isotope geochemistry sr nd pb and petrogenesis of leucite bearing volcanic rocks from colli albani volcano roman magmatic province central italy inferences on volcano evolution and magma genesis the colli albani composite volcano is made up of strongly silica undersaturated leucite bearing rocks magmas were erupted during three main periods but a complex plumbing system dominated by regional tectonics channelled magmas into different reservoirs the most alkali rich magmas restricted to the caldera forming period pre caldera are extremely enriched in incompatible trace elements and display more radiogenic sr 87sr 86sr 0 71057 0 71067 with slightly less radiogenic pb with respect to those of the post caldera period post caldera volcanic activity was concentrated in three different volcanic environments external to the caldera along the caldera edge and within the caldera the post caldera magmas produced melilite to leucitite bearing plagioclase free leucitites in contrast to the pre caldera lavas they are characterised by lower incompatible trace element abundances and less radiogenic sr 87sr 86sr 0 71006 0 71039 magmas evolved through crystal fractionation plus minor crustal assimilation in a large magma chamber during the pre caldera period the multiple caldera collapses dissected and partially obliterated the early magma chamber during the post caldera stage magmas were channelled through several pathways and multiple shallow level magma reservoirs were established a lithospheric mantle wedge previously depleted in the basaltic component and subsequently enriched by metasomatic slab derived component is suggested as the mantle source of colli albani parental magmas two different parental magmas are recognised for the pre and post caldera stages the differences may be related to the interplay between smaller degrees of melting for the pre caldera magmas and more carbonate rich recycled subducted lithologies in the post caldera magmas

uranium thorium helium dating of apatite experience with samples from different geochemical environments abstract u he dating which during its history has been very problematic might find special application in the field of hydrothermal mineralizations where other methods generally fail the mineral apatite is a potential candidate for this method in order to shed light on various aspects of apatite dating by the u th he method two samples each from different geochemical environments plutonic pneumatolytic and hydrothermal were probed isotope dilution techniques were used for concentration analyses from which 4he model dates also called helium indices after n b keevil were calculated 4he diffusion measurements were carried out for comparison of the 4he retentivities between the samples comparisons of the apatite he indices with plausible reference ages of the sampling locations in combination with results obtained from diffusion experiments show that the u th he dating method applied to apatite yields an age of cooling rather than an emplacement age the calculated closure temperatures are similar to those obtained by the fission track method 100 c our results suggest that the high temperature apatites plutonic and pneumatolytic may be more suitable for he dating than hydrothermal apatite important criteria for the dating of apatite by u th he are the presence of moderate radionuclide concentrations tens of ppm and a minimum abundance of inclusions

a review of the western part of the altaids a key to understanding the architecture of accretionary orogens abstract the altaids is one of the largest accretionary collages in the world and the tectonic styles of the accretionary processes have been interpreted in several ways including as an amalgamation of multiple terranes as a result of oroclinal bending of a long single arc or as a caledonian continental collision based on recent tectonostratigraphic analyses together with paleomagnetic data the tectonic styles of the neoproterozoic to paleozoic accretionary processes of the altaids are discussed the western altaids is the main focus of the study which was mainly composed of several independent linear components such as arcs and microcontinents with proterozoic basement and cover rocks various kinds of arcs existed in the paleo asian ocean including a complicated type of arc alaskan type which is a combination of the japan and mariana type intra oceanic arcs and the cordillera type continental arcs these linear components rotated and collided with each other with multiple subduction polarities which could have been an important result of multiple linear element amalgamation and which has contributed greatly to the architecture of the eurasian continent the basic tectonic styles of the altaids can be summarized as arc arc collision oroclinal bending and large scale rotation and multiple subductions with a complicated archipelago paleogeography these basic features of accretionary orogens in general can be attributed to the amalgamation of complicated multiple linear elements some mesozoic to cenozoic accretionary orogens in the world are also characterized by processes of multiple linear element amalgamation more attention should be paid to the multiple linear element amalgamation of ancient accretionary orogens which will shed light on lateral and vertical continental growth

mesozoic reactivation of the beishan southern central asian orogenic belt insights from low temperature thermochronology abstract the beishan orogenic collage boc is located in the southeast of the central asian orogenic belt caob and formed during final consumption of the palaeoasian ocean in the late palaeozoic this study applies low temperature thermochronology to constrain the meso cenozoic history of the boc apatite fission track and u th sm he data obtained for granitoid samples along a north south transect through the boc indicate three distinct phases of exhumation during 1 the late triassic early jurassic 225 180 ma 2 early cretaceous 130 95 ma and 3 late cretaceous early palaeogene 75 60 ma samples from the northern boc reveal a more profound early cretaceous signal and a weaker late triassic early jurassic signal than those in the southern boc a possible explanation for this discrepancy is the presence of crustal scale fault zones in the northern boc that are interpreted to have undergone repeated reactivation throughout the mesozoic exposing deeper exhumed sections of the boc these faults may thus have acted as a major control on exhumation in the region this pattern is consistent with results from elsewhere in the caob such as in the tianshan and the altai where regional widespread exhumation occurred since the early cretaceous while major fault zones record localised exhumation during the late cretaceous and cenozoic late cretaceous early palaeogene cooling ages were found only in the south of the boc suggesting that exhumation at that time was more localised and did not reach the northern margins of the study area the meso cenozoic cooling events described here are thought to be related to the progressive closure of the palaeo tethys ocean to the south associated collision and accretion of micro continental blocks and island arcs at the southern eurasian margin are interpreted to have induced more widespread reactivation and exhumation in central asia than previously anticipated extending to the northern margin of the tarim craton this observation hence refines the existing tectonic history models for central asia

tectono thermal evolution of the northwestern edge of the tarim basin in china constraints from apatite u th he thermochronology abstract the northwestern edge of the tarim basin which contains the kalpin uplift and the north bachu uplift lies between the south tianshan and tarim continental block and its tectonic evolution is closely correlated with the tianshan the pre mesozoic units are widely exposed in the northwestern edge of the tarim basin which allow detailed geological observation and sample collection in this paper the apatite u th he thermochronology was used to study the tectono thermal evolution of the northwestern edge of the tarim basin the sandstone samples from the upper neoproterozoic silurian devonian carboniferous and permian units etc on the northwestern edge of the tarim basin were collected and used to determine apatite u th he ages the apatite he ages in the kalpin uplift reveal the tectonic uplift events during the late cretaceous 96 70 ma and the miocene 22 20 ma these uplift events are linked to collisions of the lhasa block and the india plate with the southern eurasian continental margin respectively however apatite he ages in the north bachu uplift show only the eocene event at 40 ma because the west kunlun orogenic belt thrust over the southwestern edge of the tarim basin during the eocene the thermal modeling results reveal four cooling events in the kalpin uplift since the late neoproterozoic corresponding to 460 445 ma 350 320 ma 220 210 ma and 20 10 ma but only one episode with 220 210 ma in the north bachu uplift since the devonian based on the thermal modeling results the north bachu uplift did not experience the cooling events at 460 445 ma 350 320 ma and 20 10 ma shown in the kalpin uplift this paper provides a new insight into the tectonic evolution of the tianshan and the northwestern tarim basin

thermal tectonic history of the baogutu porphyry cu deposit west junggar as constrained from zircon u pb biotite ar ar and zircon apatite u th he dating abstract understanding postmineralization tectonic movements in porphyry deposits is critical to interpreting the complete thermal tectonic history this study reports new zircon u pb ages hydrothermal biotite 39ar 40ar age and zircon and apatite u th he ages from the baogutu porphyry copper deposit which in conjunction with pre existing geochronology and thermochronology and inverse modeling simulations constrain the thermal tectonic history of the deposit zircon la icp ms u pb concordia ages indicate a late carboniferous age of 320 1 2 2 ma for the diorite complex and 309 8 2 2 ma for the mineralized granodiorite porphyry hydrothermal biotite selected from a quartz biotite chalcopyrite vein yields a plateau age of 311 0 1 8 ma which agrees with the age of the granodiorite porphyry and a previously reported molybdenite re os age and suggests that hydrothermal fluid circulated about 310 m y ago diorite and granodiorite porphyry yield weighted mean zircon u th he ages of 200 6 5 7 ma and 241 1 8 1 ma respectively with ages ranging from 221 0 ma to 174 3 ma and 225 9 to 261 6 ma weighted mean diorite and granodiorite porphyry apatite u th he ages of 87 4 2 3 ma and 120 0 4 2 ma were obtained with ages ranging from 68 9 ma to 100 8 ma and from 91 0 ma to 152 0 ma respectively the wide range of zircon u th he ages may be due to the combined effects of u and th zonation and radiation damage and radiation damage effect may also account for the wide range of apatite u th he ages the combined effects of depth and cooling due to meteoric water circulation contribute to an older u th he age for the granodiorite porphyry relative to the diorite a five episode cooling rate history for the diorite can be deduced by inverse model simulation fast cooling moderate fast cooling relatively slow cooling fast cooling again very slow cooling the thermal and tectonic history of the wall rock indicates that it suffered significant far field effects from the qiangtang eurasia and lhasa qiangtang collision however no visible thermal effect from the india asia collision is observed

tectonic history of the kyrgyz south tien shan atbashi inylchek suture zone the role of inherited structures during deformation propagation 1 multimethod chronology was applied on intrusives bordering the kyrgyz south tien shan suture stss to decipher the timing of 1 formation and amalgamation of the suturing units and 2 intracontinental deformation that built the bordering mountain ranges zircon u pb data indicate similarities between the tien shan and tarim precambrian crust caledonian 440 410 ma and hercynian 310 280 ma zircon u pb ages were found at the edge of the stss related to subduction and closure of the turkestan ocean and the formation of the suture itself permian triassic 280 210 ma titanite fission track and zircon u th he data record the first signs of exhumation when the stss evolved into a shear zone and the adjacent tarim basin started to subside low temperature thermochronological apatite fission track zircon and apatite u th he analyses reveal three distinct cooling phases becoming younger toward the stss center 1 jurassic cretaceous cooling ages provide evidence that a mesozoic south tien shan orogen formed as a response to the cimmerian orogeny 2 early paleogene 60 45 ma data indicate a renewed pulse of stss reactivation during the early cenozoic 3 neogene ages constrain the onset of the modern tien shan mountain building to the late oligocene 30 25 ma which intensified during the miocene 10 8 ma and pliocene 3 2 ma the cenozoic signals may reflect renewed responses to collisions at the southern eurasian border i e the kohistan dras and india eurasia collisions this progressive rejuvenation of the stss demonstrates that deformation has not migrated steadily into the forelands but was focused on pre existing basement structures

continental growth of northwest china the northwest corner of china is the most interior part of central asia and includes parts of the siberian tarim and kazakhstan plates where the junggar and tarim basins are separated by the east west trending tianshan range a preliminary terrane map divides the area into thirteen separate areas which are classified into four types l continental 2 oceanic crust 3 island arc 4 composite the boundaries for these terranes are faults that have been previously defined either by geologic mapping or holocene activity passive consolidation of the southern margin of paleo asia is marked by accretion and subduction of the paleo tethys oceanic basins and by development of volcanic arcs paleomagnetic data indicate that the major plates and associated smaller terranes did not reach final consolidation until the permian or later the stratigraphic record reveals development of intracontinental basins in the late carboniferous followed by the pan asian thermal event that gave rise to widespread intrusion of a type granites of late paleozoic age the frontal collision of india along the southern border of asia in the paleocene reactivated nearly all of the older major strike slip and thrust faults formed during the late paleozoic consolidation of paleo asia the complex nature of the continental growth of central asia has produced and destroyed many varieties of sutures and reconstruction of the original plate and terrane configurations therefore is enigmatic

palaeozoic collisional tectonics and magmatism of the chinese tien shan central asia abstract the chinese tien shan range is a palaeozoic orogenic belt which contains two collision zones the older southern collision accreted a north facing passive continental margin on the north side of the tarim block to an active continental margin on the south side of an elongate continental tract the central tien shan collision occurred along the qinbulak qawabulak fault southern tien shan suture the time of the collision is poorly constrained but was probably in in the late devonian early carboniferous we propose this age because of a major disconformity at this time along the north side of the tarim block and because the youshugou ophiolite is imbricated with middle devonian sediments a younger probably late carboniferous early permian collision along the north tien shan fault northern tien shan suture accreted the northern side of the central tien shan to an island arc which lay to its north the north tien shan arc this collision is bracketed by the middle carboniferous termination of arc magmatism and the appearance of late carboniferous or early permian elastics in a foreland basin developed over the extinct arc thrust sheets generated by the collision are proposed as the tectonic load responsible for the subsidence of this basin post collisional but palaeozoic dextral shear occurred along the northern suture zone this was accompanied by the intrusion of basic and acidic magmas in the central tien shan late palaeozoic basic igneous rocks from all three lithospheric blocks represented in the tien shan possess chemical characteristics associated with generation in supra subduction zone environments even though many post date one or both collisions rocks from each block also possess distinctive trace element chemistries which supports the three fold structural division of the orogenic belt it is unclear whether the chemical differences represent different source characteristics or are due to different episodes of magmatism being juxtaposed by later dextral strike slip fault motions because the southern collision zone in the tien shan is the older of the two the tarim block sensu stricto collided not with the eurasian landmass but with a continental block which was itself separated from eurasia by at least one ocean the destruction of this ocean in late carboniferous early permian times represented the final elimination of all oceanic basins from this part of central asia

mongolian summits an uplifted flat old but still preserved erosion surface in gobi altay and altay mongolia several flat surfaces worn through basement rocks and uplifted during the ongoing tectonic episode to a similar altitude of 4000 m suggests disruption of a single large scale surface new thermochronology and field data show that the plateau surfaces represent uplifted parts of an ancient peneplain that formed during jurassic time the gobi altay and altay flattopped massifs are tectonically and geomorphologically unique their preservation for 150 m y implies that no further tectonic movements occurred before the onset of the last deformation episode 5 3 m y ago it also suggests that very low erosion rates were maintained by a dry climate over millions of years

permian a type granites in tarim and western part of central asian orogenic belt caob genetically related to a common permian mantle plume abstract zircon u pb ages whole rock geochemical and zircon hf isotope data are reported for the permian a type quartz syenites granites in western tarim block northwest china zircon u pb dating indicates these alkali plutons were emplaced at 270 277 ma elemental geochemistry shows their typical a type signatures i e high total alkali hfse ree contents and ga al feo tot feo tot mgo and nb y ratios isotopically their epsilon nd values vary from 2 6 to 2 0 based on zircon hf isotope compositions they could be divided into two sub groups group i exhibits high positive ehf t ranging from 3 to 11 mostly within the range of 5 to 9 which is highly comparable with the syenites in bachu area located just south of these plutons group ii has ehf t ranging from 3 to 2 the second group exhibits comparable nd hf isotope compositions with the spatially and temporally related piqiang ultramafic mafic complex integrating geological geochemical and nd hf isotope data we suggest that the a type syenite granites were formed via intensive crystal fractionation from a common plume derived parental mafic magma coupled with variable extent of crustal contamination moreover the extent of the crustal contamination increase from inner tarim towards south tianshan collisional belt a comprehensive synthesis of the coeval a type granites in the western part of the central asian orogenic belt caob shows that they exhibit distinct geochemical features e g large range of end t and 87 sr 86 sr i values high y nb yb ta and low nb la ratios all these features argue that they were derived from juvenile mafic crust with variable addition of the mafic materials from mantle sources and some old crustal materials or mixing of mantle derived mafic magma with precambrian crustal components we suggest that both the permian a type granites from tarim and western section of caob were genetically linked to the permian tarim mantle plume which is concurrent with the two mantle domains for the permian tarim large igneous province lip

apatite fission track analysis apatite fission track aft analysis has been widely used during the past 30 years to constrain the low temperature thermal histories of many igneous metamorphic and sedimentary rocks in a wide range of geological settings applicable geological settings include orogenic belts rifted margins faults sedimentary basins cratons and mineral deposits the types of geologic problems that can be addressed include the timing and rates of tectonic events sedimentary basin evolution the timing of hydrocarbon generation and ore mineralization the absolute age of volcanic deposits the effects of major climatic changes on the near surface geothermal gradient and long term landscape evolution early work by naeser 1967 and wagner 1968 1969 first established the basic procedures that enabled the fission track dating method to be applied routinely to these geologic problems fleischer et al 1975 summarized the early studies of the broader discipline of nuclear track detection in different solid state materials more recent comprehensive overviews of fission track applications have been provided by naeser and mcculloh 1989 wagner and van den haute 1992 gallagher et al 1998 van den haute and de corte 1998 dumitru 2000 and gleadow et al 2002 etched natural fission tracks in several apatite grains are shown in figure 1 successful aft analysis is limited by the following 1 the availability of apatite from which useful aft data can be obtained often due to a lack of apatite of sufficient grain size and quality within available rock types or alternatively the lack of available rock samples due to minimal outcrop exposure or other reasons 2 economic considerations in terms of the time and money required to obtain sufficient aft data 3 the inherent limitations of aft data to resolve geological thermal history information often related to limited numbers of accumulated spontaneous fission tracks due to

geochemical and geochronological studies of granitoid rocks from the western tianshan orogen implications for continental growth in the southwestern central asian orogenic belt abstract the western tianshan orogen in northwestern china is part of the southwestern margin of the central asian orogenic belt caob which represents the largest tectonic assembly of accretionary complexes and the most significant area of phanerozoic continental growth in the world granitoid rocks have a widespread occurrence and occupy about 30 of the whole exposure of the orogen ages obtained for the granitoids vary from 896 ma to 247 ma while initial 87 sr 86 sr ratios vary from 0 70329 to 0 72070 and e nd values from 14 1 to 7 3 the occurrence of neoproterozoic granitic gneisses implies the presence of precambrian basement in this area during oceanic subduction continental growth occurred as a result of several combined processes i e by addition of oceanic crustal melts adakites the intrusion of basaltic magmas derived by partial melting of the metasomatized depleted mantle wedge and the upwelling of granitic magma derived from a mixed source of basaltic magmas and old continental basement however during the post collisional period vertical accretion of underplated juvenile mantle material may have been accomplished during slab breakoff delamination this study suggests a two stage model of continental growth of syn subduction lateral accretion of arc complexes and post collisional vertical accretion of underplated mantle material which may also be relevant for the western segment of the caob in the phanerozoic the reworking and involvement of old crust are more obvious in the western tianshan orogen compared to other parts of the caob comprising the western junggar eastern junggar alataw and altay terranes

tectonic evolution of the south tianshan orogen and adjacent regions nw china geochemical and age constraints of granitoid rocks geochemical and geochronological evidence was obtained from granitoids of the south tianshan orogen and adjacent regions which consist of three individual tectonic domains the kazakhstan yili plate the central tianshan terrane and the tarim plate from north to south the central tianshan terrane is structurally bounded by the early paleozoic nikolaev line north nalati fault and late paleozoic atbashy inyl chek south nalati qawabulak fault zones against the kazakhstan yili and tarim plates respectively the meta aluminous to weakly peraluminous granitic rocks which are exposed along the kekesu river and the bikai river across the central tianshan terrane have a tholeiitic calc alkaline or high potassium calc alkaline composition i type geochemical trace element characteristics and the y versus rb nb or y versus nb discrimination diagrams favor a continental arc setting for these granitoid rocks shrimp u pb and la icp ms u pb zircon age data indicate that the magmatism started at about 480 ma continued from 460 to 330 ma and ended at about 275 ma the earlier magmatism 470 ma is considered to be the result of a simultaneous southward and northward subduction of the terskey ocean beneath the northern margin of the tarim plate and the kazakhstan yili plate respectively the later magmatism 460 330 ma is related to the northward subduction of the south tianshan ocean beneath the southern margin of the kazakhstan yili central tianshan plate the dataset presented here in conjunction with previously published data support a late paleozoic tectonic evolution of the south tianshan orogen not a triassic one as recently suggested by shrimp u pb zircon dating for eclogites

mesozoic and cenozoic exhumation history and magmatic hydrothermal events of the central tianshan mt range nw china evidence from u th he and 40ar 39ar dating abstract the thermal history of rocks in the awulale area central chinese tianshan is significant to understanding the formation and tectonothermal evolution of the tianshan orogen k feldspar 40ar 39ar and apatite u th he age data reveal at least four mesozoic cenozoic tectonothermal events in the central tianshan which occurred during the middle late triassic 244 4 216 5 ma early jurassic ca 180 3 ma early cretaceous 99 1 98 1 ma and early miocene 18 6 13 7 ma this thermal history is consistent with that throughout the tianshan orogenic belt and surrounding blocks modeling of the thermal history shows that intrusive rocks in the central tianshan have experienced four stages of cooling in the mesozoic cenozoic stage i occurred from ca 310 to 230 ma after magma emplacement and resulted in cooling to the 40ar 39ar closure temperature of k feldspar 200 180 c during this time tectonism in the central tianshan was not intense and the rapid cooling was due mainly to thermal exchange with a maximum cooling rate of 7 1 c myr stage ii occurred from ca 230 to 180 ma collision between the qiangtang microcontinent and the eurasia continental margin resulted in active tectonism in the central tianshan and cooling was caused mainly by tectonic uplift with the maximum uplift of intrusive rocks exceeding 9 km stage iii occurred from ca 180 to 45 ma when regional tectonism was weak and cooling was gradual during this stage the apatites in the syenite porphyry remained in the he partial retention zone for a long time this indicates that during this stage the central tianshan was only weakly influenced by the far field collisions of lassa and karakoram pamir with the southern margin of eurasia stage iv occurred from 45 ma to the present day and was related to the far field influence of the middle late himalayan orogeny during which tectonism in the central tianshan became more intense in this final stage many types of intrusive rock were exhumed to the surface with the maximum uplift exceeding 2 km furthermore the cooling history and regional structural trends indicate that since the mesozoic under the influence of tectonic compression there has been differential uplift on the northern and southern sides of the east awulale area the cooling and exhumation rates of intrusive rocks are much higher on the northern side of the area than the southern side this was related to reverse movement on the yuximolegai fault as exhumation rate of the hanging wall northern region was higher than that of the footwall southern region under n s compression these findings combined with the inversion model results indicate that this differential uplift occurred mainly from the late triassic to early jurassic

hercynian post collisional magmatism in the context of paleozoic magmatic evolution of the tien shan orogenic belt abstract the hercynian tien shan tianshan orogen formed during late palaeozoic collision between the karakum tarim and the kazakhstan paleo continents in order to constrain timing of hercynian post collisional magmatism 27 intrusions were sampled for u pb zircon dating along a ca 2000 km long profile in uzbekistan and kyrgyzstan the samples were dated utilizing sensitive high resolution ion microprobe shrimp ii the obtained ages together with previously published age data allowed the timing of hercynian post collisional magmatism to be constrained and interpreted in the context of the paleozoic magmatic evolution of the region apart from hercynian post collisional magmatism two older magmatic episodes have been recognized and the following sequence of events has been established 1 approximately 10 ma after cessation of continuous caledonian magmatism a number of late silurian early devonian intrusions were emplaced in the middle and northern tien shan terranes between 420 and 390 ma the intrusions probably formed in an extensional back arc setting during coeval subduction under the margins of caledonian paleo kazakhstan continent 2 the next relatively short late carboniferous episode of subduction under paleo kazakhstan was registered in the kurama range of the middle tien shan calc alkaline volcanics and granitoids with ages 315 300 ma have distinct metallogenic affinities typical for subduction related rocks and are not found anywhere outside the middle tien shan terrane west of the talas farghona fault 3 the early permian hercynian post collisional magmatism culminated after the closure of the paleo turkestan ocean and affected the whole region across terrane boundaries the post collisional intrusions formed within a relatively short time span between 295 and 280 ma the model for hercynian post collisional evolution suggests that after collision the tien shan was affected by trans crustal strike slip motions which provided suitable conduits for ascending asthenospheric material and heat influx in the crust this produced both granitoid magmas and hydrothermal fluid flow as a result post collisional intrusions and orogenic au deposits known in the region formed coevally and were tectonically controlled 4 between 240 and 220 ma a triassic thermal event affected the region resulting in resetting and growth of new zircon grains which is detected on a regional scale probably the influx of heat into the crust during the triassic was tectonically focused and varied significantly in different terranes in the region under investigation the triassic thermal event was not accompanied by any significant magmatic activity thus after cessation of hercynian post collisional magmatism ca 280 ma ago there was a long magmatically quiet period in the tien shan

effect of alpha damage annealing on apatite u th he thermochronology recent data on he diffusion challenge the temperature sensitivity of apatite u th he thermochronology the damage induced by recoil of u and th decay series during emission of particles recoil damage has been proposed to modify he diffusion properties through time however we propose that annealing of these irradiation defects may be an important phenomenon and may be significant in case of slowly cooled or reheated basement rocks to test this hypothesis we developed a quantitative model including an explicit treatment of recoil damage annealing and their effect on he diffusion kinetics and calibrate it against literature data our model is based on two hypotheses 1 helium is in equilibrium between an apatite crystal and its defects and 2 alpha recoil damage annealing can be described analogously to fission track annealing this model has been embedded into a monte carlo simulation of helium production ejection diffusion and applied to data from the french massif central a complex slowly cooled terrain with burial reheating where the thermal history has been constrained by previous fission track ft data including ft length distributions u th he ages are close to the ft ages from the same samples and are generally reproducible among replicates but some samples present he age dispersion that is not correlated with crystal size our model reproduces the massif central data very well except for three samples where he ages are older than corresponding ft ages we show that annealing of irradiation damage has an important impact on retentivity of helium and that the he content he is only a rough approximation of the damage level in particular our results show that independence of he ages on crystal sizes in case of reheated samples is a clear indication of the higher he retentivity induced by recoil defects and that an explicit treatment of defect annealing is required for a correct interpretation of u th he ages in such a case more generally a correlation with the crystal size can bring information on the thermal path only if the age of defects well represented by the fission track age is available due to the dependence of the partial retention zone on damages conversely in case of rapid cooling or for samples having low u and th contents damage effects can be ignored without significant effects on he ages

the thermal significance of potassium feldspar k ar ages inferred from age spectrum results 40ar39ar age spectrum analyses of three microcline separates from the separation point batholith northwest nelson new zealand which cooled slowly 5 c ma 1 through the temperature zone of partial radiogenic 40ar accumulation are characterized by a linear age increase over the first 65 percent of gas release with the lowest ages 80 ma corresponding to the time that the samples cooled below about 100 c the last 35 percent of 39ar released from the microclines yields plateau ages 103 99 and 93 ma which reflect the different bulk mineral ages and correspond to cooling temperatures between about 130 to 160 c theoretical calculations confirm the likelihood of diffusion gradients in feldspars cooling at rates 5 c ma 1 diffusion parameters calculated from the 39ar release yield an activation energy e 28 8 1 9 kcal mol 1 and a frequency factor grain size parameter d0l2 5 6 3 9 14sec 1 this arrhenius relationship corresponds to a closure temperature of 132 13 c which is very similar to the independently estimated temperature from the observed diffusion compensation correlation this d0l2 implies an average diffusion half width of about 3 m similar to the half width of the perthite lamellae in the feldspars the range in microcline k ar ages from the separation point batholith is the result of relatively small temperature differences within the pluton during cooling comparison of the diffusion laws determined for microcline with those for anorthoclases and other homogeneous k feldspars e 40 to 52 kcal mol 1 reveals that ar diffusion is more highly temperature dependent in the disordered structural state than in the ordered structural state previously published u shaped age spectra are probably the result of the superimposition of excess 40ar upon diffusion profiles of the kind described here

abbot ice shelf structure of the amundsen sea continental margin and the southern boundary of the bellingshausen plate seaward of west antarctica inversion of nasa operation icebridge airborne gravity over the abbot ice shelf in west antarctica for subice bathymetry defines an extensional terrain made up of east west trending rift basins formed during the early stages of antarctica zealandia rifting extension is minor as rifting jumped north of thurston island early in the rifting process the amundsen sea embayment continental shelf west of the rifted terrain is underlain by a deeper more extensive sedimentary basin also formed during rifting between antarctica and zealandia a well defined boundary zone separates the mildly extended abbot extensional terrain from the deeper amundsen embayment shelf basin the shelf basin has an extension factor of 1 5 1 7 with 80 100 km of extension occurring across an area now 250 km wide following this extension rifting centered north of the present shelf edge and proceeded to continental rupture since then the amundsen embayment continental shelf appears to have been tectonically quiescent and shaped by subsidence sedimentation and the advance and retreat of the west antarctic ice sheet the bellingshausen plate was located seaward of the amundsen sea margin prior to incorporation into the antarctic plate at about 62 ma during the latter part of its independent existence bellingshausen plate motion had a clockwise rotational component relative to antarctica producing convergence across the north south trending bellingshausen gravity anomaly structure at 94 w and compressive deformation on the continental slope between 94 w and 102 w farther west the relative motion was extensional along an east west trending zone occupied by the marie byrd seamounts key points abbot ice shelf is underlain by e w rift basins created at 90 ma amundsen shelf shaped by subsidence sedimentation and passage of the ice sheet bellingshausen plate boundary is located near the base of continental slope and rise

late paleozoic tectonic amalgamation of northwestern china sedimentary record of the northern tarim northwestern turpan and southern junggar basins sedimentary rocks contained in basins adjacent to the tian shan provide a long and complex record of the late paleozoic continental amalgamation of northwestern china complementing that provided by rocks preserved within the range this record which comprises dramatic changes in sedimentary facies sediment dispersal patterns sandstone provenance and basin subsidence rates broadly supports previous interpretations of a two part evolution of the tian shan late devonian to early carboniferous collision of the tarim continental block with the central tian shan followed by collision of this combined block with island arcs in the north tian shan and bogda shan in late carboniferous early permian times the first collision resulted in widespread angular unconformities within the tarim basin continued convergence following the collision created a long lived flexural foredeep along the northern margin of the tarim block which received at least 2000 m of lower carboniferous through lower permian fluvial and marine sediment derived from the interior of tarim subsequent early permian continental extension of the northern tarim basin resulted in the deposition of interbedded nonmarine siliciclastic sedimentary rocks and mafic to felsicvolcanic rocks sandstone within this interval was derived from the paleo tian shan and is composed predominantly of lithic volcanic grains similar to the rhyolite in contrast to the tarim basin calc alkaline volcanic rocks and volcanogenic sedimentary rocks dominated carboniferous and permian sedimentation in the northern turpan and northwestern junggar basins volcanic arcs remained active in the north tian shan and bogda shan through the early late carboniferous depositing a kilometers thick interval of deep marine sediment gravity flows in the northwestern junggar basin major arc magmatism ceased in the late carboniferous in response to closure of the oceanic basin between the combined tarim central tian shan block and the north tian shan bogda shan arcs upper carboniferous through lower permian rocks in the northwestern junggar basin compose the sedimentary fill of a bathymetric basin of oceanic depth on the northern side of the volcanic arcs culminating in a 1000 m thick marine regressive sequence middle to upper permian sandstones were derived from the uplifted paleo tian shan and bear the distinctive provenance imprint of granitic rocks presently exposed within the range late permian subsidence of the junggar basin accommodated 5 km of nonmarine sediments however the cause of this subsidence and its relationship to regional tectonic events remain controversial

modeling of the temperature sensitivity of the apatite u th he thermochronometer apatite u th he apparent ages will generally reflect residence for extended periods at temperatures where helium is neither quantitatively retained nor lost by diffusion to characterize the response of apatite he ages to thermal histories involving partial he retention we explored solutions to the he production diffusion equation under thermally static conditions the analytical solution to this equation coupled with published diffusivity data demonstrates that the zone of partial he retention extends from about 40 c to 85 c this zone lies at temperatures 35 c cooler than the analogous fission track partial annealing zone he ages within the partial retention zone ultimately achieve a balance between he production and loss yielding a steady state age both the ultimate age and the time it takes to achieve this age are temperature dependent for example an apatite held at 75 c equilibrates to an age of 2 ma after 17 myr regardless of whether equilibrium is approached from a higher or a lower initial he age for representative dynamic thermal histories we evaluated apatite he ages using a numerical solution to the ingrowth diffusion equation the results illustrate the sensitivity of he ages to various geologic histories and are useful for understanding he age elevation relationships and for testing time temperature paths derived from apatite fission track length distributions in addition although he diffuses rapidly from apatite at shallow crustal temperatures modeling of ambient temperature fluctuations indicates that he ages are nearly unaffected by surficial processes

new 40ar 39ar dating of the grande ronde lavas columbia river basalts usa implications for duration of flood basalt eruption episodes abstract grande ronde basalt grb lavas represent the most voluminous eruptive pulse of the columbia river snake river yellowstone hotspot volcanism with an estimated eruptive volume of 150 000 km 3 grb lavas form at least 66 of the total volume of the columbia river basalt group new 40 ar 39 ar dates for grb lavas reveal they were emplaced within a maximum period of 0 42 0 18 my a well documented stratigraphy indicates at least 110 grb flow fields or individual eruptions and on this basis suggests an average inter eruption hiatus of less than 4000 years isotopic age dating cannot resolve time gaps between grb eruptions and it is difficult to otherwise form a picture of the durations of eruptions because of non uniform weathering in the top of flow fields and a general paucity of sediments between gr lavas where sediment has formed on top of grb lavas it varies in thickness from zero to 20 30 cm of silty to fine sandy material with occasional diatomaceous sediment individual grb eruptions varied considerably in volume but many were greater than 1000 km 3 in size most probably eruptive events were not equally spaced in time some eruptions may have followed short periods of volcanic repose perhaps 10 2 to 10 3 of years whilst others could have been considerably longer many 1000 s to 10 4 years recent improvements in age dating for other continental flood basalt cfb lava sequences have yielded estimates of total eruptive durations of less than 1 my for high volume pulses of lava production the grb appears to be a similar example where the main pulse occupied a brief period even allowing for moderate to long duration pahoehoe flow field production the amount of time the system spends in active lava producing mode is small less than c 2 6 based on eruption durations of approximately 10 000 years compared to the duration of the entire eruptive pulse of c 420 000 years a review of available 40 ar 39 ar data for the major voluminous phases of the columbia river basalt group suggests that activity of the steens basalt imnaha basalt grb may have at times been simultaneous with obvious implications for climatic effects resolving intervals between successive eruptions during cfb province construction and durations of main eruptive pulses remains vital to determining the environmental impact of these huge eruptions

apatite u th he thermochronometry using a radiation damage accumulation and annealing model helium diffusion from apatite is a sensitive function of the volume fraction of radiation damage to the crystal a quantity that varies over the lifetime of the apatite using recently published laboratory data we develop and investigate a new kinetic model the radiation damage accumulation and annealing model rdaam that adopts the effective fission track density as a proxy for accumulated radiation damage this proxy incorporates creation of crystal damage proportional to production from u and th decay and the elimination of that damage governed by the kinetics of fission track annealing the rdaam is a version of the helium trapping model hetm shuster d l flowers r m and farley k a 2006 the influence of natural radiation damage on helium diffusion kinetics in apatite earth planet sci lett 249 148 161 calibrated by helium diffusion data in natural and partially annealed apatites the chief limitation of the hetm now addressed by rdaam is its use of he concentration as the radiation damage proxy for circumstances in which radiation damage and he are not accumulated and lost proportionately from the crystal by incorporating the rdaam into the hefty computer program we explore its implications for apatite u th he thermochronometry we show how u th he dates predicted from the model are sensitive to both effective u concentration eu and details of the temperature history the rdaam predicts an effective he closure temperature of 62 c for a 28 ppm eu apatite of 60 m radius that experienced a 10 c ma monotonic cooling rate this is 8 c lower than the 70 c effective closure temperature predicted using commonly assumed durango diffusion kinetics use of the rdaam is most important for accurate interpretation of u th he data for apatite suites that experienced moderate to slow monotonic cooling 1 0 1 c ma prolonged residence in the helium partial retention zone or a duration at temperatures appropriate for radiation damage accumulation followed by reheating and partial helium loss under common circumstances the rdaam predicts u th he dates that are older sometimes much older than corresponding fission track dates nonlinear positive correlations between apatite u th he date and eu in apatites subjected to the same temperature history are a diagnostic signature of the rdaam for many but not all thermal histories observed date eu correlations in four different localities can be explained with the rdaam using geologically reasonable thermal histories consistent with independent fission track datasets the existence of date eu correlations not only supports a radiation damage based kinetic model but can significantly limit the range of acceptable time temperature paths that account for the data in contrast these datasets are inexplicable using the durango diffusion model the rdaam helps reconcile enigmatic data in which apatite u th he dates are older than expected using the durango model when compared with thermal histories based on apatite fission track data or other geological constraints it also has the potential to explain at least some cases in which u th he dates are actually older than the corresponding fission track dates

lithospheric structure and dynamic processes of the tianshan orogenic belt and the junggar basin abstract located at the center of the eurasian continent and accommodating as much as 44 of the present crustal shortening between india and siberia the tianshan orogenic belt tob is one of the youngest 7000 m orogenic belts in the world it provides a natural laboratory for examining the processes of intracontinental deformation in recent years wide angle seismic reflection refraction profiling and magnetotelluric sounding surveys have been carried out along a geoscience transect which extends northeastward from xayar at the northern margin of the tarim basin tb through the tianshan orogenic belt and the junggar basin jb to burjing at the southern piedmont of the altay mountain we have also obtained the 2d density structure of the crust and upper mantle of this area by using the bouguer anomaly data of northwestern xinjiang with these surveys we attempt to image the 2d velocity and the 2d electric structure of the crust and upper mantle beneath the tianshan orogenic belt and the junggar basin in order to obtain the small scale structure of the crust mantle transitional zone of the study area the wavelet transform method is applied to the seismic wide angle reflection refraction data combining our survey results with heat flow and other geological data we propose a model that interprets the deep processes beneath the tianshan orogenic belt and the junggar basin located between the tarim basin and the junggar basin the tianshan orogenic belt is a block with relatively low velocity low density and partially high resistivity it is tectonically a shortening zone under lateral compression a detachment exists in the upper crust at the northern margin of the tarim basin its lower part of the upper crust intruded into the lower part of the upper and the middle crust of the tianshan near the korla fault its middle crust intruded into the lower crust of the tianshan and its lower crust and lithospheric mantle subducted into the upper mantle of the tianshan in these processes the mass of the lower crust of the tarim basin was carried down to the upper mantle beneath the tianshan forming a 20 km thick complex crust mantle transitional zone composed of seven thin layers with a lower than average velocity the thrusting and folding of the sedimentary cover the intrusive layer in the upper and middle crust and the mass added by the subduction of the tarim basin into the upper mantle of the tianshan are probably responsible for the crustal thickening of the tianshan due to the important mass deficiency in the crust and the upper mantle of the tianshan buoyancy must occur and lead to rapid ascent of the tianshan the episodic tectonic uplift of the tianshan and tectonic subsidence of the junggar basin are closely related to the evolution of the paleozoic mesozoic and cenozoic tethys

late oligocene early miocene initiation of shortening in the southwestern chinese tian shan implications for neogene shortening rate variations abstract the cenozoic tian shan is one of the preeminent examples of an intracontinental orogen however there remains a significant controversy over when deformation related to the india asia collision commenced and therefore how shortening within the mountains has been partitioned over time one approach has been to look at the modern shortening rate as measured by geodetic studies combined with estimates of the total shortening across the range and extrapolate backwards this approach suggests that the onset of range construction was ca 10 ma k y abdrakhmatov s a aldazhanov b h hager m w hamburger t a herring k b kalabaev k b kalabayev v i makarov p molnar s v panasyuk m t prilepin r e reilinger i s sadybakasov b j souter y a trapeznikov v y tsurkov a v zubovich relatively recent construction of the tien shan inferred from gps measurements of present day crustal deformation rates nature 384 6608 1996 450 453 an alternate method is to determine the age of the onset of exhumation using thermochronology we present 19 new apatite fission track aft results from the southwestern chinese portion of the belt this region represents the first area exhumed during the late tertiary along a transect at ca 76 e exhumation commenced at the oligocene miocene boundary 24 ma along the maidan and muziduke thrusts which bound the southern side of the kokshaal range subsequently deformation propagated ca 20 km south to the kashi basin bounding thrust kbt which was exhumed by no earlier than 18 9 3 3 ma three detrital aft samples from plio pleistocene strata deposited ca 20 km farther south contain fission track grain age peaks that young monotonically upwards from 20 9 7 0 5 3 ma to 15 9 5 4 4 0 ma with a fairly constant lag time of 16 to 18 ma these ages combined with structural data suggest that both the hanging wall and the footwall of the kbt experienced a renewed episode of exhumation during the latest cenozoic the discrepancy between the late oligocene miocene initiation of significant exhumation shown herein and the 10 ma initiation estimate from geodesy suggests that the tian shan has undergone a complex late cenozoic shortening history assuming that the present shortening rate could account for the total amount of cenozoic shortening in 10 ma and realizing that shortening initiated at least 15 myr earlier we conclude that the shortening rate must have varied over time possibly in pulsed southward migrating events and that the present rate may not reflect the average rate since initiation of range uplift

pb diffusion in zircon abstract chemical diffusion of ti under anhydrous conditions at 1 atm and under fluid present elevated pressure 1 1 1 2 gpa conditions has been measured in natural zircon the source of diffusant for 1 atm experiments was a zro2 tio2 zircon mixture with experiments run in crimped pt capsules diffusion experiments done in the presence of h2o co2 fluid were run in a piston cylinder apparatus using a source of ground tio2 zrsio4 and sio2 with oxalic acid added to produce h2o co2 vapor and partially melt the solid source material yielding an assemblage of rutile zircon melt vapor nuclear reaction analysis nra with the resonant nuclear reaction 48ti p 49v was used to measure diffusion profiles for both sets of experiments the following arrhenius parameters were obtained for ti diffusion normal to cover the temperature range 1350 1550 c at 1 atm d ti 3 33 10 2 exp 754 56 kj mol 1 r t m 2 s 1 ti diffusivities were found to be similar for experiments run under fluid present conditions a fit to all of the data yields the arrhenius relation d 1 34 102 exp 741 46 kj mol 1 rt m2 s 1 these data suggest that zircon should be extremely retentive of ti chemical signatures indicating that the recently developed ti in zircon crystallization geothermometer watson e b harrison t m 2005 zircon thermometer reveals minimum melting conditions on earliest earth science 308 841 844 will be quite robust in preserving temperatures of zircon crystallization titanium diffuses somewhat faster in zircon than larger tetravalent cations u th and hf but considerably more slowly than pb the ree and oxygen hence ti crystallization temperatures may be retained under circumstances when radiometric ages or other types of geochemical information are lost

the mesozoic palaeorelief of the northern tian shan china terra nova 23 195 205 2011 abstract the tian shan range offers a natural laboratory to study orogenic processes most of the previous studies focused on either the palaeozoic evolution of the range or its cenozoic intracontinental evolution linked with the india asia collision in this study it is shown that detailed field investigations on the relationship between sedimentary cover and basement constrain the mesozoic evolution of the northern tian shan sedimentological observations argue for a limited transport distance for lower and uppermost jurassic deposits geological sections presented in this paper show that in preserved locations triassic to jurassic sedimentary series present a continuous onlap type sedimentary unconformity on the top of the basement at different scales observations clearly evidence the existence of major palaeorelief during the mesozoic according to the present study the topography of the current tian shan range and the associated movements along its northern front structures cannot be considered as the consequence of cenozoic reactivation alone

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

paleozoic tectonic evolution of the tianshan orogen northwestern china abstract the chinese tianshan orogenic belt which extends west east for about 1500 km is one of the most important paleozoic orogenic belts in central asia it includes parts of the tarim yili central tianshan and junggar plates which are microcontinental blocks with precambrian basements and meso cenozoic depositional covers the tarim plate is separated from the yili central tianshan plate by the southern central tianshan suture early carboniferous ophiolitic melanges and blueschists are widespread in this suture and are believed to be produced by the collision between a passive continental margin on the north side of the tarim plate and an active continental margin on the south side of the yili central tianshan plate the collision occurred at the end of early carboniferous the yili central tianshan and the junggar plates are separated by the northern central tianshan suture also considered as an early carboniferous collisional zone along which the middle ordovician ophiolitic melanges blueschists and late devonian early carboniferous ophiolites are exposed in early paleozoic times the subduction of the paleo junggar ocean took place in an active margin on the north side of the yili central tianshan plate along the northern central tianshan suture at the end of early carboniferous the north side of the yili central tianshan plate collided with the north tianshan arc along this suture the paleo oceanic crust in the tianshan and adjacent region was entirely consumed at the end of early carboniferous meanwhile the tarim yili central tianshan and junggar plates were accreted to eurasia epicontinental sediments were then deposited throughout the tianshan region during the middle late carboniferous times in the early permian tianshan began to uplift with accompanying volcanic eruption and granitic intrusion the uplift was accelerated in the late permian and continental molasse was extensively deposited

mesoproterozoic grenville age terranes in the kyrgyz north tianshan zircon ages and nd hf isotopic constraints on the origin and evolution of basement blocks in the southern central asian orogen abstract the north tianshan orogenic belt in kyrgyzstan consists predominantly of neoproterozoic to early paleozoic assemblages and tectonically interlayered older precambrian crystalline complexes and formed during early paleozoic accretionary and collisional events one of the oldest continental fragments of late mesoproterozoic grenvillian age occurs within the southern part of the kyrgyz north tianshan using shrimp zircon ages we document two magmatic events at 1 1 and 1 3 ga the younger event is characterized by voluminous granitoid magmatism between 1150 and 1050 ma and is associated with deformation and metamorphism the older event is documented by 1 3 ga felsic volcanism of uncertain tectonic significance and may reflect a rifting episode geochemical signatures as well as nd and hf isotopes of the mesoproterozoic granitoids indicate melting of still older continental crust with model ages of ca 1 2 to 2 4 ga the mesoproterozoic assemblages are intruded by paleozoic diorites and granitoids and nd and hf isotopic systematics suggest that the diorites are derived from melts that are mixtures of the above mesoproterozoic basement and mantle derived material their source is thus distinct from that of the mesoproterozoic rocks emplacement of these plutons into the precambrian rocks occurred between 461 and 441 ma this is much younger than previously assumed and indicates that small plutons and large batholiths in north tianshan were emplaced virtually synchronously in the late ordovician to early silurian the mesoproterozoic rocks in the north tianshan may be remnants of a once larger continental domain whose fragments are preserved in adjacent blocks of the central asian orogenic belt comparison with broadly coeval terranes in the kokchetav area of northern kazakhstan the chinese central tianshan and the tarim craton point to some similarities and suggests that these may represent fragments of a single mesoproterozoic continent characterized by a major orogenic event at 1 1 ga known as the tarimian orogeny

thermo tectonic history of the issyk kul basement kyrgyz northern tien shan central asia abstract lake issyk kul occupies a large late mesozoic cenozoic intramontane basin between the mountain ranges of the northern kyrgyz tien shan these ranges are often composed of granitoid basement that forms part of a complex mosaic assemblage of microcontinents and volcanic arcs several granites from the terskey kungey trans ili and zhetyzhol ranges were dated with the zircon u pb method shrimp la icp ms and yield concordant late ordovician silurian 456 420 ma emplacement ages these constrain the caledonian accretion history of the northern kyrgyz tien shan in the amalgamated palaeo kazakhstan continent the ancestral tien shan orogen assembled in the early permian when final closure of the turkestan ocean ensued collision of palaeo kazakhstan and tarim a late palaeozoic structural basement fabric formed and middle late permian post collisional magmatism added to crustal growth of the tien shan permo triassic cooling 300 220 ma of the ancestral tien shan was unraveled using 40 ar 39 ar k feldspar and titanite fission track ft thermochronology on the issyk kul granitoids apatite thermochronology ft and u th sm he applied to the broader issyk kul region elucidates the meso cenozoic thermo tectonic evolution and constrains several tectonic reactivation episodes in the jurassic cretaceous and cenozoic exhumation of the studied units occurred during a protracted period of intracontinental orogenesis linked to far field effects of late jurassic cretaceous accretion of peri gondwanan blocks from the tethyan realm to eurasian following a subsequent period of stability and peneplanation incipient building of the modern tien shan orogen in northern kyrgyzstan started in the oligocene according to our data intense basement cooling in distinct reactivated and fault controlled sections of the trans ili and terskey ranges finally pinpoint important miocene pliocene 22 5 ma exhumation of the issyk kul basement late cenozoic formation of the tien shan is associated with ongoing indentation of india into eurasia and is a quintessential driving force for the reactivation of the entire central asian orogenic belt

introduction to the 1997 8 el nino atlas of oceanographic conditions along the west coast of north america 23 n 50 n abstract hydrographic data collected along the west coast of north america between january 1997 and january 1999 have been compiled into a web based atlas of the 1997 8 el nino event this paper discusses the organization of the atlas describes the data that were incorporated into the atlas and explains how vertical and horizontal distributions for the different properties were constructed examples of figures from the atlas are shown

late paleozoic paleogeographic reconstruction of western central asia based upon paleomagnetic data and its geodynamic implications abstract carboniferous to permian volcanoclastic rocks have been collected from south junggar and west junggar primary magnetizations have been observed from the characteristic components of 10 sites of early permian p1 and late permian p2 red beds of south junggar area the 14 early carboniferous sites from west junggar mountains expose post folding secondary magnetizations and according to their spatial distribution 9 remagnetized sites are related to late carboniferous early permian granite emplacement whereas 5 sites are located at the vicinity of late permian mafic dykes two new paleomagnetic poles have been consequently calculated for the periods of p1 at 79 5 n 36 6 e and of p2 at 60 4 n 4 7 e with a95 of 6 8 and 5 4 respectively they yield two paleomagnetic poles at 65 3 n 329 7 e with a95 of 6 3 and 64 8 n 179 5 e with a95 of 6 9 respectively compilation of available data shows stationary and consistent poles for south junggar area during the carboniferous and permian whereas nw junggar underwent a significant anticlockwise rotation between the late carboniferous early permian and the late permian indicating that junggar was not a rigid block up to the end of the paleozoic west junggar and south junggar may have experienced contrasting tectonic evolutions comparisons of late paleozoic poles of central asia blocks show 1 counterclockwise rotation of west junggar with respect to siberia contrasting with the clockwise rotation of north kazakhstan with respect to siberia 2 no significant movements between west junggar north kazakhstan and siberia since late permian indicating that they were rigidly welded since that time and 3 anticlockwise rotations of tarim yili and south junggar with respect to the welded siberia kazakhstan west junggar block such rotations may have been accommodated by late permian to early triassic strike slip faults with an estimation of the displacements of 1570 280 km along the irtysh gornotsaev shear zone 410 380 km along the nikolaiev nalati tectonic line and 490 250 km along the chingiz alakol north tian shan fault since late permian time

structural overprints of early paleozoic arc related intrusive rocks in the chinese central tianshan implications for paleozoic accretionary tectonics in sw central asian orogenic belts abstract the chinese central tianshan constitutes the centric and vital component of the tianshan orogen it contains important information for understanding the accretionary orogeny of the central asian orogenic belt caob to better constrain the paleozoic evolution of the chinese central tianshan we present new field observations fabric analyses zircon u pb ages zircon lu hf isotopic and whole rock geochemical data of mafic intermediate and felsic intrusive rocks in the baluntai area chinese central tianshan field scale and microscopic structural patterns of the intrusive rocks reveal that the early paleozoic gabbro diorite intrusions underwent multistage structural reworking and generally exhibit gently south or north dipping foliations and microscopic kinematic fabrics indicating top to the north shearing these ductilely deformed intrusive rocks were intruded by granitic sills or dikes that are generally sub parallel or with low angle to the main foliations of the host rocks gentle foliations and top to the north kinematics can be rarely noticed within the granitic intrusions at the sites close to the baluntai fault locally intense mylonitization occurred in the felsic intrusions and deformed mafic intermediate ones where earlier shallow foliations were completely or partially transformed to steep ones bearing sub horizontal stretching lineations and asymmetric kinematic indicators suggest dextral shearing zircon la icp ms u pb datings constrain the emplacement ages of the gabbroic and dioritic intrusive rocks at 452 420 ma and the crystallization age of one mylonitic k granite at 417 ma two samples of granitic sill contain plenty of zircons inherited from the host gabbroic and dioritic rocks and a few such zircons have dark rims probably formed during granitic injection dating on these rims roughly constrains the crystallization ages of the felsic sills at ca 370 360 ma the ehf t values of a gabbro 420 ma two diorite samples 425 and 445 ma and a mylonitic granite 417 ma vary from 15 45 to 2 06 corresponding to two stage crustal model ages tdm c ranging from 2 4 ga to 1 5 ga suggesting variable degrees of involvement of the proterozoic crustal basement and the juvenile mantle components during the early paleozoic magmatic generation the geochemical data of the mafic intermediate plutons exhibit features of magmatic rocks formed in continental arcs the mylonitic granite and one undeformed granite resemble a type granite in terms of geochemical features suggesting that these granites were produced in a post collisional extensional environment considering the previously published data the early paleozoic arc type magmatic rocks within the chinese central tianshan likely resulted from the southward subduction of an oceanic basin located to the north and ceased progressively during 420 400 ma the amalgmation between chinese central tianshan and tarim most likely completed at ca 360 ma following the closure of the south tianshan back arc basin this event reworked the chinese central tianshan arc and formed the top to the north kinematics thereafter the chinese central tianshan was affected by the early permian dextral strike slip faulting due to the regional large scale transcurrent tectonics and overprinted by the permian mafic magmatism

timing of the tianshan mountains uplift constrained by magnetostratigraphic analysis of molasse deposits abstract the tianshan mountains lie in the actively deforming part of the india asia collision zone the thrust faulting and folding of the neogene and quaternary sediments forming linear roughly east west anticlines at the northern and southern borders of the orogenic belts indicate crustal shortening and thickening in late cenozoic time detailed magnetostratigraphic studies of the molasse deposits of the folded neogene and quaternary strata in the northern edge of the tianshan mountains indicate that the onset of uplift occurred at 7 ma deposition of the thick conglomerates between 7 and 2 58 ma in the studied region was mainly in response to the tectonic uplift while the accumulation of the early pleistocene conglomerates xiyu formation was mostly controlled by both tectonics and climatic cooling our results indicate that the present high relief of the tianshan mountains is the result of two dominant phases of uplift occurring at about 7 2 58 ma and the early pleistocene

radiation damage control on apatite u th he dates from the grand canyon region colorado plateau individual detrital apatite grains from the esplanade coconino and moenkopi formations in the grand canyon region of the colorado plateau yield u th he dates from 104 to 5 ma the range of dates within each unit far exceeds analytical uncertainty but correlates with both he concentration he and effective u concentration eu these dates are all signifi cantly younger than the sandstone units indicating partial to complete he loss following deposition recently published laboratory diffusion data suggest that he retentivity in apatite increases with radiation damage forward models predict that the consequences of this effect will be manifested most clearly as a correlation between u th he dates and the he and eu in suites of apatites that 1 are characterized by a large span of eu and 2 had thermal histories in which suffi cient time elapsed for the apatite he diffusion kinetics to diverge prior to reheating and partial resetting apatites in the sedimentary units investigated fi t these cri teria using geologically reasonable deposition burial and unroofi ng histories simulations that include the effect of radiation damage on apatite he retentivity can reproduce the observed distributions of apatite dates and correlations with parent and daughter concentrations these results suggest that a span of u th he dates positively correlated with eu may provide important information regarding a sample s thermal history

modelling the effect of arbitrary p t t histories on argon diffusion in minerals using the macargon program for the apple macintosh argon diffusion in mineral grains has been numerically modelled using p t t histories that may be relevant to multiply metamorphosed orogenic terranes and for rocks that have resided at high ambient temperatures in the earth s crust for long durations the macargon program1 generates argon concentration profiles in minerals assuming argon loss occurs via volume diffusion it can be run on an apple macintosh computer with arbitrary p t t histories used as input finite difference equations are used in the calculation of 40ar concentration profiles across individual diffusion domains the associated macspectrometer generates model spectra after a p t t history has been specified the form of model 40ar39ar apparent age spectra suggests that considerable caution needs to be exercised in the use of the closure temperature concept and in the interpretation of the significance of plateaux observed in many 40ar39ar apparent age spectra particularly in cases involving metamorphic rocks where complex p t t histories might apply although modelled spectra cannot be directly compared to experimentally determined 40ar39ar age spectra especially when hydrous phases are involved or in cases where loss of argon has not occurred via volume diffusion they do provide insight into theoretically expected age spectra for samples that have experienced complex p t t histories

paleozoic accretion and cenozoic redeformation of the chinese tien shan range central asia the tien shan range in central asia contains two late paleozoic sutures the older southern suture marks the collision of a passive margin at the north of the tarim block and an active continental margin subduction under the latter was to the north the younger northern suture separates a northern carboniferous island arc from an active continental margin developed over a south dipping subduction zone the subduction direction under the island arc is unknown mesozoic elastics were deposited over the doubly sutured orogen rate and energy of sedimentation waned until deposition of oligocene conglomerates above a regional unconformity interpreted as marking the onset of deformation induced by the india asia collision molasse deposition accelerated in pliocene and quaternary time and deposition continues today as active thrusts generate relief paleozoic structures control the gross divergence of cenozoic thrusts across the orogen

apatite u th sm he thermochronology of rapidly cooled samples the effect of he implantation apatite u th sm he ahe thermochronology is increasingly used for reconstructing geodynamic processes of the upper crust and the surface results of ahe thermochronology however are often in conflict with apatite fission track aft thermochronology yielding an inverted age relationship with ahe dates older than aft dates of the same samples this effect is mainly explained by radiation damage of apatite either impeding he diffusion or causing non thermal annealing of fission tracks so far systematic age inversions have only been described for old and slowly cooled terranes whereas for young and rapidly cooled samples too old ahe dates are usually explained by the presence of undetected u and or th rich micro inclusions we report apatite u th sm he results for rapidly cooled volcanogenic samples deposited in a deep ocean environment with a relatively simple post depositional thermal history robust age constraints are provided independently through sample biostratigraphy all studied apatites have low u contents 5 ppm on average while aft dates are largely in agreement with deposition ages most ahe dates are too old for leg 43 where deposition age of sampled sediment is 26 5 29 5 ma alpha corrected average ahe dates are up to 45 ma indicating overestimations of ahe dates up to 50 this is explained by he implantation from surrounding host u th rich sedimentary components and it is shown that ahe dates can be corrected by mechanically abrading the outer part of grains we recommend that particularly for low u th apatites the possibility of he implantation should be carefully checked before considering the degree to which the alpha ejection correction should be applied

underplating related adakites in xinjiang tianshan china abstract intermediate to acidic porphyries and dacites in the awulale and sanchakou area of the xinjiang tianshan region china conform to the definition of high sio2 adakite hsa these volcanic or subvolcanic rocks are characterized by high na2o na2o k2o high sr y 51 327 and strong depletions in hree and hfse nb ta and ti despite these characteristics they are distinct from a suite of adakites in the same area that are inferred to be derived from partial melting of subducted slab differences between the two suites are evident mainly in their rock association isotopic ages and compositions of sr and nd isotopes the subducted slab related adakites are associated with common island arc rocks niobium enriched basalt neb and high mg andesite hma formed in the late carboniferous 320 334 ma and are characterized by relative high nd t 3 4 11 6 and low 87sr 86sr i ratios 0 7032 0 7063 in contrast the awulale and sanchakou adakites were generated during the middle to late permian 260 278 ma possess a wide range of low nd t 0 75 5 69 a greater range of 87sr 86sr i 0 7039 0 7054 and have young nd model ages t2dm 472 699 ma in addition their mg 35 56 and compatible element contents mgo 0 93 2 22 wt cr 4 95 16 41 ppm ni 2 9 25 8 ppm are relative low compared to the subducted slab related adakites mg 55 71 mgo 1 22 6 78 wt cr 24 132 ppm ni 2 28 45 61 ppm based on these characteristics the regional association of igneous rocks and evidence from a global geoscience transect ggt the awulale and sanchakou adakites represent a genetically distinct suite in the xinjiang region we proposed that the source of these adakites was basaltic rock underplated to the base of the lower crust that partial melted at the rutile bearing amphibole eclogite facies 650 c and 1 5 gpa minimum pressure 50 km

continental vertical growth in the transitional zone between south tianshan and tarim western xinjiang nw china insight from the permian halajun a1 type granitic magmatism article i nfo the south tianshan collisional belt stcb and northern margin of the tarim block ntb are key areas for understanding the prolonged tectonic evolution of the central asian orogenic belt caob the halajun region in xinjiang province nw china is located within the tectonic transition zone between stcb and tarim blocks several granitic intrusions and one mafic ultramafic complex piqiang complex are exposed in this region zircon u pb dating whole rock major oxide trace element and nd isotopic data are presented for the huoshibulake tamu kezi ertuo and halajun ii granitic intrusions in this area new la icp ms u pb age for kezi ertuo intrusion coupled with previously published shirmp u pb ages for huoshibulake and halajun ii intrusions and piqiang complex reveals that all the igneous rocks in the halajun region are coeval 275 ma geochemically the four granitic intrusions show high contents of sio2 k 2o and total alkalis and possess trace element patterns characterized by rb nb ta zr and hf enrichment and significantly negative ba sr p eu and ti anomalies these features strongly favor an a1 type affinity for the halajun granitic intru sions among the four intrusions the kezi ertuo tamu and halajun ii intrusions possess positive to slightly negative end t values ranging from 0 9 to 0 6 whereas the huoshibulake intrusion displays less deplet ed end t values of 2 6 to 2 9 our new elemental and isotopic data suggest that the four granitic intru sions were generated by the partial melting of a common neoproterozoic gabbroic source probably as a result of the 275 ma underplating of the asthenosphere mantle derived magmas the variable involvement of the mantle components accounts for the range of end t values after the generation of the parental magma alkali feldspar arfvedsonite biotite fe ti oxides and zircon seem to have fractionated prior to the final emplacement of the granitic magmas in combination with the regional geological history we propose that the 275 ma a1 type granitic magmatism in the halajun region and other areas of the ntb provides a good proxy record for the vertical continental crustal growth in the southern margin of the caob during the permian our study in combination with other geological evidence indicates that these a1 type felsic and oib like mafic ultramafic rocks with ages from 282 ma to 275 ma in the southern margin of the caob are parts of the permian tarim large igneous province and could be genetically related to the tarim mantle plume

interpretation of u th he single grain ages from slowly cooled crustal terranes a case study from the transantarctic mountains of southern victoria land low temperature thermochronologic techniques e g apatite fission track aft thermochronology and u th he dating constrain near surface t t paths and are often applied to uplift denudation and landscape evolution studies samples collected in vertical profiles from granitic walls on either side of the ferrar glacier southern victoria land antarctica were analyzed using aft thermochronology and apatite u th he dating to further constrain the lowest temperature thermal history of this portion of the transantarctic mountains aft central ages vary systematically with elevation and together with track length information define a multi stage cooling denudation history in the cretaceous and early tertiary apatite u th he single grain age variation with elevation is not as systematic with considerable intra sample age variation although many complicating factors e g u and th rich micro inclusions fluid inclusions variation in crystal size particle ejection correction zonation and particle ejection correction implantation of he into a crystal or impediment of he diffusion out of a crystal and 147sm derived particles may contribute to age dispersion we found that variation in single grain ages correlated with cooling rate samples that cooled relatively quickly have less variation in single grain ages whereas samples that cooled relatively slowly 3 c m y or resided within an u th he partial retention zone heprz prior to more rapid cooling have a comparatively greater variation in ages decay of u and th via particle emission creates a 4he concentration profile dependent upon the initial parent u th within a crystal variation of single grain ages for samples with non homogeneous u th distributions will be enhanced with long residence time in the partial retention zone i e slow cooling because of the relative importance of loss via volume diffusion and loss via particle ejection with respect to the u th zonation and the grain boundary correction of ages for particle ejection ft correction factor typically assumes uniform u and th distribution within the crystal and when applied to a population of crystals with different u and th distributions will enhance the variation in ages most complicating factors listed above for apatite u th he ages result in ages that are too old we propose that if considerable variation in u th he single grain ages exists that a weighted mean age is determined once outlier single crystal ages are excluded using the criterion of chauvenet or a similar approach we suggest that the true age or most representative age for that age population lies between the minimum u th he age and the weighted mean age we apply this approach coupled with composite age profiles to better constrain the t t history of the profiles along the ferrar glacier significant intra sample variation in single crystal apatite u th he ages and other minerals dated by the u th he method should be expected especially when the cooling rate is slow the variation of u th he single crystal ages is therefore another parameter that can be used to constrain low temperature thermal histories

fluorite u th he thermochronology constraints on the low temperature history of yucca mountain nevada abstract fluorite is one of the secondary minerals precipitated in pore spaces at the future nuclear waste repository site at yucca mountain nevada the authors have conducted u th he dating of this fluorite in an attempt to constrain the temperature and timing of paleo fluid flux into the site repeated analysis of colourless fluorite yielded a weighted average age of 9 7 0 15 ma 2 younger than previously determined sanidine 40ar 39 ar ages 12 8 ma for deposition of the tuff laboratory he diffusion experiments conducted on the yucca fluorite yield a preliminary he closure temperature tc of 90 10 c cooling rate of 10 c ma and previous studies have determined that the fluorite precipitated from warm fluids 65 80 c at depths of it was observed that the purple coloured outer portion of the fluorite nodule yielded non reproducible and invariably older ages than colourless fluorite several possible reasons are suggested