fluid absent melting of a layered crustal protolith implications for the generation of anatectic granites we report the result of h2o undersaturated melting experiments on charges consisting of a layer of powdered sillimanite bearing metapelite hq36 and a layer of powdered tonalitic gneiss agc150 experiments were conducted at 10 kbar at 900 925 and 950 c when run alone the pelite yielded 40 vol strongly peraluminous granitic melt at 900 c while the tonalite produced only 5 vol weakly peraluminous granitic melt at 950 c the pelite and the tonalite yielded 50 vol and 7 vol granitic melt respectively when run side by side the abundance of melt in the tonalite was 10 times higher at all temperatures than when it was run alone in the pelite the melt abundance increased by 25 vol when run alone biotite dehydration melting in the tonalite yielded orthopyroxene and garnet in addition to granitic melt when run side by side only garnet was produced in addition to granitic melt experiments of relatively short duration however also contained al rich orthopyroxene we suggest that the large increase in melt fraction in the tonalite is mainly a result of increased activity of al2o3 in the melt which lowers the temperature of the biotite dehydration melting reaction in the pelite the increase in the abundance of melt is caused by transport of plagioclase component in the melt from the tonalite layer to the pelite layer this has the effect of changing the bulk composition of this layer in the direction of minimum temperature granitic liquids our results show that rocks which are poor melt producers on their own can become very fertile if they occur in contact with rocks that contain components that destabilize the hydrous phase s and facilitate dehydration melting because of this effect the continental crust may have an even greater potential for granitoid melt production than previously thought our results also suggest that many anatectic granites most likely contain contributions from two or more different source rocks which will be reflected in their isotopic and geochemical compositions

boudinage and composite boudinage in superposed deformations and syntectonic migmatization abstract the banded gneisses of the jasidih deoghar area of east india behaved as a complex geological multilayer which evolved through the combined effects of deformation and leucosome emplacement the early foliation and banding were transposed and retransposed by two generations of isoclinal folds the intensity of foliation development the layer thickness ratios and the rheological contrasts within the gneisses changed both in space and in time under layer normal compression these complex multilayers gave rise to a variety of boudins and of their pegmatite filled separation zones the geometrical relations with successive generations of folds can distinguish the closely associated pre f 1 syn f 1 and syn f 2 boudins d 2 boudinage occurred at an advanced stage of f 2 folding so that boudinage induced subhorizontal extension was considerably less than the total subhorizontal extension during d 2 composite boudinage occurred during progressive syntectonic leucosome emplacement in the course of which a group of boudinaged layers with a profusion of coarse quartz feldspar leucosome became more competent than the adjoining rocks and underwent boudinage as a whole

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

an extensive crustally derived 3325 to 3310 ma silicic volcanoplutonic suite in the eastern pilbara craton evidence from the kelly belt mcphee dome and corunna downs batholith abstract new sensitive high resolution ion microprobe zircon u pb dating indicates that the eastern pilbara craton contains an extensive suite of variably deformed 3325 to 3310 ma rhyolites high level quartz feldspar porphyries and granitoids these range in composition from na rich trondhjemite low k rhyolite to k rich monzogranite with silica rich 68 biotite hornblende bearing porphyritic granodiorites and monzogranites most abundant initial volcanism accompanied regional extension at 3325 ma and was followed by intrusion of high level porphyritic stocks and emplacement of massive to foliated batholithic granitoids at 3315 ma the compositions of this volcano plutonic suite suggest that the most likely source for parent magmas is older continental crust with tonalite trondhjemite granodiorite suite compositions crustally derived silicic volcanic rocks and granitoids have traditionally been considered rare in pre 3 0 ga granitoid greenstone terranes evidence from the eastern pilbara indicates that magmatic recycling of continental crust has occurred at least as early as 3 3 ga

pressure temperature fluid evolutionary history of the polymetamorphic sandmata granulite complex northwestern india abstract an assemblage of granulite facies metamorphosed pelitic quartzofeldpathic basic and calc silicate rocks is exposed within amphibolite facies tonalitic gneisses amphibolites and pelitic schists belonging to the banded gneissic complex at sandmata northwestern india the granulite facies rocks are bounded by a ductile shear zone and are intruded by enderbite and metanorite reaction textures geothermobarometric data and phase equilibria considerations show that a the granulite facies rocks were metamorphosed at ca 8 kbar 820 c during an early phase of metamorphism b enderbite and metanorite were emplaced subsequent to this metamorphism c all the rocks including the surrounding amphibolite facies rocks were subsequently metamorphosed at ca 6 5 7 kbar 650 700 c when coronal garnet appeared in pyroxene bearing lithologies and d the rocks were affected by non pervasive hydrous fluid flux either prior to or during the later metamorphism it is suggested that the deep crustal granulite facies rocks were juxtaposed against the amphibolite facies rocks along the ductile shear zone during this later phase of metamorphism interpretation of available geochronologic data suggests that the granulite facies metamorphism occurred prior to 1 7 ga and that the entire terrain was possibly reworked at ca 1 ga grenvillian orogeny

timescales and significance of high pressure high temperature metamorphism and mafic dike anatexis snowbird tectonic zone canada new geochronological isotopic and geochemical data for a spectacular swarm of deep crustal migmatitic mafic dikes offer important insight into processes operative during 1 9 ga high pressure high temperature metamorphism along the snowbird tectonic zone in northern saskatchewan high precision u pb zircon dates reveal anatexis of chipman mafic dikes at 1 896 2 0 3 ma during syntectonic and synmetamorphic intrusion at conditions of 1 0 1 2 gpa 750 c u pb zircon dates of 1 894 1 891 ma for cross cutting pegmatites place a lower bound on major metamorphism and deformation at the currently exposed crustal levels the persistence of elevated temperatures for 14 m y following peak conditions is implied by younger u pb titanite dates and by sm nd whole rock isotopic data that suggest the derivation of the pegmatites by melting of a mafic source limited melting of the host felsic gneiss at 1 9 ga despite high temperature is consistent with evidence for their previous dehydration by granulite facies metamorphism in the archean spatial heterogeneity in patterns of mafic dike and tonalitic gneiss anatexis can be attributed to lateral peak temperature and compositional variability we correlate 1 896 ma chipman mafic dike emplacement and metamorphism with substantial 1 9 ga mafic magmatism over a minimum along strike extent of 1 200 km of the snowbird tectonic zone this suggests a significant continent wide period of asthenospheric upwelling that induced incipient continental rifting extension was subsequently terminated by hinterland contraction associated with trans hudson accretion and orogenesis little activity in the lower crust for ca 650 m y prior to proterozoic metamorphism and mafic magmatism implies an extended interval of cratonic stability that was disrupted at 1 9 ga this episode of destabilization contrasts with the record of long term stability in most preserved cratons and is important for understanding the lithospheric characteristics and tectonic circumstances that control the destruction or survival of continents

melting of continental crust during subduction initiation a case study from the chaidanuo peraluminous granite in the north qilian suture zone the chaidanuo granite batholith is a similar to 500 km 2 homogeneous high level intrusion in the north qilian oceanic suture zone three types of enclaves have been recognized including 1 supracrustal xenoliths 2 biotite gneiss that represents restite of the upper crustal melting and 3 coeval mantle derived mafic magmatic enclave mme the batholith consists dominantly of peraluminous biotite monzogranite with sio2 69 73 wt and a cnk 1 05 1 28 and shows geochemical affinity with the upper continental crust e g enrichment of large ion lithophile elements liles k rb u th pb depletion of high field strength elements hfses nb ta ti p eu and sr and enriched sr sr 87 sr 86 i 0 731 to 0 744 nd epsilon nd t 6 0 to 7 1 isotopes zircon u pb dating indicates that this batholith formed at 516 505 ma coeval with the mmes 510 ma which represent the early stage of seafloor subduction in the north qilian suture zone the hf isotopic composition of the mme epsilon hf t 6 0 to 2 1 is more depleted than that of the host granite epsilon hf t 14 8 to 4 7 suggesting mixing of mantle and crust derived melts the inherited zircon cores in the host granite yield an age peak at similar to 750 ma with a few detritals of 591 1683 ma zircons from a biotite gneiss enclave yield a protolith age of 744 ma and a metamorphic melting event at 503 ma sr nd isotopic modeling reveals that the batholith was generated by melting of neoproterozoic granitic rocks with the addition of 10 18 mantle derived magma therefore peraluminous granite can be produced by melting of crustal materials heated by mantle derived magmas during the early stage of subduction initiation at the site of a prior passive continental margin c 2014 elsevier ltd all rights reserved

late collisional granites in the variscan erzgebirge germany the late collisional erzgebirge granites 325 318 ma were introduction emplaced at shallow crustal levels in the variscan metamorphic the erzgebirge is a ne sw trending antiformal structure basement shortly after large scale extension caused by orogenic exposing variscan crystalline basement rocks at the northcollapse these granites comprise mildly peraluminous transitional ern margin of the bohemian massif the region is famous i s types and strongly peraluminous s type rocks which can be among economic geologists for its extensive and highly subdivided into three major groups low f biotite granites low f variable metallic ore deposits which have been mined two mica granites and high f high p2o5 li mica granites the since the middle ages e g stemprok seltmann 1994 highest degree of differentiation is reached in the li mica granites tischendorf forster 1994 and for the classic studies which exhibit strongly elevated concentrations of p f li rb cs in the fields of economic geology mining and metallurgy ta sn w and u but very low ti mg co ni sr ba y zr chemistry and mineralogy e g werner 1791 breithaupt hf th and rare earth elements crystal melt fractionation is the 1849 agricola 1974 dominant process controlling the bulk composition of all groups of as in many of the other variscan provinces in central granites however metasomatic processes involving late stage residual europe granitic rocks make up a large proportion of the melts and high t orthomagmatic fluids became increasingly more exposed rocks in the erzgebirge and a great variety of important in highly evolved units and have modified the abundances compositional and textural types occur fig 1 knowof mobile elements p f li rb cs ba sr in the li mica ledge of the processes involved in the generation and granites particularly isotopic and geochemical characteristics suggest evolution of these silicic magmas is essential for underthat the three granite groups cannot be derived from a common standing heat and mass transport during the variscan precursor magma their discrete compositions are source related orogeny and also for the formation of metallic ore and are attributed to melting of quartzo feldspathic and pelitic deposits several lines of evidence indicate that early crustal lithologies in different proportions granites are common in high temperature sn w deposits are associated with the the central european variscides but the erzgebirge is unusual for emplacement of highly evolved volatile rich granitic the predominance of evolved li mica granites associated with melts tischendorf 1986 forster tischendorf 1992 economically important sn w and u deposits the abundance of stemprok 1993 the role of the granites in formation of li mica granites is attributed to a combination of favourable factors later permian u pb zn and younger ore mineralization 1 low degrees of anatectic melting of crustal protoliths 2 wide f ba is controversial but good evidence exists that the distribution of fertile lithologies rich in large ion lithophile elements vein type u deposits owe their uranium to leaching from and ore elements 3 extended magmatic differentiation by crystal the uraninite rich granites e g tischendorf forster melt fractionation and subsequent autometasomatism 1994

late quaternary alluvial stratigraphy of whitewater draw arizona implications for regional correlation of fluvial deposits in the american southwest the alluvial history of whitewater draw an arroyo in the sulphur springs valley southeastern arizona is characterized by numerous degradational and aggradational events shifts in climate appear to be responsible for the major changes in depositional environments recognized in whitewater draw over the past 15 000 yr however the degradation and aggradation documented during apparently stable climatic periods were primarily controlled by geomorphic parameters comparison between the alluvial records of arroyos in the adjacent upper san pedro valley and whitewater draw shows that periods of degradation and aggradation were out of phase in number character and timing these differences indicate that the fluvial systems in the sulphur springs valley and the san pedro valley responded differently to external climate shifts and that both systems were influenced by local geomorphic parameters this demonstrates that regional correlation of late quaternary deposits from one valley to the next should not be attempted without absolute temporal control and that intervalley correlations must take into consideration the complexity of fluvial processes

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

multiple zircon growth within early archaean tonalitic gneiss from the ancient gneiss complex swaziland abstract u pb age determinations by ion microprobe reveal multiple episodes of zircon growth and recrystallization within a single sample of tonalitic gneiss from the ancient gneiss complex the oldest episode at3644 4ma 2 produced the dominant type of zircon characteristically purplish brown and massive in texture this probably constitutes unaltered zircon precipitated from the original magma recrystallization accompanied and obscured by early pb loss took place within the oldest grains at3504 6ma and3433 8ma whole new grains grew at these times also we interpret the post 3644 ma growth as due to episodic deformational and metamorphic events that transformed the tonalite pluton into foliated banded gneiss in addition many grains are visibly overgrown by two layers of younger zircon of different colour and texture dated at2986 20ma and2867 30ma euhedral finely zoned whole grains having the 2986 ma age are present also evidently contributed by very thin felsic veins associated with the nearby lochiel granite the age of3644 4ma combined with precise zircon u pb dating of volcanics from the onverwacht group reported elsewhere demonstrates that at least part of the ancient gneiss complex is older than the barberton greenstone belt

neoproterozoic high k granites produced by melting of newly formed mafic crust in the huangling region south china abstract the huangling igneous complex in the central yangtze block south china consists of the 850 ma tonalite and trondhjemite which were intruded by the 826 13 ma high k granite the granites are composed of plagioclase 30 60 alkali feldspar 10 30 quartz 20 30 biotite 2 63 1 73 3 wt k 2 o 2 58 4 79 wt and na 2 o 3 90 5 89 wt and low mgo 0 49 1 78 wt and are magnesian alkali calcic in composition their chondrite normalized ree patterns are characterized by enrichment of lree la yb n 14 5 32 8 and mostly negative eu anomalies eu eu 0 5 1 1 they have moderately high sr 202 698 ppm and low y 6 13 18 7 ppm with sr y ratios of 14 64 these large elemental variations probably resulted from fractional crystallization they also have negative whole rock nd 9 75 to 11 73 and zircon hf values 7 1 to 15 8 which are similar to those of the neoproterozoic mafic rocks in the same region suggesting that the granites may have been produced by melting of newly emplaced mafic rocks tonalites and trondhjemites of the huangling complex previously were considered as products by differentiation of water rich basaltic magmas and melting of the thickened archean continental crust in an arc setting respectively therefore the crustal growth and reworking of the yangtze block were probably wholly controlled by the subduction system which resulted in formation of the stable continent during the neoproterozoic

origins and ages of proterozoic granitoids in the bothnian basin central sweden isotopic and geochemical constraints abstract the paleoproterozoic bothnian basin in central sweden and south central finland is dominated by a 10 km thick sequence of metasediments mainly metaturbidites these have been intruded by granitoids which have traditionally been classified as early late and postorogenic relative to the c 1 9 1 8 ga s vecofennian orogeny the early orogenic granitoids are part of a calcalkaline suite ranging in composition from ultramafic to granitic while the late orogenic granitoids mainly are purely granitic and associated with abundant pegmatites the postorogenic group forms large massifs of mainly granitic composition and associated pegmatites are rare an early orogenic tonalite yields a u pb zircon age of 1877 6 ma which is within the age range for s vecofennian granitoids of this tectonic position elsewhere in sweden and finland a late orogenic granite belonging to the harno granites from the southern part of the bothnian basin gives a u pb monazite age of 1822 5 ma this provides a minimum age for the main phases of svecofennian deformation in the region and a correlation with the 1 84 1 83 ga old granite migmatite zone of southern finland is suggested a postorogenic granite belonging to the revsund granitoids gives a u pb zircon age of 1798 8 ma together with previously published age data this indicates that the revsund granitoids were emplaced successively during an age interval of at least between 1 80 and 1 77 ga ree patterns and trace element model calculations indicate that late orogenic granites could have been produced by partial melting of metasediments similar to those exposed at the surface a contribution from the early orogenic granitoids is possible but not discerned by these calculations the postorogenic granites on the other hand have too high concentrations of sr ba y and hree to be derived from such a metasedimentary source the initial e nd values for the early orogenic granitoids range between 2 and 1 corresponding values for both late and postorogenic granites range between 1 and 3 which is on average slightly higher than those for surrounding metasediments the initial sr isotope ratios for the harno granites are intermediate between those for the metasediments and the early orogenic granitoids while the initial sr isotope ratios for the revsund granites are distinctly lower the geochemical and isotopic data support a model in which the harno granites were derived mainly from bothnian basin metasediments with a contribution from early orogenic granitoids the composition of the revsund granitoids is however inconsistent with such a derivation for these rocks a deeper as yet unknown crustal source with a significant pre 1 9 ga crustal component is indicated

late mesozoic post collisional intermediate to silicic magmatism in the badjal area far east of russia abstract the late cretaceous badjal intrusive suite at the far east of russia includes a spectrum of rocks having emplaced successively in four phases from i diorite and quartz diorite to ii granodiorite iii granite and iv high silica granite various facies being distinguished within the phases the generation of these rocks took place after the collision of a number of island arcs and a terrane now locally preserved in sikhote alin region with the asian continent that had happened in the end of early to the beginning of late cretaceous the massifs intrude in early mesozoic sedimentary and volcanic sedimentary rocks topping the basement and in comagmatic volcanic rocks chemically the granitoid rocks have high k calc alkaline character and form continuous and regular trends of most of major oxides and trace elements with the sio2 contents ranging from 55 to 77 wt that favors the concept of crystal fractionation major oxides and ree have a break at 70 wt sio2 pointing to a change of precipitating mineral assemblage such change is also documented by the negative eu anomaly we believe that the fractionation of mafic minerals had to take place at the first stage of fractionation whereas plagioclase and possibly biotite began precipitating later as supposed by abrupt decrease of ba this hypothesis is in accordance with the mass balance calculations diorite magma compositionally similar to the first intrusive phase rocks should be taken for parental for the entire badjal suite small volume of these oldest rocks makes us suggest that the parental magma was a mixture of silicic liquid and restite mineral phases that would result from the partial melting of a heterogeneous metapelite basaltic or metapelite amphibolitic protolith almost total absence of basalt and gabbro of close age in the area makes a suggestion of crustal silicic contamination of a mantle produced basic magma unlikely after the melting the differentiation of the derived magmatic mixture during ascent and the separation of mineral phases acquired from the protolith restite phases and of newly formed ones are believed to produce the entire range of rocks of the badjal intrusive suite provided the progressive succession of emplacement from less to more silicic species

evidence from mineral assemblages for infiltration of pelitic schists by aqueous fluids during metamorphism prograde mineral assemblages and compositions have been predicted for pelitic schist in the 10 component system na2o k2o cao mno feo mgo al2o3 sio2 co2 h2o for three cases of prograde metamorphism and fluid rock interaction 1 increasing temperature t at constant pressure p and constant pore fluid volume 1 without infiltration no infiltration case 2 increasing t at constant p accompanied by sufficient fluid infiltration that fluid composition is at all times constant large flux case and 3 increasing t at constantp accompanied by a timeintegrated fluid flux f 104 cm3 cm 2 intermediate flux case stable mineral assemblages and compositions were calculated by solving a system of non linear equations that specify mass balance and chemical equilibrium between minerals and fluid the model pelitic system includes quartz muscovite plagioclasc chlorite ankerite siderite biotite garnet staurolite andalusite kyanite sillimanite k feldspar and a coexisting binary h2o co2 fluid specifically prograde thermal metamorphism was modelled for shaw s 1956 average low grade pelite and for a moderate range of bulk rock compositions at p 3 5 and 7 kb and initial fluids with xco 2 o 0 02 0 40 the model predicts a carbonate bearing mineral assemblage for average pelite under chlorite zone conditions composed of quartz muscovite albite chlorite ankerite and siderite the mineral assemblages predicted for the noinfiltration case are unlike those typically observed in regional metamorphic terranes simulations of metamorphism for the large flux and intermediate flux cases however reproduce the sequence of mineral assemblages observed in normal barrovian regional metamorphic terranes these results suggest that regional metamorphism of pelitic schists is typically associated with infiltration of significant quantities of aqueous fluid

insights into the complexity of crustal differentiation k2o poor leucosomes within metasedimentary migmatites from the southern marginal zone of the limpopo belt south africa differentiation of the continental crust is the result of complex interactions between a large number of processes which govern partial melting of the deep crust magma formation and segregation and magma ascent to significantly higher crustal levels the anatectic metasedimentary rocks exposed in the southern marginal zone of the limpopo belt represent an unusually well exposed natural laboratory where the portion of these processes that operate in the deep crust can be directly investigated in the field the formation of these migmatites occurred via absent incongruent melting reactions involving biotite which produced cm to m scale k2o poor garnet bearing stromatic leucosomes with high ca na ratios relative to their source rocks field investigation combined with geochemical analyses and phase equilibrium modelling designed to investigate some aspects of disequilibrium partial melting show that the outcrop features and compositions of the leucosomes suggest several steps in their evolution 1 melting of a portion of the source with restricted plagioclase availability due to kinetic controls to produce a magma melt entrained peritectic minerals in variable proportions relative to melt 2 segregation of the magma at near peak metamorphic conditions into melt accumulation sites mas a k a future leucosome 3a reequilibration of the magma with a portion of the bounding mafic residuum via chemical diffusion h2o k2o which triggers the co precipitation of quartz and plagioclase in the mas 3b extraction of melt dominated magma to higher crustal levels leaving peritectic minerals entrained from the site of the melting reaction and the minerals precipitated in the mass to form the leucosome in the source the key mechanism controlling this behaviour is the kinetically induced restriction of the amount of plagioclase available to the melting reaction this results in elevated melt h2o and k2o and chemical potential gradient for these components across the leucosome mafic residuum contact the combination of all of these processes accurately explains the composition of the k2o poor leucosomes these findings have important implications for our understanding of melt segregation in the lower crust and minimum melt residency time which according to the chemical modelling is 5 years we demonstrate that in some migmatitic granulites the leucosomes constitute a type of felsic refractory residuum rather than evidence of failed magma extraction this provides a new insight into the ways that source heterogeneity may control anatexis this article is protected by copyright all rights reserved

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

hornblende dehydration melting in mafic rocks and the link between massif type charnockite and associated granulites eastern ghats granulite belt india a massif type intrusive charnockite body in the eastern ghats granulite belt india is associated with hornblende bearing mafic granulite two pyroxene granulite and enderbitic granulite the charnockite is characterised by pervasive gneissic foliation s1 this is axial planar to the folded layers of hornblende bearing mafic granulite f1 folds indicating that the granulite protoliths were present before the development of s1 two pyroxene granulite and enderbitic granulite occur as lenticular patches disposed along the foliation and hence could be syngenetic to s1 the tonalitic to granodioritic metaluminous to weakly peraluminous compositions and relatively high sr rb of the charnockite are consistent with its derivation by partial melting of a mafic protolith strong y depletion lack of sr depletion and strongly fractionated ree patterns with high la yb n ratio but relatively lower hree gd lu fractionation with marked positive eu anomalies suggest major residual hornblende as well as garnet but not plagioclase consistent with the hornblende dehydration melting in the source rocks such a residual mineralogy is broadly similar to those of some of the hornblende bearing mafic granulite inclusions which have compositional features indicative of a restitic nature quantitative modelling supports an origin for the charnockite melts by partial melting of a hornblende rich mafic granulite source although source heterogeneity is very likely given the rather variable trace element contents of the charnockite the whole rock and mineral compositions of the two pyroxene granulites and enderbitic granulites are consistent with them representing peritectic phase segregations of hornblende dehydration melting a clockwise p t path implies that melting could have occurred in thickened continental crust undergoing decompression

evolution of high mg al granulites from sunkarametta eastern ghats india evidence for a lower crustal heating cooling trajectory a suite of high mg al granulites from sunkarametta eastern ghats belt india shows contrasting prograde assemblages of extremely aluminous orthopyroxene cordierite sapphirine and similarly aluminous orthopyroxene ti rich spinel in closely associated domains textural and compositional characteristics indicate that both were derived from prograde dehydration melting of biotite plagioclase quartz bearing protoliths the former assemblage was stabilized at relatively more magnesian bulk composition geothermobarometric data and petrogenetic grid considerations place peak metamorphic conditions at c 950 c and 9 kbar subsequent to peak metamorphism the rocks cooled to c 700 750 c with slight lowering of pressure and the retrograde reactions also involved melt solid interaction the inferred p t trajectory is one of heating cooling at lower crustal 25 30 km depths

calculated relationships between activity of alumina and phase assemblages of silica saturated igneous rocks petrogenetic implications of magmatic cordierite garnet and aluminosilicate abstract many peraluminous igneous rocks are characterized by the presence of al rich minerals such as cordierite garnet and aluminosilicate polymorphs clearly one of the important thermodynamic intensive variables controlling the stability of these minerals relative to al poor phases such as ortho and clinopyroxene must be the activity of al 2 o 3 a al 2 o 3 in the melt calculated phase relationships of anhydrous mineral assemblages in equilibrium with quartz saturated liquids show that at crustal pressures less than about 10 kbar garnet cordierite and aluminosilicate are only likely to crystallize from melts in which a al 2 o 3 is up to one order of magnitude higher than that required for orthopyroxene clinopyroxene crystallization in spite of this peraluminous granitoid liquids saturated with orthopyroxene garnet or orthopyroxene cordierite do not ordinarily contain significantly larger amounts of excess al al in excess of that required to form feldspars than peraluminous liquids saturated with orthopyroxene clinopyroxene these observations indicate that there must be profound compositional controls on the activity coefficient of al in silicic melts and the strongest of such controls appears to be the total alkali content of the melt the activity coefficient of al in silicic melts varies directly with total alkali oxide content so that alkali poor melts can become markedly peraluminous without ever becoming saturated in aluminous phases whereas alkali rich melts of similar normative corundum content can crystallize garnet cordierite or even aluminosilicate because activity of alumina in a silicic melt is not a simple function of excess al content classification schemes for silicic igneous rocks based on model mineralogical assemblages which reflect the value of the thermodynamic effective concentration of al during crystallization convey more petrologic information than those based on chemical parameters such as normative corundum content or alumina saturation index

rb sr and u pb isotope studies on migmatites from the schwarzwald germany constraints on isotopic resetting during variscan high temperature metamorphism mineral and isotope studies were undertaken on migmatites from the schwarzwald moldanubian zone of the variscan belt the aims of the study were to date the migmatite formation and to determine the processes involved in migmatization in order to evaluate their influence on isotopic resetting textural evidence and the comparison of mineral compositions from leucosomes and mesosomes of two centimetre scale migmatite profiles respectively suggest that migmatitic textures and mineral assemblages were formed by metamorphic segregation deformation enhanced mass transport rather than by partial melting anatexis the results of rb sr thin slab dating on these profiles indicate that sr isotopes were not completely reset during migmatization no true isochron ages but ages of approximate isotopic homogenization were obtained on the thin slabs by calculating 87sr 86sr ratios back to various stages in their evolution the coincidence of these rb sr data with u pb ages of monazites from migmatites and non migmatitic gneisses shows that gneisses and migmatites were formed during the same high temperature event in the carboniferous 330 335 ma the observation that high temperature metamorphism failed to equilibrate sr isotopes on the centimetre scale imposes limitations on the use of conventional whole rock isochron techniques in dating migmatites

geochronology petrogenesis and tectonic significance of peraluminous granites from the chinese altai nw china abstract the paleozoic granites in the chinese altai are important for the study of tectonic evolution and crustal growth in the central asian orogenic belt caob four representative peraluminous granitic intrusions were selected for systematic studies of zircon u pb and hf isotopic compositions and whole rock geochemical and nd sr isotopic analyses these rocks have high asi alumina saturation index al 2 o 3 cao na 2 o k 2 o 1 01 1 46 molecular ratios with 0 6 5 6 wt of normative corundum and are characterized by moderately negative eu anomalies eu eu 0 38 0 98 and strong depletion in ba nb and sr elements our data suggest that these intrusions were emplaced from 419 to 393 ma consistent with a period of intensive magmatic activities and high temperature metamorphism in the chinese altai while in situ zircon hf isotopic analyses for these granites give predominantly positive e hf t values 0 8 to 12 8 a few inherited zircons yield negative e hf t values from 12 5 to 1 53 the u pb age and hf isotopic data of these inherited zircons are similar to that of the widespread metasediments in addition the peraluminous granitic rocks have near zero or negative e nd t values 3 3 to 0 5 and relatively high initial 87 sr 86 sr ratios 0 7079 0 7266 distinct from those of the i type granites in the study region but similar to the early paleozoic habahe sediments these isotopic compositions suggest that the newly accreted metasediments of habahe group may be the major source rock of the peraluminous granites the geochemical compositions indicate that their precursor magmas were derived from a relatively shallow crustal level p 5 kbar and zircon saturation temperatures suggest that these granitic intrusions were emplaced at 672 861 c the peraluminous granitic magmas were generated by dehydration melting of newly accreted materials which were possibly brought to at least middle crustal depth by subduction related processes in an active margin and were subsequently molten by strikingly high ambient temperature probably caused by upwelling of the hot asthenosphere associated with ridge subduction in the paleozoic

petrology and geothermobarometry of granulite facies metapelites from the hisoy torungen area south norway new data on the sveconorvegian p t t path of the bamble sector the high grade rocks metapelite quartzite metagabbro of the hisoy torungen area represent the south westernmost exposures of granulites in the proterozoic bamble sector south norway the area is isoclinally folded and a metamorphic p t t path through four successive stages m1 m4 is recognized petrological evidence for a prograde metamorphic event m1 is obtained from relict staurolite chlorite albite staurolite hercynite ilmenite cordierite sillimanite fine grained felsic material quartz and hercynite biotite sillimanite within metapelitic garnet the phase relations are consistent with a pressure of 3 6 0 5 kbar and temperatures up to 750 850 c m1 is connected to the thermal effect of the gabbroic intrusions prior to the main m2 sveconorwegian granulite facies metamorphism the main m2 granulite facies mineral assemblages quartz plagioclase k feldspar garnet biotite sillimanite are best preserved in the several metre wide al rich metapelites which represent conditions of 5 9 9 1 kbar and 790 884 c these p t conditions are consistent with a temperature increase of 80 100 c relative to the adjacent amphibolite facies terranes no accompanying pressure variations are recorded up to 1 mm wide fine grained felsic veinlets appear in several units and represent remnants of a former melt formed by the reaction bt sil qtzgrt lq this dehydration reaction together with the absence of large scale migmatites in the area suggests a very reduced water activity in the rocks and xh2o 0 25 in the c o h fluid system was calculated for a metapelitic unit a low but variable water activity can best explain the presence or absence of fine grained felsic material representing a former melt in the different granulitic metapelites the strongly peraluminous composition of the felsic veinlets is due to the reaction grt former melt silcrd bt qtz h2o which has given poorly crystalline cordierite aggregates intergrown with well crystalline biotite the cordierite and biotite producing reaction constrains a steep first stage retrograde relative to m2 uplift path decimetre to metre wide strongly banded metapelites quartz plagioclase biotite garnet sillimanite inter layered with quartzites are retrograded to m3 amphibolite facies assemblages a p t estimate of 1 7 5 6 kbar 516 581 c is obtained from geothermobarometry based on rim rim analyses of garnet biotite plagioclase sillimanite quartz assemblages and can be related to the isoclinal folding of the rocks m4 greenschist facies conditions are most extensively developed in millimetre wide chlorite rich calcite bearing veins cutting the foliation

the ghingee granite tamil nadu south india geochemistry and petrogenesis abstract the palaeo proterozoic ghingee granite is an anatectic granite formed in high grade granulite terrain by ultrametamorphism the compositional variations both in major and trace elements observed in this granite sio2 64 16 73 81 fe2o3 0 12 2 19 feo 0 12 2 80 mgo 0 10 2 19 cao 1 66 4 71 k2o 1 09 5 09 ba 223 1883 ppm cr 4 60 ppm are attributed to a source rock heterogeneity and b the tectonic disturbances that might have abruptly ended the anatectic melting process the granite is compositionally similar to perur closepet and hyderabad granites and is formed during archaean proterozoic transition by anatectic and crustal remelting processes

raman spectroscopic characterization of h2o in co2 rich fluid inclusions in granulite facies metamorphic rocks abstract water h2o has been identified during raman analyses at 150 200 c of fluid inclusions from granulites that were previously thought to contain pure co2 at room temperature h2o in a co2 rich fluid inclusion forms an optically unresolvable liquid wetting the inclusion walls during heating the h2o phase evaporates into the co2 phase to produce a homogeneous h2o co2 fluid throughout the inclusion volume and the h2o raman peak at 3640 cm 1 is clearly resolvable at elevated temperatures to test and confirm the method synthetic h2o co2 fi of known composition 87 5 mol and 95 mol co2 were also analyzed previously studied granulite samples from the adirondack mountains usa and from the kerala khondalite belt sittampundi complex and sevitturangampatti india were tested for the presence of h2o in co2 rich fluid inclusions traces of h2o were found in co2 rich fluid inclusions in quartz crystals from the adirondacks and from the kerala khondalite belt h2o was not detected in co2 rich secondary fluid inclusions in garnet from the sittampundi complex and from sevitturangampatti rather hydrogarnet was detected along the walls of inclusions in garnet as evidenced by a peak at 3661 cm 1 the hydrogarnet is interpreted to be a step daughter phase that formed by reaction of the h2o in the fluid inclusion with the garnet host during retrogression these observations confirm the presence of an h2o bearing co2 rich fluid at some time during granulite petrogenesis but do not resolve the question of whether these co2 rich fluids were trapped at peak metamorphic conditions or during retrogression or whether the fluid inclusions have reequilibrated following entrapment

geochemical signatures of mid crustal melting processes and heat production in a hot orogen the kerala khondalite belt southern india abstract garnet bearing migmatites and associated leucogranites and leucosomes of the trivandrum block in the kerala khondalite belt were formed through granulite facies dehydration melting of metasedimentary protoliths significant trace element depletions in cs zr nb and the compatible transition metal elements v cr ni cu and zn relative to average post archaean shales and model middle and upper crust recorded in all samples require their sedimentary protoliths to have been impure sandstones and greywackes rather than shales leucogranites 70 75 wt and leucosomes sio2 68 70 wt which are uniformly peraluminous and classed as s type on the basis of their a nk and asi relations form a compositional array that shows strong correlated variations between tio2 and fe2o3 and tio2 or fe2o3 with co and y these reflect coupled variations in modal garnet and ilmenite and require the presence of up to 15 20 wt of entrained peritectic garnet in the higher y and hree leucosomes the leucosomes have ree patterns with normalised la sm of 10 negative eu anomalies eu yb collectively support petrological and field observations that the leucosomes are mixtures between former melt and entrained peritectic garnet and ilmenite the leucogranites are the products of melt extraction and migration on metres to several metres lengthscales leucogranite nb y ta yb eu and sr ba relationships demonstrate that their chemical diversity is principally controlled by variation in peritectic phase assemblage i e proportion of entrained garnet ilmenite and k feldspar with near situ feldspar dominated fractional crystallisation and accumulation coupled with the loss of fractionated melt being secondary controls age corrected heat productions of the best segregated leucogranites that most closely approach melt compositions are 2 7 0 8 wm 3 somewhat greater than the weighted average 1 7 0 5 wm 3 of preserved migmatite in situ leucosomes and leucogranites of all types correction of the 550 ma heat production to account for an estimated 30 of migration and loss of melt from this middle crustal migmatite complex yields a syn orogenic heat production of c 2 0 0 6 wm 3 which though elevated compared with model middle crust is unlikely to have been high enough to have facilitated the near uht to uht metamorphism in the trivandrum block without the addition of heat external to the orogenic crust itself

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

introduction to special section mechanisms and consequences of melt segregation from crustal protoliths much of petrological effort has been concentrated on attempts to discern define and interpret regularly recurring patterns of igneous and metamorphic petrogenesis i do not wish to deny the existence of discernable order in petrological phenomena but every geological event is unique if we concentrate too heavily on discernment of order and pattern may we perhaps overlook or underrate the unique quality of each igneous or metamorphic episode possibly uniqueness may have as great significance as conformity to pattern when we attempt to fit the phenomena of petrology into the broad framework of geology francis j turner presidential address to the mineralogical society of america november 11 1969

petrogenesis of two granites from the nilgiri and madurai blocks southwestern india implications for charnockite calc alkaline granite and charnockite alkali a type granite link in high grade terrains abstract igneous charnockites share some of the characteristics of calc alkaline and alkali a type granites and together constitute important rock types in many high grade terrains the neoproterozoic kalpatta and munnar biotite hornblende granites intrude the upper amphibolite to granulite facies terrain of southwestern india geochemical characteristics of the kalpatta granite are similar to high k calc alkaline magnesian granitoids whereas those of the munnar granite are similar to alkali ferroan granitoids within the constraints imposed by the high temperature k rich nature of the magmas comparison with experimental studies on various granitoid source compositions and trace and rare earth element modeling the distinctive features of both granites reflect a source rock of charnockitic nature in this context the northern kerala nkm and cardamom hill cm charnockite massifs occurring near the granites were considered as probable source rock compositions both the nkm and cm charnockites consists of an intermediate low sio2 type and silicic high sio2 type with the intermediate type showing similarities to high ba sr granitoids with low k2o na2o ratios and the silicic type showing similarities to high ba sr granitoids with high k2o na2o ratios the proposed petrogenetic model involves partial melting of the intermediate nkm charnockite forming the kalpatta calc alkaline granite in contrast the munnar alkali a type granite was the product of melting followed by fractional crystallization of the intermediate cm charnockite it is suggested that granitoids that formed by melting of intermediate type charnockites show high k calc alkaline magnesian geochemical characteristics whereas those formed by intermediate type charnockite melting fractional crystallization show alkali ferroan geochemical characteristics

neoarchean to paleoproterozoic continental growth in the southeastern margin of the north china craton geochemical zircon u pb and hf isotope evidence from the huoqiu complex abstract the huoqiu complex in the southeastern margin of the north china craton ncc is dominated by neoarchean gray gneisses amphibolites and voluminous metasediments here we report the occurrence of neoarchean to paleoproterozoic rocks from drill core samples the gneisses are similar to ttg tonalite trondhjemite granodiorite in composition and show close spatial association with amphibolites geochemical characteristics such as high sr y and la yb n with steep ree patterns and trace element modeling suggest that these rocks were generated by partial melting of hydrous meta basalts amphibolites at the base of a thickened mafic continental crust leaving a rutile bearing eclogite residue la mc icp ms u pb age data from magmatic zircon grains show protolith emplacement ages of 2 76 2 71 ga subsequently widespread migmatization took place at 1 91 1 82 ga generating voluminous migmatites and high k granites hf isotopic compositions of zircon grains from the amphibolite and gneiss show ehf t values of 2 4 15 5 and 3 0 1 5 respectively the tdm2 hf model ages of the gneisses range from 2 87 to 3 14 ga and are identical to the tdm1 hf ages of amphibolites 2 84 3 16 ga within analytical uncertainty suggesting that the gneisses formed by partial melting of amphibolite and attest to large scale reworking of the ancient continental crust during neoarchean the zircon grains from the granites define two groups with regard to their hf isotopic composition the older group 1916 42 ma has ehf t values and tdm2 hf ages of 10 5 2 4 and 2 40 3 20 ga respectively whereas the younger one 1823 41 ma shows a large variation in ehf t values ranging from 18 1 to 12 5 with tdm2 hf model ages of 1 70 3 59 ga a couple of zircon grains from the younger group display consistent u pb ages and tdm2 indicating accretion of juvenile crust from depleted mantle sources during 1 82 to 1 91 ga however the dominant hf isotope features are consistent with the reworking of preexisting continental crust we therefore infer that only limited accretion of juvenile crust occurred during this time and that the paleoproterozoic 1 82 to 1 91 ga tectonics in the southeastern margin of the ncc witnessed extensive reworking of older continental crust

high resolution environmental reconstruction in sw peloponnese greece covering the last c 6000 years evidence from agios floros fen messenian plain a paleolimnological record from the central messenian plain southwestern peloponnese southern greece indicates rapid changes in the water level and chemistry of a transient lake on the flanks of the taygetos mountains during the last c 6000 years the analyses are based on diatoms as well as carbon and nitrogen isotopes from bulk sediments in a 7 5 m long sediment core retrieved from the drained fen of agios floros at the northern banks of the ancient river pamisos the sequence consists of fen peat in the uppermost section underlain by lacustrine sediments which are punctuated by two layers of clay with diatomaceous silt bands high accumulation rate is recorded in the oldest part of the section up to 23 mm yr particularly during two decadal long periods centered at c 5700 and c 5300 cal bp the diatom record reveals pronounced peaks in the planktonic taxon cyclotella distinguenda which correspond to the laminated sequences reflecting the rapid development of a deep lake with an open water

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

sediment recycling and crustal growth in the central asian orogenic belt evidence from sr nd hf isotopes and trace elements in granitoids of the chinese altay felsic igneous rocks are common constituents of volcanic arcs and contain valuable information about subduction related magmatism in this study we investigate nine granitoids with s type volcanic arc affinity from the chinese altay emplaced from 507 to 391 ma in an active subduction zone during the early middle paleozoic these granitoids are characterized by moderate to high sio2 contents 61 01 75 30 wt moderate total alkalis na2o k2o 3 43 7 64 wt and high al2o3 contents 13 29 17 18 wt negative epsilon nd t values 6 1 to 1 0 the wide range of epsilon hf t values 7 0 to 9 0 and enrichment of liles such as pb th and u all suggest that the granitoids were probably derived from the partial melting of subducting oceanic sediments and the associated mantle wedge this inference is further supported by the nd isotope data the high initial sr 87 sr 86 ratios 0 703963 0 719428 low ba th ratios 7 00 118 93 and uniformly negative end t values 6 1 to 1 0 indicate that slab derived aqueous fluids were vital in generating the initial magma of these granitoids and assimilation played only a minor role our data demonstrate that residual zircon retains a substantial amount of hf during the partial melting of oceanic sediments therefore hf may not be an effective tracer for the input of recycled sediments we conclude that sediment recycling played an important role in the generation of arc magmatism and the growth of the central asian orogenic belt caob c 2016 international association for gondwana research published by elsevier b v all rights reserved

interplay between geochemistry and magma dynamics during magma interaction an example from the sithonia plutonic complex ne greece orogenic granitoids often display mineralogical and geochemical features suggesting that open system magmatic processes played a key role in their evolution this is testified by the presence of enclaves of more mafic magmas dispersed into the granitoid mass the occurrence of strong disequilibrium textures in mineralogical phases and or extreme geochemical and isotopic variability in this contribution intrusive rocks constituting the sithonia plutonic complex northern greece are studied on the basis of mineral chemistry whole rock major trace element geochemistry and sr and nd isotopic composition sithonia rocks can be divided into a basic group bearing macroscopic mafic enclaves microscopic disequilibrium textures geochemical and isotopic evidence of magma interaction and an acid group in which most geochemical and isotopic features are consistent with a magma mixing process but macroscopic and microscopic features are lacking a two step mixing plus fractional crystallization mfc process is considered responsible for the evolution of the basic group the firststepexplainsthechemicalvariationinthemaficenclavegroup abasicmagma representedbytheleastevolvedenclaves interacted

vapour absent melting from 10 to 20 kbar of crustal rocks that contain multiple hydrous phases implications for anatexis in the deep to very deep continental crust and active continental margins

accretionary orogenesis of the chinese altai insights from paleozoic granitoids zircon u pb dating and whole rock major oxide trace element and nd sr isotope compositions have been determined for four representative granitic intrusions in the sw chinese altai in order to understand the orogenesis and history of crustal growth in the central asian orogenic belt caob the ashile and halatas bodiesare relatively small intrusions formed in the late carboniferous 318 6ma andpermian 267 4ma respectively thelargertarlang tb andkeketuohaibatholiths kb havemagmaticagesof 359 to 412 ma the granitoids consist chiefly of tonalite granodiorite and granite and are metaluminous to weakly peraluminous in composition asi 0 8 1 1 the ashile pluton is characterized by relatively high mgo 2 73 3 54 wt and sr 395 456 ppm contents with low k2o na2o ratios 0 31 0 42 and mantle like nd sr isotopic compositions endt 2 85 to 3 26 initial 87 sr 86 sr 0 7047 0 7051 it probably formed by fractional crystallization of a mantle derived magma with limited assimilation of oldcrustalmaterial theotherintrusionshavenear zeroornegative endtvalues 0 67to 4 41 andhighersrisotopiccompositions initial 87 sr 86 sr 0 7067 0 7092 indicating a mixture of juvenile and old components in their genesis these intrusions are characterized by relatively low k rb cs contents and low rb sr ratios mostly b0 6 and generally exhibit moderate to high heavy rare earth element hree concentrations ybn 8 26 and weak hree fractionation dy ybnb1 5 with or without negative europium anomalies eu eu 0 5 1 0 these characteristics are consistent with an origin by dehydration melting of a hornblendebearing mid crustal source above the garnet stability field dominated by mafic to intermediate rocks this suggests that the deep crust of the chinese altai may contain a considerable proportion of juvenile material mantle derived magmatism probably in an active extensional continental margin setting played an important role in the formation of the chinese altai acting not only as a heat sourceforcrustalmeltingbutasasourceofjuvenilecomponents therelativelysmallvolumesanddistinctgeochemicalcompositions of the ashile and halatas plutons may imply a substantial change in the geodynamic setting in the late paleozoic 2007 elsevier b v all rights reserved

geochemical and isotopic nd o evidence bearing on the origin of late to post orogenic high k granitoid rocks in the western superior province implications for late archean tectonomagmatic processes abstract the granitoid rock dominated central wabigoon subprovince of the superior province records low k trondhjemite tonalite granodiorite ttg type magmatic episodes at

vapor absent melting at 10 kbar of a biotite and amphibole bearing tonalitic gneiss implications for the generation of a type granites vapor absent melting experiments on a biotite and amphibole bearing archean tonalitic gneiss agc150 at 10 kbar and 875 to 1050 c show that amphibole breaks down from 900 to 950 c producing garnet orthopyroxene and granitic melt biotite dehydration melting produces 10 wt melt up to 950 c via incongruent melting reactions that produce garnet orthopyroxene and titanomagnetite widespread biotite dehydration melting occurs between 950 and 975 c and produces orthopyroxene magnetite titanomagnetite and 20 wt fluorine rich melt up to 0 31 wt f minor f rich 2 7 wt biotite is present even at 1000 c our experiments show that under vapor absent conditions intrusion of hot mantle derived magmas into the lower crust is necessary to initiate widespread biotite dehydration melting in rocks with compositions like agc150 we propose that the high thermal stability of biotite in agc150 suggests that this rock is residual after a previous episode of partial dehydroxylation that left behind somewhat f enriched biotite we show that dehydration melting of such f enriched biotite produces f rich granitic liquids with compositions within the range of a type granite and leaves behind a granulitic residue consisting of orthopyroxene plagioclase quartz titanomagnetite and magnetite

late neoarchean monzogranitic syenogranitic gneisses in the eastern hebei western liaoning province north china craton petrogenesis and implications for tectonic setting abstract the medium grained monzogranitic syenogranitic gneisses expose as roughly northeast southwest trending and cover 1500 km 2 in the southern part of the eastern hebei western liaoning province north china craton these plutonic gneisses have leucocratic features and weakly gneissic to massive structures and were metamorphosed under greenschist to lower amphibolite facies la icp ms zircon u pb isotopic dating reveals that the magmatic precursors of these monzogranitic syenogranitic gneisses were emplaced during 2527 2511 ma they are characterized by high contents of sio 2 71 64 78 08 and k 2 o 3 55 6 40 and low contents of cao 0 25 1 71 mgo 0 01 0 42 and na 2 o 2 29 4 96 according to their chondrite normalized ree patterns the monzogranitic syenogranitic gneisses can be subdivided into a high ree group and a low ree group studies of petrogenesis reveal that magmatic precursors of the high ree group samples were derived from partial melting of the regionally widespread porphyritic or medium grained dioritic quartz monzonitic granodioritic monzogranitic gneisses under low pressure conditions less than ca 10 kbar the magmatic precursors of the low ree group were formed by fractionated crystallization of the high ree group magma with titanite allanite epidote apatite and zircon as the major crystallization fractionation phases combined with previous investigations on the late neoarchean metavolcanic rocks and granitoid orthogneisses in the northwestern part of the eastern hebei western liaoning province the magmatic precursors of these monzogranitic syenogranitic gneisses were most likely formed in a back arc basin along the north of the eastern block north china craton

petrogenesis of archean ttgs and potassic granites in the southern yangtze block constraints on the early formation of the yangtze block abstract the southern yangtze is underlain by mesoarchean igneous rocks that were emplaced in two pulses including trondhjemites at ca 3092 3071 ma followed by potassic granites at 2920 ma these represent the oldest ttg suites identified in the southern yangtze and the oldest potassic granites in the entire yangtze block hf isotopic analysis reveals ehf t values of 2 49 7 49 and hf crustal model ages tdmc of 3601 3000 ma for zircons from the trondhjemites whereas those from the potassic granites show ehf t values of 1 79 4 18 and tdmc ages of 3453 3043 ma the trondhjemites have high sio2 mg and low al2o3 and k2o na2o and are highly evolved with low ni cr and co the trondhjemites are characterized by low sr y eu gd and la yb n and high y and hree typical of high hree and low pressure ttgs their geochemical and isotopic compositions indicate that they are derived from melting of juvenile mafic crust with little contribution from older crustal materials consistent with a plagioclase bearing basaltic source the multielement patterns of potassic granites are similar to that of the trondhjemites albeit with lower hree and y contents their high k2o values strongly fractionated ree patterns and low y contents indicate the presence of garnet and or amphibole as residual phases in the source region which indicates they formed at higher pressure than the trondhjemites the isotopic and geochemical data imply that the formation of the potassic granites could be related to reworking of older ttgs the increasing k2o na2o of the 3092 3071 ma trondhjemites to potassic 2920 ma granites corresponds with stabilization maturation and thickening of the archean continental crust in the southern yangtze the geochemical data of the ttgs in the kunming region show important differences from that of contemporaneous ttgs from the kongling complex suggesting that southern yangtze is composed by an independent old nucleus which is exotic to the northern yangtze a synthesis of petrogenesis geochronology geochemistry and isotope data of archean granites of the yangtze block provides important insights into the architecture of its archean crust we propose that the yangtze block can be subdivided into discrete northwestern northeastern and southern domains but which are all affected by late paleoproterozoic tectonothermal events suggesting that they had been assembled together by then to form the united yangtze block

petrological evolution of a suite of spinel granulites from vizianagram eastern ghats belt india and genesis of sapphirine bearing assemblages a suite of spinel cordierite granulites from viziangram eastern ghats belt india preserve mineral assemblages and reaction textures indicative of peak metamorphic conditions of 1000 c 8 10 kbar followed successively by near isobaric cooling down to 750 800 c near isothermal decompression to 4 5 kbar and late hydration p t conditions of each stage are evaluated through a combination of petrogenetic grid approach and thermobarometry sapphirine is developed in sillimanite bearing acid pegmatite veins that intruded the spinel cordierite granulite close to peak metamorphic conditions and also in the host rock in immediate contact with the pegmatite both sillimanite and sapphirine in the pegmatite are considered to be magmatic phases field observations and textural characteristics suggest that al metasomatism of the spinel cordierite granulite due to the intrusion of pegmatite was responsible for sapphirine formation in the spinel granulite

petrogenesis of mesozoic granitoids in the dabie uhp complex central china trace element and nd sr isotope evidence post collisional magmatism in the dabie orogen is characterized by the near contemporaneous intrusion of voluminous granitoids and minor mafic magmas in the late mesozoic 135 110 ma the granitoids are represented by three major batholiths in northern dabie zhubuyuan baimajian and tiantangzhai the largest zhubuyuan batholith comprises hornblende bearing intermediate rocks penghe suite and hornblende free granitic rocks huangbai suite the huangbai granites show limited variation in chemical composition but are highly variable in textures ranging from medium to coarse grained they have rather homogeneous sr and nd isotopic compositions with 87sr 86sr i 0 708 0 709 and end t 19 to 24 the penghe intermediate suite covers a wide spectrum of rock types including diorite monzodiorite quartz monzonite to monzogranitic rocks and all of them contain hornblende and mafic enclaves the existence of compositional and textural disequilibrium e g calcic and resorbed cores of plagioclase and the nature of chemical variation in these rock types indicate that magma mixing between an evolved mafic and a granitic magma was involved in their genesis fractional crystallization fc could have coupled with the process of magma mixing but with minor contamination by the upper continental crust during magma ascent isotopically the intermediate rocks show a variation in end t values from 13 7 to 18 while their 87sr 86sr i ratios vary from 0 7074 to 0 7088 the genesis and evolution of the dabie post collisional magmatic rocks may be summarized as follows intrusion of mantle derived mafic ultramafic magmas into thickened continental crust induced dehydration melting of the lower crust rocks with intermediate compositions and subsequent production of large amounts of granitic melts huangbai suite extensive interaction between the crustal and mantle derived melts through mixing produced a hybrid parental magma which subsequently underwent a fc process en route to higher crustal levels producing a wide variety of rock types as shown by the penghe intermediate rock suite injection of successive pulses of basic magma into upward mobile magma chambers could have resulted in the formation of the mafic enclaves as well as mafic dykes

midcrustal emplacement of the sausfjellet pluton central norway ductile flow stoping and in situ assimilation midcrustal 25 30 km emplacement of the dioritic sausfjellet pluton into rocks of the helgeland nappe complex central norway occurred in two stages stage 1 consists of two pyroxene hornblende gabbro and diorite stage 2 is asymmetrically zoned with a modally layered central zone of diorite and anorthosite and a western annular zone of quartz bearing monzodioritic rocks igneous layering was locally attenuated folded and boudinaged in the hyper solidus state the magmatic foliation trajectory pattern in the pluton defi nes a shallowly southwest plunging synform that crosscuts compositional zones mineral lineations plunge shallowly to moderately to the southwest the pluton intruded a major lithologic boundary within the nappe migmatitic pelitic gneisses are the main host rocks to the western part of the pluton whereas the eastern and central parts are hosted predominantly by metacarbonate rocks calc silicate marble quartzo feldspathic and dioritic xenoliths up to 200 m in length are present throughout the pluton they are most common in stage 1 and the central zone of stage 2 metapelitic xenoliths are conspicuously absent ductile fl ow during emplacement produced an 1 kmwide structural aureole in which host rock structures were defl ected into subparallelism with the steeply inward dipping margin of the pluton tight antiforms developed along the northeastern southeastern and southwestern margins amphibolite grade shear zones in the host rocks preserve plutonside up kinematic indicators in addition to the abundance of dioritic calc silicate and quartzo feldspathic gneiss xenoliths and geochemical evidence for assimilation in the western annular zone regional discordance of the pluton host rock contacts indicates that stoping was also an important emplacement process at midcrustal depths following magma emplacement foundering of the central portion of the chamber combined with possible ca 445 ma regional contraction produced the map scale synform defi ned by magmatic foliations and igneous layers this study demonstrates that stoping and assimilation may occur simultaneously with host rock ductile fl ow during magma chamber evolution at midcrustal levels and offers an explanation of why xenolith preservation may be compositionally dependent

contrasting granitic magmatism of the kalba fold belt east kazakhstan evidence for late paleozoic post orogenic events abstract the paper presents new original data and a review of previous studies on the late carboniferous early permian granitoids of the kunush and kalguty intrusive complexes from the kalba fold belt east kazakhstan these rocks formed at the initial post collisional stage of the irtish zaisan orogen in the western central asian orogenic belt caob the granitoids form isometric or nw linear intrusions inside the late devonian early carboniferous metasediments which overlap the accretionary lithology of the kalba fold belt in front of the altai active margin of the siberian continent these granitoids contain zircon grains with u pb ages of ca 308 291 ma synchronous with the peak post orogenic magmatic event at 300 ma in the caob the kunush high na granitoids with high sio2 67 72 and al2o3 15 18 contents and the na2o k2o ratio 1 94 6 43 and low hrees yb 0 22 0 93 ppm but moderate sr y ratios 47 179 mg 35 55 and ni cr are generally common to high al ttg series with non subduction geochemical signatures the kalguty granitoids mostly belong to calc alkalic to subalkalic high k series have non corundum cipw norm compositions and mainly weakly peraluminous asi 0 97 1 09 characteristics their fe 0 7 1 at 62 74 wt sio2 and relatively high y nb 2 4 and rb nb 9 18 ratios are similar to transitional compositions reported for fractionated i and a2 type post orogenic granites worldwide the kunush high na and kalguty high k granitoids formed at different depths in the crust and the parent melts were derived from metabasaltic morb p 10 15 kbar and metagranitic ttg like p

ductile nappe extrusion in constrictive strain at the origin of transverse segments of the allochthon boundary thrust in the manicouagan imbricate zone central grenville province quebec abstract separating the parautochthonous and allochthonous belts the allochthon boundary thrust abt is a major lithotectonic boundary that generally runs parallel with the length of the grenville province but with some transverse segments in this contribution we investigate one of these segments the manicouagan imbricate zone miz by adding new field based data supplemented by new u pb deformation ages to a compilation of previous research conducted on both sides of the abt in the region our results reveal striking differences between the footwall and hanging wall of the abt in terms of strain and structures and concludes that the transverse orientation of the abt is not related to folding an updated strain analysis allows us to determine that deformation of the miz was dominated by a constrictive type of strain we build on a previously proposed conceptual model to document ductile extrusion over basement ramps driven by tectonic forcing and lateral density contrast in the upper crust at ca 990 ma the insertion of the hot ductile nappe into partially molten metasediments of the surrounding parautochthonous belt was accommodated by constrictive strain within the former and flattening and transverse folding in the latter

p t conditions mechanism and timing of the localized melting of metapelites from the petronella shear zone and relationships with granite intrusions in the southern marginal zone of the limpopo belt south africa

grenville magmatism in west texas petrology and geochemistry of the red bluff granitic suite zone of regional extension as suggested by recent geophysical and the red bluff granitic suite rbgs underlies part of the franklin petrological studies mountains in west texas it was emplaced into a middle proterozoic shelf sequence at 1120 35 ma the suite is predominantly metaluminous it consists of early granitic to quartz syenitic sills a main mass of alkali feldspar granite small bodies of alkali feldspar syenite leucogranitic dikes and late stage peralkaline