geochronology of very low grade mesozoic andean metabasites an approach through the k ar 40ar 39ar and u pb la mc icp ms methods multiple geochronological methods using different metamorphic minerals were combined to date the regional very low grade metamorphism affecting upper jurassic lower cretaceous volcano sedimentary successions in the andes of central chile early late cretaceous metamorphic ages between 82 and 108 ma were obtained by the k ar and u pb methods for celadonite and titanite a much younger thermal event is responsible for actinolite formation at 8 ma most probably related to the intrusion of proximal miocene granitoids previous models for the metamorphism should be reinterpreted taking into account the absence of a greenschist facies event the combination of different metamorphic minerals and chronometers is regarded as a powerful analytical tool to date the very low grade metamorphism associated with the mesozoic extensional regime developed within the andes

synorogenic hydrothermal origin for giant hamersley iron oxide ore bodies geologic mapping basin analysis and calculated fluid compositions indicate that giant orebodies of microplaty hematite and possibly martite goethite in the hamersley province of western australia were formed by heated fluids driven by early paleoproterozoic orogenesis detrital grains of microplaty hematite in the mcgrath trough a foreland basin in front of the northward advancing ophthalmian fold belt constrain the age of the earliest microplaty hematite ore formation to 200 c and locally up to 400 c were involved regional circulation of hydrothermal fluids including heated surface water through reduced banded iron formations occurred during or soon after the ophthalmian orogeny we speculate that martite goethite orebodies previously considered mesozoic cenozoic could also be related to heated paleoproterozoic meteoric fluids migrating northward away from the ophthalmian fold belt

magnetic properties and potential field modeling of the peculiar knob metamorphosed iron formation south australia an analog for the source of the intense martian magnetic anomalies an extremely intense 120 a m 1 remanence directed steeply upward this ancient remanence reinforces the local earth s field inclination 63 a simple geological model constrained by drilling and physical property measurements explains both the observed magnetic and gravity anomalies consistent with the poisson theorem koenigsberger ratios qs of 10 and greater as found here are rare in nature we postulate that acquisition of a thermoremanent magnetization trm by the ore during

palaeomagnetism and magnetic anisotropy of late neoproterozoic strata south australia implications for the palaeolatitude of late cryogenian glaciation cap carbonate and the ediacaran system abstract the ediacaran global stratotype section and point gssp is defined near the base of the nuccaleena formation the cap carbonate overlying the terminal cryogenian glaciogenic elatina formation at a basin wide sequence boundary in the adelaide geosyncline south australia previous palaeomagnetic data for 205 specimens from the elatina formation yielded a structurally corrected mean declination d 208 3 and inclination i 12 9 95 4 2 palaeolatitude 6 5 2 2 and pole position at latitude p 47 3 s longitude p 359 3 e dp 2 1 and dm 4 2 new palaeomagnetic data for 141 specimens from the nuccaleena formation yield a structurally corrected mean direction of d 208 3 i 34 9 95 3 4 19 2 2 3 2 2 and pole position at p 32 3 s p 350 8 e dp 2 2 and dm 3 9 there is a substantial difference in inclination between the elatina and nuccaleena formations despite their juxtaposition positive fold tests for both formations seem to rule out remagnetisation of one or both formations fold tests on tepee like structures in the nuccaleena formation accord with the magnetisation having been acquired at or near the time the structures formed during early diagenesis magnetisation therefore variably lags deposition causing the three identified magnetostratigraphic horizons to appear highly bedding transgressive whereas local correlations may seem satisfactory regional correlations are not nevertheless the number and duration of polarity chrons at various localities appear valid implying that the nuccaleena formation accumulated over an interval of 10 5 years the question of compaction related inclination shallowing was addressed by examining the anisotropy of remanence using a superconducting 12 t magnet the effective flattening factors f determined were nuccaleena dolostones n 4 f 0 98 0 02 elatina tidal rhythmites n 6 f 0 97 0 01 elatina arenites n 9 f 0 92 0 02 the mean elatina inclination is corrected to 14 0 palaeolatitude of 7 1 and the mean nuccaleena inclination to 35 4 palaeolatitude of 19 6 inclination shallowing also was investigated using the elongation inclination method the distributions of directions for the elatina were somewhat skewed with respect to bedding possibly invalidating use of this method nevertheless for the elatina the method yields an inclination of 19 palaeolatitude of 10 for the nuccaleena the corrected inclination is 41 9 palaeolatitude of 24 these corrections do not materially alter the near equatorial palaeolatitude for the elatina formation which is supported by data for other neoproterozoic strata in southern australia however the substantial increase in palaeolatitude between the late cryogenian and the early ediacaran is maintained the sequence boundary below the nuccaleena formation is a disconformity to low angle unconformity that in places cuts deeply into the 1500 m thick late cryogenian glaciogenic succession and may represent a significant time gap marked by rapid polar wander such a hiatus may be an important factor in the juxtaposition of glacial deposits and cap carbonates

palaeomagnetism and magnetic anisotropy of proterozoic banded iron formations and iron ores of the hamersley basin western australia abstract rock magnetic properties and palaeomagnetism of weakly metamorphosed banded iron formations bifs of the palaeoproterozoic hamersley group western australia and proterozoic bif derived iron ores have been investigated the bif units sampled here are slightly younger than 2500 ma at paraburdoo mount tom price and mount newman iron ore formation was completed before 1850 ma sampling was mainly from the mount tom price and paraburdoo mining areas and for the first time a palaeomagnetic fold test on fresh unweathered and unaltered bif samples has allowed the nature of the remanence of the bifs to be defined the remanence of the bifs is carried by late diagenetic low grade metamorphic magnetite after primary haematite this remanence is pre folding and is unlikely to be greatly affected by the high anisotropy because the palaeofield inclination was demonstrably low determination of palaeofield directions from measured remanence directions is complicated by self demagnetization effects in strongly magnetic highly anisotropic bif specimens we present a method for correcting measured directions for the effects of self demagnetization and anisotropy for typical bifs the effect of magnetic anisotropy on measured remanence inclinations and inferred palaeolatitudes is minor for low palaeolatitudes but can lead to large errors in calculated palaeopoles for intermediate to moderately steep palaeolatitudes anisotropy also causes cones of confidence to be underestimated due to compression of the range of inclinations in principle deflection of post folding remanence towards the bedding plane by high magnetic anisotropy can produce an apparent syn folding signature with best agreement between directions from different fold limbs after partial unfolding thus high anisotropy cannot only bias estimated palaeofield directions and cause underestimation of errors but can also mislead interpretation of the time of remanence acquisition the anisotropy of anhysteretic remanent magnetization arm probably yields an upper limit to the anisotropy of the chemical remanent magnetization crm carried by the bifs therefore from the anisotropy of arm a maximum inclination deflection of 9 is suggested for the sampled bifs this corresponds to less than 5 change of palaeolatitude the palaeomagnetic pole position calculated for bifs at paraburdoo is 40 9 s 225 0 e dp 2 9 dm 5 8 after tilt correction but without correction for anisotropy other pole positions reported include that from flat lying bifs from wittenoom at 36 4 s 218 9 e dp 4 6 dm 9 1 from mount tom price iron ore at 37 4 s 220 3 e dp 5 7 dm 11 3 and from paraburdoo ore at 36 4 s 209 9 e dp 4 7 dm 8 8 the poles from the bifs the paraburdoo ore and the part of the tom price deposit that was sampled in this study are indistinguishable from each other and from the mount jope volcanics overprint pole the magnetization of the bifs was probably acquired during burial metamorphism of the hamersley group soon before the main folding and uplift event in the southern part of the hamersley province this tectonic event exposed magnetite rich bifs to near surface oxidizing conditions producing extensive martite goethite orebodies and also appears to have produced the syn folding overprint magnetization recorded by the mount jope volcanics of the underlying fortescue group ages of magnetization are tentatively interpreted as 2200 100 ma for the bifs 2000 100 ma for the supergene enrichment of bif to martite goethite ore recorded by the parabudoo and mount tom price orebodies and 1950 100 ma for the metamorphic martite microplaty haematite ore recorded as an overprint by the tom price orebody and as the only surviving magnetization of the mount newman orebody

syntectonic emplacement of the middle jurassic concon mafic dike swarm coastal range central chile 33 s abstract the concon mafic dike swarm cmds consists of basaltic to andesitic dikes emplaced into deformed late paleozoic granitoids during the development of the jurassic arc of central chile the dikes are divided into an early group of thick dikes 5 12 m and a late group of thin dikes 0 5 3 m two new amphibole 40ar 39ar dates obtained from undeformed and deformed dikes constrain the age of emplacement and deformation of the cmds between 163 and 157 ma based on radiometric ages field observations ams studies and petrographic data we conclude that the emplacement of the cmds was syntectonic with the jurassic arc extension and associated with sinistral displacements along the nw trending structures that host the cmds the common occurrence of already deformed and rotated xenoliths in the dikes indicates that deformation in the granitoids started previously the early thick dikes and country rocks appear to have been remagnetized during the exhumation of deep seated coastal rocks in the early cretaceous around 100 ma the remanent magnetization in late thin dikes is mainly retained by small amounts of low ti magnetite at high temperature and pyrrhotite at low temperature the magnetization in these dikes appears to be primary in origin paleomagnetic results from the thin dikes also indicate that the whole area was tilted 23 to the nnw during cooling of the cmds the nnw sse extension vectors deduced from the paleomagnetic data and internal fabric of dikes are different with respect to extension direction deduced for the middle late jurassic of northern chile pointing to major heterogeneities along the margin of the overriding plate during the mesozoic or differences in the mechanisms driving extension during such period

ages and cooling history of the early cretaceous caleu pluton testimony of a switch from a rifted to a compressional continental margin in central chile the caleu pluton in the coastal range of central chile represents the last magmatic event related to the early cretaceous rifting along the western margin of south america the pluton was emplaced into a c 10 km thick pile of mainly basalts and basaltic andesites deposited in an early cretaceous subsiding basin and affected by very low grade metamorphism the cooling history of the pluton is documented on the basis of u pb 40ar 39ar step heating and fission track dating the u pb date suggests an age of emplacement in the interval 94 2 97 3 ma rapid subsolidus cooling between 550 500 c and 250 c is documented by 40ar 39ar plateau ages on amphibole biotite and plagioclase between 94 9 1 8 and 93 2 1 1 ma slower subsolidus cooling to c 100 c is identified at the 94 90 ma interval by the fission track thermal model the geochronological data show that the emplacement of the pluton is coeval with the very low grade metamorphism of the host rocks therefore this metamorphism is probably not the result simply of burial but also of a regional thermal gradient related to the plutonism exhumation of the pluton started coevally with its emplacement and continued to about 90 ma being associated with the closure of the early cretaceous rifting the caleu plutonism represents an asthenospheric derived event during maximum extension and marks a turning point between extensional and compressional related magmatism

40ar 39ar dating of volcanism and subsequent very low grade metamorphism in a subsiding basin example of the cretaceous lava series from central chile abstract 40ar 39ar geochronological method is applied to date both emplacement and subsequent very low grade metamorphism of a thick lava series the veta negra formation from the cordon de chacana area in the coastal range of central chile on the same lava series and even on the same rock sample it was possible to measure apparently valid ages of both the emplacement of lava flows by dating transparent plagioclase and the very low grade metamorphic event by dating sericite and or adularia sericite was dated on step heated single grains of strongly altered plagioclases the radiogenic argon from remaining fresh plagioclase appearing as negligible on most part of the age spectrum the concordance between one plateau age of 118 7 0 6 ma displayed by fresh plagioclase and a previously measured plateau age on plagioclase from one lava flow from a region due west from santiago bustamante hill aguirre l feraud g morata d vergara m robinson d 1999 time interval between volcanism and burial metamorphism and rate of basin subsidence in a cretaceous andean extensional setting tectonophysics 313 433 447 may indicate that the volcanic activity of the veta negra formation was synchronous along the 80 km long latitudinal segment studied ages of 96 8 0 2 and 97 0 1 6 ma displayed by adularia and sericite respectively are slightly older than ages of 93 94 ma given by adularia from the bustamante hill these data allow to define a period of about 22 ma between volcanism and metamorphism because of the contemporaneity of some metamorphic minerals and cretaceous plutons of the region the very low grade metamorphic event may be the result of the additional effect of burial and a regional thermal event nevertheless a significant effect of burial relatively to the contribution of high thermal gradient event s seems revealed by an apparent age gradient on very low grade minerals with the stratigraphic depth

linear and planar structure in ordered multivariate data as applied to progressive demagnetization of palaeomagnetic remanence summary consider a sequence of say 10 to 20 vector observations in threedimensional space it is suspected that a few subsets of consecutive observations are made up of collinear points the purpose of this paper is to construct a statistically based algorithm to find such linear segments and to assess their accuracy a similar assessment is made for coplanar sets of points this algorithm is applied here to palaeomagnetic data and is claimed to be superior to previous methods of palaeomagnetic analysis in terms of completeness and balance of analysis treatment of measurement errors and other sources of scatter criteria for identification of linear and planar sets of points and statistical rigour stability spectra with statistically based confidence limits are obtained as a by product 1 palaeomagnetic analysis background to the geometrical problem the natural remanent magnetization nrm in rocks is known in general to have originated in one or more relatively short time intervals during their formation and subsequent history i e during rock forming or metamorphic events during which nrm is frozen in stabilized by falling temperature grain growth etc the nrm acquired during each such event is a single vector magnetization parallel to the then prevailing geomagnetic field and is called a component of nrm rocks which have suffered more than one event of this kind are therefore liable to carry multi component nrm in the laboratory these components can be identified by demagnetizing the rock step by step usually by thermal alternating field or chemical demagnetization which involves the analysis of the distribution of blocking rzmperature coercivity and chemical leaching respectively resistance to these treatments is called stability of remanence and the range of treatment over which a particular component is removed is called the stability range of the component the various discrete

the productora cu au mo deposit chile a mesozoic magmatic hydrothermal breccia complex with both porphyry and iocg affinities the productora cu au mo deposit is hosted by a cretaceous hydrothermal breccia complex in the coastal cordillera of northern chile the current resource which includes the neighboring alice cu mo porphyry deposit is estimated at 236 6 mt grading 0 48 cu 0 10 g t au and 135 ppm mo local wall rocks consist of a thick sequence of broadly coeval rhyolite to rhyodacite lapilli tuffs 128 7 1 3 ma u pbzircon and two major intrusions the cachiyuyito tonalite and ruta cinco granodiorite batholith 92 0 1 0 ma u pbzircon previous studies at productora concluded the deposit had strong affinities with the iron oxide copper gold iocg clan and likened the deposit to candelaria based on new information we document the deposit geology in detail and propose a new genetic model and alternative classification as a magmatic hydrothermal breccia complex with closer affinities to porphyry systems hydrothermal and tectonic breccias veins and alteration assemblages at productora define five paragenetic stages stage 1 quartz pyrite cemented breccias associated with muscovite alteration stage 2 chaotic matrix supported tectonic hydrothermal breccia with kaolinite muscovite pyrite alteration stage 3 tourmaline pyrite chalcopyrite magnetite biotite cemented breccias and associated k feldspar albite alteration stage 4 chalcopyrite pyrite muscovite illite epidote and chlorite veins and stage 5 calcite veins the productora hydrothermal system crosscuts earlier formed sodic calcic alteration and magnetite apatite mineralization associated with the cachiyuyito stock main stage mineralization at productora was associated with formation of the stage 3 hydrothermal breccia chalcopyrite is the dominant hypogene cu mineral and occurs predominantly as breccia cement and synbreccia veins with pyrite the alice cu mo porphyry deposit is characterized by disseminated chalcopyrite and quartz pyrite chalcopyrite molybdenite vein stockworks hosted by a granodiorite porphyry stock alice is spatially associated with the silica ridge lithocap which is characterized by massive fine grained quartz altered rock above domains of alunite pyrophyllite and dickite rhenium os dating of molybdenite indicates that main stage mineralization at productora occurred at 130 1 0 6 ma and at 124 1 0 6 ma in the alice porphyry chalcopyrite and pyrite from productora have 34ssulfide values from 8 5 to 2 2 consistent with a magmatic sulfur source and fluids evolving under oxidizing conditions no significant input from evaporite or seawater sourced fluids was detected stage 3 tourmalines have average initial sr of 0 70397 consistent with an igneous derived sr source the productora magmatic hydrothermal breccia complex formed as a result of explosive volatile fluid release from a hydrous intrusive complex metal bearing fluids were of magmatic affinity and evolved under oxidizing conditions despite sharing many similarities with the andean iocg clan strong structural control regional sodic calcic alteration locally anomalous u fluid evolution at the productora cu au mo deposit is more consistent with that of a porphyry related magmatic hydrothermal breccia sulfur rich acid alteration assemblages and relatively low magnetite contents

mesozoic arc magmatism along the southern peruvian margin during gondwana breakup and dispersal abstract a high resolution u pb zircon geochronological study of plutonic units along the south peruvian margin between 17 and 18 s allows the integration of the geochemical geodynamic and tectonic evolution of this part of the andean margin this study focuses on the composite jurassic early cretaceous ilo batholith that was emplaced along the southern peruvian coast during two episodes of intrusive magmatism a first period between 173 and 152 ma with a peak in magmatic activity between roughly 168 and 162 ma and a second period between 110 and 106 ma emplacement of the jurassic part of the composite ilo batholith shortly post dated the accumulation of the volcanosedimentary succession it intruded chocolate formation which allows to estimate a subsidence rate for this unit of 3 5 km ma the emplacement of the main peak of jurassic plutonism of the ilo batholith was also closely coeval with widespread and repeated slumping during deposition of the cachios formation in the back arc region suggesting a common causal link between these phenomena which is discussed in the context of an observed 100 km trenchward arc migration at 175 ma and the relation with extensional tectonics that prevailed along the central andean margin during pangaea break up

protracted late magmatic stage of the caleu pluton central chile as a consequence of heat redistribution by diking insights from zircon data and thermal modeling abstract zircon u pb geochronology and geochemistry are combined with whole rock composition and thermal modeling to decipher the late magmatic stage of the composite cretaceous caleu pluton which consists of four lithological zones gabbro diorite zone gdz quartz monzodiorite zone qmdz granodiorite zone gz and monzogranite zone mgz the four lithological zones include felsic dikes and veins of variable thickness and distribution zircons of four representative samples each from the mentioned zones were dated and chemically analyzed the u pb ages exhibit sample scale scatter derived from protracted zircon crystallization at pluton scale the ages are substantially overlapped with a subtle decrease of ages from mafic to felsic sample the latter has a normal age span distribution with a mean age of 94 68 0 71 2 confidence and a mswd of 0 95 zircon grains from the uppermost zone of the pluton where the qmdz is emplaced have the highest ree and hfse contents zircon crystallization temperatures oscillate between 680 and 850 c regardless of the zircon age and sample composition differences in temperature and age of zircon crystallization of up to 185 c and 2 6 myr were identified at sample scale respectively numerical modeling indicates that the melts from which zircon crystallized are highly crystalline mostly higher than 60 crystal and resemble mgz in compositions time dependent thermal models were performed to account for preservation of the system above solidus temperature for long time intervals consistent with those of zircon crystallization two non exclusive scenarios for the late stage development of caleu pluton were considered i pluton construction by magma pulses assembled incrementally and ii upward transport of residual melts by diking through a mush system to yield heat redistribution to the levels where the samples collected the first scenario does not preserve residual melts for intervals as long as 2 6 myr unless an extremely thick magma reservoir is considered the second scenario is more favorable because it could provide enough heat that allows preserving residual melts above the solidus temperature depending on i dike width ii melt transport velocity and iii dike intensity vol dike for a dike width of 0 2 m and dike intensity of 10 consistent with field observations a transport velocity of 300 m yr is required to maintain the upper mush zone above 700 c the melt transport would have occurred as successive events to allow developing the protracted late magmatic stage of the caleu pluton

palaeomagnetism of australian hematite ore bodies i the middleback ranges of south australia summary the results of a palaeomagnetic study on high grade hematite ore bodies of south australia are reported the two major ore bodies iron monarch and iron prince gave consistent directions of magnetization with either steep to moderate negative and positive inclinations the positive and negative groups of iron monarch are not antiparallel because of the presumably long time interval represented by the sampling this is interpreted as being due to polar wandering with a superimposed reversal of the earth s field the remanent magnetization is carried by hematite and is extremely stable against alternating field and thermal demagnetization the intensity of the nrm is very weak compared to the trm which suggests it to be a crm in origin lack of a detailed polar wandering curve for the pre cambrian of australia prevents precise palaeomagnetic dating of the ores but the results clearly suggest a pre cambrian age the magnetic anisotropy of the ores is low and the distribution of the principal susceptibility axes reflects in part the structure of the host rock the iron duke orebody did not yield a reliable palaeomagnetic result

reversal asymmetry in mesoproterozoic overprinting of the 1 88 ga gunflint formation ontario canada non dipole effects or apparent polar wander abstract eighty two palaeomagnetic samples of calcareous and jaspilitic grainstones iron formation or taconite and chert carbonate were collected from the 1 88 ga gunflint formation at 22 sites in the thunder bay area ontario twenty clasts of gunflint taconite also were sampled from the basal conglomerate of the overlying mesoproterozoic sibley group anisotropy of magnetic susceptibility measurements indicate the gunflint formation in the sampling area has not experienced regional dynamic metamorphism analyses by variable field translation balance and x ray diffraction show that the predominant magnetic mineral is hematite but a small amount of magnetite also is present in some samples altogether 213 gunflint specimens and 59 sibley conglomerate specimens were subjected to stepwise thermal demagnetisation and 74 gunflint specimens to stepwise alternating frequency demagnetisation the following components were isolated for the taconites gunflint magnetite normal declination d 293 4 inclination i 30 8 95 7 2 n 21 reverse d 86 7 i 54 6 95 5 8 n 29 gunflint hematite normal d 243 6 i 23 6 95 6 0 n 11 reverse d 70 3 i 51 4 95 3 2 n 79 sibley clasts magnetite normal d 282 7 i 33 4 95 7 6 n 20 sibley clasts hematite normal d 254 5 i 56 2 95 8 4 n 13 reverse d 110 6 i 55 7 95 8 3 n 11 none of these sets passed the reversal test with the normal component generally being the shallower fold tests were negative or inconclusive and the conglomerate test also was negative chert carbonate at one other site appears to have acquired a remanence carried by magnetite d 97 3 i 78 2 95 6 3 n 18 prior to folding related to keweenawan 1 1 ga logan diabase sill emplacement most of the components we identified match components for keweenawan sills volcanic rocks intrusions and baked contact rocks in the thunder bay area indicating that keweenawan magmatism caused widespread chemical remagnetisation of the proterozoic country rock in our sampling area although others have argued that asymmetry was a feature of the keweenawan geomagnetic field the declinations of our gunflint and sibley hematite and magnetite components are different suggesting that the components were acquired at significantly different times we conclude that the reversal asymmetry shown by our gunflint and sibley data may best be ascribed to apparent polar wander during keweenawan times

jurassic and early cretaceous island arc volcanism extension and subsidence in the coast range of central chile more than 2000 km 3 of acid and 9000 km 3 of basic volcanic rocks formed during the jurassic and early cretaceous in the coast range of central chile between 32 30 s and 34 s these rocks which constitute the major part of an 15 km thick pile of alternately marine and continental deposits issued from volcanic arcs situated between a land area with paleozoic basement in the west and a marginal sea in the east asthenospheric upwelling led to extension and bimodal volcanism the volcanic products were deposited in intra arc basins subsiding at high rates 100 300 m m y the source of the magmas became more depleted with time due to an increase in degree of partial melting and their compositions were modified by subduction related fluids and contamination with a progressively thinner and younger crust the basic lavas are of high k calc alkaline to shoshonitic affinity chemically resembling the lavas found in some mature island arcs in the western pacific the extension and subsidence resulted in a low relief topography close to sea level in contrast with the present day convergent type of andean volcanism at the same latitude where calc alkaline intermediate lavas erupt from volcanoes at great height above a thick crust

rock magnetic properties related to thermal treatment of siderite behavior and interpretation detailed analyses of rock magnetic experiments were conducted on the oxidation products of high purity natural crystalline siderite that were thermally treated in air atmosphere susceptibilities increase sharply between 400 and 530 c indicative of some new ferrimagnetic mineral phase generation both a drop between 540 and 590 c on the heating cycle and a dramatic increase from 590 c to 520 c on the cooling cycle occurred and are well consistent with the characteristic of magnetite a distinct hopkinson type susceptibility peak indicates that hematite is the terminal product if siderite is heated to 700 c over and over it has been revealed in detail that the original inverse magnetic susceptibility fabric contributed by the crystalline anisotropy of siderite in siderite bearing specimens is changed to a normal magnetic fabric during incremental heating over 410 490 c this is a result of dominant contributions from the distribution anisotropy of newly transformed ferromagnetic minerals a strong chemical viscous remanent magnetization could be produced during siderite oxidation in an external field rock magnetic experimental results show that magnetite maghemite and hematite are the transformation products of high temperature oxidation of siderite in air maghemite was not completely inverted to hematite even at temperature as high as 690 c during incremental thermal treatments the mineral transformation processes were confirmed by conventional optical microscopic observation x ray diffractometry and mossbauer spectroscopic analyses these results indicate that the rock magnetic methods used here are reliable and highly sensitive in detecting very small magnetic phase changes in rocks we conclude that these temperaturedependent variations of magnetic properties can be used as criteria for identification of siderite in rocks and sediments furthermore it is clear that great care should be exercised in thermal demagnetization of siderite bearing rocks in paleomagnetic magnetic anisotropy and rock magnetic studies

contrasting tectonic settings and sulfur contents of magmas associated with cretaceous porphyry cu mo au and intrusion related iron oxide cu au deposits in northern chile porphyry cu mo au and iron oxide copper gold iocg deposits share many similarities e g fe cu and au contents but also have important differences e g the predominance of sulfide minerals in porphyry deposits and iron oxides in iocg deposits genetic comparisons are complicated by the broad definition of iocg deposits here we restrict our study to iocg deposits that are related to igneous intrusive systems in the mesozoic coastal cordillera of northern chile both porphyry and iocg deposits occur in close spatial and temporal proximity offering the chance to examine what controls their different modes of formation from detailed examination of the timing geochemistry and tectonic setting of associated igneous rocks based on new and published data we find that rocks associated with mid cretaceous iocg deposits 125 110 ma are largely indistinguishable from those associated with slightly earlier 125 ma and later 110 ma porphyry cu mo au deposits magmas related to iocg deposits were formed during a brief period of back arc transtension in the mid cretaceous and are on average somewhat more mafic dioritic locally alkaline and isotopically primitive compared to granodioritic magmas associated with porphyry deposits formed during normal contractional arc tectonics in the later cretaceous however these compositional ranges overlap and the differences are not clear enough to be diagnostic we measured the so3 content of igneous apatite from selected samples of these rocks to test the hypothesis that the difference in sulfur content of the ore deposits was due to differences in sulfur content of the associated magmas early igneous apatite crystals occurring as inclusions in silicate phenocrysts from the carmen de andacollo porphyry cu au deposit re os molybdenite ages of 103 9 0 5 ma 103 6 0 5 ma are significantly richer in s 0 25 0 17 wt so3 n 69 than similar apatite crystals from two iocg deposits candelaria casualidad and a sample of regional mid cretaceous igneous rock from near productora 0 04 0 02 wt so3 n 76 using published partition coefficients for s between apatite and oxidized silicate melt we semi quantitatively estimate corresponding magmatic sulfur contents of 0 02 wt s in the carmen de andacollo magmas versus 0 001 to 0 005 wt s in the iocg associated magmas this is an order of magnitude difference and the opposite of what would be expected if the difference were due to bulk magma composition sulfur solubility is generally higher in mafic magmas whereas here the s content is higher in the more felsic porphyries we conclude that the porphyry forming magmas indeed had higher s contents than the iocg related magmas and suggest that these differences reflect different petrogenetic processes during normal subduction magmas derived from the metasomatized mantle wedge are hydrous moderately oxidized and s rich and have the potential to generate s rich porphyry type deposits in contrast in back arc extensional settings upwelling asthenospheric melts carry a weaker subduction signature including lower s contents interaction of these s poor magmas with previously subduction modified upper plate lithosphere is more likely to give rise to s poor iocg deposits

the genetic relationship between mafic dike swarms and plutonic reservoirs in the mesozoic of central chile 30 33 45 s insights from ams and geochemistry five mafic dike swarms between 30 and 33 45 s were studied for their geochemical signature and kinematics of magma flow directions by means of ams data in the coastal range of central chile 33 33 45 s middle jurassic dike swarms concon and cartagena dike swarms cmds and crmds respectively and an early cretaceous dike swarm el tabo dike swarm etds display the presence of dikes of geochemically enriched high ti and depleted low ti basaltic composition these dikes show geochemical patterns that are different from the composition of mafic enclaves of the middle jurassic papudo quintero complex and this suggests that the dikes were injected from reservoirs not related to the plutonic complex the mantle source appears to be a depleted mantle for jurassic dikes and a heterogeneous enriched lithospheric mantle for cretaceous dikes in the etds vertical and gently plunging magma flow vectors were estimated for enriched and depleted dikes respectively which suggest together with variations in dike thickness that reservoirs were located at different depths for each dike family in the elqui dike swarm eds and limari mafic dike swarm lmds geochemical patterns are similar to those of the mafic enclaves of the middle jurassic plutons in the lmds east to west magma flow vectors are coherent with injection from neighbouring pluton located to the east in the eds some dikes show geochemical and magma flow patterns supporting the same hypothesis accordingly dikes do not necessarily come from deep reservoir they may propagate in the upper crust from coeval shallow pluton chamber

palaeomagnetism of iron formation from the late palaeoproterozoic frere formation earaheedy basin western australia palaeogeographic and tectonic implications abstract oriented cores of unweathered frere formation 1 9 1 8 ga including shaly and peloidal iron formation were obtained from two holes drilled to 116 m depth on opposite limbs of an anticline in the southern earaheedy basin at the northeastern margin of the yilgarn craton anisotropy of magnetic susceptibility measurements indicated that the frere formation in this area was unaffected by regional metamorphism the predominant magnetic mineral in the iron formation is hematite but a small amount of magnetite also is present in some samples stepwise thermal demagnetisation was conducted for 263 specimens many specimens acquired spurious components of magnetisation after treatment to 300 500 c ascribable to the oxidation of siderite to magnetite on heating however other specimens displayed a stable high temperature 680 c dual polarity component a carried by hematite that gave a bedding dip corrected direction of d 314 3 i 24 8 k 38 5 95 2 6 n 78 a fold test was positive at 99 level of confidence indicating acquisition of remanence prior to stanley folding in the earaheedy basin that occurred at 1 78 ga near the end of capricorn orogenesis the presence of polarity reversals in the stratigraphic section from one drill hole and within some specimens argues for the identification of a dipole field axis and the sufficient averaging of secular variation to define a palaeopole for the frere formation located at 45 2 s 220 0 e dp 1 3 dm 2 4 component a is ascribed to a chemical remanent magnetisation crm acquired soon after deposition and indicates a palaeolatitude of 13 0 1 5 for the frere formation the frere pole plots adjacent to but is still significantly different from late to post capricorn orogeny overprint poles from the hamersley province of the southern pilbara however vertical axis relative anticlockwise rotation of 10 15 for the frere pole brings it close to these pilbara overprint poles the data imply that the earaheedy basin and possibly the yilgarn craton underwent clockwise rotation of 10 15 relative to the pilbara craton during stanley folding this finding accords with evidence for tectonic settings marked by sinistral strike slip deformation adjacent to the yilgarn pilbara cratonic boundary during deposition and subsequent folding in the earaheedy basin the data also imply that the west australian and north australian cratons were in close proximity by 1 8 ga

palaeomagnetic cleaning strategies abstract cleaning is a term used in palaeomagnetism to refer to laboratory methods designed to demagnetize preferentially the less stable components of magnetization although the diversity of methods suggests that there is a wide choice available this is illusory except in simple cases the wrong cleaning strategy may yield misleading results cleaning methods include thermal magnetic alternating field or a f low temperature chemical shock and microwave cleaning a cleaning strategy involves one or more cleaning methods applied in a specific sequence the design of a strategy is made easier with the a priori knowledge of magnetic mineralogy and granulometry or in other words rock magnetic properties although not a substitute for other rock magnetic experiments the variation of low field susceptibility with temperature k t not only provides information on magnetic mineralogy and granulometry but it also draws attention to chemical alteration that may occur during thermal cleaning apart from the rapid though reversible decrease of k associated with curie temperatures other diagnostic k t features include the low temperature about 140 c peak typical of multidomain md magnetite the high temperature hopkinson peak typical of fine grained magnetite and other magnetic minerals and the classic lepidocrocite maghemite haematite profile which is irreversible examples are given where the remanence caused by lightning can be effectively eliminated only by applying a f or low temperature lt pre treatment such remanence may be an isothermal remanent magnetization irm or a combination of irm and anhysteretic remanent magnetization arm as the unblocking temperature of relatively low coercivity md grains may extend to high temperature it is often desirable to suppress md remanence an example is given of palaeointensity determination with the without lt pre treatment the nature of the lt transition is briefly addressed in the light of a rare double low temperature peak which may reflect both the isotropic point and the verwey transition

alteration patterns and structural controls of the el espino iocg mining district chile the el espino iocg mining district is characterized by several mineralized bodies the largest of which is the el espino deposit which has an estimated geologic resource of 123 mt at 0 66 cu and 0 24 g t au mineralized bodies are distributed in a 7 10 km2 area throughout a 1 000 m vertical section they range from single veins to stockworks and breccias to manto type deposits the ore bodies are hosted primarily by volcanic volcaniclastic and sedimentary rocks of the early cretaceous arqueros and quebrada marquesa formations with a few mineralized zones within late cretaceous dioritic intrusions the fault and vein architecture shows that el espino iocg system was localized within a dilatational jog along a major transtensional dextral fault system sodic alteration albite is the most extensive style of alteration in the district and it is bounded by major ns nne trending faults sodic calcic epidote albite alteration occurs at deep to medium elevations 1 000 500 m and grades inward into calcic alteration calcic alteration surrounds dioritic intrusions of the llahuin plutonic suite significant iron oxides are associated with later calcic alteration associations actinolite epidote hematite the upper portions of the alteration system 0 500 m display hydrolytic alteration associations with abundant hematite hydrolytic veins are feeders to zones of manto type alteration and mineralization within favorable volcano sedimentary lithologies that formed el espino deposit sulfides are largely confined to calcic and hydrolytic alteration associations hydrothermal fluids responsible for hematite and sulfide mineralization had salinities between 32 and 34 wt nacleq and temperature of approximately 425 c at an estimated depth of 3 4 km geochronological u pb and 40ar 39ar data indicate that hydrothermal alteration was coeval with magmatic intrusive activity one particular dioritic intrusion 88 5 ma preceded the calcic stage 88 4 ma which was accompanied by iron oxide copper and gold mineralization hydrolytic alteration related to economic iron oxide copper and gold mineralization came immediately after at 87 9 ma

growth of granite greenstone terranes at convergent margins and stabilization of archean cratons abstract archean granite greenstone terranes represent juvenile continental crust formed in a variety of plate tectonic settings and metamorphosed through a complex series of structural and magmatic events most archean granite greenstone terranes appear to have acquired their first order structural and metamorphic characteristics at convergent plate margins where large accretionary wedges similar in aspect to the chugach makran and altaids grew through offscraping and accretion of oceanic plateaux oceanic crustal fragments juvenile island arcs rifted continental margins and pelagic and terrigenous sediments buoyant slabs of parts of archean oceanic lithosphere may have been underplated beneath these orogens forming thick crustal roots characterized by interleaving between the depleted slabs and undepleted asthenosphere back stepping of the subduction zones after accretion of plateaux and arcs caused the arcs magmatic fronts to migrate trenchward through the accretionary wedges dehydration of the subducting slabs hydrated the mantle wedges below the new arcs and generated magmas sanukitoid suite in the mantle wedge whereas other magmas tonalite trondhjemite granodiorite or ttg suite appear to have been generated by melting of hot young subducted slabs eventual collision of these juvenile orogens with other continental blocks formed anatectic granites then thickened the crust beyond its ability to support its own mass which initiated gravitational collapse and decompressional release of syn to late tectonic granitoids from wedges of fertile mantle trapped between underplated oceanic lithospheric slabs and aided in the cratonization of the granite greenstone terranes deeply penetrating structural discontinuities such as shear zones and sutures provided pathways for fluids and granitoids to migrate into the mid and upper crust forming ore deposits and plutons most preserved granite greenstone terranes have been tectonically stable since the archean and form the cratonic interiors of many continents

genesis of proterozoic iron formation sulphur isotope evidence seven units of carbonaceous shale or sulphide facies iron formation have been sampled they are associated with proterozoic iron formations that range in age from 1 9 to 2 5 ga sokoman and gunflint canada riverton united states penge south africa and brockman australia sulphur isotope ratios have been determined on the sulphides removed from these shales by both physical and chemical means the mean 34s composition of the seven units varies between 4 9 and 6 6 and the sample variance is low within each unit these distributions are more characteristic of hydrothermal sulphide than sulphide produced by biogenic reduction this hydrothermal sulphide is believed to have originated from high temperature reduction of seawater sulphate and from magmatic sulphide a model is suggested whereby this sulphide was exhaled into stratified anoxic oxic basins the sulphide and associated base metals were deposited in the reduced sediments beneath the anoxic waters while some iron and manganese was deposited on oxygenated shelves the data support but do not prove a hydrothermal exhalative origin for lower proterozoic iron formation

petrogenetic evolution of felsic volcanic sequences associated with phanerozoic volcanic hosted massive sulphide systems the role of extensional geodynamics abstract felsic volcanic hosted massive sulphide deposits form in many different geotectonic environments usually associated with episodes of extension however empirical geochemical links between felsic volcanism geotectonic environment and extensional geodynamics have not been adequately documented so as to distill the fundamental relationships between these features and formation of volcanic hosted massive sulphide vhms mineralization which is important considering most vhms deposits are hosted in felsic volcanic bearing sequences using felsic geochemical data from six different vhms mineralized phanerozoic geotectonic settings it seems that the felsic geochemical data are consistent with the overall geotectonic interpretations for these areas which in turn helps in stratigraphic and geotectonic interpretations for detailed vhms exploration in extensional geodynamic settings bimodal volcanism is common compared to the continuum of compositions observed under compressional to neutral high stress regimes in calc alkaline arc building environments the empirical geochemical metallogenic vhms relationships evident in the archean felsic volcanic rocks in particular the low zr y n yb n 2 o conditions in different source areas and geotectonic environments they seem to be the predominant controls on the compositional evolution of felsic magmas in extensional settings because of the limited degree of assimilation and fractional crystallization afc affecting their compositions with rapid intrusion into the uppermost crust to positions were they achieve neutral buoyancy in the volcanic to subvolcanic environment within plate felsic or a type anorogenic type and anomalous ocean ridge or type felsic magmas that have tholeiitic affinities relatively low alkali elements are common within parts of the volcanic sequences hosting massive sulphide deposits reflecting in part a relationship to extension the high contents of high field strength elements zr ga lree hree y nb etc and other lines of evidence indicate they are relatively high temperature relatively reduced magmas probably derived from previously dehydrated granulitic or melted oceanic simatic or continental sialic crust low p granulites or amphibolites but also in some cases possibly mantle rocks such that they are significantly water undersaturated during melting i e a h 2 o 1 reduced viscosities typical of high temperature melts enable rapid intrusion to higher levels diking vs diapirism higher rates of heat advection from mantle mafic magmas into the thickened oceanic and or continental crust during lithospheric extension are needed to facilitate melting of these rock to form a type and or or type partial melts tholeiitic felsic this usually results in bimodal magmatism as magma mixing is not favored in extensional settings especially at high magma production intrusion and extrusion rates i e high heat flow this would inhibit ferromagnesian phase in particular hornblende and magnetite fractionation especially at low f o 2 in the felsic magmas that would deplete the melt in y hree as well as ore related elements particularly zn which is 2 to 3 times higher in a type and or type rocks compared to other magma types therefore the empirical felsic igneous geochemical metallogenic relationships observed by others low zr y n yb n

tectonic evolution of proterozoic australia proterozoic australia has long been interpreted as a single intact continent in which all tectonic and magmatic activity was intracratonic this paper proposes an alternative hypothesis in which numerous fragments of continental crust were assembled by plate tectonic processes the assembly was completed between 1300 and 1100 ma when the crustal fragments were combined as an early component of the rodinian supercontinent rifting and fragmentation of archaean continents began in the late archaean and continued into the proterozoic passive margin deposits such as those of the hamersley basin accumulated on isolated fragments of archaean crust these numerous fragments were subsequently assembled into three cratons by 1830 ma a west australian craton was established by collision of the archaean pilbara and yilgarn cratons which were joined along the capricorn orogen similarly a south australian craton developed by amalgamation of the proto gawler and proto curnamona cratons along the kimban orogen a north australian craton appears to have formed by accretion of numerous crustal fragments including the kimberley pine creek lucas and altjawarra cratons with sutures marked by the king leopold halls creek tennant creek and proto isan orogens the southern margin of the north australian craton was the site of repeated terrane accretion and orogenic activity between 1880 ma and 1400 ma this included an orogenic event at 1880 1850 ma the strangways 1780 1730 ma argilke 1680 1650 ma and chewings 1620 1580 ma orogenies and the intracratonic anmatjira uplift 1500 1400 ma intracratonic rifting at 1750 to 1710 ma and 1640 to 1600 ma produced the mcarthur basin and related minor basins parts of which were deformed by the isan orogeny at 1600 and 1530 ma rifting along the line of the capricorn orogen led to deposition in the overlying intracratonic bangemall basin between 1630 and 1300 ma along the eastern margin of the south australian craton the 1670 to 1600 ma olarian orogeny marks interaction with now obscured continental crust to the east tectonic activity between 1300 and 1100 ma led to the assembly of proterozoic australia as an early component of the supercontinent of rodinia this first involved the amalgamation of the west australian and north australian cratons followed by collision with the south australian craton the centralian superbasin developed over the junction of the north south and west australian cratons between 830 and 750 ma rifting to the east formed the adelaide geosyncline at 830 ma this was followed by the breakup of rodinia with the rifting apart of laurentia and gondwanaland along the eastern margin of proterozoic australia at 750 ma and the subsequent formation of the palaeo pacific ocean after the breakup of rodinia a series of northeast southwest compressional events followed by periods of relaxation reflect the assembly of a new supercontinent old lines of weakness were reactivated culminating in the intracratonic king leopold paterson petermann ranges and pinjarra orogenies between 620 and 540 ma subsequent reactivation continued into the phanerozoic with the widespread eruption of continental flood basalts and the formation of intracratonic basins 540 530 ma

age of mineralization by basinal fluids at the el soldado manto type copper deposit chile 40ar 39ar geochronology of k feldspar abstract manto type copper deposits of the southern central andes are strata bound epigenetic mineral concentrations hosted by bimodal volcanic and volcaniclastic rock suites they are economically important but since they are not obviously related to intrusions their genesis has been controversial to some extent because it has proven difficult to date the ore forming event and distinguish it from the age of low grade regional metamorphism el soldado the largest manto type copper deposit in central chile is hosted by marine volcanic and subvolcanic intrusive rocks of the lower cretaceous extensional arc to discriminate between regional metamorphism and mineralization we dated 11 samples of k feldspar by the stepwise heating 40 ar 39 ar method hydrothermally precipitated k feldspar associated with copper sulfides precipitated at temperatures of 300 c and range in age from 100 5 1 5 to 106 1 1 1 ma mean 103 1 6 ma n 8 syn mineralization k feldspar gave an age of 102 5 2 0 which is interpreted to be the age of mineralization k feldspar unrelated to ore yields older ages between 109 4 1 1 and 112 2 ma mean 110 3 1 4 ma n 3 is variably reset by the younger ca 103 ma mineralization episode the above ages are interpreted to represent the age of alkali redistribution and crystallization due to burial metamorphism of the host sequence fission track dating of apatite in the host rocks yields an age of ca 90 ma indicating fast cooling of the system to less than ca 100 c after mineralization time temperature modeling of fission track length data are compatible with initial fast exhumation followed by slow exhumation and cooling to surface temperatures high levels of atmospheric ar were detected in all k feldspar samples and are tentatively attributed to connate meteoric fluids incorporated into the basinal metamorphic mineralizing fluids

magmatic gradients in the cretaceous caleu pluton central chile injections of pulses from a stratified magma reservoir the caleu pluton central chile extending over 338 km2 and with more than 1 400 m of vertical relief intrudes the n s trending lower cretaceous volcano sedimentary and volcanic successions at a depth equivalent to a pressure of 2 kb the host stratified volcanic successions are tilted about 30 40 e whereas the pluton shows paleomagnetic evidence of either tilting of 15 e or clockwise rotation by few degrees a gradient of westward increasing sio2 content is recognized across the pluton giving rise to three n s elongated zones gabbro diorite zone gdz tonalite zone tz and granodiorite zone gz biotite and hornblende compositions also exhibit a westward decreasing gradient in mg mg fe indicating that the more mafic the zone is the more oxidizing is its crystallization condition horizontal inward gradients of progressively less evolved rocks are recognized across gdz and tz whereas no horizontal gradients were found in the gz vertical compositional gradients are recognized in the gdz and tz which consist of an upward increase in sio2 and decrease in mgo feo fe2o3 and compatible trace elements a vertical compositional boundary was recognized along a traverse across the tz separating two magma pulses with similar trends of compositional variations the three zones of the caleu pluton were derived from a common isotopically sr nd depleted source each zone probably evolved independently as their compositional characteristics would have not been acquired in situ the resulting compositional characteristics of the zones would have been developed prior to the intrusion in a subjacent stratified reservoir placed at about seven kilometers below the pluton

u pb ages from the nimish formation and montagnais glomeroporphyritic gabbro of the central new quebec orogen canada three u pb zircon dates were obtained from two igneous suites associated with cycle 2 sedimentation in the central new quebec orogen labrador trough in the dyke lake area of the western part of the orogen schefferville zone the nimish formation includes a polymictic conglomerate containing quartz syenite cobbles that crystallized at 1877 8 1 3 ma these cobbles are petrographically and geochemically linked to the nimish volcanics which are intercalated with the sokoman iron formation consequently the syenite date gives an approximate age for both nimish magmatism and ironstone deposition in the schefferville zone a date of ca 2 65 ga obtained from a nimish trachyte in the same area indicates that zircons in the sample are xenocrysts farther east at howse lake howse zone a montagnais plagioclase glomeroporphyritic gabbro sill crystallized at 1884 0 1 6 ma the howse lake sill which intrudes turbidites of the menihek formation is considered comagmatic with the basalts that cap the form

paleomagnetism of the 1 88 ga sokoman formation in the schefferville knob lake area quebec canada and implications for the genesis of iron oxide deposits in the central new quebec orogen abstract thirty nine oriented block samples of iron formation were collected at 13 sites including opposite limbs of major folds from the 1 88 ga sokoman formation knob lake group in the schefferville knob lake area of the central new quebec orogen northern quebec the samples assayed up to 80 24 fe2o3t 54 08 fe implying fe enrichment of the iron formation up to ore grade anisotropy of magnetic susceptibility measurements on 245 standard specimens indicate a well preserved bedding parallel fabric in the iron formation suggesting minimal alteration of the magnetic mineralogy since deposition and or a mimetic secondary magnetic mineralogy the iron formation has not been internally deformed since the magnetic mineralogy was established analyses by variable field translation balance and x ray diffraction showed that the predominant magnetic mineral is hematite but a small amount of magnetite also is present in most samples following low temperature pre treatment as appropriate stepwise thermal and alternating field demagnetization of 218 specimens revealed a low temperature post folding component maximum tub 400 c d 27 1 i 20 1 95 10 9 from seven sites pole position of 40 6 s 257 0 e and components carried by magnetite maximum tub 580 c d 35 8 i 3 9 95 9 1 from 10 sites pole position of 29 6 s 250 9 e and hematite maximum tub 680 c d 40 0 i 1 6 95 18 6 from seven sites pole position of 26 8 s 247 0 e the components carried by magnetite and hematite are pre syn and post folding depending on the sampling site indicating that the magnetization was acquired continuously with deformation in the new quebec orogen at 1 84 1 83 ga no evidence was found for acquisition of magnetization during the mesozoic when many of the iron oxide orebodies in the schefferville knob lake area are thought to have formed our findings imply that an episode of fe enrichment of iron formation in the sokoman formation involved the circulation of hydrothermal fluids related to late paleoproterozoic orogenesis such orogenic circulation of fluids may have contributed to the development of hematitic orebodies in the central new quebec orogen

the paleomagnetic significance of new u pb age data from the molson dyke swarm cauchon lake area manitoba u pb geochronology paleomagnetism and petrography indicate that the molson dyke swarm along the western margin of the superior craton is a composite of at least two ages of intrusion the more extensive younger dyke set the molson swarm sensu stricto generally has a 030 trend is mainly pyroxenitic to noritic with subordinate diabase and has been related to rifting in a back arc environment during closure of the manikewan ocean at about 1920 1800 ma which culminated in the trans hudson orogen a u pb zircon age from one of these dykes located at cauchon lake indicates emplacement at 1877 7 150 4 similar to two previous u pb age determinations on molson dykes another dyke from cauchon lake yields a baddeleyite zircon u pb date of 2091 2 ma and appears to be part of an older mainly diabasic suite of east northeast trending dykes that may represent a continental rifting episode that preceded the opening of the manikewan ocean the new u pb age data require a revision to the interpretation of

mercury evidence for pulsed volcanism during the end triassic mass extinction the central atlantic magmatic province camp has long been proposed as having a causal relationship with the end triassic extinction event 201 5 ma in north america and northern africa camp is preserved as multiple basaltic units interbedded with uppermost triassic to lowermost jurassic sediments however it has been unclear whether this apparent pulsing was a local feature or if pulses in the intensity of camp volcanism characterized the emplacement of the province as a whole here six geographically widespread triassic jurassic records representing varied paleoenvironments are analyzed for mercury hg concentrations and hg total organic carbon hg toc ratios volcanism is a major source of mercury to the modern environment clear increases in hg and hg toc are observed at the end triassic extinction horizon confirming that a volcanically induced global hg cycle perturbation occurred at that time the established correlation between the extinction horizon and lowest camp basalts allows this sedimentary hg excursion to be stratigraphically tied to a specific flood basalt unit strengthening the case for volcanic hg as the driver of sedimentary hg toc spikes additional hg toc peaks are also documented between the extinction horizon and the triassic jurassic boundary separated by 200 ky supporting pulsatory intensity of camp volcanism across the entire province and providing direct evidence for episodic volatile release during the initial stages of camp emplacement pulsatory volcanism and associated perturbations in the ocean atmosphere system likely had profound implications for the rate and magnitude of the end triassic mass extinction and subsequent biotic recovery

the updip and downdip limits to great subduction earthquakes thermal and structural models of cascadia south alaska sw japan and chile this study examines thermal and structural controls of the updip and downdip rupture limits of great subduction thrust earthquakes data on past great earthquake seismic limits have been compiled for four continental subduction zones cascadia sw japan south alaska and chile these limits have then been compared to the predictions of several models for what constrains great earthquake rupture temperatures on the subduction thrusts have been estimated by finite element numerical models the landward limits of the observed updip aseismic zones correspond to the position where the thrust temperature reaches about 100 c that is depths of about 2 to 10 km for the subduction zones studied this temperature agrees with the dehydration of stable sliding smectite clays to illite chlorite the temperatures in this region are controlled mainly by the thickness of sediment on the incoming crust and by the crustal age and thus heat flow the downdip limits correspond to the depth on the thrust where either 1 the temperature reaches about 350 c which corresponds to thermally activated stable sliding behavior for crustal rocks with a transition to 450 c or 2 about 40 km depth if 350 c is reached at greater depth depths of about 40 km approximately correspond to the intersection of the thrust with the continental forearc moho and this downdip limit may be a consequence of stable sliding serpentinite or talc and other hydrated forearc mantle rocks the primary temperature controls on the downdip region are the age of the subducting oceanic plate and the thrust dip profile secondary control comes from the thickness of incoming sediment the convergence rate and the radioactive heat generation in the overlying forearc the 100 c updip limit occurs near the trench for young subducting plates with a thick sediment section such as cascadia 6 8 ma and up to 80 km landward for older oceanic crust such as south alaska 50 ma the 350 c downdip thermal limit is applicable for young oceanic plates e g cascadia and nankai whereas the forearc mantle limit applies for older plates e g south alaska and chile except near the chile rise for the margins studied that have experienced great earthquakes there is generally good agreement between the postulated thermal and moho limits and the rupture or seismogenic zone as defined by the distribution of aftershocks and by waveform tsunami and dislocation modeling the downdip limit of the interseismic locked zone from dislocation modeling is also in agreement with these limits

ion microprobe analyses of δ18o in early quartz cements from 1 9ga granular iron formations gifs a pilot study abstract the low 18 o values of precambrian cherts have been widely used to infer that temperatures were higher and or seawater 18 o was lower compared to today s oceans however the precambrian cherts presented as evidence for these temperatures are neomorphosed from amorphous precursors that originally precipitated from ocean water suggesting diagenetic alteration is an important cause of the widespread low 18 o values many pores between sand size clasts in granular iron formations gifs are filled with coarsely crystalline quartz cements that are texturally primary in unmetamorphosed gif samples from the 1 9 ga gunflint and sokoman iron formations synsedimentary clasts of gif and high minus cement porosities the volume of pore space occupied by recognizable cement inversely proportional to the degree of compaction indicate such cements precipitated directly from pore water near the depositional interface growth bands identified in quartz cement crystals using cathodoluminescence cl imaging further confirm their void filling and unrecrystallized nature in order to test the effect of neomorphism on the 18 o of silica in these rocks unrecrystallized primary quartz cements and neomorphic quartz in adjacent clasts were both analyzed by ion microprobe values of 18 o range from 23 5 to 26 4 vsmow in cements and 21 3 26 8 in clasts in samples from the gunflint iron formation samples from the sokomon iron formation record deeper more evolved pore waters and in some cases contain lower 18 o cements in individual pores of gunflint samples cement values vary only slightly more than analytical uncertainty from edge to center and no consistent increasing or decreasing trends were observed overall the 18 o of the cements are similar to the most elevated 18 o reported from cherts of similar age but span a substantially smaller range this suggests most of the lower values reported previously reflect late diagenetic conditions at greater burial depths the shallow possibly restricted seas above the gunflint and sokoman gifs appear to have been warmer and may have been lower in 18 o than present day seawater but they may not have been representative of global seawater

tectono stratigraphic evolution of the andean orogen between 31 and 37 s chile and western argentina fil charrier reynaldo universidad de chile facultad de ciencias fisicas y matematicas departamento de geologia chile universidad andres bello chile

volcanic stratigraphy and geochronology of the cretaceous lancones basin northwestern peru position and timing of giant vms deposits a 10 km thick sequence of basaltic to rhyolitic volcanic rocks forms the arc component of the cretaceous lancones basin in northwestern peru and underlies part of the huancabamba deflection the marine volcanic successions show markedly different compositional features and depositional facies consistent with a maturing arc within a shallowing marine basin the earliest volcanism accompanying rifting was dominated by basaltic pillow lava and breccia with lesser aphyric to feldspar quartz porphyritic felsic volcanic rocks these volcanic successions filled the lowest exposed portion of the basin and were accompanied by volcanogenic massive sulfide vms deposits which are inferred to have formed in a localized but relatively deep marine setting u pb zircon dating of felsic volcanic rocks associated with vms deposits at tambogrande indicates ages from 104 8 1 3 to 100 2 0 5 ma for the ore bearing volcanic sequence the timing of onset of rift related volcanism is not well constrained but is therefore of middle albian age or older subsequent latest albian to turonian volcanism is composed of successions of relatively more felsic rich volcaniclastic rocks and yields u pb zircon ages of 99 3 0 3 to 91 1 1 0 ma these later volcanic successions are intercalated and overlain by siliciclastic and carbonate sedimentary sequences prevalent in the western forearc section of the lancones basin finally the basin was intruded by late cretaceous to tertiary granitoids of the coastal batholith the genesis of the cretaceous lancones basin and other equivalent volcanic rift related marginal basins in western south america including the western peruvian trough is related tectonically to the break up of gondwana early volcanism and associated vms deposits formed in the lancones basin during the albian coincided with the initial rifting stage prior to active oceanic spreading between south america and africa during this time the relatively stationary western margin of continental south america was undergoing extension and rifting due to a westward and oceanward retreating arc resembling a mariana arc type setting the mochica orogeny marks the termination of rifting subsidence and related volcanism along the western margin of south america this orogenic event also broadly coincides with the onset of spreading of the south atlantic and westward drift of the south american continent subsequent volcanism in the lancones basin was more continental arclike under an andean type scenario

northeast trending early proterozoic dykes of southern superior province multiple episodes of emplacement recognized from integrated paleomagnetism and u pb geochronology integrated paleomagnetic and u pb geochronologic studies have been conducted to establish the paleomagnetic directions and ages of early proterozoic tholeiitic dykes of northeast trend in the southern superior province previously referred to collectively as preissac dykes it is demonstrated that they are readily separated on the basis of paleomagnetism into subsets referred to as the biscotasing and senneterre swarms in addition a pair of unnamed dykes may be associated with the north and northwest trending matachewan swarm farther west biscotasing dykes have a down west magnetization of single polarity with a corresponding paleopole at 27 8 n 136 7 w dm 12 3 and dp 9 4 senneterre dykes carry an up north or occasionally down south direction with corresponding paleopole at 15 3 s 75 7 w dm 7 0 dp 4 4 the senneterre direction is indistinguishable from the primary n1 remanence direction that dominates the magnetization of nipissing sills of the southern province paleomagnetic fiel

magnetic fabrics and compositional evidence for the construction of the caleu pluton by multiple injections coastal range of central chile the caleu pluton which is located in the coastal range of central chile consists of three n s elongated lithologic zones which define an across pluton compositional variation characterized by a westward increase in sio2 content the gabbro diorite gdz tonalite tz and granodiorite gz zones a total of 547 cores were drilled at 68 sites within the different zones to carry out magnetic fabric and paleomagnetic studies the highest values of magnetic susceptibility and remanent magnetization are found in the gdz 0 04 0 1 si whereas the lowest values are observed in the gz 0 01 0 02 si this is consistent with the significant decrease in magnetite content from the gdz to the gz the dips of the magnetic foliation planes are steep within gdz and tz and shallower dips are observed in gz magnetic foliations along some borders of the pluton are parallel to the corresponding contacts the parallelism observed between the magnetic foliations and the contacts between zones suggests that these zones have recorded the same mechanism of emplacement at the magmatic state the peculiar foliation pattern observed in the central parts of tz and gz is attributed to a lobe like structure that was acquired during the late stages of emplacement and inflation the distinct compositional characteristics of these three zones were not acquired in situ rather they were emplaced successively from mafic to felsic from a subjacent stratified sill like magma reservoir which was filled after tilting of the host volcanic succession the rapid upward transport of these melts of different compositions up to a shallow depth is attested by both the equilibration pressure of late magmatic hornblende 200 mpa and the pressure of early magmatic epidote crystallization 500 600 mpa we conclude that space was created by lateral flow propagation in which the intrusion of later pulses pushed earlier ones away from the feeder system followed by roof lifting and possibly floor depression

in situ la icp ms trace elemental analyses of magnetite the late palaeoproterozoic sokoman iron formation in the labrador trough canada abstract the sokoman iron formation in the labrador trough canada a typical granular iron formation gif is coeval with the 1 88 ga nimish volcanic suites in the same region it is composed of the lower middle and upper iron formations in addition to primary and altered magnetite in iron formations of the hayot lake rainy lake and wishart lake areas magnetite in volcanic breccia associated with the iron formation is identified for the first time in the stratigraphy trace elemental compositions of the most primary altered and volcanic brecciated magnetite of the sokoman iron formation were obtained by la icp ms commonly detected trace elements of magnetite include ti al mg mn v cr co and zn these three types of magnetite have different trace elemental compositions primary magnetite in the iron formation has a relatively narrow range of compositions with the depletion of ti pb mg and al magnetite from volcanic breccia is rich in ti al v mn mg zn cu and pb indicative of crystallization from mantle derived magmas altered magnetite in the iron formation shows a relatively wide range of trace elemental compositions mineralizing fluids associated with magmas that generated the 1 88 ga nimish volcanic suites circulated through the sedimentary piles to further enrich the iron formations and to form magnetite with variable compositions the comparisons of different types of primary and altered magnetite in the iron formation in the region show distinct provenance discrimination our findings also support the origin of iron formations in association with multiple stages of exhalative volcanic and hydrothermal processes

3430 to 3417 ma calc alkaline volcanism in the mcphee dome and kelly belt and growth of the eastern pilbara craton abstract the mcphee dome and kelly belt in the eastern pilbara craton contain well preserved proximal shallow marine dominantly intermediate volcanic successions these have shrimp u pb zircon ages of 3430 3 and 3417 9 ma respectively which are younger than geochemically similar 3 52 and 3 47 to 3 45 ga suites in other eastern pilbara greenstone belts basalts andesites and dacites from the mcphee dome are similar in composition to modern arc rocks whereas dacites and rhyodacites from the kelly belt have geochemical characteristics of archaean tonalite trondhjemite granodiorite ttg magmas the geochemistry of the mcphee dome basalts basaltic andesites and dacites most likely reflects infracrustal fractional crystallization of a mantle derived magma combined with assimilation of older crust or a silicic melt in contrast the kelly belt dacites and rhyodacites were most likely derived by melting of subducted mafic crust the extensional submarine environment of pre 3 4 ga dominantly mafic and intermediate volcanism in the eastern pilbara differs from that of approximately coeval granitoid magmatism the pre 3 4 ga granitoids have the major and trace element features of ttg suites and appear to have been emplaced into already thick crust culminating in core complex formation available data are most consistent with growth of the eastern pilbara craton in convergent tectonic settings involving pulses of extensional shallow marine arc like volcanism between 3 52 and 3 42 ga overlapping with and or punctuated by periods of crustal thickening and the intrusion of complex ttg granitoid batholiths

extremely negative and inhomogeneous sulfur isotope signatures in cretaceous chilean manto type cu ag deposits coastal range of central chile abstract chilean manto type cmt cu ag hydrothermal deposits share a characteristic association of volcano sedimentary jurassic to lower cretaceous host rocks style of mineralization ore and associated mineralogy and geochemistry with ore grades typically 1 cu that make this family of deposits significant and interesting both academically and economically although often stratabound geological evidence supports an epigenetic origin for these deposits we present a detailed stable isotope study of la serena and melipilla naltahua lower cretaceous deposits central chile which reveals extremely negative 34 s values to 50 which are among the lowest values found in any ore deposit in addition the range of 34 s values from sulfides in the two areas is very wide 38 3 to 6 9 in la serena and 50 4 to 0 6 in melipilla naltahua these new data significantly extended the reported range of 34 s data for cmt deposits co existing sulfates range from 7 9 to 14 3 and are exclusive to la serena deposit the wide sulfide isotopic range occurs at deposit and hand specimen scale and suggests a polygenic sulfur source for these deposits where bacteriogenic sulfide dominates while sulfur isotope data for the bulk of jurassic cmt deposits northern chile suggests a predominant magmatic source in their origin mean 2 7 1 9 1 contributions of a magmatic component is only likely to be involved at melipilla naltahua deposit the 13 c values obtained for calcites associated with the mineralization range from 20 1 to 0 2 also suggesting polygenic carbon sources with the likely strong involvement of degradation of organic matter and leaching of limestone two different genetic models with involvement of hydrocarbon are proposed for both areas for melipilla naltahua a two step model can be developed as follows 1 framboidal pyrite growth with very low 34 s formed by bacterial sulfate reduction in an open system and with diagenetic degradation of oil related brines leaving pyrobitumen 2 cu bearing stage replacing of framboidal pyrite inheriting depleted sulfur as low as 50 4 together with sulfides directly precipitated from a hydrothermal fluid with 34 s close to 0 for la serena a single step model fits best without framboidal pyrite generation cu bearing sulfides were precipitated mainly in veins where cu plus base metal bearing hydrothermal fluids mixed with h 2 s generated by bacterial sulfate reduction in the host rocks isotopic evidence clearly illustrates that bacterial activity perhaps enhanced by hydrothermal activity was fed by hydrocarbon brines and sulfate remobilized from continental evaporites it is possible that variable ecological conditions led to different extents of isotopic fractionation adding to the typical sulfur isotopic heterogeneity of such bacterial systems for both areas the cu bearing stage occurred during the peak to waning stages of the very low grade metamorphism that affected the lower cretaceous sequence

multiple sources for the coastal batholith of central chile 31 34 s geochemical and sr nd isotopic evidence and tectonic implications abstract the coastal batholith of central chile between 31 and 34 s represents a locus of long lived c 200 ma plutonic activity it is composed of four plutonic complexes santo domingo limari papudo quintero and illapel these complexes which occur as north trending discontinuous belts that decrease in age eastward were emplaced during four major episodes of arc magmatism the santo domingo complex carboniferous comprises hornblende biotite bearing tonalites and granodiorites with abundant mafic enclaves and enclave free granites the association of enclave bearing granitoids and enclave free granites resulted from different degrees of mixing of crustal and mantle derived magmas the limari complex lower jurassic is a bimodal association of crustal leucogranites and mantle derived gabbros the lithology of the papudo quintero complex middle jurassic varies from hornblende pyroxene diorite to biotite granite the main rock types namely tonalites and granodiorites commonly contain mafic enclaves the granitoids and the enclaves have remarkably similar ree patterns and are similar in sr nd isotope composition which suggest that they represent a cogenetic suite of mantle origin the illapel complex cretaceous consists of hornblende bearing tonalites trondhjemites and granodiorites that resemble the rocks of ttg suites the end values and initial 87 sr 86 sr ratios sri for the mafic rocks of the carboniferous complex end 2 0 and 3 5 sri c 0 7057 differ markedly from those of the mesozoic mafic rocks end 1 9 to 5 4 sri 0 7033 to 0 7039 the cretaceous rocks being the most depleted with regard to sr nd a change from carboniferous lithosphere dominated sources to mesozoic asthenosphere dominated sources of the magmas that started in the early jurassic is recognized slab components can be recognized in the source of the cretaceous rocks the crustal source of the lower jurassic leucogranites was more isotopically depleted and refractory than that of the carboniferous granites on the other hand no crustal participation in the origin of the middle jurassic and cretaceous felsic granitoids is recognized they are differentiated from mantle derived magmas progressive removal of old lithospheric mantle during the mesozoic due to a process of lithospheric delamination would explain the profound change in the subcrustal source as well as the melting and modification towards a more refractory nature of the continental crust