forty years of ttg research abstract ttgs tonalite trondhjemite granodiorite are one of the archetypical lithologies of archaean cratons since their original description in the 1970s they have been the subject of many studies and discussions relating to archaean geology in this paper we review the ideas concepts and arguments brought forward in these 40 years and try to address some open questions both old and new the late 1960s and the 1970s mark the appearance of grey gneisses ttg in the scientific literature during this period most work was focused on the identification and description of this suite and the recognition that it is a typical archaean lithology ttgs were already recognised as generated by melting of mafic rocks this was corroborated during the next decade when detailed geochemical ttg studies allowed us to constrain their petrogenesis melting of garnet bearing metamafic rocks and to conclude that they must have been generated by archaean geodynamic processes distinct from their modern counterparts however the geodynamic debate raged for the following 30 years as many distinct tectonic scenarios can be imagined all resulting in the melting of mafic rocks in the garnet stability field the 1990s were dominated by experimental petrology work a wealth of independent studies demonstrated that melting of amphibolites as well as of mafic eclogites can give rise to ttg liquids whether amphibolitic or eclogitic conditions are more likely is still an ongoing debate from 1990s onwards one of the key questions became the comparison with modern adakites as originally defined these arc lavas are reasonably close equivalents to archaean ttgs pending issues largely revolve around definitions as the name ttg has now been applied to most archaean plutonic rocks whether sodic or potassic irrespective of their hree contents this leads to a large range of petrogenetic and tectonic scenarios a fair number of which may well have operated concurrently but are applicable only to some of the rocks lumped together in the ever broadening ttg bin

eoarchean contrasting ultra high pressure to low pressure metamorphisms 1000 c gpa explained by tectonic plate convergence in deep time abstract greenland s itsaq gneiss complex igc shows eoarchean 3600 ma 250 400 c gpa low t p high pressure and 1000 c gpa high t p metamorphic regimes demonstrating a similarity of contrasting metamorphic t p regimes from the phanerozoic back to the start of earth s rock record low t p metamorphism produced i deep crustal eclogitised mafic rocks which upon partial melting formed the tonalites dominating the igc ii 550 c 2 6 gpa conditions 250 c gpa demonstrated by an olivine antigorite titano chondrodite titano clinohumite relict assemblage within mantle slivers showing geochemical and crystallographic features of a suprasubduction environment that were exhumed into the crust by 3712 ma iii rare vestiges of 3658 ma high pressure garnet clinopyroxene granulite and iv barrovian style kyanite staurolite assemblages high t p metamorphism is shown by 3669 ma crustal melts equilibrated with orthopyroxene this was coeval to the youngest juvenile tonalitic crust in the complex latter derived by anatexis under low t p conditions and a 3670 3570 ma history of deep crust migmatisation under low pressure garnet free conditions structural geology of the igc indicates its low t p regimes coincide with crustal imbrication by compression of arc like tholeiites boninite like lavas andesites felsic intermediate volcano sedimentary rocks and chemical sedimentary rocks whereas post 3660 ma high t p metamorphism was marked by late orogenic extension exhumation and deep crustal flow with mafic underplating and partial melting generating granites thus the diversity of earth s earliest recorded geodynamic settings resembles more those of modern geodynamics than the lithological and structural relationships expected from theoretical non uniformitarian scenarios like drip tectonics in a stagnant lid regime the recognition of an ultra high pressure 250 c gpa metamorphic regime at 3700 ma in the igc removes the last argument against a form of plate tectonics operating throughout the archean hence since the start of the rock record a mobile lid plate tectonic regime contributed to interior heat loss facilitating chemical communication and feedbacks between earth s surface and its deep interior

late cretaceous subduction of the continental basement of the maya block rabinal granite central guatemala tectonic implications for the geodynamic evolution of central america the rabinal granite is a peraluminous s type composite pluton formed upon partial melting of a metasedimentary source region that fringes the southernmost north america plate in central guatemala it is therefore considered together with the intruded metasedimentary sequences to be part of the continental basement of the maya block this leucocratic k feldspar plagioclase quartz muscovite biotite granite shows increasing deformation along its southern margin where it is cut across by the dextral late cretaceous top to the ne baja verapaz shear zone although it has been recently dated at 562 453 ma isotope dilution thermal ionization mass spectrometry the new data presented here including laser ablation inductively coupled plasma mass spectrometry la icp ms u pb and 40 ar 39 ar geochronology and electron probe mineral chemistry allow us to more precisely establish the timing of intrusion and metamorphic overprinting of the rabinal granite the zircons dated by la icp ms indicate a crystallization age of 471 3 5 ma early ordovician as well as abundant inherited cores with pan african and mesoproterozoic dates laser total fusion ar ar analyses of magmatic low silica muscovite si 6 2 6 4 atoms per formula unit indicate cooling following magmatic crystallization during the mid late paleozoic and variable extents of resetting of ordovician micas during cretaceous metamorphism and deformation the pressure temperature p t conditions of the inferred ordovician metamorphism that produced partial melting of the metasedimentary source of the rabinal granite and the ascent and crystallization of the granitic melt are uncertain but a clockwise p t time path with maximum p and t of p and t of 8 5 kbar and 300 c respectively this second event dated at 70 1 0 6 ma by means of laser total fusion 40 ar 39 ar analyses on high si muscovite grains is interpreted to be the result of subduction and accretion of the basement of the maya block during the latest cretaceous likely in a transpressional tectonic regime related to the lateral collision of the maya block with the pacific farallon derived caribbean arc this finding represents the first direct evidence for latest cretaceous subduction of the metamorphic paleozoic basement of the maya block north of the baja verapaz shear zone

ttgs in the making natural evidence from inyoni shear zone barberton south africa despite the consensus that ttgs the main constituents of the archaean continental crust worldwide originated by partial melting of garnet bearing amphibolites natural evidence is scarce a large variety of archaean amphibole rich rocks including migmatitic amphibolites and hornblende rich cumulates was exhumed as a tectonic melange in the inyoni shear zone isz near barberton south africa likely at the time of emplacement of the 3 2 ga nelshoogte badplaas plutons this unique collection provides the opportunity to investigate partial melting of garnet amphibolites as well as the differentiation processes occurring in the ttg magmas en route to the surface the isz migmatitic amphibolites are characterized by quartz plagioclase leucosomes in equilibrium with garnet amphibole titanite epidote garnet compositions are characterized by high almandine and grossular contents actually the leucosomes and neosomes likely lost part of their melt component and mesosomes may have been also modified towards restite rich compositions due to melt segregation restite bearing rocks are very iron rich according to their high contents in garnet besides the mg rich compositions of the hornblende cumulates and their ree distribution patterns resemble those of the nearby tt tonalite trondhjemite plutons thermobarometric estimates from the isz migmatitic rocks cluster in the range 720 800 c and 1 1 1 2 gpa for the melting reaction and the derived geothermal gradient is ca 17 22 c km these conditions are consistent with either water present melting or more likely water absent epidote dehydration melting such a geothermal gradient is therefore assigned to the genesis of large volumes of medium pressure ttg magmas that will likely form at greater depth unmelted garnet bearing amphibolites correspond to slightly lower conditions the magmatic cumulates crystallized at ca 0 6 gpa the cumulate rocks evidence that the ttg parental magmas en route to the surface experienced fractional crystallization of a large amount of amphibole before crystallizing as tonalite trondhjemite plutons at slightly shallower depths ca 0 5 gpa

petrogenesis of triassic granitoids in the fengxian taibai ore cluster western qinling orogen central china implications for tectonic evolution and polymetallic mineralization abstract the petrogenesis and tectonic setting of the triassic granitoids and their relationship with the coeval au and pb zn polymetallic mineralization in the western qinling orogen wqo in central china remain debated the fengxian taibai ore cluster ftoc in the eastern part of the wqo hosts large scale triassic granitoids and numerous polymetallic deposits providing an excellent opportunity to constrain the controversies mentioned above in this work we choose the huahongshuping granodiorite pluton guji biotite monzogranite pluton taoerling biotite monzogranite dike dongtangzi granite porphyry dike and dagou dioritic dikes in the ftoc for a combined study on the zircon u pb geochronology whole rock major trace elements and sr nd isotopes la icp ms zircon u pb dating yields ages of 225 ma 224 ma and 227 217 ma for the huahongshuping pluton guji pluton and granitoid dikes respectively all these granitoids have metaluminous and high k calc alkaline series affinities and are enriched in large ion lithophile elements lile light rare earth elements lree and depleted in high field strength elements hfse with a marked lree hree fractionation la yb n 10 66 32 42 fractionation of lree la sm n 2 72 6 60 and negative eu anomalies eu 0 45 0 99 they have initial sr isotopic ratios of 0 7062 to 0 7082 and negative end t values of 9 42 to 8 09 the above features indicate that the ftoc granitoids were mainly derived from the partial melting of the neoproterozoic meta volcanic rocks induced by the decrease in pressure and the addition of water dehydrated from biotite with direct contribution of mantle materials combined with previous data on sedimentation deformation and metamorphism and tectonics we suggest that the wqo underwent the early mesozoic tectonic evolution from the subduction at 248 235 ma through syn collision at 235 225 ma to post collision at 225 195 ma integrating with the ore geology geochronology mineralogy and quartz and carbonate h o and in situ s isotope data of sulfides from different mineralization stages at representative au and pb zn deposits in literatures we proposed a conceptive model that the post collision process and granitic magmatic activity played an important role in the polymetallic mineralization within the ftoc

synthesis of staurolite in melting experiments of a natural metapelite consequences for the phase relations in low temperature pelitic migmatites we document experiments on a natural metapelite in the range 650 775 c 6 14 kbar 10 wt of added water and 700 850 c 4 10 kbar no added water staurolite systematically formed in the fluid present melting experiments above 675 c but formed only sporadically in the fluid absent melting experiments the analysis of textures phase assemblages and variation of phase composition and fe mg partitioning with p and t suggests that supersolidus staurolite formed at near equilibrium during fluid present melting reactions the experimental results are used to work out the phase relations in the system k2o na2o feo mgo al2o3 sio2 h2o appropriate for initial melting of metapelites at the upper amphibolite facies the p t grid developed predicts the existence of a stable p t field for supersolidus staurolite that should be encountered by aluminous fe rich metapelites during fluidpresent melting at relatively low temperature and intermediate pressures 675 700 c 6 10 kbar for xh2o 1 in the knfmash system but not during fluid absent melting the implications of these findings for the scarcity of staurolite in migmatites are discussed

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

dissolution of tonalitic enclaves in ascending hydrous granitic magmas an experimental study dissolution of natural mafic magmatic enclaves in a hydrous leucogranitic synthetic melt has been tested experimentally results suggest that the mechanism of enclave dissolution is a potential hybridization process in granitic systems experiments performed in decompression simulating ascending magmas show interesting results from 10 kbar to 4 kbar for a given temperature enhanced dissolution of the tonalitic enclave into the melt has been observed compared with experiments at constant pressure furthermore the composition of the melt changed to higher cao feo and mgo contents dissolution textures on relict crystals from the tonalitic enclave were monitored these results have implications for the generation of peraluminous monzogranites and granodiorites and an example for the cabeza de araya caceres spain from the serie mixta of the iberian massif is given the tonalitic enclaves that are partially dissolved may be early or coeval intrusions into the granite magma or even into the source migmatitic area many mineralogical geochemical and isotopic features of monzogranites e g reverse zoning in plagioclase low sr isotopic ratios are accounted for by this mechanism of enclave dissolution during ascent and emplacement

leucogranites of the teton range wyoming a record of archean collisional orogeny abstract leucogranitic rocks formed by crustal melting are a prominent feature of collisional orogens of all ages this study describes leucogranitic gneisses associated with an archean collisional orogeny preserved in the teton range of northwestern wyoming usa these leucogneisses formed at 2 68 ga and initial nd isotopic compositions suggest they are derived from relatively juvenile sources two distinct groups of leucogneisses both trondhjemitic are identified on the basis of field relations petrology and geochemistry the webb canyon gneiss forms large sheet like bodies of hornblende biotite trondhjemite and granodiorite this gneiss is silica rich sio2 70 80 strongly ferroan comparatively low in alumina and is characterized by high zr and y low sr and high ree contents that define seagull shaped ree patterns the bitch creek gneiss forms small sills dikes and plutons of biotite trondhjemite silica zr y and ree are lower and alumina and sr are higher than in the webb canyon gneiss these differences reflect different melting conditions the webb canyon gneiss formed by dehydration melting in which amphibole and quartz breaks down accounting for the low alumina high feo high silica content and observed trace element characteristics the bitch creek gneiss formed by h2o excess melting in which plagioclase breaks down leaving an amphibole rich restite producing magmas higher in alumina and sr and lower in feo and hree both melt mechanisms are expected in collisional environments dehydration melting accompanies gravitational collapse and tectonic extension of dramatically thickened crust and water excess melting may occur when collision places a relatively cool hydrous lower plate beneath a hotter upper plate the archean leucogranitic gneisses of the teton range are calcic trondhjemites and granodiorites whereas younger collisional leucogranites typically are true granites the difference in leucogranite composition reflects the geochemically immature archean crust that partially melted in the teton range compared to the more geochemically evolved rocks typically involved in younger collisional orogens

tectonometamorphic evolution of the rhodope orogen 1 this study combines new data on tectonostratigraphy macrostructures and microstructures petrology and geochronology to propose a comprehensive model for the tectonometamorphic evolution of the rhodope orogen from the jurassic to the early paleogene rocks from two study areas in the central and eastern greek rhodope represent a continental suture zone rhodope suture zone with the included material most likely forming an extensional allochthon south of the european continent during permo triassic times that was subsequently subducted beneath europe in the early jurassic 180 ma on the basis of comparable metamorphic ages and coherent structures but differences in metamorphic conditions and lithologies the rocks of the rhodope suture zone are subdivided into an upper and a lower part the prograde history is linked with subduction related structures in the lower part uniaxial stretching deformation stage d1 in metapelites the earliest stage of metamorphism recorded at circa 180 ma occurred at least under ultrahigh pressure metamorphic conditions the rocks of the upper part experienced isothermal decompression with partial anatexis exhumation paths of both parts differ in temperature because of the relative tectonic position within the exhuming wedge exhumation was forced by the nestos shear zone that controlled the early phase of normal displacement by sw shearing at the base lower part and ne shearing on top upper part from the late jurassic to the late cretaceous deformation stage d2 an intervening stage of mineral recrystallization and thermal reequilibration in the upper part was followed by a common exhumation history of both parts at pressures lower than about 12 kbar 35 40 km depth during this stage exhumation was controlled by southwest directed shearing and folding deformation stage d3 slab retreat to the south led to subsequent extension deformation stage d4 and final exhumation coeval with the formation of basement domes thus making the rhodope a classic core complex juxtaposed with former structurally higher units

multistage anatexis during tectonic evolution from oceanic subduction to continental collision a review of the north qaidam uhp belt nw china abstract the north qaidam ultra high pressure uhp metamorphic belt in the northern tibetan plateau is considered as a typical alpine type uhp metamorphic belt due to the early paleozoic subduction of the qaidam block beneath the qilian block the well preserved paleozoic metatexite migmatite diatexite migmatite felsic sheet dyke and anatectic granite in the north qaidam uhp metamorphic belt provide us with an excellent opportunity to study the generation transport and final fate of crustal melt and geodynamic processes associated with orogenesis based on structural relationships petrology geochronology whole rock geochemistry and sr nd isotope data the early paleozoic anatexis during oceanic subduction to continental collision in the north qaidam uhp metamorphic belt can be further divided into three stages of development 470 ma 446 428 ma and 432 420 ma the first stage granitic leucosomes are rich in k poor in na with low sr y and enrichment of large ion lithophile elements liles which were probably derived from partial melting of ancient felsic gneiss in continental arc circumstances during oceanic subduction the second stage na rich leucosomes and tonalite plutons are characterized by high na sr sr y and la yb and low heavy rare earth elements hrees with positive end t values of 0 1 4 3 and zircon ehf t values of 8 3 10 2 similar to typical tonalite trondhjemite granodiorites ttgs these ttg like magmas were produced through partial melting of newly emplaced gabbroic rocks with arc affinity under high pressure hp granulite facies conditions in a thickened lower crust during continental collision the volumetrically significant migrated plutons evolved from ttg like melt will become segments of continental crust and contribute to crustal growth with partial residual products of hp granulite and or garnet pyroxenite to the mantle by delamination the third stage of anatexis was preserved in both migmatitic uhp gneiss and eclogite in the xitieshan and luliangshan terranes the kfs rich felsic leucosomes inside uhp gneisses in the third stage are characterized by high alkali contents and low mafic component with feot mgo tio2 2 0 2 high la yb and sr y and low y and hrees 3 high al2o3 and low mg values and 4 enriched in liles e g rb ba k sr pb and poor in high field strength elements hfses partial melting of uhp eclogite and felsic gneiss during the initial retrogression stage with pl rich and kfs rich leucosome formation was triggered by dehydration melting involving predominant zoisite and muscovite the anatectic melts from partial melting of deeply subducted uhp gneiss have accumulated and migrated outside the deeply subducted crustal slice in the form of syn collisional plutons the melt evolution from the leucosomes produced at the early exhumation stage to syn collisional granitoids produced at the late exhumation might contribute greatly to the exhumation of the north qaidam uhp metamorphic belt from mantle depths to the lower crustal levels

adakite differentiation and emplacement in a subduction channel the late paleocene sabzevar magmatism ne iran this study describes the structural setting petrogenesis and geochronology of a suite of acidic magmatic rocks that are intruded in the metamorphic core of the tertiary ophiolitic suture zone of the sabzevar range ne central iran these intrusive bodies show tabular geometries with solid state fabrics documenting syntectonic emplacement during crustal shortening in the total alkalis silica tas diagram their compositions define a medium k calc alkaline suite spanning from basaltic andesite to the dacite and rhyolite fields they show characteristic low mgo 0 15 0 60 wt and ni 40 ar 39 ar white mica and amphibole geochronology constrains the sabzevar magmatism to the late paleocene at ca 58 ma genesis of the sabzevar magmatic suite is interpreted in terms of prograde wet amphibolite melting during oceanic subduction within a pressure temperature range between a plagioclase out and a hornblende out boundary magma differentiation and high pressure amphibole fractionation of pristine slab melts are proposed as the dominant factors that imparted the adakite signature in the sabzevar structural zone implications in terms of the regional tectonic scenario are discussed and framed within the advancing and retreating evolution of the neotethyan subduction during the mesozoic tertiary time span

initiation of changjiang yangtze delta implication from sedimentary facies radiocarbon dates and fossils information of the cores pk01 and xj03

high pressure melting of eclogite and the p t x history of tonalitic to trondhjemitic zoisite pegmatites munchberg massif germany summary the zoisite pegmatites provide an excellent example of theonset of metabasite melting at eclogite facies conditions such conditions can be expected in a down going oceanicslab as well as in collision zones the newly produced meltin a metabasaltic rock of morb composition is tonalitic to trondhjemitic and shares some characteristics withthe ttg series or with adakites the amount of melt forming at a temperature of less than 7508c however is small as a result of the combined effect of relatively lowtemperature and restricted availability of water and themelt does not separate from the rock unit only if themelt fraction increases will the melt segregate and mightmigrate upwards influencing melting in the overlyingmantle wedge because of the poor outcrop situation ofthe mu nchberg area in the variscan chain of centraleurope it is unfortunately impossible to give an estimateof the amount of melt produced this will be a crucialquestion to solve at what degree of melting at whattemperature in a given composition andat what geological mechanical conditions the melt will effectively separate our data suggest the following model high pressurepartial melting and pegmatite formation occurred at con ditions only slightly above the water saturated solidus andcan be a fluid conserving or fluid recycling process any fluid released from hydrous minerals such as zoisite epidote or amphibole is trapped in small fractions offully to nearly water saturated siliceous melt for truedehydration melting a free fluid phase never exists neither the fluid nor the siliceous melt will separate andmigrate into the overlying rock units during uplift cool ing and crystallization the water which is stored in thesiliceous melt is again liberated and encounters a dehy drated eclogite facies assemblage here it hydrates theeclogite facies assemblage leading to crystallization ofhydrous minerals such as amphibole or mica this modelof fluid conservation or fluid recycling implies also thatthe trace element budget of the whole rock does notchange during melting and subsequent crystallization the pegmatites from the hangendserie of the mu nchbergarea still preserve some trace element signatures of theirindividual protoliths which means that the partial meltsdid not mix andwere not homogenized element transportoccurs onlyona restricted scale of probably much lessthanthe present thickness of the hangendserie the coarse grain size of the pegmatitic rocks is how ever a good condition to preserve textural disequilibriumand to record different stages of the uplift and coolinghistory reheating is indicated in the pegmatites by partialresorption of zoisite and crystallization of a second andthird generation with distinctly different composition this observation is in line with the idea that upliftoccurred during the collision event and shortly afteror simultaneously with stacking of the nappes that is in a short time frame which is in many cases close to thetime resolution of available geochronometric methods

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

multiphase solid inclusions in zoisite bearing eclogite evidence for partial melting of ultrahigh pressure metamorphic rocks during continental collision abstract multiphase solid ms inclusions in both garnet and omphacite were investigated for zoisite bearing eclogite from the ultrahigh pressure uhp metamorphic zone in the sulu orogen the results provide petrological evidence for local anatexis during exhumation of deeply subducted continental crust there are three types of ms inclusions 1 plagioclase quartz 2 plagioclase quartz k feldspar and 3 barite plagioclase k feldspar zoisite epidote the host minerals mostly exhibit radial fractures surrounding the ms inclusions the first and second types of ms inclusions were analyzed for their bulk compositions yielding high sio 2 and na 2 o but very low feo mgo tio 2 with variable k 2 o for the second type trace element analyses of representative ms inclusions yield generally very low concentrations except such large ion lithophile elements as sr ba and rb these features suggest different origins for the three types of ms inclusions the first type of ms inclusions would be primarily derived from dehydration melting of paragonite whereas the second type of ms inclusions would be derived from dehydration melting of both paragonite and phengite the third type of ms inclusion may result from the interaction between metamorphic fluid and host mineral in view of the occurrence of barite as a filling phase in the fractures of host minerals zoisite breakdown is also indicated by its highly cuspate shape in coexistence with quartz providing components for growth of garnet during the exhumation therefore the anatexis of zoisite bearing uhp eclogite is primarily driven by the breakdown of hydrous minerals such as phengite and paragonite the major and trace element compositions of ms inclusions in the eclogites mainly depend on the species of hydrous minerals involved in the mineralogical reactions of dehydration melting in subduction channel

granulite facies regional and contact metamorphism of the tasiuyak paragneiss northern labrador textural evolution and interpretation the tasiuyak paragneiss at the western margin of the nain plutonic suite has been subjected to two granulite facies metamorphic events i regional metamorphism during the paleoproterozoic torngat orogeny and ii contact metamorphism due to emplacement of the mesoproterozoic nain plutonic suite regional metamorphism led to partial melting of pelitic rocks and the development of a locally well preserved sequence of prograde and retrograde textures these textures are partly controlled by bulk composition and formed in the pressure temperature p t field of the continuous reaction biotite sillimanite plagioclase quartz garnet k feldspar melt along a hairpin p t path with peak conditions of 8 10 kbar 0 8 1 0 gpa and up to 870 c in the nakfmash na2o k2o feo mgo al2o3 sio2 h2o system these textures controlled the development of the contact metamorphic assemblages contact metamorphism of the pelitic rocks between the tessiarsuyungoakh intrusion and the makhavinekh lake pluton led to

regional metamorphism at extreme conditions implications for orogeny at convergent plate margins abstract regional metamorphism at extreme conditions refers either to alpine type metamorphism at low geothermal gradients of 30 c km extreme pressures refer to those above the polymorphic transition of quartz to coesite so that ultrahigh pressure uhp eclogite facies metamorphism occurs at mantle depths of 80 km extreme temperatures refer to those higher than 900 c at crustal depths of 80 km so that ultrahigh temperature uht granulite facies metamorphism occurs at medium to high pressures while crustal subduction at the low geothermal gradients results in blueschist eclogite facies series without arc volcanism heating of the thinned orogenic lithosphere brings about the high geothermal gradients for amphibolite granulite facies series with abundant magmatism therefore uhp metamorphic rocks result from cold lithospheric subduction to the mantle depths whereas uht metamorphic rocks are produced by hot underplating of the asthenospheric mantle at the crustal depths active continental rifting is developed on the thinned lithosphere in response to asthenospheric upwelling and this tectonism is suggested as a feasible mechanism for regional granulite facies metamorphism with the maximum temperature depending on the extent to which the mantle lithosphere is thinned prior to the rifting while lithospheric compression is associated with subduction metamorphism in accretionary and collisional orogens the thinned orogenic lithosphere undergoes extension due to the asthenospheric upwelling to result in orogen parallel rifting metamorphism and magmatism thus the rifting metamorphism provides a complement to the subduction metamorphism and its operation marks the asthenospheric heating of the orogenic lithosphere because of the partial melting and melt extraction of the lower continental crust contemporaneous granite migmatite granulite associations may serve as a petrological indicator of rifting orogeny that is superimposed on precedingly accretionary and collisional orogens the uht metamorphic rocks have occurred since the archean suggesting that the hot underplating has operated very early in the earth s history in contrast the uhp metamorphic rocks primarily occur in the phanerozoic indicating that the thermal regime of many subduction zones has changed since the neoproterozoic for the cold subduction

late quaternary sedimentary evolution of the outer shelf of the east china sea abstract this study investigated the late quaternary sedimentary evolution of the outer shelf of the east china sea ecs using borehole core data dh02 three unconformity bounded sedimentary facies namely prodelta facies of marine isotopic stage mis 3 intertidal to subtidal flat facies of the deglacial period and offshore tidal sand ridge facies of the holocene were identified from the base upward according to combined evidence of lithology microfossil assemblages and geochronology through comparative analysis with other neighboring borehole cores these three facies along with the mis 3 nearshore facies and the last glacial maximum lgm river facies were found to constitute the late quaternary sediment strata sedimentary processes of these five facies were interpreted based on a synthetic consideration of sea level change climate variation paleotopographic setting and paleoriver sea interactions in particular the development of the delta was attributable to integration of the slow sea level decline during the later stage of the mis 3 and abundant sediment supply resulting from the warm wet climate the gentle shelf topography suggests the presence of shallow and wide rivers in the delta and the lateral shift of river channels might have induced the local formation of nearshore facies at the abandoned river mouth the lgm river facies which was rarely observed presumably due to marine erosion with deglacial transgression was inferred to have been deposited during the latter half of the lgm in response to the rise in river base level occurrences of tidal flat facies imply the formation of a tide dominated estuary with the deglacial transgression furthermore tidal flat sediments were reworked by enhanced tidal currents in the estuary front environment forming mud ridges during subsequent deglacial transgression formation of tidal sand ridges in offshore environments of the holocene was attributable to the uneven base caused by the underlying estuarine erosion mud ridges and this promoted the occurrence of tidal depositional processes the sedimentary evolution in association with sea level change was constructed using the above mentioned results overall this study systematically elucidates the late quaternary sedimentary processes and evolution of the outer ecs shelf and provides an insight into the sedimentary development of analogous shelves around the world

molybdenite re os dating of biotite dehydration melting in the rogaland high temperature granulites s norway abstract the ability of the re os system in molybdenite to record and preserve the age of granulite facies metamorphism in polymetamorphic belts is tested using the orsdalen w mo district rogaland s norway a low pressure high temperature granulite facies domain displaying osumilite and pigeonite isograds is exposed around the 931 2 ma rogaland anorthosite complex available u pb monazite and zircon data show that a 0 93 ga contact metamorphism overprints a 1 03 0 97 ga regional sveconorwegian metamorphism in the gneiss basement molybdenite and scheelite in the orsdalen district occur in orthopyroxene bearing leucocratic veins parallel to the regional foliation the veins are interpreted as migmatic leucosomes formed by fluid absent incongruent melting of the biotite rich host rock above ca 800 c producing a granulite facies orthopyroxene garnet residual assemblage molybdenite is interpreted as a product of the melting reaction crystallized from trace amounts of mo released from biotite in the host rock during partial melting four re os analyses of molybdenite from three samples representing two mines yield an isochron age of 973 4 ma the isochroneity of the data indicates that the precipitation of molybdenite and the partial melting event are recorded on the district scale the re os system in molybdenite was not affected by subsequent 0 93 ga contact metamorphism corresponding to formation of garnet quartz coronitic textures around molybdenite and other minerals in the deposits the results indicate that granulite facies conditions prevailed at 973 4 ma near the end of a protracted event of regional metamorphism 1 03 0 97 ga biotite dehydration melting recorded in orsdalen took place at a pressure of ca 5 5 kbar orthopyroxene garnet plagioclase quartz thermobarometry possibly in association with regional decompression the study shows that granulite facies metamorphism 0 97 ga took place before intrusion of massif type anorthosites in rogaland 0 93 ga the study suggests that the high temperature osumilite bearing assemblage may be related to the 0 97 ga event and that massif type anorthosites may not be a cause for the high temperature thermal anomaly in the crust but a late expression of it

reactive bulk assimilation a model for crust mantle mixing in silicic magmas bulk assimilation of small millimeters to 1 km fragments of crust driven and ultimately masked by reactions during xenolith melting and magma crystallization is an important mechanism for crust mantle mixing xenoliths containing mica or amphibole undergo dehydration melting when incorporated into a host magma yielding mainly plagioclase pyroxene fe ti oxides and hydrous melt the xenolith is physically compromised by partial melting and begins to disintegrate xenolithic melt and crystals are mixed into the host magma xenocrystic zircon is liberated at this stage the cryptic character of assimilation is greatly enhanced in any hydrous magma by hydration crystallization reactions the reverse of dehydration melting all pyroxenes and oxides phenocrysts xenocrysts or crystals having a hybrid signature will be subject to these reactions producing feldspars amphiboles and micas that incorporate material from several sources a particularly effective mixing mechanism implicit in the model is a reduced energy penalty for bulk assimilation much of the assimilant remains in solid form compared to melt assimilation models a large role for bulk assimilation supports stoping as a credible mechanism for the ascent of magmas while the assimilation of low density crust and concomitant fractionation provide the isostatic impetus for ascent the wholesale incorporation and processing of crustal rocks in the magma chamber helps create the room for ascent

insights into the causes of arc rifting from 2 d dynamic models of subduction author s billen mi abstract 2017 american geophysical union all rights reserved back arc spreading centers initiate as fore arc or arc rifting events when extensional forces localize within lithosphere weakened by hydrous fluids or melting two models have been proposed for triggering fore arc arc rifting rollback of the subducting plate causing trench retreat or motion of the overriding plate away from the subduction zone this paper demonstrates that there is a third mechanism caused by an in situ instability that occurs when the thin high viscosity boundary which separates the weak fore arc from the hot buoyant mantle wedge is removed buoyant upwelling mantle causes arc rifting drives the overriding plate away from the subducting plate and there is sufficient heating of the subducting plate crust and overriding plate lithosphere to form adakite or boninite volcanism for spontaneous fore arc arc rifting to occur a broad region of weak material must be present and one of the plates must be free to respond to the upwelling forces

two phases of paleoproterozoic metamorphism in the zhujiafang ductile shear zone of the hengshan complex insights into the tectonic evolution of the north china craton abstract the zhujiafang ductile shear zone is located in the central hengshan complex a key area for understanding the paleoproterozoic tectonic evolution of the trans north china orogen north china craton detailed metamorphic and geochronological analyses were carried out on metapelite and garnet amphibolite from the zhujiafang ductile shear zone the metapelite preserves two phases of mineral assemblages early kyanite rutile bearing assemblage and late chlorite staurolite bearing assemblage in garnet mica schist and inclusion type muscovite high si kyanite assemblage and late sillimanite bearing assemblage in sillimanite mica gneiss garnet in the metapelite occasionally exhibits pronounced two stage zoning characteristic of a diffusion core with irregular pyrope xpy and grossular xgr contents and a growth rim with xpy and xgr increasing outwards the isopleths of the maximum xgr in garnet core and si content in inclusion type muscovite in the p t pseudosections suggest that the early mineral assemblages underwent medium high pressure type metamorphism with pressures up to 12 14 kbar at 700 750 c the late assemblages and the growth zoning of garnet rim predict a late separated clockwise p t path with peak conditions of 6 5 0 2 kbar 620 10 c medium low pressure type the garnet amphibolite is mainly composed of garnet hornblende plagioclase ilmenite and quartz without overprinting of late mineral assemblages except for localized corona textures phase modeling suggests that the rock has experienced high amphibolite facies metamorphism with peak conditions of 10 5 0 8 kbar 770 50 c which is broadly consistent with the early phase metamorphism of metapelite zircon u pb dating on metapelite yields two metamorphic age groups of 1 96 1 92 ga and 1 87 1 86 ga which are interpreted to represent the timing of the two separated phases of metamorphism two separated orogenic events may have occurred respectively at 1 95 ga and 1 85 ga in the hengshan wutai area the older phase of orogeny was resulted from continental collision and the younger phase of orogeny may be caused by within plate deformation the final exhumation of the high grade rocks formed in the older i e 1 95 ga orogeny should be related with the younger deformation metamorphic event

timescales of disequilibrium melting in the crust constraints from modelling the distribution of multiple trace elements and a case study from the lesser himalayan rocks of sikkim a numerical code has been developed to track the distribution of trace elements in crustal rocks undergoing melting the model handles diffusion with moving boundaries and accounts for the processes of diffusion dissolution and precipitation in a partially molten system among the various input parameters for modelling source composition i e modal abundance variations diffusion coefficients and partition coefficients are found to exert a significant control on the melt chemistry the other inputs such as melt reaction stoichiometry kinetics of melting and grain size of protolith have lesser influence exploration of the general behaviour indicates that for systems in which disequilibrium melting of the kind considered in this paper occurs trace element concentrations may be used to constrain the composition of the protolith or the timescales of melting depending on the specific circumstances after exploring some general features of melting in a pelitic system the model is applied to calculate trace element distributions in migmatites from the lesser himalayan rocks in sikkim india we focus on the distribution of trace elements during the initial stages of melt formation these partially molten rocks show disequilibrium distribution of trace elements and the numerical code is capable of quantitatively reproducing many of the observed patterns the results of the modelling indicate that the observed melts in this zone were formed within 50 000 years and that segregation of melts into leucosome and restite was complete between 50 000 and 250 000 years these short timescales may point to deformation enhanced melt segregation at least on a hand specimen scale it is important to distinguish between timescales of segregation over these scales and timescales of removal of melt on an outcrop scale to form plutons the latter requiring higher degrees of melting and larger distances of migration take longer

formation processes and evolution of holocene sediments in the yangtze offshore area a review and new interpretation on the response to sea level rise and deltaic progradation abstract seismic profiles integrated with borehole cores from offshore of the yangtze river delta characterize three distinctive holocene deposition units in correspondence to estuarine shallow marine and prodelta facies respectively units to sedimentary processes and evolution of three units in response to sea level rise and yangtze delta development was studied estuarine facies occurred as a cluster of mud ridges unit was considered to be formed during the later stage of the period 10 8 cal ka bp with the prevalence of tidal erosion processes at the same time however the area south of the ridge field experienced rapid deposition possibly due to shelter effects of the zhoushan archipelago unit characterized by homogeneous mud composition rapid accumulation rates and a seaward dipping structure indicated a gravity flow deposition most likely related to the wave and or tidal current induced fluid mudflows during the period of 8 5 cal ka bp intrusion of a mud diapir during 5 3 cal ka bp related to gravity subsidence localized near the river mouth lead to deformation of unit during the 3 ka to present the delta advanced to the study area depositing subaqueous clinoform unit meanwhile erosional processes dominated by tidal currents were promoted by the southeastward progradation of the delta sediments deposited in the north of the area were transformed into tidal sand ridges we predict that the delta will likely reduce in area with a further progradation in the recent future

magmatic garnet in the cordilleran type galileia granitoids of the aracuai belt brazil evidence for crystallization in the lower crust abstract magmatic garnet together with epidote is a rare mineral association in cordilleran i type granitoids and of special petrogenetic significance the metaluminous to slightly peraluminous asi 0 97 1 07 galileia batholith brazil is a large ca 30 000 km 2 neoproterozoic ca 632 570 ma weakly foliated calc alkaline granitoid body characterized by the widespread occurrence of garnet grossular 25 43 mol and epidote pistacite 9 3 22 7 mol field petrographic and mineral chemical evidence indicates that garnet epidote biotite as well as white mica crystals low si phengite are magmatic there is no difference in bulk rock major and trace element composition between the galileia granitoids and other garnet free cordilleran type granitoids worldwide this evidence strongly suggests that the origin of the uncommon garnet epidote parageneses is related to the conditions of magma crystallization such as pressure temperature and water content comparison between the mineral assemblages and mineral compositions from this study and those recorded in crystallization experiments on metaluminous calc alkaline magmas as well as within garnet bearing metaluminous volcanic rocks and granitoids indicates that the supersolidus coexistence of grossular rich garnet epidote and white mica is consistent with magma crystallization at pressures greater than 0 8 gpa above 25 km depth and at temperatures below 700 c i e near the water saturated solidus furthermore resorption textures around garnet plagioclase quartz coronas and epidote suggest that these minerals have been partially consumed prior to complete crystallization these findings demonstrate that at 630 ma the crust underneath the aracuai orogen was already at least 25 30 km thick and relatively cool however this contrasts with the marked high heat flow registered from the neighbour carlos chagas batholith located 50 km to the east in fact such granitoids record granulite facies metamorphism at the same pressure and time ca 570 ma of galileia granitoids crystallization thus a more suitable geodynamic scenario is required in order to explain these two contrasting thermal regimes within the same orogen eventually field petrographic and mineral chemical analogies with similar garnet bearing granitoids located in the fore arc settings of the british columbia subduction zone possibly imply that the galileia granitoids represent rare garnet and epidote bearing metaluminous cordilleran i type granites which can only form in a fore arc setting

early cretaceous migmatitic mafic granulites from the sabzevar range ne iran implications for the closure of the mesozoic peri tethyan oceans in central iran terra nova 22 26 34 2010 abstract the ophiolitic melange of the sabzevar range northern iran is a remnant of the mesozoic oceanic basins on the northern margin of the neotethys that were consumed during the arabia eurasia convergence history occurrence of km scale dismembered mafic hp granulitic slices is reported in this study granulites record an episode of amphibole dehydratation melting and felsic tonalite throndhjemite melt segregation at c 1 1 gpa and 800 c in situ u th pb geochronology of zircon and titanite grains hosted in melt segregations points to an early cretaceous albian age for the metamorphic climax results of this study i impose reconsideration of the current palaeotectonic models of the neothetyan convergent margin during the early cretaceous and ii argue that punctuated events of subduction of short lived back arc oceanic basins accompanied the long lasting history of the neotethyan subduction in the region

magmatic and anatectic history of a large archean diapir insights from the migmatitic core of the yalgoo dome yilgarn craton abstract the genesis of domes in the archean was fundamentally different than those on modern earth as crustal reworking was far more voluminous and triggered by extensive reheating of the crust dictated by major pulses of mantle magmatic activity in this study we investigate how such mantle magmatism reflected by extrusion of mantle derived mafic ultramafic greenstones sequences 2825 2760 ma in the youanmi terrane of the yilgarn craton australia caused a cascade of crustal processes between 2760 and 2740 ma to form the 40 60 km diameter yalgoo dome voluminous deposition of the greenstone sequences initiated at 2800 2760 ma and followed by voluminous felsic magmatism lasting between 2760 and 2740 ma this process melted older basalts at depth and formed the goonetarra granodiorite in turn magmas caused water fluxed melting of older tonalite and amphibolite 2970 2900 ma that are preserved in the migmatitic core of the dome thermodynamic and geobarometry results on these rocks show that anatexis occurred between 3 and 9 kbar and 700 800 c with 0 5 2 wt h2o provided by the goonetarra granodiorite boosted melt production up to 15 30 vol the growth of this voluminous magmatic body accompanied by migmatization of country rocks caused dramatic crustal weakening allowing for sagduction of the thick overlying greenstones and the consequent diapiric rise of the buoyant hot and voluminous granitoid bodies partial convective overturn collins et al 1998 thus the internal evolution and the doming process of the yalgoo dome resulted from the arrival of a major thermal i e magma induced mantle anomaly into the crust this event first gave rise to a gravitationally unstable crustal stratigraphy greenstone over older felsic crust followed by a wave of conductive heat traveling upward through the crust this heat first produced voluminous and hydrous magmas as the goonetarra granodiorite then this melt advected heat and h2o to mid crustal levels causing water fluxed anatexis of older rocks and triggering partial convective overturn including diapiric doming of the felsic middle crust in a manner quite unlike the formation of phanerozoic domes that develop due to orogenic driven processes

gabbroic pegmatite intrusions iberia abyssal plain odp leg 173 site 1070 magmatism during a transition from non volcanic rifting to sea floor spreading the unroofed mantle grows younger outboard just as is the case for on the iberia abyssal plain ocean drilling program site 1070 normal volcanic sea floor gabbroic pegmatites and related rocks 127 4 ma u pb zircon intrude upper mantle that was subsequently exposed and serpentinized during early cretaceous non volcanic rifting the pegmatites include a 3 4 m dike or sill the main pegmatite numerous dikelets of

on the origin and fluid content of some rare crustal xenoliths and their bearing on the structure and evolution of the crust beneath the bakony balaton highland volcanic field w hungary rarely occurring clinopyroxene plagioclase bearing felsic granulite and skarn xenoliths were studied from the mantle and crustal xenolith bearing alkaline basaltic and pyroclastic localities of the bakony balaton highland volcanic field w hungary geobarometry and geothermometry of the xenoliths made it possible to categorise them in three groups according to their depth of formation the first group formed in the lower crust together with mafic and metasedimentary granulites the second group represents magmatic intrusions of the middle crust and the third one comprises contact metamorphic rocks of relatively shallow origin the calculated pressure difference from the core and rim compositions of plagioclase and clinopyroxene as well as garnet breakdown reactions in some xenoliths show evidence for pressure decrease due to crustal thinning both in lower crustal and middle crustal xenoliths during formation of the pannonian basin fluid inclusion studies reveal the dominance of the co2 rich fluids in the whole crustal section in contrast with fluids found in mafic garnet bearing xenoliths crustal stratigraphy was constructed for the periods prior to the extension and after the extension on the basis of geobarometry and geophysical data on the basis of mineral stabilities and geothermo barometry we estimated that the pre extensional thickness of the lower and upper crust may have been 27 34 and 26 28 km respectively comparison of pre extensional and present day thickness of the lower and upper crust indicate that thinning affected both the lower and the upper portion of the crust but on a different scale the calculated thinning factors are between 2 25 and 3 4 for the lower crust and 1 3 1 56 for the upper crust

paleoproterozoic p t t evolution in the hengshan wutai fuping area north china craton evidence from petrological and geochronological data abstract this contribution carries out zircon dating and a review of the previous metamorphic and geochronological work on the hengshan wutai fuping area in the central trans north china orogen for elucidating its paleoproterozoic orogenic evolution the hengshan complex can be subdivided into north hengshan and south hengshan complexes separated by the strike slip zhujiafang shear zone high pressure hp mafic granulite and garnet absent amphibolite collected from the north hengshan complex yield metamorphic ages of 1963 1769 ma and 1882 1793 ma respectively both with weighted mean ages of 1 85 ga in the hp mafic granulite zircons of 1 90 ga and those of

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

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

partial melting of metapelites at ultrahigh pressure conditions greenland caledonides eclogite orthogneiss and by association metapelite from an island at 78 n in north east greenland experienced ultrahigh pressure uhp metamorphism at approximately 970 c and 3 6 gpa at the end of the caledonian collision 360 350 ma hydrous metapelites contain abundant leucocratic layers and lenses composed of medium grained anhedral equigranular quartz antiperthitic plagioclase and k feldspar with minor small garnet and kyanite crystals leucosomes are generally parallel to the matrix foliation are interlayered with residual quartz bands anastomose around residual garnet and commonly cross cut micaceous segregations textures suggest that the leucosomes crystallized from a syntectonic melt but crystallized at the end of local high grade deformation the metapelite outcrop is 1 5 km from kyanite eclogites with confirmed coesite but the metapelites lack coesite and palisade textures diagnostic of coesite pseudomorphs they do contain highly fractured garnet megacrysts with polycrystalline quartz inclusions some surrounded by radial fractures and ti rich phengite inclusions that suggest the former presence of coesite polyphase inclusions in garnet contain reactants and products of the inferred dehydration melting reaction phe qtz ky kfs rt melt the reactants are thought to have been early inclusions of hydrous phases within garnet that melted and then crystallized new phases garnet surrounding these inclusions has patchy zoning with elevated ca consistent with experiments that produced similar patchy microstructures in garnet around inclusions with an unequivocal melt origin the peak uhp metamorphic assemblage in these rocks is inferred to have been phengite coesite garnet kyanite rutile fluid omphacite epidote phase diagrams indicate that dehydration melting of phengite in this assemblage would have occurred after decompression from peak pressure but still above the coesite to quartz transition unusual crown and moat like textures in garnet around some polycrystalline quartz inclusions are also consistent with the inference that melting took place at uhp conditions

lithium and nd isotopic constraints on the origin of li poor pegmatite with implications for li mineralization abstract although volumetrically minor in the upper continental crust pegmatites host lots of rare metal mineralization e g li be pegmatites and associated mineralization were generally thought to originate from extreme differentiation of a parent granite system we propose an alternative model to interpret the genesis of pegmatite and factors controlling li mineralization based on combined nd and li isotopic studies on the li poor pegmatites barren from the qinghe pegmatite field of the altai orogen nw china the qinghe pegmatites show significantly varied nd isotopic compositions end t 4 3 1 7 which overlap the surrounding schists 6 5 1 5 but lower than the granites 2 2 9 of the orogen these data together with the low ree 3 3 41 ppm and absence of biotite in the pegmatites suggest they were not derived from extreme differentiation of a parent granite as traditionally thought but from low degrees of partial melting of the schists involving muscovite dehydration melting under amphibolite facies conditions the pegmatites are characterized by heavy li 7li 4 1 14 5 and low li abundance 3 6 50 ppm which contrast to that of local schist 7li 0 9 3 0 li 24 123 ppm and granite 7li 0 9 3 0 li 24 70 ppm and also to that of global li rich pegmatites 7li 1 0 10 li 500 ppm modeling study indicates that li poor pegmatites always have heavy li signature compared with li rich pegmatites due to the abundance of biotite in the source the formation of li rich pegmatite with li mineralization is attributable to the distinct source characteristics with abundant fluxing components like li na co32 and hco3 and sufficient alumina rich clay phases and the fluxing components probably originated from evaporate interlayers in the source

tomography of the westernmost ryukyu subduction zone and the serpentinization of the fore arc mantle 1 three dimensional tomographic images of the subducting philippine sea slab and the fore arc mantle beneath ne taiwan and the westernmost ryukyu were generated in this study more than 5600 events recorded simultaneously by seismic networks in taiwan and japan were relocated for the inversion for variations in vp vs and vp vs analysis of the trade off between the data variance reduction and model variance helps to determine the appropriate strictness for regularization to avoid either overinterpretatioin or underinterpretation of data the regularization parameters were also chosen to ensure suppression of artificial vp vs anomalies the subducting slab is characterized by high vp high vs and intermediate to low vp vs notable in the mantle wedge is the high vp vs anomalies that abut the surface of the subducting slab at depths of 30 80 km the previously identified positive vp vs channel connecting the slab and the arc volcano interpreted to be a melt pathway is not reproduced in this study we convert vs and vp vs to perturbations of temperature and serpentinization of the mantle the slab is cooled by 200 c 400 c relative to the mantle in accord with the estimates from theoretical modeling of subduction the serpentinization reaches 15 or 2 water content at 50 km depth in the fore arc mantle we interpret the peak serpentinization as hydrated by the water released from the basalt eclogite metamorphic reaction in the oceanic crust of the subducting philippine sea plate the spatial limitation of the present seismic networks in this region with respect to subduction zone events hinders a full description of the pattern of melting in much of the mantle wedge resolution tests of the tomographic inversion provide a basis to guide our interpretation to better resolved regions

pinwarian 1 50 ga volcanism and hydrothermal activity at the eastern margin of the wakeham group grenville province quebec volcanic belts developed along the southeastern continental margin of laurentia between 1 70 and 1 30 ga and subsequently metamorphosed at high grade are today largely concealed among gneiss complexes of the grenville province at the eastern end of the wakeham group and in the la romaine supracrustal belt to the east four 1 50 ga volcanic centres were found among gneissic synvolcanic intrusions typical of the 1 52 1 46 ga pinwarian continental magmatic arc upper amphibolite to granulite facies rhyolitic to dacitic lavas and coarse lapillistone overlie or are intimately associated with arenites typical of the wakeham group garnetite ironstone carbonate rock calc silicate rock and sillimanite bearing nodules veins and gneiss locally preserving lapilli are also present the distribution paragenesis and modes of most of these latter units differ from those of normal metasediments but are diagnostic of metamorphosed exhalites and hydrothermal alteration zones in the la romaine supracrustal belt

105 million years of igneous activity wrangell alaska to prince rupert british columbia the coast mountains orogen which lies along coastal british columbia and southeastern alaska records extensive mesozoic and cenozoic igneous activity here we focus on the two youngest periods 105 50 ma and 30 ma present the history of subduction orogen uplift and finally extension associated with this magmatic activity can be related to plate motion since the mid cretaceous the 105 50 ma period is related to terrane accretion and subduction the northwest southeast trending coast shear zone divides the resulting coast mountains calc alkaline continental margin batholith into a western 105 90 ma and an eastern 80 50 ma arc melting of hydrous mantle overlying a dehydrating slab generated the plutons of the western arc the plutons of the eastern arc show a wider range of compositions their origin consists of mantle melts modified by melts from lower crustal rocks of continental affinity and possibly amphibolitic hydrated basalt in both parts of the arc igneous bodies also resulted from crustal melting these are very abundant in the eastern arc during the younger period bi modal igneous suites consisting of mantle derived mafic magmas and coeval crustal melts are associated with crustal extension and block faulting

new shrimp u pb zircon ages and chime monazite ages from south harris granulites lewisian complex nw scotland implications for two stages of zircon formation during palaeoproterozoic uht metamorphism abstract sensitive high resolution ion microprobe shrimp u pb zircon age dating was applied to several types of granulites and gneisses from the lewisian complex in south harris nw scotland to clarify the timing of palaeoproterozoic tectono thermal events for comparison monazite was dated using the chemical th u total pb isochron method chime the obtained u pb zircon age data were evaluated under the constrains of the modal amount of constituent minerals and whole rock compositions of analysed samples the zircon internal texture based on th u ratios and the monazite chime ages zircons in three samples characterised by zoned overgrowth rims sector zoned grains and structureless high u cores yielded concordant ages of approximately 1900 ma 1909 3 ma 1899 10 and 1888 15 ma respectively the age of 1899 10 ma obtained from melanocratic granulite garnet and kyanite sillimanite 76 91 vol is interpreted to represent the timing of zircon crystallisation prior to the formation of restitic garnet and kyanite sillimanite therefore the age of 1900 ma is proposed to represent the timing of early metamorphism close to the thermal peak m1 in the leucocratic gneiss sample 93921 7 the age population at 1868 35 ma obtained from low u and low th overgrowth rims represents a second stage of zircon growth from crystallising anatectic melt m3 whereas the high u cores represent the timing of zircon growth and recrystallisation prior to anatexis monazites from two leucocratic gneisses yield a relatively narrow range of chime ages 1858 7 ma and 1840 7 ma these ages probably represent the timing of zircon and monazite crystallisation from partial melt and resetting of the u th pb system these results suggest that the timing of zircon formation was controlled by metamorphic processes such as restite formation and crystallisation from partial melt

the composite archaean grey gneisses petrological significance and evidence for a non unique tectonic setting for archaean crustal growth abstract the geodynamic context of formation of the archaean continental crust is a matter of debate the crust is largely made of grey gneiss complexes a composite rock assemblage dominated by granitoids that are generally regarded as belonging to the ttg tonalite trondhjemite granodiorite series using a large database of published ttg and grey gneiss compositions it is possible to show that the granitoids forming grey gneiss complexes actually belong to at least four main geochemical groups i a potassic component made of granodiorites and formed by melting of existing crustal lithologies and ii three sodic groups ttg proper that comprise low medium and high pressure groups the geochemistry of the low pressure group is consistent with derivation from a plagioclase and garnet amphibolite the medium pressure group was formed in equilibrium with a garnet rich plagioclase poor amphibolite whereas the high pressure group derived from a rutile bearing eclogite as the temperature of melting of metamafic rocks is largely independent from pressure this corresponds to melting along a range of contrasting geothermal gradients in turn reflecting a range of tectonic sites for the formation of the archaean continental crust

geochronological constraints on the evolution of high pressure felsic granulites from an integrated electron microprobe and id tims geochemical study abstract a combined geochronological geochemical and nd isotopic study of felsic high pressure granulites from the snowbird tectonic zone northern saskatchewan canada has been carried out through the application of integrated electron microprobe and isotope dilution thermal ionization mass spectrometry id tims techniques the terrane investigated is a 400 km 2 domain of garnet kyanite k feldspar bearing quartzofeldspathic gneisses monazite in these granulites preserves a complex growth history from 2 6 to 1 9 ga with well armored high y and th grains included in garnet yielding the oldest u pb dates at 2 62 to 2 59 ga in contrast matrix grains and inclusions in garnet rims that are not well armored are depleted in y and th and display more complicated u pb systematics with multiple age domains ranging from 2 5 to 2 0 ga 1 9 ga monazite occurs exclusively as matrix grains zircon is typically younger 2 58 to 2 55 ga than the oldest monazite sm nd isotope analysis of single monazite grains and whole rock samples indicate that inclusions of archean monazite in garnet are similar in isotopic composition to the whole rock signature with a limited range of slightly negative initial nd in contrast grains that contain a paleoproterozoic component show more positive initial nd most simply interpreted as reflecting derivation from a source involving consumption of garnet and general depletion of hree s our preferred interpretation is that the oldest monazite dates record igneous crystallization of the protolith the ca 2 55 ga dates in zircon and monazite record an extensive melting event during which garnet and ternary feldspar formed very high pressure 1 5 gpa metamorphism during the paleoproterozoic at 1 9 ga produced kyanite from garnet breakdown and resulted in limited growth of new monazite and zircon in the case of monazite this is likely due to the armoring and sequestration of early formed monazite such that it could not participate in metamorphic reactions during the high pressure event as well as the depletion of the ree s due to melt loss following the early melting event

genesis of intermediate igneous rocks at the end of the sveconorwegian grenvillian orogeny s norway and their contribution to intracrustal differentiation abundant ferroan metaluminous granitoids 970 950 ma emplaced at the end of the sveconorwegian collisional orogeny 1130 900 ma are dominated by intermediate to silicic compositions with rare mafic facies both 73 fractional crystallization of an amphibole bearing gabbroic cumulate substracted from the parent mafic composition and 30 non modal batch melting of an amphibolitic source equivalent in composition to the mafic facies produce a monzodioritic liquid with appropriate trace element composition a better fit is obtained for the partial melting process both processes could have occurred simultaneously to produce mafic cumulates and restites as there is no evidence for large volumes of dense mafic rocks in the sveconorwegian upper crust these dense mafic rocks were probably produced in the lower crust formation of these granitoids thus contributed to the vertical stratification of the proterozoic continental crust and also to the transfer of water from the lower crust to the surface

a view into crustal evolution at mantle depths abstract crustal foundering is an important mechanism in the differentiation and recycling of continental crust nevertheless little is known about the dynamics of the lower crust the temporal scale of foundering and its role in the dynamics of active margins and orogens this particularly applies to active settings where the lower crust is typically still buried and direct access is not possible crustal xenoliths derived from mantle depth in the pamir provide a unique exception to this the rocks are well preserved and comprise a diverse set of lithologies many of which re equilibrated at high pressure conditions before being erupted in their ultrapotassic host lavas in this study we explore the petrological and chronological record of eclogite and felsic granulite xenoliths we utilized accessory minerals zircon monazite and rutile for coupled in situ trace element analysis and u th pb chronology by laser ablation split stream inductively coupled plasma mass spectrometry each integrated analysis was done on single mineral zones and was performed in situ in thin section to maintain textural context and the ability to interpret the data in this framework rutile thermo chronology exclusively reflects eruption 11 17 0 06 ma which demonstrates the reliability of the u pb rutile thermo chronometer and its ability to date magmatic processes conversely zircon and monazite reveal a series of discrete age clusters between 55 11 ma with the youngest being identical to the age of eruption matching age populations between samples despite a lack of overlapping ages for different chronometers within samples exhibit the effectiveness of our multi mineral approach the ree systematics and age data for zircon and monazite and ti in zircon data together track the history of the rocks at a million year resolution the data reveal that the rocks resided at 30 40 km depth along a stable continental geotherm at 720 750 c until 24 20 ma and were subsequently melted densified and buried to 80 90 km depth 20 km deeper than the present day moho at 930 35 c the material descended rapidly accelerating from 0 9 1 7 mm yr 1 to 4 7 5 8 mm yr 1 within 10 12 myr and continued descending after reaching mantle depth at 14 13 ma the data reflect the foundering of differentiated deep crustal fragments 2 9 3 5 g cm 3 into a metasomatized and less dense mantle wedge through our new approach in constraining the burial history of rocks we provided the first time resolved record of this crustal recycling process foundering introduced vestiges of old evolved crust into the mantle wedge over a relatively short period c 10 myr the recycling process could explain the variability in the degree of crustal contamination of mantle derived magmatic rocks in the pamir and neighboring tibet during the cenozoic without requiring a change in plate dynamics or source region

thermal petrological controls on the location of earthquakes within subducting plates we find that in young and warm subducting plates earthquakes occur just below the moho in older plates earthquakes occur throughout the subducting oceanic crust as well as the subducting mantle we document this behavior in several subduction zones where there are independent constraints on earthquake locations and slab structure specifically for northern and southern japan alaska and cascadia the differences in earthquake depth relative to subducting crust may reflect large differences in temperature and thus locations of major dehydration reactions in colder slabs the crust passes through blueschist facies dehydration reactions while in cascadia and nankai the major dehydration reactions in crust may be due to zoisite and amphibole breakdown or associated melting the cold paths allow more mineral bound h2o to be retained within the crust at shallow depths eventually released upon dehydration over shorter time intervals than warm paths the cold path dehydration reactions also result in net positive volume changes of solid fluid with solid volume decreasing less than the volume of h2o produced on hot paths the net volume changes are negative with solid volumes decreasing more than the volume of h2o produced the difference in behavior could drive a net increase in pore pressure upon dehydration for the cold but not the hot crustal paths the difference in rate of release in h2o and difference in sign of net system volume change may promote seismogenesis in cold subduction zones but inhibit it in the crust of warm slabs within the mantle of the downgoing plate earthquakes mostly occur where serpentine is stable or breaks down in both settings

garnet bearing low sr and high sr singelele leucogranite a record of neoarchean episodic melting in collisional setting and paleoproterozoic overprint in the beit bridge complex southern africa abstract the study presents field petrographic mineral chemical bulk rock geochemical u pb zircon and 40ar 39ar amphibole biotite geochronologic characteristics of the singelele leucogranite which constitutes a prominent neoarchean magmatic event in the beit bridge complex terrane of the limpopo complex southern africa the singelele leucogranite is magnesian to ferroan calcic to alkalic and peraluminous two groups of granites are distinguished on the basis of bulk rock sr content and garnet chemistry low sr with low xmg garnet and high sr with high xmg garnet major and trace element characteristics show that these two groups were probably generated by a fluid absent melting of crustal rocks like metagreywacke at different depths in collision tectonic setting spatial distribution and available geochronologic data indicate that these two groups represent two episodes of singelele leucogranite magmatism low sr at 2 68 2 65 ga and high sr at 2 63 2 62 ga the singelele leucogranite occurring close to major fold structures and faults shear zones are locally affected by anatectic and metasomatic paleoproterozoic overprint at 2 0 ga 2037 21 ma and 2015 15 ma u pb for zircon overgrowths zoning of coarse garnet grains with a decrease of xca in the melting overprinted rocks reflects decompression of about 3 4 kbar the metasomatic overprint produces cl f rich biotite 2 and amphibole phase equilibria modeling shows a lack of equilibrium between these minerals and earlier garnet and indicates modification of the garnet composition by metasomatism 40ar 39ar ages for amphibole 2043 18 ma 1994 12 ma and biotite 2 1985 11 ma 1967 11 ma are consistent with the zircon overgrowth ages

geochronological and geochemical constraints on the petrogenesis of middle paleozoic kwangsian massive granites in the eastern south china block abstract to achieve a better understanding of the kwangsian orogenic event of the eastern south china block this paper documents a set of new zircon u pb geochronological and hf isotopic data and whole rock elemental and sr nd isotopic analytical results for the representative massive granite intrusions across the jiangshan shaoxing fault the studied samples are classified into two groups representing the rocks from the cathaysia block to the east of the jiangshan shaoxing fault group 1 and those from the eastern yangtze block between the anhua luocheng and jiangshan shaoxing faults group 2 the group 1 samples gave the zircon u pb ages of 405 454 ma and e hf t values of 3 6 to 15 2 with hf model ages of 1 6 2 4 ga group 2 yielded the zircon u pb ages of 400 432 ma and e hf t values of 0 2 to 12 7 with hf model ages of 1 3 2 2 ga geochemically the group 1 samples a cnk 1 02 1 43 have relatively lower al 2 o 3 mgo cao p 2 o 5 and e nd t but higher k 2 o na 2 o than those of group 2 a cnk 0 93 1 44 both groups show similar chondrite normalized patterns of rare earth elements with eu eu values of 0 15 0 92 and strongly negative ba sr nb p and ti anomalies in primitive mantle normalized spider diagrams their e nd t values range from 11 1 to 8 0 for group 1 and 8 9 to 5 0 for group 2 generally similar to those of precambrian paragneiss and contemporaneous gneissoid granites in the eastern south china block our geochronological results indicate that the kwangsian massive granites in the eastern south china block were crystallized between 400 ma and 454 ma synchronous to the kwangsian gneissoid granites along the wugong and wuyi baiyun yunkai domains in the eastern south china block the synthesis of these whole rock geochemical and in situ zircon hf isotopic data suggests that both the group 1 and 2 granites across the jiangshan shaoxing fault were predominantly derived from a crustal source with some proportional metapelitic and metaigneous components from the precambrian basement the input of juvenile mantle derived magma is insignificant taking into account other available evidences it is proposed that the jiangshan shaoxing fault might be a pre existing boundary between the yangtze and cathaysia blocks which was reactivated during the middle paleozoic kwangsian orogenic event the massive granitic magmatism was probably resulted from the doubly crustal thickening and subsequent isostatic readjustment in an intracontinental tectonic regime

diffusion controlled biotite breakdown reaction textures at the solid liquid transition in the continental crust two phase quartz intergrowths with garnet cordierite and tourmaline occur commonly in prograde high temperature migmatites granulites as well as in the last crystallization stages of biotite granites structural microtextural and mineralogical data show that they result from the breakdown of biotite in the presence of a melt phase associated with incongruent dissolution of feldspars into the melt and silica release giving quartz in silica saturated rocks biotite breakdown and growth of al rich ferromagnesian minerals occurring at the solid liquid transition in the crust early melting or final crystallization is kinetically controlled by fe and mg mass transport the network forming cations si and al being locally compensated for by feldspar dissolution crystallization this process leads to significant changes with respect to equilibrium dehydration melting reactions wherein quartz is a reactant and k feldspar a reaction product therefore quartz inclusions commonly occurring in garnets from granulite facies metapelites and metagraywackes are not simply grains passively included during garnet growth they may also correspond to newly crystallized phases resorption of feldspar may lead to more alkaline melt and to crystalline residue richer in al than expected under equilibrium conditions hence excess alumina in granulite facies rocks is not necessarily related to initial alumina rich whole rock compositions as currently considered but may be due at least partly to kinetics of melting

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

shearing of magma along a high grade shear zone evolution of microstructures during the transition from magmatic to solid state flow abstract syntectonic plutons may record short lived geological events related to crustal melting and deformation of the continental crust therefore interpretation of microstructure and orientation of fabrics is critical in order to constrain space time temperature deformation relationships during pluton crystallization here we describe the transition from magmatic to solid state deformation in the late variscan diorite granite suite dgs emplaced along the santa lucia shear zone the systematic collection of meso microstructural and quartz axis data allow us to examine the spatial distribution and the mode of superposition of different fabrics we identify three magmatic microfabric types thought to reflect the microstructural evolution at decreasing melt content during pluton crystallization our data suggest that diffusion creep dislocation creep and grain scale fracturing cooperated in accommodating the shearing of the partially molten quartzofeldspathic aggregate we suggest that the switch from upward to horizontal magmatic flow occurred at melt fractions gt 0 40 and that most of the hypersolidus fabrics formed during horizontal flow reflecting the stress field imposed by the shear zone and preserving no memory of the ascent stage

oxygen isotope heterogeneities and diffusion profile in composite metamorphic magmatic garnets from the pyrenees unusually large 1 8 o heterogeneities 4 within single crystals are reported in garnets from dioritic migmatites in the pyrenees these heterogeneities together with contrasting ca and p zoning allow the identification of different growth zones garnet cores with high 1 8 o values 12 14 are relatively poor in ca 7 9 mol grs and rich in p 400 900 ppm p 2 o 5 in contrast garnet rims with lower 1 8 o values 7 12 o are richer in ca 12 14 grs and poorer in p 100 200 ppm these growth zones can be ascribed to a metamorphic event followed by crustal partial melting and contamination by magmas from the mantle high 1 8 o intra crystalline contrasts result from mineral growth in an open magmatic system involving the interaction of partial melts with distinct 1 8 o signatures at the garnet core rim interface compositional profiles in major divalent cations are consistent with the relaxation of an initial sharp step in ca fe and mg by ca fe mg interdiffusion at the same interface an o isotope profile is documented the analogy of ca and o isotope profiles suggests that the 1 8 o distribution may also result from a diffusion process in this particular case temperature garnet composition oxygen fugacity o diffusion appears to be of the same order of magnitude as ca fe mg interdiffusion considering a duration of 10 ma for the plutono metamorphic event in the pyrenees ca and o diffusivities in the range 10 2 2 m 2 s at 850 c are retrieved from the measured profiles like ca o diffusion in garnet at magmatic temperatures 850 900 c is both slow enough to preserve large 1 8 o heterogeneities and fast enough to generate relaxation profiles

mobility of trace elements and oxygen in zircon during metamorphism consequences for geochemical tracing this study addresses the mobility of the rare earth elements ree o u and pb during the formation of metamorphic zircon by overgrowth and coupled dissolution reprecipitation for this purpose the texture and ree 18o and u pb compositions of zircon crystals from a metabasite an anatectic granite and a metapelite sampled on naxos island greece have been characterised the behaviour of o and ree during the formation of the zircon metamorphic domains shows that two end members can be distinguished i closed or partially closed system behaviour for oxygen and trace elements in zircon formed by coupled dissolution reprecipitation metamorphic domains partially preserve the textural and geochemical characteristics of the precursor zircon chemical or isotopic equilibrium between the metamorphic zircon and the matrix is not completely reached for all the elements such zircons have to be used with caution for geochemical tracing when re equilibration is not complete the u pb system is also not completely reset and the metamorphic domains yield old scattered ages ii open system behaviour for oxygen and trace elements in zircon formed by overgrowth these metamorphic zircons do not show an inherited shape and are devoid of relic magmatic zoning their composition reflects growth in an open system at the scale of the zircon grain with the fluid phase and the surrounding minerals because equilibrium between the metamorphic zircon and the matrix was thus favoured zircon crystallisation can be more confidently related to the p t history and fluid circulation events by geochemical tracing

petrology and u pb geochronology of high grade metavolcano sedimentary rocks from central xolapa complex southern mexico abstract the xolapa complex xc is a high grade metamorphic belt exposed along the southern margin of the north american plate in mexico its evolution includes an episode of widespread anatexis related to crustal thickening and subsequent uplift to upper crustal levels which culminated in the paleocene on the basis of field and petrographic work this study integrates a petrological modelling approach with la icp ms u pb zircon and monazite geochronology to elucidate the tectonothermal evolution of this metamorphic belt the study area in central xc includes a suite of alternating migmatitic paragneiss and garnet bearing mafic schist petrological evidence and phase equilibria calculations demonstrate that a staurolite kyanite grade metamorphism occurred at 640 670 c and 8 9 kbar before extensive migmatization took place differential anatexis of fertile and refractory layers occurred progressively via biotite and amphibole dehydration melting reactions and continued during peak metamorphism at granulite facies conditions of 800 820 c and 5 7 kbar according to ti in zircon thermometry melt generation mobilization progressed during cooling until crystallize at 700 c the age of prograde metamorphism was likely recorded by monazite cores that give mid to late cretaceous ages cooling and melt crystallization could be constrained by leucosome zircon dated at 61 8 0 6 ma both zircon and low y monazite from whole rock and melanosome portions respectively are consistent with an episode of growth around 60 9 0 5 ma monazite however would have continued its crystallization process for a period of ca 10 ma thin zircon and high y monazite rims show ages between 34 25 ma and are interpreted to reflect a stage of reheating by magmatic advection related to large oligocene plutonism in the region age and nature of central xc basement show good correlation with other early cretaceous volcano sedimentary basins of central and southern mexico a period of crustal thickening would have buried the sequence at depths of 30 km causing barrovian type metamorphism during mid to late cretaceous time and eventually anatexis that evolved at granulite facies conditions during orogenic collapse in the paleocene 62 ma the time span from leucosome crystallization to late stage reheating in central xc would imply a protracted high temperature evolution indicating a middle to upper crustal residence time of at least ca 30 ma

kwangsian crustal anatexis within the eastern south china block geochemical zircon u pb geochronological and hf isotopic fingerprints from the gneissoid granites of wugong and wuyi yunkai domains abstract gneissoid granites were traditionally thought to be the components of the precambrian basement in the eastern south china block but twenty four gneissoid granite samples from the wugong northern wuyi southern wuyi and yunkai domains gave zircon u pb ages of 424 456 ma 410 457 ma 426 438 ma and 415 450 ma respectively the existence of the abundant kwangsian gneissoid granites which have incorrectly been assigned to the precambrian basement indicates that the so called precambrian stratigraphic sequence of the cathaysia block should be termed complex rather than group these gneissoid granites are peraluminous granites with relatively high al 2 o 3 mgo tio 2 feot and cao na 2 o but low cao al 2 o 3 mgo feot and rb sr ratios they exhibit strongly negative ba sr nb p and ti and positive pb anomalies in primitive mantle normalized diagrams and have initial 87 sr 86 sr ratios ranging from 0 70924 to 0 72935 and negative end t values from 6 4 to 11 4 similar to those of paragneissic enclaves and precambrian sedimentary rocks in the eastern south china block the zircons crystallized in the kwangsian gneissoid granites gave ehf t values ranging from 2 4 to 19 4 with the peak at 4 almost all grains clustering at 1 to 12 and hf model ages of 1 1 2 1 ga the synthesis of these geochemical and in situ zircon hf isotopic data indicates that the kwangsian gneissoid granites dominantly originated from proterozoic metapelitic and meta igneous components with insignificant input of juvenile mantle derived magmas a model of two stage crustal anatexis is proposed for the kwangsian granitic magma in the eastern south china block with 1 the formation of 460 430 ma granite through the breakdown of hydrous minerals under the condition of the doubly thickened crust and 2 the generation of the 430 400 ma granite accompanying promoted melting along a path of isothermal decompression due to the increasing thermal weakening for the collapse of the thickened crust the crustal anatexis has probably occurred in an intracontinental tectonic regime that was geodynamically linked to the far field response to the assembly of the australian indian plate with the cathaysia block during middle paleozoic kwangsian time

determination of the fluid absent solidus and supersolidus phase relationships of morb derived amphibolites in the range 4 14 kbar experiments on a morb mid ocean ridge basalt derived amphibolite have been conducted in order to determine fluid absent solidus and supersolidus phase relationships the solidus has been determined in the range 4 14 kbar by using the incremental heating technique tested against classical procedures peritectic clinopyroxene was produced below 10 kbar and epidote was the stable peritectic phase at pressures greater than 10 kbar for temperatures near the solidus garnet was the peritectic phase for temperatures above 800 c and pressures of 10 to 14 kbar the shape of the solidus was similar to previous estimations but the temperature interval between the high and low pressure regions was strongly reduced with these new estimations the possibilities for melting of the oceanic crust at p below 10 kbar are increased and consequently the number of tectonic scenarios needed for magma generation from amphibolite sources is enlarged in the absence of free water the solidus in the pressure region above 10 kbar is at higher t than previous estimations

anatexis during high pressure crustal metamorphism evidence from garnet whole rock ree relationships and zircon rutile ti zr thermometry in leucogranulites from the bohemian massif in situ mineral garnet zircon trace element and garnet rock ree distribution data obtained on leucogranulites from the bohemian massif are consistent with an origin of these now strongly deformed rocks as migmatitic leucogranites formed through dehydration melting of muscovite bearing protoliths at high pressures during the variscan orogeny partial melting at p t conditions of 900 9408c and 1 6 1 8 gpa consumed mica and feldspar to produce peritectic kyanite and high ca 20 24 mol grossular high eu garnet along with leucogranitic melt dree grt rock values calculated from this garnet and its host rock are consistent with garnet melt equilibrium at these p t conditions ternary feldspar and rutile 2000 3200 ppm zr crystallized from the leucogranite melts in the migmatites at minimum temperatures of 4880 9208c these formed along with apatite prior to and accompanying the crystallization of garnet rims and the majority of the garnet that occurs in the leucogranulites this garnet is typified by strong depletion in eu and dree grt rock values consistent with garnet melt equilibrium only after the fractionation of apatite 50 3 wt and ternary feldspar 5 15 wt zircons in one leucogranulite are dominated by oscillatory zoned grains that yield pre variscan u pb ages 495 and 433 ma and crystallized from ti in zircon thermometry at 700 8108c in their original host magmas these xenocrystic grains are potentially preserved because of the short timespan of the high p t variscan event at 340 ma and the moderate amounts of leucogranitic partial melt present in the rocks estimated as at least 30 wt melt from zr mass balance the production of new zircon associated with the variscan event in this leucogranulite is limited to weakly zoned zircon rims characterized by flat heavy ree patterns that approach ree equilibrium with garnet rims this post peak zircon was formed at minimum p t conditions of 8408c and 0 8 1 0 gpa along the decompression cooling path defined by previous work for the bohemian massif granulites the garnet whole rock ree relationships observation of eu depletion between early and later garnet growth phases and zr ti thermometric estimates indicating temperatures of48408c for this evolution cannot be explained by the alternative interpretation of the leucogranulites as reworked and recrystallized pre variscan low p granitoids but support a model in which the bohemian leucogranulites were high p variscan age migmatites containing significant amounts of melt

open system fractional melting of archean basalts implications for tonalite trondhjemite granodiorite ttg magma genesis tonalite trondhjemite granodiorite ttg gneiss forms a major component of archean continental crust resolving the origin of ttg gneisses and the secular change in their compositions is critical for better understanding how the continental crust has evolved and when a global plate tectonic regime began on earth archean ttgs were generated by partial melting of hydrous basalts although the geodynamic setting in which this occurred is still debated in this study an integrated modeling including thermodynamic modeling accessory mineral solubility modeling and trace element modeling is conducted on coucal basalts from pilbara craton and averaged archean arc like basalts along various thermal gradients for both closed and melt drained systems the results show the amount and composition of melts would be primarily controlled by source compositions although geothermal gradient and the ability for melt to leave the system also contribute of both bulk compositions averaged archean arc like basalts generate more melts with a medium to low pressure signature by fluid absent melting during intracrustal loading high pressure ttgs may be generated by mixing residua and melts derived by fluid absent melting or just fluid present melting in a hot subduction zone most ttgs produced before the neoarchean were likely generated at the lower levels of thickened basaltic crust however rare eoarchean and mesoarchean ttgs with high pressure signatures suggest subduction occurred on the early earth in isolated localities we interpret a secular increase in the proportion of high pressure ttgs at c 3 0 2 5 ga as recording a transition from localized subduction to a global plate tectonic regime

decompressional anatexis in the migmatite core complex of northern dabie orogen eastern china petrological evidence and ti in quartz thermobarometry the voluminous diatexites in the dabie orogen of eastern china indicate spectacular anatexis in the orogen basement before exhumation to shallow depths but determination of the pressure temperature p t conditions for anatexis in the core complex is problematic because of the lack of suitable mineral assemblages in the diatexite and of re equilibrations during the retrograde stages to overcome this problem we studied an amphibolitic migmatite which occurs as a raft in the diatexites of the central dome and has been partially melted at a very low degree in such rocks with low degree of melting the microstructures still reveal the high temperature melting reactions and thus mineral analyses can be used to constrain anatectic p t conditions and interpret the geochemical characteristics of partial melts we demonstrate from petrological evidences that incongruent partial melting of amphibole plagioclase quartz lead to the formation of clinopyroxene and silicate melt trace element analyses of clinopyroxenes confirm that they have been equilibrated with anatectic melts two types of amphiboles can be identified and the amphibole cores have high al2o3 9 0 wt and tio2 1 5 wt contents suggesting a nearly constant temperature of 850 850 c with a pressure varying from 6 to 2 kbar in contrast the rims of some amphibole grains have low al2o3 8 0 wt and tio2 1 2 wt contents indicating temperatures lower than 750 c at low pressures 2 kbar we propose two distinct phases in the tectonic evolution of the migmatite core complex in the northern dabie orogen the early stage is a near isothermal exhumation and the investigated rocks are characterized by a decompression from more than 20 km to 6 km low degree partial melting in the infertile quartz poor amphibolite occurs at low pressure during this exhumation phase the second phase is characterized by a shallow nearly isobaric cooling stage these two tectonic phases are recorded in quartz grains using ti in quartz thermobarometry the quartz in contact with or enclosed in clinopyroxene and plagioclase is characterized by ti concentrations with an average value of 50 ppm these quartz grains have been equilibrated at high temperatures and pressures during the anatexis and were chemically isolated in ti poor phases during the low pressure cooling phase on the other hand the quartz enclosed in amphibole has much higher ti concentrations up to 140 ppm which is the result of further incorporation of ti released from the host mineral during the low pressure cooling diffusion modeling on one ti concentration profile in quartz enclosed in amphibole suggests fast cooling within 800 650 c with a rate of ca 0 2 c yr this study demonstrates that the application of the ti in quartz thermobarometer to lithologies with low fertility may be extremely helpful to constrain anatectic p t conditions and exhumation history of migmatized orogen basements

generation and evolution of palaeoarchaean continental crust in the central part of the singhbhum craton eastern india abstract palaeoarchaean granitoids are exposed over wide area in the singhbhum craton whose origin and role in crustal evolution are not well constrained this study presents whole rock and mineral chemical data coupled with zircon u pb dating and hf isotope results on such granitoids from the central part of the craton to understand their petrogenesis tectonic setting and role in continental crustal evolution the first phase of granitoid magmatism in this area is represented by a 3 47 ga na rich gneissic tonalite belonging to the archaean ttg tonalite trondhjemite granodiorite suite this rock is characterized by high hree heavy rare earth elements negative eu anomaly low sr y ratio and positive zircon hf isotope signature ehft 2 1 to 4 8 it is interpreted to be formed by shallow melting of a juvenile mafic source at 3 35 ga a silicic ferroan porphyritic biotite granite formed it shows variable k na low y and high sr y moderately fractionated hree and positive zircon ehft 1 8 to 4 0 and is explained as a product of high temperature melting of a heterogeneous juvenile source consisting of tonalites and mafic rocks at lower crustal depth the final phase of granitoid magmatism is marked by a 3 30 ga non porphyritic ferroan silica rich biotite granite geochemical characteristics like moderately high k moderately fractionated hree low ca and sr y and zircon ehft 0 8 to 3 7 suggest that the granite was formed by high pressure melting of a tonalite dominated source with short crustal residence all the three granitoid phases display low mg ni and cr contents and magnesium number mg precluding direct involvement of mantle in their origin rather crustal reworking caused by episodic plume related mafic ultramafic magma underplating and intraplating in an oceanic plateau setting is suggested as the possible mechanism for formation of the granitoids successive crustal reworking events involved progressively greater amount of previously formed felsic crust generating more evolved k rich granitoids it appears to be a globally important process which led to effective crustal differentiation and maturing of the cratons during palaeoarchaean

field and model constraints on silicic melt segregation by compaction hindered settling the role of water and its effect on latent heat release to investigate how large volumes of silicic melts segregate to form granitic plutons we conducted a case study of a zoned pluton in which sio 2 increases from intermediate 69 wt to highly silicic compositions 74 wt toward the contact with metasedimentary wallrock in the outer 25 m of the pluton all other major minor and trace elements vary systematically with sio 2 and indicate that outward increasing sio 2 is due to a decrease in mafic elements and minerals whole rock oxygen isotopes and elemental variation diagrams do not support mixing with wallrock as an explanation for the si rich boundary layer instead mafic enclaves which are common in the pluton also decrease in abundance in the outer 25 m of the pluton suggesting a mechanical origin for the si rich boundary layer the coupling of mechanical and geochemical boundary layers combined with geochemical modeling indicate that the silica rich enclave poor boundary layer formed by hindered settling or compaction of a crystal rich crystal fractions 60 magmatic mush segregation of melts at high crystal fraction is known to be a slow process however petrography and zr based thermometry indicate that the residual si rich liquids were water saturated water decreases melt viscosity which helps expulsion but equally importantly water also delays much of the latent heat release to late in the thermal and crystallization history of a cooling magma we show that the higher the water content the longer the time interval over which a magma chamber resides at the stage when water saturated high silica liquids form allowing sufficient time for exfiltration of silicic liquids before the magma body freezes

melting of subducted continental crust during collision and exhumation insights from granitic rocks from the north qaidam uhp metamorphic belt nw china abstract the formation and reworking of continental crust have close connections with orogeny syn and post collisional magmatism is key to understanding crustal growth and reworking during continental collision and post collisional extension here we report a series of syn collisional felsic magmatic rocks intruded into migmatized gneiss and amphibolite eclogite in the north qaidam ultrahigh pressure uhp metamorphic belt to provide insights into melting reworking of uhp metamorphosed continental crust during continental collision and exhumation the syn collisional magmas are mainly tonalite trondhjemite and mica granite that occur as plutons and dykes and high potassium granite veins in migmatites they were formed by decompression melting of subducted oceanic and continental crust at different levels during exhumation without any discernible mantle contribution the tonalite trondhjemites show adakitic signatures of both oceanic crust and mafic continental crust with varying nd isotopic compositions the mica granite plutons have high silica and alkali contents are weakly to strongly peraluminous and have crustal sr nd isotope compositions these mica granites were formed by partial melting of previously subducted continental crust and formed mostly in high pressure conditions in the garnet stability field the protolith is predominantly granitic gneiss with minor meta sediment and metabasite a compilation of published ages and our results show that these syn collisional magmatic rocks formed between 437 and 397 ma overlapping with uhp metamorphic ages 440 420 ma and suggests that exhumation occurred synchronously with continental subduction individual zircon grains show metamorphic cores and magmatic rims with a 20 myr time interval interpreted as the time between peak uhp metamorphism and melting during exhumation such long lived and episodic melting of uhp metamorphic continental crust should result from periodic processes involving melt weakening and decompression by upward movement within the subduction channel rather than coherent motion of the entire uhp belt this process represents crustal reworking during collision and exhumation without input of mantle melts

late neoproterozoic ordovician and carboniferous events recorded in monazites from southern central madagascar abstract the evolution of the basement of southern madagascar north and south of the ranotsara shear zone was investigated using u th pb electron probe monazite age dating in combination with petrographic constraints several monazite grains show a stepwise progression of younger ages towards the rim indicating partial and complete resetting during tectonic metamorphic and or fluid events the oldest ages ranging from 630 2400 ma occur relatively rare in relic cores a first clear age population is dated at 550 560 ma most ages fall in two populations at 420 460 and 490 500 ma which in some samples overlap in error we interprete these ages as dating low pressure and high temperature metamorphism we have also clear evidence for carboniferous 300 310 ma monazite overgrowth rims which can not directly be related to macroscopic structures or metamorphic parageneses in combination with literature data we propose that the observed monazite age populations are related to gondwana amalgamation and subsequent rifting events during the break up of gondwana our study confirms that only the electron or ion microprobe yields sufficient spatial resolution to date individual shells of multiple zoned monazites in the polymetamorphic basement of madagascar

co existing fluid and silicate inclusions in mantle diamond we document the compositions of co existing silicate macro inclusions and fluid micro inclusions in the fibrous coats of eight coated diamonds from the panda kimberlite canada the mineral inclusions in the diamond coats come from either the peridotite suite cr pyrope orthopyroxene olivine and cr diopside or the eclogite suite omphacite therefore fibrous diamonds grow in the same paragenetic environments as octahedral diamonds the inclusions document a more fertile source composition lower mg and higher cao than for equivalent phases in octahedral diamonds from panda and worldwide however moderate to high cr2o3 contents in garnet and clinopyroxene inclusions suggest that this apparent fertility is due to a secondary process geothermometry of the silicate inclusions yields low equilibration temperatures of 930 to 1010 c the co existing fluid micro inclusions are dominated by h2o carbonate and kcl fluid inclusions in both the peridotitic and eclogitic samples fall along linear arrays between fe ca mg carbonate and kcl inclusions in the one eclogitic sample also contain quartz we suggest that the diamonds have trapped both metasomatised minerals and the metasomatic fluid and so provide a snap shot of a metasomatic event in the mantle

metamorphism and melting of picritic crust in the early earth abstract partial melting experiments with models of archean oceanic crust maoc with 11 13 and 15 wt mgo are used to investigate the role of metamorphism and melting of primary picritic compositions in the formation of ttg like melts and continental crust on the early earth the approach investigates the possibility that the average early crust composition was comparatively mgo rich and evolved to lower magnesium content during the secular cooling of the earth high pressure partial melting experiments indicate a transition of melt compositions from aluminous basaltic melts in maoc 15 to predominantly tonalitic melts in maoc 11 and higher melting temperatures with increasing magnesium in the bulk composition tonalitic melts were generated in maoc 11 and 13 at pressures 12 5 kbar along with the residual phases garnet clinopyroxene plagioclase quartz orthopyroxene in the presence of quartz and at lower pressures in the absence of amphibole basaltic melts were generated at pressures 15 kbar predominantly in the presence of granulite facies residues such as amphibole garnet plagioclase orthopyroxene that lack quartz in all maoc compositions the tonalitic melts generated in maoc 11 and 13 indicate that thicker oceanic crust with more magnesium than that of a modern morb is a viable source for the generation of early archean high si low mgo melts and therefore ttg like continental crust in the archean the favoured settings for the generation of these melts at pressures up to 15 kbar are the base of oceanic crust much thicker than today or melting of slabs in shallow subduction zones both without interaction of the melts with the mantle during passage to the surface tonalitic melts may have formed in deeper subduction zones at 20 kbar beyond plagioclase stability but it is unlikely that such melts could migrate to shallower levels without further mantle interaction this process may have become more important during the progressive cooling of the earth

formation of cordierite bearing lavas during anatexis in the lower crust beneath lipari island aeolian arc italy cordierite bearing lavas cbl 105 ka erupted from the mt s angelo volcano at lipari aeolian arc italy are high k andesites displaying a range in the geochemical and isotopic compositions that reflect heterogeneity in the source and or processes cbl consist of megacrysts of ca plagioclase and clinopyroxene euhedral crystals of cordierite and garnet microphenocrysts of orthopyroxene and plagioclase set in a heterogeneous rhyodacitic rhyolitic groundmass containing abundant metamorphic and gabbroic xenoliths new petrographic chemical and isotopic data indicate formation of cbl by mixing of basaltic andesitic magmas and high k peraluminous rhyolitic magmas of anatectic origin and characterize partial melting processes in the lower continental crust of lipari crustal anatectic melts generated through two main dehydration melting peritectic reactions of metasedimentary rocks 1 biotite aluminosilicate quartz albite garnet cordierite k feldspar melt 2 biotite garnet quartz orthopyroxene cordierite k feldspar melt their position into the petrogenetic grid suggests that heating and consequent melting of metasedimentary rocks occurred at temperatures of 725 t 900 c and pressures of 0 4 0 45 gpa anatexis in the lower crust of lipari was induced by protracted emplacement of basic magmas in the lower crust 130 ky crustal melting of the lower crust at 105 ka affected the volcano evolution impeding frequent mafic magma eruptions and promoting magma stagnation and fractional crystallization processes

geophysical evidence for the east antarctic plate boundary in the weddell sea an improved gondwanaland reconstruction compatible with geological and geophysical information from the surrounding oceans and continents seems to require microplates to solve the enigmatic pre early mesozoic tectonic relation between west and east antarctica1 new multi channel seismic reflection data from the southeastern weddell sea acquired during the 1984 85 norwegian antarctic research expedition nare have outlined a linear wsw ene trending basement ridge buried below the continental slope over a distance of 700 km this structural high truncates the trend of the large sedimentary basins below the filchner and ronne ice shelves and may continue to within a few hundred kilometres of the antarctic penninsula we interpret the basement ridge as part of the east antarctic plate boundary during the break up of gondwana the morphology and structure of this boundary show greater apparent similarity to a rifted or obliquely rifted margin than to the sheared margin which is predicted by current reconstructions2 3 a linear east antarctic plate margin extending to the vicinity of the antarctic peninsula makes any post rift micro plate motion in the weddell embayment unlikely

distinct sources for syntectonic variscan granitoids insights from the aguiar da beira region central portugal abstract the variscan syntectonic granitoid plutons from the aguiar da beira region central portugal were emplaced into metasediments of late proterozoic early cambrian age during the last variscan ductile tectonic event d 3 which is related to dextral and sinistral shearing the older intrusion with a u pb id tims zircon age of 321 8 2 0 ma consists of porphyritic biotite granodiorite granite with transitional i s type geochemical signature relatively low 87 sr 86 sr 322 ratios 0 7070 0 7074 end 322 values of 3 9 to 4 6 and whole rock and zircon 18 o values of 10 6 and 8 0 respectively by contrast the younger intrusion is an s type muscovite biotite leucogranite emplaced at 317 0 1 1 ma showing more radiogenic 87 sr 86 sr 317 0 7104 0 7146 lower end 317 values of 7 7 to 8 7 and higher 18 o wr 11 3 and 18 o zr 9 5 the combined isotopic and geochemical evidence supports a lower crustal origin for the biotite granodiorite granite involving the anatexis of lower crustal metaigneous protoliths and possible hybridization with mantle derived magmas a shallower origin at mid crustal levels from pure crustal derivation through moderate degrees of partial melting of proterozoic cambrian metasediments is proposed instead for the muscovite biotite leucogranite

macro and microstructural analysis of the zhujiafang ductile shear zone hengshan complex tectonic nature and geodynamic implications of the evolution of trans north china orogen