the 8 2 ka event from greenland ice cores we present a collection of high resolution chemistry and stable isotope records from the plateau of the greenland ice cap during the cold event 8200 yr ago using a composite of four records the cold event is observed as a 160 5 yr period during which decadal mean isotopic values were below average within which there is a central event of 69 yr during which values were consistently more than one standard deviation below the average for the preceding period four cores in north south and central greenland show differences at decadal and shorter timescales it is not yet clear if this represents significant spatial differences in response the results show clear evidence for colder temperatures and a decrease in snow accumulation rate however the changes in chemical concentrations for the ions looked at here are small suggesting only minor changes in atmospheric circulation for this event apart from the decrease in methane concentration greenland ice cores give only weak evidence for effects outside the north atlantic region

the accretion of oceanic crust by episodic sill intrusion seismic refraction data from mature oceanic regions show remarkable consistency in crustal thickness between sites yet the characteristics of different spreading centers show great variation this leads to the paradox that the similarity in mature oceanic crust suggests that the igneous processes in crustal accretion are the same across a range of spreading rates but that the differences in the spreading centers themselves imply the opposite we have developed a simple two dimensional model for crustal accretion at oceanic spreading centers by the episodic addition of sills at a high level within the crust as implied by recent geophysical studies of spreading centers using our model we have calculated velocity and temperature fields in the crust including the effects of hydrothermal cooling we show that oscillations in the temperature field due to episodicities of 200 20000 years have an effect localized to the region within 50 500 m of the intrusion by assuming that all latent heat and excess specific heat introduced by the injection of sills is convected away by hydrothermal circulation we estimate that the maximum heat transfer associated with hydrothermal cooling in the upper crust is of the order of 10 times greater than the conductive flux we have calculated seismic velocities associated with the model temperature field in the crust and we show that these are consistent with results from both seismic refraction and reflection experiments the model may be applied to both fast and slow spreading ridges and it accounts for the apparent increases in both the mean crustal temperature and the frequency of presence of melt with increasing spreading rate hence resolving the paradox we also show that the implications of this model are consistent with geological observations of layering in ophiolites

progressive alteration associated with auriferous massive sulfide bodies at the dumagami mine abitibi greenstone belt quebec pyritite and massive sphalerite galena bodies are surrounded by two distinct concentric alteration zones within the metafelsites of the late archean blake river group at the dumagami au ag cu deposit southern abitibi belt northwestern quebec the inner alteration zone is characterized by andalusite bearing rocks and is interpreted as a metamorphosed advanced argillic alteration zone the outer zone is represented by white mica phyllonite and phyllonitic breccia and is interpreted as a metamorphosed sericitic alteration zone surrounding least altered rocks represented by massive quartziferous augen rocks and ribboned micaceous augen schists are composed of blue quartz and albite phenocrysts quartz biotite ilmenite and chlorite the sericite zone is characterized by the metamorphic mineral assemblage quartz sericite rutile pyrite or andesine and by the absence of albite phenocrysts biotite and ilmenite the andalusite zone is characterized by the metamorphic mineral assemblage quartz andalusite sericite rutile pyrite or kyanite and by the absence of albite phenocrysts andesine porphyro blasts biotite and ilmenite petrographic and geochemical features of least altered sericitic and andalusite bearing rocks indicate that felsic effusive rocks were the principal protolith strong leaching of alkali and alkaline earths especially mg within the inner alteration zone has resulted in a notable absence of chlorite a feature atypical of the alteration associated with archean massive sulfide deposits chemical and mineralogical changes from least altered to altered rocks suggest that highly acidic hydrothermal fluids were responsible for the premetamorphic formation of alteration assemblages at the dumagami deposit however gold transport in these fluids would be minimal and gold emplacement is linked instead to a late structural event

the recent history of the galapagos triple junction preserved on the pacific plate abstract at the galapagos triple junction the cocos and nazca plates are broken by a succession of transient rifts north and south of the cocos nazca c n rift modeling has suggested that each rift initiated at the east pacific rise epr its location controlled by the distance of the c n rift tip from the epr evidence on the pacific plate confirms that each transient rift formed a true rrr triple junction with the epr and clarifies the history of the region at 1 5 ma the triple junctions began jumping rapidly toward the c n rift suggesting that the c n rift tip moved closer to the epr pacific abyssal hills became broad and shallow indicating enhanced magma supply to the region at 1 4 ma the galapagos microplate developed when extension became fixed on the southern transient rift to form the south scarp of the future dietz rift basin lavas flooded the area and a galapagos nazca magmatic spreading center initiated at the epr we suggest that a hotspot was approaching the southern triple junction from the west the hotspot crossed to the nazca plate 1 25 ma dietz seamount formed within the young spreading center dikes intruded dietz rift basin and eruptions built volcanic ridges since 0 8 ma magmatic spreading has jumped northward twice most recently to dietz volcanic ridge amagmatic extension to the east has formed the large north scarp of dietz rift basin northward jumping of the southern triple junction has maintained the microplate boundary close to the proposed hotspot

orbitally driven insolation forcing on holocene climate trends evidence from alkenone data and climate modeling a global spatial pattern of long term sea surface temperature sst trends over the last 7000 years is explored using a comparison of alkenone derived sst records with transient ensemble climate simulations with a coupled atmosphere ocean circulation model under orbitally driven insolation forcing the spatial trend pattern both in paleo sst data and in model results shows pronounced global heterogeneity generally the extratropics cooled while the tropics experienced a warming during the middle to late holocene we attribute these divergent holocene climate trends to seasonally opposing insolation changes furthermore climate mode changes similar to the arctic north atlantic oscillation are superimposed on the prevalent pattern it is concluded that nonlinear changes in the entire seasonal cycle of insolation played a dominant role for the temporal evolution of holocene surface temperatures for understanding of marine proxy data apart from the dominance of summer insolation in high latitudes a notable shift in the maximum insolation of the year in low latitudes has to be taken into account which may influence timing of phytoplankton production and thus alters the seasonal origin of temperature signals recorded in the proxies

a new mg in plagioclase geospeedometer for the determination of cooling rates of mafic rocks abstract we present a new mg in plagioclase geospeedometer based on the diffusive exchange of mg between plagioclase and clinopyroxene that allows cooling rates of a wide range of mafic rocks to be determined the major element composition of plagioclase is shown to play an important role in driving the flux of mg as well as the partitioning of mg between plagioclase and clinopyroxene therefore commonly used analytical solutions to the diffusion equation are not applicable in this system and an alternate approach is developed practical aspects of the method such as the role of anorthite zoning patterns and influence of shapes of cooling paths and grain geometries in controlling the mg concentration profile shapes are discussed propagation of uncertainties in the knowledge of diffusion and partition coefficients shows that the method is capable of resolving small differences in cooling rates e g log d t d t c yr 1 3 0 3 in one example the approach is illustrated by application to two samples from the lower oceanic crust

rapid crustal accretion and magma assimilation in the oman u a e ophiolite high precision u pb zircon geochronology of the gabbroic crust 1 new high precision u pb zircon geochronology from the oman united arab emirates u a e ophiolite provides insight into the timing and duration of magmatism and the tectonic setting during formation of the lower crust the new data come from a well preserved and exposed crustal section in the center of the wadi tayin massif single grain and grain fragment 206pb 238u dates from upper level gabbros tonalites trondhjemites and gabbroic pegmatites corrected for initial th exclusion range from 112 55 0 21 to 95 50 0 17 ma with most data clustered between 96 40 0 17 to 95 50 0 17 ma zircon dates from upper level gabbros are most consistent with the ophiolite forming at a fast spreading ridge with half rates of 50 100 km ma dates from tonalites trondhjemites and from a gabbroic pegmatite associated with a wehrlite intrusion overlap with dates from adjacent upper level gabbros suggesting that any age differences between these three magmatic series are smaller than the analytical uncertainties or intrasample variability in the dates three of the dated upper level gabbros and a single gabbroic pegmatite from the base of the crust have 1 ma intrasample variability in single grain dates suggesting assimilation of older crust during the formation or crystallization of the magmas whole rock end t of seven samples including the upper level gabbros with variable zircon dates have tightly clustered initial values ranging from end 96 ma 7 59 0 23 to 8 28 0 31 the end values are similar to those from other gabbros within the ophiolite suggesting that any assimilated material had a similar isotopic composition to primitive basaltic magmas the new dates suggest that the studied section formed at a fast spreading mid ocean ridge between 96 4 95 5 ma the large intrasample variability in zircon dates in some samples is unexpected in this setting and may be related to propagation of a younger ridge into older oceanic lithosphere

chromite geochemistry and pge fractionation in the campo formoso complex and ipueira medrado sill bahia state brazil two districts of stratiform chromite deposits are situated near the northern margin of the sao francisco craton the ultramafic host rocks are remnants of early precambrian layered complexes that intruded archean gneissic blocks prior to transamazonian collisional events the intrusions contain abundant chromite exhibit orthopyroxene rich sequences and are similar geochemically with highly magnesian bulk compositions mg mg fe 0 91 0 67 and fractionation trends consistent with komatiitic parent magmas the intrusion form alteration history and current metamorphic grade of each are distinct at the ipueira medrado mines representative of the jacurici valley district a folded segment of a 200 to 250 m thick ultramafic mafic sill contains a 5 to 8 m thick highly massive chromitite layer green spinel in the orthopyroxene rich peridotite and ubiquitous granoblastic textures indicate recrystallization during upper amphibolite or granulite facies metamorphism now with incomplete retrograde alteration and serpentinization in campo formoso a deeply eroded stratiform complex contains at least seven different economic chromite seams comprising varying proportions of disseminated net textured layered and massive ores here greenschist alteration of the ultramafic host rocks is complete apart from orthopyroxene relicts in an upper unit at the cascabulhos mine microprobe studies of chromite rich specimens from representative stratigraphic sections of each intrusion suggest that unusually low tio2 levels 0 9 wt combined with high cr2o3 contents 45 55 wt reflect the primitive parental magma composition in both intrusions core compositions of chromite from cascabulhos retain high xcr 0 57 0 72 but show a wide range of xmg 0 21 0 47 consistent with fe mg exchange at grades at or below amphibolite facies some relict igneous fractionation is preserved since tio2 increases 0 2 0 4 wt while cr al or xcr decreases 0 70 0 55 from the massive chromitites hosted by serpentinite through the disseminated seams above to the two high level seams in pyroxenite chromite from ipueira medrado is more magnesian xmg 0 57 0 64 but also more aluminous xcr 0 60 0 64 when massive types are compared here the submassive minor seams stratigraphically above and below the main seam both have relatively lower xmg and xcr 0 36 0 45 and 0 45 0 57 this suggests that combined cr al and mg fe exchange occurred between chromite and silicates during postemplacement progressive metamorphism of the seams close spaced sampling of representative drill core sections at 2 or 4 m intervals has allowed the precious metal potential of each intrusion to be assessed in both cases the highest total platinum group element pge concentrations occur exclusively in chromitite illustrating the well known phenomenon of chromite control the ipueira medrado chromitite contains lower concentrations 350 ppb and has unfractionated pge patterns chalcophile element analyses provide little evidence of precious metal scavenging by immiscible sulfide segregation and so indicate little potential for mineralization elsewhere in the sill at campo formoso the alternation of pge enriched chromitite and barren silicates is suggestive of repeated influxes of primitive magma during open system fractionation an increase in the pt pd and rh tenor of the chromitite to ca 1 ppm occurs from the lower massive group through the upper disseminated group to the high level seams hosted by pyroxenite thus upward evolution of the host chromite mineral compositions is matched by increasing pt pd ru os ir ru ratio the covariation of pge ratios with chromite mineral chemistry is analogous to that occurring over the sequence of chromitite seams in the lower critical zone of the bushveld complex this provides evidence for a fundamental link in the fractionation processes controlling chromite and pge mineralization in stratiform intrusions which has clear implications for exploration models

last deglaciation in the okinawa trough subtropical northwest pacific link to northern hemisphere and tropical climate 1 detailed deglacial and holocene records of planktonic delta o 18 and mg ca based sea surface temperature sst from the okinawa trough suggest that at similar to 18 to 17 thousand years before present kyr b p late spring early summer ssts were approximately 3 degrees c cooler than today while surface waters were up to 1 practical salinity unit saltier these conditions are consistent with a weaker influence of the summer east asian monsoon eam than today the timing of suborbital sst oscillations suggests a close link with abrupt changes in the eam and north atlantic climate a tropical influence however may have resulted in subtle decoupling between the north atlantic and the okinawa trough eam during the deglaciation okinawa trough surface water trends in the holocene are consistent with model simulations of an inland shift of intense eam precipitation during the middle holocene millennial scale alternations between relatively warm salty conditions and relatively cold fresh conditions suggest varying influence of the kuroshio during the holocene

radiocarbon dating and the reconstruction of late quaternary sea level changes in hong kong abstract radiocarbon dating is used widely for the reconstruction of late quaternary sea level changes in the present study eight case studies in hong kong have been chosen to highlight the difficulty in obtaining reliable radiocarbon dates pre holocene radiocarbon dates exceeding about 8100 yr bp have been found to show a young age bias a shell sample dated by two separate radiocarbon laboratories gave last interstadial ages differing by almost 9500 years while an oxygen isotope stage 5e age was obtained using uranium series dating other evidence including stratigraphy have shown that only the last interglacial age is acceptable the young age bias is attributed mainly to the post depositional introduction of young carbon into the sample for the reconstruction of sea level changes it is essential to select samples immediately above of the base of the holocene transgressive unit in offshore boreholes and excavations to satisfy the criteria of rapid burial these samples should preferably include mud plant matter and shells to facilitate interpretation furthermore as many samples as possible from a range of water depth down to about 130 m below the present sea level should be dated to reduce error introduced by counting statistics so the true signal of sea level changes can be reflected

a mid ocean ridge thermal model constraints on the volume of axial hydrothermal heat flux the depth to the axial magma chamber at three oceanic spreading centers as indicated by seismic reflection surveys is greater than that predicted by conductive cooling thermal models of ridge crests the additional cooling at the ridge axis is owed to the circulation of seawater through the shallow crust a theoretical model for the temperature distribution at spreading centers which includes distributed heat sources and sinks is presented by representing the hydrothermal heat loss as a series of heat sinks the depths to the axial magma chamber at the east pacific rise at latitude 9 n the southern juan de fuca ridge and the lau basin spreading center are modeled heat sinks are added to the input model to increase the solidus depth until the computed magma chamber depth matches the magma chamber depth determined by seismic reflection surveys the thermal modeling indicates that the axial hydrothermal heat flux is between 7 1 106 and 13 8 106 mj m2 or 10 to 20 of the total missing heat at the ridge axis requiring extensive low temperature circulation off axis high temperature vents that are spaced 1 km apart and are active between 4 and 10 of the time would account for the axial hydrothermal heat loss

the roof of an axial magma chamber a hornfelsic heat exchanger the heat flux from an axial magma chamber into the convecting hydrothermal system at mid ocean ridges is governed by the conductive boundary layer separating them the nature of this critical interface has been modeled and seismically imaged but its characteristics have hitherto rarely been documented in ocean crust here hornfelsic rocks from the sheeted dike gabbro transitions at two tectonic exposures of fast spreading east pacific rise crust at the pito and hess deeps and in the oman and troodos ophiolites are described these rocks record thermal metamorphism to hornblende and pyroxene hornfels at 700 1000 c these temperatures in conjunction with their geological relationships imply that the hornfels are preserved fragments of conductive boundary layers their distribution at the base of sheeted dike complexes in narrow contact aureoles and dikes intruding the uppermost gabbros tracks the vertical migration of axial magma chambers over minimum depth ranges of 200 400 m the heat flux across the hornfelsic aureoles 11 44 mw km is comparable to hydrothermal fluxes released along fast spreading ridges confirming that the heat driving hydrothermal convection is transferred across conductive boundary layers

where are the eburnian transamazonian collisional belts the reconstruction of early proterozoic crustal evolution and geodynamic environments in africa and south america is incomplete if cratonic areas alone are studied if the presence of high grade

the itiuba alkaline syenite massif bahia state brazil mineralogical geochemical and petrological constraints relation to the genesis of rapakivi magmatism abstract numerous alkaline massifs occur throughout the state of bahia brazil isotopic dates fall into two age groups brasiliano 0 45 0 70 ga and trans amazon 1 8 2 1 ga ones brasiliano alkaline provinces comprise a silica undersaturated association with related volcanic rocks their emplacement is always controlled by fault zones trans amazon alkaline massifs are characterized by large plutons more than 100 km2 of k rich syenite and granite associated with mafic cumulates and abundant dyke swarms no associated volcanic rocks have been so far recorded the shape of the plutons varies as a function of their location within the sao francisco craton in its northeastern and southern parts syenite massifs are elongated trending n s while in its western part they are emplaced as circular bodies displaying contact metamorphic aureoles the presence of older archaean alkaline rocks cannot be ruled out as some granulitic facies display syenite compositions the itiuba massif provides a good example of proterozoic alkaline syenite located in the northern part of the sao francisco craton this 150 km long pluton covers 1800 km2 in area whole rock rb sr isotopic data yield a lower proterozoic age a n s trending foliation at the margins is gradually replaced by isotropic textures toward the core two sets of faults both of trans amazonian age have been defined n s trending reverse faults accompanied by intense mylonitization and younger nw trending arcuate transcurrent faults related to a ne sw compressive regime alkaline syenites constitute 98 of the exposures and are medium to coarse grained clinopyroxene amphibole hypersolvus leucratic rocks cumulates are represented by mafic layers and clinopyroxene apatite enclaves dykes are composed of alkaline syenite hypersolvus and transsolvus quartz syenites and alkaline granites syenites are metaluminous and rich in k mg p ti and ba and their geochemical trends are controlled by alkali feldspar clinopyroxene apatite and fe ti oxide fractionation oxidizing conditions are reflected by the weak variation in the mg ratio the reverse fe mg mineral zonation the synchronous precipitation of oxides and clinopyroxene in mafic layers and the late development of uralitic amphibole and low ti phlogopite temperatures for oxide equilibration of 930 880 c are compatible with the hypersolvus feldspar mineralogy the evolution of feldspar mineralogy is a good indicator of varying thermodynamical conditions during late stage crystallization of the pluton minimum values for initial crystallization temperature have been evaluated at 950 c structural and petrological results indicate that the presently exposed syenite massif represents the roof of a proterozoic magma chamber emplaced in granulite gneiss formations rapakivi magmatism may represent disrupted and floating portions of the roof of earlier magma chambers refilled by new syenite granite melts

network of off axis melt bodies at the east pacific rise at faster spreading mid ocean ridges the creation of new oceanic crust through magmatism usually occurs within a narrow zone on the ridge axis three dimensional seismic images of the fast spreading east pacific rise reveal a network of magmatic bodies 4 8 km away from the ridge axis that seem to be connected to the axial magma chamber

the development of late holocene coastal cooling in the northern south china sea abstract paired high resolution sea surface temperature sst records based on the long chain alkenone unsaturation index u 37 k were reconstructed to decipher the sst evolution over the past 8000 years and its possible driving mechanisms at the hong kong coastal region the two sst records derived from a sediment core in the pearl river estuary and an offshore core in the northern south china sea scs show different patterns of sst changes the coastal record shows relatively stable warm temperatures during the mid holocene 7 4 2 6 ka and substantial cooling 2 c thereafter the cooling was most pronounced in the past 0 6 ka likely associated with the little ice age lia the offshore record generally reveals a warming trend from 8 ka to 3 5 ka and relatively warm temperatures since then as a result the sst difference sst between the two sites increased over time suggesting that the sst gradient gradually developed in the northern scs the declining coastal sst with the increasing sst likely indicates and results from the intensified east asian winter monsoon since 7 ka especially during the late holocene consistent with previous terrestrial records revealing winter monsoon variability and modern observations sst between coastal and offshore sites could thus be used as an indicator of winter monsoon variability

near conductive cooling rates in the upper plutonic section of crust formed at the east pacific rise abstract a new geospeedometer based on diffusion modeling of mg in plagioclase is used to determine cooling rates of the upper section of the lower oceanic crust formed at fast spreading mid ocean ridges the investigated natural sample suites include gabbroic rocks formed at three different locations along the fast spreading east pacific rise these samples cover a depth interval of 0 840 m below the sheeted dike gabbro boundary and therefore allow the variation of cooling rate as a function of depth within the upper plutonic sequence to be determined we demonstrate that the cooling rates we obtained are robust reproducible and consistent across different vertical sections at fast spreading ridges and decrease significantly with increasing sample depth covering almost 4 orders of magnitude ranging from 1 c y 1 for the shallowest samples to 0 0003 c y 1 for the deepest samples both the absolute cooling rates and the rate of decrease of cooling rate with depth are consistent with conductive thermal models in contrast the absolute cooling rates determined from the deeper samples 300 m below dgb and the large decrease in cooling rate with depth are inconsistent with thermal models that include substantial cooling by off axis hydrothermal circulation within the upper plutonic section of the crust

archean foreland basin tectonics in the witwatersrand south africa the witwatersrand basin of south africa is the best known of archean sedimentary basins and contains some of the largest gold reserves in the world sediments in the basin include a lower flysch type sequence and an upper molassic facies both of which contain abundant silicic volcanic detritus the strata are thicker and more proximal on the northwestern side of the basin which is at least locally bound by thrust faults these features indicate that the witwatersrand strata may have been deposited in a foreland basin and a regional geologic synthesis suggests that this basin developed initially on the cratonward side of an andean type arc remarkably similar phanerozoic basins may be found in the southern andes we suggest that the continental collision between the kaapvaal and zimbabwe cratons at about 2 7 ga caused further subsidence and deposition in the witwatersrand basin regional uplift during this later phase of development placed the basin on the cratonward edge of a collision related plateau now represented by the limpopo province similarities are seen between this phase of witwatersrand basin evolution and that of active basins north of the tibetan plateau e g the tarim and tsaidam basins the geologic evidence does not appear so compatible with earlier suggestions that the witwatersrand strata were deposited in a rift or half graben

the geologic evolution of the ancient granite greenstone terrane of central southern bahia brazil abstract the granite greenstone terrain of central southern bahia belongs to the sao francisco craton and is a key area for the understanding of the archaean and lower proterozoic of south america in addition it is in an area of great metallogenic potential the granitic migmatitic gneissic basement complex is intimately associated with low to medium grade metasedimentary and metavolcanic sequences such as the brumado umburanas and contendas mirante in all of these sequences the metasedimentary rocks predominate although acid metavolcanics constitute a significant fraction of the contendas sequence and ultrabasic rocks occur in several isolated units early archaean granitoid rocks rb sr isochron ages older than 3 ga occur in some regions such as mata verde boa vista lagoa do morro and sete voltas extensive granitic intrusive activity with associated medium grade metamorphism and metasomatism occurred in most of the area 2 7 ga ago and the resulting terrain is taken to be the basement at least for the contendas mirante sequence the relationships to the brumado and umburanas metamorphic complexes are not clear and these sequences may be of archaean age the contendas mirante sequence was affected by strong lower proterozoic deformation 2 ga ago and contemporaneous granitic intrusions like the gameleira granite large domal structures interpreted as mantled gneiss domes such as those associated with the boa vista and sete voltas granitoid rocks also formed at this time a late proterozoic to early palaeozoic thermal event affected the southwestern part of the region at 0 5 0 7 ga ago as revealed by mica k ar ages

constraints on long lived mesoproterozoic and palaeozoic deformational events and crustal architecture in the northern mount painter province australia abstract meta sedimentary and igneous rocks preserved in the hanging wall of the paralana fault northern mount painter province record a protracted ca 1600 myr multi staged metamorphic and poly deformational evolution related to mesoproterozoic and palaeozoic orogenic events the mount painter province was located near the margin of both the south australian craton and the north australian craton which allows comparisons of the tectonic evolution of these margins and provides context for the evolution of the eastern proterozoic australia for both the columbia and gondwana supercontinents within the mount painter province meta sedimentary rocks form part of a basin system developed adjacent to the margins of the gawler craton and curnamona province following the olarian wartakan orogenic system sedimentation was followed by ca 1590 1552 ma ductile deformation d 1 d 3 at mid to upper crustal levels preserving a rapid ca 6 myr burial and exhumation cycle initial deformation is characterised by the development of a layer parallel fabric s 1 s 1 is overprinted by recumbent north verging folds f 2 formed during mid to upper amphibolite facies peak metamorphism m 2 which are overprinted by tight upright shallow ne sw plunging folds f 3 d 3 folds developed contemporaneous with sinistral oblique shearing and the development of proto mylonitic to ultra mylonitic fabrics along the paralana fault zone the timing of d 3 deformation is constrained by the development of a penetrative steeply dipping foliation s 3 in the ca 1585 1569 ma shallowly emplaced mount neill granite and relatively undeformed granites dated at ca 1552 ma further constraint on the absolute timing of mesoproterozoic tectonism is provided through the dating of zircon growth from samples of the meta sedimentary radium creek metamorphics this dating reveals a 1555 16 ma metamorphic event interpreted to be the consequence of m 3 amphibolite facies regional metamorphism and accompanying metasomatism the transition between d 2 and d 3 is interpreted to reflect a shift from ca 1591 1585 ma n s shortening to ca 1585 1552 ma nw se shortening this episode of orogenesis is correlated with the ca 1585 1540 ma kararan orogeny in the northern gawler craton which distinctly postdates the ca 1600 1590 ma olarian orogeny as previously interpreted the mesoproterozoic structural architecture was reactivated during neoproterozoic extension d 4 and the ca 500 ma delamerian orogeny delamerian structures are formed in a predominantly brittle regime early thin skinned thrusting was followed by thick skinned deformation d 5 d 6 during which open f 5 folds developed above oblique lateral ramps and produced type 0 overprinting patterns with f 3 mesoproterozoic folds steeply plunging f 6 buckle folds with se trending axial traces are the last phase of deformation recorded both mesoproterozoic ductile deformation phases and the palaeozoic ductile brittle to brittle overprints were focussed along the paralana fault emphasising the repeated long lived evolution of this crustal scale fault system

paleoarchean record of the detrital pyrite bearing jacobina au u deposits bahia brazil abstract u pb and lu hf isotope analyses were performed on detrital zircon grains from eight samples distributed along a 1000 m thick section of the au u pyrite deposits of jacobina to identify the primary source areas and define the maximum age for the basin sedimentation these deposits contain detrital pyrite similar to that observed in the witwatersrand gold province although the sequence was deformed by paleoproterozoic orogeny all of the detrital zircon grains n 785 yielded paleoarchean ages between 3 2 and 3 5 ga with primary peaks of approximately 3 3 3 4 ga most of the grains have nearly chondritic compositions with hf t values between 5 6 and 0 1 and hf model ages between 3 5 and 3 8 ga the presence of detrital au u pyrite in the sio 2 rich metasedimentary rocks provide unequivocal evidence that the basin formed in an o 2 poor atmosphere the u pb data set and lithogeochemistry suggest that its infill material was derived from an exclusively paleoarchean source specifically sediments derived from the ttg greenstone assembly of the gaviao block 3 2 3 5 ga these source rocks were strongly hydrothermally altered and weathered before deposition the geochemical composition of the sedimentary rocks and the isotopic signature of hf indicate that the sediments were derived from a juvenile source with minor crustal contamination and a short duration of crustal residence between 150 and 350 myr

mesoarchaean to palaeoproterozoic growth of the northern segment of the itabuna salvador curaca orogen sao francisco craton brazil the geology of the northern segment of the itabuna salvador curaca orogen sao francisco craton is reviewed and new u pb ages and nd isotope and major and trace element data are combined to improve understanding of its tectonic evolution the results indicate that oceanic crust and island arc sequences accreted at about 3 30 ga to form the mundo novo greenstone belt and between 2 15 and 2 12 ga to form the rio itapicuru and rio capim greenstone belts at about 3 08 2 98 ga mafic crust underwent partial melting to form the retirol ndia and jacurici tonalite trondhjemite granodiorite belts of the serrinha block

rapid high temperature metamorphism of east pacific rise gabbros from hess deep abstract metamorphosed oceanic gabbros provide a record of the cooling history of the lower crust near mid ocean ridges but the temperature range rate and location of subsolidus events are poorly known we combine hornblende plagioclase thermometry statistical analysis and thermal models to estimate precisely the temperature time distance from axis and duration of metamorphism in east pacific rise gabbros from hess deep odp hole 894g metamorphic hornblende and plagioclase which formed during microfracturing and sea water penetration equilibrated at a mean temperature of 716 8 c 90 confidence level comparison of the properties of the observed temperature distribution with those of model events indicates that metamorphism spanned 60 c when combined with thermal models of fast spreading centers this implies that metamorphism was rapid 6000 yr and occurred 1 4 km off axis application of this approach to other gabbros will allow comparison of spatial and temporal characteristics of deformation and fluid flow in the lower oceanic crust as a function of ridge setting

petrology of the east pacific rise crust and upper mantle exposed in hess deep eastern equatorial pacific the hess deep a rifted oval shaped depression located east of the galapagos triple junction at the tip of the cocos nazca ridge about 101 w 2 n was explored in 1988 during 21 submersible dives a total of 11 dives were devoted to the exploration of the e w trending intrarift ridge 15 km in length 3000 5400 m in depth north of the hess deep depression the intrarift ridge represents an outcrop of recent 1 m y crustal and subcrustal material created at the axis of the east pacific rise epr and emplaced during the lithospheric extention responsible for the westward propagation of the cocos nazca rift francheteau et al 1990 the lithospheric block has undergone cataclastic deformation and was dislocated by tectonic activity as attested to by the mixed and erratic distribution of rock types and by the occurrence of polygenic breccias and gabbroic mylonites the samples are metamorphosed to varying degrees but their protolith textures are generally well preserved their relic mineralogy indicates that they consist of harzburgites dunites gabbroic cumulates gabbronorites and olivine gabbros isotropic gabbros dolerites and basalts some samples of refractory harzburgites and most dunitic cumulates with local accumulation of chromite have been impregnated by wehrlitic and gabbroic primitive melts similar to those described from the mantle crust transition zone of the samail ophiolite complex oman the mineral chemistry indicate that the ultramafics partly reequilibrated with the magmatic impregnations in the liquidus solidus temperature range of 980 1100 c the dolerites and basalts have been derived from mid ocean ridge basalt primary melts presenting a broad range of incompatible element composition which suggests intermittent cycles of magmatic processes involving a progressive melting of a composite source with discontinous extraction of liquids as proposed for the epr volcanics near 13 n hekinian et al 1989 most of the rocks underwent partial retrograde metamorphism and recorded several episodes of recrystallization from the upper greenschist facies ultramafics and gabbros to diagenetic alteration volcanics the cumulate gabbronorites the isotropic gabbros and some dolerites were partially albitized and amphibolitized during the penetration of seawater in the ocean crust prior to serpentinization several samples of unfoliated amphibolites are believed to be completely metamorphosed gabbroic rocks the gabbroic cumulates and the plagioclase rich melt impregnations were variably rodingitized presence of various ca silicates such as epidote prehnite hydrogarnet and zeolite in relation to the serpentinization of the peridotites one dive located on the scarps forming the northern wall of the hess deep to the east of the explored area revealed the presence of in situ outcrops of isotropic gabbros doleritic dikes and extrusives and permitted to observe the contact between the sheeted dike complex and the high level isotropic gabbros

foreland basins and gold bearing conglomerates a new model for the jacobina basin sa o francisco province brazil abstract from a lithological structural and metamorphic viewpoint the jacobina basin in the sa o francisco province brazil is representative of the foreland basins that mark a characteristic evolutionary stage in the development of the palaeoproterozoic orogenic belts around 2000 ma ago in addition it contains gold bearing conglomerates of economic interest that are presently mined the general structure of the basin is one of stacked detrital and schist formations along sinistral strikeslip overthrusting shear zones with exposed deep seated units in the east and shallower and younger rocks in the west new 40ar 39ar geochronological data give minimum ages of 1940 1910 ma for the main thrusting and wrenching events shearing was accompanied by metamorphic recrystallization that indicates a high geothermal gradient and a progressive loading of the underthrust units despite this final major tectonic imprint at least five stages of basin evolution are indicated by traces of tectonic instability such as debris flow deposits chaotic breccias and early dyke swarms in this context the conglomerate layers found in the basin formations correspond to periods of erosion of a preceding stage in the basin development multistage hydrothermal activity is shown by widespread alteration within mineralized conglomerates sulphide fuchsite tourmaline chromite rutile leaching of ultrabasic rocks and phlogopite and or emerald mineralization in contact metamorphic aureoles around late to post tectonic granites this tectonic evolution is coherent with the development of the jacobina basin in a foreland position and its progressive involvement in the transamazonian collision belt resedimentation processes preservation of earlier stages of hydrothermal alteration in pebbles extensive hydrothermal activity and a high geothermal gradient favour a model of epigenetic mineralization for the gold bearing conglomerates with enrichment during the progressive propagation of the basin comparison with similar mineralized conglomerates shows that this model can be applied to other precambrian foreland basins

influence of the kuroshio current intrusion on depositional environment in the northern south china sea evidence from surface sediment records the influence of the kuroshio current kc intrusion on the depositional environment of the northern south china sea scs is discussed based on multi proxies of surface sediments including planktonic foraminifera clay minerals and geochemical elements the results reveal that four typical planktonic foraminifera globigerinoides ruber g sacculifer neogloboquadrina dutertrei and pulleniatina obliquiloculata can be used to trace the kc flow route in the northern scs globigerinoides ruber is mainly distributed in shallower water than the south china sea branch of the kuroshio scsbk flow route while p obliquiloculata is mostly distributed in deeper water than the scsbk flow route there are high percentages of g sacculifer and n dutertrei at the scsbk flow route along the 1000 m isobath where bottom sediments consist of clayey silt and silty clay the scsbk also affects sediment distribution of various sources taiwan sourced sediments characterized by high illite and chlorite contents are transported southwestward into the deep sea environment by the scsbk and or deep water current the pearl river sediments characterized by high kaolinite and titanium ti contents are mostly transported southwestward to be distributed in the sea environment between the pearl river mouth and southeast of hainan island but hardly entered the abyssal basin the luzon island sediments characterized by high smectite content are generally deposited at the northwest of luzon island under the effect of an eddy west of luzon island these sediments were hardly transported northwestward onto the northern shelf of the scs throughout the scsbk flow route the sedimentary environment at the northwest of luzon island is relatively oxidized and higher in salinity than the northern scs which is probably connected with the kc intrusion through the luzon strait

structure of uppermost fast spread oceanic crust exposed at the hess deep rift implications for subaxial processes at the east pacific rise the uppermost 2 km of the oceanic crust created at the fast spreading 135 mm yr 1 full rate equatorial east pacific rise epr is exposed for tens of kilometers along escarpments bounding the hess deep rift mosaics of large scale digital images from the remotely operated vehicle rov argo ii and direct observations from the submersible alvin document a degree of geological complexity and variability that is not evident from most studies of ophiolites or prevailing models of seafloor spreading dramatic variations in the thickness and internal structure are documented in both the basaltic volcanic and sheeted dike rock units these rock units are characterized by extensive faulting fine scale fracturing and rotations of coherent crustal blocks meters to tens of meters across the uppermost basaltic lavas are essentially undeformed and have overall gently inclined flow surfaces through most of the basaltic lava unit however lava flow contacts dip 20 70 w toward the epr and generally increase in dip downward in the section dikes cutting the lavas and in the underlying sheeted dike unit generally dip 90 40 e away from the epr deeper level gabbroic rocks show little evidence of the intense fracturing typical of the overlying units we interpret this upper crustal structure as the result of subaxial subsidence within 1 2 km of the epr that accommodated the thickening of the basaltic lava unit to 500 m variations in the thickness of lava and dike units and spatially related structures along the rift escarpments suggest temporal fluctuations in magma supply these results indicate that substantial brittle deformation accompanied waxing and waning volcanism during the accretion of the crustal section exposed at the hess deep rift if this type of structure is typical of uppermost oceanic crust generated at the epr these processes may be common along fast spreading mid ocean ridges

cooling of the lower oceanic crust thermal models of mid ocean ridges that balance the influx of heat from magmatic sources with the removal of heat by conduction and hydrothermal circulation allow quantification of cooling of young oceanic crust these models reproduce key observations relating to crustal accretion and hydrothermal cooling at fast spreading ridges the rate of cooling is constrained both by the bathymetry of ridge axes and by olivine compositions from ophiolite gabbros successful models involve extensive hydrothermal cooling of the lower crust within 20 km of the ridge with 50 70 kw of hydrothermal cooling for every 1 m of ridge axis at crustal ages of 0 1 0 4 ma these timates can be used to refine global models of geochemical and thermal fluxes close to spreading ridges

constraining the cooling rate of the lower oceanic crust a new approach applied to the oman ophiolite abstract knowledge of the cooling rate of newly formed lower oceanic crust is critical in constraining models of crustal accretion and hydrothermal circulation at mid ocean ridges in this study we calculate the cooling rate of the lower oceanic crust based on the closure temperature of down temperature diffusive exchange of ca from olivine to clinopyroxene in gabbros from the oman ophiolite modelling of the closure temperatures shows that the cooling rates for the lower oceanic crust decrease with depth changing rapidly in the upper third of the gabbros but more slowly in the lower two thirds there is several orders of magnitude variation in cooling rate between the top and bottom of the lower crust the cooling rate versus depth profile closely matches that of a conductive cooling model or a model in which conductive cooling is enhanced by a constant factor due to hydrothermal circulation

the witwatersrand controversy revisited recent revisionist criticism of the modified placer theory of the origin of the witwatersrand ores is singularly unconvincing in response to that criticism it is shown that the so called enigma of the sulfidation of black sands is a myth sulfide assemblages viewed as unusual are quite plausibly attributed to a source effect involving detrital transport prolonged weathering and recycling under oxygen deficient atmospheric conditions have favored preservation of detrital sulfides native elements and uraninite although many of these minerals have been reconstituted to varying degrees as a result of diagenesis and metamorphism a supposed example of structural control of mineralization is invalid evolution of the mineralogy of the witwatersrand ores is intimately linked to extensive reworking and recycling of constituents derived from deep weathering of source rocks these rocks are composed of about 60 percent arenaceous sequences 15 to 20 percent greenstone belt rocks and 20 to 25 percent metamorphic and felsic igneous rocks of the sialic crust revisionist arguments for a widespread hydrothermally induced metasomatic origin of reported precambrian paleosols associated with the ore bearing conglomerates are shown to be ill founded and wrong in summary revisionist criticism of the modified placer hypothesis proves to be at best premature the placer hypothesis as presently modified is alive and well enjoying a formidable predictive capacity as the favored working hypothesis

proterozoic first order sedimentary sequences of the sao francisco craton eastern brazil abstract the sao francisco craton in eastern brazil hosts sedimentary sequences deposited between the paleoarchean 3300 ma and late neoproterozoic 580 ma proterozoic successions occurring in this region comprise five 1st order sedimentary sequences which besides episodes of global significance record major basin forming events the ca 8000 m thick minas itacolomi 1st order sequence exposed in the brazilian mining district of the quadrilatero ferrifero and containing as marker bed the lake superior type caue banded iron formation tracks the operation of a wilson cycle in the paleoproterozoic era the quartz arenite dominated espinhaco i and ii sequences record at least two major rift sag basin forming events which affected the host continent of the sao francisco craton at around 1 75 ga and 1 57 ga the macaubas sequence and its correlatives in the extracratonic domains witness the individualization of a sao francisco congo plate in synchronicity with the break up of rodinia in the cryogenian period the sao francisco congo plate together with various fragments derived from rodinia reassembled to form gondwana in the ediacaran period in the course of the gondwana amalgamation orogenic belts developed along the margins of the craton its interior converted into foreland basins received the shallow water carbonates and pelites of the bambui 1st order sequence and its correlatives

structural and metamorphic evolution of the gabon orogenic belt collision tectonics in the lower proterozoic abstract the archaean granite gneiss terrain of central gabon comprises medium to high grade gneisses metamorphosed at 3 15 ga forming the monts cristal and underlying the mitzic region intruded by and enclosed as enclaves in granitoids 3 0 2 6 ga old that form the extensive chaillu and north gabon blocks these rocks are unconformably overlain in the east of gabon by early proterozoic francevillian sedimentary rocks between 2 3 and 2 0 ga old forming the franceville and booue basins the francevillian is overthrust on its west side by the ogooue metamorphics themselves considered to be of early proterozoic age these rocks crop out in a broad synclinorium consisting essentially of paragneisses with orthogneisses appearing locally at the base the overall tectonic setting is discussed in terms of the evolution of the various domains from west to east 1 1 on the west flank of the synclinorium is a migmatitic granulitic dome the abamie dome in which structures diverge away from the core metamorphism is of high temperature intermediate to low pressure type 2 2 on the east flank of the ogooue synclinorium the medium grade ogooue metamorphics sillimanite biotite kyanite staurolite staurolite muscovite biotite have been thrust eastwards on to the low grade kyanite sericite francevillian 3 3 the francevillian is parautochthonous being widely detached from its archaean basement owing to the effects of the same eastward directed tangential tectonics 4 4 locally slices of archaean basement have been tectonically incorporated the structural cross section illustrates a history of diapiric uprise passing eastward into tangential tectonics that is typical of classical collision belts the suture marking the collision is interpreted to be located in the abamie migmatite dome where the nappes are rooted the gabon orogenic belt is correlated with the collision belts of about the same age on the eastern margin of the congo craton and with the trans amazonian orogenic belt of the guyana shield

emerald dating through 40ar 39ar step heating and laser spot analysis of syngenetic phlogopite emerald occurring in k metasomatic rocks developed at the contact of the carnaiba leucogranite with serpentinite bahia state brazil has been dated using an original 40ar 39ar procedure it combines step heating and spot fusion experiments on two types of phlogopite crystals 1 bulk samples and individual grains extracted from the enclosing k metasomatic host rocks and 2 syngenetic solid inclusions precipitated along growing zones of the emerald host crystals the second procedure uses in situ laser probe experiments on rock sections in spite of the huge amounts of excess 40ar detected in adjacent emerald we could measure reliable ages of 1951 8 ma and 1934 8 ma for the trecho velho and braulia occurrences respectively spot fusion data had higher discrepancy than the step heating data but minute crystals of phlogopite included in emeralds bearing excess argon do not reveal excess argon a muscovite belonging to the same granite hydrothermal complex gave a plateau age of 1976 8 ma which may correspond to a higher closure temperature of the kar system during the cooling of the whole pluton and associated hydrothermal halo these accurate measurements lead to the following conclusions 1 direct emerald dating is possible 2 in spite of a polyphase history during the transamazonian orogenesis 2 ga combined step heating and spot fusion experiments give a better precision for granite related emerald mineralization than the scattered ages obtained by rb sr and k ar methods 3 the late transamazonian tectonothermal retrograde event which probably caused the dispersion of previous rb sr and k ar data is not revealed by our procedure 4 the emerald mineralization and k metamorphism appear to be linked with the thermal history of the leucogranite 5 in addition to its use in polyphase crustal domains accurate 40ar 39ar dating is of major interest in the field of metallogenic models even for instance for mineralizations characterized by disturbed isotopic systems which record effects as excess argon

reconstruction of holocene monsoon history from the pearl river estuary southern china using diatoms and carbon isotope ratios this study employs a multiproxy approach in the reconstruction of holocene monsoon history from sedimentary sequences obtained from the pearl river estuary southern china a set of modern surface sediment samples were collected within and outside the estuary and a sediment core was obtained from the mouth of the estuary a range of modern environmental variables were recorded including water salinity summer winter and annual average and water depth the diatom results show a good relationship with water salinity with marine diatoms dominating the more distal samples from outside the main estuary and freshwater diatoms dominating the proximal samples influenced by higher freshwater flux this pattern is matched by the bulk organic carbon isotopes the isotope values vary between 21 1 0 3 0 for samples from the fully marine environment between 23 2 0 8 and 23 7 0 8 for samples from the mid to outer estuary brackish water environment and less than 24 7 1 3 for samples from the inner estuary close to the freshwater sources similarly the c n ratios vary from below 7 0 6 in the marine end members to over 14 8 3 0 in the freshwater end members both the diatom and carbon data from the sediment core reveal a significant increase in freshwater flux from 8500 cal yr bp resulting from the enhanced summer monsoon regime in early holocene the strength of freshwater flux reached its highest between 7500 and 6000 cal yr bp in the last 6000 years freshwater flux decreased towards present reflecting a gradual weakening of the summer monsoon

textural and geochemical investigation of pyrite in jacobina basin sao francisco craton brazil implications for paleoenvironmental conditions and formation of pre goe metaconglomerate hosted au u deposits abstract the jacobina basin has a well preserved sedimentary record including continental and marine deposits and hosts au u py mineralization in metaconglomerate beds similar to the witwatersrand gold province based on petrographic observations in situ trace elements and multiple sulfur isotope analyses 32s 33s 34s and 36s on pyrite from alluvial and marine facies several types of pyrite were recognized the pyrite grains identified in the alluvial metaconglomerates resemble those found in several pre goe gold bearing metaconglomerates including detrital and epigenetic varieties detrital inclusion bearing pyrite is enriched in several redox sensitive trace metals as well as au which indicate an association with carbonaceous shales on the other hand the sources of detrital massive pyrite are more variable and include igneous metamorphic and hydrothermal sources from the paleoarchean hinterland of jacobina basin epigenetic pyrite in metaconglomerates formed during metamorphism by the recrystallization of detrital pyrite with negligible contributions from external hydrothermal fluids to the basin diagenetic and epigenetic pyrite are found in marine lithologies in a metapelite sample the pyrite grains formed near the sediment water column interface with s sourced from the photolytic sulfate so42 33s 0 that accumulated in sediment pore water during diagenesis significantly the pyrite associated with terrestrial metasediments shows a wide range in 34s values but quite restricted ranges in 33s and 36s values whereas pyrite associated with the marine metasedimentary rocks exhibits limited 34s values but has a wide range in 33s values and correlated 36s values close to the archean array these data suggest distinct preservation routes for mif s from atmospheric so42 and s8 in terrestrial and marine environments conditions on the terrestrial surface resulted in re equilibration of distinct s sources diminishing the amplitude of the archean atmospheric signal in contrast so42 dissolved in shallow marine settings while s8 settled to the floor favoring the preservation of mif s isotopic signatures such processes may also explain the apparent differences in interpretations of atmospheric conditions derived from uncharacterized pyrites from archean sources our data suggest that the earth s atmosphere remained anoxic and terrestrial conditions were such as to allow the syngenetic accumulation of gold as recently proposed for the witwatersrand gold deposits

chinese loess record of monsoon climate during the last glacial interglacial cycle the thick loess paleosol succession of central china contains a detailed record of changes in east asian monsoon climate since the late miocene epoch the alternating loess paleosol stratigraphy closely resembles the marine oxygen isotope record implying that episodic dust deposition and pedogenesis are in phase with global ice volume fluctuations and controlled by variations in solar radiation reaching the land surface deposits equivalent to marine isotope stages mis 5 through 1 have been dated by 14c 10be luminescence and magnetic susceptibility and grain size models malan loess mis 4 2 decreases in thickness southeastward from 30 m in the western loess plateau to 5 m east of xi an downwind asian dust is found in japan the north pacific ocean and the greenland ice sheet where it has a chemical signature that points to a primary source in western china during glacial times the mean dust flux in the central loess plateau reached 0 35 mm year whereas in interglacial times it fell to 0 1 mm year times of high dust influx and reduced soil development coincided with strengthened winter monsoon conditions which are inferred from variations in loess grain size times of decreased dust accumulation and strong pedogenesis marked by high values of magnetic susceptibility represent times of strengthened summer monsoon conditions high frequency fluctuations in dust influx during deposition of the malan loess include peaks that likely correlate with north atlantic heinrich events the younger dryas oscillation is recorded in chinese eolian and organic deposits by a shift from cool humid late glacial conditions to cold and dry conditions followed by a return to milder humid climate at the beginning of the holocene warm moist climate associated with a strong summer monsoon characterized the first half of the holocene epoch when a widespread paleosol complex developed on the loess plateau increased dust deposition during the middle to late holocene marked a shift to cooler drier conditions

slow cooling of the lowermost oceanic crust at the fast spreading east pacific rise the cooling history of the lowermost ocean crust formed at the east pacific rise and exposed at the hess deep rift is constrained using fresh primitive olivine gabbro and troctolite cores recently recovered by integrated ocean drilling program expedition 345 the mg in plagioclase and ca in olivine geospeedometers constrain the initial subsolidus cooling rates down to closure temperatures in the range of 900 750 c these independent geospeedometers yield similar slow cooling rates 0 005 0 0001 c yr 1 with a mean value of 0 0011 c yr 1 making these results robust despite the uncertainties inherent in the methods the slow cooling rates derived using natural samples provide a crucial test of predictions of the thermal structure of the lower crust based on remote imaging techniques and thermal modeling the cooling rates are most consistent with thermal models that are dominated by conductive cooling within the plutonic complex this implies that lower crust formed at fast spreading rates remains hot in the near axis environment indicating limited hydrothermal fluid ingress and reaction

retrieving timescales of oceanic crustal evolution at oceanic core complexes insights from diffusion modelling of geochemical profiles in olivine abstract the building of oceanic crust at oceanic core complexes occ has been described as a complex process involving multiple intrusions of magma over a protracted period of time the migration of primitive magmas i e mid ocean ridge basalts morbs can lead to melt rock interactions during reactive porous flow processes through the lithosphere the timescales of these reactive processes and the subsequent cooling of the modified crystal matrix remain unconstrained diffusion modelling has been widely used to retrieve timescales of magmatic processes in this study we use diffusion models to constrain i the minimum timescales of melt rock interactions and ii the cooling rates of the gabbroic sequence forming the oceanic crust at an occ we chose samples of the most primitive olivine rich troctolites from the gabbroic sequence sampled in iodp hole u1309d atlantis massif occ mid atlantic ridge 30 n olivine rich troctolites were interpreted as marking local partial assimilation of mantle intervals into the oceanic crust and thus allowed to understand the dynamics of mantle assimilation and the formation of slow spreading oceanic crust at the atlantis massif occ olivines in olivine rich troctolites represent relicts of pre existing mantle olivine while clinopyroxenes and plagioclases are crystallized during reactive percolation olivine chemical compositions show that olivine rich troctolites inherit chemical heterogeneity from the mantle precursor flat geochemical profiles in olivine indicate complete chemical re equilibration of olivine crystals with the locally modified percolating melt exception is made for most ca profiles that show lower ca contents at the olivine rim compared to the relative crystal core as the result of subsolidus cooling three dimensional 3d diffusion models at magmatic conditions t 1210 1300 c and p 2 kbar reveal that complete chemical re equilibration of 4 mm size mantle derived olivine with percolating morb type melts can be attained within durations of less than 300 yr the ca in olivine geospeedometer reveal that cooling rates from 1200 c to 1050 c are constant downhole and on average 0 004 c yr they are comparable with lower temperature cooling rates 850 c 250 c estimated at the atlantis massif occ the minimum timescales from 3d models point to rather fast re equilibration of olivine the downhole chemical heterogeneity inherited from the precursor mantle coupled with the timescales of diffusive re equilibration suggest that the partial assimilation of the upwelling mantle and its incorporation into the oceanic crust occurred in the time frame of a single melt input our cooling data are consistent with building of the oceanic crust at occs controlled by continuous uplift in turn governed by long lived detachment faults the latter contribute to the rapid cooling of the assimilated mantle intervals and the magma bodies diffusion models of geochemical profiles in olivine from a single crustal section allow to reconstitute the early magmatic processes leading to mantle assimilation and early crystallization of gabbros and the cooling history of the oceanic crust at occ from magmatic conditions to hydrothermalism

hydrothermal fault zones in the lower oceanic crust an example from wadi gideah samail ophiolite oman abstract hydrothermal circulation is a key process for chemical and isotopic exchange between the solid earth and oceans and for the extraction of heat from newly accreted crust at mid ocean ridges however due to a dearth of samples from intact oceanic crust or continuous sample suites from ophiolites there remain major shortcomings in our understanding of hydrothermal circulation in the oceanic crust especially in the lower plutonic crust in particular it remains unknown whether fluid recharge and discharge occurs pervasively or if it is mainly channelled within discrete zones such as faults here we describe a hydrothermally altered fault zone that crops out in the wadi gideah in the layered gabbro section of the samail ophiolite of oman a one metre thick normal fault comprising deformed chlorite epidote fault rock with disseminated chalcopyrite and pyrite offsets gently dipping layered olivine gabbros the chlorite rich fault rocks surround strongly altered clasts of layered gabbro 50 to 80 cm in size layered gabbros in the hanging wall and the footwall are partially altered and abundantly veined by epidote prehnite laumontite and calcite veins in the wall rocks igneous plagioclase an82 2 is partially altered towards more albitic compositions an75 81 and chlorite tremolite partially replaces plagioclase and clinopyroxene clinopyroxene is moderately overgrown by mg hornblende whole rock mass change calculations show that the chlorite rich fault rocks are enriched in fe mn h2o co2 co cu zn ba and u but have lost significant amounts of si ca na cr ni rb sr cs light rare earth elements lree eu and pb gabbro clasts within the fault zone as well as altered rock from the immediate hanging wall show enrichments in na volatiles sr ba and u and depletions of si ti al fe mn mg ca cu zn rb cs lree and pb chlorite thermometry suggests a formation temperature of 300 350 c for the fault rock and based on the si loss and solubility of silica in hydrothermal fluids the intensity of alteration requires a fluid to rock ratio of up to 900 1 strontium isotope whole rock data of the fault rock yield 87sr 86sr ratios of 0 7043 0 7048 which is considerably more radiogenic than fresh layered gabbro from this locality 87sr 86sr 0 7030 0 7033 and similar to black smoker hydrothermal signatures based on epidote measured from epidote veins in the footwall and elsewhere in the ophiolite 87sr 86sr 0 7043 0 7051 altered gabbro clasts within the fault zone show similar values with 87sr 86sr ratios of 0 7045 0 7050 in contrast the hanging and footwall gabbros display values only slightly more radiogenic than fresh layered gabbro the elevated strontium isotope composition of the fault rock and clasts together with the observed secondary mineral assemblages and calculated mass changes strongly supports the intense interaction with seawater derived up welling hydrothermal fluids active during oceanic spreading assuming that such a fault zone is globally representative of faulting in the lower crust an extrapolation of our results from mass change calculations to elemental fluxes shows a significant contribution to the global hydrothermal budgets of si ti fe mn mg ca h2o cu zn sr and cs

variation of cooling rate with depth in lower crust formed at an oceanic spreading ridge plagioclase crystal size distributions in gabbros from the oman ophiolite 1 analysis of crystal size distributions csd of plagioclase in gabbros from the oman ophiolite indicates that cooling rates in the lower crust beneath a medium to fast spreading oceanic ridge did not vary smoothly with depth cooling rates in the upper half of the gabbro section were approximately 1 5 2 times faster than in the lower half of the gabbro section during igneous crystallization and or high temperature grain growth processes our results are inconsistent with thermal models for oceanic spreading ridges that approximate the effect of hydrothermal convection by using an enhanced thermal conductivity together with a conductive heat transfer mechanism instead the oman csd data are consistent with recent seismic and modeling studies that suggest that hydrothermal convection leads to closely spaced near vertical isotherms in the upper half of the plutonic section within a few kilometers of the ridge axis

cooling rates in the lower crust of the oman ophiolite ca in olivine revisited debate over the mechanism of accretion of the lower oceanic crust has centered around the gabbro glacier and the sheeted sills models the thermal profile of the crust specifically the roles of hydrothermal circulation and cooling rate is a key component in distinguishing between these two models results of this study show no systematic variation of cooling rate with depth in the lower crust of the khafifah section in the wadi tayin massif of the oman ophiolite additionally very high rates recorded near the base of the crust suggest that hydrothermal circulation plays an important role in the removal of heat throughout the crust while the conclusions presented here do not rule out the possibility of accretion by gabbro glacier they do contradict the initial tenet of the gabbro glacier model convective heat loss by hydrothermal circulation can penetrate to moho depths and crystallization is not by necessity constrained to the upper crust on the basis of these results it can be concluded that the sheeted sills model is a thermally viable mechanism for accretion of oceanic lower crust

archean and paleoproterozoic crust of the sao francisco craton bahia brazil geodynamic features abstract recent geological geochronological and isotopic studies allow the identification of four important crustal segments in the basement of the sao francisco craton in bahia the oldest is the gaviao block in the wsw part of the studied area and comprises granitic granodioritic and migmatitic continental crust including remnants of 3 4 ga ttgs which are amongst the oldest rocks in south america and are associated with archean greenstone belts the youngest segment is exposed in the itabuna salvador curaca belt which extends from se bahia along the atlantic coast to salvador then northwards into ne bahia it is mainly composed of a low k calc alkaline plutonic suite and also contains strips of intercalated metasediments and ocean floor back arc basin gabbro and basalt in the ssw part of the area the jequie block comprises granulitic migmatites with inclusions of supracrustal rocks intruded by many charnockite plutons in the ne the serrinha block is composed of orthogneisses and migmatites which form the basement for paleoproterozoic greenstone belts during the paleoproterozoic transamazonian cycle these four crustal segments collided resulting in the formation of an important mountain belt the regional metamorphism resulting from the crustal thickening associated with the collision occurred at around 2 0 ga major mineralizations were formed during the evolution of the four archean blocks and also during and after the paleoproterozoic collision

a comparison of alteration assemblages associated with archean gold deposits in western australia and paleozoic gold deposits in the southeast united states a comparison of high alumina mineral assemblages spatially associated with the haile and brewer gold deposits in the paleozoic carolina slate belt csb of the southeast united states and the sons

oxygen isotope evidence for short lived high temperature fluid flow in the lower oceanic crust at fast spreading ridges millimeter scale amphibole veins in the lower oceanic crust record fracture controlled fluid flow at high temperatures but the importance of this fluid flow for the thermal and chemical evolution of the lower oceanic crust is unclear in the section of lower oceanic crust recovered at hess deep from odp hole 894g which formed at the fast spreading east pacific rise these veins are randomly distributed with an average spacing of 1 m we unravel the history of fluid flow through one of these veins by combining in situ o isotope analyses of wall rock plagioclase with major element analyses geothermometry and diffusion modeling thermometry indicates vein sealing by amphibole at 720 c over a narrow temperature interval 20 c in situ oisotope analyses by ion microprobe with a precision of b0 5 reveal zoning of o isotopes in plagioclase adjacent to the vein the zoning profiles can be reproduced using a diffusion model if the duration of o isotope exchange was 100 yr a similar interval of fluid rock exchange is suggested by modeling potassium depletion in plagioclase adjacent to the vein if representative of fracture controlled fluid flow in the lower oceanic crust the limited duration of fluid flow and its occurrence over a narrow temperature interval suggest that high temperature fluid flow in this porosity network does not transport significant heat 2007 elsevier b v all rights reserved

hydrothermal cooling along the eastern lau spreading center no evidence for discharge beyond the neovolcanic zone heat transported from the mantle beneath spreading centers creates an astonishingly narrow ribbon of convective heat discharge at plate boundaries as apparently demonstrated by exhaustive exploration for hydrothermal discharge sites over the last three decades recent observations and models are now challenging this assumption of exclusively axis centric high temperature venting one example is the proposal that intense cooling along the vertical boundaries of a broad low velocity volume lvv of hot crust could generate high temperature fluids several kilometers off axis to test the hypothesis that substantial hydrothermal discharge might occur beyond the lvv we conducted a dense survey grid of the ridge and surrounding seafloor up to 5 km along 175 km of the eastern lau spreading center and valu fa ridge 1800 km of track line our sampling array extended from 50 to 400 m above bottom and included light scattering oxidation reduction potential and hydrographic sensors attached to the tow line and beneath the imi120 sonar mapping system the surveys successfully mapped plumes from several vent fields in the neovolcanic zone 1 5 km about the axis but did not detect evidence of significant discharge anywhere farther off axis at a few locations on the valu fa ridge however we did record oxidation reduction potential anomalies with hydrographic density signatures that imply low temperature hydrothermal sources on the axial flank although these sites are hundreds of meters deeper than the adjacent crest they are above not beyond the previously mapped lvv our results thus do not support a simple picture of high temperature fluids ascending undiluted through the crust to the seafloor several kilometers off axis however we cannot exclude the possibilities that the largely unmapped lvv is narrower here than seen on other ridges that hydrothermal fluids formed beyond the lvv are channeled to the axis or that discharge beyond the neovolcanic zone occurs only as dispersed very low temperature fluids our observations do demonstrate that high temperature discharge predominantly exits the seafloor within a narrow 1 5 km axial ribbon regardless of the presence or absence of an axial magma chamber

architecture of on and off axis magma bodies at epr 9 37 40 n and implications for oceanic crustal accretion abstract crustal accretion at fast spreading mid ocean ridges is believed to be concentrated in a narrow zone up to a few kilometers wide centered beneath the ridge axis however there is increasing evidence for off axis magmatism occurring beyond this narrow zone here we present 3d multichannel seismic mcs images from the east pacific rise 9 37 40 n extending to 11 km on the ridge flanks in the axial region two offset axial magma bodies underlie a small ridge axis discontinuity at 9 37 n displaying an overlapping geometry similar to that of the seafloor structures above on the ridge flanks a series of off axis magma lenses oaml are imaged they are located 2 10 km from the ridge axis at 700 to 1520 ms two way travel time below seafloor bsf 1 6 to 4 5 km bsf with variable areas ranging from 0 5 km2 to 5 2 km2 the largest body is centered 4 km east of the ridge axis and is composed of a large continuous flat topped lens and a series of small discontinuous westward dipping bodies along its western edge the flat crest of the oaml lies at approximately the same depth beneath layer 2a as the axial magma lens and we infer that this oaml has formed by aggregation of ascending melts that accumulate at the base of the sheeted dike section a cluster of reflections underlying the oaml at 1260 1510 ms bsf are observed that may be deeper lenses feeding melts to the upper lens this largest oaml is associated with moho travel time anomalies of 120 260 ms within a zone that extends up to 2 km from the edge of the oaml suggesting a lower crust that is partially molten with lower crustal velocities reduced by 8 18 and or thicker than normal by up to 1 km local volcanic edifices are found above two of the three oamls imaged in our study area and are inferred to be the eruptive products of the oamls from the volume of these edifices and the moho travel time anomalies we estimate the potential contribution of off axis magmatism to the total volume of the crust to be 0 01 3 the oamls imaged in our study area are present over roughly the same distance range as the zone of formation of near axis seamounts we speculate that oamls and the volcanic edifices found above them are small scale manifestations of the off axis magmatism that gives rise to near axis seamounts

chronometry and speedometry of magmatic processes using chemical diffusion in olivine plagioclase and pyroxenes the magmatic processes that fuel volcanism crustal growth ore formation and discharge of volcanic gases and aerosols to the atmosphere occur across a range of timescales from millions of years to just a few seconds for example the production of new oceanic crust at mid ocean ridges is a near continuous process that can operate in any one ocean basin on timescales of more than 100 m y however the driving force for such processes is the spreading of the ocean plates that happens on a cm yr timescale at the other end of the spectrum explosive volcanic eruptions involve the ascent and fragmentation of magma at velocities of the order of 100 m s such that the journey from a magma chamber to an ash cloud may take place in a matter of minutes in this case the driving force is the rapid expansion of magmatic gas in response to changes in pressure at intermediate timescales magmatic processes may give rise to hydrothermal ore deposits on timescales of less than a million years for an individual deposit while growth of giant granite batholiths may require piecemeal assembly of magma batches on timescales of a few million years although each of these processes has a characteristic time averaged timescale on which it operates this is typically the end result of one or more natural processes that operate on much shorter timescales for example mid ocean ridges do not extrude magma continuously onto the ocean floor mineralising fluids do not discharge continuously through the shallow crust and granitic magmas do not dribble continuously into evolving batholithic chambers in some cases it is the long term timescales that are important for example the spreading rate of ocean basins in others it is the short term timescales that are important for example the episodic growth of lava domes at active volcanoes although

identification of the scales of differential element mobility in a ductile fault zone two fundamentally different scales of element mobility have been identified in the upper amphibolite facies hunts brook fault zone of south central connecticut usa field relations mineral chemistry and 80 bulk rock analyses provide overwhelming evidence that the blastomylonitic schists and gneisses of the fault zone were derived from the enclosing granodioritic rope ferry orthogneiss two sample mass balance calculations between the rope ferry gneiss and fault rocks strongly suggest that the rope ferry gneiss was segregated into biotite rich schists and quartz plagioclase rich gneisses through the centimetre scale transfer of si al ca na ba and zr from schists to gneisses three and four sample mass balance calculations provide convincing evidence that al ca na and ba were lost and si and k gained by the entire fault zone both scales of metasomatism have been identified at two localities separated by 20 km implying that the metasomatic processes were pervasive throughout the fault zone from the mass balance calculations an apparent disparity in k mobility arises k appears to be gained by the entire fault zone but is immobile during the centimetre scale segregation this is because mass balance calculations can only detect differential element mobility differential element mobility requires that the mechanism of metasomatism produces chemical potential gradients therefore the identification of differentially mobile elements provides insight into the mechanisms of metasomatism the immobility of k during segregation of schists and gneisses implies that the segregation was not achieved through the extraction of partial melt however the element mobility is reconcilable with deformation driven metamorphic differentiation the gains of si and k and losses of ca na al and ba for the entire fault zone are also inconsistent with the intrusion or extraction of partial melt but may reflect the infiltration of a metasomatizing aqueous fluid

the archaean terranes of central eastern brazil a review abstract this paper compiles and describes several lithostratigraphic sequences which have characteristics typical of granite greenstone and granite gneiss terranes the archaean high grade metamorphic assemblages are also described and are considered to form part of the mobile belt areas three major archaean domains appear to be present in central eastern brazil i e the sao francisco craton and the jequie and goias mobile belts in addition several other small but no less important ancient nuclei occur within the tocantins province especially in central goias the sao francisco craton is mostly covered by thick platform sequences of proterozoic age and as a result it can be examined only around the margins where granite greenstone terranes are partially exposed there appears to be sufficient evidence to recognise a stable archaean basement domain except in the northeast where the craton has been largely affected by the transamazonian cycle 2 2 1 8 ga and therefore may represent a separate province the available data for the jequie and goias belts suggest that these domains have been subjected to high grade regional metamorphism chiefly granulite facies during the jequie tectono thermal event of 2 7 ga equivalent to the liberia limpopo events in africa the differentiated mafic to ultramafic complexes of central goias of uncertain age may also be related to the evolution of the goias mobile belt in general the archaean terranes described here appear to display a rather fragmentary pattern with poorly defined boundaries due to substantial reworking which makes them partially different from better preserved major ancient domains in other continents

geologic evidence for a mantle superplume event at 1 9 ga 1 both preserved and restored areal distributions of proterozoic marine intracratonic passive margin and platform sediments show a prominent peak at 1 9 ga indicating that shallow marine sediments were widespread on the continents and that sea level was high at this time the chemical index of alteration in shales deposited at this time was high suggesting warm climates possibly due to enhanced co2 levels in the atmosphere high sea level and warms climate may also explain an abundance of black shale banded iron formations and shallow marine phosphate deposits and an increase in the number of occurrences and diversity of stromatolites in general and microdigitate stromatolites at 1 9 ga all of these observations are consistent with a 1 9 ga superplume event the occurrence of only a minor positive carbon isotope shift in marine carbonates at 1 9 ga indicates that the relative rates of burial of organic and oxidized carbon remained about the same as at present slightly low 87sr 86sr isotopic ratios in seawater at 1 9 ga reflect increased mantle input of sr from the proposed superplume event whereas higher ratios at 1 85 1 75 ga may reflect increased input of continental sr from a growing supercontinent the first massive sulfate evaporites in the geologic record at 1 8 1 6 ga follow the possible 1 9 ga superplume event this may reflect an increase in both oxidation state and carbonate deposition in the oceans as plume related volcanism wanes

hydrothermal gold mineralization in the witwatersrand basin the origin of the gold mineralization in the witwaterstrand basin of south africa the largest known gold province has been controversial for decades with arguments favouring detrital1 2 placer modified placer3 4 and hydrothermal5 6 origins here we present the results of an extensive geological study of witwatersrand rocks which show that the gold and associated uranium mineralization is hydrothermal in origin and postdates a regional high temperature alteration event alteration processes identified on a small scale can be mapped out regionally as roughly strata bound zones of acid metasomatism extending far into the basin the fluid flow responsible for this alteration was concentrated in small scale structures localized along lithological boundaries we find that the gold precipitated as a consequence of interactions of the fluid with shale derived hydrocarbons present within the basin

a high resolution absolute dated holocene and deglacial asian monsoon record from dongge cave china abstract we present a continuous record of the asian monsoon over the last 16 ka from 18o measurements of stalagmite calcite over 900 oxygen isotopic measurements providing information on shifts in monsoon precipitation are combined with a chronology from 45 precise 230th dates 18o and therefore asian monsoon intensity generally follows changes in insolation although changes in 18o are generally accommodated in abrupt shifts in contrast to smoothly varying insolation indicating that threshold effects may be important 18o decreased dramatically 3 at the start of the holocene 11 5 ka and remained low for 6 ka four positive 18o events centered at 11225 97 yr bp 1 05 10880 117 yr bp 1 15 9165 75 yr bp 1 4 and a double event centered at 8260 64 yr bp 1 1 and 8080 74 yr bp 1 0 punctuated this period of high monsoon intensity all four events correlate within error with climate changes in greenland ice cores thus the relationship between the asian monsoon and the north atlantic observed during the glacial period appears to continue into the early holocene in addition three of the four events correlate within error with outburst events from lake agassiz the decline of monsoon intensity in the mid late holocene is characterized by an abrupt positive shift in 18o which occurs at 3550 59 yr bp 1 1 in 100 yr in addition the holocene is punctuated by numerous centennial and multi decadal scale events amplitudes 0 5 to 1 up to half the amplitude of the glacial interstadial events seen in the last glacial period thus holocene centennial and multi decadal scale monsoon variability is significant although not as large as glacial millennial scale variability the monsoon shows a strong connection with northern south american hydrological changes related by changes in itcz position spectral analysis of the 18o record shows significant peaks at solar periodicities of 208 yr and 86 yr suggesting variation is influenced by solar forcing however there are numerous other significant peaks including peaks at el nino frequencies observed for high resolution portions of the record between 8110 and 8250 yr which suggest that changes in oceanic and atmospheric circulation patterns in addition to those forced by solar changes are important in controlling holocene monsoon climate in addition for this high resolution portion we observe a distinctive biennial oscillation of the asian monsoon which has been associated with the tropospheric biennial oscillation tbo

re os geochronological evidence for multiple paleoproterozoic gold events at the scale of the west african craton the archean proterozoic craton of west africa hosts numerous gold deposits which are spatially and temporally related to the eburnean orogeny that took place between 2250 and 1980 ma and included multiple deformation events the majority of these gold deposits are located along shear zones the structural history is relatively well established for most gold deposits but absolute timing of the mineralization is commonly lacking five deposits hosted in the baoule mossi domain located in the southwestern part of the west african craton were studied to better constrain timing of gold events nassara and kiaka in burkina faso and wassa damang and obuasi in southern ghana gold mineralization was structurally characterized at each deposit and dated by the re os method on pyrite arsenopyrite and pyrrhotite grains that were coeval with the gold deposition combined structural and re os geochronological constraints allow two groups of gold deposits to be distinguished early orogenic gold formed during the eoeburnean orogeny i e between 2190 and 2125 ma as represented by kiaka1 at 2157 24 ma and wassa1 at 2164 14 ma these early gold occurrences are expressed as early disseminated gold enrichment 1 3 g t au at kiaka and gold hosted in folded and boudinaged quartz veins that are transposed along the primary s foliation at wassa these re os ages on the early sulfides at wassa and kiaka are significant because they definitively provide the first direct age constraints on gold only eoeburnean mineralization in the birimian of west africa late orogenic gold hosted by brittle structures and with higher gold grades up to 60g t at kiaka formed during late eburnean deformation d d events between 2120 and 2000 ma as represented by the re os dating of these ores at wassa at 2055 18 ma and obuasi at 2045 40 ma similar style ores dated at nassara at 2131 99 ma and damang at 2080 135 ma but both with large uncertainties and an undated younger event at kiaka are also suggested to be part of this later mineralizing episode the high uncertainties on the nassara and damang ages may be directly linked to the low rhenium and osmium contents of the studied samples these results highlight the polyphase character of the widespread gold mineralization in the west african craton although the late stages of the eburnean orogeny constitute a prolific period for the formation of high grade gold mineralization identification of less well studied early stage gold deposits which can also contain large quantities of gold is critical for mineral exploration in the west african craton

the major precambrian terranes of eastern south america a study of their regional and chronological evolution abstract the archaean of south america comprises two types of domain the low grade granite greenstone assemblages of cratonic type and the high grade granulite gneiss terranes of mobile zones related to the 2 7 ga old jequie event equivalent to the limpopo liberia event in africa the geological evolution of the proterozoic terranes of eastern south america is intimately linked to three major tectogenetic cycles and their related tectono thermal events i e the transamazonian the uruscuano and the brasiliano cycles of early middle and late proterozoic age respectively the first equivalent to the eburnian of africa is characterized by the reworking of archaean crust and accompanied by intense magmatism several major geotectonic provinces such as the amazonian province the sao luis province the paramirim province and the rio de la plata province were stabilized after this event the uruacuano belt seems to have been the only mobile zone active during the middle proterozoic and consequently the uruacuano cycle equivalent to the kibaran of africa has a relatively restricted regional imprint over the south american platform conversely the late precambrian brasiliano cycle equivalent to the pan african affected extensive areas adjacent to the atlantic coast and in central brazil the development of all these major mobile zones appears to be related to ensialic intracratonic processes including intense reworking of pre existing crust and within them there seems to be an almost complete absence of oceanic volcanism except for minor basaltic to andesitic volcanics found in the ribeira belt of late precambrian age

the 2 ga peraluminous magmatism of the jacobina contendas mirante belts bahia brazil geologic and isotopic constraints on the sources abstract a 500 km north south alignment of granitic intrusions cross cuts the central part of the sao francisco craton these late to post tectonic granitic bodies are emplaced between two archean blocks jequie and gaviao blocks and cross cut the contendas mirante volcano sedimentary sequence and the metasedimentary rocks of the serra de jacobina they are two mica or muscovite garnet bearing peraluminous granites rb sr systematics show that these granites were emplaced during the transamazonian orogeny 1 9 ga the high initial 87 sr 86 sr ratios 0 706 0 748 and the very low nd t 13 to 5 indicate a crustal origin possible sources are 1 the jequie block 2 the contendas mirante sequence and 3 the gaviao block including some early archean domes trondhjemitic tonalitic granodioritic ttg boa vista type which were tectonically emplaced within the volcano sedimentary sequence the currently available rb sr and sm nd data are not consistent with reworking of the 3 5 ga ttg the combination of field and isotopic constraints preclude the jequie block and the contendas mirante sequence and favour instead the gaviao block medium grade terrains as the source for the peraluminous granitic line the available data suggest that a continent continent collision occurred during the transamazonian orogeny which followed subduction obduction marked by volcanism arc tholeiitic calc alkaline and shoshonitic and related plutonism

holocene climate variability as derived from alkenone sea surface temperature and coupled ocean atmosphere model experiments holocene climate modes are identified by the statistical analysis of reconstructed sea surface temperatures ssts from the tropical and north atlantic regions the leading mode of holocene sst variability in the tropical region indicates a rapid warming from the early to mid holocene followed by a relatively weak warming during the late holocene the dominant mode of the north atlantic region sst captures the transition from relatively warm cold conditions in the eastern north atlantic and the western mediterranean sea the northern red sea to relatively cold warm conditions in these regions from the early to late holocene this pattern of holocene sst variability resembles the signature of the arctic oscillation north atlantic oscillation ao nao the second mode of both tropical and north atlantic regions captures a warming towards the mid holocene and a subsequent cooling the dominant modes of holocene sst variability emphasize enhanced variability around 2300 and 1000 years the leading mode of the coupled tropical north atlantic holocene sst variability shows that an increase of tropical sst is accompanied by a decrease of sst in the eastern north atlantic an analogy with the instrumental period as well as the analysis of a long term integration of a coupled ocean atmosphere general circulation model suggest that the ao nao is one dominant mode of climate variability at millennial time scales