comments and reply on implications of magmatic epidote bearing plutons on crustal evolution in the accreted terranes of northwestern north america and magmatic epidote and its petrologic significance

experimental calibration of the aluminum in hornblende geobarometer with application to long valley caldera california volcanic rocks a new geobarometer based on the al content of igneous hornblendes in equilibrium with melt fluid biotite quartz sanidine plagioclase sphene and magnetite or ilmenite has been calibrated experimentally the calibration was performed by equilibrating the required phase assemblage over the pressure range 2 8 kbar at 740 780c and then analyzing euhedral hornblendes in equilibrium with glass melt experiments were performed on natural samples of both volcanic and plutonic rocks earlier empirical calibrations of this geobarometer relied on analyzing natural hornblendes from plutons with the required phase assemblage and inferring pressure from nearby metamorphic country rocks the experimental calibration differs from the empirical calibrations especially above 5 kbar and shows that the al content of hornblendes in equilibrium with the required phase assemblage is greater for a given total pressure than previously thought the geobarometers uncertainty is dramatically reduced the derived equation is p plus minus 0 5 kbar 3 minus 3 46 plus minus 0 24 4 23 plus minus 0 13 al sup t the geobarometer is applied to post bishop tuff volcanic rocks from long valley caldera california and reveals that most rhyodacites in this complex erupted from depths of about 6 km these eruptions occurred over 500 000 yr suggesting that the rhyodacitic magma reservoir beneath longmore valley had reached a steady p depth t state less

magmatic epidote and its petrologic significance epidote is a major magmatic mineral in tonalite and granodiorite in a belt coextensive with the mesozoic accreted terranes between northern california and southeastern alaska textural and chemical evidence indicates that epidote crystallized as a relatively late but magmatic mineral that formed through reaction with hornblende in the presence of a melt phase the observed relations concur with experimental data on crystallization of epidote from synthetic granodiorite at 8 kbar total pressure plutonic rocks bearing magmatic epidote must have formed under moderately high pressures corresponding to lower crustal depths under fairly oxidizing conditions

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

epidote in calcalkaline magmas an experimental study of stability phase relationships and the role of epidote in magmatic evolution experiments on tonalite and granodiorite were performed at conditions ranging from 2 1 to 18 kbar and 550 to 850 c to establish the magmatic stability field of epidote as a function of p t and f02 at water saturated conditions and f02 buffered by nno epidote has a wide magmatic stability field in tonalite at 13 kbar this field extends from the wet solidus at 630 to 790 0c the low pressure intersection of the magmatic epidote crystallization reaction and the h20 saturated tonalite solidus and hence the minimum pressure for magmatic epidote occurs at about 5 kbar the clapeyron slopes of epidote melting reactions are moderately positive in p t space at pressures below the intersection of the epidote melting and the gamet in reactions at 13 kbar 790 c at this intersection epidote reaches its maximal thermal stability 790 c in tonalite h20 in the presence of garnet that is above 14 kbar epidote melting reactions have steep negative clapeyron slopes in p t space at p t conditions near the low pressure intersection of the epidote melting reaction with the water saturated solidus experiments were also performed with f02 buffered by hm at these oxidizing conditions the magmatic stability field of epidote is enlarged down to a pressure of about 3 kbar in granodiorite the stability of epidote has been investigated at pressures from 7 5 to 18 kbar with respect to the tonalite the epidote out reaction is located almost at the same p t conditions at 10 kbar but the maximum thermal stability is about 50 c lower toward higher pressures although few experiments indicate that epidote stability does not increase dramatically with decreasing water activity at present state it is not possible to predict melting and crystallization behavior of epidote at waterundersaturated conditions modal abundances of epidote are low 05 vol in the presence of major 40 vol melt fractions thus epidote in calc alkaline magmatic rocks is not likely to affect major elements strongly by fractional crystallization

reply to comment by michard et al on the mesozoic margin of the maghrebian tethys in the rif belt morocco evidence for polyphase rifting and related magmatic activity

correlation between magnetic fabrics strain and biotite microstructure with increasing mylonitisation in the pretectonic wyangala granite australia abstract the wyangala granite is a foliated porphyritic silurian granite from the palaeozoic circum pacific type eastern lachlan orogen elo of australia it is a paramagnetic ilmenite bearing s marginal i type two mica to mainly biotite granite with different biotite contents and local chlorite alteration very highly strained quartz epidote bands are present in this contribution anisotropy of magnetic susceptibility ams is compared with independently measured intensity and 3d style of strain biotite microstructure and degree of mylonitisation for low strain granites with weak s foliations through medium strain protomylonitic granites with moderate s and c foliations to a high strain altered granite with a strong single foliation the samples are further analysed for possible contributions from sample heterogeneity magmatic flow and sub magmatic deformation a good correlation p ams 1 02 0 04 ln p e qtz is obtained between site average degree of ams p ams in the granite and degree of finite strain anisotropy p e qtz from aspect ratios of quartz aggregates in s foliations in hand specimen and outcrop p ams 1 03 1 14 p e qtz 1 4 19 the magnetic fabric ellipsoids agree with a kinematic regime between neutral and pure oblate predicted by the march model the observed quartz strains however exceed the ams march strains and are near neutral plano linear character the geological factors that may have contributed to these differences include intra and inter crystalline deformation of biotite and bimodality in s and c magmatic fabric is not clearly evident in either the ams or the biotite data new data for synkinematic oligoclase low titanium biotite and low sodium k feldspar show that conditions during deformation were approximately transitional greenschist amphibolite facies i e well below solidus this agrees with published age data that put the granite emplacement in an extensional back arc setting in already deformed country rocks and before later convergent regional deformation in middle devonian tabberabberan event to early carboniferous kanimblan event time

constraints on the origin of archean trondhjemites based on phase relationships of nuk gneiss with h2o at 15 kbar we report the t x h2o phase relations for the trondhjemitic nuk gneiss which comprises the principal component of the second phase of archean 3 0 2 8 by igneous activity in the godthab region of southwestern greenland a pressure of 15 kbar was chosen to place constraints on possible protoliths for trondhjemitic melts at lower crustal depths under h2o saturated conditions a melting interval of 135 c separates the solidus at 610 c from the liquidus at 745 c h2o saturation at 15 kbar occurs at approximately 15 5 wt h2o the h2o undersaturated liquidus extends along a curved path from 745 c at 15 5 wt h2o to 1100 c at 2 h2o lower h2o contents were not investigated at low h2o contents 7 mafic residues consisting of some combination of hb ga ep and bi are possible and could correspond to amphibolitic source rocks at lower melt h2o contents 5 possible residues consist of na pl qz ga and could correspond to an earlier generation of tonalitic trondhjemitic rocks however such residues would not impart the highly fractionated ree patterns characteristic of archean trondhjemites if a first generation of tonalitic trondhjemitic melts was generated by higher pressure partial fusion of eclogite and emplaced at 55 km depth it would crystallize to an assemblage consisting almost entirely of na pl qz with highly fractionated ree patterns these rocks in turn could be partially melted to yield a second generation of trondhjemites which would inherit the highly fractionated ree patterns because neigher pl nor qz is capable of significantly fractionating hree from lree

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

archaean cratonization and deformation in the northern superior province canada an evaluation of plate tectonic versus vertical tectonic models abstract the archaean minto block northeastern superior province is dominated by tonalite trondhjemite enderbite pyroxene tonalite granodiorite and granite with subordinate mafic rocks and supracrustal belts the plutons have been interpreted as the batholithic roots of andean type plate margins and intra oceanic arcs existing horizontal tectonic models propose that penetrative recrystallization and transposition of older fabrics during terrane assembly at 2 77 and 2 69 ga produced a n nw tectonic grain in the douglas harbour domain northeastern minto block tonalite and trondhjemite dominate the faribault thury complex 2 87 2 73 ga and enderbite constitutes 50 100 km scale ovoid massifs troie and qimussinguat complexes 2 74 2 73 ga magmatic muscovite and epidote in tonalite trondhjemite have corroded edges against quartz plagioclase suggesting resorption during ascent of crystal charged magma foliation maps and air photo interpretation show the common development of 2 10 km scale ovoid structures throughout the douglas harbour domain outcrop and thin section scale structures imply that many plutons experienced a phase of syn magmatic deformation typically followed by high temperature sub magmatic overprints thermobarometric data for plutons indicate near solidus recrystallization at 4 6 kbar pressures the common preservation of syn magmatic fabrics in plutons of different ages seems incompatible with the origin of these fabrics through superimposed regional orogenesis the broad uniformity of intrusion ages and lithologies throughout the minto block and the rarity of shallowly dipping planar fabrics also seem inconsistent with accretion of disparate older terranes each of which should preserve distinct histories a possible alternative explanation for these features is provided by vertical tectonic models whereby buoyant felsic magmas ascended as crystal slurries while dense supracrustal rocks and solidified felsic intrusions emplaced into them subsided as cold fingers 10 20 km scale instabilities shear between upwelling and downwelling limbs would have concentrated in the weak intrusions generating steeply plunging syn magmatic fabrics and producing ductile overprints in solidified rocks

the timing of sub solidus hydrothermal alteration in the central zone limpopo belt south africa constraints from titanite u pb geochronology and ree partitioning abstract in the central zone of the limpopo belt south africa palaeoproterozoic granulite facies metamorphism was superimposed on an earlier archaean orogenic history previously determined ages of 2030 2020 ma obtained from high temperature chronometers zircon garnet monazite are generally thought to provide the best estimate of the peak of palaeoproterozoic granulite facies metamorphism in the central zone whereas ages as young as 2006 ma from late melt patches suggest that temperatures remained above the wet solidus for an extended period we present a new mc icp ms 207 pb 206 pb age of 2030 9 1 5 ma for titanite found in amphibolite to greenschist facies alteration zones developed adjacent to quartz vein systems and related pegmatites that cut a strongly deformed central zone metabasite this age could potentially date cooling of rocks at this locality to temperatures below the wet solidus alternatively the titanite could be inherited from the metabasite host and the age determined from it date the peak of metamorphism integration of the geochronology with la icp ms trace element data for minerals from the metabasite the hydrothermal vein systems and comparable rocks elsewhere shows that the titanite formed during the amphibolite facies hydrothermal alteration not at the metamorphic peak or during the greenschist facies phase of veining this suggests that high grade rocks in the central zone have cooled differentially through the wet solidus and provides timing constraints on when palaeoproterozoic reworking in the central zone began this study illustrates the potential of combined geochronological and high resolution geochemical studies to accurately match mineral ages to distinct crustal processes

allanite and other ree rich epidote group minerals epidote group minerals rich in rare earth elements ree in particular allanite are common accessory phases in igneous metamorphic metasomatic and sedimentary rocks small amounts of ree are present in most epidote group minerals but in allanite and the related minerals dissakisite ferriallanite dollaseite khristovite and androsite the ree are essential structural constituents an important characteristic of ree rich epidote group minerals is that their octahedrally coordinated m sites contain major amounts of divalent cations this paper summarizes literature data for these minerals and discusses their chemistry occurrence phase relations and petrologic and geologic significance the chapter emphasizes allanite because it is the most common and best studied of the ree rich epidote group minerals epidote group minerals contain isolated silicon tetrahedra and corner sharing groups of two tetrahedra and are thus assigned to the disilicate or sorosilicate structural family for a detailed description of the structure see franz and liebscher 2004 the epidote group structural formula is a2m3 sio4 si2o7 o f oh or in a simplified form a2m3si3o11 o f oh in which a ca sr pb2 mn2 th ree3 and u and m al fe3 fe2 mn3 mn2 mg cr3 and v3 deer et al 1986 there are two structurally different a sites a 1 and a 2 with different coordination numbers and there are three different m sites m 1 m 2 and m 3 which are all octahedrally coordinated ueda 1955 dollase 1971 in epidote group minerals trivalent ree are accommodated in the a sites which in endmember epidote both contain ca the incorporation of ree3 is commonly charge balanced by a divalent cation fe2 mn2 mg substituted for a trivalent one in the m sites table 1

the petrogenesis and emplacement of the new hampshire plutonic suite a longstanding problem associated with the acadian orogeny in new england is whether or not the orogeny involved a redistrubution of mass and energy within the midcrust without the addition of magmas and or anomalous heatfrom the mantle and or lower crust several closed system models suggest the new hampshire plutonic suite nhps resulted from essentially in situ partial melting of the metasedimentary rocks of the central maine terrane cmt at midcrustal levels crustal thickening models shear heating along the decollement and the presence of u enriched metasedimentary rocks have been suggested as heat sources for anatexis while applicable to numerous nhps plutons there are several difficulties with these models as explanations for the nhps as a whole 1 the syntectonic bethlehem kinsman and spaulding members are too cao and na 2 o rich to have been derived from the exposed metapelites deeper seated metagraywackes may account for some of the nhps source rocks but not for the metaluminous spaulding tonalite and other more mafic nhps rocks 2 some nhps 1 8 o nd and sr isotopic values are outside the range of cmt and bronson hill terrane metasedimentary rocks requiring a more primitive mantle like component 3 the temperatures of the kinsman and the spaulding magmas were higher than those attained during regional metamorphism associated with crustal thickening greater than 850 c 4 magmatic epidote and hornblende geobarometry for the winnipesaukee pluton of the spaulding tonalite suggest intratelluric crystallization initially at pressures 6 kbars deeper than the inferred decollement between the allochthonous cmt terrane metasedimentary rocks and underlying basement these data suggest that while the nhps contains a significant component from the cmt metasedimentary rocks the metaluminous spaulding rocks mafic enclaves in the kinsman and mafic rocks in northeastern vermont western maine and southeastern new hampshire provide evidence of heat and mass beyond that associated with closed midcrustal system processes

bulk rock and mineral chemistries and ascent rates of high k calc alkalic epidote bearing magmas northeastern brazil abstract a manifestation of the pan african brasiliano orogeny 700 550 ma in northeastern brazil was the emplacement of widespread neoproterozoic granitoids in diverse tectonic terranes among these plutons are the magmatic epidote bearing conceicao das creoulas caldeirao encantado murici and boqueirao plutons located close to the boundary between the alto pajeu and cachoeirinha salgueiro terranes the plutons are high k calc alkalic granodiorites to monzogranites with tabular k feldspar megacrysts pistacite atomic fe 3 fe 3 al in epidote in these granitoids ranges from 21 to 27 high oxygen fugacity log f o 2 19 to 13 and the preservation of epidote suggest that the magma was oxidized al in hornblende barometry indicates hornblende solidification between 6 and 8 kbar at 620 to 780 c according to the hornblende plagioclase thermometer zircon saturation thermometry attests to a near liquidus temperature range from 794 to 853 c partial corrosion of magmatic epidote in these four plutons occurred during an interval of no more than 10 30 years which corresponds to maximum magma ascent rates of 650 1000 m year diking associated with regional shearing probably facilitated rapid transport of granitic magma through hot continental crust at peak metamorphism and permitted survival of epidote that was out of equilibrium at the low pressure of final emplacement similarities between mineralogical composition chemistry and isotopic compositions end 0 60ga between 2 and 5 t dm from 1 2 to 1 3 ga 18 o values 10 v smow of these four plutons and neoproterozoic magmatic epidote bearing plutons elsewhere in northeastern brazil argue for similar metabasaltic mafic sources that had previously experienced low temperature alteration

influence of fore arc structure on the extent of great subduction zone earthquakes 1 structural features associated with fore arc basins appear to strongly influence the rupture processes of large subduction zone earthquakes recent studies demonstrated that a significant percentage of the global seismic moment release on subduction zone thrust faults is concentrated beneath the gravity lows resulting from fore arc basins to better determine the nature of this correlation and to examine its effect on rupture directivity and termination we estimated the rupture areas of a set of mw 7 5 8 7 earthquakes that occurred in circum pacific subduction zones we compare synthetic and observed seismograms by measuring frequency dependent amplitude and arrival time differences of the first orbit rayleigh waves at low frequencies the amplitude anomalies primarily result from the spatial and temporal extent of the rupture we then invert the amplitude and arrival time measurements to estimate the second moments of the slip distribution which describe the rupture length width duration and propagation velocity of each earthquake comparing the rupture areas to the trench parallel gravity anomaly tpga above each rupture we find that in 11 of the 15 events considered in this study the tpga increases between the centroid and the limits of the rupture thus local increases in tpga appear to be related to the physical conditions along the plate interface that favor rupture termination owing to the inherently long timescales required for forearc basin formation the correlation between the tpga field and rupture termination regions indicates that long lived material heterogeneity rather than short timescale stress heterogeneities are responsible for arresting most great subduction zone ruptures

constraints on archean trondhjemite genesis from hydrous crystallization experiments on nuk gneiss at 10 17 kbar we present crystallization experiments on archean nuk gneiss from west greenland and relate these to conditions for early continental crustal genesis the studied composition is a trondhjemite from a voluminous felsic intrusive phase dated at 2 800 3 000 m y in a detailed set of experiments at 10 kbar the liquidus mineralogy is observed to change with increasing water content from plagioclase above 1000 c and 7 wt water water saturation of the melt occurs with 10 5 wt water at 800 c mafic liquidus minerals at 15 kbar differ from those at 10 kbar in that garnet replaces pyroxene and that epidote replaces hornblende below 690 c at 13 wt water and at water saturation 15 5 wt garnet crystallization is never observed at pressures below 13 kbar using additional experiments at 12 5 13 and 17 kbar the liquidus phase variation for trondhjemite with water is accurately defined between 8 18 kb

contrasting sources and p t crystallization conditions of epidote bearing granitic rocks northeastern brazil o sr and nd isotopes abstract the 618 ma curral de cima tonalite and 577 ma lourenco monzodiorite northeastern brazil are magmatic epidote bearing plutons that carry ferrohornblende biotite titanite and epidote major trace and isotope chemistry suggests that the major magmas of the two plutons followed similar differentiation trends but derived from source rocks that differed in age and isotopic composition the mineral phases of the curral de cima tonalite the presence of amphibole rich clots and juvenile component average end 3 55 point to an i type source for these rocks these data and high calculated 18 o w r 10 0 for the tonalite and high 18 o value for a clot 9 3 argue that the clots are fragments of a metabasaltic source rock that has been hydrothermally altered at a low temperature in contrast average calculated 18 o w r for the lourenco monzodiorite 7 8 87 sr 86 sr 0 7083 end 14 6 and t dm 1 92 ga these data are compatible with a long crustal residence time of lower crust amphibolites source epidote in the curral de cima pluton crystallized close to the nno buffer and hornblende chemistry due to al reequilibration yielded sub solidus temperature and pressure in contrast in the lourenco pluton epidote crystallized close to the hm buffer and al in hornblende points to near solidus solidification 685 c around 4 4 kbar this study confirms that magmatic epidote in granitic plutons can crystallize at pressures lower than 5 5 kbar under higher f o 2 as experimentally foreseen rapid magma transportation through hot continental crust during the peak of metamorphism in early stages of an orogenic cycle prevents epidote dissolution

granite emplacement by crustal boudinage example of the calmayo and el hongo plutons cordoba argentina this study deals with the structure and emplacement of the calmayo and el hongo trondhjemite plutons famatinian belt of cargentina it provides structural data from the granites and the country rocks and a study of the magnetic fabric in the plutons new u pb geochronological data yield intrusion ages of 512 1 3 4 ma and 500 6 4 5 ma for the calmayo and el hongo plutons respectively the el hongo massif and the southern part of the calmayo trondhjemite preserve magmatic structures whereas the northern domain of calmayo shows the imprint of solid state deformation the main foliation in the country rocks outlines a boudin like pattern at the map scale and the granites are located along boudin necks suggesting that the emplacement of these tron dhjemite plutons was linked to large scale boudinage of the country rocks

petrology of the high mg tonalites and dioritic enclaves of the ca 2130 ma alto maranhao suite evidence for a major juvenile crustal addition event during the rhyacian orogenesis mineiro belt southeast brazil abstract combined field observations petrography mineral chemistry geochemistry nd isotopes and u pb zircon data are presented for the ca 2130 ma alto maranhao suite the suite composed by allanite bearing biotite hornblende tonalites and commingled dioritic mafic magmatic enclaves mme occupies an estimated area of 300 km 2 in the southern quadrilatero ferrifero region minas gerais brazil tonalites and commingled mme are medium k rocks with high mg mg number 0 46 and high cr 55 ppm contents they are also enriched in large ion lithophile elements lile sr ba some high field strength elements hfse th u and light rare earth elements ree but depleted in nb ta and heavy ree t dm model ages for tonalites and mme are similar and range from 2 3 to 2 4 ga the nd t values are grouped around chondritic values 0 1 0 field relations and geochemical data indicate that tonalites and commingled dioritic mme crystallized as synchronous independent magma pulses with limited mixing both magmas are compositionally equivalent to high mg andesites in modern subduction zones implying that the suite was issued from the partial melting of a mantle wedge metasomatised to different degrees by slab derived melts geobarometry aluminium in hornblende indicates crystallization at mid crustal depths i e 0 59 0 42 gpa the igneous suite evolved by fractional crystallization of hornblende biotite and accessory minerals zircon apatite allanite and fe ti oxide the ubiquitous occurrence of hornblende and allanite in tonalites and dioritic mme suggests that this mineral pair was largely responsible for the observed changes of light ree heavy ree and eu eu ratios within the members of the suite the geographical extent 1000 km 2 and paleoproterozoic ages of the alto maranhao suite ca 2130 ma and associated granitoids of the mineiro belt 2 1 2 2 ga constitute a major event of juvenile addition to the continental crust in the context of the southern sao francisco craton and the ca 2 2 2 1 ga rhyacian orogenesis of the south american platform the age and general geochemical characteristics of the alto maranhao suite and its emplacement after a ca 2 35 ga juvenile tholeiitic source derived high al ttg suite indicate a tectonic setting akin to that of late archaean high mg granitoids

distinguishing magmatic from subsolidus epidote laser probe oxygen isotope compositions epidote occurs in four textural varieties in the mount lowe intrusion of southern california euhedra anhedra intergrowths and cross cutting veins of these conspicuous elongate euhedra which range up to 3 cm in length meet most of the established textural criteria for magmatic epidote equant anhedral grains which are 5 mm in diameter are texturally ambiguous although rare allanite cores are suggestive of a magmatic origin epidote intergrowths with hornblende and biotite also meet certain textural criteria for crystallization from a magma notably euhedral faces against biotite finally late stage veins of epidote cross cut all phases in the rock and are likely subsolidus oxygen isotopic compositions of these four textural varieties of epidote determined using the laser probe indicate that the use of textural criteria alone in establishing epidote parageneses can be misleading intracrystalline 18o variations in the euhedra document both magmatic and subsolidus compositions oxygen isotope compositions are bimodal averaging 5 36 0 13 n 11 and 4 66 0 23 n 21 these data combined with plagioclase and zircon 18o values are interpreted to indicate that magmatic oxygen isotopic compositions have been preserved where epidote 18o is greater than 5 significant portions of each euhedral crystal have been affected by subsolidus exchange and are characterized by epidote 18o values less than 5 intracrystalline 18o values of epidote anhedra range from 4 50 to 6 08 and thus also document both magmatic and subsolidus compositions subsolidus exchange is volumetrically less significant in the anhedra than in the euhedra values of 18o for intergrowths and late stage cross cutting veins of epidote average 3 81 0 22 and document clearly subsolidus growth the data confirm that in the mount lowe intrusion large euhedra of epidote are magmatic in origin consistent with textural evidence the data also indicate that equant anhedra of epidote are also magmatic in origin and thus the absence of good crystal faces does not necessarily indicate subsolidus growth the subsolidus origin of intergrowths of epidote with euhedral faces against biotite indicates that well developed crystal faces do not require growth from a magma finally the subsolidus origin of the vein epidote is consistent with textural evidence the increased resolution available with laser based oxygen isotope measurements offers an important test for documenting whether epidote is of magmatic or subsolidus origin given the barometric significance of magmatic epidote oxygen isotope compositions can be used to aid in the interpretation of epidote bearing plutons

contrast of metamorphic and structural histories across the work channel lineament coast plutonic complex british columbia a prominent topographic lineament along the western margin of the coast plutonic complex of southeast alaska and adjacent parts of british columbia separates terranes of contrasting metamorphic histories where it enters british columbia near prince rupert as the work channel lineament the western terrane preserves a regional medium high pressure metamorphic facies series increasing from chlorite grade in the west to kyanite muscovite migmatite near the lineament the metamorphic structural and age relations of the western terrane are consistent with a model of crustal thickening accompanied by westward transport of higher over lower grade metamorphic units during the jurassic this terrane was uplifted and cooled by cretaceous time 85 ma east of work channel lineament high temperature relatively low pressure assemblages are overprinted onto high pressure assemblages the low pressure metamorphic assemblages developed in the eocene during rapid uplift 2 mm yr of rocks which initially may have been metamorphosed at the same time and along the same pt gradient as those of the western terrane structures along the work channel lineament suggest vertical motion during uplift of the eastern terrane and after metamorphism of the western terrane because the metamorphic discontinuity across the lineament is 3 5 kbar at about 700 the lineament in the prince rupert area marks a major crustal break

mineralogy geochemistry and petrology of neoarchean ferroan to magnesian granites of carajas province amazonian craton the origin of hydrated granites associated with charnockites abstract 2 75 to 2 73 ga old granitic intrusions associated with coeval charnockitic rocks were emplaced in the northern domains of the carajas province the vila jussara suite was recognized recently and its geologic mineralogical and geochemical characteristics are presented in this paper and compared with the data available in the literature on the estrela complex serra do rabo and igarape gelado granites and planalto suite monzogranites and syenogranites are dominant in most units but in the igarape gelado granite and vila jussara suite granodiorites and tonalites are also relatively abundant the main mafic phases in all these granitoids are amphibole potassian hastingsite with subordinate magnesiohastingsite 0 95 fe fe mg 0 47 and biotite 0 88 fe fe mg 0 52 with rare occurrence of clinopyroxene relict crystals magmatic epidote of pistacite component from 26 to 31 occurs only in the vila jussara suite the rocks of estrela serra do rabo and planalto units contain only ilmenite generally with coronas of titanite or ilmenite associated with magnetite and are essentially reduced ferroan granites whole rock feot feot mgo 0 89 that evolved at low f o 2 fmq 0 5 the igarape gelado granite needs additional studies but it is formed apparently by reduced ferroan and oxidized ferroan granites the vila jussara suite is also composed of reduced ferroan granites that are similar to those of the other granite units however it has additionally oxidized ferroan and magnesian granitoids which contain magnetite ilmenite as the main fe ti oxide phase and evolved respectively at moderate f o 2 nno 0 5 and at comparatively higher f o 2 nno to nno 1 the magmas of these granites were formed at temperatures 900 c and derived from a heterogeneous dominantly reduced and tholeiitic granulitic crust locally varying to oxidized and calc alkaline in composition plagioclase orthopyroxene clinopyroxene and fe ti oxides were the main phases in the melt residues and the liquids were enriched in ba zr y and ree except for eu but not in sr compared to their sources the magmas should have relatively high water content 4 wt the magmatic evolution was strongly influenced by partial melting processes the different varieties of the vila jussara suite are not linked by fractional crystallization and derived from distinct magmas the neoarchean granites of the carajas province were emplaced in a collisional tectonic setting they are syntectonic granites that were deformed and sometimes intensely recrystallized during their emplacement and subsequent subsolidus cooling the magmas were generated in the crust at pressures of 800 to 1000 mpa and emplaced at pressures of 200 to 500 mpa the studied granitoids are similar geochemically to the ferroan and magnesian groups of charnockites the magnesian variety of the vila jussara suite is similar to the pyroxene free granitoids of the neoarchean matok pluton of the collisional limpopo belt the neoarchean granites of carajas are hydrated granites emplaced at comparatively higher crustal levels when compared with the matok pluton

zircon dating and mineralogy of the mokong pan african magmatic epidote bearing granite north cameroon we present the mineralogy and age of the magmatic epidote bearing granite composing most of the mokong pluton in the central africa orogenic belt north cameroon this pluton intrudes neoproterozoic 830 to 700 ma low to high grade schists and gneisses poli maroua group and is crosscut or interleaved with bodies of biotite granite of various sizes the pluton is weakly deformed in its interior but solid state deformation increases toward its margins marked by narrow mylonitic bands trending nne ssw the magmatic epidote granitic rocks are classified as quartz monzodiorite granodiorite monzogranite and syenogranite they are medium to coarse grained and composed of k feldspar plagioclase biotite amphibole epidote magnetite titanite zircon apatite in these granites the pistacite component atomic fe 3 fe3 al in epidote ranges from 16 to 29 high oxygen fugacity log o2 14 to 11 and the preservation of epidote suggest that the magma was oxidized al in hornblende barometry and hornblende plagioclase thermometry indicate hornblende crystallization between 0 53 and 0 78 gpa at a temperature ranging from 633 to 779 c zircon saturation thermometry gives temperature estimates ranging from 504 to 916 c the latter being obtained on samples containing inherited zircons u pb geochronology by la icp ms on zircon grains characterized by magmatic zoning yields a concordia age of 668 11 ma 2 the mokong granite is the only known occurrence magmatic epidote in cameroon and is an important milestone for the comparison of the central africa orogenic belt with the brasiliano fold belt where such granites are much more abundant

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

thickness and composition of the bonanza arc crustal section vancouver island canada the composition and origin of continental crust is in part constrained by evidence preserved in exhumed arc crustal sections the bonanza arc on vancouver island canada is a jurassic aged arc crust section exhumed in the eocene we use al in hornblende geobarometry of felsic plutons in the bonanza arc to assess the depths of crystallization of its exposed arc crust plutons of the unfoliated island plutonic suite are emplaced as sheets at depths of 2 10 km within a stratigraphy of devonian to triassic supracrustal rocks the west coast complex is more generally strained and intrudes below the pre jurassic supracrustals at depths of 10 18 km previous work shows that rare ultramafic units within the west coast complex crystallized at depths of 14 26 km the bonanza arc crustal section has a present structural thickness of at least 15 km the average composition for each arc component volcanic and plutonics when apportioned into the crustal section derived by stratigraphy and hornblende barometry shows the bulk composition of the entire arc is basaltic andesite 56 sio 2 mg mg fe mg 50 not unlike some estimates for bulk continental crust although only vestiges of ultramafic rocks occur in the bonanza arc today a significant component 6 20 km of this rock type must have been present in the section for the bulk arc to have been a melt in equilibrium with mantle olivine and may have been lost by foundering or tectonic thinning

clockwise p t paths for crustal melting and h2o recycling in granite source regions and migmatite terrains abstract clockwise pressure temperature p t paths of metamorphism pass through two kinds of melting reactions with increasing temperature the first generates h 2 o saturated partial melts at the h 2 o saturated solidus only if excess h 2 o is present usually this accounts for less than 3 melt the second generates h 2 o undersaturated melts through dehydration melting reactions dmrs of micas or other hydrous minerals and occurs at temperatures higher than the h 2 o saturated solidus if h 2 o undersaturated melts remain in situ and the source regions are not exhumed then the crystallisation reactions encountered along the near reverse p t path may produce the same mineralogy as in the source rock in regions of exhumation along the decompression segments of clockwise p t paths two other types of crystallisation reactions occur the better known type occurs at the recrossing of the pelite h 2 o saturated solidus at shallower depth this can occur at mid crustal depths or much shallower at pressures lower than the appropriate invariant point which relates all of these reactions and will result in granitic pegmatites at the other type remaining mica can react with hydrous melt to generate anhydrous granulite with free h 2 o fluid such suprasolidus decompression dehydration reactions sddrs result in freezing of pelitic melts at mid crustal depth to k feldspar quartz assemblages and release of h 2 o which can induce h 2 o saturated melting in adjacent fertile lithologies lacking h 2 o e g psammite older granite distinct mineral proportions and textures will result from each reaction the actual reactions occurring are related through specific invariant points whose locations are known in the p t space the displacement of these invariant points due to additional components in the solid melt and fluid phases is reasonably well known the flat lying sddr very low d p d t will extend to much higher pressures if a low ah 2 o fluid is present sddr involving amphibole or epidote will occur in mafic to intermediate compositions over a large depth range depending on fe mg ca na etc of the rock compositions

emplacement petrological and magnetic susceptibility characteristics of diverse magmatic epidote bearing granitoid rocks in brazil argentina and chile abstract magmatic epidote mep bearing granitoids from five neoproterozoic tectonostratigraphic terranes in northeastern ne brazil early palaeozoic calc alkalic granitoids in northwestern nw argentina and from three batholiths in chile have been studied the elongated shape of some of these plutons suggests that magmas filled fractures and that dyking was probably the major mechanism of emplacement textures reveal that in many cases epidote underwent partial dissolution by host magma and in these cases may have survived dissolution by relatively rapid upward transport by the host magma in plutons where such a mechanism is not evident unevenly distributed epidote at outcrop scale is armoured by biotite or near solidus k feldspar aggregates which probably grew much faster than epidote dissolution preventing complete resorption of epidote by the melt al in hornblende barometry indicates that in most cases amphibole crystallized at p 5 kbar kyanite bearing thermal aureoles surrounding plutons that intruded low grade metamorphic rocks in ne brazil support pluton emplacement at intermediate to high pressure mep show overall chemical variation from 20 to 30 mol mole percent pistacite ps and can be grouped into two compositional ranges ps20 24 and ps27 30 the highest ps contents are in epidotes of plutons in which hornblende solidified under p 3 0 10 3 si typical of magnetite series granitoids crystallized under higher oxygen fugacity in ne brazil argentina and chile it seems that mep is more common in precambrian to palaeozoic ilmenite series granitoids while its occurrence in magnetite series granitoids is more restricted to mesozoic to tertiary granitoids

sulfide and oxide minerals from s type and i type granitic rocks abstract in the ohmine i type and s type granitic rocks of miocene age kii peninsula central japan ilmenite rutile pyrrhotite pyrite and chalcopyrite are common but no magnetite is present the primary paragenetic relations of these minerals are determined by their modes of occurrence as inclusions within major plagioclase orthoclase quartz hornblende and biotite although their parageneses changed during cooling significant subsolidus reactions for sulfide minerals took place in both the groundmass and the inclusions until the rocks cooled below 300 c the loss of s and cu through silicate crystals was too easy to preserve the chemistry of sulfides included in silicates however the removal of ni through silicate crystals as well as along grain boundaries was restricted as indicated by the fact that the distribution frequency of ni bearing pyrrhotite is higher in inclusions than in the groundmass and significantly higher in i type cognate enclaves than in host i type rocks nickel bearing pyrrhotite is common in i type but rare in s type rocks sedimentary enclaves restates and country rocks this eliminates the possibility that the ohmine s type rocks were formed by interaction of i type magma and sedimentary rocks or by mixing between i type and an ideal s type magmas as interpreted from the chemistry of biotite and the chronology of these rocks on the basis of mineralogy and isotopic data for s o sr c and rare gases it was shown that i type granitic rocks were derived from partial melting of intermediate igneous rocks containing hornblende and plagioclase and s type from partial melting of sedimentary rocks containing biotite and orthoclase

epidote phenocrysts in dacitic dikes boulder county colorado epidote ps21 crystallized early as elongate phenocrysts in late cretaceous rhyodacitic dikes in the vicinity of ward boulder county colorado other unusual phenocryst phases are garnet gr17 24 and muscovite in a xenolith containing kyanite corundum biotite and plagioclase magmatic garnet grew as a rim around xenocrystic pyrope rich py37 garnet the xenolith was derived from a granulite facies zone not represented at the present day erosion surface which is composed of upper amphibolite facies cordierite and sillimanite bearing gneisses the dike magmas were fed not from an immediately underlying batholith but from a magma chamber at a depth corresponding to a pressure of 8 13 kilobars phenocrysts cystallized in the temperature range 800 to 700 c under h2o and o2 activities greater than normal for silicic magmas this occurrence shows convincingly not only that epidote can be magmatic but that it is a possible early crystallization phase in silicic magmas

time space relationships among structural and metamorphic aureoles related to granite emplacement a case study from the serre massif southern italy tectonic and thermal perturbations related to emplacement of granodiorite in the upper continental crust have been investigated in the late hercynian basement exposed in southern calabria italy here the structural aureole is marked by the presence of a major rim fold adjacent to the intrusive contact for a length of at least 20 km geometrical analysis of the structural aureole and related foliations lineations and crenulations reveals that the perturbed zone is at least 3000 m wide and characterized by an open synform trending nearly parallel to the intrusive contact this pattern is compatible with a laccolith like mode of magma emplacement related to the accretion of the pluton that shouldered weak phyllitic and slaty wall rocks the metamorphic aureole about 1800 m wide is characterized by biotite cordierite and andalusite that appear sequentially in spotted schists and hornfelses approaching the intrusive contact the peak assemblage equilibrated between 535 and 590 c at pressures between 175 and 200 mpa confirmed by al in hornblende barometry on granodiorite microstructural analysis allowed the inference of a time lag between the thermal and tectonic perturbations with the aid of thermal modelling it was possible to quantify the time required to reach the peak temperature at a distance from the intrusive contact where cordierite spots and andalusite porphyroblasts clearly overprint crenulations this estimate represents the time limit to accomplish deformation in the inner portion of the aureole and thus indicates a minimum strain rate of 4 10 14 s 1 within the country rocks during granodiorite intrusion

petrology geochemistry and la icp ms u pb geochronology of paleoproterozoic basement rocks in bangladesh an evaluation of calc alkaline magmatism and implication for columbia supercontinent amalgamation abstract the paleoproterozoic 1 7 ga basement rocks from maddhapara bangladesh show a large range of chemical variations e g sio2 50 7 74 7 and include diorite quartz diorite monzodiorite quartz monzonite and granite the pluton overall displays metaluminous calc alkaline orogenic suite mostly i type suites formed within subduction related magmatism the observed major elements show general trends for fractional crystallization trace element contents also indicate the possibility of a fractionation or assimilation explain the entire variation from diorite to monzonite even granite the pluton may have evolved the unique chemical features by a process that included partial melting of calc alkaline lithologies and mixing of mantle derived magmas followed by fractional crystallization and by assimilation of country rocks the pluton shows evidence of crystal fractionation involving largely plagioclase amphibole and possibly biotite some of the fractionated magmas may have mixed with more potassic melts from distinct parts of the continental lithosphere to produce granites and or pegmatites new geochronological results of granitic pegmatite 1722 10 ma are indisputably consistent with diorite and tonalite and those data showing credible geochronological sequence i e diorite tonalite granitic pegmatite identical paleoproterozoic age 1 7 ga with distinctive magmatism of the maddhapara basement rocks have agreeable relationship with the citz india the consistent magmatism is also common in the transamazonian of south america trans hudson orogeny in north america bohemian massif and the svecofennian poland have identified the sequential growth of the continent through the amalgamation of juvenile terrains succeeded by a major collisional orogeny such paleoproterozoic subduction related orogens in australia have similar counterparts in antarctica and other part of the world these types of paleoproterozoic magmatism dominantly contributed to assemble amalgamation and breakup of the enormous columbia supercontinent

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

oxygen isotope compositions and magmatic epidote from two contrasting metaluminous granitoids ne brazil oxygen isotope compositions of mineral separates were determined from two metaluminous granitoids emas and sao rafael plutons from northeastern brazil the i type emas pluton has high 18o wr values 11 5 11 8 whereas the sao rafael pluton has low 18o wr values 7 5 8 1 but sr and nd are characteristics of s type granitoids measured mineral mineral fractionations suggest continuous sub solidus inter mineral isotope exchange among all minerals except zircon there is a large and consistent quartz epidote fractionation that gives apparent temperatures that are much lower than anticipated closure temperatures for epidote oxygen isotope fractionation between natural zircon and magmatic epidote is opposite to that predicted from theoretical determinations as 18o epidote 18o zircon an empirical calibration based on these results would suggests a closure t for oxygen in epidote of 500 c and qtz epi 5 19 at 500 c

the stability of amphibole in andesite and basalt at high pressures reinvestigation of earlier work on mt hood andesite and kilauea olivine tholeiite under n no conditions and with silver capsules the revised curve for the appearance of garnet is significantly lower in pressure no orthopyroxene appeared in the run products in contrast to the results of the earlier work the high temperature segment of the amphibole out curve for the tholeiite is at least as high as 1040 degrees c at 13 kbar and 1050 degrees c at 16 kbar and the high pressure part of this curve is at about 27 kbars about 6 kbar higher than in the earlier work amphibole is the sole silicate phase on the vapor saturated liquidus in the andesite at pressures below the garnet in curve 15 5 kbar revised phase relationships are consistent with the derivation of andesite by amphibole liquid equilibria in basaltic magma and reveal stability to depths of nearly 100 km the great depths to which amphiboles are stable make it probable that they are a source of h 2 o for martial melting in deeply subducted oceanic crust modified journal abstract

petrological constraints on crystallization conditions of mesoarchean sanukitoid rocks southeastern amazonian craton brazil we report petrological and geochemical data for the 2 87 ga rio maria sanukitoid granodiorite and associated rocks from mesoarchean granite greenstone terranes of the eastern amazonian craton brazil the dominant rocks have granodiorite to subordinate monzogranitic compositions with minor proportions of intermediate quartz diorites or quartz monzodiorites in addition to mafic endmembers occurring as layered rocks or as enclaves the mineral assemblage is dominated by amphibole plagioclase biotite and epidote minerals all of inferred magmatic origin pyroxenes being notably absent textural and compositional criteria indicate that amphibole is a principal mineral on the liquidus of all the rio maria rocks crystallization conditions have been derived from a comparison between natural phase assemblages proportions and compositions and experimental studies carried out on similar magma compositions the comparison shows that the parental magmas were water rich with more than 7 wt dissolved h2o with crystallization temperatures in the range 950 680 c the mg mg fe ratios of both amphibole and biotite indicate fo2 conditions in the range nno 0 5 to nno 2 5 where nno is nickel nickel oxide buffer therefore pointing to both water rich and oxidizing conditions for sanukitoid magmas amphibole compositions indicate emplacement at around 200 mpa and record a high pressure stage of magma crystallization around 600 900 mpa sanukitoid magmas share two of the principal characteristics of modern arc magmas elevated redox state and volatile contents which suggest that they may have formed in a geodynamic environment broadly similar to present day subduction zones

paleoproterozoic granitoids of the losevo terrane east european craton age magma source and tectonic implications abstract the losevo terrane lies between the sarmatia and volgo uralia segments of the east european craton and is considered to be part of the east sarmatian orogen eso here we report the results from field petrologic geochemical sm nd isotopic and zircon u pb geochronological studies on a suite of granitoids from the losevo terrane lt the lt granitoids are divided into four types 1 migmatite leucosome from quartz diorite to granodiorite 2 tonalite trondhjemite and granodiorite ttg with high si na sr contents high sr y la yb ratios and low mg k y yb contents 3 trondhjemite granodiorite granite tgg spatially related to the ttg suite but showing higher k negative eu anomalies and flat ree patterns and 4 high k calc alkaline monzogranite and granodiorite of i type the early 2115 ma migmatites occur as lensoid and stromatic layers the ttg and tgg 2100 2075 ma constitute about 20 of the lt and form batholiths 150 540 km 2 and stocks small massifs of the i type granites 2081 12 ma intrude the migmatites the migmatites ttg and tgg contain inherited zircon cores that yield ages between 2130 and 2172 ma the granitic suite shows positive end values ranging from 2 1 to 5 7 for the ttg and tgg and 2 1 and 2 2 for the migmatites suggesting magma derivation from juvenile paleoproterozoic sources model ages 2255 2450 ma in contrast the i type granite shows slightly negative end value of 0 5 suggesting that the magma sources involved a mixture of archean and paleoproterozoic components all the granitoids from lt record varying degrees of fractional crystallization with or without crustal contamination the temperature of parental melts of the granites are estimated to be in the range of 793 33 c to 878 16 c the migmatites were derived mainly from metaigneous sedimentary protoliths under p 5 6 kbar whereas the ttgs were derived from lower crustal metabasites under p 15 kbar the tgg were derived from mid crustal metagreywackes under p

crystallization and resorption in plutonic plagioclase implications on the evolution of granodiorite magma gesiniec granodiorite strzelin crystalline massif sw poland abstract granodiorite from the gesiniec intrusion strzelin crystalline massif sw poland contains complexly zoned plagioclases five chemically and structurally distinct zones can be correlated among crystals cores 25 35 an inner mantles 40 45 an outer mantles 40 25 an resorption zones 35 50 an and rims 35 30 an good structural and chemical major and trace elements correlation of zones between crystals indicates that zonation was produced by changes in conditions of crystallization on a magma chamber scale plagioclase being the liquidus phase records a time span from the beginning of crystallization to emplacement and rapid cooling of granodiorite as thin dykes crystallization began with the formation of inner mantles the paucity and different sizes of inner mantles suggests slow crystallization in high temperature magma normally zoned inner mantles were formed under increasing undercooling compositional trends in mantles suggest closed system crystallization the major resorption zones were caused by injection of less evolved magma as indicated by the strontium increase in plagioclase the injection triggered a rapid rise of magma and plagioclase crystals facilitating mixing but also inducing fast kinetically controlled growth of complex multiple oscillatory zonation within resorption zones the ascent of magma caused decompression melting of plagioclase and produced melt inclusions within inner mantles the cores the decompression range is estimated at a minimum of 2 kbar emplacement of granodiorite as thin dykes allow rapid cooling and preservation of magmatic zonation in plagioclases melt inclusions crystallized completely during post magmatic cooling the zonation styles of plutonic plagioclase differ markedly from volcanic ones suggesting different magma evolution zones in plutonic plagioclase are well correlated indicating crystallization in quiescent magma where crystals accumulation and compositional magma stratification may occur crystals probably did not travel between different regimes resorption occurred but as single albeit complex episodes good correlation of zones in plutonic plagioclases allows a distinction between the main processes controlling zonation and superimposed kinetic effects

comments and reply on implications of magmatic epidote bearing plutons on crustal evolution in the accreted terranes of northwestern north america and magmatic epidote and its petrologic significance comment

magmatic gradients in the cretaceous caleu pluton central chile injections of pulses from a stratified magma reservoir the caleu pluton central chile extending over 338 km2 and with more than 1 400 m of vertical relief intrudes the n s trending lower cretaceous volcano sedimentary and volcanic successions at a depth equivalent to a pressure of 2 kb the host stratified volcanic successions are tilted about 30 40 e whereas the pluton shows paleomagnetic evidence of either tilting of 15 e or clockwise rotation by few degrees a gradient of westward increasing sio2 content is recognized across the pluton giving rise to three n s elongated zones gabbro diorite zone gdz tonalite zone tz and granodiorite zone gz biotite and hornblende compositions also exhibit a westward decreasing gradient in mg mg fe indicating that the more mafic the zone is the more oxidizing is its crystallization condition horizontal inward gradients of progressively less evolved rocks are recognized across gdz and tz whereas no horizontal gradients were found in the gz vertical compositional gradients are recognized in the gdz and tz which consist of an upward increase in sio2 and decrease in mgo feo fe2o3 and compatible trace elements a vertical compositional boundary was recognized along a traverse across the tz separating two magma pulses with similar trends of compositional variations the three zones of the caleu pluton were derived from a common isotopically sr nd depleted source each zone probably evolved independently as their compositional characteristics would have not been acquired in situ the resulting compositional characteristics of the zones would have been developed prior to the intrusion in a subjacent stratified reservoir placed at about seven kilometers below the pluton

comments and reply on implications of magmatic epidote bearing plutons on crustal evolution in the accreted terranes of northwestern north america and magmatic epidote and its petrologic significance reply

u th pb and 230th 238u disequilibrium isotope systematics precise accessory mineral chronology and melt evolution tracing in the alpine bergell intrusion in order to investigate the potential of combined th u pb isotope and 230th 238u disequilibrium systematics for tracing magmatic crystallization and melt evolution conventional high resolution single crystal tims techniques have been applied to zircon titanite and fragments of geochemically characterized growth zones of allanite these minerals were extracted from a single tonalite specimen collected from the feeder zone of the tertiary bergell pluton southern steep belt s switzerland n italy the isotopic results document an extended history of crystallization and melt evolution of at least 5 ma with well resolved zircon ages defining an early interval of 33 0 to 32 0 ma followed by crystallization of zoned allanite from 32 0 to 28 0 ma and formation of magmatic epidote possibly as late as 26 ma trace and major element patterns in zoned allanite closely mirror melt evolution characterized by increase of u concentration and sharp decrease of th and lree reflecting early crystallization of phases low in u and in particular the dominating control by allanite precipitation preservation of substantial quantities of excess 206pb derived from initial excess 230th in all analyzed allanite grains indicates that their isotopic systems have not been reset by loss of radiogenic pb during prolonged residence at magmatic conditions and regional metamorphic cooling and that the measured sequence of 208pb 232th dates translates into a real age sequence major loss of radiogenic pb from compositionally zoned allanite by volume diffusion would have resulted in a data pattern grossly different from the observed one as demonstrated by numerical modeling of 232th 208pb 238u 230th 206pb isotopic evolution the results therefore suggest closure temperatures 700 c for magmatic allanite quantification of 230th 238u disequilibrium relationships reveals a smooth initially steep decrease of th u in the magma from values of 2 9 at 32 0 ma to 0 1 at 28 0 ma in equilibrium with sequential allanite zoning comparison of calculated th u magma with measured total rock th u 0 79 requires fractional crystallization of allanite at an early stage this process of removal of allanite and thus th from the melt was operative at least until 31 5 ma providing an upper limit for the time of emplacement of the studied magma batch the fact that bulk zircon and a large part of allanite crystallization predate emplacement cautions against equating ages determined on refractory minerals from deep seated plutons with intrusion ages the persistence of magmatic conditions over a period of 5 ma concentrated in a narrow collision zone between the european and the african plate has been a major controlling factor for the oligocene evolution of the adjacent insubric fault the main tectonic lineament of the alps being active from late cretaceous to miocene times

geochronology and petrogenesis of neoarchean potassic meta granites from huai an complex implications for the evolution of the north china craton abstract neoarchean potassic meta granites occurring in the huai an complex within the north central part of the north china craton ncc comprise leucocratic syenogranites biotite monzogranites and garnet bearing syenogranites high pressure garnet granulites occur as lenses sheets or dismembered dikes with the granitoids here we present shrimp zircon u pb data from the leucocratic syenogranites which show an age of 2493 6 ma mswd 1 and la icp ms u pb data from the biotite monzogranites and garnet syenogranites revealing their formation ages of 2437 10 ma mswd 2 0 and 1977 2003 ma respectively we also report metamorphic ages of ca 1800 ma from zircons in the garnet syenogranites geochemical studies show low mgo 0 5 0 06 wt cr 6 19 0 53 ppm and ni 27 66 2 01 ppm with varying sio2 62 3 to 76 8 wt contents indicating their derivation from a crustal source the leucosyenogranites exhibit elevated positive eu anomalies with decreasing sr th u and bulk ree concentrations and th u ratio whereas rb and ba values increase with decreasing th u ratio indicating fluid melt interaction the leucosyenogranites have high sr and ba contents and sr y and la yb n ratios and low y and hrees with zircon ehf t values ranging from 1 7 to 5 8 and hf model ages of 2 60 2 73 ga reflecting their derivation by partial melting of overthickened juvenile lower crust of eclogitic composition or an eclogitic mineral assemblage in the residue geochemical compositions of the biotite monzogranites with positive end t values 2 3 to 2 5 and model ages

age origin and cooling history of the coronel joao sa pluton bahia brazil received april 2 2002 accepted august 8 2004advance access publication october 22 2004in north east brazil archean and paleoproterozoic cratonic blocksare enclosed within a network of brasiliano age 0 7 0 55ga metasedimentary foldbelts the unfoliated coronel joa o sa grano diorite pluton which contains magmatic epidote and stronglyresorbed clinopyroxene intrudes the sergipano foldbelt zirconsyield a concordant u pb crystallization age of 625 2ma titanite ages are approximately 621ma discordant zircons suggestinheritance from at least two magma sources of ages 2 2ga model calculations based on diffusion parameters andrb sr isotope data from separated minerals indicate that the plutoncooled ata rate of 36 c myr whole rock element compositionsandinitialsr ndisotopic compositionsthat areheterogeneous onalllength scales suggest magma mixing trace element concentrationsand nd isotope data argue against a contribution from a contem poraneous mantle derived magma values of magmatic e

th pb ion microprobe dating of allanite allanite which is a common accessory mineral in a wide variety of rock types typically contains high concentrations of th and u thus an in situ method of u th pb dating of this phase would have broad application we describe a method to permit th pb ages of allanite to be determined with approximately 10 accuracy using a high resolution ion microprobe knowledge of the composition and substitution mechanisms of this complex mineral is key to understanding the relative ion ization efficiencies of th and pb the chemical compositions of three allanite samples used as age standards cima d asta pluton 275 5 1 5 ma atesina volcanic complex 276 3 2 2 ma la posta pluton 94 2 ma were determined using an electron microprobe permitting an assessment of matrix effects on ionization an ion microprobe calibration curve involving elemental and oxide species of th and pb i e 208 pb th vs tho 2 th yields highly scattered apparent ages when allanite age standards with different fe contents are used however a three dimensional plot of 208 pb th vs tho 2 th vs feo sio improves the accuracy of the calibration to about 10 even though this level of uncertainty is substantially greater than that expected for u th pb ion microprobe analyses of zircon or monazite th pb ages of allanite can still be used to address important geologic questions we used this method to date two metamorphic allanite grains from the footwall of the main central thrust nepal himalaya and an allanite grain from the pacoima canyon pegmatite california allanite inclusions in garnet from nepal yield significantly older ages than the coexisting monazite indicating that allanite formation in these rocks records a previous metamorphic cycle that predates slip along the fault the pacoima canyon allanite grain yields a younger age than that reported for zircon implying pb loss during cooling of the pegmatite

high ti amphibole as a petrogenetic indicator of magma chemistry evidence for mildly alkalic hybrid melts during evolution of variscan basic ultrabasic magmatism of central iberia central iberian variscan granite batholiths and anatectic complexes are punctuated by coeval stocks of hydrous high k calc alkaline ultrabasic to intermediate rock series despite their overall calc alkaline affinity the mafic ultramafic members contain high ti amphibole oikocrysts rimmed by lower ti amphibole cummingtonite and high ti amphibole replacing early phlogopite to understand the factors controlling the saturation of high ti amphibole in the parental magmas clinopyroxene melt phlogopite melt and amphibole melt relationships are reviewed this analysis reveals that for melts with intermediate compositions the affinity of tio2 for amphibole rises in alkalic magmas accordingly mildly alkalic trachytoid to subalkaline medium to high k andesite and dacite compositions are estimated for interstitial high ti amphibole saturated melts amphibole ce pb ratios reveal a mantle crust hybrid nature for interstitial melts with subalkaline trachytoid compositions the hydrous character of the variscan basic magmas favoured an overall magmatic evolutionary trend with a low rate of variation of na2o with respect to silica during amphibole crystallization

cao mgo al 2 o 3 sio 2 h 2 o to 35 kb amphibole talc and zoisite dehydration and melting reactions in the silica excess part of the system and their possible significance in subduction zones amphibolite melting and magma fractionation

evolution of magmatic afm mineral assemblages in granitoid rocks the hornblende melt biotite reaction in the liberty hill pluton south carolina

magnetic fabrics and compositional evidence for the construction of the caleu pluton by multiple injections coastal range of central chile the caleu pluton which is located in the coastal range of central chile consists of three n s elongated lithologic zones which define an across pluton compositional variation characterized by a westward increase in sio2 content the gabbro diorite gdz tonalite tz and granodiorite gz zones a total of 547 cores were drilled at 68 sites within the different zones to carry out magnetic fabric and paleomagnetic studies the highest values of magnetic susceptibility and remanent magnetization are found in the gdz 0 04 0 1 si whereas the lowest values are observed in the gz 0 01 0 02 si this is consistent with the significant decrease in magnetite content from the gdz to the gz the dips of the magnetic foliation planes are steep within gdz and tz and shallower dips are observed in gz magnetic foliations along some borders of the pluton are parallel to the corresponding contacts the parallelism observed between the magnetic foliations and the contacts between zones suggests that these zones have recorded the same mechanism of emplacement at the magmatic state the peculiar foliation pattern observed in the central parts of tz and gz is attributed to a lobe like structure that was acquired during the late stages of emplacement and inflation the distinct compositional characteristics of these three zones were not acquired in situ rather they were emplaced successively from mafic to felsic from a subjacent stratified sill like magma reservoir which was filled after tilting of the host volcanic succession the rapid upward transport of these melts of different compositions up to a shallow depth is attested by both the equilibration pressure of late magmatic hornblende 200 mpa and the pressure of early magmatic epidote crystallization 500 600 mpa we conclude that space was created by lateral flow propagation in which the intrusion of later pulses pushed earlier ones away from the feeder system followed by roof lifting and possibly floor depression

coalescence of lateral spreading magma ascending through dykes a mechanism to form a granite canopy el hongo pluton sierras pampeanas argentina this study deals with the structure of the el hongo trondhjemite a ene wsw elongate pluton belonging to the famatinian magmatic arc that developed from late cambrian to silurian times in the sierras pampeanas argentina anisotropy of magnetic susceptibility ams measurements combined with structural and microstructural data permit the correlation of magmatic and magnetic fabrics within this ferromagnetic pluton magnetic foliations strike north south and dip steeply west but in the marginal facies the foliation becomes parallel to the ene wsw pluton contacts magnetic lineations mostly trend north south and plunge se gravity data reveal that the pluton is in the form of a very thin horizontal sheet resting on two north south trending root zones detected below the central portion of the pluton these root zones represent feeder dykes that exploited tension fractures the emplacement of the el hongo pluton took place during the action of a buried sinistral shear zone parallel to the elongation of the pluton corresponding to a transfer fault associated with the late extensional exhumation of the famatinian domain the emplacement of other famatinian granites of the sierras pampeanas could be related to similar late shear zones

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