first early permian paleomagnetic pole for the yili block and its implications for late paleozoic postorogenic kinematic evolution of the sw central asian orogenic belt we conducted a paleomagnetic study on the early permian volcanic and sedimentary rocks and the neoproterozoic mafic dikes in the yili block nw china magnetite and hematite were proven to be the principal magnetic remanence carriers demagnetizations revealed stable characteristic remanence magnetizations with a sole reversed magnetic polarity the magnetic remanence of only the early permian strata turned out to be primary based on positive fold tests meanwhile the magnetic remanence age of the mafic dikes is ambiguous accordingly the first early permian paleomagnetic pole for the yili block is calculated at 81 5 n 256 5 e n 11 and a95 10 9 comparisons of this new pole with published ones from the yili tarim and south junggar blocks provide new quantitative constraints on late paleozoic kinematic evolution of the sw central asian orogenic belt 1 between the yili and tarim blocks significant relative movement took place along major strike slip faults during the late carboniferous to early permian 580 290 km and the early to late permian 585 340 km and the displacement rate increased from the late carboniferous to early permian 19 3 9 7 mm yr to the early to late permian 29 3 17 0 mm yr 2 a significant relative rotation of 28 3 18 3 in the late permian and a lateral displacement of 630 295 km after the late permian occurred between the yili and south junggar blocks the significant strike slip movements played an important role in the formation of the central asian orogenic belt and should be considered with great attention in tectonic and paleogeographic reconstructions

mineralogy and mineral chemistry related to the au mineralization in the dunde fe zn deposit western tianshan abstract the dunde deposit which is located in the awulale fe metallogenic belt xinjiang china is a uniquely large fe zn deposit and also contains considerable au resources we present the results of a comprehensive study of this deposit based on fieldwork mineralogy and mineral geochemistry to constrain the au mineralization our study indicates that the mineralization of fe zn and at least some au precipitated in the same hydrothermal system the au mineralization which occurs in the sulfide stage is characterized by invisible au at high temperatures and by electrum at low temperatures unusual sulfide is recognized in the deposit such as cobaltite and bismuth minerals pyrrhotite compositions indicate that the early precipitation of sulfide generated in high temperatures 410 570 and high sulfur fugacities 2 47 to 6 68 arsenopyrite compositions indicate that the late precipitation of sulfide formed in low temperatures 326 393 and low sulfur fugacities 7 41 to 12 00 bismuth minerals exhibit a close relationship with the au mineralization in high temperatures and high oxygen fugacities bismuth dissolved in the hydrothermal fluid as bi3 and precipitated as galenobismuthite and cosalite accompanying precipitation of the invisible au accompanying the decrease of oxygen fugacities and temperatures the bi melt directly separated from the hydrothermal fluid and scavenged au the reduced condition further enriched au in bi melts through the decomposition of the invisible au bearing bi pb sulfide the decrease of temperatures caused the decomposition of the bi au melt and the formation of native bismuth bismuthinite and electrum in addition to the mineralization of fe zn and au considerable cu co and mn bearing sulfide precipitated in the zn ores integrated utilization of the zn ores may get greater economic values

magnetic properties of sedimentary greigite fe3s4 an update greigite fe3s4 is an authigenic ferrimagnetic mineral that grows as a precursor to pyrite during early diagenetic sedimentary sulfate reduction it can also grow at any time when dissolved iron and sulfide are available during diagenesis greigite is important in paleomagnetic environmental biological biogeochemical tectonic and industrial processes much recent progress has been made in understanding its magnetic properties greigite is an inverse spinel and a collinear ferrimagnet with antiferromagnetic coupling between iron in octahedral and tetrahedral sites the crystallographic c axis is the easy axis of magnetization with magnetic properties dominated by magnetocrystalline anisotropy robust empirical estimates of the saturation magnetization anisotropy constant and exchange constant for greigite have been obtained recently for the first time and the first robust estimate of the low field magnetic susceptibility is reported here the curie temperature of greigite remains unknown but must exceed 350 c greigite lacks a low temperature magnetic transition on the basis of preliminary micromagnetic modeling the size range for stable single domain behavior is 17 200 nm for cubic crystals and 17 500 nm for octahedral crystals gradual variation in magnetic properties is observed through the pseudo single domain size range we systematically document the known magnetic properties of greigite at high ambient and low temperatures and with alternating and direct fields and illustrate how grain size variations affect magnetic properties recognition of this range of magnetic properties will aid identification and constrain interpretation of magnetic signals carried by greigite which is increasingly proving to be environmentally important and responsible for complex paleomagnetic records including widespread remagnetizations

pyrite framboid evidence for oxygen poor deposition during the permian triassic crisis in kashmir abstract the khunamuh formation of the guryul ravine section in kashmir provides one of the most detailed deep water records of the end permian mass extinction in perigondwanan palaeolatitudes examination of pyrite framboid size distributions and sediment microfabric reveals a distinct change in the uppermost permian strata from nonlaminated pyrite free silty mudstones to finely laminated silty shales rich in framboidal pyrite around 1 m above the base of the formation the size frequency distribution of the framboids is identical to populations from severely oxygen restricted modern depositional environments this change to dysoxic benthic conditions coincides with the rapid demise of diverse permian brachiopod and bryozoan assemblages thus the link between extinction and anoxia is clearly seen at guryul ravine and it provides an interesting contrast to other perigondwanan sections developed in shallower water where both the onset of anoxia and the extinction occurred somewhat later in the earliest triassic

geological characteristics and metallogenesis of iron deposits in western tianshan china abstract the western tianshan metallogenic belt is one of the most significant polymetallic iron metallogenic belts in china important advances have been achieved recently in iron exploration in the awulale mountain in western tianshan china these newly discovered iron deposits are mainly hosted in the basic medium andesitic lavas and volcaniclastics often comprising a number of high grade ores magnetite is predominated in ore mineral assemblages and pyrite chalcopyrite pyrrhotite or sphalerite increase in certain deposits wallrock alterations are intensively developed exemplified as sodic calcic and potassic alterations which display in different patterns as country rocks and ore controlled structures vary skarn assemblages are commonly developed in ore districts like beizhan dunde and chagangnuoer and pyroxene albite k feldspar epidote actinolite alterations are dominated around ore bodies in zhibo deposit whereas the shikebutai deposit develops alteration assemblages comprising of jasper barite sericite and chlorite thus iron deposits can be divided into three types including volcanic sedimentary type volcanic magmatic hydrothermal type and iron skarn type our preliminary interpretation about the tectonic background of this iron mineralization in this area is in the late stage of a collisional accretional orogenic belt around carboniferous with some extrusional extensional tectonic transition locally iron mineralization is likely to have a close genetic relationship with volcanic subvolcanic activity syn or slightly post the volcanism which took place besides continental arc volcanic eruption contributes to majority of mineralizing iron with minor extracted from hydrothermal replacement from wall rocks

magnetite composition of zhibo iron deposit in western tianshan mountains and its genetic significance located in the eastern awulale metallogenic belt of western tianshan mountains the large size zhibo iron deposit is hosted in basaltic andesite andesite and pyroclastic rocks of the lower carboniferous dahalajunshan formation the zhibo iron deposit consists of four main areas the eastern zone the middle zone the western zone and no 13 ore body in which the ore bodies occur as layers stratoid bodies or lenses mineralization consists of massive magnetite associated with minor disseminations of pyrite and veinlets of hematite and trace chalcopyrite the ore structures include mainly massive and disseminated subordinately brecciated and occasionally banded or vein stockwork forms whereas the ore textures consist of ubhedral anhedral grainular metasomatic relict and platy forms the principal alteration mineral assemblages in zhibo include diopside albite k feldspar epidote and actinolite together with calcite quartz chlorite and titanite on the basis of observed mineral assemblages and ore textures as well as mineral analyses two periods of ore deposition have been recognized magmatic period and hydrothermal ore forming period which could be further subdivided into three metallogenic stages i e magnetite diopside epidote stage a1 magnetite k feldspar epidote stage b1 and quartz sulfide stage b2 electron microprobe analyses show that the magnetite with magmatic features has higher content of feot and lower content of al2o3 cao mgo sio2 in comparison with the magnetite with hydrothermal features the content of v2o5 in brecciated and disseminated ore is relatively high similar to the content of volcanic rocks suggesting that part of iron of the ore forming stage came from the wall rock massive and disseminated magnetite has varied feot values mostly above 90 feot content of magnetite from brecciated veined and dendritic ores varies between 90 and 92 the feot content of magnetite from laminated ore varies between 88 and 92 with higher average content of other oxides such as cao and sio2 both ternary plot of tio2 al2o3 mgo and ca al mn versus ti v discriminant diagram suggest that the formation of the zhibo iron deposit was associated with volcanic activity and hydrothermal metasomatism of magmatic hydrothermal fluid

flow of variably fluidized granular masses across three dimensional terrain 2 numerical predictions and experimental tests numerical solutions of the equations describing flow of variably fluidized coulomb mixtures predict key features of dry granular avalanches and water saturated debris flows measured in physical experiments these features include time dependent speeds depths and widths of flows as well as the geometry of resulting deposits three dimensional 3 d boundary surfaces strongly influence flow dynamics because transverse shearing and cross stream momentum transport occur where topography obstructs or redirects motion consequent energy dissipation can cause local deceleration and deposition even on steep slopes velocities of surge fronts and other discontinuities that develop as flows cross 3 d terrain are predicted accurately by using a riemann solution algorithm the algorithm employs a gravity wave speed that accounts for different intensities of lateral stress transfer in regions of extending and compressing flow and in regions with different degrees of fluidization field observations and experiments indicate that flows in which fluid plays a significant role typically have high friction margins with weaker interiors partly fluidized by pore pressure interaction of the strong perimeter and weak interior produces relatively steep sided flat topped deposits to simulate these effects we compute pore pressure distributions using an advection diffusion model with enhanced diffusivity near flow margins although challenges remain in evaluating pore pressure distributions in diverse geophysical flows riemann solutions of the depth averaged 3 d coulomb mixture equations provide a powerful tool for interpreting and predicting flow behavior they provide a means of modeling debris flows rock avalanches pyroclastic flows and related phenomena without invoking and calibrating rheological parameters that have questionable physical significance

mo deposits in northwest china geology geochemistry geochronology and tectonic setting abstract northwest china covering northern xinjiang northern gansu and westernmost inner mongolia mainly includes junggar basin and its surrounding mountains such as chinese altay junggar chinese tianshan and beishan it lies at the junction of siberia tarim and kazakhstan plates and is a key sector of the central asian orogenic belt caob characterized by multistage phanerozoic continental growth herein at least nine mo only or mo dominated fourteen cu mo two w mo and one be mo deposits have been discovered these 27 deposits occur in altay west jungar west tianshan east tianshan and beishan areas and have been formed during accretionary or collisional orogenies the majority of the deposits are porphyry type followed by the skarn and quartz vein types the orebodies occur mainly as veins lens pods in the positions from inner intrusions through contact zones to the hostrocks distal to causative intrusions the host rocks are variable in lithologies including granites porphyries volcanic breccias and tuffs and sedimentary rocks outward from orebodies to hostrocks the wallrock alteration is zoning from potassic k feldspar quartz mica through phyllic quartz sericite chlorite epidote to propylitic or argillic alterations with skarn specifically occurring in skarn type systems hydrothermal mineralization generally includes four stages from early to late represented by 1 potassic feldspar quartz veins or veinlets 2 quartz molybdenite stockworks 3 quartz polymetallic sulfide stockworks and 4 quartz carbonate fluorite veins or veinlets the ore forming fluids were initially magmatic in origin and shew high temperature and high salinity containing daughter mineral and or co2 bearing fluid inclusions and eventually evolved to low temperature low pressure low salinity and co2 poor meteoric water the porphyry mo deposits can be further subdivided into two subtypes i e dabie and endako types the endako type mo deposits e g suyunhe and hongyuan together with all the cu mo systems were formed in the palaeozoic subduction related magmatic arcs the dabie type porphyry mo deposits represented by giant donggebi and baishan together with the mo only mo dominated and w mo or be mo deposits were formed in syn to post collisional tectonic setting with isotope ages ranging 260 213 2 ma with the kumutage skarn type mo system being an exception aged 319 ma the dabie type porphyry mo deposits are characterized by the co2 bearing fluid inclusions that cannot be observed in the endako type porphyry mo systems the re contents in molybdenites from porphyry and porphyry skarn cu mo systems are mainly 100 ppm suggesting a source significantly contributed by the mantle whereas the re contents in molybdenites from the mo only or w mo deposits are mainly

geological features mineralization types and metallogenic setting of late paleozoic iron deposits in western tianshan mountains of xinjiang the western tianshan metallogenic belt is one of the significant polymetallic iron metallogenic belts in china notable advances have been made in the exploration of iron ore resources in western tianshan mountains especially in the awulale metallogenic belt where several iron deposits have been discovered or explored such as chagangnuoer beizhan zhibo dunde songhu wuling and nixintage akesayi this region has therefore become an important large scale iron ore development base area these newly found iron deposits mostly of medium to large size with high grade are mainly hosted in the mafic intermediate andesitic lavas and volcaniclastics however these regional iron ore deposits have been insufficiently studied and hence their mineralization environment and metallogenic regularity are poorly known due to lack of a comprehensive overall study based on the previous researches combined with lots of field investigations and indoor collations in recent three years done by the authors this paper tentatively makes a comprehensive elucidation of the geological characteristics distribution patterns and mineralization types of iron deposits in western tianshan mountains of xinjiang these iron deposits can be divided into two types namely marine volcanic type and skarn type and the former can be further divided into three subtypes i e volcanic sedimentary volcanic magmatic hydrothermal and skarniod subtypes the authors preliminarily discuss the geological setting of iron metallogeny in western tianshan mountains and hold that this area might belong to the late stage of a collisional orogenic continental environment in carboniferous period with the development of the extrusion extensinal tectonic transition somewhere and these geological settings were favorable for iron mineralization based on comparing the characteristics between iron deposits in this region and typical volcanics hosted iron deposits both in china and abroad the authors consider that iron deposits were genetically closely related to volcanic intrusive activity and formed contemporaneously with or slightly later than volcanic activity in the early ore forming stage filling metasomatism of iron rich fluid melt played the major role and mineralization became intensified by hydrothermal metasomatism in the late stage accompanied by lots of hydrothermal rock alteration in the whole metallogenic process mineralizing iron material was probably mainly derived from the depth and from arc magmatism with minor iron material extracted from volcanic subvolcanic gas and fluid wall rock metasomatism iron rich fluid melt might be created by the mafic magmatic differentiation generated in the subduction nevertheless the detailed formation mechanism magmatic origin and evolutionary process remain the keynote questions deserve further research

iron rich fragments in the yamansu iron deposit xinjiang nw china constraints on metallogenesis abstract volcanic rock hosted iron deposits are among the important iron ores in china however the nature of primary magma and petrogenesis associated with these iron ores remains controversial here we report iron rich fragments irf from the yamansu iron deposit in eastern tianshan mountains nw china which occurs in association with volcanic breccia submarine volcanic breccia and ignimbrite the irf is composed of five types including oligoclase iron oxide type oio oligoclase albite iron oxide type oaio albite iron oxide type aio albite k feldspar iron oxide type akio and k feldspar iron oxide type kio these fragments display typical volcanic fabric features such as porphyritic texture hyalopilitic texture of the groundmass and vesicles filled by minerals to form amygdales the feldspar phenocrysts of irf are dominantly albite the groundmass of irf consists of magnetite and feldspar the magnetite is distributed in between the feldspar laths and together display hyalopilitic texture which could be observed only in volcanic rocks the vesicles are filled with magnetite feldspar chlorite and calcite from the margin to the interior the irf has high si al fe ca ti na and k contents and low mg content the average total fe is 26 wt the magnetite is mostly titanium vanadium magnetite with the tio2 content ranging up to 4 86 wt and v2o3 content up to 3 20 wt the irf probably came from iron rich melts and represent the products of the fenner magma evolution the basaltic magma evolved into the fe na rich residual melts by crystallization under low oxygen fugacity condition in a closed magma chamber after intruding into the shallow crust the fe na rich residual melts were emplaced in hypabyssal environments or erupted generating the orebodies or providing the material source for the generation of the high grade iron ores which were subsequently enriched by the late stage hydrothermal fluids

an experimental method for testing soil mobility of landslides debris flows can travel great distances from their sources and pose hazards to communities downstream identification of soils susceptible to liquefaction during slope failure could aid in locating potential debris flow sources we used a modified version of the slump test typically used to measure the consistency of wet concrete mix to test the proclivity of soil to flow the method can be used to identify a threshold density below which a soil will flow under field saturated conditions we performed 51 soil mobility tests on 13 sandy and gravelly soils with 0 16 silt and clay fines and one sample with 72 fines collected from landslide and nonlandslide sites results are consistent with corresponding site porosity and failure mode for coarse grained soils threshold porosity for flow ranged from 0 39 to 0 65 highest failure velocities were associated with the loosest soils and soils with the lowest permeability failure velocities and modes observed in our tests were consistent with test results for identical soils in a large scale flume on thicker samples 0 65 m that approach average failure depths measured in the field 0 75 m this novel test was successfully designed as a rapidly conducted 24 h to prepare and test samples including overnight saturation low cost alternative to large diameter triaxial tests capable of testing gravelly soils that does not require a predefined shear rate allowing pore pressure to evolve as soil fails under self weight and drained conditions

replacement of pyrite framboids by magnetite in limestone and implications for palaeomagnetism limestones are an important source of palaeomagnetic data with magnetite the dominant carrier of the magnetization because the magnetite is present in low concentrations however it has been directly observed only rarely and in trace amounts in acid treated separates such residues are composed primarily of spheres the ultimate origin of which has been enigmatic7 yet interpretation of the palaeomagnetic data depends on a knowledge of the origin of the magnetite which has been inferred to carry either a chemical or a viscous remanent magnetization here we present electron microscope data obtained from a sequence of samples of onondaga limestone from an east west traverse across northern new york which unambiguously show that magnetite spheres are derived by alteration and replacement of framboidal pyrite remagnetization is thus caused by chemical rather than viscous processes this chemical remanent magnetization is compatible with a fluid mediated event occurring on a regional scale induced by tectonic stress of alleghenian late palaeozoic age

a review of the cu ni sulphide deposits in the chinese tianshan and altay orogens xinjiang autonomous region nw china principal characteristics and ore forming processes abstract several cu ni sulphide deposits and occurrences have recently been discovered along parallel deep faults in the chinese tianshan and altay orogenic belts xinjiang province nw china the kalatongke and several other cu ni mineralized intrusions are located along the irtysh fault which separates the altay orogenic belt from the junggar basin the huangshan huangshandong xiangshan tudun erhongwa tula ergen and hongling deposits occur along the kanggurtag suture which separates the jueluotage orogenic belt from the turpan hami tuha basin the baishiquan tianyu and tianxiang deposits are located along the arqikekuduke fault which separates the jueluotage orogenic belt from the central tianshan precambrian terrane the poyi poshi and luodong deposits are located along the baidiwa fault which separates the central tianshan precambrian terrane from the beishan paleozoic rift re os dating of cu ni sulphide ores reveals that these cu ni ore belts formed in a narrow age range of 298 282 ma this age range is about the same as those of associated intrusions and dykes dated by the shrimp zircon u pb method tectonic and geochronological constraints suggest that the amalgamation of continental blocks mainly occurred during the late carboniferous in the central asian orogenic belt large scale hydrothermal and magmatic metallogenesis in the region occurred during post collisional stages of latest carboniferous to early permian age the cu ni sulphide deposits are part of this metallogenic event the cu ni orthomagmatic sulphide deposits in northern xinjiang are represented by 1 net textured type deposits by segregation of a cu ni metal sulphide melt and 2 magma conduit type some mafic ultramafic suites exhibit lithological zoning caused by strong differentiation stratiform orebodies are hosted by ultramafic rocks at the base of the magma chamber good examples are the kalatongke huangshandong and poshi deposits others such as the xiangshan baishiquan tianyu and tula ergen deposits are hosted by magma conduits consisting of peridotite troctolite and pyroxenite these ultramafic rocks either occur within faults or are surrounded by gabbroic and dioritic intrusions these two types of orthomagmatic cu ni sulphide deposits are also distributed along the same ore belts for instance the differentiated sill related huangshandong deposit co exists with the magma conduit type tula ergen deposit in the jueluotage orogenic belt orthomagmatic cu ni sulphide deposits in northern xinjiang formed during post collisional extension and are possibly related to a late carboniferous early permian mantle plume event the mafic ultramafic suites and associated cu ni deposits are commonly accompanied by dyke swarms and are characterized by elongated outcrops occurring along parallel e w trending regional faults these mafic ultramafic suites and accompanying dyke swarms are generally fractionated implying that they were feeders of presently eroded flood basalts

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

effects of dilatant hardening on the development of concentrated shear deformation in fissured rock masses this study examines the effects of dilatant hardening on the development of concentrated shear deformation specifically the analysis considers the shear of an inelastic ally deforming rock mass containing a weakened layer of thickness h the presence of the weakened layer causes localization instability characterized by an unbounded ratio of a strain increment in the weakened layer to that in the far field to occur earlier than it would be predicted from the response of the material surrounding the embedded layer the development of instability in time depends on the ratio of the rate of imposed shear strain to that for fluid mass exchange between the layers c h2 where c is an effective diffusivity in the limit the pressure in the weakened layer is equal to that in the surrounding material and localization instability occurs at the peak of the drained stress strain curve for finite instability is delayed until the material in the weakened layer has been driven to the peak of its undrained dilatantly hardened stress strain curve the time delay between final instability and the time at which the weakened layer passes the peak of its drained stress strain curve is called the precursor time tpr because rapid straining of the weakened layer occurs during this period for small as appropriate for tectonic applications and most laboratory experiments a nonlinear asymptotic analysis predicts that where is the half width of the peak of the stress strain curve is the difference in the peak stresses of the rock mass and the weak layer divided by times the elastic shear modulus and is a nondimensional measure of the strength of dilatant hardening for a wide range of numerical values the precursor times are very short less than a few hours for tectonic strain rates and less than a few tens of seconds for typical laboratory strain rates

mineralogy fluid inclusions and isotopes of the cihai iron deposit eastern tianshan nw china implication for hydrothermal evolution and genesis of subvolcanic rocks hosted skarn type deposits abstract most skarn deposits are closely related to granitoids that intruded into carbonate rocks the cihai 100 mt at 45 fe is a deposit with mineral assemblages and hydrothermal features similar to many other typical skarn deposits of the world however the iron orebodies of cihai are mainly hosted within the diabase and not in contact with carbonate rocks in addition some magnetite grains exhibit unusual relatively high tio 2 content these features are not consistent with the typical skarn iron deposit different hydrothermal and or magmatic processes are being actively investigated for its origin because of a lack of systematic studies of geology mineral compositions fluid inclusions and isotopes the genetic type ore genesis and hydrothermal evolution of this deposit are still poorly understood and remain controversial the skarn mineral assemblages are the alteration products of diabase three main paragenetic stages of skarn formation and ore deposition have been recognized based on petrographic observations which show a prograde skarn stage garnet clinopyroxene disseminated magnetite a retrograde skarn stage main iron ore stage massive magnetite amphibole epidote ilvaite and a quartz sulfide stage quartz calcite pyrite pyrrhotite cobaltite overall the compositions of garnet clinpyroxene and amphibole are consistent with those of typical skarn fe deposits worldwide in the disseminated ores some magnetite grains exhibit relatively high tio 2 content 1 wt which may be inherited from the diabase protoliths some distinct chemical zoning in magnetite grains were observed in this study wherein cores are enriched in ti and magnetite rims show a pronounced depletion in ti the textural and compositional data of magnetite confirm that the cihai fe deposit is of hydrothermal origin rather than associated with iron rich melts as previously suggested fluid inclusions study reveal that the prograde skarn garnet and pyroxene formed from high temperature 520 600 c moderate to high salinity 8 1 23 1 wt nacl equiv and 46 wt nacl equiv fluids massive iron ore and retrograde skarn assemblages amphibole epidote ilvaite formed under hydrostatic condition after the fracturing of early skarn fluids in this stage had lower temperature 220 456 c and salinity 8 4 16 3 wt nacl equiv fluid inclusions in quartz sulfide stage quartz and calcite also record similar conditions with temperature range from 128 to 367 c and salinity range from 0 2 to 22 9 wt nacl equiv oxygen and hydrogen isotopic data of garnet and quartz suggest that mixing and dilution of early magmatic fluids with external fluids e g meteoric waters caused a decrease in fluid temperature and salinity in the later stages of the skarn formation and massive iron precipitation the 18 o values of magnetite from iron ores vary between 4 1 and 8 5 which are similar to values reported in other skarn fe deposits such values are distinct from those of other iron ore deposits such as kiruna type and magmatic fe ti v deposits worldwide taken together these geologic geochemical and isotopic data confirm that cihai is a diabase hosted skarn deposit related to the granitoids at depth

the marcona magnetite deposit ica south central peru a product of hydrous iron oxide rich melts marcona the preeminent andean magnetite deposit 1 9 gt 55 4 fe and 0 12 cu is located in the iron oxide copper gold iocg subprovince of littoral south central peru fe oxide and cu zn pb sulfide mineralization was controlled by northeast striking faults transecting a middle jurassic aalenian to oxfordian andesitic shallow marine arc and a succession of contiguous plate boundary parallel late jurassic to mid cretaceous volcanosedimentary basins at marcona hydrothermal activity was initiated in the earliest middle jurassic 161 177 ma by high temperature mg fe metasomatism represented by cummingtonite and phlogopite magnetite assemblages subsequently during the terminal eruptions 156 162 ma of the arc widespread albite marialite alteration na cl metasomatism was followed by the emplacement of an en echelon swarm of massive magnetite ore bodies with subordinate overprinted magnetite sulfide assemblages hosted largely by paleozoic metasilici clastics the magnetite orebodies exhibit abrupt smoothly curving contacts dike like to tubular apophyses and intricate amoeboid interfingering with dacite porphyry intrusions there is no convincing megascopic or microscopic evidence for large scale fe metasomatism associated with the main sulfide poor mineralization the largest 400 mt minas 2 3 4 orebody is interpreted as a bimodal magnetite dacite intrusion comprising commingled immiscible melts generated through the dissolution of metasedimentary quartz in parental andesitic magma oxygen and sulfur stable isotope geothermometry indicates that the evolution at ca 159 ma from magnetite biotite calcic amphibole phlogopite fluorapatite to magnetite phlogopite calcic amphi bole pyrrhotite pyrite assemblages coincided with quenching from above 800 c to below 450 c and the concomitant exsolution of dilute aqueous brines subsequently chalcopyrite pyrite calcite pyrrhotite sphalerite galena assemblages in part metasomatic were deposited from lower temperature 360 c brines the cu poor marcona kiruna type magnetite and cu rich iocg deposits in the district therefore although spatially contiguous represent contrasting ore deposit types the former are interpreted as the product of fe oxide melt coexisting with dacite magma within an andesitic arc which failed during the closure of a back arc basin the weak associated magmatic hydrothermal cu sulfide mineralization at marcona was generated through melt vesiculation and contrasts with the considerably higher grade cu and ag rich orebodies of the major cu rich iocg deposits in the central andes e g la candelaria punta del cobre mantoverde raul condestable and mina justa which were the products of cool oxidized hydrothermal fluids plausibly expelled from the adjacent basins during tectonic inversion

review of geology alteration and origin of iron oxide apatite deposits in the cretaceous ningwu basin lower yangtze river valley eastern china implications for ore genesis and geodynamic setting abstract in the cretaceous ningwu volcano sedimentary basin in the yangtze river valley metallogenic belt eastern china there are three areas with a dense distribution of magnetite or hematite deposits the meishan deposit in the north washan nanshan and taocun deposits in the center and the zhongjiu and gushan deposits in the south the mineralization in the ningwu basin is associated mainly with subvolcanic intrusions consisting of gabbro diorite porphyry and or gabbro diorite alteration zoning of these deposits is pronounced and includes 1 an upper light colored zone of argillic kaolinite silica carbonate and pyritic alteration 2 a middle dark colored zone of diopside fluorapatite magnetite phlogopite and garnet with fluorapatite magnetite 3 a lower light colored zone of extensive albitic alteration however at the gushan iron deposit the lower light colored zone and the middle dark colored zone are absent whereas the principal alteration is represented by silicification kaolinization and carbonatization the iron oxide apatite deposits in the ningwu basin are typically magmatic metasomatic origin and are similar to the kiruna type deposits in scandinavia particularly with respect to mineral assemblages fabric and structure of the iron ores occurrence of the orebodies and wall rock alteration the iron oxide apatite deposits of the ningwu basin contain magnetite and or hematite with diopside or actinolite and apatite gangue they were formed in a rift or extensional environment and the mineralization is associated with alkaline magmatism the time interval between magmatism and related mineralization is very short

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

the implications of non suppressed geomagnetic secular variation during the permo carboniferous reversed superchron abstract we have investigated variations in the natural remanent magnetisation nrm in red beds which were deposited during the permo carboniferous reversed superchron pcrs near the paleo equator these red beds are reliable recorders of the geomagnetic field because the nrm is carried by detrital hematite we analysed the angular standard deviation asd and spectral content of the directional records to quantify the amount of paleosecular variation psv during the time of deposition the asd of a high resolution directional record of the dome de barrot which spans approximately 27 kyr is 10 this is comparable to recent 0 5 ma low latitude lavas from the psvrl database mcelhinny and mcfadden 1997 and to recent lake sediments from the secvr database mcelhinny and lock 1996 the frequency spectra of declination and inclination are also similar to those from the recent lake sediments spectral power significant at the 95 level is concentrated in periods of 1 5 kyr higher frequencies are not significant from the asd and spectral analyses it appears that the amount of psv during at least part of the pcrs was comparable to that of recent times several heuristic and numerical geodynamo models are discussed in the context of superchrons and psv there is a discrepancy between the amount of sv shown by our data and that suggested by numerical models which use lateral heat flux variations at the core mantle boundary cmb however computational limitations still force some model parameters to have values that are unrealistic for the earth for the phenomenological geodynamos low energy state models which suggest that psv is suppressed during a superchron are not supported by the dome de barrot data rather a high energy state of the geodynamo in which outer core convection is vigorous is favoured as a prerequisite for a superchron to occur

flow of variably fluidized granular masses across three dimensional terrain 1 coulomb mixture theory rock avalanches debris flows and related phenomena consist of grain fluid mixtures that move across three dimensional terrain in all these phenomena the same basic forces govern motion but differing mixture compositions initial conditions and boundary conditions yield varied dynamics and deposits to predict motion of diverse grain fluid masses from initiation to deposition we develop a depth averaged three dimensional mathematical model that accounts explicitly for solid and fluid phase forces and interactions model input consists of initial conditions path topography basal and internal friction angles of solid grains viscosity of pore fluid mixture density and a mixture diffusivity that controls pore pressure dissipation because these properties are constrained by independent measurements the model requires little or no calibration and yields readily testable predictions in the limit of vanishing coulomb friction due to persistent high fluid pressure the model equations describe motion of viscous floods and in the limit of vanishing fluid stress they describe one phase granular avalanches analysis of intermediate phenomena such as debris flows and pyroclastic flows requires use of the full mixture equations which can simulate interaction of high friction surge fronts with more fluid debris that follows special numerical methods described in the companion paper are necessary to solve the full equations but exact analytical solutions of simplified equations provide critical insight an analytical solution for translational motion of a coulomb mixture accelerating from rest and descending a uniform slope demonstrates that steady flow can occur only asymptotically a solution for the asymptotic limit of steady flow in a rectangular channel explains why shear may be concentrated in narrow marginal bands that border a plug of translating debris solutions for static equilibrium of source areas describe conditions of incipient slope instability and other static solutions show that nonuniform distributions of pore fluid pressure produce bluntly tapered vertical profiles at the margins of deposits simplified equations and solutions may apply in additional situations identified by a scaling analysis assessment of dimensionless scaling parameters also reveals that miniature laboratory experiments poorly simulate the dynamics of full scale flows in which fluid effects are significant therefore large geophysical flows can exhibit dynamics not evident at laboratory scales

high resolution record of the upper olduvai transition from po valley italy sediments support for dipolar transition geometry abstract a detailed record of the upper olduvai polarity transition composed of 100 transitional directions has been obtained from the crostolo section in northern italy a careful examination of the rock magnetic properties of the sediments using standard paleomagnetic techniques thermomagnetic scanning electron microscopy microprobe x ray diffraction and mossbauer analysis shows that greigite is the main magnetic carrier of the remanence the correlation of a shift in the magnetic record to a small scale sedimentary feature observed in the section indicates that the magnetization was acquired at deposition or very shortly after the virtual geomagnetic pole vgp path is largely confined along a great circle over north and south america 90 west of the site and consists of three stages first the vgp moves to southern mid latitudes then after a period of standstill it comes back to almost true north and finally the south pole is reached in a third step although different from a record from the southern indian ocean this path is virtually identical to those obtained for the same transition from north pacific deep sea cores and partly coincides with a north atlantic record which suggests that a dipolar component may be present in the transitional field during the upper olduvai reversal a review of the recently obtained records of various transitions shows that in more than two thirds of the cases the vgp paths are similarly confined along a meridian over the americas or antipodal to them irrespective of the sampling site and of the sense of the transition although not deterministic this tendency suggests that a similar dipolar component might be present in the transitional fields of other reversals of different ages

the cretaceous superchron geodynamo observations near the tangent cylinder abstract if relationships exist between the frequency of geomagnetic reversals and the morphology secular variation and intensity of earth s magnetic field they should be best expressed during superchrons intervals tens of millions of years long lacking reversals here we report paleomagnetic and paleointensity data from lavas of the cretaceous normal polarity superchron that formed at high latitudes near the tangent cylinder that surrounds the solid inner core the time averaged field recorded by these lavas is remarkably strong and stable when combined with global results available from lower latitudes these data define a time averaged field that is overwhelmingly dominated by the axial dipole octupole components are insignificant these observations suggest that the basic features of the geomagnetic field are intrinsically related superchrons may reflect times when the nature of core mantle boundary heat flux allows the geodynamo to operate at peak efficiency

petrogenetic interpretation of granitoid rock series using multicationic parameters abstract granitoid rock compositions from a range of tectonic environments are plotted on a multicationic diagram devised by de la roche and his coworkers this shows that there is a systematic change through an orogenic cycle which leads progressively to the ultimate development of alkaline magmas possible source materials and mechanisms of magma generation are considered from analysis of mineral compositional vectors these suggest that most granitoid series result from a two stage process first fractional crystallisation of clinopyroxene olivine and calcic plagioclase from a basic source with tholeiitic affinities produces a magma of intermediate composition this magma then undergoes periodic mixing with a felsic magma followed by in situ fractionation to generate individual intrusions within granitoid series

post middle oligocene origin of paleomagnetic rotations in upper permian to lower jurassic rocks from northern and southern peru abstract we report paleomagnetic data from 28 sites of upper permian to lower jurassic strata from northern and southern peru in northern peru 6 s a stable magnetic component from six permo triassic sites passes fold and reversal tests the overall mean pole agrees well with late permian to triassic poles from cratonal south america suggesting this part of peru has experienced neither significant rotation nor latitudinal transport since the permo triassic in southern peru 13 to 16 s thermal demagnetization isolates stable magnetic components in 16 of 22 upper permian to lower jurassic sites collected along the transition between the altiplano and the eastern cordillera these 16 sites are rotated 14 to 147 counterclockwise and pass an inclinations only fold test within the same structural zone three other permo triassic sites as well as 10 paleocene sites also show important counterclockwise rotations roperch and carlier j geophys res 97 1992 17233 17249 butler et al geology 23 1995 799 802 the large magnitude and exclusively counterclockwise sense of rotation suggest that the tectonic regime included an important sinistral shear component no correlation exists between rotation amount and rock age suggesting the rotations are post paleocene in age because the rotations occur along the fringe of the eastern cordillera they were likely produced during its structural formation hence from the late oligocene to present sinistral shear acting in the northern part of the bolivian orocline appears much more pronounced than that north of the abancay deflection which likely arises from differences in convergence obliquity

noise in the quiet zone abstract we have carried out a detailed paleomagnetic investigation of two stratigraphically overlapping sections from the scaglia bianca formation 85 89 5 ma in the umbria marche area in central italy sampling was conducted over 32 m and 7 m intervals at la roccaccia and furlo respectively after af cleaning the majority of specimens show the expected normal magnetic field orientation however a number of specimens are directionally anomalous some of these deviant specimens are accompanied by apparent spikes or dips in normalized intensity a detailed investigation of rock magnetics shows that most of these deviations are not a sign of excursionary geomagnetic field behavior but rather correspond to specimens with distinct rock magnetic characteristics and are therefore rock magnetic noise such specimens should not be interpreted as records of the geomagnetic field our experience suggests that detailed rock magnetic and magnetic fabric analysis should be done on all anomalous directions prior to interpreting them as geomagnetic field behavior after elimination of rock magnetic noise in the scaglia bianca data sets there is a high degree of agreement in direction and to a lesser extent relative intensity between correlative portions of the two sections we therefore offer this data set as a robust record of geomagnetic field behavior during the 4 5 myr interval represented by the la roccaccia section a statistical analysis of the relative intensity observations suggests that this period of the cretaceous normal superchron is characterized by a normalized variability in paleointensity standard deviation about 28 of the mean value that is significantly lower than seen during the oligocene over intervals in which reversals or tiny wiggles occur typically about 50 the directional stability results in virtual geomagnetic pole dispersion compatible with that found in volcanic rocks from around the same latitude and ranging in age from 80 to 110 ma

geomagnetic secular variation recorded by sediments deposited during the cretaceous normal superchron at low latitude abstract paleosecular variation of the earth s magnetic field reflects the convective vigor of the geodynamo to investigate the energy regime of the geodynamo during the cretaceous normal superchron we collected 534 samples for a paleosecular variation study from a 1400 m long paleontologically well described section in northern peru thermal demagnetization isolates stable magnetization directions carried by greigite arguments are equivocal whether this remanence is syn diagenetic acquired during the cretaceous normal superchron or a secondary overprint acquired during a chron of solely normal polarity in the upper cenozoic yet pre bruhnes 800 kyr we explore the ramifications on the s value which quantifies paleosecular variation that arises from directional analysis sun compass correction bedding correction sampling frequency outlying directions and different recording media the sum of these affects can readily raise the s value by more than 20 s values from northern peru are indistinguishable from other s values for the cretaceous normal superchron as well as those for the last 5 ma summing over all the potential uncertainties we come to the pessimistic conclusion that the s value is an unsuitable parameter to constrain geodynamo models alternatively no statistical difference in paleosecular variation exists during much of the cretaceous normal superchron and during the last 5 ma

magnetic properties of sedimentary greigite fe3s4 new data are presented here that document the previously poorly known magnetic characteristics of sedimentary greigite fe3s4 several diagnostic magnetic properties can be used in conjunction to rapidly screen sediments to detect the presence of greigite such discriminants are necessary because greigite displays a similar or higher range of coercivities compared to most ferrimagnetic minerals and the commonly used method of alternating field demagnetization cannot be used to distinguish between palaeomagnetic remanence components due to greigite and other ferrimagnetic minerals sedimentary greigite has high ratios of the saturation isothermal remanent magnetization to magnetic susceptibility mrs which are accompanied by hysteresis ratios of mrsms 0 5 and bcr bc 1 5 ms is the saturation magnetization bc is the coercive force and bcr is the coercivity of remanence greigite has no low temperature phase transition whereas the ferrimagnetic iron sulphide pyrrhotite undergoes a major phase transition at about 34 k and unoxidized magnetite displays the verwey transition at 118 k low temperature measurements may therefore prove valuable in distinguishing between magnetite pyrrhotite and greigite although the absence of a low temperature transition cannot be taken as definitive evidence for the presence of greigite greigite also displays characteristic high temperature behaviour with a major drop in magnetization between 270 and 350 c determination of the mrs ratio is an excellent means of rapidly screening sediments to identify stratigraphic intervals that may contain greigite subsequent low and high temperature analyses can then provide unambiguous identification of greigite and enable the evaluation of the presence of other magnetic minerals

the abagong apatite rich magnetite deposit in the chinese altay orogenic belt a kiruna type iron deposit abstract the abagong apatite rich iron deposit is located in the kelang volcano sedimentary basin in the southern margin of the chinese altay orogenic belt aob which is an important polymetallic belt in china the ore bodies are interbedded with the upper silurian to lower devonian metamorphosed calc alkaline felsic volcanic rocks that show well preserved porphyritic and volcaniclastic textures the ore mineralization is aligned along faults that controlled their emplacement and occurs as lenses veins and stratoids bodies that generally cut the host rocks at a high angle and show sharp contacts the ore bodies and host rocks have undergone greenschist facies metamorphism deformation and folding the iron ores have massive densely disseminated banded and brecciated structures magnetite from the abagong deposit is low in ti tio 2 0 002 0 129 wt and the apatites from different ore types p poor and p rich ores are typical igneous fluorapatites with minor cl contents apatites of different forms and from different ore types show similar rare earth element ree and trace element normalized patterns with weak to moderate enrichment in light ree th u and pb depletion in sr ba nb ta and ti and negative eu anomalies indicating a common source and genesis the similar ree patterns for the magnetite apatite and volcanic host rocks suggest their close genetic linkage and support a magmatic origin for the deposit the abagong deposit shows the typical characteristics of kiruna type deposits with regard to the mineral assemblages ore texture and structure and the apatite and magnetite geochemistry we propose that the kiruna type abagong apatite rich iron deposit was derived from fe p rich melt through liquid immiscibility and the activity of hydrothermal fluids

cretaceous to early paleogene tectonic evolution of the northern central andes 0 18 s and its relations to geodynamics the tectonic phases subsidence patterns and location of the magmatic arc along the peruvian margin during late cretaceous and paleogene times are compared with the main geodynamic parameters absolute plate motion convergence velocity and direction age of the subducted slab and accretional events for the same periods during the late cretaceous long termed compression seems to be controlled by the absolute trenchward motion of the overriding plate and to a minor extent by the young age of the subducted lithosphere short lived contractional or extensional phases are mainly linked to the acceleration or deceleration of the convergence between the oceanic and continental plates and probably to changes in the convergence direction periods of high and low convergence rates coincide with increased and decreased subsidence rates along the margin respectively and seem to be independent of compressional tensional regimes during the paleogene in addition to the mentioned processes the eastward shift of the magmatic belt and subsidence in the forearc zone are interpreted as the result of the subduction erosion of the margin this process seems to be related to the decrease of the dip of the benioff zone expressed by the drastic widening of the magmatic arc

architecture and mineral deposit settings of the altaid orogenic collage a revised model abstract the altaids are an orogenic collage of neoproterozoic paleozoic rocks located in the center of eurasia this collage consists of only three oroclinally bent neoproterozoic early paleozoic magmatic arcs kipchak tuva mongol and mugodzhar rudny altai separated by sutures of their former backarc basins which were stitched by new generations of overlapping magmatic arcs in addition the altaids host accreted fragments of the neoproterozoic to early paleozoic oceanic island chains and neoproterozoic to cenozoic plume related magmatic rocks superimposed on the accreted fragments all these assemblages host important many world class late proterozoic to early mesozoic gold copper molybdenum lead zinc nickel and other deposits of various types in the late proterozoic during breakup of the supercontinent rodinia the kipchak and tuva mongol magmatic arcs were rifted off eastern europe siberia and laurentia to produce oceanic backarc basins in the late ordovician the siberian craton began its clockwise rotation with respect to eastern europe and this coincides with the beginning of formation of the mugodzhar rudny altai arc behind the kipchak arc these earlier arcs produced mostly cu pb zn vms deposits although some important intrusion related orogenic au deposits formed during arc arc collision events in the middle cambrian and late ordovician the clockwise rotation of siberia continued through the paleozoic until the early permian producing several episodes of oroclinal bending strike slip duplication and reorganization of the magmatic arcs to produce the overlapping kazakh mongol and zharma saur valerianov beltau kurama arcs that welded the extinct kipchak and tuva mongol arcs this resulted in amalgamation of the western portion of the altaid orogenic collage in the late paleozoic its eastern portion amalgamated only in the early mesozoic and was overlapped by the transbaikal magmatic arc which developed in response to subduction of the oceanic crust of the paleo pacific ocean several world class cu mo porphyry cu pb zn vms and intrusion related au mineral camps which formed in the altaids at this stage coincided with the episodes of plate reorganization and oroclinal bending of magmatic arcs major pb zn and cu sedimentary rock hosted deposits of kazakhstan and central asia formed in backarc rifts which developed on the earlier amalgamated fragments major orogenic gold deposits are intrusion related deposits often occurring within black shale bearing sutured backarc basins with oceanic crust after amalgamation of the western altaids this part of the collage and adjacent cratons were affected by the siberian superplume which ascended at the permian triassic transition this plume related magmatism produced various deposits such as famous ni cu pge deposits of norilsk in the northwest of the siberian craton in the early mesozoic the eastern altaids were oroclinally bent together with the overlapping transbaikal magmatic arc in response to the northward migration and anti clockwise rotation of the north china craton the following collision of the eastern portion of the altaid collage with the siberian craton formed the mongol okhotsk suture zone which still links the accretionary wedges of central mongolia and circum pacific belts in the late mesozoic a system of continent scale conjugate northwest trending and northeast trending strike slip faults developed in response to the southward propagation of the siberian craton with subsequent post mineral offset of some metallogenic belts for as much as 70 400 km possibly in response to spreading in the canadian basin india asia collision rejuvenated some of these faults and generated a system of impact rifts

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

the generation of a stratiform skarn and volcanic exhalative pb zn deposit sawusi in the southern chinese altay mountains the constraints from petrography mineral assemblage and chemistry abstract numerous pb zn and fe ore deposits occur in the volcanic sedimentary basins along the southern chinese altay mountains the mineralization of these deposits is hosted by early middle devonian volcanics and sedimentary rocks and is directly associated with skarns the genesis of these deposits is uncertain or hotly debated this study considers the sawusi pb zn deposit from the southern chinese altay the host rock of the sawusi deposit is rhyolitic crystal tuff with intercalated bands of fe mn calcic carbonates and a small amount of rhyolite within the upper section of the early devonian kangbutiebao formation on the basis of a field investigation the petrographic features of the skarn were described detailed electron microprobe analysis was performed on the skarn minerals and the o h and c isotopic compositions of quartz and its fluid inclusions were determined petrographic observation revealed a progressively intensifying alteration from the tuff and its intercalated bands to the stratiform skarn characterized by alternate bands of dark garnet biotite skarn and light grunerite biotite skarn mineral chemical analysis shows that the stratiform skarn of the sawusi deposit consists mainly of spessartine almandine mn rich grunerite biotite and fe mn calcic carbonate minerals a mass balance consideration of the cations in the skarn minerals suggests that the stratiform skarn formed through dimetasomatism between the tuff and intercalated bands of sedimentary fe mn calcic carbonates however the scale of the metasomatism is limited to a few centimeters laser 40 ar 39 ar dating was performed on the skarn grunerite yielding an isochron age of 412 15 ma the 40 ar 39 ar age is identical within analytical error to the zircon u pb age 401 2 7 ma of the rhyolite from the kangbutiebao formation that hosts the skarn and pb zn mineralization and the age 401 3 1 ma of the biesisala porphyry sawusi deposit the o h and c isotopic compositions show that the ore forming fluid includes two components one is h 2 o co 2 which has volatilized from carbonate and dehydrated from tuff and the other is meteoric derived surface water carrying minor meteoric co 2 or organic matter derived co 2 therefore the stratiform skarn and pb zn mineralization of the sawusi deposit are quite different from the contact metasomatic skarn and related deposits and suggest a volcanic exhalation sedimentation origin instead this study has broad relevance to the pb zn and fe deposits along the southern altay mountains in the areas of ore genesis study and ore prospecting

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

sedimentary evolution of an active margin during middle and upper cretaceous times the north peruvian margin from late aptian up to senonian the sedimentary evolution of the north peruvian margin during upper cretaceous can be summarized as follows

post oligocene rotations in southern ecuador and northern peru and the formation of the huancabamba deflection in the andean cordillera abstract a paleomagnetic study of more than 410 samples from 41 sites from paleocene to early oligocene volcanic and intrusive formations in southern ecuador and northern peru reveals a pattern of rotations in opposite senses on both sides of the huancabamba deflection the amplitude of the rotations is 25 clockwise north of the deflection and 20 anticlockwise south of it north of the deflection the results do not allow us to ascertain whether the rotation arises from oroclinal bending or from block rotation in a distributed shear south of it the absence of geological evidence for widespread strike slip faulting suggests that the results reflect a rotation of the peruvian margin which could be related to the shortening documented in southern peru and to the uplift of the central andes

middle cambrian to permian subduction related accretionary orogenesis of northern xinjiang nw china implications for the tectonic evolution of central asia abstract middle cambrian to permian subduction related records are widely distributed in northern xinjiang which can be grouped into the chinese altay east junggar eastern tien shan west junggar yili and tarim domains by integrating paleogeographic and geological data we suppose that the chinese altay east junggar eastern tien shan domain was more closely located to siberia while the west junggar and yili domains occupied an intermediate position near the kazakhstan block in the early paleozoic paleoasian ocean distribution of andean type magmatic arcs island arcs accretionary wedges ophiolitic slices and or microcontinents shows an archipelago paleogeography forming a huge accretionary active margin sequences the tarim domain was on the opposite side of the early paleozoic paleoasian ocean remaining passive margin these tectonic units drifted northwards and approached the southern active margin of the siberian craton in the late paleozoic leading to termination of the paleoasian ocean and formation of a complicated orogenic collage between siberian craton and the tarim block between the end permian and triassic these multiple accretion processes significantly contributed to the lateral growth of central asia

molybdenite re os and albite 40ar 39ar dating of cu au mo and magnetite porphyry systems in the yangtze river valley and metallogenic implications abstract the area of the middle lower yangtze river valley eastern china extending from wuhan hubei province to western zhenjiang jiangsu province hosts an important belt of cu au mo and fe deposits there are two styles of mineralization i e skarn porphyry stratabound cu au mo fe deposits and magnetite porphyry deposits in several nne trending cretaceous fault bound volcanic basins the origin of both deposit systems is much debated we dated 11 molybdenite samples from five skarn porphyry cu au mo deposits and 5 molybdenite samples from the datuanshan stratabound cu au mo deposit by icp ms re os isotope analysis nine samples from the same set were additionally analyzed by ntims on re os results from the two methods are almost identical the re os model ages of 16 molybdenite samples range from 134 7 2 3 to 143 7 1 6 ma 2 the model ages of the five samples from the datuanshan stratabound deposit vary from 138 0 3 2 to 140 8 2 0 ma with a mean of 139 3 2 6 ma their isochron age is 139 1 2 7 ma with an initial os ratio of 0 7 8 1 mswd 0 29 these data indicate that the porphyry skarn systems and the stratabound deposits have the same age and suggest an origin within the same metallogenic system albite 40 ar 39 ar dating of the magnetite porphyry deposits indicates that they formed at 123 to 125 ma i e 10 20 ma later both mineralization styles characterize transitional geodynamic regimes i e the period around 140 ma when the main ns trending compressional regime changed to an ew trending lithospheric extensional regime and the period of 125 115 ma of dramatic ew trending lithospheric extension

new field evidence bearing on the origin of the el laco magnetite deposit northern chile a reply the el laco magnetite deposit has been interpreted as lava flows and feeder dikes formed from iron oxide magma but more recently as a product of metasomatic replacement open pit exposure created at laco sur during the 1990s reveals that the massive magnetite contains magnetite veined blocks and smaller fragments of altered andesitic volcanic rock clearly supporting the replacement origin open space growth of magnetite and pyroxene along the walls of chimney structures and veins within magnetite indicates precipitation from aqueous fluid rather than from gas alone the native sulfur bearing cristobalite alunite alteration widespread at el laco and broadly contemporaneous with magnetite formation is recognized as part of a steam heated horizon generated in the vadose zone above a paleowater table consequently the magnetite replacement must have taken place beneath the paleowater table but probably 300 m beneath the paleosurface thus magnetite deposition at or immediately beneath the paleosurface is precluded the main outstanding question is how hot hypersaline brine the probable ore forming fluid at el laco attained such shallow crustal levels within a high volcanic edifice without undergoing the extensive groundwater dilution documented from most shallowly preserved hydrothermal systems

petrogenesis and tectonic implications of the late carboniferous calc alkaline and shoshonitic magmatic rocks in the awulale mountain western tianshan abstract the widespread late carboniferous calc alkaline and shoshonitic magmatic rocks in the awulale mountain provide crucial constraints on the tectonic evolution of the western tianshan here we perform detailed petrological investigations as well as zircon u pb chronological whole rock geochemical and sr nd isotopic analyses on these magmatic rocks from two geological sections along the duku road magmatic rocks in the section i with zircon shrimp u pb ages of 306 8 ma and 306 4 ma are composed of medium k calc alkaline to shoshonitic basalt trachy andesite and trachyte while those in the section ii consist of shoshonitic trachy andesite trachyte with a u pb age of 308 1 ma and monzonite with a u pb age of 309 6 ma all these magmatic rocks are characterized by strong enrichments in large iron lithophile elements with depletions of nb ta and ti indicating the origination from subduction modified lithospheric mantle the end t values of the rock samples collected from the section i 2 80 5 45 and section ii 3 34 5 37 are generally higher than those of the devonian to early carboniferous arc type magmatic rocks in the yili central tianshan suggesting that depleted asthenosphere might also be involved in their generation based on these geochemical data and petrological observations we suggest that the early stage 308 1 309 6 ma shoshonitic monzonite trachy andesite and trachyte in the section ii were generated by mixing between mafic magmas and trachytic melts while the late stage 306 4 306 8 ma medium k calc alkaline to shoshonitic basalt trachy andesite and trachyte in the section i were produced by partial melting of depleted asthenospheric and metasomatized lithospheric mantle followed by the processes of fractional crystallization and crustal contamination taking into account the available regional geological data the subduction of south tianshan ocean was probably ceased at 310 ma and these calc alkaline and shoshonitic magmatic rocks in the awulale mountain formed in a post collisional setting subsequent to slab break off

geomagnetic secular variation in the cretaceous normal superchron and in the jurassic abstract it is now widely thought that geomagnetic polarity reversals occur spontaneously as a result of normal dynamo action rather than being externally triggered if this is the case then it may well be that periods of time in which the geomagnetic reversal frequency was dramatically different were characterised by different styles of secular variation two such periods were the cretaceous normal superchron cns 84 125 ma when the field was dominantly of a single polarity for 40 myr and the jurassic period 145 200 ma when reversals occurred at an average rate of as much as 4 6 myr 1 here we analyse a database of new and published palaeomagnetic directions from lavas emplaced during these periods in order to obtain first order descriptions of the palaeosecular variation psv during these times we then compare these records with one another and with that produced for the period 0 5 ma with average reversal frequency of 4 0 myr 1 our results are more equivocal than those obtained in a previous similar study mcfadden p l merrill r t mcelhinny m w lee s h 1991 reversals of the earths magnetic field and temporal variations of the dynamo families journal of geophysical research solid earth and planets 96 3923 3933 we demonstrate that this is probably a result of the previous study being affected by an artefact of their correction for within site scatter the usefulness of our jurassic record is severely limited by the restricted palaeolatitudinal span of the available data however our record for the cns is sufficient to allow us to conclude that it was likely that secular variation then was different from that in the 0 5 ma period this supports the hypothesis of a link between psv and reversal frequency and therefore endorses psv analysis as a first order tool for determining geomagnetic stability in the past

stress strain behaviour of a loosely compacted volcanic derived soil and its significance to rainfall induced fill slope failures the stress strain behaviour of a soil is crucial to the recognition of the mechanism of slope failure triaxial tests composed of isotropically consolidated drained icd and undrained compression icu tests and anisotropically consolidated constant deviatoric stress path cqd tests were carried out with the aim of investigating the stress strain behaviour of loosely compacted volcanic derived soils the fact that the critical states are the same for icu icd and cqd tests may show that the critical state is independent of the above three stress paths the critical state line as defined in critical state soil mechanics is obtained from the e log p and q p plots based on the results of the above tests the initial state of the consolidated specimens at initiation of failure may be classified as dilative or contractive in the light of the locations of the soil state relative to the critical state line for contractional soils the increased pore water pressure generated by rainfall infiltration leads to a contractive failure in a drained manner giving rise to high excess pore water pressure the excess pore water pressure caused by contraction cannot be dissipated instantly resulting in a decrease in the shear resistance of the soil the failure process is rapid the failed soil mass is prone to flow after failure under the action of gravity due to its high moisture content and inflow of surface runoff and rainwater for dilational soils the increased pore water pressure resulting from infiltration leads to dilation which reduces pore water pressure and thus increases the shear resistance of the soil however continued rainfall infiltration may be able to equilibrate the reduction in pore water pressure caused by dilation and therefore the dilation or displacement can continue in hong kong volcanic derived soil is characterized by high permeability both the high permeability of volcanic derived soil and a shallow failure surface make it possible for the reduction in pore pressure to equilibrate relatively quickly therefore the failure is also rapid at least for poorly compacted fill slopes c 1999 elsevier science b v all rights reserved

closing history of the southern tianshan oceanic basin western china an oblique collisional orogeny abstract the tianshan tien shan range is an important paleozoic collisional orogenic belt and the key to understand the central asia tectonic evolution this paper integrates our research results with the existing chinese and international literature on sedimentology geochemistry isotopic geochronology paleontonology and paleomagnetism of the tianshan and tarim regions to propose that the oblique collision may have played an important role in the late paleozoic closing of the southern tianshan oceanic basin as a result of the sinian latest proterozoic younger than 800 ma continental extension and rifting process the tarim and yili blocks separated from their parent continent in the late cambrian ordovician the southern tianshan oceanic crusts between the two blocks subducted northward beneath the southern margin of the yili block in the silurian during the devonian early carboniferous the tarim block rapidly drifted to the north and rotated about 46 clockwise this process induced the collision of the yili micro continent with the eastern segment present geographical position of the tarim continent in the late devonian and the southern tianshan oceanic crust evolved to be a west facing remnant oceanic basin during the late carboniferous early permian the tarim block located within an almost constant latitude range rotated about 26 clockwise with respect to the yili micro continent which ultimately closed the remnant oceanic basin in a scissors like manner from east to west and completed the tarim yili collision subsequent a type subduction of the tarim continental crust and lithosphere scale sinistral shearing generated a magmatic arc on the southern margin of the tarim yili suture zone the late permian early triassic clastics deposited in a peripheral foreland basin developed above the arc

whole rock geochemistry and sr nd pb isotope systematics of the late carboniferous volcanic rocks of the awulale metallogenic belt in the western tianshan mountains nw china petrogenesis and geodynamical implications abstract the awulale metallogenic belt amb of the western tianshan nw china includes late carboniferous ca 320 ma ore bearing volcanic rocks of the dahalajunshan formation the petrogenesis and tectonic setting of these volcanic rocks are important for the understanding of the tectonic evolution and metallogeny of the western tianshan this paper presents new major and trace elements and sr nd pb isotope data from the dahalajunshan volcanic rocks which are mainly calc alkaline basaltic trachy andesite and trachy andesite with subordinate basalt trachy basalt and rhyolite the variations of major and trace elements in the mafic and intermediate volcanic rocks indicate the fractionation of pyroxene and magnetite or hornblende magnetite apatite and plagioclase respectively during their petrogenesis the dahalajunshan volcanic rocks have similar primitive mantle normalized diagrams and chondrite normalized rare earth element ree patterns suggesting their similar mantle source s they are characterized by enrichment in large ion lithophile elements liles and light rees lrees depletion in heavy rees lan ybn 2 80 to 9 59 and high field strength elements hfses and end t ranging from 1 2 to 6 0 at 86sr 87sr t 0 7047 0 7063 and 206pb 204pbi 17 49 18 19 both the geochemical and isotopic data indicate that the volcanic rocks were probably derived by low degree melting of sub arc lithospheric mantle modified by fluids in a continental arc setting our obtained results in conjunction with previous published data allow us to suggest that the southward subduction of junggar oceanic crust continued until the late carboniferous and was followed by a tectonic shift from continental arc to post collisional extension environment

petrogenesis of the kekesai composite intrusion western tianshan nw china implications for tectonic evolution during late paleozoic time abstract the late carboniferous to permian is a critical period for final amalgamation of the central asian orogenic belt caob which is characterized by voluminous igneous rocks particularly granitoids the kekesai composite granitoid porphyry intrusion situated in the chinese western tianshan southwest part of caob includes two intrusive phases a monzogranite phase intruded by a granodiorite phase la icpms u pb zircon analyses suggest that the monzogranitic rocks were formed at 305 5 1 1 ma with a wide age range of inherited zircons 358 488 ma and 1208 1391 ma whereas the granodioritic rocks formed at 288 7 1 5 ma the monzogranitic and granodioritic phases have similar geochemical features and sr nd hf isotopic compositions they exhibit high and variable sio 2 66 71 wt and mgo 0 41 2 14 wt contents with some arc like geochemical characteristics e g enrichment of large ion lithophile elements and negative anomalies of nb ta and ti and relatively high initial 87 sr 86 sr ratios i sr 0 7055 0 7059 low positive e nd t 0 84 to 1 03 as well as a large variation in hf isotopic compositions with e hf t between 3 43 and 14 8 implying both of them were derived from similar source materials these geochemical characteristics suggest that they might be mainly derived from the partial melting of arc derived mesoproterozoic mafic lower crust with involvement of a mantle derived component in variable proportions by mantle derived magma underplating the presence of late ordovician to earliest early carboniferous inherited zircons and the hf isotopic compositions in the monzogranitic sample similar to that of the widespread juvenile arc rocks indicates some crust contamination during magma emplacement our new data combined with previous studies imply that extensive post collisional magmatism due to underplating of mantle derived magma could plausibly be explained by slab break off regime

velocity structure of the andes of central peru from locally recorded earthquakes arrival times of local events recorded during two short term seismic fied investigations in central peru are used to determine the p and s velocity structures above the slab in a segment of the andes where the subduction is horizontal the velocities in the upper crust can be related to the sedimentary cover the moho depth is estimated to be about 50 km below the western part of the sub andean zone and deepens gently to the west reaching 60 km under the western cordillera an uplift of high velocity material is observed under the coastal zone associated with a cluster of earthquakes in and above the slab the asymmetrical structure of the crust below the eastern cordillera reflects the convergence process in this part of the andes the shortening occurs along active reverse steeply west dipping faults involving the crust in its entirety on the eastern border of the cordillera the compression then decreases toward the east

primary carboniferous and permian paleomagnetic results from the yili block nw china and their implications on the geodynamic evolution of chinese tianshan belt in order to better understand the tectonic role of the yili block on the paleozoic evolution of the chinese tianshan belt we performed a primary paleomagnetic study on carboniferous and permian rocks from different areas in the yili block nw of china more than 320 sedimentary and volcanic samples were collected from 39 sites except for the ordovician samples showing a weak and unstable magnetic remanence the majority of this collection presents characteristic remanent magnetization carried by magnetite and hematite in the study area though positive fold test has been observed on the early carboniferous rocks a general remagnetization of these rocks has been identified and attributed to the late carboniferous magmatism moreover all early and late carboniferous samples from the interior of the yili block yield stable and coherent magnetic directions with exhaustively reverse magnetic polarity the late carboniferous c2 is considered as the magnetic remanence age since these rocks are covered or intruded by synchronous magmatic rocks of the yili arc which lasted until to 310 ma the c2 paleomagnetic pole is therefore calculated at 68 6 n 290 6 e with 95 6 4 and n 15 the late carboniferous rocks located close to a deformation zone present a consistentmagnetic inclination but significant different declination with respect to other areas and are suspected to have probably experienced a local rotation although no fold test can be performed due to the monoclinal bedding stable magnetic components are isolated from late permian p2 red beds in the interior of the yili block with also a solo reverse magnetic polarity the p2 paleomagnetic pole of the yili block has been therefore calculated from the characteristic remanent magnetization 79 7 n 172 0 e with 95 11 3 and n 5 keeping important uncertainties in mind comparisons of the c2 and p2 paleomagnetic poles of the yili block with available coeval poles of junggar tarim and siberia indicate 1 no significant relative motion between the yili and junggar blocks since the late carboniferous 2 no significant or weak latitudinal relative motion occurred since the late carboniferous among these blocks but 3 the 46 2 15 1 and the 31 6 15 1 counterclockwise rotations of the yili junggar blocks with respect to tarim and siberia took place during c2 to p2 these rotations are accommodated by the permian dextral strike slip faults along the northern and southern sides of tianshan belt and sinistral strike slip faulting along the erqishi fault of altay belt resulting in about 1000 km and 600 km lateral displacements in the tianshan and altay belts respectively

provenance of late paleozoic strata in the yili basin implications for tectonic evolution of the south tianshan orogenic belt this research was supported by the 973 program under grant no 2015cb453000 the national natural science foundation of china 41502109 the cultivating program of middle aged key teachers of chengdu university of technology kygg201718 innovation team of sedimentary geology of chengdu university of technology kytd201703 and by the china postdoctoral science foundation 2015m582528 cawood acknowledges support from the australian research council grant fl160100168

paleomagnetic study of an arcuate fold belt developed on a marginal orogen the cajamarca deflection northern peru abstract a paleomagnetic study of over 500 cores from 48 sites sampled in albian to turonian sedimentary formations along the cajamarca deflection in northern peru indicates that these lithologies carry a stable remanent magnetization in the two major segments of the deflection positive fold tests establish that the magnetization predates the folding except for five sites in the western part which appear to be remagnetized by the intrusion of paleocene granites the eastern part of the deflection does not appear to have undergone any significant rotation whilst a post late eocene counterclockwise rotation of about 25 is recorded in the central and western parts this angular difference only accounts for about half of the difference in the strikes of structures in the two sections of the deflection when considered with the available tectonic sedimentological and geodynamic data these results indicate that the arcuate shape of the cajamarca fold belt results from complex processes early multi phased compressional events of paleocene eocene age created the folds and built an arcuate structure with non rotational mechanisms then in post late eocene times the structure reached its present form after rotation of its central and western parts the bearing of these results on the geodynamic evolution of the andean cordillera is discussed

from magma replenishment to epithermal mineralization combined evidence from zircon u pb u th he and trace element data abstract the dahalajunshan formation volcano volcanoclastic rocks in chinese western tianshan host specific paleozoic epithermal gold deposits as well as volcanogenic fe cu mineralization although numerous studies have been taken to constraint its geological geochemical geochronological features as well as petrogenesis the link between volcanism and mineralization is not well constrained in this contribution accessory zircon was collected from the intermediate to felsic volcanic rocks outcropped at the jingxi yelmand gold deposit detailed cathodoluminescence imaging illustrates ubiquitous resorption surfaces and embayments and in situ la icpms trace element analysis further indicates periodic recharge of magma chamber by mafic magma input from depths u pb geochronology constrains two main pulses of magmatism at 370 ma and 365 362 ma respectively the duration of magmatic activity in the magma chamber is estimated to 52 6 myrs using the u pb age of the oldest zircon antecryst 406 9 5 4 ma and the youngest zircon autocryst 354 6 3 0 ma as cutting edge and taking the analytical uncertainty into consideration u th he dating of zircon from the first pulse yields an age of 354 15 ma which is identical to u pb age within analytical uncertainty however zircons from the second pulse of volcanic rocks yield much younger u th he centered at 292 6 7 6 ma due to significant post eruption reheating events we propose that the intermediate to acid volcanic rocks are constructed by incremental addition of small magma batches the periodic injection of more mafic magma and subsequent mixing may act as important trigger of not only volcanic eruption but also epithermal mineralization gold mineralization at the deposit should be no later than 292 6 ma

geology geochemistry and geochronology of the dunde iron zinc ore deposit in western tianshan china abstract the dunde iron zinc deposit 185 mt at 35 fe located in the tianshan orogenic belt xinjiang northwest china is hosted in late carboniferous volcanic volcaniclastic rocks characterized by prograde skarn formation and retrograde alteration but the ore lacks any clear spatial link with intrusive rocks four stages of skarn formation and ore development can be recognized 1 a prograde skarn stage that formed grossularitic garnet gr 39 80 ad 15 58 and diposidic pyroxene di 63 97 2 a retrograde skarn stage dominated by the formation of magnetite with minor epidote ferropargasite apatite hematite and trace amounts of titanite and spinel 3 a sulfide stage dominated by the formation of arsenide loellingite and arsenopyrite and sulfide minerals sphalerite pyrrhotite pyrite chalcopyrite and minor galena calcite and traces of quartz and 4 a chlorite calcite stage mainly characterized by the formation of chlorite calcite and traces of sericite exsolution of droplet like and patchy chalcopyrite is developed within sphalerite of the sulfide stage indicating an exsolution temperature of 350 400 c as such the crystallization temperature of early stage magnetite may have been 400 c abundant fluid inclusions occur in calcite which include daughter mineral bearing h 2 o h 2 o and pure h 2 o inclusions the h 2 o inclusions have a wide range of homogenization temperatures from 147 c to 367 c with salinities of 2 4 23 4 wt nacl equivalent the daughter mineral bearing h 2 o inclusions have homogenization temperatures from 172 c to 347 c with salinities of 31 9 33 0 wt nacl equivalent using the chlorite geothermometer the temperature of chlorite formation is constrained to be between 152 c and 222 c average 194 c sulfur isotope compositions of pyrrhotite sphalerite pyrite and loellingite have a narrow range of 34 s values from 3 8 to 8 1 average 34 s 6 8 suggesting that the sulfur was magmatic derived zircon la icp ms u pb dating of wall rock dacite yields a weighted mean 206 pb 238 u age of 316 0 1 7 ma combined with previous dating results of magnetite mineralized diorite stocks diorite dikes and garnet skarn it can be inferred that the dunde iron zinc deposit formed in the late carboniferous after 316 ma and is genetically related to deep dioritic intrusions during the late carboniferous the tectonic setting of this region changed from subduction collision to extension accompanied by mantle derived magma underplating in deep after the formation of the iron zinc ore deposit the dunde district was intruded by an early permian k feldspar granite that yields a zircon la icp ms u pb age of 295 75 0 71 ma

stationary and nonstationary behaviour within the geomagnetic polarity time scale we analyse the geomagnetic polarity time scale gpts since the upper jurassic by displaying the successive lengths of polarity intervals as a function of their order of occurrence the sequence consists of three segments between the upper jurassic and the lower cretaceous segment a comprises intervals of short duration with a mean duration of about 0 29 my and no clear long term evolution segment b begins around 130 ma displays a sudden increase of the duration of the magnetic intervals an interval of maximum duration the normal cretaceous superchron and a long and erratic sequence of intervals with decreasing average duration between 85 ma and about 25 ma from 25 ma to the present segment c consists of intervals of short duration with a mean value of 0 23 my this description suggests that the earth s magnetic field could have experienced a fairly stationary regime until slightly before the onset of the cretaceous superchron when the regime has been rapidly and strongly perturbed before progressively returning to another stationary regime about 25 ma ago a geophysical explanation for this sequence of events could be that the geodynamo has been perturbed by the arrival of some cold material at the core mantle boundary as this material would have heated up the geodynamo would have been brought back to its stationary regime

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

a review of the geological characteristics and mineralization history of iron deposits in the altay orogenic belt of the xinjiang northwest china abstract in this review we describe the geological characteristics and metallogenic tectonic origin of fe deposits in the altay orogenic belt within the xinjiang region of northwestern china the fe deposits are found mainly within three regions ordered from northwest to southeast the ashele kelan and maizi basins the principal host rocks for the fe deposits of the altay orogenic belt are the early devonian kangbutiebao formation the middle to late devonian altay formation with minor occurrences of lower carboniferous and early paleozoic metamorphosed volcano sedimentary rocks the principal mineral forming element groups of the deposits are fe fe cu fe mn fe p fe pb zn fe au and fe v ti the fe deposits are associated with distinct formations such as volcanic rocks skarn deposits pegmatites granite related hydrothermal vein mineralization and mafic pluton related v ti magnetite deposits the fe deposits are most commonly associated with volcanic rocks in the upper kangbutiebao formation in the volcano sedimentary kelan basin and in skarn deposits at several localities including the lower kangbutiebao formation in the volcano sedimentary maizi basin and the altay formation at jiaerbasidao kekebulake region homogenization temperatures of fluid inclusions in the prograde retrograde and sulfide stages of the skarn type deposit are mainly medium to high temperature cluster between 200 and 500 c medium temperature cluster between 200 and 340 c and low to medium temperature cluster between 160 and 300 c respectively ore fluids in the sedimentation period in the volcano sedimentary type deposit are characterized by low to medium temperature with a peak around 190 c low to moderate salinity 3 23 to 22 71 wt nacl equiv ore fluids in the pegmatite type deposit are characterized by low to medium temperature with a peak at 240 c low salinity with a peak around 9 wt nacl equiv an analysis of the isotopic data for fe deposits from the altay orogenic belt indicates that the sulfur was derived from several sources including volcanic rocks and granite as well as bacterial reduction of sulfate from seawater the present results indicate that different deposit types were derived from various sources the ree geochemistry of rocks and ores from the fe deposits in the altay orogenic belt suggests that the ore forming materials were derived from mafic volcanic rocks based on isotopic age data the timing of the mineralization can be divided into four broad intervals early devonian 410 384 ma middle devonian 377 ma early permian 287 274 ma and early triassic c 244 ma the ore forming processes of the fe deposits are closely related to volcanic activity and the emplacement of intermediate and felsic intrusions we conclude that fe deposits within the altay orogenic belt developed in a range of tectonic settings including continental arc post collisional extensional settings and intracontinental settings

paleomagnetic tracking of mountain building in the peruvian andes since 10 ma 1 we report paleomagnetic data from 73 sites 628 samples of upper oligocene to pliocene rocks from central and northern peru the data indicate that the subandean zone has not experienced vertical axis rotation since the upper oligocene whereas the coast and the western cordillera record a coherent pattern of counterclockwise rotations emplaced in the last 10 ma this pattern can best be explained by two competing hypotheses 1 a propagation in rotations from the bolivian orocline toward the north or 2 a more punctual and widespread event linked to subduction of the nazca ridge that caused the rotations on the basis of the time space relationship of paleomagnetic rotations deformation magmatism and nazca south america plate convergence the latter appears more likely moreover because the rotations north of the abancay deflection 15 5 s are synchronous with deformation and exhumation they indicate that major mountain building in the peruvian andes has occurred since 10 ma finally we suggest that a succession of aseismic ridge subduction events played an important role in shaping the modern topography of the andean chain

global time scale and regional stratigraphic reference scales of central and west europe east europe tethys south china and north america as used in the devonian carboniferous permian correlation chart 2003 dcp 2003 the boundaries of the devonian carboniferous and permian stages of the global stratigraphic reference scale abbreviated to global stratigraphic scale gss are described in relation to the biostratigraphic and or lithostratigraphic units of the regional stratigraphic reference scales abbreviated to regional stratigraphic scales rss of central and west europe east europe tethys south china eastern tethys and north america in their type regions the boundaries of gss units rarely coincide with those of homonymous rss units moreover the definitions of some rss units have changed several times over the last decades and subsequent misunderstanding of the stratigraphical significance of these changes has often introduced errors into proposed global correlation charts the stratigraphic framework proposed in our global devonian carboniferous permian correlation chart 2003 dcp 2003 devonian carboniferous permian correlation chart 2003 menning m schneider j w alekseev a s amon e o becker g von bitter p h boardman d r bogoslovskaya m braun a brocke r chernykh v chuvashov b i clayton g dusar m davydov v i dybova jachowicz s forke h c gibling m gilmour e h goretzki j grunt t a hance l heckel p h izokh n g jansen u jin y g jones p kading k ch kerp h kiersnowski h klets a klug ch korn d kossovaya o kotlyar g v kozur h w laveine j p martens th nemyrovska t l nigmadganov a l paech h j peryt t m rohn r roscher m rubidge b schiappa t a schindler e skompski s ueno k utting j vdovenko m v villa e voigt s wahlman g p wardlaw b r warrington g weddige k werneburg r weyer d wilde v winkler prins c f work d m 2004 abschlusskolloquium dfg schwerpunktprogramm 1054 evolution des systems erde wahrend des jungeren palaozoikums im spiegel der sedimentgeochemie abstracts univ erlangen germany 2004 p 43 herein abbreviated to dcp 2003 and cited as dcp 2003 in references is an attempt to reduce these errors the dcp 2003 is the stratigraphic base for project 1054 of the deutsche forschungsgemeinschaft dfg the evolution of the late palaeozoic in the light of sedimentary geochemistry this composite time scale has been carefully balanced as far as data allows to remove unnecessary artificial compression and expansion of time intervals biozonations and depositional events the ages selected in dcp 2003 are markedly different to those in the geologic time scale 1989 gts 1989 harland w b armstrong r l cox a v craig l e smith a g smith d g 1990 a geologic time scale 1989 cambridge univ press cambridge harland w b armstrong r l cox a v craig l e smith a g smith d g 1990 a geologic time scale 1989 cambridge univ press cambridge pp 1 263 and in gradstein and ogg gradstein f m ogg j 1996 a phanerozoic time scale episodes 19 1 2 3 4 insert whereas they are closer to those of the geologic time scale 2004 gts 2004 gradstein f m ogg j g smith a g 2004 a geologic time scale 2004 cambridge univ press cambridge pp 1 589 mostly the ages are rounded to the nearest 0 5 ma in order to avoid estimates of questionable accuracy whereas ages of 0 1 ma in the gts 2004 and their error bars of 0 4 ma to 2 8 ma for the devonian to permian stage boundaries suggest an improved accuracy in contrast in the dcp 2003 questionable ages and positions of stratigraphic boundaries are marked by arrows c 2006 elsevier b v all rights reserved

petrogenesis of carboniferous adakites and nb enriched arc basalts in the alataw area northern tianshan range western china implications for phanerozoic crustal growth in the central asia orogenic belt abstract carboniferous volcanic rocks in the alataw area northern tianshan range xinjiang consist of early carboniferous ca 320 ma adakites and nb enriched arc basalts and basaltic andesites nebs and late carboniferous ca 306 310 ma mainly high k calc alkaline andesites dacites and rhyolites the adakites are calc alkaline and characterized by high na2o k2o 1 52 3 32 ratios negligible to positive eu anomalies strong depletion of heavy rare earth elements e g yb 0 74 1 47 ppm and y 6 7 14 9 ppm positive sr and ba but negative nb and ti anomalies and relatively constant end t values 3 4 6 6 and 87sr 86sr i ratios 0 7035 0 7042 some andesitic and dacitic adakite samples exhibit high mgo contents similar to magnesian andesites the nebs are sodium rich na2o k2o 2 03 8 06 and differ from the vast majority of arc basalts in their higher nb zr tio2 and p2o5 contents and nb th nb la and nb u ratios and minor negative to positive anomalies in ba nb sr zr and ti they have the highest end t values 6 4 11 6 but varying 87sr 86sr i ratios 0 7007 0 7063 the high k calc alkaline suite is similar to typical normal arc volcanic rocks in terms of moderately fractionated rare earth abundance and distinctly negative eu nb sr and ti anomalies they have end t values 1 2 6 4 and 87sr 86sr i ratios 0 7018 0 7059 geochemically they are similar to coeval i type granitoids in the alataw area given the presence of early carboniferous ophiolites in the northern tianshan range and the isotopically inappropriate compositions of proterozoic metamorphic basement in the alataw area we argue that the alataw adakites were most probably related to the melting of young subducted crust of the northern tianshan ocean the nebs likely originated from mantle wedge peridotites metasomatized by adakites and minor slab derived fluids the later high k calc alkaline suite was generated by afc processes that acted on melts derived from a mantle wedge metasomatized by hydrous fluids the larger range of isotopic compositions exhibited by both the neb and high k suite relative to the adakites suggests that the mantle wedge was heterogeneous prior to slab or fluid mediated metasomatism continental crustal growth of the central asian orogenic belt was dominated by contributions of the juvenile materials from the depleted mantle prior to 270 ma and possibly afterwards the results of this study suggest that other carboniferous nb enriched basalts in the tianshan range were generated by subduction processes rather than by intraplate tectonics as previously proposed

the secular variation of earth s magnetic field models of the magnetic field at the core mantle boundary at selected epochs from 1715 0 to 1980 0 reveal novel features in the field at the core these suggest that the flow of core fluid is coupled to the mantle and that magnetic diffusion is significant

application of in situ titanite u pb geochronology to volcanic hosted magnetite deposit new constraints on the timing and genesis of the zhibo deposit western tianshan nw china abstract the awulale metallogenic belt within the western tianshan orogenic belt of northwestern china includes four large iron oxide deposits with a total resource of 1000 million metric tons mt fe among these the zhibo deposit is a large 337 mt at 26 68 wt fe volcanic hosted magnetite deposit where massive ti poor magnetite ores are hosted in the carboniferous volcanic and volcaniclastic sequences here we use in situ u pb analyses of titanite and zircon by laser ablation icp ms to place tight constraints on the timing and genesis of iron mineralization at zhibo titanite in the magnetite ore are closely associated with magnetite and ca alteration assemblages consisting of actinolite epidote and calcite the dated titanite exhibit strongly fractionated ree patterns with heavy ree enrichment neutral to negative eu anomalies and have low th and u concentrations and low th u ratios the textural and geochemical characteristics indicate that the titanite are hydrothermal in origin and coeval with magnetite in the paragenetic sequence titanite from three magnetite ores yield weighted mean 207pb corrected 206pb 238u ages of 310 3 1 8 ma mswd 0 17 310 1 1 8 ma mswd 0 30 and 315 3 2 5 ma mswd 0 26 constraining the iron mineralization at zhibo to a time interval between 315 ma and 310 ma magmatic zircon from a host andesite sample yield u pb age of 316 3 3 4 ma mswd 0 079 the overlapping ages for magnetite ores and the host volcanic rocks confirm a genetic relationship between them and are consistent with a magmatic contribution to the mineralization system as also indicated by fe and o isotope data of magnetite in previous work these new u pb results are also consistent with age estimates for mineralization and igneous activity in other major magnetite deposits in the awulale iron metallogenic belt indicating a significant iron mineralization event related to the ca 315 300 ma volcanism combined with previous geological and geochemical evidence we conclude that the zhibo magnetite deposit was formed mainly by iron rich fluids derived from a mafic to intermediate magma in a volcano plutonic structure

brief reversed polarity interval during the cretaceous normal polarity superchron a striking feature of the geomagnetic time scale is the 34 m y period of normal polarity field bias known as the cretaceous normal polarity superchron new paleomagnetic data from three deep sea drilling project cores sites 317a 402a and 463 indicate that a brief interval of reversed polarity occurred during the superchron this reversed polarity interval may help refine past motions of the pacific plate and aid resolution of the statistical structure of the geomagnetic reversal chronology

alteration mineralogy mineral chemistry and genesis of zhibo iron deposit in western tianshan mountains xinjiang the zhibo iron deposit is one of the recently discovered large magnetite deposits in the awulale iron metallogenic belt which is located at the eastern corner of western tianshan mountains the ore bodies are hosted mainly by volcanic and volcaniclastic rocks of the lower carboniferous dahalajunshan formation the wall rocks of the zhibo iron deposit have been strongly altered three stages of alteration assemblages and associated mineralization have been recognized stage dominantly composed of pyroxene albite magnetite stage characterized by amphibole k feldspars epidote magnetite pyrite and stage dominated by epidote chlorite calcite quartz pyrite hematite chalcopyrite electron microprobe analyses indicate that the zhibo deposit have compositional features of mineral assemblages similar to many other magmatic hydrothermal iron oxide deposits pyroxene consists mainly of diopside di 62 97 83 56 with minor hedenbergite hd 16 44 36 45 igneous plagioclase ab47 57 57 82 an41 5 51 87 or0 56 0 68 has been altered into albite ab77 89 99 33 an0 46 2 48 or0 21 20 3 k feldspar related to late hydrothermal alteration is superimposed upon the pre existing associations amphiboles are entirely of actinolite composition abundant fe epidote fe fe al 0 22 0 36 and minor chlorite have pervasively replaced early mineral assemblages in contrast with high titanium 3 08 tio2 magnetite in volcanic rocks most magnetite from the ore bodies has lower w tio2 0 23 a small amount of disseminated magnetite has a w v2o5 similar to the magnetite from volcanic rocks suggesting minor iron movement from the wall rock during the early metasomatism mineralogy and mineral chemistry of the altered minerals indicate that metasomatic replacement played an important role in mineralization in addition the zhibo iron deposit also has some textural features suggestive of an fe rich melt origin for the deposit such as the abrupt contact between the wall rock and massive magnetite ore the existence of significant amounts of breccia the arrangement of magnetite that defines a flow texture and platy magnetite combined with regional geology of the awulale iron metallogenic belt it might be suggested that fe rich magmatic liquids constituted the most plausible origin for the mineralization producing massive magnetite ore bodies associated with extensive wall rock alteration there existed a genetic relationship between late carboniferous continental arc magmatism and massive magnetite mineralization

andean tectonics related to geometry of subducted nazca plate seismological and geological data show that tectonic segmentation of the andes coincides with segmentation of the subducted nazca plate which has nearly horizontal segments and 30 east dipping segments andean tectonics above a flat subducting segment between 28 s to 33 s are characterized by from west to east 1 a steady topographic rise from the coast to the crest of the andes 2 no significant quaternary and possibly neogene magmatism 3 a narrow belt of eastward migrating apparently thin skinned neogene to quaternary shortening of the andes and 4 plio pleistocene uplift of the crystalline basement on reverse faults in the pampeanas ranges from about 15 to 24 s over a 30 dipping subducted plate a west to east andes cross section includes 1 a longitudinal valley east of coastal mountains 2 an active neogene and holocene andesitic volcanic axis 3 the altiplano puna high plateau 4 a high neogene but inactive thrust belt eastern cordillera and 5 an active eastward migrating subandean thin skinned thrust belt tectonics above a steeply subducting segment south of 33 s are similar west of the volcanic axis but quite different to the east early cenozoic tectonics of western north america were quite similar to the neogene andes however duration of segmentation was longer and the width of deformation was greater in the western united states patterns of crustal seismicity are systematically related to plio quaternary structural provinces implying that current deformational processes have persisted since at least the pliocene horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench above flat subducting segments crustal seismicity occurs over a broad region whereas over steep segments it is confined to the narrow thrust belt strain patterns in the forearc region are complex and perhaps extensional and a broad region of the altiplano puna and eastern cordillera appears to be aseismic

paleozoic northward drift of the north tien shan central asia as revealed by ordovician and carboniferous paleomagnetism abstract three new middle late ordovician and two new early carboniferous paleomagnetic poles have been obtained from the north tien shan zone ntz of the ural mongol belt in kyrgyzstan and kazakhstan paleolatitudes for the carboniferous are unambiguously northerly and average 15 5 n whereas the ordovician paleolatitudes 6 9 and 9 are inferred to be southerly given that a very large 180 rotation of the ntz would be necessary during the middle paleozoic if the other polarity option was chosen thus the ntz drifted northward during much of the paleozoic east west drift cannot be determined as is well known from paleomagnetic data in addition detailed thermal demagnetization analysis reveals two overprints one of recent age and the other of permian age which is a time of strong deformation in the ntz the paleolatitude of the combined permian overprint is 30 5 2 n the paleolatitudes collectively track those predicted for the area by extrapolation from baltica very well but are different from those of siberia for ordovician times this finding is compatible with sengor and natal in s sengor a m c natal in b a 1996 paleotectonics of asia fragments of a synthesis in yin a harrison m eds the tectonic evolution of asia cambridge univ press cambridge pp 486 640 model of tectonic evolution of the ural mongol belt and disagrees with the models of other researchers declinations of the ordovician and early carboniferous results range from northwesterly to northeasterly and are clearly affected by local relative rotations which seem characteristic for the entire ntz because the permian overprint declinations also show such a spread apparently the important latest paleozoic triassic deformation involved shear zone related rotations as well as folding and significant granitic intrusions

do superchrons occur without any palaeomagnetic warning abstract the mainly dipolar geomagnetic field generated by the geodynamo within the earth s liquid core has reversed its polarity many times in the past this succession of intervals of alternate polarity defines the geomagnetic polarity time scale gpts usually interpreted as resulting from a gamma renewal process the rate of which would be controlled by the boundary conditions imposed by the mantle on the core in this interpretation boundary conditions would have occasionally evolved towards being unfavourable to reversals leading the reversal rate of the process to progressively decrease and reach zero at least twice causing the onset of the so called kiaman and cretaceous very long polarity intervals superchrons here we reconsider this causal link analysing the latest and best constrained gpts thanks to the continuous record provided by marine magnetic anomalies describing the past 160 myr we found that contrary to earlier claims no long term behaviour over the 40 myr preceding the cretaceous superchron can be seen in the reversal rate that could explain its onset at 120 ma more generally it turns out that hardly any special behaviour can be identified in the gpts which could have announced this superchron only the occurrence of the longest of all pre superchron intervals cm1n 3 myr before the onset of the superchron could be identified as a precursor such a behaviour could possibly be the consequence of medium term on the 10 myr time scale changes in the boundary conditions imposed by the mantle on the core but we note that the sole analysis of the gpts does not allow this to be tested in fact it appears that a single stationary process could also explain the entire pre superchron gpts with the only possible exception of cm1n this suggests that the occurrence of superchrons does not necessarily require changes in boundary conditions and could simply attest for a sudden non linear transition between a reversing and a non reversing state of the geodynamo

fundamental transitions in the geodynamo as suggested by paleomagnetic data abstract analysis of the reversal chronology shows that the reversal process effectively ceases during the cretaceous superchron the geodynamo appears to have two basic states a reversing states and a non reversing state this must reflect a fundamental transition in the geodynamo process at the onset and termination of the superchron we suggest that this transition and indeed the long term variations in reversal rate may reflect a change from a predominantly oscillatory dynamo to a steady one which might for example be caused by a change in the relative roles of the and effects in 2 dynamos if so this should probably have been reflected in reduced secular variation during the superchron unfortunately data are not available to test this prediction directly but we use the surface integral of the angular standard deviation of paleomagnetic field directions as a proxy this shows a significantly reduced secular variation during the superchron as predicted

geodynamo energy source constraints from palaeomagnetic data abstract analyses of palaeo geomagnetic data can provide surprisingly strong constraints upon models of the geodynamo in particular it is shown here from an analysis of the geomagnetic reversal sequence that long term variation in reversal rate does not occur as a result of change in any inherent inhibition or encouragement in the reversal process instead there must be some physical process that actually triggers reversals and this process must derive from an energy source effectively independent of the energy source powering the main geomagnetic field from this two very simple generalised models referred to as the hot blob and cold blob models are suggested data from individual reversal transitions are used to discriminate between these two models the preferred model is that the geodynamo is driven primarily by convection related to freezing of the outer core at the inner core outer core boundary cold blobs descending from the core mantle interface are identified as the instabilities that trigger reversals

geomagnetic paleointensity and direct age determination of the isea m0r chron abstract a combined paleomagnetic and geochronologic study has been conducted on an andesite lava sequence at jianguo liaoning province northeastern china thermal demagnetization and thermomagnetic analysis revealed that natural remanent magnetization is carried by both magnetite and hematite stepwise thermal demagnetization up to 675 c isolated well defined reverse characteristic remanent magnetization chrm in three time independent lava flows with a mean direction of d i 179 2 59 7 with 95 3 0 it also showed that a high temperature component 585 c has the same chrm direction as that of the low to medium temperature ca 170 300 585 c components a modified version of the thellier thellier paleointensity method coe j geophys res 72 1967 3247 3262 with systematic partial thermoremanent magnetization checks was used for paleointensity determinations twelve out of 39 samples yielded reliable results in the temperature interval of 170 550 c virtual dipole moment values range from 3 9 to 4 7 1022 am2 with an average of 4 2 0 1 1022 am2 40ar 39ar age determination on one lava flow is 116 8 3 0 myr 2 error relative to fish canyon sanidine 28 02 0 28 myr the magnetic reversed polarity with well defined age could correspond to the isea within the older part of the cretaceous normal superchron cns but we cannot rule out the possibility that this reversal interval could also correspond to the m0r due to many ambiguities on previous published ages on isea and m0r intervals as well as the new monitor age correction we applied it also suggests that a weak magnetic field nature already documented before the cns extended at least into the very beginning of the cns

la mc icp ms u pb dating of low u garnets reveals multiple episodes of skarn formation in the volcanic hosted iron mineralization system awulale belt central asia volcanic hosted iron deposits of the eastern awulale metallogenetic belt in central asia possess a reserve of over 1 2 billion tons of iron ores and constitute one of the most important basements for high grade iron resources in china skarns are widespread in these deposits and closely associated with iron mineralization the ages of these skarns are unclear and their genesis remains debated preventing further investigation into their metallogenic processes we focused on garnets in nine ore bearing skarns from three large scale iron deposits chagangnuoer dunde and beizhan in the eastern awulale belt u pb dating was conducted on these garnets using our in house reference material the taochong garnet tc 13 pb pb isochron age 126 2 23 ma initial pb 207 pb 206 ratio 0 845 0 022 laser ablation multicollector inductively coupled plasma mass spectrometry la mc icp ms was employed in the garnet u pb dating and high precision u pb ages 03 1 6 were obtained highlighting the advantages of la mc icp ms in dating low u minerals the garnet u pb ages of the nine skarn samples fall into three groups i e 329 0 5 1 326 3 3 ma two samples 316 3 2 9 311 2 2 4 ma six samples and 295 6 1 0 ma one sample implying three episodes of skarn alteration in the volcanic hosted iron mineralization system the first and second episodes of skarns formed as a result of contact metasomatism between coeval volcanic rocks and limestone and they have economically important iron mineralization the third was likely caused by a local postcollision granitic intrusion but its metallogenic potential deserves further assessment

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

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

asymmetry in the reversal rate before and after the cretaceous normal polarity superchron abstract the geomagnetic reversal chronology shows a decreasing reversal rate from about 160 ma to about 118 ma a cessation in the reversal process the cretaceous normal polarity superchron until about 83 ma and then an increasing until recent times visually the rate of decrease in leading into the superchron appears larger than the rate of increase in following the superchron statistical tests developed here confirm the visual perception of an asymmetry in the rate of change in this asymmetry could be caused by a relatively rapid breakdown of d with thermal energy being advected away followed by a slower thickening of the layer through thermal conduction however in the absence of good thermal models as to how different core mantle boundary conditions affect the geodynamo there are probably many apparently viable speculations as to the origin of the observed rate asymmetry

in situ u pb zircon dating using laser ablation multi ion counting icp ms high resolution in situ u pb zircon geochronology on zoned grains can obtain isotope signatures from multiple growth or thermal events we present a method using laser ablation multicollector inductively coupled plasma mass spectrometry la mc icp ms to overcome complications associated with intricately zoned zircon crystals through in situ sampling of zircon volumes as small as 12 m 25 m and 40 m in diameter by about 10 m in depth high precision u pb age of a series of zircon standard covering a wide age range of 30 to 1 065 ma was acquired using la mc icp ms the precision of measured pb u ratios in homogeneous zircon is about 2 2 resulting in routinely achieved precision of u pb ages obtained by external calibration of 1 2 or better all masses of interest can be simultaneously recorded with a multi ion counting system mic operating in static mode and the short ablation required to achieve such precision results in spatial resolution that is superior to comparable u pb zircon analyses by single collector icp ms the resulting present u pb age for five zircon reference samples and two geological samples show an excellent agreement with the previously reported id tims or shrimp data

the role of the earth s mantle in controlling the frequency of geomagnetic reversals a series of computer simulations of the earth s dynamo illustrates how the thermal structure of the lowermost mantle might affect convection and magnetic field generation in the fluid core eight different patterns of heat flux from the core to the mantle are imposed over the core mantle boundary spontaneous magnetic dipole reversals and excursions occur in seven of these cases although sometimes the field only reverses in the outer part of the core and then quickly reverses back the results suggest correlations among the frequency of reversals the duration over which the reversals occur the magnetic field intensity and the secular variation the case with uniform heat flux at the core mantle boundary appears most earth like this result suggests that variations in heat flux at the core mantle boundary of the earth are smaller than previously thought possibly because seismic velocity anomalies in the lowermost mantle might have more of a compositional rather than thermal origin or because of enhanced heat flux in the mantle s zones of ultra low seismic velocity

geochronology and geochemistry of granitoid rocks from the zhibo syngenetic volcanogenic iron ore deposit in the western tianshan mountains nw china constraints on the age of mineralization and tectonic setting abstract several large syngenetic volcanogenic iron ore deposits including the chagangnuoer fe cu the zhibo fe and the beizhan fe deposits occur in the awulale fe cu metallogenetic belt amb which is located in the southeastern corner of the yili block in the western tianshan mountains nw china in the present study we report geochronological and geochemical data for granitoid rocks associated with the zhibo iron ore deposit in order to constrain the age of the stratiform fe mineralization and tectonic setting of this deposit laser ablation icpms la icpms u pb zircon ages of 318 9 1 5 ma and 304 1 1 8 ma were obtained for a diorite and a granite pluton respectively both of which intruded the volcanic volcaniclastic rocks of the early carboniferous dahalajunshan formation additional la icpms u pb zircon ages of 320 3 2 5 ma and 294 5 1 6 ma were obtained for two crosscutting granite dikes respectively the ca 319 ma diorite has a high potassium calc alkaline weakly peraluminous composition and is characterized by large ion lithophile element lile and light rare earth element lree enrichment and flat heavy rare earth element hree patterns it further shows distinct negative anomalies of nb ta and ti in contrast the ca 304 ma granite classifies as a2 type granite and is characterized by an enrichment of lile and lree flat hrees and distinct negative anomalies of ba nb ta and ti the ca 320 ma granite dike has a low potassium and weakly peraluminous composition and is characterized by lile and lree enrichment and a distinct depletion of nb ta ti and hree the ca 295 ma granite displays a high potassium calc alkaline and weakly peraluminous composition and is characterized by a slight enrichment of lile and lree flat hrees and a distinct depletion of nb ta and ti the data presented here in conjunction with previously published data suggest a submarine syngenetic volcanogenic mineralization age of ca 320 ma for the zhibo iron ore deposit the zhibo and other submarine volcanogenic iron ore deposits of the amb are inferred to have formed in a continental arc setting related to subduction tectonics furthermore post collisional granitic magmatism from 304 to 295 ma was superimposed on the entire amb

recent investigations of the 0 5 ma geomagnetic field recorded by lava flows we present a synthesis of 0 5 ma paleomagnetic directional data collected from 17 different locations under the collaborative time averaged geomagnetic field initiative tafi when combined with regional compilations from the northwest united states the southwest united states japan new zealand hawaii mexico south pacific and the indian ocean a data set of over 2000 sites with high quality stable polarity and declination and inclination measurements is obtained this is a more than sevenfold increase over similar quality data in the existing paleosecular variation of recent lavas psvrl data set and has greatly improved spatial sampling the new data set spans 78 s to 53 n and has sufficient temporal and spatial sampling to allow characterization of latitudinal variations in the time averaged field taf and paleosecular variation psv for the brunhes and matuyama chrons and for the 0 5 ma interval combined the brunhes and matuyama chrons exhibit different taf geometries notably smaller departures from a geocentric axial dipole field during the brunhes consistent with higher dipole strength observed from paleointensity data geographical variations in psv are also different for the brunhes and matuyama given the high quality of our data set polarity asymmetries in psv and the taf cannot be attributed to viscous overprints but suggest different underlying field behavior perhaps related to the influence of long lived core mantle boundary conditions on core flow psv as measured by dispersion of virtual geomagnetic poles shows less latitudinal variation than predicted by current statistical psv models or by previous data sets in particular the brunhes data reported here are compatible with a wide range of models from those that predict constant dispersion as a function of latitude to those that predict an increase in dispersion with latitude discriminating among such models could be helped by increased numbers of low latitude data and new high northern latitude sites tests with other data sets and with simulations indicate that some of the latitudinal signature previously observed in vgp dispersion can be attributed to the inclusion of low quality insufficiently cleaned data with too few samples per site our matuyama data show a stronger dependence of dispersion on latitude than the brunhes data the taf is examined using the variation of inclination anomaly with latitude best fit two parameter models have axial quadrupole contributions of 2 4 of the axial dipole term and axial octupole contributions of 1 5 approximately 2 of the octupole signature is likely the result of bias incurred by averaging unit vectors