inherited smnd isotope components preserved in monazite inclusions within garnets in leucogneiss from east antarctica and implications for closure temperature studies abstract application of the sm nd dating technique using garnet can be hampered by the presence of minor microscopic to submicroscopic ree rich inclusions in particular monazite and to a lesser extent zircon an acid leaching method using a mixture of hno 3 hcl has been used to tackle the monazite contamination problem with this leaching technique we demonstrate that inherited monazite inclusions have been preserved in garnet formed under high grade metamorphic conditions even very low levels of monazite contamination 0 001 wt have a profound effect on the sm nd system in the garnet analysis of a garnet from a leucogneiss dyke in the northern prince charles mountains eastern antarctica reveals that unleached garnet concentrates and the monazite inclusions as represented by the leachates define an apparent isochron of 1150 30 ma mswd 1 88 which represents a mixing age of inclusions preserving their inherited sm nd isotopic systematics in contrast analyses of the leached garnet concentrates yield much younger ages of 630 to 555 ma model calculations show that the presence of as little as 0 01 wt of monazite inclusions would swamp the sm nd isotope signature of the garnet concentrates this very low level of contamination is beyond the reach of physical purification processes and without leaching contamination is inevitable in inclusion bearing garnets such contamination can lead to older apparent ages than those of garnet formation and thus has profound implications for sm nd as well as u pb garnet chronometry

franciscan subduction off to a slow start evidence from high precision lu hf garnet ages on high grade blocks lu hf analyses of garnet from metabasic amphibolite glaucophane schist and eclogite facies blocks from the franciscan complex give highly precise ages that allow us to place new constraints on the early thermal history of the franciscan subduction zone garnets yield 176 lu 177 hf ratios ranging from 1 5 to 28 with the highest ratios from garnets with high spessartine pyrope ratio sulphuric acid leaching sal of garnets revealed the presence of inclusions with significantly higher lu hf ratios than those of garnet itself most likely apatite their removal by sal brings the 176 lu 177 hf ratios in garnets down by as much as 40 this suggests that 176 lu 177 hf ratios of apparently pure garnets can be greatly overestimated due to the presence of such inclusions sm nd garnet analyses were dominated by inclusions mainly sphene and failed to provide precise and accurate age information the oldest lu hf ages are 168 7f0 8 and 162 5f0 5 ma on plagioclase bearing garnet amphibolite from panoche pass and the berkeley hills respectively which suggests initiation of the subduction zone at about 169 ma coeval with the formation of the tectonically overlying coast range ophiolite relatively high temperature conditions persisted for about 14 ma as indicated by 153 4f0 8 ma garnet growth recorded in epidote amphibolite and 157 9f0 7 in eclogite from ring mountain and jenner respectively a 146 7f0 7 ma age was obtained from garnet glacuophane schist metamorphosed at around 400 jc the sequence of ages from central and northern california shows a younging trend with decreasing metamorphic grade which supports previous suggestions that the high grade metamorphic blocks and slices resulted from progressive underthrusting and underplating in a cooling subduction system combining geothermometry with geochronological data allow us to estimate cooling rate along the subduction zone interface from amphibolite to blueschist facies conditions as ca 15 jc ma the thermal history requires high initial geothermal gradients within both the footwall and the hangingwall of the subduction zone and a relatively slow subduction rate of the order of 10 km ma during the initial stages of franciscan subduction such conditions are consistent with initiation of the subduction zone at or close to an oceanic spreading centre the data also suggest slow exhumation rates and significant residence time at depth of the earliest franciscan rocks

diffusion closure temperature and age of a mineral with arbitrary extent of diffusion theoretical formulation and applications the commonly used expression of the closure temperature tc of a diffusing species in a mineral as derived by m h dodson contrib mineral petrol 40 1973 259 264 is applicable only to systems which have undergone sufficient diffusion so that even the composition at the center of individual grains is significantly removed from that established at the onset of cooling we have extended dodson s formulation to include cases with arbitrarily small amount of diffusion and applied it to calculate tc and age profiles which would develop in single crystals of different geometries these results permit evaluation of the extent of resetting of mineral age and of ion exchange geothermometers during cooling the measured age profile of a mineral can also be used to constrain its cooling rate these applications have been illustrated for the cases of garnet biotite fe mg exchange geothermometer and the cooling age and closure temperatures of the sm nd and lu hf decay systems in garnet

intraplate extension prior to continental extrusion along the ailao shan red river shear zone left lateral movement of the ailao shan red river shear zone lends support to the hypothesis of continental extrusion resulting from the collision of india with asia our new observations from northwestern yunnan china and northwestern vietnam on different sides of the shear zone demonstrate that the sinistral offset was 600 km according to correlations of permian triassic flood basalt successions and late paleogene highly potassic mafic magmas we conclude that the shear was propagating on the south china continental margin and does not correspond to a suture between south china and indochina furthermore the highly potassic magmas were emplaced from ca 40 to 30 ma before the shear movement which was caused by the late oligocene to early miocene ca 27 22 ma extrusion activity this suggests that a late eocene to early oligocene intraplate extension possibly induced by delamination of thickened continental lithosphere took place in northwestern yunnan or eastern tibet as a response to the india asia collision this extension and sea floor spreading of the south china sea that began ca 30 ma could have accounted for the initiation of the ailao shan red river shear zone

delineating alteration footprints from field and aster swir spectra geochemistry and gamma ray spectrometry above regolith covered base metal deposits an example from abra western australia

population wide garnet growth zoning revealed by la icp ms mapping implications for trace element equilibration and syn kinematic deformation during crystallisation the strong partitioning of many trace elements into garnet and their slow diffusivities in both garnet and the rock matrix means that their distribution may record valuable petrogenetic information not documented by major elements in metamorphic rocks complex trace element growth zoning in garnet porphyroblasts from a garnet grade metapelite from the barrovian sequence of the sikkim himalaya is assessed using quantified la icp ms raster mapping coupled with x ray micro computed tomography the data document systematic changes in the zoning patterns from early to late nucleated crystals and also suggest that the ree y chemistry incorporated into garnet is dependent on persistent disequilibrium in the rock volume there is evidence for hree y diffusion haloes surrounding growing garnets and a heterogeneous hree y distribution in the rock matrix annuli superimposed on oscillatory zoning are not consistent with formation during some rock wide event but are dependent on the spatial disposition of the garnet annuli are interpreted to reflect an integrated history of varying growth rates and the incorporation of pre existing heterogeneities due to relatively slow matrix diffusivities conversely smooth zoning of many transition metals indicate that their distribution in garnet may be controlled by equilibrium partitioning between garnet and the rock matrix significant rotation of garnet porphyroblasts during growth is revealed due to immobility of cr over the duration of the crystallisation interval and overprinting of the heterogenous precursor cr distribution strain rate estimates derived from this zoning are on the order of 10 11 10 12 hbox s 1

evaluating chemical equilibrium in metamorphic rocks using major element and sm nd isotopic age zoning in garnet townshend dam vermont usa abstract a quantitative assessment of metamorphic chemical equilibrium derived from correlation of spessartine content and garnet sm nd ages suggests that major element matrix equilibrium was maintained to a first order throughout a ca 40 cm wide rock sample during garnet growth however cm scale sm nd isotopic heterogeneity limits the sm nd age precision required to evaluate more subtle age differences within individual garnet crystals central wafers from 1 3 cm diameter garnet grains within a 1 21 104 cm3 block of pelitic schist were used to document concentric growth zoning of major elements with decreasing mn and ca and increasing fe and mg from cores to rims garnets also preserve growth zoning patterns for hree and mree and show evidence for resorption and partial recrystallization of the outermost rims similar garnet core compositions and identical garnet rim compositions for large like sized porphyroblasts throughout the sample suggest that garnet growth occurred at near equilibrium p t x conditions for major elements over the sample volume comparison of 28 rock sm nd isotope values from the sample indicates substantial cm scale heterogeneity which precludes meaningful use of local garnet rock isotope pairs for isochron age calculation therefore sm nd isotopic compositions of thirty eight concentric core to rim garnet segments from ten large 1 3 cm garnets and two small 1 4 mm bulk garnets with narrow ranges of mn content are paired with sixteen matrix whole rock sm nd isotopic compositions collected over the rock volume to define a range of isochron ages from 383 1 6 8 ma to 324 5 3 3 ma four of the garnets have anomalously young rims that likely result from post growth alteration chlorite quartz and xenotime haloes around garnet suggest that anomalously young garnet rim ages reflect post growth resorption recrystallization effects excluding these young rims yields a range of ages from 383 1 6 8 oldest core to 374 9 1 8 ma youngest rim sm nd age precisions 1 5 m y and high mswd result primarily from isotopic heterogeneity in the finely layered metasedimentary rock matrix however garnet cores with high mn n 7 mantles with intermediate mn n 14 and rims with low mn n 8 including the 2 smaller bulk garnet analyses define three distinct multi grain isochrons of 380 3 2 0 ma n 23 mswd 14 377 3 1 4 ma n 30 mswd 18 and 376 5 1 0 ma n 24 mswd 18 respectively yielding an average garnet growth duration of 3 8 2 2 m y these three composite mn age zones define a mn vs age relationship that reflects depletion of mn in the rock matrix as it is sequestered by growing garnet correlation of garnet major element compositions throughout the sample suggests that major element matrix equilibrium was generally maintained to a first order throughout the ca 4 m y duration of garnet growth

direct dating of left lateral deformation along the red river shear zone china and vietnam 1 exposures of high grade midcrustal rocks within the red river shear zone rrsz which separates the indochina and south china blocks exhibit clear evidence of left lateral ductile deformation assuming that the south china sea represents a pull apart basin formed at the southeastern termination of the rrsz it has been argued that seafloor magnetic anomalies constrain the timing of sinistral slip accommodated by the rrsz between 32 and 17 ma at a rate of 4 cm yr while 40ar 39ar thermochronometry indicates that left lateral slip occurred along the rrsz between 25 and 17 ma the timing of earlier high temperature deformation has not been directly constrained in situ th pb ion microprobe dating of monazite inclusions in garnets allows direct assessment of the timing of amphibolite grade metamorphism and synchronous left lateral shearing results from northern segments of the rrsz in yunnan china indicate that synkinematic garnet growth occurred between 34 and 21 ma and are the first to document late oligocene metamorphism and left lateral shearing data from the southern rrsz within vietnam are complicated by tertiary overprinting of rocks that experienced amphibolite facies metamorphism during the indosinian orogeny 220 ma the period during which sinistral deformation is now constrained to have occurred along the rrsz i e 34 17 ma is essentially coincident with spreading of the south china seafloor 32 17 ma this temporal and kinematic link between left lateral shearing along the rrsz and opening of the south china sea supports the view that indochina was extruded from asia as a block along lithospheric scale strike slip faults

new constraints on the structure thermochronology and timing of the ailao shan red river shear zone se asia new structural petrographic and 40ar 39ar data constrain the kinematics of the asrr ailao shan red river shear zone in the xuelong shan xls geochronological data reveal triassic early tertiary and oligo miocene thermal events the latter event 33 26 ma corresponds to cooling during left lateral shear in the fansipan fsp range thrusting of the sapa nappe linked to left lateral deformation and cooling of the fsp granite occurred at 35 ma rapid cooling resumed at 25 29 ma as a result of uplift within the transtensive asrr in the daynuiconvoi dncv foliation trends nw se but is deflected near large scale shear planes stretching lineation is nearly horizontal on steep foliations shear criteria indicate left lateral shear sense zones with flatter foliations show compatible shear senses petrographic data indicate decompression from 6 5 kbar during left lateral shear temperatures 700 c 40ar 39ar data imply rapid cooling from above 350 c to below 150 c between 25 and 22 ma without diachronism along strike along the whole asrr cooling histories show two main episodes 1 rapid cooling from peak metamorphism during left lateral shear 2 rapid cooling from greenschist conditions during right lateral reactivation of the asrr in the nw part of the asrr xls diancang shan we link rapid cooling 1 to local denudations in a transpressive environment in the sw part ailao shan and dncv cooling 1 resulted from regional denudation by zipper like tectonics in a transtensive regime the induced cooling diachronism observed in the ailao shan suggests left lateral rates of 4 to 5 cm yr from 27 ma until 17 ma dncv rocks always stayed in a transtensive regime and do not show cooling diachronism the similarities of deformation kinematics along the asrr and in the south china sea confirms the causal link between continental strike slip faulting and marginal basin opening

ar ar and fission track ages in the song chay massif early triassic and cenozoic tectonics in northern vietnam the song chay massif is the northeasternmost metamorphic complex in vietnam to the east of the red river shear zone it shows a large antiformal structure involving orthogneisses and migmatites overlain on its northern flank by muscovite bearing marbles an e w striking fault bounds the dome to the south kinematic indicators along a s n section reveal top to the n shear sense along the interface between the orthogneissic core and the overlying metasediments radiometric ages were obtained by the 40ar 39ar method using purified mica separates across the dome ages range from 236 ma at the southern edge to 160 ma in the core attesting to a strong imprint in the early triassic time a clear difference is seen between these mesozoic ages and the eocene to miocene ages from 40 to 24 ma that obtained in the nearby red river shear zone using the same method these data show that the song chay massif was already high in the crust when the high temperature deformation of the red river shear zone took place the final exhumation of the song chay orthogneiss constrained by fission track analysis on samples along the same transect occurred during the early miocene and could be interpreted as the consequence of a first normal sense of motion along the fault which bounds the massif to the south timing is similar to that of exhumation in the red river shear zone

trace element mapping by la icp ms assessing geochemical mobility in garnet a persistent problem in the study of garnet geochemistry is that the consideration of major elements alone excludes a wealth of information preserved by trace elements particularly the rare earth elements rees this is despite the fact that trace elements are generally less vulnerable to diffusive resetting and are sensitive to a broader spectrum of geochemical interactions involving the entire mineral assemblage including the growth and or dissolution of accessory minerals we outline a technique for the routine acquisition of high resolution 2d trace element maps by la icp ms and introduce an extension of the software package xmaptools for rapid processing of la icp ms data to visualise and interpret compositional zoning patterns these methods form the basis for investigating the mechanisms controlling geochemical mobility in garnet which are argued to be largely dependent on the interplay between element fractionation mineral reactions and partitioning and the length scales of intergranular transport samples from the peaked hill shear zone reynolds range central australia exhibit contrasting trace element distributions that can be linked to a detailed sequence of growth and dissolution events trace element mapping is thus employed to place garnet evolution in a specific paragenetic context and derive absolute age information by integration with existing u pb monazite and sm nd garnet geochronology ultimately the remarkable preservation of original growth zoning and its subtle modification by subsequent re equilibration is used to see through multiple superimposed events thereby revealing a previously obscure petrological and temporal record of metamorphism metasomatism and deformation

yttrium zoning in metamorphic garnets abstract yttrium zoning has been measured in garnets from the straits schist connecticut usa using the synchrotron x ray fluorescence sxrf microprobe sxrf compositional maps for mn and fe show that the major elements are concentrically zoned about the gannet core in the classic bell shaped profiles considered indicative of growth zoning yttrium however displays oscillatory zoning which varies three orders of magnitude in abundance from about 3 to 4000 ppm and is correlated with textural zoning in garnet the major element abundances by contrast display no visible inflection related to textural zoning at this time it is not possible to unequivocally identify the mechanism which has produced this oscillatory zoning most likely this reflects either the breakdown of y enriched phases changes in the rate of garnet growth with time or changes in the effective y concentration of matrix phases due to breakdown and reprecipitation of garnet as the equilibrium reactions involving gannet change this study demonstrates that the sxrf microprobe provides excellent spatially resolved compositional maps for trace elements in porphyroblasts something other analytical techniques have been unable to do it also suggests that the diffusion rates for y in garnet are probably significantly slower than those for major cations such as mn fe mg and ca as a result trace element mapping potentially provides a clearer record of the reaction history of the rock provides details of how chemical zoning in garnet is produced and allows evaluation of assumptions about chemical equilibrium between minerals

coupling of p t t d histories of eclogite and metagreywacke insights to late ordovician silurian crustal folding events recorded in the beishan orogen nw china

hydrothermal plumes at the rodriguez triple junction indian ridge water column anomalies of light transmission mn fe al and ch4 concentrations were searched in the central southeastern and southwestern indian ridge segments centered on the rodriguez triple junction rtj 25 32 s 70 02 e for the purpose of locating hydrothermally active areas in july to august 1993 we found an active zone in the central indian ridge segment 25 18 20 s approximately 12 miles north of the rtj where significant hydrothermal plumes were observed at 2 200 2 400 m depth intensive tow yo observations using a ctd rosette multi sampling system equipped with a transmissometer revealed that the plumes show temporal as well as spatial variations discrete water samples within the plumes were enriched in mn fe and ch4 with maximum concentrations of 9 8 nm 40 2 nm and 3 3 nm respectively judging from the spatial and chemical characteristics of the plumes especially from transmission anomalies andc h4mn ratios we speculate that the hydrothermal venting site might be not in the rift valley but on the eastern off axis zone several miles distant from the rift valley

timing of prograde metamorphism in the zanskar himalaya the timing of the prograde metamorphism of continental crust buried during the collision of india and eurasia is of importance to our understanding of the mechanics of mountain building and crustal metamorphism our knowledge of the himalaya indicates that continental collision at 55 50 ma culminated in crustal melting and leucogranitic magmatism at 24 18 ma however the timing of events in the intervening 30 m y is poorly understood here we present sm nd isotope analyses of garnets from the zanskar himalaya that also preserve information on crustal and orogenic evolution during that period the prograde p t t paths indicate that heating and burial from 6 to 10 kbar continued until at least 25 30 ma and imply peak temperatures 700 c close to those necessary to cause the crustal melting in the himalaya at ca 20 ma without a requirement for additional heat sources such as shear heating on major crustal shear zones the persistence of high temperatures as late as 20 ma in the himalaya may be related to postcollisional thrusting south of the suture zone and within the indian plate at 40 35 ma

age and origin of magmatism along the cenozoic red river shear belt china to decipher the geodynamic significance of cenozoic magmatism along the red river shear belt geochemical analyses u pb and rb sr dating and pb sr nd isotope tracing were undertaken zircon monazite titanite and a ti u oxide from foliated granitoid intrusions in the shear belt gneisses yield u pb emplacement ages of 33 1 0 2 2 31 9 0 3 25 8 0 2 and 24 7 0 2 ma and an age of 35 0 0 3 ma was obtained for the roughly 100 km long adjacent jinping phan si pang alkali granite together with our previous data the new ages suggest that magmatism and left lateral strike slip movements occurred coevally during latest eocene oligocene times from 33 to 22 ma the rb sr dating of muscovite and biotite from the northernmost gneisses indicates that cooling to 500 c occurred at 52 6 1 1 ma pre dating the onset of magmatism whereas further cooling to 300 c took place at 28 9 0 6 this shows that unroofing in the north took place almost 9 million years earlier than in the central gneiss segments of the shear zone geochemical data substantiate two types of magmas 1 amphibole bearing intrusions of alkaline trend which are derived from sources with isr 0 7065 0 7089 and i nd 3 7 to 6 6 2 leucogranitic layers and bodies having isr 0 7084 0 7354 and i nd 3 3 to 13 4 the former type of intrusion is found in both the gneisses and the adjacent unmetamorphosed cover rocks whereas leucogranites are restricted to the shear belt gneisses source signatures of the alkaline intrusions lie adjacent to the those of oib plotting at the lower end of the mantle array contamination of these melts by continental material seems to be very limited on the other hand the leucogranitic layers are essentially crustal derived but none of the them has country rock isotope signatures requiring melting of crust different from the actually exposed gneisses magma sources similar to those of ocean island basalt indicate magmatism to involve melting of light rare earth element and large ion lithophile element enriched mantle domains most likely present in the lithosphere underneath the region since lithospheric thickening or subduction can be ruled out to produce both types of magmas the presence of an important thermal anomaly is required which is coevally active with left lateral strike slip shear adiabatic decompression and melting within the rising anomaly is the most plausible mechanism to produce the mantle magmas which successively migrate through the crust to induce anatectic melting at 20 15 km crustal depth alkaline magmas largely dominate the volume of magmatism along the belt being continuously present in the shear zone for millions of years such lubrication potentially explains how very large amounts of displacement can be absorbed in surprisingly narrow shear zones such as the red river belt possibly also playing a role for where and when zones of plate scale lateral extrusion develop

intraplate tectonics in asia a precise age for large scale miocene movement along the ailao shan red river shear zone china tertiary left lateral movement along the 1000 km long ailao shan red river shear zone appears to have played an important role in absorbing post collisional northward penetration of india into asia crustal strike slip shear along this zone caused the formation of a gneiss belt metamorphosed to amphibolite grade including anatectic melting both metamorphism and melting were induced by ductile deformation yielding the possibility to date the major tectonometamorphic event that shaped the ailao shan red river belt 17 u pb isotope analyses were performed on small size fractions of zircon monazite and xenotime extracted from two different leucogranitic layers the two samples are located about 50 km apart in the central segment of ailao shan in structurally well controlled settings where the melts crystallized within the strongly foliated gneisses during late stages of deformation all mineral u pb analyses had to be corrected for excess or deficit amounts of radiogenic 206 pb originating from initial 230 th disequilibrium in the 238 u decay series in monazite such disequilibrium 206 pb reaches 20 of total radiogenic 206 pb the corrected u pb ages of monazite and xenotime lie between 22 1 and 23 9 ma whereas zircon yields significantly discordant u pb ages between 30 5 and 33 9 ma pointing to precambrian material in the magma source region inherited components could also be detected in monazite the set of u pb data shows that monazite and xenotime formed simultaneously in both localities substantiating an early miocene age of 23 0 0 2ma for late kinematic crystallization of anatectic melts in the metamorphic belt of the ailao shan red river shear zone

onset timing of left lateral movement along the ailao shan red river shear zone 40ar 39ar dating constraint from the nam dinh area northeastern vietnam left lateral motion along the ailao shan red river asrr shear zone has been widely advocated to be the result of the collision between the indian and eurasian plates and to account for sea floor spreading in the south china sea our new 40 ar 39 ar data on the south easternmost outcrop of the day nui con voi metamorphic massif northern vietnam suggest that the exhumation of metamorphic massif by shearing along the asrr zone began 027 ma and lasted until 022 ma a perfect correlation between location and cooling path for the samples along the shear zone suggests that the transtensional deformation may have propagated northwestward at a rate of 0 6c m y 1 such a good correlation also indicates that the onset of the leftlateral movement of the shear zone may have occurred later than 027 5 ma this conclusion is consistent with our previous interpretation that collision induced southeastward extrusion of indochina along the asrr shear zone postdates the opening of the south china sea and that extrusion tectonics in se china may not be responsible for the opening of the south china sea 2000 elsevier science ltd all rights reserved

duration of strike slip movements in large shear zones the red river belt china abstract sixty four size fractions of zircon monazite xenotime and titanite were dated by the u pb method to temporally and spatially constrain left lateral movements in the red river shear zone between the depths of 15 and 20 km the shearing led to the formation of the high grade metamorphic gneisses that form the bulk of the 4200 m high diancang shan and 3100 m high ailao shan mountain ranges the omnipresence of melt layers and pockets in these gneisses shows that temperatures associated with tectonometamorphism lie at and above the solidus of continental crust and deformation patterns are very homogeneous over the entire length of the belt north of the gneisses subvolcanic bodies intrude unmetamorphosed cover rocks in particular eocene red beds along the projected trend of the red river shear zone for a leucogranitic layer in the diancang shan gneisses an age of 24 2 0 2 ma 2 is obtained for monazite and 22 4 0 2 ma is obtained for xenotime whereas zircon formed through two growth stages at 24 23 ma these ages record solidification of the anatectic melts over a period of at least 1 8 0 4 ma during which left lateral shear was continuously active in the ailao shan gneisses zircon and titanite ages are 26 3 0 3 and 26 1 0 2 ma for two monzonitic bodies and 24 1 0 2 and 22 4 0 2 ma for two pegmatitic layers that occur within the monzonitic intrusions these results show that zircon monazite titanite and probably also xenotime are closed systems for u and pb under melting conditions 650 c therefore the ages date mineral formation in the melt rather than cooling below certain closure temperatures a significantly older age of 35 0 0 1 ma is obtained for a monzonitic subvolcanic intrusion north of the diancang shan gneisses because these intrusions follow the trend of the shear zone they most likely represent the surface expression of early activity along that zone indicating that strike slip movement had already occurred in latest eocene time the total set of u pb ages shows that magmatic and metamorphic activity occurred from 35 to 20 ma strengthening the idea that the red river shear zone absorbed most of the intraplate movements required to compensate the opening of the south china sea between 32 and 16 ma the existence of monzonitic syenitic intrusions along the red river zone implies that melting affected source rocks other than the diancang shan and ailao shan gneiss protoliths potential sources to produce such rocks lie in the deep crust possibly including melts from the mantle consequently the red river shear zone must be a deep rooted structure along which melts migrated to the surface moreover the contemporaneity of shear motion and magma generation suggests that melting occurred in direct relation to strike slip activity

the origin of mn and y annuli in garnet and the thermal dependence of p in garnet and y in apatite in calc pelite and pelite gagnon terrane western labrador x ray compositional maps and zoning profiles for major and trace elements have been measured in garnet apatite and epidote group minerals from calc pelitic and pelitic schists from western labrador by electron microprobe and by laser ablation icp ms high y abundance in apatite adjacent to resorbed garnet and a decreasing modal abundance of apatite with garnet growth indicate that apatite participates in major rock forming mineral reactions phosphorus concentration in garnet porphyroblasts coexisting with apatite decreases smoothly from core to rim and depends moderately on metamorphic grade apatite coexisting with xenotime shows high y cores and the concentration of y in apatite increases with metamorphic grade many of the analyzed garnet porphyroblasts exhibit y annuli which provide information about different physicochemical processes operating during garnet growth we describe criteria to fingerprint three of these processes in the analyzed garnets 1 yttrium annuli produced by garnet resorption and regrowth are characterized by an asymmetric shape with a steeper slope on the inner side of the annulus and by a decoupled variation between garnet compatible and garnet incompatible elements at the resorption margin 2 y annuli produced by breakdown of y and ree rich trace phases such as xenotime monazite apatite and epidote group minerals are indicated by ree peaks in lree monazite allanite mree epidote or hree xenotime zircon 3 y annuli associated with diffusion controlled growth occur in garnet with oscillatory textural zoning defined by alternating inclusion rich and inclusion poor zones with increases in concentrations of garnet compatible elements such as y and hree correlating with inclusion free zones mn annuli in some garnets were produced by sporadic local breakdown of mn rich epidote and in several cases indicate that thin section scale equilibrium during annulus formation was not achieved raising questions concerning the use of mn concentration as a time marker in garnet growth modeling

dynamic recrystallization of garnet and related diffusion processes abstract electron backscatter diffraction and compositional data of permian metapegmatite garnet from the koralpe basement eastern alps austria show the evolution of distinct intracrystalline shear zones in c 10 mm sized almandine spessartine garnet orientation maps reveal continuous distortion of the crystal lattice new garnet grains grain size 20 30 m within intragranular deformation zones oriented subparallel to the main foliation of the metapegmatite have similar crystal orientations to the host garnet whereas recrystallized grains along fractures or along growth heterogeneities show a much weaker crystallographic preferred orientation cpo needles of kyanite as well as inclusions of apatite xenotime and rutile occur along the deformation zones together with the ca variation across the deformation zones these inclusions give evidence of material diffusion related to crystal plastic deformation of garnet the individual garnet porphyroclasts record the evolution of recrystallization microfabrics new grains once formed by subgrain rotation recrystallization are able to deform partly by diffusion accommodated grain boundary sliding resulting in a weakening of the cpo whereas magmatic garnet growth had taken place at lp conditions in equilibrium with andalusite the occurrence of kyanite in some deformation zones triggered by the localized crystal plasticity of garnet indicates that the deformation was related to the cretaceous hp metamorphic event

timing duration and inversion of prograde barrovian metamorphism constrained by high resolution lu hf garnet dating a case study from the sikkim himalaya ne india abstract we investigated lu hf isotopic systematics in garnets from gradually higher grade metamorphic rocks from the first appearance of garnet at c 500 c to biotite dehydration melting at c 800 c in the sikkim himalaya india exceptionally precise lu hf ages obtained for the barrovian metasedimentary sequence in the lesser himalaya lh correspond to the time of early garnet formation on a prograde path and show remarkable correlation with increasing metamorphic grade and decreasing structural depth the youngest age is reported for the garnet zone 10 6 0 2 ma and then the ages become progressively older in the staurolite 12 8 0 3 ma kyanite 13 7 0 2 ma and sillimanite 14 6 0 1 ma zones the oldest age of 16 8 0 1 ma was recorded at the top of the sequence in the zone marking the onset of muscovite dehydration melting directly below the main central thrust mct these ages provide a tight constraint on the timing and duration of the barrovian sequence formation which lasted about 6 ma the age pattern is clearly inverted with respect to structural depth but shows normal correlation with the metamorphic grade i e earlier garnet growth in higher grade rocks the timing of the peak metamorphic conditions estimated for the garnet and kyanite zones suggests that thermal climax occurred in all zones within a relatively short time span about 10 13 ma ago the metamorphic inversion in the sikkim himalaya must therefore have occurred after the metamorphic peak was attained in all grades lu hf garnet ages in the overthrust higher himalaya hh continue to be older but do not show a clear progression there is about 6 ma jump to 22 6 0 1 ma within the mct zone and even older ages of 26 28 ma were obtained for migmatites from the lower part of the hh the lu hf system in garnets from the hh records the time of melting at or near the thermal peak rather than dates initial garnet growth as in the lh the age pattern obtained in this study provides precise time constraints on the continental collision models which must take into account early and prolonged anatexis in the hh progressive delay in initial garnet growth in different barrow zones in the lh nearly contemporaneous metamorphic peak in all grades and post peak inversion of the coherent lh block during exhumation

comparison of laser ablation icp ms and isotope dilution ree analyses implications for sm nd garnet geochronology abstract high precision garnet chronometry using the sm nd u pb rb sr or lu hf isotope systems is gaining widespread application for constraining the prograde and high temperature history of metamorphic belts however due to the low concentrations of the chronometrically important elements garnet analyses are often susceptible to incorporation of small quantities of accessory phases such as zircon monazite and allanite here we assess the extent to which such inclusions affect garnet analyses by comparing in situ analyses of sm and nd concentrations in the crystal lattice obtained by laser ablation inductively coupled mass spectrometry la icp ms with those obtained by isotope dilution id on separates of the same garnets the results show that the garnets studied here are largely free from inclusions in general however the low concentrations of sm and nd in the garnet lattice mean that the incorporation of even very small quantities of accessory phases will result in a significant change in the measured garnet sm and nd concentration simple modelling of the effect of such inclusions on age information shows that garnet inclusion disequilibrium and the extent to which inclusions decrease the apparent parent daughter ratio are critical such modelling demonstrates that the incorporation of old lree enriched minerals such as monazite will yield an apparently lower age than the true garnet age however this study indicates that erroneous age information can be identified if duplicate garnet analyses are performed in addition la icp ms is shown to be a useful technique in verifying the purity of samples for isotope analysis

closure of the east paleotethyan ocean and amalgamation of the eastern cimmerian and southeast asia continental fragments abstract the phanerozoic record of southeast asia preserves the history of opening and subsequent consumption of the paleotethyan ocean and the assembly of continental fragments into asia as part of the broader scale reconstruction of pangea however uncertainty remains as to which of the many suture zones in southeast asia represents the relict of the main ocean when final ocean closure occurred and the assembly history of the eastern cimmerian and southeast asia continental fragments our overview of the geological features of the suture zones the bounding continental fragments and their magmatic metamorphic and sedimentary records resolves many of these key issues the sedimentary biogeographical structural lithological geochemical and geochronological data from the changning menglian inthanon and bentong raub suture zones argue for their linkage with the longmu co shuanghu suture zone in central tibet and together constitute the main east paleotethyan ocean relict the eastward subduction of the ocean resulted in the development of a series of magmatic arc back arc basin and continental fragments in southeast asia including from west to east the lincang sukhothai east malaya arc the jinghong nan sa kaeo back arc basin the simao west indochina fragment the luang prabang loei back arc basin the south indochina fragment the wusu and truong son back arc basins the north indochina fragment the jinshajiang ailaoshan song ma branch back arc basin and the south china block assembly of these fragments resulted in indosinian high temperature and high pressure metamorphism and related tectonothermal event available data indicate a switch from subduction of the main east paleotethyan ocean to the collision of the sibumasu with simao indochina blocks at 237 ma with subsequent syn and post collisional events at 237 230 ma and 230 200 ma respectively along the changning menglian inthanon and bentong raub suture zones the timing of initial syn and post collision events along the jinshajiang ailaoshan song ma suture zone with its record of back arc basin closure is at 247 ma 247 237 ma and 237 200 ma generally 10 ma older than that along the changning menglian inthanon and bentong raub suture zones our synthesis of all available data enables establishment of a comprehensive geodynamic model for the east paleotethyan evolution this model links the spatial temporal pattern across southeast asia into a series of tectonic events including ocean back arc basin opening subduction closure subsequent assemblage and orogenic collapse along with associated igneous metamorphic and sedimentary activities

paleo to mesoarchean polymetamorphism in the barberton granite greenstone belt south africa constraints from u pb monazite and lu hf garnet geochronology on the tectonic processes that shaped the belt the barberton granite greenstone belt bggb of south africa is an exceptionally well preserved meso paleoarchean metamorphic supracrustal belt one of only a few in the world studies of metamorphism in the bggb have considerable potential to advance our understanding of tectonic processes in the archean crust two current hypotheses persist to explain the origin of amphibolite facies metamorphism in the southern bggb the first interprets these rocks to be the consequence of accretionary tectonics while the second proposes a dome and keel vertical tectonic process driven by sinking of greenstone layers and the doming of the underlying granitoid crust in this study metamorphic pressure temperature p t analysis has been combined with garnet lu hf and monazite u pb geochronology to directly date the amphibolite facies metamorphism within the stolzburg terrane of the bggb a garnet biotite chlorite bearing sample yields a lu hf garnet age of 3233 17 ma and a garnet staurolite kyanite bearing sample produces a u pb monazite age of 3191 9 ma whereas an andalusite kyanite bearing sample produces a u pb monazite age of 3436 18 ma phase diagrams and garnet compositional modeling produce a clockwise p t evolution with rocks reaching peak p t conditions of 8 5 kbar and 640 c for the ca 3200 ma event and minimum peak p t conditions of 4 5 kbar and 550 c for the ca 3435 ma event the duration of metamorphism for the ca 3200 ma event is estimated to be 50 20 m y based on differences in age between u pb and lu hf systems and durations needed to fit models of diffusionally modified garnet chemical zoning similarly shaped p t paths over the stolzburg terrane indicate that the metamorphism occurred in response to crustal thickening due to an accretionary tectonic process thus the stolzburg terrane constitutes an orogenic core exhumed along the komati fault

diffusional homogenization of light ree in garnet from the day nui con voi massif in n vietnam implications for sm nd geochronology and timing of metamorphism in the red river shear zone abstract high grade migmatitic and mylonitic gneisses from the day nui con voi massif in northern vietnam record temperatures of 760 810 c at pressures of 6 10 kbars high temperature conditions have resulted in the development of major element diffusional profiles in garnet laser ablation icp ms analyses of trace elements indicate that ree and hf closely followed rayleigh like fractionation trends but underwent significant post crystallization modification light ree and to a lesser extent hree rim to rim zonation profiles show progressive flattening with the decreasing garnet size nd and sm are completely homogenized in the crystals smaller than 1 5 mm while lu always preserves variable degrees of core to rim concentration gradient the observed ree patterns are interpreted as resulting from the combination of protracted garnet growth of progressively smaller crystals and intracrystalline diffusion this had profound influence on sm nd geochronology and resulted in isochron ages ranging from 50 to 32 ma the youngest age was obtained for a sample where all garnet crystals are smaller than 2 mm in which light ree profiles are completely or nearly completely homogenized thus only the youngest age represents geologically meaningful event and 31 7 0 9 ma age is interpreted as the best estimate of the resetting episode due to high temperature diffusional homogenization of light ree during early oligocene metamorphism older sm nd ages reflect mixed analyses of variably reset individual garnet crystals lu hf isotopic analyses of bulk garnet fractions despite yielding high parent daughter isotopic ratios appeared very scattered and did not allow defining isochron ages instead apparent ages defined by whole rock and individual garnet fractions range from c 80 to 160 ma very old apparent ages are interpreted as being the consequence of intracrystalline lu diffusion and preservation of the original hf distribution which leads to lower 176 lu 177 hf ratios and thus steeper older isochrons back diffusion of lu during commonly observed resorption played a subordinate role in modifying isotope systematics

yttrium zoning in garnet coupling of major and accessory phases during metamorphic reactions the concentration of yttrium in pelitic garnets as a function of metamorphic grade has been examined in relation to the distribution of xenotime ypo 4 in samples from new england and british columbia samples with xenotime present only as inclusions in garnet generally possess high y cores and concentrations that drop off discontinuously along zoning shoulders of variable width to low y outboard regions samples with matrix xenotime are restricted to the garnet zone y concentration of these garnets generally decreases smoothly from core to rim xenotime may also be present in reaction zones around garnet in xenotime bearing samples y g r t is strongly temperature dependent and ranges from 5000 ppm in the garnet zone to 150 ppm in the sillimanite zone measured yttrium zoning profiles in xenotime absent samples are reproduced with both rayleigh fractionation and diffusion models but p t histories of the samples examined favor the rayleigh model with garnet volume bulk rock yttrium and mode of y hree accessory phases controlling the profile shape high yttrium annuli in staurolite zone samples may form by garnet overgrowth of proximal matrix enriched in yttrium due to garnet consumption during discontinuous staurolite forming reactions an increase in y g r t and hree g r t in garnet from anatectic samples is related to dissolution of phosphates in vapor absent peraluminous melt with partitioning of highly compatible y and hree into garnet grown during anatexis textural analysis reveals that phosphates are absent from regions of garnet grown in equilibrium with melt a main result of this study is identification of an intimate coupling between major pelite phases and accessory phases during reaction progress this coupling is of great advantage in that it may be used to 1 calibrate sensitive geothermometers and geobarometers 2 identify particular regions of garnet grown in different garnet producing reactions over a range of grades and 3 reveal portions of pelite reaction history invisible to major elements

p t t paths and post metamorphic exhumation of the day nui con voi shear zone in vietnam abstract the day nui con voi belt in vietnam is the southeasternmost part of the red river shear zone in asia it is a narrow high grade metamorphic core complex consisting of garnet sillimanite biotite gneisses mylonite bands amphibolite layers and migmatites geothermobarometric study of the complex revealed that the peak metamorphism took place under amphibolite facies conditions of 690 60 30 c and 0 65 0 15 gpa and the subsequent mylonitization occurred under greenschist facies conditions of 480 c and under 0 3 gpa fifteen synkinematic hornblende and biotite separates from gneisses amphibolites and mylonites were dated with the k ar method hornblende separates from the day nui con voi give k ar ages of 26 4 28 5 ma and the biotite separates do give 24 5 24 7 ma combination of thermobarometric and geochronological data yields the cooling history of 500 c at 28 ma and 300 c at 24 ma with a cooling rate of 70 110 c ma 1 and 23 km post metamorphic exhumation of the core complex the first 16 km exhumation from the peak of metamorphism at probably 31 ma to 28 ma was triggered by the left lateral strike slip displacement of the red river shear zone

exhumation history of the red river shear zone in northern vietnam new insights from zircon and apatite fission track analysis abstract a new set of zircon and apatite fission track ages from the ailao shan and day nui con voi dncv metamorphic massifs of the red river shear zone rrsz and neighboring rocks in northern vietnam is presented a complex along strike diachronous denudation history is revealed the southern sector of the dncv cooled to about 100 c by the late oligocene whereas its central compartment was affected by the later thermotectonic evolution of the song chay dome to the e of the rrsz whose final exhumation occurred during the early miocene the northern sector of the rrsz is characterized by the 35 ma phan si pang pre to synkinematic intrusion fission track ages from a vertical section within the phan si pang granite indicate rapid exhumation and cooling the paleozoic tectonic block to the west of the rrsz fission track ages between 40 and 30 ma was exhumed and cooled earlier than the fault mylonite belt fission track ages of 30 ma and younger and also than the eastern block its structural level is consistent with field observations that suggest the rrsz in northern vietnam to be a transtensional system with a regional ne sw oriented extension component

combined lu hf and sm nd geochronology of the marianske lazně complex new constraints on the timing of eclogite and granulite facies metamorphism abstract lu hf and sm nd garnet whole rock geochronology combined with petrographic observations minero chemical variations thermodynamic modelling and structural data was used to constrain the p t t d evolution of eclogites from the marianske lazn complex bohemian massif boudins of mostly isotropic eclogite with relict steep eclogite facies fabric are affected by steep migmatitic foliation which is followed on a regional scale by the development of almost pervasive predominantly se dipping extensional foliation the structural succession shows continuous transition from eclogite to garnetiferous migmatitic amphibolite and to amphibolite migmatite a least retrogressed sample of eclogite shows clusters of fine grained inclusion poor garnet omphacite relicts surrounded by a fine grained clinopyroxene plagioclase symplectite with minor amphibole biotite plagioclase intergrowths after white mica kyanite with plagioclase spinel coronas and accessory rutile rare potassic white mica occurs as inclusions in omphacite a more retrogressed eclogite with no omphacite or kyanite relicts contains inclusion poor garnet surrounded by amphibole plagioclase corona in a matrix dominated by plagioclase amphibole symplectite with minor clinopyroxene in places the symplectite is overgrown by coarse grained amphibole peak p t conditions inferred from combined conventional thermobarometry and phase equilibria modelling and based on inclusions of white mica up to 3 33 si p f u matrix omphacite jd33 36 and garnet core alm33 38prp38 42grs22 25sps1 compositions are 25 kbar at 650 750 c a ht overprint occurred at 14 18 kbar and 800 c based on coexisting clinopyroxene jd18 24 plagioclase an18 35 and amphibole na b lu hf and sm nd garnet geochronology has been applied to both samples an older age c 390 ma obtained by the lu hf method is interpreted as the timing of hp metamorphism while c 15 ma younger ages were obtained by the sm nd method as temperatures for the ht overprint exceed the empirically and experimentally determined closure temperature of the sm nd system the sm nd ages are interpreted to date cooling following the ht overprint combined together contrasting eclogite and amphibolite facies migmatite fabrics the mineral textures calculated p t conditions and distinct lu hf and sm nd ages provide a complete p t t d path characterised by rapid 15 ma transition from hp subduction crustal thickening to extensional ht shearing this unconventional exhumation path does not fit to models of monocyclic exhumation in a subduction channel proposed for the munchberg and zev devonian hp units to the west in contrast the post peak extensional low angle shearing shortly after subduction and collision resembles more the geodynamic model typical for the iberian subduction system both in time scales and sequence of tectonic events

pressure temperature paths from p t pseudosections and zoned garnets potential limitations and examples from the zanskar himalaya nw india the extraction of p t histories from metamorphic rocks provides a valuable dataset for the elucidation of the tectonic mechanisms for orogeny while continued re equilibration frequently obliterates early information garnet zonation and inclusion assemblages can often surmount this problem the task is more difficult in high variance assemblages or if inclusions are not preserved but one approach is to use pseudosections that are specific to the bulk composition of a given rock in the latter case the compositions and abundances of all the minerals are fixed at a given p t point such that if the effective bulk composition is known the garnet composition alone can be used to reconstruct the history here we explore this approach using examples from the zanskar himalaya nw india pseudosections have been calculated for four pelitic to semipelitic rocks from the zanskar himalaya and have been contoured for garnet composition the calculations adequately model the mineral assemblages in the rocks and predict the presence of chlorite in the early assemblage where chlorite is found as inclusions within garnet moreover the pseudosections successfully model the garnet core compositions with all three independent compositional contours overlapping at a single pressure and temperature this occurs at 550 c and at pressures varying from 3 7 kbar for the four rocks studied we have been less successful however at modelling garnet compositions beyond the cores because fractionation of the effective bulk composition is caused by garnet growth itself however in this case a combination of the pseudosection and conventional thermobarometry using fe ti inclusions and matrix phases allows us to reconstruct the entire p t history the resulting p t paths record burial of 3 5 kbar without significant temperature increase followed by isobaric heating of 50 100 c this evolution is consistent with himalayan collision in the early tertiary but a combination of the p t data presented here and published geochronological data suggests renewed thrusting south of the suture zone in the oligocene in addition the data demonstrate that no extra heat source is required to cause melting of the himalayan crust in the miocene while melting could have occurred both by dehydration during decompression or in the presence of a fluid the lack of garnet resorption does suggest decompression was rapid and followed quickly by cooling this scenario favours melting by decompression

models of garnet differential geochronology abstract rayleigh distillation models are developed to describe theoretical growth zoning of lu hf rb sr sm and nd in typical garnet crystals from metapelites and metabasites effects of diffusion limited transport within the matrix and intracrystalline diffusion are also considered qualitatively theoretical zoning profiles show strong depletions of lu in garnet rims compared to cores but virtually invariant hf rb sr sm and nd profiles generally consistent with natural profiles for lu and hf and previously published models theoretical isochron diagrams for lu hf exhibit distinctive arcuate distributions and high mswds consistent both with himalayan data and with expectations that garnet growth durations exceed chronologic resolution by as much as an order of magnitude predicted isochron diagrams for sm nd and rb sr exhibit vertical arrays for garnet and high mswds that are generally lower than for lu hf in metapelites inherent chronologic resolution for bulk separates is best for lu hf in metapelites and rb sr but analytical considerations favor rb sr or sm nd for chronologic zoning studies diffusion limited transport in the rock matrix strongly influences zoning profiles but does not change the main trends on isochron diagrams intracrystalline diffusion will initially rotate lu hf isochrons to steeper slopes giving older apparent ages the natural himalayan data indicate growth of garnet in one rock from the greater himalayan sequence at 34 ma consistent with previously measured monazite ages from the same rock data from another himalayan rock suggest polymetamorphism that includes a paleozoic component

rare earth diffusion kinetics in garnet experimental studies and applications abstract we determined the diffusion coefficient of sm in almandine garnet as function of temperature at 1 bar and fo 2 corresponding to that of wustite iron buffer and to a limited extent that of a few other selected rare earth elements in almandine and pyrope garnets both garnets were demonstrated to have metastably survived the diffusion annealing at conditions beyond their stability fields the experimental diffusion profiles were analyzed by secondary ion mass spectrometry and in addition by rutherford back scattering spectroscopy for two samples transmission electron microscopic study of an almandine crystal that was diffusion annealed did not reveal any near surface fast diffusion path using reasonable approximations and theoretical analysis of vacancy diffusion the experimental data were used to develop an expression of rare earth element ree diffusion coefficient in garnet as a function of temperature pressure fo 2 ionic radius and matrix composition calculation of the closure temperature for the sm nd decay system in almandine garnet in a metamorphic terrain shows very good agreement with that constrained independently modeling of the ree evolution in melt and residual garnet suggests that for dry melting condition the ree pattern in the melt should commonly conform closely to that expected for equilibrium melting however for much lower solidus temperatures that would prevail in the presence of a h 2 o co 2 fluid the concentration of light ree in the melt could be significantly lower than that under equilibrium melting condition a reported core and rim differences in the ree content of a garnet crystal in a mantle xenolith in kimberlite have been reproduced by assuming that the ree zoning was a consequence of entrapment in a magma derived from an external source for 32 000 yr before the eruption

structure and shearing conditions in the day nui con voi massif implications for the evolution of the red river shear zone in northern vietnam 1 the day nui con voi massif bears a record of the red river shear zone rrsz activity in north vietnam it forms a large scale antiformal core complex type structure bounded by the song hong and song chay faults the kinematics of both faults are identical and reflect transtensional shear initiated under upper amphibolite facies conditions and propagated into greenschist facies microfabric analysis establishes that both extensional and strike slip shearing initiated between 700 and 500 c the rrsz evolved from a single subvertical fault which due to strike perpendicular extension underwent progressive dilation the created space was intruded by already metamorphosed and deformed ductile middle crust in the form of a gneissic dome both strike slip and extensional shearing were accommodated in the limbs of the antiform while its core was uplifted from midcrustal level bearing only a minor record of sinistral shear

lu hf geochronology and trace element distribution in garnet implications for uplift and exhumation of ultra high pressure granulites in the sudetes sw poland combining lu hf garnet geochronology with in situ trace element analyses in garnet allowed its to gain new insight into the metamorphic evolution of uhp uht rocks in the stary gieraltow region in the polish sudetes prograde garnet growth recorded by rayleigh type heavy ree hree zoning in the felsic granulites indicates that the obtained 386 6 4 9 ma lu hf age represents the time of garnet crystallization on a prograde uhp metamorphic path the surrounding rocks were metamorphosed at the same time as indicated by 381 2 6 7 ma sm nd garnet age obtained for the mid crustal metapelites the second metamorphic episode which affected most of the lower crust in the orlica snieznik massif osm occurred at ca 340 ma as determined by u pb zircon and sm nd garnet dating of granulites in this and previous studies is interpreted as a high temperature event which took place on a retrograde path trace element distribution in garnets from the layered granulites showed significant differences in distribution of medium and hree in garnets from mafic and felsic protoliths over the course of the metamorphic evolution this had strong impact on the isotopic dating results and led to decoupling of the sm nd and lu hf clocks which recorded timing of the two different metamorphic episodes separated by as much as 40 ma moreover the preservation of the hree growth zonation profile in garnets from the felsic granulites whose minimum metamorphic temperature was established at 900 degrees c implies that the lu hf system under relatively dry conditions does not undergo significant diffusional re equilibration even at such extreme temperatures and therefore it sill provides the age of prograde garnet growth under hydrous conditions at least some resetting will take place as documented by the partially relaxed hree zonation profile in the amphibolitised mafic granulite which yielded a 10 ma younger age the hree distribution study appeared to be a particularly valuable and essential tool which allowed us to distinguish garnet growth from post growth complexities and hence provide improved age interpretation medium ree on the other hand did not show any obvious correlation with the isotopic signature of garnet two distinct metamorphic episodes recorded in the stary gieraltow region show that buoyancy driven uplift of uhp rocks can be arrested at the base of a continental crust if not supported by any additional force in our case study the uhp rocks would have never reached the surface if their uplift had not been resumed after a long pause under a different tectonic regime the multistage discontinuous uplift revealed by the uhp rocks of the osm provides a new scenario for the exhumation of continental crust from mantle depths distinct from the fast track exhumation histories recognized in uhp terranes elsewhere c 2006 elsevier b v all rights reserved

oligo miocene midcrustal subhorizontal shear zone in indochina new structural observations of oligo miocene deformation in north vietnam along the red river shear zone suggest that left lateral strike slip shear was restricted to the upper and middle crust above a horizontal shear zone left lateral shear deformation is associated with low pressure low temperature parageneses high temperature deformation is restricted to zones where the foliation has a low dip in the core of the dai nui con voi antiformal dome these observations complete those made earlier in the bu khang extensional dome farther south above the horizontal shear zone left lateral transpression was active during the first stage and changed to transtension some 33 myr ago purely extensional metamorphic domes bu khang or transtensional domes dai nui con voi were exhumed during this younger stage our observations plead for caution when interpreting strike slip structures at lithospheric scale