geochronology and geochemistry of late paleozoic volcanic rocks on the western margin of the songnen zhangguangcai range massif ne china implications for the amalgamation history of the xing an and songnen zhangguangcai range massifs abstract we here elucidate the tectonic evolution of the xing an and songnen zhangguangcai range massifs during the early carboniferous early permian based on zircon u pb dating and whole rock geochemical analyses of volcanic rocks of the songnen zhangguangcai range massif in the sunwu area heilongjiang province ne china euhedral subeuhedral zircons from three rhyolites and one dacite from the study area display fine scale oscillatory growth zoning indicating a magmatic origin zircon u pb dating by la icp ms indicates that these acidic volcanic rocks formed in the early carboniferous early permian i e early carboniferous 351 ma early late carboniferous 319 ma and early permian 295 293 ma the early carboniferous rhyolites exhibit chemical affinities to a type rhyolites implying an extensional environment their positive ehf t values 8 67 to 13 4 except for one spot of 1 63 and hf two stage model ages t dm2 562 988 ma indicate that the primary magma was possibly derived from partial melting of newly accreted continental crust the early late carboniferous rhyolites and dacites 319 ma exhibit calc alkaline peraluminous signature molar al 2 o 3 cao k 2 o na 2 o ratio or a cnk 1 04 1 22 the ehf t values and t dm2 ages of zircons from the 319 ma dacites are in the range of 5 33 to 9 32 and 907 1268 ma respectively suggesting that the primary magma was derived from partial melting of newly accreted crust the early permian rhyolites 295 293 ma show chemical affinities to a type rhyolites implying an extensional tectonic environment their positive ehf t values 8 82 to 13 8 and hf two stage model ages 484 743 ma indicate that the primary magma was derived from partial melting of newly accreted crust combined with the geochemical features of coeval igneous rocks from the eastern margin of the xing an massif these data reveal the late paleozoic tectonic history and relationships of the xing an and songnen zhangguangcai range massifs i e early carboniferous westward subduction of the paleo asian oceanic plate beneath the xing an massif followed by early late carboniferous collision and amalgamation of microcontinental blocks and early permian post collisional extension

a short term post lomagundi positive c isotope excursion at c 2 03 ga recorded by the wooly dolomite western australia it is generally accepted that carbon isotope variations in seawater were muted between c 2 06 ga after the end of the lomagundi carbon isotope excursion lcie and c 1 3 ga evidence is presented here that c 30 myr after the end of the lcie the biogeochemical cycle of carbon experienced a short term c 2 myr high amplitude up to 8 4 v pdb perturbation recorded in the horseshoe rift basin western australia the basin was initiated at c 2 03 ga with deposition of fluvial and shallow marine sandstones followed by the eruption of flood basalt and culminated with the deposition of platform carbonates and accompanying volcaniclastic and siliciclastic sediments wooly dolomite the horseshoe rift basin during deposition of the wooly dolomite was fault compartmentalized but connected to an ocean six depositional sequences make up the wooly dolomite sequence 1 records establishment of a carbonate platform conformably on basalt and coevally with volcaniclastic sedimentation all other sequences have dominant carbonate platform deposits and are unconformity bounded sequence 3 contains a c 57 m thick section with 13 c enriched carbonates bracketed between carbonates with close to 0 carbon isotope values further high resolution chemostratigraphic studies may reveal a more complex pattern of carbon isotope variations during the boring billion years but without precise geochronology similar short term carbon isotope excursions in carbonate successions could be incorrectly correlated to the lcie supplementary material table 2 including chemical and isotopic data sample locations and their position in figure 8 is available at https doi org 10 6084 m9 figshare c 2868055

upb geochronology of layered mafic intrusions in the eastern baltic shield implications for the timing and duration of paleoproterozoic continental rifting abstract paleoproterozoic layered mafic ultramafic intrusions are widespread in the eastern baltic fennoscandian shield and mark the initial stages of continental rifting and outpouring of voluminous flood basalts u pb zircon baddeleyite ages have been determined for seven layered mafic intrusions and a subvolcanic felsic unit associated with two paleoproterozoic volcanic sedimentary belts the pechenga imandra varzuga belt in the kola peninsula and the sumi sariola belt in karelia the layered intrusions in the sumi sariola belt show an age progression from 2449 0 1 1 ma for the burakovka complex located at the southeastern edge of the belt to 2442 1 1 4 ma and 2441 3 1 2 ma for the lukkulaisvaara and tsipringa intrusions of the olanga complex in northern karelia northeastern part of the belt these age results indicate a total duration of mafic magmatism along the ca 800 km belt in karelia to be less than 10 m y the pechenga imandra varzuga belt kola peninsula contains two distinct age groups of layered mafic intrusions and related rocks the younger of these overlaps with the ages obtained for the olanga intrusions and includes the 2441 0 1 6 ma imandra lopolith and a spatially associated felsic subvolcanic unit dated at 2442 2 1 7 ma the older group of mafic layered intrusions in the kola peninsula includes the 2501 5 1 7 ma pana tundra intrusion the 2504 4 1 5 ma monche pluton and the 2505 1 1 6 ma general skaya mountain intrusion the temporal and spatial distribution of the 2 50 2 44 ga rift related igneous activity in the baltic shield is consistent both with the model of starting mantle plume and with the model of passive rifting controlled by pre existing lithospheric weaknesses

geochemical and geochronological constraints on the tectonic setting of the xinlin ophiolite northern great xing an range ne china abstract the xinlin ophiolite located in the great xing an range domain of the central asian orogenic belt caob is considered to represent a fragment of an oceanic plate the ophiolite consists of several discontinuous blocks composed dominantly of serpentinized ultramafic rocks with subordinate cumulate gabbros basalts and diabase dikes which are intruded by younger leucogranite dikes here we characterize the geochemical features and formation age of the ophiolite the ultramafic rocks are dominated by harzburgite with minor dunite and are enriched in large ion lithophile elements liles and light rare earth elements lrees consistent with remnants of oceanic mantle the mafic rocks show depletion in some high field strength elements hfses and have relatively low tio2 and mgo contents and likely originated from an e morb like source in a back arc setting this suggests that the ophiolite is of supra subduction zone ssz origin the cross cutting leucogranite dikes most of which are trondhjemitic have high al2o3 contents low k2o na2o ratios and negative zircon ehf t values 8 5 to 3 8 average 5 8 which are features distinct to other granites e g fractionation shear and subduction types within the ophiolite and to coeval granitoids in the northern great xing an range zircon u pb dating on samples of gabbro and a leucogranite dike yielded ages of ca 480 and 324 1 ma respectively suggesting that the ophiolite formed in the early ordovician and was emplaced at ca 320 ma together with the ages of ca 480 ma for the duobaoshan arc and ca 340 ma for subduction related high mg diorites we suggest that ophiolite formation was probably related to the opening of an ordovician back arc basin whereas formation of the younger leucogranite dikes was probably related to closure of the back arc basin

the changing nature of mantle hotspots through time implications for the chemical evolution of the mantle the melting products of mantle plumes can place constraints on the geochemical structure of the mantle and show how it has varied through time the highest temperature melts komatiites and picrites derived from each plume are assumed to represent the best available sample of the thermal boundary layer from which the plume originates a survey of the geochemistry of picrites and komatiites shows that they underwent a fundamental change at about the archean proterozoic boundary archean komatiites have depleted or neutral geochemistry whereas most protero zoic and younger picrites have originated from enriched oib type mantle although rare depleted plumes persist to the tertiary e g gorgona if plumes originate from a thermal boundary layer near the core mantle interface the composition of this layer must have changed from a mixture of depleted and neutral mantle during the archean to mainly enriched mantle in the proterozoic this change coincides with a drop in the maximum mgo content of the ma

dating the termination of the palaeoproterozoic lomagundi jatuli carbon isotopic event in the north transfennoscandian greenstone belt abstract existing radio isotopic age constraints indicate that the global palaeoproterozoic lomagundi jatuli large positive carbonate carbon isotopic excursion with 13 c values 5 occurred between 2 2 and 2 06 ga in the north transfennoscandian greenstone belt of the kola peninsula nw russia northern norway and finland the lomagundi jatuli event is recorded in the carbonate rocks of the umba and kuetsjarvi sedimentary formations in the imandra varzuga and pechenga greenstone belts in both areas thick mafic volcanic units umba and kuetsjarvi volcanic formations overlie the carbon isotopic excursion bearing sedimentary units overlying and younger sedimentary units contain carbonate rocks with 13 c values typically ranging between c 1 and 3 signalling the termination of the lomagundi jatuli excursion two new u pb id tims isotope dilution thermal ionisation mass spectrometry zircon dates constrain this termination in both successions the lower unit of the il mozero sedimentary formation is a cross and parallel bedded volcaniclastic greywacke derived largely from erosion of the underlying umba volcanic formation it has yielded detrital zircons with 207 pb 206 pb dates as young as 2055 5 2 3 ma which is a maximum age for deposition and is inferred to date part of the underlying umba volcanics in pechenga the kolosjoki sedimentary formation was intersected by a drill hole obtained by the icdp international continental scientific drilling program supported far deep fennoscandian arctic russia drilling early earth project drilling programme zircons from a mafic fine tuff in this drill core have yielded a 207 pb 206 pb age of 2056 6 0 8 ma this age is interpreted as an eruption age contemporaneous with sedimentation the new age determinations overlap each other within uncertainty and is within error of previously published detrital zircon ages of 2058 2 ma from the kolosjoki sedimentary formation and 2049 28 ma from the kuetsjarvi volcanic formation combined these indicate that the lomagundi jatuli excursion terminated across fennoscandia by 2056 6 0 8 ma and may correlate with similar termination ages in fennoscandia and the transvaal south africa

petrology of the axial ridge of the mariana trough backarc spreading center abstract the axial ridge of the mariana trough backarc basin between 17 40 n and 18 30 n rises as much as 1 km above the floor of a 10 15 km wide rift valley physiographic segmentation with minor ridge offsets and overlaps coincides with a petrologic segmentation seen in trace element and isotope chemistry analyses of 239 glass and 40 aphyric basalt samples collected with alvin and by dredging show that the axial ridge is formed largely of olivine hypersthene normative tholeiitic basalt about half of these are enriched in both lil elements and volatiles but are depleted in hfs elements like other rocks found throughout much of the mariana trough the lil enrichments distinguish these rocks from n morb even though nd and sr isotope ratios indicate that much of the crust formed from a source similar to that for n morb in addition to lil enriched basalt there is lil depleted basalts even more closely resembling n morb in major and trace elements as well as sr nd and pb isotopes both basalt varieties have higher al and lower total fe than morb at equivalent mg level mg ranges from relatively primitive e g mg 65 70 to more highly fractionated e g mg 45 50 highest parts of the axial ridge are capped by pinnacles with elongated pillows of basaltic andesite e g 52 56 sio 2 these are due to extreme fractional crystallization of basalts forming the axial ridge active hydrothermal vents with chimneys and mats of opaline silica barite sphalerite and lesser amounts of pyrite chalcopyrite and galena formed near these silicic rocks the vents are surrounded by distinctive vent animals polychaete worms crabs and barnacles isotope data indicate that the mariana trough crust was derived from a heterogeneous source including mantle resembling the morb source and an arc source component the latter was depleted in hfs elements in previous melting events and later modified by addition of h 2 o and lil elements

reassessment of continental growth during the accretionary history of the central asian orogenic belt we argue that the production of mantle derived or juvenile continental crust during the accretionary history of the central asian orogenic belt caob has been grossly overestimated this is because previous assessments only considered the palaeozoic evolution of the belt whereas its accretionary history already began in the latest mesoproterozoic furthermore much of the juvenile growth in central asia occurred in late permian and mesozoic times after completion of caob evolution and perhaps related to major plume activity we demonstrate from zircon ages and nd hf isotopic systematics from selected terranes within the caob that many neoproterozoic to palaeozoic granitoids in the accreted terranes of the belt are derived from melting of heterogeneous precambrian crust or through mixing of old continental crust with juvenile or short lived material most likely in continental arc settings at the same time juvenile growth in the caob occurred during the latest neoproterozoic to palaeozoic in oceanic island arc settings and during accretion of oceanic island arc and precambrian terranes however taking together our data do not support unusually high crust production rates during evolution of the caob significant variations in zircon ehf values at a given magmatic age suggest that granitoid magmas were assembled from small batches of melt that seem to mirror the isotopic characteristics of compositionally and chronologically heterogeneous crustal sources we reiterate that the chemical characteristics of crustally derived granitoids are inherited from their source s and cannot be used to reconstruct tectonic settings and thus many tectonic models solely based on chemical data may need re evaluation crustal evolution in the caob involved both juvenile material and abundant reworking of older crust with varying proportions throughout its accretionary history and we see many similarities with the evolution of the sw pacific and the tasmanides of eastern australia

metallogenesis of the late pan african gold bearing east ouzzal shear zone hoggar algeria in the w hoggar algeria the major transcurrent n s east ouzzal shear zone eosz hosts several world class gold deposits over a 100 km length the late pan african eosz separates two contrasting precambrian domains the archaean in ouzzal block to the west orthogneisses with subordinate metasediments reworked and granulitized in the c 2 ga eburnean event and a middle proterozoic block to the east again orthogneisses and metasediments involved in the c 600 ma pan african event the eosz is a mylonite belt 1 3 km wide with a 50 m wide ultramylonite belt hosting numerous quartz veins and lenses giant hydrothermal quartz system associated with a quartz sericite pyrite carbonate beresite alteration these hydrothermal events occurred under ductile evolving towards brittle conditions between 500 and 300 mpa at 500 300 c with aqueous carbonic fluids derived both from underlying devolatilized metamorphic rocks and a mantle source as recorded by stable c o isotope data no gold mineralization was associated with these typical mesothermal events following a pressure drop to 130 mpa related to the inception of extensional tectonics the eosz was later percolated by a new set of hydrothermal fluids evolved from basinal waters that deeply penetrated into the in ouzzal basement these fluids were ca bearing brines up to 25 wt eq nacl characterized by high d 9 to 18 range mobilized by the thermal energy released by the late pan african granite magmatism taourirt granites as demonstrated by pb isotope data the brines leached au from the in ouzzal granulites which contain 3 ppb au fluid inclusion studies indicate that gold was deposited from these brines in the eosz at a depth of c 5 km due to mixing and cooling with descending diluted fluids

mantle plumes and continental tectonics abstract it is now recognised that plumes originate from a thermal boundary layer deep in the mantle a new plume consists of a large head with a diameter of approximately 800 to 1200 km and it is followed by a smaller tail which has a diameter of 100 to 200 km at the top of its ascent the head flattens to form a disk 2000 to 2500 km in diameter that may melt to produce a continental flood basalt or oceanic plateau the tail melts to produce chains of volcanic activity such as the hawaiian emperor chain the ascent of a new plume leads to surface uplift of 500 to 1000 m over a length scale of 2000 km this places the lithosphere under tension but the horizontal forces produced do not result in runaway extension unless the continental lithosphere is already anomalously hot and therefore weak if the lithosphere is cold and strong slow extension leading to a new spreading ridge may postdate plume arrival by 10 to 40 m y the time difference being the time taken to conduct heat from the plume to the overlying lithosphere plume heads may control the timing and position of new ocean ridges but are not the fundamental force that drives plate tectonics the heat from plume tails may weaken the overlying plate and pin the position of spreading the ascent of a mantle plume can also have an important influence on subduction large oceanic plateaus such as the ontong java plateau are more buoyant than normal oceanic crust because they have a thicker basaltic layer and because they are underlain by a 200 km thick layer of hot material from a mantle plume head as a consequence oceanic plateaus are more difficult to subduct than normal oceanic crust this can lead to jamming of plates at subduction zones resulting in a flip in the sense of subduction as appears to have happened in the caribbean region or to obduction of the plateau finally when a plume head is emplaced beneath continental crust heat released from the plume may lead to melting of the overlying crust the granites associated with many greenstone belts and intercontinental magmatic provinces such as the paleozoic lachlan fold belt of eastern australia and the middle proterozoic anorogenic granites of the central and southwestern united states may have formed in this way

multiple palaeoproterozoic carbon burial episodes and excursions organic rich rocks averaging 2 5 total organic carbon and positive carbonate carbon isotope excursions view the mathml source and locally much higher i e the lomagundi jatuli event are hallmark features of palaeoproterozoic successions and are assumed to archive a global event of unique environmental conditions following the c 2 3 ga great oxidation event here we combine new and published geochronology that shows that the main palaeoproterozoic carbon burial episodes cbes preserved in russia gabon and australia were temporally discrete depositional events between c 2 10 and 1 85 ga in northwest russia we can also show that timing of the termination of the lomagundi jatuli event may have differed by up to 50 ma between localities and that ni mineralisation occurred at c 1920 ma further cbes have traits in common with mesozoic oceanic anoxic events oaes both are exceptionally organic rich relative to encasing strata associated with contemporaneous igneous activity and marked by organic carbon isotope profiles that exhibit a stepped decrease followed by a stabilisation period and recovery although cbe strata are thicker and of greater duration than oaes 100 s of metres versus metres 106 years versus 105 years their shared characteristics hint at a commonality of cause s and feedbacks this suggests that cbes represent processes that can be either basin specific or global in nature and a combination of circumstances that are not unique to the palaeoproterozoic our findings urge circumspection and re consideration of models that assume cbes are a deep time singularity

detrital zircon ages of the jiageda and woduhe formations constrains on the tectonic attribute of the xing an terrane in the central great xing an range ne china abstract to ascertain the late early paleozoic tectonic attribute of the xing an terrane and the distribution and nature of the precambrian basement in ne china we studied the geochronology petrology and geochemistry of the metamorphic rocks of the jiageda and woduhe formations in the central great xing an range u pb ages of detrital zircons from the jiageda and woduhe formations vary from 963 to 374 ma and can be divided into three groups 963 679 570 468 and 457 374 ma the youngest detrital zircons of the three samples from the jiageda formation yield weighted mean ages of 413 3 414 9 and 416 1 ma respectively indicating that this formation was formed during the late silurian early devonian the youngest detrital zircons of the sample from the woduhe formation yield a weighted mean age of 429 4 ma indicating that this formation was formed during the middle late silurian the protoliths of the metamorphic rocks from the jiageda and woduhe formations are terrigenous clastic rocks with some mafic intermediate volcanic rocks characterized mainly by volcanic sedimentary formation considering the geochemical characteristics of the rocks and the regional tectonic setting we suggest that both the woduhe and jiageda formations were formed in an island arc active continental margin environment and that the xing an terrane was located in an active continental margin related to the subduction of the paleo asian ocean during the middle silurian early devonian a comparison between the jiageda and woduhe formations with respect to their protoliths geochemical properties sedimentary environment and geochronological characteristics shows that the jiageda formation does not constitute precambrian crystalline basement but instead consists of metamorphosed and deformed late silurian early devonian sedimentary strata intercalated with volcanic rocks by combining these results with the regional geology we conclude that the precambrian basement in ne china is not extensively distributed being limited to precambrian metamorphic rocks in the northern erguna massif and xing an terrane eastern songliao terrane and southern jiamusi massif and the protolith ages are younger than neoproterozoic

u pb age and hf isotopic data of detrital zircons from the devonian and carboniferous sandstones in yimin area ne china new evidences to the collision timing between the xing an and erguna blocks in eastern segment of central asian orogenic belt abstract new u pb age data and hf isotope systematics on zircons from middle devonian and lower carboniferous units from ne china provide evidence for the depositional age of these poorly constrained units as well as for the timing of the collision between the erguna ergun in some references and xing an blocks in the eastern end of central asian orogenic belt caob the detrital zircons from the devonian and carboniferous sandstones yielded similar age spectra in probability diagrams showing three distinct peaks at 450 ma 500 ma and 540 ma respectively along with proterozoic and archean age groups except for a relatively young age population at 360 ma detected for zircons from the hongshuiquan fm the dating results suggest that the depositional time of the niqiuhe fm should be younger than late silurian as indicated by the youngest 422 ma detrital zircon from this fm and that of the hongshuiguan fm with a youngest detrital age of ca 338 ma is younger than middle mississippian visean which is in accordance with those inferred from biostratigraphic data these dominant age groups are widely found in the erguna and xing an blocks it is suggested that the sediments of the studied sandstones from niuqiuhe and hongshuiquan fms come mainly from the erguna and xing an blocks the e hf t value of dated zircons from our sandstones has a wide range 21 to 16 which shows a mixed signature of that from paleozoic granitoids in xing an block 6 17 and erguna block 2 to 6 indicating that erguna and xing an blocks had connected before the middle devonian deposition of the niqiuhe fm 422 5 ma combined with the age data we suggest that the erguna and xing an blocks started to accrete at 540 ma reached the peak of collision at 500 ma and went into the post collision stage from 490 to 450 ma the study provides more evidences and constraints for the early paleozoic evolutions of the inter continental block amalgamations and accretions in caob

temporal constraints on the paleoproterozoic lomagundi jatuli carbon isotopic event the paleoproterozoic lomagundi jatuli positive 13c excursion in sedimentary carbonates represents an event whose magnitude and duration is unique in earth history although precise absolute chronology of this event remains poorly constrained in northeastern fennoscandia an 1300 m thick sedimentary volcanic succession of the pechenga greenstone belt records decline of this isotopic excursion zircons from sedimentary rocks that occur within the decline have yielded 207pb 206pb dates at 2058 2 ma 6 ma including u decay constant uncertainties and provide the first maximum age constraint on the termination of the lomagundi jatuli event combined with existing constraints these data indicate an 140 m y interval characterized by 13c rich carbonate accumulation

isotope provinces mechanisms of generation and sources of the continental crust in the central asian mobile belt geological and isotopic evidence abstract the available geological geochronological and isotopic data on the felsic magmatic and related rocks from south siberia transbaikalia and mongolia are summarized to improve our understanding of the mechanisms and processes of the phanerozoic crustal growth in the central asian mobile belt camb the following isotope provinces have been recognised precambrian t dm 3 3 2 9 and 2 5 0 9 ga at the microcontinental blocks caledonian t dm 1 1 0 55 ga hercynian t dm 0 8 0 5 ma and indosinian t dm 0 3 ga that coincide with coeval tectonic zones and formed at 570 475 420 320 and 310 220 ma continental crust of the microcontinents is underlain by or intermixed with juvenile crust as evidenced by its isotopic heterogeneity the continental crust of the caledonian hercynian and indosinian provinces is isotopically homogeneous and was produced from respective juvenile sources with addition of old crustal material in the island arcs or active continental margin environments the crustal growth in the camb had episodic character and important crust forming events took place in the phanerozoic formation of the camb was connected with break up of the rodinia supercontinent in consequence of creation of the south pacific hot superplume intraplate magmatism preceding and accompanying permanently other magmatic activity in the camb was caused by influence of the long term south pacific plume or the asian plume damping since the devonian

a widespread positive 13ccarbanomaly at around 2 33 2 06 ga on the fennoscandian shield a paradox carbon isotope measurements carried out on 201 carbonate samples from the early proterozoic of the kola peninsula n karelia and norway yield 13c pdb spanning 20 5 to 11 a general 13c secular trend shows that prior to 2 33 ga values are typically normal marine averaging around 3 0 between 2 33 and 2 06 ga in jatulian time there follows a rapid excursion to positive 13c of around 6 post jatulian time is characterized by 13c of sedimentary carbonates fluctuating between 5 and 3 also it is remarkable for the first pronounced development of diagenetic carbonates which have 13c between 14 and 6 the c 6 positive 13c shift with a duration of about 270 myr coincides with a maximum in the diversity and abundance of stromatolites and with widespread development of red beds but does not coincide with the maximum of buried corg mass the fennoscandian shield represents the largest isotoically anomalous carbonate province yet reported and the positive 13c excursion together with a series of major global palaeoenviromental changes seems to be more intense than the precambrian cambrian transition events however it is still not clear what kind of mechanism this phenomenon could be attributed to an increase of the ronov ratio and or broecker ratio and other possible models are discussed as the target for future investigations

the hydrothermal fluid of archaean lode gold deposits at different metamorphic grades compositional constraints from ore and wallrock alteration assemblages a low salinity mixed aqueous carbonic fluid is common to all archaean lode gold deposits throughout the range of mineralising conditions from sub greenschist to lower granulite facies temperatures alteration assemblages and fluid inclusion data give constraints on the fluid composition fluid xco2 is 0 1 0 3 in typical greenschist facies mesothermal deposits at higher temperatures the assemblages are consistent with formation from a fluid of similar composition but slightly higher or lower xco2 cannot be ruled out and fluid inclusion data indicate that ch4 may be an important component in ore fluids at these temperatures fluid ph is neutral or weakly alkaline at all conditions a range of relative oxidation states of four orders of magnitude fo2 is indicated at any temperature with deposits more oxidising relative to qfm at lower temperature sulphur contents of the fluids vary from 10 to 10 3 5m s with a trend towards lower sulphur contents at lower temperatures the relative concentrations of major cations in solution are similar at all conditions with na k ca although ca may be less abundant at low temperatures the broad similarities in ore fluid composition at all temperatures give support to crustal continuum models in which archaean lode gold mineralisation involved either a single fluid moving through the middle and upper crust or derivation of ore fluids by similar processes at different crustal levels many of the compositional differences between high and lowtemperature ore fluids may be attributed to evolution of deep sourced hydrothermal solutions as they rise along structurally controlled conduits the constancy of major ore fluid component concentration e g co2 cl k suggests fluid buffering and high fluid rock ratios along fluid pathways fluid buffered conditions can also explain the ore fluid fo2 temperature relations with equilibria between oxidised and reduced aqueous carbon or sulphur species controlling the oxidation state in contrast the concentrations of components present in lesser abundance in archaean gold ore fluids e g s ca h were probably controlled either by saturation of one or more mineral phases brought on by decreasing temperature or were rock buffered through fluid rock reactions extrapolation to high temperatures of the k na and ca contents of the gold bearing fluids indicates that their composition is consistent with derivation from or final equilibration with rocks of intermediate granitic composition thus giving support to isotopic and geological arguments for ore fluid source regions external to the greenstone belts the fluid oxidation states are characteristic of a wide range of potential source rocks including mantle derived igneous rocks calc alkaline granitoids and magmas and seaflooraltered metabasalts strongly oxidised magmatic sources or unusually oxidising source processes e g co2 streaming during granulitisation of the lower crust are therefore not required in the genesis of arch an lode gold deposits

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

precambrian metallogeny related to tectonics in the eastern part of the baltic shield abstract the bulk of the eastern baltic shield is underlain by archaean continental crust composed of three segments with varying tectonic settings the kola peninsula province consists of granulite gneiss terrain with fe ni cu deposits greenstone belts with ni cu bif and ree deposits a highly deformed volcanic sedimentary basin with kyanite deposits and late archaean gabbroanorthosite intrusions hosting fe ti v ores the karelian province is a typical granite greenstone belt terrain the greenstone belts were formed during the lopian orogeny 2 9 2 6 ga in tectonic environments resembling rifts and island arcs they contain ni cu and massive pyrite deposits with cu mineralizations and bif deposits late orogenic granites host mo cu mo deposits of the porphyry type the belomorian province is a region of granulite gneiss and granite greenstone belt terrains reworked tectonically and metamorphically during the lopian and svecofennian orogenies 2 0 1 75 ga which involved continent continent collision here muscovite and rare metals muscovite pegmatite deposits are related to svecofennian igneous and metamorphic processes in the early proterozoic the main metallogenic events were associated with intracontinental rift structures of the karelian stage 2 5 2 0 ga in the kuolajarvi vetrenny poyas zone and the monchegorsk fedorova pana toundri zone and of the svecofennian stage 2 0 1 8 ga in the pechenga imandra varzuga zone the above mentioned zones contain major ni cu pge and ti deposits hosted by layered intrusions 2 45 2 35 ga and gabbro wehrlites 2 0 1 9 ga and cu and au mineralizations hosted by metavolcanic rocks epicontinental sediments of the jatulian group 2 2 2 0 ga in a slightly deformed within plate basin contain cu cu co and schungite deposits the early proterozoic svecofennian province is represented by the ladoga belt only metamorphic zoning thrusts deformation and granite gneiss domes developed and some cu cu zn fe ti w and graphite mineralizations formed there the tectonic setting of the ladoga belt is interpreted as that of a deformed shelf deposit on a rifted and passive continental margin in addition to this an abortive continental rift belt with rapakivi granite intrusions 1 65 1 55 ga developed along the southern boundary of the baltic shield sn and sn cu zn fe deposits and fluorite and rare metal mineralizations are associated with these granites

paleozoic to mesozoic micro block tectonics in the eastern central asian orogenic belt insights from magnetic and gravity anomalies abstract the ne china region involves a complex accretionary belt formed by multiple stages of assembly of micro blocks from the paleozoic to mesozoic the tectonics of which provides important clues on the evolution of the east asian continental margin with a view to evaluating the micro block evolution in ne china we used gravity and magnetic methods to analyze the geophysical characteristics of the micro blocks and their boundaries combining with regional geology paleomagnetism and paleontology we reconstructed the micro block assembly history in the eastern central asian orogenic belt five paleozoic to mesozoic micro blocks are recognized in ne china the erguna xing an songnen jiamusi and nadanhada and their boundaries show significant differences in gravity and magnetic fields the boundary between the erguna block eb and the xing an block xb shows as a continuous tectonic line in the gravity field whereas the one between the xb and the songnen block sb exhibits a rather discrete but strong magnetic anomaly belt the boundary between the sb and the jiamusi block jb is characterized by both distinct gravity and magnetic anomalies the nadanhada micro block nmb appears as an independent micro block in the fourth order detail field of the bouguer gravity anomaly where it shows as a short axis or circinate structure in this anomaly field the paleozoic to mesozoic tectonic evolution of these micro blocks in ne china is closely related to five sutures in the eastern central asian orogenic belt the xinlin xiguitu suture was first formed at 500 ma leading to the assembly of the eb and the xb the heihe hegenshan suture was formed at 320 290 ma marking the amalgamation of the xb and the sb the mudanjiang yilan suture was formed at 200 ma marking the collision of the jb and the sb the paleo asian ocean closed during 250 230 ma in a scissor style the subduction of the paleo pacific ocean was initiated at 200 ma based on the deep structural architecture imaged from gravity and magnetic anomalies we propose an evolutionary model of the micro blocks in the ne china which provides important insights into the tectonic evolution of the ne china region during the paleozoic to mesozoic

dehydration melting and water saturated melting of basaltic and andesitic greenstones and amphibolites at 1 3 and 6 9 kb

intra oceanic arcs of the paleo asian ocean abstract the paper reviews and integrates geological geochronological geochemical and isotope data from 21 intra oceanic arcs ioa of the paleo asian ocean pao which have been identified in the central asian orogenic belt the world largest accretionary orogeny the data we discuss structural position of intra oceanic arc volcanic rocks in association with back arc terranes and accretionary complexes major periods of intra oceanic arc magmatism and related juvenile crustal growth lithologies of island arc terranes geochemical features and typical ranges of nd isotope values of volcanic rocks four groups of ioas have been recognized neoproterozoic early cambrian early paleozoic middle paleozoic and late paleozoic the neoproterozoic early cambrian or siberian group includes eleven intra oceanic arcs of eastern and western tuva sayan southern siberia russia northern and southwestern mongolia and russian altai the early paleozoic or kazakhstan group includes selety urumbai bozshakol chingiz and baydaulet aqastau arc terranes of the kazakh orocline the middle paleozoic or southern group includes six arc terranes in the tienshan orogen chinese altai east kazakhstan west junggar and southern mongoia only one late paleozoic intra oceanic arc has been reliably identified in the caob bogda in the chinese tienshan probably due to pao shrinking and termination the lithologies of the modern and fossil arcs are similar although the fossil arcs contain more calc alkaline varieties suggesting either their more evolved character or different conditions of magma generation of special importance is identification of back arc basins in old accretionary orogens because boninites may be absent in both modern and fossil ioas the three typical scenarios of back arc formation active margin rifting intra oceanic arc rifting and fore arc rifting were reconstructed in fossil intra oceanic arcs some arcs might be tectonically eroded and or directly subducted into the deep mantle therefore the structural and compositional records of fossil intra oceanic arcs in intracontinental orogens allow us to make only minimal estimations of their geometric length life span and crust thickness

timing and nature of the xinlin xiguitu ocean constraints from ophiolitic gabbros in the northern great xing an range eastern central asian orogenic belt jifeng ophiolitic melange ultramafic rocks meta basalts and gabbros crops out in the northern segment of the great xing an range the eastern segment of the central asian orogenic belt which marks the closure of the xinlin xiguitu ocean associated with the collision between the erguna block and xing an block in order to investigate the formation age and magma source of the jifeng ophiolitic melange the gabbros from newly discovered the jifeng ophiolitic melange are studied with zircon u pb ages whole rock geochemistry and zircon hf isotopes zircon u pb dating from the ophiolitic gabbros yields u pb age of 647 5 3 ma which may represent the formation age of the ophiolitic melange the gabbros display low sio2 tio2 k2o contents high na2o lree contents and indistinctive ree fractionation la yb n 1 97 2 98 it shows an e morb like affinity while the element concentrations of the jifeng samples are lower than that of e morb more importantly nb displays negative anomaly in comparison with th which shows a transitional ssz type ophiolite signature moreover the e hf t values of 647 ma zircons in the gabbros range from 8 4 to 13 4 and the corresponding hf single stage ages t dm1 are between 687 and 902 ma which is obviously older than the crystallization age of 647 ma these geochemical features can be explained as melts from the partial melting of a depleted mantle source meta somatized by fluids derived from a subducted slab accordingly we conclude that the jifeng ophiolitic melange is probably related to transitional ssz type ophiolite and developed in an intra oceanic subduction which indicates that an ocean the xinlin xiguitu ocean existed between the erguna block and xing an block the ocean s formation might be no later than the neoproterozoic 647 ma and it was closed in the late cambrian because of the collision between the erguna block and xing an block

the timing of crustal extension and the eruption of continental flood basalts the association between mantle plumes continental flood basalt eruption and the extension and rifting of the crust is well recognized1 3 but there has always been controversy as to whether the plume or the rifting was the initiating factor4 5 was rifting triggered by doming above the thermal plume2 3 6 or was rifting the driving force preceding the associated volcanicity7 8 here i show that recent flow by flow mapping of two continental flood basalt provinces the columbia river basalt group and the deccan basalt group allows one to infer the relative timing of volcanicity and crustal extension in both provinces eruption of the main tholeiitic phase preceded significant extension and crustal thinning this suggests that the primary event in the formation of these provinces was the upwelling of a mantle plume and that the subsequent crustal extension rifting and decompression was in part a consequence of the initiation of that plume

is there the paleoproterozoic komatiite related to mantle plume in the jifeng area northern great xing an range ne china

accretionary growth and crust formation in the central asian orogenic belt and comparison with the arabian nubian shield kroner a windley b f badarch g tomurtogoo o hegner e jahn b m gruschka s khain e v demoux a and wingate m t d 2007 accretionary growth and crust formation in the central asian orogenic belt and comparison with the arabian nubian shield in hatcher r d jr carlson m p mcbride j h and martinez catalan j r eds 4 d framework of continental crust geological society of america memoir 200 p 181 209 doi 10 1130 2007 1200 11 for permission to copy contact editing geosociety org 2007 the geological society of america all rights reserved kroner kroener mail uni mainz de windley brian windley btinternet com tomurtogoo igmr magicnet mn hegner hegner lmu de jahn jahn earth sinica edu tw khain khain ginras ru demoux demoux uni mainz de wingate mwingate tsrc uwa edu au

petrography and the ree composition of apatite in the paleoproterozoic pilgujarvi sedimentary formation pechenga greenstone belt russia the first globally significant phosphorous rich deposits appear in the paleoproterozoic at around 2 ga however the specific triggers leading to apatite precipitation are debated we examine phosphorous rich rocks up to 8 wt p2o5 in 1 98 1 92 ga old pilgujarvi sedimentary formation pechenga greenstone belt russia phosphates in these rocks occur as locally derived and resedimented sand to gravel pebble sized grains consisting of apatite cemented muddy sediments phosphatic grains can be subdivided into four petrographic types a d each has a distinct ree signature reflecting different early to late diagenetic conditions and or metamorphic overprint pyrite containing petrographic type d which typically has a flat ree pattern negative ce anomaly and positive eu anomaly is the best preserved of the four types and best records conditions present during apatite precipitation type d phosphatic grains precipitated under sub oxic basinal conditions with a significant hydrothermal influence these characteristics are similar to zaonega formation phosphates of nw russia s onega basin and consistent with phosphogenesis triggered by the development of anoxic sulfidic sub oxic redoxclines at shallow sediment depth during the paleoproterozoic

late paleozoic to early triassic multiple roll back and oroclinal bending of the mongolia collage in central asia abstract the architecture and mechanics of an orogen can be understood in terms of a system of collages that are characterized by a complex assemblage of multiple components but the fundamental paleogeographic framework and the tectonic relationships between the different components are often insufficiently defined because of unavailable data the central asian orogenic belt caob provides an ideal opportunity to address the fundamental framework of paleogeography and tectonic relationships between the diverse and many components in this huge collage in this paper we review several lines of available evidence which enable us to propose a new tectonic model of huge roll back in the formation of the accretionary tectonics of the mongolian collage in central asia in the early paleozoic the mongolia collage comprised the southern siberian and the tuva mongol oroclines the siberia craton and the mongolia collage jointly formed a giant tadpole shape within the paleo asian and panthanlassic oceans its head siberia was to the south and the tail tuva mongol to the northwest the structures and tectonic zonation of the mongolia collage are characteristic of multiple arcs which have been separately described in detail in different segments southwards from the southern siberia east sayan west sayan gorny altai chara via the lake zone junggar tianshan gobi altai beishan alxa to the manlay hegenshan baolidao solonker segments almost all segments underwent early paleozoic to permian or even triassic frontal subduction and accretion while rifting in the late carboniferous to permian or triassic occurred in the outward oceanward westward advancing mongolian collage therefore we suggest that a huge complex roll back active from the carboniferous to permian or even to late triassic facilitated the formation of the mongolian collage the outward multiple roll back process was compatible and almost coeval with the start of the tuva mongol orocline and rotation of the siberian craton as confirmed by paleomagnetic and structural data during the roll back processes an archipelago paleogeography was formed behind the frontal subduction and accretion in which independent arcs or terranes were amalgamated or collided to form composite arcs or terranes either simultaneously or at slightly different times the roll back process was affected by the collision of the kazakhstan collage along the chara and karamay zones in the early permian the collision of the tarim craton along the south tianshan zone in the early permian the collision of the dunhuang block along the liuyuan zone in the early permian triassic the collision of the alxa block along the qugan qulu zone in the permian and the collision of the north china craton along the solonker zone in the middle late triassic the tectonic styles and architecture of accretionary orogenic belts like the caob are characterized both by the amalgamation of multiple terranes and by oroclinal bending the systematic anatomy of the multiple roll back processes and their interactions with the adjacent collages shed light on the evolving orogenic architecture and the crustal accretionary history of orogens

continental rifting at pre existing lithospheric weaknesses continental rifting is the poorly understood thermomechanical process by which continents break up and new ocean basins form two dominant styles of continental rifting are recognized1 crevice style rifts are characterized by initial graben formation local uplift of the graben shoulders and little or no volcanism arch volcanic rifts exhibit an initial doming of the crust extensive volcanism and late stage graben formation the prevailing expla nation for the two styles is that crevice rifts form in response to regional horizontal stresses principally derived from the inter action of the lithospheric plates along their edges whereas arch volcanic rifts are thought to be the result of mantle convection currents which upwell directly beneath the rift axis2 we propose an alternative working model of continental rifting in which both styles initiate in response to regional horizontal stresses the different surface expressions of the two styles are explained in terms of differences in the mode of failure at different kinds of pre existing weaknesses in the continental lithosphere rather than differences in the nature of the driving forces involved

ophiolites in the xing an inner mongolia accretionary belt of the caob implications for two cycles of seafloor spreading and accretionary orogenic events the xing an inner mongolia accretionary belt in the southeastern segment of the central asian orogenic belt caob was produced by the long lived subduction and eventual closure of the paleo asian ocean and by the convergence between the north china craton and the mongolian microcontinent two ophiolite belts have been recognized the northern erenhot hegenshan xi ujimqin ophiolite belt and the southern solonker linxi ophiolite belt most basalts in the northern ophiolite belt exhibit characteristics of normal type to enriched type mid ocean ridge basalt affinities with depleted nd isotopic composition epsilon nd t 5 comparable to modern eastern pacific mid ocean ridge basalts most basaltic rocks in the southern belt show clear geochemical features of suprasubduction zone type oceanic crust probably formed in an arc back arc environment the inferred back arc extension along the solonker linxi belt started at circa 280 ma statistics of all the available age data for the ophiolites indicates two cycles of seafloor spreading subduction which gave rise to two main epochs of magmatic activity at 500 410 ma and 360 220 ma respectively with a gap of similar to 50 million years myr the spatial and temporal distribution of the ophiolites and concurrent igneous rocks favor bilateral subduction toward the two continental margins in the convergence history with final collision at similar to 230 220 ma in the whole belt signals of continental collision and himalayan style mountain building are lacking we thus conclude that the xing an inner mongolia segment of the caob experienced two cycles of seafloor subduction back arc extension and final appalachian type soft collision

magmagenesis in a subduction related post collisional volcanic arc segment the ukrainian carpathians abstract calc alkaline magmatism in the south west ukraine occurred between 13 8 and 9 1 ma and formed an integral part of the neogene subduction related post collisional carpathian volcanic arc eruptions occurred contemporaneously in two parallel arcs here termed outer arc and inner arc in the ukrainian part of the carpathians outer arc rocks mainly andesites are characterized by lile enrichment e g k and pb nb depletion low compatible trace element abundances high 87sr 86sr high 18o and low 143nd 144nd isotopic ratios 0 7085 0 7095 7 01 8 53 0 51230 0 51245 respectively inner arc rocks are mostly dacites and rhyolites with some basaltic and andesitic lavas they also show low compatible element abundances but have lower 87sr 86sr 18o and higher 143nd 144nd ratios 0 7060 0 7085 6 15 6 64 0 5125 0 5126 respectively than outer arc rocks both high nb and low nb lithologies are present in the inner arc based on the lile enrichment especially pb a higher fluid flux is suggested for the outer arc magmas compared with those of the inner arc combined trace element and sr nd o isotopic modelling suggests that the factors which controlled the generation and evolution of magmas were complex compositional differences between the inner and outer arcs were produced by introduction of variable proportions of slab derived sediments and fluids into a heterogeneous mantle wedge and by different extents of upper crustal contamination degrees of magmatic fractionation also differed between the two arcs the most primitive magmas belong to the inner arc isotopic modelling shows that they can be produced by adding 3 8 subducted terrigenous flysch sediments to the local mantle wedge source up to 5 upper crustal contamination has been modelled for fractionated products of the inner arc the geochemical features of outer arc rocks suggest that they were generated from mantle wedge melts similar to the inner arc primitive magmas but were strongly affected by both source enrichment and upper crustal contamination assimilation of 10 20 bulk upper crust is required in the afc modelling assuming an inner arc parental magma we suggest that magmagenesis is closely related to the complex geotectonic evolution of the carpathian area several tectonic and kinematic factors are significant 1 hydration of the asthenosphere during subduction and plate rollback directly related to collisional processes 2 thermal disturbance caused by ascent of hot asthenospheric mantle during the back arc opening of the pannonian basin 3 clockwise translational movements of the intracarpathian terranes which facilitated eruption of the magmas

the western central asian orogenic belt a window to accretionary orogenesis and continental growth abstract the architecture of accretionary orogens is a key to understand continental growth here we present an overview of the orogenic components and their amalgamation in the western central asian orogenic belt caob the caob records the convergence and interactions among various types of orogenic components including the japan type mariana type and alaska aleutian type arc systems as well as the active marginal sequences of the siberia craton which incorporated wide accretionary complexes and accreted arcs and terranes during construction of the caob the kazakhstan arc chain was characterized by multiple subduction whereas the northern fringe of the tarim craton remained mostly as a passive margin the multiple convergence and accretions among these various orogenic components generated huge orogenic collages in the late paleozoic and even in the early triassic involving parallel amalgamation circum microcontinent amalgamation and oroclinal bending the preservation of trapped basins played a significant role in orogenesis with some parts of the oceanic plate being subducted and others behaving as rigid units the orogenesis in the caob was long lived lasting for more than 800 m y involving multiple subduction and long continuous accretion and featuring the complexity of accretionary orogenesis and continent growth

new isotopic and geochemical data from the palaeoproterozoic pechenga greenstone belt nw russia implication for basin development and duration of the volcanism abstract the north pechenga group of the pechenga greenstone belt comprises a thick supracrustal succession developed within a time interval of ca 500 ma in the early part of the palaeoproterozoic era it records several important global events leading to the onset of modern style earth system but the temporal evolution of the belt is still insufficiently established new nd isotope data clearly indicate an abrupt change from a strong crustal signature in the lower two volcanic units to a depleted mantle like signature in the upper two volcanic units which conforms with the coeval shift of the volcanism from subaerial to submarine indicating a significant thinning of the lithosphere due to continental rifting the timing of the change in the geotectonic regime can be bracketed between 2058 6 ma u pb data and 2018 54 ma sm nd data in situ ion microprobe analyses of zircon grains from a thin felsic tuff member in the upper part of the succession pilgujarvi volcanic formation produced an age of 1988 3 ma which is within error the same as has been reported previously for the ferropicritic magmatism that produced the pechenga ni cu ores it is concluded that the upper 3 km thick volcanic pile dominated by tholeiitic basalts and minor ferropicrites was generated within a short time span unresolvable with current geochronological tools geochemical and isotopic data suggest that the felsic tuffs were generated from a ferropicritic parental magma via extensive fractional crystallisation and the presence of gneiss xenoliths in the tuffs reveals that this happened in a magma chamber located in the underlying archaean basement thus although the mafic subaqueous volcanism of the upper part of the belt does not bear nd isotopic or trace element evidence for contamination with ancient crustal material the rifting of the basement did not yet proceed to a total break up of the archaean craton

juvenile helium in ancient rocks ii u he k ar sm nd and rb sr systematics in the monche pluton ratios frozen in uranium free ultramafic rocks abstract the important geodynamic parameter the 3 he 4 he ratio in rocks and fluids of the continental crust is generally decreasing from the mantle values 10 5 to the radiogenic ratio 10 8 on the time scale of about 1 ga or less however the ratios observed in some ancient rocks and minerals are much higher than the radiogenic value due to a preferential retention of trapped he when compared with radiogenic helium and or a low u 3 he ratio in a sample the distribution of he ar nd and sr isotopes k rb sm and u in ultrabasic rocks in rock forming minerals in ores from the 2 49 ga monche pluton and in basic rocks of the main range the kola peninsula enables us to distinguish sources of the rocks and trapped fluids and outline some peculiarities of petrogenetic and fluid processes the initial values of nd t 0 9 0 5 87 sr 86 sr t 0 7021 0 0002 for the 2 49 ga monche pluton are rather similar to these for other layered intrusions of the baltic shield they differ considerably from the model values for the depleted 2 5 ga old mantle probably the melts originated in the specific enriched subcontinental mantle 4he concentrations of 10 480 10 6 cm 3 g and u concentrations of 0 01 0 001 ppm in rocks of the pluton are translated into atomic ratio of 4 he u up to 500 and u he age up to 10 ga these rocks contain dominantly trapped he despite the fact that u in gabbros from the main range is about two orders of magnitude higher this conclusion appears to be valid because of better retention of trapped he as compared with radiogenic one the 3 he 4 he ratios 0 16 0 74 10 6 exceed considerably the radiogenic value but are much lower than those in the mantle due to a contribution of crustal he isotopic and geochemical evidence implies that a melt from which the ultrabasic rocks were formed originated from the mantle thus crust mantle fluid occurred in the mantle melt geological observations and the relationship between 3 he 4 he and mg fe mg values show that the high magnesium divines formed during melt ascent i e under higher temperatures and probably at slightly greater depth as compared with those parameters for the magma chamber the observed and rather different sources of3 he and 4he as well as the con siderable constancy of their ratio in different minerals separated from both the ultramafic rocks and gabbros implies 1 1 an intensive process of mixing between mantle and crustal components a melt convection in the chamber may have occurred 2 2 the two types of rocks could originate by crystallization differentiation of one and the same melt moreover similar ratios of 3 he 4 he 0 2 10 6 are observed in the enclosing gneisses of the kola belomorian series practically all 3he and 4he are concentrated in secondary amphiboles hence the fluid which stimulated the metamorphic process was probably released from the ultramafite bearing melt the source of trapped 4he and 40ar corrected for air appears to be the continental crust but their ratio varies over a wide range in contrast to approximately constant 3 he 4 he ratio degassing of the melt is probably responsible for extremely high 4 he ar values in some minerals

analysis of petrologic hypotheses with pearce element ratios pearce element ratios can test whether the members of a rock suite are comagmatic and can illustrate the causes of chemical diversity in comagmatic suites comagmatic rocks have constant ratios for elements conserved in the system during changes that led to the chemical diversity in basaltic systems the incompatible elements ti k and p are often conserved the slope of the trend on a pearce element ratio diagram is sensitive to the stoichiometry of the crystallizing and segregating phases a judicious choice of ratios as axes for the diagram provides a signature for the phases involved and estimates of their compositions in basaltic rocks diagrams with ti k vs p k can provide a test of the comagmatic hypothesis diagrams with 0 5 mg fe k vs si k have trends that are distinct for each comagmatic suite and different mineral assemblage different suites are distinguished by the intercepts in diagrams whereas mineral assemblages are recognized by the slopes of the trends for example if olivine is the sole crystallizing and segregating phase the trend will have a slope of 1 diagrams with 2ca na k vs al k distinguish plagioclase from augite assemblages and in conjunction with 0 5 mg fe k diagrams unravel the crystallization sequences of suites that have suffered three phase crystallization and segregation analyses from the uwekahuna laccolith kilauea the 1955 and 1967 68 eruptions of kilauea diamond craters volcanic field oregon and experimental data on morb glasses provide illustrations of the interpretations that can be obtained from pearce element ratios

genetic implications of stable isotope characteristics of mesothermal au deposits and related sb and hg deposits in the canadian cordillera the canadian cordillera hosts a large number of mesothermal au or ag vein deposits the deposits are scattered throughout accreted and suspect terranes of the cordillera and often have a distinct spatial association with strike slip faults these vein deposits possess a striking degree of homogeneity in delta 18 o values on vein district and regional scales in most districts delta 18 o values for mineralized quartz veins vary only by 2 to 3 per mil across the entire district over the canadian cordillera as a whole delta 18 o values for such veins range from 13 to 18 per mil smow in contrast to the high degree of homogeneity evident in the delta 18 o values of the veins alteration zones in adjoining wall rocks possess distinctive zoning patterns indicating isotopic exchange between ore fluids and host units deposits of hg and sb are also closely associated with strike slip faults in the canadian cordillera area the delta 18 o studies of quartz from these deposits indicate average values of 29 or 2 0 per mil smow for quartz from the hg deposits and 21 0 or 1 5 per mil for quartz from the sb deposits when corrected for isotopic fractionation assuming equilibrium conditions average delta 18 o values for ore forming fluids of the various deposit types all fall within a 5 per mil range indicating a probable common fluid source for the three deposit types all three deposit types have delta 13 c values for carbonates in the range of 10 to 2 per mil pdb which are believed to reflect a heterogeneous source for the carbon the delta d values from serpentinites associated with au deposits and inclusion fluids from all three deposit types are lighter than 100 per mil and latitudinally dependent indicating the involvement of chemically evolved meteoric water in the ore forming process a model for formation of these deposits is presented involving the convection of meteoric water in the brittle crust to depths of 12 to 15 km and temperatures of 300 degrees to 400 degrees c during descent the fluid evolves in delta 18 o and acquires constituents involved in the later ore forming process fluids ascend in highly permeable zones associated with strike slip faults depositing quartz sulfides and au at a depth of approximately 10 km and a temperature of 300 degrees or 50 degrees c at shallower levels and cooler temperatures sb quartz or au veins are formed and near the surface cinnabar and carbonate are deposited at temperatures near 150 degrees c similarities in geology and geochemistry of mesothermal au deposits of the canadian cordillera and deposits of the mother lode california and the archecan cratons suggest that the model proposed herein may also be applicable to those deposits

early proterozoic ore deposits and tectonics of the birimian orogenic belt west africa abstract the early proterozoic of the west african craton comprises a series of volcanic troughs and sedimentary basins the birimian with granitic terranes accreted on archean nuclei in the man and reguibat shields studies of the birimian of the man shield indicate a model of polycyclic evolution with a major collision event d 1 2 1 ga thrusting part of the proterozoic terrane over the archean before individualization of numerous volcanic troughs and clastic infill basins the proposed evolution for the birimian orogenic belt comprises 1 deposition of the sedimentary lower birimian b1 with minor tholeiitic volcano sedimentary intercalations containing chert and or mn formations and with most of the detritus being derived from early proterozoic sources apart from contamination near the proterozoic archean contact 2 pre b2 crustal thickening related to d 1 thrusting 3 formation over about 40 ma of the upper birimian b2 with numerous volcanic troughs of different composition tholeiitic and rare komatiitic bimodal tholeiitic to calc alkaline volcano plutonic and tarkwaian clastic infill basins and 4 major transcurrent tectonics with sinistral d 2 and dextral d 3 strike slip faults such a tectonic evolution from a collision d 1 phase to a transcurrent d 2 d 3 phase is typical of collision belts and in the present case the evolution of the birimian orogenic belt can be extended into guyana the metallogenic history of the birimian shows a three phase evolution coinciding with the orogenic evolution and extends over almost 150 ma from the perkoa massive zn ag sulfides 2 12 ga with a clear mantle affinity to the latemesothermal au quartz veins 2 ga with according to lead isotopes a high crustal participation the economic mineralization of belt thus consists of 1 1 pre orogenic pre d 1 deposits related to early extension zones this was diverse with stratiform au tourmalinite type 1 au loulo in mali dorlin in guyana stratiform fe cu faleme in senegal and mn nsuta in ghana tambao in burkina faso and a single massive zn ag sulfide deposit perkoa in burkina faso associated with regional volcanosedimentary variably tholeiitic stratigraphic marker beds 2 2 syn orogenic post d 1 to syn d 2 d 3 deposits with disseminated au sulfides type 2 au yaoure in the ivory coast in extensional zones of the b2 followed by auriferous paleoplacers type 3 au in b2 extensional zones tarkwaian banket conglomerate or syn d 2 transtensional zones debris flow of orapu in guyana 3 3 late orogenic post peak d 2 d 3 deposits with mesothermal au mineralization evolving from a disseminated gold bearing arsenopyrite and au quartz lode type type 4 au ashanti in ghana to a quartz vein type with free gold and cu pb zn ag bi paragenesis most of the gold in west africa comes from this phase finally the metallogeny of the birimian appears as rich in au and mn formations and poor in volcanogenic mineralization and bif it differs from the archean metallogeny of west africa and other regions through the presence of certain deposit types such as au stratiform tourmalinite gold bearing conglomerate with au and fe ti oxides but no uranium or sulfides and au arsenopyrite rich shear zone deposits that are very common in but not exclusive to the proterozoic and paleozoic

petrology of the hegenshan ophiolite and its implication for the tectonic evolution of northern china abstract petrographic observations and mineralogical and geochemical analyses have revealed that the hegenshan ophiolite is of mid ocean ridge origin and has been subjected to dynamothermal metamorphism at medium p t conditions the metamorphism is characterized by a prograde change in paragenesis from the greenschist to epidote amphibolite facies with peak temperature conditions of 570 640 c at pressures of 4 10 kbar the amphiboles formed by this metamorphism show k ar ages of 110 130 ma the metamorphic conditions and k ar ages suggest that the hegenshan ophiolite is located at the suture between the siberian and north china continental blocks where the continental collision in this area took place in middle mesozoic time given the temporal and spatial distribution of the igneous activity around the da hinggan ling mountains it is suggested that the extensive yanshanian magmatism in this region resulted predominantly from a southward subduction of an oceanic plate prior to collision alternatively it may possibly have resulted from the collision itself at the final stage

geochronology geochemistry and hf isotopic compositions of early permian syenogranite and diabase from the northern great xing an range northeastern china petrogenesis and tectonic implications geodynamic evolution in the late paleozoic is significant for understanding the final amalgamation of the central asian orogenic belt caob no consensus has yet been reached regarding the late pa

geochemistry and u pb zircon dating of the toudaoqiao blueschists in the great xing an range northeast china and tectonic implications abstract the toudaoqiao complex is a sequence of high pressure metamorphic rocks located along the suture zone that separates the xing an and erguna blocks in northeast china the rocks were metamorphosed up to epidote blueschist facies at p t conditions of approximately 0 9 1 1 gpa and 320 450 c lithological associations and major and trace element compositions indicate that the blueschists are metabasalts with ocean island basalt oib and normal mid ocean ridge basalt n morb affinities similar to those of the philippines intraoceanic accretionary complex formed by subduction of oceanic crust magmatic zircons extracted from two samples of the epidote blueschist facies metabasalts from the north mountain in the toudaoqiao complex exhibit 206pb 238u ages of 511 5 and 516 17 ma whereas a greenschist from the south mountain has a 206pb 238u age of 511 2 ma these data show that the protoliths of the toudaoqiao complex are of early paleozoic age in addition a granitic dike from the south mountain exhibits a weighted mean 206pb 238u age of 492 1 ma these ages constrain the timing of high pressure metamorphism in the toudaoqiao complex to 490 510 ma the new data support the view that the suture zone between the xing an and erguna blocks is the toudaoqiao xinlin fault rather than the derbugan fault as previously thought this suture which extends from xinlin through toudaoqiao and southern mongolia named the south mongolia toudaoqiao xinlin suture zone may be an important boundary between the central mongolia erguna and xing an blocks

granoblastic olivine aggregates as precursors of type i chondrules an experimental test abstract chondrule formation models involving precursors of granoblastic olivine aggregates goa of either planetesimal or nebular origin have recently been proposed we have therefore conducted chondrule simulation experiments using mixtures of 100 h thermally annealed goa and an en to test the viability of goa as predecessors of porphyritic olivine po chondrules isothermal runs of less than 5 min at 1350 1550 c result in goa disaggregation and fe mg exchange runs of 0 5 4 h show textures superficially similar to granular and po chondrules but with reversely zoned olivine charges isothermally heated at 1550 c for 1 and 4 h before being cooled at 10 and 100 c h undergo olivine crystallization and yield classical po textures although most evidence of origin from goa is erased the cores of normally zoned euhedral crystals are relict as phenocrysts in type i chondrules can be relict such chondrules could have experienced similar peak temperatures to those of type ii chondrules chondrules containing goa with olivine triple junctions resemble experimental charges heated for minutes at temperatures between 1350 and 1450 c and type i chondrules with subhedral to anhedral olivine plus goa relicts resemble charges heated at the same temperatures but for longer duration type i chondrules with a mass of granular olivine or irregular anhedral olivine grains in the center and much glass nearer the margin on the other hand require limited heating at high temperature 1550 c while type i chondrules with euhedral olivines resemble charges heated at 1550 c for 4 h the majority of type i chondrules in cv chondrites display evidence of derivation from goa many finer grained chondrules in cr and uoc on the other hand could not have been derived from such coarse grained precursors but could have formed from fine grained dustballs as stipulated in the standard paradigm thus both goa and dustballs represent viable chondrule precursors of coarser and finer grained type i po chondrules respectively

the crustal accretion history and tectonic evolution of the ne china segment of the central asian orogenic belt abstract the basement rocks in parts of ne china constitute a khondalitic sequence of sillimanite and garnet bearing gneisses hornblende plagioclase gneiss and various felsic paragneisses zircon u pb dating of garnet sillimanite gneiss samples from the erguna xing an jiamusi and khanka blocks indicates that high grade metamorphism occurred at 500 ma evidence from detrital zircons in paleozoic sediments from the songliao block also indicates the former presence of a 500 ma component this uniformity of u pb ages across all crustal blocks in ne china establishes a 1300 km long late pan african khondalite belt which we have named the ne china khondalite belt this indicates the blocks of ne china were amalgamated prior to 500 ma contrary to current belief one scenario is that this amalgamated terrane had a tectonic affinity to the siberia craton once forming part of the late pan african 500 ma sayang baikal orogenic belt extensively developed around the southern margin of the siberia craton this belt was the result of collision between currently unidentified terranes with the southeastern angara anabar province at about 500 ma where the rocks were deformed and metamorphosed to granulite facies it appears likely that at sometime after 450 ma the combined ne china blocks rifted away from siberia and moved southward to form what is now ne china the combined block collided with the north china craton along the solonker xar moron changchun suture zone at 230 ma rather than in the end permian as previously thought local rifting at the eastern extremity of the developing central asian orogenic belt caob resulted in the splitting away of the jiamusi khanka bureya blocks however this was only transient and sometime between 210 and 180 ma these were re united with the caob by the onset of pacific plate subduction which has dominated the tectonic evolution of the region since that time

gold potential of the pan african trans sahara belt and prospect for further exploration abstract the trans sahara belt is one of the mobile belts formed during the pan african orogeny that spans between 870 and 550 ma the trans sahara belt was formed as result of the collision between the converging west african craton congo craton and the sahara metacraton continental blocks the belt with major exposure in the tuareg and dahomeyan consists of pre neoproterozoic basement which were thoroughly reworked during the pan african orogeny neoproterozoic metasedimentary and metavolcanics and syn tectonic pan african granitoids which intrude archean and paleoproterozoic rocks the trans sahara belt though less endowed when compared to older archean orogenic greenstone belts in terms of gold mineralization displays reasonable gold mineralization in both the tuareg shield and dahomeyan shield with about 160 tons of gold reported so far across the belt mineralization is associated with varying lithologies across the belt and appears to have been regionally controlled by n s trending pan african shear zones in the tuareg shield mineralized quartz veins are associated with syntectonic granitoids mylonitic and meta volcanosedimentary rocks conversely gold mineralization is only associated with metasediments and migmatite gneisses in the dahomeyan shield with occurrences largely confined to metasedimentary sequence in benin and nigeria the prevailing tropical climate in the dahomeyan shield has also facilitated secondary concentration of gold in alluvial bed of river draining supracrustal rocks hosting primary gold mineralizations the occurrence of primary mineralization in variety of rocks across the tuareg and dahomeyan shield and widespread occurrence of placer gold deposits in the dahomeyan shield justifies the need for further exploration and evaluation of mineralization potential the emplacement of gold which occur between 535 and 500 ma is controlled by n s trending pan african mega shear zones however at the district scale it is evident that the pan african mega shear zones are not the only important structures lower order faults and shear zones are important structures to consider in exploration targeting exploration particularly at district scale might benefit from gis based prospectivity mapping considering the mineralizations are generally structural controlled gis based prospectivity mapping which integrates multifaceted digital data sets can provide a foundation for vectoring towards explorations targets in many mineralized districts within the trans sahara belt

geochemical and tectono magmatic evolution of the volcano sedimentary rocks of pechenga and other greenstone fragments within the kola greenstone belt russia abstract the early proterozoic volcano sedimentary pechenga and imandra varzuga regions form the two larger fragments within the kola greenstone belt a prominent feature of the north eastern baltic shield this belt also contains three smaller fragments at polmak pasvik and ust ponoy the geology of the larger fragments consists of an older northern and a younger southern part separated by prominent conformable deformation zones accordingly the 2350 1850 ma lithological succession of the pechenga greenstone fragment is subdivided into the northern 2550 1905 ma and southern pechenga complexes 1905 1700 ma likewise the imandra varzuga fragment comprises the northern and southern imandra varzuga complexes the relatively better known northern pechenga complex is subdivided into four units which in turn are subdivided into 11 formations in contrast southern pechenga complex comprises only one unit subdivided into four volcano sedimentary and two volcanic formations both the volcanic and volcaniclastic components of these complexes are characterised by magmatic compositions ranging from basaltic andesite to trachybasalt trachyandesite basalt ferropicrite basalt picrite basalt andesite picrite basalt and andesite dacite rhyolite the gabbro wehrlite intrusions hosting the pechenga ni cu sulphide deposits lower down in the stratigraphy are petrogenetically related to ferropicritic volcanics volcanic rocks of the southern pechenga complex extruded from six autonomous contemporaneous volcanic centres the volcanic rocks belong to two geochemical cycles each starting with tholeiitic and ending with calc alkaline magmas these changes reflect magmatic responses to major tectonic phases of crustal rifting and closure changes in the sr and nd isotope ratios indicate that the magmatic melts originated in various deep mantle and mantle crustal source areas a systematic gradual geochemical change is observed between subsequent volcanic formations notwithstanding the fact that the deposition of such volcanic formations is commonly interrupted by sedimentary events changes in the magmatic source were therefore gradual and the magmatic processes were continuous the kola greenstone belt formed as an intracontinental rift kalevian rifting event that has undergone a complex tectono metamorphic evolution during the karelian orogenic megacycle 2550 1700 ma of the northern baltic shield the history of two complete tectono magmatic cycles the lapponian cycle and the svecofennian cycle within this megacycle is recorded in the volcano sedimentary successions of the pechenga fragment the complete lapponian cycle is embodied within the volcanic sequence of the imandra varzuga fragment where the composition of the volcanic and volcano sedimentary rocks range from tholeiitic and boninitic basalts to basaltic andesites calc alkaline rhyodacite boninitic picrite basalts and basaltic andesites subalkalic trachybasalts and trachyandesites here the subsequent svecofennian cycle is represented by tholeiitic basalts and ferropicrites followed al rich basalts and concluded by dacite rhyolites trachyandesites and trachytes during the final phases of the kalevian rifting event tholeiitic and ferropicritic magmas were generated from two distinct mantle sources the tholeiitic magma was derived from a subabyssal depleted mantle source in contrast the ferropicritic magma came from an abyssal metasomatically enriched mantle source the larger degree of mantle contamination in the latter was probably critical in the formation of the ni cu ores of the pechenga gabbro wehrlite intrusions

paleozoic multiple accretionary and collisional tectonics of the chinese tianshan orogenic collage abstract subduction related accretion in the junggar balkash and south tianshan oceans paleo asian ocean mainly in the paleozoic gave rise to the present 2400 km long tianshan orogenic collage that extends from the aral sea eastwards through uzbekistan tajikistan kyrgyzstan to xinjiang in china this paper provides an up to date along strike synthesis of this orogenic collage and a new tectonic model to explain its accretionary evolution the northern part of the orogenic collage developed by consumption of the junggar balkash ocean together with paleozoic island arcs northern ili issyk kul and chatkal located in the west which may have amalgamated into a composite arc in the paleozoic in the west and by addition of another two roughly parallel arcs dananhu and central tianshan in the east the western composite arc and the eastern dananhu and central tianshan arcs formed a late paleozoic archipelago with multiple subduction zones the southern part of the orogenic collage developed by the consumption of the south tianshan ocean which gave rise to a continuous accretionary complex kokshaal kumishi which separated the central tianshan in the east and other paleozoic arcs in the west from cratons tarim and karakum to the south cross border correlations of this accretionary complex indicate a general southward and oceanward accretion by northward subduction in the early paleozoic to permian as recorded by successive southward juxtaposition of ophiolites slices of ophiolitic melanges cherts island arcs olistostromes blueschists and turbidites which are mainly paleozoic in age with the youngest main phase being late carboniferous permian the initial docking of the southerly tarim and karakum cratons to this complicated late paleozoic archipelago and accretionary complexes occurred in the late carboniferous early permian in the eastern part of the tianshan and in the late permian in the western part which might have terminated collisional deformation on this suture zone the final stages of closure of the junggar balkash ocean resembled the small ocean basin scenario of the mediterranean sea in the cenozoic in summary the history of the altaids is characterized by complicated multiple accretionary and collisional tectonics

gold mineralization in the west hoggar shear zone algeria the amesmessa gold prospect is located along a vertical n s trending crustal scale ductile shear zone stretching lineations are subhorizontal this major shear zone is a late pan african dextral strike slip fault of the pharusian belt of the tuareg shield algeria the amesmessa shear zone is asymmetric strong thermal and deformational gradients are present along its western border where biotitic ultramylonites are in contact with a rigid archean complex in ouzzal block whereas there is a progressive gradation through mylonite then protomylonite to the proterozoic gneiss of the eastern block which displays co axial pan african structures the amesmessa shear zone is characterized by the presence of a felsic dike complex emplaced during shearing and forming the most important parent material for ultramylonites basic magmas and carbonatites also intruded within the shear zone the gold rich quartz veins are located within the ultramylonitic western part of the shear zone these n s trending laminated quartz veins formed during the late increments of shearing plastic brittle transition by repeated syntectonic hydraulic fracturing along zones of rheological contrast parallel to foliation the ore mineral association pyrite galena native gold sphalerite crystallized in the deformed quartz matrix along late shear planes undeformed e w trending banded quartz veins are present in the mylonitic eastern part of the shear zone their gold content is low and no native gold has been observed a strong hydrothermal alteration resulted in the development along the walls of the n s gold bearing quartz veins of a 5 m wide carbonate sericite albite pyrite secondary mineral association which implies an important co2 supply and moderate temperature conditions there is no alteration halo around the e w quartz veins ultramylonites hydrothermally altered rocks and quartz veins display similar ree patterns characterized by strong lree enrichments shear related fluids could be likely parental fluids for the amesmessa gold mineralization and the associated hydrothermal alteration hydrothermal fluids were drawn into dilation zones and filled opening fractures along the main planar discontinuity of the most deformed rocks the supply of co2 may come from a deep seated source as suggested by the presence of carbonatite dikes in the shear zones and the existence of co2 h2o rich fluid inclusions in quartz the location of the gold bearing quartz veins in the western part of the shear zone can be explained by the presence of strong thermal and rheological gradients

the pre devonian tectonic framework of xing an mongolia orogenic belt xmob in north china abstract a new tectonic division of the xing an mongolia orogenic belt xmob in north china has been presented according to our research and a lot of new data of tectonics geochronology and geochemistry four blocks and four sutures have been recognized in the xmob including the erguna eb xing an airgin sum xab songliao hunshandake shb and jiamusi jb and xinlin xiguitu xxs xilinhot heihe xhs mudanjiang ms and ondor sum yongji sutures oys the framework of the xmob is characterized by a tectonic collage of the blocks and orogenic belts between them different precambrian basements have been found in the blocks including the neoproterozoic metamorphic rocks and plutons in the eb the neoproterozoic metamorphic rocks in western and eastern of segments of the xab mesoproterozoic and neoproterozoic metamorphic rocks in middle segments of the xab respectively the neoproterozoic metamorphic rocks and mesoproterozoic volcanic rocks and plutons in the shb and neoproterozoic metamorphic rocks in the jb the xxs resulted from a northwestward subduction of the xab beneath the eb during the cambrian which was followed by the forming of the xhs and oys in the northwest and south margins of the shb in the silurian respectively the ms was caused by a westward subduction of the jb beneath the east margin of the shb during the middle devonian the three cambrian silurian and middle devonian events indicate that the xmob belongs to a pre middle devonian multiple orogenic belt in the central asian orogenic belt caob forming of the xmob suggests that the southeast part of the paleo asian ocean closed before the middle devonian

silurian magmatism on the eastern margin of the erguna block ne china evolution of the northern great xing an range abstract there are few early paleozoic volcanics and mafic rocks in the xing an mongolia orogenic belt in spite of the extensive occurrence of granites caused by a synchronous orogenic event despite the fact that this was a major orogen the properties of the lithosphere and its associated orogenic processes are poorly known in this paper a comprehensive study of u pb zircon ages lu hf isotopes and whole rock elemental analyses was carried out on three intermediate mafic intrusions which were recently identified in the northern great xing an range representative samples of the yikete yalu and xinglong intrusions yielded the weighted mean 206pb 238u ages of 435 1 ma 430 8 ma and 447 4 ma respectively this indicates the presence of silurian intermediate mafic rocks in the region the dioritic rocks exhibit a na rich a calc alkaline affinity and enrichment in liles rb ba k and sr and depletion in hsfes ta ti zr and hf indicating that the yikete dioritic rocks were a mantle wedge that had previously been subjected to metasomatism by fluids and melts related to a subducted oceanic slab and its overlying sediments the gabbroic rocks are tholeiitic in composition with variable mgo 3 14 8 31 and low k2o 0 16 1 0 and tio2 0 25 2 12 contents they are characterized by enrichment of liles depletion of hfses and highly positive zircon ehf t 5 8 to 13 3 values in addition they possess geochemical features of morb with subordinate oib the geochemical features indicate that the gabbroic rocks were derived from an upwelling of asthenospheric mantle that had been into the supra subduction wedge facilitated by subduction rollback before their generation combined with the coeval arc type magmatic rocks widely exposed on the eastern margin of the erguna block we link the generation of the dioritic rocks to the northwestern subduction of the nenjiang oceanic plate during the early paleozoic the generation of the gabbroic rocks might result from rifting of the back arc basin consequently an arc back arc model is proposed to explain the subduction evolution of the nenjiang ocean during the early paleozoic

experiments on silicate melt immiscibility in the system fe2sio4 kalsi3o8 sio2 cao mgo tio2 p2o5 and implications for natural magmas the effect of cao and mgo with or without tio2 and p2o5 on the two melt field in the simplified system fe2sio4 kalsi3o8 sio2 has been experimentally determined at 1 050 1 240 c 400 mpa despite the suppressing effect of mgo cao and pressure on silicate melt immiscibility our experiments show that this process is still viable at mid crustal pressures when small amounts 0 6 2 0 wt of p2o5 and tio2 are present our data stress that the major element partition coefficients between the two melts are highly correlated with the degree of polymerisation nbo t of the sio2 rich melt whatever temperature pressure or exact composition experimental immiscible melt compositions in natural systems at 0 1 mpa from the literature lunar and tholeiitic basalts plot on similar but distinct curves compared to the simplified system these relations between melt polymerisation and partition coefficients which hold for a large range of compositions and fo2 are extended to various volcanic and plutonic rocks this analysis strengthens the proposal that silicate melt immiscibility can be important in volcanic rocks of various compositions from tholeiitic basalts to lamprophyres however the majority of proposed immiscible compositions in plutonic rocks are at least not coexisting melts but may have suffered accumulation of early crystallized minerals

evolution of the precambrian in ouzzal block central sahara algeria abstract new structural and geochronological data are used to constrain the evolution of the precambrian in ouzzal block situated on the eastern side of the west african craton the results of this study can be summarized as follows 1 i the block can be subdivided into two series one is mainly composed of archaean granitic gneisses 3 2 2 5 ga old while the other contains metasedimentary and basic to ultrabasic rocks which can be compared with those described in archaean supracrustal terrains 2 ii the entire block is affected by very high temperature granulite facies metamorphism 3 iii the metamorphism is associated with two main deformational phases the first of which is responsible for a regional foliation foliation refolded during the second phase 4 iv the major tectono metamorphic event in the area is related to a 2 0 ga old eburnian orogeny which ended with carbonatite emplacement nearly all evidence of the earliest crustal evolution events i e the initial emplacement and deformation of the granitic and supracrustal materials has been erased by the eburnian event

compilation of west african mineral deposits spatial distribution and mineral endowment abstract the west african craton is highly endowed in minerals and their spatial and temporal distribution varies from single to multi phase mineralization events they are broadly related to three major tectono metallogenic elements and formed during distinct mineral epochs 1 in both archean shields kenema man and reguibat and paleoproterozoic domains baoule mossi eglab these are characterized by giant iron ore deposits that formed between ca 2 5 2 3 ga nearly all gold porphyry copper lead zinc and sedimentary manganese ore that developed between 2 2 and 2 1 ga and primary diamonds that formed between two intervals at ca 2 2 2 0 ga and in the mesozoic 2 across pan african and variscan belts these are distinguished by major precambrian iocg s copper gold that formed at ca 2 1 ga and approximately 680 ma and neoproterozoic sedimentary iron ore and phosphate deposits 3 within intracratonic and coastal basins these include the development of cenozoic lateritic bauxites over mesozoic dolerites tertiary quaternary mineral sands deposits oolitic iron ore and sedimentary phosphate deposits geological spatial and temporal correlations using the multi commodity west african mineral deposit database highlight that gold and non gold commodities formed in multiple phases this commenced in the liberian orogeny 2 9 2 8 ga with the enrichment of iron ore nickel sulphides diamonds and gold in the earth s crust the pre eburnean or tangaean eoeburnean eburnean i event yielded gold and the major eburnean orogeny yielded gold iron ore manganese diamonds magmatic nickel sulphides copper gold lead zinc and ree minerals throughout the pan african event sedimentary manganese deposits lead zinc ree minerals sedimentary phosphates and again gold were formed primary diamonds and magmatic nickel sulphides are related to the break up of gondwana followed by an intense lateritic weathering period that formed bauxite deposits along the craton margin

mesothermal gold mineralization in a proterozoic greenstone belt western flin flon domain saskatchewan canada the flin flon domain trans hudson orogen canada is an example of a proterozoic greenstone belt that hosts a large number of small mesothermal gold occurrences the greenstone belt consists of tholeiitic to calc alkaline volcanic rocks amisk group approximately 1900 1880 ma unconformably overlain by molasse type sedimentary rocks missi group 1850 1840 ma the supracrustal rocks are intruded by gabbroic to granitic rocks ranging in age from synvolcanic to late tectonic 1890 1835 ma metamorphic grade varies from prehnite pumpellyite facies to amphibolite facies and peak thermal conditions generally were attained locally during granitoid intrusion many of the shear zones and the dominant regional foliation developed during ductile deformation coeval with peak metamorphism these shear zones were reactivated and mineralized under brittle ductile conditions during postpeak metamorphic uplift mesothermal gold mineralization is hosted in quartz veins that developed at jogs or zones of competency contrasts along brittle ductile shear zones in all lithologies in the region alteration envelopes which consist of quartz carbonate chlorite albite muscovite pyrite are usually less than a few meters wide and overprint regional metamorphic assemblages the veins consist of milky colored quartz and may also contain tourmaline ankerite chlorite and muscovite pyrite and arsenopyrite are the dominant sulfides and gold grade is proportional to the modal abundance of sulfides the dominant fluids associated with gold mineralization were h 2 o co 2 nacl 0 6 14 7 wt nacl equiv in composition with generally uniform h 2 o co 2 phase ratios in a given plane or zone but variable phase ratios between zones oxygen isotope mineral pairs indicate temperature of mineralization of between 360 degrees and 420 degrees c which when combined with fluid inclusion density estimates indicate that most of the gold mineralization occurred at a pressure of about 2 kbars rapid pressure release in dilatant zones with related phase separation and change in fluid composition and properties and local fluid wall rock interaction are the likely gold precipitation mechanisms the o h s c and sr isotope compositions of hydrothermal minerals in the shear zones indicate that the mineralizing fluids interacted extensively with proterozoic metamorphic and igneous rocks similar in composition to those presently exposed at the surface at high temperatures and at low water rock ratios the o and h isotope compositions are compatible with formation of the fluids from devolatilization reactions during prograde metamorphism at depth the o isotope composition of barren quartz veins varies with the host rocks indicating that these fluids were either locally derived or interacted extensively with the immediate host rocks an rb sr isochron age of 1760 or 9 ma on tourmaline and muscovite from the rio deposit is similar to 40 ar 39 ar plateau ages for muscovites from the premetamorphic laurel lake deposit in the flin flon domain this suggests that fluid advection through the rio deposit was contemporaneous with the thermal event that affected laurel lake muscovites these ages along with an age from the tartan lake mesothermal deposit 1791 or 4 ma in the flin flon domain indicate that fluid flow related to hydrothermal activity along the shear zones in the western flin flon domain occurred periodically over a period of about 30 ma these proterozoic gold occurrences are similar to archean mesothermal gold deposits in terms of geologic structural and tectonic setting alteration and vein mineralogy and fluid composition pressure and temperature however the limited extent of alteration the low gold content the isotopic systematics and the lack of either s type granites or mantle derived lamprophyres suggest that the shear zones hosting the western flin flon domain occurrences sampled limited quantities of fluids and thus were not transcrustal features capable of hosting giant gold deposits

tectonic implications of new zircon u pb ages for the xinghuadukou complex erguna massif northern great xing an range ne china abstract the nature and the timing of formation of the xinghuadukou complex of ne china are important to further our understanding of the precambrian geological evolution of the erguna massif and the eastern segment of the central asian orogenic belt caob here we present the results of zircon u pb dating of a metavolcanic rock and 11 granitoid basement samples collected from the complex we use these data to determine the age and tectonic origin of the complex and to examine the linkages between this complex and the surrounding tectonic domains and older cratonic areas igneous zircons from the metavolcanic rock yield a magmatic age of 850 ma indicating that the supracrustal rocks that represent the original xinghuadukou group formed during the neoproterozoic petrological data and new zircon u pb ages for these samples enabled the identification of multiple stages of granitoid magmatism within different sections of the xinghuadukou group indicating that this group which was originally thought to have formed contemporaneously actually records the emplacement of granitoids at 957 786 479 418 265 and 209 192 ma this result indicates that the xinghuadukou complex records multiple stages of magmatism and contains various neoproterozoic supracrustal components combining the new age data with regional geological data we propose that the erguna massif should be considered part of the mongol domain as it has a close affinity to other blocks that formed part of this domain during the neoproterozoic these data suggest that all of the complexes and terranes within the mongol domain belong to the caob and were finally accreted to the southern margin of the siberian craton in the early paleozoic

timing of formation and tectonic nature of the purportedly neoproterozoic jiageda formation of the erguna massif ne china constraints from field geology and u pb geochronology of detrital and magmatic zircons abstract this paper reports new secondary ion mass spectrometry and laser ablation inductively coupled plasma mass spectrometry u pb age data for detrital and magmatic zircons from metasedimentary and igneous rocks from the jiageda formation jf of the erguna massif ne china these data constrain the timing of formation and tectonic nature of the jf as well as the tectonic affinity of the erguna massif the jf within the erguna massif is dominated by metasedimentary rocks and volcanics that have a gneissic or schistose structure the majority of zircons from metasedimentary volcanic and intrusive rocks in this area show striped absorption and fine scale oscillatory growth zone patterns and relatively high th u ratios 0 11 11 39 that are indicative of a magmatic origin four samples from the base of the type section of the jf in the village of jiageda yield detrital zircons with similar age populations that peak at 790 850 890 and 960 ma in addition to a few mesoproterozoic to paleoproterozoic and neoarchean zircons whereas a single sample from the upper part of the jf yields zircons that include a youngest age group of 764 ma zircons from granites and mafic dikes that intruded the type section of the jf yield weighted mean ages of 261 238 183 and 160 ma in comparison zircons from four volcanic samples collected 15 km northwest of the village of jiageda and one metasedimentary sample from the jf in the badaguan area all yield late mesozoic crystallization ages 158 132 126 and 128 ma for the four volcanic rocks and a youngest age of 178 2 ma for the metasedimentary rock combining these results with knowledge of the field relationships of the rocks in this area and previously published ages we infer that 1 the originally determined neoproterozoic jf does not represent a standard stratigraphic sequence but instead consists of neoproterozoic to mesozoic metasedimentary and volcanic rocks while the true neoproterozoic jf is exposed only in the village of jiageda and 2 nearly all of the sediments within the neoproterozoic jf were sourced directly from units that were exposed within the study area and adjacent regions our new data also provide precise geochronological evidence for the existence of a precambrian terrane within the erguna massif the ages of detrital zircons from neoproterozoic units within the erguna massif and precambrian geochronological data from adjacent cratons and massifs near the southern siberian craton indicate that the erguna massif and the massifs of the southern siberian craton e g the tuva mongolian and central mongolian massifs are not linked to the siberian and the north china cratons but instead most likely represent a fragmented precambrian terrane derived from the tarim craton

sm nd and pb isotopic study of mafic rocks associated with early proterozoic continental rifting the peropohja schist belt in northern finland the perapohja schist belt in northern finland rests unconformably on archaean granitoids and marks the early stages of proterozoic crustal evolution in the fennoscandian baltic shield 2440 ma old layered mafic intrusions predate the supracrustal and ca 2200 ma old sills of the gabbro wehrlite association intrude the lowest quartzites and volcanics runkaus of the sequence the sm nd mineral isochron of the penikat layered intrusion gives an age of 2410 64 ma the initial nd values of the penikat intrusion nd 2440 1 6 and the runkausvaara sill nd 2200 0 suggest that these mafic magmas were contaminated by older crustal material the sm nd and pb isotopic results on the 2 44 2 2 ga old runkaus volcanics indicate mobility of pb fractionation of sm nd during late greenschist facies metamorphism and crustal contamination the pb pb data provide an age of 1972 80 ma with a high initial 207pb 204pb ratio 1 8 49 while scattered sm nd data result in an imprecise age of 2330 180 ma with an initial nd value of about zero secondary titanite gives an u pb age of ca 2250 ma the jouttiaapa basalts in contrast ascended from the mantle without interaction with older crust these lree depleted tholeiites mark a break in continental sedimentation and yield a sm nd age of 2090 70 ma their initial nd 4 2 0 5 implies that the subcontinental early proterozoic mantle had been depleted in lree for a long period of time the first lava flows are strongly depleted in lree suggesting that their source was significantly more depleted than the source of mid ocean ridge basalts today

lithospheric electrical structure between the erguna and xing an blocks evidence from broadband and long period magnetotelluric data abstract the central asian orogenic belt caob is the largest accretionary orogen on earth and its evolution has been controversial for a long time northeast china and adjacent regions form the eastern portion of the caob which is an important area for exploring the tectonic evolution of both the northeast asia and the caob the erguna and xing an blocks are two micro blocks in the west of northeast china the magnetotelluric mt method can study subsurface electrical structures a dense broadband and long period magnetotelluric mt profile across the collision belt of the two blocks has been completed based on the results of data analysis a three dimensional 3 d resistivity model of the crust and upper mantle has finally been obtained the mt results show that the derbugan fault is not a suture zone whereas the xinlin xiguitu fault is considered the boundary of the erguna and xing an blocks by westward subduction the conductive bodies in the middle lower crust of the erguna and xing an blocks may have been formed in an extensional environment and imply that the crust is weak the collision belt includes a prominent high conductivity anomaly from the lower crust to the upper mantle and indicates the existence of mantle upwelling associated with probable delamination

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

the kola alkaline province of the cis and finland precise rbsr ages define 380 360 ma age range for all magmatism abstract twenty one new rb sr mineral and biotite whole rock isochrons are presented for intrusive rock complexes of the palaeozoic kola alkaline province in the cis and finland samples from the ultrabasic alkaline and nepheline syenite complexes khibina lovozero afrikanda turiy mys ozernaya varaka sokli and iivaara all yield ages in the very narrow range from 361 7 to 377 3 ma i e late middle devonian to upper devonian these data represent crystallization rather than intrusion ages but are nevertheless argued as marking very closely the time of intrusion there is no indication of bimodal age distribution the emplacement of the alkaline magmas is related temporally to the subsidence of the kontozero graben of the central kola peninsula sr initial ratios of the agpaitic nepheline syenites of the giant khibina and lovozero intrusions scatter between 0 703 and 0 704 because of high rb sr ratios of the rocks these isotope ratios exclude long term pre enrichment of alkalis in the sources of the magmas prior to segregation and intrusion and thus episodic enrichment during earlier rifting events in the northern baltic shield the magma formation may be related to wrench movements in the surroundings of the major trollfjord komagelv fault which forms part of the late caledonian arctic north atlantic megashear system

a tectonic transition from closure of the paleo asian οcean to subduction of the paleo pacific plate insights from early mesozoic igneous rocks in eastern jilin province ne china abstract the timing of tectonic transition from the paleo asian oceanic regime to the paleo pacific regime in the easternmost central asian orogenic belt remains controversial to constrain the event we present new zircon u pb ages hf isotopic compositions and whole rock major and trace element data for early mesozoic igneous rocks from eastern jilin province ne china these new ages together with existing age data indicate that early mesozoic magmatism in the kaiyuan yanji area can be subdivided into four stages early triassic ca 250 ma early middle triassic 249 235 ma late triassic 229 208 ma and latest triassic to early jurassic 207 175 ma the early triassic igneous rocks constitute a bimodal igneous rock association that formed in an extensional environment caused by break off of the subducting slab during final closure of the paleo asian ocean the early middle triassic magmatism is dominated by adakitic granitoids whose primary magmas were derived from partial melting of thickened mafic lower crust suggesting that the north china craton and songliao xilinhot block were still undergoing collisional orogenesis at 249 235 ma the late triassic igneous rocks comprise a typical bimodal association of rocks in an e w trending belt formed in a post orogenic extensional environment related to final closure of the paleo asian ocean the latest triassic to early jurassic igneous rocks have calc alkaline characteristics were emplaced as a ne sw trending belt parallel to the eastern margin of eurasia and formed in an active continental margin setting associated with initial subduction of the paleo pacific plate we conclude that the tectonic transition between the paleo asian and the paleo pacific regimes took place during the latest triassic

sources of phanerozoic granitoids in the transect bayanhongor ulaan baatar mongolia geochemical and nd isotopic evidence and implications for phanerozoic crustal growth abstract the central asian orogenic belt caob is renowned for massive generation of juvenile crust in the phanerozoic mongolia is the heartland of the caob and it has been subject to numerous investigations particularly in metallogenesis and tectonic evolution we present new petrographic geochemical and sr nd isotopic analyses on phanerozoic granitoids emplaced in west central mongolia the data are used to delineate their source characteristics and to discuss implications for the phanerozoic crustal growth in central asia our samples come from a transect from bayanhongor to ulaan baatar including three tectonic units the baydrag cratonic block late archean to middle proterozoic the eo cambrian bayanhongor ophiolite complex and the hangay hentey basin of controversial origin the intrusive granitoids have ages ranging from ca 540 to 120 ma the majority of the samples are slightly peraluminous and can be classified as granite s s including monzogranite syenogranite and alkali feldspar granite most of the rocks have initial 87sr 86sr ratios between 0 705 and 0 707 late paleozoic to mesozoic granitoids 250 ma are characterized by near zero end t values 0 to 2 whereas older granitoids show lower end t values 1 5 to 7 the data confirm the earlier observation of kovalenko et al geochemistry international 34 1996 628 who showed that granitoids emplaced outside of the pre riphean basement rocks are characterized by juvenile positive end t values whereas those within the pre riphean domain and the baydrag cratonic block as for the present case show a significant effect of contamination by precambrian basement rocks nevertheless mass balance calculation suggests that the granitoids were derived from sources composed of at least 80 juvenile mantle derived component despite our small set of new data the present study reinforces the general scenario of massive juvenile crust production in the caob with limited influence of old microcontinents in the genesis of phanerozoic granitoids

a revised geodynamic model for the lapland kola orogen abstract the paleoproterozoic lapland kola orogen in fennoscandia has been studied for decades and several plate tectonic models have been proposed including one sided subduction zone either towards sw or nw or two opposite verging subduction zones before the collision based on new structural and isotope data from finland and recently published data from russia we propose a revised tectonic model for the paleoproterozoic lapland kola orogen the main components are foreland in the ne followed by cryptic suture inari arc retro arc basin and retro arc foreland in the sw the latter three constitute the inari orocline subduction towards present sw and subsequent arc magmatism inari arc started at ca 1 98 ga followed by voluminous sedimentation in the deepening retro arc basin underplating of a mid ocean ridge caused flat subduction and magmatic flare at 1 92 ga over a broad distance in the retro arc basin rapid heating led to melting of the retro arc basin sediments and voluminous amounts of granulite facies diatexites formed during collision d1 at 1915 1910 ma large thrust nappes formed on the foreland deformation in the retro arc basin is seen as recumbent folding and shearing of diatexites in the lower parts of the basin and thrusting of metatextite diatexite packages in the upper parts a post collisional stage is seen as 1904 ma appinites and decompression derived granites at 1 90 1 89 ga renewed shortening d2 due to far field effects in sw at 1 88 1 87 ga led to thick skin shortening of the archean middle crust large scale crustal duplexing of already cooled granulites towards the retro arc foreland and inclined upright folding of granulites in the opposite direction towards the inari arc a switch in the stress field from ne sw to nw se led to orogen parallel contraction and buckling started along a dextral strike slip fault zone to form the inari orocline buckling is seen in the bending of pre orocline fabrics and formation of syn orocline fabrics radial conical folds d3 radial fractures a strike slip fault zone and thrusting at the hinge zone the end result is a mega scale parallel multi layer fold composed of the inari arc retro arc basin and possibly also the heated retro arc foreland

permian geodynamic setting of northeast china and adjacent regions closure of the paleo asian ocean and subduction of the paleo pacific plate abstract northeast china and adjacent regions are located in the central east asian continent and consist tectonically of both the paleo asian and paleo pacific orogens between the siberian platform and sino korean north china block this paper discusses some hotly debated issues concerning the permian geodynamic setting of these regions based on a comprehensive analysis of available geological geochemical paleobiogeographical and paleomagnetic data spatial and temporal distribution of ophiolites and associated continental marginal sequences permian sedimentary sequences spatial distribution and geochemistry of permian magmatic rocks and the evolution of paleobiogeographical realms imply 1 that the permian marine basins in northeast china and adjacent regions include remnants of the paleo asian ocean in southeastern inner mongolia and central jilin province and active continental margins of the paleo pacific ocean 2 that the suture between the siberian and sino korean paleoplates was finally emplaced in the permian and is located in areas from suolunshan solonker eastwards through regions north to the xar moron river in southeastern inner mongolia and then central jilin province to the yanji area and 3 that the permian crustal evolution of northeast china and adjacent regions as well as parts of the siberian paleoplate was influenced by subduction of the paleo pacific oceanic plate finally the permian tectonic framework and paleogeography of northeast china and adjacent regions in central east asia are discussed briefly and early and late permian palinspastic reconstruction maps are provided

early geochemical evolution of an oceanic island arc and backarc fiji and the south fiji basin the initial 25 to 35 ma of oceanic arc volcanism in fiji the early arc stage was dominated by eruption of lree depleted low k tholeiitic basalts and andesites the oldest volcanics include the highest hfse group morb like in composition plus high mg basalts and basic andesites with flat ree patterns as low as 4x chondritic concentrations these erupted either within the volcanic arc during its first 5 ma or in an adjacent backarc during its first ma of formation subsequent volcanics of the early arc stage apparently erupted onto early backarc basin crust most are intermediate between earlier extremes in hfse contents but at least one volcano produced strongly lree depleted island arc tholeiites in contrast medium k tholeiitic and calc alkaline andesites erupted during the mature arc stage parental basalts were enriched in lree but depleted in hree relative to earlier basalts due to a change in source composition counterflowing morb source asthenosphere within the mantle wedge dominated vol

recognizing oib and morb in accretionary complexes a new approach based on ocean plate stratigraphy petrology and geochemistry abstract we present a new approach for recognizing the origin of accreted basaltic rocks based on ocean plate stratigraphy ops and on the petrology and geochemistry of basalts from mid oceanic ridges morb and oceanic islands oib using examples from four accretionary complexes ac in sw japan akiyoshi mino tamba chichibu and shimanto the key to the problem is the model of ops which includes an association of igneous and sedimentary rocks that form on an oceanic plate during its travel from a mid oceanic ridge to a subduction zone we propose the reconstruction of the tectonic settings of basalts according to their relationships with associated ops sediments their petrogenesis and their geochemical features five types of ops are recognized in the accretionary complexes of sw japan 1 sandstone shale 2 sandstone shale and chert 3 sandstone shale chert and morb 4 sandstone shale chert morb and gabbro peridotite 5 seamount ops including oib cap carbonates slope clastics and basal shale chert the alkaline tholeiitic or calc alkaline composition of basaltic melts which are typical of oceanic islands mid oceanic ridges and island arcs respectively can be identified by the sequence in crystallization of their major phenocrysts i e olivine ol clinopyroxene cpx and plagioclase pl and by their compositions alkaline and calc alkaline mafic lavas are characterized by an ol cpx pl succession whereas tholeiitic melts by their ol pl cpx succession titanium rich minerals e g ti augite kaersutite ti biotite are typical of alkaline lavas the application of geochemistry based tectonic discrimination diagrams is also a powerful tool if not supported by geological and petrological data may result in confusion due to magma contamination post magmatic alteration and secular change of mantle thermal conditions we propose that a direct comparison of normalized multi element patterns and key binary plots from older volcanic rocks with their modern analogues provides a more viable and reliable method of basalt discrimination our ops petrology geochemistry method allows us to confirm the above conclusions that the lavas of the akiyoshi mino tamba and southern chichibu ac formed in oceanic islands because they are associated with seamount ops sediments crystallized from ol to cpx and pl contain ti augite and kaersutite and are enriched in tio2 lree and nb in this paper we present geochemical data from the inuyama basalts of the mino tamba ac and from the toba complex in the huge mikabu greenstone belt of the chichibu ac the inuyama basalts are in contact with jurassic pelagic cherts but their geochemical features are confusing they contain phenocrysts of ol ti augite and kaersutite and therefore probably formed in seamounts the toba volcanic rocks are a part of the huge ophiolite belt they have flat to slightly lree enriched ree patterns are characterized by an ol cpx succession of phenocrysts and they plot in the oib field in binary plots suggesting they formed in an oceanic plateau

world class base and precious metal deposits a quantitative analysis over 62 percent of the 193 000 metric tons of gold discovered to date is located in four countries and more than 68 percent occurs in four types of mineral deposits about 55 percent of the 1 740 000 metric tons of silver found is in four countries and 45 percent is in four types of deposits fifty six percent of the 1 52 billion metric tons of discovered copper is from four countries and four types of deposits contain 88 percent of the total over 50 percent of both the 713 000 000 metric tons of zinc and 349 000 000 metric tons of lead discovered to date come from four countries and 70 percent of both metals occur in four types of deposits all discovered gold would fit in a cube with a height of 22 m silver in a 55 m cube copper in a 550 m cube zinc in a 460 m cube and lead in a 310 m cube at least 74 percent of gold silver zinc and lead is in deposits having average grades above the respective median grades and 44 percent of copper is in deposits with average grades above the median grade of all deposits lower grade deposits contain less total metal than higher grade deposits tonnage of mineralized rock is an even better predictor of contained metal with over 96 percent of each metal s total residing in deposits having greater than median size and between 47 and 79 percent of metal contained in the largest 10 percent of deposits world class deposits defined as the upper 10 percent of deposits in terms of contained metal account for over 86 percent of all gold 79 percent of silver 84 percent of copper 71 percent of zinc and 73 percent of lead these giant deposits contain at least 100 metric tons 3 2 moz gold 2 400 metric tons 77 moz silver 2 million metric tons copper 1 7 million metric tons zinc or 1 million metric tons lead mineral deposits occur rarely in the earth s crust and large ones are especially uncommon this analysis shows that only the unusually large deposits can significantly affect supply

general geology and evolutionary history of the early proterozoic polmak pasvik pechenga imandra varzuga ust ponoy greenstone belt in the northeastern baltic shield abstract the early proterozoic polmak pasvik pechenga imandra varzuga ust ponoy greenstone belt is situated in the northeastern part of the baltic shield and is discontinuously developed over a length of ca 1000 km it comprises five main zones which are separated by longitudinal and a number of transverse syndepositional faults each zone contains a number of lithostratigraphic units at the rank of groups that are subdivided into several sedimentary and volcanic formations with a total thickness over 16 000 m the sequence comprises a series of sedimentary volcanic rhythms usually separated by stratigraphic breaks marked by palaeo weathering the groups are separated by major disconformities which may mark record rift inversion or orogenic episodes at least seven distinctive lithostratigraphic markers occur in the belt including two komatiitic volcanic series separated by immature conglomerates two red coloured proto evaporitic formations separated by the alkaline volcanics and two different black shale formations perhaps the main characteristic of the belt is the apparently long history of sedimentary volcanic development at the same site spanning a time period of 2500 to 1800 ma a three stage rift development model is considered 1 an intracontinental rift stage 2500 2100 ma separated by the major inversion 2400 2330 ma and comparable with present day afar triangle and east african rifts 2 a transitional from intracontinental to to intercontinental rift stage 2100 1970 ma with possible short lived spreading about 1990 1970 ma 3 a collision related intercontinental rift stage 1970 1800 ma followed by the svecofenian orogeny 1800 1700 ma the observed distribution of platinum group elements pge mineralization is related to layered gabbro norite complexes fedorova and panskie tundry gora general skaya intrusions dated at 2470 ma which are apparently co magmatic and coeval with the first komatiitic volcanic series seidorechka formation found in the eastern part of the belt only the major chromite occurences are connected with a layered gabbro norite intrusion imandra lopoloth which is co magmatic and coeval between 2440 2330 ma with the second komatiitic volcanic series rusinga polisarka akhmalahti and malbekk formations the ni cu deposits of the belt are connected with a differentiated gabbro werhlite complex which is co magmatic and coeval ca 1970 ma with the specific fe and ti rich ferropicrites with komatiitic affinities

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

geodynamic setting of mesothermal gold deposits an association with accretionary tectonic regimes mesothermal gold provinces of phanerozoic age are characteristically associated with regional structures along which allochthonous terranes have been accreted onto continental margins or arcs a recurring sequence of transpressive deformation uplift late kinematic mineralization and shoshonitic magmatism is consistent with thermal reequilibration of tectonically thickened crust mesothermal gold camps in the superior province are spatially associated with large scale structures that have been interpreted as zones of transpressive accretion of individual subprovinces or allochthonous terranes these boundary structures are characterized by the sequence of significant horizontal shortening uplift late kinematic mineralization and shoshonitic lamprophyres and therefore may have the same geodynamic significance as phanerozoic counterparts in this model thermal re equilibration of underplated and subducted oceanic lithosphere and sediments in a transpressive regime over time scales of 10 to 40 m y is a necessary precursor to gold mineralization hydrothermal fluids are released along boundary faults and their splays during uplift the uniform temperature low salinity and mole co2 signify uniform source conditions whereas the variable o c sr and pb isotopic compositions of fluids reflect lithological complexity of the source regions and conduits ou the basis of this model it is suggested that mesothermal lode gold deposits are the product of subduction related crustal underplating and deep late metamorphism rather than magmatic or metamorphic events in the supracrustal rocks secular variations in the generation of archean proterozoic and phanerozoic mesothermal au provinces reflect the timing of collisional orogenies within terranes of these eras

a 1300 km late pan african metamorphic belt in ne china new evidence from the xing an block and its tectonic implications abstract the xing an block is one of several fault bounded crustal units in northern china located along the southeastern margin of the central asian orogenic belt caob and includes the great xing an range the basement rocks constitute a khondalitic sequence of sillimanite and garnet bearing gneisses hornblende plagioclase gneiss and felsic paragneiss named the xinghuadukou complex la icp ms zircon u pb dating of a sillimanite gneiss from hanjiayuan indicates high grade metamorphism occurred at 496 3 ma with several older detrital zircon grains with ages extending from 678 8 to 1373 17 ma a sample of hornblende plagioclase gneiss from the northern part of the xinghuadukou complex yields a metamorphic age of 496 7 ma whereas igneous zircon core ages range from 601 15 to 1637 23 ma another sample of hornblende plagioclase gneiss from farther south in the xinghuadukou complex yields a metamorphic age of 495 5 ma and magmatic core ages of 546 4 ma these data indicate that high grade metamorphism occurred at 500 ma and that several earlier magmatic events are recorded in the area including some in the neoproterozoic these new age data together with other recent data obtained from the erguna songliao jiamusi and khanka blocks establish a 1300 km pan african khondalite belt along the southern margin of the caob

ages geochemistry and tectonic implications of the cambrian igneous rocks in the northern great xing an range ne china abstract the xinlin xiguitu suture zone located in the great xing an range ne china in the eastern segment of the central asian orogenic belt caob represents the boundary between the erguna and xing an micro continental blocks the exact location of the xinlin xiguitu suture zone has been debated especially the location of the northern extension of the suture zone in this study based on a detailed field geochemical geochronological and sr nd hf isotope study we focus our work on the cambrian igneous rocks in the erguna xing an block the xinglong granitoids mainly include 520 ma diorite 470 ma monzogranite and 480 ma pyroxene diorite the granitoids show medium to high k calc alkaline series characteristics with post collision granite affinity the circa 500 ma granitoids have low ehf t values 16 6 to 2 2 and ancient two stage model tdm2 ages between 1317 ma and 2528 ma these results indicate the primary magmas of the xinglong granitoids were probably derived from the partial melting of a dominantly paleo mesoproterozoic old crustal source with possible different degrees of addition of juvenile materials and formed in a post collision tectonic setting after the amalgamation of the erguna and xing an blocks compared with the xinglong granitoids the duobaoshan igneous rocks are consisted of the approximately coeval rhyolitic tuffs 491 5 ma and ultramafic intrusions 497 5 ma within the duobaoshan formation they are generally enriched in large ion lithophile elements liles and depleted in high field strength elements hfses e g nb ta and ti consistent with the geochemistry of igneous rocks from island arcs or active continental margins the ultramafic rocks have high positive ehf t values 1 3 to 15 and end t 1 86 to 2 28 and relatively young two stage model tdm2 ages and low initial 87sr 86sr ratios 0 70628 0 70853 indicating the partial melting of a depleted mantle source from a subducted slab in the ocean basin between the erguna xing an and songliao blocks the rhyolitic tuffs contain a group of phanerozoic zircons with ehf t values 4 6 to 15 0 suggesting that the rhyolitic tuffs were derived from juvenile lower crustal material with some ancient crustal material coupled with our previous geochemical and isotopic studies on early paleozoic igneous rocks we proposed that the collision of the erguna and xing an blocks at least took place ca 500 ma ago and that there exist in a westward subduction of an oceanic plate between the eruguna xing an and songliao blocks took place during the early ordovician up to now there are more evidences and constraints that the northern extension location of the xinlin xiguitu suture zone is located in the jifeng xinglong areas

deep penetration of sedimentary fluids in basement rocks from southern norway evidence from hydrocarbon and brine inclusions in quartz veins abstract this paper presents evidence for fluid flow and fluid rock interaction at upper crustal levels within the crystalline basement of southern norway in the high grade modum complex postmetamorphic veins of quartz occur in association with albitisation of metagabbros and metasediments pressure temperature conditions for the formation of these veins are in the range 250 300 c and ca 1 2 kbar primary and pseudosecondary fluid inclusions in the quartz veins show two fluids 1 hydrocarbon c0 2 inclusions and 2 aqueous inclusions with variable salinities dark carbonaceous solid inclusions are also present the hydrocarbon inclusions are methane dominated ca 80 100 mot and the presence of higher complex hydrocarbons is demonstrated the aqueous inclusions in the metagabbro hosted veins show more saline compositions than the metasediment hosted veins a salinity range of ca 23 0 wt nacl eq is found some of the aqueous inclusions may contain nahco 3 crush leach analyses of inclusion fluids show na ca k ci dominated compositions with na ca k hydrocarbon poor concentrates yield br cl ratios close to seawater while hydrocarbon rich concentrates show higher values the types and chemistry of the hydrocarbons in these veins indicate a biogenic origin of the hydrocarbon fluids the inclusion fluids are interpreted as derived from an overlying sedimentary basin in late precambrian or permian times the veining is interpreted as the deep expression of the percolation of basinal fluids into the metamorphic basement during crustal extension

the crustal continuum model for late archaean lode gold deposits of the yilgarn block western australia most archaean gold ores belong to a coherent genetic group of structurally controlled lode deposits that are characteristically enriched in au with variable enrichments in ag as w sb bi te b and pb but rarely cu or zn and are surrounded by wallrock alteration haloes enriched in k lile and co2 with variable na and or ca addition evidence from the yilgarn block of western australia combined with similar evidence from canada and elsewhere indicates that such deposits represent a crustal continuum that formed under a variety of crustal regimes over at least a 15 km crustal profile at pt conditions ranging from 180 c at 1 kb to 700 c at 5 kb individual deposits separated by tens to hundreds of kilometres collectively show transitional variations in structural style of mineralisation vein textures and mineralogy of wallrock alteration that relate to the pt conditions of their formation at varying crustal depths specific transitions within the total spectrum may be shown also by deposits within gold camps although nowhere is the entire continuum of deposits recorded from a single gold camp or even greenstone belt