ultramafic complexes and associated mineral deposits in the precambrian of eastern minas gerais brazil based on field work prospection and petrographic studies ultramafic complexes and their mineral deposits from eastern minas gerais in southeastern brazil are generally outlined to form a basis for future investigations in this region the bodies dealt with occur at ipanema corrego novo bela vista de minas rio pomba and liberdade these ultramafic bodies are generally enclosed in high grade gneisses and consist of serpentinized peridotites and harzburgites which were metamorphosed together with their country rocks in upper amphibolite to granulite facies weathering of these rocks gives rise to nickeliferous laterite while metamorphism has resulted in anthophyllite asbestos and talc deposits

two diamictites two cap carbonates two δ13c excursions two rifts the neoproterozoic kingston peak formation death valley california comment and reply the prima facie evidence for the age of the proterozoic kingston peak formation cited by prave 1999 1 is the 13c value of carbonate in metalimestones in the middle and upper parts of the formation those data and the c isotope composition of the overlying noonday dolomite were used by prave

polarimetric radar modeling of mixtures of precipitation particles with the advances of dual polarized radar techniques in meteorology it is now possible to deduce precipitation microphysical characteristics in far more detail than possible with reflectivity measurements alone radar parameters such as differential reflectivity and differential phase between horizontal and vertical polarizations have been studied in detail as well as linear depolarization ratio copolar correlation coefficient and backscatter differential phase while these parameters can be linked to certain microphysical properties of specific classes of precipitation such as raindrops or hail very little study has been directed at the practically important cases of mixtures of different types of precipitation particles such as rain hail graupel ice crystals and snow each type can have different size shape orientation and dielectric constant distributions the present treatment is rigorous and is based on the mueller matrix formulation radar parameters are derived from the averaged mueller matrix computations careful consideration is given to the orientation and size distributions of the different particle types after calculating single particle scattering characteristics some simple two component mixtures such as rain hail and ice crystals snow are considered finally a 2d numerical cloud model is used to simulate the rain hail graupel and snow fields of an evolving convective storm from which the radar parameters are derived for the initial peak and dissipating stages of the storm model computations are performed at c and s band frequencies

polymetamorphism in the panamint mountains california a 39ar 40ar study metamorphosed upper precambrian rocks in the panamint mountains show textural evidence for polymetamorphism previous isotopic studies have established the age of metamorphism as middle to late mesozoic this study was undertaken to try to determine the ages of the two metamorphic events using the 39ar 40ar stepwise degassing technique of the k ar method on biotite muscovite and hornblende separates the resulting spectra indicate that metamorphism occurred during the intervals 170 150 ma ago and 80 70 ma ago many analyses yield ambiguous results because of slow cooling following metamorphism contamination of the separates by refractory phases containing extraneous argon and in some cases the superposition of the two metamorphic events the ages of metamorphism correspond to periods of emplacement of granitic rocks in the vicinity of the panamint mountains suggesting that the high heat flux responsible for metamorphism was associated with the emplacement of the sierra nevada batholith the ages of metamorphism and granite emplacement are diachronous with the development of major thrust faults in the western great basin which suggests that metamorphism might have been triggered by the formation of nappes rather than causing the formation of nappes

properties of tropical and midlatitude ice cloud particle ensembles part i median mass diameters and terminal velocities abstract this is part i of a study that characterizes several bulk properties of ice particle populations sampled in midlatitude and tropical cirrus and deep stratiform ice clouds for the purpose of developing an understanding of how particles evolve in ice clouds and to derive empirical and analytical relationships that describe microphysical properties for use in cloud and climate models the effort focuses on describing the microphysical properties of ice cloud layers in the vertical the size distribution data and particle imagery were obtained during lagrangian spiral descents and balloon borne ascents through cloud layers that formed in association with synoptic scale lifting midlatitude and deep convection tropics temperatures ranged between 20 and 63 c for the midlatitude clouds and between 0 and 50 c for the tropical clouds optical depths spanned the range 0 4 7 for the midlatitude clouds and 20 30 for the tropical clouds this part of the study characterizes median mass diameter dm

brazilian structural provinces an introduction abstract the territory of brazil coincides almost entirely with the south american platform the crystalline core of the continent its basement is composed of ancient metamorphic and igneous rocks and it has not suffered any tectonic regeneration since the beginning of the phanerozoic sedimentary rocks with almost horizontal bedding cover this crystalline basement this latter shows ages as old as early precambrian although ages between 500 and 1000 m y are conspicuously frequent the cratonic areas became consolidated more than 1700 m y ago whereas the fold belts formed essentially between 500 and 1700 m y ago the sedimentary cover accumulated from the early silurian in three large intracratonic basins until the platform became completely stabilized a late jurassic to early cretaceous reactivation caused by the break up of the ancient gondwana continent created another basin sequence chiefly along the atlantic continental margin based on the nature of the crystalline basement rocks and the sedimentary cover ten structural provinces can be distinguished 1 rio branco province in the north of the country occupied by the guyana shield and still only poorly known with an important fold belt developed during the so called transamazonian cycle 2000 200m y with high grade metamorphic rocks and only slight influence of later events 2 tapajos province in central brazil corresponding to the amazonas or guaporecraton showing chiefly precambrian crystalline rocks and behaving as an cratonic area during the phanerozoic 3 sa o francisco province located on the atlantic shield with its basement covered by rocks of different ages chiefly affected by the brasiliano cycle between 1000 and 500 m y and constituting another cratonic area 4 tocantins province between the amazonas and sa o francisco cratons with the oldest rocks in its centre ages over 2600 m y and at the eastern and western borders metamorphic sequences of various fold belts and almost no phanerozoic deposits 5 mantiqueira province located along the southern part of the atlantic coast affected chiefly by the brasiliano folding cycle 6 borborema province in the northeast brazilian fold belt affected by the brasiliano cycle in a very complex way and with important faulted zones it was reactivated in phanerozoic times when also sedimentary covers accumulated 7 amazonas province represented by the amazonas sedimentary basin syneclise and subdivided into four parts by three important arcs 8 parnaiba province coinciding with the piauf maranha o syneclise and filled with a rather thick sedimentary sequence 9 paranaprovince the sedimentary basin of southern brazil in which the well known late paleozoic glaciation features are found and which possesses an extensive cover of basaltic rocks of late jurassic to early cretaceous age 10 coastal province and continental margin the youngest structural unit developed during the separation of the continent and represented by rift valleys and coastal basins filled with mesozoic cenozoic deposits of various kinds

the 40ar 39ar thermochronology of the eastern mojave desert california and adjacent western arizona with implications for the evolution of metamorphic core complexes mesozoic thickening and cenozoic extension resulted in the juxtaposition of upper and middle crustal rocks in the eastern mojave desert southeastern california and western arizona the application of 40ar 39ar thermochronology to rocks in this region provides information about the timing and nature of thrusting plutonism metamorphism denudation and detachment faulting the 40ar 39ar ages of 175 to 125 ma from the clipper piute turtle mohave bill williams and hualapai mountains are interpreted to be the result of a middle mesozoic thermal event s caused by crustal thickening the 40ar 39ar data from the clipper and piute mountains suggest that this thermal event was followed by a period of cooling at rates of 1 5 c m y orogenesis culminated during the late cretaceous when rocks exposed in the old woman piute chemehuevi and sacramento mountains attained temperatures 500 c which reset the k ar systems of minerals from proterozoic rocks high grade metamorphism in the old woman mountains area was caused by the intrusion of the old woman piute batholith at 73 1 ma cooling rates following batholith emplacement in the old woman mountains were 100 c m y between 73 and 70 ma and 5 10 c m y from 70 to 30 ma between 65 and 25 ma the entire eastern mojave desert underwent a period of cooling at a rate of 2 10 c m y by 30 ma rocks exposed in the old woman piute marble ship clipper and turtle mountains were below 100 c the 40ar 39ar ages from the sacramento mountains suggest that mylonitization caused by the onset of regional extension occurred at 23 1 ma when extension started in the chemehuevi mountains rocks exposed in the southwestern and northeastern portions of footwall to the chemehuevi detachment fault were at 180 c and 350 c respectively this suggests that the exposed part of the chemehuevi detachment fault initiated at a dip of 5 30 or as a series of higher angle faults that cut to a depth of 10 12 km and were later rotated to their present dip unroofing of the footwalls to detachment faults in the sacramento and chemehuevi mountains resulted in average cooling rates of 10 50 c m y between 22 and 15 ma

direct surface thermodynamic observations within the rear flank downdrafts of nontornadic and tornadic supercells abstract despite the long surmised importance of the hook echo and rear flank downdraft rfd in tornadogenesis only a paucity of direct observations have been obtained at the surface within hook echoes and rfds in this paper in situ surface observations within hook echoes and rfds are analyzed these mobile mesonet data have unprecedented horizontal spatial resolution and were obtained from the verifications of the origins of rotation in tornadoes experiment vortex and additional field experiments conducted since the conclusion of vortex the surface thermodynamic characteristics of hook echoes and rfds associated with tornadic and nontornadic supercells are investigated to address whether certain types of hook echoes and rfds are favorable or unfavorable for tornadogenesis tornadogenesis is more likely and tornado intensity and longevity increase as the surface buoyancy potential buoyancy as measured by the convective available potential energy and equivalent potential temperature in the r

evolution of extensional basins and basin and range topography west of death valley california neogene extension in the death valley region se california has produced a variety of sedimentary basins diachronous movements on an array of strike slip and normal fault systems have resulted in the uplift and preservation of older basins in modern ranges one of the best exposed of these is the nova basin on the western flank of the panamint mountains the nova basin includes over 2000 m of sedimentary and volcanic rocks deposited during denudation of the panamint mountains metamorphic core complex in late miocene early pliocene time the principal growth structure for the basin was the emigrant detachment which initiated and moved at a low angle modern panamint valley west of the range developed as a consequence of late pliocene recent kinematically linked movement on the right slip high angle hunter mountain fault zone and the low angle panamint valley detachment detailed mapping of the intersection between the emigrant and panamint valley detachments demonstrates that segments of the earlier system remained active during development of panamint valley and thus during development of modern basin and range topography as well these results indicate that large scale extension in the death valley region accommodated by movement on low to moderate angle normal fault systems and high angle strike slip fault systems is a continuing process basin and range topography in the panamint valley death valley area was generated at least in part by displacements on low angle detachments rather than high angle normal faults

post taconic tilting and acadian structural overprint of the classic barrovian metamorphic gradient in dutchess county new york field mapping and 40ar 39ar age spectrum dating of white mica across the classic metamorphic gradient in dutchess county new york indicate that post peak metamorphic and structural events complicate the interpretation of this sequence as an intact taconic metamorphic field gradient oriented samples were collected and fabrics were measured along two 7 km transects these transects extend from west of the biotite zone west of clove mountain to the sillimanite zone swamp river valley in eastern dutchess county field and petrographic analysis reveals three pervasive foliations s1 s2 and s3 s1 is folded in the lower grade rocks of the sequence but is increasingly overprinted at garnet and higher zones by a penetrative foliation s2 s2 dips steeply to the se and is axial planar to folds in s1 s2 is overgrown by porphyroblasts of biotite biotite chloritoid biotite garnet staurolite kyanite sillimanite at respective metamorphic grades that appear to approximate peak temperature conditions s1 s2 and peak temperature conditions are constrained as late taconic based on a depositional age of 460 ma potter 1972 for the walloomsac formation and a staurolite age of 454 ma lanzirotti and hanson 1997 from kyanite sillimanite grade rocks s3 is a greenschist facies foliation composed of muscovite biotite chlorite that truncates or overprints s2 in garnet through sillimanite grade rocks in the eastern part of the study area s3 dips gently to the s se and becomes more penetrative to the east such that in the garnet zone s2 is overprinted by a weakly developed spaced s3 cleavage while in many samples in the staurolite and higher metamorphic zones s2 micas are significantly overprinted by s3 micas s3 is observed wrapping around and truncating peak temperature porphyroblasts three samples of lepidoblastic white mica with varying proportions of s1 s2 and s3 from the biotite through staurolite zone all produce 40ar 39ar age spectra suggesting closure ages of 385 ma these white mica ages are interpreted to represent the time of cooling through white mica closure 350 c the 40ar 39ar ages in conjunction with field mapping and petrographic analysis suggest that s3 developed in the devonian acadian orogeny furthermore the 40ar 39ar data constrains the timing of tilting and uplift of the ordovician barrovian sequence from the middle silurian to early devonian this study suggests mid crustal wedging during the salinic orogeny as a possible mechanism for tilting the rocks of this region and exposing the taconic aged barrovian field gradient at the surface today we conclude the low grade acadian metamorphic overprint of these rocks involved deformation that produced a new third cleavage that was coincident with regional cooling from peak taconic metamorphic conditions and thus required strain but no reheating during the acadian

uranium lead isotopic ages from the sierra nevada batholith california this study provides new information on the timing and distribution of mesozoic magmatic events in the sierra nevada batholithic complex chiefly between 36 and 37 n latitude u pb ages have been determined for 133 zircon and 7 sphene separates from 82 samples of granitoid rocks granitoid rocks in this area range in age from 217 to 80 m y triassic intrusions are restricted to the east side of the batholith jurassic plutons occur south of the triassic plutons east of the sierra nevada as isolated masses within the cretaceous batholith and in the western foothills of the range cretaceous plutons form a continuous belt along the axis of the batholith and occur as isolated masses east of the sierra nevada no granitic intrusions were emplaced for 37 m y east of the sierra nevada following the end of jurassic plutonism however following emplacement of the eastern jurassic granitoids regional extension produced a fracture system at least 350 km long into which the dominantly mafic calc alkalic independence dike swarm was intruded 148 m y ago the dike fractures probably represents a period of regional crustal extension caused by a redistribution of the regional stress pattern accompanying the nevadan orogeny intrusion of cretaceous granitic plutons began in large volume about 120 m y ago in the western sierra nevada and migrated steadily eastward for 40 m y at a rate of 2 7 mm y this slow and constant migration indicates remarkably uniform conditions of subduction with perhaps downward migration of parent magma generation or a slight flattening of the subduction zone such steady conditions could be necessary for the production of large batholithic complexes such as the sierra nevada the abrupt termination of plutonism 80 m y ago may have resulted from an increased rate of convergence of the american and eastern pacific plates and dramatic flattening of the subduction zone u pb ages of the giant forest alaskite sequence in sequoia national park are all in the range 99 3 m y indicating a relatively short period of emplacement and cooling for this nested group of plutons u pb ages of a mafic inclusion and its host granodiorite indicate that both were derived from a common source or that the mafic inclusion was totally equilibrated with the granodioritic magma comparison of isotopic ages determined by different methods such as zircon u pb sphene u pb hornblende k ar and biotite k ar suggests that zircon u pb ages generally approximate the emplacement age of a pluton however some plutons probably contain inherited or entrained old zircons and the zircons of some samples are disturbed by younger thermal and metamorphic events the ages reported here are consistent with u pb age determinations previously made on granitic rocks to the north stern et al 1981 the age distribution of granitic belts determined here is in general agreement with those established by k ar dating evernden and kistler 1970 but does not differentiate the five epochs of plutonism determined in their study

lateritic nickel deposits of brazil many nickel deposits are known in brazil accounting for about 350 106 tons of ore with an average of 1 5 ni all are of the lateritic type these deposits are scattered throughout the country being rarer in the northeastern region and in the south below 25 s latitude they are mainly associated with mafic ultramafic massifs of large dimensions and ultramafic alkaline complexes and occur in climatic regions of contrasting seasons the weathering profile developed over the fresh rock consists from bottom to top of the following horizons altered rock coarse saprolite argillaceous saprolite ferruginous saprolite and lateritic overburden the thickness of each horizon varies from one deposit to another the whole profile generally exceeding 20 m the saprolitic horizons with inherited minerals serpentine chlorite or neoformed minerals smectites constitute the silicated nickel ore and are thicker were climatic conditions are drier the ferruginous upper horizons made up of iron oxide hydroxides are more developed in more humid regions in brazil the silicated ore generally prevails over the oxidized ore the main ni bearing minerals are serpentine smectite garnierite and goethite the lateritic nickel deposits of brazil may be correlated with two erosion surfaces corresponding to the sul americano lower tertiary and velhas upper tertiary levelling cycles the degree of dismantling of the higher and more ancient surface and the consequent development of the velhas surface control the position of the nickel accumulation in the landscape thus the deposits may be found either in the lowlands or in the highlands where they are always covered by a silcrete layer the alteration profiles in the brazilian lateritic nickel deposits are broadly similar to those described elsewhere in the world however they present two characteristic features the silicated ore prevails over the oxidized ore and a silicified layer covers the profies developed on the highlands

characteristics of deep tropical and subtropical convection from nadir viewing high altitude airborne doppler radar abstract this paper presents observations of deep convection characteristics in the tropics and subtropics that have been classified into four categories tropical cyclone oceanic land and sea breeze vertical velocities in the convection were derived from doppler radar measurements collected during several nasa field experiments from the nadir viewing high altitude er 2 doppler radar edop emphasis is placed on the vertical structure of the convection from the surface to cloud top sometimes reaching 18 km altitude this unique look at convection is not possible from other approaches such as ground based or lower altitude airborne scanning radars the vertical motions from the radar measurements are derived using new relationships between radar reflectivity and hydrometeor fall speed various convective properties such as the peak updraft and downdraft velocities and their corresponding altitude heights of reflectivity levels and widths of reflectivity cores are estimated the most significant

comparison of disdrometer and x band mobile radar observations in convective precipitation abstractmicrophysical data from thunderstorms are sparse yet they are essential to validate microphysical schemes in numerical models mobile dual polarization x band radars are capable of providing a wealth of data that include radar reflectivity drop shape and hydrometeor type however x band radars suffer from beam attenuation in heavy rainfall and hail which can be partially corrected with attenuation correction schemes in this research the authors compare surface disdrometer observations to results from a differential phase based attenuation correction scheme this scheme is applied to data recorded by the national oceanic and atmospheric administration noaa x band dual polarized noxp mobile radar which was deployed during the second verification of the origins of rotation in tornadoes experiment vortex2 results are presented from five supercell thunderstorms and one squall line 183 min of data the median disagreement radar disdrometer in attenuation corrected reflectivity z and

the thermal significance of potassium feldspar k ar ages inferred from age spectrum results 40ar39ar age spectrum analyses of three microcline separates from the separation point batholith northwest nelson new zealand which cooled slowly 5 c ma 1 through the temperature zone of partial radiogenic 40ar accumulation are characterized by a linear age increase over the first 65 percent of gas release with the lowest ages 80 ma corresponding to the time that the samples cooled below about 100 c the last 35 percent of 39ar released from the microclines yields plateau ages 103 99 and 93 ma which reflect the different bulk mineral ages and correspond to cooling temperatures between about 130 to 160 c theoretical calculations confirm the likelihood of diffusion gradients in feldspars cooling at rates 5 c ma 1 diffusion parameters calculated from the 39ar release yield an activation energy e 28 8 1 9 kcal mol 1 and a frequency factor grain size parameter d0l2 5 6 3 9 14sec 1 this arrhenius relationship corresponds to a closure temperature of 132 13 c which is very similar to the independently estimated temperature from the observed diffusion compensation correlation this d0l2 implies an average diffusion half width of about 3 m similar to the half width of the perthite lamellae in the feldspars the range in microcline k ar ages from the separation point batholith is the result of relatively small temperature differences within the pluton during cooling comparison of the diffusion laws determined for microcline with those for anorthoclases and other homogeneous k feldspars e 40 to 52 kcal mol 1 reveals that ar diffusion is more highly temperature dependent in the disordered structural state than in the ordered structural state previously published u shaped age spectra are probably the result of the superimposition of excess 40ar upon diffusion profiles of the kind described here

surface in situ observations within the outflow of forward flank downdrafts of supercell thunderstorms abstract in the long standing conceptual model of a supercell thunderstorm the forward flank downdraft ffd and its associated negative buoyancy originate from precipitation loading and the latent chilling of air due to the melting and evaporation of precipitation the horizontal buoyancy gradient within the outflow of the ffd has been identified as an important source of low level streamwise vorticity in three dimensional numerical simulations of supercells these simulations have demonstrated that the formation of low level mesocyclones is critically dependent on the baroclinic generation of horizontal vorticity within the ffd outflow despite the implied dynamical importance of the ffd outflow in the evolution of supercell thunderstorms only a very limited number of thermodynamic observations have been obtained within ffd outflow the range of thermodynamic conditions within ffd outflow is not well known nor is it known whether any systematic relationship exists between the thermodynamic character

measurement and interpretation of hailstone density and terminal velocity abstract the mass density fall velocity and internal structure have been determined for low density hailstones from an truly spring storm in boulder colorado and drag coefficients were calculated the density of the permeable hail is determined from a new method of volume measurement the results are compared to those of particles grown in a one dimensional parcel model using the appropriate sounding the implications of the results are discussed with reference to hail growth in convective storms