what can we learn about crustal structure from thermal data thermal data represent a valuable remote sensing aid in studying crustal evolution heat flow q results from the heat loss by the cooling earth and from the heat production a of the radiogenic elements brought to the upper crust by magmatic intrusives heat flow is often observed to be linearly related to heat production the slope of this relation or thermal depth d has been used to infer a global upward enrichment in heat producing elements this thermal depth has been equated with the thickness of granites but such an interpretation has not been confirmed everywhere the depth to which granitic plutons are rooted can be computed from the inversion of gravity data it averages 7 2 km and is much smaller than the thermal depth granulite facies rocks are assumed to be present in the lower crust on the basis of seismic and geochemical data these rocks are generally depleted or initially poor in radiogenic elements u th k it is suggested that the thermal depth reflects the depth to the depleted layer in continental regions and that it corresponds to the granulitic layer in most places worldwide thermal and seismic data support this relationship although surface heterogeneities introduce complications thermal data can therefore be used to constrain the structure of the crust and its evolution through time

global systematics of unaveraged mid ocean ridge basalt compositions comment on global correlations of ocean ridge basalt chemistry with axial depth and crustal thickness by e m klein and c h langmuir

thermophysical properties of the earth s crust in situ measurements from continental and ocean drilling knowledge of the variation of thermal conductivity and heat production with depth is critical to understanding the thermal state of the crust continental and oceanic drill holes provide unique opportunities for the calibration of empirical and theoretical models relating crustal thermal properties to physical and chemical properties these calibrations are based on more than 30 000 well log measurements of acoustic velocity both compressional and shear density porosity radioelement concentrations k u th and major element chemistry al si ca fe s mg na in formations ranging in age from 5 to 620 ma and in composition from granitic to gabbroic the mineralogical data available from induced gamma ray spectroscopy logs when combined with known mineral thermal conductivities in a model for the thermal conductivity of a crystalline aggregate enable the in situ application of a laboratory estimation technique and yield continuous conductivity profiles however mineralogy based conductivity models cannot account for the anisotropic conductivity observed in foliated metamorphic rocks a phonon conduction model which utilizes acoustic velocity and bulk density measurements is accurate within 15 in both isotropic and anisotropic formations of the upper continental crust and in oceanic crustal layers 2c and 3 in oceanic crustal layers 2a and 2b however the effects of fracturing on compressional and shear velocities lead to inaccurate results the heat flow heat production relationship which provides one method of determining subsurface heat production has well documented limitations allis 1979 and rybach and buntebarth 1982 1984 on the basis of laboratory measurements have proposed quantitative relationships between heat production and rock compositional and physical properties the well log k u and th data indicate that heat production can be predicted from compositional data within a factor of 2 or 3 predictions of heat production from acoustic velocity and bulk density measurements are only accurate to within an order of magnitude three factors account for these results 1 the natural variability of velocity and density for a given rock composition 2 the tendency of u and th to associate with secondary rather than primary minerals and 3 u and th redistribution through fluid flow and metamorphism thus seismic velocity based heat production profiles can at best be taken as first order estimates which may misrepresent the complexity of the continental crust however the correlation of formations of known heat production with subsurface structures when combined with a suite of existing techniques for estimating heat production may lead to more accurate results

heat flow heat production and crustal structure in peri atlantic regions abstract a linear relationship is commonly observed between heat flow and heat generation it provides information about the distribution of radioelements with depth over large regions the linear relationship is no more tenable but points still cluster within an elongated trend in which a regression analysis provides the bulk reduced heat flow and a gross estimate of the heat production distribution with depth over the whole considered region when geological provinces share an identical heat flow heat generation relation they form a geothermal province data measurements in western europe suggest that at least two provinces should be considered one can be related to the hercynian crust and the second to the older caledonian crust we suggest that the depth obtained from thermal data corresponds to the depth of the mafic granulite layer western france south and central england share identical geothermal parameters which can be correlated with the depth of the granulitic layer down to 17 18 km data from scotland do not fit the relationship of south and central u k and present a lower slope indicating that the radiogenic enriched layer is shallow 8 10 km this correlates with the seismic structure of the caledonian crust where the granulite layer is observed at this depth data from scotland can be compared to those from norway newfoundland new hampshire and eastern appalachian crust where the same crustal composition is observed

linear relationship between heat flow and heat production in panxi paleorift zone southwestern china the panzhihua xichang panxi area of southwestern china approximately 26 00 n to 28 48 n and 101 00 e to 103 00 e is a paleorift zone of hercynian age the last major tectono thermal event occurred 200 to 250 ma ago with extensive basaltic eruptions regional surface heat flow values vary from 40 to 90 mw m with the average of 63 15 mw m the u th contents increase with the acidity of igneous rocks the highest contents u 9 57 ppm th 27 67 ppm are observed in biotite granite the u th contents are well correlated with each other and the correlation coefficient is 0 9646 indicating the poor migration of u in epimagmatic state on the basis of heat flow and heat production data a linear relationship was obtained for the region

heat flow and heat production in brittany western france new heat flow determinations in brittany western france allow mapping with a good coverage 1 point per 1000 km2 over that region a high heat flow zone oblique to the present geological structures trends from northern central brittany to the south and then turns to the ese towards the massif central heat production a and heat flow q are tested against a linear relationship the data do not fit a strict linear relationship but the points are distributed within an elongated cluster in which a linear regression can be approximated yielding the following equation q 26 6 3 5 mw m2 15 6 0 6 km a several tests of significance corroborate the linear cluster and do not alter the computed parameters the mapping of the residual values shows a geographical distribution which can be correlated with filtered gravimetric and magnetic data those residuals reflect local variations in heat production distribution or in the heat transport mechanism the high value of the slope 15 6 km computed during the linear regression reflects the influence of the upper to middle crust this value is too elevated to represent the thickness of the granitic intrusions which are known to be shallow rooted 4 6 km from gravity data interpretation compared to adjacent hercynian countries the data lie within the range of those of central and southwestern england

relationships between the petrophysical properties density seismic velocity heat generation and mineralogical constitution the petrophysical properties density p compressional wave velocity v p and heat generation a exhibit considerable variation with rock type to characterize a given rock quantitatively a single parameter the cation fraction expressed by the k value has been found to be suited best 100 crystalline rocks have been sampled to cover the widest possible range in k value from silicic types like granophyres and granites through granodiorites tonalites diorites gabbros amphibolites hornblendites pyroxenites to ultrabasic rocks p k a and v p were determined experimentally the latter at 0 5 kbar 50 mpa the interrelations between these properties were established by regression analysis the relation k and v p k show a linear dependence whereas a k a and a v p follow exponential laws in a w m 3 12 20 230 5 k mole cm 3 in a 22 5 8 15 g cm 3 and in a 16 5 2 74 v p km s all correlation coefficients are better than 0 9

investigations and interpretations of the vertical distribution of u th and k south africa and canada investigations of the distribution of u th and k in deep profiles exposed by precambrian doming have been presented for vredefort south africa and have recently been completed for a similar structure near sudbury ontario canada an evaluation of the vredefort data shows evidence of a major crustal repetition of the vertical radioelement distribution with depth a simple explanation is that the south african basement is geochemically layered with repetition occurring from layer to layer this implies a sawtooth or serrated distribution with depth preliminary results from the sudbury investigation show a rise in heat production with distance from the sudbury structure corresponding to decreasing depth in the basement rocks the repetition of the radioelement distribution observed at vredefort does not appear in the sudbury data although th and k make a jump at nearly the same depth the u data show no corresponding increase u loss is suspected in the project area affecting the overall distribution of heat production in the sudbury basement given its complex history and geographic location the material in the sample area may not be representative of the superior province

a heat generation model for continental crust based on deep drilling in the baltic shield abstract this paper presents a heat generation model for the old continental crust on the basis of a new petrological model on the origin evolution and deep structure of the crust in which petrological geochemical and geophysical data obtained from the kola superdeep borehole and other deep boreholes drilled into the crystalline basement of the east european platform are taken into account the petrological section obtained from the borehole observations was extrapolated down to the moho boundary taking into consideration the evolution and generalized seismic model of the continental crust in our model the continental crust is subdivided into three layers a granite gneiss layer 0 17 km a charnokite enderbite layer 17 30 km and a basic layer including autochthonous restites of the protocrust 30 40 km the boundaries between the layers correspond to seismic surfaces k1 and k2 the crustal contribution to heat flow is estimated to be about 25 mw m2 52 of the total heat flow for the baltic shield and the mantle heat flow is 22 24 mw m2 a comparison is made between the heat generation model obtained in this study and the ones estimated from seismic velocity structure the difference in the heat generation value between these models may originate from the differing geological and tectonic history of the sampling localities

radioelement concentrations in the deep profile through precambrian basement of the vredefort structure the vredefort basement granite in south africa forms the core of an undomed and overturned sequence of precambrian strata slawson 1976 proposed that the granite core has also been overturned exposing to view a thick section through the archean crust incrasing in depth toward the center of the dome profiles across the vredefort basement provide an opportunity for studying the distribution of u th and k with depth in the upper and middle crust detailed radioelement concentration profiles across the granitic basement showed that 1 the central part of the basement core is depleted in th and u relative to the perimeter and the concentration of th and u falls off regularly from the basement margin inward 2 the distribution of the radioelements and the distribution of heat production with depth over the outer 8 km are consistent with an exponential depth function 3 the felsic rocks of the central basement are characterized by high k u and th u ratios and the ratios k u k th and th u increase from the margin inward and 4 mafic granulites which occur in the central regions of the dome have normal k u k th and th u ratios high grade metamorphic environment resulted in the depletion of bothmore u and th relative to k in the greater proportion of the felsic rocks from the central regions of the vredefort basement depletion of u was far greater than that of th the normal th u k u and k th ratios in mafic rocks in close proximity to the felsic rocks suggest that mineral assemblages have a strong control on the behavior of the radioelements during regional metamorphism in the vredefort basement less

vertical distribution of heat production in the continental crust abstract using an experimentally established formula seismic velocity vp was converted to heat production a to investigate the vertical distribution of heat sources the conversion performed for a number of characteristic 1 d seismic crustal sections from various tectonic units in central europe yields a general decrease of a with depth z the pattern of decrease depends on geological age young tectonic units with high heat flow exhibit a more rapid decrease the depth dependence of a does not exactly follow the simple exponential law a z a 0 exp z d in general the logarithmic decrement d decreases with depth from 20 30 km at a depth of 10 km to about 10 km at the base of the crust in the uppermost 10 km layer of the crust a certain redistribution of u and th can occur by deep groundwater migration through microcrack networks at depths greater than 10 km geochemical constraints fairly constant th u and k u ratios suggest that the rate of decrease of the heat producing radioelements u th and k is similar

k rb sr ba u and th geochemistry of the lapland granulites fennoscandia lile fractionation controlling factors the lile geochemical patterns of the three main lithological units graywacke shale metasedimentary sequence tholeiitic metaigneous rocks and migmatitic rocks of the lapland granulite belt are described k ba sr and th concentrations in metasediments are nearly similar to average continental crust whereas rb and u are unevenly impoverished in particular graphitic metashales and calcsilicate rocks are not significantly depleted in uranium tholeiitic metaigneous rocks comprises metavolcanics which present k rb ratios similar to metasediments and metaplutonics with lile abundances close to those of the low k tholeiites migmatites show wide range in lile content metatexites and diatexites have higher k rb th and u concentrations and similar k rb ratios with respect to equivalent unmobilized rocks potassic pegmatoids are strongly enriched k rb ba and th but moderately in sr and u plagioclasic pegmatoids and ferromagnesian restites are rich in sr and poor in other lil elements

temperatures and duration of crystallization within gem bearing cavities of granitic pegmatites abstract crystallization within miarolitic cavities in granitic pegmatites begins with the primary igneous minerals feldspars quartz micas and lesser others along the margins of pockets and ends with the formation of zeolites carbonates and finally dense clay the euhedral and gem quality crystals for which many pegmatites are mined form within this interval and they are commonly suspended in the dense clay with no points of attachment to a substrate the sequence of mineral assemblages reflects a history of crystallization with cooling that continues well after and to temperatures below the initial magmatic stage low temperature assemblages of zeolites and clays are normally restricted to the miarolitic cavities themselves in otherwise fresh and unaltered pegmatite of igneous origin this study utilizes feldspar solvus thermometry to delineate the temperature interval over which the euhedral and gem quality crystals form additional data include a survey of al si order in k feldspar and the mineralogy of the clay size fraction from miarolitic cavities as further constraints on the conditions of crystallization the primary feldspars at pocket margins record an average crystallization temperature of 431 c 1 68 c approximately the same temperature at which crystallization takes place across the massive portions of thin miarolitic pegmatite dikes the average calculated temperature of closure of perthite exsolution occurred at 338 c 1 60 c pairings of glassy non perthitic rims on k feldspar with radial albite of cleavelandite habit on the k feldspar surfaces record the final stage of crystallization of the primary silicates in pockets at an average temperature of 359 1 64 c the glassy rims are not perthitic because they form close enough to the closure temperature of perthite that exsolution does not occur the zeolites which include stilbite ca laumontite and heulandite are restricted in their stability to less than 300 c and they follow the precipitation of the gem minerals in all known cases thus the gem forming stage likely falls between 435 355 c based on the feldspar solvus temperatures cookeite kaolinite montmorillonite and illite enclose zeolites and formed after them in a thermal model a dike 1 meter thick emplaced as hydrous granitic liquid at 700 c at a depth of 7 km ambient temperature of 160 c cools to 435 c at its center in 12 days and from 435 c to 355 c the interval over which the primary silicates crystallize in pockets in another 14 days increasing the host rock temperature to 300 c only extends the crystallization interval of the principal pocket silicates by another 5 2 months the dike center reaches 275 c at which temperature laumontite could crystallize approximately 55 days after emplacement clay minerals likely form and perhaps reform below 200 c the upper thermal stability limit of stilbite ca as the devitrified products of a remnant and flux depleted hydrosilicate glass that partially fills the miarolitic cavities the pegmatite achieves thermal equilibrium with the host rocks at 160 c more than 200 000 years later

is there a relationship between seismic velocity and heat production for crustal rocks abstract several investigators proposed an empirical relationship between heat production and seismic velocity for rocks typical of the continental crust these relationships could allow for indirect resolution of the distribution of heat producing elements with depth new heat production data for rock samples from the pikwitonei granulite domain and the sachigo granite greenstone subprovince of the superior province do not follow these relationships indicating that heat production cannot be reliably predicted from seismic velocity these two parameters are not likely to be related as velocity depends upon the major mineral constituents of rocks whereas heat production results from radiogenic components concentrated in both major and trace minerals in rocks

radiogenic heat production of crustal rocks an assessment based on geochemical data a survey of the geochemical literature and unpublished data has resulted in the classification of u th and k concentrations by rock type over 2500 data entries have been compiled permitting calculation of their radiogenic heat production in the igneous rocks mean heat production ranges from highs of 12 20 heat production units hpu wm 3 in some peralkaline intrusives through 4 hpu in acidic 2 in intermediate and 1 in basic rocks to a low of 0 3 hpu in ultramafic rocks siliceous clastic rocks generally have greater heat production 2 to 4 hpu than do chemical sedimentary rocks including the carbonates 0 4 to 2 hpu the heat production of metamorphic rocks generally depends on the radioelement contents of their igneous and sedimentary predecessors modified by metamorphic processes based on estimates of the proportion of the continental crust that specific rock types occupy the weighted mean radiogenic heat production of the upper continental crust estimated from this data base is 3 hpu

relationship between seismic velocity and heat production comparison of two sets of data and test of validity abstract an empirical relationship between compressional wave velocity v p and radiogenic heat generation a is frequently used to estimate heat production distributions in the continental crust two data sets from rybach and buntebarth 1 and kern and siegesmund 2 have been re evaluated and made comparable bulk density was included as a further parameter to investigate the relationships v p a v p and a statistical analysis shows reasonably high and comparable regression coefficients for the first two relationships but much less agreement for the a relationship

heat flow heat production and thermo tectonic setting in mainland uk new heat flow data for the united kingdom together with additional heat flow and heat production determinations for caledonian age granites have led to a revision of the uk heat flow map and a re examination of the relationship between heat flow q 0 and heat production a 0 for granites and basement rocks previously recognized broad belts of above average heat flow are now resolved into separate zones which reflect to a greater extent the geological structure and tectonic history of the uk the zones of highest heat flow are spatially associated with voluminous high heat production granitoid batholiths in sw england northern england and the eastern highlands of scotland a single linear correlation between q 0 and a 0 is no longer tenable and an analysis in terms of broad heat flow provinces each with a characteristic upper crustal heat production distribution and deep heat flow contribution is also considered to be an oversimplification on the q 0 a 0 plot the data form four separate clusters three corresponding to the granite batholiths in sw england northern england and the eastern highlands of scotland and the fourth to the basement rocks of central england and wales an explanation of the q 0 a 0 data is proposed in terms of the crustal structure and thermo tectonic setting of each area in the case of the granite batholiths the data reflect the contrasting depth extent and radioelement depth functions of the intrusions these parameters in turn are related to the magmatic evolution and emplacement history of each batholith and the nature of the crust into which they were emplaced

exposed cross sections through the continental crust implications for crustal structure petrology and evolution abstract the continental crust is exposed in cross section at numerous sites on the earth s surface these exposures which appear to have formed by obduction along great faults during continental collision may be recognized by exposures of deep crustal rocks exhibiting asymmetric patterns of metamorphic grade and age across the faults and by distinctive bouguer anomaly patterns reflecting dipping basement structure and an anomalously deep mantle from an examination of five complexes which meet these criteria it is concluded that the most prominent layering in the crust is not compositional but metamorphic the lower crust consists of granulite facies rocks of mafic to intermediate composition while the intermediate and shallow levels consist predominantly of amphibolite facies gneisses and greenschist facies supracrustal rocks respectively post metamorphic granitic intrusions are common at intermediate to shallow levels position of discontinuities in refraction velocity where present commonly correspond to changes in composition or metamorphic grade with depth the continental crust is characterized by lateral and vertical heterogeneities of varying scale which are the apparent cause of the complex seismic reflections recorded by cocorp field observations coupled with geochemical data indicate a complex evolution of the lower crust which can include anatexis multiple deformation polymetamorphism and reworking of older crustal material the complexity of the crust is thus the result of continuous evolution by recycling and metamorphism through time in a variety of tectonic environments

selective heat producing element enrichment in a crustal segment of the mid european variscan chain abstract heat production has been calculated from u th and k content measurements on selected rock from the armorican massif western france geographical geochemical and age criteria have been introduced during the selection in order to obtain a representative sampling of the outcropping formations this procedure allows the delineation of a high heat production zone with values nearly twice the average upper crustal heat production ones the high heat production zone laterally ends through a high gradient belt indicating an upper crustal origin this zone cross cuts the geological structures and encompasses various granitoid plutons of different geochemical typologies and ages however among the granites belonging to the same geochemical type only those plutons located in this zone are enriched in radioelements those located outside with the exception of a few massifs present normal radiogenic element concentrations the enrichment affects all intruded granitoids or orthogneisses irrespective of their age when geographical features are restored prior to the late carboniferous shear deformation then the high heat production zone no longer appears as fortuitous it shapes a region intruded by numerous granitoids and structured during the early proterozoic we therefore propose that the high heat production zone reflects a previous selective enrichment of this crustal segment related to the late cambrian plutons

metallogenesis in the french part of the variscan orogen part i u preconcentrations in pre variscan and variscan formations a comparison with sn w and au abstract in the french part of the variscan orogen the pre variscan metamorphic rocks and the variscan granites resulting from pure crustal recycling are enriched in uranium and tin data on tungsten and gold are too scarce for a reasonable estimation of the average concentrations of these elements to be obtained the spatial distribution of uranium preconcentrations is transverse to the main variscan structural domains and the distribution of the sn w mineralizations prompts the same conclusions u sn w and au metal enrichment in crustal domains in europe seems to result from late proterozoic to early palaeozoic basic and acid magmatism for uranium at least two main stages of partial melting early palaeozoic and late carboniferous lead to enrichment of the metal in peraluminous leucogranites these leucogranites represent the main source of late variscan uranium and tin within the peraluminous granitic complexes the main metal source is related to the late intrusion of small granitic cupolas strongly enriched in metals in the case of uranium the efficiency of the hydrothermal remobilization depends on the proportion of metal located in uraninite which is easily leachable by hydrothermal solutions the emplacement of these cupolas is structurally controlled in well defined areas by large shear zones these shear zones are continuously active during the successive emplacement of the granitic magmas most of the hydrothermal uranium mineralization is related to the permian reactivation of the shear zones in a brittle stage these zones then channel the hydrothermal fluids involved in the last metal concentration step leading to the formation of the orebodies the efficiency of metal remobilization is dependent on a close spatial relationship between magmatic structures enriched in uranium and brittle structures channeling the hydrothermal fluids

a test of the relationship between seismic velocity and heat production for crustal rocks new heat production data of 40 rock samples comprising a wide variety of crustal rocks are compared with respective p wave velocities and densities in order to test the empirical velocity density heat production systematics proposed by rybach and buntebarth 1 2 which have been used by several investigators to determine heat production with depth in the crust our laboratory data do not convincingly fit any of the proposed relationships the lack of confirmation is attributed to the fact that the physical rock properties are mainly related to the single crystal properties of the respective major minerals whereas heat production in the rocks results from radiogenic components which are largely concentrated in accessory minerals

evidence for variable upper mantle temperature and crustal thickness in and near the australian antarctic discordance abstract the southeast indian ridge seir in and near the australian antarctic discordance aad exhibits at a constant spreading rate almost the full range of the many geophysical and geochemical parameters characteristic of the slow mid atlantic ridge and fast east pacific rise we used satellite derived gravity data in combination with seamarc ii bathymetry in and near the aad to examine regional density variations in the upper mantle beneath the aad through three dimensional gravity analysis we found that at least two end member models satisfy the gravity observations regional crustal thickness variations of at least 3 km along the seir near the aad or a temperature anomaly of the order of 150 c in the upper mantle beneath the seir these new observations combined with other geophysical and geochemical characteristics of the australian antarctic discordance provide further evidence that the temperature structure of a mid ocean ridge is a controlling factor in addition to spreading rate in the crustal accretionary process numerical models of mantle flow beneath mid ocean ridges offer one means of investigating the dynamic effect of a variable upper mantle temperature on the accretionary process our results indicate that temperature is important especially at intermediate and slower spreading rates where thermal effects can dominate mantle flow beneath a mid ocean ridge and result in increasing crustal production with decreasing spreading rate at the constant intermediate spreading rate of 37 mm yr characteristic of the seir in and near the aad our numerical models show that significant crustal thinning 2 4 km can occur with relatively small variations in upper mantle temperature all else being equal thus combined with our end member gravity models these observations and results suggest that both anomalously cool upper mantle and thin crust exist beneath the aad

heat production and thermal conductivity of rocks from the pikwitonei sachigo continental cross section central manitoba implications for the thermal structure of archean crust the pikwitonei and sachigo subprovinces of central manitoba provide a cross sectional view of the superior province crust in cross section the upper to mid level crust is composed of synformal greenstone belts surrounded by tonalitic gneisses both of which are intruded by granitoid plutons this crustal structure persists downward into the granulite facies where keels of the greenstone belts can be found to constrain thermal models of the crust we measured heat production and thermal conductivity in 60 rocks from this terrain using standard gamma ray spectrometry and divided bar techniques large vertical and lateral heterogeneities in heat production in the upper crust are evident heat production is high in granites and metasedimentary rocks intermediate in tonalite gneisses and low in the portions of greenstone belts dominated by mafic meta igneous rocks in the deeper granulite facies rocks heat production decreases by a factor of two in the tonalitic gneisses and remains low in the high grad

heat production in an archean crustal profile and implications for heat flow and mobilization of heat producing elements we have measured concentrations of heat producing elements th u and k in 58 samples representative of the main lithologies in a 100 km transect of the superior province of the canadian shield from the michipicoten wawa greenstone belt near wawa ontario through a domal gneiss terrane of amphibolite grade to the granulite belt of the kapuskasing structural zone near foleyet this transect has been interpreted as an oblique cross section through some 25 km of crust uplifted along a major thrust fault and thus provides an opportunity to examine in detail a continuous profile into deep continental crust of archean age mean heat production values for these terranes based on aereal distribution of major rock types and calculated from their th u and k concentrations are michipicoten greenstone belt 0 72 w m 3 wawa domal gneiss terrane amphibolite grade 1 37 w m 3 kapuskasing granulites 0 44 w m 3 among the silicic plutonic rocks tonalites granites and their derivative gneisses the relatively large variation in heat production correlates with modal abundances of accessory minerals including allanite sphene zircon and apatite we interpret these variations as primary pre metamorphic the relatively high weighted mean heat production of the domal gneiss terrane can be accounted for by the larger proportion there of late stage th u and k rich granitoid plutons these may have been derived from the underlying kapuskasing granulite terrane leaving it slightly depleted in heat producing elements transport of th u and k therefore could have taken place in silicate melts rather than in aqueous or carbonic metamorphic fluids this conclusion is supported by the lack of a statistically significant difference in heat production between tonalites tonalite gneisses and mafic rocks of amphibolite versus granulite grade the pre metamorphic radioactivity profile for this crustal section is likely to have been uniformly low with a mean heat production value less than 1 w m 3 this result is distinctly different from measured profiles in more silicic terranes which show decreasing heat production with depth this implies fundamental differences in crustal radioactivity distributions between granitic and more mafic terranes and may be an important factor in selective reactivation of lithologically different terranes possibly resulting in preferential stabilization of basic terranes in the geological record our results indicate that a previously determined apparently linear heat flow heat production relationship for the kapuskasing area does not relate to the distribution of heat production with depth low but significant heat production 0 4 0 5 w m 3 continues to lower crustal depths with no correlation to the depth parameter from the linear relationship this low heat production may be a minimum average granulite heat production and suggests that in general heat flow through the moho is 8 10 mw m 2 lower than the reduced heat flow calculated from the heat flow heat production regression