submarine metamorphism of gabbros from the mid cayman rise an oxygen isotopic study abstract oxygen isotopic compositions of minerals in 22 samples of submarine gabbros were determined the gabbros were collected using the submersible alvin from the 700 m vertical section of the rift valley wall of the mid cayman spreading center our study indicates that in the mid cayman rise seawater barely reached the bottom of the plutonic layer abundant seawater penetration water rock mass ratio 1 was limited to the upper part of the plutonic layer from the observed oxygen isotopic compositions of coexisting minerals and from the experimental and empirical determinations of equilibrium fractionation of oxygen isotopes for mineral water and mineral mineral pairs we show the following 1 pyroxene and olivine did not exchange oxygen with seawater 2 plagioclase is in isotopic disequilibrium with pyroxene 3 the rate of oxygen exchange in plagioclase was not slowed by the absence of cation exchange 4 plagioclase and amphibole have exchanged oxygen with seawater or isotopically modified seawater 18o 3 and 5 amphibole has exchanged or acquired during formation hydrogen from seawater at 380 c t 600 c the decrease in extent of isotopic exchange of plagioclase and the decrease in amphibole abundance with depth indicate that seawater flux decreased rapidly with depth water rock mass ratio falling from 1 7 to 0 2 over a 300 m interval

rheological evolution of the ocean crust a microstructural view rheological studies of the oceanic lithosphere have used direct observations of rocks and inferences based on thermal mechanical and experimental models modeling studies help to constrain an average crust and mantle rheology and the deep seated processes that control regional scale strength and stress field variations the smoothing of fine scale variations in such models can obscure many of the geological processes that influence strain localization and deformation partitioning in the ocean crust examination of deformation mechanisms and histories in ocean crust rocks provides a complementary approach to modeling examples of structures and deformation histories in diabases from deep sea drilling project ocean drilling program site 504b the hayes and atlantis fracture zones mid atlantic ridge and the troodos ophiolite cyprus are presented in conjunction with a synthesis of microstructural studies of the ocean crust over the last 25 years a survey of brittle quasi plastic and synmagmatic viscous deformation is used to demonstrate the influence of primary compositional and textural characteristics and variable magmatic and hydrothermal histories on deformation mechanisms and strain localization in the ocean crust geological evidence indicates that hydrothermal fluids strongly influence the nature of deformation and that effective stresses may be low due to fluid overpressures melt distribution will strongly influence strain localization at the base of the crust and synkinematic hydration during crystal plastic deformation plays a key role in the relative strengths of polyphase oceanic lithologies a schematic distribution of failure mechanisms in the ocean crust is used to discuss the controls on variations in lateral and vertical strength profiles and their possible relation to spreading rates although microstructural studies of ocean crust are still in their infancy they provide valuable constraints for rheological models and further insights to explain the distribution of seismicity at spreading centers and acoustic signatures

a suggestion for the origin of mesoscopic fabric in accretionary melange based on features observed in the chrystalls beach complex south island new zealand the chrystalls beach complex a probable accretionary melange of triassic age cropping out in south island new zealand contains a distinctive set of mesoscopic structures all of which contribute to the typically chaotic appearance of the melange these structures are 1 soft sediment bedding pull aparts 2 weakly developed slaty cleavage 3 folds in dismembered bedding with slaty cleavage as the axial surface foliation and 4 quartz fiber lineated shear surfaces analysis of overprinting relations indicates that these structures formed during a single progressive deformation rather than in several discrete events as such they record a change in response to incremental strain from initial ductile behavior toward one dominated by brittle failure it is suggested that this variation resulted from progressive dewatering during shallow level accretion at the base of an inner trench slope

mineralogy of a scapolite bearing rock from rajasthan northwest peninsular india abstract electron microprobe analyses of coexisting phases from a scapolite garnet epidote calcite plagioclase hornblende pyroxene sphere hematite magnetite rock of the aravalli group early precambrian at karera district bhilwara rajasthan reveal that chlorine is an important constituent of both scapolite me 71 3 and hornblende making the latter a dashkesanite variety the amphibole also contains an unusually high k 2 o 3 7 wt and is a chlor potassium hastingsite the epidote contains 41 pistacite and shows complete substitution of al by fe 3 in the al site and to some extent also in aio and or al oh sites the garnet is also rich in ferric iron and has a mol composition pyr 23 alm 8 5 gro 13 and 54 5 sp 1 0 the pyroxene is dominantly a hedenbergite the phase relations and textural as well as geological criteria exclude metasomatic processes and favour equilibrium recrystallization of the scapolite bearing assemblage the chlorine presumably derived from an evaporite component of the aravalli metasediments the geothermometry based on the fractionation of na and ca between scapolite eqan 43 3 and plagioclase an 85 90 yields metamorphic temperatures of around 700 c for the scapolite bearing rock which are in agreement with those obtained by other mineral equilibria in the associated pelitic assemblages

controls on hydrothermal alteration in a section of fast spreading oceanic crust abstract young oceanic crustal sections that formed at the fast spreading east pacific rise epr were mapped by submersible along steep fault scarps that bound the hess deep rift valley excellent exposures allowed the documentation of the spatial distribution of alteration within the lava sequence sheeted dike complex and uppermost plutonic sequence alteration assemblages record the initiation and subsequent cooling of an axial hydrothermal system s that was periodically perturbed by the injection of sheeted dikes mineralogical and geochemical data indicate that temperatures increased across the lava dike transition 50 180 c and that the grade of metamorphism was heterogeneous within the sheeted dike complex in that temperatures varied laterally from 100 300 c to 600 800 c within the upper dikes and were 400 450 c within the lower sheeted dikes during vigorous hydrothermal circulation at the epr the base of the hydrothermal system was located within or at the top of a sequence of gabbros that is interpreted to be the root zone of the sheeted dike complex the base of the hydrothermal cell did not penetrate into the subjacent plutonic sequence until the section of crust had migrated off axis and the magma lens had solidified the vigor of the hydrothermal system would have waned by this time and hydrothermal fluids migrated into the lower crust along broadly distributed microfracture networks at temperatures between 450 and 650 c although locally temperatures may have reached 800 c the highest temperature alteration assemblages within the sheeted dikes formed in response to the injection of a dike or swarm of dikes that initiated a short lived hydrothermal system the sheeted dikes and plutonic rocks experienced very little retrograde alteration and there is no evidence for a discrete off axis hydrothermal event s incipient alteration conditions in crustal sections that formed at mid ocean ridges with slower rates of spreading than the epr are similar to hess deep although there are differences in the distribution and mineralogical expression of incipient alteration in plutonic sequences the evolution of alteration as a section of crust cools and moves off axis reflects its particular magmatic tectonic history and thus may vary significantly in ridges with all spreading rates

geotectonic significance of gneissic amphibolites from the vema fracture zone equatorial mid atlantic ridge a large collection of gneissic amphibolites was recovered by two close dredge hauls from the deepest part of the north facing slope of the transverse ridge forming the south wall of the vema fracture zone serpentinites and various types of gabbroic rocks ranging from undeformed slightly uralitized gabbros and norites to flaser and mylonitic gabbros were associated with the gneissic amphibolites in the shallowest of the dredge hauls petrological studies indicate that the amphibolites were derived from similar gabbroic rocks that reequilibrated under stress in the conditions of the amphibolite facies on the other hand the associated meta gabbros display sequences of secondary minerals indicating complex cooling and cataclastic histories without reaching metamorphic equilibrium we tentatively suggest that the gneissic amphibolites and associated metagabbros formed in a vertical shear zone generated in oceanic layer 3 by tectonism associated with the vema fracture zone hydrothermal circulation of seawater along the highly permeable shear zone was activated by magmatic intrusions k ar dating suggests within relatively large analytical uncertainties that the amphibolite metamorphism took place 10 m y ago i e at a time when the dredging sites were located in the vicinity of the spreading center

provenance of triassic cretaceous sandstones in the antarctic peninsula implications for terrane models during gondwana breakup abstract the antarctic peninsula is the largest of twelve micro continental fragments between southern south america east antarctica and new zealand their original positions and movement history following gondwana breakup are poorly constrained recent work on the antarctic peninsula fore arc arc back arc suggests it consists of two suspect terranes that originated elsewhere and collided with the pacific margin of gondwana in the cretaceous this study reviews provenance models for three laterally extensive kilometers thick sedimentary units in the antarctic peninsula using previous studies and new sandstone detrital modes and chemistry 1 the trinity peninsula group accretionary complex triassic was possibly derived from a glaciated continental margin and may consist of several accreted tracts the deformed quartzose view point and legoupil formations were eroded from a deeply eroded stable continental margin containing an old arc and mature quartzose sandstones with provenance ages suggesting sources in north patagonia and local antarctic peninsula basement the weakly deformed and more feldspathic hope bay formation was derived from an eroded arc possibly the early permian arc of the north patagonian massif 2 the lemay group accretionary complex middle triassic middle cretaceous is dominated by arkoses derived from an eroded continental margin of carboniferous and permian age possibly in the marie byrd land sector with several belts of volcanilithic sandstones representing episodes of arc magmatism the youngest part of the lemay group contains abundant metamorphic detritus derived from basement uplifted during mid cretaceous terrane docking 3 the latady formation in the southern antarctic peninsula has a completely different petrology comprising mature sublitharenites 190 ma of gondwanan provenance deposited in restricted rift basins younger volcanic litharenites 180 ma are dominated by ree enriched low eu eu rhyolitic material eroded from silicic calderas associated with gondwana breakup abundant dacitic to andesitic detritus in the youngest lf 160 145 ma was derived from a contemporaneous arc possibly on the thurston island block as the latady evolved into a back arc basin

controls of the altyn tagh fault on the early middle miocene sedimentation in the honggouzi area qaidam basin western china abstract the altyn tagh fault atf has important controls on the sedimentation in the qiadam basin in this study detailed sedimentological and geochemical analyses were presented to deduce the impact of the atf on deposition in the honggouzi area during the early middle miocene which is a significant period for atf strike slip movements the early middle miocene strata of the honggouzi area are deposits of a fan delta and show retrogradation by sedimentary characteristic of drilled wells the elementary geochemical analysis indicates that the early middle miocene deposits of the honggouzi area was mainly composed of felsic rocks formed in the continental island arc or active continental margin setting and derived from a segment of the altyn tagh mountains atm in addition the seismic data suggests that 1 the nearly nw se directed anticlines which were caused by the flexural deformation and uplift of the piedmont area in the northwestern qaidam basin occurred simultaneously during the early middle miocene and 2 retrogradational sedimentation was caused by greatly increased accommodation during the early middle miocene when the subsidence amount of flexural deformation was larger than the uplift amount the flexural deformation uplift of the piedmont area of the northwestern basin and uplift of the atm were all controlled by the atf strike slip movements and therefore were the ways in which the atf strike slip movements affected the early middle miocene deposition of the honggouzi area

a view of the lower crustal component of hydrothermal systems at the mid atlantic ridge an extensive suite of hydrothermally altered gabbros was recovered by alvin and dredging from the mid atlantic ridge in the vicinity of the eastern ridge transform intersection rti of the kane fracture zone mark where tectonic extension has provided a window into the lower crustal component of hydrothermal cells four alteration types are distinguished on the basis of metamorphic assemblage mineral composition and deformation textures a conceptual model is presented that places temperature fluid rock ratio relationships in conjunction with styles of deformation into a spatial and temporal framework exsolution of late stage magmatic fluids in the vicinity of the magma hydrothermal interface at temperatures 700 c marks the onset of fluid rock interaction in the mark gabbros kelley et al this issue with subsequent alteration involving seawater derived fluids being strongly influenced by deformation mechanisms in zones of ductile shear hydration was initiated at temperatures between 550 and 700 c and moderate fluid rock ratios elsewhere in the lower crust the onset of seawater penetration occurred at temperatures of 400 550 c and was facilitated by brittle fracturing early vein networks indicate very low initial fluid rock ratios and are interpreted as being related to the solidification of plutons later fractures indicate higher water rock ratios and may be related to the downward propagation of shallow fault systems throughout the crust fluid rock interaction ceased at temperatures 180 300 c subsequent cataclastic deformation associated with the unroofing and emplacement of crustal blocks in the rti massif produced moderate fluid rock ratios within localized zones hydrothermal alteration in the crustal section exposed at mark was initiated at lower temperatures and water rock interaction proceeded to lower temperatures than many other plutonic suites these differences must reflect the temporal and spatial evolution of magmatic and tectonic extension that is particular to each section of sampled crust

geochemical characterization evolution and source of a mesozoic accretionary wedge the torlesse terrane new zealand whole rock major and trace element compositions of quartzofeldspathic permian cretaceous sandstones of the torlesse terrane new zealand display progressive changes which compare well with published petrographic data chemical indices in three permian early late triassic rakaia subterrane petrofacies show small contrasts as do the modes reflecting original source compositions a late triassic rakaia petrofacies displays sharp increases in sio 2 al 2 o 3 k 2 o na 2 o th sc la sc ce sc la n y n and decreases in ti zr and v la consistent with partial cannibalistic recycling and a small influx of lithic volcanic detritus in the post collisional late jurassic early cretaceous pahau subterrane sio 2 al 2 o 3 remains high but other indices decrease to levels seen in the permian due to recycling of the earlier rakaia petrofacies and influx of 10 30 mafic intermediate volcaniclastic detritus derived from inboard volcanogenic terranes the results are confirmed by additional suites including mudstones which cover a larger area but also show that some spatial variation occurs within the terrane as a whole whole rock data and chemical index of alteration indices indicate that the torlesse was derived from a relatively unweathered source with granodioritic bulk composition comparisons with data from a proposed rakaia source in the new england and hodgkinson orogens of eastern australia suggest that the observed chemical and modal compositions cannot be simply derived from that area as presently exposed

submarine metamorphism of gabbros from the mid cayman rise petrographic and mineralogic constraints on hydrothermal processes at slow spreading ridges the coarse grained submersible collected rocks from the mid cayman rise in our study span a vertical section of about 700 m and range from amphibolite to gabbro petrographic and mineralogic studies showed that 1 some deformation is present in all rocks 2 the distribution of deformation is not uniform within a specimen and 3 alteration has taken place preferentially in the deformed zones the abundance of amphibole decreases with sample depth and argues that seawater flux into the oceanic crust decreased with depth the compositional changes in rocks indicate that partially exchanged seawater was the metamorphic fluid that supplied na k and h2o to and removed ca from the rock in all about 15 of the original rock has been transformed to amphibolite in the 700 m vertical section our study indicates that deformation of oceanic crust is necessary for providing pathways for seawater penetration which in turn is necessary for the submarine metamorphism to occur if deformation continues to take place during the spreading of seafloor alteration may also continue to occur along newly formed fractures and cracks

origin of the torlesse terrane and coeval rocks south island new zealand the carboniferous to lower cretaceous torlesse terrane and haast schist derivatives constitute the major part of the complexly deformed facies of the eastern province of new zealand strata consist mainly of quartzofeldspathic graywacke and mudstone intercalated with minor but widely distributed conglomerate and volcanics with associated chert and limestone clastic rocks were deposited largely by sediment gravity flow mechanisms in a deep marine environment also present are a few highly fossiliferous shallow marine and terrestrial deposits of limited areal extent which rest unconformably on or in fault contact with torlesse flysch several periods of deformation are recognized and melange is present on both local and regional scales metamorphism ranges from zeolite to greenschist facies the bulk of the rocks fall into five areally extensive and mutually exclusive fossil zones of the following ages permian atomodesma middle triassic early late triassic torlessia late triassic monotis and late jurassic early cretaceous contacts between major fossil zones are mainly tectonic petrographic analysis permits subdivision of torlesse sandstones into five major petrofacies that correspond in age to the five major fossil zones the sandstone petrofacies mainly arkosic together with the composition of conglomerate clasts mainly indurated torlesse rocks indicate that the source terrane was a continental volcano plutonic arc probably part of gondwanaland coupled with autocannibalistic reworking of older uplifted torlesse rocks in contrast to the quartzofeldspathic nature of the torlesse coeval sedimentary rocks of the eastern province are volcanogenic they are thought to represent related forearc basin maitai murihiku terranes and trench complex caples terrane deposits derived from a volcanic island arc brook street terrane three petrofacies are established for maitai murihiku and caples sandstones the petrofacies indicate a common evolving immature to submature volcanic island arc source for these terranes a reconstruction of new zealand s eastern and western provinces is proposed in permian and triassic times the torlesse was deposited in trench slope or borderland basins along a trench transform margin fronting a continental volcano plutonic arc source western province gondwanaland deposition was spasmodic but voluminous and was accompanied by concurrent deformation and accretion resulting in parallel belts of torlesse rock younging outward from the gondwanaland margin at the same time the brook street terrane volcanic arc and associated terranes were forming to the west of the torlesse site separated from gondwanaland by a marginal sea in latest triassic or early jurassic times the torlesse was rafted into the volcanic arc system via transform faulting approximately parallel to the gondwana margin the collision event resulted in tectonic thickening of torlesse and caples rocks at the plate interface and metamorphism to haast schist the source was then dominated by older partly metamorphosed torlesse terrane newly uplifted along the collision front closing of the marginal sea behind the brook street terrane in late jurassic early cretaceous times resulted in juxtapositioning with the western province gondwanaland along the median tectonic line

metabasalts axial hot springs and the structure of hydrothermal systems at mid ocean ridges knowledge of the chemical transfer and mineralogical transformations that occur when sea water reacts with basalt at elevated temperatures and pressures can be used along with geological and geophysical data to deduce the typical structure and evolutionary sequence for hydrothermal systems within the oceanic crust along the axis of a mid ocean ridge studies of metabasalt and metadiabase dredged from fault scarps along the axial valley of the mid atlantic ridge reveal consistent relationships among the bulk chemistry of the altered rocks and their secondary mineralogy mineral abundances and mineral compositions especially for chlorite these relationships can be interpreted in terms of the distribution of alteration with respect to time temperature and water rock ratio in young crust assemblages of chl ab ep act chl ab ep act qtz chl ab qtz and chl qtz are produced at successively higher effective sea water to rock ratios within the temperature range of the greenschist facies 250 to 450 c toward higher ratios chlorites tend to become more mg rich pillow basalts of layer 2 are commonly altered under these conditions by sea water on the descending rather than the ascending limb of a convection system this type of alteration which reflects moderately high water rock ratios may characteristically occur above a still partially molten magma chamber as the magma chamber solidifies and is then penetrated more pervasive alteration of the deeper part of layer 2 and of layer 3 occurs at lower water rock ratios producing solutions such as those recovered from springs on the galapagos rift and the east pacific rise at 21 n localized upwelling limbs of the convection system can produce veins filled with quartz sulfides and fe rich chlorite

characterisation and tracing of permian magmatism in the south western segment of the gondwanan margin u pb age lu hf and o isotopic compositions of detrital zircons from metasedimentary complexes of northern antarctic peninsula and western patagonia abstract metasedimentary rocks in the antarctic peninsula and south western patagonia record detrital zircon evidence for significant permian magmatic events along the palaeo pacific margin of south west gondwana however it is unclear where and how this magmatism formed due to the lack of outcropping permian igneous sources at similar latitudes combined u pb o and lu hf isotope analyses of detrital zircon grains in permo triassic metasedimentary rocks indicate that the permian magmatism resulted from the interaction of crust and mantle derived sources in an active continental margin permian detrital zircons from the trinity peninsula group in the antarctic peninsula range from crustal signatures in the northern part 18 o of 8 initial e hf of 6 to mantle like values in the south 18 o of 5 initial e hf of 3 zircons from the northern domain have isotopic features similar to those from the patagonian duque de york complex they also share a secondary ordovician component of ca 470 ma the middle jurassic cape wallace beds in low island record a ca 250 ma igneous source with stronger crustal signatures 18 o and initial e hf values of 7 5 to 10 8 and 3 2 to 14 2 respectively in contrast zircons from the upper jurassic miers bluff formation on livingston island and cretaceous sediments on james ross island have similar permian u pb ages o and hf trends to their trinity peninsula group counterparts suggesting reworking after the late jurassic our results provide evidence for a permian subduction related magmatic arc partly located in patagonia and extending to west antarctica the southerly decrease in 18 o coupled with increasing initial e hf indicate fewer sedimentary components in the magma source and is consistent with a glaciated cold and dry climate these conditions are comparable with west antarctica climate settings located close to the south pole during the carboniferous and permian

30 million years of permian volcanism recorded in the choiyoi igneous province w argentina and their source for younger ash fall deposits in the parana basin shrimp u pb zircon geochronology evidence abstract we present four shrimp u pb zircon ages for the choiyoi igneous province from the san rafael block central western argentina dated samples come from the yacimiento los reyunos formation 281 4 2 5 ma of the cochico group lower choiyoi section andesitic breccias dacitic to rhyolitic ignimbrites and continental conglomerates agua de los burros formation 264 8 2 3 ma and 264 5 3 0 ma and cerro carrizalito formation 251 9 2 7 ma upper choiyoi section rhyolitic ignimbrites and pyroclastic flows spanning the entire permian succession of the choiyoi igneous province a single zircon from the el imperial formation that is overlain unconformably by the choiyoi succession yielded an early permian age 297 2 5 3 ma while the main detrital zircon population indicated an ordovician age 453 7 8 1 ma the new data establishes a more precise permian age artinskian lopingian for the section studied spanning 30 ma of volcanic activity volcanological observations for the choiyoi succession support the occurrence of explosive eruptions of plinian to ultraplinian magnitudes capable of injecting enormous volumes of tephra in the troposphere stratosphere the new shrimp ages indicate contemporaneity between the choyoi succession and the upper part of the parana basin late paleozoic section from the irati up to the rio do rasto formations encompassing about 24 ma geochemical data show a general congruence in compositional and tectonic settings between the volcanics and parana basin permian ash fall derived layers of bentonites thickness and granulometry of ash fall layers broadly fit into the depletion curve versus distance from the remote source vent of ultraplinian eruptions thus we consider that the choiyoi igneous province was the source of ash fall deposits in the upper permian section of the parana basin data presented here allow a more consistent correlation between tectono volcanic permian events along the paleo pacific margin of southwestern gondwana and the geological evolution of neighboring paleozoic foreland basins in south america and africa

halogen bearing minerals from airport hill visakhapatnam india coexisting biotite hastingsite and apatite in a granulitic charnockite from eastern ghats india contain large amounts of chlorine and fluorine the chlorine content in biotite is one of the highest ever recorded in this mineral the hastingsite composition is quite similar to that of other chlorhastingsites reported elsewhere except that it contains fluorine the apatite crystals contain rare earth elements in addition to halogens

revised tectonic implications for the magnetic anomalies of the western weddell sea abstract ten years after the usac u s a rgentina c hile project which was the most comprehensive aeromagnetic effort in the antarctic peninsula and surrounding ocean basins questions remain regarding the kinematics of the early opening history of the weddell sea key elements in this complex issue are a better resolution of the magnetic sequence in the western part of the weddell sea and merging the usac data set with the other magnetic data sets in the region for this purpose we reprocessed the usac data set using a continuation between arbitrary surfaces and equivalent magnetic sources the equivalent sources are located at a smooth crustal surface derived from the existing bathymetry topography and depths estimated by magnetic inversions the most critical area processed was the transition between the high altitude survey over the antarctic peninsula and the low altitude survey over the weddell sea that required downward continuation to equalize the distance to the magnetic source this procedure was performed with eigenvalue analysis to stabilize the equivalent magnetic source inversion the enhancement of the mesozoic sequence permits refining the interpretation of the seafloor spreading anomalies in particular the change in shape and wavelength of an elongated positive in the central weddell sea suggests that it was formed during the cretaceous normal polarity interval the older lineations in the southwestern weddell sea are tentatively attributed to susceptibility contrasts modeled as fracture zones numerical experimentation to adjust synthetic isochrons to seafloor spreading lineations and flow lines to fracture zones yields stage poles for the opening of the weddell sea since 160 ma to anomaly 34 time the corresponding reconstructions look reasonable within the known constraints for the motions of the antarctic and south america plates however closure is not attained between 160 and 118 ma if independent published east antarctica africa south america rotations are considered the lack of closure may be overcome by considering relative motion between the antarctic peninsula and east antarctica until 118 ma time an important component of convergence

microstructures of the cayman trough cabbros many different deformation textures are observed in submersible collected gabbros from the mid cayman rise spreading center indicating considerable variation in conditions of deformation in this slowly spreading ridge features suggesting flow range from mechanical twins and bent crystals low strain to complete recrystallization with a well developed foliation high strain textures suggesting fracture include zones 0 1 10 mm wide of extremely reduced grain size cracked grains with undulose extinction and crosscutting fractures these textures occur separately next to and overprinting each other of the variables controlling the deformation features temperature and water pressure are most easily estimated minimum temperature during deformation is suggested by minimum temperature during deformation of mineral assemblages unaffected by this deformation this temperature is above 500 550 c for the majority of the features observed observations suggest enhanced recrystallization where primary minera

prediction of some weathering trends of plutonic and volcanic rocks based on thermodynamic and kinetic considerations abstract the exposed crust consists mainly of plagioclase 35 quartz 20 k feldspar 11 volcanic glass 12 biotite 8 and muscovite 5 quartz is a resistate thus feldspars and glass represent approximately 75 percent of the labile minerals the weathering characteristics of these constituents are summarized in the context of thermodynamic mass balance and kinetic considerations experimentally determined release rate constants were used to predict the proportions of ca na and k released by feldspars of plutonic rocks granites to gabbros to weathering solutions the chemical weathering trends of the weathered residues calculated from the kinetic data conform closely to the initial trends observed in some recent weathering profiles demonstrating the accuracy of the predictions since the weathering of feldspars is controlled by processes that should not change through geological time the relative release rates of ca na and k from the feldspars of granitic rocks can be calculated for future and past episodes of continental weathering experimentally determined release rate constants are not available for a wide range of volcanic glass compositions but the limited data indicate that compositional trends are predictable in weathering profiles developed on volcanic rocks the kinetic data available for rhyolitic glasses accurately predict the initial weathering trends observed in a recent rhyolite weathering profile

cathodolumenescence spectra of quartz as provenance indicators revisited abstract we present a discrimination scheme for cathodoluminescence cl spectra of quartz as a tool in provenance studies we analyzed 1000 quartz crystals from 58 samples of different plutonic volcanic metamorphic and pegmatitic rocks the technique is based on the measurement of the relative intensity of the two main emission centers in visible light at 470 490 nm and 600 640 nm the results confirm a red violet or bright to medium blue luminescence for volcanic phenocrysts mainly bright blue colors for felsic plutonic and high temperature metamorphic quartz as well as brown to dark blue cl for quartz of low temperature metamorphic origin these and additional results for mafic plutonic dark blue and pegmatitic quartz bright blue lead to a possible discrimination with the following rock grouping 1 volcanic quartz 2 low temperature metamorphic and mafic plutonic quartz and 3 felsic plutonic high temperature metamorphic and pegmatitic quartz in the proposed scheme 87 of the spectra are classified correctly the three quartz groups can be taken as an estimate for the amount of volcanic metamorphic and plutonic quartz in sediments unless major input from plutonic high temperature metamorphic or pegmatitic quartz has taken place hence despite criticism within the literature during the last decades the cl colors of detrital quartz still can be used as provenance indicators we finally conclude that the measurement of wavelength spectra is a fast and straightforward method to determine quartz bearing source rocks of siliciclastic sediments

the late palaeozoic to early mesozoic evolution of southern south america a plate tectonic interpretation the limits of 3 distinct upper palaeozoic to lower mesozoic tectono stratigraphic zones of the ancestral pacific margin of gondwana in southern south america are documented these are a fore arc region consisting in part of oceanic components a magmatic arc that is coincident with carboniferous to triassic continental sedimentation and a back arc region comprising the upper palaeozoic epicratonic sequences of the samfrau geosyncline these zones traceable 2300 km from 29 56 s cape horn document the semi continuous subduction of the ancestral pacific floor from the middle devonian to the triassic between 29 s and 37 s the palaeo subduction zone was roughly coincident with the present pacific coast line between 37 s and cape horn the subduction zone migrated progressively westward as an alaskan style accretionary prism grew to a width of 250 km while in the northern sector jurassic are intrusions are coincident with the upper palaeozoic are intrusions in the s they intruded the upper palaeozoic to lower mesozoic accretionary prism 200 km w of their palaeozoic equivalents no post middle palaeozoic sutures have been identified in southern south america hence there is no evidence of late palaeozoic mesozoic or cenozoic accretion of discrete microplates

provenance of the sedimentary rakaia sub terrane torlesse terrane south island new zealand the use of igneous clast compositions to define the source abstract the permian to late triassic rakaia sub terrane part of the torlesse superterrane of the new zealand eastern province is an accretionary complex that comprises an enormous volume of quartzofeldspathic sandstones and mudstones with subsidiary conglomerates plus minor oceanic assemblages the field of provenance analysis has undergone a revolution with the development of single crystal isotope dating techniques using mainly silt to sand sized single mineral grains fine sand and mud may be transported over thousands of kilometres with potential problems for provenance studies reinforcing the need for caution when interpreting provenance evidence from single heavy mineral grains this study focuses on coarser grained rocks notably conglomerates which involve much shorter transport distances and therefore may be used to trace proximal sources a detailed rock sampling programme and geochronological geochemical and sr nd isotope analysis of igneous clasts from four conglomerates boundary creek te moana mckenzie pass and lake hill have broadly characterised the igneous source for the rakaia sub terrane shrimp u pb zircon ages of rakaia sub terrane igneous clasts define three distinct periods of magmatic crystallisation the first period ranges in age from 292 to 243 ma permian to middle triassic with two clusters recognisable a minor early permian one ranging in age from 292 to 277 ma and a major late permian to middle triassic one from 258 to 243 ma all these clasts are confined to the kazanian permian te moana the dorashamian permian mckenzie pass and the carnian late triassic lake hill conglomerates the clasts which are subduction related calc alkaline to high k calc alkaline and metaluminous to peraluminous range in lithology from andesite to rhyolite and their plutonic equivalents many clasts at lake hill have chemical compositions characteristic of partial melts of a source of basaltic composition in equilibrium with amphibole pyroxene garnet adakites indicative of a thick crust in the clast source area the second period comprises carboniferous calc alkaline metaluminous to weakly peraluminous clasts of the permian boundary creek conglomerate ranging in age from 356 to 325 ma the third group consists of two cambrian clasts a monzogranite from te moana c 497 8 ma and a dacite from lake hill c 517 ma geochronology geochemistry and sr nd isotopes of rakaia igneous clasts correlate broadly with those of permian to triassic plutons and volcanics from the amundsen and ross provinces of marie byrd land thus the antarctic sector of the panthalassan margin of gondwana is the probable source for the rakaia sub terranes as opposed to the other postulated sources in eastern australia

using element geochemistry and detrital zircon geochronology to constrain provenance of the eocene sediments in the deep water area of the pearl river mouth basin south china sea abstract as the largest sag in the zhu ii depression within the pearl river mouth basin the baiyun sag bys lies in an area of interaction among the eurasian plate pacific plate and indian plate the bys mainly experienced the paleogene rift stage and neogene quaternary post rift stage thick syn rift sediments in the bys have recorded the nature of provenance and provide an excellent record of regional tectonic evolution prior to the opening of the south china sea in this study elemental geochemical analysis combined with published zircon detrital u pb dating results are applied to better understand the provenance change and tectonic setting of the bys during the eocene in this respect the chondrite normalized ree patterns of eocene samples from six wells are highly similar to upper continental crust ucc and post archean australian shale paas particularly with respect to lree enrichment significantly negative eu anomalies and slight ce anomalies geochemical results confirm that the bys eocene sediments were derived from a continental island arc to an active continental margin in addition the rock types of sources and the relatively narrow detrital zircon ages show that mesozoic intermediate felsic igneous and metamorphic rocks derived from local topographic highs contributed to infilling most of the bys furthermore the results show that the panyu low uplift began to accept large sediments transported by the paleo pearl river from the south china coast during the active syn rift stage prior to the opening of the south china sea

u pb ion microprobe zircon geochronology from the basement inliers of eastern graham land antarctic peninsula new geological mapping combined with u pb ion microprobe zircon geochronology on the isolated but locally extensive exposures of crystalline basement inliers of eastern graham land has greatly improved our understanding of the region s early crustal evolution and has allowed a more thorough evaluation of patagonia antarctic peninsula connections prior to gondwana break up at eden glacier diorite gneisses yield early ordovician protolith ages of 487 3 and 485 3 ma and represent the oldest in situ rocks recorded on the antarctic peninsula and indicate a significant spatial extension of famatinian age magmatism of patagonia zircon overgrowths in the early ordovician protoliths and granitic leucosomes developed within them record two phases of permian metamorphism at c 275 and c 257 ma coincident in part with diorite plutonism of the area at 272 2 ma at adie inlet granitic leucosomes from paragneiss have been dated at 276 3 ma and these are in turn cut by 257 3 ma xenolith rich diorite gneiss the diorite intruded during a second phase of deformation which folded the paragneiss leucosomes into tight folds this whole assembly is cut by intensely brecciated megacrystic granodiorite which yielded a 259 3 ma age south of cabinet inlet a very different sequence of events is evident with triassic magmatism at c 236 ma extensive along the joerg peninsula migmatitic leucosomes are dated at c 224 ma and magmatism and deformation events apparently continued to c 209 ma at cape casey our data indicate that the devonian and carboniferous magmatism at target hill considered to represent the classic basement complex of the antarctic peninsula is not representative regionally the target hill crustal block contains a major break along cabinet inlet to the north ordovician and permian protoliths were variably migmatized during two episodes of permian deformation and metamorphism whereas to the south triassic protoliths and triassic metamorphism are encountered supplementary material u pb isotopic data are available at http www geolsoc org uk sup18526

petrology and mineralogy of dredged gabbro from gettysburg bank eastern atlantic gabbros have been dredged from gettysburg bank 110 km west of portugal on the azores gibraltar fracture zone primary minerals in olivine pyroxene and brown hornblende gabbros are partially replaced by metamorphic minerals igneous textures are inhomogeneously overprinted by a granular polyhedral deformation and a cataclastic deformation amphiboles show characteristics which indicate a transition from crystallisation in a magma chamber to formation of amphibole in solid gabbro under metasomatic conditions of the amphiboles analysed chlorine was present in the green amphiboles but below 0 05 in the brown suggesting the penetration of sea water after the formation of the brown amphibole but during the formation of the green

initiation and collapse of active circulation in a hydrothermal system at the mid atlantic ridge 23 n gabbro and basalt collected from an area south of the kane fracture zone along the mid atlantic ridge have three stages of alteration which record the cooling of a hydrothermal system 1 stage 1 penetration of seawater began between 400 and 550 c altering pyroxene to fibrous green amphibole 2 stage 2 propylitic alteration formed along connected fractures between 250 and 300 c as fracture density increased the fe mg ratio of chlorite increased the final result being an fe chlorite quartz sulfide breccia 3 stage 3 late smectite veinlets formed at low temperatures 200 c after active circulation ceased the study focuses on stage 2 alteration by assuming local equilibrium between alteration minerals and the hydrothermal fluid constraints can be placed on the fluid composition responsible for stage 2 alteration the stage associated with deposition of sulfides the following activities of species in solution were determined for the system feo na2o cao mgo al2o3 sio2 h2o at 350 bars and 250 c log a ca a2 h 8 0 log a na a h 5 0 log a fe a2 h 1 7 log a mg a2 h 6 0 log a sio2 was set at quartz saturation 2 3 at 350 bars and 250 c fluid inclusions record the introduction of a low temperature seawater salinity fluid during formation of the latest quartz veins associated with stage 2 alteration mixing of this and the hydrothermal fluid caused a drop in temperature and increase in oxidation state resulting in increased precipitation of quartz pyrite and chalcopyrite the salinities of fluid inclusions trapped in quartz during stage 2 alteration are as much as 3 times that of seawater concentration of a fluid initially of seawater salinity may be the result of boiling at 350 c and 3000 m depth

algal and archaeal polyisoprenoids in a recent marine sediment molecular isotopic evidence for anaerobic oxidation of methane 1 analyses of 13c contents of individual organic molecules in a marine sediment show that crocetane 2 6 11 15 tetramethylhexadecane an isomer of phytane is produced by microorganisms that use methane as their main source of carbon the sediments lie at a water depth of 68 m in the kattegat the strait between denmark and sweden crocetane appears first 185 cm below the sediment water interface in the zone marking the transition from sulfate reduction to methanogenesis its 13c value is 90 10 versus vienna pee dee belemnite vpdb its structure which includes four isoprene units arranged symmetrically around a tail to tail linkage suggests that it is produced by a member of the archaea growing at the intersection of the diffusion gradients for sulfate and methane in sedimentary pore waters the source organism apparently functions as a methane consuming member of the microbial consortium responsible for the anaerobic oxidation of methane hoehler et al 1994 in which as first demonstrated quantitatively in these sediments iversen and jorgensen 1985 electrons are transferred from methane to sulfate the presence of archaeal biomass throughout the sediment section is indicated by significant concentrations of 2 6 10 15 19 pentamethylicosane pmi and of ether bound phytane and biphytane the pmi reaches a minimum value of 47 well below the transition zone its isotopic depletion could reflect either methanogenic or methanotrophic sources the ether bound lipids are isotopically uniform throughout the section and are presumed to derive from archaea that utilize a carbon source unaffected by the oxidation of methane

amphibolitized sheared gabbros from ophiolites as indicators of the evolution of the oceanic crust bay of islands newfoundland abstract in the bay of islands ophiolite complex both the basal part of the cumulate gabbro sequence and the underlying peridotites have been plastically deformed higher in the series the cumulate gabbros are undeformed except in localized shear zones slightly to strongly oblique to the layering the resulting shear bands are narrow and composed either of mylonitic amphibole bearing gabbros or of highly foliated amphibolites the amphibole composition and nature and the chemistry of coexisting minerals combined with structural evidence imply that a first episode of shearing subhorizontal in the crust occurred at decreasing temperatures ranging from 750 to 450 c this shearing probably represent the response of the lithosphere to stresses also responsible of asthenospheric flow in the upper mantle a second episode of shearing oblique to the first one and probably related to cooling of the crust away from the ridge occurred at lower temperatures ranging from 500 to 300 c the surrounding gabbros have undergone static hydrothermal metamorphism at temperatures decreasing from 650 to 450 c all metamorphic events occurred in hydrous conditions in the vicinity of the ridge axis

basement chronology of the antarctic peninsula recurrent magmatism and anatexis in the palaeozoic gondwana margin abstract a revised pre jurassic chronology is presented for the antarctic peninsula on the basis of new u pb zircon dating using both conventional 11 samples and shrimp microprobe data 10 samples the age range for plutonism and high grade metamorphism is from 435 8 ma silurian to c 206 ma approximately the triassic jurassic boundary with peaks of activity in devonian carboniferous permian and mid triassic times although some are represented by rocks with limited outcrops the new data confirm the importance of mid and latest triassic magmatic events previously identified using rb sr geochronology the adie inlet gneiss previously thought to be neoproterozoic is recognized as a permian migmatite derived from paragneiss with a provenance dominated by cambrian granitoids granite gneiss from nw palmer land previously dated as cambrian is shown to be triassic detrital zircons in metasedimentary rocks and inherited zircons in granitoids are dominated by mesoproterozoic to cambrian components with sparse palaeoproterozoic to archaean grains suggesting sources within gondwana at times these sources were nearby as shown by an archaean cobble sized granite clast from a permo triassic conglomerate another clast in the same conglomerate previously thought to be devonian in age has an ordovician crystallization age

recognition and significance of inherited quartz overgrowths in quartz arenites abstract inherited quartz overgrowths represent remnants of cement adhering to a grain s surface from a previous sedimentary cycle and can be recognized in petrographic studies on the basis of textural criteria they may occur as rounded double or penetrating overgrowths since these features occupy only a part of a grain s surface and are randomly positioned when studied in two dimensional sections the likelihood of their being transected and observed is significantly less than their whole rock abundance this has resulted in an underestimation of their importance as a provenance indicator probability that a random cut will transect a cap of inherited cement on any particular grain depends upon proportion of the grain s surface covered by the cap position of the cap on the grain s surfac and orientation of the cut through the grain an experiment was devised to analyze the relationship between size of a grain coating cap and likelihood that it will be transected in a random cut this involved using an eccentric stereonet one in which the origin is not at the projection sphere s center to project random planes results of the experiment were plotted as a curve and then derived mathematically since size of a coating cap is related to its projection angle measured from the sphere s center a second experiment was devised to determine the relationship between projection angles and equivalent apparent projection angles seen when a cut does not pass through a sphere s center it was found that the true projection angle equals the mode of apparent projection angles relati nships determined in these two experiments enable calculation of the whole rock proportion of grains with inherited overgrowths from petrographic data although at the present time results must be considered as estimates this capability leads to a more precise understanding of importance of inherited overgrowths as an indicator of sedimentary provenance the procedure was applied to samples of upper precambrian quartz arenites from the uinta mountain group the grand canyon supergroup and the belt supergroup resulting in calculated whole rock proportions of grains with inherited overgrowths that are in general twice the values observed in thin section this procedure may also be useful in a variety of other applications including studies of sediment distribution intrabasinal sedim nt maturation lithostratigraphic correlation and rates of grain surface abrasion

permo carboniferous conglomerates in the trinity peninsula group at view point antarctic peninsula sedimentology geochronology and isotope evidence for provenance and tectonic setting in gondwana field observations from the trinity peninsula group at view point on the antarctic peninsula indicate that thick southward younging and overturned clastic sedimentary rocks comprising unusually coarse conglomeratic lenses within a succession of fine grained sandstone mudstone couplets are the deposits of debris and turbidity flows on or at the foot of a submarine slope three detrital zircons from the sandstone mudstone couplets date deposition at 302 3 ma at or shortly after the carboniferous permian boundary conglomerates predominantly consist of quartzite and granite and contain boulders exceeding 500 mm in diameter zircons from granitoid clasts and a silicic volcanic clast yield u pb ages of 466 3 ma 373 5 ma and 487 4 ma respectively and have corresponding average ehft values between 0 3 and 7 6 a quartzite clast conglomerate matrix and sandstone interbedded with the conglomerate units have broadly similar detrital zircon age distributions and hf isotope compositions the clast and detrital zircon ages match well with sources within patagonia however the age of one granite clast and the ehf characteristics of some detrital zircons point to a lesser south africa or ellsworth mountain like contribution and the quartzite and granite dominated composition of the conglomerates is similar to upper palaeozoic diamictites in the ellsworth mountains unlike detrital zircons large conglomerate clasts limit possible transport distance and suggest sedimentation took place on or near the edge of continental crust comparison with other upper palaeozoic to mesozoic sediments in the antarctic peninsula and patagonia including detrital zircon composition and the style of deformation suggests deposition of the trinity peninsula group in an upper plate basin on an active margin rather than a subduction related accretionary setting with slow extension and rifting punctuated by short periods of compression

ocean floor hydrothermal metamorphism in the limousin ophiolites western french massif central evidence of a rare preserved variscan oceanic marker the limousin ophiolite is located at the suture zone between two major thrust sheets in the western french massif central this ophiolitic section comprises mantle harzburgite mantle dunite wehrlites troctolites and layered gabbros it has recorded a static metamorphic event transforming the gabbros into undeformed amphibolites and the magmatic ultramafites into serpentinites and or pargasite bearing chloritites with various thermobarometric methods it is possible to show that the different varieties of amphibole have registered low p c 0 2 gpa conditions with temperature ranging from high t late magmatic conditions to greenschist zeolite metamorphic facies the abundance of undeformed metamorphic rocks which is typical of the lower oceanic crust the occurrence of ca al mg metasomatism illustrated by the growth of ca al silicates in veins or replacing the primary magmatic minerals the p t conditions of the metamorphism and the numerous similarities with oceanic crustal rocks from ocean drilling program and worldwide ophiolites are the main arguments for an ocean floor hydrothermal metamorphism in the vicinity of a palaeo ridge among the west european variscan ophiolites the limousin ophiolites constitute an extremely rare occurrence that has not been involved in any hp subduction related or mp orogenic metamorphism as observed in other ophiolite occurrences i e france spain and germany

very low and low grade metamorphism in the trinity peninsula group permo triassic of northern graham land antarctic peninsula the permo triassic trinity peninsula group is a widespread regionally metamorphosed metasedimentary sequence in northern graham land antarctica which forms the local basement to the mainly jurassic cretaceous antarctic peninsula magmatic arc the metamorphic grade thermal evolution and pressure series of this major tectono stratigraphical unit are largely unknown determining the nature of the metamorphism has relied hitherto on conventional optical identifications of the major phases mainly in rare volcanic beds however diagnostic mineral parageneses are generally absent and the precise metamorphic grade is unknown or has to be inferred over large areas using white mica illite crystallinity of interbedded mudrocks the trinity peninsula group is now shown to have been pervasively altered mainly at anchizonal and epizonal grades conditions ranged from upper anchizonal in the northeast to thoroughly epizonal in the southwest outwith thermal aureoles near plutonic intrusions the alteration temperatures ranged mainly from 250 to 325 c exceeding 300 c in the highest grade epizone greenschist facies parts of the sequence the facies series k white mica b cell dimension measurements and mineral phases present are characteristic of an intermediate pressure series altered under moderate geothermal gradients 35 c km corresponding to burial depths of c 7 10 km unroofing and substantial erosion of the trinity peninsula group took place during polyphasal vertical tectonic movements linked to the development of the magmatic arc in northern graham land the geological setting of the trinity peninsula group is ambiguous and could have been a foreland or back arc basin or the mid to upper levels of an accretionary prism

crustal growth of the antarctic peninsula by accretion magmatism and extension a subduction accretion model incorporating new geophysical data is presented to explain the geology of the antarctic peninsula from late palaeozoic to cenozoic time according to the model the peninsula consists of overlapping accretionary magmatic and extensional regimes that were diachronous across the peninsula and have built the crust to its present form the crust which contains a small proportion of sialic basement was mainly formed by accretionary and magmatic processes and modified to its present shape by extension the gondwanide orogeny for the antarctic peninsula is interpreted in terms of the accretionary processes

provenance of late palaeozoic metasediments of the patagonian proto pacific margin southernmost chile and argentina in this provenance study of late palaeozoic metasediments of the eastern andean metamorphic complex eamc along the south patagonian proto pacific margin of gondwana the palaeogeological setting of the continental margin in devonian carboniferous and permian times is reconstructed the study is based on detrital heavy mineral contents chemical compositions of tourmaline grains and whole rock element and nd sr isotopic compositions element and isotopic compositions reveal that devonian carboniferous metaturbidites deposited before the development of a late carboniferous permian magmatic arc along the margin were mainly fed from felsic recycled old continental rocks the last recycling phase involved erosion of metasediments that were exposed in patagonia feeder systems to the basin cut either through epidote rich or garnet rich metasediments in permian time eamc metaturbidites were deposited next to the evolving magmatic arc and were derived from felsic crustal rocks two provenance domains are recognised the metasediments of the northern one are chemically similar to those of the devonian carboniferous metasediments this domain was fed from the metasedimentary host rocks of the magmatic arc the southern domain probably was fed from the arc proper as indicated mainly by the dominance of metaplutonic lithic fragments abundant detrital biotite and the major element composition of the metasediments

stretching of the deep crust at the slow spreading southwest indian ridge abstract the 500 metres of gabbro drilled in the southwest indian ocean during odp leg 118 allow direct observation of early high temperature extensional deformation in the deep crust of a slow spreading rift valley about 30 of the recovered gabbros have been plastically stretched at progressively decreasing temperatures granulite to lower amphibolite facies metamorphic conditions increasing water rock ratio and probably increasing deviatoric stresses the early solid state strain in granulite facies metamorphic conditions produced low dipping foliations and low strain gradients at this stage mylonitic shear zones only formed in oxide rich gabbros strain localization in the deepest and hottest part of the oceanic crust thus appears to have been influenced by the magmatic stratigraphy in upper to lower amphibolite facies metamorphic conditions the ductile strength of the gabbro was probably higher and deformation concentrated in mylonitic shear bands which may have provided access to seawater derived fluids strain localization at this stage was accompanied by brittle failure and boudinage in the mylonitic foliation of individual porphyroclasts or of thick gabbro intervals the influence of such brittle failure on hydrothermal circulations was critical it allowed for extensive crust seawater interactions at high temperatures upper amphibolite facies conditions in a spreading context where continuous magma supply provides heat and new crustal material to the axial region the mechanical behaviour of the ridge axis lithosphere should be controlled by continuous ductile deformation in granulite facies conditions this may lead to the steady state necking configuration described by tapponnier and francheteau 1978 alternatively a discontinuity of the magma supply allowing the lower crust of the axial region to cool below granulite facies temperatures should cause mylonitic shear zones characteristic of amphibolite facies deformation to extend deeper into the axial domain eventually leading to the detachment fault configuration proposed by karson in press as magma supply to slow spreading oceanic ridges is likely to be discontinuous necking and faulting periods should alternate through time

the gneiss of zabargad island deep crust of a rift abstract the gneisses of the zabargad island are in contact with lherzolites that were emplaced by mantle diapirism during the early rifting of the red sea the gneisses and the peridotites at their contact develop similar structures kinematics and common granulite metamorphism away from the contact the shear strain and associated recrystallizations decrease rapidly thus the zabargad gneisses are metamorphosed and deformed by the mantle diapiric intrusion this is confirmed by preliminary isotopic dates the gneisses have a tripartite polygenic composition of felsic granulites metagabbros and amphibolites the metagabbros and amphibolites are metamorphosed only by the mantle diapir intrusion which also induces a partial anatexis of the gneisses the felsic granulites record a proceeding high p granulite facies metamorphism they are assumed to represent screens of ancient continental crust

paleomagnetism and tectonics of the south shetland islands and the northern antarctic peninsula new paleomagnetic data presented here from 61 sites in paleozoic mesozoic and cenozoic igneous and sedimentary rocks from the antarctic peninsula and the south shetland islands constrain the relative motion of the antarctic peninsula since the mid cretaceous and allow the quantification of tectonic rotation between the different blocks recognized within the area paleozoic and jurassic results failed the fold test and suggest an important remagnetization in the area the similarity between these results and those obtained from cretaceous intrusives indicates a mid cretaceous age for the remagnetization the paleopoles obtained for the different blocks and for different ages suggest that there is no relative rotation among them these combined results allow us to obtain a cretaceous 90 ma and paleocene 60 ma paleopole these paleopoles document a very low apparent polar wandering of the antarctic peninsula for the last 100 ma

the eastern palmer land shear zone a new terrane accretion model for the mesozoic development of the antarctic peninsula a major ductile fault zone the eastern palmer land shear zone has been identified east of the spine of the southern antarctic peninsula this shear zone separates newly identified geological domains and indicates that during late jurassic terrane accretion and collision two and possibly three separate terranes collided resulting in the palmer land orogeny the orogeny is best developed in eastern palmer land and eastern ellsworth land there shallow marine sedimentary rocks of the latady formation and a metamorphic and igneous basement complex of possible lower palaeozoic to pre early jurassic age are thrust and folded this forms an arcuate east directed foreland fold and thrust belt up to 100 km wide and 750 km long parallel to the axis of the antarctic peninsula the newly identified antarctic peninsula domains include 1 a parautochthonous eastern domain that represents part of the margin of the gondwana continent comparable to the western province of new zealand the ross province superterrane of marie byrd land the eastern series of south central chile the pampa de agnia and tepuel rocks of north patagonia and the cordillera darwin rocks of tierra del fuego 2 a suspect central domain that represents an allochthonous microcontinental magmatic arc terrane comparable to the median tectonic zone of new zealand the amundsen province superterrane of marie byrd land and coastal cordillera of north chile and 3 a suspect western domain with strong similarities to the eastern province of new zealand western series of south central chile and chonos metamorphic complex of north patagonia that represents either a subduction accretion complex to the central domain or another separate crustal fragment although an allochthonous terrane hypothesis for the antarctic peninsula remains to be fully tested this has much in common with models for the new zealand and south american parts of the pacific margin of gondwana the identification of a potential allochthonous terrane continent collision zone allows us to define the edge of the gondwana continent in the antarctic peninsula sector of the supercontinent margin which has implications for mesozoic reconstructions of gondwana

evolution of oceanic gabbros from dsdp leg 82 influence of the fluid phase on metamorphic crystallizations abstract gabbro breccias were recovered from an anomalously shallow level of the ocean crust during dsdp leg 82 the rocks display evidence of metamorphic crystallization related either to localized deformation or to hydrothermal circulation of a seawater derived fluid under static conditions secondary phases consist of plagioclase amphibole and minor clinopyroxene ilmenite sphene and chlorite petrological study indicates that deformation took place at high temperature under anhydrous conditions and was followed by hydrothermal circulation the compositions of secondary minerals i e strong zonations presence of chlorine in amphiboles varying compositions of secondary plagioclase indicate that reactions of the gabbros with the fluids occurred at a low water rock ratio relations between cl na and k in amphiboles suggest penetration of at least two distinct fluids of different compositions metamorphic crystallization stopped when greenschist facies conditions were reached 350 c probably because hydrothermal circulation faded out

tectonic environment of the devonian gramscatho basin south cornwall framework mode and geochemical evidence from turbiditic sandstones the portscatho formation within the allochthonous unit of the middle and upper devonian gramscatho group is a thick sequence of deep water sandstones and interbedded slates deposited by southerly derived turbidity currents into the gramscatho basin of south cornwall throughout an approximately 3 5 km thick sequence the portscatho formation is petrographically and chemically coherent except that the upper section shows a higher proportion of metamorphic clasts high but variable cr and low uniform zr abundances complementary framework mode and bulk geochemistry indicate that the sandstones were derived from a dissected continental magmatic arc of predominantly acidic composition similar to average upper continental crust but with an admixture of minor intermediate basic material flysch deposition took place in a fore arc setting the presence of an arc to the south of cornwall during the devonian implies that there was subduction at the margin of the gramscatho basin whose ultimate closure was accommodated by the northward stacking of flysch ophiolite nappes

ridge axis deformation and coeval melt migration within layer 3 gabbros evidence from the lizard complex u k ridge parallel extensional shear zones within the layer 3 gabbros of the lizard ophiolite have recorded varied and numerous chemical and mineralogical changes under a variety of p t conditions these changes accompany tectonic exhumation of lower crustal levels by a listric fault mechanism during amagmatic extension at a slow spreading centre deformation within the mid crustal level of layer three resulted in the migration and impregnation of an evolved iron titanium rich silica poor melt along the shear zones this syntectonic infiltration occurred along the lowermost portions of listric faults which rooted in a partially molten zone beneath the ridge axis residual melt was tapped from this underlying zone and tectonically redistributed about deforming gabbros situated immediately above the partially molten zone the occurrence of sub solidus synkinematic assemblages within the shear zones which were generated at various temperatures and water rock ratios is explained in terms of their simultaneous development on listric fault planes in the presence of a hydrothermal fluid phase the sequential overprinting of pre existing assemblages arises from repeated block rotation during amagmatic episodes of extension within an oceanic slow spreading environment

composition and classification of paleozoic flysch mudrocks of eastern australia implications in provenance and tectonic setting interpretation abstract the mudrock suites of paleozoic flysch sequences of eastern australia can be characterised on the basis of the relative abundances of phyllosilicates and tectosilicates mainly feldspar and quartz a mudrock maturity index is defined as the ratio of total phyllosilicates to phyllosilicates feldspar quartz mudrocks are classified into tectic phyllo tectic and phyllic types on the basis of increasing maturity based on studied examples of eastern australia preliminary characteristics of mudrocks of various tectonic settings are identified oceanic island arc mudrocks are of the tectic type characterised by high feldspar and low phyllosilicate content they are derived from calc alkaline andesitic rocks of the magmatic arc mudrocks of the continental island arc and active continental margins andean type and strike slip basins are characterised by a higher phyllosilicate content and are mainly phyllo tectic in type passive margin mudrocks are of the phyllic type characterised by extremely high phyllosilicate and low feldspar contents suggesting their recycled nature mudrock maturity is accompanied by an increase in the k2o na2o ratio from oceanic island arc to continental island arc and active continental margin to passive margin type mudrocks

late palaeozoic early mesozoic fore arc basin sedimentary rocks at the pacific margin in western antarctica we infer that the trinity peninsula group of the antarctic peninsula was deposited in a fore arc basin s of composite type located between the gondwanian arc massif consisting of plutonic and older gneissose basement rocks and a subduction complex now represented by greenschists blueschists and albite epidote hornblende bearing rocks of the pre jurassic metamorphic complex

the formation and evolution of the paleo pearl river and its influence on the source of the northern south china sea abstract the paleo pearl river has been suggested as one of the major source terrains for the northern south china sea which headstreams in the hinterland of the tibetan plateau and flows through the south china block eventually discharges into the pearl river mouth basin prmb of the south china sea scs however the drainage history of the paleo pearl river remains a matter of debate detrital zircon u pb ages trace elements nd and sr isotopes and clay minerals from the eocene to miocene sediments in the pearl river mouth basin of the south china sea have been used to investigate the sedimentary provenance and reconstruct the drainage history of the paleo pearl river the eu eu la sc la co th sc and th co ratios the hf la th and la sc co th diagrams and the chondrite normalized ree patterns of the samples suggested that the eocene to miocene sediments were mainly derived from felsic source rock the lower eocene sediments were mainly provided by the intrabasinal uplift regions the dong river and bei river initiated since upper eocene supplied large amounts of felsic detritus from the southern china coast to zhu depression and then transported sediments into the zhu depression during the upper early oligocene the nd and sr isotopes of the sediments suggested that the liu river may not provide sediments for the prmb however the gui river may have made a very small contribution the nd and sr isotopes of the sediments also suggested that the paleo pearl river began to extend westward to form xi river and eventually the hongshui river in the late oligocene the trace element ratios of the samples also suggested that the miocene sediments may have received more input from an intermediate rock area indicating the main provenance switch from the coast of south china to a northern source during this time this conclusion has been supported by the detrital zircon u pb ages nd and sr isotopes and clay minerals data of the samples which show that the provenance of the sediments supplied by the paleo pearl river had an abrupt shift in the middle miocene the southeastern tibetan plateau rapid uplifted and experienced erosion in the middle miocene leading to the progressive headward erosion of the pearl river system into the continental interior

provenance of late palaeozoic metasediments of the sw south american gondwana margin a combined u pb and hf isotope study of single detrital zircons combined u pb and lu hf isotope measurements of single detrital zircon grains in carboniferous metasediments from patagonia delineate the source areas of the sediments the detritus represented by four metasandstone samples was deposited prior to onset of subduction in late carboniferous time along the south patagonian proto pacific gondwana margin a broad series of detrital zircon age peaks 0 35 0 7 ga 0 9 1 5 ga and a large spread 0 3 3 5 ga in the age spectra require numerous sources a fifth metasediment was deposited after the onset of subduction this syncollisional sample shows two distinct u pb age peaks at c 290 ma and 305 ma this points to a few sources only patagonia west antarctica initial hf isotope compositions of selected u pb dated zircons from the carboniferous metasediments reveal zircon protoliths originating from both recycled crust and juvenile sources ehf t 0 4 3 5ga 14 to 12 a comparison with crustal compositions of possible source areas indicates that the detritus mainly originated from the interior of gondwana extra andean patagonia the argentine sierra de la ventana southernmost africa east antarctica as well as northern chile and northwestern argentina the sediment transportation paths are consistent with an autochthonous palaeogeographical position of patagonia with respect to gondwana in carboniferous time

response of two phase fluids to fracture configurations within submarine hydrothermal systems submarine hydrothermal systems at mid ocean ridges evolve within a region of brittle deformation overlying a region of ductile deformation the interplay between brittle and ductile modes of deformation and the geometry of fracture systems within the brittle zone can exert a significant influence on fluid flow especially under conditions leading to aqueous two phase separation within the brittle region fractures vary between large conduits directly open to the seafloor and small veins and microcracks with constricted or indirect connections to the seafloor in the ductile region fluids may be trapped under near lithostatic pressure while fluids within throughgoing veins in the brittle region will experience hydrostatic pressure in a narrow transition zone between these two extremes a steep pore fluid pressure gradient must exist in the brittle region itself fluid pressures will vary between lithostatic and hydrostatic as fluid occupies interstices varying from isolated pores through tortuous microcracks to channels directly connected to the main upflow conduits a hydrostatic gradient modified by the decreased density of hot fluid and the effects of the flow itself should be applicable in major conduits open to the seafloor fluctuating pressure conditions in the transition zone may induce phase changes in a fluid remaining at constant depth as a result of the large volume increase that accompanies two phase separation a two phase fluid must either accelerate upward or exert an increased pressure on its surroundings possibly leading to enhanced fracturing the fracture configuration will also affect the behavior of the mixed fluid after phase separation a two phase fluid experiencing rapid turbulent flow in a large open conduit is more likely to remain a well mixed suspension of vapor and liquid whereas a two phase mixture in a constricted branching series of channels moving more slowly in a laminar flow regime is more likely to undergo physical isolation of the phases leading to venting of a vapor phase and retention of a saline brine phase at depth we propose a possible configuration for the fracture pattern within a submarine hydrothermal system consisting of a feeder zone comprised of anastomozing microcracks and veinlets progressively increasing in diameter and decreasing in abundance feeding into a main upflow stockwork zone in which flow directly to the surface takes place in relatively large unconstricted conduits fluid movement within a feeder zone may be modified by capillary flow tortuous channelways disconnected fractures blocking of cracks by mineral deposition and opening of new fractures

petrography and geochemistry of the carboniferous triassic trinity peninsula group west antarctica implications for provenance and tectonic setting the carboniferous triassic trinity peninsula group is a metasedimentary sequence that crops out widely in the northern antarctic peninsula these are some of the most extensive outcrops in the area and hold the key to evaluating the connections of the antarctic peninsula in gondwana however they are still poorly understood here we present our provenance study of the trinity peninsula group using petrographic and geochemical approaches in combination with cathodoluminescence of detrital quartz in order to constrain its source characteristics and tectonic setting using differences in modal composition and quartz cathodoluminescence characteristics we define three petrofacies derived from the progressive uplift and erosion of a volcano plutonic continental arc which exposed the plutonic metamorphic roots as indicated by major and trace elements the source is felsic with a composition ranging from tonalitic to granodioritic the relatively unweathered condition of the source area points to a dry and cold climate at the time of deposition but this does not necessarily mean that it was glaciated deposition of the sediments occurred within an active continental margin relatively close to the source area probably along the south patagonia antarctic peninsula sector of gondwana strong chronological petrological and chemical similarities with the sediments of the duque the york complex in patagonia suggest that they were derived from the same source

nile river sediment fluctuations over the past 7000 yr and their key role in sapropel development the provenance pattern of nile river sediments can be used as a proxy for paleoclimatic changes in east africa the 87sr 86sr ratios are particularly appropriate for such provenance investigations because the white nile drains predominantly crystalline basement rocks whereas the blue nile and atbara flow off the ethiopian highlands which consist of tertiary volcanic rocks a high resolution profile of 87sr 86sr and ti al ratios from a well dated core in the nile delta shows a close correspondence with known changes in nile flow over the past 7000 yr at times of higher river flow there was markedly decreased input of blue nile derived and total sediment this change was caused by northward movement of the inter tropical convergence zone resulting in increased vegetative cover in the ethiopian highlands due to higher rainfall and a longer wet season this inverse relationship between nile river flow and sediment flux may have had important implications in the development of agricultural technology in ancient egypt the marked minimum in 87sr 86sr at 4200 4500 yr b p is coincident with the end of the old kingdom in egypt and provides independent evidence that demise of the old kingdom might have been associated with an extended period of catastrophic low floods during the quaternary and late neogene there was periodic deposition of organic rich sediments sapropels in the eastern mediterranean that represent important indicators of major environmental change evidence from the ti al ratio suggests that the pattern of erosion and sediment supply from the nile catchment observed in this study also occurred throughout much of the neogene and quaternary the reduced inputs of blue nile sediment during times of sapropel formation contributed to the increased primary productivity by reducing the amount of phosphate removed on particles and to the observed change to n limitation in the eastern mediterranean which are important characteristics of sapropel deposition