heat producing elements and the thermal and baric patterns of metamorphic belts the character of sedimentary basins before they are deformed and metamorphosed may strongly influence the thermal and baric patterns of metamorphic belts crustal thickening of anoxic sedimentary basins and subsequent thermal reequilibration may produce large areas of high grade metamorphic rocks and granites because the concentrations of the heat producing elements are high in such basins in new england there is a spatial association among granites and high grade metasedimentary rocks rich in u and th that now form the central maine terrane the high content of heat producing elements in these rocks is attributed to fixing of u and th in highly reduced sediments that were deposited in an anoxic basin that formed in the silurian when the basin was thickened during the devonian acadian orogeny the thermal energy generated by the u and th rich sediments produced the observed broad zone of high grade rocks and anatectic granites this hypothesis was tested with thermal calculations that reproduce most of the first order thermal and baric patterns of the acadian appalachians if pretectonic lateral variations in heat production are assumed

role of plutonism in low pressure metamorphic belt formation wickham and oxburgh1 recently proposed that low pressure high temperature low p high t metamorphism in the eastern pyrenees and possibly all low p high t metamorphic belts resulted from anomalously high mantle heat flow brought about by rifting their model is largely constrained by the presence of nearby synmetamorphic rift related sedimentary rocks and the interpretation that the migmatites and granites are the product of in situ melting in the presence of an anomalously steep geotherm here we present an alternative model in which low p high t metamorphism pro grade reactions at pressures near or below the al2sio5 triple point results from contact effects near sill like igneous intrusions at intermediate crustal levels low p high t conditions can be achieved through this process in regions of continent continent collision with normal mantle heat flux as well as in zones of extension our model is based on studies of the low p high t metamorphic terrane in the new england appalachians

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

low pressure facies series metamorphism in an accretionary sedimentary prism southern alaska the low pressure high temperature metamorphism of the chugach metamorphic complex alaska occurred in an ocean continent convergence zone to achieve the high temperatures at a relatively shallow depth in an accretionary prism we propose the large scale transport of heat by fluids which preheated the metamorphic belt by tectonic focusing of fluids followed by injection of melts both of which were generated downdip in a shallow subduction zone

estimation of mineralogy and lithology from geochemical log measurements mineralogy chemistry and physical properties of core samples from a research well in grand county utah have been determined to assess the applications of geochemical logging tool measurements to reservoir characterization the cored interval over 827 ft or 252 m of continuous core samples a wide range of lithologies including subarkose sublitharenite litharenite feldspathic litharenite quartz wacke lithic wacke siltstone mudstone and coal total clay mineral contents range from 2 to almost 60 wt and consist primarily of illite and kaolinite with subordinate ordered mixed layer illite smectite dolomite cement is pervasive locally large amounts up to 25 wt of siderite or ankerite are present total feldspar minerals comprise 5 18 wt with sodium ri h plagioclase slightly more abundant than potassium feldspar continuous geochemical well logs of 12 elements k u th al si ca fe s gd ti cl and h were obtained using logging tools that measure neutron capture neutron activation and natural gamma ray activities applications of these geochemical log measurements to assess mineralogy and lithology for the research well are complicated by 1 the near compositional colinearity of the assemblage illite kaolinite k feldspar 2 the presence of dolomite and iron bearing carbonates 3 the predominance of na plagioclase over k feldspar and 4 the abundance of sedimentary and metamorphic rock fragments in the litharenite group of lithologies because of these mineral and textural features successful application of the log measurements to reservoir characterization requires calibration to core analyses mineral logs calculated using least squares inversion programs accurately describe changes in mineralogy and mineral proportions once the mineral assemblage is identified lithology is best estimated based on these mineral logs alternatively up to seven general lithology types can be discriminated using the lithology ratio al 2 o 3 fe 2 o 3 sio 2 al 2 o 3 fe 2 o 3 when chemical data are calibrated with petrographic data

geochemical logging in the cajon pass drill hole and its application to a new oxide igneous rock classification scheme a new elemental oxide classification scheme for crystalline rocks is developed and applied to geochemical well logs from the cajon pass drill hole this classification scheme takes advantage of measurements of elements taken by a geochemical logging tool string it uses k 2o versus sio 2 al 2o 3 to distinguish between granites granodiorites tonalites syenites monzonites diorites and gabbros oxide measurements from cores are used to calibrate the elemental abundances determined from the well logs from these logs a detailed lithologic column of the core is generated the lithologic column derived from the well log classification scheme is compared with a lithologic column constructed from core samples and well cuttings in the upper 1295 m of the well agreement between the two columns is good discrepancies occur from 1295 to 2073 m and are believed to be caused by the occurrence of rock types not distinguished by the classification scheme and or the occurrence of secondary minerals despite these discrepancies the well log based classification scheme helps to distinguish changes in rock type and shows potential as an aid to the construction of lithologic columns in boreholes of crystalline rocks

elemental and mineralogical analyses using geochemical logs from the cajon pass scientific drillhole california and their preliminary comparison with core analyses estimates of elemental and mineralogical abundances from geochemical logs are compared to preliminary chemical and modal analyses from cores in the cajon pass scientific drillhole accuracies of log computed weight percent oxide and mineralogy determinations range from 10 to 30

reservoir characterization from downhole mineralogy abstract it is now a relatively established procedure with recent advances in logging technology to be able to generate a comprehensive continuous measurement of major element chemistry in the subsurface concurrent with these advances strategies have been developed which transform elemental data derived from nuclear logging measurements into a set of mineral modes that accurately represent the mineralogy of a rock resulting mineralogy logs are potentially valuable on their own especially for creating a geological model in the absence of or to enhance core measurements mineralogy logs can also be used for the determination of other petrophysically useful formation descriptors such that in wells where core recovery is poor they can help to extend evaluation across poorly defined zones a selection of applications are investigated these show that with good acquisition derived mineralogy may be used effectively for inter well correlation and for the determination of enhanced estimates of matrix density and porosity

the origins of granulites a metamorphic perspective although many recent reviews emphasize a uniformity in granulite pressure temperature p t conditions and paths granulites in reality preserve a spectrum of important petrogenetic features which indicate diversity in their modes of formation a thorough survey of over 90 granulite terranes or occurrences reveals that over 50 of them record p t conditions outside the 7 5 1 kbar and 800 50 c average granulite regime preferred by many authors in particular an increasing number of very high temperature 900 1000 c terranes are being recognized both on the basis of distinctive mineral assemblages and geothermobarometry petrogenetic grid and geothermobarometric approaches to the determination and interpretation of p t histories are both evaluated within the context of reaction textures to demonstrate that the large range in p t conditions is indeed real and that both near isothermal decompression itd and near isobaric cooling ibc p t paths are important amphibolite granulite transitions promoted by the passage of co 2 rich fluids as observed in southern india and sri lanka are exceptional and not representative of fluid related processes in the majority of terranes it is considered on the contrary that fluid absent conditions are typical of most granulites at or near the time of their recorded thermal maxima itd granulites are interpreted to have formed in crust thickened by collision with magmatic additions being an important extra heat source erosion alone is not however considered to be the dominant post collisional thinning process instead the itd paths are generated during more rapid thinning 1 2 mm yr exposure related to tectonic exhumation during moderate rate or waning extension ibc granulites may have formed in a variety of settings those which show anticlockwise p t histories are interpreted to have formed in and beneath areas of voluminous magmatic accretion with or without additional crustal extension ibc granulites at shallow levels very rapid 5 mm yr extensional thinning subsequent to collision it is suggested that the preservation of ibc paths rather than itd paths in many granulites is primarily related to the rate and timescale of extensional thinning of thickened crust and that hybrid itd to ibc paths should also be observed most ibc granulites and probably many itd granulites have not been exposed at the earth s surface as a result of the tectonic episodes which produced them but have resided in the middle and lower crust for long periods of time 100 2000 ma following these events the eventual exhumation of most granulite terranes only occur through their incorporation in later tectonic and magmatic events unrelated to their formation

the synchronism of crustal thickening and low pressure facies metamorphism in the mount isa inlier australia 2 fast convective thinning of mantle lithosphere during crustal thickening the mount isa inlier is like some other northern australian early to middle proterozoic fold belts characterized by an anti clockwise p t t path i e regional prograde low p facies metamorphic assemblages grew during crustal thickening p t and were overprinted by a phase of essentially isobaric cooling p 0 t in this study two possible models for the synchronism of regional low p facies metamorphism and crustal thickening are quantitatively evaluated using a one dimensional finite difference technique 1 extreme elevation of isotherms immediately before the thickening event due to e g lithospheric extension magmatic activity or crust mantle delamination and 2 crustal thickening accompanied by convective thinning of the mantle lithosphere only the latter results in prograde low p facies metamorphism during crustal thickening it is the preferred model for the mount isa inlier

the stress heat flow paradox and thermal results from cajon pass conventional friction models predict a substantial thermal anomaly associated with active traces of strike slip faults but no such anomaly is observed from over 100 heat flow determinations along 1 000 km of the san andreas fault the cajon pass well is being drilled to bring deep heat flow and stress data to bear on this paradox preliminary heat flows to 1 7 km in the neighboring arkoma well indicated a positive anomaly of about 30 as would be expected if frictional resistance of the fault were 50 mpa or so but the pliocene and pleistocene history of the transverse ranges particularly high rates of uplift and erosion could also explain the anomaly preliminary stress results from cajon pass and a new interpretation of regional data by mark d zoback and colleagues suggests that the maximum compressive stress near the fault is almost normal to the trace and hence the resolved shear stress is low and the fault weak although this is consistent with long standing evidence from low heat flow vexing questions remain regarding the physics of strike slip faulting the heat flow data show large variability with depth probably from three dimensional structure and an overall decrease from over 90 mw m in the upper kilometer to less than 80 mw m in the lower 300 m with no evidence of advective heat transfer whether this decreasing trend is significant and whether it continues downward to the background value 70 mw m less the radiogenic heat from overlying rocks as expected for a weak fault must be verified with data from greater depth

some remarks on high temperature low pressure metamorphism in convergent orogens many high temperature low pressure high t low p metamorphic terranes show evidence for peak mineral growth during crustal thickening strain increments at pressures near the maximum attained during the heating cooling cycle such terranes are not readily explained as the conductive response to crustal thickening since the resulting moho temperatures would greatly exceed the crustal liquidus and because heating due to conductive equilibration on length scales appropriate to lithospheric scale strains must greatly outlast the deformation consequently high t low p metamorphism may be generated during crustal thickening only when significant heat is advected within the crust as for example may occur during the segregation of granitic melts we show that without the addition of asthenospheric melts and at strain rates appropriate to continental deformation the conditions required for significant lower crustal melting during deformation are only likely to be attained if heat flow into the lower crust during crustal thickening is increased substantially for example by removing the mantle part of the lithosphere a simple parameterization of lithospheric deformation involving the vertical strain on the scale of the crust c and the lithosphere 1 respectively allows the potential energy of the evolving orogen to be readily evaluated using this parameterization we show that an important isostatic consequence of the deformation geometries capable of generating such high t low p metamorphism during crustal thickening with c1 is an imposed upper limit to crustal thicknesses which is much lower than for homogeneous deformations fc f1 for the same initial lithospheric configuration

adaptive whole earth tomography 1 an approach for seismic tomography is presented which allows the parameterization to be refined during the inversion the objective is to use the data to refine the mesh and the velocity model together and hence both are considered part of the solution some simple rules are used to identify the volumes of a three dimensional model in need of refinement the self adaptive parameterization is applied to an initial mesh built from uniformly distributed spherical triangles and delaunay tetrahedra application of the technique to a typical summary ray p wave arrival time data set shows it to be both feasible and practical for large scale whole earth tomography a noticeable trend in the resulting models as the parameterization is refined is the thinning of the farallon and tethys subduction features imaged in the mid mantle together with an increase in amplitude of the velocity perturbation

thermal modelling in low pressure high temperature metamorphic belts abstract low pressure high temperature metamorphic belts occur throughout the world they display a characteristic metamorphic style which has remained unchanged with time they are found in disparate tectonic settings including magmatic arcs regions of extension regions of thickened crust in continent continent collision zones and accretionary wedges they are commonly characterized by an abundance of granitoid plutons pressures of metamorphism are primarily less than or equal to that of the aluminosilicate triple point approximately 4 kbar while temperatures are in the range 500 750 c with rare exceptions inferred ptt paths are nearly isobaric and recrystallization was rapid and prograde metamorphic thermal gradients are commonly in the range 60 150 c km these conditions necessitate the presence of a large component of advective heat transfer and or anomalously high basal heat flow during metamorphism the large magmatic fluxes associated with the formation of magmatic arcs provide an adequate heat source for low pressure metamorphism lpm lithospheric extension is likely to produce lpm only if the degree of thinning is large 3 and there is additional heat from magmas the generation of lpm by crustal thickening requires either a large component of advective heating by magmas anomalously high basal heat flow or pre thickening lithospheric extension lpm in this environment must necessarily be accompanied by plutonic activity advection of heat by aqueous fluids may provide a significant contribution to the thermal budget but acting along is unlikely to produce lpm regional lpm induced by advective sources rarely occurs along a geotherm which increases monotonically with depth or synchronously across the region at a given depth it occurs over time as a series of overlapping temporally separate and spatially localized thermal events

orogeny associated with anticlockwise p t t paths evidence from low p high t metamorphic terranes in the arunta inlier central australia low pressure high temperature metamorphic terranes from the proterozoic northern arunta inlier central australia are characterized by anticlockwise pressure temperature time paths the thermal peak was reached before or during the earliest folding and it was associated with abundant repeated intrusion of granite sheets we generalize our observations in the area to infer that low p high t metamorphism is induced principally by focusing of mid crustal heat from rapid repeated granite sheet intrusion which generates large metamorphic aureoles intrusion and associated metamorphism produce melting reactions which increase the fluid pressure and thermally soften the mid crust resulting in accelerated melt enhanced deformation shortening is accommodated generally by thrusting in the mid crust and by upright folding in the upper crust compression occurs during cooling of the terrane as indicated by near isobaric cooling paths and the preservation of upper crust sections the result is the production of discrete low p high t terranes that are localized around abundant granitoids show thin skinned deformation and have anticlockwise p t time paths continued granitoid intrusion produces posttectonic discordant plutons at high crustal levels within low p metamorphic belts

mechanical consequences of granite emplacement during high t low p metamorphism and the origin of anticlockwise pt paths high temperature t low pressure p metamorphic belts preserve evidence of metamorphism under extremely perturbed and hence transient thermal regimes pt paths are commonly anticlockwise that is the maximumt was attained prior to or at the same time as the maximump with cooling at near constant or increasingp and in many terrains peak metamorphic temperatures prevailed during convergent deformation using a simple coupled thermal mechanical model that assumes a strongly temperature dependent rheology for the continental lithosphere we show that the coincidence of metamorphism with convergent deformation in high t low p terrains may reflect the thermal weakening attendant with rapid advective movement of heat within the lithosphere via granite magma ascent the model shows that the magnitude of the thermal weakening effect is sensitive to the critical temperature for granite segregation tm crit the depth of emplacement of the magmas and most importantly the granite forming mechanism during the prograde cycle material points attain maximum temperatures during a transient 3 ma high strain rate pulse withe up to 10 14 s 1 at levels near the site of granite emplacement the attainment of maximum pressures follows peak temperatures with rapid cooling paths that are close to isobaric or involve slight compression

an extended episode of early mesoproterozoic metamorphic fluid flow in the reynolds range central australia the products of metamorphic fluid flow are preserved in zones within the marbles and metamorphosed semipelites of the upper calcsilicate unit in the granulite portion of the late palaeoproterozoic reynolds range group northern arunta block central australia the zones of retrogression characterized by minerals such as wollastonite grossular and clinohumite local resetting of oxygen isotopic compositions and local major element metasomatism were channelways for water rich fluids derived from granulite facies metapelites u th pb isotopic ages measured by the shrimp ion microprobe on zircon and monazite from a granulite facies semipelite an early semiconcordant aluminous quartz rich fluid flow segregation and a late discordant quartz rich segregation record some of the extended thermal history of the area zircon cores from the semipelite show its likely protolith to be an igneous rock 1812 11 ma old itself derived from a source containing zircon as old as 2 2 ga low th u overgrowths on the zircon grew during granulite facies metamorphism at 1594 6 ma monazite cooled to its blocking temperature at 1576 8 ma zircon cores from the semiconcordant segregation are dominantly 2 3 ga old indicating that the source of the fluids was not the particular metamorphosed semipelite studied two generations of low th u overgrowths on the zircon give indistinguishable ages for the older and younger of 1589 8 and 1582 8 ma respectively the monazite age is the same 1576 12 ma zircon from the late discordant segregation gave 1568 4 ma fluid flow occurred for at least 18 3 ma and ended 26 3 ma after the peak of metamorphism suggesting a very slow cooling rate of 3 c ma 1 the last regional high grade metamorphism in the reynolds range occurred at 1 6 ga not 1 78 ga as previously thought the high grade event at 1 78 ga is a separate event that affected only the basement to the reynolds range group

upper mantle shear velocity beneath eastern australia from inversion of waveforms from skippy portable arrays in 1993 the research school of earth sciences of the australian national university commenced a nationwide seismometry project skippy for a comprehensive study of the upper mantle beneath the australian continent we applied a waveform inversion technique to broad band data recorded while the skippy portable arrays were positioned in eastern australia in order to construct a 3 d model of shear velocity in the upper mantle and transition zone beneath eastern australia and the adjacent oceanic regions the skippy data were augmented by data from the permanent seismological observatories in the region the first step of the waveform inversion used involved the matching of the waveforms of fundamental and higher mode rayleigh waves with waveforms synthesized from radially stratified models in the second stage the linear constraints on radial variations in shear velocity were combined in a tomographic inversion for aspherical variations in shear velocity the preferred model reduces the data variance by 90 per cent owing to the dense data coverage structural features with dimensions larger than 250 km laterally and 50 km vertically are resolved the major structural features inferred from the tomographic images are 1 a substantial increase in the thickness of the high velocity lid from the oceanic region to the phanerozoic continental and proterozoic continental regions 2 a pronounced low velocity zone centred at 140 km depth beneath the part of eastern australia often referred to as the tasman fold belt which confirms previous inferences from rayleigh wave dispersion curves 3 a prominent zone of low wave speeds beneath the eastern margin of the continent that coincides with locations of cenozoic volcanism and regions of enhanced heat flow 4 deep low wave speed anomalies beneath both the coral and tasman seas and 5 localized high wave speed perturbations in the mantle transition zone 410 660 km depth beneath eastern australia in particular beneath the new england fold belt and the mount isa block from the images we infer that the eastern boundary of the precambrian shields does not coincide with a sharp seismic contrast unless this boundary is located further to the east than is commonly assumed the structure of the upper mantle and transition zone beneath eastern australia as inferred from the images is more complex than that beneath the precambrian cratons of central and western australia which could be due to subduction beneath the eastern continental margin prior to opening of the tasman sea if correct this interpretation would imply that part of the upper mantle has moved along laterally with the lithosphere during the relatively fast northward motion of the plane

petrological and geochemical investigations at the cajon pass deep drillhole chemical analyses of cores from the cajon pass deep drillhole confirm vertical lithologic diversity of the basement rocks they are metaluminous and calcalkaline weak major element correlations with wide ranging sio 2 suggest they are not a simple cogenetic suite quartz contents correlate with the thermal gradient profile supporting conduction as the principal thermal transport mechanism abundances of heat producing elements are variable and somewhat lower than average crustal values cross calibration with other measures of th u and k are underway ages of surface granitoids and core 17 are late cretaceous other plutonic rocks appear older but late mesozoic initial lead isotopic signatures of the plutonic rocks change dramatically with depth in the hole

on the stability of isostatically compensated mountain belts the formation of convergent mountain belts is invariably accompanied by an increase in gravitational potential energy due to part of the work done by the forces driving convergence the evolution of potential energy stored in an orogen is dependent on 1 the density structure 2 the thermal evolution and 3 the way convergent deformation is partitioned between crust and mantle lithosphere it is now well recognized that this increase in potential energy associated with the mountain building process raises the possibility that significant extension or collapse may accompany the relaxation of the forces driving convergence provided the lithosphere is thermally weakened e g england 1987 in this paper we evaluate the stability of isostatically compensated mountain belts under the assumption that the strength of continental lithosphere is governed by a combination of frictional sliding and creep processes using the brace goetze model for the rheology of the lithosphere the reference lithosphere defined to be in potential energy and isostatic balance with the mid ocean ridges changes with different thermal parameters of the lithosphere the instantaneous extensional strain rate for thermally mature mountain belts is calculated by balancing the horizontal buoyancy force stored in the mountain belts measured relative to the reference state with the vertically integrated strength of the lithosphere for initial strengths spanning the probable natural range it is shown that horizontal buoyancy forces arising in isostatically balanced mountain belts are sufficiently large to induce the collapse at significant rates greater than a few times 10 16 s 1 and leading to significant finite extension providing the moho temperatures exceed about 650 700 c a condition only likely to be attained if the mantle lithosphere has not been thickened to the same extent as the overlying crust consequently processes that thin the mantle lithosphere as a consequence of convergent deformation such as the convective instability of a thickened lower thermal boundary layer greatly increase the possibility of collapse the calculations presented here suggest that near complete destruction of the mountain system by extensional collapse may be possible if such processes can reduce total lithospheric thickening to less than half the contemporary crustal thickening i e fl fc 2 where fl and fc are the lithospheric and crustal thickening factors

the metamorphic evolution of granulites at fyfe hills implications for archaean crustal thickness in enderby land antarctica granulites at fyfe hills in enderby land antarctica crystallized at temperatures in excess of 850 c and possibly as high as 1000 c and at pressures of 8 10kbar during the mid to late archaean a number of features including repeated retrograde metamorphism at 5 5 8kbar retrograde reaction textures and rimward zoning in pressure sensitive systems suggest that following peak metamorphism the granulites stabilized at a depth of 18 26 km after stabilization the granulites cooled near isobarically to temperatures of 600 700 c assuming a total crustal thickness of 35 40 km during this late archaean interval of isobaric cooling the peak metamorphic crustal thickness is estimated at 35 56 km this estimate is significantly less than the 60 70 km obtained by summing the depths of the present levels of exposure 26 34 km and the thickness of the crust presently beneath fyfe hills approxi mately 35km and is therefore consistent with independent evidence for extensive post archaean thickening of the enderby land crust

petrologic and age constraints on the origin of a low pressure high temperature metamorphic complex southern alaska the interrelationships between metamorphism deformation magma intrusion and 40ar 39ar geochronology were determined for a low pressure high temperature metamorphic complex which formed from an accretionary prism in the chugach mountains southern alaska compressional deformation which first produced south verging folds and associated thrusts was followed by magma intrusion and development of north verging folds synmetamorphic southward directed thrusting of metamorphosed flysch over flysch produced increased load in the footwall as documented by the distribution of mineral assemblages and by pressure temperature modeling of garnet growth the initial heating to the greenschist facies may have been accomplished by a combination of advective heating from aqueous fluids and of conductive heating from subducted young oceanic crust regionally developed amphibolite facies metamorphism followed intrusion of felsic sills the peak metamorphic conditions derived from geothermobarometry mineral assemblages and fluid inclusions ranged from 400 to 600 c at a depth of 10 km the increased heat from associated synmetamorphic concordant felsic sills raised the ambient temperatures to produce a regional distribution of andalusite and cordierite with a core zone of sillimanite bearing migmatites subsequent cooling was initially rapid 55 c ma to 350 c based on 40ar 39ar dates of 53 ma for hornblende and 50 ma for biotite and may have slowed to 11 c ma until 200 c based on an 40ar 39ar date of 35 ma for plagioclase intrusions of felsic sills and at least one pluton were along the initially north dipping foliations associated with south verging folds this suggests the source region for the melts may have been downdip in the subduction zone rather than from directly below within the accretionary prism

the case for simultaneous deformation metamorphism and plutonism an example from proterozoic rocks in central arizona abstract the syntectonic 1 70 ga crazy basin monzogranite provides an example of the complex spatial and temporal interactions between metamorphism deformation and plutonism synchronous plutonism and deformation is indicated by syn shortening dikes sills and veins parallel magmatic and solid state fabrics fabrics in xenoliths and a foliation triple point synchronous plutonism and metamorphism is indicated by a systematic increase from 400 c to 630 c towards the pluton at a constant pressure of 300 mpa 3 kb temperatures are consistent with a conductive cooling model in which a 700 c pluton was emplaced into country rocks undergoing greenschist facies regional metamorphism synchronous deformation and metamorphism is indicated by porphyroblast inclusion geometries that document the synmetamorphic development of the s2 cleavage the pluton was emplaced adjacent to the shylock shear zone during progressive shortening emplacement of granite as ne trending sheets was facilitated by temporal partitioning of transpressional convergence into strike slip and dip slip components at the scale of the pluton s aureole and on the relatively rapid time scale of 103 106 y regional deformation and metamorphism were punctuated by thermal softening and increased diffusion rates data suggests that accretion of proterozoic arcs in arizona involved diachronous pluton enhanced deformation and associated high temperature low pressure regional metamorphism