paleoproterozoic tectonic transition from collision to extension in the eastern cathaysia block south china evidence from geochemistry zircon u pb geochronology and nd hf isotopes of a granite charnockite suite in southwestern zhejiang abstract the badu complex and associated paleoproterozoic granitoids are among the oldest known rocks in the cathaysia block in south china the paleoproterozoic units of the badu complex are dominantly composed of metapelitic rocks and meta greywackes here we report la ss icp ms laser ablation split stream inductively coupled plasma mass spectrometry zircon u pb data from a newly discovered garnet bearing granite which show an emplacement age of 1929 15 ma and metamorphism at 1872 34 ma we also report u pb ages of 1886 16 ma 1858 7 ma 1848 11 ma from a gneissic granodiorite and two charnockites respectively the garnet bearing granite is peraluminous with a cnk range from 1 1 to 1 3 the rock shows relatively high sio2 k2o and rb contents and low total ree sr cao and ferromagnesian components typical of leucogranites the whole rock nd two stage model age tdm2 nd of this rock is ca 2 7 ga zircon hf crustal model ages tdmc hf peak at about 2 7 ga and abundant inherited zircons occur with u pb ages in the range of 2044 to 2803 ma evidences from zircon u pb age and hf isotope compositions whole rock nd isotopes and whole rock major and trace elements suggest a metasedimentary protolith and that the garnet bearing granite leucogranite was derived by partial melting in a thickened crust at about 1 93 ga the gneissic granodiorite and charnockites show indistinguishable major and trace element features as well as zircon hf and whole rock nd isotope compositions indicating that they were generated from the same source rocks the gneissic granodiorite and charnockites are ferroan calc alkalic and metaluminous with a cnk range from 0 80 to 0 98 they display relatively low sio2 contents and ga al ratios suggesting their a type affinity their zircon tdmc hf age peak is 2 9 ga and whole rock tdm2 nd ages range from 2 8 to 2 9 ga these high temperature rocks were generated possibly through the partial melting of ancient amphibolites 2 9 ga with heat input from asthenosphere upwelling and magmatic underplating in an extensional setting at about 1 85 1 88 ga combined with data from previous studies we suggest three episodes of major crustal growth in this region at 1 8 1 9 ga 2 7 ga and 2 9 ga during the 1 8 1 9 ga event extensive reworking also took place from the 1 93 ga syn collision leucogranite garnet bearing granite the 1 88 1 85 ga post collision a type granites and charnockites the granite charnockite suite and the1 85 1 77 ga intraplate basaltic rocks li 1997 and xiang et al 2008 in the region a paleoproterozoic orogenic cycle can be deciphered in the cathaysia block the rapid transition of tectonic regime from collision to post collisional extension can be correlated to the assembly of the supercontinent columbia

geology of panamint valley saline valley pull apart system california palinspastic evidence for low angle geometry of a neogene range bounding fault geological studies support the interpretation that northern panamint valley and saline valley southeast california form paired pull apart basins on opposite sides of the right slip hunter mountain fault zone eight to ten km of late cenozoic net slip can be established on the hunter mountain fault zone palinspastic reconstruction of northern panamint valley indicates that the valley was formed by movement on a shallow crustal low angle normal fault of 0 15 degree west dip during the last 3 0 ma this interpretation appears to contradict the notions that little extension is accomodated in the uppermost crust by low angle faulting and that the most recent extension in the basin and range province is accomodated exclusively by high angle faulting saline valley however is interpreted to have formed by extension on closely spaced rotated planar normal faults thus within one geometric system of paired pull apart basins extension appears to have been accommodated in the shallow crust in two different ways

thermal gradient and geochronology of a paleozoic high grade terrane in the northeastern cathaysia block south china abstract the chencai terrane in the northeastern cathaysia block forms part of the northeast extension of the paleozoic orogeny in south china recent field investigations have revealed the occurrence of high grade rocks in this terrane including granulite facies metapelites khondalites and orthopyroxene bearing granulites however the thermal gradients and metamorphic history of this region have not been well constrained thus impeding a better understanding of the geodynamic history of the paleozoic orogeny in south china here we report results from a systematic study on the metamorphic pt conditions and geochronology of the orthopyroxene granulites and metasedimentary rocks metapelites from the chencai area the orthopyroxene granulites preserve granulite facies peak assemblage of opx grt pl kfs qtz and a retrograde p t trajectory peak pt conditions are 845 860 c and 0 83 0 86 gpa corresponding to a thermal gradient of 30 c km which is much hotter than the normal thermal gradient of continental crust mineral compositions of the metasedimentary rocks re equilibrated during retrograde metamorphism so that the peak pt conditions recorded by these rocks are much lower than those obtained from the orthopyroxene granulite peak pt conditions are 710 720 c 0 57 0 58 gpa for garnet biotite gneiss and 760 780 c 0 60 0 62 gpa for biotite garnetite zircon u pb dating shows that this episode of granulite facies metamorphism occurred at 430 450 ma and the detrital population shows minor or no contribution from the basement rocks of the cathaysia block the neoproterozoic rocks occurring along the jiangnan belt are considered to have provided the sedimentary source as indicated by the two main age peaks of 0 79 ga and 0 83 ga from the detrital zircon cores lu hf isotope data on the zircons indicates extensive alteration and little new zircon growth during the paleozoic metamorphism the ehf t values and model ages tdm1 of zircons suggest that the source rocks involved juvenile crustal components with hf model ages of 1 1 1 5 ga sedimentation of such a continental shelf sequence as well as the high thermal gradient of 30 c km confirm the previously proposed continental arc formation and maturation model in the chencai region

origin of paired high pressure ultrahigh temperature orogens a ridge subduction and slab window model introductionmetamorphism at elevated pressure temperature conditions and distinct uid regimes generates ultrahigh pres sure uhp high pressure hp andultrahigh temperature uht rocks harley 2004 brown 2006 2007 2009 kelsey 2008 santosh andomori 2008a b santos et al 2009 santosh et al 2009a zhang et al 2009 although previous studies havecorrelated these di erent orogens todistinct tectonic settings recent inves tigations record high pressure anduht conditions from the same tec tonic environment within the sameorogenic belt and in some cases evenwithin the same samples mostoftheseexamples come from major collisionalzones along which continental blocksweresuturedatdi erenttimesinearthhistory including from the neoarch ean hp uht rocks of the limpopobeltinsouthafrica tsunogaeandvanreenen 2006 the palaeoproterozoichp guo et al 2004 obrien et al 2005 and uht santosh et al 2007a b 2008 2009b belts in thenorth china craton and the hp uht belts in the neoproterozoiceastern ghats belt as well as thecambrian gondwana suture in south ern india shimpo et al 2006 tsuno gaeet al 2008a santoshet al 2009c notably the thermal regimes for thesepaired hp and uht rocks are similar 1000 c and they also share acommon uid history characterizedby co

response of the u pb chronometer and trace elements in zircon to ultrahigh temperature metamorphism the kadavur anorthosite complex southern india abstract zircon from quartzites in the contact aureole and wider environs of the kadavur anorthosite complex se india was studied by laser ablation icp ms to assess the response of the u pb isotope system and trace element concentrations to ultrahigh temperature uht contact metamorphism 1000 c combined cathodoluminescence imaging and la icp ms analyses show that zircon grains contain detrital cores which yield ages between 3 4 ga and 1 8 ga and exhibit a large spread in ree concentrations these cores are associated with one or two rims that provide concordant age populations at 955 16 ma and 810 7 ma 2 and relatively uniform ree patterns the older ages of ca 955 ma record the imprints of regional early neoproterozoic metamorphism related to rodinia assembly the younger age is interpreted to date anorthosite emplacement and its associated contact metamorphism during a second regional metamorphic episode coeval with the intrusion of a type granites in the area zircon grains from the country rocks away from the anorthosite show additional rims yielding ages from 590 to 490 ma corresponding to regional pan african tectonometamorphism the zircon rims are either newly grown domains or represent recrystallized and re equilibrated parts of precursor zircon discordance is only observed in 25 of the detrital cores these cores have relatively high u concentrations 400 ppm which strongly suggests that pb loss occurred under u ht conditions by partial recrystallization annealing of zircon that had become metamict the preservation of old concordant ages and source ree characteristics by the low u i e weakly to non metamict detrital cores shows that pristine zircon is extremely robust to thermal disturbance and that its u pb systematics can remain unaffected despite multiple u ht metamorphic episodes these observations imply that u pb ages in zircon cannot be reset by volume diffusion under any crustal conditions disturbance of the u pb system in individual zircon crystals or parts thereof can only be achieved by recrystallization of radiation damaged zircon

experiments on simultaneous faulting and folding above a basement wrench fault abstract wrench zones commonly show en echelon patterns of folds in the stratified cover above a basement fault examples are the darien basin in panama and the inglewood fault trend in california folds are often associated with synthetic and antithetic wrench faults but the time relationships between folds and faults may not be obvious we have done experiments where simultaneous faulting and folding occur above a basement fault four experiments were performed using a simple apparatus capable of imposing localised strike slip motions this apparatus consists of two plastic plates one can be moved laterally past the other by a piston thus modelling lateral motion on a single basement fault models were properly scaled to account for gravitational forces brittle behaviour of sandstone and ductile behaviour of clay or shales in the upper crust each model is a vertical succession of silicone layers and sand layers each layer is 2 mm thick except for the basal silicone layer which is 2 4 6 or 8 mm thick in experiments 1 2 3 or 4 respectively all models were deformed at the same slip rate on the basal fault 1 cm h 1 experiments 1 2 and 3 with 8 cm total horizontal sinistral offset were designed to investigate the influence of basal silicone layer thickness on the deformation of a multilayered model experiment 4 with 20 cm of total offset was designed to investigate the influence of an increasing left lateral offset on folding and faulting with a 2 mm thick basal silicone layer experiment 1 synthetic strike slip faults appeared en echelon above the basement fault no folds were visible at the surface of the experiment but the interfaces between sand and silicone layers showed millimetric folds en echelon above the basement fault with a 4 mm thick basal silicone layer experiment 2 en echelon synthetic strike slip faults and folds appeared above the basement fault faults and folds rotated progressively anticlockwise and synclines were activated as dextral antithetic shear zones with a 6 mm thick basal silicone layer experiment 3 synthetic faults and en echelon folds appeared as before but the fold amplitude was larger than in experiment 2 an increasing left lateral offset produced anticlockwise rotation of faults and folds as before and synclines were activated as dextral antithetic shear zones in experiment 4 after 16 cm of left lateral offset the anticlockwise rotation of the folds was such that antithetic dextral shear zones became almost parallel to the basement fault and were then reactivated as synthetic sinistral shear zones

geochronology and geochemistry of a suite of mafic rocks in chencai area south china implications for petrogenesis and tectonic setting abstract paleozoic crust mantle interaction and geodynamic setting of the south china block have remained largely elusive due to the scarcity of mafic rock suites here we report data from a suite of mafic rocks including norite mafic granulite garnet bearing amphibolite biotite bearing amphibolite and amphibolite from the chencai region in the northeastern cathaysia block of south china that mostly record amphibolite to granulite facies metamorphism u pb dating of zircons from the mafic granulite and biotite bearing amphibolite show broadly coeval magmatism and amphibolite to granulite facies metamorphism during 435 438 ma the garnet bearing amphibolite also records metamorphism at 436 ma and the magmatic age of its protolith is constrained to be between 436 ma and 496 ma the norite has an emplacement age of 422 ma geochemically the amphibolite and granulite have high al2o3 contents and belong to the high alumina group the garnet bearing amphibolite belongs to the intermediate alumina group whereas the norite and biotite bearing amphibolite belong to the low alumina group a cumulate origin is proposed for the norite with orthopyroxene as the main cumulus phase based on their similar trace element and lu hf isotopic signatures it is inferred that the protoliths of amphibolite and granulite belonging to high alumina group were likely generated from the same source furthermore their lree enrichment relative to hree positive eu anomaly and negative nb ta and zr anomalies suggest derivation from the same enriched mantle source perhaps in a continental arc environment zircons from the garnet bearing amphibolite belonging to intermediate alumina group have high positive hf t values with a peak of 14 and trace element signatures similar to n morb suggesting a depleted asthenospheric mantle source the rock suite from chencai suggests formation in an active continental margin with the garnet bearing amphibolite representing relics of the paleozoic oceanic crust in northeastern south china and the high and low alumina mafic rocks representing layered cumulates that developed in the arc root the amphibolite to granulite facies metamorphism recorded by these mafic and supracrustal rocks of chencai complex is consistent with high heat flow in an arc root

zircon u pb geochronology and geochemistry of two episodes of granitoids from the northwestern zhejiang province se china implication for magmatic evolution and tectonic transition abstract granitoids 175 80 ma representing a prominent yanshanian jurassic to cretaceous magmatic event in south china widely intrude the precambrian crystalline basement and paleozoic strata here we report zircon u pb age data geochemical characteristics and sr nd isotopes of the late jurassic and early cretaceous granitoids from the northwestern zhejiang province zxb of southeastern china our results reveal two distinct episodes for the yanshanian magmatism the jiemeng and datongkeng granodiorites formed at 148 6 1 1 ma whereas the huangshitan jiuligang and ruhong aluminous a type granites were generated between 129 0 0 6 ma and 126 1 1 1 ma the two magmatic phases represent a tectonic transition from an active continental margin to post orogenic setting during the late jurassic ca 150 ma to early cretaceous ca 128 ma geochemically these intrusions are granodioritic to granitic in composition and show an affinity of s type and a type granitoids respectively the s type granodiorites of jiemeng and datongkeng are characterized by moderate sio 2 65 0 69 6 wt high k 2 o na 2 o 5 0 7 6 wt k 2 o na 2 o 1 2 1 5 zr 31 109 ppm sr 71 190 ppm and high field strength elements low to intermediate mg and moderate nb depletion the a type granites of huangshitan jiuligang and ruhong are characterized by high sio 2 72 7 77 2 wt k 2 o na 2 o 6 9 8 8 wt k 2 o na 2 o 1 3 2 1 fe t fe t mg ga 17 29 ppm 20 ppm commonly zr 96 197 ppm and sr 8 45 ppm with slight nb depletion the s type granodiorites have higher mg a nk sr sr ba sr y la yb n and lree hree and lower sio 2 k 2 o na 2 o ga and zr with weak negative eu anomalies compared to those of the a type granites with negative eu anomalies all these rocks show y nb ratios 1 2 high initial 87 sr 86 sr i sr ratios and low e nd t and are depleted in nb ti and sr indicating crustal origin with subduction zone signatures we suggested that the zxb s type granitic bodies might have been derived from the mesoproterozoic metamorphic basement rocks through partial melting induced by mantle derived magma followed by limited fractional crystallization the zxb aluminous a type granites were also derived from a similar magma source but underwent fractional crystallization at higher crustal levels the a type granites in the zxb correlate with a post orogenic tectonic setting a geological comparison between zxb and adjacent areas indicates that the geochemical features of the zxb a type granites are comparable with the baijuhuajian a type granites and also the adjacent areas a type granites but are distinct from the late cretaceous a type granites 105 90 ma distributed along the southeastern coastal area of south china we correlate the formation of the zxb s type granodiorites to inland compression associated with the subduction and collision of the paleo pacific plate in the late jurassic 170 145 ma in contrast the a type granites formed under a post orogenic setting during the early cretaceous period 145 120 ma resulting from lithospheric thinning and continent extension accompanied by slab roll back of the paleo pacific plate following the subduction collision event

the behaviour of monazite from greenschist facies phyllites to anatectic gneisses an example from the chugach metamorphic complex southern alaska monazite is a common accessory mineral in various metamorphic and magmatic rocks and is widely used for u pb geochronology however linking monazite u pb ages with the pt evolution of the rock is not always straightforward we investigated the behaviour of monazite in a metasedimentary sequence ranging from greenschist facies phyllites into upper amphibolites facies anatectic gneisses which is exposed in the eocene chugach metamorphic complex of southern alaska we investigated textures chemical compositions and u pb dates of monazite grains in samples of differing bulk rock composition and metamorphic grade with particular focus on the relationship between monazite and other ree bearing minerals such as allanite and xenotime in the greenschist facies phyllites detrital and metamorphic allanite is present whereas monazite is absent in lower amphibolites facies schists 550 650 c and 3 4 kbar small medium y monazite is wide spread mnz1 indicating monazite growth prior and or simultaneous with growth of garnet and andalusite in anatectic gneisses new low y high th monazite mnz2 crystallised from partial melts and a third high y low th monazite generation mnz3 formed during initial cooling and garnet resorption u pb shrimp analysis of the second and third monazite generations yields ages of 55 50 ma monazite became unstable and was overgrown by allanite and or allanite epidote apatite coronas within retrograde muscovite and or chlorite bearing shear zones this study documents polyphase complex monazite growth and dissolution during a single relatively short lived metamorphic cycle

jiangnan orogen in south china developing from divergent double subduction abstract the jiangnan orogen is considered as a continent continent collisional belt resulting from the closure of a meso neoproterozoic ocean separating the southeastern margin of the yangtze block from the northwestern margin of the cathaysia block recent data indicate the existence of early neoproterozoic 1000 825 ma volcanic arc assemblages on both sides of the orogen suggesting that the ocean lithosphere between the yangtze and cathaysia blocks must have undergone divergent double sided subduction during the period of 1000 825 ma the divergent double subduction eventually resulted in the closure of the ocean basin at 825 ma leading to the soft collision of the yangtze and cathaysia blocks to form the jiangnan orogen without involvement of continental deep subduction high grade metamorphism of continental crust and uplift exhumation of high grade metamorphic rocks shortly after the collision the initial detachment of the ocean lithosphere from the overlying crust and sedimentary sections induced underplating of mantle magmas triggering partial melting of accretionary wedge strata to form some peraluminous s type granites in the period 825 815 ma finally the sinking of the oceanic slab pulled down the overlying strata to form some basins in which the banxi group and its equivalent strata including bimodal volcanic rocks were formed in the period 815 750 ma

structural development of the lower paleozoic belt of south china genesis of an intracontinental orogen abstract an extensive structural study of wuyishan and surrounding areas south china brings data on the structures formed prior to the devonian unconformity building the lower paleozoic belt an ordovician tectonic event was responsible for i south directed structures in the wuyishan proper and its southern border related to both thin and thick skinned tectonics accompanied by metamorphism and crustal thickening ii north directed structures to the west of ganjiang fault and north of jiangshan shaoxing fault where only thin skinned tectonics is visible the southward deformation accommodating the shortening includes emplacement of thrust sheets involving deep crustal material and some mantle peridotite and likely repetition of continental material both responsible for crustal thickening the orogeny was due to the underthrusting of the southern part of the south china block beneath the northern part of this block closing the pre existing nanhua rift created at around 850 800 ma and involving the rift sedimentary fill in the southward thrusting the syn metamorphic tectonic piling was followed by anatexis and granite emplacement at around 440 390 ma our new u th pb epma monazite dating gives ages of 433 9 ma and 437 5 ma early silurian for the main anatectic event and a younger age of 412 5 ma late silurian early devonian for a late re heating process the lower paleozoic belt is an intracontinental orogen without oceanic suture record it shows some similarities with the cenozoic european pyrenees chain with a strong asymmetry

provenance of late ordovician sedimentary rocks in the se yangtze block implications for deposition in an active continental margin abstract the jiangnan orogen separates the yangtze block from the cathaysia block to the southeast several competing models have been proposed for the evolution of both blocks and the intervening orogen which mainly focus on aspects of igneous and metamorphic rocks limited research has been conducted on the sedimentary rocks in this region provenance of the late ordovician sedimentary rocks in the northeastern jiangnan orogen has been constrained based on systemic analyses of petrology heavy mineral assemblages geochemistry and detrital zircon u pb ages which provide vital clues for reconstructing the evolution of this belt during the early paleozoic late ordovician deposits in the northeastern jiangnan orogen consist of conglomerates sandstones siltstones shales calcareous mudstones and limestones these strata were deposited in a shallow to deep sea environment showing water depth deepening to the northwest the sandstone contains lithic graywackes and feldspar lithic graywackes with low compositional and textural maturities they consist on average of quartz clasts 27 feldspar clasts 18 and lithic fragments 55 the lithic fragments are angular including andesites rhyolites tuffs granites slates phyllites quartzites siltstones and chert which also occur in conglomerate form minor pyroxene chromite magnetite and garnet grains occur in the sandstone geochemical analyses show that the sandstone and siltstone have widely variable fe2o3t mgo 4 43 12 05 cs v ni and ti contents they also exhibit a distinct negative nb ta trough compared to the average upper continental crust and have light rare earth element lree enrichment patterns with obvious eu anomalies similar to subduction related rocks in an arc setting detrital zircon u pb ages demonstrate a mixed source from ca 784 ma and ca 455 ma paleocurrent indicators point to a southeastward origin these results demonstrate that late ordovician sedimentary deposition occurred in a northwestward facing active continental margin along the southeastern yangtze block

adakitic granites within the ne jiangxi ophiolites south china geochemical and nd isotopic evidence geochemical and nd isotopic data are reported for the ca 970 ma albite granites within the neoproterozoic ne jiangxi ophiolite south china these granites are high in al2o3 15 17 na2o 7 3 8 5 mg 43 62 sr 383 920 ppm sr y 166 444 and y yb 12 14 but low in k2o 0 2 0 6 tio2 0 3 p2o5 0 1 furthermore they display consistently high morb like initial end values of 4 6 to 6 7 at ca 970 ma these geochemcial and isotopic characters are similar to those of the modern adakites compared with the modern adakites the xiwan albite granites are depleted in middle to heavy ree with y yb ratio of 12 14 which can be attributed to residual amphibole and garnet in the source geological and geochemical features suggested that the granites were generated by low degrees of partial melting of subducted spilitized oceanic crust at pressures high enough to stabilize garnet and amphibole these are the first adakitic granites identified within the ne jiangxi ophiolite their presence shows that granitic rocks that intrude ophiolites can be generated not only during the processes of sea floor spreading oceanic crust shearing and obduction of fragmented oceanic lithosphere as previously thought but also during the process of subduction of oceanic lithosphere the shrimp u pb zircon age of 968 23 ma for the xiwan adakitic granite geochimica 23 1994 117 should be re interpreted as the timing of subduction of oceanic crust rather than as the age of oceanic crustal formation as previously interpreted

valid garnet biotite gb geothermometry and garnet aluminum silicate plagioclase quartz gasp geobarometry in metapelitic rocks abstract at present there are many calibrations of both the garnet biotite gb thermometer and the garnet aluminum silicate plagioclase quartz gasp barometer that may confuse geologists in choosing a reliable thermometer and or barometer to test the accuracy of the gb thermometers we have applied the various gb thermometers to reproduce the experimental data and data from natural metapelitic rocks of various prograde sequences inverted metamorphic zones and thermal contact aureoles we have concluded that the four gb thermometers perchuk l l lavrent eva i v 1983 experimental investigation of exchange equilibria in the system cordierite garnet biotite in saxena s k ed kinetics and equilibrium in mineral reactions springer verlag new york berlin heidelberg pp 199 239 kleemann u reinhardt j 1994 garnet biotite thermometry revised the effect of alvi and ti in biotite european journal of mineralogy 6 925 941 holdaway m j 2000 application of new experimental and garnet margules data to the garnet biotite geothermometer american mineralogist 85 881 892 model 6av kaneko y miyano t 2004 recalibration of mutually consistent garnet biotite and garnet cordierite geothermometers lithos 73 255 269 model b are the most valid and reliable of this kind of thermometer more specifically we prefer the holdaway holdaway m j 2000 application of new experimental and garnet margules data to the garnet biotite geothermometer american mineralogist 85 881 892 and the kleemann and reinhardt kleemann u reinhardt j 1994 garnet biotite thermometry revised the effect of alvi and ti in biotite european journal of mineralogy 6 925 941 calibrations due to their small errors in reproducing the experimental temperatures and good accuracy in successfully discerning the systematic temperature changes of the different sequences in addition after applying the gasp barometer to 335 natural metapelitic samples containing one kind of aluminosilicate and 33 natural metapelitic samples containing two kinds of aluminosilicate as well as to rocks in limited geographic areas and thermal contact aureoles we propose that the calibrations of holdaway holdaway m j 2001 recalibration of the gasp geobarometer in light of recent garnet and plagioclase activity models and versions of the garnet biotite geothermometer american mineralogist 86 1117 1129 and newton and haselton newton r c haselton h t 1981 thermodynamics of the garnet plagioclase al2sio5 quartz geobarometer in newton r c navrotsky a wood b j eds thermodynamics of minerals and melts new york springer verlag 131 147 based on kleemann and reinhardt s kleemann u reinhardt j 1994 garnet biotite thermometry revised the effect of alvi and ti in biotite european journal of mineralogy 6 925 941 thermometer are the most valid gasp barometers other gb thermometers and gasp barometers are not recommended

basin formation along the bocono moron el pilar fault system venezuela within the boundary zone between the caribbean south america plates which includes the bocono moron el pilar right lateral fault system that has been active since the late tertiary there are several sedimentary basins in the venezuelan andes and the caribbean mountains these were formed in zones of crustal depression where faults diverge and as pull apart basins in zones of en echelon stepovers or along releasing bends in the faults in addition there are pull apart basins along the avila and la victoria faultzones in the caribbean mountains which are associated with the plate boundary the basins are filled with alluvial lacustrine fluvial and marine sediments mainly of quaternary age on the basis of model studies dimensions of the basins and age of their sedimentary fill it is estimated that right lateral offset necessary to form them along the bocono moron el pilar fault system varies from 2 to 125 km along the avila and la victoria fault zones this right lateral offset is estimated to range from 2 to 30 km on this basis the maximum right lateral offset along the southern caribbean plate boundary that can be documented along the bocono moron el pilar fault system has not exceeded 100 125 km if there has been larger offset it has to be documented by other means or it has taken place along other fault zones within the system

role of a ductile decollement in the development of pull apart basins experimental results and natural examples abstract traditional models describe pull apart basins as graben or half graben basins with normal or normal oblique slip master faults analogous to death valley california yet many pull aparts are characterized by asymmetric basins with strike slip master faults indicating that not all pull apart basins conform to the simple death valley models we present analogue modelling results that show developmental sequences and structural styles of pull aparts are dramatically different when overburden rides over a ductile horizon and that thickness of the ductile horizon exerts control on basin development in our models synthetic and antithetic strike slip faults control basin geometries while localized normal faulting and local oblique slip on strike slip faults accommodate basin subsidence faults evolve from initial strike slip to normal oblique and normal dip slip to form a system of either isolated sub basins in the case of thick ductile layers or coalescing sub basins in the case of thin ductile layers these results demonstrate distinct differences between non ductile and ductile decollement pull apart structures basin boundaries dominated by normal faults suggest a decollement within or at the base of a non ductile layer similar to death valley california basin bounding faults dominated by strike slip and oblique slip faults indicate basin formation over a ductile layer similar to the gulf of elat aqaba or gulf of paria venezuela and trinidad

intracontinental subduction a possible mechanism for the early palaeozoic orogen of se china the early palaeozoic orogen of se china consists of three lithotectonic elements from top to bottom a sedimentary upper unit a metamorphic lower unit and a gneissic basement the boundaries between these units are flat lying south directed ductile decollements the lower one is coeval with an amphibolite facies metamorphism m1 the belt is reworked by migmatite granite domes high temperature metamorphism m2 and granitic plutons related to post orogenic crustal melting we date here the syn m1 ductile shearing at 453 7 ma by u th pb method on monazite previous ages and our new 40 ar 39 ar ages of biotites and muscovites show that the metamorphic rocks experienced syn m2 exhumation from 440 to 400 ma the early palaeozoic orogen of se china is an intracontinental belt in which decollements accommodated the north directed subduction of the cathaysian continent this orogen is an example of intracontinental subduction that was not preceded by oceanic subduction

zircon u th pb hf isotopes of the basement rocks in northeastern cathaysia block south china implications for phanerozoic multiple metamorphic reworking of a paleoproterozoic terrane abstract the cathaysia block records a complex tectonic history the understanding of which is central to the debate on the evolution of the south china block and its position in various supercontinent assemblies here we investigate two key formations from this block the mayuan group in the northern fujian province northeastern cathaysia block and its equivalent the badu group in southwestern zhejiang province previous studies traced the paleoproterozoic records from the badu group whereas a neoproterozoic age was proposed for the mayuan group the rocks sampled in this study from both groups show similar mineral assemblages of garnet sillimanite biotite plagioclase quartz k feldspar muscovite graphite as well as high contents of sio 2 and al 2 o 3 typical of amphibolite to granulite facies metapelitic rocks zircon u pb data yield two discordia intercept ages of 1990 ma and 2450 ma from one sample and discordia intercept ages of 3 5 ga 2 5 ga 1 86 ga and 233 ma from another in the badu group zircons in two samples from the mayuan group yield intercept ages of 1859 ma and 249 ma in one sample and 2 6 ga 1 87 ga 257 ma and a weighted mean 206 pb 238 u age of 248 ma in the other the ca 1 86 1 87 ga and 230 250 ma ages are interpreted to represent the time of metamorphic reworking because zircon grains of these ages tend to have low th u ratios flat hree patterns and unzoned internal texture as revealed by cathodoluminescene cl images these results confirm that the badu group is a paleoproterozoic lithostratigraphic unit and also suggest that at least part if not all of the mayuan group is paleoproterozoic evidence for paleozoic metamorphic reworking that is considered to have affected the whole of cathaysia block is not revealed in this study in contrast our data clearly show obvious mesozoic metamorphic reworking at ca 230 250 ma zircon hf t values range from 19 to 11 with a peak at 5 5 and show t dm c hf ranging from 1 9 to 4 1 ga with a peak at ca 2 7 3 0 ga suggesting that a major crustal growth took place during this time this interpretation is consistent with the previously suggested crustal growth peaks of 2 7 ga and 2 9ga a synthesis of the reliable geochronological data gathered so far on phanerozoic metamorphic reworking of the northeastern cathaysia block reveals that the imprints of these tectonothermal events is differently distributed in the different zones rocks metamorphosed during the paleozoic tectonothermal event dominantly occur in the western zone whereas those reworked by high grade metamorphism during the mesozoic tectonothermal event mainly outcrop in the eastern zone our study alerts the previous notion of a uniform distribution of the reworked rocks by high grade metamorphism all across the northeastern cathaysia block and provides new insights on the evolution of the south china block

structural interpretation of hydrocarbon traps sealed by basement normal block faults at stable flank of foredeep basins and at rift basins closures sealed by basement normal block faults are most commonly formed by a grid of faults with multiple orientations and by a favorable dip of the reservoir strata hydrocarbons are trapped updip by structural upturns along the edge of the downdropped blocks or at the upthrown or downthrown edge of blocks where block tilt is away from the sealing faults closure parallel with reservoir strike is provided by 1 updip junction of primary oblique or oblique and longitudinal block bounding faults 2 internal secondary faults that trend transverse or oblique to the block boundaries 3 positive bend of reservoir strata 4 plunge of block edge folds and or 5 stratigraphic terminations of the reservoir fault seal upward along the faults is most likely where a thick section of impervious strata above the reservoir is in contact with itself across the fault or where the fault does not extend upward into shallower permeable zones seal across the fault surface is most certain where impermeable rocks are laterally juxtaposed against the reservoir impervious material within the fault zone such as shaly smear gouge cataclastic gouge mineral deposits and asphalt or tar impregnations can be effective barriers where reservoir facies are juxtaposed across the fault the traps can form a major hydrocarbon play on the stable shelves of foredeep or foreland basins as demonstrated by the san joaquin valley and eastern venezuela basins the traps provide secondary prospects in rift basins uch as the viking graben and vienna basin

cathodoluminescence studies and u pb dating of zircons in pre mesozoic gneisses of the tauern window implications for the penninic basement evolution the pre mesozoic basement in the eastern and central part of the tauern window eastern alps consists mainly of anatectic plagioclase gneisses and amphibolites intruded and migmatized by carboniferous granitoids the gneisses are geochemically similar to calcalkaline dacites of modern primitive island arcs cathodoluminescence investigations and u pb data on zircons indicate an uppermost proterozoic or cambrian extrusion age of the dacitic protoliths and a carboniferous syngranitoid anatexis around 320 ma high k calcalkaline meta rhyolites in the north central tauern window form a separate part of the basement not affected by carboniferous anatexis and plutonism their protoliths extruded in the lower carboniferous in the permian high k granitoids intruded the anatectic basement

formation and tectonic evolution of southeastern china and taiwan isotopic and geochemical constraints abstract the southern part of china consists of the proterozoic yangtze craton and the phanerozoic south china foldbelts including the himalayan foldbelt of taiwan models for continental growth have been many and controversial isotopic and geochemical data from mesozoic and younger granitoids and sediments are used here to place constraints on the tectonic evolution of southeastern china and to evaluate whether the young phanerozoic foldbelts are representative of old rejuvenated landmass whether they characterize crustal accretion through successive subduction processes hence suggesting a net growth of continental mass or whether they represent some intermediate situation available sm nd isotopic data for phanerozoic granitoids and metasediments from the south china foldbelts and taiwan invariably show proterozoic model ages t dm ranging from 1 to 2 5 ga with a mean of 1 54 0 30 1 ga all rocks have negative nd t values 2 to 15 suggesting variable but important contributions of old continental materials to the sources of the phanerozoic rocks obviously they do not represent wholesale growth of continental mass the granitoids of the south china foldbelts have multiple origins as viewed from their chemical and isotopic characteristics secular geochemical variation has been established for inland granitoids of proterozoic to mesozoic age their high i sr values 0 710 0 737 suggest that most granitoids are essentially anatectic products of ancient continental crust this is compatible with remelting via microcontinental collision or terrane accretion repeated intracrustal reprocessing by partial fusion and differentiation has undoutedly contributed to important mineralization mesozoic granitoids excluding a type alkaline granites in the coastal region and taiwan show relatively low i sr 0 705 0 710 and high nd values implying that greater amounts of mantle components have been added to ancient continental material in the generation of these rocks this means that mesozoic magmatism in the coastal region resulted from subduction of a paleo pacific plate kula beneath the asian continent the overall geochemical evidence seems to be consistent with a model of serial collisions between microcontinental blocks for the bulk of the south china foldbelts the latest arc continent collision began about 4 ma ago in eastern taiwan

post orogenic extension in the eastern part of the jiangnan orogen evidence from ca 800 760ma volcanic rocks we present a systematic geochronological and geochemical study on ca 800 760 ma volcanic rocks in the eastern part of the jiangnan orogen the xucun composite dykes are dated at ca 805 ma the mafic components have oib like trace element patterns and positive anomalies in zr and hf the least contaminated sample has relatively depleted nd isotopic features suggesting the xucun mafic dykes may have been generated from the partial melting of oib like asthenosphere with later crustal contamination the xucun felsic dykes have decoupled nd hf isotopes and the hf isotope compositions of zircons indicate that the dykes may be derived from the partial melting of the early neoproterozoic juvenile crustal materials with minor incorporation of paleoproterozoic crustal components the ca 800 790 ma shangshu volcanics include two compositional series calc alkaline and tholeiitic the shangshu calc alkaline volcanics in the minjiawu area have low abundances of lile hfse and high na2o contents and sr y ratios similar to adakitic rocks the evident arc like geochemical features and radiogenic nd isotopes nd t values of 3 7 to 4 8 suggest that these rocks may have been generated from the partial melting of juvenile lithospheric mantle metasomatized by na rich melts released from the subducted slab the tholeiitic mafic rocks from the shangshu bimodal volcanics represent two different magma sources the partial melting of metasomatized lithospheric mantle led to the formation of arc like basalts with low tio2 contents negative anomalies in zr and hf and high values of mg and nd t 6 2 whereas the partial melting of asthenospheric mantle generated volcanic rocks with high tio2 contents and low positive nd t 1 4 to 2 7 without negative anomalies of nb ta zr and hf the shangshu felsic rocks were formed by the reworking of early neoproterozoic juvenile arc crustal materials the ca 760 ma mafic rocks from the puling bimodal volcanics generally have low tio2 contents 0 9 wt nearly flat ree distributions and arc like trace element patterns they may have been generated from the high degree partial melting of metasomatized lithospheric mantle one sample has a high tio2 content 2 41 wt and high nd t 6 2 with overall oib like trace element patterns implying the local partial melting of asthenospheric mantle the occurrence of significant volumes of bimodal volcanics in the eastern part of the jiangnan orogen suggests an extensional setting in the period ca 800 760 ma the evident partial melting of newly metasomatized lithospheric mantle and subordinate partial melting of asthenosphere suggest that post orogenic extension shortly after the neoproterozoic orogenesis may be a better explanation for the genesis of the mid neoproterozoic magmatic rocks in the eastern part of the jiangnan orogen post orogenic extension may be diachronous along the whole orogenic belt and probably has no direct relationship with the rodinia rifting event a more detailed model is presented to illustrate the evolution of the eastern part of the jiangnan orogen

archaean to proterozoic crustal evolution in the central zone of the limpopo belt south africa botswana constraints from combined u pb and lu hf isotope analyses of zircon a combined set of u pb and lu hf in situ laser ablation icp mc ms zircon analyses were obtained from orthogneisses and granitoids in the central zone of the limpopo belt which comprises the beit bridge and mahalapye complexes the results indicate that by combining the two isotope systems primary magmatic zircon domains can be distinguished from those formed during later metamorphic events even if the distinct zircon domains underwent multiple pb loss and the texture age relationships as obtained by cathodoluminescence images and u pb analyses are ambiguous furthermore the applied technique allows distinction of zircon grains formed in juvenile magmas from those generated by melting of older continental crust or affected by substantial crustal contamination the combined u pb and lu hf data reveal that the sand river gneiss suite of the beit bridge complex was emplaced at 3283 8ma and formed from melting of an older archaean crust which was derived from a depleted mantle source at around 3 65 ga the hafnium model age tdm is significantly older than those obtained from zircons from numerous neoarchaean granitoids of the beit bridge complex comprising the singelele gneiss 2647 12ma the bulai granite 2612 7ma the regina gneiss 2649 9ma and two samples of the zanzibar gneiss 2613 6ma these granitoids show initial ehf t values between 0 5 and 7 1 which correspond to initialtdm between 3 46 and 3 01ga these variable tdminitial and ehf t initial values are interpreted to be the result of different mixtures of reworked 3 65 ga palaeoarchaean crust with juvenile magmas extracted from the depleted mantle during the neoarchaean at 2 65 ga this conclusion is supported by results obtained from the mahalapye complex which was affected by migmatization and granite intrusions during the palaeoproterozoic at 2 02 2 06 ga the mokgware granite 2019 9ma contains zircon xenocrysts with pb pb ages of 2 52 2 65 ga and 2 93 ga and hafnium model ages of 3 0 3 4 ga indicating that this granite is derived from remelting of archaean crust in contrast uniformtdm hf initial ages of 2 61 2 67 ga obtained from a diorite gneiss 2061 6ma of the mahalapye complex indicate that its protolith may have been formed from remelting of a neoarchaean juvenile crust variable ehf t initial values from 3 7 to 6 3 of zircon cores 2711 11ma in an adjacent leucosome also support a model of mixing of juvenile mantle derived matter with older crust in the neoarchaean

two new geobarometers for garnet amphibolites with applications to southeastern vermont two new geobarometers for the assemblage garnet hornblende plagioclase quartz have been calibrated on the basis of the equilibrium 6 anorthite 3 tremolite 2 grossular 1 pyrope 3 tschermakite 6 quartz and its fe end member equivalent data representing 37 natural samples that equilibrated at conditions of 2 5 to 13 kbar and 500 to 800 c were fitted to the general equation rt in keq a bt p i c by using a weighted least squares method multiple correlation coefficients are high 0 98 and 0 97 lines of constant keqhave extremely shallow slopes in p t space 2 to 8 bars per degree suggesting that pressures may be deduced precisely even where temperatures are only poorly constrained typical analytical errors and temperature imprecisions propagate to very small errors in pressure about 500 bars caution is advised in applying the barometers outside the range of calibrant mineral compositions application of the calibrations to samples from southeastern vermont near the strafford chester and athens domes documents relatively high pressure metamorphism 7 to 10 kbar for these structures

ca 825 ma komatiitic basalts in south china first evidence for 1500 c mantle melts by a rodinian mantle plume mantle plume or superplume activities have repeatedly been invoked as a cause for the breakup of the neoproterozoic supercontinent rodinia with supportive evidence including radiating dike swarms globally synchronous anorogenic igneous activity large scale lithospheric doming and unroofing and geochemical signatures similar to recent plume related magmatism however high temperature magmas such as picrite or komatiite a requisite product of mantle plume activities have not previously been identified for those events we present here geochronological and geochemical data from a suite of pillow lavas in central south china sensitive high resolution ion microprobe shrimp u pb dating of zircons from an evolved member of andesitic composition within the suite indicates that the lavas were erupted at 823 6 ma all but a few highly evolved crust contaminated basaltic rocks are characteristically high in mgo 10 2 17 5 ni 183 661 ppm and cr 677 1672 ppm but low in tio2 0 5 0 7 al2o3 10 6 12 7 and feot total fe as feo 7 4 10 5 typical of komatiitic basalts our geochemical modeling which removes the effect of olivine crystallization suggests that their primary magma has typical komatiitic compositions with mgo 20 feot 11 sio2 47 tio2 0 48 al2o3 10 ni 860 ppm and cr 1780 ppm such a high mgo content in the primary melts implies a melt temperature of 1500 c suggesting that the yiyang komatiitic basalts were generated by melting of an anomalously hot mantle source with potential temperature t p 260 50 c higher than the ambient mid oceanic ridge basalt morb source mantle similar to that of modern mantle plumes the yiyang komatiitic basalts are thus the first solid petrological evidence for the proposed ca 825 ma mantle plume that played a key role in the breakup of the supercontinent rodinia

distribution of fe2 and mg between coexisting garnet and hornblende in synthetic and natural systems an empirical calibration of the garnet hornblende fe mg geothermometer abstract multiple regression analysis of a compilation of the fe2 mg distribution between garnet and hornblende from experimental runs on basaltic to intermediate compositions n 22 and coexisting garnet clinopyroxene hornblende from natural intermediate to basaltic rocks n 43 has been performed to define ln kd fe2 mg grt hbl as a function of temperature and garnet composition the regression of data covering a large span in pressure 5 16 kbar temperature 515 1025 c and composition yields the ln kd fe2 mg grt hbl p t compositional relationship r2 0 93 t c 1504 1784 x ca grt x mn grt ln k d fe 2 mg grt hbl 0 720 273 where k d fe 2 mg grt hbl fe 2 mg grt fe 2 mg m 1 m 3 hbl x ca grt ca ca mn fe 2 mg in garnet x mn grt mn ca mn fe 2 mg in garnet application of this expression to natural garnet hornblende pairs in intermediate to basaltic and semipelitic rock types from various settings gives temperatures that are consistent with other methods

metagabbros of the gangdese arc root south tibet implications for the growth of continental crust abstract the gangdese batholith of the southern tibet is regarded as a principal component of the pre himalayan collision andean type continental margin however the gangdese arc root remains poorly understood because most of previous studies concentrated on granitoids from the middle and upper crust of the arc in this paper we report new petrochemical and geochronological characteristics of metagabbros occurred as a part of the late cretaceous lilong batholith in the eastern gangdese arc the gabbros with protolith ages of 82 95 ma underwent slightly late 68 77 ma granulite facies metamorphism and nearly coeval anatexis typical of lower arc crusts the metagabbros and diorites of the lilong batholith show geochemical features of juvenile crust in continental magmatic arcs and an original magmatic differentiation trend with gabbros formed as the cumulates whereas diorites as the crystalline products of evolved magmas the lilong batholith forms an arc crustal section with a possible thickness of 20 30 km and may have an overall andesitic composition providing a support for the andesite model of continental crust growth

emplacement and metamorphism of the mafic rocks from the chencai terrane within the cathaysia block implications for the paleozoic orogenesis of the south china block abstract understanding the early paleozoic tectonic evolution of the south china block scb is critical in constraining its relationship with super continent configurations such as the rodinia and gondwana high grade mafic and supracrustal rocks occurring in the chencai terrane of the cathaysia block are the uplifted lower crustal components of the kwangsian early paleozoic orogen which extensively reshaped many parts of the scb two amphibolites three biotite bearing amphibolites one norite and one mafic granulite were collected from the chencai terrane in previous studies these rocks or their protoliths have been suggested to belong to the same cumulate series and the mafic granulite records metamorphic p t conditions of a high geothermal gradient environment 30 km our sims zircon u pb results indicate that protoliths of these meta mafic rocks were emplaced at 434 445 ma and most of them have experienced subsequent almost synchronous metamorphism at 428 439 ma the norite sample 434 ma which experienced the least crustal contamination and the least metamorphic alteration has the lowest zircon 18o values of 6 7 v smow whereas the mafic granulite which experienced the highest grade metamorphism has the highest zircon 18o values of 9 4 v smow zircon 18o values of other samples mostly scatter between them most metamorphic zircons have younger u pb ages and higher 18o values than magmatic zircons high 18o crustal materials e g sedimentary rocks have possibly been involved during the metamorphism which resulted in the contrasting 18o values of magmatic and metamorphic zircons the large variations in 18o values shown by magmatic zircons of each sample like the amphibolite and biotite bearing amphibolite indicate that the high 18o value crustal materials sedimentary rocks must have started to affect these mafic rocks since the magmatic stage zircon 176hf 177hf values of each of these samples scatter within small ranges initial hf t values of most zircons fall in between the chur line and the dm linear model line indicating that these mafic rocks were derived from an enriched mantle source combined with previous geochronological metamorphic and geochemical data we propose that these paleozoic mafic magmatism might have contributed thermal energy to the later thermal relaxation of the orogeny this episode of mafic magmatism may represent a transition of the kwangsian orogen from early compression to post orogenic extension metamorphism of these mafic rocks and the synchronous anatexis indicated by the high grade supracrustal rocks from chencai terrane might be related with extension rather than orogenic compression

structural architecture of the central apennines interpretation of the crop 11 seismic profile from the adriatic coast to the orographic divide 1 we present an interpretation of the eastern half portion of the crop 11 line a deep reflection seismic profile 265 km long that cuts across the central apennines from the tyrrhenian coast to the adriatic coast in the study area the line cuts across a pile of thrust sheets that underwent tectonic transport between the messinian and the pleistocene in its easternmost part the line runs through the plio pleistocene deposits of the adriatic foredeep in the foreland region the crop 11 line integrates previous information on the crustal structure derived from petroleum exploration and from deep seismic sounding refraction experiments in particular the crop 11 line confirms the existence of a very thick sedimentary sequence underlying the mesozoic tertiary carbonates of the apulia platform interpreted as the paleozoic triassic sedimentary cover of a pre cambrian crystalline basement in the mountain chain where the base thrust of the orogenic wedge reaches a depth of about 25 km this sedimentary sequence appears to be the deepest geological unit incorporated in the thrust system this interpretation of the crop 11 profile suggests an unusual thin skin tectonic style implying the detachment from the original basement and the incorporation in the post tortonian tectonic wedge of a very thick paleozoic triassic sedimentary sequence possibly affected by low grade metamorphism in the lower part other new suggestions from the crop 11 seismic data concern the origin of the fucino basin one of the most remarkable plio pleistocene intramontane basins the normal faults bordering this structural depression as other important normal faults present in the central apennines e g the caramanico fault system in the majella region seem to have been controlled by gravitational collapse processes driven by uplift during crustal shortening rather than by a generalized extension subsequent to the apennine compression as usually reported in the geological literature if this interpretation is correct the strong seismic activity in correspondence to the apennine watershed may be related to the very recent increase in the structural relief caused by an out of sequence propagation of the active thrusts

intracontinental deformation in a frontier of super convergence a perspective on the tectonic milieu of the south china block abstract since mesozoic the south china region has been located at the center of a triangular area surrounded by westward subduction of the pacific plate northward subduction of the india plate beneath the eurasia plate and collision of the north and south china blocks along the central china orogen this region thus marks the frontier of a super convergent regime within the super convergence domain the compressional structures in the center of the south china block are mainly characterized by shortening thrusting and decollement the block underwent inhomogeneous rejuvenation of the pre existing crust and lithospheric structures through reactivation of late stage activities the surface deformation of the xuefeng intracontinental tectonic system within the south china block is possibly derived from intraplate tectonics particularly the distinct magmatism between the west and east limbs of the xuefeng precambrian uplift is a possible response of the rheological structure of the lithosphere the deep structures as revealed from tomographic studies show marked difference of lithosphere architecture between the eastern and western sub blocks of the south china block these data illustrate the long term mosaic of multi block convergence which led to present day inhomogeneity in the continental lithosphere in south china a comparison of the intensive contraction of the south china block with the distinct features of rifting of the north china block brings out the contrasting structural and tectonic signatures developed in the same frontier of one of the largest super convergent systems on the globe during the mesozoic to cenozoic

partial melting and the formation of granulite facies assemblages in namaqualand south africa abstract dehydration melting reactions in which water from a hydrous phase enters the melt leaving an anhydrous solid assemblage are the dominant mechanism of partial melting of high grade rocks in the absence of externally derived vapour equilibria involving melt and solid phases are effective buffers of ah2 o the element partitioning observed in natural rocks suggests that dehydration melting occurs over a temperature interval during which for most cases ah2o is driven to lower values the mass balance of dehydration melting in typical biotite gneiss and metapelite shows that the proportion of melt in the product assemblage at t 850 c is relatively small 10 20 and probably insufficient to mobilize a partially melted rock body granulite facies metapelite biotite gneiss and metabasic gneiss in namaqualand contain coarse grained discordant unfoliated anhydrous segregations surrounded by a finer grained foliated matrix that commonly includes hydrous minerals the segregations have modes consistent with the hypothesis that they are the solid and liquid products of the dehydration melting reactions bt sil qtz pi grt crd kfs l metapelite bt qtz pl opx kfs l biotite gneiss and hbl qtz opx cpx pl l metabasic gneiss the size shape distribution and modes of segregations suggest only limited migration and extraction of melt growth of anhydrous poikiloblasts in matrix regions development of anhydrous haloes around segregations and formation of dehydrated margins on metabasic layers enclosed in migmatitic metapelites all imply local gradients in water activity also they suggest that individual segregations and bodies of partially melted rock acted as sinks for soluble volatiles the preservation of anhydrous assemblages and the restricted distribution of late hydrous minerals suggest that retrograde reaction between hydrous melt and solids did not occur and that h2o in the melt was released as vapour on crystallization this model combined with the natural observations suggests that it is possible to form granulite facies assemblages without participation of external fluid and without major extraction of silicate melt

anatomy of zircon growth in high pressure granulites sims u pb geochronology and lu hf isotopes from the jiaobei terrane eastern north china craton abstract granulite facies rocks that witnessed high pressure and high temperature metamorphism preserve important records of the growth and recrystallizaton of zircons during the multiple stages of tectonic evolution of orogenic belts developed in complex subduction collision settings here we investigate zircons in the paleoproterozoic high pressure hp granulites from jiaobei terrane in the eastern north china craton that experienced a multi stage metamorphic evolution we present sims zircon u pb ages for zircons from garnet amphibolites mafic granulites and pelitic granulites our data show four age peaks at 2 5 ga 1 92 ga 1 87 1 85 ga and 1 83 ga zircons belonging to the latter three age groups show similar characteristics of low th and u contents low th u ratios 176 lu 177 hf ratios and low 176 hf 177 hf ratios of zircons from the protoliths the youngest u pb age of ca 1899 ma is interpreted as the maximum estimate of the time of peak metamorphism zircons formed through intergranular melt growth during low to medium pressure retrogression display homogeneous lu hf isotope compositions low 176 lu 177 hf and high 176 hf 177 hf ratios the u pb age of 1870 1850 ma yielded by zircons crystallized from intergranular melt is interpreted to represent the time of low to medium pressure granulite facies retrogression some zircon grains with prismatic morphology and low 176 lu 177 hf and 176 hf 177 hf ratios display heterogeneous lu hf isotopic compositions possibly related to coupled dissolution reprecipitation the u pb ages of these zircons are identical to those formed from intergranular melt and these two groups of zircons formed during the same metamorphic stage zircons formed through subsolidus growth show large variations in lu hf isotopic compositions and their ca 1833 ma age represents the timing of amphibolite facies retrogression the hp granulite facies metamorphism in jiaobei is thus constrained between 1870 and 1899 ma with low to medium pressure granulite facies retrogression at ca 1850 1870 ma followed by amphibolite facies retrogression at ca 1833 ma our data show that several zircon cores and recrystallized grains were entrained from basement rocks during the emplacement of the mafic magmatic protoliths in the case of mafic granulites for the pelitic granulites the older zircons are detrital grains of magmatic origin the hf crustal model ages of these zircons that show peaks at ca 2 7 ga and 2 9 ga are consistent with the timing of mafic magmatism in the region as identified in previous studies the lu hf isotopes of newly grown metamorphic zircons which have equilibrated chemical compositions trace the nature of the protolith the zircon hf crustal model age of ca 2 5 2 7 ga from the pelitic granulites is consistent with that of the basement rocks in the jiaobei terrane and the zircon hf model ages t dm of ca 2 0 ga and 2 2 2 3 ga from the mafic granulites might represent the time of magma generation from the mantle with crustal contamination the geochronology of the basement rocks as revealed by the old zircon cores and the metamorphic evolution of the jiaobei terrane are comparable with the tectonic history of the central and western north china craton and provide insights into the complex tectonothermal events associated with the neoarchean paleoproterozoic crustal evolution history of the craton also the pervasive 2 5 ga zircons in a variety of rocks seem to suggest that the jiaobei terrane was already part of a coherent tectonic unit at such time

u pb geochronology and hf nd isotopic geochemistry of the badu complex southeastern china implications for the precambrian crustal evolution and paleogeography of the cathaysia block abstract the oldest rocks of the cathaysia block south china comprise the badu complex and paleoproterozoic granites in the wuyishan area southern zhejiang and northwestern fujian provinces new zircon u pb ages hf isotopes and trace elements for metamorphic rocks from the badu complex and bulk nd isotope compositions of these rocks and granites in the wuyishan area provide important constraints on the precambrian crustal evolution of the cathaysia block inherited cores of zircon grains from the metamorphic rocks are of magmatic origin predominantly formed at ca 2 5 ga while overgrowth rims reflect two episodes of granulite facies metamorphism related to collisional orogeny at 1 89 1 88 ga and 252 234 ma the unimodal age distribution 2 5 ga of detrital zircons and the positive hf t of most neoarchean zircons suggest that the detritus of these sedimentary protoliths of the badu complex came from a proximal volcanic arc and that they were deposited in an arc basin synchronously with 2 5 ga volcanism zircon u pb ages and hf isotopes with whole rock nd isotopes suggest that the juvenile crust of the eastern cathaysia block was generated mainly at 2 5 ga and 2 8 ga and minor at 3 5 3 3 ga with some evidence for the generation at 3 7 3 6 ga and 4 0 ga a strong 1 9 ga orogeny and the 3 3 3 0 ga thermal event only involved the reworking of older crust material paleoproterozoic 1 89 1 86 ga granitic magmatic activity and high grade metamorphism in the eastern cathaysia block were synchronous with the assembly of the columbia supercontinent using integrated geochronological hf nd isotopic and petrologic data as a barcode we compare the cathaysia block with other paleoproterozoic orogens worldwide and argue that its closest affinity is with the south korean peninsula and the lesser himalaya of nw india consequently the eastern cathaysia block and the south korean massif as the united cathay south korea terrane was close to the lesser himalaya terrane in the paleoproterozoic configuration of the columbia supercontinent the spatial linkage was maintained for ca 1 ga until the fragmentation of the rodinia supercontinent during neoproterozoic time

neoproterozoic bimodal magmatism in the cathaysia block of south china and its tectonic significance shrimp u pb zircon age geochemical and sm nd isotopic results are reported for the mamianshan volcanic rocks in the cathaysia block of southeastern south china the mamianshan volcanic rocks are bimodal in composition and are dominantly transitional to mildly alkaline basalts and subordinate alkaline rhyolite with an eruption age of 818 9 ma the basaltic samples are characterized by lree enriched and humped trace element patterns similar to many alkali basalts in continental rifts variablend t values between 3 33 and 4 35 indicate that the primary magma of these basalts was derived from an oib like mantle source and underwent fractional crystallization plus crustal contamination the rhyolitic rocks are highly enriched in th ta nb ree zr hf and y and depleted in sr p eu and ti sharing affinity to a 1 type granites combined with their slightly positive nd t values 0 22 to 0 92 the mamianshan felsic rocks were most likely generated by partial melting of the regional paleoproterozoic mayuan amphibolites the mamianshan bimodal volcanic rocks in the cathaysia block are coeval with the widespread intraplate magmatism around the yangtze block our results support the idea that a coherent south china craton was formed during the ca 1 0 ga sibao orogeny and it subsequently underwent extensive continental rifting related to mantle plume or superplume activities beneath rodinia since ca 825 ma 2004 elsevier b v all rights reserved

challenges in high pressure granulite metamorphism in the era of pseudosections reaction textures compositional zoning and tectonic interpretation with examples from the bohemian massif high pressure granulites are an important record of geodynamic processes in overthickened or subducted continental crust orthopyroxene free assemblages in granitic ternary feldspar s quartz garnet kyanite rutile intermediate ternary feldspar s quartz garnet clinopyroxene kyanite rutile titanite and basic garnet clinopyroxene plagioclase quartz rutile compositions indicate formation conditions at mantle depths clinopyroxene compositions in variscan high pressure granulites are unusual in that they include omphacite in plagioclase bearing rocks thus not eclogite and al rich diopside i e indicating high ca tschermak content with both yielding temperatures above 900 c problems such as compositional zoning multiple generations of key phases in reaction domains and unmixing of high temperature solid solution phases during cooling ternary feldspars omphacite clearly indicate disequilibrium and require very careful interpretation as to which phases and compositions possibly represent a former equilibrium association pressure temperature p t determination by the pseudosection method although allowing prediction of mineral assemblages compositions and molar proportions for a fixed bulk composition for modelled p t conditions still requires reliable activity composition information for the key phases feldspar and clinopyroxene as well as an interpretation of former equilibrium compositions in the investigated samples i e the same restrictions applying to conventional thermobarometry the interpretations of some recently determined pseudosections for the composition of variscan clinopyroxene bearing high pressure granulites contradict numerous published p t paths however quantitative information from thermobarometry or pseudosections must be integrated with key petrographic observations in the case of the variscan example it is argued that petrographic observations and published p t paths are consistent with mineral assemblages predicted in pseudosections and support existing tectonometamorphic models

high pressure granulites formation recovery of peak conditions and implications for tectonics high pressure granulites are characterised by the key associations garnet clinopyroxene plagioclase quartz in basic rocks and kyanite k feldspar metapelites and felsic rocks and are typically orthopyroxene free in both basic and felsic bulk compositions in regional metamorphic areas two essential varieties exist a high to ultrahigh temperature group and a group representing overprinted eclogites the high to ultrahigh temperature type formerly contained high temperature ternary feldspar now mesoperthite coexisting with kyanite is associated with garnet peridotites and formed at conditions above 900 c and 1 5 gpa clinopyroxene in subordinate basic rocks is al rich and textural evidence points to a high pressure high temperature melting history the second variety contains symplectite like or poikilitic clinopyroxene plagioclase intergrowths indicating former plagioclase free i e eclogite facies assemblages this type of rock formed at conditions straddling the high pressure amphibolite high pressure granulite field at around 700 850 c 1 0 1 4 gpa importantly in the majority of high pressure granulites orthopyroxene is secondary and is a product of reactions at pressures lower than the peak recorded pressure in contrast to low and medium pressure granulites which form at conditions attainable in the mid to lower levels of normal continental crust high pressure granulites of nonxenolith origin mostly represent rocks formed as a result of short lived tectonic events that led to crustal thickening or subduction of the crust into the mantle short times at high temperature conditions are reflected in the preservation of prograde zoning in garnet and pyroxene high pressure granulites of both regional types although rare are known from both old and young metamorphic terranes e g c 45 ma namche barwa e himalaya 400 340 ma european variscides 1 8 ga hengshan china 1 9 ga snowbird saskatchewan and 2 5 ga jianping china this spread of ages supports proposals suggesting that thermal and tectonic processes in the lithosphere have not changed significantly since at least the end of the archean

major lithologies of the high grade zhoutan terrane within the cathaysia block and their tectonic implications for the neoproterozoic paleozoic south china abstract pre devonian crystalline basement of the cathaysia block was united through the wuyi yunkai orogeny the uplifted orogenic core is dominantly composed of paleozoic high grade terranes occurring in a northeast southwest trending belt the zhoutan terrane in the middle part of this belt is one of the key terranes to reveal the tectonic architecture of the south china block however how to correlate it with other high grade terranes what kind of rock records are preserved in this terrane and their tectonic implications for the wuyi yunkai orogen are still unresolved this paper carries out an integrated study on the petrology zircon u pb geochronology geochemistry and zircon lu hf isotopic compositions of the main rock types in the zhoutan terrane with a purpose of better understanding the wuyi yunkai orogeny mafic granulite noritic of high t and low p conditions occurs as lenses within the zhoutan formation the zircon u pb geochronological data give magmatic and metamorphic ages of 460 450 ma and 430 ma respectively neoproterozoic and paleoproterozoic 1 89 ga xenocrysts also occur meta pelites of the zhoutan formation experienced high t p amphibolite facies metamorphism and ages of their detrital zircons are dominantly neoproterozoic 810 820 ma with some subordinate paleo to mesoproterozoic and archean signatures amphibole gneiss layers of this formation give a paleozoic metamorphic age of 450 ma consistent with the high p amphibolite facies metamorphism of the neighboring longyou terrane fine grained garnet mica schist of the wanyuan formation consists of greenschist to amphibolite facies mineral assemblage and detrital zircons of these meta sediments display both neoproterozoic and early precambrian ages at 770 ma 820 ma 850 ma 1 84 ga and 2 48 ga metamorphosed calc silicates of the wanyuan formation show similar detrital zircon age peaks of 624 ma 780 790 ma 810 ma 840 850 ma 1 8 1 98 ga and 2 48 ga and a paleozoic metamorphic age of 447 ma the cizhu pluton is a composite granitoid body intruding the zhoutan and wanyuan formations with emplacement ages ranging at least from 485 ma to 446 ma ehf t values of zircons from the mafic granulite range from 10 9 to 8 5 indicating its protolith derived from enriched mantle sources with or without crustal contaminations the cizhu magmatic event is a crustal reworking event as indicated by the coherently negative zircon ehf t values and ancient hf crustal model ages of 1530 1830 ma zircon ehf t values of the metamorphosed calc silicates show large variations ranging from 21 2 to 11 consistent with its complex provenances the deposition of the sedimentary rocks of the zhoutan and wanyuan formations is consistent with the development of the neoproterozoic paleozoic nanhua rift the development of continental margin sequences like the zhoutan and wanyuan formations the evolution of the paleozoic metamorphism from early higher p t metamorphic stage to later lower p t stage as well as the generation of the paleozoic mafic ultramafic rocks and the intermediate cizhu pluton are best explained by collision to post collision settings we therefore conclude that the zhuotan terrane as well as other high grade terranes witnessed a whole geological cycle from early rifting to the closure of a rifted basin nanhua basin and the final lithospheric delamination following the rejoining of continental blocks flanking the rift

development of asymmetric basins along continental transform faults abstract many basins associated with wrench or transform faults are asymmetrical both longitudinally and laterally the sense of basin asymmetry changes along the strike of transform faults geophysical data from the dead sea rift and other transforms indicate that wrench induced asymmetric basins are bounded on only one side by a transform fault this implies that during the evolution of such basins maximum compression was parallel to the transform the direction of extension can change during the geologic evolution of a basin a possible explanation of this observation takes the strength of the fault into into consideration in the case of a weak fault in a strong crust the horizontal principal stresses would probably rotate to orientations parallel and perpendicular to the fault thus minimizing the shear stress on the main fault basins formed by regional extension such as in the east african rift system sometimes show similar features recent studies have shown that these basins often undergo oblique extension this suggests that the asymmetry in these basins may result from simultaneous strike slip motion and fault normal extension similar to the way asymmetry is produced in large transform basins periods of oblique extension can alternate with periods of orthogonal extension the asymmetry formed during periods of oblique extension is often preserved in the stratigraphic record

intracrustal detachment within zones of continental deformation geologic mapping of active to recently active geologic structures in panamint valley western united states and in the haiyuan region and northern tibetan plateau china suggests detachment in the middle and lower crust on a scale of tens of kilometres to at least several hundred kilometres detachment occurs similarly in predominantly extensional panamint valley and in predominantly compressional china environments it involves structures with displacements of more than 10 km and displacement rates of more than 3 mm yr perhaps more than 10 mm yr the steeply dipping strike slip faults present in all three areas hunter mountain fault haiyuan fault and altyn tagh fault zone terminate in zones of extension or compression and geometric relations indicate that all structures including strike slip faults are thin skinned and restricted to the upper crust in panamint valley and in the haiyuan region deformation within these systems can be reconstructed in three dimensions displacement on the strike slip faults is absorbed by extension or compression occurring at the termination of the faults so strike slip displacement is roughly equal to and in the same direction as shortening or extension we propose that left slip on the altyn tagh fault zone in northern tibet is similarly absorbed by shortening southeast of the fault zone within the qaidam basin and the nan shan

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

recalibration of the gasp geobarometer in light of recent garnet and plagioclase activity models and versions of the garnet biotite geothermometer the garnet al silicate plagioclase gasp geobarometer has been recalibrated using four recent garnet activity models four analogous garnet biotite temperature models and two recent plagioclase activity models a typical sillimanite bearing sample that formed at about 5 25 kbar 575 c shows a possible p range of 0 7 kbar due to t error 1 kbar due to range of garnet activity model 0 9 kbar due to range of plagioclase activity model and 5 4 kbar due to range of experimental end member reversals extended by one sigma calibrations were further constrained with the kyanite sillimanite k s phase boundary such that the best fit of 76 pelitic schist samples from 11 localities provides an individual end member calibration for each of the eight possible combinations of garnet and plagioclase activity models with the appropriate geothermometer samples with low grossular or anorthite component were rejected the end member calibrations are constrained to pass through the the best determined portion of the gasp experimental reversals at 1230 c 26 6 kbar these individual end member calibrations provide self consistent models that tend to compensate for error in the garnet and plagioclase activity expressions the models were also tested on a set of 59 samples from the alps the recommended calibration is the average garnet activity model and average garnet biotite t model of holdaway 2000 the fuhrman and lindsley 1988 plagioclase activity model and h grs 6628521 s grs 258 76 to combine with the remaining phases in the berman database to produce the optimum end member gasp curve these thermodynamic data are for the gasp geobarometer only error is about 0 8 kbar absolute and about 0 6 kbar relative geological error is the largest component of error in many of these samples care should be taken to be sure that analyzed plagioclase and biotite are near analyzed garnet that the peak t portions of garnet and plagioclase are selected that the peak t al silicate is determined and that the t calculated is the most accurate possible these calibrations represent an improvement over previous published gasp calibrations these eight models are available for distribution as three programs t p p t intersection for the dos based personal computer

p t t evolution of orogenic belts and the causes of regional metamorphism barrow 1893 introduced three important ideas that furthered understanding of metamorphic processes i the use of critical index minerals in argillaceous rocks to define metamorphic zones and elucidate spatial features of regional metamorphism ii the concept of progressive metamorphism and iii the concept of magmatic advection of heat as a possible cause of regional metamorphism this article expands upon these themes by reviewing our understanding of the dynamic evolution of orogenic belts as interpreted from the p t t paths of metamorphic rocks and by considering the likely causes of the different kinds of regional metamorphism that we observe within orogenic belts understanding metamorphic rocks allows the distinction of two fundamentally different types of orogenic belt defined by relative timing of maximum t and maximum p orogenic belts characterized by clockwise p t paths achieved maximum p before maximum t the metamorphic peak normally post dated early deformation within the belt and additional heating above the normal conductive flux has been related to the amount of overthickening by contrast orogenic belts characterized by counterclockwise p t paths achieved maximum t before maximum p the metamorphic peak normally pre dated or was synchronous with early deformation within the belt and additional heating above the normal conductive flux has been related to the emplacement of plutons techniques used to constrain portions of p t t paths include the use of mineral inclusion suites in porphyroblasts and reaction textures thermobarometry the use of fluid inclusions thermodynamic approaches such as the gibbs method radiogenic isotope dating fission track studies and numerical modelling we can utilize specific mineral parageneses in suitable rocks to determine individual p t t paths and a set of p t t paths from one orogenic belt allows us to interpret the spatial variation in dynamic evolution of the metamorphism recent advances are reviewed with reference to collision metamorphism high temperature low pressure metamorphism granulite metamorphism and subduction zone metamorphism and some important directions for future work are indicated

internally consistent recalibrations of mineral equilibria for geothermobarometry involving garnet orthopyroxene plagioclase quartz assemblages and their application to the south indian granulites abstract three reactions are calibrated as geothermobarometers for garnet orthopyroxene plagioclase quartz assemblages namely 1 2 ferrosilite 1 3 pyrope 1 2 enstatite 1 3 almandine a ferrosilite anorthite 2 3 almandine 1 3 grossularite quartz b and enstatite anorthite 2 3 pyrope 1 3 grossularite quartz c the internally consistent geothermobarometers based on reactions a b and c are calibrated from experimental data only the thermodynamic parameters of reaction a are derived from published experimental data in the fmas system n 104 in the range 700 1400 c and 5 50 kbar while those for reaction b are derived by summation of the existing reversed experimental data of the mineral equilibria ferrosilite fayalite quartz d and anorthite fayalite 2 3 almandine 1 3 grossularite e the retrieved thermodynamic parameters for reactions a b and c are respectively h0 cal 3367 209 2749 350 and 3985 545 s0 cal k 1 1 634 0 163 8 644 0 298 and 5 376 0 391 and v01 298 cal bar 1 0 024 0 60946 and 0 5614 on a one cation basis the derived margules parameters of the ternary ca fe mg in garnet are wfe mg 1256 1 0 0 23 t k wmg fe 2880 1 7 0 13 t k wca mg 4047 77 1 5 t k wmg ca 1000 77 1 5 t k wca fe 723 0 332 0 02 t k wfe ca 1090 cal and the ternary constant c123 4498 1 516 0 265 t k cal subregular solution model of non ideal mixing and fe mg al in orthopyroxene wfe mg 948 200 0 34 0 10 t k wfe al 1950 500 and wmg al 0 cal regular solution model of non ideal mixing the anorthite activity in plagioclase is calculated by the al avoidance model of subregular ca na mixing commonly used for geobarometry based on reactions b and c when the geothermobarometers are applied to garnet orthopyroxene plagioclase quartz assemblages n 45 of wide compositional range from the precambrian south indian granulites temperature ranges of 690 860 c x 760 45 c and pressure ranges of 5 10 kbar were obtained the p t values were estimated simultaneously and there is no difference in the pressure calculated from pmg reaction c and pfe reaction b in the existing calibrations this difference is 1 kbar or more furthermore there is no compositional dependence of the ln k of the experimental data in the fmas n 104 and the cfmas n 78 systems at different temperatures and the estimated temperatures of the south indian granulites