regolith formation rate from u series nuclides implications from the study of a spheroidal weathering profile in the rio icacos watershed puerto rico abstract a 2 m thick spheroidal weathering profile developed on a quartz diorite in the rio icacos watershed luquillo mountains eastern puerto rico was analyzed for major and trace element concentrations sr and nd isotopic ratios and u series nuclides 238 u 234 u 230 th 226 ra in this profile a 40 cm thick soil horizon is overlying a 150 cm thick saprolite which is separated from the basal corestone by a 40 cm thick rindlet zone the sr and nd isotopic variations along the whole profile imply that in addition to geochemical fractionations associated to water rock interactions the geochemical budget of the profile is influenced by a significant accretion of atmospheric dusts the mineralogical and geochemical variations along the profile also confirm that the weathering front does not progress continuously from the top to the base of the profile the upper part of the profile is probably associated with a different weathering system lateral weathering of upper corestones than the lower part which consists of the basal corestone the associated rindlet system and the saprolite in contact with these rindlets consequently the determination of weathering rates from 238 u 234 u 230 th 226 ra disequilibrium in a series of samples collected along a vertical depth profile can only be attempted for samples collected in the lower part of the profile i e the rindlet zone and the lower saprolite similar propagation rates were derived for the rindlet system and the saprolite by using classical models involving loss and gain processes for all nuclides to interpret the variation of u series nuclides in the rindlet saprolite subsystem the consistency of these weathering rates with average weathering and erosion rates derived via other methods for the whole watershed provides a new and independent argument that in the rio icacos watershed the weathering system has reached a geomorphologic steady state our study also indicates that even in environments with differential weathering such as observed for the puerto rico site the radioactive disequilibrium between the nuclides of a single radioactive series here 238 u 234 u 230 th 226 ra can still be interpreted in terms of a simplified scenario of congruent weathering incidentally the u th ra disequilibrium in the corestone samples confirms that the outermost part of the corestone is already weathered

formation and palaeozoic evolution of the gorny altai altai mongolia suture zone south siberia zircon u pb constraints on the igneous record abstract the altai sayan fold belt epitomises how phanerozoic island arc continental crust contributed to the growth of the palaeo eurasian continent its formation is dominated by the closure of the palaeo asian ocean pao when island arc systems and gondwana derived terranes accreted to the siberian margin relics of the pao related tectonic units and associated granitoids occur along the ophiolitic charysh terekta ulagan sayan suture ctuss between gorny altai and altai mongolia zircon la icp ms u pb dating of this igneous record constrains the multi stage geodynamic pao evolution primitive kuznetsk altai island arc crust formed at the siberian margin during the ediacaran early cambrian 525 555 ma this island arc matured during the middle late cambrian 510 ma and was consumed by pao subduction in the late cambrian early ordovician 480 490 ma forming the siberian early caledonian accretion collision belt south of the ctuss granitic magmatism occurred within gondwana derived altai mongolia during the middle late ordovician 450 470 ma palaeo kazakhstan assembly and during a silurian early devonian 400 425 ma andean type collision middle late devonian 360 395 ma granitoids were emplaced as a result of the collision of altai mongolia with siberia during final pangaea amalgamation the suture was strike slip reactivated associated magmatism 295 ma the youngest 220 255 ma sampled granitoids originated in an intra plate setting

venus express mission 1 venus express is well and healthy and has now been providing exciting new data from venus our nearby twin planet for over 2 years many of the new results are presented and discussed in the subsequent papers in this special section the overall scientific objective of venus express is to carry out a detailed study of the atmosphere of venus including the interaction of the upper atmosphere with the solar wind and the interaction of the lowest part of the atmosphere with the surface of the planet in addition the plasma environment and magnetic fields as well as some aspects of the surface of the planet are addressed for the first time investigations make systematic use of the transparent infrared spectral windows in order to probe the atmosphere in four dimensions three spatial dimensions plus time the spacecraft design is taken from mars express with some modifications necessary owing to the specific environment around venus the payload is composed of three spectrometers a camera a magnetometer an instrument for detecting energetic particles and a radio science package the orbit is polar and highly elliptic with a pericenter altitude of about 200 km over the northern polar region and an apocenter altitude of 66 000 km presently the coverage of the southern hemisphere is very good but important gaps still do exist the coverage of the northern hemisphere is much less dense venus express is a part of the european space agency s program for the exploration of the inner solar system which includes missions to study the sun mercury venus the moon mars and comets and asteroids

reassessment of continental growth during the accretionary history of the central asian orogenic belt we argue that the production of mantle derived or juvenile continental crust during the accretionary history of the central asian orogenic belt caob has been grossly overestimated this is because previous assessments only considered the palaeozoic evolution of the belt whereas its accretionary history already began in the latest mesoproterozoic furthermore much of the juvenile growth in central asia occurred in late permian and mesozoic times after completion of caob evolution and perhaps related to major plume activity we demonstrate from zircon ages and nd hf isotopic systematics from selected terranes within the caob that many neoproterozoic to palaeozoic granitoids in the accreted terranes of the belt are derived from melting of heterogeneous precambrian crust or through mixing of old continental crust with juvenile or short lived material most likely in continental arc settings at the same time juvenile growth in the caob occurred during the latest neoproterozoic to palaeozoic in oceanic island arc settings and during accretion of oceanic island arc and precambrian terranes however taking together our data do not support unusually high crust production rates during evolution of the caob significant variations in zircon ehf values at a given magmatic age suggest that granitoid magmas were assembled from small batches of melt that seem to mirror the isotopic characteristics of compositionally and chronologically heterogeneous crustal sources we reiterate that the chemical characteristics of crustally derived granitoids are inherited from their source s and cannot be used to reconstruct tectonic settings and thus many tectonic models solely based on chemical data may need re evaluation crustal evolution in the caob involved both juvenile material and abundant reworking of older crust with varying proportions throughout its accretionary history and we see many similarities with the evolution of the sw pacific and the tasmanides of eastern australia

the thermo tectonic history of the song kul plateau kyrgyz tien shan constraints by apatite and titanite thermochronometry and zircon u pb dating abstract the song kul basin sits on a plateau at the northern and middle kyrgyz tien shan junction it is a lacustrine basin occupied by lake song kul and predominantly developed on igneous basement this basement was targeted for a multi method chronological study to identify the different magmatic episodes responsible for basement formation and to constrain the timing of the development of its present day morphology zircon u pb dating by la icp ms revealed four different magmatic episodes a late cambrian 500 ma island arc system a late ordovician 450 ma subduction related intrusion an early permian 290 ma collisional stage and a middle to late permian 260 ma post collisional magmatic pulse middle to late triassic 200 230 ma titanite fission track ages and late triassic early jurassic 180 210 ma apatite fission track ages and thermal history modeling indicate the song kul basement was already emplaced in the shallow crust at that time an exhumed fossil apatite fission track partial annealing zone is recognized in the bordering song kul mountain ranges the area experienced only minor post early mesozoic denudation the igneous basement was slowly brought to apatite u th he retention temperatures in the late cretaceous palaeogene miocene to present reactivation of the tien shan does not manifestly affect this part of the orogen

aspects of the tectonic evolution of china the subject of this special publication is one of the most interesting in global geoscience the tectonic evolution of china the assemblage of terranes that underlie this part of the world provides outstanding opportunities to elucidate global processes and many of the factors that shape the earth s lithosphere are best exemplified by the geology of china and its immediately adjacent areas in addition there are geological features that are particular and unique to the region some have been the focus of recent attention and have attracted international interest because of their global importance this volume provides accounts of up to date research by chinese and international geological teams on key aspects of the tectonic evolution of china and its surrounding areas the papers describe the formation of the geological terranes that make up this part of east asia place constraints on plate tectonic models for their assembly and provide accounts of unique geological feature of the subcontinent

precambrian geology of the kazakh uplands and tien shan an overview abstract a comprehensive review of new data on geology and geochronology of precambrian terranes in the western central asian orogenic belt reveals new insights into its evolution at the present surface these terranes mostly consist of meso to neoproterozoic sedimentary magmatic and metamorphic assemblages with insignificant paleoproterozoic rocks archean material is represented exclusively by detrital and xenocrystic zircons in younger strata meso to neoproterozoic felsic magmatic rocks were mostly sourced from neoarchean and paleoproterozoic continental crust indicating its reworking and potential wider presence at deeper crustal levels most meso to neoproterozoic assemblages are of intraplate origin the supra subduction assemblages of neoproterozoic and mesoproterozoic ages are of limited extent we propose to recognize the issedonian and ulutau moyunkum groups of terranes separated by early paleozoic z shaped ophiolitic suture based on their different tectono magmatic evolution in the mesoproterozoic and neoproterozoic distinctly different are the mesoproterozoic and early neoproterozoic assemblages with lithological variations at the beginning of the late neoproterozoic and practically no differences at the end of the neoproterozoic the issedonian group of terranes could be part of a mesoproterozoic ca 1100 ma orogen between the siberian north china and laurentian cratons the pre mesoproterozoic crust of these terranes was completely reworked during the younger events the ulutau moyunkum group of terranes appear to be lithologically and geochronologically similar to the tarim craton both the issedonian and ulutau moyunkum groups of terranes were metamorphosed during the ulutau moyunkum event at 700 25 ma the breakup into currently mappable precambrian terranes took place during end ediacaran to early paleozoic times after opening of oceanic basins whose relics are preserved in numerous paleozoic ophiolitic sutures

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

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

multiple zircon growth during fast exhumation of diamondiferous deeply subducted continental crust kokchetav massif kazakhstan diamondiferous rocks from the kokchetav massif kazakhstan represent deeply subducted continental crust in order to constrain the age of ultra high pressure uhp metamorphism and subsequent retrogression during exhumation zircons from diamondiferous gneisses and metacarbonates have been investigated by a combined petrological and isotopic study four different zircon domains were distinguished on the basis of transmitted light microscopy cathodoluminescence trace element contents and mineral inclusions mineral inclusions and trace element characteristics of the zircon domains permit us to relate zircon growth to metamorphic conditions domain 1 consists of rounded cores and lacks evidence of uhp metamorphism domain 2 contains diamond coesite omphacite and titanian phengite inclusions providing evidence that it formed at uhp metamorphic conditions p 43 kbar t 950 c domain 3 is characterised by low pressure mineral inclusions such as garnet biotite and plagioclase which are common minerals in the granulite facies overprint of the gneisses p 10 kbar t 800 c this multi stage zircon growth during cooling and exhumation of the diamondiferous rocks can be best explained by zircon growth from zr saturated partial melts present in the gneisses domain 4 forms idiomorphic overgrowths and the rare earth element pattern indicates that it formed without coexisting garnet most probably at amphibolite facies conditions p 5 kbar t 600 c the metamorphic zircon domains were dated by shrimp ion microprobe and yielded ages of 527 5 528 8 and 526 5 ma for domains 2 3 and 4 respectively these indistinguishable ages provide evidence for a fast exhumation beyond the resolution of shrimp dating the mean age of all zircons formed between uhp metamorphic conditions and granulite facies metamorphism is 528 3 ma indicating that decompression took place in less than 6 ma hence the deeply subducted continental crust was exhumed from mantle depth to the base of the crust at rates higher than 1 8 cm year we propose a two stage exhumation model to explain the obtained p t t path fast exhumation on top of the subducted slab from depth 140 to 35 km was driven by buoyancy and facilitated by the presence of partial melts a period of near isobaric cooling was followed by a second decompression event probably related to extension in a late stage of continental collision

tectonic evolution of the paleozoic barluk continental arc west junggar nw china abstract the barluk arc situated in west junggar is an important component of the kazakhstan domain in the central asian orogenic belt caob however the nature of the barluk arc active continental margin or oceanic island arc is in debate the igneous rocks in the shiwu area barluk arc have diagnostic petrologic geochronologic and geochemical signatures that uncover the attributes of the barluk arc in this paper we report detailed petrology whole rock geochemistry and sr nd isotopic zircon u pb dating and hf isotopic data of igneous rocks in the shiwu area our results reveal that 1 except for 302 2 ma gabbro igneous rocks from barluk arc in the shiwu area are dioritic and granitic have ages ranging from 323 8 to 310 5 ma and they have geochemical signatures originated from typical continental arcs including calc alkaline series rocks enrichment of lile depletion of hfse and higher th yb and ta yb values 2 the granitic porphyry and rhyolite in barluk arc have old tdm2 hf ages up to 1091 ma suggesting the existence of precambrian crust or similar materials as their sources and 3 the 302 2 ma gabbro dike has high values of end t 7 54 and ehf t 12 8 to 15 5 and a young t2dm nd age 455 ma and therefore it was possibly related to upwelling of asthenosphere mantle in a post subduction extensional tectonic setting based on these parameters we suggest that the barluk arc is defined as a late paleozoic continental margin arc along the margin of the kazakhstan continent westward subduction of the junggar oceanic plate in the barluk area started in early devonian and continued to the late carboniferous 310 ma subsequent post collisional tectonic processes between the junggar oceanic plate and kazakhstan continent continued to the late carboniferous 302 ma

shrimp zircon chronology of hp uhp rocks of the makbal metamorphic complex in the northern tien shan kyrgyzstan abstract in this paper we present the first results of sensitive high resolution ion microprobe shrimp u pb zircon dating obtained for the ultra high pressure uhp metamorphic rocks of the makbal metamorphic complex in the northern tien shan kyrgyzstan zircon grains from two hp eclogite samples and one sample of uhp garnet talk chloritoid schist have similar bright rims identified on cathodoluminescence cl images the rims are interpreted as zircon overgrowth during uhp metamorphism dating of zircon rims from the three samples yielded similar ages of 509 7 ma 502 10 ma and 498 7 ma the time span of 510 500 ma is considered as the minimum age of uhp metamorphism of the makbal complex this age is in accordance with middle cambrian faunal age of the kara archa ophiolite in the makbal area it is suggested that the makbal rocks were metamorphosed in the middle cambrian subduction zone dipping north under the northern tien shan microcontinent a deformed granodiorite associated with the makbal metamorphic rocks yielded an age of 514 5 ma indicating development of a magmatic arc simultaneously with the uhp metamorphism cores of zircon grains from the garnet talk chloritoid schist yielded a wide spectrum of ages 1850 800 ma characteristic for detrital zircons of continental origin a depositional age

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

palaeozoic arc magmatism in the central asian orogenic belt of kazakhstan shrimp zircon ages and whole rock nd isotopic systematics abstract early palaeozoic tonalite to granodiorite intrusions in northern kazakhstan are associated with lode gold mineralization and have shrimp zircon ages of 457 3 6 6 ma aksu 452 9 5 6 ma and 447 4 5 4 ma both zholymbet the stepnyak intrusion contains large xenoliths with an age of 480 6 5 0 ma one early palaeozoic zircon from a porphyritic diorite at stepnyak has a core with a near concordant 207pb 206pb age of 3888 1 5 ma whereas other xenocrystic grains are between 983 and 2698 ma old the early archaean age is probably inherited from unexposed basement of the kokchetav massif and represents the oldest crustal material so far known from the asian continent it appears that the aksu zholimbet and stepnyak granitoids were emplaced in the late ordovician in an andean or japan type continental arc environment on the margin of the kokchetav massif late palaeozoic granitoids in central kazakhstan have devonian zircon ages of 407 1 3 9 381 1 3 1 and 369 2 4 9 ma whereas the youngest sample from the topar massif has a late carboniferous emplacement age of 314 1 5 1 ma initial end values range from 5 to 1 corresponding to nd model ages of 1 1 0 6 ga the isotopic data are similar to those of other phanerozoic granitoids of the central asian orogenic belt and corroborate melting of predominantly juvenile crustal protoliths our data also confirm that devonian and carboniferous arc magmatism in central kazakhstan assimilated older and already accreted island arc systems yet is derived from primitive sources probably in a subduction setting the range of end values with corresponding nd mean crustal residence ages in the range of 0 6 1 1 ga for most samples indicates variable recycling of late proterozoic largely juvenile continental crust in the formation of the palaeozoic magmatic arcs of central kazakhstan the orogenic evolution terminated with permian anorogenic rift related alkaline granite magmatism end 2 to 8 which reflects melting of juvenile mantle derived lower crust

metamorphic growth and recrystallization of zircon distinction by simultaneous in situ analyses of trace elements u th pb and lu hf isotopes in zircons from eclogite facies rocks in the sulu orogen simultaneous in situ analyses of trace elements u th pb and lu hf isotopes were carried out on distinct domains of zircons in ultrahigh pressure uhp eclogite facies metamorphic rocks from the main hole of the chinese continental scientific drilling ccsd in the sulu orogen for the first time trace elements are directly linked to lu hf isotopes in metamorphic zircons with reference to their u pb dates this enables methodological integration to distinguish four types of metamorphic zircon solid state replacement and dissolution recrystallizations of protolith zircons and new growth from the aqueous fluid metamorphically grown zircons are characterized by concordant u pb ages for the metamorphism flat hree patterns typical of the garnet effect low contents of ree especially hree y nb ta and th u high contents of hf low lu gd n lu hf and th u 0 1 ratios and elevated 176hf 177hf ratios relative to solid state recrystallized zircons this suggests the effects of both garnet and fluid on the growth of metamorphic zircons in contrast metamorphic recrystallization has reset the u th pb isotope system of protolith zircons to different extents depending on the extents of fluid action during metamorphism solid state recrystallized zircons exhibit the lowest degrees of resetting and thus almost inherit all geochemical features from the protolith zircons which are characterized by discordant u pb ages close to or below the protolith age steep mree hree patterns typical of magmatic origin high contents of trace elements and their ratios and low 176hf 177hf ratios on the other hand dissolution recrystallized zircons show the highest degrees of reworking and thus have concordant or nearly concordant u pb ages for the metamorphism steep mree hree patterns lowered contents of trace elements such as ree th u y nb ta and ti relative to the protolith zircons and almost unchanged hf isotope ratios replacement recrystallized zircons display intermediate degrees of reworking and thus have their many features of elements and isotopes in between while the metamorphic growth in the presence of both garnet and fluid is characterized by both depletion of hree with flat pattern and the low contents of trace elements the metamorphic recrystallization in the presence of aqueous fluid is indicated by gradual decreases of mree to hree without the flat hree pattern therefore the simultaneous in situ analyses of metamorphic zircons have the advantage over single term analyses in making distinction between the new growth and the different types of recrystallization

geochemistry and age of ultrahigh pressure metamorphic rocks from the kokchetav massif northern kazakhstan isotopic and geochemical data of the zerenda series metamorphic rocks from the kokchetav massif are reported some of these rocks contain microdiamond inclusions in garnets and other indicators of ultrahigh pressure metamorphism p 40 kbar t 900 1000 c the diamond bearing rocks exhibit distinctive geochemical characteristics compared to typical crustal rocks the ree patterns range from lree depleted to slightly lree enriched chondrite normalized la yb n 0 1 5 4 with a negative eu anomaly they are depleted in incompatible elements e g sr ba u th with respect to the upper crust in contrast non diamondiferous rocks of the zerenda series exhibit normal crustal geochemistry all rocks of the zerenda series have very radiogenic lead isotopes the measured values 238u 204pb compared with those calculated for the interval between crust formation and ultrahigh pressure uhp metamorphism suggest a decrease by factors of up to 200 during the uhp metamorphism the sm nd mineral isochrons from the diamond bearing rocks and other rock types of the zerenda series give a middle cambrian 524 535 ma age of metamorphism the nd model ages show that crust formation occurred about 2 3 ga ago significant fractionation of sm and nd and loss of incompatible elements may be due to partial melting of the protoliths the ar ar age determinations of secondary biotite and muscovite from the diamond bearing rocks yield an age of 517 5 ma this cooling age requires a short time interval between uhp metamorphism and uplift to a crustal level ultrahigh pressure metamorphism might be a significant source of pb for the mantle we propose that the radiogenic pb of the oceanic array is the contamination traces of numerous uhp events beside the geological aspect we demonstrate a method of dating a high grade metamorphic terrain using nd isotopes we compare whole rock isochrons and mineral isochrons and in this way get some insight into the behaviour of the sm nd system during very high grade metamorphic events

deciphering caledonian events timing and geochemistry of the caledonian magmatic arc in the kyrgyz tien shan abstract in the kyrgyz tien shan also known as tian shan in literature the caledonian cambro silurian intrusions comprise an extensive magmatic arc stretching from east to west for more than 1000 km the characteristic feature of the arc is its relatively homogeneous composition of rock types over the whole structure the kichy naryn and djetim intrusions presented in this study are slightly elongated in an east west direction and occupy an area of ca 100 km2 the main rock types are diorite granodiorite and granite geological and geochemical features of the kichy naryn and djetim intrusions demonstrate characteristics of i type granite series rocks of the two intrusions define a continuous high k calc alkaline series diorite and granite of the kichy naryn intrusion yielded early silurian crystallisation ages of 436 2 ma u pb zircon diorite from pebbles in the conglomerate sampled close to the contact of the kichy naryn intrusion yielded a significantly older early ordovician crystallisation age of 466 10 ma the obtained ages of 466 and 436 ma match ages of two major regional magmatic pulses at ca 435 440 and 460 470 ma which took place during continuous subduction from the cambrian to the silurian the amount of granites in the northern tien shan their prolonged history of formation and pronounced i type geochemical characteristics suggest their formation in an andean type active margin environment

tectonic history of the irtysh shear zone ne kazakhstan new constraints from zircon u pb dating apatite fission track dating and palaeostress analysis abstract the irtysh shear zone isz is an important structure in the framework of the central asian orogenic belt caob it represents the site of final collision of kazakhstan with siberia during hercynian times and records up to 1000 km of lateral displacement during subsequent reorganization in the caob edifice we present new zircon u pb apatite fission track and fault kinematic data along the isz and consequently derived its tectonic history with emphasis on its formation and reactivation episodes carboniferous 340 320 ma zircon u pb ages were obtained for the syn and post collisional kalba narym intrusives dating their emplacement in the framework of the siberia kazakhstan collision during this period the isz experienced an early brittle left lateral mainly transtensional stress regime late carboniferous early permian post collisional intrusives were emplaced and the stress regime changed to a late brittle regime characterized by more compressional conditions indicating rheological strengthening as a response to cessation of ductile shearing and cooling of the isz crust apatite fission track data and thermal history modeling reveal late cretaceous 100 70 ma cooling of the isz basement rocks as a response to denudation of a bordering late mesozoic altai orogen after this denudation event the tectonic activity ceased during the late mesozoic early cenozoic a final step of cooling from 25 ma exhibited by some of the thermal history models may reflect reactivation of the isz and initiation of cenozoic altai mountain building the late plio pleistocene phase of mountain building coincides with a new change in the palaeostress field characterized by minor transpressional right lateral shear conditions

zircon response to diamond pressure metamorphism in the kokchetav massif ussr ultrahigh pressure metamorphism of the diamond bearing gneisses in the kokchetav massif ussr has been dated at 530 7 ma 2 sgr by shrimp ion microprobe analysis of metamorphic zircons zircon xenocrysts as old as 2000 ma have retained original isotopic compositions through the extreme metamorphic conditions and indicate that the protolith that was buried to diamond forming pressures included early proterozoic components highly variable reset ting of u pb isotopic systems in zircon xenocrysts at the time of metamorphism suggests that diffusive lead loss had commenced but was incomplete a disequilibrium observation consistent with relatively transient exposure of the rocks to extreme crustal depths

geometry kinematics and tectonic models of the kazakhstan orocline central asian orogenic belt abstract the central asian orogenic belt caob is one of the largest accretionary orogens on earth and is characterized by the occurrence of tight oroclines kazakhstan and tuva mongolian oroclines the origin of these large scale orogenic curvatures is not quite understood but is fundamentally important for understanding crustal growth and tectonic evolution of the caob here we provide an outline of available geological and paleomagnetic data around the kazakhstan orocline with an aim of clarifying the geometry kinematics and geodynamic origin of the orocline the kazakhstan orocline is evident in a total magmatic image and can be traced by the continuation of high magnetic anomalies associated with the devonian volcanic belt and the late devonian to carboniferous balkhash yili arc paleomagnetic data show 112 126 clockwise rotation of the northern limb relative to the southern limb in the late devonian to early carboniferous as well as 15 28 clockwise rotation of the northern limb and 39 40 anticlockwise rotation of the southern limb relative to the hinge of the orocline during the late carboniferous to permian we argue that the kazakhstan orocline experienced two stage bending with the early stage of bending late devonian to early carboniferous 112 126 driven by slab rollback and the later stage late carboniferous to permian 54 68 possibly associated with the amalgamation of the siberian tarim and baltic cratons this new tectonic model is compatible with the occurrence of rift basins the spatial migration of magmatic arc and the development of large scale strike slip fault systems during oroclinal bending

metasedimentary rocks of the angara kan granulite gneiss block yenisey ridge south western margin of the siberian craton provenance characteristics deposition and age abstract the angara kan granulite gneiss block yenisey ridge is one of the main basements uplift within the south western margin of the siberian craton the major part of the angara kan block is composed of early precambrian high grade metamorphic rocks metasedimentary association of the kan granulitic complex is composed of garnet bearing garnet orthopyroxene and orthopyroxene bearing gneisses garnet and orthopyroxene bearing gneisses with cordierite and sillimanite studied paragneisses were formed at the expense of granulite metamorphism of terrigenous rocks ranging from graywacke to pelitic rock or mudstone to estimate the time of sedimentation and metamorphism of the terrigenous deposits the u pb zircon dating has been performed using the shrimp ii ion microprobe detrital zircon cores from the biotite orthopyroxene and high alumina gneisses yield ages of 2 6 1 94 and 2 4 1 94 ga respectively together with the age of the magmatic zircons formed during high grade metamorphism and partial melting 1 89 ga and metamorphic rims 1 87 ga it defines the time of sedimentation between 2 0 1 94 and 1 89 1 87 ga detrital zircon ages indicate both archean and paleoproterozioc rocks in provenance source that agrees with the nd model ages of metasediments ranging in interval 2 4 2 8 ga potential source of the archean detrital zircons was the exposed basement of the southwestern siberian craton whereas the paleoproterozoic juvenile crustal source seems to be buried basement of the tungus province of the siberian craton deposition of the kan terrigeneous rocks was coeval with sedimentation in the southeastern part of the sharyzhalgay uplift where ages of detrital zircon cores and metamorphic rims from paragneisses bracket sediment deposition between 1 95 and 1 85 ga

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

an early cambrian plume induced subduction initiation event within the junggar ocean insights from ophiolitic melanges arc magmatism and metamorphic rocks abstract the central asian orogenic belt caob is one of the largest accretionary orogens in the world the mechanism of continental growth and tectonic evolution of the caob remain debated here we present an overview of early paleozoic ophiolitic melanges calc alkaline intrusions and metamorphic rocks in west junggar with an aim to provide constraints on the time and mechanism of subduction initiation in the junggar ocean a branch of the southern paleo asian ocean pao the early paleozoic ophiolitic melanges are composed of ultramafic mafic rocks cherts pelagic limestones basaltic breccias and tuffs the mafic rocks from these ophiolitic melanges are divided into morb like and oib like types the morb like rocks were formed in a fore arc setting but the oib like mafic rocks were formed by the intra plate magmatism related to mantle plume activities the early paleozoic intrusions are occurred as small stocks with a dominant composition of diorite trondhjemite and granodiorite these granitoids display high k calc alkaline affinities and have high and positive end t and ehf t values formed in an arc related setting the metamorphic complex is mainly composed of blueschist and amphibolite blocks with metamorphism ages ranging from 500 ma to 460 ma their protoliths are calc alkaline andesite and tholeiitic and alkaline basalts formed in an arc related and seamount setting respectively it is clearly show that the west junggar was under an intra oceanic subduction regime during the early paleozoic and the initial subduction of the southern pao might have occurred in the early cambrian based on our observations and in combination with previous work we propose the plume induced subduction initiation model for the early middle cambrian tectonic evolutionary of the junggar ocean our new model not only shed light on subduction initiation dynamics of the southern pao but also contribute to tectonic evolution of the caob

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

formation of the kokchetav subduction collision zone northern kazakhstan insights from zircon u pb and lu hf isotope systematics abstract the kokchetav subduction collision zone is located in the western part of the ancestral central asian orogenic belt this zone is built up by the precambrian kokchetav microcontinent which includes a hp uhp metamorphic belt and the north kokchetav tectonic zone nktz which represents an accretionary complex between the kokchetav microcontinent and the adjacent stepnyak island arc the entire region is widely intruded by palaeozoic granitoids which were emplaced after the collision of the stepnyak island arc with the kokchetav microcontinent we present zircon u pb ages and lu hf systematics in zircon to better characterize the tectonic evolution of the kokchetav subduction collision zone the lu hf results indicate that the kokchetav basement rocks are derived from late neoarchaean early palaeoproterozoic 2 5 ga crust for the granite gneiss basement of the kokchetav microcontinent early mesoproterozoic grenville age 1 17 1 14 ga zircon u pb crystallization ages were obtained for the nktz two main age components were recognized 1 an oldest mesoproterozoic age component 1 20 1 05 ga similar as for the kokchetav microcontinental zircons and 2 a younger early cambrian 540 520 ma or late cambrian early ordovician 490 480 ma age component th u ratios 0 1 and are typical of a magmatic protolith the distinction between both zircon types is supported by characteristic cathodoluminescence images these results confirm previous observations indicating early palaeozoic high grade metamorphism 540 520 ma and collisional deformation 490 480 ma of the mesoproterozoic kokchetav zone as a result of subduction accretion and collision of the stepnyak island arc to the kokchetav microcontinent for two additional samples of the balkashin granitic complex early devonian 415 395 ma zircon crystallization ages magmatic th u ratios were obtained the occurrence of a mesozoic xenocryst within these leucogranites may indicate that they were emplaced in a continental arc setting

late paleozoic early mesozoic geodynamics of central asia abstract correlation and synthesis of published and new structural paleomagnetic and geochronological data from central asia show the important role of strike slip faulting in their evolution the pattern of major strike slip faults outlines a terrane collage produced by a late devonian early carboniferous collision of the gondwana derived altai mongolia tuva composite microcontinent with siberia and a late carboniferous permian collision of east europe and kazakhstan with siberia the accreted continental margins were cut by strike slip faults and conjugate thrusts into numerous terranes which mixed with one another and disturbed the previous structural and facies framework those complex and multi stage deformations resulted from the late devonian early carboniferous collision of gondwana derived terranes the deformations reached their peak in the late carboniferous permian due to the collision of the kazakhstan east european baltica and siberian continents a system of sinistral strike slip faults formed a mosaic block structure of central asia along the margin of the siberian continent as a result of the late carboniferous permian collision this resulted in the formation of the northern eurasia continent early mesozoic strike slip faulting and conjugate thrusting resulted from the rotation of the siberian and east european cratons

diamond inclusions in garnets from metamorphic rocks a new environment for diamond formation diamonds commonly occur in kimberlites lamproites and alluvial sediments derived from these rocks more recently diamonds or their graphite pseudomorphs have been discovered in ultramafic massifs1 and picrites2 here we report the occurrence of diamonds in situ in crustal rocks highly retrograded high pressure metamorphic garnet pyroxene and pyroxene carbonate garnet rocks biotite gneisses and schists from the kokchetav massif northern kazakhstan ussr the diamonds are cubo octahedral averaging 12 m in size and occur in zircons and with euhedral graphite as inclusions in unzoned garnets we believe that the zircon and garnet matrices protected these diamonds from retrogressive transformation to graphite mica rutile titanite clinopyroxene kyanite and zircon also occur as inclusions in garnet often intergrown with the diamonds equilibration relations of inclusions and host garnets indicate that both diamonds and graphite crystallized from a fluid phase under static conditions at pressures of 40 kbar and temperatures 900 1 000 c

microdiamonds frontier of ultrahigh pressure metamorphism a review abstract this is a comprehensive review paper devoted to microdiamonds from ultrahigh pressure metamorphic uhpm terranes incorporated in orogenic belts formed at convergent plate boundaries in paleozoic mesozoic alpine time when in 1980 the first small diamonds were discovered within amphibolite granulate facies metamorphic rocks it came as a great surprise that buoyant continental crust could be subducted to depths of hundreds of kilometers and then subsequently exhumed since then much progress has been made in understanding the mechanism of these diamonds formation and the number of new diamond bearing uhpm terranes was significantly increased especially within european orogenes moreover new variations in tectonic settings in which uhp rocks can be formed and exhumed came to the attention of geologists simply due to the finding of diamonds in places previously forbidden for their formation e g oceanic islands ophiolites and forearc environments over the past decade the rapidly moving technological advancement has made it possible to examine microdiamonds in detail and to learn that part of them has a polycrystalline nature that they contain nanometric multiphase inclusions of crystalline and fluid phases and that they keep a crustal signature of carbon isotopes scanning and transmission electron microscopy focused ion beam techniques synchrotron infrared spectroscopy micro x ray diffraction and nano secondary ion mass spectrometry studies of these diamonds provide evidence that they keep traces of fluid originated from both crustal and mantle reservoirs and that they probably interacted with deep mantle plumes hypotheses proposed for diamond formation in subduction zones founded on both analytical and experimental studies are discussed the paper also emphasizes that the discovery of these microdiamonds as well as coesite triggered a major revision in the understanding of deep subduction processes leading to a clear realization of how continental materials can be recycled into the earth s mantle and geochemically rejuvenate it