluminosilicate complexes are preferentially stabilized relative to trivalent lanthanides in melts of appropriate composition and their presence minimizes the enthalpy of the melt the influence of these complexes is particularly pronounced in melts with a high feldspathic component and a strongly polymerized structure their petrogenetic influence is best documented through the behaviour of europium relative to the other lanthanides during anhydrous anatexis in high grade metamorphic terrains and in the anomalous europium partition coefficients of phenocryst phases which formed during the crystallization of highly silicic magmas

sedimentology of the archean dore metasediments arliss lake area southern michipicoten greenstone belt superior province dore metasediments in the arliss lake area of the michipicoten greenstone belt predominantly represent submarine fan sedimentation in mid to outer fan settings subaerial deposition was rare occurring in a sandy braided stream environment and as mud poor debris flows in a probable fan delta setting the arliss lake subbasin was likely small and of an irregular configuration which restricted down fan sediment distribution promoting aggradational lobe development it existed on a convergent plate margin felsic pyroclastic volcanism was periodically synchronous with epiclastic sedimentation but only rarely dominated paleogeographic development addressed by comparing juxtaposed thrust fault bounded lithological packages of similar facing suggests that the subbasin experienced several episodes of instability which caused dramatic changes in relative sea level

an ion microprobe study on trace element composition of clinopyroxenes from blueschist and eclogitized feti gabbros ligurian alps northwestern italy some petrologic considerations abstract detailed microtextural investigations coupled with ion microprobe trace element determinations have been performed on clinopyroxenes from blueschist and eclogitized fe ti gabbros from ligurian alps northwestern italy ion microprobe analyses have also been carried out on garnets from eclogitized fe ti gabbros the aim is to understand the relations between metamorphic reactions and trace element compositions of clinopyroxenes in high pressure low temperature terranes trace and major element analyses on relicts of igneous diopsides indicate similar protoliths for both eclogites and blueschists metamorphic clinopyroxenes have variable trace element compositions which reflect the compositions of original mineral phases in particular the clinopyroxenes replacing the igneous diopside commonly retain geochemical signatures of the precursor mineral such as relatively high sc y and hree contents the trace element variations among high pressure clinopyroxenes in different textural domains are greater for blueschists than for eclogites this reflects the more limited major element equilibration of blueschists relative to eclogites in deformed eclogites the clinopyroxenes lack the geochemical signatures of the original igneous minerals most probably due to the enlargement of reaction domains caused by synmetamorphic deformation trace element partitioning between omphacite and garnet suggests that in both undeformed and deformed eclogites chemical equilibrium is locally approached and therefore the trace element composition of clinopyroxenes is controlled by the coexisting minerals for instance clinopyroxenes in eclogites are strongly hree and lree depleted i e with bell shaped pattern due to the equilibration with garnet and allanite respectively the partition coefficients for rees between omphacite and garnet are markedly lower than those reported in the literature on the basis of crystal chemical considerations we suggest that this is probably due to lower temperature conditions during formation

implications of retrograde metamorphism for gold mineralisation in the greenstone belts of northern dharwar craton karnataka india abstract gold mineralisation in the archaean hutti mangalur and gadag greenstone belts of the northern dharwar craton is characterised by hydrothermal wall rock alterations with ubiquitous presence of chlorite sericite muscovite biotite carbonates sulphides and quartz veins indicating greenschist facies mineral assemblages such wall rock alterations associated with gold mineralisation in these belts post date the peak of regional metamorphism and are retrograde with respect to regional metamorphism there is a salient relationship between gold mineralisation and retrograde greenschist facies metamorphism in these belts

alternatives to core mantle boundary topography abstract nuclear explosions at the french test sites in the south pacific have been recorded at the regional seismic networks in the western united states a total of approximately 600 good quality short period seismograms were selected from an initial set of 900 the dense receiver spacing particularly in california allows the division of the networks into numerous subarrays and the data from each subarray can be analysed using conventional array techniques such as beam forming and frequency wave number analysis the seismic rays provide dense sampling of a small area of the core mantle boundary cmb beneath the eastern pacific from 5 to 13 n and 133 to 126 w pcp has a shallow incidence angle on the cmb at an epicentral distance of 65 and d is sampled over a large swathe from 2 s to 24 n and from 134 to 123 w beam traces have been formed for the p and pcp phases for each of the subarrays using slowness and azimuth values derived from array methods good quality beams have been selected for further analysis by applying a matched filter to the beamed traces an accurate set of pcp p differential travel times has been determined and compared with iasp91 the residuals show a smooth trend over the area sampled for the given epicentral distances p and pcp have similar paths through the crust and upper mantle so we make the assumption that near source and near receiver structures have negligible effect on the differential times we also assume that the midmantle is relatively homogenous many previous studies have interpreted pcp p residuals in terms of cmb topography by mapping the residuals directly to topography a range in elevation of 40 km over a lateral extent of about 700 km results alternatively we attempted to interpret the residuals in terms of velocity heterogeneity in d a range of d models have been tested including lateral velocity variation in a d layer with constant thickness the pcp p residuals may be explained by allowing the p wave velocity to vary laterally by a factor of 2 over the region sampled with most of the region being marginally fast this suggests that d heterogeneity has large spectral power at relatively short wavelengths in this scenario previous tomographic inversions for cmb topography will have large errors since in these inversions corrections for d structure are based on low order spherical harmonic models

the onaman prospect ontario an unusual occurrence of cu bearing au mineralization in a shear zone the onaman prospect is an unusual example of mesothermal gold mineralization in which native gold electrum and minor chalcopyrite are disseminated in metavolcaniclastic rocks in shear zones or concentrated in felsite dikes that intrude the metavolcaniclastic rocks mineralization was contemporaneous with carbonatization and involved early replacement of iron bearing minerals in the wall rock by ankerite and pyrite and later replacement of pyrite by arsenopyrite pyrrhotite chalcopyrite electrum and native gold oxygen isotope ratios of ankerite suggest a metamorphic origin for the fluid and carbon isotope ratios are similar to those of large mesothermal gold deposits in the superior province for which a mantle origin has been proposed sulfur isotope ratios correlate positively with gold grades and may reflect decreasing f o2 during mineralization a model is proposed in which deformation of the volcanic pile led to the formation of a shear zone and created dilational zones allowing dike intrusion metamorphic fluids rose along the shear zones and the dikes pyritization of the host rock was the major control on gold and chalcopyrite deposition reducing a h 2 s and destabilizing the bisulfide complexes of au and cu the onaman occurrences provide an uncommon example of a mesothermal system in which ph was relatively low and the correspondingly low solubility of copper allowed the fluid to saturate with and deposit chalcopyrite in most other mesothermal gold systems copper is too soluble to saturate the fluid with a copper mineral and is therefore flushed through the system

is plagioclase removal responsible for the negative eu anomaly in the source regions of mare basalts the nearly ubiquitous presence of a negative eu anomaly in the mare basalts has been suggested to indicate prior separation and flotation of plagioclase from the basalt source region during its crystallization from a lunar magma ocean lmo are there any mare basalts derived from a mantle source which did not experience prior plagioclase separation crystal chemical rationale for ree substitution in pyroxene suggests that the combination of ree size and charge m2 site characteristics of pyroxene fo2 magma chemistry and temperature may account for the negative eu anomaly in the source region of some types of primitive low tio2 mare basalts this origin for the negative eu anomaly does not preclude the possibility of the lmo as many mare basalts still require prior plagioclase crystallization and separation and or hybridization involving a kreep component

seismic tomography in boreholes summary synthetic seismic tomographic data sets for cross hole and hole to surface arrays are computed and inverted inversion imaging is performed through an iterative projection solution to a matrix equation the orientation as well as the position of an anomaly relative to the source and recorder arrays affects both the rate of convergence of the solution and the resolution obtained data density and distribution determine the physical validity of the image obtained at convergence borehole tomography is apparently a viable method for imaging inter hole structure

the effect of temperature on the seismic anisotropy of the perovskite and post perovskite polymorphs of mgsio3 using elastic constants determined by ab initio molecular dynamics over a range of temperatures we show that the seismic anisotropy of mgsio3 perovskite is significantly temperature dependent at 90 gpa the direction of greatest shear wave splitting changes from 001 at 0 k to 010 at 3500 k in addition there is no shear wave splitting in the 100 direction at 0 k but a significant amount of splitting is exhibited at 3500 k we have also calculated the first set of high temperature elastic constants of the new post perovskite phase by ab initio molecular dynamics methods we find that in contrast to mgsio3 perovskite temperature has little effect on the acoustic anisotropy of this phase we determine ln vs ln vp for the post perovskite phase and find it to be very low about 0 9 this may be an important parameter for mapping out the post perovskite phase in the lowermost mantle

δs at high pressure and dlnvs dlnvp in the lower mantle s a measure of the sensitivity of the adiabatic bulk modulus to isobaric density changes controls lateral variations of p wave velocities to estimate the effect of pressure on s we have performed lattice dynamic calculations of the cubic alkaline earth oxides we used a many body parametric hamiltonian that reproduces the available laboratory data on thermoelastic properties the calculations were carried out for p t conditions that correspond to those of the lower mantle we find that s decreases from 4 1 at standard conditions to 2 at lower mantle conditions the latter value is consistent with d l anderson s estimates based on geophysical data with s 2 and a corresponding parameter for shear modulus not decreasing with compression no partial melting is required to account for the observation that lnvs lnvp 2 in the lower mantle although more data and calculations on additional minerals are needed to draw any firm conclusions our results indicate that the relatively small lateral variations of lnvp when compared to lnvs are due entirely to compressional effects

long wavelength s wave velocity structure throughout the mantle summary long wavelength s wave velocity anomalies are obtained for the entire mantle by inverting 6ooo long period 40 100s sh body waves and lo00 long period 100 500 s love waves in the shallow mantle the patterns of fast and slow velocity anomalies correlate well with age and tectonics in the depth range of 400 1000 km we find two antipodal fast velocity anomalies one in the western pacific and the other in south america they form a fairly strong i 2 anomaly moreover the 1 2 anomaly is the largest spherical harmonic component throughout the entire mantle except for in the layer of 1000 1300 km depth a fast velocity anomaly near the caribbean previously reported by several other investigators is found between the depths of 1000 and 1300km with diminishing amplitudes it can be traced to canada and the aleutians and may be related to old subducted slabs in the same depth interval similar size anomalies can be found in the middle of the pacific and in the indian ocean these could be checked by more detailed studies to judge the reliability of this study heterogeneity power has a peak at the surface and a secondary peak at the bottom of the mantle d consistent with the existence of chemical and or thermal boundary layers at the base of the mantle we also find a fast velocity anomaly which surrounds the pacific ocean this feature has been found by other researchers previously but its interpretation is difficult due to the trade off between velocity anomaly and core mantle boundary deformation

a review of the superior province of the canadian shield a product of archean accretion abstract the superior province consists of northern and southern high grade gneiss subprovinces and a central region of alternating granite greenstone and metasedimentary belts subprovinces are commonly fault bounded and display contrasting lithological assemblages metamorphic and structural styles geophysical characteristics and ages they are comparable to phanerozoic suspect terranes the middle and late archean rocks of the superior province are of mantle and juvenile crustal origin with little evidence of inheritance of components older than ca 3 1 ga high grade gneiss subprovinces are mainly deeper level equivalents of the supracrustal rich belts but one minnesota river valley is part of an older ca 3 6 ga terrane that was juxtaposed late in the tectonic history granite greenstone terranes are characterized by low grade volcano sedimentary sequences tholeiitic komatiitic submarine lava plain tholeiitic calc alkalic submarine to subaerial central complexes early platform type quartz arenite stromatolitic carbonate late timiskaming type shoshonitic alkalic volcanic fluvial sediment that range in age from ca 3 0 ga to 2 7 ga the volcanics represent oceanic island arc and continental arc volcanism the quartz arenite carbonate sequences deposition on relatively stable early formed ca 3 0 ga crust and the timiskaming type sequences deposition in pull apart basins developed along strike slip faults late archean ca 2 75 2 70 ga greenstone sequences occur throughout the superior province middle archean ca 3 0 2 8 ga sequences are common in northern and central superior province rare or absent in the south volcanism was accompanied by mafic to trondhjemitic plutonism the metasedimentary belts consist of variably metamorphosed volcanogenic turbidites with detrital zircons ranging in age from ca 3 0 ga to 2 7 ga the metasediments were deposited after major volcanism and probably represent accretionary prisms late archean orogenesis accompanied and followed by calc alkalic plutonism youngs southward from about 2 73 2 72 ga to 2 70 2 68 ga it involved large scale north south compression and dextral transpression and displays thin to thick skinned transitions the contemporaneity of events along the lengths of the belts and their southward younging coupled with the evidence of major compression are consistent with subduction driven oblique accretion of oceanic and continental volcanic arcs accretionary sedimentary wedges older microcontinental fragments etc in a convergent margin setting analogous to parts of the pacific rim differences eg abundance of komatiites and tonalitic plutonic suites are attributable to a hotter archean mantle

vesicles amygdales and similar structures in fault generated pseudotachylytes amygdales in fault generated pseudotachylytes from the outer hebrides thrust scotland and the ikertoq shear belt west greenland contain mineral assemblages dominated by k spar sphene epidote quartz and carbonates respectively these contrasting assemblages indicate that vesicle infilling took place under differing environmental conditions other filled cavities resembling amygdales in form and mineralogy are recognized as replacement pseudomorphs after porphyroclasts using published data relating vesicle abundance and solubility functions for h2o and co2 to pressure a palaeoseismic depth estimate equivalent to the depth of vesiculation of about 1 6 km is inferred for the greenland pseudotachylytes described in this paper

the relationship between shear and compressional velocities at high pressures reconciliation of seismic tomography and mineral physics the value of the parameter from low pressure laboratory measurements disagrees with the value from seismic observations the parameter relates the isobaric change in shear velocity vs to the change in compressional velocity vp seismic evidence indicates exceeds 2 0 in the lower mantle whereas data on a variety of minerals at high temperature t and ambient pressure p result in lower values we reconcile these differences our ab initio model calculations on mgo show that increases with p and is 2 0 2 5 at lower mantle pressures there is no need to assume partial melting to explain the seismic data these calculations also provide insight into the p and t dependence of the dimensionless parameter 1 g g t p where g is the isotropic shear modulus and is the volume thermal expansivity using measured values of thermoelastic parameters coupled with thermodynamic identities we seek constraints on s 1 ks ks t p where ks is the adiabatic bulk modulus and confirm that p causes s to decrease we find poisson s ratio increases with p and t altogether these results show that for mgo increases from 1 3 at ambient conditions to over 2 at lower mantle conditions we expect other mantle minerals to behave similarly therefore we find that reconciliation of the mineral physics approach with that of seismic tomography concerning does not require special assumptions about the state of the lower mantle

production of hybrid granitic magma at the advancing front of basaltic underplating inferences from the sesia magmatic system south western alps italy abstract the permian sesia magmatic system of the southwestern alps displays the plumbing system beneath a permian caldera including a deep crustal gabbroic complex upper crustal granite plutons and a bimodal volcanic field dominated by rhyolitic tuff filling the caldera isotopic compositions of the deep crustal gabbro overlap those of coeval andesitic basalts whereas granites define a distinct more radiogenic cluster sri 0 708 and 0 710 respectively afc computations starting from the best mafic candidate for a starting melt show that nd and sr isotopic compositions and trace elements of andesitic basalts may be modeled by reactive bulk assimilation of 30 of partially depleted crust and 15 30 gabbro fractionation trace elements of the deep crustal gabbro cumulates require a further 60 fractionation of the andesitic basalt and loss of 40 of silica rich residual melt the composition of the granite plutons is consistent with a mixture of relatively constant proportions of residual melt delivered from the gabbro and anatectic melt chemical and field evidence leads to a conceptual model which links the production of the two granitic components to the evolution of the mafic complex during the growth of the mafic complex progressive incorporation of packages of crustal rocks resulted in a roughly steady state rate of assimilation anatectic granite originates in the hot zone of melting crust located above the advancing mafic intrusion upward segregation of anatectic melts facilitates the assimilation of the partially depleted restite by stoping at each cycle of mafic intrusion and incorporation residual and anatectic melts are produced in roughly constant proportions because the amount of anatectic melt produced at the roof is a function of volume and latent heat of crystallization of the underplated mafic melt which in turn produces proportional amounts of hybrid gabbro cumulates and residual melt such a process can explain the restricted range in isotopic compositions of most rhyolitic and granitic rocks of the permo carboniferous province of europe and elsewhere

constraints on lower mantle physical properties from seismology and mineral physics the ratio of relative shear s to compressional p lateral seismic velocity variations is potentially an important constraint on the mineral physics of the earth s lower mantle recent seismic results show that there is an increase in this ratio from 1 7 to 2 6 in the lower mantle to 2000 km depth which is significantly greater than published values in the mineral physics literature we calculate in terms of s ln ks ln and a ln ln s and show that the seismic variation of within the depth range that p and s models appear proportional is consistent with scalings computed using recent results from geodynamics and mineral physics studies furthermore we find that s 1 at 1900 300 km depth irrespective of a

correlations between compressional and shear wave velocities and corresponding poisson s ratios for some common rocks and sulfide ores abstract the correlations between v p and v s and corresponding poisson s ratios are important in modeling and interpreting seismic data in terms of chemistry and lithology data from high pressure laboratory measurements of compressional and shear wave velocities v p and v s are analyzed for 12 common categories of rocks amphibolite anorthosite basalt diorite eclogite felsic gneiss gabbro diabase granite intermediate gneiss limestone mafic gneiss and peridotite and 4 types of massive sulfide ores chalcopyrite pyrite sphalerite and pyrrhotite the analysis reveals that the linear correlation provides good descriptions for the v s v p and ln v s ln v p relationships poisson s ratio is linearly correlated with v s v p shear modulus g and young s modulus e for these rocks and sulfide ores a decrease in poisson s ratio is associated with increases in v s g and e however the variation of poisson s ratio with v p depends on the logarithmic ratio r s p i e ln v s ln v p poisson s ratio increases or decreases with v p when r s p 1 r s p varies systematically with lithology 0 300 for granite 0 573 for diorite 0 602 for felsic gneiss 0 631 for intermediate gneiss 0 721 for gabbro diabase 0 768 for mafic gneiss 0 866 for eclogite 0 890 for amphibolite and 1 391 for peridotite it is suggested that r s p can be used as a proxy for the composition of the deep continental crust and the upper mantle

thermoelasticity of a mineral composite and a reconsideration of lower mantle properties a re examination of thermal and elastic properties of a mineral composite emphasising derivative properties especially pressure and temperature dependences of incompressibility resolves residual doubts about the application to the lower mantle of thermodynamic estimates of these properties the only counter intuitive conclusion is that ks p s 4 21 at p 0 which is significantly higher than the values for component minerals this leads to a revised lower mantle equation of state and also to a new one which requires as input only k0 4 21 and k 1 425 k k0 k k0 k p k these equations and three other finite strain theories fit the lower mantle prem tabulation very well and agree on the extrapolation to p 0 k0 203 1 1 6 gpa 0 128 2 1 2 gpa 0 3972 5 4 0 kg m 3 a further six finite strain equations are rejected as incompatible with homogeneity of the lower mantle application of the theory to thermal properties gives s 1 ks ks t p decreasing with depth from 2 7 to 1 6 over the lower mantle depth range conversely e 1 t p increases with depth from 7 1 to 8 7 corresponding values of lnvs lnvp p vary from 1 7 to 2 3 this conclusion is not consistent with a simple thermal interpretation of tomographic observations of lower mantle velocity anomalies compositional variations must be invoked

sensitivities of seismic velocities to temperature pressure and composition in the lower mantle we calculated temperature pressure and compositional sensitivities of seismic velocities in the lower mantle using latest mineral physics data the compositional variable refers to the volume proportion of perovskite in a simplified perovskite magnesiowustite mantle assemblage the novelty of our approach is the exploration of a reasonable range of input parameters which enter the lower mantle extrapolations this leads to realistic error bars on the sensitivities temperature variations can be inferred throughout the lower mantle within a good degree of precision contrary to the uppermost mantle modest compositional changes in the lower mantle can be detected by seismic tomography with a larger uncertainty though a likely trade off between temperature and composition will be largely determined by uncertainties in tomography itself given current sources of uncertainties on recent data anelastic contributions to the temperature sensitivities calculated using karato s approach appear less significant than previously thought recent seismological determinations of the ratio of relative s to p velocity heterogeneity can be entirely explain by thermal effects although isolated spots beneath africa and the central pacific in the lowermost mantle may ask for a compositional origin 2001 elsevier science b v all rights reserved

influence of stretching and density contrasts on the chemical evolution of continental magmas an example from the ivrea verbano zone the southern ivrea verbano zone of the italian western alps contains a huge mafic complex that intruded high grade metamorphic rocks while they were resident in the lower crust geologic mapping and chemical variations of the igneous body were used to study the evolution of underplated crust slivers of crustal rocks septa interlayered with igneous mafic rocks are concentrated in a narrow zone deep in the complex paragneiss bearing belt and show evidence of advanced degrees of partial melting variations of rare earth element patterns and sr isotope composition of the igneous rocks across the sequence are consistent with increasing crustal contamination approaching the septa therefore the paragneiss bearing belt is considered representative of an assimilation region where in situ interaction between mantle and crust derived magmas resulted in production of hybrid melts buoyancy caused upwards migration of the hybrid melts that incorporated the last septa and were stored at higher levels feeding the upper mafic complex synmagmatic stretching of the assimilation region facilitated mixing and homogenization of melts chemical variations of granitoids extracted from the septa show that deep septa are more depleted than shallow ones this suggests that the first incorporated septa were denser than the later ones as required by the high density of the first injected mafic magmas it is inferred that density contrasts between mafic melts and crustal rocks play a crucial role for the processes of contamination of continental magmas in thick under plated crust the extraction of early felsic hybrid melts from the lower crust may be required to increase the density of the lower crust and to allow the later mafic magmas to penetrate higher crustal levels

aspherical earth structure from fundamental spheroidal mode data free oscillation data reveal heterogeneity in the earth s mantle whose geographical pattern is dominated by spherical harmonics of angular degree two and correlates well with the hydrostatically referenced geoid the heterogeneity can be modelled as localized in the transition zone 420 670 km depth and may be related to a large scale component of mantle convection

alteration pattern and fluid inclusions of gold bearing quartz veins in archean trondhjemite near wawa ontario canada systems of auriferous quartz veins occur in archean granitic rocks within 1 6 km of the wawa greenstone belt 80 km northeast of wawa ontario the vein system at the renabie mine strikes west for approximately 850 m dips steeply to a depth of at least 950 m and hosts orebodies up to 30 m wide and 210 m long the six vein systems at the braminco prospect strike north to northwest dip steeply and are up to 20 m wide alteration of feldspar to quartz calcite white mica albite and of biotite amphibole to chlorite intensifies within 2 to 4 m of quartz vein systems bearing native gold mass balance calculations suggest that the alteration of granitic rocks added k co2 h20 rb and ba and removed na three types of primary fluid inclusions coexist in quartz from the auriferous vein systems type i the most abundant type is an aqueous solution with 10 to 25 vol percent co2 with density of 0 75 g cm3 homogenization temperatures range from 220 o to 360oc type ii is nearly 100 vol percent liquid co2 with a density of 0 95 g cm 3 type iii the least abundant type is an aqueous liquid with 10 to 50 vol percent h20 gas homogenization temperatures range from 120 o to 220oc apparent salinities of types i ii and iii based on clathrate and ice melting points are estimated to be 4 to 8 0 and 11 to 14 equiv wt percent naci respectively the preferred interpretation is that the gold bearing quartz veins formed from co2 1aden fluids of less than 10 equiv wt percent naci during regional greenschist metamorphism of granitic rocks the three types of fluid inclusions may represent two or three different generations of fluids trapped over a range of temperature and pressure another possibility is that the fluid inclusions represent samples of a fluid comprised of immiscible co2 and nacih20 phases entrapped during quartz crystallization the source of the gold is not known

repetitive hydraulic fracturing and shear zone inflation in an archean granitoid hosted ribbon banded au quartz vein system renabie area ontario canada abstract economically significant tonalite 2722 ma and trondhjemite 2728 ma hosted archean au quartz vein mineralization in the renabie area ontario located in a marginal part of the 10 15 km thick wawa domal gneiss terrane shows strong structural control major ore zones occur in e w and nw nnw striking ductile brittle high strain zones that are superimposed on an earlier regional foliation mineralized zones are up to 20 30 m wide mineable to a depth of 1300 vertical metres and dip typically at 60 80 angles production and reserves for the renabie mine totalled 6 million tonnes at 6 6 g t au and 2 g t ag net shear from kinematic indicators is oblique reverse south side up sinistral on e w structures e g nudulama renabie main zone depth 2475 level and west side up sinistral on nw nnw structures e g braminco 21 zone the strain features of the shear zone hosted mineralization together with evidence of reaction enhanced ductility associated with hydrothermal fluid wallrock interaction indicate that mineralization and hydrothermal alteration occurred synchronously with shear deformation the characteristic banded appearance book ribbon structure of the renabie veins which is typical of many archean and cordilleran au quartz vein systems is interpreted to be the result of inflationary vein formation by repeated increments of hydraulic fracturing and mineral deposition 250 750 associated with hydrothermal fluid overpressuring within and below the actively deforming ductile brittle shear zones crack seal type process fracture orientation was largely controlled by the anisotropy in rock tensile strength associated with the approximately planar shear fabric the element mineral association of renabie is typical of oxidized archean au quartz vein systems containing anhydrite hematite carbonate and tourmaline with a chalcophile association consisting of mos 2 cu pb sulphides native au ag and pb bi au ag tellurides key factors in the development of significant mineralization at renabie as compared with other usually small low grade archean granitoid hosted au quartz vein systems include the repetitive nature of the mineralizing process and the focusing of au bearing fluids within the actively deforming shear zones a specific and sufficiently large reservoir of overpressured hydrothermal archean au bearing fluid was generated at some location not yet identified in the archean crustal structure represented by the kapuskasing section

broad band trajectory mechanics author s vasco dw nihei k abstract 2019 published by oxford university press on behalf of the royal astronomical society we present a trajectory based solution to the elastodynamic equation of motion that is valid across a wide range of seismic frequencies that is the derivation of the solution does not invoke a high frequency assumption or require that the medium have smoothly varying properties the approach adopted from techniques used in quantum dynamics produces a set of coupled ordinary differential equations for the trajectory the slowness vector and the elastic wave amplitude along the ray path the trajectories may be determined by a direct solution of the governing equations or derived as the by product of a numerical wavefield simulation synthetic tests with interfaces and layers containing increasingly narrow transition zones indicate that the conventional high frequency trajectories associated with the eikonal equation bend too sharply into high velocity regions as the wavelength exceeds the transition zone width tests in a velocity model based upon mapped structural surfaces from the geysers geothermal field in california indicate that discrepancies between high frequency and broad band trajectories can exceed several hundred metres at wavelengths of 1 hz an application to a crosswell tomographic imaging experiment demonstrates that the technique provides a basis for the seismic monitoring of fluid flow along narrow features such as fracture zones

the structural evolution of the missanabie renabie gold district pre orogenic veins in an orogenic gold setting and their influence on the formation of hybrid deposits

a tomographic image of mantle structure beneath southern california we determined the variations in seismic structure beneath southern california by using a tomographic method of inversion on teleseismic p delays recorded with the southern california array the algorithm employed was a modified form of an algebraic reconstruction technique art used in medical x ray imaging deconvolution with an empirically estimated point spread function was also used to help in focusing the image the inversion reveals two prominent features beneath the region the first is a thin vertical wedge directly beneath the transverse ranges that is 2 3 faster than the surrounding region this feature deepens to the east attaining a maximum depth of about 250 km beneath the san bernardino mountains the second feature is a major zone of low velocity material that is 2 4 slow under the salton trough rift valley extending to a depth of about 125 km two possible explanations for the spatial association of the transverse ranges with the velocity anomaly below are lithospheric subduction or small scale sublithospheric convection in the region of the big bend of the san andreas fault the low velocity anomaly beneath the salton trough is consistent with convective upwelling there

trace element redistribution in high temperature deformed gabbros from east ligurian ophiolites northern apennines italy constraints on the origin of syndeformation fluids c n r centro di studio per la cristallochimica e la cristallografia via abbiategrasso 209 1 271 00 pavia abstract mg gabbros from east ligurian ophiolites northern apennines italy display a high temperature low pressure recrystallization localized along ductile shear zones in deformed gabbros the igneous diopside is recrystallized into granoblastic aggregates of neoblastic diopside and minor red brown amphibole the latter displays a pargasitic composition with high amounts of ai na a and ti 1 8 0 7 and 0 4 atoms per formula unit respectively major element composition of neoblastic minerals highlight equilibration temperature conditions in the range 800 950 c red brown ti pargasite also occurs as a minor interstitial constituent presumably growing from a residual trapped liquid in the differentiated lithologies fe ti diorites of the plutonic ophiolitic complex by means of ion microprobe sims technique rare earth la ce nd sm eu gd dy er yb and selected trace elements sr y cr v sc zr ti have been analysed in igneous and neoblastic diopside as well as in ti pargasites ti pargasites have also been analysed for f and c1 and compared with the halogen composition of the amphiboles mainly hornblendes to actinolites which are related to the subsequent low temperature brittle evolution neoblastic ti pargasite from deformed mg gabbros bears close compositional similarities with igneous ti pargasite from undefonned fe ti diorites whereas it is geochemically distinct from the amphiboles post dating the ductile event in particular ti pargasites have relatively high contents of f ree y zr and sr which are not consistent with crystallization in the presence of seawater derived hydrothermal fluids high grade recrystallization probably developed in the presence of volatile rich igneous fluids either trapped between the cumulus minerals or injected into the ductile shear zones from outside the local system an alternative hypothesis comprises the absence of fluid phase and the development of ductile shear zones in ti pargasite rich layers the petrological features of deformed gabbros recovered from present day slow spreading ridges and fracture zones bear close similarities with the investigated ophiolitic metagabbros in east ligurian ophiolites high grade ductile shear zones have been related to the initial stages of the uplift of the gabbro peridotite complex to the sea floor

travel times and station corrections for p waves at teleseismic distances approximately 3300 shallow focus earthquakes and 1000 seismic stations have been used in a study of p wave travel times and station residuals including azimuthal effects the events were selected from a catalog containing 160 000 earthquakes and those having uniform distance and azimuthal coverage were systematically relocated and used to refine p wave travel times and station corrections station corrections are provided for 994 seismic stations the station corrections involve three terms the static effect and two cosine terms with appropriate phase shifts they exhibit general consistency over broad geographic areas and where coverage is dense often show abrupt changes from one geological province to another the cos 2 terms appear to be due to upper mantle anisotropy and they correlate with the stress direction in the crust

robust inversion of iasp91 travel time residuals for mantle p and s velocity structure earthquake mislocations and station corrections using both p and s arrival time information 41 108 events in the international seismological centre isc catalog for the years 1964 to 1987 are relocated relative to the iasp91 velocity model the mean absolute horizontal relocation is 7 7 km and the mean absolute depth relocation is 9 1 km the mean absolute origin time shift is 1 2 s the relocation procedure increased the p residual standard deviation slightly from 2 3 s to 2 5 s while decreasing the s residual standard deviation from 6 8 s to 6 1 s when plotted as bottoming point averages the resulting iasp91 p and s arrival time residuals show coherent patterns as a function of geographic location an iterative lp residual norm minimization algorithm is used to estimate the set of p and s velocity variations as well as the earthquake relocation and seismographic station parameters which best explain the travel time residuals the procedure is robust in that extremely large travel time residuals which are common in the isc data do not unduly influence the velocity estimates both the p and s models of lateral heterogeneity contain prominent circum pacific low velocities 1 to 2 perturbations underlying the back arc basins between 35 and 200 km depth this ring of negative deviations continues into the depth interval 200 400 km the continental cratons are underlain by high velocity anomalies with maximum amplitudes of 2 iceland and the azores are underlain by low velocity mantle material that extends down to at least 400 km the benioff zones are only intermittently imaged as 1 2 high velocity regions in the uppermost 400 km they are best resolved in the p velocity variations both the p and s velocity models contain a circum pacific ring beneath the benioff zones of 1 2 positive velocity deviations in the depth range 660 870 km coherent high velocity features are seen below south america borneo tonga fiji the marianas and the northern kuriles the anomalies beneath south america and borneo extend into the 870 1070 km depth range below depths of 1270 km for p variations and 1070 km for s variations the amplitude of the heterogeneity has decreased significantly it is only in the lowermost mantle 2670 km to the core mantle boundary that the level of p heterogeneity rises significantly above the estimated noise level in this depth range a partial ring around the pacific basin is observed although this

3 dimensional structure of the indian ocean inferred from long period surface waves to improve the lateral resolution of the first global 3 dimensional models of seismic wave velocities regional studies have to be undertaken the dispersion of rayleigh waves along 86 paths across the indian ocean and surrounding regions is investigated in the period range 40 300 s the regionalization of group velocity according to the age of the sea floor shows an increase of velocity with age up to 150 s only similar to the results in the pacific ocean but here this relationship vanishes more quickly at long period therefore the correlation of the deep structure with surface tectonics seems to be shallower in the indian ocean than in the pacific ocean a tomographic method is applied to compute the geographical distributions of group velocity and azimuthal anisotropy and then the 3 d structure of s wave velocity horizontal wavelengths of 2000 km for velocity and 3000 km for azimuthal anisotropy distribution can be resolved except for the central part of the south east indian ridge which displays high velocities at all depths the inversion corroborates a good correlation between lithospheric structure down to 120 km and surface tectonics low velocities along the central and southeast indian ridges velocity increasing with the age of the sea floor high velocities under african indian and australian shields at greater depths the low velocity zones under the gulf of aden and the western part of the southeast indian ridges hold but the low velocity anomaly of the central indian ridge is offset eastward the low velocity anomalies suggest uprising material and complex plate boundary

whole mantle p wave travel time tomography abstract a method of tomographic inversion to obtain three dimensional velocity perturbations in the earth s whole mantle has been developed and applied to more than two million p wave arrival time data reported by international seismology center isc the model is parameterized with 32 768 blocks the divisions in latitude longitude and radius are 32 64 and 16 respectively horizontal cell size is 5 6 5 6 the layer thicknesses vary with depth 29 km just below the surface and 334 km just above the core mantle boundary starting from a spherically symmetric earth model we obtained a three dimensional model using the following iterative procedures first we relocated all the events second we backprojected the residual into the whole mantle third we refined the spherically symmetric earth model the solutions have been converged in five iterations we adopted the following techniques in the backprojection procedure the first order smoothness was introduced as a damping which makes the solution independent of the starting model and its apparent fluctuation minimal the basic equations for delay times and smoothness are solved using the conjugate gradient method an iterative method which guarantees the convergence of solution into the exact least squares solution the weight on the smoothness i e the damping factor was objectively determined by a simplified cross validation technique the final solution was obtained as an average of the ten solutions each of which was derived from one tenth of the total data set the reliability of the solution is examined in two ways 1 mapping the resolution given by the reconstruction of checkerboard patterns and 2 mapping the variance given from the gaussian noise input this is the first result which delineates the whole mantle structure with its reliability mapping the longest wavelength anomalies of the lower mantle are similar to those of previous studies by dziewonski 1984 and hager and clayton 1989 however the shorter wavelength patterns have many discrepancies the fast anomalies of the uppermost mantle beneath the shield and the slow anomalies beneath the active tectonic regions are clearly seen we also revealed the l 2 pattern at the transition zone reported by the previous surface wave and free oscillation studies the most striking feature is that of the fast anomalies lying along the extension of the large subducting plate up to depths of about 1000 km or more the resolving powers in these regions are generally good

an inversion for radial viscosity structure using seismic tomography using a uniform velocity density scaling we have inverted the seismically inferred 3 dimensional structure of the whole manfie for the radial viscosity structure which best fits the geoid we are able to obtain 60 72 variance reductions for three different s wave tomographic models the resulting viscosity structures are remarkably similar showing a high viscosity lid a low viscosity zone in the transition region and a high viscosity lower manfie a resolution analysis indcates that the viscosity structure in the upper manfie is well resolved by the data however the resolution in the lower manfie is poorer our models axe in general agreement with previous studies except that our inversions prefer a low viscosity layer at 400 670 km as opposed to 100 400 kin

normal mode and free air gravity constraints on lateral variations in velocity and density of earth s mantle with the use of a large collection of free oscillation data and additional constraints imposed by the free air gravity anomaly lateral variations in shear velocity compressional velocity and density within the mantle dynamic topography on the free surface and topography on the 660 km discontinuity and the core mantle boundary were determined the velocity models are consistent with existing models based on travel time and waveform inversions in the lowermost mantle near the core mantle boundary denser than average material is found beneath regions of upwellings centered on the pacific ocean and africa that are characterized by slow shear velocities these anomalies suggest the existence of compositional heterogeneity near the core mantle boundary

shrimp u pb zircon triassic intrusion age of the finero mafic complex ivrea verbano zone western alps and its geodynamic implications interstitial subhedral slightly pink zircons have been separated from a diorite of the external gabbro unit of the finero mafic complex which crops out in the northernmost tip of the ivrea verbano zone ivz southern alps they are characterized by oscillatory zoning under cathodoluminescence cl a highth u ratio 0 9 0 4 and associated high u th pb rare earth element ree ta nb li and p contents consistent with a magmatic origin the mean of the single concordant ages is 232 3 ma which is here proposed to constrain the timing of intrusion of the finero mafic complex most of these magmatic zircons exhibit an outer rim up to 50 mm thick with a bright emission with transgressive and or sub concordant contacts to the internal oscillatory zoned domains in which the th u ratio is decreased as are the u th pb ree ta and nb contents these rims give younger slightly discordant pb u ages between 219 3 ma and 205 3 ma which are interpreted to highlight a fluid assisted recrystallization event the gabbros also have an interstitial zircon population consisting of rounded small colourless bright to milky crystals with blurred zoning patterns in cl these zircons have low u th pb nb and ta contents and lowth u ratio down to 0 08 the overall metamorphic character together with field evidence of the presence of blocks of paragneiss belonging to the crystalline basement at the roof of the external gabbro unit indicates that these zircons are inherited from local crustal rocks they have u and pb isotopic compositions that are strongly altered and discordant showing an upper limit of the timing of recrystallization consistent with the middletriassic emplacement age obtained from the magmatic zircons it is concluded that the finero mafic complex can no longer be considered as a part of the huge permian mafic complex of the central ivz it follows that the intrusive record preserved in the ivz allows us to look at the interplay between the continental crust and mantle melts through time i e permian totriassic in this scenario the modest crustal assimilation shown by the finero mafic complex can be interpreted in the first instance as the consequence of the limited volume of intruded melt less than 2 km in thickness however the possible role played by the decreased fertility of the metamorphic country rocks owing to a inferred permian magmatic event must be better constrained the intrusion of the finero mafic complex may represent the deep crustal counterparts of the ladinian volcanism that is widespread throughout the southern alps the geodynamic

deep crustal oxygen isotope variations the wawa kapuskasing crustal transect ontario we have studied the oxygen isotopic composition of rocks from a 100 km transect through the central superior province of ontario representing progressively the shallower terrains of the kapuskasing structural zone ksz the wawa gneiss terrane wgt and the michipicoten greenstone belt mgb these rocks range in age from 2 76 to 2 60 ga and correspond to a section through approximately 20 km of crustal thickness equivalent lithologic types have similar range of 18o values at each crustal level tonalitic to granodioritic rocks 6 4 to 9 5 dioritic and anorthositic rocks 5 5 to 7 6 mafic gneisses group 1 majority 5 7 to 7 1 group 2 8 1 to 9 5 18o values exhibit a remarkable correlation with sio2 values similar to that observed in unaltered plutonic rocks of equivalent composition paragneisses have significantly higher 18o values 9 3 to 12 2 low grade metavolcanic and metasedimentary rocks of the mgb are 18o enriched compared to their high grade equivalents in the ksz and wgt 7 4 to 13 3 for mafic to felsic metavolcanic rocks 11 4 to 14 7 for clastic metasediments coexisting minerals exhibit 18o fractionation consistent with equilibrium but corresponding to uniform isotopic temperatures about 553 to 584 c across the entire transect lower than the inferred metamorphic temperatures in the highest grade ksz terrane the lack of distinctive isotopic differences between equivalent rock types in the ksz wgt and mgb suggests that there is no significant gradient in 18o with depth in the crust the majority of mafic gneisses group 1 appear to have been emplaced either as subaerial extrusives intrusive sills or less likely as submarine extrusives that were hydrothermally altered at high temperatures the less abundant group 2 mafic rocks have the 18o values typical of greenstones that were altered at low temperature by seawater and isotopically resemble low grade rocks in the michipicoten and abitibi belts in general no major changes in whole rock isotopic composition appear to have occurred during granulite facies metamorphism implying limited flux of water or co2 the continuous linear gradient in 18o versus sio2 in the high grade rocks cannot be due to differentiation of a mafic source magma a model involving an association between mantle derived mafic magma and 18o enriched crustal materials is more consistent with the oxygen isotopic and the ree data

re os molybdenite ages from the archean yellowknife greenstone belt comparison to u pb ages and evidence for metal introduction at 2675 ma the late archean yellowknife greenstone belt in the southwestern slave province hosts world class orogenic gold deposits with total production of 15 million ounces moz of gold the timing of gold mineralization is estimated to be ca 2600 ma based on the relative timing of d 2 structures and associated formation of deposit hosting shear zones the majority of studies on the deposits indicate that gold was emplaced by metamorphically driven hydrothermal fluids depositing gold in chemical and structural traps alternatively based on ore mineral compositions proximity to plutons and increased gold grades in the vicinity of quartz feldspar porphyry dikes it has been suggested that the gold deposits may also be related to the intrusions in this study we present two new re os molybdenite ages from two intermediate to felsic plutonic rocks and compare them with u pb zircon crystallization ages the duckfish aplite contains intergrown molybdenite and yields a re os model age of 2611 7 ma the same as a u pb zircon crystallization age of 2611 2 1 5 ma for the aplite and within assigned error of a previously reported 2608 3 ma u pb titanite age for the duckfish pluton the subvolcanic ryan lake pluton has synplutonic quartz veining with intergrown molybdenite that yields an re os model age of 2679 8 ma repeat analysis of 2675 7 within assigned error of the 2671 6 5 ma u pb zircon crystallization age of the pluton in both cases the re os and u pb chronometers survived later low to moderate temperature low pressure contact and regional metamorphism the ca 2675 ma ryan lake pluton and associated quartz feldspar porphyry dikes have gold contents significantly higher than the least altered ore hosting mafic volcanic rocks but the pluton is significantly older than the ca 2600 ma relative age of the gold deposits the age data and the fact that an early pluton carries elevated gold suggest that similar plutons and associated quartz feldspar porphyry dikes may have fertilized the greenstone belt prior to formation of the orogenic gold deposits we suggest that when ca 2600 ma metamorphically derived fluids interacted with these older rocks the gold budget in the hydrothermal fluid could have been increased as evidenced by increased gold grades near porphyries an intrusion related early introduction of gold into the yellowknife greenstone belt may have implications for other archean gold deposits where there is a spatial relationship between gold and early plutonic rocks

mapping the upper mantle three dimensional modeling of earth structure by inversion of seismic waveforms a method is presented for the inversion of waveform data for the three dimensional distribution of seismic wave velocities the method is applied to data from the global digital networks international deployment of accelerometers global digital seismograph network the selected data set consists of some 2000 seismograms corresponding to 53 events and 870 paths the moment tensors of the earthquakes are determined through an iterative procedure which minimizes the corrupting influence of lateral heterogeneity a global model is constructed for shear wave velocity expanded up to degree and order 8 in spherical harmonics and described by a cubic polynomial in depth for the upper 670 km of the earth s mantle although no a priori information is incorporated the model predictions reproduce much of what is known about the dispersion of mantle waves for example high phase velocities for shields low velocities at ridges and a strong degree 2 pattern for rayleigh waves since the method makes use of complete waveforms overtone data are also included it is shown that the model is reproducible in that substantially the same model can be constructed from each half of the total data set considered independently the model shows that shields and ridges are major features in the depth interval 25 250 km the ridges of the southern pacific and the larger shields persist to 350 km but the southeast indian rise is underlain by a high velocity anomaly at this depth as is much of the mid atlantic ridge at 450 650 km the major features are a broad region of high velocities incorporating south america much of the south atlantic and parts of west africa a broad region of low velocities in the central and eastern pacific high velocities in the western pacific and a low velocity anomaly beneath the red sea and the gulf of aden in the absence of a crustal correction degrees 2 and 3 show a high positive correlation with the geoid paradoxically this is largely destroyed when the distribution in crustal thickness is taken into account spherical harmonic degrees 4 7 show a significant negative correlation

a hypothesis for base and noble metal deposits chemical deposits in the red sea salton sea and cheleken peninsula have concentrations of cu zn pb ag and some au brines emanating from these base metal systems have over 10 cl and have chalcophile metals in excess of sulphide in solution siliceous sinters laden with au ag hg and sb exist around vents and wells in california western nevada and new zealand hot co2 h2o fluids that exhale from these noble metal systems have less than 2 cl and sulphide in excess of chalcophile metals some geothermal systems around the world have features intermediate between these two end members cu fe and ag au concentrations occur around the margin of an archean felsic stock in the wawa greenstone belt in ontario canada the interpretation is that an initial seawater dominated system concentrated exhalites rich in base metals this culminated with regional metamorphism and the concentration of precious metal bearing veins from low salinity fluids

isotopic evidence from the ivrea zone for a hybrid lower crust formed by magmatic underplating the large mafic complex overlying mantle peridotites in the ivrea zone of northern italy originated by underplating of mantle derived magma onto the base of continental crust and may be a unique outcrop of typical mafic lower crust sampled elsewhere only by mafic xenoliths neodymium and strontium isotope data show that the magmas assimilated large and roughly uniform proportions of crustal material the resulting hybrid magmas have the same isotopic signature as and may represent the lower crustal roots of upper crustal hercynian granitoid intrusions

chemical evolution of a large mafic intrusion in the lower crust ivrea verbano zone northern italy the ivrea verbano and adjacent strona ceneri zones have been described collectively as a section through the continental crust while resident in the lower crust amphibolite to granulite facies paragneiss of the ivrea verbano zone was intruded by huge volumes of mafic to intermediate plutonic rocks grouped as the mafic complex growth of the mafic complex involved hypersolidus deformation in an extensional environment isotopic and trace element variations close to the axis of this structure indicate crystallization from mantle derived melts that were extensively contaminated by crustal material previous investigations determined that the contaminant was fingerprinted by 87sr 86sr 0 71 18o 10 12 5 and a positive eu anomaly in the present study the contaminant is also shown to have been enriched in ba with respect to rb and k charnockites associated with paragneiss septa in the lower part of the mafic complex have the appropriate chemistry to be samples of the contaminating material these chemical features can be explained by melting of granulite facies paragneiss which had previously been depleted in k and rb by an earlier melting event the ba enrichment in the core of the mafic complex can be modeled by a replenishment tapping fractional crystallization rtf process operating within a small magma chamber is repeatedly replenished by mantle melts and contaminated by ba rich charnockite very high ba k in the lower part of the complex are tentatively attributed to chemical exchange between the cumulate framework and infiltrating anatectic melts from underlying paragneiss septa in contrast to the mafic complex the chemistry of coeval granites in the adjacent strona ceneri zone reflect a component derived from crustal rocks that had not been significantly depleted by a previous melting event significantly the incompatible trace element abundances in the mafic complex and strona ceneri granites are similar to model compositions for the lower and upper crust respectively

tectonic setting of late archean bimodal volcanism in the michipicoten wawa greenstone belt ontario the tectono stratigraphic relationships depositional environments rock associations and major and trace element compositions of the late archean 2744 2696 ma bimodal basalt rhyolite volcanic rocks of the michipicoten wawa greenstone belt ontario are compatible with an origin along a convergent plate margin that varied laterally from an immature island arc built on oceanic crust to a more mature arc underlain by continental crust this environment is similar to that of the cenozoic taupo kermadec tonga volcanic zone michipicoten basaltic rocks most of which are proximal deposits compositionally similar la yb n 0 63 1 18 to modern oceanic island arc tholeiites are interpreted as having formed along the largely submerged island arc voluminous michipicoten rhyolitic pyroclastic rocks la yb n 4 3 18 7 ybn 5 7 15 9 probably erupted subaerially from the continental arc with distal facies deposited subaqueously on the adjacent oceanic island arc and proximal facies deposited in subaeri

regional 3 d heterogeneities by waveform inversion application to the atlantic area the waveform inversion method described in woodhouse dziewonski 1984 was modified to retrieve regional scale 3 d heterogeneities by using the minor arc part of seismograms the lateral heterogeneities are expanded horizontally into blocks 10 10 and radially into legendre polynomials up to order 3 0 670 km and thus the results show much fine details of lateral variation than previous global scale studies we assumed that the heterogeneities produce the perturbation of eigenfrequencies which are the minor arc average of local eigenfrequency shift we applied the method to the upper mantle beneath the atlantic ocean and its environments care was taken about the weighting of the data set we found that the fit of each seismogram became better when the weighting of each seismogram is proportional to the inverse of initial data residuals resolution is good in the triangular region surrounded by south america europe and north america resolution is not good in the south atlantic because of the poor path coverage depth resolution is not clear because of the use of legendre polynomials though the results suggest a broad half width of the order of 200 km or more we found some similarities between previous global studies and our results for example low velocities beneath the east pacific rise chile rise and azores triple junction and a high velocity canadian shield are obtained however there are also differences the high velocity zone beneath the brazilian shield at shallow depth is not a prominent feature in this study instead we found a somewhat unexpected feature near the romanche and vema fracture zones where shallow positive anomalies exist smoothed results calculated by the spherical harmonic expansion are also shown for the purpose of comparison with global studies

a worldwide comparison of predicted s wave delays from a three dimensional upper mantle model with p wave station corrections abstract this paper is a comparison on a worldwide scale between p station corrections deduced from isc residuals dziewonski and anderson and synthetic s station corrections computed using a three dimensional upper mantle model obtained from mantle wave data woodhouse and dziewonski the upper mantle s velocity model is described by a spherical harmonic expansion up to degree 8 the p station corrections are smoothed using a similar expansion in order that the two data sets can be compared correlations between p station corrections t p and synthetic s station corrections t s relevant to various depths of integration indicate that a station correction contains information about structures down to at least 670 km for this depth of integration the correlation coefficient of the two data sets is 0 59 the slope a of the relation t s a t p b obtained for the worldwide distribution of stations is in good agreement with results of previous regional studies using direct readings of p and s arrival times a 3 61 0 13 an analysis of regional variations of the relation t s a t p b is carried out on the basis of two published global tectonic patterns okal jodan results for oceanic regions are not reliable due to the lack of data on continental areas a significant difference appears between mountains a 2 7 0 3 and shields a 4 5 0 4 for okal s pattern a 5 7 1 5 for jordan s pattern the largest a value for shields rules out an explanation by partial melting as proposed in previous studies thermal heterogeneities lead to low a values undulations of the lithosphere asthenosphere boundary appear to be the most feasible explanation of the high slope beneath shields they are also able to explain the range of variation of the station corrections the lowest values of the station corrections correspond to a total vanishing of the low velocity zone beneath the oldest shields for mountains the mean values of the station corrections as well as the low a value can be accounted for by a slight increase of poisson s ratio together with a significant density increase

depletion of heavy rare earth elements in metamorphic minerals from adirondack anorthosites repartitioning of rare earth elements ree during the grenville granulite facies metamorphism of adirondack anorthosite has produced metamorphic minerals depleted in the heavy ree relative to concentrations predicted by igneous partition coefficients the heavy ree depletion is observed in garnet bearing metamorphosed anorthosites and results from closed system repartitioning of ree among heavy ree rich garnet and other recrystallizing minerals the resulting heavy ree depleted patterns contrast with primary igneous mineral patterns which conform to predictions from igneous partition coefficients whole rock ree patterns of adirondack anorthosites are not affected by the local ree rearrangement anorthosite ree patterns reflect their primary igneous mineralogy and are independent of any secondary metamorphic minerals formed during the grenville metamorphic event consequently the ree did not move more than a few centimetres during the granulite facies metamorphism

signature of remnant slabs in the north pacific from p wave tomography summary a 3 d ray tracing technique was used in a global tomographic inversion in order to obtaintomographicimagesofthenorthpacific thedatareportedbythegeophysical surveyofrussia 1955 1997 wereusedtogetherwiththecataloguesoftheinternational seismologicalcenter 1964 1991 andtheusgeologicalsurveynationalearthquake information center 1991 1998 and the recompiled catalogue was reprocessed the final data set used for following the inversion contained 523430 summary ray paths the whole of the earth s mantle was parametrized by cells of 2 2 and 19 layers the large and sparse system of observation equations was solved using an iterative lsqr algorithm asubhorizontalhigh velocityanomalyisrevealedjustabovethe660kmdiscontinuity beneaththealeutiansubductionzone thishigh velocityfeatureisobservedatlatitudes of up to 70 n and is interpreted as a remnant of the subducted kula plate which disappeared through ridge subduction at about 48ma a further positive velocity perturbation feature can be identified beneath the chukotka peninsula and okhotsk sea extending from 300 to 660km depth and then either extending further down to 800km chukotka or deflecting along the 660km discontinuity okhotsk sea this high velocity anomaly is interpreted as a remnant slab of the okhotsk plate accreted to siberia at 55ma

the gold carbonate association source of co2 and co2 fixation reactions in archaean lode deposits abstract abundant carbonate is a ubiquitous feature of archaean lode gold deposits both in domains of alteration enveloping veins and in rocks enclosing auriferous sediments detailed studies of chemical mass balance in metabasalts progressively altered towards gold bearing quartz carbonate veins disposed within shear zones at yellowknife reveal massive additions of co2 k si and fe accompanying mineralisation with concomitant depletions of na coherent behaviour of al sc zr also v and nb provides a reference for constraining the volume relations during hydrothermal alteration in the peripheral regions of alteration depletions of ca and mg result in overall volume reduction but these elements are added to veins and their immediate alteration envelopes where there is a large positive volume change precious metals together with cr ni cu zn pb rb cd and ba have been added to the veins and altered wall rocks whereas quartz noble metals and other trace elements have been precipitated from hydrothermal fluids solution of ca mg and some fe indigenous to peripheral alteration regions combining with co2 from the mineralising reservoir appears to be the process for forming the abundant ferri dolomite gangue observations of many archaean au deposits reveals that carbonate chemistry reflects the nature of wall rocks with wall rocks donating the bivalent metal cations and hydrothermal fluids the co2 alteration reactions at yellowknife involved hydrolysis of albite accompanied by fixation of kaqueous to produce muscovite with na loss and hydrolysis of chlorite epidote with co2 fixation to form ferri dolomite studies of oxygen isotopes fe 2 fe 2 fe 3 and structure reveal that au quartz and carbonate were precipitated in the presence of fluids of probable metamorphic origin 18 o 8 9 at low redox potential and at ambient temperatures of 400 450 c during episodes of hydrofracturing the abundant co2 and k required for extensive carbonate muscovite replacement alteration could be supplied by fluids released during prograde metamorphism under greenschist or greenschist amphibolite facies conditions where the relative proportion of co 2 h 2 o is in the order of 0 2 to 0 5 and k na 1 given high co2 co in the hydrothermal reservoir these molecules may act as complexing agents for transport of au and other rare elements e g w pd ni cr

how deep can we see the high velocity anomalies beneath the japan islands high velocity anomalies are found in the lower mantle beneath the japan islands and their vicinity through an analysis of detailed three dimensional p wave velocity structure based on a tomographic inversion of isc data beneath the sea of japan the high velocity anomalies in the lower mantle are slab like and appear to extend below the deep focus earthquakes of the high velocity pacific slab descending from the japan trench this slab like high velocity body extending into the lower mantle steepens below a depth of 500 600 km resolution analyses confirms the existence of a slab like high velocity region in the lower mantle though the depth extent is not well resolved on the other hand the pacific slab descending from the izu bonin trench seems to be fingering that is high velocity anomalies are penetrating into the lower mantle beneath latitude 26 n and not penetrating but horizontally bending in the deeper portion of the upper mantle around 29 n

3 d spatial variation of b values of magnitude frequency distribution beneath the kanto district japan summary a method is developed for estimation and interpolation of 6 values in space a 3 d spline function is considered for the logarithm of the 6 value at each location in the space since many parameters for the spline coefficients are required to obtain a sensible estimate of the spatial variation of 6 values we consider the penalized log likelihood with the standard roughness penalties for the spline function further the error bands of the b value estimation at each location can be calculated using the current method the spatial distribution of 6 values beneath the kanto district down to the depth of 100km is determined based on hypocentral data of microearthquakes from the kanto tokai observational network of the national research center for disaster prevention the stability of the estimated pattern is checked by comparing with the results using alternative cut off magnitudes this is further ensured by comparison with the result obtained by an alternative model using equally divided blocks on the whole the vertical change in 6 value is greater than the horizontal one it is high in the crust of the eurasian plate especially above the upper boundary of the subducting pacific plate and in the northwest part or the volcanic area in the kanto district a steep decrease of the 6 values is seen to take place in perpendicular direction to the subducting pacific plate boundary also a similar change is seen in the boundary between the eurasian and phillippine sea plates especially beneath the southern part of the kanto plain the 6 value is low in the upper boundary of the subducting plates but high in the lower plane of the double seismic zone in the pacific plate it appears that even within a narrow area of aftershocks the 6 value can change significantly it is also found that the variation of the 6 value estimate is in good agreement with the structure of seismic wave fractional velocity perturbations the regions of high and low 6 values correspond respectively to the lower and higher parts of the p wave velocity the similar relationship is seen with the spatial structure of the seismic wave attenuation

multi stage melting in the lower crust of the serre southern italy the lower crustal section exposed in the serre southern italy consists part of the section the most depleted metagreywackes underwent melt extraction of about 9 13 vol the two stage melting mainly of al rich metasediments which underwent granulite facies metamorphism partial melting and melt extraction the paper occurred during the prograde metamorphism and continued during the isothermal decompression considers the formation of melts in metapelites and metagreywackes leucosomes and host rocks have been studied to investigate the melting process biotite rich and biotite free melanosomes with scarce felsic components are present the biotite rich types are widespread in the upper part of the section and the two types may occur side

primary positive eu anomaly in clinopyroxenes of low crust gabbroic rocks abstract positive eu and sr anomalies have been found in whole rocks and clinopyroxenes as well as feldspars in gabbros of the deep crustal mafic intrusion of the ivrea zone northwestern italy these anomalies are particularly pronounced at one stratigraphie level of the intrusion the upper part of the upper zone of the layered series huz the eu eu ratios range from 19 to 40 in plagioclases and from 3 to 14 in the coexisting clinopyroxenes mass balance calculations show that metamorphic redistribution of eu from plagioclase cannot account for the positive eu anomalies of the clinopyroxenes and that a corresponding anomaly in the parent melt is required speculatively this feature is attributed to mixing of a mantle component with crustal highly polymerized melts deriving from advanced anatexis of granulite facies metasediments the existence in the deep crust of hybrid melts having a positive eu anomaly may be important for the eu balance between the lower and upper crust

seismic wave dispersion and attenuation in aheim dunite an experimental study summary a machine has been developed which provides for the study of both shear modulus dispersion and internal friction in geological materials through the observation of forced torsional oscillations of low frequency 10 1000 mhz and strain amplitude 1000 c the eventual goal is an understanding of the dispersion and attenuation of seismic shear waves in the earth s upper mantle measurements have been made on cylindrical specimens of an olivine rich rock from aheim norway which contains in addition to olivine about 10 per cent pyroxene and 5 10 per cent of hydrous silicate phases dominantly clinochlore serpentine and talc in order to separate aspects of the mechanical behaviour associated with the olivine aggregate from those attributable to the hydrous phases and or their dehydration products the specimens were either previously fired under controlled oxygen fugacity at 1200 c for 24 hours in order to effect complete dehydration within the olivine stability field or simply oven dried at 110 c linearity of the mechanical behaviour has been demonstrated by the amplitude independence of the results for the strain amplitude range 10 8 10 6 and by the quantitative consistency between the observed modulus dispersion and that calculated from the measured internal friction through the kramers kronig relations of linear theory both the pressure dependence at room temperature and the temperature dependence at 300 mpa of the shear modulus and internal friction have been investigated much of the variation of the shear modulus is attributed to changes of crack porosity associated with changes in pressure and temperature and with in situ dehydration temporal evolution of q 1 towards a lower asymptotic value over periods of hours of exposure to given conditions of high pressure and temperature is tentatively attributed to the gradual diminution of enhanced anelastic relaxation associated with regions of decaying stress concentration at asperities on cracks and or grain boundaries at the highest pressures and temperatures achieved in this study 300 mpa and 1000 c marked dispersion of the shear modulus amounting to 5 per cent between periods of 3 and 100 s and concomitant strong internal friction varying with oscillation period to approximately as 0 011to1 6 are observed the energy dissipation appears to be concentrated within rather than at the boundaries between the olivine grains these results provide the clearest indication yet that solid state probably intragranular anelastic relaxation in ultramafic rocks gives rise to losses comparable with those observed seismologically in the earth s upper mantle although the mechanistic basis for the observed anelasticity remains to be established

three dimensional velocity map of the upper mantle beneath the pacific ocean as determined from rayleigh wave dispersion abstract a three dimensional shear wave velocity model of the upper mantle beneath the pacific is determined from rayleigh wave phase velocities at periods 60 200 s without any a priori regionalization the results clearly indicate that the velocity distribution in the uppermost 150 km is correlated with the surface tectonics the seismic velocities of the young regions such as the east pacific rise and the west coast of north america are lower than the average pacific structure by 2 5 and those of the old regions such as the northwest pacific are higher by 2 4 as the depth increases the low velocity region in the southeast pacific seems to move to the northwest a wide area of the central pacific shows low velocity below depths of 200 250 km an area off the coast of western canada exhibits a velocity lower by 4 5 above 200 km but changes to a velocity higher by 4 5 below that depth cross sections of the marginal sea regions around the pacific the tasman sea the philippine sea the sea of japan the okhotsk sea and the bering sea have quite different features from each other the bering sea has a velocity lower by 1 2 above 100 km and below 300 km and higher by 1 at depths 150 250 km the philippine sea and the sea of japan have a velocity lower by 1 2 at depths 50 150 km and a velocity higher by 1 2 at depths 200 400 km the bismark islands region and the south fiji basin show similar velocity changes at a depth of about 250 km and 150 km respectively the tasman sea is characterized by low velocity in the upper 400 km there is a good correlation between high velocity and seismicity below 150 km in the marginal sea regions

remobilization in high grade metamorphic environments abstract most preserved massive sulphide ores have undergone regional metamorphism and attendant deformation and do in consequence express various degrees of remobilization particularly in high grade metamorphic environments deformation produces elongation effects chaotic folding and foliation development together with flowage structures and ductile injections of sulphides many such structures are however the product of deformational events concurrent with the waning portion of the metamorphic cycle this is commonly indicated by an intense retrograde schistosity in wallrocks adjacent to piercement sulphide bodies irrespective of whether deformation accompanies prograde or retrograde metamorphism there is evidence that most mechanical remobilization is fluid facilitated on the microscale equidimensional coarse grained annealed sulphide fabrics are characteristic the annealed texture is commonly retextured by later events which result in preferred orientations kinking and related evidence of crystal plastic behaviour in massive sulphide deposits an appreciation of the rheological contrast between most sulphides altered silicate host rocks and unaltered host rocks will enable the mine geologist to see through deformation effects and recognize gross relationships characteristic of a submarine exhalative genesis recognition of such gross relationships is opposed to the concept of grossly discordant remobilization maximum relative movements are usually in the order of 100 m although transgressive remobilization in the order of 1000 m has been recognized metamorphism promotes chemical redistribution of components along local chemical gradients this can result in migration of components across the wallrock ore interface there is substantial support for metahydrothermal redistribution into veins of certain components of an orebody undergoing high grade metamorphism there is some support for the formation of sulphide neomagmas and sulphide silicate pegmatitic magmas but the evidence is equivocal and remains contentious most chemical remobilization is on a scale of tens of metres but it is also possible over considerably greater distances in the broken hill area australia chemical remobilization by hydrothermal processes accompanying regional metamorphism has occurred over distances of kilometres and has formed new economic deposits

geodynamical implications from the correlation of surface geology and seismic tomographic structure abstract we have investigated the relationship between the lateral seismic structure in the mantle and the geological features associated with major mantle upwellings the oceanic ridge system and hotspots using the technique of correlation analysis we have identified that these features extend quite deeply into the mantle to a depth of around 1300 km this finding is surprising for the ridges because it indicates that the style of upwelling is rather active and is somewhat two dimensional in character the confidence level of the correlation between ridges and slow seismic regions is maintained at a level of greater than 90 to a depth of 1300 km this is followed by a sharp decrease to 50 at 1700 km a similar correlation pattern is found for the hotspots in both cases the correlation maxima lie close to a depth of 1000 km where remnants of old slabs have recently been revealed by correlation analysis these findings suggest that dynamical processes at a depth of around 1000 km can be linked with surface tectonic activity this correlation disappears at a depth of around 1500 1700 km certain features of the surface tectonic pattern can again be recognized around a depth of 2000 km and in the deepest mantle where major hotspot plumes and the remnant old lithosphere are the dominant features there may be some dynamical relationship between the ridge system and the two megaplumes we have proposed a physical mechanism based on viscous heating within megaplumes to explain the two peaks in the correlation between the hotspots and the seismic heterogeneities

insights into the genesis of orogenic gold deposits from the zhengchong gold field northeastern hunan province china abstract the zhengchong gold deposit is located in the central jiangnan orogen belt where gold ore occurs mainly as quartz veins altered slates and minor breccias hosted by shear zones in the lengjiaxi group based on its vein crosscutting relationships mineral assemblages and paragenetic sequences mineralization in this deposit is divided into four stages stage 1 quartz q1 pyrite stage stage 2 quartz q2 pyrite arsenopyrite stage stage 3 quartz q3 pyrite polymetallic sulfide mineral stage and stage 4 quartz q4 carbonate stage two types of fluid inclusions have been recognized in the quartz grains co2 bearing fluid inclusions c type and aqueous fluid inclusions w type fluid inclusions in hydrothermal quartz yield homogeneous temperatures and salinities of 312 406 c and 4 65 9 21 wt naclequiv stage 1 252 324 c and 2 20 8 10 wt naclequiv stage 2 193 296 c and 2 24 14 04 wt naclequiv stage 3 and 127 209 c and 2 57 6 59 wt naclequiv stage 4 the co occurrence of these inclusion types despite their variable fluid salinities can be explained by fluid boiling during stages 2 and 3 of mineralization possibly as a result of episodic pressure decreases linked to fault valve activity laser raman spectroscopic analysis results reveal that small quantities of ch4 and n2 are present in the stage 2 3 fluid inclusions fluid inclusion data and microthermometric results show that the ore fluids were low salinity and co2 rich yielding both co2 inclusions c type and aqueous inclusions w type the trapping pressures estimated from the c type inclusions are 80 180 mpa for stages 2 and 3 yielding gold mineralization depths of 7 8 km the mineralizing fluids had calculated 18oh2o values ranging from 3 7 to 8 2 and d values from 75 2 to 61 8 indicating derivation mainly from metamorphic fluids the sulfide minerals yield 34s values of 8 9 to 0 1 and 206pb 204pb 207pb 204pb and 208pb 204pb ratios of 17 83 18 12 15 67 15 76 and 38 03 38 62 respectively which differ significantly from the isotopic composition of the granodiorite 34s 2 4 to 1 0 206pb 204pb 18 57 19 28 207pb 204pb 15 69 15 76 208pb 204pb 38 97 40 34 however the s pb isotopic compositions of the sulfide minerals are similar to those of slates of the lengjiaxi group in the study area the zhengchong gold deposit probably originated from a metamorphic fluid system characterized by low salinity low density and co2 enrichment with limited admixture of magmatic fluids based on similarities to other orogenic gold deposits we infer that the zhengchong orebody represents an orogenic gold deposit formed during the late ordovician jiangnan orogeny caused by collision of the yangtze and cathaysia blocks

prospecting for water in the transition zone d ln vs d ln vp abstract we report the values of r d ln vs d ln vp due to water as a function of pressure for mg 2 sio 4 olivine wadsleyite ringwoodite the results show that ringwoodite has significantly lower r than olivine and wadsleyite due to the reason that bulk modulus of ringwoodite is more sensitive to water than shear modulus r of ringwoodite decreases with pressure while r of olivine and wadsleyite increases with pressure r reported here was calculated using theoretical method derived equation of states r due to temperature is also reported as a comparison of r due to water the local value of r in the upper mantle and transition zone can discriminate between lateral variations of water content and temperature as the origin of lateral seismic variations

the greenschist metamorphism of archaean synvolcanic stocks near wawa ontario canada two archaean synvolcanic stocks with contact aureoles occur in the wawa greenstone belt near wawa ontario canada the gutcher lake and jubilee stocks consist mainly of granitoid trondhjemite with feldspar laths mottled by white mica calcite epidote and rimmed by clear albite biotite is partly or wholly pseudomorphosed by chlorite sphene some epidote is partly altered to calcite chlorite the granitoid phase grades into a foliated phase of quartz albite white mica calcite chlorite near fracture zones traversing the stocks the alteration of the gutcher lake stock along its foliated margin involved addition of k2o h2o co2 mno plus rb loss of cao plus sr and a shift in fe 2 fet from 0 66 to 0 81 the alteration of the jubilee stock along the darwin shear involved addition of h2o co2 loss of sr and no significant shift in fe 2 fet the greenschist alteration also modified the contact aureoles bordering both stocks one interpretation is that regional metamorphism in the archaean overprinted a greenschist assemblage on both stocks the alteration was intense near fracture zones and sporadic remote from fractures lower integrated water to rock ratios along the darwin shear compared to the margin of the gutcher lake stock may explain the comparatively lower perturbation of the element abundances and redox state of iron

the genesis and tectonic control on archaean gold deposits of the western australian shield a metamorphic replacement model abstract gold deposits occur in greenstone belts world wide and contribute to anomalously high gold production from archaean terranes as in other cratons archaean gold mineralization of western australia represents a complex array of deposit styles despite this most deposits are clearly epigenetic and large deposits have a number of features in common including their strong structural controls distinctive wallrock alteration fe sulphide k mica albite ca mg fe carbonates consistent metal associations au ag as sb w b low base metals commonly fe rich host rocks great depth extension and lack of appreciable vertical zonation these shared characteristics combined with their ubiquitous occurrence indicate that archaean gold deposits had a common origin related to the tectonic evolution of greenstone belts auriferous hydrothermal systems were broadly synchronous with regional metamorphism and emplacement of synkinematic granitoids and felsic porphyry intrusions although these gold systems involved low salinity lowdensity reduced near neutral h 2 o co 2 fluids carrying gold as reduced sulphur complexes the origin of the fluids is equivocal most timing evidence and stable isotope data cannot distinguish metamorphic from magmatic granitoid or felsic porphyry orggins but the lack of consistent spatial relationships between specific volumetrically significant intrusive phases and large gold deposits in a number of cratons strongly favours metamorphic derivation of fluids the metamorphic replacement model for gold mineralization involves devolatilization of the lower portions of the greenstone pile with high geothermal gradients inhibiting significant melting co 2 possibly formed by the decarbonation of early alteration related to mantle degassing along crustal scale synbasinal fault zones auriferous fluids were channelled along greenstone scale faults in part developed during reactivation of crustal scale faults in a strike slip regime gold deposition occurred largely under greenschist facies conditions about 300 400 c 1 2 kb in response to decreasing gold solubility with declining temperature however a major control on gold deposition was fluid wallrock interaction many large deposits formed by sulphidation of fe rich host rocks with synchronous deposition of fe sulphides and gold however the variable nature of gold depositing reactions including lowering of fo 2 and ph allowed a multitude of small and some large deposits to form wherever that fluid circulation occurred in consequence several of the relatively small deposits currently worked from open pit are hosted by ultramafic and felsic rocks there are few constraints on the source of components au s k co 2 added to gold deposits but even giant deposits such as the golden mile kalgoorlie could have formed from a realistic greenstone source volume ca 8 8 5 km convective circulation of fluids could have contributed to the generation of high fluid rock ratios on the regional scale the markedly heterogeneous distribution of large gold deposits gold productivity and host rocks to deposits can be accommodated by the metamorphic replacement model the most favourable conditions for development of auriferous hydrothermal systems operated in younger ca 2 7 0 1 ga rift phase greenstones where greatest extension and crustal thinning produced high geothermal gradients crustal scale synbasinal faults and rapid extrusion and burial of volcanics including abundant komatiites iron rich tholeiitic basalts and dolerites were preferred host rocks for large gold deposits the least favourable conditions existed in older ca 3 5 3 4 ga platformphase greenstones where gentle sagging on submerged continental crust produced eruption of mainly mafic volcanics with few komatiites commonly in very shallow water environments this allowed intense synvolcanic alteration of both gold source rocks and potential host rocks the generally smaller gold deposits formed mainly in ultramafic or greywacke hosts younger ca 3 0 ga platform phase greenstones appear intermediate in nature but unlike other greenstones have significant epigenetic gold deposits in originally oxide facies bif which were deposited on relatively deep water platforms similar controls appear to exist on a world scale with gold mineralization peaking at ca 2 7 0 1 ga in response to development of major rift zones in thickened relatively mature continental crust interestingly the giant witwatersrand goldfield formed at about the same time

measurements of mantle wave velocities and inversion for lateral heterogeneities and anisotropy 3 inversion lateral heterogeneity in the earth s upper mantle is investigated by inverting dispersion curves of long period surface waves 100 330 s models for seven different tectonic regions are derived by inversion of regionalized great circle phase velocity measurements from our previous studies we also obtain a representation of upper mantle heterogeneities with no a priori regionalization from the inversion of the degree 6 spherical harmonic expansion of phase and group velocities the data are from the observation of about 200 paths for love waves and 250 paths for rayleigh waves for both the regionalized and the spherical harmonic inversions corrections are applied to take into account lateral variations in crustal thickness and other shallow parameters these corrections are found to be important especially at low spherical harmonic order the trench region and fast velocities down to 250 km under shields below 200 km under the oceans both s velocity and s anisotropy support a model of small scale convection in which cold blobs detach from the bottom of the lithosphere when its age is large enough the spherical harmonic models clearly demonstrate a posteriori the relation between surface tectonics and s velocity heterogeneities in the first 250 km all shields are fast most ridges are slow below 300 km a belt of fast mantle follows the pacific subduction zones however at greater depths large scale heterogeneities that seem to bear no relationship to surface tectonics are observed the most prominent feature at 450 km is a fast velocity region under the south atlantic ocean smaller scale heterogeneities that are not related to surface tectonics are also mapped at shallower depths an anomalously slow region centered in the south central pacific is possibly linked to intense hot spot activity a very fast region southeast of south america may be related to subduction of old pacific plate between 200 and 400 km a belt of sv sh anisotropy follows part of the ridge and subduction systems indicating vertical mantle flow in these regions the spherical harmonic results open new horizons for the understanding of convection in the mantle perspectives for the improvement of the models presented are discussed

three dimensional velocity structure and resolution of the core mantle boundary region from whole mantle inversions of body waves the nature of the core mantle boundary cmb has important implications for deep earth processes particularly those which have their origin in the lower mantle or outer core the possibility of the existence of lateral p wave velocity variations in the outermost core is explored here by obtaining models of three dimensional cmb region velocity structure using an inversion of waveform data the numerical inversions involve determination of the three dimensional structure of the earth s outermost core and lower mantle using over 5000 long period seismic waveforms collected from global digital seismic networks spheroidal modes with periods between 33 and 100 s were selected to model the body wave portion of seismograms recorded from global earthquakes first order perturbations in s wave velocities in 11 mantle layers 200 350 km thick and p wave velocities in one outermost core layer comprise the least squares solutions to the inverse problem although the statistical f test supports the additional layer in the inversions this layer is not required by the f ratio to occur in the outermost core pattern retrieval resolution tests in which synthetic data constructed from an initial synthetic earth model are inverted for model parameters are used to compare parameters for one region to those of another in which some coefficient power smearing is suspected depth resolution tests indicate that the best radial resolution at the cmb is 500 km thus this method does not produce the resolution required for outermost core lateral velocity variation analysis an examination of the limitations of this approach demonstrates the need for waveform inversion techniques which more accurately predict raypaths in the deep mantle improvements in radial resolution are needed before whole mantle velocity models can begin to distinguish whether significant heterogeneity related to core mantle interactions occurs in the cmb region

magnetic mineralogy in an archean crustal cross section implications for crustal magnetization a traverse across precambrian rocks from the kapuskasing uplift in ontario provided 31 samples from a crustal cross section that is now exposed at the surface about half the samples are dominantly paramagnetic magnetite accompanied in a few samples by minor amounts of pyrrhotite is the only significant magnetic mineral in the other half magnetic susceptibilities of these samples are constant between room temperature and about 540 c above which temperature they drop rapidly to zero magnetic susceptibility does not depend on rock type apparent original depth in the crust or degree of metamorphism the average induced magnetization of the most magnetic units in this section is less than 1 a m which is much lower than the source requirements for the deep crust deduced from aeromagnetic and satellite magnetometer surveys

fluid evolution of the neoarchean pampalo orogenic gold deposit e finland constraints from la icpms fluid inclusion microanalysis abstract the sources of fluids and precious metals in orogenic gold deposits remain controversial and a key question is whether auriferous magmatic hydrothermal fluids are a decisive ingredient for economic orogenic gold mineralization contributions of magmatic hydrothermal fluids have been proposed based on indirect arguments such as isotope data or emplacement of granites coevally with gold deposits by contrast complete elemental concentration data for orogenic gold fluids which could be used to compare them directly with those of magmatic hydrothermal fluids are not yet available this study reports the major and trace element chemistry of fluid inclusions from the neoarchean pampalo orogenic gold deposit located in the hattu schist belt in eastern finland three successive generations of gold mineralized and barren veins provide the paragenetic and chronological framework for establishing the fluid evolution history of the pampalo deposit based on petrography microthermometry and la icpms fluid inclusion microanalysis five major fluid types were identified the data shows that all fluid types despite characteristic differences carry the signature of orogenic gold fluids i e low to intermediate salinity presence of co 2 ch 4 or n 2 high s contents enrichment in b as sb and w and low pb and zn concentrations gold concentrations vary significantly between the oldest ca 0 6 g g fluid related to the zones with highest gold grades in the deposit and later fluid types

mapping the lower mantle determination of lateral heterogeneity in p velocity up to degree and order 6 the data from the international seismological centre bulletins for the years 1964 1979 are used to derive a three dimensional model of lateral variations of the p velocity in the lower mantle unlike previous studies in which such perturbations were represented by a blocklike parameterization we seek the solution in the form r k 0k l 0l m 0l k r kalm cos m kblm sin m plm cos some 500 000 travel time residuals for teleseismic distances from 5000 earthquakes are used in an iterative procedure to derive the coefficients a and b the maximum k and l used are 4 and 6 respectively in each iteration in addition to solving for the three dimensional structure we also relocate all the earthquakes perturb the average travel time curve and determine station corrections for over 1000 stations particular attention is given to the problem of weighting the individual observations in order to avoid as much as possible the bias due to the uneven distribution of sources and receivers the resulting model shows a high level of perturbations just below the 670 km discontinuity and just above the core mantle boundary where the maximum perturbations reach 1 1 5 of the average velocity even for this highly smoothed model at a depth of 2000 km the rms perturbations are some 3 to 4 times lower than at either of the two boundaries the model predicts well the large scale pattern of observed travel time residuals for various source regions except for the distinct effects of the subduction zones the most striking large scale three dimensional feature of the model is a ring of high velocities circumscribing the pacific basin within a depth range from 1000 km to the core mantle boundary if the proportionality between perturbations in the velocity and temperature is assumed the derived pattern of anomalies could be consistent with a two boundary layer model of convection in the lower mantle some caution however should be exercised in interpreting the results near the 670 km discontinuity where the resolving power of our data set is limited by the lack of adequate control of the upper mantle structure at the same time a significant global increase in the level of perturbations in the lowermost mantle is established beyond question the results of dziewonski et al 1977 on the correlation between the equivalent geoid and the observed geoid for the angular order numbers 2 and 3 is confirmed but there is no significant correlation for degrees from 4 to 6 these calculations are made for the rigid earth richards and hager this issue show that introduction of finite viscosity can change the sign of a geoid anomaly in addition recent studies of lateral variations in the shear velocity reveal the presence of large scale anomalies in the upper mantle thus while the inferences made here with respect to the origin of the gravest orders of the geopotential field are not conclusive they point the way in which results from seismology can be used to address some of the basic questions of geodynamics

seismic structure of the continental crust based on rock velocity measurements from the kapuskasing uplift the michipicoten greenstone belt mgb wawa gneiss terrane wgt and kapuskasing structural zone ksz in ontario canada are regarded as a 25 km thick partial cross section through the crust of the superior province we measured compressional wave velocities vp at confining pressures up to 600 mpa for representative rock samples from these terranes vp for mgb is variable 6 1 7 km s and depends on composition and anisotropy compressional wave velocities for low anisotropy quartzofeldspathic gneisses and intrusive rocks from the wgt and the ksz show little variation over a wide range of silica content but mafic gneisses and anorthositic rocks show a wide velocity range over a narrower silica range a trend corroborated by data for other high grade rocks a model of the continental crust based on the laboratory data shows three megalayers with differing seismic characteristics the shallowest levels mgb are seismically heterogeneous with high average velocities 6 6 km s at 100 mpa because of the dominance of metabasalts the balance of the upper crust wgt is seismically homogeneous as it is dominated by a variety of isotropic quartzofeldspathic rocks with average velocities of 6 35 km s at 100 mpa and 6 6 km s at 600 mpa the uppermost lower crust ksz is layered seismically heterogeneous and transversely isotropic the average velocity at 600 mpa is 6 84 km s the boundary between wgt and ksz is marked by a velocity increase of between 0 2 and 0 3 km s an increase similar to that commonly associated with midcrustal discontinuities i e conrad in shield areas here this boundary corresponds to the amphibolite granulite facies transition and to a change from a tonalitic middle crust to a lithologically heterogeneous uppermost lower crust refraction results show an anomalous high velocity zone in the upper crust under the ksz that on the basis of laboratory velocity data can be correlated with ksz lithologies these data indicate that ksz rocks can be traced to the west to depths as great as 20 km levels deeper than 20 km not exposed at the surface in the ksz are characterized by velocities greater than 7 0 km s and may be dominated by mafic and anorthositic lithologies or by more garnet rich rocks the velocity model of the superior province crust based on geological reconstruction and laboratory velocity data is in general agreement with the velocity structure to depths of about 25 km determined for the crust below the mgb from refraction experiments the velocity structure of this model is also generally similar to the velocity structure of shield areas

the greenschist facies of an archean assemblage near wawa ontario a stock of trondhjemite intrudes an archean succession metamorphosed to the greenschist facies at gutcher lake 30 km north of wawa in ontario the stock is 4 km2 in plan and is partly enveloped by

sphalerite geobarometry in the cu fe zn s system phase relations have been determined among sphalerite pyrite pyrrhotite and intermediate solid solution iss from 439 degrees to 725 degrees c at 1 bar and 353 degrees to 760 degrees c at 5 kb the extents of solid solution in the four phase assemblage are described as functions of temperature in kelvins by the following equations log mole zns in iss 1 bar 2 795 1 632 t super 1 log mole zns in iss 5 kb 2 659 1 608 t super 1 log mole cus in pyrrhotite 1 bar 2 546 1 866 t super 1 log mole cus in sphalerite 1 bar 4 202 3 755 t super 1 and log mole cus in sphalerite 5 kb 4 084 3 791 t super 1 the pressure dependence in kb of the composition of sphalerite in equilibrium with pyrite and pyrrhotite with or without iss can be expressed for the temperature independent portion of the sphalerite geobarometer as p 42 30 32 10 log mole fes because of the low solubility of copper in sphalerite in the four phase assemblage below 650 degrees c the presence of chalcopyrite in natural ores should be of no consequence to sphalerite geobarometry over its temperature independent portion nevertheless sphalerites which are inhomogeneous in fes and which contain abundant blebs of chalcopyrite in excess of the amount that can be reasonably accounted for by an equilibrium process of solubility and subsequent exsolution are a common feature of many sulfide ores such sphalerite grains should be avoided in applying the geobarometer the chalcopyrite blebs may be a product of a premetamorphic sphalerite chalcopyrite intergrowth or represent true exsolution from sphalerite in any one of 1 a former sphalerite pyrrhotite iss equilibrium 2 a stable four phase assemblage at temperatures above the temperature independent portion of the geobarometer or 3 a metastable four phase assemblage in which pyrite was inert use of the sphalerite geobarometer requires a thorough understanding of the textural relationships and discrimination of equilibrium from disequilibrium or low temperature reequilibrated assemblages

the growth of large mafic intrusions comparing niquelândia and ivrea igneous complexes abstract the niquel ndia complex brazil is one of the world s largest mafic ultramafic plutonic complexes like the mafic complex of the ivrea verbano zone it is affected by a pervasive high t foliation and shows hypersolidus deformation structures contains significant inclusions of country rock paragneiss and is subdivided into a lower and an upper complex in this paper we present new shrimp u pb zircon ages that provide compelling evidence that the upper and the lower niquel ndia complexes formed during the same igneous event at ca 790 ma coexistence of syn magmatic and high t subsolidus deformation structures indicates that both complexes grew incrementally as large crystal mush bodies which were continuously stretched while fed by pulses of fresh magma syn magmatic recrystallization during this deformation resulted in textures and structures which although appearing metamorphic are not ascribable to post magmatic metamorphic event s but are instead characteristic of the growth process in huge and deep mafic intrusions such as both the niquel ndia and ivrea complexes melting of incorporated country rock paragneiss continued producing hybrid rocks during the last vanishing stages of magmatic crystallization this resulted in the formation of minor late stage hybrid rocks whose presence obscures the record of the main processes of interaction between mantle magmas and crustal components which may be active at the peak of the igneous events and lead to the generation of eruptible hybrid magmas

seismic reflection profiles across deep continental crust exposed in the kapuskasing uplift structure seismic imaging of precambrian cratons holds the promise of deep structural mapping and interpretation of fundamental crustal construction processes deep structures may be identified by tracing reflections to the surface1 particularly in exposed crustal cross sections2 elevated along deeply penetrating faults but many such cross sections have structural orientations too steep for successful reflection imaging like the ivrea zone3 or occur in plate boundary settings not representative of continental interiors by contrast the archaean greenstone terrain exposed in the kapus kasing uplift of canada4 5 has gently dipping structures and an intracratonic setting within the superior province thus providing an opportunity to examine the third dimension of continental crust down to 25 30 km palaeodepth6 here we present new seismic reflection data which although generally supportive of structural models based on surface geology4 gravity modeling5 and seismic refraction studies7 8 also indicate that the kapuskasing structure is a relatively thin thrust sheet containing seismically reflective sequences of high grade rock which may be analogues of seismi cally reflective in situ lower crust

inaa idms and sims comparative ree investigations of clinopyroxenes from mantle xenoliths with different textures abstract clinopyroxenes from mantle xenoliths from the massif central france have been chosen for a comparative study of rare earth element ree abundances obtained by means of conventional and in situ analyses xenoliths from hungary and the canary islands showing unusual trace element or mineralogical features have also been investigated secondary ion mass spectrometry sims has been shown to be a powerful technique for both in situ and bulk investigations competitive with instrumental neutron activation analysis inaa and isotope dilution mass spectrometry idms in the quantification of ree in clinopyroxenes in addition the application of sims for microspot analysis provides reliable information about ree concentrations with respect to texture when sims is applied for bulk investigations its major advantages are the simple and fast sample preparation and the small quantity of material required the results obtained in this paper indicate that grain by grain and internal homogeneity of clinopyroxenes which is related to texture determines the degree of agreement between sims inaa and id the data for clinopyroxenes from equigranular xenoliths which recrystallized to almost equilibrium conditions show the best agreement for clinopyroxenes from protogranular lree depleted xenoliths the agreement is good when the whole clinopyroxene population has the same degree of depletion and the metasomatic la and ce enrichment is negligible when complex geochemical processes are investigated such as subsolidus reactions and interactions with fluids and melts conventional bulk techniques are often inadequate and the sims technique is the only one capable of thoroughly describing the effects of element migration at a microscale for a proper understanding of geochemical processes in the mantle it is here suggested that sims investigations in thin section are made prior to mineral selection for subsequent elemental and isotopic analysis by id

formation of gold deposits a metamorphic devolatilization model a metamorphic devolatilization model can explain the enrichment segregation timing distribution and character of many goldfields such as those found in archean greenstone belts slate belts and other gold only provinces in this genetic model hydrated and carbonated greenschist facies rocks particularly metabasic rocks are devolatilized primarily across the greenschist amphibolite facies boundary in an orogenic setting devolatilization operates on the scale of individual mineral grains extracting not just h2o and co2 but also s and in turn au elevated gold in solution is achieved by complexing with reduced s and by h2co3 weak acid buffering near the optimal fluid ph for gold solubility the buffering is more important than being at the point of maximum gold solubility low salinity ensures low base metal concentrations in the auriferous metamorphic fluid migration of this fluid upwards is via shear zones and or into hydraulic fracture zones in rocks of low tensile strength the geometry of the shear zones dictates the kilometre scale fluid migration paths and the degree of fluid focusing into small enough volumes to form economic accumulations of gold deposition of gold from solution necessitates breakdown of the gold thiosulphide complex and is especially facilitated by fluid reduction in contact with reduced carbon bearing host rocks and or by sulphidation of wallrocks to generate iron bearing sulphide and precipitated gold as such black slate carbon seams banded iron formation tholeiitic basalt magnetite bearing diorite and differentiated tholeiitic dolerite sills are some of the important hosts to major goldfields gold deposition is accompanied by carbonation sulphidation and muscovite biotite alteration where the host rock is of suitable bulk composition the correlation of major gold deposits with rock type even when the gold is primarily in veins argues for rock dominated depositional systems not fluid dominated ones as a consequence a general role in gold deposition for fluid mixing temperature decrease and or fluid pressure decrease and boiling is unlikely although such effects may be involved locally several geological features that are recorded at gold only deposits today reflect subsequent modifications superimposed upon the products of this generic metamorphic devolatilization process overprinting by higher grade metamorphism and deformation and or palaeo weathering may provide many of the most obvious features of goldfields including their mineralogy geochemistry geometry small scale timing features geophysical response and even mesoscopic gold distribution

late kinematic timing of orogenic gold deposits and significance for computer based exploration techniques with emphasis on the yilgarn block western australia abstract orogenic gold deposits are a widespread coherent group of epigenetic ore deposits that are sited in accretionary or collisional orogens they formed over a large crustal depth range from deep seated low salinity h2o co2 ch4 n2 ore fluids and with au transported as thio complexes regional structures provide the main control on deposit distribution in many terranes first order faults or shear zones appear to have controlled regional fluid flow with greatest ore fluid fluxes in and adjacent to lower order faults shear zones and or large folds highly competent and or chemically reactive rocks are the most common hosts to the larger deposits focusing of supralithostatic ore fluids into dilatant zones appears to occur late during the evolutionary history of the host terranes normally within d3 or d4 in a d1 d4 deformation sequence reactivation of suitably oriented pre existing structures during a change in far field stress orientation is a factor common to many deposits and repeated reactivation may account for multiple mineralization episodes in some larger deposits absolute robust ages of mineralization support their late kinematic timing and in general suggest that deposits formed diachronously towards the end of the 100 to 200 m y long evolutionary history of hosting orogens for example in the yilgarn block a region specifically emphasised in this study orogenic gold deposits formed in the time interval between 40 and 90 m y with most about 60 to 70 m y after the youngest widespread basic ultrabasic volcanism and towards the end of felsic magmatism the late timing of orogenic gold deposits is pivotal to geologically based exploration methodologies this is because the present structural geometries of i the deposits ii the hosting goldfields and iii the enclosing terranes are all essentially similar to those during gold mineralization at least in their relative position to each other thus interpretation of geological maps and cross sections and three dimensional models can be used to accurately simulate the physical conditions that existed at the time of ore deposition it is particularly significant that the deposits are commonly related to repetitive and predictable geometries such as structural heterogeneities within or adjacent to first order structures around rigid granitoid bodies or in specific locked up fold thrust structures importantly the two giant greenstone hosted goldfields kalgoorlie and timmins show a remarkably similar geometry at the regional scale computer based stress mapping and gis based prospectivity mapping are two computer based quantitative methodologies that can utilize and take advantage of the late timing aspect of this deposit type to provide important geological aids in exploration both in broad regions and more localized goldfields both require an accurate and consistent solid geology map stress mapping requires knowledge of the far field stresses during mineralization and the empirical prospectivity mapping requires data from a significant number of known deposits in the terrane the kalgoorlie terrane in the yilgarn block meets these criteria and illustrates the potential of these methodologies in the exploration for orogenic gold deposits low minimum stress anomalies interpreted to represent dilational zones during gold related deformation coincide well with the positions of known goldfields rather than individual gold deposits in the terrane and there are additional as yet unexplained anomalies the prospectivity analysis confirms that predictable and repetitive factors controlling the siting of deposits are i proximity to and orientation and curvature of granitoid greenstone contacts ii proximity to segments of crustal faults which strike in a preferred direction iii proximity to specific lithological contacts which have similar preferred strike iv proximity to anticlinal structures and v the presence of preferred reactive host rocks e g dolerite the prospectivity map defines a series of anomalous areas which broadly conform to those of the stress map 78 correspondence the most prospective category on this map covers less than 0 3 of the greenstone belts and yet hosts 16 of the known deposits which have produced 80 of known gold thus it discriminates in favour of the larger economically more attractive deposits in the terrane the successful application of stress mapping and prospectivity mapping to geology based exploration for orogenic gold deposits indicates that more quantitative analysis of geological map data is a profitable line of research the computer based nature of these methodologies is ideal for the production of an ultimate integrated deposit target map which can be compared to other more conventional targeting parameters such as geophysical and geochemical anomalies such an integrated strategy appears the way forward in the increasingly difficult task of cost effective global exploration for orogenic gold deposits in poorly exposed terranes

aspherical heterogeneity of the mantle from phase velocities of mantle waves long period surface waves are used to map the lateral heterogeneity of the upper 100 600 km of the mantle there is good correlation of velocity with surface tectonics and heat flow convergence regions are generally slow for love waves and fast for rayleigh waves back arc basins have slower than average shallow mantle some island arcs show evidence of fast material at greater depth deep seated slow anomalies underlie the red sea afar region of north east africa western north american northern east pacific rise indian ocean triple junction and the tasman sea campbell plateau regions the fastest regions are in the north central australia new guinea and the south atlantic

mercury deposits of western california an overview mercury deposits in western california are near a thrust fault that separates two groups of mesozoic rocks the franciscan assemblage a metamorphosed melange with serpentine and graywacke is structurally overlain by the great valley sequence a sedimentary series resting on oceanic crust these mesozoic rocks are partly covered by volcanic and sedimentary rocks of cenozoic age cinnabar with silica minerals dolomite native mercury and bituminous matter occurs around the fractured margins of serpentine bodies and around hot springs that emanate from the franciscan assemblage fluid inclusions and hot springs suggest that cinnabar precipitated from co2 h2o fluids with 2 wt chlorine at t 250 c prograde metamorphism of mesozoic sediments expulsed mercury bearing fluids that migrated up serpentine related fractures and exhaled onto the surface

deep crustal structure and tectonic history of the northern kapuskasing uplift of ontario an integrated petrological geophysical study the northeast trending kapuskasing uplift transects the east west belts of the central superior province over a distance of some 500 km granulite to upper amphibolite facies rocks of the uplift form three distinct geological geophysical entities from south to north the chapleau groundhog river and fraserdale moosonee blocks uplift of the granulites along a moderately northwest dipping crustal scale thrust fault is attributed to an early proterozoic compressional event major northeast striking faults that bound the kapuskasing zone on the west were examined by modelling of geophysical anomalies to determine dip and by geobarometry of garnet orthopyroxene plagioclase quartz assemblages to determine vertical displacement granulites in the kapuskasing zone have 7 to 9 kbar signatures whereas those in the quetico belt to the west indicate metamorphic pressure of 4 6 kbar individual calibrations of the barometer yield consistent pressure differences of 2 3 kbar suggesting 7 10 km of west side down movement on the faults modelling of gravity and aeromagnetic gradients indicates westerly dips of 60 65 with west side down offset of up to 14 km these major normal faults probably formed as collapse structures in response to crustal thickening which occurred during the preceding compressional uplift stage differences in the configuration of individual blocks of the kapuskasing zone can be related to variable fault slip and intersection angles between normal and reverse faults thus the groundhog river and southern fraserdale moosonee blocks are perched thrust tips analogous to the sangre de cristo range of the laramide uplift province whereas the southern chapleau block is a tilted slab with similar configuration to the laramide wind river range pop up geometry deduced for the northern fraserdale moosonee block resembles the structure of the laramide uinta mountains a normal fault crosses the surface trace of the basal thrust fault between the groundhog river and fraserdale moosonee blocks and causes a 65 km gap without granulites this article contains supplementary material