neogene tectonostratigraphy and kinematics of calabrian basins implications for the geodynamics of the central mediterranean abstract a new structural model for the calabrian arc southern italy is presented based on a systematic analysis of basin kinematics combined with a review of the literature on basement structure three tectonic patterns can be distinguished a nw se trending oblique thrust zones determining a set of n130 trending segments b sw ne to e w trending thrust zones and c radial and concentric tensional fault systems in the southern tyrrhenian back arc area these faults combined with dome shaped uplift centres determine the actual arc patterns a and b can be regarded as the on shore representation of the calabrian accretionary wedge system the kinematics of various types of basins such as piggy back pull apart and complex oblique strike slip basins are connected to these patterns the patterns a and b can be linked to the middle miocene early pliocene development of oblique convergent crustal shear zones strike slip cycle whereas pattern c can be linked to the pleistocene collapse of the southern tyrrhenian basin and concentric uplift of the arc our structural model has considerable implications for the understanding of the kinematic evolution of the central mediterranean we propose a model in which the evolution is characterized by an alternation of the translation of the calabrian lithosphere element either to the southeast gravitationally or to the northeast compression combining a our analyses of basin evolution b the structural model and c some considerations concerning kinematics of the central mediterranean with the geodynamic mechanisms as proposed in literature we present a model which consists of a geodynamic cycle comprising the following three stages 1 a tension stage with back arc basin opening related to the diapiric inflow of asthenosphere material and roll back of the subducted ionian slab resulting in gravitational displacement of the calabrian element to the southeast 2 a transpression stage with oblique overthrusting towards the northeast upon the southern apennines and 3 a compression stage with ne sw compression due to oblique dextral shear along the north african margin plate rupture lithosphere split and back arc basin collapse

lode gold mineralization in the tanami region northern australia the tanami region of northern australia has emerged over the last two decades as the largest gold producing region in the northern territory gold is hosted by epigenetic quartz veins in sedimentary and mafic rocks and by sulfide rich replacement zones within iron formation although limited geochronological data suggest that most mineralization occurred at about 1 805 1 790 ma during a period of extensive granite intrusion although structural relationships suggest that some deposits predate this period there are three main goldfields in the tanami region the dead bullock soak goldfield which hosts the world class callie deposit the granites goldfield and the tanami goldfield in the dead bullock soak goldfield deposits are hosted by carbonaceous siltstone and iron formation where a late d5 structural corridor intersects an early f1 anticlinorium in the granites goldfield deposits are hosted by highly sheared iron formation and are interpreted to predate d5 the tanami goldfield consists of a large number of small mostly basalt hosted deposits that probably formed at a high structural level during d5 the d5 structures that host most deposits formed in a convergent structural regime with 1 oriented between e w and ene wsw structures active during d5 include ne trending oblique thrust dextral faults and ese trending sinistral faults that curve into n to nnw trending reverse faults localized in supracrustal belts between and around granite complexes granite intrusions also locally perturbed the stress field possibly localizing structures and deposits forward modeling and preliminary interpretations of reflection seismic data indicate that all faults extend into the mid crust in areas characterized by the n to nw trending faults orebodies also tend to be n to nw trending localized in dilational jogs or in fractured competent rock units in areas characterized by ese trending faults the orebodies and veins tend to strike broadly east at an angle consistent with tensional fractures opened during e w to ene wsw directed transpression many of these deposits are hosted by reactive rock units such as carbonaceous siltstone and iron formation ore deposition occurred at depths ranging from 1 5 to 11 km from generally low to moderate salinity carbonic fluids with temperatures from 200 to 430 c similar to lode gold fluids elsewhere in the world these fluids are interpreted as the product of metamorphic dewatering caused by enhanced heat flow although it is also possible that the fluids were derived from coeval granites lead isotope data suggest that lead in the ore fluids had multiple sources hydrogen and oxygen isotope data are consistent with both metamorphic and magmatic origins for ore fluids gold deposition is interpreted to be caused by fluid unmixing and sulfidation of host rocks fluid unmixing is caused by three different processes 1 co2 unmixing caused by interaction of ore fluids with carbonaceous siltstone 2 depressurization caused by pressure cycling in shear zones and 3 boiling as ore fluids move to shallow levels deposits in the tanami region may illustrate the continuum model of lode gold deposition suggested by groves mineralium deposita 28 366 374 1993 for archean districts

distribution patterns of live planktic foraminifers as related to regional hydrography and productive systems of the mediterranean sea abstract distribution patterns of live planktic foraminifers are examined at two times of the year late summer and late winter through horizontal plankton towing along a nw se transect across the west and east mediterranean basins and in the alboran sea french oceanographic cruises vicomed i ii and iii respectively foraminiferal production is significantly different in the two sampling periods and across the west and east mediterranean foraminiferal density is higher in winter than summer e g in the western basin 3700 specimens 1000 m3 in winter against 200 600 specimens 1000 m3 in summer the production averaged for the two sampling sets is higher in the western basin than in the eastern one however in late summer peaks of maximal production are observed in the central mediterranean strait of sicily and ionian basin 1000 to 2000 specimens 1000 m3 where the assemblages are dominated by globigerinoides ruber rosea and g ruber alba in the mediterranean sea frontal regions and semipermanent eddies which are essentially controlled by regional hydrography represent a significant portion of the regional production they also elicit a higher diversity of species their influence is observed in summer as well as in winter at the end of summer the rather shallow mixed layer hosts spinose symbiont bearing species such as g ruber alba g ruber rosea o universa and g trilobus at that season in the western basin the nutrient content of the mixed layer is generally low and the surface layer is rapidly nutrient depleted summer foraminiferal production remains low at the surface with respect to the winter production in the eastern basin late summer and autumn correspond to the biological spring summer foraminiferal production in surface waters is higher than in winter but standing crops are lower than in the western basin for the same species at the same period in summer however frontal structures and eddies are generally better developed than in winter in some particular cases frontal boundaries in the western basin eddies south of cyprus nutrients from intermediate layers can be upwelled through isopycnal or diapycnal mixing to the photic zone and allow for secondary blooms in the western basin these blooms favour the development of grazers at depth globorotalia truncatulinoides g inflata south of cyprus in the eastern basin oligotrophic conditions promote the development of heterotrophic organisms and predatory foraminifers have higher standing stocks g ruber alba g ruber rosea in winter the break down of the thermocline combined with deep turbulent mixing in the northwestern basin and the deepening of the mixed layer in the alboran sea and along the north african coast result in increased phytoplankton biomass and favour reproduction and proliferation of species such as g truncatulinoides or g inflata our observations indicate that the geographic and depth distribution of live foraminiferal species cannot be entirely explained by the general temperature and salinity differences among the different mediterranean basins but that they are strongly related to regional hydrographic patterns hydrography controls the seasonality and depth of the turbulent mixing which differ between the western and the eastern basins the depth of the mixed layer and its seasonal stratification and the strength of the pycnocline nutrient supply and primary production are mostly regulated by these dynamics one may therefore expect that the species assemblages on the sea floor and in deep sea cores will also correlate to the various hydrographic parameters of the overlying water masses

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

peralkaline granitoid magmatism in the mongolian transbaikalian belt evolution petrogenesis and tectonic significance article i nfo the central asian orogenic belt caob is well known for its massive generation of juvenile crust in the phanerozoic in eastern caob voluminous peralkaline and alkaline alkali feldspar granitoids and genetically related bimodal volcanics were emplaced in three stages from early permian to late mesozoic 295 270 ma 230 190 ma and 150 120 ma they occur as huge granitoid belts in extensional tectonic settings and form a complex network of about 12 million km 2 in area among them the 2500 km long mongolian transbaikalian belt mtb is the most spectacular and it comprises more than 350 granite syenite plutons and stocks with numerous co genetic volcanic fields the three stages of granitoids have similar chemical compositions but show temporal variation in nd isotopic composition initial end t values range from 1t o 5 for the early permian 0 to 4 for the early mesozoic and 2t o 3 5 for the late mesozoic granitoids the negative end t values observed in the mtb are not typical of the caob granitoids which are generally characterized by positive values however several pieces of evidence suggest that the mtb peralkaline and alkali feldspar granitoids were produced from enriched mantle derived sources the evidence includes 1 felsic and mafic rocks formed in the same stage have similar initial nd sr isotopic ratios and sm nd model ages tdm 2 the granitoid belts extend over thousands of kilometers and intersect distinct crustal provinces but no correlation is found in chemical compositions between the granitoids and country rocks 3 the abundant syenites are considered to be cogenetic or in some cases parental to the granites experimental and isotope data argue for the derivation of syenites from an enriched mantle source thus the granites and comendites are regarded as mantle derived 4 a study of melt inclusions in quartz phenocrysts of comendite indicates a high liquidus temperature of 1000 1100 c for the magma generation this suggests that the silicic magma was generated in an unusually high temperature condition which is likely produced by basaltic magma that underplated the lower crust the generation of voluminous peralkaline and alkaline alkali feldspar granitoids and genetically related bimodal volcanic rocks represents an important addition of juvenile crustal mass to the earth s continental crust during a time span of about 150 ma from late paleozoic to late mesozoic

oceanic anoxia at the precambrian cambrian boundary the precambrian cambrian pc c boundary separates fossils representing two discrete evolutionary phases the neoproterozoic soft bodied ediacarian biotas and cambrian small shelly faunas the biological discontinuity is suspected to have been a result of mass extinction however recent discoveries of the ediacarian biotas in cambrian sediments have led to an understanding that the faunal change was gradual through the pc c transition th u ratios which are high in oxidizing conditions and low in reducing conditions show a considerable positive correlation with 13c values at all studied sites of the pc c boundary this correlation indicates that reported 13c variation across the pc c boundary from numerous localities corresponds to redox variation in the depositional environment the negative 13c anomaly that occurs worldwide at the pc c boundary therefore corresponds to the widespread development of an oxygen deficient shallow marine environment this finding suggests that widespread oceanic oxygen deficiency which has been interpreted to reflect phanerozoic mass extinction events also occurred immediately before the cambrian explosion

early paleozoic amalgamation of the songnen zhangguangcai range and jiamusi massifs in the eastern segment of the central asian orogenic belt geochronological and geochemical evidence from granitoids and rhyolites abstract to constrain the early paleozoic tectonic evolution of the songnen zhangguangcai range and jiamusi massifs we undertook zircon u pb dating and geochemical analyses major and trace elements and hf isotopic compositions of early paleozoic igneous rocks in the lesser xing an zhangguangcai range of eastern heilongjiang province ne china the zircons from one diorite two rhyolites and four granitoids are euhedral subhedral and display fine scale oscillatory growth zoning indicating a magmatic origin la icp ms zircon u pb age data indicate the occurrence of magmatic events in the late ordovician 443 451 ma and middle silurian 424 430 ma the late ordovician igneous rocks are composed mainly of diorite tonalite and rhyolite whereas the middle silurian igneous rocks consist mainly of monzogranites the early paleozoic igneous rocks have sio2 59 65 76 11 wt mgo 0 13 1 23 wt and na2o k2o 4 68 7 77 wt and are chemically a calc alkaline series they are characterized by enrichment in light rare earth elements and large ion lithophile elements and depletion in heavy rare earth elements and high field strength elements such as nb ta and ti consistent with the chemistry of igneous rocks from an active continental margin setting in situ hf analyses of zircons show that ehf t values of zircons that formed in the early ordovician captured and inherited zircons in late ordovician igneous rocks range from 0 1 to 7 1 those of zircons that formed in the late ordovician including captured and inherited zircons in middle silurian monzogranite vary from 2 6 to 5 6 and those of zircons that formed in the middle silurian vary from 3 0 to 3 2 the progressive decrease in zircon ehf t values from the early ordovician to the middle silurian suggests an increasing involvement of ancient crust in the magma sources corresponding to the transition from subduction to collision take together these data suggest that the formation of late ordovician diorite tonalite and rhyolite in the zhangguangcai range adjacent to the mudanjiang fault may have been related to the westward subduction of an oceanic plate between the songnen zhangguangcai range and jiamusi massifs in contrast coeval rhyolites from the lesser xing an range located far from the mudanjiang fault may have formed in the supra subduction zone the occurrence of a middle silurian monzogranite association combined with geochronological data for detrital zircons from early devonian sedimentary rocks in eastern heilongjiang province suggests that the amalgamation between the songnen zhangguangcai range and jiamusi massifs occurred in the middle silurian

asthenospheric and lower lithospheric mantle contributions to continental extensional magmatism an example from the british tertiary province abstract the compositions of palaeocene basic magmas in skye and mull british tertiary igneous province btip shed light on the general problem of the relative importance of lithospheric and asthenospheric mantle as sources of continental magmatism during extension after the effects of crustal contamination in some magmas have been detected and the samples showing it eliminated from consideration the remainder show a significant diversity of trace element compositions basic lavas from skye and mull which lack crustal contamination have contents of compatible and moderately compatible elements e g ni cr y yb and also of moderately incompatible elements e g ti p zr hf sr mree which are within the range shown by the tholeiitic and mildly alkalic ocean island basalts oib as from iceland in contrast their contents of strongly and very strongly incompatible elements e g k rb ba u th nb ta lree are within the range shown by normal mid ocean ridge basalts these btip magmas are interpreted as second stage melts from mantle which had already lost a very small melt fraction the latter is thought to be represented by the strongly alkalic permo carboniferous dyke swarms associated with rifting at that time in nw scotland residual uppermost asthenosphere from the late palaeozoic magma genesis event is postulated to have become attached to the thickening lithosphere beneath nw scotland and thus to have remained coupled to it throughout the mesozoic until reactivated by palaeocene stretching occasional hawaiites in the top third of the skye lava pile remnant and also basalt dykes of the swarm cutting the granites of the central complex the last significant magmatic event at this centre have the elemental characteristics of typical north atlantic oib notably la ta 10 12 these compositions are thought to demonstrate input from asthenospheric source basic magmas throughout the lithospheric source dominated skye and mull magmatism it is suggested that all the basic magmas in this province are ultimately derived from asthenospheric sources but that the larger magma batches became contaminated during uprise by melting and assimilating the most fusible parts of overlying lithospheric mantle followed by lower crust in many cases smaller magma batches traverse the overlying lithosphere with little or no contamination perhaps the difference between the two lies in their flow behaviour laminar or turbulent during uprise

permian bimodal volcanism in the zhangguangcai range of eastern heilongjiang province ne china zircon u pb hf isotopes and geochemical evidence abstract in this paper we report on zircon u pb dating hf isotopes major and trace elements and sr nd isotope data with the aim of constraining the petrogenesis and regional tectonic evolution of late paleozoic volcanic rocks in the zhangguangcai range of eastern heilongjiang province ne china located in the eastern segment of the central asian orogenic belt caob between the siberian and north china cratons the late paleozoic volcanic rocks in the zhangguangcai range are composed mainly of basalt basaltic andesite rhyolite and minor dacite the results of la icp ms zircon u pb dating for two basaltic rocks three rhyolites and one dacite indicate that they formed in the early permian ca 292 ma the mafic rocks have sio2 50 13 53 80 wt k2o 0 98 2 28 wt mg 0 51 0 71 cr 144 541 ppm ni 74 260 ppm 87sr 86sr i 0 7044 and end t 4 28 whereas the felsic rocks have sio2 69 12 77 98 wt k2o 3 09 5 33 wt mg 0 17 0 36 87sr 86sr i 0 7032 and end t 4 32 these data are typical of bimodal volcanism the mafic volcanic rocks are characterized by a strong enrichment in large ion lithophile elements liles such as rb ba sr and pb depletion in high field strength elements hfses such as nb ta and ti depletion in heavy rare earth elements hrees and weak negative eu anomalies eu eu 0 88 0 94 on the other hand the felsic rocks show a strong depletion in nb ta sr p and ti enrichment in th u and k and relatively large negative eu anomalies eu eu 0 28 0 95 taken together these data suggest that the mafic magma was derived from the partial melting of a depleted lithospheric mantle modified by subducted slab derived fluids and that the felsic magma originated by partial melting of newly accreted crust the early permian bimodal volcanic rocks together with the coeval a type granites indicate an extensional environment similar to a back arc basin such a setting was possibly related to subduction of the paleo asian oceanic plate beneath the jiamusi and songnen zhangguangcai range massifs

zirconolite allanite and hoegbomite in a marble skarn from the bergell contact aureole implications for mobility of ti zr and ree zirconolite allanite and hoegbomite are present as accessory phases in a metasomatically altered spinel calcite marble from the contact with the bergell intrusives switzerland italy textural relationships indicate a step wise alteration of spinel to 1 hoegbomite or corundum magnetite 2 margarite and 3 chlorite replacement of spinel by hoegbomite can be described by the substitution 1 94 mg2 fe2 zn2 mn2 ca2 ti4 0 12 oh where al3 and fe3 are held constant the average composition of the bergell hoegbomites is given by the formula fe 0 97 2 mg0 69zn0 04ti0 17al3 94fe 0 06 3 o7 98 oh 0 02 and seems to be imposed by the composition of pre existing spinel during the first two steps of spinel alteration calcite was replaced by anorthite phlogopite and the rare earth element ree bearing minerals zirconolite allanite and sphene were formed allanites have characteristic chondrite normalized ree patterns with enrichment in the light ree the zirconolite patterns show a marked increase in concentration from la to ce followed by an almost constant section sphene lacks detectable la and its ree patterns vary from grain to grain contemporaneous formation of phlogopite ree bearing minerals and hoegbomite during replacement of the spinel calcite marble indicates that the metamorphic fluid introduced potassium along with ree and other high valence cations ti4 zr4 u4 th4a3804265 nb5a3804265 y3a3804265 possibly as polynuclear complexes the abundance of fluorine bearing phlogopite and fluor apatite as well as their close association with ree bearing minerals and hoegbomite suggests f and po 4 3 as likely ligands for complexing of the above mentioned elements

nd hf o isotopic evidence for subduction induced crustal replacement in ne china abstract convergent plate margins represent an ideal setting to understand the formation and evolution of the earth s crust nevertheless the geodynamic processes involved in crustal evolution remains highly debatable based on a large geochemical dataset of permian cretaceous granitic rocks from the yanbian area in ne china we propose a new geodynamic model involving crustal replacement to decipher crustal evolution during a superimposed multi stage subduction system these phanerozoic granitic rocks from two adjacent zones show contrasting hf nd isotopic variation trends with decreasing age of magmatism whole rock end t and zircon ehf t decrease gradually in the north granite zone ngz but show the opposite trends in the south granite zone sgz zircon 18o shows a slight rise in the ngz but it initially increases from permian to jurassic and then decreases to cretaceous in the sgz the nd hf o isotopic compositions of the cretaceous granitic rocks in both zones converge four crustal components are identified in the source of granites across the yanbian area including 1 juvenile crust 2 subduction accretionary complex 3 new arc crust and 4 precambrian crust the contributions made to the bulk crustal melting source of granites by the juvenile crust component in the ngz and by the precambrian crust in the sgz were both gradually decreased whereas the proportion of the new arc crust increased in both zones pre existing crustal components were extensively eroded destructed diluted and replaced by the new arc crust formed during paleo pacific slab subduction considering the prevalence of these two nd hf o isotopic variation trends in many orogenic belts subduction induced crustal replacement represents an important mechanism for crustal evolution in convergent plate margins

mud volcanoes along the inner deformation front of the calabrian arc accretionary wedge ionian sea abstract we present geophysical data integrated with the analysis of well targeted sediment samples in order to contribute to a better understanding of fluid circulation in the calabrian arc accretionary wedge two cores bs81 ii 5 and bs81 ii 10 collected on a swell in the hangingwall of the inner deformation front show upper pleistocene mud breccias that include cretaceous to late miocene rock fragments mechanically incorporated into the eruption deposit by the upward transport of overpressured fluids an additional core cala 21 came from the summit of a topographic high in the footwall of the inner deformation front of the accretionary wedge and contains a mud breccia patchy cloudy facies where sediment disturbance is caused by fluid expulsion integration of the entire data set provides evidences of the interplay among tectonics fluid emissions and sedimentation and enables the identification of two new volcanoes in the accretionary prism where overpressuring due to pliocene and pleistocene sediment accumulation and the evolution of active fault systems triggered fluid circulation and the formation of those structures

geochronology and geochemistry of late paleozoic volcanic rocks from eastern inner mongolia ne china implications for igneous petrogenesis tectonic setting and geodynamic evolution of the south eastern central asian orogenic belt abstract here we report new petrography geochemistry u pb zircon geochronology and hf isotope results for volcanic rocks of the dashizhai formation in the dashizhai area keerqin right wing front banner inner mongolia the volcanic rocks have mafic to felsic bulk rock compositions and include basalt rhyolite basaltic andesite andesite dacite and rhyolitic tuff two dacite and rhyolitic tuff samples yield laser ablation inductively coupled plasma mass spectrometry la icp ms 206pb 238u zircon ages of 332 1 ma and 300 1 ma respectively suggesting that the timing of felsic volcanism ranged from the early to late carboniferous mafic volcanic rock samples are enriched in large ion lithophile elements lile and light rare earth elements lree la nb 1 58 4 84 and are depleted in high field strength elements hfse th yb ratios

plate detachment asthenosphere upwelling and topography across subduction zones this study analyzes the topography across subduction zones considering the separate contributions of the crust and the mantle lithosphere to the observed surface elevation we have found a transition from a region where the overriding plate is coupled to the descending slab and pulled down along with it to a region where the overriding plate floats freely on the asthenosphere when the subducting slab retreats oceanward rapidly this transition is abrupt and the edge of the overriding plate is uplifted we propose that at some point during rapid slab rollback the overriding plate detaches and rebounds like a boat released from its keel this event is associated with suction of asthenospheric material into the gap that is opened between the plates up to the base of the crust as a result the forearc uplifts and magmatism in the arc increases

geochronology and geochemistry of the heilongjiang complex and the granitoids from the lesser xing an zhangguangcai range implications for the late paleozoic mesozoic tectonics of eastern ne china abstract an integrated study on two subduction related units the heilongjiang complex and magmatic rocks in the lesser xing an zhangguangcai range was carried out to explore the paleozoic to early mesozoic tectonic framework of the eastern northeast china with a focus on the jiamusi block the heilongjiang complex which is considered as the suture between the jiamusi and songliao blocks is composed of blueschist amphibolite and ultramafic rocks the blueschist from the yilan area shows geochemical features similar to oib and has a 206pb 238u age of 288 2 ma which is interpreted as the crystallization age of protolith formed in an ocean island setting in contrast amphibolite from the xiachengzi area shows geochemical features similar to n morb zircons in this amphibolite show magmatic character and yield a 206pb 238u age of 248 4 ma which is interpreted as the protolith age calcic amphiboles in the amphibolite give a 40ar 39ar plateau age of 195 3 ma which is interpreted as the metamorphic age of the heilongjiang complex magmatic rocks in the lesser xing an zhangguangcai range mostly belong to i type granites that were formed in an active continental margin setting zircon u pb ages of these granites range from 200 ma to 174 ma with a westward younging trend shown as two groups of emplacing ages of 200 ma in the east and 180 ma in the west our new data support an early occurrence likely since the early permian of a large ocean namely the mudanjiang ocean in the area whose subduction resulted in the metamorphic unit the heilongjiang complex and the magmatic unit the lesser xing an zhangguangcai range in the jurassic

triassic volcanism in eastern heilongjiang and jilin provinces ne china chronology geochemistry and tectonic implications abstract with the aim of constraining the early mesozoic tectonic evolution of the eastern section of the central asian orogenic belt caob we undertook zircon u pb dating and geochemical analyses major and trace elements sr nd isotopes of volcanic rocks of the luoquanzhan formation and daxinggou group in eastern heilongjiang and jilin provinces china the analyzed rocks consist mainly of dacite and rhyolite with sio2 contents of 68 52 76 65 wt three samples from the luoquanzhan formation and one from the daxinggou group were analyzed using laser ablation inductively coupled plasma mass spectrometry la icp ms u pb zircon techniques three zircons with well defined oscillatory zoning yielded weighted mean 206pb 238u ages of 217 1 214 2 and 208 1 ma and one zircon with oscillatory zoning yielded a weighted mean 206pb 238u age of 201 1 ma these ages are interpreted to represent the timing of eruption of the volcanic rocks the triassic volcanic rocks are characterized by high sio2 and low mgo concentrations enrichment in large ion lithophile elements liles and light rare earth elements lrees depletion in high field strength elements hfses and heavy rare earth elements hrees 87sr 86sr i 0 7040 0 7050 luoquanzhan formation and 0 7163 0 7381 daxinggou group and end t 1 89 3 94 luoquanzhan formation and 3 42 3 68 daxinggou group these geochemical features indicate an origin involving the partial melting of juvenile lower crust nd model ages tdm2 of 651 821 ma and that compositional variation among the volcanic rocks arose from mineral fractionation and minor assimilation these volcanic rocks formed within an extensional environment following collision of the ncc and jiamusi khanka massif during the late paleozoic early triassic

early jurassic mafic magmatism in the lesser xing an zhangguangcai range ne china and its tectonic implications constraints from zircon u pb chronology and geochemistry abstract la icp ms zircon u pb dating and geochemical data have been obtained from five representative mafic ultramafic intrusions in the lesser xing an zhangguangcai range ne china with the aim of improving our understanding of the mesozoic tectonic evolution in the region and in particular determining the time of initiation of the circum pacific tectonic system the selected zircons exhibit striped absorption in cathodoluminescence cl images and have high th u ratios 0 20 3 16 indicating a magmatic origin the zircon u pb dates indicate that most of these magmatic zircons other than a few relics that were captured and entrained in the magma formed in the late early jurassic 186 182 ma and not as previously supposed in the middle caledonian the five mafic ultramafic plutons are composed of olivine gabbro hornblendite gabbro hornblende gabbro and gabbro diorite the olivine gabbro and hornblendite display cumulate textures implying that fractional crystallization of olivine and plagioclase took place during magma evolution these mafic ultramafic igneous rocks have sio2 37 3 55 7 mgo 3 05 13 3 al2o3 11 8 23 8 mg 42 69 mg 100mg mg fe2 total and eu 0 88 1 32 and they display three types of rare earth element ree distribution patterns right slipped flat patterns or dome like the trace element spider diagrams show that the rocks are enriched in large ion lithophile elements liles such as ba k and sr and depleted in high field strength elements hfses such as nb ta zr and hf the zircons have ehf 186 182 ma 2 7 to 12 0 and tdm1 366 732 ma the geochemical data indicate that the early jurassic mafic magma originated in an extensional environment from the partial melting of a depleted mantle wedge that had been metasomatized by fluids released from a fossil subducted slab these data combined with information on the spatial variation of coeval igneous rocks indicate that the formation of the early jurassic mafic ultramafic rocks in the lesser xing an zhangguangcai range was related to the subduction of the paleo pacific plate beneath the eurasian continent and this event would mark the beginning of the circum pacific tectonic system

a new geomagnetic polarity time scale for the late cretaceous and cenozoic we have constructed a magnetic polarity time scale for the late cretaceous and cenozoic based on an analysis of marine magnetic profiles from the world s ocean basins this is the first time since heirtzler et al 1968 published their time scale that the relative widths of the magnetic polarity intervals for the entire late cretaceous and cenozoic have been systematically determined from magnetic profiles a composite geomagnetic polarity sequence was derived based primarily on data from the south atlantic anomaly spacings in the south atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles fine scale information was derived from magnetic profiles on faster spreading ridges in the pacific and indian oceans and inserted into the south ariantic sequence based on the assumption that spreading rates in the south atlantic were smoothly varying but not necessarily constant a time scale was generated by using a spline function to fit a set of nine age calibration points

constraints on the timing of uplift of the yanshan fold and thrust belt north china this paper reports in situ u pb ages and hf isotopic data on detrital zircons from paleozoic to late mesozoic strata in the xishan area near beijing that provide important constraints on the evolution and paleogeography of the northern part of the north china craton ncc zircons from carboniferous to late mesozoic sandstones are characterized by three major groups of u pb ages 2 5 2 3 ga 1 9 1 7 ga and late phanerozoic ages the neoarchean and paleoproterozoic zircons and some phanerozoic zircons have negative ehf t values similar to igneous zircons from intrusive rocks known from the ncc indicating that the source of the sandstones was locally derived however late triassic to late jurassic 205 158 ma sandstones contain a group of phanerozoic zircons with positive ehf t values distinct from known igneous zircons in the ncc suggesting an exotic source the nearest potential source is the xing meng orogenic belt xmob to the north of the ncc where considerable amounts of juvenile crustal materials exist it is concluded that there are two sudden shifts in source provenances of the xishan sediments from the ncc before the late triassic to a mixed source from the ncc and xmob during the late triassic to late jurassic and then from the ncc again during the late jurassic 158 ma the first shift in source resulted from the exhumation and denudation of rocks due to uplift of the xmob which coincided with post collisional extension in the late triassic and intraplate extension of the ncc in the middle to late jurassic the second source shift resulted from the uplift of the yanshan fold and thrust belt thus constraining the timing of this to the late jurassic

geological framework of the archean and paleoproterozoic tanami region northern territory the tanami region a poorly exposed mostly paleoproterozoic province within the north australian craton hosts a number of significant gold deposits in diverse settings rare exposures of 2 520 2 500 ma amphibolite facies archean gneiss and metasedimentary rocks form basement to the thick overlying metasedimentary succession of the 1 880 1 830 ma tanami group the basal unit of the tanami group is the dead bullock formation a fining upward deep water succession dominated by siltstone carbonaceous siltstone iron rich siltstone and mafic sills carbonaceous and iron rich lithologies in the upper dead bullock formation represent important hosts for gold mineralization the conformably overlying killi killi formation represents turbiditic sedimentary rocks that are correlated with the widespread lander rock beds of the arunta region sedimentation of the tanami group was terminated by regional deformation and greenschist to amphibolite facies metamorphism during the tanami event d1 m1 at around 1 830 ma the tanami group is unconformably overlain by rhyolite siliciclastic sedimentary rocks and felsic ignimbrite of the ware group that were deposited at about 1 825 1 810 ma subsequent ese wnw to se nw directed shortening d2 followed by ne sw to e w directed shortening d3 has resulted in open ne f2 and nw f3 trending folds in both the tanami and ware groups voluminous granitoids dominated by i type biotite granodiorite and monzogranite were intruded in the interval 1 825 1 790 ma and have been subdivided using geochemical criteria into the birthday frederick and grimwade suites basalt and immature sedimentary rocks of the mount charles formation are restricted in extent to the tanami mine corridor and are interpreted to reflect a continental rift succession that was deposited around 1 800 ma with an early archean sedimentary provenance steep s to se dipping f4 fold structures of tanami and ware group metasedimentary rocks many spatially associated with 1 825 1 790 ma granitoid intrusions indicate a period of sse directed regional shortening d4 syn to post the regional granitoid intrusive phase a network of n to nw striking faults several of which are interpreted as oblique thrusts with a component of left lateral movement indicates a period of d5 convergence during wsw ene to e w directed shortening the tanami mine corridor fault system comprises a network of n ne to ene striking d5 faults that merge with n to nw striking faults and probably accommodated movement between granite core domains d5 faulting is associated with the main phase of gold mineralization in suitable structural lithological traps the paleoproterozoic basement of the tanami region is unconformably overlain by quartz sandstone lithic arenite and conglomerate of the pargee sandstone pargee sandstone may represent syn tectonic sedimentation related to the 1 730 ma strangways orogeny and is unconformably overlain by the late paleoproterozoic platform cover succession of the birrindudu group the paleoproterozoic basement and cover sequences have subsequently undergone several episodes of faulting collectively termed d6 the paleoproterozoic evolution of the tanami region is interpreted to have occurred in an intracratonic setting but was fundamentally influenced by tectonic events in the adjacent halls creek orogen 1 835 1 805 ma halls creek orogeny and arunta region 1 815 1 800 ma stafford event the boundaries between the tanami region and kimberley region to the northwest and the arunta region to the southeast are transitional and are largely defined by the presence or absence of identifiable dead bullock formation

narrow subducting slabs and the origin of backarc basins abstract both tectonic and seismic data suggest that horizontally wide in the direction parallel to the axis of the trench dipping subducting slabs typically move stably at fixed angles to the horizontal and that their associated trenches migrate very slowly or not at all in contrast where backarc basins babs exist the associated trenches typically migrate at rates of several cm year this behavior may be related to the finite width of the slabs under babs analysis shows that hydrodynamic suction created by the downdip motion of a wide slab is sufficient to balance the pull of gravity on the heavy slab however for a narrow slab sideways asthenospheric flow into the slab overriding plate corner effectively eliminates the suction which results in the rapid sinking of the slab hydrodynamic modeling is used to quantify this hypothesis modeling is used also to show that wide slabs are stable in their inclined position not because of the gravitational wing like sliding but due to the lifting force produced by the negative pressure suction in the slab overriding plate corner finally the model results confirm that for relatively narrow slabs this lifting force is small and cannot counterbalance gravity therefore when bending moments in a narrow slab exceed the plastic yield limit the slab becomes unstable this instability can cause the rapid migration of the slab s hinge and the associated arc at rates of 10 20 cm year and thus form the bab

contemporaneous formation of vein hosted and stratabound gold mineralization at the world class dead bullock soak mining camp australia we investigate the relative and absolute timing associated with the formation of the world class dead bullock soak dbs mining camp the study area is situated in the paleoproterozoic granites tanami gold province in the northern territory australia and consists of five orogenic gold deposits including the callie deposit with a resource of 14 2 moz au at dbs the ore is hosted in paleoproterozoic metasedimentary rocks and consists of two contrasting styles of mineralization vein hosted and stratabound the vein hosted mineralization consists of visible gold in quartz chlorite carbonate veins preferentially hosted in finely laminated decarbonized siltstones in contrast the stratabound mineralization consists of free gold associated with arsenopyrite concentrated in bedding parallel sulfide rich layers hosted in iron rich siltstones this study combines structural and petrographic analysis with detailed u pb geochronological data on hydrothermal and metamorphic xenotime and monazite to constrain the relative and isotopic timing of ore formation in the dbs mining camp the results of this study show that both mineralization styles developed within a series of ne trending and se dipping vein corridors or structural pathways associated with the development of axial planar fabric related to small scale f2 folds u pb geochronology shows that both mineralization styles formed at c 1805 ma approximately 35 m y after peak metamorphism at c 1840 ma a post mineralization hydrothermal event is locally remobilizing gold and yields an age of c 1790 ma which provides a minimum age for gold deposition

the tectonic expression slab pull at continental convergent boundaries examination of five thrust belt systems developed at continental subduction boundaries suggests that they comprise two distinct groups that display pronounced and systematic differences in structural style topographic elevation denudation metamorphism postcollisional convergence and foredeep basin geometry and facies the distinctive geological features developed within each thrust belt group appear to be causally linked to the relative rates of subduction and convergence via the magnitude of horizontal compressional stress transmitted across the subduction boundary at subduction boundaries where the rate of overall plate convergence is less than the rate of subduction termed here retreating subduction boundaries the transmission of horizontal compressive stress across the plate boundary is small and regional deformation of the overriding plate is by horizontal extension the tectonic expression of these retreating subduction boundaries includes topographically low mountains little erosion or denudation low grade to no metamorphism little to no involvement of crystalline basement in shortening little to no postcollisional convergence anomalously deep foredeep basins and a protracted history of flysch deposition within the adjacent foredeep basin analysis of deflection and gravity data across three retreating subduction boundaries apennine carpathian and hellenic systems shows that subduction is driven by gravitational forces acting on dense subducted slabs at depths between about 40 and 80 km carpathians 50 and 150 km apennines and 50 and 250 km hellenides the total mass anomalies represented by the slabs are approximately 3 1012 6 1012 and 12 1012 n m respectively the slabs are partially supported by flexural stresses transmitted through the subducted lithosphere to the foreland and partially supported by dynamic viscous stresses in the asthenosphere at subduction boundaries where the rate of overall plate convergence is greater than the rate of subduction termed here advancing subduction boundaries the transmission of horizontal compressive stress across the plate boundary is large and regional deformation of the overriding plate is by horizontal shortening the tectonic expression of these advancing subduction boundaries includes topographically high mountains antithetic thrust belts large amounts of erosion and denudation exposure of high grade metamorphic rocks at the surface extensive deformation of crystalline basement to midcrustal depths protracted postcollisional convergence tens of millions of years and a protracted history of molasse deposition within the adjacent foredeep basins analysis of gravity and deflection data across two advancing subduction boundaries developed within the continental lithosphere western to eastern and southern alps and himalayas shows that the thrust sheets have been translated for great distances over the foreland lithosphere relative to the point at which the subduction forces are applied thus obscuring any flexural and gravity signals from the subducted slab however it appears that far field stresses presumably related to global plate motions drive most of the convergent motion across these subduction boundaries the concept that orogenic belts formed above retreating subduction boundaries have recognizable tectonic signatures that differ from those of orogenic belts formed above advancing subduction boundaries suggests that it may be possible to interpret the plate boundary settings in which ancient orogenic belts evolved appendix b is available with entire article on microfiche order from the american geophysical union 2000 floridaavenue n w washington d c 20009 document t92 004 2 50 payment must accompany order

shrimp u pb analyses of detrital zircon a window to understanding the paleoproterozoic development of the tanami region northern australia poorly exposed paleoproterozoic turbiditic to shallow marine sedimentary rocks of the tanami basin nt australia are largely the erosional products of either the 1 87 1 85 hooper orogeny and or magmatism associated with the 1 87 ga nimbuwah event dating of detrital zircon from six of the principal sedimentary units shows that deposition spanned at least the period 1 84 1 77 ga collectively the detrital zircon ages reveal a progression in provenance that is a record of the development of the orogen the basal dead bullock formation contains only zircon derived from archean basement and no contemporaneous products of orogeny its deposition age is inferred to be 1 87 1 84 ga orogenic 1 86 ga zircon appears in the overlying killi killi formation deposited between 1 84 1 82 ga and persists probably due to recycling in all overlying units except one the mount charles formation the accepted stratigraphic position of this unit might be incorrect

history of subduction and back arc extension in the central mediterranean summary geological and geophysical constraints to reconstruct the evolution of the central mediterranean subduction zone are presented geological observations such as upper plate stratigraphy hp lt metamorphic assemblages foredeep trench stratigraphy arc volcanism and the back arc extension process are used to define the infant stage of the subduction zone and its latest back arc phase based on this data set the time dependence of the amount of subducted material in comparison with the tomographic images of the upper mantle along two cross sections from the northern apennines and from calabria to the gulf of lyon can be derived further the reconstruction is used to unravel the main evolutionary trends of the subduction process results of this analysis indicate that 1 subduction in the central mediterranean is as old as 80 myr 2 the slab descended slowly into the mantle during the first 20 30 myr subduction speeds were probably less than 1 cm year x1 3 subduction accelerated afterwards producing arc volcanism and back arc extension and 4 the slab reached the 660 km transition zone after 60 70 myr this time dependent scenario where a slow initiation is followed by a roughly exponential increase in the subduction speed can be modelled by equating the viscous dissipation per unit length due to the bending of oceanic lithosphere to the rate of change of potential energy by slab pull finally the third stage is controlled by the interaction between the slab and the 660 km transition zone in the southern region this results in an important re shaping of the slab and intermittent pulses of back arc extension in the northern region the decrease in the trench retreat can be explained by the entrance of light continental material at the trench

the role of decarbonization and structure in the callie gold deposit tanami region of northern australia the callie deposit is the largest 6 0 moz au of several gold deposits in the dead bullock soak goldfield of the northern territory s tanami region 550 km northwest of alice springs the callie ore lies within corridors up to 180 m wide of sheeted en echelon quartz veins where they intersect the 500 m wide hinge of an ese plunging f1 anticlinorium the host rocks are the blake beds of the paleoproterozoic dead bullock formation which consist of a 350 m thick sequence of lower greenschist facies graphitic turbidites and mudstones overlying in excess of 100 m of thickly bedded siltstones and fine sandstones the rocks are fe rich and dominated by assemblages of chlorite and biotite both of which are of hydrothermal and metamorphic origin a fundamental characteristic of the hydrothermal alteration is the removal of graphite a process which is associated with bleaching and the development of bedding parallel bands of coarse biotite augen gold is found only in quartz veins and only where they cut decarbonized chloritic rock with abundant biotite augen and no sulfide minerals auriferous quartz veins differ from barren quartz veins by the presence of ilmenite apatite xenotime and gold and the absence of sulfide minerals the assemblage of gold ilmenite apatite xenotime indicates a linked genesis and mobility of ti p and y in the mineralizing fluids geochemical analysis of samples throughout the deposit shows that gold only occurs in sedimentary rocks with high feo feo fe2o3 and low c c co2 ratios 0 8 and 0 2 respectively this association can be explained by reactions that convert c from reduced graphitic host rocks into co2 and reduce ferric iron in the host rocks to ferrous iron in biotite and chlorite these reactions would increase the co2 content of the fluid facilitating the transport of ti p and y from the host rocks into the veins both co2 and ch4 produced by reaction of h2o with graphite effervesced under the lower confining pressures in the veins this would have partitioned h2s into the vapor phase destabilizing au bisulfide complexes the loss of co2 and h2s from the aqueous phase caused precipitation of gold ilmenite apatite and xenotime it is proposed that this process was the main control on gold precipitation oxidization of iron in the very reduced wall rocks resulting in reduction of the fluid provided a second mechanism of gold precipitation in previously decarbonized rocks contributing to the high grades in some samples although sulfide minerals especially arsenopyrite did form during the hydrothermal event host rock sulfidation reactions did not play a role in gold precipitation because gold is absent near rocks or veins containing sulfide minerals sulfide minerals likely formed by different mechanisms from those associated with gold deposition both the fold architecture and subsequent spatially coincident sinistral semibrittle shearing ensured that the ore fluids were strongly focused into the hinges of the anticlines within the anticlines a reactive cap of fine grained graphitic reduced fe rich turbidites above more permeable siltstones and fine sandstones impeded fluid flow ensuring efficient removal of graphite and the associated effervescence of co2 from the fluid caused the precipitation of gold exploration for similar deposits should focus on the intersection of east west shear zones with folds and fe rich graphitic host rocks

sedimentary evolution of the neogene quaternary crotone basin southern italy and relationships with large scale tectonics a sequence stratigraphic approach abstract the crotone basin is a neogene quaternary depocenter developed along the ionian margin of calabria southern italy the basin opening was concomitant with the onset of spreading of the tyrrhenian backarc basin during late serravallian and the subduction of the ionian lithosphere below the calabrian arc the adoption of modern sequence stratigraphic principles to subdivide the basin fill into stratigraphic sequences of different rank bounded by subaerial unconformities has allowed cycles of creation and loss of accommodation mainly related to tectonics to be recognized in particular an alternation between phases of basin subsidence concomitant with spreading of the tyrrhenian backarc basin and forward migration of the arc and phases of compressional or transpressional tectonics accompanied by basin uplift and stopping of arc migration are well recognizable in the basin fill in particular for the late messinian to late pleistocene part of the succession the present study highlights the effectiveness of the sequence stratigraphic method in basin analysis to decipher the effects of large scale tectonic events on sedimentation and ultimately to reconstruct the geodynamic evolution of the central mediterranean

chronology of the middle upper pliocene succession in the strongoli area constraints on the geological evolution of the crotone basin southern italy the aim of this study is to reconstruct the evolution of the strongoli area a critical sector of the crotone basin calabria southern italy where a thick middle upper pliocene marine succession is present the strongoli succession shows prominent changes in the sedimentary environment that are partly forced by tectonics major tectonostratigraphic events have been recognized that might correlate with spreading pulses in the back arc tyrrhenian sea in particular we demonstrate that a dramatic basinal collapse at c 2 3 ma correlates with the so called calabrian transgression auctorum and is close in age to the oceanization of the marsili basin

orogenic gold formation in an evolving decompressing hydrothermal system genesis of the samut gold deposit eastern desert egypt abstract samut is a vein type gold mineralization in the eastern desert of egypt which shares many geologic features with other orogenic deposits in this region the deposit has a long history of mining activity dating back to ancient times but so far has received little attention from the standpoint of geologic study here we present new field petrographic microthermometric and microanalytical data from the samut gold deposit to evaluate its genesis and fluid evolution vein quartz at samut shows abundant evidence of precipitation coinciding with early phases of ductile deformation that show transitions to increasing brittle fracturing and fragmentation through time in some samples brecciation of the vein quartz is intense and shows evidence of repeated episodes of fragmentation this brittle style of deformation culminates with latest stage brittle opening and loss of confining pressure resulting in open space filling textures backscattered electron images and electron microprobe analysis reveal three varieties of pyrite in the mineralized quartz veins py i py ii and py iii laser ablation icpms analysis of pyrites from mineralized quartz veins reveals that the earlier py i is low in arsenic and gold whereas py ii and py iii are enriched in both elements anhedral py ii is particularly gold rich and gives way to blocky euhedral py iii that is rich in arsenic but not as rich in gold compared to py ii fluid inclusions in the least brittle deformed vein quartz indicate that the veins formed at minimum temperatures of 300 c early in the hydrothermal system the calculated fluid pressures at homogenization of the aqueous carbonic inclusions furnish a wide range throughout the paragenesis perhaps on the order of 1 to 3 kbar the 34s values of pyrites imply sulfur isotopic composition of the mineralizing fluid on the order of 0 6 to 4 9 overlapping with the broad range of 34s values calculated for ore fluids in orogenic gold deposits worldwide together these results support a general model in which gold and sulfide deposition was coinciding with an ongoing syn deformational vein formation and tending towards the late stages of vein growth prior to loss of confinement free milling gold appears to be mostly related to the later stages of brittle fracturing and re sealing of the veins during repeated fluid release as the background fluid pressure evolved from earlier dominantly lithostatic to later fully hydrostatic conditions

icp ms a powerful tool for high precision trace element analysis in earth sciences evidence from analysis of selected u s g s reference samples abstract inductively coupled plasma mass spectrometry icp ms is a powerful analytical technique with considerable application to geochemistry in this paper we demonstrate the power of icp ms as a tool in petrogenetic studies data for 28 elements in seven u s g s basalt and andesite reference samples agv 1 bcr 1 bhvo 1 bir 1 dnc 1 w 1 and w 2 are presented accuracy and precision standard deviation for 26 of these elements is either excellent a detailed description of the icp ms technique we have developed is given a simple hf hno 3 dissolution in screwtop teflon bombs was used for sample preparation icp ms instrumental sensitivity was measured by external solutions with surrogate calibration for 4 elements matrix correction was made by use of standard addition naturally occurring internal standards rb y ce and pb were used to correct for instrumental drift between sample and spiked sample measurements we report data for the elements li cs rb sr ba zr hf nb ta y sc u th pb and all 14 of the ree in addition the procedure acquired data for be mo w tl and bi many elements in this array are crucial for the evaluation of petrogenetic models and source tracing herein lies the real power of icp ms one technique one dissolution small sample size good limits of detection good to excellent accuracy and good precision for many geochemically important elements

late quaternary changes in mediterranean intermediate water density and formation rate three recently acquired eastern mediterranean cores containing late quaternary sediments display a marked faunal contrast between the holocene and older sapropels it is suggested that the absence of neogloboquadrinids in the holocene sapropel and their abundance in older sapropels reflect differences in food availability related to the extent of development of a deep chlorophyll maximum layer dcm and the intensity of new primary production associated with this layer the depth of such a layer which consists of phytoplankton with a characteristic taxonomic composition is determined by the vertical hydrodynamical structure during deposition of the older sapropels the pycnocline was apparently positioned well above the base of the euphotic layer a situation that favors a pronounced dcm and an associated relatively high rate of new production shallowing of the pycnocline implies a relatively low density of the mediterranean intermediate water miw on the other hand during deposition of the holocene sapropel the pycnocline had apparently vanished due to termination of miw formation

arsenopyrite pyrite association in an orogenic gold ore tracing mineralization history from textures and trace elements laser ablation inductively coupled plasma mass spectrometry la icp ms spot analysis and element mapping coupled with high resolution focused ion beam scanning electron microscopy fib sem imaging has been performed on au hosting sulfides from a proterozoic orogenic gold deposit tanami gold province north central australia gold distribution patterns in the arsenopyrite pyrite assemblage which probably crystallized toward the end of the au forming event suggest a process of initial scavenging of au into arsenopyrite and subsequent remobilization of that au following brittle fracture gold expelled from the sulfide lattice during the remobilization event is reconcentrated around the margins of the same arsenopyrite grains and within swarms of crosscutting microfractures the distribution of pb bi and ag closely mimic au indicating that these elements were also reconcentrated preservation of oscillatory zonation patterns for co ni sb se and te in arsenopyrite imply however that no significant degree of sulfide recrystallization took place residual concentrations of invisible gold in grain centers are 5 ppm in arsenopyrite and 1 ppm in coexisting pyrite the presence of submicron sized pores is suggestive of a fluid driven process rather than solid state diffusion micron scale 2 10 m remobilized gold is accompanied by fine particles 200 nm 2 m and nanoparticles 200 nm of gold the extensive variation of au concentrations within single arsenopyrite grains underlines the significance of textures when using trace element data to understand how gold ores evolved

late jurassic to early early cretaceous tectonic nature on the ne asian continental margin constraints from mesozoic accretionary complexes abstract it is widely accepted that subduction of the paleo pacific plate beneath eurasia began in the early mesozoic however the tectonic nature and processes of the ne asian continental margin in the late jurassic to early early cretaceous remain unclear in this contribution lithological provenance and metamorphic deformation data are used to constrain the formation of mesozoic accretionary complexes ac along the ne asian continental margin and determine the tectonic relationship between the ne asian continental margin and the paleo pacific plate in the late jurassic to early early cretaceous the mesozoic ac formed during the late jurassic including the raohe ac in ne china the khabarovsk samarka and taukha acs in the russian far east and the mino tamba ac in japan and early cretaceous including the shimanto ac in japan and the kiselevka manoma ac in the russian far east the jurassic ac underwent a complex evolution including the formation of seamounts middle late jurassic terrigenous clastic sedimentation adjacent to the trench collision of seamounts with eurasia northward migration of the ac early early cretaceous terrigenous clastic sedimentation a second period of accretion and final emplacement the earliest mesozoic ac arrived at the trench at ca 195 ma combined with the occurrence of early mesozoic calc alkaline igneous rocks along the ne asian continental margin we suggest that subduction of the paleo pacific plate beneath eurasia began in the early jurassic jurassic ac in japan was affected by a regional high p t metamorphic event at 160 ma which might have been related to subduction collision of seamounts with eurasia at the trench ages and hf isotopic compositions of detrital zircon grains from the middle upper jurassic and cretaceous ac sandstones together with paleontological and paleomagnetic data indicate that jurassic clastic sediments were sourced mainly from the south china block scb whereas the lower cretaceous sandstones were sourced from the eastern central asian orogenic belt caob the inferred northward migration of jurassic ac in the late jurassic to early early cretaceous indicates that the boundary between the ne asian continental margin and the paleo pacific plate was characterized by strike slip tectonics at this time likely due to a small angle of obliquity in the subduction of the paleo pacific plate beneath eurasia in the late early cretaceous westward subduction of the paleo pacific plate beneath eurasia resulted in the emplacement of the raohe and khabarovsk acs at 137 130 ma and the formation of acs in the russian far east and japan

exploration targeting for orogenic gold deposits in the granites tanami orogen mineral system analysis targeting model and prospectivity analysis abstract a major challenge for mineral exploration geologists is the development of a transparent and reproducible approach to targeting exploration efforts particularly at the regional to camp scales in terranes under difficult cover where exploration and opportunity costs are high in this study a three pronged approach is used for identifying the most prospective ground for orogenic gold deposits in the paleoproterozoic granite tanami orogen gto in western australia a key input to the analyses is the recent development of a 4d model of the gto architectural evolution that provides new insights on the spatio temporal controls over orogenic gold occurrences in the area in particular on the role of pre mineralization pre 1795 ma d gtoe d gto1 d gto2 architecture in localization of gold deposits and the spatial distribution of rock types in 3d this information is used to build up a model of orogenic gold minerals system in the area which is then integrated into the three mutually independent but complementary mineral prospectivity maps namely a concept driven manual and fuzzy analysis and a data driven automated analysis the manual analysis involved 1 generation of a process based gold mineral systems template to aid target selection 2 manual delineation of targets 3 manual estimation of the probability of occurrence of each critical mineralization process based on the available information and 4 combining the above probabilities to derive the relative probability of occurrence of orogenic gold deposits in each of the targets the knowledge based geological information system gis analysis attempts to replicate the expert knowledge used in the manual approach but queried in a more systematic format to eliminate human heuristic bias this involves representing the critical mineralization processes in the form of spatial predictor maps and systematically querying them through the use of a fuzzy logic model to integrate the predictor maps and to derive the western gto orogenic gold prospectivity map the data driven empirical gis analysis uses no expert knowledge instead it employs statistical measures to evaluate the spatial associations between known deposits and predictor maps to establish weights for each predictor layer then combines these layers into a predictive map using a weights of evidence wofe approach application of a mineral systems approach in the manual analysis and the fuzzy analysis is critical potential high value targets identified by these approaches in the western gto lie largely under cover whereas traditional manual targeting is biased to areas of outcrop or sub crop amenable to direct detection technology such as exploration geochemistry and therefore towards areas that are data rich the results show the power of combining the three approaches to prioritize areas for exploration while the manual analysis identifies and employs human intuition and can see through incomplete datasets it is difficult to filter out human bias and to systematically apply to a large region the fuzzy method is more systematic and highlights areas that the manual analysis has undervalued but lacks the intuitive power of the human mind that refines the target by seeing through incomplete datasets the empirical woe method highlights correlations with favorable host stratigraphy and highlights the control of an early set of structures potentially undervalued in the knowledge driven approaches yet is biased due to the incomplete nature of exploration datasets and lack of abundant gold deposits due to the extensive cover the results indicate that the most prospective areas for orogenic gold in western gto are located in the central part of the study area largely in areas blind to previous exploration efforts according to our study the procedure to follow should be to undertake the analyses in the following order manual prospectivity analysis followed by the conceptual fuzzy approach followed by the empirical gis based method undertaking the manual analysis first is important to prevent explorationists from being biased by the automated gis based outputs it is however emphasized that all of the prospectivity outputs from these three methods are possible and they should not be treated as treasure maps but instead as decision support aids therefore a final manual prospectivity analysis redefined by the mutual consideration of output from all of the methods is required the strategy employed in this study constitutes a new template for best practice in terrane to camp scale exploration targeting that can be applied to different terranes and deposit types particularly in terranes under cover and provides a step forward in managing uncertainty in the exploration targeting process

episodic back arc extension during restricted mantle convection in the central mediterranean abstract in the central mediterranean during the last 80 myr africa has been slowly converging toward eurasia with an average relative velocity of less than 1 cm yr 1 for 50 myr this slow convergence led to the build up of the alpine orogenic belt and to the initiation of consumption of the former tethys ocean in the last 30 myr however the whole central western mediterranean has been swept by rapid episodes of trench migration up to 6 cm yr 1 and back arc opening which consumed the whole former tethys ocean we combine plate tectonic history geological timing and laboratory modelling to reconstruct the subduction history and the evolution of the central mediterranean over the last 80 myr we find that the dynamic evolution of the subducting oceanic lithosphere can reconcile the rapid episodic back arc migration with the slow african convergence but only if the subduction process is restricted to the upper mantle

paleozoic accretionary and collisional tectonics of the eastern tianshan china implications for the continental growth of central asia this paper deals with the various tectonic units in the chinese eastern tianshan orogenic collage in the central asian orogenic belt and discusses the paleozoic geological history of the several periods of accretion and collision of archipelago systems lying between the tarim and southern angaran continental margins the late ordovician silurian to early devonian eastern tianshan archipelago was characterized by a the harlik dananhu subduction system with a s dipping polarity in the north b a southerly n dipping subduction system beneath the central tianshan arc in the middle and c the south tianshan ocean against tarim in the south during the devonian to early carboniferous n dipping subduction led to the harlik dananhu arc and the kanggurtag forearc basin accretionary complex in the early to mid carboniferous the magmatic front associated with the n dipping subduction beneath the dananhu harlik arc migrated southwards forming the yamansu arc constructed upon the kanggurtag accretionary forearc by the late carboniferous the dananhu harlik arc was attached northwards to the angaran margin resulting in lateral enlargement of the angaran continent in the latest carboniferous to early permian a multiple soft collision left wide suture zones in the south that include the ophiolite strewn aqikkuduk shaquanzi and kumishi accretion collision complexes which were stitched by early permian post collisional plutons by re defining and re interpreting the various tectonic terranes this paper presents a new improved model for the paleozoic evolution of this part of central asia

titanium mobility in metamorphic systems a review abstract titanium is a dispersed lithophile element commonly regarded as chemically immobile at sub magmatic temperatures however a growing number of field studies demonstrate that ti is somewhat mobile in certain environments this review examines the current state of knowledge concerning the mobility of ti during alteration and metamorphism of crustal rocks also considered are relevant ti oxide solubility data from experiments and determinations of ti concentrations in natural waters where ti mobility is well documented its length scale is 10 m however ti may move greater distances in deep crustal shear zones the solubility of ti oxides in geologic fluids is strongly temperature dependent under low to medium grade metamorphic conditions ti mobility is limited by the low solubility of ti oxides in the associated fluids under eclogite conditions ti mobility is probably limited not by ti oxide solubility which reaches 1 ti but by restricted fluid flow in low permeability rocks

geochronology and isotope geochemistry studies of an epithermal gold deposit in the northern lesser khingan range ne china the gaosongshan example abstract the gaosongshan epithermal gold au deposit is located in the northern margin of the lesser khingan range polymetallic ore belt northeast ne china and contains au resources of more than 23 tons at an average grade of 6 3 g t gold orebodies are mainly hosted in intensely altered volcanic rocks of the lower cretaceous banzifang formation and as exploration has continued gold has also been discovered in extensively altered porphyritic intrusions with the highest au grade of 70 g t however the ages of metallogenesis and related magmatism have not been well constrained yet which have hindered the summary of metallogenic regularities in this study laser ablation inductively coupled plasma mass spectrometry la icp ms zircon u pb dating from the granite porphyry yields a weighted mean 206pb 238u age of 101 6 1 2 ma slightly older than the rb sr isotope age of 98 1 6 ma for the auriferous quartz samples indicating that the gaosongshan au mineralization is genetically related to the granite porphyry the fluid d and 18oh2o values vary from 136 to 111 and from 9 3 to 2 5 respectively and the 4he and 40ar isotope compositions range from 15 7 10 9 to 35 5 10 9 cm3 stp g and 13 4 10 7 cm3 to 34 1 10 7 cm3 stp g respectively the obtained isotopic data show that the ore forming fluids were mixtures of magmatic and meteoric water but were dominated by meteoric water the 206pb 204pb 207pb 204pb and 208pb 204pb ratios of ores in the gaosongshan deposit are 17 752 18 856 15 514 15 674 and 38 005 39 921 respectively which are similar to those of the gaosongshan granite porphyry implying that pb and possibly au originated primarily from late early cretaceous magmatic intrusions combining these results with data from previous studies we propose that the gaosongshan au deposit formed in an extensional tectonic setting during the rollback of the paleo pacific oceanic plate in ne china

age constraints on late mesozoic lithospheric extension and origin of felsic volcanism in the songliao basin ne china abstractlate mesozoic lithospheric extension in ne asia resulted in the development of a large extensional province and widespread formation of volcanosedimentary basins songliao basin is the biggest in that region situated between the siberian and sino korean north china cratons the songliao basin contains voluminous volcanic rocks as a major part of the basin fill volcanic successions form a significant but unexposed part of the late mesozoic magmatic province in ne china here we report zircon u pb ages and geochemical data for felsic volcanic rocks from the songliao basin zircon populations of two types of rhyolites with distinct i and a type affinities analyzed by laser ablation inductively coupled plasma mass spectrometry yielded similar u pb ages of 114 2 and 113 2 ma respectively whole rock nd and zircon hf isotope data of the felsic rocks indicate an origin from newly formed crustal protoliths the i type dacites and rhyolites show geochemical signatures with subduction related af

residual topography lithospheric structure and sunken slabs in the central mediterranean abstract the central mediterranean comprises subduction collision and backarc extension all in a relatively small area in this paper we analyze the topography of this region examining the influence of subducting slabs on overriding plates we take the observed surface elevation remove the contribution of the crust to it and use the residual topography to identify regions that are unusually low unusually high or normal then we calculate lithospheric thickness where local isostasy applies the results provide information about the structure of the lithosphere in this complex region and about the vertical tectonics in particular we found a difference between the south tyrrhenian subduction zone and the apennines consistent with the processes of slab rollback and slab break off respectively in the south tyrrhenian the edge of the oceanic plate is strongly pulled down whereas the edge of the overriding plate the calabrian peninsula is uplifted this opposite trend indicates weak plate coupling that is the subducting slab which is rapidly rolling back hangs almost entirely on the oceanic plate and the overriding plate is free in contrast in the apennines high residual topography and uplifting are detected on both sides of the mountain belt consistent with complete break off of the subducted lithosphere

zircon u pb age and sr nd hf isotope geochemistry of permian granodiorite and associated gabbro in the songliao block ne china and implications for growth of juvenile crust abstract post orogenic granitic and associated mafic rocks from northeastern ne china consist of granodiorite and gabbro intrusions we report here upon new u pb zircon ages geochemical data and sr nd hf isotopic data for these rocks la icp ms u pb zircon analysis yields an age of 262 8 1 0 ma for the granitic rocks and a uniform age of 262 1 0 7 ma for the gabbro most of the granitic and mafic rocks are characterised by low k 2 o na 2 o and pertain to the subalkaline series in the total alkali silica diagram the granodiorite samples show low 87 sr 86 sr i ranging from 0 700 to 0 705 positive e nd t values from 0 3 to 0 8 and large variation in e hf t values of between 4 0 and 2 5 indicating that both newly underplated basalt 70 80 and ancient lower crustal sources 20 30 contributed to their origin furthermore positive e hf t values with two stage model ages t dm2 of 1123 1260 ma together with nd model ages 960 1000 ma suggest an important episode of crustal growth during the meso neoproterozoic beneath the songliao block in contrast the investigated gabbro is characterised by relatively high 87 sr 86 sr i ratios 0 707 0 708 negative e nd t 5 9 to 5 3 and e hf t values 5 0 to 2 3 implying that this was derived from an enriched mantle source the geochemical data indicate that the granitic magmas underwent separation of clinopyroxene hornblende k feldspar plagioclase ti bearing phases e g rutile ilmenite titanite apatite and zircon during their evolution whereas the gabbro is characterised by low mgo 2 92 3 92 wt mg 35 41 and compatible elements content such as cr 10 68 ppm co 16 31 ppm and ni 5 7 33 ppm features of a more evolved mafic magma there is no evidence that the granitic and mafic rocks were affected by crustal contamination during emplacement our interpretation is that the two coeval intrusive suites were both formed in a post orogenic extensional setting related to lithospheric delamination or collapse of the central asian orogenic belt caob xingmeng orogenic belt in china

collision belt between the khanka block and the north china block in the yanbian region northeast china the pre jurassic geotectonics in northeast china and its adjacent regions are characterized by collision of microcontinental blocks the yanbian region lies between the north china block ncb and the khanka block kb along the dashanzui antu kaishantun collision belt the yanbian pre jurassic fold belt can be divided into two parts one part represents the western margin of the kb which consists of late proterozoic metamorphic basement and a permian active continental margin the other one is located at the eastern margin of the ncb and is composed of proterozoic passive margin deposits where the seluohe and qinglongcun groups are developed and rocks of the permian passive margin collision of these two parts took place along the dashanzui antu kaishantun collision zone during the late permian to early triassic as evidenced by the emplacement of syncollision granites and a type granites in northeast china and adjacent regions a series of microcontinental blocks collided with one other from the end of early paleozoic to the early triassic the collision event that took place in the yanbian region may represent the latest collision event in the paleo asian oceanic domain c 2003 elsevier ltd all rights reserved

geochronology and geochemistry of igneous rocks in the southeastern lesser xing an range northeastern china petrogenesis and implications for the early mesozoic tectonic evolution we present systematic u pb age data collected by laser ablation multi collector inductively coupled plasma mass spectrometry precise geochemical data and nd isotope data for igneous rocks from th

a jurassic garnet bearing granitic pluton from ne china showing tetrad ree patterns abstract ne china is characterized by the massive distribution of phanerozoic granitoids most of them are of i and a type granites whereas s type granites are rarely documented the present work deals with the dongqing pluton a small granitic body emplaced in the southern zhangguangcai range the pluton comprises a two mica garnet granite and a garnet bearing muscovite granite the latter occurs as veins in the former the pluton shows a gradational contact with the surrounding host granites rb sr and sm nd isotope analyses on whole rocks and minerals reveal that the two types of granites were emplaced synchronously at about 160 ma the pluton was emplaced coeval with the surrounding i type granitic pluton and had a rapid cooling history it is characterized by an initial sr isotopic ratio of 0 706 slightly negative end t values 0 5 to 1 9 and young depleted mantle model ages 970 1090 ma this suggests that the parent magma originated from partial melting of relatively juvenile crust which is largely compatible with the general scenario for much of the phanerozoic granitoids emplaced in the central asian orogenic belt geochemically the granites of the dongqing pluton are peraluminous with a shand index molar ratio a cnk of 1 0 1 1 for the two mica granites and 1 2 1 3 for the garnet bearing granites all the garnet bearing granites and some of the two mica granites show tetrad ree patterns tetrad group whereas most two mica granites show normal granitic ree patterns normal group the normal group granites exhibit depletion in nb ta p and ti in spidergrams and generally weak positive eu anomalies in ree patterns by contrast the tetrad group granites manifest depletion in ba nb ta sr p and ti and significant negative eu anomalies the trace element data constrain the parental magmas to having undergone extensive magmatic differentiation during their late stage magmatic evolution intense interaction of residual melts with aqueous hydrothermal fluids resulted in the non charac charge and radius controlled trace element behavior and the tetrad effect in ree distribution patterns this in turn leads to the invalidation of the commonly used tectonic discrimination criteria derived from trace element abundances of normal granites in view of this and previous studies we conclude that there were probably no s type granites produced in ne china during the phanerozoic consequently weathered sedimentary material did not play an important role in the genesis of the strongly peraluminous granites in the zhangguangcai range

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

odp leg 107 in the tyrrhenian sea insights into passive margin and back arc basin evolution leg 107 of the ocean drilling program drilled a west northwest east southeast transect of seven sites across the tyrrhenian sea the youngest of the sub basins of the mediterranean sea sites 654 653 652 and 656 document the rifting and subsidence of the sardinia passive continental margin on the upper margin site 654 we cored a classic transgressive sequence subaerial conglomerates overlain by oyster bearing sands overlain by marine marl comparison between the recovered lithologies and seismic reflection profiles suggests that the synrift sediments on the upper margin are tortonian late miocene to messinian latest miocene in age whereas synrift sediments on the lower margin are messinian to pliocene in age during the messinian desiccation of the mediterranean sites 654 and 653 now on the upper sardinian margin apparently occupied a basinal setting where they received nannoplankton bearing clays interbedded with laminated gypsum sites 656 and 652 now on the lower sardinia margin were apparently higher standing during the desiccation event their messinian facies are subaerial and lacustrine respectively we infer from these lines of evidence that tilting and subsidence occurred more than a million years earlier on the upper margin than on the lower margin such diachroneity can be interpreted in terms of migration of the zone of maximum extension above a rolling back subduction zone or in terms of extension of continental crust by shear along a deep detachment fault sites 655 651 and 650 were drilled into two small basalt floored basins of the central and eastern tyrrhenian emplacement of basaltic crust in the central tyrrhenian vavilov basin apparently began more than a million years before emplacement of basaltic crust in the eastern tyrrhenian marsili basin this observation is compatible with previous suggestions that the tyrrhenian has grown southeastward in response to rollback of the down going slab that currently dips northwestward under the toe of italy at the easternmost site high vesicularity of the basalt and benthic foram assemblages in the oldest sediments imply that the basalt erupted in water shallower than 2 500 m it has apparently subsequently subsided to its present depth of 4 100 m below sea level nearly three times as fast as normal subsidence of crust formed at a mid ocean ridge

a major change in the sedimentation regime in the crotone basin southern italy around 3 7 3 6 ma abstract the lower pliocene succession of the crotone basin calabrian arc southern italy is mainly comprised of blue grey marly clay with good magnetic properties here the bio magnetostratigraphic data indicate a mean sedimentation rate of about 12 15 cm kyr around 3 7 3 6 ma a major change in the sedimentation regime occurred the blue grey hemipelagic marls grade rapidly into silty marls with a significant increase in the terrigenous fraction and with abundant siliceous remains throughout the whole interval magnetic properties of these sediments are very poor but an integrated calcareous plankton biostratigraphy foraminifera and nannofossils infers a high average sedimentation rate about 50 60 cm kyr the abrupt onset of this sedimentation regime in the crotone basin is contemporaneous with a major unconformity already recognized in the northern sector of the basin part of a major reorganization phase in the whole apenninic maghrebid chain known as globorotalia puncticulata event reports of coeval siliceous sediments in other marginal basins of the apennines southern calabria southern and northern apennines suggest that this siliceous event might have been regionally extensive having important palaeoceanographical implications we infer that the siliceous event is characterized by a combined tectonic and climate induced change in palaeoceanographic conditions the tectonic triggering factors may have been linked to two synchronous events in the tyrrhenian apennine system 1 the shortening event also known as g puncticulata event and 2 the coeval opening of the vavilov basin in the tyrrhenian sea which yielded profound influences in terms of physiography and characteristics of the crotone basin the consequent uplift of the southern apennines would have increased sediment supply and availability of silica resulting in eutrophication and enhanced silica preservation strong winter mixing and possibly upwelling conditions could have increased primary productivity during heavy isotope stages gi4 gi2 and mg8 at the onset of the siliceous event this important event lasting from ca 3 6 ma to ca 3 2 ma would have recorded a peculiar transitional period before further climatic deterioration and more drastic palaeoceanographic changes occurred around 3 1 ma leading to cyclic sapropel deposition in the whole of the mediterranean sea

the valle di manche section calabria southern italy a high resolution record of the early middle pleistocene transition mis 21 mis 19 in the central mediterranean abstract the on land marine valle di manche section crotone basin calabria southern italy one of the candidates to host the gssp of the middle pleistocene ionian stage preserves a manifold record of independent chronological paleoclimatic and stratigraphic proxies that permit a straightforward correlation with marine and terrestrial reference records at the global scale in particular the section holds an excellent record of the matuyama brunhes magnetic reversal which occurs in the midst of marine isotope stage mis 19 we report on a complete revision of the section that improves dramatically the available dataset especially in the stratigraphic interval straddling the lower middle pleistocene boundary our benthic 18o record provides evidence that the matuyama brunhes transition the stratigraphic position of which is marked by a prominent tephra the pitagora ash occurred during full mis 19 in agreement with many records worldwide we obtained an age of 786 9 5 ka for the matuyama brunhes magnetic reversal and pinpointed the paleomagnetic transition of to a 3 cm thick interval indicating that the event was very fast since the section fulfills all the requirements to host the gssp of the ionian stage we propose that the boundary should be placed at the base of the pitagora ash ca 12 5 cm below the midpoint of the matuyama brunhes reversal

neogene and quaternary rollback evolution of the tyrrhenian sea the apennines and the sicilian maghrebides reconstruction of the evolution of the tyrrhenian sea shows that the major stage of rifting associated with the opening of this basin began at similar to10 ma it involved two episodes of back arc extension which were induced by the rollback of a west dipping subducting slab the first period of extension 10 6 ma was prominent in the northern tyrrhenian sea and in the western part of the southern tyrrhenian sea the second period of extension mainly affected the southern tyrrhenian sea began in the latest messinian 6 5 ma and has been accompanied by subduction rollback at rates of 60 100 km myr 1 slab reconstruction combined with paleomagnetic and paleogeographic constraints indicates that in the central apennines the latest messinian 6 5 ma arrival of a carbonate platform at the subduction zone impeded subduction and initiated a slab tear and major strike slip faults these processes resulted in the formation of a narrow subducting slab beneath the ionian sea that has undergone faster subduction rollback and induced extreme rates of back arc extension

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

zircon u pb and molybdenite re os dating and geological implications of the shimadong porphyry molybdenum deposit in eastern yanbian northeastern china the shimadong porphyry mo deposit is located in eastern yanbian in the eastern part of the north margin of the north china craton northeastern china here we present the whole rock major and tra

dynamics of benthic marine communities across the early middle pleistocene boundary in the mediterranean region valle di manche southern italy biotic and stratigraphic implications abstract the valle di manche vdm section calabria southern italy offers the opportunity to investigate the ostracod turnover along a continuous shelf succession straddling the early middle pleistocene boundary and compare it against other paleoenvironmental i e mollusks and paleoclimatic uvigerina peregrina 18o records high resolution ca 1 sample m ostracod fauna quantitative data coupled with gradient analysis detrended correspondence analysis and non metric multi dimensional scaling document a strong relationship between changes in faunal composition and lithofacies vertical stacking patterns the comparison between the mollusk and ostracod derived ordination data demonstrates that the meio and macro faunal turnover is guided by a common complex gradient bathymetry the integrated ostracod mollusk gradient analysis also provides a trend in water depths along the section highlighting to what extent such multivariate approach can improve the paleoenvironmental and sequence stratigraphic interpretation of ancient shallow marine successions when plotted stratigraphically ordination major axis sample scores reveal two increasing decreasing patterns in water depth that fit well with the t r cycles previously identified paleobathymetric estimates combined with the vertical distribution of key ostracod groups i e epiphytic taxa on sandy substrates vs deep sea mud related taxa allow refining depositional trends stratal stacking patterns and position of previously not well resolved sequence stratigraphic surfaces within each t r cycle e g transgressive surface ts indeed two rapid increases in water depth values mark the tss that separate shallowing upward progradational rst from deepening upward retrogradational tst stratal stacking patterns of shelf deposits the tss which underline fine grained successions dominated by deep sea mud lover taxa are invariably constrained to the inception of interglacials marine isotope stages mis 21 and 19 identified within the vdm section by benthic foraminifera 18o values within both the vdm t r cycles the deepest conditions ca 140 m of water depth are invariably identified within the neopycnodonte unit slightly above the lightest 18o intervals the overlying decreasing bathymetric trend coupled with shifts in ostracod ecological groups allows to identify in the bryozoans lithofacies the stillstand falling of the relative sea level also tracked by a progressively heavier 18o record more stable paleobathymetric conditions around 40 45 m of water depth characterize the overlying silt sand deposits dominated by epiphytic species and showing the heaviest 18o values

a mediterranean perspective on 10be sedimentation and climate around the matuyama brunhes boundary les liaisons dangereuses abstract the 10be 9be ratio is commonly employed as a tool for establishing the stratigraphic position of paleomagnetic excursions and reversals whenever the traditional paleomagnetic approach fails to provide conclusive results in particular it is held that 10be production rates in the atmosphere depend on the strength of the earth s magnetic field and the fallout and deposition of cosmogenic beryllium at the surface happen on a very short time scale however investigations performed on terrestrial and marine successions demonstrate that the 10be record and the paleomagnetic signal are often asynchronous mechanisms that control the conveyance and deposition of cosmogenic 10be to the seafloor are still ambiguous and poorly documented here we discuss the dynamics of 10be in a central mediterranean marginal marine depositional scenario characterized by a pervasive terrigenous influx our data show that a very close correlation exists between 10be concentrations and the local proxy of rainfall rates and regimes pollen indicating that a considerable 10be transport from the mainland may occur in response to the remobilization of terrestrial reservoirs during periods of increased runoff superimposed is a dynamic oceanographic setting that further controls the preservation potential of 10be at the bottom in terms of changing water chemistry and or composition of the sedimentary flux to the seafloor results of our investigation suggest that in particular environmental and depositional settings the interplay between climate terrigenous yield and oceanography may jeopardize the sedimentary depiction of the meteoric 10be contribution thus challenging the use of 10be for tracking the stratigraphic position of geomagnetic reversals

sedimentary geochemistry geochronology and sequence stratigraphy as tools to typecast stratigraphic units and constrain basin evolution in the gold mineralised palaeoproterozoic tanami region northern australia abstract rare earth elements rees high field strength elements and detrital zircon populations typecast and establish the provenance of siliciclastic sedimentary rocks of the palaeoproterozoic tanami group this group is the main host of gold in the tanami region an important proterozoic gold province in northern australia these data also define chronostratigraphic surfaces which provide a framework for constructing sequence stratigraphic models for the tanami basin this basin model provides insight into the likely spatial distribution of deep water facies which are the best trap rocks for epigenetic gold deposition the largest gold deposits are hosted in the well bedded carbonaceous siltstone of the dead bullock formation which is the basal unit of the tanami group at the callie deposit which is the largest deposit in the region the host dead bullock formation is characterised by 1 enrichments of mgo tio2 cr sc 2 light ree depletion relative to paas 3 a dominant population of detrital zircons with an age of 2500 ma in contrast turbidites of the overlying killi killi formation which is less strongly mineralised are characterised by 1 enrichments in zr la sc la sm 2 relatively flat ree pattern and 3 the presence of a major population of detrital zircons with ages of 1880 1840 ma these characteristics are independent of grain size and are interpreted to indicate that the dead bullock formation was derived from a provenance dominated by archean mafic and felsic rocks whereas the killi killi formation was derived from a provenance dominated by felsic rocks of palaeoproterozoic age these characteristics which are best shown using zr sc versus th sc diagrams can be used as an exploration tool to differentiate the more favourable dead bullock formation from the less prospective killi killi formation these data combined with regional geologic data provide a first order understanding of the make up of the tanami basin fill with implications for the distribution of siltstone dominated facies that are the best host rocks for gold

tectonically driven mud volcanism since the late pliocene on the calabrian accretionary prism central mediterranean sea abstract mud volcanoes recently discovered on the offshore calabrian arc are investigated at two sites 60 km apart in water depths of 1650 2300 m using swath bathymetry 2d 3d multichannel seismic and cores the seabed and subsurface data provide information on their formation and functioning in relation to tectonic activity during the rapid plio quaternary advance of the accretionary prism fore arc extension and thrust belt compression are seen to have involved two main phases of activity separated by a regional unconformity recording a mid pliocene 3 5 3 0 ma tectonic reorganization the two sites of mud volcanism lie in contrasting tectonic settings inner fore arc basin vs central fold and thrust belt and record differing forms of seabed extrusive activity twin mud cones and a caldera vs a broad mud pie at both sites subsurface data show that mud volcanism took place throughout the second tectonic phase since the late pliocene differing forms of mud extrusion were accompanied by subsidence to form depressions beneath and within extrusive edifices up to 1 5 km thick the basal subsidence depressions point to sources within the succession of thrusts underlying the inner to central arc consistent with microfossils within cored mud breccias from both sites that are derived from strata as old as late cretaceous these results are argued to support a model of mud volcanism in which deeply rooted fluid conduits drive a process of near surface sediment mobilization the onset of extrusive activity is inferred to have been triggered by the mid pliocene tectonic reorganization the change in stress fields which post dates the migration of frontal compression across the area by 4 ma opening pathways for the release of overpressured fluids from deep within the accretionary prism the rising fluids interacted with shallow

a type granites in northeastern china age and geochemical constraints on their petrogenesis abstract a type granites are widely distributed in northeastern china ne china they were emplaced during three major episodes the permian late triassic to early jurassic and early cretaceous and evolved in different tectonic regimes according to their mineralogical and geochemical characteristics two subgroups of a type granites aluminous and peralkaline can be recognized the peralkaline subgroup contains alkali mafic minerals such as riebeckite arfvedsonite and sodic pyroxene while the aluminous subgroup contains annite and fe rich calcic or sodic calcic amphibole with respect to the aluminous subgroup the peralkaline granites contain higher rb ga and total rare earth elements ree but lower mgo cao al 2 o 3 ba and sr based on the discrimination criteria of eby geology 20 1992 641 the permian and late triassic to early jurassic a type granites belong to the a 2 post orogenic type whereas the early cretaceous granites are of the a 1 anorogenic type nd isotopic compositions of these a type granites indicate their derivation from a dominantly juvenile crustal source their origin is thought to have involved partial melting of an underplated lower crustal source because the generation of a type granites requires high melting temperature we propose models involving slab break off lithospheric delamination and extension the permian a type granites have the same age range as those in eastern junggar southern mongolia and central inner mongolia they occur along a major suture and form a narrow belt between the north china and siberian cratons we suggest that their formation was associated with post collisional slab break off the late triassic to early jurassic a type granites are likely to be the product of lithospheric delamination after the final collision of the major crustal blocks in the late paleozoic to early triassic the early cretaceous a type granites have an anorogenic affinity and were possibly associated with rifting in eastern china at this time associated with the onset of paleo pacific subduction consequently we conclude that the a type granites in ne china were generated at three different times involving multiple processes operative in different tectonic environments

wallrock alteration associated with turbidite hosted gold deposits examples from the palaeozoic lachlan fold belt in central victoria australia abstract the study of wallrock alteration phenomena around slate belt hosted gold deposits can provide valuable information regarding the physico chemical conditions of the ore bearing fluids and the timing relationships between deformation peak metamorphism granitoid emplacement and mineralisation thus studying the processes involved in the interaction of the hydrothermal fluids with the host rock is critical in order to constrain genetic models and ultimately as a tool for exploration largely due to the characteristics of the host rocks turbidite hosted mesothermal gold deposits were historically believed to lack such diagnostic alteration haloes and their study has long been neglected however recent studies in the palaeozoic lachlan fold belt in southeastern australia and in similar tectonic settings elsewhere have demonstrated that alteration around many of these deposits is more widespread and pervasive than previously thought a review of existing information and new data strongly support the viability of studying hydrothermal alteration as a guide to turbidite hosted mineralisation and as an indicator of the character of the solutions associated with ore deposition characteristically the auriferous reef structures are surrounded by broad bleached zones while disseminations of carbonate spots pyrite and arsenopyrite porphyroblasts are the most obvious features of alteration depending on parameters such as fault geometry fluid pressure and reactivity and permeability of the host rocks these disseminations can occur up to a kilometre away from the auriferous structures sericitisation and carbonatisation are commonly discernible within 3 to 10 m wide haloes wallrock alteration chemistry of turbidite hosted gold deposits in the lachlan fold belt shows consistent enrichments of k2o co2 s and as and the anomalous values of these elements also provide the most useful dispersion haloes for turbidite hosted gold exploration due to both their regularity and extent

hf isotopic compositions of the standard zircons and baddeleyites used in u pb geochronology abstract zircon and baddeleyite u pb geochronological dating is widely used in solid earth sciences and the advent of rapid in situ methods of analysis such sims and icp ms has re emphasized the importance of having uniform standards recently it has been shown that hf isotopic data can provide important information on these minerals since they contain high concentrations of hf but have low lu hf ratios which results in negligible age correction however the complex internal structures that result from multiple thermal events such as inherited cores and metamorphic overgrowths require that the hf isotopic data be measured with high spatial resolution however the isobaric interferences of 176 yb and 176 lu on 176 hf hamper the precise determination of the 176 hf 177 hf ratio during in situ laser ablation mc icpms analysis it is shown here that mass biases of yb yb and hf hf change with time during analyses and behave differently for solutions and solid material therefore it is suggested that the mean yb value of the individual spot be used to obtain the precise isotopic composition for in situ zircon and baddeleyite hf isotopic analyses for low yb hf 176 yb 177 hf yb value have little effect on the accuracy of the hf isotopic composition however the appropriate yb isotopic abundance is necessary for high yb hf 176 yb 177 hf 0 001 zircons and baddeleyites since the interference of 176 yb on 176 hf is significant using the mean yb value of the individual spot and newly published yb isotopic abundance data six standard zircons and two standard baddeleyites have been investigated using a neptune mc icpms with 193 nm laser for zircons the obtained 176 hf 177 hf ratios are 0 282307 31 2sd for 91500 0 282680 31 2sd for temora 0 281729 21 2sd for cz3 282177 17 2sd for cn92 1 0 282983 17 2sd for fm0411 and 281234 11 2sd for phalaborwa the baddeleyites from phalaborwa and sk10 2 have 176 hf 177 hf ratios of 0 281238 12 and 0 282738 13 2sd these results agree well with the values obtained by the solution method and indicate that these standards have different hf isotopic compositions in which the extremely low 176 lu 177 hf and 176 yb 177 hf values of cz3 zircon and phalaborwa baddeleyite make them excellent standards for machine calibration during in situ zircon hf isotopic measurement with the other standards being more suitable for the development of the correction method

are iron rich sedimentary rocks the key to the spike in orogenic gold mineralization in the paleoproterozoic paleoproterozoic rocks with ages from 2100 to 1800 ma are recognized as belonging to one of the three main periods in earth history that host orogenic gold deposits in contrast to older archean orogenic gold deposits which are hosted predominantly in greenstone terranes supracrustal sedimentary rocks became increasingly important as hosts in the paleoproterozoic unusually iron rich 1840 ma marine mudstones in the tanami region host one world class gold deposit and many other large gold deposits fluid rock modelling at 300 and 325 c suggests a strong correlation between gold grade and these iron rich fine grained sedimentary rocks new regional stratigraphic correlations for similar iron rich rocks to those in the tanami region are suggested with 1860 ma gold bearing stratigraphy in the pine creek region these northern australian paleoproterozoic iron rich sedimentary rocks could be linked globally to iron rich and gold bearing sedimentary rocks deposited between 2100 to 1800 ma in homestake south dakota united states and in ghana and brazil worldwide from about 2400 to 1800 ma the paleoproterozoic is also marked by the deposition of mainly superior style bifs which are attributed to the progressive oxygenation of the deep oceans resulting in the global scrubbing of iron from the oceans the high iron concentrations noted in pre 1800 ma marine sediments in northern australia could also be related to this same process occurring globally and help explain a worldwide peak in orogenic au deposits hosted in rocks aged from 2100 to 1800 ma

the hulan group its role in the evolution of the central asian orogenic belt of ne china the hulan group of central jilin province northeastern ne china consists of felsic volcanic and sedimentary rocks that underwent sub greenschist to amphibolite facies metamorphism and multiple phases of deformation during formation of the central asian orogenic belt young nd model ages of 1100 ma obtained in this study do not support earlier models that the hulan group was either rifted from the north china craton to the south or from the jiamusi block to the north since both these blocks have much older nd model ages detrital zircon u pb ages from a meta sedimentary rock of the hulan group define a maximum age of deposition of 287 6 ma not early palaeozoic as previously believed rb sr mineral isochron data indicate that metamorphism of the hulan group occurred at 250 ma this conclusion is further supported by an age of 248 4 ma for the dayushan granite which intruded the hulan group these new isotopic data together with evaluation of the regional geology indicate that final oceanic closure in the chinese segment of the central asian orogenic belt took place in the late permian the hulan area is located along the solonker xra moronchangchun suture that extends from inner mongolia across to central jilin province it is this zone that marks the final ocean closure resulting in the amalgamation of the north china and siberian cratons 2007 elsevier ltd all rights reserved