highly fractionated i type granites in ne china ii isotopic geochemistry and implications for crustal growth in the phanerozoic ne china is the easternmost part of the central asian orogenic belt caob the area is distinguished by widespread occurrence of phanerozoic granitic rocks in the companion paper part i we established the jurassic ages 184 137 ma for three granitic plutons xinhuatun lamashan and yiershi we also used geochemical data to argue that these rocks are highly fractionated i type granites in this paper we present sr nd o isotope data of the three plutons and 32 additional samples to delineate the nature of their source to determine the proportion of mantle to crustal components in the generation of the voluminous granitoids and to discuss crustal growth in the phanerozoic despite their difference in emplacement age sr nd isotopic analyses reveal that these jurassic granites have common isotopic characteristics they all have low initial 87 sr 86 sr ratios 0 7045f0 0015 positive end t values 1 3 to 2 8 and young sm nd model ages 720 840 ma these characteristics are indicative of juvenile nature for these granites other late paleozoic to mesozoic granites in this region also show the same features sr nd and oxygen isotopic data suggest that the magmatic evolution of the granites can be explained in terms of two stage processes 1 formation of parental magmas by melting of a relatively juvenile crust which is probably a mixed lithology formed by pre existing lower crust intruded or underplated by mantle derived basaltic magma and 2 extensive magmatic differentiation of the parental magmas in a slow cooling environment the widespread distribution of juvenile granitoids in ne china indicates a massive transfer of mantle material to the crust in a post orogenic tectonic setting several recent studies have documented that juvenile granitoids of paleozoic to mesozoic ages are ubiquitous in the central asian orogenic belt hence suggesting a significant growth of the continental crust in the phanerozoic d 2003 elsevier science b v all rights reserved

age and cooling history of the manaslu granite implications for himalayan tectonics abstract the manaslu granite is one of the high himalayan leucogranites which are believed to be derived by partial melting of quartzofeldspathic gneissess of the high himalayan crystallines hhc and emplaced at the contact between the hhc and the tethyan sedimentary series we have analyzed a suite of muscovite 15 biotite 3 and alkali feldspar 10 by the 40 ar 39 ar method to help constrain the age and cooling history of the granite 40 ar 39 ar ages ranges are 18 4 to 13 3 ma 17 0 to 14 7 and 16 4 to 3 4 for muscovite biotite and k feldspar respectively based on the muscovite analyses and a re interpretation of a previously published u pb monazite age we conclude that the crystallization age of the manaslu granite is everywhere 20 ma the postcrystallization cooling history given by the micas and the feldspars suggests the presently exposed parts of the pluton were emplaced at depths of 8 to 15 km these data place a minimum age of movement on the mct in this area of 20 ma we estimate the time of magma segregation and transport to be no more than 5 ma

formation of low δ 18 o magmas of the kangerlussuaq intrusion by addition of water derived from dehydration of foundered basaltic roof rocks the kangerlussuaq intrusion in east greenland is concentrically zoned from quartz nordmarkite quartz syenite at the margin through pulaskite to foyaite nepheline syenite in the centre with no apparent intrusive contacts the 18o values of coexisting minerals are consistent with oxygen isotope equilibrium at magmatic temperatures most of the intrusion formed from low 18o magma magma 18o values generally increased upwards from about 3 3 in the quartz nordmarkites to 5 6 in the foyaites the lowest magma 18o value of about 1 0 is from the upper part of the nordmarkites where there is a high concentration of foundered basaltic xenoliths stoped from the roof of the intrusion the amphiboles in the syenites have d values that range from those typical of hydrous mantle derived minerals to much lower values 86 to 157 as do whole rock samples of xenolith and country rock 125 to 148 the low magma 18o and d values are consistent with continuous incorporation exchange and upward escape of low 18o and d fluids released from stoped basaltic roof material mass balance suggests that the integrated amount of water involved was 7 wt of the volume of the magma but locally reached 30 wt water the requirement for large amounts of water with low 18o value is satisfied only if the foundered basalt contained most of its water in cavities as opposed to hydrous minerals even with this requirement the volume of stoped basalt would have been equal to the volume of the magma repeated recharge of the residual magma with progressively less contaminated silica undersaturated melt resulted in a gradual shift across the low pressure thermal divide crystallisation was suppressed by the depression of the liquidus due to water saturation of the residual magma ph2o 1 kbar

andesitic water a phantom of the isotopic evolution of water silicate systems comment on isotopic shifts in waters from geothermal and volcanic systems along convergent plate boundaries and their origin by w f giggenbach on account of the low porosity of the lithosphere intracrustal fluids behave very differently from surface fluids in that they are changing their geochemical and isotopic labels according to the geological environment given a heat source meteoric waters can be supplied plentifully and their rates of throughput in geothermal systems are sufficiently high to exhaust the compositional signals of a given rock buffer in contrast fluids exsolved from magma and subducted fluids would be supplied at less than about one tenth of the meteoric rate over the life time of a system based on up to date flow models the isotopic evolution of meteoric water interacting with crustal rock follows a curved to l shaped track in the d versus 18o plot instantaneous present day tie lines between recharge and discharge are secants of such tracks and can have a range of slopes at the start of an interaction waters have d and 18o values approaching equilibrium with the original rock composition water w1 using known hydrogen isotope fractionation factors w1 values generally plot in the region of andesite or andesitic waters of various authors since the w1 waters have d values that are on average more positive and also less variable than those of the meteoric recharges most tracks and tie lines have positive slopes and the plotting of a large number of tie lines will produce a focus on the field of w1 waters regardless of the original water source

micrometer scale chemical and isotopic criteria o and si on the origin and history of precambrian cherts implications for paleo temperature reconstructions abstract oxygen and silicon isotopes in cherts have been extensively used for the reconstruction of seawater temperature during the precambrian these reconstructions have been challenged because cherts can have various origins hydrothermal sedimentary volcanic silicification and their isotopic compositions might have been reset by metamorphic fluid circulation existing criteria used to assess the pristine sedimentary origin of a chert are based on petrography criterion 1 chert is composed mostly of microquartz on the bulk oxygen isotopic composition criterion 2 bulk 18 o has to be close enough to the maximum 18 o value previously measured in other cherts of the same age and on the presence of a large 18 o range at the micrometer scale criterion 3 18 o range of 10 at 2 m however these criteria remain incomplete in determining precisely the origin and degree of preservation of ancient cherts we report in situ si and o isotope compositions and trace element concentrations in seven chert samples ranging from 1 88 to 3 5 ga in age correlations between 30 si and al 2 o 3 and k 2 o tio 2 reveal that microquartz is of three different origins i e diagenetic hydrothermal or silicification moreover chert samples composed mostly of diagenetic microquartz show a large range of 30 si at the micrometer scale 1 7 4 5 consistent with the large range of 18 o previously found in the gunflint diagenetic cherts we propose two further quantitative criteria to assess the origin state of preservation and diagenetic history of cherts criterion 4 uses trace element concentrations coupled with 30 si to ascribe the origin of cherts among three possible end members diagenetic hydrothermal and silicified criterion 5 is the presence of a large range of 30 si in pure diagenetic microquartz in the seven samples analyzed in this study only one from the gunflint iron formation at 1 88 ga passes all the criteria assessed here and can be used for paleo temperature reconstructions the criteria outlined here will help guide to the identification of pristine diagenetic cherts and or to better constrain seawater paleo temperature reconstructions by taking into account the effect of diagenesis

hydrogen and oxygen isotope geochemistry of a type granites in the continental margins of eastern china hydrogen and oxygen isotope studies of both whole rock and mineral separates were carried out for five representative atype granites in eastern china from north to south the whole rock d d and d 18 o values vary from 2145 to 282 and from 21 2 to 5 0 for nianzishan pluton from 2135 to 2110 and from 1 6 to 6 7 for shanhaiguan pluton and from 2145 to 2111 and from 1 5 to 9 1 for laoshan massif respectively the highest whole rock dd and d 18 o values are observed in suzhou pluton and range from 281 to 259 and from 3 5 to 9 2 respectively homogeneous d d and d 18 o values are found in kuiqi massif ranging from 2113 to 299 and from 5 6 to 8 5 respectively the results show that the large variabilities in the hydrogen and oxygen isotope ratios are mainly related to the diverse processes of magma evolution such as magma degassing fractional crystallization as well as post magmatic alteration by local meteoric water under subsolidus conditions the initial d d and d 18 o values of the a type granites in eastern china are constrained to be about 250 5 and 7 5 1 0 respectively this is consistent with mantle like protoliths for granitoid rocks and thus precludes the possibility that the a type granites is derived from the anatexis of the 18 o rich upper crust however recycling of crustal materials by earlier subduction cannot be excluded in the processes of generating the a type granitic magmas the variation in the ddepletion of the granites with latitude implies that the paleogeographic localities of the a type granites have not changed significantly since the mesozoic q 2000 elsevier science b v all rights reserved

origin of the fumarolic fluids of vulcano island italy and implications for volcanic surveillance variations in d and 18o values with h2o contents and outlet temperatures indicate that the fumaroles of la fossa crater have discharged mixtures of magmatic water and marine hydrothermal water since 1979 the contribution of meteoric water was low in the period 1979 1982 and very low afterwards the 18o values of the marine hydrothermal component of 5 to 7 2 are due to isotopic exchange with the 18o rich silicates of the rocks under high temperature and low permeability conditions the 18o value of the magmatic end member is generally 3 5 to 4 3 although values as high as 5 5 to 6 5 were reached in the summer of 1988 when magma degassing appears to have extended into the core of the magma body the d values of the end member were close to 20 typical of andesitic waters both the isotopic values and chemical data strongly support a dry model consisting of a central magmatic gas column and a surrounding hydrothermal envelope in which marine hydrothermal brines move along limited fracture zones to undergo total evaporation on approaching the conduits of magmatic fluids the vents at the eastern and western boundaries of the fumarolic field are fed by fluids whose pressure is governed by the coexistence of vapor liquid and halite giving rise to a high risk of phreato magmatic explosions should magma penetrate into these wet environments most la fossa eruptions were triggered by an initial hydrothermal blast and continued with a series of phreatomagmatic explosions the fluids discharged by the forgia vecchia fumaroles are mixed with meteoric water which is largely evaporated although subordinate loss of condensed steam may be responsible for scrubbing most of the acidic gas species the temperatures and pressures and the risk of a sudden pressure increase are low a boiling hydrothermal aquifer at 230 c is present underneath the baia di levante beach this area has a minor risk of hydrothermal explosions

comment on geochemistry of magmatic gases from kudryavy volcano iturup kuril islands by yu a taran j w hedenquist m a korzhinsky s i tkachenko and k i shmulovich

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

brittle tectonothermal evolution in the forsmark area central fennoscandian shield recorded by paragenesis orientation and 40ar 39ar geochronology of fracture minerals in this paper we show how studies of fracture mineral paragenesis their orientation and ar 40 ar 39 geochronology can be applied in order to recognise the brittle tectonothermal evolution in an area samples were selected from nearly 18 km of drill cores from the upper i km of the fennoscandian shield obtained during the site investigation for a repository of spent nuclear fuel in forsmark central sweden four major events of fracturing and or reactivation of fractures associated with fracture mineralisation have been distinguished the first event was characterised by precipitation of epidote quartz and chlorite along preferably sub horizontal and gently dipping fractures or steep wnw ese to nw se fractures precipitation occurred between 1 8 and 1 1 ga possibly during the late stage of the svecokarelian orogeny close to 1 8 1 7 ga the second event is associated with precipitation of hematite stained adularia and albite prehnite laumontite calcite and chlorite preferably along steep ene wsw to nne ssw and nnw sse fractures precipitation occurred around 1107 7 to 1034 3 ma probably due to effects from the sveconorwegian orogeny this event was followed by a period with dissolution of fracture minerals and subsequent precipitation of mainly calcite quartz pyrite and asphaltite during the palaeozoic the formation fluid emanated from an overlying organic rich sedimentary cover precipitation occurred during reactivation of proterozoic fractures but formation of new fractures is also inferred possibly due to far field effects of the caledonian orogeny or elevated hydrostatic pressure due to its foreland basin the latest event is dominated by clay minerals chlorite and calcite along hydraulically conductive fractures these minerals are prominent along sub horizontal and gently dipping fractures but also occur in sets of steeply dipping fractures it is inferred that the hydraulically conductive fractures are proterozoic structures in which mineral precipitation has occurred during a long period however some of the near surface sub horizontal fractures may be quaternary in age c 2009 elsevier b v all rights reserved less

sources of hercynian granitoids from the french massif central inferences from nd isotopes and consequences for crustal evolution the french massif central is characterized by a great abundance and variety of granitoids emplaced during hercynian times 360 280 ma distinct nd i and 87sr86sr i are not associated with particular petrographic types however lower nd i values 8 5 occur among the younger leucogranites from the western part of the massif intermediate nd i 6 correlate with aluminous monzogranites and higher values 6 to 3 with calc alkaline granodiorites in the eastern massif central nd i are homogeneous at the regional scale in the western massif central the leucogranites and the monzogranite granodiorites display different trends of nd i with emplacement ages from these results extreme models involving a depleted mantle or an old archean or early proterozoic source may be ruled out the trend of increasing nd i with decreasing age observed in the monzogranite granodiorite group suggests either an increasing contribution of a mantle component or the progressive melting of more mafic more refractory crustal sources or both the reverse trend displayed by the leucogranites might be explained in terms of the upward migration of partial melting across an isotopically zoned metasedimentary pile the large amount of granites may be the result of favourable processes during the collision tectonics combined with the unusual abundance of fertile materials enriched in water and heat producing elements in the pre carboniferous crust

oxygen isotope composition of the denchai sapphire thailand a clue to its enigmatic origin alluvial gem sapphires from the denchai area northern thailand have o isotope compositions ranging from 4 7 per mil to 6 1 per mil 15 samples with one outlier 8 4 per mil it is suggested that these sapphires are not in o isotope equilibrium with and must be xenocrysts from the nearby alkali basalts which contain olivine phenocrysts with delta 18 o 4 9 per mil to 5 1 per mil and are the presumed sources of these gem sapphires homogeneous delta 18 o values within individual sapphire crystal indicate limited interactions with the carrier magmas during transport to the surface the o isotope compositions of these sapphires suggest that crustal rocks might have been involved in the genesis of those sapphires with delta 18 o values higher than 5 5 per mil a hybrid origin model for the denchai sapphires is therefore postulated which involves different degrees of interactions between a fractionated partial melt from metasomatized mantle and a lower mid crust al rich rock

character of fluids associated with hydrothermal alteration and metamorphism of palaeoproterozoic submarine volcanic rocks baffin island nunavut canada the bravo lake formation on central baffin island canada formed in a palaeoproterozoic 1 9 ga intrabasinal submarine volcanic rift the rocks of which preserve mineral assemblages formed during submarine hydrothermal alteration amphibolite facies regional metamorphism and retrograde metamorphism both stable isotope and cation exchange geothermometry of co existing mineral assemblages indicate preservation of hydrothermal alteration at 300 c amphibolite facies metamorphism at 590 c and retrograde metamorphism at 300 c fluid isotopic compositions calculated from 18 o and d values measured from minerals suggest that three isotopically distinct fluids interacted with the bravo lake formation during its evolution 1 palaeoproterozoic seawater infiltration during hydrothermal alteration 2 regional metamorphic fluids at low water rock ratios and 3 retrograde fluids indicating increasing water rock ratios hydrothermal fluids have 18 o and d values near 0 similar to modern seawater values the similarity in the isotopic composition of palaeoproterozoic and modern seawater is consistent with the buffering of the isotopic composition of ocean water by hydrothermal circulation at mid ocean ridges by similar processes as operate today the results of this study show that the isotopic composition of prior fluid events can be preserved in amphibolite facies submarine mafic volcanic rocks if there is minimal penetrative structural deformation and dry prograde metamorphism 2005 elsevier b v all rights reserved

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

hydrothermally induced changes in mineralogy and magnetic properties of oxidized a type granites abstract the changes in magnetic mineralogy due to the hydrothermal alteration of a type granitic rocks have been thoroughly investigated in samples from the granite of tana corsica france and compared with other a type granites meruoca ne brazil bushveld south africa mount scott wichita mountains oklahoma usa and the stratoid hypersolvus granites of madagascar the altered red colored samples and their non altered equivalents were magnetically characterized by means of magnetic susceptibility measurements hysteresis loops remanent coercivity spectra and lowrie test it is shown that hydrothermalization in magnetite bearing granites is related to the formation of fine grained magnetite and hematite and to coeval depletion in the content of primary low coercive coarse grained magnetite these mineralogical changes give typical rock magnetic signatures namely lower susceptibility magnitudes and anisotropy degrees prolate ams anisotropy of magnetic susceptibility fabrics and increased coercivities optical microscopy and sem scanning electronic microscopy images suggest that the orientation of the secondary magnetic minerals is related to fluid pathways and micro fractures formed during the hydrothermal event and therefore may be unrelated to magma emplacement and crystallization fabrics changes in magnetic mineralogy and grain size distribution have also to be considered for any paleomagnetic and iron isotope studies in granites

biotites as indicators of fluorine fugacities in late stage magmatic fluids the gardar province of south greenland the major element chemistry and fluorine contents of biotites from the gardar alkaline province of south greenland provide an insight into the f contents of late stage fluids associated with the magmatism biotites were taken from composite intrusions ranging from alkali gabbro to syenites nepheline syenites and alkali granites in each complex they show a large range of fe fe mg from 02 to 1 and exhibit strong fe f avoidance there is considerable variation in ffor any value of fe fe mg but for each centre maximum f values plotted against fe fe mg define a nearly straight line of characteristic slope towards low or zero f at pure annite micas in the sio2 undersaturated centres have higher f contents than those from oversaturated centres cl is low 0 69 wt except in the kungnat intrusion where it reaches 1 4 wt phase equilibrium and textural considerations suggest that most or all biotites grew subsolidus in a pervasive deuteric fluid 5i8o values suggest that these fluids were largely magmatic in character and that extensive reactions with envelope fluids did not occur metasomatic resetting of f in biotites in early intrusive units in the aureole of later units can be demonstrated experimental data of munoz mineralogical society of america reviews in mineralogy 13 469 494 1984 were used to calculate families of curves showing variation in f with fe fe mg for biotites in equilibrium with fluids of fixed fugacity ratio i hf i h2o at fixed p andt the resulting curves cut sharply across the maximum fluorine lines observed in the natural examples as it seems highly unlikely that changes in fluid composition andt acting together could produce the observed linear relationship we conclude that the partitioning of f between fluid and mica in the plutonic environment is not well modelled by the experiments possible explanations are short range order sro of fe and mg on octahedral sites in biotite mason canadian mineralogist 30 343 354 1992 or effects resulting from different f speciation in alkaline fluids if perfect fe f avoidance is assumed fe mg sro can increase maximum f content the f levels seen in the silica saturated centres are broadly consistent with a cooling rate related control it is possible that the higher f in biotites in the undersaturated centres reflects different spedation in the fluid rather than higher f contents with strongly bound sif complexes more common

fluid rock interaction in the miocene post caldera tejeda intrusive complex gran canaria canary islands insights from mineralogy and o and h isotope geochemistry the intra caldera volcaniclastic deposits of the miocene tejeda caldera on gran canaria host an similar to 12 km diameter intrusive complex comprising a peralkaline trachytic to phonolitic cone sheet swarm surrounding a central core of hypabyssal syenite stocks both intrusive rock types display textural and mineralogical features indicative of secondary fluid rock interaction including 1 deuteric mineral phases e g aegirine alkali amphibole analcime 2 turbid alkali feldspars and 3 hydrothermal mineral phases phyllosilicates fe ti oxides mn oxides and quartz altered cone sheets have whole rock delta o 18 values ranging from 0 center dot 1 to 10 center dot 0 parts per thousand n 22 and whole rock delta d values between 62 and 149 parts per thousand n 28 three altered syenite samples have whole rock delta o 18 values of 2 center dot 5 1 center dot 5 and 0 center dot 9 parts per thousand and corresponding delta d values of 91 99 and 121 parts per thousand the h2o concentrations of the altered cone sheets range from 0 center dot 4 to 0 center dot 8 wt n 28 and the altered syenites have h2o concentrations of 0 center dot 5 0 center dot 5 and 0 center dot 6 wt respectively the majority of altered samples are depleted in o 18 relative to the typical delta o 18 range for unaltered trachytes and syenites delta o 18 6 8 parts per thousand indicative of interaction with local meteoric water delta o 18 c 8 parts per thousand at temperatures epsilon 150 degrees c only one cone sheet sample appears petrographically unaltered and has a typical igneous isotopic composition delta o 18 7 center dot 1 parts per thousand delta d 48 parts per thousand and a relatively high h2o concentration 2 center dot 2 wt a weak correlation r 0 center dot 55 between delta d and h2o is observed in the cone sheets reflecting the combined effects of magmatic h2o exsolution and subsequent deuteric and hydrothermal alteration no systematic variation in delta o 18 or delta d was detected across the cone sheet swarm most likely reflecting overprinting of isotopic compositions during successive intrusive events however the highest delta o 18 values 8 center dot 2 10 center dot 0 parts per thousand occur in clay bearing cone sheets from the central part of the intrusive complex suggesting enhanced infiltration of relatively cool meteoric water in this area overall at least three phases of fluid rock interaction can be distinguished 1 deuteric alteration c 300 500 degrees c by late magmatic fluids expelled from a solidifying crystal mush 2 hydrothermal alteration epsilon 150 300 degrees c by meteoric water during the final stages of crystallization and or immediately following solidification of the intrusive complex 3 retrograde alteration related to the influx of relatively cool 150 degrees c meteoric waters

oxygen isotope geochemistry of the mesozoic anorogenic complexes of damaraland northwest namibia evidence for crustal contamination and its effect on silica saturation fifty six new oxygen isotope analyses of minerals separated from nine mesozoic anorogenic complexes of damaraland in northwest namibia have been used to estimate the 18o values of the original magmas magma these complexes range in composition from nepheline syenite to alkaline and peraluminous granites often with a variety of rock types present at a single centre the silica undersaturated rock types show a relatively small spread of values of magma from 6 0 to 7 0 mean 6 6 which is consistent with their derivation from the mantle with little or no subsequent crustal contamination the silica oversaturated rocks show a wide range of magma values from 4 9 to 12 0 with a mean value of 8 3 the high values of magma in the silica oversaturated rocks can only be explained with considerable involvement of the continental crust in their petrogenesis and those rocks with magma 10 are interpreted as essentially crustal melts it is generally accepted that the damaraland complexes were generated as a result of rifting across the tristan plume with the plume providing both mantle derived magma and with it the heat required for crustal melting in addition to their mantle like oxygen isotope ratios the undersaturated rocks of the damaraland complexes possess the trace element characteristics e g low zr nb ratios of ocean island basalt suggesting that their parental magmas were produced from the plume itself in contrast the oversaturated complexes generally have higher zr nb ratios that are consistent with a larger crustal input the highest values of magma in the damaraland complexes are found in granitic rocks that intrude the central zone of the pan african damara orogen where presumably there is a substantial component of sedimentary origin in the lower to middle crust

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