trading partners tectonic ancestry of southern africa and western australia in archean supercratons vaalbara and zimgarn original connections among the world s extant archean cratons are becoming tractable by the use of integrated paleomagnetic and geochronologic studies on paleoproterozoic mafic dyke swarms here we report new high quality paleomagnetic data from the similar to 2 41 ga widgiemooltha dyke swarm of the yilgarn craton in western australia confirming earlier results from that unit in which the primary origin of characteristic remanent magnetization is now confirmed by baked contact tests the corresponding paleomagnetic pole 10 2 degrees s 159 2 degrees e a 95 7 5 degrees in combination with newly available ages on dykes from zimbabwe allow for a direct connection between the zimbabwe and yilgarn cratons at 2 41 ga with implied connections as early as their cratonization intervals at 2 7 2 6 ga the proposed zimgarn supercraton was likely distinct from vaalbara kaapvaal plus pilbara at 2 4 ga but both of those entities independently fragmented at ca 2 1 2 0 ga reassembling into the kalahari and west australian cratons by 1 95 1 8 ga c 2012 elsevier b v all rights reserved

synthetic fluid inclusions in natural quartz iii determination of phase equilibrium properties in the system h2o nacl to 1000 c and 1500 bars phase equilibria in the system h2o nacl have been determined to 1000 c and 1500 bars using synthetic fluid inclusions formed by healing fractures in inclusion free brazilian quartz in the presence of the two coexisting immiscible h2o nacl fluids at various temperatures and pressures petrographic and microthermometric analyses indicate that the inclusions trapped one or the other of the two fluids present or mixtures of the two salinities of the two coexisting phases were obtained from heating and freezing studies on those inclusions which trapped only a single homogeneous fluid phase results of the present study are consistent with previously published data on the h2o nacl system at lower temperatures and pressures and indicate that the two phase field extends well into the p t range of most shallow magmatic hydrothermal activity as a consequence chloride brines exsolved from many epizonal plutons during the process of second boiling should immediately separate into a high salinity liquid phase and a lower salinity vapor phase and produce coexisting halite bearing and vapor rich fluid inclusions this observation is consistent with results of numerous fluid inclusion studies of ore deposits associated with shallow intrusions particularly the porphyry copper deposits in which halite bearing and coexisting vapor rich inclusions are commonly associated with the earliest stages of magmatic hydrothermal activity

hypocentral trend surface analysis probing the geometry of benioff zones a hypocentral trend surface is a continuous function of latitude and longtitude fitted by least squares to a set of hypocenters so that it predicts depth to the middle of a benioff zone in this paper we take a relatively simple approach to hypocentral trend surface analysis the hypocentral trend surface is constructed from a spherical surface harmonic expansion whose coefficients are selected so as to minimize the standard vertical deviation between hypocenters and trend surface the vertical deviation of a hypocenter from the hypocentral trend surface is called its residual consideration of a hypocentral trend surface cannot be divorced from consideration of the associated residuals and their spatial distribution any analysis of benioff zone geometry trades off presumed thickness of the zone with its presumed shape this trade off can be investigated by examining suites of candidate hypocentral trend surfaces together with their associated residual distributions generated by varying the number of degrees of freedom available to the trend surface hypocentral trend surfaces are generated for three high quality sets of hypocenters obtained by local seismic networks in honshu japan cook inlet alaska and south peru hypocenters beneath honshu generate a bimodal distribution of residuals about their trend surface the vertical separation of the upper and lower sheets of this double benioff zone averages about 33 km no double benioff zone configuration is discovered below cook inlet or south peru regional trend surfaces are established for intermediate depth teleseismic data from south and middle america in south america 0 40 s teleseismic data suggest that along strike transitions between flat and moderately steeply dipping sections of the benioff zone are achieved by flexure of a coherent slab rather than by fragmentation of the slab into tear bound flaps or separate tongues of lithosphere hypocentral trend surface analysis of teleseismic data from south peru suggests a geometry similar to that indicated by hypocentral trend surface analysis of the local data of hasegawa and sacks an eastward shallowing benioff zone beneath southwest mexico is closely associated with the geometry of the trans mexican volcanic arc the benioff zone beneath central america is more steeply dipping the flat slab steep slab transition near 96 w may relate to the difference in age of oceanic lithosphere on either side of the tehuantepec ridge while the strong flexure of the subducted nazca plate beneath ecuador is closely associated with the carnegie ridge the nazca ridge extropolates slightly north of the flexure in subducted lithosphere beneath south peru the flexure appears instead more closely associated with the pronounced inflection in the map view shape of the leading edge of the overriding south american plate

fluid assisted interaction of peraluminous metapelites with trondhjemitic magma within the petronella shear zone limpopo complex south africa abstract the principal problem concerning the evolution of precambrian granulite complexes located between granite greenstone cratons is their interaction with the underthrusted greenstone belts during exhumation from the lower crust besides evidence for pervasive and localized fluid fluxes arising from devolatilization of the greenschist and amphibolite facies rocks in the course of their burial underneath the granulites this interaction might also be expressed in formation of diverse magmas of granitic trondhjemitic and granodioritic composition we present results of a petrological fluid inclusion and thermobarometric study of interaction between fluidized trondhjemitic magma and peraluminous metapelitic granulites associated with the regional high grade petronella shear zone located in the southern marginal zone smz of the limpopo granulite complex south africa the hot t 1000 c trondhjemitic magma which presumably originated from partial melting of a basaltic amphibolite material at the base of the granulite complex or at the top of the underthrusted greenstone blocks intruded granulites at p 8 0 9 5 kbar 24 28 km depth at 2 667 0 9 ga during the exhumation of the smz the magma heated and assimilated orthopyroxene cordierite metapelites and dragged them to a depth of 18 20 km 6 3 6 5 kbar the magma was heterogeneously saturated with mgo feo al 2 o 3 by the dissolved metapelites this process provoked crystallization of several garnet generations from the trondhjemitic melt various mineral assemblages included in the different generations of garnet allowed application of twq method combined with perple x pseudo sections to trace sub isobaric cooling of the magma from t 900 600 c at 5 5 6 5 kbar the isobaric cooling also affected the associated metapelites fluid inclusions trapped in garnet and quartz in the trondhjemite show that the magma transported carbonic fluid with densities corresponding to the late stages of magma cooling 600 650 c and 5 5 6 5 kbar carbonic fluids coexisted with aqueous salt fluids preserved as inclusions with salinity up to 20 58 wt nacl eq these low water activity fluids a h 2 o 0 3 bearing na k and ca salts being exsolved from the magma on cooling and solidification provoked formation of complex na gedrite biotite sillimanite quartz staurolite plagioclase bearing assemblages after cordierite in metapelites at temperatures 630 570 c and pressures 5 5 6 5 kbar these data provide evidence that hot trondhjemitic melts played a critical role in the exhumation of granulites onto the adjacent granite greenstone craton the trondhjemite transferred heat from the lower to the middle crust and transported large volumes of external aqueous carbonic salt fluids that participated in the rehydration of a significant portion of the smz

geochronological evidence for an extension of the northern lobe of the bushveld complex limpopo province south africa abstract we have conducted the first geochronological study of the ultramafic mafic succession the waterberg project located in the southern marginal zone of the limpopo belt north of the northern lobe of the bushveld complex using u pb dating on zircons extracted from both mineralized reefs as well as on detrital zircons of the waterberg group unconformably overlying the mafics this contribution is the first in a series of publications describing the well mineralized succession and will therefore give an overview of the local geology u pb baddeleyite ages for the main mafic sill intruding the area supplement the data the succession is extremely well mineralized and therefore of interest to the wider geological community the intrusion ages for the lobe are 2059 3 and 2053 5 ma and within error coeval with the intrusion ages of the bushveld complex detrital zircon age probability density distributions indicate a maximum age of deposition of c 2045 ma for the waterberg group and show abundant peaks of archean age paleoproterozoic zircons are interpreted to represent zircon from the detritus of the now eroded roof rocks of the lobe while the archean zircons are most likely sourced from the limpopo complex to the northeast the major dolerite sill intruding the project area has been confirmed to belong the umkondo igneous province with a baddeleyite age of 1125 5 ma the combination of these new age dates local geology and existing literature is used to argue that i the ultramafic mafic succession is related to the bushveld complex ii a significant unconformity is present between igneous rocks and the overlying sediments and iii the area was exposed to a level of paleoproterozoic tectonism not previously observed in the bushveld complex

tectonic trigger for the formation of late miocene cu rich breccia pipes in the andes of central chile during the late miocene the decreasing angle of subduction below the andes of central chile caused a reduction in the influx of new magma and heat into the base of magmatic systems that had been active throughout the miocene this led to their cooling and solidification triggering the release of large volumes of metal rich magmatic fluids from middle and upper crustal magma chambers and generating a group of economically significant cu rich breccia pipes the sr and nd isotopic compositions of the magmatic fluids that formed these brecccas determined from breccia matrix minerals were variable implying that these fluids were not exsolved from a specific cu porphyry magma as is often invoked to explain andean cu deposits instead the formation of the late miocene deposits in central chile was tectonically triggered by cooling of a variety of magma types during the last stages of existence of long lived andean magmatic systems as both subduction angle and as a result subarc magma supply decreased

seismicity and shape of the subducted nazca plate an updated compilation of earthquake locations and focal mechanism solutions from the international seismological centre and preliminary determination of earthquakes is the basis of a comprehensive study of the geometry of the wadati benioff zone beneath western south america the new data support previous mapping of a sharp flexure rather than a tear in the subducted nazca plate beneath southern peru and provide evidence for a similar flexure in the southward transition from nearly horizontal subduction to a slab with 30 dip at latitude 33 s in contrast the transition from 30 slab dip beneath bolivia to a nearly horizontal dip in the region between 28 s 32 s is more gradual occurring over several hundred kilometers of along strike distance between 20 s and 32 s this southward flattening corresponds to a broadening of a horizontal benchlike part of the subducted plate formed between 100 and 125 km depth the transition in continental tectonic style near 27 s 28 s from a wide volcanically active plateau to a narrow nonvolcanic cordillera appears not to be associated with the main slab flattening which begins to the north of these latitudes but with a more abrupt change in curvature of the subducted slab from convex upward to concave upward immediately below the plate boundary interface the concept of gaussian curvature is applied to slab bending to explain how subduction geometry is affected by the shape of the south american plate we hypothesize that the polarity of vertical curvature in the subducting slab is governed by the orientation of lateral curvature of the plate margin focal mechanism solutions for intermediate and deep earthquakes are grouped by geographic region and inverted for the orientation and relative magnitudes of the principal stresses results of the inversion indicate that downdip extension dominates in the slab above 350 km while downdip compression dominates at greater depths

immiscibility and continuous felsic melt fluid evolution within the rio blanco porphyry system chile evidence from inclusions in magmatic quartz the 5 2 to 3 9 ma late porphyries at rio blanco are spatially associated with hydrothermal tourmaline cemented breccias and a significant cu mo deposit magmatic inclusions in quartz from these rocks are used to better understand late stage magmatic processes particularly those involving melt melt and melt fluid immiscibility inclusions are represented by glass of common rhyolitic composition hereafter type 1 crystallized volatile rich felsic melt type 2 h2o and salt rich fluids and microphenocrysts coexistence of types 1 and 2 material within the same growth plane and within a single inclusion suggests that crystallizing rhyolitic magma separated into two silicate melts volatile poor and volatile rich the volatile rich silicate melt had a prolonged evolution extending below the solidus of volatile poor silicate melt with a progressive reduction of the silicate volatile ratio and ultimately to highly saline fluids the latter may have been the source of hydrothermal fluids that had the potential to transport metals the occurrence of immiscibility in the rio blanco rhyolites may be in some ways comparable to that in volatile rich pegmatites and strongly suggests that this process may be common in other se normal felsic magmas and important in the origin of mineralized porphyries

geochronology of the hout river shear zone and the metamorphism in the southern marginal zone of the limpopo belt southern africa abstract in this paper monazite u pb and zircon evaporation dates stepleaching pb pb results on garnet staurolite and kyanite and hornblende ar ar data are presented which constrain the timing of granulite facies metamorphism in the southern marginal zone of the limpopo belt and its thrusting onto the kaapvaal craton the southern marginal zone of the limpopo belt is considered to be a lower crustal equivalent of the northern kaapvaal craton granulite exhumation is associated with southward thrusting along the hout river shear zone which is a set of thrust and strike slip shear zones zircon ages for the matok intrusive complex which was emplaced within the zone during this thrusting charno enderbites 2671 4 ma granodiorites between 2667 and 2664 ma have previously been interpreted as evidence for rapid exhumation of the southern marginal zone within only 7 ma we have obtained a u pb date of 2691 7 ma for monazite from the bandelierkop quarry in the southern marginal zone interpreted as the age of high grade metamorphism a single zircon evaporation pb pb date of 2643 1 ma from a leucosome band at the same locality may indicate longer lasting metamorphism or decompression melting during exhumation anatexis of metapelitic xenoliths within the matok intrusive complex was dated at 2663 4 ma by u pb on monazite indistinguishable from existing zircon ages for this complex pb pb step leaching dates obtained on synkinematically grown garnet 2691 20 ma staurolite 2712 37 ma and kyanite 2672 51 ma from the khavagari hills in the giyani greenstone belt in the immediate footwall of the hout river shear zone indicate that early thrusting was contemporaneous with peak metamorphism in the southern marginal zone ar ar dating and geochemistry on syntectonic hornblende separates from the same shear zone system yielded disturbed spectra and indicated multiple populations probably reflecting repeated or continuous tectonic activity of the hout river shear zone up to about 2600 ma

hydration of cordierite and hypersthene and a description of the retrograde orthoamphibole isograd in the limpopo belt south africa

exhumation of limpopo central zone granulites and dextral continent scale transcurrent movement at 2 0 ga along the palala shear zone northern province south africa abstract the palala shear zone is part of a major tectonic lineament in southern africa which extends over 1000 km from central botswana to the soutpansberg in south africa the lineament is situated between the central zone of the limpopo belt and the kaapvaal craton at its main exposure in the koedoesrand window 50 km e of ellisras northern province the palala shear zone comprises a c 15 km wide mylonite zone which can be subdivided into a northern central and southern domain this study addresses the tectono metamorphic history of the palala shear zone in the koedoesrand window and its role in the tectonics of the limpopo belt in order to do this structural and petrographical work was combined with geochronology age data from relic mineral parageneses show that metapelites from the central domain underwent granulite facies metamorphism around 2 6 ga however because of an intense proterozoic overprint field relationships of the archean event are largely disrupted during a second high grade event at c 2 0 ga granulite facies metamorphism was reached in the northern domain and the adjacent limpopo central zone charnockites were emplaced in the northern domain after 2 1 ga and were affected by retrograde amphibolite and greenschist facies metamorphism around 1 97 ga post orogenic transcurrent faulting under sub greenschist facies conditions characterizes the central and southern domains field observations and microstructural investigations indicate a dextral shear sense for the retrograde mylonitisation these findings document that the palala shear zone played an important role during the exhumation of the proterozoic granulites in the limpopo central zone associated with a transpressive orogeny between 2 05 and 1 95 ga low grade post orogenic transcurrent faulting in the palala shear zone at c 1 9 ga may be associated with the transtensional opening of the soutpansberg and palapye grabens on the basis of the tectono metamorphic history in the koedoesrand window and regional geophysical geochemical and sequence stratigraphic data the palala shear zone is interpreted as a proterozoic suture zone along which the archean kaapvaal and zimbabwe provinces were juxtaposed

mineralogy geochemistry and petrology of neoarchean ferroan to magnesian granites of carajas province amazonian craton the origin of hydrated granites associated with charnockites abstract 2 75 to 2 73 ga old granitic intrusions associated with coeval charnockitic rocks were emplaced in the northern domains of the carajas province the vila jussara suite was recognized recently and its geologic mineralogical and geochemical characteristics are presented in this paper and compared with the data available in the literature on the estrela complex serra do rabo and igarape gelado granites and planalto suite monzogranites and syenogranites are dominant in most units but in the igarape gelado granite and vila jussara suite granodiorites and tonalites are also relatively abundant the main mafic phases in all these granitoids are amphibole potassian hastingsite with subordinate magnesiohastingsite 0 95 fe fe mg 0 47 and biotite 0 88 fe fe mg 0 52 with rare occurrence of clinopyroxene relict crystals magmatic epidote of pistacite component from 26 to 31 occurs only in the vila jussara suite the rocks of estrela serra do rabo and planalto units contain only ilmenite generally with coronas of titanite or ilmenite associated with magnetite and are essentially reduced ferroan granites whole rock feot feot mgo 0 89 that evolved at low f o 2 fmq 0 5 the igarape gelado granite needs additional studies but it is formed apparently by reduced ferroan and oxidized ferroan granites the vila jussara suite is also composed of reduced ferroan granites that are similar to those of the other granite units however it has additionally oxidized ferroan and magnesian granitoids which contain magnetite ilmenite as the main fe ti oxide phase and evolved respectively at moderate f o 2 nno 0 5 and at comparatively higher f o 2 nno to nno 1 the magmas of these granites were formed at temperatures 900 c and derived from a heterogeneous dominantly reduced and tholeiitic granulitic crust locally varying to oxidized and calc alkaline in composition plagioclase orthopyroxene clinopyroxene and fe ti oxides were the main phases in the melt residues and the liquids were enriched in ba zr y and ree except for eu but not in sr compared to their sources the magmas should have relatively high water content 4 wt the magmatic evolution was strongly influenced by partial melting processes the different varieties of the vila jussara suite are not linked by fractional crystallization and derived from distinct magmas the neoarchean granites of the carajas province were emplaced in a collisional tectonic setting they are syntectonic granites that were deformed and sometimes intensely recrystallized during their emplacement and subsequent subsolidus cooling the magmas were generated in the crust at pressures of 800 to 1000 mpa and emplaced at pressures of 200 to 500 mpa the studied granitoids are similar geochemically to the ferroan and magnesian groups of charnockites the magnesian variety of the vila jussara suite is similar to the pyroxene free granitoids of the neoarchean matok pluton of the collisional limpopo belt the neoarchean granites of carajas are hydrated granites emplaced at comparatively higher crustal levels when compared with the matok pluton

comment on ultrahigh temperature granulites and magnesian charnockites evidence for the neoarchean accretion along the northern margin of the kaapvaal craton by rajesh et al

geology of the post collisional porphyry copper molybdenum deposit at qulong tibet porphyry deposits are usually thought to form from subduction related calc alkaline magmas in magmatic arc settings although some porphyry deposits also occur in post collisional extensional settings the post collisional deposits remain poorly understood here we describe the igneous geology alteration mineralogy and mineralization history of qulong a newly discovered porphyry cu mo deposit in southern tibet that belongs to the post collisional class the deposit is associated with miocene monzogranite granodiorite intrusions and is hosted partly by jurassic andesitic dacitic volcanic rocks the deposit contains 7 1 mt cu and 0 5 mt mo metal the 19 5 ma granodioritic monzogranitic rongmucuola pluton with diorite enclaves is the earliest miocene intrusive unit it was intruded by a regularly shaped stock p porphyry and then thin dikes x porphyry of monzogranite at about 17 7 ma the main cu mo mineralization is associated with the p porphyry a barren diorite porphyry intruded the p and x porphyries around 15 7 ma petrologic trends of the intrusions suggest that the miocene intrusions have similar origins and probably formed by fractionation of a deeper magma chamber emplacement of miocene porphyries controlled by the qulong anticline is in direct response to the rapid uplift erosion of the gangdese arc batholiths in southern tibet miarolitic cavities and unidirectional solidification textures key evidence for volatile separation have been recognized in the p and x porphyries respectively early potassic alteration characterized by quartz k feldspar anhydrite pervades the p porphyry and rongmucuola pluton laterally this alteration grades into quartz biotite anhydrite k feldspar which affects all miocene intrusions except the latest dioritic porphyry wall rocks of rongmucuola pluton and jurassic andesitic dacitic volcanics within 1 1 5 km of the porphyries are dominated by pervasive potassic alteration an outer halo of propylitic alteration epidote chlorite calcite extends up to 2 km away from the deposit feldspar destructive alteration sericite chlorite clay minerals has overprinted most of the potassic and part of the propylitic alteration the alteration is strongly pervasive in the interior of the porphyry bodies and occurs as vein halos away from the porphyry bodies the earliest quartz k feldspar alteration and veins are barren whereas approximately 60 of the cu reserves is associated with slightly later quartz biotite anhydrite alteration barren assemblages are related to irregular quartz k feldspar anhydrite veins which are truncated by the x porphyry cu sulfide bearing assemblages are associated with discontinuous chalcopyrite biotite and continuous quartz anhydrite chalcopyrite molybdenite veins deposition of cu mo with abundant anhydrite occurred during or between emplacement of closely related porphyries from high temperature magmatically derived fluids and was probably caused by the disproportionation of so

controls of subduction geometry location of magmatic arcs and tectonics of arc and back arc regions most variation in geometry and angle of inclination of subducted oceanic lithosphere is caused by four interdependent factors combinations of 1 rapid absolute upper plate motion toward the trench and active overriding of the subducted plate 2 rapid relative plate convergence and 3 subduction of intraplate island seamount chains aseismic ridges and oceanic plateaus anomalously low density oceanic lithosphere cause low angle subduction under conditions of low angle subduction the upper surface of the subducted plate is in contact with the base of the overlying plate the wedge of low density asthenosphere is replaced by subducted lithosphere and the width of the arc trench gap either is significantly increased or a magmatic arc is not developed within the overlying plate the fourth factor is age of the subducting lithosphere subduction of young lithosphere produces two opposing tendencies 1 low angle subduction and increased arc trench distance owing to its low density and 2 decreased arc trench distance owing to its higher temperature two factors of secondary importance contribute to variation in subduction zone geometry and arc trench distance accretion of sediment in trenches depresses the upper portion of the subducting oceanic plate and causes the trench axis to migrate seaward prolonged subduction thickens the upper plate depresses the isotherms in the subducted plate and may create a broader arc both factors increase the arc trench gap the four primary factors also control development of other tectonic elements such as regional subsidence for example the amazon basin and a portion of the cretaceous interior seaway of western united states intra arc extension for example the basin and range province foreland fold and thrust belts and laramide style tectonics

metal fractionation between magmatic brine and vapor determined by microanalysis of fluid inclusions the major and trace element compositions of individual fluid inclusions from a range of magmatic hydrothermal ore deposits were analyzed by laser ablation inductively coupled plasma mass spectrometry to explore the behavior of ore forming components during fluid phase separation boiling in high temperature saline fluid systems data from 13 samples showing unambiguous evidence for coeval trapping of a liquid brine and a coexisting vapor phase identify two groups of elements with drastically different geochemical behavior na k fe mn zn rb cs ag sn pb and tl preferentially partition into the brine probably as cl complexes whereas cu as au probably as hs complexes and b selectively partition into the vapor fluid phase separation is probably a major previously underestimated process in the chemical differentiation that contributes to the extreme range of selective element enrichments in magmatic hydrothermal systems from deep plutons through porphyry style and greisen deposits to epithermal mineralization and volcanic fumaroles

plate reconstructions aseismic ridges and low angle subduction beneath the andes low angle subduction segments beneath peru and chile are believed to represent interaction of the subduction zone with the aseismic nazca and juan fernandez ridges on the basis of symmetric sea floor spreading models and plate hotspot reconstructions predicted continuations of the relatively buoyant ridges correspond well with the limits of the low angle subduction segments further the history of interaction of the aseismic ridges with the subduction zone as predicted by plate reconstructions is not inconsistent with available information on volcanic episodes of the andes if the contemporary correlation of volcanic gaps and low angle subduction is also applicable to the rest of the late cenozoic the occurrence of deep seismicity beneath the eastern flanks of the andes is also believed to reflect the effects of low angle subduction the deep seismic segments are adjacent to segments which are presently or were recently experiencing low angle subduction thus the deep seismicity is interpreted as representative of remnants of normal oceanic lithosphere subducted before interaction of aseismic ridges with the subduction zone other geologic phenomena are associated with low angle subduction beneath south america including landward shift in loci of tectonism surficial subsidence and laramide style deformation the aseismic ridge buoyancy hypothesis appears to be a valuable predictive tool for the interpretation of such geologic effects

melting carbonic fluids and water recycling in the deep crust an example from the limpopo belt south africa the proposed retrograde orthoamphibole isograd in the southern marginal zone of the limpopo belt separates hydrated amphibolite grade metapelites from their granulite grade precursors and provides an intriguing geological dilemma widespread rehydration of metapelitic granulites under conditions of 660 600 c and 0 6 gpa and co2 dominated fluid inclusion populations appear to suggest thorough flushing of the high grade crust with an externally derived carbonic fluid however past studies of the carbon and oxygen isotope geochemistry of the hydrated rocks have not demonstrated the involvement of any voluminous out of equilibrium fluid in the evolution of the rocks this contribution proposes a model wherein the hydrating fluids are derived from crystallizing anatectic leucosomes generated by in situ fluid absent biotite melting along the prograde path model equilibrium fluid compositions suggest that reaction between this melt derived h2o and biogenic graphite produced co2 rich fluid compositions and potentially high fluid rock ratios at the wet granite solidus declining temperature resulted in fluid compositions shifting to higher xh2o with the precipitation of graphite essentially at the sites of initial fluid generation thereby preserving original pre metamorphic isotopic heterogeneities the hydration pattern of the southern marginal zone appears to be a function of melt migration in the hydrated zone leucosomes generally approximate minimum melt compositions and in this zone h2o was effectively recycled between the prograde and retrograde assemblages in contrast leucosomes in the granulite grade portion of the terrane have lost a k2o and h2o rich melt fraction and although some hydration has occurred in this zone orthopyroxene is generally preserved in metapelites in a general context in situ crystallization of graphitic partially melted source rocks has the potential to produce high fluid rock ratios at temperatures close to the wet granite solidus this single process holds the potential for widespread retrogression of formerly high grade assemblages at a variety of ah2o values without external fluid input

fluid evolution in a volcanic hosted epithermal carbonate base metal gold vein system alto de la blenda farallon negro argentina alto de la blenda is a 6 6 ma intermediate sulphidation epithermal vein system in the farallon negro volcanic complex which also hosts the 7 1 ma porphyry cu au deposit of bajo de la alumbrera the epithermal vein system is characterised by a large extent and continuity 2 km 400 m open to depth 6 m maximum width and an average gold grade of 8 g t the vein is best developed within an intrusion of a fine grained equigranular monzonite interpreted as the central conduit of a stratovolcano whose extrusive activity ended prior to porphyry cu au emplacement at bajo de la alumbrera which is in turn cut by minor epithermal veins the alto de la blenda vein consists predominantly of variably mn rich carbonates and quartz with a few percent of pyrite sphalerite galena and other sulphide and sulphosalt minerals four phases of vein opening hydrothermal mineralisation and repeated brecciation can be correlated between different vein segments stages 2 and 3 contain the greatest fraction of sulphide and gold they are separated by the emplacement of a polymictic breccia containing clasts of quartz feldspar porphyry as well as basement rocks fluid inclusions in quartz related to stages 2 to 4 are liquid rich with 2 4 wt nacl eq they homogenise between 160 and 300 c with very consistent values within each assemblage vapour inclusions are practically absent in the epithermal vein quartz fragments in the polymictic breccia contain inclusions of intermediate to vapour like density and similar low salinity 3 wt nacl eq besides rare brine inclusions containing halite laser ablation inductively coupled plasma mass spectrometry la icp ms analyses of epithermal inclusions indicate high concentrations of k fe as sb cs and pb that significantly vary within and through subsequent vein stages careful consideration of detection limits for individual inclusions shows high gold concentrations of 0 5 to 3 ppm dissolved in the ore fluid which contains variably high sulphur concentrations in excess over fe and other chalcophile metals compositional variations are interpreted to reflect cooling and contraction of lower density magmatic fluids at depth like those preserved in porphyry clasts that were mechanically transported up by the polymictic breccia ore mineral precipitation from the magmatic fluid occurred by further cooling and possibly minor mixing with surface derived water leading to sulphide saturation de sulphidation of the magmatic fluid and consequent gold precipitation the absence of flash boiling and or reduction by carbonaceous host rocks has led to relatively modest but constant gold grades in the carbonate base metal gold veins of alto de la blenda

sodium calcium metasomatism chemical temporal and spatial relationships at the yerington nevada porphyry copper deposit tilting associated with extensional tectonics has exposed 1 800 m of continuous palcovertical relief through the jurassic age yerington porphyry copper deposit the rarely observed root zones of a porphyry system are characterized at yerington by sodium calcium metasomatism the principal hydrothermal reactions involve the replacement of primary k feldspar by oligoclase and of primary biotite by actinolite copper has been removed sodic calcic alteration developed at deep intrusive contacts nearly simultaneously with potassic alteration and copper mineralization at shallow intrusive contacts the reactions accompanying potassic alteration are the reverse of those accompanying sodic calcic alteration plagioclase reacted to form k feldspar hornblende reacted to form biotite the potassic and sodic calcic alteration couplet principally accompanied each of two separate intrusive events 1 luhr hill granite stock and associated porphyry dikes and 2 walker river granite porphyry dikes the younger walker river intrusion was emplaced at slightly deeper levels than the older luhr hill complex vertically stacked and partially superimposed alteration and mineralization patterns resulted from differences in depth and in timing of emplacement at lower temperatures late sodic alteration was superimposed on early sodic calcic alteration at deeper levels in the system whereas late quartz sericite pyrite alteration was superimposed on potassic alteration at higher levels sodic alteration is characterized by the replacement of k feldspar by albite and of biotite by chlorite copper deposited during potassic alteration was remobilized the transition to lower temperature alteration is associated only with the walker river intrusion the hydrothermal system associated with the emplacement of the luhr hill complex had not evolved to lower temperature assemblages prior to emplacement of the walker river dikes phase equilibria and geologic reconstruction of depth of emplacement suggest that temperatures during early sodic calcic alteration were limited to the range of 360 degrees to 480 degrees c at pressures between 300 and 800 bars late sodic alteration took place at similar pressures but at temperatures less than 360 degrees c nearly simultaneous development of potassic and sodic calcic alteration resulted from the flow of fluids away from intrusions at shallow levels potassic alteration and from the flow of fluids toward intrusions at deeper levels sodic calcic alteration because k na and na ca activity ratios of a fluid in equilibrium with granitic rocks decrease with decreasing temperature cooling of high temperature fluids can generate potassic alteration and heating of fluids to high temperatures can generate sodic or sodic calcic alteration

origin of the charnockites of the louis lake batholith wind river range wyoming the 2 63 ga louis lake batholith a calc alkalic pluton exposed opxand fayalite fay bearing felsic igneous rocks inin wind river range of western wyoming consists of minor diorite clude opdalite opx granodiorite enderbite opx tonalquartz diorite granodiorite and granite at shallow structural levels ite and mangerite opx monzonite le maitre et al the batholith is pyroxene free but at deeper levels all units of the 1989 many of these rock names are not commonly batholith contain pyroxenes on its northern margin the batholith used the term charnockite sensu lato is typically applied was emplaced at p 5 6 kbar t 775 800 c fo2 at to any opx bearing granitoid regardless of its bulk fmq fayalite magnetite quartz 1 5 to fmq 1 8 and composition for example the charnockite suite of south ah2o 0 1 along the southern margin of the batholith the india includes enderbites opdalites and charnockites emplacement pressure was 3 kbar the batholith includes two howie 1955 the nigerian suite includes charnockite compositional series the peraluminous high ree series is rich in and opdalite olarewaju 1987 and that of lofoten is k2o and displays larger feo feo mgo the metaluminous associated with mangerites malm ormaasen 1978 low ree series has less k2o and smaller feo feo mgo because the terms enderbite and opdalite are so rarely in the boulder canyon area most of the rocks belong to the lowused in this paper we use the term charnockite for all ree series the nd pb and sr initial isotopic compositions of opx bearing granitic rocks where it is important to both series vary and do not correlate with sio2 abundance relatively describe a specific rock type we use opx as a varietal constant fe mg ratios over a wide range of sio2 and the large modifer to the rock name e g opx granodiorite instead variation in initial isotopic compositions indicate that the sources of opdalite for the louis lake batholith include older wyoming province crustin the 1980s workers in southern india recognized and mantle derived mafic melts the calc alkalic nature of the areas where biotite bearing granitic gneiss had been boulder canyon charnockites indicates that there is no special cconverted to pyroxene bearing gneiss along fluid pathtype magma instead charnockites will form in any rock type if ways janardhan et al 1982 friend 1985 this led to the water activity is low enough the term charnockitization srikantappa et al 1985 and the concept that charnockite is a metamorphic rather than an igneous rock as a result charnockite has been used incorrectly by some workers as a synonym for

the timing of successive fluid events in the southern marginal zone of the limpopo complex south africa constraints from 40ar 39ar geochronology abstract we have carried out a geochronological study in the high grade metamorphic southern marginal zone of the limpopo complex south africa aimed at providing time constraints on episodes of fluid activity this work chiefly comprises 40 ar 39 ar dating with accompanying petrography of amphibole and mica and further incudes zircon u pb geochronology on a post peak metamorphic trondhjemite intrusion the so called zone of rehydration within 30 km n of the hout river shear zone where the southern marginal zone is thrust over the kaapvaal craton was studied in particular but sampling also included areas of patchy metasomatism in the granulite zone further north where no post peak metamorphic rehydration on a regional scale occurred amphibole in a mafic granulite and an older generation of zircons in the petronella trondhjemite yielded dates in the 2700 2730 ma range corresponding to the published age of peak granulite metamorphism in the southern marginal zone a cluster of 40 ar 39 ar dates on amphiboles from the zone of rehydration indicate an age between 2660 and 2670 ma for this event in accord with results from previous work on the hout river shear zone the main population of u pb zircon dates from the trondhjemite dating its intrusion also falls in this age range as does the published age of the matok pluton these data suggest that rehydration at 2660 2670 ma was a discrete episode a further 40 ar 39 ar age cluster defined by dates on both amphiboles and micas is seen at 2010 2040 ma which is the age of a major tectonometamorphic event in the central zone of the limpopo complex and previously reported from the southern marginal zone as rb sr biotite dates events we found in the 2010 2040 ma bracket are strike slip shearing dated by fabric forming amphibole as well as localized potassic metasomatism dated by both amphiboles and micas the latter was not confined to the zone of rehydration and appears in one case to be associated with gold mineralization a number of 40 ar 39 ar dates intermediate between 2600 and 2000 ma were obtained but these are diverse and can be accounted for by mixing incomplete resetting and or excess 40 ar they do not indicate events in this intermediate age bracket

exsolution of immiscible vapor and liquid phases from a crystallizing silicate melt implications for chlorine and metal transport abstract the compositional variation of a cl bearing aqueous fluid exsolving from a crystallizing silicate melt is evaluated for the system haplogranitic melt na k c1 h 2 o with attention to the effect of vaporliquid immiscibility on the aqueous fluid above critical pressures for the aqueous fluid p 1 5 kb at 800 c where the fluid is single phase the cl concentration in the aqueous fluid c cl aq gradually decreases with crystallization of a silicate melt under rayleigh fractionation as demonstrated by previous studies however the variation of ca is quite different below critical pressures where vapor liquid immiscibility occurs in particular at low pressures p cl h 2 o ratio in a melt is small and ca will increase with crystallization a high cl h 2 o ratio at low pressure results in liquid exsolution and decreasing ca with crystallization eventually both vapor and liquid phases will exsolve simultaneously regardless of the initial exsolving phase during this simultaneous exsolution the composition of the exsolving vapor and liquid phases the mass ratio of these phases and the cl concentration of the silicate melt will all remain constant comparison of the model and published experimental data indicates that the simultaneous exsolution of vapor and liquid may occur from a normal rhyolite crystallized at pressures below the critical pressure for the aqueous fluid therefore simultaneous exsolution of the immiscible aqueous phases is likely to be a common phenomenon in magmatic hydrothermal systems at conditions for the vapor liquid immiscibility model calculations at 0 6 and 1 2 kb indicate that major amounts of cl in the system will be distributed to the liquid phase particularly at the lower pressure this suggests that the majority of chloridecomplexed metals exsolving from a magma may be transported in the liquid phase under shallow crustal conditions

p t evolution of a variscan lower crustal segment a study of granulites from the schwarzwald germany pressure temperature time p t t paths of orogenic granulites introduction provide important information on the thermal and chemical structure general background of the lower continental crust through time and constraints on tectonic granulites are typical rocks of the earth s middle to processes we present the first detailed petrological investigation of lower crust under high temperature conditions they are granulites from the variscan schwarzwald pelitic granulites from the found as xenoliths in basaltic volcanic rocks mainly central schwarzwald gneiss complex csgc are characterized by within continental rifts but most granulites occur as the peak assemblage garnet rutile kyanite antiperthite complexes or terranes in orogenic settings orogenic quartz felsic to intermediate granulites from the southern granulites display a wide compositional range harley schwarzwald gneiss complex ssgc exhibit different peak as1989 1998 and are known from a variety of collisional semblages with clinopyroxene orthopyroxene ternary feldspar garnet belts that formed during different episodes since the quartz and sillimanite and manifold retrograde reaction textures archaean e g harley 1998 clarke et al 2000 moller peak p t conditions were calculated by two feldspar thermometry et al 2000 determination of bulk rock and mineral garnet orthopyroxene thermometry and various geobarometers minicompositions calculation of peak equilibration conmum estimates for peak conditions are 950 1010 c and 1 4 1 8 ditions and dating of orogenic granulites thus provide us gpa for the granulites of the csgc which followed a clockwise with important constraints on the thermal and chemical p t path the retrograde path is characterized by initial isothermal structure of the earth s continental crust at different decompression associated with partial melting followed by isobaric geological times cooling peak conditions for the ssgc are 1015 c and 1 5 gpa harley 1989 showed that equilibration conditions minimum temperature maximum pressure no prograde relics are deduced from natural orogenic granulites cover a wide preserved and isothermal decompression was less pronounced than range in particular pressures are highly variable and in the csgc other variscan hp ht granulites from central the granulite field may be divided into a low pressure europe show similar lithologies equilibration temperatures and lp a medium pressure mp and a high pressure hp ages 340 335 ma the heat for widespread high temperature facies according to green ringwood 1967 peak metamorphism in the variscan lower crust could have been supplied temperatures of many granulite terranes scatter around by repeated intrusion of subduction related basic magmas rapid 800 c bohlen 1987 but an increasing number of near isothermal decompression of the granulites may have been ultra high temperature uht granulite complexes facilitated by considerable volumes of partial melt and by orogenic extension 900 1100 c and 0 7 1 3 gpa harley 1998 are being recognized e g harley hensen 1990 dasgupta et al 1995 klemd brocker 1999 hokada 2001 regarding retrograde p t paths harley 1989 distinguished two

partial melting and the origin of metapelitic granulites in the southern marginal zone of the limpopo belt south africa abstract two potentially realistic models exist to explain the origin of the granulite facies rocks of the southern marginal zone i co2 fluxing inducing the necessary high temperature dehydration reactions and ii melting of hydrous minerals to yield an h2o undersaturated granitic melt in equilibrium with a dehydrated granulitic assemblage petrographic and field evidence from the bandelierkop quarry in the southern marginal zone indicate a sequence of melting reactions involving muscovite and biotite relict textural evidence indicates that the reaction responsible for the generation of most of the melt bi sil qz plag melt ga ksp was fluid absent the melting reactions can be shown to have occurred during the prograde metamorphic path with the highest temperature reaction bi qz plag hy cord melt occurring at approximately peak metamorphic conditions 850 c and less than 9 5 kbar this suggests a fluid absent mode of granulite formation a subsequent retrograde reaction cord ksp h2o ky qz related to a rise in h2o during in situ crystallisation of the melt did not destroy the granulite assemblages and hypersthene remains stable in areas of the southern marginal zone that record pressures of greater than 6 5 kbar during retrogression

gold in porphyry copper deposits experimental determination of the distribution of gold in the cu fe s system at 400 to 700 c experiments in the system au cu fe s were carried out at temperatures of 400 to 700 c to determine how much gold could be accommodated by bornite and chalcopyrite the two most common ore minerals in porphyry copper gold deposits our results show that for all temperatures bornite contains one order of magnitude more gold than chalcopyrite or intermediate solid solution iss its high temperature equivalent the range of gold concentrations in bornite and chalcopyrite or iss decreases with decreasing temperature from 1 280 to 8 200 ppm au in bornite and 100 to 125 ppm au in iss at 600 c to 235 to 364 ppm au in bornite and 5 to 16 ppm au in chalcopyrite or iss at 500 c and to 13 to 80 ppm au in bornite and 2 to 4 ppm au in chalcopyrite or iss at 400 c the amount of gold in bornite is also strongly dependent on the composition of bornite being highest in stoichiometric bornite compositions cu5fes4 and decreasing toward cu rich and cu poor compositions phase equilibrium constraints for solutions with geologically reasonable reduced sulfur contents indicate that high temperature porphyry copper gold deposits will contain bornite and magnetite whereas lower temperature deposits whether primary or overprinted by phyllic alteration will contain chalcopyrite and pyrite if gold is present in the ore forming solutions more of it will be deposited in high temperature porphyry copper gold deposits where it will be closely associated with bornite coexisting magnetite in these deposits should generate magnetic anomalies lower temperature deposits will contain less gold which is hosted by pyrite as well as chalcopyrite and will lack magnetic anomalies comparison of the amount of gold hosted by natural porphyry copper gold ores to that hosted by bornite and chalcopyrite in our experiments suggests that significant amounts of gold can be lost from these deposits into surrounding hydrothermal systems

vulcanian explosions in the process of building destruction of the lava dome of andesitic volcano insight from the seismic signals recorded at volcan de colima mexico abstract the seismic signals associated with the vulcanian explosive events at volcan de colima mexico during its 1998 2014 eruption were discriminated into three types according the waveforms of their initial phases this classification includes about 85 90 of all seismic signals associated with explosive events it was shown that the classification of the explosive seismic signals gave a useful tool for analysis of the eruption development analysis was performed for two 6 month stages of the eruption the 2003 post effusive stage culminated with large explosions and the 2007 co effusive stage during the beginning of new lava dome growth in the crater of the volcano the predominance 95 of the type 1 and 3 events with low frequency lf and high low frequency hf lf initial phases in the sample of the 2003 post effusive stage explosive events identifies the termination of the lava effusion and preparation to the stage of large explosions the high participation 37 of the type 2 events with high frequency hf initial phases in the sample of the 2007 co effusive stage explosive events indicates the process of the lava dome growth beginning the introduction of multi stage conceptual model of explosive process allows the interpretation of peculiarities of the seismic signals associated with explosive activity analysis of the spectral content of the initial lf phases of the seismic signals associated with the explosions showed that the sources of lf initial phases corresponding to the ascent of magma within the volcanic conduit above the fragmentation zone were common for post effusive and co effusive stages the source of events with the hf initial phases differed from the source of events with the lf first phases the sources of explosions at explosion level were situated at different sites during post effusive and co effusive stages

the late archean record a puzzle in ca 35 pieces abstract the global archean record preserves ca 35 large cratonic fragments and a less well defined number of smaller slivers most archean cratons display rifted margins of proterozoic age and therefore are mere fragments of supercratons which are defined herein as large ancestral landmasses of archean age with a stabilized core that on break up spawned several independently drifting cratons the tectonic evolution of individual archean cratons such as the slave craton of north america or the kaapvaal craton of southern africa should therefore always be considered in the context of their ancestral supercratons this is particularly true for many of the smaller cratons which are too limited in size to preserve the complete tectonic systems that led to their formation these limitations not only apply to the crustal geology of archean cratons but also to their underlying lithospheric mantle keels if these keels are archean in age as their broad correlation with ancient surface rocks suggests they also are rifted and drifted remains of larger keels that initially formed below ancestral supercratons the study of archean cratons and their lithospheric keels should thus be global in scope in the search for late archean supercratons a craton like the slave with three to four rifted margins has a ca 10 maximum probability of correlating with any of the ca 35 remaining cratons around the globe assuming wholesale recycling of archean cratons has been limited since ca 2 0 ga alternatively if the original number of independently drifting cratons was significantly larger the probability of successful correlations is much less due to repeated cycles of break up and plate tectonic dispersal since ca 2 0 ga the probability of correlation is probably independent of present day proximity unless there is independent evidence that two neighbouring cratons were only separated by a narrow ocean nevertheless most previously proposed craton correlations rely erroneously on present day proximity implicitly extrapolating relatively recent paleogeography back to the archean considering the fundamental differences between some of the better known cratons such as the slave superior and kaapvaal it seems likely that these cratons originated from independent supercratons sclavia superia vaalbara with distinct amalgamation and break up histories this is contrary to widely held opinion based largely on an idealized view of the supercontinent cycle that all cratons shared a common history in a single late archean supercontinent in contrast to the superior and kaapvaal cratons which are radically different from the slave craton and each other the dharwar craton of peninsular india the zimbabwe craton of southern africa and the wyoming craton of north america all show significant similarities to the slave craton it thus seems likely that at least some of these were nearest neighbours to the slave craton in the ca 2 6 2 2 ga sclavia supercraton

andean tectonics related to geometry of subducted nazca plate seismological and geological data show that tectonic segmentation of the andes coincides with segmentation of the subducted nazca plate which has nearly horizontal segments and 30 east dipping segments andean tectonics above a flat subducting segment between 28 s to 33 s are characterized by from west to east 1 a steady topographic rise from the coast to the crest of the andes 2 no significant quaternary and possibly neogene magmatism 3 a narrow belt of eastward migrating apparently thin skinned neogene to quaternary shortening of the andes and 4 plio pleistocene uplift of the crystalline basement on reverse faults in the pampeanas ranges from about 15 to 24 s over a 30 dipping subducted plate a west to east andes cross section includes 1 a longitudinal valley east of coastal mountains 2 an active neogene and holocene andesitic volcanic axis 3 the altiplano puna high plateau 4 a high neogene but inactive thrust belt eastern cordillera and 5 an active eastward migrating subandean thin skinned thrust belt tectonics above a steeply subducting segment south of 33 s are similar west of the volcanic axis but quite different to the east early cenozoic tectonics of western north america were quite similar to the neogene andes however duration of segmentation was longer and the width of deformation was greater in the western united states patterns of crustal seismicity are systematically related to plio quaternary structural provinces implying that current deformational processes have persisted since at least the pliocene horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench above flat subducting segments crustal seismicity occurs over a broad region whereas over steep segments it is confined to the narrow thrust belt strain patterns in the forearc region are complex and perhaps extensional and a broad region of the altiplano puna and eastern cordillera appears to be aseismic

think outside the box there is no limpopo orogeny reply to comment by laurent et al on paper by rajesh et al 2014

thrust exhumation of the neoarchean ultrahigh temperature southern marginal zone limpopo complex convergence of decompression cooling paths in the hanging wall and prograde p t paths in the footwall

constraints on the form of the p t loop in the southern marginal zone of the limpopo belt south africa abstract a generalised clockwise p t path consisting of a prograde portion a high grade isothermal decompression portion and an essentially isobaric cooling portion has been suggested for the metamorphic evolution of the southern marginal zone previous studies have concluded that the high grade decompression path started in the kyanite stability field peak metamorphic mineral compositions have been reset during the equilibrium decompression event and hydration during the cooling dominated isobaric portion occurred in the kyanite stability field the present petrographic study from the bandelierkop quarry a locality in the granulite zone of the southern marginal zone identified a series of sequential anatectic reactions which record a period of prograde heating from approximately 700 c to 850 c during this period pressure is constrained to within the sillimanite stability field by the production and preservation of prograde generations of sillimanite the maximum temperature melting event is closely followed by two divariant reactions ga qz sil cord and ga qz cord hy which occur in response to a period of general isothermal decompression a subsequent cooling dominated rehydration period is indicated by a crystallisation hydration reaction cord ksp melt bi ky qz which occurred at a minimum temperature of approximately 640 c and pressure greater than 6 5 kbar the present data confirms the clockwise form of the loop and also supports the previously suggested flat slope of the retrograde cooling portion additional mineral chemistry data relating to the second decompression reaction indicate that the zonation profiles of cordierite and garnet as well as the growth of small quantities of garnet after the reaction are in strong support of the suggested p t path the present mineral compositions in the southern marginal zone reflect a near total re equilibration during the high grade cooling dominated portion of the p t path and as a result reflect p t conditions considerably lower than those of the peak metamorphism

ultrahigh temperature granulites and magnesian charnockites evidence for neoarchean accretion along the northern margin of the kaapvaal craton abstract shrimp u pb analysis of zircon grains from an ultrahigh temperature mg al granulite from the northern margin of the kaapvaal craton southern marginal zone shows ages of 2 72 ga for the peak metamorphism geochemical characteristics of syn tectonic charnockites in the area are similar to magnesian charnockites typically formed in subduction settings in contrast to the prevailing models on peak metamorphism in the context of neoarchean continent collision between archean zimbabwe and kaapvaal cratons resulting in the formation of the limpopo complex we relate the neoarchean events to the accretion of this marginal block to the rest of the kaapvaal craton in this model the subduction event is marked by the formation of magnesian charnockites and the ultrahigh temperature metamorphism dates the collision event

tectonic development southeastern border of the puna plateau northwestern argentine andes the puna altiplano a 500 000 km2 area at an average elevation of 3 7 km in the central andes occurs in a non collisional plate setting over a 30 east dipping segment of the benioff zone and is bounded to the north and south by nearly flat segments of the subducted nazca plate a 120 km long portion of the southeastern boundary of the puna in northwestern argentina was studied to gain insight into the structures associated with uplift of the plateau and into the nature of structures over the transition from flat to steeper subduction the structural geometry and kinematic evolution of the region was determined from geologic mapping and kinematic analysis of minor fault arrays the present southeastern margin of the puna was uplifted between 5 10 and 2 ma northwest southeast horizontal shortening and vertical extension during uplift were produced by northeast striking en echelon thrust faults dipping 17 55 northwest four to seven en echelon structural blocks each 20 30 km long constitute the border of the puna their structural geometry precludes large scale overthrusting as an important mechanism of plateau uplift along the studied segment in the past 2 m y there has been a 45 change in the direction of shortening and locally a 90 change in the extension direction indicating that the structures accompanying uplift are no longer active this change could be due to a change either in the geometry of the subducted plate or in convergence directions although neither of these hypotheses is testable given presently available data