moho signature from wide angle reflections preliminary results of the 1993 trans hudson orogen refraction experiment abstract in 1993 a combined crustal seismic refraction and wide angle reflection experiment was conducted over the northern saskatchewan and manitoba portion of the trans hudson orogen by lithoprobe the program included three seismic lines extending over 1900 km in length the exceptionally high quality data collected during the field experiment included seismic reflections and refraction events from the moho and upper lithospheric mantle the relatively high seismic fold and signal to noise ratio of the field observations made it possible to process the wide angle portion of the data set with standard reflection signal enhancement techniques the moho is revealed as a complex interface with significant structural relief 40 52 km that correlates in first order fashion with crustal geology and structure the moho topography is interpreted as a result of variable ductile reworking and eclogitization of the lower crust

metamorphic interpretation of high pressure temperature metapelites with preserved growth zoning in garnet eastern grenville province canadian shield high pressure temperature metapelites that occur in close proximity to eclogitized mafic rocks in the southern part of the gagnon terrane parautochthonous belt eastern grenville province were investigated in order to constrain depths of burial and p t paths mineral assemblages and partial melting relationships in these metapelites are consistent with peak temperatures in the range between 700 and 800 c however growth zoning is apparently well preserved in garnets and only narrow rims width 100 500 m are obviously affected by diffusional retrograde resetting despite uncertainties regarding mineral assemblages and compositions of matrix minerals at early stages of garnet growth it can be shown that the observed growth zoning profiles of garnets imply increase of both pressure and temperature up to a common maximum at pressures between 1300 and 1600 mpa and that thermal relaxation did not occur during the initial stages of unloading on the other hand calculated retrograde p t conditions are consistent with steep decompression paths the inferred hair pin shaped p t path is consistent with independent evidence of rapid tectonically driven exhumation resulting in the preservation of growth zoning in garnets from such a high temperature regime

high salinity fluid inclusions formed from recycled seawater in deeply subducted alpine serpentinite abstract the origin of high pressure brines has been investigated in the erro tobbio peridotite western alps a mantle slice that was exposed to the seafloor of the mesozoic ligurian piedmontese tethys and was later involved in alpine subduction hydrothermal alteration by seawater derived fluids led to replacement of the mantle assemblage by cl bearing serpentine 0 35 wt cl brucite 0 2 wt cl and alkali bearing phyllosilicates 0 2 wt cl 0 2 0 55 wt na2o 1 5 wt k2o relics of these hydrous phases occur in olivine titanian clinohumite antigorite assemblages formed at 2 5 gpa and 550 600 c during partial devolatilization and veining of the hydrothermally altered peridotite the high pressure phases lack chlorine and alkalis and are coeval with fluid inclusions trapped in the syn eclogitic veins the inclusions are salt saturated and contain up to 50 wt cl na k mg and fe high fluid chlorinity was probably achieved during rehydration of relict mantle minerals and deposition of hydrous phases in the veins the data presented suggest that the seafloor hydrothermal signature was inherited by the eclogitic fluid due to partitioning of chlorine and alkalis into the fluid phase the presence of salty brines in eclogitized hydrous peridotites can indicate deep recycling of seawater derived fluids hydrous ultramafic systems can therefore act as large scale carriers of seawater into the mantle

reaction enhanced ductility the role of solid solid univariant reactions in deformation of the crust and mantle abstract in collision belts and subduction zones penetrative deformation of thrust sheets may be catalyzed by metamorphic reactions if the reactions have a significant effect on the mechanical properties of rocks reaction enhanced ductility may result if a reaction produces a significant decrease in grain size thus allowing the rock to deform superplastically the product grain size of a reaction is a function of the ratio of nucleation and growth rates this ratio varies along any pressure temperature time ptt path and only attains a significant value at a certain distance from an equilibrium boundary a comparison of rates of change of p and t along typical segments of ptt paths for crustal rocks leads to the conclusion that a high nucleation rate a small grain size and resulting superplastic deformation are more likely to be achieved when reaction boundaries are crossed during underthrusting rather than during the thermal readjustments of a perturbed geotherm following crustal thickening or during uplift an example of reaction enhanced ductility occurs in eclogite facies metagranitoids in the sesia lanzo zone western alps where medium grained plagioclase reacted to fine grained jadeite quartz during early alpine underthrusting microstructural evidence from shear zones shows that the jadeite quartz aggregates were highly ductile and it is inferred that the small grain size allowed superplastic deformation further evidence suggests that this process operated during regional deformation of metagranitoids rubie 1982 the reaction of olivine to spinel or the beta phase by metastable overstepping of the two phase stability fields in a subducting plate ringwood 1975 may result in fine grained products and thus may be analogous to the albite jadeite quartz reaction the consequent decrease in effective viscosity is likely to have important consequences for plate dynamics at and below the depths of major phase transitions

structure of a paleoproterozoic continent continent collision zone a lithoprobe seismic reflection profile across the trans hudson orogen canada abstract an 800 km reflection seismic profile across the trans hudson orogen northern saskatchewan and manitoba images crustal scale tectonic imbrication in an unprecedented picture of paleoproterozoic crustal accretion and continent continent collisional tectonism the profile is crudely symmetric about a crustal scale culmination in the western part of an accreted juvenile collage reindeer zone geologic and isotopic data suggest that this culmination is cored by microcontinental archean basement west of the culmination highly reflective juvenile crustal elements dip westward into the lower crust beneath the wathaman batholith and archean continental crust of hearne craton to the east strong reflections in the juvenile reindeer zone crust and reworked archean foreland of the thompson belt have eastward dips persisting into the middle crust and extending beneath the superior craton a continuous reflection moho well defined for 500 km in the western part of the profile shows marked relief 12 15 s including a prominent root below the crustal culmination these imaged structures give evidence of substantial crustal shortening and thickening via large scale imbrication consistent with collisional orogeny w dipping structures below the wathaman batholith and reworked hearne craton may reflect subduction polarity in this part of the orogen however geological evidence suggests that e dipping structures below superior craton are largely related to late post collisional deformation rather than to prior oceanic subduction polarity

coincident vibroseis and dynamite surveys across the western flank of the trans hudson orogen abstract coincident dynamite and vibroseis reflection surveys provide comparable seismic signals along a 200 km segment of the trans hudson orogen transect on single fold field records signal amplitudes from explosive sources are consistently 40 db higher than on the corresponding vibroseis records however the vibroseis final stack exhibits higher crustal reflectivity due primarily to the difference in data acquisition fold of the two surveys within the crust distinctive seismic patterns such as dominant westerly dipping reflections on the western end of the profile and a 3 s depression in the moho beneath a crustal scale culmination are clearly visible on both sections the advantages of the dynamite source include deeper signal penetration and mapping of a number of previously unrecognized subcrustal reflections moreover the dynamite data also indicate that diffraction patterns detected at lower crustal and moho depths have large apertures and extend to greater two way traveltimes than were recorded in the vibroseis data thus proper migration of lower crustal reflections requires recording greater traveltimes than were recorded during the 1991 lithoprobe vibroseis acquisition program

alpine metamorphic evolution of ligurian alps north west italy chemography and petrological constraints inferred from metamorphic climax assemblages the up to date petrological and microtextural information on the ligurian alps indicates that the metamorphic rocks from the oceanic lithosphere and the paleo european continental margin underwent an alpine type metamorphic evolution characterized by low dt dp gradients in particular rocks from the ligurian piedmontese oceanic lithosphere underwent an alpine metamorphism typical of alpine type blueschist rocks the distribution of the alpine metamorphic facies in paleo european continental margin is closely related to the structural position of the different tectonic units the prograde evolution frequently preserves paragenetic and textural relics of the earlier parageneses if relics of the earlier parageneses are preserved the rock exhibits continuous prograde reactions confirmed by strong compositional zoning of the metamorphic minerals therefore these reactions lead to chemical and microtextural equilibrium relations between the minerals in limited domains of the rocks microtextural sites the main compositional aspect of coronitic textures is the mineral zoning particularly when the minerals of the coronas are the consequence of a wide range of solid solutions in such cases the reacting minerals are armored and the kinetics are lowered the prograde metamorphic evolution which involved the rocks from the oceanic lithosphere and the paleo european continental margin is quite consistent with a subduction type geodynamic process in different ages during alpine times

isotopic evidence for closed system anatexis at midcrustal levels an example from the acadian appalachians of new england we have investigated the sr and o isotope systematics of granitoid and metasedimentary samples from the central maine terrane cmt of new england granitoid samples were taken from interior and contact zones within the acadian aged 410 m y synmetamorphic and syntectonic kinsman quartz monzonite kqm which is a member of the new hampshire plutonic series metasedimentary samples were taken from silurian and devonian formations hosting the kqm initial sr isotope ratios sri and 18o values for the kqm range from 0 70799 to 0 71246 and 1 6 to 12 9 respectively and sri and 18o values of the metasedimentary rocks range from 0 70770 to 0 75008 and 6 2 to 14 1 respectively we observe a linear and slightly positive correlation between sri and 18o for interior kqm samples that can be duplicated by a mixing curve calculated for metasedimentary endmembers whereas the sri and 18o values of contact kqm samples cluster near the sri and 18o values of the metasedimentary rocks with which they are in contact mixing calculations provide no evidence for a measurable primitive mantle component in either interior or contact kqm samples and we conclude that the sr o isotopic composition of the kqm is most likely a reflection of isotopic heterogeneities inherited from a complex package of midcrustal metasedimentary source rocks we propose that the kqm is the product of midcrustal partial melting that was initiated due to excess thermal energy from the decay of anomalously high concentrations of heat producing elements in silurian source rocks within the cmt because we see no isotopic evidence for a lower crustal or mantle component in the kqm we suggest that midcrustal anatexis may have occurred as a closed system process requiring no accompanying mantle derived magma or above normal mantle heat flow

progressive eclogitization under fluid present conditions of pre alpine mafic granulites in the austroalpine mt emilius klippe italian western alps metabasites from the austroalpine mt emilius klippe preserve different stages of the evolution from pre alpine granulite to alpine eclogite facies assemblages the pre alpine protolith includes layered garnet i clinopyroxene i plagioclase hornblende granulites and gabbros the eclogitic paragenesis comprises garnet ii clinopyroxene ii epidote ca amphibole chlorite eclogitic deformation is composite and involves two main episodes d1a d1b d1a produces a layer parallel foliation and incomplete transformation of granulites and is concluded by an episode of brittle deformation leading to the formation of eclogitic veins d1b gives rise to widespread s l tectonites and mylonites the eclogitic veins and surrounding haloes enriched in fluids during veining control the nucleation of d1b shear zones deformation is accomplished through ductile flow of reaction weakened plagioclase and hornblende sites and cataclasis of garnet i and clinopyroxene i both meso and microscale observations indicate that fluids play a primary role during eclogitization in both enhancing deformation and the kinetics of metamorphic reactions fluids influence plastic flow rates in matrix aggregates and promote fracturing of hard minerals the extent of metamorphic re equilibration is closely related to the density of the fluid pathways

structurally controlled growth of fibrous amphibole in tectonized metagabbro constraints on asbestos concentrations in non serpentinized rocks the nucleation and concentration of asbestos in rocks are mostly associated with mechanisms of fibre formation combined with the water dependent mineralogical alteration produced during the serpentinization of ultramafic masses very little is known about the structural settings and tectonic histories that influence and control the occurrence of asbestos in non serpentinized rocks diffusely embedded within tectonized ophiolitic suites focusing on a case history provided by a tectonized metagabbro from the ligurian alps northern italy a multiscale structural petrographic approach is used to investigate the relationships between rock fabric and fibrous amphibole growth within the metagabbro meso to microstructural observations are used to document the role of structurally controlled fluid rock interactions in localizing the growth of fibrous amphibole during ductile to brittle shearing mylonitic foliation to shear veins a qualitative structural scenario is provided to illustrate the growth of asbestos amphiboles in shear veins during the progression of shear deformation towards semi brittle rheological conditions the mechanisms of the structurally controlled growth of fibrous amphibole in non serpentinized rocks imply an examination of the tectonic boundary conditions that are at the origin of the concentration of asbestos in ophiolitic rocks involved in orogenic belt construction

p t conditions for the eclogitic re equilibration of the metaophiolites from val d ala di lanzo internal piemontese zone western alps abstract a study has been made of the high pressure early alpine re equilibration in the eclogites and metasedimentary cover of the val d ala di lanzo ophiolite all of the main high pressure minerals have been analysed and their compositions used to determine re equilibration temperatures the minimum conditions proposed p 1 3 gpa t 450 460 c are also indicated by the presence of a jadeite quartz bearing metagranite the temperatures are compared with those reported for similar eclogites from the voltri group the aosta valley and the valais comparison of recalculated temperatures shows that the temperature and probably the pressure of the eclogitic re equilibration increased in the aosta valley and the valais in keeping with what has been observed in the internal penninic basement of the gran paradiso and monte rosa crystalline massifs

high pressure metamorphism in mafic schist from northern vermont the only confirmed occurrence of glaucophane and omphacite in the appalachian mountains of new england is in mafic schist from tillotson peak 44 48 72 33 north central vermont mineral assemblages observed all with epidote sphene magnetite phengitic muscovite carbonate calcite or dolomite apatite sulfide are 1 actinolite glaucophane chlorite garnet quartz 2 actinolite glaucophane chlorite albite garnet 3 actinolite glaucophane omphacite garnet 4 glaucophane chlorite garnet quartz 5 glaucophane omphacite quartz 6 albite glaucophane chlorite quartz and 7 albite paragonite chlorite quartz electron microprobe investigations show that plagioclase chlorite sphene white mica carbonate and magnetite grains are relatively homogeneous however amphibole pyroxene epidote and garnet grains are zoned indicating incomplete equilibration and changing physical conditions during metamorphism glaucophane in assemblage 6 has the general formula rim composition na 1 9 m4 ca 0 1 mg 1 8fe 1 2 2 al 1 0 vi fe 0 3 3 si 7 9 al 0 1 iv o 22 oh 2 glaucophane in assemblages 1 to 5 is somewhat poorer in the glaucophane endmember and richer in actinolite ca rich amphibole grains are zoned extensively with barroisite cores and actinolite rims omphacite grains in assemblages 3 and 5 have the general formula rim composition na 0 5 ca 0 5 mg 0 4 fe 0 1 2 al 0 4 vi fe 0 1 3 si 2o 6 in spite of the significant zoning the partitioning of elements among the minerals at their grain margins suggests that all seven assemblages above may coexist at the same physical conditions of metamorphism in mafic rocks with a relatively small range in bulk composition correlation of mineral compositions and assemblages with mineral stability data suggests high pressure facies series metamorphism at about 9 2 kb and 450 100 c with p h2o p total it is proposed that the metamorphism occurred during the ordovician

valid garnet biotite gb geothermometry and garnet aluminum silicate plagioclase quartz gasp geobarometry in metapelitic rocks abstract at present there are many calibrations of both the garnet biotite gb thermometer and the garnet aluminum silicate plagioclase quartz gasp barometer that may confuse geologists in choosing a reliable thermometer and or barometer to test the accuracy of the gb thermometers we have applied the various gb thermometers to reproduce the experimental data and data from natural metapelitic rocks of various prograde sequences inverted metamorphic zones and thermal contact aureoles we have concluded that the four gb thermometers perchuk l l lavrent eva i v 1983 experimental investigation of exchange equilibria in the system cordierite garnet biotite in saxena s k ed kinetics and equilibrium in mineral reactions springer verlag new york berlin heidelberg pp 199 239 kleemann u reinhardt j 1994 garnet biotite thermometry revised the effect of alvi and ti in biotite european journal of mineralogy 6 925 941 holdaway m j 2000 application of new experimental and garnet margules data to the garnet biotite geothermometer american mineralogist 85 881 892 model 6av kaneko y miyano t 2004 recalibration of mutually consistent garnet biotite and garnet cordierite geothermometers lithos 73 255 269 model b are the most valid and reliable of this kind of thermometer more specifically we prefer the holdaway holdaway m j 2000 application of new experimental and garnet margules data to the garnet biotite geothermometer american mineralogist 85 881 892 and the kleemann and reinhardt kleemann u reinhardt j 1994 garnet biotite thermometry revised the effect of alvi and ti in biotite european journal of mineralogy 6 925 941 calibrations due to their small errors in reproducing the experimental temperatures and good accuracy in successfully discerning the systematic temperature changes of the different sequences in addition after applying the gasp barometer to 335 natural metapelitic samples containing one kind of aluminosilicate and 33 natural metapelitic samples containing two kinds of aluminosilicate as well as to rocks in limited geographic areas and thermal contact aureoles we propose that the calibrations of holdaway holdaway m j 2001 recalibration of the gasp geobarometer in light of recent garnet and plagioclase activity models and versions of the garnet biotite geothermometer american mineralogist 86 1117 1129 and newton and haselton newton r c haselton h t 1981 thermodynamics of the garnet plagioclase al2sio5 quartz geobarometer in newton r c navrotsky a wood b j eds thermodynamics of minerals and melts new york springer verlag 131 147 based on kleemann and reinhardt s kleemann u reinhardt j 1994 garnet biotite thermometry revised the effect of alvi and ti in biotite european journal of mineralogy 6 925 941 thermometer are the most valid gasp barometers other gb thermometers and gasp barometers are not recommended

metamorphism of the maclean lake and central metavolcanic belts la ronge domain trans hudson orogen pressure temperature variations and tectonic implications metamorphic assemblages differ between the metasedimentary maclean lake belt and the adjacent central metavolcanic belt in the la ronge domain trans hudson orogen the former consists of meta arkoses psammitic gneisses metaconglomerates and calc silicate gneisses of upper amphibolite facies 600 740 c 440 660 mpa with local migmatization whereas the latter is comprised mainly of metavolcanic and plutonic rocks with minor metasedimentary schists of greenschist to lower amphibolite facies 480 630 c 520 560 mpa petrographic evidence indicates that peak metamorphic conditions were reached towards the end of d1 deformation during which the central metavolcanic belt was thrust onto the maclean lake belt along the mclennan lake tectonic zone which separates the two belts peak metamorphic assemblages did not undergo retrograde alteration during d2 deformation indicating that high temperature was maintained during d2 deformation differences in pressure p and temperature t between the northeastern and southwestern parts of the central metavolcanic belt may have resulted from tilting along strike after peak metamorphism peak temperatures increase gradually from the central metavolcanic belt to maclean lake belt across the mclennan lake tectonic zone peak pressures in the two belts are similar implying that the central metavolcanic belt thrust sheet was probably thin the p t data for the maclean lake belt indicate a relatively high thermal gradient 40 50 c km similar to that in the metasedimentary kisseynew domain in the orogen

fracture geometry of pseudotachylyte generation zones a study of shear fractures formed during seismic events abstract the orientation and movement sense of shear fractures in pseudotachylyte generation zones developed in the ikertoq shear belt west greenland are described and the fracturing sequence is inferred the generation zones occur in strongly foliated gneisses and consist of concordant usually paired principal displacement shear fractures paired shears on which most melt was generated and a system of minor shear fractures minor shear fracturing was most intense between principal displacement shear fractures and most melt intruded from these paired shears inwards with respect to the zone intrusion of injection veins of pseudotachylyte off the principal displacement shear fractures is believed to be contemporaneous with minor shear fracturing in the generation zones generation zones affect intact rocks and individual principal displacement shear fractures have a maximum displacement of 4 6 m paired principal displacement shear fractures are up to 3 m apart and have a maximum observed length of 1 km in all the zones described the fracturing appears to be the result of a single episode of slip both dextral and sinistral generation zones occur within the study area and have different geometries in each case the displacement is strike slip

redistribution of high pressure fluids during retrograde metamorphism of eclogite facies rocks voltri massif italian western alps abstract high pressure clinopyroxenes in metasomatic mafic rocks rodingites and metagabbros containing titanoclinohumite and eclogites from the voltri massif ligurian western alps retain primary two phase aqueous inclusions h 2 o nacl kcl in the metasomatized rocks the inclusions align with the c axes and the cleavage planes of diopside the eclogites display mylonitic textures primary fluid inclusions are hosted in early porphyroclasts of omphacite omphacite i and are absent in later fine grained omphacite omphacite ii that forms the mylonitic foliation omphacite i has been frequently replaced by retrograde symplectites clinopyroxene plagioclase amphibole in such cases the fluid inclusions are either absent or occur in much lower number the textural observations suggest that formation of the symplectites has been controlled by availability of the fluid inclusions inside omphacite i the symplectites have been in turn overgrown by glaucophanic and winchitic amphiboles indicating that breakdown of the omphacite i to symplectites still occurred at relatively high pressure post entrapment changes have caused a large range of densities in the analyzed inclusions although the textural evidence constrains the entrapment of fluid inclusions to high pressure conditions the highest density isochores in the eclogites indicate pressure temperature conditions 8 kbar and 550 c lower than those attained during the high pressure stage such p t estimates approach those of symplectite formation after omphacite i thus the fluid inclusions preserved in omphacite were formed by low pressure re equilibration of a fluid presumably trapped at eclogite facies conditions the inclusions kinetically triggered the retrograde breakdown of the eclogitic clinopyroxenes the compositional overlap between the analyzed inclusions and those related to the latest greenschist evolutionary stage suggests further redistribution of the high pressure fluids and close system behaviour of these rocks during their overall exhumation path

alpine olivine and titanian clinohumite bearing assemblages in the erro tobbio peridotite voltri massif nw italy the microstructures in the erro tobbio peridotite indicate several stages of recrystallization of olivine titanian clinohumite bearing assemblages the development of these assemblages is closely associated with serpentinite mylonites in which they occur in shear bands and foliations and are inferred to have grown synkinematically in veins and as post kinematic radial aggregates in the peridotite wall rock adjacent to these mylonites the same assemblages have recrystallized statically at the expense of original olivine and pyroxenes mesh textured chrysotile and antigorite veins in addition the olivine bearing assemblage occurs in widespread vein systems the brittle deformation of the peridotite resulting in the development of these vein systems is closely related to ductile deformation of metagabbroic dykes in the peridotite although early metasomatism resulted in extensive rodingitization of the gabbros some dykes show an eclogitic assemblage of na clinopyroxene garnet chloritoid chlorite f talc these observations the microstructures and the mineral chemistry all suggest that the assemblages in the ultramafic rocks and metagabbros developed during a prograde evolution towards high pressures 13 16 kbar 450 550 c and during subsequent decompression this metamorphic evolution is considered to be related to late cretaceous intraoceanic subduction in the alps apennine system and closure of the piedmont ligurian basin

the importance of serpentinite mylonites for subduction and exhumation of oceanic crust abstract in the ultramafic erro tobbio unit voltri massif western alps a set of overprinting structures in serpentinite mylonites is related to alpine subduction to about 80 km depth and to subsequent exhumation antigorite mylonites are cut by en echelon olivine veins which in turn are dissected by multiple sets of shear bands containing olivine and titanian clinohumite the transition from olivine free to olivine bearing structures indicates recrystallization during prograde metamorphism all structures display the same top to the nw kinematics providing evidence for a continuous non coaxial deformation the serpentinite mylonites surround km scale bodies of pre alpine peridotite which show only minor alpine overprint this indicates that during subduction related deformation recrystallization and fluid flow were strongly localized within serpentinite mylonites olivine bearing discontinuous shear planes with top to the se sense of movement crosscut the prograde structures the inversion of shear sense suggests a change in position of the serpentinites relative to the downgoing slab i e from the subducted slab to the upper plate during accretion thus the shear sense inversion marks the change from burial to exhumation of the serpentinites the low density of antigorite serpentinites 2 75 g cm3 causes strong buoyancy thus providing a mechanism for the exhumation of deeply subducted rocks it is suggested that serpentinites may act as carriers for the uprise of eclogite bodies which have higher densities than the peridotitic upper mantle

geologic setting of the jolu gold mine saskatchewan u pb age constraints on plutonism deformation mineralization and metamorphism structurally hosted lode gold deposits in the early proterozoic reindeer zone saskatchewan have many attributes in common with archcan counterparts worldwide the detailed field relationships at the jolu mine indicate that the development of shear zones and mineralization in the star lake pluton clearly postdates eraplacement of the host footwall granodiorite at 1846 or ma and predates the time of regional metamorphism in the star lake area estimated at 1783 ma using a u pb titanitc age obtained from the star lake pluton the best estimate for the timing of gold mineralization is 1807 ma based on u pb dating of hydrothermal titanitc found in a mafic dike sample that exhibits a close spatial and temporal relationship to quartz vein mineralization this age of mineralization is identical to high grade metamorphism at deeper crustal levels in the trans hudson orogen e g in the kisseynew domain and as has been shown for some archcan lode gold deposits provides a temporal link between gold mineralization at high crustal levels and high grade metamorphism and devolatilization at depth

geodynamic evolution and thermal history of the central flin flon domain trans hudson orogen constraints from structural development 40ar 39ar and stable isotope geothermometry shear zones in the flin flon area record five episodes of development these have been studied using structural analysis and 40ar 39ar thermochronology deformation occurred as a result of collision between the archean rae hearne and archean superior plates during the trans hudson orogeny 1900 to 1700 ma the geodynamic evolution encompasses a transition from island arc supracrustal development 1900 ma to tectonic mountain building under brittleductile conditions 1860 ma two episodes of ductile lineation schistosity l s shear zones on the prograde metamorphic path 1860 1810 ma and two episodes of retrograde displacement between 1790 and 1690 ma during final suturing of the archean cratons with paleoproterozoic assemblages metamorphism evolved in three stages contact aureoles to granitoid bodies formed at 1860 1840 ma hornblende homfels facies 40ar 39ar ages indicate cooling through 500 c at 1840 ma this was followed by regional heating burial which produced regional peak thermal recrystallization at 1820 1790 ma under lower greenschist to lower amphibolite conditions lastly retrograde metamorphism took place during unroofing between 1790 and 1690 ma with a regional cooling rate of 2 c m y

seismically induced serpentine dehydration as a possible mechanism of water release in subduction zones insights from the alpine corsica pseudotachylyte bearing monte maggiore ophiolitic unit abstract the monte maggiore ophiolitic unit of alpine corsica consists of an intact to variably hydrated spinel lherzolite intruded by jurassic gabbro dykes a widespread serpentinization marked by low pressure serpentine polymorphs results from sea water interaction with mantle rocks during jurassic sea floor spreading of the piemonte liguria ocean a second serpentinization event marked by the presence of antigorite is related to the cretaceous to paleogene subduction of the piemonte liguria oceanic lithosphere tectonic pseudotachylyte veins locally crosscutting the unit display several original characteristics 1 they are hosted by serpentinite showing that seismic ruptures can propagate through hydrated mantle rocks 2 the host serpentinite in contact with pseudotachylyte veins shows a thin 500 m or less rim of secondary olivine newly crystallized at the expense of serpentine the heat necessary for serpentine dehydration is likely provided by the frictional melt antigorite bearing clasts themselves partly dehydrated along their boundaries reworked in pseudotachylyte veins on one hand and antigorite veins crossing pseudotachylyte veins on the other hand show that frictional melting took place at pressure and temperature conditions compatible with antigorite stability this indicates that frictional meting occurred at pressures between 0 7 and 0 85 gpa that is at 20 to 30 km depths in the subducting piemonte liguria oceanic slab since the dehydration of serpentine into olivine is only observed at the host rock selvages of pseudotachylyte veins it cannot be related to the crossing by the subducting slab of the regional 610 100 c isotherm of the subduction zone temperature at which antigorite starts to dehydrate into olivine an estimate of the water released by the dehydration reaction suggests that for a magnitude 6 earthquake the average amount is about 1 36 l m2 or 1 36 105 m3 for the entire rupture surface assumed to be 100 km2 if the released water is not incorporated in the nearby frictional melt it can flow away from the slip zone and contribute to the various fluid rock interactions active in subduction zones alternatively if the water cannot escape from the slip zone and if the pore pressure is locally high enough it could trigger aftershocks following the initial seismic rupture

the synchronism of crustal thickening and low pressure facies metamorphism in the mount isa inlier australia 2 fast convective thinning of mantle lithosphere during crustal thickening the mount isa inlier is like some other northern australian early to middle proterozoic fold belts characterized by an anti clockwise p t t path i e regional prograde low p facies metamorphic assemblages grew during crustal thickening p t and were overprinted by a phase of essentially isobaric cooling p 0 t in this study two possible models for the synchronism of regional low p facies metamorphism and crustal thickening are quantitatively evaluated using a one dimensional finite difference technique 1 extreme elevation of isotherms immediately before the thickening event due to e g lithospheric extension magmatic activity or crust mantle delamination and 2 crustal thickening accompanied by convective thinning of the mantle lithosphere only the latter results in prograde low p facies metamorphism during crustal thickening it is the preferred model for the mount isa inlier

coronitic reactions and ductile shear zones in eclogitised ophiolite metagabbro western alps north italy abstract the objective of this work is to study the most significant static and syn tectonic transformations which accompanied the eclogite metamorphism 450 50 c 12 15 kbar of an ophiolite metagabbro from the western alps in partly undeformed volumes abundant igneous mineral relics occur and the eclogite metamorphism produced coronitic or incomplete pseudomorphic reactions the disequilibrium or local equilibrium conditions implied by these features suggest that slow diffusion and interface kinetics were the rate limiting factors which prevented a large scale eclogite equilibration the undeformed volumes are bounded by syn eclogite ductile shear zones where the magmatic minerals and textures are obliterated and a new fabric foliation develops the enlargement of equilibrium domains observed in the shear zones indicates that a close approach to eclogite equilibration of these metagabbros was not attained without the catalytic effect of pervasive deformation the bulk rock chemistry also played an important role in determining the kind of eclogite assemblage in particular only sheared gabbros rich in fe can produce bimineralic eclogites

two new geobarometers for garnet amphibolites with applications to southeastern vermont two new geobarometers for the assemblage garnet hornblende plagioclase quartz have been calibrated on the basis of the equilibrium 6 anorthite 3 tremolite 2 grossular 1 pyrope 3 tschermakite 6 quartz and its fe end member equivalent data representing 37 natural samples that equilibrated at conditions of 2 5 to 13 kbar and 500 to 800 c were fitted to the general equation rt in keq a bt p i c by using a weighted least squares method multiple correlation coefficients are high 0 98 and 0 97 lines of constant keqhave extremely shallow slopes in p t space 2 to 8 bars per degree suggesting that pressures may be deduced precisely even where temperatures are only poorly constrained typical analytical errors and temperature imprecisions propagate to very small errors in pressure about 500 bars caution is advised in applying the barometers outside the range of calibrant mineral compositions application of the calibrations to samples from southeastern vermont near the strafford chester and athens domes documents relatively high pressure metamorphism 7 to 10 kbar for these structures

upper and middle crustal response to delamination an example from the lachlan fold belt eastern australia a delamination model applied to the lachlan fold belt in eastern australia shows that lithospheric delamination began in the early silurian 430 ma after crustal thickening in the wagga omeo zone the delamination induced isostatic uplift farther east to produce localized transient extensional basins and coeval silicic magmas which formed by partial melting of the lower crust during asthenospheric upwelling however the voluminous granitoid intrusions were emplaced during and after compression reflecting reestablishment of horizontal compression associated with plate convergence continued sinking of the slab produced an east ward widening orogenic belt where the granite batholiths and associated thermally softened zones of deformation tracked the direction of delamination once the slab was completely detached at 400 ma the compressive stresses were transmitted across the belt and delamination recommenced about 1000 km westward against the cratonic margin the process was repeated at 400 370 ma within 60 m y the two sinking slabs had converted a 1700 2000 km wide continental margin into a 700 km wide orogenic belt important effects of delamination on the upper and middle crust are 1 production of regional scale low pressure metamorphic belts of limited crustal thickness 2 widespread voluminous silicic magmatism 3 lack of deep crustal exposures even with high degrees of crustal shortening and 4 partial replacement of the lower crust by a mafic underplate that cooled completely after deformation ceased

chloritoid bearing assemblages in eclogitised metagabbros of the lanzo peridotite body western italian alps abstract dykes of metagabbro from the lanzo peridotite body in the western italian alps contain mg chloritoidtalc garnet na pyroxenes chlorite or fe chloritoid glaucophane garnet na pyroxenes eclogitic assemblages of cretaceous age in these metagabbros the metamorphic equilibration is incomplete and controlled by local equilibrium conditions here the chemical gradients which document the magmatic microstructural domains and the diffusion patterns during metamorphism survive the eclogitic crystallisation concentration of elements like al mg and fe near the boundaries between the magmatic domains favours the crystallisation in reaction coronas of mg rich chloritoid in mg metagabbro and of fe rich chloritoid in fe ti metagabbro phase compatibilities in the al2o3 feo mgo sio2 h2o system suggest temperatures around 400 500 c at approximate pressures of 15 18 kbar for the eclogitic metamorphism of the lanzo rocks in spite of the difficulties and of the inaccuracies of these thermobarometric estimates the inferred p t values are in good agreement with determinations involving na pyroxenes and garnets

calculated mineral equilibria in the greenschist blueschist eclogite facies in na 2 o feo mgo al 2 o 3 sio 2 h 2 o the new greatly expanded internally consistent dataset of holland and powell includes thermodynamic data for a wide range of mineral end members in common rock forming minerals in particular including femg 1 substitutions in glaucophane garnet chloritoid and carpholite and femg 1 and mgsial 1al 1 substitutions in talc and chlorite moreover we have the uncertainties and correlations for these data with the data we have calculated the full pressure temperature phase diagram for the system na2o feo mgo al2o3 sio2 h2o nfmash for quartz or coesite and h2o in excess in the range 300 800 c and 5 50 kbars by solving the set of non linear equations formed by the equilibrium relationships for an independent set of equations between the end members in an assemblage in nfmash the compositions of the minerals and pt can be calculated thus the changes in mgsial 1al 1 along nmash reactions and fesial 1al 1 along nfash reactions are calculated and the changes in femg 1 and mgsial 1al 1 along nfmash reactions are calculated from this information it is straightforward to generate pt diagrams for specific rock compositions mineral assemblages and mineral compositional changes in the phase diagram are discussed in relation to greenschist blueschist and eclogite facies assemblages in metapelitic rocks it is found that the correspondence between the predictions of the phase diagrams and the observations on rocks is remarkably good when semiquantitative extensions of the phase diagram to include ca mgfe 1 nasica 1al 1 fe3 al 1 and kna 1 substitutions are taken into account the agreement is essentially complete

comment on subduction polarity reversal at the junction between the western alps and the northern apennines italy by g vignaroli c faccenna l jolivet c piromallo f rossetti abstract we discuss the model for the reversal of the subduction polarity in the alps apennine system proposed by vignaroli et al vignaroli g faccenna c jolivet l piromallo c rossetti f 2008 subduction polarity reversal at the junction between the western alps and the northern apennines italy tectonophysics 450 34 50 this model integrates recently published tomographic data with a re interpretation of the surface geology of the voltri massif to suggest a tertiary extensional regime of nappe excision the structural reconstruction proposed by vignaroli et al vignaroli g faccenna c jolivet l piromallo c rossetti f 2008 subduction polarity reversal at the junction between the western alps and the northern apennines italy tectonophysics 450 34 50 individuates in the voltri massif two major tectonic complexes an eclogite facies unit the lower tectonic complex ltc underneath a low grade tectonic element the upper tectonic complex utc which attained significantly lower p t conditions the two complexes are separated by a major extensional contact most of the arguments exposed by vignaroli et al vignaroli g faccenna c jolivet l piromallo c rossetti f 2008 subduction polarity reversal at the junction between the western alps and the northern apennines italy tectonophysics 450 34 50 to substantiate such a nappe pile reconstruction and the proposed extensional regime do not properly consider the field data the datasets available in the literature and the published geological maps also the available p t data and metamorphic histories of rocks pertaining to the ltc and utc do not provide evidence for a significant pressure jump between the two complexes in addition the expected thermal effects achieved during metamorphism in a regime of crustal thinning and astenospheric uprise do not occur in the study area we conclude that the evidence supporting the geodynamic model proposed by vignaroli et al vignaroli g faccenna c jolivet l piromallo c rossetti f 2008 subduction polarity reversal at the junction between the western alps and the northern apennines italy tectonophysics 450 34 50 is poorly convincing and not supported by the field and petrologic observations both the re interpretation of the surface geology and the resulting crustal reconstruction have several major flaws

pseudotachylyte bearing strike slip duplex structures in the fort foster brittle zone s maine abstract detailed 1 60 scale mapping of the fort foster brittle zone in the mylonitic rye formation of southernmost maine has revealed the intricate internal duplex structure of a system of probable paleozoic age dextral strike slip faults that have produced abundant pseudotachylyte and minor breccia the internal configuration of this brittle zone consists of a mosaic of individual pseudotachylyte generation zones as slab duplex structures individual duplex zones are up to 100 m in length and 1 m or less in width and are defined by pairs of layer parallel slip surfaces along which frictional melts were produced these slab duplex structures are interpreted as zones of displacement transfer between long overlapping layer parallel en echelon strike slip fault surfaces contractional duplexes develop layer parallel compressional structures that tend to shorten and thicken the fault bounded slabs by the formation of lateral ramps and conjugate faults kinks and asymmetric folds extensional duplexes develop layer parallel stretching and thinning by the formation of oblique dextral shears high angle conjugate pairs and localized fault breccias the production of pseudotachylyte by friction melting along layer parallel fault surfaces in these exposures is attributed to rapid slip during paleoseismic events the rupture structures developed during these events may be characteristic of fault structure and mechanics at near focal depths in a strike slip seismogenic zone

experimental sytectonic dehydration of serpentinite under conditions of controlled pore water pressure we have studied the mechanical behavior of serpentinite under conditions favouring dehydration to olivine talc water at temperatures ranging between 300 and 600 and at total pressures from 100 to 300 mpa constant stress stress relaxation and constant displacement rate testing techniques permitted a wide range of deformation rates and paths to be accessed effective confining pressure was held constant during the dehydration process by means of a controlled pore water pressure system stress supported by the samples was almost independent of deformation rate at temperatures below those required for the onset of the breakdown reaction at 500 and 600 c however the strength was linearly dependent on the inverse of the deformation rate and independent of effective confining pressure over the range investigated microstructural studies revealed that the weakening was associated with sliding in narrow shear zones containing ultrafine grained 0 1 m olivine produced in the breakdown reaction the enhanced deformability is interpreted in terms of flow of the olivine in these shear zones by diffusion accommodated grain boundary sliding these experimental results suggest that metamorphic transformations affect the deformability of rocks and demonstrate enhancement of deformability through the transient existence of fine grained reaction products the results may also be applicable to the mechanics of deformation processes in oceanic transform faults and give some indication of the degree of weakening that might be expected from grain size reduction by dynamic recrystallization in plastically deforming rocks

kisseynew metasedimentary gneiss belt trans hudson orogen canada back arc origin and collisional inversion the kisseynew metasedimentary gneiss belt initially developed in a back arc setting the ages of detrital zircons and crosscutting plutons constrain turbidite sedimentation to 1 855 1 841 ga postdating early deformation and metamorphism associated with arc arc and arc continent collisions affecting much of the trans hudson orogen these turbidites are interpreted to have been deposited in a back arc basin behind a retreating subduction boundary the kisseynew domain and its southern flank are marked by 1 84 1 83 ga magmatic rocks that may be related to subduction of back arc basin oceanic crust collapse of the kisseynew basin by ductile fold and thrust deformation began during 1 84 1 83 ga magmatism and continued through peak metamorphism at ca 1 820 1 805 ga from authors

thermal evolution of the kisseynew sedimentary gneiss belt manitoba metamorphism at an early proterozoic accretionary margin

proterozoic collisional tectonism in the trans hudson orogen saskatchewan isotopic and structural data from the juvenile reindeer zone of the trans hudson orogen northern saskatchewan indicate a pre 1 85 ga thermotectonic event possibly reflecting arc continent collision followed by a more extensive nappe forming ca 1 83 1 80 ga thermotectonism during terminal continent continent collision preliminary data from the adjacent ensialic cree lake zone suggest high grade reworking of archean crust by the pre 1 85 ga event in the rae province to the west high grade metamorphism and reworking of archean crust occurred about 2 0 ga and may be related to the formation of the coeval taltson magmatic zone

dehydration induced fracturing of eclogite facies peridotites implications for the mechanical behaviour of subducting oceanic lithosphere structural analysis of eclogite facies peridotites from the voltri massif in northwest italy shows that deformation in the upper 60 km of the alpine serpentinite peridotite dominated subduction zone was strongly localized in antigorite rich serpentinite mylonites deformation in the peridotite wall rock was dominated by the development of sub planar shear fracture systems and veins with olivine titanian clinohumite bearing assemblages fracturing of the peridotite wall rock was closely associated with synkinematic antigorite breakdown in serpentinite mylonites during peak subduction hp lt metamorphism and is inferred to have occurred as a result of hydraulic fracturing the deformational structures observed allow to place some constraints on the mechanical behaviour of the erro tobbio peridotites during subduction it is inferred that fracturing of the peridotite may have occurred at transient fluid pressures in excess of 0 9 times the lithostatic pressure the shear stresses along the subducting plate contact were controlled by the rheology of serpentinite mylonites which suggests that maximum values of about 60 mpa were reached at 30 km depth while below 40 km depth they decreased to less than 5 mpa these shear stresses are distinctly lower than those inferred for basalt dominated subduction zones

titanian clinohumite and geikielite in marbles from the bergell contact aureole the assemblage titanian clinohumite forsterite spinel calcite is widespread in marbles from the eastern bergell contact aureole switzerland italy the bergell titanian clinohumites vary considerably in composition tio2 0 19 to 2 05 wt f 2 2 to 3 4 wt electron microprobe analyses show that the titanian clinohumites contain less than detectable amounts of co cu ni zn al cr and cl no trace of ferric iron could be detected by moessbauer spectroscopy moreover the moessbauer spectra indicate that fe2 occurs only in one of the five octahedral positions in the crystal structure of the studied titanian clinohumite under the conditions of the contact metamorphism 600 650 c 3 kb total pressure the compositional variation along the exchange vector tio2m 1 oh f 2 takes place at constantxoh which is fixed by the pore fluid titanian clinohumite sometimes contains geikielite inclusions which strongly fractionate fe and mn relative to titanian clinohumite the geikielites from the bergell marbles are poor in cr2o3 fe2o3 and mno and thus different from those found in carbonatites kimberlites and serpentinized ultramafic rocks

metamorphism of high grade pelites in the brabant lake area eastern la ronge domain northern saskatchewan the brabant lake area mostly comprises early proterozoic clastic metasediments of the maclean lake belt eastern la ronge domain reindeer zone the belt underwent upper amphibolite facies metamorp

thermotectonic studies in the paleoproterozoic glennie domain trans hudson orogen canada abstract the glennie domain of the trans hudson orogen tho is characterized by paleoproterozoic island arc volcanogenic assemblages and granitoid rocks thrust as sheets over the archean saskatchewan craton prior to terminal collision between the archean rae hearne and superior cratons four discrete deformation events have been recorded in the domain the d1 and d2 events are associated with the southwestward directed imbrication of paleoproterozoic allochthons over the saskatchewan craton while the d3 and d4 events are associated with terminal collision and transpression respectively discernable metamorphic episodes are associated with the d2 and d3 events with peak thermal metamorphism during d3 using inveq thermobarometry metamorphic conditions during d3 in the santoy lake area were ca 655 705 c and 6 1 7 3 kbar and ca 530 595 c and 3 2 4 8 kbar for the brownell lake area 40ar 39ar hornblende and biotite ages suggest the santoy lake area remained above ca 490 c until ca 1720 ma when it underwent rapid cooling through the biotite ar closure temperature of ca 330 c 40ar 39ar biotite ages from the brownell lake area suggest moderate cooling from peak thermal metamorphism at ca 1810 1755 ma when it passed through the ca 310 c biotite ar closure temperature differences between cooling rates for the two areas are interpreted as differential exhumation of smaller lithotectonic blocks bound by domain wide deep crustal faults and shear zones zircon pb evaporation ages of variably deformed granitoid rocks from the santoy lake area show that the d2 event occurred after ca 1857 ma the oldest granitoid in the santoy lake area provides a minimum age of mafic volcanism in the area of ca 1886 ma as well as a significant hiatus ca 50 ma between deposition of the two supracrustal assemblages in the domain

textural evolution during high pressure dehydration of serpentinite to peridotite and its relation to stress orientations and kinematics of subducting slabs insights from the almirez ultramafic massif abstract the almirez ultramafic massif se spain preserves the transformation of high p antigorite atg serpentinite to chlorite chl harzburgite 1 6 1 9 gpa 680 710 c a metamorphic reaction that is the primary source of water at the intermediate depth of subducting slabs we present a detailed ct and ebsd study of oriented samples across the atg serpentinite dehydration isograd to investigate the textural evolution during serpentinite dehydration to peridotite and its relation to stress orientations and the kinematics of subducting slabs above the atg out isograd atg serpentinite shows a prograde mylonitic foliation and a weak shape preferred orientation spo of oxide aggregates defining a n s stretching lineation the antigorite crystal preferred orientation cpo is characterized by 001 atg perpendicular to the foliation and the poles to 100 atg and 010 atg distributed in a girdle like symmetry with 100 atg nearly parallel to the stretching lineation the antigorite microstructure and cpo are consistent with deformation by dislocation creep twinning and dissolution precipitation creep these textures record the long term shear deformation near the slab interface where the main compressive stress 1 was at an acute angle to the foliation below the atg out isograd atg serpentinite dehydrated to unfoliated coarse grainedchl harzburgite with granofels or spinifex textures distributed in alternating decameter sized lenses crystallization of granofels and spinifex chl harzburgite records respectively a sequence of slow and fast fluid draining events during serpentinite dehydration under the same orientation of the principal stresses that resulted in the shear deformation of the atg serpentinite both textural types exhibit coarse grained textures with systematic mineral cpos and spos and microstructures without evidence of major ductile deformation the texture of the granofels chl harzburgite formed by a topotactic dehydration reaction after atg serpentinite coupled to compaction leading to an olivine layering subparallel to the atg serpentinite foliation the olivines of granofels chl harzburgite are rounded and display a weak cpo that can be accounted for by the topotactic reaction 100 atg 100 ol and 001 atg 010 ol after atg serpentinite similarly orthopyroxene shows a marked cpo consistent with the topotactic reaction 100 opx 001 atg and 001 opx 100 atg spinifex chl harzburgite displays systematic mineral spo and cpo spinifex olivines are tabular on 100 ol and elongated along 001 ol c b a and define a ese wnw platelet lineation the average texture is characterized by 001 ol opx subparallel to a strong ese wnw oxide aggregate lineation and 100 ol opx and 001 ol opx within a plane of similar orientation to the atg serpentinite foliation the spos and cpos of spinifex chl harzburgites are composed of up to four orientation populations of tabular olivines where one population is volumetrically dominant in all samples relative to the main orientation population the cpo of olivine shows clustered distribution of 100 ol and 010 ol maxima rotated at systematic angles around 001 ol these clustered cpos and spos show a remarkable correlation with the orientation of the principal paleostresses the preponderant orientation population of tabular olivines lies on the plane of maximum compression 2 3 plane and has 010 ol and its 100 ol tabular faces nearly perpendicular to the atg serpentinite foliation this population can be accounted for by oriented growth of platy crystals perpendicular to 1 the other populations lie in the plane perpendicular to the least compressive stress 3 and the planes of maximum shear the driving force and causes for the oriented crystallization of tabular olivine in these planes is uncertain but their correlation with paleostresses suggests a cause effect relationship spinifex and granofels chl harzburgites show a marked ese wnw oxide aggregate lineation that differs in orientation from that of atg serpentinite and is approximately parallel to the intermediate compressive stress 2 these lineations and the platelet lineation of spinifex chl harzburgite may be due to along strike fluid flow below the permeability barrier that constituted the atg out dehydration isograd our study shows that the kinematics of the slab paleostresses and fluid flow exert a dynamic control on the textures of atg serpentinite dehydrating to peridotite in subducting slabs

u pb geochronology in the trans hudson orogen northern saskatchewan canada we have obtained u pb ages on zircons from volcanic and plutonic units in several lithotectonic domains of the southern trans hudson orogen in northern saskatchewan these data constrain the timing of early proterozoic orogenic events in the region and enhance our understanding of both the relationships among local domains and the relationship of the trans hudson orogen to other early proterozoic orogens in north america with one exception all units studied so far yield zircon ages of 1890 1835 ma most of which are systematically earlier than previously reported rb sr isochron ages on the same or similar units suggesting open system behavior in the rb sr systems five metarhyolites from volcanic sequences in the la ronge domain glennie domain and hanson lake block give ages ranging from 1888 to 1876 ma most of the plutons we dated ranging from gneissic syntectonic tonalites and granodiorites to less deformed late intrusions such as the wathaman batholith and other smaller bodies yield ages of 18

geochemistry of high grade eclogites and metarodingites from the central alps analyses for major minor and trace element contents of metamorphosed variably rodingitized mafic rocks demonstrate substantial removal of na and as much as three fold gains in ca as a consequence of rodingitization modest declines in si and fe can be explained in terms of dilution effects losses in k and ba do not correlate with ca and may have been caused by an alteration process not related to the rodingitization the ca metasomatism was not accompanied by a gain in sr the relative contents of ti zr hf y co sc and heavy ree show no readily detectable changes despite the rodingitization other alteration and subsequent metamorphisms namely eclogite facies t 800 c p 20kbar followed by amphibolite facies sillimanite zone protoliths were tholeiitic basalt or diabase and gabbro with trace element contents indicative of a spreading center origin trace element and ree patterns indicate low pressure fractionation of this magma with plagioclase stable this petrogenesis is consistent with prior conclusions on the shallow crustal origin of the protolith of the eclogite metarodingite garnet lherzolite suite in the cima lunga adula nappe central alps based on their bulk chemical composition the mafic rocks in this suite could be the equivalent of mesozoic ophiolitic rocks in the more external parts of the alps