petrology of garnet cordierite sillimanite gneisses from the el tormes thermal dome iberian hercynian foldbelt w spain the core of the el tormes thermal dome situated in the central part of one of the main metamorphic belts of the iberian peninsula is formed by garnet cordierite biotite sillimanite pelitic gneisses these rocks that very often are cut by minor intrusions of al rich s type granites are metatexitic gneisses in which there exists garnet showing different stages of resorption and transformation into an aggregate of cordierite plagioclase biotite the garnet mantled and corroded mainly by cordierite has never been found to occur in contact with the prismatic sillimanite of the matrix thus indicating that the continuous reaction gr sill q cd has taken place the presence of corroded biotite inside the garnet rimming cordierite of the aggregates as well as inside the cordierite of the matrix which usually includes remmants of sillimanite indicates that the continuous reaction bi sill q cd fk h2o has occurred too therefore a realistic net reaction for these aggretates should be represented by the univariant at a given x text h text 2 text o equilibrium biotite sillimanite garnet quartz cordierite k feldspar h2o 1

geologic thermobarometry of retrograded metamorphic rocks an indication of the uplift trajectory of a portion of the northern scandinavian caledonides kyanite grade pelitic schists from the efjord area 68 15 n 16 45 e northern norway show abundant textural evidence of retrograde reequilibration during late orogenic uplift and cooling of this part of the scandinavian caledonides graphical analysis of mineral compositions in seven samples that show a range of retrograde textures indicates that they reached final chemical equilibrium under nonuniform pressure temperature conditions three element partitioning geothermometers and geobarometers yield consistent pt estimates in individual samples which together define a trend in pt space the sample showing the fewest signs of retrogression yields the highest pressure and temperature estimates whereas samples exhibiting abundant reequilibration textures give lower pt estimates we suggest that the trend indicated by these samples reflects a portion of the pt trajectory assumed by the efjord area during uplift thus the judicious use of element partitioning geothermometers and geobarometers on samples showing signs of retrograde metamorphism may yield important new information concerning the late tectonic evolution of metamorphic terrains

controls on the apparent thermal and baric structure of mountain belts the apparent thermal and baric structures of metamorphosed terranes are expressed in the map patterns of isograds and bathograds potential controls on such map patterns include 1 thermal properties of the lithosphere 2 mechanism amount and history of crustal thickening and 3 variation in the rates of uplift and erosion simplified tectonic models of a single burial and uplift event combined with one dimensional models for the conductive transfer of heat have been used to examine the effect of these controls on the relationship of apparent isotherms and isobars of peak metamorphic conditions in hypothetical mountain belts intersecting apparent isotherms and isobars are difficult to explain unless there are important variations in uplift times or thermal parameters across a metamorphic belt differences in original burial depth alone are unlikely to produce such intersections variation in thermal parameters across an orogen may lead to large variations in peak metamorphic temperature with li

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

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

the thermotectonic evolution of a proterozoic low pressure granulite dome west uusimaa sw finland three fold generations have been recognized in svecofennian rocks 1 800 ma from west uusimaa sw finland the first one f1 might be related to thrusting and imbrication tectonics at plate collision contacts the main generation f2 is due to a n s horizontal crustal shortening which created at first e w trending upright folds in the whole region and later tightened these f2 folds in the western part of the belt whereas conjugate shear zones and tectonic lenses of competent rock bodies developed in the eastern part simultaneously the metamorphic conditions rose from amphibolite to granulite facies in this eastern part which is known as the west uusimaa complex the amphibolite to granulite facies transition zone along the western boundary of the granulite facies complex is studied in detail a number of prograde mineral reactions are telescoped in this transition zone the breakdown of biotite and amphibole to ortho clino pyroxene in metaigneous rocks the appearance of garnet in cordierite bearing metapelites and the appearance of scapolite in calcareous rocks distinct mineralogical changes also occur in this zone which cross cuts all major structures and rock units and are only affected by late f3 folding open disharmonic folds with approximately n s trending axial planes and young shear zones associated with pseudotachylite generation the absence of any evidence of block faulting and tilting of the crust that could be associated with the granulite complex suggests that the whole region represents one crustal level

granulites at winding stair gap north carolina the thermal axis of paleozoic metamorphism in the southern appalachians granulite facies rocks in the thermal axis of paleozoic taconic metamorphism are exposed in southwestern north carolina in a 370 m long road cut at winding stair gap lithologic units are composed of metasedimentary and metaigneous rocks some of which are now migmatites structural analysis of the exposure indicates that deformation occurred before during and after the peak of metamorphism interpretation of the predeformation stratigraphy suggests five of the six lithologic units may correlate with intervals in the late precambrian tallulah falls formation or the upper part of the late precambrian or possibly early paleozoic coweeta group the sixth unit an orthopyroxenite may be related to presumed ophiolites elsewhere in the hayesville thrust sheet prograde metamorphic conditions of t 750 775 c p 6 5 7 0 kbar were determined from mineral stability fields and cation exchange geothermometry geobarometry partitioning of water into anatectic melts of quartzofeldspathic gneiss and garnet sillimanite schist units resulted in ph2o ptotal during the prograde event an isobaric cooling path from the metamorphic peak to 600 c may be related to expulsion of partial melts that may be represented by pegmatites and trondhjemite dikes at higher structural levels limited retrograde effects were likely facilitated by water released from crystallization of the remaining in situ melts at t 700 c

numerical modelling of mud volcanoes and their flows using constraints from the gulf of cadiz it is estimated that the total number of submarine mud volcanoes is between 1000 and 100 000 because many are associated with greenhouse gases such as methane it is argued that the global flux of these gases to the atmosphere from the world s terrestrial and submarine mud volcanoes is highly significant clues to the processes forming submarine mud volcanoes can be found in variations to their height shape surface morphology physical properties and internal structure a model of isostatic compensation between the mud column and the sediment overlying the mud source is used to predict a depth to the mud reservoir beneath mud volcanoes once erupted the general behaviour of an individual mud flow can be described and predicted using a viscous gravity current model the model shows that conical shaped mud volcanoes comprise multiple superimposed radial flows in which the thickness eruption rate and speed of individual mud flows strongly depends on the viscosity density and over pressure of the erupted mud using these parameters the model predicts the lowermost flows will be the oldest thickest and have the greatest length of run out while the uppermost flows will be the youngest thinnest and shortest this model is in contrast to more traditional models of stratiform mud volcano construction in which younger flows progressively bury older ones and travel furthest from the summit applying the model to the two mud volcanoes studied in the gulf of cadiz quantitative estimates are derived for the depths to mud sources exit and flow velocities eruption duration and volume fluxes flow thickness and conduit radii for example with an average kinematic viscosity of 1 5 m2 s 1 for the erupted mud a density of 1 8 103 kg m 3 and a thickness for the youngest flows of about 0 5 m the model predicts a lowermost flow thickness of 3 6 m an average eruption duration of 7 h and a conduit radius of about 9 m to construct a conical shaped mud volcano of 260 m height similar to those studied in the gulf of cadiz is estimated to require a mud source at 4 6 km depth and a total of at least 100 individually erupted flows

titanium aluminum and interlayer cation substitutions in biotite from high grade gneisses west greenland detailed electron microprobe study pelitic to mafic metamorphic rocks from two localities higher ti k k na and possible k na similar al iv and lower al vi in granulite vs amphibolite grade samples modified journal abstract

a valid margules formulation for an asymmetric ternary solution revision of the olivine ilmenite thermometer with applications abstract the olivine ilmenite thermometer of andersen and lindsley 1979 was based on an incorrect formulation for the excess free energy of an asymmetric ternary solution a valid formulation is derived and used to revise the parameters of the olivine ilmenite thermometer for olivine and ilmenite that have equilibrated above 700 c temperature can be calculated from t c 273 12549 p 0 03x fa 0 01099 x gk x il 0 062 10496 x fa 5767 x gk x il x hem 38602 141550x il 47183x gk 5 67 r ln k d 6 52x fa 3 09 x gk x il x hem 16 49 109 46 x il 36 49x gk with k d x il x fo x gk x fa the revised model gives wil gkg 5767 3 09t 0 011p and gexch 7301 8 9t 0 047p t in k p in bars applications include apollo 17 breccias and kimberlites

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

low pressure cordierite bearing migmatites from kelly s mountain nova scotia the kelly s mountain gneiss complex of cape breton island nova scotia is a migmatitic paragneiss dominated by biotite and cordierite bearing assemblages metamorphic grade throughout the complex is in the upper amphibolite facies with garnet absent and only retrograde muscovite present in the high grade core of the complex the reaction biotite andalusite quartz cordierite k feldspar sillimanite ilmenite h2o is preserved the pelitic migmatites contain cordierite and k feldspar rich leucosomes and biotite rich melanosomes minor clinopyroxene bearing amphibolite in the complex does not show migmatitic textures the migmatites are interpreted as in situ peraluminous partial melts on the basis of phase relations and textural criteria retrograde metamorphism under conditions of high fluid pressure locally produced muscovite after k feldspar and muscovite green biotite chlorite after cordierite in paragneiss and sphene after ilmenite in amphibolite peak metamorphic conditions of 1 3 5 kb and 580 700 c are estimated the high geothermal gradient inferred from these conditions was probably caused by the intrusion of diorites associated with the gneiss complex the kelly s mountain complex represents a rare example of migmatites formed in the low pressure facies series and illustrates some of the reactions involving melting in high grade pelitic rocks

evolution of p t conditions during a high grade metamorphic event in the maniwaki area grenville province pelitic and basic rocks of the grenville supergroup in the maniwaki area 100 km north of ottawa were selected for a detailed investigation of the metamorphic history of the central metasedimentary belt of the grenville province mineral assemblages suggest metamorphic conditions of the granulite facies with local and irregular persistence of the amphibolite facies chemical analysis of minerals reveals a lack of equilibrium on the thin section scale systematic variation of mineral compositions is explained as the net result of several mineral equilibria established at different stages during the metamorphic evolution of the area peak temperatures obtained by biotite garnet thermometry cores of isolated grains are between 760 and 860 c whereas clinopyroxene garnet thermometry gives 740 820 c retrograde temperatures as estimated by various thermometers range between 550 and 750 c biotite garnet 498 and 670 c cordierite garnet and 690 and 720 c clinopyroxene garnet peak pressures were 6

petrology of biotite cordierite garnet gneiss of the mccullough range nevada i evidence for proterozoic low pressure fluid absent granulite grade metamorphism in the southern cordillera proterozoic migmatitic paragneisses exposed in the mccullough range southern nevada consist of cordierite almanditic garnet biotite sillimanite plagioclase k feldspar quartz ilmenite hercynite this assemblage is indicative of a low pressure facies series at hornblende gr anulite grade textures record a single metamorphic event involving crystallization of cordierite at the expense of biotite and sillimanite thermobarometry utilizing cation exchange between garnet biotite cordierite hercynite and plagioclase yields a preferred temperature range of 590 750 c and a pressure range of 3 4 kb equilibrium among biotite sillimanite quartz garnet and k feldspar records ahlo between 0 03 and 0 26 the low ah o together with low fol qfm and optical properties of cordierite indicate metamorphism under fluid absent conditions preserved mineral compositions are not consistent with equilibrium with a melt phase earlier limited partial melting was apparently extensive enough to cause desiccation of the pelitic assemblage the relatively low pressures attending high grade metamorphism of the mccullough range paragneisses allies this terrane with biotite cordierite garnet granulites in other orogenic belts closure pressures and temperatures require a transient apparent thermal gradient of at least 50 c km during part of this proterozoic event in the southern cordillera

the spinel and quartz associations in high grade xenoliths from tallante s e spain and their potential use in geothermometry and barometry remarkable spinel quartz associations are observed in some xenoliths found in the basaltic rocks from tallante the main primary parageneses are quartz plagioclase sillimanite spinel garnet ilmenite graphite quartz plagioclase garnet orthopyroxene spinel ilmenite

magma mixing in a main stage of formation of montagne pelee the saint vincent type scoria flow sequence martinique f w i contrasting with the first stage of activity at montagne pelee which built an andesitic volcano chiefly formed of brecciated lava flows and pyroclastic breccias the second main stage 40 000 to 20 000 years b p is characterized by the prevalence of scoria flows of more mafic composition many cross sections of the deposits of stage 2 show an upward magmatic evolution from early acid andesites plinian air fall and pumice flow deposits to late basaltic andesites scoria flow deposits heterogeneous banded rocks complex mineralogical associations and mineralogical disequilibrium features in the whole series are explained by mechanical mixing of contrasting magmas reversed zoning characterized plagioclase phenocrysts of both the acid and basic terms and also pyroxene phenocrysts of the mafic terms grossly uniform composition of residual glass in most macroscopically homogeneous rocks implies that chemical exchanges between liquids and local hybridization occurred plutonic rocks with cumulate textures are found in upper units of the sequence their phenocrysts anorthite salitic clinopyroxene pargasitic hornblende and ti magnetite are in equilibrium with a subalkaline basaltic interstitial liquid the same phases are commonly present as xenocrysts in the basaltic andesites it is inferred that basaltic andesites of stage 2 were firstly derived from a subalkaline basalt by crystal fractionation mixing of the basaltic andesites 52 sio2 with acid andesites 62 sio2 occurred in late stage of evolution of the series producing heterogeneous rocks of intermediate composition successive eruption of acid up to mafic magmas dominant amount of mafic rocks in the sequence and short duration of the magma mixing event shown by narrow rims of the reversely zoned phenocrysts suggest that mixing was produced by intrusion of mafic magma into a differentiated magma body we propose that hydrostatic overpressure induced by a high rate of intrusion triggered the eruption thus magma mixing would be due to a large volume of mafic magma passing through the resident magma rapid admixture of the two batches increased gas exsolution resulting in a highly explosive eruption magma mixing can thus explain the explosive dynamics of the basaltic andesites eruptions which surprisingly contrast with the relatively effusive style of the preceding andesitic stage