petrographic and geochemical variation along the reykjanes ridge 57 n 59 n abstract high density sampling of the neo volcanic zone along the reykjanes ridge has been used to test the relationship between bathymetric features and geochemistry the axial region has short wavelength features which correspond to elongate en echelon volcanic ridges these ridges sit atop intermediate wavelength features termed swells and troughs a broad correspondence is found between the sections containing more fractionated samples and the intermediate wavelength swells conversely the troughs are dominated by more primitive lavas this suggests that swells represent areas of greater melt flux resulting in thicker crust and hence generally more fractionation progressively higher concentrations of incompatible elements are found northwards along the ridge with increases occurring as a series of steps that do not correlate with short or intermediate wavelength bathymetric features the northward increase in ratios such as nb zr suggests that sections of the ridge are fed by similar aggregate melt fractions and that melt fractions become slightly smaller northwards changes or consistency in ratio particularly rb nb cannot be explained by progressively greater northward incorporation of a component from the icelandic plume mantle

a detailed gravity field over the reykjanes ridge from seasat geosat ers 1 and topex poseidon altimetry and shipborne gravity the method of least squares collocation lsc was used to construct a detailed gravity field over the reykjanes ridge using seasat geosat ers 1 topex poseidon altimeter data and shipborne gravity data mean altimeter derived geoid gradients along with their standard deviations were used in order to avoid cross over adjustment of sea surface heights the ship data were adjusted to a satellite only gravity field using a quadratic polynomial in time and then merged with the satellite data to produce a combined gravity field the necessary covariance functions were derived using the law of covariance propagation in a test case a comparison between a combined gravity field and ship data not included in the calculation yielded an rms difference of 2 31 mgals the combined gravity field contains a wealth of short wavelength information and clearly reveals the detailed tectonic structures associated with the reykjanes ridge

evolution of the guerrero composite terrane along the mexican margin from extensional fringing arc to contractional continental arc the western margin of mexico is ideally suited for testing two opposing models for the growth of continents along convergent margins accretion of exotic island arcs by the consumption of entire ocean basins versus accretion of fringing terranes produced by protracted extensional processes in the upper plate of a single subduction zone we present geologic and detrital zircon evidence that the zihuatanejo terrane of the guerrero composite terrane originated from the latter mechanism the evolution of the zihuatanejo terrane can be explained by extensional and compressional processes operating entirely within the upper plate of a long lived subduction zone that dipped east under the mexican margin this process controlled crustal growth by continental margin rifting and addition of new igneous and volcaniclastic material during extension followed by accretion and thickening of the crust during contraction prior to this study all mesozoic rocks in the western part of the guerrero composite terrane were considered to be part of a single arc however we divide it into four distinctive tectonostratigraphic assemblages 1 a triassic early jurassic accretionary complex arteaga complex 2 a jurassic to earliest cretaceous extensional volcanic arc assemblage 3 an early cretaceous extensional arc assemblage and 4 a santonian maastrichtian compressional arc assemblage 1 the arteaga subduction complex forms the basement to the zihuatanejo terrane and includes grenville pan african and permian detrital zircon suites that match the potosi fan of the mexican mainland 2 the jurassic to earliest cretaceous extensional volcanic arc assemblage shows a callovian tithonian ca 163 145 ma peak in magmatism extensional unroofing began in this time frame and continued into through the next 3 the early cretaceous extensional arc assemblage has two magmatic peaks one in the barremian aptian ca 129 123 ma and the other in the albian ca 109 ma in some localities rapid subsidence produced thick mainly shallow marine volcano sedimentary sections while at other localities extensional unroofing of all older assemblages resulted in recycling of zircon from all older units 1 2 3 4 for the santonian maastrichtian compressional arc assemblage our new detrital zircon dates show for the first time that arc volcanic rocks of this age are present in the coastal zihuatanejo terrane the contractional arc developed atop assemblages 1 3 which were shortened between turonian and santonian time ca 93 and 84 ma taken together the western zihuatanejo terrane records a more protracted history of arc magmatism than has yet been dated in other terranes of western mexico but it closely matches the history of baja california to the northwest

the guerrero suspect terrane western mexico and coeval arc terranes the greater antilles and the western cordillera of colombia a late mesozoic intra oceanic arc accreted to cratonal america during the cretaceous the guerrero suspect terrane composed of late jurassic early cretaceous sequences extends from baja california to acapulco and is considered to be coeval with the late mesozoic igneous and sedimentary arc sequences of the greater antilles the west indies venezuela and the western cordillera of colombia these sequences represent the remnants of an arc which accreted to the north american and northern south american cratons at the end of the cretaceous in western mexico the arc sequences built on continental crust consist of high k calc alkaline basalts andesites and rhyolites enriched in lree with abundant siliceous pyroclastic rocks interbedded either with aptian albian reefal limestones or red beds they do not show magmatic changes during the arc development in contrast the arc sequences built on oceanic crust show an evolution with time arc activity began with the development of depleted low k tholeiitic mafic suite guanajuato igneous sequence followed first by mature tholeiitic basalts and then by calc alkaline olivine basalts interbedded with micritic limestones and radiolarian oozes of early cretaceous age at the end of the arc growth during aptian albian times calc alkaline pillow basalts and and esites poured out in the volcanic front while shoshonitic olivine basalts extruded in the back arc the tholeiitic and shoshonitic mafic rocks as well as the calc alkaline lavas are mildly enriched in lree y and nb and show high nd ratios typical of oceanic arcs in contrast the calc alkaline mafic suite enriched in lree y and nb exhibits lower nd ratios suggesting that it was derived by the partial melting of a mantle source contaminated either by paleozoic subducted sediments or old source enrichments oib the cretaceous arc rocks of the greater antilles interbedded with and or capped by aptian albian limestones the cretaceous andesites of northern colombia the cretaceous tholeiitic and calc alkaline volcanic rocks of venezuela and the cretaceous volcano plutonic arc assemblage of tobago share a similar magmatic evolution with the western mexican oceanic arc the tholeiitic plutono volcanic assemblage of tobago depleted in lree and characterized by high nd values is similar to the guanajuato volcano plutonic sequence of mexico considered to represent the pristine stage of the arc the mature tholeiitic sequences exposed in the proto caribbean arc show flat to moderately enriched lree patterns like those of the guerrero terrane however felsic plutonic and volcanic rocks prevail in the caribbean calc alkaline suites accompanied locally by shoshonitic lavas characterize the end of arc magmatic activity in both places thus the geochemical features of the late jurassic cretaceous arc series of the guerrero terrane and the proto caribbean are consistent with the following plate tectonic model the guerrero terrane and the proto caribbean probably belonged to the same intra paleo pacific arc system the development of which was related to the subduction of oceanic basins fringing the north and northern south american cratons this subduction zone was wsw dipping while subduction was going on these magmatic arcs drifted moved closer to the north and south american cratons and finally collided with the american borderlands at different periods during the cretaceous the late mesozoic guerrero and proto caribbean arc sequences show striking similarities with the miocene calc alkaline lavas dredged from the banda ridges the north marianas seamount province and the halmahera and philippine arcs we suggest that the diverse but mostly submarine segments of this late mesozoic intra pacific arc rimmed the north and south american cratons as much as these tertiary arcs rim southeast asia

a kuroko type polymetallic sulfide deposit in a submarine silicic caldera manned submersible studies have delineated a large and actively growing kuroko type volcanogenic massive sulfide deposit 400 kilometers south of tokyo in myojin knoll submarine caldera the sulfide body is located on the caldera floor at a depth of 1210 to 1360 meters has an area of 400 by 400 by 30 meters and is notably rich in gold and silver the discovery of a large kuroko type polymetallic sulfide deposit in this arc front caldera raises the possibility that the numerous unexplored submarine silicic calderas elsewhere might have similar deposits

detrital zircon u pb geochronology of southern guerrero and western mixteca arc successions southern mexico new insights for the tectonic evolution of southwestern north america during the late mesozoic late jurassic cretaceous arc related volcaniclastic rocks from the southern guerrero and western mixteca terranes of mexico were analyzed by u pb detrital zircon geochronology laser ablation multicollector inductively coupled plasma mass spectroscopy to place constraints on the depositional history and provenance of the rocks pre middle jurassic basement rocks and sandstone from the upper cretaceous mexcala formation were also analyzed to defi ne the origin and provenance of the prevolcanic substratum and the time of accretion of guerrero composite terrane sequences data from the taxco taxco viejo teloloapan and arcelia assemblages indicate that the youngest 129 141 ma zircon fraction in each sequence was derived from local volcanic sources whereas older populations ca 247 317 365 459 530 617 712 878 947 964 1112 1188 1350 1420 1842 1929 2126 2439 and 2709 3438 ma show sediment infl ux from varied sources most likely through grain recycling the major zircon clusters in these sequences match the populations recorded in the nearby acatlan complex in contrast the huetamo sample is dominated by lower cretaceous ca 126 ma zircons of local volcanic provenance and the zihuatanejo sample contains zircon clusters ca 259 ca 579 and ca 947 1162 ma comparable to major populations recorded in the underlying arteaga complex a sample from the middle triassic middle jurassic arteaga complex at tzitzio contains zircon clusters ca 202 247 ca 424 ca 600 ca 971 and ca 2877 ma consistent with an ultimate derivation from both north american and south american sources the sample from the las ollas suite contains comparable zircon populations ca 376 475 ca 575 ca 988 1141 and ca 2642 2724 ma and it is interpreted to be part of the prevolcanic basement in contrast the youngest zircon cluster ca 105 ma in the mexcala formation coincides with the major volcanic events in the taxco taxco viejo teloloapan and arcelia assemblages whereas the older clusters ca 600 ca 953 ca 1215 ca 1913 and ca 2656 2859 ma broadly match the major populations recorded in rocks from the acatlan complex these new data combined with available geochemical and isotopic data indicate that the taxco taxco viejo arc assemblage developed on continental crust the acatlan complex is the most plausible candidate the teloloapan and arcelia arc assemblages were developed on oceanic crust as offshore arcs facing the acatlan complex the zihuatanejo terrane assemblages were developed on the arteaga complex and evidence no infl uence from the acatlan complex this suggests that these assemblages were formed farther away or in a restricted basin the guerrero composite and mixteca arc successions are coeval with the alisitos arc of northern mexico and in part with the nevada and klamath ranges of the southwestern united states and with the arc series from the greater and lesser antilles and northern south america data indicate that during late mesozoic time southwestern north america was a site of intensive volcanism in a complex arc trench system similar to that of the east pacifi c our data are consistent with a diachronic accretion of the guerrero composite terrane sequences beginning during late cenomanian time with the amalgamation of the teloloapan and probably the arcelia assemblages and fi nishing at the end of cretaceous time with the accretion of the zihuatanejo terrane assemblages

anisotropy and mantle flow in the chile argentina subduction zone from shear wave splitting analysis 1 we examine shear wave splitting in teleseismic phases to observe seismic anisotropy in the south american subduction zone data is from the charge network which traversed chile and western argentina across two transects between 30 s and 36 s beneath the southern and northwestern parts of the network fast polarization direction j is consistently trench parallel while in the northeast j is trench normal the transition between these two zones is gradual we infer that anisotropy sampled by teleseismic phases is localized within or below the subducting slab we explain our observations with a model in which eastward nazca entrained asthenospheric flow is deflected by retrograde motion of the subducting nazca plate resulting southward flow through this area produces n s j observed in the south and northwest e w j result from interaction of this flow with the local slab geometry producing eastward mantle flow under the actively flattening part of the slab index terms 7203 seismology body wave propagation 7218 seismology lithosphere and upper mantle 8123 tectonophysics dynamics seismotectonics 8150 tectonophysics plate boundary general 3040 9360 information related to geographic region south america citation anderson m l g zandt e triep m fouch and s beck 2004 anisotropy and mantle flow in the chile argentina subduction zone from shear wave splitting analysis geophys res lett 31 l23608 doi 10 1029 2004gl020906

pyrite deformation textures in the massive sulfide ore deposits of the norwegian caledonides abstract the norwegian caledonides represent a belt of late precambrian to early palaeozoic rocks emplaced as a series of nappes onto the fennoscandian basement massive sulfide orebodies occur along virtually the whole length of the orogenic belt making it possible to study deposits of a similar origin chemistry and mineralogy but variably deformed at different metamorphic conditions samples from seven pyrite rich ore deposits deformed at temperatures from 320 c to 610 c have been investigated using reflected light petrographic observation orientation contrast oc imaging and electron backscatter diffraction ebsd while a range of brittle annealment and growth textures are preserved in the deposits all preserve evidence for lattice misorientation and low angle 2 sub grain boundary development indicative of dislocation creep these results are at odds with those implied from the published pyrite deformation mechanism map which suggest that pyrite does not deform by plastic mechanisms at natural geological strain rates and that the brittle ductile transition is at 425 c our results imply that pyrite is more ductile than generally inferred and the current deformation mechanism map needs to be revised with plastic deformation mechanisms operating at a much wider range of temperature and strain rate conditions

development of a brine dominated hydrothermal system at temperatures of 400 500 c in the upper level plutonic sequence troodos ophiolite cyprus abstract microthermometric analyses of quartz hosted fluid inclusions in fresh and epidotized plagiogranites from the troodos ophiolite cyprus indicate that brine rich fluids 46 56 wt nacl equivalent at temperatures 400 500 c were pervasive throughout many of the plagiogranite bodies high salinity fe bearing inclusions along healed microfractures homogenize by halite dissolution at temperatures of 400 500 c liquid dominated low salinity 2 7 wt nacl secondary fluid inclusions are ubiquitous in all plagiogranite and gabbro samples studied homogenization of the low salinity inclusions occurs at uncorrected temperatures of 200 400 c the quartz hosted high salinity fluids represent either phase separation of hydrothermal seawater or an exsolved magmatic aqueous phase at uncorrected temperatures of 400 500 c migration and segregation of brine and vapor phases into fractures near the margins of the crystallizing plagiogranite bodies resulted in preferential entrapment of the fe rich brines in the deep seated high temperature portions of the hydrothermal system subsequent fracturing of the upper plutonic rocks at temperatures of 200 400 c allowed penetration by seawater which pervasively altered the plagiogranites hydration reactions involving formation of secondary minerals and or mixing with phase separated fluids resulted in fluid salinities two times that of seawater

on the accuracy of marine gravity measurements we have assessed the accuracy of lamont doherty geological observatory s global marine gravity data bank by examining the crossover errors coes at intersecting ship tracks more than 63 000 coes were found having a standard deviation of 22 43 mgal mgal 10 5 m s 2 we use the coes to find and remove linear drifts and dc shifts present in the data set this adjustment reduces the standard deviation to 13 96 mgal coes generally decrease with latitude which we attribute to uncertainties in the eotvos correction high coes occur in areas of high gravity gradients these two features point to poor navigation as the principal source of error in marine gravity surveys coes have generally been decreasing during the last two decades reflecting improvements in instrumentation and quality of navigation a comparison of the shipboard gravity data to seasat derived gravity revealed a 9 mgal bias in the terrestrial data which probably reflects uncertainties in choice of reference field the adjusted data set was used to generate a gravimetric geoid for the nw atlantic ocean by removing this geoid from the seasat sea surface heights a residual geoid was obtained a special feature of this map is an elongate ene trend that appears to correlate with the edge of the gulf stream

trench parallel flow beneath the nazca plate from seismic anisotropy shear wave splitting of s and sks phases reveals the anisotropy and strain field of the mantle beneath the subducting nazca plate cocos plate and the caribbean region these observations can be used to test models of mantle flow two dimensional entrained mantle flow beneath the subducting nazca slab is not consistent with the data rather there is evidence for horizontal trench parallel flow in the mantle beneath the nazca plate along much of the andean subduction zone trench parallel flow is attributale utable to retrograde motion of the slab the decoupling of the slab and underlying mantle and a partial barrier to flow at depth resulting in lateral mantle flow beneath the slab such flow facilitates the transfer of material from the shrinking mantle reservoir beneath the pacific basin to the growing mantle reservoir beneath the atlantic basin trenchparallel flow may explain the eastward motions of the caribbean and scotia sea plates the anomalously shallow bathymetry of the eastern nazca plate the long wavelength geoid high over western south america and it may contribute to the high elevation and intense deformation of the central andes

antarctic marine gravity field from high density satellite altimetry summary closely spaced satellite altimeter profiles 5 km collected during the geosat geodetic mission geosat gm and those planned for the extended ers 1 mission are easily converted to grids of vertical gravity gradient and gravity anomaly as profile spacing decreases it becomes increasingly difficult to perform a crossover adjustment on the original geoid height profiles without introducing large cross track gradients if one is only interested in the horizontal and vertical derivatives of the gravitational potential however adjustment of the profile is unnecessary the long wavelength radial orbit error is suppressed well below the noise level of the altimeter by simply taking the along track derivative of each profile ascending and descending slope profiles are then interpolated onto separate uniform grids these two grids are summed and differenced to form comparable grids of east and north vertical deflection using laplace s equation the vertical gravity gradient is calculated directly from the vertical deflection grids fourier analysis is required to construct gravity anomalies from the two vertical deflection grids these techniques are applied to high density 2 km profile spacing geosat gm profiles in antarctic waters 60 s to 72 s gridding and interpolation are performed using the method of projection onto convex sets where the smoothness criteria corresponds to upward continuation through 4 km of ocean the resultant gravity grids have resolution and accuracy comparable to shipboard gravity profiles after adjustment of a dc shift in the shipboard gravity profiles 5 mgal the rms difference between the ship and satellite gravity is 5 5 mgal many interesting and previously uncharted features are apparent in these new gravity maps including a propagating rift wake and a large leaky transform along the pacific antarctic rise

fluid temperature and salinity characteristics of the matagami volcanogenic massive sulfide district quebec fluid inclusions hosted within various lithologic units of the 40 million metric ton mt matagami district abitibi greenstone belt preserve samples of archean volcanogenic massive sulfide vms and postvolcanogenic massive sulfide hydrothermal fluid microthermometric measurements on ore hosted primary two phase liquid vapor inclusions from matagami s south limb deposits indicate that the vms hydrothermal fluid was highly saline 16 2 4 7 wt nacl cacl2 equiv 1 n 230 and of moderate temperature trapping temperature tt 208 32 c 1 n 230 a fluid with these characteristics is capable of transporting 5 10 4 m 30 ppm zn as zncl3 and zncl4 2 chloride complexes however the low temperature of this fluid precluded efficient cu transport 3 ppm which may in part explain the relatively cu poor nature of the matagami deposits calculated densities of this ore fluid as high as 1 10 g cm3 are consistent with a bottom hugging brine model however a subset of the data indicate a fluid less dense than ambient seawater suggesting that buoyant hydrothermal plumes were also present a microthermometrically determined high cacl2 content xnacl 0 55 molar na ca 2 3 1 for the vms ore hosted primary fluid is consistent with an archean hydrothermal fluid more ca rich than modern day seawater quartz epidote veins located in the hydrothermal cracking zone of the bell river complex host primary liquid vapor halite inclusions these inclusions are interpreted to be samples of the deep seated equivalent to the vms ore hosted hydrothermal fluid described above microthermometry indicates that these inclusions trapped a high temperature brine tt 373 44 c 1 n 92 38 2 1 9 wt nacl equiv 1 n 92 we interpret this brine to be a phase separated product of modified model seawater 3 2 wt nacl an exsolved magmatic fluid or a combination thereof deep within the hydrothermal system at 650 to 670 c and a near lithostatic pressure of 90 mpa phase separation and subsequent convection lowered the temperature of the brine prior to its entrapment within the hydrothermal cracking zone the occurrence of high temperature brine overlain by lower temperature salinity fluid suggests a two cell convection model consistent with metal mass balance calculations for the south limb the high salinity of the ore hosted fluid inclusions indicates two possibilities 1 a significant amount of brine was incorporated into the upper cell and mixed with heated seawater during convection 2 archean seawater itself was very saline and of variable salinity with the cooling of the bell river complex lower temperature fluids dominantly of seawater origin circulated deep within the hydrothermal system modified by water rock interaction yet not phase separated these fluids sealed off the remnant permeability of the fracture network of the hydrothermal cracking zone and were locally trapped as secondary liquid vapor fluid inclusions homogenization temperature th 242 17 c 9 1 1 6 wt nacl equiv 1 n 14 hosted within the bell river complex quartz epidote vein material post and or waning stage vms hydrothermal activity is evident from the presence of quartz epidote veins crosscutting wabassee group hanging wall rocks microthermometry on quartz hosted primary liquid vapor fluid inclusions suggests that this activity occurred at relatively low temperatures th 76 177 c n 212 over a wide range of salinity 6 0 32 4 wt nacl cacl2 equiv n 212 and with a high apparent cacl2 content xnacl 0 06 these fluid inclusion data illustrate the importance of subsea floor chemical and physical processes directly related to metal transport and deposition in vms systems in particular phase separation deep within the hydrothermal system is interpreted as a key process for generating saline brines capable of forming significant ore deposits

the rhyolite hosted volcanogenic massive sulfide district of cuale guerrero terrane west central mexico silver rich base metal mineralization emplaced in a shallow marine continental margin setting the cuale mining district situated in the cordillera madre del sur 30 km southeast of puerto vallarta mexico comprises a number of small polymetallic pb zn cu volcanogenic massive sulfide deposits vms and vein and stockwork hosted deposits with low sulfidation epithermal characteristics massive sulfide mineralization contains high ag au grades when compared to similar deposits in western mexico the massive sulfide bodies are hosted by an interbedded rhyolitic volcanic and volcaniclastic succession herein termed the cuale volcanic sequence regionally the cuale volcanic sequence is part of the zihuatanejo subterrane belonging to the guerrero terrane which is interpreted to be a complex mesozoic accreted arc terrane hosting the majority of vms deposits in mexico the rocks of the cuale volcanic sequence are not metamorphosed and are only weakly deformed but exhibit intense chlorite sericite and quartz hydrothermal alteration three stratigraphic units are identified within the cuale volcanic sequence 1 a more than 400 m thick footwall unit of quartz and plagioclase phyric rhyolite flows related hyaloclastites and subordinate heterolithic volcaniclastic breccias 2 an ore horizon characterized by sedimentary rocks ranging from volcaniclastic conglomerates to black shales as well as aphyric to quartz and plagioclase phyric rhyolite and 3 late intrusions composed of quartz and plagioclase phyric rhyolite and subordinate andesitic dikes intruding all units the sedimentary rocks of the ore horizon are intercalated with rhyolite tuff horizons some of which exhibit welding or accretionary lapilli that are interpreted as evidence for shallow submarine or subaerial deposition the rhyolites of the cuale volcanic sequence are calc alkaline with only limited compositional variations however the trace element signatures reveal a weak trend from the footwall rhyolites which are characterized by low zr y 1 1 9 and low chondrite normalized la yb ratios 9 of 11 samples between 1 24 and 3 3 to slightly higher values in the ore horizon and late intrusive phase zr y 1 6 7 6 and chondrite normalized la yb ratios between 2 25 and 15 this compositional evolution can be explained by slightly decreasing degrees of partial melting related to a transition from amphibole free assemblages for the footwall to amphibole bearing assemblages in the residuum for the ore horizon and intrusive rhyolites this interpretation is supported by the observed volcanic stratigraphy and indicates increasing pressure at the site of melt generation as the volcanic edifice was built zircons from five samples of the cuale volcanic sequence have been dated by the u pb method using isotope dilution thermal ionization mass spectrometry id tims although some samples show evidence of a minor inherited pb component an age range of 157 2 0 5 to 154 0 0 9 ma has been established cuale represents the oldest dated vms camp in western mexico as other vms districts of western mexico are between 151 3 and 138 ma two styles of mineralization are recognized volcanogenic massive sulfide mineralization vms and late lowsulfidation epithermal vein and stockwork mineralization characterized by high au 1 89 ppm and zn 2 35 but low cu 0 2 contents the vms mineralization is contained in more than 15 small orebodies which exhibit a range of mineralization styles orebodies interpreted as proximal are characterized by stockwork vein networks with high au and cu contents underlying massive pyrite bodies which grade laterally and vertically into sphalerite and galena rich massive sulfide ore stockwork mineralization is locally hosted by aphyric rhyolite more distal orebodies rich in pb zn and ag are hosted by black shale and locally exhibit laminated or brecciated sedimentary textures in the galena sphalerite pyrite ore the sedimentary textures observed in some massive sulfide orebodies suggest that the sulfide has been transported and deposited in anoxic basins adjacent to the rhyolite domes some of the more significant orebodies show evidence of both distal sedimentary ore textures and proximal features indicated by intense quartz sericite pyrite alteration in the immediate footwall

multistage hydrothermal alteration of gabbroic rocks from the failed mathematician ridge oxygen isotopic mineralogical and fluid inclusion data are presented for fresh and hydrothermally altered gabbroic rocks from the failed mathematician ridge the mathematician ridge though initially a fast spreading ridge presently has a deep central rift valley more typical of slow spreading rifts the inner rift walls expose rare plutonic rocks uncomplicated by proximity to deep large offset fracture zones undeformed mathematician ridge gabbros preserve most of the original texture and phase compositions and have been only slightly mineralogically or isotopically modified by post crystallization hydrothermal alteration metagabbros have been subjected to a pervasive ductile deformation and recrystallization to foliated rocks composed of aluminous amphibole low calcium plagioclase and ilmenite oxygen isotope and phase chemical data suggest this event began at temperatures of up to 700 c water rock ratios of less than one and produced modified seawater fluids with isotopic compositions of 3 5 6 0 the amphibolites are cross cut by multiple generation quartz epidote veins forming a network replacement of the amphibole grade mineralogies by greenschist grade mineralogies fluid oxygen isotopic compositions of 2 4 and a two phase separation in the fluid captured in conjugate vapor fluid inclusions are all associated with the formation of this network fluid inclusion data for the quartz indicate initial temperatures of over 500 c these samples may represent rocks from the 1 2 km deep plumbing system of black smoker type venting

age and paleotectonic setting of volcanogenic massive sulfide deposits in the guerrero terrane of central mexico constraints from u pb age and pb isotope studies the guerrero terrane of central and western mexico comprises volcanic and sedimentary strata of mainly mesozoic age with geochemical signatures indicating formation in juvenile back arc to slightly more evolved arc environments at least 60 volcanogenic massive sulfide vms deposits have been discovered thus far within the guerrero terrane they comprise two main belts the coastal belt mainly comprising deposits in the zihuataneo subterrane including the cuale bramador district and the central belt which includes deposits in the teloloapan subterrane campo morado tlanilpa azulaquez tizapa rey de plata etc the guanajuato subterrane el gordo las gavalinas and the zacatecas subterrane san nicolas and el salvador deposits the age of host rocks for the vms mineralization in the various subterranes was poorly constrained prior to this study eleven new u pb zircon ages for volcanic host rocks from the campo morado tlanilpa azulaquez leon guanajuato and san nicolas el salvador vms districts together with recently determined u pb ages from the cuale area demonstrate that vms deposits in the guerrero terrane range from latest middle jurassic to early early cretaceous callovian to valanginian in age the oldest ages were obtained from the cuale district total age range of 157 4 4 1 ma for three units and the youngest are from the tlanilpa azulaquez district total age range of 139 7 2 5 ma for two samples volcanogenic massive sulfide deposits in the central belt resemble those of the bimodal siliciclastic deposit type whereas those in the coastal belt are more similar to the bimodal felsic type published geochemical and radiogenic isotope data from the various subterranes suggest that vms deposits in the coastal belt and most of those in the central belt are hosted within juvenile to slightly evolved arc settings the san nicolas and el salvador deposits of the zacatecas subterrane are the only deposits that unequivocally formed in a back arc setting our new u pb dating results together with existing stratigraphic geochemical and radiogenic isotope data from throughout the guerrero terrane are consistent with the guerrero terrane having developed as a west facing continental margin arc that was built on mainly oceanic crust and overlying early mesozoic siliciclastic fans shed from the east along the western margin of nuclear mexico slab rollback in the middle jurassic led to the development of a rocas verdes type continental back arc inboard of the arc which is now preserved as the arperos basin in the central and southern guerrero terrane and the host rocks for the san nicolas and el salvador deposits farther to the north in southeastern zacatecas state we have determined pb isotope compositions of sulfides from vms deposits and several other styles of mineralization from throughout the guerrero terrane a compilation of these and previously published data shows that isotopic compositions from the various vms deposits fall in a relatively confined field that overlaps the average upper crustal growth curve this suggests that metals were mainly sourced from continentally derived sedimentary strata that are interlayered with or immediately underlie the host volcanic section and or from the host volcanic rocks themselves or subvolcanic intrusive equivalents that had assimilated significant quantities of upper crustal material either at the point of origin or during subsequent ascent and eruption lead isotope compositions from the francisco i madera deposit in zacatecas state indicate that most of the sulfides in this deposit are epigenetic rather than syngenetic in origin and are likely tertiary in age

a combined fluid inclusion and s h o he ar isotope study of the devonian ashele vms type copper zinc deposit in the altay orogenic belt northwest china abstract the altay orogenic belt of kazakhstan and china incorporates one of the world class volcanogenic massive sulfide vms polymetallic metallogenic belt more than 12 large and superlarge vms deposits have been found in this belt the ashele basin is located at the western end of the chinese altay bordering kazakhstan and hosts the large ashele copper zinc deposit which is a typical vms deposit and the largest one in this basin the principal orebodies are stratabound and located between basalt and tuff units in the ashele formation the mineralization has a two layered structure that consists of an upper stratiform concordant massive cu zn or barite orebody and a lower veinlet disseminated and stockwork cu or cu pb zn ag orebody associated with silicic alteration several exhalative rocks such as barite hematitic jasper pyrite and silicified units also occur here we investigate the ore forming fluids of the exhalative sedimentary units including jaspilte stratiform barite and cu zn orebodies which carry fluid inclusions that show homogenization temperatures of 100 410 c with two peaks around 230 and 150 c and low salinity clustering between 2 and 8 wt nacl equiv corresponding to nacl h2o fluids the ore forming fluids of the vein orebody and silicified zones are characterized by low to medium temperature peak homogenization temperatures between 120 and 280 c low to medium salinity 0 7 12 3 wt nacl equiv and h2o co2 ch4 n2 nacl fluids the 34s values of barite associated with the stratiform barite orebody range from 20 4 to 21 4 indicating that the sulfur was derived from the seawater the sulfur isotope values of sulfide 3 7 to 7 7 with a mean of 3 1 are similar to those of vms deposits from other parts of the world the host volcanic rocks are inferred to be the major source from which the mineralizing fluid leached the ore forming elements the 3he 4he ratios of fluid inclusions in pyrite are in the range of 0 136 0 260 r ra broadly similar to the helium ratios of the crust the 40ar 36ar ratios range from 394 to 9515 and are higher than those of atmospheric argon the 40ar 4he ratios of the ore fluids range from 0 002 to 0 064 with a mean of 0 034 the helium and argon isotope compositions of fluid inclusions suggest that the ore fluids of the ashele deposit were mainly derived from the crust and were mixed with a minor amount of mantle component the 18osmow values of quartz from the silicified zones and vein orebody range from 8 3 to 11 1 with corresponding 18ofluid values of 4 45 to 2 24 whereas the d values of fluid inclusions are between 140 and 90 the 18osmow values of barite from the stratiform barite orebody range from 7 7 to 9 1 with corresponding 18ofluid values of 3 03 to 1 79 with d values of fluid inclusions showing a range of 162 and 135 the combined isotopic data h o and he ar suggest that the ore forming fluids of the ashele deposit were mainly derived from deep circulation of seawater and mixed with magmatic fluids the decrease in temperature and pressure water rock exchange and changes in the composition of ore fluids played important roles in the ore forming processes of the ashele cu zn deposit

mesozoic volcanogenic massive sulfide vms deposits in mexico abstract volcanogenic massive sulfide vms deposits are the most conspicuous type of deposits that formed during the mesozoic in mexico many mexican vms deposits display classical kuroko type mineral zonation and structure and some of them as cuale and la minita formed in shallow submarine environments the most prospective time window for the formation of vms deposits in mexico comprises the late jurassic and the early cretaceous vms stopped forming during the progressive continentalization of mexico since its metallotectonic processes dominated by extensive tectonics changed giving way to compression during the late early cretaceous new vms deposits did not form until after the opening of the gulf of california mesozoic vms deposits in mexico occur in submarine volcano sedimentary sequences that deposited essentially in association with back arc basins now found roughly along the boundaries between tectonostratigraphic terranes or within juvenile and slightly evolved arcs at the internal parts of the terranes while few others occur on the epicontinental seafloor and hinterlands of eastern mexico vms deposits are especially abundant in the guerrero composite terrane and are also present in the alisitos and parral terranes however new evidence referred in this paper indicate the western continental edge of the cratonic block of oaxaquia as a promising new prospective region for vms deposits interestingly no vms deposits are found in the northern part of the guerrero composite terrane despite the occurrence of marine volcano sedimentary sequences similar to those in the south such absence can be related to differential extensional unroofing much larger in the southern part of the guerrero composite terrane than in the northern part many vms deposits occur along or close to terrane boundaries especially around the guerrero composite terrane this distribution reflects the association between vms deposits and back arc basins which represented the frontal part of terranes or sub terranes that were ultimately accreted to the oaxaquia cratonic block as a consequence vms deposits usually display strong deformation and thrusting and their mineral and compositional zonation can be found overturned due to such common association between these deposits and terrane boundaries that reactivated during the cenozoic vms deposits are especially susceptible to overprinting by later metallogenic processes unrelated to vms producing environments this susceptibility may be held accountable for the complex mineral associations found in the francisco i madero deposit the inclusion fluids in the cuale la minita el rubi tizapa and campo morado deposits have salinities that range from 2 5 to 20 0 wt nacl equiv and temperatures of homogenization from 110 to 420 c although a particular fluid inclusion assemblage with daughter halite and sylvite from a stockwork at the tizapa deposit ranged salinities from 39 7 to 64 7 wt nacl and from 35 9 to 43 5 wt kcl and temperatures of homogenization from 440 to 550 c the general characteristics listed above may account for mineralizing fluids from magmatic marine and modified marine sources the entrainment of cool and oxidizing seawater within upwelling fluids increased as the paleohydrothermal systems waned the waning stages are normally represented by barite rich mineralizations and in the case of the la minita deposit this notoriously outlasted by mn oxide mineralization that reflects even more oxidizing conditions a similar evolution is deduced in cuale which is the only known mesozoic vms deposit in mexico that formed in a shallow submarine environment together with la minita the shallower the formation of vms deposits is the more quickly mineralizing fluids evolve into highly oxidized end members once the paleohydrothermal systems cooled down the extremely high salinity and hot inclusion fluids in tizapa despite the unavailability of further geochemical data are likely to represent magmatic brines that did not undergo significant mixing with seawater as for the temporal evolution of the studied deposits they generally exhibit 1 an increasing magmatic contribution from early to middle stages for both fluids as mineralizing fluids grew hotter and more saline and sulfur and 2 the waning of hydrothermal activity towards the last stages on mineralization which is characterized by the prevalence of seawater as fluids grew cooler and more diluted and oxidizing thus reflecting quite a typical evolution for most vms deposits elsewhere new 34s values for the same vms deposits reflect multiple sources of sulfur in the mineralizing fluids magmatic sedimentary metasedimentary and marine different degrees of dominance of either source for sulfur in these deposits and in their stages of mineralization have been inferred in a way that also responds to a schematic evolution from magmatic to seawater dominated paleohydrothermal systems

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

melanges in southern mexico geochemistry and metamorphism of las ollas complex guerrero terrane las ollas complex loc is a subduction complex spatially associated with the early cretaceous zihuatanejo huetamo subterrane guerrero terrane in southern mexico loc tectonic melanges compose of a stack of east dipping west vergent tectonic sheets containing blocks of metabasalt metadolerite metagabbro ultramafics volcaniclastics quartz rich sandstone and chert enveloped in a highly sheared clastic or serpentinitic matrix most ig neous and igneous derived metamorphic blocks show geochemical and isotopic features typical of island arc tholeiitic suites i l ow tio 2 0 13 to 0 91 and zr 5 to 57 ppm contents ii high lfse hfse n ratios low lan ybn 0 5 to 4 values and high end t 7 9 to 8 0 ratios petrographical and mineral chemistry evidence indicates that blocks underwent early recrystallization under high pressure and low temperature hp lt blueschist facies conditions during subduction typical assemblages include blue sodic through calco sodic to na rich calcic amphibole lawsonite tremolite mg chlorite white mica albite quartz phase relations and chlorite thermometry suggest temperatures of about 200 330 o c and pressures of 5 7 kbar it is proposed that sedimentary blocks were generated by in situ remobilization and mixing whereas igneous blocks most probably derived from the chemically and isotopically identi cal zihuatanejo island arc suite our data suggest that loc represents part of a subduction complex formed by east ward directed subduction related with the evolution of the early cretaceous zihuatanejo island arc resume le complexe de las ollas loc est un complexe de subduction associe dans l espace au sous terrane zihuatanejo huetamo datant du cretace et localise dans le sud du mexique les melanges tectoniques du loc com prennent un empilement de feuillets tectoniques a vergence ouest et a pendage est qui contiennent des blocs de meta basalte metadolerite metagabbro roches ultramafiques roches volcanoclastiques gres riches en quartz et du chert enveloppes dans une matrice clastique ou serpentineuse hautement cisaillee la plupart des blocs ignes et metamorphi ques provenant de roches ignees presentent des caracteristiques geochimiques et isotopiques typiques de suites tholeiiti ques d arc insulaire i faibles concentrations de tio 2 0 13 a 0 91 et de zr 5 a 57 ppm ii rapports eleves

ore metamorphism in sulfide mineralizations from the cinco villas massif western pyrenees spain massive stratiform and vein type zn pb fe f ba mineralizations associated with namuro westfalian metasediments occur in the cinco villas paleozoic massif these mineralizations were previously considered to be the result of the precipitation of hydrothermal fluids liberated by the aya granitic intrusion however the orebodies show several metamorphic features typical of similar mineralizations described in the geologic literature suggesting a preintrusive genesis of probably exhalative sedimentary nature the sedimentary sequence consists of two formations 1 the aranaz limestones dated as namurian and 2 a westphalian schistose formation composed of pelitic semipelitic and psammitic rocks with some conglomeratic tuffs albite rich laminae tourmalinites and interlayered basic sills these materials have been affected by the hercynian deformation and a low pressure regional metamorphism ranging from anchizone to biotite zone mesoscale structural data and microscopical observations suggest three periods d 1 d 2 and d 3 of progressive deformation accompanied by fracturing and shearing phenomena d 2 the main deformation developed isoclinal to recumbent folds and a regional foliation s sub 0 2 the more important metamorphic traits are produced during this deformation and at the same time the aya granitoid intrusion was emplaced the orebodies exhibit a lenticular to tabular morphology concordant with host rocks or forming a variable angle with s sphalerite galena and different proportions of pyrite pyrrhotite and chalcopyrite represent the major sulfide minerals quartz carbonates fluorite and barite are the main gangue minerals regional deformation and metamorphism have caused uneven but important effects on the geometry and primary textures of the orebodies however there is evidence of some premetamorphic textures such as debris flow breccias slump folds and particularly in pyrite framboidal relicts and colloform textures folding foliation transposition shearing durchbewegung and vein type ores are common metamorphic features in ore layers where the sulfides and gangue minerals have responded differently to deformation thus textures in pyrite denote a fundamentally brittle behavior but in some cases there is evidence of cataclastic flow kinking deformation bands and recrystallization in contrast sphalerite galena chalcopyrite and pyrrhotite exhibit different textures indicating that dislocation glide and creep mechanical twinning and recrystallization have generally been important mechanisms these processes suggest deformation under temperatures 400 degrees c which is consistent with the degree of metamorphismon in the host rocks micro and mesoscale remobilization of gangue and ductile sulfides due to a mixture of both mechanical and chemical processes is a generalized phenomenon in all deposits probably a major scale remobilization has been operative this would explain the existence of important vein type mineralizations in this region

gravity fields of the southern ocean from geosat data in august 1990 the u s navy declassified all geodetic mission gm radar altimeter data acquired by the geosat satellite over oceanic regions south of 60 s we have used these gm data in conjunction with the unclassified lower resolution geosat exact repeat mission erm altimeter data to construct high resolution gravity fields on a 5 km grid covering the annular region of the southern ocean which lies between 60 s and 72 s and encircles antarctica during the gm a complete mapping of the marine geoid between 72 and 72 n was accomplished the gm produced more densely spaced ground tracks typically 2 or 3 km at 60 s than those of either the erm or seasat consequently we were able to use the gm data to map the marine gravity field at a higher resolution than was previously possible using satellite altimeter data this paper describes the techniques we used to derive these gravity fields and image them these techniques involve 1 computing along track sea surface height slopes 2 gridding of these ascending and descending slopes 3 converting the slopes to conventional deflections of the vertical 4 transforming the deflections to gravity anomalies in the frequency domain and 5 imaging the resulting images of the marine gravity field reveal much that is new about the seafloor and the tectonic fabric of the southern ocean a region which includes large expanses of seafloor that have never been surveyed by ships

evolution of the xialiao basin the xialiao depression is one of several small depressions that comprise the north china basin formed during the cenozoic era the xialiao depression is a wedge shaped rift basin with a central horst situated between two grabens the basin is approximately 65 km wide and the grabens reach maximum depths of 6 7 km we present a quantitative analysis of the evolution of the xialiao depression hereafter the xialiao basin within the framework of simple models of crustal extension we find that both uniform and modified two layer extension modelling techniques provide reasonable regional estimates of the rate of extension and subsidence of the basin however estimations of heat flow and subsidence based on the parameters calculated using either modelling technique are not consistent with either the present basin configuration of a central horst flanked by two grabens or the high heat flow in the xialiao basin combining the results of our attempts to model the extension of the basin with results from both body and surface wave studies of the crust and upper mantle beneath northeast china we infer that the xialiao basin has been extended at least 30 of its initial width

sks splitting beneath continental rift zones we present measurements of sks splitting at 28 digital seismic stations and 35 analog stations in the baikal rift zone siberia and adjacent areas and at 17 stations in the east african rift in kenya and compare them with previous measurements from the rio grande rift of north america fast directions in the inner region of the baikal rift zone are distributed in two orthogonal directions ne and nw approximately parallel and perpendicular to the ne strike of the rift in the adjacent siberian platform and northern mongolian fold belt only the rift orthogonal fast direction is observed in southcentral mongolia the dominant fast direction changes to rift parallel again although a small number of measurements are still rift orthogonal for the axial zones of the east african and rio grande rifts fast directions are oriented on average nne that is rotated clockwise from the n s trending rift all three rifts are underlain by low velocity upper mantle as determined from teleseismic tomography rift related mantle flow provides a plausible interpretation for the rift orthogonal fast directions the rift parallel fast directions near the rift axes can be interpreted by oriented magmatic cracks in the mantle or small scale mantle convection with rift parallel flow the agreement between stress estimates and corresponding crack orientations lends some weight to the suggestion that the rift parallel fast directions are caused by oriented magmatic cracks

metallogenesis and fluid evolution of the huangtupo cu zn deposit east tianshan xinjiang nw china constraints from ore geology fluid inclusion geochemistry h o s isotopes and u pb zircon re os chalcopyrite geochronology abstract the recently discovered huangtupo cu zn deposit is located in the dananhu tousuquan arc east tianshan nw china orebodies are hosted in volcano sedimentary rocks of the late ordovician early silurian daliugou formation the metallogenic physicochemistry ore forming fluids and metallogenesis of the deposit were investigated through orebody features fluid inclusion microthermometry laser raman spectroscopy and synchrotron radiation x ray fluorescence srxrf the results indicate that the deposit includes an upper sedimentary exhalative ore belt stage i as well as a lower vein stockwork ore belt in a hydrothermal fluid supply channel stage ii associated with silicification chloritization epidotization pyrophyllitization and beresitization alteration the re os isochron age of 429 5 10 ma obtained from primary chalcopyrite the la icp ms zircon u pb age of 423 3 2 9 ma for the host hangingwall tuff fluid inclusion fi microthermometry and geological characteristics were applied to constrain hydrothermal processes and ore genesis quartz and barite were collected from the massive sulfide and vein ores for fi study four types of fi were identified liquid rich l type vapor rich v type daughter mineral bearing s type and c type containing co2 h2o stage i barite fis yield homogenization temperatures of 232 364 c peak value 265 c with salinities of 4 85 10 94 wt nacl equiv whereas stage ii quartz fis yield temperatures of 270 400 c peak value 350 c with salinities of 2 41 13 72 wt nacl equiv peak value 6 28 wt laser raman spectroscopy analyses of fi gas compositions indicate that the fis contain mainly h2o with minor co2 and n2 initial ore forming fluids comprise a medium high temperature low salinity system of hot evolved seawater that was cooled and diluted by mixing with cold seawater fluid mixing and local boiling would have facilitated mineralization srxrf analyses indicate that the liquid phase is enriched in cu and zn the 34s values of sulfide indicate that the sulfur was derived from the magma host volcanic rocks oxygen and hydrogen isotopic data also imply that the ore forming fluids were mainly derived from deep circulation of seawater and mixed with minor magmatic fluids the estimated trapping pressures for vein stockwork mineralization fluids range from 20 to 60 bar average 40 bar at a depth of 0 4 km the ore fluids ascenting along faults and fracture networks at or near the seafloor played important roles in oreforming processes of the huangtupo vms cu zn deposit

the evolution of crustal deformation in an oceanic extensional environment abstract fault statistics collected along the slow spreading reykjanes ridge are used to investigate its complex tectonic evolution the reykjanes ridge is oriented oblique to the spreading direction and is located close to the iceland hotspot a wide distribution in fault strike and fault throw occurs across the axis of the reykjanes ridge with a sharp change in mean strike and mean throw occurring at the edge of the axial zone within the axial zone there is no significant change in the distribution of faults with increasing distance from iceland proximity to the hotspot does not influence the detailed tectonics near the axis we infer that the fault development within the axial zone is controlled by the plate tectonic generated regional stress field outside the axial zone the faults strike oblique to the spreading normal direction this change in fault strike coupled with the lack of any evidence for strike slip movement suggests that these faults develop as a passive response to the thermal evolution of the lithosphere our results indicate that at a slow spreading ridge the crust is deformed by two stress fields one responsible for the fault formation within the axial zone the other for the faults which bound the axial zone

anisotropic structure under a back arc spreading region the taupo volcanic zone new zealand 1 we investigate anisotropy surrounding a continental back arc spreading region the central volcanic region cvr in the north island of new zealand and present a simple method for spatial averaging to examine heterogeneous anisotropy s phases from local earthquakes yield consistent trench parallel fast directions in the southern compressional region but varied directions in the northern extensional region in the forearc east of the cvr is nearly trench parallel suggesting shear in the mantle or trench parallel flow in the western cvr local phases from 50 to 80 km depth give parallel to extension 120 and phases from 150 to 220 km depth yield similar 150 these results suggest asthenospheric flow with olivine a axes oriented in the extension direction down to 100 150 km depth in the eastern cvr local phases show mixed suggesting that it is a transition between the western cvr and the forearc trench parallel for shallow local events within the cvr may be caused by fluid filled cracks trench parallel for deeper local events and for sks phases can be explained by standard olivine fabrics developed by lithospheric shearing by trench parallel flow and by fossil anisotropy in the subducting oceanic lithosphere alternatively in the cvr they could be caused in part by hydrous minerals at the slab mantle upper interface which could cause olivine a axes to orient parallel to the trench due to a maximum shear stress oriented perpendicular to the trench along the slab we suggest that trench perpendicular flow in the extensional region helps to drive fluids away from the slab while in the compressional region trench parallel flow fails to distribute the fluids explaining the location of changes in geophysical properties

seismic characterization of mantle flow in subduction systems can we resolve a hydrated mantle wedge this study provides new constraints on the resolvability of mantle flow in subduction zone settings as inferred by observations of seismic anisotropy we are motivated by the broad range of shear wave splitting observations in subduction systems that suggest complex flow geometries changes in the deformation state of mantle minerals or a combination of both while shear wave splitting fast polarization directions are typically interpreted as a proxy for flow or maximum finite extension experimental studies suggest that olivine slip systems change under higher stress and hydration states conditions likely appropriate for subduction systems in this study we predict shear wave splitting as a result of mantle silicate lattice preferred orientation development in steady state two dimensional mantle flow models using a textural development theory that incorporates the combined effects of intracrystalline slip and dynamic recrystallization we utilize the resulting textures to predict shear wave splitting for populations of seismic raypaths traversing the model within the subduction zone mantle wedge the results of our study demonstrate that combined observations of variations in fast polarization directions and splitting times make it possible to resolve a shift from anhydrous to hydrous mantle insubduction zone settings provided very good sampling of the mantle wedge our models are generally consistent with observed splitting variations for several subduction zones with dense data sampling including tonga japan and kamchatka the implications of our work suggest that provided adequate data sampling shear wave splitting measurements can provide the necessary information to evaluate potential competing effects between variations in mantle flow direction and changes in the stress and hydration states of subduction zone mantle wedges

hydrothermal anorthitization of plagioclase within the magmatic hydrothermal transition at mid ocean ridges examples from deep sheeted dikes hole 504b costa rica rift and a sheeted dike root zone oman ophiolite abstract the unique petrological record from the deep sheeted dikes of odp hole 504b gives insight into the magmatic hydrothermal transition and the hydrothermal reaction zone at mid ocean ridges a high temperature peak hydrothermal event at hole 504b defined as stage 1 corresponds to an early stage in the evolution of an axial reaction zone and perhaps to an initial high temperature phase of black smoker activity the temperatures and or the a ca a na 2 ratios of hydrothermal fluids overlapped and even exceeded those inferred for the reaction zones of many modern black smoker systems the volume of hydrothermally altered rock at this stage of evolution of the reaction zone was much less than 10 stage 1 is manifest in hand specimen by reaction halos adjacent to hornblende veins and by irregular alteration patches there is no epidote associated with stage 1 wallrock reactions involved the replacement of magmatic clinopyroxene by magnesiohornblende and of magmatic plagioclase by hydrothermal plagioclase these secondary minerals were not in equilibrium hydrothermal plagioclase varies in composition from an 54 to an 95 and has low mgo and feo tot it has distinctive bright cathodoluminescence and a porous appearance stage 1 at hole 504b occurred at pressures between 38 and 85 mpa 0 38 and 0 85 kbar and temperatures in excess of about 425 c these p t conditions closely approach the two phase field for seawater stage 1 is overprinted by retrograde alteration that caused further replacement within halos and patches as well as 10 40 alteration mostly to chlorite and albite of all the fresh sheeted dikes that were unaffected during stage 1 stage 1 alteration at hole 504b is unique among oceanic and ophiolitic samples however we report one area from the oman ophiolite that exhibits features similar to stage 1 at hole 504b early halos around hornblende veins contain secondary hornblende and porous hydrothermal calcic plagioclase an 55 an 78 with low mgo and feo tot fluid inclusions in quartz show that some early hydrothermal fluids had 5 6 2 3 wt nacl equivalent at temperatures constrained to be above 320 and below 510 c as well as a saline brine with 31 to 40 wt equivalent nacl and similar temperature constraints although evidence for stage 1 alteration of sheeted dikes is far from ubiquitous it is now identified in an oceanic setting hole 504b and in an ophiolitic example from oman

calcium rich brines and other hydrothermal fluids in fluid inclusions from plutonic rocks oceanographer transform mid atlantic ridge abstract fluid inclusions in metagabbro plagiogranite and epidosite from the oceanographer transform mid atlantic ridge were investigated using microthermometric measurements and laser raman spectroscopy inclusions in quartz reveal the presence of a high salinity average 40 wt nacl cacl 2 high temperature 340 c calcium rich na ca about 1 by weight fluid in metagabbro and plagiogranite at room temperature these fluids bear a halite daughter crystal and may also contain small crystals of calcite titanite and hematite a low density vapor rich fluid bearing a small amount of co 2 is closely associated with the briney fluids in one sample all samples contain a population of high temperature 150 to 400 c aqueous liquid dominated fluid inclusions commonly of secondary origin having seawaterlike salinity 3 to 8 wt nacl equivalent we conclude that the calcium rich brines are produced by a combination of two processes fluid phase separation and segregation of brines from vapor prior to trapping to generate the high salinities and extensive fluid mafic rock interaction at low water rock ratio to promote the calcium enrichment later introduction along microfractures of low salinity modified seawater at high water rock ratio is recorded by the secondary fluid inclusions the chemical complexity and association with co 2 may indicate that there is a magmatic component to the brines in addition to a seawater source differences between the oceanographer transform brines and those reported in previous studies of oceanic and ophiolite samples are attributed to variations in the degree of water rock interaction the host rock lithology and the influence of a magmatic fluid component

oceanwide gravity anomalies from geos 3 seasat and geosat altimeter data three kinds of satellite altimeter data have been combined along with 5 5 bathymetric data to calculate a 0 125 ocean wide gridded set of 2 3 106 free air gravity anomalies the procedure used was least squares collocation that yields the predicted anomaly and standard deviation the value of including the bathymetric data was shown in a test around the dowd seamount where the root mean square rms difference between ship gravity measurements decreased from 40 mgal to 20 mgal when the bathymetry was included comparisons between the predicted anomalies and ship gravity data is described in three cases in the banda sea the rms differences were 20 mgal for two lines in the south atlantic rms differences over lines of 2000 km in length were 7 mgal for cruise data in the antarctica region the discrepancies were 12 mgal comparisons of anomalies derived from the geosat geodetic mission data by marks and mcadoo 1992 with ship data gave differences of 6 mgal showing the value of the much denser geosat geodetic mission altimeter data

mantle anisotropy beneath northwest pacific subduction zones to assess the location strength and orientation of seismic anisotropy in the southern kuril japan and izu bonin subduction zones we analyzed shear wave splitting in local s phases and teleseismic sks phases fast directions from these phases are roughly parallel to the direction of absolute pacific plate motion wnw in izu bonin roughly parallel to the strike of the trench nne in central honshu and roughly n in the southern kuril back arc in the vicinity of sakhalin island assuming that the orientation and strength of anisotropy does not vary with depth modeling of splitting times from local s and teleseismic phases recorded at sakhalin island requires that the maximum depth of anisotropy lies between 480 km and 950 km in contrast splitting times for local s phases from events in izu bonin rule out anisotropy in the transition zone all the data are consistent with a model in which the lower transition zone 520 660 km and lower mantle are largely isotropic and in which anisotropy occurs intermittently in the upper transition zone 410 520 km possibly due to preferred orientation of spinel assuming that splitting in the upper mantle is produced by preferred orientation of olivine the observed fast directions indicate that the geometry of back arc strain varies systematically between subduction zones the relationship of fast directions and plate motions suggests that the subducting slab exerts significant control on back arc flow but that flow correlated with upper plate deformation must also exist

multiple plume events in the genesis of the peri caribbean cretaceous oceanic plateau province the oceanic crust fragments exposed in central america in north western south america and in the caribbean islands have been considered to represent accreted remnants of the caribbean colombian oceanic plateau ccop on the basis of trace element and nd sr and pb isotopic compositions we infer that cumulate rocks basalts and diabases from coastal ecuador have a different source than the basalts from the dominican republic the latter suite includes the 86 ma basalts of the duarte complex which are light rare earth element ree enriched and display relative to normal mid ocean ridge basalts nmorb moderate enrichments in large ion lithophile elements together with high nb ta pb and low th contents moreover they exhibit a rather restricted range of nd and pb isotopic ratios consistent with their derivation from an ocean island type mantle source the composition of which includes the himu high 238u 204pb component characteristic of the galapagos hotspot in contrast the 123 ma ecuadorian oceanic rocks have flat ree patterns and relative to nmorb are depleted in zr hf th and u moreover they show a wide range of nd and pb isotopic ratios intermediate between those of ocean island basalts and nmorb it is unlikely on geochemical grounds that the plume source of the ecuadorian fragments was similar to that of the galapagos in addition because of the nne motion of the farallon plate during the early cretaceous the ecuadorian oceanic plateau fragments could not have been derived from the galapagos hotspot but were likely formed at a ridge centered or near ridge hotspot somewhere in the se pacific

the origin and evolution of the mexican cordillera as registered in modern detrital zircons abstract continents are thought to be built at convergent margins by mantle derived mafic inputs but the processes that transform them into more felsic and long lasting constituents are still unclear here we use u pb geochronology trace elements and hf isotopic compositions of modern detrital zircons from western mexico to illustrate how a typical mafic island arc built on an oceanic basin can be rapidly transformed into a mature intermediate to felsic continent by the combined effects of subduction erosion and rear arc sediment underthrusting prompted by the closure of a back arc basin in western mexico the most significant mass transfer from mantle to crust occurred during the early cretaceous in the context of an intra oceanic island arc locally dubbed alisitos guerrero subsequently the process of continentalization was achieved by the deep reprocessing of the newly formed lithologies once they were dynamically incorporated into the melting source during the development of the mexican orogen the mexican continental crust did not only grow by arc related inputs however as there was at least one magmatic episode in the jurassic where extensional driven mantle upwellings triggered melting of upper crustal sediments in this view mafic island arcs and back arc basins are ephemeral geologic features that can be easily consumed into trenches but the geochemical evidence of their existence and possibly even a significant portion of their mass can be preserved in younger and more felsic continental igneous rocks

shear wave splitting and mantle flow associated with the deflected pacific slab beneath northeast asia 1 a total of 361 sks and five local s wave splitting measurements obtained at global and regional seismic network stations in ne china and mongolia are used to infer the characteristics of mantle fabrics beneath northeast asia fast polarization directions at most of the stations in the western part of the study area are found to be consistent with the strike of local geological features the dominant fast directions at the eastern part beneath which seismic tomography and receiver function studies revealed a deflected slab in the mantle transition zone mtz are about 100 from north which are almost exactly the same as the motion direction of the eurasian plate relative to the pacific plate and are independent of the direction of local geological features the splitting times at those stations are about 1 s which correspond to a layer of about 150 km thickness with a 3 anisotropy the shear wave splitting observations complemented by the well established observation that most of the eastern part of the study area is underlain by a lithosphere thinned by delamination in the paleozoic era can be best explained by the preferred alignment of metastable olivine associated with the subduction of the deflected pacific slab in the mtz or by back arc asthenospheric flow in the mantle wedge above the slab

salinity of oceanic hydrothermal fluids a fluid inclusion study abstract an extensive microthermometric study of quartz epidote plagioclase anhydrite and sphalerite hosted fluid inclusions from ophiolitic semail oman and trinity california ophiolites and oceanic east pacific rise hydrothermal vents gorringe bank odp leg 111 hole 504b crust has been carried out in order to constrain a model accounting for wide salinity variations measured in the oceanic hydrothermal fluids recorded salinities in fluid inclusions vary between 0 3 and 52 wt nacl eq however more than 60 of the mean standard deviation salinities of the samples are within the range 3 2 0 3wt nacl eq microthermometric error and the mean salinity of all fluid inclusions without the brines is 4 0 wt nacl eq with a standard deviation of 1 6 wt nacl eq whereas most samples display slightly higher salinities than seawater several samples exhibit very high salinities more than two times that of seawater these high salinities are restricted to the plagiogranites semail and trinity ophiolites which mark the top of the fossil magma chamber in the transition zone between the plutonic sequence and the sheeted dyke complex the fluid inclusion population studied in the plagiogranites is characterized by the occurrence of four major fluid inclusion families 1 low to medium salinity liquid vapor fluid inclusions which homogenize into the liquid phase 2 low salinity liquid vapor fluid inclusions with pseudocritical homogenization 3 low to medium salinity liquid vapor fluid inclusions which homogenize into the vapor phase and 4 high salinity liquid vapor halite fluid inclusions which homogenize into the liquid phase by halite dissolution and exhibit salinities as high as 52 wt nacl eq these fluid inclusion families are interpreted as resulting from phase separation occurring in hydrothermal or magmatic fluids within the transition zone between the hydrothermal system and the magma chamber at temperatures higher than 500 c very low salinities less than half that of seawater have been found only in one dioritic sample from the gorringe bank mineralogical and petrological data from the semail ophiolite shows the absence of cl rich minerals furthermore hydration of the crust although very important cannot account for the large salinity variations thus the large salinity variations measured are interpreted as resulting from phase separation and generation of brine rich fluids at the top of the magma chamber this phase separation and subsequent phase segregation and brine accumulation may result in a double diffusive convection of the fluids in the hydrothermal system

mineral assemblages of the francisco i madero zn cu pb ag deposit zacatecas mexico implications for ore deposit genesis abstract the francisco i madero deposit central mexico occurs in the mesozoic guerrero terrane which hosts many ore deposits both cretaceous volcanogenic massive sulfides and tertiary epithermal and skarn deposits it is hosted by a 600 m thick calcareous pelitic unit of lower cretaceous age crosscut by porphyritic dikes that strike nw se a thick felsic volcanic tertiary sequence consisting of andesites and rhyolitic ignimbrites unconformably overlies the cretaceous series at the base the mineralization consists of several mantos developed within calcareous beds they are dominantly composed of sphalerite pyrrhotite and pyrite with minor chalcopyrite arsenopyrite and galena at the top of the orebody there are calcic skarns formed through prograde and retrograde stages the resulting mineral assemblages are rich in manganoan hedenbergite hd 75 28 di 40 4 jh 40 20 andraditic garnets adr 100 62 grs 38 0 epidote ep 95 36 czo 60 5 pie 8 0 chamosite calcite and quartz the temperature of ore deposition estimated by chlorite and arsenopyrite geothermometry ranges from 243 to 277 c and from 300 to 340 c respectively the pressure estimated from sphalerite geobarometry averages 2 1 kbar this value corresponds to a moderately deep skarn and agrees with the high cu content of the deposit paragenesis p t conditions and geological characteristics are compatible with a distal dike related zn skarn deposit its style of mineralization is similar to that of many high temperature carbonate replacement skarn deposits in the southern cordillera

melting behavior of fluid inclusions in laboratory grown halite crystals in the systems naclh2o naclkclh2o naclmgcl2h2o and naclcacl2h2o abstract the chemical composition of aqueous fluid inclusions in crystals of halite can be accurately determined from observed melting behaviors of ice hydrohalite and sylvite some fluid inclusion melting behaviors observed in laboratory grown halite crystals systems nacl h 2 o nacl kcl h 2 o nacl mgcl 2 h 2 o and nacl cacl 2 h 2 o differ from predicted stable equilibrium relations in the nacl h 2 o and nacl kcl h 2 o systems observed first melt temperatures are up to 15 c below the equilibrium eutectic temperatures of 21 2 and 22 9 c respectively the final melting temperature of ice in the presence of hydrohalite and the final melting temperature of hydrohalite are reproducible and match predicted melting temperatures the limit of detection of sylvite daughter crystals in the nacl kcl h 2 o system is approximately 5 wt 1 molal kcl final melting temperatures of sylvite match published equilibrium data to within 0 3 c in the nacl kcl h 2 o system at halite saturation m kcl can be determined from the final sylvite dissolution temperature or from the final melting temperature of hydrohalite fluid inclusions in the nacl mgcl 2 h 2 o and nacl cacl 2 h 2 o systems that form stable salt hydrates mgcl 2 12h 2 o and cacl 2 6h 2 o during freezing first melt within 3 c of predicted eutectic temperatures 37 and 52 c however fluid inclusions with mgcl 2 or cacl 2 may also start melting at temperatures as low as 80 c such low first melt temperatures indicate the presence of metastable salt hydrates presumably mgcl 2 8h 2 o mgcl 2 6h 2 o or cacl 2 4h 2 o the formation of metastable phases during freezing of fluid inclusions can lead to misinterpretation of the chemical composition of fluid inclusions in natural samples this is especially true for fluid inclusions with first melt temperatures below 37 c which may be erroneously interpreted as being rich in cacl 2 the final melting of ice in the presence of hydrohalite may vary by more than 15 c in fluid inclusions of different size but identical bulk composition and occurs at lower temperatures than predicted in fluid inclusions from the nacl mgcl 2 h 2 o and nacl cacl 2 h 2 o systems however the final melting temperature of ice in inclusions which fail to nucleate hydrohalite and the final melting temperature of hydrohalite are reproducible to within 0 1 c and can be used to determine mgcl 2 and cacl 2 molalities

speculative tectonic model for the early mesozoic arc of the southwest cordilleran united states the early mesozoic continental arc of arizona california and western nevada is here interpreted to have occupied a continuous graben depression more than 1000 km long similar to the modern extensional are of central america high calculated rates of subsidence the presence of syndepositional normal faults and fault talus breccias the abundance of calderas and high level intrusions and interstratification of cale alkalic and alkalic volcanic rocks provide support for this model the arc graben depression acted as a long lived more than 40 m y trap for craton derived quartz sands which were funneled along the length and across the width of the depression

processes of brine generation and circulation in the oceanic crust fluid inclusion evidence from the troodos ophiolite cyprus detailed temporal thermal and compositional data on aqueous fluid inclusions from a suite of plutonic and diabase samples from the troodos ophiolite cyprus provide the first documentation that generation of high temperature brines may be common at depth in the oceanic crust anastomosing arrays of fluid inclusions in rocks of the upper intrusive sequence record episodic fracturing events the earliest fracturing event at temperatures 450 600 c resulted in entrapment of brine rich aqueous fluids with salinities of 36 61 wt nacl equivalent homogenization of the brine inclusions by haute dissolution the virtual absence of vaporrich fluid inclusions throughout the upper level plutonic sequence and the restriction of brine inclusions to the most evolved plutonic rocks suggests that exsolution of brines off of the late stage gabbro and plagiogranite melts played a significant role in generating the quartz hosted high salinity inclusions cooling of the fluids during pulses of fluid migration associated with episodic fracturing events resulted in entrapment of the brines in the deep seated high temperature portion of the hydrothermal system in localized areas the high temperature brines nacl kcl cacl2 caused extreme alteration of the plagiogranite bodies and in the formation of podiform epidosites arrays of low temperature low salinity fluid inclusions which in some samples crosscut fractures dominated by brine inclusions indicate downward propagation of a cracking front subsequent to collapse of the high temperature magmatic system resulting in penetration of seawaterlike fluids into the plutonic sequence at temperatures 200 400 c hydration reactions under greenschist facies conditions or limited mixing with brine rich fluids may have resulted in salinity variations from 70 below to 200 above seawater concentrations temperatures and compositions of the low salinity inclusions are similar to those found in stockwork systems beneath troodos ore deposits and to those of fluids exiting active submarine hydrothermal vents at mid ocean ridge spreading centers the low temperature fracture networks may represent an extensive deep seated feeder system which coalesced to form zones of concentrated hydrothermal upflow