identification of inherited radiogenic pb in monazite and its implications for u pb systematics the simplest interpretation of uranium lead mineral ages in igneous rocks embodies the assumption that melting of pre existing rocks leads to homogenization of the lead isotopes thus resetting the radiometric clock zircons often violate this assumption by only partially dissolving and preserving a component of lead inherited from before the melting episode but until now such behaviour has not been observed in monazite here we report u pb data for zircons and monazites from a tertiary granite from the himalayas in which zircons and monazites contain inherited radio genic lead an analysis of a single entirely igneous zircon crystal plots on concordia at 19 5 0 4 myr eight monazite fractions lie on a line from an upper intercept of 471 10 myr to the youngest fraction which plots slightly above concordia at 21 myr the presence of excess 206pb in the youngest monazite fractions indi cates that the monazite data represent a mixing line between young magmatic monazite and an older inherited component rather than a resetting of 471 myr monazites at 20 myr these data suggest a closure temperature for pb in monazite significantly higher than previously published estimates

u pb dating of monazite and its application to geological problems monazite is an underutilized mineral in u pb geochronological studies of crustal rocks it occurs as an accessory mineral in a wide variety of rocks including granite pegmatite felsic volcanic a

a 700 ma sm nd garnet whole rock age from the granulite facies central kaoko zone namibia evidence for a cryptic high grade polymetamorphic history continental collision of the kalahari and the congo craton in africa and the rio de la plata craton in south america resulted in a structurally complex neoproterozoic belt system the kaoko dom feliciano ribeira belt it is uncertain whether these three cratons collided more or less simultaneously during one single orogenic event at 580 550 ma or whether the belt owe its structural and metamorphic features to several so far poorly constrained events the kaoko belt nw nambia representing the belt system between the southern congo craton and the rio de la plata craton is an ideal object to study these complexities within this belt high grade meta igneous and metasedimentary rocks of the sillimanite k feldspar zone contain large garnet porphyroblasts that grew at ca 730 10 c at 6 7 1 2 kbar during peak metamorphic granulite facies conditions a sm nd garnet whole rock errorchron n 7 mswd 6 0 obtained on a siliceous metasediment yielded an unexpected old age of 692 14 ma which is interpreted as an inherited metamorphic age of an older granulite facies event it is evident that the dated garnets survived the younger high grade granulite facies metamorphism that occurred between ca 570 and 520 ma and preserved their old sm nd isotope systematics implying that the closure temperature for sm nd in garnet is higher than ca 730 c in this case these results imply fast cooling rates at different times during the pan african orogeny that prevent isotopic homogenization at sample scale moreover it is suggestive that trace element ree diffusion in garnet is considerably slower than major element diffusion from a regional point of view it is possible that these specific siliceous metasedimentary rocks may be unrelated to the pan african metamorphic evolution of the kaoko belt between 570 and 520 ma and may represent lithological units that belong to so far unrecognized terranes in the kaoko belt 2007 elsevier b v all rights reserved

geochronology and petrogenesis of pan african syn tectonic s type and post tectonic a type granite namibia products of melting of crustal sources fractional crystallization and wall rock entrainment abstract the oetmoed granite migmatite complex ogmc central damara orogen namibia consists mainly of 526 to 516 ma garnet and cordierite bearing granite and subordinate 488 to 494 ma hornblende and titanite bearing granite in the form of planar sheets and dykes additionally a slightly elongated granite body occurs in the center of the complex the garnet and cordierite bearing granite has major and trace element characteristics of s type granite but the hornblende and titanite bearing granite has higher hfse and ree contents similar to a type granite whereas the garnet and cordierite bearing granite contains numerous restitic xenoliths the hornblende and titanite bearing granite is xenolith free the country rocks are cordierite sillimanite k feldspar garnet bearing metasedimentary rocks and migmatite cordierite and garnet rich xenoliths in the s type granite do not represent primary restite their depleted chemical composition is best explained by varying and large degrees of partial melting of incorporated country rocks most chemical variations among the garnet and cordierite bearing granite can be explained by processes linked with fractional crystallization of plagioclase biotite and accessory phases mostly monazite and zircon major and trace element data and high 18 o values suggest that the least evolved members of the garnet and cordierite bearing granite were derived from metapelitic rocks at ca 800 c as inferred from monazite and apatite dissolution thermometry higher cao and na2o but lower sio2 contents and lower rb sr ratios as well as lower 18 o values of the hornblende and titanite bearing granite suggest that they are more likely generated by partial melting of non pelitic sources metagranitoids at temperatures in excess of 900 c decreasing tio2 na2o feotot mgo cao total ree content but increasing al2o3 and k2o indicate fractionation of mainly hornblende and titanite in the case of the hornblende and titanite bearing granite the differing compositions of the garnet and cordierite bearing granite and the hornblende and titanite bearing granite are attributed to different source rocks metapelite instead of metagranitoid and different temperatures during melting as inferred from accessory phase dissolution thermometry furthermore significant entrainment of country rock in some samples played a major role during petrogenesis of the garnet and cordierite bearing granite but was not important during the evolution of the hornblende and titanite bearing granite intrusion of such hot felsic magmas close to the inferred peak of metamorphism has probably caused in part the high temperature metamorphism and anatexis of the country rocks at relatively low pressures

major and trace element and isotopic sr nd o constraints for pan african crustally contaminated grey granite gneisses from the southern kaoko belt namibia abstract light grey to dark grey fine grained to medium grained granitic to granodioritic gneisses are substantial part of the nk formation in the southern kaoko belt namibia based on field relations these rocks can be subdivided into two distinct groups one group belongs to a pluton with an areal extent of approximately 6 km2 the other group represents gneissic granitic veins for all samples sio2 varies between 68 1 and 74 7 wt the lree enriched granite gneisses of the nk pluton are slightly peraluminous a s i 1 05 1 25 with low rb concentrations 86 133 ppm and high concentrations of sr and ba 187 683 and 644 1535 ppm respectively the gneisses have initial end 500 ma values ranging from 4 1 to 10 7 and initial 87sr 86sr 500 ma ratios ranging from 0 70695 to 0 71108 and whole rock oxygen isotope values ranging from 9 4 to 10 7 the gneissic granitic veins can be subdivided into three different groups groups a b c most geochemical data are similar to those obtained for the pluton only the 18o values for group b and the rb sr ratios for group c are distinct from the pluton linear variation of major and trace elements in the nk granite gneisses from the pluton can be explained by igneous fractionation involving plagioclase 10 wt biotite 1 wt alkali feldspar 13 wt and lree enriched accessory minerals the 87sr 86sr ratios become more radiogenic and 18o values increase with increasing sio2 indicating that in addition to crystal fractionation processes the nk granite gneisses were affected by assimilation processes involving contaminants similar to basement derived granites from the central damara orogen which show lower end 500 ma ratios the protolith of the nk granite gneisses is most likely a biotite and or amphibole bearing but muscovite free rock most probably an intermediate igneous rock i e diorite to granodiorite or a metagreywacke concerning the isotopic differences between the granite gneisses from the pluton and the gneissic granitic veins partial melting probably took place in a heterogeneous compositionally layered middle to lower crust at temperatures of 845 10 c as calculated by monazite and zircon saturation equations the final stages of continental collision during westward dipping subduction of the kaoko belt beneath the ribeira belt took place between ca 590 and 520 ma in view of such a pan african subduction zone related geotectonic situation the nk granite gneisses are products of melting events in arc derived rocks produced in an earlier stage of the pan african orogeny

onset and duration of zn pb mineralization in the talate pb zn fe skarn deposit nw china constraints from spessartine u pb dating abstract distal pb zn skarn commonly displays ambiguous relationships with the nearby intrusions leading to poorly constrained and often controversial skarn metallogenic models in this study we discuss the potential usage of spessartine a common u bearing gangue mineral in many pb zn skarn deposits in u pb dating spessartine from the talate skarn pb zn deposit xinjiang nw china can be divided into two types gi and gii based on mineral assemblage gi spessartine sp56 5 al28 3 gr12 4 to sp66 0 al22 3 gr8 5 is coarse grained with core rim texture occasionally well preserved disseminated magnetite and magmatic ti titanite grains are hosted within the spessartine gi core indicating that the spessartine gi is slightly postdate the magmatic titanite ti and disseminated magnetite gii massive spessartine sp64 2 al24 8 gr2 7 to sp69 6 al20 1 gr1 2 occurs with hydrothermal titanite tii and shows a close relationship with coarse grained hydrothermal magnetite and is cut by quartz or fine sulfide e g pyrite pyrrhotite and chalcopyrite veins the flat time resolved signals obtained from the depth profile analyses and inclusion free trace element mapping of u in spessartine indicate that the u is structurally bounded in the lattice both gi and gii spessartine show clear positively correlation between u al and mn but no apparent correlation is present between u and fe which indicates that al and mn are more important in the u incorporation in spessartine integrating with the positive correlations with u and lrees in spessartine which suggest u is incorporated in the lattice by a substitution of u4 viii 2 al3 iv ca2 viii 2 si4 iv in situ u pb dating of the two spessartine types yielded a weighted average 206pb 238u age of 231 7 7 2 ma gi mswd 0 56 n 35 and 211 5 5 8 ma gii mswd 1 1 n 47 considering the errors caused by common pb and matrix mismatch the corrected gi and gii spessartine ages are consistent with the ti 228 0 4 6 ma mswd 2 n 28 and tii 209 8 3 6 ma mswd 1 03 n 39 titanite ages suggesting that spessartine u pb dating is robust and reliable the least 8 1 my gap between the formation of the two titanite and spessartite types reflects two mineralization events suggesting an influx of magmatic fluids during the retrograde alteration mineralization our study represents a new way of directly constraining the timing of skarn alteration mineralization

isotopic chronometry of zoned garnets growth kinetics and metamorphic histories basic information on the chronological and pressure temperature evolution of regional metamorphic terrains may in principle be derived from metamorphic garnets because of the similarly low diffusivities of sm nd and major cations in this mineral we report here smnd and rbsr isotopic and major element data on prograde garnets from regionally metamorphosed pelites from newfoundland the garnets preserve a prograde major element zonation as well as a sympathetic variation in sm nd ratio smnd data for separated portions of the garnet from core to rim provide both upper limits on the time for garnet growth and demonstrate synchronous growth of different garnet grains on a hand specimen scale the rbsr data on the same garnet fractions are in general agreement with these results but in some cases cannot be interpreted in terms of growth a minimum heating rate of 3k ma 1 is derived by combining the estimates for garnet growth time with the apparent temperature interval over which the garnet grew deduced from the major element zonation this value is similar to the minimum suggested by theoretical models for the thermal evolution of thickened continental crust the growth rate is within the range of 1 3 19 mm ma 1 set respectively by the isotopic data and the likely upper limit for heating rate during regional metamorphism these growth rates appear too slow to be controlled by surface reaction and suggest that other factors such as transport may be rate limiting in this case the limits set on the effective diffusion coefficient for material transport to the growth site 0 4 6 1 10 17 m2 s 1 suggest that grain boundary diffusion is probably the transport mechanism for supply of material to the growing garnet

trace element zoning in a metamorphic garnet trace element zoning has been measured in an amphibolite garnet from the tauern window austria by using an ion microprobe humps in the zoning profiles of na sc v y and the heavy rare earth elements mark a period of open system behavior these humps correspond to a part of the major element zoning profile that is interpreted as a p t reversal the source of the mass excesses of these elements remains ambiguous they were derived either on a thin section scale by the breakdown of trace element enriched refractory minerals or externally from unusual trace element enriched fluids p t paths determined from garnet zoning may require modification if open system behavior is important during garnet growth

geochronology and petrology of migmatites from the proterozoic damara belt importance of episodic fluid present disequilibrium melting and consequences for granite petrology abstract the oetmoed granite migmatite complex ogmc central damara orogen namibia consists of grt and crd bearing s type granites and hbl and titanite bearing a type granites that intruded into crd sil kfs bearing metasedimentary rocks stromatic migmatites and nebulites stromatic migmatites formed by limited in situ partial melting of metapelites under h2o saturated conditions at 700 c and 4 5 kbar this partial melting event took place close to the peak of regional metamorphism at 510 ma as revealed by sm nd garnet whole rock ages as well as u pb monazite ages the newly formed melt remained more or less at the site of origin melanosomes of the stromatic migmatites do not resemble true residues instead they represent reaction zones between in situ melt and the metasedimentary host rock leucosomes from the stromatic migmatites are lree and hfse depleted which is typical for the low melt fractions generally observed in migmatite terranes mass balance calculations suggest that these leucosomes may be disequilibrium melts similar 18 o whole rock values between 13 and 14 observed in leucosomes and corresponding melanosomes suggest homogenization of oxygen isotopes between the different rock types via the melt and an internal fluid phase during melting other leucosomes with a major element chemistry similar to the leucosomes from the stromatic migmatites but higher ree contents suggest significant incorporation of lree enriched phases monazite from the country rocks for these leucosomes sm nd garnet whole rock ages are 473 ma indicating a second phase of melting nebulites mainly result from injection of granitic melts into the country rocks but their residual chemistry indicates that partial melting and limited melt removal must have occurred monazites from these nebulites record concordant u pb ages between 540 ma and 470 ma indicating episodic migmatization during high grade regional metamorphism intrusive peraluminous granites are likely generated by partial melting of pelitic sources in the lower crust and different source rocks are probably not the controlling factor for the different chemistry of leucosomes and granites therefore different melting conditions fluid absent vs fluid present and different modes of entrainment and solubilities of accessory phases control the elemental budgets of the leucosomes large scale migmatite granite complexes are a substantial part of some high grade terrains but the distinctive geochemical composition of some leucosomes make these rocks unlikely precursors of large scale granitic bodies

sm_nd garnet ages from the uluguru granulite complex of eastern tanzania further evidence for post metamorphic slow cooling in the mozambique belt abstract two samples from the uluguru granulite complex yield garnet sm nd ages of 633 7 and 618 16 ma similar to previously published hornblende 40ar 39ar and k ar ages the similarity of the sm nd to the k ar age suggests that the closure temperature of garnet to nd diffusion is similar to that of hornblende to ar diffusion assuming that published zircon u pb ages of about 700 ma date peak granulite facies metamorphism a mean post metamorphic cooling rate of 2 3 c ma can be calculated for the time interval 700 to 630 ma such slow cooling rates imply thermal relaxation with a thickness length scale greater than the thickness of average continental crust this in turn implies that the thermal perturbation responsible for metamorphism was preceded by regional crustal thickening probably in a collisional orogen

initiation of zn pb mineralization in the pingbao pb zn skarn district south china constraints from u pb dating of grossular rich garnet abstract in this study we discuss the application of u pb dating on u rich common pb bearing grossular rich garnet and the issues of common pb correction grossular rich garnet samples were collected from the baoshan and huangshaping areas of the pingbao skarn district hunan south china the baoshan garnet comprises gr64 3 and34 0 to and99 0 gr0 2 gr grossular and andradite with minor spessartine 0 43 1 45 almandine 0 0 61 and pyrope 0 01 0 91 the huangshaping 304 garnet comprises gr41 7 and56 0 to gr70 2 and25 9 with minor spessartine 1 42 2 10 whereas the huangshaping 301 garnet comprises gr62 0 and26 6 spe11 2 to gr74 5 and20 9 pyr2 5 spe2 1 spe spessartine pyr pyrope the baoshan garnet grains are subhedral to euhedral with well developed core rim textures and contain approximately 1 10 ppm u the huangshaping 304 garnet is also euhedral with grains size ranging from several to tens of millimeters the core rim textured garnet grains are compacted together with interstitial quartz and contain about 5 10 ppm u the huangshaping 301 garnet is anhedral to tabular with interstitial quartz and magnetite and contains up to 579 ppm u in situ u pb dating of the baoshan huangshaping 304 and 301 garnet samples yielded consistent anchored y intercept ages at 162 6 2 9 ma n 60 mswd 1 5 160 2 1 5 ma n 59 mswd 1 9 and 160 9 0 3 ma n 60 mswd 4 1 respectively these ages are consistent with the new and published zircon and titanite u pb ages 162 160 ma indicating that the garnet u pb ages are reliable and the technique is robust we suggest that the non u bearing mineral correction has involved in the third party common pb ratio which is believed to be a better correction for the system common pb

metamorphic response and crustal architecture in a classic collisional orogen the damara belt namibia abstract the damara belt is a well exposed mid crustal section through a collisional orogen of cambrian age that closed the khomas ocean basin between passive margins on the congo and kalahari cratons collision resulted in a bi vergent orogen with a distinct paired metamorphic pattern of foreland vergent high p low t orogenic margins and a broad high grade low p high t orogen core spatial and temporal patterns of the metamorphic response to collision have been characterized for all parts of the belt using a large dataset n 240 of internally consistent quantitative pt determinations evolution of mineral parageneses and semi quantitative p t paths metamorphic mapping and quantitative metamorphic field gradients integration with deformation history structural profiles metamorphic chronology magmatic history and stratigraphy constrains a dynamic model of crustal architecture during peak metamorphic events the pattern of zonal metamorphic response is demarcated by three major metamorphic discontinuities md with steep pressure gradients inferring crustal scale structures that accommodated lateral exhumation of crustal wedges discontinuities are confirmed by deformation features in the field and metamorphic mineral growth indicates that vertical flattening at the peak of metamorphism progressing through ductile to brittle extensional structures crustal wedges along the orogenic margins experienced steep clockwise p t paths with peak pt conditions terminated by isothermal decompression during rapid exhumation in isostatic response to deep burial metamorphic chronology and over printing metamorphic fabrics constrain a sequence of foreland propagating out wedging of crustal thrust wedges that resulted in telescoping of the orogenic margins peak metamorphism at a geothermal gradient of 20 25 c km and 8 9 kb in the southern zone wedge i was attained between 517 530 ma followed by south directed out wedging at the uis pass suture accommodated by relative extension md1 at high structural levels near the boundary with the high grade orogen core out wedging of the southern zone further buried the southern margin zone wedge ii in the footwall below the uis pass suture peak metamorphism at 17 c km and 9 5 11 5 kb in the southern margin zone was attained at 517 4 ma and followed by out wedging on basal thrusts accommodated by vertical flattening and extensional reactivation of the uis pass suture md2 peak metamorphism at 17 c km and 10 5 kb in a high p low t crustal wedge in the northern margin wedge iii occurred at 510 4 ma peak metamorphism in this wedge was terminated by isothermal decompression during north directed thrusting accommodated by extension at higher structural levels near the high grade orogen core md3 granulite facies metamorphism in the high grade orogen core which formed the upper plate to the deeper thrust wedges occurred at significantly lower pressures 4 3 6 0 kb higher t depth ratios 38 47 c km and low p t clockwise p t paths indicating only moderate burial and protracted high grade conditions metamorphic chronology confirms high grade conditions 540 505 ma persisted beyond isostatic adjustment of the high p low t orogenic margins high heat flow conditions and long thermal lag are best explained by lithospheric breakoff during collision in contrast to the high p low t margins that experienced lateral exhumation in a convergent system the high grade orogen core was exhumed vertically as a broad core complex during extensional collapse of the orogen peak metamorphic conditions in the central part of the orogen core record higher pressures than marginal lower grade zones to both the north and south indicating vertical exhumation as a broad massif this post peak vertical exhumation of the central core occurred after 505 ma was accommodated by brittle extensional detachments at high stratigraphic levels with km scale omission of crustal section

sm nd garnet dating of polyphase metamorphism northern coast mountains south eastern alaska usa sm nd ages of garnet from the northern coast mountains of south eastern alaska usa constrain the timing of thermal events in polyphase metamorphic rocks of the western metamorphic belt and provide new data on the spatial extent of cretaceous regional metamorphism bulk garnet whole rock sm nd ages for a sillimanite zone amphibolite taku inlet and a biotite zone metapelite tracy arm are 77 17 ma and 59 12 ma respectively garnet core whole rock 80 9 ma core matrix 84 9 ma rim whole rock 59 4 ma and rim matrix 62 4 ma ages were obtained from a sample collected 200 m west of a palaeocene coast plutonic metamorphic complex sill like pluton that separates medium grade metamorphic rocks from high grade metamorphic rocks and voluminous tertiary plutons in the core of the orogen the garnet core ages of c 80 ma indicate that the regional metamorphic grade reached garnet zone prior to the intrusion of the plutons and high grade metamorphism of rocks to the east similar ages for the younger plutons the youngest garnets and the rim of a multistage garnet c 59 ma indicate a later episode of contact metamorphic garnet growth documentation of pre 71 ma garnet zone metamorphism along the western edge of the coast plutonic metamorphic complex confirms that albian to late cretaceous metamorphism associated with crustal thickening affected this part of the orogen the similarity of garnet sm nd ages to independent age estimates for metamorphic events confirms that this technique provides useful estimates for the timing of late cretaceous to tertiary thermal events the c 20 myr difference between garnet core and rim ages suggests that the sm nd isotope systematics of a single garnet grain can be used for distinguishing between multiple metamorphic events

ion microprobe determinations of trace element concentrations in garnets from anatectic assemblages abstract trace element data for garnet cordierite plagioclase alkali feldspar and tourmaline have been obtained by ion microprobe analysis of assemblages from a pelite derived leucogranite a sillimanite grade anatectic gneiss and a kyanite grade sub solidus schist from the high himalaya results confirm that for equilibrium melting residual plagioclase will cause depletion in sr and eu in the melt and residual garnet will result in strong depletion of y and hree consistent with their published partition coefficients all trace elements of interest are strongly incompatible in cordierite implying that for equilibrium fractional melting of pelites at shallow depths hree concentrations in garnets from metasediments are independent of metamorphic grade but the metamorphic garnets are significantly less enriched in hree than is the leucogranite garnet this suggests either that the leucogranite is derived from a source with lower modal abundances of garnet than the analysed metasediments or that garnet from the anatectic gneiss has not reached chemical equilibrium with a granitic melt the latter possibility is consistent with sluggish self diffusion rates in garnets for ree at temperatures of 5 kbar

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

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

u pb monazite ages in amphibolite to granulite facies orthogneiss reflect hydrous mineral breakdown reactions sveconorwegian province of sw norway in the rogaland vest agder terrain of the sveconorwegian province of sw norway two main sveconorwegian metamorphic phases are reported a phase of regional metamorphism linked to orogenic thickening m1 and a phase of low pressure thermal metamorphism associated with the intrusion of the 931 2 ma anorthosite charnockite rogaland igneous complex m2 phase m1 reached granulite facies to the west of the terrane and m2 culminated locally at 800 850 c with the formation of dry osumilite bearing mineral associations monazite and titanite u pb geochronology was conducted on 17 amphibolite to granulite facies orthogneiss samples mainly from a suite of 1050 2 8 ma calc alkaline augen gneisses the feda suite in these rocks prograde negatively discordant monazite crystallized during breakdown of allanite and titanite in upper amphibolite facies at 1012 1006 ma in the feda suite and other charnockitic gneisses concordant to slightly discordant monazite at 1024 997 ma probably reflects breakdown of biotite during granulite facies m1 metamorphism a spread of monazite ages down to 970 ma in biotite hornblende samples possibly corresponds to the waning stage of this first event in the feda suite a well defined monazite growth episode at 930 925 ma in the amphibolite facies domain corresponds to major clinopyroxene formation at the expense of hornblende during m2 growth or resetting of monazite was extremely limited during this phase in the granulite facies domain up to the direct vicinity of the anorthosite complex the m2 event was shortly followed by cooling through ca 610 c as indicated by tightly grouped u pb ages of accessory titanite and titanite relict inclusions at 918 2 ma over the entire region a last generation of u poor monazite formed during regional cooling below 610 c in hornblende rich samples at 912 904 ma this study suggests 1 that monazite formed during the prograde path of high grade metamorphism may be preserved 2 that monazite ages reflect primary or secondary growth of monazite linked to metamorphic reactions involving redistribution of rees and th and or fluid mobilisation 3 that the u pb system in monazite is not affected by thermal events up to 800 850 c provided that conditions were dry during metamorphism

40ar 39ar thermochronology of the pan african damara orogen namibia with implications for tectonothermal and geodynamic evolution abstract forty three 40ar 39ar step heating experiments on white mica biotite hornblende and whole rocks from the damara orogen namibia revise and refine the regional perspective on cooling exhumation and tectonic reactivation across the orogen these data also document post orogenic motion on major shear zones and the thermal effects of late syn to post tectonic granite intrusions these data show regional cooling through temperatures of 300 350 c from 545 to 520 ma in the central kaoko zone 530 510 ma in the eastern kaoko zone and 495 480 ma in the southern zone of the damara belt discordant 40ar 39ar age spectra relate to 1 partial resetting of k ar isotopic system in micas by late stage activity in the major kaoko belt shear zones such as the purros and three palms mylonite zones giving apparent ages of 467 6 481 3 and 492 ma and 2 thermal effects from granitic plutions particularly in the ugab domain of the southern kaoko belt 500 475 ma and in the granite dominated central zone of the damara belt 490 460 ma phyllites of the lower grade flank regions of the orogen record apparent 40ar 39ar mica crystallization ages of 517 ma eastern kaoko zone 526 ma northern zone damara belt and 568 553 ma southern foreland zone damara belt the southern foreland samples however are maximum ages due to variable influence of detrital mica rates of cooling vary between zones within the kaoko belt in the central kaoko belt hornblende biotite and white mica apparent 40ar 39ar ages are similar suggestive of rapid cooling through the 300 550 c temperature range whereas in the orogen core or western kaoko belt hornblende and biotite apparent ages are older than the white mica ages because the white micas record the reactivation of the main structures these data support models of diachronous deformation and metamorphism of the component parts of the orogen and suggest that pan african orogenesis in the damara is broadly bracketed between 580 and 500 ma with final movements through 480 ma and spans through the terminal amalgamation of gondwana in the kuunga orogeny through the initiation of subduction under the gondwana continental margin

microfabrics indicating granulite facies metamorphism in the low pressure central damara orogen namibia abstract the reconstruction of the fluid evolution during progressive metamorphism in the central damara orogen by hoernes and hoffer indicated that fluid present conditions were only periodically attained among these rocks and that the highgrade metamorphism along the west coast of namibia occurred under essentially water absent conditions the characteristic mineral assemblage in the metapelites is plagioclase quartz alkali feldspar biotite cordierite garnet sillimanite apatite zircon ore orthopyroxene bearing parageneses in the metapelites are missing and granulite facies metamorphism may not be detected on the basis of mineral assemblages a detailed study along a 100 km traverse across the migmatite area in the central damara orogen namibia reveals several microfabric features which indicate in analogy to many other well known granulite facies terrains where similar microfabrics have been described that the pan african regional metamorphism in this area culminated at conditions of low pressure granulite facies the observed features are 1 a the activation of basal a and prism c glide systems in quartz under conditions of plastic deformation and the development of equivalent subgrain boundaries parallel prism faces and parallel basal planes 2 b the orientated exsolution of rutile needles in quartz 3 c the exsolution of string perthites mesoperthitic string perthites and antiperthites in feldspars 4 d in addition the homogeneous distribution of major elements in garnet crystals and garnet biotite thermometry suggest granulite facies conditions

the global surface roughness of 433 eros from the near laser rangefinder abstract we performed the first global surface roughness assessment of the asteroid 433 eros at baselines horizontal distances of 4 200 m we measured surface roughness using the root mean square rms deviation over a variety of baselines after first detrending the height to remove long wavelength slope effects the global surface roughness of eros is found to be self affine at all baselines investigated the surface roughness is statistically correlated with crater density at baselines of 100 200 m and boulders at a baseline of 5 m no global spatial statistical correlation was found for baselines of 4 200 and mapped tectonic lineaments ponds slope or geopotential elevation the surface roughness of the crater shoemaker charlois regio is controlled by the interplay of a high boulder density producing higher surface roughness values at small baselines and low crater density lowering surface roughness values at long baselines we estimated the mobile regolith thickness regolith that moves around and infills topography to be 0 2 6 2 m from the difference in the surface roughness values at the baseline of 4 m furthermore we find that the change in rms deviation as a function of baseline compares favorably with the moon and differs significantly from existing results for rubble pile asteroid itokawa

monazite paragenesis and u pb systematics in rocks of the eastern mojave desert california u s a implications for thermochronometry abstract studies of the paragenesis and u pb systematics of monazite in rocks from the eastern mojave desert california corroborate its potential usefulness as a prograde thermochronometer and in dating granite inheritance unmetamorphosed latham shale and its equivalents at grades ranging from greenschist to upper amphibolite facies are virtually identical in composition monazite is absent in the shale and low grade schists but it is abundant in schists at staurolite and higher grades lower grade schists instead include minute th and ce oxides and unidentified ce poor lree phosphates that apparently are lower temperature precursors to monazite thus monazite originates when the pelite passes through lower amphibolite facies conditions monazites from three upper cretaceous granites yield ages that are strongly discordant upper intercepts of 1 6 1 7 ga are similar to those defined by u pb data for coexisting zircons and coincide with a period of copious magmatism in the mojave crust as the host upper cretaceous granitic magmas were all above 700 c effective closure of the restitic monazites to pb loss must be well in excess of this temperature u pb compositions of monazite from proterozoic granitoids and schist also indicate high pb retentivity taken together these studies support the suggestion that monazite can be an effective prograde thermochronometer at least in pelites it is not usually retained as a detrital mineral but rather forms during moderate temperature metamorphism its u pb system should not be reset by subsequent higher grade metamorphism

monazite u pb dating of staurolite grade metamorphism in pelitic schists a study of the occurrence of and relations between rare earth element ree minerals in pelitic schists indicates that monazite forms at or near the p and t of the staurolite isograd samples at staurolite grade from the silurian perry mountain formation in the rumford quadrangle of maine yield monazite in sufficient quantities to permit accurate dating of the metamorphic events forming the monazites the bulk chemistry of the metapelites as seen in the major element abundances and ree patterns does not vary significantly across the study area thus the appearance and disappearance of ree phases is assumed to reflect changes in metamorphic grade in a sample from the biotite zone scanning electron microscope and microprobe studies show allanite and monazite intimately associated on a 10 m scale the texture suggest that metastable detrital monazite breaks down distributing its ree components to allanite from samples below staurolite grade in which monazite is not present our observations suggest that rees are partitioned into allanite at or near the staurolite isograd monazite forms as a metamorphic mineral initiating its role as a geochronometer garnet biotite geothermometry on samples at this grade from this and other studies places constraints on the minimum temperature necessary to form monazite 525 c 25 c at 3 1 0 25 kbar a total of 15 separates from nine schist samples ranging up to sillimanite grade have been dated each date is remarkably concordant even though petrologic and textural studies by previous workers have shown that the rocks in the area have been affected by at least three metamorphic episodes calculations indicate insignificant th disequilibrium in these monazites the conditions associated with the metamorphic events suggest that monazite remains closed to lead loss provided that subsequent metamorphisms are at or below sillimanite grade two distinct metamorphic events are resolved one at around 400 ma and one at about 370 ma the latter was due to thermal effects of a nearby pluton that yields concordant monazite ages of 363 ma this work suggests that in addition to dating plutonism and high grade metamorphism monazite should be viewed as a reliable geochronometer for moderate metamorphism of pelitic schists

u pb age of the baltoro granite northwest himalaya and implications for monazite u pb systematics the baltoro granite is a major late to post tectonic plutonic phase of the karakoram batholith of the northwest himalaya in northern pakistan u pb zircon analyses indicate both emplacement at 21 0 0 5 ma and significant precambrian zircon inheritance dates on monazite are 17 19 ma and are interpreted to have remained near their closure temperature of about 700 c for several million years after emplacement resulting in pb loss by diffusion we suggest that the granite was emplaced into rocks which were at high temperature and that they remained so until late miocene northeast tilting rapid uplift and or tectonic denudation and cooling

phobos regolith and ejecta blocks investigated with mars orbiter camera images four flybys of phobos by the mars global surveyor spacecraft in 1998 allowed imaging at resolutions 2 7 m better than obtained for any other satellite or asteroid except earth s moon the images show the interior and vicinity of the large crater stickney there are 2000 identifiable ejecta blocks the largest about 85 m across the great majority of these blocks come from stickney as part of low velocity ejecta spread eastward by the influence of phobos s rapid rotation they appear to have experienced only a few meters at most of burial or modification since emplacement the images also show materials of different albedos within stickney that mark downslope motion of regolith the number of craters on the steep slopes of stickney that are subject to the downslope motion suggests that 4 km have been degraded to the point that they are no longer recognizable

petrology of basement dominated terranes ii contrasting isotopic sr nd pb and o signatures of basement derived granites and constraints on the source region of granite damara orogen namibia abstract major and trace element and nd sr o and pb isotope data from granites of the high grade central part of the damara orogen khan and outlet gorge areas namibia indicate a dominantly lower crustal origin based on their appearance in field the granites can be divided into red granites white granites and grey granodiorites red granites and some of the white granites from the khan area are isotopically evolved initial e nd 12 5 to 18 9 and were likely derived from metaigneous sources with late archaean to early proterozoic crustal residence ages other white granites are less evolved initial e nd 6 5 to 8 8 and were likely derived from metasedimentary sources that are similar to the country rock metapelites grey granodiorites from the khan and outlet gorge area are also isotopically evolved initial e nd 9 9 to 13 1 but are derived from metaigneous sources with younger late proterozoic crustal residence ages major and trace element data do not support closed system fractional crystallization processes for all samples however some chemical features i e decreasing rare earth element ree abundances with increasing sio 2 underline the importance of crystal fractionation processes for each distinct magmatic pulse isotope data do not support mixing of different crust derived melts or assimilation of crustal rocks by a mafic magma instead highly evolved pb isotope compositions strongly negative e nd values and radiogenic sr isotope compositions argue for an undepleted basement as a potential source the most likely model involves mainly partial melting of different basement rocks of archaean to proterozoic age at different levels within the crust only some granites could be derived by melting of metasedimentary rocks of the pan african cover sequence the consistency of the chemical data with a crustal anatectic origin and the range in radiometric ages suggests that they intruded simultaneously with crustal thickening however some of them may have been emplaced during extensional tectonics c 40 50 ma later than the main period of crustal thickening the heating events that promoted melting of fertile deep crustal rocks might have been caused either by the inferred high heat productivity together with crustal thickening during the main periods of orogeny or by delamination of mantle lithosphere during the final extensional stages of the pan african orogeny

electron microprobe observations of pb diffusion in metamorphosed detrital monazites electron microprobe analyses have been made on monazite grains from paragneiss samples in the andalusite sillimanite transition 620 15 c and sillimanite orthoclase 680 15 c zones of the cretaceous ryoke metamorphic belt southwest japan monazites from pelitic gneisses are of metamorphic origin euhedral to subhedral and chronologically homogeneous giving chemical th u total pb isochron chime ages of 98 8 3 3 98 0 3 2 ma two psammitic gneisses of individual metamorphic grade contain both metamorphic monazite grains and detrital ones as old as ca 1700 ma most detrital monazite grains are heterogeneous in the tho2 and uo2 concentrations and have multiple or single rims as young as ca 100 ma several detrital monazite grains are well rounded in form exhibit homogeneous th and u distributions and show a pb diffusion profile in the margin the width of the diffusion zones is approximately constant throughout grains from each psammitic gneiss 18 22 m for 620 c and 48 58 m for 680 c assuming the isothermal diffusion of pb from homogeneous monazite spheres during a 5 ma duration of peak metamorphism we obtain diffusion coefficients of 1 9 0 3 10 21 and 1 5 0 3 10 20 cm2 s at 620 c and 680 c respectively these data derive an activation energy of 2 44 2 85 1 26 105 j mol and a frequency factor of 3 4 10 7 8 5 10 12 2 2 107 cm2 s taking account of uncertainties of 15 c in the temperatures and of 20 in the diffusion coefficients the diffusion parameters obtained from natural samples in this study provide a reliable insight into the closure temperature for pb in monazite that has been poorly understood so far

trace element fractionation during high grade metamorphism and crustal melting constraints from ion microprobe data of metapelitic migmatitic and igneous garnets and implications for sm nd garnet chronology abstract rare earth element ree and other trace element y sr ti cr v na abundances in garnet from a garnet bearing metapelite a pelitic migmatite a syn tectonic granite and a post tectonic leucogranite were measured by secondary ion mass spectrometry sims in order to identify the effective variables on the trace element distribution between garnet and the host rock garnet from the garnet bearing metapelite the pelitic migmatite and the syn tectonic granite is zoned with respect to ree the cores are enriched by a factor of 2 3 relative to the rims for the garnets from the garnet bearing metapelite equilibrium distribution following a simple rayleigh fractionation is responsible for the decreasing concentrations in ree from core to rim garnet from the pelitic migmatite shows a more complex trace element pattern following distinct enrichment and depletion patterns for ti v cr and ree from core to rim these features suggest disequilibrium between garnet and the associated melt in which the enrichment of trace elements probably correspond to a period of open system behaviour in these rocks at a time when the garnet originally nucleated in the metamorphic environment was incorporated into the melt the garnet from the syn tectonic granite shows stepwise decreasing concentrations in ree from core to rim a ree rich core can be distinguished from a broad ree depleted rim notably from core to rim an inflection of the yb er and yb dy ratios is visible whereas the decrease of hree abundance in the core region of the garnet from the syn tectonic granite may arise from equilibrium partitioning during garnet growth the inflection can be interpreted as a result of partial melting garnet cores with high yb er and yb dy 1 nucleated in the metamorphic environment without the presence of a melt whereas the rims with lower yb er and yb dy

boulders and ponds on the asteroid 433 eros abstract there are 300 features on the asteroid 433 eros that morphologically resemble ponds flat floored and sharply embaying the bounding depression in which they sit because boulders on eros are apparently eroding in place and because ponds with associated boulders tend to be larger than ponds without blocks we propose that ponds form from thermally disaggregated and seismically flattened boulder material under the assumption that repeated day night cycling causes material fatigue that leads to erosion of the boulders results from a simple boulder emplacement thermal erosion model with boulders emplaced in a few discrete events i e large impacts match well the observed size distribution under this scenario the subtle color differences of ponds somewhat bluer than the rest of the surface might be due to some combination of less space weathered material and density stratification of silicate rich chondrules and more metal rich matrix from a disaggregated boulder volume estimates of ponds derived from near laser rangefinder profiles are consistent with what can be supplied by boulders ponds are also observed to be concentrated in regions of low slope and high elevation which suggests the presence of a less mobile regolith and thus a contrast in the resistance to seismic shaking between the pond material and the material that makes up the bounding depression future tests include shake table experiments and temperature cycling fatigue of ordinary chondrites to test the thermal erosion mechanism

lead isotopic heterogeneities within alkali feldspars implications for the determination of initial lead isotopic compositions abstract the mobility of uranium during metamorphism and alteration places constraints on the use of lead isotopic compositions as petrogenetic tracers in older rocks the problem of uranium mobility may be partially circumvented by analysis of alkali feldspar which because of its low u pb ratio should have a lead isotopic composition similar to the initial lead isotopic composition of the rock previous leaching and volatilization experiments however have demonstrated that alkali feldspars contain radio genic lead which can be removed preferentially to nonradiogenic lead this paper reports on the results of progressive acid leaching of a single alkali feldspar phenocryst and on albitic perthites separated from the phenocryst this experiment suggests that a number of distinct lead isotopic reservoirs are present within the alkali feldspar including a nonradiogenic reservoir approximating the initial lead isotopic composition of the system a radiogenic reservoir corresponding to albitic exsolution lamellae within the alkali feldspar loosely bound labile lead presumably associated with uranium and thorium occupying fractures and finally a radiogenic component corresponding to unidentified uranium thorium rich phases the commonly observed decrease in radiogenicity with progressive leaching of alkali feldspars is interpreted to be largely the result of preferential leaching of the albitic perthites relative to the host alkali feldspar containing the nonradiogenic reservoir the results of this experiment confirm that initial lead isotopic compositions may be estimated from alkali feldspars through severe acid leaching however this experiment also illustrates the complexity of the lead isotopic systematics of alkali feldspars the determination of the initial lead isotopic composition of an alkali feldspar requires experimental procedures which recognize and compensate for the presence of multiple lead reservoirs within typical alkali feldspars

a metamorphic history from micron scale 207pb 206pb chronometry of archean monazite ion microprobe measurements of pb isotope ratios in monazites have been obtained in situ from thin sections using the cambridge isolab 120 molecular interferences are sufficiently resolved at an rp of 6500 to allow 2 7pb z 6pb dating of monazite with precisions as low as 4 5 ma 2 r the results presented here provide important information on the chronological history of the late archean metamorphism of the wind river range wyoming usa matrix monazites and monazite inclusions in garnets from a metapelite from the northern wind river range have been analysed by sims in a previous study peak metamorphic conditions t 800 c p 8 1 kb were estimated using inclusion assemblages in garnets from this same sample isolated monazite inclusions in garnet yield 2 7pb z 6pb age estimates of 2781 6 to 2809 10 ma those along fractures yield lower ages 2603 2687 ma which are similar to tims and sims ages of matrix monazites a single large 500 xm monazite grain locally preserves growth zoning but has a recrystallised core and a resorbed recrystallised rim age estimates for these three regions are 2788 9 ma 2663 4 and 2523 6 ma respectively thus the inclusion assemblages of sharp and essene may record peak metamorphic conditions at ca 2 8 ga and indicate a phase of metamorphism that predates by over 100 ma the emplacement of the bridget batholith the major lithologic component of the northern wind river range the analysed monazite grains appear to preserve ca 300 ma history even within a single grain monazite inclusions in garnet that are fully armoured may provide estimates for the time of garnet growth even in high grade terranes where most chronometers are reset the age pattern preserved by the large monazite grain cannot be simply related to diffusion controlled closure instead a chronology is preserved which can be related to the petrographic setting of individual grains through in situ analysis

dating magmatic and hydrothermal processes using andradite rich garnet u pb geochronometry andradite rich garnet is a common u bearing mineral in a variety of alkalic igneous rocks and skarn deposits but has been largely neglected as a u pb chronometer in situ laser ablation inductively coupled plasma mass spectrometry u pb dates of andradite rich garnet from a syenite pluton and two iron skarn deposits in the north china craton demonstrate the suitability and reliability of the mineral in accurately dating magmatic and hydrothermal processes two hydrothermal garnets from the iron skarn deposits have homogenous cores and zoned rims ad86gr11 to ad98gr1 with 22 118 ppm u whereas one magmatic garnet from the syenite is texturally and compositionally homogenous ad70gr22 to ad77gr14 and has 0 1 20 ppm u all three garnets have flat time resolved signals obtained from depth profile analyses for u indicating structurally bound u uranium is correlated with ree in both magmatic and hydrothermal garnets indicating that the incorporation of u into the garnet is largely controlled by substitution mechanisms two hydrothermal garnets yielded u pb dates of 129 2 2 mswd 0 7 and 130 1 ma 2 mswd 0 5 indistinguishable from zircon u pb dates of 131 1 and 129 1 ma for their respective ore related intrusions the magmatic garnet has a u pb age of 389 3 ma 2 mswd 0 6 consistent with a u pb zircon date of 388 2 ma for the syenite the consistency between the garnet and zircon u pb dates confirms the reliability and accuracy of garnet u pb dating given the occurrence of andradite rich garnet in alkaline and ultramafic magmatic rocks and hydrothermal ore deposits our results highlight the potential utilization of garnet as a powerful u pb geochronometer for dating magmatism and skarn related mineralization

phobos deimos and the moon size and distribution of crater ejecta blocks abstract the size and radial distributions of ejecta blocks around craters d 0 8 to 10 km on phobos and deimos have been compared to those around lunar craters d 0 2 to 3 5 km the radial distribution of blocks was found to be similar on phobos and the moon but more dispersed on deimos for the best imaged crater on deimos d 800 m the size distributions of blocks and the fraction of excavated volume present as blocks are similar to those on the moon the wider dispersal of blocks on deimos is consistent with other findings on the spread of finer ejecta over the satellite

the petrogenesis of some migmatites and granites central damara orogen namibia evidence for disequilibrium melting wall rock contamination and crystal fractionation

textural geochronological and chemical constraints from polygenetic titanite and monogenetic apatite from a mid crustal shear zone an integrated epma sims and tims study abstract titanites extracted from a 520 9 ma syn orogenic syenite at otjimbingwe damara orogen namibia show shape colour and u pb systematics that vary with position in the syenite relative to a major shear zone most titanite fractions from syenite cropping out outside the shear zone are characterized by a medium brown anhedral core overgrown by a pale brown subhedral rim empa and sims analyses show that the cores are enriched in al fe and sometimes na mg and mn but also in ree nb and zr relative to the rim tims u pb ages from these titanites gave relatively imprecise 207pb 206pb ages between 680 21 ma and 695 20 ma as well as 570 8 ma to 604 9 ma mainly due to their low 206pb 204pb ratios combined th u pb relationships suggest that these titanites contain a component similar to ca 715 750 ma old titanite populations observed so far only in neoproterozoic syenites from the northern part of the orogen titanites from syenites within the shear zone are dark brown mostly euhedral to subhedral and display only occasionally a pale brown rim these titanites show decreasing concentrations of na mg al and fe and increasing concentrations of mn from core to rim interpreted to reflect growth zonation abundances of mree hree y nb and zr are higher in the rim than in the core 207pb 206pb ages obtained on these titanites range from 470 2 ma to 538 3 ma for most samples only the samples with the highest 206pb 204pb ratios between 350 and 230 have u pb ages similar to the previously published u pb zircon age of 520 9 ma from the same complex other titanite fractions with lower 206pb 204pb ratios have slightly older ages than ca 520 ma indicating some inheritance inherited titanite survived multiple metamorphic episodes in the damara orogen between ca 750 ma and 470 ma and the igneous event that lead to the intrusion of the otjimbingwe syenite at 520 ma these data imply a high closure temperature of u pb in titanite of 650 700 c consistent with previous studies coeval apatite has no apparent inheritance low 206pb 204pb ratios between 55 and 170 and u pb ages similar to or only slightly younger than the u pb zircon or titanite age obtained on the titanite fractions with the highest 206pb 204pb ratios assuming a closure temperature for u pb of 900 c for zircon 700 c for titanite and 450 c for apatite initial fast cooling of the complex can be inferred our data demonstrate that titanite can contain multiple age domains preserving a chronological and chemical record of previous igneous events

isotopic constraints on pan african metamorphism in dronning maud land antarctica abstract garnet sm nd isotopic data are presented for selected samples from the metamorphic terrane of the h u sverdrupfjella western dronning maud land and yield pan african ages ranging from 680 to 415 ma reproducibility of the garnet data is outside analytical error but this does not affect the consistency of ages obtained the garnet ages are considerably younger than the inferred initial crystallization age based on petrographical and fabric analysis structural and whole rock and zircon isotopic data which support initial growth during the grenvillian orogeny 1200 1100 ma the garnet ages however are indistinguishable from a large number of rb sr mineral ages the possibility that the garnets remained above the blocking temperature until pan african exhumation is discounted polymetamorphism of the metamorphic terrane associated with both the grenbillian and pan african orogenes is favoured the garnet ages are interpreted as reset ages but the possibility that the garnets crystallized during the pan african event cannot be discounted in this case the data imply that the pan african orogeny attained lower granulite to amphibolite facies metamorphic conditions as indicated by geothermometry and geobarometry calculations involving garnet in either case the h u sverdupfjella area was subjected to significant pan african heating to temperatures of the order of 600 650 c

the effect of fluid and deformation on zoning and inclusion patterns in poly metamorphic garnets within the bergen arcs of w norway caledonian eclogite facies assemblages t 650 c p 15 kbar have formed from grenvillian granulites t 800 900 c p 10 kbar along shear zones and fluid pathways garnets in the granulites grti pyr56 40 alm45 25gro19 14 are unzoned or display a weak ca 1 wt feo over 1000 m zoning the eclogite facies rocks contain garnets inherited from their granulite facies protoliths these relict garnets have certain areas with compositions identical to the garnets in their nearby granulite but can be crosscut by bands of a more almrich composition grtii pyr31 41alm40 47gro17 21 formed during the eclogite facies event these bands orientated preferentially parallel or perpendicular to the eclogite foliation may contain mineral filled veins or trails of eclogite facies minerals omphacite amphibole white mica kyanite quartz and dolomite steep compositional gradients up to 9 wt feo over 40 m separate the two generations of garnets indicating limited volume diffusion the bands are interpreted as fluid rich channels where element mobility must have been infinitely greater than it was for the temperature controlled volume diffusion at mineral interfaces in the granulites the re equilibration of granulite facies garnets during the eclogite facies event must therefore be a function of fracture density deformation and fluid availability the results cast doubts on modern petrological and geochronological methods that assume pure temperature controlled chemical re equilibration of garnets

geochronology and geochemistry of multiple generations of monazite from the wepawaug schist connecticut usa implications for monazite stability in metamorphic rocks u pb isotope analyses rare earth and trace element analyses and petrographic observations are presented for monazites from the wepawaug schist in southern connecticut usa two samples of kyanite zone schist were collected less than a meter apart each sample contains a different variety of monazite with distinct morphology chemistry and pb isotopic composition one sample preserves a largely amphibolite facies mineralogy including kyanite staurolite garnet biotite and chlorite with little textural evidence of later shearing monazites from this sample are xenoblastic with about 1 wt tho2 0 3 wt cao and a more lree enriched pattern than monazites from the second sample these xenoblastic monazites preserve textural evidence of a retrograde reaction to apatite which involves chlorite indicating that these monazites became unstable during retrograde chloritization of biotite these monazites give strongly discordant u pb ages which fit a chord with an upper intercept age of 411 18 ma interpreted as the minimum growth age of these xenoblastic monazites perhaps during amphibolite facies metamorphism the second sample contains s c banding evidence of dynamic recrystallization and abundant retrograde chlorite this sample contains idioblastic monazites with about 3 wt tho2 0 8 wt cao and with less fractionated ree patterns these monazites give close to concordant u pb ages with a mean 207pb 206pb age of 388 2 ma this age is interpreted as probably representing the time of monazite growth during retrogression of the sample from an amphibolite to greenschist facies mineralogy

smnd isotopic systematics of a gabbro eclogite transition abstract sm nd isotopic systems have been studied in 1 a sequence of coronitic olivine gabbros showing different degrees of transition to eclogite 2 coronitic leuco gabbro norite and 3 country rock eclogite all from the nordoyane brattvag area western norway isochrons defined by combinations of whole rock samples and relict igneous phases give ages of 1198 56 and 1289 48 ma olivine gabbro and 926 70 ma leuco gabbro norite and have been interpreted in terms of two different episodes of igneous intrusion in gabbro with advanced corona formation relict augite tends to retain its original nd isotopic composition while plagioclase may show selective disturbance related to recrystallization and partial replacement by garnet in completely eclogitized samples there is a correlation between the degree of isotopic equilibrium and microstructural equilibrium whole rock eclogites occurring within gabbro are characterized by pseudomorphs after igneous phases and preserve original igneous nd isotopic composition results for eclogite minerals suggest caledonian ages but these phases have not achieved complete isotopic equilibrium in contrast nd isotopic systems in texturally equilibrated and strongly deformed eclogite within adjacent gneisses have been disturbed on a whole rock scale minerals from an external eclogite define a sm nd isochron with an age of 400 16 ma which is interpreted to date synkinematic eclogite equilibration at high p and t conditions

a damara orogen perspective on the assembly of southwestern gondwana the pan african damara orogenic system records gondwana amalgamation involving serial suturing of the congo sao francisco and ro o de la plata cratons north gondwana from 580 to 550 ma before amalgamation with the kalahari antarctic cratons south gondwana as part of the 530 ma kuunga damara orogeny closure of the adamastor ocean was diachronous from the aracuao belt southwards with peak sinistral transpressional deformation followed by craton overthrusting and foreland basin development at 580 550 ma in the kaoko belt and at 545 530 ma in the gariep belt peak deformation metamorphism in the damara belt was at 530 500 ma with thrusting onto the kalahari craton from 495 ma through to 480 ma coupling of the congo and ro o de la plata cratons occurred before final closure of the mozambique and khomas damara belt oceans with the consequence that the kuunga suture extends into africa as the damara belt and the lufilian arc and zambezi belt of zambia palaeomagnetic data indicate that the gondwana cratonic components were in close proximity by c 550 ma so the last stages of the damara kuunga orogeny were intracratonic and led to eventual out stepping of deformation metamorphism to the ross delamerian orogen c 520 500 ma along the leading edge of the gondwana supercontinental margin

combined high precision chronometry and geochemical tracing using accessory minerals applied to the central alpine bergell intrusion central europe abstract the behaviour of nd u th and pb isotopes in granitic rocks is usually controlled by accessory minerals in this study an approach is adopted which uses the initial 143 nd 144 nd as a geochemical tracer together with high precision u th pb geochronology for accessory phases and their respective whole rock this combination of techniques is applied to zircon allanite sphene apatite and thorite from both a tonalite and a granodiorite from the tertiary alpine bergell intrusion inherited isotopic signatures have been identified in bulk zircon fractions by inverse discordance in the u pb system in the case of the granodiorite the initial nd in the zircon is 1 77 units lower than the bulk rock this result indicates that nd isotopes in zircon may serve as a useful tool for tracing crustal precursors in silicic rocks all other accessories yield an initial nd equal to their respective whole rock and u th pb ages equal to or lower than the respective intrusion ages this indicates either the absence of inherited cores or that such cores have exchanged their nd and pb in the case of apatite also sr by diffusion where available experimental mineral melt equilibria and diffusion models are consistent with this behaviour u pb geochronology of carefully selected core free zircons yields a precise intrusion age of 31 88 0 09 ma for the tonalite in contrast granodioritic zircons are partially reset due to radiation damage caused by high u concentrations however an alternative means of dating young intrusions is to use 232th 208pb ages of allanite in the case of allanite the presence of a large excess of 206pb derived from 230th indicates that no total pb loss has occurred since crystallization and the nd isotopes indicate the absence of inheritance using both the allanite age and a 238u 206pb age for sphene an intrusion age of 30 13 0 17 ma is indicated for the granodiorite taken together with an allanite th pb age for the tonalite these results strongly support the high closure temperatures for the allanites studied here 700 800 c at high post magmatic cooling rates all other accessory mineral ages are reset to values similar to hornblende 39ar 40ar ages and reflect the effect of post magmatic cooling superimposed on post metamorphic cooling

nd and sr systematics in an eclogite xenolith from tanzania evidence for frozen mineral equilibria in the continental lithosphere combined isotopic nd sr and petrographic analyses are reported for an eclogitic xenolith from the vissury mine tanzania an exceedingly large difference in measured nd values in garnet and clinopyroxene 200 units suggest a major mineral equilibration at 1 75 0 014 ga nd isotopic heterogeneities are demonstrated in different generations of garnet and zonation in sr and nd isotope compositions is present within the clinopyroxene the variation in measured nd isotope compositions in different generations of garnet corresponds to 16 units exsolution of garnet from clinopyroxene occurred in an essentially closed system with respect to sr and nd petrographically different generations of garnet are present one garnet with rutile inclusions as well as the rims of clinopyroxene are clearly off the isochron features like this can originate either if a system is not closed e g affected by a migrating fluid or if a remanent isotopic heterogeneity survived the homogenisation at 1 75 ga the subsolidus partitioning of sm and nd between garnet and clinopyroxene results in higher fractionation of the smnd ratio in garnet than in the liquid crystal partitioning since the closure temperature for the sm nd exchange between garnet and clinopyroxene may be around 850 c in a dry system many of the lithospheric ultramafic rocks may exhibit frozen mineral equilibria the isotopic data suggest that this eclogite was emplaced at 1 75 ga into the lithosphere mantle at temperatures of about 1040 c and then cooled as a closed system at a rate of 6 50 c my down to 840 c where all the exchange reactions were efficiently frozen in at 1 75 ga ago the coincidence of the emplacement of granites 1 7 1 8 ga into the central part of the tanzanian craton with the age of the eclogite suggests that lower crustal melting was triggered by a thermal event in the upper lithosphere

trace element and isotopic sr nd pb o arguments for a mid crustal origin of pan african garnet bearing s type granites from the damara orogen namibia geochronological data major and trace element abundances nd and sr isotope ratios 18o whole rock values and pb isotope ratios from leached feldspars are presented for garnet bearing granites locality at oetmoed and outcrop 10 km north of omaruru from the damara belt namibia for the granites from outcrop 10 km n omaruru reversely discordant u pb monazite data give 207pb 235u ages of 511 2 ma and 517 2 ma similar to previously published estimates for the time of regional high grade metamorphism in the central zone based on textural and compositional variations garnets from these granites are inferred to be refractory residues from partial melting in the deep crust because p t estimates from these xenocrystic garnets are significantly higher 800 c 9 10 kbar than regional estimates 700 c 5 kbar the monazite ages are interpreted to date the peak of regional metamorphism in the source of the granites sm nd garnet whole rock ages are between 500 and 490 ma indicating the age of extraction of the granites from their deep crustal sources for the granites from oetmoed both sm nd and pb pb ages obtained on igneous garnets range from 500 to 490 ma these ages are interpreted as emplacement ages and are significantly younger than the previously proposed age of 520 ma for these granites based on rb sr whole rock age determinations major and trace element compositions indicate that the granites are moderately to strongly peraluminous s type granites high initial 87sr 86sr ratios 0 716 high 18o values of 13 8 negative initial end values between 4 and 7 and evolved pb isotope ratios indicate formation of the granites by anatexis of mid crustal rocks similar to the exposed metapelites into which they intruded the large range of pb isotope ratios and the lack of correlation between pb isotope ratios and nd and sr isotope ratios indicate heterogeneity of the involved crustal rocks evidence for the involvement of isotopically highly evolved lower crust is scarce and the influence of a depleted mantle component is unlikely the crustal heating events that produced these granites might have been caused by crustal thickening and thrusting of crustal sheets enriched in heat producing elements very limited fluxing of volatiles from underthrust low to medium grade metasedimentary rocks may have also been a factor in promoting partial melting furthermore delamination of the lithospheric mantle and uprise of hot mantle could have caused localized high t regions the presence of coeval a type granites at oetmoed that have been derived at least in part from a mantle source supports this model

redistribution of rare earth elements thorium and uranium over accessory minerals in the course of amphibolite to granulite facies metamorphism the role of apatite and monazite in orthogneisses from southwestern norway the amphibolite to granulite facies transition has been studied in a high k calc alkaline hornblende biotite k feldspar megacrystic augen gneiss series from the rogaland vest agder rog va sector of the sveconorwegian province southwest norway hornblende begins to break down mainly to clinopyroxene at the cpx in isograd and biotite mainly to orthopyroxene at the opx in isograd abbreviations for minerals following kretz 1983 the magmatic accessory mineral association comprises titanite and allanite which begin to break down before the cpx in isograd titanite is preserved as relict inclusions in other minerals at higher grade monazite and thorite formed in the breakdown of allanite monazite abundance reaches a maximum between the cpx in and opx in isograds the middle and heavy rare earth element m hrees except eu content of apatite steeply increases with increasing metamorphic grade and is directly correlated to the decrease of the modal abundances of titanite hornblende and biotite the u contents of apatite are low and do not increase with metamorphic grade the light rare earth elements lrees and th content of apatite are not correlated to the breakdown of allanite around the cpx in isograd but increase around the opx in isograd a simplified equation is proposed for monazite crystallisation in the vicinity of the cpx in isograd that accounts for the allanite titanite and hornblende breakdowns and the m hree substitution in apatite 3 m hree 2o3in hornblende and litanite 3 lree 2in allaniteo3 2ca5 po4 3apatite f oh 6sio2quartz 6 lree pmonaziteo4 2ca2 m hree 3 sio4 3 f oh lessingite in apatite 6caoin plagioclase at the opx in isograd the increase of lrees and th content of apatite results either from the breakdown of some monazite 3 lree po4monazite 3sio2quartz 4caoin plagioclase f2 h2o fluid ca5 po4 3 f oh apatiteca2 lree 3 sio4 3 f oh less ingite in apatite or the breakdown of the remaining allanite 3 lree 2o3in allanite 6sio2quartz 4caoin plagioclase f2 h2o fluid 2ca2 lree 3 sio4 3 f oh lessingite in apatite the release of fluorine from the breakdown of biotite at the opx in isograd may increase apatite stability relatively to monazite in the granulite facies when compared to amphibolite facies the granulite facies augen gneisses do not show any u or th depletion this means that changes in the accessory mineral associations and the simultaneous breakdown of hydrous minerals at the amphibolite granulite facies transition do not inevitably result in depleted granulite facies rocks but rather to an isochemical element redistribution the coexistence of small amounts of metamorphic monazite and of relict inclusions of titanite in upper amphibolite facies augen gneisses suggests that high k calc alkaline orthogneisses are a suitable material to date with the u pb method prograde path of amphibolite facies regional metamorphism on monazite and the cooling path on titanite

petrology of an intrusion related high grade migmatite implications for partial melting of metasedimentary rocks and leucosome forming processes intrusion related migmatites comprise a substantial part of the high grade part of the southern damara orogen namibia which is dominated by al rich metasedimentary rocks and various granites migmatites consist of melanosomes with biotite sillimanite garnet cordierite hercynite and leucosomes are garnet and cordierite bearing metamorphic grade throughout the area is in the upper amphibolite to lower granulite facies 5 6 kbar at 730 750 c field evidence petrographic observations chemical data and mass balance calculations suggest that intrusion of granitic magmas and concomitant partial melting of metasedimentary units were the main processes for the generation of the migmatites the intruding melts were significantly modified by magma mixing with in situ partial melts accumulation of mainly feldspar and contamination with garnet from the wall rocks however it is suggested that these melts originally represented disequilibrium melts from a metasedimentary protolith the occurrence of lile hfse and lree enriched and depleted residues within the leucosomes implies that both quartzo feldspathic and pelitic rocks were subjected to partial melting isotope ratios of the leucosomes are rather constant 143nd 144nd 500 ma 0 511718 0 511754 e nd 500 ma 3 54 to 5 11 and sr 87sr 86sr 500 ma 0 714119 0 714686 the metasedimentary units have rather constant nd isotope ratios 143nd 144nd 500 ma 0 511622 0 511789 e nd 500 ma 3 70 to 6 93 but variable sr isotope ratios sr 87sr 86sr 500 ma 0 713527 0 722268 the most restitic melanosome mel 4 has a sr isotopic composition of 87sr 86sr 500 ma 0 729380 oxygen isotopes do not mirror the proposed contamination process due to the equally high 18o contents of metasediments and crustal melts however the most lile depleted residue mel 4 shows the lowest 18o value 10 mass balance calculations suggest high degrees of partial melting 20 40 it is concluded that partial melting was promoted by heat transfer and release of a fluid phase from the intruding granites high degrees of partial melting can be reached as long as the available h2o derived from the crystallization of the intruding granites is efficiently recycled within the rock volume due to the limited amounts of in situ melting it seems likely that such regional migmatite terranes are not the sources for large intrusive granite bodies the high geothermal gradient inferred from the metamorphic conditions was probably caused by exhumation of deep crustal rocks and contemporaneous intrusion of huge masses of granitoid magmas the davetsaub area represents an example of migmatites formed at moderate pressures and high temperatures and illustrates some of the reactions that may modify leucosome compositions the area provides constraints on melting processes operating in high grade metasedimentary rocks

the effect of allanite inclusions on u pb step leaching ages and sm nd isotope systematics of garnet from the ogcheon metamorphic belt south korea abstract the ogcheon metamorphic belt consists primarily of metasedimentary and metavolcanic rocks that have experienced polyphase tectonometamorphism since the neoproterozoic peak metamorphism reaching up to lower amphibolite facies produced ubiquitous garnet porphyroblasts in pelitic and mafic schists to determine the timing of their formation step leaching experiments were undertaken for five garnet fractions separated from pelitic and quartz hornblende garnet schists the u pb ages from three samples are identical within 2 errors ranging from 291 41 ma to 276 29 ma the quasi linearity of leachates in 238u 206pb and 208pb 206pb diagrams suggests that u and pb are released from a single mineral phase and that minor chemical fractionation between u and pb may have occurred during the leaching experiment deviations of residues and bulk garnet fractions from the linear trend are attributed to partial dissolution of refractory inclusions of detrital zircon th u ratios of leachates are in the range of 3 4 12 much higher than those of pure garnet and suggest the contribution of allanite negative relationships in the sm nd isochron diagram and similar 147sm 144nd ratios between whole rock and garnet corroborate the influence of light rare earth element lree rich allanite on the sm nd isotopic system simple mass balance calculations indicate that only a trace amount 0 35 modal of allanite inclusions should govern the u th pb systematics of garnet petrographic evidence together with the consistency in u pb ages suggests that allanite is a product of prograde metamorphism thus peak metamorphism responsible for the growth of allanite bearing garnet porphyroblasts in the ogcheon metamorphic belt is best estimated to be early permian

u pb dating of major and accessory minerals formed during metamorphism and deformation of metapelites abstract u pb isotope analyses on major and accessory minerals from pelitic schists in of southwestern connecticut are used to date metamorphism and deformation most of the major minerals have 206 pb 204 pb ratios less than 100 however u pb mineral isochron and mineral whole rock isochrons constructed using aliquots of the same mineral yield concordant ages with uncertainties as low as 5 ma staurolite kyanite and garnet are older than metamorphic monazites and zircons these data are summarized below 5 mineral 206 pb 204 pb system the straits schist trap falls fm monazite zircon 207 pb 206 pb 379 384 ma 380 2 ma staurolite 48 623 238 u 206 pb 394 400 ma 398 5 ma garnet 22 46 238 u 206 pb 383 ma 414 10 ma 235 u 207 pb 418 ma 207 pb 206 pb 623 ma kyanite 21 27 238 u 206 pb 399 ma chlorite anatase ilmenite 20 27 238 u 206 pb 350 370 ma apatite quartz kspar 18 19 transposed pegmatitic intrusions in the straits schist yield monazites with u pb ages analytically indistinguishable from those of metamorphic monazites and zircons in the schists growth of monazite and zircon in the schists may be related to high t fluid influx associated with pegmatite intrusion age discordance in the garnets from the straits schist is most consistent with continuous loss of intermediate daughter products of the 238 u decay chain from about 420 ma to 380 ma garnets from the adjacent trap falls formation do not display this behavior

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

electron microprobe dating of monazites from high grade gneisses and pegmatites of the kerala khondalite belt southern india monazites of five samples one leptynitic garnet biotite gneiss one khondalite one augen gneiss and two pegmatites from the central and northern part ponmudi unit of the kerala khondalite belt kkb in southern india were analyzed with the electron microprobe dating technique monazites in the augen gneiss and the pegmatites yield grain sizes between 200 800 m th abundances are rather low 10 wt and the distribution of th u and pb within single grains is fairly homogeneous contrasting to this monazites in the leptynitic gneiss and the khondalite are small 150 m they often display very complex th u pb patterns and contain high th concentrations up to 20 wt the statistical treatment of individual ages from the investigated samples revealed three populations of lower proterozoic 1 9 ga upper proterozoic 580 ma and ordovician age 470 ma as well as mid proterozoic ages between 0 8 1 7 ga which are not regarded to be of geological significance lower proterozoic ages are preserved in the cores of monazites from leptynitic gneisses and khondalites they fairly agree with sm nd model ages for similar rocks of the kkb and give a minimum age for first monazite growth or complete homogenization the prominent pan african population with mean values between 540 and 580 ma is present in the leptynitic gneiss the khondalite and the augen gneiss and in line with published isotope ages for the kkb the ordovician population finally marks the emplacement of granitic pegmatites subsequent to the pan african high grade metamorphic event there is an obvious discrepancy between khondalites and leptynitic gneisses on the one hand and augen gneisses on the other concerning the presence of lower proterozoic ages while these are abundant in the former often rimmed by upper proterozoic ages they are completely absent in the latter it appears unlikely that lower proterozoic ages were completely reset during a pan african event exclusively in the augen gneisses while they were preserved in leptynitic gneisses and khondalites it is further concluded that the augen gneisses are of magmatic origin and were derived from porphyritic granites thus the upper proterozoic age of 605 37 ma calculated for the investigated sample approximates the time of magma emplacement which slightly precedes the peak stage of pan african high grade metamorphism in the kkb and of monazite crystallization from the granitic melt a characteristic feature of the investigated monazites is the resetting of lower proterozoic and pan african ages to significantly younger values due to partial pb loss monazites not shielded by other minerals e g garnet suffered selective mobilization of pb along fractures or at their rims while th and u concentrations remained almost unchanged the results presented in this study indicate that this was mainly due to fluid rock interaction it is concluded that thermal diffusion of pb even at the suggested temperatures of 900 c only had minor influence on the th u pb composition in monazite and that the closure temperature for this system must be significantly higher than previously assumed 750 c

detecting crater ejecta blanket boundaries and constraining source crater regions for boulder tracks and elongated secondary craters on eros abstract we present results of a numerical model of the dynamics of ejecta emplacement on asteroid 433 eros ejecta blocks represent the coarsest fraction of eros regolith and are important readily visible tracer particles for crater ejecta blanket units that may be linked back to specific source craters model results show that the combination of irregular shape and rapid rotation of an asteroid can result in markedly asymmetric ejecta blankets and it follows ejecta block spatial distribution with locally very sharp distinct boundaries we mapped boulder number densities in near shoemaker msi images across a portion of a predicted sharp ejecta blanket boundary associated with the crater valentine and confirm a distinct and real ejecta blanket boundary significant at least at the 3 sigma level using our dynamical model we back track the landing trajectories of three ejecta blocks with associated landing tracks in an effort to constrain potential source regions where those blocks were ejected from eros surface in impact events the observed skip distances of the blocks upon landing on eros surface and the landing speeds and elevation angles derived from our model allow us to estimate the coefficient of restitution e of eros surface for impacts of 10 m scale blocks at approximately 5 m s 1 impact speeds we find mean values of e of approximately 0 09 0 18

u pb monazite xenotime and titanite geochronological constraints on the prograde to post peak metamorphic thermal history of paleoproterozoic migmatites from the grand canyon arizona the petrology and u pb geochronology of pelitic migmatite and calc silicate gneiss reveal a detailed prograde to post peak metamorphic thermal history for a single outcrop of paleoproterozoic supracrustal rocks in the eastern part of the grand canyon metamorphic monazite from paleosomal pelitic schist grew on the prograde path beginning at about 1708 ma and continued to grow until about 1697 ma the u pb dates for magmatic xenotime and monazite from peraluminous granite and pegmatite leucosomes indicate that partial melting which involved the breakdown of muscovite to sillimanite commenced at about 1702 ma prior to the metamorphic peak partial melting continued until about 1690 ma the youngest u pb date from magmatic monazite in the leucosomes field and petrographic evidence as well as inheritance patterns in monazites from the leucosomes suggest that some of the leucosomes appear to represent in situ partial melts that did not escape the source region between 1702 and 1690 ma the migmatite package heated to peak metamorphic conditions of about 720 c and 6 kbar cooled to about 675 c at a cooling rate 30 c million years and decompressed to about 4 kbar the u pb geochronological data for metamorphic titanite from a calc silicate gneiss exhibit a clear relationship between grain size and the 207pb 206pb date indicating that the titanite crystals record cooling ages these data combined with the titanite pb diffusion data of cherniak 1993 yield a cooling rate of 5 4 0 9 1 7 c million years integrated over the interval 1690 to 1676 ma and suggest that by 1675 ma the cooling rate slowed to less than 2 c million years the rapid decompression during the peak of metamorphism and the change in cooling rate immediately following peak metamorphism are interpreted to reflect large scale tectonic processes associated with the accretion of juvenile crust to the margin of laurentia juvenile arc crust appears to have been assembled accreted and stabilized into laurentian lithosphere in less than 30 million years

contrasting ages between isotopic chronometers in granulites monazite dating and metamorphism in the higo complex japan abstract the higo complex of west central kyushu western japan is a 25 km long body of metasedimentary and metabasic lithologies that increase in metamorphic grade from schist in the north to migmatitic granulite in the south where granitoids are emplaced along the southern margin the timing of granulite metamorphism has been extensively investigated and debated previously published sm nd mineral isochrons for garnet bearing metapelite yielded ca 220 280 ma ages suggesting high grade equilibration older than the lower grade schist to the north which yielded ca 180 ma k ar muscovite ages ion and electron microprobe analyses on zircon have yielded detrital grains with rim ages of ca 250 ma and ca 110 ma electron microprobe ages from monazite and xenotime are consistently 110 130 ma two models have been proposed 1 high grade metamorphism and tectonism at ca 115 ma with older ages attributed to inheritance and 2 high grade metamorphism at ca 250 ma with resetting of isotopic systems by contact metamorphism at ca 105 ma during the intrusion of granodiorite these models are evaluated through petrographic investigation and electron microprobe th u total pb dating of monazite in metapelitic migmatites and associated lithologies in situ investigation of monazite reveals growth and dissolution features associated with prograde and retrograde stages of progressive metamorphism and deformation monazite th u pb isochrons from metapelite diatexite and late deformational felsic dykes consistently yield ca 110 120 ma ages earlier and later stages of monazite growth cannot be temporally resolved the preservation of petrogenetic relationships coupled with the low diffusion rate of pb at

ejecta blocks on 243 ida and on other asteroids abstract seventeen positive relief features 45 150 m across are identified as probable blocks in galileo high resolution images of ida their presence provides direct evidence for regolith retention on asteroids the spatial distribution maximum size and integrated volume of the blocks are consistent with those of blocks associated with craters on the earth the moon phobos and deimos the concordance suggests that the features are impact ejecta fragments and that cratering mechanics on ida an object of average diameter 32 km are similar to those applying on previously studied rocky bodies the blocks that lie within or near the rims of craters lascaux and mammoth were likely mobilized in the low velocity tail portion of the excavation flow that formed those craters a few blocks located near smaller craters may have migrated some distance away from their source possibly by impact induced spallation hopping rolling and or sliding some blocks on ida could be surviving fragments from the koronis parent body accreted after its breakup the lifetime of 102 m sized boulders against collisional disruption is estimated to be in the 108 109year range consistent with ages considered for the largest and oldest craters on ida extrapolation of successful ejecta scaling laws to other asteroids suggests that blocks 15 and 70 m across could be present on dactyl and gaspra respectively in both cases too small to be identified in available galileo images blocks 100 m in size could be present on 433 eros and km sized megablocks on 4 vesta

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

u pb zircon and sm nd geochronology of mafic and ultramafic rocks from the central part of the tauern window eastern alps geochronological and geochemical analyses were carried out in order to identify the pre variscan basement of the tauern window eastern alps maficultramafic rocks from the central part of the tauern window have been studied by ree analysis and u pb and sm nd isotopic analyses on whole rock zircons garnets and sphene u pb and sm nd zircon dating define both magmatic pan african and cambro ordovician events from 650 ma to 486 ma within the alpine fold belt this indicates a time span of 150 ma for magmatic activities in the tauern window of the eastern alps the ages of 657 ma u pb zircon and 644 sm nd zireon obtained from an amphibolite are the oldest dates of the eastern alps they may be related to the pan african orogeny and imply an early cycle of magmatic intrusion before major activity started at around 500 ma sm nd whole rock analyses of the precambrian rocks do not define an isochron reflecting heterogenities within the mantle source the initial end values 1 2 to 4 7 are very low implying an enrichment of the magma source the second main phase of magmatic activity 539 486 ma is characterized by the emplacement of mafic ultramafic rock sequences as no ophiolitic relies are observed in these domains the early paleozoic magmatism was likely associated with extensional tectonics obtained ages of 301 3 and 314 4 3 ma point to a variscan metamorphism the first combined u pb zircon sm nd zircon data for an amphibolite from the basal amphibolite formation baf favoured the sm nd zircon isochron age as a magmatic age whereas the low initial end value point to an enriched magma source as well as to heterogenities within the magma source the obtained ages suggest that parts of the pre variscan basement within the alpine fold belt were formed during the pan africa cycle the detection of pre variscan ages within the alpine basement must reffect a complex history involving significant pre variscan activity

petrology of basement dominated terranes i regional metamorphic t t path from u pb monazite and sm nd garnet geochronology central damara orogen namibia abstract mineral chemistry combined with sm nd garnet whole rock and u pb monazite ages provides important constraints on the geological history of metapelites migmatites and granites from the high grade basement dominated part of the damara orogen namibia based on thermobarometric data the peak of regional metamorphism reached c 700 750 c and c 5 kbar u pb monazite and sm nd garnet mineral ages obtained from unmigmatized garnet and cordierite bearing metapelites indicate that these peak metamorphic conditions were reached between 525 2 and 504 3 ma some migmatites with restitic melanosomes record sm nd garnet ages of 518 3 ma indicating that high grade regional metamorphism also culminated in partial melting some other migmatites and granites give older sm nd garnet whole rock ages between 530 3 and 540 4 ma different initial nd isotope compositions of granites leucosomes and melanosomes indicate that in this case migmatization is intrusion related crust derived granites intruded at 515 2 512 3 and 509 5 ma and are probably related to the second peak of regional metamorphism the youngest granites intruded at 469 3 ma indicating that intrusion of melts is superimposed upon the regional metamorphic history age constraints for the period between c 540 and 510 ma provide substantial evidence for crustal thickening together with igneous activity whereas the high temperature event which occurred at c 470 ma must be related to extensional tectonics the concordance between the monazite and garnet ages suggest that monazite grew at high grade metamorphic conditions it is further demonstrated that combined monazite and garnet chronometry are useful in identifying the timing and nature of particular tectonometamorphic and intrusive events in polymetamorphic high grade terranes

shoemaker crater as the source of most ejecta blocks on the asteroid 433 eros the loose material regolith on the surfaces of asteroids is thought to represent ballistically emplaced ejecta from impacts but the identification of source craters and the detailed study of the regolith modification have been hampered by the limited spatial resolution and area coverage of the few asteroids imaged by spacecraft here we report the results of global mapping of the asteroid 433 eros from high resolution images obtained by the near shoemaker spacecraft based on the images and ejecta emplacement models we suggest that most large ejecta blocks on eros originate from a relatively young 7 6 km diameter crater a large fraction of the ejecta from impacts pre dating that crater has apparently been buried or eroded the images also show evidence for the action of a variety of sorting environments for regolith particles after they are deposited on the surface

early paleoproterozoic archean dykes and gneisses in russian karelia of the fennoscandian shield new paleomagnetic isotope age and geochemical investigations abstract we present here new palaeomagnetic isotopic age and geochemical data from archean and early palaeoproterozoic rocks in the eastern fennoscandian shield we have studied ne sw trending gabbronorite dyke sets and their host archean basement rocks in the vodlozero block near the 2449 ma burakovka layered intrusion in southern russian karelia both dyke sets are genetically related to the burakovka intrusion the other ca 25 km long avdeev dyke locating a few kilometers south from the burakovka intrusion yields a stable single component remanence direction that is in agreement with the direction previously obtained from the burakovka intrusion another ne sw trending dyke 0 8 m wide shalskiy diabase dyke about 30 km south of the burakovka intrusion yields a similar remanence direction as the avdeev dyke the overall mean remanence direction has a palaeopole at plat 12 3 n plong 243 5 e a 95 15 4 4 sites 28 samples the thin shalskiy diabase dyke transects a similarly ne sw trending 500 m wide coarse grained gabbronorite dyke which has now been dated by sm nd method as 2608 56 ma geochemically all the dykes are quite similar showing slight calc alkaline affinity and low tio 2 and high sio 2 with moderate mgo and low cr and ni furthermore the dykes are geochemically identical to the 2 45 ga dyke swarm in the northern karelian province the remanence direction of the thin shalskiy diabase dyke differs significantly from the high temperature and high coercivity remanence component of the unbaked archean gabbronorite dyke which yields a palaeopole at plat 22 7 n plong 222 1 e dp 8 2 dm 16 2 five samples on the basis of different remanence directions of the diabase dyke and the unbaked archean gabbronorite dyke the baked contact test for the diabase dyke is positive in addition to the high temperature and high coercivity component of the baked and unbaked archean gabbronorite dyke in low temperatures and coercivities we isolated a similar component as in the diabase dyke a comparable remanence component was also obtained from the archean basement at ca 8 km from the dykes we propose that in the studied area the archean basement and the archaean dyke were partly remagnetized due to emplacement and subsequent uplift and cooling of the large burakovka layered intrusion and related dykes at about 2 40 ga ago this interpretation lends support from a new 40 ar 39 ar dating of hornblende from another area lake paajarvi area in northern karelia there a negative baked contact test was previously obtained for the remanence of the dated ca 2 45 ga dyke rocks related to the ca 2 45 ga oulanka layered intrusion the 40 ar 39 ar dating of the unbaked archean basement which yields the same remanence component as the dykes shows a plateau age of ca 2 6 ga but in addition it also shows resetting of the basement at ca 2 4 ga ago the dating thus supports reactivation and partial remagnetization of the archean basement at ca 2 4 ga ago our new palaeomagnetic results from the burakovka dykes and the new 40 ar 39 ar dating from the lake paajarvi area give support to our previous interpretation that at lake paajarvi area the remanence component suggested to be 2 4 ga despite to negative baked contact test is indeed of this age therefore it is implied that the results can be used for continental reconstructions

u pb systematics of monazite and xenotime case studies from the paleoproterozoic of the grand canyon arizona monazite is accepted widely as an important u pb geochronometer in metamorphic terranes because it potentially preserves prograde crystallization ages however recent studies have shown that the u pb isotopic system in monazite can be influenced by a variety of processes that partially obscure the early growth history in this paper we attempt to interpret complex monazite and xenotime u pb data from three paleoproterozoic granite dikes exposed in the grand canyon single crystal monazite analyses from an unfoliated granite dike spread out along concordia from the crystallization age of the dike defined by u pb zircon data to be 1685 1 ma to 1659 2 ma a span of 26 million years back scattered electron bse imaging reveals that magmatic domains within most crystals from this sample are truncated by secondary domains associated with prominent embayments at the grain margin fragments of a single crystal yield contrasting concordant dates and fragments from the edges and tips of crystals yield the youngest dates based on these observations we suggest that the secondary domains formed at least 26 million years after the crystal formed monazite and xenotime dates from the second sample a sheared dike that cross cuts the previous dike spread out along concordia over 16 million years and range up to 2 4 normally discordant again bse imaging reveals secondary domains that truncate both magmatic zoning and xenocrystic cores fragments sliced from specific domains of a previously imaged monazite crystal demonstrate that the secondary domain is 13 million years younger than the core domain textures revealed in bse images suggest that the secondary domains formed by fluid mineral interaction normal discordance appears to result from both radiation damage accumulated at temperatures below 300 c and water mineral interaction monazite data from the third sample exhibit dispersion in both the 207pb 206pb dates 1677 1690 ma and discordance 1 6 to 3 1 reverse discordance in these monazites cannot be explained by incomplete dissolution or excess thorogenic 206pb sliced fragments from several crystals reveal dramatic intragrain u pb disequilibrium that does not correlate with either th or u concentration or position within the crystal we suggest that reverse discordance resulted from mechanisms that involve exchange or fractionation of elemental u or elemental pb and that neither the u pb dates nor the 207pb 206pb dates are reliable indicators of the rock s crystallization age given the large number of processes proposed in the recent literature to explain monazite u pb systematics from rocks of all ages our results can be viewed as another cautionary note for single crystal and multi crystal monazite geochronometry however we suggest that because individual crystals can preserve a temporal record of primary and secondary monazite growth micro sampling of individual monazite crystals may provide precise absolute ages on a variety of processes that operate during the prograde peak and or retrograde history of metamorphic terranes

evidence for inherited sm nd isotopes in granitoid zircons this study presents evidence to show that in addition to preserving u pb isotope systematics refractory zircons also preserve at least in part an inherited sm nd isotope component the zircons analyzed during this study were taken from the strontian granitoid nw scotland the inner intrusion of this composite pluton is known from a previous study to contain substantial u pb zircon inheritance whereas the outer part has only minor inheritance zircons from the inner intrusion were found to have significantly lower end425 values than either their host rock separated apatite or monazite it is argued that this isotopic disequilibrium is due to the presence of an inherited sm nd isotope component rather than being due to a post crystallization disturbance of the zircons the preservation of inherted sm nd isotopes within refractory zircons implies that they remain closed systems with respect to the diffusion of sm and nd and presumably the other ree to temperatures in excess of 700 c the fact that zircons commonly have high sm nd ratios relative to sialic crustal material means that the nd isotopic evolution of inherited zircons will be very different to that of much of the continental crust

petroleum prospects of lebanon some remarks from sedimentological and diagenetic studies of jurassic carbonates abstract only seven exploration wells have been drilled in lebanon to date failing to encounter oil or gas in commercial quantities and to penetrate into unexposed pre jurassic reservoirs this paper shows that the sedimentological and diagenetic characteristics of the major lebanese jurassic carbonate rock unit the kesrouane formation thickness 1000 m negate jurassic hydrocarbon prospectivity in northern mount lebanon the jurassic platform in this region was subjected to a first emergence and invasion by meteoric waters in the late jurassic causing a pervasive cementation a second phase of uplift and meteoric flushing occurred in the cenozoic thus only the areas located far from the jurassic and post cretaceous phreatic realms e g offshore tripoli nw lebanon may have been less affected by meteoric diagenesis and still provide promising prospects a 1993 seismic survey offshore tripoli confirmed the presence of late triassic evaporites in northern lebanon this also adds confidence to the results of the nahr ibrahim electrical survey which earlier suggested the presence of these evaporites in the subsurface of central lebanon the pre jurassic sequence hence is sealed and protected from the meteoric invasion observed in the overlying jurassic rocks accordingly the qartaba horst structure central northern lebanon would be equally if not more attractive for a pre jurassic play

sm nd dating and cooling history of scourian granulites sutherland the scourian gneisses of the lewisian complex of north west scotland have been the subject of many geochronological investigations sm nd whole rock measurements suggest that the protoliths of the lewisian complex differentiated 2 92 0 05 gyr ago from an approximately chrondritic mantle1 rb sr and pb pb whole rock ages between 2 8 and 2 6 gyr reflect subsequent depletion in rb and u refs 2 3 which has generally been associated with granulite facies metamorphism zircons which are thought to have crystallized during this metamorphism have also given u pb ages of 2 66 gyr ref 4 this granulite facies metamorphism involving pressures in excess of 10 kbar and peak temperatures estimated between 820 50 c ref 5 and 1 250 c refs 6 7 affects a terrain including supracrustal rocks7 and thus implies a major tectonic event we report here the first geochronological data measured on mineral assemblages that have also provided estimates of the conditions of metamorphism such measurements are possible because the major minerals of granulite facies assemblages show an unusually favourable spread in sm nd ratios garnet especially is strongly enriched in sm ref 8 data for three garnetiferous basic gneiss assemblages suggest that closure of the sm nd system occurred significantly later than the peak of the metamorphism

the role of inclusions in u pb and sm nd garnet geochronology stepwise dissolution experiments and trace uranium mapping by fission track analysis the u pb and sm nd dating of garnet are important tools for understanding rates of tectonometamorphic processes and have been widely applied in studies of metamorphic terranes however the budgets of uranium lead samarium and neodymium in garnet separates from metamorphic rocks may be dominated by contributions from inclusions of monazite or zircon a combined fission track and stepwise dissolution technique is proposed for evaluating the role of inclusions of monazite and zircon in the budgets of uranium lead samarium and neodymium in garnets used for u pb and sm nd chronology variations of th u u nd and sm nd ratios between successive dissolution steps reveal the contributions of monazite and zircon inclusions the stepwise dissolution procedure does not induce any apparent artifacts on 207pb 206pb and sm nd ages the technique has been applied to garnets from three metamorphic terranes almandine garnet samples from the high grade pikwitonei granulite domain manitoba canada and the wind river range wyoming usa have the majority of their uranium samarium neodymium and radiogenic lead pb hosted by micrometer scale inclusions of monazite fission track densities reveal that uranium is 108 times more abundant in inclusions than in garnets stepwise dissolution of the samples shows that neodymium and uranium are associated with the dissolution of monazite in contrast grossularandradite garnets from cascade slide adirondack highlands ny usa show little variation in fission track density and similar isotopic ratios between dissolution steps indicating that the budgets of uranium samarium neodymium and pb are not significantly influenced by inclusions the demonstrated success of these techniques leads us to recommend similar procedures as a routine matter in u pb and sm nd garnet geochronology of metasedimentary rocks

lithospheric structure of an archean craton and adjacent mobile belt revealed from 2 d and 3 d inversion of magnetotelluric data example from southern congo craton in northern namibia 1 archean cratons and the stitching proterozoic orogenic belts on their flanks form an integral part of the southern africa tectonic landscape of these virtually nothing is known of the position and thickness of the southern boundary of the composite congo craton and the neoproterozoic pan african orogenic belt due to thick sedimentary cover we present the first lithospheric scale geophysical study of that cryptic boundary and define its geometry at depth our results are derived from two dimensional 2 d and three dimensional 3 d inversion of magnetotelluric data acquired along four semiparallel profiles crossing the kalahari craton across the damara ghanzi chobe belts dgc and extending into the congo craton two dimensional and three dimensional electrical resistivity models show significant lateral variation in the crust and upper mantle across strike from the younger dgc orogen to the older adjacent cratons we find damara belt lithosphere to be more conductive and significantly thinner than that of the adjacent congo craton the congo craton is characterized by very thick to depths of 250 km and resistive i e cold lithosphere resistive upper crustal features are interpreted as caused by igneous intrusions emplaced during pan african magmatism graphite bearing calcite marbles and sulfides are widespread in the damara belt and account for the high crustal conductivity in the central zone the resistivity models provide new constraints on the southern extent of the greater congo craton and suggest that the current boundary drawn on geological maps needs revision and that the craton should be extended further south

deciphering polymetamorphic episodes in high grade metamorphic orogens constraints from pbsl sm nd and lu hf garnet dating of low to high grade metasedimentary rocks from the kaoko belt namibia abstract three dating techniques for metamorphic minerals using the sm nd lu hf and pb isotope systems are combined and interpreted in context with detailed petrologic data from crustal segments in nw namibia the combination of isochron ages using these different approaches is a valuable tool to testify for the validity of metamorphic mineral dating here pbsl lu hf and sm nd garnet ages obtained on low to medium grade metasedimentary rocks from the central kaoko zone of the neoproterozoic kaoko belt nw namibia indicate that these samples were metamorphosed at around 550 560 ma on the other hand granulite facies metasedimentary rocks from the western kaoko zone underwent two phases of high grade metamorphism one at ca 660 625 ma and another at ca 550 ma providing substantial evidence that the 660 625 ma event was indeed a major tectonothermal episode in the kaoko belt our age data suggest that interpreting metamorphic ages by applying a single dating method only is not reliable enough when studying complex metamorphic systems however a combination of all three dating techniques used here provides a reliable basis for geochronological age interpretation

structure and strain variation at mid crustal levels in a transpressional orogen a review of kaoko belt structure and the character of west gondwana amalgamation and dispersal abstract the neoproterozoic cambrian kaoko belt is an orogen scale 800 180 km transpressional system important in the amalgamation of west gondwana mid crustal transpression at amphibolite to granulite facies conditions is dominated by two major 400 km exposed strike slip shear zones bounding a 20 40 km wide high grade orogen core to the east a deeply buried nappe dominated escape zone has inverted metamorphic sequence and verges outwards onto a platformal foreland to the west an arc like neoproterozoic coastal terrane was amalgamated and variably reworked during transpression the major purros and three palms mylonite zones have calculated shear displacements on the order of 120 180 km these shear zones are moderately to steeply dipping mylonite zones of 1 5 km width are arcuate and curvilinear in map view and show along strike variation in slip kinematics also highly curved in vertical section the shear zones define a flower to half flower geometry for the orogen core an oblique network of mylonitic shear zones akin to riedel shears links the major shear zones and defines regional scale shear lozenges internally deformed by tight upright folding and shear fabrics these shear zones create domains in the orogen core with varying dominance of pure shear in shear lozenges and simple shear in shear zones however absence of dip slip domains and the smoothly continuous traces of sub horizontal to shallow and acute oblique stretching lineations across all parts of the belt preclude marked kinematic partitioning and the internal part of the belt resembles large scale triclinic shear clast aspect ratios boudin train extension sheath fold aspect geometry degree of rotation of planes producing flanking folds composite s c foliations pressure fringes on pyrite and garnet porphyroclasts provide a semi quantitative measure of strain intensity average strain ratios are x z 40 1 for the major shear zones x z 12 1 for the orogen core x z 8 1 for the escape zone and x z 3 1 for the coastal terrane a more continuous pattern of strain intensity across the whole belt is mapped using a qualitative foliation intensity index foliation traces have a sigmoidal pattern in the orogen core swinging from sub parallel to the boundary shear zones to higher acute angles in the internal parts deformation character also varies from upright open folding in amphibolite facies domains in the north upright tight chevron folding in a low grade central domain to a high grade domain of tight to isoclinal inter folded basement and cover with inclination decreasing towards the south the kaoko belt is a well exposed sector of an extensive 3000 km long broad 400 km arcuate orogenic system adamastor orogen that amalgamated west gondwana bringing the south american sao francisco and rio de la plata cratons and african kalahari and congo cratons components together though a complex system most sectors involved oblique collision and accretion of magmatic arcs of 660 610 ma age followed by peak metamorphism and main phase transpressional orogenesis between 585 and 560 ma with shear zones remaining active until 530 ma this e w amalgamation immediately pre dates the final n s amalgamation of gondwana along the kuunga orogen between 535 and 510 ma the large scale adamastor orogen consisting of kaoko dom feliciano ribeira aracuai and west congo mobile belts also shows broadly similar and symmetric structural architecture throughout the high grade thermally softened core partitioned intense wrench dominated strains and networks of transcurrent shear zones that dip inwards with listric form either side of the internal zone containing amalgamated arcs and high grade core are nappe fold and thrust belts that rework attenuated passive margin basement and adamastor ocean sediments and structures verge outward at moderate to high angles onto both foreland margins the kaoko belt well illustrates the highly partitioned nature of transpressional systems in general and patterns in common throughout the greater adamastor orogen such as metamorphic zonation and heterogeneous distribution of deformation style flow regime and highly variable degrees of reworking strain and recrystallization this highly partitioned and steep structural grain localized lithospheric extension and rifting 415 ma later during breakup and dispersal of gondwana

geochronology in migmatites a sm nd u pb and rb sr study from the proterozoic damara belt namibia implications for polyphase development of migmatites in high grade terranes sm nd garnet u pb monazite and rb sr biotite ages from a composite migmatite sample damara orogen namibia constrain the time of high grade regional metamorphism and the duration of regional metamorphic events sm nd garnet whole rock ages for a strongly restitic melanosome and an adjacent intrusive leucosome yield ages of 534i5 528i11 and 539i8 ma these results provide substantial evidence for pre 500 ma pan african regional metamorphism and melting for this segment of the orogen other parts of the migmatite yield younger sm nd ages of 488i9 ma for melanosome and 496i10 492i5 and 511i16 ma for the corresponding leucosomes garnet from one xenolith from the leucosomes yields an age of 497i2 ma major element compostions of garnet are different in terms of absolute abundances of pyrope and spessartine components but the flat shape of the elemental patterns suggests late stage retrograde equilibration rare earth element compositions of the garnet from the different layers are similar except for garnet from the intrusive leucosome suggesting that they grew in different environments monazite from the leucosomes is reversely discordant and records 207 pb 235 u ages between 536 and 529 ma indicating that this monazite represents incorporated residual material from the first melting event monazite from the mesosome mes 2 and the melanosome mel 3 gives 207 pb 235 u ages of 523 and 526 ma and 529 and 531 ma respectively which probably indicates another thermal event previously published 207 pb 235 u monazite data give ages between 525 and 521 ma for composite migmatites and 521 and 518 ma for monazite from neosomes monazite from granitic to granodioritic veins indicates another thermal event at 507 505 ma these ages are also recorded in 207 pb 235 u monazite data of 508 ma from the metasediment met 1 from the migmatite and also in the sm nd garnet ages obtained in this study taken together these ages indicate that high grade metamorphism started at c 535 ma or earlier and was followed by thermal events at c 520 ma and c 505 ma the latter event is probably connected with the intrusion of a large igneous body donkerhoek granite for which so far only imprecise rb sr whole rock data of 520i15 ma are available rb sr biotite ages from the different layers of the migmatite are 488 469 and 473 ma these different ages indicate late stage disturbance of the rb sr isotopic system on the sub sample scale nevertheless these ages are close to the youngest sm nd garnet ages indicating rapid cooling rates between 13 and 20u cm a x1 and fast uplift of this segment of the crust similar sm nd garnet and u pb monazite ages suggest that the closure temperatures for both isotopic systems are not very different in this case and are probably similar or higher than the previously estimated peak metamorphic temperatures of 730i30uc the preservation of restitic monazite in leucosomes indicates that dissolution of monazite in felsic water undersaturated peraluminous melts can be sluggish this study shows that geochronological data from migmatites can record polymetamorphic episodes in high grade terranes that often contain cryptic evidence for the nature and timing of early metamorphic events

boulder size and shape distributions on asteroid ryugu abstract in 2018 the japanese spacecraft hayabusa2 arrived at the small asteroid ryugu the surface of this c type asteroid is covered with numerous boulders whose size and shape distributions are investigated in this study using a few hundred optical navigation camera onc images with a pixel scale of approximately 0 65 m we focus on boulders greater than 5 m in diameter smaller boulders are also considered using five arbitrarily chosen onc close up images with pixel scales ranging from 0 7 to 6 cm across the entire surface area 2 7 km2 of ryugu nearly 4400 boulders larger than 5 m were identified boulders appear to be uniformly distributed across the entire surface with some slight differences in latitude and longitude at 50 km 2 the number density of boulders larger than 20 m is twice as large as on asteroid itokawa or bennu the apparent shapes of ryugu s boulders resemble laboratory impact fragments with larger boulders being more elongated the ratio of the total volume of boulders larger than 5 m to the total excavated volume of craters larger than 20 m on ryugu can be estimated to be 94 which is comparatively high these observations strongly support the hypothesis that most boulders found on ryugu resulted from the catastrophic disruption of ryugu s larger parent body as described in previous papers watanabe et al 2019 sugita et al 2019 the cumulative size distribution of boulders larger than 5 m has a power index of 2 65 0 05 which is comparatively shallow compared with other asteroids visited by spacecraft for boulders smaller than 4 m the power index is even shallower and ranges from 1 65 0 05 to 2 01 0 06 this particularly shallow power index implies that some boulders are buried in ryugu s regolith based on our observations we suggest that boulders near the equator might have been buried by the migration of finer material and as a result the number density of boulders larger than 5 m in the equatorial region is lower than at higher latitudes

isotopic equilibrium disequilibrium in granites metasedimentary rocks and migmatites damara orogen namibia a consequence of polymetamorphism and melting abstract the overwhelming part of the continental crust in the high grade part of the damara orogen of namibia consists of s type granites metasedimentary rocks and migmatites at oetmoed central damara orogen two different s type granites occur their negative e nd values 3 3 to 5 9 moderately high initial 87 sr 86 sr ratios 0 714 0 731 moderately high 206 pb 204 pb 18 21 18 70 and 208 pb 204 pb 37 74 37 89 isotope ratios suggest that they originated by melting of mainly mid proterozoic metasedimentary material metasedimentary country rocks have initial e nd of 4 2 to 5 6 initial 87 sr 86 sr of 0 718 0 725 206 pb 204 pb ratios of 18 32 18 69 and 208 pb 204 pb ratios of 37 91 38 45 compatible with their variation in rb sr u pb and th pb ratios some migmatites and residual metasedimentary xenoliths tend to have more variable e nd values initial e nd 4 2 to 7 1 initial sr isotope ratios 87 sr 86 sr 0 708 0 735 and less radiogenic 206 pb 204 pb 18 22 18 53 and 208 pb 204 pb 37 78 38 10 isotope compositions than the metasedimentary rocks on a rb sr isochron plot the metasedimentary rocks and various migmatites plot on a straight line that corresponds to an age of c 550 ma which is interpreted to indicate major fractionation of the rb sr system at that time however initial 87 sr 86 sr ratios of the melanosomes of the stromatic migmatites calculated for their u pb monazite and sm nd garnet ages of c 510 ma are more radiogenic 87 sr 86 sr 0 725 than those obtained on their corresponding leucosomes 87 sr 86 sr 0 718 implying disequilibrium conditions during migmatization that have not lead to complete homogenization of the rb sr system however the leucosomes have similar nd isotope characteristics than the inferred residues melanosomes indicating the robustness of the sm nd isotope system during high grade metamorphism and melting on a rb sr isochron plot residual metasedimentary xenoliths show residual slopes of c 66 ma calculated for an u pb monazite age of 470 ma again indicating major fractionation of rb sr at c 540 ma however at 540 ma these xenoliths have unradiogenic sr isotope compositions of c 0 7052 indicating depleted metasedimentary sources at depth based on the distinct pb isotope composition of the metasedimentary rocks and s type granites metasedimentary rocks similar to the country rocks are unlikely sources for the s type granites moreover a combination of sr nd pb and o isotopes favours a three component mixing model metasedimentary rocks altered volcanogenic material meta igneous crust that may explain the isotopic variabilty of the granites the mid crustal origin of the different types of granite emphasises the importance of recycling and reprocessing of pre existing differentiated material and precludes a direct mantle contribution during the petrogenesis of the orogenic granites in the central damara orogen of namibia

drainage pits in cohesionless materials implications for the surface of phobos viking orbiter images show grooves and chains of pits crossing the surface of phobos many of which converge toward the large crater stickney or its antipode although it has been proposed that the pits and grooves are chains of secondary craters their morphology and geometric relations suggest that they are the surface traces of fractures in the underlying solid body of phobos several models have been proposed to explain the pits of which the most plausible are gas venting and drainage of regolith into open fractures the latter mechanism is best supported by the image data and is the mechanism studied in this investigation drainage pits and fissures are modeled experimentally by using two rigid substrate plates placed edge to edge and covered by uniform thicknesses of dry fragmental debris simulated regolith fracture extension is simulated by drawing the plates apart allowing drainage of regolith into the newly created void a typical drainage experiment begins with a shallow depression on the surface of the regolith above the open fissure increased drainage causes local drainage pits to form continued drainage causes the pits to coalesce forming a cuspate groove the resulting experimental patterns of pits and grooves have pronounced similarities to those observed on phobos characteristics such as lack of raised rims linearity of grooves and chains of pits uniform spacing of pits and progression from discrete pits to cuspate grooves are the same in the experiments and on phobos in contrast gas venting pits occur in irregular chains and have raised rims these experiments thus indicate that the phobos grooves and pits formed as drainage structures the pit spacing in an experiment is measured at the time that the maximum number of pits forms prior to groove development the average pit spacing is compared to the regolith thickness for each material regression line fits indicate that the average spacing of drainage pits in unconsolidated noncohesive regolith is nearly equal to the thickness of regolith and appears to gbe independent of the angle of repose within the resolution of our experiments this provides a simple means of estimating regolith thickness where drainage pits are present on phobos two locations differing by 90 degrees in longitude have average pit spacings that suggest regolith thicknesses of 290 and 300 m suggesting that large areas of phobos have a nearly uniform regolith thickness of approximately 300 m

u pb garnet chronometry in high grade rocks case studies from the central damara orogen namibia and implications for the interpretation of sm nd garnet ages and the role of high u th inclusions garnets from different migmatites and granites from the damara orogen namibia were dated with the u pb technique after bulk dissolution of the material measured 206pb 204pb ratios are highly variable and range from ca 21 to 613 variations in isotope 208pb 204pb 206pb 204pb and trace element th u u nd sm nd ratios of the different garnets show that some garnets contain significant amounts of monazite and zircon inclusions due to their very low 206pb 204pb ratios garnets from pelitic migmatites from the khan area yield pb pb ages with large errors precluding a detailed evaluation however the 207pb 206pb ages ca 550 500 ma appear to be similar to or older than u pb monazite ages 530 1 517 1 ma and sm nd garnet ages 523 4 512 3 ma from the same sample it is reasonable to assume that the pb pb garnet ages define growth ages because previous studies are consistent with a higher closure temperature for the u pb system in garnet relative to the u pb system in monazite and the sm nd system in garnet for igneous migmatites from oetmoed pb pb garnet ages 483 15 492 16 ma and one sm nd garnet whole rock age 487 8 ma are similar whereas the monazite from the same sample is ca 30 40 ma older 528 1 ma these monazite ages are however similar to monazite ages from nearby unmigmatized granite samples and constrain precisely the intrusion of the precursor granite in this area although there is a notable difference in closure temperature for the u pb and sm nd system in garnet the similarity of both ages indicate that both garnet ages record garnet growth in a migmatitic environment restitic garnet from an unmigmatized granite from omaruru yields similar u pb 493 30 506 30 ma and sm nd 493 6 488 7 ma garnet ages whereas the monazite from this rock is ca 15 25 ma older 516 1 514 1 ma whereas the monazite ages define probably the peak of regional metamorphism in the source of the granite the garnet ages may indicate the time of melt extraction for igneous garnets from granites at oetmoed the similarity between pb pb 483 34 474 17 ma and sm nd 492 5 484 13 ma garnet ages is consistent with fast cooling rates of granitic dykes in the lower crust differences between garnet and monazite u pb ages can be explained by different reactions that produced these minerals at different times and by the empirical observation that monazite seems resistant to later thermal re equilibration in the temperature range between 750 and 900 c e g braun et al 1998 for garnet analyses that have low 206pb 204pb ratios the influence of high inclusions is small however the relatively large errors preclude a detailed evaluation of the relationship between the different chronometers for garnet with higher 206pb 204pb ratios the overall similarity between the pb pb and sm nd garnet ages implies that the inclusions are not significantly older than the garnet and therefore do not induce a premetamorphic pb signature upon the garnet the results presented here show that garnet with low 238u 204pb ratios together with sm nd garnet data and u pb monazite ages from the same rock can be used to extract geologically meaningful ages that can help to better understand tectonometamorphic processes in high grade terranes

controls on the mineralogy of s type volcanic and plutonic rocks abstract far from being simply two mica granites s type rocks contain highly diverse mineral assemblages the most important chemical factors influencing the mineralogy of an s type rock are peraluminosity bulk feo mgo mg mg fe k na ca and the fugacities of volatile species such as h2o hf h3bo3 and o2 however it is not just initial melt chemistry that determines the mineral assemblage the pressure and temperature of crystallization exert major effects as magmas evolve the p t conditions volatile fugacities and melt chemistry change often leading to changes in stable mineralogy many s type granites and silicic volcanic rocks contain partly or completely re equilibrated mineral assemblages recognition of these may provide important information about early crystallization conditions and the nature of the magma source terrains

ree u th and hf distribution in zircon from western carpathian variscan granitoids a combined cathodoluminescence and ion microprobe study a cathodoluminescence cl investigation on individual zircon crystals from variscan s type granites from the western carpathians has been combined with ion microprobe rare earth element ree and trace element analyses to explore possible connections between the cl emission of natural zircon crystals and their chemical composition the work was supported by single zircon u pb dating both by thermal ionization mass spectrometry tims and ion microprobe which helped to identify inherited zircon cores derived from related protoliths and to constrain the ages of inheritance and anatexis neither the ree nor trace elements such as y or th were found to have any direct correlation with the cl brightness the intensity of the cl signals was not directly reflected in the ree concentrations or the shape of the ree patterns uranium is probably the element responsible for quenching or suppressing the cl signals zircon cores inherited from different protoliths were distinguished by their ree patterns this enabled within a particular geological area the identification of inherited cores which possibly originate from the same protolith and therefore might lie on the same discordance line thereby improving the accuracy of inheritance ages inferred from tims single zircon analyses

ponded deposits on asteroid 433 eros in late january 2001 the near shoemaker spacecraft performed low altitude passes over the surface of 433 eros coordinated observations of the asteroid surface were obtained at submeter resolution by the near laser rangefinder and the multispectral imager this paper presents three independent coordinated observations of a 90 m pond adjacent to a granular debris flow including the highest resolution altimetric measurements of ponded deposits on eros the ponded deposits appear to have been emplaced by fluid like motion of dry asteroidal regolith a simple model of seismic agitation from impacts is developed to account for pond formation on eros the model predicts that ponds should form readily on eros but not on the moon where ponds are not observed the model also suggests that the absence of observable ponds in the largest craters of eros as well as on phobos and deimos may be related to regolith depth

parageneses and th u distributions among allanite monazite and xenotime in barrovian type metapelites imjingang belt central korea the paragenetic relationships and th u distributions among allanite monazite and xenotime were investigated in a progressive sequence of garnet to kyanite zone metapelites of the imjingang belt korea allanite is predominant in the garnet and staurolite zones whereas monazite and xenotime predominate in the kyanite zone epidote grains in the lower garnet zone are commonly zoned from allanite core to relatively y rich rare earth element ree epidote to clinozoisite rim although both ree epidote and clinozoisite disappear in higher grade metapelites moreover allanite and ree epidote often contain minute inclusions of thorium silicate the isogradic distributions and similarity of ree patterns between allanite and monazite suggest that the latter has grown at the expense of the former in addition the discontinuous th zoning in monazite is apparently inherited from heterogeneous th distribution and thorium silicate inclusions in allanite thus thorium silicate possibly provided the additional th and u necessary for the monazite formation paragenetic relationships of allanite and monazite inclusions within various index minerals suggest that at amphibolite facies conditions allanite is stable at higher pressures than monazite xenotime grains in the staurolite zone are rarely produced by the breakdown of clinozoisite and ree epidote whereas those in the kyanite zone are grown primarily at the expense of garnet incorporation of th and u into monazite and xenotime is governed mainly by the brabantite and thorite substitutions respectively taken together our results suggest that the allanite to monazite transformation is primarily responsible for the distributions of rees th and u among metapelitic phases and that the xenotime formation was facilitated by the contribution from major silicates particularly garnet

the influence of high u th inclusions on the u th pb systematics of almandine pyrope garnet results of a combined bulk dissolution stepwise leaching and sem study abstract the isotope systematics of garnet are increasingly being used to define prograde thermal histories in metamorphic belts this approach to geochronology has the distinct advantage that time information is directly related to the pressure temperature deformation history recorded by the same mineral one potential problem with the technique however is that the inventories of radiogenic isotopes apparently measured in garnet separates may be located in inclusions rather than the garnet lattice itself inclusions of old refractory minerals such as zircon may bias the ages to the extent that they are rendered meaningless in terms of dating garnet growth establishing pt histories and constraining tectonic mechanisms in this contribution we present u th pb data obtained on bulk dissolutions of garnet from the himalaya as well as the results of a detailed sem and u th pb stepwise leaching study in order to further clarify these issues our experiment has the advantage that the age difference between the garnets and potential inclusions is very large so that the influence of the latter is highlighted the u th pb data from the bulk dissolutions are highly complex and yield apparent ages ranging from 1000 ma to 30 ma the stepwise leaching study demonstrates that this complexity results from small inclusions of allanite and zircon detected with the sem the bulk of the radiogenic 206pb in these samples was released only by treatment at high pressure in a bomb in addition the 208pb 206pb ratio of the radiogenic pb released in earlier leach steps is inconsistent with the available constraints on the th u ratio of garnet lattices as measured by sims in this case the u pb ages of these garnets is dependent purely on the age of the included high u th phases and the degree to which they have isotopically re equilibrated with the whole rock during the subsequent metamorphism that grew garnet simple mass balance arguments show that in this particular instance very small amounts of these inclusions are required amounts that escape detection during conventional sample preparation these data also have implications for current thinking on the closure temperature for the sm nd and u pb systems in garnet

monazite controlled sm nd fractionation in leucogranites an ion microprobe study of garnet phenocrysts rare earth element ree abundances in four garnet phenocrysts from two monazite bearing leucogranites from southern british columbia have been measured with an ion microprobe this study examines the effect of monazite crystallization on the lrees by using garnet as a monitor of elemental variations in the liquid this approach virtually eliminates uncertainties associated with source induced lree fractionation the garnets examined are spessartine rich alm70 75sps18 22pyp3 4grs3 4 phenocrysts that show primary igneous zoning and lack monazite inclusions garnet rims have zoning profiles i e eu to ce that are depleted by factors of five to ten relative to cores of the three models investigated to constrain processes controlling lree zoning in garnet 1 1 variations in the garnet melt distribution coefficients 2 2 liquid diffusion controlled partitioning and 3 3 crystal diffusion controlled partitioning the latter model best explains the observed zoning this model is consistent with experimental studies suggesting that monazite has a low solubility in granitic melts these results suggest that nd model ages calculated using measured 147sm144nd in leucogranites should be viewed with caution as is demonstrated by neodymium isotopic data from southern british columbia and the central nepalese himalaya

la mc icp ms u pb dating of low u garnets reveals multiple episodes of skarn formation in the volcanic hosted iron mineralization system awulale belt central asia volcanic hosted iron deposits of the eastern awulale metallogenetic belt in central asia possess a reserve of over 1 2 billion tons of iron ores and constitute one of the most important basements for high grade iron resources in china skarns are widespread in these deposits and closely associated with iron mineralization the ages of these skarns are unclear and their genesis remains debated preventing further investigation into their metallogenic processes we focused on garnets in nine ore bearing skarns from three large scale iron deposits chagangnuoer dunde and beizhan in the eastern awulale belt u pb dating was conducted on these garnets using our in house reference material the taochong garnet tc 13 pb pb isochron age 126 2 23 ma initial pb 207 pb 206 ratio 0 845 0 022 laser ablation multicollector inductively coupled plasma mass spectrometry la mc icp ms was employed in the garnet u pb dating and high precision u pb ages 03 1 6 were obtained highlighting the advantages of la mc icp ms in dating low u minerals the garnet u pb ages of the nine skarn samples fall into three groups i e 329 0 5 1 326 3 3 ma two samples 316 3 2 9 311 2 2 4 ma six samples and 295 6 1 0 ma one sample implying three episodes of skarn alteration in the volcanic hosted iron mineralization system the first and second episodes of skarns formed as a result of contact metasomatism between coeval volcanic rocks and limestone and they have economically important iron mineralization the third was likely caused by a local postcollision granitic intrusion but its metallogenic potential deserves further assessment