regional retrograde metamorphism of a high grade terrain the willyama complex broken hill australia abstract retrograde metamorphism has been a major influence in the development of the presently observed lithologies of the willyama complex broken hill two broad types of retrogression are distinguished pseudomorphous and kinematic retrogression the former type of retrogression involves replacement of prograde phases without complete loss of the high grade fabric hence the prograde assemblage can frequently be inferred kinematic retrogression involves the development of a new schistose fabric and like pseudomorphous retrogression is commonly related to f3 deformation retrogression was initiated during the waning stage of prograde metamorphism and was accompanied by an influx of aqueous fluid at similar pressure but lower temperature conditions than prograde metamorphism the source of the water is believed to be the crystallization of cooling partial melts the regional nature of this willyama style of retrogression is attributed to the metasediment rich nature of the sequence the high geothermal gradient and the considerable amount of in situ partial melting

tectonothermal evolution of the mayuan assemblage in the cathaysia block implications for neoproterozoic collision related assembly of the south china craton the mayuan assemblage in the cathaysia block southeast china consists of felsic paragneiss pelitic schist greenschist amphibolite marble calcsilicate and quartzite that underwent three episodes of deformation d1 d3 and four episodes of metamorphism m1 m4 in the early neoproterozoic the m1 assemblage consists of mineral inclusions defining an early foliation s1 within porphyroblasts represented by chlorite muscovite biotite plagioclase quartz in pelitic schist and actinolite chlorite epidote albite quartz enclosed in amphibolite m2 coincides with the development of the regional schistosity s2 and represents the formation of the porphyroblasts and growth of matrix minerals resulting in development of prograde metamorphic zones chlorite biotite garnet staurolite and kyanite zones m3 is simultaneous with the third phase of deformation d3 and produced sillimanite bearing mineral assemblages in pelitic schist and hornblende bearing assemblages in amphibo lite the last metamorphic episode m4 gave rise to the retrogressive assemblage chlorite muscovite in pelitic rock and actinolite chlorite epidote in amphibolite the sequence of mineral assemblages and history of metamorphic reactions built from the petrogenetic grid of pelites suggest a near isothermal decompression clockwise p t path for the mayuan pelitic schists using the tweequ software program the garnet biotite thermometer and garnet muscovite biotite plagioclase barometer yield p t conditions for m1 of 5 5 to 6 0 kb and 450 to 500 c and conditions for the garnet staurolite and kyanite zones of m2 of 6 0 to 7 0 kb and 550 to 600 c 6 0 to 7 5 kb and 600 c and 11 0 to 11 5 kb and 600 c the p t conditions of m3 were estimated at 570 to 625 c and 4 0 to 4 5 kb using the muscovite biotite thermometer and hornblende plagioclase geothermo barometer the garnet chlorite thermometer yields temperatures of 300 to 400 c for m4 but the pressures of m4 cannot be quantitatively estimated because of the lack of a suitable geobarometer these p t estimates also define an near isothermal decompression clockwise p t path which is involved in initial crustal thickening followed by rapid exhumation and final cooling and is related to amalgamation of the cathaysia and yangtze blocks to form the south china craton

garnet orthopyroxene bearing granulites from enderby land antarctica metamorphic pressure temperature time evolution of the archaean napier complex

horizontal structures in granulite terrains a record of mountain building or mountain collapse in many high temperature 800 c granulite terrains the development of characteristic horizontal structures occurred during the metamorphic culmination and was followed by isobaric cooling the absolute magnitude of isobaric cooling commonly 300 c implies cooling intervals of the order of the thermal time constant of the continental lithosphere 100 m y such prolonged isobaric cooling implies that no significant erosional denudation followed the development of the horizontal structures and thus precludes the prograde deformation being responsible for significant crustal thickening rather the prograde deformation more probably records bulk crustal thinning during extensional collapse of a previously thickened crust possibly triggered by detachment of a thickened thermal boundary layer at the base of the lithosphere

successive overprinting granulite facies metamorphic events in the anmatjira range central australia the anmatjira range and adjacent reynolds range central australia comprise early proterozoic metasediments and othogneisses that were affected by three and possibly four temporally distinct metamorphic events m1 4 and deformation events d1 4 in the period 1820 1590 ma the north western portion of the range around mt stafford preserves the effects of 1820 ma m1 d1 and shows a spectacular lateral transition from muscovite quartz bearing schists to interlayered andalusite bearing migmatites and two pyroxene granofelses that reflect extremely low pressure granulite facies conditions over a distance of less than 10 km orthopyroxene cordierite garnet k feldspar quartz bearing gneisses occur at the highest grade implying peak conditions of 750 c and 2 5 0 6 kbar an anticlockwise p t path for m1 is inferred from syn to late d1 sillimanite overprinting andalusite petrogenetic grid considerations and quantitative estimates of metamorphic conditions for inferred overprinting assemblages the effects of m1 have been variably overprinted to the south east by a c 1760 ma m2 d2 event much of the central anmatjira range around ingellina gap comprises orthogneiss deformed during d2 and metapelites that have m1 andalusite and k feldspar overprinted by m2 sillimanite and muscovite the south eastern portion of the range around mt weldon comprises metasediments and orthogneisses that were completely recrystallized during m2 d2 with metapelitic gneisses characterized by spinel sillimanite k feldspar quartz bearing assemblages that suggest peak m2 conditions of 750 c and 5 5 1 kbar overprinting parageneses in metapelitic gneisses imply that d2 occurred during essentially isobaric cooling a third granulite facies event m3 affected rocks in the reynolds range immediately to the south of the anmatjira range at c 1730 ma a possible fourth event m4 with a minimum age of c 1590 my affected both ranges but resulted in only minor overprinting of m1 3 assemblages the superimposed effects of m1 4 mapped for the entire anmatjira reynolds range area indicate that only minor or no dislocation of the regional geology occurred during any of the metamorphic and accompanying folding events although the immediate cause of each of the metamorphic events involved advection the ultimate causes were external to the metasediments and most probably external to the crust

metamorphic evolution and shrimp u pb geochronology of mafic granulites with double symplectites in the fuping metamorphic complex middle palaeoproterozoic trans north china orogen abstract garnet bearing mafic granulites occur as lenses within ttg gneiss or metapelite in the fuping complex middle palaeoproterozoic trans north china orogen they are composed mainly of garnet clinopyroxene orthopyroxene hornblende plagioclase quartz and accessary zircon and titanite four stages of metamorphic mineral assemblages have been found in these mafic granulites a the prograde assemblages m1 are inclusions preserved in the garnet porphyroblasts including clinopyroxene hornblende plagioclase and quartz b the metamorphic peak assemblages m2 consist of garnet porphyroblasts plus matrix minerals clinopyroxene orthopyroxene hornblende plagioclase quartz zircon titanite c the first retrograde assemblages m3 are represented by clinopyroxene orthopyroxene plagioclase quartz symplectites rimming the garnets and 4 the final retrograde assemblages m4 are hornblende plagioclase quartz symplectites rimming or adjacent to the garnets the retrieved metamorphic p t paths pass from 530 600 c 6 5 7 0 kbar m1 through 680 820 c 7 2 11 7 kbar m2 to 730 760 c 3 2 3 9 kbar m3 and finally to 600 640 c 3 2 3 7 kbar m4 all the p t paths of the mafic granulites are clockwise two of these even contain near isothermal decompression itd and subsequent near isobaric cooling ibc segments shrimp u pb dating of metamorphic zircons of the mafic granulites yields a mean weighted age of 1849 9ma which is consistent with the metamorphic time of surrounding ttg gneisses therefore the fuping complex records the collision event between the eastern and western blocks of the north china craton that led to the final assembly of this craton during the late paleoproterozoic

transposed high pressure granulite fabrics cabo ortegal nw spain implications on the scales of deformation localization abstract high pressure granulite facies rocks of the bacariza formation cabo ortegal nw spain were syn metamorphically deformed at the contacts with the bounding units peridotite and eclogite massifs this enabled the formation of meter thick spectacular shear zones with reworked and transposed foliations and lineations the texturally stable mineral assemblage of the new fabrics records an intense ductile deformation of the mineral aggregate at temperatures of 700 800 c associated with amalgamation of eclogite high pressure granulitic rocks and ultramafic sheets in deep portions of a subduction channel the lattice preferred orientation of the main constituent minerals garnet augite amphibole plagioclase quartz and biotite discloses the active deformation mechanisms at the scale of the mineral grains and the relationships with the deformation at larger scales overprinting relationships of the metamorphic assemblages demonstrates that partitioning and deformation localization occurred at different scales under similar high grade conditions complete macroscopic transposition in the shear zones was complementary to meso and microscopic partitioning of deformation intensity and mechanisms between different lithological layers and mineral species

metamorphic evolution and zircon u pb geochronology of the mts huashan amphibolites insights into the palaeoproterozoic amalgamation of the north china craton abstract exposed in the southernmost terminal of the palaeoproterozoic trans north china orogen tnco the garnet bearing amphibolites in the mts huashan metamorphic complex preserve three generations of mineral assemblages including the prograde peak and retrograde metamorphic stages the prograde metamorphic stage m1 is recognized as inclusion type mineral assemblages hornblende plagioclase quartz biotite ilmenite preserved in the garnet porphyroblasts the peak metamorphic assemblages m2 are represented by the garnet porphyroblasts and matrix minerals hornblende plagioclase quartz biotite ilmenite the retrograde metamorphic assemblages m3 are characterized by the symplectitic intergrowth minerals hornblende plagioclase quartz orthopyroxene biotite ilmenite rimming decomposed relict garnet porphyroblasts known as the white eye socket reaction textures calculated by geothermometers and geobarometers the p t conditions of these three metamorphic stages are estimated to be of 5 7 6 0 kbar 660 760 c for the prograde stage m1 6 6 7 7 kbar 700 820 c for the peak stage m2 and 4 5 6 7 kbar 720 760 c for the retrograde stage m3 respectively therefore these three metamorphic stages define clockwise p t paths including nearly isothermal decompression itd processes geochronological studies of zircons from the amphibolites indicate that the protolith of the amphibolites formed at 2 3 ga and the metamorphism occurred at 1 87 1 85 ga and the metamorphic period was further deciphered to be of 1 96 1 80 ga when considering our previous reports it is therefore proposed that the southernmost terminal of the tnco was also involved in the subduction and collision processes between the eastern and western blocks of the north china craton at least as early as 1 96 ga and continued as late as 1 80 ga followed by rapid uplift and exhumation the available data suggest that the palaeoproterozoic tectono metamorphic event of the southernmost terminal of the trans north china orogen not only started earlier but also continued longer than the other terranes in the middle and northern sections elsewhere in the orogen

duration of igneous activity in the sesia magmatic system and implications for high temperature metamorphism in the ivrea verbano deep crust abstract the ivrea verbano zone and serie dei laghi of northern italy constitute a virtually complete section of the adriatic continental crust which was intruded at different levels by coeval magmas of permo carboniferous age collectively referred to as the sesia magmatic system at the deepest levels of the section the relationship between magmatic underplating and granulite facies metamorphism in the ivrea verbano zone may be studied we present new zircon u pb age data which significantly extend the record of igneous activity in this terrane and which demonstrate that igneous pulses began intruding the deep and middle crust no later than 314 ma an age corresponding to that inferred for regional granulite facies metamorphism and predating by more than 20 m y the well documented main magmatic pulse at about 292 to 282 ma a gap in age information from 316 3 ma to 276 4 ma observed in the deep crustal granulite facies paragneiss of the ivrea verbano zone overlaps a gap from circa 314 to 283 ma between igneous and reset zircons in a closely associated mafic ultramafic sill however zircon ages demonstrate intrusive activity in the middle and upper crust during this time gap we infer that prolonged maintenance of temperatures above zircon saturation in the deep crustal paragneiss could explain the gap in zircon ages in those rocks viewed in this light a gap in a geochronological record within high grade terranes might not necessarily mean an absence of events but instead may record the occurrence and duration of a thermal peak

a structural metamorphic study of the broken hill block nsw australia the prograde pressure temperature p t path for the complexly polydeformed proterozoic broken hill block australia has been reconstructed through detailed structural analysis in conjunction with calculation of compositionally specific p t pseudosections of pelitic rock units within a high temperature shear zone that formed early in the tectonic evolution of the terrane whilst the overall p t path for the broken hill block has been interpreted to be anticlockwise the prograde portion of this path has been unresolved our results have constrained part of this prograde path showing an early heating event m1 at p t conditions of at least c 600 c and c 2 8 4 2 kbar associated with an elevated geothermal gradient c 41 61 c km 1 this event is interpreted to be the result of rifting at c 1 69 1 67 ga or at c 1 64 1 61 in the broken hill block early rifting was followed by an episode of lithospheric thermal relaxation and burial during which time sag phase sediments of the upper broken hill stratigraphy paragon group were deposited following sedimentation a second tectonothermal event m2 d2 occurred this event is associated with peak low pressure granulite facies metamorphism c 1 6 ga and attained conditions of at least 740 c at c 5 kbar a regionally pervasive high temperature fabric s2 developed during the m2 d2 event and deformation was accommodated along lithology parallel high temperature shear zones the larger scale deformation regime extensional or shortening of this event remains unresolved the m2 d2 event was terminated by intense crustal shortening during the olarian orogeny during which time the first mappable folds within the broken hill block developed

composite nature of the north china granulite facies belt tectonothermal and geochronological constraints granulite facies rocks are intermittently exposed in a roughly e w trending belt that extends for approximately 2000 km across the north china craton from the helanshan qianlishan wulashan daqingshan guyang and jining complexes in the western block through the huai an hengshan xuanhua and chengde complexes in the trans north china orogen to the jianping western liaoning eastern hebei northern liaoning and southern jilin complexes in the eastern block the belt is generally referred to as the north china granulite facies belt previously interpreted as the lowest part of an obliquely exposed crust of the north china craton recent data indicate that the north china granulite facies belt is not a single terrane instead it represents components of three separate terranes the eastern and western blocks and trans north china orogen each of these units records different metamorphic histories and reflect the complex tectonic evolution of the ncc during the late archean and paleoproterozoic mafic granulites in the eastern block and the yinshan terrane western block underwent medium pressure granulite facies metamorphism at about 2 5 ga with anticlockwise p t paths involving near isobaric cooling following peak metamorphism reflecting an origin related to intrusion and underplating of mantle derived magmas pelitic granulites in the khondalite belt western block underwent medium pressure granulite facies metamorphism at about 2 0 1 9 ga with clockwise p t paths which record the paleoproterozoic amalgamation of the yinshan and ordos terranes to form the western block mafic and pelitic granulites in the trans north china orogen experienced high to medium pressure granulite facies metamorphism at 1 85 ga with clockwise p t paths involving nearly isothermal decompression following peak metamorphism which are in accord with the final collision between the eastern and western blocks to form the north china craton at 1 8 ga the ncgb cannot therefore represent a separate unique terrane instead it reflects the amalgamation of three separate granulite terranes that evolved independently and at different times d 2005 international association for gondwana research published by elsevier b v all rights reserved

exhumation of high grade terranes a review high grade granulite facies terranes are brought to the surface by a combination of uplift and erosion exhumation the reported mechanisms and durations of exhumation are variable and depend pa

characterization and p t evolution of melt bearing ultrahigh temperature granulites an example from the anapolis itaucu complex of the brasilia fold belt brazil between garnet orthopyroxene and sillimanite and coronae or sapphirine quartz ultrahigh temperature metamorphism symplectites involving corundum sapphirine spinel cordierite plagioclase sillimanite orthopyroxene biotite and ilmenite the retrograde segment of the p t path was further constrained by biotite producing reactions a composite p t path was constructed

reinterpretation of the tectonic context of high temperature metamorphism in the broken hill block nsw and implications on the palaeo to meso proterozoic evolution abstract the origin of high temperatures during regional low pressure granulite facies metamorphism within the proterozoic broken hill block australia has been reinterpreted to be the result of burial of anomalously hot rock packages for which the lithospheric geothermal gradient was initially elevated during early rifting ca 1 71 1 67 ga and then maintained during a ca 1 62 ga short lived mid crustal extensional event mid crustal extension at ca 1 62 ga was associated with amphibolite facies metamorphism and elevated lithospheric geothermal gradients and occurred prior to activity along d2 high temperature shear zones peak low pressure granulite facies metamorphism ca 1 60 ga and crustal shortening during the olarian orogeny ca 1 60 1 59 ga this reinterpretation places the broken hill block within an environment in which multiple episodes of transient extension ca 1 71 1 62 ga were followed by a switch to shortening at ca 1 60 ga the interpreted tectonic environment is a continental back arc setting located in the over riding plate of a subduction zone along the southern margin of palaeoproterozoic australia

correlations between the eastern block of the north china craton and the south indian block of the indian shield an archaean to palaeoproterozoic link abstract recent tectonometamorphic data reveal that the north china craton formed by amalgamation of an eastern block eb and a western block wb along the trans north china orogen tnco and the indian shield formed by amalgamation of the south indian block sib and the north indian block nib along the central indian tectonic zone citz the eb of the north china craton and the sib of the indian shield share many similar magmatic and sedimentary features the early archaean supracrustal sequences in both the eb and sib consist of 3 6 3 4 ga fuchsite quartzites pelitic gneisses calc silicates marbles banded magnetic quartzites and amphibolites called the caozhuang group in the eb and the older metamorphic group omg in the sib the middle archaean assemblages in both the blocks can be divided into an early 3 4 3 3 ga phase represented by the chentaigou supracrustal rocks and orthogneisses in the eb and the holenarsipur supracrustal rocks and the gorur orthogneisses in the sib and a late 3 1 2 9 ga phase represented by the qianan supracrustal rocks and the yangyashan orthogneiss in the eb and the sargur group and the peninsular gneiss in sib the late archaean taishan group and its equivalents in the eb are stratigraphically similar to the dharwar supergroup in the sib moreover both the eb and sib are characterised by near contemporaneity of late archaean granitoid intrusive and metamorphic events in addition dome and basin structures are dominant in both the eb and sib and anticlockwise p t paths charaterise the metamorphic evolution of the late archaean granulites in the two blocks the palaeoproterozoic sedimentary volcanic rocks in both blocks comprise lower clastic rich middle volcanic rich and upper clastic carbonate sequences the strong resemblance of the archaean to palaeoproterozoic magmatic and sedimentary records suggests that it is possible that the eb and sib are dispersed remnants of what was once a single continent a possible fit is proposed for the reconstruction of the eb and sib where the northern margin of the eb is placed adjacent to the western margin of the sib with the tnco and wb representing the continuations respectively of the citz and nib this fit suggests that the eb sib and the wb nib represent two discrete continental blocks that joined along the tnco citz orogen during a palaeoproterozoic collisional event fragmentation of the eb and sib may have occurred at the beginning of the mesoproterozoic and by 1 4 ga the eb and sib may have separated into two different continental blocks as evidenced by striking differences in the mesoproterozoic sedimentary successions and associated bimodal volcanics in the two blocks

high pressure granulites from the sudetes south west poland evidence of crustal subduction and collisional thickening in the variscan belt two high grade gneissic complexes of the western sudetes the gory sowie block and the nieznik area complex contain small predominantly felsic granulitic inliers with minor cpx bearing intercalations the p t conditions of the granulite facies events and of the subsequent re equilibration are estimated using the ternary feldspar thermometer and the geo calc computer program version twq jan 92 in the gory sowie granulites the peak granulitic event occurred at c 18 20 kbar and 900 c and the late decompressive re equilibration within a range of 4 10 kbar and temperatures decreasing to 600 700 c the latter event is thought to have coincided with the main metamorphic phase in the surrounding gneisses the p t estimates are more scattered in the nieznik granulites but the peak conditions for the granulitic event are estimated at pressure over 22 kbar possibly around 30 kbar and temperature exceeding 900 c the analysed samples from the nieznik area bear no significant evidence of lower pressure re equilibration integrating the thermobarometric data and some age constraints indicates that the gory sowie granulites belong to the early stage type i granulites of the variscan belt c 400 ma old which are interpreted as fragments of continental crustal materials subducted to mantle depths in the earliest stages of the variscan orogeny the nieznik granulites are more problematic they may belong to a younger high p suite c 350 ma old widespread in the southern and eastern parts of the bohemian massif and possibly related to the climax of the variscan continent continent collision

are granites and granulites consanguineous an important question in petrology is whether the production of granite magma in orogens is a closed system process with respect to mass input from the mantle this is commonly addressed by inversion of geochemical data from upper crustal granites but a complementary approach is to assess the kinship of residual granulites and associated granites in exhumed orogenic crust here we report geochemical data for a suite of contemporaneous metasedimentary granulites and granites from the eastern ghats province india part of a grenville age orogen the prograde metamorphic evolution involved increasing temperature t and pressure p to a metamorphic peak at 1000 c at 0 7 gpa followed by slow close to isobaric cooling variations in the composition of granites are interpreted to be due to local processes including fractionation during melting or crystallization and or peritectic mineral entrainment the nd and sr isotope compositions of the granites can be matched by mixing between different granulites suggesting that they may have been derived solely from sedimentary protoliths leaving behind granulite facies residues however by including geochemical data from an adjacent area to the north it becomes clear that an increasingly important mass input from the mantle was involved in granite genesis from southwest to northeast in the eastern ghats province as confirmed by modeling assimilation fractional crystallization between an exemplar mantle derived melt at 1000 ma and the residual granulites the extreme peak metamorphic temperature and p t evolution suggest extended lithosphere that relaxed thermally to its former thickness during slow cooling we postulate that the spatial variation in mantle input to the granites was related to changing feedback between the rates of extension and flux of mantle melt

contrasting metamorphic p t time paths of the kohistan granulites and tectonics of the western himalayas in the swat indus kohistan area lower crustal rocks of the kohistan arc including amphibolites granulites and ultramafic rocks are widely exposed detailed petrological structural studies reveal that the chilas jijal metamorphic complex comprises essentially intermediate to high pressure granulites with a granulite amphibolite transition controlled by extensive retrograde metamorphism and ductile deformation contrasting patterns of compositional zoning in garnet and orthopyroxene suggest complicated p t paths in the granulites geothermobarometry based on garnet bearing granulites identify two distinct p t time paths a high pressure anticlockwise path of the jijal complex and a low pressure retrograde path of the chilas complex these p t time paths represent the evolution of middle to lower crustal rocks of the kohistan arc the anticlockwise p t time path combined with published isotopic ages reveals that i the high pressure granulitefacies metamorphism is related to the kohistan asia collision and ii the crust of the kohistan arc had been thickened at least 55 km before the india asia collision

proterozoic granulites from the rauer group east antarctica i decompressional pressure temperature paths deduced from mafic and felsic gneisses

origin of granulite terranes and the formation of the lowermost continental crust differences in composition and pressures of equilibration between exposed regional granulite terranes and suites of granulite xenoliths of crustal origin indicate that granulite terranes do not represent exhumed lowermost crust as had been thought but rather middle and lower middle crustal levels application of well calibrated barometers indicate that exposed granulites record equilibration pressures of 0 6 to 0 8 gigapascal 20 to 30 kilometers depth of burial whereas granulite xenoliths which also tend to be more mafic record pressures of at least 1 0 to 1 5 gigapascals 35 to 50 kilometers depth of burial thickening of the crust by the crystallization of mafic magmas at the crust mantle boundary may account for both the formation of regional granulite terranes at shallower depths and the formation of deep seated mafic crust represented by many xenolith suites

tectono metamorphic events in the north atlantic region in the palaeoproterozoic from the view point of high grade metamorphic rocks in the lewisian complex south harris nw scotland abstract the tectono thermal history of the lewisian complex in south harris south harris complex was inferred from its geologic and metamorphic characteristics the lithological assemblages and geochemical features of the complex suggest that its precursory rocks were composed of the subduction related accretionary complex formed in the palaeo convergent margin the complex has suffered the ultra high temperature uht metamorphism that was contemporaneous with the igneous activity to make the south harris igneous complex shic and the subsequent continent continent collisional activity a similar complex recording the geological processes of the subduction the uht metamorphism and the collision has been recognized in the lapland kola belt and new quebec in the palaeoproterozoic this suggests an assembly of micro continents to form the palaeoproterozoic supercontinent in the north atlantic region

dehydration melting fluid buffering and decompressional p t path in a granulite complex from the eastern ghats india a suite of metapelites charnockites calc silicate rocks quartzo feldspathic gneisses and mafic granulites is exposed at garbham a part of the eastern ghats granulite belt of india reaction textures and mineral compositional data have been used to determine the p t x evolutionary history of the granulites in metapelites and charnockites dehydration melting reactions involving biotite produced quartzofeldspathic segregations during peak metamorphism however migration of melt from the site of generation was limited subsequent to peak metamorphism at c 860 c and 8 kbar the complex evolved through nearly isothermal decompression to 530 650 c and 4 5 kbar during this phase coronal garnet grew in the calc silicates while garnet in the presence of quartz broke down in charnockite and mafic granulite fluid activities during metamorphism were internally buffered in different lithologies in the presence of a melt phase the p t path of the granulites at garbham contrasts sharply with the other parts of the eastern ghats granulite belt where the rocks show dominantly near isobaric cooling subsequent to peak metamorphism

the application of zircon cathodoluminescence imaging th u pb chemistry and u pb ages in interpreting discrete magmatic and high grade metamorphic events in the north china craton at the archean proterozoic boundary abstract two distinct late neoarchean to early paleoproterozoic magmatic events are recognized at the northern margin of the eastern block of the north china craton based on interpretation of zircon structure cathodoluminescence signature th u pb chemistry and age the first event is represented by emplacement of the chaihe trondhjemite and xianjinchang tonalite with magmatic zircon core ages ranging from 2559 2553 7 ma respectively these rocks experienced a subsequent high grade metamorphic event constrained by partial recrystallization of primary magmatic zircon and growth of new zircon rims between 2517 and 2490 ma a second magmatic episode generated by partial melting of crustal rocks during this metamorphic event is represented by the hongshilazi syenogranite with an age of 2502 11 ma the short time interval between initial magmatism and high grade regional metamorphism signifies they were related to the same major tectonothermal event when considered along with evidence of a similar sequence of events at this time 800 km to the south in eastern hebei and the previously established 2500 ma age of granulite facies metamorphism in mafic rocks across the craton it is proposed that a large mantle plume developed beneath the north china craton at the close of the archean

granulite formation during continental extension in fiordland new zealand the formation of the high pressure high temperature rocks known as granulites has traditionally been attributed to metamorphism in the deeper parts of continental collision zones and magmatic arcs1 3 more recently greater consideration has been given to the possibility of granulite formation during continental rifting4 6 and in particular4 to whether granulites are forming today beneath the north american basin and range province here we report thermobarometric data and single zircon u pb ages for an analogous but much more deeply exhumed extensional terrain in fiord land new zealand7 8 where early cretaceous granulites underwent renewed granulite facies metamorphism at significantly shallower crustal levels following the extensional collapse of magmatically thickened continental crust our data show that granulite facies metamorphism can take place during continental extension and demonstrate that in western new zealand it was accompanied at higher structural levels by the emplacement of one or more metamorphic core complexes7 9 in keeping with the model postulated4 for the basin and range province

garnet forming reactions in mafic granulites from enderby land antarctica implications for geothermometry and geobarometry

zoned monazite and zircon as monitors for the thermal history of granulite terranes an example from the central indian tectonic zone the growth and dissolution behaviour of detrital metamorphic and magmatic monazite and zircon during granulite facies anatexis in pelitic and psammo pelitic granulites and in garnetiferous granite from the southern margin of the central indian tectonic zone citz have been investigated using reconstructed metamorphic reaction history monazite electron microprobe dating and sensitive high resolution ion microprobe shrimp u pb zircon geochronology whereas the pelitic granulites record medium pressure granulite facies metamorphism bm1 stage the psammo pelitic granulite reached ultrahigh temperatures tmax 880 c at 8 7 kbar the meta psammite additionally records two stages of granulite facies recrystallization bm2 and bm3 irrespective of variations in the bulk rock compositions and peak metamorphic conditions monazite is highly reactive during the bm1 event producing complex chemically zoned crystals textural compositional and chemical ages of these grains indicate the stability of six compositional domains cd1 to cd6 in the paragenetic sequence of which cd1 represents pre metamorphic detrital cores of paleoproterozoic age cd2 and cd3 combined mean age of 1612 14 ma mark two stages of recrystallization of detrital monazite cores during prograde events rims of cd4 monazite ages between 1615 14 and 1586 14 ma on partially to completely equilibrated cores indicate melt crystallization at or immediately following peak bm1p metamorphism cd5 monazite age of 1574 7 ma is restricted to the psammo pelitic granulites and marks final melt crystallization at the solidus during post peak cooling bm1r stage where r represents retrograde metamorphism the metamorphic rim of cd6 monazite age of 1539 24 ma around partially resorbed cd5 domains is linked to the decomposition of bm1 garnet during the terminal hydration event as part of a granulite facies recrystallization event compositionally homogeneous monazite and rims of chemically zoned monazite grains in granite together record a magmatic crystallization age of 1604 9 ma shrimp u pb zircon dating of the psammo pelitic granulite and garnetiferous granite indicates detrital or inherited cores of paleo to neoarchean age 3584 3 to 2530 3 ma which have been variously recrystallized and overgrown by new zircon 1 at 1658 12 ma 2 between 1595 5 and 1590 6 ma 3 at 1574 9 ma these zircon dates are correlated with the timing of the following 1 the protoliths of precursor sediments of the metasedimentary granulites deposited between 2530 and 1658 ma 2 a short lived high grade event 65 70 myr before the culmination of the bm1 granulite facies event 3 a high t anatectic event corresponding to the peak bm1p metamorphism at tmax 900 c 4 final crystallization of anatectic melt at the solidus cf bm1r metamorphic stage these chronological constraints from monazite and zircon when integrated with the metamorphic reaction history and published geochronological data allow recognition of three episodes of granulite facies metamorphism in the citz at 1658 ma pre bm1 event between 1612 and 1574 ma bm1 event and between 1572 and 1539 ma combined bm2 and bm3 events as part of a latest paleoproterozoic to early mesoproterozoic orogenic event this orogeny is linked to the growth of the proto greater indian landmass

hf nd isotopic evolution of the lower crust we report hf isotopic data for over 50 well studied lower crustal samples from three proterozoic and phanerozoic regions in southwest europe eastern australia and southern mexico we use these data to characterize the lu hf isotopic composition of the lower crust and in combination with existing sm nd data to constrain coupled hf nd isotopic behavior and evolution within this reservoir although most of these samples have present day parent daughter p d ratios consistent with hf nd evolution within the terrestrial hf nd array some samples have divergent p d ratios that would evolve out of the terrestrial array in 1 ga or less the present day 176hf 177hf and 143nd 144nd isotopic compositions of all samples with one lone exception plot within the terrestrial array this indicates that 1 some present day p d ratios may be a relatively recently acquired characteristic through magmatic or metamorphic processes not related to the time integrated sm nd and lu hf ratios of their sources and or 2 the lu hf and sm nd p d variations exist on a small hand size scale but not necessarily on a larger scale the lower crust from this initial data set is broadly similar to the upper crust in terms of both its present day p d values and time integrated lu hf and sm nd evolution as a result the lower crust appears to have a hf and nd isotopic composition similar to that of all other crust and mantle reservoirs so far characterized

the role of synmetamorphic igneous rocks in the metamorphism and partial melting of metasediments northwest adirondacks field and petrologic studies along the adirondack lowlands highlands boundary near harrisville ny indicate that heat from the synmetamorphic intrusion of the diana syenite complex intrusion temperature of 1 050 c played a major role in the local metamorphic thermal regime and was responsible for extensive partial melting of adjacent metasedimentary units major paragneiss of engel and engel metamorphic temperatures inferred from two feldspar and spinel quartz assemblages decrease from 850 950 c along the diana metasediment contact to 650 700 c 2 3 km away from the contact metamorphic pressures are 7 0 5 kb as determined from coexisting plagioclase garnet sillimanite quartz kyanite sillimanite and garnet rutile ilmenite sillimanite quartz grail in the paragneiss migmatites consisting of quartz microcline perthite sodic plagioclase leucosomes are generally concordant with the melanosome consisting of biotite sillimanite garnet spinel plagioclase corundum cordierite qualitatively the amount of partial melt and occurrences of corundum bearing assemblages decrease away from the diana contact activity of h2o inferred from coexisting biotite sillimanite quartz garnet k feldspar ranges from 0 01 to 0 17 and is five to ten times lower in corundum bearing rocks

petrography and zircon u pb isotopic study of the bayanwulashan complex constrains on the paleoproterozoic evolution of the alxa block westernmost north china craton abstract the bayanwulashan metamorphic complex bmc exposes along the eastern margin of the alxa block the westernmost part of the north china craton ncc bmc is principally composed of metamorphic rocks with amphibole plagiogneiss biotite plagioclase gneiss and granitic gneiss our research has been focused on the petrography and zircon u pb geochronology of the bmc to better understand the evolution of the alxa block and its relationship with the ncc evidences from field geology petrography and mineral chemistry indicate that two distinct metamorphic assemblages the amphibolite and greenschist facies had overprinted the preexisting granitic gneiss and suggest that the bmc experienced retrograde metamorphic episodes the la icp ms zircon u pb ages reveal that the primary magmatic activities of bmc were at ca 2 30 2 24 ga and the two metamorphic events were at ca 1 95 1 91 ga and ca 1 88 1 85 ga respectively these ages indicate that bmc initially intruded during paleoproterozoic not as previously suggested at archean period the early paleoproterozoic metamorphic records and the magmatic thermochronological data in bmc exhibit different evolution paths between the alxa block and the ncc the alxa block was most likely an independent early paleoproterozoic terrain following different amalgamation processes the alxa block combined with western block at ca 1 95 ga and then united with ncc at ca 1 85 ga

la icp ms u pb zircon ages of the liaohe group in the eastern block of the north china craton constraints on the evolution of the jiao liao ji belt abstract as a major lithostratigraphic unit of the palaeoproterozoic jiao liao ji belt in the eastern block of the north china craton the liaohe group consists of an assemblage that is transitional from a lower arkose and volcanic rich sequence langzishan li eryu and gaojiayu formations through a middle carbonate rich sequence dashiqiao formation to an upper argillaceous sequence gaixian formation tectonically associated with the liaohe group are large volumes of granitoid rocks named the liaoji granite whose emplacement age has been dated at about 2 1 2 2 ga regionally the liaohe group and the associated liaoji granite extend along a nee trending belt that is flanked by 2 45 2 55 ga basement gneisses u pb zircon data and cathodoluminescence cl imaging are used to constrain the maximum depositional and metamorphic ages of the liaohe group which are critical in understanding the tectonic setting and history of the jiao liao ji belt cathodoluminescence images reveal that most detrital zircon grains from the liaohe group are characterized by concentric oscillatory zoning with comparatively low luminescence and high th u ratios suggesting magmatic origin but minor zircon grains have structureless overgrowth or recrystallization rims that are characterized by low th u ratio indicating metamorphic origin results of this study show that the detrital zircon grains of the langzishan formation the lowest sequence of the liaohe group have concordant u pb ages ranging from 2 05 to 2 24 ga suggesting that the liaohe group was deposited at some time after 2 05 ga although most overgrowth or recrystallization rims are too narrow to be analyzed some rims yield concordant or upper intercept ages around 1 93 ga interpreted as the peak metamorphic age of the liaohe group detrital zircon grains from the dashiqiao formation in the middle sequence of the liaohe group have concordant u pb ages ranging from 2 25 to 2 53 ga some of which are similar to those of the basement gneisses in the area this study indicates that the liaohe group was deposited after the emplacement of the liaoji granite and the latter may be a major component of the provenance for the langzishan formation in contrast the protoliths of the middle sequence of the liaohe group were most probably derived from the basement gneisses these new zircon ages for the liaohe group and recent data for the liaoji granite combined with lithological structural and metamorphic considerations support a rifting model for the evolution of the jiao liao ji belt

the origins of granulites a metamorphic perspective although many recent reviews emphasize a uniformity in granulite pressure temperature p t conditions and paths granulites in reality preserve a spectrum of important petrogenetic features which indicate diversity in their modes of formation a thorough survey of over 90 granulite terranes or occurrences reveals that over 50 of them record p t conditions outside the 7 5 1 kbar and 800 50 c average granulite regime preferred by many authors in particular an increasing number of very high temperature 900 1000 c terranes are being recognized both on the basis of distinctive mineral assemblages and geothermobarometry petrogenetic grid and geothermobarometric approaches to the determination and interpretation of p t histories are both evaluated within the context of reaction textures to demonstrate that the large range in p t conditions is indeed real and that both near isothermal decompression itd and near isobaric cooling ibc p t paths are important amphibolite granulite transitions promoted by the passage of co 2 rich fluids as observed in southern india and sri lanka are exceptional and not representative of fluid related processes in the majority of terranes it is considered on the contrary that fluid absent conditions are typical of most granulites at or near the time of their recorded thermal maxima itd granulites are interpreted to have formed in crust thickened by collision with magmatic additions being an important extra heat source erosion alone is not however considered to be the dominant post collisional thinning process instead the itd paths are generated during more rapid thinning 1 2 mm yr exposure related to tectonic exhumation during moderate rate or waning extension ibc granulites may have formed in a variety of settings those which show anticlockwise p t histories are interpreted to have formed in and beneath areas of voluminous magmatic accretion with or without additional crustal extension ibc granulites at shallow levels very rapid 5 mm yr extensional thinning subsequent to collision it is suggested that the preservation of ibc paths rather than itd paths in many granulites is primarily related to the rate and timescale of extensional thinning of thickened crust and that hybrid itd to ibc paths should also be observed most ibc granulites and probably many itd granulites have not been exposed at the earth s surface as a result of the tectonic episodes which produced them but have resided in the middle and lower crust for long periods of time 100 2000 ma following these events the eventual exhumation of most granulite terranes only occur through their incorporation in later tectonic and magmatic events unrelated to their formation

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

geochemistry of exposed granulite facies terrains and lower crustal xenoliths in mexico the trace element compositions of 13 samples from exposed granulite facies terrains and 17 granulitic lower crustal xenoliths were determined using an inductively coupled plasma mass spectrometer to constrain the composition and evolution of the mexican lower crust most xenoliths found are pyroxeneplagioclase orthogneisses although quartzo feldspathic paragneisses form up to 50 of the xenolith population in some areas lithologies in the exposed terrains consist of pelitic and carbonate rich paragneisses charnockites and less abundant basic intermediate and felsic orthogneisses both xenoliths and exposed granulites generally have low abundances of incompatible elements such as k rb th and u the xenoliths however often have lower abundances of these elements than the exposed granulites high k rb ratios coupled with an inverse correlation between k2o and k rb indicate that most samples from the mexican lower crust are depleted in both k and rb furthermore th u ratios similar to upper crustal materials and high la th ratios suggest depletion in both th and u relative to average crustal material removal of a melt phase either after the crystallization of cumulates or following a partial melting event can explain many of the chemical features observed and may be responsible for the elemental depletions some samples however appear to have been unaffected during their residence in the lower crust and chemically resemble their presumed protoliths available geobarometric data suggest that xenoliths equilibrated near the crust mantle boundary 10 kbar and represent the lowermost crust while the exposed granulites equilibrated at shallower crustal levels 7 kbar thus the differences observed between the xenoliths and exposed granulites indicate that there may be chemical and lithologic zonation in the mexican lower crust the lowermost crust in mexico consists of crystallized basaltic liquids cumulates and or residue after the removal of a partial melt as well as garnet rich metasediments

zoning patterns in orthopyroxene and garnet in granulites implications for geothermometry compositional maps of orthopyroxene and garnet of contrasting grain size and in contact with different minerals were made from two paragneiss granulites from the minto terrane of northern quebec the compositional maps provide clear evidence of late exchange of fe fe mg after ca in garnet and al in orthopyroxene had been quenched in the extent of late fe mg exchange was controlled by neighbouring minerals with negligible fe mg gradients against plagioclase and quartz and substantial gradients against exchangeable fe mg minerals cores of grains in contact with exchangeable fe mg neighbours are progressively more reset in fe fe mg as grain size decreases whereas cores of even small grains surrounded by only plagioclase and quartz are not significantly different in fe fe mg than cores of the largest grains gradients of ca in garnet and of al in orthopyroxene in grains of uniform fe fe mg preserve a high temperature retrograde history during which intergranular exchange effected compositional uniformity of mineral rims and intragranular fe mg diffusion in garnet and orthopyroxene was rapid enough to homogenize fe fe mg the transition from efficient intergranular exchange at relatively high temperatures to local fe mg exchange at lower temperatures may have been controlled by loss of an intergranular exchange medium in the rock possibly an internally generated dehydration melt phase implications for geothermometry of granulites include the following numerical values are particular to this study 1 core compositions of garnet and orthopyroxene grains in contact with exchangeable neighbours may be reset in fe fe mg relative to the most refractory compositions by an amount equivalent to 120d c 2 fe mg exchange thermometry using even the most refractory fe fe mg compositions may not record peak granulite conditions possibly recording instead the temperature at which an intergranular exchange medium was lost from the rock and 3 temperature sensitive net transfer equilibria involving al solubility in orthopyroxene yield temperatures up to 150d c higher than maximum fe mg exchange temperatures even in grains with flat fe fe mg compositional profiles making them a better means of estimating peak granulite temperatures than fe mg exchange thermometry

tectonothermal history of the basement rocks in the western zone of the north china craton and its tectonic implications the basement of the north china craton can be divided into the eastern central and western zones based on lithological structural metamorphic and geochronological data the western zone comprises two different petrotectonic units archaean tonalitic trondhjemitic granodioritic ttg grey gneisses and metamorphic mafic rocks and palaeoproterozoic khondalite series the former is characterized by isobaric cooling ibc type anticlockwise p t paths in the north northwestern part of the zone and near isothermal decompression itd type clockwise p t paths in the eastern part adjacent to the central zone on the other hand the tectonothermal evolution of palaeoproterozoic khondalite series rocks is characterized exclusively by nearly isothermal decompression following the peak of metamorphism and then cooling defining clockwise p t paths the archaean ttg gneisses and associated mafic rocks with anticlockwise metamorphic p t paths reflects an origin related to underplating and intrusion of mantle derived magmas which may be derived from mantle plumes they represent a late archaean continental block in the western part of the north china craton the palaeoproterozoic khondalite series rocks represent passive continental margin deposits they were metamorphosed and deformed in the late palaeoproterozoic during the amalgamation of the western continental block with another continental block in the east part of the north china craton the itd type clockwise p t t paths of the palaeoproterozoic khondalite series rocks record the tectonothermal histories of the collision of the western and eastern continental blocks which resulted in the final assembly of the north china craton at c 1800 ma

proterozoic anticlockwise p t path of the lewisian complex of south harris outer hebrides nw scotland the leverburgh belt and south harris igneous complex in south harris northwest scotland experienced high pressure granulite facies metamorphism during the palaeoproterozoic the metamorphic history has been determined from the following mineral textures and compositions observed in samples of pelitic quartzofeldspathic and mafic gneisses especially in pelitic gneisses from the leverburgh belt 1 some coarse grained garnet in the pelitic gneiss includes biotite and quartz in the inner core sillimanite in the outer core and is overgrown by kyanite at the rims 2 garnet in the pelitic gneiss shows a progressive increase in grossular content from outer core to rims 3 the alvi aliv ratio of clinopyroxene from mafic gneiss increases from core to rim 4 retrograde reaction coronas of cordierite and hercynite cordierite are formed between garnet and kyanite and orthopyroxene cordierite and orthopyroxene plagioclase reaction coronas develop between garnet and quartz 5 a p t path is deduced from inclusion assemblages in garnet and from staurolite breakdown reactions to produce garnet sillimanite and garnet sillimanite hercynite with increasing temperature and 6 in sheared and foliated rocks hydrous minerals such as biotite muscovite and hornblende form a foliation modifying pre existing textures the inferred metamorphic history of the leverburgh belt is divided into four stages as follows m1 prograde metamorphism with increasing temperature m2 prograde metamorphism with increasing pressure m3 retrograde decompressional metamorphism with decreasing pressure and temperature and m4 retrograde metamorphism accompanied by shearing peak p t conditions of the m2 stage are 800 30 c 13 14 kbar pressure increasing from m1 to m2 suggests thrusting of continental crust over the south harris belt during continent continent collision the inferred p t path and tectonic history of the south harris belt are different from those of the lewisian of the mainland

high temperature reduced granulite facies nature of garnetites in the khabarny mafic ultramafic massif southern urals evidence from fluid and mineral analyses

u pb zircon geochronology of the adirondack mountains and implications for their geologic evolution u pb zircon studies of metamorphosed igneous rocks in the adirondack mountains have yielded preliminary ages within the range 1420 990 ma several geochronologically and geochemically distinct episodes of igneous intrusion and at least one pre granulite facies dynamothermal metamorphic event are documented this information is consistent with recent field and geochronological studies throughout the grenville province and suggests that a complex sequence of events occurred in the adirondack mountains prior to the widespread deformation and metamorphism commonly attributed to the approximately 1100 1000 ma ottawan phase of the grenvillian orogenic cycle

high pressure granulites in the sanggan area north china craton metamorphic evolution p t paths and geotectonic significance high pressure basic granulites are widely distributed as enclaves and sheet like blocks in the huaian ttg gneiss terrane in the sanggan area of the central zone of the north china craton four stages of the metamorphic history have been recognised in mineral assemblages based on inclusion exsolution and reaction textures integrated with garnet zonation patterns as revealed by compositional maps and compositional profiles the p t conditions for each metamorphic stage were obtained using thermodynamically and experimentally calibrated geothermobarometers the low ca core of growth zoned garnet along with inclusion minerals defines a prograde assemblage m1 of garnet clinopyroxene plagioclase quartz yielding 700 c and 10 kbar the peak of metamorphism at about 750 870 c and 11 14 5 kbar m2 is defined by high ca domains in garnet interiors and inclusion minerals of clinopyroxene plagioclase and quartz kelyphites or coronas of orthopyroxene plagioclase magnetite around garnet porphyroblasts indicate garnet breakdown reactions m3 at conditions around 770 830 c and 8 5 10 5 kbar garnet exsolution lamellae in clinopyroxene and kelyphites of amphibole plagioclase around garnet formed during the cooling process at about 500 650 c and 5 5 8 kbar m4 these results help define a sequential p t path containing prograde near isothermal decompression itd and near isobaric cooling ibc stages the clockwise hybrid itd and ibc p t paths of the hp granulites in the sanggan area imply a model of thickening followed by extension in a collisional environment furthermore the relatively high pressures 6 14 5 kbar of the four metamorphic stages and the geometry of the p t paths suggest that the hp granulites together with their host huaian ttg gneisses represent the lower plate in a crust thickened during collision the corresponding upper plate might be the tectonically overlying khondalite series which was subjected to medium to low pressure mp lp 7 4 kbar granulite facies metamorphism with a clockwise p t path including an itd segment both the hp and the mp lp granulite facies events occurred contemporaneously at c 1 90 1 85 ga in a collisional environment created by the assembly process of the north china craton

low p high t metamorphism in the okiep copper district western namaqualand south africa the okiep copper district part of the 1 2 1 0 ga high grade terrane in western namaqualand is composed of a mid proterozoic supracrustal sequence and several pre to post orogenic intrusive suites affected by two high grade events m2a m2b m3 of kibaran and one low grade event m4 of pan african age peak assemblages in quartz bearing pelites are characterized either by garnet cordierite coexisting with sillimanite biotite or by biotite sillimanite garnet a difference controlled by bulk composition and variation in water activities 0 1 0 7 during dehydration melting maximum p t conditions were reached during m2a coevally with the major deformational event d2a and are estimated at 750 820 c and 5 6 kbar a counterclockwise p t path is indicated by regionally occurring pseudomorphs of sillimanite after andalusite and by prograde reaction textures preserved as relics in m2a porphyroblasts two stages of retrograde metamorphism are distinguished m2a garnet cordierite bearing assemblages were retrogressed to biotite sillimanite quartz m2b along discontinuous foliation planes and shear zones d2b retrograde m3 corona assemblages formed at similar p t conditions 580 660 c and 5 8 0 5 kbar to the m2b assemblages but m3 crystallization postdates penetrative d2 deformation intrusion of 1 06 ga granitoids and formation of associated w mo deposits it is concluded that a kibaran high grade metamorphism in the okiep copper district is thermally punctuated and b reaction textures documenting apparent isobaric cooling of this low p high t terrane must be interpreted with caution

thermobarometry in granulites pitfalls and new approaches abstract computer models of cooling granulite facies rocks with the assemblage garnet biotite sillimanite k feldspar plagioclase quartz are presented that incorporate simultaneous reaction net transfer and exchange and multicomponent diffusion in garnet in models where cooling is accompanied by exchange reactions plus diffusion only the calculated garnet core matrix biotite temperature is always lower than the metamorphic peak by an amount that depends on the cooling rate the size of the garnet and the volumetric ratio of garnet to biotite maximum calculated core temperatures of approximately 700 750 c are to be expected from typical granulite facies garnet biotite assemblages and calculated core temperatures can easily be as low as 600 c in models where both net transfer and exchange reactions are possible calculated garnet core matrix biotite temperatures are typically higher than the metamorphic peak temperatures by more than 150 c for large garnets r 2 mm but may be lower than the metamorphic peak for small garnets r the composition of matrix plagioclase in equilibrium with garnet is very sensitive to the amount of modal plagioclase that reacts and hence calculated p t trajectories are very sensitive to the petrologic interpretation of this recrystallization history for models in which all the modal plagioclase reacts the core and rim pressures may be faithfully recovered provided that the temperature estimate for the core is reliable for models in which only a small percentage of the modal plagioclase reacts very small modal amounts of plagioclase may be in equilibrium with garnet rim which if incorrectly interpreted may lead to grossly erroneous p t path interpretations a method to test an inferred p t path model is presented that involves attempting to match the observed garnet and plagioclase zoning information with a forward model that involves simultaneous reaction and diffusion an example from western new hampshire usa is presented that closely matches the measured profile

thermal evolution of two textural types of mafic granulites in the north china craton evidence for both mantle plume and collisional tectonics mafic granulites from the north china craton can be divided into two textural types referred to as a and b types a type mafic granulites display garnet quartz symplectic coronas and outcrop in the eastern and western zones of the craton whereas b type mafic granulites exhibit orthopyroxene plagioclase clinopyroxene symplectites or coronas and are mainly exposed in the central zone of the craton most a type mafic granulites preserve the prograde m 1 peak m 2 and post peak near isobaric cooling m 3 assemblages which are represented respectively by inclusions of hornblende plagioclase quartz a peak mineralogy of orthopyroxene clinopyroxene plagio clase quartz garnet and overprinted by garnet quartz symplectic coronas these mineral assem blages and their p t pressure temperature estimates define anticlockwise p t evolutionary paths the b type mafic granulites preserve the peak m 1 post peak near isothermal decompression m 2 and cooling m 3 assemblages which are represented by the peak assemblage of orthopyroxene clinopyroxene plagioclase quartz garnet hornblende post peak orthopyroxene plagioclase clinopyroxene symplectites or coronas and later hornblende plagioclase magnetite symplectites respectively these mineral assemblages and their p t estimates define clockwise p t paths the anticlockwise p t paths of the a type mafic granulites in the eastern and western zones of the north china craton are consistent with a model of underplating and intrusion of mantle derived magmas in combination with lithological structural and geochronological data the eastern and west ern zones of the north china craton are considered to represent two continental blocks that devel oped through the interaction of mantle plumes with the lithosphere from the palaeoarchaean to the neoarchaean era the b type mafic granulites and associated rocks in the central zone represent a magmatic arc that was metamorphosed and deformed during amalgamation of the eastern and west ern continental blocks in the late palaeoproterozoic era the mineral reaction relations and clockwise p t paths of the b type mafic granulites from the central zone record the tectonothermal history of the collision that resulted in the final assembly of the north china craton at c 1800 ma

origin and evolution of granulites in normal and thickened crusts rocks buried in the upper part of a doubly thickened crust during orogeny can show prograde reactions with transitional features from lower grade rocks up to granulite facies they can be exposed at the earth9s surface within this one erogenic cycle for example the granulites exposed in british columbia have concordant u pb zircon ages of 65 85 ma in contrast rocks buried in the lower part of a thickened crust will crystallize granulites that typically show only retrograde relations with younger lower grade rocks at the cessation of orogeny such rocks will undergo near isobaric cooling from the peak metamorphism and can have very long residence times at the base of a crust of normal thickness these granulites require a second orogeny to uplift and expose them at the surface as an example the archean granulites of enderby land antarctica were metamorphosed at 3070 ma at 1000 c at 8 10 kbar pressure they then followed at least a 400 c isobaric cooling path they also had a prolonged residence time of 2000 m y near the base of the crust before uplift no doubt a continuum exists between these extremes but it is proposed that many intermediate and high pressure granulites form in the lower parts of thickened crust and therefore require a two stage cycle of tectonism to be uplifted and exposed at the earth9s surface although most granulites probably form in thickened crust at active plate margins underplating of normal 30 40 km thickness crust by mantle melts during extension can also produce granulites these granulites would also undergo isobaric cooling and require a later orogeny to expose them an example is the paleozoic lachlan fold belt of eastern australia which has undergone only minor uplift since the major ultrametamorphism at about 400 ma however isotopic petrologic and geochemical differences are to be expected between these extremes of granulite formation

very high temperature moderate pressure metamorphism in the new russia gneiss complex northeastern adirondack highlands metamorphic aureole to the marcy anorthosite the new russia gneiss complex in the northeastern adirondack highlands of new york includes meta anorthosite gneiss and anatectic gneiss derived from metagabbro mangerite and charnockite metamor

metamorphic evolution and thermobaric structure of the subduction related bacariza high pressure granulite formation cabo ortegal complex nw spain abstract the high pressure bacariza granulite formation comprises various lithostratigraphic units of granulite orthogneisses ultramafic mg rich mafic intermediate and common mafic granulites as well as of more exotic intercalations mineral assemblages in equilibrium in ultramafic to intermediate rocks contain garnet clinopyroxene and plagioclase with different amounts of zoisite clinozoisite kyanite quartz scapolite rutile and ilmenite depending on the granulite lithotype whereas granulite orthogneisses contain garnet phengite biotite k feldspar antiperthitic plagioclase quartz and rutile as primary phases thermobarometry of these rocks supports the existence of a high pressure metamorphism for which near peak p t conditions have been estimated at ca 790 c and 1 6 gpa the preserved fabrics and structures enable us to relate the metamorphism to coeval polyphasic deformational processes dynamic retrogression began under still high pressure granulite facies conditions 1 4 gpa and ca 740 c and is postdated by symplectitization 1 3 gpa and ca 715 c subsequent retrogression under medium pressure amphibolite facies conditions at similar temperature was either widespread and static or localized and dynamic as a result of intense deformation partitioning during uplift loading heating and subsequent decompression cooling are related to a single cycle in a subduction conduit setting this study suggests that high pressure granulite metamorphism might not be as uncommon in the high pressure metamorphic series as previously thought moreover it might constitute a diagnostic feature of convergent lithospheric settings whether or not associated with eclogite facies metamorphism in adjacent units

p t grids for silica undersaturated granulites in the systems mas n 4 and fmas n 3 tools for the derivation of p t paths of metamorphism abstract considering the minerals cordierite cd sapphirine sa hypersthene hy garnet ga spinel sp sillimanite si and corundum co in the system feo mgo al2o3 sio2 fmas the stable invariant points are co ga cd and sa constraints imposed by experimental data for the system mas indicate that under low ph2o conditions the invariant points occur at high temperature 900 c and intermediate pressure 7 10 kbar this temperature is higher than that commonly advocated for granulite facies metamorphism in granulites fe mg exchange geothermometers may yield temperatures of 100 150 c below peak metamorphic conditions and evidence for peak temperatures is best preserved by relict high temperature assemblages and by al rich cores in orthopyroxene application of the fmas grid to some well documented granulite occurrences introduces important constraints on their p t histories rocks of different bulk compositions occurring in close proximity in the field may record distinct segments of their p t paths this applies particularly to rocks with evidence for reaction in the form of coronas symplectites and zoned minerals consideration of curved reaction boundaries and xms isopleths may explain apparently contradictory results for the stability of cordierite obtained from low temperature experiments and thermochemical calculations on the one hand and hightemperature experimental data on the other

archean blocks and their boundaries in the north china craton lithological geochemical structural and p t path constraints and tectonic evolution an examination of lithological geochemical geochronological structural and metamorphic p t path data suggests that the basement of the north china craton can be divided into eastern and western blocks separated by major crustal boundaries that roughly correspond with the limits of a 300 km wide zone called the trans north china orogen the eastern block consists predominantly of late archean domiform tonalitic trondhjemitic granodioritic ttg batholiths surrounded by anastomosing networks and linear belts of open to tight synforms of minor volcanic and sedimentary rocks metamorphosed from greenschist to granulite facies at 2 5 ga with anticlockwise p t paths some early to middle archean rocks are locally present in the eastern block but their tectonic history is unclear due to reworking by the 2 5 ga tectonothermal event the western block has a late archean assemblage structural style and metamorphic history similar to that of the eastern block but it differs in the absence of early to middle archean assemblages and in being overlain by and interleaved with paleoproterozoic khondalites which were affected by a 1 8 ga metamorphic event involving clockwise p t paths a mantle plume model is proposed for the formation and evolution of late archean basement rocks in the eastern and western blocks based on a combination of extensive exposure of ttg gneisses affinities of mafic rocks to continental tholeiitic basalts presence of voluminous komatiitic rocks dominant diaprism related domiform structures anticlockwise p t paths and a short time span from the primary emplacement of ttg and ultramafic to mafic rocks until the onset of regional metamorphism between the two blocks is the trans north china orogen which is bounded by two major fault systems and is composed of late archean to paleoproterozoic ttg gneisses and granitoids interleaved with abundant sedimentary and volcanic rocks that are geochemically interpreted as having developed in magmatic arc and intra arc basin environments these rocks underwent multiple phases of compressional deformation and peak high pressure metamorphism followed by rapid exhumation during the late paleoproterozoic at 1 8 ga as a result of collision between the eastern and western blocks resulting in the amalgamation of the north china craton

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

late neoarchean magmatic and subsequent metamorphic events in the northern north china craton shrimp zircon dating and hf isotopes of archean rocks from yunmengshan geopark miyun beijing abstract the miyun geopark is located in the northern part of the north china craton and includes a variety of archean granitoids and metamorphic rocks shrimp zircon dating documents their emplacement ages at 2567 2508 ma metamorphic events at ca 2 50 ga 2 44 ga and 1 82 ga late neoarchean basement rocks elsewhere in the north china craton document a similar evolutionary history with intrusion of granitoid rocks commencing at ca 2590 ma and reaching three peaks at about 2550 2520 and 2500 ma these rocks then underwent regional metamorphisms at ca 2 50 ga 2 44 ga 1 92 ga and 1 82 ga positive e hf t values of 1 7 to 7 6 with model ages younger than 3 0 ga for magmatic zircons from these late neoarchean intrusive rocks indicate that they are predominantly derived from juvenile crustal sources and suggest that significant crustal growth occurred in the northern north china craton during the neoarchean the intrusive rocks in the miyun area are part of an extensive province of late neoarchean magmatism and appear to be related to mantle plume activity at this time

the dunhuang block is a paleozoic orogenic belt and part of the central asian orogenic belt caob nw china abstract the dunhuang region is located in the east of the tarim craton and has long been considered as part of a precambrian terrane of the tarim or north china craton aiming to further understand the tectonic evolution of the dunhuang region and its relations with the adjacent tectonic units this study is to investigate the metamafic rocks in the sanweishan area dunhuang region and in turn to summarize the tectono thermal events and re examine the tectonic characteristics of the region the metamafic rocks in the sanweishan area include three types of amphibolites they are garnet clinopyroxene amphibolites garnet free amphibolites and garnet mica amphibolites the whole rock major and trace element compositions suggest that their protoliths probably are e morb type tholeiitic basalts with some crustal addition the la icp ms zircon u pb data of the garnet clinopyroxene amphibolite and the garnet free amphibolite yielded lower intercept ages of 410 11 ma and 403 8 ma respectively while the garnet mica amphibolite gave three metamorphic ages of 441 3 ma 408 12 ma and 370 2 ma three suites of metamorphic conditions have been estimated at 1 14 1 55 gpa and 458 548 c 0 67 0 82 gpa and 590 600 c and 0 47 0 66 gpa and 557 586 c for garnet clinopyroxene amphibolite using different mineral thermobarometers which illustrate a collisional orogeny related clockwise p t path integrated with the study on the high pressure granulite in the mogutai area it can be inferred that the metamafic rocks in the sanweishan area obviously experienced amphibolite facies metamorphism at ca 410 400 ma and they could also underwent high grade metamorphism at ca 440 430 ma therefore we suggest that the e w trending collisional orogeny of ca 440 400 ma could sweep cross the dunhuang region the dunhuang region records multiple tectono thermal events occurring at ca 3 1 2 5 ga ca 2 3 1 6 ga ca 440 400 ma and ca 370 310 ma however compared with the limited exposure of the precambrian complex the early silurian late carboniferous ca 440 310 ma orogeny related metamorphic and magmatic rocks are widely distributed in the dunhuang region making up nearly 70 of the total exposure thus it is proposed that the geological bodies in the dunhuang region were mainly produced by a paleozoic orogeny and represent a paleozoic orogen while the limited precambrian complex probably is a microcontinental fragment or part of the margin of the tarim or north china cratons that was involved in this paleozoic orogeny during the early silurian late carboniferous we infer that the dunhuang orogenic belt is part of the central asian orogenic belt caob and thus the caob probably extends beyond the beishan orogenic collage and southward into the dunhuang region

metamorphic evolution of aluminous granulites from labwor hills uganda sapphirine cordierite quartz and spinel cordierite quartz form relic assemblages of probable archaean age in fe rich aluminous metapelites from labwor hills uganda and reflect an unusually high temperature metamorphism 1 000 c at pressures in the vicinity of 7 9 kbars and a o2 near the magnetite hematite buffer subsequent reaction textures include the replacement of spinel and cordierite by sillimanite and hypersthene and formation of sapphirine hypersthene k feldspar quartz symplectites which are interpreted as pseudomorphs after osumilite a petrogenetic grid appropriate to these assemblages suggests these reaction textures may be due to cooling at constant or increasing pressure and constant a o2 or decreasing a o2 at constant temperature and pressure the former interpretation is supported by the coexistence of ilmenohematite and magnetite during the development of the reaction textures and by the comparatively low al2o3 contents of secondary hypersthene this pressure temperature path implies that 1 metamorphism occurred at deep levels within normal thickness crust probably less than 40 45 km thick due to an extreme thermal perturbation induced either by emplacement of mantle derived magmas or by thinning of the subcontinental lithosphere in an extensional tectonic regime 2 the excavation and surface exposure of the granulites is due to a subsequent postgranulite facies metamorphism crustal thickening most probably involving their incorporation into an allochthonous upper crustal thrust sheet during the formation of the mozambique foldbelt

crustal trajectory of sapphirine bearing granulites from ganguvarpatti south india evidence for an isothermal decompression path abstract ganguvarpatti is part of a precambrian terrane characterized by granulite facies rocks including charnockites mafic granulites sapphirine bearing granulites leptynites and gneisses a sequence of reactions deduced from the multiphase reaction textures provide information on the metamorphic history of this area as they formed in response to decompression during uplift geothermobarometry and constraints from reaction textures define a segment of a p t path traversed by the granulites of ganguvarpatti near peak metamorphic conditions of c 800 c and 8 kbar were succeeded by a symplectitic stage at a significantly lower pressure c 700 c and 4 5 kbar documenting a nearly isothermal decompression p t path and rapid uplift c 12 km followed by cooling the presence of many fluid inclusions of extremely low density in the charnockites is consistent with a nearly isothermal uplift path attainment of a maximum pressure of c 8 kbar indicates c 27 km depth of burial during metamorphism this would imply a total crustal thickness of c 65 70 km at 2 6 2 5 ga such a profound crustal thickness and a clockwise decompressive p t path is interpreted as a consequence of tectonic thickening of crust accomplished by collision tectonics of the southern granulite terrane against the dharwar craton along the palghat cauvery shear zone via northward subduction

thermal controls on early tertiary short lived rapid regional metamorphism in the nw himalaya pakistan abstract during tertiary collision in the nw himalaya the leading edge of the indian plate was subducted beneath the kohistan island arc along the main mantle thrust mmt metamorphism within indian plate cover sediments was synchronous with ductile shearing and took place along a path of increasing pressure during subduction beneath the island arc initial collision cannot have pre dated 65 ma and probably shortly pre dated 50 ma radiometric data constrain the metamorphic peak as shortly post dating 50 ma as firstly initially subducted units are now probably located beneath tibet secondly the subduction thrust separating the kohistan arc terrane from the indian plate was probably cooled by continued underthrusting and thirdly the heat producing indian plate cover sediments were delaminated from the basement during collision metamorphism was more rapid than can be predicted by purely conductive models of thermal relaxation although dissipative shear heating along the mmt doubtless contributed to early stages of heating of the footwall rocks the temperatures attained in the footwall are too high to support the shear stresses required to generate them solely through shear heating a model is derived to account for both the rapid regional metamorphism and the equally rapid post metamorphic cooling dissipative shear heating along the mmt generated an early inverted thermal profile in the upper units of the indian plate as the hanging wall mafic rocks have a low thermal conductivity they would have acted as a thermal reflector and the heat would have been conducted away only slowly as footwall temperatures increased through the brittle ductile transition the role of dissipative shear heating decreased and continued heating became a function of internal heat generation within the footwall rocks together with hanging wall thermal reflectivity the metamorphic inversion was reinforced by imbrication of the metamorphic stack as it accreted onto the mmt footwall during early stages of uplift and exhumation dissipative shear heating within thrust systems provides a potentially important mechanism by which areas of large regional extent can undergo regional metamorphism over short timescales in the absence of magmatic heating

cooling and exhumation history of the northeastern gawler craton south australia abstract we present biotite 40ar 39ar ages and from the palaeo to mesoproterozoic mount woods inlier in the northeastern gawler craton south australia in combination with garnet biotite fe mg exchange modelling using the thermal history program these data are used to constrain a post peak metamorphic p t path cooling rates and a temperature time path for the inlier consistently well defined plateaus at 1 53 ga of biotite 40ar 39ar age spectra indicate that the rocks cooled through 300 c at this time cooling rates within the inlier are 3 9 c ma from peak high temperature low pressure metamorphism at 1 59 ga to 1 53 ga cooling rates subsequently decreased to 4 c ma cooling is suggested to be a function of cessation of voluminous magmatism and mantle heat input in conjunction with exhumation initiated along the southwestern margin of the inlier ca 1 592 1 582 ga the temperature time path of the inlier has a concave shape the peak metamorphic and ensuing cooling history of the mount woods inlier are akin to the nearby curnamona province coober pedy ridge and mabel creek ridge within the south australian craton deformation during high grade metamorphism within these terranes was accommodated at mid crustal levels and large scale exhumation did not occur until the terranes cooled and were able to be exhumed as coherent rigid blocks along discrete shear zones and faults

microblock amalgamation in the north china craton evidence from neoarchaean magmatic suite in the western margin of the jiaoliao block abstract the archaean earth is considered to have been characterized by microcontinents that formed dominantly through the accretion of oceanic arcs and plateaus the north china craton ncc provides a typical case where at least seven ancient microcontinental nuclei with distinct lithological features and independent tectonic histories were amalgamated into the cratonic framework at the end of the archaean here we investigate a suite of magmatic rocks developed at the periphery of one of these microblocks the jiaoliao block that forms part of the composite eastern block of the ncc we present petrological geochemical and zircon u pb geochronological data from the taipingzhai charnockite suite and associated amphibolites metagabbros and orthogneisses from the qianxi complex geochemically the rocks show a wide range of sio 2 charnockite suite 52 57 75 50 wt metagabbro 43 71 wt amphibolite 50 24 wt garnet bearing biotite 63 73 wt and mgo charnockite suite 0 89 5 01 wt metagabbro 3 99 wt amphibolite 6 23 wt garnet bearing biotite 2 08 wt the composition of the felsic units straddle from diorite through syeno diorite to granite with both alkalic and subalkalic affinity with dominantly magnesian composition and arc related features their immobile trace element relationships suggest calc alkaline affinity they show positive pb ba la nd and gd and negative nb ta sr th and ti anomalies with slightly negative anomalies of ce and y attesting to arc related features in tectonic classification diagrams the rocks plot in the vag syn colg field or the vag area suggesting subduction related origin the dominant population of zircons in all these rocks displays magmatic crystallization features including high th u values with core rims textures indicating subsequent thermal events the zircon u pb data yield upper intercept ages of 2587 10 ma to 2543 17 ma and 207 pb 206 pb mean ages of 2578 7 3 ma to 2536 8 ma for the charnockite suite marking the timing of emplacement of the arc magmas the overgrowth rims as well as discrete neoformed grains are interpreted as dating subsequent metamorphism and yield 207 pb 206 pb ages between 2533 ma to 2490 ma zircons in the metagabbro preserve upper intercept ages of 2556 20 ma representing the crystallization age of this rock the younger ages of 2449 58 ma upper intercept age and 1845 25 ma 207 pb 206 pb spot age are interpreted to represent subsequent multiple thermal events in this area zircons in the amphibolite preserve the 207 pb 206 pb mean age of 2539 9 ma representing the crystallization age of this rock the garnet bearing biotite gneiss shows an upper intercept age of 2562 10 ma mswd 0 66 n 36 and the 207 pb 206 pb mean age of 2561 9 ma mswd 0 63 n 33 which is taken to represent the crystallization age of this rock some inherited zircons are also identified with 207 pb 206 pb ages of 2664 26 ma and 2628 26 ma zircon lu hf data show dominantly positive ehf t values and combined with crustal residence ages the results suggest mesoarchean to neoarchean juvenile crust formation in the ncc we interpret the data presented here to represent a phase of major late neoarchaean arc magmatism along the western margin of the jiaoliao block related to the birth of microcontinental nuclei within the ncc our data suggest that the western and eastern blocks might not have existed as discrete crustal blocks and that the construction of the ncc is a result of the assembly of several microblocks or terranes at the end of archaean similar archean cratonic nuclei in other regions of the world might have formed part of a primitive supercontinent in the early earth

petrogenesis and geodynamics of late archean magmatism in eastern hebei eastern north china craton geochronological geochemical and nd hf isotopic evidence abstract the eastern part of the north china craton contains large volumes of late archean hornblenditic tonalitic dioritic granodioritic and granitic rocks la icpms zircon u pb dating of three plutons from eastern hebei eastern north china craton record emplacement ages of 2526 2515 ma and metamorphic ages of 2500 2490 ma for the diorite granodiorite and monzogranite and 2440 ma for the k feldspar granite hornblendites have high mgo 8 6 10 6 wt cr 674 1126 ppm ni 125 159 ppm and v 224 315 ppm at low basic silica contents sio2 45 7 49 0 wt they are enriched in light rare earth elements lrees and large ion lithophile elements liles and depleted in high field strength elements hfses with positive nd t values 1 2 to 2 0 indicating they were derived from an enriched mantle source the monzogranites have high sr and ba contents and sr y and la yb ratios low y and hrees abundances and positive nd t and zircon hf t values with nd and hf model ages less than 3 0 ga reflecting that they were derived from partial melting of a juvenile lower crust at high pressure however the k feldspar granites with low sr and ba concentrations and high hree and y contents are considered to have originated from partial melting of juvenile crustal materials at shallow crustal levels the diorites and granodiorites have variable major and trace element concentrations magmatic zircons from these rocks have heterogeneous hf t values but record uniform homogeneous u pb ages indicating they resulted from mixing of mafic and felsic magmas coupled with fractional crystallization because of the widespread distribution of late archean magmatism and evidence for magma mixing possibly related to mafic magma underplating it appears likely that this event in the eastern north china craton may relfect mantle plume activity that resulted in significant growth of continental crust during the late archean

thermal history vs fabric anisotropy in granitoids emplaced at different crustal levels an example from calabria southern italy the sila and serre granitoids of calabria were emplaced in the late carboniferous at depths ranging from 6 to 23 km in a postcollisional extensional regime their fabric which developed during and after final crystallization up to the solid state strongly increases in intensity with emplacement depth this relationship is attributed to the thermal history of the calabrian basement cooling histories of granitoids constrained by geological data and rb sr cooling ages on micas demonstrate that residence times of rocks at temperatures greater than those of the brittle ductile transition vary greatly as a function of initial emplacement depth this explains why shallow level granitoids that remained for about 10 myr at temperatures above those of the brittle domain retain their original magmatic fabric by contrast the strong fabrics of the deep seated granitoids are explained by solid state strain overprint that lasted more than 100 myr at temperatures above those of the brittle domain

porewater geochemical and lipid biomarker signatures for anaerobic methane oxidation in the hydrate bearing system from the taixinan basin the south china sea abstract anaerobic oxidation of methane aom is one of the most important biogeochemical processes associated with hydrate dissociation and carbon cycling though aom has been extensively investigated the variations of its microorganisms and lipid biomarkers are still not fully understood in this study the porewater geochemical and lipid biomarker evidence for aom was systematically investigated in the hydrate bearing system from the taixinan basin the south china sea aiming to better expound the vertical variations of aom related biomarker signatures and causes of the weak lipid biomarker patterns accordingly the sulfate methane interface smi depth of 1 000 cmbsf and onset of aom were determined on the basis of the vertical profiles of porewater parameters the occurrence of 13c depleted 2 6 10 15 19 pentamethylicosane pmi with the 13c ratios of 77 7 to 131 4 near the smi are in accord with the porewater parameters despite the absence of archaeol and sn2 hydroxyarchaeol and low abundances of pmi meanwhile the strongly 13c enriched iso c15 0 and anteiso c15 0 fatty acids suggest that non methane hydrocarbons or oil derivatives are speculated as the probable carbon sources for sulfate reducing bacteria due to the presence of unresolved complex mixtures ucms and odd even predominance oep values about 1 0 the weak in situ signal of aom related biomarkers may be attributed to the low flux methane supply and dilution effect from the high input of allochthonous and autochthonous organic matters the great difference in biomarkers between the taixinan basin and previously reported sites should be further clarified on the basis of more extensive studies in various aom related settings

neoarchean metamorphic evolution and geochronology of the miyun metamorphic complex north china craton abstract mafic and felsic granulite from the miyun metamorphic complex northeast beijing north china craton occur as either interlayers or lenses within the late archean ttg gneiss in the mafic granulite the metamorphic peak assemblages m1 are represented by porphyroblast minerals plus matrix minerals orthopyroxene clinopyroxene plagioclase quartz k feldspar hornblende biotite as well as accessory minerals of zircon apatite ilmenite magnetite the post peak assemblages m2 are characterized by red eye socket symplectites garnet clinopyroxene plagioclase quartz rimming the orthopyroxene or clinopyroxene porphyroblast as well as accessory minerals of rutile and the final retrograde assemblages m3 are marked by the hornblende biotite intergrowth as for the felsic granulite the peak assemblages m1 are represented by the porphyroblast and matrix minerals garnet orthopyroxene biotite plagioclase quartz k feldspar as well as accessory minerals in the matrix zircon monazite apatite pyrite the post peak assemblages m2 are characterized by red eye socket symplectites garnet plagioclase quartz rimming the orthopyroxene porphyroblast and the final retrograde assemblages m3 are marked by the biotite calcite intergrowth geothermobarometric computation suggests that the m1 m2 and m3 assemblages of the mafic granulite were formed under p t conditions of 800 to 860 c 6 to 10 kbar 620 to 700 c 11 to 12 kbar and 590 to 660 c 4 to 6 kbar respectively suggesting that these granulites record anticlockwise p t paths including final nearly isothermal decompression itd segments indicative of orogenic process for the felsic granulite metamorphic p t conditions of the m1 and m3 assemblages were estimated to be 680 to 750 c 6 8 kbar and 670 to 680 c 5 kbar respectively also including itd process the widely penetrative gneissosity of the granulite and these p t paths suggest a collisional event of cold nappes overthrusting on hot ones or hot nappes being buried subducted into cold ones and finally experienced fast tectonic exhumation high precision sims and la icp ms u pb dating of metamorphic zircon of the granulite suggest that the metamorphism occurred at 2 5 ga these data confirm that the miyun metamorphic complex experienced a neoarchean collisional event within the eastern block of the north china craton

carbonic metamorphism at ultrahigh temperatures evidence from north china craton abstract the north china craton ncc constitutes one of the fundamental precambrian nuclei of asia the paleoproterozoic khondalite belt along the northern margin of the ncc shows evidence for extreme crustal metamorphism at temperatures exceeding 1000 c and pressures of 10 12 kbar with the formation of diagnostic ultrahigh temperature uht mineral assemblages including sapphirine quartz high alumina orthopyroxene sillimanite and high temperature mesoperthites here we record for the first time the common occurrence of co2 rich fluid inclusions in peak uht minerals in the mg al granulites of ncc and trace the fluid evolution history from microthermometric and laser raman spectroscopic studies we show that the syn metamorphic fluid in the uht rocks was pure co2 while very high density carbonic fluids are preserved in some cases corresponding to entrapment at peak conditions many of the inclusions underwent density modification during the rapid and near isothermal exhumation of the rocks following peak uht metamorphism and initial cooling at depth the present study area within the northern margin of the ncc defines a collisional orogenic belt where continental collision and deep subduction were possibly followed by extensional collapse of the orogen evidence for the extensional phase is provided by the several mafic magmatic intrusions of paleoproterozoic age that are seen emplaced within the granulite facies supracrustals the thermal anomaly displayed by the uht granulites may thus correlate with asthenospheric upwelling and emplacement of mafic magmas possibly associated with plume activity which provided the heat and co2 input to achieve extreme crustal metamorphism and generation of dry granulite facies assemblages our study is the first report of syn metamorphic co2 rich fluids in uht granulites from a paleoproterozoic ca 1 92 ga terrane and provide evidence for carbonic metamorphism at ultrahigh temperatures as north china craton joined the columbia supercontinent amalgam

metamorphic and tectonic evolution of granulites arunta block central australia petrographical and chemical evidence presented here indicate a prolonged metamorphic evolution of the granulite terrain in the northern strangways range arunta block central australia nsr in fig 1 beginning with migmatization followed by dry granulite metamorphism at high temperatures t and moderate pressures p estimated at 850 920 c 8 1 kbar1 this occurred at 1 800 myr refs 2 3 very shortly after deposition of the protolith4 a bimodal volcanic suite5 and associated sediments granulite metamorphism was followed by isobaric cooling during which there was an episode of pervasive but partial hydration later retrogression and hydration localized along shear zones occurred at the normal geothermal gradient in the kyanite stability field a tensional rift like regime best describes the geological environment in which the protolith formed was deeply buried metamorphosed with high heat flow and then remained in isostatic equilibrium metamorphic parageneses recording uplift may be matched with mid palaeozoic tectonic events in central australia if this correlation is correct the rocks of the northern strangways range may have remained deeply buried for 1 400 myr during which the shear zones were reactivated several times fig 2

on the formation of granulites the tectonic settings for the formation and evolution of regional granulite terranes and the lowermost continental crust can be deduced from pressure temperature time p t time paths and constrained by petrological and geophysical considerations p t conditions deduced for regional granulites require transient average geothermal gradients of greater than 35 c km 1 implying minimum heat flow in excess of 100 mwm 2 such high heat flow is probably caused by magmatic heating tectonic settings wherein such conditions are found include convergent plate margins continental rifts hot spots and at the margins of large deep seated batholiths however particular p t time paths do not allow specific tectonic settings to be distinguished at this time under different conditions both clockwise cw pmax attained before tmax and anticlockwise acw pmax attained slightly after tmax paths are possible in the same tectonic setting both cw and acw end member paths can yield nearly isobaric cooling ibc paths such cooling paths are clearly not an artefact of thermobarometry but can be constrained by solid solid and devolatilization equilibria and geophysical modelling in terms of understanding the evolution of the deep crust a potentially significant group of regional granulite terranes are those that show evidence for acw ibc paths such paths are the likely result of i episodic igneous activity resulting in intrusions within all levels of the crust ii thickening of the crust by magmatic underplating iii slow uplift as a result of the formation of a deep garnet rich crustal root and iv excavation resulting from a later tectonic event unrelated to that resulting in the formation of the granulites the later event might be triggered by the delamination of the garnet rich lowermost crust

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

precambrian multi stage crustal evolution in the bamble sector of south norway pb isotopic evidence from a sveconorwegian deep seated granitic intrusion abstract the precambrian ubergsmoen augen gneiss in the bamble sector of south norway is a dry syntectonic charnockitic intrusion which was emplaced at amphibolite to granulite facies conditions in the sveconorwegian orogeny 1120 ma the present day 206 pb 204 pb and 207 pb 204 pb ratios show restricted variation 206 pb 204 pb 17 566 17 938 207 pb 204 pb 15 459 15 590 in whole rocks and 206 pb 204 pb 17 159 17 350 207 pb 204 pb 15 451 15 487 in feldspar whereas the 208 pb 204 pb ratio has a wider range of variation in whole rocks 36 571 42 200 but not in feldspars 36 487 37 054 these values reflect a 238 u 204 pb of 2 6 since the sveconorwegian and a variable th u ratio 2 20 at the time of final u pb differentiation homogenization of the pb isotopes was imperfect the data therefore do not allow pb pb isochron dating but are well suited for model calculations the isotopic evolution of pb can be reproduced by a three stage model involving 1 a mantle stage ending at 1 9 1 6 ga 2 extraction of a crustal precursor at 1 9 1 6 ga and differentiation of a lile enriched component 14 22 and 3 anatexis igneous differentiation emplacement and subsequent metamorphism at 1 12 ga the granitic intrusion acquired its low u pb ratio by magmatic differentiation processes rather than by fluid induced u loss at high grade metamorphic conditions

p t t evolution of orogenic belts and the causes of regional metamorphism barrow 1893 introduced three important ideas that furthered understanding of metamorphic processes i the use of critical index minerals in argillaceous rocks to define metamorphic zones and elucidate spatial features of regional metamorphism ii the concept of progressive metamorphism and iii the concept of magmatic advection of heat as a possible cause of regional metamorphism this article expands upon these themes by reviewing our understanding of the dynamic evolution of orogenic belts as interpreted from the p t t paths of metamorphic rocks and by considering the likely causes of the different kinds of regional metamorphism that we observe within orogenic belts understanding metamorphic rocks allows the distinction of two fundamentally different types of orogenic belt defined by relative timing of maximum t and maximum p orogenic belts characterized by clockwise p t paths achieved maximum p before maximum t the metamorphic peak normally post dated early deformation within the belt and additional heating above the normal conductive flux has been related to the amount of overthickening by contrast orogenic belts characterized by counterclockwise p t paths achieved maximum t before maximum p the metamorphic peak normally pre dated or was synchronous with early deformation within the belt and additional heating above the normal conductive flux has been related to the emplacement of plutons techniques used to constrain portions of p t t paths include the use of mineral inclusion suites in porphyroblasts and reaction textures thermobarometry the use of fluid inclusions thermodynamic approaches such as the gibbs method radiogenic isotope dating fission track studies and numerical modelling we can utilize specific mineral parageneses in suitable rocks to determine individual p t t paths and a set of p t t paths from one orogenic belt allows us to interpret the spatial variation in dynamic evolution of the metamorphism recent advances are reviewed with reference to collision metamorphism high temperature low pressure metamorphism granulite metamorphism and subduction zone metamorphism and some important directions for future work are indicated

geochemical constraints on the genesis of the algoma type banded iron formation bif in yishui county western shandong province north china craton abstract the yishui iron deposit is a typical algoma type banded iron formation bif located in yishui county taishan group shandong province in the eastern block eb of the north china craton ncc the bif is interlayered with amphibolite migmatitic granite gneiss and schist integrated petrological mineralogical and geochemical interpretations of the iron ores were used to reconstruct the genesis of the yishui bifs and their tectonic evolution the iron ore samples were collected from surface outcrops banding textures are not always present in the iron ores because the bif experienced complex high grade metamorphism after precipitation interpretations of the major elements and rare earth elements plus yttrium rey data of the iron ores suggests that the ore forming materials of the yishui bif precipitated from a mixture of seawater and high temperature hydrothermal fluids 2 and fe 2 o 3 t sio 2 fe 2 o 3 t 85 8 95 8 wt are consistent with the major minerals such as quartz and iron oxide which represent chemical precipitates chlorite amphibole plagioclase biotite and calcite are present as minor minerals the low contents of tio 2 0 01 0 09 wt al 2 o 3 0 42 1 18 wt and high field strength elements hfses indicate little to no detrital assimilation which implies that the iron ores preserved their original geochemical signatures based on geochemical interpretations we conclude that the yishui bif can be explained by a mantle plume model that describes not only the tectonic evolution but also the sources of iron and silica of the yishui bif

anticlockwise p t path of granulites from the monte castelo gabbro ordenes complex nw spain the study of mafic and aluminous granulites from the monte castelo gabbro ordenes complex nw spain reveals an anticlockwise p t path that we interpret as related to the tectonothermal activity in a magmatic arc probably an island arc the p t path was obtained after a detailed study of the textural relationships and mineral assemblage succession in the aluminous granulites and comparing these with an appropriate petrogenetic grid additional thermobarometry was also performed the granulites are highly heterogeneous with distinct compositional domains that may alternate even at thin section scale garnets are generally idiomorphic to subidiomorphic and in certain domains of the aluminous granulites they show overgrowths forming xenomorphic coronas around a more or less idiomorphic core both types of garnets show significant ca enrichment at the crystal rims which together with the other mineralogical and textural characteristics is compatible with a pressure increase with low t variation p t estimations indicate a peak of t 800 c and p 9 5 kbar attained after a significant increase in pressure that took place at high temperatures in the sillimanite field we suggest that this kind of trajectory probably anticlockwise is compatible only with a terrane heated by an intense magmatic activity after or during tectonic crustal thickening magmatic injection at the base and or into the crust which is characteristic of magmatic arcs

petrology and geochemistry of the neoproterozoic guaxupe granulite facies terrain southeastern brazil abstract geochemical and oxygen isotopic data on the neoproterozoic granulite facies rocks enderbites charnockite and charnokitic augen gneisses of guaxupe brazil are assembled geochemical discrimination function calculations show an igneous origin for majority of the rock types in the area major and trace element data especially ree are similar to tonalite and granodiorite oxygen isotope composition for charnockitic gneisses and mafic granulites yield 18o values in the range of 8 8 to 11 and fall in the range of quartzofeldspathic rocks of medium to high grade terrains elsewhere a small scale variation of 18o values compatible with the chemical data imply the preservation of pre metamorphic oxygen isotope compositions minimum peak pressure temperature conditions are 8 5 kbar and 850 c the high grade metamorphism did not appear to have altered significantly the geochemical and oxygen isotope compositions of the protoliths approximately 2 5 kbar of pressure decrease accompanied by 200 c cooling has been recorded for a portion of the cooling path the p t path is consistent with a single event namely the formation and uplift of the guaxupenappe in the neoproterozoic brasiliano cycle