origin of fine scale sheeted granites by incremental injection of magma into active shear zones examples from the pilbara craton nw australia in the archaean pilbara craton of western australia three zones of heterogeneous centimetre to metre scale sheeted granites are interpreted to represent high level syn magmatic shear zones evidence for the syn magmatic nature of the shear zones include imbricated and asymmetrically rotated metre scale orthogneiss xenoliths that are enveloped by leucogranite sheets that show no significant internal strain at another locality granite sheets have a strong shape preferred alignment of k feldspar suggesting magmatic flow while the asymmetric recrystallisation of the grain boundaries indicates that non coaxial deformation continued acting upon the sheets under sub solidus conditions elsewhere randomly oriented centimetre wide leucogranite dykes are realigned at a shear zone boundary to form semi continuous layer parallel sheets within a magma dominated dextral shear zone it is proposed that the granite sheets formed by the incremental injection of magmas into active shear zones magma was sheared during laminar flow to produce the sheets that are aligned sub parallel to the shear zone boundary individual sheets are fed by individual dykes with up to 1000s of discrete injections in an individual shear zone the sheets often lack microstructural evidence for magmatic flow either because the crystal content of the magma was too low to record internal strain or because of later recrystallisation

the role of hybridization in the genesis of hercynian granitodis in the gredos massif spain inferences from sr nd isotopes the gredos massif is one the better exposed granitoid complexes of the iberian massif it is composed mainly of peraluminous granitoids with subordinate basic and ultrabasic complexes the massif also contains mega enclaves of migmatites with which the granitoids show transitional contacts two major magmatic associations have been distinguished in this study 1 one comprises the granitoids with microgranular enclaves the enclaves and basic rocks 2 the other is formed by leucogranites intrusive into the former series and free of microgranular enclaves field relationships and microstructures indicate that the rocks of the first series are related by a dominant hybridization process the sr nd isotopic study reveals that this process is complex relating different end members of mantle and crustal affinities and occurred around 295 ma ago late with respect to the main deformation phases of the hercynian orogeny the granitoids with microgranular enclaves gme are part of an overall mixing trend involving palaeozoic mantle derived magma and melts of older crustal material amphibole bearing gme in general contain greater proportions of the mantle derived component than the cordierite bearing gme the actual mixing processes took place on a variety of scales sometimes between melts which were themselves hybrids on a local scale this hybridization process can be modelled by simple binary mixing as documented in the case of a composite dyke the isotopic signatures of the basic rocks are probably to a large degree the result of interaction with crustal melts though additionally the presence of an enriched mantle source cannot be elmininated microgranular enclaves and their immediate hosts have differing initial sr and nd isotopic signatures indicating that isotopic equilibrium was not attained this suggests that the enclaves did not reside in their final granitic melt for long before cooling of the whole system the enclaves are considered to have been derived from basaltic melts which had fractionated and hybridised to varying degrees late stage peraluminous leucogranites have similar initial nd isotopic compositions to the evolved gme a crustal source with a radically different nd isotopic composition or age does not need to be invoked in their petrogenesis

the quartz dioritic hospitais intrusion sw iberian massif and its mafic microgranular enclaves evidence for mineral clustering abstract the hospitais pluton is an elliptical body with its long axis oriented wnw ese that belongs to the evora massif iberian variscan belt this intrusion has a quartz dioritic composition with plagioclase quartz amphibole hornblende but locally also cummingtonite and biotite as the main mineral phases mafic microgranular enclaves in the quartz diorite show the same minerals as the host but with a greater abundance of amphibole and lower proportions of felsic minerals especially quartz u pb zircon ages obtained in one sample of the quartz diorite and one sample of a mafic microgranular enclave gave identical ages within error of ca 337 ma geochemical data show that the quartz diorite has a calc alkaline signature with moderate enrichment in lile hfse ratio thn yn 2 71 11 43 and clear negative ti and nb ta anomalies thn nbn 1 33 6 22 the composition of the enclaves shows that they do not represent direct melt compositions instead their geochemical features namely the abundances of fe and mn in variation diagrams and ree patterns in primitive mantle normalized diagrams suggest that they correspond to mineral clustering formed during the previous crystallization stage of quartz dioritic magma overlapping of mineral chemistry and sr nd isotope compositions in the quartz diorite and the enclaves provides further support to that conclusion the values for 87sr 86sr i and endi calculated for 337 ma in the quartz diorite vary from 0 706147 to 0 706491 and from 1 87 to 3 22 respectively this isotope composition may be explained either by differentiation of a mafic magma extracted from a mantle wedge enriched by long lasting subduction processes or by mixing between a mafic magma from a depleted mantle source and a crustal melt in the second case the most likely source for the felsic component should be meta igneous rocks like the lower cambrian alcacovas orthogneisses that outcrop in neighbouring areas

residence of ree y th and u in granites and crustal protoliths implications for the chemistry of crustal melts a systematic study with laser ablation icp ms scanning electron microscopy and electron microprobe revealed that 70 95 wt of ree except eu y th and u in granite rocks and crustal protoliths reside within reeythu rich accessories whose nature composition and associations change with the rock aluminosity the accessory assemblage of peraluminous granites migmatites and high grade rocks is composed of monazite xenotime in low ca varieties apatite zircon thorthosilicate uraninite and betafite pyrochlore metaluminous granites have allanite sphene apatite zircon monazite and thorthosilicaie peralkaline granites have aeschinite fergusonite samarskite bastnaesite fluocerite allanite sphene zircon monazite xenotime and th orthosilicate granulite grade garnets are enriched in nd and sm by no less than one order of magnitude with respect to amphibolite grade garnets granulitegrade feldspars are also enriched in lree with respect to amphibolite grade feldspars accessories cause non henrian behaviour of ree y th and u during melt solid partitioning because elevated fractions of monazite xenotime and zircon in common migmatites are included within major minerals their behaviour during anatexis is controlled by that of their host settling curves calculated for a convecting magma show that accessories are too small to settle appreciably being separated from the melt as inclusions within larger minerals biotite has the greatest tendency to include accessories thereby indirectly controlling the geochemistry of ree y th and u we conclude that ree y th and u are unsuitable for petrogenetical modelling of granitoids through equilibrium based trace element fractionation equations

experimental partitioning of sr and ba in kiglapait feldspars new electron microprobe analyses of sr and ba in feldspars and coexisting experimental glasses show very strong correlations of the partition coefficients with the ternary or content of the feldspar the correlations therefore scale inversely with temperature as also found in the literature the sr content of feldspars is maximized in mesoperthite and decreases with evolution of feldspar to the ternary minimum the relationship of d sr l to decreasing x an is initially flat at a value of 1 8 but curves sharply upward past 6 5 after the arrival of apatite at an 38 a similar concave up curve at much higher values of d is found in the literature the value of d nine points scales linearly with ternary or r 2 0 99 experimental feldspars are consistently richer in sr than natural ones at comparable an they jump from 200 ppm higher to 400 ppm higher at ap experimental glass compositions are correspondingly low in sr so the material balance is conserved inversion of natural feldspars to liquid compositions using the experimental partition coefficients fails because the experimental crystals contain enhanced values of sr from the apatite and feldspar components of the melted rock compositions the partitioning of ba five points range 0 4 to 6 1 also scales with ternary or r 2 0 99 and there is no systematic difference between the natural and experimental concentrations of ba in the feldspars the inversion of d to liquid compositions may be safe for ba but must be approached with caution for sr

dissecting complex magmatic processes an in depth u pb study of the pavia pluton ossa morena zone portugal the build up of large magmatic complexes can proceed piecemeal over periods of several million years through sequences of complex processes of magma production differentiation assimilation final crystallization and subsequent metasomatic modification all these stages can produce or modify minerals used as geochronometers such as zircon monazite and titanite the present study exemplifies such complex relationships also demonstrating how a systematic approach with comprehensive sampling and careful high resolution u pb analyses can yield a coherent picture of the entire magmatic process the study was conducted on the pavia pluton an elongated variscan intrusion in the ossa morena zone of portugal the geochronological data show that the pavia pluton was emplaced by the amalgamation of multiple magma pulses into the crust over a period of c 11 myr an early event at 340 ma revealed by xenocrystic zircon preceded the magmatic activity at the exposed level of the pluton but is recognized as the main magmatic event elsewhere in the ossa morena zone a second event at 328 ma formed tonalite trondhjemite and granodiorite and subordinate differentiates in the central domains of the pluton units i and ii a third event at c 324 ma emplaced granodiorite in the flanking domains iii v and the contemporaneous and widespread two mica granite in domainvi together with late rhyodacite porphyries microgranodiorites aplite pegmatite and pegmatite dikes a fourth event at 319 317 ma was characterized by the emplacement of some microgranites and pegmatite dikes these two last magmatic events also had an effect on the previously emplaced rocks causing local overgrowths and isotopic resetting of minerals the occurrence of a fifth magmatic event at depth at 313 ma is the inferred cause of the hydrothermal activity responsible for local zircon monazite and titanite resorption and or recrystallization and for some of the textures exhibited by the main rock forming minerals the magmatic episodes were interspersed with periods of quiescence this cyclicity presumably reflects an external control by the transtensional tectonic regime of the ossa morena zone

emplacement mechanisms of late orogenic granites structural and geochemical evidence from southern finland the country rock in southern finland formed mainly during the svecofennian orogeny ca 1 9 ga ago the middle and lower crust was partially melted 1 83 ga ago due to crustal thickening and subsequent extension during this event s type migmatites and granites were formed along a 100 500 km zone this late svecofennian granite migmatite zone lsgm zone is a large crustal segment characterised by roughly e w trending sub horizontal migmatites and granites combined ductile e w shear movements and nnw sse compressional movements defined a transpressional tectonic regime during the emplacement partial melts that moved through the crust pooled as granite sheets or froze as migmatites major transpressive shear zones border the lsgm zone which forms a tectonic and metamorphic zone that crosscuts the earlier svecofennian granitoids based on field observations and geochemical data from two sets of outcrops we show that the great volumes of late orogenic granites and migmatites in southern finland were transported and emplaced as small chemically variable batches possibly extracted from different protoliths these melt batches were transported along repeatedly activated channels and collected at some horizontal level in the crust in the nagu area the melt batches were trapped under a roof layer of amphibolite and the whole complex was synchronously folded into open folds with steep axial surfaces and e w trending fold axes the sheets of microcline granite are in places strongly sheared the microcline phenocrysts are imbricated and subsequent deformation of the microcline phenocrysts indicates syn tectonic movements of the layers as well as a syn tectonic mechanism for the late magmatic fractionation depending on the degree of crystallisation of the individual melt batches during shearing at different intensities the granites have slightly different appearances some sheared zones show a cumulate like trace element geochemistry indicating that melt fractions were expelled from the system producing layers of deformation enhanced fractionated granites and cumulate layers our interpretation is that the nagu area shows shear assisted fractionation mechanisms in granitic melts and that similar processes are responsible for the fractionation trends seen in the sub horizontal sheeted granites in hameenlinna at higher levels in the crust

cambrian ensialic rift related magmatism in the ossa morena zone evora aracena metamorphic belt sw iberian massif sm nd isotopes and shrimp zircon u th pb geochronology abstract the late ediacaran c 560 550 ma serie negra sediments of the evora aracena metamorphic belt ossa morena zone sw iberian massif preserve a record of the erosion of an avalonian cadomian magmatic arc and subsequent related turbiditic sedimentation detrital zircon from the serie negra is characterized by predominantly ediacaran and cryogenian ages with few paleoproterozoic and archean cores and a marked lack of grenvillian ages these features when combined with the metasediments enrichment in lree la yb 14 negative eu anomalies low 147 sm 144 nd values 0 121 and negative e nd 550 5 5 indicate that the protolith serie negra sediments were derived from a continental magmatic arc a period of late cadomian ca 560 540 ma tectonism was followed by an extended episode of widespread bimodal magmatism related to cambrian ca 540 500 ma rifting this tectonic inversion is expressed in the geological record by a regional early cambrian unconformity shrimp zircon u th pb ages from four felsic orthogneisses from the evora massif record cambrian 527 10 ma 522 5 ma 517 6 ma and 505 5 ma crystallization ages for their igneous protoliths this confirms the existence of widespread lower paleozoic igneous activity in the ossa morena zone i a lower cambrian ca 535 515 ma igneous felsic dominated sedimentary complex with calc alkaline signature and associated carbonate and siliciclastic deposition and ii a middle cambrian ordovician ca 515 490 ma igneous bimodal sedimentary complex with calc alkaline and tholeiitic signatures and associated dominant siliciclastic deposition but also carbonate sediments the cambrian felsic magmatism was characterized by negative eu anomalies la lu n 0 8 11 147 sm 144 nd 0 1289 0 1447 and e nd 500 ranging from 1 5 to 0 8 a tendency towards peraluminous compositions suggests late fractionation low degrees of partial melting or the mixing of crustal and mantle derived material in the magma source region some felsic rocks possibly represent the last residual melts of high temperature zircon undersaturated mafic magmas later affected by crustal contamination while others indicate partial melting of crustal metasediments variably contaminated by basaltic liquids the transition from early felsic dominated to later more mafic magmatism suggests the gradual opening of the system to tholeiitic n e morb products th n ta n 147 sm 144 nd 0 1478 0 1797 and e nd 500 values ranging from 6 4 to 7 3 while the n morb amphibolites have 147 sm 144 nd 0 1818 0 1979 and e nd 500 values of 5 8 and 7 0 reaching a maximum of 9 1 in contrast other amphibolites have a negative ta anomaly 1 35 n ta n 147 sm 144 nd values ranging from 0 1639 to 0 1946 and e nd 500 values of 3 5 to 5 2 suggesting derivation by crustal assimilation processes the sub alkaline igneous precursors of the amphibolites were most likely generated in a rift setting by asthenospheric upwelling these results strengthen the proposed geodynamic scenarios for the sw iberian massif by which cadomian accretion gave rise to an ensialic rift that developed into a proto oceanic basin and incipient spreading opening of the rheic ocean a similar transition from a convergent to a divergent plate boundary during the ediacaran to cambrian ordovician has also been reported in other segments of the northern gondwana margin

time space distribution of silicic plutonism in a gneiss dome of the iberian variscan belt the evora massif ossa morena zone portugal abstract in the iberian variscan belt polyphasic deformation has been recognized as comprising an early phase of crustal thickening followed by an intermediate phase of crustal extension and doming and a later phase of shortening the evora massif is a gneiss dome of the westernmost domains of the ossa morena zone sw iberia which provides a remarkable insight into the late paleozoic deep crustal structure of the variscan continental crust of northern gondwana in this study we bring new structural and geochronological u pb data for the northern hanging wall of the evora massif we describe the existence of low dipping d2 extensional shear zones associated with buchan type metamorphism m2 this enables three tectono metamorphic units to be distinguished the lower gneiss unit the intermediate schist unit and the upper slate unit d2 m2 structures experienced subhorizontal shortening d3 and were transposed by low plunging folding thrusting and strike slip faulting zircon grains extracted from pavia quartz feldspathic gneiss of the lower gneiss unit yielded a crystallization age of ca 521 ma cambrian stage 2 3 which establishes a correlation with tectono metamorphic units of the footwall and southern hanging wall of the evora massif u pb zircon dating of divor foliated quartz diorite 339 7 ma malarranha weakly foliated biotite rich granite 322 3 ma and undeformed porphyritic granite of the pavia pluton 314 4 ma constrain the timing of emplacement of granitic magmas synchronously with doming carboniferous magmatism initiated with doming me1 ca 343 335 ma continued through d2 m2 me2 ca 328 319 ma and lasted until the waning stage of crustal extension me3 ca 317 313 ma the evora massif gneiss dome probably formed as result of the combined effect of gravitational collapse of the thickened crust and buoyancy driven gravitational instability developed in the partially molten continental crust influenced by the transfer of heat from rising mantle derived i e dioritic gabbroic magmas rocks found in the footwall of the evora massif

syn tectonic magmatism and the development of compositional layering ungava orogen northern quebec canada abstract layer and foliation parallel emplacement of granitic veins was an important process in the regional development of compositional layering in now exhumed middle crustal sections of both archean and paleoproterozoic age in the northern ungava peninsula quebec canada in the archean superior province diorite and tonalite plutons were penetratively deformed and metamorphosed at granulite facies conditions coeval with voluminous granitic magmatism the paleoproterozoic narsajuaq arc contains evidence for contemporaneous magmatism transpressional deformation and granulite facies metamorphism prior to its collision with the superior province basement in both plutonic domains regional compositional layering is defined by 1 metre to kilometre scale alternation of generally well foliated tonalite and quartz diorite bodies and 2 centimetre to kilometre scale variably deformed granitic veins and sheets that lie parallel to layering tectonic foliation in the host rocks syn tectonic intrusion of a substantial portion of the veins along extension fractures sub parallel to layering foliation i e at high angle to the regional shortening direction is interpreted to have occurred due to the combination of a strong anisotropy and high magma pressures compositional layering generated and or enhanced by this process may contribute to the overall seismic reflectivity of the middle and lower crust

geophysical evidence of a mantle derived intrusion in sw iberia 1 deep seismic reflection data acquired as part of the sw iberia europrobe project across the transpressional variscan orogen sample three tectonic terranes the south portuguese zone the ossa morena zone and the central iberian zone the seismic data reveal the existence of a mid crustal reflective body 140 km long and of variable thickness up to 5 km the iberian reflective body the conductivity image provided by coincident mt soundings the amplitude characteristics of the seismics mineralization studies related to magmatic ore deposits and the surface geology suggest that the irb is a mantle derived mafic intrusion the geophysical geological and petrological data suggest that the irb is most probably an early carboniferous approximately at 350 340 ma mantle derived intrusion possibly linked to plume activity that took place in europe in the carboniferous and permian

87 sr 86 sr applied to age discrimination of the palaeozoic carbonates of the ossa morena zone sw iberia variscides four distinct palaeozoic episodes of marine carbonate sedimentation are reported in the ossa morena zone sw iberian variscides among them the cambrian and the devonian episodes are those that are well represented in the portuguese domains of the ossa morena zone in this zone the strong variscan deformation and metamorphic recrystallization obliterate the biostratigraphic contents of some carbonate local successions and their ages have been based on lithostratigraphic correlations the 87sr 86sr values for the ossa morena zone carbonate rocks allow to make an isotopic lithostratigraphic correlation based on the comparison between strontium ratios of marine carbonate rocks without chronological data and carbonate rocks with a good age control moreover the established 87sr 86sr fingerprints of the marine carbonate episodes from ossa morena zone should be simultaneously correlated with the worldwide seawater 87sr 86sr curve despite the interaction of the marine carbonate sedimentation with late secondary dolomitization processes or with high temperature metamorphic metasomatic fluids which increases the primary 87sr 86sr ratios this methodology allows to define two main distinct clusters of 87sr 86sr for the ossa morena zone marine carbonates a devonian cluster with strontium ratios lower than 0 708000 and a cambrian cluster ranging from 0 708777 to 0 708299 0 708575 0 000161 that can reach 0 709227 0 708866 0 709087 0 000118 if incipient late secondary dolomitization or high temperature metamorphism metasomatism operated the obtained 87sr 86sr isotopic ratios corroborate the lithostratigraphic correlations between the carbonate rocks of abrantes assumar estremoz viana alvito ficalho and escoural successions with the ovetian marianian limestones of alter do chao elvas succession moreover this methodology allows to establish the isotopic discrimination between the upper silurian devonian limestones of ferrarias barrancos and bencatel successions and the marbles of estremoz succession

in search of homelands using strontium isotopes to identify biological markers of mobility in late prehistoric portugal this study uses strontium isotopes 87 sr 86 sr in dental enamel from burial populations related to the fortified chalcolithic settlement site of zambujal c 2800e1800 bc to distinguish the presence of nonlocal individuals zambujal is located in the estremadura region of portugal near the atlantic coast and has long been considered a central location of population aggregation craft production and trade during a time of increasing political centralization and social stratification until its eventually abandonment during the bronze age while it is assumed that population migration and long distance trade played an important role in the region s development little is known about the migration patterns of individuals or groups the results of this study find that nine percent 5 out of 55 of the total surveyed individuals can be classified as non local based on 87 sr 86 sr values distinct from the local bioavailable

experimental partitioning of rb cs sr and ba between alkali feldspar and peraluminous melt hydrous partial melting experiments performed between 650 and 750 c at 200 mpa h20 on synthetic metapelite compositions quartz albite muscovite biotite mineral mixtures doped with ba sr rb and cs yielded alkali feldspar crystals with a wide range of compositions in equilibrium at their rims with peraluminous melt measured partition coefficients for normally trace lithophile elements between feldspar and melt d m fsp gl m ba sr rb cs do not depend on either temperature or bulk composition of melt for the compositions studied values of d sr fsp gl are between 10 and 14 and appear to be independent of the albite and orthoclase contents of the feldspar crystals in contrast values of d ba fsp gl and d rb fsp glare strongly dependent on the orthoclase content of feldspar relationships that can be expressed by the following linear equations d ba fsp gl 0 07 0 25 orthoclase and d rb fsp gl 0 03 0 0 1 orthoclase where orthoclase is in mole percent these equations reproduce the range of previously reported values for d ba and d rb determined on natural and synthetic samples a single partition coefficient for cs was also determined at d cs fsp gl 0 13 these data can be used in conjunction with recently published partition coefficients for muscovite biotite and plagioclase feldspars blundy and wood 1991 icenhower and london 1995 to model quantitatively the trace element signatures of peraluminous magmas during anatexis and crystallization depending on bulk composition and pressuretemperature trajectories during episodes of partial melting ba and sr and to a lesser extent rb are retained at the source while cs and li are enriched in the first melts these results explain why peraluminous granitic magmas with s type sedimentary sources carry a distinctive enrichment in li and cs

trace element constraints on the differentiation and crystal mush solidification in the skaergaard intrusion greenland new major and trace element analyses of plagioclase and clinopyroxene in gabbros from the skaergaard intrusion are used to understand the mechanisms of crystallization in the main magma body and in the crystal mush crystal cores show a continuous chemostratigraphic evolution in the layered series with compatible elements e g cr and ni in clinopyroxene being progressively depleted from the bottom up whereas incompatible elements e g sr ba ree in plagioclase and clinopyroxene become progressively enriched we performed numerical models and showed that these trends can be explained by a simple process of fractional crystallization except for the upper 20 of the intrusion where plagioclase and clinopyroxene trace element compositions depart from fractional crystallization trends at this stage of magma chamber solidification fractional crystallization becomes less efficient and is replaced by a major proportion of in situ equilibrium crystallization trace elements also show significant variations in interstitial overgrowths on plagioclase and clinopyroxene cumulus crystals they result from crystallization of the interstitial liquid in the liquid crystal mush however incompatible elements and especially ree show a degree of enrichment in plagioclase rims 20 ppm ce that strongly exceeds the highest concentrations observed in plagioclase cores up to 4 ppm ce at the top of the layered series such a strong enrichment is difficult to reconcile with a simple process of fractional crystallization of the interstitial liquid in the crystal mush but may be related to the development of silicate liquid immiscibility in the crystal mush or to delayed nucleation of apatite after the crystallization of plagioclase and clinopyroxene overgrowths diffusive re equilibration during a period of 0 1 0 25 myr significantly changed the original zoning profiles ce flux from clinopyroxene into the plagioclase lattice of adjacent crystals could also have contributed to the strong ce enrichment observed in plagioclase rims

formation of diatexite migmatite and granite magma during anatexis of semi pelitic metasedimentary rocks an example from st malo france petrological and geochemical variations are used to investigate the introduction formation of granite magma from diatexite migmatites derived from partial melting experiments show that protoliths of pelite metasedimentary rocks of pelitic to greywacke composition at st and greywacke composition yield up to 40 vol malo france anatexis occurred at relatively low temperatures granite melt over the temperature range 650 900 c and pressures 800 c 4 7 kbar principally through muscovite typical of anatexis of the continental crust conrad et dehydration melting biotite remained stable and serves as a tracer al 1988 vielzeuf holloway 1988 patino douce for the solid fraction during melt segregation the degree of partial johnston 1991 vielzeuf montel 1994 patino douce melting calculated from modal mineralogy and reaction stoichiometry harris 1998 therefore to make a granite magma was 40 vol there is a continuous variation in texture the melt fraction must be segregated from a solid fraction mineralogy and chemical composition in the diatexite migmatites of much greater volume and then collected together mesocratic diatexite formed when metasedimentary rocks melted the type of melt residuum separation process depends sufficiently to undergo bulk flow or magma flow but did not on the melt fraction present experience significant melt residuum separation mesocratic diatexite melt residuum separation at low degrees of partial that underwent melt segregation during flow generated 1 memelting f and consequently low melt fraction mf lanocratic diatexites at the places where the melt fraction was has been documented from metatexite migmatites in removed leaving behind a biotite and plagioclase residuum enriched these rocks melt segregation produced centimetre scale in tio2 feot mgo cao sc ni cr v zr hf th u and leucosomes distributed in a host composed of layers that ree and 2 a complementary leucocratic diatexite enriched in have melted and others that have not e g henkes sio2 k2o and rb where the melt fraction accumulated leucocratic johannes 1981 gupta johannes 1982 dougan 1983 diatexite still contained 5 15 vol residual biotite mg number sawyer 1987 tait harley 1988 barbey et al 1990 40 44 and 10 20 vol residual plagioclase an22 anatectic brown 1994 leucosomes represent small volumes of granite magma developed from the leucodiatexite first by further felsic magma formed when melt was squeezed out of the melt residuum separation then through fractional crystallization deforming matrix mclellan 1988 sawyer 1991 1994 most biotite in the anatectic granite is magmatic mg number brown 1994 brown et al 1995 and collected in low18 22 pressure sinks such as shear bands and inter boudin

are strongly peraluminous magmas derived from pelitic sedimentary sources igneous compositions that require the presence of a phase more aluminous than biotite are here designated as strongly peraluminous ps such compositions are common but subordinate in felsic rocks and exceedingly rare in more mafic rocks the term s type has been applied to many igneous rocks based largely or even solely upon their ps compositions neglecting or placing less emphasis on other criteria for sedimentary parentage in chappell and white s i and s type classification system the s type designation was in fact initially restricted to granites which were thought to have been derived from a chemically mature strongly weathered pelitic shaly source application of chappell s and white s criteria together with others based upon isotopic and phase equilibria considerations leads to the following conclusions about ps igneous rocks worldwide 1 very few ps rocks are derived entirely from pelitic sources 2 a majority are derived largely from intermediate to felsic crustal sources including

kiglapait geochemistry iii potassium and rubidium abstract the regular geometry and completeness of the kiglapait intrusion permit its bulk composition to be obtained by summation and the composition of successive liquids to be obtained by subtraction the summations for k and rb give 1806 and 1 08 ppm yielding r frsol k rb 1670 for the intrusion taken as equal to the parent magma r increases slightly from this initial value to 2000 at the end of crystallization where mgo approaches zero in the rocks k and rb are therefore closely coherent and their distribution coefficients can differ only by a small amount in the kiglapait system apparent feldspar liquid distribution coefficients d f l can be estimated from detailed plots of feldspar and liquid compositions against f l the kiglapait data imply that these coefficients are linear 1 1 functions of plagioclase composition within experimental error having values given by d k f l 1 42 x an d rb f l 1 13 x an with minimum values of 0 75 and 0 49 respectively the ratio r f r l lies in the range of 1 53 0 03 for the plagioclase composition range x an 0 34 to 0 67 showing that high r rocks such as anorthosite crystallized from high r liquids the apparent feldspar distribution coefficients are much closer to 1 0 than common literature values they can be reduced by assuming that the cumulate pile was continuously recharged by the circulating magma until an advanced stage of differentiation was reached and assuming that alkalies were exchanged to the feldspars from the magma when such an aquifer recharge model is calibrated using olivine liquid equilibria as a time marker for the liquid the inferred minimum equilibrium values of the distribution coefficients are d k f l 0 42 d rb f l 0 25 at the base of the intrusion their variation is given by d k f l 1 66 1 88x an d rb f l 1 17 1 41x an the equilibrium values are considered to be appropriate for deducing liquid compositions in plutonic bodies where alkali exchange can be shown or inferred to have been inhibited such as in small intrusions the apparent values are considered to be appropriate even though they may be artificial for large intrusions similar to the kiglapait the bulk k and rb concentrations in the kiglapait intrusion are consistent with a plagioclase rich abyssal tholeiite magma clinopyroxene and olivine fractionation in the mantle may contribute to the production of such high r magmas

regional implications of geochemistry and style of emplacement of miocene i type diorite and granite delos cyclades greece abstract the miocene plutons of the cyclades were emplaced in a subduction setting during regional back arc extension of continental crust that led to flat lying mid crustal detachment faulting mapping of the island of delos shows that quartz diorite and tonalite were emplaced as dykes in country rock of schist and marble within shear zones parallel to the extension direction mafic magmas were followed by numerous small batches of felsic magma with magmatic and ductile deformation synchronous with magma emplacement late granite dykes occupy brittle fractures in the more deformed rocks mafic and intermediate rocks show a bimodal distribution of incompatible trace elements with one group of broadly tholeiitic character and the other with substantial enrichment in sr nb and hfse but low th and ba these differences appear to be inherited from two distinct mafic sources that are different from the mafic source for the plutons of the eastern cyclades voluminous granodiorite results from these mafic magmas fractionating and or mixing with felsic crustal material some of which was derived by anatexis of a sedimentary protolith indicated by high b and mn some late granites appear derived from partial melting of hercynian paragneiss regionally the shear zones appear to be feeders to more extensive granitic plutons located at space produced at ramps in detachment fault zones the shear zones parallel the mid cycladic lineament a broad zone of displacement between two crustal blocks rotating in opposing directions as rollback took place at the hellenic subduction zone distinctive geochemical features in miocene igneous rocks suggests that these two blocks had quite different geological histories the localisation of plutonism and core complexes near the mid cycladic lineament suggests that this crustal scale shear played a role in bringing subduction derived magmas to mid crustal levels the heat supplied by the mafic magmas promoted ductile deformation high in the crust where extension was concentrated leading to the formation of core complexes the regional extension resulted in progressive shallowing of the position of the granite solidus within the crust leading to welding of the mid cycladic lineament which is no longer seismically active

melt segregation structures in granitic plutons when melt fraction increases beyond a critical value migmatites behave as magmas conversely when melt fraction decreases crystallizing magmas form solid framework mushes and behave as solids the richness in preserved mesoscopic melt segregation structures in migmatites is not matched by melt segregation structures in granitic bodies this paper identifies melt segregation structures in granites and compares them to structures in migmatites the rarity of segregation structures in granites is a consequence of the solid framework mush allowing for cryptic melt extraction from its pores the relatively short duration of solidification of the mush and the fragile nature of the solid framework identifying segregation features in granitoids at outcrop scale provides the basis for understanding the physical processes that lead to chemical and mineralogical differentiation in granitic magmas

temporal and compositional differences between subsolidus and anatectic migmatite leucosomes from the quetico metasedimentary belt canada migmatites in the quetico metasedi mentary belt contain two types of leucosome 1 layer parallel leucosomes that grew during deformation and prograde metamorphism these are enriched in si02 sr and eu but depleted in ti02 fe203 mgo cs rb ree sc th zr and hf relative to the quetico metasediments 2 discordant leucosomes that formed after the regional folding events when metamorphic temperatures were at their peak these are enriched in rb ba sr and eu but display a wide range of lree th zr and hf contents relative to the quetico metasediments layer parallel leucosomes formed by a sub solidus process termed tectonic segregation this stress induced mass transfer pfocess began when the quetico sediments were deformed during burial and continued whilst the rocks were both stressed and heterogeneous sub solidus leucosome compositions are consistent with the mobilization of quartz and feldspar from the host rocks by pressure solution the discordant leucosomes formed by partial melt ing of the quetico metasediments possibly dur ing uplift of the belt the range of composition displayed by the anatectic leucosomes arises from crystal fractionation during leucosome emplacement some anatectic leucosomes pre serve primary melt compositions and have smooth ree patterns but those with negative eu anomalies represent fractionated melts and others with positive eu anomalies represent accumulations of feldspar plus trapped melt

experimental equilibrium tested by plagioclase loop widths are solid and liquid respectively and an is the mole fraction or per cent an an ab it is measured from the plagioclase composition and an estimate of the liquid composition based on standard assumptions rooted in the norm recalculation routine a survey of a large new database allows study of the 1 atm experimental literature involving 819 experiments the approach to equilibrium is tested empirically by the loop width and its transform the linear partitioning function kd this function is the exchange coefficient formed as the intercept at pure an s from the array over all x s of the partition coefficient d when defined as x s x l w herex is the mole fraction d and kd are thus limited to lie between zero and 1 0 the results show a 1atm loop width ranging from 0 2t o 68 mol a n with associated kd 0 03 0 98 these extrema represent loops both far too small and far too large to represent stable equilibrium between plagioclase and liquid some of the higher values lie near the persodic limit because the an component of the liquid is scarce and the liquid composition is ab rich the loops from ne normative liquids span the range from 40 to 90 mol an in the crystal with loops 2 50 mol wide a baseline for loop width is derived from the precise synthetic study of the system di an ab at 1atm and an array of high pressure data that extrapolates to the same value of kd 0 26 at 1atm many of the 1atm experimental data fall far outside any reasonable uncertainty limits on the baseline causes of departure include crystal disequilibrium that can lead to errors in both directions and compositional problems comparison of the values of kd with liquid compositions in silica oxygen fugacity temperature kinetics and sodium reveals the common presence of sodium deficiency a crystallinity study shows a definite decrease in sodium content with liquid evolution whereas the sodium content should increase or at least remain nearly constant with evolution of the melt a selection of self consistent 1atm data from a few laboratories shows a close correspondence to the standard distribution of loop widths and indeed most literature data lie within the limited range of kd 0 26 0 4 experiments known to be hydrous show very low values of kd and corresponding large loop widths as previously understood from literature data when data limited to nearbaseline kd values are plotted against temperature they yield expected thermal slopes and concave up curves of temperature plotted against plagioclase composition literature data show a positive correlation of kd with dry pressure and a separate database from 2 to 15 kbar shows that more than half the data comport with this pressure effect a different group ranges down to kd as low as 0 2 with increasing loop widths some of these are alkalic and hence rich in the ab component of the liquid others were obtained in fe containers that control the oxygen fugacity to low values and convert the fayalite component of the liquid to iron oxygen and silica this reaction increases the activity of silica and in turn the activity of ab in the melt thus widening the loop thermodynamic studies concerned with plagioclase melt equilibria will profit from an awareness of these effects

the role of strain localization in magma injection into a transtensional shear zone variscan belt sw iberia this study deals with the interaction between deformation and magmatism in mid to deep crustal domains the relation is analysed between migmatites and shear zones and the spatial distribution of leucogranitoid veins and dykes running through a footwall migmatite system and reaching a transtensional shear zone operated under amphibolite to greenschist facies metamorphic conditions boa fe shear zone variscan belt sw iberia statistical results show that the frequency of width and spacing of the leucogranitoid dykes conform to power law distributions comparable with observations in volcanic systems the fractal geometry of the distribution of leucogranitoid dykes highlights the development of a dense framework of thinner weakly or non mineralized veins and dykes formed at higher nucleation growth ratios in the footwall migmatite system that contrasts with the emplacement of thicker dykes associated with strongly mineralized thinner veins within the shear zone the volume of injected leucogranitoid dykes in the shear zone is lower as compared with the footwall and is comparable with an expanding footwall shear zone with non coaxial flow and volume increase the boa fe shear zone seems to form a physical barrier to the transport of magma to the hanging wall

building up of a nested granite intrusion magnetic fabric gravity modelling and fluid inclusion planes studies in santa eulalia plutonic complex ossa morena zone portugal the santa eulalia plutonic complex sepc located in the ossa morena zone south portugal is composed of a medium to coarse grained pink granite g0 type and a central grey medium grained biotite granite g1 type available rb sr data indicates an age of 290 ma an emplacement model for the sepc is proposed taking into account magnetic fabric 2d gravity modelling and fluid inclusion planes studies the g0 and g1 types demonstrate different magnetic behaviour g0 is considered a magnetite type granite and g1 is an ilmenite type granite the formation of g0 required oxidized conditions related to the interaction of mafic rocks with a felsic magma the 2d gravity modelling and subvertical magnetic lineations show that the feeder zone of the sepc is located in the eastern part of the pluton confirming the role of the assumar and messejana variscan faults in the process of ascent and emplacement the magma emplacement was controlled by ene wsw planar anisotropies related to the final brittle stages of the variscan orogeny the emplacement of the two granites was almost synchronous as shown by their gradational contacts in the field the magnetic fabric however suggests emplacement of the g0 type first closely followed by emplacement of the g1 type pushing the g0 laterally which becomes more anisotropic towards the margin the g1 type became flattened acquiring a dome like structure the sepc is a nested pluton with g0 type granite assuming a tabular flat shape and g1 type forming a rooted dome like structure after emplacement sepc recorded increments of the late variscan stress field documented by fluid inclusion planes in quartz

partition coefficients of hf zr and ree between zircon apatite and liquid concentration ratios of hf zr and ree between zircon apatite and liquid were determined for three igneous compositions two andesites and a diorite the concentration ratios of these elements between zircon and corresponding liquid can approximate the partition coefficient although the concentration ratios between apatite and andesite groundmass can be considered as partition coefficients those for the apatite in the diorite may deviate from the partition coefficients the hree partition coefficients between zircon and liquid are very large 100 for er to 500 for lu and the hf partition coefficient is even larger the ree partition coefficients between apatite and liquid are convex upward and large d 10 100 whereas the hf and zr partition coefficients are less than 1 the large differences between partition coefficients of lu and hf for zircon liquid and for apatite liquid are confirmed these partition coefficients are useful for petrogenetic models involving zircon and apatite

age constraints to the relationships between magmatism metamorphism and tectonism in the aracena metamorphic belt southern spain the aracena metamorphic belt amb southwest iberian peninsula is characterized by the following geological elements a a high temperature low pressure ht lp metamorphic belt a few kilometres wide and more than 200 km long b a linear belt of oceanic amphibolites with a low pressure inverted metamorphic gradient c crustal scale ductile shear zones and d mafic noritic intrusions of high mg andesite boninite composition the relationships between these elements led to the proposal of a model of ridge subduction for this sector of the hercynian belt of europe this interpretation is supported by the age relationships displayed between the main rock units considered representative of the main tectonic and petrological processes responsible for the geological elements mentioned previously the results of a geochronological study ar ar rb sr and sm nd clearly support a late paleozoic tectonic evolution at an active continental margin the time evolution of the metamorphism in the oceanic domain ranging from 342 6 0 6 ma in the west to 328 4 1 2 ma in the east over a distance of 70 km along the metamorphic belt support a tectonic model of triple junction migration responsible for the creation at depth of a slab free window with decisive consequences for the thermal evolution of the region the origin of the linear metamorphic belt of ht lp regime may be explained by the migration along a continental margin of a punctual thermal anomaly induced by the creation of a triple junction at the continental margin

melt segregation in the continental crust distribution and movement of melt in anatectic rocks the grain and outcrop scale distribution of melt has been mapped in anatectic rocks from regional and contact metamorphic environments and used to infer melt movement paths at the grain scale anatectic melt is pervasively distributed in the grain boundaries and in small pools consequently most melt is located parallel to the principal fabric in the rock typically a foliation short branched arrays of linked melt bearing grain boundaries connect melt depleted parts of the matrix to diffuse zones of melt accumulation protoleucosomes where magmatic flow and alignment of euhedral crystals grown from the melt developed the distribution of melt leucosome and residual rocks normally melanocratic in outcrop provides different but complementary information the residual rocks show where the melt came from and the leucosomes preserve some of the channels through which the melt moved or sites where it pooled different stages of the melt segregation process are recorded in the leucosome melanosome arrays regions where melting and segregation had just begun when crystallization occurred are characterized by short arrays of thin branching leucosomes with little melanosome a more advanced stage of melting and segregation is marked by the development of residual rocks around extensive branched leucosome arrays generally oriented along the foliation or melting layer places where melting had stopped or slowed down before crystallization began are marked by a high ratio of melanosome to leucosome because most of the melt has drained away very few leucosomes remain to mark the melt escape path this is common in melt depleted granulite terranes many migmatites contain abundant leucosomes oriented parallel to the foliation mostly these represent places where foliation planes dilated and melt drained from the matrix via the branched grain boundary and larger branched melt channel leucosome arrays collected melt collected in the foliation planes was partially or fully expelled later when discordant leucosomes formed leucosomes or veins oriented at high angles to the foliation layering formed last and commonly lack melanocratic borders hence they were not involved in draining the matrix of the melting layer discordant leucosomes represent the channels through which melt flowed out of the melting layer

tectonic evolution of variscan iberia gondwana laurussia collision revisited an integrated interpretation of the late paleozoic structural and geochronological record of the iberian massif is presented and discussed under the perspective of a gondwana laurussia collision giving way to the variscan orogen compressional and extensional structures developed during the building of the variscan orogenic crust of iberia are linked together into major tectonic events operating at lithosphere scale a review of the tectonometamorphic and magmatic evolution of the iberian massif reveals backs and forths in the overall convergence between gondwana and laurussia during the amalgamation of pangea in late paleozoic times stages dominated by lithosphere compression are characterized by subduction both oceanic and continental development of magmatic arcs over and under thrusting of continental lithosphere and folding variscan convergence resulted in the eventual transference of a large allochthonous set of peri gondwanan terranes the iberian allochthon onto the gondwana mainland the iberian allochthon bears the imprint of previous interaction between gondwana and laurussia including their juxtaposition after the closure of the rheic ocean in lower devonian times stages governed by lithosphere extension are featured by the opening of two short lived oceanic basins that dissected previous variscan orogenic crust first in the lower middle devonian following the closure of the rheic ocean and then in the early carboniferous following the emplacement of the peri gondwanan allochthon an additional major intra orogenic extensional event in the early middle carboniferous dismembered the iberian allochthon into individual thrust stacks separated by extensional faults and domes lateral tectonics played an important role through the variscan orogenesis especially during the creation of new tectonic blocks separated by intracontinental strike slip shear zones in the late stages of continental convergence

alongshore momentum balance analysis on a cuspate foreland 1 nearshore measurements of waves and currents off cape hatteras north carolina u s a are used to investigate depth averaged subtidal circulation and alongshore momentum balances in the surf and inner shelf region around a cuspate foreland data were collected on both sides of the cape representing shorefaces with contrasting shoreline orientation north south vs northwest southeast subjected to the same wind forcing in the nearshore the subtidal flow is aligned with the local coastline orientation while at the cape point the flow is along the existing submerged shoal suggesting that cape associated shoals may act as an extension of the coastline alongshore momentum balance analysis incorporating wave current interaction by including vortex and stokes coriolis forces reveals that in deep waters surface and bottom stress are almost in balance in shallower waters the balance is complex as nonlinear advection and vortex force become important furthermore linearized momentum balance analysis suggests that the vortex force can be of the same order as wind and wave forcing farther southwest of cape hatteras point wind and wave forcing alone fail to fully explain subtidal flow variability and it is shown that alongshore pressure gradient as a response to the wind forcing can close the momentum balance adjacent tide gauge data suggest that the magnitude of pressure gradient depends on the relative orientation of local coastline to the wind vector and in a depth averaged sense the pressure gradient generation due to change in coastline orientation even at km length scale is analogous to the effect of alongshore variable winds on a straight coastline

petrology and u pb geochronology of the telohat migmatites aleksod central hoggar algeria the aleksod region is composed of metasedimentary rocks and large areas of biotite and hornblende bearing migmatites anatexis associated with the main deformation stages occurred under high pressure and temperature conditions estimated at 13 2 kbar and 750 50 c the bulk mineralogical composition of the telohat migmatites shows that their protolith was granodioritic internal structures of zircons and u pb data suggest a polyphased evolution with a 2131 12 ma age for the protolith and a 609 17 ma age for the pan african tectono metamorphic evolution thus precluding any kibaran event in the aleksod area leucosomes are richer in sr and display lower rb zr nb y th u and ree contents than melanosomes wherein accessory phases are stored eu contents are also lower in the leucosomes but in lesser proportion than the other rare earth s leading to a significant positive anomaly petrogenetic modelling accounting for accessory mineral phases clearly shows that the trace element contents of leucosomes and melanosomes follow a distribution law consistent neither with equilibrium nor fractional melting their trace element patterns are best explained by the model of disequilibrium melting with mixing of a few residual phases the present results and previous sr isotopic data as well raise the question of disequilibrium melting in anatexis of crustal material

geochemistry and tectonostratigraphy of the basal allochthonous units of sw iberia evora massif portugal keys to the reconstruction of pre pangean paleogeography in southern europe abstract the basal allochthonous units of nw and sw iberia are members of an intra gondwana suture zone that spreads across the iberian massif and was formed during the collision of gondwana and laurussia in the late paleozoic this suture zone is made of allochthonous terranes and is currently preserved as a tectonically dismembered ensemble a multi proxy analysis is applied to the basal allochthonous units of iberia to test their affinity and potential usage for tracing a suture zone a comparison of the lithostratigraphy tectonometamorphic evolution geochronology and geochemical characteristics of the ediacaran series of these units reveals striking affinities they derive from rather similar immature sedimentary successions deposited along the same continental margin and in relation to a cadomian magmatic arc sm nd systematics indicates that the isotopic sources are among the oldest of the iberian massif ca 2 15 1 5 ga suggesting a very strong contribution from the west african craton these ediacaran series were affected by high p and low to medium t metamorphism blueschist to eclogite facies during the late devonian ca 370 ma they occur below allochthonous ophiolitic sequences and on top of autochthonous or parautochthonous domains lacking of high p and low to medium t devonian metamorphism i e tectonically sandwiched between lithosphere scale thrusts the combination of all these characteristics makes these particular ediacaran series different from the rest of the terranes of the iberian massif such singularity could be useful for tracing more occurrences of the same suture zone along the variscan orogen particularly in cases where its preservation and recognition may be cryptic it also contributes to improve the paleogeographic reconstruction of the margin of gondwana during the ediacaran

solidification of trapped liquid in rocks and crystals trapped liquid in an igneous cumulate solidifies over a range of time and temperature that can be retrieved by use of the lever rule in binary solutions applied to plagioclase using the range in the an content found for the individual rock studied the an range so called when measured in sufficient detail defines the solidification history the instantaneous solid composition along the solidus defines the zoning of the plagioclase as it follows the trapped liquid on the liquidus the reference bulk composition of the trapped liquid is given by an intercept on the initial solid liquid lever defined by the fraction of plagioclase in the trapped parent magma times the residual porosity the mafic fraction is assumed to solidify by reaction independently of the plagioclase zoning the residual porosity is estimated from the content of evolved components in the trapped liquid and can also be calculated from the an range itself when that is calibrated to a value independently determined from the evolved components examples from a recent treatment of residual porosity are given for the solidification of selected rock compositions from the kiglapait and skaergaard intrusions the same principles apply to the solidification of melt inclusions with the difference that the latter tends to sample an evolved sheath by capture rather than a parent magma trapped by closure of a cumulate because only the cumulate has had time to exchange the evolved rejected solute owing to its slow solidification experimental examples of melt inclusions trapped at liquidus conditions demonstrate the highly evolved composition of the sheath compared to the bulk glass composition the application of the solidification principle to presumed liquid immiscibility in the skaergaard intrusion suggests that the melt inclusions are evolved during the capture process and then solidified as given by the an range in the plagioclase the hypothesis of liquid immiscibility is not needed in this analysis the very different end stage feldspar histories of the two intrusions is attributed to the sequestering of low melting feldspar components in the trapped liquids of the skaergaard cumulates compared to the lesser amounts trapped in the kiglapait cumulates

magma flow within the tavares pluton northeastern brazil compositional and thermal convection crystallization coupled with gravity removal of depleted interstitial melt has long been recognized as a mechanism of magma differentiation similarly heat released by synplutonic basaltic magma intrusions has long been recognized as capable of driving convection in granite chambers direct evidence of these processes has seldom been described in granites in the tavares pluton we mapped a number of melt extraction structures from pores of a crystal liquid mush an effectively solid magma where crystals form an interconnected skeleton and a variety of flow structures such as 1 meter scale tear or mushroom shaped blobs representing within chamber diapirs 2 meter scale ellipsoids representing frozen thermal plumes of granite driven by heat released from disrupted diorite intrusions and 3 ladder dikes and snail structures representing cross sections of several superposed cylindrical magma channels possibly feeders of diapirs and plume heads a fundamental feature of the structures in the tavares pluton is that they are well delineated by mafic schlieren developed at active channel margins we postulate a new model for the origin of marginal schlieren which combines shear flow sorting and melt escape from the flowing magma into an effectively solid surrounding mush extraction structures representing melt extraction from mush pores into melt pockets and schlieren representing regions where melt escaped into surrounding mush pores are both favored by magmas that form an interconnected solid framework at low crystal fractions 50 because these mushes are ductile and permeable favorable magmas are those with a high wetting angle between melt and solid 60 and a propitious crystal size and shape distribution we propose a model of compositional and thermal convection that accounts for all described structures

the multistage crystallization of zircon in calc alkaline granitoids u pb age constraints on the timing of variscan tectonic activity in sw iberia cl imaging and u th pb data for a population of zircons from two of the evora massif granitoids ossa morena zone sw iberia show that both calc alkaline granitoids have zircon populations dominated by grains with cores and rims either showing or not showing differences in th u ratio and having ages in the range ca 350 335 ma early carboniferous multistage crystallization of zircon is revealed in two main growth stages ca 344 342 ma and ca 336 335 ma well represented by morphologically complex zircons with cores and rims with different ages and different th u ratios that can be explained by 1 crystallization from melts with different compositions felsic peraluminous to felsic intermediate metaluminous 0 001 837 c granulite facies and or because they were derived from inheritance poor felsic and mafic rocks from a previous cycle as suggested by the internal structures of zircon cores these variscan magmatic rocks with crystallization ages estimated at ca 336 335 ma are spatially and temporally related to high temperature metamorphism anatexis processes of interaction between crustal and mantle derived magmas and intra orogenic extension that acted in sw iberia during the early carboniferous

origin of peraluminous granites and granodiorites iberian massif spain an experimental test of granite petrogenesis the discrimination between potential source materials involved in the genesis of iberian granites and granodiorites as well as the role of mantle crust inter actions are examined using constraints imposed by melting experiments melting assimilation experiments and sr nd isotope systematics the sr nd isotope re lationships indicate the existence of diaerent genetic trends in which juvenile mantle materials are involved by diaerent mechanisms 1 a source trend traced by a particular evolution of the pre hercynian basement and indicating mantle participation at the time of sedimen tation 2 a set of magmatic trends traced by gabbro tonalite enclave granodiorite associations implying the incorporation of new mantle material at the time of granite generation these relationships strongly support a pure crustal origin for the peraluminous leucogranites derived from partial melting of crustal protoliths and a hybrid origin for the peraluminous granodiorites these granodiorites are the most abundant granitic rocks of the central iberian zone ciz of the iberian massif implying that processes of hybridisation by assimilation and or magma mixing played an important role in granitoid production during the hercynian orogeny these hypotheses have been tested by means of melting and assimilation experiments melting experiments in the range 800 900 c and at pressures of 3 6 10 and 15 kbar indicate that 1 several potential source materials such as bt ms gneisses and metagreywackes are suitable for the production of peraluminous leucogranite melts 2 the melt compositions are always leucogranitic re gardless of pressure 3 pressure exerts a strong influ ence on the fertility of the source experiments at 3 kbar produce more than 20 vol of melt compared with less than 5 vol of melt produced at 10 and 15 kbar and at the same temperature the melting assimilation experi ments carried out at 1000 c and 4 7 and 10 kbar and using a proportion of 50 gabbro and 50 gneiss give high melt proportions more than 50 vol and noritic residues these melts have the composition of le ucogranodiorites and overlap with part of the compo sitional range of peraluminous granodiorites of the iberian massif the generation of more mafic granodiorites may be explained by the incorporation of some residual orthopyroxene to the granodiorite mag mas the low solubility of fe mg prevents the gen eration of granodiorite melts with more than 3 wt of mgo feo at all crustal pressures the large volumes of peraluminous hybrid granodiorites produced by assimilation of crustal rocks by mantle magmas imply that an important episode of crustal growth took place during the late palaeozoic hercynian orogeny in the iberian massif

schlieren formation in diatexite migmatite examples from the st malo migmatite terrane france schlieren are trains of platy or blocky minerals typically the ferromagnesian minerals and accessory phases that occur in granites and melt rich migmatites such as diatexites they have been considered as 1 unmelted residue from xenoliths or the source region 2 mineral accumulations formed during magma flow 3 compositional layering and 4 sites of melt loss in order to help identify schlieren forming processes in the diatexites at st malo differences in the size shape orientation distribution and composition of the biotite from schlieren and from their hosts have been investigated small biotite grains are much less abundant in the schlieren than in their hosts schlieren biotite grains are generally larger have greater aspect ratios and have except in hosts with low 10 biotite contents a much stronger shape preferred orientation than host biotite the compositional ranges of host and schlieren biotite are similar but schlieren biotite defines tighter sharper peaks on composition frequency plots hosts show magmatic textures such as imbricated tiled unstrained plagioclase some schlieren show only magmatic textures tiled biotite no crystal plastic strain features but many have textures indicating submagmatic and subsolidus deformation e g kinked grains and these schlieren show the most extensive evidence for recrystallization magmas at st malo initially contained a significant fraction of residual biotite and plagioclase crystals smaller biotite grains were separated from the larger plagioclase crystals during magma flow since plagioclase was also the major early crystallizing phase the plagioclase rich domains developed rapidly and reached the rigid percolation threshold first forcing further magma flow to be concentrated into narrowing melt rich zones where the biotite had accumulated hence increasing shear strain and the degree of shape preferred orientation in these domains schlieren formed in these domains as a result of grain contacts and tiling in the grain inertia regime final amalgamation of the biotite aggregates into schlieren involved volume loss as melt trapped between grains was expelled after the rigid percolation threshold was reached in the biotite rich layers

ree distribution in some layered migmatites constraints on their petrogenesis abstract the ree distributions in mesosomes neosomes leucosomes and melanosomes of four layered migmatites have been investigated in one example arvika migmatites the ree patterns in adjacent paragneisses the presumed parent rock of the migmatites were also determined ree patterns of neosomes and mesosomes of arvika migmatites are similar to the finegrained layers and coarse grained layers respectively observed in the adjacent paragneiss this is in agreement with the layer by layer paragneiss migmatite transformation model the ree patterns of mesosomes and neosomes indicate that these lithologies may have been closed systems for ree during the formation of the migmatites no indication of metasomatic reactions melt segregation or injection could be detected within the neosomes leucosomes are depleted and melanosomes enriched in ree contents this is interpreted to be due to separation and concentration of accessory minerals monazite epidote allanite zircon sphene apatite garnet into the melanosomes the behaviour of accessory minerals during migmatite formation is closely allied to that of biotite which is also concentrated in the melanosomes

nd isotope disequilibrium during crustal anatexis a record from the goat ranch migmatite complex southern sierra nevada batholith california geological and geochemical studies of a pelitic migmatite complex within the isabella pendant of the southern sierra nevada batholith california suggest that the leucosomes represent the products of partial melting of the metapelite host driven by the emplacement of the adjacent goat ranch pluton ca 100 ma the leucosomes preserve a record of largemagnitude nd isotope disequilibrium with respect to their pelitic source the leucosomes have a wide range of nd 100 ma from 26 0 to 211 0 as compared to 28 7 to 211 3 for the source they can be subdivided into two groups based on their major elements and sr and nd isotope geochemistry group i leucosomes have higher p2o5 contents and nd 100 ma values than those of group ii the nd 100 ma values of group i leucosomes are significantly higher than those of metapelites and migmatites by two to four epsilon units suggesting that group i leucosomes are in nd isotope disequilibrium with their sources correlations among p2o5 contents nd 100 ma values and sm nd ratios in the leucosomes suggest that apatite or monazite has played a dominant role in fractionating sm from nd and generating nd isotope disequilibrium dissolution of apatite or monazite might play a critical role in regulating the behavior of the sm nd isotope systems and thus the nd isotope compositions of melts generated during crustal anatexis especially in metasedimentary protoliths

layered granitoids interaction between continental crust recycling processes and mantle derived magmatism examples from the evora massif ossa morena zone southwest iberia portugal abstract in this paper field petrographic mineralogical geochemical and isotopic rb sr and sm nd information from three areas within the evora massif iberian variscan orogen is presented and discussed aiming at to unravel the relationships between granitoids and units mapped as migmatites and also to evaluate the interplay between mantle and crustal derived magmas one of the areas almansor displays a well developed compositional layering concordant with the regional variscan structure which was considered in previous works as an alternation of leucosome and melanosome in this study the layering is described as intercalation of diatexites weakly foliated granitoids and trondhjemitic veins diatexites have characteristics of crustal melts plus restitic material and according to geochemical and isotopic evidence result from anatexis of ediacaran metasediments weakly foliated granitoids and trondhjemitic veins from almansor have calc alkaline signatures and may be related to each other by crystal fractionation processes however the mixing between mafic mantle derived and felsic diatexitic melt magmas revealed by the isotopic data may also explain their genesis in the alto de sao bento area several igneous lithologies tonalites granodiorites porphyritic granites and leucogranites are present and show typical isotropic igneous textures despite structural and textural differences geochemical data support for most rocks an origin from the same calk alkaline suite also present at almansor the alto de sao bento leucogranites have an isotopic signature that although different from that obtained in the almansor diatexites is still compatible with an origin involving melting of ediacaran metasediments compositions with very low contents of usually incompatible elements flat normalized ree patterns and strong negative eu anomalies suggest that the anatectic melt has undergone crystal fractionation processes before reaching the composition of the leucogranite magma the almansor outcrop is then interpreted as the remnants of a shear zone that operated as a pathway for melts that moved upward through the crust providing the locus for differentiation and mingling mixing of magmas whilst alto de sao bento would correspond to the zone at a higher crustal level where magmas were trapped and forced to spread horizontally at valverde the third area foliated and non foliated granitoids are spatially related and field criteria links these rocks to metamorphic protolith and anatectic melt respectively however petrographic geochemical and isotopic information shows that they all are compositionally identical trondhjemites with no evidence of metamorphic fabric in the foliated rocks mesoscopic features are interpreted as resulting from melt segregation structures formed in a crystallizing mush in contrast to the previous areas the valverde trondhjemites probably do not belong to the main calc alkaline plutonic suite of the evora massif since they have a distinct sr and nd isotope signature

neodymium and strontium isotope content of microdiorite enclaves points to mantle input to granitoid production microdiorite enclaves are arguably the most poorly understood of the more prominent features of granitoids surprisingly they have been invoked as support for completely different models for the origin of chemical zonation in granitoids depending on whether they are interpreted as restites syn plutonic blobs of magma remobilized cumulate or basified country rock xenoliths here we present the first detailed neodymium and strontium isotope study of suites of such enclaves from two plutons in both cases we find relatively high nd values most readily explained if enclaves represent syn plutonic injections of mafic magma into the host granitoid magma as such they are the tangible remains of a very important process of mantle derived heat and mass transfer operating in the generation of granitic magmas

inherited arc signature in ediacaran and early cambrian basins of the ossa morena zone iberian massif portugal paleogeographic link with european and north african cadomian correlatives abstract geochemical data from clastic rocks of the ossa morena zone iberian massif show that the main source for the ediacaran and the early cambrian sediments was a recycled cadomian magmatic arc along the northern gondwana margin the geodynamic scenario for this segment of the avalonian cadomian active margin is considered in terms of three main stages 1 the 570 540 ma evolution of an active continental margin evolving oblique collision with accretion of oceanic crust a continental magmatic arc and the development of related marginal basins 2 the ediacaran early cambrian transition 540 520 ma coeval with important orogenic magmatism and the formation of transtensional basins with detritus derived from remnants of the magmatic arc and 3 gondwana fragmentation with the formation of early cambrian 520 510 ma shallow water platforms in transtensional grabens accompanied by rift related magmatism these processes are comparable to similar cadomian successions in other regions of gondwanan europe and northwest africa ediacaran and early cambrian basins preserved in the ossa morena zone portugal and spain the north armorican cadomian belt france the saxo thuringian zone germany the western meseta and the western high atlas morocco share a similar geotectonic evolution probably situated in the same paleogeographic west african peri gondwanan region of the avalonian cadomian active margin

kiglapait mineralogy v feldspars in a hot dry magma the lithology of the 1 305 ga kiglapait intrusion is dominated by a lower zone of troctolite succeeded by an upper zone of olivine gabbro ferrodiorite and syenite with olivine composition of pure fayalite the feldspar composition of the intrusion varies from an 68 to an 9 over a thickness of 8 4 km from the base to a sandwich horizon under an upper border zone the anhydrous nature of the kiglapait syenites is shown by their high temperature by the loss of minor biotite up stratigraphy in the intrusion and the absence of amphibole the end stage feldspar of the kiglapait syenites is that of a solidus embedded in a solvus in a 3 kbar eutectic at 1000 c the end member assemblage at temperature and pressure is invariant the final bulk composition is relatively an rich an 11 with a composition of x or 1 3 when projected to the ab or sideline the experimental feldspar solvus when corrected for the effects of an and ba and referred to 3 kbar penetrates the solidus and fits the experimental tie lines these conditions precede a stage of local coarsening under subsolidus conditions that is found in colloform symplectites invading mesoperthite the oligoclase orthoclase symplectites are iso compositional with their host mesoperthites the coarsening is assumed to be related to a plausibly f rich vapor phase that is locally consumed with time the observed phase compositions indicate the end of exsolution at 800 c at 3 kbar on the binodal solvus

adakitic like magmatism in western ossa morena zone portugal geochemical and isotopic constraints of the pavia pluton abstract granitic rocks are a major component of the earth s continental crust and occur in a wide variety of tectonic settings their chemical and isotopic characterization is crucial to the recognition of the potential sources and mechanisms involved in their generation in this study we present the first whole rock chemical and isotopic sr nd o data for the pavia pluton 328 317 ma located near the western border of the ossa morena zone evora massif portugal major and trace element geochemistry suggests that the different granitic phases composing this intrusive body enclaves granites s l and crosscutting dikes represent independent magma pulses and the majority is similar to ttgs and adakites the little sr nd o isotopic variation with 87 sr 86 sr 328 0 70428 0 70560 end 328 ranging between 3 4 and 0 4 and 18 o varying from 5 6 to 8 4 implies an isotopically similar protolith for all phases the most viable mechanism for the generation of the pavia pluton adakitic like magmatism is assimilation fractional crystallization of a mantle derived magma this mechanism was also invoked to explain the genesis of other plutons within the evora massif but they have a distinct chemistry typical arc calc alkaline rocks the chemical differences between them and the pavia pluton granitic rocks are interpreted as the result of lower degrees of crustal assimilation and higher degrees of contamination of mantle derived magmas by the sinking slab after subduction blocking and subsequent slab break off

the petrogenesis of some migmatites and granites central damara orogen namibia evidence for disequilibrium melting wall rock contamination and crystal fractionation