structural and geochronologic data from the shin jinst area eastern gobi altai mongolia implications for phanerozoic intracontinental deformation in asia abstract the complex geology of central asia records significant phanerozoic growth and deformation of continental lithosphere yet many questions about the timing and mechanisms of these processes remain the gobi altai of southwestern mongolia preserves much of this history in exposed mountain ranges and sedimentary basins the shin jinst area near latitude 44 21 to 44 32 longitude 99 35 to 99 25 contains important evidence of paleozoic growth and accretion as well as subsequent mesozoic and cenozoic deformation detailed mapping and structural data including data sets on folds and cross cutting faults point to repeated episodes of deformation and add to recent work that documents triassic and jurassic compression as well as cenozoic tectonic activity in this region new 40ar 39ar data are interpreted to record permian to jurassic convergent margin processes in particular these data in combination with previous studies suggest that southern mongolia records protracted deformation throughout the jurassic that likely resulted from compression associated with the subduction zone at the southern margin of asia at that time

re os isotopic dating of molybdenite and pyrite in the baishan mo re deposit eastern tianshan nw china and its geological significance the baishan mo re deposit is located in the eastern section of the eastern tianshan orogenic belt nw china the deposit has a grade of 0 06 mo and a high content of rhenium of 1 4 g t rhenium and osmium isotopes in sulfide minerals from the baishan deposit are used to determine the age of mineralization rhenium concentrations in molybdenite samples are between 74 and 250 g g analysis of eight molybdenite samples yields an isochron age of 224 8 4 5 ma 2 pyrite samples have rhenium and osmium concentrations varying in the range 33 4 330 6 ng g and 0 08 0 81 ng g respectively isotope data on seven pyrite samples yield an isochron age of 225 12 ma 2 on the 187re 188os versus 187os 188os plot and an age of 233 14 ma 2 on the 187os versus 187re correlation diagram the ages of molybdenite and pyrite are consistent within the analytical errors combined with field observations the data indicate that mo re mineralization in the baishan deposit is produced by a magmatic hydrothermal event in an intracontinental extensional setting after late paleozoic orogeny the initial 187os 188os ratio of pyrite is 0 3 0 07 the 34s values of molybdenite vary from 0 5 to 3 6 both data indicate that mineralization is derived mainly from a mantle source

u pb shrimp and 40ar 39ar geochronological constraints on the evolution of the xingxingxia shear zone nw china a triassic segment of the altyn tagh fault system the development of structures and their age along the segment of the altyn tagh fault system and the eastward extension of the tianshan orogenic belt remain speculative recent investigations on the structural framework granitic intrusions and metamorphic rocks in the eastern tianshan and adjacent areas show that the ne striking xingxingxia sinistral ductile shear zone nw china is subparallel to the altyn tagh fault zone and is superposed on the eastern tianshan orogenic belt u pb zircon sensitive high resolution ion microprobe shrimp dating and muscovite biotite and k feldspar 40 ar 39 ar thermochronology indicate that sinistral shear along the xingxingxia shear zone initiated at 240 235 ma broadly at the same time as initial formation of the altyn tagh fault zone but later than initiation of dextral strike slip motion along ew trending eastern tianshan orogenic belt at 270 245 ma formation of the xingxingxia ductile shear zone was associated with gondwanaland convergence along the southern margin of the eurasian continent during the late permian early triassic

major types characteristics and geodynamic mechanism of upper paleozoic copper deposits in northern xinjiang northwestern china abstract syn and post orogenic cu mineralization in the northern xinjiang paleozoic accretionary and collisional orogenic belt in northwestern china is of great economic interest about 40 cu bearing deposits have been discovered in the area making it one of the most important metallogenic belts in china according to their host rocks these deposits can be classified into four principal types 1 volcanic hosted massive sulfide cu pb zn deposits 2 porphyry cu mo au deposits 3 magmatic cu ni sulfide deposits and 4 cu mo au ag skarn deposits tectonically the development of these upper paleozoic deposits was closely associated with subduction and collision of the ancient asian ocean between the siberian block and the tarim craton an important metallogenic epoch involving formation of cu pb zn polymetallic deposits formed during southwestern rollback and associated slab detachment during devonian time 400 370 ma in southern altay southwestern direct rollback and associated slab detachment occurred during the devonian 400 370 ma in response to later collision at the accreted continetal margin xiao w j windley b f badarch g sun s li j l qin k z wang z h 2004 palaeozoic accretionary and convergent tectonics of the southern altaids implications for the lateral growth of central asia journal of the geological society of london 161 339 342 during which the ashele cu pb zn and keketale pb zn deposits formed subduction led to the accretion of the junggar ocean arc system in the middle devonian in the lower to middle carboniferous north dipping subduction beneath the dananhu arc triggered the emplacement of granitic porphyries in the tousuquan and dananhu island arc belt in the east tianshan leading to the formation of the tuwu and yandong porphyry cu mo au ag deposits in the upper carboniferous to lower permian large mafic ultramafic complexes were emplaced during closure of the ancient tianshan ocean resulting in the formation of several magmatic cu ni sulfide deposits including the kalatongke cu ni deposits in north junggar and cu ni sulfide deposits in the huangshan jing erquan area of east tianshan mountains also associated with this extensional event were emplacement of voluminous anatectic granitoids and eruption of volcanic rocks which led to formation of skarn cu mo suoerkuduk cu mo deposit and cu ag deposits weiquan cu ag deposit the tectonic settings geological features and temporal and spatial distribution of these different types of cu deposits reflect to a great extent the accretionary and collisional tectonics that occurred between the siberian block and tarim block

zircon u pb dating and sr nd pb hf isotopes of the ore associated porphyry at the giant donggebi mo deposit eastern tianshan nw china abstract the giant donggebi porphyry mo deposit is located in the jueluotage metallogenic belt of eastern tianshan xinjiang northwest china mo mineralization mainly occurs as numerous veinlets in the altered sandstone wall rocks with the development of potassic phyllic argillic and propylitic alteration assemblages outward from a buried porphyritic granite stock zircon crystals from the buried porphyritic granite yield a weighted average 206 pb 238 u age of 236 2 2 ma mswd 1 2 1 n 17 slightly older than the molybdenite re os isotope ages of 231 234 ma suggesting that the donggebi porphyry mo deposit was formed in the triassic post collisional tectonism subsequent to termination of the paleo asia ocean the samples from porphyritic granite show high contents of sio 2 k 2 o and al 2 o 3 low contents of tio 2 mgo and cao and peraluminous high k calc alkaline to shoshonite affinity with obvious lree enrichment and negative eu anomalies they have high initial 87 sr 86 sr ratios of 0 70618 to 0 70821 e nd t values of 0 60 to 1 62 and t dm2 nd ages of 0 88 to 0 96 ga their 206 pb 204 pb t 207 pb 204 pb t and 208 pb 204 pb t values are 17 122 18 577 15 493 15 574 and 37 887 38 006 respectively zircons from the porphyritic granite yield ehf t values of 1 58 to 4 82 and t dm2 hf ages of 0 96 1 36 ga these geochemical and isotopic data imply that the donggebi porphyritic granite originated mainly from partial melting of lower continental crust derived from a depleted mantle the donggebi mo deposit is unique based upon the crustal source for the causative porphyry the distal position of mineralization and the previously revealed co 2 rich ore forming fluids and thus belongs to the collisional or dabie type porphyry deposits as exemplified by the qiane chong and yaochong deposits in dabie shan the donggou deposit in qinling orogen china

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

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

the plinian fallout associated with quilotoa s 800 yr bp eruption ecuadorian andes abstract large volcanic eruptions at dacitic or rhyolitic volcanoes often generate exceptional volumes of fine ash that mantles an area up to a million km 2 these eruptions are characterized by extreme fragmentation of the magma and hence extraordinary dispersal of ash and are categorized as plinian ultraplinian or phreatoplinian events large volume co ignimbrites or co plinian ashes are often produced by such eruptions high fragmentation indices of 90 are attributed to the violent eruption of silicic magma especially if augmented by fuel coolant reactions produced when abundant external water interacts with the magma the present study documents a case where the fine ash 1 mm diameter fall deposit related to the plinian phase of the eruption comprises the overwhelming bulk about 87 wt of the eruptive products this is another example demonstrating the predominance of a widespread fine grained co plinian ash which follows the initial coarser lapilli fall historical eruptions at two other andean volcanoes quizapu chile and huaynaputina peru and at santa maria guatemala and novarupta alaska produced similar ash fall sequences quilotoa s 800 yr bp eruption in the ecuadorian andes is an example of a powerful plinian eruption at a small dacitic volcano it produced a crystal rich fine grained ash layer which mantled an extrapolated 810 000 km 2 with 1 mm thickness and has a bulk magma volume of about 18 3 km 3 stratigraphic and petrographic studies as well as granulometric and density analyses were employed to investigate these eruptive products particularly the distal co plinian fall deposits the eruptive cycle consisted of a phreatomagmatic triggering the establishment of a powerful sustained plinian column and subsequently partial column collapses leading to widespread surge and ash flow activity quilotoa s 800 yr bp eruption has a vei volcano explosivity index 6 and generated a maximum column height of about 35 km and a mass discharge rate of 2 10 8 kg s in comparison pinatubo s 1991 eruption had similar column heights and discharge rates but smaller erupted volumes factors that possibly contributed to the extreme explosiveness and resulting fine grained ash production include inferred volatile overpressures in the crystal dominated dacite magma a rapid eruption onset a forceful and sustained plinian column possibly through a crater lake thus promoting extreme fragmentation of the magma but without leaving evidence of wet phreatoplinian type deposits

large scale porphyry type mineralization in the central asian metallogenic domain a review abstract a series of porphyry type cu mo au deposits including some giant world class deposits such as kounrad aktogai kal makyr oyu tolgoi and chalukou occur in the central asian metallogenic domain camd the outline of the camd almost overlaps with that of the central asian orogenic belt caob which is located between the east europe siberia tarim and north china cratons the camd comprises sparsely exposed tectonic slices of precambrian high grade metamorphic basement rocks that are sandwiched between phanerozoic accretionary complexes and arc rocks according to geological metallogenic geochronological and tectonic setting characteristics the camd comprises three metallogenic provinces 1 the kazakhstan cu au mo 2 the mongolia cu au and 3 the northeast china mo cu metallogenic provinces early paleozoic 490 to 440 ma and late paleozoic 330 to 295 ma large scale porphyry type mineralization in the kazakhstan metallogenic province is intimately associated with the evolution of the central tianshan yili northern tianshan kokchetav chingiz paleozoic arc system which was terminated by the late permian closure of the western segment of the paleo asian ocean in contrast the 370 ma and 240 ma porphyry type mineralization of the mongolia metallogenic province is related to the tuva mongolia arc collage system that was formed during the evolution of the paleo asian ocean in the paleozoic and the mongol okhotsk ocean in the early mesozoic furthermore the northeast china metallogenic province was affected by three distinct geodynamic processes that were associated with the paleo asian mongol okhotsk and paleo pacific oceans during the paleozoic 250 ma the early mesozoic 200 ma and the late mesozoic 200 90 ma respectively except for the early paleozoic cu mineralization at duobaoshan 480 ma and bainaimiao 440 ma the porphyry type mineralization of this metallogenic province is predominantly characterized by widespread mesozoic mo mineralization in general porphyry type deposits of the camd have geological structural metallogenic alteration and fluid characteristics which are consistent either with those related to the oceanic subductional arc settings of the circum pacific metallogenic domain or with those of the continental collisional environment in the tethyan metallogenic domain the involvement of highly oxidized intermediate to felsic magmas is fundamental for the formation of porphyry deposits even though the prevalent mash and slab melting models can be employed to interpret the genesis of some porphyry cu deposits in the camd a here newly suggested model which is based on pre enriched mafic lower crust and the subduction of a relictic mid oceanic ridge is thought to explain the large scale copper mineralization in the balkhash west junggar metallogenic belt of the kazakhstan metallogenic province nevertheless most of the porphyritic suites associated with the mo deposits in the northeast china metallogenic province have relatively depleted mantle like neodymium and hafnium isotopic compositions suggesting that the ore forming granitic magma may have predominantly been derived from juvenile lower crust upwelling of asthenosphere may have triggered the melting of the lower crust during post collisional extension subsequent to the closure of the paleo asian ocean in the triassic the subduction of the paleo pacific ocean in the jurassic and the thinning of the lithosphere caused by rollback of the paleo pacific plate in the cretaceous ancient crustal components were only locally found to have been involved in the magma source of several medium large mo deposits

isotope provinces mechanisms of generation and sources of the continental crust in the central asian mobile belt geological and isotopic evidence abstract the available geological geochronological and isotopic data on the felsic magmatic and related rocks from south siberia transbaikalia and mongolia are summarized to improve our understanding of the mechanisms and processes of the phanerozoic crustal growth in the central asian mobile belt camb the following isotope provinces have been recognised precambrian t dm 3 3 2 9 and 2 5 0 9 ga at the microcontinental blocks caledonian t dm 1 1 0 55 ga hercynian t dm 0 8 0 5 ma and indosinian t dm 0 3 ga that coincide with coeval tectonic zones and formed at 570 475 420 320 and 310 220 ma continental crust of the microcontinents is underlain by or intermixed with juvenile crust as evidenced by its isotopic heterogeneity the continental crust of the caledonian hercynian and indosinian provinces is isotopically homogeneous and was produced from respective juvenile sources with addition of old crustal material in the island arcs or active continental margin environments the crustal growth in the camb had episodic character and important crust forming events took place in the phanerozoic formation of the camb was connected with break up of the rodinia supercontinent in consequence of creation of the south pacific hot superplume intraplate magmatism preceding and accompanying permanently other magmatic activity in the camb was caused by influence of the long term south pacific plume or the asian plume damping since the devonian

astronomical theory of the pleistocene ice ages a brief historical review abstract this paper traces the evolution of the astronomical theory of the pleistocene ice ages from its origin in the work of adhemar and croll through its quantitative transformation in the hands of milankovitch to its current renaissance emphasis is given to the interplay between climatic theory and geologic observation from 1941 when milankovitch published his final version of the theory until the early 1970s most attempts to test the theory empirically were carried out in the time domain by comparing the geologic record of climate with predicted climate curves these attempts ran into serious difficulties particularly the inaccuracy of the geologic time scale and the ambiguity of climate predictions early attempts to evaluate the theory with simple radiation balance models appeared to indicate that the climatic response to secular changes in the orbit would be too small to be observed geologically but by the mid 1970s the advent of an improved time scale for pleistocene marine deposits and the availability of long closely sampled records made it possible to arrange generalized but unambiguous tests of the theory in the frequency domain by comparing orbital and climatic spectra these results decisively confirmed the idea that orbital variations significantly influenced the earth s climate during the pleistocene but they did not tell us how this influence was exercised the emphasis of current work has therefore shifted to the identification and quantitative modeling of physical mechanisms by which the climate system responds so dramatically to the predicted rather subtle changes in radiative boundary conditions

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

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

in situ u pb hf and re os isotopic analyses of the xiangshan ni cu co deposit in eastern tianshan xinjiang central asia orogenic belt constraints on the timing and genesis of the mineralization abstract the timing and genesis of the major ni cu co sulfide deposit in the xiangshan intrusion have been studied based on newly obtained in situ u pb hf and re os isotopic analyses the sims u pb zircon ages of the gabbro hosting the ni cu co sulfide deposit indicate that the xiangshan intrusion was emplaced at 279 6 1 1 ma 95 confidence level mswd 1 30 n 15 on the basis of combined geological and geochronological evidence we suggest that the xiangshan and other adjacent ni cu deposits were formed in the same period sulphides have low common os concentrations and high re os ratios similar to sulphide ores from the duluth sally malay and voisey bay complexes the re os isotopic data from the disseminated and massive ores from the xiangshan intrusion do not form a single isochron as they have different initial os ratios the hf and os isotopic data suggest that the xiangshan intrusion and associated ni cu co mineralization were derived from crustally contaminated mantle melts the geochemical data show a tholeiitic affinity and a strong suprasubduction zone signature with negative nb sr and ti anomalies similar to n morb and e morb we suggest that the mafic ultramafic rocks and associated ni cu mineralization of the eastern tianshan orogen formed in an alaska type subduction zone arc setting some diagnostic features of ridge trench interaction are present in the chinese east tianshan orogen e g granites adakites high mg andesites near trench magmatism alaskan type mafic ultramafic complexes high temperature metamorphic belts that prograde rapidly from low grade belts and orogenic gold deposits the above distinctive rock groups are probably related to the same thermal event ridge subduction as in the cenozoic orogen of alaska we suggest that ridge subduction is the most plausible mechanism to provide the necessary heat ridge subduction provides an important promising model for understanding many aspects of the evolution of the chinese eastern tianshan orogen and by inference throughout much of central asia

re os dating of chalcopyrite from selected mineral deposits in the kalatag district in the eastern tianshan orogen china the kalatag cu zn au district contains a number of economically important cu deposits in eastern tianshan in xinjiang nw china due to the lack of precise mineralization ages the metallogenesis of this area has long been a matter of debate in this study chalcopyrite re os isotope methods are used to date the south meiling cu zn and hongshi cu deposits in the eastern part of kalatag area the south meiling cu zn deposit is hosted in volcanic sedimentary rocks of the late ordovician to early silurian daliugou formation the deposit consists of two parts a concordant massive sulfide ores and discordant vein type ores located in the footwall strata the principal ore minerals are pyrite chalcopyrite sphalerite minor tetrahedrite galena and pyrrhotite gangue minerals include quartz sericite and barite and minor chlorite plagioclase and carbonate minerals the hongshi cu deposit represents a hydrothermal vein system hosted in the mafic volcanic rocks of daliugou formation the orebodies are associated with quartz veins and controlled by subsidiary faults of the kalatag fault the ore forming process can be divided into the early middle and late stages and is characterized by quartz pyrite quartz chalcopyrite pyrite and quartz carbonate gypsum veins respectively re os analyses of chalcopyrite from the south meiling cu zn deposit yield an isochron age of 434 2 3 9 ma and initial 187os 188os ratio of 0 647 0 098 mswd 0 59 re os analyses of chalcopyrite from the hongshi cu deposit yield an isochron age of 431 8 2 7 ma and initial 187os 188os ratio of 0 165 0 075 mswd 0 77 since chalcopyrite is the primary copper mineral we interpret these isochron ages as the timing of cu mineralization based on field geology and petrographic evidence these results suggest that the re os ages presented here provide for the first time a direct constraint on an early paleozoic cu mineralization event of the eastern tianshan orogen the high initial 187os 188os ratios 0 647 0 098 ratio of 434 ma chalcopyrite from the south meiling deposit suggest that the metal was sourced from a two end member mixing of crust and mantle materials moreover we propose that the vms mineral system and hydrothermal vein system of the kalatag district were related to the south dipping subduction of the kalamaili oceanic plate during the late ordovician silurian

copper isotopic zonation in the northparkes porphyry cu au deposit se australia abstract significant systematic cu isotopic variations have been found in the northparkes porphyry cu au deposit nsw australia which is an orthomagmatic porphyry cu deposit copper isotope ratios have been measured in sulfide minerals chalcopyrite and bornite by both solution and laser ablation multi collector inductively coupled plasma mass spectrometry mc icp ms the results from both methods show a variation in 65cu of hypogene sulfide minerals of greater than 1 relative to nist976 significantly the results from four drill holes through two separate ore bodies show strikingly similar patterns of cu isotope variation the patterns are characterized by a sharp down hole decrease from up to 0 8 0 29 0 56 1 n 20 in the low grade peripheral alteration zones phyllic propylitic alteration zone to a low of 0 4 0 25 0 36 1 n 30 at the margins of the most mineralized zones cu grade 1 wt in the high grade cores of the systems the compositions are more consistent at around 0 2 0 19 0 14 1 n 40 the cu isotopic zonation may be explained by isotope fractionation of cu between vapor solution and sulfides at high temperature during boiling and sulfide precipitation processes sulfur isotopes also show an isotopically light shell at the margins of the high grade ore zones but these are displaced from the low 65cu shells such that there is no correlation between the cu and s isotope signatures fe isotope data do not show any discernable variation along the drill core this work demonstrates that cu isotopes show a large response to high temperature porphyry mineralizing processes and that they may act as a vector to buried mineralization

mesozoic metallogeny in east china and corresponding geodynamic settings an introduction to the special issue abstract the giant east china mesozoic metallogenic province hosts some of the world s largest resources of tungsten tin molybdenum antimony and bismuth ores of gold silver mercury lead zinc copper uranium and iron are also of major importance the province and its constituent metallogenic belts or regions south china middle lower yangtze river valley east qinling dabie interior of north china craton yan liao and north east china are the products of several pulses of igneous activity and mineralisation between 240 and 80 ma each successive stage has produced a distinctive suite of deposits that can be readily related to the geodynamic evolution of the region during the mesozoic this geodynamic evolution is linked to a complex series of tectonic events involving far field subduction plate collisions crustal thickening post collision collapse and rifting

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

re os ages for the shameika porphyry mo deposit and the lipovy log rare metal pegmatite central urals russia the ages for pegmatite rare metal and beryl emerald deposits as well as porphyry mo deposits in the hercynian uralide orogen are not well known five molybdenite samples from the lipovy log pegmatite ta nb mo deposit and 11 molybdenite samples from the shameika porphyry mo deposit were selected for re os dating both mineral occurrences are spatial temporally associated with the adui composite granite pluton a well known rare metal related granite intrusion a re os isochron age of 262 0 7 3 ma was obtained for the lipovy log pegmatite ta nb mo deposit the shameika porphyry mo deposit associated with the malyshevo leucogranitic stock and surrounding hornfels provided isochron ages of 273 5 and 282 6 ma for two groups of molybdenite within stock and within hornfels all of these re os ages are consistent with presumed hercynian ages for the granite intrusions formed in a post collisional setting within the uralide orogen

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

eruptive history and geochronology of the mount baker volcanic field washington mount baker a steaming ice mantled andesitic stratovolcano is the most conspicuous component of a multivent quaternary volcanic field active almost continuously since 1 3 ma more than 70 packages of lava flows and 110 dikes have been mapped 500 samples chemically analyzed and 80 k ar and 40ar 39ar ages determined principal components are 1 the ignimbrite filled kulshan caldera 1 15 ma and its precaldera and postcaldera rhyodacite lavas and dikes 1 29 0 99 ma 2 60 intracaldera hydrothermally altered andesite dacite dikes and pods remnants of a substantial early postcaldera volcanic center 1 1 0 6 ma 3 unaltered intracaldera andesite lavas and dikes including those capping ptarmigan and lasiocarpa ridges and table mountain 0 5 0 2 ma 4 the long lived chowder ridge focus 1 29 0 1 ma an andesite to rhyodacite eruptive complex now glacially reduced to 50 dikes and remnants of 10 lava flows 5 black buttes stratocone basaltic to dacitic and several contemporaneous peripheral volcanoes 0 5 0 2 ma and 6 mount baker stratocone and contemporaneous peripheral volcanoes 0 1 ma to holocene glacial ice has influenced eruptions and amplified erosion throughout the lifetime of the volcanic field although more than half the material erupted has been eroded liberal and conservative volume estimates for 77 increments of known age yield cumulative curves of volume erupted vs time that indicate eruption rates in the range 0 17 0 43 km3 k y for major episodes and long term background rates of 0 02 0 07 km3 k y andesite and rhyodacite each make up nearly half of the 161 56 km3 of products erupted whereas basalt and dacite represent only a few cubic kilometers each representing 1 3 the total during the past 4 m y the principal magmatic focus has migrated stepwise 25 km southwestward from the edge of the chilliwack batholith to present day mount baker

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

geochronology and eruptive history of the katmai volcanic cluster alaska peninsula abstract in the katmai district of the alaska peninsula k ar and 40ar 39ar ages have been determined for a dozen andesite dacite stratocones on the arc front and for 11 rear arc volcanoes 10 of which are monogenetic tied to mapping and stratigraphic studies our dating emphasized proximal basal lavas that rest on basement rocks in order to estimate ages of inception of each polygenetic cone oldest among arc front cones is alagogshak volcano 690 43 ka succeeded in the holocene by the active mount martin cone mount mageik consists of four overlapping subedifices basal lavas of which give ages of 93 71 and 59 ka and holocene the three small prehistoric cones of trident volcano yield ages of 143 101 58 and 44 ka falling mountain and mount cerberus dacite domes near the 1912 novarupta vent are related compositionally to the trident group and give ages of 70 ka and 114 ka mount katmai which underwent caldera collapse in 1912 consists of two subedifices that overlapped in space and time and is the only arc front center here to include basalt and rhyolite one cone began by 90 ka the other by 47 ka snowy mountain also consists of two contiguous cones which started around 200 and 171 ka respectively the younger remaining active into the holocene devils desk the only mafic cone on the arc front was short lived at about 245 ka in the rear arc 1 mount griggs produced mafic to silicic andesite in several episodes between 292 ka and the holocene 2 the savonoski river cluster includes a pliocene dacite dome and five small mafic cones 390 88 ka 3 gertrude creek cone 49 8 sio2 yields an age of 500 ka and 4 the saddlehorn creek cluster includes five pliocene basalt to andesite remnants eruptive volumes were reconstructed permitting estimates of average eruption rates for edifice lifetimes since the mid pleistocene total volume erupted along the arc front here is 210 47 km3 and in the rear arc 39 6 km3 of which mount griggs alone accounts for 35 5 km3 most productive has been mount katmai at 70 18 km3 yielding a rate of 1 km3 kyr followed by mount mageik 0 33 km3 kyr and mount griggs 0 3 km3 kyr since 50 ka

geology re os and u pb geochronology and sulfur isotope of the the donggebi porphyry mo deposit xinjiang nw china central asian orogenic belt abstract the the donggebi porphyry mo deposit in the eastern section of the eastern tianshan orogenic belt in the central asian orogenic belt contains mo metal reserves of 0 5 mt the deposit is hosted in early carboniferous metasedimentary rocks namely metasandstone meta sandy mudstone multiple hydrothermal activities have resulted in propylitic phyllic and argillic alteration in this deposit four stages i iv of hydrothermal activity are identified stage i is represented by a mineral assemblage of k feldspar quartz and wolframite stage ii consists of quartz magnetite pyrite chalcopyrite veinlets veins with phyllic halos stage iii consists of quartz molybdenite pyrite galena sphalerite chalcopyrite veins that are commonly related to phyllic alteration in the altered rocks stage iv has an assemblage of calcite gypsum molybdenite mainly occurs in stages iii re os dating results for molybdenite samples from these two stages yielded an isochron age of 234 2 1 6 ma 2 mswd 0 25 n 8 porphyritic granites have a sims u pb zircon age of 236 ma and it was probably related to the triassic felsic magmatism in this area values of 34s of sulfides range from 1 5 to 3 8 with an average value of 2 81 2 24 n 22 reflecting a deep sulfur source most molybdenite samples have high 34s values 3 36 relative to other sulfide minerals i e pyrite and chalcopyrite of stages i to iii 34s 1 5 3 8 n 18 based on the geological history and spatial temporal distribution of the granitoids it is proposed that the mo deposits in the eastern part of the east tianshan orogenic belt formed in a post collision extensional setting in the early mesozoic

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

late paleozoic strike slip shear zones in eastern central asia nw china new structural and geochronological data 1 new structural studies and 40ar 39ar dating in northwest china provide information about late paleozoic strike slip motions subsequent to accretional events which built eastern central asia during the paleozoic two principal areas were affected by these large transcurrent motions first in the tianshan range main east west ductile shear zones are dextral and coeval with an eastward decreasing greenschist retrograde metamorphism associated biotites give ages ranging from 290 ma to 245 ma the earlier n110 shearing occurred in western tianshan while the last one dextral in whole tianshan occurred 250 245 myr ago second in the chinese altay region several nw se shear zones structured the area the main motion is sinistral and occurred along the erqishi zone at 280 290 ma it is followed by a complex succession of dextral and sinistral shearing episodes leading to the northwestward structuring dated at 245 ma of a metamorphic zone that was folded during a compressive event

geodynamics and metallogeny of the central eurasian porphyry and related epithermal mineral systems a review abstract major porphyry cu au and cu mo deposits are distributed across almost 5000 km across central eurasia from the urals mountains in russia in the west to inner mongolia in north eastern china these deposits were formed during multiple magmatic episodes from the ordovician to the jurassic they are associated with magmatic arcs within the extensive subduction accretion complex of the altaid and transbaikal mongolian orogenic collages that developed from the late neoproterozoic through the palaeozoic to the jurassic intracratonic extension the arcs formed predominantly on the palaeo tethys ocean margin of the proto asian continent but also within two back arc basins the development of the collages commenced when slivers of an older proterozoic subduction complex were rifted from an existing cratonic mass and accreted to the palaeo tethys ocean margin of the combined eastern europe and siberian cratons subduction of the palaeo tethys ocean beneath the karakum and altai tarim microcontinents and the associated back arc basin produced the overlapping late neoproterozoic to early palaeozoic tuva mongol and kipchak magmatic arcs contemporaneous intra oceanic subduction within the back arc basin from the late ordovician produced the parallel urals zharma magmatic arc and separated the main khanty mansi back arc basin from the inboard sakmara marginal sea by the late devonian the tuva mongol and kipchak arcs had amalgamated to form the kazakh mongol arc by the mid palaeozoic the two principal cratonic elements the siberian and eastern european cratons had begun to rotate relative to each other drawing in the two sets of parallel arcs to form the kazakh orocline between the two cratons during the late devonian to early carboniferous the palaeo pacific ocean began subducting below the siberian craton to form the sayan transbaikal arc which expanded by the permian to become the selanga gobi khanka arc by the middle to late permian as the kazakh orocline continued to develop both the sakmara and khanty mansi back arc basins were closed and the collage of cratons and arcs were sutured by accretionary complexes during the permian and triassic the north china craton approached and docked with the continent closing the mongol okhotsk sea an embayment on the palaeo pacific margin to form the mongolian orocline subduction and arc building activity on the palaeo pacific ocean margin continued to the mid mesozoic as the indosinian and yanshanian orogens significant porphyry cu au mo and au cu deposits were formed during the ordovician in the kipchak arc e g bozshakol cu au in kazakhstan and taldy bulak porphyry cu au in kyrgyzstan silurian to devonian in the kazakh mongol arc e g nurkazgan cu au in kazakhstan and taldy bulak levoberezhny au in kyrgyzstan devonian in the urals zharma arc e g yubileinoe au cu in russia devonian in the kazakh mongol arc e g oyu tolgoi cu au and tsagaan suvarga cu au in mongolia carboniferous in the kazakh mongol arc e g kharmagtai au cu in mongolia tuwu yandong cu au in xinjiang china koksai cu au kounrad cu au and the aktogai group of cu au deposits in kazakhstan carboniferous in the valerianov beltau kurama arc e g kal makyr dalnee cu au in uzbekistan benqala cu au in kazakhstan late carboniferous to permian in the selanga gobi khanka arc e g duobaoshan cu au in inner mongolia china triassic in the selanga gobi khanka arc and jurassic in the selanga gobi khanka arc e g wunugetushan cu mo and jiguanshan mo in inner mongolia china in addition to the tectonic geologic and metallogenic setting and distribution of porphyry cu au mo mineralisation within central eurasia the setting geology alteration and mineralisation at each of the deposits listed above is described and summarised in table 1

eruptive history of chimborazo volcano ecuador a large ice capped and hazardous compound volcano in the northern andes abstract new fieldwork radiometric and whole rock chemical data permit the reconstruction of the main eruptive stages of the chimborazo compound volcano the highest summit of the northern andes chimborazo is composed of three successive edifices the basal edifice ch i was active from 120 to 60 ka and resulted in a large mostly effusive edifice which was built up during two stages of cone building terminating with the formation of a dome complex this edifice was affected by a huge sector collapse around 65 60 ka which produced a major debris avalanche that spread out into the riobamba basin covering about 280 km2 with an average thickness of 40 m and a total volume of 10 12 km3 after the emplacement of the riobamba debris avalanche eruptive activity resumed at the eastern outlet of the avalanche scar and was responsible for the construction of a less voluminous intermediary edifice ch ii whose current remnants are the politecnica and martinez peaks this edifice developed from 60 to 35 ka lastly eruptive activity shifted to the west leading to the construction of the morphologically well preserved young cone ch iii which currently forms the highest summit whymper the average eruptive rate of chimborazo volcano is 0 5 0 7 km3 ka however looking at the three successive edifices individually we estimate that there has been a progressive decrease in magma output rate from the basal edifice 0 7 1 0 km3 ka through the intermediary edifice 0 4 0 7 km3 ka to the young cone 0 1 km3 ka however during the main cone building stages the peak eruption rates are markedly higher indicating significant variations in the magma output rate during the lifespan of this arc volcano during the holocene the chimborazo eruptive activity consisted of small volume explosive events that occurred at quite regular intervals between about 8000 and 1000 yr ago since the last eruption occurred between the early part of the 5th century and the end of the 7th century and the average time interval between the events is about 1000 yr chimborazo must be considered as a potentially active volcano the presence of a thick ice cap covering the summit its steep flanks and its position above the populated lowland area of riobamba and ambato are factors that result in a high potential risk

the mo deposits of northeast china a powerful indicator of tectonic settings and associated evolutionary trends abstract northeast ne china lies in the eastern sector of the central asian orogenic belt caob that connects with the circum pacific orogenic belt in the east and accommodates multistage magmatism crustal growth and mineralization in this region 69 mo only or mo dominated and 9 cu mo deposits have been discovered including 65 deposits ranking medium size 10 kt mo metal or larger in tonnage and containing a total resource of 10 5 mt mo metal six giant and seventeen large deposits have total reserves of 6 7 and 3 1 mt mo respectively these deposits occur in the areas surrounding the songliao basin including the northern margin of the north china craton the great hingan range and the ji hei fold belt and have been formed during paleozoic and mesozoic tectono magmatic events all the mo only or mo dominated deposits were formed in the mesozoic postdating the closure of the paleo asia ocean and in a series of pulses around 250 200 ma 200 160 ma 160 130 and

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

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

dating the xiaobaishitou skarn w mo deposit eastern tianshan nw china constraints from zircon u pb muscovite 40ar 39ar and molybdenite re os system abstract the xiaobaishitou deposit 36 264 tonnes 0 78 wt wo3 in the eastern tianshan orogenic belt is a medium sized skarn deposit characterized by w mo ore bodies that developed along contacts between triassic biotite granite and mesoproterozoic carbonaceous rocks scheelite and molybdenite are the dominant metallic minerals in this deposit and mostly precipitated during the retrograde stage and or quartz sulfide stage respectively laser ablation multi collector inductively coupled plasma mass spectrometry la mc icp ms zircon u pb dating of the mineralization related biotite granites from underground openings and outcrops yields ages of 247 9 1 7 ma 1 n 22 mswd 0 69 and 252 2 3 0 ma 1 n 12 mswd 1 60 implying they were emplaced in the early triassic a muscovite 40ar 39ar weighted plateau age of 247 6 2 3 ma 2 mswd 2 03 and molybdenite re os model ages ranging from 235 8 5 7 ma to 249 6 3 6 ma weighted mean age 251 1 1 6 ma 2 mswd 0 70 are interpreted as the time of the hydrothermal activity mineralization the excellent agreement among the u pb 40ar 39ar and re os chronometers confirms that the xiaobaishitou deposit formed in the early triassic the re contents of the molybdenites range from 18 477 to 130 447 ng g suggesting that the ore forming materials were derived from continental crust together with the involvement of abundant mantle components as one of the representative triassic deposit in eastern tianshan the xiaobaishitou provides important chronological evidence for metallogenic regularity in the eastern tianshan and the central asian orogenic belt the new ages reported here confirm that the xiaobaishitou deposit represents one of the magmatic hydrothermal mineralization events that occurred in the eastern tianshan orogenic belt in lithospheric extension during the triassic it is suggested that mantle material played a role in producing the triassic mineralization and associated magmatism

regional geologic setting and metallogenesis of central inner mongolia china guides for exploration of mesothermal gold deposits abstract the central inner mongolia district forms one of the most important metallogenic provinces in china because the district has been influenced by repeated distinct tectonomagmatic activity the genetic relationships and spatial temporal evolution of ancient tectonism and associated geologic structures and metallogeny are discussed in this paper an integrated genetic model for gold metallogeny is proposed during the late archean to paleozoic repeated north southward accretional tectonics between the paleo mongolia oceanic plate and the north china paleoplate led to the formation of not only the e w trending deep crustal faults but also a series of sedimentary volcanic and metamorphic events numerous stratiform fe cu au pb zn and ree ores formed in association with sedimentation and volcanism within the platform environments at this time during the late paleozoic to early mesozoic the central inner mongolia district was largely influenced by northwestward subduction of the circum pacific plate beneath the eurasia plate a series of ne sw trending faults together with s type granitoid intrusions formed as a result of this subduction mesothermal type gold deposits e g the saiyingwusu laoyanghao shibaqin wulashan and donghuofang deposits formed during the permian to jurassic 158 5 to 275 4 ma preferentially at or near the intersections between ne sw trending and e w trending fault systems at these intersections the areas of not only late paleozoic to early mesozoic granitoids but also late archean to early proterozoic metamorphic rocks especially the gold rich seertengshan wulashan and bayan obo group rocks may be promising exploration targets for mesothermal gold deposits