protracted evolution of the maranon valley au belt magmatic complex in the peruvian andes using zircon oxygen isotopes lu hf and u pb analyses abstract the maranon valley located in the eastern cordillera of the northern peruvian andes represents a 160 km long section of the continental margin that was mainly active during the paleozoic a combination of whole rock geochemistry and u pb o and hf isotope microanalysis of zircon on selected igneous rocks reveals the timing and magmatic source variations along and across strike of the paleozoic proto andean margin the igneous rocks exhibit a typical arc related continental margin signature with strong enrichment in lile e g cs rb and th depletion of hfse e g nb and a negative ti anomaly the overlapping hf o isotopic compositions together with similar inherited zircon ages are consistent with the lavasen volcanics the esperanza subvolcanic complex and the pataz batholith sharing the same crustal source components in about the same proportion a genetic relationship is further supported by the strong depletion in sr nb ta ti and eu in the subvolcanic and volcanic rocks from the esperanza subvolcanic complex and lavasen volcanics compared to the intrusive rocks from the pataz batholith which is consistent with evolution of the erupted magmas by fractional crystallization the new zircon u pb la icp ms age determinations in combination with previously published geochronological data suggest that the pataz batholith the esperanza subvolcanic complex and the lavasen volcanics were emplaced contemporaneously between ca 342 ma and 332 ma the new mississippian u pb zircon u pb la icp ms age determination for the lavador pluton 336 332 ma and the callangate enaben pluton 341 337 ma suggest that magmatism was coeval with the emplacement of the pataz batholith the lavador pluton is aligned with the main n nw trend of the pataz batholith and shares a similar range of hf o isotopic values i e ehf t values from 5 3 to 1 3 and 18o values from 6 4 to 7 1 and is therefore believed to represent a northern extension of the pataz batholith the isotopic geochemistry and lithogeochemistry record in the maranon valley illustrates the regional scale tectonic switches that occurred during arc formation from syn collisional and compression in the pataz parcoy district between ca 342 ma and 332 ma to post collisional and extensional in the montanitas district between ca 332 ma and 319 ma the delineation of the timing and location of tectonic regime switches in the maranon valley may provide a basis for future mineral exploration

tectonic setting of synorogenic gold deposits of the pacific rim abstract more than 420 million oz of gold were concentrated in circum pacific synorogenic quartz lodes mainly during two periods of continental growth one along the gondwanan margin in the palaeozoic and the other in the northern pacific basin between 170 and 50 ma these ores have many features in common and can be grouped into a single type of lode gold deposit widespread throughout clastic sedimentary rock dominant terranes the auriferous veins contain only a few percent sulphide minerals have gold silver ratios typically greater than 1 1 show a distinct association with medium grade metamorphic rocks and may be associated with large scale fault zones ore fluids are consistently of low salinity and are co2 rich in the early and middle palaeozoic in the southern pacific basin a single immense turbidite sequence was added to the eastern margin of gondwanaland deformation of these rocks in southeastern australia was accompanied by deposition of at least 80 million oz of gold in the victorian sector of the lachlan fold belt mainly during the middle and late devonian lesser devonian gold accumulations characterized the more northerly parts of the gondwanan margin within the hodgkinson broken river and thomson fold belts additional lodes were emplaced in this flyschoid sequence in devonian or earlier palaeozoic times in what is now the buller terrane westland new zealand minor post devonian growth of gondwanaland included terrane collision and formation of gold bearing veins in the permian in australia s new england fold belt and in the jurassic early cretaceous in new zealand s otago schists collision and accretion of dozens of terranes for a 100 m y long period against the western margin of north america and eastern margin of eurasia led to widespread latest jurassic to eocene gold veining in the northern pacific basin in the former location late jurassic and early cretaceous veins and related placer deposits along the western margin of the sierra nevada batholith have yielded more than 100 million oz of gold additional significant ore forming events during the development of north america s cordilleran orogen included those in the klamath mountains region california in the late jurassic and early cretaceous the klondike district yukon by the early cretaceous the nome and fairbanks districts alaska and the bridge river district british columbia in the middle cretaceous and the juneau gold belt alaska in the eocene gold bearing veins deposited during the late jurassic and early cretaceous terrane collision that formed the present day russian far east have been the source for more than 130 million oz of placer gold the abundance of gold bearing quartz carbonate veins throughout the gondwanan north american and eurasian continental margins suggests the migration and concentration of large fluid volumes during continental growth such volumes could be released during orogenic heating of hydrous silicate mineral phases within accreted marine strata the common temporal association between gold veining and magmatism around the pacific rim reflects these thermal episodes melting of the lower thickened crust during arc formation slab rollback and extensional tectonism and subduction of a slab window beneath the seaward part of the forearc region can all provide the required heat for initiation of the ore forming processes

a hf isotope perspective on continent formation in the south peruvian andes convergent continental margins are the primary host of both growth and loss of continental crust continental growth largely occurs via subduction driven magmatism whereas continental loss largely occurs via subduction erosion and sediment subduction because the latter typically involves partial recycling into magmas both growth and loss of continental crust can be represented in the magmatic record the degree of crustal recycling can be estimated from the initial hf isotope signatures in both magmatic and detrital zircon grains recent insights into the geodynamic evolution of the peruvian margin in combination with a new dataset of hf isotopic data on zircon from the carboniferous to early cretaceous enable us to 1 compare the geodynamic history of the southern peruvian margin with its hf isotopic evolution and 2 quantify the crustal growth between 500 and 135 ma the data exhibit a correlation with trends in isotope composition v time and reflect the dominantly extensional regime that prevailed from the onset of subduction from 530 ma to c 135 ma this study demonstrates that the peruvian margin experienced continental growth with juvenile input to arc magmatism of 30 45 on average and illustrates the use of u pb and hf isotopes in zircon as a tool to trace episodes of crustal growth through time supplementary material hf istopic analyses on zircon a1 and a2 and new u pb zircon ages a3 are available at http www geolsoc org uk sup18661

the isotopic composition of yb and the precise and accurate determination of lu concentrations and lu hf ratios by isotope dilution using mc icpms 1 we report yb isotopic data for the purpose of accurately correcting measurements of lu and lu hf ratios during multicollector inductively coupled plasma mass spectrometer mc icpms analysis the yb isotopic ratios agree with recent results of chu et al 2002 and amelin and davis 2004 when corrected for mass fractionation to the same reference values protocols for lu measurement involving simultaneous subtraction of yb interference and application of a fractionation correction based on yb are described it is shown that lu concentrations and lu hf ratios determined by mc icpms using this methodology are more precise and potentially more accurate than results based on analysis of lu by thermal ionization mass spectrometry tims a number of mc icpms lu hf isotopic analyses of standard rocks and other geological samples previously analyzed by tims are presented these document reproducibility of lu hf within 0 2 where most discrepancies can be assigned to the unknown lu mass fractionation during tims analysis

crustal growth along a non collisional cratonic margin a lu hf isotopic survey of the eastern cordilleran granitoids of peru abstract an extensive in situ lu hf isotopic study of zircon from the eastern cordilleran batholiths of peru using laser ablation multi collector inductively coupled mass spectrometry la mc icpms reveals a systematic covariance between granitoid magma sources and tectonic regimes which shaped the proto andean margin of central western amazonia since 1 15 ga the hf isotope systematics are characterised by a range in initial 176hf 177hf compositions for a given intrusive event suggesting mixing of material derived from the paleoproterozoic crustal substrate and variable addition of juvenile sources from neoproterozoic to cenozoic time intrusives associated with phases of regional compressive tectonism correspond to mean initial ehf values of 6 73 2 43 1 57 for the ordovician famatinian carboniferous permian and late triassic respectively suggesting a minimum crustal contribution between 40 and 74 by mass the average initial hf systematics from granitoids associated with periods of regional extension such as the middle neoproterozoic permian triassic and cenozoic andean back arc plutonism are consistently shifted towards positive values mean initial ehf 0 7 to 8 0 indicating systematically larger inputs of juvenile magma 25 to 38 of ancient crust by mass in the absence of evidence for significant lateral accretion of exotic crust the time integrated hf record from the central proto andean margin of western amazonia suggests crustal reworking was the dominant process during episodes of arc magmatism implying that most crustal growth took place vertically via crustal underplating of isotopically juvenile mantle derived melts during intervals of crustal extension

chronostratigraphy and paleoclimatology of the lodeve basin france evidence for a pan tropical aridification event across the carboniferous permian boundary abstract paleosols preserved within the carboniferous permian succession of the lodeve basin massif central france change stratigraphically from histosols to calcic vertisols and calcisols to gypsic vertisols and ultimately back to calcic vertisols and calcisols new high precision u pb zircon ages ca idtims for tuff beds within the lodeve and adjacent graissessac basins significantly revise the chronostratigraphy of these and correlated permian terrestrial basins of eastern euramerica under the newly revised chronostratigraphy presented here these stratigraphic changes in morphology indicate a substantial drying of paleoenvironments across the carboniferous permian boundary with a trend toward progressively more arid and seasonal climates through most of the early permian this newly realized chronology provides a paleoenvironmental and paleoclimatic timeline for eastern tropical pangea that is contemporaneous with similar records in western pangea and suggests pan tropical progressive climate change toward aridity and seasonality occurred from the late carboniferous through early permian

new insights into the hydrothermal evolution of skarn deposits a case study of the dongzhongla pb zn deposit in tibet sw china abstract revealing hydrothermal evolution from the early quartz sulfide to the late fluorite carbonate sulfide stages of skarn sulfide deposits is significant for understanding their ore formation processes but the detailed studies are rare in this study we choose the dongzhongla skarn pb zn deposit 0 38 mt pb zn located in central lhasa tibet as a case study and present detailed ore deposit geology and dataset of in situ chemical compositions of quartz in situ pb isotopic ratios of galena sulfur isotopic compositions of sulfides and c o isotopic compositions of hydrothermal calcite and wall rocks to address this issue there are two types of hydrothermal quartz in the early stage 1 the early phase euhedral quartz qz1 crosscut by quartz sulfide veins and 2 the late phase subhedral quartz qz2 coexisting with sulfides titaniq thermometer of qz1 and qz2 limits the two phases with a temperature range of 336 427 c mean 373 c and 357 461 c mean 386 c respectively the decreasing al concentrations of qz1 from core to rim suggest that magmatic fluids mixed with acidic meteoric water prevented the precipitation of sulfides during the early phase the increasing al and decreasing sulfur concentrations of qz2 indicate that the increasing ph values of hydrothermal fluids made principal sulfides precipitate during the late phase in addition fluid inclusions and c o isotopic studies reveal that the late stage minerals were deposited from low salinity

mantle and slab contributions in arc magmas destructive plate margins are major sites of terrestrial magmatism that have long had a key role in models for the generation of continental crust and the development of chemical heterogeneities in the upper mantle in these models it is important to evaluate the nature and size of contributions from preexisting crust in the generation of magmas above subduction zones and to constrain the size of the fluxes of different elements from the subducted crust and the overlying mantle wedge in arc magmas kay 1 980 hawkesworth ellam 1 989 hawkesworth et al 1 99 1 a mcculloch gamble 1 99 1 the more mobile components of the subducted oceanic crust are the products of element fractionation processes in the upper continental crust the ocean basins and hydrothermal systems on the ocean floor they therefore have distinctive isotope and even trace element ratios that in principle should be readily distinguished from those in peridotite in the mantle wedge prior to subduction in practice however element ratios are almost certainly fractionated during shallow level dehydration of sub ducted material and at greater depths where material from the slab appears to contribute in the generation of arc magmas moreover the isotope signal of slab derived material may be heavily diluted by exchange with

new geochronological results and structural evolution of the pataz gold mining district implications for the timing and origin of the batholith hosted veins abstract the paleozoic pataz parcoy gold mining area is located in a right stepping jog on the regional cordillera blanca fault in northern peru most of the 8 million ounces of gold production from this area has come from quartz carbonate sulfide veins hosted by the pataz batholith despite a subduction zone setting since at least the cambrian the area records several periods of extension and its present structure is that of a rift and graben terrain the pataz district the northern part of the pataz parcoy area is dominated structurally by northwest to north northwest striking nw nnw faults and northeast to east northeast striking ne ene lineaments both of which have been active periodically since at least the mississippian early carboniferous nw nnw faults control the margins of a central horst that exposes basement schist and the pataz batholith and step across ne ene lineaments the lavasen graben to the east of the central horst contains the lavasen volcanics and the chagual graben to the west contains an allochthonous sedimentary sequence derived from the western andean cordillera new shrimp zircon geochronological data indicate emplacement of the pataz batholith during the middle mississippian at around 338 336 ma approximately 10 ma earlier than previous estimates based on 40 ar 39 ar geochronology the calc alkaline i type batholith comprises diorite and granodiorite the latter being the major component of the batholith and was emplaced as a sill complex within the moderately ne dipping sequence of the eastern andean cordillera moderate to high temperature ductile deformation took place on the batholith contacts during or shortly after emplacement following emplacement of the batholith differential uplift occurred along nw nnw faults forming the lavasen graben into which the lavasen volcanics were deposited shrimp u pb in zircon ages for the lavasen volcanics and the esperanza subvolcanic complex which was intruded into the western margin of the graben are within error of one another at ca 334 ma the ductile batholith contacts were cut by renewed movement on nw nnw faults such that the margins of the batholith are now controlled by these steep brittle ductile faults the nw nnw faults were oriented normal to the principal axis of regional shortening ene wsw during formation of the batholith hosted gold bearing quartz carbonate sulfide veins the misoriented faults were unable to accommodate significant displacement leading to high fluid pressures vertical extension in the competent batholith and formation of gold bearing veins brittle failure of the batholith was most extensive in the northern pataz district where the fault controlled western contact of the batholith is offset by a swarm of ne ene lineaments the timing of vein formation is not established despite published 40 ar 39 ar ages of 312 to 314 ma for metasomatic white mica which are interpreted as minimum ages of formation gold bearing veins formed during or shortly after uplift of the pataz batholith and formation of the lavasen graben they were therefore broadly coeval with deposition of the lavasen volcanics and emplacement of the esperanza subvolcanic complex these k rich weakly alkalic ferroan a type magmas may provide a viable source for the ore fluid that deposited gold in the pataz batholith

carboniferous orogenic gold deposits at pataz eastern andeancordillera peru geological and structural framework paragenesis alteration and 40ar 39ar geochronology the pataz province forms the central part of a 160 km long orogenic gold belt extending along the eastern andean cordillera in northern peru and has produced a total of 6 million ounces moz gold from vein type deposits during the last 100 yr the deposits present several recurrent and typical field characteristics including 1 at a regional scale location of the mineralization in low order structures within a 1 to 5 km wide structural corridor east of a major north northwest striking lineament and in spatial association with the northnorthwest striking margins of the 330 to 327 ma pataz batholith 2 at the mine scale strong lithological controls of the vein geometries and styles the lodes occurring as fairly continuous 5 km long quartz veins inside or along the margins of the batholith or as branching and bedding concordant narrow ore shoots within adjacent folded ordovician turbidite sequences 3 consistent orientations of veins in particular within the batholith where more than 80 percent of the quartz veins are emplaced in north to northwest striking eastdipping brittle ductile deformation zones 4 a consistent au ag as fe pb zn cu sb bi te w metal association and a sulfide rich paragenetic sequence with a first stage composed of milky quartz pyrite arsenopyrite and ankerite and a second stage of blue gray microgranular quartz galena sphalerite chalcopyrite sb sulfosalts electrum and native gold followed by barren calcite dolomite quartz veinlets and 5 hydrothermal alteration of the vein wall rocks consisting of pervasive muscovite alteration with minor chlorite carbonate minerals and pyrite associated with strong bleaching in plutonic rocks and of weak muscovite and chlorite alteration in sedimentary rocks structural analysis of the deposits outlines four synchronous sets of mineralized fractures in the pataz district the predominant north to northwest striking east to northeast dipping system which is generally located in reactivated reverse faults accounts for more than 80 percent of the gold resource of the district three subordinate systems include in decreasing order of economic importance 1 east west striking flat extensional veins 2 bedding concordant veinlets in east west striking and north dipping to north south striking and east dipping limbs of long wavelength folds in ordovician sedimentary rocks and 3 weakly mineralized roughly east west striking sinistral vertical faults the vein orientations of the four structural sets are compatible with a triaxial strain model with the main shortening axis p oriented at 080 15 an intermediate axis oriented at 165 00 and a subvertical extensional t axis oriented at 255 80 under these conditions the richest ore shoots are preferentially sited in sinistral pull aparts which occur at the intersection of either north south striking lodes or extensional lodes with roughly east west striking vertical faults 40ar 39ar dating of the granodiorite monzogranite bodies of the pataz batholith provides good plateau ages at 329 2 1 4 and 328 1 1 2 ma for biotite separates which are similar to a published 329 ma u pb age for the granodiorite a muscovite and a biotite sample from an aplite dike yielded plateau ages at 322 1 2 8 and 325 4 1 4 ma respectively muscovite samples from alteration intimately associated with the gold mineralization yielded three 40ar 39ar spectra with low temperature staircase shaped patterns followed by plateau segments at 314 to 312 ma these ages analytically indistinguishable at the 2 level are considered to be the most probable ages for the mineralization event three other plateaulike ages between 305 and 288 ma have been obtained and are interpreted to reflect partial argon loss during late fluid circulation associated with the intrusion of late cretaceous monzonite porphyries the age determinations are inconsistent with a genetic link between the 314 to 312 ma pataz gold deposits and the 330 to 327 ma calc alkaline pataz batholith or with the 327 to 319 ma aplite dikes or the late cretaceous porphyry magmatism instead the overall homogeneity of the structural mineralogical and geochemical characteristics of the deposits over the 160 km long mineralized belt and the geotectonic evolution suggest that gold mineralization is linked to a large scale thermal event that occurred in a thickened collisional belt undergoing uplift tectonics

paleozoic orogenic gold deposits in the eastern central andes and its foreland south america abstract in the eastern central andes and its foreland 6 34 s abundant quartz veins emplaced along brittle ductile deformation zones in ordovician to carboniferous granites and gneisses and in saddle reefs in lower paleozoic turbidites represent a coherent group of middle to late paleozoic structurally hosted gold deposits that are part of three major au sb w metallogenic belts these belts extending from northern peru to central argentina along the eastern andean cordillera and further south in the sierras pampeanas include historical districts and mines such as pataz parcoy ananea santo domingo yani aucapata amayapampa sierra de la rinconada and sierras de cordoba on the basis of the available isotopic ages two broad mineralization epochs have been identified with devonian ages in the sierras pampeanas au belt 26 to 33 30 s and carboniferous ages for the pataz maranon valley au belt in northern peru 6 50 to 8 50 s the absolute timing of the southeastern peruvian bolivian and northwestern argentinian turbidite hosted lodes which form the au sb belt of the southern eastern andean cordillera 12 to 26 s is poorly constrained field relationships suggest overlap of gold veining with carboniferous deformation events the northernmost belt which includes the pataz province is over 160 km long and consists of sulfide rich quartz veins hosted by brittle ductile shear zones that have affected carboniferous granitic intrusions gold mineralization at least in the pataz province occurred a few million years after the emplacement of the 329 ma host pluton and an episode of molassic basin formation during a period of rapid uplift of the host units the two southern belts are associated with syn to post collisional settings resulting from the accretion of terranes on the proto andean margin of south america the au sb belt of the southern eastern andean cordillera presumably formed in the final stages of the collision of the arequipa antofalla terrane and the sierras pampeanas au belt is considered concurrent with the late transpressional tectonics associated with the accretion of the chilenia terrane the three devono carboniferous andean belts are the south american segments of the trans global orogenic gold provinces that were formed from late ordovician to middle permian in accretionary or collisional belts that circumscribed the gondwana craton and the paleo tethys continental masses a paleogeographic map of the gondwana supercontinent in its middle cambrian configuration appears as a powerful tool for predicting the location of the majority of the paleozoic orogenic gold provinces in the world as they develop within mobile belts along its border the three south american belts are sited in the metallogenic continuation of the paleozoic terranes that host the giant eastern australian goldfields such as bendigo ballarat and charters towers with which they share many features when compared to deposits in the french massif central direct counterparts of the andean deposits such as pataz and ananea yani are respectively the saint yrieix district and the salsigne deposit considering the ubiquity of the au sb w vein type deposits in the eastern cordillera and sierras pampeanas and the relatively little attention devoted to them the devonian and carboniferous orogenic gold deposits in the eastern section of the central andes constitute an attractive target for mineral exploration

permian tetrapod footprints from the spanish pyrenees abstract paleozoic tetrapod footprints are a common and well known phenomenon in almost all large european countries except for spain here we report on hitherto unpublished vertebrate tracks from permian red beds of the south central pyrenees that with regard to their relative abundance diversity and quality of preservation are suitable to fill this gap of knowledge the described tracks come from two localities in muddy to fine grained sandy alluvial plain deposits in the lower third of the peranera formation of the erill castell basin near les iglesies northern catalonia the tracks can be assigned to five ichnogenera i e batrachichnus woodworth 1900 limnopus marsh 1894 varanopus moodie 1929 hyloidichnus gilmore 1927 and dromopus marsh 1894 that we interpret as footprints of temnospondyls captorhinomorphs and araeoscelids or similarly sized sauropsids with a lacertoid foot pattern this ichnofossil assemblage is most similar to ichnofaunas from the hermit formation of the grand canyon arizona the upper abo and robledo mountains formations of new mexico and the main trace fossil site of the tiddas basin morocco suggesting a late early permian artinskian age for this stratigraphic level of the peranera formation considering the relative abundance and diversity of captorhinomorph footprints the new spanish tracefossil localities may cover the onset of the early permian radiation of non diapsid eureptiles the thick late paleozoic red bed successions of the south central pyrenees have the potential to also bear footprints of otherwise unknown early therapsids so systematic fossil prospecting of this area is strongly recommended

multiple gold mineralizing styles in the northern pataz district peru gold production from the northern pataz district in the eastern andean cordillera of peru has been sourced mainly from mesothermal quartz carbonate sulfide veins hosted by the mississippian pataz batholith gold is also found in basement hosted veins underlying the batholith in the vijus santa filomena area of the district both are located within a central horst similar vein mineralogy and proximal phengitic white mica alteration are common to both however comb textured quartz the chemical compositions of bulk ore and sulfide minerals and the presence of barite and siderite veins suggest that the basement hosted veins formed at a shallower crustal level similar expressions of hydrothermal alteration associated with anomalous gold as sb and tl are also present in the adjacent lavasen graben where alteration is intimately associated with volcanic processes that deposited the mississippian lavasen volcanics k ar and 40 ar 39 ar ages for hydrothermal illite from all three locations range between mississippian and late triassic but are consistent with a single mississippian hydrothermal event if the data record a minimum age for original illite formation the geologic setting mineralization styles and chemical data suggest a range of crustal depths ranging from mesothermal batholith hosted veins through shallow to intermediate depths for the vijus santa filomena area to a near surface epithermal setting for the misquichilca area telescoping of this 10 to 13 km crustal range into a 3 km topographic section of the andes is attributed to syn and postmineralization uplift and erosion sulfide rich high grade ore shoots and moderately saline fluid inclusions in the batholith hosted veins are inconsistent with the original orogenic gold model and suggest a magmatic source component for the ore fluid consistent with stable isotope o h c and s compositions of quartz illite carbonates and sulfides the isotopic data suggest a mixed magmatic meteoric ore fluid in the basement hosted deposits of the vijus santa filomena area and the volcanic hosted misquichilca area both the pataz batholith and the esperanza subvolcanic complex are of the same mississippian age as the hydrothermal alteration and mineralization the esperanza subvolcanic complex comagmatic with the lavasen volcanics contains cognate mineral clots from which a subjacent magma chamber can be inferred it exhibits potassic calc silicate and argillic alteration and evidence for the evolution of an fe rich volatile phase the lavasen esperanza magma suite is ferroan and weakly alkaline with a type affinities these features provide a conceptual genetic link with hydrothermal alteration associated with gold mineralization including fe as sulfides phengitic white mica celadonite fe rich carbonates and less common fe oxides an oxidized intrusion related model is proposed for gold and hydrothermal alteration in the northern pataz district

improvements of precision and accuracy in in situ hf isotope microanalysis of zircon using the laser ablation mc icpms technique abstract we have developed a new analytical technique for precise and accurate in situ hf isotope ratio measurements for zircons by means of laser ablation multiple collector inductively coupled plasma mass spectrometry la mc icpms newly designed sample cell and n 2 mixing technique provided higher elemental sensitivity with smoother signal intensity profiles in order to improve the accuracy of the hf isotope ratio analysis previously reported correction methods for isobaric interferences on 176 hf by 176 yb and 176 lu were rigorously evaluated and we found that the use of independent mass bias factors for hf and yb in the isobaric interference correction resulted in more reliable hf isotopic data in order to test the reliability of the hf isotopic data from 35 m diameter ablation pits that are smaller than the typical size of most zircon grains lu hf isotopes for three zircons harvard 91500 antarctic zircon pma7 and acasta zircon ay199 were measured the present 176 hf 177 hf isotope ratio of 0 282321 46 2 s d and initial 176 hf 177 hf isotope ratio of 0 282315 47 2 s d were obtained for zircon standard 91500 the resulting present 176 hf 177 hf isotope ratio shows an excellent agreement with the previously reported data our in situ isotopic data revealed that the lu hf ratio varied significantly within the 91500 grain 176 lu 177 hf 0 000130 0 000345 suggesting the existence of slight isotopic heterogeneity in present 176 hf 177 hf approximately 15 ppm for antarctic zircon pma7 despite the smaller size of the single grain 50 100 m wide 80 300 m long and the presence of various mineral inclusions resulting analytical precision for the initial 176 hf 177 hf ratio was 128 ppm 0 281208 36 2 s d for acasta zircon ay199 despite the high yb hf 0 05 and lu hf 0 01 isotope ratios with the early archean crystallization age 3 74 ga resulting analytical precision for the initial 176 hf 177 hf ratio was 187 ppm 0 280160 52 2 s d these results demonstrate clearly that in situ hf isotope ratio microanalysis developed here has the potential to become a significant tool for zircon lu hf isotopic study

an oxygen isotopic profile through the upper kilometer of the oceanic crust dsdp hole 504b dsdp hole 504b is the deepest basement hole in the oceanic crust penetrating through a 571 5 m pillow section a 209 m lithologic transition zone and 295 m into a sheeted dike complex an oxygen isotopic profile through the upper crust at site 504 is similar to that in many ophiolite complexes where the extrusive section is enriched in18o relative to unaltered basalts and the dike section is variably depleted and enriched basalts in the pillow section at site 504 have 18o values generally ranging from 6 1 to 8 5 smow mean 7 0 although minor zeolite rich samples range up to 12 7 rocks depleted in18o appear abruptly at 624 m sub basement in the lithologic transition from 100 pillows to 100 dikes coinciding with the appearance of greenschist facies minerals in the rocks whole rock values range to as low as 3 6 but the mean values for the lithologic transition zone and dike section are 5 8 and 5 4 respectively oxygen and carbon isotopic data for secondary vein minerals combined with the whole rock data provide evidence for the former presence of two distinct circulation systems separated by a relatively sharp boundary at the top of the lithologic transition zone the pillow section reacted with seawater at low temperatures near 0 c up to a maximum of around 150 c and relatively high water rock mass ratios 10 100 water rock ratios were greater and conditions were more oxidizing during submarine weathering of the uppermost 320 m than deeper in the pillow section the transition zone and dikes were altered at much higher temperatures up to about 350 c and generally low water rock mass ratios 1 and hydrothermal fluids probably contained mantle derived co2 mixing of axial hydrothermal fluids upwelling through the dike section with cooler seawater circulating in the overlying pillow section resulted in a steep temperature gradient 2 5 c m across a 70 m interval at the top of the lithologic transition zone progressive reaction during axial hydrothermal metamorphism and later off axis alteration led to the formation of albite and ca zeolite rich alteration halos around fractures this enhanced the effects of cooling and18o enrichment of fluids resulting in local increases in 18o of rocks which had been previously depleted in18o during prior axial metamorphism

geodynamic evolution and tectonostratigraphic terranes of northwestern argentina and northern chile in ordovician time gondwana in the area of northwestern argentina and northern chile had a west facing active margin the evolution of this margin culminated in the ocloyic orogeny at the end of ordovician time this orogeny was caused by the collision of the allochthonous arequipa antofalla terrane with this margin the early paleozoic evolution of northwestern argentina and northern chile contrasts markedly with the accretionary history of central argentina and central chile where the precordillera and chilenia terranes docked in the late ordovician and late devonian periods respectively an inspection of the available stratigraphic and geochronological data on sedimentary volcanic and plutonic units of the southern central andes of northern chile and northwestern argentina reveals a lull in magmatic and metamorphic activity lasting for 100 m y from early silurian to early late carboniferous time this is interpreted as corresponding to a tectonic scenario in which the present andean margin was a passive margin of gondwana this passive margin developed in response to the rifting off of a part of the arequipa antofalla terrane the present location of this block is unknown late carboniferous time marks the renewed onset of subduction initiating the andean plate tectonic setting still prevalent today recently proposed models explain the late ordovician orogeny by the collision of laurentia with western south america during laurentia s clockwise motion around south america and away from its position in the neoproterozoic supercontinent in its present form this hypothesis is difficult to reconcile with the paleozoic tectonostratigraphic evolution of the southern central andean region

a routine high precision method for lu hf isotope geochemistry and chronology a method for chemical separation of lu and hf from rock meteorite and mineral samples is described together with a much improved mass spectrometric running technique for hf this allows i geo and cosmochronology using the176lu 176hf decay scheme and ii geochemical studies of planetary processes in the earth and moon chemical yields for the three stage ion exchange column procedure average 90 for hf chemical blanks are 0 2 ng for lu and hf from 1 g of hf a total ion current of 0 5 10 11 ampere can be maintained for 3 5 h yielding 0 01 0 03 precision on the ratio176hf 177hf normalisation to179hf 177hf 0 7325 is used extensive results for the johnson matthey hf standard jmc 475 are presented and this sample is urged as an international mass spectrometric standard suitable aliquots prepared from a single batch of jmc 475 are available from denver lu hf analyses of the standard rocks bcr 1 and jb 1 are given the potential of the lu hf method in isotope geochemistry is assessed

sedimentology and vertebrate taphonomy of the moradi formation of northern niger a permian wet desert in the tropics of pangaea abstract pangaean paleogeographic models place the tim mersoi basin of northern niger in a 5000 km wide corridor between gondwana and laurasia approximately 15 degrees south of the paleoequator late permian paleoclimate models position this basin between tropical summer wet to the north and desert to the south recent investigations of the fossil vertebrates and paleosols in the late permian lopingian moradi formation confirm that the climate was warm and hyperarid with highly seasonal monsoonal rainfall possibly as a result of these unusual wet desert conditions the tetrapod fauna shows a high degree of endemism this study tests existing paleoclimate models by providing additional data on sedimentary environments and vertebrate taphonomic processes the moradi red bed sequences accumulated in a gently subsiding sag basin to the west of the tectonically active massif de l air low angle gravelly alluvial fans prograded westward from the massif and at times impinged on a large stable northward flowing meandering channel system the interchannel mudrock sequences are over thickened by the accumulation of loessic silts and preserve isolated skull and post crania of amphibians nigerpeton and saharastega as well as semi articulated captorhinids moradisaurus detailed surface mapping of a fossil rich exposure revealed an anastomosed network of loess filled distributary channels incised into the floodplain mudrocks this provided a locus for the accumulation and rapid burial of at least 15 associated skeletons of the pareiasaurian bunostegos semi permanent ponds are evidenced by patches of fissile red mudstone containing rare bivalves and spiral coprolites in the distal floodplains away from the main river channels the combination of a generally high groundwater table warm mean annual temperatures and deflation of fines from the floodplain surface promoted the formation of gypsiferous paleosols and end point playa lakes carbonate rich mud accumulated around the lake margins and provided ideal conditions for the imprinting and preservation of tetrapod trackways

geologic setting paragenesis and physicochemistry of gold quartz veins hosted by plutonic rocks in the pataz region the pataz region is comprised of different low grade metamorphic proterozoic to early paleozoic basement series overlain by folded late paleozoic to early tertiary sediments and almost undeformed volcaniclastic rocks of late tertiary age the basement is intruded by the pataz batholith along a north northwest trending fracture zone this is the most important intrusive body of the region and consists of a geochemically varied calc alkaline magma suite the pataz batholith hosts numerous auriferous quartz veins with a simple two stage sulfide paragenesis microthermometric measurements indicate that stage i is characterized by medium chlorine k rich fluids probably related to magmatic dewatering stage ii displays medium to high chlorine solutions with significant amounts of na k ca and or mg this fluid composition is consistent with the strong interaction with the host rock as evidenced by the observed alteration features the initial chloride contents of stage ii are similar to those of stage i fluid inclusions further show a wide range of homogenization temperatures that probably reflect tectonically induced changes of pressure native gold with bimodal au ag ratios was found to be related chronologically to the second stage and spatially to pyrite that formed in the first stage as did most of the quartz and some arsenopyrite fractured pyrite was an important control on gold deposition frequently accompanied by galena sphalerite and chalcopyrite there is much evidence that vein formation occurs at considerable depth and is geometrically as well as temporally and genetically linked with the cooling stage of the pataz batholith local physicochemical factors account for quantitative mineralogic differences and for the deposition of either electrum or fine gold with less than 15 wt percent ag the early k bearing chloride solutions appear to be essentially magmatogenic fluids of the gold stage represent either new magmatogenic pulses or solutions recycled in a convective circulation system that included the nearby basement series gold leaching from precambrian to paleozoic country rocks appears appropriate in order to explain the regional distribution of the metal

the gerola valley site orobic basin northern italy a key for understanding late early permian tetrapod ichnofaunas abstract a taxonomic study has been carried out on the historical gerola valley locality pizzo del diavolo formation late cisuralian which represents one of the best tetrapod ichnosites of the whole southern alps north italy with respect to previous studies the ichnoassociation is now enlarged and lists the following taxa amphisauropus dromopus erpetopus hyloidichnus limnopus varanopus the optimal preservation of the material which in some cases shows even traces of the skin allowed the recognition of a new morphotype of varanopus the invertebrate ichnoassociation is newly described and is representative of impoverished scoyenia and mermia ichnofacies a detailed facies analysis of the uppermost arenitic pelitic lithofacies of the pizzo del diavolo formation which revealed the most abundant fossil content locality 1 and of two new localities pelitic facies localities 2 3 shows a transition from shallow lacustrine to floodplain and alluvial fan environments under a generally dry climate besides the taxonomic revision of vertebrate tracks the final aims of our study are i the depositional palaeoenvironmental and climatic reconstruction of this sector of the orobic basin through an integration of palaeontological data and facies analysis and ii the regional correlation of this key stratigraphic succession with other coeval ichnofaunas in pangaea

early permian fossil vertebrate footprints and their stratigraphic setting in megacyclic sequence ii red beds prim point prince edward island vertebrate trace fossils are reported for the first time from red beds near the top of megacyclic sequence ii at prim point in southwestern prince edward island they occur as casts of tetrapod trackways the ichnocoenose also includes a rich invertebrate ichnofauna the trackmakers thrived in an area of sparse vegetation and occupied out of channel river sediments most likely crevasse splay deposits amphisauropus latus represented by three trackways has been previously reported from germany poland and czechoslovakia it is here interpreted as the track of a cotylosaur it occurs together with the track of gilmoreichnus kablikae which is either a captorhinomorph or possibly a juvenile pelycosaur these facilitate the assignment of a late early permian late autunian age to the strata the third set of footprints those of a small herbivorous pelycosaur compare most closely with ichniotherium willsi known hitherto from the keele beds latest stephanian of the english midlands this ichnocoenose oc

systematic use of trace element distribution patterns in log log diagrams for plutonic suites trace element modelling has been widely used for petrogenetic interpretation of basaltic systems this paper indicates how to select trace element pairs having very different bulk distribution coefficients d which when plotted on simple log log diagrams permit the identification of the main magmatic process magma mixing partial melting fractional crystallization involved in the genesis of plutonic rocks fractional crystallization gives a straight line on such a diagram with a strong decrease of the compatible element whereas the concentrations of the element with d 1 increase slowly a similar evolution of the solids in equilibrium is observed and when data of at least one of the cumulates are directly available it is possible to calculate the d and f parameters of the sequence of fractional crystallization an example of this procedure is shown for a french hercynian plutonic suite the basic suite of variscan corsica

hot orogens tectonic switching and creation of continental crust many granulite terrains were too hot to have formed during continental collision rather along with many high grade metamorphic terrains that typify continental crust most formed in accretionary orogens during tectonic switching when prolonged lithospheric extension was interrupted by intermittent transient contraction based on modern and ancient examples tectonic switching occurs when slab retreat induces upper plate extension causing arc splitting formation of microcontinent slivers and backarc basins then intermittent arrival of buoyant oceanic plateaus induces transient flat subduction or slab flip and crustal thickening during slab retreat basaltic magmas produced from decompressed asthenosphere advect into the extending orogen causing granulite facies metamorphism and granite generation but subsequent thickening during flat subduction cools the region thickening is focused in the thermally softened backarc region and if sediment filled a hot short lived 10 m y narrow 50 100 km orogenic belt forms such thickening is often misleadingly ascribed to arc or microcontinent collision once slab retreat mode is reestablished lithospheric extension recommences and a new arc backarc system forms generally outboard arrival of another plateau will reverse the procedure and another short lived hot orogen will form within the orogenic system cycles of tectonic switching efficiently produce continental crust

revisional notes on the permian tetrapod ichnofauna from the tiddas basin central morocco during the mid 1980s structural geological field work in the up to 300 m thick red bed sequence of the permian tiddas basin central morocco revealed the first occurrence of palaeozoic tetrapod footprints in nw africa preservational ambiguity of part of the few published trace fossils and resulting taxonomic inconsistency necessitate a long needed reanalysis of the type locality based on 110 recently collected specimens the revised tetrapod ichnofauna of the study area comprises tracks of batrachichnus woodworth 1900 limnopus marsh 1894 amphisauropus haubold 1970 dimetropus romer and price 1940 varanopus moodie 1929 hyloidichnus gilmore 1927 and dromopus marsh 1894 which can be referred to small and medium sized temnospondyl seymouriamorph basal synapsid pelycosaur captorhinomorph and araeoscelid trackmakers this assemblage is interpreted to represent a late early permian artinskian kungurian tetrapod ichnofauna due to the relative abundance of footprints assigned to semiaquatic tetrapods and sedimentological evidence for pronounced humid palaeoclimatic conditions we propose correlation of the track bearing strata with the late artinskian early kungurian wet phase regarding the quality quantity and peculiar diversity of its fossils including the earliest record of seymouriamorph and advanced captorhinomorph tracks from the african continent the tiddas basin could be of great importance for the reconstruction of late early permian terrestrial ecosystems in proximity to the interface of euramerican and gondwanan biotic provinces

oxidized magnetite series rapakivi type granites of carajas brazil implications for classification and petrogenesis of a type granites abstract the varying geochemical and petrogenetic nature of a type granites is a controversial issue the oxidized magnetite series a type granites defined by anderson and bender anderson j l bender e e 1989 nature and origin of proterozoic a type granitic magmatism in the southwestern united states of america lithos 23 19 52 are the most problematic as they do not strictly follow the original definition of a type granites and approach calc alkaline and i type granites in some aspects the oxidized jamon suite a type granites of the carajas province of the amazonian craton are compared with the magnetite series granites of laurentia and other representative a type granites including finnish rapakivi and lachlan fold belt a type granites as well as with calc alkaline i type orogenic granites the geochemistry and petrogenesis of different groups of a types granites are discussed with an emphasis on oxidized a type granites in order to define their geochemical signatures and to clarify the processes involved in their petrogenesis oxidized a type granites are clearly distinguished from calc alkaline cordilleran granites not only regarding trace element composition as previously demonstrated but also in their major element geochemistry oxidized a type granites have high whole rock feo t feo t mgo tio 2 mgo and k 2 o na 2 o and low al 2 o 3 and cao compared to calc alkaline granites the contrast of al 2 o 3 contents in these two granite groups is remarkable the cao feo t mgo tio 2 vs cao al 2 o 3 and cao feo t mgo tio 2 vs al 2 o 3 diagrams are proposed to distinguish a type and calc alkaline granites whole rock feo t feo t mgo and the feo t feo t mgo vs al 2 o 3 and feo t feo t mgo vs al 2 o 3 k 2 o na 2 o diagrams are suggested for discrimination of oxidized and reduced a type granites experimental data indicate that besides pressure the nature of a type granites is dependent of o 2 conditions and the water content of magma sources oxidized a type magmas are considered to be derived from melts with appreciable water contents 4 wt originating from lower crustal quartz feldspathic igneous sources under oxidizing conditions and which had clinopyroxene as an important residual phase reduced a type granites may be derived from quartz feldspathic igneous sources with a metasedimentary component or alternatively from differentiated tholeiitic sources the imprint of the different magma sources is largely responsible for the geochemical and petrological contrasts between distinct a type granite groups assuming conditions near the nno buffer as a minimum for oxidized granites magnetite bearing granites formed near fmq buffer conditions are not stricto sensu oxidized granites and a correspondence between oxidized and reduced a type granites and respectively magnetite series and ilmenite series granites is not always observed

triggers for the formation of porphyry ore deposits in magmatic arcs porphyry ore deposits supply much of the copper molybdenum gold and silver used by humans a review of the main processes that trigger porphyry ore formation suggests that sulphide saturation of the magmas that supply the metals could be the overriding mechanism that helps control the temporal and spatial distribution of the ore deposits

isotopic evidence for rapid continental growth in an extensional accretionary orogen the tasmanides eastern australia the trace element signature of earth s continental crust resembles that of arc lavas but the continents may have formed during ancient igneous pulses that are hard to reconcile with supra subduction zone magmatism we explore the role of coupled arc back arc accretionary processes in crust generation by considering the tectonic context of whole rock nd isotope and zircon hf o isotope data from igneous rocks of the australian tasmanides 515 230 ma which is thought to have evolved by the repeated opening and closure of sediment filled back arc basins behind a long lived subduction zone the significance of this process for continental crust formation has yet to be evaluated from an isotopic perspective granitic rocks in this area define striking secular end ehf 18o trends that correlate with the pattern of deformational events and register changes in magma source during tectonic activity these trends reveal that juvenile magmatic input was enhanced during extensional back arc rifting episodes that immediately followed crustal thickening suggesting a relationship between slab rollback and continental growth interaction between juvenile magma and sedimentary units deposited during a preceding back arc rifting cycle was integral to the formation of stable continental material this highlights the importance of back arc environments for both the generation and differentiation of continental crust the juvenile component within the tasmanide igneous rocks increased from the cambrian to the triassic consistent with a diminished input from cratonderived metasedimentary material as the subduction zone migrated outboard of the gondwana margin subduction zone retreat formed large tracts of new crust in eastern australia at comparable rates to crust generation at modern island arcs providing a mechanism for rapid continental growth at convergent margins using isotopic tracers to link tectonic evolution and crust generation in modern and ancient orogens can lead to a better understanding of the geodynamic controls on planetary differentiation