episodic and short lived granitic pulses in a post collisional setting evidence from precise u pb zircon dating through a crustal cross section in corsica abstract in western corsica a large granitoid batholith was emplaced during carboniferous and permian times these rocks display an evolution through time from high mg k calc alkaline to calc alkaline then alkaline affinities moreover the magmatic processes can be observed at different crustal levels new and precise isotope dilution and thermo ionisation mass spectrometry id tims u pb dating suggests that granite magmatism in corsica did not simply reflect the continuous thermal evolution of a thickened crust a precise timing for the post collisional magmatism is documented with four distinct short lived episodes respectively at 345 338 305 and 280 ma separated by periods of inactivity these four magma pulses display variable nd isotopic signatures related to mantle crust interactions granite magma generation and emplacement is strongly connected to the thermal and tectonic activity of the lithosphere an increasing contribution of a depleted mantle to the magmatic processes is documented for the last igneous event this is interpreted to reflect the emplacement of depleted mantle materials in the lower crust which enhanced heat flow and triggered partial melting of previously underplated mafic rocks this increasing contribution of mantle materials marks the onset of a new geodynamic cycle characterised by an extensional tectonic regime preluding to the opening of the alpine orogenic tract

seasonal and interannual variations of the energy flux equator and itcz part ii zonally varying shifts of the itcz the itcz lies at the ascending branch of the tropical meridional overturning circulation where near surface meridional mass fluxes vanish near the itcz column integrated energy fluxes vanish forming an atmospheric energy flux equator efe this paper extends existing approximations relating the itcz position and efe to the atmospheric energy budget by allowing for zonal variations the resulting relations are tested using reanalysis data for 1979 2014 the zonally varying efe is found as the latitude where the meridional component of the divergent atmospheric energy transport aet vanishes a taylor expansion of the aet around the equator relates the itcz position to derivatives of the aet to a first order the itcz position is proportional to the divergent aet across the equator it is inversely proportional to the local atmospheric net energy input nei that consists of the net energy fluxes at the surface at the top of the atmosphere and zonally across longitudes the first order approximation captures the seasonal migrations of the itcz in the african asian and atlantic sectors in the eastern pacific a third order approximation captures the bifurcation from single to double itcz states that occurs during boreal spring in contrast to linear efe theory during boreal winter in the eastern pacific northward cross equatorial aet goes along with an itcz north of the equator efe and itcz variations driven by enso are characterized by an equatorward poleward shift in the pacific during el nino la nina episodes which are associated with variations in equatorial ocean energy uptake

alaska landfast sea ice links with bathymetry and atmospheric circulation 1 using radarsat synthetic aperture radar sar imagery of northern alaska and northwestern canada we calculated a mean climatology of the annual landfast ice cycle for the period 1996 2004 we also present the monthly minimum mean and maximum landfast ice extents throughout the study area these data reveal where and when the landfast is most stable and which sections of the coast are susceptible to midseason breakout events stabilization of landfast ice is strongly related to the advance of the seaward landfast ice edge slie into waters around 18 m deep isobaths near this depth are a good approximation for midseason landfast ice extent comparison with work from the 1970s suggests a reduced presence of landfast ice in this region of the arctic due to later formation and earlier breakup this will likely lead to an increase in coastal erosion and may also have profound effects upon subsistence activities which are intimately linked to the timing of marine mammal migration patterns interannually landfast ice formation correlates with the incursion of pack ice into coastal waters suggesting that the later mean date of formation in recent years may be related to the increasingly northward location of the perennial sea ice edge the timing of breakup correlates well with onset of thawing air temperatures analysis of regional data shows a multidecadal trend toward earlier thaw onset which suggests that the observed change in breakup dates may be part of a longer term trend

climate at high obliquity abstract the question of climate at high obliquity is raised in the context of both exoplanet studies e g habitability and paleoclimates studies evidence for low latitude glaciation during the neoproterozoic and the snowball earth hypothesis states of high obliquity are distinctive in that for 54 the poles receive more solar radiation in the annual mean than the equator opposite to the present day situation in addition the seasonal cycle of insolation is extreme with the poles alternatively facing the sun and sheltering in the dark for months the novelty of our approach is to consider the role of a dynamical ocean in controlling the surface climate at high obliquity which in turn requires understanding of the surface winds patterns when temperature gradients are reversed to address these questions a coupled ocean atmosphere sea ice gcm configured on an aquaplanet is employed except for the absence of topography and modified obliquity the set up is earth like two large obliquities 54 and 90 are compared to today s earth value 23 5 three key results emerge at high obliquity 1 despite reversed temperature gradients mid latitudes surface winds are westerly and trade winds exist at the equator as for 23 5 although the westerlies are confined to the summer hemisphere 2 a habitable planet is possible with mid latitude temperatures in the range 300 280 k and 3 a stable climate state with an ice cap limited to the equatorial region is unlikely we clarify the dynamics behind these features notably by an analysis of the potential vorticity structure and conditions for baroclinic instability of the atmosphere interestingly we find that the absence of a stable partially glaciated state is critically linked to the absence of ocean heat transport during winter a feature ultimately traced back to the high seasonality of baroclinic instability conditions in the atmosphere

tibetan tectonic evolution inferred from spatial and temporal variations in post collisional magmatism cenozoic magmatism on the tibetan plateau shows systematic variations in space and time that must be considered in models concerning tibetan tectonic evolution after the india asia collision which started in the early tertiary and terminated the gangdese arc magmatism in the lhasa terrane made of the southern tibetan plateau widespread potassium rich lavas and subordinate sodium rich basalts were generated from f50 to 30 ma in the qiangtang terrane of northern tibet subsequent post collisional magmatism migrated southwards producing ultrapotassic and adakitic lavas coevally between f26 and 10 ma in the lhasa terrane then potassic volcanism was renewed to the north and has become extensive and semicontinuous since f13 ma in the western qiangtang and songpan ganze terranes such spatial temporal variations enable us to elaborate a geodynamic evolution model which depicts when and how the indian continental lithospheric mantle started thrusting under asia by involving rollback and breakoff of the subducted neo tethyan slab followed by removal of the thickened lhasa lithospheric root we propose that only after the lithospheric removal which occurred at f26 ma could the indian mantle lithosphere have commenced its northward underthrusting and henceforth served as a pivotal control to the himalayan tibetan orogenesis consequently the tibetan plateau is suggested to have risen diachronously from south to north whereas the southern part of the plateau may have been created and maintained since the late oligocene the northern plateau would have not attained its present day elevation and size until the mid miocene when the lower part of the western qiangtang and songpan ganze lithospheres began to founder owing to the push of the underthrust indian mantle lithosphere d 2004 elsevier b v all rights reserved

on the seasonality of the hadley cell abstract the annual march of the climatological mean meridional circulations mmcs in the ncep ncar reanalyses is dominated by two components of roughly comparable mean squared amplitude 1 a seasonally invariant pair of hadley cells with rising motion centered near and just to the north of the equator and subsidence in the subtropics and 2 a seasonally reversing sinusoidally varying solsticial cell with ascent in the outer tropics of the summer hemisphere and subsidence in the outer tropics of the winter hemisphere the meridional structure and seasonal evolution of the solsticial cell are suggestive of a close association with the monsoons these results are consistent with previous analyses of the mean meridional circulation based on radiosonde data

magmatic hydrothermal processes within an evolving earth iron oxide copper gold and porphyry cu mo au deposits iron oxide copper gold iocg deposits formed by magmatic hydrothermal fluids mh iocg share many similarities with but have important differences from porphyry cu mo au porphyry deposits mh iocg deposits predominantly occur in precambrian rocks are fe oxide rich and have volumetrically extensive high temperature alteration zones whereas porphyry deposits occur almost exclusively in phanerozoic rocks are fe sulfide rich and have narrower high temperature alteration zones we propose that these deposit types are linked by common subduction modified magmatic sources but that secular changes in oceanic sulfate content and geothermal gradients at the end of the precambrian caused a transition from the predominance of s poor arc magmas and associated s poor mh iocg systems to s rich arc magmas and associated s rich porphyry deposits in the phanerozoic phanerozoic mh iocg and rare precambrian porphyry deposits are explained by local or periodic fluctuations in oceanic oxidation state and sulfate content or remobilization of previously subduction modified lithosphere in post subduction tectonic settings

age and genesis of the pusangguo skarn cu dominated polymetallic deposit gangdese metallogenic belt tibet abstract the pusangguo skarn cu polymetallic deposit is a newly discovered deposit in the western gangdese porphyry copper belt gpcb the orebodies are hosted along the contact between limestone of the early cretaceous takena formation and a biotite granodiorite intrusion the mineralization is associated with the biotite granodiorite intrusion and is divided into four stages 1 a prograde skarn stage 2 a pre ore retrograde skarn stage 3 a quartz sulfides stage and 4 a post ore carbonate stage these skarns have the characteristics of both calcareous and manganese skarns the mineral compositions of the garnets and pyroxenes indicate that the ore forming environment gradually evolved from a high temperature oxidative and alkaline environment into a low temperature and reductive environment the sulfur and lead isotopic compositions of the ore sulfides suggest a magmatic source for the sulfur and a crust dominated source for the ore forming metals the hydrogen and oxygen isotopic compositions of the quartz and garnets in the ores indicate that the ore forming fluids may have originated from a mixture of magmatic and meteoric fluids the geochronological data show that both the magmatism 14 8 0 4 ma and mineralization 13 2 0 7 ma occurred during the miocene in a post collisional tectonic setting the ore forming biotite granodiorite is a high k calc alkaline i type granite and has an adakitic geochemical affinity similar to the gangdese miocene adakites which were generated from thickened lower crust in a post collisional tectonic setting the low positive ehf t values 0 to 2 6 young two stage hf model zircon ages and whole rock geochemical and lead isotopic compositions indicate that the magma of the ore related biotite granodiorite may have originated from the thickened lower crust in the lhasa terrane

geology and geochronology of naruo large porphyry breccia cu deposit in the duolong district tibet abstract the duolong district is located in the south qiangtang terrane of tibet and is the most significant ore cluster within the bangongco nujiang metallogenic belt duolong contains one giant three large and two medium to small sized porphyry epithermal breccia copper deposits and several other mineralized porphyry bodies all deposits are closely associated with early cretaceous 123 115 ma intermediate felsic intrusions naruo is a poorly studied porphyry breccia copper deposit in the north of the duolong district hydrothermal alteration surrounding the ore bearing granodiorite at naruo is characterized by an inner potassic zone and an outer propylitic zone overlapped locally by minor phyllic and argillic alteration assemblages a detailed paragenetic study has identified five distinct hydrothermal veins m a b c d within the porphyry system hydrothermal b veins are strongly related to copper mineralization strong propylitic alteration is also observed throughout the hydrothermal breccias identified at naruo sandstone breccia diorite bearing breccia and granodiorite bearing breccia were identified according to the distribution and composition of clasts u pb zircon dating has determined the ages of the ore bearing granodiorite 121 6 1 3 ma and a barren intrusion 115 5 1 1 ma within the porphyry system diorite clasts 122 3 0 9 ma and later diorite matrix 120 5 1 0 ma in the hydrothermal breccia system suggesting that with the exception of the late barren intrusion they all belong to the same mineralizing event at duolong the geological and geochemical evidence presented in this study suggest that the porphyry and breccia systems may have originated from the same magma source but are now spatially independent

effects of rotation rate and seasonal forcing on the itcz extent in planetary atmospheres the authors study a wide range of atmospheric circulations with an idealized moist general circulation model to evaluate the mechanisms controlling intertropical convergence zone itcz migrations they employ a zonally symmetric aquaplanet slab ocean of fixed depth and force top of atmosphere insolation to remain fixed at the pole for an eternal solstice simulation and also vary seasonally for a range of rotation rates keeping all other parameters earth like for rotation rates e 8 and slower a transient maximum in zonal mean precipitation appears at the summer pole however the itcz associated with the ascending branch of the hadley circulation lies at 60 the authors assess how widely used predictors of the itcz position perform in this wide parameter space standard predictors based on different estimates of the hadley cell s poleward extent are correlated with but overestimate off equatorial itcz locations interestingly in the eternal solstice case for earth s rotation rate the itcz remains at subtropical latitudes even though the lower level moist static energy maximizes at the summer pole while seemingly at odds with convective quasi equilibrium arguments this can happen because at earth s rotation rates the thermal stratification set in convective regions can only be communicated within the tropics where temperature gradients are constrained to be weak the authors therefore develop an understanding of the itcz s position based on top of atmosphere energetics and the boundary layer momentum budget and argue that friction and pressure gradient forces determine the region of maximum convergence offering a modified dynamical perspective on the monsoon like seasonal weather patterns of terrestrial planets

the transition from porphyry to epithermal style gold mineralization at ladolam lihir island papua new guinea a reconnaissance study the exceptionally large gold resource at ladolam 1 300 metric tons of gold lihir island resulted from the transition of an early stage low grade porphyry gold system to a low sulfidation epithermal gold event this transition was probably triggered by rapid decompression during the partial slope failure of luise stratovolcano and accompanied by the ingress of seawater the original porphyry stage is indicated by remnant hydrothermal breccia clasts of strongly biotite magnetite altered monzodiorite with disseminated pyrite chalcopyrite and poorly developed pyrite quartz stockwork veins the breccias are overprinted by biotite magnetite alteration and their matrix is strongly mineralized with disseminated auriferous pyrite the breccias are cut by late stage epithermal quartz chalcedony illite adularia pyrite veins and associated illite adularia alteration that locally contain bonanza gold grades of up to 120 g t isotope data suggest a magmatic source of sulfur in the gold bearing fluids at ladolam electronic supplementary material to this paper can be obtained by using the springer link server located at http dx doi org 10 1007 s00126 001 0230 y

coastal retreat and shoreface profile variations in the canadian beaufort sea the coastline of the southern canadian beaufort sea consists primarily of unconsolidated bluffs although the sea is ice free for 3 months of the year and wave energy is restricted by pack ice the coast is undergoing regional retreat with erosion rates as high as 10 m a 1 in some locations simple and multiple regression analyses were carried out to determine the degree of correlation between the mean retreat rate measured at various locations and the different parameters that may control shoreline recession sediment texture ground ice content cliff height wave energy and shoreface gradient revealed medium to poor correlation with erosion rates showing that the recessive evolution of the coastline can not be explained solely by wave induced and subaerial processes the comparison of nearshore echo sounding records from 1987 with bathymetry from 1971 showed substantial erosion up to 1 m of the submarine profile between 12 and 15 m of water there is strong evidence that this erosion has been caused by sea ice gouging on the seafloor from depths of 5 to 9 m accretion has taken place possibly induced by ice push processes and inshore of the 5 m isobath wave and current erosion of the shoreface has occurred these results suggest that the erosion of the inner shelf by ice gouging drives the erosion observed inshore on the coastal bluffs and nearshore zone as the shoreface profile strives for a state of dynamic equilibrium

cenozoic k rich adakitic volcanic rocks in the hohxil area northern tibet lower crustal melting in an intracontinental setting it is generally accepted that the cenozoic potassic volcanic rocks of northern tibet were derived from a lithospheric mantle source here we report new chronological geochemical and isotopic data for the miocene ca 18 15 ma k rich adakitic volcanic rocks from the hohxil area of the songpan ganzi block in northern tibet we contend that these rocks were generated by partial melting of the mafic lower crust in an intracontinental setting unrelated to subduction of oceanic crust the hohxil rocks exhibit high sr y and la yb ratios high sr and la contents but low yb and y concentrations similar to adakites formed by slab melting associated with subduction however their relatively low e n d values 2 09 to 4 58 high 8 7 sr 8 6 sr 0 7072 0 7075 th u th ba and rb ba ratios and distinctive potassium enrichments k 2 o na 2 o are very different from the composition of typical adakites in addition those k rich adakitic rocks with the highest sio 2 contents 61 wt exhibit the lowest 8 7 sr 8 6 sr ratios and highest e n d values and are the oldest cenozoic volcanic rocks exposed in the songpan ganzi block suggesting that they were derived neither directly from a mantle source nor by differentiation of a shoshonitic magma taking into account the composition of lower crustal mafic xenoliths in tibet as well as the tectonic and geophysical evidence we conclude that the hohxil adakitic magmas were produced by partial melting of amphibole bearing eclogites with a k rich mafic bulk composition in the lower part 55 km of the thickened northern tibetan crust partial melting of the lower crust may have been triggered by dehydration release of fluids from sedimentary materials in the southward subducted continental lithosphere

contribution of mantle components within juvenile lower crust to collisional zone porphyry cu systems in tibet most porphyry cu mo au deposits are found in magmatic arcs worldwide and are associated with hydrous high fo2 calc alkaline magmas derived from a mantle wedge that was metasomatized by the fluids from a subducted oceanic slab recently such deposits have been documented as occurring widely in collisional settings where they are associated with potassic magmas generated during the collisional process but the genesis of the fertile magmas and the mechanism of metallic enrichment remain controversial here we present new geochemical and sr nd hf isotopic data from the post collisional fertile and barren porphyries of the miocene gangdese porphyry belt in the tibetan orogen an orogen formed by the collision of india and asia in the early cenozoic both types of porphyry are characterized by high k2o contents and have geochemical affinities with adakite but the fertile magmas were most likely derived from the melting of a thickened juvenile mafic lower crust formed by the underplating of earlier asthenospheric melts at the base of crust whereas the derivation of the barren magmas involved variable amounts of old lower crust in tibet the melting of sulfide bearing phases in the juvenile mantle components of the tibetan lower crust probably provided cu au and s to the fertile magmas the breakdown of amphibole during melting at the source released the fluids necessary for the formation of the porphyry cu deposits in tibet the thickened crust up to 70 80 km due to collision is thought to be responsible for a decrease in the fo2 of the fertile magmas during their ascent to the upper crust thus preventing the generation of more porphyry cu au and epithermal au deposits in this collisional zone

u pb and re os geochronological evidence for the jurassic porphyry metallogenic event of the xiongcun district in the gangdese porphyry copper belt southern tibet prc abstract two large copper gold porphyry deposits no i and no ii were discovered in the last decade within the xiongcun district which is located along the south margin of the gangdese porphyry copper belt these two deposits are hosted by two porphyries that were emplaced into the early jurassic volcano sedimentary rock sequences of the xiongcun formation the porphyries are both quartz diorite bodies with one of these distinguished by its large quartz eyes our new molybdenite re os and zircon u pb dates for no i and no ii deposits and host intrusions reveal the presence of two middle jurassic ore forming episodes in the xiongcun district the no ii deposit is related to the 181 175 ma quartz diorite porphyry and formed ca 172 6 2 1 ma the no i deposit is related to the 167 161 ma quartz diorite porphyry with large quartz eyes and formed ca 161 5 2 7 ma geochronological and geochemical features of the jurassic ore bearing porphyries in the xiongcun district indicate that the porphyry copper gold mineralization formed in an island arc setting which is related to the northward subduction of the neo tethys oceanic plate these observations in combination with geochronological data reported at gangdese porphyry copper belt indicated four major metallogenic events existed in the gangdese porphyry copper belt at 172 161 ma 65 48 ma 40 23 ma and 20 12 ma we have identified two mineralization episodes comprise middle jurassic metallogenic event 172 161 ma in the xiongcun district which provides new constrains for the middle jurassic metallogenic event in the gangdese porphyry copper belt

the relationship between epithermal and porphyry hydrothermal systems within the tavua caldera fiji the shoshonitic tavua caldera in viti levu fiji defines the volcanic center of the tavua volcano the caldera occurs at the intersection of the northeast trending viti levu lineament and a northwest trending lineament locally manifested by the nasivi shear zone fault related subsidence which contributed to the evolution of the tavua caldera the central inner caldera and the structural preparation for mineralization occurred on these two regional fault systems and local concentric normal faults a continuum of mineralization and alteration styles is evident along the nasivi shear zone caused by a northwesterly evolution and migration of intrusion and hydrothermal activity porphyry style subeconomic copper gold mineralization and associated alteration occurred at depth in the nasivi shear zone at the center of the caldera approximately contemporaneously with the development of the inner caldera acid sulfate style epithermal alteration and minor gold mineralization immediately postdated and were superimposed on this porphyry style alteration farther northwest along the nasivi shear zone transitional ac d sulfate to adularia sericite epithermal alteration and minor gold mineralization occur variation in hydrothermal style is considered to reflect an increasing distance from magmatic source combined with an increasing proportion of meteoric water mixing with the hydrothermal fluid northwest of this system at the intersection of the nasivi shear zone and the viti levu lineament the 4 4 m oz adularia sericite style epithermal emperor gold mine occurs high level epithermal mineralization and alteration occur 4 km northeast of the emperor gold mine in an area which has a demonstrable present day hydraulic connection with the mine variations in fluid inclusion homogenization temperatures from the mine indicate that fluids entered the structurally prepared mine area via the nasivi shear zone emperor epithermal mineralization postdated magmatism by approximately 0 4 ma stable isotope data the presence of albitic alteration beneath mineralization and high salinities within the mine indicate that the emperor northern prospects hydrothermal system is related to an intrusion into the nasivi shear zone beneath the mine this is consistent with a demonstrable northwesterly migration of intrusions with time along the nasivi shear zone albitic phyllic and propylitic assemblages comprise the main porphyry style alteration types in the caldera center calculated temperatures of formation of chlorite in these assemblages indicate that these three alteration facies formed in the temperature range 270 degrees to 340 degrees c in good agreement with available fluid inclusion data and the stability ranges of the mineral assemblages kubler indices of illite likewise imply that these assemblages formed at relatively high temperature epithermal lode alteration mineralization at emperor is shown by fluid inclusion data to have formed around 200 degrees c kubler indices of illite confirm that this alteration formed at significantly lower temperatures than did porphyry alteration and that chlorites in the emperor mine have calculated temperatures of formation mostly below 200 degrees c high level epithermal alteration occurring northeast of emperor is inferred to have formed at even lower temperatures

regime change behavior during asian monsoon onset abstractas the itcz moves off the equator on an aquaplanet the hadley circulation transitions from an equinoctial regime with two near symmetric significantly eddy driven cells to a monsoon like regime with a strong thermally direct cross equatorial cell intense low latitude precipitation and a weak summer hemisphere cell dynamical feedbacks appear to accelerate the transition this study investigates the relevance of this behavior to monsoon onset by using primitive equation model simulations ranging from aquaplanets to more realistic configurations with earth s continents and topography a change in the relationship between itcz latitude and overturning strength is identified once the itcz moves poleward of about 7 monsoon onset is associated with off equatorial ascent in regions of non negligible planetary vorticity and this is found to generate a vortex stretching tendency that reduces upper level absolute vorticity in an aquaplanet this causes a transition to the cross equatorial therm

geochronology and geochemistry of the bangpu mo cu porphyry ore deposit tibet abstract we have determined the ages of the ore bearing bangpu porphyries and the age of formation of an associated mo cu ore deposit using the la icpms zircon u pb and the molybdenite re os methods zircons from two adamellite porphyries give ages of 14 07 0 08 ma and 14 96 0 16 ma and zircons from a diorite porphyry give an age of 15 30 0 25 ma the ages of three molybdenite samples are 14 96 0 23 ma 15 08 0 23 ma and 16 61 0 23 ma light rare earth elements lree are enriched in the ore bearing adamellite which is shoshonitic and peraluminous while heavy rare earth elements hree and y are strongly depleted indicating an adakitic affinity in contrast to other deposits formed in the post collisional gangdese belt the rb sr ratio in bangpu is high demonstrating more crustal contamination the geochemistry of the ore bearing diorite porphyry differs significantly from that of the ore bearing adamellite the diorite porphyry displays a reverse slope in its ree rare earth element pattern and is shoshonitic and metaluminous the geochronology and geochemistry of both ore bearing rocks indicate that the bangpu ore deposit is related to two different source regions in a post collisional environment the metallic minerals formed mainly during the older diorite porphyry stage but they were recycled and reactivated during adamellite magmatism the sources of the mineralising fluids and the evolution of the northern gpcb gangdese porphyry copper belt where the bangpu ore deposit is located are more complex than in the southern gpcb

bulldozing and resuspension of shallow shelf sediment by ice keels implications for arctic sediment transport trajectories abstract the orientations and termination directions of newly formed ice gouges identified in a 5 year study of two offshore corridors in eastern harrison bay alasaka indicate a bimodal distribution of sediment transport directions west southwest and southeast due to ice keel bulldozing the westerly sediment transport results from the dominant westward drift of sea ice and ocean currents whereas the southeasterly transport results from episodic fall storms with winds from the northwest transport associated with ice gouging occurs by bulldozing and by resuspension during the bulldozing processes fine grained m sediment transport may also involve transport of resuspended sediment by intensified currents found near the grounded floes leaving behind a lag of coarser sediments in harrison bay about 6000 m 3 of sediment is reworked each year by ice gouging for every square kilometer of the seafloor between water depths of 5 and 18 m over 50 of this sediment is moved onshore to the southeast whereas 35 is moved alongshore to the west the actual distance of sediment transport is dependent upon the grain size of the seafloor sediment coarse grained material 63 m is bulldozed as far as 7 m in the direction of ice movement whereas sediment finer than 63 m may be transported by intensified bottom currents up to 80 times this distance

baroclinic eddies and the extent of the hadley circulation an idealized gcm study thehadleycirculationhaswidenedoverthepast30years thiswideninghasbeenqualitativelyreproduced in general circulation model gcm simulations of a warming climate comprehensive gcm studies suggest this widening may be caused by a poleward shift in baroclinic eddy activity yet the limited amplitude of the climate change signals analyzed so far precludes a quantitative comparison with theories this study uses two idealized gcms one with and one without an active hydrologic cycle to investigate changes in the extent of the hadley circulation over a wide range of climates the climates span global mean temperatures from 243 to 385k and equator to pole temperature contrasts from 12 to 100k baroclinic eddies control the extent of the hadley circulation across most of these climates a supercriticality criterion that quantifies the depth of baroclinic eddies relative to that of the troposphere turns out to be a good indicator of where baroclinic eddies become deep enough to terminate the hadley circulation the supercriticality depends on meridional temperature gradients and an effective stability that accounts for the effect of convective heating on baroclinic eddies astheequator to poletemperaturecontrastweakensortheconvectivestaticstabilityincreases convective heating increasingly influences the thermal stratification of the troposphere and the supercriticality consistent with the supercriticality criterion the hadley circulation contracts as meridional temperature gradients increase and it widens as the effective static stability increases the former occurs during el nino and may account for the observed hadley circulation contraction then the latter occurs during global warming

the response of the itcz to extratropical thermal forcing idealized slab ocean experiments with a gcm using a comprehensive atmospheric gcm coupled to a slab mixed layer ocean experiments are performed to study the mechanism by which displacements of the intertropical convergence zone itcz are forced from the extratropics the northern extratropics are cooled and the southern extratropics are warmed by an imposed cross equatorial flux beneath the mixed layer forcing a southward shift in the itcz the itcz displacement can be understood in terms of the degree of compensation between the imposed oceanic flux and the resulting response in the atmospheric energy transport in the tropics the magnitude of the itcz displacement is very sensitive to a parameter in the convection scheme that limits the entrainment into convective plumes the change in the convection scheme affects the extratropical tropical interactions in the model primarily by modifying the cloud response the results raise the possibility that the response of tropical precipitation to extratropical thermal forcing important for a variety of problems in climate dynamics such as the response of the tropics to the northern hemisphere ice sheets during glacial maxima or to variations in the atlantic meridional overturning circulation may be strongly dependent on cloud feedback the model configuration described here is suggested as a useful benchmark helping to quantify extratropical tropical interactions in atmospheric models

intraplate type magmatism in a continent island arc collision zone porgera intrusive complex papua new guinea cogenetic volatile rich alkali basalts and gabbros hawaiites and mugearites occur in the late miocene age epizonal porgera intrusive complex which is temporally and spatially associated with rich gold silver mineralization the least evolved rocks show enrichments in light rare earth elements la yb sub cn 15 19 and other incompatible elements e g ba la approx 8 10 la nb approx 0 6 0 7 sr nd approx 25 characteristic of intraplate alkalic basalts and have isotopic compositions sup 87 sr sup 86 sr approx 0 7035 epsilon sub nd approx 6 sup 206 pb sup 204 pb approx 18 66 sup 207 pb sup 204 pb approx 15 56 sup 298 pb sup 204 pb approx 38 55 consistent with derivation from a time averaged depleted mantle source the porgera intrusive complex was emplaced at 6 ma in jurassic cretaceous shelf facies sedimentary rocks near the edge of the australasian plate intrusion appears to have occurred in a back arc environment during subduction of an oceanic microplate segment on two sides beneath the continental margin and an island arc we suggest that this unusual tectonic setting promoted partial melting of asthenospheric source materials that were perhaps modified by deep 150 km subduction processes to form alkalic magmas with intraplate character

examination of the deep levels of an island arc evidence from the tonsina ultramafic mafic assemblage tonsina alaska the tonsina ultramafic and mafic assemblage located in the northern chugach mountains along the trans alaska crustal transect tact is interpreted to be the remains of a magma chamber that crystallized at the base of a mature interoceanic island arc it is one of a number of isolated masses of peridotite and gabbro that comprise the basal part of the peninsular terrane along the northern fringe of the chugach mountains in southern alaska these masses the roots of a mid jurassic island arc have been uplifted along the north dipping border ranges fault system and crop out sporadically for 700 km from kodiak island to eastern alaska the tonsina ultramafic mafic assemblage is a layered sequence whose basal rocks are dunite and harzburgite 1 km thick overlain by a narrow zone of websterite and clinopyroxenite above these ultramafic rocks is a 5 to 6 km thick layered gabbro that is garnet bearing at its base textural relationships and chemical trends throughout the series of dunite pyroxenite garnet bearing gabbro and spinel gabbronorite point to an origin by igneous accumulation from a hydrous tholeiitic parent magma at deep levels i e at the base of a mature island arc before being emplaced along the border ranges fault the rocks were crystallized from a melt at pressures between 9 5 and 11 kbar 28 to 33 km depth and equilibrated at temperatures of 800 to 90o c based on phase relationships in the cumulus assemblages pyroxene thermobarometry and olivine spinel thermometry transitional sequences from ultramafic to mafic rocks are characterized by increasingly al rich pyroxenes and spinels or garnet and the absence of olivine plagioclase assemblages deep level fractionation of al poor phases such as olivine and pyroxene at moderately high pressures and temperatures resulted in concentration of al in the evolving magma as crystallization continued the phases recorded this alumina concentration and the resultant gabbros are al rich this observed trend supports the theory of high alumina basalt generation by fractional crystallization of olivine and pyroxene comparison of the tonsina rocks with aleutian lower crustal xenoliths as well as other exposures of arc related lower crustal magma chambers in japan pakistan and western north america indicates that this process is a common mechanism for island arc magma evolution at depth

ore genesis of skarn mineralization in continental collision orogens a case study from the pusangguo co bearing cu pb zn deposit in tibet abstract the gangdese belt in southern tibet is renowned for hosting numerous collision related porphyry cu mo deposits the recent discovery of pusangguo co bearing cu pb zn skarn deposits is the first reported co bearing deposit in gangdese belt as well as in tibet researches on the geological and metallogenic characteristics of pusanguo indicate that this co bearing cu pb zn deposit was formed in a collisional setting we document mineralogy fluid inclusion studies and h o s pb isotope systematics of the skarns and ores of the pusangguo deposit intense endoskarn alteration dominated by green and pink epidote and quartz is widely disseminated throughout the host granodiorite as well as in veinlets exoskarn alteration is characterized by the presence of prograde grandite andradite32 100 and grossular0 68 with minor pyroxene diopside0 46 hedenbergite38 74 and johannsenite1 62 retrograde epidotes from both the endoskarn and exoskarn show a narrow range of fe3 fe3 al ratios 0 25 0 34 ore minerals include chalcopyrite sphalerite and galena with lesser pyrite pyrrhotite bornite co minerals and falkmanite four paragenetic stages of skarn formation and ore deposition are recognized in the pusangguo cu pb zn deposit prograde stage i retrograde stage ii early sulfide stage iii and late sulfide stage iv fluid inclusions in all stages are dominated by two phase liquid rich type l inclusions during the major phase of mineralization stage iii fluid inclusions formed at low temperatures 180 240 c and with low salinities 0 5 8 6 wt nacl equiv this is in contrast to other skarn deposits within the gangdese porphyry copper belt e g the zhibula jiama and bangpu deposits that formed from highly saline fluids at high temperatures hydrogen oxygen isotopic values obtained from garnet epidote quartz and calcite are dh2o 179 to 213 and 18oh2o 9 1 to 9 3 whereas s pb isotopic values for sphalerite chalcopyrite pyrite and galena are 34s 1 0 to 1 2 estimated 34sfluid 0 05 such data are indicative of ore forming fluids and metals that originated from a magmatic hydrothermal source related to granodiorite emplacement during the miocene post continental collision extensive mixing between the magmatic hydrothermal and meteoric fluids led to the co precipitation of cu pb and zn minerals from low salinity fluids at relatively low temperatures the pusangguo co bearing cu pb zn skarn deposit shows consistent features with other porphyry related skarn deposits in the gangdese belt except for relatively low temperature and salinity the research is of great significance for understanding cobalt as an associated mineral in the gangdese sulfide skarn deposit in tibet

the fault controlled skarn w mo polymetallic mineralization during the main india eurasia collision example from hahaigang deposit of gangdese metallogenic belt of tibet abstract the hahaigang w mo polymetallic skarn deposit is located in the central eastern part of gangdese tectono magmatic belt in lhasa terrane tibet the deposit was discovered in 2007 with currently proven 46 million tons of wo 3 ores 12 million tons of mo ores and 1 31 million tons of combined cu pb zn ores at an average grade of 0 20 wo 3 0 07 mo 0 026 cu 0 49 pb and 3 1 zn ore bodies occur in veins or disseminations and are confined within the ne striking dalong fault zone which is hosted by the lower permian pangna group of dominantly quartz sandstone and slate several granitic plutons are exposed in the area or known from drill holes ages of these granitic plutons are determined by using zircon u pb la icp ms method for example the biotite monzogranite yields a 206 pb 238 u 207 pb 238 u concordia age of 58 66 0 90 ma and a weighted mean 206 pb 238 u age of 57 02 0 42 ma the granite porphyry yields a 206 pb 238 u 207 pb 238 u concordia age of 109 1 8 9 ma and a weighted mean 206 pb 238 u age of 114 0 2 6 ma the biotite monzogranite yields a weighted mean 206 pb 238 u age of 56 1 1 1 ma re os isochron age of 63 2 3 2 ma from 5 molybdenite samples collected from the w mo skarn ores is also obtained in this study the zircon u pb and molybdenite re os geochronological data suggest that the w mo mineralization was not temporally associated with any of the dated igneous plutons however the molybdenite re os age of 63 2 3 2 ma indicates that the w mo mineralization might have occurred during the main india eurasia collision that was initiated around 65 ma microprobe analysis of ilvaite that occurs in two generations in the w mo skarn ores reveals a close relationship to ca fe f rich hydrothermal fluids which were probably derived from deeply seated magmas we suggest that ascent of the fluids was strictly controlled by the ore controlling dalong fault zone and that chemical interaction and metasomatism between the fluids and the lower permian pangna quartz feldspathic host rocks produced the ilvaite and the w mo polymetallic skarn deposit during the main india eurasia collision although the majority of the polymetallic deposits in the gangdese belt are reported to be either pre or post main collision it is evident from this study that the main collision also produced w mo polymetallic mineralization within the belt

improved general circulation models of the martian atmosphere from the surface to above 80 km we describe a set of two new generation general circulation models of the martian atmosphere derived from the models we originally developed in the early 1990s the two new models share the same physical parameterizations but use two complementary numerical methods to solve the atmospheric dynamic equations the vertical resolution near the surface has been refined and the vertical domain has been extended to above 80 km these changes are accompanied by the inclusion of state of the art parameterizations to better simulate the dynamical and physical processes near the surface boundary layer scheme subgrid scale topography parameterization etc and at high altitude gravity wave drag in addition radiative transfer calculations and the representation of polar processes have been significantly improved we present some examples of zonal mean fields from simulations using the model at several seasons one relatively novel aspect previously introduced by wilson 1997 is that around northern winter solstice the strong pole to pole diabatic forcing creates a quasi global angular momentum conserving hadley cell which has no terrestrial equivalent within such a cell the coriolis forces accelerate the winter meridional flow toward the pole and induce a strong warming of the middle polar atmosphere down to 25 km this winter polar warming had been observed but not properly modeled until recently in fact thermal inversions are generally predicted above one and often both poles around 60 70 km however the mars middle atmosphere above 40 km is found to be very model sensitive and thus difficult to simulate accurately in the absence of observations

reduced fluids in porphyry copper gold systems reflect the occurrence of the wall rock thermogenic process an example from the no 1 deposit in the xiongcun district tibet china abstract high fo2 conditions characterize the majority of global porphyry copper deposits and contain highly oxidized minerals such as magnetite and anhydrite in contrast the no 1 porphyry cu au deposit in the xiongcun district tibet china has abundant pyrrhotite reduced fluids ch4 co2 and a relative lack of highly oxidized minerals which are indicative of low fo2 conditions scanning electron microscopy fluid inclusion c h o he ar isotopes and whole rock organic carbon contents and isotope analysis were used to constrain the evolution of ore forming fluid the origin of ch4 and metal deposition mechanisms for the no 1 deposit the he ar isotopic compositions 3he 4he 0 11 0 96 ra 40ar 36ar 418 7 2920 2 suggest that the ore forming fluids predominantly derived from crust source with minor mantle input the h o isotopic analysis results 18oh2o 1 8 to 5 2 d 106 to 89 9 indicate that the ore forming fluids were derived from a magmatic source that mixed with some meteoric waters the element compositions of zircons and fluid melt inclusions from the mineralized middle jurassic quartz diorite porphyry reveal that the primary magma was characterized by high log fo2 nno conditions the quartz diorite porphyry intruded into the carbon bearing wall rocks produced abundant ch4 by thermal decomposition of organic matter 13cch4 26 3 to 28 5 which changed the redox state of the porphyry copper system from oxidized to reduced condition ore elements were deposited via fluid boiling as a consequence of the rapid decrease in temperature and pressure

a genetic linkage between subduction and collision related porphyry cu deposits in continental collision zones the genesis of continental collision related porphyry cu deposits pcds remains controversial the most common hypothesis links their genesis with magmas derived from subduction modified arc lithosphere however it is unclear whether a genetic linkage exists between collision and subduction related pcds here we studied jurassic subduction related cu au and miocene collision related cu mo porphyry deposits in south tibet the jurassic pcds occur only in the western segment of the jurassic arc which has depleted mantle like isotopic compositions e g 87 sr 86 sr i 0 7041 0 7048 e nd t as high as 7 5 and e hf t as high as 18 by contrast no jurassic pcds have been found in the eastern arc segment which is isotopically less juvenile e g 87 sr 86 sr i 0 7041 0 7063 e nd t 4 5 and e hf t 12 these results imply that incorporation of crustal components during underplating of jurassic magma induced copper accumulation as sulfides at the base of the eastern jurassic arc inhibiting pcd formation at this time miocene pcds are spatially confined to the jurassic arc and the giant miocene pcds cluster in its eastern segment where no jurassic pcds occur this suggests that the arc segment barren for subduction related pcds could be fertile for collision related pcds miocene ore forming porphyries have young hf model ages and sr nd hf isotopic compositions overlapping with those of the jurassic rocks in the eastern segment whereas contemporaneous barren porphyries outside the jurassic arc have abundant zircon inheritance and crustlike sr nd hf isotopic compositions these data suggest that remelting of the lower crustal sulfide bearing cu rich jurassic cumulates triggered by cenozoic crustal thickening and or subsequent slab break off led to the formation of giant miocene pcds the spatial overlap and complementary metal endowment between subduction and collision related magmas may be used to evaluate the mineral potential for such deposits in other orogenic belts

monsoons as eddy mediated regime transitions of the tropical overturning circulation monsoons are generally viewed as planetary scale sea breeze circulations caused by contrasts in the thermal properties between oceans and land surfaces that lead to thermal contrasts upon radiative heating1 2 but the radiative heating evolves gradually with the seasons whereas the onset of monsoon precipitation and the associated circulation changes such as reversal of surface winds occur rapidly3 4 here we use reanalysis data to show that the onset of the asian monsoon marks a transition between two circulation regimes that are distinct in the degree to which eddy momentum fluxes control the strength of the tropical overturning circulation rapid transitions of the circulation between the two regimes can occur as a result of feedbacks between large scale extratropical eddies and the tropical circulation5 using simulations with an aquaplanet general circulation model we demonstrate that rapid eddy mediated monsoon transitions occur even in the absence of surface inhomogeneities provided the planet surface has sufficiently low thermal inertia on the basis of these results we propose a view of monsoons in which feedbacks between large scale extratropical eddies and the tropical circulation are essential for the development of monsoons whereas surface inhomogeneities such as land sea contrasts are not

tectonic evolution of the qinghai tibet plateau abstract the qinghai tibet plateau composed of several continental slivers within the eastern tethyan domain is one of the pivotal sites to examine to better understand the theory of plate tectonics and the orogenic evolution on earth this plateau is generally inferred to be a collage of several continental blocks that rifted from gondwanaland and subsequently accreted to the asian continent however recent recognition of over twenty ophiolite melange zones and their associated island arcs indicates that the traditional model of tectonic evolution requires revision based on 177 recently finished 1 250 000 scale geological maps and related studies we summarize the main tectonic context of the qinghai tibet plateau and propose a new integrated model to account for the new findings the complex orogen of the immense qinghai tibet plateau consisting of multiple island arc basin systems that developed at different stages while surrounded by the north china yangtze tarim and indian plates is emphasized the entire orogen surrounded by suture zones that mark the locations of oceanic closure is investigated by examining i the first order tectonic units and ophiolitic melanges including arc arc continent collision zones and ii their internally enclosed blocks as the second order tectonic units therefore the qinghai tibet plateau is divided into three major orogenic systems namely from northeast to southwest the early paleozoic qinling qilianshan kunlunshan qin qi kun the late paleozoic triassic qiangtang sanjiang and the late paleozoic to cenozoic gangdese himalaya orogenic systems which are separated by the kangxiwa muzitagh maqin mianxian and the bangong shuanghu changning menglian sutures respectively we propose that the formation and evolution of the qinghai tibet plateau to have been intrinsically related to those of the eastern tethys recorded by the longmu co shuanghu ophiolite melange zone the southern qiangtang paleozoic accretionary arc basin system the bangong nujiang suture zone and their associated composite island arc basin systems the present day bangong shuanghu changning menglian suture system marks the final closure of the tethyan ocean the yarlung zangbo ocean opened as a back arc basin in response to the southward subduction of the tethyan ocean lithosphere in the middle triassic and closed as a result of the india asia collision at the end of cretaceous followed by the northward indention of the indian plate that resulted in significant intra continental deformation and plateau uplift in the cenozoic

differential exhumation histories between qulong and xiongcun porphyry copper deposits in the gangdese copper metallogenic belt insights from low temperature thermochronology abstract the qulong and xiongcun porphyry deposits in the gangdese copper molybdenum metallogenic belt gcmb are both giant porphyry deposits they were located in the eastern and the central gcmb about 400 km apart and were generated in the miocene and the jurassic respectively in order to reveal why the two asynchronous deposits could occur in the same metallogenic belt the in situ zircon u pb zircon and apatite fission track zft and aft and apatite u th he ahe dating were conducted their associated exhumation amounts and histories were compared and finally the prospecting significance was evaluated ahe ages of the qulong deposit indicate it underwent an amount of at least 1 3 2 8 km exhumation since 13 8 13 1 ma however aft data and modeling results reveal that the xiongcun deposit experienced two stages of rapid cooling at 20 13 ma and 13 8 ma respectively the temperature decreases from 160 c to 40 c after 20 ma and the corresponding exhumation amount is about 4 6 km integrated the regional thermochronological data exhumation amounts of 4 6 km and

geology of the jiama porphyry copper polymetallic system lhasa region china abstract the jiama deposit located in the eastern part of the well known gangdese metallogenic belt on the tibetan plateau is the largest porphyry cu polymetallic system in the region with the largest exploration budget and is economically viable in the gangdese belt to undergo large scale development the deposit is well preserved and has experienced little erosion the proven resources of the deposit are 7 4 mt cu 0 6 mt mo 1 8 mt pb zn 6 65 moz au and 360 32 moz ag the results presented in this paper are based on geological and tectonic mapping geological logging and other exploration work performed by members of the jiama exploration project team over a period of 6 years we propose that the jiama porphyry cu polymetallic system is composed of skarn cu polymetallic hornfels cu mo porphyry mo cu and distal au mineralization the development of skarn cu polymetallic orebodies at the jiama deposit was controlled mainly by the contact zone between porphyries and marbles an interlayer detachment zone and the front zone of a gliding nappe structure the hornfels cu mo and porphyry mo cu orebodies were controlled mainly by a fracture system related to intrusions and the distal au mineralization resulted from late stage hydrothermal alteration on the basis of field geological logging optical microscopy and chemical analysis we verify that the alteration zones in the jiama deposit include potassic phyllic propylitic and argillic alteration with a local lithocap as well as endoskarn and exoskarn zones the endoskarn occurs mainly as epidote alteration in quartz diorite porphyry and granite porphyry and is cut by massive andradite veins the exoskarn includes garnet pyroxene and wollastonite skarn in which the mineralogy and mineral chemical compositions display an outward zonation with respect to the source porphyry from the proximal skarn to the intermediate skarn to the distal skarn the garnet pyroxene ratio varies from 20 1 to 10 1 to 5 1 the garnet color varies from red brown to brown green to green yellow and the average composition of garnet varies from ad80 1gr18 9 sp py 1 0 to ad76 3gr23 sp py 0 7 to ad59 5gr39 5 sp py 1 0 respectively the pyroxene is not as variable in composition as the garnet and is primarily light green to white diopside with a maximum hedenbergite content of 20 and an average composition of di88 6hd8 9jo2 5 from the proximal skarn to the intermediate skarn to the distal skarn the mineralization changes from cu mo to cu mo to pb zn cu au ores respectively the wollastonite skarn displays no zonation and hosts mainly bornite mineralization the cu and mo mineralization is closely related to the potassic and phyllic zones in the porphyry hornfels zircons from four mineralized porphyries yield u pb ages of 15 96 0 5 ma 15 72 0 14 ma 15 59 0 09 ma and 15 48 0 08 ma the re os ages of molybdenite from the skarn hornfels and porphyry are 15 37 0 15 ma 14 67 0 37 ma and 14 66 0 27 ma respectively the present results are consistent with the findings of previous research on fluid inclusions isotopes and other such aspects on the basis of the combined evidence we propose a porphyry cu polymetallic system model for the jiama deposit and suggest a regional exploration strategy that can be applied to prospecting for porphyry skarn mineralization in the lhasa area

geology of the post collisional porphyry copper molybdenum deposit at qulong tibet porphyry deposits are usually thought to form from subduction related calc alkaline magmas in magmatic arc settings although some porphyry deposits also occur in post collisional extensional settings the post collisional deposits remain poorly understood here we describe the igneous geology alteration mineralogy and mineralization history of qulong a newly discovered porphyry cu mo deposit in southern tibet that belongs to the post collisional class the deposit is associated with miocene monzogranite granodiorite intrusions and is hosted partly by jurassic andesitic dacitic volcanic rocks the deposit contains 7 1 mt cu and 0 5 mt mo metal the 19 5 ma granodioritic monzogranitic rongmucuola pluton with diorite enclaves is the earliest miocene intrusive unit it was intruded by a regularly shaped stock p porphyry and then thin dikes x porphyry of monzogranite at about 17 7 ma the main cu mo mineralization is associated with the p porphyry a barren diorite porphyry intruded the p and x porphyries around 15 7 ma petrologic trends of the intrusions suggest that the miocene intrusions have similar origins and probably formed by fractionation of a deeper magma chamber emplacement of miocene porphyries controlled by the qulong anticline is in direct response to the rapid uplift erosion of the gangdese arc batholiths in southern tibet miarolitic cavities and unidirectional solidification textures key evidence for volatile separation have been recognized in the p and x porphyries respectively early potassic alteration characterized by quartz k feldspar anhydrite pervades the p porphyry and rongmucuola pluton laterally this alteration grades into quartz biotite anhydrite k feldspar which affects all miocene intrusions except the latest dioritic porphyry wall rocks of rongmucuola pluton and jurassic andesitic dacitic volcanics within 1 1 5 km of the porphyries are dominated by pervasive potassic alteration an outer halo of propylitic alteration epidote chlorite calcite extends up to 2 km away from the deposit feldspar destructive alteration sericite chlorite clay minerals has overprinted most of the potassic and part of the propylitic alteration the alteration is strongly pervasive in the interior of the porphyry bodies and occurs as vein halos away from the porphyry bodies the earliest quartz k feldspar alteration and veins are barren whereas approximately 60 of the cu reserves is associated with slightly later quartz biotite anhydrite alteration barren assemblages are related to irregular quartz k feldspar anhydrite veins which are truncated by the x porphyry cu sulfide bearing assemblages are associated with discontinuous chalcopyrite biotite and continuous quartz anhydrite chalcopyrite molybdenite veins deposition of cu mo with abundant anhydrite occurred during or between emplacement of closely related porphyries from high temperature magmatically derived fluids and was probably caused by the disproportionation of so

jurassic u pb and re os ages for the newly discovered xietongmen cu au porphyry district tibet prc implications for metallogenic epochs in the southern gangdese belt the gangdese belt in southern tibet includes the cretaceous to early eocene gangdese continental magmatic arc a newly recognized middle jurassic magmatic arc and post gangdese arc magmatism of miocene age that is well known for its association with important copper molybdenum deposits although the tectonic history of the gangdese continental arc is relatively well constrained the tectonomagmatic setting of the jurassic and miocene magmatic events was poorly constrained previously geochronological analysis of samples from the xietongmen copper gold porphyry district has identified a previously unknown metallogenic event related to middle jurassic arc magmatism the occurrence of several metallogenic epochs within the southern gangdese belt highlights the importance of systematic metallogenic studies of this belt

modeling tropical convergence based on the moist static energy budget abstract the vertically integrated moist static energy equation provides a convenient starting point for the construction of simple models of the time mean low level convergence in the tropics a vertically integrated measure of the moist static stability the gross moist stability proves to be of central importance minima in this quantity mark the positions of the tropical convergence zones we argue that the positions of these minima are determined by the time mean moisture field which is in turn closely tied to the time mean surface temperature

quaternary sedimentation of the alaskan beaufort shelf influence of regional tectonics fluctuating sea levels and glacial sediment sources abstract the offshore stratigraphy of the quaternary gubik formation of arctic alaska has been studied on high resolution seismic profiles with a maximum sub seafloor penetration of about 100 m in general marine transgressive subunits of the gubik formation are wedge shaped on the shelf thickening slightly seaward to the shelf break beyond which they are offset by landslides and slumps beneath the eastern third of the alaskan beaufort shelf active folding has created two persistent structural depressions the eastern and western wedge terranes in which the wedge morphology is especially well developed the youngest transgressive marine wedge which was deposited in such a way as to fill these depressions leaving a generally flat present day shelf surface is inferred to be late wisconsin or younger in age because it overlies a prominent disconformity interpreted to have been formed during the late wisconsin glacial sea level minimum the thickness of this youngest wedge unit a locally exceeds 40 m on the outer shelf yet apparently relict gravel deposits collected from its seabed surface indicate that the depositional rate is presently quite low on the middle and outer shelf lithologies of the gravels are exotic to alaska but similar to suites exposed in the canadian arctic islands these observations suggest a depositional scenario in which the retreating laurentide ice sheet shed sediment laden icebergs from the canadian arctic islands into the arctic ocean following the late wisconsin glacial maximum these bergs were then rafted westward by the beaufort gyre and grounded on the alaskan shelf by northeasterly prevailing winds especially large numbers of bergs accumulated in the wedge terrane embayments created as sea level rose and melted there filling the embayments with their sedimentary cargo as glacial retreat slowed depositional rates on the shelf dwindled this mode of deposition in the alaskan beaufort wedge terranes may be typical of early post glacial transgressive phases throughout quaternary time it has resulted in the preservation of disconformities that apparently formed during glacioeustatic lowstands and whose seaward termination depths appropriately corrected may yield estimates of lowstand magnitudes knowledge of global sea level fluctuations back through the sangamon interglacial oxygen isotope stage 5e and possible correlations with dated onshore deposits have facilitated a tentative correlation of major disconformities in the beaufort sea record with major 18 o enrichment maxima in the oxygen isotope curve back through stage 8 in this tentative scheme close similarities between the two data sets occur both in magnitudes and in numbers of fluctuations intervening between major correlation points further testing of the quaternary depositional model suggested here and of the resulting sea level curve awaits the collection and dating of core samples from the beaufort wedge terranes

description and numerical model of pleistocene iceberg scours and ice keel turbated facies at toronto canada near toronto in southern ontario canada glaciolacustrine muds and shoreface sands of late pleistocene age early wisconsin glaciation 45 ka were deformed by icebergs ploughing the floor of an ancestral ice dammed lake ontario outcrop mapping and a ground penetrating radar survey identify ice scour structures up to 4 m deep up to 10 m wide and at least several hundred metres long with high standing lateral margins berms ploughing of lake floor sediment is also recorded by ice keel turbated facies in which storm deposited shoreface sands have been mechanically mixed with underlying muds palaeoenvironmental information for one prominent scour such as water depths estimates of the mass and draft of the ice mass and scouring forces can be determined using a simple finite element numerical model a mass of 0 04 million tonnes a keel draft of between 10 and 30 m and a scouring force of 5 103 kn is consistent with scouring by a small iceberg bergy bit or pressure ridged lake ice the former is preferred because of the considerable depth of the scour and the overall glacial depositional setting estimates of ice mass dimensions agree with independent reconstructions of palaeobathymetry no greater than 20 m derived from storm deposited hummocky and swaley cross stratified facies that fill the scour data presented here aid the identification of ice scour structures and turbated sediment in the ancient record despite being a common process on modern high latitude shelves descriptions of ancient structures and facies are few

increasing magmatic oxidation state from paleocene to miocene in the eastern gangdese belt tibet implication for collision related porphyry cu mo au mineralization in the gangdese belt of southern tibet paleocene eocene magmas record the final stage of neo tethyan subduction and are associated with few economic porphyry deposits in contrast magmas formed during later stages of the india asia collision in the oligo miocene are associated with several large porphyry cu mo au deposits especially in the eastern part of the belt 89 e 93 e in a previous study we showed that these oligocene miocene magmas were more hydrous than the earlier paleocene eocene magmas in this study we show that the later magmas were also more oxidized paleocene eocene rocks from the eastern gangdese belt are characterized by low zircon ce4 ce3 ratios mostly 50 32 3 141 9 average 74 3 30 1 n 33 and higher f o2 values fmq 0 8 2 9 average 1 8 0 8 n 6 estimated from magnetite ilmenite pairs estimates of magmatic oxidation state from amphibole compositions also show an increase from fmq 1 2 to 2 1 average 1 6 0 2 n 40 in the paleocene eocene to 2 0 to 2 8 average 2 5 0 2 n 58 in the miocene sparse whole rock fe3 fe2 ratios show the same general trend the amphibole results are systematically shifted to higher fmq compared to data from magnetite ilmenite pairs but their trend is internally consistent the higher oxidation states and water contents of miocene igneous rocks from the eastern gangdese belt may explain their unique association with large porphyry deposits in tibet because it has been shown from other studies that the potential for hydrous calc alkaline magmas to transport au is maximized near fmq 1 0 and cu at higher f o2 fmq 1 0 in comparison the somewhat less oxidized and less hydrous paleocene eocene magmas would have been less fertile for the formation of such deposits we suggest that these differences reflect the tectonomagmatic evolution of the cenozoic gangdese collisional orogen from early collision related magmatism in the paleocene eocene to late collisional magmatism in the miocene asthenospheric upwelling following slab breakoff in the miocene caused interaction between mantle derived magmas and previously subduction modified tibetan lithosphere the resultant evolved magmas had high oxidation states and water contents which are favorable properties for the subsequent formation of magmatic hydrothermal porphyry cu mo au deposits

extent of hadley circulations in dry atmospheres the subtropical terminus of the hadley circulation is interpreted as the latitude poleward of which vertical wave activity fluxes meridional eddy entropy fluxes become sufficiently deep to reach the upper troposphere this leads to a sign change of the upper tropospheric divergence of meridional wave activity fluxes convergence of meridional eddy angular momentum fluxes and marks the transition from the tropical hadley cell to the extratropical ferrel cell a quantitative formulation for determining the depth of vertical wave activity fluxes and thus the terminus of the hadley circulation is proposed based on the supercriticality a measure of the slope of isentropes the supercriticality assumes an approximately constant value at the terminus of the hadley circulation in a series of simulations with an idealized dry general circulation model however it is unclear how to generalize this supercriticality based formulation to moist atmospheres

ice erosion of a sea floor knickpoint at the inner edge of the stamukhi zone beaufort sea alaska abstract in 1981 and 1982 detailed bathymetric and side scan sonar surveys were made of an area of the sea floor north of prudhoe bay alaska to study the changing characteristics of the seabed at the inner boundary of the stamukhi zone the coast parallel zone of grounded ice ridges that occurs in water depths between 15 and 50 m in the arctic the fathograms and sonographs resolved 10 cm features and electronic navigation gave relocations accurate to about 10 m year after year an ice boundary develops at the inner edge of the stamukhi zone where major shear and pressure deformation occur in about the same location associated with this ice boundary the bathymetry shows a pronounced break in slope the knickpoint on the shelf profile at about 20 m depth the 2 3 m high knickpoint is cut in a consolidated gravelly mud of pre holocene age a well defined gravel and cobble shoal a few meters high usually occurs at the inshore edge of the knickpoint the sonograph mosaic shows that seaward of the knickpoint ice gouges saturate the sea floor and are well defined inshore the gouges are fewer in number and are poorly defined on the records few gouges can be traced from the seaward side of the knickpoint across the shoals to the inshore side of the knickpoint studies of ice gouging rates in two seabed corridors that cross the stamukhi zone reveal the highest rates of gouging seaward of the knickpoint we believe that the knickpoint results from ice erosion at the inner boundary of the stamukhi zone intensified currents associated with this boundary winnow away fine sediments ice bulldozing and currents shape the shoals which perch atop the knickpoint the knickpoint helps to limit ice forces on the seabed inshore of the stamukhi zone

porphyry copper deposit formation by sub volcanic sulphur dioxide flux and chemisorption the processes that create economic grade accumulations of metals above magma chambers are unclear high temperature laboratory experiments show that rapid reactions between magmatic gases and earth s crust can trigger efficient metal deposition

re os systematics of sulfides chalcopyrite bornite pyrite and pyrrhotite from the jiama cu mo deposit of tibet china abstract the jiama cu mo deposit one of the largest skarn porphyry deposits in china currently being mined is located in the eastern part of the gangdese porphyry cu belt tibet china chalcopryite and bornite concentrate in the skarn ore zone at jiama with molybdenite concentration whereas molybdenite occurs in the porphyry ore zone and hornfels ore zone relatively by itself cu and mo have different distributions in the porphyry ore zone with cu concentrating above the porphyry and with mo inside of porphyry intrusion the direct re os dating on cu sulfides will be helpful for us to understand the cu and mo mineralization at jiama this study is the first attempt to use the re os isotope system to directly date chalcopyrite bornite pyrite and pyrrhotite at jiama the contents of re common os and 187 os in these four sulfides are 0 145 32 9 ppb 0 0022 0 0669 ppb and 0 0003 0 0118 ppb respectively they have very low ratios of 187 os to common os 0 09 0 55 therefore they are different from low level highly radiogenic llhr sulfides the plot of 187 re 188 os and 187 os 188 os did not yield an isochron age based on the scatter points even if age of 15 ma based on molybdenite re os age is used to calculate the initial 187 os 188 os the ratios show a wide range and low ratios of 187 re 188 os 187 os 188 os the geology suggests the cu and mo mineralizations at jiama are both related to porphyry intrusions and the cu mineralization should also have occurred in miocene based on the molybdenite re os age and porphyry zircon u pb ages

on large scale circulations in convecting atmospheres the dominant thinking about the interaction between large scale atmospheric circulations and moist convection holds that convection acts as a heat source for the large scale circulations while the latter supply water vapour to the convection we show that this idea has led to fundamental misconceptions about this interaction and offer an alternative paradigm based on the idea that convection is nearly in statistical equilibrium with its environment according to the alternative paradigm the vertical temperature profile itself rather than the heating is controlled by the convection which ties the temperature directly to the subcloud layer entropy the understanding of large scale circulations in convecting atmospheres can therefore be regarded as a problem of understanding the distribution in space and time of the subcloud layer entropy we show that the subcloud layer entropy is controlled by the sea surface temperature the surface wind speed and the large scale vertical velocity in the convecting layer and demonstrate how the recognition of this control leads to a simple physically consistent view of large scale flows ranging from the hadley and walker circulations to the 30 50 day oscillation in particular we argue that the direct effect of convection on large scale circulations is to reduce by roughly an order of magnitude the effective static stability felt by such circulations and to damp all of them

new 40ar 39ar and u th he dating for the zhunuo porphyry cu deposit gangdese belt southern tibet implications for pulsed magmatic hydrothermal processes and ore exhumation and preservation understanding the magmatic hydrothermal cooling and exhumation history is fundamental for investigating porphyry deposit formation and preservation in the zhunuo porphyry copper deposit of southern tibet hydrothermal biotite related to early potassic alteration likely formed from 14 7 0 3 ma zircon u pb age to 14 23 0 13 ma molybdenite re os age and cooled below the closure temperature for 40ar 39ar system at 13 91 0 05 ma hydrothermal biotite 40ar 39ar plateau age the 40ar 39ar spectra of another hydrothermal biotite sample that was selectively replaced by chlorite record the timing of late phyllic alteration at 13 53 0 08 ma zhunuo hydrothermal alteration and mineralization likely ceased at 13 09 0 07 ma based on the 40ar 39ar plateau age of the fresh magmatic biotite crystals the biotite 40ar 39ar systematics have not been reset by 12 0 ma post mineralization granite porphyry within ore district which is likely due to rapid cooling and or absence of fluid exsolution for the granite porphyry in combination with previous zircon u pb and molybdenite re os data two pulses of magmatic hydrothermal activity from 14 78 0 11 to 14 44 0 05 ma and from 14 23 0 13 to 13 53 0 08 ma can be recognized at zhunuo u th he thermochronological data on zircon and apatite from the inter mineralization monzogranite porphyry suggest three cooling events characterized by first stage magmatic cooling at a rate of 1850 c m y and two later phases of uplift and exhumation at 70 c m y and 10 c m y approximately 1 4 km thickness of materials have been removed from the zhunuo miocene monzogranite porphyry during uplift and erosion including a large volume of ore

dynamic ice wallow relief of northern alaska s nearshore abstract contour maps with 0 5 m depth interval were prepared for a small area seaward of reindeer island a barrier island in the beaufort sea alaska by repeated surveys with very accurate navigation and very close trackline spacing the maps reveal numerous closed depressions and mounds about 50 to 100 m in diameter and 2 to 3 m in relief presumably related to grounded ice floes common in the area year round some of the features were obliterated over the course of three seasons while new ones formed although the depressions resemble kettles they are formed by very different mechanisms we believe that these bedforms represent erosion and deposition caused by a intensified flow around stationary ice floes serving as obstacles and b pulsating currents generated by vertical oscillation or rocking motions of grounded floes in a seaway because sediment transport occurs around the ice not where it directly touches the sea floor the depressions are much larger than the base of the acting floes ice wallow bedforms although not found everywhere are characteristic of arctic nearshore regions with non cohesive sediments and most likely occur in other ice stressed coastal environments in differing degrees the bedforms studied here are highly active and must be considered in planning nearshore construction activities

volcanogenic massive sulfide deposits and the missing complement to the calc alkaline trend evidence from the jurassic talkeetna island arc of southern alaska

magmatic and structural controls on the tonnage and metal associations of collision related porphyry copper deposits in southern tibet abstract porphyry cu deposits pcds in continental collision settings are new targets for modern mineral exploration and the relationships among locations tectonic environments tonnages and metal associations are not yet understood the gangdese pcd belt formed in a typical continental collision setting between the indian and asian plates in the present study new elemental and isotopic data from fertile porphyries cu mo cu mo pb zn and cu w mo and barren coeval and pre ore porphyries of the gangdese pcd belt were combined with previously published data to provide new insights into the metallogenesis of pcds in continental collision settings factors that contributed to the formation of collision related pcds in the lhasa terrane tibet include the nature of the magmatic sources magmatic evolution during ascent through the crust and regional tectonics these pcds were not formed by partial melting of ancient lower crust instead attributed to the subduction of neo tethyan oceanic slab arc magmas and slab derived fluids resulted in the accumulation of sulfides and hydrous minerals at the base of the continental crust partial melting of this refertilized juvenile lower crust produced oxidized water and cu rich magmas with the potential to produce large pcds although only under favorable regional tectonic conditions regional compressional stress is not conducive to the formation of large magma chambers so the collision related adakite like magmas that developed during the early stages of india asia collision commonly lack alteration and associated mineralization in contrast relatively large magma chambers can develop in extensional settings within the continental collision orogen which facilitates the formation of large and giant pcds small cu w mo deposits formed during the transition between compressional and extensional settings thus the regional tectonic setting and structural framework of the host orogen influenced the tonnage of pcds the metal associations of collision related pcds in the lhasa terrane were controlled mainly by the extent of assimilation of the upper crust and the degree of fractional crystallization of the ore forming magmas lead zinc mineralization in cu mo deposits is attributed to the assimilation of metapelites by fertile magmas in the upper crust and highly fractionated fertile magmas are required to produce w mineralization

petrology and geochemistry of post collisional middle eocene volcanic units in north central turkey evidence for magma generation by slab breakoff following the closure of the northern neotethys ocean the eocene volcano sedimentary units of northern anatolia are confined into a narrow zone trending parallel to the intra pontide and izmir ankara erzincan sutures along which the northern branch of the neotethys ocean was closed during a period between late maastrichtian and paleocene the middle eocene formations overlie both the imbricated and highly deformed units of the suture zone which are paleocene or older in age as well as the formations of adjacent continental blocks with a regional disconformity therefore they can be regarded to be post collisional these units are composed of subaerial to shallow marine sedimentary beds i e the orencik formation at the base and a subaerial volcanic unit i e the hamamozu formation in the middle and at the top this sudden facies change from marine to subaerial environment in the middle eocene is a common phenomenon across northern turkey implying that a regional uplift event occurred possibly across the suture zone before the initiation of the volcanism during lutetian the middle eocene lavas span the whole compositional range from basalts to rhyolites and display a calc alkaline character except for alkaline to mildly alkaline lavas from the top of the sequence all lavas display a distinct subduction signature our geochemical data indicate that calc alkaline lavas were derived from a subduction modified source whereas alkaline to mildly alkaline lavas of the late stage were possibly sourced by an enriched mantle domain magmas evolved in magma chambers emplaced possibly at two different crustal levels magmas in deeper 13 km and possibly larger chambers fractionated hydrous mafic minerals e g amphibole and biotite two pyroxenes and plagioclase and assimilated a significant amount of crustal material intermediate to acid calc alkaline lavas and pyroclastics were derived from these chambers magmas in the shallower chambers on the other hand 12 km crystallized anhydrous mineral assemblages assimilated little or no crustal material and fed basic to intermediate lavas in the region both deep and shallow chambers were periodically replenished by mafic magmas we argue that a slab breakoff model explains better than any alternative model i why the volcanism during the middle eocene was confined into a rather narrow belt along the suture zone ii why it initiated almost contemporaneous with a regional uplift after the continental collision event iii why it postdated arc volcanism along the pontides in the north by 15 20 my iv why it assimilated significant amount of crustal material and v why alkalinity of lavas increased in time

subduction arc reversal and the origin of porphyry copper gold deposits in island arcs the tertiary quaternary porphyry copper gold deposits of the southwest pacific rim particularly in luzon mainland papua new guinea bougainville guadalcanal and fiji mostly formed after a reversal of arc polarity where this reversal has not occurred e g new zealand and japan porphyry copper gold deposits are absent or scarce hence older porphyry copper gold deposits may be useful clues to tectonic history enrichment of magmas in gold may have resulted from two stage melting the tectonic setting of porphyry copper gold deposits differs from that inferred for volcanic hosted massive sulfide deposits

the nature and timing of crustal thickening in southern tibet geochemical and zircon hf isotopic constraints from postcollisional adakites abstract rising as the roof of the world the tibetan plateau is now underlain with the thickest continental crust on earth how and when was this crust formed which would have exerted pivotal controls to the formation of the plateau has long been an issue of hot debates this paper reports zircon u pb ages and hf isotope data for postcollisional 30 9 ma adakites in the southern lhasa terrane southern tibet a comparative analysis of whole rock rare earth element geochemistry and zircon hf isotopes between the adakites and associated gangdese igneous rocks suggests that the tibetan crust underwent a major phase of tectonic thickening between ca 45 and 30 ma in the region the lower part of the thickened crust consisted prevailingly of mafic lithologies which we argue to have resulted from intense basaltic underplating and subsequent remelting that took place during the late cretaceous and eocene time related to the neotethyan subduction processes including breakoff of the subducted slab at ca 50 ma in the early stage of the india asia collision these processes were responsible for not only the juvenile crust formation but also for the creation of a thermally softened lithosphere in the southern lhasa terrane the indentation of india consequently caused distributed lithospheric thickening with formation of an orogenic root beneath southern tibet root foundering during the oligocene gave rise to the adakitic magmatism regional topographic uplift and onset of northward underthrusting of the indian plate that has since played a key role in forming the entire tibetan plateau

bottom currents sedimentation and ice sheet retreat facies successions on the mac robertson shelf east antarctica abstract measurements of water turbidity currents seafloor sediment samples and geophysical data document the sedimentary processes and the late quaternary sedimentary history of a continental shelf valley system on the east antarctic continental margin the valley is up to 1200 m in depth and strikes across the shelf it is interpreted as having formed by glacial erosion processes on the outer shelf sill of the valley northwestward offshore currents with speeds of up to 0 47 m s referenced to 100 cm above the seabed were recorded over a 10 month period such currents are competent to initiate bedload transport of medium sand and formation of small ripple sized bedforms which explains the occurrence of ripple cross bedding observed in some x radiographs of cores a nepheloid layer with concentrations of up to 3 5 mg l was noted at two stations and the available evidence suggests that density flows resulting from the sea ice brine rejection mechanism are competent to entrain shelf bottom sediments episodically sediment cores taken from this environment document the succession of facies resulting from retreat of glacial ice from the shelf during the holocene transgression under low sea level glacial conditions the ice sheet is interpreted to have been grounded on the outer shelf where it deposited a grounding line moraine rising sea level and global warming caused the ice shelf to retreat leaving a sub ice shelf glacial marine mud this is overlain by a laminated to massively bedded ice rafted debris rich facies interpreted as being deposited at the glacial calving front above this is a siliceous mud and ooze facies having a basal age of about 10 000 years bp characterising modern open marine conditions two types of iceberg keel marks on the outer part of the shelf valley are recognised from sidescan sonographs one relict arcuate type formed during the retreat of the ice shelf prior to 10 000 years bp and a second modern elongate type that is presently being formed near the shelf break by iceberg grounding processes this facies succession has been described from other antarctic shelf environments and is probably representative of facies deposited during holocene glacial retreat from the antarctic shelf

the lhasa terrane record of a microcontinent and its histories of drift and growth article i nfo the lhasa terrane in southern tibet has long been accepted as the last geological block accreted to eurasia before its collision with the northward drifting indian continent in the cenozoic but its lithospheric architecture drift and growth histories and the nature of its northern suture with eurasia via the qiangtang terrane remain enigmatic using zircon in situ u pb and lu hf isotopic and bulk rock geochemical data of mesozoic early tertiary magmatic rocks sampled along four north south traverses across the lhasa terrane we show that the lhasa terrane has ancient basement rocks of proterozoic and archean ages up to 2870 ma in its centre with younger and juvenile crust phanerozoic accreted towards its both northern and southern edges this finding proves that the central lhasa subterrane was once a microcontinent this continent has survived from its long journey across the paleo tethyan ocean basins and has grown at the edges through magmatism resulting from oceanic lithosphere subduction towards beneath it during its journey and subsequent collisions with the qiangtang terrane to the north and with the indian continent to the south zircon hf isotope data indicate significant mantle source contributions to the generation of these granitoid rocks e g 50 90 0 70 and 30 100 to the mesozoic magmatism in the southern central and northern lhasa subterranes respectively we suggest that much of the mesozoic magmatism in the lhasa terrane may be associated with the southward bangong nujiang tethyan seafloor subduction beneath the lhasa terrane which likely began in the middle permian or earlier and ceased in the late early cretaceous and that the significant changes of zircon ehf t at 113 and 52 ma record tectonomagmatic activities as a result of slab break off and related mantle melting events following the qiangtang lhasa amalgamation and india lhasa amalgamation respectively these results manifest the efficacy of zircons as a chronometer u pb dating and a geochemical tracer hf isotopes in understanding the origin and histories of lithospheric plates and in revealing the tectonic evolution of old orogenies in the context of plate tectonics

ring structures made by shore ice in a muddy tidal flat st lawrence estuary canada abstract large size ring sedimentary structures occurring in a muddy tidal flat on the south shore of the st lawrence estuary are reported for the first time they range from 15 to 50 m in diameter and occur as isolated features or in groups of 3 to 5 in plan they form well defined circles scattered throughout the higher mud flat some structures are characterized by an inner ring and by a chaotic surface of alternating micro ridges and depressions the outer surrounding mud ridge is a few decimeters high and wide 25 60 cm ring structures occur at the emplacement of icefoot pustules and are related to vertical pressures exerted on the substrate by ice blocks moved up and down by tide motion icefoot pustules form every winter but not necessarily at the same location it is likely that the related ring structures can be fossilized and preserved in the sedimentary sequence consequently they can be of assistance in palaeoenvironmental interpretation

sea ice scouring on the inner shelf of the southeastern canadian beaufort sea abstract about 2200 ice scours were observed and analyzed over a distance of approximately 500 km on the inner shelf of the southeastern canadian beaufort sea ice scours were divided into two types based on their morphology multiple scours consisting of a series of parallel scours and ridges and single scours single scours are the dominant type representing more than 85 of the total observations the mean scour depth and width are 0 3 m and 11 m respectively but scour depths of more than 2 m and a scour width up to 345 m were documented the magnitude of ice scouring processes increases with water depth in water depths less than 10 m less than 25 of the seafloor surface is reworked by ice scours this percentage increases significantly seaward of the 10 m isobath being more than 75 in water depths in excess of 12 m a break in the seabed slope at about 10 to 12 m water depth marks a boundary between a nearshore zone moderately influenced by ice processes and an outer zone affected by intense ice scouring this morphological boundary could be due to intense erosion by the keels of pressure ice ridges at the inner edge of a zone of grounded ice ridges most of the observed ice scours appear to be reworked especially in water depths of less than 10 m and represent small scale sediment sinks inshore of the 10 m isobath scour reworking is believed to be mainly due to frequent bottom disturbance by wave orbital currents and mean near bottom flows during the open water season scour orientations show that the dominant motion of ice during scouring events is east or west which is subparallel to the bathymetric contours and coastline scour terminal push mounds however suggest a dominant east to southeast movement that may contribute to onshore sediment transport during ice push events

early cretaceous subduction related adakite like rocks of the gangdese belt southern tibet products of slab melting and subsequent melt peridotite interaction abstract the limited geochronology and geochemistry data available for the early cretaceous igneous rocks of the southern gangdese belt southern tibet has resulted in the proposal of conflicting geodynamic models for the generation of the widespread cretaceous igneous rocks in the middle and northern parts of the belt to explore this issue we present shrimp u pb zircon data and geochemical and sr nd pb hf isotopic data for the mamen andesites from the southern margin of the gangdese belt the mamen andesites emplaced at 136 5 ma are sodic na2o k2o 1 2 2 3 and have geochemical characteristics typical of adakites i e high al2o3 high la yb ratios and sr contents low y and hree contents and positive eu anomalies except for high cr ni and mgo contents the andesites have initial 87sr 86sr t ratios of 0 70413 0 70513 positive end t values of 3 7 5 8 and 206pb 204pb t ratios of 18 37 18 51 207pb 204pb t ratios of 15 59 15 65 and 208pb 204pb t ratios of 38 43 38 72 in situ hf isotopic analyses of zircons that had previously been dated by shrimp yielded positive initial ehf t values ranging from 11 0 to 15 5 a model calculation using trace element and sr nd pb isotopic data indicates that several percent of subducted sediment is required to generate the mamen andesites which were derived via the partial melting of subducted neo tethyan slab morb sediment fluid and subsequently hybridized by peridotite in the mantle wedge our data indicate that the neo tethyan oceanic crust was subducted northward beneath the gangdese belt during the early cretaceous at a high angle our results are inconsistent with a tectonic model that advocates the low angle or flat slab subduction of neo tethyan oceanic crust in generating the widespread cretaceous magmatism recorded in the gangdese belt

hot paleocene eocene gangdese arc growth of continental crust in southern tibet abstract the 1600 km long gangdese magmatic belt features extensive paleocene eocene i type intrusive rocks and coeval volcanic successions which can be divided into group i 69 53 ma group ii 53 49 ma and group iii 49 43 ma corresponding to neo tethyan slab rollback neo tethyan slab breakoff and ongoing indian asian collision respectively the magmas from these three groups show significant variations in geochemical and isotopic compositions which provide the information of the growth of continental crust in southern tibet the most voluminous magmatism in the gangdese belt occurred during 53 49 ma high zircon saturation temperature up to 800 c and ti in zircon temperature up to 980 c estimations suggest there is a period of thermal anomaly during 53 49 ma starting from 53 ma magmas have increased k2o contents and their zircons have decreased th u ratios and y and yb contents zircons from group ii have the most heterogeneous hf isotopic compositions ehf t 5 3 to 15 1 these are evident of ingress of asthenosphere mantle in the arc extensive crustal melting and magma mixing magma underplating during this time is the main mechanism for the growth of continental crust with the indian asian collision going on the magmas in group iii show high th y and la yb ratios and k2o contents but significantly low tzr and t ti zr values mostly below 750 c these features suggest the water fluxed melting of early arc residues occurred in the late stage of growth of continental crust the crust has been thickened and nearly mature at this stage this study has great implication on understanding of growth of continental crust in orogenic belts

tectonic magmatic and metallogenic evolution of the tethyan orogen from subduction to collision abstract this paper reviews the tectonic magmatic and metallogenic history of the tethyan orogen from the carpathians to indochina focus is placed on the formation of porphyry cu mo au deposits as being the most characteristic mineral deposit type formed during both subduction and collisional processes in this region relatively little is known about the history of the paleotethys ocean which opened and closed between gondwana and eurasia in the paleozoic and few ore deposits are preserved from this period the neotethyan ocean opened in the permian early triassic as the cimmerian continental fragments the cores of turkey iran tibet and indochina rifted from the northern gondwana margin and drifted northwards these microcontinents docked with the eurasian margin at various points in the mesozoic and cenozoic and formed a complex archipelago involving several small back arc basins and remnants of the paleotethyan ocean the main neotethyan ocean and these smaller basins were largely eliminated by collision with india and africa arabia in the early eocene and early mid miocene respectively although neotethyan subduction continues beneath the hellenic arc and the makran the majority of porphyry type deposits are found in association with neotethyan subduction mainly in the mesozoic and paleogene and syn to post collisional events in the mid paleogene to neogene they are found throughout the orogen but some sections are particularly well endowed including the carpathians balkans rhodopes eastern turkey lesser caucasus nw iran se iran sw pakistan southern tibet and se tibet indochina other sections that appear barren may reflect deeper levels of erosion young sedimentary cover or lack of exploration although there may also be real reasons for low prospectivity in some areas such as minimal subduction e g the western mediterranean region or lithospheric underthrusting as proposed in western tibet over the last decade improved geochronological constraints on the timing of ore formation and key tectonic events have revealed that many porphyry deposits that were previously assumed to be subduction related are in fact broadly collision related some forming in back arc settings in advance of collision some during collision and others during post collisional processes such as orogenic collapse and or delamination of subcontinental mantle lithosphere while the formation of subduction related porphyries is quite well understood collisional metallogeny is more complex and may involve a number of different processes or sources these include melting of orogenically thickened crust previously subduction modified lithosphere including metasomatized mantle underplated mafic rocks or lower crustal arc plutons and cumulates or upwelling asthenosphere e g in response to delamination slab breakoff back arc extension or orogenic collapse the most fertile sources for syn and post collisional porphyry deposits appear to be subduction modified lithosphere because these hydrated lithologies melt at relatively low temperatures during later tectonomagmatic events and retain the oxidized and relatively metalliferous character of the original arc magmatism unusually metallically enriched lithospheric sources do not seem to be required but the amount of residual sulfide phases in these rocks may control metal ratios e g cu au in subsequent magmatic hydrothermal ore deposits relatively au rich deposits potentially form in these settings as observed in the carpathians e g rosia montan turkey kisladag copler and iran sari gunay dalli although the majority of syn and post collisional porphyries are cu mo rich and resemble normal subduction related deposits e g in the gangdese belt of southern tibet this similarity extends to the associated igneous rocks which being derived from subduction modified sources largely retain the geochemical and isotopic character of those original arc magmas while still retaining a broadly calc alkaline character these rocks may extend to mildly alkaline shoshonitic compositions and may display adakite like trace element signatures high sr y and la yb ratios reflecting melting of deep crustal garnet amphibolitic sources but they are otherwise hard to distinguish from normal subduction related magmas small post collisional mafic alkaline volcanic centers are common throughout the orogen but for the most part appear to be barren however similar rocks in other post subduction settings around the world are associated with important alkalic type porphyry and epithermal au cu deposits and the potential for discovery of such deposits in the tethyan orogen should not be overlooked

tertiary structural evolution of the gangdese thrust system southeastern tibet structural and thermochronological investigations of southern tibet xizang suggest that intracontinental thrusting has been the dominant cause for formation of thickened crust in the southernmost tibetan plateau since late oligocene two thrust systems are documented in this study the north dipping gangdese system gts and the younger south dipping renbu zedong system rzt west of lhasa the gangdese thrust juxtaposes the late cretaceous forearc basin deposits of the lhasa block the xigaze group over the tethyan sedimentary rocks of the indian plate whereas east of lhasa the fault juxtaposes the late cretaceous eocene andean type arc the gangdese batholith over tethyan sedimentary rocks near zedong 150 km southeast of lhasa the gangdese thrust is marked by a 200 m thick mylonitic shear zone that consists of deformed granite and metasedimentary rocks a major south dipping backthrust in the hanging wall of the gangdese thrust puts the xigaze group over tertiary conglomerates and the gangdese plutonics north of xigaze and west of lhasa a lower age bound for the gangdese thrust of 18 3 0 5 ma is given by crosscutting relationships the timing of slip on the gangdese thrust is estimated to be 27 23 ma from 40 ar 39 ar thermochronology and a displacement of at least 46 9 km is indicated near zedong the age of the gangdese thrust gt is consistent with an upper age limit of 24 ma for the initiation of movement on the main central thrust in places the younger renbu zedong fault is thrust over the trace of the gt obscuring its exposure the rzt appears to have been active at circa 18 ma but had ceased movement by 8 1 ma the suture between india and asia has been complexly modified by development of the gts rzt and locally strike slip and normal fault systems

mantle contributions to crustal thickening during continental collision evidence from cenozoic igneous rocks in southern tibet the tibetan crust is twice as thick as average continental crust crustal compression and shortening as a result of indian asian collision isoftenconsideredtobetheprimarycauseforthecrustalthickening inthispaper weshowthatmagmaticcontributionisalsoimportant we come to this conclusion by documenting the paleogene linzizong volcanic succession lvs its coeval granitoid batholiths and the miocene adakitic rocks along the gangdese magmatic belt in southern tibet it has been widely accepted that the indian asian collision proceeded from a soft phase at 65 70 ma to a hard phase at 45 40 ma followed by continued post collisional convergence to the present in response to the collision and post collision convergence are a series of tectono magmatic events recorded in the gangdese magmaticbelt theseinclude 1 thesyn collisionallvsvolcanism 65 40ma andtheemplacementofsoutherngangdesebatholiths a peakageof 50ma 2 aperiod 40mato25ma thatismagmaticallyquiescent yettectonicallydominatedbyactivecompressionand crustal shortening and 3 the emplacements of post collisional adakitic rocks 25 12 ma potassic ultrapotassic volcanics 25 10 ma and peraluminous muscovite bearing granites between 24 and 18 ma these three major events contribute in different ways to the crustal thickness phase i formation of the lower juvenile crust from 65 ma to 50 ma with crustal thickening largely concentrated at 50 40maviainputofmantle derivedmagmas phaseii crustalthickeningbytectonicshorteningat 40 25ma andphaseiii retaining crustal thickness but thinning of the lithospheric mantle since 25 ma in response to crustal extension and upwelling and lateral flow of asthenosphericmantle weemphasizethatcollision inducedcrustalthickeningtookplacemainlyintheperiodof 50 40maand 25ma i e the period between the late stage of the lvs volcanism and the beginning of the adakitic rock emplacement most of the lvs rocks and the collision related granitoids in southern gangdese have end 0 attesting to the significance of mantle input most likely through remelting of mantle derived basaltic rocks including the subducted neo tethyan ocean crust the petrologic and geochemical characteristics of the miocene potassic adakitic rocks support the idea that the lower portion of the thickened tibetan crust is mafic and is genetically associated with the earlier lvs magmatism we estimate that the mantle material input contributed about 30 of the total thickness of the present day tibetan crust by assuming a pre collision crustal thickness of 35 km then the tectonic contribution would be about 20 km

deep thermal state on the eastern margin of the lhasa gangdese belt and its constraints on tectonic dynamics based on the 3 d electrical model abstract since the mesozoic the lhasa gangdese block has undergone several stages of tectonism magmatism caused by the closure of the new tethys ocean and the collision between the indian and eurasian plates which led to the lithospheric shortening and crustal uplift through the careful processing and analysis of the magnetotelluric data on the eastern margin of the lhasa gangdese block near 92 e a reliable 3d electrical model was obtained to examine the relationship between the lithospheric electrical structure and geodynamics some conductive and resistive layers were obtained which may be formed in some tectonic processes the thermal model of the lithospheric uppermost mantle 75 100 km were calculated using the arrhenius equation and the hashin shtrikman hs boundary condition combined with the terrestrial heat flow point and curie isothermal in the study area the temperature distribution at the 30 100 km depths was calculated the melt fractions of the mid lower crust calculated by the classical archie s law pressure calculated by density and temperature at different depths can be obtained in a meantime the resistor beneath the tethys himalaya terrane represented the subduction of the indian plate with a deep angle the subduction of the indian crust may not exceed the indus yarlung zangbo suture while the indian lithospheric mantle may not reach the luobadui milashan fault additionally our electrical and thermal structure indicated that the indian lithospheric mantle probably detached from the indian crust our study further discussed the underplating of the mantle derived materials and the southern extrusion along the main himalaya trust and offered a subsidiary evidence for the possible south eastward crustal flow in a local area finally the relationship between the electrical structure and the deep metallogenic mechanism was briefly established

nearshore erosion by combined ice scouring and near bottom currents in eastern hudson bay canada abstract a study of nearshore sedimentary processes was carried out in manitounuk sound a sheltered environment protected by rocky islands along eastern hudson bay high resolution seismic profiles suggest that up to 10 m of holocene sediments were eroded in less than 15 m water depth although these shallow depths are affected by low energy waves and currents according to our measurements of near bottom currents at 3 7 and 5 6 m water depths the threshold velocity for sediment motion 20 cm s 1 is only seldom exceeded and consequently the significant erosion of the nearshore deposits can hardly be explained by hydraulic processes alone geophysical data seismic and echosounding profiles suggest that the observed erosion is mainly due to bottom scouring by dragging ice keels these data show that the zone of maximum ice scouring between 8 and 12 m water depths corresponds to the zone of maximum erosion of holocene sediments because ice scouring in this area occurs when the sea is largely ice free wind induced near bottom currents probably contribute to the dispersal of the sediments resuspended by ice

genesis of collision volcanism in eastern anatolia turkey late cenozoic volcanism in eastern anatolia extends in a broad sw ne trending belt across the arabia eurasia collision zone from the arabian foreland basin in the southwest and to the kars plateau and lesser caucasus in the northeast foreland volcanism is dominated by basaltic shield and fissure eruptions of transitional tholeiitic alkaline composition volcanism on the thickened crust north of the bitlis thrust zone varies from the mildly alkaline volcano nemrut and older mus volcanics in the south through the transitional calc alkaline alkaline volcanoes bingol and suphan and the alkaline volcano tendurek to the calc alkaline volcano ararat and older kars plateau volcanics in the north isotope sr nd and trace element systematics indicate that the lavas from the foreland were derived from the mantle lithosphere of the arabian continent which had been enriched by small volumes of asthenospheric melts over a period of time and that lavas from the alkaline volcanic area around mus and the volcanoes nemrut and tendurek north of the bitlis thrust zone were derived from a lithospheric source of similar composition either from the same underthrust arabian continent or from the bitlis massif microcontinent by contrast the transitional lavas from bingol and suphan and the calc alkaline lavas from ararat and kars were derived from lithosphere carrying a distinct subduction signature inherited from pre collision subduction events positive correlations between 87sr 86sr and sio2 and rb nb and sio2 in the alkaline and transitional lavas suggest that combined assimilation and fractional crystallization was an important process within at least part of the thickened crust of the collision zone trace element covariation diagrams such as y rb indicate the importance of hornblende crystallization at depth and orthopyroxene at shallow levels within the calc alkaline provinces in contrast to the consistently anhydrous crystallization sequences of the alkaline lavas trace element diagrams based on the covariation of compatible and incompatible elements point to moderate low degrees of partial melting with residual clinopyroxene throughout and residual garnet in the foreland province consideration of mineral stabilities mantle solidi and geothermal gradients before and after collision suggest that lithospheric thickening should both increase the thickness of metasomatized lithosphere and depress the metasomatized zone to greater depths probably beneath the amphibole and dolomite breakdown curves perturbation of the thickened lithosphere by delamination of the thermal boundary layer perhaps coupled with local stretching associated with pull apart basins on strike slip fault systems is then sufficient to generate melt the composition of that melt being largely dependent on the enrichment history of the lithosphere in question in eastern anatolia volcanism appears to have started between about 8 and 6 ma ago some 5 ma after the start of rapid uplift of the east anatolian plateau

deep root of a continent continent collision belt evidence from the chinese continental scientific drilling ccsd deep borehole in the sulu ultrahigh pressure hp uhp metamorphic terrane china abstract the chinese continental scientific drilling ccsd deep borehole which reached a depth of 5158 m in the sulu ultrahigh pressure uhp metamorphic terrane provides a new window into the deep root of a continent continent collision belt and the tectonic processes by which supracrustal material is recycled into the mantle by subduction and then uplifted to the surface major research themes of the ccsd project were to 1 determine the three dimensional composition structure and geophysical character of the deep root of this orogenic belt 2 investigate the nature and timing of the uhp metamorphism 3 investigate the processes of crust mantle interaction involved in the formation and exhumation of the uhp rocks 4 study the process of fluid circulation and mineralization during subduction and exhumation 5 study the rheological properties of the various rocks during subduction and exhumation 6 develop and refine dynamic models for deep subduction and exhumation of crustal rocks and 7 establish a long term natural laboratory for the study of present day crustal dynamics e g stress strain fluid activity the ccsd has developed precise oriented profiles of the main borehole in terms of lithology geochemistry oxygen isotopes zircon shrimp u pb ages 40ar 39ar ages deformation rheology mineralization physical properties of the rocks petrophysical logs seismic reflections and underground fluids the present paper summarizes the integrated research results of this project especially the new findings concerning the deep root of a continent continent collision

experimental determination of ree partition coefficients between amphibole and basaltic to andesitic liquids at high pressure abstract amphibole liquid partition coefficients for the ree damph liqree obtained from natural rocks increase systematically with bulk rock compositional change from basalt to rhyolite and are higher for the middle to heavy ree five new experimentally determined sets of dree la sm eu2 ho lu and 4 published sets are similar to these data and provide values for use in geochemical modelling of magmatic processes involving amphibole over a range of temperature pressure and oxygen fugacity the partition coefficients increase significantly with decreasing temperature and increase slightly with increasing oxygen fugacity pressure does not appear to have a major effect although one data set suggests increased pressure lowers damph liqree in a basaltic composition

observations of sar polarimetric parameters of lake and fast sea ice during the early growth phase abstract this study examines links between two sets of parameters obtained from radarsat 2 quad pol data and geophysical information of lake ice and landfast sea ice in the resolute bay area nunavut canada the first set includes the three orthogonal backscatter coefficients hh0 vv0 vh0 in addition to the total backscatter power span the second set includes polarimetric decomposition parameters obtained using cloude pottier and yamaguchi decomposition methods data were collected during the early freezing period from 20 september until 28 december 2017 meteorological and ice climatological data from the weather station of resolute bay were used to facilitate this task ice thickness and salinity were obtained from established empirical models results show that single bounce and multiple bounce scattering mechanisms are active in lake ice during the entire freezing period while multiple bounce scattering is active in sea ice only during the initial freezing backscatter parameters can be used to determine the thickness of lake ice up to any depth while both backscatter and the tested decomposition parameters are related to the thickness of young sea ice 0 6 is found to be associated with comparable single and multiple bounce scattering mechanisms hence can be used to identify new ice orthogonal backscatter coefficients and polarimetric decomposition parameters provide different information yet their combination offers clues to better identify ice conditions and investigate questions about scattering mechanisms

response of idealized hadley circulations to seasonally varying heating 1 the response of hadley circulations to displacements of the latitude of maximum heating is investigated in idealized axisymmetric and eddy permitting models consistent with an earlier study and with theory for the nearly inviscid limit lindzen and hou 1988 the strength of the hadley circulation is sensitive to displacements of heating the winter cell strengthens and summer cell weakens when the maximum heating is displaced off the equator however in conflict with the nearly inviscid limit but consistent with observations of earth s atmosphere the strength of an annually averaged hadley circulation is comparable to the hadley circulation driven by an annually averaged heating the disagreement between these results and the nearly inviscid limit is ascribed to vertical diffusion of momentum and dry static energy in the axisymmetric model and to baroclinic eddy fluxes in the eddy permitting model nonlinear amplification of the annually averaged hadley circulation is only seen near the upper boundary in simulations with a rigid lid near the tropopause suggesting that the amplification is an artifact of the upper boundary condition

distribution of porphyry deposits in the eurasian continent and their corresponding tectonic settings in the eurasian continent there are three huge metallogenic belts of cu and mo porphyry deposits comprising the paleozoic central asian ore belt in the north the tethyan eurasian ore belt of jurassic to cenozoic age in the southwest and the east margin ore belt of the eurasian continent of jurassic to cretaceous age in the east the latter is considered to be part of the vast circum pacific ore belt some of the main features of the spatial temporal distribution of cu and mo porphyry systems and related geodynamic processes of the three metallogenic belts are described in particular the key role of postsubduction related porphyry ore systems is emphasized comprising collisional and post collisional cu mo porphyry deposits during the geological history of the eurasian continent the recurrent feature of these ore systems and related felsic rocks is their derivation from partial melting of stagnant or residual oceanic slabs and mixing with a variable amount of crustal material during magma ascent to shallower levels 2013 published by elsevier ltd

titanite in situ sims u pb geochronology elemental and nd isotopic signatures record mineralization and fluid characteristics at the pusangguo skarn deposit tibet the pusangguo skarn is a newly discovered cu pb zn deposit in the gangdese metallogenic belt tibet in order to constrain the age of the deposit and the source of hydrothermal fluids various types of titanite and coexisting minerals in the pusangguo deposit were studied the titanite occurring interstitially in fresh granodiorite is magmatic and characterized by high ree y mn and lu hf ratios and high crystallization temperatures 700 to 750 c the titanite growing with or included in diopside in the prograde endoskarn or occurring with epidote calcite and quartz in the retrograde exoskarn is hydrothermal titanite characterized by low ree y mn and lu hf ratios the titanite in the retrograde exoskarn typically contains a ree enriched early core characterized by a negative eu anomaly and an overgrowth ree depleted rim with a positive eu anomaly the secondary ion mass spectrometry u pb dating of titanite drilled from thin sections of retrograde exoskarn shows intercept ages of 14 91 0 31 ma which are consistent with the granodiorite age 14 63 0 30 ma indicating coeval emplacement of granodiorite and skarn mineralization at 14 7 ma in addition in situ epidote and calcite sr isotopes and titanite nd isotopes all analyzed in the same retrograde exoskarn thin section show similar sr nd isotopic compositions to magmatic plagioclase and granodiorite whole rock indicating that the retrograde hydrothermal fluids were directly derived from or had the same origin as the granodiorite

mineral chemistry of magmatic and hydrothermal biotites from the bangpu porphyry mo cu deposit tibet abstract the bangpu deposit located in the eastern part of the gangdese porphyry copper belt tibet is a porphyry mo cu deposit in a continental collisional orogenic belt the mo cu mineralization occurs mainly in monzogranite porphyry whereas the cu mo mineralization occurs mainly in diorite porphyry most magmatic and hydrothermal biotites from the bangpu deposit are mg rich plotting in the phlogopite and mg biotite compositional fields the bangpu magmatic biotite has high bao 0 1 wt and tio2 3 wt while the hydrothermal biotite has low bao

postsubduction porphyry cu au and epithermal au deposits products of remelting of subduction modified lithosphere porphyry cu mo au and some epithermal au deposits are formed from hydrothermal fluids exsolved from cooling water rich calc alkaline magmas emplaced in volcanoplutonic arcs above subduction zones these magmas originate by partial melting of the metasomatized asthenospheric mantle wedge however there is increasing evidence for the existence of a suite of porphyry cu au and epithermal au deposits related to magmas generated after subduction beneath the arc has ceased associated magmas tend to be mildly alkaline relatively sulfur poor and emplaced as isolated complexes rather than in voluminous volcanoplutonic arcs they are likely formed by remelting of previously subduction modified arc lithosphere triggered by postsubduction lithospheric thickening lithospheric extension or mantle lithosphere delamination metasomatized mantle lithosphere or hydrous lower crustal cumulates residual from first stage arc magmatism contain small amounts of chalcophile and siderophile element rich sulfides and constitute a fertile source for hydrous au rich but relatively sulfur poor magmas during later remelting the recognition that porphyry cu au and related epithermal au systems can also form in postsubduction and collisional tectonic settings expands the range of geological environments and geographical terranes that are prospective for such deposits

increased magmatic water content the key to oligo miocene porphyry cu mo au formation in the eastern gangdese belt tibet the eastern gangdese magmatic belt between 88 e and 94 e in southern tibet preserves a series of cenozoic collision related igneous rocks that have formed since the start of the india asia collision at 55 to 50 ma the paleocene eocene intrusive rocks have intermediate la yb n ratios average 12 0 8 6 n 150 range 1 15 64 0 intermediate to low sr y ratios mostly 40 and weak or absent eu anomalies oligocene average eun eu 0 99 0 27 n 35 range 0 67 1 58 miocene average eun eu 0 95 0 13 n 93 range 0 44 1 46 and their mineralogy consists mainly of plagioclase quartz and amphibole as phenocrysts these geochemical and mineralogical characteristics suggest that the paleocene eocene magmas were relatively dry and evolved primarily by fractionation of pyroxene and plagioclase whereas the oligo miocene magmas were more hydrous and fractionated significant amounts of hornblende and lesser plagioclase prior to upper crustal emplacement oligocene and miocene magmatic rocks crop out as small volume intrusions and are associated with several large to giant sized porphyry cu mo au deposits as well as numerous smaller porphyry and skarn deposits in contrast earlier more voluminous paleocene eocene magmatism is only known to be associated with three small porphyry deposits erosional loss of subvolcanic porphyry systems from the older paleocene eocene sequence is not thought to be a controlling factor in this temporal difference of porphyry deposit distribution because exhumation rates after 55 ma were relatively low and coeval linzizong volcanic rocks are extensively preserved in the gangdese belt instead we suggest that differences in magmatic history and petrogenesis led to this restricted temporal distribution of porphyry deposits the paleocene eocene magmas were generated during the onset of collision between india and asia and were triggered by rollback of the neo tethyan oceanic slab this magmatism records the final stage of subduction of oceanic lithosphere beneath southern tibet and the relatively low water contents of these magmas may reflect final dehydration of the remnant neo tethyan slab after oceanic slab breakoff at 40 to 38 ma the greater india slab began to subduct beneath the gangdese belt until hard collision with the indian continent at 35 ma and breakoff of the greater india slab at 25 to 10 ma this last event is suggested to have caused asthenospheric mantle upwelling and partial melting of subduction modified hydrated tibetan lithosphere the resultant partial melts may have remobilized metals residual in the deep lithosphere from previous arc magmatism giving rise to a suite of postsubduction porphyry type ore deposits in the oligo miocene