sector trilling in cordierite and equilibrium overstepping in metamorphism the origin of sector trilling in cordierite is due to the hexagonal orthorhombic al si ordering transformation which under non equilibrium conditions proceeds via a short range ordered modulated structure the growth of these distortion waves associated with progressive ordering produces a strain field which is minimized by a cyclic distribution of symmetrically equivalent modulations sector and complexly trilled cordierites in metamorphic rocks grew as the hexagonal polymorph with a considerable degree of al si disorder the enthalpy and entropy of disorder are evaluated from recent experimental work the implication is that in metamorphic rocks substantial overstepping of stable equilibrium phase boundaries is required to nucleate hexagonal cordierite moreover its composition coexisting with other phases will also be significantly different from that of the stable ordered form

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

sapphirine bearing orthopyroxene kyanite sillimanite granulites from south harris nw scotland evidence for proterozoic uht metamorphism in the lewisian sapphirine bearing orthopyroxene kyanite opx ky and sillimanite opx sil granulites have been found in the lewisian complex of south harris in northwest scotland in the opx ky granulites orthopyroxene and kyanite are intergrown in a stable mineral assemblage which indicates metamorphic condition at 800 900 c 12 kbar sillimanite inclusions within orthopyroxene suggest that sillimanite formed earlier conditions are estimated at 950 30 c at 10 kbar from orthopyroxene isopleths for aluminous orthopyroxene 8 kbar according to the fmas petrogenetic grid and similar conditions were obtained by using orthopyroxene garnet geothermobarometers the two types of orthopyroxene aluminosilicate granulites indicate that the peak metamorphic conditions were over 900 c compatible with ultra high temperature metamorphism as accessory sapphirine occurs in several assemblages and with different compositions it is interpreted to be formed at different stages of the metamorphism these granulites were formed during early proterozoic high grade metamorphism due to the emplacement of the south harris igneous complex at c 2170 1870 ma and are not related to the major metamorphic episode of the badcallian inverian metamorphism at c 2700 2500 ma in the mainland lewisian

equilibration and reaction in archaean quartz sapphirine granulite xenoliths from the lace kimberlite pipe south africa ultrahigh temperature quartz sapphirine granulite xenoliths in the post karoo lace kimberlite south africa comprise mainly quartz sapphirine garnet and sillimanite with rarer orthopyroxene antiperthite corundum and zinc bearing spinel constant accessories are rutile graphite and sulphides comparison with assemblages in the experimentally determined fmas and kfmash grids indicates initial equilibration at 1040 c and 9 11 kbar corona assemblages involving garnet sillimanite and minor cordierite developed on a near isobaric cooling p t path as both temperature and to a lesser extent pressures decreased garnet orthopyroxene fe mg exchange thermometers record temperatures of only 830 916 c these estimates do not indicate the peak metamorphic conditions but instead reflect the importance of post peak fe mg exchange during cooling correction of mineral fe mg compositions for this exhange using a convergence approach of fitzsimons harley 1994 leads to retrieved p t estimates from garnet orthopyroxene thermobarometry c 1000 c and 10 5 0 7 kbar that are consistent with the petrogenetic grid constraints u pb dating of a single zircon grain gives an age of 2590 83 ma interpreted as the age of the metamorphic event protolith major and trace element chemistries of the xenoliths differ from sapphirine quartzites typical of the napier complex antarctica but are comparable to less siliceous high cr and ni sapphirine granulites reported from several ultrahigh temperature granulite terranes

site effects in port au prince haiti from the analysis of spectral ratio and numerical simulations abstract to provide better insight into seismic ground motion in the port au prince metropolitan area we investigate site effects at 12 seismological stations by analyzing 78 earthquakes with magnitude smaller than 5 that occurred between 2010 and 2013 horizontal to vertical spectral ratio on earthquake recordings and a standard spectral ratio were applied to the seismic data we also propose a simplified lithostratigraphic map and use available geotechnical and geophysical data to construct representative soil columns in the vicinity of each station that allow us to compute numerical transfer functions using 1d simulations at most of the studied sites spectral ratios are characterized by weak motion amplification at frequencies above 5 hz in good agreement with the numerical transfer functions a mismatch between the observed amplifications and simulated response at lower frequencies shows that the considered soil columns could be missing a deeper velocity contrast furthermore strong amplification between 2 and 10 hz linked to local topographic features is found at one station located in the south of the city and substantial amplification below 5 hz is detected near the coastline which we attribute to deep and soft sediments as well as the presence of surface waves we conclude that for most investigated sites in port au prince seismic amplifications due to site effects are highly variable but seem not to be important at high frequencies at some specific locations however they could strongly enhance the low frequency content of the seismic ground shaking although our analysis does not consider nonlinear effects we thus conclude that apart from sites close to the coast sediment induced amplification probably had only a minor impact on the level of strong ground motion and was not the main reason for the high level of damage in port au prince

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

regional uht metamorphism with widespread primary co2 rich cordierite in the bakhuis granulite belt surinam a feldspar thermometry study abstract a metapelite area in the paleoproterozoic bakhuis granulite belt bgb in western surinam shows mineral assemblages diagnostic of ultrahigh temperature metamorphism uhtm high al orthopyroxene sillimanite quartz and sapphirine quartz metapelites elsewhere in the belt contain cordierite sillimanite locally associated high al orthopyroxene might indicate that the cordierite sillimanite assemblage was formed by uhtm the mg rich cordierite has an elevated birefringence up to blue second order which is evidence for a considerable amount of co2 in its lattice given the absence of a reliable model for co2 bearing cordierite phase equilibrium modelling cannot be applied to determine the metamorphic conditions of these rocks instead ternary feldspar thermometry using the fuhrmann and lindsley model was applied to mesoperthite and antiperthite in 25 samples of mainly metapelite throughout the bgb the temperatures determined range between 900 and 1050 c the temperatures are minimum estimates for the metamorphic temperature because the feldspar crystallized upon cooling from melts generated during peak metamorphism these results imply that most of the bgb witnessed uht metamorphism and that the widespread assemblage cordierite sillimanite high al orthopyroxene indeed formed under uhtm conditions the widespread formation of primary cordierite is unusual for uhtm terranes and is probably the result of its considerable co2 content

very high resolution seismic stratigraphy of pleistocene eustatic minima markers as a tool to reconstruct the tectonic evolution of the northern latium shelf tyrrhenian sea italy this paper analyzes depth variations of seismically detected lowstand features i e paleo shelf break and lowstand submerged depositional terraces lsdts in order to define vertical movements along a continental margin narrowly spaced high resolution two dimensional reflection seismic data were used collected along a segment of the continental shelf of latium eastern tyrrhenian sea italy seismostratigraphic analysis allowed us to identify the six most recent fourth order depositional sequences formed in the past 500 k y they have a different degree of preservation in the outer and middle shelf with almost no continuation in the inner shelf some wedge shaped bodies interpreted as lsdts can be identified in these units the depth of paleo shelf break and lsdts of marine isotope stage 12 ca 435 ka was measured and gradually varies along the margin between 220 m and 131 m below current sea level differential subsidence rates have been estimated for the area varying from 0 to 0 21 mm a the spatial variations agree with studies of uplifted coastal terraces on land controlled by localized volcanic activity along the latium section of the tyrrhenian coast the comparison of deformation onshore and offshore has proven to be a promising approach for understanding the evolution of uplifting continental margins

theoretical phase relations involving cordierite and garnet revisited the influence of oxygen fugacity on the stability of sapphirine and spinel in the system mg fe al si o the theoreticalp t grid for stability relations of the phases cordierite cd sapphirine sa hypersthene hy garnet ga spinel sp sillimanite si and quartz qz of hensen 1971 has proved useful in the interpretation of metamorphic mineral assemblages formed at low oxygen fugacity both experimental data and evidence from natural rocks indicate that at high oxygen fugacity compatability relations change as a result of the enlargement of the stability field of spinel which causes a topological inversion and the stabilisation of the invariant points sa ga and cd this implies the stable existence of the univariant equilibria for f text o text 2 buffered conditions sp qz ga hy si o2 sa cd cd sp qz hy si o2 sa ga and sa sp qz hy si o2 ga cd and the divariant reaction sp qz hy si o2 sa ga cd these redox equilibria are restricted to conditions of high oxygen fugacity the proposed theoreticalp t grids for both low and high oxygen fugacity satisfactorily explain all experimental data and metamorphic mineral assemblages so far found in granulites

pyroxene exsolution in granulites from fyfe hills enderby land antarctica evidence for 1000 degrees c metamorphic temperatures in archean continental crust reply

paleoproterozoic uht metamorphism with isobaric cooling ibc followed by decompression heating in the khondalite belt north china craton new evidence from two sapphirine formation processes

stability of the assemblage orthopyroxene sillimanite quartz in the system mgo feo fe2o3 al2o3 sio2 h2o the stability of coexisting orthopyroxene sillimanite and quartz and the composition of orthopyroxene in this assemblage has been determined in the system mgo feo fe2o3 al2o3 sio2 h2o as a function of pressure mainly at 1 000 c and at oxygen fugacities defined mostly by the hematite magnetite buffer the upper stability of the assemblage is terminated at 17 kbars 1 000 c by the reaction opx al silicate gar qz proceeding toward lower pressures with increasing fe fe mg ratio in the system the lower stability is controlled by the reaction opx sill qz cord which occurs at 11 kbars in the iron free system but is lowered to 9 kbars with increasing fe fe mg spinel solid solutions are stabilized besides quartz up to 14 kbars in favour of garnet in the iron rich part of the system fe fe mg 0 30 ferric ferrous ratios in orthopyroxene are increasing with increasing ferro magnesian ratio at least part of the generally observed increase in al content with fe2 in orthopyroxene is not due to an increased solubility of the mgalalsio6 component but rather of a mgfe3 alsio6 component the data permit an estimate of oxygen fugacity from the composition of orthopyroxene in coexistence with sillimanite and quartz

osumilite in the sapphirine quartz terrane of enderby land antarctica implications for osumilite petrogenesis in the granulite facies

mobilization of gold in the deep crust evidence from mafic intrusions in the bamble belt norway abstract the location of mesothermal gold deposits in the upper part of major transcurrent shear zones suggests that gold was derived from the deep wider portions of the shears the 30 km wide bamble belt south norway is a deep shear belt metamorphosed to upper amphibolite and granulite grade at about 1 1 ga preliminary studies showed that its rocks are strongly depleted in gold and other chalcophile elements mafic rocks were chosen for more detailed study of the mobilization of gold during metamorphism gold in mafic rocks largely partitions into the sulphide phase which is mainly comprised of pyrrhotite modification of this phase was required to liberate gold bamble has two main groups of mafic rock the first metabasites intruded before peak metamorphism are uniformly low in gold their o2 at peak metamorphism obtained from ilmenite orthopyroxene equilibria indicates a strongly oxidized mineral assemblage well above fmq pyrrhotite was stable at peak temperature of 800 c but on cooling below 600 c internal mineralogical buffering by the oxidized assemblage caused the o2 to cross into the stability field of pyrite following conversion of pyrrhotite to pyrite the latter was partly replaced by magnetite these serial changes in the mineralogical form of iron sulphide was conducive to the extraction of gold as was the oxidized nature of the mineral assemblage that would have buffered metamorphic fluids to high o2 the second group of mafic rocks hyperites intruded after peak metamorphism record intermediate steps in the extraction of gold these intrusions of coronitic gabbro were metamorphosed in whole or in part to amphibolite there is a decrease in gold and other chalcophile elements across a hyperite dyke from the pyrrhotite bearing coronitic gabbro interior to the pyrite bearing amphibolite margin

sapphirine bearing granulites from the sipiwesk lake area of the late archean pikwitonei granulite terrain manitoba canada sapphirine occurs in the orthopyroxene cordierite and feldspar sillimanite granulites in the sipiwesk lake area of the pikwitonei granulite terrain manitoba 97 40 w 55 05 n the orthopyroxene cordierite granulites have extremely high al2o3 24 5 wt and mgo 24 6 wt contents and contain sapphirine up to 69 2 wt al2o3 aluminous orthopyroxene up to 8 93 wt al2o3 cordierite spinel phlogopite and corundum sapphirine forms coronas mantling spinel and corundum corona sapphirine is zoned and its composition varies through the substitution mg fe mn si 2 al as a function of the phases with which it is in contact textural and chemical relationships of sapphirine with coexisting phases indicate that spinel cordierite reacted to form orthopyroxene sapphirine under conditions of increasing pressure moreover decreasing core to rim variation of al2o3 in orthopyroxene porphyroblasts suggests decreasing temperature during sapphirine formation on the basis of experimentally determined p t stability of the assemblage enstatite sapphirine cordierite and the al content of hypothetical fe2 free orthopyroxene associated with sapphirine and cordierite metamorphic temperatures and pressures are estimated to be 860 890 c and 3 0 11 2 kbar

continental thermal and tectonic regimes during the archaean metamorphic pressures estimated from mineral assemblages in archaean high grade terrains may be used to constrain geothermal regimes in the archaean continental crust in two ways first they may be combined with temperature estimates to yield bounds on the heat flux during the thickening events that produced the metamorphism the pressure temperature data do not require any greater supply of heat to the base of the continental crust in the archaean than is supplied at the present day second the pressures alone may be used to infer the height of archaean mountains these mountains appear to have had elevations comparable with those of present day mountain belts it seems probable that the elevation of present day belts is limited by the creep strength of the lithosphere and in view of the strong temperature dependence of creep in earth materials this second observation also implies that archaean continental thermal regimes were similar to present day ones these observations together with the common

geo tectonic framework of the himalaya of n pakistan in the karakorum range there is a structurally complicated cretaceous are comprising the kohistan sequence on its northern side the northern suture consists of a mega melange and is bounded to the s by tightly folded pillow bearing volcanics and sediments to the s the kohistan plutonic belt consists of southwards a early foliated and late post tectonic tonalites and diorites b aplites and pegmatites up to 30 of rock volume c basic dykes up to 10 m thick d the chilas complex a stratiform cumulate body over 300 km long and 8 km thick chromite layered dunites gabbros and norites with a low pressure granulite facies mineral fabric of tectonic origin e an amphibolite belt with a complex mixture of other rocks and f the jijal complex a 200 km 2 tectonic wedge of high pressure granulites and chromite layered dunites cumulate graded units in the chilas complex show that it is folded by an isoclinal anticline f 1 the mid upper crust of the are is folded by a 50 km half wavelength f 2 syncline the whole kohistan sequence with its two phases of isoclinal folds was tilted during himalayan collision so that the structures are now subvertical the southern suture main mantle thrust has a wedge of glaucophane schists the indian plate contains a basement of psammites and schists intruded by cambrian granites and overlain by isoclinally folded and metamorphosed carbonates and shales

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

the metamorphic evolution of granulites at fyfe hills implications for archaean crustal thickness in enderby land antarctica granulites at fyfe hills in enderby land antarctica crystallized at temperatures in excess of 850 c and possibly as high as 1000 c and at pressures of 8 10kbar during the mid to late archaean a number of features including repeated retrograde metamorphism at 5 5 8kbar retrograde reaction textures and rimward zoning in pressure sensitive systems suggest that following peak metamorphism the granulites stabilized at a depth of 18 26 km after stabilization the granulites cooled near isobarically to temperatures of 600 700 c assuming a total crustal thickness of 35 40 km during this late archaean interval of isobaric cooling the peak metamorphic crustal thickness is estimated at 35 56 km this estimate is significantly less than the 60 70 km obtained by summing the depths of the present levels of exposure 26 34 km and the thickness of the crust presently beneath fyfe hills approxi mately 35km and is therefore consistent with independent evidence for extensive post archaean thickening of the enderby land crust

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

metamorphism of an obducted island arc example of the kohistan sequence pakistan in the himalayan collided range in northern pakistan the extraordinary 40 km thick kohistan sequence of metamorphosed mafic ultramafic and calc alkaline layered plutonic and volcanic mainly andesitic to rhyodacitic rocks has been recently reinterpreted as the only complete vertical section of an intra oceanic island arc presently exposed anywhere in the world plate scale models have been suggested to explain the origin and the tectonic evolution of this arc in the himalayan collision range despite some noticeable differences in the models there is general agreement that the kohistan sequence represents the crust of an arc obducted onto the northern edge of the indian plate before the oligocene collision of india against asia new data on the tectonometamorphic evolution of the kohistan sequence have led to the conclusion that a first major d 1 event developed with increasing metamorphic grades oriented downward the kohistan pile but also toward ensandwiched basic ultrabasic granulitic rocks rare geochronological data and petrological evidence indicate that the latter granulites pyriclasites and various plagiopyrigarnites pyrigarnites metatroctolites metawebsterites metadunites etc were parts of enormous lower to middle cretaceous layered calc alkaline plutons emplaced during the arc building stages as the d 1 metamorphic event is correlated with the upper cretaceous obduction process of the kohistan onto india it is proposed that the d 1 thermal structure was strongly controlled by remnant magmatic heat source s within the obducted arc this interpretation fits nicely with recent theoretical thermal models since the obduction of the arc was probably shortly after 10 20 ma the emplacement of the former plutonic arc rocks a blueschist tectonic melange underlying the obducted arc was possibly synchronous to the obduction and not clearly linked to the older subductive process the oligocene collision of india against asia was associated with a barrovian overprinting metamorphism in the kohistan arc during this second event the dynamo thermal structure was much more classical and not controlled by the occurrence of the cretaceous metaplutonic rocks a tentative model of the thermal structures during the major events which affected the kohistan arc is proposed