ordovician alluvial fan to marine shelf transition in sw sardinia western mediterranean sea tectonically sardic phase influenced clastic sedimentation abstract a localized lower ordovician early caledonian structural event sardic phase is recorded in sw sardinia by an angular and erosional unconformity faulting and mild folding this event was accompanied by sedimentation of a transgressive clastic unit puddinga beds which is particularly well developed along a major fault system gonnesa fault zone there the puddinga beds range from basal interfingering fans some formed by coarse carbonate and schist clasts others by schist clasts only to fluvio lacustrine mud and gravelly sandstone to coastal muddy and sandy deposits and to fossiliferous restricted marine shelf sediments the existence of the slightly compressive sardic phase in sw sardinia and quasi coeval calc alkaline volcanism and an extensional structural phase sarrabese phase in central east sardinia suggest transcurrent faulting the calc alkaline acid volcanism is also interpreted by some authors to indicate that the rifted basins developed in backarc position the non volcanic sequence of sw sardinia analyzed in this paper was probably formed in basins developed on the northern margin of the gondwanaland craton this sardinian system may be the eastward extension of transcurrent rifts which have also been recently reported from nw spain structural activity has been the predominant control on sedimentation however worldwide recurring low sea level stands may have influenced the sedimentation of the lower part of the puddinga beds and the deglaciation of the saharan ice sheet has contributed to the uppermost ordovician early silurian deepening of the euxinic shelf

geochronology of the mount read volcanics tasmania australia the late middle cambrian mount read volcanics tasmania australia comprise submarine rhyolite to basalt lavas intrusions and volcaniclastic rocks which host significant polymetallic massive sulfide deposits isotopic u pb and 40 ar 39 ar dating of magmatic zircons and hornblendes respectively from different parts of the mount read volcanics yields a concordant age of 502 6 or 3 5 ma 1sigma which constrains the time of volcanogenic massive sulfide mineralization the comstock tuff from the tyndall group in the upper part of the volcanic sequence and the mount black dacite from the central volcanic complex have weighted mean 206 pb 238 u ages of 494 4 or 3 8 ma and 494 9 or 4 3 ma respectively and may be slightly younger than the other rocks fauna in the mount read volcanics is diagnostic of a late middle cambrian middle to late menevian stage age and the isotopic dates therefore imply that the late cambrian is younger than 500 ma inherited zircons in the mount read volcanics have ages similar to those found in granites and sediments in the tasman fold belt southeastern australia which suggests similar source material for the two areas

counterclockwise rotation of late eocene oligocene fore arc deposits in southern peru and its significance for oroclinal bending in the central andes the results of a paleomagnetic study along the fore arc of southern peru 15 18 s and northern chile 18 19 s are reported from middle to late miocene ignimbrites 7 sites late oligocene to early miocene ignimbrites 72 sites paleogene sediments 20 sites and mesozoic and paleocene volcanics and intrusions 31 sites comparison of locality mean directions with expected paleomagnetic directions indicates vertical axis rotations ranging from 5 2 11 3 clockwise to 55 6 7 0 counterclockwise spatially the magnitude of counterclockwise rotations increases northward from 0 within the chilean fore arc south of 18 30 s to 45 north of 16 30 s in southern peru paleomagnetic rotations recorded in paleogene red beds decrease from late eocene to late oligocene whereas miocene ignimbrites display no evidence of rotation these new results confirm that the rotations recorded in the fore arc of southern peru were acquired at least before 15 ma and probably before 25 ma and thus prior to the late neogene shortening of the sub andes the onset of major andean shortening in the eastern cordillera during the latest eocene earliest oligocene is interpreted to have triggered the bending of the peruvian fore arc the region of the peruvian fore arc with the largest rotations appears to be the fore arc counterpart of the abancay deflection a remarkable ne sw offset in the axis of the eastern cordillera induced by a major regional preorogenic structure we underline that the abancay deflection should be seen as the northwestern boundary and therefore as a key element of the bolivian orocline

tectonic rotations within the bolivian altiplano implications for the geodynamic evolution of the central andes during the late tertiary paleomagnetic results from 61 sites in tertiary red beds and volcanic rocks from the bolivian andes allow quantification of tectonic rotations within the altiplano domain a total of 16 sites were also obtained in lower miocene ignimbrites that cover the forearc of the north chilean andes near arica in the southern altiplano lipez region a local clockwise rotation of up to 38 is recorded in lower miocene volcanic rocks farther north near the salar de uyuni there is no evidence of significant rotations counterclockwise rotations are observed in the northern altiplano the largest counterclockwise rotation 28 is found in eocene oligocene red beds tiwanaku formation along the eastern border of the corque syncline middle miocene sediments located within the center of the corque basin record a counterclockwise rotation of only 10 about 200 km north of the corque basin eocene oligocene sedimentary rocks near the locality of viacha record only 10 counterclockwise rotation paleomagnetic results in lower miocene ignimbrites from the forearc near the arica bend do not show evidence for late cenozoic rotation of the forearc these new results suggest that the 20 to 25 counterclockwise rotation of the southern peruvian forearc occurred prior to the miocene anisotropy of magnetic susceptibility ams data show a magnetic fabric mostly controlled by bedding but slight ams lineations are found almost parallel to the major regional structural trends such as fold axes the apparent relationship between tectonic rotations ams lineations and structural trends suggests that rotations occured after or during the final stage of folding paleomagnetic data obtained in paleozoic rocks show evidence of clockwise rotation of the southern sub andean ranges tectonic rotations during the neogene are mostly localized on the eastern side of the central andes the curvature of the occidental margin near arica was likely acquired prior to the last stage of andean deformation although the magnitude of the rotations may vary from one locality to the other there is a consistent pattern showing counterclockwise rotations to the north and clockwise to the south a large scale regional mechanism is needed to explain this pattern of tectonic rotations within the central andes we propose a model in which along strike segmentation of the andean foreland and indentation of the altiplano by a curved forearc are the major factors controlling tectonic rotations within the altiplano eastern cordillera and sub andean ranges

geology genesis and exploration implications of the footwall and hanging wall alteration associated with the hellyer volcanic hosted massive sulfide deposit tasmania australia hellyer is a large 16 2 million metric tons high grade 13 9 zn 7 1 pb 0 4 cu 168 g t ag 2 5 g t au sea floor mound style polymetallic volcanic hosted massive sulfide vhms deposit located in the mount read volcanics of western tasmania the deposit is hosted by the que hellyer volcanics a sequence of late middle cambrian mafic to felsic coherent volcanics and polymict volcaniclastics hydrothermal alteration occurs in the regional footwall immediate footwall and hanging wall alteration in the regional footwall is confined to patchy quartz albite and chlorite with minor sericite epidote and hematite underlying hellyer is a zoned alteration pipe with a central siliceous core quartz sericite which passes into zones of chlorite chlorite carbonate sericite chlorite and finally sericite quartz stringer envelope zone on the margin overlying the central part of the deposit within the hanging wall basalt is a distinctive and zoned alteration plume five alteration zones have been identified fuchsite chlorite carbonate quartzalbite and sericite fuchsite dominated alteration occupies the central portion of the hanging wall alteration plume chlorite and carbonate alteration surrounds the fuchsite zone with carbonate zones forming near to the ore deposit and chlorite zones extending above and lateral to the carbonate outward is quartz ablite alteration which extends laterally into distal sericite alteration mass change calculations for the footwall and hanging wall indicate that in general the footwall alteration zones are depleted in cao na2o la sr ni cr and v but have enrichments of fe2o3 mno mgo k2o s and most metals compared to the host basalt the hanging wall alteration has gained cao k2o na2o co2 s in rb ba ag as mo sb cs and tl while fe2o3 mno mgo p2o5 la sr pb zn th u cd and nd are depleted cao na2o cr v and ni are depleted in the footwall andesite but enriched in the hanging wall alteration plume this relationship suggests that these elements were sourced from the breakdown of feldspars pyroxenes and andesitic groundmass of the footwall lithologies and transported in the hydrothermal fluid into the overlying basalt and precipitated as albite calcite and white micas in the hanging wall alteration the development of alteration associated with the hellyer vhms deposit occurred in three stages stage 1 regional footwall alteration was formed by unfocused hydrothermal convection of seawater down into the recently deposited volcanic pile at temperatures between approximately 250 degrees celsius and 200 degrees celsius and at low to moderate water rock ratios stage 2 alteration formed by structurally controlled fluid flow from a deep intensifying hydrothermal convection system and created the footwall alteration pipe decreasing water rock ratios and temperatures over a range of 350 degrees celsius to less than 200 degrees celsius led to the development of the concentric alteration pipe mineral zones based on modeling of whole rock d18ofr values geochemical modeling and mineral assemblages the siliceous core is interpreted to have formed at temperatures near 350 degrees celsius the chlorite rich alteration zone at 300 degrees celsius to 250 degrees celsius and the outer sericite rich alteration at temperatures of 250 degrees celsius to 150 degrees celsius the final synmineralization fluid flow event stage 3 created the hanging wall alteration plume after rapid burial of the deposit by basalt continuation of upward hydrothermal fluid flow created the zoned hanging wall alteration distribution of hanging wall alteration assemblages suggests a temperature gradient from approximately 250 degrees celsius for the fuchsite zone to lower temperatures 150 degrees celsius for the distal quartz albite and sericite alteration zones alteration mineralogy mineral chemistry lithogeochemistry and stable isotope characteristics of the footwall and hanging wall alteration have been combined into a comprehensive set of vectors which can be used in exploration for vhms deposits in similar geologic settings

new late permian paleomagnetic data from argentina refinement of the apparent polar wander path of gondwana the late paleozoic early mesozoic apparent polar wander path of gondwana is largely constructed from relatively old paleomagnetic results many of which are considered unreliable by modern standards paleomagnetic results derived from sedimentary sequences which are generally poorly dated and prone to inclination shallowing are especially common here we report the results of a joint paleomagnetic geochronologic study of a volcanic complex in central argentina u pb dating of zircons has yielded a robust age estimate of 263 0 1 6 2 0 ma for the complex paleomagnetic analysis has revealed a pretilting primary permian magnetization with dual polarities rock magnetic experiments have identified pseudo single domain titano magnetite and hematite as the mineralogic carriers of the magnetization lightning induced isothermal remagnetizations are widespread in the low coercivity magnetic carriers the resulting paleomagnetic pole is 80 1 s 349 0 e a95 3 3 n 35 and it improves a late permian mean pole calculated from a filtered south american paleomagnetic data set more broadly this new high quality igneous based paleomagnetic pole falls between the previously distinct late permian segments of the laurussian and gondwanan apparent polar wander paths suggesting that the long recognized disparity between these large paleomagnetic data sets may be primarily due to the inclusion of low quality or systemically biased data

paleomagnetism of the upper silurian and lower devonian carbonates of new york state evidence for secondary magnetizations residing in magnetite abstract paleomagnetic directions for the upper silurian and lower devonian carbonates of the helderberg escarpment new york state differ from expected late silurian and early devonian directions for cratonic north america the mean direction d 165 i 10 paleopole at 50 n 129 e is similar to late carboniferous and early permian results negative fold tests and a lack of reversals suggest that the magnetization is secondary however low coercivities low blocking temperatures the thermomagnetic curves tc near 570 c and the acquisition of isothermal remanent magnetizations all suggest that the remanence is carried by magnetite if a detrital origin of these magnetites is assumed the secondary nature of the remanence would argue for thermal resetting as a result of deep burial of the rocks however no evidence for such thermal resetting is seen in the alteration of conodonts more likely perhaps is a chemical or thermochemical origin of the remanence this would require the magnetites to be authigenic

stratigraphy and chronology of upper cretaceous lower paleogene strata in bolivia and northwest argentina integration of sequence stratigraphy magnetostratigraphy ar ar dating and paleontology considerably advances knowledge of the late cretaceous early paleogene chronostratigraphy and tectonic evolution of bolivia and adjacent areas the partly restricted marine el molino formation spans the maastrichtian and danian 73 60 0 ma deposition of the alluvial to lacustrine santa lucia formation occurred between 60 0 and 58 2 ma the widespread erosional unconformity at the base of the cayara formation is 58 2 ma this unconformity separates the upper puca and corocoro supersequences in bolivia and is thus coeval with the zuni tejas sequence boundary of north america the thick overlying potoco and camargo formations represent a late paleocene oligocene foreland fill the onset of shortening along the pacific margin at 89 ma initially produced rifting in the distal foreland santonian campanian eastward onlapping deposits indicate subsequent waning of tectonic activity along the margin significant tectonism and magmatism resumed along the margin at 73 ma and produced an abrupt increase in subsidence rate and other related phenomena in the basin subsidence was maximum between 71 and 66 ma due to the early maastrichtian global sea level high marine waters ingressed from the northwest into this underfilled basin subsidence decreased during the late maastrichtian and was low during the danian it increased again in the latest danian for which a slight transgression is recorded and peaked in the early selandian tectonism between 59 5 and 58 2 ma produced a variety of deformational and sedimentary effects in the basin and correlates with the end of emplacement of the coastal batholith the subsequent 58 2 ma major unconformity marks the onset of continental foreland basin development which extended into andean bolivia during the late paleocene oligocene interval this basin underwent internal deformation as early as eocene time in the altiplano and cordillera oriental these early structures previously assigned to the late oligocene early miocene orogeny probably accommodated observed tectonic rotations in the eocene oligocene

paleomagnetism of upper devonian reefal limestones canning basin western australia two conflicting models have been proposed for the middle paleozoic position of gondwana with respect to euramerica a pole from morocco msissi norite that may be of late devonian age suggests that a broad ocean 3 000 km separated the northern continents from gondwana whereas results from mauritania gneiguira supergroup indicate that this ocean had closed by devonian time detailed thermal and alternating field demagnetization experiments have been carried out on red and gray limestones from seven sites 89 samples in an essentially undeformed late devonian reef complex northern canning basin western australia after minor tilt correction the site mean characteristic directions average 42 7 28 9 declination inclination with k 61 6 and 95 7 8 these results position northwestern australia at 15 s latitude in the late devonian a paleolatitude consistent with the presence of a well developed reefal sequence an early age is inferred for these magnetizations on the basis of the occurrence of mixed polarities and the fact that no post devonian directions for australia resemble the observed declinations and inclinations if the smith and hallam reconstruction for gondwana is used the paleomagnetic pole calculated for the late devonian of western australia agrees with the msissi norite result from morocco and suggests that an ocean still separated gondwana from euramerica in the late devonian the pole measured from the gneiguira supergroup in mauritania may be based on remagnetizations of late paleozoic age the directions determined from canning basin limestones are similar in inclination to but differ by 40 in declination from paleomagnetic results from the upper devonian comerong volcanics lachlan fold belt southeastern australia canning basin samples are from the stable craton and comerong volcanics samples are from the tasman fold belt the discrepancy between the two australian results may be best explained by tectonic rotation of the comerong volcanics locality

volcanology and sedimentology of the host succession to the hellyer and que river volcanic hosted massive sulfide deposits northwestern tasmania the hellyer and que river high grade polymetallic volcanic hosted massive sulfide deposits of western tasmania are hosted by a sequence of late middle cambrian subaqueous mafic to felsic volcanics and volcaniclastics known as the que hellyer volcanics these volcanics occur toward the top of a thicker sequence of mainly felsic volcanics also hosting mineralization known as the mount read volcanics the base of the footwall sequence within the que hellyer volcanics is marked by massive to pillowed basaltic lavas and volcaniclastics of variable thickness these are conformably overlain by a thick sequence of andesitic basaltic and minor dacitic lavas and associated volcaniclastics eruption of these footwall sequences was rapid with breaks in volcanism represented only by thin discontinuous epiclastic horizons a hiatus in volcanism followed the emplacement of the footwall sequences this hiatus is represented by the deposition of locally derived coarse grained mass flows the distribution and geometry of these epiclastics suggest that they accumulated in local syndepositional depressions sedimentation was also accompanied by syngenetic zn pb cu ag au rich mineralization in the central or axial area of these depressions rapid burial by further mass flows assisted in the preservation of mineralization shallow intrusive to extrusive dacitic lava flows and domes in the host horizon were emplaced during and immediately after mineralization autoelastic material from these dacitic domes was then resedimented and incorporated in the epiclastic horizons overlying the orebodies eruption of massive to pillowed basaltic and minor andesitic lavas of the hellyer basalt occurred soon after mineralization and was the final phase of volcanism in the que hellyer volcanics the basalts were emplaced as extrusions and as shallow intrusions into wet fine grained sediments which began to accumulate above the coarse grained epiclastic horizons these basalts buried the remaining parts of the sulfide mound not covered by epiclastics immediately after the cessation of volcanic activity in the area a 100 m thick sequence of black shales and minor sandstones known as the que river shale was deposited the general lack of products of explosive volcanism within the que hellyer volcanics is most likely a function of the composition of the lavas combined with their emplacement in deep water under high confining pressures although the absence of such products does not rule out the possibility of emplacement in shallow water depths the absence of abundant large scale tractional sedimentary structures the presence of the overlying black anoxic que river shale and the mass flow characteristics of elastic units favor a relatively deep water setting for mineralization in the que hellyer volcanics

post middle oligocene origin of paleomagnetic rotations in upper permian to lower jurassic rocks from northern and southern peru abstract we report paleomagnetic data from 28 sites of upper permian to lower jurassic strata from northern and southern peru in northern peru 6 s a stable magnetic component from six permo triassic sites passes fold and reversal tests the overall mean pole agrees well with late permian to triassic poles from cratonal south america suggesting this part of peru has experienced neither significant rotation nor latitudinal transport since the permo triassic in southern peru 13 to 16 s thermal demagnetization isolates stable magnetic components in 16 of 22 upper permian to lower jurassic sites collected along the transition between the altiplano and the eastern cordillera these 16 sites are rotated 14 to 147 counterclockwise and pass an inclinations only fold test within the same structural zone three other permo triassic sites as well as 10 paleocene sites also show important counterclockwise rotations roperch and carlier j geophys res 97 1992 17233 17249 butler et al geology 23 1995 799 802 the large magnitude and exclusively counterclockwise sense of rotation suggest that the tectonic regime included an important sinistral shear component no correlation exists between rotation amount and rock age suggesting the rotations are post paleocene in age because the rotations occur along the fringe of the eastern cordillera they were likely produced during its structural formation hence from the late oligocene to present sinistral shear acting in the northern part of the bolivian orocline appears much more pronounced than that north of the abancay deflection which likely arises from differences in convergence obliquity

evolution and source of ore fluids in the stringer system hellyer vhms deposit tasmania australia evidence from fluid inclusion microthermometry and geochemistry the hellyer deposit is a classic large tonnage high grade mound style volcanic hosted massive sulphide vhms deposit in the cambrian mt read volcanic belt of western tasmania in the footwall directly underlying the deposit there is an extensively altered pipe which cootains a well developed and preserved stringer zone the vein paragenesis at hellyer indicates that premineralization stage i veins consist entirely of quartz and occur throughout the alteration pipe the synmineralization stage 2 veins are the most abundant veins in the stringer zone and consist of three sub stages stage 2a veins of crustiform quartz pyrite and carbonate with minor amounts of chalcopyrite sphalerite and galena stage 2b veins with abundant base metal sulphides minor quartz carbonate and barite gangue and stage 2c veins of coarsely crystalline barite with variable amounts of pyrite sphalerite galena and carbonate stages 3 6 veins are postmineralization veins and are related to the devonian tabberabberan orogeny textural petrographic and microthermometric investigations of fluid inclusions in the hellyer stringer system indicate that type i primary liquid vapour inclusions occur along growth planes of crustiform quartz crystals or within colour banding of zoned sphalerite these inclusions are 10 15 m in size and yielded homnogenisation temperatures of 170 220 c in early 2a veins 165 322 c in main stage 2b veins and 190 256 c io late stage 2c veins these data suggest a waxing and waning thermal history however the average salinity remained between 8 11 naci equiv wt in all stage 2 veins chalcopyrite bearing primary fluid inclusions have been recognised in the base metal rich stage 28 veins no evidence for presence of co2 e g formation of clathrates was recorded by rnicrothermometry however laser raman spectroscopic lrs analysis indicates the presence of co2 1 mole in the stage 2b veins and no detectable co2 in 2a and 2c vein stages semi quantitative sem wds microprobe analyses of fluid inclusion decrepitates indicate that the hellyer ore fluid was enriched in potassium and calcium but depleted in magnesium relative to seawater pixe microanalysis of fluid inclusions in quartz indicates that the stage 2b ore fluids have a significantly higher base metal concentration compared to the stage 2a veins the postmineralization stage 4 veins have a variable but lower base metal content in this study there was no fluid inclusion evidence of boiling cation composition higher salinities relative to seawater and the presence of co2 suggest that recycled seawater alone cannot be the sole source of the ore fluids this interpretation is in agreement with previous isotopic studies in the hellyer stringer system although direct input of bulk ore constituents from a magma chamber cannot be demonstrated from the present fluid inclusion data such a contribution of ore fluids from a magmatic source cannot be ruled out the possible input from the magmatic source may have occurred during the base metal rich stage 2b vein formation characterised by the intensifying temperature of deposition higher base metals and co2 contents

the mobility of zirconium and other immobile elements during hydrothermal alteration abstract development of zircon and other zr phases in hydrothermal deposits indicates that zr can be highly mobile in these systems mobility is most common in but not restricted to f rich hydrothermal systems related to alkalic f rich igneous suites these suites can range from peralkaline through metaluminous to peraluminous a few examples are neither alkalic nor f rich three locations in the trans pecos magmatic province texas u s a demonstrate this hydrothermal zr mobility all three igneous systems are alkalic and f rich but vary in alkali al ratios peralkaline rhyolites and trachytes in the christmas mountains contain as much as 2100 ppm zr mostly in aegirine or arfvedsonite zircon is rare or absent fluorspar replacement deposits in limestone at contacts with the rhyolites contain as much as 38 000 ppm zr occurring as small disseminated zircons the deposits also are enriched in a variety of incompatible elements including be rare earth elements ree y nb mo hf pb th and u the sierra blanca intrusions a series of mildly peraluminous f rich rhyolite laccoliths contain as much as 1000 ppm zr mostly as zircon hydrothermal zircon occurs as overgrowths on magmatic grains as veinlets connected to overgrowths and in fluorspar replacement bodies in adjacent limestone the highest zr concentrations in fluorspar are 200 ppm metaluminous quartz monzonite from the infiernito caldera contains 400 600 ppm zr mostly as zircon euhedral zircon in quartz fluorite veins in the quartz monzonite indicates mobility of zr zirconium concentrations in the veins are unknown but the paucity of zircon suggests little zr enrichment relative to the host zircon and more rarely zirconolite occur in skarn in the ertsberg district of irian jaya indonesia unlike in texas related igneous rocks are metaluminous and the hydrothermal system was f poor worldwide hydrothermal zircon other zr phases and ti and al bearing phases occur in skarn epithermal precious metal veins volcanogenic massive sulfide deposits and mylonites we propose that differences in zr mineralogy of igneous source rocks is an important factor in determining the availability of zr to hydrothermal fluids although zr concentrations in the sierra blanca and christmas mountains rhyolites are similar zr enrichment in fluorspar was much greater in the christmas mountains we suggest that hydrothermal solutions could easily break down aegirine and arfvedsonite to release zr but that zircon was only moderately attacked therefore far more zr was available for transport and subsequent deposition in the christmas mountains than at sierra blanca availability of other trace elements probably is also governed by their mineral host although zr mobility is most common in f rich hydrothermal systems related to alkalic and f rich igneous systems mobility at ertsberg may have been promoted by sulfate complexing

the behaviour of so called immobile elements in hydrothermally altered rocks associated with volcanogenic submarine exhalative ore deposits evidence is available that some elements notably zr tio2 y sc ce and nb are largely immobile during the alteration of volcanic rocks owing to metamorphism hydrothermal events and weathering e g floyd and winchester 1978 however it is shown by reference to analyses of rocks from the environment of five volcanogenic massive sulphide bodies that while zr tio2 and ce are mostly immobile even during intense hydrothermal alteration y and particularly sc and nb may be extremely mobile when elements are removed by solution in a hydrothermal fluid it seems that reaction rates are such that these elements are almost totally removed from the rock therefore of the so called immobile trace elements only zr and tio2 may be used with any reliability to identify the degree of magmatic differentiation in an hydrothermally altered rock however if an element has been mobile it is usually readily identified as having moved

palaeomagnetism and age of mid palaeozoic ring complexes in niger west africa and tectonic implications summary detailed af and thermal demagnetization has been performed on 185 oriented samples from 36 sites in a series of basic to acid subvolcanic ring complexes in central and southern niger published rb sr whole rock and mineral isochrons from acid members of the suite range north to south from 480 ma to 320 ma two distinct vectors have been isolated by linear regression of demagnetization data 1 a due north and negative inclination rarely reversed component is present in basic rocks from 12 sites in four distinct northern complexes published rb sr ages date from 480 ma to 420 ma this component is carried by single domain magnetite occasionally also by titanhaematite and is considered to be primary acquired during the terminal stages of this igneous episode 40ar 39ar dating of biotite from three samples used in the palaeomagnetic study yield 435 myr ages 2 a due north and positive inclination rarely reversed direction is present as a second component in some of the basic suite sites and is the predominant component in most sites in syenitic to granodioritic rocks primary fe ti oxides are rare in these acid igneous rocks and the feldspars within them are usually pervasively altered this component carried primarily by haematite is believed to be either a modern day vrm or a secondary tcrm acquired during hydro thermal activity associated with widespread quaternary volcanism in the southern air the primary component gives a mean south pole at 43 4s 08 6e n 12 k 50 95 6 2 located southwest of cape town this position requires a rapid southward excursion of the gondwana a p w path in early silurian time but it is generally consistent with most of the other silurian devonian palaeomagnetic data from gondwana these however are from the lachlan fold belt in southeast australia whose relation to the australian craton in early palaeozoic time is not clear integrating all stratigraphic palaeoclimatic tectonic and other palaeomagnetic data for early and middle palaeozoic time suggests that the proto atlantic closed during the silurian period if a late devonian early carboniferous gondwana pole in central africa is valid the ocean may have opened again before the final closure and reassembly of gondwana in the late carboniferous

district scale alteration associated with massive sulfide deposits in the mount read volcanics western tasmania the distribution of secondary mineral assemblages in six mineralized areas of the mount read volcanics defines a district wide zonation which is subconformable with massive sulfide host horizons and the contacts of sill like granitoids where present the relationships between alteration patterns and elements of cambrian geology strongly suggest that the alteration is related to cambrian hydrothermal circulation responsible for massive sulfide mineralization there is a later greenschist facies metamorphic overprint the major components of the principal assemblages are quartz albite chlorite epidote quartz albite chlorite calcite quartz sericite chlorite and quartz k feldspar chlorite several complex assemblages involving the above minerals biotite magnetite pyrite and tourmaline are characteristic of granite contacts secondary biotite is also present sporadically in the rosebery area probably related to devonian granitoids at depth there is a districtwide zonation of sodic or calcic versus nonsodic noncalcic assemblages in felsic volcanic rocks in preore volcanics nonsodic noncalcic assemblages occur immediately beneath the host horizon and may persist several hundred meters before passing into a zone including granite contacts where calcium minerals and albite occur sporadically the nonsodic noncalcic assemblages are predominantly sericitic but assemblages with prominent k feldspar occur locally in postore volcanics sericitic non sodic noncalcic assemblages may persist up to 200 m above the ore horizon before passing into relatively fresh volcanics with common albite and calcic greenschist minerals preore assemblages probably largely reflect downward circulation upwelling zones are characterized by intense sericite chlorite or chlorite development with or without k feldspar lateral spreading of solutions immediately beneath the ore horizon is common chlorite compositions preserve local patterns due to cambrian hydrothermal processes but they do not vary systematically with alteration assemblages throughout the province chlorite mg to fe ratios appear to have been preserved through metamorphism a combination of structural and alteration data is useful to mapping in the mount read volcanics this approach indicates that most syngenetic deposits and sedimentary lenses in the rosebery hercules area may lie on a single host horizon and suggests a continuity of geology from mount farrell through red hills to mount sedgwick

australian volcanic hosted massive sulfide deposits features styles and genetic models the cambro ordovician period has yielded the major development of volcanic hosted massive sulfide vhms deposits in australia resulting in about 12 million metric tons of contained metal copper lead and zinc concentrated in the mount read volcanics tasmania and the mount windsor volcanics queensland the archean 4 million metric tons of metal and the silurian 3 5 million metric tons of metal constitute the next important episodes while the devonian and permian have produced isolated deposits the deposits range from cu type to zn cu type to zn pb cu type the zn cu type deposits are restricted to the archean and silurian whereas the cu type and zn pb cu type occur sporadically throughout the time span from early archean to permian in general terms the major vhms bearing districts are calc alkaline in character and display a thick basal pile of rhyolitic volcanics 1 3 km thick including lavas epiclastics and subvolcanic intrusions which is overlain by a polymodal sequence containing various proportions of rhyolite dacite andesite basalt and sediments the major deposits are commonly located at the top contact of the rhyolite pile or within the lower part of the overlying polymodal sequence there is a wide range in variability of styles of australian vhms deposits including mounds pipes sheets layered deposits stacked deposits stockwork and disseminated deposits distal reworked deposits and cyclic layered deposits although these various styles show a range of features there is a consistent theme across the spectrum of metal zonation alteration mineralogy alteration chemistry sulfur isotopes macrotextures microtextures and host volcanic relationships which strongly suggests that they all belong to the one genetic group of ore deposit the classic mound style deposits such as hellyer have a series of key features subsets of which are represented in the other deposit styles the mound deposits are considered to form from deposition of metal sulfides on the sea floor immediately around the hydrothermal vent growth of the mound occurs by upward replacement of sulfide assemblages stable at higher fluid temperatures leading to zone refining and lead zinc silver gold enrichment in the outer and upper parts of the mound departure from the classic mound style of deposit is related principally to three key factors the chemistry of the ore fluid salinity temperature f02 and ah2s the nature permeability and chemistry of the volcanic pile and the sea hoor environment sea hoor topography and seawater depth these factors control the aspect ratio of the deposit the extent of stringer zone development the degree of subsea floor replacement mineralization the nature and spatial development of footwall alteration and the development and style of related distal mineralization sulfur isotope studies of australian vhms deposits indicate that reduced seawater sulfate is a major source of sulfur in the deposit whereas lead and strontium isotope results are compatible with the metals being derived by seawater convection and leaching from the volcanic pile and basement rocks this conclusion is supported by the application of various leaching models which demonstrate the availability of an adequate source of both base and precious metals in the footwall volcanic sequences the input of metals directly from volcanic magma chambers is not precluded by the available data and one likely scenario involves the contribution of gold and copper from a magmatic vapor plume rising to mix with convective seawater fluids which contribute lead zinc silver and gold leached from the footwall volcanics and basement rocks the relative importance of the magmatic input compared with the seawater convective input may help to explain the spectrum of deposit styles arid their spatial relationship to volcanic centers or adjacent sedimentary basins

stratigraphic volcanic setting of massive sulfide deposits in the cambrian mount read volcanics tasmania the cambrian mount read volcanics of western tasmania host the volcanic hosted massive sulfide vhms deposits at hellyer que river rosebery hercules and mount lyell the volcanics were erupted along and partly onto the western margin of a precambrian basement terrane now represented by the tyennan region the volcanic belt is asymmetrical in section with a main eastern zone of relatively massive lava rich sequences accompanied by abundant intrusions and a broader western zone of volcano sedimentary sequences rich in volcaniclastic mass flow sandstone and breccia the rocks have been strongly deformed regionally metamorphosed to lower greenschist facies and hydrothermally altered in many areas stratigraphic and facies relationships within the highly complex volcanic belt are critically reviewed and reassessed the main volcanic zone is considered to comprise two interfingering lava rich associations the quartz feldspar porphyritic eastern sequence and the largely feldspar porphyritic central volcanic complex this lava rich zone interfingers with the western volcano sedimentary sequences herein referred to as the yolande river sequence the dundas group and the mount charter group new term recent volcanologic studies indicate that the lava rich sequences and indeed the bulk of the preserved mount read volcanics are submarine in character despite the presence of abundant ignimbritelike pumiceous deposits the early volcanism was predominantly rhyolitic dacitic in composition but was followed by a period of active andesitic basaltic volcanism during which the massive sulfide deposits at hellyer and que river and some of those at mount lyell were formed this widespread andesitic basaltic volcanism appears to coincide with a period of active extension and rifting along the belt focused to some extent on the henty fault system the rifting episode is evidenced by abundant tholeiitic dikes along the henty fault and throughout the northern central volcanic complex and by tholeiitic pillow lavas within the henty fault wedge the andesitic basaltic volcanism appears to have culminated in distinctive p 2 o 5 and light rare earth element lree enriched basalts at hellyer and similar rocks at lynch creek and howards plains near queenstown may be correlates the hellyer basalts were buried by black shale and the final volcanic phase was marked by a return to active felsic volcanism which produced widespread crystal and pumice rich mass flow deposits of the southwell subgroup and tyndall group

revised and synthetic apparent polar wander paths of the african eurasian north american and indian plates and true polar wander since 200 ma we have reviewed paleomagnetic data available for the eurasian african north american and indian plates over the last 200 ma selection criteria are those generally accepted with an emphasis on evidence for lack of remagnetization accurate dating and proper structural analysis this results in 23 35 51 and 2 poles for eurasia africa north america and india respectively we believe that this limited set of higher quality data is more likely to reveal key features of apparent polar wander apw paths than averaging of larger data sets involving less stringent selection criteria we propose and describe revised apw paths but more importantly we next use relative motion models to transfer all data in a common reference frame we find good agreement between transferred data when they are averaged separately for each plate in independent 20 ma windows this is a check on consistency of paleomagnetic data kinematic models and of the geocentric dipole hypothesis transferred data from all plates are averaged in 20 ma windows to generate synthetic apw paths for all plates studied these synthetic paths are in agreement with the original revised apw paths that use only data from a single plate moreover both geographic and time resolution are improved spatial confidence intervals are of the order of 5 the synthetic paths display interesting features such as a previously ill recognized apw loop for eurasia paleomagnetic and hotspot apw are next compared and a determination of true polar wander tpw is derived we find significant tpw amounting to over 20 in the last 200 ma tpw appears to be episodic with a standstill between 180 and 110 ma there is general agreement with a previous study of livermore et al 1984 the tpw standstill appears to correlate with a time of decreasing reversal frequency ending with the cretaceous long normal superchron other periods of fast tpw would seem to correspond to increasing reversal frequency however it is suggested that the major tpw hairpin at 50 ma might correspond to the collision of india with eurasia tentative correlations with core or mantle indicators can be understood as a result of couplings between the core mantle and lithosphere we believe primarily related to episodic evolution of the d layer

mass change calculations in altered rock series a technique is presented whereby precursors of altered rocks in a fractionated rock series can be identified and changes in chemical components mass and volume accurately calculated the procedure utilizes elements with high degrees of immobility in hydrothermal and associated interactive water rock systems igneous rock series and sedimentary and metamorphic rocks with primary continuity of chemical composition are amenable to this treatment in the procedure for igneous rocks best fit fractionation lines are established for an immobile igneous compatible incompatible element pair the latter element functioning as the fractionation monitor alteration paths are linear and pass through the origin intersections of the fractionation and alteration lines yield the immobile element concentrations in the precursors to the altered rocks mobile components are determined from fractionation lines and monitor values total mass changes are proportional to the displacement of altered samples from the fractionation line masses of other elements in an altered sample reconstructed values are recalculated to the concentration of the monitor element in the precursor chemical changes are the differences between the calculated precursor compositions and the reconstructed altered rock compositions errors are mainly a function of the positioning of the fractionation lines

paleomagnetic determinations of vertical axis tectonic rotations from late cretaceous and paleocene strata of bolivia structural development of the altiplano during the past 10 m y involved crustal thickening and enhanced curvature of the central andes oroclinal bending of 30 explains reasonably the pattern of vertical axis rotations observed from units with ages 10 to 30 ma new paleomagnetic data from late cretaceous through paleocene strata of southeastern peru and the eastern cordillera of bolivia document rotations that are also counterclockwise on the north limb of the orocline and clockwise on the south limb however the rotations of units with ages 30 to 70 ma are more variable nearby areas have undergone differential vertical axis rotations of up to 60 these larger rotations that have affected 30 70 ma units in the central andes may represent 1 local rotations during late oligocene to present orogeny 2 local rotations during eocene orogeny and or 3 oroclinal bending during eocene orogency

tertiary remagnetization of paleozoic rocks from the eastern cordillera and sub andean belt of bolivia paleomagnetic samples were collected from 98 sedimentary horizons in eight different devonian to permian sedimentary units at eight localities in the eastern cordillera and the sub andean belt of bolivia for 77 sites thermal demagnetization allowed determination of a characteristic magnetization chrm with site mean 95 confidence limit 95 15 the chrm is carried predominantly or entirely by hematite fold and reversal tests from two of the sampled localities indicate that the characteristic magnetization is synfolding likely acquired during the earliest stages of deformation additionally a modified conglomerate test at one locality and the nearly uniform direction of chrm across the devonian to permian age units clearly reveals the secondary nature of the characteristic magnetization finally the chrm directions are discordant from any expected paleozoic directions paleomagnetic poles calculated from the chrm directions fall near the cenozoic portion of the apparent polar wander path for south america we interpret these observations to indicate widespread chemical remagnetization of these paleozoic strata during but prior to completion of cenozoic andean folding

paleomagnetic evidence for rotation in the precordillera of northern chile structural constraints and implications for the evolution of the andean forearc abstract palaeomagnetic results are reported for two late mesozoic to early cenozoic continental sedimentary sequences from the precordillera cordillera de domeyko of northern chile comparison of isolated components of magnetisation from the late cretaceous palaeocene purilactis formation and the unconformably overlying oligo miocene paciencia group with reference poles for south america indicate that significant post palaeocene and post lower miocene clockwise rotation has taken place a structural study of the area has revealed the presence of a number of thrust sheets that were periodically active throughout the cenozoic stratigraphie structural and palaeomagnetic data indicate that thrust sheet movement took place following deposition of both the purilactis formation lower eocene incaic orogeny and the paciencia group lowest miocene peheunche orogeny and resulted in the folding uplift and clockwise rotation of the two sequences differences in the amount of rotation between the two units reflect incremental movement of the thrust sheets 12 of rotation took place prior to deposition of the paciencia group followed by 17 of post lower miocene clockwise rotation this study demonstrates that palaeomagnetically detected rotation in the precordillera of northern chile 250 km inland of the peru chile trench is related to the periodic movement of an easterly propagating thin skinned thrust front between 22 30 and 23 30 s the north chilean forearc contains areas of non rotation thin skinned compressional related rotations and large block fault rotations indicating that the simplistic models previously presented to account for forearc rotation in the central andes should be modified in particular evidence implies that rotation could be related to a number of tectonic events and may not simply be due to one deformation event

the occurrence and origin of late paleozoic remagnetization in the sedimentary rocks of north america although it has been known for 25 years that some paleozoic sedimentary rock units in europe and north america were remagnetized during pennsylvanian or permian time it is only very recently that the complex and widespread nature of the late paleozoic remagnetization phenomenon has been generally acknowledged it is now recognized that many paleozoic paleomagnetic poles for north america that were once considered reliable are in fact the result of remagnetization and as a consequence the paleomagnetic data base for the paleozoic is undergoing rapid and drastic revision the causes of late paleozoic remagnetization in north america are currently the focus of much interest and active research the remagnetization can reside in either hematite or magnetite and different remagnetization mechanisms have been important in different settings chemical remagnetization processes some related to specific diagenetic events are dominant in hematite bearing sandstones and carbonates in magnetite bearing carbonates both chemical and thermoviscous remagnetization processes appear to have been important but it is difficult to determine which process is the dominant one in some settings some of the observed remagnetizations can be linked to the migration of chemically active and perhaps hot fluids during the mountain building events that affected much of the continent during the late paleozoic paleomagnetic studies promise to be important in assessing the role of orogeny in driving fluid migrations within sedimentary basins and in constraining the age of the migrations and the nature of the fluids

formation on the sea floor of the hellyer volcanogenic massive sulfide deposit new fluid inclusion data from the stockwork veins beneath the hellyer orebody a relatively undeformed late cambrian polymetallic volcanogenic massive sulfide deposit allow semiquantitative modeling of the behavior of the ore fluids on the sea floor and the manner of growth of the massive sulfide the earliest cool fluids may have been denser than seawater and ponded in a basin identified by sedimentological and structural studies subsequent vent fluids buoyant with respect to the brine pool and subsequently at higher temperature with respect to seawater mixed with the cooler brine during and after filling the basin to the level of its lowest outlet density inhomogeneities within the brine pool were reduced by double diffusive convection across the brine seawater interface and internal boundaries and mixing by currents within the brine pool estimates of likely temperatures and salinities in the brine pool for the first main mineralization stage avg 200 c 11 wt salinity assuming a steady or quasisteady state had been established and using volume fluxes similar to those observed in modern oceans indicate temperatures 100 c sulfides mostly pyrite sphalerite and galena precipitated from the vent fluids after mixing with cooler basin fluid settled from the gravity current to the basin floor a pattern that would have continued whether or not a true steady state was reached if the early fluids were buoyant in seawater they ponded after mixing forming a hybrid brine pool that would have required longer periods to homogenize at the peak temperature 300 c and volume flux the salinity of the vent fluids fell to 7 5 wt percent but the flow paths would have been maintained unless the brine pool had been drained at this time that part of the massive sulfide body over the vent the cu core grew by dilatational veining together with leaching cavity infilling replacement and recrystallization chalcopyrite was deposited both in the cu core and probably at the same time in a layer over the flanking massive sulfide in the final stage as temperature fell and salinity rose avg 250 c 11 wt salinity the fluid oxidation potential increased so that barite precipitation dominated resulting in a formation of a discontinuous barite rich cap over the massive sulfide the brine pool model explains several features of the hellyer ore that differ from those of the ores of the hokuroko basin namely the large metal content high zn cu ratio low aspect ratio absence of chimney fragments presence of mineral banding and high barite sulfur isotope values features seen in many other massive sulfide orebodies

the age and timing of folding in the central appalachians from paleomagnetic results comparison of the paleomagnetic pole positions of late paleozoic secondary magnetizations with a time averaged reference apparent polar wander path for north america shows that sedimentary rocks within the central appalachians were remagnetized in the later half of the permian between ca 255 and 275 ma ages of remagnetization do not vary for sites distributed along or across strike of the central appalachian fold and thrust belt or across the appalachian plateau in new york nor do they vary with stratigraphic position of the host rocks however the pattern of folding relative to remagnetization across the valley and ridge and great valley in pennsylvania maryland and west virginia shows a distinct temporal variation individual fold tests yield postfolding magnetizations near the hinterland margin of the fold and thrust belt synfolding magnetizations in the central part of the belt and prefolding magnetizations near the foreland because of the similar age of the late paleozoic magnetization we conclude that rocks across the central appalachians were remagnetized in a relatively short period of time as folding and thrusting propagated more slowly toward the foreland we interpret this pattern to indicate that older folds near the hinterland grew prior to remagnetization folds in the central region grew during remagnetization and folds near the foreland grew after remagnetization rapid remagnetization relative to folding is also evident from the physical characteristics and magnetic properties of secondary hematite in remagnetized synfolding folds in such folds rocks were apparently remagnetized during an increment of fold growth in which there was partial and possibly negligible fold limb rotation collectively these results may constrain the mechanism for basinal fluid mobilization during deformation current models for remagnetization involve the migration of chemically active brines into the foreland during tectonism if realistic our results bear on this model by suggesting that brine migration was not directly related to the successive emplacement of individual thrust sheets as has been suggested for other fold and thrust belts

paleomagnetism of mesozoic rocks from the central andes of southern peru importance of rotations in the development of the bolivian orocline a paleomagnetic study has been undertaken in jurassic volcanics and cretaceous intrusions from coastal southern peru a secondary magnetization in the jurassic volcanics was identified at 21 sites from three main localities along the southern coast of peru mean virtual geomagnetic pole 59 5 n 190 1 e k 111 95 3 0 this secondary magnetization was acquired during a tectonomagmatic event that may be related to the emplacement of the peruvian coastal batholith and or to the first andean phase of compressional deformation in late cretaceous if we assume that the remagnetization is of late cretaceous age a regional counterclockwise rotation of about 20 is recorded by the forearc of southern peru from 16 s to 18 s primary remanent magnetizations recorded by the volcanics and the batholith reveal a more complex tectonic history of block rotations counterclockwise rotations of 50 to 80 are documented near the localities of chala eight sites ocona two sites and arequipa four sites preliminary results based on three sites of late permian to early triassic sediments from northern bolivia also show a large counterclockwise rotation the observed differences between the primary and the secondary magnetization indicate that large block rotations occurred prior to the acquisition of the secondary magnetization perhaps this first episode of deformation partially structured the andean chain in the central andes and induced an incipient orocline during the cenozoic a differential shortening along the chain gave the present day shape of the bolivian deflection and resulted in a counterclockwise rotation of the forearc system by about 20 as indicated by the remagnetization

an early cretaceous paleomagnetic pole from ponta grossa dikes brazil implications for the south american mesozoic apparent polar wander path paleomagnetic results from the early cretaceous ponta grossa dike swarm are reported these dikes crosscut sediments and basement rocks associated with the ponta grossa arch a tectonic feature in the eastern border of the paleozoic mesozoic parana basin these dikes are vertical or subvertical and trend mainly nw and ne samples from 127 dikes were submitted to both alternating field and thermal demagnetizations 121 of them yielded characteristic directions interpreted as original thermal remanent magnetizations both normal and reversed polarities as well as intermediate directions 6 dikes are recorded the paleomagnetic pole is located at 30 3 e 82 4 s n 115 95 2 0 k 43 8 this pole along with the available paleomagnetic poles for the parana basin volcanics serra geral formation recalculated in this paper in the light of reported 40 ar 39 ar ages describe an apparent polar wander path segment corresponding to a clockwise rotation 6 9 of the south american plate this movement is compatible with the initial phase of the south atlantic opening with rifting progressing from south to north however the calculated plate velocity is higher than the predicted velocities reported so far based on oceanic floor magnetic anomalies the new data reported in this paper allowed the calculation of a mean early cretaceous pole 60 3 e 83 8 s n 7 95 2 9 which can be discriminated from the middle late jurassic 191 3 e 86 2 s n 3 95 7 8 and late cretaceous 346 5 e 84 1 s n 5 95 4 9 mean poles for south america indicating a more complex movement of the continent than the simple east west drift generally proposed

late oligocene early miocene major tectonic crisis and related basins in bolivia recent advances in biochronology geochronology magnetostratigraphy structural geology and basin analysis make untenable the traditional correlation of the first major andeanage deformational episode in bolivia with the middle eocene incaic tectonic phase defined in central peru and instead demonstrate that the episode took place in late oligocene and early miocene time this major tectonic crisis resulted in contemporary development of the subandean external foreland basin and altiplano intermontane basin which were separated by the initiation of thrusting in the present cordillera oriental area the deformation suggests that the bolivian orocline began to develop at that time it is likely that this tectonic upheaval is genetically linked to the marked increase in rate of plate convergence produced by the contemporaneous breakup of the farallon plate

geochemistry of the cambrian volcanic hosted massive sulfide rich mount read volcanics tasmania and some tectonic implications the cambrian mount read volcanics belt in western tasmania hosts five major gold rich massive sulfide deposits and remains the focus of intense exploration activity and geologic research a geochemical study of least altered lavas and shallow intrusions with emphasis on basaltic to andesitic compositions was carried out to determine the primary magmatic affinities and tectonic setting of eruption of the mount read volcanics to aid in internal correlations within the belt and to provide starting material estimates for mineralization related alteration studies three calc alkaline suites one extending to shoshonitic compositions and two tholeitiic suites have been distinguished within the mount read belt suite i is voluminous and includes the eastern sequence central volcanic complex tyndall group the intrusive quartz feld spar porphyries and granitoids and the andesitic lavas of the que hellyer footwall sequence suite i andesites are transitional medium to high k calc alkaline rocks la yb n 5 12 avg 8 1 and are the least light ree enriched calc alkaline lavas in the mount read volcanics suite ii comprises intrusive and extrusive often hornblende porphyritic andesites and dacites mainly from the upper part of the southern central volcanic complex they are more p 2 o 5 and light ree enriched la yb n 10 26 avg 16 7 than suite i and have high k calc alkaline affinities suite iii includes basaltic and andesitic lavas from the que hellyer hanging wall sequence in the northern part of the belt the lynch creek basalts near queenstown and intrusive basalts in the howards plains area northwest of queenstown suite iii rocks show a striking compositional range from low tio 2 0 4 0 5 low p 2 o 5 0 1 basalts with la yb n values from 8 to 12 and almost flat heavy ree patterns to low tio 2 0 4 0 8 but strongly to exceptionally p 2 o 5 and light ree enriched basalts with up to 350 times chondritic la and la yb n values up to 34 the latter are regarded as shoshonites tholeiitic pillow basalts of the henty fault wedge and the closely related dikes of the henty dike swarm constitute suite iv these have tio 2 contents from 1 0 to 1 6 percent very low nb contents 3 ppm and only slightly light ree enriched ree patterns la yb n 1 4 3 4 they are considered to be most similar to suprasubduction zone basalts erupted during the early phase of arc splitting and back arc basin development and indicate a period of tension and rifting of the mount read belt in post central volcanic complex but pre tyndall group time suite v is constituted by the miners ridge basalts and includes low tio 2 0 7 basalts with strong light ree depletion la yb n 0 5 and higher tio 2 to 2 6 lavas with only slight light ree depletion la yb n 0 9 suite v basalts are tentatively correlated with rift tholeiite sequences associated with the crimson creek formation and therefore are regarded as part of the pre mount read volcanics basement the three calc alkaline suites occur in a general stratigraphic order with suites i and ii being more or less coeval near queenstown they indicate a magmatic evolution from transitional medium to high k rocks through high k rocks to strongly enriched shoshonitic rocks closest modern analogues are postcollisional volcanics in regions of arc continent collision e g east papua and the roman province the occurrence of boninite derived chromites probably derived from the allochthonous mafic ultramafic complexes outcropping just west of the mount read belt in two sedimentary units within the mount read volcanics sequence below suite iii basalts supports a postcollisional origin for at least the major part of the mount read volcanics other similar postcollisional sequences particularly if formed in a submarine setting must be considered highly prospective for vhms deposits

major crustal rotations in the andean margin paleomagnetic results from the coastal cordillera of northern chile paleomagnetic analyses of mesozoic lavas and dike swarms from the northern chilean coastal cordillera between 25 4 s and 26 4 s reveal a clockwise rotation of about 42 magnetizations from lava flows of andesitic basaltic composition of the middle jurassic la negra formation pass both fold and reversal tests and are interpreted as prefolding remanences five dike swarms of middle jurassic to early cretaceous age yield similar directions to that obtained from the la negra formation four of the five swarms have mixed polarity suggesting that they too carry a primary or very early remanence the structural setting of the dikes suggests that they have not suffered any substantial tilting about nonvertical axes since acquisition of the remanence the clockwise rotation of the area is believed to have been the consequence of transpressional deformation of mid late cretaceous age post 100 ma associated with abandonment of the jurassic early cretaceous magmatic arc in this region and its eastward migration to form a new mid late cretaceous magmatic arc in the former back arc region this younger arc is located east of the coastal cordillera and lies in the central valley region the clockwise sense of rotation is consistent with other paleomagnetic data from northern chile and southern bolivia south of the arica deflection in the andean margin although it is the largest yet reported to the north of the arica deflection paleomagnetic studies report counterclockwise rotations and several large scale models have been proposed to explain the overall pattern of rotations models include oroclinal bending of an originally straight margin differential shortening across the margin at a preexisting bend which is subsequently tightened by the passive rotation of the limbs of the bend and distributed shear throughout the margin as a consequence of oblique convergence at a preexisting bend in contrast to these models several workers have argued that rotation is better explained in terms of localized in situ rotations we review these models in light of our results and present a domino type model with blocks bounded by left lateral faults and rotating clockwise in response to mid late cretaceous transpression within a crustal scale shear zone this is consistent with the observed strike slip fault systems identified in the coastal cordillera

vertical axis rotations determined from paleomagnetism of mesozoic and cenozoic strata of the bolivian andes 1 thermal demagnetization and principal component analysis allowed determination of characteristic remanent magnetization chrm directions from 256 sites at 22 localities in mesozoic and cenozoic sedimentary strata of the bolivian altiplano and eastern cordillera an inclination only fold test of site mean chrm directions from cenozoic units principally the santa lucia formation indicates optimum unfolding at 97 1 unfolding consistent with a primary origin for the chrm for mesozoic strata optimum unfolding occurred at 89 2 perhaps indicating secondary remagnetization at some locations for cenozoic units comparison of locality mean directions with expected paleomagnetic directions indicates vertical axis rotations from 33 counterclockwise to 24 clockwise euler pole analysis of along strike variation in crustal shortening within the subandean and interandean zones indicates 18 clockwise rotation south of the axis of curvature of the bolivian andes and 6 counterclockwise rotation northwest of the axis during the past 10 m y along strike variation of shortening within the eastern cordillera indicates 8 clockwise rotation south of the axis and 8 counterclockwise rotation northwest of the axis from 35 to 10 ma these vertical axis rotations produced by along strike variations in crustal shortening during development of the bolivian fold thrust belt agree well with observed rotations determined from paleomagnetism of cenozoic rocks in the eastern cordillera and in the subandean and interandean zones however local rotations are required to account for complex rotations in the cochabamba basin and within the altiplano the curvature of the bolivian andes has been progressively enhanced during cenozoic fold thrust belt deformation

phyllosilicates and associated minerals in some paleozoic stratiform sulfide deposits of southeastern australia the pb zn ores of eight of the nine deposits are characterized by one or the other of the phyllosilicate assemblages chlorite talc phlogopite biotite or chlorite muscovite the other contains chlorite phlogopitic trioctahedral mica but no talc phyllosilicate mineral chemistry is highly systematic and is related to ore type and ore stratigraphy in all cases chemical equilibrium between the phyllosilicates has been approached on a small scale equilibria were established substantially at the sedimentary early diagenetic stage the phyllosilicates were deposited as chemical precipitates in the hydrothermal aprons at the time of sea floor exhalative activity subsequent metamorphism has done little more than perfect crystal structures and coarsen grain size modified journal abstract

paleozoic paleogeography of north america gondwana and intervening displaced terranes comparisons of paleomagnetism with paleoclimatology and biogeographical patterns new paleomagnetic data have become available in the past 5 yr that require modifications in previously published paleogeographic reconstructions for the silurian and devonian in this paper the new paleopoles are compared to published paleogeographic models based on paleoclimatologic and biogeographic data the data from the three fields of paleomagnetism paleoclimatology and biogeography are generally in excellent agreement and an internally consistent paleogeographic evolutionary picture of the interactions between north america gondwana and intervening displaced terranes is emerging during the interval of the ordovician silurian and devonian north america stayed in equatorial paleoposition while rotating counter clockwise the northwest african part of gondwana was in high southerly latitudes during the late ordovician and was fringed by peri gondwanide terranes such as southern europe armorica and avalonian basement blocks now found in eastern newfoundland nova scotia the boston basin the appalachian piedmont and northern florida subsequently gondwana and the peri gondwanide terranes displayed rapid drift with respect to the pole this drift translates into the following pattern of movement for northwest africa during the latest ordovician early silurian this area moved rapidly northward from polar to subtropical latitudes followed by equally rapid southward motion from subtropical to intermediate about 50 s paleolatitudes during the late silurian middle devonian it is likely that significant east to west motion accompanied the latter shift in paleolatitudes with the caledonian acadian orogeny the result of silurian to early devonian convergence and collision between gondwana and north america this collision sandwiched several of the intervening displaced terranes between gondwana and north america subsequent to this collision gondwana was separated in the late devonian by a medium width ocean from north america and the avalonian and southern european blocks which were left behind adjacent to north america this new ocean closed during the carboniferous and the resulting convergence and collision were the cause of the hercynian alleghanian orogenic belt problems remaining for future research besides the further gathering of reliable paleopoles involve the uncertain pre devonian position of the southern british isles in this scenario and the very rapid velocity with respect to the pole that results from the rapid late ordovician silurian apparent polar wander for gondwana