a new indicator of movement direction during orogenesis measurement technique and application to the alps the orientation of axes of sigmoidal staircase or spiral inclusion trails within porphyroblasts provides an indicator of the direction of movement during deformation that is synchronous with metamorphism a simple technique is presented in detail to find this axis in 3d by radially sectioning a horizontal slab cut from an oriented sample when viewed from one direction the switch in asymmetry of the porphyroblast inclusion trails in these sections defines the trend of the axis further radial sectioning of a vertical slab cut parallel to this trend determines the plunge of this axis this technique is independent of the model adopted for the formation of sigmoidal or spiral shaped inclusion trails and can be used to evaluate the mechanism by which they form it can also be used to evaluate theories of folding and orogenesis measurements of spiral and sigmoid axes in garnet porphyroblasts from the european alps show that they reflect the movement of the african plate relative to europe better than linear indicators preserved within the same rocks

strain rates from snowball garnet spiral inclusion trails in garnet porphyroblasts are likely to have formed due to simultaneous growth and rotation of the crystals during syn metamorphic deformation thus they contain information on the strain rate of the rock strain rates may be interpreted from such inclusion trails if two functions are known 1 the relationship between rotation rate and shear strain rate 2 the growth rate of the crystal we have investigated details of both functions using a garnetiferous mica schist from the eastern european alps as an example the rotation rate of garnet porphyroblasts was determined using finite element modelling of the geometrical arrangement of the crystals in the rock the growth rate of the porphyroblasts was determined by using the major and trace element distributions in garnet crystals thermodynamic pseudosections and information on the grain size distribution for the largest porphyroblast size fraction size l 12 mm we constrain a growth interval between 540 and 590 c during the prograde evolution of the rock assuming a reasonable heating rate and using the angular geometry of the spiral inclusion trails we are able to suggest that the mean strain rate during crystal growth was of the order of 6 6 10 14 s 1 these estimates are consistent with independent estimates for the strain rates during the evolution of this part of the alpine orogen

constraints on earth evolution from antimony in mantle derived rocks abstract we have analyzed sb in a variety of mantle derived volcanic rocks peridotites and in the ci chondrite orgueil by spark source mass spectrometry concentrations vary from 0 02 to 0 8 ppm in oceanic basalts mid ocean ridge basalts morb oceanic island basalts oib antimony is a moderately siderophile element which behaves like the incompatible lithophile element pr during igneous processes in the mantle both morb and oib samples have similar sb pr ratios of about 0 02 which are different from those in continental crustal rocks antimony resembles pb in that it behaves like a highly incompatible element during formation of continental crust whereas it behaves only moderately incompatible during formation of oceanic basalts morb or oib consequently sb pb ratios of oceanic basalts agree within error limits with those of the continental crust and with the ci chondritic value indicating that sb pb is not strongly fractionated during crust mantle core differentiation from the sb pb ratios we estimate a sb concentration of 11 5 ppb for the primitive mantle an alternative estimate for the primitive mantle abundance is obtained by assuming the sb pr ratio of the primitive mantle to be intermediate between the morb 01b value 0 02 and that of the continental crust 0 05 this approach yields sb 8 4 ppb for the primitive mantle antimony is depleted in the bulk silicate earth by a factor of 45 the volatility corrected depletion factor of 7 is similar to other moderately siderophile elements

trace element systematics of pyrite from submarine hydrothermal vents abstract submarine hydrothermal sulfide ores occur at mid ocean ridges intra oceanic arcs and in back arc basins associated with host rocks of highly variable composition pyrite is the dominant sulfide mineral in most samples presented within this study and thus the trace element composition of pyrite may reflect the different metal sources and precipitation processes here we report on a systematic study of minor and trace element contents in pyrite from active and inactive submarine hydrothermal vent fields at different plate tectonic settings including the indian and mid atlantic ridges the tonga kermadec intra oceanic arc the lau back arc and the central okinawa trough our results show that the trace element contents of pyrite from different locations vary significantly but for most elements without any systematic correlation to the concentration in the magmatic host rocks only as appears to be depleted in pyrite from ultramafic hosted vent systems that are affected by serpentinization these as depleted pyrites preferentially host au 0 micro or nano particles bismuth is enriched in pyrite from hydrothermal systems that contain a sedimentary component and micro inclusions of sphalerite are either due to fluid sediment interaction or phase separated fluid venting pyrites from individual locations have highly variable concentrations of elements like au co cu se mo ag and sb that are most likely related to fluid evolution and changes in fluid composition sub seafloor hydrothermal fluid seawater mixing influences the distribution of au co cu se and mo in pyrite elements like au ag sb and pb often have a characteristic affinity to as while cd correlates closely with zn a magmatic volatile contribution to the hine hina hydrothermal system may result in the precipitation of cu enriched pyrite our results show that the concentrations of most trace metals in pyrite are a function of the physicochemical parameters of the fluid phase rather than a reflection of the magmatic host rock composition

the three dimensional geometry of simulated porphyroblast inclusion trails inert marker viscous flow models a fluid dynamic model for a rigid spherical porphyroblast growing in a viscous fluid matrix undergoing simple shear deformation predicts an arrangement of captured inert inclusions that is remarkably similar to the spiral pattern observed in synkinematic rolled garnets the model assumes only creeping stokes flow of the matrix and a kinetic model for the growth of the porphyroblast of the form rm km where r is the crystal radius is the amount of shear induced rotation and m and k are kinetic parameters for rotation less than 18d j b thompson and j l rosenfeld s simple ring model provides a surprisingly accurate description of the three dimensional geometry of the central inclusion surface in the simulated porphyroblasts although marker planes originally parallel to the shear plane are deflected around the porphyroblast their intersections with its surface remain approximately circular for much larger amounts of rotation than anticipated by rosenfeld the polar coordinate equation t r of the double spiral formed by the trace of the central inclusion surface in a section through the centre of simulated porphyroblasts is also surprisingly simple as follows rm 6 5k m this relationship was observed over the complete range of rotation angles investigated 0 720d and could form a reasonable basis for estimating the growth rotation history of rolled garnets from the shape of their spiral inclusion trails two dimensional sections through the simulated porphyroblasts parallel to the rotation axis yield clamshell rosenfeld s term inclusion geometries similar to the controversial millipede patterns observed in many natural porphyroblasts

distinguishing and correlating multiple phases of metamorphism across a multiply deformed region using the axes of spiral staircase and sigmoidal inclusion trails in garnet schists from the appalachian orogen in south east vermont have undergone multiple phases of garnet growth these phases can be distinguished by the trend and relative timing of foliation inflexion or intersection axes fias of foliations preserved as inclusion trails in garnet porphyroblasts the relative timing of different generations of fias is determined from samples containing porphyroblasts with two or three differently trending fias developed outwards from core to rim multi fia porphyroblasts schists from south east vermont show a consistent pattern of relative clockwise rotation of fia trends from oldest to youngest four populations or sets of fias can be distinguished on the basis of their relative timings and trends from oldest to youngest the four sets have modal peaks trending sw ne w e nnw sse and ssw nne these peaks show that each of the four fia sets has a statistically consistent trend at all scales across a 35 125 km area containing numerous mesoscopic and macroscopic folds the fias of set 4 are defined by inclusion trails that are continuous with matrix foliations have trends subparallel to most folds and are inferred to have developed contemporaneously with these structures conversely sets 1 to 3 are oblique to and pre date most matrix foliations and folds all four fia sets occur in siluro devonian rocks and must have formed in the acadian orogeny the lack of statistically significant differences in the distribution of fia trends across the study area and their consistent relative timings in multi fia porphyroblasts despite a complex regional deformation history involving numerous phases of folding at all scales suggest the porphyroblasts have not rotated relative to one another the change in fia trend with time resulted from rotation of the kinematic reference frame of bulk flow possibly as a consequence of the reorganization of lithospheric plates responsible for acadian orogenesis recognition of distinct generations of fias provides a means of distinguishing different phases of porphyroblast growth four periods of garnet porphyroblast growth occurred in the schists of south east vermont this growth was heterogeneously distributed on the cm2 m2 scale no single porphyroblast records all stages of growth and adjacent samples from the same or dissimilar rock types commonly contain porphyroblasts that preserve different sequences of growth factors that may have been responsible for switching porphyroblast growth on and off at this scale include i subtle differences in bulk chemical composition ii oscillating levels of heat owing to the buffering effect of endothermic garnet forming reactions iii channelized infiltration of fluids with localized fluid buffering of bulk composition and iv cyclic controls on the rates of diffusion and material transport of reactants either by channelized fluid flow or by a changing pattern of microfracturing during foliation development consistency in fia trend and relative timing provide a new method for potentially distinguishing and correlating successive metamorphic events or even phases of metamorphism within a progressive tectonothermal event along and across orogens using a consistent pattern of core to rim changes in fia trend multiple phases of growth of a single porphyroblastic mineral can be quantitatively distinguished allowing correlation of different phases of growth around and across macroscopic folds the relative timing of growth of different porphyroblastic minerals can also be quantitatively determined using fia data and correlated around and across macroscopic folds conceptually the paragenetic history preserved in each generation of porphyroblast growth in the form of chemical zoning and the minerals in inclusion trails could be combined to produce a more detailed p t t deformation path than previously determined

numerical simulations of spiral shaped inclusion trails can 3d geometry distinguish between end member models of spiral formation numerical 3d simulations of the development of spiral inclusion trails in porphyroblasts were conducted in order to test the proposals that a 3d spiral geometry differs between the rotation and nonrotation end member models of spiral formation proposed in the literature and b 3d spiral geometry can be used as a criterion to distinguish between the two end member models in rocks four principal differences are identified between the two sets of simulations smoothness of spiral curvature spacing of foliation planes alignment of individual foliation planes either side of the sphere representing the porphyroblast and spiral asymmetry with respect to matrix shear sense of these differences only spiral asymmetry and possibly the alignment of individual foliation planes are diagnostic criteria for distinguishing between the end member models in the absence of a readily applied test to distinguish the end member models interpretation of spiral inclusion trails is problematic it is necessary to determine complementary evidence to distinguish porphyroblast rotation or nonrotation during spiral formation

petrogenetic implications of tholeiitic basalt glasses from the east pacific rise and the galapagos spreading center basalt glasses dredged from the east pacific rise and the galapagos spreading center show a wide range of composition with fm values feototal feototal mgo from 0 49 to 0 72 analyses for major elements of 40 unaltered glasses and of 17 of these glasses for trace elements demonstrate an analogous line of differentiation over a considerable distance in the east pacific using data from the experiments of peridotite partial melting and their implication for primary melts the composition of the initial basalt magma in equilibrium with the residual phase is extrapolated to be fm 0 40 0 48 the importance of apatite introduced from the parental peridotite for the incompatible elements ree f and p2o5 within various basalt differentiation series is demonstrated

trace elements in the gas emissions from the erta ale volcano afar ethiopia high temperature 1084 c fumarolic emissions from a spatter cone at the erta ale north pit crater granted a rare opportunity to collect giggenbach bottles condensates and sublimates from the fumarolic gases of a rift plume volcano in the immediate proximity of degassing lava these results represent the highest temperature gases sampled directly for trace elements and the first high t fumarolic samples for a rift plume volcano sixty seven metals and metalloids were measured quantitatively by icp aes and icp ms in liquid gas condensates and solid sublimates the total amount of trace elements in the gas without si exceeded 170 ppm the ten most abundant elements transported in nonsilicate modes were na 61 ppm fe 41 ppm k 32 ppm br 12 4 ppm cd 3 0 ppm se 2 3 ppm cu 2 0 ppm zn 2 0 ppm te 1 7 ppm and as 1 2 ppm the gases contained 27 ppb re 1 5 ppb au 0 5 ppb pt and 0 27 ppb os rh and pd containing phases were observed in solid sublimates the only metal not detected by all analytical methods was ruthenium chalcophile elements cd cu zn se te as were transported primarily in the gaseous form alkali metals were emitted at least partially as liquid micron size sulfate droplets whereas alkali earths aluminum and ree had a high affinity to silicate aerosol in the form of basaltic microspheres the silicate aerosol content was evaluated at 59 ppm on average but varied within a 7 5 fold range during sampling because of the extremely high temperature refractory elements ca ba ta u and lree showed evidence of transport in the form of gaseous species the so2 flux in january 2011 was evaluated at 91 24 t d for the entire volcano including 31 t d from the north crater and 60 t d from the lava lake the daily output of trace elements from both erta ale craters without si was estimated at 53 kg d including 20 kg of na 13 5 kg of fe 10 5 kg of k 4 5 kg of chalcophile elements 9 g of re 0 5 g of au and 0 17 g of pt the trace element pattern of erta ale fumarolic gases do not possess any distinct geochemical specialization and resemble te patterns from fumarolic emissions of studied arc and plume volcanoes

antimony isotope variations in natural systems and implications for their use as geochemical tracers abstract multiple collector inductively coupled plasma mass spectrometry mc icp ms has been used for the precise measurement of sb isotopic composition in geological samples as well as sb iii and sb v species in aqueous samples sb is chemically purified prior to analysis by using cation exchange resin and cotton impregnated with thioglycollic acid tcf purification through cation exchange resin is required for the removal of matrix interfering elements such as transitional metals whereas tcf is required for the separation of other hydride forming elements such as ge and as the analyte is introduced in the plasma torch using a continuous flow hydride generation system instrumental mass fractionation is corrected with a standard sample bracketing approach using this technique the minimum sb required per analysis is as low as 10 ng for an estimated external precision calculated for the 123 sb 121 sb isotope ratio of 0 4 e units 2 sb isotope fractionation experiments reported here indicate strong fractionation 9 e units during sb v reduction to sb iii seawater mantle derived rocks various environmental samples deep sea sediments and hydrothermal sulfides from deep sea vents have been analyzed for their sb isotope composition we define a continental and oceanic crust reservoir at 2 1 e units seawater e 123 sb values do not vary significantly with depth and yield a restricted range of 3 7 0 4 e units sb deposited in hydrothermal environments has a significant range of sb isotopic composition up to 18 e units these variations may reflect not only contributions from different sb sources such as seawater and volcanic rocks but also kinetic fractionation occurring at low temperature in aqueous media through the reduction of seawater derived sb v in more reducing environment our results suggest that sb isotopes can be extremely useful tracers of natural processes and may be useful as paleoredox tracers in oceanic systems

reference frame angular momentum and porphyroblast rotation abstract rotation of small rigid objects in a deforming ductile matrix can produce two different types of microstructure a shape fabric due to alignment of the principal axes of a population of elongate objects and the inclusion trail microstructure preserved in syntectonic porphyroblasts we use numerical modeling to show that inclusion trails of elongate porphyroblasts are expected to be extremely complex in contrast snowball garnets are readily interpretable but misuse of reference frame and kinematic misconceptions have obfuscated the discussion on the formation of porphyroblast inclusion trails in general and snowball garnet inclusion trails in particular we clarify this point models for snowball garnet formation that are based on the notion of garnets being irrotational with respect to the earth can be rejected on a geometrical and kinematic basis further the notion that rigid objects embedded in a deforming ductile matrix generally do not rotate is unsound it violates the fundamental physical law of balance of angular momentum

getting the inside story using computed x ray tomography to study inclusion trails in garnet porphyroblasts this study demonstrates how high resolution x ray computed tomography can be used to determine the geometry of curved inclusion trails in garnet porphyroblasts for the first time the three dimensional geometry of these features is shown as an animation a stereo pair a vrml model and two dimensional sections the spatial resolution of the data in this study approximately 10 m is high enough to resolve inclusions and finer resolutions are possible comparisons of scan imagery with thin section and microprobe data demonstrate that many typical inclusion phases in garnet porphyroblasts can be clearly distinguished including quartz ilmenite and monazite a sodium metatungstate solution with a density of 1 19 g cm3 was used for a wedge calibration during scanning reducing artifacts to a minimum and allowing maximum image contrast the overall conclusion of this study is that high resolution x ray computed tomography is a quick non destructive technique that is invaluable in the study of porphyroblast microstructure the three dimensional data generated by this technique can be compared with geometries predicted by the varying theoretical models for the formation of curved inclusion trails in garnet porphyroblasts they can also be analyzed to determine the three dimensional orientation of curvature axes in individual porphyroblasts which has not been possible until now

the reflectivity of magmatic underplating using the layered mafic intrusion analog abstract the reflection characteristics of layered mafic intrusions provide an analog for characterizing the reflectivity of magmatic underplates one dimensional seismic models of a variety of mafic intrusions with different parent magmas demonstrate that cumulate layering may typically be strongly reflective with reflection coefficients as high as 0 18 a seismic profile across the bjerkreim intrusion in southern norway shows that even a moderately reflective cumulate sequence with reflection coefficients on the order of 0 05 produces detectable reflections two dimensional models demonstrate that the subhorizontal cumulate layering in typical mid to upper crustal intrusions is conducive to seismic imaging but more complicated geometries such as concentric layering created during synmagmatic deformation complicate the seismic image reflection coefficients predicted for granulite facies mineral assemblages are similar to those predicted for the igneous assemblages reflection coefficients predicted for cumulates intruded or re equilibrated at eclogite facies conditions may be either decreased or enhanced with respect to the igneous assemblages depending on the abundance of plagioclase in the cumulate layers and the pressure results suggest that magmatic underplating should not be invoked as an explanation for non reflective crust except in cases where eclogitization has occurred

hydrothermal lead transfer from mantle to continental crust the role of metalliferous sediments abstract the amount of lead annually transferred from oceanic crust to metalliferous sediments was estimated in order to test the hypothesis that a non magmatic flux of lead causes the pb surplus in the continental crust a pb surplus has been inferred from global crust mantle lead mass balances derived from lead concentration correlations with other trace elements and from lead isotope systematics in oceanic basalts dsdp odp data on the amount of metalliferous sediments in the pacific ocean and along a south atlantic traverse are used to calculate the mean worldwide thickness of 3 1 m for purely metalliferous sediment componens lead isotope ratios of 39 metalliferous sediments from the pacific define mixing lines between continent derived seawater and mantle derived basaltic lead with the most metal rich sediments usually having the most mantle like pb isotope composition we used this isotope correlation and the pb content of the 39 metalliferous sediments to derive an estimate of 130 70 g g for the concentration of mantle derived lead in the purely metalliferous end member mass balance calculations show that at least 12 8 of the lead annually transferred from upper mantle to oceanic crust at the ocean ridges is leached out by hydrothermal processes and re deposited in marine sediments if all of the metalliferous lead is ultimately transferred to the continental crust during subduction the annual flux of this lead from mantle to continental crust is 2 6 2 0 106 kg assuming this transfer rate to be proportional to the rate of oceanic plate production one can fit the lead transfer to models of plate production rate variations through time integrating over 4 ga hydrothermal lead transfer to the continental crust accounts for a significant portion of the pb surplus in the continental crust it therefore appears to be one of the main reasons for the anomalous behavior of lead in the global crust mantle system

time constraints on deformation and metamorphism from epma dating of monazite in the proterozoic robertson river metamorphics ne australia epma dating of monazite confirms the relative timing of a succession of foliation intersection axis trends in porphyroblasts fias and two periods of progressive metamorphism intervened by retrogression in the robertson river metamorphics ne australia a complete pressure temperature deformation time p t d t path has been deciphered involving an early clockwise p t loop followed by high temperature low pressure metamorphism successive generations of monazite were identified using microstructure and fia trends and then dated the succession of four fias trending ene wsw fia1 e w fia2 n s fia3 ne sw fia4 plus four matrix foliations s1 s4 were accompanied by three periods of garnet and two periods of staurolite growth monazite grew in response to most of these events isopleth thermobarometery using p t pseudosections in the mnnckfmash system for garnet porphyroblasts selected based on fia trends in their core reveal that pressure increased progressively from 3 4 kb at 530 550 c to 6 7 kb at 600 620 c during medium pressure temperature metamorphism this was accompanied by changing bulk shortening directions from nnw sse to n s to e w perpendicular to fias 1 2 and 3 the development of fia1 and fia2 occurred between 1655 2 2 ma and 1625 15 ma fia3 developed over 30 50 ma periods from 1625 15 ma to 1586 6 ma retrogressive metamorphism decompression and exhumation occurred over a not vert similar45 ma period further metamorphism accompanied regional granite intrusion and generated lower pressure higher temperature metamorphism during the development of fia4 nw se bulk shortening at 1542 8 ma this period possibly extended for another 30 50 ma based on the monazite ages obtained from the youngest foliations in the matrix correlation of these data with other ne australian proterozoic inliers suggests that they were all once part of a single orogen that developed from ca 1650 to 1500 ma

mechanism of emplacement of the cannibal creek granite with special reference to timing and deformation history of the aureole abstract the ovoid cannibal creek granite consists of a minor phase emplaced during the regional d 3 deformation and a major phase emplaced during the regional d 4 deformation it possesses an intense contact parallel foliation on its margin and in the adjacent porphyroblastic schist that formed through partitioning of the shearing component of deformation against the crystallized pluton margin during synchronous e w d 4 shortening during this process a pre existing shallow s 3 foliation was updomed around the pluton and re used reactivated by d 4 shear with associated dissolution of quartz resulting in the formation of an ovoid closed foliation around the pluton thus the granite is located within a domal shaped approximately n s trending pluton scale f 3 4 antiform that formed synchronous with emplacement three successive stages of porphyroblast growth record the history of processes operating during progressive emplacement and the structures that developed space problems were solved through upward movement of the rocks above the granite due to movement on the s 4 cleavage as well as on pre existing subvertical bedding and cleavages and by reactivation of steep d 2 faults

near orthogonal foliation development in orogens meaningless complexity or reflection of fundamental dynamic processes abstract orogenic belts are geometrically complex owing to repeated deformation within this complexity there is evidence that may suggest a common pattern of sequential steeply dipping and gently dipping foliations seven possible explanations are presented for the sequential development of these foliations which can probably be reduced to four of general importance 1 the passage of thrust sheets over flats and ramps 2 switching of the maximum and minimum compressive stress orientations during orogenesis 3 reversal of structural development owing to strong rheological anisotropy and 4 back rotation of crenulation hinges during crenulation cleavage development it is suggested that all four of these reflect fundamental dynamic processes at work to build an orogen and therefore that sequentially overprinted steeply and gently dipping foliations also reflect such fundamental processes this illustrates that detailed geometrical analysis at the meso and microscales can provide valuable input to dynamic models of orogenesis

porphyroblast rotation eppur si muove in a number of recent papers the theory has been postulated that porphyroblasts as a rule do not rotate with respect to geographical coordinates and can be used to determine the original orientation of older foliations complex inclusion patterns in spiral garnets have even been used to advocate a new model of orogenesis involving several alternating phases of horizontal shortening and extension critical assessment of the assumptions and data used to support the theory of irrotational porphyroblasts reveals numerous flaws millipede structures used as proof for flow partitioning can also form by other flow geometries evidence quoted to support irrotational behaviour of porphyroblasts is unsound porphyrobl asts do occur in sets with a preferred orientation of the internal foliation trace but these cannot be shown to represent original orientations microstructures which resemble truncation planes in spiral garnets are used as evidence that these structures developed by several phases of deformation and as proof for periodic extension and horizontal shortening in orogenesis they can however also be explained by intermittent growth of a rotating porphyroblast during a single phase of deformation finally porphyroblast sets in which orientation is a function of aspect ratio indicate that porphyroblast rotation with respect to kinematic axes does occur in at least some situations

a theory of finite strain variation through contrasting layers and its bearing on cleavage refraction abstract a theory of finite strain variation in contrasting viscous layers is presented the theory is applicable to layers which are oblique to two principal strains but parallel to the third and is not restricted to plane strain results may be obtained algebraically or by use of the mohr diagram for strain solutions are given for various examples of layering attitude and viscosity ratio measured with respect to a reference layer it is shown that the finite strain ellipsoid changes in shape and orientation across contrasting layers and in some cases the principal axes may be exchanged low strain is indicated in relatively more viscous layers and high strain with extension at a smaller angle to the layering in less viscous layers the latter may approximate simple shear parallel to layering if the strain is sufficiently high the geological implications of the theory are strong strain variations in layered rock sequences it is suggested that in general strain will be inhomogeneous from layer to layer both in orientation and amount the relationship of strain and cleavage are reviewed in the introduction to establish the validity of predicting cleavage patterns from strain data qualitative comparisons between the theoretical results of strain variation and natural cleavage refraction in layered rocks would appear to justify the assumption that cleavages of varied morphologies are sub parallel to the xy planes of strain with this assumption some three dimensional features of planar and linear fabric refraction in contrasting rocks are predicted

garnet porphyroblast timing and behaviour during fold evolution implications from a 3 d geometric analysis of a hand sample scale fold in a schist detailed 3 d analysis of inclusion trails in garnet porphyroblasts and matrix foliations preserved around a hand sample scale tight upright fold has revealed a complex deformation history the fold dominated by interlayered quartz mica schist and quartz rich veins preserves a crenulation cleavage that has a synthetic bulk shear sense to that of the macroscopic fold and transects the axis in mica rich layers garnet porphyroblasts with asymmetric inclusion trails occur on both limbs of the fold and display two stages of growth shown by textural discontinuities garnet porphyroblast cores and rims pre date the macroscopic fold and preserve successive foliation inflection intersection axes fias which have the same trend but opposing plunges on each limb of the fold and trend nne ssw and ne sw respectively the fias are oblique to the main fold which plunges gently to the wsw inclusion trail surfaces in the cores of idioblastic porphyroblasts within mica rich layers define an apparent fold with an axis oblique to the macroscopic fold axis by 32 whereas equivalent surfaces in tabular garnet adjacent to quartz rich layers define a tighter apparent fold with an axis oblique to the main fold axis by 17 this potentially could be explained by garnet porphyroblasts that grew over a pre existing gentle fold and did not rotate during fold formation but is more easily explained by rotation of the porphyroblasts during folding tabular porphyroblasts adjacent to quartz rich layers rotated more relative to the fold axis than those within mica rich layers due to less effective deformation partitioning around the porphyroblasts and through quartz rich layers this work highlights the importance of 3 d geometry and relative timing relationships in studies of inclusion trails in porphyroblasts and microstructures in the matrix

an x ray computed tomography study of inclusion trail orientations in multiple porphyroblasts from a single sample inclusion trails in 58 garnet porphyroblasts in a single sample from the cram hill formation in southeast vermont were imaged using high resolution x ray computed tomography texturally the porphyroblasts have a large core with a sub vertical foliation that has a well defined geometry with a mean orientation of 113 72e for a 95 confidence cone semi angle of 5 3 this steep foliation curves into a sub horizontal foliation the foliation intersection inflection axes fias defined by hinge lines for this curvature are tightly clustered with a mean plunge of 11 towards 201 with a 95 confidence cone semi angle of 4 1 eigenvalue analysis indicates that the clustered distribution of the foliation and fia data are unlikely to be the result of a random event there is evidence in the specimen for multiple foliation forming events subsequent to garnet nucleation and the preservation of these clustered distributions in their wake strongly suggests that the porphyroblasts have not rotated with respect to a geographic reference frame fias represent a measurable structural element that can provide information on tectonic events at the time the porphyroblasts grew a comparison of fia data collected using high resolution x ray computed tomography with data collected using the asymmetry method demonstrates that the asymmetry method is a valid technique for defining the mean fia orientations in a sample

the internal inclusion trail geometries preserved within a first phase of porphyroblast growth the inclusion trail geometry within a first phase of porphyroblast growth can differ significantly from that preserved by further enlargement because the porphyroblast forms a rigid mass up against which the rock preferentially strains during ensuing events the geometry of the first overgrown inclusion trails is affected by their primary orientation including any pre existing curvature combined with any heterogeneous rotation of this foliation about the developing stretching lineation this can impact the apparent timing of foliation intersection inflection axes preserved within porphyroblasts fias that nucleated during the development of a sub horizontal foliation but is readily resolved 3 d computer analysis of sigmoidal inclusion trails reveals that the asymmetry method for fia determinations is unaffected by the cut location relative to the porphyroblast core significantly perfect spiral inclusion trail geometries can be produced from a sigmoidal shape in cuts up 30 away from the fia therefore since fias in most porphyroblasts bear no relation to matrix structures there is a 17 chance that thin sections cut relative to the foliation lie within 30 of a fia and could contain such an apparent spiral fias maintain consistent trends for the first phase of porphyroblast growth accompanying horizontal bulk shortening but may vary in plunge fias have sub horizontal plunges for porphyroblasts nucleating during gravitational collapse but may vary in trend for all periods of porphyroblast regrowth the data available indicates that fias remain consistently trending and sub horizontal until the relative direction of plate motion causing orogenesis changes

partitioning of deformation along an orogen and its effects on porphyroblast growth during orogenesis an identical succession of foliation inflection intersection axis trends in porphyroblasts fias is present in three separate areas affected by acadian deformation and metamorphism along 130 kms of the appalachians from east central vermont to north central massachusetts the fias trend successively nw se sw ne w e nnw sse and ssw nne and appear to have formed during the same succession of deformations which have been dated in se vermont as follows not yet dated pre 425 ma 425 to 404 ma 404 to 385 ma and 385 to 360 ma respectively the two areas that are furthest apart in east central vermont and north central massachusetts have strikingly similar total distributions of fias in garnet porphyroblasts that show a somewhat inverse relationship to those observed in the chester athens dome area of se vermont that separates them that is the number of samples containing the first formed fia set is greatest for the two regions lying furthest apart and decreases in younger sets whereas the number of samples containing the first fia set is the least in the central region and increases for more recently developed sets partitioning of deformation into zones of progressive shearing and shortening has been demonstrated to control the sites of development of differentiated crenulation cleavage versus porphyroblast growth and sites of porphyroblast growth on one limb of a regional fold versus the other we argue that partitioning of the deformation around competent feldspathic gneiss bodies at an orogen scale produced the variation in the number of samples containing the various fia sets between the central and outer areas described above porphyroblasts most commonly grew during sw ne directed shortening to the north and south of the chester athens dome region of the appalachians but only rarely grew in the latter region this suggests that deformation at this time partitioned around an outlier of competent basement feldspathic gneiss that underlies the chester athens dome region and this protected the rocks above and in the strain shadow to the nw and se from the effects of the great bulk of the deformation that occurred over the period of time during which nw se trending fias formed we attribute the increase in the number of samples containing progressively younger fias in the chester athens dome area during subsequent periods of nw se n s wsw ene and wnw ese directed shortening to the rise in temperature and pressure recorded in the rocks above the chester athens dome with peak conditions occurring during the development of the nnw sse oriented fia set between 404 and 385 ma as the temperature and pressure increased the gneiss below would have become less competent allowing the deformation associated with the succession of fia sets to partition more pervasively through the dome core this provided more sites for porphyroblast nucleation and growth and hence the number of samples recording these fia sets increased the concomitant decrease in the number of samples in which porphyroblasts grew in the rock within the areas to the north and south with successive fias possibly resulted from a progressive increase in competency of these rocks as the number of porphyroblasts increased

multiple deformations with successive subvertical and subhorizontal axial planes in the mount isa region their impact on geometric development and significance for mineralization and exploration the first orogeny to affect most rocks exposed in the mount isa province caused significant repetition of stratigraphy through north south shortening geological maps are dominated however by younger folds with subvertical north south striking axial planes these folds were produced by east west shortening during a second orogeny both orogenies produced several foliations however some foliations have negligible geometric consequences allowing the geometry of the region to be described generally in terms of just the three deformations d 1 d 2 and d 3 which produced macroscopic folds and the younger deformations which produced brittle faults one of the weak events that appears superficially to have had little or no geometric impact played a highly significant role in the generation of structurally controlled mineralization that role has been identified following advances in conceptual understanding of fold development and associated foliation the deformation which occurred between the d 2 and d 3 events produced geometric effects that were masked by overprinting of north south trending d 2 folds by similarly trending d 3 folds significantly these d 3 folds which control the localization or upgrading of much mineralization in the mount isa province did not form in rocks unaffected by this intermediate event because the near parallelism of s 2 and s 3 in such rocks resulted in intensification of s 2 rather than development of a new s 3 the intermediate deformation event called d sub 2 5 locally produced highly asymmetric deflections in bedding and s 2 with subhorizontal axial planes that were subsequently overprinted by d 3 folds and an s 3 cleavage no mesoscopic foliation was produced during d sub 2 5 and its rotational effects on bedding were commonly subsumed within younger d 3 structures thus d 3 disguises the presence of d sub 2 5 except in relatively rare locations where d 2 folds were isoclinal such as in the tommy creek block and on the northern end of the snake creek anticline southeast of cloncurry and northwest of mount isa in these three locations in the latter locations both limbs were affected resulting in large scale overturning of bedding on one limb prior to overprinting by d 3 the asymmetric folds developed during d sub 2 5 created sites where bedding could be pulled apart during d 3 resulting in implosion because of the high fluid pressure in the surrounding country rock with brecciation jostling fluid access and mineralization large scale zones of mineralization form where d sub 2 5 was localized at an earlier formed heterogeneity such as a thrust surface e g at mount isa mine or a granite country rock contact in these locations early formed angular bedding relationships were locally rotated into orientations which created sites that could implode during d 3 this role of d sub 2 5 in the generation of d 3 allows recognition of regional zones for exploration that previously have received little attention recognition of the role of d sub 2 5 reveals a cyclicity in successive subvertical and subhorizontal events not recognized by most workers in this terrain that may occur in other orogens around the world but has similarly gone unnoticed after the first orogeny finished a succession of three near vertical and two subhorizontal events occurred during the east west shortening orogeny at mount isa these were succeeded by younger kinks with east west to northeast southwest trending axial planes that appear to be the youngest ductile deformation to have affected the mount isa province