precambrian alluvial fan and braidplain sedimentation patterns example from the mesoproterozoic rjukan rift basin southern norway abstract an integrated approach of facies analysis geochemistry and paleohydrology provides new insight into the sedimentology and paleogeography of alluvial sediments in the precambrian here alluvial fan and braidplain sedimentation patterns are documented in a mesoproterozoic rift basin called known as the rjukan rift basin in southern norway the studied formation heddersvatnet formation consists of volcanoclastic breccias and conglomerates cross bedded sandstones with associated mudstones and subaerial basaltic lava beds deposited in a rift basin during the syn rifting phase based on genetically related major lithofacies associations and individual minor lithofacies the sedimentation is characterized by colluvial and screen apron deposits subaerial debris flows hyperconcentrated or sheet flood deposits in the proximal part representing waning flood cycles the distal part contains shallow water traction current deposits and associated subaerial continental flood basalts that flowed down the rift valleys the synsedimentary intra rift faulting formed seasonal or climatically controlled intrafan ponded lake with playa lake type cycles and shrinkage cracks the geochemical composition of the of the studied sandstones and mudstones suggests a passive rifted continental margin with minor to moderate chemical weathering possible in a semi arid arid paleoclimate clastic petrofacies of the sandstones indicate a locally uplifted and syn rift paleotectonic setting during the main sedimentation stage passing toward a post rifting stage of the depositional basin small scale structures within the sedimentary basin affected the depositional patterns of the heddersvatnet formation by creating local sites of uplift and erosion controlling the sediment transport and defining the accommodation space for deposition the sedimentation of the heddersvatnet formation reflects a tectonic base level fall which resulted in an accumulation of thick alluvial fan or braidplain sediments near uplifted flank s of the sedimentary basin this was combined with intensive erosion of the basement in the pre vegetation landscape which resulted in changes in the hydraulic geometry of flow and sediment discharge of stream channels the following criteria observed from the heddersvatnet formation are suggestive of an ancient alluvial fan and associated braidplain sedimentation 1 deposition close to the source area 2 slightly divergent paleoflows 3 high energy flows with waning flood cycles 4 abundance of poorly sorted and laterally extensive subaerial debris flows and related streamflows possible sheetfloods hyperconcentrated flows and their associations 5 great distribution of grain size and lithofacies changes in proximal vs distal parts 6 limited suite of sedimentary structures cross stratification ripple marks and desiccanation cracks 7 fault bounded basin graben with hanging wall close to uplifted flank s 8 paleoslope estimation indicating at least moderate paleotopographic highs 9 colluvial and scree apron breccias 10 lack of any typical braided stream channel fill conglomerates and floodplains and 11 channelized sediment bodies lacking extensive lateral continuity

sheetfloods sheetwash sheetflow or abstract following mcgee s 1897 use of the term sheetflood there developed a variety of conflicting opinions and numerous terminologies relating to this process many of which are ill considered and poorly defined a classification system is devised on both a hydrologic and geomorphic basis which dispenses with many superfluous terms that have been used with respect to sheetfloods magnitude of a storm is expressed in terms of the amount of surface runoff generated from a given storm due to the surface conditions a sheetflood is defined as a sheet of unconfined flood water moving down a slope the frequency of a sheetflood is relatively low while its magnitude is relatively great sheetflow is defined as relatively high frequency low magnitude overland flow occurring in a continuous sheet and is restricted to laminar flow conditions sheetwash a term of geomorphic origin is considered to be redundant and is superseded by the more meaningful term rainwash defined as the washing action of rain on slopes

stratigraphy and facies architecture of the fluvial aeolian lacustrine sergi formation upper jurassic reconcavo basin brazil abstract the sergi formation encompasses an upper jurassic fluvial eolian lacustrine succession deposited within a wide intracratonic basin its sand bodies represent the largest and more important reservoirs in the reconcavo basin hosting 362 million m 3 of oil in place the main goal of this paper is to provide a detailed stratigraphic analysis of the sergi formation based on core and outcrop data it was achieved through the recognition and correlation of regional surfaces that have allowed the subdivision of this unit into distinct depositional sequences and the reconstruction of its depositional history the studied succession can be subdivided into three unconformity bounded sequences sequence i is composed of lacustrine mudstone at its base which is overlain by fine to medium grained sandstone related to aeolian dune and sand sheet and ephemeral fluvial stream deposits fluvial strata indicate northeastward flowing streams whereas aeolian dune deposits suggest the action of southwestward blowing winds the regional unconformity bounding sequences i and ii denotes both a climate change and tectonic rearrangement of the basin this surface delineates a change in the depositional style from fluvial aeolian lacustrine sequence i to entirely fluvial sequence ii the latter includes coarse grained to conglomeratic sandstone deposited within northwestward flowing braided channel belts changes in fluvial palaeocurrent from sequence i to ii indicate rearrangement of the drainage system related to basin tectonism furthermore a change in the fluvial discharge regime took place as a result of a change from a drier to a wetter climate fluvial deposition in sequence i was related to ephemeral streams whereas fluvial facies architecture of the sequence ii deposits indicates perennial braided streams with significant discharge variation another regional unconformity this time related to a stratigraphic base level fall and consequent widespread basinwide aeolian deflation separates the braided fluvial facies of sequence ii from the fine to medium grained sandstones ascribed to sheet floods aeolian dunes and aeolian sand sheets of sequence iii the resumption of aeolian sedimentation indicates a return to drier conditions in the basin the abrupt change from fluvial aeolian deposits to the lacustrine deposits of the overlying itaparica formation suggest a rapid rise of the water table and consequent basinwide flooding

facies controlled shrinkage crack assemblages in middle proterozoic mudstones from montana usa shrinkage crack morphotypes in the libby formation upper belt supergroup are confined to distinct environmental facies the lower facies is characterized by flat rip up clasts stromatolites oolites small scale symmetrical ripples and fenestral fabric these rocks were deposited above fair weather wave base on a periodically exposed mudflat shrinkage cracks in this facies are predominantly branching incompletely connected features in plan view except for local examples of completely connected polygonal cracks on purple argillite bed tops and rare long spindle shaped cracks on bed tops of dark grey argillite the upper facies was deposited below fair weather wave base and contains mainly unconnected short spindle shaped shrinkage cracks and rare slightly branching cracks restriction of some crack types to certain facies better constrains interpretation of the origin of these shrinkage cracks the cracks in the upper facies were strongly influenced by sediment loading and may have formed by compaction induced expulsion of water from pore space resulting in synaeresis cracks in the underlying shallower facies polygonal cracks formed by desiccation elsewhere in this facies incomplete partially connected cracks and long spindle shaped cracks on the same bedding plane are interpreted as having formed by desiccation shrinkage cracks are an under used source of environmental information but confusion as to their origin sometimes restricts their potential more intensive analysis of properties of host sediment and crack fills may further our understanding of depositional and diagenetic influence on crack morphology crack cross sections which are often more commonly exposed than bedding plane cracks may provide critical additional information on crack genesis better understanding of crack genesis will strengthen our ability to interpret unfossiliferous muddy sequences common in precambrian and lacustrine settings

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

provenance of triassic cretaceous sandstones in the antarctic peninsula implications for terrane models during gondwana breakup abstract the antarctic peninsula is the largest of twelve micro continental fragments between southern south america east antarctica and new zealand their original positions and movement history following gondwana breakup are poorly constrained recent work on the antarctic peninsula fore arc arc back arc suggests it consists of two suspect terranes that originated elsewhere and collided with the pacific margin of gondwana in the cretaceous this study reviews provenance models for three laterally extensive kilometers thick sedimentary units in the antarctic peninsula using previous studies and new sandstone detrital modes and chemistry 1 the trinity peninsula group accretionary complex triassic was possibly derived from a glaciated continental margin and may consist of several accreted tracts the deformed quartzose view point and legoupil formations were eroded from a deeply eroded stable continental margin containing an old arc and mature quartzose sandstones with provenance ages suggesting sources in north patagonia and local antarctic peninsula basement the weakly deformed and more feldspathic hope bay formation was derived from an eroded arc possibly the early permian arc of the north patagonian massif 2 the lemay group accretionary complex middle triassic middle cretaceous is dominated by arkoses derived from an eroded continental margin of carboniferous and permian age possibly in the marie byrd land sector with several belts of volcanilithic sandstones representing episodes of arc magmatism the youngest part of the lemay group contains abundant metamorphic detritus derived from basement uplifted during mid cretaceous terrane docking 3 the latady formation in the southern antarctic peninsula has a completely different petrology comprising mature sublitharenites 190 ma of gondwanan provenance deposited in restricted rift basins younger volcanic litharenites 180 ma are dominated by ree enriched low eu eu rhyolitic material eroded from silicic calderas associated with gondwana breakup abundant dacitic to andesitic detritus in the youngest lf 160 145 ma was derived from a contemporaneous arc possibly on the thurston island block as the latady evolved into a back arc basin

early eocene longmala skarn pb zn cu deposit in tibet china geochemistry fluid inclusions and h o s pb isotopic compositions abstract the longmala deposit is a representative of the skarn pb zn cu deposits found in the nyainqentanglha area of tibet china the mineralization is hosted by skarn and marble of the permian wululong formation intruded by an early eocene biotite monzogranite which is associated with the pb zn cu mineralization the genesis of the deposit is related to a four stage progression involving stage i involves the development of prograde skarn stage ii is a pre ore retrograde skarn stage iii is the emplacement of quartz sulfides ore and stage iv is the emplacement of post ore carbonate the biotite monzogranite is a high k calc alkaline highly fractionated i type granite characterized by its elevated sio2 68 38 72 62 wt fet 4 09 6 19 wt and k2o na2o 7 82 8 33 wt contents its aluminum saturation index of 0 97 0 99 and a high differentiation index of 79 86 are indicative of a last stage fractionation of the magma the monzogranite is relatively enriched in light rare earth elements th u and rb depleted in sr ba k nb p and ti and has a negative eu anomaly the ehf t values 6 2 to 1 5 two stage hf model ages average tdm2 1224 ma for zircons and whole rock pb isotopic compositions 206pb 204pb 18 806 18 938 207pb 204pb 15 676 15 697 and 208pb 204pb 39 234 39 376 indicate that the source of the magma is the crust of lhasa terrane the study of fluid inclusions from each of the four stages mentioned above show that garnet and diopside from the prograde skarn stage i are characterized by high temperatures and high oxygen fugacity indicative of magmatic fluids the succeeding mineralization stage iii is characterized by moderate to high temperature and a reducing fluid and final carbonate veining stage iv with a decreased fluid temperature and salinity the 18ow values range from 1 3 to 10 8 and the dw values are 123 5 to 93 4 indicating that the ore forming fluids were derived from mixture magmatic and meteoric fluids and the 34s values of sulfides have a relatively narrow range of 0 3 3 6 suggestive of a magmatic source the 206pb 204pb 18 571 18 722 207pb 204pb 15 707 15 796 and 208pb 204pb 39 116 39 448 values of sulfides show that ore forming metals were derived from the crust in the lhasa terrane major and trace elements characteristics of the ore associated biotite monzogranite combined with the dating results suggest that the longmala deposit were formed within the india asia continental collision tectonic setting

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

structure and evolution of the himalaya tibet orogenic belt the 1981 french chinese expedition to tibet focused on the lhasa block extending earlier coverage 400 km north of the tsangpo suture the lhasa block stood between 10 and 15 n latitude over most of the upper cretaceous and eocene and if gondwanian in origin had detached from gondwana by early permian seismic profiles reveal a complex moho topography resulting both from multiple continental thrusting and large scale strike slip faulting subduction related granitoids representing mixtures of mantle and crustal components and anatectic granitoids have been analysed and dated this study emphasizes the role of smaller blocks in the accretion of the continental mosaic

formation mechanism of magmatic rocks in narusongduo lead zinc deposit of tibet evidence from magmatic zircon the narusongduo pb zn deposit located in the longgeer gongbujiangda fault block uplift belt of middle lhasa terrane is characterized by two types of mineralization namely the cryptoexplosive breccia type in the eastern part and skarn type in the western part the magmatic rock related to the skarn type mineralization includes two kinds of quartz syenite porphyries with coarse and fine phenocrysts respectively the results of zircon u pb dating by la icp ms show that the age of quartz syenite porphyry with coarse phenocrysts is 62 54 0 77 ma while that with the fine phenocrysts is 62 47 0 91 ma the ree patterns of zircon from both porphyries have conspicuous fractionation with low ratios of lree hree and distinct anomalies of negative eu and positive ce with the zircons similar to continental zircons in u yb y discriminant diagram the hf isotopic compositions of zircon from the coarse porphyry display 176hf 177hf ratios ranging from 0 282577 to 0 282803 and ehf t values ranging from 5 58 to 2 21 suggesting that the magma was mainly derived from partial melting of the crust with the mixture of some materials from depleted mantle in different proportions the features of zircon group indicate that both quartz syenite porphyries are hybrid granites that origina ted from the crust and the mantle all the data above suggest that the emplacement and related mineralization of quartz syenite porphyries in the narusongduo pb zn deposit occurred in the main collisional stage of indo asia continental collision which resulted from the upwelling of mixed magma derived from partial melting of the asthenospheric mantle and overlying crust due to the tremendous heat and fluid flow released from the asthenosphere during the steep subduction of indian continental crust with the roll back of the subducted neo tethyan oceanic slab

the origin and pre cenozoic evolution of the tibetan plateau different hypotheses have been proposed for the origin and pre cenozoic evolution of the tibetan plateau as a result of several collision events between a series of gondwana derived terranes e g qiangtang lhasa and india and asian continent since the early paleozoic this paper reviews and reevaluates these hypotheses in light of new data from tibet including 1 the distribution of major tectonic boundaries and suture zones 2 basement rocks and their sedimentary covers 3 magmatic suites and 4 detrital zircon constraints from paleozoic metasedimentary rocks the western qiangtang amdo and tethyan himalaya terranes have the indian gondwana origin whereas the lhasa terrane shows an australian gondwana affinity the cambrian magmatic record in the lhasa terrane resulted from the subduction of the proto tethyan ocean lithosphere beneath the australian gondwana the newly identified late devonian granitoids in the southern margin of the lhasa terrane may represent an extensional magmatic event associated with its rifting which ultimately resulted in the opening of the songdo tethyan ocean the lhasa northern australia collision at similar to 263 ma was likely responsible for the initiation of a southward dipping subduction of the bangong nujiang tethyan oceanic lithosphere the yarlung zangbo tethyan ocean opened as a back arc basin in the late triassic leading to the separation of the lhasa terrane from northern australia the subsequent northward subduction of the yarlung zangbo tethyan ocean lithosphere beneath the lhasa terrane may have been triggered by the qiangtang lhasa collision in the earliest cretaceous the mafic dike swarms ca 284 ma in the western qiangtang originated from the panjal plume activity that resulted in continental rifting and its separation from the northern indian continent the subsequent collision of the western qiangtang with the eastern qiangtang in the middle triassic was followed by slab breakoff that led to the exhumation of the qiangtang metamorphic rocks this collision may have caused the northward subduction initiation of the bangong nujiang ocean lithosphere beneath the western qiangtang collision related coeval igneous rocks occurring on both sides of the suture zone and the within plate basalt affinity of associated mafic lithologies suggest slab breakoff induced magmatism in a continent continent collision zone this zone may be the site of net continental crust growth as exemplified by the tibetan plateau c 2012 international association for gondwana research published by elsevier b v all rights reserved

the nature and timing of ore formation in the budunhua copper deposit southern great xing an range evidence from geology fluid inclusions and u pb and re os geochronology abstract the budunhua copper deposit is located in the southern great xing an range where the stockwork ore bodies are mainly hosted in the lower permian dashizhai formation the mineralization occurs in two blocks the jinjiling and kongqueshan and both are characterized by four main stages arsenopyrite pyrite molybdenite quartz chalcopyrite pyrrhotite quartz galena sphalerite chalcopyrite quartz and calcite fluorite quartz wall rock alteration which is related to mineralization is dominated by silicification sericitization and chloritization fluid inclusion studies show that the ore forming fluids from cu stages in both blocks are broadly identical with trapping temperatures of 280 to 400 c for the kongqueshan and 300 to 420 c for the jinjiling estimated trapping pressures for the kongqueshan are 15 0 to 26 6 mpa and for the jinjiling are 14 9 to 29 4 mpa corresponding to entrapment depth of 1 5 to 2 7 km and 1 5 to 2 9 km below the paleowater table respectively zircon grains from the tonalite porphyry in the deposit yielded a weighted 206 pb 238 u mean age of 152 0 7 ma which is consistent with a molybdenite re os model age of 150 2 2 ma indicating that both the porphyry intrusion and the copper deposit are of late jurassic age our data suggest that the budunhua deposit is an atypical porphyry deposit related to a late jurassic magmatic hydrothermal system

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

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

metallogeny of the northeastern gangdese pb zn ag fe mo w polymetallic belt in the lhasa terrane southern tibet abstract the northeastern gangdese pb zn ag fe mo w polymetallic belt ngpb characterized by skarn and porphyry deposits is one of the most important metallogenic belts in the himalaya tibetan continental orogenic system this belt extends for nearly four hundred kilometers along the luobadui milashan fault in the central lhasa subterrane and contains more than 10 large ore deposits with high potential for development three major types of mineralization system have been identified skarn fe systems skarn breccia pb zn ag systems and porphyry skarn mo cu w systems in this study we conducted a whole rock geochemical u pb zircon geochronological and in situ zircon hf isotopic study of ore forming rocks in the ngpb specifically the jiangga jiaduopule and rema skarn fe deposits and the yaguila pb zn ag deposit although some of these deposits porphyry mo systems formed during the post collisional stage 21 14 ma the majority these three systems developed during the main soft collision stage of the india asia continental collision 65 50 ma the skarn fe deposits are commonly associated with granodiorites monzogranites and granites and formed between 65 and 50 ma the ore forming intrusions of the pb zn ag deposits are characterized by granite quartz porphyry and granite porphyry which developed in the interval of 65 55 ma the ore forming porphyries in the sharang mo deposit formed at 53 ma the rocks from fe deposits are metaluminous and have relatively lower sio2 and higher cao mgo feo contents than the intrusions associated with mo and pb zn ag mineralization while the pb zn ag deposits are peraluminous and have high sio2 and high total alkali concentrations they all exhibit moderately fractionated ree patterns characterized by lower contents of heavy ree relative to light ree and they are enriched in large ion lithophile elements and relatively depleted in high field strength elements ore forming granites from fe deposits display 87sr 86sr i 0 7054 0 7074 and end t 4 7 to 1 3 whereas rocks from the yaguila pb zn ag deposit have 87sr 86sr i 0 7266 0 7281 and end t 13 5 to 13 3 in situ lu hf isotopic analyses of zircons from fe deposits show that ehf t values range from 7 3 to 6 6 with tdm hf c model ages of 712 to 1589 ma and yaguila pb zn ag deposit has ehf t values from 13 9 to 1 3 with tdm hf c model ages of 1216 to 2016 ma combined with existing data from the sharang mo deposit we conclude that the ore forming intrusions associated with the skarn fe and porphyry mo deposits were derived from partial melting of metasomatized lithospheric mantle and rejuvenated lower crust beneath the central lhasa subterrane respectively melting of the ancient continental material was critical for the development of the pb zn ag system therefore it is likely that the source rocks play an important role in determining the metal endowment of intrusions formed during the initial stage of the india asia continental collision

prediction of some weathering trends of plutonic and volcanic rocks based on thermodynamic and kinetic considerations abstract the exposed crust consists mainly of plagioclase 35 quartz 20 k feldspar 11 volcanic glass 12 biotite 8 and muscovite 5 quartz is a resistate thus feldspars and glass represent approximately 75 percent of the labile minerals the weathering characteristics of these constituents are summarized in the context of thermodynamic mass balance and kinetic considerations experimentally determined release rate constants were used to predict the proportions of ca na and k released by feldspars of plutonic rocks granites to gabbros to weathering solutions the chemical weathering trends of the weathered residues calculated from the kinetic data conform closely to the initial trends observed in some recent weathering profiles demonstrating the accuracy of the predictions since the weathering of feldspars is controlled by processes that should not change through geological time the relative release rates of ca na and k from the feldspars of granitic rocks can be calculated for future and past episodes of continental weathering experimentally determined release rate constants are not available for a wide range of volcanic glass compositions but the limited data indicate that compositional trends are predictable in weathering profiles developed on volcanic rocks the kinetic data available for rhyolitic glasses accurately predict the initial weathering trends observed in a recent rhyolite weathering profile

sedimentology and tectonic implications of cretaceous fan delta conglomerates queen charlotte islands canada the honna formation of coniacian age consists of several hundred metres of polymictic clast supported conglomerate associated with sandstone and mudstone five conglomerate facies are recognized ungraded beds inverse graded beds normal graded beds inverse to normal graded beds and parallel stratified beds these facies are interpreted as the deposits of subaqueous cohesionless debris flows and or high density turbidity currents the depositional environment was a deep water gravelly fan that draped a fault controlled basin margin slope the fan is inferred to have passed upslope directly into an alluvial fan unpreserved hence the name fan delta can be applied to the overall depositional system this type of fan delta of which the brae oilfield in the north sea is an example is defined here as a deep water fan delta the lack of a shelf is in marked contrast to other types of fan delta three facies associations are recognized in the honna formation subaqueous proximal fan conglomerates distal fan turbiditic sandstones and pro fan interfan mudstones with thin sandy turbidites the proximal fan is envisaged as an unchannelled gravel belt with a downslope length of at least 20 km such a long subaqueous gravel belt lacks a known modern analogue the distal fan was an unchannelled sandy extension of the proximal gravel belt it is postulated that the honna formation accumulated in a foreland basin which migrated westwards from the coast mountains where the wrangellia alexander terrane was colliding with north america in this model the honna fan delta was sourced by a west verging thrust sheet whose sole thrust was the sandspit fault immediately to the east deep water fan deltas appear to develop preferentially when eustatic sea level is relatively high so that the feeder alluvial fan is small and gravelly throughout in petroleum exploration and field development care should be taken to distinguish deep water fan deltas from base of slope canyon fed submarine fans because the two systems differ significantly in terms of coarse sediment distribution

the pb isotope geochemistry of granitoids from the himalaya tibet collision zone implications for crustal evolution abstract pb isotopic data on 80 k feldspars from six individual granitoid belts of the himalaya tibet collision zone are reported the isotopic trends observed in the trans himalayan complex which outcrops along the indus zangpo suture zone are best explained by a mixture of mantle derived pb component s with crustal derived pb these confirm that the batholith was built upon sialic continental crust as a result of the northward subduction of mesozoic oceanic crust beneath southern tibet the pb isotopic compositions of k feldspars from the three granitoid belts located within the lhasa block to the north of the indus zangpo suture zone are all radiogenic and indistinguishable from those of k feldspars extracted from basement rocks no pristine mantle derived component was identified and the granitoids are interpreted as being wholly derived from the partial melting of the continental crust furthermore the isotopic compositions of the granitoids are remarkably similar over the entire lhasa block implying that they were equilibrated by fluid remobilization during granulite facies metamorphism in the lower crust to the south of the indus zangpo suture zone and within the indian shield the lead isotopic composition of k feldspars from two granitoid belts although significantly more radiogenic than those of the lhasa block are identical to that of the basement gneisses indicating also that the granitic melts are the products of the anatexis of the continental crust in both the lhasa block and the indian shield the generation of granitic melts occurred as a consequence of large scale intracontinental thrusting events that created crustal repetition and induced the development of thermal anomalies at depths due to the burial of meta sedimentary rocks rich in water and radioactive heat producing elements the lhasa block and the indian shield define two distinct pb isotopic provinces which have undergone complex episodes of u th pb fractionation and are largely composed of precambrian material furthermore the correspondence between distinct pb isotopic provinces and discrete continental segments might indicate that the blocks originated from different sectors of the gondwanian continent

provenance tectonic setting and geochemistry of greywackes and siltstones of the late precambrian hammamat group egypt abstract greywackes and siltstones from the neoproterozoic hammamat group in the wadi hammamat and wadi bali areas eastern desert of egypt were analyzed for major and trace elements the sedimentary rocks are characterized by na 2 o k 2 o 1 a wide range of both al 2 o 3 na 2 o 2 5 7 and al 2 o 3 tio 2 13 33 plus a chemical index of alteration between 60 and 44 these data suggest that the hammamat sedimentary rocks are chemically immature and that there was minimal chemical weathering involved in their erosion and deposition the sedimentary units show two broad compositional groupings in the two studied areas the wadi bali sedimentary rocks are characterized by low mgo cr ni v ba nb y and zr and high na2o cao and sr relative to the wadi hammamat sedimentary rocks these geochemical differences suggest contrasting provenance for the sedimentary rocks in the two areas the wadi hammamat sedimentary rocks were derived from 30 mafic rocks 25 granodiorite 25 intermediate volcanics and 20 felsic volcanics whereas those of the wadi bali area were derived from a relatively homogeneous source of 90 intermediate volcanics and 10 felsic volcanics the wadi hammamat sedimentary rocks were deposited in an intra arc basin that developed as a result of the creation of considerable topographic and structural relief after development of a continental arc the source rocks were continental arc volcanics dokhan volcanics and the uplifted older oceanic tholeiites and island arc assemblages the wadi bali sedimentary rocks were formed by the same scenario as the wadi hammamat sedimentary rocks but the absence of rock fragments older than the dokhan volcanics indicates a closed intra arc basin where the dokhan volcanics were the main source of clasts

zircon u pb and hf isotope constraints on the mesozoic tectonics and crustal evolution of southern tibet the first in situ hf and u pb isotope analyses of zircon separates from mesozoic granites in southern tibet identify a significant previously unknown stage of magmatism igneous zircons n 34 from a granite within the gangdese batholith show a weighted mean 206 pb 238 u age of 188 1 1 4 ma and e hf t the parts in 10 4 deviation of initial hf isotope ratios between the zircon sample and the chondritic reservoir values between 10 4 and 16 8 suggesting predominantly early jurassic intrusive activity with a juvenile mantle contribution of 40 inherited zircons from two cretaceous s type granites in the northern magmatic belt 23 delineate a slightly older 206 pb 238 u age cluster between 188 and 210 ma these zircons have e hf t values from 3 9 to 13 7 yielding crustal hf model ages from ca 1 4 to 2 1 ga suggesting a major episode of crustal growth in proterozoic time and remelting of this crust in early jurassic time combining these with literature data we interpret the jurassic gangdese magmatism as an early product of the neo tethyan subduction that played a long lasting role in the tectonic evolution of southern tibet prior to the india asia collision

alluvial sandstone composition and paleoclimate i framework mineralogy abstract systematic variations in compositional maturity of first cycle fluvial sandstones are found in the cutler formation permian and fountain formation permian pennsylvanian in colorado and in the gondwana supergroup permian triassic of peninsular india these variations reflect changing climate during deposition climate is considered to be the critical factor affecting maturity because source rocks granite granite gneiss and tectono environmental setting alluvial systems associated with basement cored block uplifts were similar and remained relatively unchanged throughout deposition of all three of the units the roughly 3 200 m thick gondwana succession consisting of five sandstone petrofacies is characterized by the following sequence of change oldest to youngest in compositional maturity expressed in q f r percentages 54 42 4 88 10 2 62 34 4 82 16 2 99 1 0 this sequence is a function of the changing paleoclimate glacial arid temperate humid warm semiarid warm semihumid warm humid associated with the overall global limatic change and the changing latitudinal location of india as it moved from a location close to the south pole northward during the interval of time represented by gondwana deposition lithologic and fossil evidence indicates that during deposition of the fountain formation climate gradually changed from relatively wet and warm to at least semiarid and warm this climatic change is the main factor responsible for the difference in compositional maturity of the older fountain sandstone q f r 65 26 9 relative to the younger q f r 54 36 10 cutler sandstone was deposited in an arid climate and has maximum compositional immaturity q f r 49 44 7 a log log plot of the ratio of total quartz to total feldspar plus rock fragments against the ratio of total polycrystalline quartz to to al feldspar plus rock fragments is a sensitive discriminator of first cycle sandstones with differing climatic heritage bulk chemical composition data support interpretation of climate from framework mineralogy but by themselves are not sensitive enough to be unequivocal indicators the same is true for the ztr index and observations of degree and abundance of solution pits embayments on detrital quartz results of this study support the conclusion of earlier empirical and theoretical analyses which suggest that the optimum conditions for the production and preservation of a distinctive climatic signature on sand composition are met in extensional plate tectonic settings such settings are characterized by coarse grained crystalline parent rocks short transport distances in low order streams deposition in nonroutine environments and shallow burial diagenesis however even in such optimum settings only rarely will climate be a more important determinant of sandstone composition than the tectonic setting itself

genesis of the banxi sb deposit south china constraints from wall rock geochemistry fluid inclusion microthermometry rb sr geochronology and h o s isotopes abstract the banxi sb deposit is located in the world class central western hunan sb belt in south china with quartz vein type sb only mineralization hosted by neoproterozoic epimetamorphic clastic sedimentary rocks in this contribution microthermometric rb sr geochronological and h o isotopic analyses on fluid inclusions trapped in quartz s isotopic studies on stibnite and major trace elemental measurements on fluid altered and unaltered wall rocks are presented to constrain the genesis of the deposit the fluid inclusions are characterized by low homogenization temperature 170 260 c and low salinity 3 7 wt nacl equiv with a low fluid density of 0 72 0 93 g cm3 and a ph of 5 59 the low dw fluid 2h 140 to 107 whereas positive 18ov smow 15 9 17 2 and 18ow fluid 18o 5 9 8 4 values in quartz indicate that the mineralizing fluid was generated from the low grade metamorphic basement rocks and received a certain input of meteoric water the rb sr isotopic compositions of quartz samples yielded an isochron age of 196 4 ma 1 mswd 0 70 for quartz stibnite ores a highly radiogenic 87sr 86sr value 0 72663 0 74002 initial 87sr 86sr 0 72640 indicates that ore forming materials are derived mainly from basement rocks and a direct material contribution from local granitic rocks can be ruled out the 34sv cdt values of stibnite range narrowly from 4 81 to 6 72 and are in the 34sv cdt range of the metamorphic rocks indicating that the sulfur was sourced from the basement rocks by sulfate reduction compositional differences between fluid altered decolorized and unaltered wall rocks are obvious the migrated sio2 so3 lree hf nb zr ta th u mo sn w into altered wall rocks suggests a deep basement metamorphic rocks derived si rich alkaline brine for the fluid rock interaction while the leached fe2o3 mgo mno tio2 ba cs rb as co v ni sb sc cu pb and zn from the altered rocks imply a material contribution of banxi group wall rocks during the sb mineralization process consequently we conclude that the quartz stibnite stage of the banxi sb deposit was formed during the late triassic orogeny in the xuefeng mountain region heated by granitic magmatism sb and s were leached from the basement rocks forming an initial si rich and alkaline ore bearing fluid precipitation of stibnite from the fluid resulted from interacting with wall rocks and mixing of meteoric water coupled by releasing of stress decreasing of temperature and lowering of the ph at higher crustal levels

architecture of pre vegetation sandy braided perennial and ephemeral river deposits in the paleoproterozoic athabasca group northern saskatchewan canada as indicators of precambrian fluvial style abstract the late paleoproterozoic athabasca basin contains more than 1 5 km of predominantly sandy strata most of which are of braided fluvial origin in the eastern part of the basin at mcclean lake sandstones and minor conglomerates of the read formation at the base of the succession are preserved within a steep walled valley system they consist predominantly of meter scale sheet elements characterized by massive and flat laminated fine to very coarse grained sandstone with minor discontinuous cobble and boulder conglomerate along lower set boundaries these are interpreted as sheet flood and stream flood deposits of a terminal dry land system deposited in an arid climatic setting analysis of closely spaced drill core indicates that late stage run off was confined to small shallow channels typically only a few tens of meters wide and a few meters deep overlying strata of the bird member of the manitou falls formation are predominantly coarse to very coarse grained sandstones with abundant small scale cross stratification and minor granule and small pebble conglomerate at mcclean lake these appear to have been deposited as sheet elements by semi ephemeral to perennial braided rivers under more humid conditions at key lake 160 km to the southwest architectural analysis of a 184 m wide section indicates that at least part of the bird member was deposited in deeper sandy braided rivers characterized by seasonally varied flow the closest modern analogue is the south saskatchewan river in which large simple flow transverse bedforms become exposed and dissected during falling stage and act as nuclei for sand flat development the presence of numerous low relief 4th order surfaces suggests continued reworking of bar tops during rising stage the incremental character of downstream accretionary elements suggests periodic migration of barforms during peak flood stages separated by periods of low flow this indicates similarities with modern perennial braided systems and is counter to the idea that all pre devonian fluvial systems should consist of stacked sheets formed by individual flood events thin gravel layers accumulated as lags on fourth order surfaces with discontinuous mud layers suggesting deposition within temporary ponds in channel thalwegs after major floods strata in overlying units indicate a return to semi perennial conditions

metamorphic overprinting of smnd isotopic systems in volcanic rocks the telemark supergroup southern norway abstract the telemark supergroup of southern norway consists of a series of metamorphosed mesoproterozoic volcanics and sediments the volcanic pile records three metamorphic events a pervasive regional greenschist facies event ca 1050 ma starmer 1993 a more localised early pre regional metamorphism burial type event and a late thermal metamorphism related to granite intrusion ca 900 ma priem et al 1973 in the present study major and trace element data are employed to illustrate the influence of these metamorphic events on apparent initial nd isotopic compositions in metamorphosed volcanics in the case of the basaltic volcanics burial and thermal metamorphism caused substantial fractionation of the ree and alteration of nd isotopic compositions by interaction with open system fluids such that spurious ages might result from age calculations using the nd isotopic data during the regional metamorphism element mobility was limited to the lile rb sr k and ba the ree were not fractionated and the original nd isotopic signatures were preserved the regional metamorphic effects on the acid volcanics are related to the degree of matrix recrystallisation since this matrix contains a large fraction of the ree where groundmass recrystallisation was limited the original nd isotopic signatures were retained but where the groundmass has been extensively recrystallised the ree have been fractionated resulting in a sm nd isochron age menuge 1985 ca 300 ma younger than the u pb zircon age dahlgren et al 1990 the results of this study emphasise the importance of distinguishing the nature of metamorphic fluid rock reactions the potential for ree mobility during low grade metamorphism and the need for careful selection and geochemical screening of samples for nd isotopic studies in metamorphosed volcanic terrains

fluid immiscibility in natural processes use and misuse of fluid inclusion data ii interpretation of fluid inclusion data in terms of immiscibility abstract phase equilibrium analysis of the relevant systems together with the application of the principles of chemical equilibrium put severe constraints on the interpretation of fluid inclusion data in terms of immiscibility in part i following from that point the major limits on the accuracy and even the validity of fluid inclusion quantitative data and their interpretation in terms of fluid composition and density are briefly discussed the practical implications of the general constraints temperature pressure topology of the fluid systems are envisaged emphasis is laid on some important consequences such as the use of isochore intercepts and the possible resulting interpretation of fluid mixing rather than unmixing the case of highly saline inclusions the identification and interpretation of heterogeneous trapping the composition and density constraints on coexisting fluids are presented and illustrated by natural examples taking into account all the measurable parameters in fluid inclusions volume temperature and nature of phase transitions more or less complete individual spectroscopic analyses all the available experimental data and all the theoretical constraints may be long and difficult however it is most generally very informative and productive although part of these data is often sufficient to deny unmixing nevertheless a final example on metamorphic fluids demonstrates how such an approach can prove and also characterize a fluid unmixing during a geologic process

paleohydraulic reconstruction of flash flood peaks from boulder deposits in the colorado front range nine watersheds in the colorado front range with steep bedrock channels were used to test the accuracy of paleohydraulic reconstruction of large flash floods using boulder deposits the nine basins consist of eight small ungauged basins ranging in size from 1 6 to 29 km2 and the big thompson river at the mouth of the big thompson canyon draining 790 km2 between 1923 and 1976 all nine basins had had one catastrophic flash flood the magnitude of which has been estimated by the conventional slope area method in each basin coarse boulder deposits of the large flash floods were identified and three axes of the five largest boulders were measured along with at least two profiles of the valley cross section a simple arithmetic average of two theoretical and two empirical relationships was used to estimate average flood velocity using boulder size and shape average depth was estimated as the arithmetic average of four values computed from the manning equation a regression equation for boulder size and unit stream power a relative smoothness equation and a modified shields relationship the appropriate flood width for the estimated average depth was found by iteration using the valley cross sections the paleohydraulic discharges thus computed generally underestimate conventional slope area discharge estimates on small streams by as much as 75 although the average amount is only 28 too low and the reconstructed discharge in one stream was 31 too large the big thompson river flood of 1976 was overestimated by 76 reasons for discrepancy in reconstructed peaks could include 1 the possibility that floods may have been able to move boulders larger than those available to be moved 2 overestimation of the slope area discharge because high water marks were set prior to erosion of the channel 3 underestimation of original roughness coefficients and 4 macroturbulent effects during fast deep flows the paleohydraulic technique is applied to two other streams in colorado with sedimentological evidence of large flash floods but no conventional indirect discharge estimates a small tributary to the big thompson river draining 1 8 km2 has a paleohydraulic reconstructed flood peak of about 60 m3 s from a flood in 1976 using boulders excavated from a foundation site in holocene alluvium along boulder creek in boulder colorado a paleohydraulic reconstructed flood peak of between 860 and 1 512 m3 s is calculated this is 1 4 to 2 4 times the magnitude of the estimated 500 yr flood

collision related genesis of the sharang porphyry molybdenum deposit tibet evidence from zircon u pb ages re os ages and lu hf isotopes article i nfo sharang is a low fluorine calc alkaline porphyry mo deposit hosted mainly in a granite porphyry of a multi stage plutonic complex in the northern gangdese metallogenic belt largely with stockwork and ribbon textured mineralization the observed age estimates suggest that the formation of the magmatic host complex 52 9 51 6 ma and the ore deposit itself 52 3 ma occurred during the main stage of the india asia collision the host rocks are characterized by lower zircon ehf t values than those of the pre ore and post ore rocks this suggests that the lhasa terrane basement might play an important role in the formation of sharang ore forming intrusions in view of the framework of magmatic metallogenic events we suggest that slab roll back may have induced melting of juvenile crust and ancient continental complexes during the india asia collision this proposal focuses exploration for additional molybdenum deposits on the collision zone

geological characteristics and genesis of the jurassic no i porphyry cu au deposit in the xiongcun district gangdese porphyry copper belt tibet abstract the xiongcun district located in the western segment of the gangdese porphyry copper belt gpcb hosts the only known jurassic mineralization in the gpcb tibet prc the no i deposit in the xiongcun district is related to the middle jurassic quartz diorite porphyry 167 161 ma and the mineralization was formed at ca 161 5 2 7 ma ore bearing middle jurassic quartz diorite porphyry emplaced into the early jurassic volcano sedimentary rock sequences of the xiongcun formation veinlets and disseminated mineralization developed within the middle jurassic quartz diorite porphyry and the surrounding metamorphosed tuff hosting a measured and indicated resource of 1 04 mt copper 143 31 t gold and 900 43 t silver with an average grade of 0 48 copper 0 66 g t gold and 4 19 g t silver the mineralization can be assigned to four stages including three main stages of hypogene mineralization and one epigenetic stage the main alteration associated with mineralization is potassic seven mineralization related hydrothermal veins have been recognized including quartz sulfide biotite sulfide magnetite sulfide quartz molybdenite sulfide chalcopyrite pyrite pyrrhotite pyrite and polymetallic veins the s and pb isotopic compositions of the ore sulfides and the re contents of the molybdenite suggest a mantle source for the ore forming materials with minor contamination from the subducted sediments hydrogen and oxygen isotope compositions of quartz in the ores suggest that both magmatic and meteoric waters were involved in the ore forming process the ore bearing porphyry 167 161 ma and ore forming 161 5 2 7 ma ages of the no i deposit correspond to the time of northward subduction of neo tethys oceanic slab the geochemical data of the ore bearing porphyry indicate that the no i deposit formed in an intra oceanic island arc setting and the ore bearing porphyry originated from the partial melting of mantle with limited contribution of subducted sediments the genesis of the ore bearing porphyry and no i deposit is interpreted as being related to northward intra oceanic subduction of neo tethys oceanic slab in the middle jurassic time 167 161 ma

geology geochronology and geochemistry of the sinongduo ag pb zn deposit in the gangdese metallogenic belt implications of intermediate sulfidation mineralization in the linzizong volcanic succession abstract the sinongduo ag pb zn deposit was first defined as a typical low sulfidation ls epithermal deposit in the widespread linzizong volcanic succession lvs gangdese belt tibet this paper we describe the lithology mineralization and hydrothermal alteration based on current advances in exploration and use multiple chemical and geochronology methods including whole rock geochemistry major and trace elements of ores and the lithocap the mineral chemistry of sphalerite and the la mc icp ms zircon u pb dating of host volcanic rocks to better constrain the genesis of the sinongduo deposit and give some exploration advice in the sinongduo deposit the host crystal tuff yields a zircon u pb age of 61 9 0 4 ma indicating nearly simultaneous diagenesis and mineralization the metallic minerals are mainly fes poor sphalerite galena pyrite and minor chalcopyrite along with silver and silver based precious minerals these minerals particularly fes poor sphalerite pyrite and chalcopyrite represent is states mineral chemistry shows that the content of fes in sphalerite is lower than 4 94 mol and the ag concentration is quite high ranging from 16 to 1798 ppm with a mean concentration of 245 ppm low ga in ratios

mineralogy fluid inclusions and c h o s pb isotopes of the palaeocene longgen pb zn deposit in the western nyainqentanglha belt tibet abstract the medium sized longgen pb zn deposit deposit proven pb zn resources of 0 13 mt 3 21 pb and 2 43 zn is located in the western nyainqentanglha belt in southern tibet the orebodies occurring as stratiform lenticular and vein types are hosted in the skarn or marble along the contacts between the limestones and the granite porphyries alteration zonations are observed from granite porphyry to limestone the proximal garnet is reddish brown and becomes more pale brown and green with distance the values of grossular gradually increase while the andradite end members gradually decrease with distance and 92 5 99 5 gro 0 5 7 5 pra 0 0 0 1 to and 9 0 36 5 gro 63 4 90 9 pra 0 1 0 5 similarly the values of diopside di9 2 30 4hd65 9 89 9 jo0 6 3 7 to di65 2 77 6hd21 33 2jo1 3 1 7 gradually increased close to the limestone retrograde metasomatic minerals overprint the prograde skarn and are dominantly composed of epidote actinolite ilvaite and chlorite the ore minerals consist mainly of galena and sphalerite followed by magnetite pyrite pyrrhotite chalcopyrite molybdenite and bornite at least five stages have been recognized and from stage i to stage v the 18ofluids values are 7 7 to 8 3 6 0 to 4 4 8 8 to 6 4 8 7 to 3 3 and 13 2 respectively the d values from stages i to v are 141 1 to 129 4 136 5 to 124 2 129 5 to 106 4 126 6 to 101 4 and 106 8 respectively the d and 18ofluids indicate that the ore forming fluids gradually evolved from magmatic to meteoric in origin the main types of fluid inclusions for the longgen deposit are liquid rich inclusions laser raman analysis indicates that the fluid inclusions within the studied minerals are dominated by h2o although some contain co2 gas given the fluid inclusion data the ore forming fluids of stages i and ii were most likely high temperature 460 480 and 340 360 c and low moderate salinity 7 9 15 0 and 2 4 12 6 wt nacl eq fluids which were formed in a magmatic hydrothermal system in stage i and as a mixture of magmatic and meteoric waters in stage ii in the subsequent stages iii and iv the fluids changed to moderate temperature 230 270 and 160 200 c and low moderate salinity 0 2 14 8 and 0 9 10 6 wt nacl eq mixed fluids of magmatic and meteoric waters in the last stage v the fluids changed to low temperature 130 160 c and low salinity 0 2 5 6 wt nacl eq meteoric water the 13c values in calcite lie within a narrow range of 6 1 to 4 4 similar to that for the magmatic source which indicates that the carbon was sourced from the granite porphyry sulfur 34s 0 4 5 estimated 34sfluids 2 77 and lead isotopic compositions of sulfides 206pb 204pb 18 549 18 776 207pb 204pb 15 685 15 839 and 208pb 204pb 38 94 39 592 similar to the whole rock values for the granite porphyry indicate that the ore forming materials were derived mainly from the granite porphyry which originated from the nyainqentanglha group consequently taking the data together suggests that the longgen pb zn deposit is a typical skarn deposit related to palaeocene magmatic hydrothermal systems and that fluid cooling caused by fluid mixing is the most crucial mechanism for pb zn deposition

u pb geochronology geochemistry and h o s pb isotopic compositions of the leqingla and xin gaguo skarn pb zn polymetallic deposits tibet china abstract the leqingla and xin gaguo deposits are two representative skarn pb zn polymetallic deposits of the gangdese pb zn polymetallic belt tibet china la icp ms zircon u pb dating of the mineralization related biotite granites from both the leqingla and xin gaguo deposits yielded weighted mean ages of 60 8 ma and 56 5 ma respectively which can be inferred as their mineralization ages the leqingla biotite granite is characterized by high al 2 o 3 total fe na 2 o and low k 2 o in comparison the xin gaguo biotite granite is characterized by relative higher k 2 o but lower al 2 o 3 total fe and na 2 o geochemical and mineralogical characteristics indicate that the leqingla and xin gaguo biotite granites are calc alkaline i type granite and high k calc alkaline i type granite respectively both the leqingla and xin gaguo biotite granites are enrichment in lree and liles and depletion in hfses and they were formed at the india asia collision stage 18 o and d values for the leqingla and xin gaguo deposits are 8 8 to 5 3 and 140 4 to 90 1 4 5 to 7 0 and 117 3 to 81 0 respectively indicating magma fluids mixed with meteoric water in ore forming fluids 34 s values 11 6 to 0 3 of ore sulfides from the leqingla deposit show characteristics of biogenetic sulfur isotope compositions suggesting sulfur for the leqingla deposit were sourced from wall rocks of the mengla and luobadui formation which are rich in organic materials 34 s values of ore sulfides from the xin gaguo deposits show bimodal distribution 5 0 to 1 6 and 1 6 2 1 indicating sulfur in the xin gaguo deposit were derived from both wall rocks and magma in the leqingla deposit most ore sulfides have the similar pb isotopic compositions with that of the mineralization related biotite granite suggesting the biotite granite supplied most of the ore forming metals pb isotopic compositions of ore sulfides and hf isotopic compositions of biotite granite show that the majority of ore forming metals are derived from mantle components of partial melting of the neo tethys ocean slab with some upper crust materials of the lhasa terrane pb isotopic compositions of ore sulfides from the xin gaguo deposit are similar to that of the leqingla deposit indicating they have the similar sources of ore forming metals

provenance analysis and tectonic setting of the ordovician clastic deposits in the southern puna basin nw argentina provenance studies on early to middle ordovician clastic formations of the southern puna basin in north western argentina indicate that the sedimentary detritus is generally composed of reworked crustal material tremadocquartzrich turbidites tolar chico formation mean composition q t 89 f7 l4 are followed by volcaniclastic rocks and greywackes tolillar formation mean q t 33 f42 l25 these are in turn overlain by volcaniclastic deposits mean qt24 f30 l46 of the diablo formation late arenig early llanvirn that are intercalated by lava flows all units were deformed in the ocloyic orogeny during the middle and late ordovician sandstones of the tolar chico formation are characterized by th sc ratios 1 la sc ratios 10 whereas associated fine grained wackes show slightly lower values for both ratios lree light rare earth elements enrichment of the arenites is 50 chondrite eu eu values are between 0 72 and 0 92 and flat hree heavy rare earth elements patterns indicate a derivation from mostly felsic rocks of typical upper crustal composition the e n d t s e d values scatter around 11 to 9 the calculated nd t d m residence ages vary between 1 8 and 2 0 ga indicating contribution by a palaeoproterozoic crustal component the th sc and la sc ratios of the tolillar formation are lower than those of the tolar chico formation normalized ree rare earth elements patterns display a similar shape to paas post archaean average australian shale but with higher abundances of hrees eu eu values range between 0 44 and 1 17 where the higher values reflect the abundance of plagioclase and feldspar bearing volcanic lithoclasts average e n d t s e d values are less negative at 5 1 and nd t d m are lower at 1 6 ga this is consistent with characteristics of regional rocks of upper continental crust composition which most probably represent the sources of the studied detritus the rocks of the diablo formation have the lowest th sc and la sc ratios lower lree abundances than the average continental crust and are slightly enriched in hrees eu eu values are between 0 63 and 1 17 the nd isotopes e n d t s e d 3 to 1 t d m 1 2 ga indicate that one source component was less fractionated than both the underlying early ordovician and the overlying middle ordovician units synsedimentary vulcanites in the diablo formation show the same isotopic composition our data indicate that the sedimentary detritus is generally composed of reworked crustal material but that the diablo formation appears to contain 80 of a less fractionated component derived from a contemporaneous continental volcanic arc there are no data indicating an exotic detrital source or the accretion of an exotic block at this part of the gondwana margin during the ordovician

u pb hf and o isotope evidence for two episodes of fluid assisted zircon growth in marble hosted eclogites from the dabie orogen abstract a combined study of internal structure u pb age and hf and o isotopes was carried out for metamorphic zircons from ultrahigh pressure eclogite boudins enclosed in marbles from the dabie orogen in china cl imaging identifies two types of zircon that are metamorphically new growth and recrystallized domain respectively the metamorphic zircons have low th and u contents with low th u ratios yielding two groups of 206 pb 238 u age at 245 3 to 240 2 ma and 226 4 to 223 2 ma respectively anomalously high 18 o values were obtained for refractory minerals with 9 9 to 21 4 for garnet and 16 9 for zircon this indicates that eclogite protolith is sedimentary rocks capable of liberating aqueous fluid for zircon growth during continental subduction zone metamorphism most of the zircons are characterized by very low 176 lu 177 hf ratios of 0 000001 0 000028 indicating their growth in association with garnet recrystallization a few of them falling within the older age group have comparatively high 176 lu 177 hf ratios of 0 000192 0 000383 suggesting their growth prior to the formation of garnet in the late stage of subduction the variations in the lu hf ratios for zircons can thus be used to correlate with garnet growth during eclogite facies metamorphism in either case the zircons have variable e hf t values for individual samples suggesting that their protolith is heterogeneous in hf isotope composition with localized fluid availability in the bulk processes of orogenic cycle nevertheless a positive correlation exists between 206 pb 238 u ages and lu hf isotope ratios for the metamorphically recrystallized zircons suggesting that eclogite facies metamorphism in the presence of fluid has the identical effect on zircon lu hf and u th pb isotopic systems we conclude that the zircons of the older group grew in the presence of fluid during the subduction prior to the onset of peak ultrahigh pressure metamorphism whereas the younger zircons grew in the presence of fluid released during the initial exhumation toward high pressure eclogite facies regime

modelling the baseline geochemistry of groundwater in a chalk aquifer considering solid solutions for carbonate phases the chalk aquifer of champagne france baseline geochemistry has been determined using a solid solution approach for the modelling of calcite dissolution the water rock interactions are modelled by the speciation code chess from field data and ca mg and sr aqueous concentrations in groundwater the stoichiometries of solid solutions are defined in each stratigraphic unit of the chalk aquifer from bulk geochemistry and chalk mineralogy of samples taken from boreholes the initial mineralisation of water at the bottom of the unsaturated zone and the characterisation of the theoretical evolution of groundwater chemistry along the flow lines associated with incongruent calcite dissolution are calculated from this approach

zircon u pb geochronology and hf isotopic constraints on petrogenesis of the gangdese batholith southern tibet abstract during the mesozoic cenozoic northward neotethyan subduction and subsequent india asia collision gave rise to the extensive transhimalayan magmatism that stretches from burma and western yunnan through southern tibet to the ladakh and kohistan complexes to understand the age distribution and petrogenesis of the gangdese batholith the largest intrusive exposure along the transhimalayan magmatic belt fifty granitic samples were selected for in situ zircon u pb and hf isotopic analyses the u pb data suggest four discrete stages of magmatic activity i e 205 152 109 80 65 41 and 33 13 ma respectively with the 65 41 ma stage being the most prominent the hf isotopic data indicate that the gangdese batholith is overwhelmed by positive e hf t values which are comparable to those of the kohistan ladakh batholiths in the west but differ markedly from those of the chayu burma batholiths in the east most of the gangdese granites show similar and young hf model ages 1000 300 ma indicating their derivation from juvenile crust however those formed in the 65 41 ma stage exhibit more heterogeneous hf isotopic ratios with negative e hf t values being observed in some granites younger than 50 ma suggesting the involvement of old indian continental crust in their petrogenesis this age may thus mark the onset of the india asia collision the newly established zircon u pb age and hf isotope database of the gangdese batholith can be used as a powerful tracer or fingerprint when studying the source to sink relation of the sediments eroded from the southern tibetan plateau

braidplain sedimentation in a proterozoic rift basin the sao joao da chapada formation southeastern brazil abstract the sao joao da chapada formation 100 200 m thick is the basal unit of the middle proterozoic espinhaco supergroup this up to 3000 m thick and slightly metamorphosed sedimentary succession is exposed in the central part of the southern serra do espinhaco southeastern brazil detailed lithofacies studies were carried out to characterize the sedimentation and paleogeography of the sao joao da chapada formation six lithofacies are present horizontal to low angle cross stratified sandstones lithofacies sh sl trough cross stratified sandstones lithofacies st planar cross stratified sandstones lithofacies sp ripple cross laminated sandstones lithofacies sr massive to horizontally stratified conglomerates lithofacies gm and fine grained rocks lithofacies f these are usually arranged in fining and thinning upward successions characterizing waning flood cycles which are always bounded by erosional surfaces no complete cycles have been found however and the cycles are normally composed only of lithofacies sh sl and st the other lithofacies are rare the observed sedimentary structures and their arrangement the geometry of the bodies mainly sheet like to slightly lenticular the low relief morphology of the erosion surfaces bounding the waning flood cycles the coarse and poorly sorted texture of the sediments and the measured paleocurrent pattern unidirectional with little dispersion throughout the study area point to deposition on a wide ephemeral braidplain characterized throughout by relative uniformity of facies and paleocurrent directions water depths on this subsiding braidplain were maintained by high discharge streams debouching onto it from the upland catchment areas as these rivers spread onto the depositional plain the changes in gradient and substratum transformed them into channels of remarkable width and very large width depth ratios these rivers ran in braided tracts and floods were temporarily confined within banks in these tracts channel switching events associated with tilting and subsidence of the basin floor resulted in the accumulation of extensive sandstone sheets interfingering laterally with one another and succeeding one another vertically which built up a braidplain succession extending up to 35 km transverse to the paleoslope the sao joao da chapada braidplain flanked the west margin of a n s elongated rift basin

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

geochemistry of fine grained clastic sedimentary rocks of the neoproterozoic ikorongo group ne tanzania implications for provenance and source rock weathering abstract the neoproterozoic ikorongo group which lies unconformably on the late archaean nyanzian supergroup of the tanzania craton is comprised of conglomerates quartzites shales siltstones red sandstones with rare flagstones and gritstones and is regionally subdivided into four litho stratigraphic units namely the makobo kinenge sumuji and masati formations we report geochemical data for the mudrocks i e shales and siltstones from the ikorongo basin in an attempt to constrain their provenance and source rock weathering these mudrocks are compositionally similar to paas and ps indicating derivation from mixed mafic felsic sources however the siltstones show depletion in the transition elements cr ni cu sc and v and attest to a more felsic protolith than those for paas and ps the chemical index of alteration cia 52 82 reveal a moderately weathered protolith for the mudrocks the consistent ree patterns with lree enriched and hree depleted patterns la yb cn 7 3 38 3 coupled with negative eu anomalies eu eu 0 71 on average which characteristics are similar to the average paas and ps illustrate cratonic sources that formed by intra crustal differentiation geochemical considerations and palaeocurrent indications suggest that the provenance of the ikorongo group include high mg basaltic andesites dacites rhyolites and granitoids from the neoarchaean musoma mara greenstone belt to the north of the ikorongo basin mass balance calculations suggest relative contributions of 47 42 and 11 from granitoids high magnesium basaltic andesites and dacites respectively to the detritus that formed the shales corresponding contributions to the siltstones detritus are 53 43 and 4

molybdenite re os dating constrains gravitational collapse of the sveconorwegian orogen sw scandinavia abstract re os dating of molybdenite from small deposits is used to define crustal domains exhibiting ductile versus brittle behaviour during gravitational collapse of the sveconorwegian orogen in sw scandinavia a 1019 3 ma planar quartz vein defines a minimum age for brittle behaviour in central telemark in rogaland vest agder molybdenite associated with deformed quartz and pegmatite veins formed between 982 3 and 947 3 ma in the amphibolite facies domain three deposits and between 953 3 and 931 3 ma west of the clinopyroxene in isograd two deposits in the vicinity of the 0 93 0 92 ga rogaland anorthosite complex the data constrain the last increment of ductile deformation to be younger than 0 95 and 0 93 ga in these two metamorphic zones respectively molybdenite is the product of an equilibrium between biotite oxide and sulfide minerals and a fluid or hydrated melt phase after the peak of 1 03 0 97 ga regional metamorphism molybdenite precipitation is locally episodic a model for gravitational collapse of the sveconorwegian orogen controlled by lithospheric extension after 0 97 ga is proposed in the west of the orogen the rogaland vest agder sector is interpreted as a large shallow gneiss dome formed slowly in two stages in a warm and structurally weak crust the first stage at 0 96 0 93 ga was associated with intrusion of the post collisional hornblende biotite granite suite the second stage at 0 93 0 92 ga restricted to the southwesternmost area was associated with intrusion of the anorthosite mangerite charnockite suite most of the central part of the orogen was already situated in the brittle upper crust well before 0 97 ga and did not undergo significant exhumation during collapse in the east of the orogen situated against the colder cratonic foreland exhumation of high grade rocks of the eastern segment occurred between 0 97 and 0 95 ga and included preservation of high pressure rocks but no plutonism

debris flow and hyperconcentrated flood flow deposits in an alluvial fan northwestern part of the cretaceous yongdong basin central korea abstract the alluvial fan deposits in the cretaceous yongdong basin korea consist of conglomerates with a muddy or sandy matrix and sandstones with thick or thin laminations the conglomerates and sandstones occur commonly in couplets constituting apparently single sedimentation units facies transitions in the deposits can be summed up in a tripartite facies sequence 1 a clast supported conglomerate with a muddy or sandy matrix and parallel clast alignment facies a and b 2 a matrix supported coarse tail normally graded conglomerate with random clast orientation facies c and thickly stratified pebbly sandstone facies d and 3 laminated sandstone facies e the clast supported conglomerate is interpreted as deposits of debris flows dominated by frictional grain interactions development of pervasive parallel clast alignment lacking large floating clasts and inverse grading suggests deposition via incremental aggradation rather than en masse freezing the matrix supported conglomerate an

the miocene gangdese porphyry copper belt generated during post collisional extension in the tibetan orogen abstract the mid miocene gangdese orogenic belt in southern tibet contains porphyry cu deposits that developed after the indian asian continental collision in the early cenozoic field work confirms that these porphyry cu deposits are temporally and spatially associated with mid miocene monzogranitic and quartz monzonitic stocks which intruded mesozoic cenozoic gangdese granitoid batholiths and surrounding triassic tertiary volcano sedimentary sequences in the lhasa terrane and formed a 50 km wide 400 km long e w belt magmatism occurred during the interval 11 2 to 19 7 ma peaking at 16 ma for the porphyry stocks this is consistent with the age 10 to 18 ma of the east west extension and subsequent ns striking normal fault systems 13 5 ma in the tibetan orogen the stocks typically occur as multiple small diameter intrusions 50 km breakdown of hornblende in the juvenile lower crust source during melting is regarded as fundamental for the formation of the fertile adakite and porphyry cu systems in south tibet the gangdese belt contains four large and three intermediate and small sized porphyry cu mo deposits and shows a continuous spectrum from typical porphyry type cu mo to skarn type zn pb mineralization twenty three samples collected from four deposits yielded a limited range of molybdenite re os isochron ages from 15 2 to 16 0 ma similar to previously reported re os ages 13 8 to 14 9 ma for other porphyry cu mo deposits in the belt suggesting that cu mo mineralization in the belt occurred in a post collisional extension environment the characteristics of porphyry cu mo deposits in the collisional zone are similar in many aspects to those in arc settings i e mineralization style alteration zoning and metal association preliminary fluid inclusion studies indicate that the early hydrothermal fluids are preserved as high temperature 450 c high salinity 40 to 47 wt nacl equiv and high pressure 1100 to 1500 bar fluid inclusions these fluids are interpreted to represent an orthomagmatic fluid that boiled episodically due to fluid overpressuring and hydrofracturing resulting in potassic alteration and precipitation of chalcopyrite molybdenite assemblages on the basis of comparison with porphyry cu deposits in arc settings we proposed that melting of thickened newly formed basaltic crust resulted in generation of cu bearing magmatic systems in the tibetan collision zone and both orogen transverse normal faults and orogen parallel thrust zones during the miocene post collisional extension constrain the temporal and spatial localization of the porphyry cu systems in the collision zone

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

discovery of late triassic mineralization in the gangdese metallogenic belt tibet the banduo pb zn deposit somdo area abstract the newly discovered banduo pb zn deposit in the somdo area of tibet is hosted by a late triassic granodiorite a combination of zircon u pb dating hf isotopic compositions sulfide rb sr dating and whole rock geochemistry was used to determine the age of the mineralization and characterize the metallogenesis of the deposit the weighted mean u pb ages of zircon from the granodiorite 203 8 1 7 ma mswd 0 53 206 1 0 4 ma mswd 0 098 and the weighted mean rb sr isochron age of sulfides from the pb zn mineralization 202 7 2 3 ma mswd 1 3 suggest that the timing of mineralization was close to that of granodiorite crystallization during the late triassic the average 87sr 86sr i value of 0 71026 implies that the ore forming metals were sourced from a mixture of mantle and crustal material the weakly altered granodiorite of the deposit is characterized by sio2 66 78 67 85 wt k2o 2 57 3 37 wt al2o3 13 53 15 15 wt and mg 45 5 51 3 it is enriched in lrees and liles and depleted in hrees and hfses and it exhibits a negative eu anomaly the granodiorite has positive and negative ehf t values 2 5 to 1 7 and crustal model ages tc dm of 1 14 1 40 ga hf isotopic analyses and whole rock geochemistry indicate that the late triassic granodiorite was likely sourced from of a mixture of mantle and ancient lower crust material that had undergone variable degrees of fractional crystallization based on regional data the banduo deposit is the oldest pb zn deposit in the gangdese belt and likely formed as a result of the northward subduction of the neo tethys ocean the metallogenic epoch indicates that deposits related to the northward subduction of the neo tethys oceanic crust exist in this area the late triassic age of granitoid hosted mineralization provides additional prospects for exploration in the gangdese metallogenic belt

geochemistry of the sedimentary rocks from the nanxiong basin south china and implications for provenance paleoenvironment and paleoclimate at the k t boundary cretaceous and tertiary clastic sedimentary rocks from the nanxiong basin south china have been analyzed to constrain their provenance depositional climate and environment evidence from discrimination diagrams for sedimentary provenance and tectonic setting show that the nanxiong basin sediments were derived from typical continental sources geochemical signatures e g eu eu th ti la ti ta ti y b ti and y ti ratios of the claystone are nearly constant suggesting the provenance of the nanxiong basin remained similar throughout the late cretaceous to early paleocene 83 56 ma in contrast rb ti cs ti ratios and toc and caco3 concentrations require an obvious change in climate across the late cretaceous and early paleocene boundary singularly higher caco3 contents and lower toc values and rb ti cs ti ratios in the late cretaceous indicate that a long period extreme dry climate occurred at that time in south china rb ti cs ti ratios and toc values escalated and caco3 contents decreased in the early paleocene suggesting that the climate became relatively wet which resulted in greater vegetation cover the lasting extreme dry climate in the late cretaceous may provide a clue to the extinction of the dinosaurs in the nanxiong basin c 2006 elsevier b v all rights reserved

provenance of the qingshuijian formation late carboniferous ne china implications for tectonic processes in the northern margin of the north china block the upper carboniferous qingshuijian formation in the western hill ne china was deposited disconformably on a lower paleozoic platformal carbonate sequence the provenance and the tectonic setting of the source area of the qingshuijian formation were studied using sandstone petrography and mudstone geochemistry sandstones consist mainly of quartz and meta sedimentary lithic fragments and are classified as litharenite feldspar is lacking in sandstones and negative eu anomalies similar to those displayed by post archean australian shale paas along with depleted ca na cs and rb and low k2o al2o3 ratios in mudstones all suggest that feldspar was scarce in the source terrane abundant sedimentary lithic fragments in sandstones low values of the index of compositional variability 1 values of chemical index of alteration higher than paas depleted transition metals higher total ree abundances and more fractionated ree patterns than paas all indicate that the source area was dominated by sedimentary rocks geochemical and sandstone tectonic discrimination diagrams suggest that the provenance of the qingshuijian formation was a recycled orogen composed of quartzose to lithic sedimentary rocks that were deposited originally in a passive margin setting the yinshan uplift located in the northern margin of the north china block that formed as a result of the collision between the north china block and the mongolian continent is interpreted to be the provenance for the qingshuijian formation during collision the intervening sea floor was subducted northward and an accretionary prism was elevated forming a barrier to arc derived sediments

messinian fanglomerates the colombacci formation in the pietrarubbia basin italy abstract during the late miocene in the central italian apennines sedimentation occurred in a few distinct basins resulting from synsedimentary tectonics alluvial fan sediments of the messinian colombacci formation accumulated in the most western of these basins the pietrarubbia basin the colombacci formation consists of coarse grained predominantly clast supported conglomerates pebbly sandstones sandstones and marls which were deposited by high viscosity debris flows and braided streams the alluvial fan deposits owe their development to an area of high relief that was caused by synsedimentary folding the mode of deposition and the occurrence of caliche nodules point to a semiarid climate

patterns of sedimentation in the precambrian abstract the principle of uniformitarianism may be applied to precambrian basin evolution and to the sedimentary record as a whole the major difference in the precambrian eon lay in variability of rates and intensities of processes controlling weathering erosion transport deposition lithification and diagenesis this paper examines precambrian sedimentation patterns within the larger framework of earth evolution pre rock record sedimentation probably comprised deep water oceanic realms within which meteoritic and cometary impact events generated very large tsunamis resulting in very coarse volcaniclastic detritus combined with fine dust settling out of suspension all reworked by marine current systems and localised turbidites from c 4 to 3 2 ga greenstone belts provided the predominant settings for the thin passive margin carbonates bif stromatolitic evaporites pelites and quartzites and lesser synorogenic turbidites conglomerates and sandstones that accompanied the volcanic and volcaniclastic rocks typical of these settings common palaeoenvironments were high gradient alluvial fans low sinuosity braided rivers and relatively shallow marine settings subject to wave and tidal action and turbidity currents although continental crustal growth continued largely through greenstone belts until c 2 7 ga the witwatersrand basin c 3 0 2 7 ga kaapvaal craton south africa reflects initial stabilisation of the oldest craton with an epeiric sea accumulating largely fluvial detritus subject to tidal inland and storm wave craton marginal reworking within a retroarc foreland basin setting neoarchaean palaeoproterozoic sedimentation is discussed within a framework of two global superevents at c 2 7 ga and 2 2 1 8 ga each encompassing major changes in earth s evolution related to the supercontinent cycle mantle superplumes peaks in crustal growth rates and significant biochemical changes within the atmosphere hydrosphere system concomitant globally raised sea levels led to chemical and clastic epeiric seas within which the first giant carbonate platforms developed and deposition of iron formation peaked globally at c 2 5 ga the first global glaciation at c 2 4 2 2 ga provides little support for the snowball earth theory but does suggest a negative feedback loop model whereby intraglacial co 2 related warming and synglacial decreases in weathering alternated up to three times models for palaeo atmospheric and oceanic evolution are mutually exclusive but by c 1 8 ga the existence of large landmasses and free oxygen in earth s atmosphere enabled erg development and red bed sedimentation globally the full spectrum of phanerozoic modern sedimentary environments was thus present on earth a third postulated superevent at c 0 8 0 6 ga essentially recreated conditions experienced at c 2 2 1 8 ga with additionally at least three global refrigeration events

syn collisional magmatism at the longgen pb zn deposit western nyainqentanglha belt tibet petrogenesis and implications for regional polymetallic metallogeny abstract syn collisional orogeny generally lacks adequate fluids and has a low potential to form magmatic hydrothermal ore deposits however some syn collisional pb zn deposits have been recognized in the nyainqentanglha polymetallic belt npb central lhasa subterrane and ore related magma petrogenesis and the metallogenic geodynamic setting is limited in this study detailed petrology geochemical and geochronological data of ore related syn collisional granite porphyries are evaluated to constrain the timing of mineralization and discuss the genesis and evolution of ore related magma using the longgen skarn pb zn deposit as a case study zircon u pb dating of ore related porphyries provides a weighted mean age of 61 2 0 8 ma which is slightly earlier than the sphalerite rb sr age of 59 1 3 0 ma indicating that a paleocene pb zn mineralization event occurred during syn collisional orogeny the porphyries have high sio2 k2o na2o and are enriched in rb th u pb lrees and a cnk 0 94 1 15 but have low p2o5 and are depleted in nb ta and ti this suggests that the longgen granite porphyries are metaluminous to weakly peraluminous high k calc alkaline arc like granites these porphyries have highly radiogenic isotopic compositions with 87sr 86sr i 0 709513 0 713587 207pb 204pb i 15 657 15 713 and 208pb 204pb i 38 964 39 163 but low end t values from 6 7 to 6 0 as well as negative zircon ehf t values from 5 1 to 3 2 corresponding to tdm2 nd ages of 1 37 to 1 42 ga and zircon hf tdmc ages of 1 30 to 1 42 ga this demonstrates that the longgen granite porphyries may have been generated by the partial melting of mesoproterozoic basement continental crust materials with significant amounts 30 50 of mantle derived melt input paleocene eocene 65 45 ma magmatic rocks associated with pb zn and fe cu deposits are compiled in the npb the pb zn ore related granitoids have relatively lower whole rock end t and zircon ehf t values higher 87sr 86sr i values and older tdmc ages than the coeval granitoids associated with fe cu mineralization this indicates that mantle material is more critical for the development of fe cu ore related granitoids than granitoids associated with pb zn mineralization the final results show that different sources of paleocene eocene syn collisional magma in the npb are beneficial for different mineralization events during the syn collisional orogeny of the indian and asian plates

sheet flow deposits of graded beds and mudstones on an alluvial sandflat playa system upper triassic blomidon redbeds st mary s bay nova scotia the 400 m of blomidon redbeds accumulated in a semi arid rift valley in the subtropics at st mary s bay these redbeds are 64 sandy mudstone playa mudflats 25 graded beds sandflats at the toes of alluvial fans 10 fissile claystone playa lakes and 1 channel sandstone stream channels flash floods in mountains south of the valley flowed down alluvial fans to spread out to the north east as sheet flows on sandflats and playa mudflats deceleration of the sheet flows deposited graded beds 2 83 cm in thickness on the sandflats and thin layers of mud on the playas nine sequences consisting of arrangements of six lithologies compose 90 of the graded beds in order of decreasing abundance these are ripple cross laminated siltstone horizontally laminated mudstone fining upward ripple cross laminated siltstone ripple cross laminated sandstone horizontally laminated mudstone cross bedded sandstone horizontally laminated mudstone ripple cross laminated sandstone ripple cross laminated siltstone rippledrift cross laminated siltstone horizontally laminated mudstone fining upward ripple drift cross laminated siltstone cross bedded sandstone ripple cross laminated siltstone and cross bedded sandstone ripple cross laminated siltstone horizontally laminated mudstone the sheet flows perhaps up to 1 m in depth had a high concentration of suspended load deposition was dominantly during lower flow regime conditions and moderate to rapid flow deceleration there are 32 thinning and fining up cycles where a sandflat package of graded beds is transitionally followed by a playa package of sandy mudstone the cycles range in thickness from 1 3 to 13 3 m averaging 4 6 m each cycle is initiated by avulsion to a new active channel network on a fan gradual abandonment of the channel network produces the thinning and fining up cycle the cycles are grouped in three 60 70 m fining up megacycles upwards within each megacycle the packages of sandy mudstone compose a progressively larger proportion of the cycles each megacycle evidently was initiated by a brief period of tectonic movement on the border faults that produced greater relief of the highlands relative to the valley floor subsequent erosion gradually lowered the relief to yield a fining up megacycle