synorogenic melting of mafic lower crust constraints from geochronology petrology and sr nd pb and o isotope geochemistry of quartz diorites damara orogen namibia quartz diorites represent the earliest ca 540 ma and most primitive plutonic rocks in the pan african damara belt and they pre date the main phase of high t regional metamorphism two suites of synorogenic quartz diorites are unusual among damaran intrusive rocks in their elemental and isotopic features comparison of the diorite compositions with melts from amphibolite dehydration melting experiments points to a garnet bearing meta tholeiite probably enriched in k2o as a likely source rock partial melting processes generated mafic ca 50 wt sio2 quartz diorites in the deep crust at temperatures of between 1 000 and 1 100 c based on comparison with experimental results and similar temperature estimates based on p2o5 solubility in mafic rocks subsequently the quartz diorites evolved by multistage polybaric differentiation processes including fractional crystallization of mainly hornblende and plagioclase and assimilation of felsic basement gneisses although their chemical characteristics high lile low hfse resemble those of other quartz diorites with calc alkaline affinities they differ in their enriched sr initial 87sr 86sr 0 70943 0 71285 nd initial e nd 9 1 to 15 2 and o 18o 6 8 8 1 isotope compositions neodymium model ages tdm that range from 1 7 to 2 2 ga and large variation in 207pb 204pb relative to 206pb 204pb indicates involvement of ancient crustal material lead 206pb 204pb 17 08 17 23 207pb 204pb 15 53 15 62 208pb 204pb 37 71 38 16 isotope compositions are strongly retarded indicating that the source underwent a pre pan african u pb fractionation and u depletion it is proposed that the quartz diorites originated by synorogenic high temperature melting of mafic lower crust this contrasts with previous suggestions favouring an origin of these rocks by melting of an enriched mantle during pan african times with characteristics modified by subduction of oceanic crust and sedimentary rocks

dehydration melting of amphibolite at 10 kbar the effects of temperature and time we have simulated the dehydration melting of a natural low k calcic amphibolite 67 4 hornblende 32 5 anorthite in piston cylinder experiments at 10 kbar and 750 1000 c for 1 9 days the solidus temperature is lower than 750 c garnet appears at 850 c the overall reaction is hb p l cpx al hb ca hb ga opx three stages of reaction are 1 melting dominated by the growth of clinopyroxene and garnet with little change in composition of liquid or garnet 2 a reversal of this reaction between 875 c and 900 c with decreases in the amounts of liquid and garnet and 3 a large increase in liquid along with the loss of hornblende and decrease of plagioclase while clinopyroxene and garnet increase garnet is enriched in pyrope and zoned from fe cores to mg edges range 3 mol pyrope liquid composition is enriched first in an to 950 c and then in ab the liquids are more calcic and aluminous than natural tonalites which is attributed to the plagioclase composition an90 the formation of peraluminous liquid from the metaluminous amphibolite is caused by anorthite not h2o saturated conditions the results are consistent with an amphibolite phase diagram with relatively high solidus temperatures in the garnet absent field 900 1000 c but with a solidus backbend at 7 9 kbar coincident with the garnet in boundary hornblende breakdown due to garnet formation in a closed system must make h2o available for h2o undersaturated melting right down to the h2o saturated solidus below 700 c which defines a large low temperature pt area where hydrous granitoid melts can be generated with residual garnet and hornblende

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

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

mafic magmatic enclaves and mafic rocks associated with some granitoids of the central sierra nevada batholith california nature origin and relations with the hosts abstract the calc alkaline granitoids of the central sierra nevada batholith are associated with abundant mafic rocks these include both country rock xenoliths and mafic magmatic enclaves mme that commonly have fine grained and less commonly cumulate textures scarce composite enclaves consist of either xenoliths enclosed in mme or of mme enclosed in other mme with different grain size and texture enclaves are often enclosed in mafic aggregates and form meter size polygenic swarms mostly in the margins of normally zoned plutons enclaves may locally divert schlieren layering mafic dikes which also occur in swarms are undisturbed composite or largely hybridized in central sierra nevada with the exception of xenoliths that completely differ from the other rocks host granitoids mafic aggregates mme and some composite dikes exhibit a bulk compositional diversity and at the same time important mineralogical and geochemical including isotopic similarities mme and host granitoids display distinct major and trace element compositions however strong correlations between mme host granitoid pairs indicate interactions and parallel evolution of mme and enclosing granitoid in each pluton identical mafic mineral compositions and isotopic features are the result of these interactions and parallel evolution mafic dikes have broadly the same major and trace element compositions as the mme although variations are large between the different dikes that are at distinctly different stages of hybridization and digestion by the host granitoids the composition of the granitoids and various mafic rocks reflects three distinct stages of hybridization that occurred respectively at depth during ascent and emplacement and after emplacement the occurrence and succession of hybridization processes were tightly controlled by the physical properties of the magmas the sequential thorough or partial mixing and mingling were commonly followed by differentiation and segregation processes unusual mme that contain abundant large crystals of hornblende resulted from disruption of early cumulates at depth whereas those richer in large crystals of biotite were formed by disruption of late mafic aggregates or schlieren layerings at the level of emplacement mme and host granitoids are considered cogenetic because both are hybrid rocks that were produced by the mixing of the same two components in different proportions the felsic component was produced by partial melting of preexisting crustal materials whereas the dominant mafic component was probably derived from the upper mantle however in the lack of a clear mantle signature the origin of the mafic component remains questionable

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

geochronology geochemistry and nd sr and pb isotopes of syn orogenic granodiorites and granites damara orogen namibia arc related plutonism or melting of mafic crustal sources highlights geochemical data from high t granodiorites and granites imply lower crustal amphibolite melting new u pb zircon ages imply syn orogenic intrusion new sr nd pb isotope data imply ancient crustal sources and constrain afc processes abstract the gawib pluton damara belt namibia consists of two main intrusive rock types magnesian calc alkaline mostly metaluminous hornblende and titanite bearing granodiorites and magnesian to ferroan metaluminous to slightly peraluminous calc alkaline granites uranium pb zircon data obtained on the granodiorites gave concordant ages of 548 5 5 6 ma indicating that the pluton belongs to the early syn orogenic magmatism in the damara orogen major and trace element variations indicate that fractional crystallization was the major rock forming mechanism for the granodiorites in the absence of high precision geochronological data the granites may represent more advanced fractionation products of the granodiorites although distinct ba sr rb relationships preclude a direct derivation of the granites from the exposed granodiorites if the granites originated by extensive fractional crystallization from similar granodiorites they can only be derived from high ba high sr low rb granodiorites crustal contamination was also important in the petrogenesis of both rock types granodiorites e nd init 7 to 13 87sr 86sr init 0 708 0 713 granites e nd init 14 to 18 87sr 86sr init 0 712 0 726 in contrast to the granodiorites the granites show more radiogenic 87sr 86sr ratios and less radiogenic e nd values indicating different contaminants for both rock types e nd vs mgo relationships imply some genetic link to isotopically unevolved quartz diorites similar to those observed at the palmental complex this pluton however is located c 80 km ne from the gawib pluton and probably cannot be viewed as the direct source of the gawib granodiorites if such a relationship is allowed the granodiorites must be viewed as hybrid rocks containing a juvenile component because they were derived from unevolved quartz diorites by fractional crystallization in addition afc processe have also played a role implying that the granodiorites contain also a reprocessed crustal component alternatively comparison with experimentally derived melts imply that the granodiorites are generated by dehydration melting of a mafic amphibole bearing lower crustal source chemical parameters of the granodiorites compared to experimental results indicate high temperatures of c 1040 c zirconium saturation temperatures obtained on the most primitive samples gave c 830 c whereas apatite saturation temperatures obtained on the same samples give temperatures of c 960 980 c the latter seems to be a more reliable temperature estimate interpretation of geochemical and isotope data from the complex suggest that the early synorogenic pan african igneous activity in this part of the damara belt was a high temperature intra crustal event in contrast to igneous processes along active continental margins that produce also intermediate plutons with calc alkaline affinities this igneous event was not a major crust forming episode and the granodiorites represent mostly reprocessed crustal material

origin of high potassium talc alkaline i type granitoids many i type granitoid magmas are generated through partial melting of older metaigneous rocks and the compositions of such melts are broadly calc alkaline and metaluminous these melts are granitic to tonalitic and result from thermal extremes in their lower crustal source regions data on the experimental partial melting of common crustal rocks suggest that high k i type granitoid magmas can be derived only from the partial melting of hydrous calc alkaline to high k calc alkaline mafic to intermediate metamorphic rocks in the crust because of their low k2o contents metabasaltic rocks of all kinds are unsuitable as sources and models that propose mixing of mantle derived basaltic magmas and crustal melts are also inadequate there is no requirement that i type calc alkaline magmatism be related in any way to subduction processes

isotopic variation in the tuolumne intrusive suite central sierra nevada california granitoid rocks of the compositionally zoned late cretaceous toulumne intrusive suite in the central sierra nevada california have initial87sr 86sr values sri and143nd 144nd values ndi that vary from 0 7057 to 0 7067 and from 0 51239 to 0 51211 respectively the observed variation of both sri and ndi and of chemical composition in rocks of the suite cannot be due to crystal fractionation of magma solely under closed system conditons the largest variation in chemistry ndi and sri is present in the outer most equigranular units of the tuolumne intrusive suite sri varies positively with sio2 na2o k2o and rb concentrations and negatively with ndi al2o3 fe2o3 mgo feo cao mno p2o5 tio2 and sr concentrations this covariation of sri ndi and chemistry can be modeled by a process of simple mixing of basaltic and granitic magmas having weight percent sio2 of 48 0 and 73 3 respectively isotopic characteristic of the mafic magma are sri 0 7047 ndi 0 51269 and 18o 6 0 and of the felsic magma are sri 0 7068 ndi 0 51212 and 18o 8 9 the rocks sampled contain from 50 to 80 of the felsic component an aplite in the outer equigranular unit of the tuolumne intrusive suite apparently was derived by fractional crystallization of plagioclase and hornblende from magma with granudiorite composition that was a product of mixing of the magmas described above siliceous magmas derived from the lower crust having a maximum of 15 percent mantle derived mafic component are represented by the inner prophyritic units of the tuolumne intrusive suite

elemental and sr nd isotopic compositional disparity of riverine sediments around the yellow sea constraints from grain size and chemical partitioning to obtain a better understanding of the source compositions of the river sediments around the yellow sea and their relationship with source rocks elements and strontium neodymium sr nd isotopes of different grain sizes silt and clay populations and chemical labile and residual phases fractionations in riverine sediments were studied extensively these results clearly revealed a systematic compositional disparity between korean river kr and chinese river cr sediments especially in the residual detrital fraction the geochemical dissimilarity between these might reflect inherited signatures of their source rocks but with minor control from chemical weathering in particular the remarkable enrichment of some elements iron fe and magnesium mg and the behavior of large ion lithophile elements e g barium ba potassium k and sr during weathering as well as less radiogenic sr isotopic compositions implies that cr sediments might be weathering products of relatively more mafic rocks with abundant ferromagnesian and plagioclase feldspar minerals compared with kr sediments derived from silicic granites with relatively higher quartz and potassium feldspar contents this different petrological rationale is clearly evident in an a cn k diagram which estimated the source rock of cr sediments as granodioritic a composition that reflects accurately the average composition of weathered continental crust in china the recognition of such geochemical systematics in two river sediments especially in grain size and chemically partitioned data may contribute to the establishment of provenance tracers for the yellow sea and east china sea sediments with multi sources as well the dust deposition in the western pacific c 2015 elsevier ltd all rights reserved

the system tonalite h_2o at 15 kbar and the genesis of calc alkaline magmas experimental phase relations and well characterized mineral and melt compositions have been determined for a sierra nevada tonalite andesite with 2 5 to 10 wt h 2o added at 15 kbar and 850 to 1100 c all results are from experiments conducted in au capsules minimal fe loss to capsule at oxygen fugacities measured to be near ni nio with increasing water content the liquidus mineral changes from clinopyroxene at 3 wt h20 to garnet followed within 50 c by clinopyroxene at 3 to 9 wt h 2o to hornblende at water contents greater than 9 wt microprobe analyses of glasses quenched from within the tonalite melting interval show an enrichment in ca relative to mg fe where fe represents total fe as fe 2 with increasing sio 2 instead of the typical calcalkaline trend of approximately constant ca mg fe from basalt through rhyolite the results document the importance of garnet and clinopyroxene in andesitic to dacitic compositions at deep crustal to uppermost mantle pressures and they indicate that it is not possible to directly produce magmas of calc alkaline composition at 15 kbar and the investigated range of water contents by either 1 partial melting of tonalitic to gabbroic eclogitic lower crust or 2 crystal fractionation from melts of andesitic to basaltic composition melting or fractionation at lower pressures or with lower water contents may produce calc alkaline liquids as plagioclase replaces garnet as the major al bearing phase cale alkaline rocks showing evidence of garnet liquid equilibration in their ree patterns must also have undergone lower pressure fractionation in order to explain their major element compositions

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

fluid present melting of meta igneous rocks and the generation of leucogranites constraints from garnet major and trace element data lu hf whole rock garnet ages and whole rock nd sr hf o isotope data pan african high grade metamorphism in the proterozoic damara orogen namibia led to formation of garnet bearing leucosomes in potassic meta igneous gneisses producing a meta igneous migmatite in addition the migmatite gneiss mesosome plus leucosome was intruded by small scale leucogranitic melts with a high amount of accumulated biotite and garnet u pb zircon ages obtained on the mesosome and the leucosome indicate late proterozoic ca 850 ma concordia upper intercept ages which are interpreted as minimum ages of the precursor rock of the migmatite u pb monazite ages obtained on the leucogranite give a concordant age of 512 1 ma and two reversely discordant ages with pb 207 u 235 ages of 544 1 and 534 1 ma indicating the growth of monazite before or close to the age of high grade metamorphism in the damara orogen high precision lu hf garnet whole rock dating gave ages of 492 6 1 7 ma for the mesosome 497 6 1 7 ma for the leucosome and 494 0 1 7 ma for the garnet and biotite bearing leucogranite indicating that the growth of garnet postdates the growth of monazite during high grade metamorphism in addition it is suggested that melting and intrusion was coeval and occurred probably shortly after the main peak of metamorphism which occurred at c 512 1 ma p t estimates obtained by conventional thermobarometry c 690 720 degrees c and accessory mineral dissolution thermometry on the leucogranite c 730 degrees c suggest that partial melting occurred through limited fluid present melting of biotite via the reaction bt kfs pig qtz h2o double left right arrow grt melt outcrop evidence diffuse relationship between the gneiss domain and the leucosomes similar size of the leucosomes homogeneous distribution of leucosomes on the sample scale suggests that minor melt segregation had occurred whole rock sr nd hf and o isotope data of the mesosome indicate that it belongs to basement rocks from this area geochemical and isotope data obtained on the leucosomes argue for derivation by in situ melting of the mesosome both leucosome and leucogranite originated from the same source rock but the leucogranite represents an accumulated melt that was able to segregate and to intrude the gneiss domain the similar isotope features of the mesosome leucosome and leucogranite indicate a direct relationship for the gneiss and the melts chemical and mineral data favour a derivation of both types of melt through fluid present melting of isotopically and chemically comparable biotite plagioclase k feldspar quartz bearing gneisses c 2008 elsevier b v all rights reserved

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

generation of sodium rich magmas from newly underplated basaltic crust sodium rich rocks of trondhjemite tonalite dacite ttd or granodiorite ttg suites form much of precambrian continental crust1 they are thought to have formed by partial melting of subducted oceanic crust2 3 a process that would have been much more widespread early in earth history than at present owing to the higher thermal gradients prevailing at that time4 phanerozoic ttd suites do exist however and seem also to relate to subduction zones5 defant and drummond6 proposed that these suites form where young 25 myr hot oceanic lithosphere is subducted and melts thus locally simulating the conditions that led to widespread crustal growth in the archaean here we describe plutonic and volcanic rocks from the cordillera blanca complex in peru which have characteristics of the high al ttd suite but which were produced above a subduction zone containing a 60 myr old slab we present evidence that the complex formed by partial melting of newly underplated basaltic crust and argue that this mechanism should be considered more generally as an additional way of generating sodium rich arc magmas

dehydration melting and water saturated melting of basaltic and andesitic greenstones and amphibolites at 1 3 and 6 9 kb

lower crustal melting and the role of open system processes in the genesis of syn orogenic quartz diorite granite leucogranite associations constraints from sr nd o isotopes from the bandombaai complex namibia abstract the bandombaai complex southern kaoko belt namibia consists of three main intrusive rock types including metaluminous hornblende and sphene bearing quartz diorites allanite bearing granodiorites and granites and peraluminous garnet and muscovite bearing leucogranites intrusion of the quartz diorites is constrained by a u pb zircon age of 540 3 ma quartz diorites granodiorites and granites display heterogeneous initial nd and o isotope compositions end 540 ma 6 3 to 19 8 18o 9 0 11 6 but rather low and uniform initial sr isotope compositions 87sr 86srinitial 0 70794 0 70982 two leucogranites and one aplite have higher initial 87sr 86sr ratios 0 70828 0 71559 but similar initial end 11 9 to 15 8 and oxygen isotope values 10 5 12 9 the geochemical and isotopic characteristics of the bandombaai complex are distinct from other granitoids of the kaoko belt and the central zone of the damara orogen our study suggests that the quartz diorites of the bandombaai complex are generated by melting of heterogeneous mafic lower crust based on a comparison with results from amphibolite dehydration melting experiments a lower crustal garnet and amphibole bearing metabasalt probably enriched in k2o is a likely source rock for the quartz diorites the granodiorites granites show low rb sr 1 and are most likely generated by biotite dehydration melting of heterogeneous felsic lower crust all segments of the lower crust underwent partial melting during the pan african orogeny at a time 540 ma when the middle crust of the central damara orogen also underwent high t medium p regional metamorphism and melting geochemical and isotope data from the bandombaai complex suggest that the pan african orogeny in this part of the orogen was not a major crust forming episode instead even the most primitive rock types of the region the quartz diorites represent recycled lower crustal material

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

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

phase relationships of gabbro tonalite granite water at 15 kbar with applications to differentiation and anatexis phase relationships have been determined at 15 kbar with variable h 2o contents for three rocks a gabbro a tonalite and a muscovite granite representing the compositional trend of the calc alkaline rock series experiments were conducted in 0 5 in piston cylinder apparatus using ag pd or pt capsules the results obtained are acceptable representations of the phase relationships for elucidation of petrological processes selected runs with 5 h 2o were completed using au capsules to overcome the problem of iron loss up to the temperature limit of au for reliable electron microprobe analyses of glass garnet clinopyroxene amphibole and plagioclase no glasses could be analyzed in the gabbro the data provide k d values as a function of temperature for pairs of coexisting minerals and liquid the chemical variations are used to evaluate 1 the fractionation trends in hydrous magmas at 55 km in thickened continental crust or uppermost mantle and 2 the products of anatexis of the thickened crust at this depth fractionation of hydrous basalt or anatexis of gabbro and tonalite or their metamorphosed equivalents appears to produce liquids diverging from the calc alkaline trend anatexis of granite produces a liquid less siliceous than itself with some chemical characteristics of syenite

evidence from the lamarck granodiorite for rapid late cretaceous crust formation in california strontium and neodymium isotopic data for rocks from the voluminous 90 million year old lamarck intrusive suite in the sierra nevada batholith california show little variation across a compositional range from gabbro to granite data for three different gabbro intrusions within the suite are identical within analytical error and are consistent with derivation from an enriched mantle source recognition of local involvement of enriched mantle during generation of the sierran batholith modifies estimates of crustal growth rates in the united states these data indicate that parts of the sierra nevada batholith may consist almost entirely of juvenile crust added during cretaceous magmatism

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

rare earth element systematics in hydrothermal fluids abstract rare earth element concentrations have been measured in hydrothermal solutions from geothermal fields in italy dominica valles caldera salton sea and the mid atlantic ridge the measured abundances show that hydrothermal activity is not expected to affect the ree balance of either continental or oceanic rocks the ree enrichment of the solutions increases when the ph decreases high temperature solutions 230 c percolating through different rock types may show similar ree patterns

exposure of a late cretaceous layered mafic felsic magma system in the central sierra nevada batholith california new u pb zircon ages for the lamarck granodiorite associated synplutonic gabbro and diorite plutons and two large mafic intrusive complexes that underlie them in the sierra nevada batholith are 92 1 ma these ages establish the late cretaceous as a period of extensive mafic felsic magmatism in the central part of the batholith and confirm the significance of mafic magmatism in the evolution of the voluminous silicic plutions in the sierran arc the lack of significant zircon inheritance in any of the units analyzed supports isotopic evidence that the lamarck and other late cretaceous sierran plutons were derived predominantly from young crust recognition of an extensive mafic felsic magma system in the sierra nevada batholith emphasizes the importance of basaltic liquids in the evolution of continental crust in arc settings

magma mixing in granitic rocks of the central sierra nevada california abstract the el capitan alaskite exposed in the north american wall yosemite national park was intruded by two sets of mafic dikes that interacted thermally and chemically with the host alaskite comparisons of petrographic and compositional data for these dikes and alaskite with published data for sierra nevada plutons lead us to suggest that mafic magmas were important in the generation of the sierra nevada batholith specifically we conclude that 1 intrusion of mafic magmas in the lower crust caused partial melting and generation of alaskite rhyolitic magmas 2 interaction between the mafic and felsic magmas lead to the observed linear variation diagrams for major elements 3 most mafic inclusions in sierra nevada plutons represent chilled pillows of mafic magmas related by fractional crystallization and granitoid assimilation that dissolve into their felsic host and contaminate it to intermediate granodioritic compositions 4 vesiculation of hydrous mafic magma upon chilling may allow buoyant mafic inclusions and their disaggregation products to collect beneath a pluton s domed ceiling causing the zoning mafic margins to felsic core that these plutons exhibit

petrogenesis of sierra nevada plutons inferred from the sr nd and o isotopic signatures of mafic igneous complexes in yosemite valley california mafic complexes in the central sierra nevada batholith record valuable geochemical information regarding the role mafic magmas play in arc magmatism and the generation of continental crust in the intrusive suite of yosemite valley major and trace element compositions of the hornblende bearing gabbroic rocks from the rockslides mafic complex and of the mafic dikes in the north america wall are compositionally similar to high alumina basalt of these rocks two samples have higher ni and cr abundances as well as higher end values than previously recognized for the intrusive suite plagioclase crystals in rocks from the north america wall and the rockslides have prominent calcic cores and sharply defined sodic rims a texture commonly associated with mixing of mafic and felsic magmas in situ analyses of 87sr 86sr in plagioclase show no significant isotopic difference from the cores to the rims of these grains we propose that the high 87sr 86sr 0 7067 and low end 3 4 of bulk rocks the homogeneity of 87sr 86sr in plagioclase and the high 18o values of bulk rocks 6 6 7 3 and zircon lackey et al in j petrol 49 1397 1426 2008 demonstrate that continental crust was assimilated into the sublithospheric mantle derived basaltic precursors of the mafic rocks in yosemite valley contamination 20 40 likely occurred in the lower crust as the magma differentiated to high alumina basalt prior to plagioclase and zircon crystallization as a consequence the isotopic signatures recorded by whole rocks plagioclase and zircon do not represent the composition of the underlying lithospheric mantle we conclude that the mafic and associated felsic members of the intrusive suite of yosemite valley represent 60 80 new additions to the crust and include significant quantities of recycled ancient crust

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

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

magnetochronology of the oligocene mammalian faunas in the lanzhou basin northwest china abstract the fluvio lacustrine sequence in the lanzhou basin located at the northeastern margin of the tibetan plateau is a rich source of oligocene miocene mammalian fossils critical to our understanding of the terrestrial asian mammal and environmental evolution while the miocene mammalian faunas have been dated with magnetostratigraphy the numerical age of the oligocene faunas remains controversial here we present new high resolution magnetostratigraphic records from two fluvio lacustrine sections in the central lanzhou basin which provide age constraints for two oligocene mammal assemblages in the lower xianshuihe formation the nanpoping and xiagou faunas the lower xianshuihe formation is suggested to span from polarity chrons c12r to c7n ranging from ca 31 ma to 24 ma the early oligocene nanpoping fauna correlates to the c12r c11n 2n polarity chron intervals yielding an age of 31 30 ma while the late oligocene xiagou fauna correlates to chrons c7an c8n 2n with an age of 26 25 ma this magnetostratigraphy provides a more accurate age than biochronology for the oligocene giant mammal indricotheriinae contained in the nanpoping fauna and sheds new light on the chronology and distribution of indricotheriinae in eurasia the faunas in particular the nanpoping fauna suggest a mixed setting of woodlands and grasslands associated with a humid environment in the lanzhou basin during early oligocene in contrast to its present arid environment with few woodlands

origin of meso proterozoic post collisional leucogranite suites kaokoveld namibia constraints from geochronology and nd sr hf and pb isotopes leucocratic granites of the proterozoic kaoko belt northern namibia now preserved as meta granites define a rock suite that is distinct from the surrounding granitoids based on their chemical and isotopic characteristics least evolved members of this 1 5 1 6 ga old leucogranite suite can be distinguished from ordinary calc alkaline granites that occur elsewhere in the kaoko belt by higher abundances of zr y and ree more radiogenic initial end values and unradiogenic initial 87sr 86sr the leucogranites have high calculated zircon saturation temperatures mostly 920 c for least fractionated samples suggesting that they represent high temperature melts originating from deep crustal levels isotope data i e endi 2 3 to 4 2 demonstrate that the granites formed from different sources and differentiated by a variety of processes including partial melting of mantle derived meta igneous rocks followed by crystal fractionation and interaction with older crustal material most fractionation corrected nd model ages tdm are between 1 7 and 1 8 ga and only slightly older than the inferred intrusion age of ca 1 6 ga indicating that the precursor rocks must have been dominated by juvenile material epsilon hf values of zircon separated from two granite samples are positive 11 and 13 and hf model ages 1 5 and 1 6 ga are similar to the u pb zircon ages again supporting the dominance of juvenile material in contrast the hf model ages of the respective whole rock samples are 2 3 and 2 4 ga demonstrating the involvement of older material in the generation of the granites the last major tectonothermal event in the kaoko belt in the proterozoic occurred at ca 2 0 ga and led to reworking of mostly 2 6 ga old rocks however the presence of 1 6 ga post collisional granites reflects addition of some juvenile mantle derived material after the last major tectonic event the results suggest that similar a type leucogranites are potentially more abundant in crustal terranes but are masked by afc processes in the case of the kaoko belt it is suggested that this rock suite indicates a yet unidentified period of mantle derived crustal growth in the proterozoic of south western africa

what controls chemical variation in granitic magmas abstract consideration of the models that have been applied to explain the chemical variations within granitic rock suites shows that most are inadequate to account for the main variations this stems from a variety of model deficiencies ranging from physical or energetic inadequacies to incompatibility with the chemical data or internal inconsistency between models based on for example isotope or trace element data and major element data we contend that any model that fails any of these tests of internal consistency cannot be considered further thus although we can point to examples in which many of the traditionally accepted mechanisms have played secondary roles in producing variation there presently remains but one viable choice for the primary mechanism by which most granitic magmas acquire compositions beyond the range defined by the compositions of crustal melts that primary mechanism is peritectic assemblage entrainment pae we infer that once a partial melt has formed in a crustal protolith it may segregate from its complementary solid residue carrying small crystals of the peritectic phase assemblage formed in the melting reaction and that the ratios of individual peritectic minerals in the entrained assemblage remains fixed in the ratio decreed by the stoichiometry of the melting reaction for those elements with low solubilities in granitic melts pae in varying degrees accompanied by co entrainment of accessory minerals is responsible for most of the primary elemental variation in granitic magmas in contrast the concentrations of elements with high solubilities in silicic melts reflect the protolith compositions in a simple and direct way the source is the primary control on granite magma chemistry it dictates what is available to dissolve in the melt and what will be formed as the entrainable peritectic assemblage the apparent complexity in granitic rock suites is largely a consequence of these processes in the source all other mechanisms contribute only as a secondary overlay

relationship between the present day stress field and plate boundary forces in the pacific northwest the relationship between plate boundary forces and the observed stress field in the pacific northwest is established using numerical models of continental deformation because the orientation of the greatest horizontal principal stress throughout the pacific northwest differs considerably from the direction of convergence between the juan de fuca and north american plates the relationship between the stress field and forces acting along the subduction zone has been unclear to address this relationship a two dimensional finite element model developed by bird 1989 is used that incorporates critical aspects of continental deformation such as a stratified rheology and interaction between thermal and mechanical components of deformation boundary conditions are specified in terms of either velocity or shear traction depending on whether the computed shear stress at the plate boundary is less than or exceeds respectively a prescribed limit shear stress limits on the subduction and transform plate boundaries are independently varied to determine the relative effect of forces along these boundaries on intraplate deformation results from this study indicate that the shear stress limit of both subduction and transform boundaries is low and that the intraplate stress field is attributed in part to the normal component of relative plate motion along the transform boundaries however the models also indicate that although the subduction zone fault is weak a minimum shear strength 10 mpa for the fault is necessary to explain the observed stress field the balance among forces along the tectonic boundaries of north america results in a surprising degree of variation in the present day stress field

pressure temperature time paths and a tectonic model for the evolution of granulites petrologic studies and application of well calibrated mineralogic thermometers and barometers reveal several important features common to many granulite terrains 1 peak metamorphic pressure and temperature conditions cluster around values of 7 5 pm 1 kbar and 800 pm 50 c implying average geothermal gradients of 30 35 c km 2 in comparison with upper amphibolite facies rocks especially in paired amphibolite granulite terrains it is evident that granulites are distinguished from amphibolites by the former s higher metamorphic temperatures but not higher pressure 3 initial cooling of granulite terrains from peak conditions is nearly isobaric for garnet closure temperatures of 600 pm 50 c the dp dt retrograde paths inferred from compositionally zoned garnet rims are 2 8 b c in most granulites 4 primary coarse sillimanite is found in most granulites and retrograde kyanite is known in a few these data suggest that granulites form as a result of anomalous thermal gradients

neodymium and strontium isotope content of microdiorite enclaves points to mantle input to granitoid production microdiorite enclaves are arguably the most poorly understood of the more prominent features of granitoids surprisingly they have been invoked as support for completely different models for the origin of chemical zonation in granitoids depending on whether they are interpreted as restites syn plutonic blobs of magma remobilized cumulate or basified country rock xenoliths here we present the first detailed neodymium and strontium isotope study of suites of such enclaves from two plutons in both cases we find relatively high nd values most readily explained if enclaves represent syn plutonic injections of mafic magma into the host granitoid magma as such they are the tangible remains of a very important process of mantle derived heat and mass transfer operating in the generation of granitic magmas

a geochemical traverse across the north chilean andes evidence for crust generation from the mantle wedge major and trace element and sr and nd isotope analyses are presented on 186 0 ma magmatic rocks along an east west traverse across north chile at 22 s esr ranges from 25 to 100 and end from 6 to 9 but the low end and high esr values are in rocks generated in the last 15 ma it is argued that previous discussions of petrogenesis in north chile have been hampered because the changes in magma chemistry in this area of unusually thick crust reflect not one but two processes one results in a progressive shift of esr from 25 to 20 and end from 6 to 6 in jurassic to recent rocks which is accompanied by increasing ta sm and sr and decreasing th ta the second is largely confined to the younger rocks and it is characterised by esr increasing up to 100 with increasing sio2 and decreasing sr and it results in relatively shallow trends on an end esr diagram the preferred interpretation is that trend 1 is due to the mobilisation of old late proterozoic mantle lithosphere as magmatism migrated eastwards and that trend 2 is due to crustal melting and contamination with differentiation in this area of thickened continental crust it follows that the mantle wedge is the principal site of crust generation and it is argued that 20 of the sr in the recent northern chile rocks is derived from the subducted ocean crust

petrogenesis and tectonic setting of the early mesozoic xitian granitic pluton in the middle qin hang belt south china constraints from zircon u pb ages and bulk rock trace element and sr nd pb isotopic compositions abstract the xitian pluton in southeast hunan province is one of the early mesozoic indosinian granitic plutons in the south china block it is composed of biotite adamellite with k feldspar megacrysts biotite adamellite and biotite granite that have u pb zircon ages of 229 9 1 4 ma 223 6 1 3 ma and 224 0 1 4 ma respectively the indosinian granitoids in the xitian pluton belong to s type granites with highly radiogenic initial 87sr 86sr ratios 0 71397 0 71910 negative end t values ranging from 10 1 to 9 4 and old nd model ages 1858 1764 ma they are enriched in radiogenic pb isotopes with 206pb 204pb t ranging from 18 130 to 18 903 207pb 204pb t from 15 652 to 15 722 and 208pb 204pb t from 38 436 to 39 037 respectively these features indicate that the granitoidswithin the xitian pluton were formed from magmas generated by remelting of metapelite and metapsammite of the paleoproterozoic metamorphic basement at temperatures of ca 800 c with low oxygen fugacity the sr nd isotopic compositions of the rocks from xitian pluton indicate that the granitic magmas were mixed with less than 10 mantle derived magmas we suggest that the xitian pluton was emplaced in an extensional tectonic setting related to release of compressional stresses within the thickened crust during the early mesozoic

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

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

field chemical and physical constraints on mafic felsic magma interaction in the lamarck granodiorite sierra nevada california evidence of magma interactions resulting in both hybridization and mingling are preserved in a diverse suite of gabbroic to intermediate rocks associated with the compositionally zoned hornblende biotite lamarck granodiorite of the eastern sierra nevada california ellipsoidal mafic enclaves were formed by quenching of small amounts of high alumina basaltic magma upon injection into and dispersal through granodiorite magma early in its crystallization synplutonic intrusions of hornblende gabbro through hybridized mafic granodiorite represent injection of mafic magma at a later stage of crystallization of the granodiorite as they crosscut regional trends in foliation and compositional zoning in the host pluton where compositional contrasts between intrusion and host granodiorite are large contacts are sharp and abundant enclaves derived from the mafic intrusion are present in the granodiorite where the host is relatively mafic or where the local scale proportion of mafic magma is large contacts are zones of extensive hybridization that contain both enclaves and hybrid schlieren uncontaminated mafic intrusions have high alumina basaltic compositions whereas hybridized intrusions have silica contents as high as 63 5 mafic intrusions locally contain coarse grained cumulus gabbro inclusions mafic schlieren in granodiorite far from mafic intrusions represent localized accumulations of hornblende fe ti oxides and biotite from the granodiorite intrusion of late mafic dikes mobilized and entrained granitic residue from the granodiorite and formed composite dikes of aplite and pillowed diorite whether interacting magmas mix or mingle is a function of the heat contents and mass fractions of the end members calculations that account for compositions heats of fusion heat capacities of liquids and crystals and a range of initial temperatures crystallinities crystal sizes and magma water contents indicate that in most circumstances the basalt end member quenches the resulting large viscosity contrast between the end members prevents hybridization homogenization is likely only if the compositional difference between host and injected mafic magma is less than 10 sio2 or if the mass fraction of mafic magma is greater than 0 5 resulting mixtures have the composition of tonalite or mafic granodiorite thus the more silicic rocks of the granodiorite pluton must represent differentiation products rather than direct hybrids of mafic or intermediate magma and felsic magma

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

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

petrology of the chilliwack batholith north cascades washington generation of calc alkaline granitoids by melting of mafic lower crust with variable water fugacity calc alkaline granitoid rocks of the oligocene pliocene chilliwack batholith north cascades range from quartz diorites to granites 57 78 sio2 and are coeval with small gabbroic stocks modeling of major element trace element and isotopic data for granitoid and mafic rocks suggests that 1 the granitoids were derived from amphibolitic lower crust having ree rare earth element and sr nd isotopic characteristics of the exposed gabbros 2 lithologic diversity among the granitoids is primarily the result of variable water fugacity during melting the main effect of fh 2 o variation is to change the relative proportions of plagioclase and amphibole in the residuum the ree data for intermediate granitoids quartz diorite granodiorite eu eu 0 84 0 50 are modeled by melting with fh 2 o 1 kbar leaving a plagioclase pyroxene residuum in contrast data for leucocratic granitoids leuco granodiorites and granites eu eu 1 0 0 54 require residual amphibole in the source and are modeled by melting with fh 2 o 2 3 kbar consistent with this model isotopic data for the granitoids show no systematic variation with rock type 87sr 86sri 0 7033 0 7043 end 0 3 3 to 5 5 and overlap significantly with data for the gabbroic rocks 87sr 86sri 0 7034 0 7040 end 0 3 3 to 6 9 the fh 2 o variations during melting may reflect additions of h2o to the lower crust from crystallizing basaltic magmas having a range of h2o contents chillwack gabbros document the existence of such basalts one dimensional conductive heat transfer calculations indicate that underplating of basaltic magmas can provide the heat required for large scale melting of amphibolitic lower crust provided that ambient wallrock temperatures exceed 800 c based on lithologic and geochemical similarities this model may be applicable to other cordilleran batholiths

assimilation of felsic crust by basaltic magma thermal limits and extents of crustal contamination of mantle derived magmas basaltic magma is the most widely used probe of mantle chemistry and structure chemical characterization of the sources for mantle derived magma is possible only after quantitative evaluation of possible contamination by crustal wall rocks the most commonly invoked contamination mechanisms are the combined processes of crustal assimilation and magmatic fractional crystallization afc because of the energetic link between these processes predicting assimilation and crystallization progress requires modeling both heat and mass transfer in the magma country rock system using the melts software package we modeled isenthalpic heat balanced afc between two common types of basaltic magma and several types of felsic crust at 1 kbar our simulations show that during the early stages of isenthalpic afc the ratio of rates of assimilation to crystallization r may be substantially greater than one 2 0 2 7 allowing assimilation of a mass of country rock up to 5 18 of the initial magma mass with only 3 7 crystallization the second stage of afc beginning with plagioclase and or pyroxene saturation is characterized by lower r 0 5 1 0 the initial high r stage results from suppression of crystallization associated with the change in magma composition as assimilation progresses under certain conditions even small degrees of crystallization of olivine alone coupled with cooling of the magma can accommodate relatively large amounts of crustal assimilation and cause large shifts in isotopic and trace element geochemical indices with little differentiation

early paleozoic intracontinental granites in the guangzhou region of south china partial melting of a metasediment dominated crustal source abstract whether the formation of early paleozoic granites in the south china block was related to intracontinental orogenic processes or oceanic subduction continues to be debated here we present whole rock major element trace element and sr nd isotopic data as well as secondary ion mass spectrometry and laser ablation inductively coupled plasma mass spectrometry zircon u pb geochronological and hf o isotopic data from biotite two mica and muscovite granites in the baiyunshan and maofengshan areas of guangzhou city south china zircon u pb ages reveal that these rocks were formed during the early paleozoic 446 438 ma these muscovite and biotite bearing granites have high sio2 69 0 80 4 wt and k2o 3 19 5 32 wt contents and low mgo 0 46 1 17 wt contents the granites are enriched in rb th u and pb and depleted in ba sr nb and ti the baiyunshan muscovite granites and maofengshan biotite granites are slightly to moderately enriched in light rare earth elements lrees la yb cn 2 27 15 1 with slight to pronounced negative eu anomalies eu eu 0 27 0 72 whereas the baiyunshan two mica granites are significantly enriched in lrees la yb cn 19 6 32 1 with moderate negative eu anomalies eu eu 0 42 0 58 these muscovite and biotite bearing granites have enriched whole rock nd and zircon hf isotopic compositions end t 12 1 to 7 6 ehf t 11 4 to 0 8 and high zircon 18o values 8 5 10 0 we suggest that the baiyunshan granites were formed by partial melting of metasedimentary rocks as they contain primary muscovite and have high zircon 18o values the maofengshan granites were formed by partial melting of a hybridized crustal source containing metasedimentary rocks with subordinate amphibolites as they have high zircon 18o values variable whole rock sio2 contents and biotite compositions similar to those crystallized in i and s type magmas these granites along with the widespread early paleozoic s type granites in the wuyi yunkai orogen are distributed across a broad area rather than in a narrow belt in addition early paleozoic s type granites in the south china block generally do not show younging trends from east to west or south to north moreover the baiyunshan maofengshan granites show geochemical characteristics similar to syn or post collisional granites and different from volcanic arc granites we therefore propose that these muscovite and biotite bearing granites were formed in an intracontinental orogenic setting