long chain alkenone patterns in the baltic sea an ocean freshwater transition two different patterns of long chain alkenones are found in surficial sediments of the baltic sea which is the largest brackish water body on earth one pattern occurs in surficial sediments from the western baltic sea where surface water salinitiy is in excess of 7 7 psu it corresponds to the pattern produced by the marine coccolithophorid emiliania huxleyi with a suite of c37 di to tetra unsaturated methyl ketones and c38 di and tri unsaturated methyl and ethyl ketones a second pattern resembling that found in lake sediments in lacking c38 methyl ketones and having distinctly higher c37 4 methyl ketone concentrations dominates in surficial sediments of the eastern and northern baltic sea where salinities are lower than 7 7 psu correspondence of sea surface temperature sst estimates from the u37 and u379 indices using marine calibrations with mean sst in the euphotic zone from july august the main haptophyte growth season is poor thus these indices are not applicable as sedimentary thermometers in surficial sediments of the baltic sea the different patterns may either reflect the salinity dependent occurrence of specific alkenone producers or changes in the alkenone biosynthesis due to physiological stress caused by salinity variations furthermore advection of saline and oxygenated north sea water may transport marine algal material characterized by a marine e huxleyi like alkenone pattern into the western baltic sea thus covering the signature of the local alkenone producers with a baltic sea pattern copyright 2000 elsevier science ltd

tracing the mobility of individuals using stable isotope signatures in biological tissues locals and non locals in an ancient case of violent death from central california abstract stable isotope analyses of biological tissues that grow during different phases of life can be used to trace the geographic location of individuals during different windows of time we apply this principle and reexamine three prehistoric skeletons excavated in 1964 from archaeological site ca yol 117 in central california field evidence suggests they were killed as part of a single violent event we report new radiocarbon dates and strontium oxygen carbon and nitrogen isotopic data from first molars third molars and bone to examine circumstances surrounding their death data suggest two of the three individuals were born and all three lived their teenage years near the site they were buried in however as adults they lived elsewhere likely to the north along the sacramento river around ad 1450 upon returning to the territory of their childhood years they were killed and placed in a mass grave the data invite a more nuanced interpretation of how we view local and non local individuals archaeologically overall the analysis provides an interesting glimpse into the nature of warfare and violence among pre contact hunter gatherers of northern california

the sandstone derived provenance record of the gualala basin northern california u s a abstract detrital zircon u pb ages and pb isotope compositions of detrital feldspars from the late cretaceous eocene gualala basin in northern california indicate that the basin received sediments from the southern sierra nevada the mojave block and the salinian block mesozoic detrital zircon age components and pb isotope compositions of detrital feldspars from the basin are similar to u pb zircon ages and pb isotope compositions from previously studied coeval conglomerate clasts the detrital zircons and feldspars however also include additional age and pb isotope components that are unobserved or poorly represented in the conglomerate based provenance record these new data suggest a more complex provenance history than that inferred from previously studied conglomerate clasts and strongly imply a connection to the mojave desert provenance evolution of the sediments in the basin indicates that significant contribution of sediment from the mojave desert began during the paleocene when it is inferred that the drainage network expanded eastward from its previous extent the presence of mojave derived late permian zircons as well as feldspars that have high 208pb 204pb ratios in the eocene age german rancho formation indicates that the basin remained in the vicinity of the mojave desert until the eocene this result demonstrates that the gualala basin had undergone very little northward motion at that time and consequently requires that no significant northward translation took place prior to 50 ma the results of this study highlight the value of additional provenance constraints combined with age distributions when identifying sediment source terranes

changes in the c n p burial rates in some baltic sea sediments over the last 150 years relevance to p regeneration rates and the phosphorus cycle three major depositional basins the gdansk bornholm and eastern gotland basins of the baltic proper which together account for 50 of the depositional areas in the entire baltic sea have accumulated increased amounts of sediment and organic carbon during the last 50 years as is shown in 210 pb dated sediment cores the shallow arkona basin has had constant accumulation rates and rate increases of bulk sediment organic carbon and aluminium in the bornholm basin are parallel and are interpreted to reflect increased material input from land or from erosion of shallow water areas in the gdansk and eastern gotland basins the accumulation rates of mineral matter have risen at lower rates than those of organic carbon and point to preferential enrichment of organic matter at increasing rates of burial the material in the gdansk and eastern gotland basins appears to have been depleted in phosphorus relative to nitrogen and organic carbon using c n p ratios from a sediment trap mooring in the gotland basin and from fluffy layer material in the pomeranian bight as characteristic ratios for sedimenting material we find evidence for significant increases in the ratios of c to p in sediments younger than 15 25 years in cores from the eastern gotland basin coincident with h2s build up in deep waters paired oxygen and phosphorus concentration data from the sub halocline water column of the gotland basin covering the time since 1970 suggest that the phosphate reflux is fed from two sources at the transition from oxic to anoxic conditions iron bound phosphate is suddenly liberated at the sediment water interface and results in a concentration jump of approximately 2 mmol dm 23 phosphate in deep water during anoxic periods with h2s in bottom waters phosphate diffuses out of the sediment and adds to the dissolved phosphate pool our data imply that the sediment contributed approximately 14 000 t a 21 of phosphorus to the water column of the deep gotland basin since the early 1970s compared to total p input to the baltic sea 1993 39 000 t the reflux of p from anoxic sediments in the gotland basin thus is a major source of p for the baltic sea on time scales of years and decades the reflux of phosphorus from the sediments may be the reason for discrepancies between the ratios of nitrogen and phosphorus in the baltic sea and for a lack of effects of declining phosphate discharge into this large ecosystem q 2000 elsevier science b v all rights reserved

proterozoic supercontinent its latest precambrian rifting breakup dispersal into smaller continents and subsidence of their margins evidence from asia several large cratons and numerous smaller blocks all with precambrian sialic crust are seen clearly in the present day structure of asia the outer parts of larger precambrian terranes and entire smaller ones are covered with vendian early paleozoic shallow water carbonate sedimentary rocks coupled with the so called old late vendian early cambrian phosphorites in siberia southern china india kazakhstan and mongolia these sedimentary rocks are interpreted to represent a global belt of near equatorial sedimentation on passive continental margins of numerous vendian early paleozoic continents with free access to the world ocean later precambrian rifting is recorded clearly in many precambrian terranes of asia rifting was repeated during cenozoic time with the same pattern and in the same places it was preceded and accompanied by various magmatic activity from 750 to 600 ma tillites filling rifts are about 650 ma and are believed to be a feature of a single proterozoic super continent rift to drift transition as attested by the beginning of thick and laterally continuous carbonate sedimentation on passive margins seems to be diachronous it started in early vendian time or even earlier in some places but not earlier than cambrian in others

early mesozoic southward subduction history of the mongol okhotsk oceanic plate evidence from geochronology and geochemistry of early mesozoic intrusive rocks in the erguna massif ne china abstract in this paper we present new zircon u pb ages hf isotope data and whole rock major and trace element data for early mesozoic intrusive rocks in the erguna massif of ne china and we use these data to constrain the history of southward subduction of the mongol okhotsk oceanic plate and its influence on ne china as a whole the zircon u pb dating indicates that early mesozoic magmatic activity in the erguna massif can be subdivided into four stages at 246 ma 225 ma 205 ma and 185 ma the 246 ma intrusive rocks comprise a suite of high k calc alkaline diorites quartz diorites granodiorites monzogranites and syenogranites with i type affinities the 225 ma intrusive rocks consist of gabbro diorites and granitoids and they constitute a bimodal igneous association the 205 ma intrusive rocks are dominated by calc alkaline i type granitoids that are accompanied by subordinate intermediate mafic rocks the 185 ma intrusive rocks are dominated by i type granitoids accompanied by minor amounts of a types these early mesozoic granitoids mainly originated by partial melting of a depleted and heterogeneous lower crust whereas the coeval mafic rocks were probably derived from partial melting of a depleted mantle modified by subduction related fluids the rock associations and their geochemical features indicate that the 246 ma 205 ma and 185 ma intrusive rocks formed in an active continental margin setting related to the southward subduction of the mongol okhotsk oceanic plate the 225 ma bimodal igneous rock association formed within an extensional environment in a pause during the subduction process of the mongol okhotsk oceanic plate every magmatic stage has its own corresponding set of porphyry deposits in the southeast of the mongol okhotsk suture belt taking all this into account we conclude the following 1 during the early mesozoic the mongol okhotsk oceanic plate was subducted towards the south beneath the erguna massif but with a pause in subduction at 225 ma and 2 the southward subduction of the mongol okhotsk oceanic plate not only caused the intense magmatic activity but was also favorable to the formation of porphyry deposits

jurassic earliest cretaceous tectonostratigraphic evolution of the erlian basin northeast china records of polyphase intracontinental deformation in northeast asia abstract the erlian basin is an important part of the late mesozoic extensional basin system in northeast asia the basin contains thick jurassic lower cretaceous rocks and provides excellent opportunities to understand intracontinental deformation in northeast asia during the late mesozoic however the jurassic early cretaceous evolution of the basin and associated driving mechanisms remain disputed based on 13 new zircon u pb ages for the jurassic lowest cretaceous strata in the erlian basin the correlation of jurassic lowest cretaceous strata between the erlian basin and the yan shan is reestablished and a high precision chronostratigraphic framework is presented the aqitu qiha formations are ascribed to the aalenian early bathonian ca 174 167 ma the xing anling group to the oxfordian early kimmeridgian ca 163 153 ma the hugejiletu formation to the tithonian early valanginian ca 151 139 ma and the aershan formation to the valanginian ca 138 ma combined with seismic reflection profiles the jurassic earliest cretaceous tectonostratigraphic evolution of the erlian basin is reconstructed and five major phases of deformation are revealed these deformation phases include aalenian early bathonian initial extension late bathonian callovian compression oxfordian early kimmeridgian extension late kimmeridgian earliest valanginian compression and valanginian extension integrated with geological evidences from peripheral regions of the erlian basin potential driving mechanisms related to far field tectonic events along asian margins are suggested for each of these deformation phases

zircon u pb and hf isotopic study of gneissic rocks from the chinese altai progressive accretionary history in the early to middle palaeozoic abstract gneissic rocks in the chinese altai mountains have been interpreted as either paleozoic metasedimentary rocks or precambrian basement this study reports geochemical and geochronological data for banded paragneisses and associated gneissic granitoids collected along a ne sw traverse in the northwestern chinese altai petrological and geochemical data suggest that the protoliths of the banded gneisses were possibly immature sediments with significant volcanic input and that the gneissic granitoids were derived from i type granites formed in a subduction environment three types of morphological features can be recognized in zircons from the banded gneisses and are interpreted to correlate with different sources zircons from five samples of banded paragneiss cluster predominantly between 466 and 528 ma some give neoproterozoic ages and a few yield discordant paleoproterozoic to archean ages zircon hf isotopic compositions indicate that both juvenile mantle and crust materials were involved in the generation of the source rocks from which these zircons were derived in contrast zircons occur ubiquitously as elongated euhedral prismatic crystals in the four samples of the gneissic granitoids and define single populations for each sample with mean ages between 380 and 453 ma the general absence of precambrian inheritance and positive zircon hf values for these granitoids suggest insignificant crustal contribution to the generation of the precursor magmas our data can be interpreted in terms of a progressive accretionary history in early to middle palaeozoic times and the chinese altai may possibly represent a magmatic arc built on a continental margin dominated by neoproterozoic rocks

granitic plutonism as an indicator of microplates in the palaeozoic of central and eastern maine abstract oxygen and strontium isotope ratios have been used to characterize source regions for granitic magmas for a transect across the northern appalachian orogen in central and eastern maine the northwestern plutons katahdin and seboeis have 18 o values of 10 3 13 3 and initial 87 sr 86 sr ratios of 0 7083 and 0 7066 respectively the central plutons bottle lake and center pond have lower 18 o values 8 2 9 9 and initial 87 sr 86 sr ratios 0 7043 0 7055 the southeastern plutons lucerne and deblois have 18 o values 9 0 11 0 but initial 87 sr 86 sr ratios 0 7077 and 0 7041 respectively which are intermediate between the northwestern and central plutons source models derived from these results and other petrological and geochemical data reflect the juxtaposition of discrete source regions by transcurrent faulting which may be related to oblique plate motions this model illustrates the importance of microplate accretion in the palaeozoic history of the northern appalachian orogen

high resolution paleomagnetic dating of sediments deposited in the central baltic sea during the last 3000 years abstract sediment cores from the main sedimentary basin in the baltic sea the gotland deep provide a high resolution record of changes in the holocene paleo environment and paleoclimate due to the inherent inaccuracy reservoir effect redeposited material of the 14 c dating method in dating marine sediments the known changes in the earth s magnetic field recorded in three sediment cores from the gotland deep were used to produce a more detailed and accurate timescale the variations in the inclination and declination were compared to the paleosecular variations over the last 3000 years recorded in annually laminated lake sediments from finland this correlation made it possible to construct for the late holocene gotland deep sediments a high resolution time scale with an estimated absolute accuracy of better than 50 years this new timescale made it possible to determine the mean annual rates of sedimentation with an unprecedented accuracy the sedimentation rates were found to have varied considerably both in time from 0 2 up to 4 3 mm year and space indicative of the patchy nature of accumulation in the gotland deep during the past 3000 years

the granite problem as exposed in the southern snake range nevada a geochemically and mineralogically diverse group of granitoids is present within an area of 900 km2 in the southern snake range of eastern nevada the granitoids exposed range in age from jurassic through cretaceous to oligocene and include two calcic intrusions two different types of two mica granites and aplites the younger intrusions appear to have been emplaced at progressively more shallow depths all of these granitoid types are represented elsewhere in the eastern great basin but the southern snake range is distinguished by the grouping of all these types within a relatively small area

internal structure and emplacement of the papoose flat pluton an integrated structural petrographic and magnetic susceptibility study the papoose flat pluton of eastern california is commonly cited in the geologic literature as a classic example of a forcibly emplaced pluton although the relative importance attributed to magmatic versus tectonic processes in controlling the structural evolution of the pluton has been controversial reexamination of this late cretaceous 83 ma pluton using a combination of new structural petrographic microstructural and anisotropy of magnetic susceptibility ams data on the pluton wall rock system has shown that the pluton is an inclined and internally zoned tabular structure assembled by forcible intrusion of successive pulses of magma at a crustal depth of 12 15 km initial pluton formation involved magma ascent in a vertical west northwest striking feeder dike which was arrested at a stratigraphically controlled mechanical discontinuity in the overlying cambrian metasedimentary rocks leading to formation of a southwest dipping sill subsequent sill inflation accompanied by horizontal infilling from the feeder dike at the base of the sill resulted in deformation and vertical translation of previously emplaced magma pulses and local raising of the sill roof facilitated by thermal weakening as the wall rock temperatures progressively rose during emplacement of successive magma pulses cooling from the roof of the pluton downward resulted in cessation of vertical inflation on the west side of the pluton and promoted lateral expansion toward the northeast and floor depression below the eastern part of the pluton we have been unable to document any regional scale structures e g of equivalence to similar age synplutonic strike slip shear zones in the sierra nevada batholith to the west that may have controlled emplacement of the papoose flat pluton however this fact does not preclude the likelihood that the country rocks were subjected to a regional deviatoric stress field at this time indeed tectonic overpressuring was probably essential for producing sufficiently high magma pressures for laccolith formation at this midcrustal level simple thermal modeling using microstructural and thermobarometric data indicates that the total duration of emplacement of the pluton did not exceed 30 000 yr this rapid emplacement rate may explain why the pluton appears to be anorogenic even though it was emplaced during a period of regional scale deformation

terrestrial organic matter in surface sediments of the baltic sea northwest europe as determined by cuo oxidation abstract we studied the distribution and composition of terrestrial organic matter in sediments of the baltic sea northwest europe to this end surface sediments from all basins of the baltic sea were analyzed for their lignin oxidation product yields and compositions after cuo oxidation lignin oxidation product yields depend on the concentration of organic carbon and range from 0 4 to 10 2 mg g 1 total organic carbon toc on the basis of an average of 13 mg g 1 toc in two river sediments we estimate that the upper limit of terrestrial organic matter in baltic sea sediments is 30 of toc the contribution of terrestrial organic matter differed between the individual basins depending on the distance from runoff discharge areas and on the area occupied by each submarine catchment lignin composition showed a relative decrease of angiosperm tissue from the southwest to the northeast reflecting the shift from temperate to boreal vegetation type the gotland and the bornholm seas which have no significant river input were characterized by high relative contributions of nonwoody strongly altered material the source may either be a mixture of pollen and peat being eroded from geologically older strata at the seafloor or laterally advected material from the other basins however the pronounced compositional differences between the basins indicated that interbasin transport of terrestrial organic matter is less important than direct river input although river signals can only be traced at a few places in the baltic sea

nd o and sr isotopic constraints on the origin of precambrian rocks southern black hills south dakota abstract the nd o and sr isotopic characteristics of precambrian metasedimentary metavolcanic and granitic rocks from the black hills of south dakota are examined two late archean granites 2 5 2 6 ga have t dm ages of 3 05 and 3 30 ga suggesting that at least one of the granites was derived through the melting of significantly older crust early proterozoic metasedimentary rocks have t dm ages that range from 2 32 to 2 45 ga these model ages in conjunction with probable stratigraphic ages ranging from 1 9 to 2 2 ga indicate that mantle derived material was added to the continental crust of this region during the early proterozoic previous studies of the harney peak granite complex have reported u pb and rb sr ages of about 1 71 ga and most granite samples examined in this study have sr isotopic compositions consistent with that age two granite samples taken from the same sill however give two point rb sr and sm nd ages of 2 08 0 08 and 2 20 0 20ga 2200 nd 15 5 respectively in addition whole rock and apatite samples of the spatially associated tin mountain pegmatite give a sm nd isochron age of 2000 100 ma 2200 nd 5 8 1 8 the sm nd o and rb sr isotopic systematics of these granitic rocks have been complicated to some degree by both crystallization and post crystallization processes and the age of the pegmatite and parts of the harney peak granite complex remain uncertain processes that probably complicated the isotopic systematics of these rocks include derivation from heterogeneous source material assimilation mixing of ree between granite and country rock during crystallization via a fluid phase and post crystallization mobility of sr the nd isotopic compositions of the pegmatite and the harney peak granite indicate that they were not derived primarily from the exposed metasedimentary rocks

isotopic variation in the tuolumne intrusive suite central sierra nevada california granitoid rocks of the compositionally zoned late cretaceous toulumne intrusive suite in the central sierra nevada california have initial87sr 86sr values sri and143nd 144nd values ndi that vary from 0 7057 to 0 7067 and from 0 51239 to 0 51211 respectively the observed variation of both sri and ndi and of chemical composition in rocks of the suite cannot be due to crystal fractionation of magma solely under closed system conditons the largest variation in chemistry ndi and sri is present in the outer most equigranular units of the tuolumne intrusive suite sri varies positively with sio2 na2o k2o and rb concentrations and negatively with ndi al2o3 fe2o3 mgo feo cao mno p2o5 tio2 and sr concentrations this covariation of sri ndi and chemistry can be modeled by a process of simple mixing of basaltic and granitic magmas having weight percent sio2 of 48 0 and 73 3 respectively isotopic characteristic of the mafic magma are sri 0 7047 ndi 0 51269 and 18o 6 0 and of the felsic magma are sri 0 7068 ndi 0 51212 and 18o 8 9 the rocks sampled contain from 50 to 80 of the felsic component an aplite in the outer equigranular unit of the tuolumne intrusive suite apparently was derived by fractional crystallization of plagioclase and hornblende from magma with granudiorite composition that was a product of mixing of the magmas described above siliceous magmas derived from the lower crust having a maximum of 15 percent mantle derived mafic component are represented by the inner prophyritic units of the tuolumne intrusive suite

the record of late pleistocene biogenic sedimentation in the eastern tropical pacific ocean we have generated 300 kyr records of biogenic opal calcite and organic carbon corg for three cores in the eastern and central equatorial pacific ocean and have compared the records to determine whether common periods of biogenic sedimentation have occurred throughout the region we find that corg has been deposited in common pulses throughout the area while opal has a much more local pattern of variation calcite varies regionally but the record is shaped by superimposed dissolution and productivity processes the most intense corg peak occurs at 18 ka and can have greater than 2 times the holocene corg content other major corg peaks occur 150 ka and perhaps at 280 ka we have compared the corg record in one of the cores v19 28 to a model deepwater oxygen record developed from 13c data in the nearby v19 30 to test whether the corg record has been mostly shaped by degradation or by the rain of organic matter from the euphotic zone we found no coherence between the two records implying that the corg record is primarily a measure of productivity by comparing the opal calcite and corg records in v19 28 a core which is at or above the lysocline we found that both increased calcite and opal deposition matches high corg accumulation we also found however that the calcite and opal records were uncorrelated so that episodes of high opal deposition do not necessarily accumulate calcite rapidly we hypothesize that at least two different plankton communities have been dominant in the waters above this site one rich in opal secreting plankton and one more dominated by calcite producers the opal rich plankton community was dominant during the intervals 10 15 ka and 35 60 ka supplement data table 2 is available with entire articleon microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document p88001 2 50 payment must accompany order

fluid enhanced deformation transformation of granitoids to banded mylonites western sierra nevada california and southeastern australia abstract deformation of tonalite and adamellite from the western sierra nevada california and southeastern australia has produced mylonite zones that show a broad range of microstructural development varying from strongly banded and or foliated to locally non foliated accompanying these fabrics are new mineral assemblages commonly rich in quartz and epidote whereas the banded foliated varieties appear to develop from progressive deformation of the granitic host they also occur in close proximity cm m to domains of non banded and or weakly to non foliated fabrics of similar mineralogy suggesting a complex timing and partitioning of deformation fluid flow and metamorphism in the mylonite zone whole rock major and trace element analyses of the tonalites show large enrichments of sio 2 cao fe 2 o 3 total fe and sr and strong depletions in na 2 o k 2 o feo mgo ba and rb with increasing stages of deformation although substantial chemical change appears to occur under static conditions locally chemical changes in the adamellite are much less dramatic all suites show systematic decrease in 18 o and convergence of oxygen isotopic compositions of whole rock and quartz with increasing deformation minimum fluid rock is estimated to lie between 0 1 and 10 increasing with intensity of deformation and depending on host composition isotopically exchanged seawater or connate water are the most plausible fluids chemical data from a number of mylonite zones in various orogenic belts show widely varying behavior gain loss of major and minor elements eleven factors control which and how much elements are gained lost during mylonitization of the granitoids the most influential of which are primary composition mineralogy and primary texture of the host as well as several aspects of fluid interaction active during deformation these influences may be a function of local rather than regional geological conditions

water rock interactions in fault gouge measurements were made of the amounts of d 18o and h2o in fault gouge collected over a depth of 400 m in the san andreas fault of california the amounts and isotopic compositions of the pore fluids also analyzed suggest that formation waters from adjacent franciscan rocks have migrated into the gouge and mixed with local meteoric water thus the gouge is an open system permeable to fluid flow this permeability has important implications concerning heat flow along the fault zone

source and tectonic implications of tonalite trondhjemite magmatism in the klamath mountains in the klamath mountains voluminous tonalite trondhjemite magmatism was characteristic of a short period of time from about 144 to 136 ma early cretaceous it occurred about 5 to 10 m y after the 165 to 159 ma josephine ophiolite was thrust beneath older parts of the province during the nevadan orogeny thrusting from 155 to 148 ma the magmatism also corresponds to a period of slow or no subduction most of the plutons crop out in the south central klamath mountains in california but one occurs in oregon at the northern end of the province compositionally extended members of the suite consist of precursor gabbroic to dioritic rocks followed by later more voluminous tonalitic and trondhjemitic intrusions most plutons consist almost entirely of tonalite and trondhjemite poorly defined concentric zoning is common tonalitic rocks are typically of the low al type but trondhjemites are generally of the high al type even those that occur in the same pluton as low al tonalite the suite is characterized by low abundances of k2o rb zr and heavy rare earth elements sr contents are generally moderate 450 ppm by comparison with sr rich arc lavas interpreted to be slab melts up to 2000 ppm initial 87sr 86sr 18o and nd are typical of mantle derived magmas or of crustally derived magmas with a metabasic source compositional variation within plutons can be modeled by variable degrees of partial melting of a heterogeneous metabasaltic source transitional mid ocean ridge to island arc basalt but not by fractional crystallyzation of a basaltic parent melting models require a residual assemblage of clinopyroxene garnet plagioclase amphibole residual plagioclase suggests a deep crustal origin rather than melting of a subducted slab such models are consistent with the metabasic part of the josephine ophiolite as the source because the josephine ophiolite was at low t during nevadan thrusting an external heat source was probably necessary to achieve significant degrees of melting heat was probably extracted from mantle derived basaltic melts which were parental to the mafic precursors of the tonalite trondhjemite suite thus under appropriate tectonic and thermal conditions heterogeneous mafic crustal rocks can melt to form both low and high al tonalitic and trondhjemitic magmas slab melting is not necessary

oxygen 18 oxygen 16 and d h studies of plutonic granitic and metamorphic rocks across the cordilleran batholiths of southern british columbia hydrogen and oxygen isotope ratios of 500 samples mainly from granitic plutons were measured along a 700 km e w traverse across the accreted terranes of southern british columbia latitudes 49 52 n despite the geological complexity and range of intrusive ages late triassic to tertiary and although there are steps in the isotopic values at some geologic boundaries e g across the strait of georgia two clear patterns emerge 1 the 18 o 16 o and d h ratios of the waters involved in hydrothermal interactions with the granitic rocks show a regular eastward trend of depletion in d and 18 o enormous areas were affected by the hydrothermal processes but the most intense alteration is localized along major north trending lineaments e g okanagan lake 2 independent of the hydrothermal effects the primary 18 o values of the granitic rocks also change systematically eastward from 7 0 to 8 5 in vancouver island reaching a minimum of 5 5 to 7 0 in the western and central coast plutonic complex then increasing progressively from the eastern coast batholith to the okanagan batholith and attaining a maximum of 10 0 to 12 0 in the nelson batholith two groups of samples are unique in their high d values the first group is represented by two geographically isolated batholiths guichon and thuya that were not affected by the tertiary meteoric hydrothermal systems and that have therefore preserved their early jurassic to triassic k ar ages the second group is represented by the jurassic plutons of vancouver island there the hydrothermal fluids were both d rich and 18 o rich 18 o 0 as evidenced by the fact that feldspars in the altered granites are enriched in 18 o relative to coexisting quartz both anomalies can be explained if these terranes were located closer to the equator and or in a maritime environment at the time of intrusive and hydrothermal activity in agreement with available paleomagnetic data excluding these anomalous areas two distinct ages of meteoric hydrothermal activity can be identified cretaceous in the west and early to mid tertiary in the east the isotopic trends in the rocks are similar to the present day patterns of meteoric waters in the region with one primary difference the paleowaters are enriched in d by about 20 compatible with a northward translation of these terranes a climatic change or both the similarities of the patterns suggest a topography similar to that of the present day a mountain chain along the coast during the early tertiary the whole rock 18 o values of the granitic rocks determined by extrapolation of the trends of 18 o quartz versus 18 o feldspar to correct for hydrothermal alteration effects suggest that the main part of the coast plutonic complex formed from magmas similar to those in oceanic island arcs derived from basaltic crust or from an upper mantle source toward the east an increasingly larger component of sedimentary rocks must be present in the source regions of the granitic magmas it may be significant that right at the edge of the north american craton there is a drop in the 18 o of the plutons from 11 down to 9 5 suggesting involvement of old granitic crust instead of sedimentary rocks these new data from southern british columbia confirm and amplify previous studies that have documented 1 the existence of widespread and pervasive hydrothermal effects throughout the north american cordillera and 2 regular west to east asymmetries in the amounts of different kinds of parent rocks in the source regions from which these mesozoic and cenozoic granitic magmas were derived

large scale porphyry type mineralization in the central asian metallogenic domain a review abstract a series of porphyry type cu mo au deposits including some giant world class deposits such as kounrad aktogai kal makyr oyu tolgoi and chalukou occur in the central asian metallogenic domain camd the outline of the camd almost overlaps with that of the central asian orogenic belt caob which is located between the east europe siberia tarim and north china cratons the camd comprises sparsely exposed tectonic slices of precambrian high grade metamorphic basement rocks that are sandwiched between phanerozoic accretionary complexes and arc rocks according to geological metallogenic geochronological and tectonic setting characteristics the camd comprises three metallogenic provinces 1 the kazakhstan cu au mo 2 the mongolia cu au and 3 the northeast china mo cu metallogenic provinces early paleozoic 490 to 440 ma and late paleozoic 330 to 295 ma large scale porphyry type mineralization in the kazakhstan metallogenic province is intimately associated with the evolution of the central tianshan yili northern tianshan kokchetav chingiz paleozoic arc system which was terminated by the late permian closure of the western segment of the paleo asian ocean in contrast the 370 ma and 240 ma porphyry type mineralization of the mongolia metallogenic province is related to the tuva mongolia arc collage system that was formed during the evolution of the paleo asian ocean in the paleozoic and the mongol okhotsk ocean in the early mesozoic furthermore the northeast china metallogenic province was affected by three distinct geodynamic processes that were associated with the paleo asian mongol okhotsk and paleo pacific oceans during the paleozoic 250 ma the early mesozoic 200 ma and the late mesozoic 200 90 ma respectively except for the early paleozoic cu mineralization at duobaoshan 480 ma and bainaimiao 440 ma the porphyry type mineralization of this metallogenic province is predominantly characterized by widespread mesozoic mo mineralization in general porphyry type deposits of the camd have geological structural metallogenic alteration and fluid characteristics which are consistent either with those related to the oceanic subductional arc settings of the circum pacific metallogenic domain or with those of the continental collisional environment in the tethyan metallogenic domain the involvement of highly oxidized intermediate to felsic magmas is fundamental for the formation of porphyry deposits even though the prevalent mash and slab melting models can be employed to interpret the genesis of some porphyry cu deposits in the camd a here newly suggested model which is based on pre enriched mafic lower crust and the subduction of a relictic mid oceanic ridge is thought to explain the large scale copper mineralization in the balkhash west junggar metallogenic belt of the kazakhstan metallogenic province nevertheless most of the porphyritic suites associated with the mo deposits in the northeast china metallogenic province have relatively depleted mantle like neodymium and hafnium isotopic compositions suggesting that the ore forming granitic magma may have predominantly been derived from juvenile lower crust upwelling of asthenosphere may have triggered the melting of the lower crust during post collisional extension subsequent to the closure of the paleo asian ocean in the triassic the subduction of the paleo pacific ocean in the jurassic and the thinning of the lithosphere caused by rollback of the paleo pacific plate in the cretaceous ancient crustal components were only locally found to have been involved in the magma source of several medium large mo deposits

geochemical trends in metal contaminated fiord sediments near a former lead zinc mine in west greenland abstract disposal of sulfidic waste in marine environments implies an environmental risk due to potential release and spreading of heavy metals to sediments and biota on a regional scale however tailings disposal in marine systems is practised in several places fiord sediments near black angel mine in west greenland are contaminated by pb and zn as a result of mining activity and marine disposal in the period 1973 1990 chemical analyses were performed on 6 fiord sediment cores collected up to 10 km away from the disposal area and included heavy metal analysis high resolution stable pb isotope stratigraphy and radiochemical dating 210pb the results were used for evaluating spatial and temporal distribution of anthropogenic heavy metals from the marine disposal a significant accumulation of pb and zn on a regional scale is still observed 9 years after mine closure stable pb isotope data provide a unique fingerprint of the mining related pb sources in the area today spreading of mining related pb up to 10 km away from the mining area accounts for more than 80 of total pb in sediment deposited within the last 100 years

oxygen isotopic compositions of central andean plutonic and volcanic rocks latitudes 26 29 south abstract oxygen isotope data are reported for 27 igneous rocks of mesozoic to quaternary age from the central andes 26 29 s the plutonic rocks and most of the volcanics have 18 o values between 6 2 and 8 3 the whole rock 18 o values show a weak correlation with initial 87 sr 86 sr data this o sr array differs from documented trends for calc alkaline plutonic suites from california scotland and northern italy but overlaps with data for volcanic and plutonic rocks from ecuador northern chile and southern peru the oxygen isotope results indicate that the magmas evolved without significant contamination from supracrustal rocks e g rocks that experienced 18 o enrichment during surficial weathering the available o sr and pb isotopic data for these rocks are best explained by magma generation in the upper mantle or lower crust from the late mesozoic on the 87 sr 86 sr values were modified at depth by isotopic exchange between the magma and a continually thickening crust of plutonic rocks of late precambrian to early mesozoic age

shrimp zircon u pb ages and tectonic implications of igneous events in the ereendavaa metamorphic terrane in ne mongolia abstract the ereendavaa metamorphic terrane in ne mongolia has long been considered as a pre altaid block or a precambrian cratonic terrane with a paleoproterozoic basement overlain by neoproterozoic cambrian rocks but the idea has not been supported by any isotopic dating sensitive high resolution ion microprobe shrimp zircon u pb dating on gneisses amphibolite and schists mylonites of the ereendavaa terrane suggests that the terrane mainly formed during early paleozoic 495 464 ma and late paleozoic early mesozoic 295 172 ma a minor amount of precambrian rocks might have been involved in the formation of the protoliths of these rocks as shown by precambrian inherited zircons 1796 794 ma the new age data also suggest that the ereendavaa terrane experienced at least two periods of magmatism 1 early paleozoic 495 464 ma and 2 late paleozoic early mesozoic 295 172 ma which are probably produced by the subduction of the paleo asian ocean in the south and the subduction of the mongol okhotsk ocean in the north respectively the mylonitized granite 172 ma and undeformed pegmatite 163 ma are interpreted to be syn and post kinematic products the new age data constrain the closure of the mongol okhotsk ocean at mid jurassic

deciphering the source and contamination history of peraluminous magmas using δ 18 o of accessory minerals examples from garnet bearing plutons of the sierra nevada batholith peraluminous granitoids provide critical insight as to the amount and kinds of supracrustal material recycled in the central sierra nevada batholith california major element concentrations indicate sierran peraluminous granitoids are high sio2 68 9 76 9 and slightly peraluminous average molar al2o3 cao na2o k2o 1 06 both major and trace element trends mimic those of other high silica sierran plutons garnet grt in the peraluminous plutons is almandine spessartine rich and of magmatic origin low grossular contents are consistent with shallow 4 kbar depths of garnet crystallization metasediments of the kings sequence commonly occur as wallrocks associated with the plutons including biotite schists that are highly peraluminous a cnk 2 25 and have high whole rock wr 18o values 9 6 21 8 average 14 5 2 9 n 26 ultramafic wallrocks of the kings kaweah ophiolite have lower average 18o 7 1 1 3 n 9 the 18o wr of the kings sequence is variable from west to east higher 18o values occur in the west where quartz in schists is derived from marine chert values decrease eastward as the proportion of quartz from igneous and metamorphic sources increases peraluminous plutons have high 18o wr values 9 5 13 consistent with supracrustal enrichment of their sources however relatively low initial 87sr 86sr values 0 705 0 708 indicate that the supracrustal component in the source of peraluminous magmas was dominantly altered ocean crust and or greywacke also plutons lack or have very low abundances 1 of grains of inherited zircon zrc cores average 18o zrc is 7 9 in peraluminous plutons a higher value than in coeval metaluminous plutons 6 7 diorites associated with peraluminous plutons also have high 18o zrc 7 4 8 3 which is consistent with the diorites being derived from a similar source magmatic garnet has variable 18o 6 6 10 5 avg 7 9 due to complex contamination and crystallization histories evidenced by multiple garnet populations in some rocks comparison of 18o zrc and 18o grt commonly reveals disequilibrium which documents evolving magma composition minor 5 7 contamination by high 18o wallrocks occurred in the middle and upper crust in some cases although low 18o wallrock may have been a contaminant in one case overall oxygen isotope analysis of minerals having slow oxygen diffusion and different times of crystallization e g zircon and garnet together with detailed textural analysis can be used to monitor assimilation in peraluminous magmas moreover oxygen isotope studies are a valuable way to identify magmatic versus xenocrystic minerals in igneous rocks

paleomagnetic constraints on the geodynamic history of the major blocks of china from the permian to the present paleomagnetic study of china and its environs has been the center of a major international effort for the last 10 years in this paper we critically review all available paleomagnetic poles of upper permian to tertiary age from the main blocks of china with the goal of placing constraints on models of the formation and the subsequent deformation of the region after selecting reliable poles by applying objective criteria we divide our analysis into first order motions of blocks and second order deformation within blocks for first order analysis apparent polar wander paths are constructed for the major blocks we discuss the compatibilities and contradictions between the geological and paleomagnetic records a sequence of paleogeographic configurations taking into account geological constraints but remaining within paleomagnetic uncertainties is presented in general the major blocks were probably in contact throughout the permian and triassic but the jurassic was the key age during which most of the movement toward china s present configuration took place our reconstructions include certain details which are suggested by the paleomagnetic record but whose geological signatures seem to have been hidden by subsequent events during the cretaceous chinese poles agree with poles from other continents transferred onto eurasia at the second order we observe that for almost each period with sufficient data the paleomagnetic poles are streaked along a small circle centered on the sampling region indicating that much of china has been affected by small 20 differential rotations this we interpret as deformation caused in part by the extrusion of the chinese blocks away from the indian collision the complete annotated list of poles is given as an appendix

sapropelic deposits in a sediment from the guinea basin south atlantic until recently it has been generally thought that a stagnant water column is a prerequisite to the formation of organic rich deposits in deep water marine sediments a belief best exemplified by studies on the eastern mediterranean sapropels many workers have assumed that following the hydrological isolation of the deep waters stagnation of the eastern mediterranean basins and the concomitant development of anoxic conditions occurred before the deposition of the sapropelic layers1 10 opposing arguments suggesting a role for increased marine productivity have however been put forward both specifically with respect to the eastern mediterranean sapropels11 12 and more generally regarding deep water organic rich deposits13 15 we report here the discovery of sapropelic layers 0 5 organic carbon 16 in a recent sediment core from a deep water site in the guinea basin it is thought most unlikely that stagnant conditions could prevail in the water column at this site and preliminary analyses of the sediments suggest that the organic rich deposits have been a direct result of increased productivity in the overlying water column the most recent occurring about 18 000 yr ago and coinciding with the last glacial maximum

early paleozoic metamorphic rocks of the erguna block in the great xing an range ne china evidence for the timing of magmatic and metamorphic events and their tectonic implications abstract the erguna block is a crustal unit located along the eastern margin of the central asian orogenic belt caob in the far north of china it contains a variety of khondalitic rocks that include sillimanite and garnet bearing gneiss biotite plagioclase gneiss and carbonate in association with hornblende plagioclase gneiss and a variety of granitic orthogneisses these rocks are collectively referred to as the mohe complex and make up the basement rocks in this poorly exposed terrane four samples were analyzed for this study including two samples of biotite plagioclase gneiss one sample of garnet sillimanite gneiss and a sample of hornblende plagioclase gneiss all samples provide evidence of metamorphism in the form of discrete zircon grains or metamorphic rims around detrital cores and they record remarkably consistent 206 pb 238 u ages between 495 2 ma and 497 6 ma detrital zircon cores with oscillatory zoning from the two biotite plagioclase gneiss samples yield magmatic ages that range from 608 8 ma to 1015 12 ma whereas those in the garnet sillimanite gneiss range from 678 8 to 1373 17 ma these results establish that a range of meso to neoproterozoic magmatic rocks were present in the source area and contributed to the sedimentary protoliths the youngest zircon core with an age of 608 8 ma limits the maximum possible age of deposition indicating that the rocks of the mohe complex cannot be late archean to early proterozoic as previously considered the 500 ma age for the granulite facies metamorphic event in the mohe complex is identical to that recorded in the mashan complex of the jiamusi block and the hutou complex of the khanka block 1000 km to the south southeast implying that an extensive early paleozoic khondalite belt extended across much of the eastern caob rocks of similar age and metamorphic grade have also been recorded from the sayang baikal orogen along the southern margin of the siberia craton 1000 km to the west it is unclear at present whether these two belts are directly related but whatever the relationship it is evident that extensive tracts of khondalitic rocks were present in the eastern caob they were subsequently affected by high grade metamorphism in the early paleozoic forming collisional accretionary terranes that were accreted during the late pan african global event

paricutin re examined a classic example of crustal assimilation in calc alkaline magma following its birth on the 20th of february 1943 the mexican volcano paricutin discharged a total of 1 38 km3 of basaltic andesite and andesite before the eruption came to an end in 1952 until 1947 when 75 of the volume had been erupted the lavas varied little in chemical or isotopic composition all were basaltic andesites with 55 to 56 sio2 18o of 6 9 to 7 0 and 87sr 86sr ratios close to 0 7038 subsequent lavas were hypersthene andesites with silica contents reaching 60 18o values up to 7 6 and 87sr 86sr of 0 7040 to 0 7043 the later lavas were enriched in ba rb li and k2o and depleted in mgo cu zn cr ni sr and co the isotopic and other chemical changes which appeared abruptly over a few months in 1947 are interpreted as the result of tapping a sharply zoned and density stratified magma chamber xenoliths of partially fused felsic basement rocks in the lavas have silica contents greater than 70 18o of 5 6 to 9 9 and 87sr 86sr between 0 7043 and 0 7101 in many respects they resemble samples of basement rocks collected from nearby outcrops three analysed samples of the latter have silica contents of 65 to 67 18o of 7 7 to 8 6 and 87sr 86sr between 0 7047 and 0 7056 these new data provide strong support for the original interpretations of wilcox 1954 who explained the chemical variations by a combination of fractional crystallization and concurrent assimilation of up to 20 weight continental crust except for a few trace elements particularly ba sr and zr the chemical and isotopic compositions of the xenoliths and basement rocks that crop out nearby match the type of contaminant required to explain the late stage lavas some of the discrepancies may be explained by postulating a contaminant that was older and richer in ba sr and zr than those represented by the analysed xenoliths others can be attributed to chemical changes accompanying disequilibrium partial melting contact metamorphism and meteoric hydrothermal alteration of the country rock many of the xenoliths show evidence of having been affected by such processes the lavas were erupted from a zoned magma chamber that had differentiated by liquid fractionation prior to the eruption the order of appearance of the lavas can be explained in terms of withdrawal of stratified liquids of differing densities and viscosities

sub recent nitrogen isotope trends in sediments from skagerrak north sea and kattegat changes in n budgets and n sources we determined 15 n 14 n ratios of total nitrogen in surface sediments and dated sediment cores to reconstruct the history of n loading of the north sea the isotopic n composition in modern surface sediments is equivalent to and reflects the isotopic mixture of oceanic nitrate on the one hand delta 15 n 5 parts per thousand and the imprint of river borne nitrogen input into the se north sea delta 15 n up to 12 parts per thousand in estuaries of the se north sea on the other hand we compare the results with 615 n records from pre industrial sediment intervals in cores from the skagerrak and kattegat areas which both constitute significant depositional centres for n in the north sea and the baltic sea north sea transition as expected isotopically enriched anthropogenic nitrogen was found in the two records from the kattegat area which is close to eutrophication sources on land enrichment of delta 15 n in cores from the skagerrak the largest sediment sink for nitrogen in the entire north sea was not significant and values were similar to those found in sediment layers representing pre industrial conditions we interpret this isotopic uniformity as an indication that most riverine reactive nitrogen with its characteristic isotopic signature is removed by denitrification in shallow shallow water sediments before reaching the main sedimentary basin of the north sea c 2008 elsevier b v all rights reserved

a mid holocene geochemical record of saline inflow to the gotland deep baltic sea the formation of mn enrichments in gotland deep sediments has been linked to the occurrence of periodic inflows of saline water from the north sea in turn these saline inflow events are very strongly linked to variation in north atlantic atmospheric conditions here sedimentary mn concentrations in mid holocene sediments are measured with a 0 5 mm sampling resolution using scanning electron microscope techniques as the sedimentation rate in core 20001 5 was estimated to be approximately 0 5 1 mm per year examining the variation in mn enrichments may potentially provide an annual record of variation in saline inflow and by extension north atlantic climate on interannual timescales there are many processes that can affect mn cycling in the gotland deep when considered together these processes could potentially act to remove or significantly weaken the transmission of the primary saline inflow signal to the measured geochemical record producing an effectively random mn record analysis of the mn rec

nd sr and pb isotopic characteristics of cretaceous intrusive rocks from deep levels of the sierra nevada batholith tehachapi mountains california the cretaceous gabbroic to granitic intrusive rocks of the tehachapi mountains were emplaced at depths of 25 30 km and thus afford a view of deep processes in the sierra nevada batholith they consist of the 115 ma tehachapi suite and the 100 ma bear valley suite new zircon u pb age data reveal the presence of the latter as far west as grapevine canyon the nd sr pb and o isotopic whole rock data and zircon pb inheritance patterns for the bulk of the suites suggest an origin by mixing between depleted mantle derived magmas and metasedimentary material with a substantial component of old continental material however this mixing is not evident in variations between isotopic ratios and chemical and lithologic parameters this implies that isotopic hybridization of magmas took place deeper than 30 km and that fractionation processes are likely responsible for the bulk of the chemical variation in this part of the sierra nevada batholith consideration of the isotopic data in the context of the sierra nevada batholith as a whole suggests that the well known east to west isotopic gradients in the batholith may reflect a change in the average isotopic character of the preintrusive framework rather than a change in amount of crustal component on the other hand the lack of areal gradients in sr and nd isotopic ratios in the main study area may indicate a lack of pronounced gradation at deep levels at least within the western batholith

oxygen isotope systematics in an evolving geothermal system coso hot springs california abstract oxygen isotope and clay mineralogy studies have been made on whole rock samples and feldspar separates from three wells along the high temperature west flank of the coso geothermal system california the reservoir rocks have experienced variable 18o 16o depletion with 18o values ranging from primary values of 7 5 down to 4 6 spatial patterns of clay mineral distributions in the three wells are not closely correlated with the distributions expected from measured pre production temperature profiles but do correlate with spatial patterns of 18o 16o depletion indicating that the stability of clay minerals in the three wells is a function of fluid rock interaction in addition to temperature detailed 18o measurements in the three wells identify a limited number of localized intervals of extensive 18o 16o depletion these intervals document localized zones of higher permeability in the geothermal system that have experienced significant fluid infiltration water rock interaction and oxygen isotopic exchange with the geothermal fluids the local zones of maximum 18o 16o depletion in each well correspond closely with current hot water production zones most feldspar separates have measured 18o values too high to have completely attained oxygen isotope exchange equilibrium with the reservoir fluid at pre production temperatures in general the lower the 18o value of the feldspar the closer the feldspar approaches exchange equilibrium with the geothermal fluid this correlation suggests that fracture induced increases in permeability increase both fluid infiltration and the surface area of the host rock exposed to geothermal fluid promoting fluid rock interaction and oxygen isotope exchange the two most 18o 16o depleted feldspar samples have 18o values too low to be in exchange equilibrium with the pre production reservoir fluid at pre production temperatures these discrepancies suggest that the reservoir fluid in the west flank of the coso geothermal system was hotter and or had a lower 18o value due to fluid rock interaction at higher permeability in the past

strontium and oxygen isotopic variations in mesozoic and tertiary plutons of central idaho regional variations in initial 87sr 86sr ratios r i of mesozoic plutons in central idaho locate the edge of precambrian continental crust at the boundary between the late paleozoic mesozoic accreted terranes and precambrian sialic crust in western idaho the r i values increase abruptly but continuously from less than 0 704 in the accreted terranes to greater than 0 708 across a narrow 5 to 15 km zone characterized by elongate lens shaped highly deformed plutons and schistose metasedimentary and metavolcanic units the chemical and petrologic character of the plutons changes concomitantly from ocean arc type diorite tonalite trondhjemite units to a weakly peraluminous calcic to calcalkalic tonalite granodiorite granite suite the idaho batholith plutons in both suites yield late cretaceous ages but permian through early cretaceous bodies are confined to the accreted terranes and early tertiary intrusions are restricted to areas underlain by precambrian crust the two major terranes were juxtaposed between 75 and 130 m y ago probably between 80 and 95 m y oxygen and strontium isotopic ratios and rb and sr concentrations of the plutonic rocks document a significant upper crustal contribution to the magmas that intrude precambrian crust magmas intruding the arc terranes were derived from the upper mantle subducted oceanic lithosphere and may have been modified by anatexis of earlier island arc volcanic and sedimentary units plutons near the edge of precambrian sialic crust represent simple mixtures of the precambrian wall rocks with melts derived from the upper mantle or subducted oceanic lithosphere with r i of 0 7035 rb sr varies linearly with r i producing pseudoisochrons with apparent ages close to the age of the wall rocks measured 18o values of the wall rocks are less than those required for the assimilated end member by sr o covariation in the plutons however indicating that wall rock 18o was reduced significantly by exchange with circulating fluids metasedimentary rocks of the belt supergroup are similarly affected near the batholith documenting a systematic depletion in 18o as much as 50 km from the margin of the batholith plutons of the bitterroot lobe of the idaho batholith are remote from the accreted terranes and represent mixtures of precambrian wall rocks with melts dominated by continental lower crust r i 0 708 rather than mantle pseudoisochrons resulting from these data are actually mixing lines that yield apparent ages less than the true age of the wall rocks and meaningless ri assimilation fractional crystallization models permit only insignificant amounts of crystal fractionation during anatexis and mixing for the majority of plutons of the region

timescales of orogeny jurassic construction of the klamath mountains an electronic supplement of this material may be obtained on a diskette or anonymous ftp from kosmos agu org login to agu s ftp account using anonymous as the username and guest as the password go to the right directory by typing cd apend type ls to see what files are available type get and the name of the file to get it finally type exit to leave the system paper 94ycj2454 timescales of orogeny jurassic construction of the klamath mountains b r hacker m m donato c g barnes m o mcwilliams and w g ernst diskette may be ordered from american geophysical union 2000 florida avenue n w washington dc 20009 15 00 payment must accompany order classical interpretations of orogeny were based on relatively imprecise biostratigraphic and isotopic age determinations that necessitated grouping apparently related features that may in reality have been greatly diachronous isotopic age techniques now have the precision required to resolve the timing of orogenic events on a scale much smaller than that of entire mountain belts forty five new 40ar 39ar ages from the klamath mountains illuminate the deformation metamorphism magmatism and sedimentation involved in the jurassic construction of that orogen leading to a new level of understanding regarding how preserved orogenic features relate to ancient plate tectonic processes the new geochronologic relationships show that many jurassic units of the klamath mountains had 200 ma or older volcanoplutonic basement subsequent formation of a large 170 ma arc was followed by contractional collapse of the arc collision with a spreading ridge may have led to large scale nw se extension in the central and northern klamaths from 167 to 155 ma coincident with the crystallization of voluminous plutonic and volcanic suites marked cooling of a large region of the central klamath mountains to below 350 c at 150 ma may have occurred as the igneous belt was extinguished by subduction of colder material at deeper structural levels these data demonstrate that the klamath mountains and perhaps other similar orogens were constructed during areally and temporally variant episodes of contraction extension and magmatism that do not fit classical definitions of orogeny

records of southern and central baltic sea eutrophication in δ 13 c and δ 15 n of sedimentary organic matter abstract the baltic sea catchment area has seen considerable agricultural and industrial development over the last century but observational evidence of eutrophication is ambiguous we examined organic carbon accumulation rates and the stable isotope signature of 13 c and 15 n in organic matter in four 210 pb dated short sediment cores from the southern and central baltic sea to search for sedimentary records of increased nutrient input and biological production organic carbon accumulation rates vary little through time in the cores from the oder estuary and the arkona basin but increase significantly in the bornholm basin and in the gotland deep records over the last 40 years 15 n in the sediments show distinct increases over the last 80 50 years in the oder estuary arkona basin and bornholm basin with the greatest increase near the mouth of river oder the gotland deep record shows increasing 15 n values from 1900 to 1975 and decreased 15 n values during the last 20 years the 15 n records are interpreted to trace the effects of eutrophication in nearshore and basinal sediments over the last 100 years and the magnitude of 15 n increase may be a measure for its extent the switch to lower 15 n values in the gotland basin sediment core after initially increasing values coincides with increasing abundance of summer blooms of nitrogen fixing cyanobacteria as observed in monitoring programs 13 c toc values of organic matter increase in the oder estuary over the last 80 years and over the last 30 years in the bornholm basin and gotland basin the tendency to heavier 13 c indicates decreasing isotope fractionation during massive plankton blooms that this effect is not well documented in the sediments of the arkona basin may be due to enhanced degradation processes in the permanently mixed water column a comparison of organic carbon concentrations and 13 c toc and 15 n from early litorina stage around 7 000 years bp and modern sediments from the gotland sea suggests that the trophic state of the baltic sea was comparable to that of the modern environment productivity was nitrate based in spring and based on nitrogen fixation in summer much higher organic carbon concentrations and 13 c enrichment in modern sediments are indications of enhanced productivity compared to early litorina times

early middle paleozoic volcanic rocks from the ereendavaa terrane tsarigiin gol area ne mongolia with implications for tectonic evolution of the kherlen massif abstract we report our newly obtained geochronological and geochemical data of volcanic rocks in the tsarigiin gol area ereendavaa terrane to better understand of the early middle paleozoic tectonic evolution of the kherlen massif ne mongolia la icp ms zircon u pb dating reveals two stages of magmatism in the southern part of the ereendavaa terrane one in late ordovician 462 to 455 ma and the other in early devonian 418 ma the late ordovician tsarig volcanic rocks are medium to high k calc alkaline and intermediate to felsic in composition and characterized by relative enrichment of lree and liles e g rb and u and also by relative depletion of hfses e g nb ta p and ti a typical feature of subduction related magmas these rocks have mostly positive whole rock nd t 1 65 to 1 35 and zircon hf t 0 1 to 4 07 values with mesoproterozoic nd and hf model ages of 1 59 1 09 ga they were likely generated in a continental arc setting and their parental magmas were originated from a common source as the product of extensive fractional crystallization of mafic melts and their interaction with arc crust the early devonian chandmana volcanic rocks consist of rhyolite and minor trachybasalt displaying a typical bimodal distribution the trachybasaltic rocks are characterized by low lan smn 2 08 2 60 tbn ybn 1 42 1 50 and la yb 7 51 8 96 ratios and relatively flat mree to hree patterns with nd t values being close to zero 1 36 to 0 13 the rhyolites are high k calc alkaline and possess variably negative hf t 4 96 to 0 29 and nd t 2 14 to 0 57 values exhibiting a strong affinity to a2 type granites considering the distinct compositional gap between the end members and geochemical and isotopic signature of the bimodal suite the felsic magma is interpreted to have derived from partial melting of a hybrid crustal source composed of both pre neoproterozoic and juvenile crustal materials while the mafic rocks were probably derived from a mantle source previously metasomatized by slab derived melt the two phases of magmatism in the southern part of the ereendavaa terrane reflected two stages of tectonic evolution early middle paleozoic subduction of the paleo asian ocean plate that gave rise to the late neoproterozoic early cambrian undur khaan island arc and late cambrian to late ordovician ereendavaa active continental margin silurian amalgamation of the ereendavaa and idermeg terranes kherlen massif caused the break off of the subducted slab and the early to middle devonian extension in the area

recycling of manganese from anoxic sediments in stagnant basins by seawater inflow a study of surface sediments from the gotland basin baltic sea abstract the occurrence of mn carbonates in sapropelic sediments has been proposed to indicate that the host sediment originally accumulated under oxygenated bottomwaters in the central baltic sea ca rich rhodochrosite layers in sapropelic sediments have been related to inflows of oxic seawater from the north sea into the brackish predominantly anoxic deeps of the central baltic this study attempts to verify the model of authigenic mn carbonate formation by comparing oceanographic records directly with the composition of surficial sediments of the gotland basin surface sediments of the gotland deep sampled in 1997 displayed significant ca rich rhodochrosite enrichments at 6 5 11 and 15 cm sediment depth which reflect periods of intense seawater inflows in 1969 76 1948 56 and 1931 39 respectively however the latest major seawater inflow detected in 1993 was not reflected in the surface sediment the topmost 6 cm of the sediment was totally depleted in mn and 210pb data implied a disturbed top layer down to 5 cm calculations of saturation indices indicate that the porewaters were undersaturated with respect to mn and ca carbonate phases in the uppermost 8 cm in 1997 at greater depths the porewaters were close to equilibrium with respect to calcite and rhodochrosite the ca rhodochrosite layers corresponded well to a sediment core sampled at the same location in 1994 but that core displayed a further enrichment of ca rich rhodochrosite close to the sediment surface which can be related to a major inflow of north sea water in 1993 the significant decrease of about 2 3 mol m2 mn in the uppermost 6 cm of a disturbed sediment surface from 1994 to 1997 corresponds to an enrichment of about 20 mol l of dissolved mn within the 90 m thick anoxic water column in 1997 we conclude that several inflows of dense seawater into the gotland deep monitored between 1994 and 1997 led to a resuspension of the unstable fluffy surface sediment layer authigenic ca rich rhodochrosite which had formed immediately after the major inflow of oxic seawater in 1993 was therefore redissolved in undersaturated bottomwaters and mn was recycled from surficial sediments into the water column this in turn means that in contrast to the established model of ca rhodochrosite formation as a result of seawater inflow the occurrence of mixed mn depleted layers does not prove that seawater inflow has not occurred during the deposition of these sediments

late jurassic early cretaceous continental convergence and intracontinental orogenesis in east asia a synthesis of the yanshan revolution abstract the basic tectonic framework of continental east asia was produced by a series of nearly contemporaneous orogenic events in the late middle jurassic to early cretaceous commonly the late mesozoic orogenic processes were characterized by continent continent collision large scale thrusting strike slip faulting and intense crustal shortening crustal thickening regional anatexis and metamorphism followed by large scale lithospheric extension rifting and magmatism to better understand the geological processes this paper reviews and synthesizes existing multi disciplinary geologic data related to sedimentation tectonics magmatism metamorphism and geochemistry and proposes a two stage tectono thermal evolutionary history of east asia during the late middle jurassic to early cretaceous ca 170 120 ma in the first stage three orogenic belts along the continental margins were formed coevally at ca 170 135 ma i e the north mongol okhotsk orogen the east paleo pacific coastal orogen and the west bangong nujiang orogen tectonism related to the coastal orogen caused extensive intracontinental folding and thrusting that resulted in a depositional hiatus in the late jurassic as well as crustal anatexis that generated syn kinematic granites adakites and migmatites the lithosphere of the east asian continent was thickened reaching a maximum during the latest jurassic or the earliest cretaceous in the second stage ca 135 120 ma delamination of the thickened lithosphere resulted in a remarkable 120 km lithospheric thinning and the development of mantle derived magmatism mineralization metamorphic core complexes and rift basins the middle jurassic early cretaceous subduction of oceanic plates paleo pacific meso tethys and mongol okhotsk and continent continent collision e g lhasa and qiangtang along the east asian continental margins produced broad coastal and intracontinental orogens these significant tectonic activities marked by widespread intracontinental orogeny and continental reconstruction are commonly termed the yanshan revolution movement in the chinese literature

recent salt marsh development and natural regeneration of reclaimed areas in the plentzia estuary n spain the distribution of benthic foraminifera a range of elements and 137cs and 210pb were studied in surface sediments and 0 5 m cores from the plentzia estuarine marshes northern spain to determine the recent development of these previously reclaimed coastal areas these marshes receive abundant calcium carbonate from regional rocks and this allows calcareous foraminifera to colonize all marsh subenvironments good preservation of the original foraminiferal assemblages in the buried sediments is also favoured by high regional background carbonate benthic foraminifera are shown to be sensitive to both elevation above mean sea level exposure and salinity results from the chemical analyses indicate moderate anthropogenic pollution in the estuary and maximum input seems to have occurred in the 1960s recent marsh foraminiferal assemblages are good analogues for the palaeoenvironmental interpretation of historical and holocene cores and the combined micropalaeontological geochemical approach is demonstrated to be successful for the identification of short term anthropogenic impacts on this coastal region

neoproterozoic 800 ma orogeny in the tuva mongolia massif siberia island arc continent collision at the northeast rodinia margin abstract the tuva mongolia massif is a composite precambrian terrane incorporated into the palaeozoic sayany baikalian belt its neoproterozoic amalgamation history involves early 800 ma and late baikalian 600 550 ma orogenic phases two palaeogeographic elements are identified in the early baikalian stage the gargan microcontinent and the dunzhugur oceanic arc they are represented by the gargan glyba block and the island arc ophiolites overthrusting it the gargan glyba is a two layer platform comprising an early precambrian crystalline basement and a neoproterozoic passive margin sedimentary cover the upper part comprises olistostromes deposited in a foreland basin during the early baikalian orogeny the dunzhugur arc ophiolite form klippen fringing the gargan glyba and show a comprehensive oceanic arc ophiolite succession the dunzhugur arc faced the microcontinent as shown by the occurrence of forearc complexes the arc continent collision followed a pattern similar to phanerozoic collisions when the marginal basin lithosphere had been completely subducted the microcontinental edge partially underthrust the arc and the forearc ophiolite overrode it continued convergence caused a break of the arc lithosphere resulting in the uplift of the submerged microcontinental margin with the overthrust forearc ophiolites sliding into the foreland basin owing to the lithospheric break a new subduction zone inclined beneath the gargan microcontinent emerged initial melts of the newly formed continental arc are represented by tonalites intruded into the gargan microcontinent basement and its cover and into the ophiolite nappe the tonalite rb sr mineral isochron age is 812 18 ma which is similar to a u pb zircon age of 785 11 ma a period of tonalite magmatism in meso cenozoic orogenic belts is recognized some 1 10 m y after the collision accordingly the dunzhugur island arc gargan microcontinent collision is conventionally dated at around 800 ma it is highly probable that in the early neoproterozoic the gargan continental block was part of the southern in modern coordinates margin of the siberia craton it is suggested that a chain of precambrian massifs represents an elongate block separated from siberia in the late neoproterozoic the tuva mongolia massif is situated in the northwest part of this chain these events occurred on the ne neoproterozoic margin of rodinia facing the world ocean

organic geochemical signals of freshwater dynamics controlling salinity stratification in organic rich shales in the lower permian irati formation parana basin brazil abstract the lower permian irati formation in the northeastern and central eastern parana basin southern brazil was investigated in terms of bulk and molecular organic geochemistry in order to enlighten the complex depositional paleoenvironment during deposition of irati black shales the geochemical data reflect temporal and spatial variations of freshwater incursions promoting salinity stratification and high primary productivity in surface waters with the establishment of photic zone euxinia overall highly concentrated c17 long chain alkylnaphthalenes point to algal blooms as a result of freshwater inflows into the irati sea mostly in the central eastern basin segment and resemble concentration variations of pristane phytane and nc17 the freshwater inflows were also retraced by increasing chroman ratios toward the top of the investigated units along with the general decrease of further palaeosalinity indicators e g gammacerane tetracyclic c24 carotane and the herein suggested carotane and lexane indexes the existence of a water column stratification promoting organic matter preservation is supported by the detection of tetrahydrophenanthrene at high concentrations in addition to the known prominent occurrence of gammacerane the detection of c10 to c31 aryl isoprenoids in addition to c16 and c18 pseudohomologue aryl isoprenoids points to the presence of photic zone euxinia in general samples from the central eastern basin were deposited in a deeper marine setting with lower salinity than samples from the northeastern basin which displays signals of a shallower marine environment with reducing bottom water conditions both deposited under freshwater influxes

the response of benthic foraminifera to productivity cycles in the eastern equatorial pacific faunal and geochemical constraints on glacial bottom water oxygen levels the stage 2 glacial maximum in eastern equatorial pacific ocean sediments is characterized by a pronounced increase in the organic carbon content which has been attributed either to increased upwelling and higher production along the equatorial divergence during the last glacial period or to enhanced preservation as a consequence of a lower concentration of dissolved o2 in glacial bottom water several lines of evidence are presented based on data from two gravity cores collected from the central panama basin which support the former interpretation first both the abundance and the size of benthic foraminifera of the genus uvigerina increase in stage 2 coincident with the increase in organic carbon concentration most probably because of the greater availability of organic detritus during the glacial period the increase in foraminiferal size and associated greater requirement of oxygen for respiration argues against the probability that oxygen was substantially depleted in the upper centimeter where uvigerina spp are thought to live second the number of meiofaunal faecal pellets increases markedly during the stage 2 maximum indicating a more populous and active infauna which again argues against oxygen depletion third there is no significant decrease in the iodine organic carbon i corg ratio in the stage 2 sediments because iodine is depleted relative to carbon in anoxic basins but enriched in association with organic matter in oxic environments the absence of a depletion in the i corg ratio in stage 2 is evidence that bottom water remained oxygen replete during the last glacial fourth molybdenum is not enriched in sediments deposited in stage 2 the presence of anoxic or dysaerobic bottom water or shallow pore waters during that time would have been recorded by the addition of mo to the sediments via coprecipitation with authigenic iron sulphides the combined faunal and geochemical data are consistent with the notion that panama basin deep waters remained oxygen replete during the last glacial maximum it is concluded that the late stage 2 carbon enrichment in the eastern equatorial pacific more probably reflects higher productivity rather than enhanced preservation

evolution of the mongol okhotsk ocean as constrained by new palaeomagnetic data from the mongol okhotsk suture zone siberia summary this paper presents new data from palaeomagnetic investigations on the upper palaeozoic and mesozoic geological units from the siberian platform and the mongol okhotsk suture zone within the southern portion of the mongol okhotsk suture we collected palaeomagnetic samples from the late permian belektuy formation borzya region 50 7un 116 9ue and the middle late jurassic shadaron formation unda daya 51 5un 117 5ue we sampled the late permian alentuy formation khilok region 50 8un 107 2ue the early to middle jurassic irkutsk sedimentary basin isb 52 0un 104 0ue the late jurassic badin formation mogzon region 51 8un 112 0ue and the early cretaceous gusinoozesk formation gusinoe lake region 51 2un 106 5ue additionally in the northern region of the mongol okhotsk suture apart from the results of the isb and gusinoozersk formations which show very large ellipses of confidence and might be the present day geomagnetic field overprint our results allow us to constrain the evolution of the mongol okhotsk ocean palaeomagnetically from the late permian to the middle late jurassic they confirm that this large permian ocean closed during the jurassic ending up in the late jurassic or the beginning of the cretaceous in the eastern end of the suture zone as suspected on geological grounds however although geological data suggest a middle jurassic closure of the mongol okhotsk ocean in the west trans baikal region our data show evidence of a still large palaeolatitude difference between the amuria and siberia blocks this is interpreted as a result of the quite fast closure of the ocean after the middle jurassic finally our new palaeomagnetic results exhibit very large tectonic rotations around local vertical axes which we interpret as probably arising both from collision processes and from a left lateral shear movement along the suture zone due to the eastward extrusion of mongolia under the effect of the collision of india into asia

can sediment geochemistry record changes in coastal upwelling palaeoproductivity evidence from northwest africa and the arabian sea abstract the causes and consequences of organic carbon c org burial in sediments underlying regions of coastal upwelling are of profound importance to studies of palaeoceanography and palaeoclimatology alike the primary control on coastal upwelling wind stress and resulting ekman transport is linked intimately to variation in the earth s insolation record via a variety of feedback mechanisms in comparison the nutrients required to sustain productivity and the factors affecting the c org content of the sediment are related to variation in global glacial boundary conditions using a variety of geochemical indicators it is apparent that the record of upwelling off northwest africa driven by the northeast trade winds is characterized by arid glacial conditions and high wind stress sustaining elevated euphotic zone productivity the resulting c org accumulation record reflects higher palaeoproductivity during glacial episodes in contrast the arabian sea upwelling is seasonal faunal and geochemical ba al u th ratios indicators suggest strong interglacial 23 ka cyclicity in the southwest monsoon the c org record is spatially and temporally consistent but in disagreement with other palaeoproductivity indicators these results point to the decoupling of surface ocean productivity in areas of coastal upwelling from deep ocean circulation and nutrient supply this result has important consequences for driving atmospheric p co 2 changes on timescales less than that of oceanic mixing further applications of geochemical tracers to areas of coastal upwelling such as northwest africa and peru are needed urgently

petrogenesis of the cretaceous cassiar batholith yukon british columbia canada implications for magmatism in the north american cordilleran interior the ca 100 ma cassiar batholith is the single largest plutonic body in the hinterland of the canadian cordillera and is part of widespread middle cretaceous to eocene magmatism that occurred in the omineca crystalline belt the cassiar batholith is dominated by muscovite biotite granite and biotite muscovite granodiorite along with subordinate biotite hornblende granodiorite quartz monzodiorite and quartz monzonite the batholith has a wide range of strontium neodymium and oxygen isotope compositions 87 sr 86 sr i 0 706 to 0 734 nd 2 7 to 17 1 t dm 2000 to 980 ma 18 o 7 6 to 11 6 but the batholith9s lead isotope compositions are limited 206 pb 204 pb 19 14 to 19 24 207 pb 204 pb 15 70 to 15 72 208 pb 204 pb 39 14 to 39 26 these data require that the cassiar magmas have been generated from at least two distinct sources an isotopically evolved felsic source and an isotopically more primitive source of mafic to intermediate composition we propose that the felsic source comprised mesoproterozoic to neoproterozoic metasedimentary rocks and paleoproterozoic basement rocks of the north american cratonic margin determining the mafic source is more problematic but most likely it was proterozoic basement rocks of mafic to intermediate composition such as have been reported from various parts of the omineca crystalline belt the cassiar batholith is lithologically geochemically and isotopically similar to other major batholiths in the cordilleran interior including the batholiths of southeastern british columbia and the idaho batholith we interpret these similarities to reflect melting of common crustal source materials namely proterozoic basement and cover rocks of western laurentia the cassiar batholith and other cordilleran interior granitoids are distinct from granitoids generated in known subduction zone environments in terms of their mineralogical geochemical and isotopic characteristics these differences reflect a predominantly or exclusively crustal source region for the cordilleran interior granitoids in contrast subduction zone magmatism contains a significant mantle input even in cases where subduction takes place beneath thick continental crust these results strongly suggest that cordilleran interior granitoids did not form directly from a subduction setting rather an intracontinental collisional setting seems more appropriate in which magma generation occurred in response to crustal thickening followed in some cases by extension

new paleomagnetic results from northern china collision and suturing with siberia and kazakhstan abstract we report new results from our paleomagnetic studies of the inner mongolia autonomous region in northeastern china and the xinjiang autonomous region in northwestern china primary magnetizations are isolated in tertiary cretaceous late jurassic and late permian rocks and secondary components of magnetization of late jurassic and recent ages are seen in strata of inner mongolia western junggar basin and part of the tarim block suffered widespread complete tectono thermal remagnetization at about 280 ma permian the paleomagnetic evidence suggests that the sampled areas in inner mongolia have been part of the north china block at least since late permian time and the junggar block collided with the tarim block by late permian time our own paleomagnetic results together with available data from other parts of eastern eurasia suggest a tectonic model in which kazakhstan europe and siberia collided with each other and formed the europe siberia kazakhstan plate eur sib kaz in late permian time to the east mongolia outer mongolia inner mongolia and the north china block were also wedged together forming the mongolia north china plate mon nc by this time these two composite continental plates were not a stable unit in the late permian the geometrical relationship of the paleomagnetic poles for these two plates suggests that collision initially occurred in the late permian near the western end 50 n 75 e of the southern siberian fold belt and progressed eastward as the mon nc plate rotated and clockwise about 117 relative to the eur sib kaz plate during the early mesozoic suturing and rotation were probably complete by the late jurassic significant relative motion between the tarim and siberia blocks continued from early triassic to late jurassic as tarim moved northeastward 1400 km relative to siberia

timing and processes of late mesozoic mid lower crustal extension in continental ne asia and implications for the tectonic setting of the destruction of the north china craton mainly constrained by zircon u pb ages from metamorphic core complexes abstract late mesozoic large scale crustal extension occurred across the northeastern section of continental asia and metamorphic core complexes mccs are the major features in this region this paper presents new zircon u pb and ar ar ages of biotite and hornblende for pre or syn mylonitized and post kinematic unmylonitized intrusions from these mccs and attempts to determine the timing of initiation duration and termination of the mid lower crustal ductile extension by integrating with geochronological data from previous studies these mccs have similar 40 ar 39 ar biotite and hornblende ages 130 to 110 ma mainly from 130 to 120 ma which are interpreted as the cooling ages and represent the timing of the late stage formation of the mccs zircon u pb ages for syn to post kinematic intrusions from some mccs suggest that the mid lower crustal extension probably initiated at ca 150 145 ma with a peak occurring 145 130 ma and continued to 130 120 ma which is older than that suggested by 40 ar 39 ar ages by 10 20 ma the early ductile extension most likely initiated in mongolia and northern china during the late jurassic 150 145 ma at a deeper level then propagated southwards 140 130 ma and finally developed throughout the entire continental ne asia 130 120 ma based on the timing and processes of the extension a geodynamic model and tectonic setting of the destruction of the north china craton ncc are proposed

stable isotope geochemistry of caledonian granitoids from the british isles and east greenland abstract the british caledonian granites bcg comprise a compositionally diverse group of intrusive rocks ranging from diorite to leucogranite which were emplaced across the british isles in and adjacent to the caledonian metamorphic belt during the early paleozoic although the largest volume of caledonian magmatic activity in britain postdates the polyphase deformation and metamorphism of the grampian orogeny a small number of plutons were emplaced prior to the main orogenic event and another group are essentially contemporaneous with it o isotope ratios for the bcg are extremely variable and essentially overlap the full range of o isotope variations documented for orogenic granitoids the margins of two plutonic complexes have been affected by subsolidus hydrothermal exchange with meteoric fluids for the older 440 ma bcg plutons 18 o 6 6 12 1 and d 83 to 45 whereas for the younger c 400 ma plutons 18 o 7 1 12 4 and d 97 to 44 there is a general positive correlation between o and sr isotope variations in the post tectonic granitoids which is absent in their pre tectonic counterparts caledonian plutonic rocks occur in east greenland in the southern half of the fold belt between 70 75 n the earliest intrusions are sheets of monzonite and granodiorite emplaced slightly before the climax of caledonian orogenic activity subsequently a suite of post tectonic granites was emplaced c 450 370 ma two of the east greenland plutons have been affected by post emplacement meteoric hydrothermal alteration the 18 o range for the unaltered east greenland plutons is 8 5 13 9

palaeomagnetism and palaeogeography of mongolia from the carboniferous to the cretaceous final report abstract the present study summarizes the results of palaeomagnetic investigations of carboniferous to cretaceous rocks with respect to global tectonic interpretations of eurasia normal and reverse directions of stable remanence components were found for the carboniferous to cretaceous rocks within single outcrops sometimes even within individual samples triassic permian and carboniferous pole positions for mongolia are strikingly different from those of the siberian platform the conclusion about the different palaeogeographical development of mongolia with respect to the siberian platform is well seen from differences in palaeolatitudes up to 50 and palaeorotations up to 150 the apparent polar wander paths for mongolia and the north china block are almost identical since the carboniferous this way palaeomagnetic and palaeogeographical data are contributing to the global tectonics model of the regions under study during the carboniferous permian and triassic the regions of mongolia investigated were not part of eurasia together with the north china block they show a palaeogeographical affinity with the pacific plate

oxygen isotope study of the long valley magma system california isotope thermometry and convection in large silicic magma bodies products of voluminous pyroclastic eruptions with eruptive draw down of several kilometers provide a snap shot view of batholith scale magma chambers and quench pre eruptive isotopic fractionations i e temperatures between minerals we report analyses of oxygen isotope ratio in individual quartz phenocrysts and concentrates of magnetite pyroxene and zircon from individual pumice clasts of ignimbrite and fall units of caldera forming 0 76 ma bishop tuff bt pre caldera glass mountain 2 1 0 78 ma and post caldera rhyolites 0 65 0 04 ma to characterize the long lived batholith scale magma chamber beneath long valley caldera in california values of 18o show a subtle 1 decrease from the oldest glass mountain lavas to the youngest post caldera rhyolites older glass mountain lavas exhibit larger 1 variability of 18o quartz the youngest domes of glass mountain are similar to bt in 18o quartz values and reflect convective homogenization during formation of bt magma chamber surrounded by extremely heterogeneous country rocks ranging from 2 to 29 oxygen isotope thermometry of bt confirms a temperature gradient between late 815 c and early 715 c bt the 18o quartz values of early and late bt are 8 33 and 8 21 consistent with a constant 18o melt 7 8 0 1 and 100 c temperature difference zircon melt saturation equilibria gives a similar temperature range values of 18o quartz for different stratigraphic units of bt and in pumice clasts ranging in pre eruptive depths from 6 to 11 km based on melt inclusions and document vertical and lateral homogeneity of 18o melt worldwide five other large volume rhyolites lava creek lower bandelier fish canyon cerro galan and toba exhibit equal 18o melt values of earlier and later erupted portions in each of the these climactic caldera forming eruptions we interpret the large scale 18o homogeneity of bt and other large magma chambers as evidence of their longevity 105 years and convection however remaining isotopic zoning in some quartz phenocrysts trace element gradients in feldspars and quartz and zircon crystal size distributions are more consistent with far shorter timescales 102 104 years we propose a sidewall crystallization model that promotes convective homogenization roofward accumulation of more evolved and stagnant volatile rich liquid and develops compositional and temperature gradients in pre climactic magma chamber crystal melt gas bubbles mush near chamber walls of variable 18o gets periodically remobilized in response to chamber refill by new hotter magmas one such episode of chamber refill by high ti sr ba zr and volatile richer magma happened 103 104 years prior to the 0 76 ma caldera collapse that caused magma mixing at the base mush thawing near the roof and walls and downward settling of phenocrysts into this hybrid melt

late jurassic early cretaceous closure of the mongol okhotsk ocean demonstrated by new mesozoic palaeomagnetic results from the trans baikal area se siberia summary we present new palaeomagnetic results from the transba ikal area se siberia from the mongol okhotsk suture zone the boundary between the amuria and siberia blocks in order to better constrain the time of closure of the mongol okhotsk ocean in the mesozoic we col lected 532 rock samples at 68 sites in six localities of basalts trachy basalts and andesites from both sides of the mongol okhotsk suture at unda river j 3 51 7 n 117 4 e kremljevka peak k1 51 8 n 117 5 e and torey lakes k 1 50 1 n 115 9 e on the southern side of the suture and at monostoy river j 1 51 1 n 106 8 e ingoda river k 1 51 2 n 112 2 e and bichura town k 1 50 6 n 107 6 e on the northern side progressive thermal demagnetization enabled us to resolve low ltc and high htc temperature components of magnetization at most sites jurassic palaeopoles computed from the htcs show a large discrepancy with respect to the apparent polar wander path of eurasia which we interpret in terms of 1700 2700 km of post late jurassic northward movement of amuria with respect to siberia although ge ological data suggest a middle jurassic closure of the mongol okhotsk ocean in the west trans baikal region our data give evidence of a large remaining palaeolatitude difference between the amuria and siberia blocks in contrast early cretaceous sites cluster remarkably well along a small circle which is centred on the average site location this implies the absence of post early cretaceous northward motion of amuria relative to siberia and demonstrates the pre early cretaceous closure of the mongol okhotsk ocean finally we interpret the very large tectonic rotations about local vertical axes evidenced by the small circle distribution of poles as arising both from collision processes and from left lateral shear movement along the suture zone due to the eastward extrusion of amuria under the effect of the collision of india into asia

the ereendavaa range north eastern mongolia an additional argument for mesozoic extension throughout eastern asia mesozoic rift basins locally bounding metamorphic core complexes have been recognized in transbaikalia and northern china numerous basement outcrops located between these two regions in eastern mongolia are considered as pre palaeozoic in age one of these the ereendavaa range appears as a gneissic core marked by amphibolite facies metamorphic conditions the range is overlain to the nw by the unmetamorphosed mesozoic onon basin below the basin the upper part of the range consists of a gently nw dipping shear zone associated with top to the nw motion the structural pattern is consistent with syn extensional exhumation of the range preliminary geochronological data indicate that the shear zone is late jurassic to early cretaceous in age coeval with the onon basin these new data from eastern mongolia constitute a link between transbaikalia and northern china indicating that nw se extensional mesozoic tectonics occurred throughout the entire region

stable isotope systematics in mesozoic granites of central and northern california and southwestern oregon 18o d and h2o contents were measured for whole rock specimens of granitoid rocks from 131 localitics in california and southwestern oregon with 41 new determinations in the klamath mountains and sierra nevada initial strontium isotope ratios are known for 104 of these samples large variations in 18o 5 5 to 12 4 d 130 to 31 water contents 0 14 to 2 23 weight percent and initial strontium isotope ratios 0 7028 to 0 7095 suggest a variety of source materials and identify rocks modified by secondary processes regular patterns of variation in each isotopic ratio exist over large geographical regions but correlations between the ratios are generally absent except in restricted areas for example the regular decrease in d values from west to east in the sierra nevada batholith is not correlative with a quite complex pattern of 18o values implying that different processes were responsible for the isotopic variations in these two elements in marked contrast to a good correlation between 87sr 86sr o and 18o observed in the peninsular ranges batholith to the south such correlations are lacking except in a few areas d values on the other hand correlate well with rock types chemistry and 87sr 86sr o except in the coast ranges where few of the isotopic signatures are primary the uniformly low d values of samples from the mojave desert indicate that meteoric water contributed much of the hydrogen to the rocks in that area even so the 18o values and 18o fractionations between quartz and feldspar are normal in these same rocks

a speculative reconstruction of the middle paleozoic continental margin of southwestern north america geologic relations and data from strontium and oxygen isotopic studies are used together to speculate on the paleogeographic positions of various crustal blocks or terranes in southwestern north america with respect to the middle paleozoic continental margin the southwestern margin of the middle paleozoic continent based on these data is quite irregular in its present configuration but most of the irregularities can be accounted for by strike slip displacement of continental fragments during several post middle paleozoic tectonic events we conclude that large scale late paleozoic sinistral displacements and mesozoic and cenozoic dextral displacements have been the major events affecting the configuration of this margin our preferred model includes the following elements 1 500 km of left lateral displacement of a large part of the margin during a late paleozoic truncation 2 slivering of the margin by right lateral faulting during the mesozoic amounting to about 275 km of total displacement and 3 further deformation of the margin during the cenozoic primarily by right lateral faulting oroclinal bending and crustal extension

87 sr 86 sr and 18 o 16 o isotopic systematics and geochemistry of granitoid plutons across a steeply dipping boundary between contrasting lithospheric blocks in western idaho a spatially abrupt geochemical boundary is preserved within four plutonic complexes along the western margin of the cretaceous idaho batholith near mccall idaho these intrusives ranging in composition from tonalite to granite were emplaced across a regional boundary between accreted oceanic arc terranes and the continental margin and their isotopic major element and trace element geochemistry provide detailed information about this change in crustal characteristics at depth indicating that the boundary is nearly vertical and extends deep into the lithosphere the hazard creek complex emplaced west of the transition in wall rock lithology has initial 87sr 86sr ri less than 0 7045 and 18o greater than 7 5 indicating little or no continental crust in its source region however elevated 18o requires some incorporation of rocks formed or altered at the earth s surface a large shift in ri and 18o is observed across the 5 8 km wide little goose creek complex which was emplaced across the wall rock boundary this is interpreted as mixing between 1 a basaltic or andesitic magma with low k2o and high na2o al2o3 and sr similar to that forming the hazard creek complex and 2 materials similar to precambrian sedimentary sedimentary rocks with low sr high 18o 15 and high ri 0 83 at 100 ma the payette river complex emplaced east of the wall rock boundary exhibits at least one additional component with low 18o 6 moderate ri 0 708 and mafic composition this component is inferred to be old basaltic material in the lower crust or upper mantle similar to that inferred to be a minor part of the peninsular ranges batholith in se california silver et al 1979 hill et al 1986 the easternmost complex in the idaho transect is made up of granites that may contain a component of granitic cratonal basement the entire west to east geochemical transition from oceanic arc magmas to cratonal magmas takes place over a lateral distance of less than 20 km although the zone of transitional protolith dominated by metasedimentary rocks is unusually narrow and may have been in part tectonically removed the striking geochemical similarities between this traverse and several other transects across much broader areas of nevada and california suggest that the craton itself was not rifted apart but that juxtaposition of the accreted oceanic arc terranes occurred along the preexisting craton margin the data confirm that the isotopic geochemistry of granitoid plutons can be used as a probe of deep lithospheric character and that major lateral variations in the lithosphere on the order of one to two kilometers in width can be recognized in favorable circumstances

timing of exhumation of the ereendavaa metamorphic core complex north eastern mongolia u pb and 40ar 39ar constraints abstract the occurrence of numerous ne sw trending rift basins that locally bound metamorphic core complexes in transbaikalia northern mongolia and north eastern china indicates that the eastern domain of the central asian orogenic belt was affected by a large scale nw se extensional tectonic event during late jurassic to early cretaceous times available geochronological information about the formation of metamorphic core complexes in the region define partly synchronous periods in transbaikalia 134 121 ma and north china 136 110 ma suggesting dome exhumation during the early cretaceous in north eastern mongolia the ereendavaa range lined by the onon shear zone was recently interpreted as one of these metamorphic core complexes of late jurassic to early cretaceous age on the basis of u pb zircons data from two synkinematic pegmatite dykes however this wide time range remains to be discussed the present work constrains the tectono thermal evolution of the range by providing new mc icp ms u pb zircons ages from pegmatite dykes and metamorphic igneous rocks and mica and amphibole 40 ar 39 ar laser step heating ages from metamorphic and magmatic rocks that crop out within the range the results show that 1 the ereendavaa range was the locus of a widespread magmatic activity prior to its exhumation from late jurassic times 2 cooling of the range began at least at 137 9 0 3 ma as revealed by a 40 ar 39 ar plateau ages obtained from an hornblende sampled within the range core and 3 exhumation along the onon shear zone occurred from 130 3 1 9 ma down to 124 8 0 3 ma as shown by ages of most of micas and amphiboles from the shear zone these more accurate ages are consistent with metamorphic core complex ages from adjacent areas furthermore they argue for an extensional dynamic paroxysm at the scale of eastern asia during a narrow time range in the early cretaceous between ca 130 ma and ca 120 ma

geodynamics of the western part of the mongolia okhotsk collisional belt trans baikal region russia and mongolia abstract after the western edge of the mongolian microcontinent joined the siberian continent in the region of central mongolia in the earliest permian these two continental blocks remained turned at an angle of about 120 with respect to each other and separated on greater extent of their present day boundary by an enormous gulf of the paleopacific called the mongolia okhotsk ocean closure of this ocean at the early middle jurassic boundary led to the complete collision of siberia and mongolia which by then had already become part of the mongolia north china continent this main collisional episode which lasted through the middle and late jurassic involved thrusting folding and magmatism and produced the mongolia okhotsk belt the onon island arc which was located in the mongolia okhotsk ocean was squeezed between the two major continents inasmuch as the third element the island arc was involved in the collision it is reasonable to distinguish two branches of the mongolia okhotsk suture these branches control the spatial distribution of gold mineralization in the trans baikal region on the southeastern periphery of siberia the crust thickened considerably after the collision and a plateau like uplift formed in the early cretaceous when compression ceased the collisional uplift collapsed and the thrusts were transformed into low angle normal faults the motions on which were responsible for the formation of rift basins and exhumation of metamorphic core complexes

vapor phase exsolution as a controlling factor in hydrogen isotope variation in granitic rocks the notch peak granitic stock utah abstract the notch peak granitic stock western utah is comprised of three concentric sequentially intruded rock types from granite at the rim to quartz monzonite i to quartz monzonite ii at the core the 18 o values of whole rocks vary about an average of 9 4 smow irrespective of the rock type and position relative to contact suggesting that the three magmas had the same parent the whole rock d values in the stock range from 100 to 55 d values increase toward the cores of both quartz monzonite i and quartz monzonite ii resulting in concentric contours the d contours of quartz monzonite ii cross cut those of quartz monzonite i suggesting little isotopic interaction between these bodies and the absence of a late pervasive fluid phase there is a positive correlation between d values and water content of the samples where samples from each body define a distinct field the positive correlation is explained by isotopic fractionation attendant on vapor exsolution from the crystallizing magma an observed increase in d with the degree of chloritization a trend opposite to that observed in systems where participation of meteoric water has been demonstrated is the result of subsolidus interaction with the exsolved fluids these results show that large variations in the hydrogen isotope ratios of a granitoid can arise by exsolution of a vapor phase from the melt on crystallization in general magmas with larger modal amount of primary hydrous phases will tend to have higher d values than those with small amounts of hydrous phases furthermore the relatively high d values of chlorites at notch peak confirm the applicability of classical concepts of closed system deuteric alteration to some granitoid bodies thus meteoric water interaction need not be always invoked to explain hydrogen isotope variation and deuteric alteration in granitoids

evolution of the solonker suture zone constraints from zircon u pb ages hf isotopic ratios and whole rock nd sr isotope compositions of subduction and collision related magmas and forearc sediments abstract the solonker zone in northern inner mongolia china is considered as the suture between the north china craton and the south mongolian microcontinent two magmatic belts are recognized along the suture zone a subduction related magmatic belt represented by the baolidao arc rocks and a younger collision related granite belt represented by the halatu granites we use zircon u pb ages zircon in situ hf isotopic analyses and whole rock nd sr isotopic data of the two magmatic belts and related forearc sediments the xilinhot metamorphic complex to constrain timing of the suturing and to discuss the petrogenesis of the magmatic rocks a gabbroic diorite bld 1 of the baolidao arc was dated at 310 5 ma by shrimp this sample shows an e nd t value of 2 5 and i sr of 0 7052 hf isotopic analyses on 25 zircons from the same sample show e hf t 5 4 to 11 5 another diorite sample xh 2 of the same arc from south of xilinhot displays even more depleted isotopic compositions with e nd t 5 6 and i sr 0 7037 the main population of zircons from this sample have highly variable and depleted hf isotopic compositions e hf t 0 18 3 the large variation in hf isotopic composition of zircons with largely the same crystallization age from a single pluton is explained by a mixing process between depleted mantle derived magma and continental crust in an active continental arc setting the halatu granite hlt 2 was dated at 234 7 ma by shrimp zircons from the granite also show a large variation of e hf t values 9 1 to 26 despite most samples having whole rock e hf t 2 the large variation in e hf t values suggests that the granite formed probably by partial melting of two source regions a dominant juvenile crust and a subordinate old continental crust most zircons from the xilinhot metamorphic complex show ages comparable with those of the baolidao arc rocks suggesting that the protolith of the metamorphic complex was probably deposited during or after arc magmatism some zircons however show precambrian ages that fall into two groups one with ages of 780 900 ma resembling those from the south mongolian microcontinent and the other with ages of 1524 2900 ma similar to those of the north china craton thus the protolith of the metamorphic complex probably formed in a forearc basin during convergence of the two continents and metamorphosed subsequently during collision in the late paleozoic our zircon age data thus constrain timing of collision between the south mongolian microcontinent and the north china craton to have been between 296 and 234 ma

tectonic setting of the taltson magmatic zone at 1 92 0 ga a granitoid based perspective the paleoproterozoic taltson magmatic zone is one of the key tectonic features of western laurentia the existing tectonic model for the belt envisions its formation by subduction of oceanic crust beneath a continental margin followed by direct collision between formerly separate crustal blocks we tested this model by comparing the large geochemical and isotopic database available for taltson magmatic zone granitoids with similar databases for phanerozoic granitoid suites from different tectonic environments the comparison reveals that the early granitoid suite of the taltson magmatic zone which had been ascribed to the subduction phase of orogenesis lacks the mantle signature apparent in granitoids of phanerozoic continental margin arc settings instead both early and late suites appear to have an intracrustal origin similar to mesozoic and cenozoic granitoids of the cordilleran interior of western north america which formed in the distant hinterland of a convergent plate margin in light of thes

redistribution and geochemical behaviour of redox sensitive elements around s1 the most recent eastern mediterranean sapropel the youngest eastern mediterranean sapropel s1 is generally believed to have formed 7 9 14c ky b p the distribution of redox sensitive elements above in and below the sharply defined organic rich unit reveal that s 1 has already suffered heavy diagenetic alteration its present thickness based on its dark colour is only a fraction of its original thickness in one core visual evidence of s1 with an inferred initial thickness of 6 cm has been destroyed and a similar thickness appears to have been lost from the tops of another two examples this is caused by oxidation of the upper reaches of s 1 by a progressive oxidation front which commenced 5 2 5 4 14c ky ago and still appears to be active in the three cores studied diagenetic relocation of many redox sensitive elements has occurred most elements expected to be enriched in sapropels or sulphide rich sediments already have maximum concentrations above or below rather than in the present s 1 layers similarities are observed in the geochemical behaviour of as fe and p of co cu mn ni and zn and of cd mo sb ti u and v the behaviour of ba is distinctive and its immobility as barite makes it an indicator for palaeoproductivity and the original high s1 corg content the elements se and i exhibit particularly large sharp concentration peaks and together are proposed as useful indicators of the present boundary between oxic and post oxic conditions

u pb zircon strontium and oxygen isotopic and geochronological study of the southernmost sierra nevada batholith california the southernmost sierra nevada offers a view into the deep levels of the mesozoic batholithic belt which constitutes much of the range to the north and represents one of the major tectonic features of western north america the main crystalline rocks of the study area are 1 the intrusive suite of bear valley a middle cretaceous tonalite batholith complex with coeval gabbroic intrusives and 2 the gneiss complex of the tehachapi mountains which consists of early cretaceous orthogneiss and subordinate paragneiss with local domains having granulite facies metamorphic assemblages the orthogneisses are dominantly tonalitic in composition with significant layers of granodioritic to granitic and lesser dioritic to gabbroic gneiss quartz rich and psammitic metasedimentary rocks with subordinate marble constitute the main framework assemblage into which the plutonic rocks were emplaced field relations demonstrate assimilation of metasedimentary material into the orthogneiss and tonalite batholith magmas and magma mixing between mafic tonalitic and granitic materials significant domains of both homogenization and inhomogenization are recognized isotopically within the mixed rocks u pb zircon studies have resolved two major igneous suites and a third suite of postdeformational intrusives all lying between 90 and 120 ma the first suite gneiss complex of the tehachapi mountains was emplaced at 115 ma and exhibits penetrative high temperature deformation developed at or near solidus conditions a number of discordance patterns along with the physical properties of the zircon suggest minor inheritance of proterozoic zircon and limited open system behavior in response to a major 100 ma plutonic event the 100 3 ma intrusive suite of bear valley crosscuts the older suite but also exhibits significant synplutonic deformation mainly concordant zircon ages indicate the igneous crystallization age but some discordances occur due to inheritance or entrainment of proterozoic zircon the high temperature deformation fabrics in these suites and within the metasedimentary framework rocks were crosscut by the granodiorite of claraville 90 ma and pegmatite dikes 95 ma the granodiorite of claraville shows strong inheritance of proterozoic zircon and high initial 87 sr 86 sr and 18 o zircon populations from paragneiss and quartzite samples are dominated by proterozoic detrital grains strontium and oxygen isotopic data on the zircon geochronology sample suite suggest simple twocomponent mixing of mantle derived gabbroic to tonalitic magmas with partial to complete melt products from the metasedimentary framework rocks sedimentary admixtures for some granitic rocks may be as high as 45 but for the tonalitic batholithic complex are no higher than about 15 modeled values of 10 20 metasediment are typical for the orthogneisses initial 87 sr 86 sr correlates directly with 18 o and generally correlates inversely with sr content some subtle complexities in the sr and o isotopic data suggest the involvement of a third cryptic component such a component could be early phanerozoic ensimatic accretionary terranes that were structurally beneath the observed metasedimentary sequence or altered oceanic crust and sediments introduced into the mantle magma source area by subduction one of the initial aims of this study was to seek out remnants of proterozoic sialic crystalline rocks within the gneiss complex of the tehachapi mountains no such remnants were found and our studies strongly suggest that sialic components within this link of the mesozoic batholithic belt were introduced into mantle derived magraatic systems by anatexis of continent derived sedimentary rocks

supracrustal input to magmas in the deep crust of sierra nevada batholith evidence from high δ 18 o zircon oxygen isotope ratios of zircon zc from intrusives exposed in the tehachapi mountains southern california reveal large inputs of high 18 o supracrustal contaminant into gabbroic and tonalitic magmas deep 30 km in the cretaceous sierra nevada batholith high 18 o zc values 7 8 0 7 predominate in the deep parts of the batholith but lower values 6 1 0 9 occur in shallower parts this indicates a larger gradient in 18 o with depth in the batholith than occurs from west to east across it oxygen sr and nd isotope data show that the supracrustal contaminant was likely young paleozoic or mesozoic hydrothermally altered upper oceanic crust or volcanic arc sediments such rocks were subducted or underthrust beneath the sierran arc during accretion of oceanic terranes onto north america this component yielded high 18 o magmas that were added to the base of the batholith on average gabbros in the southern sierra contain at least 18 of the subducted supracrustal component some tonalite and granodiorite magmas were additionally contaminated by kings sequence metasedimentary rocks as evidenced by 18 o zc and initial 87 sr 86 sr that trend toward values measured for the kings sequence besides high 18 o values in the southern sierra xenoliths in the central sierra also have elevated 18 o which confirms the widespread abundance of supracrustal material in the sub arc lithospheric mantle in contrast to 18 o zc whole rock 18 o values of many samples have undergone post magmatic alteration that obscures the magmatic contamination history of those rocks such alteration previously prevented confident determination of the mass of young hydrothermally altered mantle rocks that contributed to sierran granitoids

stable isotope geochemistry of hercynian granitoid rocks from the sarrabus massif southeastern sardinia italy abstract measurements have been made of the oxygen and hydrogen isotopic compositions of 37 samples of granitoid rocks from the sarrabus massif in several cases mineral separates have also been analyzed the 18 o values of whole rocks range from 7 0 to 10 5 and are independent of petrologic type o and sr isotopic ratios indicate that parental magmas were formed by partial melting of material which was dominantly of lower crustal origin retrograde effects are evident in the quartz biotite fractionations of which values are too large for equilibrium fractionations at igneous temperatures the d values of whole rocks range from 87 to 40 and are consistent with the conclusions reached upon the basis of the oxygen isotopic compositions the geographic distribution pattern of both 18 o and d values allows the contouring of an area in the central western part of the massif which is characterized by relatively lower 18 o and higher d content this area was affected by a strong tectonism and mineralized with fluorite by hydrothermal fluids

coupled sr o isotope variations as an indicator of source heterogeneity for the northern peninsular ranges batholith primary 18o values for tonalitic rocks from the san jacinto intrusive complex range from 9 0 to 10 6 initial87sr 86sr for the same samples varies from 0 7058 to 0 7076 rocks with low 18o tend to have higher initial87sr 86sr the majority of samples have a limited range of 18o values 10 0 10 4 that is anticorrelated with colour index this variation is compatible with crystallization of rocks with varying mineral abundances from an homogeneous for oxygen reservoir more mafic rocks show a large range of 18o values reflecting original variations in source material compositions the combined sr o isotope data demonstrate that three or more distinct components were involved in the genesis of these rocks these three components are also seen in batholithic rocks from the mojave block and sierra nevada to the north these data are interpreted as indicating a 0 35 oxygen atom basis contribution to the san jacinto rocks from an old continental lithosphere source

geochemical records of sediments in the eastern gotland basin products of sediment dynamics in a not so stagnant anoxic basin abstract geochemical data from 4 cores taken in the eastern gotland basin of the central baltic sea 68 to 243 m water depth show that depositional conditions for heavy metals in sediments were similar at all water depths prior to anthropogenic influxes after the onset of industrialization the deep anoxic parts of the basin expanded at irregular intervals and the

a critical evaluation of depositional parameters controlling the variability of organic carbon in arabian sea sediments abstract organic carbon distribution in the surficial sediments of the arabian sea to some extent mirrors the surface productivity being both high in the peripheral portions and low in the central region suggesting that the productivity is a primary source of organic matter however a critical examination reveals that the organic enrichment on the slope of the indian margin is two to four higher max 16 71 than on the slope of the arabian peninsula max 7 54 while the productivity in the former region is three to four times lower than the latter this observation suggests that the influence of productivity on organic enrichment is of secondary importance high concentrations 4 of organic carbon exactly coincide with the oxygen minimum zone 150 1500 m water depth evidently reflecting the crucial role played by the anoxic bottom waters in the preservation of organic carbon incidently the organic rich band along the western indian slope is wide long and highly concentrated as compared to that of the slope of the arabian peninsula and no organic enrichment is found on other continental slopes of the arabian sea although an equally intense oxygen minima impinge on the floor of the slope such a variability of organic carbon on the slopes is attributed to various geological parameters such as texture of sediments rates of fine terrigenous sediment deposition shelf width slope gradient bottom currents and adsorption capacity of individual clay minerals all these observations suggest that the productivity is not an ultimate control for the organic enrichment of bottom sediments but it is the bottom water anoxia in conjunction with various depositional parameters which determine the degree of preservation

magnetic susceptibility and relation to initial 87sr 86sr for granitoids of the central sierra nevada california measurement of the magnetic susceptibility of more than 6000 samples of granitic rock from the mariposa 1 by 2 quadrangle which crosses the central part of the sierra nevada batholith between 37 and 38 n latitude shows that magnetic susceptibility values are above 10 2 si units in the east and central parts of the batholith and drop abruptly to less than 10 3 si units in the western foothills in a narrow transitional zone intermediate values 10 3 to 10 2 prevail magnetic susceptibility appears to decrease slightly westward within the zones of both high and low values magnetic susceptibility in plutonic rocks is chiefly a function of the abundance of magnetite which depends in turn on the total iron content of the rocks and their oxidation ratio lower magnetic susceptibilities of felsic members of sierran intrusive suites and of some felsic rock units relative to adjacent mafic rock units commonly reflect differences in total iron content but the differences of magnetic susceptibility that define the regional pattern generally are much larger and are determined chiefly by the oxidation ratios of the rocks the relatively unaltered condition of the samples and restriction of 18o to the range of 7 to 10 3 indicate that neither hydrothermal fluids nor subsolidus alteration were important in modifying oxidation ratios correlations of magnetic susceptibility with initial 87sr 86sr suggest that oxidation ratios have been inherited from the source regions for the magmas from which the rocks crystallized reduction of fe3 to fe2 by organic carbon or other reducing substances may also have affected magnetic susceptibility

precambrian crystalline basement in southern mongolia as revealed by shrimp zircon dating single zircon ages determined by ion microprobe shrimp ii for granitoid gneisses from the southern slope of the baga bogd massif gobi altai southern mongolia reveal several episodes of zircon growth ranging from late palaeoproterozoic to late cambrian the oldest events are documented by a zircon crystallization age for a gneiss protolith at 1519 11 ma and by a xenocrystic zircon from a dark grey augen gneiss yielding an age of c 1701 ma discrete igneous events are recorded in granite gneisses with protolith emplacement ages of 983 6 956 3 and 954 8 ma these ages provide the first record of early neoproterozoic magmatic activity in this region a much younger and discrete magmatic event is recorded by several dioritic to granitic orthogneisses which are tectonically interlayered with the older gneisses and have protolith emplacement ages between 502 and 498 ma these late cambrian granitoids of calc alkaline affinity are likely to have been emplaced along an active continental margin and suggest that the baga bogd precambrian crustal fragment was either docked against the southward present day coordinates growing margin of the caob or was a large enough crustal entity to develop an arc along its margin we speculate that the precambrian gneisses of this massif may be part of a crustal fragment rifted off the tarim craton

paleoenvironmental implications of cenozoic sediments from the central arctic ocean iodp expedition 302 using inorganic geochemistry 1 integrated ocean drilling program iodp expedition 302 arctic coring expedition acex recovered a unique sediment record from the central arctic ocean revealing that this region underwent major environmental fluctuations since the late cretaceous major and trace element composition of 1 300 samples were determined using x ray fluorescence xrf the results show significant compositional variability of the sediments with depth that can be attributed to changes in a provenance and pathways of detrital material b paleoenvironmental conditions and depositional processes and c diagenetic overprint of the primary record in addition to existing lithological units we introduce new geochemical units for a more process related approach interpreting the acex record in detail via the geochemical signature of siberian flood basalts we are able to reconstruct the discontinuous rifting and deepening of the central lomonosov ridge during the paleogene accompanied by changing current regimes and the onset of sea ice eocene biosiliceous sedimentation took place in a relatively shallow setting under predominantly anoxic bottom water conditions causing a positive anoxia productivity feedback although water column stratification was repeatedly interrupted by ventilation events anoxic to sulfidic conditions were even more extreme after biosilica production ceased and significant amounts of pyrite were deposited on the lomonosov ridge especially in organic matter rich paleogene deposits diagenetic processes obscured the paleoenvironmental signals fundamental environmental changes occurred in the middle eocene but geochemical and micropaleontological proxies point not to the identical sediment depth after approximately 26 ma of non deposition or erosion the middle miocene record shows the transition to dominantly oxic bottom water conditions although suboxic diagenesis seemingly affected these deposits

contrasting18o enrichment and origins of high himalayan and transhimalayan intrusives abstract twelve analysed leucogranites of the high himalaya in bhutan chung la monlakarchung and garhwal badrinath are among the most 18 o enriched granites known 11 5 12 4 18 o smow with two exceptions and separate minerals show good isotopic concordance the data strongly support an origin of the granites by anatexis of continental basement such as the indian crystalline basement sheet or slab undercut by the main central thrust of which five samples were analysed in contrast the pre collision transhimalayan gangdese batholiths to the north of the indus tsangpo suture as exemplified by the ladakh intrusives show an initially oceanic trend of 18 o vs sio 2 that becomes gradually somewhat enriched with respect to hachijo jima while not completely outside the range of enrichment that seems possible by fractional crystallisation this could tie in with the 87 sr inhomogeneities reported by honegger et al 9 which may be due to assimilation of variably radiogenic eurasian continental basement for both the leucogranites and ladakh intrusives the 18 o levels and the concordance between minerals rule out significant cumulative water rock ratios in syn or post magmatic interaction with subsurface waters

coherent response of arabian sea upwelling and pollen transport to late quaternary monsoonal winds the indian summer monsoon causes large seasonal changes in the environments of both eastern africa and the arabian sea here we compare time series 140 kyr long of selected pollen types and foraminiferal upwelling faunas preserved in marine sediments to identify the frequencies of their variability and the coherence and phase of the terrestrial and marine responses to monsoonal circulation during interglacial intervals the pollen and upwelling records of the western arabian sea indicate stronger monsoons and are both coherent and in phase at periodicities near the precession of the earth s rotational axis 23 kyr we conclude that the strength of the monsoonal southwesterlies accounts for this high coherence of marine and terrestrial records during interglacial intervals low correlation during glacial phases indicates weaker monsoons and some decoupling of these two records these data indicate that two distinct modes of monsoon variability may exist an interglacial mode with strong monsoons and effective coupling of terrestrial and marine records and a glacial mode with both weaker monsoons and decreased linkage between arabian sea pollen and upwelling records

microfabric study of diatomaceous and lithogenic deposition in laminated sediments from the gotland deep baltic sea the deep basins of the baltic sea are commonly anoxic and finely laminated diatomaceous sediments have been deposited at intervals throughout the last 8000 yr the origin and composition of individual laminae in gotland deep sediments have often proved difficult to characterise using traditional micropalaeontological and sedimentological studies here we present a scanning electron microscope study in which lamina down to 30 m in thickness with distinct mineralogical or micropalaeontological composition have been identified and described depositional laminae sequences in the form of couplets triplets and quadruplets of diatomaceous and lithogenic laminae are observed with an average thickness of approximately 700 m diagenetic ca rhodochrosite laminae also occur within these depositional sequences examination of the diatom assemblages suggests that these bundles of laminae represent annual deposits or varves varves are relatively uncommon and typically occur in small intervals of two to five varves which are interrupted by more diffusely laminated and homogenous sediments the origin of these more massive sediments probably relates to periodic re oxygenation of the basin on inter annual time scales and destruction of varves by bioturbation

rb sr k ar and stable isotope evidence for the ages and sources of fluid components of gold bearing quartz veins in the northern sierra nevada foothills metamorphic belt california gold bearing quartz veins occur in and near major fault zones in deformed oceanic and island arc rocks west of the main outcrop of the sierra nevada composite batholith veins typically occupy minor reverse faults that crosscut blueschist to amphibolite grade metamorphic rocks whose metamorphic ages range from early paleozoic to jurassic vein micas and carbonate quartz mica assemblages that formed by hydrothermal metasomatism of ultramafic wall rocks in the alleghany grass valley washington and mother lode districts yield concordant k ar and rb sr ages the dated veins are significantly younger than prograde metamorphism penetrative deformation and accretion of their host rocks to the continental margin new and previously published mineralization ages from 13 localities in the sierra foothills range from about 140 to 110 m y ago with mean and median between 120 and 115 m y the age relations suggest that mineralizing fluids were set in motion by deep magmatic activity related to the resumption of east dipping subduction along the western margin of north america following the late jurassic nevadan collision event co 2 bearing fluids responsible for metasomatism and much of the vein mica carbonate albite and quartz deposition in several northern mines were isotopically heavy delta 18 o asymp 8 14ppm delta d between about 10 and 50ppm and do not resemble seawater magmatic or meteoric waters metasomatic and vein filling mica dolomite magnesite and quartz in altered ultramafic rocks generally formed from fluids with similar sr and o isotope ratios at a given locality consistent quartz mica delta 18 o fractionations delta 18 o sub q m 4 5 4 9ppm from various localities imply uniform equilibration temperatures probably between 300 degrees and 350 degrees c on a local mine scale fluids responsible for both carbonate alteration of mafic and ultramafic wall rocks and albitic alteration of felsic and pelitic rocks had similar sr isotope ratios samples from three veins in the central alleghany district fit a 115 7 or 3 m y rb sr isochron with a 87 sr 86 sr i value of approximately 0 7119 inferred 87 sr 86 sr ratios of metasomatic fluids from mines in different parts of the foothills region vary considerably 0 704 0 718 suggesting that sr was derived from sources ranging from western assemblage mesozoic ophiolitic or arc volcanic rocks to early paleozoic continent derived clastic rocks of the shoo fly complex systematic geographic variations in both sr and o isotopes can be rationalized by assuming extensive fluid interaction with rocks similar to the ones that are exposed within a few kilometers of the veins but the ultimate sources of the fluids and of au and other constituents may be independent of these isotopically lighter meteoric fluids deposited some late quartz overgrowths and occupied secondary fluid inclusions in earlier vein quartz