tectonics of the antarctic scotia plate boundary near elephant and clarence islands west antarctica over 5000 km of new bathymetric data collected from near the northern antarctic peninsula 60 s 63 5 s latitude 53 5 w 63 w longitude show the morphology of an irregular segment of the antarctic scotia plate boundary and nearby shetland microplate the irregular plate boundary is formed by an oblique intersection 70 of the sinistral transpressional shackleton fracture zone sfz and the sinistral transtensional south scotia ridge transform ssr near elephant ei and clarence ci islands mapped boundaries of the shetland microplate include the south shetland trench and the volcanic rift axis of bransfield strait marginal basin bathymetric data single channel seismic reflection profiles and geosat ers 1 free air gravity data show a southeast trending fault zone on the northeast side of a prominent ridge in the sfz the fault zone is defined by scarps that affect ocean floor sediments fault bounded subbasins rotated sedimentary layers angular unconformities linear gravity trends and transtensional followed by contractional deformation southeast of a termination of the sfz ridge at the south shetland trench the fault zone subdivides into segments displaying steep scarps up to 23 and canyons on the northeast margin of the ei platform these features become east west trending nearer to the western ssr south of the islands southwest trending extensional or transtensional fault zones disrupt the bransfield strait volcanic rift axis these data suggest that 1 recent 4 ma changes in the configuration of the antarctic plate near the antarctic peninsula caused a segment of the sfz transform to adjust to a more stable rectilinear geometry with the ssr transform and 2 diffuse transtension resulting from current antarctic scotia relative motion is dissecting the shetland microplate near ei and ci and transferring slivers of the scotia plate onto the antarctic plate

iron zinc magnesium and uranium isotopic fractionation during continental crust differentiation the tale from migmatites granitoids and pegmatites abstract the causes of some stable isotopic variations in felsic rocks are not well understood in particular the origin of the heavy fe isotopic compositions i e high 56 fe values deviation in of the 56 fe 54 fe ratio relative to irmm 014 of granites with sio 2 70 wt compared with less silicic rocks is still debated it has been interpreted to reflect isotopic fractionation during late stage aqueous fluid exsolution magma differentiation partial melting or soret thermal diffusion the present study addresses this issue by comparing the fe isotopic compositions of a large range of differentiated crustal rocks whole rocks of migmatites granitoids and pegmatites mineral separates with the isotopic compositions of zn mg and u the samples include granites migmatites and pegmatites from the black hills south dakota usa as well as i s and a type granitoids from lachlan fold belt australia the nature of the protolith i e i or s type does not influence the fe isotopic composition of granitoids leucosomes partial melts in migmatites tend to have higher 56 fe values than melanosomes melt residues indicating that partial melting of continental crust material can possibly fractionate fe isotopes no clear positive correlation is found between the isotopic compositions of mg u and fe which rules out the process of soret diffusion in the systems studied here zinc isotopes were measured to trace fluid exsolution because zn can easily be mobilized by aqueous fluids as chloride complexes pegmatites and some granitic rocks with high 56 fe values also have high 66 zn values in addition high sio 2 granites show a large dispersion in the zn fe ratio that cannot easily be explained by magma differentiation alone these results suggest that fluid exsolution is responsible for some of the fe isotopic fractionation documented in felsic rocks and in particular in pegmatites however some granites with high 56 fe values have unfractionated 66 zn values and were presumably poor in fluids e g a type for these samples iron isotopic fractionation during magma differentiation is a viable interpretation equilibrium fe isotopic fractionation factors between silicic melts and minerals remain to be characterized to quantitatively assess the role of fractional crystallization on iron isotopes in granitoids

early neoproterozoic crustal evolution in northern yili block insights from migmatite orthogneiss and leucogranite of the wenquan metamorphic complex in the nw chinese tianshan abstract the northern part of the yili block located in the northwest of the chinese tianshan extends westward to joint with the aktau junggar domain in kazakhstan and is one of the major continental constituents of the central asian orogenic belt caob the nature and tectonic significance of the basement of the continental domains are important for understanding the continental evolution and geodynamic processes of the caob we investigated the wenquan metamorphic complex wmc that represents the basement and metamorphosed sedimentary cover of the northern yili block the wmc is mainly composed of gneissic s type granite migmatite associated with amphibolite paragneiss micaschist quartzite marble and intruding leucogranitic dykes the migmatites display banded schlieren and ptygmatic structures containing rootless lensoid or layered leucosome and leucocratic sills which are indicative of in situ partial melting of the country rocks and variable degrees of melt migration geochemical data suggest that the protoliths of the orthogneiss and gneissic k granite belong to peraluminous s type granites characterized by wide range of i sr values 0 68324 0 72365 low nd t values 1 9 to 4 4 and two stage nd model ages t md 2 1 56 1 82 ga the s type granites might have derived from both clay poor and clay rich source rocks that are probably meta sedimentary and meta volcanic rocks in the wmc the leucogranites show relatively narrow range of i sr values 0 70273 0 70419 more negative nd t values 6 9 to 9 1 and more consistent t md 2 ages 1 51 1 55 ga the leucosome and leucocratic sills show variable sr and nd isotopic compositions probably due to localized and different degree of partial melting our new zircon shrimp and la icpms u pb ages together with previously published data indicate that the migmatization occurred at 926 909 ma the gneissic s type granites emplaced during 919 862 ma and the leucogranites emplaced in the period of 909 845 ma inherited old zircons were found in all dated samples and may have come from their source rocks association of geochronologically overlapped and genetically linked migmatites gneissic s type granites and leucogranitic rocks is interpreted as the results of anatexis of the upper crustal rocks during an early neoproterozoic tectonic metamorphic event that needs to be further studied in the northern yili block such early neoproterozoic migmatization and s type granitic magmatism are considered as an important episode of reworking and cratonization of the continental crust of the yili and adjacent continental blocks in central asia

rift propagation detachment faulting and associated magmatism in bransfield strait antarctic peninsula multichannel seismic data from bransfield strait a young 4 ma marginal basin located at the nw tip of the antarctic peninsula reveal a complex array of rifting styles short of clearly defined seafloor spreading the strait is undergoing basinwide extension while enriched mid ocean ridge basalts have been dredged from topographic highs in the ne part of the basin seismic evidence does not support the formation of oceanic crust there acoustic signatures of volcanic crust at known axial deep highs extend laterally subseafloor their distribution and style support propagation of volcanism and associated extension from ne to sw active extensional structures along both margins control a strong physiographic asymmetry the south shetland islands margin is composed of a few closely spaced large offset normal faults the gentler antarctic peninsula margin exhibits broader distributed extension to the ne normal fault polarity reversal structures nfprs dominate listric faults sole into nw dipping low angle reflectors interpreted as detachments to the sw larger nfprs and convex up geometries up to 20 km across suggest broad uplift rather than detachment based extension such uplift may precedes seafloor volcanism and rapid associated deflation along detachments nw se trending accommodation zones also segment the basin along strike bransfield strait extension may result both from plate boundary tectonics at the south shetland trench and at the evolving antarctic scotia plate boundary extension was probably initiated by onset of subduction rollback at the trench as a slab window opened from the sw augmented extension in the ne part of the basin may be a response to left lateral transtension associated with reorganization of the triple junction at the shackleton fracture zone south scotia ridge intersection

melting of plagioclase quartz assemblages at 2 kbar water pressure the melting of plagioclase and quartz has been investigated at ph2o 2kb a single crystal of plagioclase was surrounded by quartz powder and water a reaction rim consisting of glass and of an rich plagioclase developed around unchanged starting plagioclase microprobe determinations of melt and coexisting plagioclase compositions reveal a strong fractionation of plagioclase components between melt and new plagioclase for example at 850 c the approximate xan of melt is 0 3 and that of plagioclase is 0 8 the temperature interval between beginning of melting and complete melting of cotectic compositions is 100 c or more for quartz plagioclase mixtures with plagioclases between an 40 and an 75 in comparison to the system ab an h2o the plagioclase melting loop is somewhat wider in the investigated system qz ab an h2o but the temperature interval is much smaller 100 c than in the system ab an h2o 200 c the solidus data indicated by the new plagioclase compositions are practically identical with those observed in beginning of melting experiments the results show that fractionation of plagioclase components between partial melts and restite plagioclase can be more pronounced in multi component rocks than in the pure plagioclase system this finding is important for the development of albite rich rocks from more basic compositions

western scotia sea margins improved constraints on the opening of the drake passage 1 we present a revised tectonic interpretation from 28 ma to 3 2 ma of the western sector of the scotia sea incorporating new multichannel seismic reflection profiles and magnetic anomaly identifications for the continental margin off the tierra del fuego island and available complementary data for the conjugate margin of the northwestern flank of the south scotia ridge seismic profiles show a remarkable diversity of the pair of conjugate passive margins of the western scotia sea in both their morphology and structural framework the tierra del fuego continental margin can be related to a classic rifted passive margin while the southwestern margin of the scotia sea is characterized by steep slopes mostly generated by subvertical faults that abruptly separate the continental crust of the south scotia ridge from the oceanic crust of the western scotia sea this structural difference was caused by intense strike slip tectonism mostly concentrated along the modern south scotia ridge since the early development of the western scotia sea we find evidence for a previously unrecognized magnetic anomaly 10 28 ma at the foot of the tierra del fuego continental margin the same anomaly is present at the conjugate northern flank of the south scotia ridge the timing of events leading to the earliest development of the western scotia sea which determined the opening of the drake passage is important because this gateway opening had a profound effect on global circulation and climate the thickness and the distribution of the sedimentary cover overall in the abyssal plain off the two western scotia sea margins is different this is due to the different regimes of the bottom current flows which affected the western scotia sea both in the past and in the present time

the transition from an active to a passive margin sw end of the south shetland trench antarctic peninsula abstract the lateral ending of the south shetland trench is analysed on the basis of swath bathymetry and multichannel seismic profiles in order to establish the tectonic and stratigraphic features of the transition from an northeastward active to a southwestward passive margin style this trench is associated with a lithospheric scale thrust accommodating the internal deformation in the antarctic plate and its lateral end represents the tip line of this thrust the evolutionary model deduced from the structures and the stratigraphic record includes a first stage with a compressional deformation predating the end of the subduction in the southwestern part of the study area that produced reverse faults in the oceanic crust during the tortonian the second stage occurred during the messinian and includes distributed compressional deformation around the tip line of the basal detachment originating a high at the base of the slope and the collapse of the now inactive accretionary prism of the passive margin the initial subduction of the high at the base of the slope induced the deformation of the accretionary prism and the formation of another high in the shelf the shelf transition high the third stage from the early pliocene to the present day includes the active compressional deformation of the shelf and the base of slope around the tip line of the basal detachment while extensional deformations are active in the outer swell of the trench

preservation of chemical residue melt equilibria in natural anatexite the effects of deformation and rapid cooling a combined petrologic textural and chemical approach is used to interpret the relationship between chemical exchange and deformation during partial melting of a granodioritic gneiss in the contact aureole of the bergell pluton central alps in contrast to most regional metamorphic anatexites chemical equilibrium between residue and segregated melt was attained and preserved by several elements as shown by their present distribution equilibrium involved both the major and the trace elements hosted in the rock forming minerals and the light rare earth elements lree hosted primarily in allanite equilibrium with respect to lrees was achieved because allanite had a reactive behaviour during water present melting in contrast to the refractive behaviour of xenotime which mostly controls the distribution of the hree s the attainment and preservation of equilibrium between leucosome and residue requires peculiar boundary conditions we propose that the achievement of equilibrium between melt and residue was promoted by deformation operating via a mechanism of melt present granular flow i e dissolution accommodated grain boundary sliding active during partial melting microstructural observations indicate that melt present granular flow was the dominating deformation mechanism in these anatexites the preservation of residue melt equilibrium is inferred to result from rapid cooling under contact metamorphic conditions and from the lack of deformation below the solidus

small ocean basin development along the scotia antarctica plate boundary and in the northern weddell sea abstract the oceanic crust of the northern weddell sea and scotia sea contains the following principal morphostructural elements 1 the south scotia ridge and the south orkney microcontinent both with continental crust 2 the oceanic powell and jane basins and 3 jane bank which belongs to an island arc the analysis of mcs profiles and of gravimetric and magnetic data from russian italian and spanish cruises supplemented with satellite gravimetric data has enabled us to determine the relationship between these elements and to propose a model for the main stages of cenozoic evolution in the area during the early cenozoic the weddell sea oceanic crust was subducted under the southern margin of the south orkney microcontinent the subduction probably ended westwards at the south powell ridge a submarine extension of the antarctic peninsula a major transcurrent fault zone is identified in the northwestern weddell sea bounding oceanic crust of mesozoic and cenozoic ages this fault zone was probably active at least to the miocene the drifting of the south orkney microcontinent from the antarctic peninsula during the late eocene to early miocene originated the powell basin jane basin developed as a backarc related to the subduction of the weddell sea oceanic crust below jane bank the seismic stratigraphy of the depositional sequences in these two basins indicates that spreading in jane basin started simultaneously with the end of the opening in powell basin the active spreading ridge of the weddell sea collided with the trench and was subducted below jane bank at 15 20 ma drifting in jane basin and subduction below jane bank ended shortly thereafter in the middle miocene and the boundary between the antarctic scotia plates migrated north of the south orkney microcontinent along the south scotia ridge present tectonic activity in the region is minor

effects of ridge crest trench interaction on antarctic phoenix spreading forces on a young subducting plate precise measurements of spreading rates on marine magnetic profiles collected to the west of the antarctic peninsula have enabled some consideration of the forces governing plate motion since antarctic phoenix motion has been controlled by the local rather than global force balance over the last 35 m y the total effective driving force per unit length of trench is calculated to have ranged between 2 6 and 3 6 1012 n m which is much less than is commonly thought necessary to support subduction conventional calculations may overestimate slab pull for old slabs because they neglect the effect of extensional disruption in limiting the contribution to the balance of forces at the trench the low estimate of driving forces obtained here implies that resistive forces are also smaller than is generally assumed driving forces show a strong correlation with observed spreading rates which indicates that resistive forces were largely velocity dependent fluid migration up the subduction zone may elevate temperatures in and around the shear zone reducing resistive forces below the levels required by purely conductive models changes in convergence rate may affect the depths of both the brittle ductile deformation boundary and the basalt eclogite phase change causing a negative feedback which would appear as a velocity dependent resistive force the different driving forces acting on the ne and sw parts of the phoenix plate as a consequence of older oceanic lithosphere at the trench in the ne caused antarctic phoenix spreading to take place about a near pole to the sw since 21 m y ago and ultimately resulted in disruption of the phoenix plate about 9 m y ago spreading rates decreased abruptly about 6 m y ago probably because of e w compression across the long transform faults bounding the phoenix plate however spreading on the last three segments of the antarctic phoenix ridge continued at least until 4 m y ago either spreading stopped progressively from sw to ne or the final stage took place about a very near pole to the sw a magnetic quiet zone extends up to 95 km from the margin between the tula fracture zone and the north anvers fracture zone and is thought to indicate that the ridge crest became buried by terrigenous sediment prior to collision the absence of a magnetic quiet zone associated with the most recent ridge crest trench collisions suggests a change in sedimentary regime during the late miocene anomalously fast apparent spreading rates between 23 and 21 m y ago are thought to indicate an error in this part of the magnetic reversal time scale

seismic stratigraphy of the central bransfield basin nw antarctic peninsula interpretation of deposits and sedimentary processes in a glacio marine environment abstract the central bransfield basin is a deep narrow trough between the northern tip of the antarctic peninsula and the south shetland islands analyses of single channel high resolution seismic reflection data are used to characterise the seismic stratigraphy of the central bransfield basin the tectonised acoustic basement is overlain by a 1 s thick sedimentary cover composed of two main sedimentary sequences the lower sequence which shows synsedimentary deformation has only been identified on the antarctic peninsula margin the upper sequence is a complex sedimentary package composed of eight seismic units whose distribution geometry and seismic facies allow two types of seismic units to be distinguished slope and basinal units the slope units constituted by progradational stratified seismic facies form a sedimentary wedge extending from the shelf edge the basinal units fill the basin floor showing chaotic and undulated seismic facies that change basinward into stratified seismic facies both types of seismic units display an interfingering pattern at the base of the slope suggesting an alternating shift of the sedimentary depocentre from the slope to the basinfloor and vice versa this alternate pattern indicates that the sedimentary processes responsible for the infilling of the central bransfield basin followed a cyclic pattern which has likely been associated with the advance and retreat of the ice sheets over the margins during glacial and interglacial episodes during glacial periods the ice sheets advanced eroded the shallower sea floor areas and deposited diamicton and debris flow deposits along the moving grounding line resulting in a progradational sedimentary wedge on the slope at the end of glacial periods coinciding with the retreat of the ice sheets extensive sediment failures affected the continental margin during interglacial periods the ice sheets remained restricted to coastal locations and glacial troughs where processes of meltwater formation might have been significant sediment laden underflows are generated within these troughs from where they flow and spread over the shelf and down the slope to the basinfloor as sediment gravity flow deposits the combined effect of these processes is a progradational build up of the shelf and an aggradational infilling of the basin floor together with the development of the interfingering pattern at the base of the slope

autopsy on a dead spreading center the phoenix ridge drake passage antarctica new bathymetric and magnetic anomaly data from the phoenix ridge antarctica show that extinction of all three remaining segments occurred at the time of magnetic chron c2a 3 3 0 2 ma synchronous with a ridge trench collision south of the hero fracture zone this implies that the ultimate cause of extinction was a change in plate boundary forces occasioned by this collision spreading rates slowed abruptly at the time of chron c4 7 8 0 3 ma probably as a result of extinction of the west scotia ridge which would have led to an increase in slip rate and transpressional stress across the shackleton fracture zone spectacular high relief ridges flanking the extinct spreading center mapped for the first time using multibeam swath bathymetry are interpreted as a consequence of a reduction in spreading rate involving a temporary magma oversupply immediately prior to extinction

the subsolidus segregation of layer parallel quartz feldspar veins in greenschist to upper amphibolite facies metasediments layer parallel i e parallel to foliation or bedding vein formation in the graywackes and pelites of the quetico metasedi mentary belt occurred during synchronous prograde metamorphism and regional dj com pression in a traverse across metasediments which change in metamorphic grade from green schist to upper amphibolite migmatite facies layer parallel veins show the following trends 1 an increase in thickness and internal corn plexity the latter due to successive boudinage 2 low grade veins are parallel to planes of anisotropy due to the original sedimentary fabric of the host rocks but at higher grades other sites are also used and 3 a systematic increase in plagioclase quartz ratio in the veins towards higher grade adjacent mafic selvedges first exhibit quartz depletion then in the amphibolite facies plagioclase depletion mineralogical zoning is often preserved in a single vein older parts are more quartz rich than younger mass balance calculations and whole rock geochemistry based on veins mafic selvedges and country rock are consistent with a closed system subsolidus segregation origin the layer parallel veins are syntectonic and migration of the mobile components required to form their mineralogy is a stress induced mass transfer the source of these components appears to be dominantly pressure solution of the same minerals in the host rocks although metamor phic reactions may also have contributed veins nucleated first at those sites where initial sedimentary heterogeneites such as fine scale graded bedding provided gradients of normal stress across grain boundaries and hence of chemical potential necessary to drive the sub solidus segregation process the earliest veins are thus parallel to bedding later nucleation of the veins could also occur along more randomly distributed sites within the metasediments and these veins grew parallel to the schistosity rather than bedding if the two were distinct once formed the veins themselves which are more competent than the surrounding rock provide the stress heterogeneity required for their further growth the increasing plagioclase quartz ratio in the veins may be due to a tem perature dependent increase in plagioclase com ponent mobility relative to quartz alterna tively the increasing transfer distances for silica resulting from prior quartz depletion in the inner parts of the mafic selvedge may increase the relative mobility of plagioclase component

origin of the skagit migmatites north cascades range washington state stromatic and schlieren type migmatites are a major lithology in the type section of the skagit gneiss complex in the north cascades range of washington state usa migmatite mesosomes are chiefly biotite schist amphibolite and orthogneiss in decreasing order of abundance leucosomes are predominantly leucotrondhjemites with a very limited range of composition that is nearly independent of associated mesosome type melanosomes consisting mainly of biotite and or hornblende garnet are inconsistently developed and absent in places the age of migmatization is not well established but appears to be late cretaceous or early tertiary this is also the age of syntectonic tonalite to trondhjemite intrusives that are predominant in most parts of the skagit complex although temperatures in excess of 700 c and pressures as high as 10 kb occurred there is no evidence for widespread partial melting of the mesosomes with which the migmatites are closely associated mass balance calculations preclude an origin by injection of a silicate melt or hydrothermal fluid unless accompanied by metasomatic replacement reactions mass balance relationships also show that the skagit migmatites could not have formed solely by closed system processes such as partial melting or metamorphic segregation unless the mesosomes present were not the protolith from which the migmatites formed field petrographic and geochemical data indicate that an origin by migmatization of a missing mesosome is quite unlikely the most feasible process of migmatization appears to be infiltration of an aqueous fluid into a metamorphic protolith along fracture or foliation planes this triggers a variable degree of metamorphic segregation or possibly minor partial melting unmixing of leucosomes and melanosomes from the mesosome protolith must be accompanied by metasomatic replacement but the total mass transfer required is only a few wt

structure and geodynamic evolution of the central bransfield basin nw antarctica from seismic reflection data abstract the bransfield basin is a young active rift basin located between the northern margin of the antarctic peninsula and the south shetland islands margin deception and bridgeman islands divide the bransfield basin in three subbasins the western central and eastern specific morpho tectonic features and sediment fill differentiate each subbasin the structure and geodynamic evolution of the central bransfield basin which is in a stage of incipient seafloor spreading have been investigated in detail from a dense grid of single channel seismic reflection data the central bransfield basin is dominated by two families of normal faults which are oriented northeast and northwest the ne trending faults define three graben systems that are roughly parallel to the basin axis in an across basin direction the mean trend of this family of faults ranges from n71 the graben system nearest to the antarctic peninsula over n64 the intermediate graben system to n53 the graben system nearest to the south shetland islands the nw trending family of faults is responsible for the deepening of the basin from southwest to northeast both families of faults define the overall central bransfield basin structure resulting in a complex division of the basin floor additionally tens of volcanic edifices are located on the basin floor the larger ones being associated to the nw trending faults interaction of tectonics and sedimentation give place to the differentiation of three tectonostratigraphic units tu1 tu2 and tu3 from oldest to youngest tu1 occupies the southernmost graben system and it is affected by the ne trending bounding normal faults tu2 extends further northwestwards than tu1 and essentially fills the intermediate graben system tu3 represents a further extension of the sediment infill over most of the central bransfield basin and marks the initiation of the infill of the northernmost graben system faults bounding this graben also determine the straight and abrupt morphology of the south shetland islands margin the observed arrangement of the graben systems and the filling tectonostratigraphic units reveal a migration of extensional tectonics and associated depocentres from the antarctic peninsula margin to the south shetland island margin the left lateral rotation from n71 to n53 in the mean trend of the three successive graben systems could have been produced by the oblique subduction of the phoenix plate and the effect of the sinistral strike slip movement of the south scotia ridge 200 km northeastwards of the central bransfield basin

tectonic development of the bransfield basin and its prolongation to the south scotia ridge northern antarctic peninsula abstract the bransfield basin is located off the northern tip of the antarctic peninsula the analysis of multichannel seismic reflection profiles allows the shallow structure of the bransfield basin and its eastwards prolongation through the scotia sea to be imaged the bransfield basin is asymmetrical with most of the sediment input coming from the passive antarctic peninsula margin which is deformed by normal faults generally dipping less than 45 northwestwards much less sediment comes from the conjugate south shetland islands margin which is dominated by high angle normal faults generally dipping between 45 and 60 constituting the inner boundary of the south shetland block tectonic horst the detachment and moving apart of the south shetland block from the antarctic peninsula led to the opening of the bransfield basin where incipient oceanic spreading has been reported previously the south shetland block outer boundary to the northwest is made of the south shetland trench which to the east passes into the sinistral transpressive scotia antarctic plate boundary marked by the prominent south scotia ridge following cessation of spreading in the phoenix antarctic ridge northwest of the south shetland block during middle pliocene roll back played a major role in the development of the bransfield basin with maximum extension in the central subbasin in addition the eastern bransfield basin accommodates most of the sinistral transtensional deformation along the south scotia ridge normal and reverse faults were simultaneously active along the inner and outer boundaries respectively of the continental crust fragment constituting the south shetland block the development of the bransfield basin offers a good example of the tectonic processes involved in continental fragmentation associated with backarc basin development

fluid present melting of meta igneous rocks and the generation of leucogranites constraints from garnet major and trace element data lu hf whole rock garnet ages and whole rock nd sr hf o isotope data pan african high grade metamorphism in the proterozoic damara orogen namibia led to formation of garnet bearing leucosomes in potassic meta igneous gneisses producing a meta igneous migmatite in addition the migmatite gneiss mesosome plus leucosome was intruded by small scale leucogranitic melts with a high amount of accumulated biotite and garnet u pb zircon ages obtained on the mesosome and the leucosome indicate late proterozoic ca 850 ma concordia upper intercept ages which are interpreted as minimum ages of the precursor rock of the migmatite u pb monazite ages obtained on the leucogranite give a concordant age of 512 1 ma and two reversely discordant ages with pb 207 u 235 ages of 544 1 and 534 1 ma indicating the growth of monazite before or close to the age of high grade metamorphism in the damara orogen high precision lu hf garnet whole rock dating gave ages of 492 6 1 7 ma for the mesosome 497 6 1 7 ma for the leucosome and 494 0 1 7 ma for the garnet and biotite bearing leucogranite indicating that the growth of garnet postdates the growth of monazite during high grade metamorphism in addition it is suggested that melting and intrusion was coeval and occurred probably shortly after the main peak of metamorphism which occurred at c 512 1 ma p t estimates obtained by conventional thermobarometry c 690 720 degrees c and accessory mineral dissolution thermometry on the leucogranite c 730 degrees c suggest that partial melting occurred through limited fluid present melting of biotite via the reaction bt kfs pig qtz h2o double left right arrow grt melt outcrop evidence diffuse relationship between the gneiss domain and the leucosomes similar size of the leucosomes homogeneous distribution of leucosomes on the sample scale suggests that minor melt segregation had occurred whole rock sr nd hf and o isotope data of the mesosome indicate that it belongs to basement rocks from this area geochemical and isotope data obtained on the leucosomes argue for derivation by in situ melting of the mesosome both leucosome and leucogranite originated from the same source rock but the leucogranite represents an accumulated melt that was able to segregate and to intrude the gneiss domain the similar isotope features of the mesosome leucosome and leucogranite indicate a direct relationship for the gneiss and the melts chemical and mineral data favour a derivation of both types of melt through fluid present melting of isotopically and chemically comparable biotite plagioclase k feldspar quartz bearing gneisses c 2008 elsevier b v all rights reserved

sediment lithofacies processes and sedimentary models in the central bransfield basin antarctic peninsula since the last glacial maximum abstract this work provides a regional perspective on the links between glacial glacimarine and marine sedimentary processes occurring since the last glacial maximum along the central bransfield basin antarctic peninsula from shallow to deep areas by combining information provided by gravity cores from shelf to deep basin settings swath bathymetry and high resolution reflection seismic and sub bottom topas profiles seven lithofacies have been identified their textural compositional and x radiographic characteristics as well as their spatial distribution allow the differentiation of two stages of deposition involving distinct sedimentary processes the last glacial maximum was characterised by subglacial deformation by the antarctic peninsula ice sheet as it reached its most extended position on the distal mid slope platforms of the central bransfield basin proglacial debris flows occurred on the external edges of the mid slope platforms and lower slope since the last deglaciation glacimarine proglacial debris flows glacimarine proglacial processes and turbid glacial meltwater and open marine sedimentary processes turbidity currents sliding and contour current activity have predominated in the central bransfield basin glacial cyclicity is the main factor controlling the sedimentary processes in the basin but several secondary factors including physiography sediment entry points and oceanography have been identified as local controls on the sediment distribution the unusual physiography of the central bransfield basin where slope platforms form wide flat surfaces seaward from glacial troughs is a key factor in the widespread occurrence of glacial and glacimarine processes on the entire continental margin

neodymium isotope equilibration during crustal metamorphism revealed by in situ microanalysis of ree rich accessory minerals radiogenic isotopes are widely used to investigate crustal evolutionary processes however recent claims of nd and sr isotope disequilibrium during anatexis question the reliability of such information we have conducted an in situ sm nd isotope study of apatite allanite titanite xenotime and monazite in metasedimentary rocks of different metamorphic grade to test nd isotope equilibrium during metamorphism our results show that apatite retains an original probably detrital highly variable nd isotopic signature until at least 500 c before being isotopically homogenised irrespective of textural context within the rock once equilibrated apatite retains its nd isotope signature throughout anatexis in contrast allanite and titanite are equilibrated at temperatures as low as 350 400 c ree rich accessory minerals in high grade rocks 600 c show very similar initial end values at the time of metamorphism we conclude that under these metamorphic conditions nd isotope disequilibrium between crustal melts and metasedimentary sources is unlikely intra grain nd isotope zoning of monazite indicates that partial melting was open system involving the injection of externally derived melt into migmatites this process likely to be common in anatectic terranes but not always obvious at hand specimen scale or from bulk rock geochemical data can produce isotope variation that could potentially be misinterpreted as disequilibrium between the melt and its protolith

continental fragmentation along the south scotia ridge transcurrent plate boundary ne antarctic peninsula abstract the study of the south scotia ridge on the basis of swath bathymetry multichannel seismic and magnetometry profiles obtained during the hesant92 93 cruise and complemented with satellite gravimetry and seismicity data illustrates the tectonics of the region the thinned continental crust fragments of the ridge are bounded by oceanic crust of the scotia sea to the north and powell basin to the south the northern boundary represents the contact between the scotia and antarctic plates this boundary is a sinistral transpressional fault with transtensional segments and moderate recent tectonic activity another fault located at the southern boundary appears inactive and does not reveal any features that would enable the kinematics to be determined both faults have associated steep scarps since they separate oceanic and continental crust types the most significant active deformation lies in the axial depression of the ridge within a band delineated by fault systems with wsw ene and sw ne strikes these faults develop pull apart basins which separate the northern and southern blocks of the ridge the northern block is being fragmented from the antarctic plate by a zone of transtensive faults and is probably a crustal element independent of the antarctic plate the axial depression which crosses the ridge slightly obliquely is characterized by deep basins locally more than 5000 m deep and associated high seismicity the fault geometry and earthquake focal mechanisms indicate an active sinistral transtensive regime for the fault system although it may locally have transpressive regimes depending on the fault plane and the stress field orientations the internal basins are characterized by an asymmetric development showing itself as depositional wedges generally thickening northward deposits onlap the southern margins and are affected by normal faults in the northern margins the seismicity around the scotia plate shows that the present stresses are compressive along the northern boundary with the south american plate 1 sw ne and subhorizontal and along the western boundary with the antarctic plate 1 wnw ese and subhorizontal for the south scotia ridge however 1 is steeply inclined and 3 is subhorizontal with a nw se trend the stress distribution in bransfield strait is similar to the ridge and the recent extension could be partially explained by the westward continuation of the active fault system of the central south scotia ridge the fragmentation of continental crustal blocks due to the tectonic activity along the transcurrent plate boundaries is a mechanism that contributes to the deformation of the northeastern end of the antarctic peninsula this area appears appropriate for the analysis of continental plate fragmentation processes

new insights on the post rift seismic stratigraphic architecture and sedimentary evolution of the antarctic peninsula margin central bransfield basin abstract a seismic stratigraphic study of the antarctic peninsula margin of the central bransfield basin was conducted using seismic and bathymetric data the study focused on seismic sequences and units deposited after the basin opened and yield a record of the evolution of the northern most antarctic peninsula ice sheet two seismic sequences compose the post rift stratigraphic succession on the central bransfield basin slope platforms both sequences are composed of seismic units that record glacial interglacial cycles and show evidence of the antarctic peninsula ice sheet having grounded on the continental shelf and slope platform on several occasions in the past this evidence includes glacial and glaciomarine related sedimentary and erosional features within the seismic units glacial unconformities glacial troughs till wedges prograded wedges trough mouth fans glaciomarine sheet deposits and moraines which are bounded by glacial unconformities the antarctic peninsula lower slope also registered in its eastern sector the deposition of gravity flow deposits during ice sheet retreat and interglacial stages the deposition of seismic units on the middle slope produced the present day step like profile of the margin composed of wide flat slope platforms and narrow and steep upper and lower slopes seismic units show changes in thickness and distribution that record the effect of the physiography and climatic changes on margin sedimentation these variations produced an overall change from more line sourced deposition to more point sourced deposition as ice streams evolved the stacking of units shows a change in the stratigraphic architecture through time from dominantly progradational s2 to progradational aggradational s1 this change is correlated with a change in the frequency of ice sheet grounding events which was probably driven by a higher rate of eustatic rise and fall in relation with the mid pleistocene global climatic change

phase equilibria and compositions of fe mg al minerals and melts in water saturated peraluminous granitic systems phase assemblages and phase compositions were studied experimentally in water saturated biotite bearing peraluminous granitic melts as a function of alumina excess and temperature the runs were performed at 2 and 5 kbars under nno buffer biotite was stable only in composition containing 5 of normative corundum it coexisted with cordierite and hercynite at 2 kbars and with hercynite at 5 kbars in composition containing 10 of normative corundum biotite was not observed abundant cordierite and hercynite were the only fe mg al minerals these relationships show that at constant pressure the amount of cordierite increases with increasing excess of alumina simultaneously the stability of biotite decreases due to preferential partitioning of mg into cordierite and fe into biotite besides the distribution of fe mg and al among the coexisting solid phases solubility of these elements in the melts is given below 900 c melts are poor in iron and magnesium and correspond in terms of these elements to leucogranites it is suggested that the leucogranitic magmas such as parental magmas of european hercynian and himalayan leucogranites must have been formed through highly efficient separation of partial melt from restite in which ferromagnesian components are concentrated peraluminous granites rich in ferromagnesian minerals originate supposedly from restite bearing magmas

continental magnetic anomalies and the evolution of the scotia arc abstract linear belts 50 100 km in width of long wavelength positive magnetic anomalies exceeding 500 nt are observed on continental blocks of the scotia arc the most developed is the west coast magnetic anomaly which may be traced for more than 1300 km along the antarctic peninsula comparison of magnetic profile data after low pass filtering and reduction to pole reveals a striking similarity between the individual anomaly belts correlation of the anomalies with positive gravity anomalies seismic refraction data and geology indicates that the sources are linear batholiths intruded during mesozoic cenozoic subduction the anomaly belts are truncated at the block margins reflecting cenozoic fragmentation of a cuspate convergent margin an early cenozoic reconstruction based on the assumption that the batholiths once formed a continuous curvilinear feature shows a good alignment of mesozoic strato tectonic terrains and is compatible with the known history of scotia sea opening

migmatization by metamorphic segregation at subsolidus conditions implications for nd pb isotope exchange abstract we have carried out major and trace element analyses as well as nd and pb isotope measurements on a stromatic migmatite in the phikwe complex botswana in order to assess the effect of high grade metamorphic event leading to migmatization on apparent nd model ages the phikwe complex considered as an entity of the central zone of the limpopo belt underwent a high grade metamorphic event at 2 0 ga leading to migmatite formation migmatites in this area present a very good layering of migmatitic components leucocratic layers were segregated from mafic layers dominated either by hornblende or biotite on a centimeter to millimeter scale accessory minerals such as monazite apatite and zircon were not identified in hornblende rich and quartzo feldspathic layers the combination of petrography and trace element concentrations in all layers indicates that the migmatite may have been formed either by partial melting according to biotite and or hornblende dehydration reactions or by metamorphic segregation at subsolidus conditions ree modeling applying batch fractional and disequilibrium melting equations allows to discriminate between these two processes the modeled ree results of the quartzo feldspathic layers match those observed via metamorphic segregation at subsolidus conditions the end values at 2 0 ga range from 0 3 to 8 9 with lower values in the ree rich migmatitic layers thus the distribution of major and trace elements as well as the scatter nd isotope data are interpreted as reflecting the proportional amount of each mineral in the different migmatitic components the positive end value of hornblende rich layers suggests hornblende crystallization from a mantle derived source and sm nd fractionation at protolith formation age apparent nd model ages ranging from 2 3 to 3 1 ga do not reflect the crustal evolution in southern africa in archean to proterozoic time based on the geochemical features and the pb pb wr isochron at 2 0 ga mass conservation during migmatization can be assumed therefore chemical and nd isotopic compositions of the protolith prior to migmatization can be calculated the results indicate a si and k poor diorite as protolith moreover calculated protolith compositions yield a two point sm nd isochron indicating an age of 2 7 ga and very similar nd model ages of 2 73 2 75 ga those ages constrain the timing of an archean crust formation episode in the phikwe complex

the cenozoic subduction history of the pacific margin of the antarctic peninsula ridge crest trench interactions new magnetic anomaly identifications w of the shackleton fracture zone show 5 spreading sections separated by fracture zones in the 2 most southerly the ridge crest collided with a trench at the margin of the antarctic peninsula only 6 5 and 4 ma ago the latest of a series of collisions starting at the base of the peninsula so ma ago following each collision spreading and subduction both stopped opposite the south shetland trench and actively extending bransfield strait spreading also stopped 4 ma ago but before the spreading centre reached the trench the tendency of the subducting plate to continue sinking probably initiated the opening of the bransfield strait the parallelism between fracture zone orientation on the descending plate and convergence direction previously thought responsible for the tectonic segmentation of the peninsula was effective for only 10 30 ma before collision before that either the fracture zones did not extend to the trench or the subducting ocean floor formed at the farallon phoenix nazca aluk boundary with fracture zones oblique to the subduction direction the aleutian arc kula ridge model in which arc magmatism virtually ceases when ocean floor younger than 25 30 ma occurs beneath the arc fits the distribution of antarctic peninsula radiometric dates explaining a 50 60 ma gap between collisions and youngest ages in the s and possibly the migration of the youngest activity towards the trench thus gaps in the geologic record of arc magmatism need not imply cessation of subduction the progressive steepening of the peninsular margin towards the 4 ma collision site suggests tectonic erosion of the fore arc as the ridge crest approached

bransfield basin antarctic peninsula not a normal backarc basin the bransfield basin a marginal basin located northwest of the antarctic peninsula has been habitually considered as a backarc basin associated with the rollback process that took place along an inactive plate boundary the south shetland trench where the antarctic and the phoenix plates meet new geophysical and structural data discussed in this paper show that the basin opening is related to a sinistral simple shear couple between the scotia and the antarctic plates and not to the previously suggested rollback mechanism the widening of the bransfield basin and the lack of trench retreat are causing compression in the south shetland islands two different neotectonic stress directions with interchanged stress axes are found in the area of the south shetland block

what controls partial melting in migmatites the layers of six stromatic migmatites from northern western and central europe display small but systematic chemical and mineralogical differences at least five of these migmatites do not show any signs of largescale metamorphic differentiation metasomatism or segregation of melts it is concluded therefore that the compositional layering observed in most of the investigated migmatites is due to compositional differences inherited from the parent rocks almost isochemical partial melting seems to be the most probable process transforming layered paragneisses metavolcanics or schists into migmatites the formation of neosomes is believed to be caused by higher amounts of partial melts formed due to higher amounts of water moving into these layers the neosomes have less biotite and more k feldspar if k feldspar is present at all than the adjacent mesosomes these differences are small but systematic and seem to control the access of different amounts of water to the various rock portions petrographical observations chemical data and theoretical considerations indicate a close relationship between rock composition rock deformation transport of water partial melting and formation of layered migmatites

forearc tectonic evolution of the south shetland margin antarctic peninsula the main provinces of the south shetland margin antarctic peninsula characterized on the basis of multichannel seismic long range side scan sonar and swath bathymetry data include from northwest to southeast 1 the oceanic crust of the former phoenix plate flexed down toward the margin and affected by normal faulting in the upper crust 2 a narrow trench with a horizontally layered sediment fill which onlaps the thin sedimentary cover on the oceanic crust and shows incipient deformation near its landward edge 3 an accretionary prism with a complex internal structure the toe of which is overthrust above the youngest trench deposits 4 a midslope forearc basin with an asymmetric synformal structure and 5 the continental shelf which includes two distinct tectonic provinces calculations of the late cenozoic convergence history at the trench indicate a rapid decrease in convergence rate after 6 7 ma from about 60 mm yr resulting from the slowing and eventual cessation of spreading on the antarctic phoenix ridge once spreading had completely ceased 3 5 2 4 ma the convergence rate at the trench equalled the rate of extension in bransfield strait which was probably less than 10 mm yr at first but may have increased since 1 3 ma and this in turn would imply a corresponding increase in convergence rate above the basal detachment the toe of the accretionary prism is composed of a stack of thrust fault bounded wedges laterally and vertically segmented by normal faults most of the trench fill sediments are overthrust by the toe of the accretionary prism and subducted beneath it for as far as they can be traced on the seismic profiles this suggests that there may be tectonic erosion of the forearc since a large part of the trench sediment appears to be derived from erosion on the forearc slope the forearc basin records a long history of subsidence during which the depocenter migrated landward paralleling retrogradational erosion of the distal margin of the basin subduction of the hero fracture zone at approximately the same location since 10 ma may have enhanced tectonic erosion of the accretionary prism in the forearc basin a synform is coincident with a bathymetric reentrant but its axis is oblique to the trends of both the basin and the fracture zone the origin of this synform and the final phase of uplift of the blueschist facies terrane on which smith island stands are probably related to their position above the subducting fracture zone ridge the present elevation of smith island is thought to result from uplift which followed a ridge crest trench collision directly southwest of the hero fracture zone at 5 5 ma the northwestern edge of the mesozoic to early cenozoic arc batholith trending northeast southwest on the shelf is proposed to have acted as a backstop to the accretionary prism during cenozoic subduction

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

crustal model of the bransfield rift west antarctica from detailed obs refraction experiments summary the first detailed deep seismic refraction study in the bransfield strait west antarctica using sensitive obss ocean bottom seismographs was carried out successfully during the antarctic summer of 1990 1991 the experiment focused on the deep crustal structure beneath the axis of the bransfield rift seismic profile dss 20 was located exactly in the bransfield trough which is suspected to be a young rift system along the profile five obss were deployed at spacings of 50 70 km 51 shots were fired along the 310 km profile this paper gives the first presentation of the results a detailed model of the crustal structure was obtained by modelling the observed traveltimes and amplitudes using a 2 d ray tracing technique the uppermost sedimentary cover with velocities of 2 0 5 5 km s 1 reaches a depth of up to 8 km below this a complex with velocities of 6 4 6 8 km s 1 is observed the presence of a high velocity body with vp 7 3 7 7 km s 1 was detected in the 14 32 km depth range in the central part of the profile these inhomogeneities can be interpreted as a stage of back arc spreading and stretching of the continental crust coinciding with the deception bridgeman volcanic line velocities of 8 1 km s 1 characteristic of the moho are observed along the profile at a depth of 30 32 km

the behaviour of nd and pb isotopes during 2 0 ga migmatization in paragneisses of the central zone of the limpopo belt south africa and botswana we have carried out major and trace element analyses and nd pb isotope measurements on different components of two migmatite outcrops in the limpopo belt lose quarry botswana and palala south africa in order to assess the effect of migmatization during a high grade metamorphic event on sm nd systematics and nd model ages the outcrops studied are located in the mahalapye tshipise straightening zone at the south western part of the limpopo central zone close to its contact with the kaapvaal craton leucosomes of peraluminous granitic composition were formed by incongruent biotite melting in an fe rich metagreywacke lose quarry and in a metapelite palala respectively monazite u pb and garnet sm nd ages yield close to 2 0 ga for the high grade event confirming other recent work in the central zone the chemical compositions of the main mineral phases feldspar biotite and garnet indicate minimum melt formation at t 750 c and low pressure isocon plots and pb isotope data indicate that the partial melts are locally derived however lil trace element distribution between the leucosomes and paleosomes indicate the incomplete equilibration of the nascent melts with the residual feldspars and biotite in the paleosomes sm nd results on leucosomes and paleosomes of the metagreywacke migmatite show considerable scatter when calculated back to 2 0 ga the paleosomes fall in a narrow range of nd 4 and give tdm model ages between 2 45 and 2 66 ga the nd values of leucosomes are on average four units lower causing them to have apparent tdm ages of 2 7 2 92 ga on average 250 ma older than those of the paleosomes the ree th p2o5 and zr budgets of paleosomes and leucosomes indicate dominance of the lree by monazite and the hree by apatite incorporation of proportionally more monazite than apatite into the melt is inferred as the cause for the nd isotope discrepancy and hence the uncertainty of the average protolith mantle derivation age in contrast pb pb results define an apparent age of 2025 29 ma reflecting nearly full pb isotope exchange in the case of the palala metapelitic migmatite the nd values at 2 0 ga of all but two migmatitic constituents cluster between 7 and 9 5 pointing out extensive nd isotope equilibration during migmatization whereas the pb isotopes yield an errorchron date of 2311 65 ma indicating incomplete pb isotope exchange here garnet dominates the hree distribution whereas the lree are controlled by feldspar and monazite considering that nd isotope equilibration was nearly reached during migmatization nd model ages on the whole rocks represent a good approximation of those of the original protolith 3 0 3 2 ga in the regional context the pb isotope data from the lose quarry locality combined with the nd model age uncertain but within the range of 2 5 2 9 are reminiscent of the northern marginal zone of the limpopo belt and the southern zimbabwe craton whereas the nd and pb isotope data from the palala locality are typical for the limpopo southern marginal zone and the northern kaapvaal craton

the tectonic expression slab pull at continental convergent boundaries examination of five thrust belt systems developed at continental subduction boundaries suggests that they comprise two distinct groups that display pronounced and systematic differences in structural style topographic elevation denudation metamorphism postcollisional convergence and foredeep basin geometry and facies the distinctive geological features developed within each thrust belt group appear to be causally linked to the relative rates of subduction and convergence via the magnitude of horizontal compressional stress transmitted across the subduction boundary at subduction boundaries where the rate of overall plate convergence is less than the rate of subduction termed here retreating subduction boundaries the transmission of horizontal compressive stress across the plate boundary is small and regional deformation of the overriding plate is by horizontal extension the tectonic expression of these retreating subduction boundaries includes topographically low mountains little erosion or denudation low grade to no metamorphism little to no involvement of crystalline basement in shortening little to no postcollisional convergence anomalously deep foredeep basins and a protracted history of flysch deposition within the adjacent foredeep basin analysis of deflection and gravity data across three retreating subduction boundaries apennine carpathian and hellenic systems shows that subduction is driven by gravitational forces acting on dense subducted slabs at depths between about 40 and 80 km carpathians 50 and 150 km apennines and 50 and 250 km hellenides the total mass anomalies represented by the slabs are approximately 3 1012 6 1012 and 12 1012 n m respectively the slabs are partially supported by flexural stresses transmitted through the subducted lithosphere to the foreland and partially supported by dynamic viscous stresses in the asthenosphere at subduction boundaries where the rate of overall plate convergence is greater than the rate of subduction termed here advancing subduction boundaries the transmission of horizontal compressive stress across the plate boundary is large and regional deformation of the overriding plate is by horizontal shortening the tectonic expression of these advancing subduction boundaries includes topographically high mountains antithetic thrust belts large amounts of erosion and denudation exposure of high grade metamorphic rocks at the surface extensive deformation of crystalline basement to midcrustal depths protracted postcollisional convergence tens of millions of years and a protracted history of molasse deposition within the adjacent foredeep basins analysis of gravity and deflection data across two advancing subduction boundaries developed within the continental lithosphere western to eastern and southern alps and himalayas shows that the thrust sheets have been translated for great distances over the foreland lithosphere relative to the point at which the subduction forces are applied thus obscuring any flexural and gravity signals from the subducted slab however it appears that far field stresses presumably related to global plate motions drive most of the convergent motion across these subduction boundaries the concept that orogenic belts formed above retreating subduction boundaries have recognizable tectonic signatures that differ from those of orogenic belts formed above advancing subduction boundaries suggests that it may be possible to interpret the plate boundary settings in which ancient orogenic belts evolved appendix b is available with entire article on microfiche order from the american geophysical union 2000 floridaavenue n w washington d c 20009 document t92 004 2 50 payment must accompany order

dehydration melting phenomena in leptynitic gneisses and the generation of leucogranites a case study from the kerala khondalite belt southern india

origin and evolution of a migmatite the development of a stromatic migmatite exposed east and southeast of arvika western sweden is described in four stages beginning with the country rock and following evolution through three areas characterized by low medium and high amounts of leucosomes areas l m and h respectively

seismotectonics and relative plate motions in the scotia sea region source parameter inversion of p and sh waveforms and amplitudes for 20 shallow earthquakes using a singular value decomposition technique provides important constraints on the tectonics and relative plate motions of the scotia sea region the derived focal mechanisms show both thrust and strike slip faulting along the north scotia ridge and normal and strike slip faulting along the south scotia ridge two thrust faulting mechanisms south of tierra del fuego indicate diffuse convergence of the antarctic plate along the western margin of the scotia plate earthquakes are consistent with active subduction and back arc spreading along the south shetland trench and bransfield strait the depths of two earthquakes 35 km and 55 km and one shallow thrust faulting event suggest continued subduction along the south shetland trench but the underthrusting is largely aseismic due to the young plate age and slow subduction rate the magnitude and depth of normal faulting earthquakes along bransfield strait and the south scotia ridge are suggestive of diffuse extension rather than typical organized mid ocean spreading which is generally associated with smaller and shallower earthquakes eight strike slip and thrust faulting events provide well constrained slip vectors showing scotia plate motion relative to the surrounding larger plates using a four plate model scotia sc south sandwich ss antarctica an and south america sa and a fixed sa an euler vector we inverted these data together with published centroid moment tensor slip vectors along the south sandwich trench and published marine magnetic data from the ss back arc spreading center to derive the first quantitative plate motion model for the scotia region the sc sa euler pole is located in the south atlantic and shows large latitudinal uncertainties the sc an pole is better constrained and located in the weddell sea these results predict left lateral strike slip motion with a component of compression along the north scotia ridge left lateral strike slip with a component of extension along the south scotia ridge and east west compression in drake passage although no rate data have been published connecting the scotia plate with the global plate system euler vector magnitudes are obtained through closure the results though poorly constrained suggest relative motion rates of 0 5 cm yr along the north scotia ridge and 1 0 cm yr along the south scotia ridge and in drake passage

transtensional tectonics along the south scotia ridge antarctica multichannel seismic reflection profiles recorded aboard b o hesperides during the austral summer of 1991 1992 were used to identify the tectonic style of the south scotia ridge along the scotia antarctica plate boundary the ridge is composed of continental crustal fragments transported eastward from the south america antarctic isthmus 28 to 6 ma during the opening of drake passage it is made up of two highs north and south branches of the south scotia ridge separated by a central depression that contains four narrow deeps fragmentation of the ridge during and since its transport to its present position is due to transtensional sinistral motion along the scotia antarctic plate boundary this fragmentation of the ridge appears to have been in two phases during an early phase of transtension which probably took place in the oligocene a half graben fronted by a high along its northern edge was formed along the southern flank of the ridge concurrent with this transtension episode an extensive sediment prism was deposited north of the ridge the second tectonic episode during which the present plate boundary was established in its current location along the central depression may have begun about 4 ma transtensional tectonics along the sinistral transform fault plate boundary during this phase led to the creation of the present tectonic geomorphology of the south scotia ridge extension during this phase is characterized by listric faults dipping both north and south which root into a northerly dipping basal detachment surface motion along these faults caused the block above the detachment surface upper block north branch of the south scotia ridge to undergo some degree of tilting differences in morphology along the north branch suggest that this block tilting varies along strike being the least on its eastern and western ends and maximum in the center this suggests that continuity of the listric faults parallel to the plate boundary is disrupted by transverse structures structures which may have been produced by bends along the plate boundary as the blocks were transported laterally along the transtensional sinistral plate boundary they experienced some degree of counterclockwise rotation along the right lateral transverse structures creating local zones of compression and extension at their corners it is this compression created by the rotating blocks which led to the deformation of the sediments north of the eastern end of the north branch of the south scotia ridge and within the central depression itself

volcanic and tectonic evolution of the northern antarctic peninsula late cenozoic to recent abstract field investigation together with a number of geochemical petrographical analyses as well as absolute k ar age determinations and geophysical data allow the recognition of an evolutionary sequence of geodynamic events which have affected the northern region of antarctic peninsula and the adjacent islands a significant volcanic calc alkaline belt which developed on the northwestern margin of the antarctic peninsula during the cretaceous to middle tertiary is indicative of active subduction of the antarctic plate in that area this activity decreases during the lower miocene giving way to an expansive phase represented by the bransfield rift these extensional processes are dominant during the pliocene creating a rift system in southeastern bransfield towards larsen both the bransfield and larsen systems comprise one fan like rift system associated with the prince gustav rift and the scotia arc micro plate ejection of abundant pyroclastic material generated a large plateau of palagonite hyaloclastites of basaltic alkaline composition during the pleistocene recent the extensional activity continued as evidenced by the active volcanic fractures represented in bransfield by the deception penguin and bridgeman volcanic centres in the prince gustav rift by paulet islands and others and in larsen by the coley seal nunatak and argo volcanic centres the latter is characterized by basaltic olivine alkaline effusions these rifts and the continental blocks are affected by a series of fractures with a n60 70 w strike which could be directly associated with the hero fracture zone extending northwest of the south shetland islands trench

intermediate focus earthquakes under south shetland islands antarctica this study is based on data from five field surveys 1992 1996 of seismic monitoring on deception island south shetland islands antarctica in the 1994 95 and 1995 96 surveys earthquakes were recorded by a seismic array in previous surveys data were collected by a high dynamic range short period station the analysis of the events shows evidence of intermediate focus seismicity 50 h 100 km related to the underplating zone of the south shetland plate because of poor station coverage we had to use unconventional techniques to locate the events such as zero lag cross correlation method that provide information about apparent velocity and back azimuth ray tracing procedure and particle motion patterns these criteria allowed us to identify 15 earthquakes that could be considered as intermediate focus events for which some focal parameters were estimated we conclude that the subduction zone between the drake plate and south shetland microplate has a moderate level of intermediate depth microseismicity

the occurrence on carboniferous coal of anthropogenic origin in the contemporaneous vistula river sediments near cracow southern poland carboniferous pit coal is the most important anthropogenic component of the contemporaneous vistula river gravels and sands with individual fractions containing from 10 to 98 per cent coal fragments coal concentration is connected with differences in the bulk density between coal and other gravel components the lower sediments date from the first half of the nineteenth century when the coal began to appear in large quantities in the vistula channel the presence of coal is also an indicator of the depth of channel sediment reworking during floods in the nineteenth and twentieth centuries

subsolidus migmatization in high grade meta tuffs kurkijarvi southwest finland the phenomenon of migmatization was studied in precambrian metavolcanic gneisses of calc alkaline chemistry outcropping along a prograde amphibolite granulite facies transition in the west uusimaa complex of sw finland this paper discusses one of the studied gneiss levels a garnet bearing qtz plag ksp bio gneiss which was observed to transsect the metamorphic isograd pattern at almost right angle the gneiss was studied for structures whole rock chemistry major trace and ree mineral content microtextures plagioclase anorthite content and fluid inclusions data concerning the latter four subjects are presented migmatization proved to 1 have occurred parallel to compositional banding of the rocks 2 have produced identical leucosome melanosome mesosome mineral parageneses 3 have initiated feldspar garnet poikiloblasthesis and occasionally biotite porphyroblasthesis in leucosome and biotite garnet poikiloblasthesis in melanosome 4 have caused entrapment of unstrained quartz blebs carrying isolated primary two phase pure h2o fluid inclusions of unique filling degree range in the above mentioned feldspar and garnet poikiloblasts 5 have occurred post d1 pre d2 synchronous to amphibolitefacies metamorphism in the subsolidus regime 6 have been affected by d2 in the way of localized mylonitization of the melanosome and quartz migration exudation from adjacent mesosome into leucosome and 7 have had some control by the biotite content of the original compositionally banded rock initial leucosome formation appears to have been controlled by the pre leucosome biotite content the recalculated modal biotite content of the leucosome melanosome combination conspicuously is in the range of 5 20 vol of biotite final extent of the leucosome shows on its turn a marked correlation with mesosome modal biotite content because leucosomes occur carrying a recalculated modal biotite content equalling adjacent mesosome biotite content a second factor is held responsible for the onset of migmatization in the buried and sheared rock deficient water balance migmatization initiated at pt conditions fit for feldspar recrystallization and almandine formation was induced during prograde metamorphism to cancel an established zonation in water pressure or water content parallel to compositional banding zones of low ph2o or wt h2o thereby were converted into leucosomes while zones of higher ph2o or wt h2o remained unaffected and became mesosome that xh2o did not vary at the onset of migmatization is recorded in the isolated pure h2o fluid inclusions contained in the quartz blebs enclosed in the studied leucosome and melanosome poikiloblasts restore of water balance either by internally controlled factors or externally introduced ones halted migmatization and its obliteration of compositional banding

melting and diffusion processes in closed system migmatization estimated variations in mineral concentrations across leucosomes suggest that leucosomes are generated during anatexis by a diffusive exchange between the leucosome and the mesosome and not by the migration of melt from the mesosome however the presence of melt is a precondition for the diffusive exchange to take place initially a crack is formed due to shear stress the formation of a crack allows a diffusive exchange to take place through the melt which causes melting of minerals situated near the crack the diffusive exchange of material is less efficient in the mesosome where the melt is isolated at grain corners and edges the microcline enrichment of some granitic leucosomes is thought to be due to the diffusive depletion of the mesosome caused by growth of alkali feldspar during the consolidation of the migmatite in general it seems unnecessary to invoke concentrations of water in the leucosome or the intrusion of external fluids or magmas for migmatite formation

importance of melt fraction and source rock composition in crustal genesis the example of two granitic suites of northern portugal in the montalegre area northern portugal two varieties of syn to late tectonic hercynian aluminous granitoids are distinguished 1 1 muscovite rich two mica granites forming many successively emplaced bodies with little chemical variation the chemical characteristics of these units suggest partial melting of various crustal rocks under distinct p t and water saturation conditions there has been no homogenization between the different melt batches the rocks contain very few restitic minerals segregation processes may have operated either in the source rock or in an early magmatic stage 2 2 a cordierite biotite and muscovite bearing granitic body its mineralogical composition is extremely heterogeneous and it contains many xenoliths the genesis of this granite results essentially from partial melting of peraluminous orthogneisses the melt fraction was higher than the rheological critical melt percentage rcmp during anatexis the rigid framework of the source rock broke down leading to migration of both melt and residual minerals towards upper crustal levels the heterogeneity of the resulting granite is mostly related to an incomplete separation of residual crystals and melt the existence of biotite and cordierite rich samples is the result of accumulation of residual minerals during the magma emplacement this study emphasizes the role of the source rock composition which controls the degree of partial melting at given p t and water saturation conditions during crustal melting the strong geochemical differences between the two kinds of granitoids are related to the near absence of residual minerals in the muscovite rich granites and to the presence of residual minerals in the cordierite bearing granite it is suggested that the muscovite rich granites are mostly low melt fraction rcmp the presence of many residual crystals alkali feldspars and plagioclases as well as dark minerals in the cordierite granite implies a high viscosity of the melts and therefore water undersaturated melting conditions

inclusion host relations involving accessory minerals in high grade metamorphic and anatectic rocks in general accessory minerals are expected to participate in partial fusion of their host rocks to a degree determined simply by their solubilities in the melt the possibility must be recognized however that a given accessory grain may be physically isolated from the melt by inclusion within a residual major mineral because of the importance of accessory minerals to crustal rock trace element and isotope geochemistry and because of their common existence as inclusions in major phases we undertook an evaluation of the factors that affect inclusion formation during ultrametamorphism three approaches are taken 1 a review of interfacial energy considerations is used to show that the free energy of the system is lowered by location of accessory minerals at major phase grain perimeters and that the magnitude of this effect is proportional to the square of the accessory grain radius 2 annealing and partial melting experiments 1000 c 10 and 15 kbar on rock analogs are described and the results are shown to confirm the predicted tendency of accessory minerals to occupy grain boundaries and 3 the results of a study of accessory phase rutile and zircon distribution in a migmatite from the tibetan slab are reported again in confirmation of the prediction that accessories tend to be situated at majorphase grain perimeters the latter two aspects of the study reveal that although included accessory grains are common their generally small size results in only a minor contribution to the bulk rock budget of accessory mineral components most of the mass of these components is contained within populations of generally larger accessory grains located at major phase grain boundaries accordingly the assumption that accessory minerals are involved in crustal melting is generally valid

caldera unrest detected with seawater temperature anomalies at deception island antarctic peninsula increased thermal activity was detected to coincide with the onset of volcano inflation in the seawater filled caldera at deception island this thermal activity was manifested in pulses of high water temperature that coincided with ocean tide cycles the seawater temperature anomalies were detected by a thermometric sensor attached to the tide gauge bottom pressure sensor this was installed where the seawater circulation and the locations of known thermal anomalies fumaroles and thermal springs together favor the detection of water warmed within the caldera detection of the increased thermal activity was also possible because sea ice which covers the entire caldera during the austral winter months insulates the water and thus reduces temperature exchange between seawater and atmosphere in these conditions the water temperature data has been shown to provide significant information about deception volcano activity the detected seawater temperature increase also observed in soil temperature readings suggests rapid and near simultaneous increase in geothermal activity with onset of caldera inflation and an increased number of seismic events observed in the following austral summer

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

residence and redistribution of ree y zr th and u during granulite facies metamorphism behaviour of accessory and major phases in peraluminous granulites of central spain accessory minerals are thought to play a key role in controlling the behaviour of certain trace elements such as ree y zr th and u during crustal melting processes under high grade metamorphic conditions although this is probably the case at middle crustal levels when a comparison is made with granulite facies lower crustal levels differences are seen in trace element behaviour between accessory minerals and some major phases such a comparison can be made in central spain where two granulite facies terranes have equilibrated under slightly different metamorphic conditions and where lower crustal xenoliths are also found differences in texture and chemical composition between accessory phases found in leucosomes and leucogranites and those of melanosomes and protholiths indicate that most of the accessory minerals in melt rich migmatites are newly crystallized this implies that an important redistribution of trace elements occurs during the early stages of granulite facies metamorphism in addition the textural position of the accessory minerals with respect to the major phases is crucial in the redistribution of trace elements when melting proceeds via biotite dehydration melting reactions in granulitic xenoliths from lower crustal levels the situation seems to be different as major minerals show high concentration of certain trace elements the distribution of which is thus controlled by reactions involving final consumption of al ti phlogopite a marked redistribution of hree y zr between garnet and xenotime where present and zircon but also of lree between feldspars k feldspar and plagioclase and monazite is suggested

the role of water mixing processes and metamorphic fabric in the genesis of the baume migmatites ardeche france the baume migmatites arose from melting of a high grade metamorphic sequence comprising quartzofeld spathic and biotite plagioclase gneisses geochemical data and ree modelling suggest that melting occurred under open system conditions and was caused mainly either by the infiltration along shears of an alkali rich k rb aqueous fluid phase or by the injection along foliation planes of granitic magma some leucosomes result from mixing between in situ partial melts and externally derived granitic materials melting appears to be almost independent of the paleosome composition but rather is dependent on the amount of introduced material itself controlled by the metamorphic fabrics of parent gneisses and or tectonic structures

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

behaviour of boron in the generation of an anatectic complex the pen a negra complex central spain abstract the pe na negra anatectic complex central spain was formed from a pelitic protolith low in boron 3 80 ppm by dehydration reactions melting and fluid loss which expelled much of the boron residual b is present in sillimanite and muscovite migmatite melanosomes and restites sillimanite biotite ilmenite are relatively richer in b mean 29 ppm while the leucosomes are poorer 13 ppm late shears fractures and zones of retrograde alteration have permitted some reintroduction of b experiments demonstrated that b cannot be removed from these rocks by heating or by leaching indicating that very little is present in superficial adsorption and that the element occurs in mineral structures

shear zone hosted migmatites eastern india the role of dynamic melting in the generation of ree depleted felsic melts and implications for disequilibrium melting in the ranmal migmatite complex non anatectic foliated granite protoliths can be traced to polyphase migmatites structural microtextural relations and thermobarometry indicate that syn deformational segregation crystallization of in situ stromatic and diatexite leucosomes occurred at 8008c and 8 kbar the protolith the neosome and the mesosome comprise quartz k feldspar plagioclase hornblende biotite sphene apatite zircon and ilmenite but the modal mineralogy differs widely the protolith composition is straddled by element abundances in the leucosome and the mesosome the leucosomes are characterized by lower cao feo mgo mg number tio2 p2o5 rb zr and total rare earth elements ree and higher sio2 k2o ba and sr than the protolith and the mesosome whereas na2o and al2o3 abundances are similar the protolith and the mesosome have negative eu anomalies but protolith normalized abundances of ree depleted leucosomes show positive eu anomalies the congruent melting reaction for leucosome production is inferred to be 0 325 quartz 0 288 k feldspar 0 32 plagioclase 0 05 biotite 0 014 hornblende 0 001 apatite 0 001 zircon 0 002 sphene1 4melt based on the reaction large ion lithophile element ree and zr abundances in model melts computed using dynamic melting approached the measured element abundances in leucosomes for40 5 mass fraction of unsegregated melts within the mesosome disequilibrium accommodated dynamic melting and equilibrium crystallization of melts led to uniform plagioclase composition in migmatites and ree depletion in leucosome

ree fractionation and nd isotope disequilibrium during crustal anatexis constraints from himalayan leucogranites abstract the processes controlling rare earth element ree behaviour during crustal anatexis have profound implications not only for ree abundances of crustally derived granites but also for their model nd ages for typical protoliths of anatectic granites such as are exposed in metapelites from the high himalayan crystalline series hhcs a high proportion 90 of bulk rock lree and mree are sited in the accessory phases apatite and monazite and of hree in garnet and apatite mass balance constraints indicate that observed ree abundances of himalayan leucogranites cannot be generated from ree deficient reactant phases in their metasedimentary protoliths muscovite biotite plagioclase and quartz but must result from the dissolution of accessory phases ree and th abundances in anatectic melts are modelled by combining empirical expressions for monazite apatite and zircon solubilities as a function of temperature melt composition and melt water content with analysed accessory phase ree abundances this approach which allows the contribution to the melt from each accessory phase to be identified successfully predicts himalayan leucogranite trace element abundances the lree and th concentrations of a metapelite derived melt are buffered by monazite stability although apatite dissolution contributes up to 10 of lree to the melt together with 50 of the mree and 90 of the hree zircon contributes a negligible proportion of ree to the melt during anatexis dissolution of monazite and apatite in the melt results in 1 increasing ree abundances and lree hree ratios with increasing temperature of melting 2 a negative eu anomaly in the melt 3 high sm nd ratios in the melt relative to its metasedimentary protolith and 4 nd isotope disequilibrium between the melt and its protolith all of these features are observed in the ree geochemistry of himalayan leucogranites the combined effects of sm nd fractionation and nd isotope disequilibrium during anatexis imply that model nd ages calculated for anatectic granites derived from apatite and monazite bearing protoliths should be treated with considerable caution

rare earth element distributions among minerals in a granodiorite and their petrogenetic implications a study of the distribution of lanthanide rare earths in a granodiorite from the eastern peninsular ranges batholith southern california reveals that a large fraction of the ree in this rock resides in the accessory phases sphene and allanite the minerals plagioclase alkali feldspar biotite epidote and apatite each contribute approximately 1 or less of each ree to the whole rock with the exception of eu for which plagioclase contributes 7 sphene and allanite together contain 80 to 95 of each ree each of these phases is zoned in ree concentration with substantial decreases from core to margin textural observations argue for relatively early saturation and precipitation of sphene and allanite in the magma ree zoning trends in sphene and allanite and unexpectedly low ree concentrations in largely later crystallizing minerals such as feldspar indicate that the precipitation of sphene and allanite significantly reduced ree concentrations in residual melts these results illustrate the potential that sphene and allanite have for controlling the behavior of ree in granitic magmas available information collectively suggest that the sampled granodiorite existed as a complete melt that the ree contained in the assemblage of phases were derived by direct crystallization from the melt and that the melt behaved essentially as a closed system once crystallization of the phases now present began close correspondences between the major and trace element chemistries of the granodiorite and phenocryst poor lavas from similar tectonic settings support these conclusions the ree pattern of the granodiorite melt appears to have originated at depth and is characteristic of its source regions and derivation mechanism the high liquidus temperature of a granodiorite melt 1000 c indicates the importance of mantle derived components within the sources of batholithic magmas in the peninsular ranges

geochemical investigations of the origin of the manaslu leucogranite himalaya nepal the himalayan orogeny is characterized by an intraplate subduction zone the thrusting of the tibetan slab over the foreland is probably responsible for the origin of the manaslu type leucogranites this geochemical study tries to examine the crustal origin of the granite to verify whether the paragneisses of the tibetan slab are the most probable parent material and to determine the subsequent petrogenetic processes the rb sr systematics suggest that 1 1 the scatter of data points in the 87sr86srvs 87rb86sr diagram may be due to the heterogeneity of the initial 87sr86sr ratios 2 2 the initial isotopic ratios are very high 0 730 0 760 and are in agreement with those of the gneisses of the tibetan slab at the time of the melting pb isotopic ratios are homogeneous they are also consistent with a crustal prehistory the ree and th contents are unexpectedly low in comparison with the much higher contents of the parent material monazite left in the residue of melting or more probably extracted in an early stage of the magmatic history may explain the observed ree and th abundances alternatively field observations show that the emplacement of the granite has been accompanied by a very strong hydrothermal activity in consequence the low ree and th contents of the granite could also be due to the loss by solution during prolonged fluid circulation throughout the granitic melt and the sr and pb isotope distribution to the effect of late to post magmatic fluids the isotopic character of which was determined by the underlying tibetan slab

fluid inclusion geochemistry and ore genesis of the longmendian mo deposit in the east qinling orogen implication for migmatitic hydrothermal mo mineralization abstract the 1 85 ga longmendian mo deposit in east qinling is the oldest mo system recognized in china it is unique for three reasons 1 quartzofeldspathic orebodies are away from any intrusion or fault 2 it is closely associated with migmatitic rocks and 3 it has remarkably high re content in molybdenite 504 to 1660 ppm the origin of the deposit is poorly constrained in the longmendian deposit strong mo mineralization is always associated with hydrothermally altered migmatitic amphibolites to probe into ore genesis detailed fluid inclusion studies are carried out on both mineralized migmatitic amphibolites and ore barren rocks four compositional types of fluid inclusions are observed including co 2 ch 4 pc type co 2 h 2 o c type daughter mineral bearing s type and h 2 o nacl w type quartz in mesosome and melanosome of mineralized migmatitic amphibolites contains all of the four types of inclusions measurements of immiscible inclusion assemblages in the ores constrain the mo mineralization temperatures and pressures to be 225 390 c and 114 265 mpa respectively primary fluid inclusions in barren migmatitic gneisses are dominated by s type and minor of c and pc types which are identical to those in the leucosome of migmatitic amphibolites these inclusions yield lower homogenization temperatures than those in the mineralized mesosome and melanosome of migmatitic amphibolites suggesting that the ore causative injected melts should have higher temperatures and originated from depths such features of the ore forming fluids indicate that the longmendian deposit was a migmatitic hydrothermal system caused by high temperature melt injection this interpretation can also be supported by the observations below 1 tourmaline is abundant in melanosome but absent in mesosome 2 the consistent quartzofeldspathic composition of leucosome is independent of mesosome 3 leucosome in migmatitic amphibolite crosscuts each other 4 halite bearing fluid inclusions are prevalent in studied samples which is the feature of granitic rocks instead of the scenarios of in situ migmatites and 5 the trapping temperatures of fluid inclusions in mineralized migmatitic amphibolite are much lower than those required for partial melting but higher than those obtained from barren migmatitic rocks

mass balance and mass transfer in migmatites from the colorado front range metasomatic exchanges between the infiltrating fluids and wall rocks most likely initiated the formation of nine leucosomes in two large samples of the precambrian biotite quartz feldspar migmatites from the east central colorado front range the leucosomes 2 to 20 mm thick and enclosed in mafic salvages 1 to 10 mm thick are granitic to tonalitic in composition mass balance calculations suggest that each leucosome formed by local introduction of mass the net gains and losses calculated assuming that all such gains and losses were contained within the leucosome show that in general neither the gains nor the losses fit the composition of any silicate melt it is more likely that the components were transported in a fluid recalculated on constant al basis the most significant relative mass transfers were gain of k and losses of na and mg by the rocks the metasomatic reactions calculated are those for replacement of plagioclase by microcline and breakdown of biotite the reactions must have been the cause of incipient migmatization a mafic selvage formed from the paleosome by the loss of material whose composition is tonalitic to granodioritic varying systematically with the paleosome composition

migmatite formation at subsolidus conditions an alternative to anatexis this contribution discusses the formation of stromatic high grade migmatites volume considerations require that most of the leucosome material is not added from outside the system a segregation mechanism is necessary except in those cases where the protolith of the migmatite already had a banded structure although partial melting is most often advocated to provide the segregation mechanism several arguments can be raised against high degrees of melting mineral compositions and even zoning patterns are similar in both mesosomes and leucosomes sufficient degrees of melting at reasonable temperatures require more than the available amounts of water the leucosomes do not always approximate to a minimum melt composition high degrees of melting cannot occur without an appreciable volume increase etc diffusion works as a segregation mechanism at low temperatures as diffusion rates increase exponentially with temperature diffusion must become still more important as a segregation mechanism at high temperatures a model is suggested based on the diffusion of components in response to the gradient x where 1 3 3i 1 i is the mean pressure in homogeneously strained rocks 3 is larger in rock parts rich in incompetent phases than in rock parts depleted in incompetent phases accordingly mechanically competent but chemically incompetent high volume phases like quartz and feldspars stressed in micadominated parts of a rock high migrate to parts of the rock that are depleted in mica low it is suggested that hornblende occurring in many leucosomes may be premigmatitic or early syn migmatitic and due to its mechanical competency it initiates the segregation diffusion occurs along grain boundaries and is enhanced by small amounts of intergranular fluid at the best semiquantitative estimates of diffusion rates and distances indicate that the process should work over geological times

experimental determination of the solubility of ce monazite in sio2 al2o3 k2o na2o melts at 800 c 2 kbar under h2o saturated conditions the solubility of cerium monazite cepo4 in sio2 al2o3 k2o na2o melts was determined at 800 c 2 kbar under water saturated conditions dissolution experiments using synthetic ce monazite crystals and crystallization experiments using ce and po4 bearing glasses yield results in good agreement microprobe analyses of glasses using a sensitive method detection limit to 130 ppm show that the solubility of monazite is dependent on the melt composition and is especially low for peraluminous compositions application of these data to various geologic occurrences is in good agreement with petrographic and geochemical observations and shows that monazite fractionation is probably a very common phenomenon in two mica granites this provides a good explanation for the light ree and th depletion commonly observed in such rocks

is himalayan leucogranite a product by in situ partial melting of the greater himalayan crystalline a comparative study of leucosome and leucogranite from nyalam southern tibet abstract widespread leucogranites in the himalayan orogenic belt are thought to have originated by in situ partial melting of the greater himalayan crystalline ghc when it underwent high grade metamorphism during cenozoic orogenesis therefore the leucogranites and associated migmatites can be used to constrain the exhumation history of the ghc however the petrogenetic relationship between the ghc leucogranites and migmatites is not well constrained as such we carried out a detailed petrographic mineralogical geochronological and geochemical study of leucosomes and leucogranites from the nyalam region in southern tibet monazite u th pb dating indicates that anatexis of the ghc occurred during the late eocene and miocene 40 14 ma whereas leucogranite emplacement occurred from 27 to 14 ma there are marked differences between the leucosomes in migmatites and leucogranites in terms of field geology mineralogy and geochemistry suggesting different origins the leucosomes occur mainly as pockets or are interlayered with melanosomes in stromatic metatexites the leucosomes contain oligoclase and fe rich biotite and have whole rock compositions with high k2o contents 4 8 7 4 wt and k2o na2o ratios 1 35 2 97 positive eu anomalies eu eu 0 93 2 61 and low rare metal li be cs sn and ta contents these features are consistent with an origin by muscovite dehydration melting of the ghc however the leucogranites intrude the ghc and occur as small plutons along the south tibetan detachment system in contrast to the leucosomes the plagioclase and biotite in the leucogranites are albite and siderophyllite respectively the leucogranites have relatively low k2o 4 3 4 7 wt contents and k2o na2o ratios 1 04 1 24 high rare metal contents and marked negative eu anomalies eu eu 0 47 0 70 indicating an origin by extensive fractional crystallization we propose that the leucogranites were magmas produced in the deeper ghc during peak metamorphism and subsequent extensive fractional crystallization occurred during long distance upward migration of magma along the south tibetan detachment system stds during exhumation of the ghc during these processes the high grade metamorphic rocks of the ghc were partially melted resulting in the formation of leucosomes within migmatites therefore the leucosomes and leucogranites have different origins and should be considered separately in studies of himalayan orogenesis