brittle frictional mountain building 2 thermal structure and heat budget this paper describes a simple thermal model of an actively deforming critically tapered fold and thrust belt the model determines the steady state temperature distribution and heat flow as well as the pressure temperature time histories of rocks that outcrop at the surface the main parameters controlling the thermal structure are the accretion and erosion rates the undisturbed geothermal gradient at the toe and the amount of frictional heating both shear heating on the decollement fault and internal strain heating within the deforming brittle wedge are incorporated in a mechanically consistent manner and they dominate the effect of radiogenic heating except in fold and thrust belts with significantly overpressured pore fluids the mean stresses temperatures and surface heat flow all increase with an increase in the basal and internal coefficients of friction and this dependence is used to constrain the level of friction on the decollement fault beneath the steady state fold and thrust belt in taiwan rocks outcropping in the core of the central mountain range of taiwan experience maximum theoretical temperatures in excess of 400 c and maximum mean pressures in excess of 500 mpa if the coefficient of basal friction is b 0 5 qualitatively these conditions are in good agreement with the observed high greenschist facies metamorphism the theoretical surface heat flow which increases from 95 mw m2 at the front of the fold and thrust belt to 240 mw m2 at the rear is in excellent agreement with the results of a recent geothermal survey of taiwan and theoretical cooling histories are in good agreement with fission track and other geochronologic studies taken together these results provide strong evidence that sliding on the basal decollement fault beneath taiwan is governed by a coefficient of friction in the range of typical laboratory measurements b 0 5 0 2 approximately 35 of the total surface heat flux of 3 gw is heat conducted into the base of the wedge from the top of the basal decollement fault and somewhat more than 30 is heat advected into the toe by accretion the remaining heat is generated internally about 25 by internal strain heating and about 10 by radiogenic heating either an increase in the coefficient of basal friction b or a reduction in the pore fluid pressure ratio b leads to an increase in the surface heat flow because of the increased frictional heating within the wedge and on the basal decollement fault the overall balance of energy in a steady state fold and thrust belt is described by the equation e w g q where is the rate at which both mechanical and heat energy are added from external sources w g is the rate at which work is performed against gravitational body forces in a reference frame attached to the overriding plate and q is the rate at which waste heat flows out of both the upper and lower boundaries the total power input into the taiwan fold and thrust belt is approximately 4 2 gw the mechanical work being done on the base and front of the fold and thrust belt accounts for 3 of these 4 2 gw in addition 0 9 gw of heat are being advected from the subducting plate into the toe by accretion the remaining 0 3 gw are being supplied by in situ radiogenic heating the outgoing energy is dominated by the 3 gw of heat conducted out the top in the surface heat flow however another 0 8 gw are conducted down beneath the rear portion of the basal decollement fault to heat the underlying subducting slab only 0 4 of the incoming 4 2 gw do useful mechanical work against gravity within the wedge the efficiency of brittle frictional mountain building in taiwan is therefore 10

fluid infiltration and transport of major minor and trace elements during regional metamorphism of carbonate rocks wepawaug schist connecticut usa geochemical alteration of metacarbonate layers due to fluid infiltration during greenschist and amphibolite facies metamorphism was investigated based on 149 whole rock analyses of the wepawaug schist usa ankerite albite ank ab rare ankerite oligoclase ank ol biotite bt amphibole amp and diopside di index mineral zones developed during progressive metamorphism acadian orogeny and devolatilization co2 h2o s loss ank ab ank ol and bt zone rocks were altered along some lithologic contacts and in reaction selvages around veins mass changes included gains of na y and in many cases p and fe k rb and ba were typically lost and sr was lost except where calcite precipitated in and around calcite bearing veins na gain and k loss reflect muscovite destruction and albite growth in response to infiltration of fluids from surrounding metapelitic and metapsammitic metaclastic rocks at higher metamorphic grades k rb and ba were lost when biotite broke down to form amphibole or diopside these elements were transported out of metacarbonate layers down chemical potential gradients at lithologic contacts and in selvages na was lost as plagioclase reacted to form clinozoisite zoisite czo zo calc silicates cm to dm scale thickness rich in amphibole and czo zo amp zones or diopside and zo di zones formed at layer margins and in selvages via extreme metasomatism including major 1 addition of al si and in many rocks fe 2 destruction of calcite and 3 loss of ca and volatiles volatile losses near 90 percent relative to low grade precursors demonstrate that calc silicate formation resulting from al si metasomatism is an important source of co2 to be considered when assessing regional devolatilization al mass gains for calc silicates indicate that large quartz veins and some lithologic contacts were loci for massive time integrated fluid fluxes of 105 m3 m 2 heavy rare earth elements hree were gained in the ank ab through the amp zones and a di zone example gained light and mid ree including sm and nd the ree gains coincide with gains of y and in most cases p suggesting that p and y complexes transport ree volume was lost progressively from the unveined interiors of metacarbonate beds attaining 20 to 30 percent loss in the di ii zone mass addition due to veining reduced volume losses and in some cases produced volume gains reaction transport modeling indicates that fluid flow was mostly layer parallel was focused along lithologic contacts fractures and permeable horizons and was accompanied by diffusion and or mechanical dispersion at high angles to the flow directions the metasomatic fluids infiltrated from surrounding metaclastic and or syn metamorphic igneous rocks thus interaction of fluids derived from different rock types must be considered when assessing mass transfer in lithologically diverse mountain belts

a stable isotope study of serpentinization and metamorphism in the highland border suite scotland u k abstract d h and 18 o 16 o ratios have been measured for whole rock samples and mineral separates from the mafic and ultramatic rocks of the cambro ordovician highland border suite the h and o isotopic compositions of these rocks record individual stages in a relatively complex 500 myr old hydrothermal metamorphic history lizardite serpentinites d 105 18 o 6 2 record a premetamorphic history and indicate that parent harzburgites dunites and pyroxenites were serpentinized through low temperature interaction with meteoric waters during cooling the other rocks of the highland border suite record subsequent interaction with metamorphic fluids amphibolite facies hornblende schists were produced through thrust related dynamothermal metamorphism of spilitic pillow lavas during dehydration d enriched fluids were driven off from the spilites thus leaving the hornblende schists to equilibrate with a relatively d depleted internal fluid reservoir d 45 the expelled d enriched fluids may have mixed with more typical dalradian metamorphic waters which then exchanged with the remaining mafic rocks and lizardite serpentinites during greenschist facies regional metamorphism to produce antigorite serpentinites d 62 18 o 8 and greenschist metaspilites d 57 18 o 7 3 with similar h and o isotopic compositions serpentinites which have been only partially metamorphosed show intermediate h isotopic compositions between that of metamorphic antigorite d 62 and non metamorphic lizardite d 105 end members

on the association between veining and index mineral distributions in barrow s metamorphic zones glen esk scotland the concept of index mineral based metamorphic zones was first introduced by george barrow in 1912 and the barrovian metamorphic zones continue to be used as a framework for describing regional met

isotopic and chemical alteration of zircon by metamorphic fluids u pb age depth profiling of zircon crystals from barrow s garnet zone northeast scotland u pb isotopic analyses were done using ion microprobe depth profiling on zircon crystals from dalradian rocks in barrow s garnet zone northeast scotland to determine the timing of metamorphic fluid infiltration and investigate fluid zircon interactions zircon crystals collected from altered metasedimentary rock adjacent to a cross cutting quartz vein and from relatively unaltered continuously equivalent layers distal to the vein indicate that paleozoic fluid influx caused isotopic and chemical alteration along zircon crystal surfaces and internal fractures ion drilling into natural zircon crystal faces yielded u pb depth profiles with 50 100 nm spatial resolution that permitted u pb isotopic age analysis of thin grain rims affected by fluid mediated growth recrystallization at 500 550 c we interpret the measurements to indicate that fluid influx at 462 8 8 ma caused the growth recrystallization of new zircon along pitted crystal surfaces and fractures in archean zircon grains our age estimate overlaps the accepted age range ca 467 464 ma for peak barrovian garnet growth confirming that metamorphism and fluid influx were contemporaneous the new zircon was enriched in u th and common pb relative to host grains suggesting that u th and pb were actively transported by upper greenschist facies metamorphic fluids we present the first field based evidence that common pb in zircon can serve as an indicator of metamorphic fluid infiltration archean ages for some detrital zircons suggest contributions from a lewisian scourian provenance

interplay between fluid flow and fault fracture mesh generation within underthrust sediments geochemical evidence from the chrystalls beach complex new zealand the chrystalls beach complex in the otago schist on the south island of new zealand is a melange comprising sheared trench fill sediments and fragments of oceanic crust it represents an exhumed analogue for underthrust sediments actively deforming along modern subduction thrust interfaces the melange is cross cut by a fault fracture mesh comprising subvertical extension veins and subhorizontal slickenfibre coated shear surfaces both shear and extension veins have a crack seal microstructure indicating episodic growth shear veins are associated with pressure solution selvages along the shear surface whereas wall rock alteration is not observed adjacent to extension veins electron microprobe analyses of selvage seams indicate dissolution of silica from the immediate surroundings of slickenfibre shear veins and therefore these slickenfibres probably grew by local dissolution precipitation of silica on the contrary no depletion or addition of silica is detected around extension veins indicating these veins grew by precipitation from advecting fluids oxygen isotope ratios measured in vein quartz show that shear and extension veins both precipitated from an aqueous fluid with 7 18o 10 consistent with a fluid derived from low grade metamorphic dehydration reactions fluid pressure therefore probably increased as fluids were introduced to a relatively impermeable melange with increasing metamorphic grade and decreasing porosity fault fracture mesh generation therefore involved localized shear assisted by dissolution precipitation creep and concomitant extension fracturing this led in turn to transient permeability associated with a fluid pressure drop allowing episodic vein growth this process may be analogous to geophysically observed episodic tremor and slow slip which also involves a mixture of deformation styles that put together achieve shear slip along the subduction thrust interface

slab derived fluids and quartz vein formation in an accretionary prism otago schist new zealand regional quartz vein formation and the fluxes flow paths and sources of metamorphic fluids were investigated in the mesozoic accretionary prism of new zealand by using a new chemical mass balance analysis of outcrops samples were collected at meter or submeter intervals along outcrop length traverses and combined to obtain average chemical compositions of whole outcrops mass balance analysis used zr as an immobile reference frame and as a monitor of sedimentary sorting processes sio2 zr systematics produced by sedimentary processes differ greatly from those caused by metasomatic mass transfer of silica allowing evaluation of vein formation mechanisms relatively undeformed metasedimentary outcrops of low metamorphic grade mostly prehnite pumpellyite facies are nearly unveined and characterized by sedimentary compositional trends more deformed outcrops of higher metamorphic grade mostly greenschist facies contain 10 30 vol quartz veins these outcrops underwent mass addition of externally derived silica into quartz veins accompanied by addition of na and removal of k and w average silica additions suggest a time integrated fluid flux of 104 105 batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document m 3 fluid end document batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document m 2 rock end document for fluids ascending through the prism dehydration of spilitized oceanic crust subducting beneath the prism is the most probable source for this large fluid flux and could also have caused the na k metasomatism the w removed from deep levels of the prism may have been deposited in focused retrograde au w quartz veins at shallow levels by ascending fluids transfer of sio2 from subducting slabs into accretionary prisms is a plausible mechanism for long term bulk silica enrichment of the continents beyond that possible by magmatic differentiation

paleomagnetism of the 1 1 ga coldwell complex ontario canada implications for proterozoic geomagnetic field morphology and plate velocities we report new paleomagnetic data from the 1108 ma intrusive coldwell complex ontario canada to investigate the apparent reversal asymmetry observed in some midcontinent rift mcr rocks the rocks of eastern and central part of the complex are reversely magnetized with a group mean direction of d 114 8 i 63 7 95 3 6 n 30 the corresponding paleomagnetic pole at plat 47 2 n plong 206 5 e a95 4 8 is located close to the paleomagnetic poles from nearly coeval reversely magnetized rocks of the mcr system including the lower lava flows at mamainse point the rocks of western part of the complex are normally magnetized with a group mean direction d 298 0 i 56 9 95 5 8 n 10 that passes the reversal test with respect to the reversed group mean direction our results do not support the previous model in which the complex was emplaced during two periods of reversed geomagnetic field polarity separated by a period of normal polarity and hence encompasses two geomagnetic reversals instead our new data indicate that the coldwell complex records only two polarity intervals separated by a symmetrical reversal at 1102 1105 ma this reversal is likely equivalent to the lowermost reversal recorded at mamainse point and provides further evidence that the apparent reversal asymmetry reflects a plate motion rather than a persistent nondipole field geometry together with a high quality data from the 1098 ma north shore volcanics our data indicate a rapid velocity of laurentia at 25 4 cm yr the fast plate motion may reflect a decreased mantle drag due to vigorous mantle indicated by widespread intraplate magmatism at 1 1 ga

chemical and textural equilibration of garnet during amphibolite facies metamorphism the influence of coupled dissolution reprecipitation metamorphic equilibration requires chemical communication between minerals and may be inhibited through sluggish volume diffusion and or slow rates of dissolution in a fluid phase relatively slow diffusion and the perceived robust nature of chemical growth zoning may preclude garnet porphyroblasts from readily participating in low temperature amphibolite facies metamorphic reactions garnet is widely assumed to be a reactant in staurolite isograd reactions and the evidence for this has been assessed in the late proterozoic dalradian pelitic schists of the scottish highlands the 3d imaging of garnet porphyroblasts in staurolite bearing schists reveals a good crystal shape and little evidence of marginal dissolution however there is also lack of evidence for the involvement of either chlorite or chloritoid in the reaction staurolite forms directly adjacent to the garnet and its nucleation is strongly associated with deformation of the muscovite rich fabrics around the porphyroblasts cloudy fluid inclusion rich garnet forms in both marginal and internal parts of the garnet porphyroblast and is linked both to the production of staurolite and to the introduction of abundant quartz inclusions within the garnet such cloudy garnet typically has a mg rich mn poor composition and is interpreted to have formed during a coupled dissolution reprecipitation process triggered by a local influx of fluid all garnet in the muscovite bearing schists present in this area is potentially reactive irrespective of the garnet composition but very few of the schists contain staurolite the staurolite producing reaction appears to be substantially overstepped during the relatively high pressure barrovian regional metamorphism reflecting the limited permeability of the schists in peak metamorphic conditions fluid influx and hence reaction progress appear to be strongly controlled by subtle differences in deformation history the remaining garnet fails to achieve chemical equilibrium during the reaction creating distinctive patchy compositional zoning such zoning in metamorphic garnet created during coupled dissolution reprecipitation reactions may be difficult to recognize in higher grade pelites due to subsequent diffusive re equilibration fundamental assumptions about metamorphic processes are questioned by the lack of chemical equilibrium during this reaction and the restricted permeability of the regional metamorphic pelitic schists in addition the partial loss of prograde chemical and textural information from the garnet porphyroblasts cautions against their routine use as a reliable monitor of metamorphic history however the partial re equilibration of the porphyroblasts during coupled dissolution reprecipitation opens possibilities of mapping reaction progress in garnet as a means of assessing fluid access during peak metamorphic conditions

a primary magnetization fingerprint from the cretaceous laytonville limestone further evidence for rapid oceanic plate velocities paleomagnetic data from the cretaceous 101 88 ma laytonville limestone of northern california are of geodynamic interest because they suggest a rate of absolute plate motion 15 cm yr that exceeds those of all present day plates such findings assume that a primary magnetization is preserved in these accreted pelagic limestone outcrops this assumption is supported by a trend in the paleomagnetic inclination data that matches in magnitude and sign the expected sense of motion predicted by comparing the mean paleolatitude value with the north american apparent polar wander path the pervasiveness of remagnetizations seen in shallow water carbonates of north america and elsewhere however raises questions as to whether the laytonville limestone is remagnetized regardless of tests indicating a primary magnetization the remagnetized carbonates have a distinctive wasp waisted magnetic hysteresis signature thought to reflect a bimodal distribution of magnetite grain sizes this signature can be quantified by the relationship between saturation remanence mr saturation magnetization ms coercivity hc and the coercivity of remanence hcr mr ms 0 89 hcr hc 06 this power law is compared with reference data from pacific deep sea carbonates deep sea drilling project sites 167 317 and 463 ocean drilling program site 806 and tethyan pelagic carbonates valle del mis section southern alps which together define a different curve mr ms 0 53 hcr hc 1 0 the laytonville limestone falls in two categories on the basis of unblocking temperature spectra resistance to alternating field demagnetization and behavior during the acquisition of isothermal remanent magnetization red laytonville limestone is thought to contain pigmentary hematite in addition to magnetite hysteresis curves from red laytonville limestone are wasp waisted but because such curves also typify magnetite hematite mixtures these data do not provide an unambiguous test for remagnetization some white laytonville limestone however appears to contain negligible amounts of hematite and can be directly compared with the remagnetized carbonates hysteresis parameters derived from white samples are indistinguishable from those of the reference pelagic limestones such samples have a stratigraphic significance occurring where paleolatitude data indicate an equatorial crossing these samples may have depleted pigmentary hematite due to increased magnetic dissolution as the limestone entered the equatorial belt of high productivity the lack of a remagnetization fingerprint in these white samples together with paleomagnetic lithologic and paleontologic data form a consistent data set supporting the primary nature of the laytonville limestone magnetization and rapid oceanic plate velocities a small plate size coupled with the unusually vigorous mantle plume volcanism of the cretaceous pacific basin could have combined to reduce the effectiveness of asthenospheric drag accounting for the rapid motion

new evidence that the lower cambrian leny limestone at callander perthshire belongs to the dalradian supergroup and a reassessment of the exotic status of the highland border complex

uplifted neogene margin southernmost cascadia mendocino triple junction region california along a 10 km length of coast north of the mendocino triple junction a neogene accretionary complex has been uplifted 2 km and tilted northward in response to the interaction of the southern juan de fuca gorda plate with the older north american and pacific plates these plate interactions were accompanied by tectonic intercalation of miocene to pliocene deposits of the southern cascadia forearc wildcat group with penetratively deformed oligocene to miocene accretionary prism deposits of the false cape terrane new name and the cretaceous to eocene coastal terrane of the franciscan complex north of cape mendocino more than 2 km of stratigraphic section of the accretionary false cape terrane crops out at beach level in an east west trending anticline and recumbent syncline the folded false cape rocks are juxtaposed with forearc deposits to the north along the subvertical russ shear zone to the south a second subvertical shear zone truncates the false cape terrane near the mouth of the bear river this north bear river shear zone interleaves rocks of the false cape and coastal terranes with additional neogene forearc deposits the bear river beds south of the north bear river shear zone the bear river beds crop out for about 5 km in a continuous folded and imbricated section that is again truncated to the south by a third subvertical shear zone this south bear river shear zone interleaves rocks of the coastal terrane the bear river beds and pliocene to pleistocene shelf deposits folding tilting and shear zone development reflect ongoing north south crustal shortening that has occurred in response to interplate coupling and wedge thickening in the deforming miocene and younger accretionary complex the strain recorded in the accreted terranes and in the forearc overlap assemblage reflects a stress regime unlike that in the cascadia fold and thrust belt north of the eel river syncline thus the deformation pattern may be unique to the triple junction area the false cape terrane is one of only two localities along the cascadia margin where the oligocene to miocene accretionary complex is exposed on land the other locality is that of the hoh melange on the southwest olympic peninsula on land exposure of accretionary rocks in these two areas is a consequence of high uplift rates focused compression and triple junction tectonism framework grain composition data for sandstones in the hoh melange and coeval continental deposits in northernmost california the oligocene to miocene weaverville formation suggest a provenance link while sandstone of the false cape terrane presently situated directly west of the weaverville formation is dissimilar given the northward component of farallon juan de fuca plate translation with respect to north america for the past 20 m y provenance links of hoh and weaverville sandstones might reflect a substantial component of northward translation between the farallon and north american plates that was accommodated within the accretionary prism

relative motions between oceanic plates of the pacific basin appendix tables are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b84 012 2 50 payment must accompany order relative motion poles describing the displacement histories between the pacific plate and once adjacent oceanic plates farallon kula izanagi i izanagi ii and phoenix were derived for the late mesozoic and cenozoic eras because fracture zone and magnetic anomaly data are generally available from the pacific plate but not from adjacent plates a new method of analysis for onesided data was required this analysis produced stage poles and rates of relative plate motion and estimates of their confidence regions the following are the main conclusions drawn from our analysis 1 for time intervals of the order of 107 years termed stages relative motion poles for plate pairs remained nearly fixed between stages shifts in poles were commonly both large and abrupt within stages rates of plate motion were commonly observed to change markedly indicating that plates changed speed more frequently than they changed direction 2 the relative motions of all of the plates analyzed changed at about chron m11 135 ma chron 34 85 ma and chron 25 56 ma 3 during the early cretaceous there were five oceanic plates in the pacific basin rather than the four recognized by previous workers 4 to determine the number of farallon plates that existed to the east of the pacific plate during the time interval from chron 34 85 ma to chron 25 56 ma fracture zones and magnetic anomalies that record pacific farallon spreading from the northern central and southern pacific plate were analyzed separately and collectively the analysis shows that a single pacific farallon relative motion pole and a single rate are consistent with all of the data 5 spreading rates along the pacific kula ridge decreased markedly between chrons 32b and 25 72 56 ma probably in response to the arrival of buoyant kula lithosphere at a subduction zone northwest of the bering sea 6 soon after chron 25 56 ma a major reorganization is recorded along the pacific farallon boundary the kula farallon boundary is also thought to have changed during this reorganization 7 kula pacific anomalies north of anomaly 25 are modeled by assuming continued kula pacific spreading after chron 25 56 ma 8 probably by chron 18 43 ma pacific kula spreading had ceased and the kula farallon ridge system had evolved into alignment with the pacific farallon spreading direction however the timing of these events is uncertain because pacific kula data younger than chron 25 56 ma are sparse

fluid flow mineral reactions and metasomatism a general model that relates fluid flow along a temperature gradient to chemical reaction in rocks can be used to quantitatively interpret petrologic and geochemical data on metasomatism from ancient flow systems in terms of flow direction and time integrated fluid flux the model is applied to regional metamorphism quartz veins and a metasomatized ductile fault zone

field and geochemical characteristics of mesoarchean to neoarchean volcanic rocks in the storo greenstone belt sw greenland evidence for accretion of intra oceanic volcanic arcs abstract the storo greenstone belt southern west greenland consists of thrust imbricated slices of mesoarchean 3060 ma and neoarchean ca 2800 ma mafic to ultramafic volcanic rocks volcaniclastic sediments and gabbro anorthosite associations the belt underwent polyphase metamorphism at upper amphibolite facies conditions between 2650 and 2600 ma the contacts between the mesoarchean and neoarchean volcanic rocks and surrounding eoarchean to neoarchean tonalite trondhjemite granodiorite ttg gneisses are tectonic and typically bounded by high grade mylonites regardless of age the volcanic rocks are dominated by mafic amphibolites with a tholeiitic basalt composition near flat to slightly enriched light rare earth element lree patterns la smcn 0 91 1 48 relatively flat to slightly depleted heavy ree hree gd ybcn 1 0 1 28 and pronounced negative nb ta anomalies nb nb 0 34 0 73 on chondrite and primitive mantle normalized diagrams these geochemical characteristics are consistent with subduction zone geochemical signatures and partial melting of a shallow

implications of oceanic intraplate seismicity for plate stresses driving forces and rheology abstract recent studies of earthquakes occurring within the oceanic lithosphere provide valuable information about stresses in the lithosphere plate tectonic driving forces and the rheology of the lithosphere asthenosphere and mantle focal mechanisms indicate that oceanic lithosphere older than 35 million years is almost entirely in deviatoric compression both compressional and extensional events occur in younger lithosphere in young lithosphere mechanisms generally indicate compressive stress in the spreading direction or extension oblique to the spreading direction extension in the spreading direction is not observed using the constraint of compression in the spreading direction in old lithosphere models of the stresses produced by a combination of ridge push and basal drag forces require the magnitude of the drag be less than a few bars for rapidly moving plates and a few tens of bars for slow moving plates assuming that basal drag results from mantle return flow the upper limits on drag can be converted to constraints on the viscosity structure of the asthenosphere and mantle using simple two dimensional models if return flow occurs in a mantle of viscosity 10 22 poise comparable to the results from glacial rebound studies the predicted basal drag is too high unless a thin asthenosphere with viscosity less than 10 19 to 10 20 poise depending on flow depth is present the intraplate stress data thus are consistent with the idea of oceanic plates largely decoupled from the underlying mantle the strength of the lithosphere is constrained by the maximum depth of oceanic intraplate seismicity which increases with lithospheric age and appears to be bounded by a 700 800 c isotherm this limiting depth is approximately equal to the flexural elastic thickness of the lithosphere and is consistent with experimental olivine rheologies which predict rapid weakening at high temperatures similar phenomena are important for estimating the fraction of seismic and aseismic slip on transform faults and in determining the extent of rupture for large trench normal faulting events

a stable isotope study of retrograde alteration in sw connemara ireland dalradian metamorphic rocks lower ordovician meta igneous rocks mgs and caledonian granites of the connemara complex in sw connemara all show intense retrograde alteration alteration primarily involves sericitization and saussuritization of plagioclase the alteration of biotite and hornblende to chlorite and the formation of secondary epidote the alteration is associated with sealed microcracks in all rocks and planes of secondary fluid inclusions in quartz where it occurs and was the result of a phase of fluid influx into these rocks in hand specimen k feldspar becomes progressively reddened with increasing alteration mineralogical alteration in the mgs and caledonian granites took place at temperatures 275 15 c and in the mgs pfluid is estimated to be 1 5 kbar during alteration the d values of alteration phases are 18 to 29 fluid inclusions 47 to 61 chlorites and 11 to 31 epidotes chlorite 18o values are 0 2 to 4 3 while 18o values for quartz k feldspar pairs show both positively sloped mgs and highly unusual negatively sloped caledonian granites arrays diverging from the normal magmatic field on a plot the stable isotope data show that the fluid that caused retrogression continued to be present in most rocks until temperatures fell to 200 140 c the retrograde fluid had d 20 to 30 in all lithologies but the fluid 18o varied both spatially and temporally within the range 4 to 7 the fo2 of the fluid that deposited the epidotes in the mgs varied with its 18o value with the most 18o depleted fluid being the most oxidizing the d values together with low 0 18o values for the retrograde fluid in some lithologies indicate that this fluid was of meteoric origin this meteoric fluid was probably responsible for the alteration in all lithologies during a single phase of fluid infiltration the variation in retrograde fluid 18o values is attributed to the effects of variable oxygen isotope shifting of this meteoric fluid by fluid rock interaction infiltration of meteoric fluid into this area was most likely accomplished by convection of pore fluids around the heat anomaly of the galway granite soon after intrusion at 400 ma however convective circulation of meteoric water and mineralogical alteration could possible have occurred considerably later

compositional variations in metamorphosed sediments of the littleton formation new hampshire and the carrabassett formation maine at sub hand specimen outcrop and regional scales rocks from several outcrops of low grade littleton formation and laterally equivalent carrabassett formation were analyzed for major elements and zr compositional variations well outside of analytical uncertainties were observed in order of increasing magnitude for the following 10 gram subsamples from a 2 kg hand sample of visually homogeneous slate 100 gram samples taken 10 meters apart in the same outcrop 10 g subsamples from a visually homogeneous quartzite samples of relict turbidite beds in the same outcrop and single 100 g samples from seven outcrops we regard these variations as inherited from the sedimentary protolith with little if any alteration over distance scales of centimeters or greater all could be modeled successfully as mixtures mainly of clay quartz and chlorite mica although the compositions of the clay and the chloritemica components had to be adjusted for different sets of samples within reasonable bounds for sedimentary diagenetic minerals compositional variations and trends in 2 kg samples of higher grade littleton rocks are essentially the same as for the low grade samples for example sio2 and zr correlate positively as expected for sedimentary sorting of quartz and zircon from clay but not for selective metamorphic mobilization of silica also found was centimeter scale separation of k2o from al2o3 and strong covariation of zn with al2o3 in 10 g subsamples of a staurolite schist compositional evidence of element transport over distances of a few centimeters because the protolith includes at least two major al2o3 bearing components in variable proportions element ratios to al2o3 can vary such that variation in metamorphic rocks need not be a sensitive indicator of element mobility relative to al2o3 even if al is immobile the population of low grade rocks encompasses a broad compositional range thus it is not feasible to obtain a sample average with a small standard deviation it would also be difficult to demonstrate conclusively whether a set of samples was representative of its metamorphic grade or whether rocks of all metamorphic grades developed from the same combination of protolith compositions and physical characteristics

controls on the mechanisms of fluid infiltration and front advection during regional metamorphism a stable isotope and textural study of retrograde dalradian rocks of the sw scottish highlands vein controlled retrograde infiltration of h2o co2 fluids into dalradian epidote amphibolite facies rocks of the sw scottish highlands under greenschist facies conditions resulted in alteration of calcite rich marble bands to dolomite and spatially associated 18o enrichment of about 10 on a scale of metres fluid inclusion data indicate that the retrograde fluid was an h2o salt mixture with a low co2 content and that the temperature of the fluid was about 400d c detailed petrographic and textural backscattered electron imaging studies at one garnet grade locality show that advection of fluid into marbles proceeded by a calcite calcite grain edge flow mechanism while alteration of non carbonate wall rock is associated with veinlets and microcracks stable isotopic analysis of carbonates from marble bands provides evidence for advection of isotopic fronts through carbonate wall rocks perpendicular to dolomite veins and fluid fluxes in the range 2 4 28 6 m3 m2 have been computed from measured advection distances coincidence of isotope and reaction fronts is considered to result from reaction enhanced kinetics of isotope exchange at the reaction front front advection distances are related to the proportion of calcite to quartz in each marble band with the largest advection distance occurring in nearly pure calcite matrix this relationship indicates that fluid flow in carbonates is only possible along fluid calcite calcite grain edges however experimental constraints on dihedral angles in calcite fluid systems require that pervasive infiltration occurred in response to calcite dissolution initiated at calcite calcite grain junctions rather than to an open calcite pore geometry the regional extent of the retrograde infiltration event has been documented from the high 18o of dolomite ankerite carbonates from veins and host rocks over an area of least 50 50 km in the sw scottish highlands isotopically exotic 18o rich retrograde fluids have moved rapidly upwards through the crust inducing isotopic exchange and mineral reaction in wall rocks only where lithology pore geometry or mineral solubilities pressure and temperature have been appropriate for pervasive infiltration to occur

silica mobility and fluid movement during metamorphism of the connemara schists ireland quartz veins are developed in a wide range of metasediment types in the upper amphibolite facies rocks of connemara and attest to considerable migration of silica contrary to common assumptions there is clear evidence that these veins do not primarily result from movement of fluid to regions of lower p t down the regional geothermal gradient under amphibolite facies conditions a dilute chloride fluid moving down temperature has the potential to alter 60g of plagioclase to muscovite for each gram of vein quartz precipitated while cooling over the temperature interval from 650 to 500 c the absence of significant metasomatic effects in the vein walls effectively precludes a simple origin from such through flowing externally derived fluids the oxygen isotopic composition of matrix quartz shows considerable differences between different rock types quartzite pelite and marble with a range of 18osmow from c 11 5 quartzite to 18 5 marble in each rock type vein quartz compositions closely match those of the matrix quartz these results demonstrate the importance of local segregation processes in the formation of veins and suggest that fluid convection cells were not developed during metamorphism on a scale larger than the individual sedimentary formations if at all both oxygen isotope data and the absence of metasomatism indicate that veins form primarily by segregation of quartz from the host lithologies with only a relatively minor component of through flow of externally derived fluid veins are clearly not the major pathways of metamorphic dewatering it is proposed that abundant veins in the predominantly pelitic ballynakill formation formed during peak metamorphic d3 folding because the formation was embrittled by high fluid pressures but was capped by impermeable marble hence the pelitic formation fractured repeatedly and the pore fluid drained through the fractures to form veins while irreversible loss through the rest of the succession was a much less important process in the central mountains of connemara rather pure unreactive quartzites are cut by widely spaced laterally extensive quartz veins that are axial planar to d3 folds these veins may mark pathways whereby metamorphic fluid made its way through the massive impermeable quartzite from lower parts of the nappe pile but here too oxygen isotope data indicate considerable segregation of locally derived quartz reflecting the importance of pumping of fluid between wail rocks and fractures relative to the component of through flow

gradients in fluid composition across metacarbonate layers of the wepawaug schist connecticut usa metamorphic index mineral zones pressure temperature p t conditions and co2 h2o fluid compositions were determined for metacarbonate layers within the wepawaug schist connecticut usa peak metamorphic conditions were attained in the acadian orogeny and increase from 420 c and 6 5 kb in the low grade greenschist facies to 610 c and 9 5 kb in the amphibolite facies the index minerals oligoclase biotite calcic amphibole and diopside formed with progressive increases in metamorphic intensity in the upper greenschist facies and in the amphibolite facies prograde reaction progress is greatest along the margins of metacarbonate layers in contact with surrounding schists or in reaction selvages bordering syn metamorphic quartz veins new index minerals typically appear first in these more highly reacted contact and selvage zones it has been postulated that this spatial zonation of mineral assemblages resulted from infiltration largely by diffusion of water rich fluids across lithologic contacts or away from fluid conduits like fractures in this model the infiltrating fluids drove prograde co2 loss and were derived from surrounding dehydrating schists or sources external to the metasedimentary sequence the model predicts that significant gradients in the mole fraction of co2 x co 2 should have been present during metamorphism but new estimates of fluid composition indicate that differences in x co 2 preserved across layers or vein selvages were very small 0 02 or less however analytical solutions to the two dimensional advection dispersion reaction equation show that only small fluid composition gradients across layers or selvages are needed to drive prograde co2 loss by diffusion and mechanical dispersion these gradients although typically too small to be measured by field based techniques would still be large enough to dominate the effects of fluid flow and reaction along regional t and p gradients larger gradients in fluid composition may have existed across some layers during metamorphism but large gradients favor rapid reaction and would therefore seldom be preserved in the rock record most of the h2o needed to drive prograde co2 loss probably came from regional dehydration of surrounding metapelitic schists although h2o rich diopside zone conditions may have also required an external fluid component derived from syn metamorphic intrusions or the metavolcanic rocks that structurally underlie the wepawaug schist

the thermal signature of subducted lithospheric slabs at the core mantle boundary abstract we study the thermal structure around a cold deformable lithospheric slab as it sinks to the core mantle boundary and migrates along it we present analytical results for the steady thermal structure established by a steady but spatially varying motion the analysis gives a time like criterion for the thermal signature of a cold slab to persist by the time that the slab moves along the core mantle boundary the model is used to assess the feasibility of a purely thermal origin for some of the observed seismic reflectors near the core mantle boundary calculations of the time like criterion show that the dynamical conditions in our model namely the velocity and the thickness of the descending slab are hard to reconcile with observations of subduction and seismic features seismic reflections and refractions from anomalously fast regions above the core mantle boundary could be explained as thermal slabs if the thickness of the slab at subduction was larger than 200 km or somewhat less if the slab did not split at the core mantle boundary a simple thermal model also predicts from mineral physics a certain correlation between s and p wave velocity anomalies which is not observed however a purely thermal origin cannot be ruled out if the slab is buckling this process could be in agreement with the observations the amplitude of the seismic anomalies the vertical extent of high gradient zones and the p versus s comparisons chemical heterogeneities and phase transformations remain alternative or complementary explanations

retrogression of staurolite schists and the sources of infiltrating fluids during metamorphism staurolite kyanite schists from the dalradian of scotland and ireland show two types of retrograde alteration in the irish examples staurolite and kyanite are replaced by muscovite and paragonite often with margarite and sometimes chloritoid plagioclase in these samples may be albitized in contrast one scottish sample shows replacement of both staurolite and plagioclase by muscovite retrogression resulted from infiltration of fluid but while in some samples the large number of retrograde phases internally buffered the fluid composition and cation metasomatism cannot be proven in others the fluid composition was externally controlled and there was cation metasomatism indicating more extensive infiltration it is demonstrated by the use of activity diagrams showing the relationships between al silicate white mica and alkali feldspar that wholesale alteration to muscovite was most likely caused by fluids moving down temperature from a granitic or arkosic source in contrast if growth of retrograde albite is actually accompanied by na metasomatism then the infiltrating fluids are likely to have been moving up temperature through pelitic lithologies large fluid rock ratios of around 10 1 are needed to achieve significant alkali metasomatism except in extreme situations of large temperature changes and highly concentrated fluids

fast instantaneous oceanic plate velocities recorded by the cretaceous laytonville limestone paleomagnetic analysis and kinematic implications new paleomagnetic results from 101 89 ma laytonville limestone of the franciscan central belt melange of northern california confirm a southern hemisphere origin as first proposed by alvarez et al 1980 discovery and study of new laytonville limestone outcrops extend the paleomagnetic record and provide a definitive determination of age stratigraphic polarity and hemisphere of deposition paleontological determinations show that five sections are presently overturned and four are presently right side up as expected from a random distribution of nine blocks in a melange the exact accretion time is unknown but geologically reasonable estimates range between 70 and 50 ma leading to averaged estimates of poleward velocity between 30 and 14 cm yr the averaged values are obtained by dividing the total paleolatitude shift by the difference between magnetization and estimated accretion ages they reflect net poleward transport over a long time interval and are minimum estimates because they record only the poleward component of motion an instantaneous poleward velocity is recorded by the systematic inclination decrease of 160 paleomagnetic samples with respect to decreasing age over the 12 m y section conservative estimates place this minimum velocity at 15 cm yr the consistency between the values predicted by the average velocity calculations and those measured within the outcrops serves as an important check on the primary nature of the magnetization this minimum velocity is much higher than any present day plate velocity the subduction of old oceanic crust at the boundary of a relatively small plate is hypothesised as a driving mechanism to explain the measured velocities coupled with the minimum instantaneous poleward velocities this hypothesis suggests that the velocities of subducting plates may vary up to a factor of 5 similar plate geometries and resultant velocities have probably existed at other times in the past but their discovery may have been hindered by low preservation potential

mobility of gold during metamorphism of the dalradian in scotland abstract mobility of au and related metals during metamorphism has been suggested to be the source of metals enriched in orogenic au deposits this study investigates the mobility of au as and sb during metamorphism of the dalradian metasedimentary rocks of scotland the metamorphic processes in the dalradian of scotland are extremely well studied and the terrane is an ideal area to investigate mobility of these metals our results show that of the 25 major and trace elements analysed only au as sb s and volatile contents as shown by loss on ignition loi values show systematic variation with the metamorphic grade of the samples average au concentrations decrease from 1 1 0 55 ppb and 0 72 0 34 ppb in chlorite and biotite zone rocks down to 0 4 0 22 ppb and 0 34 0 13 ppb in kyanite and sillimanite zone rocks average as concentrations decrease from 4 8 ppm range 0 5 to 17 8 ppm and 1 96 1 9 ppm in chlorite and biotite zone rocks down to 0 24 0 15 ppm and 0 2 0 12 ppm in kyanite and sillimanite zone rocks average sb concentrations decrease from 0 18 0 15 ppm and 0 11 0 10 ppm in chlorite and biotite zone rocks down to 0 04 0 02 ppm in both kyanite and sillimanite zone rocks sulphur and loi concentrations also show significant decreases mass balance calculations indicate that compared to chlorite and biotite zone samples sillimanite zone samples have an average mass loss of 62 14 94 4 and 74 14 for au as and sb respectively every 1 km3 of chlorite biotite zone mixed psammitic pelitic protolith rock that is metamorphosed to sillimanite zone conditions would release 1 5 t au 8613 t as 270 t sb and 1 02 mt s the mobility of these elements is strongly controlled by the paragenesis of sulphide minerals pyrite sphalerite galena and cobaltite as well as gersdorffite decrease in abundance with increasing metamorphic grade in the dalradian metasedimentary rocks a critical aspect of the sulphide paragenesis is the transition of pyrite to pyrrhotite this transition is complete by mid greenschist facies in the loch lomond samples but is more gradual at glen esk occurring between biotite and sillimanite zones the au as and sb content of the sulphide assemblage also decreases with increasing metamorphic grade and we suggest that this is a controlling factor on the mobility of these metals from the dalradian metasedimentary rocks during metamorphism chlorite may be an important host mineral for as in the greenschist facies rocks breakdown of chlorite indirectly drives the mobility of au as and sb as this produces the bulk of metamorphic fluid that drives transition between pyrite and pyrrhotite we suggest that there is potential for significant undiscovered mineralisation in the central and sw highlands of scotland however as the total mass of gold mobilised is lower than observed in other metasedimentary terranes such as the otago and alpine schist s new zealand very efficient fluid focussing and trapping mechanisms would be required to form large deposits in the dalradian of scotland

paleomagnetism of cretaceous and eocene strata san miguel island california borderland and the northward translation of baja california the history of poleward motion of the baja california peninsula is a subject of continuing geologic investigations including paleomagnetic studies during the past decade geologic correlations seem not to necessitate northward movement while paleomagnetic data suggest movement of 1000 km or more we have made paleomagnetic measurements on cretaceous and eocene sedimentary rocks of san miguel island california which are western lithofacies of rocks cropping out on the baja peninsula a total of 97 drill cores taken from upper cretaceous turbidites though weakly magnetized were found to clean best in response to thermal demagnetization revealing a characteristic remanence the characteristic magnetization has an inclination 43 9 declination of 24 0 and an 95 of 3 9 this direction is 19 shallower and rotated 50 clockwise from the expected direction for the present site position and corresponds to a paleolatitude of 26 n a total of 117 drill cores of middle eocene rocks revealed characteristic magnetizations in response to either alternating field or thermal demagnetizations some samples suffer from a secondary thermal magnetization from early miocene volcanic activity requiring a thermal cleaning that incompletely removes the overprint a mean direction of magnetization can be derived from the intersection of planes fitted to the paths of demagnetization the characteristic magnetization for the eocene rocks has an inclination of 31 9 a declination of 71 0 and an 95 of 3 8 this direction is 19 shallower and rotated 79 clockwise from the expected eocene direction for this site and corresponds to a latitude of 17 n these paleomagnetic data in combination with recently published data from the baja peninsula and peninsular ranges of southern california indicate motion trajectories that mimic the north american craton for the period cretaceous to late eocene time during the oligocene the baja peninsula and adjoining regions to the west moved northward relative to the craton arriving at a pre gulf of california position not later than earliest miocene time a comparison of the northward motion of the baja peninsula with that of the santa lucia orocopia allochthon shows that both areas underwent rapid poleward translations but these motions occurred at different times

the influence of source rock type chemical weathering and sorting on the geochemistry of clastic sediments from the quetico metasedimentary belt superior province canada abstract the geochemistry of clastic sediments is controlled by at least four factors source rock composition weathering of the source rocks sorting of the weathered detritus and post depositional alteration during diagenesis and metamorphism in order to evaluate the relative importance of these factors major trace and rare earth element contents were determined for distal and proximal clastic turbidites from the archaean quetico metasedimentary belt the degree of metamorphism of the samples ranges from middle greenschist to upper amphibolite facies simple mixing of archaean mafic and acidic rocks of similar composition to those found in the adjacent granite greenstone terrains will account for the tio2 feot mgo cr sc y and hree contents of the clastic turbidites the metasediments are enriched in lree relative to the mafic acidic rock mixing line indicating that the acidic source was enriched in the lree compared to archaean terrains from other parts of the world some quetico turbidites have an additional fe rich source component possibly a chemical sediment incorporated at the time of turbidite slumping cao na2o k2o rb and sr form trends on bivariant plots that are oblique to the mafic acidic rock mixing line and hence cannot be accounted for by simple mixing of the source rocks chemical weathering of feldspars prior to deposition can account for cao na2o k2o rb and sr trends since cao na2o and sr are lost to the weathering solutions and k2o and rb are retained on the product clays the higher na 2 o cao k 2 o na 2 o rb k 2 o and sr cao ratios but lower cao content of the distal samples suggest that they were derived from a bulk sediment that had experienced a greater degree of chemical weathering than the proximal sediments sorting is the factor that most strongly influences the composition of individual samples within the proximal sample group the coarse grained metagraywackes with their higher quartz feldspar and heavy mineral contents are enriched in sio2 cao na2o sr lree zr and hf relative to the fine grained clay rich metapelites metapelites are enriched in tio2 al2o3 feot mgo k2o cr zn sc y hree cs and rb as a result of their higher clay contents thus the use of clastic sediment compositions in determining average crustal compositions e g for the ree can only give satisfactory results when all grain size fractions are sampled in their correct relative proportions the chemical differences observed between the high and low grade samples can be accounted for by source rock type chemical weathering of the source rocks or sorting post depositional element mobility does not appear to be an important factor in determining the composition of the quetico metasediments

high fo2 during sillimanite zone metamorphism of part of the barrovian type locality glen clova scotland introduction the redox state of sillimanite zone 650 700 c 5 6 kbar metasediments of the barrovian type area scotland was investigated the chemical and mineralogical evolution of the crust using estimates of metamorphic oxygen fugacity fo2 sulfur fugacity during orogenesis is critically dependent upon the redox fs2 and fluid chemistry based on new determinations of mineral state of metamorphic fluids james 1955 chinner 1960 and rock compositions from 33 samples a total of 94 of the buddington lindsley 1964 miyashiro 1964 french samples lack graphite contain both ilmenite hematite solid solutions 1966 eugster skippen 1967 thompson 1970 harte rhomox and magnetite and had metamorphic fo2 about 2 1975 ohmoto kerrick 1977 rumble 1978 lamb log10 units above the quartz fayalite magnetite qfm buffer the valley 1985 frost 1991a 1991b brenan et al 1995 regional variation in metamorphic fo2 for these rocks was minimal harlov et al 1997 ague 1998 the classical barrovian about 0 3 log10 units reflecting either a protolith that was type locality in glen clova angus scotland is a superb homogeneous with respect to redox state or an initially variable example of redox controls on metamorphism chinner protolith whose redox state was homogenized by metamorphic 1960 based on field relations petrography and the fluid rock interaction rhomox inclusions in garnet porchemical systematics of minerals and bulk rocks chinner phyroblasts that become richer in ilmenite from the interior to the 1960 showed that large differences in bulk rock oxedge of the host porphyroblast suggest that at least some synidation ratio or existed at local and regional scales metamorphic reduction of rock occurred significant variations in between metasediments during amphibolite facies metabulk rock oxidation ratio or that are probably inherited from morphism in glen clova or molecular 2fe2o3 sedimentary protoliths are found from one layer to the next or 100 2fe2o3 feo these differences were attributed ranges mostly between 20 and 50 or molecular 2fe2o3 to chemical contrasts inherited at least in part from the 100 2fe2o3 feo these or variations are uncorrelated sedimentary protoliths and were taken as evidence that with fo2 and do not indicate that large order of magnitude gradients significant differences in fo2 and fluid composition existed in fo2 and redox state existed or were preserved between layers during between layers during metamorphism chinner 1960 metamorphism the other 6 of the samples contain ilmenite lack rock oxidation ratio variations in glen clova have magnetite and had low fo2 0 1 order of magnitude below qfm been unequivocally established but important questions in the stability field of graphite they are characterized by comremain regarding the metamorphic fluids for example binations of the following large fluid hf h2o metasomatic what were the quantitative variations in the fugacities of tourmaline bearing veins absence or rarity of primary organic o2 and sulfur species such as s2 in the region did large matter and crosscutting late metamorphic shear zones rich in variations in fo2 and fluid composition exist between carbonaceous material such observations suggest that locally low intercalated metasedimentary layers could metafo2 conditions may have been related to the influx of reducing fluids morphic fluid rock interaction between layers have overfrom elsewhere in the area come characteristic buffer capacities inherited from

an oxygen and carbon isotopic study of multiple episodes of fluid flow in the dalradian and highland border complex stonehaven scotland the carbon and oxygen systematics of rocks north of stonehaven scotland were studied to provide new constraints on the nature and timing of metamorphic and post metamorphic fluid infiltration carbon and oxygen isotopic data were collected from carbonated spilites and carbonate bearing veins in the highland boundary fault from dalradian metacarbonate layers and pervasively carbonated schists and from three generations of carbonate bearing veins and a quartz porphyry dyke within the dalradian isotopic evidence for syn metamorphic fluid infiltration in the dalradian is preserved in quartz calcite veins in the upper chlorite zone and in staurolite zone metacarbonates 18 o values for coexisting quartz 18 o quartz 13 6 and calcite 18 o calcite 11 6 in a vein from the upper chlorite zone are consistent with precipitation of both minerals from a single fluid at c 375 c oxygen isotopic data from staurolite zone metacarbonate layers 18 o calcite 10 4 to 12 9 suggest major isotopic exchange with an external fluid equilibrated with a quartzo feldspathic reservoir such as the dalradian metasediments at metamorphic temperatures low negative carbon isotope data from these metacarbonate layers 13 c calcite 12 4 to 17 7 may indicate a component of oxidized organic material in the infiltrating fluid post metamorphic fluids derived from or equilibrated with a high temperature silicate reservoir such as felsic igneous rocks were responsible for precipitation of post metamorphic vein carbonate carbonation of pelitic schists and alteration of metamorphic feldspar porphyroblasts from barrow s famous spotted chloritoid schists in an area geographically associated with several carbonated quartz porphyry dykes some veins in the biotite and garnet zones have been conduits for multiple generations of fluid flow quartz from this generation of veins preserves a metamorphic isotopic signature 18 o quartz 11 2 to 13 2 whereas dolomite and calcite are considered to be post metamorphic based on textural evidence and isotopic data indicating extreme oxygen isotopic disequilibrium between vein quartz and carbonate minerals 18 o dolomite 23 7 to 24 2 18 o calcite 25 3 18 o quartz 11 2 to 13 2 variably depleted carbon isotopes from carbonate minerals in these veins 13 c dolomite 8 1 to 13 4 13 c calcite 12 2 may reflect input of oxidized organic material in the mineralizing fluid isotopic data from carbonate veins and variably carbonated spilites from the highland boundary fault are consistent with hydrothermal alteration of basalts by seawater which had its carbon isotope systematics modified by addition of magmatic co 2 and or oxidation of organic carbon such alteration may have occurred on the seafloor or during emplacement in the highland boundary fault

northward translation of baja british columbia along the late cretaceous to paleocene margin of western north america paleomagnetic data suggest that a large part of british columbia and southern alaska was at the latitude of baja california mexico in early late cretaceous time this allochthonous block baja bc includes the insular talkeetna in alaska and intermontane composite terranes the miniterranes between them and possibly part of the omineca metamorphic belt assuming that baja bc was adjacent to north america by middle cretaceous an analysis using plate kinematics of the pacific basin produces a model of northward movement of baja bc which is compatible with the known geology of the reconstructed margin of western north america baja bc moved north 2400 km as part of the kuia plate along a transform margin with north america from 85 to 66 ma this time interval is marked by the cessation of igneous activity in the sierra nevada development of wrench faults and basins along coastal california and initiation of laramide style tectonics to the east within baja bc diminished igneous activity to the west was coeval with possible wrench tectonic basins along its eastern margin by the end of the cretaceous baja bc was positioned from oregon to northern british columbia between 66 and 56 ma baja bc became attached to north america in a dextral transpressive stage during which the kula north america plate boundary expanded to encompass the whole block with a series of northwest trending strike slip faults a fold and thrust belt in the east and large scale uplifts and the formation of an incipient volcanic arc on the west southern alaska was the site of convergence during the whole of the period of northward translation of baja bc a late cretaceous subduction zone in alaska evolved into a major compressional belt as the basin and miniterranes on the northern margin of baja bc collided with interior alaska the collision culminated in the paleocene with formation of the mckinley granites and coeval dextral slip on the denali fault system

the correlation of reaction and isotope fronts and the mechanism of metamorphic fluid flow infiltration of a metabasite sill from islay scotland by an h2o co2 fluid caused 1 modification of delta 18 o and 2 carbonation at the sill margins maps of delta 18 o and reaction progress were constructed from a 20 x 47 7 metre sample grid across

the antrim galway line a resolution of the highland border fault enigma of the caledonides of britain and ireland the westward continuation of the highland border fault of scotland hbfz into ireland is problematic it is widely thought to follow a pronounced magnetic and gravity lineament the fair head clew bay line fcl the advantage of this interpretation is that it places all the ordovician ophiolitic complexes and associated sedimentary basins to the south of the fcl which would represent the contact between laurentia and the outboard terranes its main shortcomings are that both the deep structure and timing of strike slip are different on the hbfz and fcl in ireland the fcl is a north dipping feature that can be traced to the moho on birps profiles while the hbfz has no such signature terrane amalgamation in western ireland was completed by the late ordovician while in scotland the midland valley terrane did not finally dock until the early devonian these considerations suggest that in western ireland a branch of the hbfz exists which was active in post ordovician time and must lie south of connemara an examination of irish geological geophysical and image processed magnetic data shows that a profound lineament can be traced from antrim to galway bay the antrim galway line stitching plutons date movement on it as pre 405 ma we propose that the antrim galway line represents the continuation of the scottish hbfz while the fcl is a preserved ordovician splay of the hbfz system whose northdipping geometry is a product of ordovician collapse of the orogen in western ireland

tungsten isotope composition of the acasta gneiss complex high precision tungsten 182w 184w isotope measurements on well characterised mafic and felsic samples of the ca 3960 ma acasta gneiss complex agc northwest territories canada show radiogenic e182w values between 0 06 to 0 15 two ca 3600 ma felsic samples from this terrane have e182w 0 and are the oldest samples so far documented to have a w isotopic composition indistinguishable from that of the modern mantle the e182w data are correlated with e142nd roth et al 2014 and we attribute this variability to incomplete metamorphic homogenisation of the 3960 ma protolith with more recent material in a 3370 ma tectono thermal event notably the value of the positive e182w anomalies seen in the 3960 ma agc samples that are least affected by metamorphic homogenisation is comparable to that observed in other early archean rocks isua supracrustal belt greenland nuvvuagittuq supracrustal belt canada and the late archean kostomuksha komatiites karelia this demonstrates a globally constant signature we infer that the presence of a pre late veneer mantle represents the most straightforward interpretation of a uniform distribution of ew182 0 15 value in archean rocks of different ages we show that such a notion is compatible with independent constraints from highly siderophile element abundances in mafic and ultra mafic archean mantle derived rocks the absence of anomalous e182w and e142nd so far measured in samples younger than ca 2800 ma suggests progressive convective homogenisation of silicate reservoirs the downward mixing of an upper mantle rich in late delivered meteoritic material might account for these combined observations

assessing the geochemical and tectonic impacts of fluid rock interaction in mid crustal shear zones a case study from the intracontinental alice springs orogen central australia the reynolds anmatjira ranges central australia form part of a high grade basement terrane dissected by intensely metasomatized transpressional shear zones active during the ordovician carboniferous alice springs orogeny unlike typical retrograde structures associated with discrete fluid flow the mid crustal setting and intracontinental nature of these shear systems present significant problems for the source and ingress mechanism of the fluid involved in their rehydration to address these issues we describe two detailed traverses across deformed and metasomatized basement rocks in this region and interrogate their record of fluid rock interaction from various perspectives both traverses combine structural and petrological observations with zr in rutile and ti in quartz thermobarometry oxygen and hydrogen stable isotope analysis and major trace and rare earth element mobility trends each technique is critically evaluated for its utility in this study and its more widespread applicability to alternative field areas providing a strategic framework for the general investigation of fluid affected shear zones ultimately the integrated data sets specify pressure temperature conditions of 530 c and 4 5 kbar implying average apparent thermal gradients of 29 36 c km 1 and depths of 14 18 km other characteristic features to emerge include strongly variable element mobilities and pronounced isotopic depletion fronts consistent with the alteration effects of an externally derived non equilibrium fluid this is confirmed by calculated fluid compositions indicative of contributions from a fluid of meteoric origin with estimated 18o and d values as low as 2 3 and 59 8 respectively we propose that these surficial fluid signatures are imposed on the mid crust by the prograde burial and dehydration of hydrothermally altered fault panels produced during pre orogenic basin formation progressive fluid release with continued subsidence then leads to the accumulation of increasing fluid volumes in the vicinity of the brittle ductile transition promoting extensive hydration metasomatism and reaction softening at the locus of stress transmission from plate boundary sources the sustained injection of externally derived fluids into refractory crustal material may thus stimulate a critical reduction in the long term strength of the lithosphere providing strong impetus for the initiation and advancement of intracontinental orogenesis

paleomagnetic evidence for motions of parts of the canadian cordillera abstract paleomagnetic evidence from major terranes of the western cordillera in canada referred to collectively as baja british columbia indicates that they have moved as a whole about 2300 km from the south and rotated 60 clockwise relative to north america in the late cretaceous and earliest tertiary 85 50 ma sometime between the early jurassic and the mid cretaceous 190 100 ma these same terranes moved south over 1000 km and underwent irregular but where well constrained counter clockwise rotations the terranes amongst themselves seem to have shared much the same paleolatitude during and since the late triassic although they may have undergone relative rotations in the later jurassic and early cretaceous these motions are now known to be broadly consistent with the motions of oceanic plates to the west as calculated by independent procedures and indicate that the terranes of the western cordillera were for part of their existence coupled to oceanic plates first to the farallon plate during the late jurassic and early cretaceous and then to the kula plate during the late cretaceous and early tertiary it was this change of allegiance from farallon to kula that caused first the southerly and then the northerly motion the paleomagnetic evidence despite claims to the contrary is consistent with geologically inferred displacements along major faults in the canadian cordillera when due account is taken of the age constraints on those motions

transport mechanisms by fluid flow in metamorphic rocks oxygen and strontium decoupling in the trois seigneurs massif a consequence of kinetic dispersion

remagnetization and northward coastwise transport of franciscan complex rocks northern california a reinterpretation of the paleomagnetic data previous paleomagnetic studies of accreted oceanic rocks within the franciscan complex of northern california have concluded that these rocks originated far to the south of their present positions with respect to the north american continent based on positive fold tests the characteristic remanent magnetizations were inferred to predate accretion related deformation and metamorphism and to have been acquired during or soon after deposition of these rocks thus the paleomagnetic data were thought to provide direct information on ancient oceanic plate motions however the plate motions implied by some of these paleomagnetic data are problematic e g exceptional plate velocities and uniform polarity magnetizations in almost all of these rocks indicate the possibility of remagnetization recent work on oceanic rocks in similar subduction complexes of japan and mexico have shown that they were most likely chemically remagnetized during accretion prior to disruption of the original stratigraphic sequences modern analogs indicate that the oceanic rocks in mexico were probably remagnetized while still part of a shallow dipping subducting slab 10 at the base of an accretionary prism assuming these rocks were near horizontal at the time of remagnetization paleolatitudes at which these rocks were subducted and subsequent arc parallel displacements along the western margin of north america can be inferred in this paper franciscan rocks in northern california are reinterpreted as also having been remagnetized prior to accretion related deformation this scenario satisfies both geologic and paleomagnetic constraints for these rocks and resolves conflicts between data indicating both remagnetization and tectonic displacement transport of the laytonville limestone from the southern hemisphere is not required paleolatitudes of subduction and remagnetization in the northern hemisphere 12 to 33 appear to be inversely proportional to age of accretion middle cretaceous to oligocene for the franciscan rocks subsequent northward diplacements 800 to 3700 km and clockwise rotations 56 and 154 of these rocks inferred from the paleomagnetic data are consistent with potential displacements along the western margin of north america during late mesozoic and cenozoic time calculated using examples of modern subduction zones and current plate reconstruction models

fast tectonometamorphism and exhumation in the type area of the barrovian and buchan zones the beginning of the grampian episode of tectonometamorphism in scotland is dated by collision and obduction of the ballantrae ophiolite complex at 478 8 ma for the first time we have used zircon and garnet to radiometrically date the peak of the caledonian buchan and barrovian tectonometamorphism in their type areas as contemporaneous at 467 2 5 ma detrital garnet from the neighboring southern uplands flysch terrane has the same radiometric age and occurs in 465 2 5 ma molasse in the midland valley we show that the entire grampian episode lasted 15 2 5 m y and that it took fewer than 7 6 m y to expose high grade rocks following the peak of metamorphism those who study precambrian orogeny should note the brevity of this scottish tectonometamorphic episode and the speed of exhumation

regional metamorphic dehydration and seismic hazard we use a one dimensional model of coupled heat transport fluid transport porosity feedback and metamorphism to investigate how dehydration reactions proceeding during burial of hydrous rocks in the footwall of thrusts or the hanging wall of normal faults may influence the temporal evolution of pore fluid pressure pf based on a new interpretation of existing kinetic data for the dehydration of model serpentinite we advance the hypothesis that pf buildup to tensile failure may be possible within decades of the onset of dehydration in low permeability metamorphic rocks of the middle crust this model contrasts with both 1 typical earthquake nucleation models which involve preexisting fluids that play a passive role responding to tectonic stresses rather than generating them and 2 conventional treatments of pf generation based on equilibrium thermodynamics and constant rates of dehydration which require 10 106 years to produce significant pf increases our results suggest that repeated cycles dehydration induced seismicity are plausible in low permeability areas of the mid crust and can be sustained throughout an orogenic episode if fresh hydrous minerals descend into the metamorphic zone along the downgoing face of a thrust or normal fault system

arc related high k magmatism in the ceuta peninsula internal rif spain discovery and consequences we document the occurrence of micro diorite magmatic sills with magmatic enclaves in the ceuta peninsula within metapelites from the lower sebtides units internal rif all magmatic rocks show a primary magmatic mineralogy and geochemical signature diagnostic for high k calc alkaline to shoshonitic island arc magmatism moreover these rocks are significantly affected by secondary metamorphic transformations under greenschist to amphibolite facies conditions regionally dated at c 21 ma geometric relationships between the sills and the main regional foliation developed under intermediate pressure granulite facies conditions at c 28 ma demonstrate that the sills emplaced during the late stage of this main tectonic event new u pb in situ analyses of monazite performed on the micro diorite sills provide an age of 20 64 0 19 ma coherent with this chronological framework and interpreted as the age of greenschist facies re equilibration the discovery of pre miocene high k calc alkaline to shoshonitic arc related magmatism is clearly consistent with the subduction context proposed for the alboran basin evolution according to geophysical investigations in this framework the lower sebtides units could be considered as part of the upper plate of the subduction system while the upper sebtides must be regarded as the lower subducted plate

magnetic stratigraphy applied to synsedimentary slumps turbidites and basin analysis the scaglia limestone at furlo italy the previously studied italian scaglia sections at gubbio and moria consist entirely of pelagic limestone and marl by contrast the furlo section is characterized by synsedimentary slump folds and slide masses in the lower portion cenomanian coniacian and by intercalation of white calcarenitic turbidites in the upper part campanian paleocene demagnetization to 20 mt and 350 c or higher reveals a single stable direction after removal of an initial low coercivity component despite the stratigraphic complications the pattern of polarity zones clearly matches gubbio and the sea floor anomalies the abrupt faunal break at the end of the cretaceous was located and foram zonation shows that the very base of the paleocene is missing sedimentation rate determinations based on the magnetic polarity zonation have clarified the origin of stratigraphic complications in the furlo paleocene most samples were taken from pelagic beds but ten samples from calcarenitic turbidites yielded directions indistinguishable from those of adjacent pelagic beds detailed sampling of a turbidite 33 cm thick confirmed that this lithology is an accurate recorder of the geomagnetic field synsedimentary slumps range from coherent slide blocks with undeformed bedding to smoothly curving folds indicating flow of very soft sediment coherent slide blocks show declinations rotated by tens of degrees away from the typical regional direction but their inclinations are unaffected soft sediment slump folds yield typical regional directions indicating resetting of the remanent magnetization after deformation using stratigraphic structural and paleomagnetic data it is possible to identify in the lower part of the section a basal nonslumped unit overlain by seven slump units with various characteristics inter layered with four units of pelagic sediments which were deposited on older slump units but which themselves were not affected by synsedimentary slumping this paper emphasizes the value of paleomagnetic stratigraphy as a tool in detailed studies of sedimentology stratigraphy and basin analysis

contribution of ductile flow to exhumation of low temperature high pressure metamorphic rocks san juan cascade nappes nw washington state the san juan cascade sjc nappes were subducted to a depth of 18 km metamorphosed under low temperature high pressure conditions and then exhumed all within 16 m y during exhumation penetrative deformation by solution mass transfer smt resulted in a widespread spaced cleavage strain directions were determined for 27 sandstone samples and absolute strain measurements for a subset of 19 samples z directions generally plunge moderately to the ne and x and y directions are scattered in the plane perpendicular to z smt deformation is constrictional at the local scale but the tensor average indicates plane strain uniaxial shortening at the regional scale with sx sy and sz equaling 1 01 0 91 and 0 58 respectively the average flattening plane xy dips 30 to the ne and the average x direction plunges 20 to the north the large shortening in z was compensated by a mass loss volume strain of 47 we present a simple one dimensional model that illustrates the relationship between finite strain and ductile exhumation for a steady state convergent wedge assuming depth dependent ductile flow and no reversal of principal strain rates with depth this model indicates that ductile thinning of the sjc nappes accomplished only 13 of the total exhumation despite a vertical shortening strain of 36 there is no evidence that normal faulting contributed significantly to exhumation we conclude that erosion operating at an average rate of 1 1 km m y 1 was the dominant exhumation process

benthic foraminiferal assemblages in explorers cove antarctica a shallow water site with deep sea characteristics abstract during november and december 1993 we sampled foraminiferal populations living at a 25 28m deep site in explorers cove mcmurdo sound antarctica by means of an airlift suction apparatus and sediment coring mean standing stocks stained specimens per 10cm2 were 8 55 2 60 all specimens 1mm 0 10cm layer 62 1 27 5 all specimens 500 m 0 10cm layer 483 all specimens 63 m 0 1cm layer and 632 all specimens 28 m 0 1cm layer the fauna comprises diverse taxa including both hard shelled and soft shelled forms the coarser fractions 1mm are dominated by large agglutinated foraminifera mainly spheres and domes psammosphaerids saccamminids notodendrodiids but also contain some large calcareous taxa cibicides pyrgo the finer fractions 500 63 28 m yield numerous soft bodied monothalamous foraminifera in addition to the more commonly studied calcareous and multilocular agglutinated forms many species adopt epifaunal or shallow infaunal 0 1cm layer microhabitats but a few smaller ones are more deeply infaunal 1cm a variety of trophic strategies can be inferred from the limited available evidence some abundant large species crithionina spp gromia oviformis may be shallow infaunal or epifaunal deposit feeders but the most striking feature of the fauna is the conspicuous occurrence of large epifaunal species which appear to be suspension feeders e g astrammina astrorhiza notodendrodes cibicides some of these foraminifera can probably switch trophic mode for example from suspension feeding to osmotrophy in response to a fluctuating food supply the explorers cove fauna includes a mix of wide ranging and endemic species the main features of the fauna are consistent with environmental similarities uniformly low temperatures physical tranquillity episodic food inputs between explorers cove and deeper water settings in the northern hemisphaere comparable faunas i e those in which large agglutinated taxa are a conspicuous element are found in deep sublittoral and fjordic areas 100 1000m as well as at some bathyal and abyssal localities down to several thousands of metres which receive a substantial organic matter input from processes such as upwelling foraminifera probably play a major ecological role in these systems particularly in trophic interactions and organic carbon cycling their success may depend on two main attributes first they possess extremely efficient food gathering organelles reticulopodia and second they become quiescent when starved but can rapidly increase their metabolic rate when presented with food our scuba accessible study site in explorers cove provides a unique opportunity to clarify the role that these important protists play in shallow antarctic waters as well as in more remote deep water environments

evidence for the lower palaeozoic age of the tay nappe the timing and nature of grampian events in the scottish highland sector of the laurentian margin stratigraphical and structural evidence for latest proterozoic phanerozoic deposition of the dalradian supergroup in the scottish highlands sector of the laurentian margin is incompatible with geochronological data suggesting c 750 ma knoydartian and c 600 ma grampian orogenesis the age of the 590 2 ma ben vuirich granite is critical to the dating of grampian orogenesis fabric evidence indicates that the granite predates d2 while its relationship to d1 is unclear in this paper the d1 d2 overprint during the evolution of the tay nappe a regional grampian structure is examined documentation of the distribution style and orientation of d1 and d2 structures from well exposed sections near the highland border has allowed the development of a kinematic model for the d1 d2 deformation in this model early crustal shortening resulted in d1 upright structures that were subsequently overturned in a d2 se directed shear regime associated with underthrusting of a southern landmass the model is best explained in terms of a single continental collision event with no evidence of a time gap between d1 and d2 this analysis is consistent with pre regional d1 intrusion of the ben vuirich granite and an early palaeozoic grampian orogeny

metamorphic reactions in the biotite zone eastern scotland high thermal gradients metasomatism and cleavage formation prograde metamorphic reactions involving the growth of phyllosilicates and accompanying cleavage development have been investigated in dalradian metasediments from the biotite zone of eastern scotland crystallization of muscovite within the psammites of the southern highland group is linked to the replacement of plagioclase porphyroclasts this reaction is triggered by significant alkali metasomatism during active deformation and plays an important role in the formation of a prominent spaced cleavage within the psammites the si content of most of these early formed muscovites is partially buffered by the quartz content of the rock although close to the highland boundary fault evidence of greater influence from externally derived fluids on muscovite compositions is preserved locally higher fluid fluxes adjacent to the fault are also indicated by a relatively high 18o smow signature in the rocks the biotite producing reaction in these greenschist facies rocks is linked to the later production of a celadonite poor muscovite which formed as overgrowths around pre existing white micas this reaction is sensitive to the initial composition of muscovite and preferentially occurs in quartz rich metasediments containing a celadonite rich muscovite a systematic increase in the progress of the biotite producing reaction northwards across the biotite zone confirms the presence of high geothermal gradients along the southern margin of the dalradian block adjacent to the highland boundary fault

evidence for hfse and ree mobility during calc silicate metasomatism mesoarchean 3075ma ivisaartoq greenstone belt southern west greenland abstract the mesoarchean ca 3075 ma ivisaartoq greenstone belt consists dominantly of variably deformed and metamorphosed mafic to ultramafic flows basalts to picrites in addition there are minor serpentinites gabbros clinopyroxene rich cumulates and volcaniclastic and siliciclastic sedimentary rocks primary volcanic structures such as pillows and volcanic breccia are well preserved in low strained zones the belt underwent intense metasomatic hydrothermal alteration between 3075 and 2961 ma an early stage of calc silicate metasomatism stage i resulted in the formation of epidosites in pillow cores pillow interstitials and intrusive margins of gabbroic dykes this alteration appears to have resulted from sea floor hydrothermal alteration a second stage of calc silicate metasomatism stage ii formed centimeter to meter scale layers concordant to discordant relative to the regional foliation these layers consist of diopside garnet epidote plagioclase quartz titanite vesuvianite calcite actinolite scheelite the second stage calc silicate rocks are associated with shear zones and their origin is attributed to interaction of reactive fluids with mafic and ultramafic rocks during regional metamorphism both stages of calc silicate metasomatism caused significant mobilization of major elements si mg ca fe na k and mn lile rb cs sr ba pb and lree heavy rare earth elements hree and hfse th nb ta zr ti remained relatively immobile during stage i metasomatic alteration but were variably disturbed during the stage ii metasomatism accordingly the stage ii calc silicate rocks exhibit evidence for significant mobility of th nb and ta on a local scale 1 4 m whereas zr and ti were not significantly mobilized this resulted in calc silicates with complementary trace element patterns displaying extreme negative to positive nb anomalies nb nb 0 19 7 28 transition metals ni v co cr and sc were relatively immobile during both metasomatic events amphibolites spatially related to the second stage calc silicate rocks preserve mineralogical and major element characteristics consistent with a mafic to ultramafic composition however their near primary magmatic trace element compositions were variably modified due to strong ree mobility during the stage ii metasomatic event this gave rise to various ree characteristics including lree enriched lree depleted and u shape ree patterns some of these element patterns resemble those shown by modern boninites and mid ocean ridge basalts n morb in contrast the least altered amphibolites exhibit similar trace element patterns to those displayed by the least deformed non metasomatized mafic and ultramafic pillow flows they possess relatively flat trace element patterns with negative nb ta anomalies indicating a subduction zone geochemical signature

structurally focused fluid flow during orogenesis the islay anticline sw highlands scotland abstract displacement of isotopic compositions at boundary layers across strata of contrasting composition is commonly used to investigate hydrothermal fluid flow during orogeny this study investigates whether hydrothermal fluid flow was focused along the islay anticline islay sw highlands of scotland as shown in the axial zone of the neighbouring ardrishaig anticline four localities from the limb to the axial plane of the islay anticline were investigated for isotopic homogenization of metacarbonate units to silicate values at mull of oa on the limb of the anticline metacarbonate samples show limited isotopic resetting and the fluid flux is estimated to be 3 m 2 within the axial zone of the islay anticline metacarbonate units from port a9 chotain and bagh an da dhoruis show complete isotopic homogenization to silicate values indicating higher fluid fluxes fluid flow was enhanced along localized parasitic folds such as at port an t sruthain where metacarbonates have been isotopically reset and there are abundant quartz carbonate veins that precipitated during d 1 d 2 deformation metamorphic fluid flow was higher in the axial zone of the islay anticline and in localized antiformal structures fluid fluxes are estimated to be considerably lower than at the neighbouring ardrishaig anticline

regional fluid and metal mobility in the dalradian metamorphic belt southern grampian highlands scotland a prominent set of veins was formed during post metamorphic deformation of the caledonian dalradian metamorphic belt these veins are concentrated in dilational zones in fold hinges but apophyses follow schistosity and fold axial surface fractures the veins are most common in the cores of regional structures especially the dalradian downbend and consist of quartz calcite chlorite and metallic sulphides and oxides metals including gold have been concentrated in the veins the fluid which formed the veins was low salinity 1 5 wt nacl and kcl co2 bearing 3 16 wt co2 water of metamorphic origin the fluid varies slightly in composition within and between samples but is essentially uniform in composition over several hundred km2 vein formation occurred at about 350 50 c and 200 300 mpa pressure further quartz mineralization occurred in some dilational zones at lower temperatures 160 180 c this later mineralization was accompanied by co2 immiscibility dilution and oxidation of the metamorphic fluid occurred due to mixing with meteoric water as the rocks passed through the brittle ductile transition a similar metamorphic fluid is thought to have been responsible for gold mineralization in the nearby tyndrum fault at a later stage in the dalradian uplift

southern hemisphere origin of the cretaceous laytonville limestone of california new paleomagnetic paleontologic and stratigraphic data from outcrops of the laytonville limestone 101 to 88 million years old support a southern hemisphere origin a paleomagnetic megaconglomerate test is statistically significant and suggests magnetization at 14 5 south predating late cretaceous to eocene 70 to 50 million years ago accretion rapid kula plate movement or the existence and demise of a now vanished oceanic plate or both are required to accommodate the greater than 50 of poleward displacement implied by the paleomagnetic data this rapid motion brings into question the validity of a speed limit for absolute plate velocity based on present day plate motions

genesis and mobility of the h2o co2 fluid phase during regional greenschist and epidote amphibolite facies metamorphism a petrological and stable isotope study in the scottish dalradian during high pressure low temperature greenschist and epidote amphibolite facies metamorphism in dalradian rocks of the sw scottish highlands mineral assemblages in metabasites and calcareous metasediments were dominantly controlled by infiltration of hydrous fluids consequently mineral assemblages capable of buffering the fluid phase composition were rare equilibrium prograde h 2 o co 2 fluids usually contained less or much less than about 1 2 mol co 2 three fluid infiltration events are recognized during prograde greenschist facies metamorphism metabasic sills were infiltrated by large volumes of co 2 bearing hydrous fluid carbon isotope studies indicate that the co 2 was locally derived by widespread oxidation of graphite or other organic carbon in adjacent metasediments this may have occurred under approximately lower greenschist facies conditions as a result of mixing of fluids of varying f o 2 initiated by thermal expansion of water during heating decompression and consequent hydraulic fracturing in the epidote amphibole facies garnet zone dehydration reactions in metabasites generated large quantities of water which removed carbonate from metabasites on a regional scale and infiltrated calcareous metasediments to produce assemblages containing grossular diopside k feldspar amphibole clinozoisite and sphene a late retrograde infiltration of co 2 bearing hydrous fluid under lower greenschist facies conditions generated assemblages containing k feldspar chlorite rutile dolomite in calcareous rocks and albite prophyroblast schists in zones of intense secondary deformation large scale infiltration of fluid into greenschist facies metadolerite sills was intimately related to and possibly controlled by penetrative deformation and in the absence of a penetrative deformation grain boundary diffusion by itself was an ineffectual mechanism of fluid transport

the highland border rocks and their relation to marginal basin development in the scottish caledonides it has long been known that the highland border includes an assemblage of altered serpentinites gabbros spilitic lavas ferruginous and manganiferous cherts black phyllites quartzose clastics and amphibolites new work confirms that many of these rocks represent a dismembered tectonically emplaced ophiolite of caledonian age which originated in a narrow oceanic basin within or bordering the dalradian sedimentary pile the ophiolitic rocks were expelled from the basin as thrust sheets transecting d1 structures studies in the sw highlands suggest emplacement during d2 synchronous with se translation of the tay nappe this implies that the ophiolite originated in the n the highland border rocks are seen as marginal to a long lived northward dipping subduction zone within the southern uplands the evolution of the ballantrae ophiolitic complex the midland valley and the dalradian sedimentary basin are considered within this framework

franciscan complex limestone deposited at 24 n limestone and diabase from 11 sites in the permanente quarry the largest outcrop of calera limestone in the franciscan terranes have been studied by paleomagnetic and biostratigraphic methods observation of the upper albian uppennost albian boundary at one outcrop provides age and polarity control we propose a primary origin for a magnetic component identified in this unit another unit has been remagnetized by the diabase sill however predeformational inclinations are in agreement suggesting that remagnetization occurred shortly after deposition inclinations from these sites and another farther north at royce quarry successfully pass a megaconglomerate test and suggest a depositional paleolatitude of 24 n 4 this agrees with a plate model in which the calera limestone was deposited on the farallon plate at about 100 ma and was accreted to north america at about 70 ma

hydrothermal mobility of ti zr and ree examples from the bergell and adamello contact aureoles italy contact metamorphic marbles affected by metasomatic fluids at the contact to the bergell and adamello intrusives contain various accessory minerals zirconolite allanite titanite rutile geikielite hoegbomite which provide new evidence for the hydrothermal mobility of ti and zr in both examples ti and zr migrated along with u th y and ree in a metasomatic fluid rich in potassium the composition of the main minerals fluorine rich phlogopite pargasite and titanian clinohumite and the abundance of fluor apatite demonstrate that fluorine and phosphorus were important components of the fluid the textural relationships indicate that the formation of the accessory phases is linked to the crystallization of the hydrous minerals

regional scale fluid flow and element mobility in barrow s metamorphic zones stonehaven scotland the geochemistry of metamorphic quartz vein formation in barrow s index mineral zones north of stonehaven scotland was investigated in order to assess regional fluid flow and mass transfer metamorphic grade in the dalradian metasediments increases to the north northwest away from the highland boundary fault hbf and associated ophiolitic rocks of the highland border complex hbc passing through the chlorite chl biotite bt garnet grt chloritoid cld and staurolite st zones syn metamorphic fluid infiltration at 462 8 8 ma breeding et al in am mineral 89 1067 2004 produced considerable quartz veining vein abundance varies from about 5 to 15 volume percent of the outcrops veins tend to be more abundant in metapelitic layers than in metapsammitic ones metamorphic veins are surrounded by centimeter to decimeter wide zones of chemical and mineralogical alteration selvages porphyroblasts particularly bt grt cld and st are typically larger in selvages than in wallrocks distal to veins the altered selvages underwent fluid driven addition of na ca and sr and loss of k rb and ba alteration is most intense within 750 m of the hbf but is still very significant at the northern end of the field area some 2 km away mg fet fet total iron was either unchanged or increased due to alteration silica was added at some chl and bt zone localities near the hbf pb mass transfer was variable although pb was added at a number of locations rare earth elements ree were generally immobile but light ree and possibly heavy ree were lost at one field site the gain of na and ca and loss of k promoted the growth of plagioclase at the expense of micas particularly muscovite in selvages and wallrock inclusions throughout the field area and probably some calcite and or dolomite growth directly adjacent to the hbf the ca gains were also critical for epidote production in the bt zone gains of ca and increases in mg fet helped to stabilize grt at the expense of cld and st in some selvages hornblende and cummingtonite were discovered in strongly altered metapelitic rocks at one cld zone locality the metasomatism puts important constraints on the processes of mass transfer and suggests two models for regional fluid flow in the first model fluid flow in a direction of increasing temperature downward along the hbf added na and ca and removed k from the dalradian in the second model fluid flow upward from the hbc transported na and ca into the overlying dalradian and at the same time stripped out k the latter model is favored because it can most readily account for silica addition near the hbf but neither model can be ruled out at present in either case the veins represent fractures that transmitted very large time integrated fluid fluxes of at least 104 m3 fluid m2 rock consequently the veins were conduits for regional fluid flow that caused considerable open system chemical and mineralogical alteration during metamorphism

extreme channelization of fluid and the problem of element mobility during barrovian metamorphism geochemical profiles across chemically altered zones selvages surrounding amphibolite facies quartz kyanite veins were investigated to determine major minor and trace element mass transfer and volume strain due to channelized fluid flow the three profiles are perpendicular to two veins cutting the wepawaug schist connecticut u s a which underwent barrovian style metamorphism during the acadian orogeny selvages are highly aluminous with considerably more kyanite staurolite and garnet and less quartz plagioclase and mica than surrounding wallrocks kyanite crystals increase in size toward veins and reach several centimeters in length within veins mass balance analysis indicates 50 silica loss and 34 volume loss from selvages on average silica transferred locally from selvages to veins accounts for 40 to 80 of the vein silica the remainder must have been precipitated from fluids that flowed through the veins during regional devolatilization fluid flow also transported elements into the rock mass which were concentrated in the selvages gains of fe mn y and hree were due to the growth of selvage garnet fe zn and li were sequestered into staurolite kyanite staurolite and garnet growth resulted in al mass gains destruction of mica particularly muscovite and of plagioclase in the selvages resulted in losses of k na ba pb sn and volatiles and losses of na sr and eu respectively thin na and sr enrichment zones associated with increased modal plagioclase are found along the margins between selvages and less altered wallrock and may represent either chemical self organization produced during diffusive mass transfer and reaction or are relics from a possible earlier period of na enrichment in the selvages simple two dimensional numerical modeling of flow in a fractured porous medium indicates that fluxes vary significantly over short distances 1 m adjacent to veins fluids channelized into the high permeability fracture conduits carry the bulk of the fluid flow mass transfer to and from selvages adjacent to the conduits occurred by some combination of diffusion and flow in contrast areas distal to the conduits are impoverished in fluid and undergo much more limited infiltration as a consequence different workers can come to vastly different conclusions about the magnitude of fluid fluxes and element transfer depending on the part of a fractured outcrop studied this extreme spatial variability due to channelization can largely explain contrasting views that have arisen in the literature regarding the nature and intensity of non volatile element mass transfer during barrovian metamorphism determining average time integrated fluid fluxes and levels of element transport across outcrops remains as important research challenges due to the spatial variability of flow

cerium a chemical tracer for paleo oceanic redox conditions abstract the ce anomalies found in marine carbonates provide a sensitive indicator for the study of paleo oceanic redox conditions the dependency of cce the observed ce concentration and cepo04 the activity of the cepo4 complex to the po2 ph and pco2 of seawater can be used for deciphering some aspects of the paleo ocean chemistry and is log c ce log cepo 0 4 13 2 0 25 log p o 2 3 0 ph 1 95 0 25 log p o 2 1 50 log p co 2 1 50 log ca the ph which is related to pco2 and po2 are major factors for controlling the ce concentration in seawater therefore ce anomalies ceas ce observed ce interpolated between la and pr or nd serve as indicators for ph or pco2 or po2 changes nearly uniform ce abundances found in pacific 5000 m water columns piepgras and jacobsen 1988 and the ceas in ichthyoliths can be explained using this model a general cea shift in sediments from d s d p holes 525a 530a b 516 516f suggest that the entire south atlantic water redox condition changed from reducing to oxidizing at 56 my in the north atlantic the galicia margin samples show a redox change between 132 148 my indicating active surface currents between the atlantic ocean and the shallow sea off the iberian peninsula the carbonate samples from d s d p hole 316 in the central pacific yielded an average cea of 0 15 0 01 which is 2 3 the ceas observed in deep pacific seawater by piepgras and jacobsen 1988 the relative enrichment of ce in these samples is ascribed to coprecipitation of ce oh 4 onto fe mn o coatings the striking similarities in the trace element profiles of ree sc hf th and ceas and the low u abundances 0 08 ppm in many marine carbonates indicate insignificant ce redox and trace elemental concentration changes for the pacific ocean over the past 95 my no significant ce redox change occurred in the pacific ocean within k t boundary one 240 my china limestone deposited under oxic sedimentary conditions yielded a cea of 0 53 calculations indicate a pco2 of 1 9 present value prevailed over the 240 my tethyan ocean u th ratios and low u concentrations are diagnostic indicators for obtaining reliable ceas in marine carbonates and perhaps for biogenic apatites and conodonts as well

geology of the eel river basin and adjacent region implications for late cenozoic tectonics of the southern cascadia subduction zone and mendocino triple junction two upper cenozoic depositional sequences of principally marine strata about 4000 m thick overlie accreted basement terranes of the central and coastal belts of the franciscan complex in the onshore offshore eel river basin of northwestern california the older depositional sequence is early to middle miocene in age and represents slope basin and slope blanket deposition whereas the younger sequence late miocene to middle pleistocene in age consists largely of forearc basin deposits youthful tectonic activity related to gorda north american plate convergence indicates an active cascadia subduction zone and strong partial coupling between these plates structures of the northeastern margin of the eel river basin are principally north northwest trending east northeast dipping thrust and reverse faults that form imbricate thrust fans many faults merge downward into sole thrusts that extend to or near the gorda north american plate interface in the southern part of the region these structures have been modified by northward encroachment of the pacific plate the coastal belt fault the early tertiary accretionary suture between the franciscan central and coastal belts can be traced from arcata bay northward offshore to the southern oregon border it is tentatively extended farther northward based on aeromagnetic data to an offshore position west of cape blanco thereafter it may coincide with the offshore fulmar fault if so the franciscan coastal belt in california is correlative with the offshore fulmar terrane in oregon the cascadia subduction zone csz does not join the mendocino transform fault at the commonly depicted offshore location of the mendocino triple junction mtj instead the csz extends southeastward around the southern eel river basin and shoreward along mendocino canyon to join the petrolia shear zone similarly the mendocino fault may extend shoreward via mattole canyon and join the cooskie shear zone these two shear zones intersect onshore north of the king range and the area of their intersection is the probable location of the mtj

prograde temperature time evolution in the barrovian type locality constrained by sm nd garnet ages from glen clova scotland abstract the timing of garnet growth during metamorphism associated with the grampian orogeny in the sillimanite zone of the barrovian type locality in glen clova scotland was determined by sm nd geochronology two high precision garnet whole rock ages were achieved by employing hf partial dissolution of garnet separates to optimize purity multiple garnet growth generations were identified on the basis of the geochronology and detailed textural and chemical data an early stage at 472 9 2 9 ma during d 2 deformation under garnet zone conditions c 500 550 c and a later stage at 464 8 2 7 ma during or slightly after d 3 deformation mostly under sillimanite zone conditions peak temperature of c 660 c but possibly including some growth during kyanite zone conditions when combined with recently published garnet ages from the kyanite and garnet zones the data suggest that peak metamorphic temperatures in at least these three of barrow9s zones were achieved roughly contemporaneously the difference between garnet zone and sillimanite zone peak temperature attainment is 2 8 3 7 ma the near contemporaneity of peak temperature attainment in different metamorphic zones requires an additional source of heat beyond thermal relaxation of a variably over thickened crust we suggest that local igneous intrusions with synmetamorphic ages provided that additional heat

geochemistry of precambrian and paleozoic siliciclastic rocks from the iberian range ne spain implications for source area weathering sorting provenance and tectonic setting abstract the major and trace element chemical composition of precambrian and paleozoic shales and sandstones from the iberian range spain has been investigated to determine the provenance and tectonic setting of these rocks as well as to appraise the influence of the weathering hydraulic sorting and recycling processes upon source rock signature the samples studied belong to a prograde sequence from diagenesis to anchizonal grade and are mainly composed of quartz illite mica phases and or kaolinite with chlorite feldspars and carbonates as minor components the major element distribution chemical index of alteration cia indices reflects that recycling processes have been important in homogenizing the composition of shales and sandstones the higher cia indices observed in the silurian and devonian shales 80 and 85 respectively compared with those of precambrian to ordovician and the carboniferous age about 70 indicate that their source area underwent more intense chemical weathering processes possibly due to climatic and or tectonic variations alternatively the source area of the silurian and devonian shales may have been composed of recycled sedimentary materials mineral fractionation is mainly observed in coarser rocks through zircon apatite and xenotime accumulation although the shales are not free of these phases the zircon content of the fine grained rocks is not high enough to affect rare earth element ree contents but phosphate minerals apatite and xenotime at least partially control the ree distribution the influence of apatite and xenotime are slightly higher in the precambrian shales as the phosphate concentration produces a decrease in the la sm ratios chondrite normalized ree patterns and negative eu anomaly size of the studied rocks are similar to that of post archean australian shales paas indicating that they originally come from a differentiated silicic source the higher ree contents in the studied shales in relation to paas indicate that recycling processes in the iberian range sources were probably more intense than that of paas the slight differences among the ree patterns of the different groups of shales probably do not reflect changes in source area composition but instead suggest variations in mineral sorting chemical weathering and or sediment recycling the th sc co th cr th cr v v ni ratios support a primitive silicic source for these rocks with the higher th sc and lower co th in cambrian and ordovician shales indicating a higher proportion of felsic material in their primitive source area the compositional maturity of analyzed sandstones is typical of cratonic environments and their la sc th and zr contents reflect their passive continental margin setting

petrochemistry and tectonic significance of the highland border suite mafic rocks the metamorphosed mafic rocks of the ophiolitic highland border suite on arran and bute are of 2 types greenschist facies pillow lavas and epidote amphibolite facies hornblende schists respectively both types have been affected by post magmatic alteration processes including spilitization i e na enrichment coupled with ca depletion on the basis of 9immobile element9 chemistry and other criteria the mafic rocks are split into 2 magmatic groups a tholeiitic one which shows the effects of extensive low pressure fractionation e g fe and ti enrichment and a less fractionated alkaline group that contains relict al and ti rich clinopyroxenes the tholeiitic rocks are similar to modem ocean floor or marginal basin basalts and the alkaline rocks show affinities with 9within plate9 basalts this 9magma type9 and 9tectonic setting9 dichotomy is found at other highland border localities and is present in both the pillow lavas and hornblende schists the highland border suite mafic rocks may have formed as tholeiitic ocean basin crust upon which alkaline ocean island lavas had been erupted or alternatively as is suggested in this paper they may have formed during the early stages of development of a pre grampian marginal basin of cambrian or cambro ordovician age

franciscan complex calera limestones accreted remnants of farallon plate oceanic plateaus the calera limestone part of the franciscan complex of northern california may have formed in a palaeoenvironment similar to hess and shatsky rises of the present north west pacific1 we report here new palaeomagnetic results palaeontological data and recent plate motion models that reinforce this assertion the calera limestone may have formed on farallon plate plateaus north of the pacific farallon spreading centre as a counterpart to hess or shatsky rises in one model2 the plateaus were formed by hotspots close to the farallon pacific ridge axis on accretion to north america plateau dissection in the late cretaceous to eocene 50 70 myr could explain the occurrence of large volumes of pillow basalt and exotic blocks of limestone in the franciscan complex partial subduction of the plateaus could have contributed to laramide 70 40 myr compressional events3

a revised kinematic model for the relative motion between pacific oceanic plates and north america since the late cretaceous 1 the convergence between the pacific farallon and kula oceanic plates and the north american plate has provided a driving force for tectonic processes that have shaped the western margin of north america yet reconstructions of plate motion in the pacific basin have traditionally relied on the assumption of fixed hot spots in the last decade substantial cretaceous to paleogene motion between the pacific and atlantic hot spots has been recognized providing motivation to reevaluate the history of relative motion between pacific oceanic plates and north america by means of plate circuit reconstructions this paper presents new kinematic models based on two alternative plate circuits linking the pacific plate to north america since late cretaceous time when compared to models relying on fixity of pacific and indo atlantic hot spots our reconstructions suggest that the pacific kula and pacific farallon ridges were 600 1000 km closer to the western margin of north america and hence that the farallon and kula plates were smaller in late cretaceous to middle eocene time these findings cast significant doubt on the viability of a configuration of the farallon kula ridge that juxtaposes the kula plate with the southern part of the paleo north american margin during this time interval the results of our new reconstructions also suggest more oblique convergence between the farallon or kula plate and north america from the late cretaceous to the middle eocene related to the combination of dominantly northwest motion of the pacific plate and steady southwest motion of north america this provides an efficient mechanism for the coast parallel translation of some accreted terranes of north america it may also explain the structure of geological units in western north america such as the central belt of the franciscan complex which show a dominant coast parallel fabric formed during an interval coeval with the episode of oblique convergence

paleomagnetic results from the upper cretaceous maudlow and livingston formations southwest montana a paleomagnetic investigation of welded tuff units from the late cretaceous livingston and maudlow formations was undertaken to refine the late cretaceous to paleocene apwp for cratonic north america after progressive af and thermal demagnetization the maudlow formation 46 1 n 248 9 e yields a paleomagnetic pole at 70 n 208 e 11 sites a95 9 k 22 tectonic correction improves the directional grouping and the change is statistically significant two normal polarity intervals bounding a single reversed polarity interval are present we interpret the reversed interval to be chron 33r on the basis of 1 stratigraphic correlation of the maudlow formation with the campanian sedan formation located 40 km to the east and 2 k ar dates of 83 2 ma at the middle of the sampled section and 74 9 1 ma near the top of the sampled section the livingston formation 45 5 n 250 1 e yields a high hc magnetization with a negative fold test suggesting a post folding secondary magnetization the livingston mean direction yields a paleomagnetic pole at 72 n 218 e 10 sites a95 4 k 107 and is clearly distinguishable from a co existing lower hc component reflecting a recent viscous overprint we interpret the high hc magnetization as late cretaceous to paleocene in age neither the maudlow formation pole nor the pole derived from the high hc overprint in the livingston formation agree with previously published poles for either the late cretaceous or tertiary but they do lie on a simple continuation of the apwp between late cretaceous and paleocene mean poles secondary magnetizations found in the franciscan complex of california also yield poles which lie on a proposed extension of the apwp if the franciscan secondary poles represent an overprint event associated with early tertiary accretion and if the livingston secondary pole represents a deformational event related to laramide uplift of the nearby beartooth range then together these three poles suggest that the north american apwp may continue farther to the east and south to define a cretaceous tertiary cusp

strike slip terranes and a model for the evolution of the british and irish caledonides evidence is presented that many of the major strike faults in the british and irish caledonides were active as sinistral strike slip zones in the end silurian to pre mid devonian period some such as the highland boundary fault moved in this way at an earlier stage in the ordovician these data allow the caledonian rocks lying between the laurentian miogeocline whose basement is represented by the lewisian moine and possibly the dalradian and the gondwanaland miogeocline midland platform and welsh basin to be re analysed as a group of disorganized terranes which originated to the southwest in north america and southwest europe africa prior to the silurian the highland border terrane and northern belt terrane are interpreted as duplicated pieces of a mid ordovician sequence which was a back are to northwest subduction the midland valley terrane is interpreted as a slice of laurentian foreland onto which ophiolites were obducted in the lower ordovician but which became the basement of a continental margin arc to northwest subduction in the mid ordovician the cockburnland terrane is inferred to be part of the same arc repeated and then broken up and dispersed by continuing strike slip the connemara terrane is regarded as an allochthonous piece of the dalradian miogeocline and the south mayo terrane as a remnant of an early ordovician arc and fore arc which in mid ordovician times became a back arc marginal basin to northwest subduction the lake district wexford terrane is part of an arc to southeast subduction under gondwanaland whose activity climaxed in the mid ordovician the central terrane is interpreted as a silurian overstep assemblage which blankets the junction between laurentian and gondwanaland derived oceanic terranes and therefore iapetus is regarded as an ordovician ocean which closed prior to the silurian the model suggests that at the end of the silurian a clockwise rotating gondwanaland having carried laurentia into collision with baltica broke free and created a major sinistral strike slip zone which disrupted the ordovician palaeogeography in the british isles north american sector of iapetus

the caribbean and farallon plates connected constraints from stratigraphy and paleomagnetism of the nicoya peninsula costa rica plate kinematic reconstructions play an essential role in our understanding of global geodynamics but become increasingly difficult to constrain back in geological time due to the subduction of oceanic lithosphere here we attempt to kinematically reconstruct the cretaceous and older plate tectonic history of the caribbean plate within the mesozoic panthalassa paleo pacific ocean to this end we present new paleomagnetic data from jurassic and cretaceous oceanic sedimentary and volcanic large igneous province related rocks of the nicoya peninsula and murcielago islands of northwestern costa rica we use these data in combination with constraints from marine magnetic anomalies to infer the age of the lithospheric basement seismic tomography to locate deep mantle plume generation zones and general kinematic feasibility to test different reconstruction scenarios connecting the caribbean plate to the farallon plate as restored from pacific spreading records our resulting reconstruction implies that the western caribbean subduction zone initiated around 100 ma in an intraoceanic setting breaking up oceanic lithosphere of at least 70 myr old

assessing metasomatic mass and volume changes using the bootstrap with application to deep crustal hydrothermal alteration of marble a statistically rigorous approach for determining likely errors on estimates of mass and volume change in metasomatic systems is presented and then used to assess mass transfer resulting from hydrothermal alteration of marble during regional metamorphism analysis of metasomatic effects using standard statistical methods designed for unconstrained univariant data often fails to provide useful results for several important reasons 1 the concentration of any constituent in a composition is constrained to be between 0 and 100 wt percent 2 the sum of the concentrations of all constituents must be 100 wt percent the closure constraint compositions are multivariate and provide information only about the relative proportions of constituents 3 the maximum possible mass or volume loss is 100 percent 100 corresponds to complete mass or volume loss 4 rock bulk density must be greater than 0 g cm super 3 5 the underlying probability distributions for mass and volume changes are commonly non normal to address these issues we use statistical procedures recently developed to treat the special properties of compositional data including closure and the bootstrap method to compute accurate confidence intervals for assessing how far in error best estimates of mass and volume change are likely to be the bootstrap deals effectively with non normality and constraints 1 3 and 4 we apply our approach to gain a better understanding of synmetamorphic acadian orogeny hydrothermal alteration of upper greenschist facies and amphibolite facies marble beds of the wepawaug schist connecticut the marbles lost significant amounts of volatiles dominantly co 2 and h 2 o si ti k rb sr and ba best estimates of total mass and volume change are 27 and 32 percent respectively the bulk of the lost mass was volatiles approximately 140 g kg super 1 sio 2 approximately 60 g kg super 1 and k 2 o approximately 15 g kg super 1 numbers are g lost per kg of parent rock volatile loss was probably regional in scope loss of si ti k rb sr and ba occurred over minimum length scales on the order of typical hand sample dimensions maximum length scales remain to be determined the fortran 77 computer code that performs the calculations described herein is available from the senior author

zn and pb mobility during metamorphism of sedimentary rocks and potential implications for some base metal deposits comprehension of the genesis of pb zn ore systems is currently limited by a poor understanding of where these metals are sourced from our study of metal mobility during regional metamorphism in the mt lofty ranges south australia demonstrates that in staurolite absent siliciclastic metasedimentary rocks biotite contains 80 of the bulk rock zn as well as a considerable proportion of the total pb fluid flow through these metasedimentary rocks led to a continuous depletion of pb and zn on a mineral and bulk rock scale during prograde regional metamorphism we calculate that 80 of the bulk rock zn and 50 of the bulk rock pb were mobilised mainly through reactions involving biotite these reactions led to a calculated pb and zn loss of 2 7 and 27 mt respectively in the high grade metamorphic zone halogen contents of apatite and biotite and bulk rock zn isotope data provide evidence that cl rich metamorphic fluids were important for metal transport hence fluid flow accompanying prograde metamorphism of typical sedimentary rocks can mobilise base metals to the degree required to potentially supply significant pb zn ore systems

large scale hydration of the lithosphere above subducting slabs during subduction oceanic lithosphere capped by variably hydrated uppermost mantle oceanic crust and sediments undergoes progressive metamorphism and devolatilization the petrologic evolution of subducting oceanic lithosphere and the overriding continental oceanic lithosphere can be predicted by integrating thermal models with petrologic phase equilibria in mature subduction zones that have achieved thermal steady state the subducting slab lies at subsolidus conditions for depths 100 150 km most subducting oceanic crust passes through the blueschist to eclogite facies transition where large amounts of h2o are released by the breakdown of na amphibole lawsonite or clinozoisite and chlorite the large amounts of h2o released by continuous blueschist eclogite reactions can cause large scale hydration of the overriding lithosphere and can trigger partial melting of the mantle wedge lowermost lithosphere and asthenosphere upward migration of slab derived fluids add h2o co2 sio2 and incompatible elements to the overlying lithospheric plate that result in the formation of new phases e g serpentine amphibole and mica and chemical modification of existing phases e g trace element enrichment of clinopyroxene evidence for large scale lithospheric hydration above subducting slabs includes serpentinite diapirs observed in the marianas forearc low p wave velocities observed in the hanging wall of the alaskan and japanese subduction zones and studies of high pressure metamorphic terrains

three dimensional geometry of metamorphic fluid flow during barrovian regional metamorphism from an inversion of combined petrologic and stable isotopic data inverse calculations reveal the three dimensional geometry of time integrated fluid flux over a 120 km2 area during peak barrovian regional metamorphism in southeastern vermont prograde changes in whole rock co2 18o and 13c and calculated fluid compositions at the peak of metamorphism were inverted assuming tracer mass balance to obtain the time integrated fluid flux in three dimensions peak metamorphic fluid flow was spatially nonuniform with flux magnitudes ranging from 0 to 3 105 mol fluid cm2 rock and flux directions ranging from vertical upward and downward to horizontal averaged over the entire study area the magnitude of the time integrated metamorphic fluid flux vector is 3 4 104 mol fluid cm2 rock the average flux vector trends 45 to the southwest and points upward at 36 from the present horizontal parallel to formation boundaries on a regional scale fluids in the terrain carried 3 103 mol co2 cm2 rock toward earth s surface during the peak of metamorphism results suggest that local cross layer transport processes are secondary to terrain scale metamorphic fluid flow in driving prograde decarbonation reactions regional structure exerts a first order control on the gross geometry of peak metamorphic fluid flow