40ar 39ar evidence for 1 4 ga regional metamorphism in new mexico implications for thermal evolution of lithosphere in the southwestern usa 40ar 39ar dates recorded by hornblende and muscovite from proterozoic rocks in northern new mexico indicate pervasive mesoproterozoic metamorphism followed by a protracted uplift cooling history most hornblendes yield 40ar 39ar dates of 1 43 1 35 ga which record cooling through temperatures of ca 500 c this suggests that a regional amphibolite facies metamorphism 500 550 c 3 5 4 kbars occurred at ca 1 4 ga this metamorphism was superimposed on paleoproterozoic amphibolite grade rocks at mid crustal depths of 10 15 km contact metamorphic aureoles associated with 1 4 ga plutons emplaced at 2 5 4 kbars 8 15 km in the sandia and manzano mountains record nearly concordant hornblende and muscovite 40ar 39ar dates of 1 4 ga these suggest relatively rapid cooling to ambient conditions of 300 c following the 1 4 ga thermal event slightly younger 1 37 1 31 ga dates are recorded by muscovite throughout northern new mexico and by hornblende within high grade tectonic blocks these are interpreted to r

thermal structure beneath tohoku northeast japan combined with geophysical observations and petrological considerations we have studied the thermal models of the tohoku northeast japan subduction zone the temperature of the deeper part of the mantle wedge is constrained by the petrological requirements that demand a high temperature region 1400 c beneath the volcanic zone to satisfy this condition namely to explain the arc volcanism in the tohoku subduction zone a high temperature upper mantle beneath the japan sea possibly 1400 c at the depth shallower than 100 km is required heat flow distribution which constrains the temperature structure at shallower depth shows that low heat flow values ranging from 20 to 50 mw m2 are distributed between the trench and aseismic front and consistently high heat flow values 80 120 mw m2 prevail in the area landward of the volcanic front including the japan sea recent heat flow measurements indicate that the transition from low to high heat flow occurs between the aseismic front and volcanic front this heat flow distribution restricts the form of the flow in the mantle wedge and shear stress acting at the plate boundary a rigid non moving triangular region bounded by the subducting slab and the vertical plane beneath the aseismic front is necessary to explain the low heat flow zone between the trench and aseismic front shear stress of about 500 to 1000 bar is supposed to act at the interface between the subducting slab and rigid triangular mantle we also suggest that to explain the peak of heat flow near the volcanic front 120 mw m2 magma migration rate of about 0 05 cm3 cm 2 year 1 is necessary a comparison of the thermal structure of the tohoku subduction zone with that of the central south america which was preliminary calculated by the similar scheme shows that the temperature in the mantle wedge of the central south america is probably lower than that of the tohoku subduction zone this difference may result in that of the nature of volcanic rocks

source parameters and time dependent slip distributions of slow slip events on the cascadia subduction zone from 1998 to 2008 1 we invert for the time dependent slip history of slow slip events on the cascadia subduction zone using gps data from 1998 to 2008 the 16 slip transients have sufficient station coverage to solve for the slip distribution on the plate interface gps time series are inverted for fault slip using the extended network inversion filter limited station coverage south of portland 45 5 n latitude restricts our analysis to events on the northern half of the subduction zone slip is resolved at the base of the seismogenic zone and the slip distributions suggest a potential segment boundary near seattle 47 6 n that correlates roughly with geologic and tectonic boundaries events that initiate to the north and south tend to overlap at about this latitude we compile statistics on source parameters such as propagation rate recurrence interval and stress drop which can be used to constrain proposed models of the source mechanics over a 10 year period total strain release from slow slip events is nonuniform along strike with the greatest cumulative slip 27 cm centered beneath port angeles 48 1 n this slip patch also exhibits the most regular recurrence of mw 6 events relative to other locations along strike the spatial extent of the slip patch beneath port angeles correlates with the along strike bend of the cascadia subduction zone in northwestern washington suggesting that plate geometry plays an important role in controlling the along strike characteristics of slow slip

episodic slow slip events accompanied by non volcanic tremors in southwest japan subduction zone 1 episodic slow slip events have been recognized by means of tilt changes in the western shikoku area southwest japan the crustal tilt deformation was observed repeatedly with a recurrence interval of approximately six months coincident with the occurrences of major non volcanic deep tremor activities in this area observed tilt changes can be explained by slow slip events occurring around the source area of tremors in each episode the source of the slow slip event and tremor migrate simultaneously the spatial and temporal coincidence of tremors and slow slip events indicates that they both may be coupling phenomena reflecting the stress accumulation process at the subducting plate

pluton emplacement processes and tectonic setting of the 1 42 ga signal batholith sw usa important role of crustal anisotropy during regional shortening abstract plutons in the 1 42 ga signal batholith provide an important case study for understanding the regional tectonic setting of mesoproterozoic magmatism in the south western united states and processes of emplacement for plutons far from active plate margins mesoproterozoic plutons in the signal batholith were emplaced at 3 kbar and have a well developed magmatic fabric which is subparallel to variably oriented paleoproterozoic country rock foliations and shear zones indicating that pluton emplacement was strongly influenced by crustal anisotropy solid state deformation of mesoproterozoic granites is best developed in shallowly dipping roof domains 40 60 whereas in steep to vertical fabric domains granites are generally undeformed and locally cross cut older fabrics this suggests that roof domains remained hotter and experienced greater thermal softening and subsequent deformation structural and kinematic study of three separate plutons in the signal batholith indicate that pluton emplacement occurred in a regional contractional strain field although local complexities suggest possible stress field modification by pluton related stresses evidence to support regional contractional strain during emplacement includes 1 consistent dyke orientations throughout the batholith indicating a nne extension and se nw shortening 2 magmatic to solid state fabric transition in thrust sense deformation along the 40 60 dipping south east margin of the signal granite including melt filled shear zones and asymmetric k feldspar porphyroclasts and 3 syn tectonic textures in contact metamorphic andalusite prophyroblasts regional variation in shortening direction from se nw to n s during batholith emplacement may be related to pluton emplacement stresses locally dominating over the relatively low deviatoric stresses of regional deformation the main controlling factor for the geometry of pluton emplacement in the signal batholith was paleoproterozoic foliations and shear zones which provided conduits for movement of magma into the middle crust the flow of magma parallel to older structures may have been accommodated by a reduction in effective normal stresses by magma pressure and separation along the structural anisotropies this model for anisotropy influenced emplacement during weak regional contraction may be applicable to other 1 4 ga plutons across sw laurentia

petrogenesis of the 1 43 ga sherman batholith se wyoming usa a reduced rapakivi type anorogenic granite the 1 43 ga sherman batholith southeastern wyoming usa origin that are found throughout the proterozoic anorogenic granite provinces of the southwestern usa adirshows extreme a type petrochemical characteristics compared with other mid proterozoic granite batholiths of north america it ondacks eastern canada southern greenland and the fennoscandian shield anderson 1983 phanerozoic consists of 1 the sherman granite a coarse grained biotite hornblende granite that locally contains fayalite and pyroxenes 2 examples include paleozoic granites of southeastern australia collins et al 1982 and of corsica poitrasson et the lincoln granite a medium grained biotite granite 3 a porphyritic biotite hornblende granite that probably formed by interaction al 1995 and the mesozoic nigerian younger granites jacobsen et al 1958 and the white mountain magma of granitic and mafic magmas and 4 iron enriched mafic dikes and pods the ilmenite series metaluminous sherman granite series foland allen 1991 anorogenic granite refers to plutons that are not associated with compressional exhibits extreme values of feo feo mgo and is rich in structures and that were emplaced long after any known k ree nb and y it crystallized at temperatures exceeding orogenic event anorogenic or a type granites loiselle 900 c and a pressure of 2 5 kbar with water activity of 0 7 wones 1979 are characterized by low h2o and o2 and dlog fo2 of 0 1 to 0 5 the lincoln granite which is fugacities along with high feo feo mgo k2o peraluminous and has less extreme a type geochemical characteristics na2o and k2o contents a type granites typically are rich crystallized at temperatures as low as 750 c and dlog fo2 of in incompatible elements including rare earth elements around 0 5 units above fmq fayalite magnetite quartz the ree zr nb and ta but poor in co sc cr ni ba rocks of the sherman batholith are chemically equivalent to lavas sr and eu minimum magma liquidus temperatures are from the yellowstone hotspot like the yellowstone magmas the 900 1000 c clemens et al 1986 creaser white sherman batholith probably originated by partial melting of under1991 some workers suggest these rocks are derived plated mantle derived mafic rocks from melting of tonalitic or more felsic crust anderson cullers 1978 collins et al 1982 clemens et al 1986 creaser et al 1991 whereas others relate the granites

subduction zone anisotropy beneath corvallis oregon a serpentinite skid mark of trench parallel terrane migration 1 we studied the back azimuth dependence of ps converted phases at gsn station cor corvallis oregon using broadband p receiver functions computed from 602 teleseismic earthquakes the amplitudes and polarities of the transverse ps phases are largely two lobed which indicates anisotropy with a tilted symmetry axis a double peaked ps conversion at 4 5 6 5 s delay has the moveout of a dipping slab but is not consistent with simple deflection of the ps converted phase by the dipping interface a polarity flip on the transverse receiver function near north south back azimuth indicates an anisotropic symmetry axis aligned north south far from the convergence direction n68 e the ps phase is modeled using reflectivity synthetics with a highly anisotropic layer of depressed wave speed vp 6 0 6 5 km s near 40 km depth at the slab interface with the overriding north american plate one dimensional modeling suggests 10 anisotropy in the supraslab layer with a slow symmetry axis oriented n5 w at a 60 tilt from the vertical adjustments for the effect of slab interface dip on ps amplitude suggest a somewhat lower 7 anisotropy we infer a thin 7 km anisotropic detachment zone for the northward slippage of the siletz forearc terrane along the top of the descending slab a motion consistent with gps measurements and models of regional lithospheric dynamics serpentinite is a likely constituent for the deep anisotropic layer owing to its reduced vp and high poisson ratio the ductile rheology and hydrated composition of serpentinite make it a plausible lithology for a mechanical detachment zone

effect of three dimensional slab geometry on deformation in the mantle wedge implications for shear wave anisotropy shear wave splitting observations from many subduction zones show complex patterns of seismic anisotropy that commonly have trench parallel fast directions three dimensional flow may give rise to trench parallel stretching and provide an explanation for these patterns of seismic anisotropy along strike variations in slab geometry produce trench parallel pressure gradients and are therefore a possible mechanism for three dimensional flow in this study we quantify the effects of variable slab dip curved slabs oblique subduction and slab edges on flow geometry and finite strain in the mantle wedge of subduction zones temperature dynamic pressure velocity and strain are calculated with high resolution three dimensional finite element models these models include temperature and stress dependent rheology and parameterized slab and trench geometry thick layers 20 60 km with strong trench parallel stretching are observed in the mantle wedge when slab geometry involves a transition to slab dip less than 15 or strong curvature in the slab in these cases strong trench parallel stretching develops when flow lines have an oblique to trench normal orientation this suggests that trench parallel seismically fast directions may not indicate trench parallel flow lines in systems with large along strike variations an oblique component of stretching is confined to a 20 30 km layer above the slab in systems with oblique subduction the effects of slab edges include strong toroidal flow and focusing in the mantle near slab edges and trench parallel flow that extends 50 100 km into the core of the mantle wedge

three dimensional structure of p wave anisotropy in the presence of small scale convection in the mantle wedge 1 as a possible explanation of hot fingers in the mantle wedge below the tohoku region japan the existence of small scale convection has been proposed in this study we performed numerical calculations around the subduction zone in 3d with composite rheology i e a combination of linear and non linear rheology and found that small scale convection could arise when the rheology determined from laboratory experiments is considered we also calculated 3d structure of expected p wave anisotropy for the case with and without small scale convection based on a theory of lpo development and an assumption that anisotropy is approximately represented as hexagonal symmetry and found that the fast axis of the p wave propagation projected on the horizontal cross sections is nearly in the same direction as that of plate motion in many places it implies that effects of large scale mantle flow associated with subducting slab is still dominant even in the presence of small scale convection in mantle wedge however in vertical cross sections the projected fast axis of the p wave propagation could tilt vertically while that without small scale convection is almost horizontal therefore future seismological studies that determine the fast direction of p wave propagation in 3d would give us critical information on the possible existence of small scale convection in the mantle wedge

monazite xenotime thermochronometry and al2sio5 reaction textures in the picuris range northern new mexico usa new evidence for a 1450 1400 ma orogenic event al2sio5 reaction textures in aluminous schist and quartzite of the northern picuris range north central new mexico record a paragenetic sequence of kyanite to sillimanite to andalusite consistent with a clockwise p t loop with minor decompression near the al2sio5 triple point peak metamorphic temperatures are estimated at 510 525 c at 4 0 4 2 kbar kyanite and fibrolite are strongly deformed some prismatic sillimanite and all andalusite are relatively undeformed monazite occurs as inclusions within kyanite mats of sillimanite and centimetre scale porphyroblasts of andalusite and is typically aligned subparallel to the dominant regional foliation s0 s1 or s2 and extension lineation l1 back scatter electron images and x ray maps of monazite reveal distinct core intermediate and rim compositional domains monazite xenotime thermometry from the intermediate and rim domains yields temperatures of 405 470 c 50 c and 500 520 c 50 c respectively consistent with the prograde to peak metamorphic growth of monazite in situ ion microprobe analyses from five monazites yield an upper intercept age of 1417 9 ma near concordant to concordant analyses yield pb pb ages from 1434 12 ma core to 1390 20 ma rim we find no evidence of older regional metamorphism related to the 1650 ma mazatzal orogeny

xenolithic evidence for proterozoic crustal evolution beneath the colorado plateau tertiary diatremes of the navajo volcanic field brought a wide variety of proterozoic xenoliths to the surface of the colorado plateau examination of crustal xenoliths from the navajo volcanic field diatremes permits reconstruction of proterozoic pressure temperature p t histories beneath the colorado plateau diatremes from the northwest part of the navajo volcanic field carry the greatest variety of xenoliths including metasedimentary rocks amphibolites felsic gneisses mafic granulites and crustally derived eclogites these rock types show variable degrees of hydrous alteration and evidence for complex reaction histories in contrast diatremes from the southeast part of the navajo volcanic field contain primarily mafic and felsic granulites that show fewer reaction textures and less alteration metasedimentary and eclogitic xenoliths are absent in these diatremes the p t paths from the northwest xenoliths are counterclockwise and reach temperatures as high as 850 c and pressures equal to or greater than 10 kbar later hydrous alteration occurred around 500 c and 8 12 kbar xenoliths from the southeast diatremes preserve little evidence of their p t evolution but paths involve heating and or decompression the differing rock types p t paths and alteration histories of the northwest and southeast populations suggest that proterozoic tectonism juxtaposed two distinct crustal blocks beneath the colorado plateau previous workers have postulated that the boundary between the proterozoic yavapai and mazatzal provinces occurs in the region our data support the existence of a northeast trending boundary beneath the four corners area if the eclogitic metamorphism and hydrous alteration are proterozoic in age their restricted occurrence beneath the northwest part of the plateau suggests that subduction was northwest dipping during proterozoic accretion of the mazatzal province onto north america

stability and dynamics of serpentinite layer in subduction zone abstract the hydrous phyllosilicate serpentines have a strong influence on subduction zone dynamics because of their high water content and low strength at shallow and intermediate depths in the absence of data newtonian rheology of serpentinites has been assumed in numerical models yet experimental data show that serpentine rheology is best described by a power law rheology recently determined in subduction zone conditions hilairet n et al 2007 high pressure creep of serpentine interseismic deformation and initiation of subduction science 318 5858 1910 1913 using a simple 1d model of a serpentinized channel and as opposed to previous models in this power law rheology we examine the influence of channel thickness temperature and subduction angle on serpentine flow driven by density contrast serpentinization degree with the surroundings at temperatures of 200 500 c relevant to intermediate depths a fully serpentinized channel is unlikely to be thicker than 2 3 km for channel thicknesses of 2 km upward velocities are comparable to those using a constant viscosity of 10 18 pa s the velocity profile using power law rheology shows shear zones at the edges of the channel and a low strain rate region at its centre consistent with the frequent observation of weakly deformed hp rocks upward velocities estimated for channels 1 to 3 km thick are comparable to the serpentinization rates for maximum estimates of fluid velocities within shear zones in the literature competition between the upward flow and serpentinization may lead to intermittent behavior with alternating growth periods and thinning by exhumation at shallower levels the thickness allowed for a channel may be up to 8 10 km if the rheology has a higher dependence on stress we therefore propose that the exhumation of hp oceanic units in serpentinite channels is organized in two levels the deepest and fastest motion being driven by density contrast with the surrounding mantle and the shallowest circulation being driven by forced return flow the thicknesses estimated here for serpentinized layers at intermediated depths are similar to the precision of seismic studies the deepest serpentinite channel may thus be difficult to detect by seismic methods but it will have a strong influence on the mechanical coupling between the slab and mantle wedge

mantle wedge flow pattern and thermal structure in northeast japan effects of oblique subduction and 3 d slab geometry abstract we develop a 3 d thermal model for the northeast japan subduction margin using a realistic slab geometry for the subducting pacific plate and investigate the effects of oblique subduction and 3 d slab geometry on the mantle wedge flow pattern and the thermal structure in the tohoku region the mantle wedge flow pattern is nearly two dimensional resulting in a thermal structure similar to those obtained by a 2 d model owing to the simple slab geometry and subduction nearly perpendicular to the margin however in hokkaido oblique subduction leads to 3 d mantle wedge flow with northerly inflow and west northwestward outflow and also results in lower temperatures in the shallow part of the mantle wedge than in tohoku due to lower sinking rate of the slab between hokkaido and tohoku the slab has a hinge like shape due to a relatively sharp change in the dip direction in this hinge zone northerly mantle inflow from hokkaido and westerly mantle inflow from tohoku converge discouraging inflow from northwest and resulting in a cooler mantle wedge the model predicted mantle wedge flow patterns are consistent with observed seismic anisotropy and may explain the orientations of volcanic cross chains the predicted 3 d thermal structure correlates well with the along arc variations in the location of the frontal arc volcanoes and help to provide new insights into the surface heat flow pattern and the down dip extent of interplate earthquakes

along arc variation in the 3 d thermal structure around the junction between the japan and kurile arcs the thermal structure in subduction zones has a strong influence on seismogenesis and arc volcanism traditional 2 d models have been used to provide reasonable agreement between models and observations but in a number of cases clear 3 d effects are present one such case is in the northern japan subduction system at the junction between japan and kurile arcs surface heat flow and the occurrence of intermediate depth seismicity are different than in the tohoku and hokkaido regions we investigate the effects of 3 d slab geometry and a local deepening of slab mantle decoupling depth on the thermal structure in this region based on 3 d finite element approach we find that both effects produce the along arc variation of slab surface temperature which could reach 100 c the warmer region arises through 3 d effects of thermal conduction and the colder region arises through localized slow incoming flow in the case where 3 d slab geometry is taken into account 3 d flow arises where a local deepening of slab mantle decoupling depth is assumed which leads to both warmer and colder regions the effects on surface heat flow are small while intermediate depth seismicity in the subducted crust is suggested to be controlled by temperature dependent phase transitions the predicted changes in thermal structure are not sufficient to cause the observed deepening of seismicity this suggests that the thermal structure of this subduction zone may be more strongly influenced by time dependent deformation of the overriding crust and slab

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

split philippine sea plate beneath japan 1 the shape of the philippine sea plate subducting beneath western japan is a crucial factor in understanding earthquakes and volcanic activity we propose that the subducted plate was split along an extinct ridge due to an abrupt change of subduction direction followed by elastic deformation of the plate and an accumulation of stress near the ridge this shape is consistent with receiver function images the distribution of deep tremor sources the seismicity and focal mechanisms of intraplate earthquakes and the distribution of anomalous ratios of helium isotopes in ground water the movement history of the plate can explain active tectonics in western japan in the last 2 4 ma the location history of the volcanic front and the tear control where fluids ascend in the crust and these fluids are responsible for the generation of large earthquakes and volcanoes

determining the extent and nature of mazatzal related overprinting of the penokean orogenic belt in the southern lake superior region north central usa abstract twenty one hornblende and mica 40ar 39ar dates from central and northwest wisconsin usa provide important information on the timing spatial extent and intensity of mazatzal age metamorphism and deformation which overprinted the paleoproterozoic 1870 1820 ma penokean orogenic belt in the southern lake superior region 1760 1750 ma mica plateau ages from bedrock beneath undeformed 1750 1630 ma quartzites are interpreted as the time of lower temperature 300 350 c cooling and crustal stabilization after the penokean orogeny six mica ages from bedrock underlying deformed paleoproterozoic quartzites cluster around 1600 ma 1576 1614 ma the complete absence of 1760 1750 ma mica ages beneath regions of deformed quartzites suggests widespread heating to temperatures above 300 350 c during mazatzal related deformation and metamorphism the 1600 ma mica ages are interpreted to date the cooling phase of this metamorphism two anomalously young biotite dates are interpreted to indicate partial resetting associated with mesoproterozoic rifting at 1100 ma ten hornblende 40ar 39ar dates obtained in this study address the higher temperature overprinting effects in the southern lake superior region one latest archean age of 2503 18 ma and two ages of 1853 and 1830 ma are interpreted as remnant evidence of archean and penokean age amphibolite metamorphic events respectively the majority of hornblende ages are younger than the penokean orogeny scattering between 1796 and 1638 ma microtextural analysis indicates that similar microstructures exist in samples yielding highly discordant hornblende ages this suggests that shearing and recrystallization did not play an important role in the retention or loss of argon the 1638 ma hornblende age is concordant with the mazatzal orogeny to the south and is interpreted as representing complete thermal or fluid related resetting associated with that event six other post penokean ages scatter over a 70 million year interval 1796 1723 ma and probably reflect variable retention of radiogenic argon they are interpreted to indicate variable degrees of partial intermediate temperature 350 500 c resetting of hornblende argon systematics at 1650 1630 ma collectively these data suggest that the effects of the mazatzal orogeny in the southern lake superior region involved 350 500 c metamorphism and penetrative deformation

mesoproterozoic trans laurentian magmatism a synthesis of continent wide age distributions new sims u pb ages zircon saturation temperatures and hf and nd isotopic compositions abstract the origin and tectonic setting of the mesoproterozoic 1 50 1 34 ga granite rhyolite provinces of the midcontinent region of the united states and the coeval a type granite plutons of the southwestern united states have been an enigma for decades in this paper we review the history of this problem examine the significance of a wealth of previously published u pb zircon and whole rock sm nd data and present new u pb zircon age and lu hf isotopic data from selected zircon samples our data show that the initiation of this magmatism spans the interval 1 50 1 40 ga across the continent but there is a general younging westward younger 1 39 1 34 ga magmatism also occurred across the continent but forms a major magmatic event only in the south central mid continent there is generally a good correlation between sm nd and lu hf isotopic compositions and inferred mantle derivation model ages except possibly in southeastern oklahoma the absence of older cores in zircons is compatible with calculated zircon saturation temperatures of ca 850 c and supports mantle driven basaltic underplating and resulting crustal partial melting as a petrogenetic model for the a type magmatism our new data compilation refines the existence of the nd line that extends from southwestern texas into southeastern michigan and the western grenville province in canada it marks an isotopic boundary between granites and rhyolites of cratonal origin to the northwest 1 8 1 6 ga model ages and those of more juvenile origin to the southeast 1 5 1 4 ga model ages our suggested tectonic model for the mesoproterozoic a type magmatism across eastern and southern laurentia involves the development of an active laurentian margin extending from eastern canada across what is now the midcontinent region of the united states and at least into the current southwestern united states magmatism along this margin was likely related to asthenosphere lithosphere interactions associated with the accretion of outboard arcs and possible exotic terranes that caused complex deformational and magmatic responses in the continental interior this activity evidently spread from northeast to southwest over a period of ca 70 million years the occurrence of the younger ca 1 39 1 34 ga event indicates that similar processes occurred some 130 million years later suggesting a long lived convergent plate margin and recurring basaltic underplating in some locations perhaps due to inboard lithospheric delamination and or mantle upwelling

interpreting prograde growth histories of al2sio5 triple point rocks using oxygen isotope thermometry an example from the truchas mountains usa oxygen isotope compositions of kyanite andalusite prismatic sillimanite and fibrolite from the proterozoic terrane in the truchas mountains new mexico differ from one another suggesting that these minerals did not grow in equilibrium at the al2sio5 as polymorph invariant point as previously suggested instead oxygen isotope temperature estimates indicate that growth of kyanite andalusite and prismatic sillimanite occurred at c 575 615 and 640 c respectively temperature estimates reported in this paper are interpreted as those of growth for the different as polymorphs which are not necessarily the same as peak metamorphic temperatures for this terrane two distinct temperature estimates of c 580 c and c 700 c are calculated for most fibrolite samples with two samples yielding clear evidence of quartz fibrolite oxygen isotope disequilibrium these data indicate that locally and potentially regionally oxygen isotope disequilibrium between quartz and fibrolite may have resulted from rapid fibrolite nucleation pressures of mineral growth that were extrapolated from oxygen isotope thermometry results and calculated using petrological constraints suggest that kyanite and one generation of fibrolite grew during m1 at 5 kbar and that andalusite prismatic sillimanite and a second generation of fibrolite grew during m2 at 3 5 kbar m1 and m2 therefore represent two distinct metamorphic events that occurred at different crustal levels the ability of the as polymorphs to retain d 18 o values of crystallization make these minerals ideal to model prograde growth histories of mineral assemblages in metamorphic terranes and to understand more clearly the pressure temperature histories of multiple metamorphic events

numerical simulation of geodynamic processes with the portable extensible toolkit for scientific computation abstract geodynamics simulations are characterized by rheological nonlinearity localization three dimensional effects and separate but interacting length scales these features represent a challenge for computational science we discuss how a leading software framework for advanced scientific computing the portable extensible toolkit for scientific computation petsc can facilitate the development of geodynamics simulations to illustrate our use of petsc we describe simulations of i steady state non newtonian passive flow and thermal structure beneath a mid ocean ridge ii magmatic solitary waves in the mantle and iii the formation of localized bands of high porosity in a two phase medium being deformed under simple shear we highlight two supplementary features of petsc structured storage of application parameters and self documenting output that are especially useful for geodynamics simulations

detection of short term slow slip events along the nankai trough southwest japan using gnss data 1 we detected short term slow slip events sses previously observable only with tilt and strain data along the nankai trough southwest japan using gnss global navigation satellite system data offsets detected in gnss time series using akaike s information criterion helped automatically identify 207 episodes with a motion direction opposite to that of the relative plate motion from june 1996 to january 2012 by nonlinear inversion of the detected displacement we estimated rectangular fault models for 133 probable and 25 possible short term sses over 15 years the sse moment magnitudes range from 5 5 to 6 3 most sse fault models are located in a narrow band of non volcanic tremors on the interfaces of the subducting philippine sea plate large sses moment magnitude mw 6 often occur in western and central shikoku the cumulative slip is distributed heterogeneously along the strike generally decreasing eastward with the maximum slip 50 cm in western shikoku no definite short term sses were found in the kii channel but several short term sses occurred in ise bay both regions are known as tremor gaps the local maximum of the cumulative slip fills in the tremor gap located in ise bay the long term rate of short term sse cumulative moment increased by threefold around 2003 in eastern shikoku whereas it was almost constant in other regions comparison with short term sse catalogues using tilt data suggests that both this study and previous studies missed some sses along the nankai trough a combination of geodetic data is important in the monitoring of the spatiotemporal distribution of short term sses

detrital zircon evidence for non laurentian provenance mesoproterozoic ca 1490 1450 ma deposition and orogenesis in a reconstructed orogenic belt northern new mexico usa defining the picuris orogeny detrital zircon and igneous zircon u pb ages are reported from proterozoic metamorphic rocks in northern new mexico these data give new insight into the provenance and depositional age of a 3 km thick metasedimentary succession and help resolve the timing of orogenesis within an area of overlapping accretionary orogens and thermal events related to the proterozoic tectonic evolution of southwest laurentia three samples from the paleoproterozoic vadito group yield narrow unimodal detrital zircon age spectra with peak ages near 1710 ma igneous rocks that intrude the vadito group include the cerro alto metadacite the picuris pueblo granite and the penasco quartz monzonite and yield crystallization ages of 1710 10 ma 1699 3 ma and 1450 10 ma respectively within the overlying hondo group a metamorphosed tuff layer from the pilar formation yields an age of 1488 6 ma and represents the first direct depositional age constraint on any part of the proterozoic metasedimentary succession in northern new mexico detrital zircon from the overlying piedra lumbre formation yield a minimum age peak of 1475 ma and 60 grains 25 yield ages between 1500 ma and 1600 ma possibly representing non laurentian detritus originating from australia and or antarctica detrital zircons from the basal metaconglomerate and the middle quartzite member of the marquenas formation yield minimum age peaks of 1472 ma and 1471 ma consistent with earlier results we interpret the onset of ca 1490 1450 ma deposition followed by tectonic burial regional al 2 sio 5 triple point metamorphism and ductile deformation at depths of 12 18 km to reflect a mesoproterozoic contractional orogenic event possibly related to the final suturing of the mazatzal crustal province to the southern margin of laurentia we propose to call this event the picuris orogeny

mantle flow and deformation of subducting slab at a plate junction abstract at junctions between plate boundaries there are several general characteristic features these include trench normal fast polarization direction of s wave splitting in the mantle wedge along arc variation of the angle of subduction and the different behavior of slab stagnation along the strike of the trench in this paper we focus on the junction of the tohoku kurile arc we show the dynamical effects of the junction using a numerical model of mantle convection with a realistic curved shape of the trench we obtain 3d flow in the mantle wedge which is consistent with the observation of seismic anisotropy even with the oblique subduction we also find the along arc variation of subduction angle which agrees with the observations although the match becomes worse when we include the effect of dislocation creep that is non linear viscosity along arc variation of the angle of subduction partly contributes to the different behavior of slab stagnation in the tohoku kurile arc our results show that the shape of the trench is an important factor which considerably affects mantle flow and deformation of subducting slabs thus 3d modeling is necessary to constrain the dynamics of subduction zones near the junction

high pressure creep of serpentine interseismic deformation and initiation of subduction the supposed low viscosity of serpentine may strongly influence subduction zone dynamics at all time scales but until now its role could not be quantified because measurements relevant to intermediate depth settings were lacking deformation experiments on the serpentine antigorite at high pressures and temperatures 1 to 4 gigapascals 200 to 500 c showed that the viscosity of serpentine is much lower than that of the major mantle forming minerals regardless of the temperature low viscosity serpentinized mantle at the slab surface can localize deformation impede stress buildup and limit the downdip propagation of large earthquakes at subduction zones antigorite enables viscous relaxation with characteristic times comparable to those of long term postseismic deformations after large earthquakes and slow earthquakes antigorite viscosity is sufficiently low to make serpentinized faults in the oceanic lithosphere a site for subduction initiation

depth of seismic coupling along subduction zones underthrusting at subduction zones can cause large earthquakes at shallow depths but it is always accommodated by aseismic deformation below a certain depth the maximum depth of the seismically coupled zone or seismogenic zone is a transition from unstable to stable sliding along the plate interface we have determined the depth of this stability transition for the circurn pacific subduction zones of honshu kuriles kamchatka aleutians alaska mexico and chile these subduction zones have experienced great interplate earthquakes and the aftershock regions are well located depth estimates of interplate events that are located at the downdip edge of the aftershock regions are used to determine the maximum depth of seismic coupling for an average p wave velocity of 6 7 km s 1 above the plate interface we find that for most subduction zones the stability transition occurs at 40 5 km depth there are however several exceptions at the hokkaido trench junction where the japan trench and the kurile trench intersect seismic coupling is deep and extends down to 52 55 km deep coupling was also found in the coquimbo region in central chile the mexico subduction zone has shallow coupling the transition occurs at 20 30 km depth previous studies of micro earthquakes in honshu hokkaido the aleutians and alaska show that earthquakes within the upper plate extend no deeper than the downdip edge of the coupled zone that we find given our measurements of seismic coupling depth we then explore the mechanism that may determine coupling depth the concept of critical temperature has been used to explain the depth of seismic coupling in other tectonic environments thus we first test whether a critical temperature can explain our results temperatures at the plate interface are dependent on many variables but two that are poorly determined are shear stress and radiogenic heat generation shear stress has been constrained by inversion of heat flow data assuming a crustal radiogenic heat production rate of 3 1 exp z 8 5 wm 3 and a constant coefficient of friction we find two critical temperatures of about 400 c and 550 c the lower critical temperature may be characteristic of regions with a relatively thick continental crust and the higher temperature of regions with a relatively thin continental crust on the other hand one single critical temperature of about 250 c can explain the coupling depths if shear stresses are constant with depth

microprobe monazite geochronology putting absolute time into microstructural analysis abstract high resolution compositional mapping and dating of monazite on the electron microprobe is a powerful addition to microstructural analysis and an increasingly important tool for tectonic analysis microprobe monazite geochronology can be an efficient reconnaissance tool for evaluating metamorphic and deformational age domains but more importantly its in situ nature and high spatial resolution offer an entirely new level of structurally and texturally specific geochronologic data that can be used to put absolute time constraints on p t d paths constrain the rates of metamorphic and deformational processes and provide new links between metamorphism and deformation microprobe geochronology is particularly applicable to three persistent microstructural microtextural problem areas 1 constraining the chronology of metamorphic assemblages 2 constraining the timing of deformational fabrics and 3 interpreting other geochronological results although some monazite generations can be directly tied to metamorphism or deformation at present the most common constraints rely on monazite inclusion relations in porphyroblasts that in turn can be tied to the deformation and or metamorphic history microprobe mapping and dating allow geochronology to be incorporated into the routine microstructural analytical process resulting in a new level of integration of time t into p t d histories

anisotropic structures of oceanic slab and mantle wedge in a deep low frequency tremor zone beneath the kii peninsula sw japan 1 anisotropy is an important feature of elastic wave propagation in the earth and can arise from a variety of ordered architectures such as fractures with preferential alignments or preferred crystal orientations we studied the regional variations in shear wave anisotropy around a deep low frequency earthquake lfe zone beneath the kii peninsula sw japan using waveforms of local earthquakes observed by a dense linear array along the lfe zone the fast directions of polarization are subparallel to the strike of the margin for both crustal and intraslab earthquakes the delay time of the split shear waves in intraslab earthquakes is larger than that in crustal earthquakes and shows a down dip variation across the lfe zone this indicates that anisotropy exists in the mantle wedge and in the lower crust and or oceanic slab we explain the observed delay time of 0 015 0 045 s by suggesting that the mantle wedge consists of a deformed 1 15 km thick serpentine layer if the mantle wedge is completely serpentinized in addition to high fluid pressures within the oceanic crust the sheared serpentine layer may be a key factor driving lfes in subduction zones

shear induced permeability anisotropy of simulated serpentinite gouge produced by triaxial deformation experiments 1 we have determined the permeability of antigorite serpentinite gouge in three orthogonal directions during frictional experiments using a triaxial gas apparatus to better understand fluid flow in subduction fault zones the experiments were conducted at room temperature a confining pressure of 150 mpa a pore pressure of 100 mpa and a constant slip rate of 0 58 m s although the initial permeabilities are similar in all directions anisotropy develops during deformation with the permeability normal to the shear plane becoming one order of magnitude lower than in the other directions when the shear stress reaches steady state formation of such permeability anisotropies may enhance fluid flow along the subduction plate interface and serpentinite bearing fault zones important in evaluating potentially heterogeneous distribution of fluid pressure along the fault and in understanding the spatio temporal variation in the seismic activity

isostatic constraints on the assembly stabilization and preservation of cratonic lithosphere cratonic uplift or subsidence as recorded geologically by erosion or deposition indicates the net change in lithospheric buoyancy density change is primarily controlled by temperature change such that the geologic record reveals the net lithospheric heating or cooling our modeling shows that thermal stabilization of lithosphere can occur in 100 m y largely determined by the time for crustal radioactivity to warm the entire lithosphere longer term thermal evolution 350 400 m y is controlled by the feedback between isostatic and thermal responses crustal heat production is the dominant factor for evolution of crustal geo therms whereas the detailed initial thermal structure is less important the fact that many cratons are eroded to only modest depths indicates that they have a history of warming slightly since their formation implying a rather cool 700 1000 c for initial lithospheric mantle this is consistent with the growth of cratonic lithosphere by progressive thickening during continental assembly and or subduction stacking of oceanic lithosphere relatively warm early lithosphere due to elevated crustal heat production or higher initial lithospheric mantle temperatures may have had a lower preservation potential due to preferential deformation and destruction study of the proterozoic orogen of the southwestern united states indicates that the distinct thermal and isostatic records of adjacent crustal blocks are compatible with observed differences in crustal heat production but also with similar initial lithospheric mantle temperatures

a new regime of slab mantle coupling at the plate interface and its possible implications for the distribution of volcanoes abstract we investigate the effects of a thin low viscosity layer just above the subducting slab on 3d thermal and flow structure in the mantle wedge by taking northeast japan as an example the low viscosity layer assumed here is needed to explain the observed low surface heat flow and low seismic attenuation in the forearc by decoupling the mantle from the subducting slab we find that when the viscosity in the low viscosity layer is sufficiently low along arc component of the flow arises inside the layer and produces along arc temperature variation it can also be considered as the along arc changes in the degree of slab mantle coupling at the plate interface the onset time and the characteristic wavelength of the 3d flow depend on the viscosity and the extent of the low viscosity layer in order to explain the observed spatial and temporal changes in the distribution of quaternary volcanoes in northeast japan the viscosity and the thickness of the low viscosity layer need to be 5 10 18 pa s and 6 km respectively the model proposed here is based on an assumption which is simpler and better constrained by observations compared to previous models therefore it could be an alternative explanation of the distribution of volcanoes in northeast japan

new mexico structural zone an analogue of the colorado mineral belt abstract updated aeromagnetic maps of new mexico together with current knowledge of the basement geology in the northern part of the state sangre de cristo and sandia manzano mountains where basement rocks were exposed in precambrian cored uplifts indicate that the northeast trending proterozoic shear zones that controlled localization of ore deposits in the colorado mineral belt extend laterally into new mexico the shear zones in new mexico coincide spatially with known epigenetic precious and base metal ore deposits thus the mineralized belts in the two states share a common inherited basement tectonic setting reactivation of the basement structures in late cretaceous eocene and mid tertiary times provided zones of weakness for emplacement of magmas and conduits for ore forming solutions ore deposits in the colorado mineral belt are of both late cretaceous eocene and mid tertiary age those in new mexico are predominantly mid tertiary in age but include late cretaceous porphyry copper deposits in southwestern new mexico the mineralized belt in new mexico named the new mexico structural zone is 250 km wide the northwest boundary is the jemez subzone or the approximately equivalent globe belt and the southeastern boundary was approximately marked by the santa rita belt three groups subzones of mineral deposits characterize the structural zone 1 mid tertiary porphyry molybdenite and alkaline precious metal deposits in the northeast segment of the jemez zone 2 mid tertiary epithermal precious metal deposits in the tijeras intermediate zone and 3 late cretaceous porphyry copper deposits in the santa rita zone the structural zone was inferred to extend from new mexico into adjacent arizona the structural zone provides favorable sites for exploration particularly those parts of the jemez subzone covered by neogene volcanic and sedimentary rocks

late paleoproterozoic climate tectonics and metamorphism in the southern lake superior region and proto north america evidence from baraboo interval quartzites abstract red supermature quartzites of the baraboo interval of the lake superior region contain detrital zircon that ranges in age from 1782 to 1712 ma deposition clearly occurred after the geon 18 penokean orogeny these late paleoproterozoic sedimentary rocks consist largely of quartz kaolinite or pyrophyllite and hematite detrital feldspar and muscovite are rare or absent their chemical index of alteration ranges from 96 8 to 98 6 among the most chemically mature clastic sediments in the geological record the quartzites are underlain by mature feldspar free paleosols accounting for the absence of feldspar in the overlying sediments and indicating the presence of first cycle quartzose detritus such physical and chemical characteristics imply that late paleoproterozoic deposition in the lake superior region occurred in a stable tectonic setting with subdued topographic relief in a warm humid climate folding and low grade metamorphism of the quartzites is thought to reflect 1630 ma foreland

fracture propagation and stability of ice shelves governed by ice shelf heterogeneity tabular iceberg calving and ice shelf retreat occurs after full thickness fractures known as rifts propagate across an ice shelf a quickly evolving rift signals a threat to the stability of larsen c the antarctic peninsula s largest ice shelf here we reveal the influence of ice shelf heterogeneity on the growth of this rift with implications that challenge existing notions of ice shelf stability most of the rift extension has occurred in bursts after overcoming the resistance of suture zones that bind together neighboring glacier inflows we model the stresses in the ice shelf to determine potential rift trajectories calving perturbations to ice flow will likely reach the grounding line the stability of larsen c may hinge on a single suture zone that stabilizes numerous upstream rifts elevated fracture toughness of suture zones may be the most important property that allows ice shelves to modulate antarctica s contribution to sea level rise

low frequency tremors intraslab and interplate earthquakes in southwest japan from a viewpoint of slab dehydration 1 low frequency tremors lft recently found in the southwest japan fore arc likely occur due to hydro fracturing from fluids leaving the dehydrating philippine sea slab however there are some places without such lft they are n izu kanto e shikoku and s kyushu these are the places where island arc type crust is subducting we propose that dehydration of the subducted crust does not occur beneath these regions because it is composed mainly of tonalite lacking the quantity of hydrous minerals seen in normal subducting oceanic crust we show that almost no earthquake occurs within the subducted crust in such regions consistent with dehydration embrittlement hypothesis for intraslab seismicity the lack of dehydration from the crust would also affect the mode of occurrence of interplate earthquakes because pore fluid pressure is more difficult to rise at the thrust zone this might be reflected in the irregular occurrence of great earthquakes in these regions

subduction of the philippine sea plate beneath southwestern japan slab geometry and its relationship to arc magmatism 1 we carry out high resolution three dimensional seismic tomography of the crust and upper mantle beneath southwestern sw japan using arrival time data obtained from the nationwide seismograph network the tomographic images provide new insights into the configuration of the philippine sea slab and arc magmatism the results confirm the existence of an aseismic portion of the philippine sea slab at greater depths beyond the seismic portion the philippine sea slab is subducting aseismically down to at least a depth of 200 km in chubu 60 80 km in kinki and 60 km in chugoku and is subducting seismically to depths of 150 200 km in kyushu in the chubu district it is subducting seismically at a shallow dip and then bends downward beneath the volcanic area whereas in the kinki district it is subducting subhorizontally as far as the japan sea the differences in the slab geometry between adjacent regions correlate with the differences in volcanic activity suggesting the importance of the geometry of the philippine sea slab on arc magmatism our tomographic images further imply subduction of the philippine sea slab down to a depth of around 70 km northwest of the izu peninsula the philippine sea slab in that region might have split into western and eastern parts separated by a slab tear and collided with the pacific slab at depths of 150 200 km

the slip distribution of the 1946 nankai earthquake estimated from tsunami inversion using a new plate model abstract a new model for the shape of the philippine sea plate psp subducting beneath southwest japan was constructed by combining marine seismic survey results and seismicity data this resulted in a plate model that is generally 3 5 km shallower than previous estimates and contains new details of plate boundary structure near the nankai trough axis we estimated the slip distribution of the 1946 nankai earthquake by inverting tsunami waveforms using the new plate boundary model and found that the large slip off cape muroto and seismic moment were reduced by about 10 from the previous estimate of tanioka and satake 2001a however we also found reason to suspect the tide gauge closest to the epicenter at shimotsu and this resulted in a much more substantial reduction in the amount of slip in the eastern segment of the rupture area we also find that our new rupture model correlates well with features of the subducting plate boundary such as a large subducting seamount off cape muroto kodaira et al 2000a this suggests that such features have an important effect on either earthquake rupture propagation or the interseismic accumulation of elastic strain energy or both

an inverted continental moho and serpentinization of the forearc mantle volatiles that are transported by subducting lithospheric plates to depths greater than 100 km are thought to induce partial melting in the overlying mantle wedge resulting in arc magmatism and the addition of significant quantities of material to the overlying lithosphere asthenospheric flow and upwelling within the wedge produce increased lithospheric temperatures in this back arc region but the forearc mantle in the corner of the wedge is thought to be significantly cooler here we explore the structure of the mantle wedge in the southern cascadia subduction zone using scattered teleseismic waves recorded on a dense portable array of broadband seismometers we find very low shear wave velocities in the cold forearc mantle indicated by the exceptional occurrence of an inverted continental moho which reverts to normal polarity seaward of the cascade arc this observation provides compelling evidence for a highly hydrated and serpentinized forearc region consistent with thermal and petrological models of the forearc mantle wedge this serpentinized material is thought to have low strength and may therefore control the down dip rupture limit of great thrust earthquakes as well as the nature of large scale flow in the mantle wedge

40ar 39ar evidence for middle proterozoic 1300 1500 ma slow cooling of the southern black hills south dakota midcontinent north america implications for early proterozoic p t evolution and posttectonic magmatism 40ar 39ar total gas and plateau dates from moscovite and biotite in the southern black hills south dakota provide evidence for a period of middle proterozoic slow cooling early proterozoic 1600 1650 ma mica dates were obtained from metasedimentary rocks located in a synformal structure between the harney peak and bear mountain domes and also south of bear mountain metamorphic rocks from the dome areas and undeformed samples of the 1710 ma harney peak granite hpg yield middle proterozoic mica dates 1270 1500 ma two samples collected between the synform and bear mountain dome yield intermediate total gas mica dates of 1550 ma we suggest two end member interpretations to explain the map pattern of cooling ages 1 subhorizontal slow cooling of an area which exhibits variation in mica ar retention intervals or 2 mild folding of a middle proterozoic 1500 ma 300 c isotherm according to the second interpretation the preservation of older dates between the domes may reflect reactivation of a preexisting synformal structure and downwarping of relatively cold rocks during a period of approximately east west contraction and slow uplift during the middle proterozoic the mica data together with hornblende data from the black hills published elsewhere indicate that the ambient country rock temperature at the 3 4 kbar depth of emplacement of the hpg was between 350 c and 500 c suggesting that the average upper crustal geothermal gradient was 25 40 c km prior to intrusion the thermochronologic data suggest hpg emplacement was followed by a 200 m y period of stability and tectonic quiescence with little uplift we propose that crust thickened during the early proterozoic was uplifted and erosionally thinned prior to 1710 ma and that the hpg magma was emplaced into isostatically stable crust of relatively normal thickness we speculate that uplift and crustal thinning prior to hpg intrusion was the result of differential thinning of the subcrustal lithosphere beneath the black hills if so this process would have also caused an increase in mantle heat flux across the moho and triggered vapor absent melting of biotite to produce the hpg magma this scenario for posttectonic granite generation is supported in part by the fact that in the whole of the black hills the hpg is spatially associated with the deepest exposed early proterozoic country rock

seismic consequences of warm versus cool subduction metamorphism examples from southwest and northeast japan warm and cool subduction zones exhibit differences in seismicity seismic structure and arc magmatism which reflect differences in metamorphic reactions occurring in subducting oceanic crust in southwest japan arc volcanism is sparse and intraslab earthquakes extend to 65 kilometers depth in northeast japan arc volcanism is more common and intraslab earthquakes reach 200 kilometers depth thermal petrologic models predict that oceanic crust subducting beneath southwest japan is 300 to 500 c warmer than beneath northeast japan resulting in shallower eclogite transformation and slab dehydration reactions and possible slab melting

a silent slip event on the deeper cascadia subduction interface continuous global positioning system sites in southwestern british columbia canada and northwestern washington state usa have been moving landward as a result of the locked state of the cascadia subduction fault offshore in the summer of 1999 a cluster of seven sites briefly reversed their direction of motion no seismicity was associated with this event the sudden displacements are best explained by 2 centimeters of aseismic slip over a 50 kilometer by 300 kilometer area on the subduction interface downdip from the seismogenic zone a rupture equivalent to an earthquake of moment magnitude 6 7 this provides evidence that slip of the hotter plastic part of the subduction interface and hence stress loading of the megathrust earthquake zone can occur in discrete pulses

u pb and hf isotopic analysis of zircon in lower crustal xenoliths from the navajo volcanic field 1 4 ga mafic magmatism and metamorphism beneath the colorado plateau zircon from lower crustal xenoliths erupted in the navajo volcanic field was analyzed for u pb and lu hf isotopic compositions to characterize the lower crust beneath the colorado plateau and to determine whether it was affected by 1 4 ga granitic magmatism and metamorphism that profoundly affected the exposed middle crust of southwestern laurentia igneous zircon in felsic xenoliths crystallized at 1 73 and 1 65 ga and igneous zircon in mafic xenoliths crystallized at 1 43 ga most igneous zircon has unradiogenic initial hf isotopic compositions hf 4 1 7 8 and 1 7 1 6 ga depleted mantle model ages consistent with 1 7 1 6 ga felsic protoliths being derived from juvenile proterozoic crust and 1 4 ga mafic protoliths having interacted with older crust metamorphic zircon grew in four pulses between 1 42 and 1 36 ga at least one of which was at granulite facies significant variability within and between xenoliths in metamorphic zircon initial hf isotopic compositions hf 0 7 to 13 6 indicates growth from different aged sources with diverse time integrated lu hf ratios these results show a strong link between 1 4 ga mafic magmatism and granulite facies metamorphism in the lower crust and granitic magmatism and metamorphism in the exposed middle crust