low heat flux and large variations of lithospheric thickness in the canadian shield 1 ten new heat flux determinations have been made using measurements in 22 mining exploration boreholes located at latitudes higher than 51 n in the canadian shield they provide data in poorly sampled regions near the core of the north american craton where one expects the lithosphere to be thickest the new heat flux values are all smaller than 34 mw m 2 and are among the lowest recorded so far in the shield for all the new sites there is no relationship between heat flux and heat production in surface rocks in the canadian shield heat flux variations occur at wavelengths 100 km and are mostly of crustal origin local averages in two 250 250 km windows located on archean areas at high latitudes on either side of james bay are 29 mw m 2 and 31 mw m 2 the lowest values found so far at this scale in the canadian shield s wave traveltime delays derived from tomographic models provide the additional constraints needed to resolve differences of deep lithospheric thermal structure there is no significant correlation between average surface heat flux and traveltime delays within the canadian shield confirming that variations of the surface heat flux are mostly of crustal origin traveltime delays cannot be explained by variations in crustal heat production only and require variations of heat supply to the lithosphere and or radiogenic heat production in the lithospheric mantle these variations are associated with changes of lithospheric thickness that may be as large as 80 km the heat flux at the base of the superior lithosphere is constrained to be 11 2 mw m 2

generation of continental rifts basins and swells by lithosphere instabilities 1 continents may be affected simultaneously by rifting uplift volcanic activity and basin formation in several different locations suggesting a common driving mechanism that is intrinsic to continents we describe a new type of convective instability at the base of the lithosphere that leads to a remarkable spatial pattern at the scale of an entire continent we carried out fluid mechanics laboratory experiments on buoyant blocks of finite size that became unstable due to cooling from above dynamical behavior depends on three dimensionless numbers a rayleigh number for the unstable block a buoyancy number that scales the intrinsic density contrast to the thermal one and the aspect ratio of the block within the block instability develops in two different ways in an outer annulus and in an interior region in the outer annulus upwellings and downwellings take the form of periodically spaced radial spokes the interior region hosts the more familiar convective pattern of polygonal cells in geological conditions such instabilities should manifest themselves as linear rifts striking at a right angle to the continent ocean boundary and an array of domal uplifts volcanic swells and basins in the continental interior simple scaling laws for the dimensions and spacings of the convective structures are derived for the subcontinental lithospheric mantle these dimensions take values in the 500 1000 km range close to geological examples the large intrinsic buoyancy of archean lithospheric roots prevents this type of instability which explains why the widespread volcanic activity that currently affects western africa is confined to post archean domains

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

petrology of the great abitibi dyke superior province canada the great abitibi dyke gad which can be traced northeast for 700 km across the abitibi belt in the southeastern superior province of the canadian shield is composed of olivine gabbro to monzodiorite weakly saturated to undersaturate d in silica all rocks of the gad can be derived by mainly plagioclase and olivine fractionation from a parental magma corresponding in composition to chilled margin samples two units can be distinguished a marginal unit unit 1 representing 0 50 crystallization and a central unit unit 2 found over about half of the dyke length representing 50 70 crystallization modelling using pearce elemental ratio analysis quantifies the fractionation history and allows mass balance calculations over the present exposure level of the dyke the approximate balance between the amounts of cumulate and fractionated rocks suggests that unit 1 differentiated essentially in situ as a closed system in contrast unit 2 rocks were formed by loss of substantial plagioclase and olivine from the parental magma this fractionate must have either been lost to depth or left behind in an external chamber unit 2 rocks show depletion in plagioclase and enrichment in mafic minerals along strike towards the southwest deeper exposure level a trend explained by density stratification regional variation in unit 1 chemistry is interpreted in terms of lateral magma injection towards the northeast from the locus of keweenawan rift volcanism feldspar olivine and augite compositions are linearly correlated with equilibrium temperature and extent of magma evolution the parent magma had a trace element chemistry corresponding to a within plate setting and was probably derived from an incompatible element enriched mantle similar to the source for ocean island basalts oibs

the initiation of subduction by crustal extension at a continental margin summary we investigate how subduction may be triggered by continental crust extension at a continental margin the large topography contrast between continental and oceanic domains drives the spreading of continental crust over oceanic basement subduction requires the oceanic plate to get submerged in mantle so that negative buoyancy forces may take over and drive further descent this is promoted by two mechanisms loading by continental crust bends the oceanic plate downwards extension in the continental domain induces crustal thinning which acts to raise mantle above the oceanic plate in this model the width of the continental region undergoing extension is an important control parameter the main physical controls are illustrated by laboratory experiments and simple theory for elastic flexure coupled to viscous crustal spreading three governing dimensionless parameters are identified one involves the poorly constrained oceanic plate buoyancy we find that the oceanic plate can be thrust to depths larger than 40 km even if it is buoyant enabling metamorphic reactions and density increase in the oceanic crust another parameter is the ratio between the width of the continental extension region and the flexural parameter for the oceanic plate initiating subduction is easier if the continent thins over a short lateral distance or if the oceanic plate is strong the third important parameter is the ratio of oceanic plate thickness to initial continental crust thickness such that a weak plate and a thick crust do not favour subduction thus the change from a passive to an active margin depends on the local characteristics of the continental crust and is not determined solely by the age and properties of the oceanic lithosphere it is shown that the spreading of continental crust induces uplift of the margin as the adjacent seafloor subsides evidence for the emplacement of continental crust over oceanic basement at passive margins is reviewed

lith5 0 a revised crustal model for canada based on lithoprobe results summary during the last 15 yr numerous seismic refraction experiments have been conducted overthe canadian landmass most as part of the lithoprobe initiative the new results providethe basis for revising existing global models of crustal structure e g crust5 1 mooney et al 1998 which do not incorporate the new lithoprobe data the crustal model discussedhere lith5 0 combines the lithoprobe and crust5 1 data to provide better constraints oncrustal structure in canada the objectives of the study are to 1 compile available new datainto a single consistent database of crustal thickness and velocity including the uppermostmantle pn velocity and 2 to construct a revised 5 5 crustal model of the canadianlandmass in a format that is generally compatible with existing global models the lithoproberesults suggest a 5 5 km average root mean square rms crustal thickness difference betweenlith5 0andcrust5 1 evenwiththeincorporationofthenewdata thegeographiccoveragein north and central canada remains sparse the new model does not explicitly incorporategeological constraints but instead relies on spherical spline interpolation for extrapolatingvalues into regions that lack control to illustrate the implications of this new crustal model we use lith5 0 to correct for the isostatically compensated crustal heterogeneity in order toobtain improved estimates of the dynamic surface topography the latter provides importantconstraints on the density structure of the uppermost mantle

inter and intra group compositional variations in apollo 15 pyroclastic green glass an electron and ion microprobe study abstract representative samples of the five compositionally distinct groups a e of apollo 15 pyroclastic green glass were systematically analyzed for major and trace element abundances by electron and ion microprobe techniques these primary magma compositions define a high co trend groups a and d and a low co trend groups b and c on sr zr ba and nd versus co variation diagrams group e glasses are significantly enriched in sr relative to the glass groups of the two trends pearce element ratio analyses and sr ca ba ca systematics indicate that olivine orthopyroxene and clinopyroxene controlled green glass compositions simple crystal liquid igneous fractionation processes involving such an assemblage however cannot explain the observed inter and intra group trace element variation trends the magmas represented by the high and low co trends are interpreted to have been erupted from two distinct but compositionally variable source regions a source mixing model involving differentiated cumulate trapped residual liquid and extremely fe rich components can successfully account for the compositional ranges of the two sources required to produce the observed chemical variations upon partial melting group e glasses appear to represent a magma erupted from a source with a more limited chemical variability the apollo 15 green glasses represent multiple eruptive events from three chemically distinct but compositionally variable source regions assuming that this mixing model is correct

surface topography as key constraint on thermo rheological structure of stable cratons why some archean cratons survived for billions of years while the rest of the lithosphere has been reworked probably for several times is both enigmatic and fundamental for plate tectonics craton longetivity is mainly explained by their buoyancy and analyzed by testing gravitational stability of hardly detectable cratonic mantle keels as a function of a hypothesized thermo rheological structure destruction of some cratons suggests that buoyancy is not the only factor of their stability and many previous studies show that their mechanical strength is equally important the upper bounds on the integrated strength of cratons are provided by flexural studies demonstrating that te values equivalent elastic thickness in cratons are highest in the world 110 150 km yet the lower bounds on the integrated strength of stable cratons are still a matter of debate as well as the question on how this strength is partitioned between crust and mantle and which set of rheological parameters is most pertinent we show that primary observed cratonic features flat topography and frozen heterogeneous crustal structure represent the missing constraints for understanding of craton longevity the cratonic crust is characterized by isostatically misbalanced density heterogeneities suggesting that the lithosphere has to be strong enough to keep them frozen through time without developing major gravitational instabilities and topographic undulations hence to constrain thermo rheological properties of cratons one should investigate the stability of their topography and internal structure our free surface thermo mechanical numerical models demonstrate that craton stability cannot be warranted even by very high crustal strength so that dry olivine mantle and cold thick lithosphere are indispensable conditions we establish lower bound limits on the thermo rheological structure of cratons in particular we find that minimal te needed for long term stability of continents is approximately 90 110 km of which at least 70 km should be contributed by the lithosphere mantle

temperature and rheological properties of the mantle beneath the north american craton from an analysis of heat flux and seismic data 1 we combine heat flux data and seismic velocity models for the north american lithosphere to derive constraints on thermal conditions and deformation mechanisms in the underlying convecting mantle local heat flux averages that are not affected by shallow crustal heat production contrasts allow calculation of reliable lithospheric geotherms and uncertainty ranges for consistency with the seismic data the mantle potential temperature beneath north america must lie within a 1290 c 1450 c range close to that for the oceanic mantle sampled at mid ocean ridges the heat flux at the base of the lithosphere varies laterally from 11 3 mw m 2 beneath the 250 km thick archean core of the superior province to 15 3 mw m 2 beneath the thinner younger appalachians province it is shown that the most likely cause of such rates of heat supply into the north american continent is small scale convection in an unstable boundary layer beneath the rigid mechanical lithosphere this allows useful constraints on the mantle rheological properties we show that the most likely deformation mechanism is dislocation creep in wet mantle rocks ranges for the mantle temperature water content and rheological parameters could be tightened very significantly once strong constraints are obtained on radiogenic heat production in the lithospheric mantle

geochemical and sr nd isotopic constraints on the mantle source of neoproterozoic mafic dikes of the rifted eastern laurentian margin north central appalachians usa abstract abundant and widely distributed unmetamorphosed mafic dikes intrude mesoproterozoic rocks of the new jersey highlands the age of the dikes is imprecisely known but interpreted to fall between 615 and 576 ma which is consistent with the range of ages of mafic dikes from labrador and nova scotia south to pennsylvania that were emplaced along the rifted eastern laurentian margin new jersey highlands dikes are a few cm to 18 m wide and have lengths of as much as several km they have sharp largely discordant contacts against enclosing mesoproterozoic rocks and aphanitic chilled margins that grade into coarser grained interiors columnar joints are present locally and suggest emplacement at a shallow crustal level geochemical compositions of the dikes range from alkalic to less common tholeiitic basalt having generally high tio2 p2o5 zr nb y and la yb and low mgo cr and ni tio2 contents define high ti and low ti dikes that differ in high field strength elements hfse and light rare earth elements lree but overlap in abundances of most other elements dike magma evolved in an ascending mantle plume of oib like asthenosphere from enriched higher tio2 compositions to more depleted lower tio2 compositions subtle differences in the dike compositions are due to variations in the amount of partial melting within the plume and the depth of melt segregation sr nd isotope values of both dike compositions overlap and are characterized by end t of 1 5 to 3 8 and initial 87sr 86sr ranging from 0 7032 to 0 7077 higher sr isotope ratios are interpreted as resulting from local interaction of the dike magma with heterogeneous high 87sr 86sr lithospheric mantle having emi or emii like geochemical characteristics dikes form tabular structures that have long segments striking an average of n44 e and short segments striking about east west their regional geometries form right stepping rhomb shaped patterns due to emplacement into rift related dilational fractures likely formed through a combination of southeast directed extension and strike slip shear stresses geochemical compositions of the dikes are the same regardless of their structural trend or location implying they formed during a single magmatic event they along with other mafic dikes in the north central appalachians were emplaced in a within plate tectonic setting along the rifted margin of eastern laurentia prior to opening of the iapetus ocean

the la brena el jaguey maar complex durango mexico ii petrology and geochemistry a suite of 16 basanitic volcanic rocks representing all stages in the evolution of the la brena el jaguey lbej maar complex has been studied petrographically and analyzed for mineral compositions and whole rock major element trace element and sr nd pb isotopic compositions two feldspathic granulite xenoliths were also studied as possible lower crustal contaminants to the lbej magmas the volcanic rocks contain the stable minerals olivine plagioclase augite and titanomagnetite ilmenite plus a diverse suite of xenocrusts derived from disaggregation of mantle xenoliths of spinel lherzolite olivine orthopyroxene spinel and lower crustal granulite xenoliths plagioclase quartz augite ilmenite late stage interstitial melts rich in fe and ti migrated into vesicles in several samples forming coarse grained segregation vesicles that are dominated by ilmenite blades up to 2 mm long the whole rock elemental data are typical of intra plate basanitic rocks with strong enrichments in large ion lithophile elements i e k th u as well as high field strength elements i e nb ta relative to mid ocean ridge basalts morb and estimates of primordial mantle abundances mg increased systematically with time during the evolution of the lbej maar complex from 57 0 58 2 in the pre maar lavas to 59 1 63 8 in the post maar lavas compatible elements ca sc cr co ni correlate positively with mg whereas a large group of incompatible elements al na k p rb sr zr nb ba la ce sm hf ta th u correlate negatively with mg these trends can be closely reproduced by simple models of fractional crystallization provided that the incompatible element abundances of the parental high mg magmas are allowed minor variability all successful fractionation models demand an important role for augite despite its presence in the lbej volcanic rocks as only a late stage microphenocrystic and groundmass mineral minor garnet fractionation is necessary to produce depletion of heavy rare earth element ree abundances in the pre maar lavas whose ree patterns cross those for the rest of the suite the importance of augite and garnet fractionation indicate that the differentiation of the lbej magmas took place within the upper mantle a conclusion that is supported by the presence of spinel lherzolite xenoliths in magmas from all stages in the evolution of the maar complex isotopic data for seven lbej volcanic rocks show the following ranges 87sr 86sr 0 70327 0 70347 nd 4 2 5 0 206pb 204pb 18 60 18 81 207pb 204pb 15 58 15 65 208pb 204pb 38 19 38 58 sr nd values are negatively correlated and form a trend parallel to the mantle array overlapping the field for ocean island basalts oib the lbej rocks have similar 87sr 86sr values but lower nd compared to basanitic rocks from the us basin and range province brp pb isotopic ratios are positively correlated and overlap the braod fields for morb and oib and the small fields for mexican ore deposits and volcanic rocks from the active subduction related mexican volcanic belt the lbej rocks have slightly more radiogenic pb than basanitic rocks from the us brp despite correlations among the isotopic ratios of the lbej suite none of these ratios correlate with position in the eruption sequence mg or any other compositional parameter the two lower crustal xenoliths have high 87sr 86sr values 0 707 0 710 and low nd 1 5 8 0 compared to the lbej volcanic rocks but their pb isotopic compositions are only slightly more radiogenic than the volcanic rocks these data do not support the widely held view that the lower crust is a major reservoir of unradiogenic pb in order to further constrain the role played by crustal contamination in generating the isotopic diversity in the lbej suite we conducted an extensive investigation of sr nd pb isotopic ratios for scoria clasts from different levels of a single scoria fall horizon in the pyroclastic sequence related to the formation of la brena maar our results do not support an important role for crustal contamination in the lbej magmas rather we conclude that minor isotopic variability exists in the mantle source regions beneath the maar complex

implications of a simple mantle transition zone beneath cratonic north america many areas of old continental interiors have thick roots that extend to depths of 250 km or more in contrast to the oceans and younger continents whose lithospheric thickness is less than 100 km these cratonic roots might perturb temperatures in their surrounding mantle though the net result could be either cooling or heating both of which may lead to small scale convective flow around the root we show here using new data from a study of the seismic structure of the canadian shield that the relative positions of the 410 km and 660 km seismic discontinuities are unperturbed beneath one of the deepest and broadest cratonic roots on the earth differential arrival times and internal discontinuity structure are remarkably uniform and simple varying by little more than 0 5 s over the root s 3500 km lateral extent this implies that the root has no significant thermal effect on the underlying mantle 50 k and any small scale convection or cold mantle downwelling associated with the large free air gravity anomaly beneath the shield must be confined to the upper mantle our observations are also consistent with phase changes solely in the olivine system mg fe 2sio4 with the caveat that our method is dominantly sensitive to s wave velocity jumps alone

lillooet glacier basalts southwestern british columbia canada products of quaternary glaciovolcanism the retreat of lillooet glacier lg has exposed a succession of pillow basalt and subordinate amounts of breccia and hyaloclastite the lithofacies and physiographic setting suggest that the deposits have a glaciovolcanic origin and represent a partially dissected basaltic pillow dominated tindar chemically the lg volcanic rocks are basalt to basaltic andesite and as a group they represent the highest silica quaternary mafic products in the garibaldi volcanic belt gvb like other northern gvb alkaline basalts they lack the nb ta depletion signature typically associated with subduction related products geochemical and petrologic analysis indicates that the lg basalts are comagmatic and that chemical variations within the suite are consistent with sorting of the observed phenocryst assemblage olivine plagioclase thermodynamic modeling establishes shallow crustal pre eruptive storage conditions at 2 kbar 1 kbar 100 mpa or 7 5 km and an h2o content of 0 5 1 wt we estimate that the

the adaptation of pearce element ratio diagrams to complex high silica systems pearce element ratios pers of pearce 1968 express geochemical data in a form where variations in absolute compositions of an igneous suite can be evaluated generally the denominator value in the ratio is taken as a major element abundance but it is argued here that zr provides a more suitable choice zr remains incompatible in magmatic systems up to 68 wt sio2 because zircon fractionation can be suppressed by high melt temperatures and increased volatile contents the use of zr thus permits per modelling to be extended to much higher levels of silica than previously investigated however such systems are more complex than those just involving simple basaltic magmas besides fractionation the processes of magma mixing combined assimilation and fractional crystallization and the initial degree of partial melting in the mantle source must also be considered to distinguish and evaluate these processes a set of example suites are investigated from a complex synextensional calc alkaline province in the western usa samples within most individual suites can be modelled by fractionation however a significant trend orthogonal to the main fractionation vector is also apparent and open system processes are inferred successful modelling is achieved on an inter suite basis using diagrams with axis functions of 4 ca na 0 5 fe mg zr versus si al zr potential open system evolution paths between mafic end members and crustal contaminants are also displayed and evaluated on these same diagrams the encouraging results suggest that such per diagrams may be employed as a versatile tool for investigating the systematics of related igneous suites over a wide area

heat flow gravity and structure of the abitibi belt superior province canada implications for mantle heat flow the results of new heat flow measurements are presented for eleven sites located in the abitibi greenstone belt of the archean superior province these new and previously published heat flow data are used with gravity and seismic data to constrain the crustal structure and composition of the abitibi subprovince and to determine the mantle heat flow beneath the canadian shield in order to analyze the long wavelength variations in heat flow and bouguer gravity anomaly the crust was divided into three layers with distinctive composition and physical properties average density heat production and thickness for each layer were varied within a range compatible with measurements and yielded crustal models consistent with gravity and heat flow data the dominant contribution to crustal heat production is that of a tonalitic type layer which has the lowest density the models require mantle heat flow to range between 10 and 14 mw m 2 the westward increase in heat flow requires the tonalitic layer to thicken from the grenville front toward the kapuskasing uplift at the expense of the greenstone supracrustals long wavelength gravity data are more affected by changes in crustal thickness than by composition and the low bouguer anomaly near the eastern grenville front is caused by 5 6 km of crustal thickening in greenstone terranes the long wavelength variations in heat flow are more useful for determining average crustal composition and the thickness of supracrustals than the gravity data

diverse discrete mantle derived batches of basalt erupted along a short normal fault zone the poison lake chain southernmost cascades the poison lake chain consists of small monogenetic calc alkaline basaltic volcanoes located east of the cascade arc axis 30 km ene of lassen peak in northeastern california this chain consists of 39 distinguishable units in a 14 km long and 2 km wide zone trending nnw parallel to nearby quaternary normal faults the 39 units fall into nine coherent groups based on stratigraphy field characteristics petrography and major element compositions petrographic differences among groups are expressed by different amounts and proportions of phenocrysts mgo sio 2 k 2 o sio 2 and tio 2 sio 2 variation diagrams illustrate clear differences in compatible and incompatible elements among the groups variation of k 2 o tio 2 and k 2 o p 2 o 5 with mgo indicates that most of the basalts of the poison lake chain cannot be related by crystal fractionation at different pressures and that compositions have not been affected significantly by incorporation of low degree silicic crustal melt or interaction with sialic crust limited trace element and whole rock isotopic data also suggest little if any incorporation of upper crustal material and that compositional variation among groups primarily reflects source compositional differences precise 40 ar 39 ar determinations show that the lavas were erupted between 100 and 110 ka the migration of paleomagnetic remanent directions over 30 suggests that the entire poison lake chain could represent three short lived episodes of volcanism within a period as brief as 500 yr the diverse geologic petrographic chemical paleomagnetic and age data indicate that each of the nine groups represents a small discrete magma batch generated in the mantle and stored briefly in the lower crust a nnw normal fault zone provided episodic conduits that allowed rapid ascent of these batches to the surface where they erupted as distinct volcanic groups each aligned along a segment of the poison lake chain compositional diversity of these primitive magmas argues against widespread long lived ponding of uniform basalt magma at the base of the crust in this region and against interaction with a zone of melting assimilation storage and homogenization mash in the lower crust

petrology of recent lava flows volcano mountain yukon territory canada abstract volcano mountain yukon territory a quaternary edifice located near the junction of the pelly and yukon rivers erupted two recent nephelinite lava flows between 3000 and 7300 years bp and a third recent lava flow that is probably younger the porphyritic nephelinites have olivine phenocrysts fo 91 fo 79 and ti rich diopside microphenocrysts the groundmass consists of olivine fo 83 fo 80 ti rich diopside ulvospinel magnetite solid solutions nepheline leucite patches of residual glass and traces of calcite the nephelinites also carry spinel lherzolite xenoliths and xenocrystic olivine fo 87 fo 90 the xenocrysts have lower cao contents than the phenocrysts samples from the two older flows have similar thompson space representations whereas the youngest flow has a distinctly different representation the thompson space representations of the volcano mountain nephelinites are different from the representations of nephelinites from other centers in the cordillera thompson space representations and pearce element ratio diagrams suggest the two older flows are comagmatic whereas the youngest flow is from a separate magma batch thermodynamic modeling suggests the magmas that supplied the lava flows originated at pressures near 2 5 gpa left behind a residue containing olivine and rose along an isenthalpic path after separating from the source rocks

inference of mantle viscosity from grace and relative sea level data summary gravity recovery and climate experiment grace satellite observations of secular changes in gravity near hudson bay and geological measurements of relative sea level rsl changes over the last 10 000 yr in the same region are used in a monte carlo inversion to infer mantle viscosity structure the grace secular change in gravity shows a significant positive anomaly over a broad region 3000 km near hudson bay with a maximum of 2 5 gal yr 1 slightly west of hudson bay the pattern of this anomaly is remarkably consistent with that predicted for postglacial rebound using the ice 5g deglaciation history strongly suggesting a postglacial rebound origin for the gravity change we find that the grace and rsl data are insensitive to mantle viscosity below 1800 km depth a conclusion similar to that from previous studies that used only rsl data for a mantle with homogeneous viscosity the grace and rsl data require a viscosity between 1 4 1021 and 2 3 1021 pa s an inversion for two mantle viscosity layers separated at a depth of 670 km shows an ensemble of viscosity structures compatible with the data while the lowest misfit occurs for upper and lower mantle viscosities of 5 3 1020 and 2 3 1021 pa s respectively a weaker upper mantle may be compensated by a stronger lower mantle such that there exist other models that also provide a reasonable fit to the data we find that the grace and rsl data used in this study cannot resolve more than two layers in the upper 1800 km of the mantle

predictive regions for geochemical compositional data of volcanic systems abstract in this contribution the theoretical basis to define predictive regions on multivariate compositional data is applied to volcanological geochemical data the method defines a predictive region based on the calculation of the eigenvalues and eigenvectors of the covariance matrix of a logratio transformed data set to circumvent the issue of singularity of the covariance matrix of the raw data due to the closure problem principal components have been used to reduce dimensionality and produce a 2 d display of the transformed data and the confidence ellipse the method was tested in a well studied volcanic sequence from stromboli volcano the vancori period and in the tholeiitic differentiation of thingmuli volcano represented in the afm diagram results of the first example are strongly consistent with previous works suggesting that the methodology reproduces the conventional patterns and potentially could help in the search of hidden patterns in noisy data results of the second example depict a useful approach for the treatment of volcanic geochemical data when plotted in a ternary diagram

apollo 15 green glass compositional distribution and petrogenesis abstract we have characterized a comprehensive suite of individual green glass beads from apollo 15 soil to determine interelement behavior and to constrain petrogenetic relationships we analyzed 365 particles for trace elements by instrumental neutron activation analysis and analyzed 52 of them selected to cover the compositional ranges observed for trace elements for major elements by electron microprobe analysis we confirm the observation of delano 1979 that the beads comprise discrete compositional groups although two of the groups he denned are further split on the basis of trace element compositions each of the resulting seven groups has distinct average rare earth abundances the coherence between major and trace element data was masked in previous studies by imprecision correlated error and nonrepresentative sampling of the different groups most of the compositional characteristics of the green glasses can be explained by a model for batch equilibrium melting of a nearly homogeneous ultramafic source region when the complicating effects of high pressure and low oxygen fugacity are taken into account the previously puzzling behavior of ni and co as apparently incompatible elements may arise from partial reduction of those elements to the zero oxidation state resulting in low mineral melt partition coefficients the model also offers explanations for why the green glasses form boomerang shaped trends on many two element variation diagrams and why certain compositions groups a and d are more abundant than glasses with other compositions

the petrology of poas volcano lavas basalt andesite relationship and their petrogenesis within the magmatic arc of costa rica abstract basalt andesite and dacite from poas volcano exhibit a distinct petrogenetic evolution the basalts al 2 o 3 16 17 4 k 2 o 1 43 1 61 are hypersthene normative tholeiites with phenocrysts of labradorite olivine and two pyroxenes orthopyroxene en 67 56 is generally rimmed by clinopyroxene fs 10 18 wo 39 48 in all basalts and in a few andesites pyroxene phenocrysts have al and ti rich salite rims that surround cores of orthopyroxene or augite having lower values the al z values of tetrahedral site occupied by al for these augites are above 6 thus defining on the al z vs tio 2 diagram an intermediate trend between arc related cumulates and rift related tholeiites and ophiolite layered cumulates moreover the presence of olivines with coronae of wormy intergrowths of orthopyroxene and ti magnetite similar to those occurring in metagabbros of the granulite facies and characteristic of arenal gabbroic xenoliths together with anorthite megacrysts an 94 91 suggest a possible contamination origin for these basalts sr isotope ratios for the basalts are between 0 703816 and 0 703849 whereas andesites and dacites have lower ratios 0 703676 0 703715 these relationships together with the mineral compositions suggest that the basalts may have formed from basaltic andesites by contamination with olivine gabbros and anorthosites of the lower crust pearce element ratio analysis confirms that the basalts had a different origin from the rest of the suite and that the dacite may be derived by crystal fractionation of the less silicic basaltic andesite with removal mainly of clinopyroxene and plagioclase

upper mantle velocity temperature conversion and composition determined from seismic refraction and heat flow 1 we compile upper mantle pn velocities from seismic refraction wide angle reflection surveys in the southern superior province of the canadian shield and compare them with temperatures at the moho deduced from heat flow data calculated moho temperatures and pn velocities correlate well showing that in this area pn depends primarily on temperature the obtained values of v pn t depend weakly on the assumed value of moho heat flow and are on the order of 6 0 10 4 10 km s 1 k 1 within the range of temperature derivatives obtained in laboratory studies of ultramafic rocks comparison between observed pn velocities and predicted values for several mineralogical models at moho temperatures allows constraints on both the moho heat flow and the shallow mantle composition for all moho heat flows undepleted clinopyroxene rich mantle compositions do not allow a good fit to the data for depleted mantle compositions temperatures consistent with the observed pn velocities correspond to values of moho heat flow larger than 12 mw m 2 for our preferred moho heat flow of 15 mw m 2 the best fit mantle composition is slightly less depleted than models for average archean subcontinental lithospheric mantle this may be due to rejuvenation by melt related metasomatism during the keweenawan rifting event the similarity in pn t conversion factors estimated from this empirical large scale geophysical study and those from laboratory data provides confidence in the absolute temperature values deduced from heat flow measurements and seismic studies

early crystallization history of alkali olivine basalts diamond craters oregon abstract basaltic lavas form the isolated quaternary volcanic centers of diamond craters and occur at cow lakes in s e oregon the exposed rocks are olivine and plagioclase phyric alkali olivine basalts the holocrystalline groundmasses contain plagioclase titanaugite olivine fe ti oxides and minor apatite and occur in the high alumina greater than 17 petrographic province of the northwestern great basin the whole rock and phenocryst compositions provide enough information to define the equilibrium state of the magmas the solutions to the thermodynamic equations are valid for systems of known bulk composition and can be used to evaluate whether rock compositions are close to magma compositions two of the lavas from diamond craters have whole rock and phenocryst compositions consistent with their being natural melts rock compositions that reflect the effects of crystal separation open systems are recognized by significant differences in predicted versus observed phenocryst modes the p t conditions of phenocryst saturation and the crystallization history of the magmas can be inferred from petrologic data combined with thermodynamic modelling and mass balance constraints the initial melts are inferred to have equilibrated at 0 8 1 0 gpa and 1210 1240 c the majority of the lavas from diamond craters have compositions that reflect olivine and plagioclase crystal separation these lavas can be related to two possible parent rock compositions by small amounts 2 8 by weight of olivine and plagioclase separation

principles involved in mobilization and remobilization abstract many orebodies result from appropriate combinations of fluid flow chemical environment pressure temperature and rock structure within the context of mobilization and possibly remobilization this is largely because solid state diffusion is too slow to allow transport of materials over distances in the order of kilometres within appropriate time frames advective mass transport involving large fluid rock ratios is the only viable mechanism of solution transfer that satisfies distance time frame requirements despite removal of metal species from a source region and their emplacement in otherwise barren rock being influenced by transport mechanism and fluid chemistry it is the porosity and permeability mechanical and chemical of source region and emplacement site and the need for focussed flow that are fundamental to forming ore concentrations and constitute important exploration guides thee processes and recognition of mechanically enhanced permeability are related to crack seal vein systems critical vein systems subcritical joints and vein breccias the only significant differences between the basic principles of mobilization and remobilization are the lesser need for focussing of circulating fluids and the local role of solid state remobilization in highly ductile massive sulphide by definition the chemical environment associated with fluid state mobilization and remobilization must differ from that of previous deposition and this generally means that the associated metamorphic conditions will differ

petrology of lavas from sierra negra volcano isabela island galapagos archipelago sierra negra volcano is a voluminous active basaltic shield situated in the western galapagos archipelago a hotspot related chain adjacent to the galapagos spreading center the volcano has erupted fe rich tholeiitic basalts of very limited compositional range comparison of lavas from each of five stratigraphic units covering the entire volcano indicates that the lavas are mineralogically similar and compositionally restricted 4 4 to 6 9 wt mgo the lavas are fe rich and hypersthene normative and plot in both the alkaline and tholeiite fields of the alkali silica diagram rare earth element ree patterns are steep and parallel the major and trace element data indicate a comagmatic relationship by fractional crystallization of the observed phenocryst phases projections into pseudoternary phase diagrams suggest that the magmas cooled and crystallized at pressures between 1 and 3 kbar calculated melt densities range from 2 73 to 2 77 g cm3 which are higher than those of typical mid ocean ridge basalt morb hawaiian tholeiite and other galapagos lavas the high calculated melt densities may be caused by fractionation at greater depths than is ordinary for the other basaltic magmas suppression of plagioclase and olivine crystallization in favor of augite may eliminate the minimum density window that has been proposed for morb fractionation sierra negra lavas have the most radiogenic lead and strontium isotopic ratios in the western galapagos indicating that the magmas have a relatively large contribution of plume material and have been minimally contaminated by entrainment of morb producing mantle magmatic 3he 4he isotopic ratios from sierra negra are approximately 15 times the atmospheric ratio although these ratios clearly indicate plume helium they are not the highest 3he 4he in the archipelago suggesting that helium is decoupled from the heavier isotopes high sm yb ratios light rare earth element lree enrichment and a steep ree slope are consistent with an origin by moderate extents 5 15 of partial melting of a garnet lherzolite source with ree characteristics that are between chondrite and depleted earth mantle sources sierra negra lavas are compositionally monotonous because homogeneous plume rich material has experienced long term exposure to a well regulated sublithospheric thermal environment with additional compositional restrictions imposed in the lithosphere by high magma flux

highly varying radiogenic heat production in finland fennoscandian shield abstract radiogenic heat production in finland has been previously studied using airborne gamma ray surveys and glacial till measurements alike for the first time this paper presents a detailed survey on the spatial variation in radiogenic heat production determined using outcrop samples obtained from all important lithologies of the country the dataset of 6465 samples represents mostly mesoarchean about 2 7 ga paleoproterozoic ca 2 2 1 8 ga and mesoproterozoic ca 1 6 1 3 ga rocks nearly all data are from precambrian fennoscandian shield area but heat production appears to be highly variable and above global archean and proterozoic averages spot readings show an arithmetic average of 1 34 1 19 wm 3 and a range from 0 02 to 19 4 wm 3 the interpolated areal average of the whole area is 1 42 1 41 wm 3 the high standard deviation of data is related to the geochemical characteristics of uranium u thorium th and potassium k resulting in a skewed distribution of heat production mesoproterozoic anorogenic rapakivi granites and late paleoproterozoic svecofennian granitoids show the highest heat production values in the range of 3 5 wm 3 the results show no distinct dependencies of heat production with geological age metamorphic grade nor seismic p wave velocity but an increasing trend of heat production with sio2 content and decreasing trends of heat production with fe2o3 content and with rock density are evident surface heat flow 44 borehole data values correlates weakly with heat production r 0 35 the general heterogeneity of heat production calls for supporting information from other geophysical methods for better understanding of the thermal state of the lithosphere in finland and beyond

origins of the 1954 1960 lavas kilauea volcano hawaii major element constraints on shallow reservoir magmatic processes in the 1954 1960 period kilauea volcano hawaii erupted four times summit eruptions occurred in 1954 halemaumau and 1959 kilauea iki and flank eruptions occurred in the lower east rift zone in 1955 and 1960 our analysis demonstrates that the chemical compositions of lavas from each eruption are related solely through fractionation and sorting of the observed phenocryst assemblages we have used pearce element ratio diagrams to make the following observations and inferences 1 the conserved element ratio p k for each eruption has less variance than is expected from analytical uncertainty thus we cannot reject the hypothesis each set of lavas represents a comagmatic suite of compositions 2 where each of the four groups of lava analyses is plotted on a pearce element ratio diagram with axes of si k and 0 5 mg fe 1 5ca 2 75na 0 25al k the data fit a slope of 1 0 therefore we cannot reject the hypothesis the chemical variation in each data set is consistent with sorting of ol pl au 3 each of the 4 groups of lavas has the same mean p k ratio as a result we cannot reject the hypothesis the 1954 1960 lavas originated from a single magma batch or several magmas with identical p k values finally using the most magnesian glass analysis in the kilauea data suite 1959 sg 05 we have modelled the differentiation path with thermodynamic based calculations these calculations predict a liquid line of descent that is consistent with the path inferred from rock and glass analyses therefore the simulations provide an additional test of our hypotheses and corroborate the postulated differentiation model additionally these calculations suggest that differentiation took place at less than 0 3 gpa and extended over a temperature range of between 100 c and 150 c

lithospheric assembly and modification of the se canadian shield abitibi grenville teleseismic experiment this paper presents the results of a joint lithoprobe incorporated research institutions for seismology iris program for array seismic studies of the continental lithosphere passcal teleseismic experiment that investigates portions of the grenville and superior provinces of the canadian shield along the quebec ontario border data from a 600 km long n s array of 28 broadband seismographs deployed between may and october 1996 have been supplemented with additional recordings from an earlier 1994 deployment and from stations of the canadian national seismograph network and the southern ontario seismic network relative delay times of p and s waves from 123 and 40 teleseismic events respectively have been inverted for velocity perturbations in the upper mantle and reveal a low velocity nw se striking corridor that crosses the southern portion of the line at latitude 46 n and lies between 50 and 300 km depth multievent s k s splitting results yield an average delay time of 0 57 0 22 s and a direction of fast polarization of n93 e 18 which is consistent with an earlier interpretation as being due to fossil strain fields related to the last major regional tectonic event subtle variations in splitting parameters over the low velocity corridor may suggest an associated disruption in mantle fabric profiling of radial receiver functions reveals large and abrupt variations in moho topography specifically a gradual thickening in crust from 40 to 45 km between latitudes 45 n and 46 n which is followed by an abrupt thinning to 35 km at 46 6 n some 65 km southeast of the grenville front this structure is interpreted as a subduction suture extending the full length of the front and punctuating a major pre grenvillian archean proterozoic episode of lithospheric assembly in the southeast canadian shield the low velocity mantle corridor by contrast is better explained as the extension of the monteregian white mountain new england seamount hotspot track below the craton and is here postulated to represent interaction of the great meteor plume with zones of weakness within the craton developed during earlier rifting episodes

petrogenesis of the flood basalts from the early permian panjal traps kashmir india geochemical evidence for shallow melting of the mantle article i nfo the early permian panjal traps of northern india represent a significant eruption of volcanic rocks which occurred during the opening of the neotethys ocean basaltic basaltic andesites dacitic and rhyolitic rocks collected from guryal ravine and pahalgam show evidence for subaerial and subaqueous eruptions indicating that they are contemporaneous with the formation of a shallow marine basin the major and trace element geochemistry of the basalts is consistent with a within plate setting and there are basalts which have high ti tio2 n 2 0 wt and low ti tio2 b 1 8 wt compositions the high ti basalts are similar to oib whereas the low ti basalts are similar to continental tholeiites the identification of high and low ti basalts within the panjal traps is analogous to other large igneous provinces e g karoo deccan parana emeishan the sr nd isotopic values end t 5 3 to 1 3 isr 0 70432 to 0 71168 of both types of basalts overlap indicating that the rocks may have originated from the same ancient subcontinental lithospheric i e emii like mantle source tdm 2000 ma the two groups of basalts can be modeled by using a primitive mantle source and different degrees of partial melting where the high tirocksare producedby 1 partial melting of a spinel peridotite source where as the low ti rocks are produced by 8 partial melting trace elemental and isotope modeling indicatesthat some of the basalts assimilated 10 crustal material in contrast the basaltic andesites are likely formed by mixing be tween basaltic magmas and crustal melts which produced rocks with higher sio2 55 wt content and enriched isotopic signatures end t 6 1 isr 0 70992 the panjal trap volcanism was likely due to partial melting of the sclm within a passive extensional setting related to the rifting of cimmeria from gondwana contemporaneous volcanic and plutonic granitic rocks throughout the himalaya are probably not petrogenetically related but are likely part of the same regional tectonic regime

heat flow and structure of the lithosphere in the eastern canadian shield twenty two new determinations of heat flow and radiogenic heat production in the superior and grenville provinces of the canadian shield are presented the new data and previously published measurements strongly constrain the thermal structure of the eastern canadian shield in the abitibi greenstone belt heat flow gradually increases from 29 mw m 2 near the grenville front to 44 mw m 2 east of the kapuskasing uplift this heat flow variation is interpreted in terms of crustal thickening and increased thickness of a tonalitic layer with average heat production of about 1 1 w m 3 this interpretation based on estimated heat production of major rock types in the region is consistent with crustal models derived from recent seismic reflection and refraction studies it also leads to an estimate of about 12 mw m 2 for the mantle heat flow beneath the area the average heat flow in the grenville province 41 10 mw m 2 is the same as that of the superior province this similarity and the lack of significant variation of heat flow across the grenville front indicate that the crust on both sides of the front has similar heat production and thus composition in the western part of the grenville province heat flow reaches high values in the vicinity of the boundary between the allochtonous polycyclic and monocyclic belts where enriched granitic plutons are found in the crystalline terranes in the central part of the grenville province heat flow and heat production are related to each other the parameters of the linear heat flow heat production relationship qr 30 2 in mw m 2 and d 7 1 1 7 km are close to those of the much younger appalachian province implying that the higher appalachian heat flow is due solely to higher heat production in the upper crust the data provide no evidence for variation of mantle heat flow between the superior grenville and appalachian provinces whose tectonic ages range between 2700 and 400 ma the small value of the mantle heat flow about 12 mw m 2 implies that the depth to the 450 c isotherm which controls the effective elastic thickness of the lithosphere is very sensitive to crustal heat production

a lithospheric instability origin for the cameroon volcanic line the cameroon volcanic line cvl is an enigmatic structure that defies common dynamic models of melt generation and volcanic activity on earth there magma generation and intrusion has been sustained for more than 70 myr over a 1600 km long chain straddling the ocean continent boundary with no detectable spatial age progression the chain is nearly perpendicular to the coastline and terminates in a y shaped structure that has not been affected by absolute plate motions implying that the mantle upwelling that feeds magmatic activity is attached to the continent we propose that this form of volcanism is due to a new type of instability that may develop within the subcontinental lithospheric mantle at the edge of a continent laboratory experiments document how lithosphere beneath a continental block of finite size can become unstable due to cooling from above the instability pattern is made of linear upwellings and downwellings that converge radially towards the center of the continent in an outer region and an array of polygonal cells in a central region the pattern is characterized by branching structures that are reminiscent of the strike and y shaped outline of the cvl the instability develops over long timescales with small rates of upwelling and melting and is attached to the continent by construction downwellings adjacent to upwellings induce compression in the crust which may account for deformation in the benue trough just before the onset of cvl magmatism

devonian alaskan type ultramafic mafic intrusions and silicic igneous rocks along the southern altai orogen implications on the phanerozoic continental growth of the altai orogen of the central asian orogenic belt abstract in this contribution we have carried out an integrated study of the petrography geochronology geochemistry and mineral chemistry for the devonian alaskan type ultramafic mafic complexes and granitoids widely distributed along the southern altai orogen in order to provide a better understanding of the continental growth process and the regional tectonic evolution la icpms zircon u pb dating of three mafic samples one acidic volcanic rock sample and one granitic sample yielded the concordia ages of ca 400 ma olivine chemical compositions indicate these mafic intrusions crystallized from evolved mafic magmas and compositions of the clinopyroxene and hornblende show typical arc affinities whole rock compositions reveal their tholeiitic signature with enriched lrees and liles and intensive depleted hfses nb ta zr and hf leading to low hfse lree ratios nb la 0 1 0 4 isotopically these mafic intrusions have positive zircon e hf t 7 6 to 14 3 mostly 10 and positive whole rock e nd t 0 8 to 5 3 values which suggest that their parental magmas were derived from a time integrated depleted mantle source these features together with the presence of primary water bearing minerals biotite and or hornblende suggest that their primitive magmas were derived from a time integrated depleted lithosphere mantle which were metasomatized by fluids released from subduction slab these findings in combination with coeval voluminous granitoids and arc signature volcanic rocks indicate that the underplating of mafic magmas at the continental arc root leads to the partial melting of the continental crust it is inferred that mafic magmatic underplating in the altai orogenic belt significantly contributed to the phanerozoic vertical continental crust growth in the altai orogen

thermo mechanical controls on intra plate deformation and the role of plume folding interactions in continental topography thermo tectonic age and inherited structure exert the main controls on the bulk strength of the lithosphere in intraplate settings mechanical decoupling within the lithosphere strongly affects the interaction between deep earth and surface processes thermo mechanical models demonstrate the particular importance of the rheological stratification of the lithosphere in the preservation of ancient cratonic blocks in the surface expression of plume and mantle lithosphere interactions and their impact on the dynamic topography in general the same is true for the effect of large scale lithospheric folding on intraplate basin formation and associated differential vertical motions initiation of continental lithosphere subduction crucial for linking orogenic deformation to intraplate deformation appears to be facilitated by plume lithosphere interactions we present a discussion of these aspects focusing on better process understanding of continental deformation in the context of a number of well documented cases of intraplate deformation

a new inference of mantle viscosity based upon joint inversion of convection and glacial isostatic adjustment data we present new profiles of mantle viscosity derived on the basis of non linear occam style joint inversions of an extensive set of data associated with mantle convection and glacial isostatic adjustment gia the convection related observables include satellite derived free air gravity harmonics the geodetically inferred excess ellipticity of the cmb and tectonic plate motions the gia constraints involve two classes of observables previously shown to be relatively insensitive to errors in the late pleistocene ice history the so called fennoscandian relaxation spectrum frs and a set of site specific decay times determined from the postglacial sea level history in hudson bay and sweden the inverted viscosity profiles show a significant three orders of magnitude increase from the upper mantle mean value of f4 10 20 pa s to a high viscosity 10 23 pa s peak at

crystallization conditions of los angeles a basaltic martian meteorite the texture of los angeles stone 1 is dominated by relatively large 0 5 2 0 mm anhedral to subhedral grains of pyroxene and generally subhedral to euhedral shocked plagioclase feldspar maskelynite minor phases include subhedral titanomagnetite and ilmenite fe rich olivine olivine augite dominated symplectites some of which include a si rich phase and some which do not pyrrhotite phosphate s and an impact shock related alkali and silica rich glass closely associated with anhedral to euhedral silica grains observations and model calculations indicate that the initial crystallization of mg rich pigeonitic pyroxenes at 1150 c probably concomitantly with plagioclase was followed by pigeonitic and augitic compositions between 1100 and 1050 c whereas between 1050 and 920 to 905 c pyroxene of single composition crystallized below 920 to 905 c single composition fe rich clinopyroxene exsolved to augite and pigeonite initial appearance of titanomagnetite probably occurred near 990 c and fmq 1 5 whereas at and below 990 c and fmq 1 5 titanomagnetite and single composition fe rich clinopyroxene may have started to react producing ilmenite and olivine however judging from the most common titanomagnetite compositions we infer that most of this reaction likely occurred between 950 and 900 c at fmq 1 0 0 2 and nearly simultaneously with pyroxene exsolution thus producing assemblages of pigeonite titanomagnetite olivine ilmenite and augite we deem this reaction as the most plausible explanation for the formation of the olivine augite dominated symplectites in los angeles but we cannot preclude possible contributions to the symplectites from the shock related alkali and silica rich glass or shocked plagioclase and the breakdown of fe rich pigeonite compositions to olivine augite silica below 900 c reactions between fe ti oxides and silicate minerals in los angeles and other similar basaltic martian meteorites can control the t fo2 equilibration path during cooling which may better explain the relative differences in fo2 among the basaltic martian meteorites

formation of the jinchuan ultramafic intrusion and the world s third largest ni cu sulfide deposit associated with the 825 ma south china mantle plume we report here shrimp u pb geochronological geochemical and nd isotopic data for the jinchuan ultramafic intrusion gansu province china which hosts the world s third largest magmatic ni cu sulfide deposits u pb baddeleyite analyses yield an age of 812 26 ma for the ultramafic intrusion this age is indistinguishable within analytical uncertainties from the u pb zircon ages of 827 8 ma and 828 3 ma for the sulfide bearing ultramafic rocks and the dolerite dykes that cut the ultramafic intrusion respectively these u pb dating results show beyond doubt that the jinchuan ultramafic intrusion and associated ni cu sulfide deposit were formed at 825 ma rather than 1500 ma as has been widely believed the ultramafic rocks exhibit large negative nd t values 8 9 to 12 0 that decrease with increasing la sm suggesting that their parental magmas were derived from a long term enriched lithospheric mantle and experienced crustal contamination mineralogical petrological and geochemical data all indicate that the jinchuan intrusion was generated by melting of the enriched lithospheric mantle heated by an anomalously hot plume the u pb ages of 825 ma for igneous baddeleyites and zircons and 900 880 ma for inherited zircons in the jinchuan mafic ultramafic rocks are comparable with those in the qaidam block and qilian belt the western extension of the qinling belt that was likely derived from northern yangtze craton the jinchuan ni cu sulfide bearing intrusion along with coeval regional plume related mafic dykes and tholeiites and mafic ultramafic complexes with associated v ti and ni cu pge mineralization is interpreted to be genetically related to the 825 ma south china mantle plume

a detailed study of the distribution of heat flow and radioactivity in new hampshire u s a abstract we present a set of 39 new determinations of heat flow and radiogenic heat production for several different geological environments in the state of new hampshire u s a with the extensive data set now available for the state the linear relation of heat flow and heat production appears as a very useful generalization for the study of the heat flow field of a geological province our measurements indicate that the vertical distribution of radiogenic heat production is similar in plutonic and metasedimentary rocks our data are compatible with the linear relationship established earlier by f birch and his co workers in 1968 young granites are markedly enriched in radioactive elements and those which do not outcrop are revealed by anomalies in the general relation of heat flow versus radioactivity heat flow is high for plutons by low elsewhere the mean heat flow through metasedimentary formations is 1 15 cal cm2 s 48 mw m2 a value near the mean heat flow for old cratons the lowest heat flow measured is 0 76 cal cm2 s 32 mw m2 for a unit poor in radioactivity the heat flow field grades smoothly into the low heat flow regions of the canadian shield the new hampshire region is in thermal equilibrium its heat flow is in secular equilibrium with the heat generated by crustal sources and that supplied from the mantle in this area the thermal perturbations due to orogenic events decrease below the detection level in less than 200 275 ma the thickness of the layer which is thermally affected during continent continent collision type orogenies cannot be greater than about 190 km

modelling of igneous fractionation and other processes using pearce diagrams geochemical data can be quantitatively modelled by means of pearce diagrams these are graphs of a z vs b z where a b and z are compositional abundances e g wt sio2 wt mgo and ppm la and z has the additional property of having constant absolute abundance in the terminology of igneous petrology z the common denominator variable could be an incompatible element the numerators a and b may be complex algebraic combinations of elements or even cipw normative abundances the utility of pearce diagrams lies in the fact that slopes of data distributions equal the bulk a b ratio of minerals lost or gained from a suite of cogenetic rocks there is no distortion because these plots correct for data closure terms of the form ai z0 zi where z0 is the abundance in a reference sample remove the scaling to ai caused by the abundance of a particular choice of z subtraction of these terms for different samples e g ai z0 zi aj z0 zj quantifies mineral losses and gains mathematical analysis shows that limited compatibility of the denominator variable is permitted a bulk partition value d of 0 1 introduces an error of only 10 in values of ai z0 zi and 10 in slope angle on pearce diagrams over a crystallization interval of 50 for d 0 01 the error is minimal for a crystallization interval over 90

interpretation of open system petrogenetic processes phase equilibria constraints on magma evolution we have used a computer model traces to simulate low pressure differentiation of natural basaltic magmas in an attempt to investigate the chemical dynamics of open system magmatic processes our results in the form of simulated liquid lines of descent and the calculated equilibrium mineralogy were determined for perfect fractional crystallization fractionation paired with recharge and eruption prf fractionation paired with assimilation afc and fractionation paired with recharge eruption and assimilation fear these simulations were calculated in an attempt to assess the effects of combinations of petrogenetic processes on major and trace element evolution of natural systems and to test techniques that have been used to decipher the relative roles of these processes if the results of prf calculations are interpreted in terms of a mass balance based fractionation model e g bryan et al 1969 it is possible to generate low residuals even if one assumes that fractional crystallization was the only active process in effect the chemical consequences of recharge are invisible to mass balance models pearce element ratio analyses however can effectively discern the effects of prf versus simple fractionation the fractionating mineral proportions and therefore bulk distribution coefficients d of a differentiating system are dependent on the recharge or assimilation rate comparison of the results of simulations assuming constant d with the results calculated by traces show that the steady state liquid concentrations of some elements can differ by a factor of 2 to 5 if the prf simulation is periodic with episodes of mixing separated by intervals of fractionation parallel liquidus mineral control lines are produced most of these control lines do not project back to the parental composition this must be an important consideration when attempting to calculate a potential parental magma for any natural suite where magma chamber recharge has occurred most basaltic magmas cannot evolve to high silica compositions without magnetite fractionation small amounts of rhyolite assimilation assimilation fractionation 0 1 however can drive evolving basalts to more silica rich compositions if mass balance models are used to interpret these synthetic afc data low residuals are obtained if magnetite is added to the crystallizing assemblage this approach works even for cases where magnetite was not a fractionating phase thus the mass balance results are mathematically correct but are geologically irrelevant

early to middle miocene intra continental basaltic volcanism in the northern part of the arabian plate se anatolia turkey geochemistry and petrogenesis continental basalts ranging in age from 16 5 to 19 08 ma crop out throughout the northern part of the arabian plate the basalts have distinctive petrographic characteristics such as rounded and skeletal olivine phenocrysts with abundant melt inclusions implying the mixing of two distinct magmas all of the analysed basalts are tholeiitic in composition the presence of quartz xenocrysts with clinopyroxene rims in some samples indicates that crustal assimilation was probably an important process during magma ascent to the surface and low mg number and high sio 2 contents of the basalts clearly show that they have experienced fractional crystallization as well as crustal contamination variations of the major and trace elements versus mgo show that olivine clinopyroxene plagioclase were the main fractionating minerals in terms of incompatible trace elements the basalts have oib like signatures with a slight depletion at nb ta on primitive mantle normalized diagrams the basalts have slightly lree enriched patterns with la yb n 5 5 to 6 7 la nb ratios are close to unity suggesting the melts may have originated in the asthenospheric mantle partial melting modelling based on ree data imply that the melts were not produced from a single mantle source depth which is either purely a spinel or garnet peridotite end member the samples lie on a binary mixing line between low degree melts 10 from spinel peridotite sources on a plot of la yb v dy yb requiring interaction of melts derived from both garnet and spinel peridotite fields melts originating from both sources were initially tapped by distinct magma chambers which subsequently hybridized into a single flow hybridized magma ascended to the surface along neogene strike slip faults which are linked to the dead sea fault zone

crustal geochemistry in the wawa foleyet region ontario fifty three rock samples from the kapuskasing structural zone ksz and 56 from the wawa domal gneiss terrane wgt ontario have been analysed for major elements 32 trace elements and 18o d was measured in a few samples average chemical compositions for the ksz and wgt regions have been calculated from map unit averages weighted by regional abundance compared with estimates of the composition of the upper continental crust ucc the ksz is enriched in al fe mg ca p transition elements sc and sr depleted in si b most rare earth elements zr hf nb ta li na k u th ba and rb but similar in composition to estimates of the lower continental crust the wgt is closer in composition to the ucc these data support the interpretation on geophysical and petrological grounds that the crust here is layered and has been uplifted the wgt and ksz regions representing progressively deeper levels igneous rock and orthogneiss 18o values in the ksz and wgt show good correlation with the weigh

major element evolution of basaltic magmas a comparison of the information in cmas and alfe projections basalt petrologists disagree as to whether the commonly used projection forsterite diopside silica in the system cmas cao mgo al2o3 sio2 can adequately resolve differences in basaltic glass compositions for purposes of petrogenetic modelling here we suggest than an analogous plot the aluminium factor diagram alfe of nesbitt and cramer 1981 has greater diagnostic value than fo di sil a plot of molar al2o2 cao na2o k2o feo mgo vs feo feo mgo it produces more coherent patterns both for experimental basalt glasses and for natural lavas it is like fo di sil a projection through plagioclase but has the advantage that it monitors changes in fe fe mg in melts and associated crystalline phases and is particularly useful in assessing the timing of clinopyroxene crystallization in a suite of lavas the diagram owes its greater resolving power to the fact that the computation of its coordinates is less sensitive to analytical uncertainty than for fo di sil in the latter diagram normative quartz is calculated as a residual and thus manifests the uncertainties in all the major elements

radiogenic heat production in the continental crust abstract the thermal structure and evolution of continents depend strongly on the amount and distribution of radioactive heat sources in the crust determining the contribution of crustal rocks beneath a superficial layer is a major challenge because heat production depends weakly on major element composition and physical properties such as seismic wavespeed and density enriched granitic intrusives that lie at the current erosion level have a large impact on the surface heat flux but little influence on temperatures in the deep crust many lower crustal rocks that are poor in heat producing elements are restites from ancient orogenic events implying that enrichment of the upper crust was achieved at the expense of deeper crustal levels for the same total heat production concentrating heat sources in an upper layer acts to reduce temperatures in the lower crust thereby allowing stabilization of the crust the present day structure of the crust is a consequence of orogeny and should not be adopted for thermal models of the orogenic event itself this review summarizes information extracted from large data sets on heat flow and heat production and provides estimates of crustal stratification and heat production in several geological provinces analysis of global and regional data sets reveals the absence of a positive correlation between surface heat flow and crustal thickness showing that the average crustal heat production is not constant differences of heat flow between geological provinces are due in large part to changes of crustal structure and bulk composition collating values of the bulk crustal heat production in a few age intervals reveals a clear trend of decrease with increasing age this trend can be accounted for by radioactive decay indicating that thermal conditions at the time of crustal stabilization have not changed significantly for the average crustal thickness of 40 km moho temperatures are near solidus values at the time of stabilization suggesting an intrinsic thermal control on crustal thickness and heat production distribution crustal thickening by more than about 10 km above this mean value induces changes of gravitational potential energy that exceed the strength of the lithosphere for several provinces where strong constraints on heat production are available it is shown that prior to intracrustal fractionation only modest amounts of thickening were needed to generate the conditions of ultra high temperature metamorphism the tell tale signature of crustal heat production is anatectic and metamorphic events that lag the cessation of orogenic activity by several tens of million years crustal heat production is decreasing with time because it is due to the radioactive decay of uranium thorium and potassium its rundown is responsible for the secular cooling of lithospheric roots at a typical rate of about 100 k gy 1 implying complex thermal interactions with a convecting mantle that is not cooling at the same rate

sm nd isotopic evidence for crustal contamination in the ca 1750 ma wanapitei complex western grenville province ontario the wanapitei complex consists of a variably metamorphosed gabbronorite lying immediately adjacent to the grenville front boundary fault u pb zircon geochronology indicates a crystallization age of for a noritic component with both grenville aged ca 1000 ma metamorphism and minor older inheritance indicated geochemical evidence is consistent with plagioclase pyroxene fractionation but indicates additional open system behaviour data indicate contamination of a depleted mantle by light rare earth element enriched material during the penokean this was followed during emplacement by extensive contamination of the then isotopically near chondritic magma with variable amounts of evolved archean crustal material on the order of 40 in extreme cases generating end 1 75 values between 0 and 7 5

heat flow scaling for mantle convection below a conducting lid resolving seemingly inconsistent modeling results regarding continental heat flow numerical models of mantle convection below a conducting lid meant to mimic a continent have been used 1 to argue that heat flow variations in stable continental regions result principally from lithospheric thickness variations and 2 to argue that the relationship between lithospheric thickness and continental heat flow is too weak to allow for this we reconcile these results using a theoretical heat flow scaling which shows that the relationship between heat flow and lid thickness can take on two end member forms as the ratio of lid thickness to convecting layer depth increases the effects of lid thickness variations on heat flow move from weak to strong modeling studies that concluded that lithospheric thickness variations could lead to significant continental heat flow variations assumed a convecting layer depth appropriate to upper mantle convection and thus maximized the relative thickness of the continental lithosphere i e the conducting lid studies that concluded that the relationship is weak assumed whole mantle convection which placed them on the flatter end of the heat flow versus relative lid thickness curve

deep mantle contributions to the surface dynamics of the north american continent abstract the regional and continental scale dynamics of north america and vicinity are explored using a high resolution model of mantle flow the model is constrained by simultaneously inverting global seismic and mantle convection data sets and it includes an explicit treatment of the positive chemical buoyancy of the continental tectosphere moreover it adopts a depth dependent mantle viscosity structure which reconciles both glacial isostatic adjustment gia and convection data the flow model successfully reproduces plate velocities and observations of surface gravity and topography including the continent scale quasi linear depression after corrections for gia and crustal heterogeneity extending from northern alaska to venezuela the predictions also match lithospheric flow and stress fields inferred from local and regional measurements of seismic anisotropy and surface deformation we demonstrate that these signals are largely driven by viscous flow coupled to density anomalies in the deep portions of the upper mantle and within the lower mantle where the latter may be associated with the descent of the ancient kula farallon plate system and an active mantle upwelling below the pacific margin of the north american plate more importantly the flow calculations elucidate how these large scale heterogeneities give rise to regional scale flow and stress patterns below the southwestern u s and below the central u s

convection under a lid of finite conductivity heat flux scaling and application to continents 1 a scaling law for the heat flux out of a convective fluid covered totally or partially by a finitely conducting lid is proposed this scaling is constructed in order to quantify the heat transfer out of the earth s mantle taking into account the effect of the dichotomy between oceans and continents which imposes heterogeneous thermal boundary conditions at the surface of the mantle the effect of these heterogeneous boundary conditions is studied here using simple two dimensional models with the mantle represented by an isoviscous fluid heated from below and continents represented by nondeformable lids of finite thermal conductivity set above the surface of the model we use free slip boundary conditions under the oceanic and continental zones in order to study in an isolated way the possible thermal effect of continents independently of all mechanical effect a systematic study of the heat transfer as a function of the rayleigh number of the fluid of the width of the lid and of its thermal properties is carried out we show that estimates of continental lithosphere thickness imply a strong insulating effect from continents on mantle heat loss at least locally the heat flux below continents was low in the past and of the order of the present one if the continental thickness has remained broadly constant over the earth s history

mantle viscosity beneath the hudson bay an inversion based on the metropolis algorithm 1 we revisit the problem of inferring mantle viscosity from postglacial relative sea level rsl data across the hudson bay we invert a recently revised data set using the metropolis algorithm together with an annealing schedule this method which is well established in geophysics is applied here for the first time to the glacial isostatic adjustment problem the metropolis algorithm performs a search which is not limited to downhill moves in the model space and thus is less influenced by local minima of the misfit than traditional inverse approaches furthermore its cpu requirements are far superior to monte carlo methods the major drawbacks include slow convergence and the need for careful tuning of crucial variables such as the temperature schedule and the increment in the model space when all the hudson bay rsl data are considered and the viscosity of the upper mantle above the 670 km discontinuity is inverted the best fitting solution is characterized by a viscosity close to 2 1020 pa s however when the shallow upper mantle and transition zone viscosity are separately inverted other less traditional solutions with a more complex viscosity structure are found to be equally possible a stable feature is the lower mantle viscosity which is generally found to be close to the value of 1021 pa s in all of the stochastic inversions we have performed the solutions agree with previous findings concerning both postglacial rebound observables and global geodynamics signatures

geochemical evolution of earliest carboniferous continental tholeiitic basalts along a crustal scale shear zone southwestern maritimes basin eastern canada abstract continental tholeiites were extruded locally within the extensional wrench maritimes basin from about 375 to 330 ma late devonian early carboniferous basalts of the early carboniferous diamond brook formation are up to 1 5 km thick and overlie a succession of predominantly felsic volcanic rocks reaching several kilometres in thickness the volcanic rocks and co genetic plutons occur along a crustal scale strike slip fault that facilitated the transport of relatively primitive magma resulting in basalts with high mg number ni and cr end 4 and relatively low tio2 v y and zr fractional crystallization of plagioclase and lesser clinopyroxene in the absence of iron oxide fractionation resulted in repetitive extrusion cycles hundreds of metres in thickness that show an upward increase in plagioclase phenocrysts or megacrysts and in tio2 and other high field strength elements hfses rare andesites are interpreted as products of partial melting of underplated gabbro the relatively primitive basalts resemble low ti basalts found in other continental flood basalt provinces and have trace element and isotopic characteristics that have been ascribed in the literature either to partial melting of or contamination of asthenospheric melts by subcontinental lithospheric mantle in the maritimes basin similar continental tholeiites were extruded intermittently over a 45 ma period high feot 12 normalised to 8 mgo suggests high potential temperature in the source mantle presumably within a mantle plume the youngest eruption products geochemically and isotopically resemble himu oib plume activity may have gradually migrated eastward from late devonian to permian from the maritimes basin through scotland to norway and north germany

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

grain size sensitive seismic wave attenuation in polycrystalline olivine 1 in order to investigate the processes responsible for the attenuation of seismic shear waves in the earth s upper mantle four olivine polycrystals ranging in mean grain size d from 3 to 23 m have been fabricated characterized and mechanically tested in torsion at high temperatures and seismic frequencies both the shear modulus which governs the shear wave speed vs and the dissipation of shear strain energy q 1 have been measured as functions of oscillation period to temperature t and for the first time grain size at sufficiently high t all four specimens display similar absorption band viscoelastic behavior adequately represented for 1000 t 1200 or 1300 c and 1 to 100 s by the expression q 1 a tod 1 exp e rt with a 7 5 102 s m 0 26 and e 424 kj mol 1 this mildly grain size sensitive viscoelastic behavior of melt free polycrystalline olivine is attributed to a combination of elastically and diffusionally accommodated grain boundary sliding the latter becoming progressively more important with increasing t and or to extrapolation to the larger mm cm grain sizes expected in the earth s upper mantle yields levels of dissipation comparable with those observed seismologically implying that the same grain size sensitive processes might be responsible for much of the observed seismic wave attenuation the temperature sensitivity of vs is increased substantially by the viscoelastic relaxation allowing the lateral variability of wave speeds to be associated with relatively small temperature perturbations

thermal diffusivity of igneous rocks at elevated pressure and temperature thermal diffusivity measurements of seven igneous rocks were made to temperatures of 400 sup 0 c and pressures of 200 mpa the measuring method was based on the concept of cylindrical symmetry and periodic heat pulses the seven rocks measured were westerly rhode island granite climax stock nevada quartz monzonite pomona washington basalt atikokan ontario canada granite creighton ontario canada gabbro east bull lake ontario canada gabbro and stripa sweden granite the diffusivity of all the rocks showed a positive linear dependence on inverse temperature and excluding the east bull lake gabbro showed a linear dependence on quartz content quartz content varied from 0 to 31 by volume diffusivity in all cases rose or remained steady with increasing confining pressure the pressure effect was strongest at lowest pressures and vanished by levels between 10 and 100 mpa depending on rock type the pressure effect measured as a percentage change in diffusivity is stronger in the four rocks of granite composition than in the three of basaltic composition our results agree well with existing thermal diffusivity measurements at atmospheric pressure

seismic imaging of thickened lithosphere resulting from plume pulsing beneath iceland ocean plates conductively cool and subside with seafloor age plate thickening with age is also predicted and hot spots may cause thinning however both are debated and depend on the way the plate is defined determining the thickness of the plates along with the process that governs it has proven challenging we use s to p sp receiver functions to image a strong persistent lab beneath iceland where the mid atlantic ridge interacts with a plume with hypothesized pulsating thermal anomaly the plate is thickest up to 84 6 km beneath lithosphere formed during times of hypothesized hotter plume temperatures and as thin as 61 6 km beneath regions formed during colder intervals we performed geodynamic modeling to show that these plate thicknesses are inconsistent with a thermal lithosphere instead periods of increased plume temperatures likely increased the melting depth causing deeper depletion and dehydration and creating a thicker plate this suggests plate thickness is dictated by the conditions of plate formation

analysis of petrologic hypotheses with pearce element ratios pearce element ratios can test whether the members of a rock suite are comagmatic and can illustrate the causes of chemical diversity in comagmatic suites comagmatic rocks have constant ratios for elements conserved in the system during changes that led to the chemical diversity in basaltic systems the incompatible elements ti k and p are often conserved the slope of the trend on a pearce element ratio diagram is sensitive to the stoichiometry of the crystallizing and segregating phases a judicious choice of ratios as axes for the diagram provides a signature for the phases involved and estimates of their compositions in basaltic rocks diagrams with ti k vs p k can provide a test of the comagmatic hypothesis diagrams with 0 5 mg fe k vs si k have trends that are distinct for each comagmatic suite and different mineral assemblage different suites are distinguished by the intercepts in diagrams whereas mineral assemblages are recognized by the slopes of the trends for example if olivine is the sole crystallizing and segregating phase the trend will have a slope of 1 diagrams with 2ca na k vs al k distinguish plagioclase from augite assemblages and in conjunction with 0 5 mg fe k diagrams unravel the crystallization sequences of suites that have suffered three phase crystallization and segregation analyses from the uwekahuna laccolith kilauea the 1955 and 1967 68 eruptions of kilauea diamond craters volcanic field oregon and experimental data on morb glasses provide illustrations of the interpretations that can be obtained from pearce element ratios

origin of differentiated lavas at kilauea volcano hawaii implications from the 1955 eruption kilauea s 1955 eruption was the first major eruption longer than 2 days on its east rift zone in 115 years it lasted 88 days during which 108 106 m3 of lava was erupted along a discontinuous 15 km long system of fissures a wide compositional range of lavas was erupted including the most differentiated lavas 5 0 wt mgo from a historic kilauea eruption lavas from the first half of the eruption are strongly differentiated 5 0 5 7 wt mgo later lavas are weakly to moderately differentiated 6 2 6 7 wt mgo previous studies using only major element compositions invoked either crystal fractionation macdonald and eaton 1964 or magma mixing wright and fiske 1971 as models to explain the wide compositional variation in the lavas to further evaluate these models detailed petrographic mineralogical and whole rock major and trace element xrf analyses were made of the 1955 lavas plagioclase and clinopyroxene in the early and late lavas show no petrographic evidence for magma mixing olivines from both the early and late lavas show minor resorption which is typical of tholeiitic lavas with low mgo contents core to rim microprobe analyses across olivine augite and plagioclase mineral grains give no evidence of disequilibrium features related to mixing instead plots of an ab vs distance from the core d and fo vs d 4 5 generated essentially linear trends indicative of simple crystal fractionation least squares mass balance calculations for major and trace element data using observed mineral compositions yield excellent results for crystal fractionation sum of residuals squared 0 01 for major elements and 5 for trace elements magma mixing produced less satisfactory results especially for cr furthermore trace element plots of zr vs sr cr and a12o3 generate curved trends indicative of crystal fractionation processes there is no evidence that mixing occurred in the 1955 lavas instead the data are best explained by crystal fractionation involving a reservoir that extends at least 15 km along kilauea s east rift zone a dike was intruded into the rift zone from the summit reservoir eight days after the eruption started instead of causing magma mixing the dike probably acted as a hydraulic plunger forcing more of the stored magma to be erupted

geochemistry and petrogenesis of early carboniferous volcanic rocks in east junggar north xinjiang implications for post collisional magmatism and geodynamic process abstract early carboniferous volcanic rocks in the batamayineishan formation overlie unconformably the molasse deposits and the ophiolitic melanges and are restricted in narrow zones along both sides of the kalamaili orogenic belt in north xinjiang southern central asian orogenic belt these rocks demonstrate the post collisional setting in east junggar commenced in tournaisian and also mark an important transitional period from the final amalgamation to late paleozoic voluminous juvenile granitoids in east junggar the volcanic rocks are composed of basalt basaltic andesite andesite trachyte and rhyolite both mafic and felsic rocks are characterized by enrichments in large ion lithophile elements light rare earth elements and depletion in nb and ta low initial 87sr 86sr and high positive nd t three groups of mafic rocks have been identified shoshonitic group 1 has the highest mgo cao ni and cr and the lowest na2o al2o3 la ba la yb and ba th with primary magma features group 2 calc alkaline and high k calc alkaline mafic rocks have the lowest k2o p2o5 th and th nb and the highest tio2 and group 3 shoshonitic to potassic alkaline has the highest k2o p2o5 la ba la yb and th nb and the lowest tio2 the a type like felsic rocks were derived from the differentiation of the mafic magma geological and geochemical evidences indicate that the batamayineishan formation was generated from the process of slab breakoff detachment group 1 samples are produced by decompressional melting of the upwelling asthenosphere mainly composed of spinel and garnet 50 50 lherzolite which has been enriched by overlying metasomatized lithosphere during ascent group 2 is derived from 5 10 partial melting of shallower spinel bearing lithospheric mantle induced by the hot rising asthenosphere where the contribution of slab derived fluid is predominant low partial melting 3 5 of the mantle wedge and or thickened lithospheric mantle enriched by slab derived components generates group 3 slab breakoff as an important geodynamic process accounts for the post collisional magmatism between 343 5 ma 330 ma providing a model for post collisional crust mantle interaction in the caob