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

experimental melting of biotite plagioclase quartz muscovite assemblages and implications for crustal melting abstract in order to understand the role of mica rich rocks as a source of granite magmas a series of melting experiments was performed on two different starting materials the first composition is a model biotite gneiss consisting of 30 wt biotite 30 wt plagioclase and 40 wt quartz the second composition is a model two mica pelites consisting of 15 wt biotite 15 wt muscovite 30 wt plagioclase and 40 wt quartz experiments were performed under vapor absent conditions at 1 0 gpa and between 750 o and 950oc with only biotite in the starting material the volume of melt is always less than 15 vol below 900oc and reaches 25 vol at 950oc in experiments that involve both biotite and muscovite in the starting material the melt proportion increases up to 28 vol at 825oc and reaches 60 vol at 950oc for the biotite plagioclase quartz bpq assemblage the solidus is located at 800oc at 1 0 gpa the melting reaction produces a metaluminous granitic liquid and leaves a residuum consisting of garnet biotite orthopyroxene plagioclase quartz in addition the experiments show that at 1 0 gpa biotite can be stable above 950oc with both micas in the starting material bpqm the solidus at 1 0 gpa is located at 750oc the melting reactions produce a peraluminous granitic liquid and leave a residuum of garnet sillimanite biotite quartz plagioclase kfeldspar in experiments below 900oc at 950oc the residuum consists of garnet orthopyroxene biotite plagioclase the melt fraction is determined by the proportions of the hydrous phases and of the amount of feldspar relative to quartz mineral modes of the source rocks particularly the amount of quartz are at least as important as the amount of available h20 in controlling the melt fraction generated during crustal anatexis

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

trace element behaviour during migmatization evidence for a complex melt residuum fluid interaction in the st malo migmatitic dome france the st malo migmatitic dome represents an interesting example wherein migmatites arise from the anatexis of the surrounding gneisses petrographical and chemical data suggest that leucosome compositions are compatible with partial melting of the quartzo feldsphathic fraction of the parent gneiss the contribution of the incongruent melting of biotite to the melt does not exceed 5 of the parent rock

anatexis of ultrahigh pressure eclogite during exhumation in the north qaidam ultrahigh pressure terrane constraints from petrology zircon u pb dating and geochemistry plagioclase rich leucosomes and k feldspar rich veins are widely distributed within retrogressed eclogite in the xitieshan area of the north qaidam ultrahigh pressure metamorphic terrane western china in this contribution a combined study of petrology zircon internal structure zircon u pb age zircon trace element composition whole rock geochemistry and sr nd isotopes in plagioclase rich leucosomes and k feldspar rich veins provides insight into the nature and timing of partial melting in these rocks petrological evidence for partial melting is provided by elongated highly cuspate feldspars quartz along grain boundaries e g muscovite garnet and clinopyroxene and felsic veinlets composed of quartz and feldspar along the boundaries of garnet clinopyroxene and clinopyroxene plagioclase symplectite in the metabasite whole rock geochemistry suggests that the plagioclase rich leucosomes have lower contents of both rare earth elements rees and high field strength elements hfses but higher large ion lithophile elements liles e g rb ba k sr and pb than the metabasite hosts the plagioclase rich leucosomes may be divided into two subgroups according to their distinct ree patterns 1 higher total ree content with or without weak negative eu anomalies and 2 lower total ree content with conspicuous positive eu anomalies the eu rich group also shows higher sr content than those in the eu poor group however the k feldspar rich veins show higher ree hfse and lile concentrations than the plagioclase rich leucosomes and metabasites the inherited cores of k feldspar rich veins are of typical magmatic origin oscillatory zoning with high th u ratios enriched heavy h rees and negative eu anomalies yielding 206 pb 238 u ages ranging from 904 8 ma to 915 28 ma the zircon cores of plagioclase rich leucosomes and mantles of k feldspar rich veins exhibit no zoning or weak zoning with flat hree patterns and no negative eu anomalies and they contain mineral inclusions of garnet and clinopyroxene implying that these zircon domains were formed during an episode of eclogite facies metamorphism with 206 pb 238 u ages ranging from 444 10 ma to 452 9 ma internal textures and mineral inclusions quartz plagioclase and k feldspar and trace element systematics steep hree patterns with negative eu anomalies and low th u ratios of the zircon rims from plagioclase rich leucosomes and k feldspar rich veins are similar to anatectic zircon these anatectic zircon domains yielded 206 pb 238 u ages ranging from 430 10 ma to 440 12 ma and 406 12 ma to 430 13 ma respectively an integrated study of petrology geochronology and geochemistry demonstrated that the metabasites of the north qaidam ultrahigh pressure terrane likely experienced initial partial melting with plagioclase rich leucosome formation under eclogite facies conditions and partial melt crystallization during the granulite facies stage triggered by dehydration melting involving zoisite and rare muscovite the k feldspar rich veins within the metabasites may be the result of injected melts that were derived from partial melting of nearby gneisses

granite related tin metallogenic events and key controlling factors in peninsular malaysia southeast asia new insights from cassiterite u pb dating and zircon geochemistry

badrinath gangotri plutons garhwal india petrological and geochemical evidence for fractionation processes in a high himalayan leucogranite the gangotri leucogranite is the western end of the badrinath granite one of the largest bodies of the high himalayan leucogranite belt hhl it is a typical fine grained tourmaline muscovite biotite leucogranite the petrography shows a lack of restitic phases the inferred crystallization sequence is characterized by the early appearance of plagioclase quartz and biotite and by the late crystallization of the k feldspar this suggests that in spite of being of near minimum melt composition the granite probably had long crystallization or melting interval in agreement with previous experimental studies tourmaline and muscovite have a mainly magmatic origin even though the major element composition is homogeneous there are several geochemical trends when cao decreases there is an increase in na2o rb sn u b f and a decrease in k2o fe2o3 tio2 sr ba zr ree th which are best explained by a fractionation process with early crystallizing phases experimental solubility models for zircon and monazite in felsic melt support a magmatic origin for these two accessory phases as well rb sr isotope data show this granite to have like other hhl heterogeneous isotopic values for sr initial 87sr 86sr ratios calculated at 20 ma range between 0 765 and 0 785 therefore no mixing i e no convection occurred between the different batches of magma in contrast 18o data show little variation 13 04 0 25 implying a source with homogeneous 18o values differences in timing between fluid infiltration and the onset of melting related to differences in temperature of the source could explain why source homogenization occurred for the gangotri and not for the manaslu granite the use of experimental results for solubility and the position of the accessory minerals during melting predict a low viscosity for the melt during its extraction this in turn explains the lack of restitic phases major and accessory in the granite as well as some field features lensoid shape pronounced magmatic layering based on the petrographic and isotopic studies it is suggested that the mechanism of ascent was not diapiric but rather that the melt ascended along several fractures and the level of emplacement was partialy controlled by the density contrast between the melt and host rocks

dating the onset of variscan crustal exhumation in the core of the bohemian massif new u pb single zircon ages from the high k calc alkaline granodiorites of the blatna suite central bohemian plutonic complex abstract the variscan central bohemian plutonic complex crops out between the upper crustal tepla barrandian and the high grade moldanubian units bohemian massif much of the complex is made up of calc alkaline plutonic rocks 1 the geochemically more primitive na rich 354 4 ma sazava suite which was emplaced syntectonically during regional shortening 2 the younger more evolved potassic blatna suite which records both the shortening along its nw contact and the onset of normal shearing related to exhumation of the moldanubian unit to the se new ion microprobe u pb zircon ages for the high k calc alkaline blatna suite pinpoint this major switch in the tectonic regime both blatna and kozarovice granodiorites 346 2 ma and 347 2 ma 2 were generated by melting of heterogeneous crust and were emplaced contemporaneously to form the blatna composite pluton from age spectra preserved in inherited zircons and whole rock sr nd isotope signatures the likely crustal sources for the magmas were immature greywackes rich in neoproterozoic 615 10 ma kozarovice or late cambrian early ordovician 492 4 ma blatna volcanogenic detritus an additional important petrogenetic process was variable mixing with enriched mantle derived monzonitic magmas which may also have supplied the extra heat for crustal anatexis supplementary material analytical techniques selected whole rock major and trace element geochemical analyses and u th pb data are available at http www geolsoc org uk sup18391

mineralogical evidence for fractionation processes in the himalayan leucogranites of the ramba dome southern tibet abstract himalayan leucogranites provide some of the best examples in the world of highly felsic and strongly peraluminous granites and they have been paid special attention with regard to investigating the evolution of continental crust here we describe our mineralogical investigations of a late miocene leucogranite complex within the ramba dome of the tethyan himalaya belt that have helped us decipher the petrogenesis of these high silica peraluminous granites the ramba leucogranites range from two mica granites biotite muscovite to muscovite leucogranites muscovite garnet tourmaline with the two mica granites constituting the main part of the pluton and the muscovite leucogranites commonly occurring as late stage dikes that are located within the pluton or invaded the country rocks variations in the mineral assemblages of the two types of leucogranite are associated with systematic changes in bulk rock geochemistry and the distinctive mineralogical features demonstrate a genetic relationship between the two types of leucogranite that involved fractional crystallization with the muscovite leucogranites being more evolved than the two mica granites chemical data for the minerals in the two mica granites record a long period of differentiation during which a normal granitic melt developed into a highly evolved magmatic system with high flux components e g h2o f b in contrast the mineralogical data for the muscovite leucogranites record the crystallization of a very late residual melt i e a volatile rich highly fluid melt according to recent studies of differentiation processes in silicic magma reservoirs we can interpret the ramba leucogranites as having undergone a high degree of differentiation by in situ fractional crystallization in a crystal mush environment the two mica granites represent a congealed crystal mush that was composed of cumulate crystals and a trapped interstitial liquid and the muscovite leucogranites represent the almost pure liquid that was extracted from the crystal mush modeling using the trace elements sr and ba shows that the extraction probably occurred when the crystallinity of the mush was 59 64 at least for the most evolved muscovite leucogranite sample the two mica granite corresponds to a residual crystal mush that had a terminal porosity of 20 26 filled with a trapped interstitial liquid collectively fractional crystallization can account for the geological mineralogical petrological and geochemical variations between the two types of leucogranite this work highlights the important role of crystal fractionation in the generation of himalayan leucogranites

geochemical evolution of peraluminous paleoproterozoic bandal orthogneiss nw himalaya himachal pradesh india implications for the ancient crustal growth in the himalaya abstract paleoproterozoic 1900 100 ma granites along a 2000 km linear belt in the lesser himalaya are significant in terms of their tectonic setting and emplacement as they form the basement for younger sedimentary cover rocks the geochemistry and petrogenesis of one granitic body bandal orthogneiss 1904 70 ma from the lesser himalayan belt has been studied and correlated with the other plutons in the belt the paleoproterozoic bandal orthogneiss covering 500 sq km surface area occupies the core of the kulu rampur window himachal pradesh nw lesser himalaya the orthogneiss is s type peraluminous and can be subdivided into two suites a muscovite biotite granite suite i coarse grained porphyritic gneiss cpg and fine grained gneiss fg and a tourmaline muscovite granite suite ii leucogranite lg the two mica granites suite i form the bulk of the bandal orthogneiss whereas the tourmaline bearing granite which cuts across suite i occurs in the central part of the main pluton comparison of their bulk composition with melt experimental data suggest formation of lg from a water undersaturated magma of minimum melt composition at pressures around 3 4 kbar whereas the tourmaline free granites may have been derived by melting due to biotite dehydration at pressures 5 kbar high concentrations of mgo feo ba sr rb sr and ti of the two mica granites reflect higher extents of melting derived from biotite dehydration melting of a metasedimentary source tourmaline leucogranites with higher rb sr and low ba sr ratios represent low fraction melts extracted from their protoliths under conditions of low water activity the striking similarities in terms of the geological setting geochemical characteristics and the age data between bandal orthogneisses and the adjoining wangtu gneisses suggest that they were formed under similar tectono magmatic environment during paleoproterozic period the wide spread occurrence of paleoproterozoic gneisses in the lesser himalayan belt strongly suggest that they represent the granitic basement on which the younger sedimentary rocks were deposited the geochemical studies on the overlying sedimentary rocks chail and jutogh formations in the lesser himalaya suggest that the huge volumes of granitic magmatism played a significant role in changing the crustal composition of the himalayan rocks from mafic to felsic during the post archean

geochronology and geochemistry of the late triassic longtan pluton in south china termination of the crustal melting and indosinian orogenesis the indosinian orogeny is recorded by triassic angular unconformities in vietnam and south china and by widely occurring granitoids in the yunkai nanling and the xuefengshan belts of south china the longtan pluton in the northwestern part of the xuefengshan belt is a typical high k calc alkaline i type granitoid which can shed light on the relationship between the indosinian tectonic and magmatic activity in the region three precise zircon u pb ages yielded a mean of 218 0 8 ma which is taken as the age of crystallization the pluton consists of both granodiorite 64 59 68 01 sio2 and 3 25 4 22 k2o and granite 70 49 71 80 sio2 and 4 07 4 70 k2o the granodiorites are characterized by relatively high mg 54 57 low contents of na2o 3 2 4 3 wt low abundances of incompatible elements lile nb and p high initial 87sr 86sr 0 7175 0 7184 and negative end t 9 98 to 9 72 ree patterns show moderate fractionation la yb cn 8 07 18 80 with negative eu anomalies eu eu 0 62 0 86 compared with the granodiorite the granite has a wider range of mg 49 59 lower contents of na2o 2 8 4 2 wt higher initial 87sr 86sr 0 7232 0 7243 and more negative end t 12 07 to 11 24 values ree patterns are relatively flat la yb cn 14 73 29 37 with smaller negative eu anomalies eu eu 0 48 0 63 the granodiorite has lower k2o na2o and al2o3 mgo feotot values than the granite based on major and trace element geochemistry and sr nd isotopes we interpret the longtan granodioritic magma to have been derived by partial melting of interlayered proterozoic metabasaltic to metatonalitic source rocks whereas the granite was probably derived from a mixture of proterozoic metagraywackes and metaigneous rocks field petrographic and geochemical evidence indicate that partial melting and fractional crystallization were the dominant mechanism in the evolution of the pluton the longtan granodiorites and granites are petrologically and geochemically similar to typical indosinian varieties and are considered to have been produced in a similar manner the indosinian granitoids in the region show a magmatic peak age of 238 ma from the yunkai nanling belt in the southeast and a magmatic peak age of 218 ma of the xuefengshan belt to the northwest these early and late magmatic episodes of the indosinian granitoids also display slight variations of regular compositions end t values and t dm ages thus we propose a syncollisional extension model that indosinian granitoids were generated by decompressional partial melting of crustal materials triggered by two extensions during collision of the indochina and south china blocks the longtan pluton in the northwesternmost part of the orogenic belt marks the termination of the indosinian magmatism and orogenesis

petrology of the coyaguayma ignimbrite northern puna of argentina origin and evolution of a peraluminous high sio2 rhyolite magma abstract the coyaguayma ignimbrite is a strongly peraluminous sp sillimanite garnet bearing silicic rhyolite which erupted in the northern puna segment of the central andean plateau in the upper miocene 11 ma a period that was characterized by the eruption of voluminous 100s to 1000s km3 dacitic ignimbrites of high k calc alkaline affinity in this region the sp magmatic rocks are both rare and small but their importance is potentially much greater as rocks of this type are usually interpreted as products of crustal melting and therefore useful for addressing mantle addition vs crustal recycling in the central andes the phenocryst assemblage of the coyaguayma ignimbrite comprises plagioclase an26 18 quartz ba rich and ba poor sanidine minor al rich ferromagnesian minerals alt and alvi rich biotite almandine spessartine garnet and sillimanite as well as accessory zircon and monazite textural relations suggest that the accessory and ferromagnesian phases crystallized before quartz and feldspars mineral equilibria suggest that crystallization of the rhyolite magma began at 5 kbar and 800 c and continued almost isobarically to 720 c causing the residual liquid to increase h2o contents from 4 5 to 7 5 before eruption most major features e g high sio2 a cnk 1 3 low cao mgo tio2 and feo and trace element patterns low ba sr th lree and eu eu high rb u y and nb along with the thermobarometric constraints on magmatic p t and water contents are consistent with an origin by mica dehydration melting of metapelitic sources e g typical biotite muscovite gneisses from the outcropping s puna basement however the relatively low initial 87sr 86sr 0 7125 and high 143nd 144nd 0 512200 ratios invalidate a pure crustal origin instead we propose contamination of calc alkaline dacitic magmas similar to typical puna ignimbrites by metapelite at mid crustal settings 18 km depth geochemical modeling that satisfies the chemical and isotopic data suggests mixing of 70 dacite and 30 of metapelite partial melts followed by extensive 70 fractionation of plagioclase k feldspar quartz and biotite with minor magnetite and apatite from the hybrid magma sillimanite in the coyaguayma rhyolite is interpreted as a restite mineral or a product of incongruent melting of the metapelite which was preserved intact in the hybrid melt due to local equilibrium this petrogenetic model explains most characteristics of crystal poor sp rhyolites from the puna plateau e g tocomar ramadas rhyolites and it may be more generally applicable to occurrences of sp magmas in andean type continental arcs dominated by calc alkaline magmatism

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

eocene magmatism from the liemai intrusion in the eastern tethyan himalayan belt and tectonic implications multistage magmatic thermal events occurred in the yardoi dome and contain important information on the tectonomagmatic processes the dome has played a crucial role in understanding the collisional evolution of the tethyan himalayan we present new geochronological and geochemical data for muscovite granite exposed in the liemai area eastern tethyan himalayan belt liemai muscovite granite is strongly peraluminous with a cnk values characterized by evolved geochemical composition with high contents of sio 2 enriched large ion lithophile elements and is depleted of high field strength elements these geochemical features indicate that granites possibly derived from partial melting of metasedimentary rocks and plagioclase fractional crystallization probably played a critical role in production of peraluminous granitic melts zircon u pb dating from muscovite granite yielded ages of approximately 48 5 1 1 ma representing its crystallization ages this age is the oldest age of tethyan himalayan leucogranite from the yardoi dome and adjacent areas however the inherited zircon cores have ages of 135 7 3339 2 ma the ehf t values of all zircons vary from 6 4 to 2 3 and have varying hf isotope crustal model ages of 731 839 ma the geochemical and isotopic compositions indicate that magma of the liemai granite can most likely be interpreted as products of the break off related to thermal perturbation along the break off window associated with the subduction of neo tethyan slab these magmas were derived mainly from the anatexis of ancient crustal materials under contraction and thickening conditions due to subduction of the indian continent beneath southeastern tibet

what controls partial melting in migmatites the layers of six stromatic migmatites from northern western and central europe display small but systematic chemical and mineralogical differences at least five of these migmatites do not show any signs of largescale metamorphic differentiation metasomatism or segregation of melts it is concluded therefore that the compositional layering observed in most of the investigated migmatites is due to compositional differences inherited from the parent rocks almost isochemical partial melting seems to be the most probable process transforming layered paragneisses metavolcanics or schists into migmatites the formation of neosomes is believed to be caused by higher amounts of partial melts formed due to higher amounts of water moving into these layers the neosomes have less biotite and more k feldspar if k feldspar is present at all than the adjacent mesosomes these differences are small but systematic and seem to control the access of different amounts of water to the various rock portions petrographical observations chemical data and theoretical considerations indicate a close relationship between rock composition rock deformation transport of water partial melting and formation of layered migmatites

u pb zircon constraints on the tectonic evolution of southeastern tibet namche barwa area the eastern syntaxis of the himalayas is expressed in the crust as a pronounced southward bend in the orogen the change in strike of geologic features coincides with the high topography of the namche barwa region the exposure of granulite grade metamorphic rocks and a 180 degree bend in the yalu tsangpo we have conducted a geochronologic and geochemical investigation of several suites of granitoids collected from the namche barwa massif and subjacent terranes of southeastern tibet ranging from cm scale dikes and sills to larger outcrop scale intrusions u pb shrimp rg zircon ages establish at least five magmatic episodes 400 to 500 ma 120 ma 40 to 70 ma 18 to 25 ma and 3 to 10 ma these episodes broadly correlate to spatial patterns in sample localities as follows 400 to 500 ma ages occur in zircon cores collected from within the massif proper 120 ma granites related to early gangdese arc plutonism are primarily located northeast of namche barwa later 40 70 ma gangdese activity is expressed in granites west of namche barwa 18 to 25 ma granites occur both along the suture zone west of gyala peri and directly north of namche barwa along the area of the jiali fault zone and are attributed both to shearing within the jiali fault zone and to an early miocene gangdese thrust event exceptionally young 10 ma zircon ages are clustered near the core of the massif along the yalu tsangpo gorge trace element geochemical data indicates the presence of both fluid present and fluid absent melts with a fluid absent decompression melting regime dominating near the core of namche barwa

thermal constraints on the emplacement rate of a large intrusive complex the manaslu leucogranite nepal himalaya the emplacement of the manaslu leucogranite body nepal himalaya has been modelled as the accretion of successive sills the leucogranite is characterized by isotopic heterogeneities suggesting limited magma convection and by a thin 100 m upper thermal aureole these characteristics were used to constrain the maximum magma emplacement rate models were tested with sills injected regularly over the whole duration of emplacement and with two emplacement sequences separated by a repose period additionally the hypothesis of a tectonic top contact with unroofing limiting heat transfer during magma emplacement was evaluated in this latter case the upper limit for the emplacement rate was estimated at 3 4 mm year or 1 5 myr for 5 km of granite geological and thermobarometric data however argue against a major role of fault activity in magma cooling during the leucogranite emplacement the best model in agreement with available geochronological data suggests an emplacement rate of 1 mm year for a relatively shallow level of emplacement granite top at 10 km uninterrupted by a long repose period the thermal aureole temperature and thickness and the isotopic heterogeneities within the leucogranite can be explained by the accretion of 20 60 m thick sills intruded every 20 000 60 000 years over a period of 5 myr under such conditions the thermal effects of granite intrusion on the underlying rocks appear limited and cannot be invoked as a cause for the formation of migmatites

geochemistry nd and sr isotopes and u pb zircon ages of granitoid and metasedimentary xenoliths from the navajo volcanic field four corners area southwestern united states abstract proterozoic granitoid xenoliths from the navajo volcanic field nvf in the southwestern united states range in composition from tonalite diorite to granite with the latter greatly dominating there is no apparent relationship between chemical or isotopic composition and age geographic location or degree of alteration or deformation of the xenoliths two of the major ages of proterozoic plutonism recognized in the southwest 1 75 1 70 yavapai and 1 45 1 41 ga anorogenic are represented by the nvf xenoliths in some instances within a single diatreme we did not find any xenoliths in the 1 65 1 62 ma mazatzal range the xenoliths are dominantly i types and both nd and sr isotopes and trace element distributions indicate juvenile arc sources approximately 1 8 ga the granite magmas may have been produced by either vapor absent partial melting or fractional crystallization of a tonalite parent juvenile sources must have been available for melting in each of the three periods of magma production and hence some juvenile 1 8 ga crust in this region must have escaped melting in the first two episodes of magmatism and deformation metasedimentary xenoliths are chiefly biotite sillimanite schists garnet biotite schists and garnet paragneisses these three groups are strikingly similar in chemical and isotopic composition suggesting similar graywacke pelite protoliths the schists appear to be limited to the far northern part of the volcanic field consistent with a possible crustal boundary in the basement compositional and thermobarometric results suggest that the biotite schist xenoliths are not restites but that the paragneisses may be mixtures of melt and restite sedimentary protoliths may have been deposited in a forearc basin and later tectonically transported to the deep crust during one or more arc continent collisions in the late paleoproterozoic

fluxed melting of metapelite and the formation of miocene high cao two mica granites in the malashan gneiss dome southern tibet identifying the timing of formation and geochemical nature of the cenozoic granites along the himalayan orogen is essential to test or formulate models that link crustal anatexis with tectonic transition during the evolution of large scale collisional orogenic belts the malashan gneiss dome one of the prominent domes within the tethyan himalaya experienced barrovian type metamorphism and partial melting of pelitic rocks at relatively deep levels during the collision between india and eurasia new la mc icp ms zircon u pb analyses yielded that the malashan two mica granites formed at a time span of 17 6 0 1 to 16 9 0 1 ma the malashan two mica granites are characterized by 1 high sio2 71 3 wt al2o3 14 8 wt and relatively high cao 1 3 wt 2 relatively high sr 146 ppm but low rb sr ratios 1 3 which are nearly constant relative to large variations in ba concentrations 3 enrichment in lree depletion in hree and no or weak negative eu anomalies eu eu 0 7 0 9 4 as compared to granites in the other northern himalayan gneiss domes and high himalayan belt relatively lower initial 87sr 86sr ratios 0 7391 0 7484 and similar unradiogenic nd isotope compositions end t 13 7 to 14 4 these characteristics imply that the two mica granites were derived from fluid fluxing melting of metapelite possibly triggered by the e w extension our new data in combination with literature data indicate that there are three types of granites with diverse geochemical characteristics and distinct formation mechanisms along the himalayan orogen since the cenozoic india eurasia continental collision conceivably our new results will provide new insights on how the partial melting behavior of relatively deeper crustal rocks evolved as the tectonic evolution of large orogenic belts

petrogenesis of ca 1 95 ga meta leucogranites from the jining complex in the khondalite belt north china craton water fluxed melting of metasedimentary rocks abstract anatexis occurs in response to changing p t fluid conditions during orogenesis is thus crucial for understanding the tectonic evolution of orogenic belts here we report an integrated petrological geochemical whole rock oxygen isotopic and zircon u pb hf o isotopic study of meta leucogranites from the jining complex north china craton to unravel its petrogenesis and constrain the tectonic evolution of the paleoproterozoic khondalite belt the meta leucogranites are weakly peraluminous with homogeneous minimum melt compositions and are characterized by low rb contents and rb sr ratios high ba and sr contents and sr y ratios and moderately fractionated ree patterns without obvious eu anomalies these trace element characteristics differ from that of leucogranites derived from fluid absent melting considering their low whole rock zr saturation temperatures 746 780 c and comparable compositions with experimentally generated melts produced by water fluxed melting we infer that the meta leucogranites are low temperature granites and were generated by water fluxed melting of metasedimentary rocks at 6 8 kbar sims zircon u pb analyses suggest that the meta leucogranites were emplaced at 1 95 ga or slightly earlier and were subsequently metamorphosed and cut by a pegmatite dike at 1 92 ga the abundant inherited zircons yield nearly concordant 207 pb 206 pb ages ranging from 2 24 to 1 98 ga which are consistent with the detrital zircon age spectra of the metasedimentary rocks from the khondalite belt the hf isotopic compositions of inherited and magmatic zircons e hf t 0 2 to 7 7 closely overlap with the time evolved e hf range of detrital zircons from metasediments in the khondalite belt different types of zircons inherited magmatic and metamorphic zircons have similar 18 o values of 7 2 9 2 which is lower than that of metamorphic zircons 10 0 13 6 from surrounding metasediments thus we propose that the meta leucogranites were formed by water fluxed melting of metasedimentary rocks from the khondalite belt at 1 95 ga during the prograde upper amphibolite facies metamorphism

manaslu leucogranite a collision signature of the himalaya a model for its genesis and emplacement the manaslu pluton is one of 10 leucogranites that formed in the overthrusted higher himalaya after the indo eurasian collision field and analytical data indicate that the underlying migmatites of the tibetan slab may be where the leucogranitic melts were generated the himalayan crustal thrusting of a hot slab over a rather cold volcano sedimentary pile le fort 1975a provides the necessary release of fluids these fluids cross the main central thrust mct induce the partial anatexis of the overheated tibetan slab and produce a leucogranitic magma the emplacement of the magma at first is located along the main disharmonic plane above the mct between the infrastructure and the superstructure there it generates a convective hydrothermal system extending very far laterally according to the stratification of permeabilities progressive emplacement of the granite proceeds as the convected fluids including the fluids released by the saturated magma dissolve the mainly calcareous host rocks of tibetan sedimentaries a frozen image of this caving out progression is given by the extensive network of granitic dikes outside the pluton the two quite independant fluid cycles of the generation and of the emplacement were triggered or guided by the tectonics due to collision this two fold model dominated by fluid activity may be of importance for other leucogranites and granites

the behaviour of nd and pb isotopes during 2 0 ga migmatization in paragneisses of the central zone of the limpopo belt south africa and botswana we have carried out major and trace element analyses and nd pb isotope measurements on different components of two migmatite outcrops in the limpopo belt lose quarry botswana and palala south africa in order to assess the effect of migmatization during a high grade metamorphic event on sm nd systematics and nd model ages the outcrops studied are located in the mahalapye tshipise straightening zone at the south western part of the limpopo central zone close to its contact with the kaapvaal craton leucosomes of peraluminous granitic composition were formed by incongruent biotite melting in an fe rich metagreywacke lose quarry and in a metapelite palala respectively monazite u pb and garnet sm nd ages yield close to 2 0 ga for the high grade event confirming other recent work in the central zone the chemical compositions of the main mineral phases feldspar biotite and garnet indicate minimum melt formation at t 750 c and low pressure isocon plots and pb isotope data indicate that the partial melts are locally derived however lil trace element distribution between the leucosomes and paleosomes indicate the incomplete equilibration of the nascent melts with the residual feldspars and biotite in the paleosomes sm nd results on leucosomes and paleosomes of the metagreywacke migmatite show considerable scatter when calculated back to 2 0 ga the paleosomes fall in a narrow range of nd 4 and give tdm model ages between 2 45 and 2 66 ga the nd values of leucosomes are on average four units lower causing them to have apparent tdm ages of 2 7 2 92 ga on average 250 ma older than those of the paleosomes the ree th p2o5 and zr budgets of paleosomes and leucosomes indicate dominance of the lree by monazite and the hree by apatite incorporation of proportionally more monazite than apatite into the melt is inferred as the cause for the nd isotope discrepancy and hence the uncertainty of the average protolith mantle derivation age in contrast pb pb results define an apparent age of 2025 29 ma reflecting nearly full pb isotope exchange in the case of the palala metapelitic migmatite the nd values at 2 0 ga of all but two migmatitic constituents cluster between 7 and 9 5 pointing out extensive nd isotope equilibration during migmatization whereas the pb isotopes yield an errorchron date of 2311 65 ma indicating incomplete pb isotope exchange here garnet dominates the hree distribution whereas the lree are controlled by feldspar and monazite considering that nd isotope equilibration was nearly reached during migmatization nd model ages on the whole rocks represent a good approximation of those of the original protolith 3 0 3 2 ga in the regional context the pb isotope data from the lose quarry locality combined with the nd model age uncertain but within the range of 2 5 2 9 are reminiscent of the northern marginal zone of the limpopo belt and the southern zimbabwe craton whereas the nd and pb isotope data from the palala locality are typical for the limpopo southern marginal zone and the northern kaapvaal craton

tracing the transhimalayan magmatic belt and the lhasa block southward using zircon u pb lu hf isotopic and geochemical data cretaceous cenozoic granitoids in the tengchong block yunnan china abstract a number of cretaceous cenozoic granitoid bodies in the tengchong block were analyzed for their u pb zircon ages lu hf zircon isotopic compositions and whole rock geochemistry zircons from six different granite bodies in the central tengchong block have weighted mean 206 pb 238 u ages ranging from 76 to 65 ma and negative e hf t values ranging from 2 7 to 15 5 with t dm c ranging from 1312 to 1928 ma one dextrally sheared and gneissic granodiorite sample of the nabang shear zone in the western tengchong block yielded a weighted mean 206 pb 238 u age of 19 7 0 4 ma and positive e hf t values ranging from 0 2 to 10 7 with t dm c ranging from 418 to 1092 ma the late cretaceous granites have metaluminous and peraluminous affinities with strong enrichment of u and th and significant negative anomalies of eu ba sr nb p and ti the miocene granodiorite displays strong negative anomalies of nb ta p and ti and a weak negative eu anomaly the data suggest that the late cretaceous granites in the tengchong block were derived from partial melting of ancient most likely proterozoic continental crustal material whereas the miocene gneissic granodiorite was derived from partial melting of juvenile crustal material possibly within a supra subduction setting and intruded synkinematically into the active nabang shear zone lu hf isotopic data of late cretaceous paleocene granites are indistinguishable from those of granites of the central lhasa block thus we tentatively correlate the late cretaceous granites of the tengchong block with those of the central lhasa block the boundary between the south and central lhasa blocks in western tengchong may be exposed in the nne striking dextral nabang shear zone along the chinese myanmar border region which hosts early eocene granitoids with e hf t values 0 the lu hf data of the zircons from the gneissic miocene granodiorite indicate a juvenile crustal substrate in the western tengchong block as well the synkinematic miocene granodiorite provides an important age constraint for dextral shear at mid crustal levels which was active until at least c 19 20 ma in the nabang shear zone this miocene intrusion may be a first indication that active margin tectonics lasted 20 25 ma longer than in the gangdese belt and possibly was associated with deformation partitioning in the hinterland produced by increasing obliquity of subduction during clockwise rotation of sundaland during the india asia collision

crustal generation of the himalayan leucogranites abstract detailed studies of the himalayan two mica leucogranites such as the manaslu pluton indicate that they have very uniform mineralogical petrological and structural characteristics one can relate their occurrence to the thickest zones of the underlying tibetan slab in these zones migmatization attains its greatest development and vertical extension the granite is emplaced at first along the main disharmonic plane above the main central thrust mct at the top of the tibetan slab infrastructure ductile deformation of the granite is variable the granite being syn to late kinematic with regard to the functioning of the mct major elements are very homogeneous except for na and k implying that p t conditions of melting were relatively uniform the melted material was of a similar composition over a vast volume and the percentage of melting was small 10 15 trace elements are highly variable some are characteristic for very evolved material ta rb cs u or show the link with the tibetan slab ba sr whilst others are problematic th ree ree and th abundances being much less in the granite than in the tibetan slab imply that they have been extracted during one of the main stages of formation possibly by monazite radiogenic pb sr nd and stable o isotopes are consistent with the origin of the granite from the tibetan slab however the heterogeneous sr isotopic ratios make age dating difficult and imply poor mixing or little fluid interaction during its evolution h isotope data indicate that magmatic compositions of the main body of the manaslu granite have been preserved late or post magmatic alterations are extremely local in the main pluton to the north another belt of two mica granites occurs whose characteristics are very similar to the high himalaya belt they were probably generated in a similar way during this recent intracontinental evolution

cretaceous 100 ma high silica granites in the gajin area central tibet petrogenesis and implications for collision between the lhasa and qiangtang terranes abstract high silica granites that occur as isolated plutons are widely distributed in collisional orogenic belts and are considered a hallmark of continental collision processes the late mesozoic collision between the lhasa and qiangtang terranes along the bangong nujiang suture zone was key event in the evolution of the tibetan plateau however the timing of this collision remains debated here we report the ages and geochemical characteristics of granites from the gajin area in the north central lhasa terrane central tibet zircon u pb dating reveals that the gajin granites formed at 100 ma these granites exhibit uniformly high sio2 contents 74 9 78 4 wt are weakly peraluminous compositions and have high k2o moderate na2o and al2o3 and low fe2o3t mgo and cao contents they are characterized by enrichments in rb cs and u and by depletions in zr hf nb and ti these characteristics in combination with high rb sr ratios and pronounced negative eu anomalies indicate that the gajin granites are highly fractionated rocks the granites have enriched whole rock nd end t 4 3 to 2 9 and relatively depleted and slightly variable zircon hf ehf t 1 0 to 5 5 isotope compositions the gajin granites were probably generated by high degrees of crystal fractionations of magma produced by partial melting of mature crustal sources the cretaceous magmatic rocks in the north central lhasa terrane record a magmatic lull between 100 and 85 ma which corresponds to a transition in both geochemical composition e g hf nd isotopes and rock association at 100 ma combining the above characteristics with the development of a regional angular unconformity around the time of the formation of the granites we suggest that the initial continental collision between the lhasa and qiangtang terranes probably occurred between 110 and 90 ma therefore the 100 ma high silica granites in the gajin area were formed during the continuing continental convergence that followed the initial collision between the lhasa and qiangtang terranes the generation of discrete and monotonous high silica granites may constitute evidence for a significant tectonothermal transition in evolution of the continental crust

petrology and u pb geochronology of the telohat migmatites aleksod central hoggar algeria the aleksod region is composed of metasedimentary rocks and large areas of biotite and hornblende bearing migmatites anatexis associated with the main deformation stages occurred under high pressure and temperature conditions estimated at 13 2 kbar and 750 50 c the bulk mineralogical composition of the telohat migmatites shows that their protolith was granodioritic internal structures of zircons and u pb data suggest a polyphased evolution with a 2131 12 ma age for the protolith and a 609 17 ma age for the pan african tectono metamorphic evolution thus precluding any kibaran event in the aleksod area leucosomes are richer in sr and display lower rb zr nb y th u and ree contents than melanosomes wherein accessory phases are stored eu contents are also lower in the leucosomes but in lesser proportion than the other rare earth s leading to a significant positive anomaly petrogenetic modelling accounting for accessory mineral phases clearly shows that the trace element contents of leucosomes and melanosomes follow a distribution law consistent neither with equilibrium nor fractional melting their trace element patterns are best explained by the model of disequilibrium melting with mixing of a few residual phases the present results and previous sr isotopic data as well raise the question of disequilibrium melting in anatexis of crustal material

water fluxed melting of the continental crust a review abstract water fluxed melting also known as fluid or water present melting is a fundamental process in the differentiation of continents but its importance has been underestimated in the past 20 years during which research efforts focused mostly on dehydration melting reactions involving hydrate phases in the absence of a separate aqueous phase the presence of a free aqueous phase in anatectic terranes influences all major physical and chemical aspects of the melting process from melt volumes viscosity and ability to segregate from rock pores to melt chemical and isotopic composition a review of the literature shows that melting due to the fluxing of aqueous fluids is a widespread process that can take place in diverse tectonic environments active tectono magmatic processes create conditions for the release of aqueous fluids and deformation driven transient high permeability channels capable of fluxing high temperature regions of the crust where they trigger voluminous melting water fluxed melting can be either congruent in regions at the water saturated solidus or incongruent at suprasolidus p t conditions incongruent melting reactions can give rise to peritectic hornblende or to nominally anhydrous minerals such as garnet sillimanite or orthopyroxene in this case the presence of an aqueous phase is indicated by a mismatch between the large melt fraction generated and the much smaller fractions predicted in its absence the relatively small volumes of aqueous fluids compared to that of rocks imply that melting reactions are generally rock buffered fluids tend to move upwards and down temperature however there are cases in which pressure gradients drive fluids up temperature potentially fluxing suprasolidus terranes crustal regions at conditions equivalent to the water saturated solidus represent a natural impediment to the up temperature migration of aqueous fluids because they are consumed in melting reactions in this case continued migration into supra solidus terranes take place through the migration of water rich melts thus melts become the transport agent of water into supra solidus terranes and responsible for water fluxed melting other processes such as the relatively rapid fluid migration through fractures also allow regional aqueous fluids to by pass the water saturated solidus fluid trap and trigger melting above solidus conditions when aqueous fluids or hydrous melts flux rocks at supra solidus conditions they equilibrate with the surroundings through further melting decreasing water activity and giving rise to undersaturated melts it is in these conditions that hornblende or anhydrous peritectic phases are stabilized unlike dehydration melting the melt fraction generated in this case is not limited by the water contained in hydrous minerals but by the volume of water added to the system unlike melting at the water saturated solidus these melts are capable of rising without freezing and do give rise to upper crustal granitic bodies

geochemical character and petrogenesis of pan african amspoort suite of the boundary igneous complex in the kaoko belt nw namibia abstract in the kaoko belt of northwestern namibia syn tectonic granitoids of the boundary igneous complex intruded a nnw sse trending suture between the neoproterozoic coastal terrane and archaean neoproterozoic congo craton derived foreland typical representatives of this syn collision igneous association are ca 550 ma old k feldspar phyric bt cam granites granodiorites of the amspoort suite with minor cpx gabbro and rare two pyroxene dolerite bodies the scarce opx cpx bt dolerites could come from a chur like mantle source 87sr 86sr550 0 7045 e550nd 0 the rest of the amspoort suite is interpreted as a product of ht anatexis of a heterogeneous lower crust built mainly by immature metapsammites rich in arc derived detritus with minor metabasite and intermediate metaigneous bodies the most likely source appears to be the anatectic coastal terrane gneisses yet partial melting of so far little constrained congo craton cover if formed by immature and youthful detritus unrelated to the basement cannot be discounted in any case the rather primitive sr nd isotopic compositions of the amspoort suite apart from dolerites e550nd 3 4 to 5 3 87sr 86sr550 0 7063 0 711 rule out any major role for the congo craton basement derived material which is in sharp contrast to other more southerly pan african granitoid complexes of the kaoko belt

chemical evolution of a large mafic intrusion in the lower crust ivrea verbano zone northern italy the ivrea verbano and adjacent strona ceneri zones have been described collectively as a section through the continental crust while resident in the lower crust amphibolite to granulite facies paragneiss of the ivrea verbano zone was intruded by huge volumes of mafic to intermediate plutonic rocks grouped as the mafic complex growth of the mafic complex involved hypersolidus deformation in an extensional environment isotopic and trace element variations close to the axis of this structure indicate crystallization from mantle derived melts that were extensively contaminated by crustal material previous investigations determined that the contaminant was fingerprinted by 87sr 86sr 0 71 18o 10 12 5 and a positive eu anomaly in the present study the contaminant is also shown to have been enriched in ba with respect to rb and k charnockites associated with paragneiss septa in the lower part of the mafic complex have the appropriate chemistry to be samples of the contaminating material these chemical features can be explained by melting of granulite facies paragneiss which had previously been depleted in k and rb by an earlier melting event the ba enrichment in the core of the mafic complex can be modeled by a replenishment tapping fractional crystallization rtf process operating within a small magma chamber is repeatedly replenished by mantle melts and contaminated by ba rich charnockite very high ba k in the lower part of the complex are tentatively attributed to chemical exchange between the cumulate framework and infiltrating anatectic melts from underlying paragneiss septa in contrast to the mafic complex the chemistry of coeval granites in the adjacent strona ceneri zone reflect a component derived from crustal rocks that had not been significantly depleted by a previous melting event significantly the incompatible trace element abundances in the mafic complex and strona ceneri granites are similar to model compositions for the lower and upper crust respectively

timing and nature of the partial melting processes during the exhumation of the garnet bearing biotite gneiss in the southern altyn tagh hp uhp belt western china abstract partial melting of the garnet grt bearing biotite gneiss from the jianggalesayi area in the southern altyn tagh high pressure hp ultra high pressure uhp metamorphic belt has been identified and reported here felsic veins were distributed along the foliation plane and deformed together with the grt bearing biotite gneiss which dominated the main outcrop and preserved diagnostic features of anataxis highly cuspate and irregular quartz qz melt and plagioclase pl melt surrounded or stretched into biotite and or quartz grains with obvious eroded boundaries further provided the microstructural evidences for partial melting of the gneiss cl images showed that zircons from the grt bearing biotite gneiss were characterized by multizone structures of core rim i rim ii rim iii the different zircon zones from the gneiss had integrated recordings of the protolith ages 836 1613 ma the peak metamorphic age of 503 5 ma the retrograded age of 456 4 ma and the anatectic age of 418 3 ma zircons from felsic veins also exhibited core rim structures on cl images the felsic veins yielded similar protolith ages 1099 1072 ma and anatectic age 417 2 ma as the host gneiss the zircons from the host gneiss and felsic veins had plotted identical crustal evolution curves in the ehf t vs u pb age diagram with identical tdm ages the u pb ages and lu hf isotope both suggested that the felsic veins were internally derived by the partial melting of the host gneiss moreover the high content of al2o3 and rb in felsic veins indicated that the decomposition of biotite accounted for the partial melting of the gneiss the temperatures calculated by ti in zircon thermometer for the anatectic zircons from the felsic veins 582 694 c were identical to the conditions of the amphibolite facies stage of the grt bearing biotite gneiss t 619 646 c which indicated that the crystallization of the melt had taken place during the amphibolite facies stage of the exhumation the anatectic ages 418 3 ma and 417 2 ma of the gneiss and felsic veins were determined to be within the range of the fourth episode of the early paleozoic magmatism 420 385 ma which had formed in a post collisional extensional setting therefore the combination of the magmatic heat events and stress release may have played an important role in the partial melting of the gneiss

possible juvenile palaeoarchaean ttg magmatism in eastern india and its constraints for the evolution of the singhbhum craton high precision shrimp u pb zircon dating yields a late palaeoarchaean age 3290 8 6 ma for a large unmetamorphosed weakly peraluminous ttg body the keonjhargarh bhaunra pluton in the singhbhum craton of eastern india one inherited subhedral zircon grain gave a concordant age of 3495 9 5 3 ma and nd isotope characteristics show a juvenile trend with end t 0 and t dm 3395 3453 ma the data support a model of typical archaean crustal evolution until late palaeoarchaean times for the singhbhum craton which is in contrast to the more southerly bastar craton where palaeoarchaean non ttg granites have been identified these data demonstrate the diachronous development of continental crustal blocks now forming the basement of the eastern and central peninsular of india

melt generation and fluid flow in the thermal aureole of the bushveld complex granite sheets emplaced into the migmatite zone of the eastern contact aureole of the bushveld complex resulted from fluid enhanced incongruent biotite melting of the underlying silverton formation shales during prograde metamorphism ba concentrations are extreme in both the sheets 1000 ppm and the hornfels 800 ppm into which they have been emplaced we conclude that a ba rich hydrothermal fluid induced melting in the aureole and that fluid transport of ba2 and to a lesser extent sr2 and eu2 persisted in the melt zones under subsolidus conditions sr isotope systematics from high ba localities define an errorchron of 2161 106 ma with an initial 87sr 86sr ratio of 0 705 0 001 metasedimentary rocks unaffected by fluid infiltration were homogenized at the same time but with an increased initial ratio suggesting that whereas isotope homogenization was achieved between outcrops permeated by fluids there is no evidence of regional homogenization oxygen isotope compositions of psammitic metasediments in the aureole are uncorrelated with distance from the contact suggesting the infiltrating fluid equilibrated isotopically with the metasediments their elevated 18o values 11 3 12 1 are consistent with a fluid source from devolatilization of the sedimentary lithologies textural analysis at both outcrop and thin section scale of arkose and psammites in the aureole lakenvalei and magaliesberg formations shows that the extent of melting was highly heterogeneous even on the grain scale and resulted from heterogeneously distributed infiltration of aqueous fluid on dilatant cracks and grain boundaries cathodoluminescence imaging of quartz shows a marked difference in the amount of fine structure with that in the melted rocks having uniform luminescence in contrast to that in rocks containing little or no melt which preserve textures inherited from the regionally metamorphosed protolith simple finite difference thermal modelling of the aureole suggests that the width of the melt zone 500 m is inconsistent with conductive heat transfer and hence that the thermal structure has been modified by fluid advection

estimating abundances of volatile and other mobile components in evolved silicic melts through mineral melt equilibria to variable degrees of accuracy from source partitioning between and earths residual minerals and anatectic melts through melt fractionation partitioning between igneous minerals and residual melts to the end stages of magma solidification where higher concentrations of normally trace elements may promote saturation in their crystalline phases the magmatic abundance of rare alkalis and alkaline earths

proterozoic anorogenic magmatism and its orogenic connections there is a major belt of proterozoic so called anorogenic igneous rocks that extends from the baltic shield through southern greenland and eastern canada to southern usa all rocks are situated within a zone of 1 9 1 6 ga arc accretionary orogens that consist of juvenile mantle derived material especially accretionary prism sediments that provided the fertile source for abundant granitic crustal melts there were two main periods of formation of the anorogenic rocks 1 76 1 55 ga rocks are mainly rapakivi granites and rhyolites that formed about 60 200 ma after the last compressional events in the svecofennian ketilidian and penokean orogens the time lag being caused by the slow thermal relaxation of thickened lower crust that had commonly been depleted by the extraction of earlier granitic crustal melts 1 45 1 4 ga anorogenic rocks are mainly rhyolites and peraluminous granites in central southern usa and anorthosites in eastern canada that formed in the continental margin of the grenvillian ocean the subduction of which led to the grenville orogen these anorogenic igneous rocks formed in extensional regimes during the formation and breakup of a 1 5 ga supercontinent

the metamorphic signature of contemporaneous extension and shortening in the central himalayan orogen data from the nyalam transect southern tibet abstract geological relationships and geochronological data suggest that in miocene time the metamorphic core of the central himalayan orogen was a wedge shaped body bounded below by the n dipping main central thrust system and above the n dipping south tibetan detachment system we infer that synchronous movement on these fault systems expelled the metamorphic core southward toward the indian foreland thereby moderating the extreme topographic gradient at the southern margin of the tibetan plateau reaction textures thermobarometric data and thermodynamic modelling of pelitic schists and gneisses from the nyalam transect in southern tibet 28 n 86 e imply that gravitational collapse of the orogen produced a complex thermal structure in the metamorphic core amphibolite facies metamorphism and anatexis at temperatures of 950 k and depths of at least 30 km accompanied the early stages of displacement on the main central thrust system our findings suggest that the late metamorphic history of these rocks was characterized by high t decompression associated with roughly 15 km of unroofing by movement on the south tibetan detachment system in the middle of the metamorphic core roughly 7 8 km below the basal detachment of the south tibetan system the decompression was essentially isothermal near the base of the metamorphic core roughly 4 6 km above the main central thrust the decompression was accompanied by about 150 k of cooling we attribute the disparity between the p t paths of these two structural levels to cooling of the lower part of the metamorphic core as a consequence of continued and probably accelerated underthrusting of cooler rocks in the footwall of the main central thrust at the same time as movement on the south tibetan detachment system

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

geochemical characteristics of crustal anatexis during the formation of migmatite at the southern sierra nevada california we provide data on the geochemical and isotopic consequences of nonmodal partial melting of a thick jurassic pelite unit at mid crustal levels that produced a migmatite complex in conjunction with the intrusion of part of the southern sierra nevada batholith at ca 100 ma field relations suggest that this pelitic migmatite formed and then abruptly solidified prior to substantial mobilization and escape of its melt products hence this area yields insights into potential mid crustal level contributions of crustal components into cordilleran type batholiths major and trace element analyses in addition to field and petrographic data demonstrate that leucosomes are products of partial melting of the pelitic protolith host compared with the metapelites leucosomes have higher sr and lower sm concentrations and lower rb sr ratios the initial 87sr 86sr ratios of leucosomes range from 0 7124 to 0 7247 similar to those of the metapelite protoliths 0 7125 0 7221 however the leucosomes have a much wider range of initial end values which range from 6 0 to 11 0 as compared to 8 7 to 11 3 for the metapelites sr and nd isotopic compositions of the leucosomes migmatites and metapelites suggest disequilibrium partial melting of the metapelite protolith based on their sr nd and other trace element characteristics two groups of leucosomes have been identified group a leucosomes have relatively high rb pb ba and k2o contents rb sr ratios 0 15 rb sr 1 0 and initial end values group b leucosomes have relatively low rb pb ba and k2o contents rb sr ratios 0 15 and initial end values the low rb concentrations and rb sr ratios of the group b leucosomes together suggest that partial melting was dominated by water saturated or h2o fluxed melting of quartz feldspar assemblage with minor involvement of muscovite breakdown of quartz and plagioclase with minor contributions from muscovite resulted in low rb sr ratios characterizing both group a and group b leucosomes in contrast group a leucosomes have greater contributions from k feldspar which is suggested by 1 their relatively high k concentrations 2 positive or slightly negative eu anomalies and 3 correlation of their pb and ba concentrations with k2o contents it is also shown that accessory minerals have played a critical role in regulating the partitioning of key trace elements such as sm nd nb and v between melt products and residues during migmatization the various degrees of parent daughter fractionations in the rb sr and sm nd isotopic systems as a consequence of nonmodal crustal anatexis would render melt products with distinct isotopic signatures which could profoundly influence the products of subsequent mixing events this is not only important for geochemical patterns of intracrustal differentiation but also a potentially important process in generating crustal scale as well as individual pluton scale isotopic heterogeneities

magnesium isotopic behaviors between metamorphic rocks and their associated leucogranites and implications for himalayan orogenesis abstract magnesium isotopic compositions along with new sr nd pb isotopic data and elemental analyses are reported for 12 miocene tourmaline bearing leucogranites 15 eocene two mica granites and 40 metamorphic rocks to investigate magnesium isotopic behaviors during metamorphic processes and associated magmatism and constrain the tectonic magmatic metamorphic evolution of the himalayan orogeny the gneisses granulites and amphibolites represent samples of the indian lower crust and display large range in 26mg from 0 44 to 0 09 in mafic granulites 0 44 to 0 10 in amphibolites and 0 70 to 0 03 in granitic gneisses the average mg isotopic compositions of the granitic gneisses 0 19 0 34 mafic granulites 0 22 0 17 and amphibolites 0 25 0 24 are similar indicating the limited mg isotope fractionation during prograde metamorphism from granitic gneisses to mafic granulites and retrograde metamorphism from mafic granulites to amphibolites the eocene two mica granites and miocene leucogranites are characterized by large variations in elemental and sr nd pb isotopic compositions the leucogranites and two mica granites have their corresponding 87sr 86sr i varying from 0 7282 to 0 7860 and 0 7163 to 0 7191 143nd 144nd i from 0 511888 to 0 512040 and 0 511953 to 0 512076 207pb 204pb from 15 7215 to 15 7891 and 15 7031 to 15 7317 208pb 204pb from 38 8521 to 39 5286 and 39 2710 to 39 4035 and 206pb 204pb from 18 4748 to 19 0139 and 18 7834 to 18 9339 however they have similar mg isotopic compositions 0 21 to 0 06 versus 0 24 to 0 09 which did not originate from fractional crystallization nor source heterogeneity based on hornblende biotite muscovite dehydration melting reaction and mg isotopic variations in two mica granites and leucogranites with the proceeding metamorphism along with elemental discrimination diagrams eocene two mica granites and miocene leucogranites could be related to hornblende dehydration melting and muscovite dehydration melting respectively mg isotopic compositions of eocene two mica granites become heavier compared to the source because of residues of isotopically light garnet in the source while those of miocene leucogranites become lighter because of entrainment of isotopically light garnet from the source region thus a new model for crustal anatexis and himalayan orogenesis was proposed based on the mg isotope fractionation in the leucogranites and metamorphic rocks this model emphasizes a successive process from indian continental subduction to rapid exhumation of the higher himalayan crystalline series hhcs the former underwent high temperature ht and high pressure hp granulite facies prograde metamorphism which resulted in the hornblende dehydration melting and the formation of eocene two mica granites while the latter experienced amphibolite facies retrogression and decompression which resulted in the muscovite dehydration melting and the formation of miocene leucogranites

oligocene crustal anatexis in the tethyan himalaya southern tibet abstract recent studies in the xiaru and malashan gneiss dome of the tethyan himalaya southern tibet identify that xiaru and paiku tourmaline bearing leucogranite dike formed at 28 29 ma together with 28 ma kuday garnet bearing leucogranites it is demonstrated that the himalayan orogenic belt experienced a major episode of crustal melting in the mid oligocene geochemical data indicate that three suites of leucogranite are characterized by large variations in the major and trace element compositions as well as sr nd isotope systematics which could be explained by combined fractional crystallization and relative contributions of micas and accessory phases dissolved into a crustal melt during decompressional melting of metapelitic rocks documentation of oligocene partial melting of crustal rocks could indicate that the exhumation of deep crustal rocks in the himalayan orogenic belt could have started as early as oligocene

loss of metals from pelites during regional metamorphism in aluminous metapelites the ratio h2o k2o decreases with increasing metamorphic grade and degree of reaction this ratio is a very practical indicator for the progress of the mineral reconstitution during progressive metamorphism with decreasing values of the ratio h2o k2o the cu concentration and the following element ratios also decrease either continuously or in stepwise fashion tl k2o ba k2o pb k2o bi k2o hg k2o sr na2o zn fe2 mg cd fe2 mg rb k2o remains approximately constant in the aluminous metapelites of the damara orogen in namibia the following losses occur between the biotite isograd and anatexis 61 cu 20 tl 34 ba 59 pb 86 bi 46 hg 30 sr 25 zn 31 cd thus the potential of regional metamorphism to form hydrothermal deposits in the low grade environment should not be neglected

zircon behaviour during crustal anatexis evidence from the tibetan slab migmatites nepal abstract the morphology and internal structures of zircon populations from three distinct types of migmatites from central nepal low zr and high zr in situ migmatites and injection migmatite were studied to examine the behaviour of zircon during anatexis the zircons from mesosomes and melanosomes were all residual zircons characterized by rounded morphologies with little overgrowth and a small elongation ratio suggestive of a detrital or metamorphic origin zircons from the leucosomes showed distinctive features the low zr tonalitic leucosomes we characterized by zircon populations with the same features as those from mesosomes and melanosomes indicative of no or very few transformations during migmatization the high zr leucosome contained euhedral crystals with morphological features typical of magmatic zircons found in crustal peraluminous magmas internal zoning patterns suggested a complex evolution involving dissolution and growth the injected granitic leucosome contained well developed magmatic zircons the role of subsolidus differentiation partial melting and melt composition on the contrasted behaviour of zircons during anatexis is discussed the zircon populations from the tibetan slab migmatites are distinct from those of the manaslu granite therefore suggesting that the migmatite leucosomes are not the unsegregated equivalents to the melts which gave raise to the manaslu type granites

petrologic and geochemical links between the post collisional proterozoic harney peak leucogranite south dakota usa and its source rocks abstract the proterozoic terrane of the black hills south dakota includes the composite harney peak leucogranite and associated pegmatites that were emplaced into metamorphosed pelites and graywackes available dates indicate that granite generation post dated regional metamorphism and deformation that have been attributed to collision of the wyoming and superior cratons at 1760 ma previous radiogenic and stable isotope work indicates that the exposed metasedimentary rocks are equivalent to sources of the leucogranites in this study whole rock and mineral compositions of the metasedimentary rocks were used to calculate the likely average residue mineralogies and melt fractions that would be generated by muscovite dehydration melting of the rocks these were then used to model observed trace element compositions of the granites using published mineral melt distribution coefficients model trace element melt compositions using pelitic and graywacke protoliths yield similar results the models reproduce well the observed depletion of transition metals and ba in the granites relative to metasedimentary protoliths the depletion is due mainly to high proportion of biotite with variable amounts of k feldspar in the model residue sr is also depleted in the granites compared to source rocks but to a lesser relative extent than ba this is because of the low biotite melt distribution coefficient for sr and because high proportion of plagioclase in the residue is compensated by high sr concentrations in protoliths rubidium cs and ta behaved as slightly compatible to incompatible elements and therefore were not strongly fractionated during melting of the considered elements only b appears to have been highly incompatible relative to residue during melting the protoliths had sufficient b to allow tourmaline crystallization in those parts of the harney peak granite in which ti concentration was sufficiently low not to enhance crystallization of biotite the reproducibility of observed trace element concentrations in the harney peak granite by the models supports the often made proposition that metapelites and metagraywackes are common sources for leucogranites this argues against mass input from the mantle into metagraywacke and metapelitic crustal sources or melting of amphibolites to generate the post collisional harney peak and other similar peraluminous granite suites

leucogranites in lhozag southern tibet implications for the tectonic evolution of the eastern himalaya abstract petrogenesis of the himalayan leucogranite is strongly influenced by conditions which are associated with the tectonic evolution of himalayan orogen in this article we present petrological geochronological and geochemical results of the lhozag leucogranites that crop out alongside the south tibetan detachment system stds in the east of himalaya zircon u pb dating revealed three episodes of leucogranitic magmatism in lhozag at 17 8 0 1 ma 15 1 0 1 ma and 12 0 0 1 ma respectively the lhozag leucogranites show relatively low end t low zircon ehf t and high initial 87sr 86sr ratios which are similar to the high himalayan crystalline series hhcs indicating that they were derived from the hhcs the characteristics of relatively high na2o and rb contents high rb sr ratios and low cao mgo tfe2o3 tio2 and sr contents indicate that both the ca 18 ma lhozag tourmaline leucogranites and the ca 15 ma lhozag two mica granites were derived from fluid absent muscovite dehydration melting of metasediments the opposite geochemistry characteristics of the ca 12 ma khula kangri two mica granites imply that these granites are derived from fluid present melting of metasediments four khula kangri two mica granite samples with relatively lower tio2 tfe2o3 mgo and cao contents higher rb concentrations and rb sr ratios could be evolved from the khula kangri two mica granites with relatively lower rb sr ratios the melting behaviors of the lhozag leucogranites varied from fluid absent melting to fluid present melting implying that there were p t xh2o variations in the deep crust the tectonic evolution would give rise to variation of p t xh2o variation and subsequent transformation of melting behavior our new results display the transformation of melting behavior of the lhozag leucogranites which implies the tectonic evolution from earlier n s extension to later e w extension in the eastern himalaya at ca 12 ma

different melting conditions and petrogenesis of peraluminous granites in western qinling china and tectonic implications abstract the qinling orogen records the prolonged amalgamation history between the north china block ncb and the south china block scb accompanied by major granitoid intrusions during the triassic however the petrogenesis of these granitoids especially that of the peraluminous granites remains a matter of debate detailed study of these peraluminous granites which are the products of crustal melting will give insights into the melting process and petrogenesis of peraluminous magmas and will further constrain the regional geological evolution in this study xiahe xh tourmaline bearing two mica granite and baijiazhuang bjz two mica granite were selected for systematic analysis of their mineralogy petrology zircon u pb geochronology and geochemistry the zircon u pb dating showed that the xh and bjz granites were emplaced at approximately 242 ma and approximately 214 ma respectively the presence of muscovite and tourmaline high peraluminosity at 1 1 low oxygen fugacity o2 conditions below the fayalite magnetite quartz fmq buffer and low zircon saturation temperatures indicate sedimentary protolith s type granitic affinity the low cao na2o ratios

paleoproterozoic porphyries and coarse grained granites manifesting a vertical hierarchical structure of archean continental crust beneath the yangtze craton abstract granites generated by anatexis are ideal crustal probes to decipher the differentiation processes within deep continental crust in this study the successive emplacement of granites including the porphyries and the coarse grained granites was found in the nucleus the kongling complex of the yangtze craton the porphyries intruding at 1840 17 ma shrimp ages occur as veins and contain phenocrysts of k feldspar 15 quartz 5 plagioclase 4 and garnet 1 setting in a fine grained groundmass 75 of k feldspar plagioclase quartz and biotite the coarse grained granites emplacing at 1821 8 ma shrimp ages are outcropped as pluton and comprise k feldspar 60 quartz 30 biotite 5 plagioclase 5 they are similar to a type granites with relatively high sio2 69 2 71 5 wt 72 6 74 5 wt high fe2o3t mgo 21 5 27 7 5 9 19 5 104 ga al ratios 3 6 4 3 2 7 3 5 respectively and enriched hfses th u pb ga the porphyries contain higher total trace element contents and zirconium saturation temperature mean 892 c than those of the coarse grained granites mean 803 c magmatic zircons from the porphyries and coarse grained granites have ehf t values of 17 6 to 11 7 and 21 0 to 14 8 which yield two stage hf model ages of 3 6 3 3 ga and 3 8 3 4 ga respectively both falling into the evolutionary trend of the 2 9 2 8 ga kongling ttgs it implies both granites are anatectic crustal reworking derivatives from partial melting of the archean ttgs under sustaining extension in paleoproterozoic applying trace element modelling we further identify a unique tonalitic source for both granites which is analogous to the exposed natural kongling tonalite and find the porphyries slightly affected by the juvenile crust based on the intrusion of porphyry veins into granitic gneisses we envisage the crustal structure beneath the craton is layered from older 2 94 ga tonalites upward to younger 2 87 ga granitic gneisses such hierarchical structure could shed new light on the imperceptible vertical differentiation process as cratonization

geochemical and isotopic disequilibrium in crustal melting an insight from the anatectic granitoids from toledo spain in the hercynian anatectic complex of toledo act the anatectic granitoids include leucogranitic leucosomes leucogranitic massifs and restite rich granites they show a broad range of initial sr and nd isotope ratios from 0 711 to 0 720 and 0 51164 to 0 51203 respectively which clearly indicate the absence of isotopic homogenization in the melts broadly the ranges reflect the isotopic variation of the metapelitic protoliths if crustal melting occurs under water undersaturated conditions as is the case of the act the generated melts do not isotopically and chemically equilibrate with the granulitic residuum the preservation of heterogeneities could arise through a number of processes 1 duration of the process in which the presence of melts with disequilibrium features and the high solid content of several of the granites in the act point to a very short lived magmatic system 2 limited diffusion in which poorly segregated and volatile undersaturated granitoids have a very restricted opportunity for isotopic and chemical homogenization and 3 magma dynamics in which the low melting rates the high restite and low water contents and the silica rich composition of these granites together with the small volume and high viscosity values of the melts seriously restrict the physical processes leading to homogenization

evidence for an early archaean granite from bastar craton india abstract granitoids in the early archaean are believed to be potassium poor tonalite trondhjemite granodiorite rocks only after continental crust attained sufficient thickness did true relatively potassium rich granites form no record of true granite prior to 3 4 ga is available we report a 3 6 ga true granite from the archaean bastar craton in india in contrast to the typical early archaean granitoids which are commonly deformed into gneisses this granite is relatively undeformed the age and composition of the granite implies that continental crust of the bastar craton attained sufficient thickness to permit intracrustal melting at 3 6 ga supplementary material representative major element trace element and ree composition of the dalli rajhara granite samples and a summary of shrimp u pb zircon data for the granite sample d 9 are available at http www geolsoc org uk sup18337

new geochronological constraints on the thermal and exhumation history of the lesser and higher himalayan crystalline units in the kullu kinnaur area of himachal pradesh india new geochronological petrological and structural data from the beas sutlej area of himachal pradesh india are used to reconstruct the tectonothermal and exhumation history of this part of the himalayan orogen sm nd garnet ages at 40 5 1 3 ma obtained on a pegmatoid from the inverse metamorphic high himalayan crystalline hhc in the malana parbati area probably mark local melting during initial decompression ongoing exhumation in ductilely deformed leuco gneiss is constrained by sm nd garnet ages at 29 1 ma and white mica rb sr ages around 24 20 ma while bt rb sr ages indicate a drop of regional metamorphic temperatures below 300 c between 15 and 12 ma the deep sutlej gorge exposes medium grade paragneisses and proterozoic orthogneisses of the lesser himalayan crystalline lhc overthrust by the hhc along the main central thrust mct mica cooling ages in the hhc are in the range of 14 11 ma above the extruded wedge of the hhc the leo pargil leucogranite and associated dykes intrude the haimanta unit hu below the weakly metamorphic palaeo mesozoic sediments of the tethyan himalayas th the leo pargil leucogranite yielded a mean sm nd garnet age of 19 1 ma and rb sr muscovite and biotite cooling ages between 16 4 and 11 6 ma marked young extrusion of lhc units resulted in differentiated exhumation cooling of more frontal parts of the orogen very young ductile deformation of lhc gneisses near wangtu is constrained by late kinematic pegmatite intrusions crosscutting the main mylonitic foliation sm nd garnet and rb sr muscovite ages of these pegmatites range between 7 9 0 9 and 5 5 0 1 ma published apatite ft ages down to 0 6 ma also document accelerated diachronous sub recent exhumation of different parts of the orogen together with geochronological data from the literature the new results demonstrate that the hhc and the hu were deformed by shortening and crustal thickening during the eohimalayan phase late eocene oligocene followed by a strong thermal overprint and intrusions of granitoids during the neohimalayan phase early to middle miocene the lhc experienced amphibolite facies metamorphic conditions in the late miocene prior to extrusion between the hhc and the very low grade lesser himalayan sediments in conjunction with climate changes young tectonic activity in this central part of the himalayan orogen may have strongly influenced fluvial incision and erosion and therefore contributed to the accelerated uplift as indicated by extensive accumulation of late miocene to early pleistocene fluviatile lacustrine sediments in the zanda basin the transhimalayan headwaters of the sutlej in western tibet

tectonothermal evolution of the high himalayan crystalline sequence langtang valley northern nepal the high himalayan crystalline sequence in north central nepal is a 15 km thick pile of metasediments that is bound by the main central thrust to the south and a normal fault to the north the langtang section through the metasediments shows an apparent inversion of metamorphic isograds with high p kyanite grade rocks exposed beneath low p sillimanite grade rocks textural evidence confirms that the observed inversion is a result of a polyphase metamorphic history and phase equilibria studies indicate that thermal decoupling has occurred within a mechanically coherent section of crust rocks now exposed at the base of the high himalayan thrust sheet underwent barrovian regional metamorphism m1 prior to 34 ma in the early stages of the himalayan orogeny recording metamorphic conditions of t 710 30 c p 9 1 kbar after the activation of the main central thrust which emplaced these metapelites southwards onto the lower grade lesser himalayan formations the upper part of the thrust sheet was overprinted by a second heating event m2 resulting in sillimanite grade metamorphism and anatexis of metapelites at t 760 30 c p 5 8 0 4 kbar between 17 and 20 ma crustally derived leucogranite magmas have been emplaced into low grade tethyan sediments on the hangingwall of the normal fault that bounds the northern limit of the metapelitic sequence the cause of the selective heating of the upper section of the metasediments during m2 cannot be reconciled with either post thrusting thermal relaxation or advection models the cause of m2 remains problematical but it is suggested that heat focusing has occurred at the top of the high himalayan crystalline sequence as a result of movement on the normal fault blanketing metapelites of high heat productivity with low grade sediments of low thermal conductivity this model implies that the normal fault was active before m2 consistent with decompression textures that formed during or shortly after sillimanite grade metamorphism

the effect of accessory minerals on the redistribution of lead isotopes during crustal anatexis a model abstract the initial pb isotopic composition of anatectic granites is modelled as a mixture of pb derived from major minerals that have characteristically low 238 u 204 pb and 232 th 204 pb ratios i e feldspars biopyroboles etc with pb contributed from dissolved accessory minerals with characteristically high 238 u 204 pb and 232 th 204 pb ratios i e zircon monazite etc the initial pb isotopic composition of anatectic granites is a function of a the age type modal distribution and heterogeneity in the initial u and th content of the accessory minerals b variation in melt composition and temperature during anatexis c the fraction of the source melted and d the extent to which the melt was homogenized prior to crystallization the differential behavior of accessory minerals during anatexis can explain the common yet enigmatic decoupling of the pb isotopes from other chemical and isotopic systems in granite rocks granitic plutons derived by anatexis of the same source material are not required to have the same initial pb isotopic composition low temperature peraluminous granites are more likely to exhibit homogeneous initial pb isotopic compositions dominated by the pb contained within the major mineral assemblage of the source material in contrast the initial pb isotopic composition of higher temperature metaluminous or peralkaline anatectic granites is more likely to be heterogeneous unless the melts are homogenized during emplacement and will record a larger fraction of the radiogenic pb component contained in the accessory mineral assemblage of the source material heterogeneity in the initial pb isotopic composition of individual granites or granitic suites can preserve the weighted average age of the radiogenic pb present in the accessory mineral assemblage of the source region even if these minerals are entirely dissolved during anatexis the use of isotopic tracer studies in terrane analysis will be optimized by the isotopic characterization of granitic suites

precambrian acid volcanism in central india geochemistry and origin abstract the early proterozoic bijli rhyolites 2180 25 ma constitute an important part of the nandgaon group in the central indian craton a tectonically active block sandwiched between the dharwar craton in the south and the bundelkhand craton in the north the rhyolites are chemically heterogeneous they have high sio 2 74 4 k 2 o 4 41 ba 834 ppm and low mgo 0 22 cao 1 0 iron 2 8 v cr and ni 5 6 and 1 ppm respectively they contain moderate sr 88 ppm rb 140 ppm th 27 ppm and u 6 ppm total ree ranges from 223 ppm to 453 ppm with high lree lree 307 and all the rhyolites have low to high negative europium anomalies eu eu 0 03 to 0 77 the overall chemical signatures support a crustal anatectic origin for the rhyolites from a source of intermediate composition basalt pooling below the crust and associated rifting most likely initiated anatexis

anatexis and metamorphism in tectonically thickened continental crust exemplified by the sevier hinterland western north america the generation of granitoid magmas by partial melting of crustal rocks during continental thickening events is well documented in many geological provinces throughout the world including the late mesozoic sevier belt of western north america we present a thermal and petrologic model of anatexis and metamorphism in regions of crustal thickening where the only mantle contribution is the normal conductive supply of heat through the base of the lithosphere i e advection of mass and energy are excluded we distinguish between formation of migmatites and generation of mobile granitoid magmas and examine the temporal and spatial relationships between these two distinct anatectic processes between anatexis and regional deformation and between anatexis and metamorphism a fundamental conclusion is that if protoliths rich in hydrous minerals are present regional anatexis is the end product of classical barrovian metamorphism in thickened continental crust even in the absence of a free water rich fluid phase barrovian metamorphic facies series are predicted with thickening ratios maximum crustal thickness attained initial crustal thickness as low as 1 3 but mobile granitoid magmas are not formed if this ratio is less than approximately 1 5 above these lower bounds barrovian metamorphism and anatectic granitoid magmatism occur independently of the magnitude of thickening and of the way in which thickening is accomplished both processes are sensitive to a diminished heat supply lowering either moho heat flow or crustal radioactive heat production results in blueschist eclogite metamorphism and inhibits the formation of mobile granitoid magmas we model anatexis under fluid absent conditions and show that with such a constraint migmatization is always a syn kinematic process relative to the crustal thickening event whereas generation of mobile granitoid magmas is in most cases post kinematic relative to crustal thickening but can be syn kinematic if thickening takes more than approximately 50 myr the typical time intervals for melting are consistent with geological observations mobile granitoid magmas are predicted by most of our models within approximately 10 myr of the end of the crustal thickening event this incubation period results primarily from the temperature increase required for the dehydration melting reactions capable of producing large melt fractions to occur the energetic requirements of anatexis are relatively minor compared to conductive crustal thermal budgets as shown by the fact that once the necessary p t conditions are attained melting reactions are completed within time intervals on the order of 1 myr i e 1 2 orders of magnitude smaller than the characteristic time scales of the tectonic processes involved in crustal thickening

age and origin of post collision baltoro granites south karakoram north pakistan insights from in situ u pb hf and oxygen isotopic record of zircons abstract origin of post collision plutonism is critical to understand the tectonothermal evolution of the over thickened continental crust in collision zones this has proven difficult to reconcile with the conventional whole rock geochemical and field based studies alone we report in situ study of zircon u pb hf and o isotopes from five samples of the baltoro plutonic unit bpu in south karakoram the plutonic unit is the western part of the southern asian margin of the india asia convergent zone baltoro granites and a biotite rich enclave yielded similar and overlapping u pb ages ranging from 26 to 15 ma hafnium isotopic composition ehf 0 is very heterogeneous ranging from 17 1 to 4 4 while the oxygen isotopic composition of the granites is homogeneous with mean 18 o ranging from 7 2 to 9 4 based on u pb geochronology and hf o isotopic composition the involvement of two main sources is suggested 1 cretaceous calc alkaline karakoram crust and 2 karakoram gneisses moreover possible involvement of metasomatized asian lithospheric mantle is supported by elevated oxygen composition of granites and identical hf composition of biotite rich enclave to the mantle derived baltoro lamprophyre however direct contribution from juvenile pristine mantle is unlikely as no juvenile mantle type hf and oxygen values were obtained this also precludes the involvement of southward juvenile arc related component of kohistan ladakh batholith our new u pb and hf data are comparable to the mesozoic karakoram batholith miocene two mica leucogranites in the pangong range and magmatism from the lhasa terrane in south tibet suggesting a genetic link between the karakoram and the rocks to the east this magmatic event is best explained by lower crust partial melting promoted by both thermal equilibration following crustal thickening and heat advection by ultrapotassic magmas associated with the breakoff of the indian continental margin

water present melting in the middle crust the case of the ollo de sapo gneiss in the iberian massif spain abstract the variscan migmatitic region of sanabria in the inner zone of the iberian massif spain provides a unique place to study the geochemistry and rheology of anatectic melts formed by water present conditions in sanabria the ollo de sapo gneiss underwent partial melting at 0 5 0 66 gpa and 690 770 c leucosomes leucogranitic bodies within the migmatite and leucogranites are widespread throughout the migmatite with the leucogranitic bodies being concordant to discordant to the foliation mass balance calculations indicate that the partial melting of the gneiss produced 62 5 wt of melt experiments and pseudosections made at the peak metamorphic p t conditions under fluid absent conditions yield much lower volumes of melt and therefore a fluid present scenario is proposed major and trace element composition and structures of leucosomes and leucogranites support this scenario trace element composition of leucosomes is variable with low to high ree zr y and th content and positive to negative eu anomalies the composition of the most felsic ree depleted eu rich leucosomes is closely similar to that of experimental melts with added water at t

seasonal variation in skeletal silicification of neodenticula seminae a marine planktonic diatom sediment trap experiments in the nw pacific ocean 1997 2001 abstract neodenticula seminae is an important component of pelagic diatom floras in the subarctic north pacific and its adjacent seas we conducted an sem image analysis of four series of samples collected by sediment traps in the nw pacific in order to investigate the relations between its morphology evolution and environment seasonal variation in the skeletal morphology was common lightly silicified morphs were dominant during spring blooms and in the summer under oligotrophic conditions owing to surface water stratification and progressive nutrient limitation whereas heavily silicified morphs became abundant in the fall and winter subsequent to the onset of intensive vertical mixing these observations suggest that the morphology of n seminae is influenced by environmental factors including the nutrient availability as well as by its reproductive strategy

patterns of legacy sediment deposits in a small south carolina piedmont catchment usa abstract anthropogeomorphic changes to channels and floodplains occurred in response to destructive agricultural practices following the arrival of european settlers into the americas the southeastern piedmont physiographic region of the usa experienced severe erosion and sedimentation after settlement in the 1700s and farming up through the 1930s in places deep floodplain aggradation formed uninterrupted alluvial deposits that extend many kilometers this research examines anthropogenic impacts of land use change on valley bottom sedimentation in the chicken creek watershed of south carolina abrupt contacts between precolonial floodplain soils and a thick overburden of historical legacy sediment are common and well exposed throughout the two km study reach and provide clear evidence of extensive postcolonial sedimentation soil stratigraphic and sedimentologic evidence are presented to identify and contrast pre and postcolonial sediment characteristics and thicknesses lidar based spatial analysis is used to examine patterns of legacy sediment deposition and floodplain storage and together with field observations of bank stratigraphy examine the environments in which this occurred precolonial geomorphic stability is indicated by a buried soil epipedon ab horizon and other features on precolonial floodplain surfaces legacy sediment 2 to 4 m thick rests on top of exposed precolonial soils throughout the stream corridor with the pre postcolonial contact 1 5 m above the channel bed on average linear regression analysis shows that legacy sediment thickness varies statistically with changes in valley width and proximity to tributary sediment sources thin precolonial alluvium over bedrock suggests modest erosion and sedimentation rates prior to the arrival of euroamerican settlers mean grain sizes are similar between the pre and postcolonial alluvium but substantial contrasts in the degree of stratification and bulk density document differences between the respective depositional environments and post depositional changes to the two units channel adjustments since the time of maximum aggradation include bed incision to at least as low as the precolonial longitudinal profile and widening that is ongoing

anatomy of garnets in a jurassic granite from the south eastern margin of the north china craton magma sources and tectonic implications abstract the late jurassic jingshan granite located at the south eastern margin of the north china craton contains abundant garnets which can be subdivided into three types based on texture and composition i euhedral garnet in mafic biotite and garnet rich enclave grt i ii coarse grained garnet grt ii in the host granite and iii small euhedral garnet in aplite grt iii in general grt i has higher feo cao and lower mno contents than grt ii grt iii has higher mn but lower ca contents than others grt i has lower mree and hree contents than grt ii grt iii has prominent and distinctly negative eu anomaly as well as higher mree composition compared to the others systematic variations in oxygen isotope compositions are observed among the three garnet types with 18 o values of 4 7 in grt iii some of the grt ii and grt iii display two distinct zonings with cores having similar major and trace element compositions to grt i cathodoluminescence cl images revealed that the zircons from different garnet bearing samples possess fine scale oscillatory zoned magmatic rims with inherited cores in situ zircon u pb dating and trace element analyses show that the dark luminescent magmatic rims all have jurassic concordia ages 160 ma and similar trace element patterns most of the inherited cores also display similar triassic ages of 210 236 ma which is similar to the ages of ultrahigh pressure uhp metamorphic rocks of the dabie sulu orogen 230 ma in addition jurassic concordia ages were also found in a zircon inclusion in grt i implying that the grt i was formed shortly before the main magmatic event the age data suggest that the three different garnet types may be genetically related and modified by cogenetic magmatic events based on the zircon u pb ages from different garnet bearing samples the major element trace element oxygen isotope and zoning textures of the three kinds of garnet we suggest that grt i may be peritectic garnet whereas grt ii and iii are probably the results of magmatic dissolution precipitation processes and re equilibration of garnets with changing magmatic conditions during melting differentiation crystallization and cooling within the granite we conclude from the oxygen isotopic character of the garnets and ages of the zircons that the source rocks for the jingshan granites are from dabie sulu orogen representing the south china craton

isotopic study of the manaslu granite himalaya nepal inferences on the age and source of himalayan leucogranites a detailed isotopic study of the manaslu leucogranite was carried out a u pb age of 25 ma and a whole rock rb sr age isochron of 18 ma were obtained suggesting that the magmatic activity lasted at least 7 ma initial sr isotopic ratios are very high 0 740 to 0 760 and initial nd isotopic ratios are low nd in 13 to 16 and they show the existence of large isotopic variations even at the metre scale these are not the result of perturbations by fluids but rather they reflect the initial isotopic heterogeneity of the source material which has not been obliterated by magmatic processes e g fusion mixing by convection these results also support the crustal origin of this leucogranite the tibetan slab paragneisses whose sr and nd isotopic ratios are very similar to those of the granite at an age of 20 ma are the most probable parental material nd model ages for both the leucogranite and the gneisses are in the range 1 5 2 ga a model of formation of the manaslu granite by coalescence of different batches of magma is in agreement with the present data

mid eocene high sr y granites in the northern himalayan gneiss domes melting thickened lower continental crust abstract within the himalayan collisional belt granotoids occur along two sub parallel belts the northern himalayan gneiss domes nhgd and the high himalayan crystalline series hhcs in the yardoi area of nhgd two mica granite a new type granite occurs in the core of the yardoi gneiss dome ygd dala and quedang from north to south and extends at least 50 km long these granites have similar mineral composition elemental and radiogenic isotope geochemistry and age of formation shrimp zircon u pb dating indicates that the yardoi and the quedang two mica granites formed at 42 6 1 1 ma and 42 8 0 6 ma respectively similar to the dala pluton these two mica granites have 1 high sio2 68 wt al2o3 15 wt and a cnk 1 0 2 relatively high sr and lree but low y 40 and up to 250 and la yb 30 4 very weak or no eu anomalies and 5 as compared with those in the himalayan leucogranites low initial sr 87sr 86sr i

geochemical investigations of the origin of the manaslu leucogranite himalaya nepal the himalayan orogeny is characterized by an intraplate subduction zone the thrusting of the tibetan slab over the foreland is probably responsible for the origin of the manaslu type leucogranites this geochemical study tries to examine the crustal origin of the granite to verify whether the paragneisses of the tibetan slab are the most probable parent material and to determine the subsequent petrogenetic processes the rb sr systematics suggest that 1 1 the scatter of data points in the 87sr86srvs 87rb86sr diagram may be due to the heterogeneity of the initial 87sr86sr ratios 2 2 the initial isotopic ratios are very high 0 730 0 760 and are in agreement with those of the gneisses of the tibetan slab at the time of the melting pb isotopic ratios are homogeneous they are also consistent with a crustal prehistory the ree and th contents are unexpectedly low in comparison with the much higher contents of the parent material monazite left in the residue of melting or more probably extracted in an early stage of the magmatic history may explain the observed ree and th abundances alternatively field observations show that the emplacement of the granite has been accompanied by a very strong hydrothermal activity in consequence the low ree and th contents of the granite could also be due to the loss by solution during prolonged fluid circulation throughout the granitic melt and the sr and pb isotope distribution to the effect of late to post magmatic fluids the isotopic character of which was determined by the underlying tibetan slab

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

diverse magma sources for the himalayan leucogranites evidence from b sr nd isotopes abstract the himalayan orogen is featured by widespread s type leucogranites ca 45 to 9 ma formed primarily from the partial melting of metapelites 800 480 ma and minor from the partial melting of amphibolite with subordinate metapelites the ama drime gneiss and mabja leucogranite pluton are both located at the footwall of the ns trending xainza dinggye normal fault in the central part of the himalayan orogen with published end t values of 21 0 to 19 6 and 19 3 to 18 3 and 87sr 86sr t values of 0 90954 0 92574 and 0 84853 0 85474 respectively in this study we presented new shrimp zircon u pb ages and laser ablation multi collector inductively coupled plasma mass spectrometry la mc icp ms tourmaline boron isotope data on the metamorphic rocks and leucogranites of the himalayan orogen the weighted mean 207pb 206pb age of the zircon cores of the ama drime gneiss t0446 2 3 is 1854 4 ma the tourmaline 11b values of the ama drime gneiss t0446 1 6 are 17 6 to 14 3 similar to those of the mabja leucogranite t0436 4 18 9 to 17 4 in contrast the tourmaline 11b values of the quedang metapelite t0389 18 and its partial melting product malashan leucogranite t0659 12a 3 are substantially higher i e 15 3 to 12 5 and 16 2 to 8 0 respectively the tourmaline 11b values of the yadoi leucogranite t0321 4 derived from partial melting of amphibolite range from 8 4 to 5 4 therefore the tourmaline b isotopes and sr nd isotopes results are consistent and may have reflected an e w extension along the southern tibetan rift system indicating a late miocene anatexis in the north himalaya region the melts derived from the partial melting of metapelites and mature crustal materials e g paleoproterozoic ama drime gneiss ascended along the n s trending xainza dinggye normal fault and were subsequently emplaced in the tethyan himalaya sequence

experimental determination of the solubility of ce monazite in sio2 al2o3 k2o na2o melts at 800 c 2 kbar under h2o saturated conditions the solubility of cerium monazite cepo4 in sio2 al2o3 k2o na2o melts was determined at 800 c 2 kbar under water saturated conditions dissolution experiments using synthetic ce monazite crystals and crystallization experiments using ce and po4 bearing glasses yield results in good agreement microprobe analyses of glasses using a sensitive method detection limit to 130 ppm show that the solubility of monazite is dependent on the melt composition and is especially low for peraluminous compositions application of these data to various geologic occurrences is in good agreement with petrographic and geochemical observations and shows that monazite fractionation is probably a very common phenomenon in two mica granites this provides a good explanation for the light ree and th depletion commonly observed in such rocks

petrogenesis of migmatites in maine usa possible source of peraluminous leucogranite in plutons high t low p facies series conditions during deformation within a devonian crustal scale shear zone system defined by kilometer scale straight belts of apparent flattening strain that anastomose around lozenges of apparent constrictional strain at upper amphibolite facies grade metapelites are partially melted the onset of which is introduction