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

the peraluminous aurumina granite suite in central brazil an example of mantle continental crust interaction in a paleoproterozoic cordilleran hinterland setting abstract peraluminous granites and tonalites granodiorites of the aurumina suite crop out in the external zone of the brasilia fold belt on the western margin of the sao francisco craton rocks of this suite intrude retrograde medium grade schists and paragneisses of the ticunzal formation and display a magmatic muscovite biotite garnet assemblage and discrete graphite lamellae the suite is divided into six members au1 through au6 which grade from synkinematic to post kinematic in character with ages spanning from 2 11 to 2 16 ga according to zircon u pb data compositional features of the aurumina suite and associated coeval metaluminous mafic to intermediate plutons suggest an origin by source hybridization processes characterized by reaction between metasedimentary rocks and basaltic melts that would have taken place at shallow depths pressure

fractional crystallization of mantle derived melts as a mechanism for some i type granite petrogenesis an example from lachlan fold belt australia the mt buller igneous suite in the southeastern lachlan fold belt encompasses a large variety of rock types from gabbros through diorites to granites mafic rocks and enclaves have primitive mantle like initial 87 sr 86 sr values 0 7037 0 7045 and nd values 5 6 to 4 1 the granites do not show distinctively more radiogenic initial 87 sr 86 sr values suggesting they are derived from the same parent as the gabbros most of the mt buller rocks do not show a positive correlation between silica and initial 87 sr 86 sr or between silica and 143 nd 144 nd values indicating that neither crustal contamination nor mixing between mantle and crustal derived melts has been important and that these rock types are likely to have formed by fractional crystallization from a common mantle derived parent some scatter in compatible trace elements particularly in granites and diorites may be explained by mixing geochemical modelling confirms a comagmatic origin of the suite magma generation probably took place in the subduction modified upper mantle this mechanism may play an important role in i type granite petrogenesis in particular tectonic settings such as during an efficient slab roll back or sinking of oceanic crust in divergent subduction conditions

p t t evolution of ultrahigh temperature granulites from the saxon granulite massif germany part ii geochronology the granulites of the saxon granulite massif equilibrated at high pressure and ultrahigh temperature and were exhumed in large part under near isothermal decompression this raises the question of whether p t t data on the peak metamorphism may still be retrieved with confidence felsic and mafic granulites with geochronologically useful major and accessory phases have provided a basis to relate p t estimates with isotopic ages presented in a companion paper the assemblage garnet clinopyroxene in mafic granulite records peak temperatures of 1010 1060 c consistent with minimum estimates of around 967 c and 22 3 kbar obtained from the assemblage garnet kyanite ternary feldspar quartz in felsic granulite multiple partial overprint of these assemblages reflects a clockwise p t evolution garnet and kyanite in the felsic granulite were successively overgrown by plagioclase spinel plagioclase sapphirine plagioclase and biotite plagioclase most of this overprinting occurred within the stability field of sillimanite garnet clinopyroxene in the mafic granulite were replaced by clinopyroxene amphibole plagioclase magnetite the high p t conditions and the absence of thermal relaxation features in these granulites require a short lived metamorphism with rapid exhumation the ages of peak metamorphism 342 ma and shallow level granitoid intrusions 333 ma constrain the time span for the exhumation of the saxon granulites to about 9 ma

depletion of light rare earth elements in felsic magmas contrary to simple generalizations about their behavior light rare earth elements lree do not act as incompatible elements in very felsic magmas in fact lree concentrations typically decrease often drastically during differentiation of such magmas the simplest explanation for this depletion involves the separation of minute easily overlooked quantities of an extremely lree rich accessory mineral either monazite or allanite our data indicate that felsic liquids with 50 ppm lree may be saturated in either of these accessories and that the concentration required for saturation decreases in increasingly felsic liquids this accounts for incompatible behavior of lree even at high concentrations in mafic magmas in contrast to compatible behavior at low concentrations in felsic magmas partitioning of lree into solid rather than liquid has important implications for trace element and nd isotope modeling of crustal anatexis as well as for magma differentiation

partial melting of metagreywackes part ii compositions of minerals and melts a series of experiments on the fluid absent melting of a quartz rich aluminous metagreywacke has been carried out in this paper we report the chemical composition of the phases present in the experimental charges as determined by electron microprobe this analytical work includes biotite plagioclase orthopyroxene garnet cordierite hercynite staurolite gedrite oxide and glass over the range 100 1000 mpa 780 1025 c biotites are na and mg rich with ti contents increasing with temperature the compositions of plagioclase range from an17 to an35 with a significant orthoclase component and are always different from the starting minerals at high temperature plagioclase crystals correspond to ternary feldspars with or contents in the range 11 20 mol garnets are almandine pyrope grossular spessartine solid solutions with a regular and significant increase of the grossular content with pressure all glasses are silicic sio2 67 6 74 4 wt peraluminous and leucocratic feo mgo 0 9 2 9 wt with a bulk composition close to that of peraluminous leucogranites even for degrees of melting as high as 60 vol with increasing pressure sio2 contents decrease while k2o increases at any pressure the melt compositions are more potassic than the water saturated granitic minima the h2o contents estimated by mass balance are in the range 2 5 5 6 wt these values are higher than those predicted by thermodynamic models modal compositions were estimated by mass balance calculations and by image processing of the sem photographs the positions of the 20 to 70 isotects curves of equal proportion of melt have been located in the pressure temperature space between 100 mpa and 1000 mpa with increasing pressure the isotects shift toward lower temperature between 100 and 200 mpa then bend back toward higher temperature the melting interval increases with pressure the difference in temperature between the 20 and the 70 isotects is 40 c at 100 mpa and 150 c at 800 mpa the position of the isotects is interpreted in terms of both the solubility of water in the melt and the nature of the reactions involved in the melting process a comparison with other partial melting experiments suggests that pelites are the most fertile source rocks above 800 mpa the difference in fertility between pelites and greywackes decreases with decreasing pressure a review of the glass compositions obtained in experimental studies demonstrates that partial melting of fertile rock types in the crust greywackes pelites or orthogneisses produces only peraluminous leucogranites more mafic granitic compositions such as the various types of calk alkaline rocks or mafic s type rocks have never been obtained during partial melting experiments thus only peraluminous leucogranites may correspond to liquids directly formed by partial melting of metasediments other types of granites involve other components or processes such as restite unmixing from the source region and or interaction with mafic mantle derived materials

fracture mechanical properties of carbonate and evaporite caprocks in sichuan basin china with implications for reservoir seal integrity abstract fracture development in reservoir rocks enhances the flow and transport properties of the petroleum system yet fracture in caprocks can significantly risk reservoir seal integrity over geological time scales the slow subcritical fracture propagation can contribute considerably to the fracture system development especially under reactive fluid environments during petroleum migration and accumulation using the double torsion technique we measure the subcritical fracture growth properties of carbonate and anhydrite caprock core samples in sichuan basin china under different environments of ambient air de ionized water and formation water experimental results show that the aqueous fluids significantly promote subcritical fracture growth 1 compared to ambient air the presence of aqueous fluids dramatically reduces both the fracture toughness kic and subcritical fracture growth index sci of the caprocks 2 the kic reduction is more prominent for the anhydrite 50 caprock than the carbonate 30 caprock which suggest that mineral solubility is responsible for the fluid weakening effect 3 a systematic change of sci with varying fluid conditions is observed sci is the largest in air smallest in formation water and intermediate in de ionized water microstructural analysis suggests a predominantly opening mode for both the natural fractures and induced fractures during fracture mechanical testing our results further demonstrate that subcritical fracture growth process is also strongly related to rock composition and stress conditions these observations provide insights into the mechanisms controlling fracture growth in carbonate and evaporite caprocks which could extend to brittle failure and fluid rock interactions occurring during the seismic cycle in the upper crust

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

trace element modelling of pelite derived granites the presence or absence of a vapour phase during incongruent melt reactions of muscovite and biotite together with the composition of the protolith determines the trace element characteristics of the resulting melt provided that equilibrium melting occurs for those phases that host the tracc elements of interest for granitic melts rb sr and ba provide critical constraints on the conditions that prevailed during melting whereas ree are primarily controlled by accessory phase behaviour mass balance constraints for eutectic granites that are formed by the incongruent melting of muscovite in pelites indicate that melting in the presence of a vapour phase will result in a large melt fraction and deplete the restite in feldspar hence the melt will be characterized by low rb sr and high sr ba ratios in contrast vapour absent melting will result in a smaller melt fraction and an increase in the restitic feldspar consequently high rb sr and low sr ba ratios are predicted vapour absent melting will also enhance the negative eu anomaly in the melt granites that result from the incongruent melting of biotite in the source will be characterized by higher rb concentrations than those that result from the incongruent melting of muscovite the himalayan leucogranites provide an example of unfractionated crustally derived eutectic melts that are enriched in rb but depleted in sr and ba relative to their metasedimentary protoliths these compositions may be generated by the incongruent melting of muscovite as a low melt fraction f 0 1 from a pelitic source under vapour absent conditions

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

flow pattern and sediment transport in a braided river the torrent de st pierre french alps summary in order to bring some understanding on the mountain stream dynamics we report measurements of flow and sediment transport leveled in a proglacial gravel bed river the torrent de st pierre in the french alps the river exhibits a modest discharge 3 s during most of the glacier melting season a braiding plain developed thanks to the occurrence of a massive landslide providing a small and simplified study area quite similar to recent experimental studies a mass balance was established at the outlet of the braiding plain our measurements indicate that in the range of flows measured the dominating transport mode is suspension though less important bed load transport is far from being negligible dissolved load eventually appears to be very small compared to solid transport analysis of velocity profile measurements shows that in this highly turbulent and shallow stream the use of a logarithmic form fails to recover the velocity profile and to estimate the shear velocity of the flow a uniform chezy like relationship is shown to be valid for the velocity with friction a coefficient considered as constant over the range of our measurements accordingly the only relevant velocity for transport description is the average velocity coupled measurements of bed load and velocity show that bed load transport is related to the average velocity through a power law eventually a correlation between bed load transport and suspended load transport is evidenced and discussed

petrology and geochemistry of syn to post collisional metaluminous a type granites a major and trace element and nd sr pb o isotope study from the proterozoic damara belt namibia abstract leucocratic a type granites of the proterozoic damara orogen namibia are a distinctive groups of rocks based on their mineralogical chemical and isotopic characteristics a type granites can be distinguished from the leucocratic s type granites on the basis of fe rich amphibole and biotite and higher abundances of zr nb y and lree but lower values of 87 sr 86 sr 500 ma and 18 o in the a type granites in comparison with a type granites described in the literature the damaran a type granites are enriched in cao al2o3 mgo feo ba sr and eu but depleted in ga and zn low k rb rb sr rb ba and sr ba ratios indicate that these melts are not fractionation products of calc alkaline source rocks and therefore these chemical criteria are interpreted as a feature of the source rocks i e undepleted metatonalites the a type granites have high calculated zircon and monazite saturation temperatures mostly in excess of 800 c and it is concluded that these granites represent high temperature partial melts the relationship between major and trace element data and isotopic composition demonstrates that the a type granites originate from several sources and have undergone different processes the lack of evidence for shallow crustal contamination e g lack of country rock xenoliths the low viscosities of the melts as inferred from their high calculated temperature and their high nd concentrations relative to the country rocks suggest that the isotopic characteristics were acquired from an enriched mantle initial sr isotopic compositions vary over a narrow range 0 70486 0 70928 but the initial nd isotopic compositions vary considerably with initial end values between 0 03 and 6 35 this wide range in the nd isotope values is probably the result of interaction of mantle derived magmas with crustal rocks a type granites at baukwab have negatively correlated initial end values and 207 pb 204 pb and 206 pb 204 pb ratios indicating interaction of mantle derived magmas low initial end low 207 pb 204 pb and 206 pb 204 pb with continental crust high initial end high 207 pb 204 pb and 206 pb 204 pb a type granites from albrechtstal and oetmoed have positively correlated pb isotope ratios and show also positive correlations between nd isotopes and pb isotopes suggesting interaction of at least two different source materials the relationship between nd isotopes and nd concentrations precludes simple assimilation instead mixing of different sources seems to be more realistic for the a type granites from oetmoed oxygen isotopes are negatively correlated with sr isotopes indicating that the crustal end member has high 18 o values and rather low initial 87 sr 86 sr ratios whereas a hypothetical mantle component has lower 18 o values but higher initial 87 sr 86 sr ratios the chemical and isotope data suggest that the damaran a type granites were generated by a variety of processes including partial melting of mantle derived tonalitic sources limited crystal fractionation and interaction of these magmas with crustal material these data indicate that the damaran a type suites mark episodes of continental growth during which mantle material although limited in extent is added to the crust

nd isotopic mapping of the sri lanka basement update and additional constraints from sr isotopes abstract the high grade crystalline basement of sri lanka can be subdivided into three provinces distinguished by distinct isotopic model ages this confirms the conclusions of milisenda et al 1988 made on the basis of much more limited data high grade ortho and paragneisses from the highland complex in central sri lanka yield palaeoproterozoic to archaean nd model ages from 2 0 to 3 4 ga and reflect an extended premetamorphic crustal history the wanni complex in the western and northwestern part of the island is characterized by meso to neoproterozoic nd model ages ranging from 1 0 to 2 0 ga the vijayan complex in the southeast is occupied by neoproterozoic i type granitoids with nd model ages from 1 0 to 1 8 ga this distinct crustal province appears to fit best a detached continental margin plutonic terrane the isotopic data preclude a common premetamorphic history of the wanni and vijayan complexes with the ancient highland complex the combined nd sr and pb isotope data together with recent geochronological studies strongly suggest that the distinct crustal age provinces are separated by tectonic discontinuities and that they represent terranes that were accreted just prior to the high grade metamorphic event in neoproterozoic time

anthropic aggradation of the waiho river westland new zealand microscale modelling long term aggradation of the waiho river south westland new zealand has now raised the head of its alluvial fan to unprecedented elevations in its natural state the river would like all other major rivers in the area be somewhat incised into its fanhead the only relevant factor able to account for the aggradation is the presence of control banks stopbanks in local parlance that restrict the ability of the river to move over the whole of its natural fanhead a 1 3333 scale physical hydraulic model a microscale model was used to study this situation an alluvial fan was generated in the model and allowed to develop to equilibrium with steady inputs of water and sediment within boundaries geometrically similar to those of the natural unrestricted waiho river the boundaries were then altered to represent the presence of the stopbanks and the fan allowed to continue evolving under the same water and sediment inputs the model fanhead aggraded in a spatial pattern similar to that recorded on the waiho taking into consideration the limitations of microscale modelling these results indicate that the aggradation in the waiho is a result of the lateral restriction of the river by stopbanks this poses fundamental questions about the variables that control the behaviour of alluvial fans the results also suggest that microscale modelling can be used to make reliable quantitative predictions of the effects of engineering works on rivers in spite of the low level of dynamic similarity with the prototype compared to that in larger scale models copyright 2003 john wiley sons ltd

generation of peraluminous granitic magma in a post collisional setting a case study from the eastern qilian orogen ne tibetan plateau abstract early paleozoic peraluminous granites are abundant in the eastern part of the qilian orogen northeastern margin of the tibetan plateau a combined study involving geochronology whole rock geochemical and sr nd hf isotopic compositions for three early paleozoic peraluminous granitic plutons jishishan ledu and shichuan plutons from the eastern qilian orogen was carried out to evaluate the causes of chemical variations and generation mechanisms of peraluminous granitic magmas these granitic plutons have magma crystallization ages of 455 427 ma and are moderately to strongly peraluminous a cnk 1 03 1 18 geochemical and sr nd hf isotopic data indicate that they consist substantially of crust derived melts the jishishan and ledu peraluminous granites were dominantly produced by partial melting of precambrian orthogneisses the shichuan monzogranites with low hree contents e g yb 0 80 1 83 ppm and slightly negative end t 5 3 to 2 3 and positive ehf t 1 6 to 3 4 could be derived from immature crustal materials relatively high average zircon saturation temperatures 750 c for each pluton obvious negative eu anomalies eu eu 0 28 0 80 and low pb ba ratios 0 03 0 16 for the jishishan ledu and shichuan granites are consistent with crustal melting involving biotite breakdown under fluid absent conditions our results suggest that compositional variations of moderately to strongly peraluminous granitic magmas are mainly controlled by source compositions and melting conditions while the processes such as mixing with mantle derived magma fractional crystallization restite unmixing and peritectic assemblage entrainment were insignificant or only play secondary roles in their genesis late ordovician to middle silurian crustal anatexis in the eastern central qilian was probably linked with slab break off which may be an important mechanism in addition to lithospheric delamination for the generation of moderately to strongly peraluminous granites in a post collisional setting

nature and origin of proterozoic a type granitic magmatism in the southwestern united states of america the mountain ranges of arizona and adjacent california and nevada contain large areas underlain by proterozoic anorogenic granites comprising the southwesternmost portion of a transcontinental belt of 1 4 1 5 ga old anorogenic complexes that extends across north america northeast into labrador of these a two mica monazite bearing granitic suite resides in central and southeastern arizona as part of a peraluminous subprovince that is bordered on the south southern arizona to sonora and west western arizona and adjacent portions of california and nevada by marginally metaluminous granites bearing biotite sphene hornblende and fluorite all of these 1 4 ga granites are distinctly more potassic iron enriched relative to mg and depleted in ca mg and sr in contrast to typical orogenic granitoids in general the large ion lithophile element enriched composition is a consequence of limited melting of a water deficient crustal source at depths greater than 25 37 km for the peraluminous granites this contrast is less extreme perhaps resulting from a larger degree of melting as a consequence of a greater metasedimentary component and water in its crustal source the anorogenic granitic magmas intruded into the upper crust at depths of 8 17 km or shallower at temperatures up to 790 c the most dramatic variation in the crystallization intensive parameters resides in the oxygen fugacity which spans three orders of magnitude relative to other anorogenic suites all of the magmas crystallized at elevated levels of o2 as reflected in their assignment to the anorogenic magnetite series yet a regionally significant rise in primary o2 levels unmatched elsewhere in the transcontinental belt occurs for plutons in western arizona including the holy moses and hualapai granites the most extreme case is the hualapai granite whose biotite fe fe mg ratios drop due to high o2 to a low of 0 27 down from more typical levels of 0 54 to 0 75 such extreme variations in primary levels of oxygen fugacity must be an indirect imprint of regional changes of the level of oxidation of the lower crust the high fo2 holy moses and hualapai plutons have intruded near the regional boundary between the metaluminous and peraluminous granites and appear to be imaging a major change in the level of oxidation of the lower crust this boundary is also approximately equivalent to significant changes in the nd and pb isotopic compositions of these granites and the metamorphic and magmatic character of the older orogenic terrane on a global scale the crust forming orogenies ended by 1 6 ga ago and the continents entered a long lived era dominated by localized extension and transcontinental intrusion of anorogenic potassic rapakivi granite mafic dike swarms charnockite and anorthosite the absence of orogenic deformation implies that plate consumption became intraoceanic during this time the profuse and widespread nature of the igneous activity has no phanerozoic analogue and is considered to be unique to the proterozoic a crustal overturn model ties the magmatism to heating within a largely undepleted subcontinental mantle the eventual rise of mantle plumes and the transfer of heat into the youthful undifferentiated proterozoic crust subsequent melting and rise of potassic granitic magmas from the lower crust leads to considerable crustal reorganization a process that would continue until both the mantle and crust reached a stable configuration

selective entrainment of peritectic garnet into s type granitic magmas evidence from archaean mid crustal anatectites entrainment of restite is commonly invoked to explain both the origin of relatively mafic granites and granodiorites as well as the chemical connection between granite magmas and their sources this concept has become linked to models for magma migration out of the source as restite entrainment is considered to take place when diatexitic sources mobilise en masse this is at odds with the common occurrence of relatively mafic granites as high level intrusions in the crust or their eruptive equivalents that must have formed from markedly water undersaturated magmas that ascended through narrow conduits we investigate pelitic migmatites from the mkhondo valley metamorphic suite mvms in swaziland where a mid crustal heating event produced metatexitic migmatites with minimal post anatectic recrystallisation in these rocks all the garnet is peritectic having arisen through biotite fluid absent melting which produced garnet poikiloblasts characterised by inclusions of melt quartz and biotite leucosomes that represent sites of melt transfer carry similar smaller typically 1 mm entrained garnet poikiloblasts which were capable of amalgamating to form larger composite grains in anatectic structures where melt was present for longer entrained garnet was extensively recrystallised via a dissolution precipitation process to adopt a more magmatic character the peritectic garnet in the pelitic source appears to have grown out of equilibrium with feldspar and hree rich accessory phases while the recrystallised garnet in the larger melt filled structures became progressively better equilibrated with these minerals thus peritectic garnet in the source grew sufficiently rapidly to prevent trace element equilibrium with the bulk rock composition and concurrent rapid magma segregation prevented the development of diatexitic source conditions the segregated magma consisted of melt the peritectic assemblage principally garnet and the accessory minerals monazite and zircon these rocks illustrate that mafic granites may arise purely as mixtures of melt and the peritectic assemblage produced by the incongruent melting reaction importantly under the circumstances which produced the mvms anatectites peritectic garnet is entrained as 1 mm poikiloblasts demonstrating how mafic granitic magmas can migrate out of the source without the source becoming diatexitic

high temperature 350 c thermal histories of the long lived 500 ma active margin of ecuador and colombia apatite titanite and rutile u pb thermochronology abstract quantitative reconstruction of thermal histories can be a powerful tool to study numerous natural processes such as tectonic plate interaction cratonic stability and extra terrestrial phenomena such as asteroid ejection a majority of thermochronological studies have focused on temperatures lower than 300 c few previous studies have demonstrated that u pb data from apatite and other accessory phases can be used to recover thermal history information at t 350 c we present u pb data from apatite to constrain the thermal histories of triassic peralluminous anatectites from the northern andes between the temperatures of 350 550 c the accuracy of the thermal history models is assessed by comparisons with previous geological models and comparisons with pre existing and newly acquired u pb titanite and rutile 40ar 39ar muscovite and low temperature thermochronological data this study also examines the feasibility of using a large regionally dispersed apatite u pb data set to obtain continuous thermal history paths along a long lived 500 ma active margin a second aim of this study is to further test the hypothesis that the dominant mechanism for pb displacement through apatite is volume diffusion as opposed to aqueous fluid interaction the thermal history models derived from the triassic anatectites exposed in the andes of colombia and ecuador are entirely consistent with lower temperature thermochronological constraints and previously established geochronological and geochemical constraints they reveal and quantify trench parallel changes in the amount of jurassic early cretaceous extension significantly bolstering and adding to previous tectonic interpretations confirmation of the utility of u pb thermochronology provides geologists with a powerful tool for investigating the high temperature thermal evolution of accessory minerals

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

do gravel bed river size distributions record channel network structure 1 bed load sediment particles supplied to channels by hillslopes are reduced in size by abrasion during downstream transport the branching structure of the channel network creates a distribution of downstream travel distances to a given reach of river and thus may strongly influence the grain size distribution of the long term bed load flux through that reach here we investigate this hypothesis using mass conservation and the sternberg exponential decay equation for particle abrasion to predict bed material variability at multiple scales for both natural and artificial drainage networks we assume that over a sufficiently long timescale no net deposition occurs and that grains less than 2 mm are swept away in suspension we find that abrasion during fluvial transport has a surprisingly small effect on the bed load sediment grain size distribution for the simple case of spatially uniform supply of poorly sorted hillslope sediments this occurs because at any point in the channel network local resupply offsets the size reduction of material transported from upstream thus river bed material may essentially mirror the coarse component of the size distribution of hillslope sediment supply furthermore there is a predictable distance downstream at which the bed load grain size distribution reaches a steady state in the absence of net deposition due to selective transport large scale variability in bed material such as downstream fining must then be due primarily to spatial gradients in hillslope sediment production and transport characteristics a second key finding is that average bed load flux will tend to stabilize at a constant value independent of upstream drainage area once the rate of silt production by bed load abrasion per unit travel distance is equal to the rate of coarse sediment supply per unit channel length q bed load flux equilibrates over a distance that scales with the inverse of the fining coefficient in the abrasion rate law and can be approximated simply as q 3 thus the efficiency of particle abrasion sets a fundamental length scale shorter for weaker rocks and longer for harder rocks which controls the expression in the river bed of variability in sediment supply we explore the role of the abrasion length scale in modulating the influence of sediment supply variability in a number of channel network contexts including individual tributary junctions a sequence of tributary inputs along a main stem channel and variable basin shapes and network architecture as expressed by the width function these findings highlight the need for both data and theory that can be used to predict the grain size distributions supplied to channels by hillslopes

small is beautiful upscaling from microscale laminar to natural turbulent rivers 1 the use of microscale experimental rivers with flow depths of the order of a few millimeters to investigate natural processes such as alluvial fans dynamics knickpoints migration and channel morphologies such as meandering and braiding has become increasingly popular in recent years this raises the need to address the issue of how to extrapolate results from the experimental microscale at which flow is laminar to the scale of natural turbulent rivers we address this question by performing measurements of average flow velocity and sediment transport in an experimental laminar river the average flow velocity is correctly predicted from the navier stokes equation solved for a steady uniform laminar flow laminar sediment transport is found to be consistent with the law of meyer peter and muller 1948 commonly used to describe sediment transport in natural turbulent rivers we also show that surface tension is important only if the microscale river width is on the order of or smaller than the capillary length these results allow us to demonstrate that the evolution of longitudinal bed profiles of turbulent and laminar rivers are governed by identical dimensionless equations and therefore follow the same dynamics differences of time and length scales at work in experimental and natural rivers are mainly encoded in the expression of two parameters a diffusion coefficient and a threshold slope on the basis of this analysis we derive a set of equations allowing us to rescale bed elevation downstream distance time and uplift rate from an experimental microscale river to the field scale finally we show how this set of equations can be used to rescale these same parameters in the case of a temporally varying discharge

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

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

are strongly peraluminous magmas derived from pelitic sedimentary sources igneous compositions that require the presence of a phase more aluminous than biotite are here designated as strongly peraluminous ps such compositions are common but subordinate in felsic rocks and exceedingly rare in more mafic rocks the term s type has been applied to many igneous rocks based largely or even solely upon their ps compositions neglecting or placing less emphasis on other criteria for sedimentary parentage in chappell and white s i and s type classification system the s type designation was in fact initially restricted to granites which were thought to have been derived from a chemically mature strongly weathered pelitic shaly source application of chappell s and white s criteria together with others based upon isotopic and phase equilibria considerations leads to the following conclusions about ps igneous rocks worldwide 1 very few ps rocks are derived entirely from pelitic sources 2 a majority are derived largely from intermediate to felsic crustal sources including

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

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

mineral leucosome trace element partitioning in a peraluminous migmatite a laser ablation icp ms study abstract low pressure peraluminous migmatites from the pena negra complex central spain stayed partially molten for 5 10 ma until the latest hercynian deformations allowed the segregation of melt as discordant leucosome veins due to long residence within its source the melt had time enough to equilibrate with residual phases other than accessories included within major refractory minerals we estimated crystal melt partition coefficients as the concentration ratios between leucosome samples appearing to be pure melts and mesosome minerals which were analyzed for trace elements with a laser probe coupled to an icp mass spectrometer our data reveal that when biotite is stable li rb cs tl sc v cr ni nb and ta become strongly compatible the role of biotite in fractionating y th u and the ree is insignificant cordierite strongly fractionates li and be and also has some effect on the hree and u garnet produces extreme fractionation of sc y and the hree the ree partition coefficients for ferromagnesian silicates increase with the atomic number this effect being progressively more important through biotite cordierite and garnet k feldspar strongly fractionates ba and pb but plays a secondary role to biotite for rb and cs k feldspar fractionation does not change the lree hree ratio but plagioclase fractionation produces a significant decrease in the lree hree ratio both feldspars greatly fractionate sr and eu monazite fractionation produces a dramatic depletion in ree th and u as well as a decrease in the lree hree and th u ratios apatite also fractionates the ree although in contrast with monazite it increases the lree hree ratio and does not affect the th u ratio zircon fractionation like apatite and garnet produces a strong depletion in the hree and a concomitant increase in the lree hree ratio in contrast with monazite zircon fractionation causes the th u ratio to increase

partial melting of diverse crustal sources constraints from sr nd o isotope compositions of quartz diorite granodiorite leucogranite associations kaoko belt namibia the torrabaai koigabmond complex southern kaoko belt namibia consists of three main intrusive rock types including metaluminous hornblende and titanite bearing quartz diorites metaluminous hornblende and biotite bearing granodiorites and peraluminous garnet and muscovite bearing leucogranites uranium pb zircon data obtained on the granodiorites and leucogranites indicate concordia upper intercept ages of 553 40 ma although pb 207 pb 206 ages of ca 650 ma in zircon from the granodiorites suggest some inheritance of older material uranium pb monazite data obtained on the leucogranites give concordant ages of 550 ma 3 ma these ages are similar to the rb 87 sr 86 whole rock age of 584 35 ma obtained on the granodiorites and leucogranites although the rb sr age seems to be biased towards older ages due to limited assimilation of older material in contrast to other plutonic complexes from the kaoko belt the quartz diorites granodiorites and granites show a restricted range in their initial nd sr and o isotope compositions quartz diorites epsilon nd init 5 4 to 6 7 delta o 18 8 3 9 4 parts per thousand sr 87 sr 86 0 7081 0 7098 granodiorites epsilon nd init 6 1 to 7 7 delta o 18 9 9 10 9 parts per thousand sr 87 sr 86 0 7071 0 7105 leucogranites epsilon nd init 4 9 to 8 7 delta o 18 9 8 11 3 parts per thousand sr 87 sr 86 0 7060 0 7125 enclaves are found in the granodiorites and leucogranites but not in the quartz diorites they have a granodioritic composition with quartz plagioclase k feldspar and hornblende and some have additional garnet relative to the country rock gneisses the so called nk formation enclaves are depleted in sio2 na2o k2o sr ba and enriched in cao feo total mgo tio2 sc v cr ni rb and y rare garnet bearing enclaves are additionally depleted in lree and enriched in hree relative to the granodiorites these features are qualitatively consistent with the hypothesis that these enclaves may represent moderately depleted melting residues of nk formation gneisses in comparison with experimentally derived melts and based on low al2o3 feo mgo tio2 ratios and high al2o3 feo mgo tio2 values it is suggested that the quartz diorites are generated by dehydration melting of a mafic amphibole and plagioclase bearing lower crustal source of pan african age the granodiorites likely represent fractionation products of the quartz diorites however it is also possible that the granodiorites represent partial melting products of a mafic to intermediate lower crustal source but experienced likely slightly lower degrees of melting probably at water present conditions the leucogranites display higher al2o3 feo mgo tio2 ratios but lower al2o3 feo mgo tio2 values and are most likely generated by biotite dehydration melting of felsic crustal sources major and trace element and isotope variations indicate that fractional crystallization with only limited crustal contamination was the major rock forming mechanism it is suggested that most of the isotope variation reflects pre existing heterogeneities of the sources consequently interpretation of geochemicaland isotope data from the complex suggests that the pan african igneous activity in this part of the damara kaoko belt was not a major crust forming episode and all rock types represent reprocessed crustal material c 2008 elsevier b v all rights reserved less

a flume experiment on the development of subaqueous fine gravel dunes from a lower stage plane bed 1 determining the hydraulic conditions whereby gravel dunes first develop in subaqueous environments is fundamental as their presence may influence engineering solutions designed to maintain bed stability in addition estimates of the flow conditions associated with preserved gravel bed forms in sedimentary sequences are useful for reconstructing the depositional environments and geometries of for example oil and gas bearing geological strata consequently a series of experiments considered dune initiation in these experiments defects and latterly incipient dunes developed from lower stage plane gravel beds during near threshold conditions of motion crit 1 0 1 016 and long periods of marginal bed load transport rates the three dimensional defects were almost imperceptible positive ovoid features with heights of one or two grain diameters and lengths and spans of a few decimeters after 17 hours of flow incipient low amplitude simple two dimensional dunes developed from the defects with heights ranging between 0 029 and 0 055 m nonequilibrium wavelengths of 1 4 m and spans of 0 6 0 9 m continued development over several days with crit ratios of around 1 3 resulted in near equilibrium two dimensional dunes with wavelengths averaging 2 6 3 5 m and spans equal to the flume width 4 m the inception of incipient dunes could be predicted using bulk flow models however this approach was not suitable for the prediction of defect development near bed turbulence in the form of small scale sweep events of limited breadth controls the initiation of defects but larger scale coherent turbulent structures in the outer flow are related to dune development significantly both defects and incipient dunes can exist at the same time which indicates that the effects of sweeps on the bed morphology persist at the same time as larger scale turbulent structures are beginning to effect sediment transport

a model for monazite melt equilibrium and application to the generation of granitic magmas abstract in order to model the behaviour of zr hf ti p u th y and the rare earth elements ree in the continental crust the parameters which control the equilibria between accessory minerals and granitic melts must be determined this paper proposes equations for the equilibrium between monazite and ca poor felsic melts in a first step all the light ree are considered as a single component and an equation describing monazite solubility is deduced from the available experimental data in a second step the fractionation of ree by monazite is investigated on the basis of a natural example of coexisting monazite and obsidian from macusani peru the solubility of monazite can be used as a thermometer based on the total ree content of natural magmatic rocks application of ree thermometry to various peraluminous rock series suggests that it is a reliable tool for determining temperatures of magmatic processes the equation governing ree fractionation by monazite is used to discuss genetic relationships between a melt and a presumed parental rock using the composition of the monazite in the parental rocks and the ree pattern of the daughter rock this method has been applied successfully to high himalayan manaslu and tibetan slab granites it is suggested that detailed petrographical work on monazite including electron microprobe analyses and sem imaging of its internal structures combined with the model presented here can be a useful tool for deciphering the genesis and the differentiation of peraluminous granitic magmas in the continental crust

generation of syntectonic calc alkaline magnesian granites through remelting of pre tectonic igneous sources u pb zircon ages and sr nd and pb isotope data from the donkerhoek granite southern damara orogen namibia abstract the 541 4 ma old magnesian weakly peraluminous calc alkalic donkerhoek onanis granite is part of the ca 6000 km2 large donkerhoek batholith in the southern zone of the damara orogen of namibia linear major and trace element variations and decreasing mgo feo al2o3 cao k2o na2o ba and sr concentrations with increasing sio2 indicate that this part of the batholith represent a coherent mass and underwent fractional crystallization processes the donkerhoek onanis granites are isotopically evolved initial end 4 7 to 12 3 initial 87sr 86sr 0 7099 0 7157 with moderately radiogenic pb isotope ratios 206pb 204pb 17 26 18 22 207pb 204pb 15 59 15 67 208pb 204pb 37 60 38 06 beside heterogeneities imparted by the sources an evaluation of lree fractionation and nd isotope data suggests that afc processes also modified some samples based on the chemical and isotope data the donkerhoek onanis granites cannot be derived by partial melting of al and fe rich metasedimentary rocks of the kuiseb formation in which they intruded instead melting of meta igneous crustal sources with proterozoic crustal residence ages is more likely three igneous to meta igneous rock suites from the area matchless amphibolites proterozoic mafic to felsic gneisses from the southern kalahari craton basement syn tectonic salem granodiorites to granites are potential sources an evaluation of chemical and isotope data suggests that remelting of early syn orogenic salem type granites is the most likely process which would also explain the existence of ca 563 4 ma old zircon in the donkerhoek onanis granites comparison of the donkerhoek onanis granites with experimentally derived melt compositions from an intermediate igneous parent indicates temperatures between 800 and 850 c it is suggested that the pan african igneous activity in this part of the damara belt was a moderate temperature intra crustal event although there are some compositional similarities with juvenile granites generated in subduction zones unradiogenic pb isotope ratios and moderately radiogenic sr and unradiogenic nd isotopes suggest that reprocessed crustal rocks are more likely sources previously obtained high 18o values of the donkerhoek onanis granites ranging from 11 8 to 13 6 covering the range of 18o values obtained on salem type granites from the area 12 5 13 3 confirm this view in contrast to igneous processes along active continental margins that produce juvenile batholiths with calc alkaline affinities this igneous event was not a major crust forming episode and the donkerhoek onanis granites represent reprocessed crustal material

vertical sorting in bed forms flume experiments with a natural and a trimodal sediment mixture two sets of flume experiments were conducted to examine grain size selective transport and vertical sorting in conditions with migrating bed forms and bed load transport in the two sets of experiments we used a sediment mixture from the river rhine and a trimodal mixture respectively the vertical sorting profiles showed a downward coarsening trend within the bed forms and in some experiments an essentially immobile coarse bed layer formed underneath the migrating bed forms three mechanisms contribute to the formation of such a coarse bed layer 1 the avalanching process at the lee face 2 conditions of partial transport in which a relatively large amount of coarse material does not participate in the transport process and 3 the winnowing of fines from the trough surface and subsurface the experiments show that vertical sorting fluxes not only occur through net degradation or aggradation but also through the migration of bed forms and through the variability in trough elevations this is contradictory to the way vertical sorting processes are modeled in most existing sediment continuity models for nonuniform sediment the present study is therefore also a plea for modifying existing sediment continuity models to account for vertical sorting processes other than through net aggradation or degradation

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

bedrock topography modeling and calcrete uranium prospectivity analysis of central erongo region namibia abstract the tertiary palaeodrainage system in the namibian calcrete uranium province is deeply buried under cenozoic sediments such as fluvial alluvial and desert flash flood terrace sediments duricrusts and or aeolian and fluvially reworked aeolian sands and has no obvious surface expression because of which this province remains under explored in spite of hosting the world s largest calcrete uranium deposit at langer heinrich this paper presents first pass bedrock geology and bedrock topography models for the namibian calcrete uranium province generated using air borne magnetic data which are further used to delineate subsurface tertiary palaeochannels in the province conjunctive interpretations of surface geology bedrock geology and bedrock topography indicated that the major underlying controls on the spatial distribution and configuration of the tertiary palaeochannels in the namibian province are both lithological and structural however the basement structural fabric that formed during the damaran orogeny possibly exerts a fundamental control on both palaeodrainage as well as present drainage the analysis also indicated that the carnotite deposition in the langer heinrich palaeochannel is possibly a consequence of westward reorientation of the course of the proto swakop river finally the paper presents a manual prospectivity analysis of the province for calcrete uranium deposits which resulted in the demarcation of five prospective sections of palaeochannels in the tumas basin swakop river and khan river terraces and in the gawib flats west of the langer heinrich deposit

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

elemental dispersion and stable isotope fractionation during reactive fluid flow and fluid immiscibility in the bufa del diente aureole ne mexico evidence from radiographies and li b sr nd and pb isotope systematics the 31 6 0 3 ma old bufa del diente alkali syenite ne mexico intruded a sequence of cretaceous limestones with intercalated sub horizontal chert layers the cherts acted as aquifers that facilitated transport of brines and pegmatitic melts within the shallow level 1 kbar contact metamorphic aureole fluid driven reactions between chert and marble wallrock and the influx of late melts and various fluids gave rise to distinct chemical and isotopic signatures within the aquifer and across the zones of infiltration and fluid driven reaction aqueous brines of magmatic origin produced thick wollastonite mantles around the chert layers wollastonite formation occurred at the expense of limestone and chert and generated co2 this co2 induced fluid unmixing into an aqueous brine and a low density co2 rich fluid which was lost to the overlying marble where it oxidized organic matter and caused 13c and 18o shifts in a zone some 5 10 cm wide after wollastonite formation the chert aquifers were locally intruded by pegmatite veins carrying alkali feldspar quartz aegirine augite eudialyte zircon and apatite aqueous fluids that exsolved during crystallization of the pegmatite veins escaped along late cross fractures and migrated along the inner and outer borders of the wollastonite margins chemical dispersion patterns of u al na k p s fe and ree across the chert to marble boundary and its metasomatic rims are shown by autoradiography and neutron induced radiography scavenging of cations at mineralogical contacts and cation transport into the marbles occurred only on the mm to cm scale isotopic data for pb and sr across a simple metachert marble boundary and for pb sr nd b and li across a metachert pegmatite marble sequence demonstrate the following 1 the pb and sr isotopic signature of early fluids was buffered by the carbonate wallrock only late fluids shielded from wallrock interaction by a wollastonite mantle variably preserved a memory of their initial magmatic signature 2 since the nd isotope signature of marble and chert is bound to calcite and clay minerals systematic shifts to unradiogenic nd in marble reflect loss of carbonate bound nd as the wollastonite margin is approached nd in the wollastonite margin is dominated by nd originally bound to clay minerals the later emplacement of the pegmatite which carried the nd isotope signature of its alkali syenite source had little effect on the nd isotopic composition of the wollastonite rim 3 although the li and b isotopic compositions reflect the alkali syenite source they are also affected by isotopic fractionation and partitioning between melt fluid and solids

geochronology geochemistry and isotope compositions of piaochi s type granitic intrusion in the qinling orogen central china petrogenesis and tectonic significance abstract s type peraluminous granite can crop out in various tectonic settings the paleozoic piaochi granitic intrusion is a large s type peraluminous intrusion located in the north qinling orogen and has important implication for its tectonic evolution though intensively studied the age and tectonic setting of this intrusion are still controversial in this study we carried out an integrated study of zircon u pb age major and trace elements and sr nd hf o isotope compositions for the piaochi intrusion with aims to constrain its petrogenesis and tectonic significance sims and la icpms u pb zircon data obtained from magmatic zircons indicate that the piaochi intrusion was emplaced at 473 4 ma and contained ca 490 ma inherited zircons the granite samples from the piaochi intrusion show typical s type characteristics such as the relatively moderate aluminum saturation index 1 00 1 08 low zirconium saturation temperatures occurrences of muscovite and high zircon 18 o values 8 70 to 9 54 the relatively low sm nd 0 13 0 20 and feo t mgo ratios 2 21 3 9 suggest that the granites are unfractionated the geochemical compositions reveal that they were derived from partial melting of a clay poor but plagioclase and biotite rich psammitic source similar to metasedimentary rocks outcropping in the north qinling unit moreover the piaochi granites are moderately depleted in eu and heavy rare earth elements and have relatively high la yb n values 22 8 56 8 implying a high pressure plagioclase poor and garnet rich residual assemblage the piaochi granites have high and variable initial 87 sr 86 sr ratios 0 72167 0 72511 and low e nd t from 10 23 to 8 71 values with two stage nd model ages t dm2 of 1 91 2 03 ga which are comparable with those of supracrustal materials in the north qinling unit in addition our results show that the ca 470 ma magmatic zircons display a narrow e hf t array within those of the inherited cores these imply that the piaochi granites inherited their isotopic characteristics wholly from their sources and the hf isotopes might have been differently homogenized during the partial melting our new zircon u pb ages indicate that the piaochi granites were emplaced after the climax of the uhp metamorphism coeval with later retrograde metamorphism during exhumation therefore we suggest that the formation of the early paleozoic piaochi intrusion was triggered by the exhumation of the uhp rocks

petrogenesis of early syn tectonic monzonite granodiorite complexes crustal reprocessing versus crustal growth abstract the 563 7 6 1 ma old early syntectonic mon repos complex is a predominantly metaluminous magnesian calc alkalic granodioritic to granitic intrusion major and trace element variations imply that the rocks evolved through fractional crystallization processes involving amphibole biotite fe ti oxides zircon and apatite rocks less evolved than granodiorite show evidence of hornblende accumulation initial sr 87sr 86sr 0 7090 0 7111 and nd end 5 3 to 12 3 isotopic compositions of the granodiorites and granites are highly heterogeneous and vary with sio2 contents indicating that assimilation of older crustal components occurred initial 206pb 204pb 16 65 17 65 and 207pb 204pb ratios 15 52 15 60 are similar to other mafic intermediate complexes from the damara belt one monzodiorite and two quartz monzonites are k2o lile and hfse enriched and have similar sio2 contents as the granodiorites these samples are similar to post collisional mafic magmas from elsewhere in the world their e nd values 3 8 to 4 3 87sr 86sr ratios 0 7051 0 7073 and trace element characteristics imply that their unexposed parental melts are derived from a lithospheric mantle source that was contaminated and metasomatized by crustal material during ancient subduction processes the data are explained by assuming a flat subduction model where melting predominantly involves ancient continental mantle lithosphere and crust with limited if any melting of the underlying asthenospheric mantle during flat subduction a sliver consisting of buoyant oceanic lithosphere was amalgamated with the base of the ancient continental lithosphere of the overriding plate the oceanic mantle lithosphere and the oceanic crust dehydrated but did not melt and these fluids lowered the solidus of the overlying continental mantle lithosphere and crust this scenario can explain the occurrence of rare k2o and lile enriched monzodiorites and quartz monzonites with crustal like isotopic compositions observed in this study as well as some more alkaline rocks in the belt because the metasomatized continental lithospheric mantle has lower solidus temperatures than peridotitic mantle it is very likely that such metasomatized rocks may melt early during flat subduction thus k2o enriched monzodiorites to monzogranites are not restricted to late stages of the evolution of an orogen but may already form at the onset of an orogeny

quantitative models of sedimentary basin filling summary quantitative modelling of the filling of sedimentary basins was begun in earnest in the 1960s dozens of themes and variations have been proposed since then and have yielded an abundance of idealized stratigraphic patterns as functions of both imposed changes and basin properties post plate tectonic modelling began with rigid lid models which show the stratigraphic signature of subsidence variation this work introduced the connection between stratigraphy and the rheology of the lithosphere rigid lid models are the simplest type of geometric model in which the sediment surface is assigned prescribed geometries usually corresponding to different depositional environments these can reproduce many aspects of overall stratal geometry but are formally restricted to relatively long timescales for which quasi steady surface topography can be assumed so called dynamic models attempt to represent the morphodynamics of the sediment surface by abstracting and averaging short term transport processes most of the dynamic models proposed to date can be seen as special cases of a single general morpho dynamic equation the most important result of the first wave of quantitative basin filling models is that even relatively simple models can produce reasonable stratal patterns we now have a wide array of tools for exploring scenarios searching for general behaviours and effects and making initial quantitative predictions we have also learned that basin response to external forcing as recorded in stratigraphy can be as sensitive to the characteristics of the basin as to the forcing the main brake on the development of basin modelling is not computing power but lack of methods and data for testing the models we have already developed physical experiments which are only just beginning are one means of doing this experimental stratigraphy is a bridge to quantitative field tests which will require collaboration among academic researchers from a wide range of areas and between academia and industry on projects of greater scale and degree of integration than we have seen so far the advancement of quantitative sedimentary geology will also require significant changes in the way the subject is taught at all levels

evolution of a sediment wave in an experimental channel the routing of bed material through channels is poorly understood we approach the problem by observing and modeling the fate of a low amplitude sediment wave of poorly sorted sand that we introduced into an experimental channel transporting sediment identical to that of the introduced wave the wave essentially dispersed upstream and downstream without translation although there was inconclusive evidence of translation late in the experiment when the wave was only 10 20 grain diameters high alternate bars migrated through zones of differing bed load transport rate without varying systematically in volume celerity or transport rate sediment that overpassed migrating bars was apparently responsible for dispersion of the wave the evolution of the wave was well predicted by a one dimensional model that contains no adjusted empirical constants numerical experiments demonstrate however that the theory does not predict sediment waves that migrate long distances downstream such waves can only be explained by the following processes not represented by the theory selective bed load transport spatial variations in bar and other form roughness the mechanics of mobile armor and perhaps other mechanisms

experimental evidence of dynamic scaling and indications of self organized criticality in braided rivers the evolution of an experimental braided river produced in our laboratory has been monitored and analyzed it has been shown that in addition to the spatial scaling revealed by sapozhnikov and foufoula georgiou 1996a braided rivers also exhibit dynamic scaling this implies that a smaller part of a braided river evolves identically in the statistical sense to a larger one provided the time is renormalized by a factor depending only on the ratio of the spatial scales of those parts the small value of the estimated dynamic exponent z is interpreted as an indication that the evolution of small channels in a braided river system is to a large extent forced by the evolution of bigger channels the presence of dynamic scaling is further interpreted as indicating that braided rivers may be in a critical state and behave as self organized critical systems in a recent paper sapozhnikov and foufoula georgiou 1996a evidence was presented that natural braided rivers ex hibit anisotropic scaling self affinity in their geometrical structure within a range of scales spanning the width of the narrowest channel to the width of the braid plain in simple words within these scales if a small part of a braided river is stretched in a certain way along the mainstream direction and a certain different way along the perpendicular direction then this stretched part looks statistically the same as a bigger part of the river such anisotropically scaled objects are called self affine fractals and are characterized by two fractal exponents nx and ny the ratio nx ny characterizes the scaling anisotropy and the fractal dimension d 5 ny 2 nx 1 1 ny e g see sapozhnikov and foufoula georgiou 1995 indicates how densely the object fills the space in sapozhnikov and foufoula georgiou 1996a three natu ral braided rivers of different scales and different hydrological and sedimentological characteristics aichilik and hulahula in alaska and brahmaputra in bangladesh were analyzed for spatial scaling using the logarithmic correlation integral lci method developed by sapozhnikov and foufoula georgiou 1995 interestingly enough it was observed that despite their different scales 0 5 15 km in braid plain width slopes 7 3 10 23 8 310 25 and types of bed material gravel to sand all three rivers exhibited anisotropic spatial scaling with almost the same fractal exponents nx 5 0 72 0 74 and ny 5 0 51 0 52 the x axis being oriented along the river and the y axis being in the perpendicular direction in simple terms this im plies that if parts of a braided river are stretched by l along the mainstream direction and by l n y n x l 0 7 along the perpen dicular direction the resulting images will look statistically similar to each other similarity within a braided river at the same time the invariance of nx and ny between braided rivers of different sizes and hydrology sedimentology suggests that the same anisotropic scaling as above applied to different riv ers will result in statistically similar images apart possibly from a normalization factor to account for the different mass of each river note that mass here refers to the area i e number of pixels covered with water the presence of such a statistical scale invariance in the spatial structure of braided rivers apart from being interesting in its own right might indicate the presence of universal features in the underlying mechanisms responsible for the formation of braided rivers and deserves further theoretical and experimental investiga tion braided rivers besides their complex geometry at any in stant of time are also highly dynamic systems characterized by intensive erosion sediment transport and deposition and fre quent channel shifting as they evolve predicting the evolution of braided rivers in terms of frequency and magnitude of chan nel shifting is of paramount importance where hydraulic struc tures or land developments are planned or where field use must be made of maps and air photos it is of great practical and theoretical value therefore to study the evolution of braided rivers in addition to their spatial structure in view of the evidence for spatial static scaling in braided rivers the question is asked here as to whether they also show dynamic scaling the presence of dynamic scaling would imply that space and time can be appropriately rescaled such that the evolution of the spatial structure of parts of the river of dif ferent size would be statistically indistinguishable the pres ence of dynamic scaling would also provide a highly desirable integrated framework for studying simultaneously the spatial and temporal structure of braided rivers

constraints on melting and magma production in the crust major intrusions of granitic rocks are found in several tectonic settings and in all cases crustal melts may contribute to the volumes of granitic magma high grade metamorphism and partial melting of the crust take place predominantly under fluid absent conditions we present a model for calculating the amounts of melt that may be formed by fluid absent breakdown of micas and amphiboles in common crustal rock types pelitic quartzofeldspathic intermediate and mafic melt proportions depend mainly on the kind of source rock the pressure at which melting takes place the temperature and the hydrous mineral h2o content of the source as a consequence of the pressure dependence of water solubility in silicate melts any given source rock will produce more melt by a given fluid absent reaction at lower pressure at a given pressure higher temperature reactions can produce more melt from a given source rock based on a survey of the compositions of common rock types we show that the amounts of melt can vary from 50vol thus crustal rocks vary widely in their fertility as magma sources depending on the types and amounts of hydrous phases they contain in general muscovite breakdown in pelites will yield only small quantities of melt and lead to migmatite formation biotite breakdown in pelites occurs at higher temperature and because most high grade pelites below granulite grade are biotite rich can yield up to about 50 vol of granitoid melt rocks of intermediate composition and hornblende rich mafic rocks are potentially highly fertile magma sources also provided that the high temperatures necessary for biotite and hornblende breakdown are realized pyroxene rich mafic rocks and quartzofeldspathic rocks are much less potentially fertile data suggest that mechanisms exist for the efficient segregation of melt and restite in systems with 30and probably 20vol melt the pressure temperature history of a region can greatly influence crustal source fertility through its control over the occurrence of subsolidus dehydration and melting equilibria

autogenic response of alluvial bedrock transition to base level variation experiment and theory 1 short term autogenic processes are thought to be averaged out by long term allogenic forcing and have often been ruled out in stratigraphic interpretation it is generally difficult to resolve the scale of autogenic effects on landscape evolution and even more so on sedimentary records here we analyze short term variation in migration rate of the alluvial bedrock transition in a series of experiments the experiments developed a fan delta over a stationary sloped nonerodible basement with constant water and sediment discharges the rate of base level change varies between experiments but was constant for a given experiment the location of the alluvial bedrock transition varied autogenically in all experiments the instantaneous migration rate of the alluvial bedrock transition varied by as much as an order of magnitude more than that induced by allogenic forcing the strong autogenic pulses can be explained by sediment storage and release due to intrinsic variation in the transport efficiency of the fluvial system data from experiments with base level change indicate that the autogenic signal in the migrating alluvial bedrock transition is weakest during landward migration onlap when base level rises and strongest during basinward migration valley incision when base level falls we use a one dimensional geometric model to explain the variations in the autogenic response of the alluvial bedrock transition as a function of direction of base level change the model periodically changes fluvial slope to represent the effects of sediment storage and release with base level rise and fall the modeling result reproduces the observed pattern of autogenic variation in alluvial bedrock transition

petrogenesis of synorogenic high temperature leucogranites damara orogen namibia constraints from u pb monazite ages and nd sr and pb isotopes highlights geochemical data from high t leucogranites imply pure crustal melting new u pb monazite ages constrain intrusion time close to peak metamorphism updated sr nd pb isotope data imply metasedimentary sources two suites of leucogranites were emplaced at 508 5 9 ma in the okombahe district of the damara belt namibia synchronous with the peak of regional high temperature metamorphism the sr 87sr 86srinit 0 707 to 0 711 nd endinit 4 5 to 6 6 and pb isotopic 206pb 204pb 18 51 19 13 207pb 204pb 15 63 15 69 208pb 204pb 38 08 38 66 compositions indicate that these peraluminous s type granites were derived from mid to lower crustal rocks which are slightly different to the metapelitic rocks into which they intruded since the leucogranites are unfractionated and show no evidence for assimilation or contamination they constrain the temperature and pressure conditions of their formation calculated zr and lree saturation temperatures of ca 850 c indicate high temperature crustal melts high rb sr and low sr ba ratios are consistent with biotite dehydration melting of pelitic source rocks qz ab or systematics reveal that melting and segregation for the least fractionated samples occurred at ca 7 kbar corresponding to a mid crustal level of ca 26 km however there is no evidence for a mantle component that could have served as a local heat source for crustal melting therefore the hot felsic magmas that formed close to the time of peak metamorphism are the result of long lasting high temperature regional metamorphic conditions and intra crustal collision

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

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

compilation of alluvial channel data a computer based collection of alluvial channel data from both laboratory and field investigations has been prepared the collection contains 5 263 records of laboratory observations and 1 764 records of field observations each record contains 10 alluvial channel variables presented in a standard format in si international system units the paper presents a discussion of the contents of the data collection a review of some of the data sources and comparisons with earlier data collections information for users of the data collection is also presented

development of longitudinal profiles of alluvial channels in response to base level lowering degradation of alluvial channels in cohesive sediments was studied in 15 m and 20 m long flumes with a slope of 0 01 cm cm degradation was initiated by lowering base level to a fixed position and the development of the longitudinal profile of the channel is analysed through a model formulated as a heat diffusion equation it is based on the equation of sediment continuity combined with an assumption regarding sediment transport namely that sediment discharge is linearly proportional to the channel slope in accordance with the boundary and initial conditions imposed by the experimental setup and procedure the basic equation is amenable to an analytical solution which defines bed elevation at any distance and time as a function of the amount of base level lowering and a diffusion coefficient additional problems arising from bank erosion and channel armouring are also treated successfully within the framework of the same model the results show that in homogeneous alluvial sediments not subject to armouring the ultimate result of base level lowering by a certain amount is degradation all along the channel by the same amount the main impact of erosion is felt in the early stages after initiation of the process and mainly near the mouth the rate of degradation at any station along the channel reaches a peak and then slowly decreases with time and the peak rate is attenuated with distance from the outlet the model permits the prediction of intermediate stages of profile development at any distance from the outlet and at different times

large scale fluvial grade as a nonequilibrium state in linked depositional systems theory and experiment 1 a graded river conveys its sediment load without net aggradation or degradation grade is thought to represent the equilibrium state of a river system subject to steady allogenic forcing we address the concept of grade in linked depositional systems through a combination of mathematical modeling and flume experiments we develop a moving boundary morphodynamic model of fluviodeltaic sedimentation and use it to quantify the conditions necessary for a state of sustained grade in an alluvial deltaic system developing on a uniformly sloping shelf to test our theory we conducted a series of flume experiments in which we constructed laboratory scale fluviodeltaic systems under specific relative sea level histories on large spatiotemporal scales our theoretical and experimental results suggest an alternative view of grade as an intrinsically nonequilibrium state that requires a fall in relative sea level under conditions of stillstand or rise in relative sea level the equilibrium graded profile does not exist and the alluvial river will always aggrade with a steady sediment supply alluvial grade can be sustained if relative sea level falls with a specific square root of time dependence common sequence stratigraphic conceptualizations of alluvial river response to sea level change suggest that rivers aggrade and degrade in response to relative sea level rise and fall respectively we show that rivers can remain aggradational if the rate of sea level fall is sufficiently small the boundary between aggradational and degradational states is the graded state described here that associated with a relative sea level fall that varies with the square root of time

high precision molybdenum isotope analysis of low mo igneous rock samples by mc icp ms abstract recent advances in analytical techniques have achieved high precision 2 0 06 determinations of mo isotopic ratios in silicate rocks but few such data are as yet available especially for low mo

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

selective sorting and abrasion of river gravel ii applications most gravel rivers show a tendency for characteristic grain size to decrease in the downstream direction over scales of tens or hundreds of kilometers it has been surmised that this downstream fin

constraints on the origin of damaran granites by rb sr and δ 18 o data the mean 87sr 86sr and mean 87rb 86sr ratios of the intrusive granites from the north and south of the orogen s central zone plot on straight lines these are interpreted as areal isochrons indicating the time of last sr isotope homogenization 526 and 571 ma ago in the respective source rocks

experiments on upstream migrating erosional narrowing and widening of an incisional channel caused by dam removal 1 the present paper reports on a laboratory investigation of the erosion of a deltaic front induced by the removal of a dam we built a laboratory model of a dam and observed both the sedimentation in the reservoir due to the downstream propagation of a delta front and the erosion of the delta front during dam removal including measurement of channel morphology and flow field the experiments provide a detailed view of a phenomenon that has not been described in detail to date erosional narrowing after the sudden removal of a dam the flow incises into the reservoir deposit first rapidly and then more slowly during the initial period of rapid incision the width of the channel can in some circumstances undergo rapid and substantial narrowing all the while incorporating sediment from its sidewalls as the rate of incision slows the channel first stops narrowing and then enters a phase of slow widening this pattern of narrowing followed by widening tends to propagate upstream the minimum channel width attained at a cross section however increases with upstream distance from the dam while the period of erosional narrowing is very short the incision is so intense that large amounts of sediment are delivered downstream in a short period of time the process thus has practical implications in regard to the strategy of dam removal undistorted froude similitude is used to scale the results up to field dimensions

alluvial fans formed by channelized fluvial and sheet flow ii application the formulation of the companion paper describing the evolution of mean elevation in a radially symmetric alluvial fan is applied to the specific case of the tailings basin of an iron mine the tailings basin provides a simple field scale example of a fluvial fan delta system calculation procedures are developed to determine the effect of changing water and sediment discharge on fan evolution the analysis is used to evaluate options for the extension of the lifetime of the tailings basin

input and output mass flux correlations in an experimental braided stream implications on the dynamics of bed load transport through the development of a model experiment it is shown that there exists a correlation between input and output sediment fluxes in a micro scale braided stream that remains valid regardless of the stability of the braided river this correlation has some important consequences on the mechanics of bed load transport by braided rivers it enables the definition of both a dimensionless stream power and a dimensionless transport efficiency these dimensionless variables in turn permit the definition of a braided river stability criterion with regard to bed load transport the existence of such a correlation also suggests that the average critical shear stress or slope of motion may depend on the flux of mass input to the system using these findings together with a one dimensional exner equation for the conservation of mass a kinematic wave equation for the average evolution of the riverbed is eventually derived and its significance analyzed

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

intracrustal recycling and upper crustal evolution a case study from the pan african damara mobile belt central namibia abstract clastic sediments from the pan african damara mobile belt show a wide range in depositional nd nd 0 18 to 1 and several key trace element ratios notably th nd and la sm increase from 0 2 to 0 7 and 4 0 to 8 0 respectively with decreasing nd 0 such trace element ratio nd 0 data arrays are attributed to binary mixing within the sedimentary mass and the data array defined by the damara sediments is consistent with data from other regions the low nd 0 end member is characterised by high th nd th y la sm and hf zr ratios and reflects highly differentiated upper crust dominated by granitoids pb isotope data suggest that the low nd 0 end member also had relatively high time integrated th u 4 by contrast the high nd 0 end member reflects juvenile upper crust but this end member has la sm th nd and hf zr ratios 4 0 0 2 and 0 02 respectively slightly lower than those of taylor and mclennan s preferred model for bulk crust and significantly lower than those of the andesite model intracrustal differentiation is discussed in terms of bulk crust present day average upper crust post archaean shale and a highly differentiated low nd 0 end member trace element ratios which are not significantly fractionated during erosion and sedimentation e g th nd th y and la sm show relatively smooth increases between these reservoirs whereas other trace element ratios which are fractionated both by sedimentary and magmatic processes e g rb sr and rb zr increase dramatically in the more differentiated crustal reservoirs

experimental study of avulsion frequency and rate of deposition in existing models of alluvial architecture it is typically assumed that mean avulsion frequency is independent of sedimentation rate however if avulsion is driven by superelevation of a river bed above its surrounding flood plain one might expect avulsion rate to increase with sedimentation rate we have carried out a series of experiments with laboratory scale fluvial fans in which we measured the frequency of apical avulsions as a function of mean sedimentation rate on the fan avulsion frequency increased strongly with increasing sedimentation rate and then stabilized as mass flows began to influence deposition in the regime of increasing avulsion frequency the added volume of sediment needed to trigger an avulsion decreased with increasing sedimentation rate although our experimental results cannot simply be scaled up to natural rivers they suggest the possibility of coupling between avulsion frequency and sedimentation rate that would be strong enough to change qualitatively the results of existing models of alluvial architecture these models should be applied with caution until avulsion mechanics are better understood

alluvial fans formed by channelized fluvial and sheet flow i theory alluvial fans and fan deltas are of three basic types those built up primarily by the action of constantly avulsing river and stream channels those constructed by sheet flows and those resulting from the successive deposition of debris flows the present analysis is directed toward the first two types a mechanistic formulation of flow and sediment transport through river channels is combined with a simple quantification of the overall effect of frequent avulsion to derive relations describing the temporal and spatial evolution of mean i e averaged over many avulsions bed slope and elevation in an axially symmetric fan an example of a fan formed predominantly by the deposition of sand is compared to a similar one formed predominantly by the deposition of gravel in each example the case of channelized flow is compared to the case of sheet flow the model is applied to the tailings basin of a mine in the companion paper

genesis of the two main types of peraluminous granitoids two mica monzogranites to leucogranites and biotite rich cordierite bearing tonalites to monzogranites form two distinct groups of peraluminous granitoids they can be distinguished by mineral and rock associations and by the variation of their peraluminosity during differentiation except for the rare muscovite bearing granitoids produced by extreme fractionation or local contamination of metaluminous magmas the majority of peraluminous granitoids are produced by partial melting of crustal rocks production of either muscovite bearing granites or biotite rich cordierite bearing granitoids does not depend only on the nature of the sources but is also controlled by the physical parameters of partial melting and consequently by the way anatexis of a thickened crust is enhanced biotite rich cordierite bearing granitoids are generated where mantle derived magmas are injected into or have underplated crustal rocks two mica granites are generated where thickened crust is affected by major crustal shears or thrusts correlations between differentiation and peraluminosity indicate the dominant role of either restite unmixing or fractional crystallization in the production and evolution of the various types of peraluminous granitoids

crust mantle interaction during syn collisional magmatism evidence from the oamikaub diorite and neikhoes metagabbro damara orogen namibia abstract the early syntectonic 563 7 6 1 ma old oamikaub diorite damara orogen namibia consists of metaluminous magnesian calc alkalic to calcic diorites granodiorites and granites associated gabbro diorites and gabbros belong to the neikhoes metagabbro linear major and trace element variations imply that the rock suite evolved through fractional crystallization processes involving amphibole biotite fe ti oxides zircon and apatite initial sr 87sr 86sr 0 7058 0 7123 and nd e nd 2 1 to 18 8 isotopic compositions are highly variable and negatively correlated indicating that assimilation of crustal components occurred unradiogenic initial 206pb 204pb 16 23 17 23 and 207pb 204pb ratios 15 50 15 57 suggest derivation from or interaction with ancient crust with low u pb two gabbro diorites have mgo ni and cr abundances that are compatible with derivation of these rocks from upper mantle lithologies their initial e nd values 2 1 and 7 4 and 87sr 86sr ratios 0 7058 and 0 7076 imply derivation from an aged metasomatized lithospheric mantle other mafic samples have mgo abundances and compatible element concentrations that exceed the values commonly accepted for primary mafic melts implying some accumulation of clinopyroxene and amphibole the granodiorites form a homogenous group in which the isotope data initial e nd 12 4 to 14 1 initial 87sr 86sr 0 7083 0 7096 imply a lower crustal source the granites are also magnesian and calc alkaline but two of them are strongly peraluminous their isotope data initial e nd 13 2 to 18 8 initial 87sr 86sr 0 7099 to 0 7123 imply derivation from more ancient sources alternatively these samples gained their isotope systematics through extensive afc processes from parental granodiorites a common subduction zone environment as suggested from negative nb ta anomalies in multi element diagrams seems unlikely for all samples because of a lack of isotopically depleted signatures the data from the oamikaub diorite and other mafic complexes are better explained by a flat subduction model involving mainly continental mantle lithosphere and crust with limited if any melting of asthenospheric mantle

geochemical constraints on crustal anatexis a case study from the pan african damara granitoids of namibia major and trace element models of recently published vapour absent mica dehydration melting experiments are used to identify granitoids generated by muscovite and biotite dehydration melting and to distinguish between plagioclase limited and biotite limited biotite dehydration melting in the case of granitoids from the pan african damara mobile belt namibia many of the leucogranites and salem type granitoids may be modelled by biotite dehydration melting the low rb sr granitoids e g donkerhuk onanis salem onanis donkerhuk nomatsaus salem goas probably reflect feldspar limited biotite dehydration melting a pelitic source whereas the high rb sr suites e g bloedkoppie leucogranite stinkbank leucogranite salem swakopmund leucocratic stink bank granite reflect biotite limited biotite dehydration melting a greywacke source alaskites from the damara belt have major element compositions which are consistent with muscovite dehydration melting and their positive eu anomalies are linked to high k2o reflecting k feldspar entrainment combined zr and lree light rare earth element solubility models indicate that insufficient time probably less than 104 years had elapsed between melt generation and melt extraction to ensure that the alaskite melts attained their equilibrium concentrations of zr and the lrees in contrast the leucogranites and salem type granites have attained their equilibrium inventories of these trace elements combined fe2o3 and mgo contents in some samples from two granitoids the salem goas and donkerhuk onanis intrusions are higher than those readily attainable by biotite dehydration melting indicating either 1 that they contain a contribution from melts generated by incipient garnet breakdown or 2 that they contain small amounts of an entrained ferromagnesian phase

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

the himalayan leucogranites constraints on the nature of their crustal source region and geodynamic setting article i nfo late oligocene miocene leucogranites within southern tibet form part of an extensive intrusive igneous province within the himalayan orogen the main rock types are tourmaline leucogranites tg and two mica leucogranites 2 mg they have high sio2 70 56 75 32 wt al2o3 13 55 15 67 wt and 87 sr 86 sr i 0 724001 0 797297 and low mgo 0 02 0 46 wt and 143 nd 144 nd i 0 511693 0 511906 chondri te normalized rare earth element ree patterns display strong negative eu anomalies whole rock major and trace element and sr nd isotope data for the leucogranites suggest that their source region was a two component mixture between a fluid derived from the lesser himalayan lh crustal sequence and the bulk crust of the higher himalayan hh sequence trace element and sr nd isotope modeling indicate that the proportion of fluid derived from the lh sequence varied from 2 to 19 and the resulting metasomatised source experienced 7 16 melting the amount of fluid derived from the lh sequence increases from north to south northward underthrusting of the indian continent resulted in infiltration of the lh derived fluid into the overlying hh sequence subsequent decompression melting of this metasomatised crust mostly during the miocene 25 9 ma generated the leucogranites this may be linked to steepening of the subducted slab of indian lithosphere beneath the orogenic belt

multiple sources for the coastal batholith of central chile 31 34 s geochemical and sr nd isotopic evidence and tectonic implications abstract the coastal batholith of central chile between 31 and 34 s represents a locus of long lived c 200 ma plutonic activity it is composed of four plutonic complexes santo domingo limari papudo quintero and illapel these complexes which occur as north trending discontinuous belts that decrease in age eastward were emplaced during four major episodes of arc magmatism the santo domingo complex carboniferous comprises hornblende biotite bearing tonalites and granodiorites with abundant mafic enclaves and enclave free granites the association of enclave bearing granitoids and enclave free granites resulted from different degrees of mixing of crustal and mantle derived magmas the limari complex lower jurassic is a bimodal association of crustal leucogranites and mantle derived gabbros the lithology of the papudo quintero complex middle jurassic varies from hornblende pyroxene diorite to biotite granite the main rock types namely tonalites and granodiorites commonly contain mafic enclaves the granitoids and the enclaves have remarkably similar ree patterns and are similar in sr nd isotope composition which suggest that they represent a cogenetic suite of mantle origin the illapel complex cretaceous consists of hornblende bearing tonalites trondhjemites and granodiorites that resemble the rocks of ttg suites the end values and initial 87 sr 86 sr ratios sri for the mafic rocks of the carboniferous complex end 2 0 and 3 5 sri c 0 7057 differ markedly from those of the mesozoic mafic rocks end 1 9 to 5 4 sri 0 7033 to 0 7039 the cretaceous rocks being the most depleted with regard to sr nd a change from carboniferous lithosphere dominated sources to mesozoic asthenosphere dominated sources of the magmas that started in the early jurassic is recognized slab components can be recognized in the source of the cretaceous rocks the crustal source of the lower jurassic leucogranites was more isotopically depleted and refractory than that of the carboniferous granites on the other hand no crustal participation in the origin of the middle jurassic and cretaceous felsic granitoids is recognized they are differentiated from mantle derived magmas progressive removal of old lithospheric mantle during the mesozoic due to a process of lithospheric delamination would explain the profound change in the subcrustal source as well as the melting and modification towards a more refractory nature of the continental crust

an experimental study of the effects of base level change on fluvial coastal plain and shelf systems abstract several of the concepts of sequence stratigraphic models were tested by experiments in a large flume using different gradients for successive runs a physical model of a fluvial drainage basin coastal plain continental shelf and continental slope was constructed in all experiments initial gradients of the coastal plain and shelf were identical systematic raising and lowering of the water surface simulated the changes in relative sea level i e base level that are similar to most sequence stratigraphic models base level changes significantly affected the shelf area but they had little effect on the fluvial drainage basin during base level falls fluvio deltaic progradation occurred fluvial aggradation occurred only during periods of base level stillstand or rise because of the slow rate of head ward erosion of incised valleys a significant time gap existed from the time base level first fell below the shelfbreak and the development of a cross shelf bypass valley during this lag time deposition occurred on the exposed shelf to form a fluvial braid plain concurrently channels developed at the shelfbreak and grew by headward erosion at various rates continuous incision and widening of these channels formed valleys these valleys were not linked directly to the main drainage basin unt l ultimately one captured the flow from that drainage at that point other valleys were abandoned and ceased to grow during the subsequent base level rise the single large cross shelf valley filled with coarser sediments while the other valleys partially filled with finer suspended load sediments the main cross shelf valley also supplied coarser sediments to the slope