experimental calibration of the aluminum in hornblende geobarometer with application to long valley caldera california volcanic rocks a new geobarometer based on the al content of igneous hornblendes in equilibrium with melt fluid biotite quartz sanidine plagioclase sphene and magnetite or ilmenite has been calibrated experimentally the calibration was performed by equilibrating the required phase assemblage over the pressure range 2 8 kbar at 740 780c and then analyzing euhedral hornblendes in equilibrium with glass melt experiments were performed on natural samples of both volcanic and plutonic rocks earlier empirical calibrations of this geobarometer relied on analyzing natural hornblendes from plutons with the required phase assemblage and inferring pressure from nearby metamorphic country rocks the experimental calibration differs from the empirical calibrations especially above 5 kbar and shows that the al content of hornblendes in equilibrium with the required phase assemblage is greater for a given total pressure than previously thought the geobarometers uncertainty is dramatically reduced the derived equation is p plus minus 0 5 kbar 3 minus 3 46 plus minus 0 24 4 23 plus minus 0 13 al sup t the geobarometer is applied to post bishop tuff volcanic rocks from long valley caldera california and reveals that most rhyodacites in this complex erupted from depths of about 6 km these eruptions occurred over 500 000 yr suggesting that the rhyodacitic magma reservoir beneath longmore valley had reached a steady p depth t state less

multiple 3 8 3 1 ga tectono magmatic events in a newly discovered area of ancient rocks the shengousi complex anshan north china craton abstract near anshan city in the north china craton the oldest rocks in asia 3300 ma have been thoroughly documented at two localities baijiafen and dongshan in this paper we report the full geological context for more ancient rocks from a third anshan locality the polyphase migmatite shengousi complex shrimp u pb zircon dating indicates a protracted tectono magmatic history for the shengousi complex the oldest recognised component is banded trondhjemitic gneiss 3773 6 ma which is veined by strongly deformed granitic pegmatite these occur with a second generation of trondhjemitic rocks 3454 8 and 3448 9 ma the next generation of plutonic rocks is a composite suite of iron enriched mafic dykes 3332 6 and 3331 8 ma with broadly coeval felsic veins 3311 4 ma finally there was intrusion of monzogranite 3129 6 ma strong deformation has generally brought these 3773 3129 ma plutonic phases into close concordance to form banded rocks however locally cross cutting relationships are preserved in small lower strain domains that give a relative chronology agreeing with the absolute zircon u pb chronology all of this complex history is recorded in a single some of the the 3450 ma igneous phases discovered in the shengousi complex are new ages for igneous rocks in the anshan area the precambrian geology of the anshan area is thus marked by polyphase eoarchaean orthogneisses the newly recognised 3450 ma phases widespread 3360 3300 magmatic activity found in association with mesoarchaean plutonic rocks and the neoarchaean anshan group and palaeoproterozoic liaohe group metasedimentary rocks this shows that the geology of the anshan area has a strong similarity with the narryer gneiss complex of western australia which contains the polyphase eoarchaean meeberrie gneisses the 3490 3440 ma eurada gneiss association 3300 ma granites and migmatisation and is intercalated with younger precambrian metasedimentary rocks of different ages containing hadean detrital zircons no other ancient gneiss complexes in the world show such a close match in their history with the anshan area rocks this means that the anshan area is prospective for locating new occurrences of hadean crustal components intercalated with younger sedimentary rocks

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

late paleozoic retrograded eclogites from within the northern margin of the north china craton evidence for subduction of the paleo asian ocean abstract retrograded eclogites from the central part of the northern margin of the north china craton hebei province china occur as separate tectonic lenses or boundins within garnet biotite plagioclase gneisses of the paleoproterozoic hongqiyingzi complex characterized by amphibolite facies paragneisses the petrographic features and mineralogical compositions represent three main metamorphic stages 1 the peak eclogite facies stage p 1 40 1 50 gpa t 680 730 c 2 the granulite facies stage and 3 the amphibolite facies stage p 0 67 0 81 gpa t 530 610 c formed during decompression the major and trace element and sm nd isotopic data suggest that most of the retrograded eclogite samples had protoliths of tholeiitic oceanic crust with geochemical characteristics of mid ocean ridge basalt morb or island arc tholeiite iat environment and were contaminated by crustal components during subsequent subduction zircon shrimp isotopic dating of two different textural varieties of retrograded eclogite defines a weighted mean age of 325 ma which is interpreted as the peak metamorphic age of the eclogites and reflects the occurrence of eclogite facies metamorphism related to subduction of paleo asian oceanic crust beneath the north china craton during the late paleozoic finally we show that the retrograded eclogite from hebei province is not related to the baimashi retrograded eclogite at the northern foot of the heng mountains approximately 300 km to the southwest

neoarchean growth and paleoproterozoic metamorphism of an archean ophiolite melange in the north china craton abstract the north china craton ncc preserves important records of neoarchean crustal growth and paleoproterozoic metamorphism here we investigate a suite of metamorphic rocks belonging to the hongqiyingzi group from the northern part of the trans north china orogen tnco these rocks are dominantly composed of amphibole plagioclase gneisses with minor retrograded eclogites and serpentinited peridotites we employ zircon u pb geochronology and hf o isotopes on the amphibole plagioclase gneisses to unravel the formation age and metamorphic history the magmatic zircon cores yield upper intercept ages of 2664 ma and 2518 ma suggesting that the protoliths of the hongqiyingzi gneisses formed in the neoarchean the metamorphic rims with lower u and th concentration yield upper intercept ages of 1840 ma and 1847 ma constraining the timing of metamorphism as ca 1 85 ga the ehf t values of the magmatic zircon grains are dominantly positive whereas those of the metamorphic grains are negative both magmatic and metamorphic zircon grains display a restricted range of 18o 6 11 7 44 slightly higher than the normal mantle value these isotopic features indicate that the younger metamorphic event did not modify the hf o isotopic compositions the p t computations yield temperatures of 700 760 c and pressures to be 5 7 kbar suggesting the rocks have been retrograded to amphibolites facies the retrograded eclogites and serpentinized peridotites of the hongqiyingzi group exposed as tectonic lenses within the amphibole plagioclase gneisses are interpreted as parts of an ancient oceanic lithosphere whereas the amphibole plagioclase gneisses represent meta volcano sedimentary units we thus interpret that the hongqiyingzi group represents a tectonically dismembered neoarchean ophiolitic melange that formed on the continental margin and metamorphosed to eclogite facies during deep subduction in paleoproterozoic and exhumed during phanerozoic

shrimp u pb zircon ages of granitoid rocks in the luliang complex implications for the accretion and evolution of the trans north china orogen abstract the luliang complex is situated in the central segment of the trans north china orogen tnco a continent continent collisional belt along which the discrete eastern and western blocks amalgamated to form the basement of the north china craton the complex consists of supracrustal and granitoid rocks of which the latter can be subdivided into pre tectonic ttg gneisses including the yunzhongshan guojiazhuang and chijianling guandishan gneisses syn tectonic gneissic granites represented by the huijiazhuang gneissic granite and post tectonic granites including the luyashan charnockite luchaogou porphyritic granite and tangershang guandishan massive granite the pre tectonic ttg gneisses are mostly calc alkaline and considered to have formed in a magmatic arc environment shrimp u pb analyses reveal that the yunzhongshan gneisses were emplaced at 2499 9 ma representing the earliest arc related magmatic event in the luliang complex this was followed by the intrusion of the guojiazhuang gneisses at 2375 10 ma the most widespread arc related magmatic event in the region was the emplacement of the chijianling guandishan granitoid gneisses of which the tonalitic granodioritic and monzogranitic gneisses were emplaced at 2199 11 ma 2180 7 ma and 2173 7 ma respectively metamorphic zircon overgrowth rims from a tonalitic gneiss yielded a weighted mean 207 pb 206 pb age of 1872 7 ma consistent with the metamorphic age range of 1880 1820 ma defined by metamorphic zircons from various high grade rocks in the tnco thus the evolution of the luliang complex involved emplacement of the yunzhongshan granitoids at 2499 ma the guajiazhuang granitoids at 2375 ma and the chijianling guandishan granitoids at 2199 2173 ma with the final collision between the eastern and western blocks in this area occurring at 1872 7 ma the whole series of magmatic events lasting nearly 650 ma this suggests that the trans north china orogen represents a long lived magmatic arc the new shrimp zircon data combined with structural and petrological studies also reveal the existence of both syn and post collisional granites in the luliang complex the former is represented by the huijiazhuang gneissic granite that was emplaced at 1832 11 ma whereas the latter include the 1815 5 ma luyashan charnockite 1807 10 ma luchaogou porphyritic granite 1798 11 ma guandishan massive granite and 1790 14 ma tangershan massive granite

deglaciation of the greenland and laurentide ice sheets interrupted by glacier advance during abrupt coolings abstract understanding patterns of ice sheet deglaciation is key for predicting the rate of future ice sheet melt yet the processes underlying deglaciation remain elusive the early holocene 11 7 ka to 8 2 ka greenlandian stage represents the most recent period when the laurentide and greenland ice sheets underwent large scale recession moreover this ice sheet recession occurred under the backdrop of regional temperatures that were similar to or warmer than today and comparable to those projected for the upcoming centuries reconstructing laurentide and greenland ice sheet behavior during the early holocene and elucidating the mechanisms dictating this behavior may serve as a partial analog for future greenland ice sheet change in a warming world here we present 123 new 10be surface exposure ages from two sites on baffin island and southwestern greenland that constrain the behavior of the laurentide and greenland ice sheets and an independent alpine glacier during the early holocene on baffin island we focus on a unique area where moraines deposited by the laurentide ice sheet rest directly adjacent to moraines deposited by an independent alpine glacier sixty one 10be ages reveal that advances and or stillstands of the laurentide ice sheet and an alpine glacier occurred in unison around 11 8 ka 10 3 ka and 9 2 ka sixty two 10be ages from southwestern greenland indicate that the greenland ice sheet margin experienced re advances or stillstands around 11 6 ka 10 4 ka 9 1 ka 8 1 ka and 7 3 ka our results reveal that ice sheets respond to climate perturbations on the same centennial timescale as small alpine glaciers we hypothesize that during the warming climate of the early holocene freshening of the north atlantic ocean induced by ice sheet melt resulted in regional cooling and brief periods of ice sheet stabilization superimposed on net glacier recession these observations point to a negative feedback mechanism inherent to melting ice sheets in the baffin bay region that slows ice sheet recession during intervals of otherwise rapid deglaciation

paleoproterozoic accretionary orogenesis in the north china craton a shrimp zircon study abstract the inner mongolia suture zone imsz and the trans north china orogen tnco incorporate the major paleoproterozoic accretionary orogens in the north china craton ncc with the jiao liao ji belt jljb representing the third one here we investigate the paleoproterozoic tectonics of the imsz and tnco through zircon shrimp geochronology on a representative suite of rocks comprising metasediments and arc magmatic rocks shrimp analysis of zircons with textures indicating extreme recrystallization under ultrahigh temperature uht conditions from the metapelites at heling er in the southern domain of the imsz reveals a single population with a weighted mean 207 pb 206 pb age of 1913 17 ma the zircons in another uht granulite from this locality yield a weighted mean 207 pb 206 pb age of 1910 18 ma these data correlate with the ca 1 92 ga age reported from zircons in sapphirine bearing uht granulites further north and confirm the regional extent of the paleoproterozoic uht metamorphism within the imsz zircons in a charnockite from the southern margin of the khondalite belt fringing the uht granulites in the imsz show two distinct age groups an older population with a magmatic paragenesis and a weighted mean 207 pb 206 pb age of 1932 24 ma and a younger group of metamorphic zircons with an age of 1858 26 ma we also report zircon ages from charnockites in two localities around xing he in the huangtuyao belt belonging to the huai an complex within the westernmost domain of the tnco at the junction with the imsz the charnockite from first locality carries two distinct zircon populations with the older group yielding a weighted mean 207 pb 206 pb age of 2477 2 ma and the younger population showing an age of 1807 38 ma the internal structure as revealed from cl images and the overall high th u values up to 2 42 of the older zircons suggest their magmatic affinity whereas the younger group with extremely low th u 0 02 0 09 is of metamorphic origin zircons from the charnockite in the second locality also define two distinct age clusters with a dominant older magmatic group having a weighted mean 207 pb 206 pb age of 2147 11 ma and a minor younger group with an age of 1958 25 ma the range of ages from 2477 to 2147 ma from magmatic zircons in the charnockites from the eastern periphery of the imsz within the western margin of the tnco in combination with similar ages reported in recent studies from zircons in magmatic complexes within the imsz suggest a prolonged history of subduction related arc magmatism and accretionary tectonics analogous to those in some of the phanerozoic belts such as the central asian orogenic belt and the western pacific subsequent progressive collision and suturing of the continental blocks were accompanied by the exhumation of high pressure hp and uht metamorphic rocks the available data from the imsz and tnco suggest long lived convergent margins associated with the southward subduction of the yinshan block and westward subduction of the eastern block in a double sided subduction realm prior to the final amalgamation of the ncc and its incorporation within the columbia supercontinent in the late paleoproterozoic

the application of zircon cathodoluminescence imaging th u pb chemistry and u pb ages in interpreting discrete magmatic and high grade metamorphic events in the north china craton at the archean proterozoic boundary abstract two distinct late neoarchean to early paleoproterozoic magmatic events are recognized at the northern margin of the eastern block of the north china craton based on interpretation of zircon structure cathodoluminescence signature th u pb chemistry and age the first event is represented by emplacement of the chaihe trondhjemite and xianjinchang tonalite with magmatic zircon core ages ranging from 2559 2553 7 ma respectively these rocks experienced a subsequent high grade metamorphic event constrained by partial recrystallization of primary magmatic zircon and growth of new zircon rims between 2517 and 2490 ma a second magmatic episode generated by partial melting of crustal rocks during this metamorphic event is represented by the hongshilazi syenogranite with an age of 2502 11 ma the short time interval between initial magmatism and high grade regional metamorphism signifies they were related to the same major tectonothermal event when considered along with evidence of a similar sequence of events at this time 800 km to the south in eastern hebei and the previously established 2500 ma age of granulite facies metamorphism in mafic rocks across the craton it is proposed that a large mantle plume developed beneath the north china craton at the close of the archean

development of the north china craton during the late archaean and its final amalgamation at 1 8 ga some speculations on its position within a global palaeoproterozoic supercontinent abstract new data obtained over the past few years indicate that by the end of the archaean the north china craton consisted of two major crustal entities identified as the eastern and western blocks the formation of island and magmatic arcs along the western margin of the eastern block record the latest archaean events so far identified the location of the western block at this time is unknown however the occurrence of palaeoproterozoic passive margin sediments similar in age to those occurring in the eastern block indicates essentially similar conditions and possible separation by an ocean of unknown dimensions re initiation of east dipping subduction beneath the western margin of the eastern block resulted in closure of this palaeoproterozoic ocean bringing the western block into close proximity continued subduction resulted in a major continental continental collision and the western margin of the eastern block was again reactivated leading to extensive thrusting high pressure metamorphism and the generation of crustal melts this occurred between 1 88 and 1 79 ga ago resulting in the formation of the trans north china orogen and final amalgamation of the north china craton although information is sparse to non existent we have reviewed the possible location of the north china craton at the time of collision and note a similarity in terms of age lithology and distribution of major crustal components between the north china craton and baltica although speculative we present this model in the hope that it will stimulate debate and lead to further research in these areas

polyphase zircon in ultrahigh temperature granulites rogaland sw norway constraints for pb diffusion in zircon shrimp u pb ages have been obtained for zircon in granitic gneisses from the aureole of the rogaland anorthosite norite intrusive complex both from the ultrahigh temperature uht 900 c pigeonite in zone and from outside the hypersthene in isograd magmatic and metamorphic segments of composite zircon were characterised on the basis of electron backscattered electron and cathodoluminescence images plus trace element analysis a sample from outside the uht zone has magmatic cores with an age of 1034 7 ma 2 n 8 and 1052 5 ma 1 n 1 overgrown by m1 metamorphic rims giving ages between 1020 7 and 1007 5 ma in contrast samples from the uht zone exhibit four major age groups 1 magmatic cores yielding ages over 1500 ma 2 magmatic cores giving ages of 1034 13 ma 2 n 4 and 1056 10 ma 1 n 1 3 metamorphic overgrowths ranging in age between 1017 6 ma and 992 7 ma 1 corresponding to the regional m1 sveconorwegian granulite facies metamorphism and 4 overgrowths corresponding to m2 uht contact metamorphism giving values of 922 14 ma 2 n 6 recrystallized areas in zircon from both areas define a further age group at 974 13 ma 2 n 4 this study presents the first evidence from rogaland for new growth of zircon resulting from uht contact metamorphism more importantly it shows the survival of magmatic and regional metamorphic zircon relics in rocks that experienced a thermal overprint of c 950 c for at least 1 myr magmatic and different metamorphic zones in the same zircon are sharply bounded and preserve original crystallization age information a result inconsistent with some experimental data on pb diffusion in zircon which predict measurable pb diffusion under such conditions the implication is that resetting of zircon ages by diffusion during m2 was negligible in these dry granulite facies rocks imaging and th u y systematics indicate that the main processes affecting zircon were dissolution reprecipitation in a closed system and solid state recrystallization during and soon after m1

precise determination of phanerozoic zircon pb pb age by multicollector sims without external standardization 1 zircon has long been recognized as the best geochronometer and the most important timekeeper in geosciences modern microbeam techniques such as sims and la icpms have been successfully applied to in situ u pb zircon age determinations at spatial resolutions of 20 30 m or less matrix matched calibration by external standardization of well characterized natural zircon references is a principal requirement for precise microbeam u pb zircon age determination due to fractionation effects between pb and u which usually result in an external age error exceeding 1 alternatively zircons with a closed u pb system can be directly dated by measurement of 207pb 206pb isotopic ratio without external standardization which has been a common practice for zircons older than 1 0 ga but not for relatively young 1 0 ga and particularly phanerozoic ones because of limitations of analytical precision we describe in this paper a method of 207pb 206pb measurement on phanerozoic zircons using a new generation of large radius magnetic sector multicollector cameca ims 1280 sims in combination with multicollector mode a nuclear magnetic resonance nmr magnet controller and oxygen flooding techniques we achieve precisions of 207pb 206pb ratio of 0 1 and 0 1 0 2 propagating to pb pb age errors 0 4 and 1 3 excluding u decay constant uncertainties for zircons of latest neoproterozoic and late paleozoic to mesozoic age respectively therefore the multicollector sims is capable of direct determination of zircon pb pb ages as young as mesozoic age with uncertainties of geological significance this technique is useful for direct dating of zircons in thin sections moreover it has significance for dating of some other u rich minerals i e baddeleyite and zirconolite that are not suitable for sims u pb dating by external standardization

alternative interpretations as to the origin of the hydrocarbons of the guajira basin colombia abstract the origins of the hydrocarbon gases recovered from the chuchupa ballena and riohacha fields located in the guajira basin northeast colombia are examined these gases are composed of methane with trace amounts of wet gas components c2 the gases recovered at three onshore seep localities appear to include a thermogenic component as a result of fractionation it is unclear whether they represent pure thermogenic gases or a mixed thermogenic biogenic origin similar to chuchupa an examination of oil microseepage observed in soil samples recovered from the onshore guajira region point to another hydrocarbon system apparently not related to the three gas fields some of these microseeps include fresh nonbiodegraded oils these microseeps did not correlate with either of the tertiary oil families from the sinu uraba basin located to the southwest of the study area or with cretaceous oils from that basin a better correlation was observed with the la luna formation but significant differences remain implying that the source for the microseeps was less calcareous and deposited in a less restricted environment than associated with classical la luna facies this could indicate either a facies change or that a different source unit is present within the region

zircons from kimberlite new insights from oxygen isotopes trace elements and ti in zircon thermometry zircons found in mantle sourced kimberlite provide probes into the isotopic chemistry of the asthenosphere and subcontinental lithospheric mantle however little is known about the conditions of formation of these zircons a suite of 88 zircons found in kimberlites from africa siberia brazil and the united states have been analyzed for their ti concentration and selected zircons were analyzed for their rare earth element ree concentrations by ion microprobe in addition precise and accurate laser fluorination oxygen isotope data were obtained for zircons from brazil 5 1 0 3 1sd and the midwest united states 5 3 0 3 yielding mantle like d 18 o values similar to published data for africa 5 2 0 3 and siberia 5 3 0 2 most megacrysts in this study preserve fine scale oscillatory zoning in cl and are generally homogenous in oxygen isotopic composition consistent with preservation of primary compositions a few zircons from brazil show some evidence of chemical zoning due to recrystallization the ti content of mantle zircon is in general low with average compositions from each locality of 13 8 4 ppm 1sd kaapvaal craton 12 8 7 ppm siberian platform 18 11 ppm brazil and 4 8 4 3 ppm united states the recently calibrated ti in zircon thermometer yields an average temperature of 744 62 c 1sd for the average of 13 9 ppm ti with no correction for pressure atio2 o rasio2 the ti content of zircons found within rutile nodules from the orapa kimberlite kaapvaal craton is almost indistinguishable from those with no constraint on atio2 suggesting that reduced atio2 is not responsible for lower than expected mantle temperatures the average temperature in this study corresponds to 3 gpa on a 40 mw m 2 cratonic geotherm if correct this would suggest that zircon megacrysts from all four cratons formed in the shallow lithospheric mantle however there are several possibly confounding effects to this thermometer including a pressure correction and disequilibrium zircon growth zircons from rutile nodules have ree contents that span the range of mantle zircon ree and are similar to both zircon megacrysts and zircons from metasomatic assemblages 2007 elsevier ltd all rights reserved

assembling north china craton within the columbia supercontinent the role of double sided subduction abstract an evaluation of recent s wave receiver functions s wave velocities and two versions of p wave tomographic images along various transects in the north china craton provides some clues on the subduction collision history of the different crustal blocks and their final amalgamation within the paleoproterozoic columbia supercontinent interpretation of a n s seismic section of the craton suggests thick slab debris sinking to various depths in the mantle the w e seismic corridors show the preservation of a thick 200 km lithospheric root tectosphere beneath the ordos block and its variable and extensive erosion towards the yanliao block eastern block this zone is characterized by layers with marked velocity contrast and suggests repeated stacking of the remnants of underplated and accreted paleoproterozoic oceanic lithosphere the present day lithosphere asthenosphere boundary beneath this region probably marks the erosional plane along which decratonization occurred through subduction erosion from the east and thermal and material erosion by upwelling asthenosphere from below resulting in the partial destruction of the tectosphere and its thinning towards the east within the asthenosphere below the yanliao block younger and thinner slabs predominate in the absence of any prominent thick high velocity layers these younger slabs define a westward polarity and constitute a mega scale duplex formed by underplating through phanerozoic subduction process particularly the pacific plate subduction from the east the lithologic associations within the inner mongolia suture zone dividing the yinshan block to the north and ordos block to the south correspond to an accreted ocean plate stratigraphic sequence with the tonalite trondhjemite granodiorite ttg gneisses charnockites and calc alkaline granites representing a continental arc built up through subduction from the north the seismic transects bring out a contrasting polarity in the subduction regime with an oblique east to southward subduction of the yinshan block and a westward subduction of the yanliao block here i propose a double sided subduction history for the ncc similar to the ongoing subduction process in the western pacific such double sided subduction is considered to promote rapid amalgamation of continental fragments within supercontinents and the subduction polarities and mantle dynamics of ncc are therefore considered to be critical in evaluating the final assembly of the paleoproterozoic supercontinent columbia

paleoproterozoic arc magmatism in the north china craton no siderian global plate tectonic shutdown abstract arc magmatism in convergent plate margins has been a major contributor to continental growth following arc arc and arc continent collisions in the archean leading to the amalgamation of micro blocks the north china craton ncc witnessed major pulses of continental arc magmatism during the paleoproterozoic in this study we present geochemistry zircon u pb geochronology and lu hf isotope data from a suite of magmatic rocks sampled from the region of confluence of two major paleoproterozoic suture zones in the ncc the inner mongolia suture zone imsz and the trans north china orogen tnco our zircon u pb geochronological data indicate new zircon growth during multiple tectonothermal events as displayed in the 207pb 206pb weighted mean ages of 2410 41 ma for metagranite 2480 12 ma 2125 18 ma 1946 8 ma 1900 15 ma and 1879 12 ma from metagabbros 2446 11 ma from charnockite and 1904 6 ma and 1901 9 ma from metatuffs the 207pb 206pb upper intercept age of zircons in the khondalite shows 2102 76 ma which is identical to the age obtained from the magmatic zircons in one of the metagabbros the khondalites also carry a group of concordant metamorphic zircons with 207pb 206pb mean age of 1881 20 ma metamorphic zircons in the gabbros and charnockites also yield similar ages of 1890 14 ma and 1852 19 ma respectively the age data suggest prolonged arc magmatism in a convergent margin setting during ca 2 48 to 1 9 ga followed by metamorphism at ca 1 89 1 85 ga associated with the final collision lu hf analyses reveal that the dominant populations of zircons from all the rock types are characterized by positive ehf values 1 9 to 6 8 mean 1 8 the ehf and tdmc data suggest that the magmas were mostly derived from neoarchean and paleoproterozoic juvenile components the salient geochemical features of these rocks attest to magma generation from heterogeneous sources involving subduction derived arc components with minor input from continental crust the results presented in this study together with those from previous investigations in different domains of the imsz and tnco suggest major paleoproterozoic arc magmatic events in the ncc lasting for nearly 600 million years associated with the final assembly of the crustal blocks into a coherent craton construction of the final cratonic architecture of the ncc thus witnessed not only the arc continent amalgamations at 2 7 2 5 ga but also major crust building events in the paleoproterozoic through melts generated from juvenile and recycled components in continental magmatic arc systems along an active convergent margin followed by intense deformation and metamorphism during the final collision at 1 85 1 80 ga the prominent paleoproterozoic magmatic records in the ncc do not support the proposal of global plate tectonic shutdown in the siderian and confirm vigorous convergent margin magmatism and crust building processes throughout the paleoproterozoic

high pressure granulites in the sanggan area north china craton metamorphic evolution p t paths and geotectonic significance high pressure basic granulites are widely distributed as enclaves and sheet like blocks in the huaian ttg gneiss terrane in the sanggan area of the central zone of the north china craton four stages of the metamorphic history have been recognised in mineral assemblages based on inclusion exsolution and reaction textures integrated with garnet zonation patterns as revealed by compositional maps and compositional profiles the p t conditions for each metamorphic stage were obtained using thermodynamically and experimentally calibrated geothermobarometers the low ca core of growth zoned garnet along with inclusion minerals defines a prograde assemblage m1 of garnet clinopyroxene plagioclase quartz yielding 700 c and 10 kbar the peak of metamorphism at about 750 870 c and 11 14 5 kbar m2 is defined by high ca domains in garnet interiors and inclusion minerals of clinopyroxene plagioclase and quartz kelyphites or coronas of orthopyroxene plagioclase magnetite around garnet porphyroblasts indicate garnet breakdown reactions m3 at conditions around 770 830 c and 8 5 10 5 kbar garnet exsolution lamellae in clinopyroxene and kelyphites of amphibole plagioclase around garnet formed during the cooling process at about 500 650 c and 5 5 8 kbar m4 these results help define a sequential p t path containing prograde near isothermal decompression itd and near isobaric cooling ibc stages the clockwise hybrid itd and ibc p t paths of the hp granulites in the sanggan area imply a model of thickening followed by extension in a collisional environment furthermore the relatively high pressures 6 14 5 kbar of the four metamorphic stages and the geometry of the p t paths suggest that the hp granulites together with their host huaian ttg gneisses represent the lower plate in a crust thickened during collision the corresponding upper plate might be the tectonically overlying khondalite series which was subjected to medium to low pressure mp lp 7 4 kbar granulite facies metamorphism with a clockwise p t path including an itd segment both the hp and the mp lp granulite facies events occurred contemporaneously at c 1 90 1 85 ga in a collisional environment created by the assembly process of the north china craton

thermal evolution of two textural types of mafic granulites in the north china craton evidence for both mantle plume and collisional tectonics mafic granulites from the north china craton can be divided into two textural types referred to as a and b types a type mafic granulites display garnet quartz symplectic coronas and outcrop in the eastern and western zones of the craton whereas b type mafic granulites exhibit orthopyroxene plagioclase clinopyroxene symplectites or coronas and are mainly exposed in the central zone of the craton most a type mafic granulites preserve the prograde m 1 peak m 2 and post peak near isobaric cooling m 3 assemblages which are represented respectively by inclusions of hornblende plagioclase quartz a peak mineralogy of orthopyroxene clinopyroxene plagio clase quartz garnet and overprinted by garnet quartz symplectic coronas these mineral assem blages and their p t pressure temperature estimates define anticlockwise p t evolutionary paths the b type mafic granulites preserve the peak m 1 post peak near isothermal decompression m 2 and cooling m 3 assemblages which are represented by the peak assemblage of orthopyroxene clinopyroxene plagioclase quartz garnet hornblende post peak orthopyroxene plagioclase clinopyroxene symplectites or coronas and later hornblende plagioclase magnetite symplectites respectively these mineral assemblages and their p t estimates define clockwise p t paths the anticlockwise p t paths of the a type mafic granulites in the eastern and western zones of the north china craton are consistent with a model of underplating and intrusion of mantle derived magmas in combination with lithological structural and geochronological data the eastern and west ern zones of the north china craton are considered to represent two continental blocks that devel oped through the interaction of mantle plumes with the lithosphere from the palaeoarchaean to the neoarchaean era the b type mafic granulites and associated rocks in the central zone represent a magmatic arc that was metamorphosed and deformed during amalgamation of the eastern and west ern continental blocks in the late palaeoproterozoic era the mineral reaction relations and clockwise p t paths of the b type mafic granulites from the central zone record the tectonothermal history of the collision that resulted in the final assembly of the north china craton at c 1800 ma

cratonic reactivation and orogeny an example from the northern margin of the north china craton abstract reactivation of cratonic basement involves a number of processes including extension compression and or lithospheric delamination the northern margin of the north china craton ncc adjacent to the inner mongolian orogenic belt was reactivated in the late paleozoic to early mesozoic during this period the northern margin of the ncc underwent magmatism n s compression regional exhumation and uplift including the formation of e w trending thick skinned and thin skinned south verging folds and south verging ductile shear zones zircon u pb shrimp ages for mylonite protoliths in shear zones which show ages of 310 290 ma mid carboniferous early permian constraining the earliest possible age of deformation muscovite within carbonate and quartz feldspar muscovite mylonites from the kangbao weichang and fengning longhua shear zones defines a stretching lineation and gives 40ar 39ar ages of 270 250 ma 250 230 ma 230 210 ma and 210 190 ma deformation developed progressively from north to south between the late paleozoic and triassic exhumation of lower crustal gneisses high pressure granulites and granites occurred at the cratonic margin during post ductile shearing 220 210 ma an undeformed early jurassic 190 180 ma conglomerate overlies the deformed rocks and provides an upper age limit for reactivation and orogenesis deformation was induced by convergence between the southern mongolia and north china cratonic blocks and the location of this convergent belt controlled later deformation in the yanshan tectonic province this province formed as older e w trending archean proterozoic sequences were reactivated along the northern margin of the ncc this reactivation has features typical of cratonic basement reactivation compression crustal thickening remelting of the mid to lower crust and subsequent orogenesis adjacent to the orogenic belt

single zircon grains record two paleoproterozoic collisional events in the north china craton abstract the western block of the north china craton consists of the yinshan block in the north and the ordos block in the south which were amalgamated along the east west trending khondalite belt at 1 95 ga the western block then collided with the eastern block to form the coherent basement of the north china craton along the north south trending trans north china orogen at 1 85 ga the huaian complex a high grade terrrane located at the conjunction of the khondalite belt and trans north china orogen records metamorphic events associated with both collisions the complex consists of lithologies from both the khondalite belt and trans north china orogen of which the former consist of graphite garnet sillimanite gneiss garnet quartzite felsic paragneiss calc silicate rock and marble together called the khondalite series zircons in the graphite garnet sillimanite gneiss can be divided into three types 1 spherical grains without internal structures 2 grains with a core and rim structure and 3 grains with a dark core surrounded by double rims except for the dark core in type 3 all other types of zircon domains are structureless and highly luminescent with very low th u ratios typical of a metamorphic origin analyses on the cores of type 2 and the inner rims of type 3 from two samples yield upper intercept ages of 1946 26 and 1947 22 ma similar to previously determined metamorphic ages from the khondalite belt and thus interpreted as the time of collision between the yinshan and ordos blocks analyses on type 1 zircons rims of type 2 and the outer rims of type 3 from the same two samples give ages of 1850 15 and 1857 16 ma interpreted as the time of collision between the eastern and western blocks thus zircons in the graphite garnet sillimanite gneiss of the huaian complex record both of the paleoproterozoic collisional events in the north china craton

archean blocks and their boundaries in the north china craton lithological geochemical structural and p t path constraints and tectonic evolution an examination of lithological geochemical geochronological structural and metamorphic p t path data suggests that the basement of the north china craton can be divided into eastern and western blocks separated by major crustal boundaries that roughly correspond with the limits of a 300 km wide zone called the trans north china orogen the eastern block consists predominantly of late archean domiform tonalitic trondhjemitic granodioritic ttg batholiths surrounded by anastomosing networks and linear belts of open to tight synforms of minor volcanic and sedimentary rocks metamorphosed from greenschist to granulite facies at 2 5 ga with anticlockwise p t paths some early to middle archean rocks are locally present in the eastern block but their tectonic history is unclear due to reworking by the 2 5 ga tectonothermal event the western block has a late archean assemblage structural style and metamorphic history similar to that of the eastern block but it differs in the absence of early to middle archean assemblages and in being overlain by and interleaved with paleoproterozoic khondalites which were affected by a 1 8 ga metamorphic event involving clockwise p t paths a mantle plume model is proposed for the formation and evolution of late archean basement rocks in the eastern and western blocks based on a combination of extensive exposure of ttg gneisses affinities of mafic rocks to continental tholeiitic basalts presence of voluminous komatiitic rocks dominant diaprism related domiform structures anticlockwise p t paths and a short time span from the primary emplacement of ttg and ultramafic to mafic rocks until the onset of regional metamorphism between the two blocks is the trans north china orogen which is bounded by two major fault systems and is composed of late archean to paleoproterozoic ttg gneisses and granitoids interleaved with abundant sedimentary and volcanic rocks that are geochemically interpreted as having developed in magmatic arc and intra arc basin environments these rocks underwent multiple phases of compressional deformation and peak high pressure metamorphism followed by rapid exhumation during the late paleoproterozoic at 1 8 ga as a result of collision between the eastern and western blocks resulting in the amalgamation of the north china craton

the petrology of two distinct granulite types in the hengshan mts china and tectonic implications abstract the archean to proterozoic hengshan complex north china craton comprises tonalitic and granodioritic gneisses with subordinate mafic lenses pegmatites and granites amphibolite facies assemblages predominate although granulite facies relics are widespread and greenschist facies retrogression occurs in km wide shear zones mafic lenses locally abundant occur as strongly deformed amphibolite hornblende plagioclase boudins or sheets in contrast to previously published models we find two series of mafic rocks with distinctly different granulite facies evolutions in the north of the complex relict high pressure mafic granulites are garnet clinopyroxene bearing rocks with a secondary development of orthopyroxene around both garnet kelyphites and clinopyroxene coronas south of the newly defined central e w trending zhujiafang shear zone numerous mafic boudins and less deformed dykes exhibit a macroscopically visible magmatic texture with coronitic growth of metamorphic garnet full of quartz inclusions between the magmatic plagioclase and pyroxene domains additional orthopyroxene after magmatic augite and sodic rims to magmatic plagioclase clearly indicate medium pressure granulite facies metamorphism these findings suggest tectonic juxtaposition in this area of three different structural levels of the same proterozoic imprinted crust high pressure granulite grade in the northern hengshan medium pressure granulite grade in the southern hengshan and amphibolite to greenschist facies grade in the wutaishan to the se

hadean to neoarchean episodic crustal growth detrital zircon records in paleoproterozoic quartzites from the southern north china craton abstract the archean terranes exposed in several regions of the north china craton ncc provide important windows to evaluate the hadean archean continental crustal evolution history here we report results from sims zircon geochronology and oxygen isotopes as well as la icpms zircon hf isotopic analyses on detrital zircons from paleoproterozoic quartzites to probe the early crustal evolution in the southern ncc our data show episodic magmatism during the eoarchean paleoarchean mesoarchean neoarchean and paleoproterozoic as inferred from the concordant ages of 3 6 ga and 2 2 ga and the well defined upper intercept ages of 3404 30 ma 2919 31 ma 2772 9 5 ma 2698 4 7 ma 2652 6 7 ma and 2532 5 2 ma the hf and oxygen isotopic compositions of eoarchean paleoarchean zircons demonstrate that the cratonic nucleus of the ncc was built in the hadean 4 0 ga similar to the timing of formation of the nuclei of old cratons elsewhere on the globe such as the yilgarn craton western australia and the slave craton canada with the eoarchean paleoarchean 3 8 ga 3 6 ga and 3 4 ga marking the first phase of crustal reworking of the hadean crust the salient aspect of our data is that the meoarchean to early neoarchean 3 0 2 6 ga with a peak at 2 7 ga was a significant period for crustal growth from the mantle throughout the ncc comparable with the event of major archean crustal growth worldwide the latest neoarchean 2 5 ga marks a period of significant crustal reworking that led to remelting of the newly formed juvenile mafic crust 2 7 ga integration with the data obtained from the ttg tonalite trondhjemite granodiorite and granitoid rocks of the archean terranes and the lower crustal granulite xenoliths from various parts of the ncc suggests that the episodic eoarchean via paleoarchean and mesoarchean to neoarchean crustal growth as well as crustal reworking intensely destroyed the ancient cratonic nucleus resulting in the scarce preservation of the hadean and eoarchean remnants in both the upper and lower crust of the ncc following the paleoproterozoic sedimentation of the archean zircons apart from pb loss during the subsequent thermal events there was no major alteration in the hf and oxygen isotopic compositions thus our detrital zircon data provide convincing evidence for the repeated crustal growth and reworking of the ncc in the archean

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

shrimp u pb zircon geochronology of the huai an complex constraints on late archean to paleoproterozoic crustal accretion and collision of the trans north china orogen