seismic hazard in central southern africa summary seismic hazard maps of central southern africa where hazard has been expressed in terms of peak ground acceleration for an annual probability in excess of show relatively high values that distinguish the seismic hazard potential of the deka fault zone the mid zambezi basin luangwa rift and western central mozambique in areas such as central southern africa where little is known about the geology of the region and the fault systems have not been fully mapped seismic hazard potential may be estimated from seismicity and broad scale fault features for this region such potential is based on earthquake magnitude m 2 6 events of such magnitude have recently occurred in the mid zambezi basin southern zimbabwe and western central mozambique this paper follows the conventional probabilistic hazard analysis procedure defining seismic source zones from seismicity based on instrumental records from a cataloque that spans a period of 83 years geological and geomorphological features in the region are described on the mesoscale and are correlated with the seismicity as broad fault zones the scarcity of strong motion accelerogram data necessitated the formulation of attenuation values based on random vibration theory

neoproterozoic adakitic plutons in the northern margin of the yangtze block china partial melting of a thickened lower crust and implications for secular crustal evolution abstract numerous neoproterozoic felsic and mafic ultramafic intrusions occur in the hannan region at the northern margin of the yangtze block among these the wudumen and erliba plutons consist of granodiorites and have shrimp zircon u pb ages of 735 ma the rocks have high k 2 o 0 8 3 6 wt and na 2 o 4 4 6 4 wt and low mgo 0 4 1 7 wt they also have high sr y 32 209 and la yb n ratios 4 4 38 6 their e nd values range from 0 41 to 0 92 and zircon initial 176 hf 177 hf ratios from 0 282353 to 0 282581 these geochemical features are similar to those of adakitic rocks produced by partial melting of a thickened lower crust our new analytical results combined with the occurrence of voluminous arc related mafic ultramafic intrusions emplaced before 740 ma lead us to propose that the crustal evolution in the northern margin of the yangtze block during neoproterozoic involved 1 rapid crustal growth and thickening by underplating of mafic magmas from the mantle which was modified by materials coming from the subducting oceanic slab from 1 0 to 0 74 ga and 2 partial melting of the thickened lower crust due to a thermal anomaly induced by upwelling of asthenosphere through an oceanic slab window producing the 735 ma adakitic wudumen and erliba plutons our model suggests that the crustal thickness was more than 50 km at the northern margin of the yangtze block at 735 ma and rule out the possibility of a mantle plume impact causing the 735 ma magmatism in the region

the genesis of gold mineralisation hosted by orogenic belts a lead isotope investigation of irish gold deposits abstract lead isotope analyses have been performed on 109 gold and 23 sulphide samples from 34 irish gold occurrences including 27 placers and used to shed light on the sources of mineralising fluids and metals associated with gold mineralisation hosted by orogenic belts the pb isotope ratios of lode and placer gold range from 206pb 204pb 17 287 18 679 207pb 204pb 15 382 15 661 and 208pb 204pb 37 517 38 635 consistent with the pb isotopic data on previously reported irish sulphide mineralisation there is no evidence that gold mineralisation is associated with distinctive source regions and it appears to have been derived from similar sources to those responsible for the widespread sulphide mineralisation in ireland it is inferred that the principal controls on the au mineralisation are structural and not related to the distribution of au in their source rocks the range of pb isotope ratios favours the interaction of multiple source reservoirs predominantly during the caledonian orogeny c 475 380 ma underlying basement was the primary control on two key sources of pb gold occurrences located to the south east of the iapetus suture are characterised by pb compositions that derive predominantly from the late proterozoic crustal basement or overlying lower palaeozoic sediments whilst those located north west of the iapetus suture are characterised by less radiogenic pb signatures derived predominantly from late proterozoic or older crustal basement a third source relatively enriched in radiogenic pb also played a role in the formation of a number of irish gold occurrences and may have been associated with syn to post orogenic intrusives magmatic processes may therefore have played an important role in the formation of some orogenic gold occurrences

stable isotope compositions of serpentinite seamounts in the mariana forearc serpentinization processes fluid sources and sulfur metasomatism the mariana and izu bonin arcs in the western pacific are characterized by serpentinite seamounts in the forearc that provide unique windows into the mantle wedge we present stable isotope o h s and c data for serpentinites from conical seamount in the mariana forearc and s isotope data for torishima seamount in the izu bonin forearc in order to understand the compositions of fluids and temperatures of serpentinization in the mantle wedge and to investigate the transport of sulfur from the slab to the mantle wedge six serpentine mineral separates have a restricted range of 18o 6 5 8 5 antigorite separates have d values of 29 5 to 45 5 that reflect serpentinization within the mantle wedge whereas chrysotile has low d values 51 8 to 84 0 as the result of re equilibration with fluids at low temperatures fractionation of oxygen isotopes between serpentine and magnetite indicate serpentinization temperatures of 300 375 c two late cross fiber chrysotile veins have higher 18o values of 8 9 to 10 8 and formed at lower temperatures as low as 100 c aqueous fluids in equilibrium with serpentine at 300 375 c had 18o 6 5 9 and d 4 to 26 consistent with sediment dehydration reactions at temperatures 200 c in the subducting slab rather than a basaltic slab source three aragonite veins in metabasalt and siltstone clasts within the serpentinite flows have 18o 16 7 24 5 consistent with the serpentinizing fluids at temperatures 250 c 13c values of 0 1 2 5 suggest a source in subducting carbonate sediments the 34s values of sulfide in serpentinites on conical seamount 6 7 to 9 8 result from metasomatism through variable reduction of aqueous sulfate 34s 14 derived from slab sediments despite sulfur metasomatism serpentinites have low sulfur contents generally 164 ppm that reflect the highly depleted nature of the mantle wedge the serpentinites are mostly enriched in 34s median 34ssulfide 4 5 consistent with a 34s enriched mantle wedge as inferred from arc lavas

isotope evidence for the involvement of recycled sediments in diamond formation diamonds as chemically robust containers of material from the mantle have provided important information regarding the evolution of the earth s interior1 yet despite much effort2 9 the origins of diamonds themselves have remained controversial10 13 sulphur isotope ratios measured in most mantle or meteoritic samples exhibit minimal deviation from the meteoritic standard whereas the ratios in crustal materials deviate by as much as 70 ref 14 for this reason isotopic studies of sulphide mineral inclusions in diamonds and mantle material are an attractive means of investigating the possible role of crust mantle interaction in the formation of diamonds15 recent ion microprobe studies have found departures from mantle values in the sulphur isotope compositions of individual diamond inclusions16 and some mantle rocks17 which would be consistent with subduction of altered ocean crust into the region of diamond growth here we present new sulphur and lead isotope data from diamond sulphide inclusions which suggest that sedimentary material might also be subducted into the mantle and that the crustal recycling process has been operative for at least 1 000 million years

marymia an archean amphibolite facies hosted orogenic lode gold deposit overprinted by palaeoproterozoic orogenesis and base metal mineralisation western australia the marymia gold deposit comprising two orebodies keillor 1 and keillor 2 is at the northern end of the plutonic well greenstone belt in the marymia inlier in the southern capricorn orogen just north of the yilgarn craton the marymia inlier is a discrete fault bounded archean gneiss granitoid greenstone domain surrounded by sedimentary basins that were formed and variably metamorphosed and deformed during several palaeoproterozoic orogenic cycles the greenstone sequence at marymia is stratigraphically and geochemically similar to greenstone sequences in the yilgarn craton but was subjected to further deformation and metamorphism in the palaeoproterozoic late archean deformation d1 d2 was ductile to brittle ductile in style whereas palaeoproterozoic deformation was predominantly brittle equilibrium mineral assemblages indicate that peak amphibolite facies metamorphism 540 575 c 3 kb was overprinted by greenschist facies metamorphism 300 360 c petrographic textures indicate that prograde metamorphism was coeval with d1 d2 with peak metamorphism early to syn d2 gold mineralisation at marymia is hosted in metamorphosed tholeiitic basalts and banded iron formation on a gross scale the distribution of gold is controlled by d2 folds and shear zones lithological contacts with strong rheological or chemical contrasts provide local controls gold related alteration comprises subtle millimetre to centimetre wide zones of silicification with variable amounts of quartz hornblende biotite k feldspar plagioclase calcite siderite scheelite titanite epidote sulfide and telluride minerals quartz veins are generally narrow and discontinuous with low total volume of quartz gold is sited in the wall rock at vein salvedges or within stringers of wall rock within veins there are two distinct opaque mineral assemblages pyrite pyrrhotite chalcopyrite galena and hessite petzite altaite bi telluride galena ore samples are variably enriched in ag te pb w cu s and fe reflecting heterogeneity of the ore mineralogy structural timing and temperature of formation of alteration and ore minerals support deposition of gold during late peak amphibolite facies metamorphism from neutral to alkaline ph 5 6 moderately oxidising log o2 21 22 and co2 bearing x co2 0 2 fluids the total sulfur content of the fluid is estimated at 1m s lead isotope compositions support derivation of lead from within the local greenstone sequence gold lodes were deformed by faults and shear zones in the palaeoproterozoic with only limited remobilisation subeconomic carbonate vein and breccia hosted base metal mineralisation is locally hosted within palaeoproterozoic fault zones which clearly cut gold lodes base metal related alteration is characterised by intense carbonatisation chloritisation and albitisation of the mafic host rocks mineral assemblages are consistent with formation at greenschist facies conditions lead isotope compositions support crystallisation at ca 1 7 ga from lead that is similar in composition to earlier gold related galena

structural and geochronological constraints on the tectono thermal evolution of the danba domal terrane eastern margin of the tibetan plateau the songpan ganze terrane of the tibetan plateau is underlain by neoproterozoic crystalline basement rocks of the yangtze block these basement rocks are exposed as a series of extensional tectonic domes that form a nearly north south trending extensional belt more than 1000 km long in the eastern margin of the tibetan plateau in the danba area detachment faults separate the basement core complexes e g the gezong and gongcai complexes from the paleozoic strata which have been thinned or removed completely the cover sequences have undergone upper greenschist to lower amphibolite facies metamorphism to form the danba schist and are overlain by the triassic xikang group a thick flysch sequence both the basement rocks and the paleozoic rocks have undergone multiple stages of deformation and thus provide an excellent opportunity to study the tectono thermal evolution of the eastern margin of the tibetan plateau two stages of deformation corresponding to three generations of foliation s1 view the mathml source and view the mathml source have been recognized on the basis of structural and microscopic observations we selected amphibole and biotite separates associated with distinct generations of foliation for 40ar 39ar dating using laser microprobe incremental heating technique to place numerical constraints on the major tectono thermal events within the danba area the geochronogical results reveal an earliest metamorphic event at 258 6 0 5 ma s1 biotite and 263 6 0 8 ma s1 amphibole coinciding temporally with the mantle plume that produced the voluminous emeishan flood basalts the second event was a progressive extensional deformation first occurred at 159 166 ma view the mathml source amphibole responsible for the earlier tectonic doming of the crystalline basement and then the final tectono thermal overprint recorded by view the mathml source foliation and view the mathml source metamorphism locally in the core complexes at 47 58 ma for the gezong complex and 64 81 ma for the gongcai complex this major post orogenic extensional event is believed to be a consequence of collision between the north china and south china blocks the apparent discrepancy of the 40ar 39ar ages observed between localities suggests a slow cooling process associated with progressive uplift

optically continuous silcrete quartz cements of the st peter sandstone high precision oxygen isotope analysis by ion microprobe a detailed oxygen isotope study of detrital quartz and authigenic quartz overgrowths from shallowly buried 1 km quartz arenites of the st peter sandstone in sw wisconsin constrains temperature and fluid sources during diagenesis quartz overgrowths are syntaxial optically continuous and show complex luminescent zonation by cathodoluminescence detrital quartz grains were separated from 53 rocks and analyzed for oxygen isotope ratio by laser fluorination resulting

sulphur isotopic composition of sulphides from the beaver dam and other meguma group hosted gold deposits nova scotia implications for genetic models sulphur isotopic compositions were determined for sulphide mineral phases in meguma group hosted gold deposits nova scotia in order to resolve i potential source regions for sulphur in these de

oxygen isotope exchange and closure temperatures in cooling rocks retrograde exchange of oxygen isotopes between minerals in igneous and metamorphic rocks by means of diffusion is explored using a finite difference computer model which predicts both the zonation profile of 18o within grains and the bulk 18o value of each mineral in the rock apparent oxygen isotope equilibrium temperatures that would be observed in these rocks are calculated from the 18o values of each mineral pair within the rock in systems which cool linearly from a sufficiently high temperature or at a low enough cooling rate such that the final oxygen isotope values are not dependent upon the initial oxygen isotope values slow cooling the apparent oxygen isotope temperature derived for a rock composed of a single mineral pair can be shown to be simply related to the dodson closure temperatures tc for the two phases and the mode of the rock adding a third phase into a system which undergoes slow cooling will cause the apparent temperature derived for the two minerals already present to differ from the simple relationship for a two phase system in some systems oxygen isotope reversals can be developed if cooling is not slow then the mineral 18o values resulting from cooling will be partly dependent upon the initial temperature of the system concerned the model successfully simulates the mineral 18o values that are often observed in granitic rocks application of the model will help in assessing the validity of oxygen isotope thermometry in different geological settings and allows quantitative prediction of the oxygen isotope fractionations that are developed in cooling closed systems

crustal response to continental collisions between the tibet indian south china and north china blocks geochronological constraints from the songpan garze orogenic belt western china we report shrimp u th pb monazite conventional u pb titanite sm nd garnet and rb sr muscovite and biotite ages for metamorphic rocks from the danba domal metamorphic terrane in the eastern songpan garze orogenic belt eastern tibet plateau these ages are used to determine the timing of polyphase metamorphic events and the subsequent cooling history the oldest u th pb monazite and sm nd garnet ages constrain an early barrovian metamorphism m1 in the interval c 204 190 ma coincident with extensive indosinian granitic magmatism throughout the songpan garze orogenic belt a second higher grade sillimanite grade metamorphic event m2 recorded only in the northern part of the danba terrane was dated at c 168 158 ma by a combination of u th pb monazite and titanite and sm nd garnet ages it is suggested that m1 was a thermal event that affected the entire orogenic belt while m2 may represent a local thermal perturbation rb sr muscovite ages range from c 138 100 ma whereas rb sr biotite ages cluster at c 34 24 ma these ages document regional cooling at rates of c 2 3 c myr 1 following the m1 peak for most of the terrane however those parts of the terrane affected by the higher temperature m2 event e g the migmatite zone experienced initially more rapid c 8 c myr 1 cooling after peak m2 before joining the regional slow cooling path defined by the rest of the terrane at c 138 ma regional slow cooling between c 138 and c 30 ma is thought to be the result of post tectonic isostatic uplift after extensive crustal thickening caused by collision of the south and north china blocks the clustering of biotite rb sr ages marks the onset of rapid uplift across the entire terrane commencing at c 30 20 ma this cooling history is shared with many other regions of the tibet plateau suggesting that uplift of the tibet plateau including the songpan garze orogenic belt occurred predominantly in the last c 30 myr as a response to the continuing northwards collision of india with eurasia

multiple isotope composition s pb h o he and ar and genetic implications for gold deposits in the jiapigou gold belt northeast china the jiapigou gold belt 150 t au one of the most important gold producing districts in china is located at the northeastern margin of the north china craton it is composed of 17 gold deposits with an average grade around 10 g t au the deposits are hosted in archean gneiss and ttg rocks and are all in shear zones or fractures of varying orientations and magnitudes the 34s values of sulfide from ores are mainly between 2 7 and 10 the pb isotope characteristics of ore sulfides are different from those of the archean metamorphic rocks and mesozoic granites and dikes and indicate that they have different lead sources the sulfur and lead isotope compositions imply that the ore forming materials might originate from multiple mainly deep sources fluid inclusions in pyrite have 3he 4he ratios of 0 6 to 2 5 ra whereas their 40ar 36ar ratios range from 1 444 to 9 805 indicating a dominantly mantle fluid with a negligible crustal component 18o values calculated from hydrothermal quartz are between 0 2 and 5 9 and d values of the fluids in the fluid inclusions in quartz are from 70 to 96 these ranges suggest dominantly magmatic water with a minor meteoric component the noble gas isotopic data along with the stable isotopic data suggest that the ore forming fluids have a dominantly mantle source with minor crustal addition

mesozoic cenozoic evolution of the danba dome songpan garze east tibet as inferred from la icpms u pb and fission track data abstract the eastern margin of the tibetan plateau is a key region to understand the formation and tectonic evolution of the plateau most studies concentrated on the longmen shan range that corresponds to the tectonically very active boundary between the tibetan plateau and the sichuan basin this work addresses the deformation and exhumation pattern in the poorly understood metamorphic danba dome situated at the conjunction between the southern longmen shan range and the major strike slip xian shui he fault the early mesozoic and cenozoic evolution of the dome are then compared to the regional evolution of eastern tibet the about 830 ma zircon u pb ages obtained on the gezong and gongcai gneiss as well as u pb ages on inherited zircons in the early mesozoic granites confirm the occurrence of neo proterozoic magmatic episodes in the danba basement at 800 900 ma and 650 750 ma following the closure of the palaeotethys ocean granitoids were emplaced in the danba metamorphic basement and triassic cover from about 230 ma to about 170 ma the western part of the lian he kau granitic massif emplaced between 210 and 200 ma within the triassic cover sequence of the dome was only slightly affected by the early mesozoic deformation however in the eastern deeper level compartment of the massif ductile deformation occurred between 200 and 190 ma contemporaneous with the onset of the barrovian type metamorphism dated at 200 180 ma that deformation is compatible with the early mesozoic deformation observed in the longmen shan range and with the development of the large scale decollement level in the songpan garze area apatite fission track data then indicate that following the oligocene middle eocene onset of eastward motion of the songpan garze terrane the danba dome developed as a crustal scale antiform during the late miocene early pliocene 10 4 ma the exhumation was controlled by the lian he kau fault a major transpressive structure parallel to the xian shui he fault and the associated manai thrust faults regionally while the oligocene middle miocene exhumation phase seems to have been of regional extent the late miocene to present day phase is localised along the longmen shan range thrusts as well as along the back thrusts formed by the lian he kau manai and associated faults

lead isotope evidence for a hercynian origin of the salsigne gold deposit southern massif central france the salsigne gold deposit contains a complex association of sulphide layers gold rich disseminations quartz bearing veins and flat reefs which are hosted by folded and slightly metamorphosed paleozoic sediments on both sides of a major thrust zone it is demonstrated that these various au as ore types have similar lead isotope compositions 206pb 204pb 18 22 18 56 which are characteristic of hercynian ore deposits in the southern massif central the lower cambrian to devonian host rocks and associated pb zn ba occurrences display distinctly less radiogenic corrected isotopic signatures 206pb 204pb 17 83 17 98 which are characteristic of cambrian lead in the montagne noire concerning the controversial origin of the salsigne gold mineralization these results disagree with the former syngenetic hypothesis and support a new model of hercynian syntectonic gold concentration

patterns of late cenozoic exhumation deduced from apatite and zircon u he ages from fiordland new zealand new apatite and zircon u th he ages from the fiordland region of new zealand s south island expand on earlier results and provide new constraints on patterns of late cenozoic exhumation and cooling across this region zircon u th he cooling ages in combination with increased density of apatite ages show that in addition to a gradual northward decrease in cooling ages that was seen during an earlier phase of this study there is also a trend toward younger cooling ages to the east distinct breaks in cooling age patterns on southwestern fiordland appear to be correlated to the location of previously mapped faults the northward decrease in ages may reflect asynchronous cooling related to migration in the locus of exhumation driven by subduction initiation or it may reflect synchronous regional exhumation that exposed different structural levels across fiordland or some combination of these effects in either case differential exhumation accommodated by major and minor faults that dissect fiordland basement rocks apparently played an important role in producing the resulting age patterns

generation and emplacement of triassic granitoids within the songpan ganze accretionary orogenic wedge in a context of slab retreat accommodated by tear faulting eastern tibetan plateau china the eastern songpan ganze accretionary orogenic wedge northern tibetan plateau is characterized by extensive magmatism coeval with middle to upper triassic closure of the paleotethys ocean along a double subduction system traced by the kunlun anyemaqen suture to the north and the jinsha litang suture to the south new field petrographic and geochemical data on mesozoic plutons intrusive into triassic metasediments of the eastern songpan ganze wedge are presented and integrated in a review of available data for the region three types of granitoids are distinguished i high k calc alkaline granite to granodiorite yanggon maoergai markam sheng meng xue sheng taiyanghe menggu manai dusong xian tagong and jiulong plutons ii high k alkaline granite nyanbaoyeche and rilong plutons and the niuxingou shoshonitic syenite and iii peraluminous s type granite markam and manai leucogranites elemental and isotopic signatures of these granitoids are consistent with the implication of both crustal yangtze craton and songpan ganze metasediments and mantle asthenospheric mantle and metasomatised lithospheric mantle sources based on i the ages of the plutons that are concomitant with the end of the northern and southern subductions of the paleo tethys ii the position of the plutons intrusive in the crustal tectonically accreted units decoupled from the downgoing plate of this double subduction system iii the diversity of their petrological and geochemical signatures we propose that magma emplaced in the songpan ganze accretionary orogenic wedge were generated at the end of the paleotethys closure in a context of slab retreat accommodated by a tear fault along the passive margin of the south china block indeed we argue that this context is the most favourable to trigger synchronous partial melting of i the suprasubduction enriched mantle wedge owing to lowering of the mantle solidus by metasomatism ii the upwelled undepleted asthenospheric mantle into the tear fault owing to decompression iii the metasomatized lithospheric continental mantle of the yangtze craton owing to the temperature increase associated to the asthenospheric upwelling and iv the rocks of the songpan ganze accretionary wedge composed of metasediments and yangtze continental margin basement owing to the combined effects of increased radioactive heat production and mantle heat flux we propose that the concentration of plutons along the eastern margin of the songpan ganze results from focussed migration and emplacement of the granitic magmas guided by the development of a tear fault along the former passive eastern margin of the yangtze craton as a consequence of the paleotethys slab retreat

nature age and tectonic setting of granitoid hosted orogenic gold deposits of the jiaodong peninsula eastern north china craton china the jiaodong gold province along the southeastern margin of the north china craton has combined production and resources of 900 tonnes of gold more than 95 of the gold in this province is hosted in mesozoic granitoids which intruded late archean metamorphic basement rocks episodically at ca 160 150 and 130 126 ma the deposits of the jiaodong peninsula form one of the largest provinces of granitoid hosted lode gold deposits in the world the jiaodong gold deposits occur as massive auriferous quartz veins hosted in subsidiary second or third order faults and as disseminated and stockwork style mineralization hosted in alteration zones along major regional scale faults mineralization took place during brittle reactivation d3 of pre existing d2 ductile shear zones wall rock alteration is dominated by k feldspar and sericite with also sulfide minerals and extensive silicification closest to the orebodies the alteration was accompanied by the introduction of volatiles fe mg k rb ag as au bi and sb enrichment of base metals in the gold lodes is generally low although locally significant gold deposition took place at temperatures of 250 to 350 c for most gold deposits and the ore fluids are co2 bearing and have calculated salinities of 6 13 wt nacl equiv deposits from the zhao ye xixia and muping rushan belts have similar if not identical structural controls alteration patterns ore mineral assemblages and stable isotope signatures lead isotope data indicate that the ore stage leads were buffered by their host rocks the granitoid hosted gold deposits in the zhao ye belt are most radiogenic the greenstone hosted deposits in the xixia belt is the least radiogenic and the granitoid hosted deposits in the muping rushan belt have intermediate compositions gold mineralization in the zhao ye xixia and muping rushan belts in the jiaodong gold province appears to be broadly contemporaneously at ca 130 120 ma as constrained by shrimp u pb zircon dating on host rocks and a post gold dike as well as by ar ar rb sr and k ar dating of hydrothermal alteration minerals and ores this early cretaceous age is about 100 million years younger than the peak of ultra high pressure metamorphism in the qinling dabie sulu orogen between the north china and yangtze cratons and about 500 km south of the gold province however it overlaps with the extension and exhumation stage of the collisional orogeny and the subduction of the pacific plate beneath the eurasian continental crust whereas the majority of the jiaodong gold deposits occur on the margin of the north china craton some also occur in the suture zone between the two cratons they are significantly landward relative to the subduction zone of the pacific plate compared with typical orogenic lode gold deposits globally lode gold deposits of ca 130 120 ma occurred on both sides of the tan lu fault zone available data favor a temporal link of gold mineralization with pacific plate subduction

rb sr sm nd and pb isotope systematics of pyrite implications for the age and genesis of lode gold deposits lode gold deposits commonly consist of pyrite and lesser amounts of lead and zinc sulfides accompanied by quartz and calcite their exact origin remains controversial mainly because of the scarcity of reliable geochronological data here we present direct rb sr dating of pyrite from a lode gold deposit and propose the method as a useful geochronological technique for gold mineralization for which age data are poor a positive correlation between present day 87 sr 86 sr and 87 rb 86 sr ratios of pyrite fes 2 subsamples from the linglong gold mine of china corresponds to an age of 122 123 ma which dates the age of gold mineralization the sr nd and pb isotopic compositions of pyrites and the associated rocks suggest that the hydrothermal fluids responsible for the pyrite and gold mineralization were probably derived from a mixed source i e degassing of mafic magmas and meteoric water that had leached the country rocks

near source attenuation of peak horizontal acceleration strong motion data recorded within 50 km of the rupture zone were used to study near source attenuation characteristics of horizontal peak ground acceleration for worldwide earthquakes of magnitudes 5 0 to 7 7 the data base consisted of 229 horizontal components of peak acceleration recorded from 27 earthquakes including the 15 october 1979 imperial valley earthquake these data were found to be adequately represented by the functional relationship pga a exp b m r c m d where pga represents the mean of the peak values scaled from the two horizontal components of each recording m is richter magnitude and r is distance from the fault rupture zone peak acceleration was found to be lognormally distributed with a standard error representing a 45 per cent increase in the median estimate the regression analysis statistically confirmed the results of earthquake simulation studies that have predicted peak acceleration to become independent of magnitude and distance in the near field an extensive sensitivity study showed that predictions based on our attenuation relationships are stable with respect to reasonable model and parameter variations an analysis of residuals was used to investigate the behavior of peak acceleration with respect to various earthquake site and recording parameters the more significant findings being 1 a similarity in the level of acceleration recorded on soil or rock 2 larger than average accelerations recorded at sites located on shallow soils or in areas of steep topography 3 larger than average accelerations associated with earthquakes having reverse fault mechanisms and 4 lower than average accelerations recorded in large embedded structures

nutrient enriched waters and the early skeletal fossil record this paper examines the role of stratified oxygen depleted and nutrient enriched water masses new in transforming the fossil record over the precambrian cambrian boundary interval the following may serve as markers for the encroachment of new over low latitude cratons trace element enriched black shales stone coals and hydrocarbon source rocks phosphorites phosphatized shells and organic matter sponge spicule cherts early appearance of silica and phosphatic skeletons appearance of anabarites and coleoloides biofacies peak diversity of phosphatic species occurrence of black shale lagerstatten and light 13 c values of carbonates nutrient depleted waters ndw are inferred over inner cratonic areas and nutrient starved waters nsw may well have developed over deeper waters at times through the later cambrian related to the effects of sea level rise and salinity stratification carbon isotopic fluctuations are suggested to record the history of these water masses with major changes in carbon burial and palaeoproductivity the latter may well imply episodes of greenhouse climate with globally raised levels of carbon dioxide and relatively high ocean temperatures

fault plane solutions of the deka fault zone and mid zambezi valley an analysis of twenty one fault plane solutions from the deka fault zone and the mid zambezi valley shows normal dip slip faulting with a strike slip component the fault planes strike in a south

late triassic granitoids of the eastern margin of the tibetan plateau geochronology petrogenesis and implications for tectonic evolution abstract late triassic granitoids in the songpan garze fold belt sgfb on the eastern margin of the tibetan plateau formed at 230 to 220 ma and can be divided into two groups group 1 are high k calc alkaline rocks with adakitic affinities k adakites with sr 400 ppm y n 32 105 and high k 2 o na 2 o 1 group 2 are ordinary high k calc alkaline i types with lower sr 18 ppm and weakly fractionated ree patterns la yb n 0 50 to 0 94 and initial 87 sr 86 sr 0 70528 to 0 71086 but group 1 rocks have higher e nd t 1 01 to 4 84 than group 2 3 11 to 6 71 calculated initial pb isotope ratios for both groups are 206 pb 204 pb 18 343 to 18 627 207 pb 204 pb 15 610 to 15 705 and 208 pb 204 pb 38 269 to 3759 group 1 magmas were derived through partial melting of thickened and then delaminated ttg type eclogitic lower crust with some contribution from juvenile enriched mantle melts group 2 magmas were generated by partial melting of shallower lower crustal rocks the inferred magma sources of both groups suggest that the basement of the sgfb was similar to the exposed kangding complex and that the sgfb was formed in a similar manner to the south china basement here passive margin crust was greatly thickened and then delaminated all within a very short time interval 20 myr such post collisional crustal thickening could be the tectonic setting for the generation of many adakitic magmas especially where there is no spatial and temporal association with subduction

a type granite and adakitic magmatism association in songpan garze fold belt eastern tibetan plateau implication for lithospheric delamination abstract the songpan garze fold belt covers a huge triangular area 200 000 km2 confined by the south china yangtze north china and tibetan plateau continental blocks in the songpan garze fold belt triassic adakitic granitoids have been identified however whether there are triassic a type granites is unclear here we report our first finding of an a type granite nianbaoyeche which occurs in the central part of the songpan garze fold belt the nianbaoyeche granite 820 km2 is characterized by arfvedsonite in its mineral assemblage using both la icpms and tims u pb zircon dating methods we obtain a magma crystallization age of 211 1 ma which is slightly younger than triassic adakitic granitoids 216 221 ma in the songpan garze fold belt the nianbaoyeche granite is enriched in si k na rb ree hfse nb ta zr hf with elevated feotot feotot mgo and ga al ratios but is depleted in al mg ca ba and sr the ree compositions show moderately fractionated patterns with la yb n 2 67 7 54 and eu eu 0 09 0 34 these geochemical characteristics indicate that the nianbaoyeche granite has an a type affinity geochemical data and u pb zircon age combined with regional studies show that the nianbaoyeche granite formed in a post collisional tectonic setting sr nd isotopic data for the granite exhibit isr 0 7090 0 7123 and end t 2 72 to 4 26 with tdm 1 15 1 51 ga suggesting that the magma has a dominantly crustal source though a minor contribution from the mantle cannot be ruled out melting to produce an a type granite may have resulted from triassic lithospheric delamination after triassic crustal thickening of the songpan garze fold belt due to convergence between the yangtze north china and north tibet continental blocks the lithospheric delamination model also helps to explain the triassic adakitic magma generation in the songpan garze belt we conclude that association of a type granite and adakitic granitoids in post collisional environment could be a useful indicator of lithospheric delamination

cenozoic and mesozoic metamorphism in the longmenshan orogen implications for geodynamic models of eastern tibet new zircon u pb and mica 40ar 39ar dating combined with structural studies in the longmenshan orogen confirm that most of the upper crustal deformation in the eastern margin of tibet is mesozoic however at lower structural levels apatite u pb and monazite electron microprobe dating reveals a previously unknown domain of cenozoic ca 65 ma barrovian type metamorphism and deformation this discovery shows that the crust in the eastern margin of tibet was already a substantial thickness around the time of the india asia collision associated deformation has a n s oriented stretching lineation implying that deformation was not driven by topographic gradients in the tibetan plateau the observed moderate amounts of distributed postmetamorphic e w shortening can probably explain the present thickness of the continental crust in the area these results do not support models of crustal thickening caused by solid state lateral flow of midcrustal metamorphic rocks

concentration of platinum group elements by magmatic fluids in layered intrusions evidence leading to a model which supports the formation of pt pd reefs in a layered intrusion by the action of magmatic fluids is presented as well as some of the constraints imposed on such a model the stillwater complex is used as an example of this model within the stillwater complex no laterally continuous sulfide enriched cumulates occur above the lowermost parts of the ultramafic series until one reaches the platiniferous j m reef this sulfide poor stratigraphic interval beneath the j m reef is characterized by unusually cl rich apatite although reported sulfide modes and whole rock sulfur contents in the cumulates below the j m reef are approximately an order of magnitude lower than that expected for a sulfide saturated magma sulfide saturation prior to the formation of the j m reef is suggested by relatively high pd and pt concentrations reported in the cumulates below the j m reef as well as by the presence of sulfide as inclusions in silicate and oxide cumulus minerals hence it is suggested that the footwall cumulates to the j m reef were originally sulfide saturated but later lost sulfur to a fluid phase exsolving from inter cumulus liquid sporadic sulfide bearing pegmatoids in some of these rocks are additional evidence for late loss of sulfur these observations as well as the association of the j m reef at the first significant stratigraphic break in the cl cl f ratio of apatite and the relatively high solubility of pt and pd as aqueous cl complexes are consistent with the formation of the j m reef by exsolution of cl and volatile rich fluids from footwall intercumulus liquids which leached and transported ore components both the pge and s upward to the level of the reef using published whole rock platinum group element pge concentrations for the ultramafic and lower banded series the cumulates below the j m reef must have been enriched by a factor of at most 1 24 for pt and 1 75 for pd if these rocks were to be the source for the metals in the j m reef model raleigh fractionation calculations suggest that the cumulates beneath the j m reef could have had the required metal concentrations if during the crystallization of a parent magma containing no more than 15 ppb of either metal the average bulk solid liquid distribution coefficients for pt and pd were no more than 5 these values are consistent with assumption of sulfide saturation prior to the formation of the j m reef during solidification of intercumulus liquid the separation and loss of a cl rich fluid will deplete the residual assemblage of s and pge if 0 1 to 0 5 wt percent fluid containing 5 percent s evolved during solidification of the footwall cumulates the amount of fluid expected to be exsolved by the crystallization of approximately 2 to 10 percent fluid saturated intercumulus liquid the fluid liquid distribution coefficients for pt and pd would need to be almost as large as the sulfide liquid silicate liquid distribution coefficients to remove significant amounts of pt and pd much smaller distribution coefficients are needed if sulfide is completely lost to fluid during degassing the latter possibility is consistent with the low s contents of the footwall cumulates beneath the j m reef numerical models of the degassing of intercumulus liquids suggest that thick cumulus sequences can act as chromatographic columns to separate the pge and s during the degassing of intercumulus silicate liquids the pge being enriched at a sulfide dissolution front as upward migrating sulfide undersaturated fluids resorb cumulus sulfides zones of pge and s enrichment can occur where degassing fronts encounter stratigraphic physical chemical discontinuities such as changes in the fluid saturation temperature of the intercumulus silicate liquids

the relationship of mantle derived fluids to gold metallogenesis in the jiaodong peninsula evidence from d o c s isotope systematics abstract the largest gold district in china is the jiaodong peninsula where three types of gold deposits are recognized quartz vein fracture altered and breccia types the first two developed along a group of ne trending faults and are hosted by granitic intrusions dated at 160 to 150 ma biotite granite and 130 to 126 ma granodiorite and by metamorphic rocks of the precambrian crystalline basement the breccia type gold system is mainly located around the northern margin of the jiaolai cretaceous basin where mineralisation is controlled by both detachment fractures and ne trending faults this study is based on stable isotope determinations from ten gold deposits including linglong jiaojia sanshandao cangshang wang ershan dayigezhuang denggezhuang pengjiakuang fayunkuang and dazhuangzi as well as the linglong jurassic biotite granite guojialing cretaceous granodiorite and archean gneiss the stable isotope systematics reflect the style of the three types of gold deposits but also show that they belong to the same metallogenic system in which the hydrothermal fluids were derived from a mantle fluid reservoir and mixed with crustal fluids the ore forming age is later than both the jurassic biotite granite and cretaceous granodiotite but overlaps with the 121 to 114 ma ages of lamprophyre and diabase dykes the hydrothermal fluids that were responsible for both gold mineralisation and the retrograde alteration of the diabase and lamprophyre dykes are similar and represent a co 2 and potassium rich system this fluid system is interpreted to be the consequence of cretaceous lithospheric thinning asthenospheric upwelling and mantle degassing in eastern china

spectral estimates of seismic shear waves the scaling of seismic sources according to brune 1970 1971 with a stress drop of 100 bars allows accurate prediction of average fourier spectra of seismic shear waves in the far field for the frequency band 1 to 10 hz random vibration theory under the assumption of stationary response accurately predicts average linear elastic response spectrum amplitudes in this same frequency band these conclusions are reached through comparison of theoretical estimates with strong motion observations during a wide range of california earthquakes local magnitudes 4 0 to 7 2 distances 9 to 130 km thus these models are an accurate physically based method of estimating high frequency strong ground motion for a range of earthquake magnitudes and distances and offer the advantage over purely empirical methods of a physical basis for extrapolating to predict ground motions for magnitudes and distances which are poorly documented with data

pt and pd mobility in hydrothermal fluids evidence from komatiites and from thermodynamic modelling abstract komatiite host rocks and ores provide a natural laboratory for the study of pt and pd during hydration and carbonation of ultramafic rocks the degree of correlation between pt and pd and of those elements with lithophile elements such as ti in komatiitic rocks and ores is strongly dependent on the magmatic sulfide content of the rock pt and pd are typically very strongly correlated within s poor komatiites and in disseminated ores containing 5 volume or less magmatic sulfides in contrast they are almost completely decoupled in sulfide rich ore samples thermodynamic modelling based on the existing body of published experimental data on pt and pd solubilities suggests that pd and pt can be readily transported as bisulfide complexes in acidic neutral solutions under reduced and moderate oxidation condition at 300 c and chloride complexes are only important under extremely acidic oxidised conditions pd is much more soluble than pt in the case of fluids in sulfide rich environments in equilibrium with the characteristic assemblages found within altered and metamorphosed komatiites solubilities of both elements are much lower in sulfide poor environments especially where ph values are constrained to neutral values by the near ubiquitous presence of carbonate minerals in the alteration assemblage this accounts for the evident near immobility of pt and pd in even the most altered silicate host rocks as compared with their wide dispersion in sulfide rich ores

attenuation of strong ground motion in canada from a random vibrations approach abstract theoretical relations are developed for the attenuation of horizontal peak ground acceleration pga and velocity pgv in canada with emphasis on eastern canada the theoretical basis is a composite of several approaches at intermediate epicentral distances results from random vibration theory hanks and mcguire 1981 boore 1983 which can be used to describe pga and pgv in terms of shear radiation implied by brune9s 1970 source model are applied to canadian earthquakes motions are extended into the near source region using arguments concerning magnitude scaling joyner and boore 1981 at distances greater than two crustal thicknesses ground motions are attributed largely to the lg phase and are attenuated accordingly nuttli 1982 the resulting attenuation relations for pga and pgv in eastern canada are in excellent agreement with strong motion data from moderate eastern canadian and central united states earthquakes compared to empirical attenuation relationships based on modified mercalli intensity the theoretical model predicts a stronger dependence of peak ground motions on magnitude in other respects amplitude levels and distance decay empirical and theoretical predictions are similar the model predicts that eastern events will have higher peak accelerations but similar peak velocities in comparison to their western counterparts of the same moment magnitude this arises from the relative enrichment of eastern earthquakes in higher frequencies

fluid inclusion record of the hypozonal orogenic renco gold deposit zimbabwe during the retrograde p t evolution the hypozonal renco gold deposit in zimbabwe is hosted by a shear zone system cutting granulite facies enderbites the geologic history of the region testifies to a polyphase tectonometamorphic evolution along the exhumation path three major events are notable during which fluid inclusions could have been formed detailed petrographic work and comparison of calculated isochores with the p t evolution allows correlation of inclusion formation with the tectonometamorphic evolution fluid inclusions in the mineralized lithologies of the renco gold mine are dominated by carbonic inclusions of highly variable densities the high density carbonic inclusions are related to the peak of metamorphism their lower density counterparts show a bimodal occurrence some are re equilibrated equivalents of the inclusions trapped at peak metamorphic conditions whereas others clearly post date this event and are related to gold mineralization the latter type of carbonic inclusions occurs in distinct microstructural domains of quartz crystals i e subgrain grain boundaries therefore these inclusions are interpreted to have formed because of selective trapping of co2 from a mixed h2o co2 low salinity ore fluid all the aqueous inclusions observed at renco can be attributed to a late retrograde greenschist facies overprint

relative timing of deformation and two stage gold mineralization at the hutti mine dharwar craton india gold mineralization at hutti is confined to a series of nine parallel n s to nnw sse trending steeply dipping shear zones the host rocks are amphibolites and meta rhyolites metamorphosed at peak conditions of 660 40 c and 4 1 kbar they are weakly foliated s1 and contain barren quartz extension veins the auriferous shear zones reefs are typically characterized by four alteration assemblages and laminated quartz veins which in places occupy the entire reef width of 2 10 m and contain the bulk of gold mineralization a 1 5 m wide distal chlorite sericite biotite calcite plagioclase alteration zone can be distinguished from a 3 5 m wide proximal biotite plagioclase quartz muscovite calcite alteration zone gold is both spatially and temporally associated with disseminated arsenopyrite and pyrite mineralization an inner chlorite k feldspar quartz calcite scheelite tourmaline sphene epidote sericite alteration halo which rims the laminated quartz veins is characterized by a pyrrhotite chalcopyrite sphalerite ilmenite rutile and gold paragenesis the distal chlorite sericite and proximal biotite plagioclase alteration assemblages are developed in microlithons of the s2 s3 crenulation cleavage and are replaced along s3 by the inner chlorite k feldspar alteration indicating a two stage evolution for gold mineralization ductile d2 shearing alteration and gold mineralization formed the reefs during retrograde evolution and fluid infiltration under upper greenschist to lower amphibolite facies conditions 560 60 c 2 1 kbar the reefs were reactivated in the d3 dextral strike slip to oblique slip environment by fault valve behavior at lower greenschist facies conditions ca 300 350 c which formed the auriferous laminated quartz veins later d4 crosscutting veins and d5 faults overprint the gold mineralization the alteration mineralogy and the structural control of the deposit clearly points to an orogenic style of gold mineralization which took place either during isobaric cooling or at different levels of the archean crust from overlaps in the tectono metamorphic history it is concluded that gold mineralization occurred during two tectonic events affecting the eastern dharwar craton in south india between ca 2550 2530 ma 1 the assemblage of various terranes of the eastern block and 2 a tectono magmatic event which caused late to posttectonic plutonism and a thermal perturbation it differs however from the pre peak metamorphic gold mineralization at kolar and the single stage mineralization at ramagiri notably greenschist facies gold mineralization occurred at hutti 35 90 million years later than in the western dharwar craton

diffusion effects on oxygen isotope temperatures of slowly cooled igneous and metamorphic rocks recently obtained data on oxygen diffusion in feldspars quartz and hornblende permit the prediction of the apparent18o16o temperatures that would be measured in a rock that consisted only of those three minerals and cooled slowly from high temperature the computed temperatures would be based on the differences in the18o16o ratios between coexisting pairs of minerals the present calculation takes into account the diffusion rates for oxygen as a function of temperature the cooling rate of the rock the mineral grain sizes and the mode of the rock for mineral grains 1 mm in radius and a cooling rate of 10 c m y the minimum difference in apparent temperature between quartz feldspar and feldspar hornblende pairs will be 115 c despite the assumption of a normal uneventful slow cooling history to room temperature further the apparent quartz hornblende temperature will range over 30 c 590 620 c depending on the mode of the rock for a cooling rate of 1000 c m y the apparent difference in temperature can be as much as 400 c consequently consistency in temperatures obtained by oxygen isotope analysis should not be expected in most high grade metamorphic rocks or igneous rocks which are cooled slowly departures from the pattern of temperatures obtained in this model would imply a very rapid quench from high temperature or a complex history for the rock for some minerals including hornblende the relation between temperature and the equilibrium fractionation of oxygen isotopes between coexisting phases has been derived from observed relations in natural specimens the choice of the specimens used for such calibrations needs to be re evaluated in light of these findings this may result in a change in the equilibrium equation constants an example from the literature the san jose tonalite baja california mexico was modelled and yields 18o concentrations in the minerals that correspond closely with the measured values this suggests that the model used is appropriate that the rock has had a simple thermal history and that it cooled at 100 200 c m y over the temperature range 800 500 c the set of paleotemperatures obtained for a rock will in general yield neither the mineral closure temperatures nor the formation or crystallization temperatures on the other hand the cooling rate of the rock may be derived from the data this in turn may have important tectonic implications with regard to denudation and uplift rates

widespread delayed mixing in early to middle cambrian marine shelfal settings abstract the radiation of burrowing metazoans in the early phanerozoic dramatically altered the properties of marine sediment an event commonly referred to as the cambrian substrate revolution or agronomic revolution the advent of infaunalization and especially biogenically mediated sediment mixing profoundly impacted the development of phanerozoic biogeochemical cycling including nutrient fluxes organic carbon burial seafloor oxygenation and sediment ecology however the timing of the development of mixed seafloor sediment has historically not been well constrained mixing has been assumed in the absence of data to occur at the precambrian cambrian boundary with the appearance of the index fossil and three dimensional burrow treptichnus pedum herein we present new ichnological stratigraphic and taphonomic data suggesting that although significant developments in infaunalization paleobiologically complex animal substrate interactions particularly burrow construction occurred during the early and middle cambrian mixing remained suppressed throughout this interval we demonstrate using a novel multi proxy approach that shelfal sediment in the earliest cambrian was essentially unmixed moreover our findings indicate that even as late as the middle cambrian 30 million years after the precambrian cambrian transition and well after the appearance of supposedly deposit feeding trilobites seafloor heterolithic shelfal sediment remained largely unmixed on a global scale these findings challenge the current assumption that mixing occurred with the first appearance of three dimensional burrows and hold important implications for the advent and development of modern style biogeochemical cycling

sulfur isotopes in sediment hosted orogenic gold deposits evidence for an early timing and a seawater sulfur source we report sulfur isotopic compositions of sulfides of various paragenetic stages in the giant sukhoi log sediment hosted orogenic au deposit in russia the overall mean value and the significant variability in early pyrite indicate that the sulfur was from the reduction of seawater sulfate the later generations of sulfide have 34s values in successively smaller ranges coincident with the mode that is around the median value of the whole data set together with textural evidence sulfide trace element data and gold occurrence the data demonstrate that metamorphism has gradually homogenized the early sulfur accompanied by the segregation of quartz and the release of au from the lattice of early pyrite and its reprecipitation as inclusions in later pyrite the s isotopic compositions of sulfides in sukhoi log and many other major orogenic au deposits hosted in sedimentary rocks of various ages show a pattern generally parallel to the seawater sulfate curve through geologic time indicating that the sulfur in most sediment hosted orogenic au deposits was probably also originally from the reduction of seawater sulfate we conclude that sulfidation and gold mineralization in many sediment hosted orogenic gold deposits was early during basin evolution when seawater was the principal active fluid rather than later during or after basin inversion as proposed in current models

contribution of crustal materials to the mantle sources of xiaogulihe ultrapotassic volcanic rocks northeast china new constraints from mineral chemistry and oxygen isotopes of olivine abstract ultrapotassic igneous rocks can generally be divided into two sub groups based on the tectonic settings in which they formed the orogenic sub group occurs in subduction related tectonic settings while the anorogenic sub group is confined to stable continental regimes the pleistocene xiaogulihe ultrapotassic volcanic rocks outcrop in the western part of heilongjiang province northeast china and are of intraplate origin with respect to its tectonic settings previous elemental and isotopic investigations have suggested that the mantle source of these volcanic rocks had been modified by continental derived sediments resulting from an ancient subduction at least older than 1 5 ga in this contribution we performed in situ oxygen isotope analysis on olivine grains in these ultrapotassic rocks using secondary ionization mass spectrometry sims the olivine grains generally have higher 18 o values and cao contents than those of mantle peridotite xenoliths in the nearby keluo potassic rocks and show linear correlations between major and trace elements and fo suggesting that they are cognate phenocrysts resulted from fractional crystallization processes the restricted and non correlated variations in 18 o with the fo of these olivine grains imply that the fractional crystallization processes might have negligible influence on their 18 o values the relatively higher 18 o values of the olivine phenocrysts than the normal mantle imply the addition of an 18 o rich crustal component into their mantle source after ruling out the crustal contamination of the host magmas we propose that the high 18 o feature of the olivine phenocrysts was inherited from the subducted crustal component in their mantle source given the rapid oxygen isotopic diffusion under high temperature and the long period between mantle metasomatism event and volcanic eruption it is postulated that the high 18 o signature could only be preserved in the relatively cold and stable subcontinental lithospheric mantle such speculation is consistent with our previous inference that the xiaogulihe ultrapotassic volcanic rocks were mainly generated from the lower subcontinental lithospheric mantle which had been metasomatized by potassium rich silicate melts derived from ancient subducted continental derived sediments

stochastic prediction of ground motion and spectral response parameters at hard rock sites in eastern north america empirical predictions of ground motions from large eastern north american earthquakes are hampered by a lack of data for such events for this reason most prediction techniques have been based at least in part on data from the seismically active and well instrumented western north america concentrating on the prediction of response spectra on hard rock sites we have used a relatively new theoretical technique that does not require western data to make ground motion predictions for eastern north america this method often referred to as the stochastic model has its origins in the work of hanks and mcguire who treat high frequency motions as filtered random gaussian noise for which the filter parameters are determined by a seismological model of both the source and the wave propagation the model has been successfully applied to the predictions of ground motions in the western united states and to short period magnitudes from large to great earthquakes worldwide for our application the essential parameters of the model are estimated by using existing data from small to moderate eastern north american earthquakes a crucial part of the model is the relation between seismic moment and corner frequency the relation proposed in 1983 by nuttli for mid plate earthquakes leads to predictions of ground motions that are lower than available data by a factor of about 4 on the other hand a constant stress parameter of 100 bars gives model predictions in good accord with the data to aid in applications the ground motion predictions are given in the form of regression equations for earthquakes of magnitude 4 5 to 7 5 at distances within 100 km of the source the explanatory variables are hypocentral distance and moment magnitude m because predictions are often required in terms of m g rather than m we have used the theoretical model to establish a relation between the two magnitudes the predicted relation agrees with the sparse data available although the large uncertainties in the observed magnitudes for the larger events as well as the sensitivity of the theoretical magnitude to the attenuation model make it difficult to discriminate between various source scaling models

origin of the jiaodong type xinli gold deposit jiaodong peninsula china constraints from fluid inclusion and c d o s sr isotope compositions abstract the xinli gold deposit within the sanshandao cangshang gold belt in the jiaodong gold province is a typical jiaojia type ore deposit with high grade quartz sulfide vein veinlet stockworks that cut mesozoic granodiorite four ore stages are distinguished in the following sequences i quartz k feldspar sericite pyrite ii quartz pyrite iii quartz polymetallic sulfide and iv quartz calcite pyrite fluid inclusions in quartz and calcite in the ores contain c o h fluids dominantly of six types with varied liquid vapor ratio ore fluid in the main ore stage is characterized by medium low temperature low salinity reducing condition and nacl h 2 o co 2 ch 4 in composition the 18 o water smow of quartz in ore stages ii and iii range from 2 82 to 5 34 and the d smow are between 69 6 and 88 3 both suggest that the ore fluid was dominantly magmatic water the 13 c pdb of the calcite are from 6 4 to 2 4 indicating mantle contribution to the ore fluid the pyrite 34 s cdt from four ore stages are basically similar varying from 9 42 to 11 62 which implies significant involvement of crustal materials the initial 87 sr 86 sr ratios in the calcite ranging from 0 710657 to 0 711542 also denote a crustal contribution the fluid composition and systematic h o c s sr isotopic data are explained by a model of slab subduction induced gold mineralization in the model the ore fluid and metals were considered to associate with the dehydration and desulfidation of the subducting paleo pacific slab and the subsequent devolatilization of enriched mantle wedge the h 2 o in the fluid might have been released from the devolatilization of enriched mantle wedge the co 2 could be from mantle wedge or subcontinental lithospheric mantle and the sulfur and strontium would be initially released from the subducted seafloor sediments this process was further supported by the geochemical features of intermediate basic dykes contemporaneous to the gold ore deposits the deposit features and ore forming process of the xinli gold deposit hardly concur those of the orogenic gold deposit and the pluton related gold deposit the study supports the argument of that the gold deposits in the jiaodong belong to a unique gold type related to a complex slab subduction related process the geodynamic engine of the jiaodong type gold deposit is specific i e the slab subduction coupling with the consequent lithospheric thinning and structure adjustment

the rapid heating defocused beam technique a co2 laser based method for highly precise and accurate determination of δ18o values of quartz abstract we describe a laser fluorination technique specifically for quartz that allows both high accuracy and high precision determination of oxygen isotope ratios regardless of grain size the rapid heating defocused beam technique rhd utilizes a defocused 32 w co 2 laser at full power which delivers 17 w 20 w mm 2 to the sample to quickly react quartz in contrast other recent laser fluorination studies use a tightly focused laser beam and slow heating i e sharp and kirschner 1995 fouillac and girard 1996 kirschner and sharp 1997 and find laser analysis of fine grained quartz to be inaccurate three quartz standards nbs 28 qz bra and qz cwru were analyzed using the rhd technique at the university of wisconsin to test for grain size effects rhd analyses show no correlation between grain size and 18 o values and are in excellent agreement with 18 o values obtained using conventional fluorination techniques additional analyses of the same quartz standards performed at university of wisconsin using slow heating and a focused beam yield 18 o values that are both less precise and significantly up to 0 8 lower than the accepted values attempts to use the rhd technique with a 20 w co 2 laser which delivers 8 w 10 w mm 2 to the sample at brgm were unsuccessful probably due to insufficient power density of the defocused beam we conclude that the rhd technique yields excellent accuracy and precision but that power densities of 15 20 w mm 2 may be necessary across a large part of the sample surface we recommend the use of lasers with at least 30 w of power

paleoproterozoic structural evolution of the southern segment of the jiao liao ji belt north china craton abstract the paleoproterozoic jiao liao ji belt separates the eastern block of the north china craton into two small sub blocks the northern longgang and the southern rangrim blocks however it still remains unknown or controversial about the subduction polarity collisional deformation and kinematics between two sub blocks the southern segment of the belt consists of the paleoproterozoic fenzishan and jingshan groups and paleoproterozoic high pressure mafic granulites and serpentinites blocks which are located in the jiaodong complex all of which are separated from the jiaodong complex of neoarchean ttg gneisses by stz1 ductile shear zones structural analysis in this study indicates that most of the rocks in all the units of the southern segment of the jiao liao ji belt underwent three distinct episodes of folding d1 to d3 and two stage of ductile thrust shearing stz1 coeval to d1 and d2 stz2 between d2 and d3 the d1 deformation formed penetrative axial planar foliations s1 bedding parallel ductile shear zone mineral stretching lineations l1 and rarely preserved small isoclinal d1 folds in the jingshan and fenzishan groups in the jingshan group however penetrative deformational transposition resulted in stacking of sedimentary compositional layers which are separated by bedding parallel ductile shear zones stz1 at a period of about 1956 ma to 1914 ma the kinematic indicators of stz1 in the jingshan group with resultant prograde peak metamorphism up to granulite facies grade and the fenzishan group with peak metamorphism up to amphibolite facies grade indicate nw directed compression d2 resulted in crustal thickening with retrograded medium pressure granulite facies grade at about 1914 1893 ma the d2 deformation produced nw verging asymmetric and recumbent folds interpreted to have resulted from basement involved thicken skin structures the jiaodong complex was also involved into the development of wnw verging asymmetric tight folds associated with d2 in the jingshan and the fenzishan groups ongoing collision led to the development of transpressional ductile shearing stz2 forming the transpressional taipingzhuang dextral ductile shear zone between the jingshan group and the southern archean complex and the transpressional tading xiadian sinistral ductile shear zone between the jingshan group and the northern archean complex all three lithotectonic units were superposed during the late d3 deformation with amphibolite facies metamorphism the d3 deformation developed wnw trending open to tight upright folds at about 1893 1875 ma the structural pattern resulting from superimposition of d2 and d3 is a composite synform in the fenzishan and jingshan groups the structural events of d1 and stz1 and d2 and stz2 deformation were possibly responsible for fast syn collisional exhumation of the high pressure mafic granulites the structural patterns and deformational history of the fenzishan and jingshan groups suggest a southeastward directed oblique subduction beneath the northwestern margin of the rangrim block and that the final scissor shaped closure of the rift led to collision between the two blocks to form the coherent basement of the eastern block of the north china craton

a back projection method for imaging large scale lateral variations of lg coda q with application to continental africa summary a new method is developed which uses back projection tomography to regionalize large scale lateral variations of coda q for lg waves which have traversed long continental paths successful use of this method requires precise and stable single trace measurements of lg coda q xie nuttli 1988 the method converges rapidly and requires minimal computer storage it also allows quantitative estimation of resolution and error in imaging lateral variations of coda q the spatial resolution of this method is limited by the uneven spatial coverage of the data base by our limited knowledge of lg coda generation and by the trade off between the stability and the spatial resolution of the coda q inversion the method is applied to a large set of digital lg coda data from africa where large scale lateral variations of lg coda q are found to correlate well with major tectonic features most of africa is stable and like other stable regions has relatively high coda q values the lowest values of coda q are associated with the african rift system other regions of low q values include the atlas mountains and cape fold belt regions of mesozoic and younger deformation the lateral variation of frequency dependence of lg coda q correlates in most regions with that of q at 1 hz our analysis of the spatial resolution of this method indicates that the resolving power of lg coda q imaging is comparable to that of velocity tomography using long period surface waves using empirical approaches we estimate that uncertainties in lg coda q and its power law frequency dependence parameter are less than about 60 and about 0 2 respectively for most of africa

lower cambrian acritarchs from northern spain the precambrian cambrian boundary and biostratigraphic implications acritarchs are reported from basal cambrian rock units inthe cantabrian region of northern spain that are known to contain archaeocyathan and trilobite faunas biostratigraphic correlation of the iberian sequences with other regions has been hampered by the strong provincialism of these faunas however this report of evidently cosmopolitan acritarch taxaestablishes the time equivalence of early cambrian trilobite faunas from iberia baltoscandia and the east european platform eep our data suggest that the detrital deposition of the lower cambrian herreria formation embraces at least three and possibly four lower cambrian acritarch zones previously identified in the eep eastern siberia baltoscandia scotland greenland svalbard and western north america the early cambrian transgression in northern spain was probably initiated in talsy times schmidtiellus mickwitzi trilobite zone in baltoscandia and the eep in part corresponding to the dokidocyathus regularis archaeocyathian zone of the middle tommotian in siberia

the granite hosted gold deposit of moulin de cheni saint yrieix district massif central france petrographic structural fluid inclusion and oxygen isotope constraints the moulin de cheni orogenic gold deposit is the only granite hosted deposit of the saint yrieix district french massif central it occurs in 338 1 5 ma old peraluminous leucogranites and is characterized by intense microfracturing and bleaching of the granite in relation to pervasive sulfide crystallization formation of quartz veins and gold deposition occurred in two successive stages an early mesozonal stage of quartz sulfide fe as s deposition usually devoid of gold and a late epizonal stage of base metal and gold deposition both stages postdate peak metamorphism and granite intrusion the genesis of the deposit is the result of four successive fluid events 1 percolation of aqueous carbonic metamorphic fluids under an assumed lithostatic regime of 400 450 c at a maximum depth of 13 km 2 formation of the main quartz lodes with coeval k alteration and introduction of as and s from aqueous carbonic fluids percolating along regional faults arsenopyrite and pyrite deposition was linked to the alteration of fe silicates into k feldspar and phengite at near constant iron content in the bulk granite temperature was similar to that of the preceding stage but pressure decreased to 100 50 mpa suggesting rapid uplift of the basement up to 7 5 km depth 3 the resulting extensional tectonic leads to the deposition of gold boulangerite galena and sphalerite in brecciated arsenopyrite and pyrite from aqueous fluids during a mixing process temperature and salinity decrease from 280 to 140 c and 8 1 wt eq nacl to 1 6 wt eq nacl respectively 4 sealing of the late fault system by barren comb quartz which precipitated from dilute meteoric aqueous fluids 1 6 wt eq nacl to 0 9 wt eq nacl under hydrostatic conditions at 200 150 c

oxygen isotopes in coexisting garnets clinopyroxenes and phlogopites of roberts victor eclogites implications for petrogenesis and mantle metasomatism abstract 18 o values of coexisting garnet clinopyroxene and phlogopite for twelve compositionally and texturally diverse roberts victor eclogite xenoliths range from 3 8 to 7 1 4 0 to 7 4 and 5 9 to 7 4 respectively differences between the 18 o values of coexisting garnets and clinopyroxenes are normally zero however there is some variation in the 18 o values of different fractions of the same mineral in four samples which suggests the presence of isotopic zonation and inhomogeneity possibly resulting from the introduction of a secondary fluid which metasomatized the eclogites and resulted in the formation of phlogopite amphibole and celsian the 18 o value of the metasomatic fluid is generally buffered by the isotopic composition of the primary garnet and clinopyroxene as indicated by a correlation between the isotopic composition of phlogopite and the primary pyroxene and garnet the large range in 18 o values of the eclogites and the similarity in the isotopic composition of coexisting pyroxene and garnet support the interpretation that the roberts victor eclogites represent metamorphosed altered basalts the eclogites were subjected to infiltration metasomatism in the mantle prior to their incorporation in the kimberlite and the source of this fluid was probably unrelated to the eclogite

an investigation into earthquake ground motion characteristics in eastern north america abstract a random vibration model of the hanks mcguire type is used to predict peak ground motions at rock sites in eastern north america the assumed geometric decay and distance dependent duration approximate the propagation of direct body waves at short distances and lg waves at regional distances the model predicts peak acceleration and velocities as well as response spectra and magnitude for a given seismic moment and corner frequency to predict ground motions for a given lg magnitude m lg the model is first used to calculate the seismic moment corresponding to that magnitude assuming a source scaling law then knowing the moment and corner frequency the model is used to calculate peak ground motions available data from strong motion recordings and from the ectn and lrsm networks modified to estimate horizontal ground motions on rock where appropriate are used to verify the model9s assumption and predictions modified mercalli intensity data are also used ground motions predicted by the model with a stress drop of 50 to 200 bars agree with the ground motion data but the m lg values computed by the model for given seismic moments do not agree with nuttli9s 1983 moment versus m lg data for large earthquakes resolution of the latter disagreement awaits the collection of instrumental data from large events in eastern north america

the early paleozoic development of bioturbation evolutionary and geobiological consequences abstract bioturbation the physical and chemical mixing of sediments by burrowing animals is a critical engineering process in modern seafloor environments and exerts an important control on not only benthic ecology and sediment properties but also ocean wide biogeochemical cycling well mixed sediments have long been assumed to appear at the precambrian cambrian boundary with the first occurrence of the index fossil and three dimensional burrow treptichnus pedum however field based analyses synthesizing ichnological stratigraphic sedimentological and taphonomic data collected from lower paleozoic successions worldwide indicate that sediment mixing in marine shelf environments remained limited until at least the late silurian 120 million years after the precambrian cambrian transition in spite of early advances in complex modes of infaunalization mixed layer development had a much more gradual trajectory and did not occur in step with the first appearance of three dimensional burrows the delayed appearance of modern style intensities of sediment reworking postdating both the cambrian explosion and the great ordovician biodiversification event may be due to the late stage radiation of mobile infaunal deposit feeders biological bulldozers the organisms which are in modern marine settings the most efficient bioturbators the protracted development of the sediment mixed layer holds important implications for contemporaneous biogeochemical cycling and oxygenation of the ocean atmosphere system the delayed development of intensive sediment mixing may have also mediated exceptional preservation of both fossilized soft tissues and shallow tier trace fossils and may therefore have contributed to the anomalous preponderance of konservat lagerstatten and exceptionally preserved shallow tier trace fossils and bioglyphs characteristic of the lower paleozoic stratigraphic record

phosphogenesis around the proterozoic phanerozoic transition there was a very significant and widespread phosphogenic event around the proterozoic phanerozoic transition this is reflected in the occurrence of many major and minor latest proterozoic and cambrian phosphate deposits phosphorites in asia australia africa and south america as well as minor occurrences in many other parts of the world the major peak in phosphogenesis was in the tommotian with lesser events in the ediacaran and the lenian bracketed by other minor though significant events recent evidence on the timing of proterozoic phosphogenesis does not now support the suggestion of a phosphogenesis glaciation link these peaks in phosphogenesis are taken as evidence that there was a marked increase in the phosphate content of the shallow seas at about this time possibly as a result of the circulation of deep phosphorus rich ocean waters into the photic zone not only was this responsible for the formation of phosphorites but it also had a profound effect on the biota and may have been a driving mechanism for the cambrian faunal explosion and the initiation of biomineralization

the jurassic danba hypozonal orogenic gold deposit western china indirect derivation from fertile mantle lithosphere metasomatized during neoproterozoic subduction the genesis of hypozonal orogenic gold deposits is highly controversial with both crustal metamorphic fluids and variously derived sub crustal fluids suggested as fluid and metal sources the lower jurassic danba gold deposit a rare phanerozoic hypozonal orogenic gold deposit on the western margin of the yangtze craton china provides an opportunity to resolve these controversies danba is characterized by biotite amphibole alteration and pyrrhotite dominant ore assemblages formed at 500 600 c the 34s values of ore related pyrrhotite range from 3 1 to 9 9 in sharp contrast to those of syn sedimentary pyrrhotite from devonian host rocks which are between 6 8 and 9 5 the pb isotope compositions of ore related pyrrhotite with 206pb 204pb 17 85 18 25 and 207pb 204pb 15 48 15 67 are less radiogenic and more variable than those of unaltered host rocks these differences combined with the hypozonal feature of the deposit indicate that ore metals and sulfur were derived from a sub crustal rather than a local crustal source the pge patterns of ore related pyrrhotites are similar to those of sulfides in baltic mantle lithosphere that was metasomatized by aqueous fluids in terms of marked enrichment of pd and ru but significantly differ from those of magmatic intrusions and related magmatic hydrothermal deposits this suggests that the metals that formed the danba deposit were transported via aqueous fluid from metasomatized mantle lithosphere rather than from a magmatic source the 18o of mantle fluid calculated from hydrothermal quartz and biotite ranges from 10 to 12 the anomalously high s and o isotope ratios and variable pb isotope ratios together with pge data support a model in which the inferred mantle lithosphere source was significantly hydrated metasomatized and fertilized by fluid derived from subducted oceanic sediments with elevated 34s and 18o values geological evidence suggests that this subduction event was neoproterozoic in age this study defines a model in which hydrated and metasomatized mantle lithosphere was formed during early subduction but it was over 500 million years later during later lower jurassic asthenosphere upwelling that metal and sulfur rich fluid was released into the crust to form the danba gold deposit

calculation of oxygen isotope fractionation in hydroxyl bearing silicates abstract the modified increment method has been applied to the calculation of oxygen isotope fractionation factors for hydroxyl bearing silicate minerals the order of 18o enrichment obtained in common rock forming minerals is pyrophyllite kaolinite tourmaline talc prehnite topaz illite phengite lepidolite muscovite staurolite epidote glaucophane serpentine chlorite tremolite hornblende phlogopite biotite humite norbergite ilvaite hydroxyl bearing silicates are enriched in 18o relative to hydroxyl groups but depleted in 18o relative to anhydrous counterparts three sets of self consistent fractionation factors between quartz and the hydroxyl bearing silicate minerals between calcite and the silicate minerals and between the silicate minerals and water have been calculated for a temperature range of 0 1200 c the fractionation factors calculated for mineral pairs are applicable to isotopic geothermometry in igneous metamorphic and sedimentary petrology they can be used as a test of isotopic equilibrium or disequilibrium in natural mineral assemblages over all temperature ranges of geological interest the difference in oxygen isotope composition between the hydroxyl bearing mineral and the oh group is quantitatively demonstrated to be temperature dependent and therefore can be used as a single mineral geothermometer

carbon isotope geochemistry and palaeontology of neoproterozoic to early cambrian siliciclastic successions in the east european platform poland a well preserved stratigraphic record of the organic carbon isotopic composition across the precambrian cambrian transition has been obtained from a sequence of entirely detrital rocks underlying the lublin slope east european platform poland the observed 13 c pattern for organic carbon is comparable to isotope data determined for coeval carbonate platform sequences elsewhere these results underline the truly global nature of these secular variations within the carbon cycle during the terminal proterozoic and early cambrian largely independent of host lithology and facies the direct combination of micropalaeontological and geochemical results from sedimentary organic matter allows a detailed interpretation of the isotope record as probably caused by secular changes in the net balance of burial and erosion rates of repositories of organic carbon furthermore these variations appear to be directly correlatable with changes in the diversity and abundance of the biota and radiation extinction events

lithospheric controls on the formation of provinces hosting giant orogenic gold deposits ages of giant gold systems 500 t gold cluster within well defined periods of lithospheric growth at continental margins and it is the orogen scale processes during these mainly late archaean palaeoproterozoic and phanerozoic times that ultimately determine gold endowment of a province in an orogen a critical factor for giant orogenic gold provinces appears to be thickness of the subcontinental lithospheric mantle sclm beneath a province at the time of gold mineralisation as giant gold deposits are much more likely to develop in orogens with subducted oceanic or thin continental lithosphere a proxy for the latter is a short pre mineralisation crustal history such that thick sclm was not developed before gold deposition in constrast orogens with protracted pre mineralisation crustal histories are more likely to be characterised by a thick sclm that is difficult to delaminate and hence such provinces will normally be poorly endowed the nature of the lithosphere also influences the intrinsic gold concentrations of potential source rocks with back arc basalts transitional basalts and basanites enriched in gold relative to other rock sequences thus segments of orogens with thin lithosphere may enjoy the conjunction of giant scale fluid flux through gold enriched sequences although the nature of the lithosphere plays the crucial role in dictating which orogenic gold provinces will contain one or more giant deposits the precise siting of those giants depends on the critical conjunction of a number of province scale factors such features control plumbing systems traps and seals in tectonically and lithospherically suitable terranes within orogens

triassic granitoids in the eastern songpan ganzi fold belt sw china magmatic response to geodynamics of the deep lithosphere the songpan ganzi fold belt sgfb sw china was developed from a passive continental margin into an orogenic belt with the consumption of the paleo tethys during the evolution of the sgfb numerous late triassic granitic plutons formed and exhibited a progressive development from adakite i type granite high ba sr granite a type granite and monzonite representative late triassic plutons were studied to unravel the bewildering evolution of the eastern sgfb the menggu pluton 224 3 ma consists of granites with high alkali k2o na2o 7 85 10 4 wt and adakitic characteristics sr y 19 38 the endt values 2 77 to 5 03 initial 87sr 86sr ratios 0 7050 0 7063 and low nb ta ratios 8 10 are indicative of an origin by partial melting of amphibolitic lower crust rocks from the niuxingou pluton 215 3 ma are richer in k than na k2o na2o 1 1 1 5 and contain high sr 1006 1662 ppm and ba 1277 2009 ppm typical of shoshonite and high ba sr granite they have less enriched endt values 0 08 to 2 04 and less radiogenic 87sr 86sri ratios 0 7047 0 7048 and formed from a mixed melt derived from upwelling asthenosphere and the overlying metasomatised lithospheric mantle the taiyanghe pluton 205 3 ma consists of monzonites with high al2o3 20 wt but low mgo 0 94 1 39 wt the rocks are richer in na than k k2o na2o 0 7 contain high large ion lithophile element lile 681 834 ppm sr and 2142 9453 ppm ba and display strongly fractionated ree patterns la yb n 35 63 these features together with their enriched nd sr isotopic compositions endt 4 78 to 6 50 87sr 86sri 0 7074 0 7090 suggest that the monzonite probably formed from low degrees of partial melting of metasomatised lithospheric mantle although a continuous compressional regime during the mid and late triassic has been invoked for the sgfb the generation of crustally derived adakitic and shoshonitic plutons reflect thickening in response to an arc continental collision accompanied by fracturing of the lithosphere and an extensional regime in the deep lithosphere in the late triassic the 205 ma taiyanghe pluton was emplaced simultaneously with a rapid uplift of the lithosphere when surface deposits changed from deep water turbidite to tidal flat sediments it was therefore generated during decompression probably related to the rapid removal of the overthickened lithospheric mantle the triassic magmatism in the eastern sgfb is therefore important for probing geodynamic processes in the deep lithosphere

a classification scheme for epigenetic archaean lode gold deposits recent research has shown that archaean lode gold deposits occur in environments that range in their metamorphic grade from prehnite pumpellyite to lower granulite facies based on this data a new classification is proposed for these deposits in this classification shallow level gold deposits are classed as epizonal the so called mesothermal deposits are mesozonal and the deeper deposits commonly in mid amphibolite or lower granulite terrains are hypozonal

stabilities of laurite rus2 and monosulfide liquid solution at magmatic temperature we have reversed the equilibria ru s2 rus2 and 2feo s2 2fes o2 under controlled temperature oxygen fugacity fo2 and sulfur fugacity fs2 at 1 atm to test if the platinum group mineral laurite rus2 has a stability field at magmatic temperature outside that of magmatic monosulfide liquid the fs2 in equilibrium with ru and rus2 is given by log fs2 6 9 12190 t 1173 to 1673 k whereas the fs2 at sulfide saturation of a basaltic melt may be approximated from log fs2 6 7 12800 t 2 log xfeo log fo2 1473 to 1673 k our experiments support the notion that the sulfide mineral laurite can crystallize directly out of a sulfur bearing sulfide undersaturated silicate melt as implied by laurite chromite intergrowth textures in natural chromite ore

mantle crustal and atmospheric noble gases in ailaoshan gold deposits yunnan province china abstract helium and argon isotopic and elemental compositions of fluids released by crushing pyrite grains are reported from the ailaoshan gold province china the province is a series of gold deposits hosted on a 200 km segment of a major normal fault formed during eocene extension the deposits have been well characterised using conventional geochemical and microthermometric techniques the results of which are consistent with a predominantly high temperature magmatic fluid present in the ore minerals in contrast analyses of noble gases from three gold deposits demonstrate the palaeofluids were a mixture between a mantle derived magmatic fluid and two different low temperature fluids similar to modern groundwaters the specific isotopic and abundance characteristics of helium and argon allow unequivocal identification of both a mantle derived component and a surface derived component to these fluids the magmatic fluid dominates deposits from the northern end of the fault the second groundwater is present only in deposits in the south of the province by assuming fluid inclusion homogenisation temperatures in coexisting quartz are representative of fluid temperature 3he heat ratios can be estimated the 3he heat ratios vary systematically along the fault from 30 10 12 cm3 stp j 1 in the northern deposits to

emplacement of the longmen shan thrust nappe belt along the eastern margin of the tibetan plateau abstract the topographic descent from the eastern margin of the tibetan plateau to the sichuan basin passes through the longmen shan thrust nappe belt which is sub divided by six nw dipping major listric thrusts with accompanying duplexes and imbricate fans into five large scale nappes each nappe has its own stratigraphic and deformational features in the inner longmen shan longmen mountains allochthonous nappe units have incorporated both mesoproterozoic basement and sinian neoproterozoic to triassic cover sequences as thick skinned horses whereas in the frontal longmen shan sinian cretaceous cover sediments have been stripped from the basement as thin skinned fold and thrust sheets including the extensively distributed klippen structures pre thrusting extension during devonian to middle late triassic times resulted in syn depositional normal faults structural inversion of these faults initiated the peng xian guan xian basement complex jiuding shan and tangwangzhai nappes during the early episode of the indosinian orogeny norian to rhaetian this was followed by episodic thrusting during latest triassic to early cretaceous times to develop the guan xian an xian and southeastern marginal nappes that have incorporated sediments from the neighbouring foreland basin into the frontal part of the thrust nappe belt during a late miocene reactivation of the pre existing thrusts differential thrusting occurred across the thrust nappe belt

cross arc geochemical variations in the kurile arc as a function of slab depth lavas from transects across the kurile islands arc showed geochemical variations related to changes in the compositions of fluids derived from the subducting slab enrichment factors for boron cesium arsenic and antimony all elements with strong affinities for water decreased across the arc this decrease is presumably related to losses of water rich fluids during the dehydration of the subducting plate enrichments of potassium barium beryllium 10 and the light rare earth elements remained constant these species may move in silica rich fluids liberated from the slab at greater depths

some problems concerning the precambrian cambrian transition and the cambrian faunal radiation in siberia the so called khailakh and manykay stages of pre tommotian age are inferred to be partly coeval with the type tommotian and to be based on a misunderstanding about the control of facies on faunal distributions evolutionary patterns are being studied through this interval using a variety of approaches synoptic reviews are presented on diversity changes on changes in skeletal composition and on archaeocyathan morphology the latter group appear to have originated on the siberian platform just prior to the tommotian and migrated globally during the atdabanian and botomian to form secondary centres of diversification olenellid trilobites may have dispersed in a similar pattern but other groups such as volborthella and relatives appear to have had a separate siliciclastic centre of origin the decline in diversity of small shelly fossils after the tommotian and the coeval expansion in archaeocyathans and trilobites may be related to basinal changes connected with the break up of palaeopangea

determination of lg wave attenuation using single station seismograms a case study for zimbabwe summary in this paper we show how the quality factor q may be calculated using a single seismograph station and a number of events recorded on analogue seismograms we followed nuttli s 1973 method and extended it to one seismograph station using the single station bulawayo bul we determined a mean q value of 650 for zimbabwe furthermore we considered different propagation paths over zimbabwe as the seismic waves travelled to bul and found a low q value of 350 for the deka fault zone the q value of 650 obtained in this study agrees well within error with that of 603 reported by chow et al 1980 using the multistation multi event method and lies within the range of q values 454 759 reported by xie mitchell 1990 using a back project method to image large scale lateral variations of lg coda q the results obtained are important as the q value constitutes part of the input data in seismic hazard calculations the method may be used to determine q in regions where there may be only one station with reliable analogue seismogram data

patterns of phytoplankton radiation across the precambrian cambrian boundary late proterozoic and early phanerozoic phytoplankton comprise a wide array of microfossils of possibly bacterial and algal affinity their radiation patterns around the proterozoic cambrian boundary interval are becoming increasingly well known for numerous sequences in the east european and siberian platforms china baltoscandia and north america transgressive sequences in the east european platform and northern baltoscandia comprise alternating sandstones mudstones and claystones their sedimentology is well documented and microfossils acritarchs chitinous fossils sabelliditids shelly fossils and trace fossils are well represented rocks immediately overlying the transitional beds have yielded lower cambrian trilobite faunas that are utilized for regional correlations continuous successions of planktic microorganisms consisting of prokaryotic photoautotrophic cyanobacteria and algal protists are known from rocks comprising the transitional vendian cambrian sequence in the east european platform the rock sequence attributed to the lower cambrian platysolenites antiquissimus zone has yielded acritarchs of 9cambrian type9 abruptly replacing a morphologically simple proterozoic type microbiota recorded in beds of the upper proterozoic sabellidites vendotaenia zone the radiation of novel forms of phytoplankton appears to have been synchronous over a wide part of the late proterozoicarly cambrian shelf and is parallelled by the nearly contemporaneous emergence of cambrian shelly fossils and trace fossils along with new acritarch taxa the diagnostic acritarch assemblage of the platysolenites antiquissimus zone consists of widely distributed species occurring also in the lowermost cambrian deposits of the siberian platform baltoscandia greenland north america svalbard and scotland it is suggested that along with shelly faunas trace fossils and geochemical data the general radiation patterns of phytoplankton shed light on the events shaping the important biotic events around the precambrian cambrian boundary it is also believed that a better understanding of the regulatory mechanisms affecting the base of trophic chains may in turn be of significant importance in defining the precambrian cambrian boundary

iridium and palladium as discriminants of volcanic exhalative hydrothermal and magmatic nickel sulfide mineralization neutron activation of pd au and ir in samples from 1 economically significant ni sulfide deposits of western australia 2 the host rocks of these deposits 3 hydrothermal and or metasomatic ni sulfide mainly millerite mineralization in peridotite at nullagine western australia 4 hydrothermal replacement volcanic exhalative ni cu sulfide mineralization at sherlock bay western australia and 5 volcanic exhalative sulfides in inter flow sediments at kambalda are used to show that ir and pd can be used to discriminate between the various origins magmatic volcanic exhalative hydrothermal metasomatic and metamorphic ascribed to ni sulfide deposits all the economically significant ni sulfide deposits in western australia are strongly enriched in the precious metals relative to other crustal rocks and have similar pd ni and ir ni ratios to each other these ratios persist regardless of whether the nickel sulfides occur at the base of volcanic ultramafic piles e g kambalda or as centrally disposed interstitial sulfides in intrusive dunite bodies the strong similarities of the pd ir ratios of the ni sulfide ores and of the spinilex textured komatiites which represent ultramafic liquids from which the ni sulfide liquids were derived together with other features of the precious metal geochemistry of the ores and host rocks of the ni sulfide deposits are used to show that the deposits had magmatic origins in contrast the pd ir and ir ni ratios of the hydrothermal and or metasomatic ni sulfide mineralization at nullagine are distinctly different from those of the major ni deposits the former contains high au moderate but variable pd and exceptionally low ir similarly the volcanic exhalative sulfides at kambalda and sherlock bay contain negligible quantities of pd and ir all the ir in both the interflow sediments and the hydrothermal metasomatic mineralization is contributed by the host rocks and not by the sulfides the western australian data together with precious metal data for other examples of hydrothermal ni cu sulfide mineralization strongly suggest that although ni au and pd may be transported in aqueous solution ir cannot hence the relative abundance of ir can be used as a discriminant of magmatic sulfides

timing of granite emplacement and cooling in the songpan garze fold belt eastern tibetan plateau with tectonic implications abstract new u pb and rb sr geochronology on syn and post orogenic granites provide constraints on the timing of major tectonic events in the songpan garze fold belt west sichuan china the ma nai granite was probably syn kinematic with the main deformation and yields an age of 197 6 ma that is interpreted as an upper age limit of the indosinian event zircons and apatites from the post kinematic rilonguan granite also yield jurassic ages 195 6 and 181 4 ma the post orogenic markam massif gives two ages of 188 1 and 153 3 ma both granites are undeformed and cut structures in the triassic sedimentary rocks these results demonstrate that the major deformation and decollement tectonics in the songpan garze fold belt occurred prior to the early jurassic the wide range of ages obtained for post kinematic granites from early jurassic to late jurassic suggests that locally magmatic activity persisted for a long time at least 50 ma after the indosinian compressional tectonism no tertiary ages have been obtained suggesting that these granites were not affected strongly by the india asia collision

geochemistry of the emeishan flood basalts at yangliuping sichuan sw china implications for sulfide segregation a well developed 1 000 m thick basaltic sequence in the yangliuping region northern part of the emeishan basalt province includes the lower and middle units of tholeiitic basalts and an upper unit of both tholeiites and subalkalic basalts the basalts contain 42 55 wt sio2 and 4 1 8 3 wt mgo most of these lavas have gd yb 2 0 zr nb 12 and nd 260 ma values from 2 5 to 4 7 the platinum group elements pge are very mildly depleted in most of the basalts which contain 8 19 ppb pt and 7 27 ppb pd however a significant proportion of the middle unit basalts are strongly depleted in pge with some samples having concentrations lower than detection limits they have extremely high zr nb ratios up to 14 5 and low nd 260 ma values 3 21 to 0 65 features of extensive lower crustal contamination some samples in this unit have high ni pd 3 965 61 198 and low pd cr 410 000 3 930 000 ratios indicating sulfide segregation and pge depletion prior to eruption the primary magmas were s undersaturated and derived from partial melting at variable depths in the upper mantle the early and late stage magmas as represented by the lower and upper units underwent afc processes which induced mild s saturation and pge depletion in some of the basalts whereas the magmas represented by the middle unit experienced more extensive crustal contamination resulting in stronger s saturation and in most cases significant pge depletion

effects of temporal variations in seismicity on seismic hazard the accuracy of stationary mathematical models of seismicity for calculating probabilities of damaging shaking is examined using the history of earthquakes in china from 1350 a d to 1949 a d during this time rates of seismic activity varied periodically by a factor of 10 probabilities of damaging shaking are calculated in 62 cities in north china using 50 yr of earthquake data to estimate seismicity parameters the probabilities are compared to statistics of damaging shaking in the same cities for 50 yr following the data window these comparisons indicate that the seismic hazard analysis is accurate if 1 the maximum possible earthquake size in each seismogenic zone is determined from the entire seismic history rather than from a short time window and 2 the future seismic activity can be estimated accurately the first condition emphasizes the importance of realistically estimating the maximum possible size of earthquakes on faults the second indicates the need to understand possible trends in seismic activity where these exist or to develop an earthquake prediction capability with which to estimate future activity without the capability of estimating future seismicity stationary models provide less accurate but generally conservative indications of seismic ground shaking hazard in the united states the available earthquake history is brief but gives no indication of changing rates of activity the rate of seismic strain release in the central and eastern united states has been constant over the last 180 yr and the geological record of earthquakes on the southern san andreas fault indicates no temporal trend for large shocks over the last 15 centuries both observations imply that seismic activity is either stationary or of such a long period that it may be treated as stationary for seismic hazard analyses in the united states

formation of gold deposits a metamorphic devolatilization model a metamorphic devolatilization model can explain the enrichment segregation timing distribution and character of many goldfields such as those found in archean greenstone belts slate belts and other gold only provinces in this genetic model hydrated and carbonated greenschist facies rocks particularly metabasic rocks are devolatilized primarily across the greenschist amphibolite facies boundary in an orogenic setting devolatilization operates on the scale of individual mineral grains extracting not just h2o and co2 but also s and in turn au elevated gold in solution is achieved by complexing with reduced s and by h2co3 weak acid buffering near the optimal fluid ph for gold solubility the buffering is more important than being at the point of maximum gold solubility low salinity ensures low base metal concentrations in the auriferous metamorphic fluid migration of this fluid upwards is via shear zones and or into hydraulic fracture zones in rocks of low tensile strength the geometry of the shear zones dictates the kilometre scale fluid migration paths and the degree of fluid focusing into small enough volumes to form economic accumulations of gold deposition of gold from solution necessitates breakdown of the gold thiosulphide complex and is especially facilitated by fluid reduction in contact with reduced carbon bearing host rocks and or by sulphidation of wallrocks to generate iron bearing sulphide and precipitated gold as such black slate carbon seams banded iron formation tholeiitic basalt magnetite bearing diorite and differentiated tholeiitic dolerite sills are some of the important hosts to major goldfields gold deposition is accompanied by carbonation sulphidation and muscovite biotite alteration where the host rock is of suitable bulk composition the correlation of major gold deposits with rock type even when the gold is primarily in veins argues for rock dominated depositional systems not fluid dominated ones as a consequence a general role in gold deposition for fluid mixing temperature decrease and or fluid pressure decrease and boiling is unlikely although such effects may be involved locally several geological features that are recorded at gold only deposits today reflect subsequent modifications superimposed upon the products of this generic metamorphic devolatilization process overprinting by higher grade metamorphism and deformation and or palaeo weathering may provide many of the most obvious features of goldfields including their mineralogy geochemistry geometry small scale timing features geophysical response and even mesoscopic gold distribution

carbon isotopic evolution of the terminal neoproterozoic and early cambrian evidence from the yangtze platform south china profound geotectonic climatic and biological changes occurred during the terminal neoproterozoic and its transition into the early cambrian these changes are reflected in temporal variations of the carbon isotopic composition of seawater recorded in a sedimentary succession on china s yangtze platform however in addition to secular changes in carbon isotopes the studied succession reflects additional variations in accordance with the deposition in different sedimentary facies ranging from shallow water platformal to deeper water basinal settings likely a consequence of incorporation of variable amounts of 13 c depleted

orogenic gold common or evolving fluid and metal sources through time abstract orogenic gold deposits of all ages from paleoarchean to tertiary show consistency in chemical composition they are the products of aqueous carbonic fluids with typically 5 20 mol co 2 although unmixing during extreme pressure fluctuation can lead to entrapment of much more co 2 rich fluid inclusions in some cases ore fluids are typically characterized by significant concentrations of ch 4 and or n 2 common estimates of 0 01 0 36 mol h 2 s a near neutral ph of 5 5 and salinities of 3 7 wt nacl equiv with na k ca mg this fluid composition consistency favors an ore fluid produced from a single source area and rules out mixing of fluids from multiple sources as significant in orogenic gold formation nevertheless there are broad ranges in more robust fluid inclusion trapping temperatures and pressures between deposits that support a model where this specific fluid may deposit ore over a broad window of upper to middle crustal depths much of the reported isotopic and noble gas data is inconsistent between deposits leading to the common equivocal interpretations from studies that have attempted to define fluid and metal source areas for various orogenic gold provinces fluid stable isotope values are commonly characterized by the following ranges 1 18 o for precambrian ores of 6 to 11 and for phanerozoic ores of 7 to 13 2 d and 34 s values that are extremely variable 3 13 c values that range from 11 to 2 and 4 15 n of 10 to 24 for the neoarchean 6 5 to 12 for the paleoproterozoic and 1 5 to 10 for the phanerozoic secular variations in large scale earth processes appear to best explain some of the broad ranges in the o s and n data fluid rock interaction particularly in ore trap areas may cause important local shifts in the o s and c ratios the extreme variations in d mainly reflect measurements of hydrogen isotopes by bulk extraction of waters from numerous fluid inclusion generations many which are not related to ore formation radiogenic isotopes such as those of pb sr nd sm and os measured on hydrothermal minerals are even more difficult to interpret for defining metal source particularly as the low salinity ore fluids transport limited amounts of these elements and significant amounts of these may be locally added to the minerals during alteration reactions at the sites of gold deposition noble gas and halogen data are equally equivocal fluid exsolution from granitoids emplaced into the upper and middle crust metamorphism of the crust or fluids entering trans crustal fault zones from below the crust all remain as permissive scenarios associated with orogenic gold formation as the abundant geochemical data are equivocal however geological and geochronological data weigh heavily against a magmatic hydrothermal model in the upper to middle crust there is no universal temporal association between orogenic gold and magmatism and where there is an overlap in age there is no specific type of magmatism consistently associated with gold formation nor element zonation around any specific pluton a crustal metamorphic model for fluid and metal sources is very consistent with geological geochronological and geochemical data although metamorphism on a regional scale that releases these components into major fault zones can be associated with many processes along active continental margins these can include crustal thickening and radiogenic heating slab rollback and heating during crustal extension or subduction of a spreading ridge heating the base of an accretionary prism in rare examples where phanerozoic orogenic gold deposits are hosted in precambrian high grade metamorphic terranes fluids and metals must however enter a transcrustal fault system from a sub crustal source this could either be a devolatilized subducted relatively flat perhaps stalled slab and its overlying sediment or the corner of the fertilized mantle wedge that releases a fluid during a thermal event

quaternary phosphorites off the southeast coast of india abstract detailed petrological mineralogical geochemical and radiogenic u sr nd and stable isotope c o s studies have been carried out on the quaternary phosphorites of the continental margin off chennai southeast coast of india these phosphorites are formed as a result of trapping and binding of sediments by microbial mats and are similar to phosphate stratiform stromatolites detrital and biogenic constituents enclosed in the phosphorites controlled the major and minor element composition except for sr and u the concentrations of most of the trace metals are lower than those in average shale and phosphorite middle rare earth element mree enriched patterns are the characteristic feature the u th dating method indicates that the ages of the phosphorites are beyond 300 000 years 87 sr 86 sr ratios of the phosphorites are higher than that of present day seawater and end values are more negative than those of seawater of the atlantic ocean carbon isotope ratios are within the range expected for the oxic suboxic zone but sulfur isotope ratios indicate suboxic conditions during phosphorite formation these results imply that the benthic microbial mats thrived on the shallow shelf during the quaternary low sea level conditions periodic or episodic sedimentation onto the mats led to their death the bacteria associated with decaying microbial mats utilised phosphorus supplied by continental sources and rapidly precipitated phosphate the availability of a high percentage of phosphorus in seawater seems to be an important controlling factor for the formation of phosphate stromatolites the composition of these phosphorites differs from the modern phosphorites in upwelling regions but are similar to cambrian apatite stromatolites these phosphorites provide evidence that the replicates of ancient phosphate stromatolites do exist in the quaternary

fluid inclusion and stable isotope o h c and s constraints on the genesis of the serrinha gold deposit gurupi belt northern brazil the serrinha gold deposit of the gurupi belt northern brazil belongs to the class of orogenic gold deposits the deposit is hosted in highly strained graphitic schist belonging to a paleoproterozoic 2 160 ma metavolcano sedimentary sequence the ore zones are up to 11 m thick parallel to the regional nw se schistosity and characterized by quartz carbonate sulfide veinlets and minor disseminations textural and structural data indicate that mineralization was syn to late tectonic and post metamorphic fluid inclusion studies identified early co2 ch4 n2 and co2 ch4 n2 h2o nacl inclusions that show highly variable phase ratios co2 homogenization and total homogenization temperatures both to liquid and vapor interpreted as the product of fluid immiscibility under fluctuating pressure conditions more or less asso ciated with postentrapment modifications the ore bearing fluid typically has 18 33mol of co2 up to 4mol of n2 and less than 2mol of ch4 and displays moderate to high densities with salinity around 4 5wt nacl equiv mineralization occurred around 310 to 335 c and 1 3 to 3 0 kbar based on fluid inclusion homogenization temperatures and oxygen isotope thermometry with estimated oxygen fugacity indicating relatively reduced conditions stable isotope data on quartz carbonate and fluid inclusions suggest that veins formed from fluids with 18 oh2o and dh2o 310 335 c values of 6 2 to 8 4